Theoretical understanding of mechanical properties of low-dimensional materials

Abstract

Low-dimensional materials including carbon nanotubes (CNTs), graphene, transition metal carbides and nitrides (MXenes), transition metal dichalcogenides (TMDs) and beyond have recently become the focus of intense research because of their outstanding physical and chemical properties. Amongst those characteristics, the mechanical properties of the low-dimensional systems play a significant role in exploring their potential applications in industrial sectors. Nevertheless, the mechanical characteristics of the low-dimensional material systems have not yet been fully understood. This is because direct experimental measurements of mechanical properties of these materials face great difficulties due to the critical availability of the high-quality crystals and sophisticated facilities. To this end, the numerical methods become a promising alternative for investigating such properties of the low-dimensional materials. Firstly, this thesis presents the background and objectives of this project (Chapter 1). The recent progresses in numerically exploring the in-plane mechanical properties of low-dimensional material systems, including first-principles density functional theory (DFT), force-field based classical molecular dynamics (MD), and the finite element method (FEM) are then discussed. Some recent cases have been discussed to show the advantages and disadvantages of these multiscale simulation methods (Chapter 2). Since carbon-based materials are considered to be one of the most important family of low-dimensional materials, a comprehensive numerical study was conducted to investigate the mechanical properties (e.g. critical buckling load and vibrational properties) of carbon-based structures, such as carbon nanotubes (CNTs) and their modifications. The FEM was employed because it is able to analyse the systems consisting thousands of atoms (Chapter 3), and investigate macroscopic mechanical properties. Afterward, the mechanical properties of molybdenum disulfides (MoS2), as one of the most important TMDs, were examined by means of DFT calculations. The impacts of the structural polytypes, as well as external pressure on the mechanical response of the layer structured MoS2 were also investigated (Chapter 4). In addition, a comprehensive study was conducted to explore the energy-dependent anisotropic mechanical properties of different types of bulk TMDs using the first principles DFT calculations. Different analyses such as Density of States (DOS) and crystal orbital Hamilton population (COHP) were performed to fully understand the behaviour of the layer-structured systems (Chapter 5). Investigating the in-plane mechanical properties of monolayer two-dimensional (2D) materials was also one of the main objectives of this research. Hence, the mechanical characteristics of the lateral TMDs were thoroughly examined with consideration of the impact of the heterostructure configurations on the stability of the 2D systems (Chapter 6). Finally, the recently discovered MXenes were numerically investigated, and the mechanical properties of the functionalized systems were systematically explored. The DFT results reveals that the studied MXenes exhibit significant mechanical strength, which in-plane Young’s moduli are even larger than that of graphene. It may open the avenue for promising applications of such novel 2D materials (Chapter 7). It is worthy to note that the studies on the mechanical properties of 2D materials are still in early stages. As such, it is vital that future studies focus on exploring plausible numerical approaches for engineering the mechanical characteristics and eventually expanding the application of the low-dimensional materials.