Understanding thermal transport in nanoscale systems by atomstic simulations

Thermal transport represents an open issue for experimental and theoretical investigation in the field of nanoscale physics. Several challenges strictly depend on understanding the details related to how heat is transported at the nanoscale, how heat carriers behave when the size involved are that small and how to better exploit such features. In this scenario, the role of the present work is twofold: first, theoretical methods and atomistic simulations are adopted to investigate thermal transport is adopted and a very detail-oriented assessment have been performed; secondly, it is studied a panoply of different materials exhibiting interesting features related to thermal transport. The first aspect aims at highlighting the advantages and disadvantages of each approach, in order to delineate how these techniques can complement each other to give the best performances, reliable trends and especially detailed information about thermal conductivity and its related properties in nanostructured systems. The purpose of second element, istead, is to characterize nanostructured material in order to provide a thorough description of the nanoscale features which can influence thermal transport. Semiconductors are widely adopted to devise nanostructured materials: the first part of this thesis is focused on nanostructures based on silicon and germanium. The idea of nanoengineering a material involves the usage of interfaces, reduced dimensionality, substitutional defects and alloys, and the creation of nanovoids: these components have been deeply addressed in the present investigation aiming at possible applications in the field of renewable energies (e.g. thermoelectric conversion), and in the field of information technology (e.g. thermal rectification, thermal dissipation, etc.). In the context of complex material for technological application, organic molecules are rising due to their improved performances in several application areas: LEDs, photovovoltaic cells, thermoelectric applications, etc. The second part of this thesis aims at shredding light on the mechanisms of energy relaxation and thermal dissipation in organic and hydrogen-bonded system. Furthermore, organic molecules present several interesting characteristics when arranged in a glassy configuration: here a detailed investigation to understand the role of molecular orientation in organic glasses thermal transport is also provided.