Catalytic properties of ceria nanoparticles: CeO2 (ceria) is widely applied as an oxidation/reduction catalyst in industrial, automotive, and biomedical applications. Previous studies have connected the catalytic and autocatalytic activity of ceria to the presence of O-vacancies and associated Ce 3+ ions. Significant efforts have been dedicated to producing well-dispersed nanometer-scale ceria particles that maximize surface-to-volume ratio and surface O-vacancy concentration. Despite this, efforts to date have not yielded a definitive understanding of the atomistic mechanisms underlying the high activity of ceria nanoparticles (CNPs) as catalysts, catalyst supports, and/or anti-oxidants and O-scavengers.
Recent calculations in the Beck group have shown that CNPs, in contrast to bulk ceria samples, are super-oxidized, and are terminated with chemically absorbed O2 and O3 molecules. These results suggest the possiblity that catalytic mechanisms at ultrasmall CNPs may not depend on the the thermodynamics and kinetics of O-vacancies. Current efforts focus on exploring catalytic mechanisms and CNP structures relevant in catalysis for energy and applications as biological anti-oxidants.
Ge Surface Properties: The self-assembly of nanostructures with novel electronic and optical properties has led to intense interest in understanding and controlling the heteroepitaxial growth and morphology of crystalline thin films. Heteroepitaxial Ge on Si(100) has served as a model system for many theoretical and experimental investigations in this area. Using DFT calculations we have studied surface and interface properties in the Ge/Si quantum dot system. We studied the wetting layer thickness and strain dependence of the surface and interface excess energy of a Ge layer atop a Si (001) substrate, and the surface excess energy of dimer vacancy line (DVL) reconstructed Ge (001) surfaces and the RS reconstructed Ge (105) surface as a function of applied strain.
Results of this work have elucidated the complex relationship between surface energy and strain energy in the formation of Ge quantum dots on Si (001), pointed to the strong impact of DVLs on strain in the Ge surface, and allowed for the determination of useful fits of surface excess energy versus strain for the Ge (001) and (105) surfaces from first principles.
Electron-Phonon coupling from TDDFT: The details of thermal transport in solids are essential inputs in models for thermoelectric processes, flash or laser annealing, and heat transport in nanotubes and nanowires. While classical molecular dynamics simulations are able to explicitly treat phonon-mediated heat transport, they are unable to treat electron-mediated heat transport or the exchange of thermal energy from ions to electrons.
Recent calculations have demonstrated explicit-time integration TDDFT methods showing highly accurate treatment of electron dynamics. Since these calculations capture the full coupled dynamics of the electron-ion system, they naturally capture essential behavior such as thermal equilibration between the electronic and ionic subsystems, electrical resistance due to the scattering of electrons by phonons and the resultant Joule heating, and the thermoelectric effect, as well as modifications of this behavior due to structure of the studied system at the nanoscale. We are actively developing and applying methods for isolating the relevant atomic-scale behaviors and extracting associated parameters from novel TDDFT calculations in advanced materials systems.
Leakage Current through a-SiO2: Leakage of electrical current through oxide layers is a major limitation in mobile electronic devices (cell phones, computers, etc). In addition, radiation-induced failure of electronic devices (e.g. on satellites) is often triggered by catastrophically high leakage currents following ion strikes through dielectric layers. While the general mechanisms of leakage are well-known, the detailed, materials-specific, atomic-scale structures controlling leakage are generally unknown.
Using dynamical, atomistic structures and energy level distributions calculated from quantum mechanics as input, we have directly calculated leakage currents using both a 3D percolation method based on Mott tunneling, and a parameter-free quantum mechanical ramework. These complementary methods allow ab initio calculation of leakage currents in arbitrary device technologies based entirely on quantum mechanical calculations of defect structure and energy level distributions.
Displacement Damage Dynamics: Understanding and modeling the structural damage produced in a target solid during ion implantation or irradiation in a space or fission/fusion environment is important in electronic device and energy applications. Using first-principles dynamical calculations we study displacement damage at the atomic-scale. Focusing on low-energy (<1 keV) displacements in Si and SiO2, we work to build a physical understanding of the connection between individual atomic displacements and any resulting stable defects.
Recent results have demonstrated the importance of dynamic disorder formation and relaxation and recrystallization processes in determining stable defect profiles. This dynamic disorder has been compared to the local melting behavior observed for higher energy displacements. Disorder is also generated along the displacement tracks of recoiling atoms. In SiO2 these disordered tracks can contribute to dielectric failure of gate oxides by providing a connected pathway of defect states within the oxide bandgap.
Ion Stopping Power from TDDFT: Time-dependent Density Functional Theory (TDDFT) rigorously extends density functional theory beyond the static, time-independent regime, allowing ab-initio calculation of excited states, optical properties and electron dynamics. A recent implementation of explicit timeintegration TDDFT in conjunction with MD for atomic nuclei (TDDFT+MD) allows direct, real-time calculation of the full quantum mechanical dynamics of many atom systems. This powerful method enables ab initio calculation of electron-mediated ion-solid interactions, electron-phonon coupling, electron transport in molecules and crystals, and driven catalytic processes.
Using this method we have studied the classic problem of the stopping power of well-channelled ions in Si. Using ions from across the periodic table we have calculated the so-called Z1 oscillations in stopping power with incident ion species. These oscillations were shown to arise from the fully dynamic interaction of the ion and target electrons, and quantitative agreement between experiments and calculation has been achieved without the use of free parameters.
Frenkel Pairs in Si: Point defects control the electronic properties of semiconductor materials and devices. The Si lattice vacancy and interstitial are generally accepted as prototype semiconductor point defects, and have been well characterized by both experimental and computational studies. Based on seminal EPR experiments dating to the 1960s, it was concluded that interstitials in Si migrate athermally preventing the appearance of stable vacancy-interstitial pairs, or Frenkel pairs (FPs).
Using DFT calculations of structures, total energies and charge densities, we have shown that charge redistribution between nearby interstitials and vacancies interferes with the athermal migration of the individual defects, suppressing FP recombination.
Above: Dynamical charge density variations as a Si atom moves through an electron gas. See here for details.