Researchers have recently become interested in two-dimensional materials such as graphene, hexagonal boron nitride (h-BN), Transition Metal Dichalcogenides (TMDs), etc. Their 2D hexagonal structures result in unique properties, which make these materials attractive for scientists and engineers. In this work, we investigated the electronic properties of graphene, h-BN, and MoS2 based on density functional theory (DFT). We first studied the electronic properties of monolayers of different materials. We found a zero bandgap and observed massless Dirac Hamiltonian in graphene. For h-BN, a large bandgap at K-point was observed. Also, we observed the bandgap opening in MoS2 and a strong splitting of its bands. Then, we extended these studies to graphene and h-BN bilayers. For graphene bilayer, we observed a gapless material and massive Dirac fermions. For h-BN bilayer, an indirect bandgap was observed, smaller in comparison with its monolayer. The main focus of this study was the investigation of graphene/h-BN heterostructures for different stacking configurations. The suitability of h-BN as a substrate for graphene is due to its small lattice constant mismatch with graphene and its high insulating gap (~ 5 eV). Another important aspect to be observed in graphene/h-BN heterostructures is the gap opening brought by the h-BN layer proximity to the initially gapless graphene layer. We found the effect of bandgap opening in graphene/h- BN and determined the most stable configuration which is the AB[CB]. This work supports the findings of many researchers who demonstrate that graphene/h-BN heterostructures are very useful as building blocks for nanodevices with desirable electronic properties.

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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.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with intrinsic band gaps are considered to be prospective alternatives for graphene in the applications of emerging nano-semiconductor devices. As a significant member of the TMDs family, WSe₂ with superior optical properties attracts increasing attention, especially in the optoelectronics. In this thesis, the electronic and mechanical properties of 2D WSe₂ have been studied experimentally and theoretically. Firstly, the fabrication of substrate-supported and suspended pre-patterned WSe₂ FETs with the low-cost optical lithography and vapour HF etching technology have been realised. The subsequent electrical measurement of the fabricated WSe₂ FETs indicates that the WSe₂/dielectric interface can affect the electrical performance of 2D WSe₂ negatively. To gain more insights on the impact of field-effect on 2D WSe₂, first-principle calculations have been conducted in this research to study the evolutions of the crystal structure, electronic band structure, conductive channel size, and electrical transport property of WSe2 under various levels of field-effect. Furthermore, a layer thinning and chemical doping method of 2D WSe₂ by vapour XeF₂ exposure featured with good air-stability, scalability, and controllability has been developed to enable the layer engineering of 2D WSe₂ and integration of 2D WSe₂ to logic circuits, solar cells, and light-emitting diodes (LED). The thinning and doping mechanism has been investigated with a combination of Raman spectroscopy, photoluminescence (PL) spectroscopy, and Xray photoelectron spectroscopy (XPS) characterization techniques. Afterwards, the inplane elastic properties (including the Young's modulus, breaking strain, and etc.) of 2D WSe₂ have been measured with nanoindentation experiments implemented by atomic force microscopy (AFM). The results prove the suitability of 2D WSe₂ in the applications of flexible devices and nanoelectromechanical systems (NEMS) operating in the audio resonance frequency, such as acoustic sensors and loudspeakers. To provide a comprehensive understanding of the strain engineering of 2D WSe₂, the strain induced variations of the crystal structure, electronic band structure, and electrical transport property of 2D WSe₂ have been further studied with first-principle calculations, which paves the way for the performance tuning of 2D WSe₂ devices via strain and applications of 2D WSe₂ in strain sensors.

Semiconducting nanocrystals in open-access microcavities are promising systems in which enhanced light-matter interactions lead to quantum effects such as the modulation of the spontaneous emission process and exciton-polariton formation. In this thesis I present improvements of the open cavity platform which serves to confine the electromagnetic field with mode volumes down to the λ³ regime and demonstrate results in both the weak and strong coupling regimes of cavity quantum electrodynamics with a range of different low-dimensional materials. I report cavity fabrication details allowing a peak finesse of 5 × 10⁴ and advanced photonic structures such as coupled cavities in the open cavity geometry. By incorporating two-dimensional materials and nanoplatelets in the cavity I demonstrate the strong coupling regime of light-matter interaction with the formation of exciton-polaritons, quasi-particles composed of both photon and exciton, at room temperature. In the perturbative weak coupling regime I show pronounced modulation of the single-photon emission from CdSe/ZnS quantum dots and the two-dimensional material WSe₂ and demonstrate Purcell enhancement of the spontaneous emission rate by factors of 2 at room temperature and 8 at low temperature. The findings presented in this thesis pave the way to establish open microcavities as a platform for a wide range of applications in nanophotonics and quantum information technologies.

Sodium-ion batteries (SIBs) are rechargeable batteries analogous to lithium-ion batteries but use sodium ions (Na+) as the charge carriers. They are considered a promising alternative for lithium-ion batteries (LIBs) in renewable large-scale energy storage applications due to their similar electrochemical mechanisms and abundant sodium resources. Two-dimensional (2D) materials, with atomic or molecular thickness and large lateral lengths, have emerged as important functional materials due to their unique structures and excellent properties. These 2D nanosheets have been highly studied as sodium-ion battery anodes. They have large interlayer spacing, which can effectively buffer the big volume expansion and prevent electrode collapse during the charge-discharge process. Different strategies such as preparing composites, heterostructures, expanded structures, and chemical functionalization can greatly improve cycling stability and lead to high reversible capacity. In this dissertation, state-of-the-art SIB based on 2D material electrodes will be presented. In particular, Tin-based 2D materials and laser-scribed graphene anodes are discussed. Different strategies involving engineering both synthesis methods, intrinsic properties of materials, and device architecture are used to optimize the battery performance.

van der Waals heterostructures meet other low-dimensional materials. Stacking of about 1 nm thick nanosheets with out-of-plane anchor groups functionalized with fullerenes integrates this zero-dimensional material into layered heterostructures with a well-defined chemical composition and without degrading the mechanical properties. The developed modular and highly applicable approach enables the incorporation of other low-dimensional materials, e.g. nanoparticles or nanotubes, into heterostructures significantly extending the possible building blocks.

<div>Since the debut of graphene, many 2D materials have emerged as promising candidates for silicon alternatives to extend Moore’s Law, such as MoS₂ and phosphorene. However, some common shortcomings such as low mobility, instability and lack of massive production methods limit the exploration and applications of these materials. Here, we introduce a novel member to the 2D category – high-mobility air-stable 2D tellurium film (tellurene).</div><div><br></div><div>Tellurium (Te) is a narrow bandgap semiconductor with unique one-dimensional chiral structure. Recently, a hydrothermal synthesizing method was developed to produce large-area tellurene nanofilms with thickness ranging from tens of nanometers down to few layers. In this thesis, a thorough investigation of Te properties in 2D quantum region was first carried out by various material characterization techniques including TEM and Raman spectroscopy. Potential applications of Te-based electronics, optoelectronic and thermoelectric devices were explored, and high-performance Te FETs were achieved with record-high drive current over 1 A/mm via device scaling and contact engineering. Magneto-transport, including weak anti-localization and Shubnikov-de-Haas oscillations was studied at cryogenic temperature. Quantum Hall effect was observed for the first time in both 2D electron and hole gases with mobility of 6,000 and 3,000 cm²/Vs, and non-trivial Berry phase in Te 2D electron system was detected as the first experimental evidence of massive Weyl fermions. This work not only demonstrates the great potential of tellurene films for electronics and quantum device applications, but also expands the spectrum of topological matters into a new material species - Weyl semiconductors.</div>

Since the scotch-tape isolation of graphene, two-dimensional (2D) materials have been studied with increasing enthusiasm. Two-dimensional transition-metal dichalcogenides are of particular interest as atomically thin semiconductors. These materials are naturally transparent in their few-layer form, have direct band gaps in their monolayer form, exhibit extraordinary absorption, and demonstrate unique physics, making them promising for efficient and novel optical devices. Due to the two-dimensional nature of the materials, their properties are highly susceptible to the environment above and below the 2D films. It is critical to understand the influences of this environment on the properties of 2D materials and on the performance parameters of devices made with the materials. For transparent optical devices requiring electrical contacts and gates, the effect of transparent conducting oxides on the optical properties of 2D semiconductors is of particular importance. The ability to tune the optical properties of 2D transition-metal dichalcogenides could allow for improved control of the emission or absorption wavelength of optical devices made with the materials. Continuously tuning the optical properties of these materials would be advantageous for variable wavelength devices such as photodetectors or light emitters. This thesis systematically investigates the intrinsic structural and optical properties of two-dimensional transition-metal dichalcogenide films, the effect of substrate-based optical interference on the optical emission properties of the materials, and demonstrates methods to controllably tune the luminescence emission of the materials for future optical applications. This thesis advances the study of these materials toward integration in future efficient and novel optical devices. The specific transition metal dichalcogenides investigated here are molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2). The thickness-dependence of the intrinsic in-plane crystal structure of these materials is elucidated with high-resolution transmission electron microscopy; thickness-dependent optical properties are studied using Raman and photoluminescence spectroscopies. This thesis investigates the optical interference effects from substrates with transparent conducting oxide layers on the optical properties of few-layer MoS2 films. An understanding of these effects is critical for integrating MoS2 into efficient optical devices. We predict contributions of optical interference effects to the luminescence emission of few-layer MoS2 films. The predictions are experimentally verified. We also demonstrate the use of optical interference effects to tune the wavelength and intensity of the luminescence emission of few-layer MoS2. This thesis explores the use of electric fields applied perpendicular to the films to continuously and reversibly tune the band gap of few-layer MoS2 for future variable wavelength devices. To facilitate integration into devices, we demonstrate electric fieldinduced band gap tuning by applying electric fields with a pair of transparent or semitransparent conducting layers, and without the need for direct electrical contact to the MoS2 films. The observed band gap tuning is attributed to the Stark Effect. We discuss challenges to maximizing the effect of electric field-induced band gap tuning. We demonstrate that optical interference effects do not prevent observation of band gap tuning via applied electric fields. We successfully combine two luminescence emission tuning methods: optical interference effects and electric field effects.

van der Waals heterostructures meet other low-dimensional materials. Stacking of about 1 nm thick nanosheets with out-of-plane anchor groups functionalized with fullerenes integrates this zero-dimensional material into layered heterostructures with a well-defined chemical composition and without degrading the mechanical properties. The developed modular and highly applicable approach enables the incorporation of other low-dimensional materials, e.g. nanoparticles or nanotubes, into heterostructures significantly extending the possible building blocks.