Transition to cubic phase and enhancement of green upconversion emission by adding La3+ ions in hexagonal NaLuF4:Yb3+/Er3+ nanocrystals

Abstract

Phase transition from hexagonal to cubic was successfully achieved through La³⁺ ions tridoping in NaLuF₄:Yb³⁺/Er³⁺ nanocrystals (NCs) by a solvothermal process at 300 °C. X-Ray diffraction (XRD) patterns, scanning electron microscopy (SEM) images, transmission electron microscopy (TEM) images, Fourier-transform infrared (FTIR) absorption spectra, upconversion (UC) luminescence spectra and decay curves were used to characterize the resulting samples. Compared to cubic phase α-NaLu_0.78Yb_0.2Er_0.02F₄ NCs which are similar in size and prepared at normal temperature (280 °C), the number of the surface organic groups such as hydroxyl group (–OH) attached on NaLu_0.48La_0.3Yb_0.2Er_0.02F₄ NCs is reduced. Furthermore, the green UC emission increases and simultaneously the red UC emission decreases. The UC mechanisms were studied by power-intensity log dependence. It indicates that in both of the two samples, the red UC emission is a two-photon process when the excitation power density is low, but a three-photon process occurs when the excitation power density is high. However, the green UC emission is a two-photon process all the time both at low or high excitation power density. Through analyzing the luminescence decay curves of the ⁴S_3/2 → ⁴I_15/2 transition and ⁴F_9/2 → ⁴I_15/2 transition under the 980 nm excitation wavelength, it is concluded that the population of the ⁴F_9/2 level originates from the ⁴I_13/2 level, not from the ²H_11/2/⁴S_3/2 level in the two samples.