Thermoelectric experiments on nanowire-based quantum dots

Abstract: This thesis experimentally investigates the possibilities of using quantum effects in semiconductor nanostructures for engineering their thermoelectric properties. More specifically, heterostructured InAs/InP nanowires are used to create short InAs quantum dots (QDs) with electronic state structure resembling that found in atoms. Recently developed top-heater architecture is used to apply a temperature differential across the QDs. The nanowire-based QD devices are used for studies of thermoelectric effects at the nanoscale and for experimental demonstration of particle-exchange heat engines.This thesis first gives an overview of the most important physical effects governing the behavior of quantum dots (QDs). The Master equation approach to model the electronic transport in QDs is introduced in the sequential electron tunneling approximation. It is used to illustrative the transport behavior of QDs. The Landauer-Büttiker approach is also introduced as a reference and the differences with the sequential electron tunneling approximation are discussed. A summary of the most important literature on the thermoelectric properties of single QDs is given and discussed to provide the context for the experimental studies in this thesis. Finally, a description of the experimental methods used in this thesis is given.There are three studies included in this thesis. The first investigates the nonlinear thermoelectric response of a QD with an applied thermal bias. A strongly nonlinear behavior is observed which can be fully explained by the interplay between different QD electronic states contributing to thermocurrent in opposing directions. The second study experimentally demonstrates efficient particle-exchange heat engines based on QDs for the first time. The analysis of the heat engines' power and efficiency indicate heat-to-electric conversion efficiencies up to 70% of Carnot efficiency. The third study investigates the thermoelectric response of QDs in the presence of Kondo correlations. It verifies a previous theoretical prediction that the sign of the thermoelectric signature in QDs inverts due to the Kondo correlations.The experiments presented in this thesis have been successful in filling a gap between theory and experiments on several fronts. Future experiments could, for example, study Kondo-correlated QDs in the nonlinear thermoelectric response regime in the presence of magnetic field, where theory predictions are harder to obtain, or could employ thermoelectric characterization techniques to study entropy of various different QD states.