Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Light-Driven Chemical Creation of Fluorescent Quantum Defects(2018) Powell, Lyndsey Rae; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluorescent quantum defects are emerging as a new frontier in nanoscience. In semiconducting single-walled carbon nanotubes (SWCNTs) these symmetry-breaking defects are introduced into the sp2 carbon lattice, generating localized trap states for excitons. The emission from these states, which we call defect photoluminescence (PL), is only observed at very low defect densities, making precise control of the chemistry used to generate the defects imperative. In this dissertation I address the chemistry related to the generation of these defects in semiconducting SWCNTs. Typically, the organic reactions used to covalently modify SWCNTs are slow and imprecise, such as in the case of aryldiazonium chemistry. We use visible light that is tuned into resonance with SWCNTs to drive their functionalization by aryldiazonium salts and generate bright defect PL, accelerating the reaction and significantly improving the efficiency of covalent bonding to SWCNTs. We further expand this optical technique to another chemistry with which we demonstrate tunable switching between the inactive and reactive isomers of a diazoether compound, for highly controllable modification of nanostructures. This technique is used to selectively functionalize a SWCNT chirality within a mixture, to the near exclusion of other chiralities, even among semiconductors that are nearly identical in diameter and electronic structure. Furthermore, we address the challenge of PL self-quenching in highly concentrated systems of carbon nanomaterials, which occurs fundamentally due to the spectral overlap in their emission and absorption spectra. We demonstrate that fluorescent quantum defects extend the PL-concentration linearity over a significantly wider range than their unmodified counterparts. This optical technique and chemistry provide opportunities to chemically tailor SWCNTs at the single chirality level for improved separations, passivation, and lithography, and for generation of bright defect PL, even within highly concentrated systemsItem Exciton Photophysics at Fluorescent Quantum Defects(2018) Kim, Mijin; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluorescent quantum defect is an emerging synthetic structure that can be covalently attached to a semiconducting single-walled carbon nanotube. Incorporation of fluorescent quantum defect breaks the symmetry of carbon nanotubes at a defect center, creating new optically allowed, low-lying states in the electronic structure of carbon nanotube. Exciting electronic and optical properties arise from the defects, including the generation of new photoluminescence features, which can be used for applications, such as chemical sensing, bioimaging, and quantum light source. As excitons dominate the optical properties of carbon nanotubes, understanding the exciton photophysics in a defect-tailored carbon nanotube is essential to efficiently harness the emission properties of fluorescent quantum defects. In this dissertation, I aim to understand the exciton photophysics in fluorescent quantum defects in order to explain the origins and behavior of novel phenomena arising from them. First, the structure-property relationships of fluorescent quantum defects are discussed; these guide the systematic tuning of defect-induced emission and the binding energy of defect-trapped excitons. Then, the discussion moves to the exciton dynamics at fluorescent quantum defects. Particularly, I describe how the chemical nature of defects or the density of defects influences the thermal detrapping energy of excitons. The exciton-electron interaction at a fluorescent defect is also discussed. Our results suggest that a fluorescent quantum defect colocalizes an exciton and an electron as a tri-charge carrier and the brightening at the defect can be chemically tuned. Finally, I introduce super-resolved, hyperspectral photoluminescence spectroscopy, enabling both direct probing of a single fluorescent defect and the quantitative evaluation of the brightening of dark excitons.Item HIGH FREQUENCY GENERATION BASED ON CARBON NANOTUBE FIELD-EFFECT TRANSISTORS(2014) Song, Da; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon nanotubes (CNTs) are promising materials in radio frequency (RF) applications due to their high mobility, high current density and low capacitance. Over the past several years, extensive experimental and theoretical works have been focused on increasing the cut-off frequency of carbon nanotube field effect transistors (CNTFETs). However, there is limited study aiming for understanding the linearity of CNTFETs, which is an important aspect when radio frequency transistors are working in multiple frequency environments. In this dissertation, CNTFETs are fabricated based on horizontally aligned carbon nanotubes grown on quartz substrate. DC characterization shows three conduction regions in the transfer curve of the device, p-type and n-type linear regions, and ambipolar nonlinear region. The single tone excitation measurement shows extra harmonic generations as a result of the nonlinearity of the device. Same measurement is conducted with control devices without carbon nanotubes in the channel and confirms the nonlinearity is from the carbon nanotubes in the channel. Comparison between the 1st order harmonic amplitude and the 2nd order derivative of current with respect to gate voltage indicates that nonlinear transconductance is the cause of nonlinearity in the device. In order to understand the nonlinearity thoroughly, an elementary model based on 1D electronic transport and Drude model is built. The model can accurately predict the DC performance and nonlinearity of the device. Taking advantage of the transitions between linear and nonlinear transfer regions, we build our CNTFETs into gate controlled radio frequency mixers. Two-tone mixing measurement shows clearly that intermodulation terms in the output spectrum are strong in ampibolar regions and suppressed to noise floor in the linear regions. We further perform passive mixing (no source/drain voltage applied) in higher frequency regime and demonstrate the generation of harmonic and intermodulation signals in the output frequency range between 75 and110GHz, which is the among the highest output frequency observed from CNTFETs to date.Item Development of Carbon Nanotube Field-Effect Transistor Arrays for Detection of HER2 Overexpression in Breast Cancer(2011) Aschenbach, Konrad Hsu; Gomez, Romel D; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We developed a carbon nanotube biosensor platform that was deployed at the National Cancer Institute and successfully detected the HER2 oncogene in real cancer cells at clinically relevant levels. HER2 is a receptor protein that resides on the surface of certain cancer cells and is associated with higher aggressiveness in breast cancers. Overabundance of HER2 at the chromosomal, cell surface, and intermediate gene expression levels can all indicate a dangerous HER2 status. At the present, testing for HER2 status requires labor-intensive laboratory procedures using expensive reagents. Cost remains the major barrier to widespread screening. We propose an integrated electronic testing platform based on direct label-free gene detection. The system would integrate the various labor-intensive processes that are usually performed by skilled laboratory technicians. The heart of the system is an array of carbon nanotube field-effect transistors that can detect unlabelled nucleic acids via their intrinsic electric charges. We developed a scalable fabrication technique for carbon nanotube biosensor arrays, hardware and software for data acquisition and analysis, theoretical models for detection mechanism, and protocols for immobilization of peptide nucleic acid probes and hybridization of nucleic acids extracted from cells. We demonstrated detection of HER2 from real cell lines which express cancer genes, thereby lowering the technological barrier towards commercialization of a low-cost gene expression biosensor. The system is suitable for lab-on-a-chip integration, which could bring rapid, low-cost cancer diagnoses into the clinical setting.Item Nanoelectronic Materials(2010) Moore, Tracy; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis explores fabrication methods and characterization of novel materials used in field effect transistors, including metallic nanowires, carbon nanotubes, and graphene. Networks of conductive nanotubes are promising candidates for thin film electrode alternatives due to their desirable transparency, flexibility, and potential for large-scale processing. Silver nanowire and carbon nanotube networks are evaluated for their use as thin film electrode alternatives. Growth of silver nanowires in porous alumina membranes, dispersion onto a variety of substrates, and patterning is described. Metallic carbon nanotubes are suspended in aqueous solutions, airbrushed onto substrates, and patterned. The conductivity and transparency of both networks is evaluated against industry standards. Graphene is a two dimensional gapless semimetal that demonstrates outstanding room temperature mobilities, optical transparency, mechanical strength, and sustains large current densities, all desirable properties for semiconductors used in field effect transistors. Graphene's low on/off ratio and low throughput fabrication techniques have yet to be overcome before it becomes commercially viable. Silicon oxide substrates are common dielectrics in field effect transistors and instrumental in locating mechanically exfoliated graphene. The morphology of two different silicon oxides have been studied statistically with atomic force microscopy and scaling analysis. Tailoring the physical properties of these substrates may provide a control of graphene's electrical properties. A silicon oxide substrate may also be chemically altered to control the properties of graphene. I have modified silicon oxide with self-assembled monolayers with various terminal groups to control the field near the graphene. I characterize the monolayers with atomic force microscopy, x-ray photospectroscopy, and contact angles. I characterize graphene on these substrates using Raman microscopy and transport measurements. Finally, I examine low frequency noise in graphene field effect transistors on conventional silicon oxide substrates. As devices become smaller, the signal to noise ratio of these devices becomes important. Low frequency noise occurs on long time scales and must be controlled for device stability. I measure novel behavior of low frequency noise in multiple graphene devices. The noise may be described electron-hole puddles in the graphene that are caused by trapped charges near the surface of silicon oxide.Item Thermal and performance modeling of nanoscale mosfets, carbon nanotube devices and integrated circuits(2006-05-31) Akturk, Akin; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We offer new paradigms for electronic devices and digital integrated circuits (ICs) in an effort to overcome important performance threatening problems such as self heating. To investigate chip heating, we report novel methods for predicting the thermal profiles of complex ICs at the resolution of a single device. We resolve device and IC temperatures self-consistently, with individual device performances, while accounting for IC layout and software application details. At the device level, we calculate performance and generated heat details. We then extend these performance figures to the overall chip using a stochastic or Monte Carlo type methodology. Next, at the IC level, we solve for the device temperatures using the chip's layout and application software details. Here, we apply our mixed-mode algorithm to two-dimensional (planar) and three-dimensional ICs. To relieve thermal stresses and performance degradation in specific areas of extreme heating or hot spots, we offer design strategies using thermal contacts or different IC layouts. Moreover, we also show chips that we had designed and fabricated through IC fabrication clearing house MOSIS for experimental investigations. We also investigate carbon nanotubes (CNTs) and CNT embedded MOSFETs as new device paradigms for future electronic circuits. To examine the effects of CNTs on device performance, we develop a CNT Monte Carlo simulator, and determine scattering rates and CNT electron transport. Here, we report position-dependent velocity oscillations and length effects in semiconducting single-walled zig-zag carbon nanotubes. Our calculated results indicate velocity oscillations in the Terahertz range, which approaches phonon frequencies. This may facilitate new high frequency RF device and circuit designs, opening new paradigms in communication networks. Furthermore, to obtain device performance figures for MOSFETs that embed CNTs in their channels, our device solver determines interactions between the CNT and silicon (Si) by obtaining quantization and transport effects on the tube and the Si, and at the CNT-Si barrier. We predict that the CNT-MOSFET yields a better performance than the traditional MOSFET. Especially, CNT-MOSFETs employing lower diameter tubes exhibit improved performance capabilities. We also perform similar analyses for CNT embedded SOI-MOSFETs.Item semiconducting carbon nanotube transistors: electron and spin transport properties(2006-04-25) Chen, Yung-Fu; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Single-walled carbon nanotubes (SWNTs) have attracted great interest both scientifically and technologically due to their long mean free paths and high carrier velocities at room temperature, and possibly very long spin-scattering lengths. This thesis will describe experiments to probe the charge-and spin-transport properties of long, clean individual SWNTs prepared by chemical vapor deposition and contacted by metal electrodes. A SWNT field-effect transistor (SWNT-FET) has been shown to be sensitive to single electrons in charge traps. A single charge trap near a SWNT-FET is explored here using both electronic and scanned-probe techniques, and a simple model is developed to determine the capacitances of the trap to the SWNT and gate electrode. SWNTs are contacted with ferromagnetic electrodes in order to explore the transport of spin-polarized current through the SWNT. In some cases spin-dependent transport was observed, verifying long spin scattering lengths in SWNT. However, in many cases no spin-dependent effects were observed; these results will be discussed in the context of the present state of results in the literature. Semiconducting SWNTs (s-SWNTs) with Schottky-barrier contacts are measured at high bias. Nearly symmetric ambipolar transport is observed, with electron and hole currents significantly exceeding 25 µA, the reported current limit in m-SWNTs. Four simple models for the field-dependent velocity (ballistic, current saturation, velocity saturation, and constant mobility) are studied in the unipolar regime; the high-bias behavior is best explained by a velocity saturation model with a saturation velocity of 2 x 10^7 cm/s. A simple Boltzmann equation model for charge transport in s-SWNTs is developed with two adjustable parameters, the elastic and inelastic scattering lengths. The model predicts velocity saturation rather than current saturation in s-SWNTs, in agreement with experiment. Contact effects in s-SWNT-FET are explored by electrically heating the devices. These experiments resolve the origin of nanotube p-type behavior in air by showing that the observed p-type behavior upon air exposure cannot be explained by change in contact work function, but is instead due to doping of the nanotube. Modest doping of the SWNT narrows the Schottky Barriers and provides a high-conductance Ohmic tunnel contact from electrode to SWNT.Item Carbon Nanotube Devices: Growth, Imaging, and Electronic Properties(2006-01-11) Brintlinger, Todd Harold; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation focuses on growth, fabrication, and electronic characterization of carbon nanotube (CNT) devices. A technique for imaging CNTs on insulating substrates with the scanning electron microscope (SEM) will be described. This technique relies on differential charging of the CNT relative to the surrounding insulator. In addition, it is not only quicker than using scanning probe microscopy (SPM), but is also useful for identifying conducting pathways within an assortment of CNTs and metallic contacts. CNT field effect transitors (FETs) fabricated on strontium titanate gate dielectric show transconductances normalized by channel width of 8900 S/m, greatly exceeding that in Si FETs. Intriguingly, the transconductance cannot be explained within the conventional FET or Schottky-barrier models. To explain this, it is proposed that there is Schottky-barrier lowering due to high electric fields at the nanotube/contact interface. Exploring novel CNT-FET lithography, I demonstrate focused electron beam induced deposition (FEBID) of pure gold for CNT device electrodes. In examination of the CNT/electrode interface, equivalence between FEBID leads and leads deposited using conventional electron beam lithography is found with the majority device resistance in the CNT. Lastly, CNTs are suspended across wide trenches (>100 microns). These trenches are formed without lithography or etching and have metallic leads on either side of the trench for electrical transport measurements. Using a mechanical probe as a mobile gate, electrical transport can be performed on these suspended CNT devices, which show minimal hysteresis consistent with the absence of charge trapping.