Stabilization and Band-Gap Tuning of the 1T-MoS2 Monolayer by Covalent Functionalization

The MoS2 monolayer is the second most studied two-dimensional material after graphene. However, the covalent chemistry through the S layers has not been fully explored for controlling the properties of the MoS2 monolayer. Herein we probe the potential of chemical functionalization of monolayer MoS, in tuning its electronic properties by first-principles density functional theory. We find that the chemical bonding of the functional groups (H, CH3, CF3, OCH3, NH2) is anomalously strong (4-5 eV) on the 1T phase (in low-coverage regimes) but very weak on the 2H phase. This strong affinity of 1T-MoS2 for functional groups is closely related to its metallicity and partially filled Mo 4d states. More interestingly, 1T-MoS2, which is metastable when unfunctionalized, becomes the stable phase after a crossover coverage of functionalization. Surface functionalization also results in dramatic changes to the electronic properties of 1T-MoS2. We find that the band gap of the 1T-MoS2 monolayer strongly depends on the functional group type and its coverage and can be tuned from zero to similar to 1 eV. This work demonstrates the great potential of controlling monolayer MoS2's phase stability and electronic properties by covalent functionalization.