Physics Theses and Dissertations

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https://hdl.handle.net/1794/2048

This collection contains some of the theses and dissertations produced by students in the University of Oregon Physics Graduate Program. Paper copies of these and other dissertations and theses are available through the UO Libraries.

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Browsing Physics Theses and Dissertations by Author "Blaikie, Andrew"

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    Graphene Electromechanical Resonators and Their Use in Thermal Detectors
    (University of Oregon, 2020-09-24) Blaikie, Andrew; Alemán, Benjamín
    In the quest to probe the nanoscale, new materials have been discovered. One of these materials is graphene, a sheet of carbon a single atom thick. An especially exciting application of graphene is its use in thermal detectors. These detectors sense broadband light by measuring an optical absorption induced temperature increase in a detecting material. Modern applications require that thermal detectors work at room temperature, while maintaining high speed and sensitivity, properties which are inherently limited by the heat capacity of the detector. To this end, graphene has generated interest because it has the lowest mass per unit area of any material, while also possessing extreme thermal stability and an unmatched spectral absorbance. Yet, due to its weakly temperature-dependent electrical resistivity, graphene has failed to challenge state-of-the-art thermal detectors at room temperature. Here, in a departure from conventional bolometric thermal detection, where the temperature-dependent electrical resistance serves as a readout for photodetection, we use a graphene nanoelectromechanical system to detect light via resonant sensing. In our approach, absorbed light heats and thermally tensions a suspended graphene resonator, thereby shifting its resonant frequency. Using the resonant frequency as a readout for photodetection, we achieve a room-temperature noise-equivalent power and bandwidth challenging the state of the art. Despite great technological progress, scientific questions remain unanswered in graphene nanoelectromechanical systems, including the exact origin of their high mechanical dissipation, which could add noise in electromechanical sensing applications. Due to this high dissipation, the quality factor in suspended graphene, is orders of magnitude lower than in heavier bulk resonators. Here, we perform a large-scale study of the quality factor in suspended graphene drumheads to help further understand their mechanical dissipation properties. We find that the quality factor in graphene drumheads agrees with the predictions of a theory of dissipation dilution with a bending stiffness heavily modified by out-of-plane wrinkles. We find that Ga+ ion irradiation increases in-plane stress and reduces wrinkling in graphene drumheads, improving the quality factor by a factor of 30. This dissertation includes previously published and unpublished co-authored material.