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Hagerty, PhillipPhysical Vapor Deposition of Materials for Flexible Two Dimensional Electronic Devices
Master of Science (M.S.), University of Dayton, 2016, Chemical Engineering
Molybdenum Disulfide (MoS2) and Tungsten Disulfide (WS2) are two materials in a larger class of materials known as Transition Metal Dichalcogenides (TMDs) that have begun emerge as semiconducting materials. When their horizontal length scale is reduced from bulk to monolayer they demonstrate surprising combinations of properties including a direct electronic band gap and mechanical flexibility. Two dimensional (2D) materials have the potential to revolutionize performance and tailorability of electro-optical devices fabricated entirely from molecularly thin materials. In a departure from traditional exfoliation or high temperature chemical vapor deposition approaches for 2D materials synthesis, novel plasma-based physical vapor (PVD) techniques were used to fabricate uniform films over large areas. This experimental approach allowed unique studies. For example, vapor phase growth allowed systematically variation of the sulfur vacancy concentration in MoS2 and WS2 and subsequent correlation to electronic properties. This effort leads to controlled bottom-up assembly of 2D devices on flexible and standard substrates to experimentally couple the remarkable intrinsic mechanical and electronic properties of ultrathin materials, which are particularly appealing for molecular sensing. The pursuit of an all physical vapor deposited field effect transistor (FET) is the main priority for the 2D materials community as definitive demonstration of the feasibility of physical vapor deposition as a scalable technique for consumer electronics. PVD sputtered Titanium Nitride (TiN) and Tungsten (W) were experimentally characterized as potential back gated materials, Plasma Vapor Deposited (PLD) a-BN was electrically characterized as a uniform ultra-thin low temperature dielectric, and sputtered MoS2 and WS2 were electrically characterized as a semiconductor material. Tungsten deposition methods were previously researched and mimicked for smooth and conductive back gate material depositions. TiN was parameterized and the best room temperature deposition conditions were 70V applied to the sputtering gun with 25 sccm gas flow of 90% N2 and 10% Ar for 60 minutes. The best high temperature depositions were done at 500oC, 70V applied to the sputtering gun with 25 sccm gas flow of 90% N2 and 10% Ar for 30 minutes. Dielectric a-BN electrical characterization began to occur after 6nm which equated to 100 pulses, while 200 pulses equated to 16.5nm thickness. A dielectric constant of 5.90 ± .65 is reported for a-BN for under 20nm thickness. Soft probing techniques by conductively pasted gold wires on the probe tips were required to obtain true electrical measurements of 2D materials in a stacked structure, otherwise scratching would occur and uniformity would cease to exist in the film. Chemical Vapor Deposition (CVD) and mechanical exfoliation have provided the only working TMD semiconductor 2D materials in MOSFET structure to date with lithographic electrical connections. PVD sputtering as a new synthesis method for crystalline TMD with a stoichiometric ratio is achievable over large areas. Though, reduced area depositions are required for doped Silicon and Silicon Oxide (SiO2) based FET structures to limit the chance of encountering a pinhole. With reduced area and stoichiometric enhancement control, sputtered TMD films exhibit high sensitivity to oxygen and are electrically conductive even when exposed to a field effect. Increasing the grain size of the sputtered materials is the next driving force towards a fully recognizable TMD thin film transistor.

Committee:

Christopher Muratore, PhD (Committee Chair); Terrence Murray, PhD (Committee Member); Kevin Myers, DSc (Committee Member)

Subjects:

Aerospace Engineering; Aerospace Materials; Electrical Engineering; Engineering; Materials Science

Keywords:

PVD; materials; 2D materials; Nanoelectronics; TMDs; 2D Transistors; Molybdenum Disulfide; MoS2; WS2; Tungsten Disulfide

Lee, JaesungOptically Transduced Two-Dimensional (2D) Resonant Nanoelectromechanical Systems and Their Emerging Applications
Doctor of Philosophy, Case Western Reserve University, 2017, EECS - Electrical Engineering
Two-dimensional (2D) crystals, derived from layered materials and consisting of atomically thin sheets with weak van der Waals interlayer interactions, have been the subject of many exciting research efforts, including discoveries of new device physics and explorations of creating novel devices for future applications. In addition to excellent electrical and optical properties in their atomically thin limit, 2D crystals also intrinsically possess excellent mechanical properties (e.g., high strain limit of ~25%, and Young’s modulus of EY~1TPa for graphene), making them attractive candidates for next generation nanoelectromechanical systems (NEMS). Initial studies on 2D NEMS have mostly been focused on the semimetal graphene, where challenges remain in device performance (e.g., low quality factors) and practical applications (e.g., sensors and oscillators). Meantime, remarkable opportunities are emerging for the new 2D semiconductors. This dissertation presents investigations of both fundamental device physics and engineering of device functions and performance toward the perspective of technological applications. This dissertation includes: (i) study of frequency scaling of 2D NEMS resonators for providing an important guideline to achieve 2D resonators with desired resonance frequency; (ii) investigation of air damping in 2D NEMS to evaluate performance of resonators when they are operating in air which may be exploited for applications in gas and pressure sensing; (iii) experiments on frequency tunability for creating highly tunable resonant 2D NEMS, which may enable applications in voltage controlled oscillators; and (iv) demonstration of parametric amplification for greatly boosting the relatively low initial Q values of 2D NEMS resonators. Based on the aforementioned fundamental device physics and engineering studies, 2D NEMS have been explored and their potential has been evaluated for future applications in sensing and radio-frequency (RF) signal processing. By integrating passive 2D NEMS into an optical and electrical combined circuitry, self-sustained feedback 2D NEMS oscillators have been created; and positive feedback and feedback cooling have been explored for RF signal processing applications. In addition, as proof-of-concept studies with potential for sensing applications, the effects of pressure variations and gamma-ray radiation upon the 2D NEMS have been tested, and excellent responsivities and sensitivities for potential sensing capabilities have been achieved. The findings in this dissertation may provide import understandings of 2D NEMS, and help pave the way for transforming 2D NEMS resonators into relevant emerging applications.

Committee:

Philip Feng (Committee Chair); Christian Zorman (Committee Member); John Lewandowski (Committee Member); Hongping Zhao (Committee Member)

Subjects:

Electrical Engineering

Keywords:

NEMS; MEMS; Resonator; Oscillator; 2D Materials; Molybdenum Disulfide; MoS2; Graphene

Lee, Edwin WendellGrowth and Nb-doping of MoS2 towards novel 2D/3D heterojunction bipolar transistors
Doctor of Philosophy, The Ohio State University, 2016, Electrical and Computer Engineering
Molybdenum disulfide (MoS2) is a member of a group of layered materials called transition metal dichalcogenides (TMDs) characterized by monolayers consisting of a transition metal atom (Mo or W for example) sandwiched between chalcogen atoms (S, Se, Te) on either side. The monolayers have no out-of-plane bonds and bulk TMDs consist of many monolayers stacked and held together weakly by van der Waals forces. Bulk MoS2 exhibits an indirect band gap of 1.2 eV, but monolayer films exhibit a direct gap of 1.8 eV. MoS2 has been studied for a wide range of applications, many by utilizing micromechanically exfoliated, micron-scale flakes to study its material properties. Study of these flakes points to scaling limitations, and many groups have explored large-area growth methods to produce high-quality, continuous films. This work aims to demonstrate traditional device engineering based on MoS2 including growth, doping, heterostructure study and device design. We demonstrate single crystal growth of MoS2 by depositing Mo on sapphire substrates and sulfurizing the samples in a chemical vapor transport process. The growth process is robust, and reasonably could be scaled up to wafer-scale processing. The films exhibited excellent structural qualities, and electrical measurements showed high space-charge mobility. The MoS2 films were also doped with Nb in order to achieve p-type mobility. Degenerate doping of the films was demonstrated and confirmed by low temperature Hall measurement, and film conductivity increased by four orders of magnitude over unintentionally doped films. The degenerately doped films were shown to exhibit a Hall mobility of approximately 10 cm2V-1s-1. Heterojunction diodes were formed between degenerately doped p-MoS2¬ and n-doped SiC and GaN by direct growth and film transfer, respectively, to form 2D/3D heterojunctions. Electrical measurements were utilized to extract the conduction band offsets in MoS2/SiC (¿EC = 1.6 eV) and MoS2/GaN (¿EC = 0.2 eV) junctions. Characterization of the heterostructures showed that traditional 3D semiconductor methods are sufficient to characterize the 2D materials despite the van der Waals gaps between each MoS2 monolayer. The MoS2/GaN heterojunction was used as the base/collector junction for a tunneling heterojunction bipolar transistor (THBT) for which the emitter was atomic layer deposited Al2O3. THBTs showed small common base gain corresponding with positive transconductance in the common emitter configuration. As such, the MoS2/GaN heterojunction shows significant promise for future HBT applications.

Committee:

Siddharth Rajan (Advisor); Aaron Arehart (Committee Member); Roberto Myers (Committee Member)

Subjects:

Engineering

Keywords:

MoS2; Molybdenum disulfide; Doping; 2D-3D; Heterojunction; heterojunction bipolar transistor; transition metal dichalcogenide; 2D materials;

Gross, Carl MorrisGrowth and Characterization of Molybdenum Disulfide Thin Films
Master of Science in Electrical Engineering (MSEE), Wright State University, 2016, Electrical Engineering
Two-dimensional materials, or materials that are only one atomic layer thick, have seen much research in recent years because of their interesting electrical properties. The first of these materials, graphene, was found to have incredible electrical properties but lacked a bandgap in intrinsic films. Without a bandgap, graphene cannot create transistors that can be shut off. Molybdenum disulfide, however, is a two-dimensional semiconductor with a large bandgap. The main issue of molybdenum disulfide is that synthesized films are a much lower quality than their exfoliated counterparts. For molybdenum disulfide to be able to be used practically, a method of synthesis must be found that can reliably create quality large area monolayer films. In this thesis, three methods of molybdenum disulfide film synthesis are presented. Methods implemented used a tube furnace as a chemical vapor deposition system to evaporate source materials to synthesize thin films of molybdenum disulfide. An exploration into the different synthesis parameters shows optimal conditions for these specific methods. Then a discussion of these different methods is presented by judging films grown by using these methods on relevant criteria. This work shows methods to synthesize large area, polycrystalline, small grain, multilayer films, both intrinsic and doped, and to synthesize small area, single crystal and polycrystalline, monolayer films of molybdenum disulfide.

Committee:

Yan Zhuang, Ph.D. (Advisor); Shin Mou, Ph.D. (Committee Member); Michael Saville, Ph.D., P.E. (Committee Member)

Subjects:

Electrical Engineering; Engineering; Materials Science; Nanoscience; Nanotechnology

Keywords:

Molybdenum Disulfide; 2D Materials; Chemical Vapor Deposition; Raman Spectroscopy

Beauchamp, Damian RichardMolecular Engineering of Organic Photosensitizes for P-type Dye-Sensitized Solar Cells and the Immobilization of Molecular Catalyst for the Hydrogen Evolution Reaction
Master of Science, The Ohio State University, 2016, Chemistry
Solar energy has become an important component in the clean energy mix. There are several different kinds of solar cells that have been developed over decades. The focus of the first three chapters will be p-type dye-sensitized solar cells (DSSCs), which are omnipotent for obtaining high efficiency and cost effective tandem DSSCs. The efficiency of p-type DSSCs lags behind their n-type counterpart due to being less investigated. Herein, the attempts to increase performance of the p-type component via molecular engineering of organic photosensitizers is described. Through the addition of bulky hydrophobic alkyl chains performance can be enhanced, though it was found that the location of these alkyl chains is a critical factor. Additionally, by adopting a double-acceptor single-donor design, as described in chapter 3, when employing the commonly used triphenylamine donor moiety, one can simultaneously increase the molar extinction coefficient while reducing the synthetic steps yielding one the fields top performing photosensitzers. In addition to the conversion of solar energy to electrical energy, the storage of intermittent renewable energy is important. Energy can be stored mechanically (e.g. pumped hydro, fly wheels, compressed air, etc.), electrochemically (e.g. batteries and capacitors), or in chemical bonds (e.g. hydrolysis, carbon dioxide reduction, etc.). Of these methods hydrolysis to produce hydrogen has been identified as an attractive potential method. This is because hydrogen has high specific energy, can be transported, and used as a fuel in fuel cells emitting only water. The problem is industry currently employs steam-methane reforming to produce hydrogen, because catalysts currently employed for hydrolysis are expensive (i.e. noble metals) and/or unstable. Therefore finding a more abundant, lower cost, and stable catalyst which can be easily processed has been of importance. Molybdenum disulfide based catalysts have been identified as a good candidate because of their low Gibbs free energy of proton absorption. The molecular variants have the highest density of catalytically active sites, but suffer from desorption from electrode surfaces. Herein a molecular molybdenum disulfide catalyst is immobilized via polymer coordination yielding a catalytic material which can be easily processed into films via a resin. This produced stable catalytic films on electrode surfaces, which show good activity toward hydrogen evolution via water reduction.

Committee:

Yiying Wu, PhD (Advisor); James Cowan, PhD (Committee Member)

Subjects:

Chemistry; Energy; Gases; Organic Chemistry; Polymer Chemistry; Polymers

Keywords:

Solar energy, dye-sensitized solar cells, organic photosensitizers, molecular engineering, fuel cells, catalysts, Molybdenum disulfide, immobilized, polymer, hydrogen evolution, water reduction

Yu, Jenwei RoscoeMethane activation over molybdenum disulfide, molybdenum carbide, and silver(110). Molecular orbital theory
Doctor of Philosophy, Case Western Reserve University, 1990, Chemistry
The atom superposition and electron delocalization molecular orbital (ASED-MO) theory is used to study methane C-H bond activation over MoS2, MoC, and O/Ag(110), C1 coupling mechanisms to form C2H6 and C2H5OH on MoS2, CH2 coupling on MoC, and the binding properties of C2 olefins to 2 S in the CpMoS4MoCp (Cp = C5H5) complex and on MoS2. For methane C-H bond activation by the Mo IV oxidative insertion mechanism, the theory predicts low barriers for both MoS2 and MoC catalysts. These results suggest the possibility of incorporating methane into the Fischer-Tropsch process over these catalysts. The activation barrier for H abstraction by O on Ag(110) is calculated to be lower than over most oxides. The calculations also suggest the possibility of direct O insertion into a methane C-H bond to make methanol on the O/Ag(110) surface. It has been demonstrated by Klier and coworkers that the Fischer-Tropsch reaction over MoS2 proceeds by the CO insertion mechanism. The calculations also favor this mechanism. High barriers are found for the other C1 coupling mechanisms (CH3 + CH3, CH2 + CH3, and CH2 + CH2). Two CH2 coupling on MoC is also studied. The calculations show that the coupling barrier on MoC is smaller than that on MoS2 and the desorption of C2H4 is calculated to be easier on MoC. Complexes which have a S4 structure chelated by ligands (e.g. C2H4 and C2H2) have been insolated by DuBois and coworkers. Acetylene hydrogenation in these complexes were also observed. These seem to suggest that S2- in MoS2 basal planes would possibly have the same reactivities. This would be in contrast to the general belief that MoS2 basal planes are inert toward catalytic reactions and that hydrogenation over MoS2 occurs on the edge unsaturated Mo IV sites. Theoretical calculations carried out to study the binding strengths of C2 olefins to the sulfur anions in the CpMoS4MoCp complex and on MoS2 demonstrate that the binding of C2 olefins to MoS2 basal plane S2- is much weaker than in the complex

Committee:

Alfred Anderson (Advisor)

Subjects:

Chemistry, Physical

Keywords:

Methane activation; Molybdenum disulfide and carbide; Molecular orbital theory