Abstract
Monolayers of transition-metal dichalcogenides (TMDs) exhibit numerous crystal phases with distinct structures, symmetries and physical properties1,2,3. Exploring the physics of transitions between these different structural phases in two dimensions4 may provide a means of switching material properties, with implications for potential applications. Structural phase transitions in TMDs have so far been induced by thermal or chemical means5,6; purely electrostatic control over crystal phases through electrostatic doping was recently proposed as a theoretical possibility, but has not yet been realized7,8. Here we report the experimental demonstration of an electrostatic-doping-driven phase transition between the hexagonal and monoclinic phases of monolayer molybdenum ditelluride (MoTe2). We find that the phase transition shows a hysteretic loop in Raman spectra, and can be reversed by increasing or decreasing the gate voltage. We also combine second-harmonic generation spectroscopy with polarization-resolved Raman spectroscopy to show that the induced monoclinic phase preserves the crystal orientation of the original hexagonal phase. Moreover, this structural phase transition occurs simultaneously across the whole sample. This electrostatic-doping control of structural phase transition opens up new possibilities for developing phase-change devices based on atomically thin membranes.
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Acknowledgements
We thank S. Zhou, K. Deng and W. Yang from Tsinghua University for providing a 1T′ MoTe2 crystal for reference. This work was supported in part by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the US Department of Energy under contract no. DE-AC02-05CH11231, within the ‘Light-Material Interactions in Energy Conversion’ Energy Frontier Research Center (for optical measurements); under the ‘van der Waals Heterostructures Program’ (for transport studies); and by the National Science Foundation (NSF) under grant EFMA-154274 (for device design and fabrication). Y.L., Y.Z., and E.J.R. acknowledge support from the Army Research Office (grant W911NF-15-1-0570); the Office of Naval Research (grant N00014-15-1-2697); the NSF (grants DMR-1455050 and EECS-1436626); and from the Stanford Graduate Fellowship programme.
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Ying W., J.X. and X.Z. initiated the research and designed the experiments; Ying W., J.X. and H.Z. performed Raman measurements; J.X. and Ying W. conducted gate-dependent SHG spectroscopy and mapping; Y.A. and J.X. prepared monolayer MoTe2; and Ying W. fabricated and characterized devices. Ying W. measured the doping level with assistance from K.Y.F., S.W. and W.S. (W.S. was under the guidance of A.Z); Y.L. and Y.Z. performed theoretical calculations under the guidance of E.J.R.; Ying W., J.X., H.Z., Y.Z, W.S., Yuan W., E.J.R. and X.Z. analysed data; Ying W., J.X., H.Z., Y.Z., Yuan W., E.J.R. and X.Z. wrote the manuscript. X.Z. and Yuan W. guided the work.
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Extended data figures and tables
Extended Data Figure 1 Raman features of pristine 2H-and 1T′-phase monolayer MoTe2 at 220 K.
Raman spectra for the 2H and 1T′ phases of monolayer MoTe2 are plotted in green and red, respectively. The Raman modes at 171.5 cm−1 and 236 cm−1 are the 
and 
oscillation modes, belonging to the 2H phase. Excited by the same wavelength (632.8 nm), the 1T′ monolayer has just one dominant mode, at 166.8 cm−1. The blue curve, from bare ionic liquid, shows no Raman modes and so acts as a clear and flat background in all Raman measurements.
Extended Data Figure 2 Calculated shift in the Raman peak of the Ag mode of the 1T′-phase MoTe2 monolayer owing to strain.
The lattice constants range from (aT′ bT′) at x = 0 to (aH, bH) at x = 1. All lattice constants can be found in the Methods. Inset, top and side views of the atomic displacement pattern of the Ag mode. The magnitude of the displacements is proportional to the length of the red arrows. Purple and yellow spheres denote molybdenum and tellurium atoms, respectively.
Extended Data Figure 3 Fitting of the Lorentz function for Raman spectra at different biases.
a, b, Raman spectra at 0 V and 4.4 V are well fitted by only one Lorentz function, representing a single Raman mode (
in the 2H phase, and Ag in the 1T′ phase). c, A typical Raman spectrum taken during transition from one phase to another (for example, at a bias of 3.6 V) is well fitted by two Lorentz functions, centred at 
and Ag.
Extended Data Figure 4 Gate-dependent SHG spectroscopy of monolayer and bilayer 2H MoTe2.
a, The SHG intensity of monolayer MoTe2 displays notable hysteresis during forward and backward alteration of the positive top gate (thereby altering the doping of the sample with electrons). The large variation in SHG intensity results mainly from a change in inversion symmetry during phase transition. b, The SHG intensity of a bilayer sample shows no hysteresis under sweeping of the top gate bias. c, Doping with holes (that is, by applying a negative top gate voltage) into 2H monolayer MoTe2 slightly increases the SHG intensity by 1.5 times; a reserve gate sweep shows no hysteresis.
Extended Data Figure 5 Angular polarized Raman (Ag mode) pattern on an exfoliated 1T′-phase MoTe2 monolayer.
The position of zero degrees is arbitrary. By rotating the polarization of the excitation laser, we detected a twofold pattern, verifying the anisotropic xx and yy components in the Ag Raman tensor.
Extended Data Figure 6 SHG intensity versus excitation energy before and after phase transition.
At a gate bias of 0 V and a pump wavelength of 1,100 nm, a notable excitonic resonance is observed in a 2H-phase MoTe2 monolayer. At the same pump power but a bias of 4.4 V, the SHG intensity drops by more than one order of magnitude over the entire excitation spectrum range. The incident excitation power for each wavelength was fixed at the same level. The sample was kept at a temperature of 220 K during the measurements.
Extended Data Figure 7 SHG mapping at several typical voltage biases.
a–d, These biases correspond to before (0 V and 2.2 V; a, b), during (3.1 V; c) and after (4.1 V; d) phase transition.
Extended Data Figure 8 Analysis of the mechanism of phase transition from 2H to 1T′ by the Hall effect.
a, c, Gate-dependent Raman intensity ratio. The ratio (1T′/1T′ + 2H)) is extracted from Lorentz fitting of Raman spectroscopy at each gate (as in Fig. 2b). Red arrows show forward and backward gate sweeping. From a, we determine the threshold for transition from phase 2H to 1T′ to be 3.2 V (red dashed line) on the basis of fitting with the Preisach model. The corresponding carrier density at this threshold is higher than that predicted theoretically7, perhaps because of the presence of kinetic barriers during transition, which were not considered in the theoretical calculation. In order to compare with the predicted critical doping level, where the two phases are energetically degenerate, we determined the corresponding critical voltage in c, following a typical method50. The red dashed lines indicate that transition from phase 2H to 1T′ occurred at a bias of 2.6 V while the reverse process began at 2.2 V. The average (2.4 V) refers to the critical voltage at which the two phases are energetically degenerate. b, d, The Hall resistance [Rxy – Rxy(0)] for the same sample at 3.2 V (b) and 2.4 V (d), as a function of the magnetic field, B. Here Rxy is the transverse resistance under the magnetic field while Rxy(0) is the transverse resistance without magnetic field. The slopes of their linear fittings (dashed lines) give the corresponding carrier densities to be 2.2 × 1014 cm−2 and 8.5 × 1013 cm−2, respectively. The latter, which excludes the kinetic barrier, matches the carrier range that is predicted7 to drive this phase transition (0.4–1 × 1014 cm−2).
Extended Data Figure 9 Evidence that the phase transition is not thermally driven during Raman measurements.
a, Raman spectra of monolayer 2H MoTe2 before and after exposure to a 1 mW μm−2, 633-nm laser for 20 hours. A shift in neither peak intensity nor peak position is observed. b, Gate-dependent Raman intensity ratio (1T′/(2H + 1T′)) under different laser-power excitations: 1 mW μm−2 or 0.1 mW μm−2. At each gate voltage, the intensity for the 2H phase (or 1T′ phase) Raman mode 
(or Ag) was extracted by Lorentz fitting of the Raman mixture in the range 160–180 cm–1. The error bars represent standard errors propagated from the fitting parameters. Given that the thresholds and hysteresis loops are the same at both powers, the transition between the 2H and the 1T′ phase must be independent of laser power and is determined purely by the electrostatic doping level rather than by a thermal effect. All experiments were conducted in a vacuum of 2 × 10-6 torr and at a temperature of 220 K.
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Wang, Y., Xiao, J., Zhu, H. et al. Structural phase transition in monolayer MoTe2 driven by electrostatic doping. Nature 550, 487–491 (2017). https://doi.org/10.1038/nature24043
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DOI: https://doi.org/10.1038/nature24043
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