Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N26: Computational Nanoscience III |
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Sponsoring Units: DCOMP DMP DAMOP Chair: Brahim Akdim, AFRL Room: LACC 501B |
Wednesday, March 23, 2005 8:00AM - 8:36AM |
N26.00001: Simulations of semiconductor nanoparticles Invited Speaker: We review theoretical works concerning the optical properties of II-VI, III-V and group-IV semiconductor nanocrystals. We present simulations based on semi-empirical tight binding method. In the case of Si nanocrystals, we discuss why the time decay of the photoluminescence is characterized by stretched exponentials. The usual explanation is that excitations can migrate between neighbour nanocrystals but we discuss experimental results in which it cannot be the case. We show that stretched exponentials can be explained quantitatively by intrinsic recombination in an ensemble of isolated nanocrystals made of an indirect gap material. Optical properties of PbSe nanocrystals will be discussed in the second part of the talk. We show that the cubic lattice of the material leads to unusual properties compared to zinc-blende semiconductors. Interband and intraband optical transistions are calculated and compared to experiments. Finally we present results concerning the energy transfer between neighbor nanocrystals by Foerster-type transitions. Results for III-V and Si nanocrystals will be compared. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N26.00002: Microscopic dielectric function in semiconductor quantum dots Xavier Cartoixa, Lin-Wang Wang Dielectric function and screening effects within a quantum dot is of paramount importance in describing the optical properties and energy levels in a quantum dot. Previously it was believed that the dielectric function inside a quantum dot decreases compared to the bulk due to the increase of the band gap. Recently[1], using macroscopic electric fields and response analysis, it was suggested that the reduction of dielectric constant in a quantum dot is due to surface bond breaking, not due to the opening of the band gap. We have investigated this issue by studying the microscopic response function using plane wave ab initio calculations. Indeed, we found that the microscopic response function f(r1,r2) is identical to the bulk value when both r1 and r2 are within the quantum dot. We have provided a model which allows one to accurately approximate the quantum dot microscopic dielectric function f(r1,r2) from its bulk values without doing explicit calculations. The model also produces accurately the overall dielectric constant reduction for a quantum dot compared to its bulk value. [1] C. Delerue, M. Lannoo, G. Allan, Phys. Rev. B 68, 115411 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N26.00003: Computation of the influence of scanning probe microscope (SPM) on quantum dot eigenstates and 2DEG potential Michael Stopa We calculate the electronic structure of GaAs-AlGaAs two-dimensional electron gas (2DEG) devices, such as quantum dots and quantum point contacts (QPCs) in the presence of a tip of a scanning probe microscope at some distance above the surface. The calculation employs standard density functional theory with exchange and correlation treated in the local density approximation. The position and voltage on the tip are varied and the conditions for depletion of the 2DEG are shown to compare favorably to experiment [1]. We show that the size of the depletion region created (by a negative tip voltage) is unexpectedly small due to focusing of the potential lines by the higher dielectric. We study the interaction of the tip with an isolated quantum dot that contains one or two electrons. The raster pattern of the \textit{difference }between single particle energies reveals that the tip distorts the shape of the confining potential and suggests that excited state properties, if they can be measured experimentally, can contribute to the resolution of spatial information. [1] M.A. Topinka, R.M. Westervelt, E.J. Heller, ``http://meso.deas.harvard.edu/papers/Topinka, PT 56 12 (2003)'' (Imaging Electron Flow), Physics Today \textbf{56}, 12 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N26.00004: Quasi-ballistic and diffusive electron transport in inhomogeneous semiconductor nanostructures Dan Csontos, Sergio E. Ulloa We study nonequilibrium electron transport in inhomogeneous, nondegenerate semiconductor nanostructures using a computational approach based on the self-consistent, direct solution of the semiclassical, steady-state Boltzmann transport equation and the Poisson equation. We show that, in general, large applied and built-in fields in these systems give rise to strongly out-of-equilibrium electron velocity distributions that display interesting structure in the high-energy tail of the distribution, caused by the interplay between quasi-ballistic and diffusive contributions to the electron transport. The observed characteristics have a strong spatial dependence, related to the large inhomogeneous electric field variations in these systems, as well as a strong dependence on temperature and the detailed nature of the electron scattering where we find that the impact of a phonon threshold-energy scattering mechanism on the nonequilibrium distribution is considerable. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N26.00005: Thermodynamics and kinetics of group-IV nanoparticles under pressure Matteo Cococcioni, Francesco Mauri, Gerbrand Ceder, Nicola Marzari The kinetics and thermodynamics of phase transformations in group-IV nanoparticles during a shock compression are studied with full first-principles molecular dynamics simulations. A novel electronic-enthalpy functional is introduced to describe accurately and efficiently finite-size quantum systems under pressure. Significant differences are found in the structural response of carbon, silicon and germanium nanoparticles, depending on size, composition, and surface structure. The presence of trapped metastable amorphous configurations for Si and bigger Ge nanoparticles highlights the importance of kinetics effects in the phase transformation. It also demonstrates the possibility of using nanoparticles to study bonding rearrangements and structural transformations which are not accessible to the bulk counterparts. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N26.00006: Effect of anharmonicity of the inter-atomic potential on the built-in strain in epitaxial quantum dot structures Olga L. Lazarenkova, S. Lee, P. von Allmen, F. Oyafuso, G. Klimeck, M. Korkusinski, R. Timm, H. Eisele, M. D\"ahne It is demonstrated that the anharmonicity of the inter-atomic potential is important in covered nanostructures. Compared to the strain distribution found with the standard Keating model, corrections of over 100 meV are found in electronic band offsets, resulting in values significantly closer to the experimental data. The anharmonicity correction coefficients for Si, Ge, AlAs, GaAs, InAs, AlSb, GaSb, and InSb are presented. The simulated lattice constant profiles and deformation of the cleaved surface are shown to be in a good agreement with the data observed by XSTM measurements for InAs/GaAs and GaSb/GaAs quantum dot structures. The anharmonicity corrections can be performed without a significant increase of the computational cost, since the model remains limited to the nearest neighbor interactions. Simulations of strained systems containing up to 30 million atoms are demonstrated. This work was performed while OLL held a National Research Council Research Associateship Award at Jet Propulsion Laboratory. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N26.00007: Electronic properties of phosphorous $\delta$-doped silicon Gefei Qian, Yia-Chung Chang, John R. Tucker We present a comprehensive theoretical study of phosphorous $\delta$-doped silicon (a candidate material for quantum computation) with doping density up to $3.4 \times 10^ {14} cm^{-2}$ (which corresponds to 1/2 monolayer of doping). A microscopic model based on empirical pseudopotentials and planar Wannier orbital basis is used to calculate the delta- doped system. 1000 monolayers of silicon is included to minimize the boundary confinement effect. Self-consistent potential as well as the exchange-correlation effects due to the doping electrons have been taken into account. It is shown that the 2D band structure of the delta-doped system can be reasonably approximated by an effective mass model over a large range of doping density. However, for ultra-high doping ($> 2 \times 10^{14} cm^{-2}$), which reaches the experiment limit, the band-structure is significantly deformed, as a result of the strong confinement from the V-shape self- consistent potential. The Fermi level (relative to the conduction band minimum) as a function of the doping density is studied and its implication on the transport properties will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N26.00008: Hydrogenated silicon fullerenes and endohedral dopings Vijay Kumar, Yoshiyuki Kawazoe Empty cage fullerene structure of Si$_{20}$ can be stabilized by hydrogen capping in equi-atomic concentration [1]. We study hydrogenated silicon fullerenes Si$_n$H$_n$, ($n$ = 14-28) using {\it ab initio} ultrasoft pseudopotential method and generalized gradient approximation for the exchange-correlation energy. It is found that Si$_{20}$H$_{20}$ has the optimal size. The empty space in the cages can be filled with atoms and this allows the formation of endohedral silicon fullerenes. The interaction of the guest atom with the cage is weak as compared to metal encapsulated silicon clusters [2]. Our results show that doping can be used to manipulate the highest occupied-lowest unoccupied molecular orbital gaps of these fullerenes and prepare species with large magnetic moments and varied optical properties. Guest atoms with closed electronic shell configurations generally occupy the center of the cage while open shell atoms tend to drift towards the wall of the cage. [1] V. Kumar and Y. Kawazoe, Phys. Rev. Lett. 90, 055502 (2003). [2] V. Kumar and Y. Kawazoe, Phys. Rev. Lett. 87, 045503 (2001). [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N26.00009: Optical properties of CdTe and CdSe nanocrystals Guy Allan, Christophe Delerue The electronic structure of CdTe and CdSe nanocrystals has been calculated as a function of size using the tight-binding approximation with an sp3d5s* basis including spin-orbit coupling and taking into account d core atomic functions on the cation atoms. Two particle electron-hole interaction is also incorporated in the calculation of the photoemission decay rate. For CdSe, we compare the results for the two zinc blende and wurtzite atomic structures. Comparison is made with recent experimental results. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N26.00010: Stability of DX center in semiconductor quantum dots Jingbo Li, Su-Huai Wei, Lin-Wang Wang Semiconductor quantum dot(QD) are of great current interest for applications because the physical properties of QD such as the band gap can be tailored by size or shape. On the other hand the application of semiconductors as novel electronic devices depend critically on its doping properties. Although defect properties have been extensively studied in the past for bulk, very few studies have been done for QD. For example, it is known that DX$^ {-}$ center in Si doped GaAs is unstable in bulk, however, it is not clear whether it is stable the case in GaAs QD. Using first– principles band structure method, we study how the size of QD affects the stability and transition energy levels of DX center of GaAs:Si. We find that although Si DX center is unstable in bulk GaAs, when the dot size is small enough, it is stabilized. The critical size of QD is around 3nm of diameter. The stabilization is due to the strong quantum confinement effect, the conduction band edge of QD increases. The formation energy of the tetrahedral coordinated Si$_{Ga}^{-}$ also increases because the occupied shallow defect level is mostly CBM-like. On the other hand, the DX$^{-}$ defect level contains significant amount of non-CBM characters, so the increase of formation energy of the DX$^{-}$ center is less than the shallow Si$_{Ga}^ {-}$ defect. Our studies show that defect in QD could be significantly different from the bulk. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N26.00011: Interaction of Magic Gold Cluster with Si60 Cage Yoshi Kawazoe, Qiang Sun, Qian Wang, Puru Jena Both Au clusters and Si clusters individually are important subjects in chemistry, physics, and materials science. It is very interesting to study the interactions between these two technologically important systems. In this paper, first-principles studies are performed on Au$_{12}$W@Si$_{60}$ by using projector-augmented wave (PAW) method and generalized gradient approximation for the exchange-correlation energy. The geometry, electronic structure, orbital hybridization, and charge transfer are discussed. It has been found that the magic Au$_{12}$W cluster actively interacts with Si, and Si$_{60}$ cage structure can be stabilized. Meanwhile the metal cluster is dissociated when encapsulated in Si60 cage, and charge is transferred from Si cage to the metal atoms. The present study suggests that due to the special properties of Au itself, the magic gold clusters have both energetic stability and chemical activity and can be used to design novel nano structures and nano devices. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N26.00012: Fixed-Phase Path Integral Monte Carlo Simulations in Quantum Dots in Magnetic Fields Daejin Shin, John Shumway We have developed a fixed-phase approximation for path integral Monte Carlo (PIMC) simulations. With the fixed phase approximation, the difficulties created by phases in path integrals for magnetic systems are managed in a practical way. We first demonstrate the method on electrons in a 2-D parabolic dot in a magnetic field. The PIMC method allows us to extend the simulation to a realistic 3D model of an InGaAs/GaAs lens-shaped self-assembled dot, so we can study the deviations from an idealized parabolic model. We then apply the method to study the magnetic field dependence of biexciton binding in the different dot models. [Preview Abstract] |
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N26.00013: Ab-Initio Calculations of the Structural Properties of CdSe Nanorods and their Interaction with Organic Ligands Andrew Williamson, Aaron Puzder, Giulia Galli, Liberato Manna, Paul Alivisatos First principles electronic structure simulations are used to study the atomistic detail of the interaction between organic surfactant molecules and the surfaces of CdSe semiconductor nanoparticles.[1] These calculations provide insights into the relaxed atomic geometry of organics bound to semiconductor surfaces at the nanoscale as well as the electronic charge transfer between surface atoms and the organics. We calculate the binding energy of phosphine oxide, phosphonic and carboxylic acids and amine ligands to a range of CdSe nanoparticle facets. The calculated relative binding strengths of ligands to different facets support the hypothesis that these binding energies control the relative growth rates of different facets, and therefore the resulting geometry of the nanorods. The calculated relaxed atomic geometries of CdSe nanorods with a range of diameters and growth directions are then compared with the results of EXAFS measurements of recently synthesized nanorods to determine their atomic structure and surface relaxations. [1] A. Puzder, A.J. Williamson, N. Zaitseva, G. Galli, L. Manna and A.P. Alivisatos, Nano Lett. (2004). [Preview Abstract] |
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