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Tip-sample distance-dependent current-voltage tunneling spectroscopy on monolayers of base-free naphthalocyanine (Nc), a planar molecule, and tin-naphthalocyanine (SnNc), a nonplanar molecule, has been studied on a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface using a variable-temperature STM at 50 K under ultra-high vacuum conditions. The current-voltage curves show an unsymmetrical diode-like nature especially at large tip-sample distances in both cases. Normalized differential conductivity of all spectra has been considered for further analysis. The ionization and electron affinity levels are compared with the single-molecule local density of states (LDOS) near the Fermi energy using a theoretical calculation for Nc and SnNc. A tip-sample distance-dependent highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap shrinking is observed in the case of Nc, in which the filled levels of the molecules are pinned while the unfilled levels near the Fermi energy are shifting toward lower energy. In contrast, there is no such HOMO-LUMO gap shrinking in the case of the SnNc decreasing tip-sample distance. However, a subsequent increase in the tunneling current was observed by almost 1 order of magnitude compared with Nc. A model is proposed to explain this phenomenon where the Nc-graphite interface is considered as a pure capacitive interface.
Gopakumar et al. (2006) studied this question.