The influence of carbon on the adsorption/desorption kinetics and monolayer formation of p-quaterphenyl on Au(1 1 1)

The influence of surface carbon on the formation of highly regular thin films of p-quaterphenyl (4P) grown on a Au(1 1 1) surface has been investigated in the mono- and multilayer regime by thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS) and micro-channelplate low energy electron diffraction (MCP-LEED) under UHV conditions. A small carbon pre-coverage of 0.03 monolayers (ML) improves the surface mobility of adsorbed 4P molecules with respect to the clean surface by passivating Au surface defects. This enables a highly crystalline 4P film growth even at a low substrate temperature of 93 K. Otherwise, the thermodynamically stable monolayer structure is identical to that on the C-free surface, namely consisting of alternating edge/face packed, lying molecules. The structure of the first 4P monolayer grown on a 0.15 ML C covered Au(1 1 1) surface is different. Only edge-on oriented molecules accommodate in the corresponding surface unit cell. At a C coverage of 0.5 ML the first 4P monolayer is disordered, whereas for higher coverages a highly crystalline structure with upright standing molecules is adopted. Depending on the substrate temperature during film growth the surface carbon causes a different wetting behaviour of the 4P films, resulting in a different mesoscopic film morphology.