Browsing by Subject "Carbon nanotube"
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Item Open Access A Study of Field Emission Based Microfabricated Devices(2008-04-25) Natarajan, SrividyaThe primary goals of this study were to demonstrate and fully characterize a microscale ionization source (i.e. micro-ion source) and to determine the validity of impact ionization theory for microscale devices and pressures up to 100 mTorr. The field emission properties of carbon nanotubes (CNTs) along with Micro-Electro-Mechanical Systems (MEMS) design processes were used to achieve these goals. Microwave Plasma-enhanced CVD was used to grow vertically aligned Multi-Walled Carbon Nanotubes (MWNTs) on the microscale devices. A 4-dimensional parametric study focusing on CNT growth parameters confirmed that Fe catalyst thickness had a strong effect on MWNT diameter. The MWNT growth rate was seen to be a strong function of the methane-to-ammonia gas ratio during MWNT growth. A high methane-to-ammonia gas ratio was selected for MWNT growth on the MEMS devices in order to minimize growth time and ensure that the thermal budget of those devices was met.
A CNT-enabled microtriode device was fabricated and characterized. A new aspect of this device was the inclusion of a 10 micron-thick silicon dioxide electrical isolation layer. This thick oxide layer enabled anode current saturation and performance improvements such as an increase in dc amplification factor from 27 to 600. The same 3-panel device was also used as an ionization source. Ion currents were measured in the 3-panel micro-ion source for helium, argon, nitrogen and xenon in the 0.1 to 100 mTorr pressure range. A linear increase in ion current was observed for an increase in pressure. However, simulations indicated that the 3-panel design could be modified to improve performance as well as better understand device behavior. Thus, simulations and literature reports on electron impact ionization sources were used to design a new 4-panel micro-ion source. The 4-panel micro-ion source showed an approximate 10-fold performance improvement compared to the 3-panel ion source device. The improvement was attributed to the increased electron current and improved ion collection efficiency of the 4-panel device. Further, the same device was also operated in a 3-panel mode and showed superior performance compared to the original 3-panel device, mainly because of increased ion collection efficiency.
The effect of voltages applied to the different electrodes in the 4-panel micro-ion source on ion source performance was studied to better understand device behavior. The validity of the ion current equation (which was developed for macroscale ion sources operating at low pressures) in the 4-panel micro-ion source was studied. Experimental ion currents were measured for helium, argon and xenon in the 3 to 100 mTorr pressure range. For comparison, theoretical ion currents were calculated using the ion current equation for the 4-panel micro-ion source utilizing values calculated from SIMION simulations and measured electron currents. The measured ion current values in the 3 to 20 mTorr pressure range followed the calculated ion currents quite closely. A significant deviation was observed in the 20-100 mTorr pressure range. The experimental ion current values were used to develop a corrected empirical model for the 4-panel micro-ion source in this high pressure range (i.e., 3 to 100 mTorr). The role of secondary electrons and electron path lengths at higher pressures is discussed.
Item Open Access Beyond A Simple Composite of Metal Oxide/Graphene/Carbon Nanotubes: Controlling Nanostructured Electrodes at Macroscopic Scale(2014) Sedloff, Jennifer WedebrockThe development of electronic textiles, which have many potential healthcare and consumer applications, is currently limited by a lack of energy storage that can be effectively incorporated into such devices while having sufficient energy density, power density, and durability to perform well. The overall goal of this work was to improve the energy density and potential for use in electronic textile applications of a nanostructured composite of few-walled carbon nanotubes, manganese oxide, and reduced graphene oxide. Two approaches towards improving the desired properties by controlling the macroscopic structure of the composite were pursued: one, to make fiber or wire-shaped electrodes via wet-spinning in aqueous chitosan solutions (10% acetic acid), and the other, to make composite films with controlled porous structures using nitrocellulose as a sacrificial filler material. Both approaches yielded the desired macroscopic structures. The composite fibers were non-conductive due to the insulating nature of manganese oxide and its positioning on the surface of the fibers. Composite fibers of few-walled carbon nanotubes and reduced graphene oxide made by the same method were found to have good volumetric capacity, rate capability, stability and flexibility. Nonintuitively, electrochemical performance of composite films declined with increasing porosity due to a decrease in conductivity, highlighting the importance of balancing the interplay between properties important to device performance when designing controlled structures of complex materials.
Item Open Access Computational Study of Low-friction Quasicrystalline Coatings via Simulations of Thin Film Growth of Hydrocarbons and Rare Gases(2008-04-25) Setyawan, WahyuQuasicrystalline compounds (QC) have been shown to have lower friction compared to other structures of the same constituents. The abscence of structural interlocking when two QC surfaces slide against one another yields the low friction. To use QC as low-friction coatings in combustion engines where hydrocarbon-based oil lubricant is commonly used, knowledge of how a film of lubricant forms on the coating is required. Any adsorbed films having non-quasicrystalline structure will reduce the self-lubricity of the coatings. In this manuscript, we report the results of simulations on thin films growth of selected hydrocarbons and rare gases on a decagonal Al$_{73}$Ni$_{10}$Co$_{17}$ quasicrystal (d-AlNiCo). Grand canonical Monte Carlo method is used to perform the simulations. We develop a set of classical interatomic many-body potentials which are based on the embedded-atom method to study the adsorption processes for hydrocarbons. Methane, propane, hexane, octane, and benzene are simulated and show complete wetting and layered films. Methane monolayer forms a pentagonal order commensurate with the d-AlNiCo. Propane forms disordered monolayer. Hexane and octane adsorb in a close-packed manner consistent with their bulk structure. The results of hexane and octane are expected to represent those of longer alkanes which constitute typical lubricants. Benzene monolayer has pentagonal order at low temperatures which transforms into triangular lattice at high temperatures. The effects of size mismatch and relative strength of the competing interactions (adsorbate-substrate and between adsorbates) on the film growth and structure are systematically studied using rare gases with Lennard-Jones pair potentials. It is found that the relative strength of the interactions determines the growth mode, while the structure of the film is affected mostly by the size mismatch between adsorbate and substrate's characteristic length. On d-AlNiCo, xenon monolayer undergoes a first-order structural transition from quasiperiodic pentagonal to periodic triangular. Smaller gases such as Ne, Ar, Kr do not show such transition. A simple rule is proposed to predict the existence of the transition which will be useful in the search of the appropriate quasicrystalline coatings for certain oil lubricants. Another part of this thesis is the calculation of phase diagram of Fe-Mo-C system under pressure for studying the effects of Mo on the thermodynamics of Fe:Mo nanoparticles as catalysts for growing single-walled carbon nanotubes (SWCNTs). Adding an appropriate amount of Mo to Fe particles avoids the formation of stable binary Fe$_3$C carbide that can terminate SWCNTs growth. Eventhough the formation of ternary carbides in Fe-Mo-C system might also reduce the activity of the catalyst, there are regions in the Fe:Mo which contain enough free Fe and excess carbon to yield nanotubes. Furthermore, the ternary carbides become stable at a smaller size of particle as compared to Fe$_3$C indicating that Fe:Mo particles can be used to grow smaller SWCNTs.Item Open Access Developing an In Vivo Intracellular Neuronal Recording System for Freely Behaving Small Animals(2013) Yoon, InhoElectrophysiological intracellular recordings from freely behaving animals can provide information and insights, which have been speculated or cannot be reached by traditional recordings from confined animals. Intracellular recordings can reveal a neuron's intrinsic properties and their communication with other neurons. Utilizing this technology in an awake and socially behaving brain can bring brain research one step further.
In this dissertation, a customized miniature electronics and microdrive assembly is introduced for intracellular recording from small behaving animals. This solution has realized in vivo intracellular recording from freely behaving zebra finches and mice. Also, a new carbon nanotube probe is presented as a surface scanning tip and a neural electrode. With the carbon nanotube probe, intracellular and extracellular neural signals were successfully recorded from mouse brains. Previously, carbon nanotubes have only been used as a coating material on a cell-culturing platform or on a metal based neural electrode. This probe is the first pure carbon nanotube neural electrode without an underlying platform or wire, and it is the first one that has achieved intracellular and extracellular recordings from vertebrate cortical neurons.
Item Open Access Electron Transport through Carbon Nanotube Quantum Dots in A Dissipative Environment(2012) Mebrahtu, Henok TesfamariamThe role of the surroundings, or environment , is essential in understanding funda- mental quantum-mechanical concepts, such as quantum measurement and quantum entanglement. It is thought that a dissipative environment may be responsible for certain types of quantum (i.e. zero-temperature) phase transitions. We observe such a quantum phase transition in a very basic system: a resonant level coupled to a dissipative environment. Specifically, the resonant level is formed by a quantized state in a carbon nanotube, and the dissipative environment is realized in resistive leads; and we study the shape of the resonant peak by measuring the nanotube electronic conductance.
In sequential tunneling regime, we find the height of the single-electron conductance peaks increases as the temperature is lowered, although it scales more weakly than the conventional T-1. Moreover, the observed scaling signals a close connec- tion between fluctuations that influence tunneling phenomenon and macroscopic models of the electromagnetic environment.
In the resonant tunneling regime (temperature smaller than the intrinsic level width), we characterize the resonant conductance peak, with the expectation that the width and height of the resonant peak, both dependent on the tunneling rate, will be suppressed. The observed behavior crucially depends on the ratio of the coupling between the resonant level and the two contacts. In asymmetric barriers the peak width approaches saturation, while the peak height starts to decrease.
Overall, the peak height shows a non-monotonic temperature dependence. In sym- metric barriers case, the peak width shrinks and we find a regime where the unitary conductance limit is reached in the incoherent resonant tunneling. We interpret this behavior as a manifestation of a quantum phase transition.
Finally, our setup emulates tunneling in a Luttinger liquid (LL), an interacting one-dimensional electron system, that is distinct from the conventional Fermi liquids formed by electrons in two and three dimensions. Some of the most spectacular properties of LL are revealed in the process of electron tunneling: as a function of the applied bias or temperature the tunneling current demonstrates a non-trivial power-law suppression. Our setup allows us to address many prediction of resonant tunneling in a LL, which have not been experimentally tested yet.
Item Open Access Structure and Morphology Control in Carbon Nanomaterials for Nanoelectronics and Hydrogen Storage(2009) McNicholas, Thomas PatrickCarbon nanomaterials have a wide range of promising and exciting applications. One of the most heavily investigated carbon nanomaterial in recent history has been the carbon nanotube. The intense interest in carbon nanotubes can be attributed to the many exceptional characteristics which give them great potential to revolutionize modern mechanical, optical and electronic technologies. However, controlling these characteristics in a scalable fashion has been extremely difficult. Although some progress has been made in controlling the quality, diameter distribution and other characteristics of carbon nanotube samples, several issues still remain. The two major challenges which have stood in the way of their mainstream application are controlling their orientation and their electronic characteristics. Developing and understanding a Chemical Vapor Deposition based carbon nanotube synthesis method has been the major focus of the research presented here. Although several methods were investigated, including the so-called "fast-heating, slow-cooling" and large feeding gas flowrate methods, it was ultimately found that high-quality, perfectly aligned carbon nanotubes from a variety of metal catalysts could be grown on quartz substrates. Furthermore, it was found that using MeOH could selectively etch small-diameter metallic carbon nanotubes, which ultimately led to the productions of perfectly aligned single-walled carbon nanotube samples consisting almost entirely of semiconducting carbon nanotubes. Thiophene was utilized to investigate and support the hypothesized role of MeOH in producing these selectively gown semiconducting carbon nanotube samples. Additionally, this sulfur-containing compound was used for the first time to demonstrate a two-fold density enhancement in surface grown carbon nanotube samples. This method for selectively producing perfectly aligned semiconducting carbon nanotubes represents a major step towards the integration of carbon nanotubes into mainstream applications.
Although extremely useful in a variety of technologies, carbon nanotubes have proven impractical for use in H2 storage applications. As such, microporous carbons have been heavily investigated for such ends. Microporous carbons have distinguished themselves as excellent candidates for H2 storage media. They are lightweight and have a net-capacity of almost 100%, meaning that nearly all of the H2 stored in these materials is easily recoverable for use in devices. However, developing a microporous carbon with the appropriately small pore diameters (~1nm), large pore volumes (>1cm3) and large surface areas (≥3000m2/g) has proven exceedingly difficult. Furthermore, maintaining the ideal graphitic pore structure has also been an unresolved issue in many production means. Several microporous carbon synthesis methods were investigated herein, including inorganic and organically templated production schemes. Ultimately, thermally treating poly (etherether ketone) in CO2 and steam environments was found to produce large surface area porous carbons (≥3000m2/g) with the appropriately small pore diameters (<3nm) and large pore volumes (>1cm3) necessary for optimized storage of H2. Furthermore, the surface chemistry of these pores was found to be graphitic. As a result of these ideal conditions, these porous carbons were found to store ~5.8wt.% H2 at 77K and 40bar. This represents one of the most promising materials presently under investigation by the United States Department of Energy H2 Sorption Center of Excellence.
The success of both of these materials demonstrates the diversity and promise of carbon nanomaterials. It is hoped that these materials will be further developed and will continue to revolutionize a variety of vital technologies.
Item Open Access Synthesis, Purification and Application of Few-Walled Carbon Nanotubes and Inorganic Nanowires(2007-05-02T16:01:35Z) Qian, ChengOne-dimension (1D) nanostructures such as wires, rods, belts and tubes have become the focus of intensive researches for investigating structure-property relationships and related scientific and technological applications. Few-walled carbon nanotubes (FWNTs), a special type of small diameter multi-walled carbon nanotubes with superb structural perfection, are first discovered in our laboratory and systemically studied in this dissertation, including the synthesis by chemical vapor deposition (CVD) method, the purification and their applications. Moreover, iron phosphide nanorods/nanowires with controlled structures have been synthesized in solution phase and their magnetic properties have been investigated. The first parts of this dissertation are mainly focused on the studies of FWNTs synthesized by CVD method using binary catalyst Co (or Fe) with Mo (or W) supported on MgO made by modified combustion method. The structures of as-grown FWNTs can be controlled by three basic growth parameters: temperature, catalyst composition and carbon feeding rate. It is found that the as-grown FWNT materials prepared from W-containing catalysts are much more easily purified than those from Mo-containing catalysts. Both raw and purified FWNTs show enhanced electron field emission characteristics compared to other current commercial nanotubes. The highly pure FWNTs are then used to prepare composite materials with polymers and noble metal nanocrystals. Furthermore, the structures of FWNTs are attempted to be controlled by adjusting the growth parameters of carbon monoxide CVD. Highly pure DWNTs (over 95%) are obtained and well characterized by TEM, Raman and fluorescence spectrum. The optical properties of DWNTs and their application in bio-imaging are primarily investigated. In addition, conducing films are fabricated using highly pure FWNTs and the relationship between the structure and the conductivity is surveyed and further possible improvements are discussed. The second parts of this dissertation describe a solution-phase route for the preparation of single-crystalline iron phosphide nanorods and nanowires. The mixture of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) which are commonly used as the solvents for semiconductor nanocrystal synthesis is not entirely inert. TOP serves as phosphor source and reacts with Fe precursors to generate iron phosphide nanostructures with large aspect ratios. In addition, the morphology of the produced iron phosphide structures can be controlled by the ratio of TOPO/TOP. A possible growth mechanism is discussed.Item Open Access Tip-based Creation and Functionalization of Nanoscale Surface Patterns(2008-07-29) Woodson, Michael ENanostructures are being intensely studied due to unusual material properties and simple scaling concerns in the microelectronics industry. Fabricating useful nano-scale structures and devices, either by arranging existing nanoparticles such as carbon nanotubes or by manipulating bulk materials into nanometer-scale geometries, is a challenging prospect. One promising approach is to generate a nanometer-scale pattern and transfer that geometry into another material. The research described in this dissertation concerns the fabrication of nanometer-scale patterns, by Atomic Force Microscope-based methods and Electron Beam Lithography, on planar surfaces and the transfer of those patterns into functional materials. Differences in surface energy were used to guide the growth of bulk conducting polymer along predefined nano-scale patterns. Carbon nanotubes were assembled into an ordered and continuous material with no guidance and used to lithographically write silicon oxide nanopatterns on a silicon surface. Finally, the two previous projects were combined, and surface energy patterns were used to guide the deposition of dense carbon nanotube bundles along a planar substrate.