Properties of CdSe nanocrystal dispersions in the dilute regime: Structure and interparticle interactions

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Abstract
We report an investigation of the properties of CdSe nanocrystal dispersions using small-angle x-ray scattering. These nanocrystals are prepared from organometallic precursors using high-temperature solution chemistry. The study provides direct and accurate measures of the nanocrystal size and size distribution. The low polydispersity measured confirms the high quality of the nanocrystals prepared using a high-temperature solution chemistry route, as anticipated from optical data. The study also provides information on the interparticle interactions and their dependence on a few relevant parameters, such as nature of the capping molecules and solvent. Nanocrystal association, e.g., dimers, in dispersions characterized by weakly attractive interactions, and/or aggregation in solutions with strong attractions, can be observed. The study also unveiled other features where interactions are reversed from repulsive stabilizing to attractive as the particle size is decreased. This behavior, unexpected for colloidal dispersions, may be caused by a reduction of the cap density as the size is decreased. The general trend for the interparticle interactions in these dispersions can be understood within the framework of a van der Waals core to core attractive potential, to which are superposed effects of cap affinity to the core and to the surrounding solvent. Within these considerations, one can distinguish three types of dispersions: sterically stabilized dispersions, dispersions thermodynamically stable but governed by weak attractions, and unstable dispersions where strong attractions induce macroscopic aggregation.