Nanoparticles Form Diamond-like Crystals

Combining gold and silver may not seem like an obvious way to make diamonds. But when conditions are just right, the method works spontaneously.

Researchers have demonstrated that macroscopic crystals with diamond-like shapes and crystal lattice structures can be grown by means of a new procedure that brings together oppositely charged building blocks through electrostatic attractions. The method has been used to build three-dimensional structures from millions of gold and silver nanoparticles that were modified with alkanethiol monolayers to make them negatively and positively charged, respectively (Science, published online Feb. 23, dx.doi.org/10.1126/science/1125124).

When oppositely charged and roughly equal-sized atomic ions or micrometer-sized particles are used as building blocks in crystals, they tend to assemble in close-packed structures characteristic of NaCl and CsCl. So that's what Bartosz A. Grzybowski expected would happen when he formed crystals from the gold and silver nanoparticles.

But it didn't turn out that way. "We were quite astonished to see that they crystallize into a diamond lattice, not a close-packed structure," the Northwestern University chemical engineering professor says. So Grzybowski and other members of his research group, including Alexander M. Kalsin, Marcin Fialkowski, and Maciej Paszewski, carried out experiments to probe the relationships between particle size, electrostatic forces, and crystallization.

One of the team's key findings is that the observed structures are a direct consequence of electrostatic effects unique to the nanometer scale. Grzybowski explains that in solutions of particles measuring just a few nanometers, the size of the particles is roughly equivalent to the thickness of the so-called Debye layer, a layer of counterions that surrounds each charged particle and shields it from the electrostatic forces of neighboring charged particles. The group's calculations show that the closeness in size between the screening layer and particle diameters results in energetically favorable conditions for forming crystals that would not form under other conditions.

"The effects of electrostatic forces in the nano regime are totally different than in other size regimes," Grzybowski asserts. He intends to exploit the uniqueness "to tweak the nanoparticles" and design materials with tailored properties.

Another outcome of the study is that solutions containing a range of particle sizes (polydisperse solutions) lead to more stable and higher quality crystals than narrow-size-range (monodisperse) solutions. The enhancement results from the presence of smaller particles that serve to screen the electrostatic forces acting between larger particles, the researchers explain. That result comes at a time when some research groups are developing methods for narrowing nanoparticle size ranges for improved performance in some applications such as optoelectronics and catalysis.

It's quite interesting that the polydisperse nature of the nanoparticles plays an important role in the crystallization process, says Jennifer A. Lewis, a professor of materials science at the University of Illinois, Urbana-Champaign. Lewis adds that "the work is truly elegant and may have broad-reaching impact for assembling nanostructured materials and composite architectures."