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Thin sensors boast high-level gas, chemical detection

Posted: 25 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Graphene? Molybdenum disulphide? sensors?

Two California researchers began searching for other atomically thin materials with unique properties, getting inspiration from the discovery of graphene, a two-dimensional layered material with unusual and attractive electronic, optical and thermal properties.

Molybdenum disulphide (MoS2) has proved to be one of the most promising. Single-layer and few-layer molybdenum disulphide devices have been proposed for electronic, optoelectronic and energy applications. A team of researchers, led by engineers at the University of California, Riverside's Bourns College of Engineering, have developed another potential application: sensors.

"The sensors are everywhere now, including in smartphones and other portable electronic devices," said Alexander Balandin, UC presidential chair and professor of electrical and computer engineering, also the lead researcher on the project. "The sensors we developed are small, thin, highly sensitive and selective, making them potentially ideal for many applications."

Alexander Balandin

Balandin: Sensors are everywhere now.

Balandin and the graduate students in his lab built the atomically thin gas and chemical vapour sensors from molybdenum disulphide and tested them in collaboration with researchers at the Rensselaer Polytechnic Institute in Troy, New York. The devices have two-dimensional channels, which are great for sensor applications because of the high surface-to-volume ratio and widely tuneable concentration of electrons.

The researchers demonstrated that the sensors, which they call molybdenum disulphide thin-film field-effect transistors (TF-FET), can selectively detect ethanol, acetonitrile, toluene, chloroform and methanol vapours.

The findings were published in a recent paper, "Selective chemical vapour sensing with few-layer MoS2 thin-film transistors: Comparison with graphene devices," in the journal Applied Physics Letters. In addition to Balandin, co-authors were Rameez Samnakay and Chenglong Jiang, both Ph.D. students in Balandin's lab, and Michael Shur and Sergey Rumyantsev, both of Rensselaer Polytechnic Institute.

The selective detection did not require prior functionalisation of the surface to specific vapours. The tests were conducted with the fabricated devices and intentionally aged devices. The molybdenum disulphide sensors used in the study were aged for two months because practical applications require that sensors remain stable and operational for at least a month.

Schematic of the molybdenum disulphide

The Molybdenum disulphide (MoS2) thin-film sensor with the deposited molecules that create additional charge.

Sensors made with atomically thin layers of MoS2 revealed better selectivity to certain gases owing to the electron energy band gap in this material, which resulted in strong suppression of electrical current upon exposure to some of the gases. Graphene devices, from the other side, demonstrated selectivity when one used current fluctuations as a sensing parameter.

"Sensors implemented with atomically thin MoS2 layers are complementary to graphene devices, which is good news," Balandin said. "Graphene has very high electron mobility while MoS2 has the energy band gap."

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