Spontaneous separation of single from polycrystalline5 nm gold nanocrystals (NCs) is observed in colloidal solution. This segregation takes place upon selfassembling of single crystalline NCs at the air − solvent interface and in precipitated superlattices. Polycrystalline NCs are observed to remain in the suspension. Transmission electron microscopy analysis of the size distribution of NCs issued from the diff erent populations indicates that the NC size does not change from each other, excluding therefore any size segregation in this process. Using both low-frequency Raman scattering and X-ray diff raction provides reliable characterization of nanocrystallinity for each population of NCs, thus confi rming the crystallinity segregation. The single crystalline NCs are found by electron diff raction to self-assemble into close-packed superlattices with long-range translational and orientational ordering, while polycrystalline NCs behave like spheres with no preferential orientation. The face-to-face orientational ordering, which is only observed for single crystalline NCs, supports the relevance of the specific crystallinity-related morphologies of these NCs in their better ability to self-assemble. Exploiting this spontaneous segregation would open up a simple alternative to other demanding routes for controlling crystallinity of nanocrystals and optimizing their properties for potential applications.
Source : Crystallinity Segregation upon Selective Self- Assembling of Gold Colloidal Single Nanocrystals. H.Portales, N. Goubet, S.Sirotki, E. Duval, A. Mermet, P. Albouy, and M.P. Pileni. Nano Lett., 2012, 12, 5292−5298.