Realization of a Hidden Structural Phase in 2D TaS2 Crystals

Recently, structural phase or polymorph of 2D material is attracting growing interest as a controlling parameter to functionalize their physical properties. Various polymorphs can exist in transition metal dichalcogenides (TMDs), and polymorphism has significant impacts on electronic states and properties. Here, we report the discovery of an unprecedented polymorph of a TMD 2D material [1]. By mechanical exfoliation, we made thin flakes from a single crystal of 2Ha-type tantalum disulfide (TaS2), a metallic TMD with a charge-density-wave (CDW) phase. We fabricated microdevices for the flakes, and performed microbeam X-ray diffraction measurements and electrical transport measurements. Both measurement results indicate that some flakes possess a polymorph different from any one known in TaS2 bulk crystals. Moreover, the flakes with the unique polymorph displayed the dramatically enhanced CDW ordering temperature. The present results suggest the potential existence of diverse structural and electronic phases accessible only in 2D materials. [1. M. Yoshida et al. Nano Lett. 17, 5567 (2017)].