Lanthanide (Ln³⁺)-doped photon avalanche (PA) upconversion nanoparticles (UCNPs) have great prospects in many frontier applications, such as super-resolution bioimaging, miniaturized lasers, single-molecule tracking, and quantum optics. However, it remains challenging to realize PA in colloidal Ln³⁺-doped UCNPs at room temperature (RT), due to the deleterious quenching effect associated with the surface and lattice OH^- defects. 

In a recent study published in Nano Letters, the research group led by Prof. CHEN Xueyuan from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences (CAS) developed a novel approach based on the pyrolysis of KHF₂ for the 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 (UCL).  

Through the design of a series of controlled experiments, the researchers demonstrated that the KHF₂ precursor can effectively prevent the generation of OH^– defects during the growth of the UCNCs, which resulted in highly efficient UCL in Yb³⁺/Er³⁺ and Yb³⁺/Ho³⁺ co-doped KMgF₃ UCNPs, with UCQYs of ～3.8% and ～1.1%, respectively, under 980 nm excitation at a power density of 20 W cm^-2. 

Specifically, because of the suppressed OH^-  defects and enhanced cross-relaxation rate between Tm³⁺ ions in the aliovalent Tm³⁺-doped system, they realized efficient PA luminescence from Tm³⁺ at 802 nm in KMgF₃: Tm³⁺ UCNPs upon 1064 nm excitation at RT, with a giant nonlinearity of ～27.0, a PA rise time of 281 ms, and a threshold of 16.6 kW cm^-2. 

These findings provide a general approach for the development of highly efficient PA UCNPs with huge nonlinearities through aliovalent Ln³⁺ doping and crystal lattice engineering, which may open up a new avenue for the exploration of PA UCNPs toward various frontier applications such as single-beam super-resolution bioimaging, single-molecule tracking, and miniaturized lasers.