Persistence of Monoclinic Crystal Structure in Three-Dimensional Second-Order Topological Insulator Candidate 1T'-MoTe2 Thin Flake without Structural Phase transition

TL;DR

This study demonstrates that thin flakes of MoTe2 maintain their monoclinic 1T' structure at low temperatures without transitioning to the orthorhombic phase, enabling the exploration of second-order topological insulator states.

Contribution

It reveals that MoTe2 thin flakes thinner than ~19.5 nm do not undergo the expected structural phase transition, preserving the 1T' phase at low temperatures, which is crucial for topological applications.

Findings

Monoclinic 1T' structure persists in thin flakes below 19.5 nm at low temperatures.

Raman spectroscopy detects the absence of the out-of-plane vibration mode D in thin flakes.

High hole density from exfoliation may stabilize the 1T' phase at low temperatures.

Abstract

A van der Waals material, MoTe2 with a monoclinic 1T' crystal structure is a candidate for three-dimensional (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 would be transformed into the orthorhombic Td structure as the temperature is lowered, which hinders the experimental verification and the electronic applications of the predicted SOTI state at low temperatures. Here, we present systematic Raman spectroscopy studies of the exfoliated MoTe2 thin flakes with variable thicknesses at different temperatures. As a spectroscopic signature of the orthorhombic Td 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 K 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.