Experimental and computational investigation of two-dimensional entities

Two-dimensional (2D) transition metal dichalcogenides and 2D entities (domain walls in 3D ferroelectric single crystals) were analysed throughout this work using a combination of experimental scanning transmission electron microscopy (STEM) techniques, and computational image simulation.

Se implantation of molybdenum disulphide (MoS2) was investigated via high angle annular dark field (HAADF) STEM image simulation. Substitutional ions were accurately identified through comparison of experimental images with simulations. Further simulations of thicker 2D samples and various ion species found that as samples transition from thin to thick (from less than 1 nm to greater than 1 nm), the intensity margins become narrower. Simulation of different substitutional atoms found that image intensity can be similar between species, solidifying the need for spectroscopy to first identify all atomic species in the sample, before accurate simulations can be produced.

Examination of 2D transition metal dichalcogenides (TMDCs) including MoS2, tungsten disulphide (WS2), and tungsten diselenide (WSe2), when deposited with various metals (gold, palladium, nickel), were completed via HAADF STEM under various conditions. Palladium exhibited different particle behaviour dependant on the substrate TMDC it was in contact with when heated ex-situ. The different behaviour suggests a difference in bonding dynamics. Pd-WSe2 was the most thermally stable pair, suggesting it would be a suitable bond to investigate for future metal-TMDC contacting. Nickel oxidised when left in the atmosphere, meaning unless nickel is put onto a TMDC under vacuum, and then encapsulated, it will oxidise immediately and be of little use for electrical contacting purposes. The nickel oxide particles had a size and shape independent of the 2D substrate beneath them, and the total nickel deposited amount.

4DSTEM was used to study the strain dynamics and properties of 2D WSe2 deposited with gold nanoparticles. A strain range of 7% was found, showing nanoparticles are an effective manner of creating regions of varying strain. Patterned deposition of nanoparticles may become a method to create regions of a different strain, tailoring regions with different electrical and optoelectrical properties in 2D TMDCs. 4DSTEM also found the strain in 2D domain walls in ferroelectric Cu3B7O13Cl to range between 0.4%. These minimal ranges show the mechanical strain induced in the material in its orthorhombic form. Changing the electron beam current resulted in domain walls moving to reduce internal stress caused by changes in charge carrier concentrations.