Formation of Self-Assembled Butanethiol Monolayers on Au Substrates:  Spectroscopic Evidence for Highly Ordered Island Formation in Sub-Monolayer Films

Abstract

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...

Macroscopic Tribological Testing of Alkanethiol Self-assembled Monolayers (SAMs): Pin-on-disk Tribometry with Elastomeric Sliding Contacts

“…2,962 cm -1 ) has somewhat increased inside the sliding track, and this change is more significant for the cases of C 11 CH 3 SAM than of C 11 OH SAM. The increase in the intensity of the asymmetric methyl C-H stretch peak coincides with the occurrence of the peaks at 1,110 and 1,265 cm -1 (arising from Si-O asymmetric stretching and Si-CH 3 asymmetric bending modes, respectively [48]) inside the sliding track, as shown in Fig. 7b, which suggests that the transfer of uncrosslinked monomer species from the pin to the SAM films might have occurred.…”

“…The asymmetric and symmetric methyl C-H stretches, m a (CH 3 ) at 2,962 cm -1 and m s (CH 3 ) at 2,877 cm -1 , respectively, and the band at 2,936 cm -1 assigned to a Fermi resonance (FR) between the CH 3 symmetric stretching mode and overtones of bending modes, appear only for C 11 CH 3 and C 5 CH 3 SAM films, but are absent for C 11 OH SAM films, as expected. The position and peak width of the methylene asymmetric C-H stretching modes are particularly sensitive to the conformational order of the alkyl chains [15,16,[45][46][47][48]. As seen in Fig.…”

Observation of surface enhanced IR absorption coefficient in alkanethiol based self-assembled monolayers on GaAs(001)

“…18,21 An example of the latter is located near 2924 cm −1 and corresponds to the asymmetric CH 2 vibration adjacent to sulfur. 28 The distribution of these modes contributes to the apparent position of normal mode peak frequencies that for lower values of n weigh more significantly. Consequently, we will consider the asymmetric mode peak amplitude as a generalized feature of the SAM independent of its precise frequency, with the implicit observation that the spectral envelope contains within it certain constant factors independent of n.…”

The self-assembly of a layered material: metal-alkanethiolate bilayers

“…A similar stabilization of the alkyl chains occurs in other thiolate systems. Although some authors have suggested that shortchain systems are disordered at room temperature, there are now clear indications that it is indeed possible to have ordered thiolate monolayers and thiolate-capped nanoparticles with very short chains (9,24). Combined with results from the layered materials, one can now say that self-assembled alkanethiolate systems have an inherent stabilization, with the layered materials being the most stable.…”