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The discovery of graphene, a material just
one atom thick and possessing exceptional strength and other novel properties,
started an avalanche of research around its use for everything from electronics
to optics to structural materials. But new research suggests that was just the
beginning: A whole family of 2D materials may open up even broader
possibilities for applications that could change many aspects of modern life.
The latest “new” material, molybdenum
disulfide—which has actually been used for decades, but not in its 2D form—was
first described just a year ago (2011) by researchers in Switzerland. But in
that year, researchers at Massachusetts Institute of Technology (MIT)—who
struggled for several years to build electronic circuits out of graphene with
very limited results (except for radio-frequency applications)—have already
succeeded in making a variety of electronic components from molybdenum
disulfide. They say the material could help usher in radically new products,
from whole walls that glow to clothing with embedded electronics to glasses with
built-in display screens.
A report on the production of complex
electronic circuits from the new material was published online in Nano
Letters; the paper is authored by Han Wang and Lili Yu, graduate students
in the Department of Electrical Engineering and Computer Science (EECS); Tomás
Palacios, the Emmanuel E. Landsman Associate Professor of EECS; and others at
MIT and elsewhere.
Palacios says he thinks graphene and
molybdenum disulfide are just the beginning of a new realm of research on 2D
materials. “It’s the most exciting time for electronics in the last 20 or 30
years,” he says. “It’s opening up the door to a completely new domain of electronic
materials and devices.”
Like graphene, itself a 2D form of
graphite, molybdenum disulfide has been used for many years as an industrial lubricant.
But it had never been seen as a 2D platform for electronic devices until last
year (2011), when scientists at the Swiss university EPFL produced a transistor
on the material.
MIT researchers quickly swung into action:
Yi-Hsien Lee, a postdoctoral researcher in associate professor Jing Kong’s
group in EECS, found a good way to make large sheets of the material using a
chemical vapor deposition (CVD) process. Lee came up with this method while
working with Lain-Jong Li at Academia Sinica in Taiwan and improved it after
coming to MIT. Palacios, Wang, and Yu then set to producing building blocks of
electronic circuits on the sheets made by Lee, as well as on molybdenum
disulfide flakes produced by a mechanical method, which were used for the work
described in the new paper.
Wang had been struggling to build circuits
on graphene for his doctoral thesis research, but found it much easier to do
with the new material. There was a “hefty bottleneck” to making progress with
graphene, he explains, because that material lacks a bandgap—the key property
that makes it possible to create transistors, the basic component of logic and
memory circuits. While graphene needs to be modified in exacting ways in order
to create a bandgap, molybdenum disulfide just naturally comes with one.
The lack of a bandgap, Wang explains, means
that with a switch made of graphene, “you can turn it on, but you can’t turn it
off. That means you can’t do digital logic.” So people have for years been searching
for a material that shares some of graphene’s extraordinary properties, but
also has this missing quality—as molybdenum disulfide does.
Because it already is widely produced as a
lubricant, and thanks to ongoing work at MIT and other laboratories on making
it into large sheets, scaling up production of the material for practical uses
should be much easier than with other new materials, Wang and Palacios say.
Wang and Palacios were able to fabricate a
variety of basic electronic devices on the material: an inverter, which
switches an input voltage to its opposite; a NAND gate, a basic logic element
that can be combined to carry out almost any kind of logic operation; a memory
device, one of the key components of all computational devices; and a more complex
circuit called a ring oscillator, made up of 12 interconnected transistors,
which can produce a precisely tuned wave output.
Palacios says one potential application of
the new material is large-screen displays such as television sets and computer
monitors, where a separate transistor controls each pixel of the display.
Because the material is just one molecule thick—unlike the highly purified
silicon that is used for conventional transistors and must be millions of atoms
thick—even a very large display would use only an infinitesimal quantity of the
raw materials. This could potentially reduce cost and weight and improve energy
efficiency.
In the future, it could also enable
entirely new kinds of devices. The material could be used, in combination with
other 2D materials, to make light-emitting devices. Instead of producing a
point source of light from one bulb, an entire wall could be made to glow,
producing softer, less glaring light. Similarly, the antenna and other
circuitry of a cell phone might be woven into fabric, providing a much more
sensitive antenna that needs less power and could be incorporated into
clothing, Palacios says.
The material is so thin that it’s
completely transparent, and it can be deposited on virtually any other
material. For example, molybdenum disulfide could be applied to glass,
producing displays built into a pair of eyeglasses or the window of a house or
office.