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Laser-Shock-Induced Nanoscale Kink-Bands in WSe2 2D Crystals Maithilee Motlag,†,‡,⊥ Yaowu Hu,†,‡,⊥ Lei Tong,§,⊥ Xinyu Huang,§ Lei Ye,*,§ and Gary J. Cheng*,†,‡
School of Industrial Engineering and ‡Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States § School of Optical and Electronic Information and Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, China Downloaded via NOTTINGHAM TRENT UNIV on August 22, 2019 at 11:08:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
S Supporting Information *
ABSTRACT: The response of materials to high stress and strain rates in nature has been a long-term scientific interest. The formation of kinks is a common deformation feature under high pressure and strain rates among foliated structures, such as rock-forming minerals. Although deformation of foliated rock layers in geology has been investigated for a century, the deformation behavior of their nanoscale counterparts, such as two-dimensional (2D) layered materials, under high stress and strain rates has not been investigated. 2D transition metal dichalcogenides have very strong in-plane rigidity, whereas the interlayer shear modulus is 3 orders of magnitude lower than their in-plane Young’s modulus. Here, we study the structure and property changes in multilayer WSe2 2D layers during a 3D nanoshaping process where laser-shock pressure at the GPa level is applied to imprint the 2D multilayers into a designed nanomold, forming a large local bending strain of 5−6%. The microstructure of the 2D multilayers after laser shock is observed to be a nanoscale kink-band, similar to that of strained geological layered crystals, due to the highpressure-induced bending and shearing. The deformed kink-band structure is investigated experimentally by highresolution transmission electron microscopy and atomic force microscopy and validated by molecular dynamics simulations. The changes in the resulting electronic band structure are investigated by first-principles calculations. The laser-shock straining technology to induce kink-band structures can enrich the understanding and facilitate the applications of many 2D materials. KEYWORDS: layered 2D materials, high strain rate, microstructure, kink-band, laser shock applications in nanoelectronics and optoelectronics,6−9 piezoelectric devices,10 biosensors,11 and energy storage devices.12 To further improve the device performance of these applications, inducing nanostructure geometry of nanoscopic mechanical deformation is an effective method, as the optical and electronic properties of TMDs are largely determined by the crystal structure. A fine control of the strain distribution is crucial to engineer these optical and electrical properties of atom-thick layered materials. However, there are several challenges to precisely control the deformation of multilayer TMDs at the nanoscale. First, the strain distribution and crystal structure after deformation of multilayer TMDs highly rely on the loading and strain rate level. Currently, there are limited ways to apply high stress and strain levels on TMDs, especially
T
he study of mechanical behavior of materials at high stress and strain rates is of widespread importance. The origin of kink-bands in several metamorphic rocks, such as micas and foliated rocks, has received considerable attention for more than a century.1−3 The mechanism of kink formation has been traced to the various static and dynamic loading histories, such as regional metamorphism, meteorite impact, and nuclear explosion.4 Shocked foliated rocks tend to form kink-band structures different than those subjected to static loading conditions. Despite all of the research about mechanical deformation of macroscopic foliated materials, mechanical deformations on 2D nanoscale multilayered materials with promising physical, optical, electronic, and mechanical properties5 have not yet started. Among 2D materials, the family of transition metal dichalcogenide (TMD) materials, which are formed by weak van der Waals stacking layers of covalently bonded transition metals M (Mo, W, Sn, etc.) and chalcogens X (S, Se, Te), has attracted immense research interest for their potential © XXXX American Chemical Society
Received: June 15, 2019 Accepted: August 19, 2019 Published: August 19, 2019 A
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Figure 1. Laser-shock-induced kinked structure in a multilayer WSe2 2D material. (a) Schematic of the laser-shock deformation process of multilayer WSe2. (b) Schematic of the suspended multilayer WSe2 before laser shock and kinked multilayer WSe2 after laser shock. (c) 3D view of the WSe2 crystal after laser shock. (d) Schematic diagram representing formation of the kink-band and a twin boundary in rocks. (e,f) AFM imaging of multilayer WSe2 before laser shock and after laser shock. (g) Kink-band structure of multilayer WSe2 observed from TEM. (h) Kink-band structure in Biotite found in the Migiandone, Val’dossola, Italian Alps. PPL image, 20× (field of view = 1 mm). (i) Schematic diagram representing kink formation in rocks.
strain rates onto WSe2 2D layered crystals. The results of our molecular dynamics (MD) simulations provide further insights to reveal the deformation mechanism which is related to the flexural slip and shearing between WSe2 2D layers and the van der Waals forces between the nanolayers, leading to a locked kink-angle and large strains around the kink-bands. The kinkbands in multilayer 2D nanocrystals have also been utilized to induce electrical property changes in a kink-banded WSe2 fieldeffect transistor (FET) device, revealing the enhanced extrinsic characteristics which can be achieved by process-controlled crystal structure engineering. We have also studied the energy band structure of kink-banded multilayer WSe2 after laser shock, using a photoluminescence (PL) spectrum and theoretical verification with first-principles calculations. The understanding of mechanical behavior of multilayered 2D nanocrystals under high strain rate deformation provides guidance for nanoshaping of 2D nanocrystals and a tool to tailor the transport behavior in 2D materials.
at the nanoscale. Second, the deformation behavior of TMD 2D layered materials is much different than those of conventional thin films. TMD 2D materials have very strong in-plane rigidity, whereas the interlayer sliding between WSe2 layers occurs under slight shear loading.13 The interlayer shear modulus of the WSe2 sheets is 3 orders of magnitude lower than their in-plane Young’s modulus.14 Third, due to their high in-plane and flexural modulus and challenges to form good interfaces between the TMDs and the substrates, the generated strains and structural changes in TMDs are not permanent in traditional bending or straining techniques, which are not suitable for device fabrication. In this work, we have utilized pulsed laser processing to generate shock pressure at the GPa level within an ultrashort time scale onto multilayer WSe2 2D crystals, which are placed on top of a nanotrench. This setup can be used to study the nanoscale mechanical deformation process at high stress and strain rates and the resulting electrical and optical property changes. Laser-shock-induced nanoshaping has been used in thin metal films,15 monolayer 2D crystals,16,17 and nanowires18 to generate three-dimensional nanoscale shaping and strain engineering.19 The response of TMD 2D layers to laser-shock nanoshaping is intriguing due to their highly anisotropic mechanical properties that are similar to those of the complicated deformation mechanism of layered structures in foliated rocks in nature. A formation of a kink-band geometry is found during the shock process by applying an ultrashort laser-pulse-induced shock wave to generate high stress and
RESULTS AND DISCUSSION Figure 1a depicts the schematic process of laser-shock nanoshaping of multilayer 2D crystals. A mechanically exfoliated WSe2 multilayer is transferred onto an e-beamfabricated SiO2 nanomold (200−300 nm wide/50 nm deep nanotrenches). Multilayer WSe2 in this work is obtained using mechanical exfoliation as it contains fewer defects and grain boundaries than CVD-grown 2D material.20 A confinement layer (fused silica) and a momentum transfer layer (aluminum) B
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Figure 2. Cross-sectional TEM analysis of the deformed multilayered WSe2 by laser shock. (a) Cross-sectional TEM image of the armchairlike deformation of the flake; the inset is the top-down view of the flake. (b) Zoomed-in image of the flake at the first kink, as shown in the red dashed box in (a). The gap between the substrate and kinked multilayer WSe2 is marked by the blue dashed line. The kinked angle is ∼38°. (c,d) Zoomed-in image of the first kink structure in (b). (c) TEM image of upper kinked region showing the twin-like structure of the multilayer. (d) TEM image of the lower kinked region. (d) AFM image of successive laser shock on a WSe2 flake with increasing laser intensity from 3 to 4 GW/cm2.
processing, the multilayer WSe2 is deformed into the nanomold with sloped side walls, as shown in the 3D morphology of laser-shocked WSe2 in Figure 1f. The transmission electron microscopy (TEM) images of the WSe2 layers after laser shock are shown in Figure 1g, which is consistent with the MD simulation results. The WSe2 layers are deformed into kink-bands with a kink-boundary in the structure of twin planes. The WSe2 layers do not conformably deform into the nanomolds, whereas some areas remain in a suspended kink-band structure with a kink-angle between the kink-band plane and the unrotated plane. It is interesting to note that the kink-band structure in WSe2, as seen from Figure 1d, is very similar to the kink-bands found in many metamorphic rocks, such as the Biotite found in the Migiandone, Val’dossola, Italian Alps, as shown in Figure 1h. The schematic diagram representing kink-band formation in foliated layers of rocks is illustrated in Figure 1i; although the mechanism is much different from that in laser shock of 2D materials, the kink-band formation is still related to the shearing of the foliated layers under high strain rate deformation. The kink-band in a rock is a sharply defined zone within which the foliated layers have been deformed by gliding. The boundaries of the zone are sharp, often planar, and are inclined at a certain angle to the active glide plane. The active glide plane is flexed sharply at the kink-boundary such
are placed above the WSe2 2D crystal layers to effectively transfer a laser-induced shock wave into the 2D crystal layers. The laser source is a Q-switch Nd:YAG laser with a pulse duration time of 5 ns, and the laser intensity is defined as the average value of the pulse laser energy, which can induce shock pressure at the GPa level. Figure 1b,c depicts the atomic structure of multilayer WSe2 before and after the application of laser shock. The schematic diagram representing the kink-band and twin boundary formation in WSe2 layers is illustrated in Figure 1d. During the laser-shock interaction with the multilayer WSe2 2D crystals, the deformation of WSe2 is complex due to its large in-plane modulus,21 low shearing modulus, and substrate−WSe2 interaction, including friction and van der Waals forces. The dangling-bond-free surface of 2D materials also facilitates interfacial sliding between materials and substrates under such high pressure. The multilayer WSe2 is deformed to kink-bands with a twin boundary formed on both edges of the kink-band. The shape of the WSe2 multilayer crystals after laser shock into the predesigned trench structures is measured by AFM. The AFM results clearly verify that the WSe2 flake is fully suspended on top of the trench structure after the transfer process. As shown in Figure 1e, the multilayer WSe2 is flat and uniform after being transferred to the top of the trench, which is determined by its bending stiffness and adhesion energy.22,23 After laser-shock C
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Molecular dynamics simulations have been carried out using LAMMPS27 in two steps to give us insight into the interesting kink-band structure deformation mechanics in multilayered WSe2. Figure 3a−d depicts the molecular dynamics of the
that the angle between the glide plane in the host crystal and in the kink-band is bisected by the kink-boundary. The flakes at most of the previously conformal areas detach from the substrate and become suspended again, indicating elastic deformation during the laser-shock process. The shock pressure applied onto multilayer WSe2 is controllable.15 The relationship between laser-shock power and layer thickness is shown in the AFM imaging results in Figure S1 and is further discussed in the Supporting Information. The change in the photoluminescence (PL) of WSe2 before and after laser shock is shown in Figure S2. The PL spectra for multilayer WSe2 consist of two peaks located at 1.62 and 1.52 eV, corresponding to direct and indirect band gaps, respectively, in accord with previous works.24 Red shifts of 60 and 110 meV are observed for the respective direct and indirect band gaps after laser shock. This is mainly due to the large bending strain around the kink-hinge and the in-plane strain in the straight areas of WSe2 layers after the laser-shock process. In the multilayered WSe2 structure, the weak interlayer interactions enable facile sliding between layers under shearing and bending. Due to the high strain rate laser shock and the ultrashort picosecond laser pulse time scale, a nanogeometry with tunable strain is formed. This geometry is further studied using cross-sectional TEM analysis, as shown in Figure 2 (more details are provided in Figure S3). Figure 2a depicts the kinked deformation after laser shock, featuring ultraflat surfaces on the top and the bottom of the trenches. The multilayer WSe2 is observed to be linearly deformed along the side walls of the trenches, rather than conformally attached to the concave features. Figure 2b shows an enlarged view of the kink-band multilayer with a large interfacial gap (blue dashed line) between the side wall of the nanomold and the multilayer WSe2. The deformed WSe2 shows sharp transitions between the different straight lines, with a tilt angle of 38°, which is a deterministic parameter for the kink-band structure. The bending effect is highly localized in areas with lengths around 2 nm, as shown in Figure 2c,d, forming a perfect twin boundary, where the lattice orientation is equally 71° from both sides of the twin boundary. It is worth noting that such kink-band deformation mechanisms have been studied for anisotropic layered materials such as mica in the field of structural geology.4,25 However, kink-band deformations in 2D layered nanomaterials have not yet been widely studied. In the field of structural geology, Dewey explained that the kink-planes are plane and parallel regardless of foliation (layering in metamorphic rock) rotation, indicating that these planes are invariant during deformation or that the kink-angle is an intrinsic property. The mechanism of nucleation and growth of kinks has also been studied in card deck experiments in 1974.26 The angle of inclination of the foliation may be stabilized from a smaller angle to a characteristic large angle, which strongly depends on the direction and the value of the applied loads. In our case, the underlying complex mechanism behind the kinking of laser shocked multilayer 2D WSe2 strikingly resembles the kinks formed in foliated rocks and observations of the card deck experiments. The layer orientations around the bend areas are highly symmetric, and the strains in each layer are found to be similar, thereby reducing the potential for delamination between the layers. The friction between the interlayers of WSe2 also limits a slip and delamination between the layers, thus resulting in a kink-band structure.
Figure 3. Simulation results for the deformation of the multicrystalline aluminum film. (a−f) Deformation procedure of the multilayer at different times. (a) Initial setup of molecular dynamics of the laser-shock deformation process at 0 ps. (b) Multilayer WSe2 pressed onto the top part of the trench at 12 ps. (c) Onset of wrinkling in the multilayer WSe2 observed around 38 ps. (d) WSe2 makes complete contact with the lower part of the trench at 64 ps. (e) Relaxation of the multilayer after the ultrahigh strain rate deformation. (f) Reduction in structural wrinkles through interlayer sliding from a time step of 65 to 130 ps. (g) Layer-by-layer strain plot for relaxed configuration (f) of multilayer WSe2.
laser-shock deformation evolution of six-layer WSe2 at an initial velocity of 0.2 km/s. It is found that when the impact velocity of the laser shock is below 0.1 km/s, there is a lack of sufficient momentum to deform the WSe2. The supporting video (Movie S1) demonstrates the deformation process of a flat WSe2 layer that is deformed to a kink-band on a nanotrench mold, in which the WSe2 layers are not conformally imprinted to the nanomold but form a kink-band with a fixed angle to the side wall of the mold. Figure 3e,f shows the relaxation and selfelimination of wrinkles in the multilayer WSe2. Figure 3b is the state after equilibration and relaxation. A gap is observed between the aluminum film and the multilayer WSe2. During the dynamic deformation process, wrinkles start to form in the multilayered WSe2 at around 38 ps (Figure 3c) due to interference from the propagating tensile stress waves. The wrinkled portion begins to attach onto the bottom of the trench at 55 ps, whereas the aluminum film is only partially deformed, as observed from Figure 3d. In the second step, the wrinkles are eliminated during the adiabatic relaxation process, D
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Figure 4. Schematic diagram (a) and simulation (b) of atomic force distribution (van der Waals force and shear force between the layers) for half-profile of deformed WSe2 plotted along the location from edge to center. (b) Simulation and experimental results for the radius of curvature for different layers in multilayer WSe2; for different layers, the radius of curvature is almost constant in both simulation and experiment, indicating finite sliding in the sample during the laser shock to form the kinked twin-like structure. (d) Layer-by-layer curvature magnitude comparison from experimental TEM results and MD simulation results. (e) Twin boundary from the TEM image with an average value of the twin angle illustrating a constant twin angle. The simulated results agree well with experimental results.
plotted in Figure S5b). Atomic shear strains of up to 2.5% are observed at the kink-hinges. As expected, the shear strains are higher at the kink-hinges than at the flat region of the flake. The overall shear strains are found to be lower than tensile inplane atomic lattice strains, as shown in Figure 4b. Higher van der Waals and shear forces are observed at the sharp transitions of the kink-band structure, revealing the consistency in the magnitude of the curvature angle. In addition, the local strain29 at the corner of WSe2 can enhance van der Waals forces, leading to stiffening and locking of the structure which results in the kinked shape structure independent of flake thickness. Figure 4c depicts the close-up view of the twin boundary as observed in experiment and simulation. Interestingly, due to the high strain rate, the magnitude of the curvature across the layers is consistently independent of the trench size. The radius of curvature of each layer of WSe2 is calculated after determining the curvature angle for each layer using an image filtering algorithm.30 The radius of curvature minimizes the energy of the multilayer WSe2 and “locks” the system, resulting in discrete twining bands31 and symmetric kink formation upon straining. The values of the radius of curvature from the TEM results are compared to the values predicted by simulations, and it is found that the results are in good agreement with each other. This confirms that characteristic sliding occurs between the layers during the deformation of multilayer WSe2, as it reaches a maximum curvature angle resulting in the kink-band structure. The curvature magnitude of the six layers from experimental TEM results and MD simulation results is compared in Figure 4d. The results are in good agreement with each other. The similar curvature magnitudes of the layers suggest that the sliding between the
as seen in Figure 3e,f, to reduce the elastic strain energy stored in the wrinkles generated in the non-equilibrium laser-shock process. The elastic tensile strain along the plane stretches the WSe2 flake to form straight lines with different slopes along the trench. The simulated corner of the WSe2 flake shows sharp transitions consistent with our TEM data, as observed in Figure 2c,d. Layer-by-layer atomic strain distribution for the relaxed configuration is calculated28 and depicted in Figure 3g. The overall profile of strain remains similar for all of the layers. The strains localized near the corners of the trench are found to be lower in the flat region of the flake due to relaxation during self-elimination of the wrinkles, and more details are included in the Supporting Information (Figures S4 and S5). When 2D multilayer TMD material is deformed, sliding occurs due to the van der Waals force interactions which displace the 2D sheets instead of breaking the strong covalent bonds. This can be seen from the Movie S1. This mechanism of sliding of the layers continues until an equilibrium “kink-angle” is reached and a twin boundary is observed. The equilibrium shape of the multilayer kink is determined by the force balance between the van der Waals forces and the shear forces acting between the layers, as shown in Figure 3h, which also determines the thickness of the twin boundary. To visually analyze the atomic forces in WSe2 during kink-band deformation, a schematic diagram of atomic force comprising the van der Waals and shear forces is depicted in Figure 4a. The distribution of the atomic forces for equilibrated wrinklefree strained multilayer WSe2 is plotted in Figure 4b. The atomic zoomed-in snapshot of deformed and undeformed configurations from a top view is shown in Supporting Information Figure S5a), and the atomic shear strains are E
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Figure 5. Band structure of strained multilayer WSe2 at different locations. (a) Schematic of the three simulated locations at upper edge (location A), lower edge (location B), and center (location C). (b) Calculated band structure for multilayer WSe2 before laser shock with no strain. (c−e) Calculated band structure for locations A, B, and C, respectively. The red arrow indicates a direct band gap at the K point of the Brillouin zone, and the green arrow indicates an indirect band gap between Γ point of the valence band and K point of the conduction band. (f) Relationship between average local strains and the direct and indirect band gaps for A, B, and C in the simulation and in the experimental PL results. With increased strain, both direct and indirect band gaps are decreased.
layers takes places in a similar manner, resulting in the same bending curvature and arc length for each layer after the high strain rate deformation. Each layer has its center of arc. This is much different from the traditional bending of sheet metals, where the arcs at different positions share the same center, and arc length is increasing proportionally with their bending radius (as shown in the red marked line in Figure 4e). The twin boundary width of about 2 nm in the layered WSe2 crystal can be clearly seen from the TEM image in Figure 4e, with constant arc length and bending radius for each layer. Density functional theory (DFT) calculations have also been carried out using Quantum Espresso (Figures S6 and S7).32 The energy band structures of the deformed multilayer WSe2 are plotted at three major locations depicted in Figure 5a, viz. upper kink-hinge (location A), lower kink-hinge (location B), and central region (location C). The energy band structure for the multilayer WSe2 before laser shock is shown in Figure 5b for comparison. The energy band structures for three locations (A, B, and C) are plotted as shown in Figure 5c−e, respectively. The relationship between the local strain and band gap is plotted in Figure 5f. The band gap opening greatly depends on the level of local strain. Both direct and indirect band gap are red-shifted with an increase of strain. Location A exhibits higher strains of 5.8% compared to that at location C. Location A shows a direct band gap of 1.503 eV and an indirect band gap of 1.31 eV with strains of 5.8%, whereas location B with comparable strains of 5.4% shows a direct band
gap of 1.51 eV and an indirect band gap of 1.32 eV. The calculation results are consistent with PL measurements. To analyze the influence of crystal structure deformation on the electronic properties of WSe2, FET devices were fabricated from multilayer WSe2. The performances have also been measured before and after laser shock. After laser shock, the WSe2 channel also shows a kink-band structure, with two metal electrodes perpendicular to the wrinkle direction, as shown in Figure 6a, and its inset shows the schematic of the device structure. The FET output characteristics are shown in Figure 6b,c (line graphs in Figure S8). Before laser shock (Figure 6b), the drain current increases linearly with the increase of the source−drain bias under a fixed gate bias. With the increase of a positive gate bias, the source−drain current also increases under a fixed drain bias. After laser shock (Figure 6c), under a fixed gate bias, the source−drain current is also found to be enhanced by 2-fold under the same source−drain bias, suggesting that the contact quality has been improved. To investigate the origin of this phenomenon, the saturation drain current as a function of gate bias is shown in Figure 6d. We assume that the gate insulator capacity is unchanged because the device is not damaged after laser shock, and the channel length and width are not changed. The kink-band deformation will induce modulation of the energy band dispersed on the WSe2 crystal lattice, where strain can be seen as a modulation factor to the system according to the k·p method. Here, we focus on the conduction band of kinked WSe2; the strainF
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Figure 6. Electronic performances of multilayer WSe2 before and after laser shock. (a) AFM image of the device with kinked multilayer WSe2 as the channel. The inset shows the schematic of the device structure. (b,c) Drain current Id as a function of drain bias Vds before (b) and after (c) laser shock, with the gate voltage ranging from 0 to 40 V in steps of 5 V. The drain current is enhanced by 2-fold after laser shock, and the contact quality is improved after laser shock. (d) Saturation drain current as a function of the gate bias before and after laser shock, which indicates that the carrier mobility is increased after laser shock as a result of strained kink-band crystal structure. (e,f) Drain current Id versus gate voltage Vgs for the WSe2 FET with drain−source voltages Vds ranging from 0.1 to 1.6 V in steps of 0.3 V before (e) and after (f) laser shock, respectively. For drain−source voltages from 0.1 to 1.6 V, the drain−source current is observed to be higher after laser shock, and the on−off ratio is also increased.
where gm is the transconductance of the WSe2 device, Lch is the channel length, Wch is the channel width, CSiO2 = 115.1 μF/m2 is the capacitance of the gate oxide per unit area, and Vds is the drain bias; the kink-band-induced strain can enhance the carrier mobility of WSe2 from 861.9 to 1805.3 cm2/(V·s) after laser shock under a 0.1 V drain bias, finally leading to the increase of current after laser shock. Under a fixed source− drain bias, the device shows ambipolar characteristics, which are preserved after laser shock. The on−off ratio is further increased from ∼106 to ∼107 after laser shock. The current under a fixed gate bias is also increased after laser shock, indicating that the device performance has been improved.
induced crystal lattice change gives rise to the modulation of an electron wave vector, as described by eq 1 below: k′ = k 0 1 − κ 2ε 2
(1)
where k0 is the electron wave vector under zero strain, κ is the ratio between the shear deformation potential and energy separation between two lowest conduction bands, and ε is the strain vector along the WSe2 in-plane direction. Then the effective electron mass is determined from band dispersion characters.33,34 1 1 ∂ 2E = me ℏ ∂ 2k′ me =
(2)
CONCLUSIONS In conclusion, by using ultrafast laser shock to deform multilayer WSe2, we are able to create a tunable strain profile with low interlayer friction and sliding, which provides an ideal platform to understand the complex mechanical deformation process in multilayer 2D materials. A kink-band structure in multilayer WSe2 is observed through TEM and AFM measurements, and the strain gradient is confirmed to be uniform across all layers of WSe2. To better understand the deformation mechanism under ultrafast high strain, molecular dynamics simulations are carried out and density functional theory is used to evaluate the electronic energy band gap of the deformed multilayer WSe2. A FET device based on the kinked multilayer WSe2 is also fabricated to confirm the improved performance with a deformed crystal structure. The mobility is increased from 861.9 to 1805.3 cm2/(V·s) after laser shock under a 0.1 V drain bias, and the on−off ratio is further
m0 1 + κ 2ε 2
(3)
where m0 is the electron mass under zero strain, ℏ is the reduced Planck constant, and the effective carrier mass becomes lighter along the wrinkle kink direction, leading to larger mobility according to eq 4 below:35 q μ= τ me (4) To calculate electron mobility in detail, the transfer curves are also measured, as shown in Figure 6e,f (line graphs are included in Figure S9). Based on eq 5 g Lch μ= m × Vds WchCSiO2 (5) G
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ACS Nano increased from ∼106 to ∼107 after laser shock. Significantly, this research attempts to elucidate the formation of kink-bands via ultrahigh strain rate deformation in multilayered 2D materials, which provides an avenue to obtain a greater insight into the straining mechanisms of TMDs for further improving electronic and optical applications based on mechanical design.
shocked multilayer WSe2; zoomed-in snapshot of undeformed and deformed atomic lattice configuration of WSe2 and atomic shear strain plotted along the location for the kinked WSe2; molecular dynamics simulations of deformation of trilayer WSe2 at various impact velocities; a four-unit supercell of WSe2 in the DFT calculations; band structure of monolayer WSe2; electronic characters before and after laser shock; transfer curves before and after laser shock (PDF) Kink formation in the WSe2 crystal (MP4)
METHODS Laser Shock Nanoshaping of Multilayered WSe2. Multilayer WSe2 was obtained using mechanical exfoliation as it contained fewer defects than CVD-grown 2D materials. The silica mold (200−300 nm wide/50 nm deep nanotrenches) was fabricated by e-beam lithography. An Nd:YAG laser (Continuum Surelite III, 1064 nm wavelength and pulse duration of ∼5 ns) was used as an energy source for ablation. A glass slide was used as a confining media because of its high shock impedance, and a sacrificial coating was deposited by spraying graphite lubricant above the 4 μm aluminum foil. The graphite layer was instantaneously ablated by the high-intensity 3 mm diameter laser beam generating a shock wave which caused deformation in the WSe2 flake, thereby changing the equilibrium state for the nonlinear flake−substrate system. Molecular Dynamics Simulation. The molecular dynamics simulations were performed using LAMMPS27 to investigate the laserinduced deformation effect and interlayer sliding behavior of multilayered WSe2. The multilayered flake was modeled as a rectangular sheet of ∼50 nm × 7 nm. The interplane interactions, intraplane interactions, and interlayer interactions of two neighboring atomic layers were defined using the Lennard-Jones potential (ε = 0.3417 eV, σ = 2.93943 A; ε = 0.3417 eV, σ = 2.281 A; and ε = 0.01994 eV, σ = 3.4 A).36 To capture the friction effects, the long ends are coupled horizontally by a harmonic coupling spring.37 An initial velocity from 0.2 to 0.8 km/s was applied to the aluminum film to simulate experimental shock pressure in the range between 7 and 10.2 GPa. Density Functional Calculations. The electronic structure of the deformed multilayer WSe2 was estimated using density functional theory calculations. The coordinates of the deformed WSe2 were extracted at three major locations at the upper corner, the lower corner, and the center, viz. A, B, and C, respectively, as shown in Figure 4a. Density functional theory using the ultrasoft Perdew− Burke−Ernzerhof (PBE) potential32 was employed with help of Quantum Espresso38 to study the electronic properties of the strained WSe2. Band structure was calculated along the symmetry points Γ, M, K, and Γ. The path was depicted in the Brilluion K path selection in Supporting Information Figure S7c). A 20 × 20 × 20 Monkhrost pack k mesh was used with a cutoff energy of 400 eV. A baseline DFT simulation was carried out with a unit cell of undeformed WSe2 to obtain a direct band gap of 1.577 eV at the K point for single-layer WSe2 and an indirect band gap of 1.604 eV for two-layer unstrained WSe2. AFM and TEM Experiments. AFM images and line profiles were acquired from a Digital Instruments/Veeco system with tapping mode. TEM cross-sectional experiments were carried out with a FEI Titan transmission electron microscope. The sample was cut with a focused ion beam with a nanomanipulator.
AUTHOR INFORMATION Corresponding Authors
*Phone: 86 27- 87792461. E-mail:
[email protected]. *Phone: +1 765-494-5436. E-mail:
[email protected]. ORCID
Gary J. Cheng: 0000-0002-1184-2946 Author Contributions ⊥
M.M., Y.H., and L.T. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS Financial assistance in the form of National Research Council Senior Research Associateship (G.J.C.), NSF Grant Nos. CMMI-0547636 and CMMI 0928752 (G.J.C.), and National Natural Science Foundation of China (Grant No. 61704061) (L.Y.) has been crucial for this work. The authors appreciate the discussion and help from Tingting Shen and Dr. Joerg Appenzeller of Purdue University in the characterization of the materials. REFERENCES (1) Dewey, J. F. Nature and Origin of Kink-Bands. Tectonophysics 1965, 1, 459−494. (2) Marshall, B. Kink-Bands and Related Geological Structures. Nature 1966, 210, 1249−1251. (3) Borg, I.; Handin, J. Experimental Deformation of Crystalline Rocks. Tectonophysics 1966, 3, 249−367. (4) Hörz, F. Static and Dynamic Origin of Kink Bands in Micas. J. Geophys. Res. 1970, 75, 965−977. (5) Gupta, A.; Sakthivel, T.; Seal, S. Recent Development in 2D Materials Beyond Graphene. Prog. Mater. Sci. 2015, 73, 44−126. (6) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451−10453. (7) Akinwande, D.; Petrone, N.; Hone, J. Two-Dimensional Flexible Nanoelectronics. Nat. Commun. 2014, 5, 5678. (8) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; et al. Progress, Challenges, And Opportunities in Two-Dimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898−2926. (9) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics Of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699− 712. (10) Alyörük, M. M.; Aierken, Y.; Ç akır, D.; Peeters, F. M.; Sevik, C. Promising Piezoelectric Performance Of Single Layer TransitionMetal Dichalcogenides and Dioxides. J. Phys. Chem. C 2015, 119, 23231−23237. (11) Hu, Y.; Huang, Y.; Tan, C.; Zhang, X.; Lu, Q.; Sindoro, M.; Huang, X.; Huang, W.; Wang, L.; Zhang, H. Two-Dimensional Transition Metal Dichalcogenide Nanomaterials for Biosensing Applications. Mater. Chem. Front. 2017, 1, 24−36.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b04705. Discussion of the deformation of WSe2 in relation with laser shock power and layer thickness, kink formation mechanism, and strains in laser-shocked WSe2; AFM image of successive laser shock on a WSe2 flake with increasing laser intensity; line profiles for the sample before and after laser shock and PL spectra of the sample before and after laser shock; TEM images of laserH
DOI: 10.1021/acsnano.9b04705 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano
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