Chalcogen vacancies are common in experimentally fabricated transition metal dichalcogenides (TMDs). However, the impact of vacancies on the phase stability of 2D TMDs is not well understood. In this work, we performed theoretical studies to investigate how vacancies affect the phase energetics of Mo-based TMDs (MoS2, MoSe2, and MoTe2). We found that the chalcogen vacancies tend to be clustered in the 2H phase; by contrast, the vacancies in the 1T phase prefer to be isolated at low vacancy concentrations and form clustered vacancy lines at high vacancy concentrations. The presence of X vacancies decreases the energy difference between the 2H and 1T phases, and the 1T phase becomes stabilized against the 2H phase when the number of vacancies exceeds the critical concentration (22% for MoS2, 20% for MoSe2, and 4% for MoTe2). Since the creation of vacancies would cause lattice strain, we also investigated the role of tensile strain, which appears to be critical in decreasing the 2H/1T phase energetics. The defect-free MoX2 monolayers would require an equibiaxial tensile strain of 10-15% to make the 1T phase energetically preferable. When vacancies are present, the threshold strain needed for the phase change is significantly reduced, which decreases with increasing defect concentration. Our results revealed that vacancies and tensile strain can both decrease the phase energetics between the 2H and 1T phases, and the synergistic effect between them effectively improves the possibility of achieving the 2H to 1T transition. It is worthy to note that, among the three MoX2 systems, the phase transformation induced by vacancies and/or strain is most easily accessible in MoTe2 due to its small 2H/1T energy difference. The 2H-to-1T phase transition in MoS2, however, would require either a high concentration of S vacancies or a large lattice strain, and it is practically more likely the combination of both effects that gives rise to the stabilization of the metastable 1T phase.
