Vacuum-annealed and oxygen plasma treated ellipsometric investigations on molybdenum bronzes and measurements of their thermo optic coefficients and electronic polarizability coefficients

Abstract

In this investigation, we have measured the optical constants (n, k) of vacuum evaporated MoO3 thin films and of ZxMoO3 (Z = H+, Li+) bronze thin films being annealed at different temperatures and with different concentrations (x) over the range 298–453 K without and within an oxygen plasma environment using manual ellipsometry. We have also measured mass densities, thermo optic coefficients (TOCs), electronic polarizability coefficients (EPCs) and the coefficients of temperature dependence of density of MoO3 thin film and of its bronzes over the same temperature range. It was found that the change in density and porosity of MoO3 thin film (when annealed at 453 K for 36 h) was not more than 6.4% and 15% respectively from the room temperature data. In the case of bronzes, when the samples H0.203MoO3, Li0.076MoO3 and Li0.133MoO3 were annealed at 453 K within an oxygen plasma over different periods of times, it was found that the overall increase in mass density and decrease in the porosity was not more than 4% and 11%, respectively from the initial data at room temperature. Comparably, when the samples H0.211MoO3 and Li0.27MoO3 were annealed at 453 K within an oxygen plasma over different periods of time, the respective decrease in density and increase in porosity was found not more than 3.4% and 6% from the room temperature data. All these results are mostly due to evolution of H2O which causes diffusion of oxygen leading to shallow and deep localised states in the oxide bronze layers. TOCs and EPCs of samples HxMoO3 (x = 0.203) and LixMoO3(x = 0.076, 0.133) upon annealing at 453 K (without and within an oxygen plasma) over different periods of times have an increase in TOCs and in EPCs, and that increase is from 0.10 × 10−4 K−1 to 13.5 × 10−4 K−1 and 0.13 × 10−27 cm3 K−1 to 3.2 × 10−27 cm3 K−1, respectively. On the other hand, the thermo optic coefficient and the coefficient of temperature dependence of density of MoO3 thin films decrease from − 1.2 × 10−4 K−1 to − 8.86 × 10−4 K−1 and − 1.86 × 10−4 g cm−3 K−1 to − 15.13 × 10−4 g cm−3 K−1, respectively. In the case of H0.211MoO3 and Li0.27MoO3 when annealed at 453 K over different periods of time within oxygen plasma, the overall decrease in TOC is from − 0.3 × 10−4 K−1 to − 4.62 × 10−4 K−1, and the decrease in coefficient of temperature dependence of density is from − 0.45 × 10−4 g cm−3 K−1 to − 8.9 × 10−4 g cm−3 K−1. These results are due to linear thermal expansion effect.