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

Abstract

Hierarchically structured-lanthanide (Ln^III)-doped β-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 β-NaGdF₄:Tb^III,Tm^III,Yb^III core@shell nanoparticles emitting by UC, DS, and DC. In the NaGd_y−xTm_0.015Yb_0.20Tb_xF₄@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.