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FacultyKristen Fichthorn | Research Thin-Film EpitaxyThin films that form when atoms or molecules are deposited onto a solid surface have a variety of applications and in nearly every case the properties of the film (e.g., mechanical, catalytic, electrical, magnetic) are dependent on the interface structure. Since growth often occurs away from equilibrium, the kinetics of deposition and surface diffusion can play a significant role in determining film morphology. From a theoretical perspective, this is a challenging, multi-scale problem because film morphology (which evolves over micron lengths and times up to hours) can sensitively depend on the kinetics of deposition and surface diffusion, which occur over Angstroms and picoseconds. We have highlighted this challenge in a recent publication discussing nanohut formation in fcc(110) homoepitaxy: K. A. Fichthorn and M. Scheffler, "Nanophysics - A step up to self-assembly", Nature 429, 617 (2004).
A focus of our work has been on resolving unique atomic-scale mechanisms and growth modes. Some highlights are:
Understanding the Mechanisms of Island Growth and Bi-Layer Nanostructure Formation in Co/Cu(001) Heteroepitaxy
Support for this work is acknowledged from the National Science Foundation, grants: ECC-0085604, DMR-9617122, DGE-9987598, DMR-0514336.
Colloidal Nanoparticle Forces and DispersionNanoparticles possess desirable traits for a wide range of applications: structural, magnetic, optical, catalytic, etc. However processing colloidal nanoparticles is difficult because they tend to aggregate uncontrollably, unless passivated by surfactant or polyelectrolyte molecules. These molecules can be used to facilitate nanoparticle assembly. However, understanding of this phenomenon is in its very early stages. A large impediment is that colloidal nanoparticle forces are not well understood - theories for colloidal forces have been developed for large objects, to which continuum approximations apply. It is unclear (and unlikely) that these theories are accurate in all cases. Experiments have yet to resolve these forces. In our work, we are using molecular-dynamics to simulate solid nanoparticles in a liquid, quantify the interparticle forces, and elucidate their origins. A second component focuses on using the force laws from molecular dynamics to simulate large-scale colloidal suspensions and their assembly. Some highlights are:
Solvation Forces Between Colloidal Nanoparticles: Directed Alignment and Oriented Attachment
Support for this work is acknowledged from the National Science Foundation, grants: DGE-9987598, CCR-0303976, the Environmental Protection Agency, and the Petroleum Research Fund.
Multi-Scale Simulation of Materials and Interfacial PhenomenaA continuing challenge in materials simulation is to conduct long-time simulations of structural evolution, while accurately retaining atomic detail. Molecular-dynamics simulations can provide accurate details at atomic scales, however, they are not practical for simulating times beyond the nanosecond range. In many materials, dynamical evolution occurs through a series of "rare events", in which the system spends a long-time period in one potential-energy minimum before escaping and moving on to another. Since localized motion in the potential-energy minima is not significant, dynamical evolution can be simulated as a series of long-time jumps between potential-energy minima. This is the aim of kinetic Monte Carlo simulations. In principle, if a kinetic Monte Carlo simulation can incorporate all potential-energy minima of a system and the transition-state theory rates of all possible long-time jumps between the potential-energy minima, then this technique can reach macroscopic times while retaining the accuracy of molecular dynamics. We developed the Step and Slide method [R. A. Miron and K. A. Fichthorn, "The Step and Slide Method for Finding Saddle Points on Multidimensional Potential Surfaces", J. Chem. Phys. 115, 8742 (2001)] to find transition states of high-dimensional systems with minimal computational overhead, so that transition-state theory rate constants can be obtained and incorporated into kinetic Monte Carlo simulations. Another area of interest is in accelerating molecular-dynamics simulations of rare events. Here, we developed the Local Boost [J. - C. Wang, S. Pal, and K. A. Fichthorn, "Accelerated Dynamics of Rare Events with the Local Boost Method", Phys. Rev. B 63, 85403 (2001)] and Bond Boosting [R. A. Miron and K. A. Fichthorn, "Accelerated molecular-dynamics of rare events with the bond-boost method", J. Chem. Phys. 119, 6210 (2003)] methods, by which molecular-dynamics simulations can be made to accurately probe long-time dynamics over microseconds, or longer. Our most recent innovations have been in multiple-time scale accelerated molecular dynamics: addressing the small-barrier problem [R. A. Miron and K. A. Fichthorn, Phys. Rev. Lett. 93, 138201 (2004)]. Support for this work is acknowledged from the National Science Foundation, grants: ECC-0085604, DMR-9617122, DGE-9987598, DMR-0514336.
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