Abstract
Topological design of π electrons in zigzag-edged graphene nanoribbons (ZGNRs) leads to a wealth of magnetic quantum phenomena and exotic quantum phases1,2,3,4,5,6,7,8,9,10. Symmetric ZGNRs typically show antiferromagnetically coupled spin-ordered edge states1,2. Eliminating cross-edge magnetic coupling in ZGNRs not only enables the realization of a class of ferromagnetic quantum spin chains11, enabling the exploration of quantum spin physics and entanglement of multiple qubits in the one-dimensional limit3,12, but also establishes a long-sought-after carbon-based ferromagnetic transport channel, pivotal for ultimate scaling of GNR-based quantum electronics1,2,3,9,13. Here we report a general approach for designing and fabricating such ferromagnetic GNRs in the form of Janus GNRs (JGNRs) with two distinct edge configurations. Guided by Lieb’s theorem and topological classification theory14,15,16, we devised two JGNRs by asymmetrically introducing a topological defect array of benzene motifs to one zigzag edge, while keeping the opposing zigzag edge unchanged. This breaks the structural symmetry and creates a sublattice imbalance within each unit cell, initiating a spin-symmetry breaking. Three Z-shaped precursors are designed to fabricate one parent ZGNR and two JGNRs with an optimal lattice spacing of the defect array for a complete quench of the magnetic edge states at the ‘defective’ edge. Characterization by scanning probe microscopy and spectroscopy and first-principles density functional theory confirms the successful fabrication of JGNRs with a ferromagnetic ground-state localized along the pristine zigzag edge.
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Data availability
All data are available in the paper or the Supplementary Information. The scanning probe microscopy spectra data and theoretical calculation results are available in the Zenodo repository at https://doi.org/10.5281/zenodo.13894455 (ref. 49).
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Acknowledgements
J.L. acknowledges support from the NRF, Prime Minister’s Office, Singapore, under the Competitive Research Program Award (NRF-CRP29-2022-0004), MOE grants (MOE T2EP50121-0008 and MOE-T2EP10221-0005) and Agency for Science, Technology and Research (A*STAR) under its AME IRG Grant (Project715 number M21K2c0113). S.G.L. acknowledges the support from the US National Science Foundation under grant number DMR-2325410, which provided all the theoretical topological formulation and analyses as well as the DFT calculations. H.S. acknowledges the support from KAKENHI programme number 22H01891. T.K. acknowledges the support from KAKENHI programme number 23K04521. S.S. acknowledges the support from A*STAR under its AME YIRG Grant (M22K3c0094).
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J.L. supervised the project and organized the collaboration. S.S. and J.L. conceived and designed the experiments. S.S. and Y.T. carried out the STM and nc-AFM measurements. Z.X., S.S., T.K. and H.S. synthesized the organic precursors. S.G.L. conceived the topological equivalence of the defect edge of JGNR with the end of AGNR, and supervised the theoretical analyses and calculations. W.T., J.R. and Y.H. performed the theoretical studies and DFT calculations. F.J.G. assisted in the optimization of the nc-AFM measurements and instrumentation. W.H. participated in scientific discussion. S.S., W.T., S.G.L. and J.L. wrote the paper with inputs from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Schematic illustration of the correspondence between the end of a wide AGNR and the edge of JGNR.
The unit cell of the AGNR, commensurate with its termination, is enclosed by an orange dashed curve. The quantities \(W\) and \({W}_{{notco}}\) in Eq. (1) are to be counted within this specific unit cell. The unit cell of the finite-size JGNR is defined by the area between two grey lines, where 1, 2, …, L label the unit cells. A sufficiently wide AGNR can always be found that shares the same termination as the edge of the JGNR. Through this mapping, the associated AGNR contains L repeated units along its width direction. The unit cell of the wide AGNR, commensurate with its termination, is the area enclosed by the orange dashed curve.
Extended Data Fig. 2 JGNR edge states counting.
Sketch of the defective edge of JGNR with different m value, which is mapped to the end of a large width AGNR with unit cell enclosed by the orange dashed curve that is commensurate with its termination on the left (the topological defect edge of interest). W is the number of carbon rows forming the length of the finite-size JGNR (which is equivalent to the number of carbon rows forming the width of the associated AGNR). \({W}_{{co}}\) and \({W}_{{notco}}\) are the number of connected and unconnected carbon pairs, respectively, in the unit cell of the wide AGNR. L is the number of repeat units along the width of the associated AGNR direction (i.e., \(L\) is number of unit cells forming the length of the finite-size ZGNR of physical interest), and the repeat unit is denoted by the area between the two grey dashed lines. The relationships between L and W, Wco and Wnotco are shown. a, A defect edge with m = 1. b, A defect edge with m = 2. c, A defect edge with m = 3. d, A regular zigzag edge. The repeat unit along the width of AGNR direction is a single benzene ring. These quantities are important in the calculation of Z index.
Extended Data Fig. 3 On-surface synthesis of the 5-ZGNR and JGNRs.
a, e, f. STM topographic image of molecular precursor 1, 2, and 3 as deposited on Au(111), respectively. b, f, j. The STM topographic images of polymer chains obtained by annealing the sample of a, e, f up to 473, 473, and 423 K, respectively. c, g, k. STM topographic images of fully cyclized (5,2)-JGNR, (4,2)-JGNR and 5-ZGNR through a subsequent annealing at 623, 623, and 573 K (Vs = 1 V, It = 100 pA), respectively. d, h, l. Constant current zoom-in STM images of an individual (5,2)-JGNR, (4,2)-JGNR and 5-ZGNR, respectively (Vs = −800 mV, It = 200 pA, CO-functionalized tip).
Extended Data Fig. 4 Spatial localization of edge state of 5-ZGNR and TDZ edge state of (5,2)-JGNR.
a, Constant current STM images of 5-ZGNR, (Vs = −800 mV, It = 200 pA, CO-functionalized tip). b, Colour-coded dI/dV spectra collected as a function of position along the red dotted line (from left to right) in a (Vac = 20 mV, Vs = 1.5 V). The dashed line overlayed on b is the point dI/dV spectrum acquired at the position labelled with a cross in a. c, Constant current STM images of (5,2)-JGNR, (Vs = −800 mV, It = 200 pA, CO-functionalized tip). d, Colour-coded dI/dV spectra collected as a function of position along the red dotted line (from top to bottom) in c (Vac = 20 mV, Vs = 1.5 V). The dashed line overlayed on d is the point dI/dV spectrum acquired at the position labelled with a cross in c.
Extended Data Fig. 5 Bias-dependent dI/dV maps of (4,2)-JGNR across a wide energy window.
a, d, Constant-current dI/dV maps of (4,2)-JGNR recorded at various sample biases marked in b by the dashed vertical lines (Vac = 10 mV). b, dI/dV point spectrum of (4,2)-JGNR on Au(111) acquired at the positions marked in panel c (yellow and green cross). Grey dashed line spectrum represents the Au(111) reference spectrum (Vac = 20 mV). c, Constant current STM images of (4,2)-JGNR, (Vs = 800 mV, It = 200 pA, CO-functionalized tip).
Extended Data Fig. 6 Bias-dependent dI/dV maps of (5,2)-JGNR across a wide energy window.
a, d, Constant-current dI/dV maps of (5,2)-JGNR recorded at various sample biases marked in b by the dashed vertical line (Vac = 10 mV). b, dI/dV point spectrum of (5,2)-JGNR on Au(111) acquired at the position marked in panel c (yellow and green cross). Grey dashed line spectrum represents the Au(111) reference spectrum (Vac = 20 mV). c, Constant current STM images of (5,2)-JGNR, (Vs = −800 mV, It = 200 pA, CO-functionalized tip).
Extended Data Fig. 7 Symmetric design and synthesis of precursor 3.
The synthetic scheme towards the Z-shape backbone by simultaneously constructing the two identical methylphenanthrene branches and its single crystal X-ray diffraction (XRD) structure.
Extended Data Fig. 8 Asymmetric design and synthesis of precursor 2.
The synthetic scheme towards the Z-shape backbone by separately constructing the triphenyl branch and the methylphenanthrene branch of precursor 2 and its single crystal XRD structure.
Extended Data Fig. 9 Asymmetric design and synthesis of precursor 1.
The synthetic scheme towards the Z-shape backbone by separately constructing the biphenyl branch and the methylphenanthrene branch of precursor 1 and its single crystal XRD structure.
Supplementary information
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This file contains Supplementary Figs. 1–15 and Tables 1–3.
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Song, S., Teng, Y., Tang, W. et al. Janus graphene nanoribbons with localized states on a single zigzag edge. Nature (2025). https://doi.org/10.1038/s41586-024-08296-x
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DOI: https://doi.org/10.1038/s41586-024-08296-x