Twisting Layers of Graphene Produces More Control of Electrical Properties

‘Twisted’ layers of 2D materials produce photonic topological transition at ‘magic’ rotation angles.

Graphene has already been discovered to have excellent properties to help with conductivity but with a little “twist” – there’s more benefits.

Monash researchers are part of an international collaboration applying ‘twistronics’ concepts (the science of layering and twisting 2D materials to control their electrical properties) to manipulate the flow of light in extreme ways.

The findings, published today in the journal Nature, hold the promise for leapfrog advances in a variety of light-driven technologies, including nano-imaging devices; high-speed, low-energy optical computers; and biosensors.

This is the first application of Moire physics and twistronics to the light-based technologies, photonics, and polaritonics, opening unique opportunities for extreme photonic dispersion engineering and robust control of polaritons on 2D materials.

Applying Twistronics to Photons

The team took inspiration from the recent discovery of superconductivity in a pair of stacked graphene layers that were rotated to the ‘magic twist angle’ of 1.1 degrees.

In this stacked, misaligned configuration, electrons flow with no resistance, while separately, each of the two graphene layers shows no special electrical properties.

The discovery has shown how the careful control of rotational symmetries can unveil unexpected material responses.

Twisted Bi-Layer (tBL) α-MoO3

Twisted bi-layer (tBL) α-MoO3. Credit: FLEET

The research team was led by Andrea Alù at the Advanced Science Research Center at the Graduate Center, CUNY, Cheng-Wei Qiu at National University of Singapore and Qiaoliang Bao formerly at Monash University.

The team discovered that an analogous principle can be applied to manipulate light in highly unusual ways. At a specific rotation angle between two ultrathin layers of molybdenum-trioxide, the researchers were able to prevent optical diffraction and enable robust light propagation in a tightly focused beam at desired wavelengths.

Typically, light radiated from a small emitter placed over a flat surface expands away in circles very much like the waves excited by a stone that falls into a pond. In their experiments, the researchers stacked two thin sheets of molybdenum-trioxide and rotated one of the layers with respect to the other. When the materials were excited by a tiny optical emitter, they observed widely controllable light waves over the surface as the rotation angle was varied. In particular, they showed that at the photonic ‘magical twist angle’ the configured bilayer supports robust, diffraction-free light propagation in tightly focused channel beams over a wide range of wavelengths.

“While photons — the quanta of light — have very different physical properties than electrons, we have been intrigued by the emerging discovery of twistronics, and have been wondering if twisted two-dimensional materials may also provide unusual transport properties for light, to benefit photon-based technologies,” said Andrea Alù.

Qingdong Ou

Dr Qingdong Ou (Monash University) with s-SNOM facility at Melbourne Centre for Nanofabrication (MCN). Credit: FLEET

“To unveil this phenomenon, we used thin layers of molybdenum trioxide. By stacking two of such layers on top of each other and controlling their relative rotation, we have observed dramatic control of the light guiding properties. At the photonic magic angle, light does not diffract, and it propagates very confined along straight lines. This is an ideal feature for nanoscience and photonic technologies.”

“Our experiments were far beyond our expectations,” said Dr Qingdong Ou, who led the experimental component of the study at Monash University. “By stacking ‘with a twist’ two thin slabs of a natural 2D material, we can manipulate infrared light propagation, most intriguingly, in a highly collimated style.”

“Our study shows that twistronics for photons can open truly exciting opportunities for light-based technologies, and we are excited to continue exploring these opportunities,” said National University of Singapore graduate student Guangwei Hu, who led the theoretical component.

“Following our previous discovery published in Nature in 2018, we found that biaxial van der Waals semiconductors like α-MoO3 and V2O5 represent an emerging family of material supporting exotic polaritonic behaviors,” said A/Prof Qiaoliang Bao, “These natural-born hyperbolic materials offer an unprecedented platform for controlling the flow of energy at the nanoscale.”

John Weathersby, CEO of the National Graphene2D Association is excited for the possibilities this brings to real-world applications.

“This is another step in channelling the unique properties of 2D materials to open the door to applications we’ve only begun to imagine,” Weathersby said. “Using 2D materials to manipulate light sources will get us close to flexible display services that could be applied to almost any device, from phone and computer screens to self-adjusting contact lenses.  It’s exciting to think of the discoveries and applications that will come from this field in the near future.”

SOURCE: Sci-Tech Daily

The National Graphene2D Association (NGA2D) is the leading organization promoting the commercialization of graphene and graphene-like 2D materials in the United States.  NGA2D members represent commercial graphene development, production, manufacturing and investment stakeholders as well as academic research institutions. For additional information visit www.nga2d.com