Upconversion luminescence is an active field of research since the pioneering work by Auzel . The past two decades the discovery of upconversion nanocrystals has triggered renewed interest, also because of the potential applications in bio-imaging, solar cells, sensor and security [2-4]. The Yb3+-Er3+ ion couple is one of the most efficient systems of upconversion and is widely used for infrared to green upconversion. However, the upconversion efficiency, especially in nanocrystals, is still quite low, typically below a few percent, even with approaches aimed at enhancing the efficiency, including core-shell architectures, sensitization with dye-antennas, photonic and plasmonic enhancement [2-5]. It is of prime importance to understand the underlying reason why the upconversion process is intrinsically inefficient.
A major loss mechanism is related to the high concentrations of rare earths ions like Yb3+ and Er3+ in upconversion materials. Upconversion relies on multi-step energy transfer. For this reason high dopant concentrations are crucial to realize efficient transfer between lanthanide neighbors. On the other hand, the high dopant concentrations give rise to energy migration and cross-relaxation quenching. The trade-off between efficiency losses by concentration quenching and efficiency gain by energy transfer upconversion determines the maximum upconversion quantum yield but is not well understood. Here we present a systematic investigation on the concentration dependence of luminescence quenching for Er3+ and Yb3+ in NaYF4 nanocrystals, the upconversion model system. Yb3+ and Er3+ concentrations are varied between 1 and 60% for core and core-shell nanocrystals where an undoped isocrystalline shell is grown around the lanthanide doped core. Luminescence spectra and luminescence lifetime measurements, by both indirect and direct excitations, are reported and analyzed. The results show that the concentration quenching is strongly reduced in core-shell geometries. For Yb3+ concentration quenching is limited up to the highest concentrations of 60%. For Er3+ concentration quenching varies for different emitting levels. The strongest quenching occurs for the 4I11/2 level which is an important intermediate state in the IR to green upconversion process. Variation of the solvent reveals that a major loss mechanism is multi-phonon relaxation due to coupling with high energy vibrations of the coordinating ligands and solvent. These results give more insight of decay dynamics and concentration quenching for lanthanide ions involved in upconversion process. The results can serve to optimize the upconversion efficiency by careful tuning of concentrations and core-shell design and thus boost the application of upconversion nanomaterials.
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