Many groups have been investigating both how the choice of non-solvent and multiple steps during the purification processes of semiconductor nanocrystals (NCs) can affect both the optical properties and the ligand surface density. Based on an approach for the classification of covalent compounds suggested by M. L. H Green, molecules that are able to act in role of ligands for NCs are categorized by the specific interaction with the particle surface and include X-type, L-type, and Z-type. X-type ligands are only replaceable by ligands of the same kind owing to the fact that NCs are metal rich on the surface rendering them positively charged, so the new ligand has to be able to maintain the charge balance of the system. TOPO and alkylamines are frequently utilized ligands of the L-type variety, and in the case of II-VI nanocrystals, primary amines tend to prefer the Se sites over the metal sites. Our group has been investigating the use of ethanol (EtOH) as the non-solvent during the cleaning process of II-VI NCs coated with TOPO and stearic acid. Unlike the reports using methanol during the purification process, the photoluminescence quantum yield (PLQY) measurement on EtOH purified samples do not exhibit any degradation in the PLQY, even after many purification steps. In general, during the washing steps, the TOPO (L-type ligand) is removed in the early stages of cleaning, followed by stearate (X-type ligand) removal, although the amount of stearate removed from the NCs surface reaches a plateau after a few washing steps. The competition between both surface ligand densities with our suggested model of preferential desorption of L-type over X-type ligands from NC surfaces should influence the optical properties strongly, and thusly the charge carrier dynamics. In our current work, we have performed x-ray transient absorption spectroscopy to further understand the charge carrier dynamics of ZnSe nanocrystals as a function of surface ligand coverage. We use above band gap excitation from a femtosecond laser and probe the decay dynamics at both the Se and Zn K-edges. The x-ray data points towards evidence for a photoinduced charge transfer from Se sites to Zn sites that depends directly in the surface ligand density.

Time resolved x-ray spectroscopy of ZnSe semiconductor nanocrystals: Understanding the role of surface ligands on dynamical properties

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.