2D analysis of polydisperse core-shell nanoparticles using analytical ultracentrifugation.

Accurate knowledge of the size, density and composition of nanoparticles (NPs) is of major importance for their applications. In this work the hydrodynamic characterization of polydisperse core-shell NPs by means of analytical ultracentrifugation (AUC) is addressed. AUC is one of the most accurate techniques for the characterization of NPs in the liquid phase because it can resolve particle size distributions (PSDs) with unrivaled resolution and detail. Small NPs have to be considered as core-shell systems when dispersed in a liquid since a solvation layer and a stabilizer shell will significantly contribute to the particle's hydrodynamic diameter and effective density. AUC measures the sedimentation and diffusion transport of the analytes, which are affected by the core-shell compositional properties. This work demonstrates that polydisperse and thus widely distributed NPs pose significant challenges for current state-of-the-art data evaluation methods. The existing methods either have insufficient resolution or do not correctly reproduce the core-shell properties. First, we investigate the performance of different data evaluation models by means of simulated data. Then, we propose a new methodology to address the core-shell properties of NPs. This method is based on the parametrically constrained spectrum analysis and offers complete access to the size and effective density of polydisperse NPs. Our study is complemented using experimental data derived for ZnO and CuInS2 NPs, which do not have a monodisperse PSD. For the first time, the size and effective density of such structures could be resolved with high resolution by means of a two-dimensional AUC analysis approach.

The effects of post-processing on the surface and the optical properties of copper indium sulfide quantum dots.

In the current contribution we report on investigations regarding the surface of CuInS2 quantum dots and on different strategies to control the amount of surface ligands in a post-processing step. In particular, the reactivity of the organic components, that is, 1-dodecanthiol and 1-octadecene as ligand and solvent, respectively, during nanocrystal formation was studied. A new method to remove residuals from the reaction mixture and to detach excess organics from the surface of the nanocrystals is reported. Our new method, which is based on the utilization of acids, is compared with standard purification procedures by means of thermogravimetric analysis (TGA) with particular focus on its efficiency to remove organics. As a complement, the surface chemistry is analyzed by nuclear magnetic resonance spectroscopy (NMR) to shed light on the nature of the organic components still present after purification. Further analysis of the product by inductively coupled plasma optical emission spectroscopy (ICP-OES) is performed to verify the influence of the new purification method on surface composition and properties. Moreover, steady state and time resolved spectroscopies give insights into excitonic behavior as well as recombination processes. Finally, the new method is optimized for the purification of CuInS2-ZnS nanocrystals, which show enhanced optical properties.