Cliff McCold1 Lucas Domulevicz1 Josh Hihath1

1, University of California, Davis, Davis, California, United States

The development of large-scale, highly-efficient thermoelectric materials has emerged as one of the ultimate goals of nanoscale science and technology, and the last two decades have seen incredible progress in the advancement of thermoelectric materials. The use of superlattices, quantum-wells, nanoparticle assemblies, and nanostructured composite materials have significantly increased the thermoelectric figure of merit, ZT, of many systems. Along these lines, hybrid organic-inorganic nanostructured materials provide unique opportunities for improving thermoelectric efficiency because the electrical properties and thermal properties have the potential to be tuned independently. In this study, we demonstrate control over both the Seebeck coefficient and the type of charge carrier (n-type or p-type) in a series of hybrid 2-dimensional molecule-nanoparticle superlattices. Interestingly, by systematically changing the length and HOMO-LUMO gap of a series of electrically conductive heteroacene-ladder molecules used to interlink gold the nanoparticles in a monolayer array, we are able to measure a crossover in the sign of the Seebeck coefficient, corresponding to a crossover in the sign of the majority charge carrier (from positive to negative). This finding is confirmed with Hall-effect measurements. To understand the origins of this effect we examine the redox properties of the heteroacene molecules and find that the alignment between the chemical potential of the nanoparticles and the HOMO and LUMO levels of the molecule change throughout the series, resulting in electron-based tunneling between nanoparticles for some molecules and hole-based tunneling in others, which in turn results in the different signs of the Seebeck coefficient and Hall voltage. Our findings develop a stronger understanding of charge transport in hybrid molecule-nanoparticle monolayer arrays, establish a novel framework for maximizing the thermoelectric efficiency of these materials, and demonstrate that both electron and hole transport can be attained in these systems, thus opening a new opportunity for creating integrated thermoelectric devices.