Fig. 1. (Color online) (a) Photoluminescence (PL) intensity map of narrow emission lines centered at 1.719 eV. The dashed triangle indicates the position of the monolayer WSe₂^[18]. (b) PL spectrum of localized emitters. The left inset is a high resolution spectrum of one SPE. The right inset is an enlarged view of the monolayer excitons emission^[18]. (c) Second order correlation measurement of the PL from one SPE in (b)^[18]. (d) A histogram comparison of the linewidth between the emission from the delocalized excitons and 92 localized emitters^[18]. (e) The integrated counts of the photon emission from SPE as a function of laser power. The red line is a guide to the eye^[18]. (f) Time-resolved PL of SPE showing a single exponential with decay time of 1.79 ± 0.002 ns^[18]. (g) Typical PL spectra of 36 nm thick GaSe, recorded at a temperature of T = 295 K and T = 10 K, respectively^[19]. (h) The PL spectra of single photon emissions from hBN samples^[36].

Fig. 2. (Color online) (a) Optical microscope image of a typical device used in experiments. The dotted lines highlight the footprint of the graphene, hBN and the TMD layers individually. The Cr/Au electrodes contact the graphene and TMD layers to provide electrical bias^[42]. (b) A raster-scan map of integrated EL intensity from monolayer and bilayer WSe₂ areas of the quantum LED for an injection current. The dotted circles highlight the submicron localized emission in this device^[42]. (c) A schematic energy band diagram, including the confined electronic states of the QDs. Electro-luminescence(EL) emission from QD starts at lower bias than the conventional LED operation threshold^[42]. (d) Typical EL emission spectra for QDs in the monolayer (top) and bilayer (bottom) WSe₂. The shaded area highlights the spectral window for LED emission from the bulk WSe₂ excitons^[42]. (e) Top (bottom) spectra of PL and EL correspond to 10 K (room temperature) operation temperature^[42]. (f) Comparison of the integrated EL intensity for the WSe₂ layer and for a QD as a function of the applied current. (g) Intensity-correlation function, g⁽²⁾(t) of with a rise-time of 9.4 ± 2.8 ns^[42].

Fig. 3. (Color online) (a) Scanning electron microscope (SEM) image of nanopillar substrate, fabricated by electron beam lithography^[28]. (b) Illustration of the fabrication method: (1) mechanical exfoliation of layered materials (LM) on PDMS and all-dry viscoelastic deposition on patterned substrate; and (2) deposited LM on patterned substrate^[28]. (c) Dark field optical microscopy image (real color) of monolayer layer (1L)-WSe₂ on nanopillar substrate^[28]. (d) PL spectra taken at nanopillar in a low orderly, enclosed by the blue, green and pink rectangles, the Second-order correlation measurement were shown below respectively^[28]. (e) Schematic diagram for the strain applied in monolayer (upper part) and bilayer (lower part) geometries^[30]. (f) Defect emission lines as a function of pressure, showing a redshift at a rate of 1.31(7) (peak A) and 1.33(3) meV/GPa (peak C) initially as well as a subsequent blueshift at a rate of 0.72(4) (peak B) and 0.67(9) meV/GPa (peak D), respectively, red and blue arrows are guides to the eye^[30]. (g) Fitting data of the PL peak energies as a function of pressure^[30].

Fig. 4. (Color online) (a) Optical image of the monolayer WSe₂/hBN stack. The dashed square indicates the scanning area in the PL mapping measurements^[51]. (b) Schematic of phonon–photon entanglement. The circularly polarized states ( ) with an angular momentum of 1 = ± 1 are degenerate in WSe₂ due to time-reversal symmetry^[51]. (c) A PL spectrum at V_g = − 78 V. The splitting energy of the doublets is identical to that of the corresponding b doublets. The energy spacing between a and b doublets is the energy of the (Γ) phonon. Inset shows similar behavior for the QD D6^[51]. (d) Polarization of the D3a doublet measured in the linear basis. The lines are fits to the experimental data (dots). (e)The orange dashed line shows an example of the linearly polarized emission in the linear basis measurement, the red dashed circle with a radius of 0.5 can be either circularly polarized emission or an unpolarized light source. The green dashed line shows an example of circularly polarized emission in a circular basis measurement while the red dashed circle with a radius of 0.5 represents unpolarized emission^[51].