could make better biochips
Technology Research News
Researchers who make materials used in
electronics -- like silicon wafers and thin metal coatings that are etched
to form wires and other components -- are usually looking to keep the
materials' surfaces smooth, because stray bumps, or defects are not conducive
to conducting electrons without leading them astray.
A group of researchers from the University of Maryland, however, is taking
the opposite tack by encouraging buckles, or tips to form in oxidizing
metals. They have found that in at least four metals, they can control
the oxidation process to produce a uniform sprinkling of tips on the surface
of the materials.
Controlling this process is essentially a way of making materials self-assemble,
said Ramamoorthy Ramesh, a professor of materials engineering and physics
at the University of Maryland. This is one of several efforts to make
a material do the work of forming structures at the microlevel, rather
than physically carving out the tiny structures using relatively expensive
methods like lithography.
The tips form when the surface of a metal absorbs oxygen, causing oxidation,
or corrosion. Ordinary rust is a type of oxidation. When metals are oxidized
they expand as much as 30 to 40 percent, said Ramesh "The oxidation typically
is accompanied by huge volume change, so the crystal structure is different
and there's a lot of surface stresses. And the material in some sense
buckles up like a carpet, and it forms these blocky islands," said Ramesh.
The researchers produce uniform tips in oxidized palladium, copper, iron
and cobalt by spreading very thin films of metal on silicon or glass,
and heating them. "[If] I take a piece of glass like your window pane,
coat [it] with about 500 angstroms of metal and oxidize it, I can get
these tips at about 500 degrees," Ramesh said. An angstrom is about the
width of an atom.
The key to a useful self-assembly process is making the tip formation
predictable. The researchers have produced uniform sprinklings of tips
ranging as tall as one micron and spaced two to seven microns apart.
Because these metals have good field emission properties, meaning they
can predictably emit streams of electrons, these uniform tips could eventually
be used to channel electrons, said Ramesh.
There are several potential applications along these lines, he said. The
tips could be used to apply magnetic and electric fields to microfluidic
systems in order to control fluid flowing through tiny channels. They
could also be used to shoot electrons at the phosphors that light up in
a field emission-type display screen. The iron oxide tips may also prove
useful in future applications that require polarized electrons because
80 to 100 percent of the electrons emitted by iron oxide are spin polarized,
Electrons exist in one of two states: spin up, or spin down. A stream
of electrons is polarized when all the electrons are in the same state.
Polarized electrons could eventually prove useful in electronics because
the two states could represent the ones and zeros of binary communication.
The researchers are working to make the tips not only uniform, but, like
crystals, periodic. A periodic structure has an exact pattern, making
the exact location of each tip predictable.
The advantage of a periodic structure is that it scatters light evenly.
"Anything that's periodic means it will start scattering [radiation] in
a very coherent manner. That's why you can do x-ray diffraction or electron
diffraction in crystals, because they have a periodic structure," Ramesh
said. If the researchers can make the tips grow in a periodic manner,
they may be able to use them for applications having to do with light,
It's nice research because it shows development of uniformly sized oxide
particles on on patterned surface, said Caroline Ross, associate professor
of materials science at the Massachusetts Institute of Technology.
"In general, the self assembled processes are good for making structures...
smaller than available with conventional lithography," said Ross. However,
self assembled processes like these "suffer from the inability to precisely
control the location of the structures. Their uses will be much greater
if the nanostructure positions can be more precisely controlled," she
The researchers are working to understand the oxidation process better
in order to more fully control the tips' growth process, said Ramesh.
Ironically, they're going through literature from the '50s and '60s to
augment their understanding of the process.
"This is a very well-known field," said Ramesh. But the research into
the field has been largely aimed at preventing corrosion rather than encouraging
it, he said. "People are worried because... one [tip] in a few hundred
microns... can cause a short in their device. This is the other end of
the spectrum where every few thousand angstroms you have a [tip] and there's
a very uniform distribution."
The researchers' next steps are creating periodic structures, growing
carbon nanotubes on top of the tips, and exploring the use of the tips
as cathodes for use in batteries, Ramesh said.
Ramesh's research colleagues were Sanjeev S. Aggarwal, Satishchandra B.
Ogale, Chandan S. Ganpule, Sanjay R. Shinde, Vlad A. Novikov, A. P. Monga,
Mark R. Burr, Vincent Ballarotto and Ellen D. Williams. They published
the research in the March 5, 2001 issue of Applied Physics Letters. The
research was funded by the University of Maryland, Motorola and Telcordia.
Timeline: 5-10 years
Funding: Corporate, University
TRN Categories: Semiconductors and Materials
Story Type: News
Related Elements: Technical paper, "Oxide Nanostructures
through Self-assembly," Applied Physics Letters, March 5, 2001.
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