Orientational ordering of the second layer of C60 molecules on Au(111).

We have studied the orientational ordering of the second layer of C(60) molecules on Au(111) using scanning tunnelling microscopy (STM) at 77 K. The orientation of individual molecules within the second layer follows a regular pattern, giving rise to a 2 × 2 superlattice. The long-range order of the 2 × 2 lattice depends on the structure of the first molecular layer with the best ordering found inside the R14° domain. The second layer formed on top of the contrast-disordered R30° domain consists of patches of bright and dim molecules. The contrast between bright and dim patches shows a clear dependence on the sample bias. This bias-dependent contrast is explained by considering the contributions to tunnel current from HOMO and LUMO mediated electron transfer processes. Scanning tunnelling spectroscopic measurement reveals the narrowing of the HOMO-LUMO gap for the layer of molecules in direct contact with the Au(111) substrate.