Molybdenum disulfide has all the 2D potential of graphene, with a bandgap

August 23, 2012 — Massachusetts Institute of Technology (MIT) researchers are building various electronic components out of a 2D form of molybdenum disulfide (MoS2).

Compared to limited results with 2D graphite — graphene — the MoS2 research is a starting point for walls that glow, clothing with embedded electronics, glasses with built-in display screens, and other applications.

This is the start of a “new realm” of research into 2D materials for electronic materials and devices, according to Tomás Palacios, the Emmanuel E. Landsman Associate Professor of EECS.

Large sheets of MoS2 were fabricated by Yi-Hsien Lee, a postdoc in associate professor Jing Kong’s group in EECS, via 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, Han Wang and Yu then produced building blocks of electronic circuits on the sheets, as well as on MoS2 flakes produced by a mechanical method. 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.

Wang found the material easier to use than graphene, since graphene lacks a bandgap. Graphene must be precisely modified to create a bandgap for transistors. This is not a problem with MoS2.

MoS2 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.

The material is so thin that it’s completely transparent, and it can be deposited on virtually any other material. 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 cellphone might be woven into fabric, providing a much more sensitive antenna that needs less power and could be incorporated into clothing, Palacios says.

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.

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.

Courtesy of David Chandler, MIT News Office. Learn more at www.mit.edu.

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