The production of large area, high quality two-dimensional (2D) materials using chemical vapour deposition (CVD) has been an important and difficult topic in contemporary materials science research, after the discovery of the diverse and extraordinary properties exhibited by these materials. This thesis mainly focuses on the CVD synthesis of two 2D materials; bilayer graphene and monolayer tungsten disulphide (WS2). Various factors influencing the growth of each material were studied in order to understand how they affect the quality, uniformity, and size of the 2D films produced. Following this, these materials were combined to fabricate 2D vertical heterostructures, which were then spectroscopically examined and characterised. By conducting ambient pressure CVD growth with a flat support, it was found that high uniform bilayer graphene could be grown on the centimetre scale. The flat support provides for the consistent delivery of precursor to the copper catalyst for graphene growth. These results provide important insights not only into the upscaling of CVD methods for growing large area, high quality graphene and but also in how to transfer the product onto flexible substrates for potential applications as a transparent conducting electrode. Monolayer WS2 is of interest for use in optoelectronic devices due to its direct bandgap and high photoluminescence (PL) intensity. This thesis shows how the controlled addition of hydrogen into the CVD growth of WS2 can lead to separately distributed domains or centimetre scale continuous monolayer films at ambient pressure without the need for seed molecules, specially prepared substrates or low pressure vacuum systems. This CVD reaction is simple and efficient, ideal for mass-production of large area monolayer WS2. Subsequent studies showed that hexagonal domains of monolayer WS2 can have discrete segmentation in their PL emission intensity, forming symmetric patterns with alternating bright and dark regions. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. These results provide important insights into the spatially varying properties of these CVD-grown TMDs materials, which may be important for their effective implementation in fast photo sensors and optical switches. Finally, by introducing a novel non-aqueous transfer method, it was possible to create vertical stacks of mixed 2D layers containing a strained monolayer of WS2, boron nitride, and graphene. Stronger interactions between WS2 on graphene was found when swapping water for IPA, likely resulting from reduced contamination between the layers associated with aqueous impurities. This transfer method is suitable for layer by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, a result which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with large area coverage and controllable thickness on the atomic level.

<p dir="ltr">Two-dimensional (2D) perovskite semiconductors are an emerging family of hybrid materials featuring a built-in quantum well architecture which has gained much interest due to its potential as a promising candidate for next-generation photovoltaic and optoelectronic applications. To successfully integrate 2D perovskites as efficient devices, it is imperative that a thorough understanding of the fundamental properties these materials possess and how their complex heterostructures behave is established. However, to date, the synthetic challenges regarding high-quality crystals of these materials due to the structural complexity and the hybrid nature have impeded further progress in this area. Thus, we demonstrate a general method to construct tunable 2D organic semiconductor-incorporated perovskites (OSiP) by simultaneously manipulating slab thickness of the inorganic layers and conjugation length of the organic substituents. The energy band offsets and exciton dynamics at the organic-inorganic interfaces were elucidated using computational means and ultrafast spectroscopy, while lattice dynamics were quantified via temperature-dependent spectroscopy and X-ray diffraction studies. Results show that longer and more planar π-conjugated organic ligands induce a more rigid inorganic crystal lattice, which leads to suppressed exciton-phonon interactions and superior optoelectronic properties such as efficient lasing.</p><p dir="ltr">Furthermore, understanding ion migration in two-dimensional (2D) perovskite materials is key to enhancing device performance and stability as well. However, prior studies have been primarily limited to heat and light-induced ion migration. To investigate electrically induced ion migration in 2D perovskites, we construct a high-quality single crystal 2D perovskite heterostructure device platform with near defect-free van der Waals contact. While achieving real-time visualization of directional ion migration, we also uncover the unique behavior of halide anions inter-diffusing towards the opposite direction under prolonged bias. Confocal microscopy imaging reveals a halide migration channel that aligns with the crystal and heterojunction edges. After sustained ion migration, stable junction diodes exhibiting up to ~1000-fold forward to reverse current ratio are realized. Unraveling the fundamental properties of 2D OSiPs as well as ion migration in 2D perovskite heterostructures paves the way towards stable and efficient devices.</p>

After the experimental discovery of graphene at the beginning of the 21st century, many other interesting 2D materials have been discovered. However, the electrical and optical properties of these layers are greatly influenced by the composition and quality of the surrounding materials. In order to preserve the exceptional properties of thin films, attention has gradually been drawn to heterostructures from 2D composite materials. This thesis describes the preparation and characterization of heterostructures composed of graphene and hexagonal boron nitride. In addition, a specific focus will be placed on optimizing the production process of heterostructures by the dry thin film transfer process, prepared by micromechanical exfoliation. Characterization and quality of prepared layers are controlled by Raman spectroscopy, while morphology is examined by atomic force microscope (AFM). Furthermore, the electrical properties of the graphene-hBN device are discussed and the charge carrier of the graphene field-effect transistor is measured.