MANIPULATION OF MOLECULAR-ORIENTATION IN ULTRATHIN ORGANIC FILMS - NTCDA ON AG(111)

The bonding of ,4,5,8-naphthalene-tetracarboxylicacid-dianhydride (NTC DA) on Ag(lll) and its molecular orientation were investigated by X-ra y photoelectron (XPS) and near edge X-ray absorption fine-structure sp ectroscopy (NEXAFS). Vapour-deposited NTCDA films show high orientatio nal order in the mono-and multi-layer regime. In the monolayer regime the molecules are strongly bound to the substrate via the conjugated p i-system of the naphthalene core which leads to a parallel orientation of the molecular plane to the Ag(lll) substrate. For thicker films (e .g., 150 Angstrom) the molecular orientation strongly depends on the s ubstrate temperature during preparation. Film preparation at 285 K res ults in upright standing molecules, whereas low substrate temperatures yield a uniform molecular orientation with the molecular plane parall el to the substrate. This behaviour is explained by the weak intermole cular interaction and the adsorption kinetics.