Two-dimensional (2D) carbon materials, such as graphene, have attracted particular attention owing to the exceptional carrier transport characteristics that arise from the unique π-electron system in their conjugated carbon network structure1,2,3,4. To complement zero-bandgap graphene, material scientists have devoted considerable effort to identifying 2D carbon materials5,6,7,8. However, it is a challenge to prepare large-sized single-crystal 2D carbon materials with moderate bandgaps5,9. Here we prepare a single-crystal 2D carbon material, namely monolayer quasi-hexagonal-phase fullerene (C60), with a large size via an interlayer bonding cleavage strategy. In this monolayer polymeric C60, cluster cages of C60 are covalently bonded with each other in a plane, forming a regular topology that is distinct from that in conventional 2D materials. Monolayer polymeric C60 exhibits high crystallinity and good thermodynamic stability, and the electronic band structure measurement reveals a transport bandgap of about 1.6 electronvolts. Furthermore, an asymmetric lattice structure endows monolayer polymeric C60 with notable in-plane anisotropic properties, including anisotropic phonon modes and conductivity. This 2D carbon material with a moderate bandgap and unique topological structure offers an interesting platform for potential application in 2D electronic devices.
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J.Z. acknowledges support from the National Key Research and Development Program of China 2017YF0204700, the Key Research Program of the Chinese Academy of Sciences, grant number XDPB13, and the National Natural Science Foundation (22175184, 22105207). We thank Y. Cheng for the assistance with FIB-SEM; X. Hao and T. Liang for the assistance with the single-crystal XRD data analysis; and Y. Zou for the assistance with the UPS and LEIPS analyses.
The authors declare no competing interests.
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Extended data figures and tables
a, In qHP C60, each C60 cage forms two [2+2] cycloadditon C-C bonds and four C-C single bonds with neighbouring C60 cages. Thus there are only 52 carbon atoms possess π states in each C60 cage. b, Differing from that in qHP C60, two [2+2] cycloadditon C-C bonds and two C-C single bonds are formed with neighbouring C60 cages in qTP C60, therefore 54 carbon atoms possess π states in each C60 cage.
a, Magnesium salicylate is obtained from the reaction system by chromatographic separation. The peak at 297.02543 corresponds to [M-H]- signal of magnesium salicylate. b, XRD spectra of Mg intercalated qHP C60 before (red) and after (black) ion-exchange reaction. In Mg intercalated bulk qHP C60, interlayer distance is characterized by (200) peak in 2θ = 10.7°. After ion-exchange reaction, the (200) peak moves to lower 2θ position, which indicate the expanding of interlayer distance. The peak in 2θ = 8.8° (interlayer distance of 10 Å) is attributed to the intercalation of NBu4+ ions. As NBu4+ ions are flexible, the expanded (200) peaks in exfoliated polymeric C60 are broader than that in bulk crystals.
Extended Data Fig. 3 Statistical analysis of the lateral sizes and proportion of monolayer for exfoliated qHP C60 flakes.
a, Typical optical image of the exfoliated qHP C60 flakes on SiO2/Si substrate. Scale bar, 50 μm. b, The statistics of lateral sizes of monolayer qHP C60. c, Statistics of thickness of exfoliated qHP C60 flakes.
Tauc plot to determine the optical bandgap of qHP C60, with extrapolation of the linear region (red line) estimating of approximately 1.55 eV.
a, Transfer characteristics of monolayer qHP C60 transistors. Inset is a schematic of the device. b, Output characteristics of the device in a.
Extended Data Fig. 6 Environment stability (a, b) and thermal stability (c) of the exfoliated qHP C60.
a, b, Photograph of exfoliated qHP C60 nanosheets dispersed in NMP before (a) and after (b) standing in air for one month. The solution retained clear without sediment or aggregation. c, Raman spectra and optical images of exfoliated qHP C60 flakes before (red) and after (black) heating at 600 K for ten minutes. The Raman spectrum and optical image remain unchanged after heating, indicating that the polymeric C60 frameworks in exfoliated qHP C60 do not decompose at 600 K.
For monolayer qHP C60, peak of Mg 1s is absent, and the appearing N 1s peak at 401 eV binding energy is typical for quaternary ammonium groups. The O 1s (532 eV) and O 1s (533 eV) peaks correspond to oxygen from surrounding environment and the SiO2 substrate, respectively. Meanwhile, absence of typical carboxyl group peak in O 1s (535 eV) indicates that there is no residual salicylate on monolayer flakes.
C 1s peak can be consistently deconvoluted into several components attributed to sp2-hybridized C-C (284.6 eV), sp3-hybridized C-C (284.9 eV), sp3-hybridized C-N (286.8 eV) with approximately 52:80:24 intensity ratios, in agreement with the C60/NBu4+ ratio of 1:6 in monolayer qHP flakes. Moreover, a series of π-π* shake-up satellite peaks of C60 are identified in 286.2eV, 288.3 eV, 289.4 eV and 290.6 eV.
Extended Data Fig. 9 Zeta potential distribution of monolayer qHP C60 solution before (black) and after (red) treatment with hydrogen peroxide.
Average zeta potential of monolayer qHP C60 solution is calculated as −40.2 mV, indicating the negative charge on monolayer flakes. After oxidization, negative charges are neutralized, and average zeta potential turns to −8.4 mV.
The absence of N 1s peak indicates the removal of NBu4+ cations on the monolayer after the treatment of H2O2. Besides, C-O bonds are not formed, because typical C-O peak at 286–287 eV is not found. The O 1s peak is attributed to the adsorbed oxygen from the surrounding environment.
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Hou, L., Cui, X., Guan, B. et al. Synthesis of a monolayer fullerene network. Nature 606, 507–510 (2022). https://doi.org/10.1038/s41586-022-04771-5
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