Wu's Condensed Matter Theory Group

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2009 -- 2012 News and Highlights

  • December 14, 2012 Wu's group received NERSC (National Energy Research Scientific Computing Center) 2013 Allocation Award of 2,000,000 CPU hours for quantum mechanical simulations of complex nanostructures.

  • October 26, 2012 Huashan's paper, Optimal Size Regime for Oxidation-Resistant Silicon Quantum Dots, was published on ACS nano.

    ABSTRACT: First-principles computations have been carried out to predict that appropriately terminated silicon quantum dots with diameters in the range of 1.2 - 2 nm will offer a superb resistance to oxidation. This is because surface treatments can produce dangling bond defect densities sufficiently low that dots of this size are unlikely to have any defect at all. On the other hand, these dots are large enough that the severe angles between facets do not expose bonds that are vulnerable to subsequent oxygen attack. The absence of both surface defects and geometry-related vulnerabilities allows even very short passivating ligands to generate an effective barrier, an important consideration for charge and exciton transport within quantum dot assemblies. Our computations, which employ many-body perturbation theory using Green functions, also indicate that dots within this size regime have optical and electronic properties that are robust to small amounts of inadvertent oxidation, and that any such oxygen incorporation is essentially frozen in place.

  • May 18, 2012 William Oswald obtained his MS degree. He now works in a company belonging to Lockheed Martin at Aurora, CO.

  • April 21, 2012 Huashan successfully defended her PhD thesis proposal.

  • March 22, 2012 William published his first scientific article, Energy gaps in graphene nanomeshes, in Physical Review B.

    Abstract: We report on the band gap opening and electronic structures of graphene nanomeshes (GNMs), the defected graphene containing a high-density array of nanoscale holes, from first-principles calculations. As expected, quantum confinement at the GNM necks leads to a sizable band gap; however, surprisingly, the appearance of a gap depends sensitively on the hole arrangement and periodicity. For the simplest hexagonal zigzag-edged holes passivated by hydrogen, two-thirds of GNMs remain semimetallic while the rest are semiconductors. Furthermore, we show that the energy gap opening in GNM results from the combination of quantum confinement and the periodic perturbation potential due to perforation.

  • February 08, 2012 Huashan published her first scientific article, First principles analysis of the initial oxidation of Si(001) and Si(111) surfaces terminated with H and CH3, in Journal of Chemical Physics. This article reports a surprising finding that oxidation occurs via direct dissociative adsorption, without any energy barrier, on Si(111) and reconstructed Si(001) that have been hydrogen terminated; oxidation initiates with a barrier of only 0.05 eV on unreconstructed Si(001), from first-principles calculations. Furthermore, these results are consistent with the measured 1.6-1.7 eV effective oxidation barrier because of coverage dependency and much larger hopping barriers involved. Methyl termination, in contrast, offers an additional level of protection because oxygen must first undergo interactions with these ligands in a three-step process with significant energy barriers.

  • October 12, 2011 Prof. Wu received $60,000 research funding from Alfred P. Sloan Foundation to support his research in density functional development for aqueous fluids simulations.

  • May 06, 2011 Prof. Wu received the prestigious DOE early career research award, and the $750,000 funding will support his project Quantum Mechanical Simulations of Complex Nanostructures for Photovoltaic Applications.

    Abstract: This research project involves theoretical studies of complex semiconducting nanomaterials and their interfaces with other materials for next-neration photovoltaic cells. The central challenge in materials simulation is to address complexities in real materials rather than considering the properties of idealized materials or structures. For photovoltaic devices, it is of paramount importance to be able to accurately predict excited-ate properties of complex nanostructures that are the result of optical absorption. However, current quantum mechanical simulation methods are either computationally too demanding or not sufficiently accurate. The primary objectives of this project are (1) developing a new theoretical approach for electronic excitation calculations that is both accurate and applicable to large and complex systems and (2) applying the new methodology to investigate complex nanostructures that have great potential of opening new routes toward designing a material'transport, electronic, and optical properties for photovoltaic and other optoelectronic applications.

  • May 10, 2010 Prof. Wu's recent work, Quantum Monte Carlo computations of phase stability, equations of state, and elasticity of high-pressure silica, which was published in the Proceedings of the National Academy of Sciences, made Science News.
    ScienceDaily (May 10, 2010) -- Scientists have used quantum mechanics to reveal that the most common mineral on Earth is relatively uncommon deep within the planet. Using several of the largest supercomputers in the nation, a team of physicists led by Ohio State University has been able to simulate the behavior of silica in a high-temperature, high-pressure form that is particularly difficult to study firsthand in the lab. The resulting discovery -- reported in the early online edition of the Proceedings of the National Academy of Sciences -- could eventually benefit science and industry alike. Silica makes up two-thirds of the Earth's crust, and we use it to form products ranging from glass and ceramics to computer chips and fiber optic cables.

  • January 15, 2010 Prof. Wu published a feature review article, Charge separation in nanoscale photovoltaic materials: recent insights from first-principles electronic structure theory, in Journal of Materials Chemistry. This article focuses on the key problem of charge separation in nanoscale photovoltaic materials. Recent theoretical/computational work based on first principles electronic structure approaches is presented and discussed. We review applications of state-of-the-art electronic structure calculations to nano-scale materials that enable charge separation between an excited electron and hole in so-called excitonic photovoltaic cells. We highlight important theoretical results that provide insight into such experimentally observed processes, which are yet to be understood and do not appear to obey a single unique model but rather depend on atomistic details. Examples are provided that illustrate how computational approaches can be employed to probe new directions in materials design for inducing efficient charge separation. We also discuss the computational challenges in electronic structure theory for reliably predicting and designing new materials suitable for charge separation in photovoltaic applications.

  • September 9, 2009 Prof. Wu published a communication,Quantum Monte Carlo Simulation of Nanoscale MgH2 Cluster Thermodynamics, in Journal of the American Chemistry Society. In this paper the desorption energy of MgH2 clusters is calculated using the highly accurate quantum Monte Carlo (QMC) approach, which can provide desorption energies with chemical accuracy (within 1 kcal/mol) and therefore provides a valuable benchmark for such hydrogen-storage simulations. Compared with these QMC results, the most widely used density functional theory (DFT) computations (including a wide range of exchange-correlation functionals) cannot reach a consistent and suitable level of accuracy across the thermodynamically tunable range for MgH2 clusters. Furthermore, our QMC calculations show that the DFT error depends substantially on cluster size. These results suggest that in simulating metal-hydride systems it is very important to apply accurate methods that go beyond traditional mean-field approaches as a benchmark of their performance for a given material, and QMC is an appealing method to provide such a benchmark due to its high level of accuracy and favorable scaling (N3) with the number of electrons.

  • August 21, 2009 The starting date of Wu's research group at CSM.