Complementary Chalcogen Ordering Enables Energetically Favorable and Multifunctional Two-Dimensional Transition-Metal...

In conventional two-dimensional (2D) transition-metal dichalcogenides (TMDCs), intrinsic mirror symmetry forbids out-of-plane dipoles and suppresses symmetry-enabled properties. Janus TMDCs provide a direct route to mirror-symmetry breaking; however, their chemically inequivalent surfaces often introduce local strain imbalance and structural distortions. Designing symmetry-broken TMDC architectures that combine high structural stability with retained symmetry-derived functionality is therefore crucial. Here, we introduce complementary chalcogen ordering as a strategy for balanced symmetry breaking in 2D MSSe monolayers (M: transition metals). In this architecture, S and Se atoms are arranged into ordered, complementary patterns across the two chalcogen sublayers, thereby decoupling mirror-symmetry breaking from the severe surface and bonding imbalances characteristic of Janus monolayers. By scanning all transition metals, we find that nearly 79% of the proposed structures are energetically more favorable than their Janus counterparts. Five highly stable monolayers are identified, spanning nonmagnetic, ferromagnetic, and antiferromagnetic ground states, as well as metallic and semiconducting electronic structures. These materials exhibit diverse functionalities, including high Curie temperatures, phototunable spin textures, ferroelasticity, and enhanced thermoelectric performance. This work establishes complementary chalcogen ordering as a general design principle for stabilizing symmetry-broken 2D TMDCs and unlocking symmetry-enabled multifunctionality beyond the Janus paradigm.