Thermoelectric Transport in Two Dimensions

As modern electronics shift towards nanoscale, the device performance becomes limited by the increasing dissipated power density. Proper heat management is one of the major challenges in the design of nanoelectronic devices. Thermoelectric devices can pump heat using electrical energy and can be used to cool the chips. They are also used to convert thermal energy into electrical energy and can be used to recycle industrial waste heat into electric power or power wearable electronics.
This dissertation focuses on the characterization of thermoelectric transport in two-dimensional (2D) systems, including Si-based thin films and few-layer 2D materials, and evaluates the strategies to optimize their performance. A thermoreflectance-based method, heat diffusion imaging, is developed to measure the in-plane thermal conductivity of thin films or even few-layer samples supported on a substrate. Enhanced thermoelectric performance has been achieved by adding periodic nano-sized holes to suppress the phonon transport and by doping via surface charge transfer to improve carrier mobility. The thermal conductivity of a holey Si thin film reduces by almost 20 times compared to a high-doped bulk Si sample or Si thin film without holes. P-type doping has been realized in holey Si thin films with organic surface dopants F4TCNQ, which improves the thermoelectric power factor by two orders of magnitude.
2D transitional metal dichalcogenides (TMDs) are the other material system of interest. They possess a wide range of electronic properties, from insulating to superconducting, depending on their crystalline structures and the topology of their electronic structure. Their electronic properties are usually superior to their bulk counterparts and can be modified, for instance, by applying an electrostatic back gate bias. In this dissertation, we study the in-plane thermoelectric transport of 4-layer NbSe2 grown by molecular beam epitaxy. It includes the first thermal conductivity measurement in the low-temperature range (below room temperature) reported for a few-layer NbSe2. The charge carriers added by back gate doping are not enough to alter the behavior of the metallic NbSe2, but they can change the resistance of a semiconducting 2H-MoTe2 by one order of magnitude and improve its thermoelectric power factor by up to 89% compared to the intrinsic value. The effects of different metal contacts, i.e., Ti and TiOx, have been evaluated for WSe2 in a cross-plane geometry. In general, thermal and electrical contact resistances are particularly important in the design of 2D devices and can change the efficiency of these devices significantly.