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Playing Rubik's Cube with Nanoparticles

Recent Research from the group of Professor Andreas Stein.

Atomic and ionic crystals pack atoms in a wide range of periodic motifs. When the building blocks are larger than atoms – uniform, spherical nanoparticles or colloids with sizes between around 10 and 100 nanometers – the resulting colloidal crystals typically assume a dense face-centered-cubic (fcc) order. Such colloidal crystals are of great interest in the growing field of photonics, where they may be used to manipulate the propagation or emission of light. However, if the choice of a photonic crystal structure is limited to fcc, optical properties are also restricted.

Graduate student Fan Li and senior Sarah Delo in Professor Stein's group recently discovered an unusual approach to assemble nearly cubic nanoparticles into ordered arrays with simple cubic rather than fcc symmetry. Surprisingly, they achieved this with a template or mold consisting of fcc-packed polymer spheres. After these were infiltrated with a surfactant and with precursors for titania-phosphorus oxide composites and subsequently heated to 400 °C, the majority of the product showed periodic arrays of tiny cubes as depicted in the scanning electron micrograph in Figure 1. Multiple layers are clearly visible, in which square arrangements of particles sit directly on similarly arranged cubes in adjacent layers, forming a simple cubic structure.

Figure 1. Titania-phosphorus oxide cubes in simple cubic pattern.

On the basis of structural and compositional analyses, Stein and coworkers proposed the mechanism outlined in Figure 2 for the product formation. When the precursor hardens within the fcc colloidal crystal template it initially forms a three-dimensionally ordered macroporous (3DOM) skeleton. During sample heating the skeletal walls shrink and eventually break down into small, nearly identical building blocks, still in an fcc arrangement. However, at the processing temperature, the phosphorus oxide component forms a liquid phase. Resulting capillary forces move the particles closer together, so that adjacent layers merge, forming the simple cubic array.

Figure 2. Proposed mechanism for the structural transformation from face-centered cubic to simple cubic.

In their publication (Angew. Chem. Int. Ed. 2007, 46, 6666,, Li et al. conclude that "Even though the structures created in the present work are far away from the perfection necessary for photonic crystals, an adaptation of the self-reassembly method may provide a faster, low cost alternative to produce simple cubic photonic crystals that are normally prepared by elaborate micromachining, layer-by-layer lithography, holography and macroporous silicon etching, all expensive and time-consuming methods." This work was also highlighted in a Nature New & Views article (Nature 2007, 449, 550.

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