**3.1 The preparation of BNNSs**

2D nanosheets can be synthesized with two bottom-up and top-down approaches that relate to the synthesis of sheets from boron and nitrogen precursor [47, 48] and also the separation of layers. Chemical reaction [49] and CVD [39, 40, 50] are based on the former and micromechanical cleavage [51, 52], high-energy electron beam [53, 54], ball milling [55], and chemical [49, 56, 57]/liquid exfoliation [58–61] are based on the later approach.

Each technique has unique advantages for a specific application; however, there are always disadvantages in any way. In synthesis processes, a great amount of effort was put into preventing the formation of a strong chemical bond between the substrate and nanosheets. The crystallization process time, the nucleation on the substrate and the low density of critical nuclei are the important factors in synthetic methods. On the other hand, in the top-down view, exfoliation of layers is used. Nanosheets obtained from exfoliation usually have a higher crystallinity, but their lateral dimensions are limited by the material used. Also, due to lip-lip interactions between sheets, exfoliation of layers to isolate them is difficult. Therefore, the production of single layer is associated with a lot of problems. But in the bottomup approach, there is a lot of control over the supply of thin nanosheets with high lateral dimensions. However, the crystallinity of obtained nanosheets is less than the exfoliation process.
