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ZnO nanowires to grow out of the
circular copper substrate in all directions, as seen in this
scanning electron micrograph. Detail image shows copper droplets
at the tip of some nanowires. |
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Images � by NIST |
Chemists at the National Institute of
Standards and Technology (NIST) have been the first to measure
significant amounts of copper incorporated into zinc oxide (ZnO)
nanowires during fabrication. The issue is important because copper
plays a significant - but not well-understood
- role in important optical and electrical properties of the
nanowires. Previous experiments found only trace amounts of copper.
Although zinc oxide is best known as a strong sunblock, cold remedy,
itch reliever and paint pigment, nanotech engineers like it for its
photoluminescence (the ability to emit light after absorbing
electromagnetic radiation), field emission (the basis for advanced,
high-definition flat-panel displays) and piezoelectric properties (stressing
or changing shape when electricity is applied and producing
electricity when stressed). ZnO nanomaterials may one day be used to
improve solar cells, lasers, sensors, ultraviolet light sources, field
emission sources and piezoelectric devices.
Copper enters the ZnO nanowires during fabrication. The nanowires
- about 50 to 150 nanometers wide and up to 40 micrometers long
- are grown on a copper substrate using a chemical vapor
deposition process. The copper substrate forms droplets that absorb
the zinc and oxygen vapors and deposits the ZnO on the substrate. As
the nanowire grows, the zinc pushes the droplets up from the surface,
but some copper remains inside the nanowire�s crystal lattice.
In a new paper, NIST chemists report using a variety of measurement
techniques to learn that the ZnO wires contain a surprising amount of
copper - between 5 and 15 percent.
High-resolution imaging studies of ZnO nanowires reveal that the
copper manages to fit into zinc oxide�s regular crystalline structure
without disrupting it. �It is in there somewhere,� explains chemist
Susie Eustis. Because the copper can be easily detected when you know
what to look for, she says, researchers plan to use it to better
understand the crystal structure of ZnO nanowires with an eye toward
manipulating the nanowires to improve performance. �The copper acts
like a smart tag that you put on an animal in the wild to trace where
it travels,� says Eustis.
The role copper plays in ZnO nanowires is ambiguous. Published studies
differ on whether the copper increases or decreases the nanowires�
photoluminescence. Eustis and colleagues found that the copper in the
nanowire increases the output of visible light but at the expense of
ultraviolet emission.
In addition to determining the role copper plays in ZnO nanowires, the
researchers plan to learn how to grow uniform nanowires that may one
day be used in commercial products. This research is part of ongoing
studies to find the best methods to determine the concentration and
distribution of atoms inside nanostructures.
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