One challenge in preparing new nanomaterials is fabricating three-dimensional designer materials assembled from two different types of nanoparticles. Professor Stephen O'Brien, in collaboration with Dr. Chris Murray of the IBM T. J. Watson Research Center, MRSEC/IBM bridging postdoctoral scientist Dr. Franz Redl, and co-workers at the University of New Orleans have done precisely that by tailoring the experimental conditions so that nanoparticles with dissimilar, yet complementary properties would assemble themselves into repeating 3-D patterns. One type is composed of lead selenide, a semiconductor that has applications in infrared detectors and thermal imaging and can be tuned to be more sensitive to specific infrared wavelengths. The other material, magnetic iron oxide, is best known for its use in the coatings for certain magnetic recording media. This combination of these nanoparticles may have novel magneto-optical properties as well as properties key to the realization of quantum computing. For example, it might be possible to modulate the material's optical properties by applying an external magnetic field. To produce an ordered structure, such as that shown in the accompanying transmission electron micrograph (TEM), the particles had to be very uniform, all within 5 percent of the target size. The iron oxide particles were 11 nanometers in diameter, and contained approximately 60,000 atoms, and the lead selenide particles 6 nanometers were in diameter, and contained approximately 3,000 atoms. Forming these so-called "crystal structures," as opposed to random mixtures of nanoparticles, is essential for the composite material to exhibit consistent, predictable behavior. Such new materials with otherwise unattainable properties, are sometimes referred to as "metamaterials."