Riassunto analitico
In today’s world, expanding population and increasing industrialization generate an increasing demand for energy. Despite significant advances in energy production that also exploits renewable energy sources, like wind power or photovoltaics, a substantial amount (about 67%) of the energy generated is wasted in the form of heat during various industrial processes, transportation, and power generation. One promising solution to this critical challenge is to improve the overall energy efficiency by development of thermoelectric generators (TEGs), which can directly convert waste heat into electrical energy.
Miniaturization of TEGs down to the nanoscale size is becoming more and more attractive for their use in many devices such as smartphones, CPUs, nanoscale sensors and implantable medical devices, especially for the possibility of engineer the transport properties of electrons and phonons, which results in an increasing in the Figure of Merit (ZT factor), the adimensional parameter that quantifies the thermoelectrical efficiency of a system.
Ab initio simulations provide a strategy to study and predict the transport properties of a material, which also allows to design new structures with enhanced ZT.
In this work we present theoretical calculations on InAs-InAsSb nanowire heterostructures carried out with Density Functional Theory and Density Functional Perturbation Theory with the solution of the Boltzmann Transport Equation (BTE) in the relaxation time approximation both for electrons and phonons. This approach allows to obtain all the transport properties necessary to evaluate ZT, such as the Seebeck coefficient, the electrical conductivity and the thermal conductivity both for electrons and phonons. To this end, we employ the Quantum Espresso code in combination with Phoebe code for the solution of the electronic and phononic BTE; this provides a workflow to predict with very good accuracy and with a reasonable computational cost the figure of merit of nanostructures using a semi-classical framework within Electron Phonon Average Approximation and Wannier Transport calculation.
The theoretical results confirm previous experimental studies in twinned InAsSb superlattice nanowires. Besides they highlight differences between wurtzite (WZ) and zincblende (ZB) phases in terms of phonon distribution, for which a new herostructure is suggested, made by a combination of InAs WZ and InAsSb ZB phases. The latter nanowire heterostructure is the object of a preliminary experimental investigation, whose methods and results are also reported in this work. The thermoelectric properties of these newly engineered nanostructures are experimentally addressed resorting on nanodevice design and fabrication techniques (electron beam lithography), and performing the measurements of the transport properties of the fabricated devices. In particular field effect transistors have been realised allowing for the electronic characterization (electron
conductivity and Seebeck coefficient) as well as for the measurement of the thermal conductivity exploiting the 3 omega method. The combined theoretical and experimental approach reported in this work shines light on the electrical and thermal transport mechanisms driving the thermoelectric applications of nanoscale semiconductor materials.
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