Highly efficient up-conversion luminescence in Er3+/Yb3+ co-doped Na5Lu9F32 single crystals by vertical Bridgman method

Shinan He,Haiping Xia,Baojiu Chen,Yongsheng Zhu,Jianli Zhang

doi:10.1038/s41598-017-09222-0

Shinan He, Haiping Xia + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-09222-0

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Abstract

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.

Highlights

Considerable interests in rare-earth (RE) doped UC phosphors have burgeoned since the development of synthetic methods for creating highly processable UC materials with applications in biolabeling[1, 2], sub-band gap energy harvesting in photovoltaics[3, 4], and security printing[5]
It can be seen from the X-ray powder diffraction (XRD) spectra of Er3+/Yb3+ co-doped Na5Lu9F32 single crystal in Fig. 1(b), the structure of the single crystal was confirmed to be cubic phase, in accordance with JCPDS card (27–0725) of Na5Lu9F32 as shown in Fig. 1(c), which the diffraction peak positions of the obtained samples doped with Er3+/ Yb3+ ions are matched perfectly with those of standard Na5Lu9F32, though there exists a slight shift in the range of 0.10° to 0.45° as the dopant change, indicating that this transparent crystal has pure cubic phase
The relative amount of Yb3+ sensitizer ions tend to obviously improve the up-conversion luminescence (UCL) intensity in Er3+/Yb3+ phosphors, we have investigated the impact of various ytterbium concentrations on the intensity of the UC emission under the same experiment condition

Summary

Results and Discussions

The relative amount of Yb3+ ions tends to improve the EQE in Er3+/ Yb3+ co-doped Na5Lu9F32 single crystals obviously, we have investigated the impact of various ytterbium concentrations on the EQE of the up-conversion progress under the same experiment condition Since the observed UC emissions are located in visible wavelength area, the exact UC luminescence CIE chromaticity coordinates of all the Er3+/Yb3+ co-doped Na5Lu9F32 single crystal samples are shown in Fig. 8 to obtain the true color of the UC emissions. This characteristic is favorable for applications in UC displays for electro-optical devices

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