Monolayer films from ethanethiol (ET) and n-octadecanethiol (OT) spontaneously adsorbed onto epitaxially grown Au( 1 I I ) films on mica were examined by scanning tunneling microscopy (STM). The resulting atomically resolved images are the first reported for gold-adsorbed organothiolate molecules and reveal the packing arrangement of the overlayer. Tunneling is presumed to occur between the microscope tip and the gold-bound sulfur of the n-alkanethiolate head group. For both the ET and OT monolayers, an image that corresponds to a hexagonally packed array of adsorbates with respective nearest-neighbor and next-nearest-neighbor spacings of 0.50 f 0.02 and 0.87 f 0.04 nm was observed. This packing agrees well with the (d? X d/5)R3O0 structure determined for long-chain n-alkanethiolate monolayers on Au(l11) in recent helium diffraction' and clcctron diffraction2 studies. Furthermore, images with the above spacings were found to exhibit continuity over areas from a few square nanometers up to about 600 nm2, indicating the potential utility of STM for probing both the short-and long-range order of organic monolayer films. Structural interpretations of these images are presented and examined within the context of molecular level descriptions that have been recently developed from macroscopic characterization studies of these monolayers.

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Monolayer films formed by the chemisorption of alkanethiols (CH,(CH2)^SH, n = 1-17) at epitaxially grown Au(l 11) films were examined using atomic force microscopy (AFM). Atomically resolved images were found for films with n > 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 0.50and 0.87-nm distances of a (V3 X v/3)R30°adlayer on a Au(l 11) lattice, the two-dimensional arrangement reported in recent diffraction1 2"3 and scanning tunneling microscopy4,5 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 shortand 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 < 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.

Monolayers formed by the chemisorption of CF&FZ),(CHZ)ZSH (FT) at epitaxially grown Au(ll1) films were characterized using atomic force microscopy (AFM), electrochemistry, and infrared reflection spectroscopy (IRS). The AFM was used to probe the atomic scale arrangement of the monolayer. The electrochemical and IRS studies provided insight into the surface coverage and spatial orientation of the monolayer. The AFM images exhibit ordered domains of hexagonal periodicity with average nearest-and next-nearest-neighbor separation distances of 0.58 f 0.02 and 1.01 f 0.02 nm, respectively. These spacings agree well with a (2 X 2) adlayer at Au(lll), a two-dimensional arrangement predicted on the basis of considerations of a space-filling model. The sizes of the ordered domains typically range from a few adsorbates up to 30 nm2 and occasionally larger. The results of the surface coverage ((5.7 f 0.7) X 10-10 moUcm2) and orientational analysis (20° average tilt of the perfluorocarbon chains from the surface normal) support the presence of a (2 X 2) adlayer. Differences in the ability to image the FT monolayer and those formed from alkanethiols (CH&Hz),SH) are discussed, along with the advantages of correlating the findings from microscopic and macroscopic characterization techniques.