Raman-active modes of 1T′-WTe2 under tensile strain: A first-principles prediction

Monolayer 1T′-WTe2 attracts rapidly growing interests aiming for promising applications in spintronics, dissipationless transport, and quantum computations. Due to one-dimensional W-W chains, 1T′-WTe2 exhibits unique anisotropic structure and promising properties, which can be modified by simply applying strains. Based on first-principles calculations, we systematically study the phonon dispersion curves as well as the Raman-active modes of 1T′-WTe2 under different tensile strains. We find that one branch of acoustic phonon softens at special q points under a critical strain of a=11.55% along the a axis (with W-W chains) direction, or b=7.0% along the b-axis direction and ab=8.44% along the biaxial direction. Before reaching such critical strains, the Raman frequencies of Ag1, Ag3, and Ag4 modes, contributing to the main peaks in Raman spectra of 1T′-WTe2, show anisotropic responses to different strains. The response to biaxial strains is found to be the most sensitive. We find that the frequency shift of Ag3 mode shows parabolic characters of strained 1T′-WTe2, then we split it into two parts and it shows a Raman-shift transition at ∼5% strains. While for theAg1 and Ag4 modes, the frequencies change linearly.