Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q32: Focus Session: Self Assembly of Molecules on Surfaces |
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Sponsoring Units: DCMP Chair: Janice Reutt-Robey, University of Maryland Room: E142 |
Wednesday, March 17, 2010 11:15AM - 11:27AM |
Q32.00001: Organization of adenine on Ag(111) and correlated interfacial electronic structure measured with low temperature scanning tunneling microscopy Thomas P. Pearl, Katie M. Andrews, Bryce F. Davis Low temperature scanning tunneling microscopy and spectroscopy has been used to observe the organization of the nucleobase adenine on the Ag(111) surface as well as to resolve modifications to Ag(111) surface electronic structure. Multiple hydrogen bonding interactions between adenine adsorbates dictate the formation of dimers on the surface as well as long range order of molecular domains, which have limited commensuration with the Ag(111) lattice. Differential conductance spectroscopy recorded at 15 K reveals an upward energetic shift of the Shockley-type surface state native to Ag(111) from a band edge of -67 meV on the clean surface to+82.5 meV recorded over adenine islands. Differential conductance maps show free-electron like scattering in the adenine domains. Dispersion of the parallel wave vector of scattered electrons in the adenine domains is compared to the dispersion for electron scattering in bare silver and the ratio of effective masses for electrons in those bands is 1.1 $\pm $ 0.05. It is hypothesized that this shift occurs due to a combination of effects brought on by the adsorption of adenine including changes in work function and dipole-induced screening of the first image potential. [Preview Abstract] |
Wednesday, March 17, 2010 11:27AM - 11:39AM |
Q32.00002: Self-assembly of non-metallated tetraphenyl porphyrin molecules on noble metal substrates Geoffrey Rojas, Xumin Chen, Jie Xiao, Peter Dowben, Yi Gao, Xiao Cheng Zeng, Axel Enders Studies of non-metallated meso-tetraphenyl porphyrin (TPP) molecules adsorbed on noble metal substrates of Cu(111) and Ag(111) will be presented. STM observations have been modeled using density functional theory (GGA-DFT). The STM results are consistent with the electronic structure obtained from combined photoelectron and inverse photoemission spectroscopy that confirm substrate dependent surface interactions with H2TPP. Self-organization of H2TPP into ordered 2D networks as a result of inter-molecular bonds is found only on Ag(111), as the molecule-substrate interaction allows for molecule diffusion. It was found that those TPP adsorbed on Cu(111) interacted strongly with the substrate, resulting in charge transfer and repulsive interaction between adsorbed molecules and no ordering on the surface. Rather, on Cu(111), the molecule-substrate interaction is much larger than the weaker inter-molecule binding energies. For Ag(111) the strongest interactions between the adsorbed molecules and the substrate appear to occur at substrate step edges. [Preview Abstract] |
Wednesday, March 17, 2010 11:39AM - 11:51AM |
Q32.00003: A one-dimensional ice structure built from pentagons Javier Carrasco, Angelos Michaelides Heterogeneous nucleation of water plays a key role in fields as diverse as atmospheric chemistry, astrophysics, and biology. Ice nucleation on metal surfaces offers an opportunity to watch this process unfold, providing a molecular-scale description at a well-defined, planar interface. We discuss a density-functional theory study on a metal surface specifically designed to understand such phenomena. Together with our colleges at the University of Liverpool, we found that the nanometer wide water-ice chains experimentally observed to nucleate and grow on Cu(110) are built from a face sharing arrangement of water pentagons [1]. The novel one-dimensional pentagon structure maximizes the water-metal bonding whilst simultaneously maintaining a strong hydrogen bonding network. These results reveal an unanticipated structural adaptability of water-ice films, demonstrating that the presence of the substrate can be sufficient to favor non-conventional structural units. \\[4pt] [1] J. Carrasco {\it et al.}, Nature Mater. {\bf 8}, 427 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:03PM |
Q32.00004: Two-dimensional Supramolecular Structures by Hydrogen and Halogen Interactions Jong Keon Yoon, Howon Kim, Jeong Huem Jeon, Se-Jong Kahng Supramolecualr ordering has been actively studied due to it's possible applications to the fabrication processes of nano-electronic devices. Van der Waals interaction and hydrogen bonding are frequently studied mechanisms for various molecular structures based on non-uniform charge distributions. Halogen atoms in molecules can have electrostatic interactions with similar strength. Big halogen atoms have strong non-uniform charge distributions. To study molecular orderings formed by hydrogen and halogen interactions, we chose a molecular system containing oxygen, hydrogen, and bromine atoms, a bromo-quinone. A two-dimensional molecular network was studied on Au(111) using a low-temperature scanning tunneling microscope. Bromo-quinone molecules form self-assembled square grids having windmill structures. Their molecular orderings, chiral structures, and defects are explained in terms of hydrogen and halogen interactions. [Preview Abstract] |
Wednesday, March 17, 2010 12:03PM - 12:15PM |
Q32.00005: Benzene on Cu(111): I. Application of van der Waals-Density Functional Formalism to Determine Binding Sites and Energy Contour Map Kristian Berland, T.L. Einstein, Per Hyldgaard With a recently developed van der Waals density functional (vdW-DF)\footnote{M. Dion et al., Phys. Rev. Lett. 92 (2004) 246401} we study the adsorption of benzene on Cu(111).\footnote{KB, TLE, and PH, Phys. Rev. B 80 (2009) 155431} The vdW-DF inclusion of nonlocal correlations changes the relative stability of 8 high-symmetry binding-position options and increases the adsorption energy by over an order of magnitude, achieving good agreement with experiment. The metallic surface state survives benzene adsorption. From a contour plot of the potential energy, we find that benzene can move almost freely along a honeycomb web of ``corridors" linking fcc and hcp sites via bridge sites, consistent with the low diffusion barrier in experiment. [Preview Abstract] |
Wednesday, March 17, 2010 12:15PM - 12:27PM |
Q32.00006: Benzene on Cu(111): II. Molecular assembly due to Lateral van der Waals and Surface-State-Mediated Indirect Interactions Per Hyldgaard, Kristian Berland, T.L. Einstein Experiments show that benzene condenses into two different structural phases: a compact and a sparse phase, both of approximately hexagonal symmetry. The vdW-DF calculations demonstrate that the denser benzene-overlayer phase, with lattice constant 6.74 \AA, is due to direct benzene-benzene vdW attraction. The structure of the second, sparser phase, with lattice spacing 10.24 \AA, is attributed to the indirect electronic interactions mediated by the well-known metallic surface state on Cu(111). To support this claim, we use a formal Harris-functional approach to evaluate nonperturbatively the asymptotic form of this indirect interaction. Our extended vdW-DF scheme---which combines calculations of molecular physisorption, of direct intermolecular vdW coupling, and of indirect electronic interactions between the molecular adsorbates---accounts well for the structural phases of benzene on Cu(111). Our preliminary vdW-DF study of acene and quinone interactions provides building blocks for modeling of anthraquinone assembly on Cu(111).\footnote{G. Pawin, $\ldots$, L. Bartels, Science 313 (2006) 961} [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q32.00007: Monte Carlo Study of the Diffusion of CO Molecules inside Anthraquinone Hexagons on Cu(111) Kwangmoo Kim, T.L. Einstein, Jon Wyrick, Ludwig Bartels Using Monte Carlo calculations of the two-di\-men\-sion\-al (2D) lattice gas model, we study the diffusion of CO molecules inside anthraquinone (AQ) hexagons on a Cu(111) plane. We use experimentally-derived CO-CO interactions\footnote{K.L. Wong, $\dots$, L. Bartels, J.\ Chem.Phys.{\bf 123}, 201102 (2005)} and the analytic expression for the long-range surface-state- mediated interactions\footnote{K. Berland, TLE, and P. Hyldgaard, Phys.Rev.\ B {\bf 80}, 155431 (2009)} to describe the CO-AQ interactions. We assume that the CO-CO interactions are not affected by the presence of AQ's and that the CO-AQ interactions can be controlled by varying the intra-surface-state (ISS) reflectance $r$ and the ISS phase shift $\delta$ of the indirect-electronic adsorbate-pair interactions. Comparing our results with experimental observations, we find that not only pair but also surface-state-mediated trio interactions\footnote{P. Hyldgaard and T.L. Einstein, EPL {\bf 59}, 265 (2002)} are needed to understand the data. [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q32.00008: Tunneling Spectroscopy of Unoccupied Electronic States of Benzoate on Cu(110) Alex Pronschinske, Daniel Dougherty Benzoic acid adsorbed on Cu(110) self assembles into a variety of structurally robust and highly ordered superlattices [1]. Some of these are known to involve Cu-adatom-mediated bonding in manner similar to that proposed for the well-known thiolate SAM's [2]. We have employed scanning tunneling spectroscopy at room temperature in both constant current mode and constant height mode in order to observe the unoccupied electronic structure of benzoate adsorption structures. Electronic states are observed that are assigned as $\pi $* orbitals and image potential derived states. The energy of the $\pi $* state corresponds remarkably well with published ground state DFT calculations [3] and image potential state energies suggest substantial adsorption-induced work function changes. [1]Frederick et al., Surf. Rev. Lett. 3, 1523 (1996). [2]Maksymovych et al., Phys. Rev. Lett. 97, 046804 (2006). [3]Lennartz et al., Langmuir 25, 856 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q32.00009: Density functional theory study of the energetics of SH molecules on Au (111) and the threshold coverage for surface reconstruction lifting Yongduo Liu, Vidvuds Ozolins The structure of self-assembled monolayers of SH molecules on Au (111) has been studied within both low and high coverage regimes. A dumbbell-like structure is favorable in energy over any other possible surface morphology in the low coverage regime. At full coverage, a structure is proposed that is similar to that of the c (4x2) structure, but at twice the size. In addition, we also investigate the adsorbent-mediated Au herringbone reconstruction lifting. Results show that the threshold coverage for the lifting is 0.47 monolayer, with the mechanism being attributed to the interactions between the intrinsic dipole moments of the surface and SH molecules. [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q32.00010: Adsorption and diffusion of sexithiophene on Ag(110) Jeronimo Matos, Abdelkader Kara We use density functional theory (DFT-PBE-PAW) to calculate the absorption of sexithiophene (6T) molecules on Ag (110). Our results show that these molecules absorb 3 {\AA} above the surface with a binding energy of about 0.64 eV/molecule. The internal structure of the molecule as well as the substrate atomic positions was not altered by the absorption. The energy barrier for diffusion of a 6T molecule along the channel was found to be 66 meV. Analysis of the densities of state shows little modification of the electronic structure of the substrate. Without inclusion of Van der Waals interactions, our results point to a weak chemisorption or a strong physisorption of 6T on Ag (110). [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q32.00011: Self-assembly of photooxidatively resistant pentacene derivatives on gold surfaces Jun Wang, Irvinder Kaur, Bogdan Diaconescu, Jian-Ming Tang, Glen P. Miller, Karsten Pohl Novel pentacene derivatives that show excellent resistance to photooxidation have been prepared and show potential as semiconductors in active layers of organic thin film electronic devices. The design and fabrication of more efficient organic thin film devices require us to develop an essential understanding of the growth processes of these molecules on various substrates and how the intermolecular and molecule-substrate interactions reflect their final structure formation. Self-assembled monolayers (SAMs) are promising molecular structures with long-term stability in these devices. Here we present a combined experimental and theoretical study by STM and ab-initio calculations of the self-assembly of a photooxidatively resistant pentacene derivative --- 6,13-dichloropentacene (DCP) --- on gold surfaces. On the Au(111) surface, DCP forms self-assembled domains with various high symmetry orientations. The uniformity of the SAM improves greatly when the DCP molecules are deposited on the Au(788) vicinal surface where the presence of parallel atomic steps select only one of the possible SAM orientations due to the molecule-step interaction. Thus we observe the formation of large DCP SAM structures with perfect single domain orientation. [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q32.00012: Scanning tunneling spectroscopy of organic zwitterions assembled on Si(111)-7$\times$7 M. Silveira Rodrigues, D. Bogdan, C.A. Dutu, B. Hackens, S. Melinte, M. El Garah, E. Duverger, F. Palmino, F. Cherioux We report on the molecular assembly and local electronic properties of organic zwitterions deposited on the Si(111)-7$\times$7 surface. We analyze these molecular systems by scanning tunneling microscopy and spectroscopy down to low temperatures. The molecules adsorb at specific sites on the Si(111)-7$\times$7 surface, forming star-shaped configurations composed of three zwitterions filling up a half unit cell [1]. The regioselectivity of the process is determined by electrostatic interactions between the substrate and the ionic species, resulting in mirrored configurations in neighboring half unit cells. Thus, we probe chiral assemblies of achiral molecules on Si(111)-7$\times$7 by spatially-resolved tunneling spectroscopy. For all investigated systems, ab initio simulations of the relaxed structures and local density of states are compared to experimental data. [1] Y. Makoudi et al., Surf. Sci. 602, 2719 (2008). [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q32.00013: A molecular view of TiOPc -- C$_{70 }$interface formation Yinying Wei, Steven Robey, Janice Reutt-Robey A key strategy for the improvement of organic electronic devices involves the optimization of chemical morphology for efficient charge separation. Fundamental studies of chemical morphology - electronic property relations, particularly along crucial domain boundaries, are needed to realize these goals. We present STM/STS studies of TiOPc: C$_{70}$ films, prepared by vapor deposition on Ag (111). Sequential deposition of C$_{70}$ onto the ordered honeycomb TiOPc phase generates extended domains of a co-crystalline C$_{70(1)}$TiOPc$_{(2) }$monolayer phase, characterized as a hexagonal network (2.1 nm nearest-neighbor C$_{70}$ packing). Subsequent deposition of C$_{70}$ onto this network proceeds in a layer-by-layer growth process, generated molecularly abrupt interfaces. The transport gap of each interface is measured by STS, and correlated to the C$_{70}$ packing density. The orderly structure evolution of the C$_{70}$-TiOPc is contrasted to that of the previously reported C$_{60}$-TiOPc and the implications for electronic transport are discussed. [Preview Abstract] |
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