Characterization of Monomer-Containing Monolayer Films on Au Surfaces.

Exploration into the nature of ultrathin, surface-confined films capable of electronic conduction has been the primary purpose of this dissertation. Specifically, the confinement of pyrrole monomer to Au surfaces through organosulfur linkages has been investigated. The synthesis of these new $\omega$-(N-pyrrolyl)alkanethiols is described herein. Cyclic voltammetry was utilized in this work to polymerize the monolayers and characterize the resulting films. Voltammetric analysis yielded currents in the correct potential region for poly(N-alkylpyrrole) charging/discharging. Other electrochemical studies revealed the ability of strong nucleophiles and electroinactive diluents to quench surface-confined polymer formation though they did not affect oxidation of the monomer. Exchange experiments performed with an electroactive thiol in solution indicated a much higher degree of monolayer stability to desorption after electrochemical polymerization of the monolayer. Infrared reflection-absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS) were used to characterize the structural changes in the monolayer upon electrochemical polymerization. Infrared analysis yielded the loss of all pyrrole ring absorbance bands upon polymerization due to changes in the absorption cross-section of the resulting pyrrole units. Additional infrared studies on surface-confined oligomers indicated that some band intensity could be seen for the shorter chains. In agreement with literature reports on bulk species, the infrared intensities of oxidized (conducting) oligomers was higher than undoped oligomers of similar length. IRRAS analysis of the methylene stretching region for the alkane chains showed no change in band intensity indicating no loss of material from the surface. This was confirmed by integrating nitrogen and sulfur abundances on pristine and electrochemically oxidized surfaces using XPS. Microscopic investigations were carried out using various scanning probe microscopy (SPM) techniques but were hampered by the sensitivity of these monolayers to oxygen and airborne contaminants. These monolayers were also studied using physical and scanning electron microscopy (SEM) techniques. Pyrrole-terminated SAMs on Au electrodes were found to significantly increase the adhesion of thick poly(pyrrole) films to the electrode surface

Surface-confined monomers on electrode surfaces Part 3. Electrochemical reactions and scanning probe microscopy investigations of ω-(N-pyrrolyl) alkanethiol self-assembled monolayers on gold

The formation and subsequent polymerization of ω-(N-pyrrolyl)alkanethiol monolayers on polycrystalline gold under various conditions was performed. Polymer formation in the monolayer is suppressed by addition of H2O or pyridine to the Bu4NClO4-CH3CN or Bu4-propylene carbonate (PC) electrolyte. Exposure of gold electrodes to ethanol solutions of n-hexanethiol and N-(6-mercaptohexyl)pyrrole results in monolayers which do not reflect the composition of the dosing solution. The presence of 75% n-hexanethiol in the assembly solution results in mixed monolayers which still display surface-confined pyrrole oxidation, but no voltammetric evidence for polymer formation. Scanning tunneling microscopy of the oxidized monolayers on Au(111)/mica yields images which are not substantially different from those of the unoxidized monolayers on Au(111)/mica. © 1995 Elsevier Science B.V. All rights reserved

Surface-Confined Monomers on Electrode Surfaces. 1. Electrochemical and Microscopic Characterizaiion of ω-(N-Pyrrolyl) alkanethiol Self-Assembled Monolayers on Au

The synthesis of various ω-(N-pyrrolyl)alkanethiols and electrochemical characterization of the resulting monolayers on Au is described. Cyclic voltammograms of the monolayers on Au in BU4NCIO4/CH3CN display an irreversible wave at +1.00 V vs SSCE due to oxidation of pyrrole sites in the monolayer. There is no voltammetric evidence for polymerization of the surface-confined pyrrole units when the potential is scanned between 0 and +1.3 V in BU4NCIO4/CH3CN. Poly(pyrrole) films deposited on w-(N-pyrrolyl)-alkanethiol/Au surfaces are extremely adherent and smooth. Scanning electron micrographs of the polymers show that the films are composed of nuclei 80-100 nm in diameter. The differences between poly(pyrrole) formed on modified and unmodified Au are discussed. © 1995, American Chemical Society. All rights reserved

Tethered monolayers of poly((N-pyrrolyl)alkanethiol) on Au

The electrochemical oxidation of pyrrole-terminated, self-assembled alkanethiol monolayers on Au surfaces in dry electrolyte media (\u3c 0.005% H2O) results in tethered poly(N-alkylpyrroles) of varying monomer repeat lengths. After pristine (monomeric) monolayers of 10-(N-pyrrolyl)hexane-1- thiol (1), 7-(N-pyrrolyl)-heptane-1-thiol (2), and 6-(N-pyrrolyl)hexane-1- thiol (3) on Au have been oxidized in nonaqueous electrolyte media, electrochemistry characteristic of poly(N-alkylpyrroles) is observed. Reflection-absorption infrared spectra of the oxidized monolayers exhibit aromatic C-H out-of-plane deformation bands between 700 and 800 cm-1 that can be used to determine the effective conjugation length of the poly(N- alkylpyrroles) produced. The number of monomer repeat units in the backbone ranges from 2 to ~20 and depends on the oxidation potential applied to the monomeric monolayers. In comparison to the monomeric monolayers, the electrochemically polymerized monolayers are demonstrated to be more stable with respect to exchange by a redox-labeled alkanethiol

Mobility of the Alkanethiol-Gold(111) Interface Studied by Scanning Probe Microscopy

Scanning tunneling microscopy (STM) was used to observe the motion of pits on the CH3(CH2)nSH/Au(111) surface (n = 5, 15, and 17). These pits are shown to diffuse to step edges at rates independent of the alkanethiol chain length during STM imaging. Heating the CH3(CH2)nSH/Au(111) surface (n = 15 and 17) at 100 °C causes removal of the pits from the surface without damage to the monolayer. The proposed mechanism for pit motion suggests that the Au-S bonds are labile and that only Au atoms diffuse. The pits are shown to be defects in the Au(111) surface formed during thiol adsorption. © 1993, American Chemical Society. All rights reserved

Angle-resolved photoemission measurements of ω-(n-pyrrolyl)alkanethiol self-assembled monolayers using in-situ sample preparation apparatus

An in-situ sample preparation apparatus was developed for measuring intrinsic angle-resolved ultraviolet photoemission spectra (ARUPS) from ω-(n-pyrrolyl)alkanethiol self-assembled monolayers (pyrrolyl-SAMs) on an Ag surface. The apparatus enables the preparation of the SAMs directly from the thiol solution and the measurement of ARUPS without exposing the sample to air. The spectral features of the SAMs were analyzed with the aid of ab-initio molecular orbital calculations. It was observed that the ARUPS features associated with the π bands originating from the substituent pyrrole are distinct from the features associated with the alkyl chain