Tuning Anisotropic Optical Responses in MXenes via Topochemical Surface Modification

MXenes are a broad family of two-dimensional transition-metal carbides and nitrides with chemically programmable surface terminations. While the optical properties of conventional, mixed-termination MXenes such as Ti₃C₂T_x (T = O, OH, F) have been explored, the impact of controlled, single-termination surface modification remains unresolved. Here we combine topochemical synthesis of stacked single-crystal, surface-regulated MXenes with imaging spectroscopic ellipsometry to resolve the anisotropic optical responses of all-inorganic Ti₃C₂Cl₂ and hybrid organic–inorganic MXenes (h-MXenes) bearing alkylimido and alkylamido ligands. Stacked single-crystal Ti₃C₂Cl₂ exhibits hyperbolic behavior at wavelengths ≥950 nm, with a negative in-plane real permittivity and a positive out-of-plane component. In contrast, covalent surface modification suppresses hyperbolicity in the 350–1700 nm window: h-MXenes show no hyperbolic response over this range. Increasing ligand length in h-MXenes induces a systematic blue shift of the in-plane NIR feature and strongly reduces out-of-plane absorption, rendering long-ligand samples nearly transparent along that axis. We also observe a Fano resonance arising from coupling between the metallic Ti₃C₂ bands and C–H vibrational modes of the ligands. These results establish surface chemistry as a lever for engineering anisotropic optical responses in MXenes and provide design rules for photonic and optoelectronic applications.