Computational study of the molecular level mechanisms of the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique for thin film deposition.

摘要

There are a number of recent and emerging techniques that utilize the ability of laser ablation of a volatile matrix to entrain, eject and, if needed, deposit large macromolecules or nano-objects to a substrate. In particular, the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique shows a potential to produce uniform ultra-thin nanocomposite films with concentrations of nanoscale elements not attainable by other current methods. The lack of understanding of the fundamental underlying processes involved in laser ablation, however, hampers further optimization of the experimental parameters in MAPLE. In this dissertation I report the results of a comprehensive computational investigation of the relation between the basic mechanisms of laser interaction with multi-component target materials, the non-equilibrium processes caused by the fast deposition of laser energy, the parameters of the ejected ablation plume, and the resulting morphological characteristics of the growing film.The physical mechanisms and molecular-level picture of laser-induced material ejection from solutions of polymer molecules in a volatile matrix are analyzed in a series of coarse-grained molecular dynamics (MD) simulations. Simulations are performed for polymer concentrations up to 6 wt.% and laser fluences covering the range from the regime where molecular ejection is limited to matrix evaporation from the surface up to more than twice the threshold fluence for the onset of the collective molecular ejection or ablation. Contrary to the original picture of the ejection and transport of individual polymer molecules in MAPLE, the simulations indicate that polymer molecules are only ejected in the ablation regime and are always incorporated into polymer-matrix clusters generated in "phase explosion" of the target. Additionally, the entanglement of the polymer molecules facilitates the formation of elongated viscous droplets that can be related to nanofilament structures observed experimentally on targets and in films deposited by MAPLE.In an effort to enable coarse-grained MD simulations of MAPLE deposition of carbon nanotube (CNT)-polymer nanocomposites films, parameterization of a mesoscopic force field designed for CNT-organic matrix systems is performed based on a set of atomistic simulations. The mesoscopic model reproduces essential characteristics of CNT-CNT and CNT-solvent interactions, predicted in atomistic simulations, at a small fraction of the computational cost.