Adsorption of thioether molecules on an alumina thin film

Low-temperature scanning tunneling microscopy has been employed to study the adsorption of (bis(3-phenylthio)-phenyl)sulfane (BPPS) molecules on an aluminum-oxide film grown on NiAl(110). Large variations in the molecular coverage on incompletely oxidized samples indicate substantial differences in the binding strength of BPPS to metallic (NiAl) versus dielectric (alumina) surfaces. From atomically resolved images, we obtain possible BPPS adsorption geometries on the oxide, in which the sulfur centers and not the phenyl rings of the molecule govern the interaction. A local hexagonal ordering of BPPS, as deduced from pair correlation functions, suggests a preferential binding of the BPPS sulfur atoms to Al ions with distorted pyramidal coordination in the oxide surface. Our work provides insight into rarely explored binding schemes of organic molecules on wide-gap oxide materials.