Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S19: Mesoscopic Systems, Clusters, and Nanoscale Systems |
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Sponsoring Units: DCP Chair: Ravindra Pandry, Michigan Technological University Room: Colorado Convention Center 104 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S19.00001: First principles study of adsorption and dissociation of H$_{2}$, O$_{2}$, and CO on $\alpha $-Al$_{2}$O$_{3}$ (0001) supported Pt-Co alloy T.J. Dhilip Kumar, C. Zhou, B. Naduvalath Recently, there has been several investigations carried out to improve the electro-catalytic activity of Pt and Pt based alloys for the oxygen reduction reaction in fuel cell electrodes. In particular, Pt-Co alloy systems have been often employed. To gain physical insight into the catalytic properties of these systems we have performed a systematic study of the electronic structures, bonding and growth patterns of nanoclusters of Pt-Co alloy using first principles density functional calculations. The 3:1 ratio of Pt-Co alloy has been constructed as nanoclusters and thin film supported on $\alpha $-Al$_{2}$O$_{3}$. The geometry optimized tetrahedron, and the square planar structures of Pt$_{3}$Co are placed over the slabs of six layers $\alpha $-Al$_{2}$O$_{3 }$(0001) surface. Activity of H$_{2}$, O$_{2}$ and CO on these structures from various approaches has been explored. In all our calculations the non-locality in the exchange correlation functional is taken into account by considering spin polarized generalized gradient approximation as proposed by Perdue and Wang. Brillouin zone integrations have been performed using Monkhorst-Pack grids with (2 X 2 X 1) $k$-point meshes. The electronic structures of these systems have been analyzed by computing the electronic density of states. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S19.00002: Influence of Charge State on the Reaction of FeO$_{3}^{+/-}$ with Carbon Monoxide J.U. Reveles, S.N. Khanna, N.M. Reilly, G.E. Johnson, A.W. Castleman Jr. A synergistic study combining experiments in molecular beams and first principles electronic structure calculations within a gradient corrected density functional approach is used to investigate the reactivity of charged FeO$_{3}$ clusters with CO. It is shown that highly oxidized iron clusters are able to readily effect the oxidation of CO to CO$_{2}$ at ambient temperature. Calculated energy profiles of the reaction demonstrate that the oxidation efficiency is governed by the strength of oxygen binding to the iron atom. Results for FeO$_{3}^{+/-}$ are presented and reveal that cation clusters are more efficient than the corresponding anion clusters at effecting the oxidation reaction due to different bond energies resulting from charge distribution. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S19.00003: Atomic-scale Characterization of Free Radical Adsorption to the Si(111)-7 x 7 Surface Nathan Guisinger, Shaun Elder, Nathan Yoder, Mark Hersam Ultra-high vacuum (UHV) scanning tunneling microscopy (STM) was employed to investigate free radical chemistry on the Si(111)-7 $\times $ 7 surface with atomic-scale spatial resolution. In particular, the nitroxyl free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was explored, due to its single-site binding mechanism. UHV STM imaging of isolated molecules revealed that TEMPO covalently reacts with adatom dangling bonds with high affinity, while exhibiting a preference towards center adatom sites during the initial stages of adsorption. Adsorption to center and corner adatoms approached a ratio of 1:1 with increased surface coverage. Upon saturation, the surface exhibited long-range ordering. Following adsorption to a center adatom site, significant charge transfer occurred between TEMPO and a neighboring adatom. Scanning tunneling spectroscopy was utilized to investigate this delocalized effect by spatially mapping the local density of states. This study provides fundamental insight into free radical surface chemistry and suggests a direct pathway for forming nearly perfectly ordered organic adlayers on the Si(111)-7 $\times $ 7 surface. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S19.00004: CO$_{2}$ Sensing and CO and H$_{2}$O Interactions on Mats of Gold Nanoparticle Decorated GaN Nanowires. C.A. Berven, R. Abdelrahaman, W. Barredo, D.N. McIlroy We report on the use of mats of gold-nanoparticle-decorated GaN nanowires for the detection of CO$_{2}$ and the possible generation of CO$_{2}$ and H$_{2}$ by interactions of CO and H$_{2}$O on the surfaces of the gold nanoparticles. The sensor was constructed from a 10-20 $\mu $m thick mat of GaN nanowires grown on a 1 cm diameter sapphire substrate followed decoration of the nanowires with gold nanoparticles. Selective wet etching was then used to reduce the amount of gold on the nanowires. Electrical measurements were made of the mat under various atmospheres. When exposed to just water vapor, CO or H$_{2}$ the current-voltage curves were similar to those when in vacuum. However, when the device was exposed to CO followed by H$_{2}$O, we saw a significant reduction in the current at all values of applied voltage. Exposure to just CO$_{2}$ showed a similar response to that of mixing CO and H$_{2}$O implying that CO$_{2}$ is being generated and that what is being sensed is the product of the reaction of CO + H$_{2}$O creating CO$_{2}$. As a consequence of CO$_{2}$ being detected after the mixing of CO and H$_{2}$O, we speculate that H$_{2}$ is being generated giving a possible new mechanism for H$_{2}$ generation. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S19.00005: Sorption and condensation of Neon in MCM48 as monitored by X-ray diffraction. Paul Sokol, Duncan Kilburn In this submission we report on experiments that simultaneously measure adsorption isotherms and X-ray diffraction measurements on Neon in the mesoporous silica glass MCM 48. The isotherms are similar to those reported previously in the literature. Simultaneous X-ray measurements allow the variation of adsorbate structure to be monitored as the sorption progresses. We observe that the most rapid increase in the intensity of the first peak in the scattered spectrum occurs during the initial sorption of Neon layers, but before capillary condensation. We show that this can be explained via conventional scattering theory and that it provides a new perspective on the processes of monatomic gas sorption. We also report on modified phase transitions and structures for the confined Neon. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S19.00006: Water in Carbon Nanotubes:A New Quantum Phase of Water George Reiter, Christian Burnham, Dirar Homouz, Philip Platzman, Jeremy Mayers, Tyno Abdul-redah, Alexander Moravsky, Jichen Li, C.-K. Loong, Alexander Kolesnikov The momentum distribution of the protons in ice Ih, ice VI, high density amorphous ice and water in carbon nanotubes has been measured using deep inelastic neutron scattering. We find that, at low temperatures, the momentum distribution for the water in the nanotubes is qualitatively unlike that in any other phase of water or ice. The kinetic energy of the protons is 35mev less than that in ice Ih at the same temperature, and the high momentum tail of the distribution, characteristic of the molecular covalent bond and the stretch mode of the proton in the molecule, is not present. We observe a phase transition between 230K and 268K in the nanotube data. The high momentum tail is present in the higher temperature measurement, which resembles that of ice Ih at the same temperature. Molecular dynamics simulations show the phase transition to be associated with the reordering of the hydrogen bonds of the 2-D ice layer, coating the interior of the nanotube at low temperatures, into a 3-D structure at 268K. Although there is yet no model for water that explains the low temperature momentum distribution, our data reveals that the protons in the hydrogen bonds in the 2-D ice layer are coherently delocalized, and that the 2-D ice layer is a qualitatively new phase of ice. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S19.00007: Bulk vs. Nanoscale WS$_2$: Finite Size Effects and Solid State Lubrication S. Brown, J.L. Musfeldt, I. Mihut, J.B. Betts, A. Migliori, R. Rosentsveig, R. Tenne Metal dichalcogenide nano-structures have recently attracted attention due to their unique closed cage structures, hierarchy of length scales, and outstanding solid-state lubrication behavior. In order to understand the bulk vs. nanoscale effects, we measured the low temperature specific heat of layered and nano-particle WS$_2 $. Below 9 K, the specific heat of the nano-particles deviates from that of the bulk counterpart. Further, it deviates from the usual T$^3$ dependence below 4 K, due to both finite size effects and inter-particle interactions. This separation of nanoscale effects from T$^3$ dependence can be modeled by assuming that the phonon density of states is flexible, changing with size and shape of the nanoparticle. We invoke relationships between low temperature T$^3$ phonon term, Young's modulus, and friction coefficient to assess the difference in the tribological properties. Based on this analysis, we conclude that the improved lubrication properties of the nanoparticles are extrinsic in origin. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S19.00008: Theory of surface enhanced Raman scattering from a molecule adsorbed on a chain-like cluster of metallic nanoparticles and nanoshells Jeanne Bonner, Karamjeet Arya The Raman cross-section from a molecule is believed to enhance by more than 10 orders of magnitude when it is adsorbed on a cluster of silver nanoparticles. These large enhancements are attributed to the resonant excitation of the surface plasmon modes of the cluster those have very large localized electric field near its surface. The resonant position and the electric field of these modes are very sensitive to the structure of metal particles and the size and shape of the cluster. Using multiple scattering in the wave-vector space between the individual particles in the cluster we have calculated the resonant position of these modes and their enhanced electric field for clusters of different shape formed from two, three, and four nanospheres and nanoshells. We find the maximum enhancement in the cross-section can reach up to 10 orders of magnitude for silver particle clusters. We also find important new results for the chain like clusters of three or more particles where there is a dramatic increase in the enhancement due to very sharp resonant features of the modes. These features may be helpful in identifying the cluster shape and size in the surface enhanced Raman scattering experiments. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S19.00009: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S19.00010: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S19.00011: Assembly and Interaction of Au/C Core-Shell Nanoparticles Peter Sutter, Eli Sutter, Yimei Zhu The encapsulation of metal nanoparticles in fullerene cages has attracted much interest recently due to the expected novel and exotic properties. Despite the interest in these nanostructures, important properties, such as the thermal stability and interactions of encapsulated nanoparticles as well as the process of encapsulation itself are not entirely understood. Using real-time transmission electron microscopy we study the formation of Au/C core-shell structures from C-supported Au nanoparticles, and their thermally and electron beam induced interactions [1, 2]. At temperatures below 400C no C-shell is assembled, and closely spaced Au nanoparticles interact by coalescence. At high temperatures (400C - 800C) the Au particles are transformed into Au/C core-shell structures via encapsulation into curved, fullerene-like C shells. The shells initially passivate the Au cores and inhibit their coalescence. But under electron irradiation, the Au cores can break free from their shells, and hence can coalesce. Surprisingly, at this stage the assembled C-sheets may actually enhance the coalescence process by driving the directed motion of Au/C particles and causing the efficient contraction of widely spaced particle ensembles. \newline [1] E. Sutter, P. Sutter, Y. Zhu, Nano Lett. 5, 2092 (2005). \newline [2] E. Sutter, P. Sutter, Y. Zhu, Surf. Sci. 600, 3654 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S19.00012: DFT study on the charge density shift bucky-ferrocene vs. bucky-ruthenocene Tunna Baruah, Rajendra Zope, Mark Pederson Iron and ruthenium atoms have similar valence electron configurations. Ferrocene and ruthenocene are stable metallocenes that satisfy the 18-electron rule. Recently, Sawamura et al. [J. Am. Chem. Soc. vol. 124, pp. 9354 (2002)] have succeeded in synthesizing fused complexes of ferrrocene and ruthenocene with fullerene, known as bucky-ferrocene and bucky-ruthenocene, respectively. The experiments show shift in charge density from ferrocene to fullerene in the ground state. However, no charge transfer from ruthenocene to fullerene is observed in case of the bucky-ruthenocene. We have performed all electron density functional calculations to obtain the equilibrium structures of the bucky-ferrocenes and bucky-ruthenocenes. Both, the staggered and eclipsed geometries are examined. The differences in the electronic structure and nature of bonding in these two systems are studied by analyzing the frontier orbitals, dipole moments, and charge density in their ground and singlet excited states. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S19.00013: Temperature-dependent properties of SiC Clusters C. Ghosh, M. Yu, S. Shen, C.S. Jayanthi, S.Y. Wu Using a semi-empirical quantum mechanics based molecular dynamics simulation [Phys. Rev. B, 74, 155408 (2006)], Si$_{n}$C$_{m}$ clusters were shown to exhibit several types stable structures corresponding to different compositions and distributions of Si and C but a fixed total number of atoms [Ming Yu et al. -- APS March 2007]. Specifically, it was shown that an almost uniform admixture of Si and C atoms in a 147-atom SiC cluster exhibited a bucky diamond structure, while the Si-rich and the C-rich stable structures for the same fixed number of total atoms (n+m= 147) exhibited totally different structures with different co-ordinations, bonding, etc. In the present work, we will investigate how equilibrium structures of these different structures evolve with the increase of the temperature from 0K to the melting temperature, and quantities such as pair-distribution functions, electronic density of states, etc. will be calculated up to the melting temperature. This study will provide characterizations of both ordered and disordered SiC clusters, as well as SiC ``liquids'' in reduced dimensions. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S19.00014: Thermodynamic Properties and Grain Growth in Pt Nanoparticles Katherine Sloyan, Thomas Ekiert, Karl Unruh The evolution in the structural and thermodynamic properties of chemically prepared Pt nanoparticles has been studied by x-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and ac magnetic susceptibility measurements. Depending on the synthesis conditions, the XRD and TEM measurements indicated that nanoparticles could be prepared with mean diameters between about 5 and 10 nm. A combination of XRD, TEM, and DSC measurements also indicated that the as-prepared nanoparticles were stable with respect to grain growth to temperatures of about 300 \r{ }C. Above this temperature, grain growth resulted in an increase in the mean particle size and a slight increase in the Pt lattice parameter as well. AC susceptibility measurements as a function of the temperature indicate that with decreasing grain size there is an increase in the real part of the susceptibility and a corresponding decrease in the imaginary susceptibility. [Preview Abstract] |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S19.00015: Ligand control of solubility and capping structure of colloidal CdSe nanorods Wei Wang, Sarbajit Banerjee, Shengguo Jia, Michael Steigerwald, Irving Herman The length and functional group of the organic capping ligands of colloidal CdSe nanorods play a critical role in determining their solubility in chloroform. Optical transmission spectroscopy shows decreased solubility of CdSe nanorods capped by relatively long ligands, and increased solubility of CdSe nanorods capped by alkyl phosphonate ligands. $^{1}$H and $^{31}$P nuclear magnetic resonance (NMR) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) studies indicate that when mixtures of phosphonic acids with different lengths are used in synthesis, the shorter ligands selectively remain on the inorganic surface, but the overall number of ligands on the surface is smaller than that of the nanorods capped by long ligands. The proposed capping structure of colloidal CdSe nanorods is that there is a shell of ligands bound to the core of the nanorods and additional ligands can be trapped by this shell. The bound and the trapped ligands can strongly affect the solubility of the individual nanorods and the interactions between the nanorods that lead to aggregation. This work is supported by the MRSEC program of the National Science Foundation under Award No. DMR-0213574 and by the New York State Office of Science, Technology, and Academic Research (NYSTAR). [Preview Abstract] |
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