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

Abstract

Crystal-field perturbation is theoretically the most direct and effective method of achieving highly efficient photoluminescence from trivalent lanthanide (Ln3+) 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 Ln3+-doped luminescent nanocrystals (NCs). Herein, we report crystal-field perturbation through interstitial H+-doping in orthorhombic-phase NaMgF3:Ln3+ 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 Ln3+-substituted [MgF6]4− octahedral clusters. This effectively alleviates the parity-forbidden selective rule to enhance the 4f–4 f radiative transition rate of the Ln3+ emitter and is directly corroborated by the apparent shortening of the radiative recombination lifetime. The interstitially H+-doped NaMgF3: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 Ln3+-doped luminescent NCs with high brightness.

Near‐infrared emitting metal halide materials: Luminescence design and applications

“…The oscillating dipoles induced in the local structure led to additional ligand-field interactions around Ln 3+ and facilitated the mixing of opposite-parity states into the 4f wavefunction of Ln 3+ , thus partially breaking the parity-forbidden intra-configurational 4f transitions. 236,241,242 As a result, the oscillator strength of the electric dipole transition (f ed ) and correlated electric dipole radiative transition probability (A ed ) of intra-4f optical transitions were highly enhanced. The significantly increased downconversion luminescence of Er 3+ in the NIR-II spectral region (Figure 9G) enabled the interstitially H +doped NaMgF 3 :Yb/Er NCs to be applied in high-resolution NIR-II angiography.…”

“…(F) Schematic illustration of interstitial H + -doping and crystal-field perturbation in NaMgF 3 :Yb/Er NCs. 236 (G) PL spectra of NaMgF 3 :Yb/Er NCs synthesized with different nominal amounts of acetic acid (HAc). Reproduced with permission.…”

“…Reproduced with permission. 236 Copyright 2023, Springer Nature. two sub-peaks at 775 nm (1.6 eV) and 905 nm (1.4 eV) fell significantly lower than the band gap, suggesting the existence of relatively deep defect states, which may lead to the rapidly suppressed emission intensity.…”

“…Different from the above-mentioned local structural modification by conventional cation regulation, interstitial doping can achieve genuine crystal field perturbation without a distinct lattice distortion. Very recently, Hong's group 236 proposed an interstitial H + -doping strategy to stimulate crystal field perturbation of NaMgF 3 :Yb/Er NCs with high symmetry, which mitigated the parityselection rule of Ln 3+ and effectively enhanced both upconversion and NIR downconversion luminescence of Er 3+ . As shown in Figure 9F, the introduced H + ions formed hydrogen bonds with the neighboring F À ions, which exerted a slight effect on the local coordination environment around Ln 3+ by none-equivalently changing the bond lengths, bond angles, and differential charge density distributions.…”