本論文主要研究了磁性雜質在三種不同的石墨烯體系中的性質。這三個體系分別是：伯爾納堆垛（Bernal stacked）結構的雙層石墨烯(bilayer graphene)，包含空位的雙層石墨烯和zigzag 型石墨烯納米帶(graphene nanoribbon)。本文中主要運用的數值方法為Hirsch-Fye 量子蒙特卡羅方法 （quantum Monte Carlo method）和貝葉斯最大熵方法（Bayesian Maximum Entropy method）。 / 在論文的第二章會詳細介紹這兩種方法。 我們用量子蒙特卡羅方法得到的結果是準確的，因為原則上我們可以計算無窮大的體系， 並且不需要采用任何近似去處理多體問題。 / 第三章，我們討論磁性雜質在伯爾納堆垛結構的雙層石墨烯中的性質。我們主要考慮兩種情況：磁性雜質分別位於A 亞晶格和B 亞晶格上。我們發現類似于單層石墨烯中的情況，隨著化學勢的變化，雜質原子的磁矩在一定的能量範圍內可調。但是由於雙層石墨烯中的兩套亞晶格不等價，雜質的性質很大程度上取決於雜質的位置，並且其區別隨著s-d 混合強度的增加和d 電子的關聯能的減少而變得更加明顯。我們也討論了雜質原子的譜密度和s-d 電子之間的關聯函數。我們的所有結果都體現雜質性質對空間位置的相同依賴關係。 / 在第四章，我們研究在伯爾納堆垛結構的雙層石墨烯中磁性原子在空位附近的性質。在雙層石墨烯系統中，空位引起的局域態的性質依舊取決於空位所屬的亞晶格。這也是由於兩套晶格的不等價性。我們討論雜質在空位周圍時雜質性質對空位所屬亞晶格的依賴關係，以及兩種缺陷之間的相對位移對雜質磁性的影響。當磁性原子附著在空位的最近鄰格點上時，A 亞晶格上的空位會對雜質原子的磁矩有更強的抑制。而隨著磁性原子和空位之間的距離增加，局域態對磁性原子的影響迅速衰減，並且B 亞晶格上的空位對雜質的影響相對長程一些。 / 在第五章，我們討論兩個磁性雜質在zigzag 型石墨烯納米帶邊緣上的間接互作用隨化學式的變化。由於在zigzag 型石墨烯納米帶邊緣有局域的零能態，雜質的磁矩被嚴重抑制，兩個雜質原子之間的自旋-自旋關聯函數也與石墨烯中的行為有很大區別。有趣的是，當兩個雜質附著在兩個最近鄰的碳原子上時，隨著化學式的降低，雜質之間的反鐵磁關聯會有一個顯著增強再降低的過程。我們也討論了自旋關聯隨著距離的變化，并發現關聯強度隨著距離的變化迅速衰減。 / 最後再第六章，我們會本論文中的內容進行總結和討論。 / In this research thesis, we study the magnetic properties of Anderson impurities in three different graphene based systems: pure Bernal stacked bilayer graphene, Bernal stacked bilayer graphene with a vacancy and graphene ribbon with zigzag edges. Quantum Monte Carlo method based on the Hirsch-Fye algorithms are used to obtain the basic thermodynamic quantities, and the Bayesian maximum entropy method is used to obtain the spectral densities of the impurity sites. We discuss the numerical methods in chapter 2. / In chapter 3, we investigate the local moment formation of a magnetic impurity in Bernal stacked bilayer graphene. In the two cases we study, impurity is placed on top of the two different sublattices in bilayer system. We find that similar to the monolayer case, magnet moment of the impurity could still be tuned in a wide range through changing the chemical potential. However, the property of the impurity depends strongly on its location due to the broken symmetry between the two sublattices. This difference becomes more apparent with the increase in the hybridization and decrease in the on-site Coulomb repulsion. Additionally, we calculate the impurity spectral densities and the correlation functions between the impurity and the conduction-band electrons. All the computational results show the same spatial dependence on the location of the impurity. / In chapter 4, we address the issue of single magnetic adatom located in the vicinity of a vacancy in bilayer graphene with Bernal stacking. In bilayer system, the property of vacancy induced localized states depends on whether the vacancy belongs to A or B sublattice. The magnetic impurity is placed in the vicinity of the vacancy, and the dependence of its magnetic property on the location of the vacancy is discussed. We switch the values of the chemical potential and study the basic thermodynamic quantities and the correlation functions between the magnetic adatom and the carbon sites. When the magnetic adatom is located on the nearest site of the vacancy, the local moment is more strongly suppressed if the vacancy belongs to the sublattice A. The influence of the zero-energy localized states decays fast as the displacement between the two defects increases, and the effect of B vacancy on the local moment of magnetic adatom is relatively long ranged. / In chapter 5, we examine the behavior of the indirect magnetic interactions of two magnetic impurities on the zigzag edge of graphene ribbon as a function of chemical potential. We find that the spin-spin correlation between two adatoms located on the nearest sites are drastically suppressed at the Dirac energy, and as we lower the chemical potential, the antiferromagnetic correlation is first enhanced and then decreased in values. If the two adatoms are adsorbed on the sites belong to the same sublattice, we find similar behavior of spin-spin correlation except for a cross over from ferromagnetic to antiferromagentic correlation in the vicinity of zero-energy. We also calculated the weight of the wave functions and basic thermodynamic quantities for various values of parameters, and compare the results with their counterpart in bulk graphene. / Finally in chapter 6, we summarize our results. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Sun, Jinhua = 石墨烯體系中磁性雜質的量子蒙特卡羅研究 / 孫金華. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 92-[100]). / Abstract also in Chinese. / Sun, Jinhua = Shi mo xi ti xi zhong ci xing za zhi de liang zi Mengte Kaluo yan jiu / Sun Jinhua. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Electronic properties of graphene --- p.1 / Chapter 1.1.1 --- Band structure --- p.1 / Chapter 1.1.2 --- Dirac fermions --- p.4 / Chapter 1.1.3 --- Mesoscopic effects --- p.4 / Chapter 1.1.4 --- Superconductivity in graphene --- p.6 / Chapter 1.2 --- Potential applications --- p.7 / Chapter 1.2.1 --- In semiconductors --- p.8 / Chapter 1.2.2 --- In spintronics --- p.10 / Chapter 1.3 --- Magnetic impurities in graphene based systems --- p.11 / Chapter 1.4 --- Outline of this thesis --- p.13 / Chapter 2 --- Numerical method --- p.15 / Chapter 2.1 --- Quantum Monte Carlo method based on Hirsch-Fye algorithm . --- p.15 / Chapter 2.1.1 --- Anderson impurity model Hamiltonian --- p.16 / Chapter 2.1.2 --- Key steps of Hirsch-Fye algorithm --- p.16 / Chapter 2.1.3 --- Basic thermodynamic quantities --- p.23 / Chapter 2.1.4 --- Extended Hirsch-Fye algorithm --- p.24 / Chapter 2.1.5 --- Two-impurity case --- p.25 / Chapter 2.1.6 --- Relation to the determinant quantum Monte Carlo method based on BSS algorithm --- p.27 / Chapter 2.2 --- Bayesian maximum entropy method --- p.30 / Chapter 2.2.1 --- Bayesian inference --- p.31 / Chapter 2.2.2 --- Maximum entropy analysis --- p.34 / Chapter 3 --- Magnetic impurity in Bernal stacked bilayer graphene --- p.37 / Chapter 3.1 --- Introduction --- p.37 / Chapter 3.2 --- Density of states and local density of states --- p.38 / Chapter 3.3 --- Results --- p.43 / Chapter 3.3.1 --- Basic thermodynamic quantities --- p.43 / Chapter 3.3.2 --- Spectral densities --- p.48 / Chapter 3.3.3 --- Correlation functions --- p.50 / Chapter 3.4 --- Summary --- p.52 / Chapter 4 --- Magnetic impurity in the vicinity of a vacancy in bilayer graphene --- p.54 / Chapter 4.1 --- Introduction --- p.54 / Chapter 4.2 --- Zero-energy localized states in the vicinity of vacancy --- p.56 / Chapter 4.2.1 --- Monolayer graphene with a vacancy --- p.57 / Chapter 4.2.2 --- Bilayer graphene with vacancy --- p.58 / Chapter 4.3 --- Results --- p.60 / Chapter 4.3.1 --- Basic thermodynamic quantities --- p.61 / Chapter 4.3.2 --- Spin-spin and charge-charge correlations --- p.68 / Chapter 4.4 --- Summary --- p.69 / Chapter 5 --- Indirect exchange of magnetic impurities in zigzag graphene ribbon --- p.71 / Chapter 5.1 --- Introduction --- p.71 / Chapter 5.2 --- Density of states and local density of states --- p.73 / Chapter 5.3 --- Results --- p.76 / Chapter 5.3.1 --- Basic thermodynamic quantities --- p.77 / Chapter 5.3.2 --- Spin-spin correlation --- p.80 / Chapter 5.3.3 --- Weight of different components of the d electron wave function --- p.84 / Chapter 5.3.4 --- Comparison : two magnetic impurities in bulk graphene sheet --- p.85 / Chapter 5.4 --- Summary --- p.88 / Chapter 6 --- Summary and discussions --- p.89 / Bibliography --- p.92 / Chapter A --- Derivation of the input Green’s function G0(τ ) in Bernal stacked bilayer graphene --- p.101 / Chapter B --- Input Green’s function for U = 0 in bilayer graphene in the presence of a vacancy --- p.106

Graphene, since its first isolation, carries many promises on its superior properties. However, unlike its conventional two-dimensional (2D) counterparts, e.g. Si and GaAs systems, graphene represents the first 2D systems built on an atomically thin structure. With every atoms on the surface, graphene is severely affected by the environment and the measured properties have not reaching its full potential. Avoiding all possible external contamination sources is the key to keep graphene intact and to maintain its high quality electronic properties. To achieve this, it requires a revolution in the graphene device structure engineering, because all factors in a conventional process are scattering sources, i.e. substrate, solvent and polymer residues. With our recent two inventions, i.e. the van der Waals transfer method and the metal-graphene edge-contact, we managed to completely separate the layer assembly and metallization processes. Throughout the entire fabrication process, the graphene layer has never seen any external materials other than hexagonal boron nitride, a perfect substrate for graphene. Both optical and electrical characterizations show our device properties reach the theoretical limit, including low-temperature ballistic transport over distances longer than 20 micrometers, mobility larger than 1 million cm²/Vs at carrier density as high as 2 ×10^12 cm^-2, and room-temperature mobility comparable to the theoretical phonon-scattering limit. Moreover, for the first time, we demonstrate the post-fabrication cleaning treatments, annealing, is no longer necessary, which greatly eases integration with various substrate, such as CMOS wafers or flexible polymers, which can be damaged by excessive heating. Therefore the progress made in this work is extremely important in both fundamental physics and applications in high quality graphene electronic devices. Furthermore, our work also provides a new platform for the high quality heterostructures of the 2D material family.

Electron-electron (e-e) interactions in 2-dimensional electron gases (2DEGs) can lead to many-body correlated states such as the the fractional quantum Hall effect (FQHE), where the Hall conductance quantization appears at fractional filling factors. The experimental discovery of an anomalous integer quantum Hall effect in graphene has faciliated the study of the interacting electrons which behave like massless chiral fermions. However, the observation of correlated electron physics in graphene is mostly hindered by strong electron scattering caused by charge impurities. We fabricate devices, in which, electrically contacted and electrostatically gated graphene samples are either suspended over a SiO₂ substrate or deposited on a hexagonal boron nitride layer, so that a drastic suppression of disorder is achieved. The mobility of our graphene samples exceeds 100,000 cm²/Vs. This very high mobility allows us to observe previously inaccessible quantum limited transport phenomena. In this thesis, we first present the transport measurements of ultraclean, suspended two-terminal graphene (chapter 3), where we observe the Fractional quantum Hall effect (FQHE) corresponding to filling fraction ν=1/3 FQHE state, hereby supporting the existence of interaction induced correlated electron states. In addition, we show that at low carrier densities graphene becomes an insulator with a magnetic-field-tunable energy gap. These newly discovered quantum states offer the opportunity to study correlated Dirac fermions in graphene in the presence of large magnetic fields. Since the quantitative characterization of the observed FQHE states such as the FQHE energy gap is not straight-forward in a two-terminal measurement, we have employed the four-probe measuremt in chapter 4. We report on the multi-terminal measurement of integer quantum Hall effect(IQHE) and fractional quantum Hall effect (FQHE) states in ultraclean suspended graphene samples in low density regime. Filling factors corresponding to fully developed IQHE states, including the ν±1 broken-symmetry states and the ν=1/3 FQHE state are observed. The energy gap of the 1/3 FQHE, measured by its temperature-dependent activation, is found to be much larger than the corresponding state found in the 2DEGs of high-quality GaAs heterostructures, indicating that stronger e-e interactions are present in graphene relative to 2DEGs. In chapter 5, we investigate the e-e correlations in graphene deposited on hexagonal boron nitride using the thermopower measurements. Our results show that at high temperatures the measured thermopower deviates from the generally accepted Mott's formula and that this deviation increases for samples with higher mobility. We quantify this deviation using the Boltzmann transport theory. We consider different scattering mechanisms in the system, including the electron-electron scattering. In the last chapter, we present the magnetothermopower measurements of high quality graphene on hexagonal boron nitride, where we observe the quantized thermopower at intermediate fields. We also see deviations from the Mott's formula for samples with low disorder, where the interaction effects come into play . In addition, the symmetry broken quantum Hall states due to strong electron-electron interactions appear at higher fields, whose effect are clearly observed in the measured in mangeto-thermopower. We discuss the predicted peak values of the thermopower corresponding to these states by thermodynamic arguments and compare it with our experimental results. We also present the sample fabrication methods in chapter 2. Here, we first explain the fabrication of the two-terminal and multi-terminal suspended graphene and the current annealing technique used to clean these samples. Then, we illustrate the fabrication of graphene on hexagonal boron nitride as well as encapsulated graphene samples with edge contacts. In addition, the thermopower measurement technique is presented in Appendix A, in which, we explain the temperature calibration, DC and AC measurement techniques.

This thesis describes optical microscopical and spectroscopical studies of 2D materials, including graphite/graphene and multilayer/single layer MoS2, under strong charge transfer doping. Under this conceptually unifying umbrella lie many aspects of materials behaviors unique to each of the systems. The strong chemical doping results from intercalation and surface adsorption, and changes the electronic properties of the host 2D materials drastically. Associated with the significant electronic change, aspects such as mass transport, surface reaction, and phase transformation are covered in the following chapters. The first chapter introduces representative members of the 2D materials family, graphene and molybdenum disulphide (MoS2). It briefly reviews the history, discovery and unique properties of each materials class. The other part of the introduction focuses on the main methods utilized in the study of these materials. A concise survey of Raman spectroscopy and optical reflectance contrast spectroscopy will be presented. The second chapter investigates the intercalation process of Li into bulk graphite. This is a revisit of an extensively studied subject, with a new set of experiments and theories. Here we show that the daunting technical difficulties of disentangling complex electrochemical systems can be cleanly addressed with optical methods with well defined samples. Measuring and understanding the intrinsic transport of Li in graphite electrodes has been a difficult task. The challenge is well recognized to stem from a multitude of simultaneous electrochemical processes as well as systematic heterogeneities in the sample. We distinguish the Li intercalation process in graphite from all other processes, combining optical reflectance microscopy and Raman spectroscopy. The heterogeneity problem is circumvented by using lithography to tailor a single crystal into a defined geometry. We apply two levels of theoretical models to interpret the intrinsic information revealed in our data. Concentration dependent diffusion coefficients are measured, in agreement with theoretical results. The effects of sample geometry and electrode reaction kinetics on the overall intercalation are elucidated. The third chapter presents the study of lithiation on single and few layer graphene. Raman spectroscopy reveals a high doping level similar in strength to that of the bulk intercalated compound. The optical reflectance imaging, however, shows a different observation from the bulk case. We directly visualize the surface film formation and associated strong doping. The lithiation in single and few layer graphene progresses differently from the bulk graphite, since certain stages of the intercalation compound cannot be sustained by a single or few layer sample. The realization of strong charge transfer doping in lithiated single and few layer graphene could lead to discoveries of interesting physics. The direct visualization of surface film formation could have important implications in the design of electrochemical energy storage systems. The fourth chapter explores the structural effect of strong charge transfer doping in bulk and multilayer MoS2 with optical methods. MoS2, as a representative material of the transition metal dichalcogenide family, possesses different structural polymorphs. Strong charge transfer doping induces a structural phase change, which goes from the usual thermodynamically stable semiconducting 2H phase into the metallic 1T/1T' phase. The metallic 1T/1T' structure can remain a metastable phase without the stabilization of intercalants. We optically induce the 1T/1T' to 2H phase change and measure the temperature dependent kinetics of the structural phase transformation with in situ Raman spectroscopy. We demonstrate a photolithography technique, which efficiently patterns in-plane coherent heterojunctions between 1T/1T' and 2H MoS2. The fifth and final chapter describes the study of the structural change in single layer MoS2. More spectroscopic methods are employed for characterization, such as photoluminescence spectroscopy and second harmonic generation. The results indicate that the structural change occurs in single layer MoS2 after reaction with n-butyl lithium. The structural change can be reversed by thermal and laser annealing, similar to the case of bulk and multilayer MoS2. The annealed MoS2 exhibits reduced crystallinity. Future directions to further this work are outlined in the last section.

Since its physical isolation via the "scotch tape method," graphene (a monolayer of graphite) has attracted much attention from both the solid-state and high-energy scientific communities because its elementary excitations mimic relativistic chiral fermions. This has allowed graphene to act as a testbed for exploring exotic forms of symmetry breaking and for verifying certain longstanding theoretical predictions dating back to the very first formulation of relativistic quantum mechanics. In this dissertation I describe scanning tunneling microscopy and spectroscopy experiments that visualize ordered electronic states in graphene that originate from its unique chiral structure. Two detailed investigations of chemical vapor deposition graphene grown on copper are presented. In the first, a heretofore unrealized phase of graphene with broken chiral symmetry called the Kekulé distortion is directly visualized. In this phase, the graphene bond symmetry breaks and manifests as a (√3×√3)R30° charge density wave. I show that its origin lies in the interactions between individual vacancies ("ghost adatoms") in the crystalline copper substrate that are mediated electronically by the graphene. These interactions induce the formation of a hidden order in the positions of the ghost adatoms that coincides with Kekulé bond order in the graphene itself. I then show that the transition temperature for this ordering is 300K, suggesting that Kekulé ordering occurs via enhanced vacancy diffusion at high temperature. In the second, Klein tunneling of electrons is visualized for the first time. Here, quasi-circular regions of the copper substrate underneath graphene act as potential barriers that can scatter and transmit electrons. At certain energies, the relativistic chiral fermions in graphene that Klein scatter from these barriers are shown to fulfill resonance conditions such that the transmitted electrons become trapped and form standing waves. These resonant modes are visualized with detailed spectroscopic images with atomic resolution that agree well with theoretical calculations. The trapping time is shown to depend critically on the angular momenta quantum number of the resonant state and the radius of the trapping potential, with smaller radii displaying the weakest trapping.

Graphene, first isolated in 2004 by Andre Geim and Konstantin Novoselov, is an atomically thin two-dimensional layer of hexagonal carbon that has been extensively studied due to its unique electronic, mechanical, thermal and optical properties. Its vast potential has led to the development of a wide variety of novel devices such as, transistors, solar cells, batteries and sensors that offer significant advantages over the conventional microelectronic ones. Although graphene-based devices show very promising performance characteristics, limited has been done in order to evaluate how these devices operate in a radiation harsh environment. Undesirable phenomena such as total dose effects, single event upsets, displacement damage and soft errors that silicon-based devices are prone to, can have a detrimental impact on performance and reliability. Similarly, the significant effects of irradiation on carbon nanotubes indicate the potential for related radiation induced defects in carbon-based materials, such as graphene. In this work, we fabricate graphene field effect transistors (GFETs) and systematically study the various effects of ionizing radiation on the material and device level. Graphene grown by chemical vapor deposition (CVD) along with standard lithographic and shadow masking techniques, was used for the transistor fabrication. GFETs were subjected to different radiation sources, such as, beta particles (electron radiation), gamma (photons) and ions (alpha, protons and Fe particles) under various radiation doses and energies. The effects on graphene’s crystal structure, transport properties and doping profile were examined by using a variety of characterization tools and techniques. We demonstrate not only the mechanisms of ionized charge build up in the substrate and displacement damage effects on GFET performance, but also that atmospheric adsorbents from the surrounding environment can have a significant impact on the radiation hardness of graphene. We developed different transistor structures that mitigate these effects and performed computer simulations to enhance even further our understanding of radiation damage. Our results show that devices using a passivation layer and a shielded gate structure were less prone to irradiation effects when compared to the standard back-gate GFETs, offering less performance degradation and enhanced stability over prolonged irradiation periods. This is an important step towards the development of radiation hard graphene-based devices, enabling operation in space, military, or other radiation sensitive environments.

Since the successful isolation of graphene in 2004, two-dimensional (2D) materials have become one of the hottest research fields in material science. My research is about two kinds of popular 2D materials--graphene and transition metal dichalcogenides (TMDCs). Making graphene into nanoribbons has been predicted and demonstrated to be an effective way to open a bandgap in this pristinely zero-bandgap 2D material. But the rough edge condition of etched graphene nanoribbons has always been a big issue adversely affecting electron transport performance. The electron mean free path of this kind of devices is usually way below the channel width. By using a dual-gate structure based on bilayer graphene/hexagonal boron nitride heterostructure, we found a way to form 300nm-wide conducting channels with high aspect ratio (>15) that can achieve ballistic transport, indicating perfect edge conditions. As the first star member of TMDCs family, monolayer MoS2 is predicted to be strongly piezoelectric, an effect that disappears in the bulk owing to the opposite orientations of adjacent atomic layers. We conduct the first experimental study of the piezoelectric properties of two-dimensional MoS2 and show that cyclic stretching and releasing of thin MoS2 flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, whereas no output is observed for flakes with an even number of layers. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90 degrees. Transport measurements show a strong piezotronic effect in single-layer MoS2, but not in bilayer and bulk MoS2. Monolayer WSe2, another popular TMDC, has also attracted much recent attention. In contrast to the initial understanding, the minima of the conduction band are predicted to be spin split. Because of this splitting and the spin-polarized character of the valence bands, the lowest-lying excitonic states in WSe2 are expected to be spin-forbidden and optically dark. We show how an in-plane magnetic field can brighten the dark excitonic states and allow their properties to be revealed experimentally in monolayer WSe2. In particular, precise energy levels for both the neutral and charged dark excitons were obtained. Greatly increased emission and valley lifetimes were observed for the brightened dark states as a result of their spin configuration.

The behavior of water confined at the nanoscale plays a fundamental role in biological processes and technological applications, including protein folding, translocation of water across membranes, and water filtration and desalination processes. Remarkably, nanoscale confinement can drastically alter the properties of water. Understanding these changes in the physical behavior of water can provide new insights into many scientific questions and technical challenges. This thesis focuses on phase diagrams of water confined by graphene and graphene oxide. First, by performing Molecular Dynamic (MD) simulations, we constructed phase diagrams of water confined by graphene, a hydrophobic smooth surface. We found that the phase behaviors of water confined by graphene are complicated. In the phase diagram, monolayer square ice, bilayer square ice, liquid and vapor phases were presented. The non-monotonic cavitation pressures as a function of walls separations was unexpected. The values of cavitation pressures significantly deviated from the classical prediction for bulk water. Next, I moved to water under hydrophilic confinements. The first model used was a hydrophilic graphene-based surface where graphene C-water O interactions were tuned to create a hydrophilic surface but maintaining the geometry of the graphene. The phase diagram of water confined by hydrophilic graphene is presented. The extremely high magnitude of cavitation pressures found in this analysis suggests that energy can be converted efficiently from changes in relative humidity. Furthermore, the oscillation of cavitation pressures as a function of walls separations is relevant to water transportation. By randomly distributing hydroxyl groups on graphene, we saw similar cavitation pressures in a graphene oxide (GO) model.

Two dimensional (2D) materials have been explosively researched since the discovery of graphene but the applications of 2D materials have been extremely constrained because of a variety of shortcomings in the materials such as zero bandgap in graphene or defective growth techniques for wide-bandgap materials. Nonetheless, such novel materials are very promising in the doomed situation which Moore’s law keeps slowing down. Graphene and αMoO3 have been particularly of interest because graphene has developed large-scale growth methods and αMoO3 has wide bandgap. In case of graphene, searching for the applications with zero bandgap has been important and in the other, αMoO3 has not been developed for large-scale growth techniques yet even though the applications are strongly expected to be developed. In this thesis, unconventional CVD graphene electronics and large scale αMoO3 synthesis have been studied for very large scale integration (VLSI). A 512 flexible graphene voltage amplifier array and the highest peak-to-valley current ratio NDR devices emitting green color in graphene nanogap are presented so that large-scale CMOS compatible circuit integration can be available for bio and RF (radio frequency) applications. Having 2.8eV bandgap, a large-scale growth method for αMoO3 is developed for the first time showing ambipolar and memristive behaviors.

In this thesis, we investigate how the optical and electronic properties of graphene may be modified in proximity to various other materials. We present several examples of how modification in this way can help make graphene better suited for specific device applications. We develop a method of up-scaling the fabrication of FeCl3-intercalated few-layer graphene from micron-sized flakes to macroscopic films so that it may be used as a transparent electrode in flexible light-emitting devices. We also find that photo-responsive junctions can be arbitrarily written into FeCl3-intercalated few-layer graphene by means of optical lithography. These junctions produce photocurrent signals that are directly proportional to incident optical power over an extended range compared to other graphene photodetectors. Through theoretical analysis of these junctions, we conclude that the enhanced cooling of hot carriers with lattice phonons is responsible for this behaviour. Finally, we trial rubrene single crystals as the light-absorbing layer in a graphene phototransistor. We find that rubrene single crystal-graphene interfaces exhibit enhanced charge transfer efficiencies under illumination with extremely weak light signals. Through a comparative study with similar devices, we conclude that the wide variation in sensitivity amongst graphene phototransistors is largely due to extraneous factors relating to device geometry and measurement conditions.