Energy transfer in individual high aspect-ratio nanostructures via all-optical techniques

Nanoscale energy transfer in high aspect-ratio nanostructures plays a crucial role in engineering reliable nanostructure-based devices and optimizing heat dissipation. The ultrafast time and micro- to nano-length scales characteristic of nanoscale energy transfer require devising appropriate investigation techniques. This thesis focuses on the mechanical and thermal energy transients occurring in individual MoS2 nanotubes (NTs) and InAs nanowires (NWs), addressing the relevant time and length scales using all-optical microscopy.A nanofabrication protocol is developed to suspend single nanostructures over a trench, while matching the requirements dictated by the spectromicroscopy setup. In addition, to address the role of the environment, configurations in which the nanostructure is supported on polymethyl methacrylate (PMMA) or sapphire substrates are investigated.Energy transfer is investigated in individual multiwall MoS2 NTs. First, the MoS2 NT absolute extinction cross-section per unit length is experimentally retrieved over a wide range of wavelengths and for light polarizations both perpendicular and orthogonal to the NT main axis. The impact of NT diameters and its environment is investigated. Then, analytical and numerical simulations are implemented to disentangle the absorption and scattering contributions to the extinction cross-section. Second, the mechanical properties of MoS2 NTs are investigated, allowing for the identification of their breathing and thickness modes. Finally, supported by analytical models, the thickness of the MoS2 NT shell was accessed, found to be constant (16 nm) with respect to the NT external radius.The mechanical response of individual InAs NWs is also investigated, giving access to their breathing and longitudinal modes, rationalized both by analytical models and numerical simulations, providing a benchmark for the elastic stiffness matrix proposed for wurtzite InAs NWs. The extrinsic and intrinsic attenuation times of the acoustic modes are accessed in the hypersonic frequency range. On the thermal side, a novel method for analyzing heat dissipation out of the NWs starting from time-resolved optical traces affected by reproducibility issues is devised. The extent of the NW-to-substrate contact correlates with the rate of heat dissipation out of the individual NW. Finally, the thermal conductance at the InAs-PMMA interface G~83 MW/m2K is retrieved.