Methods and Trends in 2D van der Waals Heterostructures

Abstract

The research here will identify both thermal and electronic trends in 2D heterostructures and develop rigorous methodologies for calculating interface structures. A link will be made between common devices and the materials and interfaces from which they are constructed. A comprehensive presentation of the theories used throughout the thesis will be given, defining density functional theory, the Frozen phonon method and thermal conductivity via the Boltzmann transport equation. The failure of Anderson's rule in 2D heterostructures will be demonstrated and explained. Also demonstrated, will be, how 2D heterostructure bandgap predictions can be improved using two physically based corrections to Anderson's rule ΔE_Γ and ΔE_IF. We will show that ΔE_Γ affects the bandstructure such that for any constructed heterostructure the effective mass will always decrease and will likely exhibit an indirect bandgap. Furthermore, we will provide expressions to give the band alignments in terms of knowable material properties. It is then discussed how theory could be readily extended to other 2D heterostructures by adjustments to corrective terms ΔE_Γ and ΔE_IF. This insight allows for a method which avoids the need for advanced calculation when estimating the properties of TMDC heterostructures. This in turn will expand the possibilities for exploring the optoelectronic properties of various heterostructures to the broader research community. We will follow on by showing that the thermal conductivity of a TMDC heterostructure will be lower than either of its constituents. We will show that this is consistent even when considering a wide range of possible conductivities. Generous parameter allocation will be used to define maximum bounding conductivity values. Such an approach will improve the confidence in the results. This is expected to provide a route for artificially reducing heat flow in 2D layered materials and will demonstrate a clear trend in the thermal transport of 2D heterostructure interfaces. To facilitate methods for heterostructures and interfaces A program, ARTEMIS, well be developed. This software allows for the generation of interfaces by identifying lattice matches between two parent crystal structures. To allow for further exploration of the energetic space of the interface, multiple surface terminations parallel to the Miller plane and interface alignments are used to generate sets of potential interfaces for each lattice match. These interface structures can then be used in atomic simulations to determine the most energetically favourable interface. The software here can help to both drastically reduce the work of generating and exploring interfaces, as well as aid in understanding of how the interface structure influences subsequent properties. Using several test cases, we will demonstrate how ARTEMIS can both identify the location of an interface in existing structures, and also predict an optimum interface separation based upon the parents’ atomic structures, which aims to accelerate and inform the study of interface science. The electronic and mechanical properties of Ba₂TiSi₂O₈, an inter-grain material, will be obtained and compared using the generalised gradient approximation and hybrid functional methods. This will define procedures and limitations for the investigation of inter-grain materials. With no need to adhere to the stoichiometry of the parent crystals procedural generation of inter-grain materials cannot be achived by ARTEMIS. It will be demonstrated that for insulating inter-grain materials hybrid functionals can result in significant change. The hybrid functional corrects the bandgap from 3.79 eV to 5.72 eV and shows overall stronger ionic bonding and weaker covalent bonding within the structure. The calculated value of 131.73 GPa for the bulk modulus sits between the values of its two parent crystals (BaTiO₃ and SiO₂). Using the HSE06 functional, will give a better understanding of the optical and electron transport properties and provide better understanding of the chemical structure making this investigation helpful for further work on similar inter-grain systems. This body of work serves the purpose of identifying general trends across heterostructures and interfaces and developing sound methodologies for the investigation of these structures.