Proof of crystal-field-perturbation-enhanced luminescence of lanthanide-doped nanocrystals through interstitial H + doping.

Crystal-field perturbation is theoretically the most direct and effective method of achieving highly efficient photoluminescence from trivalent lanthanide (Ln ³⁺ ) ions through breaking the parity-forbidden nature of their 4f-transitions. However, exerting such crystal-field perturbation remains an arduous task even in well-developed Ln ³⁺ -doped luminescent nanocrystals (NCs). Herein, we report crystal-field perturbation through interstitial H ⁺ -doping in orthorhombic-phase NaMgF ₃ :Ln ³⁺ NCs and achieve a three-orders-of-magnitude emission amplification without a distinct lattice distortion. Mechanistic studies reveal that the interstitial H ⁺ ions perturb the local charge density distribution, leading to anisotropic polarization of the F ^- ligand, which affects the highly symmetric Ln ³⁺ -substituted [MgF ₆ ] ^4- octahedral clusters. This effectively alleviates the parity-forbidden selective rule to enhance the 4f-4 f radiative transition rate of the Ln ³⁺ emitter and is directly corroborated by the apparent shortening of the radiative recombination lifetime. The interstitially H ⁺ -doped NaMgF ₃ :Yb/Er NCs are successfully used as bioimaging agents for real-time vascular imaging. These findings provide concrete evidence for crystal-field perturbation effects and promote the design of Ln ³⁺ -doped luminescent NCs with high brightness.
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