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Making the most of metal oxides on the nanoscale



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On first blush, two scientists striving to exploit unique electronic and magnetic properties of metal oxides that occur in the nanoscale may seem to be working toward the same end. They're using the same equipment and some of the same methods, but their research at Pacific Northwest National Laboratory and its potential applications are quite different.

At the heart of both project is molecular beam epitaxy equipment—an instrument that researchers use to generate beams of atoms in a highly controlled vacuum environment. The instrument directs these beams onto a surface where they condense and form crystalline materials with dimensions on the nanoscale.

Here is where the similarity between Scott Chambers and Yong Liang's research ends. Chambers and his colleagues are focusing on materials with only one dimension on the nanoscale, while Liang and his team are developing three-dimensional nanostructures called nanodots that are so small that about 100,000 of them would fit on the head of a pin.

These nanodots or quantum dots are metal oxide crystals that are like artificial atoms with unique electronic properties. "Unlike normal atoms, however, the properties of the nano-dots can be changed by changing the material size, material composition and how they interact with the substrate," said Liang, a senior research scientist who works in the Environmental Molecular Sciences Laboratory, a Department of Energy user facility on Pacific Northwest's campus.

Other research institutes are working with nanodots to build more efficient lasers or memory devices in electronics, but the research at Pacific Northwest is focused on applications more directly tied to DOE's energy missions.

Liang and his team are investi-gating the potential use of nanodots for generating hydrogen on demand, which would be useful in future fuel cell applications. He hopes to demonstrate the proof of this concept within the next year.

While Liang's research may someday be used to generate energy, the extremely thin sheets of metal oxides that Chambers and his fellow researchers are developing could potentially be used in a new kind of computing system known as a quantum computer.

"Conventional computing techniques are very rapidly reaching their limits. Probably by the end of the decade we'll reach a brick wall in terms of size, speed and power dissipation," said Chambers, a chief senior scientist who was the original developer of the molecular beam epitaxy equipment in EMSL.

Unlike electronic devices that use the charge of electrons to carry signals, quantum computing would utilize the spin of electrons, or the polarization of light. This approach potentially could greatly increase speed and lower the power dissipation in computing systems.

Chambers' team is sandwiching a sheet of crystalline zinc oxide only 10 atomic layers thick between two sheets of mixed zinc and manganese oxide, each about 500 atomic layers thick. When the two outermost sheets are attached to a battery to form a circuit, spin-polarized charges are transported into the ultrathin sheet of zinc oxide.

"The recombination of these electrons and holes—or missing electrons—occurs in the middle layer," Chambers said. "The holes, which have just one spin orientation will recombine only with electrons that have the same spin orientation. As a result, polarized ultraviolet light is emitted, which may be useful in optical quantum computing."

Chambers' research could lead to advances in medical diagnosis and treatment. His ultraviolet light emitting devices could be attached to the end of a fiber-optic probe and travel through blood vessels for direct ultraviolet irradiation to treat internal organs. Similar devices could be useful in photocatalytic reactors requiring ultraviolet sources, such as those designed to destroy toxic organic waste.•

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