Thickness Dependence of the Epitaxial Structure of Vanadyl Phthalocyanine Film

We have studied the thickness dependence of the crystal structure in vanadyl phthalocyanine (VOPc) films epitaxially grown on KBr(100) surface. Optical second-harmonic generation observed in oblique incidence increases nearly quadratically with film thickness up to at least 64 nm and saturates, indicating a structural change from an epitaxial polar VO orientation to a nonpolar bulk crystal structure. The latter was assigned to monoclinic structure based on X-ray diffraction. Optical absorption spectra in visible and infrared regions and scanning electron microscopy also show the structural difference between the films thicker and thinner than about 80 nm.