Monolayer atomic crystal molecular superlattices

Abstract

Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials1,2,3. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility4,5,6,7,8. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures9,10,11 but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures12,13,14, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 107, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.

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Figure 1: In situ electrochemical–optical measurement platform to monitor electrochemical intercalation process in real time.
Figure 2: Structural and property evolution from BP to MPMS during the dynamic intercalation process.
Figure 3: TEM characterization of structure evolution from BP to MPMS.
Figure 4: Evolution of electrical properties from BP to MPMS and comparison of stability.
Figure 5: Tunable structural and physical property of MACMS intercalated with different molecules.

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Acknowledgements

The authors acknowledge the Electron Imaging Center for NanoMachines (EICN) at California NanoSystem Institute (CNSI) and Nanoelectronic Research Facility (NRF) at UCLA for technical support. Xiangfeng D. acknowledges support by National Science Foundation DMR1508144 (materials synthesis) and Office of Naval Research through grant number N00014-15-1-2368 (device fabrications). Y.H. acknowledges support by National Science Foundation EFRI-1433541. Y.L. was supported by a Resnick Prize Postdoctoral Fellowship at Caltech. L.L. acknowledges support through the 973 grant of MOST (No. 2013CBA01604). X.H.C. acknowledges support from the National Natural Science Foundation of China (Grant No. 11534010). W.A.G. and Y.L. were also supported by DOE DE-SC0014607. W.A.G acknowledges the Extreme Science and Engineering Discovery Environment (XSEDE) supported by National Science Foundation grant ACI-1053575. Y.L. acknowledges the computational resources sponsored by the DOE’s Office of Energy Efficiency and Renewable Energy and located at the National Renewable Energy Laboratory, and the Texas Advanced Computing Center (TACC). I.S. thanks the Deanship of Scientific Research at King Saud University for its funding of this research through grant PEJP-17-01.

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Contributions

Xiangfeng D., Y.H. and C.W. co-designed the research. C.W. conducted device fabrication, electrical properties measurements and data analysis. C.W., Q.H. and U.H. conducted the intercalation experiments. C.W., U.H., Z.L. and Z.F. conducted structural and optical characterizations. Y.L., H.X. and W.A.G. contributed to the superlattice atomic and electronic structure calculations. E.Z. conducted the TEM studies. Q.H., Xidong D., Y.-C.H., H.W., H.-C.C., I.S. and L.L. contributed to the initial measurement system set-up, preparation of 2D materials and data analysis. R.C. contributed to the initial BP property characterization. N.O.W. contributed to the schematic drawing. G.J.Y. and X.H.C. prepared the initial BP material. Y.H. and Xiangfeng D. supervised the research. Xiangfeng D. and C.W. co-wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to Lei Liao or Yu Huang or Xiangfeng Duan.

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Reviewer Information Nature thanks N. Guisinger, K. Loh and Q. Xiong for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Stepwise reaction mechanism and its partition map.

First derivative of the electrochemical gate current in Fig. 2a. By analysing the original current curve and local minimum of the first derivative, the stepwise reaction can be clearly identified: that is, no major intercalation for 0–1.0 V (over-potential for Br sub-reaction), 1.0–1.4 V for major bulk intercalation, 1.4–2.0 V for few-layer BP formation, 2.0–2.5 V for trilayer BP formation, 2.5–3.0 V for bilayer BP formation and beyond 3.0 V for MPMS formation, which is also consistent with bandgap evolution from bulk, few, trilayer and bilayer to monolayer phosphorene.

Extended Data Figure 2 TEM EDX spectra of BP and MPMS.

a, b, Spectra of BP and MPMS, showing the existence of Br and N after intercalation. Three average spectra gave an atomic ratio of P:N:Br as 33.2:1.2:1.0.

Extended Data Figure 3 Raman spectra characterization of BP and MPMS.

a, Raman spectra to compare the relative peak intensity and full-width at half-maximum evolution from pristine BP (black) to MPMS (red). The MPMS spectrum is multiplied by 20 for easy comparison. b–d, Ag1, B2g and Ag2 mode comparison between pristine BP and MPMS to show redshift, blueshift and blueshift after MPMS formation, respectively. Insets: schematic illustration of atomic motion of each vibration modes.

Extended Data Figure 4 The calculated electronic band structure evolution from BP to MPMS.

a, b, Electronic structure of monolayer phosphorene (a) and MPMS (b), demonstrating the enlarged bandgap from 1.94 eV in monolayer phosphorene to 2.13 eV in MPMS, as determined by the transition from VBM-1 (green) and CBM (red). The newly introduced bands of MPMS marked as grey dotted lines are mainly from bromine atomic p orbitals. The orange VBM-0 band is mainly (about 90%) from phosphorus, but those orbitals contribute little to the optical transition, owing to very small overlap with the CBM. c, d, e, Monolayer phosphorene charge-density distribution of CBM (red in a), VBM-0 (orange in a) and VBM-1 (green in a), showing the transition bandgap determined by CBM and VMB-0/VMB-1 (very close in energy). f, g, h, MPMS charge-density distribution of CBM (red in b), VBM-0 (orange in b) and VBM-1 (green in b), showing the transition bandgap determined by VBM-1 and CBM due to large overlap of charge density.

Extended Data Figure 5 The on/off ratio and mobility of the MPMS devices and the recently reported few-layer and thin BP devices.

Six MPMS devices (red star) show an average mobility of 270 cm2 V−1 s−1 and averaged on/off ratio of 8.6 × 106. For comparison, we list recent studies of few-layer BP (less than 5 nm, marked as the blue triangle) and thin BP (5 nm to 15 nm, marked as the black square) devices. The MPMS devices outperform the best few-layer BP devices in both mobility and on/off ratio, and show comparable mobility but much higher on/off ratio than thin BP devices. Data points indexed are taken from the following: data 1 from ref. 41, data 2 from ref. 28, data 3 from ref. 42, data 4 from ref. 43, data 5 from ref. 27, data 6 from ref. 42, data 7 from ref. 20, data 8 from ref. 44, data 9 from ref. 42, data 10 from ref. 45, data 11 from ref. 46, data 12 from ref. 47, data 13 from ref. 48, data 14 from ref. 49, data 15 from ref. 50, data 16 from ref. 46, data 17 from ref. 19, data 18 from ref. 46, data 19 from ref. 51, data 20 from ref. 52, data 21 from ref. 53, data 22 from ref. 54, data 23 from ref. 55, data 24 from ref. 56, data 25 from ref. 15, data 26 from ref. 26.

Extended Data Figure 6 Lateral BP–MPMS heterojunction.

a, Photoluminescence mapping (at 553 nm) of a lateral BP–MPMS heterostructure to highlight the MPMS part. Scale bar: 3 μm. The signal in the electrode area is due to a scattering-induced background. b, The corresponding Raman spectral mapping centred at 438 cm−1 to show the main BP region with stronger Raman signal. Scale bar: 3 μm. c, SEM image to show the lateral BP–MPMS heterojunction device. Scale bar: 3 μm. d, Schematic illustration of a lateral BP–MPMS heterojunction. e, Band diagram of the BP–MPMS heterojunction. f, The typical diode characteristics of a lateral BP–MPMS heterojunction; inset: optical microscope image of the corresponding BP–MPMS heterojunction. Scale bar: 3 μm.

Extended Data Figure 7 XRD patterns of MACMS obtained from six additional 2DACs.

a, XRD pattern of WSe2 and WSe2/CTAB superlattice verifying the interlayer distance expansion from 6.43 Å (13.76°) of WSe2 (002) peak (black) to 15.20 Å (5.81°) of WSe2/CTAB superlattice (002) peak (red). b, XRD pattern of SnSe and SnSe/CTAB superlattice demonstrating the interlayer distance expansion from 5.74 Å (31.16°) of SnSe (004) peak (black) to 15.62 Å (5.65°) of SnSe/CTAB superlattice (002) peak (red). c, XRD pattern of GeS and GeS/CTAB superlattice showing the interlayer distance expansion from 5.18 Å (34.58°) of GeS (004) peak (black) to 15.76 Å (5.60°) of GeS/CTAB superlattice (002) peak (red). d, XRD pattern of NbSe2 and NbSe2/CTAB superlattice revealing the interlayer distance expansion from 6.18 Å (14.31°) of NbSe2 (002) peak (black) to 14.85 Å (5.95°) of NbSe2/CTAB superlattice (002) peak (red). e, XRD pattern of Bi2Se3 and Bi2Se/CTAB superlattice exhibiting the interlayer distance expansion from 14.16 Å (18.78°) of Bi2Se3 (006) peak (black) to 23.07 Å (11.49°) of Bi2Se3/CTAB superlattice (006) peak (red). f, XRD pattern of In2Se3 and In2Se3/CTAB superlattice indicating the interlayer distance expansion from 9.50 Å (18.67°) of the In2Se3 (004) peak (black) to 15.40 Å (11.48°) of the In2Se3/CTAB superlattice (004) peak (red).

Extended Data Table 1 Key characteristics of MPMS and recently reported few-layer BP

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Wang, C., He, Q., Halim, U. et al. Monolayer atomic crystal molecular superlattices. Nature 555, 231–236 (2018). https://doi.org/10.1038/nature25774

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