Comparative study of structural order, thermal desorption behavior, and work function change of self-assembled monolayers of pentafluorobenzenethiols and tetrafluorobenzenethiols on Au(111)

Abstract

The surface and interface structures, thermal desorption behaviors, and electronic properties of pentafluorobenzenethiol (PFBT) and tetrafluorobenzenethiol (TFBT) self-assembled monolayers (SAMs) on Au(111) formed at different vapor deposition temperatures (298, 323, and 348 K) were examined to understand the effect of dipole directions in fluorinated aromatic thiol SAMs using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS), and Kelvin probe (KP) measurements. Molecular-scale STM observations revealed that PFBT SAMs on Au(111) had a well-ordered phase with a c(2 x root 3) structure at 348 K, whereas TFBT SAMs showed a well-ordered phase with a (2 root 3 x 8 root 2)R30 degrees structure at 323 K. TDS and XPS measurements also demonstrated that PFBT and TFBT SAMs showed significantly different thermal desorption behaviors and interface structures due to opposite directions of molecular dipole moments. The KP measurements showed that the extent of the work function change of SAM-modified Au substrates was lowest in most highly-ordered PFBT and TFBT SAMs formed at 348 K and 323 K, respectively, regardless of chemisorbed surface coverage. These findings revealed that the degree of structural order of SAMs is a more significant factor in causing the depolarization effect rather than chemisorbed surface coverage.