Fig. 1. Evolution of the microscopic structure of Bi-doped nitridated germanate glass with the introduction of Al₂O₃.

Fig. 2. (a) NIR PL emission spectra (λ_ex = 460 nm) of Bi-doped nitridated germanate glasses containing xAl₂O₃ (x = 0–13%); (b) corresponding emission spectra normalized to the emission peak at 1160 nm; (c) emission spectra inside the integrating sphere without and with the sample 0Al or 11Al upon 460 nm excitation; inset shows the magnified spectra in the wavelength range of 800 to 1600 nm.

Fig. 3. (a) Optical absorption spectra of the glass samples containing xAl₂O₃ (x = 0–13%); (b) contour plot of the emission-excitation map from the 6Al glass sample; corresponding PL excitation spectra monitored at (c) 930 nm, (d) 1160 nm, (e) 1260 nm, and (f) 1520 nm of the glass samples containing xAl₂O₃ (x = 0–13%).

Fig. 4. Decay curves of (a) 930 nm, (b) 1160 nm, (c) 1260 nm, and (d) 1520 nm emission peaks of 11Al glass sample (λ_ex = 460 nm).

Fig. 5. (a) Normalized XRD spectra; (b) Raman spectra; (c) FTIR spectra; and (d) ²⁷Al 1D MAS NMR spectra (400 MHz) of the glass samples containing xAl₂O₃ (x = 0–13%), showing a gradual variation with increasing Al₂O₃ content, with the vertical scales (not shown) representing the intensity in arbitrary units, normalized to the maximum peak height in each spectrum; (e) percentage distribution of AlO₄, AlO₅, and AlO₆ by integrating area calculation of Gaussian multipeak fitting; the purple, green, and blue polyhedra beside the NMR spectra stand for Al tetrahedra, hexahedra, and octahedra, respectively. (f) Visible PL emission spectra (λ_ex = 350 nm) of Bi-doped nitridated germanate glasses containing xAl₂O₃ (x = 0–13%).