Angle resolved ultraviolet photoelectron spectroscopy of oriented sexiphenyl layers

Ultra-thin films of oriented p-sexiphenyl (6P) on a GaAs(001) wafer were studied by angle resolved ultraviolet photoelectron spectroscopy (ARUPS) using UV photons in the energy range between 20 and 60 eV. The samples were prepared under ultra-high vacuum by controlled evaporation onto cleaned GaAs substrates, which were thermostated at enhanced temperature during the deposition process. The thickness of the deposited films played a very important role for the orientation of 6P molecules. For layers having a thickness of about 35 A, the molecules were oriented perpendicular to the substrate surface and for films of about 300-A thickness the molecules lost their upright orientation. The orientation of 6P molecules was monitored both by the anisotropy as visible in the measured ARUPS spectra and independently by atomic force microscopy (AFM) investigations carried out after UPS measurements. From the experimental photoelectron spectra, the full one-dimensional valence band structure of oriented p-sexiphenyl layers has been determined. While valence states between 2 and 5 eV show only weak dispersion, the bands between 5 and 11 eV show a clear momentum dispersion, which indicates the high degree of order in the investigated 6P layers.