Multi-emission processes of hierarchically structured NaGdF₄:Tm:Yb:Tb core@shell nanoparticles

Hierarchically structured-lanthanide (Ln(III))-doped beta-NaGdF4 nanoparticles emitting by upconversion (UC), downconversion (DC) and/or downshifting (DS) are currently investigated for potential applications as anti-counterfeiting agents or in energy conversion. Yet, the role of doping concentration, excitation pump power density (P-exc), and Ln(III)-Ln(III) separation on the Ln(III)-to-Ln(III) energy transfer (ET), luminescence quenching, and emission color tunability need to be further elucidated to boost their luminescence. In light of that, herein, these issues are discussed in nanostructured beta-NaGdF4:Tb-III,Tm-III,Yb-III core@shell nanoparticles emitting by UC, DS, and DC. In the NaGdy-xTm0.015Yb0.20TbxF4@NaYF4 compositions, larger Tb-III amounts induce stronger Tb-III UC while for P-exc lower than 20 W cm(-2), both cooperative sensitization upconversion (CSU) and energy transfer upconversion (ETU) mechanisms may occur. On the other hand, for P-exc larger than 50 W cm(-2), energy migration upconversion (EMU) dominates the UC. By separating Tb-III from Tm-III and Yb-III in core@shell@NaGdF4 architectures, only the CSU and ETU mechanisms occur, while for P-exc densities larger than 40 W cm(-2), saturation of upconversion takes place. Upon 488 nm excitation, Tb-III DS and Yb-III DC emissions are noticed as well. Thus, this study helps to understand how the proper design of hierarchically structured nanoparticles can control the Ln(III)-to-Ln(III) ET mechanisms while their versatile excitation and multi-emission are interesting features for use as anti-counterfeiting agents or in energy conversion.