Identifying light impurities in transition metal dichalcogenides: the local vibrational modes of S and O in ReSe₂ and MoSe₂

The transition metal dichalcogenides provide a rich field for the study of two-dimensional materials, with metals, semiconductors, superconductors and charge density wave materials being known. Members of this family are typically hexagonal, but those based on rhenium (ReSe₂ and ReS₂) and their ternary alloys are attracting attention due to their triclinic structure and their resulting, strong in-plane anisotropy.
Here, Raman spectra of dilute ReSe_2-xS_x alloys containing low levels of sulfur (x < 0.25) were obtained in order to investigate the distribution of substitutional sulfur atoms over the non-equivalent chalcogen sites of the ReSe₂ unit cell. Four different Raman bands arising from the local vibrational modes of sulfur atoms were observed, corresponding to these four sites. One local vibrational mode has a substantially in-plane displacement of the sulfur atom, two are partially out-of-plane and one is completely out-of-plane. The interpretation of the experimental data is based on calculations of the lattice dynamics and non-resonant Raman tensors of a model alloy via density functional theory. For comparison, polarization-dependent Raman spectra of pure ReS₂ are also presented; a dramatic increase in the Raman cross-section is found for the out-of-plane modes when the excitation polarization is normal to the layers and the light propagates in the layer plane. A similar increase in cross-section is found experimentally for the local vibrational modes of sulfur in dilute ReSe_2-xS_x alloys and is predicted for dilute sulfur-containing alloys based on MoSe₂. The analogous local vibrational modes of substitutional oxygen impurities in ReSe₂ were also investigated computationally.