Ultrafast interfacial charge carrier dynamics in ZnSe and ZnSe/ZnS core/shell nanoparticles:: Influence of shell formation

The post-preparative irradiation of water-soluble thiol-stabilized ZnSe colloidal nanocrystals (NCs) leads to a strong increase of the photoluminescence quantum yield of UV blue emission. The altered photoluminescence properties of the colloidal samples have been explained previously by changes in particle composition, size, and surface modification accompanying irradiation. In this work, the charge carrier dynamics in NCs before and after irradiation treatment has been examined via femtosecond transient absorbance spectroscopy in the UV-visible spectral region. Upon adsorption of the electron acceptor methylviologen (MV2+), an ultrafast heterogeneous electron transfer from the conduction band of the nanocrystals to MV2+ is observed and studied. Therefore, the employment of the MV2+ helps to assign the observed dynamics to electron or hole relaxation processes. It is shown that a ZnSe/ZnS core/shell structure is formed during the post-preparative treatment. The transient absorbance measurements of the MV2+/NC system indicate that the formation of a shell results in the confinement of the hole to the core, while the electron is still delocalized throughout the entire core/shell structure. The confinement of the hole to the core also explains the strong increase of the photoluminescence quantum yield and the photostability of the NCs after post-preparative irradiation.