Thermopower distribution of single molecule junctions with different interaction types

Abstract

The thermopower (S) distribution in single-molecule junctions with different interaction types were investigated by using a scanning tunneling microscope break-junction (STM-BJ) technique. We used 4,4’-bipyridine (BPy) and 1,2-bis(4-pyridyl)ethylene (BPyE) molecules, each having the Van der Waals (vdW) interaction between a pyridine ring and a Au atom and a donor-acceptor (DA) interaction between a nitrogen(N) atom and a Au atom, depending on the different binding geometries formed with the Au electrodes. From the full width at half maximum (FWHM) in the distribution of S, we found that S had a smaller variation for the vdW interaction compared to the DA interaction, due to the high binding stability of vdW interaction. Furthermore, we measured the molecular bonding forces which are in the range of 1.5 nN - 1.8 nN for the vdW interaction and 0.8 nN for the DA interaction. This confirms that the bonding is stronger for the vdW interaction than for the DA interaction, which is consistent with the experimental results for the S distributions as well as those for the molecular bonding stabilities.