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The behavior of fundamental particles, such as photons and electrons, and quasi-particles, including polaritons and excitons, can be influenced by their surrounding environments. The development and implementation of artificial materials characterized by their low-dimensional structures and nanometric features have facilitated the discovery of a diverse array of previously unanticipated physical phenomena. This thesis examines the control and manipulation of photons, electrons and polaritons at the nanoscale in order to realize novel optoelectronic functionalities. To do so, we focus on further developing two platforms: subwavelength-spaced arrays of nanostructures (metasurfaces) and 2D material to enhance device performance and functionalities. After motivating the use of nanoscale material patterning for wave and dispersion engineering, we highlight several emerging opportunities in electro-optical devices and functionalities. These opportunities range from shaping optical cavity modes with metasurfaces to structured light and 2D-material-based detectors and sources.

Over the past few decades, Moore’s Law has been a driving force behind the exponential increase in integrated circuits capabilities, significantly transforming people’s lives. However, the continuation of Moore’s Law is being called into question as the continuous shrinking of electric circuits is approaching physical limitations. Currently, the size of transistors and interconnects, two fundamental components in modern chips, has reached the tens of nanometers. Further downscaling leads to circuit delay, heat dissipation and poor performance of microchips, ultimately prohibiting Moore’s Law. One of the biggest challenges is the materials: conventional semiconductors and metals in transistors and interconnects exhibit undesirable resistivity increases when reduced to the nanoscale. Thus, the need for 2D semiconductors and better metals with fewer dimensional impacts has become increasingly urgent. This underscores the critical importance of understanding electronic transport in small system, which is the focus of this thesis. By utilizing first-principles calculations, the mobility and conductivity of 2D semiconductors and finite-size metals can be predicted, leading to physical insights and high-throughput screening for best candidates for electronic transport. This dissertation is organized as follows: Chapter 2 describes the Boltzmann transport equation (BTE) to study electronic transport in materials. Chapter 3 demonstrates how to obtain the transition rates, necessary for solving the BTE, from first principles. In particular, we developed an efficient approach to interpolate the electron-phonon-coupling matrix in 2D materials, which can also include the effect of free carrier screening. Based on the state-of-the-art calculations, Chapter 4 shows that reduced dimensionality results in a larger “density of scattering” in 2D semiconductors, leading to generally lower carrier mobility. We predict several high-mobility (>1400 cm²V⁻¹s⁻¹) 2D semiconductors with extremely small effective mass and/or weak EPC, thus avoiding the “dimensional curse”. Chapter 5 and 6 demonstrate the computational approaches to high-field transport properties and remote phonon scattering for 2D semiconductors, respectively. Finally, for metal in interconnects, we develop an ab initio approach to incorporating electron-phonon/surface scattering for electronic transport in finite-size metals and demonstrated it in copper films with different surfaces in Chapter 7.

Two-dimensional (2D) materials are a class of extraordinarily thin materials that are normally referred to being one or two atoms thick. Compared to their bulk counterparts, they would exhibit distinct unique physical properties due to the constrains of the size in one dimension. The remarkable advantages could be offered by 2D materials, such as the electrical, mechanical, and thermal properties, intriguing intense scientific research and great interests from various fields. Being the most well-studied 2D material of them all, graphene becomes a shining star in the family, providing a promising platform of quantum phenomena studies in a 2D regime. The later exploration of transition metal dichalcogenides (TMDs) also provides scientists new insight of the 2D devices applications in terms of the electronics and optoelectronics. Undoubtedly, the research of the novel 2D materials uncovers unprecedented advantages and provides one of the essential building blocks for next generation devices. This thesis centers itself among various exotic physics quantum phenomena and demonstrates several different approaches in terms of the quantum phenomena investigation in 2D material devices. One of the approaches is to change the materials band structure through twist and stacking. The moiré potential and the proximity effect between the layers in van der Waals structures could change the fillings of the states, leading to the emergence of strong correlation, generate charge transfer, therefore resulting into the change of the properties and observations of novel quantum phenomena. The second approach is to utilize the gate-defined structures to locally tune material band structure by changing the electrical potential or applying the displacement field. Due to the fine structure of the designed gate configurations, electronic states can be controlled within a nano-scale regime. The provided flexibility of the gate structures gives the possibility of creating, controlling, or modifying quantum phenomena, as well as achievement of the relevant applications.

Nanochannels with nanoscale characteristic dimensions have sparked broad interest across scientific and engineering disciplines. The emergence of a versatile family of two-dimensional (2D) materials enables high-throughput engineering of nanochannels with tunable layered structural and functional properties in the form of laminar membranes, which opens up new opportunities for applications in liquid molecular separation, gas separation, and ion sieving. Despite significant progress in 2D-material-based nanochannel, thorough understanding of this field is still lacking. In this thesis, I systematically and in-depth explored the mechanism of nanochannel modification on filtration. I also fabricated a series of 2D-material-based nanochannel membranes for water treatment. My research demonstrated that interlayer spacing plays a crucial role in determining the selectivity and permeability of nanofiltration membranes. Proper tuning gives these membranes high throughput, precise selectivity, and multiple uses. [...]

This dissertation examines the importance of open-sourced scientific instrumentation in two-dimensional (2D) material research. 2D materials are gaining attention due to their extraordinary electrical, mechanical, optical, and thermal properties and their potential to transform various fields. However, studying these materials often requires complex and expensive scientific instrumentation, which can limit the accessibility and progress of research. The study explores the potential of open-source software and hardware in scientific instrumentation, and its role in democratizing access, fostering collaboration, and accelerating innovation. The study presents two instruments implemented for 2D material research, using only open-source software and hardware, and demonstrates experiments conducted with these instruments. Additionally, the study explores broader implications of open-sourced scientific instrumentation by demonstrating a motorized variable filter stage and retractable leadless pacemaker. The dissertation concludes by emphasizing the critical role of open-source instruments in the advancement of material science and the broader scientific community and the need for ongoing support and engagement to fully realize their potential.

Since their invention in 1958, Integrated Circuits (ICs) have become increasingly more complex, sophisticated, and useful. As a result, they have worked their way into every aspect of our lives, for example: personal electronic devices, wearable electronics, biomedical sensors, autonomous driving cars, military and defense applications, and artificial intelligence, to name some areas of applications. These examples represent both collectively, and sometimes individually, multi-trillion-dollar markets. However, further development of ICs has been predicted to encounter a performance bottleneck as the mainstream silicon industry, approaches its physical limits. The state-of-the-art of today’s ICs technology will be soon below 3nm. At such a scale, the short channel effect and power consumption become the dominant factors impeding further development. To tackle the challenge, projected by the ITRS (International Technology Roadmap for Semiconductors) a thinner channel layer seems to be the most viable solution. This dissertation will discuss the feasibility of using 2D (two-dimensional) materials as the channel layer. The success of this work will lead to revolutionary breakthroughs by pushing silicon technology to the extreme physical limit. Starting from graphene in 2004, 2D materials have received a lot of attention associated with their distinct optical, electrical, magnetic, thermal, and mechanical properties. In the year 2010, IBM demonstrated a graphene-based field effect transistor with a cut-off frequency above 100 GHz. The major challenge of applying graphene in large-scale digital circuits is its lack of energy bandgap. Other than carbon, a variety of graphene-like 2D materials have been found in various material systems, like silicene, germanene, phosphorene, MoS2, WS2, MoSe2, HfS2, HfSe2, GaS, and InS, etc. Among all the 2D materials, silicene appears to be the most favored option due to its excellent compatibility with standard silicon technology. Similar to its counterpart graphene, silicene does not exhibit an opened energy band gap, which prevents its immediate applications in the mainstream semiconductor industry. In-plane tensile strain leads to the formation of a bandgap in bilayer silicene with low buckling. This dissertation conducts a study on energy bandgap opening in silicene and ultra-thin silicon films by engineering the strain and stress. Density functional theory has been employed to optimize the atomic structures and to calculate the energy band diagrams. Bandgap opening up to 0.17 eV has been found in the AA-stacked bilayer silicon film under in-plane strain 10.7%~15.4%. A number of common semiconductor substrates have been investigated to examine the possibility of growing bilayer silicon with excessive in-plane strain i.e., CdSe, InAs, GaSb, and AlSb. In addition, by taking into the account of magnetic spin polarization, energy bandgap 0.25 eV in AB-stacked bilayer silicon films have been observed. Without applying strain, the spin up and down state are degenerated. By applying the strain 7.56 %, the spin degeneration has been removed both in the conduction band and valence band. This opens the opportunity to build spin-based field effect transistor.

The 2D materials have drawn great interests since they have been exploited. Although they have excellent properties, the mass production has been a great challenge. To overcome this, a solution proceeded approach was introduced in 2018. The MoS2 ink is prepared by intercalating ammonia salt into the raw material and exfoliated the bulk materials into small thin flakes and get a suspension solution. This MoS2 ink can be easily spin-coated on to hydrophilic surface and form a uniform large-scaled semiconducting thin film with high performance. The thin films are made of flakes overlapped with each other by van der Waals force, so we call them van der Waals thin film (VDWTF). And my projects are main about the applications of the VDWTF. The first application is using the thin film as conventional transistors. And we manage to build an integrated pressure sensor array, by combining solution-processed two-dimensional (2D) MoS2 van der Waals (vdW) thin film transistor (TFT) active matrix and conductive micropyramidal pressure-sensitive rubber (PSR) electrodes made of polydimethylsiloxane/carbon nanotube composites, to achieve spatially revolved pressure mapping with excellent contrast and low power consumption. We demonstrate a 10 by 10 active matrix by using the 2D MoS2 vdW-TFTs with high on-off ratio, minimal hysteresis, and excellent device-to-device uniformity. The combination of the vdW-TFT active matrix with the highly uniform micropyramidal PSR electrodes creates an integrated pressure sensing array for spatially resolved pressure mapping. This study demonstrates that the solution-processed 2D vdW-TFTs offer a solution for active-matrix control of pressure sensor arrays, and could be extended for other active matrix arrays of electronic. Because the thin film MoS2 has a high light transmission rate, our thin film can applied for optoelectronic devices as transparent transistor. Here we merge our thin film transparent phototransistors (TPTs) with liquid crystal (LC) modulators to create an optoelectronic neuron array that allows self-amplitude modulation of spatially incoherent light, achieving a large nonlinear contrast over a broad spectrum at orders-of-magnitude lower intensity than most optical nonlinear materials. For a proof-of-concept demonstration, we fabricated 10,000 optoelectronic neurons, each serving as a nonlinear filter, and experimentally demonstrated an intelligent imaging system that uses their nonlinear response to significantly reduce input glares while retaining the weaker-intensity objects within the field of view of a cellphone camera. This nonlinear glare-reduction capability is important for various imaging applications, including autonomous driving, machine vision, and security cameras. Beyond imaging and sensing, this optoelectronic neuron array, with its nonlinear self-amplitude modulation for processing incoherent broadband light, might also find applications in optical computing, where nonlinear activation functions that can work under ambient light conditions are highly sought. In the next chapter, we demonstrate the application of the VDWTF as flexible electronics. The films feature a sliding and rotation degree of freedom among the staggered nanosheets to ensure mechanical stretchability and malleability, as well as a percolating network of nanochannels to endow permeability and breathability. With an excellent mechanical match to soft biological tissues, the freestanding films can naturally adapt to local surface topographies and seamlessly merge with living organisms with highly conformal interfaces, rendering living organisms with electronic functions, including leaf-gate and skin-gate transistors. On-skin transistors allow high-fidelity monitoring and local amplification of skin potentials and electrophysiological signals.

Microscopic engineering of material properties is crucial to advancing the technological needs in the age when development is pushing towards the ends of the Moore’s Law. The desire to engineer materials with properties best suited for intended applications has driven development of various modulation methods. Intercalation and the use of van der Waals materials emerged as a versatile property manipulation tool due to their unique atomic structure.In the first part of this dissertation, we developed a novel platform based on lithium ion batteries with ultrafast optical spectroscopy and electrochemical control in black phosphorus. Our investigation revealed a strong dependence in phonon-ion interactions. The thermal conductivity of black phosphorus is reversibly tunable from 2.45 to 3.86 W�m−1�K−1, 62.67 to 85.80 W�m−1�K−1, and 21.66 to 27.58 W�m−1�K−1 in the three principal directions. The phenomenon is attributed to phonon scattering introduced by intercalation. At the fully discharged state, thermal conductivity is reduced up to six times compared to the pristine crystal.Next, we modified the intercalation system to achieve reversible complementary doping. We produce a p-type and n-type transistor through lithium ion intercalation and copper zero-valent chemical intercalation. Interestingly, we found that intercalation can suppress the neutral impurity scattering from the environment, leading to enhancement of electron mobility in the system.We apply these findings in 2D systems to overcome ongoing challenges of multiplexable sensing in gas sensors. Due to the high noise levels in thin film sensors, alternate current analysis is seldom employed. We use bulk tin selenide crystal as sensing material in our sensors. The sensor-analyte interaction was investigated using electrochemical impedance spectroscopy coupled with an equivalent circuit model. We demonstrate that frequency-dependent phase signal signatures can detect methanol and ethanol within 10% accuracy and water vapor within 15% accuracy in gas mixtures.In addition, we investigated the electrical transport behavior of microshutters though investigation of the electric field effects surrounding the Next-Generation Microshutter Array. We find that that design modification to a bi-electrode design can significantly improve the electrostatic torque actuation, enabling reliable actuation at a reduced voltage difference of 70 V.

Transition-metal dichalcogenide (TMD) materials have promising properties that make them suitable for wearable bioelectronics and biosensors. Specifically, the Platinum based TMD materials, PtSe2 and PtTe2, can be fabricated at low temperatures, allowing for growth on flexible substrates, such as polyimide film. Exploration of piezoresistivity and piezoelectricity in PtSe2 and PtTe2 can enable more applications for the novel 2D material. This report focuses on the experimental setup and results for the verification of the piezoresistive and piezoelectric property in PtSe2 and PtTe2. The piezoresistivity experiment focuses on the change in resistance in the material due to strain induced by bending using a two-point bending fixture. In the piezoelectricity experiment, samples are put in a periodic strain at 2 Hz with the voltage response of the material being measured using a digital oscilloscope. An analysis of the experimental results is discussed along with a proposal for further applications for using piezoelectric materials as energy-harvesting elements of the next-generation bioelectronics.

Abstract: Single photon emitters hosted in hexagonal boron nitride have attracted significant attention over the past 5 years. This is in part due to their room temperature operation, chemical and physical stability, and high quantum efficiency. Continuous hexagonal boron nitride thin films (~ 10 – 20 nm thick) are of particular interest because of their potential to be integrated with existing silicon photonics devices. Additionally, there have been multiple reports of the deterministic activation of hBN emitters through the introduction of strain on the materials crystal lattice. This thesis describes the theory, fabrication, and characterization of hexagonal boron nitride coated nano-/micro-structures as a method of both deterministically activating defect centers as well as spatially aligning them to resonant antinodes in buckled microcavity devices. First, the design and construction of an epifluorescence spectroscope/microscope instrument crucial to the optical characterization of hBN emitters, is presented. A particular emphasis was placed on accounting for undesirable autofluorescence from microscope objectives. Next, a thin-film transfer technique for hexagonal boron nitride (hBN) multilayers was developed and refined. The technique was then used to transfer continuous hBN films to SiO2 substrates patterned with structures ~ 100 – 200 nm in height with lateral dimensions on the order of a few μm. Blanketing the film over nanoscale-height features introduces strain on the hBN crystal lattice near feature edges, causing naturally present defects to emit across much of the visible range. Notably, the brightness of emitting arrays was characterized and shown to be much greater than emission from unintentional wrinkles in the film. The transfer process was then used as a means of integrating hBN emitters inside recently demonstrated buckled microcavities, where two Bragg mirrors separated by a low-adhesion layer are thermally shocked resulting in the formation of a closed cavity structure due to the stress inherent to the thin films comprising the top mirror. Both the active and passive optical properties of the resulting devices were then measured. The scalability and deterministic placement provided by the hBN transfer process paired with the potential for cavity enhancement offers a promising platform for a real-world single photon source with broad applications in quantum information and computing.

Ultrathin materials will play a crucial role in next-generation electronic devices but defects in these materials are unavoidable. The effect of defects can permeate entire ultrathin layers and extend beyond interfaces into adjacent layers. Identifying and understanding atomic scale defects inside ultrathin electronic materials leads to new ways of controlling and eliminating defects to achieve higher performance devices and tune electronic and magnetic properties. Complex oxides and two-dimensional (2D) materials are ideal for creating ultrathin conducting layers. Both classes of materials exhibit a wide range of dielectric, optoelectronic and magnetic properties useful in a range of device applications. 2D materials can be isolated down to layers just a single atom thick leading to thinner and higher performance electronics. Virtually all applications of these materials involve interfaces with other materials which presents special challenges since defects near interfaces impact material properties on either side of the interface. Surface science techniques specialized at investigating ultrathin layers such as depth-resolved cathodoluminescence spectroscopy (DRCLS) and x-ray photoelectron spectroscopy (XPS) must be used to improve these materials. The van der Waals gap which allows individual 2D layers to be isolated from one another opens the opportunity for foreign particles such as water to reside between layers and impact electronic properties. Water sitting between layers of germanane, a graphene analogue, significantly impacts luminescence properties and points to an important effect that intercalants between layers need to be accounted for when creating heterostructures based using 2D materials. While 2D materials can be mechanically exfoliated and isolated in monolayers, better interfaces can be achieved if single 2D layers are deposited directly onto a substrate using techniques such as molecular beam epitaxy (MBE). However, conditions under which 2D materials are grown can significantly impact the interfacial properties between the 2D material and the substrate. We used XPS to study the effect of growth conditions on interface properties between MnSe2, a 2D ferromagnet and Bi2Se3, a topological insulator. The samples were transferred from MBE into XPS using a UHV suitcase to study pristine interfaces and prevent air exposure. Interfaces between complex oxides are also important such as the 2D electron gas (2DEG) that forms between two otherwise insulating complex oxides, LaAlO3 and SrTiO3. A complementary 2D hole gas (2DHG) was expected to form at these interfaces as well, but defect formation made observing that interface more challenging. Advanced growth designed to control defect formation and near-nanoscale resolved defect identification confirmed that minimizing oxygen vacancies was the key to realizing a conducting 2DHG. Cation vacancies and antisite substitutions can also play a large role in the physical properties of complex oxides and impact charge distributions extending tens of nanometers below interfaces. Identifying and controlling defects plays an important role in achieving advanced electronic features at complex oxide interfaces. DRCLS is a powerful tool to study such interfaces formed with ultrathin layers. Combining MBE with DRCLS and XPS using all UHV transfers is a powerful approach to create complex oxide interfaces and study the impact of nearby defects without external contamination.

In the first study, superconducting 2D NbSe₂ was examined towards its prototypical demonstration as a transition-edge sensor, where photoexcitation caused a thermodynamic phase transition in NbSe₂ from the superconducting state to the normal state. The efficacy of the optical absorption was found to depend on the wavelength of the incoming radiation used, which ranged from the ultra-violet (405 nm), visible (660 nm), to the infrared (1060 nm). In the second case involving WSe₂, the UV-ozone treatment revealed the presence of localized excitonic emission in 1L WSe₂ that was robust and long-lived. Our third material platform dealt with hybrid 0D-2D ensembles based on graphene and WSe₂, specifically graphene–endohedral, WSe₂–fullerene (C₆₀), and WSe₂–Au nanoparticles, and exhibited exceptional performance gains achieved with both types of hybrid structures. Next, we investigated WSe₂ based mixed dimensional hybrids. Temperature T-dependent and wavelength λ-dependent optoelectronic transport measurements showed a shift in the spectral response of 1L WSe₂ towards the SPR peak locations of Au-Sp and Au-BP, fostered through the plexciton interactions. Models for the plexcitonic interactions are proposed that provide a framework for explaining the photoexcited hot charge carrier injection from AuNPs to WSe₂ and its influence on the carrier dynamics in these hybrid systems. Last, we studied interactions of vdWs hybrid structures composed of WSe₂ with 0D buckminsterfullerene (C₆₀) spheres. Our results indicate that the C₆₀-WSe₂ vdWs hybrid heterostructure appears to be an attractive architecture for enabling charge transfer and high performance photodetection capabilities. T-dependent electrical transport measurements after C₆₀ deposition revealed a dominant p-type conduction behavior and a significant ×10³ increase in WSe₂ field-effect mobility, with a maximum field-effect mobility of 281 cm²V⁻¹s⁻¹ achieved at 350 K and room-T mobility of 119.9 cm²V⁻¹s⁻¹ for the C₆₀-WSe₂ hybrid.

A novel controlled phase gate for photonic quantum computing is proposed by exploiting the powerful nonlinear optical responses of atomically thin transition metal dichalcogenides (TMDs) and it is shown that such a gate could elicit a π-rad phase shift in the outgoing electric field only in the case of two incident photons and no other cases. Firstly, the motivation for such a gate is developed and then the implementation of monolayer TMDs is presented as a solution to previous realization challenges. The single-mode case of incident photons upon a TMD is derived and is then used to constrain the more general multimode case, where the probability of producing a nonlinear response is approximated and evaluated for the tuning of various physical parameters within the system. The implications of the variability of these parameters are then discussed.

Flexible electronics, as an emerging field, has gained tremendous attention over the past few years with the advancement in areas such as two-dimensional (2D) materials fabrication and transfer printing techniques. Mechanical peeling, where thin film materials or printed patterns are transferred from a donor substrate to a target substrate, is demonstrated to be a promising and key operation to transfer 2D material or fabricating complex flexible electronic devices, and it is shown that in many cases performing the mechanical peeling process at a desired peeling condition is needed for high quality product. However, despite the promise of mechanical peeling techniques, studies on scaling up the process and enabling the process in a high-volume manufacturing setting are lacking. In this study, a R2R mechanical peeling process that enables continuous and high-throughput transfer of 2D materials and printed electronics has been developed. The system allows for speed and tension control and is used for performing 2D materials transfer. A system model of the R2R process is derived by integrating the peeling process with the R2R system dynamics. The proposed model provides fundamental understanding of the physics involved in the process, including the interactions between peeling front and the roller dynamics. The model can be effectively used for control design and simulation. The success of the R2R peeling process is dependent on the peeling front geometry and its stability during the peeling process. A real-time supervisory control strategy is developed to enable the peeling angle control in a R2R mechanical peeling process. The supervisory control strategy utilizes a peeling front model to estimate the peeling speed and the adhesion energy between the donor substrate and the material to be transferred. The information is used to generate reference signals for the web tension controllers at the regulatory level to adjust the web tensions in order to achieve desired peeling angles. The proposed control strategy is demonstrated with both simulation and experimental results. To reject periodic disturbances originating from rotating roller shafts and adhesion energy changes in the laminate, a model-based repetitive controller is developed and demonstrated.

The ability to rationally design materials with specific properties is a major motivator in materials science and condensed matter physics. Heterostructures of two dimensional (2D) materials, weakly bonded out of plane via van der Waals forces, offer a versatile platform for creating materials by stacking old materials in new ways. In this thesis, we focus on a facet of that endeavor that has been growing considerably over the past decade: the effect of moiré patterns that form when two lattices are nearly, but not quite, aligned. The van der Waals forces are weak enough that materials are free to sit on each other with incommensurate alignment, due to twist, lattice constant mismatch, or strain. However, the van der Waals forces do serve to create local commensuration, leading to the formation of domains and dislocations at the interface. We use dark field transmission electron microscopy (TEM) to study the resulting structures, and connect to their unique electronic properties. We investigate the atomic and electronic structure of twisted bilayer graphene near the small and large angle limits of 0° and 30°. We then move beyond twist (seeing how strain produces moiré patterns of its own), beyond graphene (to sublattice-asymmetric materials that can host ferroelectric domains), and finally, beyond bilayer (to twisted trilayer and quadrilayer), getting a view of how extensive the space of moiré-based materials is.

In this work, we have explored 2D semiconducting transition metal dichalcogenides (TMDs), black phosphorus (BP), and graphene for various applications using liquid and mechanical exfoliation routes. The topical areas of interest that motivate our work include considering factors such as device integration, stability, doping, and the effect of gasses to modulate the electronic transport characteristics of the underlying 2D materials. In the first area, we have integrated solution-processed transparent conducting oxides (TCOs), specifically indium-doped tin oxide (ITO) with BP, which is a commonly used TCO for solar cell devices. Here we have found surface treatment of glass substrates with a plasma before spin-coating the solution-processed ITO, to be effective in improving coverage and uniformity of the ITO film by promoting wettability and film adhesion. The maximum transmittance obtained was measured to be ~75% in the visible region, while electrical measurements made on BP/ITO heterostructures showed improved transport characteristics compared to the bare ITO film. Within the integration realm, inkjet-printing of BP and MoS2 p-n hetero-junctions on standard ITO glass substrates in a vertical architecture was also demonstrated. To address the issue of stability which some 2D materials such as BP face, we experimented with ionic liquids (ILs) to passivation the hydrophilic surface of BP to minimize its oxidative degradation. The enhanced stability of BP was inferred through Raman spectroscopy and scanning probe microscopy techniques, where no observable changes in the A1g and A2g Raman vibrational modes were observed for the BP films passivated with ILs over time under ambient conditions. On the other hand, a blue-shift in these Raman modes was evident for unpassivated samples. Atomic force microscopy measurements on the unpassivated samples clearly revealed the difference in surface characteristics through localized regions of degradation that intensified with time which was absent in IL passivated BP samples. The electronic device measurements for IL coated BP devices showed a more stabilized Ids−Vds characteristic in the 5.4 K to 335 K temperature range. Prototypical demonstrations of stabilized ILs/BP devices at ambient printed on flexible polyimide substrates were also successfully made. At the same time, doping is one of the essential steps required for the modulation of carrier density and electronic transport in electronic and optoelectronic devices, which is the third topical area we have addressed in this work with semiconducting TMDs. Of the conventional approaches used to dope 3D semiconductors, ion-implantation is commonly adopted but given the ultra-thin nature of 2D materials, this approach is not feasible as it causes severe damage to the delicate crystalline lattice of ultra-thin 2D membranes. Instead, we have used plasma-based doping routes with UV-ozone treatement and solution processing using 1,2 dichloroethane, to characterize the temperature-dependent two-terminal and three-terminal electronic and optoelectronic transport of mechanically exfoliated 2D MoS2 and WSe2. A significant difference was seen in the optoelectronic properties between the two dopants, owing to differences in their respective doping mechanisms and the intrinsic structural attributes of the exfoliated flakes. A significant reduction in barrier height was evident after doping using both techniques in MoS2, while an increase in barrier height after soaking in 1,2 dichloroethane was seen in WSe2. Lastly, in the fourth topical area for sensing devices, we have studied the effect of gas-flow in inkjet-printed and spin-coated graphene and MoS2 to modulate the electronic transport for the 2D materials since their increased surface area is an ideal platform to observe interactions with external stimuli, in this case, in-coming gas species. Here, the chamber pressure and change in current with flow of gas was measured in the steady-state, as well as time-dependent dynamic transport toward nitrogen and carbon dioxide. We observed significant differences in the electrical response of mono-elemental graphene and binary MoS2, owing to differences in microstructure and joule heating response to the ambient gas. In conclusion, the findings obtained from our work will provide an important framework to help guide strategies in further improving integration schemes, stability, doping and sensing behavior driven by the unique structural attributes inherent to 2D materials for high-performance devices in the future.

In this work, synthesis of combinatorial library of CuxNi1-x (copper nickel) alloy thin films via co-sputtering from Cu (copper) and Ni (nickel) targets as catalysts for chemical vapor deposition (CVD) growth of graphene is reported. The gradient alloy morphology, composition and microstructure were characterized via scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD), respectively. Subsequently, the CuxNi1-x alloy thin films were used to grow graphene in a CH4-Ar-H2 (methane-argon-hydrogen) ambient in thermal CVD tube furnace. The underlying rationale is to adjust the CuxNi1-x alloy carbon solubility at a fixed temperature (~1000 oC) to control the graphene layer number as the solubility limit of carbon in Cu is approximately 75 +/- 0.5 ppm and C in Ni is 1.3 at.%. The energy dispersive x-ray spectroscopy (EDS) analysis revealed that a continuous gradient of CuxNi1-x (25.29%68.95%, bilayer growth dominates from 47.56%=3) growth occurs for x Finally, we show large area bi-layer graphene can be grown via the thin film catalyst with optimized catalyst composition.

In recent years, two-dimensional (2D) van der Waals (vdW) materials have aroused much interest for their unique structural, thermal, optical, and electronic properties and have become a hot topic in condensed matter physics and material science. Many research methods, including scanning tunneling microscopy (STM), transmission electron microscopy (TEM), optical and transport measurements, have been used to investigate these unique properties. Among them, STM stands out as a powerful characterization tool with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. This dissertation focuses on scanning tunneling microscopy and spectroscopy (STM/S) investigation of 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on the continuous interface in WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer agree with previously reported values for MoS2 monolayer and MoS2/WS2 heterobilayer on SiO2 fabricated through the mechanical exfoliation method. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers with thickness ranging from single to ~20 layers stabilized at the beta phase with a superstructure at room temperature. After cooling down to around 180 K, the beta phase converted to a more stable beta' phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven beta-to-beta' phase transformation was investigated by temperature dependent transmission electron microscopy and Raman spectroscopy, combined with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. STS measurements show that the indirect bandgap of monolayer beta' In2Se3 is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials and shed light on their numerous potential applications like non-volatile memory devices. The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in two-dimensional (2D) ferroelectric beta' In2Se3 is visualized with scanning tunneling microscopy and spectroscopy (STM/S) combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.

In a world where modern micro- and nanoelectronics are reaching their limits, aspotlight has been focused on novel two-dimensional and layered materials. It iscritically important to understand the electronic transport properties of potentiallyuseful semiconductors in order to gauge their applicability for electronic devices. Thisdissertation explores layered semiconductor materials and characterizes their gatetransport properties within the context of field effect transistor devices. One projectexamines MoO3 and its versatility as a layered gate dielectric material integrated ontoa WSe2 field effect transistor. The following project concentrates on the anisotropicand ambipolar transport properties of ReSe2. Lastly, the subthreshold regime of thegate characteristic curve is explored for ambipolar devices, illustrating how to tunethis behavior in WSe2 devices with respect to different contact metals. Overall, theelectronic characterization of these materials will aid in developing new, functionalnano-scale devices.

Atomically thin two-dimensional (2D) materials come in all necessary flavors to make semiconductor devices: conductors, semiconductors, and insulators. Graphene, transition metal dichalcogenides (TMDCs), and hexagonal boron nitride (hBN) are the quintessential building blocks. The van der Waals nature of the bonds in 2D films allows the ability to stack materials to achieve novel properties because of their exceptional mechanical, electronic, and optical properties and interactions, which enables various applications of 2D materials in transistors, biosensors, light-emitting devices, and photodetectors. Spectroscopic measurements such as Raman and photoluminescence (PL) reveal a wealth of information since 2D materials are affected by their environment and other local perturbations, e.g., strain and charge doping. My research focused on developing efficient and noninvasive optical methods to evaluate and characterize the properties of 2D materials. In particular, we investigated strain-tunable properties, the effects and signature of charge doping, and the environmental screening properties of graphene and TMDCs. Identifying the charge density and impurities in graphene is vital for graphene-based applications, which require high-quality graphene. I developed an effective optical method to determine the doping level and the local charge density variations in graphene before any fabrication process. This method differentiates charge density variations in graphene via the Raman 2D peak asymmetry that manifests at low charge 1-25 × 1010 cm-2. We explore the effect of charge inhomogeneity, "charge puddles", within the laser spot using simulated Raman 2D spectra, revealing a different signature for large or small charge puddles. Our work provides a simple and noninvasive optical method for estimating the doping level, local charge density variation, and transport properties of graphene, with up to two orders of magnitude higher precision than previously reported optical methods. Strain is another crucial factor that significantly impacts the properties of 2D materials. We studied the charge distribution and radiative efficiency of excitonic complexes in strained monolayer TMDCs, especially WSe2. Straining and electrostatic gating are combined to investigate the dynamics of quasi-particles in WSe2. We found that negative trions accumulate while positive trion emission is near zero, indicating that both conduction and valence bands are bent downwards in the strained area. Finite element analysis of strain distribution and density functional theory calculations of band structures of WSe2 support the experimental results. Hence, localized strain allows locally separating electrons and holes in WSe2 and manipulating light-matter interaction for applications in novel strained-engineered optoelectronics. I applied machine learning and deep learning techniques to improve the efficiency and accuracy of data processing and analysis since traditional methods require domain expertise and have the potential to introduce artifacts. I categorized the wealth of information and data by applying machine learning to spectroscopic information to separate different influences, e.g., strain, charge doping, and dielectric environment. We developed deep learning models to classify graphene Raman spectra according to different charge densities and dielectric environments. To improve the accuracy and generalization of all models, we use data augmentation through additive noise and peak shifting. Using a convolutional neural net (CNN) model, we demonstrated the spectra classification with 99% accuracy. Our approach has the potential for fast and reliable estimation of graphene doping levels and dielectric environments. The proposed model paves the way for achieving efficient analytical tools to evaluate the properties of graphene.

Semiconductor photonic devices enable efficient optical communication using light for fast transmission and detection of information. Conventional photonic devices primarily use group III-V semiconductors. However, these devices have limited integrability with silicon-based circuits, are bulky, and it is challenging to engineer their fundamental properties. Van der Waals (vdW) semiconductors are good candidates for the next generation of devices because they are atomically thin and feature strong emission, making them easy to integrate and efficient. Furthermore, they enable engineering of optical, electrical, and structural properties through formation of heterostructures and moir'{e} superlattices. We design and create novel photonic devices by leveraging the optical tunability of vdW semiconductors. Our transferrable distributed Bragg reflector (DBR) cavity has a high quality factor, can strongly couple with vdW materials, and constructed using cavity fabrication methods that are robust and reliable. The DBR cavity coupled with hexagonal boron nitride (hBN) encapsulated monolayer MoSe$_{2}$ forms strongly coupled exciton-polaritons. To further study photonic devices with vdW heterobilayers, we design and create the first WSe$_{2}$/MoSe$_{2}$ heterostructure laser and perform rigorous power dependence and first-order spatial coherence measurements to ensure lasing. The device shows a non-linear increase of the output intensity, linewidth narrowing, and a formation of extended spatial coherence, signifying lasing. Lastly, we study electrically controlled and cavity coupled WS$_{2}$}/MoSe$_{2}$ hybrid excitons to better understand the exciton-cavity coupling mechanism. Together, this improved understanding of vdW cavities and heterostructures sets the stage for designable photonic technologies and allows exploration of novel physics.

Light emission in the visible and infrared spectral domains is widely utilized in telecommunication and on-chip signal processing. However, further advances in the field of optical communications require the development of light sources for photonic integrated circuits. A two-dimensional materials platform enables the fabrication of on-chip compatible light-emitting devices that additionally benefit from the unique physical properties of atomically thin crystals. In this work, we present novel light-emitting devices based on a vertical assembly of two single-layer graphene electrodes separated by a few-layer hexagonal boron nitride crystal, referred to as Gr/h-BN/Gr tunnel junctions. Exploiting the Dirac cone band structure of graphene, we demonstrate twist-controlled resonant light emission from Gr/h-BN/Gr tunnel junctions driven by photon-assisted inelastic electron tunneling. Nearly aligned devices exhibit pronounced resonance features in both optical and electrical characteristics that vanish rapidly for twist angles θ ≳ 3°. While the position of the resonance peak in the current-voltage characteristics is defined by the twist angle between the two graphene electrodes and cannot be altered, the position of the photon emission peak can be spectrally tuned by means of applied bias voltage. In contrast, misaligned devices exhibit a broad emission spectrum without any resonance signature. These experimental findings can be well explained by a theoretical model based on the transfer Hamiltonian formalism, in which the resonant behavior at small twist angles is attributed to momentum conserving cone-to-cone tunneling. The spectral tunability of the photon emission combined with the uniform light intensity distribution across the entire overlap area between graphene electrodes makes twist-controlled Gr/h-BN/Gr light-emitting tunnel junctions attractive candidates for on-chip optoelectronics.

Can graphene electronic states be probed optically? Being a 2D semimetal, graphene has mostly been investigated with electronic transport techniques. In this work, we explore excitonic states of layered semiconducting transition metal dichalcogenides (TMDs) as sensors for strongly correlated electronic states in graphene. The last two decades have seen a gold rush in research of 2D materials, leading to the development of layered heterostructures allowing for nearly arbitrary combinations of materials with atomic control over the thickness. In such heterostructures, the physics of different materials can be combined to yield new emerging electronic phases. In our explorations, we use TMD excitons, graphene integer and fractional quantum Hall states, and moiré potentials as building blocks to explore new physics in our optical experiments. In a first step, we use neutral Rydberg excitons as sensors for the compressibility of proximal graphene. We demonstrate excitonic sensing of graphene fractional quantum hall states. More generally, we investigate the interactions between excitons and these strongly correlated states. Finally, we use excitons to study the charging diagram of two proximity-coupled Fermi seas under the influence of a moiré potential. We observe charge transfer steps which we tentatively assign to the formation of Mott-Wigner states in the TMD, induced via Coulomb interactions.

The ability for 2D membranes to separate water isotopes have been largely unexplored. Recently, 2D membranes made of stacked/overlapping graphene oxide (G-O) sheets have been used for isotopic water filtration for deuterium (D), while the effect for oxygen-18 (18O) remains untested. The instability of G-O membranes is one barrier for their use in water filtration. This thesis presents a thorough investigation of G-O-based membranes in water filtration with the objective of enriching D and 18O isotopes in water. The partial decomposition of G-O has been a used to control the swelling of G-O membranes in water. A one-step photochemical method using a photocatalyst, I-907, was developed to reduce G-O and decorate them with Co3O4 nanoparticles under UV irradiation. An in-depth study of metal-cation-modified G-O membranes were conducted. Metal cations with a high electronegativity and a small hydration radius had a greater binding strength which were better for constraining the G-O laminate channels. The membranes were evaluated in water vapour permeation and pervaporation, and the results revealed that for metal cations of the same valency, the relationship between permeation and interlayer spacing was positive. Amongst the different valence states, steric exclusion from cations is dominant for minimal water layers, whereas the ability for cations to attract and hold water molecules inside their outer shell has a greater affect on swollen channels. Water isotope separation was evaluated using pervaporation for G-O membranes of various thicknesses, which is controlled by a combination of the vapour pressure isotope effect (VPIE) and the membrane diffusion effect. The water isotope enrichment produced from the G-O membranes was lower or similar to that obtained from theoretical and membraneless vaporisation. Dead-end filtration with G-O membranes and UV-irradiated G-O membranes were employed to isolate the VPIE from the membrane effect. Water isotope separation was found to be caused by isotope diffusivity (molecular diffusion and adsorption separation), and adsorption separation is the dominant mechanism for improvements in D and 18O enrichment. The findings of this thesis imply that 18O water isotope enrichment via G-O membranes is achievable and suggests modifications to G-O membranes for improving water isotope separation. A liquid- to gas-phase membrane process will benefit future studies on 2D membranes for water isotope separation.

Spatially indirect excitons (IXs), also known as interlayer excitons, are bound pairs of an electron and a hole in spatially separated layers. IXs can propagate over long distances before they recombine into light, and they can cool down below the temperature of quantum degeneracy within their lifetimes, which can be controlled by gate voltage up to microseconds and beyond. These properties make IXs a promising platform for studying fundamental physics phenomena and as the medium for highly efficient signal processing devices. IXs were originally studied in gallium arsenide (GaAs) heterostructures, where IXs have shown evidence for Bose-Einstein condensation, and proof of principle has been demonstrated for excitonic transistors and excitonic integrated circuits. IXs only exist at low temperatures in GaAs systems due to the low IX binding energy on the order of 10 meV. In the transition-metal dichalcogenide (TMD) heterostructure system, the IX binding energy is predicted to be more than two orders of magnitude higher, making IXs stable at room temperature and allowing for the possibility of high temperature IX superfluidity. To date, observation of some of the key IX behaviors, namely the long-range IX transport and evidence of IX condensation, has remained elusive in the TMD system. This dissertation characterizes the IX spectrum in a MoSe2/WSe2 heterostructure, demonstrates the realization and control of long-range IX propagation using a new mechanism beyond the know mechanism for IX control in GaAs heterostructures, and separately identifies a quantum origin for the propagation of IXs generated by resonant excitation in the TMD heterostructure.

The discovery of one-atom thick graphene layer has marked the starting era of two-dimensional (2D) van der Waals (vdW) materials. Since then, various electronic properties, such as semi-metal, semiconductor, insulator, superconductor, and magnet, have been discovered with materials in the 2D form. Recently, with the rapid development in device fabrication, artificial superlattices can be created using building blocks of 2D materials. This offers researchers novel control over materials to create novel quantum states which do not exist in their bulk parent. These emergent materials offer better tunability compared with their bulk relatives, such as carrier density tuning, band structure and topology engineering.One seminal work is the observation of unexpected strong correlation effect and superconductivity in ~1.1° twisted bilayer graphene in 2018. A rising research field, moiré physics, has emerged as a platform for quantum simulation. Flat electronic bands can form in these twisted 2D samples, which can give birth to various interesting quantum phenomena, for example, superconductivity, Mott insulators, ferromagnetism, and quantum anomalous hall effect. In this thesis, I will summary our efforts in studying moiré physics using microwave impedance microscopy (MIM), including direct visualization of the moiré pattern in graphene-based heterostructures and observation of correlated insulating states at fractional carrier fillings in a WSe2/WS2 moiré superlattice. Our results demonstrate MIM as a powerful tool towards exploring emergent quantum states in such semiconducting moiré superlattices. We further explore correlated electronic states and moiré exciton in the TMD moiré system with layer degree of freedom. A robust excitonic insulator has been observed in 1L/2L WSe2/WS2 moiré superlattice with the population of correlated electron-hole pairs in different WSe2 layers. Another highly pursued topic is searching for intrinsic magnetic topological insulators (TI) where topology meets magnetism. Together with transport measurements and magnetic characterization, we perform MIM study on MnBi2Te4 thin flakes which is predicted as an intrinsic magnetic TI. This material provides a good platform to study the interplay between magnetic state and topological order in this material. The band crossing effect together with the emergence of topological edge states are observed during the magnetic-field driven topological phase transition. Our results highlight the importance of combination of multi-modal probes to determine the magnetic state, topological order, and bulk electronic property in a magnetic topological insulator.

The recent discoveries of ferroic properties, such as ferromagnetism and ferroelectricity, in two-dimensional van der Waals (2D vdW) materials have generated a surge of interest in the exploration of fundamental interaction in reduced dimensions and functional device applications. On one hand, ferromagnetism in 2D vdW crystals enables better understanding of fundamental interactions in ultra-thin condensed matter systems. However, existing material candidates face limitations, such as the lack of room-temperature magnetic ordering. Here we report the room-temperature ferromagnetism in Co-doped graphene-like ZnO, a novel chemically stable 2D material, down to single-atom thickness. Through magneto-optic Kerr effect and superconducting quantum interference device measurements, we identify the spontaneous magnetization in such systems at room temperature and above. X-ray characterization shows that the Co atoms form Co2+ states in graphitic ZnO. By varying the Co doping level, we observe an exotic manipulation among paramagnetic, ferromagnetic, and disordered phases. Our development of Co-doped 2D ZnO opens an appealing path to 2D ferromagnetism persisting at room temperature with unique controllability. On the other hand, the stabilization of 2D ferroelectricity overcomes the thickness limit. In contrast with conventional 3D ferroelectrics, 2D ferroelectrics show great advantages, including but not limited to, high mobility, perfect interface, allowing for non-destructive reading, being fatigueless. In this dissertation, we report the physical vapor deposition (PVD) of 2D vdW SnSe directly onto the silicon substrate. The synthesis process occurs at relatively low temperatures; the deposition temperature is 250 ℃. By varying the time of synthesis, we are able to manipulate the thickness of SnSe single crystals, which spans in a large range from 6 nm to the bulk. Out-of-plane ferroelectricity is detected in SnSe at room temperature (300 K) by means of piezoresponse force microscopy (PFM). More importantly, the ferroelectric switching voltage scales linearly with the sample thickness. A 6-nm-thick SnSe sheet displays sub-0.3 V switching, which is substantially smaller than those of previously reported 2D ferroelectrics and hafnium oxides. Moreover, the ferroelectric mechanisms are unraveled by transmission electron microscopy (TEM); AA stacking of vdW layers and interlayer sliding give rise to the vertical ferroelectric polarization. The ferroelectric dipole, as a new degree of freedom, can effectively manipulate the optical and electrical behaviors in SnSe. These results indicate that 2D SnSe is a promising candidate for future energy-efficient and non-volatile electronics or optoelectronics, which is highly compatible with the emergent complementary metal oxide semiconductor (CMOS) technologies.

Electronic materials represent a vast category with wide-ranging properties. Though definitionally, their electronic properties are of interest, study of their thermal properties provides additional important information. Whether to find materials with desirable thermal conductivity—high to dissipate heat or low to limit heat transport—or because the mechanisms underlying heat transport provide insight into the fundamental physics of the material, thermal characterization allows better understanding of such materials.In the study of the thermal properties of materials, optical pump-probe methods are a key tool. Laser-based measurements allow probing of small areas without requiring microfabrication, and the use of pulsed lasers and modulated beams allows measurement of high-speed effects and so small sample areas. Time domain thermoreflectivity (TDTR), which uses modulated pulsed laser beams, stands as a key tool for measurement of the thermal properties of both bulk and thin film samples. This work seeks to provide a detailed introduction to the technique and the mathematical analysis required for such measurements, apply TDTR to interesting materials systems, and push beyond the limits of traditional TDTR with the development of a transducerless time domain reflectance (tTDR) technique which uses the same equipment.Study of thermal properties can provide new insights into the fundamental properties of materials. Metallic vanadium dioxide nanobeams have a much lower thermal conductivity than would be expected given their electrical conductivity, implying that electrons are more effective at carrying charge than heat. This result is not always seen in other sample geometries, and TDTR measurements allow characterization of thin film samples, allowing further study. Unfortunately, the samples measured in this work did not provide high enough electrical conductivity to draw conclusions about electronic thermal transport, but this stands as an interesting line of investigation. One category of electronic materials of increasing interest is that of two-dimensional materials, whose ultra-thin nature offers new properties and applications. However, it also makes study of their thermal properties challenging and makes traditional TDTR infeasible. By contrast, tTDR is well suited to characterize their properties, and in this work initial measurements were made on suspended molybdenum disulfide. In addition to the fundamental properties of the materials, other effects can be engineered. For example, creating bilayers with twists between layers causes the development of moiré patterns which have novel properties. Their in-plane thermal conductivities are not well characterized, and tTDR provides an avenue for changing that. Preparation of such samples is very challenging, thus far preventing systematic study as a function of twist angle, but initial measurements demonstrated that tTDR is an applicable tool for such systems. Time domain reflectance measurements, both in the form of TDTR and tTDR, are powerful tools for the characterization of a wide range of materials, including many electronic materials. This work seeks to offer insight into and expand their applications.

Low-dimensional materials including carbon nanotubes (CNTs), graphene, transition metal carbides and nitrides (MXenes), transition metal dichalcogenides (TMDs) and beyond have recently become the focus of intense research because of their outstanding physical and chemical properties. Amongst those characteristics, the mechanical properties of the low-dimensional systems play a significant role in exploring their potential applications in industrial sectors. Nevertheless, the mechanical characteristics of the low-dimensional material systems have not yet been fully understood. This is because direct experimental measurements of mechanical properties of these materials face great difficulties due to the critical availability of the high-quality crystals and sophisticated facilities. To this end, the numerical methods become a promising alternative for investigating such properties of the low-dimensional materials. Firstly, this thesis presents the background and objectives of this project (Chapter 1). The recent progresses in numerically exploring the in-plane mechanical properties of low-dimensional material systems, including first-principles density functional theory (DFT), force-field based classical molecular dynamics (MD), and the finite element method (FEM) are then discussed. Some recent cases have been discussed to show the advantages and disadvantages of these multiscale simulation methods (Chapter 2). Since carbon-based materials are considered to be one of the most important family of low-dimensional materials, a comprehensive numerical study was conducted to investigate the mechanical properties (e.g. critical buckling load and vibrational properties) of carbon-based structures, such as carbon nanotubes (CNTs) and their modifications. The FEM was employed because it is able to analyse the systems consisting thousands of atoms (Chapter 3), and investigate macroscopic mechanical properties. Afterward, the mechanical properties of molybdenum disulfides (MoS2), as one of the most important TMDs, were examined by means of DFT calculations. The impacts of the structural polytypes, as well as external pressure on the mechanical response of the layer structured MoS2 were also investigated (Chapter 4). In addition, a comprehensive study was conducted to explore the energy-dependent anisotropic mechanical properties of different types of bulk TMDs using the first principles DFT calculations. Different analyses such as Density of States (DOS) and crystal orbital Hamilton population (COHP) were performed to fully understand the behaviour of the layer-structured systems (Chapter 5). Investigating the in-plane mechanical properties of monolayer two-dimensional (2D) materials was also one of the main objectives of this research. Hence, the mechanical characteristics of the lateral TMDs were thoroughly examined with consideration of the impact of the heterostructure configurations on the stability of the 2D systems (Chapter 6). Finally, the recently discovered MXenes were numerically investigated, and the mechanical properties of the functionalized systems were systematically explored. The DFT results reveals that the studied MXenes exhibit significant mechanical strength, which in-plane Young’s moduli are even larger than that of graphene. It may open the avenue for promising applications of such novel 2D materials (Chapter 7). It is worthy to note that the studies on the mechanical properties of 2D materials are still in early stages. As such, it is vital that future studies focus on exploring plausible numerical approaches for engineering the mechanical characteristics and eventually expanding the application of the low-dimensional materials.

Graphene has attracted enormous attention due to its unique characteristics. However, the LPCVD graphene grown on copper turns out to be polycrystalline because of the high nucleation density (ND) on the copper foil surface. In order to realize better quality LPCVD graphene, this ND needs to be significantly reduced. Based on the observations from our initial graphene growths on as-received copper, we figured that the uneven Cu surfaces with defects produce large NDs. At a large ND, the graphene flakes nucleated at different sites coalesced to produce polycrystalline graphene. Due to such issues, we have implemented an electropolishing technique to smoothen the native surface of the copper foil. We will discuss the successful implementation of the surface smoothening process to reduce nucleation site formation while limiting the surface defects (which leads to wrinkle formation). The annealing process was also helpful to flatten the surface during the growth process further. We have also observed that graphene grows across Cu grain boundaries and, in the process, produces an additional surface area for graphene growth. That later causes to form wrinkles, which affect graphene properties negatively. In the next project, the effect of multi-step copper surface oxidization, base pressure vacuum in the middle of the process, and integration of Cu enclosures on suppressing the ND will be discussed. The technique is based on the self-cleaning characteristics of copper oxides and the metal evaporation in a high vacuum at high temperatures. The ND has reduced to ~5 nucleation/cm2 on average (an improvement compared to the previously reported minimum value, ten nucleation/cm2 which was obtained using copper enclosures), and the graphene/copper surface has become smoother. The self-aligned graphene island geometry and shape of the flakes have reflected the symmetry and the single crystallinity of graphene. The final project will discuss the growth of cm-scale graphene flakes on Cu and 3D-multilayered graphene on 3D-Ni foams and used Ni's gettering carbon diffusion effect to make the Cu foil carbon-free. The Ni-foam/Cu enclosure was oxidized in situ to assist with the self-cleaning process of metal oxides. The ND has been reduced to ~0.57 nucleation/cm2 and obtained cm-scale graphene flakes.

The isolation of graphene has led to a surge of interest in other 2D materials, primarily layered semiconducting materials that can be utilized in a wide range of applications including but not limited to: optoelectronics, sensing, low-power transistors and flexible electronics. One family of 2D materials that has garnered significant interest is transition metal dichalcogenides. Currently the mechanical exfoliation of these materials is the preferred method for device fabrication. Numerous approaches have been employed to grow crystalline thin films of these materials. Despite the progress that has been made, it is still challenging to achieve growth of defect free, large-area uniform thin films. Before wide-scale application of these materials can be achieved some key issue such as monolayer controllability still need to be further investigated. Here we present our investigation on the thin film growth of 2D transition metal dichalcogenides via metal-organic molecular beam epitaxy (MOMBE), a method not widely used to study the growth of 2D materials, but one that holds promise. This work primarily focuses on the growth of tungsten-based transition metal dichalcogenides and to a lesser extent tungsten-based TMD alloys. We utilize X-ray synchrotron radiation in order to characterize the thin films during growth in situ and in real-time. We also utilize numerous ex situ characterization techniques in this work, such as atomic force microscopy and X-ray photoelectron spectroscopy. We investigate the growth of thin films, specifically the effect of various growth conditions. We consider how the ratio of the incident species affects the growth rate, nucleation and quality of thin films. We also investigated the impact of substrate temperature, the starting substrate and the carrier gas flow rate on the mode of growth, the growth rate, morphology and the composition of the thin films grown by MOMBE. We also employed this approach for the growth of tungsten-based alloys. We find that the carrier gas flow rate and substrate temperature have a significant effect on growth. We also observe that depending on the ratio of incident species, unwanted byproducts may be produced. Finally, using this growth method along with synchrotron characterization techniques, we demonstrate the near layer-by-layer growth of TMD thin films with the expected stoichiometry.

In this work, we demonstrate the mechanism for etching exfoliated graphene on SiO2 and other technological important substrates (Si, SiC and ITO), using low-energy electron sources. Our mechanism is based on helium ion sputtering and vacancy formation. Helium ions instead of incident electrons cause the defects that oxygen reacts with and etches graphene. We found that etching does not occur on low-resistivity Si and ITO. Etching occurs on higher resistivity Si and SiC, although much less than on SiO2. In addition, we studied the degradation mechanism of MoS2 under ambient conditions using as-grown and preheated mono- and thicker-layered MoS2 films. Thicker-layered MoS2 do not exhibit the growth of dendrites that is characteristic of monolayer degradation. Dendrites are observed to stop at the monolayer-bilayer boundary. Raman and photoluminescence spectra of the aged bilayer and thicker-layered films are comparable to those of as-grown films. We found that greater stability of bilayers and thicker layers supports a previously reported mechanism for monolayer degradation involving Förster resonance energy transfer. As a result, straightforward and scalable 2D materials integration, or air stable heterostructure device fabrication may be easily achieved. Our proposed mechanisms for etching graphene and ambient degradation of MoS2 could catalyze research on realizing new devices that are more efficient, stable, and reliable for practical applications.

Electron correlation effects in quantum materials are very strong. It is critical to investigate the structure of quantum materials to better understand and manipulate their physical properties. Quantum effects are prominent at the atomic microscopic length scale, which can not be examined by average long range structural measurements using traditional diffraction methods. Instead, pair distribution function (PDF) analysis, a local structure probe, can effectively unveil the mystery of local structure, which is more sensitive to local behavior than bulk average features. The first section of my dissertation will concentrate on the local structural study of the Iron oxy-chalcogenides, {\cSSe}, which are layered materials formed by stacking layered units of La$_{2}$O$_{2}$ and Fe$_{2}$O$M_{2}$ ($M$ = S, Se). Local crystal structure was studied using the PDF technique, which involves Fourier transforming the measured total scattering intensity to obtain a real space representation of inter-atomic correlations. This technique was used to study local, short range structural correlations that deviate from the average structure. PDF analysis for $M$ = S, Se shows short-scale structural distortions in a typical range of 1-2 nm, indicating nematic fluctuations. However, neutron powder diffraction (NPD) provides clear evidence that the average, long-range structure remains tetragonal throughout the high and low temperature regimes. A comparable result was obtained for Fe$_{1.1}$Te. These findings highlight the ubiquity of nematic fluctuations in iron-based superconductors and related materials. The second part of my research is focused on measuring the transport and vibrational properties of black phosphorus and related materials. Phosphorene, a novel two-dimensional (2D) material, is gaining researchers' attention due to its exceptional properties, including a unique layer structure, a widely tunable band gap, strong in-plane anisotropy, and high carrier mobility. Strain in 2D materials can tune the material properties. The effect of tensile strain on the Raman spectra of black phosphorus (BP) was studied by using a simple custom strain device revealed clear red shifting of all three phonon modes, A$^{1}_{g}$, B$_{2g}$ and A$^{2}_{g}$. We anticipate that our method of in-situ Raman spectroscopy could be an effective tool that can allow observation of strain effects directly, which is critical for future flexible electronic devices. Even though, black phosphorus has several unique properties, there are some limitations in its application in devices due to its environmental instability. Several passivation techniques have been employed, but just covering the surface may not be the long term solution. Doping engineering is found to be an efficient strategy in order to enhance the properties of BP, and its promising performance in electronic devices. We have synthesized a series of As$_{x}$P$_{1-x}$ (x = 0, 0.2, 0.5, 0.83, 1) alloys which show similar properties to black phosphorus and are more stable in comparison to BP. Temperature dependent transport properties of As$_{x}$P$_{1-x}$ alloys show that small arsenic doping greatly increases the thermoelectric power of black phosphorus. Thermoelectric properties of these materials provide an environmentally friendly solution for direct and reversible conversion between heat and electricity. They have potential applications in a wide range of fields, including transportation, industry, and power generators/solid-state refrigerators, and may also provide solutions for sustainable energy sources.

This dissertation will have three chapters. In chapter one, a comprehensive review on defects in two dimensional materials will be presented. The aim of this review is to elaborate on different types of defects in two dimensional (2D) materials like graphene and transition metal dichalcogenides (TMDs). First, different types of point and line defects, e.g. vacancies, anti-sites, guest elements, adatoms, vacancy clusters, grain boundaries, and edges, in these materials are categorized in terms of structure. Second, interactions among defects are discussed in terms of their rearrangement for low-energy configurations. Before studying the electronic and magnetic properties of defective 2D materials, some of the structures are considered in order to see how defect structure evolves to a stable defect configuration. Next, the influence of defects on electronic and magnetic properties of 2D materials is discussed. Finally, the dynamic behavior of defects and 2D structures under conditions such as electron beam irradiation, heat treatment, and ambient conditions, is discussed. Later as a case study, defects in a two dimensional transition metal dichalcogenide will be presented. Among two-dimensional (2D) transition metal dichalcogenides (TMDs), platinum diselenide (PtSe2) stands at a unique place in the sense that it undergoes a phase transition from type-II Dirac semimetal to indirect-gap semiconductor as thickness decreases. Defects in 2D TMDs are ubiquitous and play crucial roles in understanding and tuning electronic, optical, and magnetic properties. Here intrinsic point defects in ultrathin 1T-PtSe2 layers grown on mica were investigated through the chemical vapor transport (CVT) method, using scanning tunneling microscopy and spectroscopy (STM/STS) and first-principles calculations. Five types of distinct defects were observed from STM topography images and the local density of states of the defects were obtained. By combining the STM results with first-principles calculations, the types and characteristics of these defects were identified, which are Pt vacancies at the topmost and next monolayers, Se vacancies in the topmost monolayer, and Se antisites at Pt sites within the topmost monolayer. Our study shows that the Se antisite defects are the most abundant with the lowest formation energy in a Se-rich growth condition, in contrast to cases of 2D molybdenum disulfide (MoS2) family. Our findings would provide critical insight into tuning of carrier mobility, charge carrier relaxation, and electron-hole recombination rates by defect engineering or varying growth condition in few-layer 1T-PtSe2 and other related 2D materials. Also, in order to investigate the layer dependency of vibrational and electronic properties of two dimensional materials, 2M-WS2 material was selected. Raman spectroscopy and DFT calculation proved that all Raman active modes have a downshift when material is thinned to few layers (less than 5 layers). It was proven that there is a strong interaction between layers such that by decreasing the number of layers, the downshift in Raman active modes is mostly for the ones which belong to out-of-plane atomic movements and the most downshift is for the Ag2 Raman active mode. Also, I investigated the effect of number of layers on the band structure and electronic properties of this material. As the number of layers decreases, band gap does not change until the materials is thinned down to only a single monolayer. For a single monolayer of 2M-WS2, there is an indirect band gap of 0.05eV; however, with applying in-plane strain to this monolayer, the material takes a metallic behavior as the strain goes beyond ±1%.

The discovery of graphene and its unprecedented properties inspired an extraordinary increase in research progress, launching an era of two-dimensional (2D) electronic materials. These stable crystalline atomic layers enable the design of ultrathin 2D devices by combining different 2D materials as the foundational components. In order to control the properties of these devices, materials with a variety of electronic properties must be available. In this dissertation, we explore three distinct paths to achieve this goal: expanding the library of 2D materials, post synthesis defect engineering, and proximity engineering of the electrostatic environment. First, we report the MBE synthesis and STM/S characterization of a new 2D insulator, honeycomb structure BeO. In addition to determining the atomic structure and density of states, we used moiré pattern analysis to demonstrate the high crystallinity of the BeO and determined the work function modulation across the moiré pattern. We illustrate that the scalable growth, weak substrate interactions, and long-range crystallinity make honeycomb BeO an attractive candidate for future technological applications. The next focus of this work was defect engineering of monolayer WS₂ by UHV annealing. A high concentration of S vacancies was generated by UHV annealing of the WS2, leading to S vacancy defect-defect coupling. Using STM/S we determined that the interaction of nearby S vacancies leads to an increase of deep in-gap states for different divacancy geometries. This indicates that vacancy engineering can be a useful tool to controllably manipulate 2D material electronic properties. Finally, we demonstrate the creation of a nanoscale planar p-n junction within a single monolayer of MoSe₂ by modulating the electronic properties of the underlying substrate. By intercalating Se at the interface of the hBN/Ru substrate, the hBN becomes decoupled from the Ru, changing its conductivity and work function. We find that this change in the electronic landscape tunes the band gap of the overlying MoSe₂, by screening and shifting the MoSe₂ work function. Thus, this dissertation shines a light on the vast opportunities 2D materials provide for exploration of novel approaches to materials engineering, and demonstrates a tool set for manipulating the electronic properties of these fascinating materials.

We report electron transport measurements in dual-gated monolayer WS₂ encapsulated in hexagonal boron-nitride. Using gated Ohmic contacts which operate from room temperature down to 1.5 K, we measure the intrinsic conductivity and carrier density as a function of temperature and gate bias. An intrinsic electron mobility of 100 cm²/(V·s) at room temperature, and 2,000 cm²/(V·s) at 1.5 K are achieved. The mobility shows a strong temperature dependence at high temperatures, consistent with phonon scattering dominated carrier transport. At low temperature, the mobility saturates due to impurity and long-range Coulomb scattering. First principles calculations of phonon scattering in monolayer WS₂ are in good agreement with the experimental results, showing we approach the intrinsic limit of transport in these 2D layers.

The development of fast pixelated direct electron detectors allows the collection of full scattering information at every scanning point in Scanning Transmission Electron Microscopy (STEM). With two scanning directions (x, y) and two momentum dimensions (kx, ky), this results in a four-dimensional dataset, thus the technique is colloquially referred to as 4D-STEM. 4D diffraction data is not readily interpretable, so it must be processed into a more interpretable two-dimensional image. While traditional imaging modes like high-angle angular dark field and bright field images can be replicated, new methods of processing the diffraction data like Center-of-Mass (COM) imaging can quantitatively measure electric field or other properties of the sample. Thus, a better understanding of what information is stored and where in the diffraction pattern will better inform how we can extract quantitative information from the 4D diffraction dataset. In the first part of this thesis, I go over theoretical calculations for diffraction patterns for a variety of samples. By demonstrating how contrast and movement of the bright disk is affected by sample size compared to probe size, I show how the virtual detector geometry of COM imaging affects what length-scale of electromagnetic field is being measured rather than electric versus magnetic fields. Simulations of multiple scattering demonstrate the signal-to-noise efficiency of second moment imaging for thicker biological samples. Finally, a theoretical calculation of polarity contribution in nanobeam electron diffraction (NBED) from 2D materials makes explicit the origin and Z-dependence of the violation of Friedel’s Law even in monolayer samples. In the second part of my thesis, I go over new imaging modes. By understanding how tilt affects placement of diffracted disks in a 2D material, I quantitatively map the in-plane rotation, out-of-plane tilt, and strain in a corrugated sheet of MoS2. Finally, I show how supervised machine learning can be used for high-resolution phase retrieval as a faster, more transferable process than ptychography to demonstrate its capability as a powerful tool for guiding further 4D-STEM characterization development.

Enhanced light-matter interactions through a plethora of dipole-typepolaritonic excitations started to emerge in two-dimensional (2D) layered materials in recent years. 2D Van der Waals (vdWs) polar crystals sustaining phonon-polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications within the mid-infrared (mid-IR) to terahertz ranges. One of the fundamental hurdles in polaritonics is the trade-off between electromagnetic field confinement and the coupling efficiency with free-space light, a consequence of the large momentum mismatch between the excitation source and polaritonic modes. Our group recently demonstrated the fundamental problem of momentum mismatch can be overcome with a graphene acoustic plasmon resonator with nearly perfect absorption (94%) of incident mid-infrared light. This high efficiency is achieved by utilizing a two-stage coupling scheme: free-space light coupled to conventional graphene plasmons, which then couple to ultraconfined acoustic plasmons. To date, experimental demonstration of excitation of the surface phonon-polaritons (SPhPs) in transition metal dichalcogenides (TMDs) remains unexplored, so here we demonstrate novel strategies for dynamically controlling of far-infrared light using unique optical properties of TMDs in particular ZrS2. In comparison to other vdW materials, the phonon modes of these TMDs are much softer and exhibits phonon peaks within the far-infrared (far-IR) regime. Here we experimentally demonstrate the excitation of SPhPs in ZrS2 acoustic resonator by the far-IR absorption spectroscopy. The surface optical (SO) phonon modes of these TMDs were tuned and tailored to lie anywhere within the reststrahlen band, by controlling the ribbon width, which brings extreme tunability. In addition, the absorption can be pushed beyond 90% with the integration of gold reflectors and can become promising material for far-IR biosensing. The results demonstrate TMDs as a new platform for studying phonon-polaritons exhibiting good quality factors and excellent tunability which enable far-IR nanophotonics devices. Recently our group demonstrated ultra strong coupling (USC) between polar phonons and mid-IR light in coaxial nanocavities. Here we push the boundary further up to the far-IR range and we propose to demonstrate USC coupling between polar phonons and far-IR light using the coaxial nanocavity platform. Our numerical simulation predicts a level splitting of strongly coupled polaritons of 95% of the resonant frequency, enabled by epsilon-near-zero (ENZ) responses in the far-IR of the ZrS2 filled coaxial nanocavities. The ability to reach the USC regime in mass-produced nanocavity systems can open up new avenues to explore non-perturbatively coupled light-matter systems, multiphoton effects, as well as higher-order nonlinear effects, which may lead to novel applications in sensing, spectroscopy, and nanocavity optomechanics.

Two-dimensional (2D) materials have drawn much attention in the last two decades and have been investigated as an emerging material system in a wide range of applications. Recently, various 2D materials have been reported to exhibit non-volatile resistive switching (NVRS) phenomenon. However, it was not believed to be accessible in monolayer atomic sheets due to excessive leakage current. This dissertation presents the first discovery of resistive switching behavior in 2D monolayer materials in a vertical metal-insulator-metal (MIM) structure with systematic research in electrical characteristics, switching mechanisms, performance improvements, and potential applications. Chapter 2 discusses NVRS in monolayer molybdenum disulfide (MoS₂) and three other transition metal dichalcogenides, featuring low switching voltages, large on/off ratio, fast switching speed, and forming-free characteristics. The electrical performance, including retention, endurance and variability, and applications in flexible non-volatile memory and radio-frequency (RF) switch are investigated. In Chapter 3, the resistive switching phenomenon in the representative 2D insulator, hexagonal boron nitride (h-BN), is studied, which sets the record of the thinnest NVRS material to ~0.3 nm. In Chapter 4, the collection of 2D atomic sheets showing NVRS is further expanded to a dozen materials, indicating its potential universality in ultrathin non-metallic 2D materials. A Dissociation-Diffusion-Adsorption (DDA) model has been proposed to describe the common mechanism behind bipolar NVRS in 2D crystalline monolayers with both experimental support by scanning tunneling microscope (STM) measurements and theoretical support by first-principle calculations. In Chapter 5, a current-sweep method and a constant electric stress method are applied on the 2D-based memory devices to provide more insights into the switching mechanism. Multiple resistance states can be achieved in 2D memory devices, which enables the applications in multi-bit storage and neuromorphic computing. In Chapter 6, defect engineering through electron irradiation treatment has been performed on monolayer MoS₂, showing a significant improvement on yield and endurance of the fabricated devices, which is attributed to the induced sulfur vacancies that promote the formation of “conductive points”.

Spin currents and spin defects are exciting topics of research because of their applications in spintronics and quantum information. A central challenge for studying spin-dependent phenomena is creating materials with the characteristics that are ideally suited for the generation and injection of highly spin polarized carriers. Likewise, many transport phenomena rely on electron-defect interactions, where a commonly occurring challenge is in the tunability and reproducibility of the interacting defects. Heterostructures of stacked 2D materials can potentially provide the flexibility to engineer the charge and spin transport properties of a device, and can even provide defect centers that can be exploited for the device operation. In this thesis I will discuss two different types of stacked 2D material devices, both utilizing graphene as the base 2D material. In the first device, graphene is stacked on top of an assembled layer of superparamagnetic iron oxide nanoparticles (SPIONs). The interaction of the spin carriers in graphene with the assembled SPIONs gives rise to the observation of an Anomalous Hall Effect in the electron-doped regime. Demonstrating a magnetic proximity effect that can be integrated into a more complex 2D heterostructure systems.The second device studied in this thesis is a tunnel-junction with a hexagonal boron nitride (h-BN) tunnel barrier, with graphene acting as the tunneling electrode reservoirs. Tunneling transport spectroscopy shows possible spin interaction with defect states in the h-BN tunnel barrier. This device geometry demonstrates the promise of stacked 2D material systems to create new functional devices.

Two-dimensional (2D) magnetic materials have attracted substantial interest in recent years due to their potential applications in spintronic devices and due the recent discovery of 2D magnetic materials believed to harbor exotic quantum spin liquid (QSL) phases, which have potential uses in topological quantum computing. In this dissertation we present an analysis of the magnetic anisotropy that enables 2D ferromagnetism in CrI3, one of the first 2D ferromagnets discovered. We determine the most general microscopic spin interactions allowed by this material's crystal symmetries and estimate the strength and sign of these interactions by fitting angle-dependent ferromagnetic resonance data using this spin model. Surprisingly, we estimate that the Kitaev interaction is the strongest in this material with K ~ -5.2 meV, which is 25 times larger than the estimated Heisenberg exchange J ~ -0.2 meV, and responsible for opening the ~5 meV gap at the Dirac points in the spin-wave dispersion. Furthermore, we find that the symmetric off-diagonal anisotropy Γ ~ -67.5 eV, though small, is crucial for opening a ~0.3 meV gap in the magnon spectrum at the zone center and stabilizing ferromagnetism in this 2D material.Secondly, we present a symmetry analysis of the most general forms of tensors describing physical responses in 2D materials composed of a honeycomb lattice of edge-sharing octahedra, which includes materials such as CrI3 and the QSL candidate α-RuCl3, for crystals belonging to different crystallographic point groups. We focus on the magnetic-field-dependent thermal conductivity tensor κij(H), which describes a system’s longitudinal and thermal Hall responses, for the cases when the magnetic field is applied along high-symmetry directions, perpendicular to the honeycomb plane and in the honeycomb plane. We highlight some unexpected results, such as the equality of fully longitudinal components to partially transverse components in rank-3 tensors for systems with three-fold rotational symmetry, and make testable predictions for the thermal conductivity tensor.Finally, we present our results from numerical calculations using exact diagonalization to study the presence of magnon bound states in both the ferromagnetic and antiferromagnetic Kitaev honeycomb model, which are known to host QSL ground states, in the presence of an external magnetic field perpendicular to the honeycomb plane.

Advancements in spintronics rely on materials which exhibit stable magnetization or long-lived spin states that can be easily manipulated or probed. One class of two-dimensional (2D) materials, the monolayer transition metal dichalcogenides (TMDs) have gained significant interest due to their large spin-orbit coupling which leads to the existence of inequivalent, spin-split valleys in the band structure and predictions of long-lived spin-valley lifetimes. Furthermore, the TMDs exhibit unique optical selection rules which allow for selective control and manipulation of spin and valley polarization by helicity of light. In addition, due to their weak van der Waals coupling, these materials can be easily picked up and stacked into heterostructures with other 2D or three dimensional (3D) materials without the need to match lattice constant or growth conditions. Here, the strength of one material can surmount the weaknesses of another system, and such heterostructures have been utilized to control exciton diffusion, enhance spin/valley lifetimes, and even induce proximity spin-orbit coupling and ferromagnetism.In this thesis, we focus on measuring the spin/valley dynamics in TMD monolayers and heterostructure devices. Using time-resolved Kerr rotation (TRKR) microscopy, we observe long-lived (>5 ns) spin/valley lifetimes and a complex spatial dependence of spin/valley density in monolayer WS2 flakes. Comparisons to photoluminescence (PL) microscopy allow us to elucidate the roles that resident carriers and dark trion formation play in this spatial dependence and the stabilization of spin-valley lifetime. We extend these techniques to heterostructure devices of monolayer WSe2 and graphene where we reveal an ultrafast quenching of spin-valley signal at the WSe2/graphene heterojunctions. Complementary measurements of PL and photoconductivity demonstrate an efficient, electron dominated charge transfer into graphene, from which we conclude that the quenching of the spin-valley signal is due to charge mediated spin transfer into graphene. Additionally, we explore the growth of complementary magnetic materials by molecular beam epitaxy. Specifically, we demonstrate growth of MgAl0.5Fe1.5O4 (MAFO), a low damping, ferrimagnetic insulator which is an ideal candidate for the demonstration of proximity effects in 2D materials. We find that highly crystalline films of MAFO can be grown by MBE and utilize alternating layer epitaxy to alter the distribution of Fe in the crystal lattice, from which we observe a significant change in the saturation magnetic moment.

Since the discovery of graphene in 2004, much research enthusiasm has been evoked for this first discovered two-dimensional (2D) material due to its unique physical and electrical properties, such as an extraordinarily high surface-to-volume ratio, a gapless linear band structure with Fermi level tunability, bipolarity and ultra-high carrier mobility. The following discovery of transitional metal dichalcogenides (TMDCs), with direct band gaps and spin-coupled valleys, adds another crucial piece to the 2D material library. The thesis aims at 2D materials growth and utilizing their unique physical and electrical properties for real-world solid-state device applications. Chemical vapor deposition (CVD) of monolayer MoS2 is first demonstrated, with both single crystalline flake (~15 µm) and large area (centimeter level) polycrystalline continuous film growth achieved. Various characterization methods are subsequently utilized to examine growth morphology and quality such as optical microscopy, scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL), atomic force microscopy (AFM) and electrical measurements. Monolayer growth is confirmed by AFM and PL for monocrystalline flakes and polycrystalline continuous films, respectively. For the application portion, an electrically tunable lateral structured graphene-silicon-graphene bipolar junction transistor (BJT) has been fabricated with direct current gain over 20. During device operation, electrons in the graphene emitter overcome the graphene/Si barrier to diffuse into the silicon base and are subsequently collected by the collector under a large base-collector reverse bias. In contrast with traditional silicon BJTs, the current gain of graphene BJT device can be readily and smoothly tuned. With the doping concentration and Fermi level of the graphene emitter electrically controlled by a top gate electrode, the electron injection barrier height is consequently tuned, leading to tunability in the graphene BJT direct current gain. With decent performance, simplicity and extensibility, this novel graphene BJT device demonstrates a promising way forward for nanoscale BJT applications. With an ultra-high surface to volume ratio, graphene is an ideal platform for sensor applications. A graphene chemical sensor, based on a pristine graphene field-effect-transistor (FET) bonded with a µ-column gas flow channel cap, has been miniaturized for volatile compounds sensing and discrimination. With a small size of 1 cm by 0.7 cm, the miniature graphene sensor remains as sensitive (nanogram detection limit) and even faster (down to sub-second) than its pioneering large footprint counterpart. More importantly, a true label-free nanoelectronic sensing platform is pioneered by combining the electrical gate tunability of graphene sensor responsivity (or gate spectra) with the principal component analysis (PCA) technique. In contrast to conventional electronic sensors or electronic nose technology, surface functionalization is no longer needed in order to achieve chemical discrimination. 11 measured chemicals (acetone, ethyl acetate, chloroform, dichloromethane, pentane, heptane, nonane, benzene, toluene, o-xylene and p-xylene), as represented by 11 clusters of points in PCA plot, are clearly grouped into separate regions with each representing a corresponding chemical category. The identification accuracy, as verified by multiple algorithms such as k-nearest neighbors (KNN), linear discrimination analysis (LDA), support vector machine (SVM) and multi-layer perceptron (MLP), is rather satisfactory with at least 98.8% accuracy for the 11 specific chemicals and at least 93.9% accuracy for the four corresponding categories, indicating the robustness of data acquired by the graphene sensor. This work should lay the groundwork toward true label-free electronic sensor with high sensitivity and selectivity, and a novel electronic nose technology with better simplicity and higher accuracy.

Scanning probe microscopy (SPM) has emerged as an indispensable technique to image, characterize, manipulate, and create new functional properties of materials at the nanoscale. Concurrently, the tantalizing nanoscale properties of ferroelectric materials have aroused immense interest both scientifically and technologically. Here SPM techniques are used to probe and exploit the nanoscale ferroelectric functionalities in various material systems, and each chapter in this dissertation is allocated to one specific material. Polarization retention failure compromises the performance and life expectancy of non-volatile nanoelectronic devices and thus needs to be addressed. In a designer-defect-engineered epitaxial BiFeO3 system, an ultralong polarization retention with no degradation over one year has been discovered. SPM based dynamic switching measurements reveal a significantly increased activation field that provides sufficient energy for efficient domain wall pinning. Atomic resolution scanning transmission electron microscopy indicates that nanoscale defect pockets pervade the entire film thickness acting as domain wall pinning centers. The findings provide guidelines for improving polarization retention in ferroelectrics using defect engineering. Domain walls, as one type of topological defect, have given rise to the recent development of prototype domain wall nanoelectronics elements. Here, the nanoscale electrical conduction behaviour of single crystal 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 (PMN-PT) with a (001) orientation is investigated. Electrical conductivity at domain walls in these crystals is observed using conductive AFM (c-AFM). The AC-conductivity response at such walls is mapped and local preferential charge relaxation along domain walls is investigated by Kelvin probe force microscopy revealing slow dynamics in the minute's range. These findings provide new insight into domain wall properties at a.c. frequencies for domain wall based nanoelectronics. Van der Waals (vdW) thio- and seleno-phosphates have recently gained considerable attention for the use as ‘active’ dielectrics in two-dimensional or quasi-two-dimensional electronic devices. Bulk ionic conductivity in these materials has been identified as a key factor for the control of their electronic properties. However, direct evidence of specific ion species’ migration at the nanoscale, particularly under electric fields, and its impact on material properties has been elusive. Here, direct evidence of a phase-selective anisotropic Cu ion hopping mechanism is reported in copper indium thiophosphate (CuInP2S6) through detailed scanning probe microscopy measurements. A two-step Cu-hopping path including a first intralayer hopping (in-plane) and second interlayer hopping (out-of-plane) crossing the vdW gap is unveiled. Evidence of electrically controlled Cu ion migration is further verified by nanoscale energy-dispersive X-ray spectroscopy (EDS) mapping. These findings offer new insight into anisotropic ionic manipulation in layered vdW ferroelectric/dielectric materials for emergent vdW electronic device design. Overall, these studies suggest that SPM can provide detailed nanoscale insight into the functional properties of ferroelectric materials, and advance the development of prototype nanoelectronics components via the in-situ field control.

Due to the large variety and unique physiochemical properties, such as high electrical conductivity, adjustable interlayer spacing, intercalation chemistry and so on, two-dimensional (2D) materials have attracted tremendous attention for their suitability in the development of high-performing supercapacitors. Despite that great progress has been made, there is still no single 2D material that can perfectly meet all the requirements to replace the existing material, mainly activated carbon, used in commercial supercapacitors. Therefore, continuing efforts for exploring novel, high-performing 2D materials with low cost are desirable. In this dissertation, the prior studies on the development of 2D materials ranging from layered inorganic materials to organic-inorganic hybrids for supercapacitors are first reviewed. As an emerging type of 2D material, layered organic-inorganic hybrids start to show promising results to be used to fabricate high-density, nonporous, and thick electrodes for compact capacitive energy storage. However, the studies in this area are still lacking. Thus, the goal here is to explore the opportunities of 2D organic-inorganic hybrids for applications in supercapacitors. The relevant techniques and methods used throughout the study are then outlined. Next, three research chapters supporting the main findings of the investigation are included. The first research chapter describes a facile mechanical strategy to improve the kinetics and rate performance of 2D organic-inorganic hybrid electrodes at ultrahigh mass loadings (up to 30 mg cm-2). The second research chapter reports the synthesis of a new layered organic-inorganic hybrid material with excellent volumetric and areal capacitances even at mass loadings reaching 50 mg cm-2, highlighting the good electrode kinetics. The third research chapter presents the wafer-scale electrochemical deposition synthesis of 2D organic-inorganic composite films with controlled size and thickness, which are promising for the development of flexible and transparent electrochemical energy storage devices. Finally, conclusions and recommendations are given at the end of this dissertation.

Micro/nanoscaled particles represent a group of intelligent materials that can precisely and rapidly respond to biological microenvironments and improve therapeutic outcomes. In order to maximize biomedical application potentials, developing nanoscale particles that are able to catalyse in-situ substrates to address the biological problems is highly desired but still remains a critical challenge. Herein, a 2D nanosheet-based catalytic nanoparticles with enzyme mimicking behaviour is developed for enhanced drug delivery toward the tumour microenvironment as well as the antibacterial treatment. The nanoparticles are constructed via a facile one-pot method and exhibit ultrathin monolayer nanosheet morphology and could be further attaching drugs as cargo to fulfil the drug delivery task. The 2D structure of the LDHs allows high catalytic activity, leading to a responsive, sustained, and relatively high amount of substrate transformation. Herein, in this thesis, H2O2 was used as the model substrate to demonstrate the nanoparticles catalyzing ability, as well as the potential applications. The Ferrous ion-containing LDHs was tested for different medicinal purposes, tumour targeting drug deliveries and antibacterial treatment

Two-dimensional (2D) layered semiconductors, such as MoS2, have been considered as promising channel materials for field-effect transistors (FETs) in the post-silicon era. Their unique properties, such as ultrathin nature, suitable bandgaps, free of dangling bonds, make them outstanding in the dimension and power consumption scaling, where the conventional silicon-based metal-oxide-semiconductor field-effect transistors (MOSFET) have encountered significant challenges to follow Moore’s Law. Using 2D semiconductors as successors to replace the channel materials in the well-established silicon technology is not an individual task. When applying these new low dimensional materials to practices, a considerable number of processes and routines come across difficulties, because the processes that have been constantly developed and improved over the last half-century are designed for the three-dimensional (3D) silicon industry. Thus, new fabrication strategies designed for the 2D semiconductor are urgently needed, before high-performance and large-scale 2D material-based FETs (2D-FETs) can be applied to the industry. Among abundant tiny but nonnegligible obstacles, the thesis focuses on scale-up, contamination control, parameters refinement, and the creation of novel dielectric materials, where novel strategies are proposed to improve electronic device performance. After an introduction of background, an inclusive literature review, and an instrument summary, the five experimental chapters present the studies on the field of materials preparation, channel isolation, contact resistance reduction, and dielectric engineering. Detailed statements of the study motivation, state-of-the-art development status, challenges, research methodologies, materials characterization and device performance characterization are also included in each experimental chapter. At the end of the thesis, all of these studies contribute to a breakthrough of MoS2-based FETs over a wafter scale, which exhibit a high ON/OFF ratio of 10^6 and steep subthreshold swings (SS) of 120 mV dec-1. These results indicate a step has moved forward to realize the practical applications of high-performance 2D-FETs.

Two-dimensional (2D) layered materials with atomic- to nano-scale thickness have a wide variety of applications, one of which is as a solid lubricant. These materials have intrinsically low friction and good anti-wear properties, and thus are promising for a variety of emerging nanoscale technologies, but a fundamental understanding of their properties is not yet well-established. This dissertation seeks to address this from a nanoscopic point of view with atomistic simulations. First, through direct comparison of atomic-scale friction of monolayer graphene, monolayer MoS2, and a graphene/MoS2 heterostructure, we showed that graphene exhibits lower friction than MoS2, graphene, and their heterostructure. The origin of this friction contrast was shown to be the difference in surface energy barrier height (i.e., magnitude of the surface potential energy) due to the higher dispersion contribution to the sliding barrier for MoS2. We then isolated and understood the role of tip apex structure on the quality (i.e., distortion and degree of symmetry) of the surface energy landscape. In this context, we showed how different patterns of friction anisotropy can arise due to changes in quality of the tip–sample interfacial structure, an explanation that challenges previously proposed anisotropy mechanisms. Next, the effect of the elemental composition of 2D materials on single asperity friction was systematically explored by characterizing the friction and adhesion of MoS2, MoSe2, and MoTe2. We found that the presence of larger atoms within the lattice of a 2D material unexpectedly decreased friction. Our simulations and numerical analysis revealed that the lattice spacing, a parameter previously disregarded, significantly affected friction. Specifically, a larger lattice spacing due to the larger chalcogen enabled the tip to slide more easily across correspondingly wider saddle points in the potential energy landscape, resulting in lower friction. Finally, we used simulations to correlate the dynamics of a sliding contact to its electrical conduction. Both current and lateral force exhibited fluctuations corresponding to the periodicity of the substrate lattice, i.e., stick-slip, and the lateral force increased during stick events while the current decreased exponentially. We attributed this inverse correlation between current and lateral force to sliding-induced variations in atom–atom distances across the contact. Overall, the results of this dissertation contribute to establishing the framework of fundamental knowledge needed to design and optimize 2D materials for emerging applications.

The success of graphene in the field electronics has led to a growing interest in exploring other two-dimensional materials such as the transition metal dichalcogenides. Like graphene, they are capable of being thinned down to the monolayer limit. These materials have gained a major impact in the field of photonics due to their direct bandgap at the monolayer limit. Unique transitional metal dichalcogenides properties such as large spin-orbit coupling has become of great interest in the field of spintronics and valleytronics. One of the major goals for the growth of the materials is growing large films that are uniform and thin. In this thesis, we focus on the growth of TMDs using a chelant in solution. In this vein, we study the effects of different chelating agents in their binding to our WS2(MoS2) precursor. Large area growth of transitional metal dichalcogenides using a solution-based process has been well studied topic. A wafer-scale growth of Two-dimensional(2D) tungsten disulfide (〖WS〗_2) with consistent uniformity has remained a difficult challenge in all types of growth methods. Specifically, the growth of 〖WS〗_2 using a solution-based approach has been a difficult challenge due to different chemistry. Here we report the wafer scale growth of uniform 〖WS〗_2 using a spin-coat process. Previously, a solvent of ethylenediaminetetraacetic acid (EDTA) in DMSO with ammonium tetra thiomolybdate (〖〖(NH〗_4)〗_2 〖MoS〗_4), and a thermolysis step was used to produce uniform wafer scale 〖MoS〗_2. In our work, the use of Glycine in DMSO with ammonium tetra thiomolybdate (〖〖(NH〗_4)〗_2 〖WS〗_4), to produce wafer-scale growth with consistent uniformity. A study of three different chelating agents using a dimethyl sulfoxide (DMSO) solvent was done to show the chelant’s critical role in growing uniform films. The two main metrics that are evaluated are the amount film coverage and photoluminescence from the growths. This growth method provides a cost-effective reproducible synthesize of TMD films at the wafer scale.

2 Dimensional materials, i.e. van der Waals materials, are an exciting class of materials thancan be stacked one on top of another to form new and exciting metamaterials. These sheets and combinations of them have been shown to have extraordinary thermal properties such as the extremely high 3000 W/K/m in plane thermal conductivity of Graphene and signficantly low 5 MW/K/m^2 thermal conductance across MoS2 and Graphene heterostructures. Accurately calculating the thermal conductance and conductivity of these various structures/materials and understanding how to modulate these thermal properties is not a simple feat. This dissertation is meant to guide future graduate students interested in this field by showing the challenges and potentially fruitful future next steps in the field of thermal transport of 2-D materials.In Chapter 1, the dissertation is focused on introducing the unique flavor of heat transportin 2-D materials. This involves an brief history of nanoscale heat transport and how the field emerged. This is followed by a motivation for why there has been an increased interest in 2-D thermal transport and a description of two of the most studied vdW materials. Following the description of these layers, this chapter covers some unique characteristics of heat transport on 2-D materials, such as commensurability and different junction types. Finally, the chapter ends with a discussion of different methods used to study these materials and what their strengths/weaknesses are.In Chapter 2, the dissertation is focused on explaining the methods and surrounding backgroundthat are used within the following chapters. This involves explaining background concepts such as interatomic potentials, periodic boundary conditions, statistical ensembles, cell relaxation, and force constants. In addition to background concepts, the methods of NEMD and ESKM and discussed in detail, which are the two major computational methods used within the dissertation.In Chapter 3, the dissertation is focused on the best practices within the NEMD methodfor calculating thermal conductance. Moreover, this chapter shows that with a proper choice of thermostat the nonlinear part of the temperature profile should actually not be excluded in thermal transport calculations. It also compares results from NEMD and AGF in the ballistic regime to probe different transport regimes and show one should directly calculate the thermal conductance from the temperature difference between the heat source and sink. Finally, the chapter explains why the local thermostat in the sink and source should be Langevin instead of Nosé-Hoover.In Chapter 4, the dissertation is focused on heat transport across graphene step junctions. Firstthe chapter explains the relevance of graphene step junctions. The chapter then discusses experi- mental and computational results, which show that layers in this devices are thermally decoupled due the lower cross-plane thermal conductance between the layers. The results also show that no significant thermal asymmetry exists across graphene step junctions this size, which is contrary to previous calculations in literature. In addition, the chapter shows that bilayer island defects, common in CVD grown graphene, have little to no contribution to overall thermal transport.In Chapter 5, the dissertation is focused on thermal transport across graphene-MoS2 hetero-junctions. The chapter starts with a motivation to study heat transfer in heterojunctions. This is followed by experimental results that discuss the varying thermal conductance for different G/MoS2 stacking compositions. Following the experimental results is a discussion around the computational results, which lead to the conclusion that the cross-plane thermal conductance is sensitive to c-axis lattice expansions that can occur due to thermal expansion. This chapter also contains a discussion on some of the differences seen in the thermal conductance of GMGMG between experiment and theory.In Chapter 6, the dissertation is focused on wrapping up the results contained within thisdissertation.