Controlling the Thermoelectric Properties of Thiophene-Derived Single-Molecule Junctions

Thermoelectrics are famously challenging to optimize, because of inverse coupling of the Seebeck coefficient and electrical conductivity, both of which control the thermoelectric power factor. Inorganic-organic interfaces provide a promising route for realization of the strong electrical and thermal asymmetries required for thermoelectrics. In this work, transport properties of inorganic-organic interfaces are probed and understood at the molecular scale using the STM-break junction measurement technique, theory, and a class of newly synthesized molecules. We synthesized a series of disubstituted thiophene derivatives varying the length of alkylthio-linkers and the number of thiophene rings. These molecules allow the systematic tuning of electronic resonances within the junction. We observed that these molecules have a decreasing Seebeck coefficient with increasing length of the alkyl chain, while oligothiophene junctions show an increasing Seebeck coefficient with length. We find that thiophene-Au junctions have significantly higher Seebeck coefficients, compared to benzenedithiol (in the range of 7-15 mu V/K). A minimal tight-binding model, including a gateway state associated with the S-Au bond, captures and explains both trends. This work identifies S-Au gateway states as being important and potentially tunable features of junction electronic structure for enhancing the power factor of organic/inorganic interfaces.