Tunable Out-of-Plane Piezoelectricity in Thin-Layered MoTe2 by

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Surfaces, Interfaces, and Applications

Tunable Out-of-Plane Piezoelectricity in Thin-Layered MoTe2 by Surface Corrugation-Mediated Flexoelectricity Seunghun Kang, Sera Jeon, Sera Kim, Daehee Seol, Heejun Yang, Jaekwang Lee, and Yunseok Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06325 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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ACS Applied Materials & Interfaces

Tunable Out-of-Plane Piezoelectricity in Thin-Layered MoTe2 by Surface Corrugation-Mediated Flexoelectricity

Seunghun Kang,† Sera Jeon,‡ Sera Kim,§ Daehee Seol,† Heejun Yang § Jaekwang Lee,*,‡ and Yunseok Kim*,†

†

School of Advanced Materials and Engineering, Sungkyunkwan University (SKKU), Suwon

16419, Republic of Korea ‡

Department of Physics, Pusan National University, Busan, 46241, Republic of Korea

§

Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419,

Republic of Korea

* Correspondence and requests for materials should be addressed to J.L. and Y.K. (email: [email protected] and [email protected])

1 ACS Paragon Plus Environment

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ABSTRACT Piezoelectricity crystallographically exists only in the in-plane direction in two-dimensional transition metal dichalcogenides. Here, we demonstrated flexoelectricity-tunable out-of-plane piezoelectricity in semiconducting 2H-MoTe2 flakes by creating surface corrugation. In particular, the strong out-of-plane piezoelectricity and its spatial variation depending on local flexoelectricity was observed even though crystallographically there exists only in-plane piezoelectricity. Surface corrugation-mediated flexoelectricity tuning can be applied to other two-dimensional or thin-layered materials and, furthermore, the results could provide useful information on the interweaving nature between mechanical stimulus and electric dipole in lowdimensional materials.

Keywords: Two-dimensional transition metal dichalcogenides, corrugation, flexoelectricity, outof-plane piezoelectricity, piezoresponse force microscopy


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INTRODUCTION Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been extensively studied owing to their superior electronic and mechanical properties as well as their ultra-thin lowdimensional nature.1-4 The ultra-thin 2D TMD materials are mechanically flexible and have therefore been widely investigated for applications in flexible devices.4-6 Recently, their flexible nature makes it possible to explore mechanical stimulus-induced exotic phenomena such as conductivity and phase transition.7-10 In particular, physical properties between in-plane and outof-plane directions are highly asymmetric due to their ultra-thin 2D nature.11-14 As a result, it has been thought that the piezoelectricity can exist only in-plane direction in the 2D TMD materials.11-12, 15-16 In many cases, when a mechanical stimulus is applied to the 2D TMD materials along the out-of-plane direction, a strain gradient effect can be observed in the 2D materials.15, 17-18 In three-dimensional (3D) oxide materials, it has been previously reported that the strain gradient can induce electromechanical coupling, also known as flexoelectricity, in all insulators by breaking the spatial inversion symmetry and generating electric polarization,19-21 which can be used to build upon interesting physical properties.22-25 For instance, rectification of charge transport23 was realized in strain-graded dielectrics where the internal flexoelectricity was generated through strain gradient control via epitaxial growth. Also, the manipulation of oxygen vacancies in ionically active oxide thin film,22 as well as domain switching in ferroelectric oxide thin film,24,26 were demonstrated via nanoscale flexoelectricity using a scanning probe microscope (SPM) tip. Very recently, it was reported that flexoelectricity can be observed in oxide semiconductors as well.25 All these observations indicate that, even though crystallographically there exists only in-plane piezoelectricity in the semiconducting 2D TMD



materials,11, 15-16 there is a possibility that out-of-plane piezoelectricity could be generated from a strain gradient. Therefore, when the 2D TMD materials are deformed along their out-of-plane direction, the electric polarization apparently emerges along the out-of-plane direction via internal flexoelectricity. These changes can be achieved through surface-corrugation mediatedflexoelectricity as a result of the so-called corrugation of the 2D or thin-layered materials, which theoretically induces a strain gradient (Figure 1a).27-28 Therefore, the corrugation can be a simple way to tune the electric polarization because the corrugation is readily varied in flexible 2D or thin-layered materials along the out-of-plane direction. The magnitude of the out-of-plane piezoelectricity is also expected to be readily tunable by the degree of the corrugation, i.e., the degree of the surface corrugation-mediated flexoelectricity, unlike 3D oxide materials. In fact, there are already some reports on substrate-induced corrugation in graphene as well as layered structures, indicating the feasibility of this tuning method.29-30 Once the validity of the out-of-plane piezoelectricity is experimentally verified in the 2D or thin-layered materials, we expect that it could motivate exploration of new types of piezoelectric materials and broaden the scope of current device applications based on piezoelectricity.11-12 Furthermore, since the out-of-plane piezoelectricity was realized in a typical thin 2D or layered TMD material, unlike in Janus 2D TMD materials,31,32 it can be applied to various nanoelectromechanical devices more widely. In this study, we have demonstrated tunable out-of-plane piezoelectricity in 2H-MoTe2 via surface corrugation-mediated flexoelectricity. The corrugation in 2H-MoTe2 was artificially developed via substrate roughness and thickness modulation of MoTe2, and it was experimentally observed that the corrugated 2H-MoTe2 exhibited strong out-of-plane piezoelectricity which did not exist in the pristine MoTe2. Furthermore, it was found that


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spatially varied, local, out-of-plane piezoelectricity was dependent on local flexoelectricity induced by local strain gradient. Density functional theory (DFT) calculations further confirmed that

out-of-plane

piezoelectricity

was

proportional

to

non-centrosymmetric

Mo–Te

displacements along the out-of-plane direction due to the surface corrugation-mediated flexoelectricity.

EXPERIMENTAL SECTION Fabrication. 2H-MoTe2 flakes were prepared by mechanical exfoliation on Au/SiO2/Si substrates from a single crystal of bulk 2H-MoTe2, which was synthesized by the flux method. To obtain MoTe2 flakes with a thickness of tens of nanometers, the exfoliation method was carried out on the MoTe2 bulk material several times. For control of the strain in MoTe2 flakes, three types of substrate (flat, low roughness, and high roughness) were used in the experiment. The gold film of the flat substrate was transferred from mica. Both low and high roughness substrates were prepared by directly depositing gold on SiO2 via thermal deposition techniques at different deposition speeds. Measurements. AFM measurements were performed using a commercial system (Park systems NX-10) equipped with a function generator and data acquisition system (National Instruments NI-PXIe 5122/5412) with LabVIEW/MATLAB for BE measurements. The PFM amplitude map was measured on 30 × 30 grid points by applying gradually increasing AC voltages up to 1.5 Vpeak with 300–400 kHz on the MoTe2 flake. In all the AFM measurements, a conductive Pt/Cr coated tip (BudgetSensors Multi75E-G) was used. Calculations. All calculations were carried out using DFT with the plane wave-based Vienna ab-initio simulation package.28,33 We used the projector augmented wave method of Blöchl34 in



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the implementation of Kresse and Joubert.35 The generalized gradient approximation was employed for the exchange-correlation functional. We used an energy cut-off for the plane wave of 550 eV and Γ-centered 24 × 8 × 4 k-point meshes for the electronic band gap calculations, and 8 × 4 × 4 k-point meshes for the atomic-position-by-atomic-position polarization calculations. The calculations were converged in energy to 10−6 eV/cell and the structures were allowed to relax until the forces were less than 2 × 10−2 eV/Å. A vacuum layer of 20 Å was inserted perpendicular to the MoTe2 monolayer to avoid spurious inter-layer interactions.

2π Corrugation, C, was defined as C( x) = Asin( x) , where A and λ refer to the height and

λ

width of the corrugated structure, respectively, and x is the unit vector which is perpendicular to the direction of corrugation.36 In order to maintain the corrugated structure, Mo atoms were clamped and only the Te atoms were allowed to be fully relaxed. The polarization in i-direction37 was calculated using

∂Pi =

e * Z ij δd j , Ω

(1)

where Zij* refers to the Born effective charges associated with the atomic displacement (δdj) of the ions from their position in the unpolarized (un-corrugated) structure, e is the charge of an electron, and Ω refers to the cell volume considered.


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RESULTS AND DISCUSSION

Figure 1. (a) Side view of the possible corrugated crystal structures of MoTe2 flakes according to the degree of flexoelectricity (or corrugation). (b) Schematic representations of (left) AFM and MoTe2 flake corrugated by Au roughness, (right-top) BE based Vac amplitude sweep waveform and BE waveform at a single step and (right-bottom) corresponding PFM amplitude versus magnitude of the applied ac voltage.

To

explore

strain-gradient-induced

out-of-plane

piezoelectricity,

we

used

the

piezoresponse force microscopy (PFM) amplitude as a function of the magnitude of AC voltage, the so-called Vac amplitude sweep of PFM.38 Even though the lock-in technique is generally used in the Vac amplitude sweep for measuring PFM amplitude, the band excitation (BE) technique was used instead (Figure 1b). As a result of the relatively much higher signal-to-noise ratio based on the high Q factor (e.g., >100) compared to the lock-in technique, the BE technique is suitable for probing the small PFM amplitudes in the 2D TMDs.39 Since each step of the Vac amplitude sweep includes the frequency sweep by the BE technique, the system can measure the PFM amplitude depending on the Vac of the BE response. Thus, the slope of the PFM amplitude versus



Vac amplitude sweep indicates the piezoelectric coefficient in the out-of-plane direction, also known as effective piezoelectric coefficient, i.e., d33,eff.11, 40 Although the BE technique has a high signal-to-noise ratio, the PFM amplitude in ultra-thin materials such as 2D or thin-layered TMDs can be too low to be masked by the noise level, i.e., noise floor. Thus, if the noise floor is comparable to the measured PFM amplitude, the base line of the noise floor can be observed in the result of the Vac amplitude sweep, as schematically presented in Figure 1b.

Figure 2. Corrugation engineering through modulation of the substrate roughness. (a–f) Topography images of (a–c) different substrates and (d–f) corresponding 12 nm-thick multi-layer MoTe2 flakes that were transferred to each substrate: (a, d) flat (Rq = 0.44 nm); (b, e) low roughness (Rq = 1.24 nm); and (c, f) high roughness (Rq = 2.22 nm) Au substrates. (h) Roughness of MoTe2 flake versus type of substrate. Roughness of MoTe2 flakes were obtained from Figures


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(d–f). Corrugation engineering through thickness modulation of the MoTe2 flake. (g) Entire topography image of multi-layer MoTe2 flake on a low-roughness Au substrate (Rq = 1.08 nm). The inset images show multi-layer MoTe2 flakes depending on the thickness of the flakes (10, 16, and 20 nm). The surface roughness for 10, 16, and 20 nm-thick flakes is 1.16, 0.63, and 0.52 nm, respectively. (i) Roughness of the MoTe2 flake versus its thickness. The data of 12 and 25 nmthick flakes was obtained from other flakes for the comparison. (j) Raman spectra of MoTe2 flakes depending on the thickness of the flake.

In order to tune the magnitude of flexoelectricity, we controlled the corrugation of MoTe2 flakes via two different experimental methods: substrate roughness and flake thickness (Figure 2). To modulate the corrugation of MoTe2 flakes via substrate roughness, three types of Au substrates with different roughness: 0.44, 1.24, and 2.22 nm, referred to as flat, low, and high roughness substrates, respectively, were prepared as shown in Figures 2a–c. It could be clearly seen that the surface features of the three substrates were very different. After preparing the substrates, the 12-nm-thick MoTe2 flakes were transferred onto each substrate as presented in Figures 2d–f. The substrate roughness was found to significantly affect the surface features of the MoTe2 flakes owing to physical interactions between the substrate and MoTe2 flakes. In particular, not only did the shape of the flake surface resemble that of the substrate, but the surface roughness of the MoTe2 flakes increased with increasing substrate roughness (Figure 2h). The roughness of each 12 nm-thick MoTe2 flake that transferred onto the flat, low roughness, and high roughness substrates was 0.41 nm, 0.75 nm, and 1.9 nm, respectively, which varied from the substrate roughness by 60 to 93%. It is evident that the MoTe2 flake corrugation is



closely related to the substrate roughness (see more information in Figure S1). Therefore, substrate roughness can be an effective parameter for controlling corrugation. In addition to substrate roughness, we employed thickness engineering to modulate surface corrugation of the MoTe2 flakes. As shown in Figure 2g (the inset images indicate the topography of each area marked with white boxes), the surface features were clearly different for each thickness of the MoTe2 flake although the substrate was the same. Upon decreasing the thickness of MoTe2 flake from 25 nm to 10 nm, the roughness gradually increased from 0.45 nm to 1.16 nm; 36% and 93% of the substrate roughness (1.24 nm). Therefore, we concluded that the roughness of the MoTe2 flake is directly correlated to surface corrugation and can be controlled by engineering substrate roughness as well as flake thickness. In other words, manipulation of flake roughness can be used to tune the degree of flexoelectricity. It has been shown that application of high strain to the MoTe2 flake can induce phase transition from 2H to 1T’.10 However, as shown in Figure 2j, the Raman shift of 233 cm–1 indicates that the prepared multilayer MoTe2 flakes were 2H phase regardless of their thickness.41 So, even though the strain induced by the corrugation can be varied depending on the thickness of the MoTe2 flakes, their crystal structure is still intact as the semiconducting 2H phase.


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Figure 3. (a) Topography images and the corresponding (b) spatial maps of d33,eff in the 35 nmthick MoTe2 flake transferred on the high roughness substrate (Rq = 3.0 nm). The red circles in Figures (a) and (b) indicate the same regions of the measured area. (c) PFM amplitude versus magnitude of the applied AC voltage in regions 1 and 2 of Figure (b). The red and violet solid line indicates the fitting line obtained by the least square method. The inset shows schematics of the strain states in regions indicated as orange and violet solid squares.

To probe the out-of-plane piezoelectricity in corrugated 2H-MoTe2, Vac amplitude sweep based on BE technique was measured in the out-of-plane direction as mentioned above in Figure 1b. In this study, the Vac amplitude sweep was performed on 30 × 30 grid points to observe the spatial relationship depending on the degree of corrugation. The spatial map of d33,eff in 35 nmthick MoTe2 flake is presented in Figure 3b. Interestingly, the local out-of-plane effective piezoelectric coefficient is highly related to the local topographic features of the measured area (Figure 3a). As shown in the red circles, a high out-of-plane effective piezoelectric coefficient was observed in the region where the strain gradient was expected to be high, indicating that the 11 ACS Paragon Plus Environment


strain gradient-related flexoelectricity induces out-of-plane piezoelectricity. To directly compare the out-of-plane piezoelectricity according to location, we extracted two different data sets from the relatively high (region 1, orange square) and low (region 2, violet square) strain gradient locations, as shown in Figures 3a and b, respectively. The inset in Figure 3c schematically shows the two different locations and the corresponding Vac amplitude sweep results. As expected, region 1 with the higher strain gradient showed a relatively higher PFM amplitude as compared to region 2. At the same time, the measured PFM amplitude for region 2 was slightly higher than the noise floor of our measurement system. Therefore, since the out-of-plane PFM amplitude was clearly observed in the high-strain-gradient region as opposed to the nearly flat region, we were able to confirm that the strain-gradient-related flexoelectricity indeed induced the out-ofplane piezoelectricity. For quantitative analysis, we conducted the calibration taking this noise floor. In region 1 (2) with the high (low) strain gradient, the slope, i.e., d33,eff, was obtained as approximately 3.98 (0.97) pm/V when the force-distance curve-based calibration was performed.40, 42-43 We noted that the actual d33,eff can be lower than 3.98 pm/V because of the electrostatic contribution (Figure 4). Nonetheless, even consideration of the electrostatic contribution, the reduced value is still on the similar range with our theoretically calculated value (Figures 5 and S9). The details will be discussed in Figure 5.


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Figure 4. (a) d33,eff according to the MoTe2 flake thickness. The black triangles represent the average of the spatial maps of d33,eff (over areas of 0.5 µm by 0.5 µm (up triangle) and 1 µm by 1 µm (down triangle)). The blue (orange) circle represents the average value of the top 3% (5%) for the measured image. The orange solid line is a guide line for eye. (b) Scheme for flakethickness-dependent flexoelectric (red line), clamping (blue line), and electrostatic (yellow line) effects. The green line represents the total intensity of d33,eff due to the summation of flexoelectric and clamping effects. (c) Topography images and the corresponding (d) spatial maps of d33,eff in the 125 nm-thick MoTe2 flakes transferred on the high roughness substrate (Rq = 3.0 nm). (e, f) Surface potential images of the (e) 35 nm-thick and (f) 125 nm-thick MoTe2 flakes which are the same flakes in Figures 3 and 4. Each average surface potential is 274.1 ± 34.6 mV and 275.2 ± 38.9 mV, respectively.

We examined the dependence of out-of-plane piezoelectricity (or effective piezoelectric coefficient) on the degree of the surface-corrugation-mediated flexoelectricity. Since the high 13 ACS Paragon Plus Environment


piezoelectric coefficient was only observed in the high strain gradient region as shown in Figures 3a and b, we focused on the top 3% and 5% values in the spatial maps of d33,eff. As presented in Figure 4a, the piezoelectric coefficient gradually decreased then saturated as the flake thickness increased (i.e., the strain gradient decreased). These results imply that strain gradient related flexoelectricity induces changes in the out-of-plane piezoelectric coefficient of the MoTe2 flake. However, we note that, in very thin flakes such as 21 nm, the PFM amplitude was lower than expected which is probably due to the stronger clamping effect in thinner flakes.44-45 In this case, both flexoelectric and clamping effects can be competing with each other because both effects increase with a decrease in the flake thickness as shown in Figure 4b. Since the flexoelectric effect is stronger than the clamping effect based on the obtained results, there can be two different regimes: 1) In region I, the detectable clamping effect contributes to the d33,eff and 2) In region II, the flexoelectric effect dominates the d33,eff. As presented in Figures 4c and d, even though the surface roughness of the thick MoTe2 flakes was very low (e.g., Rq = 0.1 nm for 125-nm thick MoTe2 flakes), the out-of-plane piezoelectric coefficient was still measurably large and there were no significant dot-like features. We note that, although the locally different electrostatic effect can contribute to the measured PFM reponse,46-48 the electrostatic response was nearly the same over the measured surface as shown in Figure 4e. Thus, the locally different d33,eff in Figure 3b might not be related to the electrostatic contribution. Nonetheless, since there was measurable surface potential of about 275 mV for both 35 nm-thick and 125 nm-thick MoTe2 flakes as shown in Figures 4e and 4f, the electrostatic interaction could have increased the measured PFM response over the entire measured region.47-48 Accordingly, the unexpected out-of-plane piezoelectric coefficient in the thick MoTe2 flakes might be relevant to the electrostatic interaction.


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In our previous work,47 we reported that the electrostatic contribution on the PZT film surface with the same cantilever is 1.4 pm per 1 V of the DC voltage under application of 0.5 AC voltage, i.e., 2.4 pm/Vac under 1 Vdc (without charge injection contribution). If we assume that the measured surface potential fully contributes to the measured d33,eff with the similar manner in the PZT film, the electrostatic contribution to the measured d33,eff can be calculated as 0.77 pm/V. This value is much smaller than the measured average d33,eff for the thick MoTe2 flakes as shown in Figure 4a. Thus, since the average d33,eff is 2.08 pm/V for 125 nm-thick MoTe2 flake, the actual d33,eff can be 1.31 pm/V from the subtraction between two values. On the other hand, as previously reported,49 the electrostatic contribution can be reduced if the PFM measurement is performed at the sample edge. That is, non-local electrostatic effect can be reduced at the sample edge. Since we measured the PFM in the flake samples as shown in Figure 2g, the PFM measurements in the MoTe2 flakes can be considered edge measurements. This indicates that the electrostatic effect in the MoTe2 flakes can be slightly different from that in the PZT thin film. As a result, the overall electrostatic contribution to the PFM response can be even smaller than 0.77 pm/V. Therefore, in the present case, there is possibility that the small degree of the surface corrugation, the bottom part of the flake close to the substrate, or other unknown contributions might contribute to the measured d33,eff (Figures S3 and S4). Although the degree of the surfacecorrugation-mediated flexoelectricity, i.e., roughness or corrugation, on the surface is very small, out-of-plane piezoelectricity can be still generated (the dotted circle in Figure S3a). Furthermore, the bottom part of the flake close to the substrate might be still affected by the substrate roughness. In such a situation, since the electric field distribution during the PFM measurements covers a relatively larger area compared to that of the thin flake, the dot-like features on the spatial map are hard to be visible (Figure S3). As a result, the flake-thickness-dependent



behavior can be schematically drawn as presented in Figure 4b. We note that, as previously reported, a higher strain gradient region also can show a higher current due to the high strain (more details in Figure S5),9-10 indicating the feasibility of the polar conducting nature.50-51 In addition, even though the instrumental noise floor can contribute to the measured d33,eff,52 it can be excluded because we consider the base line related to this noise floor for calculating d33,eff. While the current flow can induce Joule heating expansion as reported elsewhere,53-54 this is unlikely, considering the measured current level (here, 1.5 nA under 0.25 V). Furthermore, since the Joule heating expansion is a 2nd harmonic response, there might not be a significant effect on our measurements. Finally, since we did not apply high voltage, the electrochemical reaction may not contribute to the measured PFM response.55

Figure 5. (a) Schematic diagrams of the possible corrugated crystal structures of 2H-MoTe2 flake in the armchair direction. (b) Theoretical calculation of polarization depending on the atomic position in 5% corrugated 2H-MoTe2. Atomic position is marked in Figure (a). (c) Polarization (left-axis) of 2H-MoTe2 depending on the degree of corrugation. The corrugation of


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3, 5, and 8% corresponds to strains of 0.38, 0.62, and 0.93%, respectively.

Finally, to confirm the origin of the corrugation induced out-of-plane piezoelectricity, we performed theoretical calculations on the corrugated 2H-MoTe2. Monolayer 2H-MoTe2 has a hexagonal primitive cell, which is centrosymmetric with two Te atoms equidistant from the Mo atom. From the first-principles DFT calculations for the flat monolayer 2H-MoTe2, the bond length of Mo–Te is estimated to be about 2.73 Å. Then, the corrugated 2H-MoTe2 was calculated in a similar manner. Figure 5a shows the top view and side view of 5% corrugated armchair monolayer 2H-MoTe2, respectively. Here, the corrugation C was defined as C ( x) = Asin(

2π

λ

x)

where A and λ indicate the height and the width of corrugated structure, and x corresponds to the unit vector perpendicular to the direction of corrugation. Armchair or zigzag corrugation is classified by the edge structure of extreme points where x equals zero or λ/2 (Figure S6). From the top view, it can be seen that our model structure has the armchair edge structure at the extreme points where x equals zero. The out-of-plane polarization is calculated at each atomic position in the right-handed figure of Figure 5a along the z-direction. As shown in Figure 5b, some of atomic positions indeed show out-of-plane polarization. In particular, the out-of-plane polarization as a function of atomic position clearly indicates that the out-of-plane polarization varies with the atomic position of the corrugated monolayer 2H-MoTe2. It is a zero-polarization at the nodes, i.e., “atomic position 1” and “atomic position 5”, maintaining the nonpolar centrosymmetric structure. On the other hand, “atomic position 3” (antinode) has the largest out-of-plane polarization of 3.29 pC/m because of the relatively large displacement of the Te atoms in the z-direction. Similar phenomenon can be observed in Figure 5c and this demonstrates the total polarization as a 17 ACS Paragon Plus Environment


function of the degree of corrugation, where total polarization is estimated by the summation of atomic-position-by-atomic-position polarization over the half-wavelength of corrugation. The total out-of-plane polarization is calculated to be about 5.66, 8.91, and 36.51 pC/m, in order of increasing corrugation. The average ion displacement in the z-direction is calculated to be about 0.038, 0.064, and 0.115 Å, respectively. Large corrugation yields more displacement and higher non-centrosymmetric distortion, consequently inducing larger polarization. Therefore, since the theoretical results are consistent with our experimental results (Figure S7), we confirmed that the strain gradient related flexoelectricity apparently induces the out-of-plane piezoelectricity in the corrugated thin-layered MoTe2. Even though the calculation results imply that both the valleys and peaks correspond to the maximum out-of-plane polar nature, this was only experimentally observed in the peaks of the corrugation (Figure 3b). There could be two different reasons for the observed phenomena being related to the stacking of the MoTe2 flake on the Au substrate as shown in Figure S2. One reason is that most valleys are too flat to induce a significant polarity. Another reason is that a sufficient voltage field cannot be applied to the MoTe2 flakes because of the poor physical contact between flake and substrate in the valley region.

CONCLUSION We have demonstrated that tunable out-of-plane piezoelectricity can be realized in corrugated flakes of semiconducting 2H-MoTe2 via surface corrugation-mediated flexoelectricity. The degree of the flexoelectricity in the MoTe2 flakes was systematically modulated by the degree of the corrugation by varying the substrate and flake thicknesses. In the corrugated 2H-MoTe2, we experimentally observed the out-of-plane piezoelectricity using PFM. Furthermore, the spatially


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varied local out-of-plane piezoelectricity was clearly visible and the local out-of-plane piezoelectricity was dependent on the local flexoelectricity. Our DFT calculations support that the ripple features indeed induce local polarization in the out-of-plane direction in the 2H-MoTe2. Corrugation engineering is expected to further increase out-of-plane piezoelectricity magnitude. We expect that the tunable out-of-plane piezoelectricity in 2D or thin-layered TMD materials provides an opportunity to better understand the electromechanical coupling phenomena in other low-dimensional materials, could motivate exploration of new multifunctional materials and applications, and could broaden the scope of current device applications. Furthermore, since the out-of-plane piezoelectricity was obtained in a typical thin-layered TMD material, it can be more widely applied to novel nanoelectromechanical devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Strain analysis for corrugation, line profiles of MoTe2 flake and Au substrate, topography images including wrinkles, current images, and theoretical calculation of polarization.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J. L.), [email protected] (Y. K.)

Notes The authors declare no competing financial interest.



ACKNOWLEDGMENT This work was supported by Basic Research Lab. Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2014R1A4A1008474), and also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A2B2003342). This work was partially supported by IBS-R011-D1. J. Lee acknowledges the support of the National Research Foundation of Korea (NRF) Grant funded by the Korean government (MSIP) (NRF2015R1C1A1A01053810).

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Tunable Out-of-Plane Piezoelectricity in Thin-Layered MoTe2 by

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