Person:

Kaxiras, Efthimios

The lattice Boltzmann method is increasingly important in facilitating large-scale fluid dynamics simulations. To date, these simulations have been built on discretized velocity models of up to 27 neighbors. Recent work has shown that higher order approximations of the continuum Boltzmann equation enable not only recovery of the Navier-Stokes hydrodynamics, but also simulations for a wider range of Knudsen numbers, which is especially important in micro- and nanoscale flows. These higher-order models have significant impact on both the communication and computational complexity of the application. We present a performance study of the higherorder models as compared to the traditional ones, on both the IBM Blue Gene/P and Blue Gene/Q architectures. We study the tradeoffs of many optimizations methods such as the use of deep halo level ghost cells that, alongside hybrid programming models, reduce the impact of extended models and enable efficient modeling of extreme regimes of computational fluid dynamics.

We present a procedure that makes use of group theory to analyze and predict the main properties of the negatively charged nitrogen-vacancy (NV) center in diamond. We focus on the relatively low temperature limit where both the spin–spin and spin–orbit effects are important to consider. We demonstrate that group theory may be used to clarify several aspects of the NV structure, such as ordering of the singlets in the ((e^2)) electronic configuration and the spin–spin and spin–orbit interactions in the ((ae)) electronic configuration. We also discuss how the optical selection rules and the response of the center to electric field can be used for spin–photon entanglement schemes. Our general formalism is applicable to a broad class of local defects in solids. The present results have important implications for applications in quantum information science and nanomagnetometry.

We report on the ability to modify the structure and composition of ultrathin oxides grown on Ni and Ni-Al alloy surfaces at room temperature utilizing photon illumination. We find that the nickel-oxide formation is enhanced in the case of oxidation under photo-excitation. The enhanced oxidation kinetics of nickel in 5% Ni-Al alloy is corroborated by experimental and simulation studies of natural and photon-assisted oxide growth on pure Ni(100) surfaces. In case of pure Ni substrates, combined x-ray photoelectron spectroscopy analysis, and atomic force microscope current mapping support the deterministic role of the structure of nickel passive-oxide films on their nanoscale corrosion resistance. Atomistic simulations involving dynamic charge transfer predict that the applied electric field overcomes the activation-energy barrier for ionic migration, leading to enhanced oxygen incorporation into the oxide, enabling us to tune the mixed-oxide composition at atomic length scales. Atomic scale control of ultrathin oxide structure and morphology in the case of pure substrates as well as compositional tuning of complex oxide in the case of alloys leads to excellent passivity as verified from potentiodynamic polarization experiments.

Density functional theory, informed by experimental studies, is used to investigate the interplay of surface morphology, the adsorption site of reactants, the nature of the interaction between adsorbates and the surface, the potential energy landscape for adsorbates on the surface, adsorbate coverage, temperature, and the dynamic evolution of these factors during adsorption and reaction. We summarize our current understanding of Au atom release on the (111) surface and the corresponding effects on adsorption and reactivity. Gold was selected for these investigations because of the recent intense interest in the activity of gold nanoparticles for several important catalytic reactions. Fundamental experimental studies on Au single-crystal surfaces have established that atomic O is extremely active for oxidation of CO and olefins, that the local bonding of O is an important factor in determining the reactivity and selectivity for oxidation, and that Au atom release is induced by electronegative adsorbates, such as O, Cl, and S. These experimental results guided our theoretical studies. Density functional theory is an extremely useful tool since it evaluates the energetics associated with the incorporation of gold into the adsorbate layer, while providing fundamental physical insight into the underlying cause of gold incorporation. We use our results from static DFT calculations along with ab initio molecular dynamics simulations to understand the effect of surface morphology on the activity of gold for CO oxidation. Our investigation of Au atom release and incorporation induced by electronegative atoms clearly illustrates the importance of using experiments in combination with theory to establish the importance of and the underlying reasons for metal atom release and the affect on bonding and reactivity.

Fabricating stable functional devices at the atomic scale is an ultimate goal of nanotechnology. In biological processes, such high-precision operations are accomplished by enzymes. A counterpart molecular catalyst that binds to a solid-state substrate would be highly desirable. Here, we report the direct observation of single Si adatoms catalyzing the dissociation of carbon atoms from graphene in an aberration-corrected high-resolution transmission electron microscope (HRTEM). The single Si atom provides a catalytic wedge for energetic electrons to chisel off the graphene lattice, atom by atom, while the Si atom itself is not consumed. The products of the chiseling process are atomic-scale features including graphene pores and clean edges. Our experimental observations and first-principles calculations demonstrated the dynamics, stability, and selectivity of such a single-atom chisel, which opens up the possibility of fabricating certain stable molecular devices by precise modification of materials at the atomic scale.

A fully dynamical approach using ab initio molecular dynamics (AIMD) simulations is applied to the investigation of CO oxidation on O-covered Au(111). We investigate how the activity of gold depends upon temperature, oxygen coverage, and surface structure. On clean Au(111) at 500 K, CO binds transiently on top of Au atoms, spending a small fraction (~7%) of the total simulation time adsorbed on the surface. The presence of O on the surface increases the residence time for CO by more than 4 times on a surface containing 0.22 ML of O. On the other hand, the probability for CO adsorption decreases with oxygen coverage from 31% at 0.22 ML of oxygen to 15% at 0.55 ML of oxygen. Our simulations show that the activity for CO reaction with O to yield (CO_2) decreases with increasing oxygen coverage. We attribute this decrease of activity to (1) the decrease in the CO adsorption probability as the oxygen coverage increases and (2) the decreasing amount of reactive chemisorbed oxygen (oxygen bound in a 3-fold site) with increasing total oxygen coverage. We show that oxygen bound in sites of local 3-fold coordination (chemisorbed oxygen) is nearly 2 times more reactive than other oxygen species observed on the surface, namely, surface and subsurface oxide. Our work demonstrates the value and feasibility of using AIMD to study surface reactions.

We investigate electron and hole dynamics upon photon excitation in dye-sensitized solar cells, using a recently developed method based on real-time evolution of electronic states through time-dependent density functional theory. The systems we considered consist of organic sensitizers and nanocrystalline (TiO_2) semiconductors. We examine the influence of various factors on the dynamics of electrons and holes, including point defects (vacancies) on the (TiO_2) surface, variations in the dye molecular size and binding geometry, and thermal fluctuations which result in different alignments of the electronic energy levels. Two clear trends emerge: (a) dissociated adsorption of the dye molecules leads to faster electron injection dynamics by reducing interfacial dipole moments; (b) oxygen vacancy defects stabilize dye adsorption and facilitate charge injection, at the cost of lower open circuit voltage and higher electron−hole recombination rate. Understanding of these effects at the atomic level suggests tunable parameters through which the electronic characteristics of dye-sensitized solar cell devices can be improved and their efficiency can be maximized.

Employing ab initio calculations we predict that the magnetic states of hydrogenated diamond-shaped zigzag graphene quantum dots (GQDs), each exhibiting unique electronic structure, can be selectively tuned with gate voltage, through Stark or hybridization electric-field modulation of the spatial distribution and energy of the spin-polarized molecular orbitals, leading to transitions between these states. Electrical read-out of the GQD magnetic state can be accomplished by exploiting the distinctive electrical properties of the various magnetic configurations.

Impurities segregated to grain boundaries of a material essentially alter its fracture behavior. A prime example is sulfur segregation-induced embrittlement of nickel, where an observed relation between sulfur-induced amorphization of grain boundaries and embrittlement remains unexplained. Here, (48×10^6)-atom reactive-force-field molecular dynamics simulations provide the missing link. Namely, an order-of-magnitude reduction of grain-boundary shear strength due to amorphization, combined with tensile-strength reduction, allows the crack tip to always find an easy propagation path.

We use the results of first-principles electronic structure calculations and a strong coupling perturbation approach, together with general theoretical arguments, to illustrate the differences in super-exchange interactions between the copper-oxides and iron-pnictides. We provide a possible explanation for the two magnetic ground states within the same theoretical foundation. Contrary to the emerging view that magnetic order in the iron-pnictides is of itinerant nature, we argue that the observed magnetic moment is small because of frustration introduced by the electrons of the Fe orbitals as they compete to impose their preferred magnetic ordering.