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The stacking order plays a critical role in the electronic and optical properties of two-dimensional materials. It is however of great challenge to achieve large-size and homogeneous bilayer crystals with precisely controlled stacking orders. Here, we demonstrate an optimized chemical vapor deposition strategy to grow MoSe 2 bilayers with controlled AA or AB stacking sequences. Reverse gas flow effectively suppresses the random nucleation centers, leading to uniform growth of the second layer of MoSe 2 on the first monolayer. A customized temperature profile selectively activates the growth of the MoSe 2 bilayer with different stacking orders: the AA stacking MoSe 2 bilayer tends to form at 810 °C, and the AB stacking MoSe 2 bilayer prefers to grow at a higher temperature of 860 °C. A series of characterization methods confirm that MoSe 2 bilayers with different stacking orders exhibit distinct crystal structures and physical properties. Our results demonstrate a robust and effective route for the controllable synthesis of transition metal dichalcogenide bilayers, which will benefit the development of two-dimensional materials and van der Waals heterostructures.

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A radar window for a reentry vehicle should process critical properties, such as high mechanical properties which are resistant to stress failure induced by large thermal impact, high microwave transmission efficiency at room and high temperature, and very high ablation resistance. Fused silica, BN and Si 3 N 4 ceramics are the three popular materials for such applications, but they are limited in some fields for their intrinsic disadvantages. [1] When Si 3 N 4 , [2] AlN, [3] AlN-Si 3 N 4 , [4] AlN-BN, [5] BN [6] were introduced into the SiO 2 matrix respectively, the resultant grain reinforced SiO 2 composites had the higher strength, fracture toughness than SiO 2 matrix. At the same time, the dielectric property was not influenced and the impact resistance was enhanced.The monolithic window materials, however, are susceptible to mechanical failure caused by shock wave loading at ultra high temperature, the destructive result is catastrophic. The continuous fiber reinforced ceramic matrix composites have received considerable attention for structural applications because of their excellent thermal stability, light weight, and damage tolerance imparted by the reinforcing fibers. In the field of the radar window materials the BN fibers and silica fibers are two common fibers reinforced ceramic matrix composites, such as BN f /SiO 2 [7] and BN f /(BN+SiO 2 ). [8] The fracture toughness of the composites was enhanced to a large degree. However, the fabrication of the continuous BN fibers is difficult and the fibers have not been manufactured on a large-scale. Silica fibers, with excellent ablative resistance, thermal shock damage resistance, dielectric properties, chemical stability and flexibility, are suitable for fabricating high temperature radar windows materials. [9][10][11] But the disadvantage of the silica fibers is that high temperature processing of typical inorganic systems will lead to serious fiber degradation. [11] Recently, three dimensional silica fibers reinforced Si 3 N 4 (3D SiO 2f /Si 3 N 4 ) ceramic composites have been prepared by Qi and his coworkers [12][13] through Preceramic polymer Impregnation Pyrolysis (PIP) methods at low temperature and the composites have good mechanical and dielectric properties while the ablation properties of the composites were not examined. In the present study, in order to fabricate a new radar window material for high-temperature application, the 2.5 dimensional silica fiber reinforced Si 3 N 4 -BN matrix composites were prepared by PIP method through repeated infiltration of hybrid precursor and pyrolysis at high temperature in ammonia atmosphere. The mechanical, microstructure, dielectric, thermal and ablation properties are characterized using a range of techniques and the results are discussed. Results and Discussion Densification of the compositesThe density of the samples as a function of pyrolysis cycles is plotted in Figure 1. Repeated infiltration pyrolysis cycles are required to densify the composites due to the shrinkage of precursor in firing. The ...