Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B31: Focus Session: Computational Nanoscience I: Electronic and Optical Properties of Nanoclusters |
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Sponsoring Units: DMP DCOMP Chair: James R. Chelikowsky, University of Texas at Austin Room: Morial Convention Center 223 |
Monday, March 10, 2008 11:15AM - 11:51AM |
B31.00001: Optical Properties of Free and Embedded Small Nanoparticles Invited Speaker: It is well known that the absorption spectra, as well as the effective dielectric function, of nanoparticles in vacuum or surrounded by a dielectric medium can be obtained by classical Mie and Maxwell-Garnett theories. A limit as to how the particles can be for the theory to apply has not been established. Here I present theoretical results on the optical properties of small Ag, Au, and Si and Ge nanoparticles with tens of atoms in vacuum and in an embedded dielectric medium obtained from first-principles density-functional calculations. In particular, I will discuss the role that $d$-electron play on the optical properties of Ag and Au nanoparticles, and the cases when classical Mie and Maxwell-Garnett theories can be applied for nanoparticles of just few atoms in size and whose atoms are in bulk-like and not bulk-like positions. Comparison will be made for nanoparticles in vacuum and embedded in an alumina matrix. The quantum-mechanical results indicate that small nanoparticles in alumina can have an imprint on the effective dielectric function that is several times larger than would be predicted by Maxwell-Garnett theory for same-size particles. This work was supported by a GOALI NSF grant, DOE, the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, and Alcoa Inc. Collaborators: S. \"{O}g\"{u}t, K. Jackson, J. Jellinek, A. Halabica. R. F. Haglund, R. Magruder, S.J. Pennycook and S.T. Pantelides. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B31.00002: Ab initio calculation of temperature effects in the optical response of open-shell sodium clusters Marie Lopez del Puerto, Murilo Tiago, James Chelikowsky We incorporate the temperature effect on the optical absorption spectra of open-shell sodium clusters by combining pseudopotentials, Langevin molecular dynamics and time-dependent density functional theory. We have done calculations for several open-shell sodium clusters, for which experimental data is available for comparison. We find that the positions of the lower energy peaks of the calculated spectra correspond very well to the peaks in the experimental spectra. We fit the width of the peaks in the lower temperature calculations to the corresponding experimental result to obtain the instrumental line width. We then use this same width for the high temperature calculations and find very good agreement with experiment. Finally, we analyze the transitions that contribute to the observed peaks in the absorption spectra and we plot the effective valence charge density for specific transitions for each cluster. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B31.00003: Bethe-Salpeter and Quantum Monte Carlo Calculations of the Optical Properties of Carbon Fullerenes P.R.C. Kent, M.L. Tiago, F.A. Reboredo, Randolph Q. Hood We have calculated the low energy optical excitations of the carbon fullerenes C20, C24, C50, C60, C70, and C80. Properties are calculated via the GW-Bethe-Salpeter Equation (GW-BSE) and diffusion Quantum Monte Carlo (QMC) methods. We compare these approaches with time dependent density functional results and with experiment. GW-BSE and QMC have previously shown good agreement for small molecules, but this is the first study of these methods for these larger yet prototypical nanostructures. The first ionization potentials are consistently well reproduced and are similar for all the fullerenes and methods studied. However, electron affinities and first triplet exciton show substantial method and geometry dependence. GW-BSE yields triplet energies around 1eV below the QMC results. We discuss the possible reasons for these differences. Research at Oak Ridge National Laboratory performed at the Materials Science and Technology Division, sponsored by the Division of Materials Sciences, and at the Center for Nanophase Materials Sciences, sponsored by the Division of Scientific User Facilities, U.S. Department of Energy. Research at Lawrence Livermore National Laboratory was performed under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B31.00004: Efficient first principles quasiparticle states using optimal Brillouin zone sampling David Prendergast, Steven G. Louie We present a methodology for accurate evaluation of quasiparticle states within the GW approximation [1], exploiting optimal Brillouin zone sampling [2]. This approach permits fast, efficient sampling of the Brillouin zone using a compact k-dependent Hamiltonian. Applications to systems with complicated dispersion or large numbers of atoms are permitted with favorable computational scaling and straightforward exploitation of existing parallelized numerical libraries. As input, this method requires only standard density functional theory calculations of eigenstates and eigenenergies on a very coarse k-point grid. For systems with large numbers of atoms, a single k-point is often sufficient. K-point convergence of the dielectric matrix and self-energy is readily achieved leading to accurate GW quasiparticle states. [1] M. S. Hybertsen and S. G. Louie, Phys. Rev. B {\bf 34}, 5390 (1986). [2] E. L. Shirley, Phys. Rev. B {\bf 54}, 16464 (1996). [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B31.00005: First Principles Absorption Spectra of Intermediate Size Ag$_n$ ($n=10-20$) Clusters Kopinjol Baishya, Juan Carlos Idrobo, Serdar Ogut, Mingli Yang, Koblar A. Jackson, Julius Jellinek First principles optical absorption spectra, obtained within time-dependent density functional theory (TDDFT), for the ground state and low-energy isomers of Ag$_n$ ($n=10-20$) are presented. Overall, our theoretical results exhibit quite good agreement with spectra obtained for Ag clusters trapped in rare- gas matrices. We show that the classical predictions from Mie- Gans theory using the bulk dielectric function of Ag are in rather good agreement with experimental results and TDDFT spectra for isomers of various shapes. We analyze the orbital character of the optical excitations, and unexpectedly find that the $d$ electrons of Ag$_n$ clusters in this size range have a significant contribution to low-energy optical excitations, unlike the case for smaller Ag$_n$ ($n < 9$) clusters. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B31.00006: Trends in the Electronic Structure and Vibrational Dynamics of 34 atom Ag-Cu Nanoalloy (Ag$_{n}$-Cu$_{34-n}$) H. Yildirim, A. Kara, T.S. Rahman We report results of a systematic study of the electronic and geometric structures as well as vibrational and thermodynamical properties of Ag$_{n}$Cu$_{34-n}$, using density functional theory and model interaction potentials. A detailed analysis of the effect of coordination and atomic environment shows the limitation of coordination to \textit{solely} explain the changes in the structural and vibrational characteristics. On the other hand, a combination of coordination and environment, as expressed in the elemental characteristics of the neighbors, shows systematic trends in the bond length and vibrational free energy distribution. In addition, Cu atoms in the Cu-core/Ag-shell structure, with coordination 6 show a bi-modal feature with 2 distinct ranges of bond lengths. A global analysis shows that the fluctuations in the vibrational free energy depend on the elemental environment which is more pronounced in the case of copper atoms. Correlations between the d band characteristics with coordination/bond length and environment will also be discussed. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B31.00007: Static dipole polarizabilities of icosahedral fullerenes from C$_{60}$ to C$_{2160}$ by all-electron density-functional theory Rajendra Zope, Tunna Baruah, Mark Pederson, Brett Dunlap The electronic response of C$_{60}$, C$_{180}$, C$_{240}$, C$_{540}$, C$_{720}$, C$_{960}$, C$_{1500}$, and C$_{2160}$ fullerenes is characterized by determining their static dipole polarizabilities by all-electron density- functional theory. We first determine the dipole polarizabilities of C$_{60}$, C$_{180}$, C$_{240}$, and C$_{540}$ fullerenes by the finite-field method, using 35 basis functions per atom (NRLMOL basis set), and using the PBE-GGA. In the second set of calculations the sum-over-states (SOS) polarizabilities for all fullerenes from C$_{60}$ through C$_{2160}$ are determined by our fully analytic formulation of density functional theory(ADFT). The 6-311G(d,p) basis set is used in the ADFT calculation, which amounts to 38800 basis functions for the largest fullerene in this series, namely C$_ {2160}$. The SOS polarizabilities are roughly 4 times larger than the finite-field polarizabilities. When scaled by a correction factor obtained within linear response theory, the SOS polarizabilities are within 1-3\% of the finite-field polarizabilities. The polarizability per carbon atom increases from 1.34 \AA$^3$ in C$_{60}$ to 4 \AA$^3$ in C$_{2160}$ while the ratio of fullerene polarizability to its volume approaches unity pointing to quenching of quantum size effects by C$_{2160}$. The results show previous tight-binding calculations greatly exaggerate the electronic response of large fullerenes. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B31.00008: Heusler clusters Alexey Zayak, Scott Beckman, Murilo Tiago, Peter Entel, James Chelikowsky Real space pseudopotential calculations are used in order to investigate the properties of Heusler clusters. Bulk-like clusters with various Ni-Mn-Ga compositions have been examined in the size range from 15 up to 169 atoms. Among these compositions the closest to the stoichiometric Ni$_2$MnGa are the most stable. These clusters retain tendency for tetragonal distortion, which is inhabited from the bulk properties. Although, the surface effects dominate suppressing the tetragonal structure in the smaller clusters, the bigger clusters develop a bulk-like tetragonal distortion. We predict the existence of switchable Ni-Mn-Ga clusters, which might be of great interest for the nano-Magnetic-Shape-Memory technology. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B31.00009: Tangential Ligand-Induced Strain in Icosahedral Au13 Leeor Kronik, Olga Guliamov, Anatoly Frenkel, Laurent Menard, Ralph Nuzzo A quantitative comparison of first principles calculations with extended x-ray absorption fine structure and transmission electron microscopy measurements provides strong evidence that Au$_{13}$ nanocrystals are stabilized in a slightly distorted icosahedral structure by on-top phosphine ligands and a combination of on-top and bridging thiol ligands. Importantly, the ligands change the icosahedral strain (i.e. the radial- tangential bond length ratio) significantly, with the tangential bonds within the Au core exhibiting much more disorder than the radial ones. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B31.00010: Structural Properties and Phase Transitions in Small Gold Nanoclusters Yanting Wang, Sergey Rashkeev Small gold nanoclusters (below 5 nm in diameter) exhibit good catalytic activity. Molecular dynamics simulations combined with the parallel tempering method have been used to investigate the bulk and surface structural properties and phase behavior of small gold nanoclusters (10$^{1}$-10$^{4}$ atoms). For small clusters, the transition from solid to liquid does not occur at a definite temperature. Instead, one gets a temperature range in which the transition state from solid to liquid is observed, and this range become broader for nanoclusters with smaller number of atoms. In this work, we perform an analysis of structural and dynamic properties of gold nanoparticles of different sizes and show that the nature of the solid-liquid phase transition in very small nanoparticles (with tens of atoms) is radically different from that of the mid-sized ones (10$^{2}$-10$^{4}$ atoms). The surface characteristics of the particle (the presence of low-coordinated atoms at the surface and the dynamic fluxionality, i.e., an ability of the surface to reconstruct) that define its catalytic behavior are also investigated and analyzed. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B31.00011: The peculiar distribution pattern of Pd on the PdAu bimetallic nanoclusters Dingwang Yuan, Xingao Gong, Ruqian Wu Through systematic density functional calculations, we found that Pd atoms in PdAu nanoclusters may only take the (111) facets while leave the (001) facets with pure Au. This is promoted by the tendency that Pd prefers to form bonds with Au, rather than Pd. The segregation from the (001) facet to the (111) facet appears to occur easily. The local activity of Pd somewhat depends on the size of cluster and site of substitution. The peculiar distribution pattern of the active constituent should strongly alter the chemical properties of bimetallic nanoclusters toward catalyzing reactions. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B31.00012: Computational model for the production of monodisperse silver spheres in solution Daniel Robb, Ionel Halaciuga, Vladimir Privman, Dan Goia We report the results of computational modeling of the production of monodisperse, spherical silver particles through the rapid mixing of reducing agent and silver-amine complex solutions, in the absence of a protective colloid. We find that the process can be modeled effectively by a two-stage reaction mechanism used previously to model the production of gold particles [1]. Here, we treat both the equilibrium concentration of silver atoms and the surface tension of silver particles as free parameters in our simulation, finding that the reaction time scale is fit by a narrow region of this parameter space. As in the previous work on gold particles, a kinetic `dimer suppression factor' is required to limit the number of final particles produced. In addition, we consider an extension of the two-stage reaction model which incorporates the effect of the silver-amine complexation reaction on the availability of Ag monomers. [1] J. Park, V. Privman, and E. Matijevic, \textit{J. Phys. Chem. B} \textbf{105}, 11630 (2001). [Preview Abstract] |
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