Temperature and thickness dependence of molecular orientation of α-sexithienyl on Cu(111)

Atomic and electronic structures of alpha-sexithienyl(6 T) films grown on Cu(111) were studied by near edge x-ray absorption fine structure (NEXAFS) and S 1s x-ray photoemission spectroscopies. The polarization dependent NEXAFS indicated highly ordered arrays of 6 T molecules, with their molecular long axes parallel to the surface, at substrate temperatures lower than 300 K. On the other hand, their molecular long axes became normal to the surface at 360 K, while they initially adsorbed with their long axes parallel to the surface. The film thickness and substrate temperature dependence of the molecular orientation is discussed in terms of the van der Waals interaction between 6 T and Cu(111) and the kinetics of the growth process. It is demonstrated that the molecular orientation of the promising organic semiconductor 6 T can be controlled by substrate temperature. (C) 2003 American Institute of Physics.