Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H12: Focus Session: Directed Organization of Molecular Semiconducting Films |
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Sponsoring Units: DMP DCMP Chair: Daniel Dougherty, North Carolina State University Room: 308 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H12.00001: Structural evolution of the self-assembled layers of funtionalized fullerenes on metal surfaces. Bogdan Diaconescu, Mikael Jazdzyk, Glen Miller, Karsten Pohl Self-assembled organic thin films have a great number of practical applications, ranging from sensors and biological interfaces in medical implants to organic electronics and photovoltaics. Self-assembled monolayers (SAMs) form as a result of a delicate balance between competing molecule-substrate and intermolecular interactions. To control such self-assembly processes, it is mandatory to understand how this balance reflects onto the SAM's final structure. Here, we present a STM study of the self-assembly of C60 functionalized with alkane chains of various lengths (F-C60) on Ag(111). We find that F-C60 molecules lay down on the Ag surface and form a zigzag like pattern with an oblique unit cell of size dependent on the alkyl chain lengths and two molecules per basis. The C60s are placed at a larger than van der Waals distance. The symmetry of the F-C60 SAM is dictated by the alkane-surface interaction while the size of the unit cell is a consequence of the intermolecular interactions. These results show that C60s can be assembled in 2D and non-compact molecular arrays with a surface density controllable via appropriate chemical functionalization. Funded by the NSF Center for High-rate Nanomanufacturing (NSF NSEC-425826). [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H12.00002: Controlled Chemical Morphology in TiOPc - C$_{60}$ Films Yinying Wei, Steve 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$_{60}$ films, prepared by vapor deposition on Ag (111). We show how growth conditions can be adjusted to harness anisotropic TiOPc - TiOPc interactions, leading to three dramatically distinct film structures: nanophase segregated TiOPc and C$_{60}$ domains, a co-crystalline TiOPc$_{ (2)}$ C$_{60 (1)}$ honeycomb network, and a quasi-periodic array of triangular TiOPc domains and C$_{60}$ nanoclusters. Electronic transport gaps measured by STS (Z (V)) prove to be sensitive to the phase, varying by up to about 0.5 eV along domain boundaries. We propose structural models for each hetero interface and discuss the physical origin of the observed transport characteristics. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H12.00003: Monte Carlo Study of the Honeycomb Structure of Anthraquinone Molecules on Cu(111) Kwangmoo Kim, T. L. Einstein, Ludwig Bartels Using Monte Carlo calculations of the two-di\-men\-sion\-al (2D) lattice gas model, we demonstrate a mechanism for the spontaneous formation of honeycomb structure of anthraquinone (AQ) molecules on a Cu(111) plane. Unlike the suggestion of long-range sub\-strate-me\-di\-ated repulsion,\footnote{Greg Pawin {\it et al.}, Science {\bf 313}, 961 (2006).} long-range attractions play important roles in our calculations. However, the interplay between attractions and repulsions is still integral to the spontaneous formation of AQ's honeycomb structure. We also compare the critical local coverage rate of AQ's where the honeycomb structure starts to form. Furthermore, we study the diffusion of CO molecules inside AQ honeycombs on the Cu(111) plane. The surface phase transitions of CO molecules between solid, liquid, and gas 2D phases are studied via the specific heat singularity in short-range correlation functions.\footnote{N. C. Bartelt, T. L. Einstein, and L. D. Roelofs, PRB {\bf 32}, 2993 (1985).} [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 9:12AM |
H12.00004: Patterned molecular nanostructures Invited Speaker: Surfaces and interfaces not only determine to a large extent the properties of small-scale materials due to their high surface-to-volume ratio, they are also an ideal platform for the design, fabrication and device integration of nanostructures. Both, top-down and bottom-up methods have been developed for the handling of matter at the molecular and atomic scale. In the present talk I will demonstrate how the remarkable progress in controlling atomic and molecular interactions at surfaces has provided the unique ability to engineer supramolecular architectures of well-defined size, shape, composition and functionality. Using noncovalent interactions as hydrogen bonding, ionic bonding and metal-ligand interactions, molecular building blocks can be rationally combined into desired functional architectures. The potential functionalities comprise molecular magnetism, novel heterogeneous catalysis, selective host-guest interactions and new concepts of nanoscale mechanics. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H12.00005: Reactions on surfaces for the creation of stable templates M. Matena, M. Wahl, M. Stoehr, T.A. Jung, T.-L. Lee, J. Zegenhagen, T. Riehm, L.H. Gade Molecular assemblies on surfaces can be used as templates that allow the study of host guest interactions and thus provide a starting point for the generation of complex hierarchic structures. An important prerequisite besides the regularity of such structures is their stability. We reported the formation of a molecular network generated by thermal dehydrogenation of a perylene derivative (DPDI) on a Cu(111) surface [1]. By thermal activation, these molecules become autocomplementary H-bond donors/acceptors and form a honeycomb structure. Besides utilizing this network for the incorporation of guest molecules [2], NIXSW (normal incidence x-ray standing wave) experiments were carried out to determine the height of DPDI above the substrate surface before and after the thermal activation. The formation of the network involves a lowering of the height difference between the molecular end groups and the perylene core what is required to enable H-bonding between the molecules. [1] M. St\"{o}hr et al., Angew. Chem. Int. Ed., 44 (2005) 7394; [2] M. St\"{o}hr et al., Small 3 (2007) 1336 [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H12.00006: Meandering C$_{60}$ Chains on Organic Film Substrates Wei Jin, Daniel Dougherty, Qiang Liu, William Cullen, John Weeks, Steven Robey, Janice Reutt-Robey Meandering chains of C$_{60}$ molecules are observed following vapor deposition of C$_{60}$ on a variety of organic molecular films including pentacene and zinc phthalocyanine,as well as previously reported a-sexithiophene. Such filamentous structures are in complete contrast to C$_{60}$'s typical close-packed growth habit, but are reminiscent of dipole fluids. We present STM images and a statistical analysis of chain structures observed for 0.2 - 0.9 mL C$_{60}$ on the square ZnPc overlayer $(a=13.6\mathop A\limits^\circ ,b=14.5\mathop A\limits^\circ ,\alpha =89^\circ )$on Ag (111). Large islands of meandering C$_{60}$ islands indicate $>$50 nm C60 diffusion lengths on this ZnPc film. The C$_{60}$ chain length, $\ell \sim $ 20 nm, depends weakly on coverage, reflecting the $\sim $constant C$_{60}$ density within these islands, and exhibits multiple branches. Chain structures are compared to the molecular dynamics predictions of a 2-d dipole fluid with the C$_{60}$ - C$_{60}$ interaction described by the Girifalco Potential (1 eV attraction) with additional dipole terms. A vertical moment of $\sim $0.8 D simulates filament formation. The structure of the ZnPc support, not included in this simulation, appears to be a secondary consideration in these chain structures. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H12.00007: Molecular dynamics study of polarized C60 on an organic surface Qiang Liu, Wei Jin, John D. Weeks, Janice Reutt-Robey The experimental results of vapor deposition of C60 on a variety of organic molecular films has showed unusual meandering chain structures. Here we use the Langevin molecular dynamics method to mimic the system. In the simulation, we considered interactions including the intermolecular Girifalco potential, dipole-dipole interactions, octupole interactions suggested by W. Losert$^{2}$ and substrate potentials. Choosing different parameters can give us different C60 patterns: the close-packed C60 islands, isolated C60 molecules and meandering chain islands. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:24AM |
H12.00008: Harnessing Surface Dislocation Networks for Molecular Self-Assembly Invited Speaker: The controlled fabrication of functional wafer-based nano-arrays is one of the ultimate quests in current nanotechnologies. Well-ordered misfit dislocation networks of ultrathin metal films are viable candidates for the growth of two- dimensional ordered cluster arrays in the nanometer regime. Such bottom-up processes can be very complex, involving collective effects from a large number of atoms. Unraveling the fundamental forces that drive these self-assembly processes requires detailed experimental information at the atomic level of large ensembles of hundreds to thousands of atoms. The combination of variable temperature measurements from our home-built STM correlated with 2D Frenkel-Kontorova models based on first-principle interaction parameters is used to explain how uniform arrays can form with the strain in the thin film as the driving force responsible for the surface self-assembly process. This process is generally applicable to assemble many molecular species thus opening avenues towards complex self-assembled structures based on a lock-and-key type approach. Moreover, when increasing the molecular coverage and/or decreasing the strain in the thin film the intermolecular interactions will eventually dominate the elastic effects and dictate the self-assembly process via molecular structure and functionality. We will show that controlling this delicate balance leads to a richness of structures, ranging from disperse ordered arrays of molecular clusters to patterned self-assembled monolayers (SAMs) of functionalized fullerenes and methanethiol. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H12.00009: Molecular and electronic structure of organic semiconductors on ultra-thin oxide films Brad Conrad, William Cullen, Ellen Williams We utilize scanning tunneling microscopy (STM) to molecularly image and probe the interactions of organic semiconductors. To mimic a device substrate and growth modes, ultra-thin oxide (UTO) films less than 1 nm thick are grown on Si(111) in ultrahigh vacuum at room temperature. These films are characterized by STM and display a long range RMS roughness of 0.109 nm versus a typical RMS roughness of 0.3 nm for thick SiO2. UTO films are then used as substrates for growth of pentacene, C60, and PCBM. Standing up pentacene is molecularly resolved and described by a thin-film phase unit cell with a=0.76nm and b=0.59nm in the ab-plane. The morphology and electronic structure of co-depositions of pentacene, C60, and PCBM are then deposited on UTO films and will be presented. http://arxiv.org/abs/0811.2515 [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H12.00010: Mapping Molecular States of a Co-Porphyrin Molecule on Au(111) surface Howon Kim, Won Jun Jang, Jong Keun Yoon, Se-Jong Kahng, Won-joon Son, Seungwu Han The biggest factor determining electronic transport property in molecular devices, is the character of molecular electronic states and their interaction with conducting electrodes. While there are tremendous efforts to manifest electronic devices with single molecules, the study of molecular orbitals and the molecule-metal interaction is comparatively small. In this talk, we will present our study of Co-porphyrin on Au(111) performed using low-temperature scanning tunneling microscope (STM). As the sample bias is changed, the STM images show strong spatial variations. With molecular orbital calculation and STM simulation, the bias-dependent images were well-reproduced. At +2V, the molecule shows four lobes, while at -2V, it exhibit two lobes. At intermediate biases, the molecule displays center-concentrated orbitals. Even though the molecules adsorb on Au(111) surface, the calculated orbitals from a free molecule are in good agreement with experimental data, implying marginal substrate effect. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H12.00011: Self-organized Pattern Formation in Dewetting of Elastically Confined Thin Polymer Layer Danish Faruqui, Ashutosh Sharma We report various stages of self-organized, sub-micron, surface directed patterns in a thin polystyrene (PS) layer (thickness $\sim $20nm) sandwiched between a silicon substrate and a cross-linked elastomeric layer (polydimethylsiloxane; PDMS, thickness $\sim $20nm). Morphological evolution of the self-organized surface patterns was recorded both on the elastic PDMS surface and on visco-elastic PDMS-PS interface. The instability patterns could be aligned by placing a micro-stamp (pitch 1500 nm) in conformal contact on the surface of the PDMS-PS bilayer.inducing anisotropic and regular surface pattern very similar to that on the master stamp. AFM scans of the top elastic PDMS surface and optical micrographs confirmed this contact-less transfer of master pattern, at both interfaces, uniformly over a larger area (cm$^{2})$. The anisotropic surface pattern thus formed on the PDMS and PDMS-PS interfaces was employed to explore the subsequent stages of self-organized sub micron structures. Subsequent stages of self organization in this system refers to formation of ordered assembly of sub-micron structures in initial pattern followed by their morphological and topographical evolution in terms of shape, size, separation and aerial density. [Preview Abstract] |
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