Materials Sciences
Bimetallic surface segregation of nanoparticles
To better understand catalysts in fuel cells, Wang et al. used a Monte Carlo code built around the embedded atom method to investigate the segregation of platinum atoms on the surfaces of platinum-nickel nanoparticles. They used four kinds of nanoparticle shapes (cube, tetrahedron, octahedron, and cubo-octahedron) terminated by {111} and {100} facets to examine the extent of Pt segregation. Regardless of the shape and composition of the nanoparticles, the Pt atoms segregated preferentially to the facet sites, less to edge sites, and least to vertex sites in the outermost atomic layer of the nanoparticles.
G. Wang, M. A. Van Hove, and P. N. Ross, "Segregations in Pt-Ni catalyst nanoparticles," Surface Science (submitted, 2003). BES, SciDAC
Electronic interaction of TOTA+ with DNA
Investigating the electronic interactions of compounds intercalated in DNA is important because of the biological and medical roles played by intercalators. Reynisson et al. made first-principles quantum mechanical calculations of trioxatriangulenium ion (TOTA+) binding to duplex DNA by intercalation. The electronic structure calculations revealed no meaningful charge transfer from DNA to TOTA+, but showed the importance of backbone, water, and counterion interactions, which shift the energy levels of the base and the intercalated TOTA+ orbitals significantly. The calculations also showed that the inserted TOTA+ strongly polarizes the intercalation cavity, where a sheet of excess electron density surrounds the TOTA+ (Figure 8).
J. Reynisson, G. B. Schuster, S. B. Howerton, L. D. Williams, R. N. Barnett, C. L. Cleveland, U. Landman, N. Harrit, and J. B. Chaires, "Intercalation of trioxatrianguleneium ion with DNA: Binding, electron transfer, x-ray crystallography, and electronic structure," J. Am. Chem. Soc. 125, 2072 (2002). BES, NSF, NCI
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Electronic structure of C60 monolayers
Exhibiting properties such as high transition temperature superconductivity and antiferromagnetism, C60-based fullerides are ideal model compounds for exploring key conceptual issues in strongly correlated physics. Yang et al. studied the structure and electronic structure of electron-doped C60 monolayers. The overall shape of the calculated band structure showed good agreement with the high-resolution angle-resolved photoemission experiments, indicating a reduction of the occupied bandwidth by about 40 percent due to electron-phonon interaction.
W. L. Yang, V. Brouet, X. J. Zhou, H. J. Choi, S. G. Louie, M. L. Cohen, S. A. Kellar, P. V. Bogdanov, A. Lanzara, A. Goldoni, F. Parmigiani, Z. Hussain, and Z.-X. Shen, “Band structure and Fermi surface of electron-doped C60 monolayers,” Science 300, 303 (2003). BES, ONR, NSF
Nucleation of single-walled carbon nanotubes
The nucleation pathway for single-walled carbon nanotubes has been investigated using first-principles density-functional calculations. Fan et al. have shown that incorporation of pentagons at an early stage of the nanotube growth reduces the number of dangling bonds and facilitates bonding to the metal. As a result, the formation of a capped single-walled nanotube is overwhelmingly favored compared to any structure with dangling bonds (such as graphite sheets) or to a fullerene.
X. Fan, R. Buczko, A. A. Puretzky, D. B. Geohegan, J. Y. Howe, S. T. Pantelides, and S. J. Pennycook, “Nucleation of single-walled nanotubes,” Phys. Rev. Lett. 90, 145501 (2003). BES, ORNL, NSF, MEVU
Investigating the connection between orbital and spin ordering
Medvedeva et al. studied the complex nature of the fundamental connection between orbital ordering phenomena and spin ordering alignment on a microscopic scale using their recently developed average spin state calculation scheme. This scheme allowed them to treat a paramagnetic state and to describe successfully the experimental magnetic and orbital phase diagrams of LaMnO3 , KCuF3 , La0.5 Sr1.5 MnO4 , and LaSr2 Mn2 O7 . For the first time, they investigated from first principles the stability of charge and orbital ordering upon onset of a magnetic transition in La0.5 Sr1.5 MnO4 and LaSr2 Mn2 O7 .
J. E. Medvedeva, M. A. Korotin, V. I. Anisimov, and A. J. Freeman, “Charge and orbital ordering melting in layered LaSr2 Mn2 O7 ” (in preparation, 2003). BES
Calculating the properties of an organic semiconductor
There has been increasing interest in organic semiconductors such as pentacene for applications in electronic devices. Tiago et al. have studied the optical and electronic properties of crystalline pentacene, using a first-principles Green’s function approach. They investigated the role of polymorphism on the electronic energy gap and linear optical spectrum by studying two different crystalline phases: the solution-phase structure and the vapor-phase structure. Charge-transfer excitons were found to dominate the optical spectrum. Excitons with sizable binding energies were predicted for both phases.
M. L. Tiago, J. E. Northrup, and S. G. Louie, "Ab initio calculation of the electronic and optical properties of solid pentacene," Phys. Rev. B 67, 115212 (2003). BES, NSF
Relativity and the stability of intermetallic compounds
Why are some intermetallic compounds stable while others are not? Wang and Zunger showed that the existence of stable, ordered 3d-5d intermetallics CuAu and NiPt, as opposed to the unstable 3d-4d isovalent analogs CuAg and NiPd, results from relativity. First, in shrinking the equilibrium volume of the 5d element, relativity reduces the atomic size mismatch with respect to the 3d element, thus lowering the elastic packing strain. Second, in lowering the energy of the bonding 6s,p bands and raising the energy of the 5d band, relativity enhances (diminishes) the occupation of the bonding (antibonding) bands. Raising the energy of the 5d band also brings it closer to the energy of the 3d band, improving the 3d-5d bonding.
L. G. Wang and A. Zunger, “Why are the 3d-5d compounds CuAu and NiPt stable, whereas the 3d-4d compounds CuAg and NiPd are not,” Phys. Rev. B 67, 092103 (2003). BES
Electronic structures and properties of vitamin B12
Methodologies developed for the materials sciences are being extended to the study of biomaterials and biomolecules. Kurmaev et al. have calculated the electronic structure of the vitamin B12 molecule and its co-enzymes: cyanocobalamin, methylcobalamin, and adenosyl cobalamin. For the first time, all the side chains in these complex molecules were included in the ab initio calculation.
E. Z. Kurmaev, A. Moewes, L. Ouyang, L. Randaccio, P. Rulis, W. Y. Ching, M. Bach, and M. Neumann, "The electronic structure and chemical bonding of vitamin B12," Europhys. Lett. 62, 582 (2003). BES, NSF, EU, RFBR, NSERC, NEDO, MIUR, CMR
Density functional calculation of lead on silicon
Lead and silicon can form a well-defined interface (Pb/Si) which is ideal for studying two-dimensional behavior. But despite extensive work in the literature, the structure of the different Pb phases formed on Si(111) is still not clear, especially when the Pb coverage is more than one monolayer (ML). Using first principles calculations, Chan et al. have determined the atomic configuration, energy stability, and electronic structure of the different phases of Pb/Si(111). Their calculations will help experimenters identify the atomic model and domain wall arrangement for the controversial structures of Pb/Si(111) at the Pb coverage of 1 to 4/3 ML.
T.-L. Chan, C. Z. Wang, M. Hupalo, M. C. Tringides, Z.-Y. Lu, and K. M. Ho, “First-principles studies of structures and stabilities of Pb/Si(111)," Phys. Rev. B 68, 045410 (2003). BES
Growth and formation of silicon nanoclusters
Silicon nanoclusters have been the focus of intense theoretical and experimental interest, because the combination of the unique optical properties of silicon quantum dots and their compatibility with existing silicon-based and nanobiological technologies demonstrates great promise for numerous applications. Draeger et al., using density-functional and quantum Monte Carlo calculations, have demonstrated that the optical gap is significantly reduced by the presence of oxygen and fluorine atoms on the surface. This is an important result for experimentalists, as oxygen is a common contaminant in the procedure for synthesizing nanoparticles.
E. Draeger, J. C. Grossman, A. J. Williamson, and G. Galli, “Influence of synthesis conditions on the structural and optical properties of passivated silicon nanoclusters,” Phys. Rev. Lett. 90, 167402 (2003). BES
Molecular-dynamics simulations of kinetic coefficients
Asta et al. have demonstrated a new equilibrium molecular-dynamics simulation technique for calculating the kinetic coefficient (i.e., the proportionality constant between interface velocity and undercooling) of solid-liquid interfaces in elemental metals. This work represents a necessary first step towards studying dynamic properties of alloy solid-liquid interfaces near equilibrium.
M. Asta, D. Y. Sun, and J. J. Hoyt, "Role of atomistic simulation in the modeling of solidification microstructure," in NATO-ASI Proceedings: Thermodynamics, Microstructure and Plasticity, ed. by A. Finel (in press). BES
Solving the mystery of exchange bias
Exchange bias, a shift in magnetization that occurs when two different kinds of magnet come into contact, is used in numerous electronic applications such as magnetic multilayer storage and read head devices, but the effect is still not properly understood. Canning et al. have performed first-principles spin-dynamics simulations of the magnetic structure of iron-manganese/cobalt (FeMn/Co) interfaces (Figure 9). These quantum mechanical simulations, involving 2,016-atom super-cell models, reveal details of the orientational configuration of the magnetic moments at the interface that are unobtainable by any other means, offering important clues to solve the mystery of exchange bias.
A. Canning, B. Ujfalussy, T. C. Schulthess, X.-G. Zhang, W. A. Shelton, D. M. C. Nicholson, G. M. Stocks, Y. Wang, and T. Dirks, “Parallel multi-teraflops studies of the magnetic structure of FeMn alloys,” Proc. Int. Parallel and Distributed Processing Symposium (IPDPS’03, April 2003, Nice, France), p. 256b. BES, ASCR-MICS
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Fast large-scale nanostructure calculations
Fast first-principles calculation of thousand-atom systems is a challenge in computational nanoscience. Li and Wang have found a method to solve this challenge, using the charge-patching method for the charge density and an idealized surface passivation. It takes only about one hour on 64 processors to calculate a thousand-atom system with first-principles accuracy. Thousand-atom quantum dots for various materials were calculated for the first time.
J. B. Li and L. W. Wang, "First principle thousand atom quantum dot calculations," Phys. Rev. Lett. (submitted, 2003). BES
Understanding atomic ordering of semiconductor alloys
It has long been understood that atomic ordering, widely observed in certain epitaxially grown semiconductor alloys, is driven by surface thermodynamics and/or by growth kinetics, but not by bulk thermodynamics. Batyrev et al. have proposed a microscopic growth model for the ordering of gallium arsenide antimonide (GaAsSb) to account for the observed 3D ordered structure. For the first time, an interplay between ordering and surface reconstruction during the growth was proposed to explain the experimental observations. The model is qualitatively different from the widely accepted surface reconstruction- and dimerization-induced ordering models that explain only the in-plane 2D patterns.
I. G. Batyrev, A. G. Norman, S. B. Zhang, and S.-H. Wei,
“Growth model for atomic ordering: The case for quadruple-period ordering
in GaAsSb alloys,” Phys. Rev. Lett. 90, 026102 (2003).
BES