Doctor of Philosophy, The Ohio State University, 2024, Electrical and Computer Engineering

Nano-vacuum channel field emission transistors are emerging as a promising choice for high-frequency transistors in space applications. The vacuum transistor possesses several advantages that have garnered significant interest. The vacuum channel, with its ballistic transportation, proves to be a more efficient carrier conveyer than the solid-state channel. Additionally, the breakdown field in a vacuum is known to be higher than that in a solid-state medium. Furthermore, vacuum transistors exhibit greater resilience against radiation. Advancements in semiconductor fabrication have enabled the achievement of smaller and improved field emission currents compared to previous metal-based vacuum transistors. Carbon nanotubes (CNTs), characterized by their high aspect ratio, electrical conductivity, and thermal conductivity, make them ideal candidates for field emitter tips, ensuring reliable field emission performance over extended operational durations. The fabrication of uniform silicon field emitter arrays with a 6-8nm radius has been accomplished through well-established silicon fabrication techniques, resulting in current densities exceeding 100 A/cm2 at gate voltages below 75V. Semiconductor field emitters introduce innovative approaches, such as utilizing pn junctions and surface states. Since semiconductor-based vacuum transistors involve more complex factors influencing field emission current, both theoretical and experimental studies become crucial. The initial segment of this proposal explores the impact of surface states on field emitters on field emission current through 1D mathematical modeling. A 1D AlGaN field emitter exhibits four distinct phases under applied electric fields, with the fermi level pinning on the surface state level. The study reveals that the field emission current from n-type AlGaN field emitters decreases at low applied electric fields due to fermi level pinning, suggesting higher doping concentrations or passivation as potential solu (open full item for complete abstract)

Committee: Siddharth Rajan (Advisor); Mark Ruegsegger (Committee Member); Wu Lu (Committee Member); Arehart Aaron (Committee Member) Subjects: Electrical Engineering

PhD, University of Cincinnati, 2024, Engineering and Applied Science: Electrical Engineering

This dissertation work deals with the development of refined models of field emission (FE) from carbon nanotubes (CNTs). The first project described focuses on an efficient algorithm computing the temperature distribution along a CNT during FE, considering the substrate as a perfect heat sink at temperature T₀. It incorporates Joule heating effects, radiative losses, and recently reported analytical expressions for emission current density and heat exchange at the CNT tip, including Nottingham-Heating and Henderson-Cooling effects. Temperature dependencies of CNT's electrical resistivity and thermal conductivity are also included. Simulation times for calculating CNT FE characteristics and temperature distribution were found to be about two orders of magnitude faster compared to numerical methods accounting for both current and energy exchange at the CNT tip. The algorithm was adapted to analyze the impact of thermal contact resistance on the FE properties of a CNT. Using a boundary condition from literature, thermal contact resistance effects at a CNT/chuck interface were accounted for, with the chuck assumed as a perfect heat sink at temperature T₀. Results demonstrate that current constriction at the CNT/chuck contact point induces self-heating effects, which escalate with higher thermal contact resistance values. Consequently, this increases the temperature profile along the CNT, including its tip temperature, and augments the FE current beyond values presumed with the CNT/chuck interface at T₀. The fractional change of emission current with applied external electric field was calculated for increasing thermal resistivity values of the CNT/chuck interface. Next, the effect of electrostatic forces on the FE properties of flexible emitters as a function of the strength of the applied (macroscopic) external electrostatic field are investigated. Results show that deflection of a flexible metallic CNT due to an externally applied electrostatic field inc (open full item for complete abstract)

Committee: Marc Cahay Ph.D. (Committee Chair); Tao Li Ph.D. (Committee Member); Mark Schulz Ph.D. (Committee Member); Tyson Back Ph.D. (Committee Member); Yeongin Kim Ph.D. (Committee Member); Je-Hyeong Bahk Ph.D. (Committee Member) Subjects: Electrical Engineering

PhD, University of Cincinnati, 2020, Engineering and Applied Science: Electrical Engineering

Carbon nanotube (CNT) based field emitter cathodes show considerable potential as highly efficient electron emitters. Their incredible geometrical aspect ratios facilitate field emission (FE) current at very low applied external electric fields. However, knowledge of how to effectively implement these ultra-high efficiency cathodes in real devices is still lacking. These emitters tend to exhibit both permanent and temporary degradation patterns thought to be linked to a multitude of various physical phenomena. For a complete functioning vacuum electronic system, the consequential secondary effects, such as the multipactor effect, from the electron beam impinging the anode must also be considered, especially in the high-power regime of operation. A thorough characterization of both the FE mechanisms in these volatile CNT based emitters and the consequential secondary effects of the anode could eventually lead to novel fabrication of highly efficient vacuum electronic device applications such as directed energy, ion propulsion, high power radio frequency transmission, miniature X-ray sources, electron beam lithography, terahertz sources, and many more. Our work addresses the two issues mentioned above in collaboration with workers in the Materials and Manufacturing Directorate at Wright-Patterson Air Force Base. The first part of this work focuses on the modeling of the FE properties of cold cathode carbon nanotube fiber (CNF) field emitters, improving upon an existing multiscale model developed by Weiming Zhu, a previous PhD student of Dr. Cahay. This improved model no longer uses simple analytical expressions to determine emission current density and average energy exchange per an emitted electron, as it returns a more accurate exact numerical determination for both quantities. However, the multiscale model requires determination of these quantities over at least 10^4 unique CNT emitters, taking a substantial amount of computational time. To remedy this problem, th (open full item for complete abstract)

Committee: Marc Cahay Ph.D. (Committee Chair); Tyson Back Ph.D. (Committee Member); Je-Hyeong Bahk Ph.D. (Committee Member); Punit Boolchand Ph.D. (Committee Member); Hans-Peter Wagner Ph.D. (Committee Member) Subjects: Electrical Engineering

PhD, University of Cincinnati, 2018, Engineering and Applied Science: Electrical Engineering

A multiscale model of field emission (FE) from a carbon nanotube fiber (CNF) is developed which takes into account Joule heating within the fiber and radiative cooling and the energy exchange mechanism at the tip of the individual carbon nanotubes (CNTs ) in the array located at the fiber apex. The model predicts the fraction of CNTs being destroyed as a function of the applied external electric field and reproduces many experimental features observed in some recently investigated CNFs such as, order of magnitude of the emission current (mA range), low turn on electric field (fraction of V/µm), deviation from pure Fowler-Nordheim behavior at large applied electric field, hysteresis of the FE characteristics, and a spatial variation of the temperature along the CNF axis with a maximum close to its tip of a few hundred degree Celsius. The multiscale model is in qualitative agreement with the FE characteristics of CNFs measured by collaborators at Wright-Patterson Air Force Base (WPAFB). The model is used to predict the FE characteristics of both linear and hexagonal arrays of seven CNFs taking into account the effects of electrostatic shielding between adjacent CNFs. The multiscale model described in this dissertation can provide a tool to develop cold cathodes that can provide a current density of a few times 10^5 A/m^2 and a total current of at least 10 mA for at least several hundred hours of continuous waveform (CW) operation, when operated at temperature less than 1000 degree Celsius.

Committee: Marc Cahay Ph.D. (Committee Chair); Je-Hyeong Bahk Ph.D. (Committee Member); Punit Boolchand Ph.D. (Committee Member); Rashmi Jha Ph.D. (Committee Member); Hans Peter Wagner Ph.D. (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 1984, Graduate School

Committee: Not Provided (Other) Subjects: Engineering

MS, University of Cincinnati, 2014, Engineering and Applied Science: Electrical Engineering

Field emission (FE) properties of carbon nanotube (CNT) fibers from Rice University and the University of Cambridge have been studied for use within a high current electron source for a directed energy weapon. Upon reviewing the performance of these two prevalent CNT fibers, cathodes were designed with CNT fibers from the University of Cincinnati Nanoworld Laboratory. Cathodes composed of a single CNT fiber, an array of three CNT fibers, and a nonwoven CNT sheet were investigated for FE properties; the goal was to design a cathode with emission current in excess of 10 mA. Once the design phase was complete, the cathode samples were fabricated, characterized, and then analyzed to determine FE properties. Electrical conductivity of the CNT fibers was characterized with a 4-probe technique. FE characteristics were measured in an ultra-high vacuum chamber at Wright-Patterson Air Force Base. The arrayed CNT fiber and the enhanced nonwoven CNT sheet emitter design demonstrated the most promising FE properties. Future work will include further analysis and cathode design using this nonwoven CNT sheet material to increase peak current performance during electron emission.

Committee: Marc Cahay Ph.D. (Committee Chair); Punit Boolchand Ph.D. (Committee Member); Altan Ferendeci Ph.D. (Committee Member) Subjects: Electrical Engineering

Master of Science (MS), Wright State University, 2011, Physics

In this thesis, single and multi-layered graphene films were epitaxially grown on either Si-face or C-face of SiC single crystal substrates. The film growth conditions, such as decomposition temperatures and pressures, and their surface morphologies were optimized. These films were then characterized by using surface analysis tools including SEM, TEM, AFM evanescent wave microscopy and electron educed spectroscopy. In addition to studying graphene decomposed from SiC crystals, carbon nanotube material was fabricated using a floating catalyst technique. These carbon nanotube material was then studied for potential cathode applications in this thesis. Field emission properties of these cathodes was measured and compared between carbon nanotubes grown by the floating catalyst technique and carbon nanotube material fabricated from a super acid solution spinning process. The result found that carbon nanotube material produced from the floating catalyst method supported the highest emission currents. As a result of this research, carbon nanotube field emitters fabricated from this method are now being studied in a wide range of vacuum electronic applications.

Committee: Gregory Kozlowski PhD (Advisor); Jason Deibel PhD (Committee Member); Jerry Clark PhD (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2008, Engineering : Electrical Engineering

In this thesis we have demonstrated novel nanoscale cold cathode devices based on rare-earth monosulfides, gold nano pinetree like and carbon nano pearl arrays. We first developed a MEMS based cold cathode device based on pulsed laser deposition (PLD) of lanthanum monosulfide (LaS). The device exhibited a largest field emission (FE) current of 5x10-7A which is suitable for flat panel displays. It can also be used to develop a three terminal device providing gate control of the field emission current. Then, in an effort to fabricate nanowire arrays of LaS for efficient field emission, we discovered the formation of the trimodal arrays where three different nanoscale arrays of LaS- nanodots, nanodomes and nanowires have been simultaneously fabricated by PLD of LaS on flexible self-assembled nanoporous alumina templates. The nanodots with a density of 1010/cm2 and the nanodomes with a density of 109/cm2 are formed on the surface of the template while the nanowires are formed inside the pores with a density of 1010/cm2. The LaS nanodots and the nanodomes formed on the surface acted as efficient field emitters by reducing the threshold voltage by ~3.5 times for the same amount of field emitted current compared to the LaS thin films deposited on silicon substrates. During the process of investigating the formation of trimodal arrays of LaS, we have observed multi level and multi modal self assembly on surface of the nanoporous alumina. Specifically three distinct unique nanoscale features have been observed. The first feature is the self assembly of carbon nanopearls in the form of nanotentacles and nanonecklaces on an array of nickel nanodots and nanodomes. The second feature is the nanopine tree like structures formed by e-beam evaporation of gold onto the alumina template. Both these features have emitted stable field emission current. The third features is the nanoblade like structures formed by e-beam evaporation of nickel on the alumina template. A stable field emis (open full item for complete abstract)

Committee: Marc Cahay Dr. (Committee Chair); Kenneth Roenker Dr. (Committee Member); Punit Boolchand Dr. (Committee Member); David Klotzkin Dr. (Committee Member); Vesselin Shanov Dr. (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, University of Akron, 2010, Polymer Science

The main objective of this dissertation is synthesis of carbon nanotube (CNT) based superhydrophobic materials. The materials were designed such that electrical and mechanical properties of CNTs could be combined with superhydrophobicity to create materials with unique properties, such as self-cleaning adhesives, miniature floatation devices, ice-repellant coatings, and coatings for heat transfer furnaces. The coatings were divided into two broad categories based on CNT structure: Vertically aligned CNT arrays (VA coatings) and mesh-like (non-aligned) carbon nanotube arrays (NA coatings). VA coatings were used to create self-cleaning adhesives and flexible field emission devices. Coatings with self cleaning property along with high adhesiveness were inspired from structure found on gecko foot. Gecko foot is covered with thousands of microscopic hairs called setae; these setae are further divided into hundreds of nanometer sized hairs called spatulas. When gecko presses its foot against any surface, these hairs bend and conform to the topology of the surface resulting into very large area of contact. Such large area of intimate contact allows geckos to adhere to surfaces using van der Waals (vdW) interactions alone. VA-CNTs adhere to a variety of surfaces using a similar mechanism. CNTs of suitable diameter could withstand four times higher adhesion force than gecko foot. We found that upon soiling these CNT based adhesives (gecko tape) could be cleaned using a water droplet (lotus effect) or by applying vibrations. These materials could be used for applications requiring reversible adhesion. VA coatings were also used for developing field emission devices. A single CNT can emit electrons at very low threshold voltages. Achieving efficient electron emission on large scale has a lot of challenges such as screening effect, pull-off and lower current efficiency. We have explored the use of polymer-CNT composite structures to overcome these challenges in this work. NA-C (open full item for complete abstract)

Committee: Ali Dhinojwala Dr. (Advisor); Gustavo Carri (Committee Chair); Li Jia (Committee Member); Purushottam Gujrati (Committee Member); Jutta Luettmer-Strathmann (Committee Member) Subjects: Materials Science; Physics; Polymers

Doctor of Philosophy, University of Akron, 2008, Engineering-Applied Mathematics

Field emission from conducting nanofibers has a significant importance due to its possible application in electronics like flat panel displays, x-ray machines, sensors, etc.The standard theoretical model describing field emission is the Fowler-Nordheim model, which is valid for bulk material, constant applied electric field and zero temperature. A more general theoretical model is required in the realistic cases of arbitrary electromagnetic fields and arbitrary but finite temperature. This work presents an asymptotic procedure for calculating field emission from nanofibers of finite length for static and dynamic fields at arbitrary finite temperature. It investigates the behavior of a nanofiber in the presence of electrostatic and EM fields. The resultant field potentials outside the system are obtained by employing the slender-body approximation. The total external potential is used in conjunction with the the Wentzel-Krammers-Brillouin approximation to estimate the tunneling probability of the electrons in the fiber due the total external field. Unlike the standard Fowler-Nordheim method, the current density of the field emission is obtained by using quantum wire density of states. In addition, this work investigates radiative and scattering properties of conducting nanofibers for the purpose of nanoantenna applications . The results for the distributions of the induced currents are compared to the results from the solution of Hallen's integral equation and the corresponding radiation patterns are compared. The results are extended for the case of a broadside uniform array of N aligned fibers.

Committee: Subramaniya Hariharan (Advisor) Subjects: Electrical Engineering; Electromagnetism; Physics