Increasing entropy induced the in-plane piezoelectricity and negative Poisson's ratio in two-dimensional high-entropy Janus transition metal dichalcogenide monolayers

Entropy engineering has recently emerged as an effective strategy for tailoring the physical properties of two-dimensional materials. However, the structural stability and multifunctional mechanical properties of entropy-engineered Janus transition metal dichalcogenides (TMDs) remain largely unexplored. Here, density functional theory calculations were employed to systematically investigate 28 Janus medium- and high-entropy TMDs. Comprehensive evaluations of their energetic, dynamical, mechanical, and thermodynamic stabilities identified six configurations that satisfy all stability criteria. Among them, (TiFeVMnCoCr)SSe and (VCrMnFeCoZr)SSe exhibit pronounced anisotropic negative Poisson's ratios. The calculated directional Poisson's ratios range from −0.77 to −0.61 for (TiFeVMnCoCr)SSe and from −1.43 to −0.36 for (VCrMnFeCoZr)SSe. Detailed structural analysis reveals that the auxetic behavior originates from the cooperative rotation of the transition-metal–chalcogen bonding network and the associated bond-angle rearrangement, which effectively redistributes the applied strain. Overall, both compounds are found to be narrow bandgap semiconductors and exhibit remarkable in-plane piezoelectric responses arising from the intrinsic symmetry breaking of the Janus structure. In addition, they remain elastically linear up to approximately 15% strain and possess excellent mechanical robustness, with ultimate tensile strengths reaching 6.90/7.03 GPa for (TiFeVMnCoCr)SSe and 7.07/7.38 GPa for (VCrMnFeCoZr)SSe along the x- and y-directions, respectively.