Dopant-dependent reflectivity and refractive index of microcrystalline molybdenum–bronze thin films

Abstract

Reflectivity spectra of HxMoO3 and LixMoO3 thin films were measured over the photon energy range from 0.4 to 4.2 eV. It was found that microcrystalline molybdenum bronzes have reflectances between 6% and 30% over the concentration x, range 0⩽x⩽0.64. Values for the real part of the refractive index n were also determined from the refined reflectivity data using different numerical techniques depending upon the dispersive and nondispersive regions in the data. The values of high-frequency dielectric constant εhf of ZxMoO3 (Z=H+,Li+) bronzes were determined from the refractive index data to estimate the effective electronic masses involved in the optical and/or polaronic transitions. We interpret the optical data using the modified Drude–Zener model together with a single-oscillator model to differentiate between bound and free electronic states. Using a single-oscillator model, the oscillator energy Ea and the dispersion energy Ed were found to increase and decrease, respectively, with increasing x values, opposite to what occurs in crystalline molybdenum bronzes. These findings support the fact that Bloch electrons are almost absent in the investigated microcrystalline bronzes. The small reflectivity crests and the observed shift of the minimum reflectivity (or minimum refractive index) toward higher frequency with increasing x value in ZxMoO3 appear to be consistent with polaronic hopping, which arises from the excitation of electrons from localized in-gap states of Mo5+ ions into higher impurity states of Mo6+ (or Mo4+) ions within the optical band gap.