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

Abstract

A van der Waals material, MoTe 2 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 MoTe 2 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 MoTe 2 thin flakes with variable thicknesses at different temperatures, are presented. As a spectroscopic signature of the orthorhombic T d structure of MoTe 2 , 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 MoTe 2 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 MoTe 2 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.

Emergent layer stacking arrangements in c-axis confined MoTe2

“…This proposal should be studied experimentally in thin exfoliated flakes. We note that our data provide strong evidence that surface oxidation does not stabilize ordered 1T′ stacking via hole doping, as previously suggested in the literature 20 , 21 . Alternatively, to stabilize the T d phase, the application of a strong out-of-plane electric field may promote the ferroelectric phase due to the electrostatic coupling.…”

Layer Stacking Determination in Topological Semimetal MoTe2 via STEM Imaging, Liquid He TEM, and Quantitative Electron Diffraction

“…1a): the high temperature 1T' phase (an inversion-symmetric higher order topological insulator) and the low temperature T d phase (a ferroelectric Weyl semimetal), with a bulk transition temperature of T c ~ 250 K between the two phases. In thin exfoliated flakes of MoTe 2 , the preferred layer stacking (and its temperature dependence) is hotly debated [1][2][3]. MoTe 2 flakes show exciting quantum phenomena such as exotic superconductivity and third-order non-linear Hall effect [4,5], but our understanding of this behavior is incomplete, given the uncertain sample symmetry and topology.…”

Raman Studies of Two-Dimensional Group-VI Transition Metal Dichalcogenides under Extreme Conditions

“…2.1. Low-Temperature Raman Study of 2D TMDs Although 2D TMDs have been frequently investigated using low-temperature Raman spectroscopy, the lowest temperature used has been that of liquid nitrogen temperature (77 K) [16][17][18]. Raman studies on TMDs using temperatures as low as that of liquid helium temperature (4.2 K), for example, are few.…”