At APS March last year, Pablo Jarillo-Herrero reported superconductivity in graphene bilayers. Soon after this, hundreds of publications were published on arXiv, with theorists trying to understand the phenomenon, and experimentalists trying to reproduce the effect and observe different properties on new materials.
A few weeks ago, at APS March 2019, there were more than 150 talks including the keyword “twisted bilayers” in their abstract, and google scholar finds more than 800 results for “twisted bilayer” in 2019 only.
Atomically thin materials have driven a lot of basic research in physics for the past decade, with some applications in optics and electronics. They have been proposed recently as a way to extend the Moore’s law.
Recently, new properties have been explored by assembling those atomically-thin layers each other through van der Waals bonding to form hetero-bilayers. In these structures, the electrons can no longer move freely in the planes of the atomic layers, but their behaviour (electronic states and band structure) depend on the two lattices, forming so-called Moiré-potentials, with a superlattice created at the nanometre scale. If the orientation on the two layers is changed (twisting them relative to each other), the moiré lattice changes, modifying the optical properties of the bilayers.
Interesting properties may arise with semiconducting layered materials, like TMDs (transition metal dichalcogenides). This has been reported a few weeks ago in four Nature papers. These articles investigate the impact of the moiré potential on light emission and absorption of hetero-bilayers made out of tungsten diselenide, tungsten disulphide and molybdenum disulphide. In these structures, the light emission and absorption is governed by the excitons:
- Tran et al. Evidence for moiré excitons in van der Waals heterostructures;
- Seyler et al. Signatures of moiré-trapped valley excitons in MoSe₂/WSe₂ heterobilayers;
- Jin et al. Observation of moiré excitons in WSe₂/WS₂ heterostructure superlattices;
- Alexeev et al. Resonantly hybridized excitons in moiré superlattices in van der Waals heterostructures.
The two papers from Tran et al and Seyler et al deal with interlayer excitons, where the electrons and holes composing the exciton reside in two different layers. They use this to probe light emission effects. In the first, they show that the twist angle between the two layers can influence the polarisation and the energy of the light emission. In the second, they report individual interlayer excitons, that are trapped in the moiré potential. This allows them to detect polarised light, with a splitting of the emission bands when a magnetic field is applied.
Another type of excitons is possible if the electron and the holes are located on the same layer within the hetero-bilayer. Particularly, this allows light absorption, with different signatures caused by the moiré potential. Jin et al report different absorption features, that they can tune with an applied voltage, revealing the role of intralayer excitons.
The intralayer excitons can be hybridised into interlayer excitons, as Alexeev et al have reported. These hybridised states combine the strong absorption allowed by intralayer excitons and the stability of interlayer excitons under an electric field. These intriguing properties are allowed by the delocalisation of the conduction band over the two layers. With such excitons, the impact of the moiré potential on the optical properties (both absorption and emission) is reportedly amplified.
With all of this, it’s no wonder 2D hetero-bilayers will play a major role in future light technologies. Those four papers show a better understanding of the excitons in this hot material, with the ability to control both the absorption and the emission. My take is that the applied photovoltaics research community has a lot to earn by considering this material for further studies, as it may allow surpassing the Shockley-Queisser efficiency limit for solar cells.
Finally, I’m happy to share I’ll participate in the workshop ”Moiré in Paris” on 3-4 June to discuss these incredible structures.