Tiny Crystrals in Large Quantities

Uniform-sized nanocrystals can be prepared in large batches through a new preparation method developed by researchers in South Korea. The technique may hasten development of future nanotechnology applications by providing a low-cost route to commercial quantities of uniform nanocrystals.

Researchers working in nanometer-scale science have demonstrated a variety of devices that exploit unique optical, electronic, and other size-dependent properties of nanocrystals. The key to capitalizing on those properties--for example, the unique color associated with semiconductor nanocrystals of a specific size that are suspended in a liquid--is limiting variation in particle size. Several methods for preparing monodisperse nanocrystals have already been reported, but typically, those methods yield much less than 1 g of product.

Now, scientists in South Korea have shown that 40-g batches of uniform-sized magnetite (Fe₃O₄) nanocrystals and other materials can be prepared in a single reaction without a size-sorting step [Nat. Mater., 3, 891 (2004)]. Scaling up the process to produce multikilogram quantities is expected to be straightforward, the scientists say.

The research team includes Taeghwan Hyeon, a professor of chemical engineering at Seoul National University, graduate student Jongnam Park, and their coworkers at Sungkyunkwan University and Pohang University of Science & Technology.

To prepare the nanocrystals, the researchers react metal chlorides, such as FeCl₃, with sodium oleate, which produces a metal-oleate complex. Then when the material is heated slowly in a high-boiling-point solvent such as 1-octadecene, the complexes decompose and form nanocrystals. The method was used to prepare nanocrystals of MnO, CoO, -Fe₂O₃, MnFe₂O₄, and CoFe₂O₄.

On the basis of investigations conducted with transmission electron microscopy, infrared spectroscopy, and other analytical methods, Hyeon and coworkers determined that particle nucleation tends to occur at lower temperatures (about 200–240 °C) than crystal growth (about 300 °C). That information enabled the team to separate the crystal nucleation and growth processes and thereby control the monodispersity of the products. For example, by carrying out a series of reactions using five solvents with a range of boiling points, the group produced iron oxide nanocrystals in five different sizes: 5, 9, 12, 16, and 22 nm. In each batch, particle size variation was less than 4%, the team notes. In addition, the group demonstrated that particle size can be controlled at an even finer scale by varying reagent concentrations.

The synthesis procedure is quite general, Hyeon remarks, and can be used to prepare nanocrystals of a variety of transition-metal oxides. Furthermore, the procedure relies on nontoxic, inexpensive, and environmentally friendly reagents, he adds.