CHARACTERIZATION OF THE SURFACE-STRUCTURE OF CH3 AND CF3 TERMINATED N-ALKANETHIOL MONOLAYERS SELF-ASSEMBLED ON AU(111)

Surface elemental and structural characterization of hexadecanethiol a nd heptadecanethiol (C-16 and C-17 for short) and 16,16,16-trifluorohe xadecanethiol (FC16) self-assembled monolayers (SAMs) on a Au{111} sur face have been obtained from time-of-flight scattering and recoiling s pectrometry. The clean Au surface was also characterized in order to i dentify the azimuthal orientation of the SAMs with respect to the subs trate. Classical ion trajectory simulations were used to relate the ex perimental scattering and recoiling data to the surface structure. The scattered and recoiled atoms originate from the outermost five-six at omic layers, and azimuthal anisotropy was observed in the measurements . The results provide a model for the SAMs in which the alkyl chains c hemisorb with the S atoms situated above the face-centered-cubic (fcc) threefold sites of the Au{111} surface to form a continuous film with a (root 3 X root 3)R30 degrees structure that fully covers the Au sur face. The orientation of the molecular axis azimuth of the SAMs relati ve to the Au azimuthal directions was determined. The data indicate th at the molecular chains have specific tilt and twist angles relative t o the Au surface and six coexisting domains resulting from the six equ ivalent tilt directions of the molecular axis. Dramatic changes in the anisotropic patterns of the ion scattering azimuthal scans from the s urfaces of the SAMs with different terminations were observed. These p henomena result from the different tilt angles of the CH3 and CF3 grou ps. The data are consistent with free rotation of both the CH3 and CF3 groups. The C-16 SAM exhibited the best azimuthal features and was mo re resistant to radiation damage from the incident Ar+ scattering beam than the other films. Due to the tilt angle of the SAMs, an '''ion's eye view'' of the structure, i.e., the positions of the atomic cores a s experienced by the incoming keV ions, reveals a regular array of slo ping cavities within each unit cell. (C) 1998 American Institute of Ph ysics. [S0021-9606(98)70244-2].