Resumen de Transport property measurements of nanostructured materials
The aim of this doctoral thesis is the measurement of the transport properties of nano-structures and its correlation with the physical phenomenon that give rise to those results. This thesis can be divided in two fundamental sections: a) Measurements of thermal properties and b) measurements of electrical properties.
Regarding the samples that have been analyzed, a wide variety of materials that correspond to either inorganic samples, like silicon germanium or bismuth telluride, or organic ones, like several types of polymers, are found. However, all of them present a common thread, they are thermoelectric materials. These kinds of materials are able to transform a difference of temperature into electrical energy, and vice-versa, by means of the Seebeck and Peltier effects, respectively. These materials are considered as energy harvesting devices because of their capability to transform waste heat from power plants or car exhaustion, among others, into electricity in a renewable way. In a world with an increasingly demand of energy, thermoelectric devices are very promising. However, as a counterpoint, they present a relatively low efficiency for bulk materials. It has been predicted theoretically and observed experimentally, that these materials enhance its efficiency when they reduce its dimensionality, as for instance thin films (2D structures) or nanowires (1D structures). However, in order to determine the efficiency of these structures, the transport properties must be measured and it becomes extremely difficult as its dimension is reduced. Therefore, thermoelectric materials are excellent candidates to measure transport properties and they have been chosen in this thesis because of their highly interest as energy harvesting devices.
The thesis is divided in five different chapters. In the first chapter, an introduction to thermoelectricity and a review of the characterization of transport properties of films and nanowires is presented. In the second chapter, the experimental methods used to characterize the nano-structures under study are explained. In the third chapter, the thermal transport properties of films and nanowires made of inorganic or organic materials are studied with scanning probe techniques. Once these properties are obtained, a physical explanation of the phenomenon involved in each case that give rise to that result is presented. In the fourth chapter, the electrical transport properties of films and nanowires are studied by scanning probe microscopy and other techniques, like a four probe station. It is worth mentioning, that in several sections of either the third or fourth chapter the experimental work was combined with simulations to perform the analysis of the transport properties and to elucidate the physical process involved behind. Finally, the fifth chapter summarizes the most important conclusions reached in this doctoral work.
