Origin of the High Upconversion Green Luminescence Efficiency in β-NaYF4:2Er~3+,20Yb~3+

摘要

Site-selective spectroscopy in hexagonal β-NaYF4:Er~(3+),Yb~(3+) has revealed different environments for Er~(3+)ions (multisite formation). The low-temperature S_(3/2)→I_(15/2)Er~3+green emission depends on the excitation wavelength associated with the F_(7/2) Er ~(3+) level. We have studied the effect of hydrostatic pressure on the green, red, and blue Er ~(3+) emission upon NIR excitation at ～980 nm, in order to establish the role played by energy resonance conditions and the multiple Er ~(3+) sites due to the disordered structure for the upconversion (UC) process (energy tuning). The variation of photoluminescence spectra and lifetimes as a function of pressure and temperature reveals that the origin of the high green UC efficiency of the β-NaYF4:Er ~(3+),Yb ~(3+) compound is mainly due to the multisite distribution, and the low phonon energy of the host lattice.