Ti 3 AlC 2 MAX and Ti 3 C 2 MXene Quantum Sheets for Record-High Optical Nonlinearity.

Two-dimensional (2D) transition-metal carbides (MXenes) have attracted great interest owing to their unique structures and superior properties compared to those of traditional 2D materials. The transformation of 2D MXenes into sub-5-nm quantum sheets (QSs) is urgently required but rarely reported. Herein, the Ti 3 AlC 2 MAX and Ti 3 C 2 MXene QSs with monolayer structures and sub-5-nm lateral sizes are demonstrated. Exceptionally high yields (>15 wt %) are obtained through an all-physical top-down method. The QS dispersions present unique photoluminescence, and the QSs-poly(methyl methacrylate) (PMMA) hybrid thin films demonstrate remarkable nonlinear saturation absorption (NSA). Absolute modulation depths of 30.6 and 49.9% and saturation intensities of 1.16 and 1.25 MW cm -2 (i.e., 116 and 125 nJ cm -2 ) are achieved for Ti 3 AlC 2 QSs and Ti 3 C 2 QSs, respectively. Such record-high NSA performances of MXene QSs would boost the application of MAX/MXene materials in nonlinear optics.