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Posted: Sep 06, 2007
Playing Rubik's cube with nanoparticles
(Nanowerk Spotlight) In order to exploit the unique properties of nanoscale materials for advanced applications it is often necessary to assemble nanoparticles into arrays with specific architectures. The interaction among the nanoparticles, or effects arising from their assembled larger structure, could result in interesting optical, magnetic or catalytic properties that researchers and engineers then could exploit for new materials and applications.
In recent years, there has been much interest in colloidal crystals - which are examples of periodic nanoparticle arrays - as photonic crystals, templates for photonic crystals, sensors, optical and electrooptical devices, and as model systems to study crystallization processes. The success of many of these potential applications is currently limited by scientists' ability to control the structure of colloidal crystals.
Normally, crystallization of uniform colloids produces face-centered cubic or hexagonal close-packing. A few other colloidal crystal structures have recently been reported, but they either require careful balance of electrostatic interactions between colloidal particles, or they rely on directing nanoparticles on a lithographic pattern that then dictates the geometry of a few layers in a thin film.
New research now has resulted in a completely different and novel approach of colloidal crystallization that results in simple cubic colloidal crystals extending over many unit cells in three dimensions. Simple cubic packing is quite rare, even in atomic structures. Here, it results from combined disassembly and self-reassembly of a template- directed structure in a single reaction step.
The shape of a cube is the most common and simplest shape found in crystals. Scientists differentiate between three Bravais lattice shapes which form the cubic system – simple, body-centered cubic (bcc) and face-centered cubic (fcc).
The simple cubic system consists of one lattice point on each corner of the cube. Each atom at the lattice points is then shared equally between eight adjacent cubes, and the unit cell therefore contains in total one atom (1/8 * 8). The body centered cubic system has one atom in the center of the unit cell in addition to the eight corner points. It has a contribution from 2 atoms per cell ((1/8)*8 + 1). Finally, the face centered cubic has atoms on the faces of the cube of which each unit cube gets exactly one half contribution, giving a total of 4 atoms per unit cell ((1/8 for each corner) * 8 corners + (1/2 for each face) * 6 faces).
Colloidal crystals prepared by natural assembly of uniform spherical particles form close-packed arrays, typically with fcc packing. Since many applications involving colloidal crystals (photonics, optoelectronics, combinatorial screening, etc.) depend on the specific geometry of the colloidal crystal and/or on interactions between colloidal particles, a simple and low-cost alternative to current fabrication methods for nanoparticle arrays would be good news for many researchers.
"Our approach of preparing periodic arrays of uniform nanoparticles is based on a new concept of combining disassembly (top down) and self-reassembly (bottom up)
syntheses" Dr. Andreas Stein explains to Nanowerk. "In the reassembled colloidal crystals, simple-cubic packing, which is relatively rare even among atomic crystals, extends in three-dimensions over a large number of unit cells."
Stein, a Professor in the Department of Chemistry at the University of Minnesota, together with Fan Li, a graduate student in Stein's group who first-authored the paper, has recently reported a novel disassembly synthesis of nanocubes and nanospheres composed of silica ("Shaping Mesoporous Silica Nanoparticles by Disassembly of Hierarchically Porous Structures"). These were formed by replication of void spaces in a colloidal crystal template, i.e. a mold composed of close-packed, uniform spheres.
"As we explored syntheses of shaped nanoparticles with other compositions, we discovered a novel self-assembly mechanism" says Stein. "When the disassembly synthesis was applied to titanium oxide-phosphorus oxide compositions, nanocubes could be formed and, surprisingly, these self-reassembled into a ordered arrays with simple cubic symmetry. For comparison, if we separated the cubes after their synthesis and tried to reassemble them, they did not form ordered arrays. We believe that self-reassembly occurred because the cubes were placed in the correct positions to reassemble by the original template and because capillary forces from the melting phosphorus oxide component drew the cubes together."
Schematic of the proposed disassembly and self-reassembly mechanism in situ. A porous skeleton is templated by a face-centered cubic colloidal crystal and disassembles into its building blocks during calcination. Smaller particles are assimilated by the larger cubes. Alternating layers of cubes merge, driven by capillary forces from a melted phase, producing the observed simple-cubic-packed arrays of nanocubes. For clarity, the interparticle space is exaggerated and any irregular aggregates are ignored. (Reprinted with permission from Wiley)
Stein cautions that the order of the simple cubic arrays that they obtained so far is not yet sufficient for actual photonic crystals "but we hope that we can optimize this order in the future."
"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" he says.
"We had originally been working on monolithic porous silica structures with hierarchical porosity" says Stein. "This is a mouthful, but it means that we devised methods to create silica with uniform larger pores and uniform smaller pores by combining two types of template ("Controlling the Shape and Alignment of Mesopores by Confinement in Colloidal Crystals: Designer Pathways to Silica Monoliths with Hierarchical Porosity"). The large pores permit easy transport of guest molecules through the structure and the small pores provide the material with selectivity (like a sieve) and an extremely high surface area for chemical interactions. We discovered the nanocubes in a sample where we had changed one of the components in the synthesis mixture (an acid used to solidify the precursors). The self-reassembly observed for titanium oxide-phosphorus oxide (and now also for two other compositions) was also not expected."
It is still difficult to assemble nanoparticles into three-dimensional arrays with specific symmetries other than close-packed structures. Stein's current research provides a new approach to achieve simple cubic packing, adding one more symmetry to a very small list of choices. As this list becomes larger in the future, it will be easier to manufacture functional devices from nanoparticle arrays. Stein's unique approach to simple cubic arrays of nanoparticles adds to the choices of available geometries, coordination numbers and packing densities in nanoparticle arrays.
"We plan to confirm the proposed formation mechanism, so that we can apply our method to other compositions and optimize the order of the self-reassembled arrays" Stein explains his group's next steps. "Another challenge for the future will be to obtain other nanoparticle shapes and arrange these into 1-, 2- or 3-dimensional arrays. Hopefully, the particles can self-assemble, but it we may have to lend them a hand using various external forces."