Rapid Self-Assembly of Alkanethiol Monolayers on Sputter-Grown Au(111)

Abstract

Atomically flat, sputter-grown Au(111) films allowed well-ordered alkanethiol (exemplified by octanethiol) monolayers to be self-assembled from solution markedly faster and in larger domain sizes than previously reported. An X-ray photoelectron spectroscopy analysis showed that complete monolayer coverage was reached by 0.2−60 min of incubation in 0.1−0.001 mM ethanolic solution at room temperature (∼17 °C), with single-step (0.1 and 0.01 mM) or two-step (0.001 mM) adsorption kinetics. Increasing the temperature to 35 °C was enough to cause a single-step, diffusion-controlled adsorption also from the 0.001 mM solution, yielding the full monolayer coverage in approximately 10 min. Scanning tunneling microscopy (STM) imaging proved that well-ordered islands, with the (√3 × √3)R30° structure more-or-less strongly modulated by the c(4 × 2) superlattice, begin to form at 0.6−0.7 monolayer coverage most likely by homogeneous nucleation and grow rapidly thereafter. This kinetics of ordering requiring the considerably high threshold coverage for the nucleation, but allowing the fast growth of the nuclei was independently confirmed by the infrared reflection absorption spectroscopy. A typical c(4 × 2) domain size at the saturation coverage was estimated to be no less than 10−15 nm, and the structural identity often seemed not to be disrupted even across the etch pits. This superior structural order is reflected on the highest level of molecular resolution achieved by the in-air STM imaging. The expected registry of the (√3 × √3)R30° or c(4 × 2) lattice with that of Au(111) was also confirmed. On Au films that were also sputter-grown but no longer atomically flat, we observed at least by 1 order of magnitude slower self-assembly.