UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item Solution Processing of Long Carbon Nanotubes: from Fundamentals to Applications(2019) Wang, Peng; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Single-walled carbon nanotubes (SWCNTs) are one of the most intensively studied nanomaterials due to their extraordinary mechanical, electrical, and optical properties. Attaining aqueous solutions of individual SWCNTs is the critical first step for harnessing their outstanding properties and applying them in many applications and further processing, such as sorting, imaging, and sensing. However, the current ultrasonication-then-ultracentrifugation approach inevitably introduces defects to SWCNTs and cuts the nanotubes into smaller pieces, compromising the electrical and mechanical properties of this otherwise remarkable material. In this dissertation, we introduce an unexpectedly simple approach that completely eliminates the need for ultrasonication, and nondestructively disperses SWCNTs in aqueous solution, so that the synthetic lengths of SWCNTs can be preserved. The dispersion is achieved by using surfactants to wrap and stabilize the protonated SWCNTs by simple acid-base neutralization reactions. The result is that the protons on SWCNTs are replaced by surfactants, and thus, we name this method “superacid-surfactant exchange (S2E).” In chapters 2-4, we demonstrate the length of dissolved SWCNTs by S2E can be 4-10 times longer than the sonicated controls, thereby significantly improving the optical, electrical and electromechanical properties. We further find that by tuning the concentrations of SWCNTs in this S2E process, short nanotubes can be selectively extracted out, allowing separation of the long carbon nanotubes (>10 µm). In chapter 5, we show that long SWCNTs can behave like mechanical reinforcing structures that enhance the mechanical strength of graphene through π-π interactions without sacrificing much of the outstanding transparency of graphene. This fact has enabled the fabrication of the mechanically strong yet ultrathin graphene/SWCNTs hybrid structure (G+T) for operando probing of the electrical double layer at the electrode-electrolyte interface by X-ray photoelectron. Finally, as a ramification result from the S2E process, chapter 6 describes the scalable synthesis of organic-color-center tailored SWCNTs.Item NANOTUBE-MATRIX INTERPLAY AND TUNABILITY IN ULTRAHIGH VOLUME-FRACTION ALIGNED CARBON NANOTUBE POLY(URETHANE-UREA) NANOCOMPOSITES(2017) Gair, Jeffrey Lynn; Bruck, Hugh A.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The present dissertation seeks to better understand the nature of biphasic poly(urethane-urea) (PUU) interactions in materials with densely packed, aligned carbon nanotubes (CNTs). Of particular interest are the CNT-matrix interactions with in-situ polymerized PUU of various stoichiometric ratios. A novel synthesis method for PUU which permits fabrication of PUU-based polymer nanocomposites (PNCs) has been developed. Study of the thermal and multiscale mechanical behavior of stoichiometrically varied PUU materials has been conducted to demonstrate significant interaction between the matrix and CNTs, both in terms of morphology and mechanical reinforcement. PNCs with CNT Vf up to 30% have been achieved with excellent wetting confirmed via Micro-CT. TGA and DSC have revealed that CNTs stabilize thermal degradation of PUU by inducing crystallinity and reducing phase-mixing. AFM confirmed this by visualizing the crystals present in the matrix materials. CNT-induced crystallinity and phase-separation are attributed to the binding of CNTs to hard segments, which limit chain mobility during polymerization. Higher CNT Vf PNCs were found to increase soft-segment crystallinity, though with diminishing returns. Extreme crystallinity was found at 10% Vf CNTs which is though to arise due to an optimized spacing to permit ordered crystal formation of the PUU. Enhancements to indentation modulus of up to 1600% in the transverse orientation and 3500% in the axial orientation have been recorded via quasi-static nanoindentation. Greater CNT Vf and greater hard-segment composition lead to reduced chain mobility, and in some instances, can reduce CNT effectiveness in mechanical enhancement. The 10% CNT Vf exhibits greater indentation and storage moduli arising which is thought to arise from an optimized balance of inter-CNT spacing and chain mobility. Furthermore, PUU with higher hard-segment content is highly anisotropic and highly rate-sensitive, indicating significant morphological interactions with inter-CNT spacing of ~18nm. Degradation and increased loss modulus are seen in similar PUU with 20% loading, pointing to weak chain interactions and reduced hydrogen, likely do to confinement and reduced mobility. A model has also been developed which sheds light on the evolution of CNT-matrix interactions across a wide range of CNT volume-fractions.Item EXCITON ENGINEERING THROUGH TUNALBLE FLUORESCENT QUANTUM DEFECTS(2016) Kwon, Hyejin; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis demonstrates exciton engineering in semiconducting single-walled carbon nanotubes through tunable fluorescent quantum defects. By introducing different functional moieties on the sp2 lattice of carbon nanotubes, the nanotube photoluminescence is systematically tuned over 68 meV in the second near-infrared window. This new class of quantum emitters is enabled by a new chemistry that allows covalent attachment of alkyl/aryl functional groups from their iodide precursors in aqueous solution. Using aminoaryl quantum defects, we show that the pH and temperature of complex fluids can be optically measured through defect photoluminescence that encodes the local environment information. Furthermore, defect-bound trions, which are electron-hole-electron tri-carrier quasi-particles, are observed in alkylated single-walled carbon nanotubes at room temperature with surprisingly high photoluminescence brightness. Collectively, the emission from defect-bound excitons and trions in (6,5)-single walled carbon nanotubes is 18-fold brighter than that of the native exciton. These findings pave the way to chemical tailoring of the electronic and optical properties of carbon nanostructures with fluorescent quantum defects and may find applications in optoelectronics and bioimaging.Item REAL-TIME INVESTIGATION OF INDIVIDUAL SILICON NANOSTRUCTURED ELECTRODES FOR LITHIUM-ION BATTERIES(2013) Karki, Khim Bahadur; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Silicon-based anode materials are an attractive candidate to replace today's widely-utilized graphitic electrodes for lithium-ion batteries because of their high gravimetric energy density (3572 mAh/g vs. 372 mAh/g for carbon) and relatively low working potential (~ 0.5V vs. Li/Li+). However, their commercial realization is still far away because of the structural instabilities associated with huge volume changes of ~300% during charge-discharge cycles. Recently, it has been proposed that silicon nanowires and other related one-dimensional nanostructures could be used as lithium storage materials with greatly enhanced storage capacities over that for graphite in the next generation of lithium-ion batteries. However, the studies to date have shown that the nanomaterials, while better, are still not good enough to withstand a large number of lithiation cycles, and moreover, there is little fundamental insight into the science of the improvements or the steps remaining before widespread adoption. This dissertation seeks to understand the basic structural properties and reaction kinetics of one dimensional silicon nanomaterials, including Si-C heterostructures during electrochemical lithiation/delithiation using in-situ transmission electron microscopy (TEM). I present my work in three parts. In part I, I lay out the importance of lithium-ion batteries and silicon-based anodes, followed by experimental techniques using in-situ TEM. In part II, I present results studied on three different nanostructures: Si nanowires (SiNWs), Si-C heterostructures and Si nanotubes (SiNTs). In SiNWs, we report an unexpected two-phase transformation and anisotropic volume expansion during lithiation. We also report an electrochemically-induced weld of ~200 MPa at the Si-Si interface. Next, studies on CNT@α-Si heterostructures with uniform and beaded-string structures with chemically tailored carbon-silicon interfaces are presented. In-situ TEM studies reveal that beaded-string CNT@ α-Si structures can accommodate massive volume changes during lithiation and delithiation without appreciable mechanical failure. Finally, results on lithiation-induced volume clamping effect of SiNTs with and without functional Ni coatings are discussed. In Part III, a conclusion and a brief outlook of the future work are outlined. The findings presented in this dissertation can thus provide important new insights in the design of high performance Si electrodes, laying a foundation for next-generation lithium ion batteries.Item Characterization of Physical Properties of Multi-Scale Polymer Composites Under Various Processing Conditions(2012) Lederer, Anne Catherine; Bigio, David I; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There is a great interest in using micro and nano scale ingredients as fillers to create composites with enhanced physical properties. This thesis research explores the improvements these fillers offer with focus on combining both micro and nano ingredients to make multi-scale polymer composites. This investigation reveals the interplay of ingredient mixing, microstructural evolution, and processing conditions and characterizes the improvements of thermal and mechanical properties. This data is used to develop fundamental processing-structure-property relationships of these multi-scale composites across different concentrations of microscale and nanoscale ingredients and processing conditions in order to optimize their development.Item High Frequency Generation from Carbon Nanotube Field Effect Transistors Used as Passive Mixers(2012) Tunnell, Andrew Jacob; Williams, Ellen; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The high mobilities, low capacitances (due to small sizes), and high current densities of carbon nanotube field-effect transistors (CNT FETs) make them valid candidates for high frequency applications. The high cost of high frequency measurement equipment has been the largest hurdle to observing CNT transistor behavior at frequencies above 50 GHz. One economic solution to this barrier is to use an external harmonic mixer to convert high frequency signals to lower frequencies where they can be detected by a standard spectrum analyzer. By using this detection method, a new regime of high frequency CNT FET behavior is available for study. In this dissertation, we describe the design and fabrication of CNT FETs on quartz substrates using aligned arrays of CNTs as the device channel. The nonlinear input voltage to output drain current behavior of the devices is explained and approximated to the first order by using a Taylor expansion. For the high frequency mixing experiments, two input voltages of different frequencies are sourced on the gate of the devices without any device biasing. The input frequencies are limited to 100 kHz to 40 GHz by the signal generators used. The nonlinearities of the fabricated CNT FETs cause the input frequencies to be mixed together, even in the absence of a source-drain bias (passive mixing). The device output is the drain current, which contains sum and difference products of the input frequencies. By using an external harmonic mixer in combination with a spectrum analyzer to measure the drain current, output frequencies from 75 to 110 GHz can be observed. Up to 11th order mixing products are detected, due to the low noise floor of the spectrum analyzer. Control devices are also measured in the same experimental setup to ensure that the measured output signals are generated by the CNTs. The cutoff frequencies from previous passive mixing experiments predict that our devices should stop operating near 13 GHz, however our measurement setup extends and overcomes these cutoffs, and the generation of high frequency output signals is directly observed up to 110 GHz. This is the highest output frequency observed in CNT devices to date.Item THERMAL IMAGING OF MULTIWALLED CARBON NANOTUBES(2010) Baloch, Kamal Hussain; Cumings, John P; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since early days of their discovery, it has been realized that Carbon Nanotubes (CNTs) have an unusually high thermal conductivity. Unfortunately, the amount of heat they can transfer from one medium to another can be limited by their thermal contact resistance, Rc, which in the worst case can result in thermally insulating bulk materials. Prior studies on individual nanotubes have reached various disparate conclusions, partly because many techniques employed for measuring such small samples rely on uncharacterized heat sources thus leaving fundamental uncertainties in the measurements. This has caused concerns that the true potential of these extraordinary thermal conductors will remain untapped. Relying on solid to liquid phase transition of sub-200nm Indium islands for thermometry, we report direct measurement of Rc by employing an independently characterized metallic heat source. Also we demonstrate that this contact resistance can be reduced by almost two orders of magnitude if a CNT is imbedded into metal contacts. Additionally in our preliminary data on a self-heated CNT we observe that the substrate gets hot while the CNT itself remains cold when electric current is passed through it. This observation cannot be explained by assuming joule heating to be the primary source of heat transfer. We can qualitatively explain these results by assuming that hot electrons flowing through the biased CNT can be scattered off the phonons of a dielectric substrate. Principles of the novel measurement technique, experimental results and simulations are presented in this dissertation.Item CARBON NANOTUBE THIN FILM AS AN ELECTRONIC MATERIAL(2009) Sangwan, Vinod Kumar; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon nanotubes (CNT) are potential candidates for next-generation nanoelectronics devices. An individual CNT possesses excellent electrical properties, but it has been extremely challenging to integrate them on a large-scale. Alternatively, CNT thin films have shown great potential as electronic materials in low cost, large area transparent and flexible electronics. The primary focus of this dissertation is patterning, assembling, characterization and assessment of CNT thin films as electronic material. Since a CNT thin film contains both metallic and semiconducting CNTs, it can be used as an active layer as well as an electrode material by controlling the growth density and device geometry. The growth density is controlled by chemical vapor deposition and airbrushing methods. The device geometry is controlled by employing a transfer printing method to assemble CNT thin film transistors (TFT) on plastic substrates. Electrical transport properties of CNT TFTs are characterized by their conductance, transconductance and on/off ratio. Optimized device performance of CNT TFTs is realized by controlling percolation effects in a random network. Transport properties of CNTs are affected by the local environment. To study the intrinsic properties of CNTs, the environmental effects, such as those due to contact with the dielectric layer and processing chemicals, need to be eliminated. A facile fabrication method is used to mass produce as-grown suspended CNTs to study the transport properties of CNTs with minimal effects from the local environment. Transport and low-frequency noise measurements are conducted to probe the intrinsic properties of CNTs. Lastly, the unique contrast mechanism of the photoelectron emission microscopy (PEEM) is used to characterize the electric field effects in a CNT field effect transistor (FET). The voltage contrast mechanism in PEEM is first characterized by comparing measurements with simulations of a model system. Then the voltage contrast is used to probe the local field effects on a single CNT and a CNT thin film. This real-time imaging method is assessed for potential applications in testing of micron sized devices integrated in large scale.