Absolute up-conversion quantum efficiency reaching 4% in β-NaYF4:Yb3+,Er3+ micro-cylinders achieved by Li+/Na+ ion-exchangeElectronic supplementary information (ESI) available. See DOI: 10.1039/c8tc01806e

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-NaYF 4 :Yb 3+ ,Er 3+ 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-NaYF 4 :Yb 3+ ,Er 3+ , 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-NaYF 4 :Yb 3+ ,Er 3+ 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-NaYF 4 :Yb 3+ ,Er 3+ in ratiometric thermometers is successfully demonstrated. The high relative temperature sensitivity ( S R ) of 1113/ T 2 and excellent repeatability as indicated by three heating-cooling cycles (293-753 K) suggest their promising application as a temperature sensor. Absolute up-conversion quantum efficiency (UCQE) up to 4% is achieved in Li + -doped β-NaYF 4 :Yb 3+ ,Er 3+ micro-cylinders developed via Li + /Na + ion-exchange.