Tunneling voltage polarity dependent submolecular contrast of naphthalocyanine on graphite.: A STM study of close-packed monolayers under ultrahigh-vacuum conditions

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 vapor-deposited on a graphite substrate in ultrahigh vacuum and investigated in situ at low temperature (50 K) using scanning tunneling microscopy (STM). The crystallographic monolayer parameters of the deposited films were 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 topographs with submolecular resolution, a clear difference is evident between tunneling images corresponding to occupied and unoccupied sample states. The intramolecular structure exhibited by the STM images can be understood qualitatively by considering frontier orbitals calculated for isolated molecules. Moreover, single point tunneling spectra show a distinct signature of the organic adlayer as expected for resonant tunneling.