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Abstract Optical properties of PVA:Ag thin films prepared by casting method have been investigated. Different film thicknesses have been prepared to study the effect of thickness on the optical properties of the prepared films. Transmission and absorption spectra have been recorded, in order to estimate kinds of transition and the value of the energy gap. The optical band gap were found to be decrease from 1.45eV to 1.25eV for the allowed direct transition, and it is decreases from 2.3eV to 2.0eV for the allowed indirect transition when the thickness of the films increases from 10m to 40m. Keywords: PVA, thin films, optical properties, casting method.

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. 1.25eV 1.45eV .

. m 40 m 10 2.0eV 2.3eV, Introduction Recently, studies on optical and electrical properties of polymer films have increased enormously for there extensive applications in optical and electronic devices P[1] P. The synthesizing of these materials can be customized to have exceptional optical and electrical properties. Poly (vinyl alcohol) (PVA) is one of the most important polymeric materials as it has many applications in industry and is of relatively low cost P[2] P. PVA is a potential material having a very high dielectric strength, good charge storage capacity and dopant-dependent electrical and optical properties. Vargas et al. studied the electrical conductivity of the PVA blend with inorganic acids and water P[3,4]P. It is reported that the water content in the PVA based electrolyte enhanced the conductivity while preserving the dimensional stability of the electrolyte P[5] P. Previous study P[6] P, revealed that PVA nanofibers containing Ag nanoparticles were prepared by the short heat treatment of electrospun PVA/silver nitrate (AgNOR3R) nanofiber mats at 155C for use in wound dressing applications. After the short heat treatment, the AgP+P ions therein were reduced to produce a large number of Ag nanoparticles. However, this was not a controllable process. The polymer nanofibers containing Ag nanoparticles showed very strong antimicrobial activities P[7,8]P. Literature studies reveal that the ammonium salts are excellent proton donors to the polymer matrix and ammonium salts doped with PVA are rare. In the present work, the effects of AgNO3 on the optical properties of the PVA films were studied. Experimental procedure Poly (vinyl alcohol) with molecular weight 10000 g/mol, supplied by (BDH chemicals, England) with high purity were used as basic polymeric materials in this work., the aqueous solution of this polymer were prepared by dissolving PVA with different weight in a mixed of deionzed water and ethanol and thoroughly stirred using a magnetic stirrer for about one hour at

Dr. Sami Salman Chiad (*) Dr. Saad Farhan Oboudi (**) Dr. Nadir Fadhil Habubi (*) Dr. Ghuson Hamed Mohamed (**)

, sabodi137@yahoo.com, nadirfadhil@yahoo.com, samichiad2003@yahoo.com (*) Al_Mustasiriyah University, College of Education, Physics Department/

Baghdad/ Iraq. (**) Baghdad University, College of Science, Physics Department/ Baghdad/ Iraq.

Thickness Dependence on the Optical Properties of Ag Doped PVA

mailto:nadirfadhil@yahoo.commailto:samichiad2003@yahoo.com

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room temperature until PVA was completely soluble. AgNOR3R solution was prepared by dissolving the salt in redistilled water. 5% concentrated of AgNOR3R were mixed with PVA matrix. The solution was poured into flat glass plate dishes. Homogenous films were obtained after drying in an oven for 24 hours at 313K.The thickness of the produced films was in the range of 10, 20, 30 and 40 m. The absorbance and transmittance measurements were carried out using a Shimadzu UV in/VIS-160A double beam spectrophotometer in the wavelength range (300-1100) nm. Results and discussion The absorbance spectrum for the prepared films with different thicknesses is shown in Fig. (1). The absorbance increases but not systemically with the increasing of film thickness because in the case of thicker film, more atoms are present in the film so more states will be available for the photons to be absorbed. The absorbance spectrum shifts to longer wavelengths (red shift) with increasing of films thicknesses.

Fig. (1) Absorptance of PVA:Ag thin films versus wavelength.

The dependence of the absorption coefficient on the thickness of deposited films as a function of wavelength are shown in Fig. (2). One can see from this figure that the absorption coefficient of the films is characterized by strong absorption at the shorter wavelength region between (400-600) nm and without sharp edge on the long wavelength side. Which means that there is a large probability of the allowed direct transition P[9] P, and then decreases with increasing of wavelength. The following relation could be use for calculating the absorption coefficient () P[10]P:

------------- (1) t

A303.2= Where (A) is the absorption and (k) is the film thickness. Fig. (2) Shows the dependence of on photon energy, the absorption edge shift toward lower wavelength range (blue shift). The shift in the absorption edge may be attributed to the difference in grain size P[11]P and /or carrier concentration P[12]P.

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Fig. (2) Absorption Coefficient of PVA:Ag thin films versus wavelength. The optical energy gap values (ERgR) for PVA:Ag films have been determined. The band gap energies of these films were calculated by using the following relation. In general the absorption coefficient and photon energy (h) are related as Tauc plot P[13PP] P:

------------- (2) ngEhh

A)( =

Where (A) is a constant and (n) assumes values of (1/2, 2, 3/2 and 3)for allowed direct, allowed indirect and forbidden direct and indirect transitions respectively, by using this equation which is used to find the type of the optical transition by plotting the relations 2)( h , 2/1)( h and

3/2)( h and 3/1)( h versus photon energy (h ) and select the optimum linear part. It is found that the relation for (n) =1/2 yields linear dependence. The extrapolation i.e. ERgR, of the portion at (=0) as shown in Figures (3-6).

Fig (3) 2/1)( h for PVA:Ag thin film versus photon energy.

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Fig (4) 2/1)( h for PVA:Ag thin film versus photon energy.

A plot of 2)( h versus h for the films with different thicknesses is shown in the following Figures(7-10). The plots are linear indicating the allowed indirect band gap nature of the films. Extrapolation of the line to the h axis gives the band gap. The optical energy gap

Fig (5) 2/1)( h for PVA:Ag thin film versus

photon energy.

Fig (6) 2/1)( h for PVA:Ag thin film versu

photon energy.

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decreases with increasing thickness as shown in the Figs. This is due to the increase of the density of localized states in the ERgR, which cause a shift to lower values. It is clear that the value of ERgR decreases sharply with thickness increasing from 10 to 40 m. This increment could be attributed to the increase in crystallite size which induced an increase in density.

Fig (7) 2)( h for PVA:Ag thin film versus photon energy.

Fig (8) 2)( h for PVA:Ag thin film versus photon energy.

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Fig (9) 2)h( for PVA:Ag thin film versus photon energy.

Fig (10) 2)( h for PVA:Ag thin film versus photon energy. Conclusions We have studied the effect of thickness on the optical properties of PVA:Ag films. The results showed that, The UVvisible optical studies reproduce the formation of films with different thickness and its effect on the microstructure. These microstructural changes reflect in the form of variation in the band gap as well as the other optical properties. The value of the optical energy gap ERgR decreases sharply with increasing thickness from 10 to 40 m. UReferences

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