Orientation of Thin Films Synthesized from Silicon Phthalocyanine Dichloride on a Highly Oriented Pyrolytic Graphite Investigated Using Near Edge X-ray Absorption Fine Structure

Molecular orientation of thin films of silicon phthalocyanine (SiPc) compounds on highly oriented pyrolytic graphite (HOPG) was investigated by near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The films were prepared by a casting method using solution of SiPc dichloride. XPS results showed that the chlorine atoms in SiPc dichloride were substituted by oxygen atoms when the film was heated in the air. The orientation of the molecules with respect to the substrate plane was investigated by the polarization dependences of the Si K edge NEXAFS spectra. For the sample heated in the air, two clear peaks appeared in the NEXAFS spectra at around 1847.2 and 1852.4 eV, which were assigned to Si 1s→σ_Si–N^* and Si 1s→σ_Si–O^*, respectively. The intensities of the resonance peaks showed strong polarization dependence. A quantitative analysis of the polarization dependence revealed that the Si–N bond was lying down while the Si–O bond was out of the plane.