Black phosphorus (BP), a typical mono-elemental and two-dimensional (2D) material, has gathered significant attention owing to its distinct optoelectronic properties and promising applications, despite its main obstacle of long-term stability. Consequently, BP-analog materials with long-term chemical stability show additional potential. In this contribution, tin sulfide (SnS), a novel two-elemental and 2D structural BP-analog monochalcogenide, has been demonstrated to show enhanced stability under ambient conditions. The broadband nonlinear optical properties and carrier dynamics have been systematically investigated via $Z$-scan and transient absorption approaches. The excellent nonlinear absorption coefficient of $50.5\times {10}^{-3}\mathrm{cm}/\mathrm{GW}$, 1 order of magnitude larger than that of BP, endows the promising application of SnS in ultrafast laser generation. Two different decay times of ${\tau }_1\sim 873\mathrm{fs}$ and ${\tau }_2\sim 96.9\mathrm{ps}$ allow the alteration between pure $Q$ switching and continuous-wave (CW) mode locking in an identical laser resonator. Both mode-locked and $Q$-switched operations have been experimentally demonstrated using an SnS saturable absorber at the telecommunication window. Femtosecond laser pulses with tunable wavelength and high stability are easily obtained, suggesting the promising potential of SnS as an efficient optical modulator for ultrafast photonics. This primary investigation may be considered an important step towards stable and high-performance BP-analog material-based photonic devices.

The environmental instability and uneliminable electronic trap states in black phosphorus quantum dots (BPQDs) limit the optoelectronics and related applications of BPQDs. Here, fluorinated BPQDs (F‐BPQDs) are successfully synthesized by using a facile electrochemical exfoliation and synchronous fluorination method. The F‐BPQDs exhibit robust ambient stability and limited fluorination capability, showing a nonstoichiometric fluorination degree (D_F) maximum of ≈0.68. Density functional theory calculations confirm that due to the edge etching effect of fluorine adatoms, the simulated F‐BPQDs become structurally unstable when D_F surpasses the limit. Furthermore, the trap states of BPQDs can be effectively eliminated via fluorination to obtain a coordination number of 3 or 5 for fluorinated and unfluorinated phosphorus atoms. The results reveal that the air‐stable F‐BPQDs exhibit fluorine defect‐enhanced electronic tolerance, which is crucial for nanophotonics and nanoelectronics applications.