**4.1 Mechanical exfoliation**

*Mechanical exfoliation* has been considered the simplest way to prepare high-quality 2D material. The mechanical exfoliation employs exterior force to overcome weak van der Waals interactions between material layers. The most frequently used method is scotch tape removal. In this method, the pulling action breaks down the weak van der Waals interactions between the layers. Pacile et al. used the same method to isolate BN nanosheets from the powdered h-BN pellet (hard-pressed). By repeating the peeling and pressing process, monolayered BN nanosheets were obtained. They have reported a strong relationship between h-BN nanosheet defects and captured excitons' recombination intensity [57]. Therefore, this technique is suitable for small-scale sample preparations, only for the laboratories, which is the main drawback of this technique. Another mechanical exfoliation method that uses shear force is ball milling. This method introduces fewer point defects compared to other mechanical methods. At the same time, this method is more efficient than the scotch tape method. According to Li et al., the controlled ball milling parameters are the key to producing relatively large in-plane BN nanosheets samples. They used h-BN powder with a benzyl benzoate (C12H12O2) milling agent and successfully achieved 0.3–1 μm diameter flakes with thickness between 20 and 110 nm [58].
