Engineering Commons^™

Enabling Nanoimprint Lithography Techniques Across Multiple Manufacturing Processes, Vincent Einck

Doctoral Dissertations

Advanced nanooptics in the areas of flat lenses, diffractive elements, and tunable emissivity require a route to high throughput manufacturing. Nanooptics are often demanding of high refractive index materials, nanometer precision and ease of fabrication. Nanoimprint lithography (NIL) is a low-cost, high throughput manufacturing technique beginning to be realized in commercial industry.^1,2 The NIL process is an ideal manufacturing candidate due to its ability to have a fast process time, efficient use of materials, repeatability and high precision while also having wide diversity of potential structures and material choices. Appling NIL techniques to other facets of manufacturing enable the …

Development Of Metal Halide Perovskites For Radiation Detection, Ryan Tan

Doctoral Dissertations

Metal halide perovskite (MHP) semiconductors have attracted significant interest in recent years within photovoltaic and radiation detection communities due to their inexpensive solution growths, high effective atomic number for gamma and X-ray sensing, suitable bandgap, large resistivity, and moderate mobility-lifetime products. The MHP stoichiometry can also be tuned as needed to achieve desired physical and electronic properties. Moreover, the hybrid or organometallic halide perovskite (OMHP) variants contain a large atomic fraction of hydrogen for fast neutron sensing. These qualities make MHPs an attractive low-cost option for meeting detector needs within nuclear security and imaging applications. This work presents the development …

Brain Inspired Organic Electronic Devices And Systems For Adaptive Signal Processing, Memory, And Learning., Subhadeep Koner

Doctoral Dissertations

A new class of electronic device has emerged which bear the potential for low powered brain like adaptive signal processing, memory, and learning. It is a non-linear resistor with memory coined as memristor. A memristor is a two-terminal electrical device which simultaneously changes its resistance (processing information) and store the resistance state pertaining to the applied power (memory). Therefore, it can collocate memory and processing much like our brain synapse which can save time and energy for information processing. Leveraging stored memory, it can thereby help future engineered systems to learn autonomously from past experiences. There has been a growing …

Cmos Compatible Carbonization Of Polymer For Elctrochemical Sensors, Mohammad Aminul Haque

Doctoral Dissertations

Carbon-based electrodes that are integrable with CMOS readout electrodes possess great potential in a wide range of cutting-edge applications. The primary scientific contribution is the development of a processing sequence which can be implemented on CMOS chips to fabricate pyrolyzed carbon microelectrodes from 3D printed polymer microstructures to develop lab-on-CMOS monolithic electrochemical sensor systems. Specifically, optimized processing conditions to convert 3D printed polymer micro- and nano-structures to carbonized electrodes have been explored in order to obtain sensing electrodes for lab-on- CMOS electrochemical systems. Processing conditions have been identified, including a sequel of oxidative and inert atmosphere anneals to form pyrolyzed …

Tailoring Interfaces And Composition For Stable And Efficient Perovskite Solar Cells, Hamza Javaid

Doctoral Dissertations

Metal halide perovskite solar cells (PSCs) have revolutionized the field of thin film photovoltaics. Within a decade, the power conversion efficiencies (PCEs) have increased at a phenomenal rate, rising from 3.8% to more than 25% in single-junction devices, moving them ahead of the current silicon-based technology. The high efficiencies of perovskite solar cells (PSCs) and their other unique properties arise from a combination of organic and inorganic components and electronic-ionic conduction, making them excellent candidates for a plethora of applications. However, PSCs face a significant—and ironic—roadblock to commercialization: these light-harvesting materials degrade under sunlight—the very condition they would need …

Towards Higher Power Factor In Semiconductor Thermoelectrics: Bandstructure Engineering And Potential Barriers, Adithya Kommini

Doctoral Dissertations

To keep up with the current energy demand and to sustain the growth requires efficient use of existing resources. One of the ways to improve efficiency is by converting waste heat to electricity using thermoelectrics. Thermoelectric devices work on the principle of Seebeck effect, where an applied temperature difference across the material results in a potential difference in the material. The possibility of drastic improvements in the efficiency of thermoelectric (TE) devices using semiconductor nanostructured materials renewed interest in thermoelectrics over the last three decades. Introducing confinement, interfaces, and quantum effects using nanostructures for additional control of charge and phonon …

High Resolution Electron Energy Loss Spectroscopy Of Plasmonic Nanostructures, Grace Pakeltis

Doctoral Dissertations

This dissertation discusses developing fabrication techniques to study the plasmonic phenomena of nanostructures utilizing high spatial and energy resolution of monochromated aberration-corrected scanning transmission electron. While standard lithography has been widely used to create planar nanostructures, investigation into 3-dimensional nanostructures is lacking. A robust synthesis approach utilizing focused electron beam induced deposition, atomic layer deposition, and thin film sputter deposition to fabricate complex 3D plasmonic architectures is described and characterization of single nanoresonators is presented. Additionally, this dissertation discusses the use of high-resolution electron energy loss spectroscopy to investigate the hybridization of gold nanorod oligomers. Experiment and simulation resolve magnetic …

Thermoelectric Transport In Disordered Organic And Inorganic Semiconductors, Meenakshi Upadhyaya

Doctoral Dissertations

The need for alternative energy sources has led to extensive research on optimizing the conversion efficiency of thermoelectric (TE) materials. TE efficiency is governed by figure-of-merit (ZT) and it has been an enormously challenging task to increase ZT > 1 despite decades of research due to the interdependence of material properties. Most doped inorganic semiconductors have a high electrical conductivity and moderate Seebeck coefficient, but ZT is still limited by their high lattice thermal conductivity. One approach to address this problem is to decrease thermal conductivity by means of alloying and nanostructuring, another is to consider materials with an inherently low …

Electro-Thermal Transport In Two-Dimensional Materials And Their Heterostructures, Arnab K. Majee

Doctoral Dissertations

”Smaller is better” is the mantra that has driven semiconductor industry for the past 50 years. The on-going quest for faster electronic switching, higher transistor density, and better device performance, has been driven by a self-fulfilling prophecy popularly known as Moore’s law, according to which the number of transistors per unit area of a chip doubles itself approximately every two years. A modern smartphone has about 8 billion transistors, which is as large as current earth’s population. Although each transistor dissipates negligible power, but the collective power dissipation from all the transistors in an electronic gadget and inefficient heat removing …

Spin Effects Of Excited States In Organic Semiconductors And Hybrid Perovskites For Optoelectronics And Spintronics, Miaosheng Wang

Doctoral Dissertations

Organic semiconductors and organic-inorganic hybrid perovskites have demonstrated versatile functionalities for optoelectronic and spintronic applications. Research in this dissertation focuses on the spin effects of excited states in emerging organic semiconductor and hybrid perovskite systems to understand the fundamental working principle. The investigation on the spin effects in excited states can provide insightful guidance for the development of the next-generation organic and hybrid perovskite optoelectronics and spintronics.

In organic semiconductors, the excited states, namely, excitons can be characterized by the total spin angular momentum S=S_e+S_h of the constituent electron and hole, which determines the …

Chemico-Physical Interactions In Metal Halide Perovskites, Yongtao Liu

Doctoral Dissertations

Metal halide perovskite (MHP) has attracted tremendous attention due to its success in optoelectronics, largely due to outstanding photovoltaic performance. A wide variety of characterization approaches have been used to explore the fundamentals behind the outstanding optoelectronic properties of MHP, which has yet to be unambiguously established despite considerable efforts to do so. Given the high ionic mobility in MHP, when physical phenomena are coupled with chemical changes, all behaviors will become very complex due to the strong ion migration. Therefore, chemico-physical interactions in MHP can no longer be ignored, which will be the focus of the researches in this …

Market-Conscious Strategies To Improve The Performance And Stability Of Planar, P-I-N Hybrid Organic-Inorganic Metal Halide Perovskite Solar Cells, Brandon Dunham

Doctoral Dissertations

Planar, p-i-n (inverted) hybrid organic-inorganic perovskite solar cells that use low-temperature, solution-processable charge-transport layers have garnered much attention due to their direct compatibility with flexible substrates and cost-effective roll-to-roll manufacturing. Nevertheless, this architecture has failed to repeatedly achieve the superior power conversion efficiencies frequently attained by its n-i-p counterpart. Additionally, the perovskite active layer has poor stability in the presence of prolonged light exposure, high temperatures, and moisture. In this study, we propose commercially viable strategies to improve the performance and stability of inverted methylammonium lead iodide perovskite solar cells. First, we show that a simple two-step method comprising evaporation-induced …

Exploration Of Thin Films For Neuromorphic, Electrofluidic, And Magneto-Plasmonic Applications, Walker L. Boldman University Of Tennessee Knoxville

Doctoral Dissertations

Due to the limit in computing power arising from the Von Neumann bottleneck, computational devices are being developed that mimic neuro-biological processing in the brain by correlating the device characteristics with the synaptic weight of neurons. We demonstrate a platform that combines ionic liquid gating of amorphous indium gallium zinc oxide (aIGZO) thin film transistors and electrowetting for programmable placement/connectivity of the of the ionic liquid. In this platform, both short term potentiation (STP) and long-term potentiation (LTP) are realized via electrostatic and electrochemical doping of the aIGZO, respectively, and pulsed bias measurements are demonstrated for low power considerations. Using …

Resistive Switching Characteristics Of Nanostructured And Solution-Processed Complex Oxide Assemblies, Zimu Zhou

Doctoral Dissertations

Miniaturization of conventional nonvolatile (NVM) memory devices is rapidly approaching the physical limitations of the constituent materials. An emerging random access memory (RAM), nanoscale resistive RAM (RRAM), has the potential to replace conventional nonvolatile memory and could foster novel type of computing due to its fast switching speed, high scalability, and low power consumption. RRAM, or memristors, represent a class of two terminal devices comprising an insulating layer, such as a metal oxide, sandwiched between two terminal electrodes that exhibits two or more distinct resistance states that depend on the history of the applied bias. While the sudden resistance reduction …

Quantitative Probing Of Vacancies And Ions Dynamics In Electroactive Oxide Materials, Jiaxin Zhu

Doctoral Dissertations

Oxygen vacancy and ion dynamics in functional oxides are critical factors influencing electrical conductivity and electrochemical activity of oxides assemblies. The recent advancements in deposition and fabrication of oxide heterostructured films with atomic-level precision has led to discovery of intriguing physical properties and new artificial materials. While still under debate, researchers most often attribute these observed behaviors to unique oxygen vacancy distributions in the substrate near heterointerfaces. In electroactive oxides devices such as solid oxide cells (SOCs), oxygen vacancy and ion transport at the triple-phase boundary determines the performance of the device. This complex process motivates numerous remaining questions regarding …

Probing Quantized Excitations And Many-Body Correlations In Transition Metal Dichalcogenides With Optical Spectroscopy, Shao-Yu Chen

Doctoral Dissertations

Layered transition metal dichalcogenides (TMDCs) have attracted great interests in recent years due to their physical properties manifested in different polytypes: Hexagonal(H)-TMDC,which is semiconducting, exhibits strong Coulomb interaction and intriguing valleytronic properties; distorted octahedral(T’)-TMDC,which is semi-metallic, is predicted to exhibit rich nontrivial topological physics. In this dissertation,we employ the polarization-resolved micron-Raman/PL spectroscopy to investigate the optical properties of the atomic layer of several polytypes of TMDC. In the first part for polarization-resolved Raman spectroscopy, we study the lattice vibration of both H and T’-TMDC, providing a thorough understanding of the polymorphism of TMDCs. We demonstrate that Raman spectroscopy is a …

Chemical Stability And Performance Influence Of Choice Substituents And Core Conjugation Of Organic Semiconductors, Jack Ly

Doctoral Dissertations

Realizing organic based active materials for electronic devices, such as thin film transistors and photovoltaics, has been long sought after. Advancement in the field driven by chemists, engineers, and physicists alike have bolstered organic based semiconductor performance levels to rival those of traditional inorganic amorphous silicon-based devices. Within the field of organic semiconductors (OSC), two categories of active materials may be generalized: (1) polymer and (2) small molecule semiconductors. Each class of OSC inherently have their own advantages and disadvantages. Polymer semiconductors (PSC) allow a wide range in tunability via choice monomers and side chain engineering to illicit desirable energy …

Direct Printing Of Conductive Inks For Organic Electronics And Wearable Microfluidics, Aditi Naik

Doctoral Dissertations

This dissertation examines the direct printing of conductive inks on polymeric substrates for applications in organic electronics, microfluidic valving systems, and wearable sweat sensors. The inexpensive production of solution-based electrodes with high electrical conductivity is necessary to enable the next-generation of printed, flexible, and organic electronics. Specifically, the optimization and printing of liquid-phase graphene ink and nanoparticle-based silver ink by soft nanoimprint lithography and inkjet-printing is discussed to achieve printed functional devices. Using scalable low-cost patterning systems, these flexible applications are compatible with roll-to-roll processing, enabling large-scale manufacturing. This research expands the knowledge of high-resolution printing optimization for the direct …

Fabrication Of High Refractive Index, Periodic, Composite Nanostructures For Photonic And Sensing Applications, Irene Howell

Doctoral Dissertations

This dissertation examines methods of fabricating high refractive index, periodic structures and their applications. Structures with a refractive index periodicity in one-dimensionally are fabricated by stacking layers of (high-refractive index) nanoparticle-filled and unfilled layers. More complex two- and three-dimensional structures are fabricated by direct printing of nanoparticles via solvent-assisted soft nanoimprint lithography. Polymer-nanoparticle composites are an active area of research and development especially for photonic applications. We show use of two composite formulations, first for fabrication of one-dimensional photonic crystals, and second for scalable UV-nanoimprinting. One dimensional photonic crystals, which possess a periodicity in refractive index, result in a constructive …

Probing Local Vacancy-Driven Resistive Switching In Metal Oxide Nanostructures, Jiaying Wang

Doctoral Dissertations

Novel nonvolatile memory technologies garner intense research interest as conventional ash devices approach their physical limit. Memristors, often comprising an insulating thin film between two metal electrodes to constitute a class of two-terminal devices, enable a variety of important large data storage and data-driven computing applications. In addition to nonvolatile behavior, other features such as high scalability, low power consumption, and sub-nanosecond response times make memristors among the most attractive candidate systems. Their strength in electronic storage relies on the unique properties of the tunable variations in resistance induced from the accumulation of charged defects based on the applied bias …

Parallel Algorithms For Time Dependent Density Functional Theory In Real-Space And Real-Time, James Kestyn

Doctoral Dissertations

Density functional theory (DFT) and time dependent density functional theory (TDDFT) have had great success solving for ground state and excited states properties of molecules, solids and nanostructures. However, these problems are particularly hard to scale. Both the size of the discrete system and the number of needed eigenstates increase with the number of electrons. A complete parallel framework for DFT and TDDFT calculations applied to molecules and nanostructures is presented in this dissertation. This includes the development of custom numerical algorithms for eigenvalue problems and linear systems. New functionality in the FEAST eigenvalue solver presents an additional level of …

Increasing Organic Semiconductor Performance Through Chemical And Processing Modifications, Edmund Burnett

Doctoral Dissertations

This thesis focuses on tuning molecular packing of organic semiconductors through processing or chemical modifications to increase performance and establish structure-property relationships. Chapter 2 utilizes differing processing techniques to alter the molecular packing of bistetracene in the thin film and thorough polymorph characterization to relate the modification of molecular packing to the increase in charge mobility and mechanism. Chapter 3 introduces the oligomer as a model system to resolve issues that would be difficult or impossible using polymeric systems, due to their monodispersity and increased crystallinity allows for more detailed structural characterization. In this chapter we determine a crystal packing …

Low Energy Recoil Simulations In Mgo, Linbo3, And Litao3 Using Ab Initio Molecular Dynamics, Benjamin Aaron Petersen

Doctoral Dissertations

Ab initio molecular dynamics (AIMD) was utilized to test a series of materials, MgO, LiNbO₃ , and LiTaO₃ , to determine defect structures produced due to low energy recoil events . The kinetic energy required to displace an atom from its lattice site, the threshold displacement energy, was calculated for an array of directions in each material, based on symmetry and complexity of the structure. MgO having a simple rock salt structure provided a model material for demonstrating computational techniques used later on LiTaO₃ and LiNbO₃ . The minimum values for displacing an atom were at …

Solution-Based Assembly Of Conjugated Polymers Into Nanofibers For Organic Electronics, Daniel E. Acevedo Cartagena

Doctoral Dissertations

Solution-based crystallization of conjugated polymers offers a scalable and attractive route to develop hierarchical structures for electronic devices. The introduction of well-defined nucleation sites into metastable solutions provides a way to regulate the crystallization behavior, and therefore the morphology of the material. A crystallization method for generating metastable solutions of poly(3-hexylthiophene) (P3HT) was established. These metastable solutions allow P3HT to selectively crystallize into nanofibers (NFs) on graphene-coated surfaces. It was found that the crystallization kinetics is faster with increasing P3HT molecular weight and concentration. Through in situ atomic force microscopy, it was confirmed that NFs grow vertically in a face-on …

Advanced Purification And Direct-Write 3d Nanoprinting Via Focused Electron Beam Induced Deposition, Brett Bloxton Lewis

Doctoral Dissertations

This dissertation addresses three difficulties with focused electron beam induced deposition preventing broader application; purity, spatial control, and mechanical characterization.

Focused electron beam induced deposition (FEBID) has many advantages as a nanoscale fabrication tool. It is compatible for implementation into current lithographic techniques and has the potential to direct-write in a single step nanostructures of a high degree of complexity. FEBID is a very versatile tool capable of fabricating structures of many different compositions ranging from insulating oxides to conducting metals.

Due to the complexity of the technique and the difficulty in directly measuring many important variables, FEBID has remained …

Characterization Of Electronic And Ionic Transport In Soft And Hard Functional Materials, Lawrence A. Renna

Doctoral Dissertations

Control over concurrent transport of multiple carrier types is desired in both soft and hard materials. For both types of materials, I demonstrate ways to characterize and execute governance over both electronic and ionic transport, and apply these concepts in the fabrication of devices with applications in conducting composites, photovoltaics, electrochemical energy storage, and memristors. In soft materials, such as polymers, the topology of the binary polymer mesoscale morphology has major implications on the charge/ion transport. Traditional approaches to co-continuous structures involve either using blends of polymers or diblock copolymers. In polymer blends, the structures are kinetically trapped and …

Dynamic Range Limitations Of Low-Noise Microwave Transistors At Cryogenic Temperatures, Ahmet Hakan Coskun

Doctoral Dissertations

Dynamic range is an important metric that specifies the limits of input signal amplitude for the ideal operation of a given receiver. The low end of dynamic range is defined by the noise floor whereas the upper limit is determined by large-signal distortion. While dynamic range can be predicted in the temperature range where compact transistor models are valid, the lack of large-signal models at temperatures below -55 C prevents the prediction and optimization of dynamic range for applications that require cryogenic cooling. For decades, the main goal concerning the performance of these applications was lowering the noise floor of …

Strategies For Controlling Bulk Heterojunction Morphology, Zach Daniel Seibers

Doctoral Dissertations

Organic photovoltaic devices have been extensively studied as a means to produce sustainable energy. However, the performance of organic-photovoltaic (OPV) devices is dependent upon a number of factors including the morphology of the active layer, device architecture, and processing conditions. Recent research has indicated that fullerenes in the bulk heterojunction are entropically driven to the silicon and air interfaces upon crystallization of P3HT, which occurs during thermal annealing. The first chapter of this research focuses on investigating the structure and function of end-tethered poly(3-hexylthiophene) chains to a transparent electrode as an anode buffer layer. Neutron reactivity reveals that these P3HT …

Evaporation Induced Self-Assembly And Characterization Of Nanoparticulate Films: A New Route To Bulk Heterojunctions, Yipeng Yang

Doctoral Dissertations

Polymer-based semiconducting materials are promising candidates for large-scale, low-cost photovoltaic devices. To date, the efficiency of these devices has been low in part because of the challenge of optimizing molecular packing while also obtaining a bicontinuous structure with a characteristic length comparable to the exciton diffusion length of 10 to 20 nm. In this dissertation we developed an innovative evaporation-induced nanoparticle self-assembly technique, which could be an effective approach to fabricate uniform, densely packed, smooth thin films with cm-scale area from home-made P3HT nanoparticles. Unlike the previous reports of nanoparticle-based film formation, we use a mixture of two solvents so …

Material And Process Engineering For Bulk Single Crystal Growth Of High Performance Scintillator Potassium Calcium Iodide, Adam Coleman Lindsey

Doctoral Dissertations

Protection against threats of nuclear terrorism relies on the deployment of an enormous number of radiation detection devices with energy resolution to differentiate the radiological signatures of special nuclear materials amongst naturally occurring radiation and other nuisance sources. The capabilities of these devices rely upon the availability of high performance scintillator and semiconductor materials which provide useful responses in the presence of radiation. So far, few materials have been developed to a level that can supplant the use of underperforming NaI:Tl [thallium doped sodium iodide] crystals in the field due to their high cost and/or low yields of production. KCaI …