Browsing by Subject "Carbon nanotubes"

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.

The rapid maturation of the internet of things (IoT) has led to an ever-stronger drive for large-area, flexible, and/or customizable electronics for data collection, display, and communication. The use of printing technology for IoT and thin-film electronics has shown growing promise due to its potential in low-cost and high-throughput manufacturing, as well as the capability to handle a wide array of substrates, materials, and production techniques (e.g., mass-production or customizable). At the heart of many IoT devices are thin-film transistors (TFTs). Carbon nanotubes (CNTs) have been considered promising candidates for printed TFTs due to their extraordinary electronic properties and material attributes, such as mechanical flexibility and low-temperature processability. Despite the significant research progress in printing CNT-TFTs and demonstrating CNT-TFTs in applications, process variability has still remained a major obstacle to the translation of CNT-TFTs out of the lab and into products. In addition, most printing processes reported in the literature involve post-printing thermal treatments, limiting the throughput and efficiency of printing approaches. This dissertation contains scientific discoveries, technical advancements, and innovations that reduce the process variability of CNT-TFTs and lead to the development of print-in-place processes -- a series of rapid, versatile, and streamlined printing approaches for yielding CNT-TFTs without any postprocessing. The key enabling factor of variability reduction is to understand the impact of CNT ink temperature, a commonly overlooked factor, on the resultant ink deposition via aerosol jet printing (AJP). It was discovered that an appropriately lowered ink temperature benefits both long-term (~1 hour) and short-term (~1 minute) stability of AJP, resulting in fully printed TFTs with average mobility of 12.5 cm2/V·s and mobility variation as small as 4%. The streamlining of the print-in-place processes for CNT-TFTs involved identifying low-temperature processable materials and formulating corresponding printable inks from these materials. By printing silver nanowires (AgNW) as the electrical contacts and hexagonal boron nitride (h-BN) as the gate insulator, the maximum processing temperature of the print-in-place process was reduced to 80 °C with no additional thermal treatment required. Notably, the resultant devices (known as 1D-2D TFTs) showed relatively good performance, including an on/off-current ratio up to 3.5×105, channel mobility up to 10.7 cm2/V∙s, small gate hysteresis, and superb mechanical stability under bending. In addition, another print-in-place process was demonstrated by employing a side-gate configuration with an ion gel dielectric and graphene contacts. Compared to the fabrication process for 1D-2D TFTs, this 3-step process was even more streamlined, and the resulting devices exhibited more uniform performance at the cost of the on/off-current ratio. Besides variability reduction and streamlining, the versatility and environmental friendliness of printed CNT-TFTs were also enhanced by studying the use of all-aqueous inks for CNT-TFTs. The study unveiled the printing challenges imposed by aqueous CNT inks compared to the more commonly used inks that depend on harsh solvents, such as toluene. It was discovered that the ionic surfactant in the aqueous inks hinders the CNT adhesion with the substrate, and a multi-step process with interstitial rinsing was proposed to mitigate this issue. Through the combination of aqueous CNT, graphene, and crystalline nanocellulose (CNC) inks, water-only TFTs were printed without the use of any harsh chemical solvents, and most device layers are either recyclable or biodegradable. In addition, although this dissertation is primarily focused on the printing techniques used to make CNT-TFTs, the materials and methods developed in the works were also utilized to demonstrate a surface acoustic wave (SAW) tuning device, an uncommon application for CNT-TFTs. The phase-velocity tunability (∆v/v = 2.5% ) was exceptional and close to the theoretical limit, suggesting the promise of CNT-TFTs for SAW applications and that there exist numerous unexplored areas where CNT-TFTs could potentially be advantageous. Overall, the findings and advancements contributed by this dissertation advance the printing of CNT-TFT by addressing obstacles and demonstrating possibilities. These include the developed methodologies, such as the print-in-place approaches and the multi-step aqueous printing with interstitial rinsing, along with the discoveries that may trigger follow-up studies, such as the correlation between ink temperature and process variability. Combined, these contributions help to reduce the variability, lower the process duration, and enhance the versatility of printed CNT-TFTs, pushing the field toward ubiquitous use in real-world applications.

As the cost of medical care increases, people are relying increasingly on internet diagnosis and community care rather than the expertise of medical professionals. Technological and medical advances have facilitated a partial answer through the increase in handheld sensing apparatuses. Yet even with these developments, significant further advancements are required to further drive down fabrication requirements (both in terms of cost and environmental impact) and facilitate fully-integrated and easy to use sensors. Printing electronics could be a powerful tool to accomplish this as printing allows for low-cost fabrication of high-area electronics. The vast majority of printed electronics reports focus on utilization of already developed commercial inks to create devices with new functionalities. This significantly limits development because current inks both necessitate damaging post processing—which precludes the use of delicate substrates, making skin-integration impossible—and many inks require bespoke printing processes, which increases fabrication complexity and thus cost. Further, with the proliferation of single-use medical testing, consideration must be made towards environmental compatibility. Therefore, innovations in electronic ink formulation and printing geared towards addressing the post-processing and environmental impact concerns are needed to enable continued progress towards printed POC sensors. The work contained in this dissertation centers around the development of inks intended to advance electronic biosensing applications. Focus is on using aerosol jet printing to enable the printing of nanomaterials and utilizes the unique properties of these nanomaterials—such as functionality immediately after printing, recyclability, and compatibility with deposition directly on biological surfaces (i.e., human skin)—to develop technologies intended to democratize healthcare. Notably, low temperature printable silver nanowire (AgNW) inks for the creation of biologically integrated electronics are demonstrated. Electrically conductive inks are created that are capable of achieving high conductivities when directly deposited onto living tissue at temperatures compatible with life (20 °C). The conductive lines yielded high resistance to degradation from bending strain, with a mere 8% decrease in conductivity when the plastic film on which they were printed was folded in half. As a demonstration, the AgNW ink was printed onto a human finger and used to illuminate a small light that remained illuminated even when the finger was bent. These results pave the way towards patient-specific medical diagnostics that are comfortable to wear, easy to use, and designed towards the needs of each individual patient. Next, a printing method to deposit biological sensing proteins for biomedical assays is investigated. Traditional techniques require extended time and the use of large quantities of immensely expensive proteins to make biosensors. Herein, a decade-old belief that aerosol jet printing is incompatible with the deposition of proteins is overturned, and, in doing so, highly sensitive biosensors for carcinoembryonic antigen (CEA) that compare favorably the mainstay fabrication technique that is known to impart no damage to the printed biological inks is demonstrated. Finally, the co-printing of a bio-recognition element with the previously mentioned electrically conductive AgNW ink demonstrate the potential for the future investigation of a fully aerosol-jet printed electronic biosensor. To address the environmental waste accumulation concern that plagues the advancement of ubiquitous patient-guided sensing, inks that facilitate the creation of fully-printed, all-carbon recyclable electronics (ACRE) are investigated. The combination of nanocrystalline cellulose, graphene and semiconducting carbon nanotubes enable the first fully recyclable transistor device. The ACRE transistors maintain high stability for over 10 months, display among the best performance of any printed transistor (Ion/Ioff: 104 and Ion 65 µA µm-1) and can be entirely deconstructed for recapture and reuse of the constituent CNT and graphene inks with near 100% nanomaterial retention and the biodegradation of the cellulose-based components. ACRE-based lactate sensors are used as an illustration of utility to show the versatility of the platform. Finally, as a culminating demonstration, a fully-printed chip for the handheld measurement of blood clot time (prothrombin time) was developed. Printing the entirety of the device allows for the creation of a low-cost chip for the simple, fast, and robust measurement of human blood clot times. In addition, a custom-designed, handheld control system with a 3D-printed case was developed to create a fully integrated point-of-care measurement platform towards simplifying medicine dosing strategies. The work described herein marks a significant leap in the development of printed inks to enable custom biological sensing applications. Once fully realized, these applications will mark a watershed, ushering in an era of individualized medicine with ubiquitous sensing to actively track disease progression in real-time. We are at the dawn of a new era in medicine that focuses more on prevention and control as opposed to reaction. One future direction for this work is promoting directly printed and reusable on-skin theragnostics for bespoke patient care such as the delivery and monitoring of pain medication that allows for better oversite over use and misuse.

Energy storage systems are critically important for many areas in modern society including consumer electronics, transportation and renewable energy production. This dissertation summarizes our efforts on improving the performance metrics of energy storage and conversion devices through rational design and fabrication of hybrid nanomaterial systems.

This dissertation is divided into five sections. The first section (chapter 2) describes comparison of graphene and carbon nanotubes (CNTs) on improving the specific capacitance of MnO2. We show that CNTs provided better performance when used as ultrathin electrodes but they both show similar performance with rapid MnO2 specific capacitance decrease as electrodes become thicker. We further designed ternary composite electrodes consisting of CNTs, graphene and MnO2 to improve thick electrode performance (chapter 3). We demonstrate that these electrodes were flexible and mechanically strong, had high electrical conductivity and delivered much higher capacity than electrodes made without CNTs.

Chapter 4 describes assembly of flexible asymmetric supercapacitors using a graphene/MnO2/CNTs flexible film as the positive electrode and an activated carbon/CNTs flexible film as the negative electrode. The devices were assembled using roll-up approach and can operate safely with 2 V in aqueous electrolytes. The major advantage of these devices is that they can deliver much higher energy under high power conditions compared with those designed by previous studies, reaching a specific energy of 24 Wh/kg at a power density of 7.8 kW/kg.

Chapter 5 describes our approach to improve the energy and power densities of nickel hydroxides for supercapacitors. This was done by assembling CNTs with Co-Ni hydroxides/graphene nanohybrids as freestanding electrodes. The assembled electrodes have dramatically improved performance metrics under practically relevant mass loading densities (~6 mg/cm2), reaching a specific capacitance of 2360 F/g at 0.5 A/g and 2030 F/g even at 20 A/g (~86% retention).

Finally, we discuss our efforts on designing highly active electrocatalysts based on winged nanotubes for oxygen reduction reactions (ORR). The winged nanotubes were prepared through controlled oxidization and exfoliation of stacked-cup nanotubes. When doped with nitrogen, they exhibited strong activity toward catalyzing ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance owning to their unique carbon structure.

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.

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.

bold

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.

Chiral, molecular and nanoscale assemblies are promising candidates for the development of spintronic-based devices, characterized by information processing using both electronic charge and spin, and are poised to give rise to superior computational efficiency relative to modern electronic architectures that only operate using processing of electronic charge. Essential to realizing such spintronic assemblies is the ability to isolate and engineer enantiopure, chiral nanoscale materials that feature highly-tunable and unique electronic structures. Congruent with such requirements, this work focuses on engineering molecular and nanoscale organic matter that interface with single-walled carbon nanotubes (SWNTs), and are capable of: (i) generating concentrated, enantioenriched SWNT-based chiral inks from racemic mixtures that aim to be amenable with current ink-jet printing designs for electronic device fabrication, or (ii) inducing SWNT lattice handedness in achiral SWNT platforms that depend on conjugated polymer electronic structure and polymer pitch length. Specifically, this work explores: (i) experimental and computational investigation of engineered chiral, binaphthalene-based surfactant frameworks that are able to disperse and resolve enantiomers of SWNTs via enthalpic and entropic differences in surfactant-SWNT interactions in aqueous solutions, and (ii) chiral, semiconducting aryleneethynylene-based polymers that helically wrap metallic single-walled carbon nanotubes (m-SWNTs) and give rise to m-SWNT band gap opening and a metallic to semiconducting phase transition in such assemblies. Exploitation of such unique designs will enable opportunities to develop exceptional chiro-optic and spintronic materials, and help elucidate critical structure-function relationships that broadly inform material design for such applications.

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).

As the Internet of Things (IoT) continues to expand, there is increasing demand for custom low-cost sensors, displays, and communication devices that can grow and diversify the electronics ecosystem. The benefits to society of a vibrant, ubiquitous IoT include improved safety, health, and productivity as larger and more relevant datasets are able to be generated for fueling game-changing artificial intelligence and machine learning models. Printed carbon nanotube thin-film transistors (CNT-TFTs) have emerged as preeminent devices for enabling potentially transformative capabilities from, and widespread use of, IoT electronics. Still, despite intensive research over the past 15 years, there has yet to be the development of a streamlined, direct-write, in-place printing process, similar to today’s widely used inkjet or 3D printing technologies, where the substrate never leaves the printing stage and requires little to no post-processing. The development of such a process for producing CNT-TFTs could lead to the emergence of print-on-demand electronics, where direct-write printers are capable of printing distinct IoT sensing devices or even full IoT systems with little to no user intervention.

The work contained in this dissertation describes discoveries and innovations for streamlining and optimizing direct-write printed electronics using in-line or in-place methods, with primary focus on an in-place printing process for producing CNT-TFTs at relatively low temperature. The key enabling aspect of the in-place printing of CNT-TFTs was the development of aerosol jet-printable low-temperature conductive and dielectric inks that are functional immediately after printing. Additionally, the printed semiconducting CNT films required modified rinsing procedures for in-line processing, which proved to enhance performance. Notably, the resulting CNT-TFTs exhibited promising performance metrics with on/off-current ratios exceeding 103 and mobilities up to 11 cm2V-1s-1, while also operating under mechanical strain or after long-term bias stress, despite being printed with a maximum process temperature of only 80 °C. While optimizing these devices, various contact morphologies and configurations were investigated, where it was found that there was less variability in performance between sets of top-contacted devices, compared to bottom-contacts. Additionally, it was discovered that there are processing and performance trade-offs associated with various contact morphologies, with silver nanowires holding most value for in-place printing.

Although primary focus is given to aerosol jet-printed, CNT-based devices, this work also outlines another rapid, and potentially in-line, process for improving IoT-relevant electronics printed from a widely used direct-write method: fused filament fabrication. Here, using a high intensity flash lamp, the conductance of thermoplastic filaments are enhanced by up to two orders of magnitude. It was found that high-intensity light vaporizes the topmost layer of thermoplastic on metal-composite filaments, leaving behind a metallized surface layer in a technique referred to as flash ablation metallization (FAM). FAM was then used to enhance the performance of 3D printed circuit boards, demonstrating use in an immediately relevant application.

Overall, the development of in-place printed CNT-TFTs and the FAM process establish practical and scientific foundations for continued progress toward print-on-demand electronics. These foundations include: the development of low-temperature inks, rapid and in-line compatible process methods, and investigations of the impacts of various materials, device configurations, or process steps on electronic performance. Altogether, these developments have the potential to lower the time, costs, and overhead associated with printed electronics, moving the field closer to a point that is more accessible to industrialists, academics, and hobbyists alike.

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.

Machine learning algorithms are emerging in a wide range of application domains, ranging from autonomous driving, real-time speech translation, and network anomaly detection to pandemic growth and trend prediction. In particular, deep learning, facilitated by highly parallelized processing in hardware accelerators, has received tremendous interest due to its effectiveness for solving complex tasks across different application domains. However, as Moore's law approaches its end, contemporary electronic deep-learning inferencing accelerators show diminishing energy efficiency and have been unable to cope with the performance demands from emerging deep learning applications. To mitigate these issues, there is a need for research efforts on emerging artificial intelligence (AI) accelerators that explore novel transistor technologies with high transconductance at the nanometer technology nodes and low-latency alternatives to metallic interconnects. In this dissertation, we focus on the modeling and optimization of two such technologies: (i) high-speed transistors built using carbon nanotubes (CNTs), and (ii) integrated photonic networks that parallelize matrix-vector multiplications.

CNTs are considered to be leading candidates for realizing beyond-silicon transistors. Owing to the ultra-thin body of CNTs and near-ballistic carrier transport, carbon nanotube field-effect transistors (CNFETs) demonstrate a high on-current/off-current ratio and low subthreshold swing. Integrated circuits (ICs) fabricated from CNFETs are projected to achieve an order of magnitude improvement in the energy-delay product compared to silicon MOSFETs. Despite these advances, several challenges related to yield and performance must be addressed before CNFET-based high-volume production can appear on industry roadmaps. While some of these challenges (e.g., shorts due to metallic CNTs and incorrect logic functionality due to misaligned CNTs) have been addressed, the impact of fabrication process variations and manufacturing defects has largely remained unexplored.

Silicon photonic networks have been known to outperform the existing communication infrastructure (i.e., metallic interconnect) in multi-processor systems-on-chip. In recent years, their application as compute platforms in AI accelerators has attracted considerable attention. Leveraging the inherent parallelism of optical computing, integrated photonic neural networks (IPNNs) can perform the otherwise time-intensive matrix multiplication in O(1) time. Given their competitive integration density, ultra-high energy efficiency, and good CMOS compatibility, IPNNs demonstrate order-of-magnitude higher performance and efficiency than their electronic counterparts. However, the performance of photonic components is highly sensitive to fabrication process variations, manufacturing defects, and crosstalk noise.

In this dissertation, we present the first comprehensive characterization of CNFETs and IPNNs under imperfections. In the case of CNFETs, we consider the impact of fabrication process variations in different device parameters and manufacturing defects that are commonly observed during fabrication. To characterize IPNNs, we consider uncertainties in phase angles and splitting ratios in their building blocks (i.e., Mach--Zehnder interferometers), non-uniform optical loss in the waveguides, and quantization errors due to low-precision encoding of tuned phase angles. Using detailed simulations, we show that these devices can deviate significantly from their nominal performance, even in mature fabrication processes. For example, we show that more than 90% CNFETs can fail due to a 5% change in the CNT diameter. Similarly, the inferencing accuracy of IPNNs can drop below 10% due to uncertainties in the phase angles and splitting ratios.

To ensure the adoption of accelerators based on CNFETs and IPNNs, techniques to test and mitigate the catastrophic impact of imperfections are necessary. As the nature of imperfection in CNFETs vary significantly from those in Si-MOSFETs, existing commercial test pattern generation tools are inefficient when they are applied to ICs with imperfect CNFETs. This thesis presents VADF, a novel CNFET variation-aware test pattern generation tool that significantly improves the efficiency of small delay defect testing under imperfections. Unetched CNTs in the active layer can lead to parasitic FETs that can cause resistive shorts. In addition, we propose ParaMitE, which is a low-cost optimization technique, to reduce the probability of para-FET occurrence and mitigate their impact on performance. The thesis also describes three optimization techniques to improve the power-efficiency and reliability of IPNNs under imperfections. OptimSVD leverages non-uniqueness of the singular value decomposition to minimize the phase angles in an IPNN while guaranteeing zero accuracy loss. We propose CHAMP and LTPrune, which, to the best of our knowledge, are the only photonic hardware-aware magnitude pruning techniques targeted towards IPNNs.

In summary, this dissertation tackles important problems related to the reliability and high-volume yield of next-generation AI accelerators. We show how the criticality of different imperfections can change based on their magnitude and also the location and parameters of the affected components. The methods presented in this dissertation, while targeted towards CNFETs and IPNNs, can be easily extended towards other emerging technologies leveraged for AI hardware. The insights derived from this work can help designers to develop post-silicon AI accelerators that, in addition to demonstrating superior nominal performance, are resilient to inevitable imperfections.

Light-matter interactions are fundamental to many critical emerging technologies – such as photovoltaics, photonic sensing, and information transmission – that rely upon the efficient capture of light and its conversion to useful energetic states. However, to realize these technologies as a viable future we must first understand the fundamental processes which govern and dictate the energetic, spatial, and temporal identity of materials following photoexcitation. As is suggested by the term “light-matter” both the qualities of the light and the structural composition of the material will influence these characteristics resulting from their interaction. This dissertation investigates how photoexcitation conditions and material structure can be leveraged to modulate the energetic and charged states, and the dynamics thereof, which arise following photoexcitation of nanoscale and molecular systems. Employing ultrafast pump-probe transient absorption spectroscopy, this work characterizes the transient states which arise from photoexcitation of: (i) single-walled carbon nanotubes (SWNTs) wrapped by aryleneethynylene semiconducting polymers; (ii) covalently linked ethyne bridged porphyrin donor, rylene acceptor, molecular “ratchets” and (iii) rylene chromophores covalently linked to amino acid models. In nanoscale systems, this work highlights how the electronic structure of 1-dimensional SWNTs: (i) enable a complex interplay of excitation fluence dependent multi-body interactions, arising from the multitude of photogenerated energetic states, which may be harnessed to modulate the nature and lifetime of charge separated states and; (ii) give rise to a collection of heretofore ill-defined photoexcited-states with low energy optical transitions. At a molecular level, this work demonstrates how molecular structures can be engineered to: (i) utilize quantum coherence in a donor-acceptor “ratchet” which exhibit excitation frequency dependent uphill energy transfer, via vibronic mixing, to undergo electronically irreversible charge transfer and; (ii) selectively photooxidize amino acid analogues in biologically reminiscent photoreactions. These findings presented herein may be used to guide optoelectronic designs which efficiently guide and harness the charged and energetic species which arise from photoexcitation.

With the advent of the internet-of-things (IoT) and a more connected digital ecosystem, new electronic sensors and systems are needed. Printing has been identified as a means of fabricating low-cost electronics on non-rigid, large-area substrates. Printed electronics have been demonstrated to have the required electrical and mechanical properties to facilitate new and unique flexible electronic sensors for the IoT. One printable material that has demonstrated significant promise, specifically when compared to more traditional printed semiconductors, is solution-processed carbon nanotubes (CNTs). While some work has been done to facilitate the fabrication of CNT thin-film transistors (TFTs), little work has been done to assess the viability and potential of CNT-TFTs and other CNT thin films for real-world sensing applications.

The work contained in this dissertation describes the use of aerosol jet printing to fabricate CNT-TFTs, and the resulting study of their capability for various sensing applications. Aerosol jet printing allows for printing all the materials necessary for a fully-functional CNT-TFT, including the semiconducting thin film, conducting contacts and gate, and insulating gate dielectric. Using this system, flexible and fully printed CNT-TFTs were developed and characterized. Fully printed transistors were fabricated with field-effect mobilities as a high as 16 cm2/(Vs). The transistors were also resilient to substantial bending/strain, showing no measurable performance degradation after 1000 bending cycles at a radius of curvature of 1 mm.

The printed CNT-TFTs were evaluated for several sensing applications, including environmental pressure sensing and point-of-care biological sensing. The biological sensors, which were electronically transduced immunoassays, consisted of an antifouling polymer brush layer to enhance the CNT-TFT sensitivity and printed antibodies for detection of target analytes. Unparalleled sensitivity in unfiltered biological milieus was realized with these printed biosensors, detecting protein concentrations as low as 10 pg/ml in whole blood. In addition to demonstrating an electronically transduced TFT-based biosensor, work was done to develop a stable platform with high yield that will provide the means for a deeper understanding of the biosensing mechanisms of transistor-based sensors. As part of this biosensor platform development, novel solution-gated CNT-TFTs were demonstrated, with stable operation in ionic solutions for periods as long as 5 hours.

Another important electronic sensing technique is capacitive-based sensing. Using aerosol jet printed carbon nanotubes, a capacitive sensor has been developed and demonstrated for measuring insulating material thickness. The sensors rely on the fringing field between two adjacent electrodes interacting with the material out-of-plane, and that interaction being perturbed differently based on the thickness of the overlaid material. This sensor was also demonstrated in a one-dimensional array, which can be used to map tire tread thickness from the outside of the tire.

Overall, this dissertation explores the use of printed carbon nanotubes for diverse sensing applications. While this work provides real-world demonstrations that have potential impact for the IoT, there are also substantial scientific advancements made. Namely, insight into biosensing mechanisms, operation of solution-gated nanomaterial-based transistors, and demonstration of porosity and thickness effects on printed capacitive sensor electrodes.

Chemical vapor deposition (CVD) has been recognized as one of the most promising methods to produce carbon nanotubes (CNTs) industrially. Oxygen is important in CNT high-volume production, but few of the studies propose mechanistic details for how oxygen exerts these effects. Since reported optimization conditions to generate CNTs are based on empirical results, several gray areas still exist in the CNT growth mechanism. Uncovering the CNT growth mechanism, especially for the oxygen-related CNT synthesis, is necessary to promote CNT production with atomistic control.

Here, in order to separate gas and catalyst thermal effects and allow for uniform transformation of gas as it approaches substrate, a specifically designed CVD reactor was assembled for determining the CNT growth mechanism. Two typical and promising oxygen-related CVD processes (equimolar C2H2-CO2 reaction and water-assisted CNT growth) were studied to analyze the role of oxygen. The reported equimolar C2H2-CO2 reaction process could deposit CNTs on various substrates and at relatively low temperature. The water-assisted CVD process could obtain dense, high-purity, and millimeter-scale CNTs, which is promising for mass production of CNTs.

Firstly, native CO2 (12C CO2) and 13C-labeled CO2 were individually used as feedstock with C2H2 to grow CNTs. A statistical study using the Raman spectra of the yielded CNTs in both conditions indicated that the C in CO2 was not incorporated into CNTs. Based on this conclusion, an electron-pushing CNT growth mechanism was proposed. The role of oxygen in CO2 was indicated as grasping hydrogen atom in the raw CNT lattice.

Secondly, CNTs were synthesized with various H2O concentrations ranging from 10 parts per million (ppm) to 220 ppm. Quantitative study of the CNT outer diameters by TEM imaging indicated that the outer diameter tended to increase with H2O concentration (the average outer diameter ranges from 4.8 nm at 10 ppm H2O to 6.4 nm at 220 ppm H2O). Raman spectra revealed that the dominant CNTs went from SWCNTs at 10 ppm H2O to MWCNTs at high H2O concentration, consistent with the diameter increase trend. The formed catalyst might explain the CNT quality change. The AFM images of the catalyst demonstrated that the height, size and spacing of the iron nanoparticles on the substrate increased with water concentration. The alignment property was tested by SEM imaging. The yielded CNTs at 120 ppm H2O got the best alignment. The gas composition analysis results indicated that the H2O might promote the decomposition of the main carbon precursor. The oxygen in H2O might influence the catalyst activation and carbon precursor decomposition.

Through the CNT characterization and gas composition analysis, the role of oxygen could be categorized into three areas: 1) absorb hydrogen in immature CNT lattice; 2) influence catalyst formation; 3) promote carbon precursor decomposition.