Thermal conductivity measurement in nanostructures: new perspectives for the 3ω method

In recent years, interest in studying the thermal properties of materials at the nanoscale has grown due to their relevance to nanoelectronics and heat management and recovery in integrated devices. In this context, research efforts are focused on developing new materials and devices capable of transforming wasted heat into usable electrical energy. This form of energy conversion, known as thermoelectric conversion, represents a promising strategy for improving the overall efficiency of energy generation, storage and recovery processes in various fields. Nanostructured materials are ideal candidates in this scenario, as they can be designed to exhibit high electrical conductivity σ, a large Seebeck coefficient (S) and low thermal conductivity (k). Thanks to their small size and high surface-to-volume ratio, nanostructures can suppress phonon transport at the edges (Casimir effect), providing an ideal basis for targeted interface design and modulation of state density (both phononic and electronic). In particular, Group III-V semiconductor nanowires are attracting enormous interest due to their combined electrical and thermal transport properties, offering the possibility of exploiting gate modulation and thermal property control simultaneously in the same nanostructure. The development of thermoelectric devices based on these nanomaterials requires reliable techniques for measuring thermal conductivity, which is challenging due to the various manufacturing steps involved, the small temperature gradients that occur on a nanometric scale, and the weak signals involved. A fully electrical technique is available for all samples with a high aspect ratio: the 3ω method, based on injecting an alternating current (AC) at frequency ω into the sample and measuring the 3ω component of the voltage drop (third harmonic). This component is related to the temperature variation due to the Joule effect and therefore to the thermal conductivity of the sample. To overcome the approximations introduced by conventional analytical models, a more accurate numerical solution of the 3ω problem is used, based on the finite element method (FEM). The main objective of the thesis is to explore new perspectives for the application of the 3ω method, studying the signal trend over time and also focusing on the appearance of the second harmonic of the signal measured on InAs nanowires deposited on a silicon oxide substrate. This phenomenon is probably attributable to the presence of thermal boundary resistance (TBR) between the nanowire and the substrate. In-depth study of this second harmonic could improve our understanding of thermal behaviour at nanostructured interfaces and assess its impact on thermal conductivity measurements. The experimental work will involve the nanofabrication of the device, the characterisation of its thermal properties and electrical measurements carried out in a field-effect transistor (FET) configuration, with the aim of determining the electrical conductivity, resistance and mobility of charge carriers.