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Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics

Abstract

The piezoelectric characteristics of nanowires, thin films and bulk crystals have been closely studied for potential applications in sensors, transducers, energy conversion and electronics1,2,3. With their high crystallinity and ability to withstand enormous strain4,5,6, two-dimensional materials are of great interest as high-performance piezoelectric materials. Monolayer MoS2 is predicted to be strongly piezoelectric, an effect that disappears in the bulk owing to the opposite orientations of adjacent atomic layers7,8. Here we report the first experimental study of the piezoelectric properties of two-dimensional MoS2 and show that cyclic stretching and releasing of thin MoS2 flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, whereas no output is observed for flakes with an even number of layers. A single monolayer flake strained by 0.53% generates a peak output of 15 mV and 20 pA, corresponding to a power density of 2 mW m−2 and a 5.08% mechanical-to-electrical energy conversion efficiency. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90°. Transport measurements show a strong piezotronic effect in single-layer MoS2, but not in bilayer and bulk MoS2. The coupling between piezoelectricity and semiconducting properties in two-dimensional nanomaterials may enable the development of applications in powering nanodevices, adaptive bioprobes and tunable/stretchable electronics/optoelectronics.

Figure 1: Single-layer MoS2 piezoelectric device and operation scheme.
Figure 2: Piezoelectric outputs from single-layer and multi-layer MoS2 devices.
Figure 3: Direct-current electrical characterizations of single-layer and bilayer MoS2 devices under strains.
Figure 4: Array integration of CVD single-layer MoS2 flakes.

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Acknowledgements

We thank C. Dean, P. Kim and K. Shepard for discussions, and K. Alexander and E. Reed for theoretical consultation. This research was supported by the US Department of Energy, Office of Basic Energy Sciences (DE-FG02-07ER46394) and US National Science Foundation (DMR-1122594). Z.L.W. acknowledges the ‘Thousands Talents’ programme for pioneer researcher and his innovation team, National Natural Science Foundation of China (51432005), and China and Beijing City Committee of Science and Technology (Z131100006013004 and Z131100006013005).

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Authors

Contributions

W.Z.W., L.W., Z.L.W. and J.H. conceived the idea. W.Z.W., L.W., Y.L.L., T.F.H., J.H. and Z.L.W. designed the experiments. L.W., D.C., F.Z. and Y.F.H. synthesized the material. L.W., D.C. and W.Z.W. fabricated the devices. Y.L.L. performed the SHG characterization. W.Z.W., L.W., L.L., Y.L.L. and S.M.N. conducted the experiments. W.Z.W., L.W., J.H. and Z.L.W. analysed the data and wrote the manuscript.

Corresponding authors

Correspondence to James Hone or Zhong Lin Wang.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Raman spectrum of MoS2 flakes and setup for SHG measurement.

a, Raman spectrum of MoS2 flakes with different layer numbers. b, Experimental setup for the SHG measurement.

Extended Data Figure 2 Mechanical strain applied to MoS2 device.

a, Schematic drawing for estimating strain in MoS2 device. b, Schematic plot of strain driving signal from linear motor. c, Typical configurations for linear motor.

Extended Data Figure 3 Piezoelectric open-circuit voltage and short-circuit current.

Extended Data Figure 4 Mechanism of electrical power generation in single-layer MoS2 due to the flow of electrons in external load driven by piezoelectric polarization charges.

The equivalent circuit of the piezoelectric nanogenerator is also shown.

Extended Data Figure 5 Piezoelectric output of MoS2 device with different strain parameters.

a, Short-circuit current–strain and open-circuit voltage–strain hysteresis loops. Hold time t1 = t2 = 1 s and acceleration a = 5 m s−2 for the curve of 0.4 Hz; hold time t1 = t2 = 0.5 s and acceleration a = 7.5 m s−2 for 0.8 Hz; hold time t1 = t2 = 0.1 s and acceleration a = 10 m s−2 for 2.5 Hz. b, Electrical outputs from bare PET substrate without single-layer MoS2 under periodic strain (0.53%).

Extended Data Figure 6 Circuit connection for measuring the power outputs on the external load and power delivered to the load at 0.53% strain.

Extended Data Figure 7 Stability test of voltage output from single-layer MoS2 device.

The frequency of 0.5 Hz was held for 300 min. The results demonstrate good stability of the device in mechanical energy harvesting for prolonged periods.

Extended Data Figure 8 Angular dependence of SHG intensity (perpendicular component) for three-layer and five-layer MoS2.

Samples of even layers (two, four and six layers) give vanishing SHG intensity regardless of their crystallographic orientation. θ denotes the angle between the fundamental light polarization and the mirror plane of the lattice.

Extended Data Figure 9 Transport characteristics of bulk device under different uniaxial strains.

Extended Data Figure 10 Electrical outputs when CVD devices 1 and 2 are destructively connected.

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Wu, W., Wang, L., Li, Y. et al. Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics. Nature 514, 470–474 (2014). https://doi.org/10.1038/nature13792

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