SCANNING TUNNELING MICROSCOPY OF ETHANETHIOLATE AND NORMAL-OCTADECANETHIOLATE MONOLAYERS SPONTANEOUSLY ADSORBED AT GOLD SURFACES

Monolayer films from ethanethiol (ET) and n-octadecanethiol (OT) spontaneously adsorbed onto epitaxially grown Au(111) 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 +/- 0.02 and 0.87 +/- 0.04 nm was observed. This packing agrees well with the (square-root 3BAR X square-root 3BAR)R30-degrees structure determined for long-chain n-alkanethiolate monolayers on Au(111) in recent helium diffraction 1 and electron diffraction 2 studies. Furthermore, images with the above spacings were found to exhibit continuity over areas from a few square nanometers up to about 600 nm 2, 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.