Molybdenum Disulfide Enables Flexible Transistors

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.