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.
Besides, 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.