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Web Date: July 26, 2012

Molybdenum Disulfide Enables Flexible Transistors

Electronics: Thin, bendable transistor is fast and requires only low power to turn on and off
Department: Science & Technology
News Channels: Materials SCENE, Nano SCENE
Keywords: transistor, molybdenum disulfide, ion gel, flexible electronics, band gap
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Bend It Like Molybdenum
A schematic shows a MoS2 transistor built on a plastic substrate (yellow). In the device, an ion gel (droplet) rests on a thin sheet of MoS2. This sheet connects the charge source and drain. A voltage applied at the top gate turns on or off the transistor by changing how the MoS2 and ions in the gel interact, which in turn controls the flow of electrons between the source and drain.
Credit: Nano Lett.
20120726lnj2-transistor
 
Bend It Like Molybdenum
A schematic shows a MoS2 transistor built on a plastic substrate (yellow). In the device, an ion gel (droplet) rests on a thin sheet of MoS2. This sheet connects the charge source and drain. A voltage applied at the top gate turns on or off the transistor by changing how the MoS2 and ions in the gel interact, which in turn controls the flow of electrons between the source and drain.
Credit: Nano Lett.

A future of wearable devices, smart bandages, and other flexible electronics depends on the development of transistors that can function properly when bent. Researchers have now built a fast transistor from a thin film of molybdenum disulfide and a polymeric gel that works even when bent almost in half (Nano Lett., DOI: 10.1021/nl301335q).

Many scientists think graphene will be the key to flexible electronics, but Lain-Jong Li of Academia Sinica in Taiwan turned to MoS2 because it has one thing that graphene doesn’t: a band gap. To turn off a transistor, the material in it must have a band gap. A graphene transistor would always be on, Li says.

To build MoS2 transistors, Li and his colleagues used a two-step chemical vapor deposition method, which they had recently developed (Adv. Mater., DOI: 10.1002/adma.201104798). They used the method to make three-atom-thick MoS2 sheets on a piece of the insulating plastic polyimide. Then the researchers topped the semiconductor with an ion gel—a concentrated, water-free mixture of polymer and an ionic liquid. When the researchers applied 0.1 V to the gel, ions gathered near the film’s surface, Li says, blocking electron flow through the MoS2 film. When they increased the voltage to 0.68 V, the ions dispersed, and the transistor turned on.

The electron flow was fast, Li says, on par with conventional silicon transistors. Meanwhile, the voltages needed to turn the current flow off and on are lower than those required for silicon devices.

As a test of the transistors’ flexibility, the researchers bent it over a rod with a 0.75 mm radius—thinner than a toothpick. Although it was almost bent in half, the transistor maintained its excellent performance, the researchers report.

 
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