Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy Studies of Planar and Nonplanar Naphthalocyanines on Graphite (0001). Part 1: Effect of Nonplanarity on the Adlayer Structure and Voltage-induced Flipping of Nonplanar Tin−Naphthalocyanine

Abstract

The adsorption of base-free naphthalocyanine (Nc), a planar molecule, and tin-naphthalocyanine (SnNc), a nonplanar molecule, on a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface at low sample temperature (50 K) has been studied using a variable-temperature scanning tunneling microscope in ultra-high vacuum conditions. The planar molecules form large areas of defect-free ordered monolayer with high molecular packing density while the nonplanar molecules show different phases of adsorption with lower molecular packing density. The SnNc adlayers follow the same geometry as the graphite substrate and form pure phases of adsorption with either all molecules in a Sn(2+) up or Sn(2+) down geometry. Moreover, a one-dimensional selectivity is observed in still another phase of Sn(2+) down geometry. Multilayers show a completely different kind of adsorption in each case. Nc molecules show columnar pi-stacking whereas the SnNc molecules exhibit noncolumnar stacking. Distinctly, a voltage-induced flipping of nonplanar tin-naphthalocyanine in the monolayer has been observed which can possibly be applied to single-molecular information storage.