The interest in single molecular electronics has recently grown due to the developments of scanning probe techniques and of precise synthesis of supramolecules. The electronic properties of molecules, which depends on the molecular orbital energy level relative to the Fermi level of the metal, can be modified by doping holes or electrons from a metal substrate. In order to study the electron transfer between the molecule and the metal substrate, we observed spatial distribution of effective tunnel barrier height by using scanning tunneling microscopy (STM). For completely fused porphyrin array, which is expected as a conducting molecular wire, the tunneling barrier height measured above the molecule is smaller than above the metal substrate, meaning that the molecule is positively charged. In case of deoxyribonucleic acid (DNA), whereas the molecular orbital is observed near Fermi level of the metal by photoemission analysis, the electron transfer was not detected by the STM, because deoxyribose-phosphate chains screen base pairs from the metal substrate.
