Molecular Dynamics Simulation of Laser Melting of Nanocrystalline Au

Abstract

The mechanisms and kinetics of short pulse laser melting of single crystal and nanocrystalline Au films are investigated on the basis of the results of simulations performed with a model combining the molecular dynamics method with a continuum-level description of the laser excitation and subsequent relaxation of the conduction band electrons. A description of the thermophysical properties of Au that accounts for the contribution of the thermal excitation of d band electrons is incorporated into the model and is found to play a major role in defining the kinetics of the melting process. The effect of nanocrystalline structure on the melting process is investigated for a broad range of laser fluences. At high fluences, the grain boundary melting in nanocrystalline films results in a moderate decrease of the size of the crystalline grains at the initial stage of the laser heating and is followed by a rapid (within several picoseconds) collapse of the crystal structure in the remaining crystalline parts of the film as soon as the lattice temperature exceeds the limit of the crystal stability against the onset of rapid homogeneous melting (the limit of superheating). At low laser fluences, close to the threshold for the complete melting of the film, the initiation of melting at grain boundaries can steer the melting process along the path where the melting continues below the equilibrium melting temperature and the crystalline regions shrink and disappear under conditions of substantial undercooling. The unusual melting behavior of nanocrystalline films is explained on the basis of thermodynamic analysis of the stability of small crystalline clusters surrounded by undercooled liquid.

Structural dynamics of laser-irradiated gold nanofilms

“…As in the low fluence scenario, melting is accompanied by a decrease of density and temperature. Such complex melting dynamics at intermediate fluence is consistent with recent theoretical predictions, 28,29 however, alternative theories exist. 58 Furthermore, inside the molten pockets and when the sample is entirely melted, the calculated self-diffusivity is the same as equilibrium liquid gold at equivalent conditions (see Fig.…”

“…9, 23, and 25-29, and therefore we describe it only briefly here. We calculate real-space atomistic correlations to obtain the structure factor 28,29 required for describing structural and thermal contributions in the temporal evolution of Bragg peaks. In order to characterize the effects of electronic excitations on the interatomic interaction, we carried out ab initio calculations of the phonon spectra at the free energy minimum volume for different values of electronic temperature and found the conditions where the electronic excitation significantly modifies the interatomic interactions.…”

“…This method has already been applied to simulate the atomistic dynamics of gold nanofilms excited by different laser fluences. 9,28 However, the results have not been directly compared with experimental data.…”

Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

“…The atomistic molecular dynamics (MD) simulation technique has indeed been instrumental in providing valuable information on the microscopic mechanisms of laser melting [19][20][21][22], photomechanical spallation and ablation [19,[23][24][25][26][27][28][29]. Beyond the analysis of the initial dynamic material response to the fast laser energy deposition, however, there has been little progress in extending the simulations to the investigation of processes responsible for the formation of complex surface morphologies and microstructures of the laser-processed targets.…”

Nanoscale structures generation within the surface layer of metals with short UV laser pulses

“…The rapid electron-phonon equilibration can produce ultrafast heating rates exceeding 10 14 K/s, providing, therefore, opportunities for investigation of the kinetic limits of achievable superheating of solid phases. Indeed, a combination of time-resolved optical, X-ray and electron diffraction probing of short pulse laser melting [4][5][6] with theoretical analysis and Molecular Dynamics (MD) simulations [7,8] has provided important information on the time-scales and mechanisms of the melting process occurring under conditions of strong superheating. However, the short pulse laser irradiation may cause not only the rapid heating and melting of localized surface or internal regions of the target, but also produce sharp temperature gradients leading to a rapid quenching of the transiently melted material.…”