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

Hierarchically structured-lanthanide (LnIII)-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 (Pexc), and LnIII–LnIII separation on the LnIII-to-LnIII 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:TbIII,TmIII,YbIII core@shell nanoparticles emitting by UC, DS, and DC. In the NaGdy−xTm0.015Yb0.20TbxF4@NaYF4 compositions, larger TbIII amounts induce stronger TbIII UC while for Pexc lower than 20 W cm−2, both cooperative sensitization upconversion (CSU) and energy transfer upconversion (ETU) mechanisms may occur. On the other hand, for Pexc larger than 50 W cm−2, energy migration upconversion (EMU) dominates the UC. By separating TbIII from TmIII and YbIII in core@shell@NaGdF4 architectures, only the CSU and ETU mechanisms occur, while for Pexc densities larger than 40 W cm−2, saturation of upconversion takes place. Upon 488 nm excitation, TbIII DS and YbIII DC emissions are noticed as well. Thus, this study helps to understand how the proper design of hierarchically structured nanoparticles can control the LnIII-to-LnIII ET mechanisms while their versatile excitation and multi-emission are interesting features for use as anti-counterfeiting agents or in energy conversion.