Enhancing Quantum Yield via Local Symmetry Distortion in Lanthanide-Based Upconverting Nanoparticles

Lanthanide-based upconverting nanoparticles exhibit significant promise for solar energy generation, biological imaging, and security technologies but have not seen widespread adoption due to the prohibitively low efficiencies of current materials. Weak transition dipole moments between 4f orbitals hinder both photon absorption and emission. Here, we introduce a novel way to increase the radiative transition rates in Yb,Er-based upconverting nanoparticles based on local symmetry distortion. Beginning from a host matrix of the well-studied hexagonal (β)-phase NaYF<sub>4</sub>, we incrementally remove Y<sup>3+</sup> ions and cosubstitute for them a 1:1 mixture of Gd<sup>3+</sup> and Lu<sup>3+</sup> . These two ions act to expand and contract the lattice, respectively, inducing local-level distortion while maintaining the average host structure. We synthesize a range of β-NaY<sub>0.8-2<em>x</em></sub>Gd<em>x</em>Lu<sub><em>x</em></sub>F<sub>4</sub>:Yb<sub>0.18</sub>Er<sub>0.02</sub> nanoparticles and experimentally confirm that particle size, phase, global structure, and Yb<sup>3+</sup> and Er<sup>3+</sup> concentrations remain constant as <em>x</em> is varied. Upconversion quantum yield is probed as the degree of cosubstitution is varied from <em>x</em> = 0 to <em>x</em> = 0.24. We achieve a maximum quantum yield value of 0.074% under 63 W/cm<sup>2</sup> of excitation power density, representing a 1.6× enhancement over the unmodified particles and the highest measured value for near-infrared-to-visible upconversion in sub-25 nm unshelled nanoparticles. We also investigate upconversion emission at the single-particle level and report record improvements in emission intensity for sub-50 nm particles. Radiative rate enhancements are confirmed by measuring excited-state lifetimes. The approach described herein can be used in combination with more established methods of efficiency improvement, such as adding passivating shells or coupling to plasmonic nanoattenas, to further boost the upconversion quantum yield.