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A few-layer covalent network of fullerenes

Abstract

The two natural allotropes of carbon, diamond and graphite, are extended networks of sp3-hybridized and sp2-hybridized atoms, respectively1. By mixing different hybridizations and geometries of carbon, one could conceptually construct countless synthetic allotropes. Here we introduce graphullerene, a two-dimensional crystalline polymer of C60 that bridges the gulf between molecular and extended carbon materials. Its constituent fullerene subunits arrange hexagonally in a covalently interconnected molecular sheet. We report charge-neutral, purely carbon-based macroscopic crystals that are large enough to be mechanically exfoliated to produce molecularly thin flakes with clean interfaces—a critical requirement for the creation of heterostructures and optoelectronic devices2. The synthesis entails growing single crystals of layered polymeric (Mg4C60) by chemical vapour transport and subsequently removing the magnesium with dilute acid. We explore the thermal conductivity of this material and find it to be much higher than that of molecular C60, which is a consequence of the in-plane covalent bonding. Furthermore, imaging few-layer graphullerene flakes using transmission electron microscopy and near-field nano-photoluminescence spectroscopy reveals the existence of moiré-like superlattices3. More broadly, the synthesis of extended carbon structures by polymerization of molecular precursors charts a clear path to the systematic design of materials for the construction of two-dimensional heterostructures with tunable optoelectronic properties.

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Fig. 1: Carbon allotropes.
Fig. 2: Synthesis and crystal structures of (Mg4C60).
Fig. 3: Mg deintercalation and mechanical exfoliation to produce graphullerene.
Fig. 4: Photoluminescence and scanning near-field optical microscopy.
Fig. 5: Thermal transport properties of graphullerite.

Data availability

The data that support the findings of this study are present in the paper and its Extended Data. The crystallographic data presented in this work are available through the Cambridge Crystallographic Data Centre (CCDC) referencing deposition no. 2151576. Further data are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work was primarily funded by the Columbia MRSEC on Precision-Assembled Quantum Materials (PAQM) under award number DMR-2011738, and the Air Force Office of Scientific Research under grant FA9550-22-1-0389. Surface characterization of graphullerene was supported by the NSF CAREER award DMR-1751949 (X.R.). Calorimetry measurements were supported by the NSF award CBET-2017198 (X.R.). C.N. acknowledges support from the Office of Naval Research Award N00014-20-1-2477, and thanks S. Buckler and D. Buckler for their generous support. M.M., C.J.D., A.G. and P.E.H. acknowledge support from the Office of Naval Research, grant numbers N00014-20-1-2686 and N00014-21-1-2622. Infrared nano-imaging experiments are supported under Energy Frontier Research Center on Programmable Quantum Materials funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under award DE-SC0019443. E.M. was supported in part by the Yad Hanadiv and Weizmann Women in Science Award for postdoctoral fellowship support. A.M.E. was supported by the Schmidt Science Fellows, in partnership with the Rhodes Trust. M.R and P.K. were supported by Army Research Office under Award W911NF-18-1-0366. R.A.W. and A.K.B. were supported by Arnold O. Beckman Fellowships in Chemical Sciences. T.H. acknowledges the postdoctoral fellowship support of JSPS Overseas Research Fellowships. X-ray diffraction, electron microscopy, AFM, zeta-potential and TGA measurements were performed in the Shared Materials Characterization Laboratory at Columbia University. We thank L. M. Campos and E. M. Churchill for their assistance with the DSC measurements. Raman spectroscopy measurements were supported by the Barnard College Department of Chemistry and Office of the Provost. We thank N. Cislo for assistance with early Raman spectroscopy measurements.

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Authors and Affiliations

Authors

Contributions

Conceptualization: E.M., J.Y., M.L.S., C.N., X.R. Methodology and investigation: E.M., A.M.E., S.T.B., A.K.B., R.A.W. Electron microscopy: A.M.E., A.Z. Device fabrication and electrical transport: M.Rezaee., P.K. Photoluminescence: T.H., X.Z., T.P.D, N.F.-M., P.J.S. s-SNOM: D.J.R., D.N.B. Raman spectroscopy: M.Reza., A.C.C. Thermal properties: M.M., C.J.D., A.G., P.E.H. Writing: E.M., A.M.E., M.L.S., C.N., X.R. Supervision: M.L.S., C.N., X.R.

Corresponding authors

Correspondence to Elena Meirzadeh, Michael L. Steigerwald, Jingjing Yang, Colin Nuckolls or Xavier Roy.

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Nature thanks Andreas Hirsch and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Electrical transport properties of (Mg4C60).

Electrical conductivity (σ) of (Mg4C60) versus temperature.

Extended Data Fig. 2 Powder X-ray diffraction of C60 polymers.

PXRD pattern of graphullerite (C60), (Mg4C60), and molecular C60. The red vertical lines represent the calculated peak positions for (Mg4C60), assuming a preferred orientation and 1.8 Å expansion along the a axis, the stacking direction.

Extended Data Fig. 3 Raman spectra and thermal stability of C60 polymers.

a, Raman spectra of (Mg4C60), graphullerite (C60), and mechanically exfoliated bilayer graphullerene (2L). The purple vertical line denotes the pentagonal pinch mode for molecular C60 (1,469 cm–1). The Ag(2) mode of graphullerite and graphullerene may overlap with the broad Hg(7) mode at 1,420 cm–1. b, Raman spectra of graphullerite after annealing at 400 and 600 °C for 1 h. The Ag(2) mode characteristic of molecular C60 appears at 1,469 cm–1 after annealing at 600 °C.

Extended Data Fig. 4 Calorimetry and thermogravimetric analysis of C60 polymers.

a, DSC traces of (Mg4C60) and graphullerite crystals. The heat flow was measured as a function of temperature at a constant temperature ramp of 10 °C min–1 during heating and 5 °C min–1 during cooling with a 50 ml /min1 nitrogen cell purge flow. b, Weight loss as a function of temperature for molecular C60 and (Mg4C60) under N2 at a constant temperature ramp of 10 °C min–1, measured by TGA. The 6% increase in the (Mg4C60) data is presumably due to the formation of MgO.

Extended Data Fig. 5 Powder X-ray diffraction of graphullerite after thermal treatment.

The PXRD patterns of graphullerite after annealing in vacuo for 1 h at different temperatures, along with the patterns of (Mg4C60) and molecular C60. Above 500 °C the covalently bonded sheets depolymerize, and molecular C60 peaks emerge and become more intense above 800 °C.

Extended Data Fig. 6 Zeta potential of graphullerite suspended in isopropanol.

Inset: image of the suspension used for this measurement.

Extended Data Fig. 7 Transmission electron microscopy image of graphullerene.

The layered structure of graphullerene is apparent in this low-magnification image. Our best estimate for the number of layers within the selected area (black box) is 2–4 based on how the contrast increases stepwise from the edge of the flake towards the selected area.

Extended Data Fig. 8 Photoluminescence of (Mg4C60).

a, The open circles represent the experimental data and the solid line is the fitting result based on the Franck–Condon model. b, PL intensity as a function of excitation laser power.

Extended Data Fig. 9 Thermal property characterization.

a, Thermal conductivity measurements on graphullerite were conducted using a fibre-based SSTR set-up similar to the schematic shown. b, The inverse of thermal conductivity versus the inverse of the simulation domain length.

Extended Data Table 1 Crystallographic data for (Mg4C60)

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Meirzadeh, E., Evans, A.M., Rezaee, M. et al. A few-layer covalent network of fullerenes. Nature 613, 71–76 (2023). https://doi.org/10.1038/s41586-022-05401-w

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