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 (Ti3C2Tx) via "surface terminations modulation" is explored. The surface terminations of Ti3C2Tx are modulated by electrochemical treatment, resulting in different states (viz., Ti3C2Tx(pristine), Ti3C2Tx(═O rich), and Ti3C2Tx(-OH rich)). The sign and magnitude of the effective NLO absorption coefficient (βeff) change with the surface terminations. Ti3C2Tx(═O rich) shows a relatively large saturable absorption (SA) with laser excitation at 515 nm (βeff = -1020 ± 136.2 cm GW-1), while reverse saturable absorption (RSA) is found in Ti3C2Tx(pristine) and Ti3C2Tx(-OH rich). The RSA of Ti3C2Tx(pristine) and Ti3C2Tx(-OH rich) is attributed to excited-state absorption, while the SA of Ti3C2Tx(═O rich) is associated with Pauli blocking. With laser excitation at 800 nm, the βeff of Ti3C2Tx(-OH rich) is 113 ± 3.2 cm GW-1, 1.68 times that of Ti3C2Tx(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.