Mar 1, 2013
MoS2 in new mobility record
2D films of molybdenum disulphide can work as ambipolar field-effect transistors that boast room-temperature charge-carrier densities as high as 480 cm2/Vs. This is the first time that such high mobilities and ambipolar behaviour have been seen in a fully working thin-film device – a result that could come in handy for making flexible transparent displays and other electronic and optoelectronic devices.
2D materials, like MoS2, have very different electronic and mechanical properties from their 3D counterparts and so may find use in a host of novel device applications. Until now, however, most research in this field has focused on graphene but this material does suffer from one big problem in that it lacks an electronic bandgap. This means that devices made from graphene cannot be easily switched off.
2D MoS2, for its part, does possess a bandgap (at 1.3 eV). And, when it is thinned to a single layer, the bandgap becomes direct (1.9 eV). This is a real advantage because it is easier to make optoelectronics and photonics devices that exploit electron–hole pair excitation with direct rather than indirect gap semiconductors, which indeed silicon is.
Plastic substrate improves mobilities
A team led by Michael Fuhrer, who has recently moved to Monash University in Australia from the University of Maryland in the US, constructed MoS2 transistors from thin flakes of the material that had been mechanically shaved (or exfoliated) off geological MoS2 crystals. Such a technique is also used to obtain graphene from graphite crystals. "The advance in our new work, however, was to use a thick polymer (PMMA) as the substrate, which we found greatly improved the mobility of charge carriers in MoS2," Fuhrer told nanotechweb.org.
The researchers measured a mobility of as high as 480 cm2/Vs for MoS2 on PMMA, and interestingly for both electrons and holes. This is ambipolar behaviour and implies that MoS2 could work as both an n- and p-type material in the same device. Unlike previous experiments that measured mobilities in MoS2 (which in fact yielded even higher values, but which are now thought to be flawed results), Fuhrer and colleagues measured the so-called four-probe conductivity of the material. The researchers also only measured current flowing through a back gate in their transistors rather than through a top gate dielectric and top metal gate, as previously.
"We believe that MoS2 could be prepared on large-area substrates using techniques like van der Waals epitaxy and make for a new semiconducting electronic material," said Fuhrer. "Since MoS2 is a van der Waals solid (it is made up of 2D sheets that are weakly bonded to each other), it is compatible with a variety of substrates. In our case, we showed that excellent transistors could be made on plastic, which means that the material may be ideal for making flexible transparent displays or other devices in which silicon is unsuitable."
The researchers are now busy trying to grow high-quality, large-area films of MoS2 with thicknesses that they can better control. "The properties of the material depend on its thickness and so dictate the type of potential application," explained Fuhrer. "For example, a single layer of MoS2 can be used for photovoltaics and light-emitting devices, to name but two applications."
The team is also looking at other materials in the same family as MoS2 to see if they have new and useful electronic properties too.
The present work was published in Applied Physics Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org