Here we report on the direct observation of a molecular p-n junction with a low-temperature STM. The molecular p-n junction has been realized in a phenyl-linked porphyrin dimer, in which a free-base porphyrin and a cobalt-substituted porphyrin are covalently linked with a phenyl ring. The porphyrin dimer was thermally deposited on Au(111), and then the sample was transfered to the cold STM stage. All STM images were acquired in a constant-current mode at 63 K.
We find that the porphyrin dimers are ordered into two-dimensional islands on Au(111), and the cobalt-substituted porphyrin within the porphyrin dimer are recognized by a low-bias STM imaging at Vs~0.3 V, in which the localized d-orbital of the central cobalt is clearly visible.
In a typical filled-state STM image of the porphyrin dimer, we have observed that the free-base porphyrin appears brighter, whereas the cobalt-substituted porphyrin appears darker. In contrast, the brightness of both porphyrins was inverted in a empty-state STM image. This bias-dependent image reversal should be originated from the difference in the electronic structures between the free-base and cobalt-substituted porphyrins. To understand the local electronic structures of the porphyrin dimer in more detail, we have performed tunneling spectroscopy at both of the free-base and cobalt-substituted porphyrins. The spectra show that the HOMO-LUMO pi orbitals of the free-base porphyrin are shifted higher by 0.2-0.5 eV in energy than those of the cobalt-substituted porphyrin, suggesting the formation of the molecular p-n junction. We sill present the detailed electronic structures of the free-base and cobalt-substituted porphyrin dimers on Au(111). |