In 2-D Credit: ACS Nano

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 (MoS 2 ) and tungsten diselenide (WSe 2 ) to make transistors because these 2-D materials have better electronic properties than silicon. In particular, MoS 2 and WSe 2 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, WSe 2 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.

Thin As Can Be [+]Enlarge Credit: Nano Lett.

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 MoS 2 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.

WSe 2 and MoS 2 have relative merits, both groups say. WSe 2 can conduct both electrons and holes, the corresponding positively charged vacancies in a material, as opposed to MoS 2 , 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 MoS 2 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.