Electron–Phonon Coupling and Nonthermal Effects in Gold Nano-Objects at High Electronic Temperatures

Abstract

Laser irradiation of metals is widely used in research and applications. In this work, we study how the material geometry affects electron–phonon coupling in nano-sized gold samples: an ultrathin layer, nano-rod, and two types of gold nanoparticles (cubic and octahedral). We use the combined tight-binding molecular dynamics Boltzmann collision integral method implemented within XTANT-3 code to evaluate the coupling parameter in irradiation targets at high electronic temperatures (up to Te~20,000 K). Our results show that the electron–phonon coupling in all objects with the same fcc atomic structure (bulk, layer, rod, cubic and octahedral nanoparticles) is nearly identical at electronic temperatures above Te~7000 K, independently of geometry and dimensionality. At low electronic temperatures, reducing dimensionality reduces the coupling parameter. Additionally, nano-objects under ultrafast energy deposition experience nonthermal damage due to expansion caused by electronic pressure, in contrast to bulk metal. Nano-object ultrafast expansion leads to the ablation/emission of atoms and disorders the inside of the remaining parts. These nonthermal atomic expansion and melting are significantly faster than electron–phonon coupling, forming a dominant effect in nano-sized gold.

Materials under XUV irradiation: effects of structure, size, and temperature

“…The dimensionality of the sample also changes its response to irradiation. In the simulation comparing a nano-sphere (effectively 0d-sample), nano-rod (1d), nano-layer (2d), and bulk (3d), the electronic heat capacity and the electronphonon coupling parameter were mostly affected at low electronic temperatures but were very close to each other at high ones (above some 5000-7000 K) 28 . However, the difference between the samples was more prominent in nonthermal behavior: smaller samples reacted to electronic pressure increase easier, expanding and destabilizing the atomic lattice 28 .…”

“…The atomic structure or the phase of the material seems to be influencing the electron-phonon coupling more at the low electronic temperatures, but not as strongly at high ones 22,28 . The effect is different from the pure effect of the densitythe electron-phonon coupling is inversely proportional to the material density (as long as the lattice structure is preserved), but the dependence qualitatively changes once the atomic structure changes (e.g., due to destabilization under expansion) 22 .…”

“…27 ; the effect of the shape and dimensionality of a nano sample on the electron-phonon coupling and nonthermal ablation was reported in Ref. 28 ; the effect of in situ irradiation temperature of material damage threshold was shown in Ref. 29 .…”

“…The lattice destabilization also depended on the particular shape of the nanoparticle. An octahedral gold nanoparticle is damaged faster than a cubic one 28 . This effect is expected to play a stronger role for smaller particlesas the ratio of surface atoms to the interior ones increases.…”