UC San Diego

The thermal conductivity of one dimensional nanostructures, such as nanowires, nanotubes, and polymer fibers, is of significant interests for understanding nanoscale thermal transport phenomena as well as for practical applications in nanoelectronics, energy conversion and thermal management. This has fueled a significant body of research aiming to identify causes and enable rational engineering of thermal conductivity, which has predominantly been achieved by limiting the phonon mean-free-path by the characteristic size of the nanostructure. Thermal conductivity in such small nanostructures could be further altered by way of phonon confinement as the structure approaches the phonon wavelength and reduction of the phonon velocity, signified via change in the elastic constants.

An enhanced experimental technique ~100X more sensitive than the original method to characterize the thermal properties of one-dimensional nanostructures such as semiconductor nanowires, nanotubes, and polymer fibers. The thermal conductivity and mechanical properties of semiconductor nanostructures was measured using the developed calorimetry setup and an in-situ FIB tensile testing setup. The thermal conductivities of sub-20 nm Ge and Ge-Si core-shell nanowires were measured and found to be significantly lower than bulk SiGe alloys or smooth Si NWs due to phonon confinement. Meanwhile, ~5 nm thick shell crystalline Si nanotubes were found to have thermal conductivity lower than the apparent boundary scattering limit and lower than even ~5 nm thick amorphous Si nanotubes, which is understood by elastic softening measured via tensile testing. Next, further mechanical characterization of crystalline and amorphous nanotubes is performed, and strong size-dependent viscoelastic behavior is observed in amorphous nanotubes due to a fluid-like surface, with room temperature viscosity equivalent to that of bulk glass above 1000 C. Finally, the thermal and mechanical properties of polymer nanofibers with diameters down to sub-100 nm are measured and discussed. The axial thermal conductivity of individual nanofibers is strongly correlated with the crystalline morphology while the elastic modulus stress relaxation is found to be highly size-dependent, enabling the smallest diameter fibers to break the stiffness-toughness tradeoff present in most material systems.