Copper Doped Zinc Sulfide Thin Film by Successive Inonic Layer Deposition (Silar)
Method
S. Syed Zahirullah1, J. Joseph Prince2
and M. Shahul Hameed1
1Department of
Physics, M.I.E.T. Engineering College, Tiruchirappalli–620 007, Tamilnadu, India.
2Department of
Physics, Anna University, Tiruchirappalli–620 024, Tamilnadu,
India.
ABSTRACT:
Successive ionic layer adsorption and
reaction (SILAR) method, has emerged as one of the solution methods to deposit
a variety of compound materials in thin film form. The SILAR method is simple,
inexpensive and convenient for large area deposition. In the present work the
colloidal solutions as well as thin films on glass substrates of zinc sulfide
and zinc sulfide: copper in Poly-Vinyl alcohol matrix have been prepared
through chemical route. In the present study the XRD patterns show a mixed type
structures having crystal planes of both cubic zinc blend and wurtzite hexagonal phases. The UV-Visible spectra of zinc
sulfide and zinc sulfide: copper shows the enhancement of band gap energy due
to the quantum size effect.
KEYWORDS: Zinc sulfide, copper, SILAR, thin film.
INTRODUCTION:
Research on semiconductor nanoparticles have
been attracting a great interest in recent years because of their distinctive
optical and electrical properties. Zinc sulfide is a wide-gap important II–VI
compound semiconductor material and it has been researched extensively because
of its wide spectrum of potential applications1. In particular,
copper-doped zinc sulfide shows unique properties which are different from
those of un-doped matrix materials, such as the emission spectra and decay time
of doped samples can be varied which create new opportunities for luminescent
study2.
Chemical film fabrication method involves chemical reaction and the
precursors are frequently components undergoing reaction at the substrate
surface or in the surrounding area of the substrate. One of the newest solution
methods for the deposition of thin film is successive inonic
layer adsorption and reaction (SILAR) method, which is also known as modified
version of chemical bath deposition3. Synthesis of nanoparticles by this method is quite easy and inexpensive
as compared to other methods. Keeping the above aspects the present study has
been undertaken to synthesis of zinc sulfide and copper doped zinc sulfide nanoparticles.
MATERIAL AND METHODS:
Synthesis: The chemical bath deposition method was employed to
deposit copper doped zinc sulfide thin films on to glass substrate3.
The deposition was carried out in a matrix solution and thiourea.
The matrix solution was prepared by adding sulfide ion source and zinc sulphate as zinc ion source in alkaline bath. The aqueous
solution of 3% weight of polyvinyl alcohol with constant stirring at constant
temperature was maintained for overnight. Zinc sulphate
of (0.5 Molarity) was used to get matrix solutions
and equal volume of thiourea solution of same molarity was prepared.
Equal volume of the matrix solution and thiourea
were mixed together to form zinc sulfide and simultaneously copper sulphate solution of 0.005 morality were added as doping
agent to get the copper doped zinc sulfide thin film. Ammonia solution was
added slowly to metal salt solution to form the metal complex and its pH value
was adjusted between 10 and 127.
Substrate cleaning plays an important role in the deposition of the thin
films4. Commercially available glass micro slides of dimensions 26 x
76 x 2mm were boiled in chromic acid and dried in air prior to deposition.
These glass substrates were kept immersed vertically in the as-prepared
colloidal solution of nanoparticles for nearly 24
hours at room temperature for deposition of the thin films.
Characterizations:
The structural investigation of zinc sulfide and zinc sulfide: copper
was carried out using X-ray powder diffractometer
(Model: Seifert XRD 3003 T/T) with CuKα
radiation (λ=0.15406nm) scanning 2θ in the range 200-800. The
UV-Visible absorption of the samples was recorded using an automated
spectrometer (Model: HITACI 113210) in the wavelength range 200nm -800nm.
RESULTS AND DISCUSSION:
The physical appearance of zinc sulfide and zinc sulfide: copper
indicates that layers are homogeneous, smooth, dirty white in colour and strongly stick to the substrate. The thin films
are taken for XRD characterization and for optical studies the solutions are
taken.
Structural Studies:
The prominent peak positions (2θ) of undoped
zinc sulfide are found at 23°, 27°, 51° and 62° corresponding to planes (111),
(200), (220) and (222) for cubic zinc blende phase
while the relatively low intensity peaks at 26.5, 28 and 47.30 are assigned to
(100), (002) and (110) planes of that of hexagonal type. It is observed that in
a mixed type phases of zinc sulfide the crystal planes (111) cubic is close to
(002) hexagonal as well as that of (220) is close to (110) and thereby it is
often difficult for precise identification. Similarly the XRD peaks of zinc
sulfide: copper are observed at 22.5°, 24° and 43° and their
respective cubic planes are (111), (200) and (222). The hexagonal planes are
(002) and (110) which corresponds to (2θ) at 19° and 39° respectively.
In the present study the data are compared with standard data from JCPDS
[No 39-136] clearly shows that both zinc sulfide and zinc sulfide: copper nanocrystals obtained in this study have the mixture of
cubic and hexagonal structure5. The broadening of the different
peaks of the nanoparticles is evidently the
characteristic of nanosized particles. The estimated
sizes of zinc sulfide and zinc sulfide: copper nanophosphors
from XRD using Debye Scherrer formula are found to be
5nm and 7nm respectively. In the present study other peaks are also observed
and it may be due to presence zinc oxide and polyvinyl alcohol in the samples.
Optical properties:
In case of zinc sulfide nanoparticles the
absorption peak is noted around 376nm indicating band gap development due to
quantum size effect4. In a quantum confinement regime the electrons
in the conduction band and the holes in the valence band are spatially confined
by the potential barrier of the surface. The lowest energy optical transition
from valence band to conduction band increases may be because of confinement of
both electrons and holes. The estimated band gap energy found to be 3.96eV
which is quite higher than the bulk materials ~ 3.96eV.
The additional absorption at 435nm is due to inherent native defects. In
copper doped zinc sulfide the UV-Visible spectra reveal the absorption edge at
380nm indicating red shift and its band gap energy is estimated to be ~ 3.6eV.
It may be attributed that the small red shift in band edge may be due to the
increase in particle size.
Stokes shift in nanoaggregates may also play an significant role as energy of emission is expected to be higher
in small aggregate due to edge effects6. The additional peaks at
370nm and 425nm for copper doped zinc sulfide are also observed which may be
attributed to the presence of lattice defects due to sulfur vacancy7.
However in the present study the prominence of the peaks shows some of copper
ions lying in the interstices of host zinc sulfide lattice.
CONCLUSION:
In the present findings the XRD patterns illustrate a mixed type
structures having crystal planes of both cubic zinc blende
and wurtzite hexagonal phases. The UV-Visible spectra
of zinc sulfide and zinc sulfide: copper show the enhancement of band gap
energy owing the quantum size effect. It shows a small red shift on copper
doping. The results confirmed the presence of copper ions lying in the
interstices of host zinc sulfide lattice.
REFERENCES:
1.
Liu XY, Tian BZ, Yu CZ, Tu B and Zhao DY. Microwave-assisted solvothermal
synthesis of radial ZnS nanoribbons.
Chem. Lett, 2004; 33:522-523
2.
Chen YY, Duh JG, Chiou BS and Peng CG. Luminescent mechanisms of ZnS:Cu:Cl
and ZnS:Cu:Al phosphors. Thin Solid Films,
2001; 392(1):50-55.
3.
Bodo Bhaskarjyoti
and Kalita PK. Chemical Synthesis of ZnS:Cu Nanosheets,
AIP Conf. Proceed, 2010; 1276:31-36.
4.
Alvaro A, de Quiroz A, Mayler
Martins, Demetrio AW and Soares Ecio
J. Franca, Modeling of ZnS quantum dots synthesis by
DFT techniques, Journal of Molecular
Structure, 2008; 873:121-129.
5.
Jayanthi K., Chawla
S., Chanda H. and Haranath
D., Structural optical and photoluminescence properties of ZnS:Cu nanoparticles thin
films as a function of dopant concentration and
quantum confinement effect, Cryst. Res.Technol, 2007; 42(10):976-982.
6.
Spano FC. Absorption and emission in oligo-phenylene vinylene nanoaggregates: The role of disorder and structuctural defects, J. Chem.Phys, 2002; 116:5877-5889.
7.
Hao-Ying Lu and Chu Sheng-Yuan.
The mechanism and characteristics of ZnS based
phosphor powders, Journal of Crystal
Growth, 2004; 265:476-481.
Received on 10.08.2013
Modified on 05.09.2013
Accepted on 14.09.2013
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Research
Journal of Pharmacognosy and Phytochemistry. 5(6): November
–December 2013, 297-299