Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N42: Focus Session: STM of Surface-Based Nanostructures |
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Sponsoring Units: DMP Chair: Arthur Smith, Ohio University Room: Colorado Convention Center 505 |
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N42.00001: Roles in Modulation of Molecular Structures on Metal Surfaces Invited Speaker: We studied the adsorption of organic molecules, their growth behavior, and their physical properties on silver and gold surfaces at the single molecule or sub-molecular scale by using low-temperature scanning tunneling microscopes. Combined with low energy electron diffraction and first-principles density functional theory calculations, the key parameters in modulating molecular structures on metals are analyzed. It is found that the alkyl chains of quinacridone derivatives (QA) determine the orientation of molecular overlayers on an Ag(110) substrate. The interaction of QA and the Ag substrate is primarily due to chemical bonding of oxygen to specific positions at the silver substrate, determining the molecular orientation and preferred adsorption site. However, the intermolecular arrangement can be adjusted via the length of attached alkyl chains. We are thus able to fabricate uniform QA films with very well controlled physical properties. Furthermore, by thermal and chemical control, we are able to self-assemble three dimensional molecular nanostructures, e.g. ordered PTCDA structures exclusively on flat Ag(111) facets, or DMe-DCNQI structures exclusively on stepped Ag(221) facets. It is demonstrated that bonding, the key factor for selectivity, occurs via the end-atoms, while the molecule's mid-region arches away from the substrate. Theoretical results, obtained by high-level theory, are consistent with the experimental observations, which have previously been interpreted in terms of bonding through the mid-region. \newline \newline In collaboration with D.X. Shi, S.X. Du, W. Ji, Z.T. Deng, L. Gao, Institute of Physics, and X. Lin, Chinese Academy of Sciences, China; C. Seidel and H. Fuchs, Universit\"at M\"unster, Germany; W.A. Hofer, The University of Liverpool, Britain; and S. T. Pantelides, Vanderbilt University, USA. \newline \newline [1] D.X. Shi et al., Phys. Rev. Lett. 96, 226101(2006). \newline [2] S.X. Du et al., Phys. Rev. Lett. 96, 226101(2006). \newline [3] L. Gao et al., Phys. Rev. B 73, 075424(2006). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N42.00002: Molecular organization: the role of substrate interaction Martin Oehzelt, Leonhard Grill, Stephen Berkebile, Georg Koller, Falko P. Netzer, Michael G. Ramsey The influence of the surface chemistry is studied for para-sexiphenyl (6P) grown on clean, partly and fully oxygen reconstructed Cu(110). LT-STM, NEXAFS and ARUPS measurements were carried out to determine the exact geometry (orientation and registry) and the nature of the bond of the molecules to the substrate. On clean copper and on the partly reconstructed surface the molecules lie flat on the surface, having a similar strong bond due to backdonation, but are adsorbed in opposite directions: [1-10] and [001], respectively. On the full reconstructed surface the 6P molecules are pointing in [001] direction and their aromatic planes have a significant tilt angle to the substrate arising from the weak bond to the surface and adopting the surface corrugation. Because of the flexibility of the Van der Waals interaction, dominating the growth on this surface, the resulting layer is similar to a 6P(20-3) bulk crystal plane but modified due to the constraints of commensurability. This increase in the tilt angle exhibits a new stress release mechanism unknown and inaccessible for inorganic heteroepitaxy. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N42.00003: Self-assembly of methanethiol on cluster arrays of Co/Au(111) Georgi Nenchev, Bogdan Diaconescu, Karsten Pohl Self-assembly on strained metallic interfaces is an attractive option for growing highly ordered multi-functional nanopatterns. We present a Variable Temperature STM and Auger Electron Spectroscopy study of selective adsorption of sulfur-terminated CH$_{3}$SH molecules on the lattice of Co clusters on Au(111). We investigate the growth of a uniform network of Co on the reconstructed Au(111) surface, the temperature evolution of the island height and the termination, and the onset of surface alloying. Further we will show the evolution of morphology of the CH$_{3}$SH film on Au (111) as a function of coverage and temperature, and the importance of the herringbone reconstruction for the SAM formation and orientation. Successful combination and control of these two processes leads to the creation of an ordered, stable patterned Co/CH$_{3}$SH heterostructure with nanometer-sized unit cell. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N42.00004: An STM Study of Nucleation and Growth of Co Nanostructures on Stepped Cu(775) Nader Zaki, Denis Potapenko, Richard Osgood, Jr., Peter Johnson We conduct an STM study of nucleation and growth of Co nanostructures on stepped Cu(775) surface. This surface has a relatively narrow terrace width of 1.4nm, which should allow a different growth mode than on previously studied step-edge growth of Co bilayer nanoislands on Cu(111). Growth of other metals on narrow stepped surfaces is known to favor step-edge nanowire formation. On the bare Cu(775) surface, STM imaging at 300K is blurred by Cu-atom surface diffusion; low-coverage Co deposition modifies this behavior by step pining. The effects of deposition rate and substrate temperature are investigated, and specific conditions for Co nanowire growth and stability will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N42.00005: Size dependent superconductivity of nano-sized Pb islands studied by low temperature scanning tunneling spectroscopy under magnetic field Takahiro Nishio, Masanori Ono, Toyoaki Eguchi, Hideaki Sakata, Yukio Hasegawa We performed scanning tunneling microscopy/spectroscopy at low temperature ($<$2 K) on atomically-flat nano-sized Pb islands formed on the Si(111)-7x7 substrate. The measured tunneling spectra revealed that the superconducting gap does not dependent on the sites in a single Pb island but depends on the lateral size of islands. These are consistent qualitatively with the results of a theoretical calculation which includes the fluctuation of superconductivity[1]. We also investigated superconductivity of Pb islands under magnetic fields up to 2.1 T. The superconducting gaps were still observed above the critical magnetic field of bulk Pb. In addition, the obtained spectra showed the decrease in the conductance at zero bias voltage when the island size is small. The conductance decrease can be explained with Gor'kov equations on superconducting spheres whose size is smaller than the coherence length. [1] T. Nishio \textit{et al}., APL \textbf{88}, 113115 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N42.00006: STM and LEEM Observations of Pb Growth on W(110) Shirley Chiang, Donell Hoffman We have recently used both scanning tunneling microscopy (STM) and low energy electron microscopy (LEEM) in a combined UHV system to study the growth of Pb on W(110). As seen in previous studies, Stranski-Krastanov growth occurs. STM images show rows of Pb islands. A critical mass of Pb is required before condensation of clusters into stacks occurs at room temperature, while at 200C the Pb immediately formed distinguishable stacked islands. After the completion of the first monolayer, LEEM observations of this system show the development of 3D Pb crystallites, with the island density depending strongly on temperature. For Pb deposition at a substrate temperature of 200C, the islands grow together and form larger islands with quasi-hexagonal sides. Annealing the Pb crystallites causes them to merge and reshape while maintaining long-range order. The crystallites melt at the usual Pb melting temperature of 323C, resulting in small, round, disordered islands. Upon recooling, the islands develop a hexagonal shape. They desorb from the surface at approximately 425C. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N42.00007: STM investigation of quantum size effect on adsorption and reactivity of different gases and alkali metals on thin Pb films Alexander Khajetoorians, Shengyong Qin, Murat Ozer, Chih-Kang Shih Recent work has shown that the Quantum Size Effect (QSE) plays a critical role in the catalytic behavior in reactivity. More specifically, the presence of quantum well states in thin metal systems can have profound effects on surface reactivity. Epitaxial thin Pb films on Si(111) are well known to exhibit pronounced QSE manifested by the phase matching of the Fermi wavelength and the layer thickness, giving rise to bilayer oscillation as well as a re-entrant quantum beats of longer periodicity. Such quantum oscillation phenomena have been observed in preferred film thickness, the location of quantum well states, as well as superconductivity. This work focuses on studies of adsorption and surface reactivity of different gases (hydrogen, oxygen and carbon monoxide) and alkali metal on thin Pb films grown on Si(111) surface. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N42.00008: Automated Tracking of Nanometer-Scale Feature Evolution Using an STM Russell Lake, Adam Dean, Niru Maheswaranathan, Chad Sosolik Time-resolved measurements of vacancy pits and adatom islands on monatomic metallic surfaces (e.g. Ag(111) [1]) have provided valuable insight into the underlying atomic diffusion processes that drive dynamics at nanometer length scales. Utilizing our variable temperature scanning tunneling microscope or STM, we are extending this probing method to more complex systems, such as the AuCu and NiAl alloys. To increase the rate of successful data acquisition for these measurements, we have developed automated tracking routines that allow for the continuous monitoring of evolving surface features with minimal operator involvement. Post-acquisition image analysis is further enhanced utilizing feature detection algorithms. Current proof-of-concept results spanning several hours of acquisition time on single crystal metal surfaces are presented. [1] K. Morgenstern et al., Phys. Rev. B 63, 045412 (2001). [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N42.00009: Chemical Identification in the Cu$_3$Au (100) Surface Using STM and DFT Rodrigo B. Capaz, Luis G. Dias, Alexandre A. Leit\~ao, Ralf-Peter Blum, Horst Niehus, Carlos A. Achete We describe the structure, energetics and electronic structure of the Cu$_3$Au (100) surface using a combination of scanning tunneling microscopy (STM) and first-principles calculations based on density functional theory (DFT). Our calculations show that the CuAu termination is the one with lower surface energy, in agreement with experiments. The well-known surface atomic rippling is also well reproduced by the calculations. Atomically-resolved STM images show an interesting voltage dependence, showing both types of atoms in the surface unit cell for lower voltages but just one type for higher voltages. Comparisons with theoretically-simulated STM images and cross- sectional electronic density profiles allows for an unambiguous assignment of Au atoms as the one appearing in higher voltage images, thus providing chemical identification at the surface. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N42.00010: STM characterization of a graphitized SiC(0001)surface Victor Brar, Yuanbo Zhang, Yossi Yayon, Taisuke Ohta, Jessica McChesney, Eli Rotenberg, Mike Crommie The two-dimensional electron gas in a single graphene sheet exhibits unique properties due the cone-shaped electron band structure near the Fermi energy. Recently the growth of a single layer of graphene on SiC(0001) has been demonstrated, opening new possibilities for fabricating large scale graphene-based devices. We have performed scanning tunneling microscopy and spectroscopy of single and bi-layer graphene films on SiC(0001). Atomically resolved topographs and dI/dV maps show clear differences between the single and bi-layer surfaces at different length scales. We have characterized the energy dependence and spatial distribution of the electron local density of states in these single and bi-layer films. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N42.00011: Calibrating the Bending of Molecule Adsorbed Nanoscale Si Cantilevers with a Modified Stoney Formula Ji Zang, Feng Liu Fundamental understanding of mechanical bending of molecule adsorbed nanoscale thin films is of both scientific and technological importance. Our current understanding, however, is limited within macroscopic analysis that neglects the atomic details of film structure and surface effects. Here, we report atomistic simulation and theoretical analysis of bending of freestanding nanometer-thick silicon (Si) films induced by adsorption of hydrogen and acetylene molecules. It reveals the dominant role of atomic surface structure and surface stress in governing their bending behavior. We show that the bending curvature of molecule adsorbed Si nanofilm does not follow the classical Stoney formula, and we develop a modified Stoney formula by taking into account of the effects arising from atomic surface reconstruction and surface stress. Our findings suggest that re-calibration has to be made in detecting trace amount of molecules by nanoscale Si mechanochemical sensors. [Preview Abstract] |
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