Figure 5. Optical properties of layer transition metal chalcogen-halogens.

Layered materials of the form 2H-MXY (a) where M is a group 3 transition metal, X is oxygen or a chalcogen (S, Se) and Y is a halogen (Cl, Br, I) are proposed as new plasmonic metals. The optical losses (b) shows that many of these compounds have considerable spectral ranges of low optical losses both compared to silver and to 2H-TaS₂ (the losses are normalized so that they can be directly compared between different materials, and the black bars denote the plasma frequencies.). This is due to the decreased electronegativity of (Hf, Zr, Ti) compared to Ta which increases the gap below the Fermi level. As a result, the predicted FOM (c) is orders of magnitude better than silver in the near-IR.