Ti₃C₂T_x MXene Quantum Dots with Enhanced Stability for Ultrafast Photonics

MXenes, as an emerging group of two-dimensional materials, have attracted significant interest for diverse photoelectric applications by virtue of their tunable terminated surface, excellent electrical conductivity, and strong nonlinear optical response. They are usually synthesized via strategies involving hydrofluoric acid, which may result in oxygenic and hydrophilic termination groups on the MXene surface and thus decrease their stability in air. Herein, a mild synthesis method based on electrochemical exfoliation is proposed to prepare Ti3C2Tx MXene quantum dots (QDs), which are highly fluorinated to enhance stability. The open-aperture Z-scan system was employed to measure the nonlinear transmittance of the MXene QDs and proves their broadband saturable absorption from 540 to 1550 nm. By integrating the MXene QD saturable absorber into an ytterbium-doped fiber laser, a set of highly stable 357 ps mode-locked pulses centered at 1069.17 nm was achieved under a low threshold power of 54 mW. In addition, tunable mode-locked optical spectra could be observed in the cavity when switching the polarization state. This work provides an environmentally friendly approach to synthesize stable MXene QDs and opens an avenue for the realization of low-threshold and tunable photonics applications based on MXene QDs.