Strain-Induced Electronic Structure Changes in Stacked van der

Apr 27, 2016 - Vertically stacked van der Waals heterostructures composed of compositionally different two-dimensional atomic layers give rise to inte...
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Strain-Induced Electronic Structure Changes in Stacked van der Waals Heterostructures Yongmin He,†,‡ Yang Yang,† Zhuhua Zhang,†,§ Yongji Gong,†,∥ Wu Zhou,⊥ Zhili Hu,† Gonglan Ye,† Xiang Zhang,† Elisabeth Bianco,∥ Sidong Lei,† Zehua Jin,† Xiaolong Zou,† Yingchao Yang,† Yuan Zhang,† Erqing Xie,‡ Jun Lou,† Boris Yakobson,† Robert Vajtai,† Bo Li,*,† and Pulickel Ajayan*,† †

Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu 730000, P. R. China § State Key Laboratory of Mechanics and Control of Mechanical Structures, Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China ∥ Department of Chemistry, Rice University, Houston, Texas 77005, United States ⊥ Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡

S Supporting Information *

ABSTRACT: Vertically stacked van der Waals heterostructures composed of compositionally different two-dimensional atomic layers give rise to interesting properties due to substantial interactions between the layers. However, these interactions can be easily obscured by the twisting of atomic layers or cross-contamination introduced by transfer processes, rendering their experimental demonstration challenging. Here, we explore the electronic structure and its strain dependence of stacked MoSe2/WSe2 heterostructures directly synthesized by chemical vapor deposition, which unambiguously reveal strong electronic coupling between the atomic layers. The direct and indirect band gaps (1.48 and 1.28 eV) of the heterostructures are measured to be lower than the band gaps of individual MoSe2 (1.50 eV) and WSe2 (1.60 eV) layers. Photoluminescence measurements further show that both the direct and indirect band gaps undergo redshifts with applied tensile strain to the heterostructures, with the change of the indirect gap being particularly more sensitive to strain. This demonstration of strain engineering in van der Waals heterostructures opens a new route toward fabricating flexible electronics. KEYWORDS: Stacked van der Waals heterostructures, strain, electronic band structure interaction, photoluminescence, chemical vapor deposition, controlled orientation and stacking order

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imental demonstrations of tunable band structures through strain engineering of monolayer transition metal dichalcogenides have been reported.15,16,21 However, no experimental result on van der Waals heterostructures under external strain has been reported due to the poor control in stacking the heterostructures. The effects of interlayer interactions on properties such as electronic band structure (alignment or bending), carrier transport, and their dependence on external strain or stress are strongly influenced by the stacking order, orientation of layers, and contaminants between layers.6,22−26 Most of the heterostructures reported have been achieved by transfer methods. The second layer is transferred onto the first layer by mechanical exfoliation and stamping using Scotch tape or PDMS,3−5 polymer-assisted transfer method (using polymer as a transfer media and etching the sacrificial layer underneath),23

wo-dimensional (2D) van der Waals (vdW) materials with several atomic layer thickness and tunable band gaps have attracted significant attention.1,2 Heterostructures3 from these 2D materials by vertical layer-by-layer stacking, such as MoS2/WSe2,4,5 MoS2/WS2,6 MoS2/graphene,7,8 WSe2/graphene,9 and so forth, offer a novel platform for exploring new properties and new physics (e.g., superlattice Dirac points,10 Hofstadter butterfly pattern,11,12 etc.), and thereafter innovative devices and applications (e.g., tunneling transistors memory devices,13 ultrathin photodetectors,14 and solar cells,5 etc.). The electronic band structures of 2D materials are very sensitive to thickness, as well as external strain and stress.15−19 Stacked van der Waals heterostructures, exhibiting coupled effects from different 2D materials, would provide a unique platform to study the interaction between the different monolayers under external strains. Moreover, these heterostructures have strong potential for application in the areas of flexible sensors, transistors, and photodetectors, with improved sensitivity as compared to monolayer devices (with only one type of material) due to the strong interaction between individual layers.17,20 Recently, both theoretical and exper© 2016 American Chemical Society

Received: March 3, 2016 Revised: April 25, 2016 Published: April 27, 2016 3314

DOI: 10.1021/acs.nanolett.6b00932 Nano Lett. 2016, 16, 3314−3320

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Figure 1. (a) Schematic of a CVD grown WSe2/MoSe2 heterostructure with bilayer WSe2 on the edge and stacked WSe2/MoSe2 heterobilayer in the center. The atomic models correspond to the WSe2/MoSe2 heterobilayer with a 3R stacking orientation. (b) The optical image of an intentionally chosen incomplete WSe2/MoSe2 heterostructures with uncovered MoSe2 monolayer exposed showing distinct color contrast. (c) Z-contrast image of the WSe2/MoSe2 heterobilayer region, and the inset is corresponding image intensity profile acquired along the red line in c, where the lowest contrast corresponds to one Mo atom in MoSe2, the medium contrast to two Se atoms in WSe2, and the highest one overlapping one W atom in WSe2 and two Se atoms in MoSe2, respectively.

or a combination of these two.27 In these methods, however, it is difficult to control orientation and stacking order with atomic accuracy. In addition, the transfer method can easily leave contaminants between the stacked layers leading to diminished interaction. To achieve fundamental physical insight and develop applications for heterostructures such as flexible electronics, the effects of interaction between layers on electronic band structure and carrier transport and their dependence on external strain or stress must be understood. We aimed to study the interlayer interaction in a seamless stacked van der Waals system with highly controlled orientation and stacking order. To achieve this, we have stacked MoSe2/WSe2 vdW heterostructures with 3R stacking via a two-step CVD method in which a WSe2 monolayer was grown on top of a MoSe2 monolayer. For the first time, we observed both interlayer coupled direct and indirect band structures in a vdW heterostructure via photoluminescence (PL). Moreover, both the first-principles electronic structure calculations and the experimental results show strong strain dependences of both bands, with the indirect coupling band being more sensitive to external strain. A WSe2/MoSe2 heterobilayer with tightly controlled 3R stacking is an ideal system to study the interaction between stacked WSe2 and MoSe2 monolayers. Figure 1a is the schematic of an as-grown sample with a WSe 2 /MoSe 2 heterobilayer in the center and a WSe2 bilayer on the edge. The structure of the sample is the result of a unique growth process. The MoSe2 monolayer was grown on a SiO2 (285 nm thick)/Si substrate in the first step. In the second step, both inplane epitaxial growth of WSe2 along the edge of MoSe2 and stacked growth of a second layer of WSe2 occurred, beginning from the edge and proceeding toward the center of the crystal.28 The result is a WSe2 bilayer as the selvage and the WSe2/MoSe2 heterobilayer in the center. The heterobilayer

region is the area of interest in the following discussion. It is worth mentioning that CVD grown heterobilayers usually deliver seamless stacking with controlled orientation, and in most cases, exhibit 3R ordered stacking, as shown in Figure 1a. Figure 1b is the optical image of a specifically chosen “incomplete” WSe2/MoSe2 heterostructure with all the representative structures: WSe2 bilayer, WSe2/MoSe2 heterobilayer, and uncovered MoSe2 monolayer, from edge to center. The scanning electron microscope (SEM) of a completely covered WSe2/MoSe2 heterostructure is also shown in Figure S1 and its inset. We performed atomic force microscopy (AFM, Supporting Information) and aberration-corrected scanning transmission electron microscopy (STEM) to investigate the quality of WSe2/MoSe2 heterostructures. The AFM image of a typical CVD-grown WSe2/MoSe2 heterostructure crystal on a SiO2/Si substrate is shown in Figure S2, with the joint between the WSe2 bilayer and WSe2/MoSe2 bilayer marked by a black dashed line corresponding to its optical image. The bilayer WSe2 structure on the edge has been confirmed from the line profile, and no step is observed at the boundary between heterobilayer MoSe2/WSe2 and bilayer WSe2 suggesting a smooth transition between bilayer WSe2 and the heterobilayer. Annular dark-field (ADF) imaging in an aberration corrected scanning transmission electron microscope (STEM) were performed to further explore the atomic structure of the MoSe2/WSe2 heterostructures. Figures 1c shows atomicresolution Z-contrast images from the WSe2/MoSe2 heterobilayer region and its corresponding image intensity profile, respectively. The alternating columns of atoms with distinct contrasts (the high, medium and low contrast columns are W atoms in WSe2 aligned with Se column in MoSe2, Se column in MoSe2, and Mo atoms in MoSe2, respectively) suggest the asgrown stacked WSe2/MoSe2 heterobilayer preserves the 3R stacking. 3315

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Figure 2. First-principles electronic structure and the strain dependence of a WSe2/MoSe2 heterobilayer corresponding to an optimized stacking order (3R) with minimized energy. (a) The first-principles electronic structure of the heterobilayer at 0% strain. Coupling in both direct and indirect band gaps is observed. Green and blue circles mark the band contribution of W (green) and Mo (blue) 3d orbitals, respectively. The circle size reveals the weight of the orbital components in the bands. (b) The biaxial stretching mode. Mo atoms in the bottom layer overlap with Se atoms on the top layer. (c) The electronic structure at 2.5% strain. (d) The dependence of both direct band gap (black circle) and indirect band gap (red triangles) on strain.

Figure 3. Raman and Photoluminescence characterization of a typical CVD grown WSe2/MoSe2 heterostructure. (a) Optical image of an as-grown WSe2/MoSe2 heterostructure used for Raman and PL characterizations. (b) Raman spectra taken from the three points marked in (a) and individual MoSe2 and WSe2 monolayers. (c) Raman mapping of the intensity of the E12g mode collected from the rectangle region in a. (d) PL spectra taken from the three points marked in (a). (e and f) PL position mapping (e) and intensity mapping (f) of coupling peak at the range of 900−950 nm collected transition region between WSe2 bilayer and WSe2/MoSe2 bilayer.

The interlayer interactions of the heterobilayer were predicted using first-principles electronic structure calculations based on density functional theory as implemented in VASP code. The optimal stacking order (3R) is adopted for our

model systems. Figure 2a presents the band structure of unstrained MoSe2/WSe2 heterobilayer, which has an indirect band gap (1.11 eV, red arrow) that is significantly reduced from those of individual atomic layers. The valence band maximum 3316

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Figure 4. (a) Schematics of the fabrication and measurements of the strained WSe2/MoSe2 heterobilayer. (b) PL spectra of strain dependence of the indirect-band gap and direct-band gap coupling peaks from the PL measurement. (c) Strain dependence of the indirect-band gap and direct-band gap and intensities from the PL measurement with linear fitting. (d) The dependence of peak intensity on external strain.

strain leads to the in-plane expansion of the lattice, which weakens the overlap of the metal d orbitals. As such, the straininduced change rate of the direct gap in K is relatively slow. On the other hand, the interlayer spacing decreases due to the Poisson’s ratio, which leads to a rapid upshift of the states at Γ point allowing the chalcogen pz orbitals from different atomic layers to overlap more substantially. Thus, the electronic structure changes in the heterostructure is a result of a coupled effect of in-plane expansion and out-of-plane compression. It is worth mentioning that we also projected the density of states (PDOS) onto Mo and W atoms at tensile strain = 0, 1%, and 2%, as shown in Figure S3. The results agree well with our above electronic structure analyses. To verify our simulation results, Raman and PL spectroscopies were used to characterize the heterostructures and determine the spatial distribution of MoSe2 and WSe2 with a 514.5 nm laser (spot size ∼1 μm). Raman spectra were taken from the three points marked in the optical image (Figure 3a,b). Raman spectra for both the individual MoSe2 and WSe2 monolayers were also obtained. The two characteristic peaks for monolayer MoSe2 at 241 cm−1 (A1g mode) and 286 cm−1 (E12g mode), and for monolayer WSe2 at 250 cm−1 (E12g mode) and 260 cm−1 (E12g mode) can be observed as shown by dashed lines in Figure 3b. In the pink inner area (point 1), three main peaks at 241 cm−1, 249 cm−1, and 280 cm−1 were observed corresponding to WSe2 and MoSe2 in the heterobilayer. On the other hand, Raman spectra collected from the blue selvage (point 3) only show the peaks at 250 and 259 cm−1, which correspond to the E12g and A1g modes of bilayer WSe2. The intermediate position (point 2) shows all Raman peaks from both MoSe2 (239 and 286 cm−1) and WSe2 (250 and 259 cm−1), indicating a transitional region between bilayer WSe2 and heterobilayer WSe2/MoSe2. Due to the presence of Raman peak at 250 cm−1 in the whole region, the spatial distribution of the WSe2/MoSe2 heterobilayer was further examined by Raman

(VBM) of the bilayer structure changes from the K point (monolayer of MoSe2 or WSe2) to the Γ point, while the conduction band minimum (CBM) retains at the K point. In addition, there is also a direct band gap at K that is larger than the indirect band gap by a mere 140 meV (black arrow). The indirect band coupling can be easily observed experimentally in the PL spectra of heterobilayers and homobilayers,4,27,29 even for samples fabricated by the transfer processes. However, the direct band gap has only been observed in homobilayers of MoS2, WS2 and WSe2.30−32 To our knowledge, there are no experimental reports of a direct band gap observed in heterobilayers obtained by transfer methods. In some transfer-stacked heterostructure, even diminished direct band photoresponses of both layers have been reported.4 The key structural difference lies in the stacking registry of two layers. The homobilayers obtained from CVD growth or exfoliation have an atomically accurate stacking order, which is not achievable in a heterobilayer obtained by transfer methods. Therefore, we believe that in order to reveal the missing direct band coupling of heterobilayer, stacking order must be strictly controlled and contamination must be avoided. To gain insight into the interaction of band structures, we performed detailed first-principles electronic structure calculations for the heterostructures under different biaxial strains as shown in Figure 2b. Experimental verification of these predictions by PL measurements is discussed later. As shown in Figure 2c, both coupling band gaps decrease significantly under 2.5% biaxial tensile strain. The calculated band gap shifts with respect to strain are summarized in Figure 2d. The change rate of the indirect band gap for the MoSe2/WSe2 heterobilayer was found to be much higher (181 meV/%) compared to that of the direct band gap (90 meV/%). This is because the VBM at the Γ point contains a portion of the chalcogen pz orbitals from both layers, whereas the CBM at the K point is contributed predominantly by metal d orbitals. The tensile 3317

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direct (from ∼840 to ∼794 nm) and indirect (from ∼940 to ∼892 nm) band coupling peaks as shown in Figure S4b, when the CVD-grown WSe2/MoSe2 heterobilayer was transferred from the SiO2/Si substrate (Figure 3d) to PMMA (Figure 4b). Figure 4b further shows the evolution of the PL spectra of the WSe2/MoSe2 heterobilayer with strain. Linear redshifts of the both the indirect-band gap and direct-band gap coupling peaks within strain range of 0−1.97% have been identified for all measured samples. The redshift for indirect-band gap coupling peak (22.7 meV/% strain) is about twice that of the direct-band gap coupling peak (11.2 meV/% strain), as shown in Figure 4c. This trend is in agreement with our simulations. As reflected in our calculated band structures, the reduction of the indirect-band gap by strain has a contribution from the upshift of the CBM at the Γ point, in addition to the downshift of the VBM at the K point. The change of relative intensity was also observed as shown in Figure 4d and the inset. Specifically, the intensity of the indirect-band gap peak decreases monotonically with strain while that of direct-band gap peak displays the opposite. The intensity ratio of the indirect-band gap to direct band gap peaks decreases from ∼3.1 at zero strain to 1.5 at 2% strain. The increased intensity of direct-band gap peak (peak 1) upon strain could be explained by the increase of the fraction of radiative recombination from the direct valley, the fraction of carriers in the direct valley, and the total number of carriers generated by laser illumination.38 However, the trend for indirect band is not fully clear at this moment. We believe the trend for indirect band gap is related to many factors such as stacked structure, defect, temperature, and substrate, which is beyond the scope of this work. Note that the measured slope of the band gap versus strain is lower than the value obtained in our simulations. There are two possible reasons for this discrepancy. First, the strain applied to the plastic substrate may not be fully transferred to the WSe2 layer and then to the MoS2 layer.37,39 In simulation, both layers are identically strained. Second, in simulation biaxial strain is considered in order to preserve the symmetry; while in experiment, uniaxial strain is applied since it is more accurate and easier to implement. Due to Poisson’s ratio the experimentally measured bandgap shift is decreased to some extent. In summary, we have performed the first experimental study of the interaction between monolayers of a van der Waals heterobilayer MoSe2/WSe2 through strain engineering. The atomically accurate 3R stacking of the heterobilayer prepared by a two-step CVD method enables the observation of strong couplings in both interlayer direct and indirect band structures. Our results suggest that both coupled band gaps are sensitive to external strain. This study provides insight into the interaction between stacked monolayers in TMDC heterostructures and will guide the design of future applications of this type of heterostructure in microscale sensing and actuating systems. Methods. Density-functional theory (DFT) calculations were performed with generalized gradient approximation (GGA) of the Perdew−Burke−Ernzerhof (PBE) functional as implemented in Vienna ab initio Simulation Package (VASP).40,41 Projector-augmented wave pseudopotential was used for modeling core electrons. We adopted the supercell approach by choosing a vacuum layer thickness over 15 Å to avoid spurious interactions from neighboring images. Structures were fully relaxed until residual forces on constituent atoms become smaller than 0.01 eV/Å, and total electronic energies were converged to 10−5 eV. The Brillion zone was sampled by 9 × 9 × 1 grids for geometry optimization and 15 × 15 × 1 grids

intensity mapping (Figure 3c). It is clear that the blue selvage (bilayer WSe2) has the strongest WSe2 relative signal, while the center pink area has the lowest relative signal (Figure 3b). The PL spectra were taken from the same three points, as shown in Figure 3d. PL spectra for both the MoSe2 and WSe2 monolayers obtained by individual growth process show the characteristic peaks for monolayer MoSe2 and WSe2 at 826 and 776 nm, respectively. For the PL spectrum taken at point 3 (bilayer WSe 2 ), two peaks can be obtained through deconvolution: one at 797 nm (A exciton peak, direct band gap emission), and the other one at a 824 nm (indirect band gap emission), as shown in Figure S4a. For the heterobilayer area (point 1), two main peaks were clearly observed at 840 and 940 nm. It is important to note that neither of these peaks occur at the same wavelength as those of individual monolayer MoSe2 and WSe2, and that both peaks indicate band gaps smaller than that found in monolayer MoSe2. This phenomenon agrees strongly with our theoretical predication that both indirect (1.28 eV corresponds to 940 nm) and direct band (1.48 eV corresponds to 840 nm) couplings have occurred. The difference between the experimental value and simulation originates from generalized gradient approximations (GGA) density functions used in simulation which underestimate the band gap and possible influence of the defects in the as-grown heterobilayer.33−35 PL peak position mapping of the heterobilayer/bilayer WSe2 region confirms that the coupling effect exists only in the WSe2/MoSe2 heterobilayer, while no effect is observed at bilayer WSe2 (Figure 3e). The PL intensity mapping (940 nm) also supports this finding, as the coupling peak can be identified only in the heterobilayer (Figure 3f). It is interesting to note that the most intense PL signal is given by the transition region, not from the heterobilayer region. This could be attributed to the built-in electric field at the interface between the heterobilayer and the homobilayer.36 In order to eliminate any potential influence from this transition region, the following PL measurements under strain were taken from points away from the transition region. The coexistence of two coupling peaks (indirect-band gap and direct-band gap) provides an ideal system to study the interlayer interaction in a heterobilayer and verify our theoretical prediction of band evolution under external strain. The emergence of both strong coupling peaks of the WSe2/ MoSe2 heterobilayer could be attributed to the clean and tightly controlled interface between two individual monolayers as shown in the STEM image (Figure 1e). Especially for directband gap coupling effect, this is, to the best of our knowledge, observed experimentally for the first time. To simplify the experimental setup, we employed a uniaxial tensile strain test to engineer the coupling effects raised from the WSe2/MoSe2 heterobilayer. We have designed a mechanical-photoluminescence measurement setup, integrating a micromechanical test stage with a PL measurement system that can apply controllable and repeatable strains while simultaneously collecting the PL spectra at designated strain (Figure 4a). The samples are prepared by transferring the as-grown heterobilayer to a PMMA film (PMMA 950, A9) using the standard PMMA-assisted transfer method. The PMMA film was used as a flexible and transparent substrate for the test. Due to the rapid cooling used in CVD growth of TMDCs materials, a global tensile strain in their few layers can be found, which can be relaxed by the transfer process, leading to a substantial redshift of the PL peak.37 We observed similar shifts in both 3318

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the Central Universities (NE2015104, NS2014006) and the Research Fund of the SKL-MCMS in NUAA (MCMS0415K01). Y.H. acknowledges the financial support from China Scholarship Council. Electron microscopy study was supported by the U.S. Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division (W.Z.), and through a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.

for electronic structure calculations. We took into account the van der Waals interactions using a DFT-D2 approach (a nonlocal correlation functional was added to account for dispersion interactions).42,43 In our mechanical-photoluminescence measurement, we used PMMA (950 PMMA, A9) as a flexible and transparent substrate to apply controllable and reproducible strains on the heterobilayer. First, the MoSe2/WSe2 heterobilayer grown a Si/ SiO2 substrate was spin-coated with PMMA solution, then baked to form a flexible film. Second, we employed KOH solution to remove the SiO2 sacrificial layer and release the 2D materials to the PMMA film. Finally, the mechanical-photoluminescence measurement was performed via a customized setup that combined a Gatan microtest tensile and compression stage and Raman microscope (Renishaw Co.). We used the Gatan microtest tensile and compression stage to apply uniaxial strain on WSe2/MoSe2/PMMA sample. In this process, we first fixed PMMA film on the holders in the microtest stage. The stage together with the sample was fixed onto the sample stage in Raman microscope for integrated mechanical-photoluminescence measurement. The distance between two holders at the initial position was set as the original length (L0). Each moving step (ΔLn) can be recorded by the software of microtest tensile and compression stage. So, the strain can be ΔL + ΔL 2 + ... + ΔLn × 100 where calculated by the equation: ε = 1 L

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the n denotes the number of moving steps. The photoluminescence measurements were performed immediately after each strain step.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00932. Scanning electron microscope and AFM characterizations of the WSe2/MoSe2 heterostructure; calculated density of states (PDOS) projected onto Mo (blue line) and W (green line) atoms in the heterobilayer at different tensile strains; PL spectra of WSe2/MoSe2 heterobilayer before and after transfer (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address

Y.H.: Center for Programmable Materials, School of Materials Science & Engineering, Nanyang Technological University, Singapore 639798, Singapore. Author Contributions

Yongmin He and Yang Yang contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA. Electronic structure computation analysis was supported by the US Army Research Office Electronics Division (grant ref. No. 67026-EL). Z.Z. acknowledges the financial supports from Research Funds for 3319

DOI: 10.1021/acs.nanolett.6b00932 Nano Lett. 2016, 16, 3314−3320

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DOI: 10.1021/acs.nanolett.6b00932 Nano Lett. 2016, 16, 3314−3320