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Piezoelectricity of Single-Atomic-Layer Mos2 for Energy Conversion and Piezotronics

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Piezoelectricity of Single-Atomic-Layer Mos2 for Energy Conversion and Piezotronics LETTER doi:10.1038/nature13792 Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics Wenzhuo Wu1*, Lei Wang2*, Yilei Li3, Fan Zhang4, Long Lin1, Simiao Niu1, Daniel Chenet4, Xian Zhang4, Yufeng Hao4, Tony F. Heinz3, James Hone4 & Zhong Lin Wang1,5 The piezoelectric characteristics of nanowires, thin films and bulk The strain-induced polarization charges in single-layer MoS2 can also crystals have been closely studied for potential applications in sensors, modulate charge carrier transport at the MoS2–metal barrier and enable transducers, energy conversion and electronics1–3.Withtheirhighcrys- enhanced strain sensing. In addition, we have also observed large piezo- 4–6 tallinity and ability to withstand enormous strain , two-dimensional resistivity in even-layer MoS2 with a gauge factor of about 230 for the materials are of great interest as high-performance piezoelectric mate- bilayer material, which indicates a possible strain-induced change in band 18 rials. Monolayer MoS2 is predicted to be strongly piezoelectric, an structure . Our study demonstrates the potential of 2D nanomaterials effect that disappears in the bulk owing to the opposite orientations in powering nanodevices, adaptive bioprobes and tunable/stretchable of adjacent atomic layers7,8. Here we report the first experimental electronics/optoelectronics. study of the piezoelectric properties of two-dimensional MoS2 and In our experiments, MoS2 flakes were mechanically exfoliated onto show that cyclic stretching and releasing of thin MoS2 flakes with an a polymer stack consisting of water-soluble polyvinyl alcohol and poly odd number of atomic layers produces oscillating piezoelectric volt- (methyl methacrylate) on a Si substrate, with the total polymer thickness age and current outputs, whereas no output is observed for flakes with tuned to be 275 nm for good optical contrast. Few-layer MoS2 flakes were an even number of layers. A single monolayer flake strained by 0.53% first identified under an optical microscope. The layer thickness was then generates a peak output of 15 mV and 20 pA, corresponding to a power measured by atomic force microscopy (Fig. 1a, inset) and confirmed by density of 2 mW m22 and a 5.08% mechanical-to-electrical energy Raman spectroscopy (Extended Data Fig. 1a). SHG was subsequently conversion efficiency. In agreement with theoretical predictions, the used to determine the crystallographic orientation of the MoS2 flakes output increases with decreasing thickness and reverses sign when (Methods and Extended Data Fig. 1b). Figure 1b shows a polar plot of 6 the strain direction is rotated by 90 . Transport measurements show the second-harmonic (SH) signal intensity from a single-layer MoS2 a strong piezotronic effect in single-layer MoS2, but not in bilayer flake as a function of the crystal’s azimuthal angle. Here we measured and bulk MoS2. The coupling between piezoelectricity and semicon- the SH component perpendicular to the excitation polarization. The lat- ducting properties in two-dimensional nanomaterials may enable tice orientation was determined by fitting the angle dependence of SH 2 the development of applications in powering nanodevices, adaptive intensity with I~I0 sin (3h), where h denotes the angle between the bioprobes and tunable/stretchable electronics/optoelectronics. direction of the ‘armchair’ edge and the polarization of the excitation Crystal structure and symmetry dictate the physical properties of a laser, and I0 is the maximum intensity of the SH response. Figure 1a material and its interaction with external stimuli. Materials with polar- schematically depicts the derived lattice orientation superimposed on ization domains, such as Pb(Ti,Zr)O3, or with non-centrosymmetric struc- the optical image of a single-layer MoS2 flake; the x axis is taken to be ture, such as ZnO and GaN, are piezoelectric and have wide applications along the ‘armchair’ direction, and the y axis along the ‘zigzag’ direction. in sensors, transducers, power generation and electronics1,3,9. Layered After optical characterization, flakes were subsequently transferred to materials, such as graphite, hexagonal boron nitride (h-BN) and many a polyethylene terephthalate (PET) flexible substrate using methods transition-metal dichalcogenides, are centrosymmetric in their bulk described previously19. Electrical contacts made of Cr/Pd/Au (1 nm/20 nm/ three-dimensional form but may exhibit different symmetry when thin- 50 nm) were deposited with the metal–MoS2 interface parallel to the ned down to a single atomic layer7,10,11. In graphene, inversion symmetry y axis. Figure 1c shows a typical flexible device with the single-layer is preserved because both atoms in the unit cell are identical, whereas MoS2 flake outlined by black dashed line. When the substrate was bent monolayer h-BN and transition-metal dichalcogenides become non- mechanically, uniaxial strain is applied to the MoS2 with a magnitude centrosymmetric because of the absence of an inversion centre, which proportional to the inverse bending radius (Fig. 1d and Methods). The leads to novel properties such as valley-selective circular dichroism12,13 applied strain was limited to 0.8% to avoid sample slippage20.Westudied and large second-order nonlinear susceptibility14,15, corresponding to the the piezoelectric response by applying strain to a device coupled to an optical second-harmonic generation (SHG) process16. Single-atomic-layer external load resistor (Fig. 1d). In this configuration, strain-induced h-BN, MoS2, MoSe2 and WTe2 have also been theoretically predicted polarization charges at the sample edges can drive the flow of electrons to show piezoelectricity as a result of strain-induced lattice distortion in external circuit9. When the substrate is released, electrons flow back and the associated ion charge polarization7,8,11, suggesting possible appli- in the opposite direction. cations of two-dimensional (2D) nanomaterials in nanoscale electrome- Figure 2a and Extended Data Fig. 3 show the piezoelectric current chanical devices that take advantage of their outstanding semiconducting and voltage responses of a single-layer MoS2. When strain is applied in and mechanical properties4–6,17. Here we report an experimental obser- the x direction (‘armchair’ direction), positive voltage and current out- vation of piezoelectricity in single-atomic-layer 2D MoS2 and its applica- put were observed with increasing strain, and negative output was ob- tion in mechanical energy harvesting and piezotronic sensing. Cyclic served with decreasing strain, directly demonstrating the conversion of 9 stretching and releasing of odd-layer MoS2 flakes produces oscillating mechanical energy into electricity (see also Extended Data Fig. 4 and electrical outputs, which converts mechanical energy into electricity. Methods). Both responses increased with the magnitude of the applied 1School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA. 2Department of Electrical Engineering, Columbia University, New York, New York 10027, USA. 3Department of Physics, Columbia University, New York, New York 10027, USA. 4Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA. 5Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, 100083 Beijing, China. *These authors contributed equally to this work. 470 | NATURE | VOL 514 | 23 OCTOBER 2014 ©2014 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH 90 2,500 (counts s–1) Figure 1 | Single-layer MoS2 piezoelectric device a b 120 60 c 2,000 y and operation scheme. a, Optical image of the 1,500 single-atomic layer MoS2 flake with superimposed y 50 30 1,000 x lattice orientation derived from SHG results. Blue 500 and yellow spheres represent Mo and S atoms, x respectively. Inset: atomic force microscopy image 180 0 of the flake. Scale bar 2 mm. b, Polar plot of the SH intensity from single-layer MoS2 as a function of the crystal’s azimuthal angle h. The symbols are 210 330 Electrode MoS2 10 µm experimental data and the solid lines are fits to the 2 µm symmetry analysis described in the text. c, A typical 240 300 270 flexible device with single-layer MoS2 flake and electrodes at its zigzag edges. Inset: optical image d Load resistor Load resistor of the flexible device. d, Operation scheme of the Load resistor e– e– single-layer MoS2 piezoelectric device. When the Armchair device is stretched, piezoelectric polarization charges of opposite polarity (plus and minus symbols) are induced at the zigzag edges of the Zigzag MoS flake. Periodic stretching and releasing of y 2 ε22( ) x the substrate can generate piezoelectric outputs ε11( ) in external circuits with alternating polarity Initial state Stretched Released (as indicated by the red arrows). strain: for single-layer MoS2 with width of ,5 mm and a length of axes, respectively. Symmetry analysis of the D3h point group suggested ,10 mm, the peak open-circuit voltage reached 18 mV and the peak that there was only one non-zero independent coefficient e11 for single- short-circuit current reached 27 pA (Fig. 2b), with voltage and current layer MoS2. The in-plane polarization along the x axis, sensed by the metal responsivities of 55.1 6 12.3 pA and 32.8 6 4.5 mV, respectively, for electrodes as shown in Fig. 1d, can be expressed as P1 5 e11(e11-e22), each 1% change in strain. There were no significant electrical outputs whereas P2 along the y axis is related to the pure shear
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