Thermoelectric Phenomena in Quantum Dots

Thermoelectricity is being intensively researched as it is believed to hold great promise for applications in power generation and cooling. One way to quantify the electrical power output of a thermoelectric material is the power factor, a function of electrical conductivity and thermopower. There are relationships between these relevant material properties that make efficient thermoelectric materials challenging to produce. The development of methods for creating nanostructured materials has allowed such trade-offs in material properties to be circumvented. Quantum dots are useful as model systems in this context since they have tunable energy filtering effects that are straightforward to characterize. The work described in this thesis explores thermoelectric phenomena in quantum dots. The aim of this work was to gain a better understanding of the most basic thermoelectric behavior of quantum dots. This knowledge can provide deeper insight into which mechanisms may be of interest in increasing the efficiency of a thermoelectric material. A deeper understanding also allows the measurement method itself to be used as a tool for characterization. A thermoelectric measurement can complement the more commonly used electrical conductance measurements, by both confirming and supplementing data. This could be of great importance for the investigation of physical phenomena in nanostructures. The quantum dots used in this work were defined in semiconductor nanowires. They were formed either by heterostructure growth or afterwards during fabrication of devices. The thermoelectric properties of the quantum dots were thoroughly investigated in the Coulomb blockade regime, and both linear and nonlinear responses as a function of the applied thermal gradient were observed and explained. Thermoelectric measurements were also successfully used to characterize different InAs nanowire devices, either with the nanowire as is or covered by a polymer electrolyte. Closer investigations of these devices revealed physical properties of the nanowires that could be used to improve thermoelectric efficiency. In fact, this thesis presents the first measurements demonstrating an increase in thermoelectric power factor at low temperatures.