Patrick Schelling - US grants

0435632
Joynt


This is a U.S.-Vietnam cooperative research project between Dr. Robert Joynt, University of Wisconsin and Professor Bach Thanh Cong, Hanoi University to study computational materials and device physics. This study includes four separate projects related to computational device physics: magnetic structure of manganites, photochemistry at the rutile/water interface, spin relaxation in quantum dots, and optimization of device design for quantum computing. Proposed topics are modern and relevant to ongoing research in condensed and materials physics. This is a thoughtful and carefully designed study. It can help the development of computational materials science in Vietnam and can enhance the human resource development in computational physics in both countries

TECHNICAL SUMMARY:
This award support research and education in optical interactions of lasers and semiconductor materials. This award is jointly supported by Chemical, Bioengineering, Environmental, and Transport Systems in the Division of Engineering and the Division of Materials Research. Research investigates how materials interact with lasers under conditions relevant to laser processing of covalent semiconductors. The work develops methods for predicting how processing conditions affect the resulting material structure. The work includes the development and use of novel computer simulation methods to elucidate the fundamental physical processes relevant to laser processing of covalent semiconductors. The general approach applies to intense femto-second pulses interacting with silicon.

The researchers address the fundamental technical challenges relevant to the development of a multiscale model of heat and mass transport appropriate for far-from-equilibrium conditions. Electronic heat transport is treated at the continuum level, while the lattice dynamics are treated using classical molecular-dynamics. A crucial component of the proposed work is that the interatomic interactions will depend on the local electronic temperature TE. Parameters in the interactions will be based on a new modification of the popular Tersoff potential, with the dependence of the parameters on TE established by fitting to a large database of energies from finite-temperature ab initio calculations. This novel approach will capture nonthermal effects known to be important for sub-picosecond melting. Heat transport by excited charge carriers is addressed using ab initio simulations of excited liquids and the Kubo-Greenwood method. The coupling between the continuum description of the electrons and the lattice will be driven using Langevin dynamics with the damping parameter fit to experiment. The focus of the proposed work develops a model for silicon as a test case. The model will be tested in its treatment of the fundamental physics of laser ablation of crystalline silicon and laser annealing of amorphous silicon. Comparison to experiment is used to validate the results of the model, and produce new insight into the role of bond-weakening and ultrafast non-thermal processes to melting and ablation.

Carrying out this project requires researchers to address the fundamental technical challenges relevant to the development of a multiscale model of heat and mass transport appropriate for far-from-equilibrium conditions.

The effort includes both scientific and educational elements. The theoretical and computer simulation methods will expand researchers' ability to model the fundamental physics of laser ablation and will be applied to the technologically relevant processes for crystalline silicon and laser annealing of amorphous silicon. The work as educational value in developing student skills, particularly the graduates and undergraduates who are directly involved in the research and the activities aid in recruiting new students for graduate study in materials simulation. The researchers and students engage in workshop activities that introduce students to materials simulation, including molecular-dynamics simulation and visualization which is coordinated with the Florida Society for Materials Simulation. The work integrates education and research through the development of course in materials simulation.

NONTECHNICAL SUMMARY:
This award support research and education in optical interactions of lasers and semiconductor materials. Research investigates how materials interact with intense lasers beams under conditions relevant to laser processing of semiconductors. The work develops methods for predicting how processing conditions affect the resulting material structure. The work includes the development and use of novel computer simulation methods to elucidate the fundamental physical processes relevant to laser processing of covalent semiconductors. The general approach applies to intense ultrafast laser pulses interacting with silicon. In carrying out this project, researchers will address the fundamental technical challenges relevant to the development of a multiscale model of heat and mass transport appropriate for far-from-equilibrium conditions.

The effort includes both scientific and educational elements. The theoretical and computer simulation methods will expand researchers' ability to model the fundamental physics of laser ablation and will be applied to the technologically relevant processes for laser etching of silicon. The work as educational value in developing student skills, particularly the graduates and undergraduates who are directly involved in the research and the activities aid in recruiting new students for graduate study in materials simulation. The researchers and students engage in workshop activities that introduce students to materials simulation, including computer simulation and visualization which is coordinated with the Florida Society for Materials Simulation. The work integrates education and research through the development of course in materials simulation.

TECHNICAL SUMMARY: When large temperature gradients are present in metallic alloys, compositional gradients and phase transformations can occur, potentially leading to mechanical failure or undesirable changes in properties. This effect, known as thermotransport, is not well understood at the atomic scale for solids. Currently, there are no theoretical approaches to predict the relevant transport parameters. The PIs propose to elucidate thermotranport using phase-field modeling in conjunction with a multiscale computational approach based on atomic-scale simulation that includes molecular-dynamics and kinetic Monte-Carlo simulations. To compute key transport parameters, the local free energy profiles for vacancy diffusion in the presence of a temperature gradient will be computed. This approach enables direct computation of dissipative processes relevant to transport. Theoretical predictions will be validated using the results of experiments on Cu-Ni and Ni-Al binary alloys.

NON-TECHNICAL SUMMARY: In materials relevant for many applications, including nuclear-fuel alloys, interconnects for electronic circuits, and alloys for gas-turbine engines, the relentless drive towards higher efficiencies, increased temperatures, and smaller dimensions results in very large temperature gradients during operation. As a result of large temperature gradients, constituent atoms tend to be redistributed, leading to composition gradients and phase transformations. This effect, known as thermotransport, results in property changes in engineered components. For example, constituent redistribution in turbine blades can lead to mechanical failure. In this project, the PIs will use simulation at the atomic and continuum scales, along with experiments, to elucidate thermotransport in Ni-Al and Cu-Ni binary alloys. The ability to predict and understand thermotransport is expected to have a major impact on the stability of alloys used in high-temperature environments. Ph.D. students will be trained in the experimental and computational studies. Computational tools will be accessible to students in the statewide Florida Society for Materials Simulation. A summer 'Materials Camp' for K-12 students and teachers, including students from groups under-represented in science careers, will be hosted. The results will be made available to the community and integrated into commercial software packages for the simulation of diffusion in multicomponent alloys.

During the early stages of planet formation, small dust particles stick together to form larger particles. If the particles are smaller than 1 km in size, forces on the surfaces of these particles, called van der Waals forces, are thought to cause the particles to stick together. Other forces, however, can cause bouncing or breaking apart of the smaller particles. The typical van der Waals force is not strong enough to stop these particles from breaking apart. The investigators will study the possibility that particles joining together depend on chemical forces between the grains, not the van der Waals forces, as the chemical forces are stronger than van der Waals forces. The investigators will model the interaction between chemical forces sticking grains together and physical forces breaking grains apart, in a way that represents the early Solar System formation as we now understand it. This project serves the national interest as it advances our knowledge of the early building processes of the Solar System. The investigators will work to include Bridge Program students in this research project, as part of the UCF Physics Department's participation as an APS Bridge Program site, and will also include research results in K - 12 and college-level classes.



Most current hypotheses for the formation of planets in protoplanetary disks are based on the accretion of small dust grains into macroscopic dust aggregates. Several barriers have been identified at different stages of planet formation that could lead to bouncing or fragmentation of particles instead of increased sticking together and subsequent growth of particles. At sizes less than ~1km in diameter, surface forces are thought to be responsible for accretion. These forces are typically modeled as van der Waals forces. Experimental results, however, indicate that the typical strength of the van der Waals force is not sufficient to prevent fragmentation during collisions. Consequently, the mechanisms for the initial stages of planet formation remain poorly understood. The investigators describe here a plan to demonstrate how much the forces between particles in protoplanetary disks might depend on the chemical state of the mineral grains. The objective is to model grain interactions and collisions in a chemical environment that is intended to reflect what is known about the early Solar System during the initial stages of planet formation. Preliminary results indicate that the chemical state of grain surfaces plays an extremely important role, and the usual assumption of weak van der Waals forces is often invalid especially when surfaces are not passivated. Atomic-scale simulation results will be used to develop coarse-grained simulation models that can be used to understand collisions between larger aggregates. As part of the UCF Physics Department's participation as an APS Bridge Program site, the investigators will work to mentor several Bridge Program students with the potential to involve them in this research project. They will also include research results in K - 12 and college-level classes.

When electrons move through wires, they are scattered by vibrating atoms and wire imperfections. This scattering is the source of the electrical resistance and results in power consumption. For the wires used in modern electronics (interconnects) this is already a bottleneck to computing performance and is worsening as the wires (along with the transistors) are made smaller. The aim of this program is to make the wires so small (<< 10-nm in width and height) and so structurally perfect that quantum size effects arise and the electrons can travel in a ballistic fashion without scattering. This can result in orders of magnitude improvements in resistance and computing energy efficiency and enable a revolution in electronics. In addition to the societal benefit of improved computing, the program will support education and research at the undergraduate, graduate and post-doctoral levels at four institutions, Columbia University, Massachusetts Institute of Technology, Rensselaer Polytechnic Institute and the University of Central Florida. The outreach effort will include: (1) The Harlem Schools Partnership (HSP) for STEM education at CU, (2) MIT's Materials Day for industry outreach, (3) the Discovery Engineering program for high-school girls at RPI, and (4) the techCAMP "Future of Information" at UCF.

To achieve its goal of ballistic conduction in metallic nanowires, the project will include the preparation and atomic scale characterization of single crystal metallic films and lines as well as experimental measurement of electron transport behavior, with ruthenium as the metal of choice for the initial studies. The stability of the metallic lines will also be investigated, since this is critical to the reliability of interconnects in computing systems. A theoretical and computational physics modeling effort will aim to provide a quantitative understanding of ballistic transport in the size and defect limits, and serve to identify preferred metals for ballistic conductance of interconnects for future efforts. The computational models and codes developed will be made available on the Nanohub (https://nanohub.org/). The project will additionally provide a proof-of-principle demonstration of how the proposed metallic conductors can be fabricated for technology implementation by the semiconductor industry.