Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W12: Molecules on Surfaces |
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Sponsoring Units: DCMP DMP Chair: Michael Altman, Hong Kong University of Science and Technology Room: Baltimore Convention Center 304 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W12.00001: Influence of interfacial structure on the charge transfer between adsorbed C$_{60}$ and Cu(111) Woei Wu Pai, J.H. Lin, A.D. Zhao, X.Q. Zhang, X.D. Xiao C$_{60}$ adsorption on metal surfaces typically incurs different amount of charge transfer from the substrate to the molecules. The charge transfer amount has never been found to approach that of optimally doped fulleride, e.g., K$_{3}$C$_{60}$, in which three electrons occupy the C$_{60}$ LUMO states. Here we demonstrate that C$_{60}$ adsorbed on Cu(111) render a nearly optimally doped fullerene film. The critical factor to produce such strong charge transfer is the identification of interfacial reconstruction in which a C$_{60}$ most probably resides in a monolayer pit consisting of seven removed Cu atoms. A direct comparison of low temperature (77 K, 4 K) STS on C$_{60}$ regions with and without interfacial reconstruction reveals drastic differences in charge transfer amount. Our STM/STS results are also consistent with a recent photoemission study [1] showing the optimal doping characteristic of the same system. This study thus demonstrates the importance of interfacial structure, which is often based on assumption, on prominent properties of molecular thin films. \newline [1] C. M. Cheng, K. D. Tsuei, unpublished. [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W12.00002: Coverage dependent supramolecular structures: 2D phases of C$_{60}$:ACA monolayers on Ag(111) Bo Xu, Chenggang Tao, Ellen Williams, Janice Reutt-Robey The dependence of supramolecular structures on fractional molecular coverage in a 2-component adlayer has been investigated using scanning tunneling microscopy. A series of acridine-9-carboxylic acid (ACA) surface structures emerges sequentially when deposited on Ag(111) at room temperature. At low molecular coverage ($\theta <0.4ML)$, ACA forms a two-dimensional gas phase. Ordered ACA structures appear with increased coverage: firstly a chain structure composed of ACA molecules linked by O--H$\cdot \cdot \cdot $N hydrogen bonds ($\theta >0.4ML)$, then a dimer structure composed of ACA dimers linked by carboxyl-carboxyl hydrogen bonds ($\theta \sim 1.0ML)$. The structures of the C$_{60}$:ACA binary system depend on the coverage of pre-deposited ACA. When the initial ACA coverage is between 0.4 ML and 0.8 ML, subsequent C$_{60}$ deposition results in a hexagonal cooperative structure with C$_{60}$ period nearly three times as large as the normal C$_{60}$ 2-D packing of 1 nm, and exists in enantiopure domains. A C$_{60}$ quasi-chain structure is formed when the initial ACA coverage is above 0.8 ML. Parallel C$_{60}$ chains are separated in space by the ACA dimer structure. Chemically reasonable molecular packing model are presented based on the observed STM images. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W12.00003: Step fluctuations on Ag(111) surfaces with C$_{60 }$ Chenggang Tao, Tim Stasevich, Ted Einstein, Ellen Williams STM has been used to characterize fluctuation properties of segments of step edges partly covered by C$_{60}$ on Ag(111) at room temperature. The distribution of C$_{60}$ at step edges exhibits a step orientation dependence: low-symmetry step edges are more favorable for C$_{60}$ binding. The temporal correlation functions of step segments between C$_{60}$-covered step regions scale as a power law, with an average exponent of 0.23$\pm $0.02, indicating that fluctuations of these ``confined'' steps are consistent with step-edge diffusion limited fluctuations. Parameters extracted from temporal correlation and autocorrelation analysis consistently indicate that close-packed steps have smaller fluctuation magnitude and higher step mobility than low-symmetry steps. The measured system sizes of step segments with different lengths show at most a weak step-length dependence. Fluctuation features thus yield the surprising conclusion that C$_{60}$ molecules are not acting as pinning points that constrain mass transport along the step edges. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W12.00004: Self-intermixed patterns of perylene derivatives Meike Stoehr, Markus Wahl, Tomas Samuely, Thomas A. Jung, Lutz H. Gade Self-assembled systems are in the focus of nanotechnology research because of their potential use in the ``bottom-up'' creation of functional supramolecular structures. Potential applications of such systems include several functional groups. Therefore, the intermixing of different molecular compounds will become a key issue. In our approach we made use of H-bonding to form well-ordered intermixed patterns of two different perylene derivatives - PTCDA and DPDI. In an UHV-setup thin films of DPDI and PTCDA were prepared by evaporation on Ag(111). The sample characterization was carried out with a homebuilt STM. For a ratio of 1:1 and a total coverage of about one monolayer, an ordered intermixed pattern was observed. Each PTCDA molecule is interacting via H-bonding with four neighbouring DPDI molecules and vice versa. Furthermore, different intermixed patterns were observed depending on the ratio of the molecules and on the total coverage. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W12.00005: Directed self-assembly of virus particles at nanoscale chemical templates Sung-Wook Chung, Chin Li Cheung, Anju Chatterji, Tianwei Lin, Jack Johnson, Jim De Yoreo Because viruses can be site-specifically engineered to present catalytic, electronic, and optical moieties, they are attractive as building blocks for hierarchical nanostructures. We report results using scanned probe nanolithography to direct virus organization into 1D and 2D patterns and \textit{in situ} AFM investigations of organization dynamics as pattern geometry, inter-viral potential, virus flux, and virus-pattern interaction are varied. Cowpea Mosaic Virus was modified to present surface sites with histidine (His) or cysteine (Cys) groups. Flat gold substrates were patterned with 10-100nm features of alkyl thiols terminated by Ni-NTA or meleimide groups to reversibly and irreversibly bind to the Hys and Cys groups, respectively. We show how assembly kinetics, degree of ordering and cluster-size distribution at these templates depend on the control parameters and present a physical picture of virus assembly at templates that incorporates growth dynamics of small-molecule epitaxial systems and condensation dynamics of colloidal systems. This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W12.00006: Enhanced Chemisorption of Cu(hfac)$_{2}$ on Parylene Surface by N$_{2}$ Plasma Treatment S. Pimanpang, P.-I Wang, D.-X. Ye, J. S. Juneja, G.-C. Wang, T.-M. Lu The metallization of polymers has been intensively studied due to its wide industrial applications. We report a study of interfacial interaction of metalorganic Cu(hfac)$_{2}$ with the Parylene surface. Parylene is a low k dielectric polymer prepared by a chemical vapor deposition technique. The as-deposited Parylene surface is shown to be hydrophobic with a measured water droplet contact angle $\sim $72\r{ }. However, after the N$_{2}$ plasma treatment, the water droplet contact angle decreases to $\sim $40\r{ } due to the formation of oxygen and nitrogen functional groups on the surface, as observed by x-ray photoelectron spectroscopy (XPS). These functional groups improve Cu(hfac)$_{2}$ chemisorption on the plasma treated Parylene surface. Further studies by XPS show that chemisorption of Cu(hfac)$_{2}$ is self-limiting up to 20 sec of Cu(hfac)$_{2}$ precursor exposure time. The enhancement of chemisorption of metalorganic precursors on the polymer surface is an important step for chemical vapor deposition or atomic layer deposition of metal. $^{a }$Supported by Thai govt. fellowship (SP) and SRC (JSJ). [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W12.00007: Surface Dynamics in thin films of a small organic glass-former Stephen Swallen, Marie Mapes, Kenneth Kearns, Mark Ediger, Sushil Satija Enhanced dynamics of molecules near the free surface of thin supported films have been measured using neutron reflectivity. Thin films of the small molecule glass former tris-naphthylbenzene were vapor deposited with subnanometer initial surface roughness, allowing diffusion rates to be measured between isotopically labeled layers. Measured dynamics suggest a mobile surface layer of about 4 nm with a significant decrease in Tg. This can be compared with recent measurements in polymeric systems which find thick active layers and Tg shifts up to 30 K. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W12.00008: Topography and Wetting of Dotriacontane Films on Graphite Surfaces Matthew Simpson, Mengjun Bai, Klaus Knorr, Haiding Mo, Haskell Taub, Steven Ehrlich, Ulrich Volkmann, Flemming Hansen We have used Atomic Force Microscopy (AFM) in the noncontact mode and synchrotron x-ray diffraction to investigate the structure, morphology, and wetting of dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films deposited from a heptane solution onto highly-oriented pyrolytic graphite (HOPG). Consistent with previous neutron diffraction measurements,$^{2}$ the x-ray patterns indicate one to two layers immediately adjacent to the HOPG surface in which the molecules are oriented with their long axis parallel to the interface. Above these parallel layers, the AFM images show a partial layer of C32 molecules oriented with their long axis perpendicular to the surface. Upon heating above room temperature, we observe the area occupied by the perpendicular monolayer first to increase and then to decrease. Just above the bulk melting point, the perpendicular monolayer dewets the underlying parallel layers as we have found for C32 films adsorbed on a SiO$_{2 }$substrate. $^{2}$K. W. Herwig, B. Matthies, and H. Taub, Phys. Rev. Lett. \textbf{75}, 3154 (1995). [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W12.00009: Ellipsometric Measurements of Dotriacontane Films Adsorbed on Au(111) Surfaces P. Soza, V. del Campo, E. Cisternas, M. Pino, U.G. Volkmann, H. Taub, F.Y. Hansen We have conducted ellipsometric and stray light intensity measurements on dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films adsorbed on Au(111) substrates in air as a function of temperature in order to determine their optical thickness and surface roughness. The C32 films were deposited from a heptane ($n$-C$_{7}$H$_{16})$ solution onto the gold surface. Our large, atomically flat gold substrates were produced by the method reported by Hegner \textit{et al}.$^{2}$ in which gold films grown on mica are glued onto Si(100) wafers. For films of 25 {\AA} thickness, our ellipsometry measurements show a decrease of about 75{\%} in the height of the monolayer substep compared to the same film adsorbed on SiO$_{2 }$substrates.$^{3}$ This substep is believed to be contributed by a monolayer phase in which the molecules are oriented with their long axis perpendicular to the surface. The substep decrease may be interpreted as reduction in the number of molecules in this phase or possibly a tilting of the molecules. $^{2 }$M. Hegner \textit{et al.}, Surf. Sci. \textbf{291}, 39 (1993). $^{3}$U.G. Volkmann \textit{et al}., J. Chem. Phys. \textbf{116}, 2107 (2002). [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W12.00010: Comparison of Thickness and Morphology of Dotriacontane Films on SiO$_{2}$/Si Surfaces Vapor-deposited in High Vacuum with those Deposited from Solution E. Cisternas, P. Soza, V. del Campo, E. Ramirez, U.G. Volkmann, H. Taub, F.Y. Hansen We have used Very High Resolution Ellipsometry (VHRE) and Atomic Force Microscopy in the noncontact mode to compare the thickness and morphology of dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films deposited by two different methods on Si(100) wafers coated with their native oxide. During deposition, the substrate temperature was held below the bulk melting point of C32. As monitored by VHRE, the film thickness of different samples was found to be in the range 20 {\AA} to 400 {\AA}. Films deposited by physical vapor deposition from a Knudsen cell in high vacuum are optically smooth and homogeneous, while deposition by dip-coating from a heptane solution also results in optically smooth but less homogeneous layers. Heating/cooling cycles were performed on these two sample types while conducting VHRE and stray light intensity measurements in order to compare the wetting behavior and surface roughness of C32 as a function of film thickness on both hydrophilic and hydrophobic SiO$_{2}$/Si surfaces. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W12.00011: Controlling growth kinetics and morphology of crystal surfaces through biomolecular interactions Jim De Yoreo, Roger Qiu, Selim Elhadj, Patricia Dove, Germaine Fu, Daniel Morse, Alan Salter, Andrzej Wierzbicki The complex shapes and hierarchical designs of biomineralized nanostructures arise from biomolecular controls over crystallization. One prevailing view is that mineral-associated macromolecules are responsible for nucleating and stabilizing non-equilibrium polymorphs and morphologies through interactions at crystal surfaces. Here we report results of \textit{in situ} AFM and molecular modeling investigations of calcite growth in the presence of acidic amino acids, polypeptides, and proteins associated with biomineral formation. We show how the sterochemical relationship between modifier and crystal lattice lead to step-specific interactions and how those interactions account for the changes in kinetics and morphology. We analyze the results in terms of classic physical models of crystal growth and epitaxy and show that there are important deviations from those classic models that stem, in part, from the low kink density of steps on calcite. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W12.00012: Mixing in a Solid Monolayer: SF$_{6}$ + C$_{2}$F$_{6}$ on Graphite G. B. Hess, B. L. Ellis, Yize Li We have studied mixing of SF$_{6}$ and C$_{2}$F$_{6}$ in a solid monolayer film on graphite by admitting one gas to the cell to a pressure between monolayer condensation and saturation, then slowly admitting the other gas. The total coverage is monitored by ellipsometry and the condition of the individual adsorbates is determined by infrared absorption spectra. In particular, shifts in absorption lines due to dynamic dipole coupling indicate the local environment of molecules of each species. At 75 and 80 K there is no detectable solubility of C$_{2}$F$_{6 }$in a dense SF$_{6}$ monolayer, then a layer of nearly pure C$_{2}$F$_{6 }$condenses on top. At 86 K there is limited solubility of C$_{2}$F$_{6}$ in dense SF$_{6}$, followed by appearance of a second C$_{2}$F$_{6}$--rich phase in the monolayer. At 90 K and above, C$_{2}$F$_{6}$ appears to replace SF$_{6}$ by continuous substitution; thus there appears to be a 2-D solid consolute critical point near 90 K. If SF$_{6}$ is added to a higher-temperature, low-density C$_{2}$F$_{6}$ layer, the C$_{2}$F$_{6}$ molecules are rapidly compressed from flat or tilted orientation to axes perpendicular to the substrate, then are continuously diluted. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W12.00013: Energetic Study of the Flip-flop Motion of P(VDF-TrFE) Xuewen Wang, Lei Cai, Jiandi Zhang, P. A. Dowben Ferroelectric copolymer P(VDF-TrFE) has attracted significant attention in material science and technology due to its excellent electromechanical properties and easy manipulation of the individual monomer or monomer clusters resulting in the change of the such properties. Theoretical aspect of the switching dynamics of P(VDF-TrFE) has been investigated by using the density function theory and compared with experimental results. Through calculation, we find a simple flipping of the individual monomer with lower energy that involves no change of the bond length and the bond angle of --C--C--, --C--H--, and \mbox{--C--F--.} These consist with the experimental results obtained with STM. We compare the STM image before and after the flipping of the monomers. Except the observation of the lattice shift at the boundary, we find no other structure distortion and no change in the inter- and intra- chain spacing. [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W12.00014: Exploiting Photo-induced Reactions in Polymeric Thin Films to Create Hierarchically Ordered, Defect-free Materials Rui Travasso, Olga Kuksenok, Anna Balazs Computer simulations reveal how photo-induced chemical reactions in polymeric thin films can be exploited to create long-range order in materials whose features range from the sub-micron to the nanoscale. The process is initiated by shining a spatially uniform light on a 2D photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and phase separation. When a well-collimated, higher intensity light is rastered over the sample, the system forms defect-free, spatially periodic structures, which resemble the phases of microphase-separated diblock copolymers. We then add a non-reactive homopolymer C, which is immiscible with both A and B. This component localizes in regions that are irradiated with a higher intensity light and one can effectively write a pattern of C onto the AB film. Rastering over the ternary blend with the collimated light now leads to hierarchically ordered patterns of A, B and C. The findings point to a facile, non-intrusive process for manufacturing high quality polymeric devices in a low-cost, efficient manner. [Preview Abstract] |
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