Wu's Condensed Matter Theory Group

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Wu's group applies and develops the state-of-the-art quantum-mechanical simulation methods to investigate novel materials for the next-generation electronic and optoelectronic devices, with current focus on materials at the nanoscale (1-100 nm) with complex structures for energy conversion and storage and other applications. The goal of our modeling and computations is to provide fundamental understanding, to predict new phenomena, and to design better materials with desirable properties. In addition, we investigate fundamental physics of quantum entanglement and the reality of wave functions.

News and Highlights (Last updated: 07/2015)

  • July 21, 2015 Marc's paper, Tunable many-body interactions in semiconducting graphene: Giant excitonic effect and strong optical absorption, was published on Phys. Rev. B.

    ABSTRACT: Electronic and optical properties of graphene depend strongly on many-body interactions. Employing the highly accurate many-body perturbation approach based on Green's functions, we find a large renormalization over independent particle methods of the fundamental band gaps of semiconducting graphene structures with periodic defects. Additionally, their exciton binding energies are larger than 0.4 eV, suggesting significantly strengthened electron-electron and electron-hole interactions. Their absorption spectra show two strong peaks whose positions are sensitive to the defect fraction and distribution. The strong near-edge optical absorption and excellent tunability make these two-dimensional materials promising for optoelectronic applications.

  • May 08, 2015 Prof. Wu hooded Marc Dvorak.

  • March 16, 2015 Marc successfully defended his theis, congratulations, Dr. Dvorak! He will join the research group of Prof. Patric Rinke in Department of Applied Physics, Aalto University, Finland, as a postdoc.

  • March 02-06, 2015 Prof. Wu, Marc and Roxanne attended the APS March Meeting in San Antonio, TX.

  • February 23, 2015 My former senior design student Chris Marchbanks' paper, Reduction of heat capacity and phonon group velocity in silicon nanowires, was published on J. Appl. Phys.

    ABSTRACT: We report on ab initio linear-response calculations of lattice vibrations in narrow silicon nanowires on the order of 1 nm along the [001], [011], and [111] growth directions. The confinement and nanowire structure substantially alter phonon distributions, resulting in an 15% to 23% reduction in heat capacity and an averaged decrease of 31% in acoustic velocities compared with bulk silicon. Based on these, we estimate an improvement up to 4 fold on thermoelectric performance due solely to the modified lattice vibrations in narrow silicon nanowires over bulk silicon.

  • January 23, 2015 Marc's paper, Dirac point movement and topological phase transition in patterned graphene, was published on Nanoscale.

    ABSTRACT: The honeycomb lattice of graphene is characterized by linear dispersion and pseudospin chirality of fermions on the Dirac cones. If lattice anisotropy is introduced, the Dirac cones stay intact but move in reciprocal space. Dirac point movement can lead to a topological transition from semimetal to semiconductor when two inequivalent Dirac points merge, an idea that has attracted significant research interest. However, such movement normally requires unrealistically high lattice anisotropy. Here we show that anisotropic defects can break the C3 symmetry of graphene, leading to Dirac point drift in the Brillouin zone. Additionally, the long-range order in periodically patterned graphene can induce intervalley scattering between two inequivalent Dirac points, resulting in a semimetal-to-insulator topological phase transition. The magnitude and direction of Dirac point drift are predicted analytically, which are consistent with our first-principles electronic structure calculations. Thus, periodically patterned graphene can be used to study the fascinating physics associated with Dirac point movement and the corresponding phase transition.

  • December 15, 2014 Wu's group received NERSC (National Energy Research Scientific Computing Center) 2013 Allocation Award of 5,000,000 CPU hours for quantum mechanical simulations of complex nanostructures.

  • December 15, 2014 Xinquan's paper, Comment on "d+id’ Chiral Superconductivity in Bilayer Silicene", was accepted by Physics Review Letters.

  • December 15, 2014 Prof. Wu visited China University of Mining and Technology and taught the Computational Physics class there.

  • October 17, 2014 Huashan's paper, Charge separation at nanoscale interfaces: Energy-level alignment including two-quasiparticle interactions, was published on the Journal of Chemical Physics.

    ABSTRACT: The universal and fundamental criteria for charge separation at interfaces involving nanoscale materials are investigated. In addition to the single-quasiparticle excitation, all the two-quasiparticle effects including exciton binding, Coulomb stabilization, and exciton transfer are considered, which play critical roles on nanoscale interfaces for optoelectronic applications. We propose a scheme allowing adding these two-quasiparticle interactions on top of the single-quasiparticle energy level alignment for determining and illuminating charge separation at nanoscale interfaces. Employing the many-body perturbation theory based on Green's functions, we quantitatively demonstrate that neglecting or simplifying these crucial two-quasiparticle interactions using less accurate methods is likely to predict qualitatively incorrect charge separation behaviors at nanoscale interfaces where quantum confinement dominates.

  • September 12, 2014 Marc's paper, Geometrically induced transitions between semimetal and semiconductor in graphene, was published on Physical Review B.

    ABSTRACT: How the long-range ordering and local defect configurations modify the electronic structure of graphene remains an outstanding problem in nanoscience, which precludes the practical method of patterning graphene from being widely adopted for making graphene-based electronic and optoelectronic devices, because a small variation in supercell geometry could change the patterned graphene from a semimetal to a semiconductor, or vice versa. Based on the effective Hamiltonian formalism, here we reveal that a semimetal-to-semiconductor transition can be induced geometrically without breaking the sublattice symmetry. For the same patterning periodicity, however, breaking the sublattice symmetry increases the gap, while phase cancellation can lead to a semiconductor-to-semimetal transition in non-Bravais lattices. Our theory predicts the analytic relationship between long-range defect ordering and band-gap opening/closure in graphene, which is in excellent agreement with our numerical ab initio calculations of graphene nanomeshes, partially hydrogen passivated and boron-nitride-doped graphene.

  • September 09. 2014 Rainer, Robert, Kento, Nicholas and Tyler joined the group to do senior design projetcs.

  • July 17, 2014 Huashan's paper, Tailoring the optical gap of silicon quantum dots without changing their size was published on Physical Chemistry Chemical Physics.

    ABSTRACT: The absorption of photons through the direct generation of spatially separated excitons at dot-ligand interfaces is proposed as a promising strategy for tailoring the optical gap of small silicon quantum dots independent of their size. This removes a primary drawback for the use of very small dots in broad range of applications. For instance, the strategy can be applied to solar energy technologies to align the absorption of such dots with the peak of the solar spectrum. The key is to establish both a Type-II energy level alignment and a strong electronic coupling between the dot and ligand. Our first principles analysis indicates that connecting conjugated organic ligands to silicon quantum dots using vinyl connectivity can satisfy both requirements. For a prototype assembly of 2.6 nm dots, we predict that triphenylamine termination will result in a 0.47 eV redshift along with an enhanced near-edge absorption character. Robustness analyses of the influence of oxidation on absorption and of extra alkyl ligands reveal that the control of both factors is important in practical applications.

  • July 03, 2014 Marc's paper, Quasiparticle energies and excitonic effects in dense solid hydrogen near metallization was published on Physics Review B.

    ABSTRACT: We investigate the crucial metallization pressure of the Cmca−12 phase of solid hydrogen (H) using many-body perturbation theory within the GW approximation. We consider the effects of self-consistency, plasmon-pole models, and the vertex correction on the quasiparticle band gap (Eg). Our calculations show that self-consistency leads to an increase in Eg by 0.33 eV over the one-shot G0W0 approach. Because of error cancellation between the effects of self-consistency and the vertex correction, the simplest G0W0 method underestimates Eg by only 0.16 eV compared with the prediction of the more accurate GWΓ approach. Employing the plasmon-pole models underestimates Eg by 0.1–0.2 eV compared to the full-frequency numerical integration results. We thus predict a metallization pressure around 280 GPa, instead of 260 GPa predicted previously. Furthermore, we compute the optical absorption including the electron-hole interaction by solving the Bethe-Salpeter equation (BSE). The resulting absorption spectra demonstrate substantial redshifts and enhancement of absorption peaks compared to the calculated spectra neglecting excitonic effects. We find that the exciton binding energy decreases with increasing pressure from 66 meV at 100 GPa to 12 meV at 200 GPa due to the enhanced electronic screening as solid H approaches metallization. Because optical measurements are so important in identifying the structure of solid H, our BSE results should improve agreement between theory and experiment.

  • May 09, 2014 Profs. Lusk and Wu co-hooded Huashan, congratulations, Dr. Li! She will start postdoc at MIT in June.

  • February 05, 2014 Huashan successfully defended her PhD thesis, congratulations!

  • January 20, 2014 Huashan's paper, Double Superexchange in Quantum Dot Mesomaterials was published on Energy Environmental Science.

    ABSTRACT: A new optoelectronic mesomaterial is proposed in which a network of quantum dots is covalently connected via or- ganic molecules. Optically generated excitons are rapidly dissociated with electrons subsequently hopping from dot to dot while holes transit via the connecting moieties. The molecules serve as efficient mediators for electron su- perexchange between the dots, while the dots themselves play the complementary role for hole transport between molecules. The network thus exhibits a double superex- change. In addition to enhancing carrier hopping rates, double superexchange plays a central role in mediating ef- ficient polaron dissociation. Photoluminescence, dissoci- ation, and transport dynamics are quantified from first- principles for a model system composed of small silicon quantum dots connected by organic moieties. The results demonstrate that double superexchange can be practically employed to significantly improve charge generation and transport. These are currently viewed as the critical ob- stacles to dramatic enhancements in the energy conversion efficiency of photovoltaic cells based on quantum dots.

  • December 24, 2013 Huashan Li got a postdoc position in Prof. Grossman's group in MIT.

  • December 15, 2013 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.

  • September 09, 2013, Huashan's paper, Dangling Bond Defects: The Critical Roadblock to Efficient Photoconversion in Hybrid Quantum Dot Solar Cells, was published on J. of Phys. Chem. C.

    ABSTRACT: Inorganic–organic hybrid materials based on silicon quantum dots (SiQDs) have been utilized for photovoltaic applications but suffer from rapid charge recombination and low carrier mobility. We present an ab initio investigation of charge dynamics to pinpoint the source of this severe problem, and our results indicate that such devices show great promise provided that dangling bond (DB) defects can be sufficiently removed. Without DBs, the predicted charge transfer (CT) rate is much higher than that of photoluminescence (PL), while the electron hopping (EH) proceeds more quickly than interfacial charge recombination (CR). In contrast, one DB in a SiQD leads to a dramatic enhancement, by 10 orders of magnitude, in the CR rate and a reduction of the EH rate by 4 orders of magnitude, so that the diffusion of carriers to electrodes becomes extremely difficult. Although other factors, such as dot size distribution and oxidation, also play a deleterious role in device performance, their effects are deemed much less important than the critical role played by dangling bonds.

  • October 01, 2013, Marc's paper, Origin of the Variation of Exciton Binding Energy in Semiconductors was highlighted by DOE Basic Energy Sciences (BES).

  • September 10, 2013, Paul Larson's paper, Role of the plasmon-pole model in the GW approximation, was published on Phys. Rev. B.

    ABSTRACT: Band gaps and band-edge energy levels are computed using the many-body perturbation theory within the GW approximation, with four common plasmon pole models (PPMs) and numerical integration employed to evaluate the dynamic screening matrix. Although the Hybertsen-Louie PPM is often adopted in GW calculations because it predicts band gaps best matching experimental data, we show that it is the Godby-Needs construction that agrees consistently with numerical integration on dynamic screening for materials with distinct characteristics. The variation in predicted band gaps due to different PPMs used can be as large as 1 eV in strongly localized electronic systems, and the band-edge energy levels are more sensitive to the choice of PPM than band gap even in simple semiconductors.

  • July 26, 2013, Marc Dvorak's paper, Bandgap Opening by Patterning Graphene, was published on Scientific Reports.

    ABSTRACT: Owing to its remarkable electronic and transport properties, graphene has great potential of re-placing silicon for next-generation electronics and optoelectronics; but its zero bandgap associ-ated with Dirac fermions prevents such applications. Among numerous attempts to create semi-conducting graphene, periodic patterning using defects, passivation, doping, nanoscale perfora-tion, etc., is particularly promising and has been realized experimentally. However, despite ex-tensive theoretical investigations, the precise role of periodic modulations on electronic struc-tures of graphene remains elusive. Here we employ both the tight-binding modeling and first-principles electronic structure calculations to show that the appearance of bandgap in patterned graphene has a geometric symmetry origin. Thus the analytic rule of gap-opening by patterning graphene is derived, which indicates that the if a modified graphene is a semiconductor, its two corresponding carbon nanotubes, whose chiral vectors equal to graphene's supercell lattice vectors, are both semimetals.

  • June 14, 2013 Marc Dvorak won the best poster award in the the 25th Annual Workshop on Recent Developments in Electronic Structure Methods.

  • May 10, 2013 Chris Marchbanks won the best individual senior design project award.

  • January 25, 2013 Paul Larsen was honored as 2013 APS outstanding referee.

  • January 04, 2013 Marc's first scientific paper, Origin of the Variation of Exciton Binding Energy in Semiconductors, made Physics Review Letters cover.

  • ABSTRACT: Excitonic effects are crucial to optical properties, and the exciton binding energy, Eb, in technologically important semiconductors varies from merely a few meV to about 100 meV. This large variation, however, is not well understood. We investigate the relationship between the electronic band structures and exciton binding energies in semiconductors, employing first-principles calculations based on the density functional theory (DFT) and the many-body perturbation theory using Green's functions (GW/BSE). Our results clearly show that Eb increases as the localization of valence electrons increases due to the reduced electronic screening. Furthermore, Eb increases in ionic semiconductors such as ZnO because, contrary to the simple two-level coupling model, it has both conduction and valence band edge states strongly localized on anion sites, leading to an enhanced electron-hole interaction. These trends are quantized by electronic structures obtained from the DFT; thus, our approach can be applied to understand the excitonic effects in complex semiconducting materials.

2009 -- 2012 News and Highlights