Direct observation of picosecond melting and disintegration of metallic nanoparticles

Abstract

Despite more than a century of study, the fundamental mechanisms behind solid melting remain elusive at the nanoscale. Ultrafast phenomena in materials irradiated by intense femtosecond laser pulses have revived the interest in unveiling the puzzling processes of melting transitions. However, direct experimental validation of various microscopic models is limited due to the difficulty of imaging the internal structures of materials undergoing ultrafast and irreversible transitions. Here we overcome this challenge through time-resolved single-shot diffractive imaging using X-ray free electron laser pulses. Images of single Au nanoparticles show heterogeneous melting at the surface followed by density fluctuation deep inside the particle, which is directionally correlated to the polarization of the pumping laser. Observation of this directionality links the non-thermal electronic excitation to the thermal lattice melting, which is further verified by molecular dynamics simulations. This work provides direct evidence to the understanding of irreversible melting with an unprecedented spatiotemporal resolution.

In situstructural kinetics of picosecond laser-induced heating and fragmentation of colloidal gold spheres

“…30 A recent structural resolution of gold particle excitation and fragmentation in air on a substrate has shown a stress-mediated explosion at relatively low fluence just above melting. 31 Such a scenario may not be distinguishable from spinodal decomposition with the fragment size depending on the level of overheating. 29 Stronger overheating will produce smaller fragments together with evaporation.…”

“…Recently, femtosecond-resolved diffraction experiments on single particles have witnessed such a behaviour. 31 An approach that is able to shed light on the manifold of proposed reactions needs to resolve the caloric balance of the system (i.e. the observation of educts and products as function of energy input), and as well allow for a kinetic determination of structure formation on an ultrafast time scale.…”

Complex imaging of phase domains by deep neural networks

“…With the advent of ultrafast X-ray free-electron lasers (XFELs), time-resolved single-particle-imaging experiments are more and more addressing ultrafast phenomena in physical or chemical processes within nanoparticles, such as melting, driven fluctuations, etc. (Clark et al, 2013;Gomez et al, 2014;Rupp et al, 2017;Rose et al, 2018;Ihm et al, 2019;Wen et al, 2019;Sobolev et al, 2020;Mudrich et al, 2020). These experiments would benefit enormously from the development of a fast and reliable phase-retrieval method to reconstruct live images of particles in real time during the execution of these time-resolved experiments.…”

Subnanosecond phase transition dynamics in laser-shocked iron

“…In addition, recent molecular dynamic simulations of iron suggest the formation of a face-centered cubic (fcc) form (-phase) by shock compression at ~40 GPa (30). Over the past decades, advances in extremely bright x-ray sources, like the x-ray free electron lasers (XFEL) provided previously unknown capabilities to explore not only the structural changes of compressed material but also the dynamics of these phase transitions with high temporal resolution (31,32). It is unexpected that iron has not been studied yet using any XFEL sources despite its paradigmatic importance and discrepancies in understanding its phase transition dynamics.…”