OPTICAL AND STRUCTURAL PROPERTIES OF SPIN COATED CADMIUM SULFIDE THIN FILMS

Thin films of cadmium sulfide have been synthesized using the spin coating technique. Films were fabricated on inexpensive amorphous glass substrates at spin speeds of 1500, 2200 and 2400 rpm for 30 seconds. Films were subsequently annealed at 200, 300 and 400 C for one hour in air in order to crystallize the phase of CdS. Films were characterized using X-ray diffraction patterns (XRD) and UV-visible spectroscopy. According to XRD patterns, the phase of CdS was crystallized without any secondary phases. The particle size, dislocation density and strain were also estimated using XRD patterns. All the films indicate a strong orientation in (002) direction. The optical band gap was determined using UV absorption spectroscopy. Tauc model was employed to determine the optical band gap. According to our data, the optical band gap decreases with the particle size.

and solar selective coatings (Olopade et al., 2013).Nanostructures have attracted a great interest in recent years because of their unique chemical, physical, optical, electrical and transport properties which are different from those of either the bulk materials or single atoms.Due to the vast surface area, all nanostructured materials possess a high surface energy and thus, are thermodynamically unstable or metastable.CdS nanoparticles are considered as a capable applicant for their applications in future opto-electronic devices, nanodevices and biological labeling due to availability of discrete energy levels, tunable band gap, size dependent chemical and physical properties, better chemical stability and easy preparation techniques.
There are many methods of fabricating CdS thin films.These include spray pyrolysis (Yadav et al., 2010), chemical bath deposition (Lisco et al., 2015), electro deposition (Takahashi et al., 2002), screen printing (Kumar et al., 2011), physical vapor deposition and sol-gel spin coating.Among various deposition methods, sol-gel spin coating technique is extensively studied as a matrix material method to produce nanocomposites because it gives a higher specific surface area, superior homogeneity and purity, better microstructural control of metallic particles, narrow pore size and uniform particle distribution.The main advantages of the sol-gel method are its simplicity, low cost and its ability to obtain uniform films with good adherence and reproducibility in a relatively shorter processing time at lower temperatures.

EXPERIMENTAL
For the deposition of CdS thin films two solutions have been prepared.
Polyethylene Glycol (PEG) was dissolved in ethanol (CH3CH2OH), and acetic acid (CH3COOH) was added to ethanoic solution under stirring which was continued for 1 hour (Solution 1).Cadmium nitrate (Cd(NO3)2) and thiourea (CS[NH2]2) were dissolved in ethanol under stirring which was continued for 1 hour (Solution 2).Solution 2 was mixed with Solution 1 and stirred again for 4 hours to obtain the final sols for deposition of thin films.CdS thin films were deposited on ultrasonically cleaned glass substrates by sol-gel spin-coating technique.Solution was dropped onto the glass substrates at speeds of 1500, 2200 and 2400 rpm for 30 seconds.After deposition, the glass substrates were dried on hot plate at 120 0 C for 1 hour and then annealed at temperatures of 200 0 C, 300 0 C, and 400 0 C in air for one hour.
Structural properties of film samples were determined using X ray diffraction (XRD) with Cu-K radiation of wavelength 1.54060 Å. UV-visible spectrometer was employed to investigate optical properties of samples.Table 1 shows the values of particle size, dislocation density, strain and interplanar distance calculated using XRD patterns given in figure 1 for different XRD peaks.Calculated results shows that the crystallites of the CdS thin films were larger at higher temperatures.

RESULTS AND DISCUSSION
The absorbance of film samples versus wavelength was measured using UV-visible spectroscopy.Hence the absorption coefficient at different wavelengths was calculated.Figure 2 indicates the graph of (αhv) 2 versus (hv) for CdS thin film coated at 2200 rpm and annealed at 300 0 C. The optical band gap (Eg) was determined using Tauc model by plotting (αhv) 2 versus (hv) and selecting the linear part in the relation of (αhv) 1/n =A(hv-Eg).Here A is a constant, α absorption coefficient, hv the photon energy and n is equal to 2 for direct transitions.The calculated band gap values are greater than that of the bulk CdS as shown in Table 2.The optical energy gap decreases with increasing temperature and it indicates a slight increase with increasing spin speed.This indicates the formation of nanoparticles of CdS and quantum confinement effect.The relation between particle radius and the band gap is given by Brus equation as follows.
Where Eg and Ebulk are the band gaps of CdS thin film and bulk, respectively, h is the Planck's constant, m*e and m*h are the effective masses of electron and hole, respectively , ε is the dielectric constant and r is the radius of the grains.The third term stands for the columbic interaction energy and often can be neglected due to high dielectric constant of semiconductor materials.Second term represents the additional energy due to quantum confinement having r -2 dependence on the band gap energy.
If the sphere is too small, the movements of the electron and hole are restricted and so they feel confined which raises the energy required to excite the electron into the conduction band.

CONCLUSIONS
CdS nanostructures have been successfully prepared by the sol-gel spin coating method.Compared to the powder diffraction pattern of CdS, (002) peak of XRD pattern of our CdS film becomes dominant.All CdS thin films exhibit a favored direction along (002), and crystalline size increases with the annealing temperature.However, the dislocation density gradually decreases with the annealing temperature.This is related to the fact that the dislocation density decreases with the improvement of crystallization.the sample is cooled from annealing temperature to the room temperature.Due to that reason, when the sample is cooled from a lower annealing temperature to the room temperature, the strain is not that high.On the other hand, the strain will be lower at higher annealing temperatures due to the improvement of crystallization.In other words, because the particles are compact at higher annealing temperatures, any free space (or voids) required for strain is not available.The absorption studies revealed a strong blue shift indicating the presence of quantum confinement effect.As a matter of fact, the optical band gap decreases when the particle size increases.Furthermore, CdS nanostructures prepared at this range of annealing temperatures and spin coating speeds can be recommended for photovoltaic and optoelectronic applications.

Figure 1 :Figure 1 .
Figure 1: XRD of CdS thin films annealed at (a) 200 ˚C and (b) 400 ˚C coated at 2200 rpm

Table 1 :
XRD results of CdS thin films spin coated at 2200 rpm and annealed at different temperatures

Table 2 :
Optical band gaps of CdS thin films spin coated at different spin speeds and annealed at different temperatures and then decreases with the annealing temperature.Lower strains can be observed at lower and higher annealing temperatures.Higher strains were observed at intermediate annealing temperatures.Strain mostly takes place in the film sample when