Surface Stress, Kinetics, and Structure of Alkanethiol Self-Assembled Monolayers

Abstract

The surface stress induced during the formation of alkanethiol self-assembled monolayers (SAMs) on gold from the vapor phase was measured using a micromechanical cantilever-based chemical sensor. Simultaneous in situ thickness measurements were carried out using ellipsometry. Ex situ scanning tunneling microscopy was performed in air to ascertain the final monolayer structure. The evolution of the surface stress induced during coverage-dependent structural phase transitions reveals features not apparent in average ellipsometric thickness measurements. These results show that both the kinetics of SAM formation and the resulting SAM structure are strongly influenced both by the surface structure of the underlying gold substrate and by the impingement rate of the alkanethiol onto the gold surface. In particular, the adsorption onto gold surfaces having large, flat grains produces high-quality self-assembled monolayers. An induced compressive surface stress of 15.9 ± 0.6 N/m results when a c(4×2) dodecanethiol SAM forms on gold. However, the SAMs formed on small-grained gold are incomplete and an induced surface stress of only 0.51 ± 0.02 N/m results. The progression to a fully formed SAM whose alkyl chains adopt a vertical (standing-up) orientation is clearly inhibited in the case of a small-grained gold substrate and is promoted in the case of a large-grained gold substrate.

Nanoparticle Structure by Coherent X-ray Diffraction

“…52,53) In experiments on 200-nm-sized-grained films it was also found that the stress takes several hours to build. 54,55) Such cantilevers have important medical applications as sensors. 56) The fact that such large stress differences were detected in the CXD experiment indicates that the structure of the SAM must be very different between the flat and the curved surface regions of the nanocrystal.…”

Novel Differential Surface Stress Sensor for Detection of Chemical and Biological Species

“…Furthermore, it indicates surface stress change observed during the first introduction is unambiguously associated with SAM formation on sensing cantilever. Previous reports [4] have indicated that distance of cantilever to the location where alk:anethiol droplets are introduced, condition of gold surface like cleanliness and roughness, and grain structure of the gold on the cantilever's surface affects the kinetics and magnitude of surface stress development. Among those conditions, the microstructure of gold film significantly influences the development of the surface stress during the formation of Alkanethiol SAMs [4].…”

“…al. [3], SAMs have been used as test system for almost all cantilever based sensing techniques [4][5][6]. This is because they are relatively easy to prepare, form well-ordered close packed films and offers limitless possibilities of variations in chain length, end group and ligand attachments [7].…”

“…al. [4] have shown that the kinetics of forrnationber self-assembled monolayers on gold-coated cantilevers and the resulting structure are depend of on the microstructure of the gold film and also the rate at which the SAM reaches the surface. ent…”

An analytical model for transient deformation of viscoelastically coated beams: Applications to static-mode microcantilever chemical sensors

“…18,38 In addition, the form of the present solution suggests that it may be applicable to more general coated-cantilever systems whose behaviors are governed by two competing time constants ͑not necessarily related to coating eigenstrain or viscoelasticity͒. 39 Examples of other mechanisms that could be accounted for by the current solution ͑or its extension͒ include growth kinetics/ molecular rearrangement in self-aligned monomers on goldcoated cantilevers 40,41 and cantilever actuation using polyelectrolyte brushes. 42 These applications involve relaxation phenomena that appear to contribute to observed overshoot responses in certain instances, although this behavior has yet to be placed upon a firm theoretical foundation.…”