STRUCTURAL ORIGINS OF THE SURFACE DEPRESSIONS AT ALKANETHIOLATE MONOLAYERS ON AU(111) - A SCANNING TUNNELING AND ATOMIC-FORCE MICROSCOPIC INVESTIGATION

Scanning tunneling (STM) and atomic force (AFM) microscopies have been used to examine structural features of monolayers formed from the spo ntaneous adsorption of alkanethiols at Au(111) surfaces. In addition t o the atomic scale spacings of the adlayer, the depressional features found in both large (50-250 nm) and atomic scale images have been char acterized for monolayers formed from several alkyl chain lengths. Thes e depressions, which are more predominant at monolayers from long chai n 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 wi th an explanation of the imaging mechanism, are presented for the firs t time. Atomic scale STM images reveal the presence of the adlayer lat tice within the depressions. In addition, atomic scale images show tha t these depressions occur along visible domain boundaries. Together, o ur results indicate that these depressions originate in the topmost la yer of the Au(111) surface and are not a consequence of structural inh omogeneities in the monolayer or of imaging artifacts. Based on these findings, a mechanism for the origin of the depressions is proposed.