Monday, August 18, 2008

Metals Self-Assemble Into Nanostructures

Cornell researchers have developed a way to self-assemble metals into complex nanostructures. This could lead to far more efficient conductors and breakthroughs in energy technology.

Alexander E. Braun, Senior Editor -- Semiconductor International, 8/18/2008 8:11:00 AM

Ever since mankind first began working metals, the only way to shape them has been the heat-and-beat approach. While this process may have increased in sophistication (nanotech uses e-beams or acids to cut or etch), the basic procedure has remained the same. Now, however, a group of Cornell University (Ithaca, N.Y.) researchers has developed a technique to self-assemble metals into complex nanostructures, which could radically change the traditional millenary process, possibly leading to new types of conductors that can carry more information than any other existing wire.

The research effort is headed by Ulrich Wiesner, professor of materials science and engineering; Francis DiSalvo, J.A. Newman Professor of chemistry and chemical biology; and Sol Gruner, a John L. Wetherill Professor of physics. They have developed a method of coating metal nanoparticles ~2 nm in diameter with a ligand, an organic compound that allows them to be dissolved in a liquid and mixed with a block copolymer composed of two different chemicals whose molecules link together to solidify in a predictable pattern. When the polymer and ligand are removed, the metal particles fuse into a solid structure.

According to Wiesner, this target has been pursued for over two decades. “This is a complex problem, because metals typically have very high surface energies,” he said, adding that nanoscopic metal particles tend to aggregate into clusters. “Once these clusters are formed, they cannot be rearranged. Thus, to get structure control over metals using any sort of a self-assembly process isn’t easy.”

The aggregation issues were overcome by designing particular ligand structures. Ligands are attached to the metal nanoparticles surface, making them soluble in the solvents used in the self-assembly process with the polymers. These also have charges that provide repulsive interactions, so that when they come close, they don’t necessarily click together and form irreversible aggregation states. The repulsive interactions enable them to flow past each other and accommodate structure-formation processes governed by the polymeric species added into the mixture.

By designing the metallic nanoparticles with tailored organic ligands and then working with block copolymers that structure-direct the nanoparticles, it became possible to make, for the first time, nanostructures in what the researchers refer to as the dense nanoparticle regime. The Cornell breakthrough lay in adding a ligand that creates high solubility in an organic solvent, allowing particles to flow even at high densities. The ligand layer surrounding each particle was made relatively thin to ensure that the volume of metal in the final structure would be sufficiently large to maintain its shape after the organic materials were removed.

A solution of ligand-coated platinum nanoparticles was mixed with a block copolymer. The nanoparticles solution combines with only one of the two polymers, and the two polymers assemble into a structure that alternates between small regions of one and the other, producing clusters of metal nanoparticles suspended in one polymer and arranged around the outside of the other polymer’s hexagonal shapes. Depending on the polymers, other patterns can be attained.

The material is then annealed in the absence of air, turning the polymers into a carbon scaffold that supports the shape into which the metal particles have been formed. Finally, the material is heated in air to oxidize the ligands and burn away the carbon. Because metal nanoparticles have a low surface melting point, they sinter into a solid structure.

Chemically self-assembled complex platinum nanostructure with uniform hexagonal ~10 nm pores. (Source: S. Warren and U. Wiesner, Cornell University)
Chemically self-assembled complex platinum nanostructure with uniform hexagonal ~10 nm pores. (Source: S. Warren and U. Wiesner, Cornell University)
The result was a platinum structure with uniform hexagonal pores ~10 nm across. This could be a significant energy technology development, because platinum is considered the best catalyst available for fuel cells and such a porous structure would enable fuel to flow and react over a larger surface.

Wiesner considers results obtained so far as extremely promising, because this is the first time it has been possible to structure metals in bulk ways. “What you can do with one metal, you can do with mixtures of metals,” he said. In principle, it should be possible to use the approaches developed by Cornell in thin films to lay down metallic structures in silicon solely through self-assembly processes.

“This is exciting,” Wiesner said, “but limited to the polymer’s properties. Being organic materials, polymers aren’t very etch-resistant, lack magnetic properties, and usually don’t offer high electronic conductivity.” However, if instead of working with polymers it were possible to use metal block copolymer composites, then all the desired properties — electron conduction, magnetism, etc. — would be available.

According to Wiesner, the next step is to attempt to do the same thing that they did in bulk, which lacks the surface interactions between the material and substrate, in thin films. It should be possible to lay down in a well-defined way metal lines on a substrate using such a simple self-assembly process.

This has enormous potential for Moore’s Law. “But the devil is typically in the details, and although these preliminary results are extremely promising, it yet remains to be proven whether these self-assembly processes can truly fabricate these kinds of structures on thin film and do so using multiple materials,” Wiesner warned. The researchers used platinum, because it is a good catalyst, and a mesoporous material with a high surface area that lends itself to catalysis applications.

However, for semiconductor applications, it may be necessary to use metal alloys, some of which have strong magnetic properties. There is much to be considered, such as whether these alloys would be nanostructured.