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The rare-earth nanocrystals containing Er 3+  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 Er 3+  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 Zr 4+  doped hexagonal NaGdF 4 :Yb,Er nanocrystals were prepared, and the enhancement effects of the Zr 4+  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 Er 3+  emitters was elucidated in combination with the results based on both of Zr 4+  and its lower valence counterparts, e.g., Sc 3+ , Mg 2+ , Mn 2+ . Univalent ions such as Li +  was utilized to substitute Na +  ion rather than Gd 3+ , and the synergistic effects of Zr 4+  and Li +  ions by co-doping them into NaGdF 4 :Yb,Er nanocrystals were investigated toward optimal enhancement. Upon optimization, the up-conversion emission of co-doped NaGdF 4 :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.

Doping Lanthanide Nanocrystals With Non-lanthanide Ions to Simultaneously Enhance Up- and Down-Conversion Luminescence