| Two-dimensional（2D）rhenium disulfide（Re S2）is a kind of semiconducting nanomaterial with few lattice symmetry elements and only atomic thickness in the direct direction.Due to its mechanical flexibility,optical transparency,large specific surface area,anisotropic mechanical,electrical and optical properties,Re S2 exhibits wide prospect in flexible devices,tactile sensors and micro-nano photonics.On the one hand,the application of 2D Re S2 depends on high quality synthesis and deep understanding of the real structure of materials at atomic scale.Chemical vapor deposition（CVD）is the most promising method to achieve high-quality,large-area and batch controllable preparation of 2D Re S2.However,the research field is still in the early stage,there are some problems such as small crystal domain size,inhomogeneous thickness,quite limited understanding of the regulation methods,internal mechanisms of grain orientation and single crystal preparation,which need to be further explored.Besides,the application of 2D Re S2 in flexible devices depends on the deep understanding of its fracture mechanics.On the other hand,defining failure mode and operating conditions at atomic scale and proposing feasible mechanical properties control methods are the premise to promote the practical application of such materials in the field of flexible electronics.Based on the above requirements,we chose rhenium trioxide（Re O3）and sulfur（S）as precursors to prepare monolayer Re S2 on Si O2/Si substrate through sodium-assisted chemical vapor deposition（SA-CVD）at first,exploring the effects of the amount of sodium salt adjuvant on the coverage and domain size of Re S2.The results show that the average domain size of two-dimensional Re S2 synthesized by SA-CVD method is 29.7μm,the maximum domain size is 40μm,the thickness of monolayer product is 0.86 nm,and the ratio of monolayer product area to total domain area is 89.7%.Compared with the conventional CVD method,the average domain area of the product increased from4.40μm2 to 684.6μm2.This can be attributed to the increase of surface alkalinity of Si O2/Si substrate after using sodium salt,which adsorb more precursor.Transmission electron microscopy with atomic resolution shows that the synthesized Re S2 is polycrystalline with rich grain boundary structure.Furthermore,2D Re S2 was also prepared on sapphire,STO（001）,2D molybdenum sulfide,which proved the generality of CVD method.In addition,we researched the influence of substrate type,growth temperature,precursor concentration and growth time on domain morphology.Domain shape was quantitatively described by mathematical parameters such as circularity and aspect ratio and fractal dimension.The results show that the shape of Re S2 domain varies with the type of crystalline substrate,showing the degree of anisotropy related to surface symmetry of crystalline substrate.The migration-limited model was further established to explain the shape change rule of crystal domains.Subsequently,in order to study the mechanical properties at grain boundaries of polycrystalline Re S2,we used high-resolution spherical aberration correction scanning transmission electron microscopy to produce localized and in-situ fracture near grain boundaries in monolayer Re S2 with dense grain boundaries.This approach enables accurate observation and dynamic tracking of fracture mechanics at multiple grain boundaries at atomic scale.Experimental results show that monolayer Re S2 generally exhibits characteristics of brittle fracture with sharp and smooth fracture tip at atomic level.Different from the fact that grain boundaries in brittle materials are short plates of mechanical properties,grain boundaries with different structures exhibit distinct fracture mechanics characteristics.Strength of most grain boundaries is lower than intrinsic crystal,which induces intergranular fracture.However,a small amount of grain boundaries with special structures can block and capture cracks,which enhances mechanical properties of materials.Density functional theory calculation well supports the experimental observation at the same time.It is proposed that the mechanical strength of different grain boundaries in monolayer Re S2 is controlled by relative relationship between grain boundary direction and Re chain orientation on any side of the grain boundary.The results also reflect the anisotropy of mechanical properties of low symmetry two-dimensional crystals.In summary,we prepared monolayer Re S2 on a variety of substrate surfaces by CVD method,and studied the influence of preparation parameters on the morphology of Re S2 domain.Then we proposed migration-limited model to explain the shape evolution rule of products on different types of substrates.And applied atomic resolution transmission electron microscope to revel fracture mechanics properties of Re S2 at different grain boundaries,which provided theoretical support for the application of such materials in the field of flexible devices. |
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