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2D PbS nanoplatelets (NPLs) form an emerging class of photoactive materials and have been proposed as robust materials for high-performance optoelectronic devices. However, the main drawback of PbS NPLs is the large lateral size, which inhibits their further investigations and practical applications. In this work, ultra-small 2D PbS NPLs with uniform lateral size (11.2 +/- 1.7 nm) and thickness (3.7 +/- 0.9 nm, approximate to 6 layers) have been successfully fabricated by a facile liquid-phase exfoliation approach. Their transient optical response and photo-response behavior are evaluated by femtosecond-resolved transient absorption and photo-electrochemical (PEC) measurements. It is shown that the NPLs-based photodetectors (PDs) exhibit excellent photo-response performance from UV to the visible range, showing extremely high photo-responsivity (27.81 mA W-1) and remarkable detectivity (3.96 x 10(10) Jones), which are figures of merit outperforming currently reported PEC-type PDs. The outstanding properties are further analyzed based on the results of first-principle calculations, including electronic band structure and free energies for the oxygen evolution reaction process. This work highlights promising applications of ultra-small 2D PbS NPLs with the potential for breakthrough developments also in other fields of optoelectronic devices.

MXenes,as an emerging group of two-dimensional materials, have attracted significant interest for diverse photoelectric applicationsby 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 1550nm. 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 1069.17nm was achieved under a low threshold power of 54mW. 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.