Burstein Moss effect in CdO–V₂O₅–P₂O: Er³⁺ glasses, and the Yb³⁺ concentration effect on up conversion and downshifting emissions

Er³⁺ and xYb³⁺ co-doped CdO–V₂O₅–P₂O₅ invert glasses were studied through their structural, optical and photoluminescent properties as a function of the incorporation of Yb³⁺ ions. From XRD was observed that the glass system remains amorphous up to Yb³⁺ concentration of 4.0 mol%. Raman Spectroscopy revealed that the samples are composed by amorphous Cd₂V₂O₇ and PO₄ units, with a maximum phonon energy of 923 cm⁻¹. The optical bandgap values calculated by Tauc's method were in the range of 2.47–2.70 eV, depending on the Yb³⁺ content. Furthermore, Burstein-Moss effect on the optical bandgap was observed for Yb³⁺ concentration higher than 1.0 mol%, like heavy doped semiconductors. Such fact is followed by a reduction of the Urbach energy from 0.70 to 0.35 eV. The Er³⁺:(²H_11/2, ⁴S_3/2) → ⁴I_15/2, ⁴F_9/2 → ⁴I_15/2 up conversion and ⁴I_13/2 → ⁴I_15/2 downshifting emissions upon 980 nm laser excitation, are gradually improved with the Yb³⁺ content. A maximum enhancement of 20 and 80 times for both green and red up conversion emission was achieved, respectively, whereas the NIR downshifting emission was improved about 5.3 times for 4.0 mol% of Yb³⁺, regarding the singly Er³⁺ doped glass. Such effect is partially related to the Burstein Moss effect and an efficient energy transfer from Yb³⁺ to Er³⁺.