Unravelling the anisotropic light-matter interaction in strain-engineered trihalide MoCl₃

Layered trihalides exhibit distinctive band structures and physical properties due to the sixfold coordinated 3d or 4d transition metal site and partially occupied d orbitals, holding great potential in condensed matter physics and advanced electronic applications. Prior research focused on trihalides with highly symmetric honeycomb-like structures, such as CrI₃ and α-RuCl₃, while the role of crystal anisotropy in trihalides remains elusive. In particular, the trihalide MoCl₃ manifests strong in-plane crystal anisotropy with the largest difference in Mo–Mo interatomic distances. Research on such material is imperative to address the lack of investigations on the effect of anisotropy on the properties of trihalides. Herein, we demonstrated the anisotropy of MoCl₃ through polarized Raman spectroscopy and further tuned the phonon frequency via strain engineering. We showed the Raman intensity exhibits twofold symmetry under parallel configuration and fourfold symmetry under perpendicular configuration with changing the polarization angle of incident light. Furthermore, we found that the phonon frequencies of MoCl₃ decrease gradually and linearly with applying uniaxial tensile strain along the axis of symmetry in the MoCl₃ crystal, while those frequencies increase with uniaxial tensile strain applied perpendicularly. Our results shed light on the manipulation of anisotropic light-matter interactions via strain engineering, and lay a foundation for further exploration of the anisotropy of trihalides and the modulation of their electronic, optical, and magnetic properties.