Optical properties of fully inorganic core/gradient-shell CdSe/CdZnS nanocrystals at the ensemble and single-nanocrystal levels.

We report the synthesis and optical characterization of fully inorganic gradient-shell CdSe/CdZnS nanocrystals (NCs) with high luminescence quantum yield (QY, 50%), which were obtained by replacing native oleic-acid (OA) ligands with halide ions (Cl-and Br-). Absorption, photoluminescence excitation (PLE) and photoluminescence (PL) spectra in solution were unaffected by the ligand-exchange procedure. The halide-capped NCs were stable in solution for several weeks without modification of their PL spectra; once deposited as unprotected thin films and exposed to air, however, they did show signs of aging which we attribute to increasing heterogeneity of (effective) NC size. Time-resolved PL measurements point to the existence of four distinct emissive states, which we attribute to neutral, singly-charged and multi-excitonic entities. We found that the relative contribution of these four components to the overall PL decay is modified by the OA-to-halide ligand exchange, while the excited-state lifetimes themselves, surprisingly, remain largely unaffected. The high PL quantum yield of the halide-capped NCs allowed observation of single particle blinking and photon-antibunching; one surprising result was that aging processes that occurs during the first few days after deposition on glass seemed to offer a certain increased protection against photobleaching. These results suggest that halide-capped CdSe/CdZnS NCs are promising candidates for incorporation into opto-electronic devices, based on, for example, hybrid perovskite matrices, which require eliminating the steric hindrance and electronic barrier of bulky organic ligands to ensure efficient coupling.