| dc.description.abstract | In the past two decades, nanowires have attracted a lot of attention because of their novel physical properties and promising applications. This dissertation explores thermal properties of several different kinds of nanowires of complex structures. First, to enhance the measurement sensitivity, the experimental set-up was modified based on a common-mode rejection scheme, which extended the lower limit of measurable thermal conductance down to ~10 pW/K. Utilizing this more powerful scheme, we investigated thermal transport through individual electrospun polyethylene (PE) nanofibers, boron carbide nanowires, and quasi-one-dimensional (quasi-1D) van der Waals (vdW) Ta2Pd3Se8 nanowires, together with thorough structural characterizations to establish structure-transport property relations. It has been shown that the electrospun PE nanofibers can have much higher thermal conductivity than the bulk value because of the highly ordered molecular orientation as disclosed by Raman spectroscopy. In general, the thermal conductivity of PE nanofibers increases with the electrospun voltage because the more aligned structure of nanofibers prepared at higher voltage. For single crystalline boron carbide nanowires, the measurements show a general trend of higher thermal conductivity as the carbon concentration and wire diameter increase, while no significant dependence on the stacking fault orientation and density has been observed. Importantly, the results show that kinks can pose remarkable resistance to thermal transport, which has been attributed to the combined effects of backscattering of highly focused phonons in boron carbides and required mode conversion at the kink. Interestingly, we show that defects in the kink, instead of posing resistance, actually facilitate phonon transport across the kink and reduce its resistance. Lastly, the thermal conductivity of exfoliated single crystalline quasi-1D vdW Ta2Pd3Se8 nanowires have been investigated. The results indicate an interesting size dependence of the thermal conductivity on both the wire diameter and sample length, which suggests important contributions of phonons both along and not-along the molecular chains. Up to 13 µm ballistic transport along the molecular chain at room temperature has been observed, which represents one of the longest observed ballistic transport so far. In summary, the novel observations presented in this dissertation disclose intriguing interactions between complex molecular structures and nanowire morphologies, which provides new insights into tuning the nanowire thermal properties. | |
