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SELF-MADE:Nanoparticles of gold and iron oxide

self-assemble into exotic quasicrystalline structures under

certain conditions.

Image: D. Talapin et al, Nature 2009

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Quasicrystals are a relative newcomer to the field of materials science, having been

discovered just 25 years ago. Nudging their way between amorphous solids such as

glass and crystals such as quartz, the structures exhibit an ordered structure like

crystals do, but their uniqueness lies in the fact that the ordered arrangement does

not repeatthat is to say, it is not periodic.

Quasicrystals also lack translational symmetry. To give a two-dimensionalanalogue, the pattern cannot be shifted laterally without changing its appearance

(unlike, say, an infinitely repeating checkerboard pattern, which can be shifted two

squares in any direction and retain its original format).

Since their discovery in 1984 quasicrystals have been found in many highly

synthesized materials and in one mineral of apparently natural originan alloy of

aluminum, copper and iron found in Russia. But a study in this week'sNature

shows that quasicrystals may be a fairly natural way for objects to pack themselves

together and may not require much manipulation to take shape. (Scientific

Americanis part of the Nature Publishing Group.) A team led by University of

Chicago physicist Dmitri Talapin reports that various pairings of nanoparticles,

when mixed in solution and left to evaporate, coalesce into quasicrystal structures.

That a variety of particle pairstwo different iron oxides mixed with gold as well as

lead sulfide paired with palladiumcan successfully self-assemble into

quasicrystals suggests that it may be a more common arrangement than had been

thought, the study's authors wrote. The only real common ground in the pairings

seemed to be a relatively consistent ratios among the sizes of the different

nanoparticles. "These structures can self-assemble without much human

intervention," Talapin says. "All [researchers] need is to properly design the

building blocks."

In 2006 Talapin and a group of researchers, some of whom were also involved with the new study, showed how nanoparticles can

self-assemble into more than a dozen different periodic lattice structures. The next step, he says, was to move beyond the periodic "to

try making some really weird creaturesand quasicrystals are a quite natural candidate along those lines."

A solution of spherical nanoparticles of iron oxide and gold did the trick, but Talapin and his colleagues wanted to know if

quasicrystalline structures are a general packing arrangement or if the team had simply lucked out in their choice of ingredients. So they

tried other combinations of particles and got the same result. "We found that it's really a law of nature rather than a unique combination

of many parameters that just line up in a lucky way," Talapin says.

Alfons van Blaaderen, a physicist at Utrecht University in the Netherlands who wrote a commentary inNatureon the new research, says

that quasicrystals indeed appear to be more generic than had been thought. But he notes that in the absence of simulations or theory to

show how the structures are forming, it is impossible to completely rule out a wild stroke of luck in the Talapin group's recipes. "It could

just be a weird coincidence" that the various mixtures of particles self-assembled into quasicrystals, van Blaaderen says, adding that

such a coincidental result is highly unlikely.

Van Blaaderen's group is working on replicating the findings with larger particles, which would allow researchers to observe the

Permanent Address: http://www.scientificamerican.com/article.cfm?id=quasicrystals-self-assemble

Exotic Quasicrystal Structures May Be More Normal ThanAssumed

Different nanoparticles can self-assemble into complex quasicrystal arrangements

By John Matson | Wednesday, October 14, 2009 | 3 comments

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structures taking shape in real-time. Such a window into the quasicrystal formation process could help materials scientists understand

what is necessary to build structures incorporating characteristics of both amorphous solids and crystals.

Whatever the case, Talapin and his colleagues have added to a growing number of regimes in which quasicrystals, a total unknown just a

few decades ago, can form. "Now we see these materials are much less weird than we used to believe," he says.

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