Nanometals: Identifying the Onset of Metallic Relaxation Dynamics in Monolayer-Protected Gold Clusters Using Femtosecond Spectroscopy

Abstract

Electronic relaxation dynamics were studied for a series of gold monolayer-protected clusters (MPCs) whose sizes ranged from 1.5 to 2.4 nm. Au96(mMBA)42, Au102(pMBA)44, Au115(pMBA)49, Au117(mMBA)50, Au144(pMBA)60, Au250(pMBA)98, and Au459(pMBA)170 (pMBA = para-mercaptobenzoic acid; mMBA = meta-mercaptobenzoic acid) were selected for study because they bridged the expected transition from nonmetallic to metallic electron behavior. Excitation-pulse-energy-dependent measurements confirmed Au144(pMBA)60 (1.8 nm) as the smallest MPC to exhibit metallic behavior, with a quantifiable electron–phonon coupling constant of (1.63 ± 0.25) × 1016 W m–3 K–1. Smaller, nonmetallic MPCs exhibited nanocluster-specific transient extinction spectra characteristic of transitions between discrete quantum-confined electronic states. Volume-dependent electronic relaxation dynamics for ≤1.8 nm MPCs were observed and attributed to a combination of large energy differences between electronic states and phonon frequencies and spatia...