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 "Alemán, Benjamín"

Now showing 1 - 6 of 6
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    Building and Characterizing Graphene Nanomechanical Resonator Networks
    (University of Oregon, 2024-01-09) Carter, Brittany; Alemán, Benjamín
    Networks of nanoelectromechanical (NEMS) resonators are useful analogs for a variety of many- body systems and enable impactful applications in sensing, phononics, and mechanical information processing. Two main challenges are currently limiting progress toward realizing practical NEMS networks. The first is building a platform of interconnected resonators that is scalable in both size and tunability. The second is spatially quantifying the mechanical parameters of each resonator in the network and their coupling. In this work, we address these two main challenges with a novel scalable platform to build the network and a compatible method to characterize mechanical parameters. Together, this work fills in a vital gap for the experimental realization of programmable NEMS networks.We first present a novel platform of suspended graphene resonators that hosts strong coupling and is scalable in 2D. In this platform, we suspended graphene over pillar arrays, in which large areas of suspended graphene act as drumhead resonators and shared membrane between adjacent resonators allows for direct coupling through strain. We demonstrate the versatility advantages of our graphene-based resonator network by providing evidence of strong coupling through two different tuning methods. We demonstrate the 2D scalability potential of this platform with evidence of coupling between three resonators. Finally, we show noteworthy coupling dynamics of inter-resonator higher order mode coupling that is enabled by our versatile platform. We then demonstrate a scalable optical technique to spatially characterize graphene NEMS network. In this technique, we read out the fixed-frequency collective response as a single vector. Using just two response vectors, we solve for the site-specific elasticity, mass, damping, and coupling parameters of network clusters. Compared to multiple regression, our algebraic fully characterizes the network parameters without requiring a priori parameter estimates or iterative computation. We apply this technique to single-resonator and coupled-pair clusters and find excellent agreement with expected parameter values and spectral response. Our approach offers a direct means to accurately characterize both classical and quantum resonator systems.
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    Engineering and Characterization of Single Photon Emitters in Hexagonal Boron Nitride
    (University of Oregon, 2020-09-24) Ziegler, Joshua; Alemán, Benjamín
    Single photons have broad applications across quantum information technologies including quantum computation, boson sampling, and quantum communication. Current single photon generation techniques are not ideal due to concerns such as low brightness, stochasticity, or need for low temperatures. The recently discovered single photon emitter in the 2D material hexagonal boron nitride is an interesting single photon source due to its extreme brightness and photoluminescence stability. Moreover, the nanoscale thickness of the material allows for strong coupling to hybrid photonic structures. However, integration of these single photon emitters has not yet been reliably realized due to a large amount of inhomogeneous broadening (which may be due to strain) and difficulties in consistent fabrication of these emitters. If these two problems could be solved, the viability of integrating these emitters would be greatly increased. Here, we address wavelength variation in boron nitride emitters by identifying a zero-dimensional boron nitride nanostructure that hosts single photon emitters with reduced spectral variation. We find that these emitters in this nanostructure have a wavelength variation reduced by a factor of 5. We offer reasoning that this may be due to the mechanical robustness of the zero-dimensional structure. In order to reliably fabricate these emitters, we develop a focused ion beam milling technique to create single photon emitters by cutting holes in the hexagonal boron nitride. Optimally tuning the milling parameters, we achieve a 31% yield of sites with the signature of single photon emission and a 94% yield of sites that have the signature of few photon emission. Together, these two results open the door for large-scale on-chip integration of boron nitride emitters into photonic or plasmonic structures. More engineering is likely needed for further control of emission wavelength as well as reduction of the two-photon probability, but our results will be invaluable for practical uses of hexagonal boron nitride single photon emitters. This dissertation includes previously published coauthored material.
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    Field Emission Based Displacement Sensing Using a Carbon Nanotube Enhanced Electromechanical Probe
    (University of Oregon, 2020-09-24) Resch, Rudolph; Alemán, Benjamín
    The simple measurement of a distance has long powered sensitive scientific instruments. In the case of scanning probe microscopy (SPM), instruments like Scanning Tunneling Microsopes (STM) and Atomic Force Microscopes (AFM) are powered by ultra-sensitive rulers, measuring atomic-scale distances between their probes and the surface. These instruments opened the doors to direct investigation of the nanoscale world. A decade before, however, was a long forgotten instrument known as the Topografiner. Unlike STM and AFM, the Topografiner used an incredibly sensitive long-range ruler based on a field emission current produced by a sharp metal tip. Most importantly, the long range allows for non-contact investigations of topography and surface properties. Unfortunately, the noisy dc field emission techniques used in its operation and the tip-geometry hindered the potential of the technique to reach atomic-scale resolution. We address these issues by using a high-aspect-ratio carbon nanotube as a field emitter and leverage ac electromechanical coupling to incorporate phase locked loop measurement techniques.These together form a new platform for high precision displacement sensing. We achieve vertical displacement sensing with sub-atomic resolution at room temperature, with a position sensitivity of η=700±400 fm/√Hz while the emitter is located at z~250 nm from the surface, and η~100 pm/√Hz at z~1 μm. This displacement sensitivity approaches that of AFM and STM, with the advantage of a long working distance and large dynamic range. Our electromechanical model shows this improved performance is due to the large aspect ratio and nanometer scale dimensions of the nanotube. The revived topografiner will enable atomic-resolution, high dynamic range SPM imaging, and could also be used to measure and map driven nanomechanical systems or subsurface metallic structures. This dissertation contains previously unpublished co-authored material.
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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.
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    High Surface Area Electrodes for Neurostimulation of the Retina
    (University of Oregon, 2020-12-08) Zappitelli, Kara; Alemán, Benjamín
    Retinal degenerative diseases (RDDs), which cause the gradual death of rods and cones in the retina, impact millions of people all over the world, yet there are few clinically viable treatments and no cure. Multi-electrode array (MEA) - based retinal implants have emerged over the last two decades as a viable treatment option for those blinded by RDDs. Small electronic implants placed within the degenerate retina are able to restore vision to blind patients, however restored visual acuity (VA) is currently low. To provide patients with useful, high-resolution vision, electrode sizes must be decreased and the density of MEAs increased. As sizes decrease, small electrodes must pass increasingly more charge per unit area in order to provide enough current to stimulate the remaining retinal circuitry. The amount of charge needed often exceeds well-established safety limits that prevent electrode degradation and tissue damage. In order to safely provide enough current to the tissue, one aims to maximize electrode capacitance - the amount of charge an electrode can store for use in stimulation - by increasing the overall surface area without increasing the electrode footprint. In this dissertation, I approach surface area enhancement from the microscale and the nanoscale. I first investigate how micro-patterning of electrode geometry with a fixed footprint can increase stimulation capability. I utilize an electric force microscopy (EFM) characterization technique to compare multiple different microscale electrode geometries. I then introduce vertically aligned carbon nanotubes (VACNTs) grown with chemical vapor deposition (CVD) as an ideal high surface area electrode material. I demonstrate VACNT biocompatibility with retinal neurons in vitro and show that high aspect ratio VACNTs can be integrated with silicon microfabrication processes to create an in vivo platform for rodent studies. Finally, I present work towards integrating these two approaches to create the next generation of high surface area retinal implant electrodes. Taken as a whole, this work offers promise for improving retinal neuron stimulation and restoring high VA to blind patients. This dissertation includes previously published co-authored material.
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    Shaping, Tuning, and Playing Nanodrums: Towards Scalable and High Quality Factor Graphene Nanoelectromechanical Systems
    (University of Oregon, 2020-09-24) Miller, David; Alemán, Benjamín
    Nanomechanical systems (NEMS) are some of humankinds most exquisite sensors of mass and force and have enabled the transduction of physical phenomena down to the single-phonon level. Despite incredible progress on the overall properties of mechanical resonators, development of large-scale arrays is only now beginning to be explored. Such arrays could be transformative in basic science, allowing for realization of topological metamaterials and studies of networks, and for applied devices, such as next-generation mass spectrometers and thermal imaging cameras. To make a NEMS array viable for these applications however, each NEMS device must have several desirable properties. First, all devices must have high mechanical quality factors (Q) combined with low mass, for high sensitivity. This requires both a fundamental knowledge of the origin of mechanical dissipation and viable engineering methods to maximize the Q for a given mass. Second, devices must have scalable control methods for tuning the frequency and exciting motion. No such devices that meet these requirements exist today and applications for NEMS arrays remain limited. Graphene NEMS have the potential to meet these needs, if some fundamental challenges can be addressed. Although graphene NEMS have low mass, they also have a relatively low Q. Furthermore, engineering methods to modify the shape of graphene NEMS are limited, making it difficult to tune and enhance their properties. Finally, like all other NEMS, tuning and control methods that scale to large arrays are sorely lacking. In this work, we will begin to address these needs in graphene NEMS through a compendium of studies. We will first use shape engineering to enhance the properties of graphene NEMS. Then, we will present a detailed study of the Q and demonstrate methods to enhance it. Finally, we will study actuation and control methods for graphene NEMS, including demonstration of an electo-optic method that is truly scalable. Together, these studies pave the way for future work on large-scale arrays of NEMS. This dissertation includes previously published and unpublished co-authored material.