Effect of dye concentration on the optical properties of red-BS dye-doped PVA film
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In this experimental study, the effect of dye concentration on the optical properties of red-BS dye-doped polyvinyl alcohol (PVA) thin films is investigated. Three thin film samples with different concentration of red-BS dye were prepared by spin-coating method on the glass substrate. Using transmission and reflection spectrum of films, their optical parameter such as refractive index, absorption coefficient, and dielectric function are extracted and the effect of dye impurity on theses parameters has been studied. The band gap energy of samples is calculated using Tauc method. Band gap energy of samples is decreased by increasing the concentration of dye impurity in PVA films.
KeywordsPolyvinyl alcohol Absorption coefficient Extinction coefficient Band gap
Traditionally, polymer matrix composites have been thought as insulating materials and have been used in applications like power tool handles, cable, jackets, capacitor films and electronic packaging materials. Especially the electrical and optical properties of polymers have been extensively investigated due to their applications in optical devices recently. Polymeric materials have unique properties such as low density, light weight, and high flexibility and are widely used in various industrial sectors. In recent years there has been great progress in understanding polymer optical properties and development in their theoretical description [1, 2, 3, 4].
Polyvinyl alcohol (PVA) is an important and interesting polymer because of its attractive physical and optical properties. Unlike most vinyl polymers, PVA is not prepared by polymerization of the corresponding monomer. The monomer vinyl alcohol is unstable with respect to acetaldehyde. PVA instead is prepared by first polymerizing vinyl acetate, and the resulting polyvinylacetate is converted to the PVA. Other precursor polymers are sometimes used, with formate, chloroacetate groups instead of acetate. The properties of the polymer depend on the amount of residual ester groups . PVA is a colorless polymer, so because of its low cost and volume productivity it will be one of the key materials for using instead of glass and different coatings in optics industry. As a polymer waveguide PVA has attracted much attention. In addition it is found that it can produce a large reflective index . These materials are promising candidates as non-liner optical elements [6, 7, 8, 9, 10].
Ultraviolet (UV) radiation initiates photo physical and photochemical processes in polymers but its influence is restricted to a thin surface layer. It is because our experiments are done on PMMA thin films. In this experimental research we have used red BS dye as an impurity in PMMA films as a powerful UV absorber. Organic compounds contained in polymer films, even in trace amounts, affect the reaction accruing during radiation. Interesting contributions concern the impact of the UV irradiation on the properties of PMMA thin films obtained by a spin-coating technique [1, 2].
In this manuscript we have investigated the effect of red-BS dye on the optical properties of PVA thin films. The manuscript is organized as follow, following the introduction in “Introduction”, experimental setup is presented in “Experimental setup”. “Results and discussion” includes results and discussion and “Conclusion” is devoted to conclusion.
Thickness and band gap energy of samples
Spin coater speed (s)
Films thickness (mm)
Energy band gap (eV)
Varian Carry 500 UV–Vis–NIR spectrometer was used to obtain the transmission and reflection spectrum of samples in this range.
Results and discussion
The picture of pure PVA and PVA doped with red BS dye thin films are presented in Fig. 1. PVA is colorless polymer and with adding red BS dye its color is changed to red.
In this equation α is the absorption coefficient, E is the photon energy, and B is a factor depends on the transition probability and can be assumed to be constant within the optical frequency range, and the index m is related to the distribution of the density of states. The index m has discrete values like 1/2, 3/2, 2, more depending on whether the transition is direct or indirect and allowed or forbidden. In the direct and allowed cases, the index m = 1/2 whereas for the direct but forbidden cases it is 3/2. But for the indirect and allowed cases m = 2 and for the forbidden cases it is 3 or more. Taking m = 2 corresponds to indirect allowed transitions in samples, the band gap energies of thin films were calculated, which are shown in Table 1.
As can be seen, the energy gap of pure PVA sample (S1) is equal to 3.86 eV, with increasing the concentration of the red BS dye in PVA polymer, is reduced. Thus the doped, thin films show a blue shift behavior (see Fig. 4); it can be concluded that the presence of dopant and its interaction results in the creation of new molecular dipoles, which could be results of point defects created within the band gap. Nitrogen and oxygen has a high electronegativity. Due to the presence of nitrogen in the red BS dye structure and the possibility to establish covalent bonds with hydrogen and carbon in the PVA structure, the thin films are highly capable of being polarized. It can be explained by the fact that with larger amount of red BS dye in PVA polymer we have more molecules dipoles in deposited thin films, in other words deposited thin films is closer to semiconductors, with low magnitude of band gap energy.
The extinction coefficient k describes the properties of the material with respect to light of a given wavelength and indicates the absorption changes when the electromagnetic wave propagates through the material. In Fig. 5, the extinction coefficient k of samples at UV region began to rise and increases with increasing concentration of red BS dye in PVA dielectric medium. It can be observed that there is anomalous dispersion regions when 3.5 < E < 4.5 eV and, as well as normal dispersion when 3 < E < 3.5 eV for doped samples. So the normal dispersion is seen to occur everywhere expect in the neighborhood of a resonant frequency.
In this experimental work we have investigated the effects of thickness and dye concentration on the optical properties of dye-doped PVA thin films. Using transmittance and reflectance of films, their several optical parameters have been extracted and studied. From S1 to S3 transmittance of samples specially in the range of visible spectrum is decreased, which is the effect of dye increment in the structure of films. Also the absorption coefficient of samples is increased. Adding dye in the structure of PVA has been leaded to decreasing the band gap energy of films because of smaller band gap energy of dye. Increasing the absorption of samples is due to decreasing the real part of their refractive index and increasing its imaginary parts.
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