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Dual Role of Electron-Accepting Metal-Carboxylate Ligands: Reversible Expansion of Exciton Delocalization and Passivation of Nonradiative Trap-States in Molecule-like CdSe Nanocrystals

Lawrence, Katie N., Dutta, Poulami, Nagaraju, Mulpuri, Teunis, Meghan B., Muhoberac, Barry B., Sardar, Rajesh
Journal of the American Chemical Society 2016 v.138 no.39 pp. 12813-12825
Fourier transform infrared spectroscopy, X-ray diffraction, absorption, ambient temperature, ligands, nanocrystals, nuclear magnetic resonance spectroscopy, photoluminescence
This paper reports large bathochromic shifts of up to 260 meV in both the excitonic absorption and emission peaks of oleylamine (OLA)-passivated molecule-like (CdSe)₃₄ nanocrystals caused by postsynthetic treatment with the electron accepting Cd(O₂CPh)₂ complex at room temperature. These shifts are found to be reversible upon removal of Cd(O₂CPh)₂ by N,N,N′,N′-tetramethylethylene-1,2-diamine. ¹H NMR and FTIR characterizations of the nanocrystals demonstrate that the OLA remained attached to the surface of the nanocrystals during the reversible removal of Cd(O₂CPh)₂. On the basis of surface ligand characterization, X-ray powder diffraction measurements, and additional control experiments, we propose that these peak red shifts are a consequence of the delocalization of confined exciton wave functions into the interfacial electronic states that are formed from interaction of the LUMO of the nanocrystals and the LUMO of Cd(O₂CPh)₂, as opposed to originating from a change in size or reorganization of the inorganic core. Furthermore, attachment of Cd(O₂CPh)₂ to the OLA-passivated (CdSe)₃₄ nanocrystal surface increases the photoluminescence quantum yield from 5% to an unprecedentedly high 70% and causes a 3-fold increase of the photoluminescence lifetime, which are attributed to a combination of passivation of nonradiative surface trap states and relaxation of exciton confinement. Taken together, our work demonstrates the unique aspects of surface ligand chemistry in controlling the excitonic absorption and emission properties of ultrasmall (CdSe)₃₄ nanocrystals, which could expedite their potential applications in solid-state device fabrication.