Building Tunable Matter

Scientists have a new periodic table to work with--and it's made up of nanoparticles

Particle pointillism: this electron microscopy image of a new metamaterial assembled out of nanoparticles has been colorized to highlight the positions of magnetic iron oxide particles in blue and semiconducting lead selenide quantum dots in red.

2 July 2003—Researchers from IBM, Columbia University, and the University of New Orleans have caused nanometer-sized magnetic and semiconducting particles to assemble themselves into a novel material that may have properties not found in nature. The group announced the research in the 26 June issue of Nature .

The team took advantage of a natural tendency for matter to organize itself when it is in the right physical and chemical conditions. They simmered 11-nm-wide magnetic iron oxide particles and 6-nm-wide spheres of semiconducting lead selenide particles in a soup of solvent until the particles assembled themselves into crystals with a cubic structure [see photo]. The researchers are now trying to determine whether the new crystal—a so-called metamaterial—will retain both the magnetic properties of iron oxide and the tunable optic properties of lead selenide quantum dots or whether it will show unique magnetoptical properties emergent from the assembly, which could have applications in future computing technology.

In the construction of most new materials, scientists are limited to using the atoms on the periodic table. That is, materials are made by combining atoms and molecules. In the case of the new metamaterial, however, researchers combined two types of nanoscale particles, each containing thousands of atoms and having tunable properties that are different from 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 Corp.’s Thomas J. Watson Research Center (Yorktown Heights, N.Y.).

”The most crucial step,” Murray told IEEE Spectrum, ”is tuning the size of the blocks.” The properties of nanoparticles such as semiconducting quantum dots are highly dependent on their size. Given the same starting material, a small nanoparticle would have different properties from a larger one, emitting blue light instead of red, for instance. So scientists designing metamaterials can to some degree select its properties by choosing the constituent particle size. For example, 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 as close to identical in size as possible, both to define the metamaterial’s properties and to control the final crystal structure. A structure will only self-assemble if the ratio of the particle sizes is exactly right. ”If there is even a modest distribution in size or shape or makeup, [the material] can’t assemble,” Murray says.

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 at just the right temperature and at just the right rate so that the particles assembled themselves into the crystal shape. As Murray put it, ”Once you set up conditions appropriately, you can take your hands off the wheel.”

The self-assembly process has been used before toa create new materials, but this is the first time researchers have combined two components in three dimensions to make relatively large crystals. The research proves that self-assembly can be used to build designer materials with desired properties for specific applications. Murray and his colleagues made metamaterials that conformed to three crystal configurations extrapolated from studying colloids of microspheres. But there should be many other configurations to try.

Vincent H. Crespi, a professor of physics at Pennsylvania State University (University Park, Pa.), has worked out theoretical crystal structures for metamaterials having three types of spherical nanoparticles. He says further work will include discovering self-assembled structures for nanoparticles that are not spherical, like those Murray’s group used. A team at the University of California, Berkeley, led by quantum dot pioneer Paul Alivisatos, recently reported making semiconductor nanocrystals shaped like jacks instead of spheres. ”What kinds of crystals these will self-assemble into is a great unknown,” says Crespi. ”It’s essentially redoing the periodic table.”