1. INTRODUCTION:
Cadmium sulfide has, hexagonal wurtzite structure and the cubic zinc blende structure . Cadmium sulfide is a direct band gap II-VI semiconductor (2.42 eV). CdS has discrete energy levels, tunable
band gap, size dependent optical properties and well developed synthetic
protocols, good chemical stability [1]. CdS is having many optoelectronic
applications including solar cell, photodiode, Light emitting diode, Non linear
optics, heterogeneous catalysis[2].The conductivity increases when irradiated
with light leading to uses as a photo resistor. Both polymorphs are piezoelectric and the hexagonal is also pyroelectric. Electroluminescence
CdS crystal can act as a solid state laser[3].CdS and cadmium selenide are used in manufacturing of photo resistors sensitive to visible and near infrared
light [4]. Since CdS has 2.42 eV(515nm) band gap, it is the most promising
candidate among II-VI compounds for detecting visible radiation [5]. Cadmium Sulfide is also used in the production of solar
cells where it is used as a buffer layer in the manufacture of CIGS (Copper
-Indium-Gallium-Selenide) solar cells [2].
The Optical properties were determined by using UV-Vis
spectroscopy. The absorption spectra of CdS exhibit a well defined absorption
feature at ~ 478nm. This is assigned to the optical transition of the first
excitonic state. Generally the wavelength of the maximum exciton absorption
decreases as the particle size decreases as a result of quantum confinement of
the photo generated electron- hole pairs. While considering blue shift relative
to the absorption of bulk CdS indicates quantum size effect [6]. The band gap
of nanoparticle of CdS from Absorption Spectra was found to be 2.66 eV(480nm)
while that of bulk is 2.42eV (515nm)[7]. Rare earth atom exhibit several
interesting properties, large magnetic moments and luminescence. Introducing Ho
atoms into the CdS matrix can lead to a material that shows multiple intensity
effects. Holmium ions are shown in photo stable up conversions nanoparticle,
when energy transfer between two elements results in multi color luminescence.
Holmium lasers are used in medical, dental, and fiber-optical applications.
Another important application of this element Holmium is their uses as a
magnetic resonance imaging (MRI) contrast agent [9]. Aim of this investigation
is to investigate the effect on optical properties of CdS nanoparticle doped
with magnetic rare earth element Holmium ions.
2. MATERIAL AND METHODS:
Synthesis and Characterization
CdS nanoparticles are prepared by the proper mixing of
Cadmium chloride and Sodium sulphide solutions. CdS nanoparticles were grown by
the chemical precipitation method at room temperature. In synthesis procedure
100 ml aqueous solution of the reactants was prepared. 0.1 M of CdCl2,
0.1 M of Na2S, and 0.1M of [Ho(OH2)9]3+
complexes in dilute H2SO4 were used as the reactant
materials. Freshly prepared aqueous solution of 0.1M Na2S was mixed
drop by drop in the 0.1 M CdCl2 solution using vigorous
stirring. As the reaction was started the reaction system gradually changed
from transparent to light yellow and after completion of reaction this turn to
dark yellow. Few drops of Tri Ethyle Amine (TEA) are added to the solution to
prevent the agglomeration of particles. The precipitate was then washed several
times with ethanol and then with acetone and centrifuged. The precipitate
collected from centrifugation was dried at 50ºC for 3 hours. After annealation
sample is taken and made into fine powder form. The XRD of the powder were
studied using Rigaku DMAX diffractometer using Cu -Kα radiation
monochromatised with a graphite crystal and high temperature attachment in
θ-2θ geometry. Data in JCPDS file can compare with observed values.
The surface topography and microstructure were studied using Field Emission
Scanning Electron Microscopy (FESEM). FTIR spectroscopy uses Michelson
interferometer to produce an interferogram. Energy Dispersive X-ray Spectrum
Analysis (EDX) was used to determine percentage composition of Ho in CdS. In
this investigation, The UV measurements were done with UV spectrometer JASCO
V-550 in the wavelength range of 200 nm to 850 nm. The UV absorption peak of
the prepared nanocrystal is measured and computed the optical band with energy.
For the fluorescent analysis, the excitation and emission spectra are carried
out. Excitation wavelength was fixed at 498 nm, to obtain the emission spectrum
between 350 and 670 nm.
3. RESULTS AND DISCUSSIONS:
A. Particle Size determination
XRD techniques are widely used for the particle size
determination of nanoparticles. Width of the diffraction lines and their shape
are characteristic of the crystalline size and the micro strain. If the line
broadening is only due to small crystalline size, the size of the crystalline
can be estimated from the Scherrer’s formula [8]
(1)
Where, θ is the Bragg’s angle, λ is the
wave length of the X-rays, L is the mean dimension of the crystalline size of
the powder sample, β is the full width at half maximum of the diffraction
at 2θ scale in the radius, k is the constant approximately equal to 0.9.
The diffraction peaks so obtained, fig 1(a-b), are compared with X-ray powder
data file, published by the Joint Committee on Powder Diffraction Standards
(JCPDS)
XRD results showed that the crystallite size of CdS:Ho nanopowder
was 13.39 nm at 80˚C which increased to 16.73 nm at 100˚C. From the
figure it is clear that the intensity of crystalline peaks increases with
increase in temperature. Simultaneously the peaks become narrower as the
temperature was increased showing the increase in crystallite size The
continuous increase in the particle size with temperature can be attribute to
the atomic diffusion. From the atomic perspective, diffusion is a stepwise
migration of atom from lattice site to lattice site. In Fact, the atom in solid
material are in constant motion, rapidly changing position. For an atom to make
such a move the atom must have sufficient energies to break bonds with its
neighbor atoms and then cause some lattice distortion during the displacement.
As the temperature increases the atom gain sufficient energy for diffusion
motion and thereby increasing crystallite size [16]. According to Ostwald
ripening phenomena the increase in the particle size is due to the merging of
the smaller particles into larger ones is a result of potential energy
difference between small and large particles and can occur through solid-state
diffusion[15].
B. FTIR Spectra of nanocrystalline CdS: Ho
FTIR measurements are undertaken in order to confirm the
formation of crystalline CdS nanoparticles and identify adsorbed species on to
the crystal surface. Bands at 696cm-1 is assigned to the stretching
vibrations of Cd-S [10, 11]. The stretching frequency of bulk CdS is 586cm-1Intense
band is centered at 1310cm-1 is attributed to the symmetric
deformation of sulphide group. The absorption peak centered at 3496 cm-1
and 1625 cm-1 corresponds to O-H stretching and bending frequencies
of water respectively, indicating the existence of water in the surface of
nanoparticle depicted in fig.2.
C. SEM Analysis of CdS:Ho nanoparticle
The Scanning electron microscopy spectrum of CdS: Ho is
shown in figure 3(a-b) with different magnifications clearly indicates the
formation of nanoclusters. The grains have aggregated to form nanoclusters
.Image also confirm hat the nanoclusters consists of a high density of CdS:Ho
nanoparticle with wave length varies from 500nm to 1µm with an average particle
size 13-17 nm. The formation of good spherical structure proved the formation
of Quantum Dots.
D. UV absorption Spectra of CdS: Ho
nanoparticle
The Optical properties were determined by using UV-Vis spectroscopy.
Energy band gap studies [5] of these materials have been reported using
absorption spectra depicted in Fig. 4. The material is reported as direct band
gap material. For higher values of absorption coefficient, optical absorption
showed a power load dependence on photon energy [13, 14].
(2)
Where exponent γ can take values 2,3,1/2 and
3/2 for indirect allowed ,indirect forbidden, direct allowed and direct
forbidden transition respectively.
Eg is the optical band gap. Optical energy gap is
obtained by extrapolating the linear portion of the absorption spectrum to αhω
= 0. The Band gap energy CdS:Ho is measured from the Tauc plot
,fig.4, is 5.35 eV. The band gap of nanoparticle of undoped CdS from
Absorption Spectra was found to be 2.66 eV (478nm) while that of bulk is 2.42
eV (515nm) [7].
Fig.4 Tauc plot of CdS;Ho
Fig 5. UVAbsorption SpectraCdS:Ho
Fig.5 of
the absorption spectra of CdS:Ho exhibits a well defined absorption feature
at ~ 485nm. Due to doping with Holmium the absorption peak has been from 478 to
485 nm. This is assigned to the optical transition of the first excitonic
state. Generally the wavelength of the maximum exciton absorption decreases as
the particle size decreases as a result of quantum confinement of the photo
generated electron- hole pairs. While considering blue shift relative to the absorption
of bulk CdS indicates quantum size effect [6].
The increase in the band gap energy of doped CdS
nanocrystalline is due to the Burstein-Moss effect [12]. The effect occurs when
the carrier concentration exceeds conduction band edge density of states, which
corresponds to degenerate doping in semiconductors. In nominally doped
semiconductors Fermi level lies above the donor states, just below the
conduction band. As you increase the doping concentration, more and more donor
states are produced which pushes Fermi level higher in energy and in the case
of degenerate level of doping, Fermi level lies inside the conduction band
above the occupied donor states. In the case of degenerate semiconductor an
electron from the top of the valence band can only be excited into conduction
band above the Fermi level (which now lies in conduction band) since all the
states below the Fermi level is occupied donor states [12]. Pauli's exclusion
principle forbids excitation into these occupied states. Thus we observe an
increase in the measured band gap. The absorption edge of the Ho doped CdS has
been shifted to longer wavelength side, due to the co-valent bonding of Ho with
CdS. As the absorption edge of Ho doped CdS is shifted to longer wavelength,
the band gap might be decreased compared to undoped CdS due to the introduction
of new energy in the surface band gap of CdS nanoparticle [17]. The red shift
of the absorption edge to the longer wavelength side has been attributed to the
strong exchange interaction between the d electron of Ho and s
and p electron of the CdS host band [18]. The Ho doped CdS is
highly effective and can significantly enhance the photo catalytic degradation
[19].
E. Fluorescence spectrum
Emission Curve
The bandwidth of a monochromator is defined as the span of
monochromatic settings (in units of wavelength) needed to move the image of the
entrance slit across the exit slit. The bandwidth of the spectrofluorometer can
be changed by adjusting the width of the excitation and emission slits. Fill
the quartz curette with Cadmium Sulphide, and insert it into the cell holder.
Wavelength range:660-860 nm, Em. Wavelength 625 nm Ex. wavelength:
350-670 nm. Once the parameters are correctly set, run the spectrum. The
excitation maximum is as depicted in fig 6.
Fig.6 Fluorescent Emission Curve of CdS:Ho
4. CONCLUSIONS:
CdS nanoparticles have been synthesized by aqueous medium
through chemical co-precipitation technique. The size and crystal structure of
CdS doped with Holmium was studied using XRD. The SEM images are also obtained
The XRD results is indicated that the particle size of nano CdS doped with
Holmium is much small as compared to that of pure CdS and decreases with
Holmium loading. From XRD results it is clear that as temperature increases,
particle size also increases. The change in particle size causes large
variation in the physical properties since 1nm size change may introduce a
considerable change in the number of surface atoms with lower co-ordination and
broken exchange bonds. The FTIR spectrum is used to characterize the
nanoparticle. Absorption peaks in the FTIR spectrum of CdS with different
particle size were explained. The wave numbers associated with variations of
the fundamental modes were determined. The shifting of the bands observed in
the FTIR spectra is due to quantum size effects. The Band gap energy CdS:Ho is
measured is 5.35 eV. The absorption spectra of CdS:Ho exhibits a well
defined absorption feature at ~ 485nm. Due to doping with Holmium the
absorption peak has been from 478 to 485 nm which is assigned to the optical
transition of the first excitonic state. The material is reported as
direct band gap material. The band gap energy for bulk CdS is 2.4eV. The
increase in the band gap energy of nanocrystalline CdS is due to the Burstein
effect. From PL spectra, the position of emission peak shifts towards higher
energy with the decrease of nanoparticle size.
5. ACKNOWLEDGEMENT:
We are thankful to the Dr. Alexander foundation for
financial support.
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