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June 2, 2003
Volume 81, Number 22
CENEAR 81 22 p. 9
ISSN 0009-2347


Four-armed nanocrystals could find use in solar cells or as polymer additives


A few years ago, chemistry professor A. Paul Alivisatos and his coworkers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory noticed that some nanocrystals would unexpectedly form branched structures. Now, the researchers can make those branched structures controllably and reproducibly. Using the material CdTe, they can make four-armed structures known as tetrapods, which they say could be used in solar cells or as additives in polymers [Nat. Mat., published online May 25,].

UNIFORM Tetrapods can be synthesized to be the same size to within a few percent, as shown in this transmission electron micrograph. The dark dot in the middle of each tetrapod is the fourth arm coming out of the plane of the page.
"We've learned slowly how to make those branched structures occur all the time," Alivisatos says. "We've started to try to figure out the mechanisms by which they form."

The branching happens because CdTe has two different stable crystal structures that are close--but not too close--in energy and that have atomically identical facets. Branches form by switching between conditions that favor one structure over the other. Alivisatos and coworkers manipulate the conditions so that the cubic structure is preferred during the nucleation phase and the hexagonal structure is preferred during the growth phase.

The crystal growth occurs in a mixture containing alkyl phosphonic acids. Alivisatos hypothesizes that the phosphonic acids stabilize the hexagonal structure. "The surfactant binds to the surface of the hexagonal structure and lowers its energy," he says. "We think that the cubic structure forms first. When the crystal is tiny, the surfactant can't bind there. When the crystal gets bigger, it switches structure. It's big enough that the organics can pack on there."

The length and width of the tetrapod arms are controlled by changing the ratios of cadmium to tellurium and to the surfactant. As the Cd/Te ratio increases, the arms get longer. As the Cd/surfactant ratio increases, the arms get fatter. Additionally, within a few percent, the four arms of the tetrapods are of equal length.

BRANCHED The cubic and hexagonal structures of the CdTe tetrapod share a facet. The exploded view of the arm shows the cubic (111) facet on the crystal core and the hexagonal (000) facet at the end of the arm. Changing the temperature switches which facet is preferred. Cadmium = yellow; tellurium = blue.
"Together, these results offer a new level of control over the synthesis of nanocrystal building blocks and provide new structural diversity for possible applications," Deli Wang and Charles M. Lieber of the department of chemistry and chemical biology at Harvard University write in an accompanying commentary.

"The work published by Alivisatos' group in Nature Materials represents one of the best controlled nanocrystal systems with complex shapes," says University of Arkansas associate chemistry professor Xiaogang Peng, who also studies the formation of branched nanocrystals. "The nearly monodisperse tetrapods shown in the paper are not only visually beautiful, but are also likely superior materials for certain applications, such as nanocrystal-polymer solar cells."

According to Alivisatos, such controllable syntheses could be especially useful for solar cells, where the diameter of the arms dictates the wavelength and the length determines the amount of energy absorbed. In contrast to nanorods, the tetrapods spontaneously orient themselves correctly.

Branched nanocrystals should be possible with other materials as well. "In principle, [branching] should be able to happen with any material that has two crystal structures, one cubic and one hexagonal, that are close in energy," Alivisatos says. However, finding the proper conditions for other materials might not be a simple matter. "It would take a lot of effort to switch from one material to the next," he says.


Chemical & Engineering News
Copyright © 2003 American Chemical Society

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Related Person
A. Paul Alivisatos

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