The team, led by Nobel prize-winning scientists Andre Geim and Kostya Novoselov, made its 3D device structures by stacking 2D layers of graphene (produced by exfoliation or the “sticky tape” method) on top of each other. These layers were then interlaid between atomically thin 2D hexagonal boron nitride (hBN) crystals complete with metal contacts for subsequent electrical measurements. Next, the researchers examined the structures by measuring their electron transport properties. They then extracted the best-performing regions using a focused ion beam for further study in a high-resolution scanning transmission electron microscope (TEM).

The ion beam is able to dig out trenches around the region of interest, explains team member Sarah Haigh, and removes a thin slice of material. The TEM then reveals where each individual 2D layer is located within the device.

Isolated contamination

“Our high-resolution images show that new kinds of 3D heterostructures with atomically sharp interfaces can be assembled,” she said. “More importantly, we found that any contamination segregates into isolated pockets – an important observation.”

Contaminating particles are invariably found on top of 2D crystals and become trapped between layers during their assembly. “These adsorbates not only degrade the electronic properties of finished devices but also mean that we cannot make true artificial layered crystals held together by Van der Waals forces,” explained Haigh. “We instead end up with a laminate ‘glued’ together by the adsorbates.

“That contamination appears to segregate into isolated pockets goes to show that the buried interfaces are clean and atomically flat and therefore have good electronic properties.”

Being able to engineer 3D structures from 2D materials in this way means that we can now create new electronic devices with specifically designed properties, she told “Traditionally electronic chips are mainly confined to 2D geometries but we could use these new structures to fabricate sophisticated 3D chip architectures that might allow for faster data processing and greater storage capacity.”

The team is now busy investigating even more complicated layer structures to understand more about how different compositions affect the electronic, thermal and mechanical properties of finished graphene devices.

The current work is detailed in Nature Materials.