The third top-down method is lithiu

The third top-down method is lithium intercalation, which can be traced back to 1975[324]. Single layer of MoS2 [325], WS2 [326], MoSe2 [327] has been obtained by lithium intercalation with high yield, but the product loses the semiconductor properties as it changes phase to the metallic phase. The semiconductor phase would be recovered by heating at 300 ℃. Since this method needs reaction for several days, modifying this method by electrochemical cell was used to make it more efficient [328]. However, the flammability of lithium and the requirement of an inert atmosphere are still challenging this method. Liquid exfoliation by direct sonication in different solvents has also been studied, such as in organic solvents [107], mixed solvent of water and ethanol [329], aqueous surfactant solutions [329] and a mixture of solvent and polymer [330]. This method is easy to carry out, but the yield is very low.