Synthesis and Characterization at the Atomic Scale of 2D Materials for Future Heterostructure Integration

Two-dimensional (2D) materials are of interest for a wide variety of electrical and optical applications, due to their intriguing optical, electronic, and mechanical properties. In particular, the stacking of single layer materials into a designer multilayer heterostructure is quite promising for electronic and optical devices. Device integration requires a fundamental understanding of the synthesis mechanisms, defect structure and stability of 2D materials. The work presented in this thesis investigates a diverse set of 2D materials which are of eminent technological relevance: Transition Metal Dichalcogenides (TMDs), silicene, and graphene. The investigations focused on gaining insight into synthesis mechanisms, electronically and geometrically characterizing surface reconstructions and defects, and thermal stability measurements.
The thermal stability of 2D WS2 on a Au/Ti substrate was studied with XPS, which offers unique insight into the surface reactions of a technologically relevant materials system at different annealing temperatures. These results were obtained by annealing under ultrahigh vacuum in small temperature steps and determining the chemical nature of the surface after each step. The WS2 itself is relatively stable until annealing temperatures (~600 ⁰C) are reached, with no change other than doping due to sulfur loss. Ti, however, diffuses upwards to the surface causing a number of side reactions. This result suggests that Ti is too reactive to be used as the sticking agent for Au, in this materials system.
The synthesis studies of 2D MoS2 have been explored at length, and yet, there has been much difficulty in repeatability of these synthesis techniques. This data-driven study explores the parameter space in which monolayer MoS2 is synthesized, based on literature results from a wide variety of research groups. The synthesis mechanism for 2D MoS2 is studied by collecting literature data on synthesis conditions for monolayer and bilayer/ multilayer MoS2 and analyzing the data with Machine Learning techniques. The synthesis parameters with the highest impact are the Mo precursor annealing temperature and the pressure of the reaction chamber.
Silicene, the 2D analogue of graphene, has been theoretically predicted to have intriguing electronic properties, but it is difficult to test these properties experimentally, due to difficulties synthesizing silicene that is electronically decoupled from the substrate. Here, silicene is synthesized via e-beam deposition of Mo onto Si, followed by annealing, and the characterized with STM/STS. A range of silicene-related reconstructions are observed at cryostatic temperatures with characteristic geometries, electronic properties and defect structures. The most notable being ribbon-silicene which has a similar lattice constant to silicene (~0.36 nm) and Dirac-type band edges. Structural models are suggested for future work in DFT calculations.