Thermoelectric performance of various benzo-difuran wires

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

For the purpose of creating single-molecule junctions, which can convert a temperature difference ΔT into a voltage ΔV via the Seebeck effect, it is of interest to screen molecules for their potential to deliver high values of the Seebeck coefficient S = -ΔV/ΔT. 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@C₈₀, Sc₃C₂@C₈₀, and Er₃N@C₈₀, 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 σ_S of EMF-based junctions are correlated with the geometric standard deviation σ and the charge inhomogeneity σ_q. We benchmark these molecules against C₆₀ and demonstrate that both σ_q, σ_S are the largest for Sc₃C₂@C₈₀, both are the smallest for C₆₀ and for the other EMFs, they follow the order Sc₃C₂@C₈₀ > Sc₃N@C₈₀ > Er₃N@C₈₀ > C₆₀. A large value of σ_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 σ_S are associated with distributions of Seebeck coefficients containing both positive and negative signs, which reveals that all these EMFs are bi-thermoelectric materials.

A New Approach to Materials Discovery for Electronic and Thermoelectric Properties of Single-Molecule Junctions

We have investigated a large set of symmetric and asymmetric molecules to demonstrate a general rule for molecular-scale quantum transport, which provides a new route to materials design and discovery. The rule states "the conductance GXBY of an asymmetric molecule is the geometric mean of the conductance of the two symmetric molecules derived from it and the thermopower SXBY of the asymmetric molecule is the algebraic mean of their thermopowers". The studied molecules have a structure X-B-Y, where B is the backbone of the molecule, while X and Y are anchor groups, which bind the molecule to metallic electrodes. When applied to experimentally measured histograms of conductance and thermopower, the rules apply to the statistically most probable values. We investigated molecules with anchors chosen from the following family: cyano, pyridl, dihydrobenzothiol, amine and thiol. For the backbones B, we tested 14 different structures. We found that the formulas (GXBY)(2) = GXBX*GYBY and SXBY = (SXBX + SYBY)/2 were satisfied in the large majority of the cases, provided the Fermi energy is located within the HOMO-LUMO gap of the molecules. The circuit rules imply that if measurements are performed on molecules with nA different anchors and nB different backbones, then properties of nA(nA + 1)nB/2 molecules can be predicted. So for example, in the case of 20 backbones and 10 anchors, 30 measurements (or reliable calculations) can provide a near quantitative estimate for 1070 measurements of other molecules, at no extra cost.