Photon Upconversion in the Visible Wavelengths with ZnSe/InP/ZnS Nanocrystals

Nanocrystals that can absorb strongly in the near-infrared (NIR) wavelengths for conversion to visible light are of great interest for biological imaging applications. In this work, we examine an inverse Type-I heterostructure with an inner InP shell for triplet-triplet annihilation-based photon upconversion. The InP-based nanocrystals are earth-abundant, benign, and can be synthetically tuned to absorb in the entire NIR window I. Here, a two monolayer ZnS shell was used to passivate the surface defects on inverse Type-I ZnSe/InP core/shell particles. We show that this ZnS shell increases the InP photoluminescence quantum yield (QY) by a factor of 16 to 0.43 and the transmitter triplet lifetime from 138 to 451 mu s, but decreases the rate of triplet energy transfer by a factor of 3. This results in the ZnSe/InP and ZnSe/InP/ZnS nanocrystal triplet photosensitizers producing a similar photon upconversion QY of about 4.0% (out of a maximum of 50%) when paired with 9,10-diphenylanthracene as the blue emitter. This work suggests that the ZnS shell can be further tuned to increase the photon upconversion QY. This work also shows that ZnSe/InP/ZnS nanocrystals are promising candidates for a hybrid organic-inorganic nanostructure that can convert NIR photons to visible light.