Role of annealing temperatures on structure polymorphism, linear and nonlinear optical properties of nanostructure lead dioxide thin films

The powder of as synthesized lead dioxide (PbO2) has polycrystalline structure beta-PbO2 phase of tetragonal crystal system. It becomes nanocrystallites alpha-PbO2 phase with orthorhombic crystal system upon thermal deposition to form thin films. Annealing temperatures increase nanocrystallites size from 28 to 46 nm. The optical properties of alpha-PbO2 phase were calculated from absolute values of transmittance and reflectance at nearly normal incidence of light by spectrophotometer measurements. The refractive and extinction indices were determined and showed a response to annealing temperatures. The absorption coefficient of alpha-PbO2 films is >10(6) cm(-1) in UV region of spectra. Analysis of the absorption coefficient spectra near optical edge showed indirect allowed transition. Annealing temperature decreases the value of indirect energy gap for alpha-PbO2 films. The dispersion parameters such as single oscillator energy, dispersion energy, dielectric constant at high frequency and lattice dielectric constant were calculated and its variations with annealing temperatures are reported. The nonlinear refractive index (n(2)), third-order nonlinear susceptibility (chi((3))) and nonlinear absorption coefficient (beta(c)) were determined. It was found that chi((3)), n(2) and beta increase with increasing photon energy and decrease with increasing annealing temperature. The pristine film of alpha-PbO2 has higher values of nonlinear optical constants than for annealed films; therefore it is suitable for applications in manufacturing nonlinear optical devices. (C) 2016 Elsevier B.V. All rights reserved.