Optical spectroscopy and electrical analysis of La³⁺-doped PVA composite films for varistor and optoelectronic applications

Poly(vinyl alcohol), PVA, a matrix with 0, 0.185, 0.37, 1.85, 3.7 and 18.5 wt% of lanthanum (III) nitrate were synthesized by the traditional casting method. The order of the crystal structure and the interaction between the mixtures of the investigated materials were analyzed by X-ray and Fourier transform infrared (FT-IR) spectroscopies, while the Scanning Electron Microscopy (SEM), was used to study the surface images of them. Moreover, the optical filtering via UV/Vis/NIR spectroscopy, dielectric constant as well as the D.C. resistivity measurements that arose by the composite films with various wt% of La³⁺ ion were carried out. The structure study of these samples reveals that not only a cluster arises via La³⁺ ion on the SEM surface, but also, the semi-crystalline phases were confirmed by analyzing the pattern of the XRD and FT-IR. However, due to the complex formation of La³⁺ content in PVA matrix, there is an increment in the transitions strength, E_d, and the oscillator wavelength, λ₀ as well as the index of refractions, while the band gap and the average excitation energy, E_s, were decreased. Furthermore, there is a facility of moving charge carriers across the bands that contribute to the small energy gap via La³⁺-ion contents which clearly noticed in the dielectric and nonlinear I–V characteristics. The forward I–V measurement of the samples exhibited two distinct regions with different slopes, which is typical as nonlinear behavior for varistor with high applied voltage. Therefore, we can say that our samples have properties make them suitable to use in the applications of optoelectronic and varistor device.