Scanning tunneling (STM) and atomic force (AFM) microscopies have been used to examine structural features of monolayers formed from the spontaneous adsorption of alkanethiols at Au( 11 1) surfaces. In addition to the atomic scale spacings of the adlayer, the depressional features found in both large (50-250 nm) and atomic scale images have been characterized for monolayers formed from several alkyl chain lengths. These depressions, which are more predominant at monolayers from long chain alkanethiols, appear as 2-4 nm diameter, 0.24 nm deep depressions in STM images and as 2-5 nm diameter, low frictional features in lateral force images. The lateral force images of these depressions, along with an explanation of the imaging mechanism, are presented for the first time. Atomic scale STM images reveal the presence of the adlayer lattice within the depressions. In addition, atomic scale images show that these depressions occur along visible domain boundaries. Together, our results indicate that these depressions originate in the topmost layer of the Au( 1 11) surface and are not a consequence of structural inhomogeneities in the monolayer or of imaging artifacts. Based on these findings, a mechanism for the origin of the depressions is proposed.

Alkanethiolate monolayers formed on rough gold surfaces can, somewhat surprisingly, act as stronger barriers to heterogeneous electron transfer than those on smooth gold surfaces. This paper presents a possible explanation for this observation by constructing simple geometric models of a "rough" and "smooth" gold surface to examine how microscopic roughness differences can affect the nucleation/growth of the adlayer and size/density of structural defects. Expectedly, the number of defects predicted for adlayers formed on smooth gold is lower than any of those for rough gold. The counterintuitive result is that the sizes of a small portion of the defects in the adlayer on the smooth surface are larger than any of those found on the rough surface. The potential implications of these results are discussed.