Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A26: Focus Session: Computational Nanoscience I: Inorganic Nanostructures and Interfaces |
Hide Abstracts |
Sponsoring Units: DMP DCOMP Chair: Peihong Zhang, SUNY Buffalo Room: 328 |
Monday, March 16, 2009 8:00AM - 8:36AM |
A26.00001: Interface electrostatics in ferroelectric capacitors from first principles Invited Speaker: Capacitors based on ferroelectric perovskites are potentially attractive for applications in nanoelectronics, such as non-volatile random-access memories and high-permittivity gate dielectrics. Thin-film geometries are sought after for optimal efficiency and information storage density. However, in such a regime, strong size effects arise that generally deteriorate the overall performance of the device. Understanding the properties of the oxide/electrode interface is crucial to overcoming these deleterious effects. In this talk I will present our recently-developed methodologies for working at fixed electric displacement field in first-principles density-functional calculations. I will show that application of fixed-$D$ methods to ferroelectric capacitors provides enhanced flexibility for the study of interface-specific issues. I will demonstrate this technique by presenting results for a range of systems based on PbTiO$_3$ or BaTiO$_3$ as ferroelectric, and Pt or SrRuO$_3$ as electrode. Based on a microscopic analysis of interface bonding and electrostatics, I will discuss possible routes to the realization of devices that are free from size effects. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A26.00002: Multiexciton absorption in CdSe nanocrystals Alberto Franceschetti, Yong Zhang Efficient multiple-exciton generation (MEG) has been recently reported in semiconductor nanocrystals. In this process, a single absorbed photon generates two or more electron-hole pairs. The MEG efficiency has so far been evaluated assuming that the change (bleaching) of the absorption spectrum due to MEG is linearly proportional to the number of excitons ($N_X$) that are present in the nanocrystal. We have examined this assumption using atomistic pseudopotential calculations for colloidal CdSe nanocrystals ranging in size from 3 to 4.6 nm. We found that the bleaching of the first absorption peak, $\Delta \alpha_{1S}$, depends non-linearly on $N_X$, due to carrier-carrier interactions. When a single exciton is present in the nanocrystal, the 1S exciton peak is already 65-75\% bleached. This non-linearity mandates an upper bound of 1.5 to the value of the normalized bleaching that can be attributed to MEG, significantly smaller than the limit of 2.0 predicted by the linear scaling assumption. Thus, measured values of the normalized bleaching in excess of 1.5 in CdSe nanocrystals cannot be due entirely to MEG, but must originate in part from other mechanisms. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A26.00003: Electronic Structure of Thiolate Covered Gold Nanoparticles Yan Li, Giulia Galli, Francois Gygi We present ab initio calculations of the structural, electronic, and bonding properties of thiolate-covered gold nanoparticles that have been crystallized in recent experiments (Au$_{102}$(MBA)$_{44}$)[1]. We simulated exactly the same system as investigated experimentally (1596 atoms) and the results of our structural optimization confirm the stability of the experimentally determined structure. We find that the crystallized solid is a semiconductor with a sizable energy gap ($\sim$0.5 eV, within DFT), and electronic states at the valence band maximum and conduction band minimum are extended over the gold nanoparticle core and the interface. This energy gap appears to be insensitive to the type of adsorbate. We find a tendency of the adsorbate to exert ``pull-out'' forces on the surface gold atoms, and our analysis of chemical bonding supports the hypothesis that gold s electrons are donated to the MBA radicals so as to form a highly stable 58-electron, filled electronic shell structure. Finally, comparisons between adsorption energies in the case of gold nanoparticles and flat surfaces show that a finite curvature of the interface enhances the stability of Au-S bonds.\\[0pt] [1]Yan Li, Giulia Galli and Francois Gygi, ACS Nano, 2, 1896(2008) [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A26.00004: Covalently Bonded Aromatic Molecules on Gold using a GW Approach Isaac Tamblyn, Su Ying Quek, Stanimir A. Bonev, Jeffrey B. Neaton Frontier molecular orbital energies dictate the nature of optical absorption, chemical reactivity, and charge injection at metal-organic interfaces. Recent work [1] on the conductance of benzenediamine-Au single-molecule junctions has shown that standard methods based on density functional theory fail to correctly position molecular orbital energies relative to the Au Fermi level, resulting in a pathological overestimate of the conductance for this class of systems. In this work, we use many-electron perturbation theory within the GW approximation to compute quasiparticle energies of aromatic molecules covalently bonded to a gold surface, taking particular care to assess dynamical screening beyond standard plasmon-pole approximations. We discuss results for benzene on Au(111) bonded via amine (-NH2) and thiol (-SH) link groups. These data are compared with more approximate model self-energy corrections applied to these systems [1], and also recent experiments. [1] Quek et al, Nano Lett. 7, 3477 (2007). [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A26.00005: First-principles studies of interfacial charge separation in nano-materials photovoltaic heterojunction Invited Speaker: Charge separation is a crucial process that must be understood in order to make substantial improvements in nano-materials based PV cells. In our work, first principles quantum mechanical calculations are employed to shed light on this process for some important nano-material heterojunctions. I will first present our work on the interfacial charge separation in Fullerene/P3HT and CNT/P3HT heterojunctions. Our findings indicate that in the fullerene system a two-step process is operative, involving an adiabatic electron transfer and an exciton dissociation via quasi-degenerate states localized on the fullerene. For the nanotubes, on the other hand, while such a two-step process is not necessary for efficient charge separation, the presence of metallic nanotubes lead to undesirable charge traps. Secondly, I will discuss how we are addressing the difficulty in employing standard DFT approaches for investigating inorganic-organic PV interfaces, which are composed of two distinct materials with very different electronic environments. I will discuss a QMC scheme for obtaining many-body corrections to the Kohn-Sham level alignments and its application to a CdSe/Oligothiophene hybrid PV interface, with the aim of tailoring its behavior by controlling the conjugation length. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A26.00006: Nanoscale phase stability reversals in titanium oxide polymorphs Paul Kent, Daniel Hummer, James Kubicki, Jeffrey Post, Peter Heaney A surprising discovery of nanoscience is the reversal in relative stability of materials with the same composition but different structures as particles evolve from the nanoscale to the macroscopic. These reversals can be problematic, as they frequently induce the precipitation of metastable contaminant phases during the synthesis of compounds of interest. To investigate the fundamental origins of this phenomenon, we have (1) performed a series of large scale density functional calculations to characterize the energetics of the stability reversal between rutile and anatase nanoparticles, and (2) experimentally monitored the hydrothermal crystallization of titania nanoparticles using in-situ x-ray diffraction. Although the phenomenon of stability reversal is commonly explained as a domination of the crystalline surface energy as a fraction of the total energy, we find computationally that the refined average surface structures cannot account for the stabilization of nano-anatase relative to nano-rutile, even for 3nm sized particles. Instead we find that defects associated with the edges and corners of nanocrystals contribute significantly to the energy and must be included in any description of the stability reversal. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A26.00007: Domain boundary formation in helical multishell gold nanowire Takeo Hoshi, Takeo Fujiwara Helical multishell gold nanowire (Y. Kondo and K. Takayanagi, Science 289, 606 (2000)) is studied by molecular dynamics simulation with electronic structure (``ELSES'' http://www.elses.jp/), so as to explore formation mechanism of helical domain boundary. We have proposed a model for the formation of helical multishell gold nanowires with molecular dynamics simulation with electronic structure (Y. Iguchi, T. Hoshi, T. Fujiwara PRL 99, 125507 (2007)). In this paper, we show simulation results with lager samples, of which the rod length is more than 10 nm and the number of rod atoms is more than one thousand. Unlike the results of shorter rods in the previous paper, a well-defined domain boundary between helical and (non-)helical regions appears, when an atom moves from a inner shell into rod surface. The inserted atoms on the rod surface causes a surface reconstruction on rod surface and introduces a helical region with a domain boundary. Such an inserted atom is a possible candidate of mechanism for forming a helical rod from an ideal (non-helical) one. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A26.00008: Plasmon resonance of gold nanoparticles: the effect of surfactants and solvents Jeremy Neal, Peter Palffy-Muhoray Metallic nanoparticles dispersed in host materials have many potential applications due to their unique optical properties. These properties are determined not only by the size, shape and composition of the particles, but also by their environment. Metallic nanoparticles are typically coated with surfactants to prevent aggregation; these surfactants can also significantly affect their optical response. The role of surfactant coatings has been studied previously, but the results are incomplete. We have obtained theoretical expressions to describe and have carried out numerical simulations to determine the effects of solvents and surfactants on the optical response. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A26.00009: Investigation of atomic oxygen embedment into copper surface by DFT calculation Minyoung Lee, Alan McGaughey, Susan Sinnott, Simon Phillpot, Judith Yang In the oxidation of a Cu(100) surface, the Cu$_2$O islands grow both into the substrate and parallel to the surface. To investigate the oxide growth into the copper surface, we analyzed oxygen embedment using DFT calculations. Using the nudged elastic band method, we calculated energy barriers for oxygen embedment for different oxygen coverages and different surface morphologies. As the oxygen coverage increases from 0.25 monolayers (ML) to 1.0 ML, the energy barrier decreases and we find an energetically favorable site between the top and second copper layers at an oxygen coverage of 1.0 ML. The different surface morphologies [$c$(2$\times$2), missing-row reconstruction and $c$(2$\times$2) with 0.25 ML disordered copper vacancy] have comparable energetics and no energetically favorable site for oxygen embedment is predicted. To find the energetically favorable transition states on the missing-row reconstructed Cu(100) surface, we will investigate oxygen embedding paths with and without point defects on the top copper layer. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A26.00010: \textit{Ab initio} vibrational dynamics of Ag$_{27}$Cu$_{7}$ nanoalloy. Marisol Alcantara Ortigoza, Rolf Heid, Klaus P. Bohnen, Talat S. Rahman We have carried out calculations of the vibrational dynamics of the 34-atom nanoalloy, Ag$_{27}$Cu$_{7}$, using density functional perturbation theory, which furnishes a powerful and reliable method to asses the linear response of the charge density to ionic perturbations. We find that the D$_{5h}$ core-shell structure of Ag$_{27}$Cu$_{7}$ [1,2] is dynamically stable, since all modes have non-zero frequencies affirming that the structure does not surrender itself to structural transitions as a result of the small perturbations in the charge density led by vibrations. The phonons of Ag$_{27}$Cu$_{7}$ range from 2.6 to 28.5 meV and are relatively evenly distributed. There are, however, three $\sim $3.0 meV gaps between 2.8-5.6, 15.0-18.7, and 23.6-26.8 meV. In modes whose frequency is below 7.0 meV, Ag atoms participate the most while Cu atoms show a very small displacement. The opposite is true for four modes whose frequency is above 24 meV. We present the displacement patterns of the main modes and find the mode with highest energy to be a radial \textit{breathing} mode of Cu atoms with respect to the center of the cluster. [1] G. Rossi \textit{et al}., PRL. \textbf{93}, 105503 (2004), [2] M. Alc\'{a}ntara Ortigoza and T. S. Rahman, PRB \textbf{77}, 195404 (2008). Work supported in part by U.S. DOE under Grant DE-FG02-07ER46354. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A26.00011: Clusters, Platelets, and Nanowires of Mo-S, and Their Assemblies P. Murugan, Vijay Kumar, Y. Kawazoe, N. Ota Nano-structures of MO-S are useful for removal of S in petroleum industry, as solid state lubricants in space technology, and in Pt-free fuel cell research. We study by first principles density functional calculations Mo-S nanoclusters, nanowires, their assemblies, and triangular nano-platetlets all of which have been produced in laboratory. Mo-S clusters have Mo polyhedral structures and sulfur atoms cap this metal polyhedron. These structures have high stability due to strong Mo-Mo and Mo-S bonding. Some of the polyhedral clusters have non-zero magnetic moments due to the partially occupied 4d states in Mo atoms. Mo$_{6}$S$_{8}$ octahedral cluster has ultra-high stability and it could be condensed to form Mo-S nanowire or nanorod. However, for high S contents, we show that triangular platelets become more stable. The Mo-S nanowires are good electronic conductors and are interesting for miniature devices. Assembly of nanowires stabilizes in a hexagonal structure with vdW interactions. The trigonal void between the nanowires can be occupied by Li atoms to develop materials for Li-ion battery applications. References: P. Murugan, V. Kumar, Y. Kawazoe, and N. Ota, Appl. Phys. Lett. (2008); Nano letters. (2007); J. Phys. Chem. A (2007). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700