Post on 19-Jun-2018
1
Chapter 1
Existing information about transition metal
dichalcogenides (TMDCs)
2
11 INTRODUCTION
Transition metal dichalcogenides characteristically contain layered crystal
structures Number of researchers have fascinated by the motivating properties of the
compounds of this family For the last few decade scientists are beginning to turn to
transition metal dichalcogenides shortly known as TMDCs As their name suggests these
are made up of transition metals such as molybdenum tungsten or niobium linked with
chalcogens such as sulfur selenium etc They comprise a layer of transition metal atoms
sandwiched between two layers of chalcogen atoms However the atoms in these layers
are strongly held together by covalent bonds whereas each layer sheet is only associated
to its neighbouring layer by weak van der Waals bonds allowing individual sheets to be
separated from each other [1]
Transition metal dichalcogenides (TMDCs) are layered materials with strong in plane
bonding and weak out of plane interactions permits two dimensional layers of single unit
cell thickness Although TMDCs have been studied for decades recent advances in these
materials characterization and device fabrication have opened up new opportunities for
two dimensional layers of thin TMDCs in electronics and optoelectronics TMDCs such
as MoS2 MoSe2 WS2 and WSe2 and some mix compounds of these materials have
valuable band gaps that change from indirect to direct in single layers allowing
applications such as transistors photo detectors and electroluminescent devices [1] The
historical developments of TMDCs methods for preparing atomically thin layers their
electronic and optical properties have been reviewed
The compounds of Transition metal dichalcogenides group can be represented by
the formula of the type MX2 (where M is the transition metal group VIB and X2 is the
chalcogen element such as Se S Te etc) TMDCs with various characters of metal
semiconductor and magnetic substances have been studied widely These materials are
considered structurally as strongly bonded two dimensional X-M-X layers loosely
coupled to one another by relatively weak Van der Waals type forces [1]
3
In X-M-X sandwich layer the metal atom (M) is bonded to six nearest neighbour
atoms (X) Figure 11 shows the schematic diagram of two dimensional model of
transition metal dichalcogenides
Fig11 Schematic model of atomic layers of TMDCs
The optical and electrical properties of TMDCs have been investigated by several
researchers The structural and bonding properties of transition metal have become an
important field in recent solid state research It is well known that the electronic structure
of transition metal dichalcogenides is characterized by two types of states First there is a
strong interaction between the outer sp orbitalrsquos of the metal and outer p and s chalcogen
orbital The electronic states resulting from this interaction form a broad bonding and a
broad anti bonding bond commonly referred to as the valence and the conduction band
Secondly there is a much weaker interaction between the outer d orbitalrsquos of the
transition metal and the outer p chalcogen orbital [2]
4
Ultrathin two-dimensional layered transition metal dichalcogenides (TMDCs) are
fundamentally and technologically intriguing They are found to be chemically versatile
Multi layered TMDCs are direct band gap semiconductors whose band gap energy as
well as carrier type (n- or p-type) varies between compounds depending on their
composition structure and dimensionality They have been investigated as chemically
active electro catalysts for hydrogen evolution and hydrosulfurization as well as
electrically active materials in opto-electronics devices Their morphologies and
properties are also useful for energy storage applications such as electrodes for Li-ion
batteries and super capacitors
Structure of these transition metal dichalcogenides can be described as solid
containing molecules which are in two dimensions extends to infinity and which are
loosely staked on top of each other to form three-dimensional crystals Several layered
materials have promising semiconducting properties and have attracted attention as a new
class of solar cell materials Very important optical energy electrical energy and chemical
energy conversion efficiencies have been obtained in photovoltaic and
photoelectrochemical solar cells The potential of this group of materials has not been
fully discovered yet It appears to be limited mainly by the availability of appropriate
materials Attempts have been made to produce good quality crystals and thin films of the
layered transition metal dichalcogenides for photovoltaic and photoelectrochemical solar
cells devices applications Several approaches actively pursued to produce high quality
single crystals and thin films of layered transition metal dichalcogenides The layered
transition metal dichalcogenides exhibit promising properties for quantum solar energy
conversion Few of these properties are listed below
The energy gap of TMDCs is largely fall in the range of 1 to 2 eV and therefore it is
ideal for the solar energy absorption
Due to strong metal dichalcogenides hybridization the width of valance and
conduction band is of considerably high magnitude and because of this the charge
carrier mobility are sufficiently high
5
The absorption coefficients are found to be high for TMDCs materials It largely falls
in the range of 105 cm-1
Therefore solar energy conversion devices produced form TMDCs may be
considered as a bright option to more known solar cells Out of the entire TMDCs family
single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport
technique are chosen for the present investigation The advantages of crystal growth by
direct vapour transport technique are discussed in detail in chapter 2 The elemental
information about the material molybdenum (Mo) tungsten (W) and chalcogen element
selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2
single crystals are shown in Table 11
Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)
Parameters Mo W Se
Atomic Number 42 74 34
Atomic Weight (amu) 9595 18384 7897
Group 6 6 16
Density(kg m3) 10280 19250 4810
Melting point (K) 2896 3695 494
Boiling point (K) 4912 6203 958
Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High
Thermal conductivity(WmK) 138 173 20
Molar specific heat (JmolK) 2406 2427 2536
Heat of fusion (kJmol) 3748 353 669
Heat of vaporization(kJmol) 598 774 9548
Covalence radius (pm) 130 139 120
6
12 MOLYBDENUM (Mo)
Molybdenum is a Group VI chemical element with the symbol Mo and atomic
number 42 Mo display body centered cubic structure at room temperature There is no
confirmation for face changes up to 280 Gpa in experiments It is likely that the
solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard
steady carbides in alloys and due to this reason most of world production of the element
is in making many kinds of steel alloys including high strength alloys and super alloys
[3]
Fig 12 The solid state structure of molybdenum
The majority of molybdenum compounds have low solubility in water Molybdenum
enclosing enzymes are by far the most general catalysts used by some bacteria to break
the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen
fixation Owing to the diverse functions of the various auxiliary types of molybdenum
enzymes molybdenum is a required element for life in all higher organisms in the
majority of the bacteria [3]
7
121 Application of Molybdenum
Molybdenum is used in steel alloys for its high corrosion resistance and weld ability
The ability of molybdenum to withstand extreme temperatures without significantly
expanding or softening makes it useful in applications that involve intense heat including
the manufacture of armor aircraft parts electrical contacts industrial motors and
filaments Approximately all the high strength steel enclose Mo in amounts from 025
to 8 Molybdenum improves the strength of steel at high temperatures It is used as
electrodes in electrically heated glass furnaces It is also employed in nuclear energy
applications as well as for missiles and air craft applications It is a valuable catalyst in
petroleum refining [4]
122 Electronic Configuration of Molybdenum
The following symbolize the electronic arrangement for the ground state neutral
gaseous atom of molybdenum The pattern associated with molybdenum in its compounds
is not necessarily identical
Ground state electron configuration [Kr] 5s1 4d5
Shell structure 2818131
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
8
13 TUNGSTEN
Tungsten is a chemical element with the chemical symbol W and atomic number 74
Tungsten is a hard and rare metal under standard conditions and found naturally on Earth
only in chemical compounds Tungsten exists in two major crystalline forms which are α
and β The former has a body centered cubic structure and is the most stable form The
structure of the β phase is called A15 cubic Tungsten has the highest melting point
(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile
strength among all the elements The density of tungsten is 193 times than that of water
and about 17 times than that of lead Tungsten has the lowest coefficient of thermal
expansion The low thermal expansion and high melting point and tensile strength of
tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d
electrons [5]
Fig 13 The solid state structure of tungsten
9
131 Application of Tungsten
Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens
alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both
the filament and target) electrodes in TIG welding super alloys and radiation shielding
Due to hardness and high density tungsten is widely used in military field Tungsten
compounds are also often used as industrial catalysts Tungsten alloys are sometimes used
in low temperature superconducting circuits Tungsten with some percentage of
chalcogen elements found to have semiconducting nature Semiconductor crystal of
tungsten dichalcogenides are used in PEC solar cell [5]
132 Electronic Configuration of Tungsten
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of tungsten The pattern associated with tungsten in its compounds is not
necessarily identical
Ground state electron configuration [Xe] 4f14 5d4 6s2
Shell structure 2 8 18 32 12 2
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
2
11 INTRODUCTION
Transition metal dichalcogenides characteristically contain layered crystal
structures Number of researchers have fascinated by the motivating properties of the
compounds of this family For the last few decade scientists are beginning to turn to
transition metal dichalcogenides shortly known as TMDCs As their name suggests these
are made up of transition metals such as molybdenum tungsten or niobium linked with
chalcogens such as sulfur selenium etc They comprise a layer of transition metal atoms
sandwiched between two layers of chalcogen atoms However the atoms in these layers
are strongly held together by covalent bonds whereas each layer sheet is only associated
to its neighbouring layer by weak van der Waals bonds allowing individual sheets to be
separated from each other [1]
Transition metal dichalcogenides (TMDCs) are layered materials with strong in plane
bonding and weak out of plane interactions permits two dimensional layers of single unit
cell thickness Although TMDCs have been studied for decades recent advances in these
materials characterization and device fabrication have opened up new opportunities for
two dimensional layers of thin TMDCs in electronics and optoelectronics TMDCs such
as MoS2 MoSe2 WS2 and WSe2 and some mix compounds of these materials have
valuable band gaps that change from indirect to direct in single layers allowing
applications such as transistors photo detectors and electroluminescent devices [1] The
historical developments of TMDCs methods for preparing atomically thin layers their
electronic and optical properties have been reviewed
The compounds of Transition metal dichalcogenides group can be represented by
the formula of the type MX2 (where M is the transition metal group VIB and X2 is the
chalcogen element such as Se S Te etc) TMDCs with various characters of metal
semiconductor and magnetic substances have been studied widely These materials are
considered structurally as strongly bonded two dimensional X-M-X layers loosely
coupled to one another by relatively weak Van der Waals type forces [1]
3
In X-M-X sandwich layer the metal atom (M) is bonded to six nearest neighbour
atoms (X) Figure 11 shows the schematic diagram of two dimensional model of
transition metal dichalcogenides
Fig11 Schematic model of atomic layers of TMDCs
The optical and electrical properties of TMDCs have been investigated by several
researchers The structural and bonding properties of transition metal have become an
important field in recent solid state research It is well known that the electronic structure
of transition metal dichalcogenides is characterized by two types of states First there is a
strong interaction between the outer sp orbitalrsquos of the metal and outer p and s chalcogen
orbital The electronic states resulting from this interaction form a broad bonding and a
broad anti bonding bond commonly referred to as the valence and the conduction band
Secondly there is a much weaker interaction between the outer d orbitalrsquos of the
transition metal and the outer p chalcogen orbital [2]
4
Ultrathin two-dimensional layered transition metal dichalcogenides (TMDCs) are
fundamentally and technologically intriguing They are found to be chemically versatile
Multi layered TMDCs are direct band gap semiconductors whose band gap energy as
well as carrier type (n- or p-type) varies between compounds depending on their
composition structure and dimensionality They have been investigated as chemically
active electro catalysts for hydrogen evolution and hydrosulfurization as well as
electrically active materials in opto-electronics devices Their morphologies and
properties are also useful for energy storage applications such as electrodes for Li-ion
batteries and super capacitors
Structure of these transition metal dichalcogenides can be described as solid
containing molecules which are in two dimensions extends to infinity and which are
loosely staked on top of each other to form three-dimensional crystals Several layered
materials have promising semiconducting properties and have attracted attention as a new
class of solar cell materials Very important optical energy electrical energy and chemical
energy conversion efficiencies have been obtained in photovoltaic and
photoelectrochemical solar cells The potential of this group of materials has not been
fully discovered yet It appears to be limited mainly by the availability of appropriate
materials Attempts have been made to produce good quality crystals and thin films of the
layered transition metal dichalcogenides for photovoltaic and photoelectrochemical solar
cells devices applications Several approaches actively pursued to produce high quality
single crystals and thin films of layered transition metal dichalcogenides The layered
transition metal dichalcogenides exhibit promising properties for quantum solar energy
conversion Few of these properties are listed below
The energy gap of TMDCs is largely fall in the range of 1 to 2 eV and therefore it is
ideal for the solar energy absorption
Due to strong metal dichalcogenides hybridization the width of valance and
conduction band is of considerably high magnitude and because of this the charge
carrier mobility are sufficiently high
5
The absorption coefficients are found to be high for TMDCs materials It largely falls
in the range of 105 cm-1
Therefore solar energy conversion devices produced form TMDCs may be
considered as a bright option to more known solar cells Out of the entire TMDCs family
single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport
technique are chosen for the present investigation The advantages of crystal growth by
direct vapour transport technique are discussed in detail in chapter 2 The elemental
information about the material molybdenum (Mo) tungsten (W) and chalcogen element
selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2
single crystals are shown in Table 11
Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)
Parameters Mo W Se
Atomic Number 42 74 34
Atomic Weight (amu) 9595 18384 7897
Group 6 6 16
Density(kg m3) 10280 19250 4810
Melting point (K) 2896 3695 494
Boiling point (K) 4912 6203 958
Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High
Thermal conductivity(WmK) 138 173 20
Molar specific heat (JmolK) 2406 2427 2536
Heat of fusion (kJmol) 3748 353 669
Heat of vaporization(kJmol) 598 774 9548
Covalence radius (pm) 130 139 120
6
12 MOLYBDENUM (Mo)
Molybdenum is a Group VI chemical element with the symbol Mo and atomic
number 42 Mo display body centered cubic structure at room temperature There is no
confirmation for face changes up to 280 Gpa in experiments It is likely that the
solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard
steady carbides in alloys and due to this reason most of world production of the element
is in making many kinds of steel alloys including high strength alloys and super alloys
[3]
Fig 12 The solid state structure of molybdenum
The majority of molybdenum compounds have low solubility in water Molybdenum
enclosing enzymes are by far the most general catalysts used by some bacteria to break
the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen
fixation Owing to the diverse functions of the various auxiliary types of molybdenum
enzymes molybdenum is a required element for life in all higher organisms in the
majority of the bacteria [3]
7
121 Application of Molybdenum
Molybdenum is used in steel alloys for its high corrosion resistance and weld ability
The ability of molybdenum to withstand extreme temperatures without significantly
expanding or softening makes it useful in applications that involve intense heat including
the manufacture of armor aircraft parts electrical contacts industrial motors and
filaments Approximately all the high strength steel enclose Mo in amounts from 025
to 8 Molybdenum improves the strength of steel at high temperatures It is used as
electrodes in electrically heated glass furnaces It is also employed in nuclear energy
applications as well as for missiles and air craft applications It is a valuable catalyst in
petroleum refining [4]
122 Electronic Configuration of Molybdenum
The following symbolize the electronic arrangement for the ground state neutral
gaseous atom of molybdenum The pattern associated with molybdenum in its compounds
is not necessarily identical
Ground state electron configuration [Kr] 5s1 4d5
Shell structure 2818131
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
8
13 TUNGSTEN
Tungsten is a chemical element with the chemical symbol W and atomic number 74
Tungsten is a hard and rare metal under standard conditions and found naturally on Earth
only in chemical compounds Tungsten exists in two major crystalline forms which are α
and β The former has a body centered cubic structure and is the most stable form The
structure of the β phase is called A15 cubic Tungsten has the highest melting point
(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile
strength among all the elements The density of tungsten is 193 times than that of water
and about 17 times than that of lead Tungsten has the lowest coefficient of thermal
expansion The low thermal expansion and high melting point and tensile strength of
tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d
electrons [5]
Fig 13 The solid state structure of tungsten
9
131 Application of Tungsten
Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens
alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both
the filament and target) electrodes in TIG welding super alloys and radiation shielding
Due to hardness and high density tungsten is widely used in military field Tungsten
compounds are also often used as industrial catalysts Tungsten alloys are sometimes used
in low temperature superconducting circuits Tungsten with some percentage of
chalcogen elements found to have semiconducting nature Semiconductor crystal of
tungsten dichalcogenides are used in PEC solar cell [5]
132 Electronic Configuration of Tungsten
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of tungsten The pattern associated with tungsten in its compounds is not
necessarily identical
Ground state electron configuration [Xe] 4f14 5d4 6s2
Shell structure 2 8 18 32 12 2
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
3
In X-M-X sandwich layer the metal atom (M) is bonded to six nearest neighbour
atoms (X) Figure 11 shows the schematic diagram of two dimensional model of
transition metal dichalcogenides
Fig11 Schematic model of atomic layers of TMDCs
The optical and electrical properties of TMDCs have been investigated by several
researchers The structural and bonding properties of transition metal have become an
important field in recent solid state research It is well known that the electronic structure
of transition metal dichalcogenides is characterized by two types of states First there is a
strong interaction between the outer sp orbitalrsquos of the metal and outer p and s chalcogen
orbital The electronic states resulting from this interaction form a broad bonding and a
broad anti bonding bond commonly referred to as the valence and the conduction band
Secondly there is a much weaker interaction between the outer d orbitalrsquos of the
transition metal and the outer p chalcogen orbital [2]
4
Ultrathin two-dimensional layered transition metal dichalcogenides (TMDCs) are
fundamentally and technologically intriguing They are found to be chemically versatile
Multi layered TMDCs are direct band gap semiconductors whose band gap energy as
well as carrier type (n- or p-type) varies between compounds depending on their
composition structure and dimensionality They have been investigated as chemically
active electro catalysts for hydrogen evolution and hydrosulfurization as well as
electrically active materials in opto-electronics devices Their morphologies and
properties are also useful for energy storage applications such as electrodes for Li-ion
batteries and super capacitors
Structure of these transition metal dichalcogenides can be described as solid
containing molecules which are in two dimensions extends to infinity and which are
loosely staked on top of each other to form three-dimensional crystals Several layered
materials have promising semiconducting properties and have attracted attention as a new
class of solar cell materials Very important optical energy electrical energy and chemical
energy conversion efficiencies have been obtained in photovoltaic and
photoelectrochemical solar cells The potential of this group of materials has not been
fully discovered yet It appears to be limited mainly by the availability of appropriate
materials Attempts have been made to produce good quality crystals and thin films of the
layered transition metal dichalcogenides for photovoltaic and photoelectrochemical solar
cells devices applications Several approaches actively pursued to produce high quality
single crystals and thin films of layered transition metal dichalcogenides The layered
transition metal dichalcogenides exhibit promising properties for quantum solar energy
conversion Few of these properties are listed below
The energy gap of TMDCs is largely fall in the range of 1 to 2 eV and therefore it is
ideal for the solar energy absorption
Due to strong metal dichalcogenides hybridization the width of valance and
conduction band is of considerably high magnitude and because of this the charge
carrier mobility are sufficiently high
5
The absorption coefficients are found to be high for TMDCs materials It largely falls
in the range of 105 cm-1
Therefore solar energy conversion devices produced form TMDCs may be
considered as a bright option to more known solar cells Out of the entire TMDCs family
single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport
technique are chosen for the present investigation The advantages of crystal growth by
direct vapour transport technique are discussed in detail in chapter 2 The elemental
information about the material molybdenum (Mo) tungsten (W) and chalcogen element
selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2
single crystals are shown in Table 11
Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)
Parameters Mo W Se
Atomic Number 42 74 34
Atomic Weight (amu) 9595 18384 7897
Group 6 6 16
Density(kg m3) 10280 19250 4810
Melting point (K) 2896 3695 494
Boiling point (K) 4912 6203 958
Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High
Thermal conductivity(WmK) 138 173 20
Molar specific heat (JmolK) 2406 2427 2536
Heat of fusion (kJmol) 3748 353 669
Heat of vaporization(kJmol) 598 774 9548
Covalence radius (pm) 130 139 120
6
12 MOLYBDENUM (Mo)
Molybdenum is a Group VI chemical element with the symbol Mo and atomic
number 42 Mo display body centered cubic structure at room temperature There is no
confirmation for face changes up to 280 Gpa in experiments It is likely that the
solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard
steady carbides in alloys and due to this reason most of world production of the element
is in making many kinds of steel alloys including high strength alloys and super alloys
[3]
Fig 12 The solid state structure of molybdenum
The majority of molybdenum compounds have low solubility in water Molybdenum
enclosing enzymes are by far the most general catalysts used by some bacteria to break
the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen
fixation Owing to the diverse functions of the various auxiliary types of molybdenum
enzymes molybdenum is a required element for life in all higher organisms in the
majority of the bacteria [3]
7
121 Application of Molybdenum
Molybdenum is used in steel alloys for its high corrosion resistance and weld ability
The ability of molybdenum to withstand extreme temperatures without significantly
expanding or softening makes it useful in applications that involve intense heat including
the manufacture of armor aircraft parts electrical contacts industrial motors and
filaments Approximately all the high strength steel enclose Mo in amounts from 025
to 8 Molybdenum improves the strength of steel at high temperatures It is used as
electrodes in electrically heated glass furnaces It is also employed in nuclear energy
applications as well as for missiles and air craft applications It is a valuable catalyst in
petroleum refining [4]
122 Electronic Configuration of Molybdenum
The following symbolize the electronic arrangement for the ground state neutral
gaseous atom of molybdenum The pattern associated with molybdenum in its compounds
is not necessarily identical
Ground state electron configuration [Kr] 5s1 4d5
Shell structure 2818131
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
8
13 TUNGSTEN
Tungsten is a chemical element with the chemical symbol W and atomic number 74
Tungsten is a hard and rare metal under standard conditions and found naturally on Earth
only in chemical compounds Tungsten exists in two major crystalline forms which are α
and β The former has a body centered cubic structure and is the most stable form The
structure of the β phase is called A15 cubic Tungsten has the highest melting point
(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile
strength among all the elements The density of tungsten is 193 times than that of water
and about 17 times than that of lead Tungsten has the lowest coefficient of thermal
expansion The low thermal expansion and high melting point and tensile strength of
tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d
electrons [5]
Fig 13 The solid state structure of tungsten
9
131 Application of Tungsten
Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens
alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both
the filament and target) electrodes in TIG welding super alloys and radiation shielding
Due to hardness and high density tungsten is widely used in military field Tungsten
compounds are also often used as industrial catalysts Tungsten alloys are sometimes used
in low temperature superconducting circuits Tungsten with some percentage of
chalcogen elements found to have semiconducting nature Semiconductor crystal of
tungsten dichalcogenides are used in PEC solar cell [5]
132 Electronic Configuration of Tungsten
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of tungsten The pattern associated with tungsten in its compounds is not
necessarily identical
Ground state electron configuration [Xe] 4f14 5d4 6s2
Shell structure 2 8 18 32 12 2
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
4
Ultrathin two-dimensional layered transition metal dichalcogenides (TMDCs) are
fundamentally and technologically intriguing They are found to be chemically versatile
Multi layered TMDCs are direct band gap semiconductors whose band gap energy as
well as carrier type (n- or p-type) varies between compounds depending on their
composition structure and dimensionality They have been investigated as chemically
active electro catalysts for hydrogen evolution and hydrosulfurization as well as
electrically active materials in opto-electronics devices Their morphologies and
properties are also useful for energy storage applications such as electrodes for Li-ion
batteries and super capacitors
Structure of these transition metal dichalcogenides can be described as solid
containing molecules which are in two dimensions extends to infinity and which are
loosely staked on top of each other to form three-dimensional crystals Several layered
materials have promising semiconducting properties and have attracted attention as a new
class of solar cell materials Very important optical energy electrical energy and chemical
energy conversion efficiencies have been obtained in photovoltaic and
photoelectrochemical solar cells The potential of this group of materials has not been
fully discovered yet It appears to be limited mainly by the availability of appropriate
materials Attempts have been made to produce good quality crystals and thin films of the
layered transition metal dichalcogenides for photovoltaic and photoelectrochemical solar
cells devices applications Several approaches actively pursued to produce high quality
single crystals and thin films of layered transition metal dichalcogenides The layered
transition metal dichalcogenides exhibit promising properties for quantum solar energy
conversion Few of these properties are listed below
The energy gap of TMDCs is largely fall in the range of 1 to 2 eV and therefore it is
ideal for the solar energy absorption
Due to strong metal dichalcogenides hybridization the width of valance and
conduction band is of considerably high magnitude and because of this the charge
carrier mobility are sufficiently high
5
The absorption coefficients are found to be high for TMDCs materials It largely falls
in the range of 105 cm-1
Therefore solar energy conversion devices produced form TMDCs may be
considered as a bright option to more known solar cells Out of the entire TMDCs family
single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport
technique are chosen for the present investigation The advantages of crystal growth by
direct vapour transport technique are discussed in detail in chapter 2 The elemental
information about the material molybdenum (Mo) tungsten (W) and chalcogen element
selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2
single crystals are shown in Table 11
Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)
Parameters Mo W Se
Atomic Number 42 74 34
Atomic Weight (amu) 9595 18384 7897
Group 6 6 16
Density(kg m3) 10280 19250 4810
Melting point (K) 2896 3695 494
Boiling point (K) 4912 6203 958
Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High
Thermal conductivity(WmK) 138 173 20
Molar specific heat (JmolK) 2406 2427 2536
Heat of fusion (kJmol) 3748 353 669
Heat of vaporization(kJmol) 598 774 9548
Covalence radius (pm) 130 139 120
6
12 MOLYBDENUM (Mo)
Molybdenum is a Group VI chemical element with the symbol Mo and atomic
number 42 Mo display body centered cubic structure at room temperature There is no
confirmation for face changes up to 280 Gpa in experiments It is likely that the
solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard
steady carbides in alloys and due to this reason most of world production of the element
is in making many kinds of steel alloys including high strength alloys and super alloys
[3]
Fig 12 The solid state structure of molybdenum
The majority of molybdenum compounds have low solubility in water Molybdenum
enclosing enzymes are by far the most general catalysts used by some bacteria to break
the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen
fixation Owing to the diverse functions of the various auxiliary types of molybdenum
enzymes molybdenum is a required element for life in all higher organisms in the
majority of the bacteria [3]
7
121 Application of Molybdenum
Molybdenum is used in steel alloys for its high corrosion resistance and weld ability
The ability of molybdenum to withstand extreme temperatures without significantly
expanding or softening makes it useful in applications that involve intense heat including
the manufacture of armor aircraft parts electrical contacts industrial motors and
filaments Approximately all the high strength steel enclose Mo in amounts from 025
to 8 Molybdenum improves the strength of steel at high temperatures It is used as
electrodes in electrically heated glass furnaces It is also employed in nuclear energy
applications as well as for missiles and air craft applications It is a valuable catalyst in
petroleum refining [4]
122 Electronic Configuration of Molybdenum
The following symbolize the electronic arrangement for the ground state neutral
gaseous atom of molybdenum The pattern associated with molybdenum in its compounds
is not necessarily identical
Ground state electron configuration [Kr] 5s1 4d5
Shell structure 2818131
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
8
13 TUNGSTEN
Tungsten is a chemical element with the chemical symbol W and atomic number 74
Tungsten is a hard and rare metal under standard conditions and found naturally on Earth
only in chemical compounds Tungsten exists in two major crystalline forms which are α
and β The former has a body centered cubic structure and is the most stable form The
structure of the β phase is called A15 cubic Tungsten has the highest melting point
(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile
strength among all the elements The density of tungsten is 193 times than that of water
and about 17 times than that of lead Tungsten has the lowest coefficient of thermal
expansion The low thermal expansion and high melting point and tensile strength of
tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d
electrons [5]
Fig 13 The solid state structure of tungsten
9
131 Application of Tungsten
Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens
alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both
the filament and target) electrodes in TIG welding super alloys and radiation shielding
Due to hardness and high density tungsten is widely used in military field Tungsten
compounds are also often used as industrial catalysts Tungsten alloys are sometimes used
in low temperature superconducting circuits Tungsten with some percentage of
chalcogen elements found to have semiconducting nature Semiconductor crystal of
tungsten dichalcogenides are used in PEC solar cell [5]
132 Electronic Configuration of Tungsten
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of tungsten The pattern associated with tungsten in its compounds is not
necessarily identical
Ground state electron configuration [Xe] 4f14 5d4 6s2
Shell structure 2 8 18 32 12 2
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
5
The absorption coefficients are found to be high for TMDCs materials It largely falls
in the range of 105 cm-1
Therefore solar energy conversion devices produced form TMDCs may be
considered as a bright option to more known solar cells Out of the entire TMDCs family
single crystal of W09Se2 WSe2 and MoSe2 compounds grown with direct vapour transport
technique are chosen for the present investigation The advantages of crystal growth by
direct vapour transport technique are discussed in detail in chapter 2 The elemental
information about the material molybdenum (Mo) tungsten (W) and chalcogen element
selenium (Se) used in present investigation for the synthesis of W09Se2 WSe2 and MoSe2
single crystals are shown in Table 11
Table 11 Elemental information of Molybdenum (Mo) Tungsten (W) and Selenium (Se)
Parameters Mo W Se
Atomic Number 42 74 34
Atomic Weight (amu) 9595 18384 7897
Group 6 6 16
Density(kg m3) 10280 19250 4810
Melting point (K) 2896 3695 494
Boiling point (K) 4912 6203 958
Electrical resistivity(times 10-8 Ω cm )at RT 534 528 High
Thermal conductivity(WmK) 138 173 20
Molar specific heat (JmolK) 2406 2427 2536
Heat of fusion (kJmol) 3748 353 669
Heat of vaporization(kJmol) 598 774 9548
Covalence radius (pm) 130 139 120
6
12 MOLYBDENUM (Mo)
Molybdenum is a Group VI chemical element with the symbol Mo and atomic
number 42 Mo display body centered cubic structure at room temperature There is no
confirmation for face changes up to 280 Gpa in experiments It is likely that the
solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard
steady carbides in alloys and due to this reason most of world production of the element
is in making many kinds of steel alloys including high strength alloys and super alloys
[3]
Fig 12 The solid state structure of molybdenum
The majority of molybdenum compounds have low solubility in water Molybdenum
enclosing enzymes are by far the most general catalysts used by some bacteria to break
the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen
fixation Owing to the diverse functions of the various auxiliary types of molybdenum
enzymes molybdenum is a required element for life in all higher organisms in the
majority of the bacteria [3]
7
121 Application of Molybdenum
Molybdenum is used in steel alloys for its high corrosion resistance and weld ability
The ability of molybdenum to withstand extreme temperatures without significantly
expanding or softening makes it useful in applications that involve intense heat including
the manufacture of armor aircraft parts electrical contacts industrial motors and
filaments Approximately all the high strength steel enclose Mo in amounts from 025
to 8 Molybdenum improves the strength of steel at high temperatures It is used as
electrodes in electrically heated glass furnaces It is also employed in nuclear energy
applications as well as for missiles and air craft applications It is a valuable catalyst in
petroleum refining [4]
122 Electronic Configuration of Molybdenum
The following symbolize the electronic arrangement for the ground state neutral
gaseous atom of molybdenum The pattern associated with molybdenum in its compounds
is not necessarily identical
Ground state electron configuration [Kr] 5s1 4d5
Shell structure 2818131
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
8
13 TUNGSTEN
Tungsten is a chemical element with the chemical symbol W and atomic number 74
Tungsten is a hard and rare metal under standard conditions and found naturally on Earth
only in chemical compounds Tungsten exists in two major crystalline forms which are α
and β The former has a body centered cubic structure and is the most stable form The
structure of the β phase is called A15 cubic Tungsten has the highest melting point
(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile
strength among all the elements The density of tungsten is 193 times than that of water
and about 17 times than that of lead Tungsten has the lowest coefficient of thermal
expansion The low thermal expansion and high melting point and tensile strength of
tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d
electrons [5]
Fig 13 The solid state structure of tungsten
9
131 Application of Tungsten
Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens
alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both
the filament and target) electrodes in TIG welding super alloys and radiation shielding
Due to hardness and high density tungsten is widely used in military field Tungsten
compounds are also often used as industrial catalysts Tungsten alloys are sometimes used
in low temperature superconducting circuits Tungsten with some percentage of
chalcogen elements found to have semiconducting nature Semiconductor crystal of
tungsten dichalcogenides are used in PEC solar cell [5]
132 Electronic Configuration of Tungsten
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of tungsten The pattern associated with tungsten in its compounds is not
necessarily identical
Ground state electron configuration [Xe] 4f14 5d4 6s2
Shell structure 2 8 18 32 12 2
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
6
12 MOLYBDENUM (Mo)
Molybdenum is a Group VI chemical element with the symbol Mo and atomic
number 42 Mo display body centered cubic structure at room temperature There is no
confirmation for face changes up to 280 Gpa in experiments It is likely that the
solid-solid phase transition found in shocked Mo is the bcc hcp transition It forms hard
steady carbides in alloys and due to this reason most of world production of the element
is in making many kinds of steel alloys including high strength alloys and super alloys
[3]
Fig 12 The solid state structure of molybdenum
The majority of molybdenum compounds have low solubility in water Molybdenum
enclosing enzymes are by far the most general catalysts used by some bacteria to break
the chemical bond in atmospheric molecular nitrogen allowing biological nitrogen
fixation Owing to the diverse functions of the various auxiliary types of molybdenum
enzymes molybdenum is a required element for life in all higher organisms in the
majority of the bacteria [3]
7
121 Application of Molybdenum
Molybdenum is used in steel alloys for its high corrosion resistance and weld ability
The ability of molybdenum to withstand extreme temperatures without significantly
expanding or softening makes it useful in applications that involve intense heat including
the manufacture of armor aircraft parts electrical contacts industrial motors and
filaments Approximately all the high strength steel enclose Mo in amounts from 025
to 8 Molybdenum improves the strength of steel at high temperatures It is used as
electrodes in electrically heated glass furnaces It is also employed in nuclear energy
applications as well as for missiles and air craft applications It is a valuable catalyst in
petroleum refining [4]
122 Electronic Configuration of Molybdenum
The following symbolize the electronic arrangement for the ground state neutral
gaseous atom of molybdenum The pattern associated with molybdenum in its compounds
is not necessarily identical
Ground state electron configuration [Kr] 5s1 4d5
Shell structure 2818131
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
8
13 TUNGSTEN
Tungsten is a chemical element with the chemical symbol W and atomic number 74
Tungsten is a hard and rare metal under standard conditions and found naturally on Earth
only in chemical compounds Tungsten exists in two major crystalline forms which are α
and β The former has a body centered cubic structure and is the most stable form The
structure of the β phase is called A15 cubic Tungsten has the highest melting point
(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile
strength among all the elements The density of tungsten is 193 times than that of water
and about 17 times than that of lead Tungsten has the lowest coefficient of thermal
expansion The low thermal expansion and high melting point and tensile strength of
tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d
electrons [5]
Fig 13 The solid state structure of tungsten
9
131 Application of Tungsten
Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens
alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both
the filament and target) electrodes in TIG welding super alloys and radiation shielding
Due to hardness and high density tungsten is widely used in military field Tungsten
compounds are also often used as industrial catalysts Tungsten alloys are sometimes used
in low temperature superconducting circuits Tungsten with some percentage of
chalcogen elements found to have semiconducting nature Semiconductor crystal of
tungsten dichalcogenides are used in PEC solar cell [5]
132 Electronic Configuration of Tungsten
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of tungsten The pattern associated with tungsten in its compounds is not
necessarily identical
Ground state electron configuration [Xe] 4f14 5d4 6s2
Shell structure 2 8 18 32 12 2
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
7
121 Application of Molybdenum
Molybdenum is used in steel alloys for its high corrosion resistance and weld ability
The ability of molybdenum to withstand extreme temperatures without significantly
expanding or softening makes it useful in applications that involve intense heat including
the manufacture of armor aircraft parts electrical contacts industrial motors and
filaments Approximately all the high strength steel enclose Mo in amounts from 025
to 8 Molybdenum improves the strength of steel at high temperatures It is used as
electrodes in electrically heated glass furnaces It is also employed in nuclear energy
applications as well as for missiles and air craft applications It is a valuable catalyst in
petroleum refining [4]
122 Electronic Configuration of Molybdenum
The following symbolize the electronic arrangement for the ground state neutral
gaseous atom of molybdenum The pattern associated with molybdenum in its compounds
is not necessarily identical
Ground state electron configuration [Kr] 5s1 4d5
Shell structure 2818131
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
8
13 TUNGSTEN
Tungsten is a chemical element with the chemical symbol W and atomic number 74
Tungsten is a hard and rare metal under standard conditions and found naturally on Earth
only in chemical compounds Tungsten exists in two major crystalline forms which are α
and β The former has a body centered cubic structure and is the most stable form The
structure of the β phase is called A15 cubic Tungsten has the highest melting point
(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile
strength among all the elements The density of tungsten is 193 times than that of water
and about 17 times than that of lead Tungsten has the lowest coefficient of thermal
expansion The low thermal expansion and high melting point and tensile strength of
tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d
electrons [5]
Fig 13 The solid state structure of tungsten
9
131 Application of Tungsten
Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens
alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both
the filament and target) electrodes in TIG welding super alloys and radiation shielding
Due to hardness and high density tungsten is widely used in military field Tungsten
compounds are also often used as industrial catalysts Tungsten alloys are sometimes used
in low temperature superconducting circuits Tungsten with some percentage of
chalcogen elements found to have semiconducting nature Semiconductor crystal of
tungsten dichalcogenides are used in PEC solar cell [5]
132 Electronic Configuration of Tungsten
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of tungsten The pattern associated with tungsten in its compounds is not
necessarily identical
Ground state electron configuration [Xe] 4f14 5d4 6s2
Shell structure 2 8 18 32 12 2
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
8
13 TUNGSTEN
Tungsten is a chemical element with the chemical symbol W and atomic number 74
Tungsten is a hard and rare metal under standard conditions and found naturally on Earth
only in chemical compounds Tungsten exists in two major crystalline forms which are α
and β The former has a body centered cubic structure and is the most stable form The
structure of the β phase is called A15 cubic Tungsten has the highest melting point
(3422 degC) lowest vapor pressure (at temperatures above 1650 degC) and the highest tensile
strength among all the elements The density of tungsten is 193 times than that of water
and about 17 times than that of lead Tungsten has the lowest coefficient of thermal
expansion The low thermal expansion and high melting point and tensile strength of
tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d
electrons [5]
Fig 13 The solid state structure of tungsten
9
131 Application of Tungsten
Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens
alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both
the filament and target) electrodes in TIG welding super alloys and radiation shielding
Due to hardness and high density tungsten is widely used in military field Tungsten
compounds are also often used as industrial catalysts Tungsten alloys are sometimes used
in low temperature superconducting circuits Tungsten with some percentage of
chalcogen elements found to have semiconducting nature Semiconductor crystal of
tungsten dichalcogenides are used in PEC solar cell [5]
132 Electronic Configuration of Tungsten
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of tungsten The pattern associated with tungsten in its compounds is not
necessarily identical
Ground state electron configuration [Xe] 4f14 5d4 6s2
Shell structure 2 8 18 32 12 2
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
9
131 Application of Tungsten
Tungsten with minor amounts of impurities is found to be brittle and hard Tungstens
alloys have wide applications in incandescent light bulb filaments X-ray tubes (as both
the filament and target) electrodes in TIG welding super alloys and radiation shielding
Due to hardness and high density tungsten is widely used in military field Tungsten
compounds are also often used as industrial catalysts Tungsten alloys are sometimes used
in low temperature superconducting circuits Tungsten with some percentage of
chalcogen elements found to have semiconducting nature Semiconductor crystal of
tungsten dichalcogenides are used in PEC solar cell [5]
132 Electronic Configuration of Tungsten
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of tungsten The pattern associated with tungsten in its compounds is not
necessarily identical
Ground state electron configuration [Xe] 4f14 5d4 6s2
Shell structure 2 8 18 32 12 2
Magnetic ordering Paramagnetic
Crystal structural Body Centered Cubic (BCC)
Element category Transition metal
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
10
14 SELENIUM (Se)
Selenium is a chemical element with symbol Se and atomic number 34 It is a
non-metal chalcogen element It rarely occurs in its pure state in nature Black selenium
converts in to gray selenium at the temperature of 180degC The gray selenium is the most
stable and dense form of selenium The gray selenium has hexagonal crystal lattice
consisting of helical polymeric chains Gray Se is formed by slow heating of allotropes
by slow cooling of molten Se or by condensing Se vapours just below the melting point
Red and black coloured Se are insulators while gray Se is a semiconductor showing
appreciable photoconductivity It can resists process of oxidation by air and it is not
affected by non-oxidizing acids The viscosity of selenium does not exhibit the unusual
changes at high temperature [6]
Fig14 The solid-state structure of selenium
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
11
141 Application of Selenium
The selenium exhibits photovoltaic and photoconductive properties Due to that
selenium is used in photocopying photocells light meters and solar cells Zinc selenide
was the first material used for blue LEDs Cadmium selenide has recently played an
important part in the fabrication of quantum dots Sheets of amorphous selenium convert
x-ray images to patterns of charge in xeroradiography and in solid-state flat-panel x-ray
cameras Selenium is a catalyst in some chemical reactions but it is not widely used
because of issues with toxicity In X-ray crystallography incorporation of one or more
selenium atoms in place of sulfur helps with multi-wavelength anomalous dispersion and
single wavelength anomalous dispersion phasing Selenium is used in the toning of
photographic prints and it is sold as a toner by numerous photographic manufacturers Its
use intensifies and extends the tonal range of black-and-white photographic images and
improves the permanence of prints [6]
142 Electronic Configuration of Selenium
The following correspond to the electronic arrangement of the ground state neutral
gaseous atom of selenium The configuration associated with selenium in its compound
form is not necessarily identical
Ground state electron configuration [Ar] 3d10 4s2 4p4
Shell structure 2 8 18 6
Magnetic ordering Diamagnetic
Element category Polyatomic nonmetal
Crystal structure Hexagonal
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
12
15 APPLICATION OF TMDCs
Significant growth has been made in the application areas of semiconducting
TMDCs Even a monolayer (thickness lt 1 nm) semiconducting TMDCs can provide an
equivalent electrical performance to a 10 nm thick organic or amorphous oxide
semiconductor Ultrathin TMDCs are particularly fine suited for transparent and flexible
electronics The charge transport and scattering mechanisms have been recognized and it
is found to be useful in many electronic devices This fundamental knowledge of TMDCs
has begun to convert into essential functional of an electronic circuit is well The need for
developing an inexpensive and efficient method of converting solar energy into electrical
or chemical energy inspired fast development of semiconductor electrochemistry in the
past decades The method of converting solar energy with the support of semiconductor
photo electrochemical cells has been advanced as an option to the well known energy
conversion method involving the use of solid state semiconductor solar cells TMDCs
materials are used in optoelectronics holographic recording systems switching infrared
generation and detection system [6]
Layered transition metal dichalcogenides MX2 (M = metal X2= chalcogen atom) may
be considered as an ideal model system for the investigation of fundamental aspects of
semiconductor metal interface The TMDCs appears to be an appropriate electrode for
solar energy conversion due to its ability to obtain relatively large photocurrents in
aqueous electrode TMDCs have been used as cathode in the lithium electrochemical cell
N type TMDCs can be used with p type WSe2 to form a heterojunction rectifier device
These properties suggest its stable and efficient application in the photo electrochemical
solar cell [6]
Similarly access to high-quality large area substrates is rising with the entrance of
chemical vapor deposition grown materials although improvements are needed for high
performance circuit applications In the field of optoelectronics methods for improving
light absorption fluorescence and electroluminescence quantum yields in ultrathin
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
13
materials will be desirable if they are to be feasible with conventional bulk
semiconductors Finally in the field of sensing applications chemical fictionalization
methods are preferred that impart high chemical selectivity and robustness without
unsettling the excellent electronic properties of the TMDCs semiconductor However the
chemical vapour deposition technique is the single identified method to obtain electronic
grade TMDCs over big areas Similarly solution based methods for arranging and
depositing TMDCs materials require large improvement particularly for high
performance electronic and optoelectronic use While many applications of
semiconducting TMDCs are almost the same to other electronic materials the atomically
slim nature of TMDCs presents exceptional opportunities [5] By focusing genuinely on
such exclusive opportunities the technological strength of semiconducting TMDCs can
be maximized
16 OCCURRENCE AND SYNTHESIS
Since TMDCs single crystals are not identified to occur naturally and so they have to
be produce in the laboratory For the growth of this crystal a range of growth techniques
are accessible at present These contain both growth from the melt as well as growth
from the vapour The well acknowledged methods over the years for the growth of binary
IV-VI compounds consist of Bridgman direct vapour transport (DVT) and chemical
vapour transport (CVT) techniques The VIB metal compounds have been originally
produced as single crystals by Kjekshus [7] and coworkers Since then a number of
research groups have deal with the growth of single crystals constantly from the vapour
phase The compounds are always produced from the pure elements The single crystals
are developed from the vapour phase either by conscious cooling creating sublimation or
by a mineralization Chemical transport responses are frequently used to develop single
crystals of TMDCs The developed crystals are stable in standard laboratory
circumstances [7]
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
14
17 EXISTING INFORMATION REGARDING CHARACTERISTICS
OF TMDCs
171 Structural Investigation
Small dimensional crystals are of immense importance because of their exacting
properties associated to the crystalline anisotropy Transition metal dichalcogenides that
crystallize in the form of parallel fiber represent a diversified family ranging from super
conductors to widespread band gap semiconductors Diverse physical properties initiate
from minute difference of the X-X and M-M legend lengths consequential in structures
possessing three different varieties of trigonal prismatic chains Their essential structural
units are formed of MX6 trigonal prisms that distribute trigonal faces and made of chains
parallel to the hexagonal c axis Same concept has been revealed in fig 15 and fig 16
Every chain shifted with regard to two adjacent ones by half the lattice factor all along
the c direction [7]
The chains are connected by metal-chalcogen bonds and shape layers are associated
by a lot weaker Van der Waals forces Their structure formulates them beneficial for
battery cathode intercalation and photochemical cell purposes These layered compounds
crystallize hexagonally in strong strands or filamentary strip created platelets As shown
in figure 16 a linear sequence of metal atoms is parallel to the c axis (the growth axis)
and six chalcogen (X) atoms encircle via a metal atoms forming distorted trigonal prism
The layer-type lattice has the metal ions in the middle of the distorted trigonal prisms
which distribute trigonal faces consequently forming isolated columns The columns
scuttle parallel to the crystallographic axis (c-axis) and are relocating from the
neighboring columns through one-half the unit cell along the c axis [7]
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
15
Fig 15 Crystal structure of TMDCs
Fig 16 Primitive cell of MoSe2 WSe2 single crystal
The distance among metal atoms by the side of the b axis is a lot shorter than the
inter prism distance This structure has the similarity with a bundle of metallic chains
each with an insulating covering The Van der Waals selenium-selenium bonds are in
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
16
plane perpendicular to the columns and the Van der Waals gap is almost vertical to the
c-axis As an example structure of MoSe2 and WSe2 are shown in fig15 and fig 16
The transition metal dichalcogenides (TMDCS) where M representing a transition
metal of group 4B 5B and 6B and a chalcogen element such as S Se Te etc crystallise
in layered structures consisting of sandwiches of three hexagonally formed sheets of
atoms The majority of group 4B and 6B (TMDCS) are semiconductors while group 5B
dichalcogenides are metallic and turn out to be superconductors at low temperatures The
crystal formation of these materials can be separated into two central groups Depending
on the crystal field symmetry about the metal atom it can be octahedral or trigonal
prismatically coordinated by the six nearest neighbour chalcogens The group 4B
dichalcogenides have octahedral structural symmetry the group 6B dichalcogenides have
prismatic while several group 5B compounds shows evidence of either or both structures
[7]
172 Electrical Properties of TMDCs
From the literature survey it is found that TMDCs are diamagnetic Every
characteristic of layer materials are found to be anisotropic The magnitude of the
conductivity at 300K fluctuates widely from sample to sample possibly because of
varying contamination concentrations However there is now noticeable proof that the
minimum indirect band gap in these semiconductors is higher than 03 eV and the
electrical conductivity in these materials is minimum at 300K It has been found in the
current research work that electrical conductivity of TMDCs increases with the
temperature (chapter 4)
It is found from literature review that early studies were made by hicksetal in 1967
[5] He has shown that the carrier concentration of both n and p type MoSe2 and WSe2
specimen were of the order of 1016cm-3 An intensive study has been made on mobility of
charge carriers in the layered semiconductors by fivazetal (1967) [8] He has investigated
the temperature dependence of electrical conductivity and Hall coefficient of
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
17
semiconducting compounds eg MoS2 MoSe2 and WSe2 From the investigation he has
explain various scattering mechanisms involved in these materials He also stated that in
these materials the free charge carriers have a tendency to become localized within each
layer Therefore they behave as if they are moving through a part of independent layers
Moreover it was investigated that this tendency is related by a strong interaction between
the free carriers and the optical phonons polarized vertically to the layers [8] Table 12
and table 13 display the discovered electrical data of MoSe2 and WSe2 single crystal by
various researchers
Table 12 Electrical data of Tungsten diselenide
SR
NO
Research
scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal P 0570 8 x1016 99 78 [5]
2 Fivazetal N 123 1 x1017 100 --- [8]
3 Deshpandeetal P 341 601x1015 304 103924 [9]
4 Lux-Steineretal P 40 6 x1015 250 --- [10]
5 Spah etal N 080 74x1016 236 --- [11]
6 Mahalawyetal N 0166 388 x1017 1215 --- [12]
7 Present work P 08635 6429 x 1014 1561 9709
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
18
Table 13 Electrical data of Molybdenum diselenide
SR
NO Research scholar
Carrier
type
Resistivity
ρ(Ωcm)
Carrier
concentrations
N(cm-3)
Hall
mobility
microH (cm2VS)
Hall
coefficient
RH(cm3C)
Ref
1 Hicksetal N 06 56 x1016 15 -110 [5]
2 Grantetal N 1 16 x1017 40 -078 [6]
3 Pathaketal N 01769 11 x1017 213 ---- [13]
4 Agarwaletal N 115 178 x1016 303 -650 [14]
5 Huetal N 25 35 x1016 314 ----- [15]
6 Sumeshetal N 4751 174 x1017 7558 ----- [16]
7 Present work P 09635 813 x 1014 1452 7526
173 Optical Properties of TMDCs
The investigation of optical properties of TMDCs gives important information of the
electronic properties and band structures of the crystal The wilsonetal [1] have
investigated the optical spectra of transition metal dichalcogenides having trigonal
prismatic coordination The transmission spectra and optical absorption of group VI A
materials have been investigated at liquid nitrogen temperature (77K) and room
temperature by frindtetal in 1965 [17] evansetal in1965 and 1968 [18] wilsonetal in
1969 [1] hazelwoodsetal in 1971 [19] and the transmission spectra of group VIA have
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
19
been investigated at liquid helium temperature by bealsetal [20] In the present
investigation optical band gap of WSe2 W09Se2 and MoSe2 single crystal have carried out
Optical band gap of all the samples under investigation are found to be around 14eV
The ennaouietal (1986) [21] and huangetal (2000) [22] have reported the
application of MoSe2 as photovoltaic material The curtistetal in 1986 have reported the
working of MoSe2 as dehydrosulfurization catalysts Recent investigations have revealed
that the layer type semiconducting group VI transition metal dichalcogenides (TMDCs)
which absorb visible solar energy in the neighborhood of infrared light are mainly
attractive materials for photoelectrochemical solar energy translation Among TMDCs
family the most efficient systems turned out to be MoSe2 and WSe2 The latest uses of
these materials contain polymer based TMDCs solar cells
It is seen from literature review that optical band gap of TMDCs material falls near to
maxima of solar radiation It indicates that TMDCs material like MoSe2 WSe2 and mix
compound of both can absorb maximum of the solar radiation incident on it This
increases the possibility of generation of photo electron hole pairs Therefore TMDCs
material must be useful in photo sensitive application Thus it is worth investigating the
use of TMDCs in PEC solar cell
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
20
REFERENCES
[1] J A Wilson and AD Yoffe JAdv Phys 18 (1969) 193 [2] J R Lince and P D Fleischauer J Mater Res 2 (1987) 827 [3] J Gobrecht H Gerisher and H Tributsch J Electrochem Soc 125 (1978) 1086 [4] F R Fan H S white B Wheeler and A J Bard J Electrochem Soc 127 (1980) 518 [5] WTHicks J Electrochem Soc 111 (1964) 1058 [6] A J Grant TM Griffiths GD Pitt and AD Yoffe J Phys C Solid State Physics 8 (1975) L17 [7] Bratts L and Kjekshus A Acta Cehm Scand 26 (1972) 3441
[8] R Fivaz and E Mooser Phys Rev B 163 (1967) 743 [9] MP Deshpande PD Patel MN Vashi and MK Agarwal J of Crystal Growth 197 (1999) 833-840 [10] M Ch Lux- Steiner R Spah MObergfell E Bucher 1st IPSE Conference Kobe Japan (1984) 259 [11] R Spah U Elrod M Ch Lux-Steiner E Bucher and S Wagner Appl Phys Lett 43 (1983) 79 [12] LH El Mahalawy and BL Evan Phys Stat Sol (b) 79 (1977) 713 [13] V M Pathak PhD thesis Sardar Patel University Vallabh Vidyanagar (1990) [14] MK Agarwal HB Patel and K Nagi Reddy J Cryst Growth 41 (1977) 84-86
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728
21
[15] SY Hu CH Liang KK Tiong and YS Huang J of Alloys and Compounds 442 (2007) 249 [16] C K Sumesh Ph D thesis 2008 Sardar Patel University Vallabh Vidyanagar [17] R F Frindt J Phys Chem Solids 24 (1963) 1107 [18] B L Evans and P A Young Phys Stat Solidi 25 (1968) 417 [19] B L Evans and R A Hazelwood Phys stat sol (a) 4 (1971) 181 [20] A R Beal J C Knights and W Y Liang J PhysC 5 (1972) 3540 [21] A Ennaoui S Fiechter W Jagermann and H Tributsch J ElectrochemSoc 97 (1986) 133 [22] J M Huang and D F Kelley Chem Mater 12 (2000) 2825 [23] C M Fang R A Degroot and C Haas Phys Rev B 56 (1997) 4455 [24] H Isomaki and J Vonboehm J Phys C 14 (1981) L75 [25] L F Mattheiss Phys Rev 8 (1973) 3719 [26] D G Clerc R D Poshusta and A C Hess J Phys Chem 100 (1996) 1575
[27] C Umrigar DE Ellis D S Wang H Krakauer and M Posternak Phys Rev B 26 (1982) 4935 [28] R A Bromley R B Murray and A D Yoffe J Phys C 5 (1972) 759 [29] V L Kalikhman and Ya Umanski Sov Phys Upeshki 15 (1973) 728