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 (Ti 3 C 2 T x ) via "surface terminations modulation" is explored. The surface terminations of Ti 3 C 2 T x are modulated by electrochemical treatment, resulting in different states ( viz ., Ti 3 C 2 T x (pristine), Ti 3 C 2 T x (═O rich), and Ti 3 C 2 T x (-OH rich)). The sign and magnitude of the effective NLO absorption coefficient (β eff ) change with the surface terminations. Ti 3 C 2 T x (═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 Ti 3 C 2 T x (pristine) and Ti 3 C 2 T x (-OH rich). The RSA of Ti 3 C 2 T x (pristine) and Ti 3 C 2 T x (-OH rich) is attributed to excited-state absorption, while the SA of Ti 3 C 2 T x (═O rich) is associated with Pauli blocking. With laser excitation at 800 nm, the β eff of Ti 3 C 2 T x (-OH rich) is 113 ± 3.2 cm GW -1 , 1.68 times that of Ti 3 C 2 T x (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.