Numerous demonstrations over the years have demonstrated the capabilities that atomically thin materials can bring to electronics: extremely small size, excellent performance and some distinctive properties. But nearly all of these demonstrations required the electronics to be tested to be essentially assembled by hand. Materials such as graphene are often placed on a random surface, so the wiring needed for it to function is built around that location. It’s not exactly a recipe for mass production.
To the extent that there has been some progress, it has been limited. One of the most recent efforts involved the use of graphene and molybdenum disulfide to make the transistor with the smallest gate length. In this case, the two atomically thin materials had to be positioned carefully but not precisely. Any excess material was etched away and a key feature was achieved by cutting the graphene sheet.
This week saw a somewhat different approach to building these tiny devices: chemistry. A research team linked the two materials used in the previous study, graphene and molybdenum disulfide, using a single bridging molecule that could react with either of them. The chemistry of the bridging molecule also influenced the behavior of a device made using this approach.
Two for one
Graphene and molybdenum disulfide form sheets only one atom thick: all the chemical bonds that hold the sheet together force it to form a planar structure. They make a useful combination because they have different properties. Molybdenum disulfide is a semiconductor, while graphene usually conducts electricity well (although it can be converted to a semiconductor given the right environment). Normally, devices involving the two materials are built simply by superimposing them on top of each other. Weak interactions called Van der Waals forces will hold them together.
The group behind the work here, based in Spain, decided to try and build something a little stronger. Many chemicals have been identified that can break the bonds in the plane of either of these materials and chemically attach themselves to the surface of the sheet. At sufficiently high levels, this reaction would cause the sheet to break. But as long as the levels of these reactions are kept low enough, the sheet will remain intact and have a sparse coating of the reactive chemical.
The new work aims to create a single molecule that acts as a bridge between graphene and molybdenum disulfide. At one end of the bridge is a chemical group that reacts with molybdenum disulfide. On the other hand, there is a chemical group that interacts with graphene. There is only a short non-reactive benzene ring in between.
Starting with some molybdenum disulfide flakes, the researchers performed a reaction that connected the bridge to the flake. Next, the flakes were placed with graphene sheets, where the other end of the bridging molecule reacted with the graphene. The result was a graphene sheet decorated with molybdenum disulfide flakes, with the two connected via the bridging molecule.
An altered device
To make a device from the bonded material, the graphene sheet was placed on a silicon substrate and flanked by electrodes. Silicon could be used to control the flow of current through graphene from one electrode to the other. This allowed the researchers to test their behavior in various states of chemical alteration.
With silicon carrying enough charge to convert graphene into a semiconductor, simply laying molybdenum disulfide across it without chemical bridges would lead to the presence of more electrons in graphene. This would make it an n-type semiconductor (n for negative). Connecting the bridge molecule to graphene alone, on the contrary, would lead to the extraction of electrons from graphene, converting it into a p-type semiconductor (p being positive).
With the whole combination of graphene-bridge-molybdenum disulfide, the two chemicals bonded to graphene partially offset each other. The bridging molecule still converted the graphene to a p-type semiconductor, but the effect was weaker due to the presence of the molybdenum disulfide.
Hence, the work provides a nice demonstration that it is possible to fine-tune the conductive property of graphene by attaching other chemicals to it. It may be possible to build a large library of bridging molecules that alter the behavior of graphene in different ways.
The work also suggests that it is possible to construct functional structures involving more than one atomically thin material using chemistry. For example, it might be possible to roll out a sheet of graphene, eliminate anything unnecessary, and then use chemistry to attach another atomically thin material to it. This could potentially get around the problem of materials randomly placing themselves on what should be a compact device.
But this document proves no such thing. Molybdenum disulfide flakes are tiny compared to graphene sheets. So what you get is a graphene sheet in which only a part is covered with molybdenum disulfide, a portion that is well under half. This is sufficient to influence the behavior of graphene, but not necessarily to make a device that requires extensive graphene-molybdenum disulfide interactions.
It is possible that we can increase efficiency and turn it into a manufacturing technique. But the process will need some considerable improvements before we get there.
Chemistry of nature2022. DOI: 10.1038 / s41557-022-00924-1 (Information on DOIs).