Synthesis, characterisation, and properties of monolayer MoS2, WS2, and their vertical heterostructures

Transition metal dichalcogenides (TMDs) are semiconducting two-dimensional (2D) materials with direct bandgaps for the monolayers. Recently, much attention has been attracted to its synthesis, properties and applications in nanoelectronics and optoelectronics. The DPhil project focused on growing 2D materials developing chemical vapour deposition (CVD) approaches to grow 2D materials, including molybdenum disulphide (MoS2), tungsten disulphide (WS2), and hexagonal boron nitride (hBN), based on which vertical layered heterostructures (VLHs) were fabricated via sequential transfer. Furthermore, a range of characterisation techniques were employed to investigate the structural, vibrational, optical and thermal properties of both these monolayers and multilayer stacks.
An atmospheric pressure CVD (APCVD) method was first developed to grow monolayer MoS2 crystals on silicon substrates with an amorphous oxide layer (SiO2/Si). A gradient of morphologies ranging from strictly triangular to highly dendritic shapes were attained with the growth dynamics controlled by tailoring the local concentrations of the precursors. In addition, the growth procedure was programmed and the conditions were optimized for large domain size. The monolayer MoS2 dendrites show good electrocatalytic performance toward hydrogen evolution reactions (HER). Subsequently, this manner was applied to synthesise monolayer MoS2 on crystalline strontium titanate (SrTiO3) substrate. The crystal shape was dependent on the surface terminations of the substrate, explained by the greatest interfacial van der Waals (vdW) bonding between MoS2 monolayers and SrTiO3 at the maximum commensuration of their lattices. The as-grown MoS2 was annealed either under vacuum or in sulphur environment, leading to either degradation or defect annihilation, respectively, as indicated by Raman and photoluminescence (PL) spectroscopy and X-ray photoelectron spectroscopy (XPS).
The focus of the following studies were on the interlayer interactions between TMD monolayers and the optical properties of the as-constituent WS2:hBN:MoS2 and WS2:hBN:WS2 VLHs. The involvement of hBN separators restricts the interlayer charge transfer and instead enables Förster energy transfer, switching the observation of PL quenching to enhancement. The level of PL enhancement could be determined by the layer number of hBN, the excitation power, the lattice strain, and the temperature, which were revealed by PL spectroscopy and transient absorption spectroscopy, as well as by theoretical modelling. The layer number of hBN adjusts the separation distance and thereby the interlayer coupling between the TMD monolayers; at higher excitation power, the possibility of interlayer energy transfer increases due to the higher exciton density, which results in a larger quantum yield (QY) of the heterosystem; the presence of strain induces shifts in the bandgap of TMDs and alters the relative offsets in the band structure generating the type II heterojunction where the interlayer interactions take place; the nonradiative decay channels are suppressed at cryogenic temperatures, and the efficiency of PL emission can be improved. These VLHs hold potential for advanced devices with desirable optical performance.

Advisor: Warner, J