Structure and electronic properties of CH3- and CF3-terminated alkanethiol monolayers on Au(111):: a scanning tunneling microscopy, surface X-ray and helium scattering study

The structure and the electronic properties of a series: of CH3- and CF3-terminated alkanethiol monolayers on Au(111) have been studied by scanning tunneling microscopy (STM) and surface X-ray and helium scattering. At full coverage, the CH3-terminated monolayers form long-range ordered domains of a (root3 x root3)R30degrees and a (2root3 x 3)R30degrees standing-up phase. By thermal desorption, distinct lying-down phases of intermediate density as well as the (p x root3) lying-down phase were generated. In contrast, the CF3-terminated monolayers at full coverage form a standing-up phase of hexagonal symmetry that exhibits no long-range order at room temperature. Even after annealing, the domain sizes are smaller by more than one order of magnitude compared to the CH3-terminated thiol monolayers. A comparison of the low-density lying-down phases suggests no measurable influence of the CF3-group on the ordering. The current-voltage dependence (I-V-curves) measured by scanning tunneling spectroscopy (STS) shows no voltage gap for CH3-terminated decanethiols. In contrast, in the I-V-curves for CF3-terminated decanethiol monolayers, an asymmetric voltage gap of about 2 V can be clearly observed. The latter results are discussed in terms of a microscopic model that includes the formation of an interfacial Coulomb barrier at the CF3/vacuum boundary. In addition, the effects of the tunneling conditions on the STM image contrast were examined. These studies demonstrate that the nature of the STM images and thus, the respective apparent lateral order of the films, strongly depends on the choice of the tunneling parameters. (C) 2001 Elsevier Science B.V. All rights reserved.