Compressed energy transfer distance for remarkable enhancement of the luminescence of Nd3+-sensitized upconversion nanoparticles†
This study reports the significant enhancement of upconversion luminescence (UCL) of Nd3+-sensitized upconversion nanoparticles (UCNPs) in a sandwich-like nanostructure by compressing the energy transfer distance. The architecture consists of a Nd sensitizer layer (NaYF4:Nd) and an activator layer (NaYF4:Yb/Er) that were enveloped with an inert NaYF4 inner core and inert NaYF4 outer shell. By introducing the inert core and modulating the core size, the Nd sensitizer layer and the activator layer were compressed in a 2D plane, which not only promotes the energy transfer from the Nd sensitizer layer to the activators by shortening their distance, but also effectively increases the energy transfer from Yb3+ to Ho3+ based on δ-doping. Furthermore, the outside inert shell shielded the inside layers from surface-related quenching. Compared with the classical core–shell nanostructure, our compressed nanostructures exhibit remarkably large upconversion luminescence enhancement (up to 19.6-fold) upon 808 nm excitation. Furthermore, this strategy was also applied to other activators (Er3+, Tm3+), which also displayed distinct UCL enhancement. Our developed nanostructure improved the UCL of Nd3+-sensitized UCNPs, which provides opportunities for diverse applications requiring ∼800 nm excitation, such as photodynamic therapy.