“We now have a playground for most of the effects we wanted to explore in 2D materials,” says Denis Bandurin, a researcher at Manchester University, and first author of the reported results. He and his collaborators in the UK, Russia, the Netherlands and the Ukraine were able to harness the 2D materials expertise at Manchester University to produce stable devices of isolated monolayer InSe for the first time.

The thin-layer InSe not only has a mobility comparable to black phosophorous, which far surpasses other 2D dichalcogenides, but with the approach adopted by Bandurin and colleagues it also remains stable – thin crystals of black phosphorous are not. Monolayer InSe also suppresses the recombination of electron–hole pairs on account of its crystal symmetry, so it can be pumped with excitons for optoelectronic studies. In addition, 2D InSe shows the quantum Hall effect, which is not present in most other 2D materials bar graphene.

Importantly, InSe has one thing graphene doesn’t have – a bandgap. Having a bandgap means that 2D InSe devices can be readily switched on and off. In addition, the bandgap is very tunable due to quantum confinement effects. “We can change the thickness by one layer and it changes the bandgap a lot,” says Bandurin. “The bandgap of monolayer InSe is twice as large as in InSe with eight layers.”

So does 2D InSe offer all that graphene offers and a bandgap? “That might be one way to sum it up, but while the mobility of 2D InSe is high, the mobility of graphene is still unsurpassed,” says Bandurin. “But of course InSe is a big step in high-quality 2D devices.”

Stable isolation

The very low effective mass of electrons in bulk InSe – which leads to a very high mobility that is interesting for testing fundamental physics and developing new technologies – has already led other groups to try and isolate 2D layers that might then retain this excellent mobility while presenting other interesting characteristics. However, so far the challenge has been preserving the crystal quality in the 2D form, as like many other 2D materials it is prone to degradation from the environment.

To prevent degradation, Yang Cao, a co-author of the paper, produced the devices by exfoliating the layers of InSe from a crystal in an inert argon atmosphere, using a “glove box” technology developed at Manchester University. They then encapsulate it in hexagonal boron nitride (hBN). “We checked the device months later and the quality had not changed,” adds Bandurin.

Device potential

While 2D InSe has a great deal to offer fundamental studies of 2D material properties, its wealth of attributes make it tempting for potential application in devices too. Inevitably this raises questions over the potential scalability of its production – the exfoliating “scotch tape” approach used by the Manchester group is far from scalable. However, he likens the situation to the early days with graphene, which was at first only isolated by the same scotch tape approach as devised at Manchester. Now there are many groups around the world working on other fabrication strategies. He tells nanotechweb.org that people are already looking at chemical deposition of InSe too.

Frank Koppens, who is leader of the Nano-Optoelectronics Group at the Institute of Photonics Sciences (ICFO) in Spain, leader of the Graphene Flagship Optoelectronics Work Package, and was not involved in these research results, commented: “The electronic mobility of InSe, shown in this work, is remarkably high, which makes it a champion semi-conducting 2D material. In combination with its optical properties such as thickness-dependent fluorescence, it’s an excellent platform for exploring new opto-electronic device concepts and applications.”

Full details are reported in Nature Nanotechnology.

For more on the latest developments in 2D Materials beyond graphene visit the Nanotechnology focus collection and IOP Publishing's dedicated journal 2D Materials.