The main objective of this thesis is the study of thermal properties of nanostructured materials as a mean to control heat transport. For this purpose thermal measurements with different experimental techniques on length scales covering tens of microns to sub-50 have been performed. Two experimental techniques in particular have been studied and presented in this thesis: (a) the scanning thermal microscopy technique (SThM) and (b) the two-laser Raman thermometry (2LRT). These techniques have been extensively applied to successfully measure thermal properties in various nanomaterials. In particular, two configurations of Si based materials are investigated using 2LRT: (a) Si membranes with thicknesses ranging from to and (b) periodic porous membranes with different lattice parameters and disordered pattern. The results obtained showed that the in-plane thermal conductivity of silicon and its temperature evolution from room temperature to about 1000 can be effectively reduced and tuned by (i) thickness and (ii) periodic patterning (holes). We attribute the reduction of the thermal conductivity to the shortening of the phonon mean free path Λ due to diffuse (incoherent) phonon-boundary scattering. Furthermore, we showed that the temperature dependence of the thermal conductivity of Si membranes in the high temperature range from to is governed by phonons with mean free path smaller than 200 . To investigate thermal transport in supported nanostructures and in smaller length scale, we studied heat transfer between different heated scanning probe sensors and nanomaterials, such as, (i) in-plane epitaxial Si1-xGex alloy nanowires and (ii) self-assembled block copolymer nanostructures, provided high resolution thermal images of sub-50 structures with sub- spatial resolution. The combination of the two experimental techniques was crucial for the thermal characterization of different material systems and the better understanding of fundamental aspects of thermal transport.
|Date of Award||27 Jun 2017|
|Supervisor||Jordi Mompart Penina (Director) & Francesc Alsina Sureda (Tutor)|