Abstract
We report on a nano-beam small angle X-ray scattering study on densely-packed, dried binary films made out of spherical silica particles with radii of 11.2 and 19.3 nm. For these three-dimensional thin films prepared by drop casting, only a finite number of colloidal particles contributes to the scattering signal due to the small beam size of 400 × 400 nm(2). By scanning the samples, the structure and composition of the silica particle films are determined spatially resolved revealing spatial heterogeneities in the films. Three different types of domains were identified: regions containing mainly large particles, regions containing mainly small particles, and regions where both particle species are mixed. Using the new angular X-ray cross-correlations analysis (XCCA) approach, spatial maps of the local type and degree of orientational order within the silica particle films are obtained. Whereas the mixed regions have dominant two-fold order, weaker four-fold and marginal six-fold order, regions made out of large particles are characterized by an overall reduced orientational order. Regions of small particles are highly ordered showing actually crystalline order. Distinct differences in the local particle order are observed by analyzing sections through the intensity and XCCA maps. The different degree of order can be understood by the different particle size polydispersities. Moreover, we show that preferential orientations of the particle domains can be studied by cross-correlation analysis yielding information on particle film formation. We find patches of preferential order with an average size of 8-10 μm. Thus, by this combined X-ray cross-correlation microscopy (XCCM) approach the structure and orientational order of films made out of nanometer sized colloids can be determined. This method will allow to reveal the local structure and order of self-assembled structures with different degree of order in general.
Highlights
Films of a few micrometer thickness containing 60–70 nanoparticle layers were prepared by drying mixed colloidal suspensions of particles with radii of 11.2 and 19.3 nm, respectively
One can imagine to study even the local order within molecular systems and with very small beams performing XCCM in wide angle geometry
Summary
These are known to show a rich phase diagram.[7]. Different types of structures formed by binary colloidal systems have been reported.[8,9,10,11,12,13] Especially, binary systems serve as model systems for studying the dense packing of matter as they can exhibit a larger space filling than monodisperse systems.[12,14,15] a large effort has been made to fabricate and characterize such systems within the last years.[3,5,16,17] A crucial step during structure formation is the drying process, as the resulting crystal structure is largely affected by the conditions under which the drying takes place.[17,18,19,20] In particular, special attention has been paid recently to the local order within colloidal crystals,[21,22,23] since detailed knowledge of such local order is both mandatory for the fabrication of materials with dedicated functional properties and for the understanding of self-assembly processes.[5,24,25,26]. By scanning with a nanofocus X-ray beam of 400 Â 400 nm[2] size in small angle X-ray scattering (SAXS) geometry, we study three-dimensional densely-packed thin colloidal films made out of dried binary mixtures of spherical silica particles prepared by drop-casting. The two-dimensional SAXS patterns were analyzed with the recently developed X-ray cross-correlations analysis (XCCA) technique.[32] This method allows to determine the orientational order present in materials lacking long-range order. Using this X-ray microscopy technique spatially resolved maps of the binary silica particle films are obtained.
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