**2. Top-down strategy**

In top-down nanofabrication, the crafting of ultrathin nanosheets is achieved through physi‐ cal-based or through chemical-based process. The physical top-down approach employs the use of mechanical force or ultrasonic wave to exfoliate layered van der Waals solids into singleand few-layer 2D materials, whereas the chemical top-down strategy relies essentially on chemical reaction that brought about by ion exchange or by the application of heat and so on. In this section, a brief overview is presented of the top-down synthetic process for 2D nano‐ structures, including their advantages and limitations.
