Molecular orientation of vacuum-deposited thin films of zincnaphthalocyanine

Abstract

Zinc 2,3-naphthalocyanine (ZnNc) was vacuum evaporated onto glass, NaCl, and highly oriented pyrolytic graphite (HOPG) substrates, and the molecular orientation was investigated by x-ray-diffraction, Fourier-transform-infrared- reflection-absorption spectroscopy, electronic spectroscopy, transmission electron microscopy, and scanning electron microscopy observations. Three types of molecular orientations were assigned for the deposited thin films depending on the different substrates kept at 250°C. ZnNc molecules deposited on glass formed columnar crystals taking N orientation, in which molecular planes oriented perpendicularly to the substrate surface. Unidirectional epitaxic growth was observed in the film deposited on NaCl. ZnNc molecules seemed to be stacked in parallel having P orientation, where the molecular planes were parallel to the NaCl(001) surface. The ZnNc square lattice made an angle of 45°to the NaCl[100] direction (4×4-type orientation). The film deposited on HOPG exhibited I orientation, in which ZnNc molecular planes were inclined to the HOPG basal plane. The tilting molecules stacked in the direction normal to the surface. These different orientations in ZnNc thin films resulted in significant electronic spectral change in the longer-wavelength region.