Inversion-asymmetric kagome monolayers: A platform for Rashba effect and two-dimensional piezoelectricity

The breathing kagome lattice, a variant of the kagome structure characterized by inversion-symmetry breaking and geometric frustration, provides a fertile ground for realizing unconventional electronic functionalities. Here, using first-principles calculations, we identify 18 stable two-dimensional M₃QX₇ compounds (M = Nb,Ta; Q = S,Se,Te; X = Cl,Br,I) that host transition-metal-based breathing kagome motifs. Unlike their Mott-insulating, isostructural M₃X₈ counterparts, these M₃QX₇ monolayers are nonmagnetic semiconductors driven by molecular-orbital splitting in embedded M₃X₁₂Q clusters. Remarkably, a pronounced Rashba spin splitting (RSS) appears in the lowest conduction band (LCB) and can be effectively tuned via biaxial strain. A k·p analysis shows that the electronic state of the LCB at the Γ point transforms as the E₁/₂ double-group representation of the C_3v point group, thereby permitting linear-in-k spin-orbit coupling terms that produce the observed RSS. Moreover, M₃QX₇ monolayers display the rare coexistence of in-plane and out-of-plane piezoelectric responses, in contrast to conventional two-dimensional piezoelectrics whose polarization is confined to the basal plane. The out-of-plane dipole moment (μ_z) and the surface asymmetry in Born effective charges (|Z_Bottom ^∗ −Z_Top ^∗ |) serve as quantitative descriptors of the vertical piezoelectric response, providing a microscopic understanding of its origin. Due to the exceptional mechanical flexibility of this system, both in-plane and out-of-plane responses reach sizeable magnitudes. These findings not only demonstrate the potential of M₃QX₇ monolayers for integration into spintronic and piezotronic devices, but they also offer new insights into emergent properties in intrinsically inversion-symmetry-broken breathing kagome lattices.