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Young, Justin RSynthesis and Characterization of Novel Two-Dimensional Materials
Doctor of Philosophy, The Ohio State University, 2016, Physics
As van der Waals layered materials are reduced from bulk crystals to monolayer sheets, a host of electronic, optoelectronic, and mechanical properties emerge which differ from those of the parent materials. This variety of materials properties—coupled to the atomically thin form factor—has attracted interest from all research sectors in the past decade due to potential applications in flexible, transparent, and low-power electronics. The two-dimensional nature of these materials makes them extremely sensitive to any surface interactions presenting both a unique opportunity to tune materials properties through surface modification and also a challenge whereby any surface contaminants can dramatically degrade the material quality. In this dissertation, we investigate and utilize this surface sensitivity in three different material systems. First, we investigate electronic transport in germanane, a germanium analog of graphane, through a combination of electronic measurements on multi-layer crystals and finite-element modeling. In addition to doping this 2D material, we uncover a sensitivity of this transport to the presence of water-vapor, as well as an anisotropy between inter- and intra-layer resistivity of up to eleven orders of magnitude. The strong water sensitivity and weak inter-layer coupling mean that the transport in these samples is dominated by the topmost layer and suggests that it may be possible to measure the effects of 2D materials in bulk materials by making electrical contact to only the topmost layer. Second, we report on a templated MoS2 growth technique wherein Mo is deposited onto atomically-stepped sapphire substrates through a SiN stencil with feature sizes down to 100 nm and subsequently sulfurized at high temperature. These films have a quality comparable to the best MoS2 prepared by other methodologies, and the thickness of the resulting MoS2 patterns can be tuned layer by layer by controlling the initial Mo deposition. This approach critically enables the creation of patterned single-layer MoS2 films with pristine surfaces suitable for subsequent modification via functionalization and mechanical stacking. Further, we anticipate that this growth technique should be broadly applicable within the family of transition metal dichalcogenides. Third and finally, we present progress toward understanding how local changes to graphene’s crystal structure, such as defects, adatoms, and electromagnetic fields, affect the observable electronic and spin transport. We developed experimental methods to perform scanning probe and scanning tunneling microscopy with the simultaneous measurement of electrical transport in graphene Hall bar devices synthesized from graphene grown by chemical vapor deposition. Through the combination of these powerful experimental techniques, we plan to investigate the connection between localized surface modifications of graphene and the electronic and spin transport in these devices with eventual expansion of this technique to other 2D materials.

Committee:

Ezekiel Johnston-Halperin, Prof. (Advisor); Jay Gupta, Prof. (Committee Member); Nandini Trivedi, Prof. (Committee Member); Andrew Heckler, Prof. (Committee Member)

Subjects:

Physics

Keywords:

2D Materials; graphene; MoS2; germanane; GeH; TMDs; STM; CVD; growth

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;

Lindquist, Miles T.Investigation of growth parameters for as-grown 2D materials- based devices
Bachelor of Science (BS), Ohio University, 2017, Physics
We investigate the effects of temperature, flow-rate, and growth time for a novel chemical vapor deposition process for producing self-contacted two-dimensional materials-based devices. For a specific set of growth parameters, a lateral thin film extending from the molybdenum pattern is found to monotonically increase in width with increasing growth time. We also observe indications that with an increased reactant delivery rate the film begins to show vertical growth at the expense of the lateral growth. These results will help advance a comprehensive process for the scalable production of as-grown, complex, two-dimensional materials-based device architectures.

Committee:

Eric Stinaff (Advisor)

Subjects:

Materials Science; Physics

Keywords:

MoS2; 2D Materials; semiconductor

Myers, JoshuaNANO-MATERIALS FOR MICROWAVE AND TERAHERTZ APPLICATIONS
Doctor of Philosophy (PhD), Wright State University, 2015, Engineering PhD
In this age of digital electronics the quest for faster computational devices and high speed communications have driven a need for new materials that are capable of fulfilling these goals. In both areas the need for a thinner channel in transistors, faster carrier transport characteristics, and better magnetic materials dominate the direction of research. Recently 2D materials have been realized. These single layer atomic thick materials show potential in having extremely high carrier transport velocities at room temperature and, due to their natural 2D structure, are the thinnest material possible in nature. On the other hand spin-spray ferrites have showed potential in producing high permeability, low loss materials with a low processing temperature compatible with current CMOS technology. One of the largest hindrances in the implementation of these materials are the lack of measurement capabilities. Both 2D materials and spin-spray ferrites have nm sized features that significantly change how the material behave. To further investigate these materials scanning microwave microscopy (SMM) is being developed as a possible characterization tool. SMM has the unique ability to collect the complex reflection coefficient simultaneously with the topography at nm horizontal spatial resolutions. The complex reflection coefficient is able to supply valuable information about materials such as conductivity and permittivity. This dissertation provides an in depth look at the potential applications for SMM and supplies a rigorous characterization, both experimentally and numerical simulations, of the SMM system. In detail we re- port first time SMM measurements of graphene's conductivity and permittivity along with characterization of graphene defects induced by oxygen plasma etching and graphene wrinkles. We have also experimentally show conductive grain boundaries in spin-spray ferrites leading to larger than expected losses. Lastly we show Fourier transform inferred spectroscopy measurements of graphene micro and nano ribbons. These results show the versatility of SMM and the ability to further characterize new materials. Furthermore we show the ability of the SMM to obtain calibrated conductivity and permittivity measurements on the nanoscale level leading to a more complete understanding of the effects of defects on the electrical properties of graphene and understanding of the losses in ferrimagnetic materials.

Committee:

Yan Zhuang, Ph.D. (Advisor); Marian Kazimierczuk, Ph.D. (Committee Member); Douglas Petkie, Ph.D. (Committee Member); LaVern Starman, Ph.D. (Committee Member); Shin Mou, Ph.D. (Committee Member)

Subjects:

Atoms and Subatomic Particles; Electrical Engineering; Electromagnetics; Engineering; Materials Science; Nanoscience; Nanotechnology; Optics

Keywords:

Scanning Microwave Microscope; Graphene; 2D Materials; AFM; Plasmonics; THz; Photonics; Defects; Nano-Materials, Spin-Spray, Ferrite

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

Ma, LuMo-S Chemistry: From 2D Material to Molecular Clusters
Doctor of Philosophy, The Ohio State University, 2016, Chemistry
Layered transition metal dichalcogenides (TMDs), especially molybdenum disulfide (MoS2), have been of great interest for a long time. MoS2 naturally occurs as the mineral molybdenite, which has been involved in diverse research fields, such as electronics, optoelectronics, spintronics, energy storage, lubrication, and catalysis. In MoS2 crystals, a sheet of molybdenum atoms is sandwiched between sheets of sulfur atoms. The covalent Mo-S bonding is strong, but the interaction between the sandwich-like tri-layers is weak van der Waals force, resulting in easy exfoliation of a single layer or a few layers. These MoS2 ultrathin layers (less than 10 atoms thick) belong to the family of two-dimensional (2D) materials. As dimensionality reduced, these MoS2 ultrathin layers have unique charge transport properties, which solve the limitation of conventional Si and other bulk materials as scaling down to microelectronic and nanoelectronic devices. However, the synthesis of orientated single- or few- layer TMDs with large area remains challenging. The first part of this dissertation focuses on the design and controlled synthesis of 2D TMDs and study of their electronic device applications. A facile vapor-solid method was employed to get single crystalline few-layer MoS2 films on (0001)-oriented sapphires with excellent structural and electrical properties over centimeter length scale. A carrier density of ~2.0E11 cm -2 and a room temperature mobility as high as 192 cm2/Vs were extracted from space-charge limited transport regime in the films. In addition, transition metal doped 2D MoS2 films were successfully synthesized by one-step process. These doped films enable the tuning of the properties 2D MoS2 films. By substituting to other substrates or film transferring, 2D/3D heterojunction diodes have also been made with excellent rectification. MoS2 has also been explored as catalysts such as for hydrogen evolution reaction (HER). The edges of MoS2 has been identified as the active sites for HER, but the basal planes are catalytically inert. The second part of this dissertation focuses on the design and characterization of Mo-S clusters and study of their HER catalytic activities. Different Mo-S clusters that contain the edge structures of MoS2 have been made to understand and improve the HER efficiency. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and X-ray absorption spectroscopy (XAS) has been used to reveal the properties of Mo and S atoms at different sites. The synchrotron X-ray based XAS is one of the most useful technique to determine the local geometric and electronic structure of materials. The XAS technique can characterize both bulk sample in transmission mode, and ~ 100 nm surface in total electron yield mode regardless of the crystallinity of the materials. The XAS includes X-ray absorption near edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS). The XANES is sensitive to the charge transfer, orbital occupancy and symmetry. The bond length and coordination details can be extracted from Fourier transform of EXAFS. The third part of this dissertation focuses on the application and improvement of XAS technique.

Committee:

Yiying Wu, Dr (Advisor); Patrick Woodward , Dr (Committee Member); Joshua Goldberger , Dr (Committee Member)

Subjects:

Chemistry

Keywords:

MoS2, 2D materials, Mo-S clusters, Hydrogen evolution reaction, X-ray absorption spectroscopy

Krupa, Sean J.Nonlinear Optical Properties of Traditional and Novel Materials
Doctor of Philosophy (PhD), Ohio University, 2016, Physics and Astronomy (Arts and Sciences)
Nonlinear optical processes are an excellent candidate to provide the heralded, indistinguishable, or entangled photons necessary for development of quantum mechanics based technology which currently lack bright sources of these photons. In order to support these technologies, and others, two classes of materials: traditional and novel, were investigated via optical characterization methods with goal of gaining insight into which materials and experimental conditions yield the greatest nonlinear optical effects. Optical characterization of periodically poled lithium niobate (PPLN) helped support the development of a simple, efficient photon pair source that could be easily integrated into optical networks. Additionally, an in-situ measurement of the 2nd order nonlinear optical coefficient was developed to aid in the characterization of PPLN pair sources. Lastly, an undergraduate demonstration of quantum key distribution was constructed such that students could see the primary application for PPLN photon pair sources in an affordable, approachable demonstration. A class of novel optical materials known as 2D materials has been identified as potential replacements to the traditional nonlinear optical materials discussed in Part I. Through optical characterization of second harmonic generation (SHG) the ideal conditions for spontaneous parametric downconversion were established as well as signal thresholds for successful detection. Attempts to observe SPDC produces hints that weak generate SPDC may be present in WS2 samples however this is incredibly difficult to confirm. As growth techniques of 2D materials improve, a photonic device constructed from these materials may be possible, however it will need some mechanism e.g. stacking, a cavity, etc. to help enhance the SPDC signal.

Committee:

Eric Stinaff (Advisor); Alexander Govorov (Committee Member); Savas Kaya (Committee Member); Nancy Sandler (Committee Member)

Subjects:

Optics; Physics

Keywords:

Optics; Photonics; Lithium Niobate; 2D Materials; 2D Transition Metal Dichalcogenides; Quantum Key Distribution