Browsing by Author "Glass, Jeffrey T"

The 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.

Computer simulation and modeling is a powerful tool for the analysis of physical systems; in this work we consider the use of computer modeling and optimization in improving the focusing properties of a variety of charged particle optics systems. The combined use of several software packages and custom computer code allows for modeling electrostatic and magnetostatic fields and the trajectory of particles through them. Several applications of this functionality are shown. The pieces of code which are shown are the starting point of an integrated charged particle simulation and optimization tool with focus on optimization. The applications shown are mass spectrographs and electron energy spectrographs. Simulation allowed additional information about the systems in question to be determined.In this work, coded apertures are shown to be compatible with sector instruments, though architectural challenges exist. Next, simulation allowed for the discovery of a new class of mass spectrograph which addresses these challenges and is compatible with computational sensing, allowing for both high resolution and high sensitivity, with a 1.8x improvement in spot size. Finally, a portion of this new spectrograph was adapted for use as an electron energy spectrograph, with a resolution 9.1x and energy bandwidth 2.1x that of traditional instruments.

Chemical and electrochemical processes at solid/liquid interfaces are key to diverse and wide range of applications. Hence, the investigation of these reactions is crucial to develop, advance, and improve technologies across numerous fields. The applications chosen as the focus of this dissertation are sanitation and neural stimulation. Sanitation challenges are an urgent global issue for which solutions are being actively developed and improved. This work aims to provide options to overcome some of the limitations found in current technologies in three areas. First, to address user adoption of sanitation facilities, electrochemical modulation of p-cresol was, for the first time, evaluated as an option for malodor nuisance control. It was demonstrated that the electrochemical oxidation of p-cresol can generate 4-hydroxybenzaldehyde following the introduction of chloride ions into the supporting electrolyte. Second, to address nutrient pollution caused by effluents with high levels of ammonium and phosphate from non-sewered sanitation systems and on-site wastewater treatment systems, silicate-based minerals (i.e., clinoptilolite and Polonite) were explored as scalable, affordable, and non-biological options to remove and recover nutrients from these effluents. Clinoptilolite and Polonite were installed and evaluated in our on-site wastewater treatment system, resulting in an increased removal performance of total N and total P from 47.5% to 84.1% and 32.3% to 78.9% respectively. Lastly, to improve the performance of neural stimulation devices, graphenated carbon nanotubes were investigated, for the first time, as an alternative material for neural electrodes. It was demonstrated that graphenated carbon nanotubes can be decorated with platinum nanoparticles to create platinum 3D structures with high cathodal charge storage capacitance and low impedance.

Mass spectrometry is widely considered to be the gold standard of elemental analysis techniques due to its ability to resolve atomic and molecular and biological species. Expanding the application space of mass spectrometry often requires the need for portable or hand-held systems for use in field work or harsh environments. While only requiring “sufficient” mass resolution to meet the needs of their application space, these miniaturized systems suffer from poor signal to background ratio which limits their sensitivity as well as their usefulness in field applications.

Spatial aperture coding techniques have been used in optical spectroscopy to achieve large increases in signal intensity without compromising system resolution. In this work similar computational methods are used in the application of these techniques to the field of magnetic sector mass spectrometry. Gains in signal intensity of 10x and 4x were achieved for 1D and 2D coding techniques (respectively) using a simple 90 degree magnetic sector test setup. Initial compatibility with a higher mass resolution double focusing Mattauch-Herzog mass spectrograph is demonstrated experimentally and with high fidelity particle tracing simulations. A novel electric sector lens system was designed to stigmate high order coded aperture patterned beam which shows simulated gains in signal intensity of 50x are achievable using these techniques.

Electrode degradation in harsh environments is a problem that plagues many devices. Traditional metallic and carbon-based electrodes are often reactive and can fail over time, leading to reduced efficiency and increased operational costs. Carbon nanomaterials can potentially offer improvements over traditional electrodes due to their high surface area and the robust nature of sp2 hybridization. This dissertation focuses on the study of two carbon allotropes with nanoscale features: carbon nanowalls and multiwalled carbon nanotubes, and their use as electrode materials in harsh environments. Specifically, the use of carbon nanotubes as a material for field emission electrodes in radiative environments was researched, and carbon nanowalls were explored as an electrode material for phosphoric acid fuel cells.

The use of carbon nanotubes as field emission electrodes in radiative environments is researched by characterizing the effects of gamma and proton radiation on carbon nanotube structural properties and field emission performance. It was determined that both proton and gamma radiation affect the crystalline structure of carbon nanotubes by reducing defect density, which leads to an increase in the applied field required to induce electron emission. However, the effects due to radiation are smaller in magnitude compared to the effects of adsorbates on field emission performance.

To explore the use of carbon nanomaterials as an electrode material for use in phosphoric acid fuel cells, carbon nanowalls were prepared in a microwave plasma chemical vapor deposition reactor. The ability to modify the structure of the resulting films was demonstrated by changing the ratio of the growth gases. Platinum nanoparticles were deposited from solution onto the carbon nanowalls, and their surface area and hydrogen adsorption capabilities were characterized using scanning electron microscopy and cyclic voltammetry. It was determined that while functional, the carbon nanowall electrodes had lower platinum loading compared to electrodes made from carbon black, and were more susceptible to degradation in phosphoric acid at an elevated temperature.

In urban, dense, and/or environmentally sensitive regions, underground high-pressure fluid-filled (HPFF) transmission cables are used to transport electricity at high voltages to prevent electrical losses. With age and use, these HPFF cables degrade and can leak petroleum-based dielectric fluid (DF) into the surrounding environment. Maintaining adequate DF in these cables is required for safe and reliable operation. Currently, detecting and locating underground DF leaks is challenging, time intensive, and inconsistent. One leak location method exists utilizing perfluorocarbon (PFC) tracer molecules and a mobile gas chromatograph. This instrument is sensitive enough to detect the atmospheric background levels of PFC in the ppqv (parts per quadrillion by volume, 10-12) range; however, this instrument is prone to analyte saturation, is not fully portable, nor does it produce real-time results.

An improved mobile and highly sensitive PFC detection method is required. A cycloidal coded aperture miniature mass spectrometers (C-CAMMS) could be such a method due to its integration of three core technologies that help to overcome the throughput versus resolution tradeoff that has historically hampered the miniaturization of mass spectrometers. The C-CAMMS prototype combines: a cycloidal mass analyzer, aperture coding, and a focal plane array detector, to enable a mobile instrument capable of detecting PFC tracers with high resolution, sensitivity, and selectivity. The cycloidal mass analyzer utilizes perpendicularly-oriented overlapping electric and magnetic fields to linearly separate ions by mass-to-charge ratio (m/q). The C-CAMMS platform uses aperture coding to increase throughput without sacrificing resolution which commonly occurs during miniaturization. Finally, a capacitive transimpedance amplifier (CTIA) array ion detector offers sensitive, simultaneous ion detection across a wide mass range.

A detailed understanding of the detection process for this instrument was obtained through extensive simulations and experiments. From this knowledge, a set of design considerations and a six-step design approach have been established for reproducibly developing a cycloidal mass analyzer that uses a focal plane array detector. This design knowledge relates the magnetic and electric field homogeneity of the cycloidal mass analyzer to the performance of the complete mass spectrometer. The efficacy of this design roadmap is demonstrated by designing the PFC-CAMMS instrument for the specific use case of PFC tracer detection and location.

The design knowledge was implemented to create a PFC-CAMMS instrument with high resolution, high sensitivity, a large mass range, and a small form factor. PFC-CAMMS achieved a resolution of 0.071 u at m/q 69 and 0.27 u at m/q 331 and demonstrated an overall mass range of m/q 29 – 502. Additionally, using PMCH (C7F14) as the test molecule, the PFC-CAMMS instrument achieved a detection limit of 20 ppbv (parts per billion by volume, 10-9) with a response time of less than 5 s from sample introduction using a capillary inlet. There is no evidence of a hysteresis effect when detecting PFCs for this benchtop (66 x 46 x 30 cm3 and ~50 kg) laboratory prototype. This work describes not only the understanding that was generated about designing electromagnetic components for a PFC-CAMMS, but also explains the fabrication, alignment, and assembly details that enable this improved baseline performance. The reproducible procedure for developing a cycloidal mass analyzer with an array detector facilitates improved hardware designs that produce a more consistent system response function across the intended mass range. Following reconstruction of the coded mass spectrum, the PFC-CAMMS instrument can overcome the throughput vs. resolution tradeoff due to its stronger yet more uniform field.

The development of human society strongly relies on the utilization of energy. While fossil fuels are still the main energy source of current human activity, concerns about the environment and the greenhouse effect brought by combustion of fossil fuels have led to tremendous research interest on developing renewable energy conversion and storage techniques. Electrochemical energy technologies represent a promising solution to overcome the current energy dilemma. For energy conversion, photoelectrochemical (PEC) water splitting directly converts water and solar energy into hydrogen and oxygen. The generated hydrogen can be used as a clean, sustainable and efficient fuel and recycled as water after combustion. Currently, photoelectrochemical water splitting devices are either expensive, low performance or unstable. Developing new materials and new architectures with improved PEC performance is in high demand. The first half of this dissertation explores a new earth-abundant chalcogenide material Cu2BaSn(S,Se)4 as a promising photocathode for efficient hydrogen evolution.

For energy storage, supercapacitors are indispensable energy sources for portable electronics and electric vehicles. The rapid development of wearable devices, biomedical implants and electronic skin have raised new mechanical challenges for conventional supercapacitors. Large mechanical deformability is required for supercapacitors to integrate with these stretchable electronics. The second half of the dissertation studied novel stretchable supercapacitors based on various carbon nanomaterials that can be used for the applications of wearable and stretchable electronics.

In chapter 2 and 3, a new solar water splitting photocathode, Cu2BaSn(S,Se)4, was systematically studied. The PEC performance of Cu2BaSn(S,Se)4 was found to increase with increasing Se concentration. However, low light absorption, poor electrolyte/semiconductor junction and poor stability limited the performance of the Cu2BaSn(S,Se)4 photocathode. To improve the photoelectrochemical performance, a Pt/TiO2/CdS/Cu2BaSn(S,Se)4 architecture with ~75% Se concentration was designed. With improved light absorption, enhanced charge separation and charge transfer, a world-record-high photocurrent density of 12.08 mA/cm2 at 0 V/RHE was obtained. The Pt/TiO2/CdS/Cu2BaSn(S,Se)4 photocathode also delivered a consistent photocurrent for more than 10 hours demonstrating superior stability at 0 V/RHE. The same architecture was applied to a solution-processed Cu2BaSn(S,Se)4 absorber and yielded similar PEC performance, demonstrating the feasibility of a high performance, low cost and stable Cu2BaSn(S,Se)4 based photocathode.

From chapter 4 to chapter 7, stretchable supercapacitors based on various carbon nanomaterials were designed, fabricated and characterized. A new stretchable supercapacitor based on crumpled carbon nanotube (CNT) forest was developed. The vertically aligned CNT forest structure was well preserved during the transfer process. Intertwining of neighboring tubes provided the electrical integrity across the whole forest. With large surface area and easily accessible pore structure, a crumpled CNT forest supercapacitor with high electrochemical performance and large mechanical deformability was successfully fabricated. To further reduce the resistance of crumpled CNT forest, an Au-CNT network was introduced at the base. A resistance decrease of an order magnitude was obtained using the Au-CNT network. As a result, the electrochemical performance of the crumpled CNT forest was significantly improved especially at high charge/discharge rate where conductivity is more important.

MXene, a new 2-Dimentional Metal Carbide has also been utilized as a stretchable supercapacitor but was found to crack during the stretchable electrode fabrication process, mainly because of its high mechanical stiffness, weak intersheet interaction and small flake size. Thus, reduced graphene oxide (RGO) was incorporated to overcome these issues. The as-prepared stretchable MXene/RGO composite supercapacitor maintained its structural integrity under various mechanical strains and demonstrated good electrochemical performance.

Finally, inkjet printing was introduced to fabricate a carbon nanotube-reduced graphene oxide-poly(ethylenedioxythiophene) (CNT-RGO-PEDOT) stretchable supercapacitor. The high electrochemical performance (20 F/g, 85% rate capability from 0.5 A/g to 5 A/g) and high mechanical robustness of printed CNT-RGO-PEDOT stretchable supercapacitor demonstrates the possibility of fabricating stretchable supercapacitor in a more scalable approach.

Carbon nanotubes (CNTs) have recently emerged as promising candidates for electron field emission (FE) cathodes in integrated FE devices. These nanostructured carbon materials possess exceptional properties and their synthesis can be thoroughly controlled. Their integration into advanced electronic devices, including not only FE cathodes, but sensors, energy storage devices, and circuit components, has seen rapid growth in recent years. The results of the studies presented here demonstrate that the CNT field emitter is an excellent candidate for next generation vacuum microelectronics and related electron emission devices in several advanced applications.

The work presented in this study addresses determining factors that currently confine the performance and application of CNT-FE devices. Characterization studies and improvements to the FE properties of CNTs, along with Micro-Electro-Mechanical Systems (MEMS) design and fabrication, were utilized in achieving these goals. Important performance limiting parameters, including emitter lifetime and failure from poor substrate adhesion, are examined. The compatibility and integration of CNT emitters with the governing MEMS substrate (i.e., polycrystalline silicon), and its impact on these performance limiting parameters, are reported. CNT growth mechanisms and kinetics were investigated and compared to silicon (100) to improve the design of CNT emitter integrated MEMS based electronic devices, specifically in vacuum microelectronic device (VMD) applications.

Improved growth allowed for design and development of novel cold-cathode FE devices utilizing CNT field emitters. A chemical ionization (CI) source based on a CNT-FE electron source was developed and evaluated in a commercial desktop mass spectrometer for explosives trace detection. This work demonstrated the first reported use of a CNT-based ion source capable of collecting CI mass spectra. The CNT-FE source demonstrated low power requirements, pulsing capabilities, and average lifetimes of over 320 hours when operated in constant emission mode under elevated pressures, without sacrificing performance. Additionally, a novel packaged ion source for miniature mass spectrometer applications using CNT emitters, a MEMS based Nier-type geometry, and a Low Temperature Cofired Ceramic (LTCC) 3D scaffold with integrated ion optics were developed and characterized. While previous research has shown other devices capable of collecting ion currents on chip, this LTCC packaged MEMS micro-ion source demonstrated improvements in energy and angular dispersion as well as the ability to direct the ions out of the packaged source and towards a mass analyzer. Simulations and experimental design, fabrication, and characterization were used to make these improvements.

Finally, novel CNT-FE devices were developed to investigate their potential to perform as active circuit elements in VMD circuits. Difficulty integrating devices at micron-scales has hindered the use of vacuum electronic devices in integrated circuits, despite the unique advantages they offer in select applications. Using a combination of particle trajectory simulation and experimental characterization, device performance in an integrated platform was investigated. Solutions to the difficulties in operating multiple devices in close proximity and enhancing electron transmission (i.e., reducing grid loss) are explored in detail. A systematic and iterative process was used to develop isolation structures that reduced crosstalk between neighboring devices from 15% on average, to nearly zero. Innovative geometries and a new operational mode reduced grid loss by nearly threefold, thereby improving transmission of the emitted cathode current to the anode from 25% in initial designs to 70% on average. These performance enhancements are important enablers for larger scale integration and for the realization of complex vacuum microelectronic circuits.

The primary goals of this research were to investigate the electrochemical behavior of carbon nanostructures of varying morphology, identify morphological characteristics that improve electrochemical capacitance for applications in energy storage and neural stimulation, and engineer and characterize a boron-­doped diamond (BDD) electrode based electrochemical system for disinfection of human liquid waste.

Carbon nanostructures; ranging from vertically aligned multiwalled carbon nanotubes (MWCNTs), graphenated carbon nanotubes (g-­CNTs) to carbon nanosheets (CNS); were synthesized using a MPECVD system. The nanostructures were characterized by using scanning electron microscopy (SEM) and Raman spectroscopy. In addition to employing commonly used electrochemical techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), a new technique was developed to evaluate the energy and power density of individual electrodes. This facilitated comparison of a variety of electrode materials without having to first develop complex device packaging schemes. It was found that smaller pore size and higher density of carbon foliates on a three-dimensional scaffold of carbon nanotubes increased specific capacitance. A design of experiments (DOE) study was conducted to explore the parametric space of the MWCNT system. A range of carbon nanostructures of varying morphology were obtained. It was observed that the capacitance was dependent on defect density. Capacitance increased with defect density.

A BDD electrode was characterized for use in a module designed to disinfect human liquid waste as a part of a new advanced energy neutral, water and additive-free toilet designed for treating waste at the point of source. The electrode was utilized in a batch process system that generated mixed oxidants from ions present in simulated urine and inactivated E. Coli bacteria. Among the mixed oxidants, the concentration of chlorine species was measured and was found to correlate to the reduction in E. Coli concentration. Finally, a new operating mode was developed that involved pulsing the voltage applied to the BDD anode led to 66% saving in energy required for disinfection and yet successfully reduced E. Coli concentration to less than the disinfection threshold.

Roughly 40% of the world does not have access to appropriate sanitation of human generated waste water. Lack of infrastructure and poverty in developing nations has stymied the deployment of conventional sewage treatment practices. In helping to solve this global issue requires the development of an energy efficient, cost-effective, low-maintenance, and decentralized toilet system that can remediate human liquid waste, or, blackwater. Herein, electrochemical disinfection as a means of treating blackwater is investigated using degenerately boron-doped diamond and Magnéli-phase titanium sub-oxide electrodes. It is found that both can be operated in potentiodynamic modes to control surface chemistry and improve generation of biocidal oxidants such as hydrogen peroxide and chlorine

in blackwater containing solutions. Use of a packed-bed electrochemical reactor is also studied in the treatment of blackwater using Magnéli-phase titanium sub-oxide granular electrodes. It is found that bed-height, flow-rate, and blackwater chemistry

can greatly affect the effectiveness of electrochemical disinfection and stability of a packed-bed electrochemical reactor. Overall, these results highlight how existing electrode materials can be modified or controlled in-situ to inhibit fouling, generate

oxidants using less energy, and therefore disinfect blackwater pathogens more effectively.

Electrical stimulation is capable of restoring function to a damaged or diseased nervous system and can thereby improve the lives of patients in a remarkable way. For example, cochlear and retinal prostheses can help the deaf to hear and the blind to see, respectively. Improvements in the safety, efficacy, selectivity, and power consumption of these technologies require a long-term biocompatible, chemically and mechanically stable, low impedance neural electrode interface which can rapidly store high charge densities without damaging the electrode or neural tissue.

In this study, vertically aligned multi-walled carbon nanotube films were synthesized and investigated for their potential use as a neural stimulation electrode. Materials characterization using electron microscopy, Raman, and x-ray photoelectron spectroscopy; and in-vitro electrochemical characterization using cyclic voltammetry, electrochemical impedance spectroscopy, and potential transient measurements were employed to determine material and electrochemical properties, respectively. Characterization was performed prior and subsequent to electrochemical and oxidative thermal treatments to determine if there were improvements in the desired properties.

The results indicated that electrochemical activation by potential cycling across the water window, a technique often used to activate and greatly improve the performance of iridium oxide electrodes, was also favorable for carbon nanotube (CNT) electrodes especially for thicker films. In addition, oxidative thermal treatments that did not significantly oxidize or etch the nanotubes caused a significant improvement in electrode performance. Phenomenological models were developed from these findings. Finally, growth of aligned CNTs using a platinum catalyst was demonstrated and suggested to reduce biocompatibility concerns due to otherwise highly toxic catalyst residue inherent in CNTs that may become bioavailable during chronic use.

Graphene, first isolated in 2004 and the subject of the 2010 Nobel Prize in physics, has generated a tremendous amount of research interest in recent years due to its incredible mechanical and electrical properties. However, difficulties in large-scale production and low as-prepared surface area have hindered commercial applications. In this dissertation, a new material is described incorporating the superior electrical properties of graphene edge planes into the high surface area framework of carbon nanotube forests using a scalable and reproducible technology.

The objectives of this research were to investigate the growth parameters and mechanisms of a graphene-carbon nanotube hybrid nanomaterial termed “graphenated carbon nanotubes” (g-CNTs), examine the applicability of g-CNT materials for applications in electrochemical capacitors (supercapacitors) and cold-cathode field emission sources, and determine materials characteristics responsible for the superior performance of g-CNTs in these applications. The growth kinetics of multi-walled carbon nanotubes (MWNTs), grown by plasma-enhanced chemical vapor deposition (PECVD), was studied in order to understand the fundamental mechanisms governing the PECVD reaction process. Activation energies and diffusivities were determined for key reaction steps and a growth model was developed in response to these findings. Differences in the reaction kinetics between CNTs grown on single-crystal silicon and polysilicon were studied to aid in the incorporation of CNTs into microelectromechanical systems (MEMS) devices. To understand processing-property relationships for g-CNT materials, a Design of Experiments (DOE) analysis was performed for the purpose of determining the importance of various input parameters on the growth of g-CNTs, finding that varying temperature alone allows the resultant material to transition from CNTs to g-CNTs and finally carbon nanosheets (CNSs): vertically oriented sheets of few-layered graphene. In addition, a phenomenological model was developed for g-CNTs. By studying variations of graphene-CNT hybrid nanomaterials by Raman spectroscopy, a linear trend was discovered between their mean crystallite size and electrochemical capacitance. Finally, a new method for the calculation of nanomaterial surface area, more accurate than the standard BET technique, was created based on atomic layer deposition (ALD) of titanium oxide (TiO2).

Aperture coding has been utilized in mass spectrometry to enable miniaturization of the instrument while maintaining resolution and throughput. In miniature cycloidal mass spectrometry, implementing aperture coding is difficult since uniform magnetic and electric fields are required in the sector region. This work shows through finite element simulation that the compatibility between a cycloidal mass analyzer and aperture coding has been improved by the development of a novel electric sector. COMSOL 5.2a was utilized as the primary finite element analysis tool, and MATLAB R2016b was used to generate most figures.

Effluence of organic compounds like benzene, toluene, ethylbenzene and xylenes (“BTEX”), and methane from an industrial setting can have a negative impact on human health and the environment. Miniature sector mass spectrometers have the potential to acquire desirable attributes for ideal organic compound detection such as robustness, low cost, high chemical specificity, high sensitivity, and low power requirements. However, barriers to their miniaturization exist in the form of a throughput vs. resolution tradeoff. Spatially coded apertures can break this tradeoff by increasing throughput without sacrificing resolution. Cycloidal sector mass spectrometers are ideal candidates for incorporation of spatially coded apertures when used with array detectors, since they use perfectly focus the image of coded aperture at the detector due to perpendicularly oriented uniform electric and magnetic fields.

A previous demonstration of a proof-of concept cycloidal-coded aperture miniature mass spectrometer (C-CAMMS) instrument employed aperture coding, a carbon nanotube (CNT) field emission electron ionization source, a cycloidal mass analyzer, and a capacitive transimpedance amplifier (CTIA) array detector to achieve greater than ten-fold increase in throughput without sacrificing resolution. However, the coded aperture image corresponding to each ion species was not constant due to a spatiotemporal variation in electron emission from CNTs, a non-uniformity in the electric field, and a misalignment of the detector and the ion source with the mass analyzer focal plane.

In this work, modifications to the sample inlet, ion source, and the mass analyzer design of the previous C-CAMMS instrument were made to improve its performance. A membrane inlet enhanced the organic compound detection sensitivity of the new C-CAMMS instrument and enabled low detection limits of 50 ppm for methane and 20 ppb for toluene. A thermionic filament-based ion source produced a significantly more stable coded aperture image than the CNT based ion source. The aperture image fluctuations in the CNT-based source were determined to be likely a result of adsorption and desorption of molecules on the CNT surface that caused local work function changes and induced spatiotemporal variation in electron emission and subsequent ion generation. Modifications to the mass analyzer improved the electric field uniformity, improved the alignment of the ion source and the detector with the mass analyzer focal plane, and increased the depth-of-focus to further facilitate alignment. Finally, a comparison of reconstructed spectra of a mixture of dry air and toluene at different electric fields was performed using the improved C-CAMMS prototype. A reduction in reconstruction artifacts for a wide mass-to-charge (m/z) range highlighted the improved performance enabled by the design changes.

The National Research Council’s Planetary Science 2013-2022 Decadal Survey underscores three interrelated themes pivotal to planetary science research: understanding solar system beginnings, searching for the requirements for life, and understanding the workings of solar systems. In situ mass spectrometry (MS) is the primary technique for the analysis of planetary substances, directly addressing the critical inquiries associated with these themes. The quintessential mass analyzer engineered for space exploration is envisioned to embody a suite of features: a mass range extending from 1 u to at least 500 u, capability for high-precision measurement of stable isotope ratios within a tolerance of ±1‰, and the ability to resolve distinct isobaric species at a low mass below 60 u, all with low power requirements. Incorporation of these capabilities within a single instrument is crucial for facilitating the exploration of the necessities of life and for advancing our understanding of solar system genesis and planetary development. Nevertheless, state-of-the-art existing spaceflight mass spectrometers do not fully integrate all these capabilities.In this research, three technologies are investigated to close this gap; spatial aperture coding, super-resolution, and field emission electron sources . The development of these three technologies as presented in this dissertation represent a significant step towards a mass spectrometer having all of the characteristics described above. First, Spatial aperture coding is a technique used to improve throughput without sacrificing resolution, historically in optical spectroscopy, and more recently as demonstrated by our laboratory at Duke University, in sector mass spectrometry (MS). Previously we demonstrated that aperture coding combined with a position-sensitive array detector in a miniature cycloidal mass spectrometer was successful in providing high-throughput, high-resolution measurements. However, due to poor alignment and field non-uniformities, reconstruction artifacts were present. In this dissertation, two methods were implemented to significantly reduce the presence of artifacts in reconstructed spectra. First, I employed a variable system response function across the mass range (10 – 110 u) instead of using a fixed function. Second, I modified the design by shifting the coded aperture slits relative to the center of the ionization volume to enable even illumination of the coded aperture slits. Both methods were successful in significantly reducing artifacts at low mass from above 35% of the peak height to less than 6% of the peak height. Second, higher resolution in fieldable mass spectrometers (MS) is desirable in space flight applications to enable resolving isobaric interferences at m/z < 60 u. Resolution in portable cycloidal MS coupled with array detectors could be improved by reducing the slit width and/or by reducing the width of the detector pixels. However, these solutions are expensive and can result in reduced sensitivity. In this dissertation, I demonstrate high-resolution spectral reconstruction in a cycloidal coded aperture miniature mass spectrometer (C-CAMMS) without changing the slit or detector pixel sizes using a class of signal processing techniques called super resolution (SR). I developed an SR reconstruction algorithm using a sampling SR approach whereby a set of spatially shifted low-resolution measurements are reconstructed into a higher-resolution spectrum. This algorithm was applied to experimental data collected using the C-CAMMS prototype. It was then applied to synthetic data with additive noise, system response variation, and spatial shift nonuniformity to investigate the source of reconstruction artifacts in the experimental data. Experimental results using two 1/2 pixel shifted spectra resulted in a resolution of 3/4 pixel full width at half maximum (FWHM) at m/z = 28 u. This resolution is equivalent to 0.013 u, six times better than the resolution previously published at m/z = 28 for N2+ using C-CAMMS. However, the reconstructed spectra exhibited some artifacts. The results of the synthetic data study indicate that the artifacts are most likely caused by the system response variation. Despite these artifacts, it was shown that the super-resolution algorithm is capable of resolving the isobaric interference between N2 and CO at m/z = 28. Third, Field emission electron sources for MS electron ionization have been of interest to spaceflight applications due to their low power compared to thermionic sources. However, state-of-the-art devices suffer from limitations such as high turn-on macroscopic field, low macroscopic current density, poor emission stability, and short lifetime. Field emitter arrays with a high spatial density of uniform emitters have the potential to address these problems. In this work, process development, fabrication, and testing of two novel field emission based devices are presented, including CNT array emitters and metallic nanowires. Instability in CNT emission was investigated using noise analysis and a polymer encapsulation process to reduce the effect of adsorbates on the tips of CNTs. This treatment was not successful in reducing emission noise in CNTs. Thus, electron beam lithography and templated electrodeposition were used to fabricate a high spatial density array of metallic nanowires, resulting in electron field emission with high macroscopic current density (2 A/cm2) and low turn-on macroscopic field (4.35 V/μm). Results indicate that templated electrodeposition of metallic nanowire arrays is a promising method for producing high-performance field emitters.

The realization of an energy future based on safe, clean, sustainable, and economically viable technologies is one of the grand challenges facing modern society. Electrochemical energy technologies underpin the potential success of this effort to divert energy sources away from fossil fuels, whether one considers alternative energy conversion strategies through photoelectrochemical (PEC) production of chemical fuels or fuel cells run with sustainable hydrogen, or energy storage strategies, such as in batteries and supercapacitors. This dissertation builds on recent advances in nanomaterials design, synthesis, and characterization to develop novel electrodes that can electrochemically convert and store energy.

Chapter 2 of this dissertation focuses on refining the properties of TiO2-based PEC water-splitting photoanodes used for the direct electrochemical conversion of solar energy into hydrogen fuel. The approach utilized atomic layer deposition (ALD); a growth process uniquely suited for the conformal and uniform deposition of thin films with angstrom-level thickness precision. ALD’s thickness control enabled a better understanding of how the effects of nitrogen doping via NH3 annealing treatments, used to reduce TiO2’s bandgap, can have a strong dependence on TiO2’s thickness and crystalline quality. In addition, it was found that some of the negative effects on the PEC performance typically associated with N-doped TiO2 could be mitigated if the NH3-annealing was directly preceded by an air-annealing step, especially for ultrathin (i.e., < 10 nm) TiO2 films. ALD was also used to conformally coat an ultraporous conductive fluorine-doped tin oxide nanoparticle (nanoFTO) scaffold with an ultrathin layer of TiO2. The integration of these ultrathin films and the oxide nanoparticles resulted in a heteronanostructure design with excellent PEC water oxidation photocurrents (0.7 mA/cm2 at 0 V vs. Ag/AgCl) and charge transfer efficiency.

In Chapter 3, two innovative nanoarchitectures were engineered in order to enhance the pseudocapacitive energy storage of next generation supercapacitor electrodes. The morphology and quantity of MnO2 electrodeposits was controlled by adjusting the density of graphene foliates on a novel graphenated carbon nanotube (g-CNT) scaffold. This control enabled the nanocomposite supercapacitor electrode to reach a capacitance of 640 F/g, under MnO2 specific mass loading conditions (2.3 mg/cm2) that are higher than previously reported. In the second engineered nanoarchitecture, the electrochemical energy storage properties of a transparent electrode based on a network of solution-processed Cu/Ni cores/shell nanowires (NWs) were activated by electrochemically converting the Ni metal shell into Ni(OH)2. Furthermore, an adjustment of the molar percentage of Ni plated onto the Cu NWs was found to result in a tradeoff between capacitance, transmittance, and stability of the resulting nickel hydroxide-based electrode. The nominal area capacitance and power performance results obtained for this Cu/Ni(OH)2 transparent electrode demonstrates that it has significant potential as a hybrid supercapacitor electrode for integration into cutting edge flexible and transparent electronic devices.