Measurement of the Energy Gap of Au55 Nanoparticles Using Far-infrared Transmission Spectroscopy

TL;DR

This study measures the energy gap of Au55 nanoparticles using far-infrared transmission spectroscopy, revealing a smaller-than-expected gap and behavior akin to a 3D semimetal rather than a giant atom.

Contribution

First direct measurement of the energy gap in Au55 nanoparticles using far-IR spectroscopy, challenging previous expectations and models.

Findings

Observed a broad far-IR absorption onset at ~10 cm-1

Energy gap is smaller (~1.2 meV) than predicted for a similar-sized metal sphere

Au55 nanoparticles behave like a 3D semimetal, not a giant atom.

Abstract

We have carried out far-infrared (far-IR) transmission measurements on the ligand stabilized Au55 nanoparticles dispersed in Teflon at volume fractions ranging from 0.2% to 1%. Instead of a series of peaks which might arise from the electron transition between discrete energy levels, we have observed a broad far-IR absorption coefficient that follows with an onset at {\Delta} ~ 10 cm-1. Furthermore this frequency dependence is totally different from that of the expected absorption due to the induced electric dipole. The onset of the absorption reminiscent of an energy gap at ~ 10 cm-1 (~ 1. 2 meV) is surprisingly smaller than that of the expected for a metal sphere of 5.3 {\AA} in radius and independent of temperature. In this work we did not observe the level correlation effect on the far-IR absorption predicted for an ensemble of metal nanoparticles. It is concluded that Au55 nanoparticles behave like a three-dimensional (3D) semimetal with an energy gap {\Delta} ~ 10 cm-1 rather than a giant atom.