Experimental Investigation of Phonon Localization in Nanostructures

University Of California-San Diego, La Jolla CA

Investigators

Abstract

Non-Technical Abstract: Phonons are collective vibrations of the atoms in a solid and are the dominant heat carriers in materials such as semiconductors. Phonon transport physics is important as it dictates thermal and other related properties of materials. Normally phonons either travel diffusively, randomly scattering from lattice impurities, or ballistically, in a straight line. This project aims to investigate a new regime of phonon transport termed "phonon localization" where the wave nature of the phonons prevents them from propogating. The study could lead to new approaches to engineer thermal properties beyond the current paradigm, in particular achieving ultra-low thermal conductivity, which is important for a wide range of thermal technologies such as thermal insulation and thermal to electrical energy conversion. The project broadens its impacts by providing interdisciplinary education and training opportunities to graduate and undergraduate students while attracting underrepresented groups into science. The integrated outreach activities provide education and research opportunities in nano- and energy technologies for disadvantaged, first generation high school students in the greater San Diego area. Technical Abstract Anderson localization has been experimentally demonstrated in both fermions and bosons, including electrons, photons (light), and recently with ultrasound. Theoretical studies have already predicted localization of thermal phonons, but until now there has been no experimental verification of these predictions, presumably due to the daunting challenges associated with the perceived experiments. This project aims to overcome these experimental challenges and carry out the first systematic study of phonon localization in low dimensional systems with disorders. The phonon localization regime is experimentally probed by thermal transport measurements of semiconductor nanowires with precisely controlled surface morphology and impurity concentration. Phonon localization signatures to be investigated include the dependence of thermal conductance on the nanowire geometry, surface roughness, and defect concentration as well as the temperature.