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First Transistors Made Of 2-D Materials

Electronics: The fast, ultrathin devices could enable sharp, low-power flexible displays

by Prachi Patel, special to C&EN
May 8, 2014 | A version of this story appeared in Volume 92, Issue 19

Illustration of a two-dimensional transistor made of graphene, molybdenum disulfide, and boron nitride built on a silicon wafer.
Credit: Ali Javey
Built on a silicon wafer, an ultrathin transistor made with 2-D materials uses graphene for electrodes, molybdenum disulfide as its semiconductor layer, and boron nitride as an insulator.

Two independent research groups report the first transistors built entirely of two-dimensional electronic materials, making the devices some of the thinnest yet. The transistors, just a few atoms thick and hence transparent, could lead to bright, bendable, high-resolution displays.

Both groups’ devices signal important progress, says Deji Akinwande, an electrical engineer at the University of Texas, Austin, who was not involved in either study. “Flexible and transparent transistors are important for future flexible smart devices,” he says.

In two new studies, the research teams, one at Argonne National Laboratory and the other at the University of California, Berkeley, used 2-D materials to make all three components of a transistor: a semiconductor, a set of electrodes, and an insulating layer to keep the other two parts separated in some areas. Such all-2-D transistors are smaller than their silicon-based counterparts and would allow for a super-high density of pixels in next-generation displays.

Saptarshi Das, Anirudha V. Sumant, and their colleagues at Argonne made flexible transistors using graphene for the electrodes, tungsten diselenide (WSe2) for the semiconductor, and hexagonal boron nitride as the insulator (Nano Lett. 2014, DOI: 10.1021/nl5009037). Electrons travel in the devices about 100 times faster than in amorphous silicon devices used to drive today’s flat-panel displays. The transistors’ high electron mobility means the devices can switch faster. Switching speed dictates a display’s refresh rate and is necessary for high-quality video, especially 3-D video.

The UC Berkeley group, led by electrical engineer Ali Javey, made similar transistors, except they used molybdenum disulfide (MoS2) as the semiconductor (ACS Nano 2014, DOI: 10.1021/nn501723y). Their transistors have an electron mobility about 70 times higher than that of amorphous silicon devices.

All-2-D transistors have one major limitation, says Vitaly Podzorov, a physicist at Rutgers University: No good methods currently exist for making large-area films of WSe2 and MoS2. In both studies, the research teams exfoliated flakes of the materials from crystals using Scotch tape. Nevertheless, Podzorov says, these two proof-of-concept demonstrations show the promise of 2-D transistors.



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