Switching the Nonlinear Optical Absorption of Titanium Carbide MXene by Modulation of the Surface Terminations

Surface terminations of two-dimensional materials should have a strong influence on the nonlinear optical (NLO) properties, but the relationship between surface terminations and NLO properties has not yet been reported. In this work, switching the NLO properties of MXenes (Ti₃C₂T) via "surface terminations modulation" is explored. The surface terminations of Ti₃C₂T are modulated by electrochemical treatment, resulting in different states (viz., Ti₃C₂T(pristine), Ti₃C₂T(═O rich), and Ti₃C₂T(-OH rich)). The sign and magnitude of the effective NLO absorption coefficient (β_eff) change with the surface terminations. Ti₃C₂T(═O rich) shows a relatively large saturable absorption (SA) with laser excitation at 515 nm (β_eff = -1020 ± 136.2 cm GW^-1), while reverse saturable absorption (RSA) is found in Ti₃C₂T(pristine) and Ti₃C₂T(-OH rich). The RSA of Ti₃C₂T(pristine) and Ti₃C₂T(-OH rich) is attributed to excited-state absorption, while the SA of Ti₃C₂T(═O rich) is associated with Pauli blocking. With laser excitation at 800 nm, the β_eff of Ti₃C₂T(-OH rich) is 113 ± 3.2 cm GW^-1, 1.68 times that of Ti₃C₂T(pristine); the RSA is caused by photon-induced absorption. Our results reveal a correlation between surface terminations and NLO properties, highlighting the potential of MXenes in photoelectronics.