Moore’s law holds that the number of transistors in integrated circuits doubles approximately every two years. To keep up with this pace, researchers predict that integrated electronics will soon require transistors that are less than 10 nm in size. These will be extremely challenging to make with silicon since the thickness of the transistor will then become greater than the channel length, which will make it difficult to switch the devices on and off.

As alternatives to silicon, researchers have turned their attention in recent years to 2D materials like graphene (a sheet of carbon just one atom thick) and transition metal dichalcogenides (TMDCs).

Graphene and TMDCs

Graphene is a semi-metal and has a number of outstanding electronic, physical and mechanical properties, but it lacks an electronic bandgap. It is thus unsuitable for making into a transistor channel (it cannot be easily switched off). However, since it conducts electrons extremely well, it is ideal for making into interconnects to wire advanced electronic devices.

TMDCs are van der Waals materials with the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te). They go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers.

2D materials transistor

The problem is that 2D materials are difficult to precisely assemble over nanoscale distances, which makes it challenging to controllably fabricate the conductor–semiconductor–conductor heterostructures needed to build atomically thin circuitry. Such junctions allow current to pass through devices made from these circuits

Researchers led by Xiang Zhang at the University of California at Berkeley and the Lawrence Berkeley National Laboratory in California have now succeeded in chemically assembling the junctions between molybdenum sulphide and graphene. “By creating a graphene-MoS2 structure, we can effectively make a 2D materials transistor,” explains team member Mervin Zhao. “Such heterostructures have been made before, but they were built by physically constructing the materials together - like stacking sheets of paper on top of each other. We have chemically grown them. It is as if we cut out a strip in a piece of paper and then fill it again with a different piece of paper.”

All-2D material circuits

Being able to grow such heterostructure junctions in this way means that all-2D material circuits, and indeed fully operational computers down the road, are now possible, he tells nanotechweb.org.

The researchers made their heterostructure junctions by first placing graphene on a substrate and then etching it with plasma to form channels. “Continuing with the paper analogy, if graphene is the paper, then the plasma is basically the scissors that cut out strips in it,” says Zhao. “Next, we add an organic salt to the substrate, which only poorly adheres to the graphene (because the carbon sheet is hydrophobic). The ensemble is put into a chemical furnace containing MoS2, which then nucleates on the edges of the graphene and finally fills in the bare channel areas (see figure).”

Good electronic properties

Our final structure is made up of a one-atom thick sheet of graphene, a three-atom thick MoS2 slightly overlapped with graphene, a three-atom thick MoS2, another overlapped region and finally a layer of graphene again, he adds.

Transistors made from the heterostructure have a high conductance of 10μS, an on-off ratio as high as 106 and a charge carrier mobility of 17cm2/V/s. “The graphene also lowers the contact resistance of MoS2,” says Zhao. “This is important since traditional contact metals, like gold, typically have very high contact resistances, so this is another selling point in graphene’s favour, along with the 2D chemical assembly.”

Xiangfeng Duan of the University of California at Los Angeles, who was not involved in this research, says that this is a "very exciting study" and that it "marks an important step forward towards integrated 2D electronics."

Zhang and colleagues, reporting their work in Nature Nanotechnology doi:10.1038/nnano.2016.115, have already made 2D logic circuits from their heterostructure, such as an NMOS inverter. “This further underscores the technology’s ability to lay the foundation for a chemically assembled atomic computer,” they write in a LBNL press release.