Figure 1. Design principle of JGNRs. The right edge of the JGNRs retains the typical zigzag structure, while the left edge is decorated with patterns of defects. The parameter denotes the distance (measured in number of missing six-fold rings) between the additional benzene rings. Red and blue arrows represent the spin-down and spin-up states of the electronic edge states, respectively. Red and blue balls represent carbon atoms on the two sublattice sites of the honeycomb structure of graphene. The conventional symmetric ZGNRs can be regarded as a special case where . The spin polarization of JGNR is localized entirely on the pristine zigzag edge, whereas in the ZGNRs, spin polarization exists on both edges with opposite directions.
Published in Nature, January 8, 2025
Janus graphene nanoribbons with novel magnetic quantum phases
A joint experiment-theory study, conducted by a theory team led by Professor Steven Louie (University of California Berkeley) in collaboration with experiment teams led by Professors Jiong Lu (National University of Singapore) and Hiroshi Sakaguchi (Kyoto University), has made a groundbreaking discovery in the design, synthesis and physical understanding of a novel type of graphene nanoribbons – named by the researchers as Janus graphene nanoribbons (JGNRs) – with unforeseen and tunable magnetic behaviors. These novel materials open exciting possibilities in quantum magnetism and spin-based technologies. This work was published in the journal Nature (Nature 637, 580 (2025)).
Graphene nanoribbons with zigzag edges (ZGNRs) are narrow strips of graphene (a single atomic layer of carbon in a honeycomb structure) with nanoscale width. The ZGNRs have been predicted to exhibit remarkable magnetic properties at their edges due to the behavior of specific edge-states of the π electrons. Until now, only two symmetric ZGNRs (i.e., having identical atomic structure on both edges) had been synthesized: the 6-ZGNR (Nature 531, 489 (2016)) and the nitrogen-doped 6-ZGNR (Nature 600, 647 (2021)), where “6” refers to the number of carbonrows forming the width of the ZGNR. It has been a long-sought goal of the research community to make other forms of zigzag-edge related GNRs with exotic magnetic quantum states for explorations of new science and applications.
In this study, guided by the topological classification theory from the Louie group’s previous research (Nano Lett. 21, 197 (2021)), it is predicted that unique and tunable magnetic behaviors can be achieved by having ZGNRs with asymmetric edges. These nanoribbons feature one pristine zigzag edge and another edge decorated with a pattern of topological defects.
Janus is the name of the two-faced ancient Roman god. The term “Janus” has been used in condensed matter physics and materials science to describe materials with different properties on opposite sides. However, designing and fabricating JGNRs with complex structural and magnetic properties have remained a major experimental challenge.
Using innovative "Z-shaped" precursor molecules for synthesis, the experimentalists in the collaboration have successfully made JGNRS, controlling the structure of the two edges independently. Two kinds of JGNRs were fabricated – both with one edge having a benzene motif array with a spacing of m=2 missing benzene rings in between and the other edge being the conventional zigzag edge. (Fig. 1.)
The Louie group’s topological classification theory predicts that the magnetic behavior of such JGNR may be controlled, ranging from antiferromagnetism to ferrimagnetism to ferromagnetism, by varying the value of m. It leads to a robust design principle for the JGNRs. In particular, the configuration of m=2 is predicted to exhibit a ferromagnetic ground state, with electron spin polarization localized entirely on the pristine zigzag edge. This behavior contrasts sharply with that of the symmetric ZGNRs, where spin polarization occurs on both edges and the aligned edge spins are antiferromagnetically coupled across the width of the ribbon. The results of the study, which combined topological theory with synthesis, scanning tunneling microscopy and spectroscopy measurements as well as density functional theory simulations, demonstrate the emergence of novel magnetic phases that can be controlled and tuned with structure changes. (Fig. 2)
The rational design and on-surface synthesis of this novel class of JGNRs incorporate both conceptual and experimental advances for engineering magnetism in quasi one-dimensional (1D) systems. Realizing such JGNRs expands the design space for precise engineering of exotic quantum magnetism, enabling perhaps the creation of robust spin centers as promising qubit platforms through bottom-up assembly as well as allowing potentially the fabrication of 1D spin-polarized transport channels with tunable bandgaps for carbon-based spintronics in the 1D limit.
Figure 2. Validation of the edge states. Both the experimental scanning tunneling spectroscopy measurements (left panel) and theoretical density functional theory (DFT) calculations (right panel) of electron distributions (color maps) confirm the existence of edge states on only the non-defective edge of the m=2 JGNR. Furthermore, topological classification theory and DFT calculations reveal that these edge states are fully spin-polarized, showing the realization of a single ferromagnetically spin-polarized edge in this class of JGNR systems.