Naphthalocyanine molecules onto Si(111)7x7 and Si(100)2x1: modes of adsorption investigated with XPS

We have deposited various thicknesses of metal free naphthalocyanine (H2NPc) in ultra-high vacuum onto Si(111)7 x 7 and Si(100)2 x 1 and studied the interfaces in situ by means of standard monochromatized X-ray photoelectron spectroscopy. By comparing the core level N 1s and C 1s spectra obtained for the pure molecule with those typical of the interface at the submonolayer and intermediate level of deposition, we derived information on the ways of adsorption of the isolated molecules onto the substrates used. In particular, NPc adsorbs planarly to the Si(111)7 x 7 substrate showing strong chemisorption effects involving all the atoms of the molecule. However, the chemisorption involves only two C atoms when using the Si(100)2 x 1 substrate. In this case, the NPc molecules are likely to be adsorbed in an out-of-planar mode. We also show in this paper some experimental evidence on the NPc/Si(100)2 x 1 system at the lowest coverages in the presence of oxygen that confirm, even for the Si(100)2 x 1 case, our recent findings onto Si(111)7 x 7, i.e, the preferential adsorption of molecular oxygen onto the NPc molecules. (C) 1999 Elsevier Science B.V, All rights reserved.