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When compounds of bromine or chlorine (represented in blue) are introduced into a block of graphite (shown in green), the atoms find their way into the structure in between every third sheet, thus increasing the spacing between those sheets and making it easier to split them apart. Image: Chih-Jen Shih/Christine Daniloff
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Graphene
has interested countless researchers with its unique strength and its
electrical and thermal conductivity. But one key property it lacks is the
ability to form a band gap, needed for devices such as transistors, computer
chips, and solar cells.
Now,
a team of MIT scientists has found a way to produce graphene in significant
quantities in a two- or three-layer form. When the layers are arranged just
right, these structures give graphene the much-desired band gap—an energy range
that falls between the bands, or energy levels, where electrons can exist in a
given material.
"It's
a breakthrough in graphene technology," says Michael Strano, the Charles and
Hilda Roddey Associate Professor of Chemical Engineering at MIT. The new work
is described in Nature Nanotechnology, co-authored by graduate student
Chih-Jen Shih, Professor of Chemical Engineering Daniel Blankschtein, Strano,
and 10 other students and postdocs.
Graphene
was first proven to exist in 2004, but making it in quantities large enough for
anything but small-scale laboratory research has been a challenge. The standard
method remains using adhesive tape to pick up tiny flakes of graphene from a
block of highly purified graphite—a technique that does not lend itself to
commercial-scale production.
The
new method, however, can be carried out at a scale that opens up the
possibility of real, practical applications, Strano says, and makes it possible
to produce the precise arrangement of the layers—called A-B stacked, with the atoms
in one layer centered over the spaces between atoms in the next—that yields desirable
electronic properties.
"If
you want a whole lot of bilayers that are A-B stacked, this is the only way to
do it," he says.
The
trick takes advantage of a technique originally developed as far back as the
1950s and '60s by MIT Institute Professor Mildred Dresselhaus, among others:
Compounds of bromine or chlorine introduced into a block of graphite naturally
find their way into the structure of the material, inserting themselves
regularly between every other layer, or in some cases every third layer, and
pushing the layers slightly farther apart in the process. Strano and his team
found that when the graphite is dissolved, it naturally comes apart where the
added atoms lie, forming graphene flakes two or three layers thick.
"Because
this dispersion process can be very gentle, we end up with much larger flakes"
than anyone has made using other methods, Strano says. "Graphene is a very
fragile material, so it requires gentle processing."
Such
formations are "one of the most promising candidates for post-silicon
nanoelectronics," the authors say in their paper. The flakes produced by this
method, as large as 50 square micrometers in area, are large enough to be
useful for electronic applications, they say. To prove the point, they were
able to manufacture some simple transistors on the material.
The
material can now be used to explore the development of new kinds of electronic
and optoelectronic devices, Strano says. And unlike the "Scotch tape" approach
to making graphene, "our approach is industrially relevant," Strano says.
While
it's hard to predict how long it will take to develop this method to the point
of commercial applications, Strano says, "it's coming about at a breakneck
pace." A similar solvent-based method for making single-layer graphene is
already being used to manufacture some flat-screen television sets, and "this
is definitely a big step" toward making bilayer or trilayer devices, he says.
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