Tuning Phonon Energies in Lanthanide‐doped Potassium Lead Halide 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 the size‐controlled synthesis of low‐phonon‐energy KPb2X5 (X=Cl, Br) nanoparticles and the ability to tune nanocrystal phonon energies as low as 128 cm−1. KPb2Cl5 nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of KPb2X5 nanoparticles promote upconversion luminescence from higher lanthanide excited states and enable highly nonlinear, avalanche‐like emission from KPb2Cl5 : Nd3+ nanoparticles. 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. We develop phonon‐engineered, lanthanide‐doped upconverting nanoparticles with the lowest phonon energies available to date. Low‐phonon‐energy KPb2X5 (X=Cl, Br) upconverting nanoparticles exhibit dramatically suppressed multiphonon relaxation, enhancing upconversion emission from higher lanthanide excited states and enabling the first room‐temperature observation of avalanche‐like upconversion by Nd3+ ions.