Fig. 1. The crystal structure, optical images,and photoluminescence (PL) of two-dimensional transition-metal dichalcogenide. (a) The diagram of the crystal structure of MoS₂^[35];(b) the optical images of WS_xSe_2-x monolayer^[36]; (c) the normalized PL spectra of WS_xSe_2-x monolayer with different ratios^[36]

Fig. 2. PL spectra in 2D black phosphorus (BP). (a) PL spectra of BP nanosheets with different layer numbers^[44]; (b) the relationship between the emitting photon energy and layer number^[45]; (c) the excitation polarization angle dependence of PL intensity^[49]; (d) both emission and excitation energies dependence of PL intensity mapping^[49]

Fig. 3. Elements for the composition of the 2D materials and luminescent doped ions in periodic table

Fig. 4. Transition electron microscope (TEM) images and PL spectra of the transition-metal doped 2D materials. (a) The HR-TEM image of the Mn doped MoS₂^[57]; (b) the HAADF-STEM image of the Nb-doped WS₂^[58]; (c) the PL spectra of the Mn-doped MoS₂ monolayer on sapphire, the Mn-doped MoS₂ monolayer on the silica wafer,and the pristine MoS₂ monolayer^[57]; (d) the PL spectra of the doped and undoped WS₂^[58]

Fig. 5. The microstructure and PL property of the Er doped MoS₂ monolayer^[59]. (a) The CS-STEM image of the atomic scale MoS₂∶Er; (b) the anti-Stokes emission of the MoS₂∶Er monolayer; (c) the Stokes emission of the MoS₂∶Er monolayer; (d) the anti-Stokes emission of the different position on the area (200 μm × 100 μm)

Fig. 6. The microstructure and PL properties of the Nd doped 2D In₂Se₃ nanosheets^[60]. (a) The TEM image of the Nd doped 2D In₂Se₃ nanosheets; (b) the intrinsic PL spectra of the Nd doped bulk In₂Se₃ and In₂Se₃ nanosheets; (c) the Stokes emission of the Nd doped 2D In₂Se₃ nanosheets; (d) the luminescence decay curve of the 1057 nm in as-prepared nanosheets

Fig. 7. The microstructure, Raman spectra, and the PL spectra of the Er doped 2D ZnSe nanosheets^[61]. (a) The TEM image of the ZnSe∶Er nanosheets; (b) Raman spectra of the ZnSe∶Er nanosheets; (c) the Stokes emission of the ZnSe∶Er nanosheets at 4 K; (d) the anti-Stokes emission of the ZnSe∶Er nanosheets at 4 K

Fig. 8. The microstructure and PL and EL spectra of the Yb, Er codoped 2D nanosheets,and the diagram of the LED^[62]. (a) The optical image of the Yb, Er codoped 2D nanosheets; (b) the anti-Stokes emission of the Yb, Er codoped 2D nanosheets under 980 nm excitation; (c) the EL spectrum of the tungsten selenide and molybdenum selenide heterojunction; (d) the diagram of the LED based on Yb, Er codoped tungsten selenide and molybdenum selenide heterojunction