Temperature and frequency dependent optical properties of ultra-thin Au films

TL;DR

This study explores the optical properties of ultra-thin gold films on silicon in the mid- and far-infrared range, revealing metallic behavior in thicker films and non-metallic characteristics below the percolation threshold, with temperature and frequency effects analyzed.

Contribution

It provides new insights into the optical behavior of ultra-thin gold films in the mid- and far-infrared, including temperature dependence and surface state transitions, extending understanding beyond visible wavelengths.

Findings

Thicker films follow Drude model with size effects

Sub-percolation films show non-metallic behavior

Surface state transition observed at 1100 cm-1

Abstract

While the optical properties of thin metal films are well understood in the visible and near-infrared range, little has been done in the mid- and far-infrared region. Here we investigate ultra-thin gold films prepared on Si(111)(7 x 7) in UHV by measuring in the frequency range between 500 cm-1 and 7000 cm-1 and for temperatures between 300 K and 5 K. The nominal thickness of the gold layers ranges between one monolayer and 9 nm. The frequency and temperature dependences of the thicker films can be well described by the Drude model of a metal, when taking into account classical size effects due to surface scattering. The films below the percolation threshold exhibit a non-metallic behavior: the reflection increases with frequency and decreases with temperature. The frequency dependence can partly be described by a generalized Drude model. The temperature dependence does not follow a simple activation process. For monolayers we observe a transition between surface states around 1100 cm-1.