Effects of Ca2+ doping on upconversion luminescence intensity and crystal field asymmetry of β-NaYF4:Yb3+/Er3+ microcrystals

Abstract The green and red upconversion (UC) emissions of 35 mol% Ca2+ doped β-NaYF4:Yb3+/Er3+ microcrystals were improved to 102.8 and 43.1 times higher than those of Ca2+-free counterparts. X-ray powder diffraction spectra, scanning electron microscopy, Fourier transform infrared spectroscopy, upconversion luminescence (UCL) spectroscopy and Judd-Ofelt analysis were performed to investigate the mechanism of UCL intensity enhancement caused by Ca2+ doping. The hexagonal-to-cubic phase transition was observed when Ca2+ doping concentration exceeded 35 mol%. The incorporation of Ca2+ into hexagonal microcrystals lattice induced unit cell volume enhancement and microcrystals surface to volume ratio variation, which affected the UCL intensity by hindering the non-radiative energy loss process via Yb3+/Er3+ ions and changing the amount of surface quenchers adsorbed on the microcrystals surface. The hypersensitive transition related local crystal field asymmetry around Er3+ in the grown hexagonal microcrystals was also investigated by Judd-Ofelt analysis but unexpectedly found to be the less dominant factor for the UCL intensity variation. This study helps to realize the mechanism of impurity ion doping resulted UCL intensity enhancement, which is instructive for high-performance UC microcrystals synthesis.