Investigation of upconversion and downconversion fluorescence emissions from β-NaLn1F4:Yb3+, Ln23+ (Ln1 = Y, Lu; Ln2 = Er, Ho, Tm, Eu) hexagonal disk system

Uniform hexagonal β-NaLn1F4:Yb3+, Ln23+ (Ln1 = Y, Lu; Ln2 = Er, Ho, Tm, Eu) disks were synthesized with hydrothermal method in which the sodium citrate was used as a shape modifier. The experimental observation indicated that both upconversion and downconversion fluorescence emissions were significantly stronger in β-NaLuF4 than that for β-NaYF4. It was found that the stronger hypersensitive transitions were mainly due to the increase of J–O parameter (Ω2) in β-NaLuF4 host, while the stronger insensitive transitions were mainly caused by the stronger overlap of electron cloud that was induced by the decrease of LnF bond lengths. Additionally, the larger absorption strength in the near-infrared region is another important factor for the stronger fluorescence emissions. The current research has a great potential in improving near-infrared conversion efficiency of silicon solar cells.