Phonon-modulated upconversion luminescence properties in some Er3+ and Yb3+ co-activated oxides

Abstract

The competition between ⁴I_13/2–⁴I_15/2 near-infrared downconversion emissions and visible upconversion emissions of ²H_11/2/⁴S_3/2/⁴F_9/2–⁴I_15/2 is a vital factor for obtaining efficient upconversion luminescence (UCL) in Er³⁺ and Yb³⁺ co-activated oxides. In this paper, we systemically studied the entire emission spectra of Er³⁺ ranging from 400–1700 nm in Er³⁺ and Yb³⁺ co-activated cubic Y₂O₃, tetragonal GdVO₄ and tetragonal NaGd(WO₄)₂ micro-sized powders, prepared by the same sol–gel method, which have different phonon thresholds, 600 cm⁻¹, 880 cm⁻¹ and 1000 cm⁻¹, respectively. It is interesting to observe that in these phosphors, the visible UCL of Er³⁺ relative to near-infrared emissions increase remarkably with the increase of phonon thresholds. In comparison with the UCL in the Y₂O₃ host, the UCL in YVO₄ and tetragonal NaGd(WO₄)₂ was tremendously improved by 9.5- and 120-fold, respectively. The UCL intensity of the NaGd(WO₄)₂:Yb³⁺/Er³⁺ was comparable to that of the famous UCP NaYF₄:Yb³⁺/Er³⁺. A model was proposed to explain the phonon-modulated UC process. This work is of great significance in the search for a novel, efficient UCP.