In a study conducted at beamline Bloch, the complete band structure of one-dimensional graphene nanoribbons was mapped for the first time using ARPES. The nanoribbons, grown on a substrate that is suitable for upscaling, have a width dependent bandgap important for device integration. The team of researchers from Germany, Sweden and Ireland recently published their results in Nature Communications.
“The importance of our study is twofold – it presents significant technological implications, while simultaneously showing the fascinating behaviour of the quantum world,” says Hrag Karakachian, PhD student at the Max Planck Institute for Solid State Research in Stuttgart and first author of the study.
The nanoribbons were grown on a silicon carbide so-called mesa structure. It’s a substrate patterned with ridges exposing the face of the silicon carbide needed to grow these specific nanoribbons. The resulting ribbons will have a width dependent bandgap, something that is important for future integration in electronics.
“As for the technological use – for almost two decades, lots of excitement has surrounded graphene, regarding it as the future building block of electronic circuitry in the post-silicon era,” continues Karakachian. “However, since its discovery, graphene is still absent from our daily home appliances, mainly because of its metallic character, i.e., the lack of a sizeable bandgap in its electronic structure.”
The growth substrate is important as it will play a role in device integration and upscaling to later industrial use.
“In our work, we have demonstrated that through one-dimensional (1D) confinement, a bandgap can be generated in graphene, whose size solely depends on the width of the graphene nanoribbon (GNR). So far, the synthesis of gapped GNRs has mostly been carried out on metallic surfaces such as gold, thus making them impractical for technologically relevant applications. Our ability to grow high-quality, well-aligned and densely packed GNRs on a wafer scale, while using semiconducting SiC as a substrate, is a major accomplishment. This gives the opportunity to exploit the electronic properties of GNRs, hence allowing the realization of new transistor design concepts,” continues Karakachian.
The study also confirms theoretical predictions of the graphene band structure. For those of us who have taken basic quantum mechanics – this is a real-world particle in a box problem.
“Theoretical band structure calculations have predicted the development of a width-dependent bandgap in graphene nanoribbons even before the experimental discovery of a pristine 2D graphene sheet. To date, we are the first group to experimentally provide complete mapping of the electronic structure of the nanoribbons, where distinct sets of subbands are measured, originating from semiconducting and metallic ribbons of slightly different widths,” says Karakachian. “Furthermore, the Dirac-like dispersion of the electrons along the ribbons, as opposed to their non-dispersive nature across the ribbons, is a testament to the ideal 1D character of our system, which verifies earlier theoretical predictions and deepens our understanding of quantum confinement phenomena. In layman’s physics terms, this is textbook quantum mechanics applied to Dirac fermions – a “particle in a box”-type behaviour.”
The team has taken advantage of the Bloch beamline, named after the Swiss-American Nobel laureate Felix Bloch, famous for his works on electron behaviour in crystals.
“Bloch is a newly developed beamline with state-of-the-art equipment. This presents many technical advantages when it comes to resolving intricate and essential details in the experiments, otherwise impossible to measure in a home lab or a relatively old synchrotron facility,” says Karakachian. “More importantly, the beamline is run by highly competent people. The staff managing the experiments is extremely helpful; they are genuinely excited about conducting basic scientific research and are very interested in our project. Our experiences at Bloch have always been positive, and our mutual collaboration will go beyond this single yet important publication in Nature Communications.”
The team is now looking to combine the graphene nanoribbons with other elements through a method called intercalation.
“Our interface analysis group in Stuttgart, headed by Prof. Dr Ulrich Starke, has a great deal of experience in functionalizing epitaxial graphene via intercalation. This method involves the incorporation of foreign atomic elements at the interface between graphene and the underlying SiC substrate, thereby modifying the electronic properties of the graphitic layer. Our next step, therefore, is to combine these two functionalization techniques, i.e., 1D confinement and intercalation, and explore the fascinating properties that would emerge,” concludes Karakachian. “Exciting times are ahead!”
Karakachian, T. T. Nhung Nguyen, J. Aprojanz, A. A. Zakharov, R. Yakimova, Ph. Rosenzweig, C. M. Polley, T. Balasubramanian, C. Tegenkamp, S. R. Power, and U. Starke. One-dimensional confinement and width-dependent bandgap formation in epitaxial graphene nanoribbons. Nat. Commun. 11, 6380 (2020).
Further papers from the MPG group on intercalation of graphene
Ph. Rosenzweig, H. Karakachian, D. Marchenko, K. Küster, and U. Starke. Overdoping graphene beyond the van Hove singularity. Phys. Rev. Lett. 125, 176403 (2020).
Ph. Rosenzweig, H. Karakachian, S. Link, K. Küster, and U. Starke. Tuning the doping level of graphene in the vicinity of the van Hove singularity via ytterbium intercalation. Phys. Rev. B 100, 035445 (2019).
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