Abstract
Phase transitions under strongly nonequilibrium conditions continue to attract attention of the research community due to their relevance to a wide range of applications, from laser synthesis of colloidal nanoparticles to surface modification, as well as their fundamental significance. The main objective of this dissertation research is to clarify several questions related to the kinetics and microscopic mechanisms of melting of thin films and nanoparticles driven far from the equilibrium by ultrashort pulse laser irradiation. The laser-induced melting, cooling, and resolidification of colloidal nanoparticles is also considered with the focus on revealing the channels of the heat transfer from the nanoparticles to the surrounding liquid.
The detailed computational investigation of the femtosecond laser interaction with thin Au films is used to establish connections between the strength of electron-phonon coupling, kinetics and channels of thermalization of the deposited laser energy, and the mechanisms of melting occurring under highly non-equilibrium conditions. In particular, the results of atomistic simulations reveal that a combined effect of lattice superheating and relaxation of laser-induced stresses ensures the dominance of the homogeneous melting mechanism at all energies down to the melting threshold and keeps the timescale of melting within a hundred of picoseconds. The long melting times and the major contribution of heterogeneous melting inferred from recent ultrafast electron diffraction probing of the melting process [Mo et al., Science 360, 1451, 2018] can be reconciled with neither analytical estimations nor predictions of detailed atomistic simulations by any physically-justifiable variation of the electron-phonon coupling parameter. The simulation results caution against the direct use of the experimental melting times for quantitative verification and parametrization of the theoretical models for the electron temperature dependence of electron-phonon coupling, until the observational results are confirmed by theory.
The effect of matrix on the kinetics of laser-induced melting is addressed in a series of atomistic simulations of femtosecond laser interaction with an Al thin film containing a Pb nanoparticle. The simulations of laser-induced melting reveal that the processes specific to the ultrashort pulse laser irradiation, such as ultrafast heating, nanoparticle-matrix energy redistribution, and strong elastic vibrations, have a large impact on the kinetics and mechanism of the nanoparticle melting process. In particular, the dynamic relaxation of the laser-induced stresses is found to significantly affect the stability of the crystal lattice against melting. Moreover, the material dynamics triggered by the laser heating is intertwined with other factors affecting the kinetic and thermodynamic conditions of melting, such as stresses related to the spatial confinement of the nanoparticle within the matrix and stabilization of the crystal lattice by the nanoparticle-matrix interface.
Finally, the processes controlling the heat transfer from a colloidal Au nanoparticle rapidly heated by a short laser pulse to the surrounding water are investigated with the focus on conditions where the nanoparticle structure is modified by laser-induced melting, quenching, and resolidification. The analysis of the heat transfer is extended to the largely unexplored regime where the liquid adjacent to the nanoparticle surface is transiently heated to or above the thermodynamic critical temperature of water. The dependence of the Au-water interfacial heat conductance on the nanoparticle temperature, interfacial heat flux, pressure, and curvature of the interface is first investigated in a series of nonequilibrium molecular dynamics simulations performed with a realistic representation of water. The simulations reveal an important role the formation of a layer of supercritical water plays in defining the heat transfer from the nanoparticle, as well as the strong dependence of the thermal boundary conductance on the size of the nanoparticle. The results of these simulations are then used to parametrize a hybrid atomistic–continuum model for the nanoparticle-water heat transfer, capable of computationally efficient analysis of the nanoparticle cooling rates realized in laser processing of colloidal nanoparticles. Finally, the predictions of the continuum model are verified against the results of molecular dynamics simulations of laser heating, melting, cooling, and resolidification of 7 and 20 nm Au nanoparticles irradiated in water by 10 ps laser pulses.
