AVS2001 Session SS+BI-TuM: Poirier Memorial Session: Self-Assembled Monolayers I

Tuesday, October 30, 2001 8:20 AM in Room 120

Tuesday Morning

Start	Invited?	Item
8:20 AM	Invited	SS+BI-TuM-1 Greg Poirier's STM Landscapes of Alkanethiol Monolayers on Gold: A Retrospective M.J. Tarlov (National Institute of Standards and Technology) The surface science community lost one of its bright young stars, Gregory E. Poirier, who passed away in September at the age of 39. During Greg's brief career at NIST he made many noteworthy contributions in the areas of surface science and chemical sensing, however, it was his STM studies of alkanethiol self-assembled monolayers (SAMs) on gold that earned him the greatest recognition. Greg's STM images were some of the first to reveal the structural complexity and phase behavior that governs the 2-D world of alkanethiols on gold. Through meticulous and rigorous interpretation of these images he unlocked many structural details of SAMs and gained an understanding of the molecular forces that govern the assembly of SAMs. This talk will review some of the highlights of Greg's STM studies including the rich variety of molecular-scale crystal structures of alkanethiol SAMs, their associated defect structures, and the development of a general mechanism for SAM formation.
9:00 AM	Invited	SS+BI-TuM-3 Imaging and Diffraction: Two Complementary Probes of Self-Assembled Monolayer Structure and Properties G. Scoles (Princeton University) Using Self-Assembled Monolayers (SAMs) as test systems, a few introductory examples of the well known complementarity between imaging (local SAM structure and defects) and diffraction (precise determination of overall SAM structure) will be given. Starting from the pioneering detection, by Poirier, of the c(4x2) superlattice superimposed to the basic hexagonal structure of alkylthiol SAMs on Au(111), we will review the present status of the question concerning the presence of at least two inequivalent sulfur atoms in the equilibrated monolayers. After reviewing the overwhelming experimental evidence that points in that direction, we will present recent Density Functional Theory calculations that may finally provide a way out of this ten year old controversy. Returning to the complementarity between imaging and diffraction, we will point out a few less known features of both methods. In particular, the sensitivity of atomic beam diffraction to very small vertical displacements (not detectable with either X-rays or STM) will be discussed along with the possibility, offered by X-rays, to provide information on buried SAM interfaces. We will conclude showing how the synthetic flexibility afforded by SAMs coupled with the local sensitivity of STM provides unique opportunities to understand electron transfer processes at the organic/metal interface.
9:40 AM		SS+BI-TuM-5 Characterization of Self-assembled Monolayers Using Near-edge X-ray Absorption Spectroscopy T.M. Willey (Univ. of California at Davis and Lawrence Livermore National Lab); A.L. Vance, A.W. vanBuuren (Lawrence Livermore National Lab); C.F.O. Bostedt (Univ. of Hamburg, Germany, and Lawrence Livermore National Lab); G.A. Fox, A.J. Nelson, L.J. Terminello (Lawrence Livermore National Lab); C.S. Fadley (Unive. of California at Davis and Lawrence Berkeley National Lab) We have investigated methyl and carboxyl terminated alkanethiols (hexadecanethiol and mercaptohexadecanoic acid) and other more complex self-assembled monolayers (SAMs) on Au(111). We characterize these SAMs using x-ray absorption at the Carbon K-edge, Sulfur L-edge, ans well as Oxygen and Nitrogen K-edges where applicable. Near-edge X-ray Absorption Spectroscopy (NEXAFS) gives information about chemical state, and polarization effects in these spectra probe the orientation of chemical bonds. Simple, carboxyl-terminated alkanethiols supposedly do not form well-ordered films. We currently investigate attachment, uniformity, and order of these films as a function of solvent, chemical state of the carboxyl group, and by intermixing with well-ordering methyl-terminated alkanethiols. We have also investigated attachment and order of more complex, disulfide containing thioctic acid derivatives. Preliminary results indicate that we have been successful in chemisorbing both sulfur-functionalized ends of a number of these molecules to the gold surface. T. Willey acknowledges a Student-Employee Graduate Research Fellowship through LLNL. This work is supported by the U.S. Department of Energy, BES Materials Sciences under contract W-7405-ENG-48, LLNL.
10:00 AM		SS+BI-TuM-6 Structure of Alkyl Thiol and Dialkyl Disulfide on Au(111) H. Nozoye (Nanotechnology Research Institute, AIST, Japan); T Hayashi, C. Kodama (University of Tsukuba, Japan); Y. Morikawa (JRCAT, AIST, Japan) The structure and formation process of self-assembled monolayers on Au(111) have been a long-standing unsolved problem. We determined the adsorption state of alkane thiol and dialkyl disulfide with different alkyl chain length on Au(111), which are prototypical self-assembled monolayer systems, by using temperature programmed desorption (TPD), high sensitivity low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) calculation. We concluded that the SH bond of alkane thiol and the SS bond of dialkyl disulfide break below room temperature, spontaneously desorbing hydrogen and resulting in the formation of highly ordered adsorption state of thiolate on the surface. The spectra above 500cm^-1 contain the information of the orientation of the alkyl moiety, however, those of low energy region contain the information of the bonding between S and Au. HREELS spectra of low energy region were almost the same for all the thiolate species, irrespective of the difference of the coverage of the thiolate and of the chain length of alkyl moiety, although the spectra above 500cm^-1 were changed depending on the coverage of the thiolate species. The change of the HREELS spectra above 500cm^-1 agreed with the conclusion obtained from STM measurements, that is the lying-down configuration in the low coverage region and standing-up configuration at the saturation coverage. The HREELS spectra of the low energy region were analyzed with the DFT calculation; The location of S of the thiolate on Au(111) is the bridge site and the SC bond is inclined about 50 degree from the surface normal. From these results we concluded that the structure of the root part of the thiolate species is common, at least for alkyl thiolate with relatively short chain, notwithstanding the difference of the orientation of the alkyl moiety, that is the lying-down and standing-up structures.
10:20 AM	Invited	SS+BI-TuM-7 Two-dimensional Phase Diagram of Decanethiol on Au (111) J.M. White, W.P. Fitts (The University of Texas at Austin) Based on variable temperature ultrahigh vacuum scanning tunneling microscopy data, we propose a two-dimensional phase diagram of monolayer decanethiol on Au(111). Four triple point temperatures were determined: T₁ at ~ 27 °C, T₂ at ~ 33 °C, T₃ at ~ 35 °C and T₄ ~ 56° C. T₁ defines the lowest temperature melting point and T₄ defines the temperature above which striped phases are metastable. These data provide a fundamental framework to understand and control meso-scale monolayer structure; moreover, they provide fundamental insight into two dimensional phase behavior of molecules with many degrees-of-freedom.
11:00 AM		SS+BI-TuM-9 Odd-Even Effects in the Electron-induced Damage of Biphenyl-substituted Alkanethiol Self-assembled Monolayers M. Zharnikov, S. Frey, H.-T. Rong (Universität Heidelberg, Germany); M. Buck (University of St Andrews, U.K.); K. Heister, M. Grunze (Universität Heidelberg, Germany) Self-assembled monolayers (SAMs) formed on Au(111) from biphenyl-substituted alkanethiols CH₃(C₆H₄)₂(CH₂)_nSH (BPn) exhibit odd-even changes in the packing density and the tilt angle of the biphenyl moieties with varying length of the aliphatic part. We have applied X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, infrared absorption spectroscopy, and advancing contact angle measurements to study electron beam induced damage in these systems as well as in pure aromatic biphenylthiol (BP0) SAMs on Au substrates. Although the character of the electron-induced damage in all investigated SAMs was found to be similar, the extent and rate of the observed changes exhibited a clear correlation with the packing density and orientation of the biphenyl moieties in the BPn SAMs: The densely packed BPn films are noticeably more insensitive (in terms of the orientational order, irradiation-induced desorption, and anchoring to the substrate) towards electron irradiation than the loosely packed BPn layers. The extent of the irradiation-induced damage in pure aromatic BP0 SAMs was observed to be very close to that in the loosely packed BPn films, which indicates that the former films are in some sense also "loosely packed". Considering the potential applications of the aromatic SAMs as lithographic resist or template, the introduction of the short aliphatic chain in the respective molecules provides a simple and efficient way to manipulate their reaction toward ionizing irradiation in a desirable way. This work has been supported by the German BMBF (05 SF8VHA 1).
11:20 AM		SS+BI-TuM-10 Highly Ordered Organic Monolayers with Reduced Antiphase Domain Walls Due to Growth from Two-dimensional Gas- and Fluid- Phases L. Gross, C. Seidel, H. Fuchs (University of Münster, Germany) The growth process of monolayers of perylene and coronene on the metal surfaces Ag(110), Au(110) 1x2 and Au(111) 23x√3 was investigated by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The preparation was done by molecular beam epitaxy (MBE), with the possibility of LEED or STM measurements during evaporation. Both molecules show (restricted on Au(110) 1x2) an evolution from isotropic disordered structures in the submonolayer regime to a highly (substrate-dependent) ordered monolayer. In all presented systems the adsobate begins to form highly ordered structures just before the coverage of a complete monolayer is reached. The final monolayer structures are coincident, except coronene on Au(111) which is commensurate, furthermore perylene on Au(110) and coronene on Au(111) show uniform domain orientation. The lateral ordering process of these molecules allows epitaxial growth with a reduced density of antiphase domain boundaries, because crystallization does not start from islands in the submonolayer regime, but from two-dimensional gas- and fluid- phases.