Papers

AVS1996 Session SS+AS-WeA: Kinetics and Formation of Self-Assembled Monolayers

Wednesday, October 16, 1996 2:00 PM in Room 203A

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		SS+AS-WeA-1 Growth of Self-assembled Monolayers from Vapor and Solution: Decanethiol on Au(111) A. Eberhardt, P. Fenter, P. Eisenberger (Princeton University) The kinetics of the self-assembly process have been investigated for decanethiol on Au(111) using grazing incidence x-ray diffraction. These studies provide the first quantitative description of thiol adsorption over a range of molecular impingement rates and temperature regimes, and the first quantitative comparison of adsorption from solution and vapor. Measurements were taken in-situ, in real time for vapor deposition and in an interrupted growth mode for solution deposition. Monolayer growth from vapor was found to be controlled by two different mechanisms, including a bimolecular associative growth mode previously unobserved in any system. The adsorption rate was found to depend on the temperature and impingement rate in a complex manner, but the growth curves are consistent with Langmuir kinetics, with an initial sticking coefficient of 1.6 \+-\ 0.1 X 10\super -4\ at room temperature. Monolayer growth from solution was found to be similar in the single molecule adsorption regime, but the bimolecular growth mode was not observed. These results provide insight into the complexities of the self-assembly process and their dependences upon the specific growth conditions.
2:20 PM		SS+AS-WeA-2 Nucleation and Growth of Amphiphiles on (111) Surfaces of Au and Ag G. Poirier (National Institute of Standards & Technology) Metallophilic organic amphiphiles represent a unique class of thin film materials because growth spontaneously terminates at one molecular monolayer and because the molecular conformation can influence the growth mode. In this presentation we discuss efforts aimed at characterization of spontaneous assembly of alkanethiols on metal surfaces using scanning tunneling microscopy (STM). The importance of surface defects is revealed by nucleation that is heterogeneous on the periodically faulted Au surface but is homogeneous on the perfect Ag surface. Subsequent monolayer formation on Au(111) proceeds by a two-step process that begins with condensation of low-density crystalline islands, characterized by surface-aligned molecular axes, from a lower-density lattice-gas phase. At saturation coverage of this phase the monolayer undergoes a phase transition to a denser phase by realignment of the molecular axes with the surface-normal. These studies reveal the important role of molecule-substrate and molecule-molecule interactions in the self-assembly of these technologically important material systems.
2:40 PM		SS+AS-WeA-3 Molecular Dynamics Simulations of Alkanethiols on Stepped Au(111) K. Krom, R. Bhatia, B. Garrison, P. Weiss (Pennsylvania State University) Molecular dynamics are used to study the effects of monatomic substrate steps on self-assembled monolayers of alkanethiols on Au(111). Our calculations reveal a mechanism of molecular reorientation at the step edges, leading to long-range structural ordering of adsorbates. The azimuthal tilt angle of the molecular chains appears to be determined by the direction of the step edge. Further, migration of adsorbates to the step produces defects and local disorder in the monolayers which mirrors our previous experimental STM images of these monolayers at substrate step edges. Adsorbate molecules at the step edges also produces kinks in the steps by a concerted mechanism.
3:00 PM		SS+AS-WeA-4 Adsorption of n-alkanes and 1-alkenes on Au(111) S. Wetterer, D. Lavrich, T. Cummings, S. Bernasek, G. Scoles (Princeton University) Thermal Energy Atom Scattering (TEAS) has been used to study the adsorption of a series of n-alkanes and 1-alkenes on Au(111). Using this technique, both adsorption and desorption can be observed with high sensitivity in real-time to yield desorption energies and sticking coefficients. For the n-alkanes (C\sub 7\-C\sub 12\), physisorption energies scale linearly with chain length as 0.06 eV per additional carbon. The physisorption of 1-alkenes (C\sub 8\-C\sub 11\) also show a similar linear chain length dependence but have energy values 0.16 eV higher than the corresponding alkane due to the polarizability of the double bond. For all molecules studied, the dependance on temperature of the sticking coefficient for physisorption is primarily controlled by the physisorption energy. Both TEAS and TPD results will be presented with extensive discussion of data analysis technique.
3:20 PM		SS+AS-WeA-5 Self Assembly and Electrochemical Oxidation of Pyrrole Terminated Alkanethiolate Films Studied by Surface Plasmon Resonance Spectroscopy A. Peterson, K. Peterlinz, R. Georgiadis (George Washington University) Polypyrrole and various substituted polypyrroles, commonly synthesized via electrooxidative polymerization of the appropriate monomer, are organic conducting polymers which are of interest for their potential applications. Recently, several studies have suggested that ultra-thin organic conducting polymer films on gold electrodes can be synthesized using spontaneous self-assembly of an appropriate redox active monomer to form an organized monolayer film. Irreversible electrochemical oxidation of the monolayer may form a conducting polymer, however, no spectroscopic evidence of monolayer polymerization or conductivity has been observed to date. In the first study of its kind, we use surface plasmon resonance (SPR) spectroscopy to monitor in situ the process of self assembly for several different pyrrole-terminated alkanethiolate films and the subsequent electrochemical oxidation of these films in non-aqueous electrolyte. >From the SPR measurements, we determined the thickness and optical absorbance of the films. After electrochemical oxidation, the SPR data indicate an increase in optical absorbance which is clear evidence for the creation of conduction bands in these films. SPR measurements are also used to observe how the optical properties of the films are altered by electrochemical doping (switching between the neutral and conducting states of polymer). The in situ SPR measurements are compared with ex situ UV-visible absorbance measurements of these films.
3:40 PM		SS+AS-WeA-6 Film Formation of Alkanethiols on Gold and Silver Studied by Linear and Nonlinear Vibrational Spectroscopy M. Himmelhaus, A. Lampert, M. Buck, M. Grunze (Universit\um a\t Heidelberg, Germany) IR reflection absorption (IRRAS) and IR-vis sum frequency generation (SFG) which are partially complementary with respect to their sensitivities to bond orientation and conformation of the methyl and methylene modes were applied in a comparative study of the vibrational structure of layers of varying thiol coverage. Gold and silver substrates which have been immersed in thiol solutions for different periods of time where examined ex situ. For the adsorption on gold the known two step process which takes place on two distinctly different time scales is seen by SFG. For short immersion times (minutes) the CH\sub 2\ signal is prominent but disappears in favor of the methyl bands at long immersion times (1-2 days). Since SFG is highly sensitive to centro-symmetric structures the conformational change from gauche to all trans is reflected by the SFG spectrum. In contrast to gold, thiol adsorption on silver yields SFG spectra which are dominated by the CH\sub 3\ bands even for short immersion times. No two time scales are seen. This suggests an island type growth in contrast to a more uniform film growth on gold.
4:00 PM		SS+AS-WeA-7 Dynamical Simulation of Perturbed Systems of Alkanethiols on Gold Surfaces K. Beardmore, N. Gronbech-Jensen, J. Kress, A. Parikh (Los Alamos National Laboratory) We have examined the pico-second dynamical behaviour of self assembled monolayers (SAMs) of alkanethiolates on gold surfaces, by classical molecular dynamics (MD) simulation. We employ a new empirical potential, developed from our recent ab-initio calculations for the Au-S-C interactions, and an all-atom model for the alkyl chain. The systems investigated are perturbed from a regular [111] ordered Au structure by introducing defects within the gold surface. The effects of defects such as missing surface atoms, step-edges, and grain boundaries on the orientation, packing, and chain-conformation of thiolate species are investigated. To allow relatively large systems to be studied for the required simulation times, the calculations were conducted using a newly developed parallel MD code.
4:20 PM		SS+AS-WeA-8 Controlling Surface Molecular Structure: Step by Step Construction of Multi-Component Organic Monolayers M. Cygan, L. Bumm, J. Arnold, T. Dunbar, D. Allara, P. Weiss (Pennsylvania State University) Precise control of molecular placement and structure is one of the most important prerequisites for nanofabrication on surfaces. We explore methods of designing and building surface film structures using organic thiolates attached to Au(111) substrates. We have examined the insertion of single aromatic oligomers into sites in a nominally complete alkanethiol lattice. We have also examined the replacement of significant portions of one type of alkanethiol lattice with molecules of a second type. Using high resolution scanning tunneling microscopy, we find in each of these studies that molecules preferentially adsorb or exchange at defect sites in the film (such as step edges) which offer increased access to the underlying Au substrate. We discuss the structure, stability, and lattice ordering of these monolayers as compared to similar films formed by coadsorption. We discuss how control of the number and disctribution of defects can be used to control the film structure and local composition.
4:40 PM		SS+AS-WeA-9 Self-assembly Strategies for Characterization of Single Molecule Conductivity of Oligomers of Thiophene-ethynylenes and Phenylene-ethynylenes T. Dunbar (Pennsylvania State University); D. Lombardi (Yale University); T. Burgin, J. Tour (University of South Carolina); M. Reed (Yale University); D. Allara (Pennsylvania State University) In order to facilitate ongoing measurements of the conductivity of individual molecules, we have developed methods for making self-assembled monolayers (SAMs) in which the candidate conducting molecules can be placed in a near-vertical, isolated fashion within a matrix of other, less conducting molecules. In this presentation we demonstrate, for the specific case of pi-conjugated phenylene-ethynylene oligomers, that one effective strategy involves the insertion of sulfur-terminated candidate molecules via solution exchange into pre-formed alkanethiolate SAMs. A previously reported STM study on a trimer molecule (Science 271, 1705 (1996)) shows that this insertion occurs at defect sites within the alkanethiolate SAM. For the insertion of larger molecules containing side chains for solubility enhancement, we find that steric effects slow this insertion rate. In addition, we find that steric and kinetic effects work against single component monolayers of long (~50 \Ao\) oligomers attaining film thicknesses as thick as the molecules are long, suggesting that coverage decreases with molecular length.