Quantum technology holds great promise, but it is also fraught with complexity. Quantum technology is expected to bring a series of technological advances over the next few decades, giving us more compact and more accurate sensors, more powerful and more secure communication networks, and larger-capacity computers. These advances will exceed the capabilities of current computing technologies, helping to rapidly develop new drugs and materials, control financial markets and enhance weather forecasts.
To realize these advantages, we need so-called quantum materials, which display significant quantum physical effects. Graphene is one such material. This two-dimensional structural form of carbon has unusual physical properties, such as ultra-high tensile strength, thermal and electrical conductivity, and certain quantum effects. Further confining this already two-dimensional material, such as giving it a ribbon-like shape, produces a range of controllable quantum effects.
This is exactly what Mickael Perrin's team exploits in their work: For several years, scientists at Empa's Laboratory of Nanointerfacial Transport, led by Michel Calame, have been conducting research on graphene nanoribbons. "Graphene nanoribbons are even more fascinating than graphene itself," explains Perrin. "By changing the length and width of graphene nanoribbons, the shape of their edges, and adding other atoms, you can give them a variety of electrical, magnetic, and optical properties."
Extremely accurate - down to a single atom
Studying promising nanoribbons is no easy task. The narrower the nanoribbon, the more pronounced its quantum properties, but it is also more difficult to obtain individual nanoribbons at the same time. This is necessary to understand the unique properties and possible applications of this quantum material and to distinguish them from collective effects.
In a new study recently published in the journal Nature Electronics, Perrin and Empa researcher Jian Zhang, along with an international team, successfully accessed individual long, atomically precise graphene nanoribbons for the first time. Zhang Jian said: "The width of graphene nanoribbons, which are only 9 carbon atoms wide, is only 1 nanometer. To ensure that only one nanoribbon is contacted, the researchers used electrodes of similar size: the carbon nanotubes they used were also only 1 nanometer in diameter."
For such an elaborate experiment, precision is key. The first is the source material. The researchers obtained the graphene nanoribbons through long-term and close collaboration with Empa's nanotech@Surfaces laboratory led by Roman Fasel. "Roman Fasel and his team have long worked on graphene nanoribbons and can synthesize many different types of graphene nanoribbons with atomic precision from a single precursor molecule," explains Perrin. The precursor molecules came from the Max Planck Institute for Polymer Research in Mainz.
As is often required to drive technological progress, interdisciplinarity is key, and different international research groups are involved, each bringing their own expertise: the carbon nanotubes were grown by a research group at Peking University, and to interpret the results, Empa researchers collaborated with computational scientists at the University of Warwick.
Contacting individual carbon strips with nanotubes poses a huge challenge for researchers. "Carbon nanotubes and graphene nanoribbons are grown on different substrates respectively," Zhang explained. "First, the nanotubes need to be transferred to the device substrate and contacted with metal electrodes. Then, we cut them using high-resolution electron beam lithography to separate them into two electrodes. Finally, we cut the nanotubes into two electrodes." The tapes are transferred to the same substrate. Precision is key: even the slightest rotation of the substrate significantly reduces the probability of successful contact. Having access to the high-quality infrastructure at the IBM Research Center Binnig and Rocher in Lüschlikon is crucial for testing and implementing this technology."
From computers to energy converters
The scientists confirmed the success of the experiment through charge transfer measurements. Since quantum effects are usually more pronounced at low temperatures, we performed measurements in a high vacuum environment close to absolute zero. But he quickly adds another particularly promising property of graphene nanoribbons: "Due to the extremely small size of these nanoribbons, we expect their quantum effects to be very strong and observable even at room temperature." This, the researcher says, will allow us to design and operate chips that actively exploit quantum effects without the need for complex cooling infrastructure.
Professor Hatef Sadeghi of the University of Warwick, who is involved in the project, added: "This project enables the realization of a single nanoribbon device, which not only allows the study of fundamental quantum effects, such as how electrons and phonons behave at the nanoscale, but can also exploit this effect for applications in quantum switching, quantum sensing and quantum energy conversion."
Graphene nanoribbons are not yet ready for commercial applications, and there is still much research to be done. In follow-up research, Zhang and Perrin aim to manipulate different quantum states on a single nanostrip. In addition, they plan to create devices based on two nanoribbons connected in series, forming so-called double quantum dots. Such circuits can serve as qubits, the smallest units of information in quantum computers. In addition, Perrin recently received a Starting Grant from the European Research Council (ERC) and a Sccellenza Professional Fellowship from the Swiss National Science Foundation (SNSF), where he plans to use nanoribbons as efficient energy converters. In his inaugural lecture at ETH Zurich, he pictured a world in which we could exploit temperature differences to generate electricity while losing almost no heat energy - a real quantum leap.
international cooperation
Several research groups made important contributions to the project. The graphene nanoribbons were grown by the Empa Nanotechnology@Surface Laboratory, led by Roman Fasel, from precursor molecules provided by Klaus Müllen's group at the Max-Planck Institute for Polymer Research in Mainz.
These nanoribbons were integrated into nanofabrication devices by members of Empa's Laboratory of Nanoscale Interfacial Transport, led by Michel Calame, who also included Mickael Perrin's research group. The precisely arranged, high-quality carbon nanotubes required for this particular study were provided by Zhang Jin's research group at Peking University. Finally, to interpret the findings, Empa researchers collaborated with computational scientists at the University of Warwick, under the guidance of Hatef Sadeghi.