Evolution of ultraflat band in the van der Waals kagome semiconductor Pd3P2(S1-xSex)(8)

We investigate the evolution of structural parameters, optical properties, and electronic structures of the van derWaals kagome semiconductor Pd3P2S8 with Se doping. When the doping level of Se increases, the band gaps of Pd3P2(S1-xSex)(8) single crystals decrease gradually, accompanied by the expanded unit cells. First-principles calculations show that there is a flat band (FB) near the Fermi level in bulk Pd3P2S8. This FB mainly originates from the d(z)2-like orbitals of Pd atoms in the Pd kagome lattice, which has finite interlayer electron hopping perpendicular to the PdS4 square plane. The interlayer hopping can be reinforced with Se doping, inducing a stronger interlayer coupling via the chalcogen atoms at apical sites, which reduces the band gap and enhances the cleavage energy. In contrast, the vanishing interlayer hopping in the two-dimensional limit results in the formation of an ultra-FB in the monolayers of these compounds. The easy exfoliation and the existence of a unique ultra-FB near EF make Pd3P2(S1-xSex)(8) a model system to explore the exotic physics of a FB in a two-dimensional kagome lattice.