Supercrystals formed by monodisperse nanocrystals investigated by X-ray diffraction

Full text: Colloidal nanocrystals are subject of intense research because of their high potential for applications in electronic, magnetic and optical devices. Furthermore they show a high tendency to form ordered two and three-dimensional superstructures ('supercrystals'). The precise ordering in those supercrystals affects the electronic coupling between the individual nanocrystals and it was shown to lead to new collective properties. These properties are related to, and hence strongly affected by the degree of ordering obtained in the supercrystals, which in turn depends on nanocrystal size, the capping ligands and chemical species of chosen nanocrystals, as well as the monodispersity of the used nanocrystal batch. Thus, quantifying the degree of ordering and perfectness of supercrystals is of major interest, in order to enable a quantitative correlation between ordering and the ordering-dependent physical properties. To judge the quality of self-assembly obtained in 3D nanocrystal superlattices, we use X-ray diffraction, applied to individual supercrystallites. The supercrystals have typical sizes of 10μm, whereas the nanocrystals are below 20nm in diameter. While for two-dimensional arrays of self-assembled nanocrystals, ordering can be quantified by inspecting electron microscope images, for faceted 3D nanocrystal supercrystals, much more efforts have to be undertaken to obtain characteristic numbers, which allow to assess the degree of ordering obtained by the self assembly process. 3D reciprocal space maps are recorded at a synchrotron source using a micrometer sized X-ray beam, from which we nd that the quality of ordering is strongly anisotropic. (author)