Ultrafast heat transfer in single palladium nanocrystals seen with an X-ray free-electron laser

TL;DR

This study reveals ultrafast, heterogeneous heat transfer and strain dynamics in individual palladium nanocrystals following laser heating, using X-ray free-electron laser measurements to observe structural evolution.

Contribution

It provides direct experimental evidence of transient strain states and heat distribution in single nanocrystals with femtosecond resolution, supported by a detailed lattice model.

Findings

Observation of split Bragg peaks indicating heterogeneous strain

Identification of threshold laser fluence for strain heterogeneity

Correlation between strain dynamics and heat distribution

Abstract

We report transient highly strained structural states in individual palladium (Pd) nanocrystals, electronically heated using an optical laser, which precede their uniform thermal expansion. Using an X-ray free-electron laser probe, the evolution of individual 111 Bragg peaks is measured as a function of delay time at various laser fluences. Above a laser fluence threshold at a sufficient pump-probe delay, the Bragg peak splits into multiple peaks, indicating heterogeneous strain, before returning to a single peak, corresponding to even heat distribution throughout the lattice expanded crystal. Our findings are supported by a lattice displacement and strain model of a single nanocrystal at different delay times, which agrees with the experimental data. Our observations have implications for understanding femtosecond laser interactions with metals and the potential photo-catalytic performance of Pd.