possibility to reach the surface of the as-grown first monolayer.With a suitable carrier gas flow rate, the source vapor can haveenough kinetic energy to reach the surface center of the as-grownfirst monolayer where there is an initial nucleation to trigger thegrowth of the second layer45. Therefore, the second monolayercrystals prefer to grow epitaxially and homogenously on theactivated nucleation centers of the first monolayer, finallypromoting the growth of MoS2 bilayer crystals, as shown inFig. 1b.