Scanning tunneling microscopy observation of stilbene derivative self-assembled on HOPG

Scanning tunneling microscopy (STM) was used to analyze the surface structures of trans-stilbene (1) and its derivative, trans-1-octyloxy-4-styryl-benzene (2) deposited on the basal plane of HOPG at room temperature. For compound 1, due to the high mobility of small adsorbate, an ordered domain at the interface layer was not formed under the experimental conditions. For compound 2, it was observed with near molecular resolution that adsorbed molecules form regular two-dimensional (2D) arrangement. This result demonstrates that by adding the octyloxyl group, the stability of molecular adsorption on HOPG can be greatly enhanced. The immobilization effect of alkyl substitutent is attributed to the van der Waals interaction between the alkyl chain and the graphite substrate. A molecular model based on the STM image and computer simulation is proposed and discussed.