Twisting nanoporous graphene on graphene

In a recent study, a team of researchers explores ¹ the intriguing electronic behaviours that emerge when two distinct forms of carbon-based materials—nanoporous graphene (NPG) and graphene—are layered together with a twist between them.

Graphene is a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice, renowned for its exceptional electrical conductivity, mechanical strength, and flexibility. Nanoporous graphene (NPG), on the other hand, is a modified version of graphene that contains a regular array of nanoscale pores, effectively making it a network of interconnected carbon nanoribbons. These pores introduce unique properties, such as tunable electronic characteristics and enhanced surface area, making NPG a material of significant interest for applications in sensors, catalysis, and energy storage.

The concept of twisted bilayers

When two layers of graphene are stacked with a slight rotational misalignment—a configuration known as a “twisted bilayer”—remarkable electronic phenomena can arise. One of the most notable is the emergence of superconductivity at certain “magic” twist angles, where the electronic interactions between the layers lead to new, emergent behaviours not present in the individual layers. This study extends the concept of twisted bilayers to a combination of NPG and graphene, aiming to uncover how the introduction of nanopores and the twist angle influence the electronic and transport properties of the resulting structure.

Interlayer coupling and electronic decoupling

The researchers employed an atomistic tight-binding model combined with non-equilibrium Green’s functions to simulate and analyse the electronic properties of the NPG/graphene bilayer system across various twist angles. They discovered that at small twist angles (less than approximately 10 degrees), the NPG and graphene layers are strongly coupled. This strong coupling is evidenced by the hybridization of their electronic bands, meaning that the electronic states of one layer significantly influence those of the other. As a result, when electrons are injected into the NPG layer, there is substantial transmission of these electrons into the graphene layer, leading to observable interference patterns in the current flow on both layers.

However, as the twist angle increases beyond this small-angle regime, the coupling between the layers weakens. This weakening allows each layer to retain its individual electronic properties, effectively leading to electronic decoupling. In this decoupled state, the layers behave more independently, and the unique characteristics of NPG and graphene are preserved without significant mutual interference.

Chiral currents induced by twist

An intriguing consequence of introducing a twist between the NPG and graphene layers is the breaking of mirror symmetry in the system. Mirror symmetry refers to the property of a system being indistinguishable from its mirror image. When this symmetry is broken due to the twist, it leads to the emergence of chiral features in the electronic currents. Chirality, in this context, means that the current exhibits a preferred directional flow, similar to how certain molecules can be “left-handed” or “right-handed.” This chiral behaviour in the current flow could have implications for developing devices that exploit directional electronic properties.

Resonant peaks in electronic density of states

The study also found that at small twist angles, resonant peaks appear in the electronic density of states (DOS) of the bilayer system. The DOS is a measure of the number of electronic states available at each energy level. These resonant peaks indicate energy levels where there is a high density of available electronic states, which can significantly affect the material’s electronic properties. The presence of these peaks suggests that the interlayer coupling is strongly dependent on the twist angle. The researchers propose that these features could be probed using scanning tunnelling microscopy (STM), a technique that allows for imaging and spectroscopy at the atomic scale. By using STM, scientists could experimentally observe and verify the predicted electronic structures and their dependence on the twist angle.

Future directions

This research provides valuable insights into how the interplay between twist angle and the intrinsic properties of NPG and graphene can be harnessed to tailor the electronic characteristics of bilayer systems. The ability to control interlayer coupling and induce chiral currents opens up potential avenues for designing novel electronic devices, such as transistors, sensors, and components for quantum computing, where directional control of electron flow is crucial.

Furthermore, the findings underscore the importance of considering both the geometric configuration (such as twist angle) and the specific material properties (like the presence of nanopores) when engineering two-dimensional heterostructures. As research in this field progresses, exploring other combinations of two-dimensional materials with various modifications and twist angles could lead to the discovery of new phenomena and the development of advanced materials with customized electronic properties.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.