Experimentally Probing the Effect of Electronic Structure on the Thermoelectric Properties of Molecular Junctions.

Abstract

To better understand thermoelectric properties of molecular junctions, I developed (in collaboration with others) novel tools for studying thermal and thermoelectric phenomena at the nanoscale. Specifically, I first established ultra-high vacuum scanning thermal microscopy (UHV-SThM) that is capable of quantitatively mapping temperature fields with ~15 mK temperature resolution and ~10 nm spatial resolution. In this technique, a custom nanofabricated atomic force microscopy (AFM) probe, with a nanoscale Au-Cr thermocouple integrated into the probe tip, is used to study temperature information of nanoscale devices. Operation in an UHV environment eliminates parasitic heat transfer between the tip and the sample enabling quantitative measurements of temperature fields on metallic and dielectric surfaces with excellent spatial resolution. By leveraging UHV-SThM, I performed quantitative studies of heat dissipation (Joule heating) in gold nanowires during electromigration. The experimental results unambiguously illustrated that electromigration begins at temperatures significantly lower than the melting temperature of gold. Further, it was shown that during electromigration voids predominantly accumulate at the cathode resulting in asymmetric temperature distributions, which provides novel insights into the microscopic details of hot spot evolution during electromigration. Finally, I investigated electrostatic control of the thermoelectric properties of molecular junctions, which is key to extremely efficient thermoelectric energy conversion. This was accomplished by carefully designing and nanofabricating three-terminal devices that feature temperature gradients exceeding 109 K/m across nanogaps. Using these devices I studied thermoelectric effects in Au-biphenyl-4,4’-dithiol-Au and Au-fullerene-Au junctions and demonstrated that the Seebeck coefficient and electrical conductance of molecular junctions can be simultaneously increased by electrostatic control of charge transmission characteristics—for the first time ever. In particular, the studies of fullerene junctions show that thermoelectric properties can be dramatically enhanced when the dominant transport orbital is located close to the chemical potential illustrating the intimate relationship between the thermoelectric properties and charge transmission characteristics of molecular junctions. The novel three-terminal devices developed in this work are expected to enable systematic exploration of predictions that promise extremely efficient molecular-scale thermoelectric energy conversion.