Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H25: Adsorption of Organics on Surfaces |
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Sponsoring Units: DPOLY Chair: Kookheon Char, Seoul National University Room: Morial Convention Center 217 |
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H25.00001: Polymer Physics Prize Break
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Tuesday, March 11, 2008 8:36AM - 8:48AM |
H25.00002: Examining the air-water interfacial activity of beta-peptides using molecular simulation and experiment. Clark A. Miller, Juan J. de Pablo Amphiphilic beta-peptides (oligomers of beta-amino acids) are predicted to adsorb at the air-water interface using computer simulation and verified using experiments. Molecular dynamics simulations are used to calculate the free energy of adsorption for different degrees of amphiphilicity and different display of hydrophilic groups. Adsorption of selected peptides is examined by measuring the surface tension of a solution of beta-peptides at different concentrations and agreement with simulation results is found. Simulations show that 2/3 hydrophobic residues lead to favorable adsorption at the air-water interface while 1/3 hydrophobic residues is unfavorable. We analyze changes in the conformational properties and angle with the interface to understand the manner of adsorption. We further investigate the behavior of multiple peptides at the interface using computer simulation and determine the surface pressure and peptide-peptide interactions at the interface. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H25.00003: Why are hyperactive ice-binding-proteins so active? Ido Braslavsky, Yeliz Celik, Natalya Pertaya, Young Eun Choi, Maya Bar, Peter L. Davies Ice binding proteins (IBPs), also called `antifreeze proteins' or `ice structuring proteins', are a class of proteins that protect organisms from freezing injury. These proteins have many applications in medicine and agriculture, and as a platform for future biotechnology applications. One of the interesting questions in this field focuses on the hyperactivity of some IBPs. Ice binding proteins can be classified in two groups: moderate ones that can depress the freezing point up to $\sim $1.0 $^{\circ}$C and hyperactive ones that can depress the freezing point several-fold further even at lower concentrations. It has been suggested that the hyperactivity of IBPs stem from the fact that they block growth out of specific ice surfaces, more specifically the basal planes of ice. Here we show experimental results based on fluorescence microscopy, highlighting the differences between moderate IBPs and hyperactive IBPs. These include direct evidence for basal plane affinity of hyperactive IBPs, the effects of IBPs on growth-melt behavior of ice and the dynamics of their interaction with ice. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H25.00004: Probing (bio)-organic monolayers at the metal/air and metal/liquid interface by sum-frequency generation spectroscopy Francesca Cecchet, Dan Lis, Yves Caudano, Christophe Silien, Alaa Adin Mani, Paul Thiry, Andr\'e Peremans In the present work, ordered monolayers of thiols prepared by self-assembly (SAM), and of lipids obtained by the Langmuir-Blodgett technique (LB), have been studied by sum frequency generation spectroscopy (SFG) at the metal/air and metal/liquid interface, in different sets of polarizations. This study is focused on the determination of the molecular orientation (i.e. the tilt angle and the twist angle of the axis and of the plane of the molecular groups, respectively) and on the analysis of the interactions occurring within the layers or with outer target molecules. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H25.00005: Flow Induced Growth of Striped Alkane Monolayers. M. Bai, H. Taub, A. Diama, K. Knorr, U. G. Volkmann, F. Y. Hansen We report our observation of the growth of striped monolayer phases of alkanes when deposited from a solution under flow. AFM measurements show that the structure and morphology of dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films grown from solution depend sensitively on the flow direction over a SiO$_{2}$-coated Si(100) substrate. The C32 film exhibits one or two layers adjacent to the SiO$_{2}$ surface in which the molecules are oriented with their long axis parallel to the interface followed by a striped monolayer of perpendicularly oriented molecules. The stripes form along the direction of solution flow with typical dimensions of a few micrometers wide and a few hundred micrometers long, depending on the solution concentration. A striped morphology is also observed for C29 grown under similar conditions. Grazing incident-angle x-ray diffraction measurements indicate that the C32 stripes are crystalline and can be indexed by a rectangular unit cell. We offer some speculations on the origin of the striped morphology. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H25.00006: Molecular dynamics studies of the structure and dynamics of ``perpendicular'' layers of \textit{n}-alkane molecules adsorbed on a solid substrate F.Y. Hansen, P. Soza, H. Taub, U.G. Volkmann Both AFM and ellipsometry studies of $n$-alkane films adsorbed on a solid substrate from a solution have shown interesting wetting and de-layering phenomena$^{2}$. It was found that on top of one or two ``parallel'' layers of molecules, where the long axis of the molecules is parallel to the surface, ``perpendicular'' layers of molecules are formed with the long axis of the molecules perpendicular to the surface. MD simulations of layers of tetracosane, $n$-C$_{24}$H$_{50,}$ molecules are set up to answer the following questions about the ``perpendicular'' layers: a) Is the melting transition driven by \textit{gauche} defect creations in the alkane chains like in the parallel layers?, b) can a rotator phase be identified?, c) is there a lateral translational mobility of the chains prior to melting?, and d) what is the mechanism driving the wetting and de-layering transitions in the films? $^{2}$H. Mo et al. Chem. Phys. Lett. \textbf{377}, 99 (2003) [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H25.00007: Structure and Phase Transitions of Vapor-Deposited C32 Films V. del Campo, E.A. Cisternas, I. Vergara, T. Corrales, U.G. Volkmann, M. Bai, S.-K. Wang, H. Taub, H. Mo, S.N. Ehrlich We have compared the structure, topography, and phase transitions of dotriacontane films ($n$-C$_{32}$H$_{66}$ or C32) that have been vapor-deposited onto a SiO$_{2}$-coated Si(100) wafer with those that have been deposited from solution. X-ray reflectivity measurements indicate that the as-deposited films differ in their morphology but share the following structural features at room temperature: adjacent to the substrate there is a nearly complete bilayer in which the molecules are oriented with their long axis parallel to the surface. Above the parallel film are partial layers of molecules oriented perpendicular to the surface. After a heating cycle above the bulk C32 melting point ($T_{b})$, AFM images of all films show the presence of 3D mesa-shaped bulk particles. On a second heating, AFM reveals the same succession of phase transitions for both film types in which a perpendicular monolayer spreads outward from the mesa-shaped particles below $T_{b}$ followed by a delayering transition to a 3D fluid droplets just above $T_{b}$.$^{2}$ $^{2}$M. Bai \textit{et al}., Europhys. Lett. \textbf{79}, 26003 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H25.00008: Thermodynamic Studies of n-Octane Thin Films Adsorbed on Magnesium Oxide(100) David Fernandez-Canoto, J.Z. Larese Thermodynamic properties of $n-$octane adsorbed on the MgO(100) surface were investigated using high-resolution adsorption isotherms in the temperature range of 225 K to 295 K. Two distinct adsorption steps were observed in all isotherms. The average area occupied by an $n$-octane molecule was estimated to be 139.1 {\AA}$^{2}$. The temperature variation of the two dimensional compressibility was used to identify phase transitions near 265.9 K and 271.4 K for the first and second layers, respectively. COMPASS force field has been used to calculate the minimum energy configuration of a single $n$-octane molecule sited on the MgO (100) facet. Calculations suggest that the most likely configuration for the adsorbed molecule is with the carbon backbone parallel to the (100) plane, and with the center of mass atop the Mg$^{2+}$ site. U.S. DOE, Materials Science Division under contract No. DE-AC05-00OR22725 with ORNL operated by UT-Battelle, LLC, and the NSF under grant DMR-0412231. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H25.00009: X-ray Atomic-Scale Analysis of Self-Assembled Monolayer Growth on Silicon J.C. Lin, J. Kellar, J. Kim, N. Yoder, K. Bevan, S. Datta, S. Nguyen, M. Hersam, M. Bedzyk Organic functionalization of silicon is of interest for applications ranging from biosensing to molecular electronics. The efficiency of molecular devices heavily depends on the ordering of the structure. Traditionally spectroscopy is used to characterize bonding, but often the overall structure can be ambiguous. Our strategy is to combine a compliment of techniques, including AFM, XPS, XRR(X-ray reflectivity), XSW(X-ray standing wave), XRF(X-ray fluorescence), and DFT(Density functional theory) to determine the atomic scale molecular configuration and packing density of Self-Assembled Monolayers (SAMs) grown on H-passivated Silicon. Our periodic DFT study of 4-bromo-phenyl-acetylene (BPA) predicts that the local packing density can affect the Br height by as much as 2 angstrom. XSW, which is used to measure the 3D Br distribution shows that the local structure is unchanged when the average SAM coverage is increased. This indicates the type of 2D island nucleation growth process being observed. Comparison between 4-bromostyrene (BrSty) and BPA SAMs provides direct evidence that the double bond root of the BPA contributes to a stiffer configuration than the single bond root. With the aromatic rings in the structure for conducting electrons, BrSty and BPA molecules are a starting point for future molecular electronic designs with more complex molecules. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H25.00010: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H25.00011: STM/S of Polydiacetylene Nanowires on Gold and Graphite Lili Wang, Rajiv Giridharagopal, Kevin Kelly The structural and electronic properties of 10,12--pentacosadiynoic acid (PCDA) monolayer films and polydiacetylene (PDA) nanowires on Au islands on a HOPG substrate have been studied using STM. Our results indicate that PCDA monolayer films can be formed on both HOPG and 1-2 monolayer (ML) Au islands, but arrange in small domains due to the dense Au clusters. The arrangement of PDA nanowires exhibits differences in density, length and height between Au and HOPG areas due to differing chemical and electronic interactions, which play an important role in the charge transfer between conducting polymers and electrodes in commercial devices. STM-tip induced nanowire cutting, desorption and polymerization is also observed, with the surrounding PCDA molecules restoring the packing nearly instantaneously. This implies that the interaction with Au clusters is not strong enough to weaken the intramolecular interactions that produce the reordering cascade effect, although it strongly influences the arrangement of the nanowires. Furthermore, the local work function and dI/dV images indicate electronic structure differences between PCDA monolayer films on 1 ML Au islands and those on 2 ML islands, and between PDA nanowires on HOPG and those across Au islands. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H25.00012: Thermodynamic and Neutron Scattering Investigation of Ethylene Wetting on MgO (100) Andi Barbour, Craig Brown, J. Z. Larese The adsorption properties of a molecular film on a solid substrate are governed by the relative strength of the molecule-substrate versus molecule-molecule interaction. The wetting properties of ethylene (C$_2$H$_4$) molecular thin films on graphite are of fundamental interest because the number of observed adlayers increases as the isothermal temperature increases with T$\leq$104K (bulk triple point). In adsorbate/substrate systems like C$_2$H$_4$/graphite, it is accepted that triple point wetting occurs. For our studies, we employed MgO nanocubes because they represent a prototypical metal oxide with a wide variety of technological uses including catalyst support. Of particular interest are wetting/layering transitions and the changes that take place in the neighborhood of the bulk triple point. We report our experimental investigation of the adsorption behavior of evidence C$_2$H$_4$ on MgO (100) using high-precision adsorption isotherms and neutron diffraction and scattering. We demonstrate the dominate role that molecule-molecule interaction plays in the wetting phenomena by comparing the behavior of ethylene on graphite and MgO. U.S. Department of Energy (DE-AC05-00OR22725) at ORNL managed and operated by UT-Battelle, LLC, and the NSF (DMR-0412231). [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H25.00013: Importance of Van Der Waals Interaction for Organic Molecule-Metal Junctions Priya Sony, Peter Puschnig, Dmitrii Nabok, Claudia Ambrosch-Draxl We present \textit{ab-initio} calculations to study the interface energetics of a weakly adsorbed organic molecule on metallic surfaces, which serve as model interfaces relevant for organic electronics. Thereby we focus on the role of the exchange-correlation potential and, in particular, the van der Waals interaction. To this extent the thiophene molecule is relaxed on clean Cu(110) and Cu(110)-(2x1)O, and the adsorption energy corresponding to various positions and orientations of the molecule is calculated on the search for the most favorable adsorption site. The molecule is found to be more strongly bound on the clean Cu(110) surface with an adsorption energy of $-0.50$~eV, as compared to $-0.30$~eV on Cu(110)-(2x1)O. Nonlocal correlations, i.e., the van der Waals interaction is found to be solely reponsible for the binding in such weakly bound systems, while the commonly used generalized gradient approximations not only underestimate the adsorption energyy but also provide the wrong physical picture for the binding. The adsorption of thiophene lowers the work function of the metallic substrate due to the formation of surface dipoles while no sizeable charge transfer is found. [Preview Abstract] |
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