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Self-Assembling Nanostructures

Researchers find an easy route to complex nanomaterials.

Researchers at the University of California, Berkeley, have found an easy way to make a complex nanostructure that consists of tiny rods studded with nanocrystals. The new self-assembly synthesis method could lead to intricate nanomaterials for more-efficient solar cells and less expensive devices for directly converting heat into electricity.

No assembly required: Nanorods of cadmium sulfide with silver-sulfide quantum dots (dark spots) form automatically when researchers mix together the right starting chemicals.

In the structures, the quantum dots are all about the same size and are spaced evenly along the rods–a feat that in the past required special conditions such as a vacuum, with researchers carefully controlling the size and spacing of different materials, says Paul Alivisatos, the professor of chemistry and materials science at Berkeley who led the work. In contrast, Alivisatos simply mixes together the appropriate starting materials in a solution; these materials then arrange themselves into the orderly structure.

Such solution-processing techniques can lead to manufacturing methods in which materials, such as those used in solar cells, are printed on continuous sheets, driving down costs compared with other methods. “Anytime you make something in solution, rather than in a vacuum, it becomes a lot easier and cheaper,” says Moungi Bawendi, a chemistry professor at MIT who was not involved in this work.

To make the rods, Alivisatos mixes a combination of methanol and a silver salt into a solution that already contains cadmium-sulfide nanorods. Cadmium ions have a strong affinity for methanol. As a result, when the materials are mixed, the methanol draws cadmium out of the nanorods. Silver ions then fill in the vacant spots left by the cadmium, forming areas of silver sulfide within the rod. At the same time, differences in the crystalline structures of the cadmiun-sulfide rods and the silver-sulfide quantum dots regulate the dots’ size and spacing. This is the first time such differences have been used to control the self-assembly of materials in solution.

The nanocrystal-studded rods could prove useful for solar cells and thermoelectric devices that convert heat directly into electricity. For example, in conventional solar cells, each photon only generates a single electron. But certain kinds of quantum dots convert single photons into multiple electrons, which could more than double the efficiency of solar cells. (See “Silicon and Sun.”) The problem has been capturing those electrons to create an electrical current. Embedding quantum dots inside rods of another material could help with this problem, says Alivisatos. The quantum dots would absorb the light, while the other material would capture the electrons that the dots generate.

A similar configuration is promising for thermoelectrics, devices that directly convert heat into electricity. The alternating crystal structures in the nanorods could block the transfer of heat while allowing electrons to pass–two key features of such devices.

Having demonstrated the new method for making the structures, Alivisatos and his colleagues are beginning to study the potential photoelectric and thermoelectric properties of the materials. They will likely need to turn to different compounds, such as copper sulfide and cadmium sulfide–a combination that has been used for solar cells in the past, Alivisatos says. There’s no guarantee, however, that these materials will form the same orderly structures, or indeed that the structures will perform as the researchers hope they will.

Even if these particular structures do not prove to be the key to low-cost, high-efficiency solar cells, the new self-assembly method for making nanostructures could inspire new materials that are. And Bawendi highlights the need to continue basic research like this to solve today’s energy problems. “We don’t know what the solution is going to be,” he says. But if we create high-quality, carefully described materials as Alivisatos has done, “some of them may be the answer,” Bawendi says.

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