Unravelling the energy transfer of Er³⁺-self-sensitized upconversion in Er³⁺-Yb³⁺-Er³⁺ clustered core@shell nanoparticles

Unravelling upconversion (UC) energy transfer mechanisms is significant for designing novel efficient anti-Stokes phosphors. We have studied the correlation of different lanthanide dopants within Er³⁺-self-sensitized core@shell upconversion nanoparticles (UCNPs). Here, our focus will be on high-concentration dopants that are able to sufficiently produce the clustering effect, especially within the interplay between Er³⁺ and Yb³⁺. We demonstrate that whatever the amount of the self-sensitizer (e.g., Er³⁺), abnormal absorption enhancement will occur as long as Yb³⁺ clusters are present. This effect originates from the substantial energy transfer between Yb³⁺-Yb³⁺ clusters despite the increased energy transfer from Yb³⁺ to Er³⁺. Therefore, the energy transfer efficiency is still constrained. However, we conversely used one of the aforementioned quench-paths of UC energy transfer to easily transfer the energy from the in-shell shell layer to the in-core area with the assistance of the energy potential reservoir, which was given by the homogeneous core@shell band offset at the interface region. Indirectly, we actualize the Er³⁺ UC luminescence with self-sensitization through an extended energy transfer path. This work provides a solid support and analytic theory for unraveling the energy transfer mechanism from recent works on Er³⁺ self-sensitized UC luminescence.