Resolving the 3D Orientation of Terphenylthiol Molecules on Noble Metals with Kelvin Probe Force Microscopy

The local work function is an invaluable feature for the specific analysis of the influences of atomic and molecular nanostructures on each other as well as the underlying surface. Adsorbate molecules can modify this parameter by introducing an electrical dipole moment, which affects the local contact potential. This can be accessed by Kelvin probe force microscopy (KPFM). In this paper, we demonstrate, by combining highly resolved topographic atomic force microscopy (AFM) data with the simultaneously acquired local work function signal, how each of these channels yield one angular coordinate, resulting in the three-dimensional determination of surface molecular dipole orientations. We studied the adsorption of terphenylthiol (TPT) self-assembled monolayers on Au(111) and Ag(111), as it is relevant in the light of electron radiation-induced transformation to carbon nanomembranes. We present noncontact AFM data combined with frequency-modulated KPFM in ultrahigh vacuum room temperature without any kind of deliberate tip functionalization. Our results show a surface coverage-dependent Langmuir-like evolution of phases with domains of flat lying as well as with upright molecular arrangements. Whereas we found an almost complete vertical orientation on silver, the orientation on gold was found to be tilted, corresponding to sp- and sp(3)-hybridized bond angles, respectively.