Two-dimensional layered materials such as garphene, MoS2 and WSe2 have attracted considerable interest in recent times and becoming an important material platform in condensed matter physics and modern electronics and optoelectronics. The studies to date however generally rely on mechanically exfoliated flakes which always be limtited to simple 2D materals. To fully explore the potential of this new class of materials, it is necessary to develop rational synthetic strategies of two dimensional lateral complicated struture,such as lateral heterostructure,multiheterostructure, superlattice,quantum well,etc.
With a relatively small lattice mismatch (∼4%) between MoS2 and MoSe2 or WS2 and WSe2, it is possible to produce coherent MoS2–MoSe2 and WS2–WSe2 heterostructures through a lateral epitaxial process. Our studies indicate that simple sequential growth often fails to produce the desired heterostructures because the edge growth front can be easily passivated after termination of the first growth and exposure to ambient conditions. To this end, we have designed a thermal CVD process that allows in situ switching of the vapour-phase reactants to enable lateral epitaxial growth of single- or few-layer TMD lateral heterostructures. We used this technique to realize the growth of compositionally modulated MoS2–MoSe2 and WS2–WSe2 lateral heterostructures. The WS2–WSe2 lateral heterostuctures with both p- and n-type characteristics can also allow us to construct many other functional devices, for example, a CMOS inverter.
In a typical sequential-growth process for 2D lateral heterostructure, the excessive thermal degradation or uncontrolled nucleation during the temperature swing between sequential growthsteps represents the key obstacle to reliable formation of monolayer r lateral complicated structure.We designed a modified CVD system. A reverse flow from the substrate to the source during the temperature swing between successive growth steps. A forward flow only applied at the exact growth temperature. So, the existing monolayer materials will not exposure to high temperature and chemical vapor source at the tempreture increasing and decreasing steps to minimize thermal degradation and eliminate uncontrolled homogeneous nucleation.We used our approach initially for the general synthesis of a wide range of 2D crystal heterostructures. We also grew more complex compositionally modulated superlattices or multiheterostructures, the number of periods and repeated spacing can be readily varied during growth. HADDF-STEM analysis of the atomic structure of the lateral heterostructures and Multiheterostructures show the atomically sharp interface can be clearly observed.
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2.Zhengwei Zhang,Xidong Duan, Xiangfeng Duan,et al, Science, 357, 2017,788–792.