Enhanced Nonlinear Optical Response of Rectangular MoS2 and

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Enhanced Nonlinear Optical Response of Rectangular MoS and MoS/TiO in Dispersion and Film 2

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Xiaohong Li, Kun-Hong Hu, Bosai Lyu, Jingdi Zhang, Yingwei Wang, Peng Wang, Si Xiao, Yongli Gao, and Jun He J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04974 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 4, 2016

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The Journal of Physical Chemistry

Enhanced Nonlinear Optical Response of Rectangular MoS2 and MoS2/TiO2 in Dispersion and Film Xiaohong Li, † Kunhong Hu, ‡ Bosai Lyu, † Jingdi Zhang, † Yingwei Wang, † Peng Wang, † Si Xiao, *, † Yongli Gao, †, § and Jun He*, † †

Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, 932 South Lushan Road, Changsha, Hunan 410083, P. R. China.

‡

Department of Chemical and Materials Engineering, Hefei University, Hefei 230601, P. R. China.

§

Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, United States.

*Corresponding author email: [email protected] (S. Xiao), Telephone: +86-18673123070 [email protected] (J. He), Telephone: +86-15802661974

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Abstract

The nonlinear response of the two-dimensional materials could be measured by the spatial self-phase modulation (SSPM) and Z-scan. Most of these materials are irregular shapes. In this paper, we have studied the nonlinear optical properties of rectangular layered MoS2 and the corresponding MoS2/TiO2 composite in dispersion and the film, respectively. For dispersion measured by SSPM, the nonlinear refraction index of rectangular layered MoS2 dispersion is larger than that of the MoS2/TiO2 composite dispersion. For film measured by Z-scan, the nonlinear refractive index of the MoS2/TiO2 composite thin film is larger than that of the rectangular layered MoS2 thin film. This research confirms the possibility of the nonlocal electron coherence established among the alignment in SSPM and provides two different paths for the enhancement of the nonlinear optical properties of MoS2.

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1. Introduction The nonlinear refractive index of the 2D materials dispersion could be measured by SSPM. In 2006, Ji et al. reported the optical property of thermally-induced self-diffraction in carbon nanotube1. Wu et al. measured the nonlinear optical (NLO) properties of the graphene nanoflake dispersions with the SSPM2. In recent years, the nonlinear refractive index of graphene (n2 ~ 10-5 cm2/W), MoS2 (n2 ~ 10-7 cm2/W), WS2 (n2 ~ 10-7 cm2/W), MoSe2 (n2 ~ 10-7 cm2/W), Bi2Te3 (n2 ~ 10-8 cm2/W) and black phosphorous dispersion (n2 ~ 10-5 cm2/W) were determined based on SSPM3-7. The nonlinear absorption and refractive index of the 2D materials, such as graphene (n2 ~ 10-7 cm2/W)8, Bi2Se3 (n2 ~ 10-10 cm2/W)9 and black phosphorous10, could be measured by Z-scan technique. According to the research by Zhang et al.11, as the increase of laser intensity in Z-scan, MoS2 film transits from a saturable absorption (SA) to a reverse saturable absorption (RSA). Recently, Zhou et al. reported that the nonlinear refractive index of the multilayer MoS2 film is two orders of magnitude larger than the monolayer MoS2 film measured by Z-scan in the near-infrared region12. A difference between the dispersion and the film is that the 2D materials in dispersion could be reoriented and form the alignment under the irradiation of laser. In 2015, Wu et al. reported the alignment of MoS2 nanoflakes induced by lasers in gap-dependent SSPM13. According to their theory, the wind-chime shaped alignment of the 2D nanoflakes would cause the nonlocal electron coherence. In an alignment, the regular shape and orientation of each 2D nanoflake may influence the NLO 3



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properties. But the shape or the orientation usually is arbitrary in most previous studies14-15. Furthermore, three possible mechanisms: single-photon absorption, two-photon absorption (TPA) and phonon-assisted absorption can account for the gap-dependent NLO properties at different excited wavelengths 15. The absorption mechanisms of 2D materials are possible to be modulated by coupling with the wide bandgap TiO2. The TiO2 thin film has a large third-order nonlinear susceptibility17,18. Hu et al. synthesized the MoS2/TiO2 composite which had effectively improved photocatalytic activity19. Hence, MoS2/TiO2 composite may exhibit novel nonlinear optical properties. In this work, the SSPM of the rectangular MoS2 and the MoS2/TiO2 composite was performed in dispersions. The nonlinear refractive index of rectangular MoS2 is larger than that of the MoS2/TiO2 or the arbitrarily shaped MoS2. The principal mechanism may be the laser-induced aligned reorientation of nanoflakes. By contrast, the Z-scan measurements of the corresponding thin film were carried on. The nonlinear refractive index of the MoS2/TiO2 is larger that of the rectangular MoS2. The principal mechanism may be that wide bandgap TiO2 nanoparticles modulated the band structure of the narrow bandgap MoS2. This research confirms the existence of the nonlocal electron coherence established among the alignment in SSPM and provides two different paths for the enhancement of the nonlinear optical properties of MoS2.

2. Materials and Characterizations

Firstly, the MoS3/TiO2 composite was synthesized by a quick deposition of MoS3 4


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on acid-activated TiO2. Subsequently, by calcining the MoS3/TiO2 composite, the nano-MoS2/TiO2 composite powder was obtained18. Next, 0.1029 g MoS2, 0.1034 g TiO2 and 0.1036 g MoS2/TiO2 composite powders were added to 60 mL Poly (methyl methacrylate) (PMMA) solution (in Tetrahydrofuran, 5 wt %), respectively. All the mixed solutions were bath sonicated for 3 hours in an ultrasonic oscillator (KQ-300DE) to disperse the powders in the Tetrahydrofuran solution. After the centrifugation for 15 min at 3000 rpm (TG20), the supernatant solutions were obtained as the dispersions for SSPM. The solution was spin-coated on a glass substrate at 1000 rpm for 30 s. This spin-coating process would be repeated for 4 times for each individual substrate. After the annealing process, the thin films of the MoS2, the TiO2 and the MoS2/TiO2 composite were obtained for Z-scan.

Figure 1. (a) the SEM image of nano-TiO2. (b) the TEM image of MoS2/TiO2 5



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composite. (c) the AFM image of MoS2 thin film. Inset (c1), (c2) and (c3) are the high resolution AFM images of three individual rectangular MoS2 nanoflakes. Inset (c4) is the height curve of the line highlighted by the white solid line in (c). (d) The UV-Vis absorption spectra of TiO2, MoS2 and MoS2/TiO2 composite dispersions. Figure 1a is the scanning electron microscopy (SEM) image of the original TiO2 powder. The TiO2 powder is consisted of numerous TiO2 nanoparticles. Figure 1b is the transmission electron microscope (TEM) image of the MoS2/TiO2. In the TEM image, the layered structure of the MoS2 can be identified which deposited on the TiO2. The size of the MoS2/TiO2 composite is 100-200 nm, approximately, but the composite do not have a typical rectangular shape. Figure 1c is the Atomic force microscopy (AFM) image of that MoS2 nanoflakes. Inset c1, c2 and c3 are the high resolution AFM images of three individual rectangular MoS2 nanoflakes, and inset c4 is the height curve of the line highlighted by the white solid line. From the AFM image, the length, the width and the average thickness of nanoflake are approximately 200 nm, 100 nm and 18 nm, respectively. While the average thickness of MoS2/TiO2 composite is between 38 nm and 41 nm, as shown in supporting information. The UV-Vis absorption spectra of the as-prepared MoS2, TiO2 and MoS2/TiO2 are shown in Figure 1d. The MoS2/TiO2 exhibits the merits of these two materials and has absorption peaks at 380 nm, 470 nm, 630 nm and 680 nm.

3. Results and Discussion 3.1 The NLO properties of dispersions in SSPM. The NLO properties of the 6


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dispersions were investigated with the spatial self-phase modulation. The SSPM is a coherent third-order nonlinear optical process20. The nonlinear refractive index n2 of solution dispersions can be calculated based on the SSPM results2.

Figure 2. Schematic diagram of the experimental setup. (a) The arbitrary orientation of nanoflakes without irradiation of laser. (b) The alignment of nanoflakes under laser irradiation.

Figure 2 shows the setup of the SSPM experiments. The polarization direction of laser is vertical. The femto-second laser pulse was produced by an optical parametric amplifier (TOPAS, USF-UV2), which was pumped by a Ti: Sapphire regenerative amplifier system (Spectra-Physics, Spitfire ACE-35F-2KXP, Maitai SP and Empower 30). The pulse repetition rate is 2 kHz. The wavelength in the SSPM measurements was 700 nm. The laser was focused by a lens (f = 200 mm). The quartz cuvette with a pathlength 10 mm was placed 10.8 mm in front of the focal point. After the incident laser beam passed through the dispersions of nanoflakes, a typical Gaussian spot was received by the CCD at the beginning. Then the light spot changed to symmetric diffraction rings. The radius of diffraction pattern increases and more rings emerged. The diffraction patterns whose radiuses reached their maximums were discussed in this paper exclusively13. 7



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Figure 3a-c show the diffraction patterns on the CCD after TiO2, MoS2 or MoS2/TiO2 dispersions, when the intensities of laser (700 nm) are 130 mW. For Figure 3a, only a Gaussian spot was received after the TiO2 dispersion and remains unchanged. Without the 2D quantum materials, the TiO2 nanoparticle dispersion cannot cause SSPM. Up to now, the SSPM of the dispersions of carbon nanotube, Graphene, MoS2, WS2, MoSe2, Bi2Te3 and black phosphorous have been reported, most of them are 2D materials1-7. Figure 3b,c show the diffraction patterns after MoS2 or MoS2/TiO2 dispersions, respectively. Both of them reached their maximums at 0.95 s and 1.02 s after the beginning of the dynamic SSPM processes. The formation mechanism of SSPM could be explained as the laser-induced aligned reorientation of nanoflakes13. The forming mechanism is given in supporting information. But the formation time (~ 1 s) for rectangular layered MoS2 or the corresponding MoS2/TiO2 composite dispersion is much longer than that for arbitrarily shaped MoS2 dispersion which is reported as ~ 0.3 s in reference 13. Theoretical calculation and explanation are listed in supporting information. The linear relationships between the number of rings N and the incident laser intensity I of the MoS2 and MoS2/TiO2 dispersions are shown in Figure 3d,e, respectively. By linear fitting of the results, we obtained the slopes of both lines as 0.68 cm2/W and 0.57 cm2/W. After comparing to the linear fitting results in reference 13, the single-photon absorption can be confirmed as the principal absorption mechanism. But the slope of MoS2/TiO2 was slightly less than that of MoS2, which indicates that a new absorption mechanism may be introduced. 8


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Figure 3. (a), (b), (c) Typical diffraction patterns images of TiO2, MoS2 and MoS2/TiO2 dispersions. (d), (e) The linear relationships between the number of rings N and the incident laser intensity I of the MoS2 and MoS2/TiO2 dispersions. The SSPM mechanisms of the MoS2 and MoS2/TiO2 solution dispersions are similar to the self-diffraction mechanism of nematic liquid crystal films and graphene2,3,20. In our experiment, N is approximately proportional to I, which is similar to the previous report5. We can calculate the nonlinear refractive index of MoS2 is 3.17 × 10-5 cm2/W and MoS2/TiO2 is 2.66 × 10-5 cm2/W. Theoretical calculation is given in supporting information. Comparing to the n2e = 2.88 × 10-6 cm2/W of random shaped disk MoS2 dispersion13, the n2e of the uniform rectangular MoS2 dispersions was significantly larger. It can been explained as that for rectangular MoS2 nanoflakes, the rectangular long axis would orient and parallel to the polarization of laser, the established nonlocal electron coherence substantially enhances the nonlinear refractive index. The 9



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scenario is depicted in the inset b of Figure 2. In addition, the n2e of the uniform rectangular MoS2 was also slightly larger than that of MoS2/TiO2. It can been explained as that the typical rectangular layered shape has been broken for composite (as Figure 1b), which cannot establish the nonlinear electron coherence as strong as the rectangular layered MoS2. Therefore, for SSPM, the nonlocal electron coherence established among the alignment of the 2D nanoflakes may be a dominate factor in the dispersion. In this case, the uniform rectangular shape would be useful for the enhancement of the nonlinear refractive index or other photoelectric properties. Namely, when it come to the spin-coated thin films of the MoS2 and MoS2/TiO2, the laser-induced reorientation cannot happens. Without the nonlocal electron coherence, the nonlinear refractive index is determined by other factor, such as the band changing. Further experimental proofs are given in the next parts.

3.2. The NLO properties of thin film sample in Z-scan. The Z-scan technique was used to measure the nonlinear refractive index of the thin film samples. The Z-scan can not be performed in these dispersion because of influence of SSPM which will result in uncertainty signal. In addition, the film can also not be measured by SSPM on account of no SSPM signal. Detailed explanation can be seen in supporting information. In Z-scan measurements, the wavelength of laser was fixed at 700 nm, the same as that in the SSPM. Since a low repetition rate (2 kHz) pulse laser is used, thermal effects on nonlinear absorption could be negligible. The linear transmittances of the film samples at 700 nm are 52 %, 69.8 % and 72.5 % for TiO2, MoS2 and 10


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MoS2/TiO2, respectively. The closed-aperture (CA) and open-aperture (OA) Z-scan of different films were performed. The pure PMMA film have no nonlinear response.

Figure 4. (a) The CA Z-scan curves of the sample films at 700 nm. (b) and (c) The OA Z-scan of TiO2, MoS2 and MoS2/TiO2 films at 700 nm with different input intensities. (d) Schematic diagram of the nonlinear absorption in MoS2/TiO2 composites.

Figure 4a is the CA Z-scan curves of the TiO2, MoS2 and MoS2/TiO2 thin films, the circle spots represent the experimental data and the solid curves are the theoretical fittings. What’s interesting is that MoS2/TiO2 film shows a nonlinear self-defocusing signal, as the same as TiO2, different from self-focusing signal of the rectangular layered MoS2. Both positive and negative nonlinear refractive coefficients of MoS2 have been reported12,21, and the positive value maybe originate from a higher percentage of few-layer nanosheets out of all. It means a new nonlinear absorption mechanisms caused by the coupling with TiO2 may be the dominate factor in the film. 11



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The transmittance curves of OA Z-scan at two different input intensities are shown in Figure4 b,c. In Figure 4b, under low input intensity, the responses of the TiO2 and MoS2 thin films show the characteristics of SA, and the response of the MoS2/TiO2 thin film is RSA. It proves the coupling of MoS2 and TiO2 bring in a new nonlinear absorption mechanisms. In Figure 4c, under higher input intensity, the MoS2/TiO2 keep the characteristics of RSA. In order to ensure the excitation intensity was below the optical damage threshold, multiple replica measurements were performed, as shown in supporting information. The explanation of MoS2 nonlinear absorption caused by intensity can be seen in supporting information. In term of the fascinating saturable absorption properties, MoS2 could be developed as passive model-locker and Q-switching device11. In addition, since reverse saturable absorption occurs under higher excitation intensity, MoS2 and MoS2/TiO2 composite might work as optical limiter or all-optical switching13,22,23. For the MoS2/TiO2, a possible theoretical model based on interface charge transfer and excited-state absorption is proposed. According to the previous works24, type-II band alignment is formed at interface of as-prepared composites. Herein, as-shown energy diagram is proposed in Figure 4d. During the photoexcitation, on the one hand, the electrons are firstly excited from the ground state S0 to the first singlet state S1; on the other hand, the upward band bending and the interfacial band energy build up a strong internal electric field for as-shown energy diagram. Under such driving force, the photoexcited electrons flow into TiO2. Meanwhile, due to interface charge transfer, charge accumulation in conduction band of TiO2 which would enhance the 12


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excited-state absorption process of the TiO2 and that formed a strong potential between valence band and conduction band, which also assist to ground state absorption for TiO2(GA-TiO2). This mechanism also gives explanation to the slight difference in the slope of SSPM in Figure 3d,e. After theoretical fitting the experimental Z-scan data with the saturable model that includes both the SA and TPA effects25,26, the nonlinear refractive index n2e = - 3.6 × 10-9 cm2/W of the MoS2/TiO2 film is obtained. The fitting and the calculation is given in supporting information. Different from the results of SSPM in dispersion, the absolute value n2e of MoS2/TiO2 film is larger than that of rectangular MoS2 film (2.6 × 10-9 cm2/W). Therefore, for Z-scan, the band changing may be a dominate factor in the film. In this case, the coupling of TiO2 would be useful for the enhancement of the nonlinear refractive index or other photoelectric properties. The nonlinear refractive indexes obtained from SSPM and Z-scan are shown in table 1. The order of magnitude of nonlinear refractive index for rectangular MoS2 in Z-scan was close to the reported value12, which indicated that our results are reasonable. The nonlinear refractive index determined in SSPM is larger than that in Z-scan measurement, which maybe originate from the nonlocal electron coherence. Our results are consistent with many reported works2,8,12,13. Table 1. The nonlinear refractive indexes obtained from SSPM and Z-scan SSPM

Z-scan Rectangula

Rectangular Sample MoS2

Exfoliated MoS213

MoS2/TiO2

r MoS2 film

13


Exfoliated

MoS2/TiO2

MoS2 film9

film


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(Single layer) 2

n2e (cm /W)

3.17 × 10

-5

2.88 × 10

-6

2.66 × 10

-5

2.6 × 10

-9

- 6.97 × 10-8 (Multilayer) - 1.16 × 10-10

4. Conclusions In conclusion, the SSPM measurements of the rectangular layered MoS2 and the MoS2/TiO2 in dispersions are performed. The nonlinear refraction index n2e of rectangular layered MoS2 dispersion is larger, meaning that the nonlocal electron coherence established among the alignment of the 2D nanoflakes may be a dominate factor in the dispersion. In this case of solution, the uniform rectangular shape would be useful for the enhancement of the nonlinear properties. Furthermore, the Z-scan of the corresponding films is performed. The absolute value of the n2e of the MoS2/TiO2 is larger. A new absorption mechanism was introduced as the interface charge transfer and excited-state absorption. In this case of film, the coupling of TiO2 would be useful for the enhancement. Therefore, this research confirms the possibility of the nonlocal electron coherence established among the alignment in SSPM and provides two different paths for the enhancement of the nonlinear optical properties of MoS2.

ASSOCIATED CONTENT

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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- 3.6 × 10-9

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Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (Grant Nos. 11104356, 61222406, 11174371, 51375139, 11504105 and 51272291), the Natural Science Foundation of Hunan Province (Grant Nos. 12JJ1001), the Grants from the Project of Innovation-driven Plan in Central South University (Grant Nos. 2015CXS1035), the Fundamental Research Funds for the Central Universities of Central South University (Grant Nos. 2015zzts161 and 2016zzts225).

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(19) Hu, K.; Hu, X.; Xu, Y.; Sun, J. Synthesis of Nano-MoS2/TiO2 Composite and Its Catalytic Degradation Effect on Methyl Orange. J. Mater. Sci. 2010, 45, 2640-2648. (20) Durbin, S. D.; Arakelian, S. M.; Shen, Y. Laser-induced Diffraction Rings From a Nematic-liquid-crystal Film. Opt. Lett. 1981, 6, 411-413. (21) Wang, K.; Feng, Y.; Chang, C.; Zhan, J.; Wang, C.; Zhao, Q.; Coleman, J. N.; Zhang, L.; Blau, W. J.; Wang, J. Broadband Ultrafast Nonlinear Absorption and Nonlinear Refraction of Layered Molybdenum Dichalcogenide Semiconductors. Nanoscale 2014, 6, 10530-10535. (22) Venkatram, N.; Rao, D. N.; Akundi, M. A. Nonlinear Absorption, Scattering and Optical Limiting Studies of CdS Nanoparticles. Opt. Express 2005, 13, 867-872. (23) Hernandez, F. E.; Yang, S.; Van Stryland, E. W.; Hagan, D. J. High-dynamic-range Cascaded-focus Optical Limiter. Opt. Lett. 2000, 25, 1180-1182. (24) Tao, J.; Chai, J.; Guan, L.; Pan, J.; Wang, S. Effect of Interfacial Coupling on Photocatalytic Performance of Large Scale MoS2/TiO2 Hetero-thin Films. Appl. Phys. Lett. 2015, 106, 081602. (25) Sheik-Bahae, M.; Said, A. A.; Wei, T.; Hagan, D. J.; Van Stryland, E. W. Sensitive Measurement of Optical Nonlinearities Using a Single Beam. IEEE J. Quantum Electron. 1990, 26, 760-769. (26) He, J.; Qu, Y.; Li, H.; Mi, J.; Ji, W. Three-photon Absorption in ZnO and ZnS Crystals. Opt. Express 2005, 13, 9235-9247.

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Figure 1. (a) the SEM image of nano-TiO2. (b) the TEM image of MoS2/TiO2 composite. (c) the AFM image of MoS2 thin film. Inset (c1), (c2) and (c3) are the high resolution AFM images of three individual rectangular MoS2 nanoflakes. Inset (c4) is the height curve of the line highlighted by the white solid line in (c). (d) The UV-Vis absorption spectra of TiO2, MoS2 and MoS2/TiO2 composite dispersions. 109x109mm (300 x 300 DPI)


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Figure 2. Schematic diagram of the experimental setup. (a) The arbitrary orientation of nanoflakes without irradiation of laser. (b) The alignment of nanoflakes under laser irradiation. 80x39mm (300 x 300 DPI)



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Figure 3. (a), (b), (c) Typical diffraction patterns images of TiO2, MoS2 and MoS2/TiO2 dispersions. (d), (e) The linear relationships between the number of rings N and the incident laser intensity I of the MoS2 and MoS2/TiO2 dispersions. 109x80mm (300 x 300 DPI)


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Figure 4. (a) The CA Z-scan curves of the sample films at 700 nm. (b) and (c) The OA Z-scan of TiO2, MoS2 and MoS2/TiO2 films at 700 nm with different input intensities. (d) Schematic diagram of the nonlinear absorption in MoS2/TiO2 composites. 109x85mm (300 x 300 DPI)


Enhanced Nonlinear Optical Response of Rectangular MoS2 and

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