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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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