Probing the Optical Properties and Strain-Tuning of Ultrathin Mo1–xWxTe2

Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo_0.91W_0.09Te₂, a semiconducting alloy of MoTe₂, using Raman, photoluminescence (PL), and optical absorption spectroscopy. Mo_0.91W_0.09Te₂ transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe₂ counterpart. We apply tensile strain, for the first time, to monolayer MoTe₂ and Mo_0.91W_0.09Te₂ to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton-phonon intervalley scattering. Strained MoTe₂ and Mo_0.91W_0.09Te₂ extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics.