Tuning the Optical, Magnetic, and Electrical Properties of ReSe2 by

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Tuning the optical, magnetic, and electrical properties of ReSe2 by nanoscale strain engineering Sefaattin Tongay, Shengxue Yang, Cong Wang, Hasan Sahin, Hui Chen, Yan Li, Shu-Shen Li, Aslihan Suslu, Francois M. Peeters, Qian Liu, and Jingbo Li Nano Lett., Just Accepted Manuscript • Publication Date (Web): 02 Feb 2015 Downloaded from http://pubs.acs.org on February 3, 2015

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Tuning the optical, magnetic, and electrical properties of ReSe2 by nanoscale strain engineering Shengxue Yang1,‡, Cong Wang2, 3,‡, Hasan Sahin4, Hui Chen1, Yan Li1, Shu-Shen Li1,5, Aslihan Suslu6, Francois M. Peeters4, Qian Liu2, 3,*, Jingbo Li1, 5,*, and Sefaattin Tongay6,* 1

State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China 2

National Center for Nanoscience and Technology, No. 11 Beiyitiao, Zhongguancun, Beijing 100190, China

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The MOE Key Laboratory of Weak-Light Nonlinear Photonics, TEDA Applied Physics School and School of Physics, Nankai University, Tianjin 300457, China 4

Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium

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Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

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School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287, United States

ACS Paragon Plus Environment

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ABSTRACT: Creating materials with ultimate control over their physical properties is vital for a wide range of applications. From traditional materials design perspective, this task often requires precise control over the atomic composition and structure. However, owing to their mechanical properties, low-dimensional layered materials can actually withstand a significant amount of strain and thus sustain elastic deformations before fracture. This, in return, presents a unique technique for tuning their physical properties by ‘strain engineering’. Here, we find that local strain induced on ReSe2, a new member of the transition metal dichalcogenides family, greatly changes its magnetic, optical, and electrical properties. Local strain induced by generation of wrinkle (1) modulates the optical gap as evidenced by red-shifted photoluminescence peak, (2) enhances light emission, (3) induces magnetism, and (4) modulates the electrical properties. The results not only allow us to create materials with vastly different properties at the nanoscale, but also enable a wide range of applications based on 2D materials, including strain sensors, stretchable electrodes, flexible field-effect transistors, artificial-muscle actuators, solar cells, and other spintronic, electromechanical, piezoelectric, photonic devices. KEYWORDS: transition metal dichalcogenides, 2D materials, strain engineering, photoluminescence, excitonics, and magnetism

Strain engineering is an effective and promising way to tune the physical properties of materials. This stems from the fact that energy levels (bands) around the Fermi level are highly sensitive to orbital coupling / interaction-neighboring atoms within the crystal, and the degree of orbital interaction strongly depends on the amount of strain. Despite such sensitivity, most materials cannot sustain very large elastic deformation. Therefore this technique requires use of


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ultra-thin and flexible materials with excellent elasticity and high Young’s modulus for considerable changes in their properties.1-4 Interestingly, application of strain at the nanoscale provides an ideal way to locally tune material properties as desired for various optoelectronic device applications. For example, it has been previously shown that local strain opens up an optical band gap in graphene after mechanical bending along the nano-trenches as a result of band renormalization,5 and strain-induced large pseudo-magnetic fields generate well-defined pseudo-Landau levels.6 Similarly, it has been found that electronic properties of 2D materials are also affected by the presence of wrinkles in the 2D materials obtained by exfoliation or chemical vapor deposition (CVD) technique.7,8 More recently, it has been theoretically predicted that layered NbX2 and VX2 structures (X=S, Se) may exhibit unique ferromagnetism behavior under tensile strain, which can be modulated with applied strain,9 and MoS2 phonon dispersion and optical gap can be modulated by strain.10, 11 In this work, we tailor magnetic, optical, and electrical properties of ReSe2 nanosheets by local strain engineering through formation of ReSe2 wrinkles on elastomeric substrates. Here, we have specifically focused on ReSe2 since even a few layers behave as if monolayers owing to much weaker interlayer coupling compared to other group VI transition metal dichalcogenides (TMDs) such as MoS2, WS2, MoSe2, etc. Since ReSe2 belongs to group-VII TMDs, rhenium atoms contain an extra electron in the d-orbitals and this leads to large differences compared to group-VI TMDs. For example, ReS2 and ReSe2 lack indirect to direct gap transition, but possess strong in-plane anisotropy with vastly different quantum confinement effects, rich Raman spectrum, and interlayer coupling properties. As a result of this weak interlayer coupling aspect, strain engineering technique can be applied to ReSe2 sheets with any thickness (number of layers). Our results show that around locally strained regions, (1) the vibrational spectrum


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changes in a way that degenerate Eg-like Raman peak, causing it to split into two peaks and Aglike peak is right-shifted with respect to the flat (unstrained) region; (2) the measured peak position red-shifted, and the intensity of photoluminescence (PL) shows an enhancement compared to the flat region; (3) analyzing the magnetic force microscopy (MFM) phase and the amplitude shifts, affirmed that the ReSe2 flake turns magnetic after the formation of wrinkles; and lastly (4) the electrical properties of various ReSe2 wrinkles formed under different prestrains present different variation trends through the comparison of I-V curves. Compared to the high crystal symmetry in graphene, atomically thin ReSe2 -- a new semiconducting 2D material with distorted 1T structure -- provides an excellent opportunity for strain engineering. In ReSe2 monolayer, the Re atoms in the metal sheet slip off their regular sites, forming a Re4 “diamond unit”, and these “units” are coplanar and coupled with one another to comprise a clustering of “diamond chains”, resulting in a lattice distortion.12,13 ReSe2 with triclinic symmetry is an anisotropic semiconductor, which is different from most of hexagonal layered TMDs, such as MoS2 and WS2. Its anisotropy may be exploited for the fabrication of versatile devices. The top and side views of the ReSe2 flake single crystal structure are shown in Figure 1a, in which the Re-chain direction has been marked with red line. Figure 1b and 1c respectively show the low-magnification and high-resolution transmission electron microscopy (TEM and HRTEM) images of ReSe2 flake deposited on the micro grid, with lacey support films using the micromechanical exfoliation and polymethyl methacrylate (PMMA) transfer method (The detailed method is shown in the Supporting Information). The lattice fringes show that the ReSe2 flake has good crystal quality, and the as-measured lattice spacing for the ReSe2 flake from the HRTEM image is 6.7 Å. Figure 1d shows the selected area electron diffraction (SAED) pattern of a single crystalline ReSe2 flake, indicating its orientation is along (210) zone axis,


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corresponding to the PDF card of a = 6.730Å (JCPDS 18-1086). Several distinctive physical properties containing optically biaxial, in-plane anisotropy of electrical properties, polarization sensitive behavior of the band-edge transitions, and excellent photoelectrochemical characteristics have been demonstrated about this material.13,14 In addition, since the indirect gap of ReSe2 is close to its direct gap, small strain is expected to cause indirect to direct gap transition.15 Local strain in the layered ReSe2 samples has been achieved by creating wrinkles on ReSe2. The schematic diagram of the fabrication process is illustrated in Figure 2. First, mechanical exfoliated ReSe2 flakes from ReSe2 crystal are deposited on the elastomeric substrates (Gel-PAK film WF-40-X4) which are pre-stretched by 30~100% (Figure 2a and 2b). The pre-stretched strain is defined as ε pre =∆L/L×100%, where L is the initial length of Gel-film substrate and ∆L is the relative length change after stretching.16 Subsequently, releasing the pre-strain of elastomeric substrates at different rates generates wrinkles of ReSe2 perpendicular to the initial strain axis (Figure 2c and 2d). The formation mechanism of these ReSe2 wrinkles can be attributed to the buckling-induced delamination.11,17 Figure 2e shows representative images for inducing local strain by forming wrinkles. Through the 3D optical microscopy image and scanning electron microscopy (SEM) image shown in Figure 2f and 2g, it is observed that largescale wrinkles have been reproducibly generated for multilayer ReSe2 flakes. Moreover, the width and height of the wrinkles are both micrometers, separated by more than ten micrometers. The chemical compositions of the flakes are measured by energy dispersive X-ray spectroscopy (EDX) (in the inset of Figure 2g), confirming that the flakes have a Re/Se stoichiometric ratio of ~1:2. By applying 100% pre-strain and a fast releasing process, three distinct morphologies of ReSe2 wrinkles with different thickness are obtained in Figure 3: panel a and b is the simple


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ripple geometry for multilayer flake; panel c and d is the triangle geometry for few-layer flake; panel e and f is the needle-like geometry for monolayer flake. Releasing the pre-strain (30% and 50%) step by step, the gradual evolution of wrinkles is exhibited in the Supporting Information Figure S1 and S3. As shown in Figure S1, it can be seen that small pre-strain (30%) causes the collapse and fold of wrinkles. The prepared wrinkles are examined by a 3D scanning confocal microscopy image and atomic force microscopy image (AFM) (Supporting Information Figure S2 and S4). It is noted that both the amplitudes and periodicities of the wrinkles are different under different pre-strains. The mismatch of elastic modulus between ReSe2 flake and Gel-film substrate causes the formation of wrinkles during the releasing process of pre-strain. Since applied strain changes the in-plane orbital interactions, from the orbital theory perspective this can modify the electronic structures of 2D materials before the fracture limit.18 To observe such change in the electronic structure in the monolayer ReSe2 under local strain, PL measurements on the wrinkled sample are investigated. In linear elasticity theory, the maximum strain in a wrinkled flake is related to both the thickness of the flake and its curvature.17 The local strain ε induced on top of the winkles (the maximum local strain) can be estimated by the following formula,11,17

ε ~ π2hδ/(1−υ2)λ2

(1),

where υ is the ReSe2 Poisson’s ratio (~0.2), h (1 nm) is the thickness of the flake, and δ (100 nm) and λ (1 µm) are the height and width of the wrinkle extracted from the atomic force microscopy (AFM) characterization (see Figure 5a). According to equation (1), the local strain ε on top of the


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wrinkle is about ~1.64%, in which the local strain is the largest (section 7, supplementary information) According to our theoretical calculations (Figure 4b), monolayer ReSe2 is a direct gap semiconductor with band-gap energy at 1.15 eV, and its PL spectrum is dominated by the direct gap transitions at the gamma point.11 As density functional theory (DFT) within generalized gradient approximation and local density approximation usually underestimates the band-gap to a large degree, it can be seen that the PL peak position of the unstrained ReSe2 monolayer is at 1.47eV, which is higher than the calculated value (Figure 4a).12 Interestingly, PL signal measured on the top of the ReSe2 wrinkle is red-shifted and the integrated PL is greater with respect to unstrained parts (Figure 4b). This red shift in PL peak position implies that the local strain on the top of the wrinkle changes the electronic structure of ReSe2.11 The decrease of the direct band-gap transition energy is about 70meV, corresponding to ~5% band gap change, which is close to that achieved in semiconducting quantum dots and nanowires under large applied strain.19,20. Here, we note that ReSe2 belong group VII-TMDs with an extra electron in dorbitals compared to group-VI TMDs (MoS2, MoSe2, WS2, etc.) and this lead to strong in-plane anisotropy. These measurements exclude effects of an isotropy direction and grants further studies. To provide understanding to the observed changes in the optical gap, we performed DFT calculations on relaxed and strained (~0.5%) ReSe2. In Figure 4b, we show the changes in the conduction and valance bands for relaxed, biaxial, and uniaxial strained ReSe2. According to Eq.(1), the local strain ε is positive, it may be mean that the maximum local strain obtained on the top of the wrinkle is a tensile in-plane strain parallel and perpendicular to the plane of ReSe2 flake. Because of the quasi-1D Re chain formation (Figure 1a), strain can be induced parallel


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(green line) and perpendicular (magenta) to the Re chain as well as in a&b (biaxial) directions. Even though the degree of band gap change (Egap) depends on the direction where strain is applied, we note that the band gap is always reduced. This agrees well with the red-shifted PL peak observed on strained ReSe2 flakes but does not provide an insight about enhancement in integrate PL. To explain PL enhancement, we consider the ‘funneling effect’ which occurs within strained regions due to the nonhomogeneous strain. Within this picture, photo-generated excitons are created almost uniformly on the ReSe2 flakes but majority of them drift to the top of the wrinkle where the local strain is the maximum and the gap is smallest. Subsequently, the excitons are mostly recombined on the strained region which in return red-shifts the PL peak position. In addition to the changes in the band gap of the material, local strain also modifies the phonon dispersion of ReSe2.10 Previously, experimental and theoretical studies suggested that uniaxial strain can soften the phonon modes and can even induce splitting in some of the Raman modes.21 The Raman spectra of a typical ReSe2 monolayer (ε

pre

= 100%) measured on the flat region

(black curve) and on top of a wrinkle (red curve) are shown in Figure 4e and compared to the phonon dispersion relation computed from our DFT calculations in Figure 4d. Comparison between the Raman spectra (Figure 4e) and vibrational analysis (Figure 4d) show overall good agreement on unstrained ReSe2 where our vibrational dispersion spectrum well predicts most prominent Raman modes observed at around 125 cm-1 and 155 cm-1 (supplementary section 8 and S7c for strain dependent DOS calculations). Strain dependent vibration density of states (DOS) calculations also Based on our symmetry analysis, these peaks are associated with Eg-like and Ag-like modes respectively whereas other theoretically predicted but experimentally unobservable peaks are Raman inactive modes. We note that due to symmetry considerations, it


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is not possible to assign well defined E2g and A1g peaks as observed in other members of TMDs, i.e. MoS2, WS2, etc., and observed modes are found to be a combination of various fundamental Raman modes resulting in Ag-like and Eg-like out-of-plane and in-plane lattice vibrations. Comparison between the most prominent Raman peaks before (black) and after (red) straining in Figure 4e reveals that the Ag-like more is slightly right-shifted while the Eg –like peak is split into two peaks. Since the Eg-like mode is in-plane vibration that is degenerate in frequency in a and b lattice directions, finite strain would break the degeneracy of the Eg-like mode resulting two peaks as observed in our measurements. On the other hand, out-of-plane Ag-like mode is relatively in-sensitive to such in-plane strain effects and is less affected compared to the Eg-like mode under local strain. After discussing on local strain induced changes in optical and vibrational properties of ReSe2, we consider changes in the magnetic properties which are often rather sensitive to changes in the bonding strength. In this regard, the strain engineering is a promising route to induce and manipulate the magnetic properties of 2D materials. For example, the magnetic moments and the ferromagnetic stability of graphene with a topological line defect are enhanced under tensile strain.22 For half-fluorinated BN, GaN and graphene, an antiferromagnetism to ferromagnetic coupling transition is achieved by applying strain.23 MFM is capable of detecting nanoscale magnetic domains, thus it can be used to distinguish the magnetic and nonmagnetic responses at nanometer scale. By using MFM, magnetic interactions between the magnetized tip and the nanomaterials can be effectively detected.24 Therefore, we employ MFM to characterize the magnetic response of monolayer ReSe2 wrinkles (ε

pre

= 100%) by analyzing the phase and the

amplitude shifts. The AFM topography image in Figure 5a shows that the thickness of ReSe2 flake is 1~2 nm, as measured from its height profile in Figure 5e, which is larger than the


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expected monolayer thickness (~0.6 nm), attributing to a different tip-sample interaction between the tip-Gel film substrate (which may possess a large adhesion force) and tip-ReSe2. Previously, similar height variations also have been observed in MoS2 and graphene.11,25 Figure 5b shows the phase image of the same ReSe2 flake in Figure 5a, while Figure 5c and 5d are the MFM phase and amplitude images at a lift height of 25 nm, respectively. To void the response variation caused by using different tips, the AFM and MFM images are measured with the same tip. In the MFM phase images shown in Figure 5c and 5g, a big negative phase shift is observed, indicating a strong attractive interaction between the ReSe2 wrinkles and the MFM tip. Figure 5d shows the MFM amplitude, it is observed a positive amplitude shift. In the MFM measurement, the decrease in the resonance frequency of the cantilever induced by the attractive force between the nanomaterials and tip leads to the decrease of phase signal as well as the increase of amplitude signal. As a consequence, the reverse shifts in the MFM phase and amplitude images affirm that the ReSe2 flake becomes magnetic at the wrinkled (strained) regions. This result is in accord with the previous report on the magnetic property of few-layer MoS2 and Fe3O4 nanoparticles confirmed by MFM measurement24. To provide an explanation for local strain induced magnetism, we have also performed extensive DFT calculations using the model constructed according to the structure shown in Fig. 5, while the size (containing the height and width of the wrinkle) is reduced to 1/1000, due to the large size of the wrinkled structure. This model takes rather large unit cell (240 atoms per unit cell: 80 Re atoms and 160 Se atoms) and simulates the structure as the actual experiments and therefore this model is closer to the actual measurements. According to our DFT calculation, the total energy of the magnetic state of wrinkled ReSe2 is 300 meV lower than nonmagnetic states, implying that the wrinkled ReSe2 is in the stable magnetic state. After the full convergence, the


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magnetic moment of the flat ReSe2 flake is 0 µB and this value increases to ~3.95 µB (supporting information S8) after wrinkle formation, proving an insight on the presence of magnetism after wrinkle formation. Charge density graph in S8 show that the spin polarization takes place locally at the strained regions and results in the magnetization on wrinkle. During wrinkle formation on elastomeric substrates, structural deformations on 2D materials can change their electrical conductivity and thus the related electrical properties of ReSe2 are expected to be sensitive to the applied strain. Two-terminal monolayer ReSe2 devices were first fabricated on the SiO2 (300nm)/Si substrates with gold as the metal electrodes, then transferred to Gel-film substrates with different pre-strains. Metal electrodes were obtained by using electron-beam lithography (EBL) followed by electron beam evaporation (EBE) of 2-nm-Cr/50nm-Au before ReSe2 transfer. After releasing of pre-strain, ReSe2 wrinkles were generated between the two electrodes (Figure 6a). As shown in Figure 6b, current-voltage (I-V) curves taken from the devices fabricated on Gel-film substrates at different pre-strain values show rather non-trivial behavior. It is noted that all the I-V curves are linear, implying that the metal electrodes make an Ohmic contact to ReSe2. Interestingly, the resistance across the semiconductor is reduced at 30% pre-strain value compared to the unstrained ReSe2 whereas 50% pre-strain results in greater resistance. To understand this electrical response, we have measured the changes in the material topology by AFM measurements (Figure S1-S4). For 30% pre-strained sample (Supporting information Figure S1), shape of wrinkles can be described as collapsed and folded, where the folded regions can be regarded as very weakly few-layer segments. As a result, the total resistance across the flake is reduced by geometrical considerations. However, observed wrinkles on 50% pre-strained ReSe2 display much sharper


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bending without significant folding, and as a result the scattering rate is much enhanced with greater resistance across the device. To summarize, we have demonstrated that the optical, magnetic, and electronic properties of ReSe2 can be simultaneously manipulated at the nanoscale by forming wrinkles (strain) on the 2D sheets by simple stretch-release method for materials deposited on elastomeric substrates. Presented results show that nanoscale local strain can modulate the optical band gap, induce magnetism, and can change the electronic and vibrational response of ReSe2 before reaching fracture limit. These findings show that strain engineering at the nanoscale can tune the physical properties towards creating multi-functional materials for wide range of applications including strain sensors, stretchable electrodes, and flexible FETs, solar cells, and other photonic devices.


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Figure 1. ReSe2 crystal structure and HR-TEM measurements a. The top and side views of ReSe2 crystal structure, the red line indicates the Re-chain direction. TEM images of ReSe2 flake b. at low magnification and c. at high resolution. d. Corresponding SAED pattern of ReSe2 flake.


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Figure 2. Schematic diagram of the fabrication process of ReSe2 wrinkles. a. An elastomeric Gel-film substrate is firstly pre-stretched by a different degree; b. mechanical exfoliated ReSe2 flakes are deposited on the elastomeric substrate; c. the pre-strain of elastomeric substrate is released at different rates; d. the wrinkles of ReSe2 perpendicular to the initial strain axis are produced. e. Photograph of achieving the process of local strain by fabricating wrinkles. f. 3D scanning confocal microscopy image of a wrinkled ReSe2 flake. g. Scanning electron microscopy


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(SEM) image of the ReSe2 wrinkles shown in f; the inset is the energy dispersive X-Ray spectroscopy (EDX).

Figure 3. The schematic diagram and SEM images of three distinct morphologies of ReSe2 wrinkles: a. and b. ripple geometry for multilayer flake; c. and d. triangle geometry for few-layer flake; e. and f. needle-like geometry for monolayer flake.


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Figure 4. Raman and photoluminescence spectra of strained monolayer ReSe2. a. Photoluminescence spectra measured on the flat (black) and on the wrinkle (red) regions in the same position of the ReSe2 monolayer. b. Density functional theory (DFT) theory calculation performed on unstrained (red), biaxial strain (blue), strain in parallel (green) and perpendicular (magenta) direction. c. Schematic diagram of funnel effect in the ReSe2 wrinkle explaining the enhancement in the integrated PL emission intensity and PL peak position. d. Phonon dispersion relation calculated from DFT calculations. e. Raman spectra taken on unstrained (black) and strained (red) ReSe2.


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Figure 5. Magnetic Force Microscopy Measurements on ReSe2. a. AFM topography, b. phase, c. MFM phase, and d. MFM amplitude images of monolayer ReSe2 wrinkled flake on Gel-film substrate. e.-h. The corresponding profiles in panels a-d.


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Figure 6. Electrical characterization on strained ReSe2. a. Schematic drawing of a two-terminal ReSe2 wrinkles device, b. I-V curves of a two-terminal ReSe2 wrinkles device under different pre-strain. ASSOCIATED CONTENT Supporting Information. The detailed experimental process is shown in the supporting information. The gradual evolution processes of wrinkles with releasing of the prestrain are shown in Figure S1 and S3. The AFM and 3D optical images are shown in Figure S2 and S4. Raman spectrum taken from strained and unstrained ReSe2 flakes is shown in Figure S5. Characteristic comparison of the elastomeric substrates is shown in Figure S6. Electronic band dispersion of monolayer ReSe2 is shown in Figure S7. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Correspondence and requests for materials should be addressed to Q.L (email:[email protected]), J.L. (email: [email protected]) and S.T. (email: [email protected]).


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Author Contributions S.Y. and C.W. worked on device fabrication and performed the measurements. S.Y. and S.T. analyzed the data. HS and FMP performed electronic structure and vibrational spectrum calculations, HC and YL performed simulations, created the atomic structure of Rese2 and calculated the magnetic ground state of wrinkled sheets. A.S. and S.T synthesized the ReSe2 crystals. S.T, A.S, S.S.L., and Q.L. discussed the experiment in this work. S.Y., J.L. and S.T. wrote the manuscript. ‡S. Y. and C.W. contributed equally to this work. All the authors read and commend on the manuscript. We thank Xiaoyang Zhu in National Center for Nanoscience and Technology for the MFM measurements. We thank Bin Chen at ASU for his material characterization measurements. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by Arizona State University, Research Seeding Program, the National Natural Science Foundation of China (91233120), and the National Basic Research Program of China (2011CB921901). Q. Liu acknowledges the support to this work by NSFC (10974037), NBRPC (2010CB934102), and the CAS Strategy Pilot program (XDA 09020300). S. Yang acknowledges

financial

support

from

China Postdoctoral

Science Foundation

(No.

2013M540127).


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BRIEFS: Tuning the optical, magnetic, and electrical properties of ReSe2 by nanoscale strain engineering.


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FM

Non FM ACS Paragon Plus Environment

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Tuning the Optical, Magnetic, and Electrical Properties of ReSe2 by

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