Novel synthesis technologies for two-dimensional transition metal dichalcogenides and their heterostructures

Two-dimensional (2D) materials, especially 2D transition metal dichalcogenides (TMDs), have been recently expected to play important roles in future applications due to their atomic-thickness nanostructure and various physical properties. Chemical vapour deposition (CVD) is considered to be the most promising synthesis method for two-dimensional materials, due to the best balance between yield and quality of the products. Hence, this thesis focuses on two novel CVD methods for synthesising TMDs and their heterostructures. The first one is molten salt-assisted chemical vapour deposition (Salt 2.0). By applying the Salt 2.0 technique, an abnormal anti-pyramid stacked MoS2/WS2 heterostructure and a MoS2/double-wall carbon nanotube mix-dimensional heterostructure with negative photoresponse are synthesised respectively. These heterostructures can provide new approaches for engineering two-dimensional nanoelectronic devices.Another technique is gas-phase chemical vapour deposition (GCVD). Different from other synthesis methods, the GCVD technique can continuously produce clean and large-mass TMD nanoflakes in aerosols which can be simply collected by a filter for device fabrication and integration at room temperature. A demonstration is achieved for producing MoS2 nanoflakes in the gas phase with an output of up to 24 μg.min-1. The MoS2 nanoflakes have comparable sizes and qualities to the ones from current methods, promising their potential to replace the current MoS2 materials in many applications. In addition, the synthesis without substrate provides a better understanding of the nucleation and growth mechanism in the synthesis of TMDs. The extension of an available novel method and the development of a completely new method provide new approaches for the synthesis of TMDs and other 2D materials