A Mismatch Made in Heaven


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A perfect match may be a blessing in love or marriage, but in materials science a slight mismatch may sometimes help achieve the best results. Researchers at the Weizmann Institute have shown that by manipulating the mismatch between the spacing of atoms in two materials they can control the size of microscopic semiconductor crystals.

The finding holds a possible key to the development of tiny semiconductors with new optoelectronic properties for basic research and for tomorrow's optics and electronics industries. This is because the properties of crystals smaller than approximately 10 nanometers, or 10 millionths of a millimeter, can be altered by merely changing their dimensions.

This research was conducted by Dr. Gary Hodes, Prof. Israel Rubinstein and then doctoral student Yuval Golan, all of the Materials and Interfaces Department, and was described in a series of papers, the most recent two published in Advanced Materials .

Using a technique called electrodeposition, the scientists laid down crystals of the semiconductor cadmium selenide, each measuring four to five nanometers, onto a gold substrate. The crystals were found to be oriented in a uniform manner. This configuration is highly beneficial for controlling the semiconductor properties and it occurs because the atoms of the crystals tend to align themselves with the atoms in the surface layer of the substrate. Such alignment is attributed to the good match between the interatomic spacings of cadmium selenide crystals and of gold.

"We now run into what might at first glance seem to be a paradox," says Hodes. "On the one hand, attaining a close match is the important first step leading to crystal alignment. On the other hand, the match is not perfect - but this imperfection can also be made to work to our advantage. In fact," he says, "it is the small remaining mismatch that allows for precise control over crystal size."

The substrate "stretches" the growing crystal in an attempt to minimize the mismatch, creating a strain within the crystal that eventually causes it to stop growing altogether. Therefore, by fine-tuning the mismatch the scientists were able to determine when the crystal will stop growing, in other words, its size. They showed that the addition of small amounts of a material called tellurium to the semiconductor reduced the mismatch to a predictable degree.

This allowed them to produce uniformly oriented crystals of varying sizes. "Compared with alternative approaches, our method is much simpler and cheaper and makes it possible to control the size of smaller crystals," says Rubinstein.

The study by Prof. Rubinstein and Dr. Hodes was supported by the U.S. Office of Naval Research.

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.