Moiré materials: new understandings unlock the gateway to future nanoelectronics applications

by Virginia Greco

A new research on electron conduction in double-wall carbon nanotubes finally offers an explanation for a long-ununderstood phenomenon about interlayer conductance and provides a predictive model to simulate the behaviour of such structures. A class of “switchable” nanotubes is also identified. This work, recently published in “Carbon”, was carried out in collaboration by members of the ICN2, Utrecht University and the University of Liège.

The so-called moiré patterns are motifs that emerge when two repetitive structures are overlaid. This phenomenon is well known from computer or TV screens: when looking at a finely striped pattern, e.g. on a shirt, the stripes do not look evenly spaced and seem to bend in some areas. While undesirable in this case, the moiré effect can indeed be surprisingly useful in materials science. In fact, two atomically thin materials can be overlapped to create a new material, in which the atomic structures of the two produce a moiré pattern. Some of these moiré materials exhibit astonishing properties, drastically different from those of their components, which make them great candidates for application in novel nano-electronic devices.

Among many possible moiré materials, particularly interesting in this context are double-wall carbon nanotubes. They are made up of two cylinders, each composed of a single layer of carbon atoms arranged in a honeycomb structure, inserted one into the other. Since carbon nanotubes are mechanically ultra-strong and great electron conductors, their combination in such double-wall structure (exhibiting a moiré pattern) is very appealing. However, conduction between layers in double-wall carbon nanotubes has been little explored.

A study recently published in Carbon and led by Prof. Zeila Zanolli, from Utrecht University (The Netherlands), explores how electrons can move between the walls in moiré structures such as double-wall carbon nanotubes. Not only it provides a model to simulate such materials, but also sheds light on a hitherto unsolved mystery about quantized conduction. This research was carried out in collaboration with Nils Wittemeier, doctoral students at the ICN2, Prof. Pablo Ordejón, ICN2 Director and leader of the Theory and Simulation Group, and Prof. Matthieu J. Verstraete, from the University of Liège (Belgium).

The (until now) unsolved G₀ mystery

Twenty-five years ago a group of researchers at the Georgia Institute of Technology measured electronic transport in multi-wall carbon nanotubes and stumbled upon an unexpected result. They observed a quantized conduction – which means that the conductance measured along the nanotube axis jumps from 0 to a constant value, instead of increasing gradually— which proved that the nanotubes are quantised conductors. But, according to commonly accepted theoretical predictions, they expected to measure a conductance of 4G₀, where 2G₀ is the contribution of each layer, i.e. the simple sum of the conductance of individual tubes. On the contrary, they observed a constant G₀ value, even when overlapping more than two nanotubes.

Although proposals had been made earlier, the phenomenon remained unexplained until recently. The team of researchers led by Prof. Zanolli was able to prove that in some double-wall carbon nanotubes the quantum interaction between the two layers limits the conductance between them, so that it remains G₀ (does not sum up to 4G₀). Besides answering such a longstanding open question, this work provides more important contributions to this research field.

SIESTA method, analytical model and “switchable” nanotubes

The authors of this study performed atomistic simulations of nanotubes with up to 600 atoms using SIESTA, a first-principles materials simulation program developed by the ICN2 group led by Prof. Pablo Ordejón and one of the flagship codes of the MAX (MAterials design at the eXascale) European Centre of Excellence. They were able to devise a simple predictive model, which allowed simulating hundreds of nanostructures with up to 100,000 atoms, while retaining an accurate description of the electronic interaction between the two walls at the quantum level. This powerful tool is extremely valuable for studying and simulating moiré materials.

In addition, the researchers identified a class of nanotubes that show a behaviour similar to a light switch. In practice, by moving the inner wall slightly in and out of the outer wall, it is possible to change between an on-state –in which electrons can move between the walls— and an off-state –where they are blocked. The discovery of “switchable” nanotubes opens up new avenues for the development of innovative nanoelectronics devices.

Reference article:
N. Wittemeier, M. J. Verstraete, P. Ordejón, Z. Zanolli, Interference effects in one-dimensional moiré crystals, Carbon 186, 416 (2022) DOI: 10.1016/j.carbon.2021.10.028