Web Date: May 6, 2014
First Transistors Made Entirely Of 2-D Materials
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, high-resolution displays that are power-efficient and bendable.
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.
Transistors are electronic switches that turn current on and off in many types of electronics. In flat-panel displays, thin-film transistors made with amorphous silicon drive the lighting of individual pixels. Scientists have been increasingly interested in even thinner semiconducting materials such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) to make transistors because these 2-D materials have better electronic properties than silicon. In particular, MoS2 and WSe2 have a larger band gap—the difference in energy between the materials’ conducting and nonconducting states—so transistors made from them need less power in theory to activate than silicon-based ones require.
Other researchers previously have made transistors with these 2-D semiconductors, but still used conventional materials for the other parts. In the 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 would be atoms thick, making them smaller than their silicon-based counterparts and allowing 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, WSe2 for the semiconducting channel, and hexagonal boron nitride as the insulator (Nano Lett. 2014, DOI: 10.1021/nl5009037). The group fabricated their devices on a plastic substrate following a standard protocol, depositing the materials layer by layer and using lithography and etching to pattern the layers.
Electrons travel in the devices about 100 times faster than in amorphous-silicon devices. Such a high electron mobility means faster-switching transistors, which dictates a display’s refresh rate and is necessary for high-quality video, especially 3-D video.
The Berkeley group, led by electrical engineer Ali Javey, made similar transistors, except they used 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.
WSe2 and MoS2 have relative merits, both groups say. WSe2 can conduct both electrons and holes, the corresponding positively charged vacancies in a material, as opposed to MoS2, which can shuttle only electrons. The ability to transport both electrons and holes is useful for making n-type and p-type transistors that are used in digital logic circuits, Sumant says. The Argonne researchers demonstrated both types of transistors.
Transistors made of MoS2 can carry higher current when they’re on. This translates to brighter pixels driven by less power. But Javey’s group studies devices made of both materials and says at this early stage of research, it makes sense to look at many different 2-D materials.
But all-2-D transistors won’t be powering displays any time soon. They have one major limitation, says Vitaly Podzorov, a physicist at Rutgers University: There are no good methods 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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