Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L10: Nanoclusters and Nanowires on Surfaces |
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Sponsoring Units: DCMP Chair: Phillip Sprunger, Louisiana State University Room: D221 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L10.00001: Structure and dynamics of PtSn/$\gamma$Al$_2$O$_3$ F.D. Vila, J.J. Rehr, S.D. Kelly, S.R. Bare Supported metal clusters have many industrial applications, especially in heterogeneous catalysis. Their activity and durability is determined by their internal atomic and electronic structure, as well as by their interaction with the support. We have previously shown\footnote{F. Vila \textit{et al.}, Phys. Rev. B {\bf78}, 121404(R) (2008).} that unusual phenomena such as large structural disorder and negative thermal expansion in supported Pt clusters can be understood by using a combination of MD and x-ray absorption spectroscopy simulations. Here we present results for prototypical Pt$_{10}$Sn$_{10}$ alloy clusters on $\gamma$Al$_2$O$_3$. Our simulations show that the internal structure and surface location of the clusters varies dynamically on a time scale of a few ps. While the Sn atoms are especially mobile, the clusters have well defined Pt-Pt and Pt-Sn coordination shells at $\sim$2.75\AA. Moreover, at any instant there are between 2 and 5 bonds between the Pt/Sn and the O atoms in the surface. Finally, we present simulations of the XANES spectra and their relation to charge transfers between atoms in the cluster and between the cluster and the surface. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L10.00002: Polarization dependent Pd deposition structure on LiNbO$_{3}$ \{0001\} surface Seungchul Kim, Andrew M. Rappe We investigate effects of polarization orientation on atomic structure of palladium deposited on lithium niobate (LiNbO$_{3}$) \{0001\} surface, using density functional theory (DFT) and kinetic Monte Carlo (kMC) simulations. Adsorption, diffusion, aggregation and clustering process -- include geometries, paths and energies -- of Pd clusters were calculated from DFT simulations. It has been observed that energy barriers of Pd motions on the negatively poled ($c^{-}$) surface are much larger than those on the positively poled surface ($c^{+}$), which indicates the Pd motions on the $c^{-}$ surface are much slower than that of $c^{+}$ surface. We demonstrate, using kMC with kinetic parameters from DFT, very slow motion of Pd on $c^-$ surface leads dispersed small clusters or atoms while fast motion on $c^+$ surface leads large clusters, indicating larger Pd-covered area on $c^-$ surface than $c^+$ after Pd deposition. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L10.00003: Cu/CuOx Nanoclusters on ZnO(1010): Electronic, Catalytic, Morphological Structure Ziyu Zhang, Fei Wang, Maomin Ren, Frank Womack, Minh Le, Yaroslav Losovyi, Richard Kurtz, Phil Sprunger, John Flake To develop a high performance catalyst for CO2 reduction, we tried bi-layers based on CuOx (x=0, 1, 2) and ZnO. The highest yield rate is found for Cu(I) on ZnO. The repeatability of the experiment illustrates that the Cu(I) catalytic clusters are stable in the air, due to the interface of the bilayer.STM and ARPUS results reveal that the preparation process are highly dependent on the annealing temperature and cluster size. EELS and UPS data show that CO adsorption is distinctly different between Cu and CuOx clusters on ZnO, which explains the different yield rate. Based on TDS and EELS of adsorption such as CO2, H2O, combined with DFT calculation, the mechanism of methanol synthesis is given by introducing intermediate products. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L10.00004: Role of nitrogen dopants in the stabilization of nancrystalline cubic zirconia Renat Sabirianov, G. Wang, Y.L. Soo, G. Luo, H.J. Lin, W.N. Mei, F. Namavar, C.L. Cheung The role of the nitrogen doping in the stabilizing the nanocrystalline cubic phase of zirconia films fabricated by nitrogen beam assisted deposition (IBAD) is investigated. The IBAD zirconia films have diameters three times larger than those previously reported in a sol-gel method. Confocal Raman spectroscopy study indicates that the atomic structure of these IBAD zirconia films evolve from cubic to tetragonal and then to monoclinic phase upon annealing at elevated temperatures. The presence of nitrogen in the films is confirmed by secondary ion mass spectroscopy. X-ray absorption near edge structure study of these films infers that the nitrogen atoms are incorporated at the substitutional sites of these films. Ab-initio density functional calculations suggests that the substitutional nitrogen atoms could effectively immobilize native defect including oxygen vacancies and interstitial ions in nanocrystalline cubic zirconia. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L10.00005: Preparation dependent neutralization efficiency of Pt/TiO2 nanoparticles Alex Arjad, Yarmoff Jory Metal nanoclusters resident on an oxide surface can be produced by sputtering thin films as well as by direct deposition. We previously used the neutralization of scattered low energy alkali ions to demonstrate that Au nanoclusters formed by sputtering a thin gold film on TiO$_2$ have similar electronic properties as those formed by deposition [1]. In this work, we compare Pt nanoclusters grown on TiO$_2$ by both sputtering and deposition. It is shown that Pt nanoclusters formed by deposition are more efficient at neutralizing scattered low energy Na+ ions than those formed by sputtering a thin platinum film. We attribute this difference to the strong-metal-support-interaction (SMSI) present in the Pt/TiO$_2$ system, but not with Au/TiO$_2$. \\[4pt] [1] P. Karmakar, G.F. Liu, Z. Sroubek and J.A. Yarmoff, Phys. Rev. Lett. 98, 215502 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L10.00006: Nanoparticle shape instability by Coulomb interactions Natalya Zimbovskaya Metal atoms adsorbed on few-layer graphenes condense to form nanometer-size droplets whose growth in size is limited by a competition between the surface tension and repulsive electrostatic interactions from charge transfer between the metal droplet and the graphene. Under certain conditions a growing droplet can be unstable to a family of shape instabilities. This phenomenon was observed for Yb deposited and annealed on few-layer graphenes. A theoretical model to describe it is developed. The model describes the onset of shape instabilities for nanoparticles where their growth is limited by a generic repulsive potential and provides a good account of the experimentally observed structures for Yb on graphene [1]. \\[4pt] [1] L. A. Somers, N. A. Zimbovskaya, A. T. Johnson, and E. J. Mele, PhPhys. Rev B 82, 115430 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L10.00007: Structure and Energy Stability of Metal Nanoparticles Hector Barron, Juan Pedro Palomares-Baez, Jesus Velazquez-Salazar, Jose Luis Rodriguez-Lopez, Miguel Jose-Yacaman In this work we present a theoretical model for the structural evolution and energy stability for metal nanoparticles from the small (1-2 nm) to the big ($\sim $50 nm) size ranges. We have found that the appearances of structural lattice defects as well as surface reconstructions are important factors that highly influence the growth process. A simple assembly model for a path transformation for metal nanoparticles is presented and compare with experimental evidence. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L10.00008: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L10.00009: Attachment of Quantum Dots on Zinc Oxide Nanorods Jared Seay, Huan Liang, Parameswar Harikumar ZnO nanorods grown by hydrothermal technique are of great interest for potential applications in photovoltaic and optoelectronic devices. In this study we investigate the optimization of the optical absorption properties by a low temperature, chemical bath deposition technique. Our group fabricated nanorods on indium tin oxide (ITO) substrate with precursor solution of zinc nitrate hexahydrate and hexamethylenetramine (1:1 molar ratio) at 95C for 9 hours. In order to optimize the light absorption characteristics of ZnO nanorods, CdSe/ZnS core-shell quantum dots (QDs) of various diameters were attached to the surface of ZnO nanostructures grown on ITO and gold-coated silicon substrates. Density of quantum dots was varied by controlling the number drops on the surface of the ZnO nanorods. For a 0.1 M concentration of QDs of 10 nm diameter, the PL intensity at 385 nm increased as the density of the quantum dots on ZnO nanostructures was increased. For quantum dots at 1 M concentration, the PL intensity at 385 nm increased at the beginning and then decreased at higher density. We will discuss the observed changes in PL intensity with QD concentration with ZnO-QD band structure and recombination-diffusion processes taking place at the interface. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L10.00010: Controlled Deposition of Nanocrystal Quantum Dots on Silicon Surfaces Oliver Seitz, Hue M. Nguyen, Damien Aureau, Amandeep Sra, Anton V. Malko, Yves J. Chabal Studying Forster resonant energy transfer (FRET) has constantly been a challenge because of the poor control in transferring nanocrystal quantum dots (NQDs) onto various substrates. This lack of control often resulted in formation of aggregates (3D growth), inhomogeneity, and poor adhesion. In this study, using self assembled monolayers (SAMs), dense monolayer of NQDs have been attached onto silicon substrate, with and without the presence of oxide interlayer, allowing investigating FRET effects via photoluminescence measurements. Such SAMs, directly attached to the silicon, via Si-C bonds, display an interface quality with low interface states. Moreover, the ability to be prepared with tunable thicknesses renders them ideal for FRET investigation. Such hybrid colloidal NQD/Silicon optoelectronic structures could potentially be attractive for both photovoltaic as well as light emitting applications. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L10.00011: Synthesis of a long gold atomic strand incorporated with carbon atoms Yoshifumi Oshima, Tomoya Ono, Kunio Takayanagi, Nguyen Duy Huy Single metal or carbon atomic strands have attracted much interest because of their unique properties. They have been usually fabricated by stretching or thinning the junction between both electrodes, but limited to be several atoms in length at maximum. We show that a long atomic strand can be synthesized by pulling one-dimensional reconstructed structure formed on the electrode surface. In the transmission electron microscope combined with a scanning tunneling microscope, gold atoms were observed to be pulled out one-by-one from carbon contaminated gold (111) surface layer each 0.5 nm elongation till diminishing the layer. This observation was explained by the first-principle calculation, showing that two carbon atoms are incorporated at each interval (0.5 nm) between two neighboring gold atoms aligned along the [112] direction to form the one-dimensional reconstructed structure, when the gold (111) surface is contaminated with carbon atoms. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L10.00012: Scanning Tunneling Microscopy Study of Quantum Cobalt Chains Nader Zaki, Danda Acharya, Denis Potapenko, Peter Johnson, Percy Zahl, Peter Sutter, Richard Osgood We recently reported [1] on a new surface phase of the Co-vicinal-Cu(111) system, which exhibits self-assembled uniform Co quantum wires that are stable at 300K. STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases as well as experimentally observed step-island formation. Our observations allow us to comment on the formation kinetics of the atomic-wire phase and on the fit of our data to a recently developed lattice-gas model. LT-STM measurements, taken on self-assembled Co chains, reveal a charge-density modulation that is dependent on tip bias. These charge-modulations are observed for tip-bias relatively far away from the Fermi level, both at negative and positive bias. We present arguments for the identification of these modulations as due to either charge-density-like waves (CDW) or excited states of this 1-D system. \\[4pt] [1] N. Zaki et al, Phys. Rev. B 80, 155419 (2009) [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L10.00013: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L10.00014: Characterization of the surface environment of PbSe nanoparticles by correlating calculated and measured x-ray spectra Keith Gilmore, Aaron Hammack, April Sawvel, Evelyn Rosen, D. Frank Ogletree, Jeffrey Urban, Delia Milliron, Brett Helms, Bruce Cohen, David Prendergast Given that defining characteristics of nanoparticles are often dictated by their surfaces, it is desirable to be able to control the surface environments. We seek this control through ligand exchange chemistry and investigate PbSe as a model system. We correlate calculated and measured x-ray spectra to quantify the extent of ligand exchange, validate our structural models, and characterize the optical and electronic properties induced by the new surface environment. Chemical shifts in x-ray photoelectron spectra indicate changes in atomic bonding at the surface, whereas x-ray absorption spectra reveal ligand conformation and binding coordination at the surface. The colloidal synthesis of PbSe particles is highly reliable and the resulting particles are technologically useful size-tunable IR absorbers. Such particles have Pb rich surfaces and native oleic acid coats. We replace the oleic acid with alternate ligands of choice, which may change the Pb:Se ratio at the particle surface. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L10.00015: Anchoring platinum on graphene using metallic adatoms F.G. Sen, Y. Qi, A.T. Alpas To anchor Pt on the graphene surface 25 different metallic adatoms were individually inserted into Pt(111)/graphene interface and the work of separation required to break the interface between Pt-adatom and C-adatom bonds were computed using first principles calculations. With the exception of Al, Zn and Au, all metals increased the strength of the Pt/graphene interface, while many transition metals with unfilled d orbitals, such as Sc, Ti, V, Cr, Co, Ni, Zr, Nb, Mo, Ru, Rh, Ta, W, Re, Os, Ir, could increase the Pt/graphene interface strength from 0.009 J/m$^{2}$ to above 0.5 J/m$^{2}$. The Pt-adatom bond had metallic character and its strength was proportional to the amount of charge transferred from the adatom to the Pt. The strength of carbon-adatom bond was proportional to the ratio of charge transferred to the carbon over charge transferred to the platinum from the adatom. As this ratio was $>$1.0 for Ir, Os, Ru, Rh and Re and these emerged as the most promising adatoms for anchoring Pt on graphene. [Preview Abstract] |
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