Phonon engineering in nanowire heterostructures

The exploitation of unique optoelectronic, mechanical, and thermal properties of nanostructured materials has a long history, even antecedent its deep understanding and systematic investigation. Indeed, the high surface-to-volume ratio of nanostructures, together with the possibility of quantum confinement, results in interesting behaviors for optoelectronic, thermoelectric, biomedical, and many more applications. Moreover, it allows the realization of material systems that would be extremely taxing or impossible to obtain in bulk form. Whatever the application, the operation of any device poses challenges in terms of heat management and energy efficiency. The work presented in this thesis falls in the field of Nanophononics. The majority of the thesis describes our investigation of heat carrier dynamics in several novel nanostructured semiconducting materials, namely core-shell hexagonal phase GaAs – SixGe1-x (0 ≤ x ≤ 0.59) nanowires, and nanowires containing GaAs-GaP and InAs-InP superlattices (SLs). For the core-shell nanowires, we assess the good crystalline quality of the samples, probe the energy and symmetry of electronic transitions through their coupling with phonons, highlight the effect of a novel kind of stacking fault on the Raman spectra and analyse the complex alloy lattice dynamics, while quantifying the effects of chemistry- and geometry- induced disorder. For the SL nanowires, we investigate several SL periods and architectures, unveiling the tailorability of the phonon spectrum, the emergence of Surface Optical phonon modes in the InAs-InP system, and the high quality of the heterostructures. A second set of efforts described in the thesis is directed at the long-term goal of creating purely phononic devices, i.e. circuits where information and/or energy is transported by phonons, like current does in electric circuits. We worked, both in the design and implementation phase, towards creating a versatile experimental setup, allowing for multiple experiments, all based on pulsed laser beams. These techniques allow, for example, to follow global lattice dynamics with high time resolution or to excite and then monitor in time and space the dynamics of a specific phonon mode with a defined energy and symmetry. This is crucial information for the realization of phononic devices and for the integration of phonon-based functionalities in other technologies. The materials and the structures investigated in this thesis are the result of cutting-edge synthesis processes that represent, in the case of hexagonal phase SixGe1-x, a long-sought after goal in optoelectronics and in the case of the SLs a remarkable degree of control in fabricating nanostructures with on-demand properties. Therefore, as the evidence illustrated in this thesis constitutes a step towards a better understanding of nanoscale phononics, we believe we can inform the design of ever more energy-efficient devices, capable of harnessing the potential of heat.