Structural, optical and transport properties of layered europium disulfide synthesized under high pressure

Materials readily forming stacks down to monolayer thickness and simultaneously possessing a finite bandgap are highly attractive from both fundamental and applied points of view. In this work, high-quality single-crystal samples of a novel layered compound, europium disulfide (EuS2), were synthesized under high-temperature-high-pressure conditions and characterized by complementary methods. According to single-crystal X-ray diffraction, the compound crystallizes in a monoclinic structure (space group P2(1)/a). Flakes down to 1-2 nm thick can be obtained by mechanical exfoliation; the angular dependence of the polarized Raman intensity allows determination of the flakes' orientation. Infrared spectra demonstrate a rich structure in a broad energy range, possibly arising from excitonic effects and interatomic transitions in Eu ions. Measurements of the Seebeck coefficient and ab initio modeling show that the material is a p-type semiconductor with a 0.9 eV indirect bandgap. At low temperatures, electrical conductivity follows Mott's law, implying the presence of defects, possibly related to the disordering of covalent S-S bonds.