Design Of Suspended Structure And Study Of Layer Interaction Effects In Two-dimensional Layered Transition Metal Dichalcogenides

Atomically layered two-dimensional (2D) materials, due to their electrons confined to the movement in 2D plane, usually deliver some unique mechanical, thermal, electrical and optical properties, distinctly different from those of bulk materials. Thus, in the next few years to a few decades,2D materials will attract intensive attention, and the research on them will become one of the hotspot and/or highlight in the many fields, such as physics, material science, chemistry, biology, etc. Among them, layered transition metal dichalcogenides (TMDCs), different from the zero bandgap of pristine graphene, can possess thickness-dependent energy band structure and easily-tuned physical and chemical properties, exhibiting a huge of advantages and prospects in the future.As well known, once a layered material is thinned down to a monolayer, the material itself is a surface and the properties of which are strongly influenced by the substrate and the surrounding environment, even to their stacking structure between layers. These usually result in their easily-tuned physical and chemical properties. Based on the interaction effects between the atomic layers of TMDCs, this dissertation has conducted the following two aspects:(i) To investigate the preparation of a type of suspended structures of TMDCs without external effects from the substrates and environment (from inside to outside of TMDCs structure);This dissertation demonstrats large scale suspended 2D materials over various nanoscale patterned substrates via chemical vapor deposition (CVD) method combining a wet-contact printing technique. Meanwhile, the fundamental transferring mechanism at the nanoscale solid-liquid-vapor interface has been elucidated. These studies will provide a theoretical and experimental guidance for the design of other TMDCs’ suspend structures on different nanopatterned substrates. Finally, as a proof-of-concept, a photodetector of suspended MoS2 has been demonstrated with significantly improved sensitivity.(ii) To investigate the interactions between layers, including the interactions at the boundary between the adjacent layers and coupling effects between layers (from outside to inside of TMDCs structure).As for interactions at the boundary between the adjacent layers:this dissertation has first reported a type of intrinsic junctions at the boundary between the adjacent layers (monolayer and bilayer, and bilayer and trilayer) in MoSe2, which show excellent rectification performance and photovoltic properties. Based on theoretical calculation of the adjacent boundary layers’ fine band structure and the analysis of experiment results, we have found that these intrinsic junctions are a type of semiconductor junctions similar to p-n junctions. Different from conventional semiconductor junctions, this kind of junctions is an intrinsic nature belonging to 2D TMDCs materials, and they are also thickness-dependent. Moreover, we have assembled a type of tandem solar cell arrays based on these intrinsic junctions, showing a great application potential in industrial and commercial fields.As for coupling effects between layers, this dissertation demonstrates a Van der Waals epitaxy to obtain a C7 type of stacking heterojucntion structures of MoSe2/WSe2. It was found that this C7 type of stacking heterostructures can deliver a strong coupling effect between layers, which usually results in a strong coupling photoluminescence (PL) peaks. Then, to tune the intensity of coupling effect, we have designed various tensile mechanical modes, and observed relative changes in direct bandgap structure of MoSe2 or WSe2 layers and indirect bandgap structure between layers (coupling effects). This finding might provide an important guidance for flexible photoelectric devices based on interlayer heterostructuresIn brief, based on the research on the interactions between the atomic layers of TMDCs, this dissertation would enrich the methods for tuning the physical and chemical performances of TMDCs, expanded the understanding of TMDCs at the atomic level. This research might promote the development of this type of 2D atomic layered materials in semiconductor devices.