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.