Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L42: Excitations at Surfaces |
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Sponsoring Units: DCMP Chair: Mao-Hua Du, Naval Research Laboratory Room: Colorado Convention Center 505 |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L42.00001: Low-Energy Acoustic Collective Excitations on Metal Surfaces Bogdan Diaconescu, Karsten Pohl, Luca Vattuone, Letizia Savio, Philip Hofmann, Vyacheslav Silkin, Jose Pitarke, Eugene Chulkov, Pedro Echenique, Daniel Farias, Mario Rocca Sound-like longitudinal plasma waves where thought to only exist in layered systems where spatially separated 2D electron plasmas are realized. Due to their low energy and linear dispersion such waves were proposed as possible candidates to mediate the attractive interaction leading to the formation of Cooper pairs in high TC superconductors. A new type of collective excitation mode on metal surfaces has been found. In contrast to the usual surface plasmon, it has an acoustic dispersion. For Be(0001) the mode was observed using EELS. Detailed ab-initio calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface state band with the underlying 3D continuum in the same region of space. While it exists up to high energies for Be(0001), the mode as such has a very general character, for low energies it is expected to exist on many surfaces, profoundly affecting their electron and phonon dynamics. 1. V. M. Silkin et. al., Phys. Rev. B 72, 115435 (2005) [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L42.00002: Raman and infrared study of synthetic Maya pigments as a function of heating time and dye concentration Layra Reza, Felicia Manciu, Lori Polette, Brenda Torres, Russell Chianelli Maya Blue is a famous indigo-based pigment produced by the ancient Mayas. Samples for the present work are made by a synthetic route, and demonstrate similar chemical stability as the ancient Maya Blue samples. Since no direct proof exists that the indigo chemically binds to the inorganic palygorskite lattice, there is still controversy on the resting place of the indigo molecules; i.e$.$ are they in the channels of palygorskite, on the surface, or both. Our analysis by FT-Raman and FT-IR spectroscopy proves the partial elimination of the selection rules for the centrosymmetric indigo, and shows the disappearance of the indigo N-H bonding, as the organic molecules incorporate into palygorskite material. Infrared data confirm the loss of zeolitic water and a partial removal of structural water after the heating process. Evidence of bonding between cationic aluminum and indigo through nitrogen is revealed by FT-Raman measurements. The oxygen carbonyl is also believed to interact with the metal. [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L42.00003: Infrared Spectroscopy of the Elastically-Strained Silicon Nanomembrane Bonding Interface Arnold Kiefer, Weina Peng, Donald Savage, Max Lagally We investigate the bonding interface between elastically strained silicon nanomembranes (SiNMs) and new host substrates with Fourier transform infrared spectroscopy in efforts to elucidate its chemical structure. We create SiNMs by heteroepitaxial growth of straining layers on the template layer of silicon-on-insulator (SOI) and then releasing the membrane sandwich by etching away the buried oxide. We then bond the SiNM to a new substrate, in the present case oxidized Si. Because the SiNM is only nanometers thick, the bonding interface contributes greatly to the membrane's structural and electrical properties. We probe the buried interface by attenuated total reflection via the evanescent wave from a germanium prism in intimate contact with the SiNM, which penetrates through the SiNM to interact with the interface bonds. In efforts to understand bond strength and interface electronic states, we probe the influence of different cleaning procedures, gas treatments, and annealing steps. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L42.00004: Dynamical Properties of Surface-mounted Dipolar Molecular Rotators Jason Underwood, John Price, Douglas Caskey, Josef Michl We use dielectric relaxation spectroscopy (DRS) to study the rotational dynamics of dipolar molecules mounted on fused SiO$_2$ surfaces. Each ``molecular rotor'' consists of three parts: 1) a mounting group for attachment to the substrate, 2) a rotating group having a permanent dipole moment, and 3) an axis connecting the rotor to the attachment group. Attachment is facilitated either by covalent bonding through reaction of silane groups with surface hydroxyls or by van der Waals interactions. Fused SiO$_2$ substrates are patterned with interdigitated electrode Au capacitors ($C \simeq$ 1 pF), and rotor molecule dynamics are characterized by measurement of the capacitance $C$ and loss tangent $\tan \delta \equiv \mathrm{Re}\{Z\}/\mathrm{Im}\{Z\}$. We employ a ratio-transformer bridge technique to measure these quantities, with sensitivities in $C$ and $\tan \delta$ of 1 aF and 1 ppm, respectively. A unique aspect of this work is the experimental apparatus, which allows us to prepare sub-monolayer films, determine coverage via two independent methods (DRS and XPS), and study molecule rotational motion, in-situ in ultra-high vacuum. Results will be presented on the kinetics of rotor adsorption/desorption, barrier height and asymmetry of the rotational potential of the molecules, and the effects of varying rotor coverages and adventitious H$_2$O. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L42.00005: Optical and opto-electronic based velocity and topographic measurements of a laser-ablated thin-metal layer on glass Anthony Valenzuela, George Rodriguez, Steven Clarke, Keith Thomas We report on our ability to resolve the velocity and spatial profile of ablatively launched metal with nano-scale precision. We utilize a nanosecond laser pulse to launch a thin layer of titanium metal from a glass surface. Subsequently, we use optical and opto-electronic devices to diagnose the velocity and topography of the launched metal. Our Photonic Doppler Velocimeter (PDV) utilizes the heterodyne principle that allows us to track multiple velocity components. Our topographer incorporates a Shack-Hartmann interferometer to provide details of the deformation of the surface as it is launched. We compare the experimental data to simulations to provide a feedback loop to improve our theoretical models. We also discuss possibilities to extend the sensitivity of the PDV system to provide a compact diagnostic with a broad range of capabilities. [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L42.00006: Surface Plasmon Assisted Kondo Resonances on a Metallic Nanowire Ren-Shou Huang, Yia-Chung Chang We propose an experiment to measure the Kondo effect for magnetic atoms adsorbed on the surface of a metallic nanowire. In addition to the traditional $sp$-$d$ hybridization, by introducing the strong electromagnetic field of the localized surface plasmon on the nanowire, we show that it is possible to observe additional $sp$-$d$ electron transfer processes assisted by surface plasmons. Due to the good surface-to-volume ratio of the nanowire, the Kondo resonances could be observed as multiple anti-resonances in the differential conductance versus bias voltage curve [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L42.00007: Positronium physisorption at quartz surfaces Rolando Saniz, Arthur Freeman, Bernardo Barbiellini, Phil Platzman The possibility of having positronium (Ps) physisorbed at a material surface is of great fundamental interest, since it can lead to new insight regarding quantum sticking and is a necessary first step to try to obtain a Ps$_2$ molecule on a material host. Experimental evidence for physisorbed Ps at the surface of quartz was reported some years ago,\footnote{Sferlazzo, Berko, Canter, Phys. Rev. B {\bf 3}, 6067 (1985).} but firm theoretical support for such a conclusion was lacking. With the FLAPW method\footnote{Wimmer, Krakauer, Weinert, Freeman, Phys. Rev. B {\bf 24}, 864 (1981).} we calculated the electronic structure and dielectric function of $\alpha$-quartz and obtained the interaction potential with a Ps atom on its surface. We show that there is indeed a bound state with an energy of $\sim0.19$ eV, which is reasonably close to the experimental estimates of $0.14$ - $0.17$ eV. A brief energy analysis in terms of the Langmuir-Hinshelwood mechanism further shows that the formation of a Ps$_2$ molecule at quartz surface would be possible. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L42.00008: Molecule-resolved structural and electronic properties of diamondoid self-assembled monolayers Jason C. Randel, Hari C. Manoharan Diamondoids represent an exciting new direction in the field of nanoscale carbon. While theoretically known to be stable, diamondoids have been experimentally inaccessible due to synthesis roadblocks and lack of natural sources, until recently purified from crude oil. We investigate the local structural and electronic properties of self-assembled monolayers formed from thiol-functionalized diamondoids. Using an ultra-high vacuum scanning tunneling microscope, we observe these monolayers to be robust and stable up to room temperature. Topographic data demonstrate well-ordered lattices for all molecules studied (adamantane through tetramantane), with lattice constants and angles that vary with polymantane order. Tunneling spectroscopy reveals a conductance gap in the energy spectra of each molecule, which we compare to calculated HOMO-LUMO gaps and band alignments. The hierarchical nature of these molecules, and the ability to functionalize them with specific atomic and molecular end groups, provide a new set of customizable molecular nanomaterials. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L42.00009: X-Ray Photoemission Analysis of Chemically Treated CdZnTe Semiconductor Surfaces Art Nelson, Daniel Vazquez, Ann Bliss, Cheryl Evans, Jim Ferreira, Rebecca Nikoloc, Steve Payne Device-grade Cd$_{(1-x)}$Zn$_{x}$Te was subjected to various chemical treatments commonly used in device fabrication to determine the resulting microscopic surface composition/morphology and the effect on contact formation. Br-MeOH (2{\%} Br), N$_{2}$H$_{4}$, NH$_{4}$F/H$_{2}$O$_{2}$, and (NH$_{4})_{2}$S solutions were used to modify the surface chemistry of the Cd$_{(1-x)}$Zn$_{x}$Te crystals. Scanning electron microscopy was used to evaluate the resultant surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and Zn 2p, Cd 3d, Te 3d, O 1s core lines were used to evaluate the chemistry of the chemically treated surfaces. Metal overlayers were then deposited on these chemically treated surfaces and the I-V characteristics were measured. The measurements were correlated to understand the effect of interface chemistry on the electronic structure at these interfaces with the goal of optimizing the metal/Cd$_{(1-x)}$Zn$_{x}$Te Schottky barrier for radiation detector devices. This work was performed under the auspices of the U.S. Dept. of Energy by the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L42.00010: On the role of multiple hot electron processes in STM-induced atom motion on surfaces J. W. Gadzuk The phenomenon of adsorbed atom excitation/manipulation induced by hot-electrons delivered to the surface by an STM tip is related to desorption induced by multiple electron transition (DIMET) processes in which femtosecond laser pulses excite substrate electrons, creating a flux of hot electrons incident upon the surface from within rather than externally as occurs with the STM. While the sources of the hot electrons differ, the individual inelastic electron scattering processes giving rise to atomic motion over activation barriers are identical. This facilitates an adaptation of DIMET theory [1] to multiple-electron STM surface processing. Recent relevant experiments: i) STM-induced non-local hot electron dissociation of dimethyldisulfide on Au (111) by Maksymovych and Yates; ii) ``below-one-electron-threshold'' excited motion of single Co atoms on Cu (111) by Stroscio and coworkers will be considered in the light of a DIMET-based theory that focuses on special aspects of multiple excitation processes. [1] J. W. Gadzuk, Chem. Phys. Vol.251, 87 (2000). [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L42.00011: Probing the mechanism of infrared resonant desorption of hydrogen from Si(111): anharmonicity and energy pooling Zhiheng Liu, Leonard Feldman, Norman Tolk, Zhenyu Zhang, Philip Cohen Desorption of hydrogen from a Si(111) surface by resonant infrared excitation of the Si--H vibrational stretch mode requires vibrational ladder climbing of a Si--H bond to a high level leading to associative desorption. We report recent experiments probing the mechanism of ladder climbing. H$_2$ desorption is observed when the excitation linewidths are narrower than the anharmonicity of the Si--H bond, favoring energy pooling over multiphoton absorption. The resonance width of H$_2$ desorption with an excitation linewidth of 8.7~cm$^{-1}$ is measured to be 39~cm$^{-1}$, opening a new opportunity for site--selective modification on the Si(111) surface. The desorption yield decreases when the sample temperature increases, consistent with an energy pooling process. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L42.00012: Probing Electron-Hole Pair Production in Ultrathin Film Schottky Diode Devices using Hyperthermal Energy Ion Beams. Matthew Ray, Russell Lake, Chad Sosolik We are investigating the interactions of hyperthermal energy ions with ultrathin film Schottky diode devices, probing the role of ion-surface impact events and charge transfer on electron-hole pair production. Specifically, we measure currents that arise from electron-hole pair production at a diode surface. To date, these currents have been explored only for thermal energy gas-surface impacts, where they are called ``chemicurrents''. Using a UHV beamline to produce well-collimated monoenergetic noble gas and alkali-metal beams from 10 eV to 10 keV, we have the unique flexibility to probe our in-house designed diode devices with a wide range of incident species, energies, and charge states. Preliminary results are presented and discussed in the context of basic gas-surface energy transfer processes. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L42.00013: Characterization of the Unoccupied Electronic Structure of Alkali Atoms on Noble Metal Surfaces by Time- and Angle-Resolved Two-Photon Photoemission Vahit Sametoglu, Aimo Winkelmann, Jin Zhao, Hrvoje Petek, Niko Pontius, Andrei Borisov, Pedro Echenique We perform angle-resolved two-photon photoemission (2PPE) spectral measurements on alkali atom (Li - Cs) covered noble metal (Cu(111) and Ag(111)) surfaces. The progressive evolution of 2PPE spectra with the alkali atom coverage is measured in the 0 - 0.1 monolayer range. We report on the dependence of 2PPE spectra on the alkali atom coverage, the photoemission angle, and the excitation laser polarization. The spectral measurements provide new information on the nature of chemisorption of alkali atoms on noble metals, as well as the photoinduced charge transfer excitation of alkali atoms. A general model for the image charge interactions at metal surfaces reproduces the experimental electronic structure quantitatively. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L42.00014: Phononic and nonlocality contributions to Second Harmonic Generation in NiO. Wolfgang Huebner, Georgios Lefkidis The experimentally observed second harmonic signal in centrosymmetric NiO can be explained with symmetry lowering mechanisms: (i) signal from the surface, (ii) spin-orbit coupling, (iii) nonlocalities, and (iv) lattice distortions [1]. First the intragap energy levels of both the (001) surface and the bulk of NiO are obtained with highly correlated quantum chemistry methods: single excitation configuration-interaction, and multiconfigurational complete active space, optimizing each d-level separately. Then the second-order susceptibility tensor is calculated beyond the electric-dipole approximation. The effects of spin-orbit coupling are studied, and a detailed analysis of the effects of the inclusion of nonlocalities from magnetic dipoles and electric quadrupoles is performed. Finally the effects of phonons in the bulk of NiO within the frozen phonon approximation are included, and the second order susceptibility tensor is computed both in a time resolved and time averaged manner [2]. \newline \newline [1] G. Lefkidis and W. H\"{u}bner, Phys. Rev. Lett. 95, 77401 (2005). \newline [2] G. Lefkidis and W. H\"{u}bner, Phys. Rev. B 74, 155106 (2006). [Preview Abstract] |
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