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Structural Determination and Nonlinear Optical Properties of New 1T’’’-Type MoS Compound 2
Yuqiang Fang, Xiaozong Hu, Wei Zhao, Jie Pan, Dong Wang, Kejun Bu, Yuanlv Mao, Shufen Chu, Pan Liu, Tianyou Zhai, and Fuqiang Huang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12133 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 3, 2019
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Structural Determination and Nonlinear Optical Properties of New 1T’’’-Type MoS2 Compound Yuqiang Fang1,2 §, Xiaozong Hu5 §, Wei Zhao1§, Jie Pan1, Dong Wang1, Kejun Bu1,2, Yuanlv Mao1,2, Shufen Chu4, Pan Liu4*, Tianyou Zhai5*, Fuqiang Huang1,3* 1 State
Key Laboratory of High Performance Ceramics and Super fine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 State
Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China 4 State
Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China 5
State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Technology, Huazhong University of Science and Technology (HUST), Wuhan 430074,. China
Supporting Information Placeholder ABSTRACT: Noncentrosymmetric MoS2 semiconductors (1H,
3R) possess not only novel electronic structures of spin-orbit coupling (SOC) and valley polarization but also remarkable nonlinear optical effects. A more interesting noncentrosymmetric structure - the so-called 1T’’’-MoS2 layers was predicted to be built up from [MoS6] octahedral motifs by theoreticians, but the bulk 1T’’’ MoS2 or its single crystal structure has not been reported yet. Here, we have successfully harvested 1T’’’ MoS2 single crystals by a topochemical method. The new layered structure is determined from single-crystal X-ray diffraction. The crystal crystallizes in space group P31m with a cell of a = b = 5.580(2) Å and c = 5.957(2) Å, which is a √3a × √3a superstructure of 1T MoS2 with cornersharing Mo3 triangular trimers observed by the STEM. 1T’’’ MoS2 is verified to be semiconducting and possesses a band gap of 0.65 eV, different from metallic nature of 1T or 1T’ MoS2. More surprisingly, the 1T’’’ MoS2 does show strong optical secondharmonic generation signals. This work provides the first layered noncentrosymmetric semiconductor of edge-sharing MoS6 octahedra for the research of nonlinear optics.
Recently, non-centrosymmetric transition metal dichalcogenides have attracted much attention due to their intriguing electronic and optical properties, which has great potential in developing nextgeneration electronic and photonic devices.[1-6] For example, their nonlinear optical effects play an important role in promising integrated photonics such as all-optical processing, supercontinuum generation, light modulation and all-optical switching.[7,8] In particular, the most studied MoS2 has unique electronic bands induced by the strong spin-orbit coupling (SOC) of d orbitals and valley polarizations.[9-11] Besides, MoS2 possesses rich crystal structures stacked by different MoS2 motifs including 1H, 1T, 1T’and 1T’’’ MoS2 single-layers [12-17](see in Figure 1). Among four MoS2 motifs, 1H and 1T’’’ MoS2 have no inversion symmetry. The reported single-layer 1H MoS2 and bulk 3R MoS2 exhibit remarkable nonlinear optical effects,[18-22] which are
considered to a promising candidate for 2D nonlinear optical devices.
Figure 1. The schematic structure for the four phases of MoS2. (a) 1T’’’ MoS2 with a √3 √3 superstructure. (b) 1T’ MoS2 with a √3 1 superstructure. (c) 1T MoS2 with a unit cell of 1 1. (d) 1H MoS2 with a unit cell of 1 1. Different from semiconducting 1H and 3R MoS2 (Eg = 1.8 eV for 1H; 1.4 eV for 3R),[5] other MoS2 phases like 1T and 1T’ are metallic, and their crystal structures are centrosymmetric.[23] Instead, another phase of MoS2 distorted from 1T MoS2, 1T’’’ MoS2, was calculated to be a 2D ferroelectric semiconductor with a smaller bandgap of 0.7 eV compared with 1H and 3R phases.[24,25] Notably, the centrosymmetry is broken in 1T’’’ MoS2, which is expected to exhibit the promising nonlinear optical effect. However, 1T’’’ MoS2 is metastable and direct synthesis of it is challenging, so the sample of 1T’’’MoS2 and its crystal structure has not been reported experimentally yet. Hence, it is urgent to prepare the 1T’’’ MoS2 crystals to further investigate its fantastic physicochemical properties.
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Just like the preparations of metastable 1T’ and 1T phases, 1T’’’ MoS2 is unable to be synthesized by using traditional solid-state chemistry methods. In this work, we have successfully obtained 1T’’’ MoS2 single crystals in a soft chemical method and determined its crystal structure by single crystal X-ray diffraction for the first time. The crystal structure of 1T’’’ MoS2 is verified by STEM, selected area electron diffraction (SAED) and powder Xray diffraction (PXRD). 1T’’’ MoS2 belongs to a noncentrosymmetric space group P31m. The solid-state optical absorption of 1T’’’ MoS2 revealed that its band gap is 0.65eV. The wave-length and polarization-resolved SHG measurements show the obvious nonlinear optical properties of 1T’’’ MoS2. The crystals of 1T’’’ MoS2 were prepared by deintercalation of K ions from the KMoS2 crystals. Except for the potassium, the other alkali atoms such as sodium and rubidium can also be used for the preparation of 1T’’’ MoS2 crystals. Comparing with the preparation of 1T’ MoS2 crystals [16], we adopted a stronger oxidizing K2Cr2O7 solution instead of I2 acetonitrile solution. The detailed synthetic method is described in the Supplementary information. The lateral size of the prepared 1T’’’ MoS2 flakes is about 100μm, as shown in the scanning electron microscopy (SEM) image (Figure 2a). The energy-dispersive X-ray spectroscopy (EDS) results (inset of Figure 2a) indicate that no residual K ion exists in the sample and the atomic ratio of Mo and S is about 1:2. The crystal data and the refinement details for 1T’’’ MoS2 are summarized in the Table S1-S4. 1T’’’ MoS2 crystallizes in the trigonal space group P31m with lattice parameters, a = 5.580(2) Å and c = 5.957(2) Å. The unit cell of 1T’’’ MoS2 consists of one independent Mo site and three independent S sites. As shown in Figure 1a, the layered structure is constructed by edge-sharing MoS6 octahedra, which is similar to those of 1T and 1T’ MoS2 but different from edge-sharing trigonal prisms in 2H or 3R MoS2. Each Mo atom is coordinated to six S atoms and four Mo atoms. The Mo-S bond lengths are 2.370(2) Å, 2.521(9) Å, 2.459(8) Å and 2.298(2) Å, compared with 2.413 Å in 2H MoS2, 2.384 Å in 3R MoS2, 2.389(5) Å in 1T MoS2.These neighboring Mo-Mo bonds (3.013(6) Å) form corner-sharing Mo3 trimers in the ab plane (see in the right panel of Figure 1a), compared with the zig-zag Mo-Mo chains (Mo-Mo: 2.78 Å) in 1T’ MoS2. Such arrangement of the planar Mo array is rare. Meanwhile, the formation of Mo-Mo bonds leads to the displacements of their coordinated sulfur atoms, which makes MoS2 layers rippled. Both 1T’’’ and 1T’ MoS2 are distorted from the ideal structure of 1T MoS2 due to the Peierls distortions.
Figure 2. (a) The SEM image of the 1T’’’ MoS2 crystals. Inset: the result of EDS data. (b) The HAADF-STEM image of 1T’’’ MoS2. (c) The PXRD pattern of the prepared 1T’’’ MoS2 crystals, where the red lines were theoretically calculated from the single crystal structure of 1T’’’ MoS2. (d) Raman spectrum of pristine 1T’’’ MoS2 flakes and the annealed sample.
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The microscopic atomic structure of 1T’’’ MoS2 clearly shows a √3a ×√3a superstructure, confirmed by the high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM). (Figure 2b) Moreover, the nearest Mo atoms form the MoMo trimmers (inset of Figure 2b), in which the measured Mo-Mo bond length (3.02 Å) is consistent with the value from the single Xray diffraction data. The SAED of 1T’’’ MoS2 crystal (Figure S1) also indicates an enlarged lattice structure compared to the 1T MoS2. Figure 2c shows the PXRD pattern of the sample, in which all the diffraction peaks matches well with the single-crystal diffraction data of 1T’’’ MoS2. This result exhibits the high purity of the synthesized sample. Room-temperature Raman spectrum of the 1T’’’ MoS2 crystals (Figure 2d) shows six characteristic peaks located at 177 cm-1(J1), 243 cm-1, 267 cm-1(J3), 305 cm-1(E1g), 398 cm-1(A1g) and 463 cm-1 respectively,[13] which are different from the peaks of 1T’ MoS2 and 2H MoS2. After annealed, the sample was transformed into thermodynamically stable 2H MoS2, confirmed by Raman spectrum. Differential thermal analysis (DTA) curve (Figure S2) shows an exothermic peak situated at 130⁰C, indicating phase transition in the 1T’’’ MoS2.
Figure 3. (a) Temperature dependence of resistivity of 1T’’’ MoS2. (b) The optical band gap of 1T’’’ MoS2. (c) Electric band structure of 1T’’’-MoS2 calculated by DFT. (b) Density of states (DOS) of Mo-4d, S-3p and total orbits. Subsequently we investigated the physical properties and electronic structure of 1T’’’ MoS2. The temperature dependence of resistivity was performed by normal four-probe method. The resistivity increases from 0.443 cm at 300 K to 25.856 cm at 10K (Figure 3a), which behaves a typical semiconducting behavior, strikingly different from analogous 1T and 1T’ MoS2 with metallic behaviors. The room-temperature conductivity of 1T’’’ MoS2 is 2.257 S cm-1, compared with 10-4 S cm-1 of 2H MoS2 and 618 S cm-1 of 1T’ MoS2.[26] The solid-state optical absorption spectrum of 1T’’’ MoS2 is shown in Figure S3 and Figure 3b. 1T’’’ MoS2 has a band gap of Eg=0.65eV, compared with Eg =1.8 eV for 1H MoS2 (single-layer MoS2)[5], 1.29 eV for 2H MoS2 and calculated 1.4 eV for 3R MoS2. As mentioned above, the 1T and 1T’ MoS2 are metallic. The differences of band structures of these MoS2 phases derive from different coordination environments and structure distortions of Mo atoms. For semiconducting 2H MoS2 and 3R MoS2, two Mo 4d electrons fully occupy 4dz2 orbital split by trigonal prismatic ligand field. While the 1T MoS2 is metallic due to partial occupation of the t2g orbitals by 4d electrons in an octahedral crystal field.[27,28] This feature could be maintained in the slighted distorted 1T’ MoS2.[29] However, the t2g orbitals of 1T’’’ MoS2 is split further after large distortion from 1T MoS2, which leads to pairing of two 4d electrons on the lower one of the split orbitals, leading to its semiconducting nature. Among the bulk semiconducting VIB-
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Journal of the American Chemical Society Group transition metal dichalcogenides, 1T’’’ MoS2 has not only a unique crystal structure but also the smallest band gap among the known phases. In order to further understand the electronic structure of 1T’’’ MoS2, density functional theory (DFT) calculations was performed. The band structure with spin-orbit coupling (SOC) effect reveals that 1T’’’ MoS2 has an indirect band gap of 0.39 eV (Figure 3c) lower than that of experimental value (0.65eV). The underestimated band gap is common for the LDA approximation. The density of states (DOS) shows that the valence and conduction band are primarily contributed by the Mo-4d orbitals and S-3p orbitals (Figure 3d). The indirect band of 1T’’’ MoS2 derives from the transition from the top of valence band between A and to the bottom of the conduction band located at A high symmetry point.
described as I = I0 sin2 (3θ), where I and I0 are the detected intensity and the maximum SHG intensity respectively. In summary, we have successfully prepared pure 1T’’’ MoS2 single crystals and determined its crystal structure by the singlecrystal diffraction. The triatomic clusters of 1T’’’ MoS2 was clearly observed by the HAADF-STEM image, which differs from the zigzag chains of 1T’ MoS2. The calculations and optical absorption spectra revealed that 1T’’’ MoS2 is an indirect semiconductor. Moreover, it shows obvious nonlinear optical properties. Our work provides a new layered noncentrosymmetric material for the investigation of nonlinear optical effects, valley polarization or ferroelectric.
ASSOCIATED CONTENT Supporting Information Preparation of 1T’’’ MoS2 crystals and single crystal determination; The electronic structure calculation by first principles; The detailed crystal data including structural refinement statistics, anisotropic displacement parameters, selected bond distances and bond angles of 1T’’’ MoS2; the SEM image and EDS spectrum of the sample;
AUTHOR INFORMATION Corresponding Authors
[email protected] [email protected] [email protected] Figure 4. SHG signals in bulk 1T’’’ MoS2 under different wavelengths and excitation power. (a) Wavelength dependent SHG intensity under excitation wavelength from 800 to 1300 nm. (b) Excitation power dependence of SHG intensity under different excitation power densities. (c) The excitation power dependence of SHG intensity. The plot is of natural logarithm. (d) Polarizationangle-dependent SHG intensity for bulk 1T’’’ MoS2. Nonlinear optical properties is much sensitive to crystal inversion symmetry. For example, the second harmonic generation (SHG) intensity of 2H MoS2 crystal was vanished owing to its centrosymmetric structure. Considering the noncentrosymmetric space group of 1T’’’ MoS2, we studied the evolution of SHG intensity with incident laser wavelengths and power, respectively. Figure 4a shows the SHG signals of the bulk 1T’’’ MoS2 under a large wavelength range from 800 to 1300 nm. Before the test, we have fixed the output laser power from femtosecond laser as 3 mW at all wavelengths. The SHG intensity is the highest under the incident laser of 850 nm. In order to investigate the excitation power dependence of SHG intensity, the power of incident laser is changed from 1.5 to 6.0 mW under the wavelength of λex = 850 nm. In the Figure 4b, we can observe the peaks located at 425 nm, which is certainly SHG signals. Figure 4c displays the logarithmic plots of excitation power vs SHG peak intensity. The slope for this plot is 1.8, which is close to the value of 2 calculated from the electric dipole theory. The crystalline lattice symmetry of 1T’’’ MoS2 can be revealed by the polarization-resolved SHG intensity.[30-32] The SHG signals were detected with emission field parallel to excitation field. For the parallel polarization, the SHG intensity should exhibit a 6-fold rotational symmetry with varying azimuthal angle θ due to the 3fold symmetry of 1T’’’ MoS2 crystal. Figure 4d shows the SHG polarization dependence for 1T’’’ MoS2, where second-harmonic intensity is plotted as a function of sample rotation angle. A clear hexagonal petal can be observed, which is consistent with the structure symmetry of 1T’’’ MoS2. The angle dependence can be
Author Contributions Yuqiang Fang, Xiaozong Hu and Wei Zhao contributed to this work equally.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was financially supported by National key research and development program (Grant 2016YFB0901600), “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (Grant XDB04040200), Science and Technology Commission of Shanghai (Grant 16JC1401700), National Science Foundation of China (Grant 51672301), National Natural Science Foundation of China (Grant 21825103), CAS Center for Excellence in Superconducting Electronics.
REFERENCES (1) Akinwande, D.; Petrone, N.; Hone, J. Two-dimensional flexible nanoelectronics. Nat. Commun. 2014, 5, 5678. (2) 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, 699712. (3) Yazyev, O. V.; Chen, Y. P. Polycrystalline graphene and other two-dimensional materials. Nat. Nanotechnol. 2014, 9, 755767. (4) Sun, Z. P.; Martinez, A. Wang, F. Optical modulators with 2D layered materials. Nat. Photonics 2016, 10, 227238. (5) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805.
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(6) Wu, S. F.; Buckley, S.; Schaibley, J. R.; Feng, L. F.; Yan, J. Q.; Mandrus, D. G.; Hatami, F.;Yao, W.; Vuckovic, J.; Majumdar, A.; Xu, X. D. Monolayer semiconductor nanocavity lasers with ultralow thresholds. Nature 2015, 520, 6972. (7) Boyd, R. W.; Shi, Z. M.; Leon, I. D. The third-order nonlinear optical susceptibility of gold. Opt Commun 2014, 326, 7479. (8) Kauranen, M.; Zayats, A. V. Nonlinear plasmonics. Nat. Photonics 2012, 6, 737748. (9) Xiao, D.; Liu, G. B.; Feng, W. X.; Xu, X. D.; Yao, W. Coupled Spin and Valley Physics in Monolayers of MoS2 and Other GroupVI Dichalcogenides. Phys. Rev. Lett. 2012, 108, 196802. (10) Zeng, H. L.; Dai, J. F.; Yao, W.; Xiao, D.; Cui, X. D. Valley polarization in MoS2 monolayers by optical pumping. Nat. Nanotechnol. 2012, 7, 490493. (11) Suzuki, R.; Sakano, M.; Zhang, Y. J.; Akashi, R.; Morikawa, D.; Harasawa, A. Yaji, K.; Kuroda, K.; Miyamoto, T.; Ishizaka, K.;Arit, R.; Iwasa, Y. Valley-dependent spin polarization in bulk MoS2 with broken inversion symmetry. Nat. Nanotechnol. 2014, 9, 611617. (12) Zhao, W.; Pan, J.; Fang, Y. Q.; Che, X. L.; Wang, D.;Bu, K. J. Huang, F. Q. Metastable MoS2: Crystal Structure, Electronic Band Structure, Synthetic Approach and Intriguing Physical Properties. Chem. -Eur. J. 2018, 24, 1594215954. (13) Singh, A.; Shirodkar, S. N.; Waghmare, U. V. 1H and 1T polymorphs, structural transitions and anomalous properties of (Mo, W) (S, Se)2 monolayers: first-principles analysis. 2D. Mater. 2015, 2, 035013. Wypych, F.; Weber, T.; Prins, R. Scanning Tunneling Microscopic Investigation of 1T-MoS2. Chem. Mater. 1998, 10,723727. (15) Yu, Y. F.; Nam, G. H.; He, Q. Y.; Wu, X. J.; Zhang, K.; Yang, Z. Z.; Chen, J. Z.; Ma, Q. L.; Zhao, M.; Liu, Z. Q.; Ran, F. R.; Wang, X. Z.; Li, H.; Huang, X.; Li, B.; Xiong, Q. H.; Zhang, Q.; Liu, Z.; Gu, L.; Du, Y. H.; Huang, W.; Zhang, H. High phase-purity 1T' MoS2- and 1T -MoSe2-layered crystals. Nat. Chem. 2018, 10, 638643. (16) Acerce, M.; Voiry, D.; Chhowalla, M. Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials. Nat. Nanotechnol. 2015, 10, 313318. (17) Fang, Y. Q.; Pan, J.; He, J.Q.; Luo, R. C.; Wang, D.; Che, X. L.; Bu, K. J.; Zhao, W.; Liu, P.; Mu, G.; Zhang, H.; Lin, T. Q.; Huang, F. Q. Structure Re-determination and Superconductivity Observation of Bulk 1T MoS2. Angew. Chem. Int. Ed 2018, 130, 12461249. (18) Yin, X. B.; Ye, Z. L.; Chenet, D. A.; Ye, Y.; Brien, K.; Hone, J. C.; Zhang, X. Edge Nonlinear Optics on a MoS2 Atomic Monolayer. Science 2014, 344, 488490. (19) Kumar, N.; Najmaei, S.; Cui, Q.; Ceballos, F.; Aiayan, P. M.; Lou, J.; Zhao, H. Second harmonic microscopy of monolaye r MoS2. Phys. Rev. B 2013, 87, 161403. (20) Malard, L. M.; Alencar, T. V.; Barbonza, A. P.; Mak, K. F.; Paula, A. M. Observation of intense second harmonic generation from MoS2 atomic crystals. Phys. Rev. B 2013, 87, 201401. (21) Li, Y. L.; Rao, Y.; Mak, K. F.; You, Y. M.; Wang, S. Y.; Dean, C. R.;Heinz, T. F. Probing Symmetry Properties of Few-Layer MoS2 and h-BN by Optical Second-Harmonic Generation. Nano Lett. 2013, 13, 33293333. (22) Shi, J.; Liu, F. C.; He, P.; Wang, R.; Qin, L.; Zhou, J. B.; Li, Xin.; Zhou, J. D.; Sui, X. Y.; Zhang, S.; Zhang, Y. F.; Zhang, Q.; Sun, T. Z.; Q, X. H.; Liu, Z.; Liu, X. F. 3R MoS2 with Broken Inversion Symmetry: A Promising Ultrathin Nonlinear Optical Device. Adv. Mater. 2017, 29, 1701486, (23) Guo, C. G.; Pan, J.; Li, H.; Lin, T.; Liu, P.; Song, C.; Wang, D.; Mu, G.; Lai, X. F.; Zhang, H.; Zhou, W.; Chen, M. W.; Huang, F. Q. Observation of superconductivity in 1T' -MoS2 nanosheets. J. Mater. Chem. C 2017, 5, 1085510860.
(24) Shirodkar, S. N.; Waghmare, U. V. Emergence of Ferroelectricity at a Metal-Semiconductor Transition in a 1T Monolayer of MoS2. Phys. Rev. Lett. 2014, 112, 157601. (25) Bruyer, E.; Sante, D. D.; Barone, P.; Stroppa, A. Possibility of combining ferroelectricity and Rashba-like s pin s plitting in monolayers of the 1T-type transition-metal dichalcogenides MX2 ( M = Mo, W; X = S, Se, Te). Phys. Rev. B 2016, 94, 195402. (26) Guo, C. G.; Li, H.; Zhao, W.; Pan, J.; Lin, T. Q.; Xu, J.; Chen, M.; Huang, F. Q. High-quality single-layer nanosheets of MS2 (M = Mo, Nb, Ta, Ti) directly exfoliated from AMS2 (A = Li, Na, K) crystals. J. Mater. Chem. C 2017, 5, 59775983. (27) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat. Chem. 2013, 5, 263275. (28) Enyashin, A. N.; Yadgarov, L.; Houben, L.; Popov, I.; Tenne, R.; Seifert, Gotthard. New Route for Stabilization of 1T-WS2 and MoS2 Phases. J. Phys. Chem. C 2011, 115, 2458624591. (29) Vellinga, M. B.; Jonge, R. D.; Haas, C. Semiconductor to metal transition in MoTe2. J. Solid State Chem. 1970, 2, 299302. (30) Cox, J. D.; Silveiro, I.; Garía de Abajo, F. J. Quantum Effects in the Nonlinear Response of Graphene Plasmons. ACS Nano 2016, 10, 19952003. (31) Zhou. X.; Cheng, J. X.; Cao, T.; Hong, H.; Liao, Z. M.; Wu, S.W.; Peng, H. L.; Liu, K. H.; Yu, D. P. Strong Second-Harmonic Generation in Atomic Layered GaSe. J. Am. Chem. Soc. 2015, 137, 79947997. (32) Z, H. L.; Liu, G. B.; Dai, J. F.; Yan, Y. J.; Zhu, B. R.; He, R. C.; Xie, L.; Xu, S. J.; Chen, X. H.; Yao, W.; Cui, X. D. Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides. Sci. Rep. 2013, 3, 1608.
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