Effects of organic gas on third-order optical susceptibility and morphology of vanadyl-phthlocyanine thin films prepared on polyimide substrate

Abstract

Vanadyl-phthlocyanine (VOPc) thin films were prepared by a molecular beam epitaxy on a polyimide (PI) substrate at 150°C, and then they were exposed to dichloroethane vapor for 25 hours. Their morphologies were investigated with Vis/UV spectra and the surface images observed by an atomic force microscope (AFM). The Vis/UV spectrum of VOPc thin film before the organic gas treatment has a broad absorption shoulder between 640 and 740nm and an absorption peak at 840nm in the Q band region. It has been known so far that VOPc thin film has Phase I ,II and III. Phase I has two absorption peaks at 680 and 740nm. Phase II and Phase III are characterized with absorption peaks at 820 and 830nm, respectively. Therefore, the morphology having an absorption peak at 840nm is different from that of Phase 111 . The Vis/UV spectrum of VOPc thin film after the organic gas treatment has a broad absorption shoulder between 640 and 740nm and an absorption peak at 830nm in the Q band region. The absorption peak at 840nm shifts to 830nm by the organic gas treatment. This means that the phase morphology of VOPc thin film changes into Phase III during the gas treatment. On the other hand, VOPc molecules deposited on P1 substrate before the organic gas treatment form needle crystals. This indicates that VOPc molecules deposited on P1 substrate are perpendicular to PI substrate. The surface image of VOPc thin film after the gas treatment is smooth. This suggests that VOPc molecules are parallel to PI substrate. The third-order optical susceptibilities after and before the gas treatment are 3.75x10^-9 and 3.25x10^-9 esu, respectively. The molecular orientation of VOPc thin film is improved during the organic gas treatment.