The Discovery of Tunable 2D Materials and van der Waals Heterostructures for Next-gen Technologies

To keep up with the global demand for faster, lighter, and more energy-efficient technologies, there is a pressing need to make device components thinner and smaller while at the same time enhancing their functionality. The world's largest semiconductor foundry, Taiwan Semiconductor Manufacturing Company (TSMC), is racing to do just that. However, current materials are approaching their size limits, meaning they cannot be reduced further without compromising their stability or performance. Therefore, to further drive miniaturisation, the discovery of novel materials that are stable at these sizes and have the necessary properties is not just a goal, but a requirement. One such promising class of materials is two-dimensional (2D) materials. 2D materials are confined to the nanoscale on one dimension, giving them sheet-like structures. Besides boasting a wide range of unique, unusual and novel properties compared to their bulk counterparts, these properties have been predicted to be highly tunable by applying external stimuli, such as functionalisation, strain, or an electric field. In addition, stacking 2D materials on top of each other, like atomic building blocks, creates van der Waals heterostructures (vdWHs) that can have unique properties not possessed by the individual layers. In recent years, the number of theoretically calculated and experimentally synthesised 2D materials has dramatically increased, and therefore, the number of possible stacking combinations has as well. A computational approach could accelerate the discovery of new materials by rapidly investigating a large number of 2D materials and their vdWHs to select promising candidates for synthesis. This thesis employs density functional theory (DFT) to examine the structural, optical, piezoelectric, and electronic properties of monolayer 2D materials and bilayer van der Waals heterostructures and how external factors like chemical modification, applied electric field, or mechanical strain can affect these properties. The materials considered in this thesis are monolayer 2D phosphorene, 19 bilayer van der Waals heterostructures composed of CuInP2S6 combined with another 2D material (CuInP2S6/X, where X = a 2D semiconductor) and the following 6 bilayer van der Waals heterostructures, HfS2/MoTe2, HfS2/WTe2, 1T-HfS2/WTe2, TiS2/WSe2, TiS2/ZnO, and TiSe2/WTe2. The effect of functionalisation, strain, and electric field, on the properties of these systems is investigated. 2D black phosphorus (phosphorene) has been shown to have excellent flexibility and highly tunable properties, which makes it ideal for flexible nanotechnologies. In this thesis, the interplay between the chemical and mechanical tunability of phosphorene functionalised with halogens is investigated. The coverage of halogens considered was 0.042, 0.0625, 0.125 and 0.25 monolayer (ML). After functionalisation, phosphorene was calculated to have a more sensitive electron and hole-effective mass in response to strain. A switching of the electrical conduction direction (from the armchair to zigzag direction) can be achieved using a smaller strain value after functionalisation. For pristine phosphorene this occurs with a tensile strain value of 4-5%, whereas, for halogen-functionalised phosphorene, this occurs at a lower strain value of 3-4%, resulting in more affordable conductance switching. Compared to pristine phosphorene, the band gap was shown to be highly dependent on the coverage and concentration of the halogen. Functionalisation with any halogen at a low concentration (0.0417 ML or 0.0625 ML) results in a widening of the band gap by up to 0.35 eV. For higher concentrations (0.125 and 0.25 ML), the band gap was calculated to be further modified (narrowed or widened) and, for some materials, reduced to 0 eV. Functionalisation at a coverage of 0.0625, 0.125 or 0.25 ML (except for the 0.25 ML Br-phosphorene system) was calculated to induce a piezoelectric response. The largest piezoelectric constant (e22) was calculated to be 1.31 C m-2 for the I-functionalised phosphorene (0.0625 ML), which outperforms the monolayer monochalcogenides SnSe, SnS, GeSe and GeS by 40%, and phosphorene oxide by 255%. The ferroelectric material CuInP2S6 has been proposed as a promising candidate for the creation of tunable van der Waals heterostructures. In this thesis, I present 19 new systems composed of monolayer CuInP2S6 combined with another 2D monolayer semiconductor. All 19 vdWHs were calculated to have a type-II band alignment. Due to the ferroelectric properties of CuInP2S6, switching between the ferroelectric states resulted in a range of unique effects, including changes to the band gap or band alignment, direct-indirect or indirect-direct transitions, and shifts in the peaks present in the absorption spectrum. Such differences allow these materials to be employed in a range of applications, including nanoelectronic, optoelectronic and ferroelectric memory devices. An additional 6 vdWHs, HfS2/MoTe2, HfS2/WTe2, 1T-HfS2/WTe2, TiS2/WSe2, TiS2/ZnO, and TiSe2/WTe2 were all calculated to have a type-III band alignment. The effects of strain and electric field on the band alignment are examined. The application of compressive strain along the a-direction resulted in a widening of the band gap for HfS2/MoTe2, HfS2/WTe2 TiS2/WSe2 and TiSe2/WTe2, and a type-III to type-II transition. A similar transition was shown for HfS2/MoTe2, 1T-HfS2/WTe2, TiS2/ZnO, and TiSe2/WTe2 due to the application of a positive electric field. Based on these effects, a multifunctional tunnelling device was proposed. With no strain (HfS2/MoTe2, HfS2/WTe2 TiS2/WSe2 and TiSe2/WTe2) or no applied electric field (HfS2/MoTe2, 1T-HfS2/WTe2, TiS2/ZnO, and TiSe2/WTe2), the device could be employed as a tunnel field-effect transistor or an Esaki diode, whereas using external stimuli, the device could be employed for various type-II applications. The findings presented in this thesis have provided key insights into the interplay of functionalisation and strain on phosphorene and its potential applications in flexible nano- and opto- electronics. In addition, this work provides a set of 25 novel vdWHs with a range of tuneable properties that make these systems suitable for many unique applications. Overall, the thesis presents a range of 2D materials and vdWHs that are versatile, having highly tunable properties, which makes them promising platforms for the miniaturisation of next-generation technologies.<p></p>