Much attention has been paid to elucidate transport properties in molecular junctions such as thiol molecule bridged by a pair of metal electrodes. Now it is recognized well that the current-voltage characteristics are strongly affected by the bonding nature between the molecule and metal substrate, and the electronic states induced at the molecule-metal interface. However, the experimental information of the local electronic states remains rather limited.
In the present study, we report a direct observation of interface states for alkanethiols (CnH2n+1SH, n=1-3) on a Pt(111) substrate by metastable atom electron spectroscopy (MAES). This method is based on energy analysis of electrons emitted by thermal collision of metastable atoms such as He*(1s2s, 23S) with a solid surface.[1] We have found that the S 3p-derived states of alkanethiols are considerably broadened upon chemisorption to form a sharp Fermi edge, while such a metallic feature is completely missing in the physisorbed multilayer. Based on the experimental findings and the first principle calculations, we introduce a new classification of alkanethiols adsorbed on a metal surface, i.e., insulating, metallic, and amphibious molecule. The electric classification of adsorbed molecules would provide a useful concept for understanding ordinary molecule-metal interactions, transport phenomena in molecular junctions, etc.
[1] Y. Harada, S. Masuda, H. Ozaki, Chem. Rev. 97, 1897 (1997). |