Ordered Self-Assembling of Tetrahedral Oxide Nanocrystals

Self-assembling of size, shape, and phase controlled nanocrystals into superlattices with translational and even orientational ordering is a new approach for engineering nanocrystal materials and devices. High purity tetrahedral nanocrystals of CoO, with edge lengths of 4.4 6 0.2 nm, were synthesized and separated from Co nanocrystals, using a novel magnetic field phase-selection technique. Selfassembling of the faceted CoO nanocrystals forms ordered superlattices, the structures of which are determined. [S0031-9007(97)04151-3] PACS numbers: 81.05.Ys, 61.46. + w Nanocrystal engineered materials are a new generation of materials that have attracted considerable interest in many fields [1 ‐ 10]. Recent research has successfully fabricated self-assembly passivated nanocrystal superlattices (NCS’s) or nanocrystals arrays (NCA’s) of metal [11 ‐ 16], semiconductor [17‐ 19], oxide [20], and sulfite [21] clusters, which are a new form of materials with fundamental interests and potential technological applications. Naturally, the size and shape selected nanocrystals behave like ideal building blocks for two- and threedimensional cluster self-assembled superlattice structures, in which the particles behave as well-defined “molecular matter” and they are arranged with long-range translation and even orientation order [22]. Well-defined ordered solids prepared from tailored nanocrystalline building blocks provide new opportunities for optimizing and enhancing the properties and performance of the materials. This is a new initiative of research on cluster engineered materials. Self-assembled arrays involve self-organization into monolayers, thin films, and/or superlattices of sizeselected nanoclusters encapsulated in protective compact organic coating. A key step in this process is the fabrication of size and shape controlled nanocrystals. Most of the current studies have been carried out for nanocrystals whose shape can be approximated as spherical; thus, they can be considered as large “atoms,” forming superlattices with body-centered cubic, face-centered cubic, or hexagonal close-packed structure. The study of Ag NCS by Harfenist et al. [15] has, however, clearly shown the vital role played by particle shape on the packing crystallography of the superlattice. In this Letter, we report, for the first time, the success of preparing self-assembled NCS of CoO nanoclusters with a tetrahedral shape and monosize distribution. A particle selection technique using magnetic field is introduced to collect size, shape, and phase controlled nanocrystals. Cobalt and cobalt oxide nanocrystals were processed by chemical decomposition of Co2sCOd8 in toluene [23 ‐25]. To disperse the freshly processed nanocrystals, sodium bis(2-ethylhexyl) sulfosuccinate [C20H37O2SNa, in short, Na(AOT)] was added as a surface active agent, forming an ordered monolayer passivation (called the thiolate) over the nanocrystal surface. The thiolate serves not only as the protection layer for the particles to avoid direct contact between the particles with a consequence of collapsing but the interparticle bonding. 100 mg of Co2sCOd8 and 50 mg of Na(AOT) were dispersed into 25 ml of toluene at room temperature. The mixed solution was then ultrasonically dispersed and heated at 130 ‐ C for 4 h in air. This temperature was chosen to enhance the crystallinity of the nanocrystals. By adjusting the wt. % ratio between the precursor and Na(AOT) from 1:1 to 5:1, the particle size was controlled to be smaller than 5 nm. Then the mixed solution was diluted to 1:4 by adding toluene. The as-prepared solution contained Co and CoO particles. Since Co particles are superparamagnetic when their sizes are smaller than 10 nm, while CoO is antiferromagnetic, a small magnetic field, generated by a horseshoe permanent magnet, was applied in the vertical direction (with the N pole at the top) so that the Co nanoparticles floated to the top surface of the liquid under the driving force of the magnetic field, forming aggregates, while the CoO particles were left in the solution. Relying on gravity force, the smaller size particles were suspended in the liquid, while the larger ones sank to the bottom. By picking the particles suspended at different depths of the solution after 24 h in the magnetic field, the size and phase selected nanoparticles were obtained. This magnetic field separation technique was very useful in this type of experiment, and the phase selected particles were highly monodispersive and pure. The structure of the nanocrystals was determined using transmission electron microscopy (TEM). For TEM observations, a drop of solution was deposited on an ultrathin carbon film supported by a copper grid. The TEM experiments were performed at 200 kV using a Hitachi HF-2000 TEM equipped with a field emission source, and at 400 kV using a JEOL 4000EX highresolution TEM. The particles reported here are in the size range of 4‐5 nm; thus, the particle contrast is rather weak in conventional bright-field (BF) TEM images due to