An air-stable anionic two-dimensional semiconducting metal-Thiolate network and its exfoliation into ultrathin few-layer nanosheets

The black, small-bandgap semiconducting framework Eu-dfdmat features extensive Eu³⁺-sulfur bridges from the linear linker 2,5-difluoro-3,6-dimercaptoterephthalate (dfdmt). Each Eu center is chelated to four dfdmt linkers to form an anionic coordination sphere involving four carboxyl O and four mercapto S centers (EuO₄S₄), wherein the charge buildup can be alleviated by the electron-withdrawing fluoro groups. The extensive metal-linker bonding, together with a trace of Eu²⁺ species, appears to boost electronic interaction in the 2D net, generating a small band gap of 1.31 eV (946 nm), albeit a modest conductivity (e.g., 10^-6 S m^-1). The crystals also exhibit persistent EPR signals indicative of organic radicals (g = 2.002). The Eu-dfdmt solid are stable in air and can be exfoliated into ultrathin nanosheets (ca. 5 nm; 6-8 layers).