Electronic and mechanical properties of ScXI (X = S, Se) monolayers and their heterostructures

Inspired by the successful synthesis of bulk ScSI in a recent work [Ferrenti et al., Chem. Mater. 34, 5443 (2022)], we have systematically investigated the mechanical and electronic properties of ScXI (X = S, Se) monolayers and their heterostructures by using first-principles calculations. Our calculations verify the experimental speculation that the bulk ScSI is readily exfoliatable and the monolayers of ScXI (X = S, Se) are stable. The Young's moduli with strong anisotropy (50.2-91.5 N m(-1)) of ScXI monolayers are comparable to those of phosphorene (26-105 N m(-1)), but smaller than those of isotropic graphene (349 N m-1), MoS2 (122.3 N m(-1)), and h-BN (276 N m(-1)), indicating their lower stiffness. In addition, ScSI/ScSeI monolayers show good flexibility with critical strain of 29%/33%. Application of strain can effectively regulate the band gap (Eg) and band edge of ScXI (X = S, Se) monolayers. For instance, the Eg of the ScSeI monolayer is reduced from 1.83 to 1.59 eV and the band gap type is changed from indirect to direct band gap when a compressive strain of 6% is applied along the x direction, which is attributed to the orbital hybridization between the d orbital of Sc and p orbital of the elements at the X and I sites. More importantly, ScXI (X = S, Se) monolayers can form type II vertical heterostructure with typical two-dimensional semiconductors due to the deeper energy levels of their valence band maximum and conduction band minimum. In addition, ScXI (X = S, Se) monolayers can also be used to form type I lateral heterostructure with the ScSeBr monolayer. The excellent ductility, strain-tuned electronic properties, and heterostructure design make ScXI (X = S, Se) monolayers promising candidates for the application of flexible electronic devices.