Search for a wetting layer in thin film growth of para-hexaphenyl on KCl(001)

The non-existence of a strongly bound wetting layer for the system para-hexaphenyl (6P)-KCl(001) was verified by thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The 6P films were grown by physical vapour deposition under ultrahigh vacuum conditions at 400 K. TDS showed just a single desorption peak even down to a coverage of 0.1 nm mean film thickness. The heat of evaporation for 6P was determined to 2.3 eV. From the change of the XPS K2p substrate signal as a function of the mean film thickness one can conclude that a strong de-wetting exists at the very beginning of the 6P layer growth at 400 K. Additional investigations with atomic force microscopy reveal that at this initial stage the film grows in a form of needle like islands with a high aspect ratio and subsequently terraced mounds of 6P are formed. No dissociation of 6P on KCl was observed.