PHOTOASSISTED STM OF PHOTOCONDUCTIVE ORGANIC THIN FILMS

The films of phthalocyanine and perylene derivatives found an application for solar cells [1,2], light-emitting diodes [2], gas sensors [3]. The improving of such devices can be reached by forming of nanocomposites [4,5]. The determination of correlation between the structure and electrical properties of nanocomposites requires the experimental methods of phase analysis on a nanometer scale. Photoassisted scanning tunneling microscopy (photoSTM) combines the high spatial resolution of STM with energy selectivity of optical spectroscopy and can be used in nanoscale phase analysis. However, many difficulties restrain the application of photoSTM technique in this area [6]. The main difficult is the heating due to light absorption. The heating causes thermal expansion of STM tip affecting the tunneling gap width. As we have showed in previous reports [7,8], the illumination of organic semiconductors influences on the STM images and local current-voltage characteristics, but remain not clear the contribution of thermal expansion in these effects. We present in this report the photoSTM investigation results of N,N’-bismetyl-perylene-3,4,9,10tetracarboxylic diimide (perylene pigment PTCDI) and copper phthalocyanine (CuPc) thin films.