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Singh, HarpalAn Investigation of Material Properties and Tribological Performance of Magnetron Sputtered Thin Film Coatings
Doctor of Philosophy, University of Akron, 2015, Mechanical Engineering
This dissertation is divided into two categories based upon lubrication functionality and its application. The categories are: Dry film lubrication and Fluid film lubrication with thin film coatings. Thin film coatings examined in this work were deposited using closed field unbalanced magnetron sputtering and RF-DC coupled magnetron sputtering systems. In Dry/Solid film lubrication, the mechanical, structural and tribological properties of two Molybdenum disulphide (MoS2) based coatings are examined and evaluated. Among the two coatings, one coating is doped with Ti (Ti-MoS2) and the other is a combination of metal, lubricant and oxide (Sb2O3/Au - MoS2). These coatings are known to provide low friction in vacuum environments. The goal of this work was to evaluate friction and wear performance of MoS2 doped coatings in unidirectional and reciprocating sliding contact under different environmental conditions. Sliding contact results showed friction and wear dependence on temperature and humidity. The formation and removal of transfer films and the recrystallization and reorientation of basal layers on the steel counterface was observed as the mechanism for low friction. Structural analysis revealed a relationship between the microstructural properties and tribological performance. It was also observed that the addition of dopants (Ti, Au, Sb2O3) improved the mechanical properties as compared to pure MoS2 coatings. Further, the rolling contact performance of the coatings was measured on a five ball on rod tribometer and a Thrust bearing tribometer under vacuum and air environments. The rolling contact experiments indicated that life of the rolling components depend on the amount of material present between the contacts. Fluid film lubrication with thin film coatings investigates the possibilities to improve the performance and durability of tribological components when oils and thin films are synergistically coupled. In this work, the ability of a Diamond Like Carbon coating to increase the durability of contacting surfaces under boundary lubrication were studied. The performance of highly hydrogenated Diamond Like Carbon (DLC) was evaluated in a mixed sliding and rolling contact. Experimental results show significant improvement in fatigue life of steel specimens after coating with a highly hydrogenated Diamond Like Carbon coating. The improved fatigue life is attributed to the coating microstructure and the mechanical properties.

Committee:

Gary Doll (Advisor)

Subjects:

Aerospace Materials; Engineering; Experiments; Materials Science; Mechanical Engineering

Keywords:

Ti-MoS2, Sb2O3 and Au doped MoS2, Solid Lubricant, Diamond Like Carbon, DLC, Rolling and Sliding, MoS2 characterization, wind turbines, micropitting, MPR, carbon coating, MoS2 film, MoS2 doped coating, bearing testing, fatigue, wear, friction, vacuum,

Ma, LuSynthesis and Characterization of Large Area Few-layer MoS2 and WS2 Films
Master of Science, The Ohio State University, 2014, Chemistry
Transition metal dichalcogenides (TMDs) bonded by a weak Van der Waals force between layers can be exfoliated to a single layer or a few layers. These ultrathin materials (less than 10 atoms thick) belong to the family of two-dimensional (2D) materials. As dimensionality reduced, 2D materials have unique heat and charge transport properties. With sizable band gaps, 2D TMDs arouse both fundamental and practical interest. However, the synthesis of orientated single- or few- layer TMDs with large area remains challenging. Here, a facile chemical vapor-metal conversion method was employed to get (0001)-orientated MoS2 and WS2 few-layer films with large area. In addition, by inserting another metal layer into the precursor Mo layers, doped MoS2 films (e.g. Nb doped MoS2 films) were successfully synthesized by one-step chemical vapor-metal conversion process. The thickness of the film was controlled by tuning the thickness of the starting metal layers. Doping density varied with the thickness of the doping metal layers. The atomic force microscopy (AFM), high resolution x-ray diffractometer (HRXRD), Raman spectroscopy and transmission electron microscopy (TEM) were used to characterize the layered TMD films. Transport properties were measured by the Hall Measurements, the Transmission Linear Measurement (TLM) and the Field-effect Transistor (FET) measurements. The pure few-layer MoS2 films were n-type and had a low background carrier concentration of ~1016 cm-3 with a high electron mobility of up to 12 cm2V-1 s-1. The Nb-doped MoS2 films were p-type and showed highly degenerate doped behavior. By decreasing the doping density, the hole mobility of Nb-doped MoS2 films increased from 0.5 cm2V-1 s-1to 8.5 cm2V-1 s-1. Moreover, MoS2/WS2 planar heterojunctions were made by the same chemical vapor-metal conversion method with one-step heating.

Committee:

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

Subjects:

Chemistry

Keywords:

2D material, few-layer MoS2 film, mobility, p-MoS2

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

Mutyala, Kalyan ChakravarthiInfluence of Metallic, Dichalcogenide, and Nanocomposite Tribological Thin Films on The Rolling Contact Performance of Spherical Rolling Elements
Doctor of Philosophy, University of Akron, 2015, Mechanical Engineering
A global study performed in 1966 revealed that nearly 30% of the energy produced is spent to overcome friction and associated losses. A new interdisciplinary domain "Tribology" was defined in the "Jost Report"' as the branch of science that essentially deals with friction, wear and lubrication. Increasing demand to improve efficiency of mechanical systems has stimulated the research in tribology over the last few decades. Surface engineering methods are state of art techniques that are being adapted by industries including bearing manufacturers to address friction and wear issues. Many new and novel coatings have been developed for specific applications, but few if any, have improved the tribological performance of the most widely used components: ball bearings. Thus, there is a need for new tribological research designed to understand the influence of the coatings deposited onto spherical rolling elements in tribological contacts, and minimize losses due to friction and wear. In this work, thin films were deposited onto spherical rolling elements and the performance of the coated balls was evaluated under different conditions. The study revealed that ball coatings improves the performance of bearings, but coatings need to be selected based on application requirements to avail the benefits of coated balls.

Committee:

Gary Doll (Advisor)

Subjects:

Mechanical Engineering

Keywords:

Thin films; Tribology; Rolling Element Bearings; Diamond-like carbon; Friction and Wear; Rolling Contact Fatigue; Ball Coatings; Transfer films; Me-DLC; Ti-MoS2; CrxN; Additives; MoDTC; Metallic; Dichalcogenide; Nanocomposite; Corrosion; EIS; PVD; CFUMS;

Luo, XinhangFew-Layer MoS2 Thin Films Grown by Chemical Vapor Deposition
Master of Science, The Ohio State University, 2014, Electrical and Computer Engineering
Due to their unique material properties, 2D semiconductors such as MoS2 have recently attracted significant research interests for few applications including next generation nanoelectronics, flexible and transparent electronics, and optoelectronics. In this MS thesis research, we carried out studies on carrier mobility and transport, ohmic contacts, and gate dielectrics on few layer MoS2 for the purposes of developing high performance 2D MoS2 MOSFETs. Specifically, using the space charge model, we investigated the carrier mobility of few-layer MoS2 grown by chemical vapor deposition (CVD) on sapphire substrates. An electron mobility of 118 cm2/Vs has been demonstrated on structures with transmission line model (TLM) patterns. We also developed a new method to measure and calculate the contact resistance for non-uniform few-layer MoS2 grown by CVD. A specific contact resistivity of 8.4×10-5 Ωcm2 has been demonstrated on Ti/Au contacts on essentially intrinsic CVD grown few-layer MoS2 that were processed by plasma ion bombardment before metallization and annealed by rapid thermal annealing (RTA). Furthermore, we performed a temperature dependent conductance study of MoS2 . The preliminary result suggests that the carrier transport in the CVD grown few-layer MoS2 is Efros-Shklovskii variable range hopping. Finally, an atomic layer deposition (ALD) process has been optimized for the deposition of Al2 O3 on MoS2 as the gate dielectric for future MOSFET development.

Committee:

Wu Lu (Advisor); George Valco (Committee Member)

Subjects:

Electrical Engineering

Keywords:

MoS2, CVD, Mobility, Contact Resistance

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

Zhang, ZhichunProcess Dependence of Defects and Dopants in Wide Band Gap Semiconductor and Oxides
Doctor of Philosophy, The Ohio State University, 2013, Electrical and Computer Engineering

Wide band gap semiconductors and oxides have attracted much attention in the last decade because of their various applications to next generation opto- and micro-electronics. My research focuses on the process dependence of the defects and dopants in wide band gap semiconductors and oxides of high-k dielectrics, ZnO and MoS2 .

Ultrahigh speed microelectronics demands high mobility semiconductors such as Ge. We are exploring how to process high-k oxides on Ge to minimize defects that trap charge. For Ge doped HfO2, which has higher k value, combined X-ray absorption spectroscopy (XAS) and depth-resolved cathodoluminescence spectroscopy (DRCLS) measurements identified the defects in Ge doped HfO2. Besides, DRCLS of GeO2/Ge structure demonstrated that different annealing temperature will affect the defects at the GeO2/Ge interface. Understanding the defects evolution under different process condition and controlling the defects density in the high-k dielectric gate stack will promote the development of nanoscale electronic devices.

Wide band gap semiconductor ZnO (Eg = 3.4 eV) is a leading candidate for future opto- and micro-electronics due to its high exciton binding energy, thermochemical stability, environment compatibility, availability of high quality large substrates and potential applications for light-emitting devices and photovoltaics. However, as-grown un-doped ZnO is naturally n-type and controllable p-type doping is still not stable and reliable. The origin of the n-type conductivity is also controversial, so the ability to understand and control the intrinsic and impurity related defects in ZnO is essential. We used nanoscale DRCLS, secondary ion mass spectrometry (SIMS), surface photovoltage spectroscopy (SPS), transient surface photovoltage spectroscopy (TSPS), photoluminescence spectroscopy (PLS) and temperature-dependent Hall-effect (TDHE) measurements to investigate the strong process dependence of Li configuration and electrical property of Li-doped ZnO on thermal treatment, the passivation and doping effect of implanted H in ZnO, the mechanical polishing induced defects and passivation effect of post-polishing diluted HF etching. DRCLS reveals an inverse relationship between the optical emission densities of lithium on zinc sites versus zinc vacancy sites, demonstrating the time dependence of Li interstitials to combine with zinc vacancies in order to form substitutional Li acceptors. The critical annealing time and temperature was determined for lowest defect intensity and highest donor concentration of H implanted ZnO. Post polishing HF etching not only removes/passivates zinc vacancy (VZn) related defects, but also decreases oxygen vacancy (VO) related defects by one order of magnitude. Understanding the process dependence of the H and Li configurations and properties in ZnO will provide guidance to control electrical property of ZnO, produce low resistivity in situ auto doped n-type ZnO bulk as well as realize stable p-type ZnO.

Two-dimensional materials are attractive for next generation nanoelectronic devices. Monolayer MoS2 has emerged as a very promising material due to its distinctive electronic, optical and catalytic properties. As a prospective material to substitute for graphene, properties of MoS2 need to be explored. We used DRCLS, SEM, AFM and SPS to characterize the band gap and defect states in thin layer MoS2. The results provide guidance to fabricate monolayer MoS2 and control the defects in MoS2.

Committee:

Leonard Brillson (Advisor); Wu Lu (Committee Member); Siddharth Rajan (Committee Member)

Subjects:

Electrical Engineering; Engineering; Nanoscience; Nanotechnology; Solid State Physics

Keywords:

Wide band gap semiconductor; high-K dielectric; Zinc oxide; MoS2; depth-resolved cathodoluminescence spectroscopy; surface photovoltage spectroscopy; photoluminescence spectroscopy

Waite, Adam RichardEffects of Fundamental Processing Parameters on the Structure and Composition of Two-Dimensional MoS2 Films
Doctor of Philosophy (Ph.D.), University of Dayton, 2017, Materials Engineering
The unique properties resulting from strongly anisotropic chemical bonds found in the whole family of transition metal dichalcogenide materials (TMDs) have been researched for over 50 years for various applications, with MoS2 being the most heavily researched. The pace of research has surged with the recent isolation and analysis of 2D materials exfoliated from Van der Waals solids, first reported by Novoselov and Geim. MoS2 was first identified to have interesting electrical properties in 2D form by Mak et al. in 2010, where it was discovered that at a monolayer, the band structure had shifted to a direct bandgap semiconductor and the band gap shifts from ~1.3 eV to ~1.9 eV. Coupled with mechanical flexibility, optical transparency, and many other unique properties, 2D MoS2 an ideal semiconductor candidate for nanoelectronic applications. The defect engineering that makes silicon-based technology so flexible has yet to be explored in 2D TMD materials. Grain boundaries are currently a structural defect that cannot be eliminated in wafer scale synthesis of these materials, therefore, a better understanding of how to control grain boundary density and understand the effects on properties is an urgent need. Each grain boundary acts as a scatter defect thought to adversely affect carrier mobility and density, negatively affecting device performance. This grain boundary problem is the driving force behind this study to discover the underlying basic principles and impact of sputter plasma deposition parameters, substrate processing conditions, and the nucleation and growth kinetics of 2D MoS2 films. This dissertation consists of three studies designed to tailor grain boundary density in 2D MoS2: 1) a study on the impact of intrinsic plasma parameters and characteristics of the pulsed DC magnetron sputtering discharge on the resulting 2D MoS2 films and their structure/properties, 2) a study on the impact of the partial pressure of the diatomic (S2) species in sulfur vapor, which dictates the high processing temperatures necessary for nucleation and growth kinetics of 2D MoS2 and correlation of processing conditions to the chemistry and structure of the deposited films, and 3) a study of the impact of the MoO3 precursor film grain size, thickness, structure, and morphology on the nucleation and growth kinetics of 2D MoS2 on composition and structure properties. Novel approaches to reduce grain boundary density and synthesis temperature without compromising stoichiometry in waferscale 2D films were developed based on fundamentals of thin film nucleation and growth. For the first time in 2D MoS2 literature, the identification of the S2 species in the sulfur vapor mixture has definitively been determined to be the only reactant to produce 2D MoS2. This revelation will allow for enhanced control of the 2D MoS2 reaction kinetics allowing for the growth of larger grain and higher crystallinity 2D films. Another first in 2D MoS2 literature, was the report of a new synthesis process where 1) room temperature sputtered MoO3 precursor films are sputtered onto sapphire substrates, 2) annealed to where they react to create a thin Al2(MoO4)3 interface layer, 3) the excess MoO3 was allowed to sublime away, and 4) then sulfidized at 850°C in a S/Ar environment where the Al2(MoO4)3 slowly decomposed releasing MoO3 precursor for subsequent reaction with the S2 vapor species. This process produced highly crystalline, large grain, approximately 1.5 layer thick 2D MoS2 films.

Committee:

Christopher Muratore, PhD (Advisor); Andrey Voevodin, PhD (Committee Member); Paul T. Murray, PhD (Committee Member); Antonio Crespo, PhD (Committee Member)

Subjects:

Engineering; Materials Science; Nanoscience

Keywords:

2D MoS2; MoO3; aluminum molybdate; 2D nucleation and growth; sulfur vapor; S2; plasma diagnostics

Poehler, Scott ATransport Phenomena of CVD Few-Layer MoS2 As-grown on an Al2O3 Substrate
Master of Science, The Ohio State University, 2015, Electrical and Computer Engineering
Fabricating devices with as-grown films is necessary for high volume production however challenges with adding functional gates to MoS2 films natively grown on Al2O3 have limited its usefulness for VLSI manufacturing. A more thorough understanding of this material configuration is required before we can develop effective surface treatment strategies prior to adding gate dielectrics. The scope of this study covers the transport phenomena of charge carriers in few layer MoS2 as-grown by CVD on Al2O3 substrate. We examine carrier transport mechanism dominance for temperatures ranging from 30 K to 310 K for AC conductance measurements and 5 K to 350 K for DC I-V measurements. Scaled AC conductance verses frequency indicating a one-to-one relationship between sDC and transition frequency ft, from 30 K to 90K is a signature of variable range hopping (VRH) transport as conductance becomes steadily independent of frequency with increased temperature. The scaling behavior with increasing temperature shows a saturation point indicating a shift from VRH transport. The fit of AC conductance to the universal power relation s = s0(1+f/ft)s supports the observed shift in transport mechanisms. Conductance becomes less sensitive to frequency beyond 90 K indicating dominance of band transport though there still exists a slight dispersion of carriers even at higher temperatures. The T-p relationship of conductance (Efros-Shklovskii (p=1/2) or Mott (p=1/3)) at low temperatures favors the Efros-Shklovskii model. Below the linear fit regions evidence shows resonant tunneling at localized states. Above these linear fit regions, the resistance curve derivative analysis of sDC from 100 K to 160 K is found to correspond to the shift from VRH to band transport. The mobility-temperature dependence calculated from measured DC current-voltage data using the Mott-Gurney law for space limited current correlates mobility measurements to the observed transition between transport mechanisms with a defining temperature point of about 160 K where the balance of transport mechanisms shift. We expected mobility to decrease at higher temperatures due to phonon scattering, but found that mobility only continued to increase. We determined that the modified Mott-Gurney-Hartke current model explained the experimental data better at the higher temperature indicating currents were more sensitive to the Frenkel effect and showing evidence of a high density of traps. In the transition from the hopping dominated transport at very low temperatures to the band transport at higher temperatures, the conductance continues to rise with temperature. The high density of traps increases the influence of impurity scattering, screening the effects of phonon scattering which explains the increasing mobility even at higher temperatures.

Committee:

Wu Lu, PhD (Advisor); Siddharth Rajan, PhD (Committee Member); Marvin White, PhD (Committee Member)

Subjects:

Electrical Engineering; Materials Science

Keywords:

MoS2, carrier transport mechanism, signature transport transition

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;

Yang, RuiCoupling Two-Dimensional (2D) Nanoelectromechanical Systems (NEMS) with Electronic and Optical Properties of Atomic Layer Molybdenum Disulfide (MoS2)
Doctor of Philosophy, Case Western Reserve University, 2016, EECS - Electrical Engineering
The discovery of two-dimensional (2D) materials has attracted tremendous interest and led to a great deal of investment due to their unique properties that are not present in three-dimensional (3D) or one-dimensional (1D) materials. Though graphene as the flagship 2D material has been extensively studied, it is a semimetal without a natural bandgap, and the difficulties in creating a useful bandgap has limited its applications in logic circuits, photonic devices and tunable devices. 2D semiconductors such as molybdenum disulfide (MoS2) compensate for graphene because they have a natural sizable bandgap, and thus can largely extend the applications of 2D materials. In order to fully exploit the distinct properties of these 2D semiconductors toward advantageous performance as applicable devices, it would be ideal to synthetically consider the electronic, mechanical, and optical properties of these materials. While MoS2 field-effect transistors (FETs), nanoelectromechanical systems (NEMS), and optoelectronic devices have been demonstrated, there are still numerous problems that need to be solved before applying the devices for sensing, computing, and communication applications that require high performance (sensitivity, reliability, responsivity, etc.). In this dissertation, state-of-the-art studies of MoS2 electronics are first introduced and surveyed. The electrical breakdown limit of MoS2 FETs is investigated because it determines the current carrying capability and failure modes, which are critical for integrated circuit applications. A completely-dry transfer method combined with vacuum thermal annealing is developed to fully harness the intrinsic properties of MoS2 without inducing residue on the surface. Then the mechanical properties and devices of MoS2 are presented. The first MoS2 nanomechanical resonator on a flexible PDMS substrate that is tolerant to a large amount of bending and straining is demonstrated, showing promise for flexible and foldable electronics. The temperature dependence of MoS2 resonators is also studied. Finally, the coupling of electrical and mechanical properties of MoS2 are explored using the first all-electrical readout of 1-, 2-, 3-layer MoS2 NEMS resonators, with the thickness confirmed with both Raman and photoluminescence (PL) characterization. The devices take the form of vibrating-channel transistors, with multimode resonances highly tunable by the gate voltage, which holds promises and intriguing potential for real-time sensing and signal processing applications.

Committee:

Philip Feng (Advisor); Christian Zorman (Committee Member); Hongping Zhao (Committee Member); Xuan Gao (Committee Member); Soumyajit Mandal (Committee Member)

Subjects:

Electrical Engineering; Nanotechnology

Keywords:

2D Semiconductor; MoS2; 2D NEMS Resonator; 2D Field-Effect Transistor; Electrical Readout; Optical Interferometry; Dry Transfer; Electromechanical Coupling

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

Cheiwchanchamnangij, TawinanApplications of the Quasiparticle Self-consistent GW Method
Doctor of Philosophy, Case Western Reserve University, 2014, Physics
In this dissertation, we present band structure studies of three types of materials, which are wurtzite GaAs, atomically thin MoS2, and highly correlated rare-earth nitrides, by using the quasiparticle self-consistent GW (QSGW) method. First, we report results for wurtzite GaAs, which is found to be stable and coexists with the zinc-blende crystal structure (more stable in bulk) in nanowires. We provide detailed band structure parameters, such as effective mass parameters, band gap, crystal field splitting, spin-orbit splitting, etc., of wurtzite GaAs. This information on the bulk band structure is needed for the study of the nanowire specific electronic states, which can be obtained within the envelope function or effective mass type theories. The strain effects on the band structure parameters are also studied, because a nanowire could be under strain due to surface tension and tension caused by the matching of the lattice constant between wurtzite and zinc-blende sections in the wire. The band structure parameters of more well known zinc-blende GaAs was also calculated in order to test the validity of our QSGW calculations. We also present the band structure of 4H GaAs as a guideline of how the band parameters might change when there is mixing between hexagonal structure (wurtzite) and cubic structure (zinc blende) in the same nanowire. Second, we present the results of bulk and atomically thin MoS2. Our QSGW results confirm the transition of the band gap nature from indirect gap to direct gap when the form of MoS2 changes from bulk to monolayer. However, the QSGW significantly overestimates the direct gaps at the K point of monolayer and bilayer MoS2 due to the very strong excitonic effect in this two dimensional material, which is not taken into account in the QSGW method. Therefore, we also estimated the exitonic effect by using the Mott-Wannier effective mass theory, and obtained a large ground state exciton binding energy for both monolayer and bilayer. Our final results of the transitions at the K point are in the very good agreement with the photoluminescence measurements. In addition, we also study the strain effect on the spin-orbit induced band structure splittings in the monolayer MoS2 and graphene, which provide an important piece of information for the study of spin scattering in these materials. Finally, the band structure studies of rare-earth nitrides, such as GdN, DyN, SmN, HoN, and YbN are presented. This group of materials is challenging in terms of the band structure calculations because of the highly correlated nature of open f shells. We found from our QSGW results that band splittings near the valence band maximum of these compounds are quite unique because they strongly depend on how they interact with the occupied 4f levels. We also show how good QSGW predicts the levels of the 4f states and the magnetic moments when compared to the experimental data.

Committee:

Walter Lambrecht (Advisor); Philip Taylor (Committee Member); Jie Shan (Committee Member); Clemens Burda (Committee Member)

Subjects:

Physics

Keywords:

QSGW;GaAs;MoS2;graphene;band structure;rare-earth nitride;nanowire;exciton

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

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

He, KeliangOptical Spectroscopy of Two-Dimensional Transition Metal Dichalcogenides (TMDCs)
Doctor of Philosophy, Case Western Reserve University, 2014, Physics
Atomically thin transition metal dichalcogenides (TMDCs), with the chemical stoichiometry MX2, where M is group VI transition metal atoms including Mo, W; and X is chalcogen atoms of S, Se and Te, have been emerging as a fast growing research field. Similar to graphene, a layered structure with weak van der Waals coupling between neighboring layers allows for them to be easily separated into single to few-layer thick samples. Different from graphene of zero band gap, they exhibit a band gap with magnitude between 1.5 and 2.0 eV and show current on-off ratios exceeding 108 in field-effect transistors. It has been discovered that an indirect to direct band-gap transition occurs in the monolayer limit at the K point of the Brillouin zone, which significantly enhances the photoluminescence quantum yield. Moreover, monolayer TMDCs are non-centrosymmetric arising from A and B sublattices being occupied by M and X atoms, respectively, which makes them an ideal laboratory to study valleytronics. The electronic structure of atomically thin TMDCs, mainly focusing on MoS2 and MoSe2, is investigated with optical absorption, photoluminescence and ultrafast pump-probe spectroscopic techniques in this thesis. First, we demonstrate continuous tuning of the electronic structure of atomically thin MoS2 on flexible substrates by applying a uniaxial tensile strain. A redshift at a rate of ~70 meV per percent applied strain for both mono- and bilayer MoS2 direct gap transitions, and at a rate 1.6 times larger for bilayer MoS2 indirect gap transitions, have been determined. Second, we investigate the optical response of monolayer MoS2 as a function of carrier density. Tightly bound negative trions, a quasi-particle composed of two electrons and a hole, are identified in doped monolayer MoS2. They possess a large binding energy (~ 20 meV), rendering trion effects significant even at room temperature. Third, we demonstrate that optical pumping with circularly polarized light can achieve complete dynamic valley polarization in monolayer MoS2. Moreover, we found that this polarization is retained for longer than 1 ns. Finally, preliminary results on the ultrafast dynamics in mono- and bilayer MoSe2 are presented. The photo-excited carrier dynamics is investigated via optical pump-probe technique.

Committee:

Jie Shan, Prof. (Committee Chair); Kenneth Singer , Prof. (Committee Member); Walter Lambrecht, Prof. (Committee Member); Philip Feng, Prof. (Committee Member)

Subjects:

Physics

Keywords:

Optical spectroscopy, two-dimensional crystal, Transition Metal Dichalcogenides, MoS2, MoSe2