ATOMIC SCALE IMAGING OF ALKANETHIOLATE MONOLAYERS AT GOLD SURFACES WITH ATOMIC FORCE MICROSCOPY

Monolayer films formed by the chemisorption of alkanethiols (CH3(CH2)nSH, n = 1-17) at epitaxially grown Au(111) films were examined using atomic force microscopy (AFM). Atomically resolved images were found for films with n greater-than-or-equal-to 4, directly revealing for the first time the arrangement of the alkyl chain structure. All of the images exhibit a periodic hexagonal pattern of equivalent spacings (e.g., respective nearest- and next-nearest-neighbor distances of 0.52 +/- 0.03 and 0.90 +/- 0.04 nm for n = 17 and 0.51 +/- 0.02 and 0.92 +/- 0.06 nm for n = 5). These spacings agree well with the analogous 0.50- and 0.87-nm distances of a (square-root 3 x square-root 3)R30-degrees adlayer on a Au(111) lattice, the two-dimensional arrangement reported in recent diffraction 1-3 and scanning tunneling microscopy 4,5 studies. In some instances, images with the above spacings were observed to extend continuously over areas as large as 100 nm2, suggesting the potential of AFM to reveal both the short- and long-range order of the alkyl chains of these and other model interfacial structures. The implications of these findings, including the inability to obtain well-resolved images for films with n less-than-or-equal-to 3, are examined in the context both of the current structural descriptions of alkanethiolate monolayers and of general issues related to imaging organic films with AFM.