Using a modified scanning tunnelling microscope we measure simultaneously the thermopower and conductance of single-molecule junctions of fullerenes connected to gold electrodes. We find that in contrast with C₆₀ (1) the endohedral fullerene Sc₃N@C₈₀ (2) forms jnctions in which the magnitude and sign of the thermopower depend strongly on the orientation of the molecule and on applied pressure. We demonstrate that the origin of this exceptional behavior is the presence of a sharp resonance near the Fermi level created by the Sc₃N inside the fullerene cage, whose energetic location, and hence the thermopower, can be tuned by applying pressure. These results reveal that Sc₃N@C80 is a bi-thermoelectric material, exhibiting both positive and negative thermopower, and provide an unambiguous demonstration of the importance of transport resonances in thermoelectric performance of organic materials.

(1) C. Evangeli et al, Nanoletters 2013, 13, 2141.

(2) L. Rincon-Garcia et al, Nature Materials 2015 doi:10.1038/nmat4487.