Optical Characterization of Nanoscale Thermal Transport

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0854118Cronin The objective of this research is to measure and develop an understanding of nanoscale thermal transport phenomena using optical, non-contact measurement techniques. In this study, micro-Raman spectroscopy will be used to measure heat flow in nanowires, nanotubes, and graphene. The temperature profile along individual suspended nanostructures will be determined with sub micron spatial resolution from the calibrated downshifts in the Raman spectra. Temperature gradients will be created by laser irradiation, micro-fabricated heaters, and by running high electric currents through the nanostructures themselves. These optical, non-contact methods 1.) eliminate the effect of thermal contact resistance and thermal shorting through the substrate, 2.) differentiate between diffusive and ballistic phonon transport, and 3.) enable time resolved measurements to be performed with nanosecond resolution. By measuring the heating and cooling times of suspended nanostructures, their thermal diffusivity will be obtained. These measurements will provide reliable values for the thermal contact resistance and the thermal conductivity of nanostructures with unprecedented precision and accuracy. Conventional approaches for measuring thermal transport do not provide reliable results on the micron and nanometer scales because of the large temperature drops across the contacts, which cannot be determined or excluded from the measurement. The proposed optical methods are, therefore, more suitable for measuring nanometer scale thermal transport than conventional techniques. Furthermore, the non contact, non destructive nature of these techniques enables further characterization of the same individual nanostructure by other means, such as transmission electron microscopy (TEM) and electron transport measurements. Intellectual Merit: The novel measurement techniques will quantify several thermal parameters that have, so far, eluded current state of the art nanoscale thermal transport measurements, such as heat generated per incident photon of laser light. These techniques also allow us to investigate several nanoscale thermal transport phenomena that remain poorly understood and have only been theoretically investigated. Broader Impacts: Understanding thermal transport in nanostructures is important for designing and optimizing high speed electronics. This is essential for extending Moore's law over the next 5 to 10 years. Results from the proposed work will enable thermal engineering on the nanometer scale to provide realistic thermal management solutions for extremely high power density circuits. Improved thermoelectric materials that exploit the low dimensionality could significantly impact our energy dependent economy by recovering electricity from waste heat. The novel techniques developed in this proposal can be readily applied by many other scientists and engineers working in the field and will likely lead to innovations in other fields of science and technology. The outreach program to Los Angeles area high school teachers will introduce underrepresented students to the results and, more importantly, the excitement of this research and other cutting edge research at universities in the Los Angeles area. Research projects for undergraduate students and the new curriculum, which integrates creativity into a science based course, will produce skilled and knowledgeable students able to address the next generation of challenges in nanotechnology.