Scanning tunneling microscopy of ethanethiolate and n-octadecanethiolate monolayers spontaneously absorbed at gold surfaces

Abstract

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.

The Nature of Alkanethiol Self-Assembled Monolayer Adsorption on Sputtered Gold Substrates

“…The Cr adhesion layer not only increases the adhesion of the gold to the surface but allows for 2D Stranski-Krastanov island/monolayer growth of the Au rather than the typical Volmer-Weber 3D island growth [39] We believe the boundary formation between neighboring Au grains is due to the presence of Au step edges, because it is difficult to obtain dense packing of alkanethiols at Au step edges. Alkanethiols are believed to bond at the three-fold hollow sites between the hexagonal arrangement of Au atoms on a Au{111} surface [11,12,[29][30][31][32]. Consequently, at a step edge, Au atoms on the upper terrace exist as an adlattice in the three-fold hollow sites of the lower Au terrace.…”

Selective adsorption of alkyl thiols on gold in different geometries

“…Therefore, configurations where all thiol groups are adsorbed like this are very stable and may not change significantly during long simulations. In contrast to this, experiments suggest a hexagonal overlayer of thiols with S-S spacing of 4.97 Å [8]. To circumvent any equilibration problems, our starting configurations are equilibrated using parallel tempering with respect to the Au-S interaction strength [9].…”

Electrostatic Induced Molecular Tilting in Self-Assembled Monolayers of n-Octadecylamine on Mica

“…Direct structural determination of self-assembled monolayers is commonly attained by high resolution scanning probe microscopies such as scanning tunneling (STM) and atomic force (AFM) microscopies. 3,4,5 In the particular case of long chain alkyl SAMs weakly adsorbed on a non-conducting support, STM cannot be used.…”