The molecular structure of thiol-monolayers on gold imaged with an Atomic Force Microscope

Abstract

Thiols spontaneously form monolayers on gold. An important question is which factors determine the structure of the thiols. The two extreme positions are: (a) the structure of the underlying gold determines the packing of the thiol molecules; (b) the geometry of the thiol molecules and the interaction between thiols determine their two-dimensional structure.To help answer this question two series of thiol molecules on gold (111) were imaged by atomic force microscopy. In order to reduce the force between tip and sample, and to prevent distortion of the thiol monolayer, all images were taken in ethanol at forces between 0.5 nN and 3 nN.The first series consisted of several azobenzene-thiols:Although these molecules possess a terminal alkyl chain, their lattice is different from the well known hexagonal (√3 × √3) R 30° lattice (with respect to the underlying gold (111) lattice) of alkane-thiols. Instead, we found two stable rectangular lattices which could both be described by lattice constants of 0.61 nm and 0.79 nm, and an angle of 89°. These structures were also found on polycrystalline gold. Also, varying the length of the alkyl chain (molecules (i) and (ii)) or using a disulfide instead of a thiol (molecules (ii) and (iii)) did not change the lattice constants. Thus we conclude that the lattice for azobenzenethiols is end-group dominated.