Fig. 1. SEM images of the synthesized samples: (a) Gd₂O₃: 3% Er³⁺; (b) Gd₂O₃: 3% Er³⁺, 10% Na⁺; (c) Gd₂O₃: 3% Er³⁺, 7% K⁺; (d) Gd₂O₃: 3% Er³⁺, 3% Cs⁺. (e), (h) TEM images; (f), (i) HRTEM images; (g), (j) SAED patterns of Gd₂O₃: 3% Er³⁺ and Gd₂O₃: 3% Er³⁺, 10% Na⁺ phosphors.

Fig. 2. (a) Absorption spectra of Gd₂O₃: 3% Er³⁺ and Gd₂O₃: 3% Er³⁺, yM⁺ (M = Na, K, Cs; y = 10%, 7%, 3%). UC PL emission spectra for (b) Gd₂O₃: xEr³⁺ (x = 1%, 3%, 5%, 7%, 10%, 12%); (c) Gd₂O₃: 3% Er³⁺, yNa⁺ (y = 1%, 3%, 5%, 7%, 10%, 12%); and (d) Gd₂O₃: 3% Er³⁺, zM⁺ (M = Na, K, Cs; z = 10%, 7%, 3%). (e) Luminescence intensity of the Gd₂O₃: 3% Er³⁺, 10% Na⁺ phosphors at different annealing temperatures; (f) CIE chromaticity diagram of the Gd₂O₃: 3% Er³⁺ and the Gd₂O₃: 3% Er³⁺, 10% Na⁺ phosphors.

Fig. 3. (a), (b) Logarithmic patterns of power dependence of 3% Er³⁺ and 3% Er³⁺, 10% Na⁺-doped Gd₂O₃ sample materials in red and green wavebands; (c) emission attenuation curves of Gd₂O₃: 3% Er³⁺ and Gd₂O₃: 3% Er³⁺, 10% Na⁺ phosphors were fitted by a double exponential decay function (980 nm excitation); (d) possible UC mechanism of Er³⁺ ions under 980 nm excitation.

Fig. 4. Photocurrent response of the device incorporating Gd₂O₃: 3% Er³⁺/PMMA and Gd₂O₃: 3% Er³⁺, 10% Na⁺/PMMA composite films under the square pulse with an NIR wavelength of 980 nm, the applied potential of 0 V, and 2 s off/on cycles.