Theoretical prediction of two-dimensional kagome semiconductors Pd3X2Y2 (X = S, O; Y = Cl, OH)

Kagome lattice and its inherent flat bands have attracted considerable attention for their potential to host a variety of exotic physical phenomena. Here, we predict a series of 9 stable two-dimensional (2D) kagome semiconductors, Pd₃X₂Y₂ (X = S, O; Y = Cl, OH), based on the design principle of valence charge balance. The perfect kagome lattice formed by the transition metal Pd allowing for the simultaneous emergence of flat bands (FBs), Dirac cores, and van Hove singularities (vHs) near the Fermi energy. The rotation of the Pd-d_xz/d_yz orbitals aligns their symmetry with that of the kagome lattice, leading to the formation of the ideal FBs. This ideal electronic structure is realized in the γ-Pd₃O₂(OH)₂ monolayer. Furthermore, three polarized materials with intrinsic built-in electric fields were identified in the α-Pd₃X₂Y₂ monolayers. The band edges of these monolayers not only facilitate the creation of diverse interface contacts but also meet the criteria for photovoltaic water splitting. However, due to the FBs, Pd₃X₂Y₂ monolayers possess a large effective mass and low carrier mobility. Our work offers insights into the design of novel 2D kagome materials and serves as an excellent example for subsequent research on flat-band systems.