Er3+/Yb3+ co-doped Na5Lu9F32 single crystals with different concentrations of Yb3+ ions were prepared to investigate their phase structure, up-conversion (UC) properties and mechanism of UC luminescence by Bridgman method. Under 980 nm near-infrared (NIR) excitation, three sharp UC emission bands topping at green ~525 nm, ~548 nm and red ~669 nm were obtained in Er3+/Yb3+ doped Na5Lu9F32 single crystals which are attributing to the transitions of 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. The quadratic dependence of pump power on UC emission indicated that two-photon process is in charge of the transition from excited state of Yb3+ ions to lower state of Er3+ ion in Na5Lu9F32 single crystals. The long-accepted mechanism for the production of red and green emissions through up-conversion (UC) under 980 nm excitation in Er3+/Yb3+ co-doped materials apply in the Na5Lu9F32 host was displayed. The enhancement of the red emission was observed due to a cross-relaxation (CR) process of the form: 4F7/2 + 4I11/2 → 4F9/2 + 4F9/2. Furthermore, an ideal yellowish green light performance could be achieved with 1.0 mol% Er3+ doped certain Yb3+ concentrations samples, and its external quantum efficiency approached to 6.80% under 5.5 Wcm−2 980 nm excitation which can be applied in developing UC displays for electro-optical devices.
The luminescence properties of Na 5 Lu 9 F 32 single crystals tridoped Tb 3+ , Yb 3+ , and Gd 3+ ions synthesized via a vertical Bridgman method were reported. Gd 3+ was codoped as a sensitizer in Na 5 Lu 9 F 32 :Tb 3+ , Yb 3+ , thus increasing the absorption line-width and the absorption cross sections in the ultraviolet− visible region. Effects of Gd 3+ concentration on the luminescence properties under ultraviolet and near-infrared were investigated. The energy transfer mechanism between the Gd 3+ and Tb 3+ ion was witnessed by the detailed analyses of the excited-state lifetimes, emission quantum yields, and emission and excitation spectra for the Na 5 Lu 9 F 32 :Tb 3+ , Yb 3+ single crystals codoping with various Gd 3+ concentrations ranging from 0 to 0.25 mol %. Following the excitation light in ultraviolet region, Na 5 Lu 9 F 32 single crystals tridoped Tb 3+ /Yb 3+ /Gd 3+ showed dominant green emissions in the visible region and at 980 nm which were attributed to 5 D 4 → 7 F 5 transition of Tb 3+ and 2 D 5/2 → 2 F 7/2 of Yb 3+ , respectively. Under the excitation of 980 nm, Na 5 Lu 9 F 32 single crystals tridoped Tb 3+ /Yb 3+ /Gd 3+ presented bright yellowish-green lights arising from the cooperative energy transfer between two Yb 3+ and one Tb 3+ . The practical up-conversion and down-conversion external quantum efficiency were measured by integrating spheres at ∼1.2% and ∼45%, respectively. The present results demonstrated successful Tb 3+ /Yb 3+ /Gd 3+ triply doping Na 5 Lu 9 F 32 single crystals with both up-and down-conversion luminescence properties that can be used as a potential spectrum converter in solar cells.
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