Characterization of few-layer 1T′ MoTe₂ by polarization-resolved second harmonic generation and Raman scattering

We study the crystal symmetry of few-layer 1T′ MoTe₂ using the polarization dependence of the second harmonic generation (SHG) and Raman scattering. Bulk 1T′ MoTe₂ is known to be inversion symmetric; however, we find that the inversion symmetry is broken for finite crystals with even numbers of layers, resulting in strong SHG comparable to other transition-metal dichalcogenides. Group theory analysis of the polarization dependence of the Raman signals allows for the definitive assignment of all the Raman modes in 1T′ MoTe₂ and clears up a discrepancy in the literature. The Raman results were also compared with density functional theory simulations and are in excellent agreement with the layer-dependent variations of the Raman modes. The experimental measurements also determine the relationship between the crystal axes and the polarization dependence of the SHG and Raman scattering, which now allows the anisotropy of polarized SHG or Raman signal to independently determine the crystal orientation.