A new technology that is CMOS compatible and integrates different 2D materials into a single electronic device has been developed by researchers in the US. The team constructed large-scale electronic circuits based on graphene and molybdenum-sulphide heterostructures. The fabrication process might be extended to develop such heterostructures from any type of 2D layered material, with potential applications in flexible and transparent electronics, sensors, tunnelling FETs and high-electron mobility transistors.

2D materials are creating a flurry of interest in labs around the world because they have dramatically different electronic and mechanical properties from their 3D counterparts. This means that they could find use in a host of novel device applications, such as low-power electronic circuits, low-cost or flexible displays, sensors and even flexible electronics, which can be coated onto a variety of surfaces.

The most well-known 2D materials are graphene (which is a sheet of carbon just one atom thick) and the transition metal "dichalcogenides". Such materials have the chemical formula MX2, where M is a transition metal (such as molybdenum or tungsten) and X is a "chalcogen" (such as sulphur, selenium or tellurium) – they go from being indirect band-gap semiconductors in the bulk to direct band-gap semiconductors when scaled down to monolayers. Such monolayers also efficiently absorb and emit light, and so could be ideal for making a variety of opto-electronic devices such as light-emitting diodes and solar cells.

Separate and selective etching

Researchers have now combined, for the first time, both graphene and molybdenum sulphide (MoS2) heterostructures into single electronic devices and circuits, thanks to a new technology that allows them to selectively and separately etch both 2D materials. The team, led by Tomás Palacios of the Massachusetts Institute of Technology, grew the heterostructures using chemical vapour deposition. MoS2 was used as a transistor channel, while graphene was used to make contact electrodes and circuit interconnects.

The team, which includes scientists from MIT, Harvard University and the United States Army Research Laboratory, patterned and etched MoS2 into an isolated channel on its heterostructures. Next, patterned aluminium oxide was formed on this channel by low-temperature atomic layer deposition and lift-off techniques. "The MoS2 is partially covered by the Al2O3 and this layer serves as both a dielectric layer as well as the etch-stop layer," say team members Lili Yu and Elton Santos.

Flexible and transparent electronics

The fabrication process could ultimately be used to construct heterostructures from any type of 2D layered material, and circuits made from such structures could be sued in heterojunction devices, such as lasers, tunnelling FETs and high-electron mobility transistors. "And, since every component in the circuits is extremely thin, the finished devices are flexible and transparent, and so could find use in applications like wearable electronics or sensors that could be wallpapered and attached to any type of surface," says Santos.

Yu adds that the team is now busy trying to integrate a thin insulating layer of another 2D material – hexagonal boron nitride – into its structure. "We are also trying to create seamless graphene/MoS2 junctions," she explains. "Some other applications for this type of hybrid junction, such as, for instance, photodetectors and memory devices, are under investigation too."

The current work is detailed in Nano Letters 10.1021/nl404795z.