Exploring seebeck-coefficient fluctuations in endohedral-fullerene, single-molecule junctions

For the purpose of creating single-molecule junctions, which can convert a temperature difference Delta T into a voltage Delta V via the Seebeck effect, it is of interest to screen molecules for their potential to deliver high values of the Seebeck coefficient S = -Delta V/Delta T. Here we demonstrate that insight into molecular-scale thermoelectricity can be obtained by examining the widths and extreme values of Seebeck histograms. Using a combination of experimental scanning-tunnelling-microscopy-based transport measurements and density-functional-theory-based transport calculations, we study the electrical conductance and Seebeck coefficient of three endohedral metallofullerenes (EMFs) Sc3N@C-80, Sc3C2@C-80, and Er3N@C-80, which based on their structures, are selected to exhibit different degrees of charge inhomogeneity and geometrical disorder within a junction. We demonstrate that standard deviations in the Seebeck coefficient sigma(S) of EMF-based junctions are correlated with the geometric standard deviation sigma and the charge inhomogeneity sigma(q). We benchmark these molecules against C-60 and demonstrate that both sigma(q), sigma(S) are the largest for Sc3C2@C-80, both are the smallest for C-60 and for the other EMFs, they follow the order Sc3C2@C-80 > Sc3N@C-80 > Er3N@C-80 > C-60. A large value of sigma(S) is a sign that a molecule can exhibit a wide range of Seebeck coefficients, which means that if orientations corresponding to high values can be selected and controlled, then the molecule has the potential to exhibit high-performance thermoelectricity. For the EMFs studied here, large values of sigma(S) are associated with distributions of Seebeck coefficients containing both positive and negative signs, which reveals that all these EMFs are bi-thermoelectric materials.