Optimal Sensitizer Concentration in Single Upconversion Nanocrystals.

Each single upconversion nanocrystal (UCNC) usually contains thousands of photon sensitizers and hundreds of photon activators to up-convert near-infrared photons into visible and ultraviolet emissions. Though in principle further increasing the sensitizers' concentration will enhance the absorption efficiency to produce brighter nanocrystals, typically 20% of Yb3+ ions has been used to avoid the so-called "concentration quenching" effect. Here we report that the concentration quenching effect does not limit the sensitizer concentration and NaYbF4 is the most bright host matrix. Surface quenching and the large size of NaYbF4 nanocrystals are the only factors limiting this optimal concentration. Therefore, we further designed sandwich nanostructures of NaYbF4 between a small template core to allow an epitaxial growth of the size-tunable NaYbF4 shell enclosed by an inert shell to minimize surface quenching. As a result, the suspension containing 25.2 nm sandwich structure UCNCs is 1.85 times brighter than the homogeneously doped ones, and the brightness of each single 25.2 nm heterogeneous UCNC is enhanced by nearly 3 times compared to the NaYF4: 20% Yb3+, 4% Tm3+ UCNCs in similar sizes. Particularly, the blue emission intensities of the UCNCs with the sandwich structure in the size of 13.6 and 25.2 nm are 1.36 times and 3.78 times higher than that of the monolithic UCNCs in the similar sizes. Maximizing the sensitizer concentration will accelerate the development of brighter and smaller UCNCs as more efficient biomolecule probes or photon energy converters.