Laminated Bilayer MoS₂ With Weak Interlayer Coupling

Recently,monolayer MoS₂ has attracted significant attention due to the strong interest in its fundamental physical properties,such as a direct optical bandgap of1.8eV in the visible range,which make it an ideal material for applications in next-generation nano-electronics,photonics,photovoltaics and valleytronics.For its appli-cation in electronic devices,especially optoelectronic devices,it is highly preferred to fabricate MoS 2 with multiple layers,even heterostructures/homostructures under different doping conditions which allows us to achieve high performance devices,such as lasers and photodetectors.However,the direct bandgap nature of monolayer MoS₂ is very fragile as the strong interlayer coupling between adjacent MoS₂ layers turns the bilayer MoS₂ into an indirect bandgap semiconductor.For optoelectronic device applications,it is critical to have a direct bandgap for MoS₂ layers.Although some work has beendevoted to tuning the electronic bandgap structure of bilayer MoS₂ by varying the twist angle between the two layers to change the interlayer coupling little progress has been made to achieve multiple MoS₂ layers with direct bandgaps.More recently,it was reported that adsorbates trapped between the individual layers of artificially stacked two-dimensional crystals can change the interlayer separations and prevent the formation of a true artificial layered crystal held together by van der Waals interaction,leading to a laminated structure.In this thesis,we demonstrate the fabrication of laminated bilayer MoS₂ with engineered interlayer coupling and bandgap structures.Laminated bilayer MoS₂structures are prepared with chemical vapor deposition method and MoS₂nanoparticles trapped between two individual MoS₂ layers of laminated structure which can prevent the formation of a true stacking structure held together by van der Waals interaction.The laminated bilayer MoS₂ clearly indicates a weak interlayer coupling with reduced van der Waals interaction between adjacent layers.As the interlayer coupling is insufficient to modify the band structure of MoS₂,the laminated bilayer MoS₂ can retain the direct bandgap structure of an isolated monolayer.Furthermore,by controlling the size of the MoS₂ nanoparticles trapped in between,the interlayer distance and interlayer coupling of bilayer MoS₂ structures can be engineered in a wide range,resulting in different bandgap behaviors.This finding is extremely important as it provides an effective approach to fabricate bandgap engineered bilayer MoS₂ structures,which is a crucial step forward to making multilayer MoS₂-based p-n junctions and homo/hetero-structures,and thus advanced electronic devices,especially optoelectronic devices.This approach is applied to not only bilayer MoS₂ structures,but also other layer structured two-dimensional materials.