Site-dependent electronic structures of a single molecule on a metal surface studied by scanning tunneling microscopy and spectroscopy

Single-molecule observation of the electronic structures of para-cyanobenzoate (pCB) adsorbed on Cu(1 1 0) has been performed using scanning tunneling microscopy (STM) and spectroscopy (STS). We found that pCB has two types of the adsorption site on Cu(1 1 0); i.e., two oxygen atoms of pCB are bridged between adjacent Cu atoms at the short- or long-bridge sites. STS and STS mapping revealed that the pCB adsorbed at the short-bridge site has a resonant peak at 2.0 V above the Fermi level, which is assigned to the lowest unoccupied molecular orbital (LUMO) of pCB. However, the LUMO state is shifted toward lower voltage (1.2 V) when the pCB molecule is adsorbed at the long-bridge site. The energy levels of the LUMO state, depending on the adsorption site of pCB, can thus be ascribed to the degree of the electronic interaction between pCB and the Cu substrate. The site transformation of pCB induced by the injection of tunneling electrons from the STM tip has also been presented.