Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session B22: Theory and Simulation of ExcitedState Phenomena in Semiconductors and Nanostructures IFocus

Hide Abstracts 
Sponsoring Units: DCOMP Chair: Andre Schleife, University of Illinois at UrbanaChampaign Room: 321 
Monday, March 14, 2016 11:15AM  11:51AM 
B22.00001: Hole localization, water dissociation mechanisms, and band alignment at aqueoustitania interfaces Invited Speaker: John L. Lyons Photocatalytic water splitting is a promising method for generating clean energy, but materials that can efficiently act as photocatalysts are scarce. This is in part due to the fact that exposure to water can strongly alter semiconductor surfaces and therefore photocatalyst performance. Many materials are not stable in aqueous environments; in other cases, local changes in structure may occur, affecting energylevel alignment. Even in the simplest case, dynamic fluctuations modify the organization of interface water. Accounting for such effects requires knowledge of the dominant local structural motifs and also accurate semiconductor bandedge positions, making quantitative prediction of energylevel alignments computationally challenging. Here we employ a combined theoretical approach to study the structure, energy alignment, and hole localization at aqueoustitania interfaces. We calculate the explicit aqueoussemiconductor interface using ab initio molecular dynamics, which provides the fluctuating atomic structure, the extent of water dissociation, and the resulting electrostatic potential. For both anatase and rutile TiO$_{\mathrm{2}}$ we observe spontaneous water dissociation and reassociation events that occur via distinct mechanisms. We also find a higherdensity water layer occurring on anatase. In both cases, we find that the second monolayer of water plays a crucial role in controlling the extent of water dissociation. Using hybrid functional calculations, we then investigate the propensity for dissociated waters to stabilize photoexcited carriers, and compare the results of rutile and anatase aqueous interfaces. Finally, we use the \textit{GW} approach from manybody perturbation theory to obtain the position of semiconductor band edges relative to the occupied 1b$_{\mathrm{1}}$ level and thus the redox levels of water, and examine how local structural modifications affect these offsets. [Preview Abstract] 
Monday, March 14, 2016 11:51AM  12:03PM 
B22.00002: Role of excited states in ShockleyReadHall recombination in wide bandgap semiconductors Audrius Alkauskas, Cyrus E. Dreyer, John L. Lyons, Chris G. Van de Walle Defectassisted recombination is an important limitation on efficiency of optoelectronic devices. However, since nonradiative capture rates decrease exponentially with energy of the transition, the mechanisms by which such recombination can take place in widebandgap materials are unclear. We investigate the role of electronic excited states in the recombination process, focusing on groupIII nitrides, for which accumulating experimental evidence indicates that defectassisted recombination is an important limiting factor in efficiency. Based on firstprinciples electronic structure calculations, we show that excited states of gallium vacancy complexes make these defects very efficient recombination centers. Our work provides new insights into the physics of nonradiative recombination. The mechanism discussed in this work is suggested to be very critical and ubiquitous in widebandgap semiconductors. This work was supported by DOE and the European Union. [Preview Abstract] 
Monday, March 14, 2016 12:03PM  12:15PM 
B22.00003: Auger recombination in InN from first principles Andrew McAllister, Emmanouil Kioupakis GroupIII Nitride materials are used in numerous electronic and optoelectronic devices including solidstate lighting, energy conversion, sensor technologies, and highpower electronics. Indium nitride in particular is interesting for fast electronics and optoelectronics in the infrared. Auger recombination is a nonradiative carrier recombination process that would limit the efficiency of these devices. The small band gap (0.7 eV) and the high intrinsic freeelectron concentrations in InN possibly make Auger recombination particularly important in this material. We used firstprinciples computational methods to determine the Auger recombination rates in InN. Our results suggest that direct Auger recombination is dominant in this material and that phononassisted Auger processes are not as important as in widergap nitrides such as GaN. [Preview Abstract] 
Monday, March 14, 2016 12:15PM  12:27PM 
B22.00004: Electronic coherence and the kinetics of energy transfer in lightharvesting systems Pengfei Huo, David Coker, Thomas Miller Recent 2Dspectroscopy experiments have observed transient electronic coherence in natural and artificial light harvesting systems, which raises questions about the role of electronic coherence in facilitating excitation energy transfer (EET) processes. In this talk, we introduce the recently developed partial linearized pathintegral (PLPI) method, which can accurately simulate exciton transfer dynamics across multiple reaction regimes, as well as reliably describe the electronic coherence among excitonic states. Further, we develop a strategy that enables the analysis of the relative impact of static and dynamic electronic coherence. With PLPI simulations, we find that energy transfer dynamics are almost entirely dominated by static coherence effects; dynamic coherence is found to cause only minor effects. These conclusions are consistent with the historical view that emphasizes the importance of energylevel alignment for efficient incoherent energy transfer,while suggesting a less important role for more exotic electronic coherence effects that have been recently emphasized. [Preview Abstract] 
Monday, March 14, 2016 12:27PM  12:39PM 
B22.00005: The Optical Spectrum of LaAlO$_3$: Quasiparticle Energies and the Effect of Lattice Screening Xiao Zhang, Andre Schleife Lanthanum aluminate (LaAlO$_3$) is a commonly used high$\kappa$ dielectric material but its exact optical properties are not well understood. By solving the BetheSalpeter Equation for the optical polarization function, which describes the interaction between electrons and holes, a precise prediction of the dielectric function can be obtained. However, for LaAlO$_3$, there are two major problems limiting the computational study: The first problem is that due to the complicated conduction band structure, the quasiparticle effect needs to be taken into account, which makes the calculations costly. We resolved this problem by interpolating accurate eigenenergies computed using a hybrid exchangecorrelation functional to a dense kpoint grid. Another problem is that for such high$\kappa$ materials, the lattice contribution to the dielectric screening may be important. We investigated this by computing the optical spectrum using electronic constant, static dielectric constant and the average of both and found that taking lattice contribution into account significantly reduces excitonic effects. All results are compared to available experiments. [Preview Abstract] 
Monday, March 14, 2016 12:39PM  12:51PM 
B22.00006: Electronic and optical properties of Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$ from first principles Kelsey Mengle, Emmanouil Kioupakis Wide bandgap semiconductors such as Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$ are used in numerous applications including high voltage/temperature electronics, deepUV emission, and transparent contacts. We have investigated the electronic and optical properties of Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$ with firstprinciples calculations based on density functional theory. The electronic and optical properties are calculated with manybody perturbation theory using the GW and BetheSalpeter equation methods. The semicore states of Ga are treated as valence electrons to accurately determine the band gap and band structure. We will present results for the structural, electronic, and optical properties of the various Ga$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$polymorphs, including $\beta $Ga$_{\mathrm{2}}$O$_{\mathrm{3}}$. This research was supported by the National Science Foundation through Grant No. DMR 1534221. Computational resources were provided by the DOE NERSC facility. [Preview Abstract] 
Monday, March 14, 2016 12:51PM  1:03PM 
B22.00007: Substrate Screening Induced Renormalization of ExcitedStates in 2D Materials Neerav Kharche, Vincent Meunier Twodimensional (2D) materials offer an emerging platform for exploring novel electronic phenomena in reduced~dimensionality systems.~ However, because of~their atomic scale thickness, their excitation energy levels in 2D materials are strongly renormalized due to the screening by the surrounding environment. This effect is expected to have strong impact when the materials are integrated into functional devices.~ For example, the presently available GW calculations significantly overestimate the band gaps in graphene nanoribbons (GNRs) by as much as one eV compared to experiment. Here, we outline an integrated computational approach combining DFT, the GW approximation, and a classical image charge model to include substrate screening effects in a computationally tractable manner. We investigate the band gaps and defect charge transition levels (CTLs) in a prototypical 2D material, hexagonal boron nitride (hBN) and a prototypical 1D nanostructure, GNR. The band gaps and defect CTLs are strongly renormalized by several tenths of an eV in the substratesupported versus the freestanding configurations. In the case of GNRs, the predicted band gaps are in an excellent agreement with~recent STS experiments. [Preview Abstract] 
Monday, March 14, 2016 1:03PM  1:15PM 
B22.00008: Electronic structure, transport properties, and excited states in CoTiSb, CoZrSb, and CoHfSb halfHeusler compounds Anderson Janotti, Zhigang Gui, Jason Kawasaki, Chris Palmstrom, Burak Himmetoglu CoTiSb is a member of a large family of halfHeusler compounds with 18 valence electrons. CoTiSb is semiconductor material with a band gap a little over 1 eV, and it has been considered promising for thermoelectric applications. It can be grown on conventional IIIV semiconductors, and could potentially be integrated in IIIV devices. Here we present results of firstprinciples calculations of electronic structure, transport properties, and excited states in CoTiSb, as well as CoZrSb and CoHfSb. Electronic structures are studied using density functional theory within the local density approximation, hybrid functional and quasiparticle GW methods. Both roomtemperature Seebeck coefficient and carrier mobility are calculated from firstprinciples. We also determine the band alignments to IIIV semiconductors, and all the results are presented and discussed in the light of available experimental data. [Preview Abstract] 
Monday, March 14, 2016 1:15PM  1:27PM 
B22.00009: Ab Initio Calculations of Excited Carrier Dynamics in Gallium Nitride Vatsal Jhalani, Marco Bernardi Bulk wurtzite GaN is the primary material for blue lightemission technology. The radiative processes in GaN are regulated by the dynamics of excited (or socalled “hot”) carriers, through microscopic processes not yet completely understood. We present ab initio calculations of electronphonon (eph) scattering rates for hot carriers in GaN. Our work combines density functional theory to compute the electronic states, and density functional perturbation theory to obtain the phonon dispersions and eph coupling matrix elements. These quantities are interpolated on fine Brillouin zone grids with maximally localized Wannier functions, to converge the eph scattering rates within 5 eV of the band edges. We resolve the contribution of the different phonon modes to the total scattering rate, and study the impact on the relaxation times of the longrange Fr\"{o}hlich interaction due to the longitudinaloptical phonon modes. [Preview Abstract] 
Monday, March 14, 2016 1:27PM  1:39PM 
B22.00010: Time Evolution of Charge Carriers & Phonons after PhotoExcitation by an UltraShort Light Pulse in Bulk Germanium Stephen Fahy, Felipe MurphyArmando, Mariano Trigo, Ivana Savic, Eamonn Murray, David Reis We have calculated the timeevolution of carriers and generated phonons in Ge after ultrafast photoexcitation above the direct bandgap. The relevant electronphonon and anharmonic phonon scattering rates are obtained from firstprinciples electronic structure calculations. Measurements of the xray diffuse scattering after excitation near the L point in the Brillouin zone find a relatively slow (~5 ps, compared to the typical electronphonon energy relaxation of the GammaL phonon) increase of the phonon population. We find this is due to emission caused by the scattering of electrons between the Delta and L valleys, after the initial depopulation of the Gamma valley. The relative slowness of this process is due to a combination of causes: (i) the finite time for the initial depopulation of the conduction Gamma valley; (ii) the associated electronphonon coupling is relatively weaker (compared to GammaL, GammaDelta and DeltaDelta couplings) ; (iii) the TA associated phonon has a long lifetime and (iv) the depopulation of the Delta valley suppresses the phonon emission. [Preview Abstract] 
Monday, March 14, 2016 1:39PM  1:51PM 
B22.00011: Constrained Density Functional Theory by Imaginary TimeStep Method Daniel Kidd Constrained Density Functional Theory (CDFT) has been a popular choice within the last decade for sidestepping the self interaction problem within longrange charge transfer calculations.\footnote{Q. Wu and T. Voorhis, \textbf{J. Chem. Theory Comput.} 2, 765774 (2006).} Typically an inner constraint loop is added within the selfconsistent field iterations of DFT in order to enforce this charge transfer state by means of a Lagrange multiplier method.\footnote{Q. Wu and T. Voorhis, \textbf{Phy. Rev. A} 72, 024502 (2005).} In this work, an alternate implementation of CDFT is introduced, that of the imaginary timestep method, which lends itself more readily to real space calculations in the ability to solve numerically for 3D local external potentials which enforce arbitrary given densities. This method has been shown to reproduce the proper $1/R$ dependence of charge transfer systems in real space calculations as well as the ability to generate useful constraint potentials. As an example application, this method is shown to be capable of describing defects within periodic systems using finite calculations by constraining the 3D density to that of the periodically calculated perfect system at the boundaries. [Preview Abstract] 
Monday, March 14, 2016 1:51PM  2:03PM 
B22.00012: Density functional study on a lightharvesting carotenoidporphyrinC$_{\mathrm{60}}$ molecular triad in explicit solvent Carlos Diaz, Tunna Baruah, Rajendra Zope We investigate the effect of solvent on the electronic structure of a biomimetic molecular triad that shows photoinduced charge transfer in laboratory. The supramolecular triad contains three different units  C60, porphyrin, and betacarotenoid. We have performed classical molecular dynamics simulation of the triad surrounded by 15000 water molecules using NAMD for 20 nanoseconds. Subsequently, we performed an allelectron density functional calculations (DFT) using large basis sets on the 50 snapshots taken from the molecular dynamics simulation. The solvent effects in the DFT calculations are treated using both the explicit water molecules as well as using the point charge representation of water. The excitation energies and absorption spectra show that the polar solvent induces significant changes in the electronic structure of the triad. [Preview Abstract] 
Monday, March 14, 2016 2:03PM  2:15PM 
B22.00013: AbInitio Computations of Electronic and Related Properties of cubic Lithium Selenide (Li$_{2}$Se) Abdoulaye Goita, Ifeanyi H. Nwigboji, Yuriy Malozovsky, Diola Bagayoko We present theoretical predictions, from abinitio, selfconsistent calculations, of electronic and related properties of cubic lithium selenide (Li$_{2}$Se). We employed a local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO). We performed the computations following the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZWEF). Our results include electronic energies, total and partial densities of states, effective masses, and the bulk modulus. The theoretical equilibrium lattice constant is 5.882 {\AA}. We found cubic Li$_{2}$Se to have a direct band gap of 4.363 eV (prediction), at $\Gamma $. This gap is 4.065 eV for a room temperature lattice constant of 6.017 {\AA}. The calculated bulk modulus is 31.377 GPa. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS 1003897, NSF (201015)RIISUBR] and NSF HRD1002541, the US Department of Energy  National, Nuclear Security Administration (NNSA) (Award No. DE NA0002630), LaSPACE, and LONISUBR. [Preview Abstract] 
Follow Us 
Engage
Become an APS Member 
My APS
Renew Membership 
Information for 
About APSThe American Physical Society (APS) is a nonprofit membership organization working to advance the knowledge of physics. 
© 2023 American Physical Society
 All rights reserved  Terms of Use
 Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 207403844
(301) 2093200
Editorial Office
1 Research Road, Ridge, NY 119612701
(631) 5914000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 200452001
(202) 6628700