Electron dynamics in films made of transition metal nanograins embedded in SiO₂: infrared reflectivity and nanoplasma infrared resonance

Abstract

We report on near normal infrared reflectivity spectra of ~550 nm thick films made of cosputtered transition metal nanograins and SiO₂ in a wide range of metal fractions. Co₀.₈₅(SiO₂)₀.₁₅,with conductivity well above the percolation threshold has a frequency and temperature behavior according to what it is find in conducting metal oxides. The electron scattering rate displays an unique relaxation time characteristic of single type of carriers experiencing strong electron-phonon interactions. Using small polaron fits we identify those phonons as glass vibrational modes. Ni₀.₆₁,(SiO₂)₀.₃₉, with a metal fraction closer to the percolation threshold, undergoes a metal-non metal transition at ~77 K. Here, as it is suggested by the scattering rate nearly quadratic dependence, we broadly identify two relaxation times (two carrier contributions) associated to a Drude mode and a mid-infrared overdamped band, respectively. Disorder induced, the mid-infrared contribution drives the phase transition by thermal electron localization. Co₀.₅₁(SiO₂)₀.₄₉ has the reflectivity of an insulator with a distinctive band at ~1450cm⁻¹ originating in electron promotion, localization, and defect induced polaron formation. Angle dependent oblique reflectivity of globally insulating Co₀.₃₈(SiO₂)₀.₆₂, Fe₀.₃₄(SiO₂)₀.₆₆, and Ni₀.₂₈(SiO₂)₀.₇₂, reveals a remarkable resonance at that band threshold. We understand this as due to the excitation by normal to the film electric fields of defect localized electrons in the metallic nanoparticles