Controllable Growth Of High Quality Monolayer MoS₂

Monolayer transition metal dichalcogenides(TMDs)have attracted much attention since they exhibit unique fundamental physical properties.Monolayer molybdenum disulfide(Mo S2),one of the most frequently studied TMDs,can be used as piezoelectric transistor,electric generator,ultrasensitive photodetector,chiral light-emitting transistor,hight quality integrated circuit and so on.In order to realize its full potential for practical application,it has to overcome a number of obstacles.The basic problems are how to produce scalable and high quality monolayer Mo S2 sample and make Mo S2 patterns that satisfy the requirements for transistors.In this thesis,we do some research around these two aspects:1.We grew monolayer Mo S2 on hexagonal boron nitride(h-BN)substrates,based on three-temperature zone chemical vapour deposition(CVD)method.The Mo S2 film has same lattice orientation with h-BN substrate.We found that as-grown Mo S2 domains can be tailored from triangular to hexagonal shapes by tuning the balance of the Mo-and S-source evaporation rates.These domains on h-BN basal plane have only two lattice orientations,i.e.either 0°or 60°;while misalignment occurs at the step edges of h-BN surface.Two domains with the same orientation could be stitched into a single crystal;while the stitching of two domains with the relative orientation of 60°forms 4|4P domain boundary.In this way,we minimized the grain boundaries and obtained continuous oriented monolayer Mo S2 film on the h-BN basal plane.2.We improved our CVD set-up and presented a wafer-scale epitaxy of orientated monolayer Mo S2 films on single crystal sapphire substrate.These films are of seamless,high quality,uniformity and homogeneity across the entire wafer,and more importantly,only consist of two domain orientations,either 0° or 60° aligned to the sapphire substrate.The film was n-type semiconductor and its bandgap was 2.11 e V.After transfer,these sapphire wafers are still reusable for subsequent growth.Our results represent a significant step towards the realization of monolayer Mo S2 devices in wafer scale for various electronic and optoelectronic applications.3.We developed a micromechanical cleavage method to pattern Mo S2 films on Si O2/Si substrates based on a proper interface engineering.The peel-off process utilizes the strong adhesion between gold and Mo S2 and removes the Mo S2 film contact with gold directly,leading to clean Mo S2 pattern generation without residuals.Such clean fabrication technique paves a way to high quality Mo S2 devices for various electrical and optical applications.