Exploring the Thermal Transport and Phase Nucleation Physics of Nanowires via Suspended Microdevices

The advance in nanofabrication and characterization methods enables researchers to study nanomaterials at small scales. Rich physics can emerge in nanomaterials due to their low-dimensional nature, such as boundary effect, quantum confinement effect etc. Also, nanomaterials usually have the most single-crystalline pristine qualities. This opens opportunities to study the intrinsic properties of many materials, which is hard to probe in bulk counterparts as the gain boundaries and defects in bulk samples can degrade their qualities. Combining the nanofabricated suspended microdevice and nanomaterials such as nanowires or nanoribbon, we can probe the intrinsic properties of materials which have interesting physics. Nanomaterials also have their unique structure and thermal physics properties which are very different from those of their bulk counterparts. In this dissertation, the author presents his work, in his pursuits of PhD degree at UC Berkeley, focused on exploring the thermal and transport physics of nanowire via transport measurement realized by using nanofabricated suspended microdevices./nChapter 2 and 3 are focused on vanadium dioxide (VO2), a strongly correlated materials with metal-to-insulator transition (MIT) occurring at temperature of 341 K. Although the first observation of the MIT of VO2 was more than 60 years ago, the transition mechanism and the correlation physics of VO2 is still under debate among theorists. Thanks to the advances in nanofabrication, measuring and modulating the MIT of single-crystalline VO2 nanowires are made possible and accessible to experimentalist. This provides us an efficient tool to study the intrinsic properties of VO2 and could help unveil its correlated nature. In chapter 2, a discovery of the recovery of Wiedemann-Franz (W-F) law in metallic VO2 is reported. This recovery is realized by introducing point defects using energetic ion irradiation. The experimental measurements and theoretical explanations are demonstrated to explain this full restoration of otherwise violated W-F law in strongly correlated system. In chapter 3, a deeply supercooled intrinsic VO2 metal is first realized by technique called irradiation shielding supported on suspended microdevices. Nucleation seeds created by ion surface milling are then introduced on nanowires’ surface and the critical nucleation sizes of supercooled metallic VO2 are measured for the first time. A model derived from classical nucleation theory is also presented./nIn chapter 4, experimental observations of abnormal thermal transport behaviors of twisted layered GeS nanowires with a screw dislocation core in the center is reported. The thermal conductivity of twisted GeS nanowires is measured by using suspended microdevices, which increases by decreasing diameters of nanowires. This trend is against to the prediction from boundary scattering, which is commonly observed in nanowires such as silicon, silver etc. In contrast, the thermal conductivities of non-twisted layered GeS nanowires and GeS nanoribbon are also measured. Their thermal transport behaves the same as common nanowires, that is thinner wires have lower thermal conductivity. The abnormal increasement of thermal conductivity at smaller diameters of twisted nanowires must origin from either their dislocation cores or the twisting topology between layers. New thermal physics may arise in this type of materials as classical explanation of the measured results fails to give full explanations.

Advisor: Wu, Junqiao