Compressed energy transfer distance for remarkable enhancement of the luminescence of Nd3+-sensitized upconversion nanoparticles

Abstract

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.