Absolute up-conversion quantum efficiency reaching 4% in β-NaYF4:Yb3+,Er3+ micro-cylinders achieved by Li+/Na+ ion-exchange

Abstract

Absolute quantum efficiency (QE) of up-conversion (UC) materials is still limited to ∼3%, which strongly limits their applications in many areas. Herein, we report highly efficient UC β-NaYF₄:Yb³⁺,Er³⁺ micro-cylinders prepared via a newly developed Li⁺/Na⁺ ion exchange method (IEM) with absolute QE as high as ∼4.0%. A small amount of Li⁺ is interstitially incorporated in β-NaYF₄:Yb³⁺,Er³⁺, which not only maintains well the micro-cylindrical morphology but also enhances the emission intensity by ∼40%, doubles absolute UCQE (∼3.0% under a fixed excitation power density of 15 W cm⁻²), and prolongs the lifetime. The UC excitation profile is composed of a sharp peak at 976 nm and a weak shoulder at 935 nm. The absolute UCQE of Li⁺/Na⁺ IEM β-NaYF₄:Yb³⁺,Er³⁺ is systemically studied over a wide excitation power density of 0.4–100 W cm⁻², and we obtained impressively high UCQE of 4.0% under 60 W cm⁻² power density, which is among the highest values in the literature. The application of two green UC emissions of Li⁺/Na⁺ IEM β-NaYF₄:Yb³⁺,Er³⁺ in ratiometric thermometers is successfully demonstrated. The high relative temperature sensitivity (S_R) of 1113/T² and excellent repeatability as indicated by three heating–cooling cycles (293–753 K) suggest their promising application as a temperature sensor.