Tunneling voltage polarity dependent submolecular contrast of naphthalocyanine on Graphite.A stm study of close-packed monolayers under ultrahigh-Vaccum conditions

摘要

Self-assembled monolayers of free-base naphthalocyanine are examined in the context of analogous phthalocyanine films,revealing the influence of the more extended pi-electron system on monolayer properties Naphthalocyanine was apor-deposited on a graphite substrate in ultrahigh vacuum and investigated in situ at low temperatue (50 K)using scanning tunnelin gmicroscopy (STM).The crystallographic monolayer parameters of the deposited films ware determined by means of submolecularly resolved STM images.In addition,vacancies in the monolayer have allowed for an accurate measurement of single molecule azimuthal angles.In STM topograhs with submolecular resolution,a clear difference is eident between tunneling images corresponding to occupied and unoccupied sample states.The intramoleculr structure exhibited by the STM images can be understood qsualitatively by considering formatier orbitals calculated for isolated molecules.Moreover,single point tunneling specta show a sistinct signature of the organic adlayer as expected for resonant tunneling.