The Wiedemann-Franz (W-F) law describing thermal transport by electrons is a hallmark of success of the Sommerfeld theory. For electrons confined in one- or two-dimensional nanostructures, however, several intriguing phenomena can lead to the breakdown of the W-F law. These phenomena include strong correlation effects associated with increased electron-electron interaction, as well as electron localization. Intellectual Merit. This experimental study addresses a fundamental question regarding whether the Wiedemann-Franz Law is valid in low dimensional conductors. The scientific question being addressed is of a very fundamental nature, and one that will impact multiple disciplines. The experimental study examines electronic thermal transport in copper nanowires, quasi one-dimensional polyacetylene nanofibers, and two-dimensional graphene nanoribbons. The thermal conductivity, electrical conductivity, and Seebeck coefficient of these nanostructures are measured using nanofabricated suspended measurement devices at various temperatures. Chemical doping, electrical field effects, and the thermal Hall effect are employed to tune and determine the electronic thermal conductivity and isolate the lattice contribution. The validity of the W-F law is examined by analyzing the measured Lorenz number and its temperature dependence, and comparing the measurement results with the predictions of different transport theories. Broader Impacts. The nanostructures of interest are being considered for nanoelectronics, sensors, energy conversion, and thermal management applications, all of which will benefit from a better understanding of electronic thermal transport in these nanostructures. The research provides interdisciplinary education opportunities for graduate and undergraduate students. New example materials for a graduate and an undergraduate course in thermal science are being produced, as are new materials for three K-12 outreach activities designed to attract young girls to engineering professions, and to expose public school students and teachers in Texas to engineering principles.
The Wiedemann-Franz law describing heat transport by electrons is a hallmark of success of the Sommerfeld theory of electrons in metals. There have been questions on the validity of this law for some nanoscale conductors used in current and future-generation electronic devices and thermoelectric energy conversion devices. Because these nanoscale conductors not only carry charges, but also dissipate heat away from the heat-generating hot spots in electronic devices, establishing a better understanding of the heat conduction capability of these nanoscale conductors are necessary for the design of reliable electronic devices and high-performance thermoelectric devices. In this research, highly sensitive experimental techniques have been established for measuring the thermal conductivity, electrical conductivity, and Seebeck coefficient of metal nanowires, polymer nanofibers and thin films, and graphene supported on an amorphous support. Chemical doping, electrical field effect, and thermal Hall Effect have been employed to tune and determine the electron and phonon contributions to the thermal conductivity. The experiments have led to the discovery of the mechanism behind the observed suppressed thermal conductivity in graphene supported on an amorphous support, and the layer thickness needed for supported multi-layer graphene to recover the high value found in graphite. In addition, the experimental results suggest that the Wiedemann-Franz law is still useful in describing electronic thermal conductivity in palladium nanoribbons as thin as 40 nm as well as a conducting polymer thin film that has been developed for thermoelectric energy conversion. The scientific question addressed by this research is a very fundamental one that can impact multiple disciplines. The nanoscale conductors studied are being explored for nanoelectronics, sensors, energy conversion, and thermal management applications, all of which can benefit from the obtained better understanding of the thermal transport properties in these nanostructures. The research has also advanced thermal transport measurement techniques. In addition, the research findings have been disseminated to the research community in seven journal articles, two book chapters, three conference proceeding papers, and forty-six invited presentations in technical conferences and departmental seminars of different universities. The research has provided interdisciplinary education and international collaboration opportunities for three graduate students and two undergraduate students, including one female graduate student and one female undergraduate student. Thanks to the NSF support, one graduate student has completed a PhD degree, and another graduate student has completed a MS degree and is continuing her study toward a PhD degree. Besides producing new example materials for a graduate and an undergraduate course in thermal science, the NSF support has enabled the research team to build and demonstrate two experimental modules in three K-12 outreach activities for attracting young girls to engineering professions and for exposing university research to public school students, teachers, parents, and the general public.
