High Stability of 1T-Phase MoS₂ₓSe₂₍₁–ₓ₎ Monolayers Under Ambient Conditions

The phase stability in transition metal dichalcogenides (TMDs) is very significant for their utility in various applications. In particular, the 1T phase of group VI TMDs is of high technological importance owing to its electrically conducting properties and phase stability via strain engineering and chemical functionalization has been reported in the literature. In this regard, alloying with atoms of different sizes is an effective approach to introduce inherent strain in the lattice by modifying bond parameters. In the present study, it has been shown that the 1T polymorph of layered chalcogenides gets stabilized in strained, mixed chalcogen alloys, MoS₂ₓSe₂₍₁–ₓ₎. The 2H phases of MoS₂ₓSe₂₍₁–ₓ₎ are amenable for efficient chemical exfoliation to monolayers and successful conversion to metallic 1T phase. The metallic 1T phase of MoS₂ₓSe₂₍₁–ₓ₎ with inherent strain is found to be very stable for a long duration under ambient conditions. Owing to different Mo–S and Mo–Mo bond lengths in mixed chalcogenides, effective tuning of stability is achieved by changing the S/Se ratio in the compound. The differential scanning calorimetry data reveals a shift in the thermal phase transition with a peak maximum at ∼160 °C in the case of 1T MoSSe which is different from its pristine counterparts. Ex situ XRD and Raman spectroscopic studies suggest the role of inherent strain in retaining the octahedral structure at high temperatures. The alloying approach may be extended to other transition metal chalcogenides to fully explore the potential of metastable polytypes.