Post on 17-Mar-2020
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CHAPTER 1
INTRODUCTION TO SEMICONDUCTORS, PROPERTIES OF
SnS AND ZnO SEMICONDUCTING MATERIALS
1.1 INTRODUCTION Nanoparticles are often defined as particles of size less than 100 nm in
diameter. A nanometre is extremely small, equal to one billionth of a metre. For
comparison, a human hair is approximately 80,000 nm wide, a red blood cell is
approximately 7,000 nm wide, a DNA double-helix is around 2 nm wide and a
typical carbon-carbon bond length in the range of 0.12 nm - 0.15 nm. Nanoparticles
can be considered as single crystals with a typical size of a few nanometres. They
still contain hundred to thousand of atoms. Due to the size quantization effect, these
nanoparticles preserve some bulk material properties; in addition, they exhibit more
interesting properties. From the technological point of view, the main reason for
studying nanostructured materials is the ease of tuning properties by gradually
varying the particle size and shape.
In recent years, there is a great interest in the synthesis and application of
semiconductor nanomaterials, since their properties are size and shape dependent. In
particular, research in IV-VI group narrow band gap semiconducting nanomaterials
have gained attraction because of their potential applications in field effect
transistors, thermoelectric materials, solar cells and near infrared detectors [1-4].
IV-VI group semiconductors are SnSe, SnS, SnTe, PbS, PbSe and PbTe. Among
these, much effort has been made on the synthesis of PbS and PbSe due to their
large exciton Bohr radii (20 nm for PbS to 46 nm for PbSe) [5]. Since lead is (Pb) a
toxic material, synthesis of non-toxic materials with properties similar to lead
2
chalcogenide is of interest in the new materials research. Semiconductor materials
should fulfill two requirements to be used as solar cells. (i) It should be less toxic
and its constituent elements should be abundant in nature. (ii) It should have good
electrical and optical properties [6]. SnS is a layered structure narrow band gap
semiconductor with less toxicity and its constituent elements are abundant in nature.
SnS has both direct (1.3 eV) and indirect (1.0 eV) band gaps, lying between
Si (1.12 eV) and GaAs (1.43 eV) [7]. It has high absorption co-efficient of 104 cm-1
and it usually exhibits p-type conductivity [8].
For solar cell applications, SnS requires wide band gap n-type
semiconductor as a window material. CdS is a wide band gap (2.4 eV)
semiconductor and it has been extensively used as window material for solar cell
applications. Reddy et al., have reported the highest conversion efficiency of 1.3%
for CdS/SnS heterostructure [9]. However, due to its toxicity Cd free window
material is required for the solar cell applications. ZnO is another wide band gap
(3.37 eV) semiconductor with n-type conductivity and it has potential applications
in the field of optoelectronic and electronic device fabrication and also it is
inexpensive and environmentally friendly material. Ichimura et al., have reported the
fabrication of bulk ZnO/SnS heterostructures for solar cell applications with SnS as
the light absorption layer and have achieved a photo conversion efficiency of 0.01%
[10].
1.2 SEMICONDUCTORS
Semiconductors are defined as materials with electrical resistivity lying
in the range of 10ˉ2 to 109 ohm cm1. Semiconductors occur in many different
chemical compositions in a variety of crystal structures. They are classified as
elemental semiconductors (Si, Ge, Se) and compound semiconductors (GaAs, ZnO,
3
CdS), based on their constituent elements. An optical property of semiconductor
depends on its band gap. Band gap is the characteristic property of the
semiconductors. Band gap (Eg) of the semiconductor is defined as the energy
difference between the top of the valence band (Ev) to bottom of the conduction
band (Ec) and it is given by
g c vE E E= − (1.1)
There are two types of band gap in semiconductors: (a) direct band gap
and (b) indirect band gap. Few examples for direct band gap semiconductors are:
ZnO, SnO2, SnS2, CdS and ZnS. Si, Ge and GeS are examples for indirect band gap
semiconductors. In the case of direct band gap semiconductors, the minimum of the
conduction band and maximum of the valence band occurs at the same k-point in the
Brillouin zone, whereas, for the indirect band gap semiconductors, the minimum of
conduction band and maximum of valence band occurs at different k-values.
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Figure 1.1 Energy level diagram of (a) direct and (b) indirect band gap
semiconductors.
1.3 NANOSCALE SEMICONDUCTORS
Semiconductor nanoparticles demonstrate quantized optical and
electronic properties due to their large surface to volume ratio. Such unique
properties have made these particles useful in the field of nonlinear optics,
luminescence electronics, optoelectronic devices and solar energy conversion [11].
The electronic and optical properties of semiconducting nanoparticles are discussed:
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1.3.1 Electronic and Optical Properties of Semiconductor Nanoparticles
In the case of semiconductor nanocrystals, the effect of particle size on
the optical properties is interesting. In order to understand the size dependent
optical and electronic properties of semiconductor nanoparticles, it is important to
know the physics behind what is happening at the nano level. The important
parameters of the semiconducting nanoparticles are discussed in the following
sections.
1.3.1.1 Exciton
When an electron is excited from the valence band to conduction band,
an electron-hole pair is created. This bound state electron-hole pair is called exciton
and it requires a minimum energy to excite it. Changes in the band gap due to
particles size, leads to the corresponding changes in the properties of the material.
The increase in exciton energy with respect to bulk semiconductor band
gap is given by,
22
2E
Rπ
μ⎡ ⎤Δ = ⎢ ⎥⎣ ⎦
h (1.2)
where, R is the radius of spherical quantum dots, µ is the reduced mass of electron-
hole pair and it is given as,
1 1 1
e hm mμ= + (1.3)
where, me and mh are effective mass of electron and hole respectively. The reduced
mass is generally smaller than the electron rest mass m0.
The exciton Rydberg energy is given as,
6
2
22RB
Eaμ
=h (1.4)
and exciton Bohr radius ( ) is given as, Ba
2
2Bae
εμ
=h (1.5)
where, e-electronic charge, ε-dielectric constant.
Therefore, the energy shift is written as
2
BR
aE ER
π⎡ ⎤Δ = ⎢ ⎥⎣ ⎦ (1.6)
This equation shows that for small quantum dots (R<<aB), the
confinement induced energy shift is large compared to the bulk semiconductor
energy gap [12]. Band gap energies and exciton Bohr radius of some common
semiconductors are given in Table 1.1.
Table1.1 Band gap and exciton Bohr radius of some common semiconductors
Semiconductors
Band gap [Eg (eV)]
Exciton Bohr radius [aB nm)]
Si 1.12 4.3
PbS 0.41 20
PbSe 0.26 46
CdS 2.58 2.5
CdSe 1.84 5.5
ZnO 3.37 2.34
SnS 1.3 7
GaAs 1.43 14
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1.3.1.2 Quantum Confinement Effect
The tuning of fundamental properties such as optical and vibrational
properties of nanostructured semiconductor material is possible when the size of the
nanostructured semiconductor material approaches the exciton Bohr radius. Though
significant variation in the fundamental properties is observed when the size is less
than the exciton Bohr radius. This is due to the confinement of charge carriers and
phonons within the nanoparticles. This is called quantum confinement effect. Efros
and Efros (1982) introduced three regimes of quantum confinement, depending on
the ratio of the nanocrystallite radius R to the Bohr radius of the electrons, holes and
electron-hole pair. They are:
(i) Strong confinement regime:
For quantum dots with a small radius, the large confinement induced
energy shift is given as,
BR a<< (1.7)
here the individual motions of electron and hole are strongly quantized in
all spatial directions.
(ii) Intermediate confinement regime:
When the effective mass of the holes is higher than that of the electrons
( e
h
mm
<<1), the electron and hole Bohr radius are:
2
2ee
am eε
=h and
2
2hh
am eε
=h (1.8)
Therefore, h ea R a< <
8
(iii) Weak confinement regime:
For large quantum dots ,e hR a a>> , the confinement effects in this size
regime are relatively small.
1.3.1.3 Density of States
In the nanostructured systems such as quantum well, wire and dots the
motion of electrons, holes and excitons are restricted in one, two and three directions
respectively. The dramatic modifications in their density of states due to the
dimensional confinement of electrons in nanomaterials, gives rise to shape
dependent properties. Based on dimensional confinement, the nanomaterials are
classified as two dimensional (2D), one dimensional (1D) and zero dimensional
(0D) nanostructures [13]. Quantum confinement effect arises as a result of changes
in the density of electronic states. The relation between position and momentum of
free and the confined particles explains the quantum confinement effect. For the free
particle, the energy and momentum are defined but there is uncertainty in position.
For localized particle or confined particle the energy is well defined, the uncertainty
of the position decreases, so there is an uncertainty in momentum. Therefore, the
discrete energy of the particle is viewed as superposition of bulk momentum states.
Quantum confinement effect compresses the series of nearby transition occurring at
slightly different energies in the bulk into a single, intense transition in a quantum
dot [14].
The density of states is given by the formula,
( 1
2( )D
E Eρ α)− (1.9)
Where, D is the dimensionality (D =1, 2, 3) of the system.
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Figure 1.2 Variation of density of states as a function of dimensionality of the
system.
In the bulk materials, large number of atoms forms a set of molecular
orbitals having negligible difference in their energy levels, resulting in continuum
bands. In the case of nanocrystalline materials, the bands split into discrete
electronic states in the valance and conduction bands. Therefore, photoluminescence
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(PL) and optical absorption (OA) spectra of nanoparticles show a blue shift in the
transition energy [15].
Figure 1.3 Energy level diagram of semiconductor quantum dot.
Effective mass approximation and tight binding approximation are used
to calculate the electronic band structure of semiconductor nanoparticles. For
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spatially confined structures, the effective mass approximation is used to calculate
the particle size and size dependent properties of the nanoparticles.
In the nanoparticles, the energy of the lowest excited state is given as,
22 2 2 2
12 2
1.8( )1 12
n
g nn
e h
e e SE R ER R R
Rm m
π αε
∞
=
⎛ ⎞= + − + ⎜ ⎟⎡ ⎤ ⎝ ⎠+⎢ ⎥⎣ ⎦
∑h (1.10)
(1.11)
22 2 2 2
212
1.8( )2
n
gn
e e SE R ER R R Rπ αμ ε
∞
=
⎛ ⎞= + − + ⎜ ⎟⎝ ⎠
∑hn
where, R is the radius of nanoparticles, Eg and E(R) are the bulk band gap and
modified interband transition values respectively, ε2 -dielectric constant of sphere,
αn -coefficient which depends on the ε, S -coordinates of the charge and the bar in
the third term represents the average over the wave function. The first term from
equation (1.10) represents the energy of quantum localization and it also depends on
the size. The second term corresponds to the Coulomb attraction and the third term
represents the solvation energy loss [16].
A number of reports are available on the confinement effect of charge
carriers in semiconductor nanostructures. Silicon has potential applications in the
field of optoelectronics. Silicon normally emits infrared luminescence due to its
small and indirect band gap. The observation of visible luminescence in the highly
porous electrochemically etched silicon is due to the quantum size effect [17].
Highly porous silicon contains the quantum size crystalline materials which are
responsible for the visible luminescence [18]. The room temperature PL spectra of
CdSe quantum rods with 3.7 nm width and with different lengths (9.2 nm, 11.5 nm,
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28.0 nm and 37.2 nm) show length dependent emission. This shows that the
dimension (shape) dependent emission [19]. InAs rods with different lengths along
with InAs dots with diameter similar to that of rods shows red shift in the optical
absorption and PL spectra from dots to short rods to longer rods and also shows
reduction in PL intensity. This is due to the effective mass of electron and hole
( = 0.024 m0 and = 0.4 m0, where, m0 is the free electron mass) are different.
Therefore, the electron wavefunction is delocalized over the entire rod whereas the
hole is in the medium confinement region and its wavefunction is limited. When the
length of the rod is increased, the overlap between the electron and hole
wavefunctions becomes smaller leading to the reduction in radiative rate, resulting
in the decrease of luminescence efficiency [20]. The room temperature optical
absorption spectra of CdSe quantum dots with diameters 2.8 nm, 4.1 nm and 5.6 nm
shows size dependent shift [21].
*em *
hm
The OA spectra of pure and polyvinyl pyrrolidone (PVP) capped ZnO
nanoparticles shows blue shift compared to bulk (~373 nm) value. The excitonic
absorption peak of the PVP-capped and non-capped ZnO nanoparticles are ~ 303 nm
and ~ 312 nm respectively. This may be attributed to the smaller size of the PVP
capped ZnO nanoparticles [22]. Optical absorption spectrum of SnS nanocrystals
with particle size of sub 10 nm shows indirect transition at 1.6 eV and with the
particle size of sub 200 nm shows 1.06 eV [23]. CdS normally exists as yellow
material and it becomes colorless when the particle size is less than 2.2 nm.
Cadmium phosphide is a black material but depending on the particle size, it shows
various colour [15]. The room temperature PL spectrum of thiophenol capped CdS
nanoparticles with diameter 3.6 nm exhibits band-edge emission and defect emission
at 3.0 eV and 2.56 eV respectively. These emissions were observed at higher
energies side compared to bulk CdS (2.43 eV) [24]. PbS quantum dots synthesized
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and stabilized in Nefion polymer showed a blue shift in the optical absorption
spectra from 0.41 eV to 2.3 eV due to strong quantum confinement as the particle
size decreased from 13 nm to 3 nm [25].
1.3.1.4 Phonon Confinement Effect
Phonons are quantized vibrations in crystalline solids. In single crystals,
phonons propagate as a wave. The vibrational properties of nanomaterials get
modified due to confinement of phonons within the nanoparticles. Considerable
changes in the vibrational spectra can be observed only when the particle size is
about 20 lattice parameters. Peak shift and broadening of the Raman spectra are
observed as a result of optical phonon confinement due to break down of lattice
periodicity. In nanomaterials, the periodicity of the crystal lattice is interrupted and
the selection rule q = 0 (zone center optical phonon) is relaxed. Therefore, the
phonons away from the Brillouin zone center also contribute to the Raman spectra.
This leads to changes in the peak shift and asymmetry broadening of the Raman
spectra [26]. Asymmetrical broadening in the optic-phonon spectra of ZnO
nanoparticles with size 8.4 nm and 4.5 nm have been observed due to the
confinement of optical phonons and the effect of phonon confinement depends on
the symmetry of the phonons [27].
1.4 STRUCTURAL AND OPTICAL PROPERTIES OF SnS
1.4.1 Crystalline Structure
Physical and chemical properties of a given material are derived from its
crystalline structure. SnS is a narrow band gap IV-VI group layered semiconductor.
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Figure 1.4 Crystal structure of SnS. The Sn atoms are green and the S atoms
are blue.
SnS was first reported by the German mineralogist Herzenberg. Single crystal of
SnS has been prepared by reacting stoichiometric mixture of Sn and S elements over
the temperature range of 600 - 750 °C [28]. SnS has orthorhombic structure. SnS is
stoichiometric under Sn rich conditions and it will form Sn1-xSx in sulfur rich
conditions.
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1.4.2 Optical Properties
SnS has both direct (1.3 eV) and indirect (1.09 eV) band gaps with a high
absorption coefficient of 104 cm-1 [8]. It has both p-type and n-type conductivity
depending on the departure of Sn stoichiometry from ideal. Optical properties of
SnS can be studied by a variety of experimental techniques such as optical
absorption, transmission, reflection, photoluminescence spectroscopy etc. In the
present work, optical properties of SnS nanostructures are investigated.
1.4.3 Applications
SnS has potential applications in the field of optoelectronic devices [29],
absorber layer in thin film solar cells [30], near infrared detectors [4], holographic
recording systems [31], anode material in lithium ion batteries [32-33] etc.
1.5 STRUCTURAL AND OPTICAL PROPERTIES OF ZnO
1.5.1 Crystalline Structure
Most of II-VI binary semiconductors crystallize in either cubic zinc
blende or hexagonal wurtzite structure where each anion is surrounded by four
cations at the corners of a tetrahedron. This tetrahedral coordination is typical of sp3
covalent bonding nature but these materials also have a substantial ionic character
that tends to increase the bandgap beyond the one expected from the covalent
bonding [34]. ZnO is a II-IV group semiconductor and its crystal structures are
shared by wurtzite (Figure 1.5 a) and zinc blende (Figure 1.5 b) structures. Under
ambient conditions, the thermodynamically stable phase is wurtzite. Wurtzite ZnO
has a hexagonal unit cell with lattice parameters a = 0.325 nm and c = 0.521 nm in
the ratio of c/a =1.6 and belongs to the space group of P63mc [34].
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(a) (b)
Figure 1.5 (a) Wurtzite and (b) Zinc blende crystal structures of ZnO.
1.5.2 Optical Properties
ZnO is a direct wide band gap (3.37 eV) and a transparent conductive
material with large exciton energy of 60 meV. ZnO films are transparent in the
wavelength range between 0.3 - 2.5 μm. Optical properties of ZnO can be studied
using experimental techniques such as optical absorption, reflection, PL and
cathodoluminescence spectroscopy etc. In the present work, room temperature PL
study is discussed in detail. Room temperature PL spectrum of ZnO consists of a
ultraviolet (UV) emission band (375 nm) and a broad emission band (420-700 nm).
At room temperature, the near UV-band is related to the excitonic nature of the
material and may be superimposed with the free exciton emission, its phonon
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replica, bound exciton emission, as well as biexciton emission [36]. The broad
emission band is called deep level emission (DLE) and is observed by several
defects in the crystal structure such as O-vacancy (Vo) [37-39], Zn-vacancy (VZn)
[40-42], O-interstitial (Oi) [43], Zn-interstitial (Zni) [44], and extrinsic impurities
such as substitutional Cu [45].
1.5.3 Applications
ZnO has potential applications in the field of optoelectronics, UV light
emitters, spintronics, solar cells, gas sensors, UV laser from ZnO nanowires, optical
photonic crystals and piezoelectric devices and as antibacterial agent [34, 46-50].
1.6 REVIEW OF LITERATURE
1.6.1 Literature Review of SnS Nanostructures
In recent years, considerable efforts have been made in the synthesis of
SnS nanostructures. Xu et al., have reported the synthesis of SnS quantum dots.
Where, SnBr2 is reacted with sodium sulfide in ethylene glycol (EG) at room
temperature in the presence of various stabilizing ethanolamines as ligands. The
ethanolamines were: triethanolamine (TEA), N-methlydiethanolamine (MDEA) and
N, N-dimethlyethanolamine (DMEA). Among these ethanolamines, small size and
monodispersed SnS nanoparticles with average particles size of 3.2 nm were formed
in the presence of TEA. This could be attributed to two reasons: (i) During
nucleation, the strong binding of multiple hydroxyl groups on the surface of SnS
nanoparticles and (ii) reaction of TEA with Sn2+ forms [Sn(TEA)n]2+ complex [51].
Biswas et al., have reported the synthesis of SnS nanorods and nanosheets through
thioglycolic acid (TGA) assisted hydrothermal process. The diameter of the SnS
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nanorods varied within 30-100 nm and the crystal size and shape depends on the
amount of TGA and sulfur source [52].
Chen et al., have reported the synthesis of SnS nanoflakes through
microwave-assisted polyol process. The final product was amorphous or impurity
when water and ethanol were using as solvent and for benzene no product was
obtained. SnS nanoflakes were formed in EG. There was no effect on the
morphologies and crystallinity of the SnS nanoflakes with increasing the reaction
time [53]. Peng et al., have synthesized SnS nanostructures such as nanobelts,
nanorightangles, nanorods and nanosheets via solvothermal route in EG medium.
SnS nanorods, nanobelts and nanorightangles were formed at reaction temperatures
of 140 °C, 160 °C and 180 °C for 12 hours respectively. The thickness and width of
SnS nanobelts were less than 30 nm and in the range of 50 nm -300 nm respectively.
The thickness of the nanosheets was between 10 nm and 20 nm. A nanorightangles
was composed of two nanobelts. Sheet-like structure was formed when thiourea was
used as sulfur source [54]. SnS nanoparticles were synthesized using tin metal and
elemental sulfur in diethyleneglycoldimethylether (diglyme) at 160 °C. In this
method, SnCl2 was dissolved in diglyme and Li[Et3BH] solution and tetrahydrofuran
was added. SnS nanoparticles with size of 20 nm - 40 nm were obtained [55].
Liu et al., have synthesized monodispersed, size tunable SnS
nanoparticles by using SnCl2 and TMS in oleylamine solution. The size of SnS
particles can be varied as 6 nm, 12 nm and 20 nm by adjusting the hot injection and
reaction temperatures between 120 °C, 150 °C and 210 °C respectively [56].
Hu et al., have reported the synthesis of SnS elegant 3D SnS urchin-like
architectures by solvothermal method. The morphological transformation from 3D
urchin-like architectures into 1D nanofibers with diameters of 20 nm - 60 nm and
lengths of 0.4 μm -3 μm has been obtained only by increasing the reaction time to
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96 hours. Uniform size SnS nanorods with diameter of 100 nm - 400 nm and length
of 0.6 μm - 4 μm were formed for the reaction time of 8-10 days [57].
Nanocrystalline SnS with different morphologies like flakes, sheets, rods and
granules has been synthesized over Sn metal foils via solvothermal method by using
ethylenediamine, water and their mixture as solvents [58]. Rao et al., have
synthesized SnS nanorods and nanoparticles through hydrothermal route at 180 °C
for 2 and 8 hours respectively. The length and diameter of the rods were between
55 nm -250 nm and 10 nm to 50 nm respectively. The formation of SnS nanorods in
the absence of any stabilizing agents was attributed to templating characteristic of
ammonium ion produced during hydrolysis of thiourea [59].
SnS nanoparticles have been synthesized by the reaction of powdered tin
with sulfur in paraffin oil. Room temperature PL spectrum of SnS nanoparticles
shows two strong emission peaks at 480 nm (blue emission) and at 415 nm
(UV emission) respectively. The strong UV emission was due to defects [60]. Single
crystalline SnS nanowires have been prepared in aqueous solution using
cetyltrimethylammoniumbromide (CTAB) as a surfactant together with oxalic acid
at room temperature. For SnS nanowires, three Raman modes were observed at
190.4 cm-1, 223 cm-1 and 273.7 cm-1. The mode observed at 190.4 cm-1 was assigned
to B2g and the modes observed at 223 cm-1, 273.7 cm-1 were assigned to A1g modes
of SnS [61]. SnS nanocrystals with size less than 10 nm have been synthesized
through solvothermal method by decomposition of bis (diethyldithiocarbamato) tin
(II) in oleylamine at elevated temperature. The shape and size tunability of SnS
nanocrystals can be achieved by controlling the reaction temperature, time and
nature of stabilizing ligands. Optical absorption spectrum of SnS nanocrystals with
particle size of sub 10 nm shows indirect band gap transition at 1.6 eV and for the
particle size of sub 200 nm shows 1.06 eV which is close to the bulk SnS [23]. The
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increased band gap value in SnS nanocrystals compared to its bulk value is due to
the quantum confinement effect of the charge carriers in semiconductors [62].
Zhu et al., have reported the synthesis of SnS nanoflowers by TGA
assisted hydrothermal method. The nanoflowers were assembled from more than ten
needle - like SnS nanorods which are 70 nm in width and ~1μm in length. SnS
nanoflower shows both direct and indirect band gap transition at 1.53 eV and
1.43 eV respectively [63]. Salavati-Niasari et al., have reported the synthesis of
different morphologies of nanostructured SnS including nanoparticles, nanosheets
and nanoflowers via a simple hydrothermal process in the presence of TGA. The
UV-Vis optical absorption of SnS nanoflower shows large blue shift compared to
other SnS nanostrutcures. Room temperature PL spectrum of SnS nanoflower shows
a strong peak at 553.05 nm with excitation wavelength of 200 nm [64].
SnS nanowire arrays have been synthesized by pulsed electrochemical
deposition in the porous anodized aluminium oxide template with uniform diameter
of 50 nm and a length up to several tens of micrometers. It exhibits strong
absorption in the visible and near-infrared spectral region and the direct energy gap
of SnS nanowire was 1.59 eV and it has high absorption coefficient (> 105 cm-1) in
the wavelength range from 400 to 800 nm [65]. Uniform ultralarge single crystal
SnS rectangular nanosheets have been synthesized via the pyrolysis of a single
source precursor and it exhibits good electrochemical properties. Therefore, it has
applications in lithium (Li) ion batteries [66]. SnS nanocrystals have been
synthesized using Sn6O4(OH)4 as Sn source. Sn6O4(OH)4 precursor was dissolved in
oleic acid and oleylamine and then thioacetamide. The SnS nanocrystals with
different shape and size can be produced by changing the reaction conditions such as
reaction temperature and Sn/S molar ratio. SnS nanoparticles with size 5 nm with
uniform size distribution were obtained at 150 °C with Sn/S ratio of 1:1. SnS
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nanoflowers and nanosheets were formed at 120 °C with Sn/S ratio of 2:1. The size
of nanoflowers was 13 nm and size of the nanosheet was 40 nm to 100 nm and
shape of the nanosheet was square. With increasing reaction time, the SnS
nanoflowers transforms into amorphous nanosheets. The main reason for the shape
evolution in the SnS nanocrystals is its layered crystal structure. The direct and
indirect band gaps were observed at 3.6 eV and 1.6 eV respectively for SnS
nanoparticles [67].
1.6.2 Literature Review of ZnO Nanostructures
Past few decades optical and vibrational properties of ZnO nanoparticles
have been extensively studied. Highly monodispersed ZnO nanoparticles with size
3.5 nm were prepared using PVP as capping agent. Optical absorption spectra of
PVP capped and uncapped ZnO nanoparticles were observed at 303 nm and 312 nm
respectively. This was due to small size of PVP capped ZnO nanoparticles. PL
spectrum of ZnO nanoparticles shows enhanced near band edge UV emission
(365 nm) and quenched defect related emission (530 nm). The quenching of defect
related emission was due to surface passivation of ZnO nanoparticles by PVP
molecules [22]. ZnO nanoparticles were synthesized using [ZnAc2.2H2O] as zinc
source and with capping agents as 1- octademide (OD), mixture of trioctylamine
(TOA) and OD with of 1:10 ratio and a mixture of trioctylphosphine oxide (TOPO)
and OD with ratio of 1:12. The ZnO particle sizes were 5 ±0.4 nm, 4±0.5 nm and
5±0.8 nm for pure OD, TOA/OD and TOPO/OD respectively. The strong UV
emission and a broad green emission were observed for all the samples. The
quenching of green emission was observed for the samples prepared in the mixture
of TOA/OD and TOPO/OD compared to that of pure OD. The green emission was
due to the oxygen vacancies on the surface. The oxygen vacancies near the surface
were reduced due to introduction of TOA and TOPO [68].
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Yang et al., have observed strong UV emission in ZnO nanoparticles due
to the passivation of surface states of ZnO nanoparticles by adsorption of acetate
molecules. Colloidal ZnO nanoparticles were synthesized in methanol solution using
KOH as precipitating agent [69]. ZnO nanoparticles were synthesized by
thermolysis of [EtZnOiPr] with TOPO at 160 °C for 5 hours without adding any
precipitating agents. The ZnO nanoparticles were monodispersed with average
particle size of 3 nm. The absorption peak was observed at 325 nm, blue shifted
compared to that of bulk ZnO (375 nm). A broad band at 530 nm with a minimum
shoulder was observed from the PL spectrum of ZnO nanoparticles. The broad green
emission was attributed to the transition of photogenerated electron from conduction
band to a deeply trapped hole in the valance band [70]. Zhang et al., have
synthesized ZnO nanorods by microemulsion method. The average diameter of ZnO
nanorods were 15 nm - 20 nm and lengths were in the range of 80 nm - 100 nm. UV
emission and deep-level emission was observed in the room temperature PL spectra
of ZnO nanorods. At 15 K, four peaks were observed from 3.0 eV to 3.5 eV. The
emission observed at 3.351 eV was assigned to donor-bound exciton (DBE) and the
peaks observed at 3.311 eV, 3.237 eV and 3.162 eV correspond to free to bound
transition (FB) and its 2LO phonon replicas (FB1LO and FB2LO) [71].
Phonon confinement of ZnO nanoparticles with different diameters has
been investigated using Raman scattering measurements. The Raman peaks were
found to broaden asymmetrically and also shifted compared to bulk ZnO phonons
[27]. Alim et al., have reported the origin of peak shift in optical phonon of ZnO
with diameter 20 nm using Raman spectroscopy under non-resonant and resonant
conditions. Three factors which responsible for the peaks shift: (i) optical phonon
confinement by the dot boundaries, (ii) the phonon localization by defects or
impurities and (iii) the laser induced heating in nanostructures [72]. Cheng et al.,
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have synthesized ZnO quantum dots by sol-gel method without using any ligands.
The intense UV emission was observed from room temperature PL spectra of ZnO
quantum dots and it shifted from 3.3 eV to 3.43 eV as the particle size reduces from
12 nm to 3.5 nm. From the resonant Raman scattering of ZnO quantum dots, the
intense polar A1 (LO) and E1 (LO) were observed and the non polar E2 phonon was
not observed [73].
1.6.3 Literature Review of SnS/ZnO Heterostructures
Few reports are available on the synthesis of SnS/ZnO herterojunction.
Ichimura et al., have fabricated ZnO/SnS heterostructure using electrodeposition
technique with low conversion efficiency [10]. SnS/ZnO heterojunction has been
fabricated by electrodeposition technique. The heterojunction shows a high
absorption in the visible wavelength range of 400 nm - 700 nm [74].
1.7 OBJECTIVE OF THE THESIS
In the past decade, many new methods have been developed for the
synthesis of SnS nanostructures. However, synthesis of SnS at nanoscale by
chemical route is still a challenge due to its layered structure. The main aim of this
work is to
• Synthesize SnS and ZnO nanostructures by chemical route like
room temperature wet chemical and solvothermal methods
• Study the influence of reaction temperature and time on the SnS
morphology
• Preparation of SnS/ZnO nanocomposites by chemical method
24
• Explore the optical properties of SnS, ZnO nanostructures and its
nanocomposite
1.8 LAYOUT OF THE THESIS
In this thesis, the research work carried out is organized in 8 chapters.
Chapter 1 is devoted to the introduction to semiconductors and review of literature.
Chapter 2 is focused on the synthesis and the description of the
characterization techniques used in the research work such as Powder X-Ray
Diffraction (XRD), Atomic Force Microscopy (AFM), Scanning Electron
Microscopy (SEM), Transmission Electron Microscopy (TEM), High-Resolution
Transmission Electron Microscopy (HRTEM), Photoluminescence, Raman
scattering and optical absorption spectroscopy.
Chapter 3, deals with the synthesis of SnS nanoparticles at room
temperature in aqueous solution and discussion of its structural and optical
properties.
In chapter 4, synthesis, structural and optical properties of SnS
nanosheets prepared at 80 °C in EG medium are discussed.
Chapter 5, gives the synthesis of SnS structures by solvothermal method
and the effect of reaction time on the morphologies of SnS nanostructures.
In chapter 6, synthesis of ZnO nanoparticles in non-aqueous medium
through chemical method and its structural and optical properties of ZnO
nanoparticles annealed at different temperatures are investigated.