Electron Dynamics in Films Made of Transition Metal Nanograins Embedded in SiO2:Infrared Reflectivity and Nanoplasma Infrared Resonance

TL;DR

This study investigates electron dynamics in transition metal nanograin films embedded in SiO2 using infrared reflectivity, revealing different conduction behaviors, phase transitions, and defect-related resonances influenced by metal fraction and temperature.

Contribution

It provides detailed insights into the infrared reflectivity spectra and electron relaxation mechanisms in nanograin films, highlighting phase transitions and defect-induced resonances.

Findings

Metallic behavior above percolation threshold with strong electron-phonon interactions.

Metal-non metal transition at ~77 K in near-threshold films.

Distinct infrared bands associated with electron localization and defect states.

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