CVD Growth Mechanism Of Two-Dimensional TMDs Materials And Alloys

Atomically thin two dimensional transition metal dichalcogenide(TMDs)materials,owing to their unique structures and excellent performance,are holding promising interests in electronic,optoelectronic,catalytic and bio applications.However,a few challenging tasks still remain in reality towards the potential applications.Understanding the microscopic mechanism of chemical vapor deposition(CVD)growth of two-dimensional TMDs is a fundamental issue.In the first part,a joint CVD-TEM method was used to capture the nucleation and subsequent growth kinetics of two-dimensional(2D)MoS2.The direct growth of few-and mono-layer MoS2 onto graphene based TEM grids allowed us to perform the subsequent TEM characterization without any solution-based transfer.Two forms of seeding centers were observed during characterizations:(ⅰ)Mo-oxysulfide(MoOxS2-y)nanoparticles either in multi-shelled fullerene-like structures or in compact nanocrystals for the growth of fewer-layer MoS2;(ⅱ)Mo-S atomic clusters in case of monolayer MoS2.In particular,for the monolayer case,at the early stage growth,the morphology appeared in irregular polygon shape comprised with two primary edge terminations:S-Mo Klein edge and Mo zigzag edge,approximately in equal numbers,while as the growth proceeds,the morphology further evolved into near-triangle shape in which Mo zigzag edge predominates.Results from density-functional theory calculations were also consistent with the inferred growth kinetics,and thus supportive to the growth mechanism we proposed.In general,the growth mechanisms found here should also be applicable in other 2D materials,such as MoSe2,WS2 and WSe2 etc.The influence of the graphene and BN substrate during the CVD growth of MoS2 was also investigated and these two substrates could affect the orientation of MoS2,make the orientation angle less than 5°.As the size increase of MoS2,this effect became more apparent,but not enough to make MoS2 strictly epitaxial growth on graphene and BN substrate.In the end,three different types of grain boundary structure were found:less than 30°,the structure of grain boundary was the dislocation core;more than 30°,the structure of grain boundary was the 4 | 4 p;the overlapp structure formed under special condition.The second part,monolayer MoS2(1-x)Se2y and MoS2(1-x)Te2y materials were prepared via Se,Te substitution into as-grown MoS2 monolayer through a two-step post-growth CVD process.Density functional theory calculations were used to investigate the stability of the alloy structures and their electronic properties.Comparison with experimental results indicated that the samples were unstrained and the Se,Te atoms were predominantly substituted in the top S sublattice.Importantly,such ultimately thin Janus structure of MOS2(1-x)Te2x exhibited properties which were distinct from their constituents.These results would inspire further exploration of the versatile properties of asymmetric 2D TMDs alloys.Finaly,I summaried the research on the CVD growth mechanism of TMDs and alloys,and also prospected the scientific problems for the further growth mechanism research.