NC-AFM Study of the Adsorption of Hexamethoxytriphenylene on KBr(001)

Molecular adsorption studies on atomically clean, well-defined surfaces of bulk insulators are still very scarce and have been restricted to a small number of molecules. In this study, the adsorption of 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP) on KBr(001) was investigated by atomic force microscopy in noncontact mode (NC-AFM) under ultrahigh vacuum at room temperature from submonolayer to multilayer coverage. This molecule is the first member of a new family of molecules, designed and synthesized for adsorption studies on alkali halide surfaces. They were built around a flat aromatic triphenylene core equipped with peripheral polar groups to enhance the interaction of the molecule with the surface in order to limit molecular diffusion at room temperature. Constant-frequency-shift NC-AFM imaging established the following adsorption sequence: The molecules decorate the KBr steps before forming two-molecular-layer-thick islands with a distorted hexagonal arrangement. At higher coverage, a second generation of much taller islands with the structure of the bulk HMTP crystal appears as a consequence of a dewetting transition, driven by the deformation energy accumulated in the first-generation islands during growth. High-resolution images obtained on top of these islands show submolecular contrast. These results will be useful in optimizing the next generation of molecules of this new family.