Van der Waals heterostructures consist of multiple layers of few-monolayer-thick sheets of materials that are only weakly coupled. This weak coupling allows the intrinsic properties of individual layers of the material to be preserved since they are not significantly affected by interactions with the other layers. It also provides a way to make new topologically interesting materials such as topological insulators, which are materials that are insulating in the bulk but can conduct electricity on their surface via special surface electronic states. In 2D materials, these topologically protected states are electronic edge states in which spin and momentum are locked together, which allows for dissipation-less 1D spin currents useful for making spin-selective electronics devices.

Until now, however, researchers have only managed to make a few kinds of van der Waals heterostructures and most of these have hexagonal symmetry. “There is no doubt that these ‘graphene-like’ structures are very promising, but being able to engineer heterostructures with different crystal symmetries could open up a whole new world of opportunities in terms of both fundamental physics and electronic device fabrication,” says a team of researchers led by Simon Brown of the University of Canterbury in New Zealand.

Bismuth and antimony

Bismuth (Bi) is a key ingredient in many topologically interesting materials thanks to its strong spin-orbit coupling, explains Brown. As a thin film, it exists in two forms: black phosphorus (BP)-like or α-form, which has rectangular symmetry; and the hexagonal β-form. Both of these are called “bismuthene” and researchers recently found that these allotropes are topological insulators.

Antimony (Sb), which sits just above Bi in group 15 of the periodic table has similar allotropes but has been much less studied. “We do know, however, that multilayer β-antimonene is also a topological insulator, but that coupling between the surfaces destroys the topological states in few-monolayer-thick films unless sufficient strain is applied,” says Brown. α-antimonene has never been made in the lab before and its topological properties have never been studied.”

Exploiting topologically-protected edge states

The α-phases of both Sb and Bi have structures that are very similar to those recently used to build new types of transistors from BP. “If we could build similar transistors from Sb and Bi, which have built-in strong spin-orbit coupling, we could then exploit their topologically-protected edge states in spin-selective devices,” explains Brown. “These could allow us to manipulate quantum information, for example, and make new types of transistors that exploit the inherent spin-momentum locking in these materials. If the new Sb and Bi structures can be grown on microelectronics-compatible substrates they could be an important step forward towards such devices.”

The researchers grew α-bismuthene, one-monolayer-thick sheets of β-antimonene, two monolayer-thick-sheets of α-antimonene and a new allotrope of monolayer bismuthene with rectangular symmetry. They did this by starting with Bi nano-islands as a base and depositing either Sb or Bi on top of them. “By controlling the amount of Sb, we found that we could control the amount of the two Sb phases found on top of the Bi structures,” says Brown.

Topologically non-trivial materials

“The fact that we already knew that Bi nano-islands have a very similar structure to BP was key to this work,” he adds. “This is important because the Bi islands have no dangling bonds on their top surface, which means that when other materials are deposited on top of them, they do not interact strongly with the Bi. This is what makes the structures van der Waals heterostructures.

“Our scanning tunnelling microscopy and density-functional theory calculations show that the new two-monolayer-thick sheets of α-antimonene are topologically non-trivial and energetically favoured over β-antimonene thanks to interactions with underlying Bi islands. The calculations, performed by our theory colleagues Guang Bian at the University of Missouri, Xiaoxiong Wang of Nanjing University and Tai-Chang Chiang of the University of Illinois at Urbana-Champaign, also reveal that the monolayer Bi has an interesting band structure with multiple Dirac cones – these are the features that made graphene famous.”

Results open up the field of topological nanostructures

According to the researchers, the new results are important in a wider context since they open up the field of topological nanostructures. “Despite thousands of papers each year on topological materials, very few so far have focused on nanoscale materials,” explains Brown. “We believe that nanostructures are going to be crucial for real applications of topological materials in the future because, for example, to be really useful, a transistor that exploits the topological properties of the material is going to have to be nanoscale,” he tells nanotechweb.org.

The team, reporting its work in 2D Mater. 5 011002 and Phys. Rev. B 96, 205434, says that it is now busy working on growing several other topological materials using techniques similar to the one described here.