Effect of an oxidized gold substrate on alkanethiol self-assembly

UV-cleaned gold substrates incubated in solutions of alkanethiol show islan ds on the monolayer surface when imaged with noncontact atomic force micros copy (AFM). The height of the islands above the monolayer is approximately twice the height of the alkanethiol monolayer, and the diameter of the isla nds is 20-200 nm. These islands are easily pushed aside during contact mode AFM imaging without damaging the underlying monolayer. Islands are observe d on gold substrates exposed to solutions of octadecane-, hexadecane-, and dodecanethiol and 1'-thiahexa(ethylene oxide)-1-octadecane at 0.01-1.0 mM c oncentrations in ethanol and hexadecane. AFM on samples with submonolayer c overage shows that the islands are not observed until the late stages of mo nolayer formation. Islands are not observed on freshly deposited gold subst rates or on UV-cleaned gold substrates that are exposed to ethanol for long er than 10 min prior to incubation in alkanethiol solutions. We conclude th at the island formation is associated with oxidation of the gold surface an d that the islands are primarily composed of alkanethiol. We hypothesize th at the stability of these structures may be due to the formation of multimo lecular complexes of alkanethiols.