Structural Determination and Nonlinear Optical Properties of New 1T‴-Type MoS₂ Compound

Abstract

Noncentrosymmetric MoS₂ semiconductors (1H, 3R) possess not only novel electronic structures of spin-orbit coupling (SOC) and valley polarization but also remarkable nonlinear optical effects. A more interesting noncentrosymmetric structure, the so-called 1T‴-MoS₂ layers, was predicted to be built up from [MoS₆] octahedral motifs by theoreticians, but the bulk 1T‴ MoS₂ or its single crystal structure has not been reported yet. Here, we have successfully harvested 1T‴ MoS₂ single crystals by a topochemical method. The new layered structure is determined from single-crystal X-ray diffraction. The crystal crystallizes in space group P3₁m with a cell of a = b = 5.580(2) Å and c = 5.957(2) Å, which is a √3 a × √3 a superstructure of 1T MoS₂ with corner-sharing Mo₃ triangular trimers observed by the STEM. 1T‴ MoS₂ is verified to be semiconducting and possesses a band gap of 0.65 eV, different from metallic nature of 1T or 1T' MoS₂. More surprisingly, the 1T‴ MoS₂ does show strong optical second-harmonic generation signals. This work provides the first layered noncentrosymmetric semiconductor of edge-sharing MoS₆ octahedra for the research of nonlinear optics.