Upconversion photoluminescence in colloidal CdSe/CdS nanocrystal-based solid films

Upconversion is the process based on the transformation of incoming low energy photons to higher energy emitted radiation. Upconverting materials are exploited in several novel applications like biolabeling and radiation harvesting, among others. Thanks to their intrinsic potential, it is very important to be able to fabricate functional devices based on such materials, and to do so in a controllable and affordable way. We investigated the below-bandgap stimulated radiation of heterostructures, obtained from core–shell semiconductor quantum dots, for the development of a large area upconverting system. The upconversion photoluminescence, after sintering of the nanoparticles, has been measured and the conversion mechanism investigated. The presence of intermediate states within the bandgap of the structure, derived from the specific quasi type-II configuration of the core–shell quantum dots, activated a uniform single photon upconversion mechanism on a wide surface area of the developed films, resulting in an anti-Stokes shift larger than 0.4 eV.