Abstract
In this Article, we reported the synthesis and the upconversion luminescence (UCL) properties of a series of novel (Na/Li)Bi(MoO4)2:Yb3+,Er3+ [(N/L)BMO:Yb3+,Er3+] and (Na/Li)Bi(MoO4)2:Yb3+,Ho3+,Ce3+ [(N/L)BMO:Yb3+,Ho3+,Ce3+] phosphors. X-ray diffraction patterns and Rietveld refinements for several representative samples indicated the pure phase of as-prepared samples. The Yb3+,Er3+ codoped (N/L)BMO presented bright green luminescence under 975 nm laser excitation with UCL spectra showing two main green bands around 529 nm (Er3+, 2H11/2 → 4I15/2) and 551 nm (Er3+, 4S3/2 → 4I15/2), in addition to a very weak one at 655 nm (Er3+, 4F9/2 → 4I15/2). The (N/L)BMO:Yb3+,Ho3+ mainly showed a green band around 544 nm (5S2,5F4 → 5I8) and a red band around 654 nm (5F5 → 5I8) upon 975 nm laser excitation. With increasing Yb3+ concentrations in (N/L)BMO:Yb3+,0.01Ho3+, the red/green ratios decreased monotonously corresponding to the emission color variation from light red to light yellow. Both UCL mechanisms of Yb3+,Er3+ and Yb3+,Ho3+ were determined to be two-phonons absorption processes in (N/L)BMO:Yb3+,Er3+/Ho3+. The Ce3+ ions were introduced into Yb3+,Ho3+ codoped (N/L)BMO to show the color tuning from light yellow to light red originating from the cross relaxation processes of (CR1) Ho3+ (5F4,5S2) + Ce3+ (2F5/2) → Ho3+ (5F5) + Ce3+ (2F7/2) and (CR2) Ho3+(5I6) + Ce3+ (2F5/2) → Ho3+ (5I7) + Ce3+ (2F7/2), which is based on the energy matching of Ce3+2F7/2-2F5/2 level pairs with Ho3+5I6-5I7 and 5F4,5S2-5F5 level pairs and confirmed by the decay times. These results suggest good UCL properties of (N/L)BMO:Yb3+, Er3+ and (N/L)BMO:Yb3+, Ho3+, Ce3+ materials, and color modulation is easily controlled by varying Yb3+ concentration and a cross relaxation process between Ce3+ and Ho3+, which provides efficient methods to regulate the emission color of UCL phosphors.
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