Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 157-165).

Exploring the properties of layered materials as a function of thickness has largely been limited to semiconducting materials as thin layers of metallic materials tend to oxidize readily in atmosphere. This makes it challenging to further understand properties such as superconductivity and charge density waves as a function of layer thickness that are unique to metallic compounds. This dissertation discusses a set of materials that use the modulated elemental reactants technique to isolate 1 to 3 layers of VSe2 in a superlattice in order to understand the role of adjacent layers and VSe2 thickness on the charge density wave in VSe2. The modulated elemental reactants technique was performed on a custom built physical vapor deposition to prepare designed precursors that upon annealing will self assemble into the desired heterostructure. First, a series of (PbSe)1+δ(VSe2)n for n = 1 – 3 were synthesized to explore if the charge density wave enhancement in the isovalent (SnSe)1.15VSe2 was unique to this particular heterostructure. Electrical resistivity measurements show a large change in resistivity compared to room temperature resistivity for the n = 1 heterostructure. The overall change in resistivity was larger than what was observed in the analogous SnSe heterostructure. v A second study was conducted on (BiSe)1+δVSe2 to further understand the effect of charge transfer on the charge density wave of VSe2. It was reported that BiSe forms a distorted rocksalt layer with antiphase boundaries. The resulting electrical resistivity showed a severely dampened charge density wave when compared to both analogous SnSe and PbSe containing heterostructures but was similar to bulk. Finally, (SnSe2)1+δVSe2 was prepared to further isolate the VSe2 layers and explore interfacial effects on the charge density wave by switching from a distorted rocksalt structure to 1T-SnSe2. SnSe2 is semiconductor that is used to prevent adjacent VSe2 layers from coupling and thereby enhancing the quasi two-dimensionality of the VSe2 layer. Electrical characterization shows behavior similar to that of SnSe and PbSe containing heterostructures. However, structural characterization shows the presence of a SnSe impurity that is likely influencing the overall temperature dependent resistivity. This dissertation includes previously published and unpublished co-authored materials.

Neste trabalho estudamos as propriedades eletrônicas de um sistema com dopagem planar ou delta. Resolvemos as equações de Schrodinger e Poisson autoconsistentemente na aproximação de Hatree para diferentes situações de interesse com ou sem campos magnéticos ou elétricos externos. O método empregado para resolvermos a equação de Schrodinger e baseado na técnica do split operator. Obtemos o potencial efetivo, os níveis de energia e a densidade eletrônica em função da temperatura, densidade e largura de difusão dos doadores. Para um campo magnético uniforme aplicado perpendicularmente ao plano de doadores mostramos que não ocorre nenhuma alteração na estrutura das sub-bandas eletrônicas no intervalo entre 0 e 20T. Para um campo magnético uniforme aplicado paralelamente ao plano de doadores os níveis eletrônicos são deslocados para energias mais elevadas provocando uma diminuição na população dos níveis mais excitados e um aumento significativo na massa efetiva do elétron. Para sistemas na presença de um campo elétrico externo calculamos a energia das sub-bandas eletrônicas, a ocupação, a polarização e a capacitância em função da voltagem externa, da densidade e da posição das impurezas doadoras. Nossos resultados para capacitância estão qualitativamente em acordo com recentes resultados experimentais. / In this work we have studied the electronic proprieties of a system with planar doping or &#948-doping. We have solved the Schrodinger and Poisson equations self-consistently in the Hatree approximation for several conditions with or without external magnetic or electric fields. In order to solve Schrodinger equation we have employed the split operator technique. The effective potential, the energy levels and the electronic density was calculated as a function of temperature, donor density and donor diffusion width. With external magnetic field applied perpendicular to the donors plane it is shown that the sub-bands energies are not altered for fields in the range of 0 to 20T. With magnetic fields applied parallel to the donors plane we have observed that the energy levels are shifted to higher energies and a depopulation of the excited levels occur, also an enhancement of the electron effective mass is found. With an electric field applied to the system we have calculated the sub-bands energy levels, the electron concentration, the polarization, and the capacitance as a function of the gate voltage, donor density and the position of the donor plane. Our results for the capacitance agree qualitatively quite well with recent experimental data.

Crystal symmetry and elemental composition play a critical role in determining the physical properties of materials. In layered van der Waals (vdW) heterostructures, a two-dimensional (2D) material layer can be influenced by interactions between adjacent layers, dictating that the measured properties of the combined system will be in part derived from the geometric structure within the active layers. This thesis examines active crystal symmetry tuning in composite heterostructures of two-dimensional (2D) materials, engineered via nanomechanically assisted twist angle control, and designed by careful consideration of lowest energy stacking configurations. The material systems, devices, and experimental setups described in this thesis constitute a platform featuring highly programmable properties that are on-demand and reversible. Two prototypical systems are discussed in detail. The first is graphene encapsulated between boron nitride (BN) crystals, wherein the alignment state between the three layers is controlled. The second is the same system, but with no graphene between the encapsulating BN layers. In both systems, a long-wavelength geometric interference pattern, also known as a moiré pattern, forms between the adjacent crystals as a consequence of lattice-constant mismatch and twist angle. The moiré pattern caries its own symmetry properties that are also demonstrated to be tunable, and can be thought of as an artificially constructed superlattice of periodic potential with wavelength much greater than the lattice constants of the constituent layers. In the BN-encapsulated graphene system we show drastic tunability of band gaps at primary and secondary Dirac points (PDP and SDPs) indicating reversible on-demand inversion symmetry breaking, as well as evidence of dual coexisting moiré superlattices and additional higher-order interference patterns that form between them. The all-BN system shows substantial enhancement and suppression of second harmonic generation (SHG) response from the vdW interface between the BN crystals when the quadrupole component of the SHG response is engineered to be minimal, by controlling for total layer number and layer number parity. Changes in the physical properties of each composite system are measured with a combination of electronic transport measurements, and optical measurements (Raman and SHG), as well as piezo-force microscopy (PFM) measurements that give direct imaging of the moiré pattern. A number of invented and adapted fabrication and actuation techniques for controlling the twist angle of a bulk vdW crystal are discussed, and in the latter portion of this thesis these techniques are extended to include actuation of monolayer flakes of 2D crystals. In this discussion several case studies are discussed, including twist angle control for a single sample monolayer tungsten diselenide on monolayer molybdenum diselenide, as well as twist angle control for twisted bilayer graphene and graphene on BN. Additionally, a novel in-plane bending mode for graphene on BN is demonstrated using similar techniques. Further discussion of actuation via traditional electrostatic MEMS techniques is also included, illustrating complete on-chip control for on-demand nanomechanical actuation of 2D materials in vdW heterostructures.

Metal-halide perovskites have attracted intense research endeavors because of their excellent optical and electronic properties. Different kinds of electronic and optoelectronic devices have been fabricated using perovskites. A feasible approach to utilize these properties in real device applications with improved performance and new functionalities is by fabricating heterostructures with extraneous materials. We have developed mixed-dimensional heterostructure systems using three-dimensional (3D) metal-halide perovskites and different types of different two-dimensional (2D) materials, including semimetal graphene, semiconducting phosphorus-doped graphitic-C3N4 sheets (PCN-S), and plasmonic Nb2CTx MXenes. First, selective growth of single-crystalline MAPbBr3 platelets on monolayer graphene by chemical vapor deposition (CVD) is achieved to prepare the MAPbBr3/graphene heterostructures. P-type doping from MAPbBr3 is observed in the monolayer graphene with a decreased work function of 272 meV under illumination. The photoresponse of the fabricated phototransistor heterostructure verifies the enhanced p-type character in graphene. Such kind of charge transfer can be used to improve device performance. Then, bulk-heterojunctions made of MAPbI3-xClx and PCN-S are prepared in solution. The matched band diagram and the midgap states in PCN-S present a convenient and efficient approach to reduce the dark current and increase the photocurrent of the as-fabricated photodetectors. As a result, the on/off ratio increases from 103 to 105, and the detectivity is up to 1013 Jones with an order of magnitude enhancement compared to the perovskite-only device. Last, plasmonic Nb2CTx MXenes and MAPbI3 heterostructures are prepared for photodiodes to broaden the detection band to near-infrared (NIR) lights. The use of the perovskite layer expanded the operation of the diode to the visible range while suppressing the dark current of the NIR-absorbing Nb2CTx layer. The fabricated photodiode reveals a detectivity of 0.25 A/W with a linear dynamic range of 96 dB in the visible region. In the NIR region, the device demonstrates an increased on/off ratio from less than 2 to near 103 and much faster response times of less than 30 ms. The improved performance is attributed to the passivation of the MAPbI3/Nb2CTx interface.

Graphene, a single atom thick hexagonally bonded sheet of carbon atoms, was first isolated in 2004 opening a whole new field in condensed matter research and material engineering. Graphene has hosted a whole array of novel physics phenomena as its carriers move at near the speed of light governed by the Dirac Hamiltonian, it has few scattering sites, it is easily gate-tunable, and hosts exciting 2D physics amongst many other properties. Graphene was only the tip of the iceberg in 2D research as researchers have since identified a whole family of materials with similar layered atomic structures allowing isolation into several atom thick monolayers. Monolayer material properties range from metals to semiconductors, superconductors, magnets and most other properties found in 3D materials. Naturally, this has led to making fully 2D heterostructures to study exciting physics and explore applications such as 2D transistors. It has recently been found that not only can you stack these materials at will but you can also tune their properties with an inter-layer twist between layers which at precise twist angles yields on-demand electronic correlations that can be easily tuned with experimental knobs leading to novel correlated phases. The pioneering techniques towards understanding each 2D material and heterostructures thereof have usually been with transport and optics. These techniques are inherently bulk macroscopic measurements which do not give insights into the nanoscale properties such as atomic-scale features or the nanoscale heterostructure properties that govern the systems. Atomic-scale structural and electronic insights are crucial towards understanding each system and providing proper guidelines for comprehensive theoretical understandings. In this thesis, we study the atomic-scale structural and electronic properties of various 2D systems using ultra-high vacuum (UHV) scanning tunneling microscopy and spectroscopy (STM/STS), a technique which utilizes electron tunneling with an atomically sharp tip to visualize atomic structure and low-energy spectroscopic properties. We focus on three major types of systems: twisted graphene heterostructures (magic angle twisted bilayer graphene and small angle double bilayer graphene), bulk and monolayer semiconducting transition metal dichalcogenides (TMDs), and 2D heterointerfaces (TMD - metal and graphene p-n junctions). We establish a number of state of the art methods to study these 2D systems in their cleanest, transport-experiment-like forms using surface probes like STM/STS including robust, clean, reliable contact methods and procedures towards studying micronscale exfoliated 2D samples atop hexagonal boron nitride (hBN) as well as photo-assisted STM towards studying semiconducting TMDs and other poorly conducting materials at low temperatures (13.3 Kelvin). We begin with one of the most currently mainstream topics of twisted bilayer graphene (tBG) where, near the magic angle of 1.1◦ the first correlated insulating and superconducting states in graphene were observed. A lack of detailed understanding of the electronic spectrum and the atomic-scale influence of the moir´e pattern had precluded a coherent theoretical understanding of the correlated states up til our work. We establish novel, robust methods to measure these micron-scale samples with a surface scanning probe technique. We directly map the atomic-scale structural and electronic properties of tBG near the magic angle using scanning tunneling microscopy and spectroscopy (STM/STS). Contrary to previous understandings (which predicted two flat bands with a several meV separation in the system), we observe two distinct van Hove singularities (vHs) in the local density of states (LDOS) around the magic angle, with a doping-dependent separation of 40-57 meV. We find that the vHs separation decreases through the magic angle with a lowest measured value of 7-13 meV at 0.79◦ . When doped near half moir´e band filling where the correlated insulating state emerges, a correlation-induced gap splits the conduction vHs with a maximum size of 6.5 meV at 1.15◦ , dropping to 4 meV at 0.79◦ . We find that more crucial to the magic angle than the vHs separation is that the ratio of the Coulomb interaction (U) to the bandwidth (t) of each individual vHs is maximized (as opposed to the proximity of the individual vHs’s), indicating that indeed electronic correlations are very important and suggesting a Cooper-like pairing mechanism based on electron-electron interactions. This establishes that magic angle tBG is to be understood in a single vHs picture where the band-width of the vHs is minimized. Spectroscopy maps show that three-fold (C3) rotational symmetry of the LDOS is broken in magic angle tBG, with an anisotropy that is strongest near the Fermi level, and is highly enhanced when the doping is in the vicinity of the correlated gap, indicating the presence of a strong electronic nematic susceptibility or even nematic order in tBG in regions of the phase diagram where superconductivity is observed. We next turn to twisted double bilayer graphene (tDBG), a system that is similar to tBG in phenomenology but turns out to be quite different. Correlated insulating and superconducting states were also found using transport in tDBG at a magic angle of 1.2-1.3◦ and ABC rhombohedral trilayer graphene aligned to hBN (ABC-tLG/hBN) with some stark differences such as displacement field tunable correlated states. We perform the first atomic-scale structural and electronic studies of small-angle tDBG as well as ABCA four layer rhombohedral stacked graphene and compare the findings to tBG. We first find that the moir´e pattern formed by tDBG is fundamentally different from tBG in that instead of hosting AB/BA Bernal stacking regions, it hosts BABA/ABCA (Bernal/rhombohedral) stacking domains. While we find this for small angle tDBG, these structural arguments will apply at all angles including the magic angle indicating that the flat bands and electron densities in tDBG are likely dominated at the ABCA sites. We use small angle tDBG to study large domains of four-layer ABCA graphene, revealing its displacement field dependent low energy spectroscopic structure and the flat band structure that comes with the four layer rhombohedral stacking which hosts the flattest band measured in any system of a 3-5 meV half-width. Furthermore, we measure the emergence of a 9.5 meV correlated gap in ABCA four-layer graphene at neutrality indicating that even without a hBN moir´e, ABCA graphene will likely host correlated states purely due to a flat band. These correlated states could be insulating or even superconducting in nature and the study thereof could provide crucial insight into whether superconductivity is related to Mott insulator physics as is suggested in the cuprates. When coupled to an hBN moir´e, these correlated states may be even stronger than that of magic angle tBG, magic angle tDBG and (most cer- tainly) ABC-tLG/hBN. Finally, we show that at Bernal - four-layer rhombohedral domain boundaries, there exists a topologically protected helical surface edge state. We next turn to the semiconducting TMDs. We find that semiconducting MoTe2 and MoSe2 have long range magnetic ordering as measured by muon spin resonance and SQUID at critical temperatures of 40 K and 100 K respectively. Using atomic-resolution STM/STS, we find that the semiconducting TMDs have a variety of intrinsic defects, one of which (a molybdenum substitution for a chalcogen, Mosub) we postulate using DFT is the cause of the long-range magnetism in the semiconducting TMDs which are not expected to host magnetism in their pristine structures. This finding establishes these semiconducting TMDs as magnetically ordered and adds them to the family of potential dilute magnetic semiconductor materials (the uniform robust fabrication of which has been sought-after for decades) which could have applications in spintronics. We then perform 13.3 Kelvin measurements (for the first time in these materials to our knowledge) on the same crystals using photoassisted STM, a technique that we establish to enable this low temperature measurement. The photo-assisted STM measurements reveal that not only are these defects magnetic but they host localized structural distortions which cover a large areas of the crystal surfaces. We find that these structural distortions are localized charge density waves due to a very high amount of localized doping that comes from the defects, putting the materials into a locally metallic regime and causing a phonon instability (found by phonon DFT). This finding of localized charge density waves in these high-quality semiconducting 2D materials is highly atypical for a semiconductor system and could have implications towards all techniques. The charge density waves could also be related to the measured magnetism as they have a much larger area of coverage in MoSe2 as opposed to MoTe2 which could be related to the critical temperature difference. We finally turn to two types of heterointerfaces, the first being metal-monolayer MoS2 junctions. We present measurements of the atomic-scale energy band diagram of junctions between various metals and heavily doped monolayer MoS2 using STM/STS. Our measurements reveal that the electronic properties of these junctions, at the fundamental limit of a minimized Schottky barrier, are dominated by 2D metal induced gap states (MIGS). These MIGS are characterized by a spatially growing measured gap in the local density of states (LDOS) of the MoS2 within 2 nm of the metal-semiconductor interface. Their decay lengths extend from a minimum of about 0.55 nm near mid gap to as long as 2 nm near the band edges and are nearly identical for Au, Pd and graphite contacts, indicating that this is a universal property of the monolayer 2D semiconductor. Our findings indicate that even in heavily doped semiconductors, the presence of MIGS sets the ultimate limit for electrical contact. These findings are generally applicable to any 2D semiconductor. We next look at another type of heterointerface, this time purely electronic in nature, graphene p-n junctions. Graphene p-n junctions should host interesting electron-optical properties such as electron collimation and Veselago lensing. While vague signatures of these have been observed, robust, definitive control of these properties are still lacking. We present the first atomic-scale characterization of state-of-the-art graphene p-n junctions using STM/STS revealing their current imperfections including significant electron-hole asymmetry, nonlinearity, roughness and intrinsic doping. We model the implications thereof and show that these imperfections strongly hinder electron-optical applications. Finally we explore the origin of these imperfections and potential avenues towards realizing better graphene p-n junction devices that may host much improved electron-optical properties.

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The electron-electron interaction is the origin of many interesting phenomena in condensed matter. These phenomena post challenges to theoretical physics and can lead to important future applications. Transition metal dichalcogenide heterostructures provide excellent platforms to study these phenomena because of the two-dimensional nature, large effective mass and tunable bandwidth with moiré potential. As electron bands become narrower such that the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. This dissertation describes the realization of this platform and probing of correlated phenomena with low- temperature transport measurements. As the first step, the electrical contact problem of few-layer transition metal dichalcogenides, which prohibits low-temperature transport measurements, needs to be solved. Two different contact schemes have been used to attack this problem. For p-type transition metal dichalcogenide, prepatterned platinum is used to bottom contact transition metal dichalcogenides. This method prevents channel from deterioration due to electron beam evaporation and the high workfunction platinum can place the Fermi level underneath the material valence band. Alternatively, for n-type transition metal dichalcogenides, a single layer of boron nitride is put on transition metal dichalcogenide before cobalt evaporation. This way, the boron nitride layer protects the transition metal dichalcogenide from the process of evaporation and can decrease the work function of cobalt thus putting Fermi level above the conduction band. With these contact methods, Ohmic contacts can be achieved at cryogenic temperature and probing the transition metal dichalcogenide heterostructures with transport measurements become accessible. Then, the magnetotransport properties of monolayer molybdenum disulphide and bilayer tungsten diselenide encapsulated with boron nitride with graphite dual-gate were measured. There are three unique features underlie this two dimensional electron gas system. First, the system is strong correlated. The Landau level spectrum reveals strong correlated signatures, such as enhanced spin-orbit coupling splitting and enhanced effective g-factor. Second, the longitudinal resistance/conductance at half-filling of Landau levels are found to depend on the spin orientation. The minority spin Landau level become totally localized at higher magnetic field. Third, in bilayer device the two layers are weak coupled and can be independently controlled by two gates. All this features establish transition metal dichalcogenide a unique platform for studying correlated physics. Finally, to achieve higher level of correlation, two layers of tungsten diselenide are stacked together with a small twist angle. With the help of moiré potential and layer hybridization, the bandwidth can be continuously tuned by the twist angle. In the range of 3 degree to 5.1degree, with moderate correlation strength, correlated insulating states are shown at half-filled flatband and are highly tunable with vertical electric field.

Research on heterojunction (HJ) metal oxide thin film transistors (TFTs) has accelerated remarkably over the last decade due to their superior performance over their conventional single-layer (SL) counterparts. Promising results in laboratory-scale demonstrations have further triggered an increased number of investigations into fabrication and processing techniques for the large-scale integration of HJ metal oxide TFTs. Nevertheless, a lack of consensus regarding the most appropriate scalable manufacturing technique, which combines low-cost and high-throughput fabrication, holds back new opportunities for HJ metal oxide TFTs in emerging applications. In this thesis, novel approaches and strategies are introduced to facilitate the large-scale integration of HJ metal oxide TFTs. The first study of this dissertation introduces the solution-processed In2O3/ZnO heterojunction TFTs with a high-κ bilayer dielectric consisting of Al2O3/ZrO2. Processing was carried out on rigid glass as well as flexible PEN substrates via rapid flash lamp annealing (FLA) as an alternative scalable and high-throughput processing route to conventional thermal annealing. In the second study of the dissertation, a novel 3D/2D/3D mixed-dimensional channel concept was developed with the combination of scalable spray coating and FLA techniques. The insertion of sprayed MoS2 nanoflakes between flashed SnO2/ZnO HJ results in outstanding device performance with a high mobility value of 62 cm2/Vs compared to single layers as well as heterojunction metal oxide TFTs, showing maximum mobility of 4.48 cm2/Vs. In the third study, the fabrication of In2O3/ZnO heterojunction metal oxide TFTs with solution-processed conductive Ti3C2Tx MXene contacts using a processing route that fully relies on a scalable spray coating process is demonstrated as an alternative to low-throughput vacuum-based electrodes. Notably, the proposed approach was successfully upscaled to a 4-inch glass substrate, underlining the significant potential garnered by MXene electrodes for industrial-scale electronics. The last study of the dissertation exploits the advantages of the adhesion-lithography (a-Lith) technique, which enables the development of coplanar self-aligned gate (SAG) In2O3/ZnO heterojunction TFTs and their facile integration into large-area electronics. Using the a-Lith technique, coplanar SAG architectures were fabricated where the gate and dielectric (Al and Al2O3, respectively) are located side by side with the source/drain electrodes (Au), separated from each other by nanogaps.

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