Investigations On The Growth Dynamics And Microstructure Evolution Of Transition Metal Dichalcogenides By In Situ Transmission Electron Microscopy

The great success of graphene has sparked a research boom in other similar two-dimensional（2D）materials.As a very important member of the big family of 2D materials,transition metal dichalcogenides（TMDCs）have broad application prospects in the fields of optoelectronics,electronics,electrocatalysis,energy storage and conversion,and sensing due to their unique physical and chemical properties.Therefore,TMDCs have become hot materials in 2D materials,which has attracted great attention from researchers.A deep understanding of the growth dynamics and microstructure evolution process of TMDCs is of great importance to realize their excellent performance in different fields.In situ transmission electron microscope（TEM）technique is an advantageous method to study the growth kinetics and structural transformation process of TMDCs.In this paper,using TEM as a research tool,combined with in situ technique,the nucleation process,growth dynamics and structural transformation process of TMDCs were studied in real time on the lattice scale.The main research contents and results are as follows:1.The reduction process of solid crystal precursor K₂Mo S₄ and the nucleation process of Mo S₂ were studied in real time on the lattice scale.During the chemical reaction,the K₂Mo S₄ lattices break and then disappear,while the Mo S₂ nanocrystals start to nucleate and grow.The nucleation and growth of the Mo S₂ nanocrystals initiate both along low-index planes and along high-index planes of the original precursor K₂Mo S₄.Moreover,the growth of layered Mo S₂ nanocrystals employs layer-by-layer growth mode,in which the growth of new Mo S₂ slab starts on old ones from the edge part.In addition,the growth dynamics of Mo S₂ was studied.Mo S₂ nanocrystals with different crystal orientations contact and coalesce to form a larger nanocrystal through the oriented attachment mode.The growth and coalescence of these nanocrystals lead to the formation of Mo S₂ polycrystalline film.This study provides direct evidence for the growth mechanism and nucleation process of Mo S₂ nanocrystals.2.The process of transformation from the solid amorphous precursor K₂WS₄ into vertically aligned WS₂ under the action of the thermal field was systematically studied.The growth dynamics process and growth mechanism of vertically aligned WS₂ were revealed.It is found that the growth of vertically aligned WS₂ employs hybrid growth modes,in which the growth of new WS₂ layers is initiated at old ones from both the edge part and middle part.These vertically aligned WS₂ show great stability at high temperature of 900°C.Detailed investigations and theoretical calculations indicate that the K element in solid precursor plays a critical role in the growth and evolution of vertically aligned WS₂.This mechanism is also applicable to the controlled growth of vertically aligned Mo S₂ with high-temperature stability,which suggests the universality of the growth mechanism.This research provides microscopic evidence for the growth mechanism of vertically aligned TMDCs,which is helpful for the tailored design and manufacture of materials with optimized structures.3.Through in situ heating treatment and electron beam irradiation,the transformation dynamic process from vertically aligned WS₂ into W nanocrystal has been revealed in real time,and the microstructure evolution mechanism of vertically aligned WS₂ was revealed.It is found that,during the structural transformation process,the pristine WS₂ becomes thin and some nanoscale voids appear between newly formed W particles due to the evaporation of S atoms from the WS₂ lattice framework and the migration of W atoms.Moreover,it is found that the density of vertically aligned WS₂ layers has a great influence on the transformation mechanism.Vertically aligned WS₂ with low density are first transformed into horizontal WS₂ and/or amorphous W and then are transformed into W nanocrystals,while vertically aligned WS₂with high density are directly converted to W nanocrystals.Furthermore,by controlling the location of electron beam irradiation,controllable micromachining of WS₂ and patterning of W can be achieved.This study provides basic knowledge and insights into the microstructure transformation kinetic process and mechanism of the vertically aligned WS₂.