Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z37: Mesoscopic Systems, Clusters, and Nanoscale Systems II |
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Sponsoring Units: DCP Chair: Shiv Khanna, Virginia Commonwealth University Room: 409 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z37.00001: Al$_{n}$Bi Clusters: A Transition from Aromatic to Jellium Stability Penee Clayborne, J.U. Reveles, S.N. Khanna, C.E. Jones, Jr., U. Gupta, J. Melko, A.W. Castleman, Jr. Through a synergetic effort using density functional theory and negative ion photodetachment studies we have investigated multiple aluminum-bismuth clusters. Our studies show that Al$_{3}$Bi and Al$_{5}$Bi are two very stable clusters. Further investigation of their electronic structure reveal that their stability can be explained using the aromatic and Jellium models, respectively. The Al$_{3}$Bi cluster has a large ionization potential of 7.1 eV, a low electron affinity of 1.4 eV and a HOMO-LUMO gap of 1.7 eV. The molecular orbitals of the cluster are reminiscent of an aromatic system and the cluster has a nucleus independent chemical shift (NICS) value of -32.19 ppm confirming its aromatic character. The Al$_{5}$Bi cluster has a HOMO-LUMO gap of 1.2 eV and a large ionization potential of 6.5 eV. The compact structure of Al$_{5}$Bi has 20 electrons, which indicates that this cluster's stability could be accounted for by the Jellium model. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z37.00002: Hydrogen Promoted Oxygen Activation by Free Gold Cluster Cations Robert N. Barnett, Bokwon Yoon, Uzi Landman, Sandra M. Lang, Thorsten M. Bernhardt In this contribution we present experiments and first-principles density functional theory calculations on gas-phase reaction of small gold clusters, aiming at elucidation of the role of hydrogen in the activation of molecular oxygen for the selective oxidation of hydrocarbons. Positively charged gold clusters. Au$_{4}^{+}$ and Au$_{6}^{+}$. were chosen because electronic factors and experimental data suggest them to be most suitable for promoting the oxidation of unsaturated hydrocarbons. Our investigations show that, although small gas phase gold cluster cations are inert toward molecular oxygen, the pre-adsorption of molecular hydrogen cooperatively activates the adsorption of O$_{2}$ on Au$_{4}^{+}$ and Au$_{6}^{+}$. Temperature and reaction time dependent investigations in an octopole ion trap under multi-collision conditions reveal that hydrogen promotes the activation and dissociation of molecular oxygen on the gold clusters at temperatures as low as 200 K. The detailed mechanism of the hydrogen induced oxygen activation, involving an intermediate hydro-peroxy-complex is revealed by the DFT calculations. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z37.00003: Large electric dipole moments and even-odd effects observed in Bi clusters. John Bowlan, Anthony Liang, Walter de Heer The electric and magnetic response of Bi$_N$ (N=1-40) produced in a cryogenic molecular beam (15K-30K) shows that these clusters have large electric dipole moments as previously observed in ferroelectric Nb, V, and Ta clusters. The electric deflection profiles are measured as a function of size, field, and source temperature. Bi clusters exhibit the characteristic even-odd oscillations with even N showing an enhanced response. The clusters are paramagnetic but the odd-N clusters have enhanced susceptibilities. These results will be discussed and compared with other work done on low-dimensional bismuth nanostructures. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z37.00004: Copper nanocluster growth at experimental conditions using temperature accelerated dynamics C. S. Dias, A. C. Cadilhe, A. F. Voter We study the dynamics of vapor phase cluster growth near experimental conditions of pressure at temperatures below $200$K. To this end, we carried out temperature accelerated dynamics (TAD) simulations at different vapor pressures to characterize the morphology of the resulting nanoparticles, which leads to a range of values of the flux of impinging atoms at fixed vapor temperature. At typical experimental pressures of $10^{-3}-10^{-4}\mbox{ bar}$ TAD provides substantial boost over regular Molecular Dynamics (MD). TAD is also advantageous over MD, regarding the sampling of the network of visited states, which provides a deeper understanding of the evolution of the system. We characterize the growth of such clusters at different vapor pressures. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z37.00005: Photoabsorption by Volume Plasmons in Metal Clusters Chunrong Yin, Chunlei Xia, Vitaly Kresin Metal clusters exhibit strong photoabsorption resonances in the visible part of the spectrum. These are collective ``surface plasmon'' excitations which have been extensively investigated. However, the UV part of the spectrum has remained rather unexplored. Some theoretical calculations have predicted that a measurable portion of the delocalized valence electrons' dipole oscillator strength should be located in this region. This predicted absorption feature has been ascribed to a volume-plasmon type of excitation, which in small particles can couple to light, in contradistinction to the situation in bulk metals. We have carried out a photodepletion cross section measurement on a pair of prototypical simple-metal nanoclusters, Na$_{20}$ and Na$_{92}$, finding that these systems indeed possess a broad volume-plasmon absorption peak centered at $\approx $4 eV and having an oscillator strength contents of $\approx $15-20{\%} of the total, in good agreement with theoretical calculations. These spectra provide the first experimental confirmation of the existence of optically active volume-type collective electronic excitations in metal nanocluster particles. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z37.00006: Atomistic Dipole Moments and Polarizabilities of Na$_{N}$ Clusters, N= 2-30, 38, and 55. Koblar Jackson, Li Ma, Mingli Yang, Julius Jellinek The response of Na$_{N}$ clusters, N = 2 - 30, 38, and 55 to a static external electric field is studied using a new method that decomposes the total cluster dipole moment and polarizability into contributions from non-overlapping atomic volumes (Jackson et al., J. Chem. Phys. \textbf{129}, 144309 (2008)). The atomic dipole moments and polarizabilities are in turn partitioned into local dipole and charge-transfer components, corresponding to dielectric and metallic responses, respectively. Analysis of the atomic polarizabilities indicates a strong dependence on the location of the atoms within the clusters and shows directly the effect of electrostatic screening in the clusters. We show that the relative importance of the charge-transfer component of the cluster polarizability increases with cluster size and approaches the bulk-limit on a per-atom basis for clusters as small as 20 atoms. The charge-transfer component is shown to be responsible for the structure/shape driven variations, and for shape-related anisotropies, in the cluster polarizabilities. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z37.00007: Magnetic Properties of Arsenic Cluster Assemblies Meichun Qian, A. C. Reber, S. N. Khanna, A. Sen, S. Mandal, N. K. Chaki Clusters have the potential to serve as building blocks of materials, enabling the tailoring of materials with novel properties. Recently, we synthesized a magnetic cluster assembled material using the As$_{7}$ cluster and cryptated K, which are the elements from main group. X-ray studies show that the As cluster is distorted to accompany two cryptated K. We have investigated the magnetic properties of As cluster assembly using density functional theory. We found the As$_{7}$ cluster has the (-2) valence state and possess one unpaired electron. The magnetic state is stabilized in the crystal and has a lower energy of 90 meV than the nonmagnetic state. The magnetic moments are located on the As$_{7}$ cluster and display antiferromagnetic order in the crystal. We also study the magnetic properties of the As cluster assemblies with transition metal as a linker, and these findings are possible to provide a new kind of magnetic materials. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z37.00008: Interpreting the magnetic and electric deflections of free metal clusters in molecular beams Anthony Liang, John Bowlan, Walter de Heer A short review of the analysis of magnetic and electric deflection data will be presented. Electric and magnetic beam deflection of several metal clusters, including Au$_{N}$, Al$_{N}$, Nb$_{N}$O, Na$_{N}$, Ho$_{N}$, Co$_{N}$, Rh$_{N}$ at various temperatures (20K-300K) and field strengths will be presented. The relation between the broadening of the deflected beam and the polarization distribution of the ensemble assuming a classical polar spherical rigid rotor model is demonstrated. Complicating factors including asymmetry effects and residual temperature effects will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z37.00009: Comparative study of metal clusters by Quantum Monte Carlo method Yuning Wu, Hai-Ping Cheng, Paul Kent Lithium and sodium clusters have been studied by fixed-node diffusion quantum Monte Carlo method. This stochastic wave-function-based approach can provide more accurate results and serve as benchmarks against which other techniques may be compared. We studied the binding energies and investigate different geometries to decide the ground state. Our results are compared with those derived from other method such as DFT and CI methods. Our objective is to validate current quantum Monte Carlo methods for small metal clusters that undergo size dependent geometrical transitions. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z37.00010: Electronic Structure of Bi$_{3}$Ga$_{y}^-$ Semiconductor Clusters and the Special Stability of Bi$_{3}$Ga$_{2}^{-}$ - A Gas Phase Zintl Analogue Jose Ulises Reveles, Ujjwal Gupta, Joshua J. Melko, Shiv N. Khanna, A. W. Castleman, Jr. Here we present evidence that the gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO gap) can be tuned (1.12eV-1.89eV) by changing the Ga composition of Bi$_{3}$Ga$_{y}$ neutral and anionic clusters, some of which display special stability. Collaboratively, mass spectrometry, photoelectron spectroscopy and computational results show that Bi$_{3}$Ga$_{2}^{-}$ is a very stable cluster with a large calculated HOMO-LUMO gap of 1.89 eV, and can be viewed as a gas phase Zintl analogue of Sn$_{5}^{2-}$, already synthesized in the solution phase. The stability of Bi$_{3}$Ga$_{2}^{-}$ is further attributed to the fact that it has 12 valence electrons and possesses a closo structure in agreement with Wade's rules. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z37.00011: Simulation of the Adsorption on Nano/Micro-Cantilever Sensors Padet Khosathit, Phillip Choi, P.-Y. Ben Jar Recent advances in nano/micro fabrication techniques have led to the development of biosensors utilizing nano or micro-sized cantilevers. Through chemical coating of probe molecules that exhibit strong affinity to the target molecules, the cantilever would deflect when the target molecules bind with the probe molecules. Previous simulation studies on these systems often involved the use of a multi-scale approach in which molecular models are used for the probe and target molecules while the cantilever is modeled as a continuum solid beam using surface tension and strain energy. In this work, we use a molecular approach to describe the entire system. In particular, the solid beam is modeled as a collection of molecules connected by lattice springs. The intermolecular interactions between three types of molecules (i.e., probe, target and cantilever molecules) are modeled using Lennard-Jones potentials. Our results show that the cantilever deflection depend on the combination of the Lennard-Jones parameters as well as the number and positions of the probe and target molecules. Details of their effects will be presented in the paper. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z37.00012: Origin of Structural Stability in Cubic ZrO2 Nanocrystals Studied by EXAFS Y.L. Soo, S.L. Chang, C.L. Cheung, R. Sabirianov, F. Namavar, W.N. Mei, P. Chu, J.F. Lee Local environments surrounding Zr nanocrystalline powders and thin films of cubic zirconia prepared by sol-gel and ion beam assisted deposition (IBAD) methods were investigated by using extended x-ray absorption fine structure (EXAFS) technique. These materials have shown cubic long-range-order structure and high hardness without chemical stabilizers. To understand the origin of structural stability, the short-range-order local structural information obtained from EXAFS measurements is of central importance. Powder samples with different nanoparticle sizes prepared by different sol-gel processes were analyzed. Zr k-edge EXAFS, as well as N K-edge x-ray absorption near-edge structures (XANES), will also be presented to demonstrate the evolution of O vacancies and possible N impurities due to thermal annealing in the IBAD deposited films. [Preview Abstract] |
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