The initial stages of the formation of butanethiol self-assembled monolayers on well-prepared Au/mica substrates have been studied using infrared spectroscopic analysis of the CH 2 and CH 3 stretching vibrations and real-space STM methods. The projection of the molecular axes of the chemisorbed species onto the surface normal is initially less than found with the fully formed films, indicating that in the initial stages of deposition they adopt configurations more prone to the surface; the evolution to the well-packed near-vertical molecular conformation that is usually associated with monolayer structures is observed even for sub-monolayer films that have been equilibrated in solution for several days. We conclude that film formation under lowconcentration conditions proceeds by island growth and that the evolution of these islands can be monitored by the spectral evolution of the infrared bands. We have also established that the infrared absorption of the symmetric methyl stretch (r + , 2876 cm -1 ) is linearly related to the overall surface coverage, while the absorption frequency is independent of coverage, thus supporting an island growth model in which the local molecular environment is unaffected by the macroscopic coverages. Our results indicate that thiolate desorption during film formation is not a significant process using methanol solvents and that the quality of the Au/mica substrate is crucial in determining the structural evolution of the growing film. IntroductionHighly organized organic thin-film interfaces have been the subject of enormous scientific interest in recent years due to their close structural similarities to biological membrane systems and because of their potential applications in thin-film sensor devices and optical networks; 1-3 their capacity to passivate metal surfaces against charge and mass transfers into solution has been used to study rapid electron transfer processes 4 and promises to provide new methods to provide inert interfaces between bodily fluids and metallic surfaces of implanted prosthetic devices. Controlling the structure and composition of the organic thin film is a natural prerequisite for the application of these systems; accordingly, a great deal of fundamental research on organic layers has been dedicated to the study of the surface and "bulk" properties of the fully formed monolayers. The two most commonly employed techniques for the preparation of these films are based on Langmuir-Blodgett methods (i.e., physisorption of preorganized organic films onto planar substrates) and molecular self-assembly, whereby amphiphilic molecules are chemisorbed from solution onto appropriate substrates by reactive terminal groups of the molecular adsorbate and, in the process, develop into high-density films. 5,6 Within the latter classification, self-assembled monolayers (SAMs) composed of organic thiols adsorbed onto metal surfaces (notably single-crystal Au(111) and evaporated Au/mica and Au/Si films with Au(111) surface textures) have received the most attention by far...
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