A novel strategy for markedly enhancing the red upconversion emission in Er3+/Tm3+ cooperated nanoparticlesElectronic supplementary information (ESI) available: FT-IR spectrum of the OA-capped particles; upconversion emission spectra and the CIE chromaticity diagram of the samples; luminescent photographs of the samples; decay curve of the core-inert shell sample; and quantum yield measurement of the core-active shell sample. See DOI: 10.1039/c8tc02370k

A rare-earth-based upconversion nanocrystal that exhibits anti-Stokes nature has attracted worldwide attention due to its unique physical properties. In this work, a novel core-active shell upconversion nanocrystal (NaErF 4 :Tm@NaGdF 4 :Yb) which emits super-bright red emission upon 980 nm near-infrared (NIR) laser excitation is presented. The Er 3+ ions act as both sensitizers and activators to enable an efficient upconversion process of core nanocrystals. To mitigate the luminescence quenching caused by the energy migration between the Er 3+ ions and internal lattice defects, an energy trapping center of Tm 3+ was introduced into the core to confine the energy of Er 3+ through an energy back transfer (Er 3+ → Tm 3+ → Er 3+ ) pathway. After that, an active shell of NaGdF 4 :Yb was grown on the core zone to simultaneously enable the efficient energy transfer from NIR photons to the upconverting zone and suppress the luminescence quenching caused by surface defects and surface associated ligands. It is worth noting that both the Tm 3+ and active shell can enhance the red to green (R/G) ratio of the Er 3+ emission, thus resulting in an ultra-bright red emission with high purity. The core-active shell nanoparticles possess more intense (∼20 times higher) emission intensity than core-inert shell nanoparticles (NaErF 4 :Tm@NaGdF 4 ), and their quantum yield reaches as high as 3.71%. This finding paves a convenient way to obtain red-emissive upconversion nanocrystals with high purity for special applications. A novel core-shell structured nanocrystal with energy trapping centers of Tm 3+ ions doped in the Er 3+ -hosted core zone and Yb 3+ ions doped in the shell portion is innovated to achieve super-bright red upconversion emission upon low 980 nm laser excitation.