Browsing by Author "Howe, Leslie"

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    Photogating Effect and Diffractive Optics in Low-Dimensional Structures
    Howe, Leslie (Virginia Tech, 2024-09-10)
    The development of nanostructures is the driving force of many scientific and technological fields. Among these myriad applications, important technologies such as advanced detection and ranging capabilities for infrared wavelengths and enhanced sensing of chemical molecules have been obtained recently, advancing our understanding of the earth's climate and space imaging. This kind of advancement is made possible through the thorough understanding of the performance of these devices which are fabricated from low-dimensional materials such as graphene, as well as the interaction of light with the materials at the microscopic scales, as is included in this dissertation. As such, the techniques of fabrication and theoretical understanding of graphene-field effect transistors (GFETs) as well as multi-level Fresnel zone plates (MLFZPs) are provided in detail. The photogating effect, understood as the ability of charge carriers to generate photocurrent when excited by an incident photon within a material, is crucial to the device physics of GFETs. We can utilize this property, as well as the resultant band bending of the interfacial band structure created within these transistors, to measure and predict the power of light as it interacts with the optical sensing area of the graphene. This allows for graphene transistors to be effective photodetectors, and we can accurately model the behavior of these detectors at many testing conditions, such as differing ambient temperatures, varying wavelengths, and multiple sensing area sizes. This work elucidates the capabilities and efficacy of these devices as photodetectors within both experimental and simulated conditions. In addition to photodetectors, GFETs prove to be capable biosensors as well, as graphene modulation due to the interaction of a molecule on its surface has a similar effect on the current of the channel as the photogating effect. When coupling these mechanisms, we find that there is a measurable effect due to the deposition of a photoreactive biomolecule of interest on the surface of the device. In particular, utilizing photoactive yellow protein (PYP), which has an incredibly strong reaction to blue light, allows us record concentration levels of the protein in solution down to the femtomole on the graphene surface of the detector when illuminated with the appropriate wavelength. This ability to measure the amount of PYP present in solution has many exciting implications, both to the understanding of the protein itself, as well as to the capability of the devices in detecting other proteins or biological molecules. Finally, nanostructures are an important component to diffraction, which allows for the construction of very precise diffractive lenses. This work entails the fabrication and simulation of MLFZPs, which are useful in their ability to tune the wavelength and focal length of the lenses to strict parameters. In addition, it is shown that these devices may be fabricated on thin polyimide films, allowing for flexibility and usefulness in mechanical applications. We have been able to fabricate lenses with features in precise control down to the nanometer in depth, and this results in incredibly precise and powerful optics which align well with simulated values.