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Abstract
The rare-earth nanocrystals containing Er3+ emitters offer very promising tools for imaging applications, as they can not only exhibit up-conversion luminescence but also down-conversion luminescence in the second near-infrared window (NIR II). Doping non-lanthanide cations into host matrix was demonstrated to be an effective measure for improving the luminescence efficiency of Er3+ ions, while still awaiting in-depth investigations on the effects of dopants especially those with high valence states on the optical properties of lanthanide nanocrystals. To address this issue, tetravalent Zr4+ doped hexagonal NaGdF4:Yb,Er nanocrystals were prepared, and the enhancement effects of the Zr4+ doping level on both up-conversion luminescence in the visible window and down-conversion luminescence in NIR II window were investigated, with steady-state and transient luminescence spectroscopies. The key role of the local crystal field distortions around Er3+ emitters was elucidated in combination with the results based on both of Zr4+ and its lower valence counterparts, e.g., Sc3+, Mg2+, Mn2+. Univalent ions such as Li+ was utilized to substitute Na+ ion rather than Gd3+, and the synergistic effects of Zr4+ and Li+ ions by co-doping them into NaGdF4:Yb,Er nanocrystals were investigated toward optimal enhancement. Upon optimization, the up-conversion emission of co-doped NaGdF4:Yb,Er nanocrystals was enhanced by more than one order of magnitude compared with undoped nanocrystals. The current studies thus demonstrate that the local crystal field surrounding emitters is an effective parameter for manipulating the luminescence of lanthanide emitters.
Highlights
Rare-earth (RE) nanocrystals are promising light-emitting materials as they are able to convert near infrared (NIR) photons into ultraviolet, visible, and NIR lights through either an up-conversion path or a down-conversion path (Chen et al, 2013; Dong et al, 2015)
Following our previous investigations on the synthesis and optical properties of RE nanocrystals (Liu et al, 2013; Huang et al, 2015; Li et al, 2019), we report hexagonal NaGdF4:Yb,Er nanocrystals doped with tetravalent Zr4+ ions that possess d0 configuration, and studied the Zr4+ doping level-dependent luminescence properties of Er3+ through both steady-state and transient luminescence spectroscopies
Zr4+-doped NaGdF4:Yb,Er nanocrystals were synthesized via a high temperature approach using oleic acid (OA) as the particle surface capping ligand
Summary
Rare-earth (RE) nanocrystals are promising light-emitting materials as they are able to convert near infrared (NIR) photons into ultraviolet, visible, and NIR lights through either an up-conversion path or a down-conversion path (Chen et al, 2013; Dong et al, 2015). Due to the low absorption cross-section (∼10−20 cm2) of lanthanide ions caused by the parity-forbidden transition of 4f electrons (Tu et al, 2015), and non-radiative relaxation between multi-energy levels, their up- and down-conversion luminescence efficiencies are generally low, which obviously hinders their applications. Increasing the concentration of sensitizer or emitter have been adopted to improve the luminescence upon 980 nm excitation (Chen Q.S. et al, 2017; Ma et al, 2017), but overincreasing the concentration of Yb3+ is unfavorable owing to the participation of Yb3+ ions on the particle surface in dissipating the absorbed energy (Johnson et al, 2017)
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