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 two-dimensional materials may open up
even broader possibilities for applications that could change many
aspects of modern life.
The latest “new” material, molybdenum disulfide (MoS2) — which has
actually been used for decades, but not in its 2-D form — was first
described just a year ago by researchers in Switzerland. But in that
year, researchers at 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 MoS2. 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
Diagram shows the flat-sheet structure of the material used by the MIT team, molybdenum disulfide. Molybdenum atoms are shown in teal, and sulfur atoms in yellow. Image courtesy of Wang et al.
A report on the production of complex electronic circuits from the new
material was published online this month in the journal 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 MoS2 are just the beginning of a
new realm of research on two-dimensional 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 2-D form of graphite, molybdenum disulfide has
been used for many years as an industrial lubricant. But it had never
been seen as a 2-D platform for electronic devices until last year,
when scientists at the Swiss university EPFL produced a transistor on
MIT researchers quickly swung into action: Yi-Hsien Lee, a postdoc 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 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 MoS2
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, MoS2 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 labs 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
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 2-D 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 cellphone 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, MoS2 could be
applied to glass, producing displays built into a pair of eyeglasses or
the window of a house or office.
In addition to Palacios, Kong, Wang, Yu and Lee, the work was carried
out by graduate student Allen Hsu and MIT affiliate Yumeng Shi, with
U.S. Army Research Laboratory researchers Matthew Chin and Madan Dubey,
and Lain-Jong Li of Academia Sinica in Taiwan. The work was funded by
the U.S. Office of Naval Research, the Microelectronics Advanced
Research Corporation Focus Center for Materials, the National Science
Foundation and the Army Research Laboratory.