Ultrafast heat flow in heterostructures of Au nanoclusters on thin-films: atomic-disorder induced by hot electrons

TL;DR

This study investigates ultrafast heat transfer and atomic disordering in Au nanoclusters on thin films using femtosecond electron diffraction, revealing hot-electron-induced lattice distortions below melting temperatures.

Contribution

It extends the two-temperature model to 0D/2D heterostructures and uncovers hot-electron effects on lattice disordering in supported nanoclusters.

Findings

Hot electrons induce reversible atomic disordering.

Intrinsic heat flow causes lattice distortion below melting point.

Lattice modifications influence surface chemical reactions.

Abstract

We study the ultrafast structural dynamics, in response to electronic excitations, in heterostructures composed of Au$_{923}$ nanoclusters on thin-film substrates with the use of femtosecond electron diffraction. Various forms of atomic motion, such as thermal vibrations, thermal expansion and lattice disordering, manifest as distinct and quantifiable reciprocal-space observables. In photo-excited, supported nanoclusters thermal equilibration proceeds through intrinsic heat flow, between their electrons and their lattice, and extrinsic heat flow between the nanoclusters and their substrate. For an in-depth understanding of this process, we have extended the two-temperature model to the case of 0D/2D heterostructures and used it to describe energy flow among the various subsystems, to quantify interfacial coupling constants, and to elucidate the role of the optical and thermal substrate properties. When lattice heating of Au nanoclusters is dominated by intrinsic heat flow, a reversible disordering of atomic positions occurs, which is absent when heat is injected as hot substrate-phonons. The present analysis indicates that hot electrons can distort the lattice of nanoclusters, even if the lattice temperature is below the equilibrium threshold for surface pre-melting. Based on simple considerations, the effect is interpreted as activation of surface diffusion due to modifications of the potential energy surface at high electronic temperatures. We discuss the implications of such a process in structural changes during surface chemical reactions.