viernes, 5 de octubre de 2007

Thin Carbon Is In: Graphene Steals Nanotubes’ Allure

First, it was buckyballs, molecules of carbon in the shape of soccer balls.

Then came carbon rolled up in nanotubes.
Now, the latest craze in materials science is graphene, a one-atom-thick sheet of carbon that looks like molecular chicken wire.
Graphene is the thinnest of all possible materials in the universe. It shares many of the properties that excited physicists about nanotubes a decade ago, but it is easier to make and manipulate, giving greater hope that it will make the move from laboratory to practical application. Physicists have made transistors out of graphene and used it to explore odd quantum phenomena at room temperatures.
“The hype is bigger,” said Carlo Beenakker, a professor of theoretical physics at Leiden University in the Netherlands, “because the physics is richer.”
It is also one of the few times in history that a roll of sticky tape — the same tape that sits in dispensers on office desks around the world — has played a central role in spurring scientific research. Tape is a surprisingly easy and effective way to split carbon into thin sheets.
A couple of years ago, just a handful of groups was researching anything with graphene. At an American Physical Society meeting last month in Denver, nearly 100 papers about graphene were presented, often to full sessions.
“It’s like discovering a new island” with a host of species to be catalogued and studied, Dr. Beenakker said. In one sense, graphene is not new. A nanotube is just rolled-up graphene. Graphite, the stuff of pencil lead, has long been known to consist of layers of carbon stacked on top of one another like a deck of cards. Pencils produce a black trail, because the friction on the tip rubs off graphite flakes. Beginning in the 1970s, scientists grew graphene flakes in the laboratory.
The laboratory-made flakes were too minuscule to be more than curiosities, and researchers had not mastered the sleight-of-hand needed to slide a single graphene card out of a deck of natural graphite.
Eight years ago, researchers led by Rodney Ruoff, a professor of nanoengineering who is now at Northwestern University, reported that he had rubbed tiny pillars of graphite against a silicon wafer surface, causing them to spread out like a deck of cards. He suggested that the technique could produce single-layer graphene, but he did not measure the flakes’ thickness. Philip Kim, a professor of physics at Columbia, took a similar approach in making a “nanopencil,” attaching a graphite crystal to the tip of an atomic force microscope and dragging it along a surface. He, too, found graphite cleaved into flakes. But the flakes, as thin as five-billionths of a meter, nevertheless consisted of probably at least 10 layers of atoms.
“We were pretty happy with this result back then,” said Dr. Kim, who presented the research in March 2004. “And then everything got changed a few months later.”
In September 2004, Dr. Kim saw a preprint of a paper by researchers led by Andre Geim, a physics professor at the University of Manchester in England. They had made single-layer graphene.
More surprising was the technique. They placed a graphite flake on a piece of adhesive tape, folding the tape over and pulling it apart, cleaving the flake in two. Folding and unfolding repeatedly, the graphite became thinner and thinner. Then they stuck the tape to a silicon wafer and rubbed it. Some graphite flakes stuck to the wafer, and those flakes were occasionally one atom thick.
Reading the preprint, Dr. Kim abandoned his nanopencil. “We rushed to the stationery store and bought Scotch tape,” Dr. Kim said.
Scientists now regularly call Dr. Geim’s innovation “the Scotch tape method.” Dr. Geim originally used Scotch tape but has switched to a different brand.
The utter simplicity makes it possible for almost anyone to jump into graphene research. Dr. Kim’s group pays an undergraduate $10 an hour to make graphene.
Dr. Geim says his main contribution was not the tape, but his way of spotting the single-layer graphene among the thicker flakes. The highest of high-tech microscopes can spot the bumps of a single atom, but using them to measure the thickness of each flake is impossibly slow. A one-atom-thick sheet is generally invisible, but Dr. Geim found that a sheet that thin does change the color of the silicon oxide layer atop the wafer, much as a sheen of oil on water generates a rainbow of colors.

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