67

CHAPTER 4

SYNTHESIS, STRUCTURAL AND OPTICAL PROPERTIES

OF SOME RARE EARTH (Eu, Ce, Sm) DOPED CADMIUM

SULFIDE NANOPARTICLES

4.1 INTRODUCTION

Semiconductor nanostructures have drawn much attention due to

their unique mechanical, optical, and electronic properties. Inorganic

compounds doped with lanthanide ions (Stouwdam et al 2003) are widely

used as the luminescent materials in lighting and displays (Blasse and

Grabmaier 1994) optical amplifiers (Digonet 2001) and lasers (Reisfeld and

Jorgensen 1977). Recently, optical properties of impurity doped nanocrystals

have attracted much attention as they are expected to modify both the

electronic states and electromagnetic fields. Therefore, a possible influence of

quantum size effect on the luminescence properties is expected for II–VI

semiconductor nanocrystals due to the inclusion of rare earth (RE) metal ions

(Konishi et al 2001). The confinement quantum size effects of semiconductor

nanoparticles not only create photogenerated carriers, which may have an

interaction with f-electrons but also significantly influences the optical

properties (Chowdhury et al 2004). Excitonic emission in the host and an

improvement in the luminescence intensity are also expected for the

semiconductor nanostructures after the RE doping. The effects of the reduced

dimensionality on the electronic relaxation and the phonon density-of-states

of semiconductor and insulating nanocrystals has been extensively

68

investigated by both theoretical and experimental approaches (Yang et al

2000, Simon and Geller 2001).

The spontaneous emission probability of optical transitions

(luminescence lifetime) from RE ions doped semiconductor nanostructures

may be significantly different from the bulk counterparts. In order to improve

the luminescence properties of CdS nanoparticles, lanthanide ions like Eu3+

could be incorporated. Europium doped fluorescent nanocrystals have been

extensively studied because of their properties for application in lighting and

display phosphors. It is possible that Eu3+ can substitute Cd2+ because of

similar ionic radii (ionic radii of Eu3+ and Cd2+ are 0.94 Å and 0.97 Å

respectively). It is expected that 4f electrons in the rare earth metal ions

participate in luminescence. These are hardly influenced by their ligands due

to the presence of 5s and 5p electrons surrounding them. Therefore, crystal

field effects observable in 3d transition metal ions are not feasible in RE

metal ions. However, rare earth ion doped phosphors have emission in the

visible range.

As the optical and electronic properties of the semiconductor

nanocrystals are significantly influenced by the RE doping, recently much

attention has been paid for the synthesis of RE doped semiconductor

nanostructures. Yang et al (2004) synthesised europium doped ZnS

nanocrystals and reported that the doped materials exhibit much better

luminescent properties than the pure ZnS nanocrystals. Ageeth et al (2002)

fabricated the europium doped CdS nanocrystals by both the micro-emulsion

and the precipitation methods. Zhu et al (2006) demonstrated that Eu3+ on the

surface of the semiconductor nanoparticle significantly increases the

fluorescence intensity of band gap emission.

Doping of II–VI compounds with trivalent ions like cerium,

terbium and europium etc., has been extensively studied (Okamoto et al 1988,

69

Jayaraj and Vallabhan 1991). Doping of rare earth element like cerium

reduces the particle size of nanomaterials and also increases surface area

(Liqiang et al 2004 and Guo et al 2005). Recently, Vij et al (2009) and

Sharma et al (2009) reported the luminescence studies on Ce doped SrS

nanostructures and Ce doped CaS nanoparticles synthesized by solid state

diffusion method. Vinay Kumar et al (2010) reported luminescence

investigations on Ce3+ doped CaS synthesized using the chemical

co-precipitation method.

Spectroscopic studies of trivalent samarium ions have received

much interest because of good fluorescence efficiency in the visible and

infrared region (Lin and Pun 2002). The understanding of the optical

properties of Sm3+ ions is of great importance due to its potential

technological applications (Nemec et al 2003). Mathew et al discussed the

optical (Mathew et al 2008) and dielectric properties (Mathew et al 2008a)

CdTe/Sm3+ in sol–gel silica glasses synthesised by sol-gel method. ZnS:Sm is

known to be a good red emitter in thin film electroluminescence devices

(Tohda et al 1986), and optical properties of Sm-doped ZnS bulk crystals

have been studied by several investigators (Swiatek et al 1991 and Hommel et

al 1991). Xu et al (2002) also reported that doping of La3+, Ce3+, Er3+, Pr3+,

Gd3+, Nd3+ or Sm3+ with TiO2 was beneficial for NO2 adsorption to enhance

the photocatalytic activity.

Wet chemical processes are practical approaches for the synthesis

of metal doped nanoparticles in a bulk quantity. Although the doping process

via wet chemical synthesis offers excellent electronic and magnetic properties

in the semiconducting nanostructures, the perfect size, structure, and shape

control of the doped semiconductor nanostructures remain a big challenge as

they significantly influence their electronic, magnetic, and optical properties.

70

50 ml of 0.652 g cadmium acetate in mixed solvent 50 ml of 0.008 g europium acetate in mixed solvent

Stirring at room temperature for 2 h

Dropwise addition of 50 ml solution of 0.195 g sodium sulphide

Ultrasonication for 60 minutes

washed with DI water and ethanol

Separation of nanoparticles by centrifugation at 4000 rpm

Dried in a hot air oven for 24 h

50 ml of 0.652 g cadmium acetate in mixed solvent 50 ml of 0.008 g europium acetate in mixed solvent

Stirring at room temperature for 2 h

Dropwise addition of 50 ml solution of 0.195 g sodium sulphide

Ultrasonication for 60 minutes

washed with DI water and ethanol

Separation of nanoparticles by centrifugation at 4000 rpm

Dried in a hot air oven for 24 h

Herein we report the preparation of europium doped CdS

nanostructures by co-precipitation method with isopropyl alcohol (IPA) and

ethylene glycol (EG) as solvents. We also demonstrate that the size, structure,

morphology and the optical properties of the europium doped CdS

nanostructure can be finely controlled by simply varying the nature of the

solvent used in the synthesis. We also investigate the effect of cerium and

samarium ion doping on the structural and optical properties of CdS

nanoparticles synthesized by chemical co-precipitation method.

4.2 Eu3+

DOPED CdS NANOPARTICLES

4.2.1 Materials Synthesis

The starting materials for the synthesis of Eu3+ doped cadmium sulfide

nanocrystals (Cd1−xEuxS with x = 2%) were (CH3COO)2Cd · 2H2O, Eu(CH3COO)3 ·

H2O, Na2S, which are of analytic grade. Isopropyl alcohol (IPA) and ethylene glycol

(EG) were purchased from CDH, India. Solvents were prepared by mixing equal

volume of DI water with the respective solvents such as IPA and EG. The solution

(50 ml) of 0.652 g cadmium acetate and 50 ml solution of 0.008 g europium acetate

were prepared separately using EG-DI water as mixed solvent. The solution of

europium acetate was added slowly into the cadmium acetate solution with a

constant stirring. The mixture was subjected to stirring at room temperature for 2 h

71

in order to achieve a good dispersion of Cd2+ and Eu3+ into the solvent. With this

mixture 50 ml solution of 0.05 M sodium sulfide (0.195 g) in EG-DI water was

added drop wise and it was observed that the mixed solution turned to pale yellow

initially and changes into yellowish orange color after stirring for an hour. The

precipitate obtained was subjected to ultrasonication for 60 min. and washed with

ethanol and DI water for four times in order to remove insoluble residues and

impurities. The nanoparticles were separated from solution by centrifugation (4000

rpm) and dried in a hot air oven at 60 C. The above procedure shown in flow chart was followed for the second reaction except that mixture of IPA-DI water was used

as a mixed solvent instead of EG. The sample prepared by using EG-DI mixed

solvent was denoted as Eu:CdS-1, whereas the sample prepared by using IPA-DI

was named as Eu:CdS-2.

4.2.2 Characterisation Methods

X-ray diffraction patterns were recorded using PANalytical X-ray

diffractometer with CuK radiation ( = 1.5406 Å) in the range of 20 to 60 (2)at a scanning rate of 0.05 /min. Morphology and composition of the synthesised samples was observed with a Hitachi S-3400 SEM-EDAX and confirmed with

Nippon Jarrell-Ash, IRIS Advantage ICP-OES spectrophotometer. High resolution

transmission electron microscope (HRTEM) and SAED studies were performed at

200 keV using JEOL JEM 3010 with LaB6 filament. UV-reflectance studies were

carried out using CARY 5E UV-VIS Reflectance mode-UV spectrophotometer.

XPS measurements were carried out using PHI Quantera SXM (ULVAC-PHI)

with Monochromatic Al Kα as an X-ray source. The specific surface area was

measured on a Quantachrome Autosorb-1 adsorption analyzer using Brunauer-

Emmett-Teller (BET) method. Photoluminescence spectra were recorded using

Shimadzu-5301 spectroflurometer with 450W Xenon lamp source. Raman

measurement were analysed with Horiba Jobin-Yvon T64000 photon design micro

Raman spectrophotometer using Ar ion laser with the excitation of 514.5 nm.

72

4.2.3 Results and Discussion

4.2.3.1 X-ray Diffraction Analysis

Figure 4.1 shows the powder XRD patterns of europium doped

cadmium sulfide nanocrystals prepared by using EG-DI water and IPA-DI water

as mixed solvents. It is observed from the XRD patterns that the samples are

highly crystalline but with different structure. The materials prepared with EG-DI

water exhibits several peaks which can be indexed to (100), (002), (101) and (102)

planes of the hexagonal wurtzite CdS, whereas for Eu:CdS-2 prepared using IPA-

DI water shows (111), (220) and (311) peaks indexed to cubic (zinc-blende)

structure. In addition, the crystallite size of the sample is significantly changed by

changing the mixed solvents. From the Scherrer equation the average crystallite

sizes for Eu:CdS-1 and Eu:CdS-2 were calculated to be 3.65 and 2.64 nm

respectively. This indicates that IPA-DI water solvent plays a significant role in

reducing the crystallite size, which is mainly due to the fact that density and the

viscosity of the solvent is much lower than that of EG-DI water. From these

results, it can be concluded that it is extremely important to have a low viscous

solvent with less density for the preparation of CdS crystals with small size.

It is also observed that the doping of CdS with europium does not

make much change in the structure of the CdS because no shift in the peak

positions was observed for both the sample irrespective of the usage of the

solvent. This is may be due to the doping of small percentage of europium in

the CdS lattice. However, the width of the XRD peaks is broadened after the

europium doping, revealing the decrease in the crystallite size. The absence of

peaks other than CdS confirms the purity of the sample. These results further

indicate that the dopant ions are incorporated into lattice of nanocrystals.

Similar results on the successful incorporation of the Eu3+ ion in the CdSe

nanocrystals with random ion displacement of the cadmium cation site were

also reported (Raola and Strouse 2002).

73

Figure 4.1 Powder X-ray diffraction of as synthesised europium doped

CdS nanostructures

4.2.3.2 Structural, Morphological and Compositional Analysis

Figure 4.2 (a)–(d) show the HRSEM images with different

magnifications of Eu:CdS-1 and Eu:CdS-2 samples respectively. A significant

change in the morphology was observed for the samples prepared using

different mixed solvents. Flower like morphology was observed for Eu:CdS-1

whereas clusters of rice like morphology was obtained for Eu:CdS-2. The

HRSEM images also show that the particles are highly uniform and

distributed homogeneously without any agglomeration for Eu:CdS-2. In

addition, particles are clear and each rice like particle is connected and

appears as a group of particle for the same samples. However, several

agglomerated particles with different size were observed for Eu:CdS-1, which

is mainly due to the high viscous nature of the EG. From these results, it is

concluded that the size, shape and agglomeration of the crystalline particles

can be controlled by simply varying the mixed solvent system. The doping of

CdS nanostructures with europium was also confirmed by ICP analysis and

the amount of europium present in the final product is given in Table 4.1.

74

Figure 4.2 HRSEM images of europium doped cadmium sulfide

nanostructures (a) and (b) Eu:CdS-1 and (c) and (d)

Eu:CdS-2

Table 4.1 Elemental Composition of Eu:CdS Nanostructures Calculated

from ICP Analysis

ElementObserved value (mg/l) Calculated percentage

Eu:CdS-1 Eu:CdS-2 Eu:CdS-1 Eu:CdS-2

Cd 361.60 575.9 83.2933 65.55115

S 19.04 82.9 15.3776 33.08485

Eu 7.80 16.2 1.3290 1.36399

75

4.2.3.3 High Resolution Transmission Electron Microscopy

Figure 4.3 (a) and (c) show the HRTEM images of Eu:CdS

nanocrystals of different structures indicating the shape of the particles is

approximately spherical, with slight prolate deviations similar to the earlier

report (Tong and Zhu 2006). The particles are connected each other and create

the nanospace between the particles which may give them excellent textural

properties. The mean diameter was about 5 nm determined from the HRTEM

image, which is slightly larger than the value estimated from the XRD

analysis. The crystalline domain size calculated from XRD patterns are much

smaller than those obtained from HRTEM, because of the existence of crystal

defect and lattice distortion resulted from doping (Qu et al 2002). Figure 4.3

(b) and (d) show the lattice fringe pattern of the synthesized nanocrystals,

confirming the highly crystalline nature of the final product. HRTEM images

of Cd0.98Eu0.02S illustrate the atomic resolution of these particles (showed in

round circle) signifying the particles are in nanodimension. Insets in Figure

4.3 (b) and (d) show the SAED pattern of Eu:CdS-1 and Eu:CdS-2

nanostructures respectively. The electron diffraction pattern on a number of

nanocrystals, consists of broad diffuse rings further confirming the crystalline

nature of the Eu:CdS nanostructures.

76

Figure 4.3 (a) and (b) HRTEM images of the sample Eu:CdS-1

nanostructures and (c) and (d) HRTEM images of the sample

Eu:CdS-2 nanostructures.

4.2.3.4 UV-reflectance Studies

UV-reflectance spectra for the synthesised europium doped CdS

nanostructures are shown in Figure 4.4. The band gap values of the Eu:CdS-1

and Eu:CdS-2 were estimated with the well defined excitonic peaks at 444 nm

(2.79 eV) and 462 nm (2.68 eV) respectively by extrapolating the reflectance

spectra. It is known that the excitonic absorption peak is associated with the

lowest optical transition and provides a simple way to determine the bandgap

77

of nanocrystals. A clear blue-shift is observed in the reflectance spectra. This

shift towards the shorter wavelength indicates the increase of optical band

gap. The observed difference in the band gap value for both the samples

may due to the variation in the crystallite size. It is reported for CdS that

quantum effects are more pronounced in the size range from 2 nm to 10 nm

(Brus 1986). The radius (R) of CdS NPs can be calculated using the effective

mass approximation model given in the Equation 4.1.

R

2e8.1

hm

1

em

12R2

22hE

(4.1)

where ΔE = 0.37 eV (Eu:CdS-1) and 0.26 eV (Eu:CdS-2) is the increase of the band gap energy, ε = 5.7 (Li and Du 2003) is the relative dielectric constant, and me = 0.19mo and mh = 0.8mo are the effective masses of

electrons and holes respectively, where mo is the free electron mass. The

calculated particle size was 3.59 nm for Eu:CdS-1 and 2.43 nm for Eu:CdS-2

which are in good agreement with the crystallite size calculated from the

XRD data.

Figure 4.4 Reflectance mode UV spectra of Eu doped CdS

nanostructures.

78

Figure 4.5 Photoluminescence spectra of Eu doped CdS nanostructures

(inset shows photoluminescence spectra for pure CdS).

4.2.3.5 Photoluminescence Studies

Figure 4.5 shows the PL spectra of the synthesised Eu3+ doped CdS

nanostructures excited at 350 nm. Inset figure shows the emission spectrum of

pure cadmium sulfide nanocrystals. Compared to the pure CdS, intense emission

peaks at 475 and 574 nm was observed for Eu3+ doped CdS. The emission peak at

574 nm, due to the intra-4f transitions of Eu3+ ions which correspond to the

magnetic dipole transition, 5D0 →7F1 was blue shifted compared to previous result

(Gajbhiye et al 2008 and Patra et al 1999). In europium, the (5D0 →7F1) transition

is mainly magnetically allowed (magnetic–dipole transition) while (5D0→7F2) is a

hypersensitive forced electric– dipole transition being allowed only at low

symmetries with no inversion center. The electronic dipole transition, 5D0→7F2 is

hypersensitive to Eu3+ symmetry. It could be defined by asymmetric ratio (A21) of

the integrated intensities of 5D0 →7F2 to 5D0 →7F1 and is found to be 3.6. Lei et al

79

(2005) related the PL emissions of CdS nanostructures into band-edge and surface

defects. Because of the quantum confinement effect, the PL peak positions of the

band-edge emission can be related with the size of the CdS crystallites in the

wavelength range 350–500 nm. The surface-defect emission was caused by

surface states, such as sulfur vacancies and/or sulfur dangling bonds created by

doping, in the wavelength range 500–700 nm. Blue emission at 430 nm was

clearly observed which is in agreement with the earlier report (Di et al 2009) for

CdS nanostructures. The broad emission peak between 450–480 nm, centered at 475 nm with a maximum intensity was due to the band-band and lattice defect

emission respectively due to the CdS host and was blue shifted compared to the

earlier reports (Savchuk et al 2006, Chahbouna et al 2007 and Cheng et al 2006).

These observations indicate that a part of Eu3+ ions, contained in CdS nanocrystals

and the energy can be significantly transferred from the host CdS to Eu3+ ions

(Hayakawa et al 1999 and Okamoto et al 2002). Upon excitation, the energy from

non-radiative recombination of electron-hole pairs can be transferred to the high

energy levels of the Eu ions (Hayakawa et al 2000 and Reisfeld et al 2000). The

mechanism of the intensification of (rare earth emission) REE has already been

reported (Reisfeld et al 2000), which also supports the observed results of the

present study. These results indicate that the adsorbed CdS particles significantly

influence the excitation of 4f electrons in rare-earth ion. A significant change in the

intensity of the emission bands was observed for the samples prepared using

different mixed solvents, which are due to the variation in the structures for

different mixed solvents. It may also be attributed to the change in the energy

transfer rate with particle size variation and the shape of the nanocrystals.

4.3 Ce3+

DOPED CdS NANOPARTICLES

4.3.1 Materials Synthesis

The raw materials used for the synthesis of cerium doped cadmium

sulfide [Cd1−xCexS with x = 0.01, 0.02 and 0.03] were cadmium acetate

80

20 ml solution of 0.07 M

cadmium acetate in en+DI

water mixed solvent

20 ml solution of 0.07 M

cerium nitrate in mixed

solvent

Mixture stirring at

room temperature for

30 min

Dropwise addition of 20 ml

solution of 0.07 M sodium

sulphide in en+DI water.

washed with DI water and ethanol

Separation of nanoparticles

by centrifugation at 4000

rpm

Dried in a hot air oven at 60 C

Ultrasonication for 60

minutes

20 ml solution of 0.07 M

cadmium acetate in en+DI

water mixed solvent

20 ml solution of 0.07 M

cerium nitrate in mixed

solvent

Mixture stirring at

room temperature for

30 min

Dropwise addition of 20 ml

solution of 0.07 M sodium

sulphide in en+DI water.

washed with DI water and ethanol

Separation of nanoparticles

by centrifugation at 4000

rpm

Dried in a hot air oven at 60 C

Ultrasonication for 60

minutes

dihydrate (99.99%), cerium nitrate (99.99%) and sodium sulfide (99.9%)

purchased from Alfa Aesar. Mixed solvents of Ethylene diamine (EDA) and DI

water (resistivity 10−18 mΩ) with 1:1 ratio, were used as a solvent.

20 ml solution of cadmium acetate (99 mol. %) and 20 ml solution of

cerium nitrate (1 mol. %) with 0.07 M were prepared separately with EDA+DI

water as a mixed solvent. Solution of cerium nitrate was added slowly into the

cadmium acetate solution and the mixture was subjected to stirring at the room

temperature for 30 min. to achieve the dispersion of Cd2+ and Ce3+ into the solvent.

With this, 20 ml solution of 0.07 M sodium sulfide was added dropwise and it was

observed that the mixed solution turned to pale yellowish white and then to

orange. The obtained precipitate was then ultrasonicated for 1 h to make

nanoparticles evenly dispersed in the solution. The solution was subjected to

centrifugation (4000 rpm) to settle down the nanoparticles and washed with

ethanol and DI water for three times and dried in a hot air oven. Similar procedure

was followed for other two concentrations of cerium (2 and 3 mol. %), which

schematically shown below.

4.3.2 Results and Discussion

4.3.2.1 X-ray Diffraction Analysis

The XRD patterns of pure and cerium doped CdS (Figure 4.6) show

that the synthesized nanoparticles possess hexagonal (wurtzite) structure and the

planes (100), (002), (101), (102), (110), (103) and (112) are clearly indexed. The

absence of additional peaks in the XRD patterns shows the purity of the sample

81

without noticeable traces of impurity, even with increasing doping concentration.

The prominent peaks in the diffraction pattern are (100)h, (002)h, (101)h of CdS

(JCPDS No. 75-1545). There are no such peaks of Ce, CeS, Ce2S3 detected, thus

confirmed the successful incorporation of Ce3+ ions into the crystal lattice of CdS

nanoparticles. Compared to pure CdS, the diffraction peaks shift slightly toward

smaller diffraction angle. The average crystallite size of 3.3 nm, 2.9 nm, 3.1 nm

was calculated using Scherrer equation respectively for 1%, 2% and 3% Ce:CdS

nanoparticles.

Figure 4.6 XRD pattern for pure and cerium doped CdS nanoparticles

4.3.2.2 Structural and Compositional Analysis

TEM micrograph in Figure 4.7 (a and c) shows well-formed

nanocrystallites of cerium doped cadmium sulfide with nearly spherical in

shape. It clearly shows the particles within a diameter of 4 nm, which is

consistent with the value calculated from XRD analysis. The smallest well

dispersed particles observed from high resolution TEM image in

82

(a)

20 nm

(b)

5 nm

(c)

50 nm

(d)

20 nm

(e)

10 nm

(f)

10 nm

Figure 4.7 (b) (indicated by circles) of these ordered entities are in the

nanoscale regime. HRTEM images of Ce doped CdS nanoparticles are also

shown in Figure 4.7 (d-f).

Figure 4.7 (a and c) TEM and (b & d-f) HRTEM images of cerium

doped cadmium sulfide nanoparticles

83

EDX spectrum of Ce:CdS nanoparticles in Figure 4.8 shows the

presence of major chemical elements namely cadmium, sulfur and cerium.

The elemental composition calculated using ICP-OES analysis for 3% Ce

doped CdS was 76.30, 21.15 and 2.54% of cadmium, sulfur and cerium

respectively, which also confirms that majority of cerium ions doped with

cadmium sulfide.

Figure 4.8 EDX spectrum of cerium doped cadmium sulfide

nanoparticles

4.3.2.3 UV-Reflectance Studies

UV-reflectance spectra of Ce doped CdS nanoparticles are shown

in Figure 4.9. Based on these reflectance spectra the absorption edge for each

compound was determined. Using these absorption edge values, the bandgap

energy was estimated by extrapolating the linear region of the plot. Band-to-band

absorption at 453 (2.73 eV), 500 (2.48 eV) and 507 nm (2.44 eV) respectively

for 1%, 2% and 3% Ce:CdS nanoparticles shows a blue shift in comparison

with the bulk CdS, which may be ascribed to the quantum confinement effect.

It can also be seen that the increase in dopant percentage determines the red

84

shift in the absorption shoulder. This was clearly due to the substitution of

Ce3+ (1.034 Å) ions into the Cd2+ (0.97 Å) which increase the size of the

crystal lattice. This is in compliance with Brus equation (Brus 1986).

Figure 4.9 UV-Reflectance spectra of cerium doped CdS nanoparticles

4.3.2.4 Photoluminescence Studies

Upon excitation at 350 nm, the samples show luminescence in the

blue region, with an emission peak positioned around 350–450 nm as shown

in Figure 4.10. There is no defect related emission peaks observed in the

spectra. As shown in the inset of Figure 4.10 the peak position of NBE

emission slightly shifts toward longer wavelength region with higher

intensity, from 363 nm to 375 nm when the concentration of dopant increases

to 3 mol. %. Thus it confirms that luminescence property of CdS nanoparticles

enhanced when Ce3+ was introduced into the CdS. Maleki et al (2007),

85

attributed the peak at 363 nm, to higher level excitonic transition and its

energy was calculated to be 3.38 eV. Similar explanation was given by

Devi et al (2008) for the peak at 376 nm in nanocrystalline CdS. Usually the

fluorescence emission of doping ions has higher photostability than the defect

related luminescence of semiconductive nanomaterials, because the defects

are greatly affected by synthesis conditions and environments.

Figure 4.10 Photoluminescence spectra of Ce:CdS nanoparticles excited

at 350 nm

4.4 Sm3+

DOPED CdS NANOPARTICLES

4.4.1 Materials Synthesis

In the present work, CdS nanocrystals was synthesised by chemical

co-precipitation method using cadmium acetate dihydrate (Cd(CH3COO)2).

2H2O, samarium nitrate hexahydrate (Sm(NO3)3. 6H2O) and sodium sulfide

(Na2S) as Cd, Sm and S sources respectively. Samples were prepared with

2 and 5 mol. % of samarium.

86

For 2 mol. % doping, aqueous solution of 1.305 g of 0.1 M

Cd(CH3COO)2. 2H2O and 0.044 g of 0.1 M of Sm(NO3)3. 6H2O was

prepared. Aqueous 0.1 M Na2S solution was also prepared separately. Na2S

solution was then added slowly into the mixture of samarium nitrate and

cadmium acetate solution with constant stirring to attain the orange yellow

solution. pH of the solution was maintained at ~11 by adding aqueous

ammonia. The solution was then refluxed with constant stirring at ~70 C for 60 min. to get a saturated solution containing Sm3+:CdS nanocrystals (NCs).

The precipitate was ultrasonicated for 60 min. to avoid the aggregation of

nanoparticles. It was further washed with distilled water and ethanol for

several times to remove the organic residues present in the NCs and the

sample was collected by centrifuging with 4000 rpm and then dried in a hot

air oven. For 5 mol. % of doping, 0.111 g of Sm(NO3)3. 6H2O and 1.265 g of

Cd(CH3COO)2. 2H2O were used with the same procedure discussed above.

4.4.2 Results and Discussion

4.4.2.1 X-ray Diffraction Analysis

The representative XRD spectra of the Sm3+ doped CdS

nanoparticles samples with two different concentrations are shown in

Figure 4.11. The diffraction profile reveals that the synthesised nanocrystals

show cubic zinc blende (-CdS) structure of CdS (JCPDS No. 10-0454). No diffraction peaks of any other minerals were detected. The crystallite size was

calculated from a single diffraction peak using the Scherrer’s equation applied

to the (111) reflection of cubic CdS. The corresponding crystallite size was

estimated to be 2.62 and 2.64 nm for 2% and 5% of Sm doped CdS

respectively.

It was observed that the (111) peak shifted towards the higher angle

from 26.55 to 26.85 for 2% and 26.90 for 5% Sm doped CdS nanoparticles

87

compared to bulk CdS. It reveals that the inclusion of dopant (Sm3+) ions

substitute the interstitial sites of CdS nanoparticles to alter the crystal

structure and lattice contraction takes place. The decrease in the lattice

constant is due to the substitution smaller Sm3+ (0.964 Å) ions in Cd2+

(0.97 Å) site. In addition, the broadness of the peak implies the synthesised

Sm doped CdS particles have smaller diameter in the nanoscale regime.

Figure 4.11 X-ray diffraction of samarium doped cadmium sulfide

nanocrystals

4.4.2.2 Structural, Morphological and Composition Analysis

FESEM images of the synthesized Sm3+ doped CdS nanocrystals

with two different magnifications are shown in Figure 4.12 (a, b). The

particles possess almost a spherical shape with little agglomeration and the

particles are distributed homogeneously.

For compositional analysis, the synthesised samples were studied

by EDX. For samarium doped CdS nanocrystals (Figure 4.13), the spectrum

88

(a) (b)

indicates the presence of Sm, Cd and S elements and was confirmed by the

elemental mapping shown in Figure 4.14. These results clearly reveal that the

material synthesised are exactly Sm:CdS, having compositions consistent

with the stoichiometric composition. The elemental composition of the

synthesised products was further confirmed by ICP-OES analysis (Table. 4.2).

From the results it was evident that the RE ions are incorporated into the CdS

crystal lattice.

Figure 4.12 FESEM images of Sm doped cadmium sulfide nanocrystals

Figure 4.13 EDX spectrum of Sm doped cadmium sulfide nanocrystals

89

Figure 4.14 Elemental mapping of Sm doped cadmium sulfide

nanocrystals

Table 4.2 Bandgap energy, Specific surface area and calculated mole

percentage by ICP-OES of Sm doped Cadmium Sulfide

Nanocrystals

Sample

Mole percentage calculated

by ICP-OES

Bandgap

energy

(eV)

Specific

surface area

(m2/g)Sm Cd S

2% Sm:CdS 0.38 76.62 22.98 2.63 140.7

5% Sm:CdS 0.77 76.11 23.10 2.58 132.9

Cd S

Sm

90

4.4.2.3 XPS Analysis

X-ray photoelectron spectra (XPS) analysis of samarium doped

CdS nanocrystals is shown in Figure 4.15 (a-d) for 5 mol. % Sm

concentration. The survey spectrum (Figure 4.15 a) reveals the presence of

characteristic peaks with binding energies as follows: C 1s (285.0 eV), O 1s

(531.8 eV), Cd 3d5/2 (405.38 eV), Cd 3d3/2 (412.13 eV) and S 2p (major peak

at 161.84 eV and a low intensity peak at 168.97 eV). In this case, the Sm 2p3/2

binding energy is observed at 1084.37 eV. The binding energy of S 2p at

161.84 eV also agrees well with binding energy with blue shift compared to

the observed S 2p in pure CdS nanoparticles (Winkler et al 1999). The

appearance of low intensity peak at 168.97 eV is due to oxidation of sulphur

in nanoparticles (Wagner et al 1978).

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Figure 4.15 XPS spectra for (a) wide energy (b) Cd 3d5/2 and Cd 3d3/2 in

the 5 mol. % Sm doped CdS nanocrystals

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Figure 4.15 XPS spectra for (c) S 2p3/2 and (d) Sm 3d5/2 in the 5 mol. %

Sm doped CdS nanocrystals

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20 nm

(b)

50 nm

(a)

4.4.2.4 HRTEM Analysis

Figure 4.16 (a) and (b) shows the TEM images of synthesised Sm

doped CdS nanoparticles for different magnifications. It can be seen from the

figure that the nanoparticles are nearly spherical in shape with little

agglomeration. Inset in Figure 4.16 (b) shows the SAED pattern of an area

containing some nanoparticles. The SAED pattern shows a set of rings of the

nanocrystals correspond to the (111), (220), and (311) planes of the cubic CdS

phase, respectively and also demonstrate the crystallinity of the synthesised

nanoparticles. This result is consistent with that of the XRD which composed

of pure cubic phase CdS.

Figure 4.16 (a, b) TEM images of samarium doped CdS nanocrystals.

Inset in (b) shows SAED pattern

The specific surface area (SBET) was calculated from the linear part

of BET (Brunauer–Emmet–Teller) plots from the N2-adsorption-desorption

isotherm shown in Figure 4.17 (a, b). The BET results (Table 4.2) showed

that the high specific surface area for both the Sm doped CdS nanocrystals.

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Figure 4.17 N2-adsorption-desorption isotherm of samarium doped

cadmium sulfide nanocrystals

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4.4.2.5 UV-reflectance studies

To investigate the optical absorption properties, the diffuse

reflectance UV spectra (DRS-UV) of Sm3+ -doped CdS in the range of 300–

800 nm was examined and the results are shown in Figure 4.18. From these

curves it is noticed that absorption is dominant mainly in blue region. The

observed shift in the spectra is due to the size quantization effect according to

which the bandgap value increases with the size reduction of crystallites. The

band gap energy of this RE doped CdS nanocrystals was estimated and

tabulated in Table 4.2. From the reflectance spectra, the absorption band edge

was found to be 471 nm and 480 nm for 2% and 5% of Sm doped CdS

nanocrystals respectively. While increasing the concentration, we can clearly

observe the obvious red shift in the spectra for 5% Sm:CdS, with decreasing

bandgap energy. Increase in the dopant concentration from 2% to 5%, shifts

the absorption shoulder towards higher wavelength. This shift is due to the

variation in the ionic radii of Sm3+ (0.964 Å) compared to that of Cd (0.97 Å)

where it is substituted. This is in compliance with Brus equation (Brus 1986)

which states that bandgap decreases with increasing size or that optical

absorption wavelength increases with increasing ionic size.

Figure 4.18 Reflectance UV spectra of Sm doped CdS nanocrystals

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4.4.2.6 Photoluminescence Studies

Figure 4.19 (a, b) shows the room temperature photoluminescence

(PL) spectra of 2% and 5% Sm doped CdS nanocrystals respectively, obtained

with an excitation wavelength of 350 nm. It is observed that the emission

spectra for both the samples illustrate a band-edge (NBE) emission at ~394

nm. It was reported that this kind of band-edge luminescence can be caused

by the recombination of excitons and/or shallowly trapped electron–hole pairs

(Wang et al 2001) and it was markedly blue shifted relative to that of the pure

CdS as shown in Figure 4.19 (a). Bulk CdS reported to have a maximum

broad emission in the 500–700 nm region of the luminescence spectra

(Sreekumari et al 2001). For 2% of Sm doped CdS NCs the emission spectra

in Figure 4.19 (a) includes two bands, a narrow band peaked at ~394 nm and a

low intense peak around ~420 nm. The 394 nm emission band is attributed to

the intrinsic emission of CdS NCs (or the band-to band transition in CdS

nanocrystals), while the other band centered at 420 nm is attributed to the

emission of surface states due to the small size of nanocrystals.

Figure 4.19 (a) Photoluminescence spectra for 2% of Sm doped CdS

nanocrystals. Inset shows a PL spectrum of pure CdS

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Figure 4.19 (b) Photoluminescence spectra for 5% of Sm doped CdS

nanocrystals

It is also observed that there is a slight shift in the band edge

emission towards higher wavelength (~398 nm) for 5% Sm doped CdS NCs.

With increasing concentration of Sm3+ ions (5%), the peak at ~ 420 nm gets

shifted towards longer wavelength of 524 nm. The origin of this emission

peak, in the presence of RE ions is related to the transitions between the

excitonic levels of CdS and the energy levels due to Sm3+, which cause shift

in the emission peaks towards longer wavelength side. Increased in the

concentration of RE ions improve the peak intensities, which is due to the

energy transfer from energy levels of RE ion. Mathew et al (2008) described

the enhancement in emission spectra is attributed to the energy transfer from

the modified CdTe phase to the rare earth ion. Similarly Agrawal et al (2011)

reported that, increasing La concentrations on CdS–Se films shifts the

emission spectra towards longer wavelength, which is caused due to the

transfer of energy from energy levels of La ion.

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The obtained result is consistent with variations observed in the

UV-reflectance spectra and specific surface area in the increasing RE dopant

ions in CdS nanocrystals. When a higher percentage of dopant is

introduced into the CdS crystal, changes in the band gap and lattice

distortions are expected to enhance the red shift in the emission spectra

(Shafiq and Sharif 2009).

4.4.2.7 Raman Studies

Micro Raman spectra for 2 and 5% samarium doped CdS

nanocrystals are shown in Figure 4.20. The 1LO and 2LO phonon modes of

CdS, located around 294 and 595 cm−1 respectively, are clearly identified in

the Raman spectra of both the samples, illustrating characteristic Raman shifts

analogous to those of pure crystalline CdS (Suh and Lee 1997). The observed

phonon peaks are shifted toward lower frequency as expected for bulk, likely

due to effect of small size and high surface area.

Figure 4.20 Raman spectra of the samarium doped cadmium sulfide

nanocrystals

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4.5 CONCLUSION

Eu-doped CdS nanostructures were synthesised by co-precipitation

method using mixed solvent system. The structural and morphology changes

in the samples prepared using different mixed solvents were studied by

powder XRD and FESEM analyses. Samples prepared with EG+DI water

exhibits hexagonal wurtzite phase, whereas with IPA+DI water shows cubic

zinc-blende phase of CdS. Flower like morphology was observed for

Eu:CdS-1 (EG+DI water) whereas clusters of rice like morphology was

obtained for Eu:CdS-2. It was observed that the structure, crystallite size and

the band gap of the europium doped CdS nanocrystals can be finely controlled

by simply varying the viscosity and density of the mixed solvents. The

incorporation of europium in the CdS lattice was confirmed by ICP analysis.

UV-reflectance spectra reveal the blue-shift, confirming the size quantisation

effect in the synthesized samples. In the characteristic room temperature PL

emission spectra of the Eu3+-doped CdS, besides the peaks due to intra-4f

transitions of Eu3+, emission peak due to CdS nanocrystals was also observed.

The results confirm that at least a part of the Eu3+ ions is effectively doped

into CdS nanocrystals and the energy transfer occurs from CdS nanocrystals

to Eu3+ ions.

Ce:CdS nanoparticles with different cerium concentrations (1, 2

and 3 mol. %) have been synthesized by chemical co-precipitation method.

XRD pattern reveals the formation of hexagonal structure of Ce:CdS with

shift toward lower angle which confirms the incorporation of cerium.

HRTEM images and SAED pattern adequately demonstrated the well

dispersed and crystalline nature of the synthesized nanoparticles. Compared to

pure CdS, the calculated bandgap energy value of Ce doped CdS

nanoparticles illustrates quantum confinement effect. For 3 mol. % of cerium

concentration, the emission intensity was found to be maximum with a slight

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shift towards higher wavelength compared to 1 and 2 mol. %. Intense

emission was observed due the incorporation of cerium ions into CdS NPs,

which will be useful for high efficient EL devices and also possible for

making full color device applications.

Sm3+ doped CdS nanocrystals have been prepared by chemical

co-precipitation method. The synthesised materials exhibit cubic zinc blende

phase with an average crystallite size of ~3 nm. FESEM images show the

aggregated morphology of the Sm doped CdS nanocrystals with high surface

area of ~ 140 m2/g. Variation in the bandgap with doping concentrations of

Sm3+ on CdS nanocrystals was observed using reflectance spectra. It was also

revealed that increasing dopant concentration decreases the bandgap value of

synthesised nanocrystals. Enhanced emission can be attributed to the energy

transfer from CdS crystallites to RE3+ ions. Peak shift in the PL spectra are

consistent with the successful incorporation of Sm3+ in the CdS nanocrystals.

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