Lanthanide-doped KMgF₃ upconversion nanoparticles for photon avalanche luminescence

Lanthanide (Ln³⁺)-doped photon avalanche (PA) upconversion nanoparticles (UCNPs) can be applied in super-resolution bioimaging, miniaturized lasers, single-molecule tracking and quantum optics.

However, it remains challenging to realize photon avalanche in colloidal Ln³⁺-doped UCNPs at room temperature due to the deleterious quenching effect associated with surface and lattice OH^– defects.

A research group led by Prof. Chen Xueyuan from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences has developed a novel approach based on the pyrolysis of KHF₂ for controlled synthesis of Ln³⁺-doped KMgF₃ UCNPs, which can effectively protect Ln³⁺ from luminescence quenching by surface and internal OH^– defects, and thereby boost upconversion luminescence.

The study was published in Nano Letters on Sept. 8.

The researchers demonstrated that the KHF₂ precursor could effectively prevent the generation of OH^– defects during the growth of UCNPs, which resulted in highly efficient upconversion luminescence in Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺ co-doped KMgF₃ UCNPs, with upconversion quantum yields of ～3.8% and ～1.1%, respectively, under 980 nm excitation at a power density of 20 W cm^-2.

Specifically, due to the suppressed OH^– defects and enhanced cross-relaxation rate between Tm³⁺ ions in the aliovalent Tm³⁺-doped system, the researchers realized efficient photon avalanche luminescence from Tm³⁺ at 802 nm in KMgF₃: Tm³⁺ UCNPs upon 1,064 nm excitation at room temperature, with a giant nonlinearity of ～27.0, a photon avalanche rise time of 281 ms, and a threshold of 16.6 kW cm^-2.

Additionally, the researchers revealed the distinctive advantages of KHF₂ for the controlled synthesis of KMgF₃: Ln³⁺ UCNPs, which endowed the UCNPs with tunable size, improved crystallinity, a reduced number of surface and lattice defects (typically OH^–), and concomitantly improved upconversion luminescence and near-infrared-II downshifting luminescence efficiencies.

This study provides an approach for the development of highly efficient photon avalanche UCNPs with huge nonlinearities through aliovalent Ln³⁺ doping and crystal lattice engineering.