Phonon Transport in Nanowires – Beyond Classical Size Effects
Understanding and controlling thermal transport in one dimensional nanostructures as well as at their interfaces are emerging as an essential necessity for the development of a broad variety of technologies in nanoelectronics and energy conversion. In the past two decades, the thermal conductivities of many different kinds of nanostructures have been explored and the underlying mechanisms governing the transport process have been dissected. For nanowires, beyond the well-recognized classical size effects due to phonon-boundary scattering, several new factors, such as acoustic softening, surface roughness, and complex morphology, have also been shown to be able to significantly alter the thermal conductivity of nanowires. This dissertation seeks to further the understanding of the complicated transport dynamics in thin nanostructures and at their interfaces, and to answer some of the fundamental questions on interactions between energy and charge carriers in quasi-one-dimensional systems. These questions are addressed through a number of combined experimental approaches, such as the thermal conductance and thermoelectric property measurements of suspended nanostructures, elastic property test with atomic force microscopy, and high-resolution transmission electron microscopy examination. By coupling the measured thermal conductivity and Young’s modulus of two groups Si nanoribbons with thickness of either ~30 or 20 nm, acoustic softening effect is shown to significantly suppress thermal transport in the thinner ribbons in addition to the classical size effects. Furthermore, it is demonstrated that phonons can ballistically penetrate through the van der Waals interface between two silicon nanoribbons with amorphous SiO2 layers of up to a total of 5 nm thick at the interface. This observation indicates an unexpected phonon mean free path that is one order of magnitude longer than that predicted based on Einstein random walk model. Lastly, taking advantage of the unique features of charge density waves occurring in quasi-one-dimensional NbSe3 nanowires, we demonstrate distinct signatures of electron-phonon scatterings that can only be recaptured through considering electron-phonon scattering, which provides data to distinguish the contribution of electron-phonon scattering to phonon transport from other scattering mechanisms.