Cluster-solid-surface collisions are one of the most interesting topics in the area of particle-surface interactions due to the unique combination of localized deposition of energy and the transient high density of atoms or molecules that results. Some of the possible outcomes include the scattering of cluster fragments, the scattering of particles from the surface, chemical reactions within the cluster, and adhesion of the cluster to the surface. This research has investigated the mechanical and chemical processes that occur when organic molecular clusters impact surfaces at hyperthermal (about 5-80 eV/molecule) and keV incident energies. The approach is molecular dynamics simulations which is ideally suited to study this process as the time scales of hyperthermal and higher energy cluster-solid-surface collisions are on the order of a few picoseconds. The empirical hydrocarbon potential of Brenner et al. is used in addition to the semi-empirical nonorthogonal tight-binding molecular dynamics (TBMD) method of Wu and Jayanthi. The simulations provide information about the types of chemical reactions that occur under the high pressure conditions of cluster-solid-surface impacts, detail the mechanisms of these reactions, reveal the types and yields of chemical products, and document any structural changes to the surface. They also reveal how the chemical reactions, their time-scales, mechanisms and products depend on a number of factors including cluster molecular species, cluster size, cluster incident kinetic energy and surface reactivity. The results are also compared to comparable molecular beam deposition products to distinguish the effects of the cluster on the outcomes of the deposition.

      We are also examining material modification of polymer substrates through polyatomic ion collisions. The goal is to determine how changes in the incident species affect the results. Also, a quantitative determination of the role of incident energy, incident angle, and surface type is being explored. The results are providing an enhanced understanding of the processing of materials through ion-surface and plasma-surface interactions. This could lead to improvements in thin-film growth and material modification methods used to manufacture a variety of products, from optical coatings to medical implants. This work is done in collaboration with mass-selected polyatomic ion deposition work performed at the University of Illinois at Chicago by Professor Luke Hanley. This work is supported by the National Science Foundation under Grant No. CHE-0200838. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).

      Ab Initio molecular dynamics simulations are used here to study the process of surface polymerization by ion-assisted deposition (SPIAD). In particular, the simulations consider the deposition of thiophene molecules on thin films of thiophene oligomers at different deposition energies and densities. The results show that both these factors affect the properties of the resulting chemically modified thiophene thin films. At an incident energy of 25 eV, thiophene polymerization occurs through two neighbor polymerization initiators that consist of C atoms that abstract hydrogen atoms from thiophene rings. However, eventually the thiophene oligomers are damaged and the polymerization disappears. In the case of deposition with 50 eV of incident energy, the damage to the thiophene oligomer thin film is much more severe. The results of these simulations provide insight into understanding the reaction mechanisms responsible for the SPIAD process. This work is done in collaboration with Professor Luke Hanley and is supported by the National Science Foundation under Grant No. CHE-0200838. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).

      The reaction of methanol molecules with size-selected Cu_n clusters, n=2-9, is investigated by first-principles molecular dynamics simulations. The simulations use density functional theory within the generalized gradient approximation and ultrasoft pseudopotentials. The molecules are deposited on the Cu clusters with an incident energy of 0.5 eV/molecule. The structure, dynamics, and reaction energies are studied as a function of the cluster size. In experiments it is found that that the dominant reactions are methanol chemisorption, demethanation, and carbide formation, which are very different from the interaction of methanol with bare copper surfaces, where physisorption is the dominant outcome. The simulations detail the atomic scale mechanisms that are responsible for these differing behaviors. For example, they show that the adsorption energy of methanol to Cu₇ is about 0.5 eV, which is larger than the energy of adsorption of methanol on the Cu(111) surface by about 0.25 eV. The simulations also illustrate the differences in the interaction of methanol with copper clusters of various sizes. This work is done in collaboration with Prof. Tamotsu Kondow of Toyota Technical Institute and is supported by the National Science Foundation under Grant No. CHE-0200838. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).

      Thin film deposition of SrTiO₃ is currently a popular area of research due to its widespread use in electronic applications and the motivation to shrink these. Pulsed laser deposition is a quite effective deposition process yielding dense homogeneous thin films. Currently, we are simulating SrO and TiO₂ molecule deposition with a kinetic energy between 0.1 and 1 eV/atom on a (001) surface of SrTiO₃. We will be examining the effects of impact energy, orientation of incident particles, and surface termination layer (SrO vs. TiO₃). The main surface phenomenon of interest is chemical changes occurring at the oxide surface due to the ablating particles. Future goals include investigating the types of collisions that occur between different sized metal/oxide clusters and the substrate. This work is done in collaboration with Prof. Simon Phillpot of the University of Florida and is supported by the National Science Foundation under Grant No. CHE-0200838. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).