Abstract
A ferroelectric is a material with a polar structure whose polarity can be reversed (switched) by applying an electric field1,2. In metals, itinerant electrons screen electrostatic forces between ions, which explains in part why polar metals are very rare3,4,5,6,7. Screening also excludes external electric fields, apparently ruling out the possibility of ferroelectric switching. However, in principle, a thin enough polar metal could be sufficiently penetrated by an electric field to have its polarity switched. Here we show that the topological semimetal WTe2 provides an embodiment of this principle. Although monolayer WTe2 is centro-symmetric and thus non-polar, the stacked bulk structure is polar. We find that two- or three-layer WTe2 exhibits spontaneous out-of-plane electric polarization that can be switched using gate electrodes. We directly detect and quantify the polarization using graphene as an electric-field sensor8. Moreover, the polarization states can be differentiated by conductivity and the carrier density can be varied to modify the properties. The temperature at which polarization vanishes is above 350 kelvin, and even when WTe2 is sandwiched between graphene layers it retains its switching capability at room temperature, demonstrating a robustness suitable for applications in combination with other two-dimensional materials9,10,11,12.
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Acknowledgements
We thank J. Folk, E. Sajadi, A. Levanyuk, T. Birol and A. Andreev for substantial insights. D.H.C. and X.X. were supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under awards DE-SC0002197 and DE-SC0018171, respectively. Synthesis efforts at ORNL were also supported by the same division of the Department of Energy. Z.Z. was partially supported by the CEM, and NSF MRSEC, under grant DMR-1420451. T.A.P. was supported by AFOSR FA9550-14-1-0277. Z.F., W.Z. and B.S. were supported by the above awards and also by NSF EFRI 2DARE 1433496 and NSF MRSEC 1719797.
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Nature thanks L. Bartels, T. Birol and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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D.H.C. conceived the experiments; Z.Z. and J.Y. grew the crystals; W.Z., T.A.P., Z.F. and M.K.M. fabricated the devices; Z.F., W.Z., T.A.P. and B.S. performed the measurements; D.H.C., X.X., Z.F., W.Z. and T.A.P. analysed the results; and D.H.C., Z.F., T.A.P. and X.X. wrote the paper with comments from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Bilayer WTe2 device.
a, Essential steps in device fabrication. b, Optical image of device B4. The red dashed line outlines the bilayer flake. Scale bar, 5 μm. c, AFM topography image of the central region in b. Scale bar, 2 μm. d, Line cut along the white dashed line in c. The step height matches the expected bilayer thickness, about 1.4 nm.
Extended Data Fig. 2 Thick WTe2 used as a gate.
a, Optical image of device F1, in which a thick (8 nm) WTe2 flake under 24-nm h-BN is used as a gate for a top graphene sheet. Scale bar, 10 μm. b, Schematic cross-section of the device. c, Two-terminal conductance G of the graphene as a function of voltage Vg applied to the WTe2 flake. There is no sign of switching or bistability at any temperature, indicating that no polarization reversal occurs on the WTe2 surface for fields of up to E⊥ ≈ 0.125 V nm−1. Inset, close-ups of the graphene Dirac point at 4 K and 300 K.
Extended Data Fig. 3 Switching of an additional bilayer device.
a, Conductance G versus perpendicular electric field E⊥, at temperatures from 4 K to 300 K and a gate doping level of ne = −4 × 1012 cm−2, for device B4. b, Conductance difference ΔG between the two sweep directions of Vb at 200 K, as plotted in Fig. 3d for device B1.
Extended Data Fig. 4 Additional transport measurements and removal of parasitic effects in the polarization measurements.
a, Conductance G versus Vb for the bilayer WTe2 in device B2, measured with the top graphene grounded. The hysteresis occurs in exactly the same range of E⊥ as it does in the graphene conductance in Fig. 2b. Note that both ne and E⊥ change when Vb is swept. The inset shows a schematic configuration of the measurement. b, Graphene conductance Ggr at 220 K as a function of Vb with the voltage VW on the bilayer WTe2 at 0 mV (blue) and 129 mV (red). The black curve is the difference between the blue and red curves. This subtraction removes most of the Vb dependence of the parasitic current that flows through the top graphene, which is not screened from the bottom gate by the WTe2. Inset, graphene conductance showing the minimum at VW = 129 mV.
Extended Data Fig. 5 Graphene/bilayer WTe2 heterostructure showing hysteresis up to room temperature.
a, b, Device image (a) and schematic cross-section (b). c, The two-terminal conductance G shows bistability at both 5 K and room temperature (300 K), implying that the polarization of the WTe2 is still present in this hybrid structure.
Extended Data Fig. 6 Length-dependent ferroelectric behaviour in trilayer WTe2 for temperatures from 2 K to 300 K.
All measurements are performed at Vb = 0 in two-terminal configurations, where the contact separation ranges from 200 nm to 1,490 nm. For all devices mentioned above and in the main text, the contacts are separated by 1–2 μm. However, if we reduce the contact separation to a few hundred nanometers (270 nm), the metal contacts prevent the polarization from switching. For a contact separation (L) of more than 480 nm, the transfer characteristics show similar hysteric behaviour as in Fig. 1c, d and Extended Data Fig. 3a. Because Vb is always grounded, E⊥ and ne change simultaneously as we sweep Vt.
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Fei, Z., Zhao, W., Palomaki, T.A. et al. Ferroelectric switching of a two-dimensional metal. Nature 560, 336–339 (2018). https://doi.org/10.1038/s41586-018-0336-3
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DOI: https://doi.org/10.1038/s41586-018-0336-3
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