ZnS Shells Enhance Triplet Energy Transfer from CdSe Nanocrystals for Photon Upconversion

Triplet energy transfer (TET) from semiconductor nanocrystals (NCs) to molecules is one of the bottlenecks that limits the efficiency of photon upconversion. While an inorganic shell can enhance the photoluminescence quantum yields (PLQYs), the role of the shell with respect to TET is still not clear. In this work, CdS and ZnS shells with different shell thickness are grown on 2.9 nm diameter CdSe NCs, resulting in nanostructures here that have increased radiative rates compared to the core. TET from these NCs to bound 9-anthracene carboxylic acid is investigated with linear photon upconversion measurements, time-resolved photoluminescence lifetime, and transient absorption spectroscopy. The ZnS shell enhances the photon upconversion QYs 1.6 times from 5.7% to 9.3%, with a concurrent increase of TET efficiency from 6.68% to 12.9% and the net rate of TET from 4.97 x 10(8) s(-1) to 6.67 x 10(10) s(-1). In contrast, TET is barely observed for CdSe/CdS core-shell NCs. Considering the changes in PLQYs and upconversion QYs, a sub-monolayer ZnS shell enhances TET by removing surface traps and increasing exciton lifetime, while a thick shell creates an energy barrier that diminishes TET. The shorter exciton lifetime and increased exciton-phonon coupling due to CdS explain the drastically different effects of ZnS and CdS shells.