**3. Bottom-up strategy**

Top-down strategy is useful in preparing ultrathin and high-quality nanosheets with large lateral size. Even so, it has to be mentioned that all exfoliation methods as mentioned before are only suitable to those materials whose bulk crystals are layered [25]. The mass production by methods mentioned earlier is usually very low. When using bottom-up approach, nanoscale materials are constructed from atomic or molecular precursors that are suitable to react and grow in size, or self-assemble into more complex structures. Then, 2D materials can be enlarged easily [6]. In this section, a brief overview is presented for the bottom-up synthetic process to 2D nanostructures.
