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 (TiCT) via "surface terminations modulation" is explored. The surface terminations of TiCT are modulated by electrochemical treatment, resulting in different states (., TiCT(pristine), TiCT(═O rich), and TiCT(-OH rich)). The sign and magnitude of the effective NLO absorption coefficient (β) change with the surface terminations. TiCT(═O rich) shows a relatively large saturable absorption (SA) with laser excitation at 515 nm (β = -1020 ± 136.2 cm GW), while reverse saturable absorption (RSA) is found in TiCT(pristine) and TiCT(-OH rich). The RSA of TiCT(pristine) and TiCT(-OH rich) is attributed to excited-state absorption, while the SA of TiCT(═O rich) is associated with Pauli blocking. With laser excitation at 800 nm, the β of TiCT(-OH rich) is 113 ± 3.2 cm GW, 1.68 times that of TiCT(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.