Interfacial Phonons Manipulation for Tunable Upconversion Luminescence

Lanthanide doped upconversion nanoparticles (UCNPs) generally suffer from thermal quenching, in which light emission experiences a loss with increasing temperature, seriously limiting their wide applications. On the other hand, the thermal conduction of typical upconversion materials (NaLnF4, Ln=lanthanide) depends mainly on the phonon rather than electrons. Here, we demonstrate that controlling the interfacial phonon can overcome thermal quenching of upconversion materials at the nanoscale. The typical core-shell upconversion nanoparticles (NaGdF4: Yb, Er @ NaGdF4), of which the emission decreases with increasing temperature, have actually a well-like phonon structure. If reversing the core and shell composition (NaGdF4 @ NaGdF4: Yb, Er), the interfacial phonon well will be changed into “a phonon barrier” that leads to an enhancement of upconversion emission with increasing temperature. This indicates that controlling phonon gradient can efficiently tune upconversion luminescence. A 13.01-fold enhancement of green upconversion (UC) luminescence are achieved upon raising the temperature from 298 K to 383 K. The tunable phenomenon of upconversion emission is ascribed to the structural breakdown of lattice vibration modes at the nanoscale.