Variation of Energy Level Alignment in Molecular Junction Elongation

The energy level alignment change in single-molecule junctions during junction elongation was investigated by using multi-IV measurements with a scanning tunneling micro-scope break-junction technique. We used 1,2-bis(4-pyridyl)ethylene molecule with Au electrodes, and observed a negative slope in the conductance trace during junction elongation. The trace was divided into three regions having high, middle, and low conductance in that order with increasing elongation. The values of the conductance and the thermopower obtained by multi-IV measurements with a temperature gradient were used to determine the energy level alignment changes of the molecule during junction elongation in each region, where the dominant orbital for transport is the lowest unoccupied molecular orbital. We found that the energy level alignment gets closer to the Au Fermi energy with decreasing conductance. These results thus provide quantitative information on energy level alignments in a metal-molecule interface during junction elongation and evolution.