Enhancement of up-conversion luminescence of Y₂Ti₂O₇:Yb³⁺/Ho³⁺ nanocrystals by incorporating Li⁺ ions

Abstract

Tri-doped (Y_0.915Ho_0.01Yb_0.075Li_x)₂Ti₂O₇ (x = 0–15.0 mol %) nanocrystals were fabricated via a facile glycine–combustion approach. The crystal structure, morphology and up-conversion spectra of the as-obtained products were investigated using X-ray powder diffractometer (XRD), transmission electron microscope (TEM), spectrophotometer pumped by 980 nm diode laser, and Fourier transform infrared spectrometer. XRD results demonstrated that, owing to the flux effect of Li⁺ ions, the crystallization temperature of matrix Y₂Ti₂O₇ decreased when compared to Li⁺-undoped samples. TEM results showed the average particle of (Y_0.79Ho_0.01Yb_0.075Li_0.125)₂Ti₂O₇ nanocrystals calcined at 800°C for 1.0 h was estimated to be ∼50 nm. It was also found that up-conversion emissions were strongly dependent on the calcining temperature and Li⁺ ion concentration. Compared with the Li⁺-free Y₂Ti₂O₇:Yb³⁺/Ho³⁺ samples, the green and red emission intensity of nanocrystals tri-doped with 12.5 mol % Li⁺ was enhanced by 10.2 and 4.8 times, respectively. The emission increase should be mainly ascribed to the distortion of local symmetry around Ho³⁺ ions by Li⁺-doping. In addition, the pump power dependence of integrated intensity suggested that both green and red emission was two-photon population process.