Tuning Phonon Energies in Nanocrystals for Enhanced Nonlinearity and Upconversion

Optical applications of lanthanide-doped nanoparticles require materials with low phonon energies to minimize nonradiative relaxation and promote nonlinear processes like upconversion. Heavy halide hosts offer low phonon energies but are challenging to synthesize as nanocrystals. Here, we demonstrate size-controlled synthesis of low-phonon-energy potassium lead halide nanoparticles and the ability to tune nanocrystal phonon energies. These nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of these nanoparticles promote upconversion emission from higher lanthanide excited states and enable highly nonlinear, avalanche-like emission when doped with neodymium ions. The realization of nanoparticles with tunable, ultra-low phonon energies facilitates the discovery of nanomaterials with phonon-dependent properties, precisely engineered for applications in nanoscale imaging, sensing, luminescence thermometry and energy conversion.