Merrell R. Fenske Professor Kristen Fichthorn | Research Interests & Highlights
Thin 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., mechani-cal, 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 deposi-tion and surface diffusion, which occur over Angstroms and picoseconds.
Our work has focused on resolving unique atomic-scale mechanisms and growth modes of metal, semi-conductor, and organic thin films using theoretical techniques such as density-functional theory (DFT), molecular-dynamics (MD) simulations, and kinetic Monte Carlo (KMC). We are also interested in fitting force fields for classical MD simulations using results from first-principles calculations. This work is supported by the US National Science Foundation. Recent highlights are:
Accelerated molecular dynamics of temperature-programmed desorption.
K. E. Becker, M. H. Mignogna, and K. A. Fichthorn, Phys. Rev. Lett. 102, 046101 (2009).
Patterns in strained-layer heteroepitaxy: Beyond the Frenkel-Kontorova model. J. D. Howe, P. Bhopale, Y. Tiwary, and K. A. Fichthorn, Phys. Rev. B 81, 121410(R) (2010).
An accelerated molecular dynamics study of the GaAs(001) β2(2x4) / c(2x8) surface. Y. Lin and K. A. Fichthorn, Phys. Rev. B 86, 165303 (2012).
Growth, Transformation, and Assembly
of Nanoscale Materials: In-sights from Simulation
Achieving the controlled synthesis of colloidal nanomaterials with selected shapes and sizes is an important goal for a variety of applications that can exploit their unique prop-erties (e.g., optical, catalytic, magnetic, etc.). In the past decade, a number of promising solution-phase synthesis techniques have been developed to fabricate various nanostruc-tures. A deep, fundamental understanding of the phenomena that promote selective growth and assembly in these syntheses would enable tight control of nanostructure mor-phologies in next-generation techniques.
Our research has focused on understanding interparticle forces and their impact on the aggregation, sintering, and assembly of colloidal nanoparticles. Our recent studies have also targeted the shape-selective synthesis of colloidal nanoparticles. We apply first-principles DFT, atomic-scale MD simulations and coarse-grained, meso-scale Monte Carlo (MC) simulations to these problems. This work is supported by the US Depart-ment of Energy.
Some recent highlights are:
Assembly of gold nanowires by sedimentation from suspension: Experiment and simulation.
D. A. Triplett, L. M. Dillenback, B. D. Smith, D. Hernandez Rodriguez, S. K. St. Angelo, P. Gonzalez, C. D. Keating, and K. A. Fichthorn, J. Phys. Chem. C 114, 7346 (2010).
Adsorption of polyvinylpyrrolidone on Ag surfaces:
Insight into a structure-directing agent.
W. Al-Saidi, H. Feng, and K. A. Fichthorn, Nano Letters 12, 997 (2012).
Microscopic view of nucleation in the anatase-to-rutile transition.
Y. Zhou and K. A. Fichthorn, J. Phys. Chem. C 116, 8314 (2012).
Multi-Scale Simulation of Materials and Interfacial Phenomena
A continuing challenge in materials simulation is to conduct long-time simulations of structural evolution, while accurately retaining atomic detail. Molecular-dynamics simu-lations 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 local-ized 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 kMC simulations.
In principle, if a kMC 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 MD. 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 transi-tion-state theory rate constants can be obtained and incorporated into kMC 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, "Accel-erated 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.
A recent area of interest is the "small-barrier" problem, which occurs when a group of neighboring free-energy minima connected by low free-energy barriers is separated from the rest of phase space by relatively high barriers. This type of potential-energy land-scape can lead to repetitive motion between neighboring minima. Much of the computa-tional effort is spent on simulating this repetitive motion, which then limits the ability to render the long-time evolution of the system. We have dealt with this problem in accel-erated MD simulations [R. A. Miron and K. A. Fichthorn, "Multiple-time scale acceler-ated molecular dynamics: Addressing the small-barrier problem", Phys. Rev. Lett. 93, 128301 (2004); Y. Lin and K. A. Fichthorn, "An accelerated molecular dynamics study of the GaAs(001) β2(2x4)/c(2x8) surface", Phys. Rev. B 86, 165303 (2012).]
Look for our newest work on dealing with the problem in KMC simulations!! This work is sup-ported by the US National Science Foundation.
Design of Surfaces with Controlled Wettability
The ability to create surfaces with controlled wettability is important for a wide variety of applications. Superhydrophobic surfaces, which possess a high (>150º) contact angle for water droplets and low (< 10º) contact-angle hysteresis, have a number of beneficial properties, including water repellency, self-cleaning, low drag, and antifouling character-istics. An archetypical, biologically-inspired surface for superhydrophobicity is the Lotus leaf. The success of the Lotus leaf at water repellency and self-cleaning is attributed to its chemical composition, as well as its surface structure, which is characterized by roughness over both micrometer and sub-micrometer length scales. The idea that (multi-scale) surface roughness can induce superhydrophobicity has inspired many studies aimed at synthesizing rough or patterned surfaces with superhydrophobic properties and quantifying the effect of roughness on superhydrophobicity. In our research, we employ continuum-level theory and atomic-scale MD simulations to tailor surface roughness for controlled wettability. This work is supported by the US National Science Foundation. A recent highlight is:
A theory for the morphological dependence of wetting on a physically-patterned solid surface.
A. Shahraz, A. Borhan, and K. A. Fichthorn, Langmuir 28, 14227 (2012).
Our work is supported by the US National Science Foundation, the Department of Energy, and Dow Chemical Co. We are grateful for computing time on XSEDE, as well as on Blue Gene/Q at Argonne National Laboratory.