Engineering the Thermopower of C60 Molecular Junctions

We report the measurement of conductance and thermopower of C-60 molecular junctions using a scanning tunneling microscope (STM). In contrast to previous measurements, we use the imaging capability of the STM to determine precisely the number of molecules in the junction and measure thermopower and conductance continuously and simultaneously during formation and breaking of the molecular junction, achieving a complete characterization at the single-molecule level. We find that the thermopower of C-60 dimers formed by trapping a C-60 on the tip and contacting an isolated C-60 almost doubles with respect to that of a single C-60 and is among the highest values measured to date for organic materials. Density functional theory calculations show that the thermopower and the figure of merit continue increasing with the number of C-60 molecules, demonstrating the enhancement of thermoelectric preformance by manipulation of intermolecular interactions.