An even stronger effect is observed for the H atom, when different free-energy barriers are found. This comes from the fact that the S-H binding strength is different when H species are close or far from the vacancy (see Figure ). The energy difference between H + bound close to the vacancy or far from it is only 2.4 kJ/mol, while for H atom, this energy difference is as large as 12.1 kJ/mol, leading to the asymmetric FES. We can conclude here that the defects present in MoS 2 will suppress the overall transport between the layers.