Temperature-induced molecule assembly effects on the near-edge X-ray absorption fine-structure spectra of Zn-phthalocyanine layers on Si substrates

The molecular arrangement of vacuum thermally deposited polycrystalline Zn-phthalocyanine (ZnPc) layers on Si substrates is investigated using near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy in the proximity of the carbon edge at E₀ = 287.33 eV. The data were collected as a function of the deposition substrate temperature T_S (30, 90, 150°C) and the incidence angle θ (20°, 45°, 70°, 90°) of the synchrotron beam with respect to the sample plane. Each spectrum was analysed by mathematical simulation applying an error function for the carbon edge and a set of Voigt and (asymmetric) Gaussian functions for C1s → π* and C1s → σ* transitions of ZnPc, respectively. It turned out that part of the organic layer consists of adventitious carbon, which does not contribute to the molecular transitions of ZnPc, whereas all molecular features exhibit polarization-dependent peak areas pointing to a reasonable fraction of well-assembled molecules at any T_S. The highest adventitious carbon fraction was found at T_S = 30°C, whereas the highest polarization dependence was found at T_S = 90°C. The calculated average molecular tilt angles for the three temperatures (30, 90, 150°C) were γ = 60.6°, 68.7° and 66.7°, respectively. If only the polarization-dependent fractions are considered, then the three samples can be mathematically described using a shared molecular tilt angle of γ = 68.7°, which corresponds to the average tilt angle of the T_S = 90°C sample.