Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2003, 42 (04), 413 - 417
Mini-crystals as building blocks:
highly structured materials through self-organized crystal growth
Materials scientists are interested in order. Not in their labs or on their desks; no, it is the microscopic world that gets their attention. Tiny, highly ordered structures are of importance in a variety of industrial applications, such as opto-electronic components, nanotechnology, or as catalysts. At Sandia National Laboratories in New Mexico, Jun Liu and his group have now developed a new class of hierarchically constructed materials. These consist of individual, microscopic crystals that spontaneously group together to form larger crystals and other ordered units. What is special in this case is that the form and orientation of the crystalline building blocks is retained.
The starting material is an aqueous solution of an organic silicon compound, which also contains a surfactant. The scientists dip a slide, which acts as the support for crystal growth, vertically into the solution. Initially, individual, equally sized, little crystals, octahedral in shape, form on the glass surface. If the slide was first coated with fine lines of plastic, the mini-octahedra align themselves along these lines, forming an extended pattern on the support. But the mini-octahedra can do much more; several octahedra can grow together to form larger objects, which then combine with each other -- and so on. This results in larger structures of "higher order", with defined symmetrical shapes. Because the mini-octahedra can share edges as well as entire faces, the structures are not tightly packed; they contain large gaps. Such structures are very unusual in crystals.
Different stacking or layering of the crystals on different surfaces results in a variety of geometries. Liu and his co-workers thus obtained larger octahedra, rosettes, star- and flower-like structures, rhombic shapes, and cubic cages, among others. This variety of shapes can be influenced by the reaction conditions, such as temperature and surfacant concentration.
Liu hopes that his new class of crystals will not only lead to the development of new types of materials, but will also add to our understanding of how such complex self-organizational processes, which also play an important role in biomineralization, occur.