Computational Study of Molecular Transfer and Nanoparticle Generation in Matrix-Assisted Pulsed Laser Evaporation: the Roles of Solvent and Photochemical Reactions

The matrix-assisted pulsed laser evaporation (MAPLE) technique was originally developed as a non-destructive method for the laser deposition of thin films composed of fragile polymer and bioorganic films. In MAPLE, frozen targets consisting of a volatile solvent matrix and a dilute solute are irradiated by a pulsed laser. The solvent molecules are preferentially excited due to their higher absorption. This leads to their active evaporation and removal from their target in an ablation process that desorbs the less volatile solute onto the substrate to create a film.

In the work leading to this dissertation, computational investigations of the laser irradiation of MAPLE targets are done to reveal mechanisms that lead to material ejection without removal of large amounts of the matrix from the target. The investigations are listed below:

1) Computational investigations are done to determine the minimum amount of non-absorbing volatile matrix needed for laser ablation of a molecular target. This work is motivated by unexpected results of an experimental investigation of pulsed laser deposition of the protein lysozyme: ablation of solid target leads to deposition of unfragmented (intact) lysozyme molecules. For a solid macromolecular target to undergo ablation, its molecules should fragment so as to provide the volatility needed for the collective material ejection. This requirement contradicts the observation of intact molecules in the deposited films, and the presence of inherent (or residual) water influencing the material ejection process is suspected. The computational investigations reveal that intact biomolecules can be ejected from a target having low water concentration. Based on the observed phenomenon, a non-destructive approach that provides dry conditions for the deposition of polymer and biomolecular filmsis suggested. The dry deposition conditions can be beneficial for applications that require minimization of the solvent interaction with deposited films.

2) The fundamental mechanisms responsible for the generation of metallic nanoparticles (NPs) in an experimental approach based on the laser irradiation of aqueous solutions of metalorganic precursors are explored in a computational investigation. A matrix of volatile molecules that gain energy by local heating from MOP molecules and their photodecomposition products is originally expected to provide a driving force for the ejection of NPs from the MAPLE target. However, continuum-level simulations show that the onset of the ablation process requires a laser fluence of at least an order of magnitude higher than in experiments. It is thus suggested that the ejection of NPs from the target is a result of the energy localization in the vicinity of the absorbing NPs. The ejection of NPs is strongly affected by their dispersion in the matrix and the thermal diffusion length scales that control the redistribution of the deposited laser energy. A new mechanism in which NPs are ejected from the target without active evaporation of the matrix is revealed in the investigations. This mechanism may have implications for the development of techniques enabling size-selective generation and ejection of clean solvent-free nanoparticles.

To perform the computational investigations, a coarse-grained model for molecular dynamics simulations of laser interactions with lysozyme and palladium acetate solutions in water is developed.