Persistence of Monoclinic Crystal Structure in 3D Second-Order Topological Insulator Candidate 1T′-MoTe₂ Thin Flake Without Structural Phase Transition

A van der Waals material, MoTe₂ with a monoclinic 1T′ crystal structure is a candidate for 3D second-order topological insulators (SOTIs) hosting gapless hinge states and insulating surface states. However, due to the temperature-induced structural phase transition, the monoclinic 1T′ structure of MoTe₂ is transformed into the orthorhombic T_d structure as the temperature is lowered, which hinders the experimental verification and electronic applications of the predicted SOTI state at low temperatures. Here, systematic Raman spectroscopy studies of the exfoliated MoTe₂ thin flakes with variable thicknesses at different temperatures, are presented. As a spectroscopic signature of the orthorhombic T_d structure of MoTe₂, the out-of-plane vibration mode D at ≈ 125 cm^–1 is always visible below a certain temperature in the multilayer flakes thicker than ≈ 27.7 nm, but vanishes in the temperature range from 80 to 320 K when the flake thickness becomes lower than ≈ 19.5 nm. The absence of the out-of-plane vibration mode D in the Raman spectra here demonstrates not only the disappearance of the monoclinic-to-orthorhombic phase transition but also the persistence of the monoclinic 1T′ structure in the MoTe₂ thin flakes thinner than ≈ 19.5 nm at low temperatures down to 80 K, which may be caused by the high enough density of the holes introduced during the gold-enhanced exfoliation process and exposure to air. The MoTe₂ thin flakes with the low-temperature monoclinic 1T′ structure provide a material platform for realizing SOTI states in van der Waals materials at low temperatures, which paves the way for developing a new generation of electronic devices based on SOTIs.