A hot endeavor in basic research is the creation of so-called metamaterials, which are generally thought of as engineered constructs with well-defined properties that are not readily found in nature and that are different from those of the materials they are made from. Researchers from IBM, Columbia University, and the University of New Orleans have caused nanometer-sized magnetic and semiconducting particles to automatically self-assemble into a novel metamaterial. And what is more, the new substance is one of the first of a new class whose properties should be tunable to whatever is needed for an application, simply by adjusting the size and type of nanoparticles used as building blocks.
The researchers combined two types of nanoscale particles, each particle containing thousands of atoms and having tunable properties that are different from those of both individual molecules and bulk materials of the same substance. In essence, the technique gives scientists a whole new periodic table to work from. ”We can choose a property that we are interested in and work backwards,” says Chris Murray, the manager of nanoscale materials and devices at IBM’s Thomas J. Watson Research Center (Yorktown Heights, N.Y.).
In the research announced in the 26 June issue of Nature , the scientists simmered 11-nm-wide magnetic iron oxide particles and 6-nm-wide spheres of semiconducting lead selenide particles in a soup of solvent. As the solvent evaporated, the particles assembled themselves into crystals with a cubic structure [see the electron microscopy image, upper left]. In the images, colorization highlights the positions of the magnetic iron oxide particles in blue and those of the semiconducting lead selenide quantum dots—nanometer-scale crystals of semiconductor—in red.
The researchers are now trying to determine if the new crystal will retain certain properties of both the iron oxide and the lead selenide, combining the best of both, or even manifest unique magneto-optical properties.
”The most crucial step,” Murray told IEEE Spectrum, ”is tuning the size of the blocks.” The properties of nanoparticles such as semiconducting lead selenide quantum dots are highly dependent on their size. Murray and his colleagues chose lead selenide quantum dots of a size sensitive to wavelengths of light used in optical telecommunications.
In addition, each type of particle has to be nearly identical in size, both to define the metamaterial’s properties and to control the crystal structure. A structure will only self-assemble if the ratio of the particle sizes is exactly right.
To make the metamaterial, the team coated the particles with a lubricant called oleic acid and immersed them in an organic solvent, dibutyl ether. Once the particles stabilized themselves in the liquid, the team slowly evaporated the ether so that the particles assembled themselves into the crystal shape. This kind of self-assembly process has been used before to create crystals, but this is the first time researchers have combined two components in three dimensions to make relatively large crystals.
The self-assembly process should extend to even more complex metamaterials. Vincent H. Crespi, a professor of physics at Pennsylvania State University (University Park), has worked out theoretical metamaterial structures having three types of spherical nanoparticles. He says further work will include discovering self-assembling structures for nanoparticles that are not spherical—those shaped like a child’s jacks or like diamonds, for instance.