A new approach to twistronics studies involving ribbons of graphene could provide researchers with better control over the twist angle and make it easier to study the electronic effects that arise from twisting and straining adjacent layers of two-dimensional (2D) materials. Previous twistronics studies focused on twisting two flakes of material and stacking them, but the new ribbon-based technique allows for a more continuous change in the twist angle, providing better control and allowing for easier study of the electronic effects.
By stacking layers of 2D materials and varying the angle between them, researchers have discovered that they can change the electronic properties of these materials. For instance, a bilayer of graphene develops a band gap when placed in contact with hexagonal boron nitride. This occurs because the slightly mismatched layers of graphene and hBN form a moiré superlattice, which allows a band gap to form. By twisting the layers further and increasing the angle between them, the band gap disappears.
This ability to change electronic properties without having to alter the chemical composition of the material is a fundamentally new direction in device engineering and has been dubbed “twistronics.” However, controlling twist angles and associated strain remains a challenge, as different areas of a sample may have different electronic properties. To overcome this, researchers at Columbia University placed a ribbon-shaped graphene layer on top of hBN and slowly bent one end of the ribbon using a piezo-atomic force microscope. This created a twist angle that varied continuously and a uniform strain profile that could be predicted.
The researchers were able to continuously tune both strain and twist angle, giving them unprecedented access to the “phase diagram” of twisted angles. This level of control allows for precise mapping of the dependence of the electronic band structure on twist angle, which was not previously possible. Additionally, the new technique provides the first opportunity to measure the role of strain in magic angle bilayer graphene systems in a reproducible way and opens up new ideas for controlling the electronic band structure in twisted layer systems.
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