Harmonic Generation

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Two-Dimensional Pyramid-Like WS2 Layered Structures for Highly Efficient Edge Second-Harmonic Generation Xianqing Lin, Yingying Liu, Kang Wang, Cong Wei, Wei Zhang, Yongli Yan, Yong Jun Li, Jiannian Yao, and Yong Sheng Zhao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07823 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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Two-Dimensional Pyramid-Like WS2 Layered Structures for Highly Efficient Edge SecondHarmonic Generation Xianqing Lin, Yingying Liu, Kang Wang, Cong Wei, Wei Zhang, Yongli Yan, Yong Jun Li, Jiannian Yao, and Yong Sheng Zhao* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Science, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China *E-mail: [email protected]

ABSTRACT: Two-dimensional (2D) layered materials, with large second order nonlinear susceptibility, have received much scientific interest due to their potential applications in nonlinear optical devices. However, the atomic thickness of 2D layered materials leads to poor field confinement and weak light-matter interaction at nanoscale, resulting in low nonlinear conversion efficiency. Here, 2D pyramid-like multilayer (P-multilayer) layered structures are fabricated for efficient edge SHG based on the enhanced light-matter interaction in whisperinggallery mode (WGM) cavities. The P-multilayer 2D layered materials, where the basal planes

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shrink gradually from the bottom to the top layers, exhibit efficient edge SH radiation due to the partial destructive interference of nonlinear polarizations between the neighboring atomic layers. Moreover, the well-defined 2D plate-like triangle morphology of P-multilayer WS2 forms WGM resonance cavity, which results in enhanced light-matter interaction and thus the enhancement of edge SHG, which can be further enhanced by hybridizing the WGM mode with plasmonics. These results would provide enlightenment for the construction of specific structures for efficient nonlinear processes.

KEYWORDS: two-dimensional layered materials, edge SHG, WS2, non-linear optics, nanophotonics

Two-dimensional (2D) layered materials, especially transition metal dichalcogenides (TMDs),1-7 have attracted great attention due to their unique crystal structure and layerdependent electronic and optical properties.8-11 Different from graphene, TMDs have a broken inversion symmetry structure with large second-order nonlinear susceptibility (on the order of 1 nm/V,12-14 which is much larger than most nonlinear dielectric materials) in the monolayer limit, exhibiting huge potential applications in nonlinear optical devices.15-17 Indeed, second-harmonic generation (SHG) has been widely demonstrated in monolayer TMDs.18-21 Unfortunately, the subwavelength interaction length, arising from the atomic thickness of monolayer TMDs, leads to poor nonlinear conversion efficiency.22 Thus, multilayer TMD nanostructures with long interference length are demanded for nonlinear optical applications with high conversion efficiency. However, due to the destructive interference of nonlinear polarizations between the neighboring atomic layers in multilayer TMDs,23,24 the SHG will diminish oscillation with the increasing numbers of layers, which limits its practical application.


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In this work, we demonstrate the construction of non-full covered layered TMDs structure (i.e. pyramid-like multilayer structure) for highly efficient edge SHG based on the partial destructive interference of nonlinear polarizations between the neighboring atomic layers. The pyramid-like multilayer (P-multilayer) TMDs structures, where the basal planes shrink gradually from the bottom to the top layers, were synthesized through a controllable chemical vapor deposition method. The as-prepared P-multilayer TMDs structures exhibit efficient edge SH (second harmonic) radiation. In addition, increasing atomic layers leads to the increase of light-matter interaction length and formation of whispering-gallery mode (WGM) resonance in the Pmultilayer TMDs. Thus, more than 40 times enhancement of SHG was achieved in the Pmultilayer TMDs structure in comparison to that from the monolayer TMDs under the same condition. Moreover, by hybridizing the WGM mode with plasmonics, we further realized more than 800 times enhancement of SHG. We believe that the results demonstrated here offer a comprehensive understanding of the edge SHG in 2D layered structures, and would provide guidance for the development of nonlinear optical devices with specific functionalities.

RESULTS AND DISCUSSION Tungsten disulfide (WS2, Figure 1a) was selected as the model transition metal dichalcogenides for constructing the pyramid-like structure due to its large second-order nonlinear susceptibility.25,26 Figure 1a shows the typical pyramid-like multilayer structure of WS2 (P-multilayer WS2), where the multilayer flakes are stacked by multiple concentric triangle sheets with gradually decreasing size. Due to the preferred 2H (two layers per unit cell stacked in hexagonal symmetry) stacking order of layered structure,27,28 the second-order nonlinear polarizations of the adjacent layers would have opposite orientations under the linearly polarized


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laser excitation, leading to the destructive interference between them at the full-covered region. Meanwhile, partial destructive interference of nonlinear polarizations occurs around the boundary of P-multilayer WS2, which would bring about efficient SH radiation at the edges. Moreover, with the increase of the height, the multilayer WS2 flakes can function as WGM resonators to effectively confine and circulate fundamental wave (FW) inside the 2D plane by total internal reflection at cavity boundary, which is beneficial for the enhancement of lightmatter interaction and thus enhancing the SHG.

Figure 1. (a) Schematic illustration of a pyramid-like WS2 structure for efficient edge SHG. (b) Bright-field image of the as-prepared P-multilayer WS2 obtained from chemical vapor deposition method. Scale bar is 10 µm. (c) TEM image of a typical P-multilayer WS2. Scale bar is 2 µm. Inset: SAED pattern obtained from the center of P-multilayer WS2. (d) AFM image of a Pmultilayer WS2. Scale bar is 1 µm. Inset: high-resolution AFM image of the region marked with red box.


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The P-multilayer WS2 nanostructures were fabricated via an atmospheric pressure chemical vapor deposition method (Figure S1).29-32 The key point for the construction of P-multilayer WS2 is to gradually decrease the supply of source materials by decreasing the flow rate of carrier gas. In a typical preparation, 300 mg tungsten oxide (WO3) powders were placed in a ceramic boat covered with clean SiO2/Si substrate at the center of a horizontal quartz tube furnace (heating zone 1), and another separate ceramic boat containing 500 mg sulfur powders located the upstream of the furnace 20 cm away from the WO3 powders (heating zone 2). Before heating, the tube was vacuum pumped to evacuate the air and then refilled with high-purity argon (Ar, the carrier gas) to atmospheric pressure. After that, the center of the heating zone 1 was heated to 930 oC at a rate of 30 oC/min while the heating zone 2 was ramped to 180 oC in 30 min, and both are kept for 15 min before being cooled to room temperature naturally. During the growth process, the flow rate of the carrier gas decreased from 40 sccm (standard cubic centimeters per min) to 20 sccm gradually. After the preparation, the substrates covered with WS2 nanostructures were obtained. It is noted that the flow rate of carrier gas (Figure S2) and the growth time (Figure S3) are very critical for the controlled preparation of specific WS2 nanostructures. As shown in Figure 1b, the as-prepared WS2 nanostructures have well-defined 2D plate-like triangle morphology with smooth and flat edges. The regularly shaped morphology would serve as WGM resonator to effectively confine photons for a long time by means of continuous internal reflection along the smooth surface, resulting in enhanced light-matter interaction.33 The transmission electron microscopy (TEM) image of a typical WS2 nanostructure (Figure 1c) shows that the WS2 flakes were stacked by concentric triangles. The corresponding selected area electron diffraction (SAED, inset of Figure 1c) exhibits a single set of hexagonally arranged diffraction spots, suggesting the single crystalline nature of the flake and an AB stacking growth


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mode.34-37 These are further confirmed by the Raman spectra (Figure S4) and atomic resolution STEM high-angle annular dark-field images (HAADF) (Figure S5).38-40 Atomic force microscopy image (AFM, Figure 1d, Figure S6) reveals that the basal planes of P-multilayer WS2 flake shrunk gradually from the bottom to the top layers, which is beneficial for achieving partial destructive interference of second-order nonlinear polarizations between them.

Figure 2. (a,b) Bright-field (a) and SH (b) optical microscopy images of a single monolayer WS2 excited with a pulsed laser (810 nm). Scale bars are 5 µm. (c,d) Bright-field (c) and SH (d) optical microscopy images of a typical multilayer WS2 excited with a pulsed laser (810 nm). Scale bars are 5 µm. (e) Spatial resolved spectra collected from the edge and center of the Pmultilayer WS2 shown in (d). Inset: measured SHG intensity as a function of FW laser power, which fits to a square dependence. (f) Layer-dependent SHG intensity of P-multilayer WS2. Inset: magnified view of the red box. (g) Scaled intensities of SHG signals from P-multilayer (black) and monolayer (red) WS2 excited with a pulsed laser (810 nm) at a fixed pump power.


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These as-fabricated P-multilayer WS2 structures offer an ideal platform for investigating edge SHG. As displayed in Figure 2a-d, an 810 nm femtosecond-pulsed laser beam (150 fs, 80 MHz) was focused on the center of a typical monolayer and P-multilayer WS2 (about 150 layers), respectively (see Figure S7a for the setup). Compared with the blue spot generated from the center of monolayer WS2 (i.e., the excitation point), a blue light radiated from the edges of Pmultilayer WS2, suggesting the efficient edge SH radiation. Spatial resolved spectra taken from the edge and center of P-multilayer WS2 show that both signals exhibit a single sharp peak at 405 nm (Figure 2e), which is twice the frequency of the FW at 810 nm. This is in consistence with the characteristic of SHG. Power-dependent measurements reveal that the signal intensity has a quadratic power dependence on the pump intensity, as illustrated in the inset of Figure 2e, which further confirms the second-order nature of the emitted light. Moreover, the SHG signals obtained from the edge is much stronger than that from the center of P-multilayer WS2 (Figure 2e), which verifies the edge SHG in P-multilayer WS2. Layer-dependent SHG intensity (Figure 2f) shows that the SHG intensity of multilayer WS2 demonstrates a diminishing oscillation with increasing numbers of layers at first. This can be ascribed to the restoration of inversion symmetry in 2H stacking TMDs with even layer numbers.23,24 Further increasing the numbers of layers (more than 70 layers) brings about rapid increase of SHG intensity, which is quite different from general multilayer 2H stacking TMDs. It is noteworthy that although the SHG is generated from the edges of P-multilayer WS2, the intensity of SHG signals is much stronger than that from monolayer WS2 under the same excitation conditions (Figure 2b,d). As plotted in Figure 2g, more than 40 times enhancement of SHG has been realized in P-multilayer WS2 in comparison to that from monolayer WS2, indicating the enhancement of edge SHG in the Pmultilayer structure.


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Figure 3. (a,b) Polarization dependent SHG in monolayer (a) and P-multilayer (b) WS2 measured under the parallel polarization configuration. (c) Top view of monolayer WS2 crystal structure. (d) Side view of bilayer WS2 crystal structure with AB stacking mode. (e) Schematic illustration of partial interference of SH fields in bilayer WS2. Polarization dependent SHG was performed to determine the mechanism of edge SHG in Pmultilayer WS2. As illustrated in Figure S7b (Supporting Information), a polarizer was adopted to select the parallel or perpendicular components of the SH radiation with respect to the polarization of FW. As shown in Figure 3a, the polarization-resolved SHG of an individual monolayer WS2 flake (measured under the parallel polarization configuration) shows a six-fold anisotropic pattern, which agrees well with previous works.41-43 This can be ascribed to the threefold rotational symmetry of monolayer WS2 crystal (Figure 3c), so that the SH response with a polarization parallel (perpendicular) to the polarization of FW signals would exhibit a six-fold rotational symmetry. The intensity of SHG varied as I//=Acos23φ, where φ is the azimuthal angle


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between the polarization of FW signals and the armchair direction of monolayer WS2.41 In addition, as illustrated in Figure 3b, similar result was obtained from P-multilayer WS2, which indicates that the mechanism of edge SHG in P-multilayer WS2 should be in accordance with that of monolayer WS2. Since 2H stacked multilayer WS2 is thermodynamically favorable, the two adjacent layers have opposite orientations (Figure 3d). As a result, the linearly polarized SH fields generated from the neighboring layers under linearly polarized FW excitation have opposite directions (Figure 3e), which leads to the coherent interference between them. Owing to the gradually decreasing size of WS2 nanosheets, the deconstructive interference of the SH fields only occurs at the full covered region. Thus, the resulting polarization of SHG from the non-full covered region is similar to that of monolayer WS2, which retains the six-fold rotational symmetry.44,45 This agrees well with the experimental results, which provides a direct confirmation of the mechanism of edge SHG in P-multilayer WS2. As mentioned above, although the edge SHG is resulted from the partial deconstructive interference of the SH fields, the enhancement of SHG has been realized in P-multilayer WS2 in comparison to that from monolayer WS2. Thus, further experiments were carried out to look deep into the enhancement of edge SHG in P-multilayer WS2. Figure 4a,b show the bright-field optical microscopy images of monolayer and P-multilayer WS2 excited by an 810 nm wavelength femtosecond laser, respectively. A brighter triangle shape pattern located at the edges of the P-multilayer WS2 was observed while only a bright spot at the excitation point was obtained from the monolayer WS2, indicating total internal reflection of the excitation laser along the edges of the P-multilayer WS2.46 Due to the atomic thickness of monolayer WS2 (≈0.7 nm), light cannot be confined in monolayer WS2 in spite of the high refractive index (n≈4) (Figure


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4c).47 Nevertheless, due to the increasing numbers of WS2 layers, the thickness of P-multilayer WS2 permits an increase of light-matter interaction length and effective optical confinement. Benefitting from the regular triangle shape of P-multilayer WS2, the nanoflake provides excellent optical confinement with the three lateral edges functioning as reflecting mirrors, forming the WGM resonance cavity (Figure 4d).

Figure 4. (a,b) Bright-field optical microscopy image of a single monolayer (a) and P-multilayer (b) WS2 excited with a pulsed laser (810 nm). Scale bars are 5 µm. (c,d) Simulated 2D normalized electric field distribution in monolayer (c) and multilayer (d) WS2. (e) FW wavelength dependent SHG of P-multilayer WS2 with different sizes. All scale bars are 5 µm. (f) The plot and fitted curve of 2λ2/3∆λ versus the side-length of the sheets. Inset: schematic of a typical WGM resonator. Due to the spectral modulation of WGM cavities, a highly confined optical field at the resonant wavelength can be obtained.48 As a result, the intensities of SHG signals varied


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periodically with the change of wavelength of excitation laser (Figure 4e). With the increase of cavity length, the spacing between two resonant modes decreases, which is a typical characteristic of WGM resonance. According to the WGM theory, the mode spacing ∆λ and the side length (D) of nanosheet should satisfy the equation ∆λ=λ2/nL=2λ2/n3D,48 where n is the refractive index of WS2 and λ is the wavelength of the excitation light. Based on the measured relationship between 2λ2/3∆λ and D plotted in Figure 4f, n=4.62 was obtained, which is close to the reported refractive index (n≈4),47 further verifying a three-edge reflected WGM resonance mechanism. The effective optical feedback and compact mode volume in WGM cavities which bring about highly concentrated local field resonance in P-multilayer WS2 flakes, along with the increase of light-matter interaction length, lead to the enhancement of edge SHG.

Figure 5. (a) Scheme of monolayer and P-multilayer WS2 hybrid plasmonic structure. (b,c) The normalized intensities of SHG signals collected from the monolayer (b) and P-multilayer (c)


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WS2 on different substrates at a fixed excitation power. (d) Simulated electric field distributions in P-multilayer WS2 on SiO2/Si (up) and silver (bottom) substrates, respectively. Owing to the effective WGM resonance, the tightly confined field in P-multilayer WS2 provides a possibility to further enhance edge SHG by hybridizing the WGM mode with plasmonics. The coupling between the plasmonic and WGM mode would further enhance the light-matter interaction, which is favorable for the enhancement of SHG at nanoscale size.49,50 As illustrated in Figure 5a, for comparison, both the monolayer and P-multilayer WS2 were transferred onto the top of a smooth silver film, separated with a 10 nanometer magnesium fluoride (MgF2) insulting gap (i.e. hybrid plasmonic structure).51 Figure 5b,c show the spectra collected from the monolayer and multilayer WS2 on different substrates at a fixed excitation power, respectively. The normalized SH intensity of monolayer WS2 in hybrid plasmonic structure is about 15 times larger than that on SiO2/Si substrate. Comparatively, more than 800 times enhancement of SHG has been realized from P-multilayer WS2 hybrid plasmonic structure with respect to that from P-multilayer WS2 on SiO2/Si substrate. These results suggest the different mechanisms of SHG enhancement between monolayer and P-multilayer WS2 hybrid plasmonic structures. For the monolayer WS2, due to the weak optical confinement of atomic thickness monolayer structure, the coupling between SPPs and FW signals is negligible. The tiny enhancement of SHG in hybrid plasmonic structure was resulted from the reflection enhancement of silver at wavelength of FW and SHG signals, where silver film substrate functions as reflecting mirror (Figure S8).49 In sharp contrary, arising from the strong optical confinement in the P-multilayer WS2, the effective coupling between WGM resonant modes and plasmonics leads to the ultracompact mode volume and significant optical field enhancement at nanoscale (Figure


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5d).49,50 The enhanced light-matter interaction, along with the reflection enhancement of silver film, brings about more than 800-fold enhancement of edge SHG in P-multilayer WS2 hybrid plasmonic structure.

CONCLUSIONS In summary, large enhancement of edge SHG was realized in pyramid-like multilayer WS2 nanosheets based on enhanced light-matter interaction through the tightly confined field in WGM microcavity. The pyramid-like multilayer WS2, where the basal planes shrink gradually from the bottom to the top layers, exhibit efficient edge SH radiation due to the partial destructive interference of SH fields between the neighboring atomic layers. Moreover, the welldefined 2D plate-like triangle morphology of pyramid-like multilayer WS2 nanosheet leads to the formation of WGM resonance cavity and thus enhanced light-matter interaction. Consequently, edge SHG was enhanced in P-multilayer WS2 in comparison to that from the monolayer under the same condition. In addition, by hybridizing the WGM mode with plasmonics, we have realized more than 800 times enhancement of edge SHG in P-multilayer WS2 hybrid plasmonic structure. We believe that the results demonstrated here would provide enlightenment for the construction of specific structures for efficient nonlinear optical processes.

METERIALS AND METHODS Preparation of WS2 microstructures. The tungsten oxide (WO3, 99.99%) and sulfur powders (S, 99.99%) were purchased from Sigma Aldrich, and used without further treatment. The P-


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multilayer WS2 nanostructures were fabricated with an atmospheric pressure chemical vapor deposition (CVD) method (Figure S1, Supporting Information). The CVD system with two separated heating zones was employed, so that the evaporation temperature of WO3 and S powders can be separately controlled. In a typical preparation, 300 mg tungsten oxide (WO3) powders were placed in a ceramic boat covered with clean SiO2/Si substrate at heating zone 1, and another separate ceramic boat containing 500 mg sulfur powders located the upstream of the furnace 20 cm away from the WO3 powders (heating zone 2). Before heating, the tube was vacuum pumped to evacuate the air and then refilled with high-purity argon (Ar, the carrier gas) to atmospheric pressure. After that, the center of the heating zone 1 (heating zone 2) was heated to 930oC (180oC) in 30 min and held for 15 min. Then, the furnace was cooled to room temperature naturally. It is noted that the flow rate of the carrier gas decreased from 40 sccm to 20 sccm gradually during the growth process, which is very critical for the preparation of Pmultilayer WS2 nanostructures. For the growth of monolayer WS2, 400 sccm high-purity Ar is continuously supplied as the carrier gas. Characterization. The WS2 microstructures were transferred onto different substrates for the measurements of transmission electron microscopy (TEM, JEOL, 2100F), atomic force microscopy (AFM, Bruker Multimode 8), aberration-corrected scanning transmission electron microscopy (STEM, JEM-ARM 200F) and confocal laser Raman spectrometer (Labram HR Evolution, Horiba). The optical measurements were carried out on home-built far-field micro optical systems. The schematic demonstration of the experimental setups for optical characterization is shown in Figure S7. Calculation methods. The numerical simulations were carried out with the commercial software Comsol, which can solve three dimensional Maxwell equations by the finite element method,


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and the frequency domain Wave Optics module was employed. A pyramid-like triangle structure was used to model WS2 structure with a refractive index (n) of 3.5. For hybrid plasmonic structure, the thickness of the Ag film is 1 µm with a permittivity of -31.857+0.43230i at wavelength of 810 nm. The height (h) of the calculated MgF2 layers (n=1.38) was 10 nm.

ASSOCIATED CONTENT Conflict of Interest: The authors declare no competing financial interest. Supporting Information. The Supporting Information is available online. Schematic of CVD method; bright-field optical microscopy image; AFM images; STEM-HAADF images; Raman spectra; illustration of the growth process; experimental setup for the optical characterization; electric field distribution simulation are given in Figures S1-S8. AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Author Contributions Y.S.Z. conceived the idea. Y.S.Z. and J.Y. supervised the project. X.L. designed the experiments and prepared the materials. X.L., Y.L. and K.W. performed the optical measurements. X.L., C.W., W.Z., Y.Y. and Y.J.L. put forward the theoretical model and contributed to the theoretical calculations. X.L., Y.L., C.W. and Y.S.Z. analyzed the data. X.L. and Y.S.Z. wrote the manuscript. All authors discussed the results and commented on the manuscript.


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ACKNOWLEDGMENT This work was supported financially by the Ministry of Science and Technology of China (Grant No. 2017YFA0204502), National Natural Science Foundation of China (Grant Nos. 21533013 and 21373241), and the Youth Innovation Promotion Association CAS (2014028). REFERENCES 1.

Xu, W.; Liu, W.; Schmidt, J. F.; Zhao, W.; Lu, X.; Raab, T.; Diederichs, C.; Gao, W.;

Seletskiy, D. V.; Xiong, Q. Correlated Fluorescence Blinking in Two-Dimensional Semiconductor Heterostructures. Nature 2017, 541, 62-67. 2.

Li, Y.; Zhang, J.; Huang, D.; Sun, H.; Fan, F.; Feng, J.; Wang, Z.; Ning, C. Z. Room-

Temperature Continuous-Wave Lasing from Monolayer Molybdenum Ditelluride Integrated with a Silicon Nanobeam Cavity. Nat. Nanotechnol. 2017, 12, 987-992. 3.

Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C. J.; Muller, D.;

Park, J. High-Mobility Three-Atom-Thick Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015, 520, 656-660. 4.

Zhou, Y.; Scuri, G.; Wild, D. S.; High, A. A.; Dibos, A.; Jauregui, L. A.; Shu, C.; De

Greve, K.; Pistunova, K.; Joe, A. Y.; Taniguchi, T.; Watanabe, K.; Kim, P.; Lukin, M. D.; Park, H. Probing Dark Excitons in Atomically Thin Semiconductors via Near-Field Coupling to Surface Plasmon Polaritons. Nat. Nanotechnol. 2017, 12, 856-860. 5.

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, 263-275.


16

Page 17 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6.

Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.;

Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.; Zhou, W.; Ajayan, P. M. Vertical and In-Plane Heterostructures from WS2/MoS2 Monolayers. Nat. Mater. 2014, 13, 1135-1142. 7.

Li, X.; Lin, M. W.; Basile, L.; Hus, S. M.; Puretzky, A. A.; Lee, J.; Kuo, Y. C.; Chang, L.

Y.; Wang, K.; Idrobo, J. C.; Li, A. P.; Chen, C. H.; Rouleau, C. M.; Geohegan, D. B.; Xiao, K. Isoelectronic Tungsten Doping in Monolayer MoSe2 for Carrier Type Modulation. Adv. Mater. 2016, 28, 8240-8247. 8.

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. 9.

Jie, W.; Chen, X.; Li, D.; Xie, L.; Hui, Y. Y.; Lau, S. P.; Cui, X.; Hao, J. Layer-

Dependent Nonlinear Optical Properties and Stability of Non-Centrosymmetric Modification in Few-Layer GaSe Sheets. Angew. Chem. Int. Ed. 2015, 54, 1185-1189. 10.

Sun, Z. P.; Martinez, A.; Wang, F. Optical Modulators with 2D Layered Materials. Nat.

Photonics 2016, 10, 227-238. 11.

Xia, F. N.; Wang, H.; Xiao, D.; Dubey, M.; Ramasubramaniam, A. Two-Dimensional

Material Nanophotonics. Nat. Photonics 2014, 8, 899-907. 12.

Trolle, M. L.; Seifert, G.; Pedersen, T. G. Theory of Excitonic Second-Harmonic

Generation in Monolayer MoS2. Phys. Rev. B 2014, 89, 235410. 13.

Wang, G.; Marie, X.; Gerber, I.; Amand, T.; Lagarde, D.; Bouet, L.; Vidal, M.; Balocchi,

A.; Urbaszek, B. Giant Enhancement of the Optical Second-Harmonic Emission of WSe2 Monolayers by Laser Excitation at Exciton Resonances. Phys. Rev. Lett. 2015, 114, 097403.


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14.

Page 18 of 23

Kumar, N.; Najmaei, S.; Cui, Q. N.; Ceballos, F.; Ajayan, P. M.; Lou, J.; Zhao, H.

Second Harmonic Microscopy of Monolayer MoS2. Phys. Rev. B 2013, 87, 161403. 15.

Zhang, L.; Liu, K.; Wong, A. B.; Kim, J.; Hong, X.; Liu, C.; Cao, T.; Louie, S. G.; Wang,

F.; Yang, P. Three-Dimensional Spirals of Atomic Layered MoS2. Nano Lett. 2014, 14, 64186423. 16.

Fan, X.; Jiang, Y.; Zhuang, X.; Liu, H.; Xu, T.; Zheng, W.; Fan, P.; Li, H.; Wu, X.; Zhu,

X.; Zhang, Q.; Zhou, H.; Hu, W.; Wang, X.; Sun, L.; Duan, X.; Pan, A. Broken Symmetry Induced Strong Nonlinear Optical Effects in Spiral WS2 Nanosheets. ACS Nano 2017, 11, 48924898. 17.

Li, D.; Xiong, W.; Jiang, L.; Xiao, Z.; Golgir, H. R.; Wang, M.; Huang, X.; Zhou, Y.;

Lin, Z.; Song, J.; Ducharme, S.; Jiang, L.; Silvain, J. F.; Lu, Y. Multimodal Nonlinear Optical Imaging of MoS2 and MoS2‑Based van der Waals Heterostructures. ACS Nano 2016, 10, 37663775. 18.

Yu, H.; Talukdar, D.; Xu, W.; Khurgin, J. B.; Xiong, Q. Charge-Induced Second-

Harmonic Generation in Bilayer WSe2. Nano Lett. 2015, 15, 5653-5657. 19.

Seyler, K. L.; Schaibley, J. R.; Gong, P.; Rivera, P.; Jones, A. M.; Wu, S.; Yan, J.;

Mandrus, D. G.; Yao, W.; Xu, X. Electrical Control of Second-Harmonic Generation in a WSe2 Monolayer Transistor. Nat. Nanotechnol. 2015, 10, 407-411. 20.

Zhou, X.; Cheng, J.; Zhou, Y.; Cao, T.; Hong, H.; Liao, Z.; Wu, S.; Peng, H.; Liu, K.;

Yu, D. Strong Second-Harmonic Generation in Atomic Layered GaSe. J. Am. Chem. Soc. 2015, 137, 7994-7997.


18

Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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21.

Yi, F.; Ren, M.; Reed, J. C.; Zhu, H.; Hou, J.; Naylor, C. H.; Johnson, A. T.; Agarwal, R.;

Cubukcu, E. Optomechanical Enhancement of Doubly Resonant 2D Optical Nonlinearity. Nano Lett. 2016, 16, 1631-1636. 22.

Zeng, J.; Yuan, M.; Yuan, W.; Dai, Q.; Fan, H.; Lan, S.; Tie, S. Enhanced Second

Harmonic Generation of MoS2 Layers on a Thin Gold Film. Nanoscale 2015, 7, 13547-13553. 23.

Kim, C. J.; Brown, L.; Graham, M. W.; Hovden, R.; Havener, R. W.; McEuen, P. L.;

Muller, D. A.; Park, J. Stacking Order Dependent Second Harmonic Generation and Topological Defects in h‑BN Bilayers. Nano Lett. 2013, 13, 5660-5665. 24.

Li, Y.; Rao, Y.; Mak, K. F.; You, Y.; Wang, S.; 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, 3329-3333. 25.

Xiao, J.; Ye, Z.; Wang, Y.; Zhu, H.; Wang, Y.; Zhang, X. Nonlinear Optical Selection

Rule Based on Valley-Exciton Locking in Monolayer WS2. Light: Sci. Appl. 2015, 4, e366. 26.

Janisch, C.; Wang, Y.; Ma, D.; Mehta, N.; Elias, A. L.; Perea-Lopez, N.; Terrones, M.;

Crespi, V.; Liu, Z. Extraordinary Second Harmonic Generation in Tungsten Disulfide Monolayers. Sci. Rep. 2014, 4, 5530. 27.

Li, X.; Basile, L.; Yoon, M.; Ma, C.; Puretzky, A. A.; Lee, J.; Idrobo, J. C.; Chi, M.;

Rouleau, C. M.; Geohegan, D. B.; Xiao, K. Revealing the Preferred Interlayer Orientations and Stackings of Two-Dimensional Bilayer Gallium Selenide Crystals. Angew. Chem. Int. Ed. 2015, 54, 2712-2717. 28.

Zheng, S.; Sun, L.; Zhou, X.; Liu, F.; Liu, Z.; Shen, Z.; Fan, H. J. Coupling and Interlayer

Exciton in Twist-Stacked WS2 Bilayers. Adv. Opt. Mater. 2015, 3, 1600-1605.


19

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29.

Page 20 of 23

Chen, J.; Liu, B.; Liu, Y.; Tang, W.; Nai, C. T.; Li, L.; Zheng, J.; Gao, L.; Zheng, Y.;

Shin, H. S.; Jeong, H. Y.; Loh, K. P. Chemical Vapor Deposition of Large-Sized Hexagonal WSe2 Crystals on Dielectric Substrates. Adv. Mater. 2015, 27, 6722-6727. 30.

Rong, Y.; Fan, Y.; Leen Koh, A.; Robertson, A. W.; He, K.; Wang, S.; Tan, H.; Sinclair,

R.; Warner, J. H. Controlling Sulphur Precursor Addition for Large Single Crystal Domains of WS2. Nanoscale 2014, 6, 12096-12103. 31.

Li, X.; Dong, J.; Idrobo, J. C.; Puretzky, A. A.; Rouleau, C. M.; Geohegan, D. B.; Ding,

F.; Xiao, K. Edge-Controlled Growth and Etching of Two-Dimensional GaSe Monolayers. J. Am. Chem. Soc. 2017, 139, 482-491. 32.

Ji, Q.; Zhang, Y.; Zhang, Y.; Liu, Z. Chemical Vapour Deposition of Group-VIB Metal

Dichalcogenide Monolayers: Engineered Substrates from Amorphous to Single Crystalline. Chem. Soc. Rev. 2015, 44, 2587-2602. 33.

Zhang, W.; Peng, L.; Liu, J.; Tang, A.; Hu, J. S.; Yao, J.; Zhao, Y. S. Controlling the

Cavity Structures of Two-Photon-Pumped Perovskite Microlasers. Adv. Mater. 2016, 28, 40404046. 34.

Shaw, J. C.; Zhou, H.; Chen, Y.; Weiss, N. O.; Liu, Y.; Huang, Y.; Duan, X. Chemical

Vapor Deposition Growth of Monolayer MoSe2 Nanosheets. Nano Res. 2015, 7, 511-517. 35.

Xia, J.; Huang, X.; Liu, L. Z.; Wang, M.; Wang, L.; Huang, B.; Zhu, D. D.; Li, J. J.; Gu,

C. Z.; Meng, X. M. CVD Synthesis of Large-Area, Highly Crystalline MoSe2 Atomic Layers on Diverse Substrates and Application to Photodetectors. Nanoscale 2014, 6, 8949-8955. 36.

Ly, T. H.; Zhao, J.; Kim, H.; Han, G. H.; Nam, H.; Lee, Y. H. Vertically Conductive

MoS2 Spiral Pyramid. Adv. Mater. 2016, 28, 7723-7728.


20

Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

37.

Zheng, J.; Yan, X.; Lu, Z.; Qiu, H.; Xu, G.; Zhou, X.; Wang, P.; Pan, X.; Liu, K.; Jiao, L.

High-Mobility Multilayered MoS2 Flakes with Low Contact Resistance Grown by Chemical Vapor Deposition. Adv. Mater. 2017, 29, 1604540. 38.

Yan, J.; Xia, J.; Wang, X.; Liu, L.; Kuo, J. L.; Tay, B. K.; Chen, S.; Zhou, W.; Liu, Z.;

Shen, Z. X., Stacking-Dependent Interlayer Coupling in Trilayer MoS2 with Broken Inversion Symmetry. Nano Lett. 2015, 15, 8155-8161. 39.

Puretzky, A. A.; Liang, L.; Li, X.; Xiao, K.; Wang, K.; Mahjouri-Samani, M.; Basile, L.;

Idrobo, J. C.; Sumpter, B. G.; Meunier, V.; Geohegan, D. B., Low-Frequency Raman Fingerprints of Two-Dimensional Metal Dichalcogenide Layer Stacking Configurations. ACS Nano 2015, 9, 6333-6342. 40.

Lu, X.; Utama, M. I.; Lin, J.; Gong, X.; Zhang, J.; Zhao, Y.; Pantelides, S. T.; Wang, J.;

Dong, Z.; Liu, Z.; Zhou, W.; Xiong, Q., Large-Area Synthesis of Monolayer and Few-Layer MoSe2 Films on SiO2 Substrates. Nano Lett. 2014, 14, 2419-2425. 41.

Malard, L. M.; Alencar, T. V.; Barboza, A. P. M.; Mak, K. F.; de Paula, A. M.

Observation of Intense Second Harmonic Generation from MoS2 Atomic Crystals. Phys. Rev. B 2013, 87, 201401. 42.

Clark, D. J.; Senthilkumar, V.; Le, C. T.; Weerawarne, D. L.; Shim, B.; Jang, J. I.; Shim,

J. H.; Cho, J.; Sim, Y.; Seong, M. J.; Rhim, S. H.; Freeman, A. J.; Chung, K. H.; Kim, Y. S. Strong Optical Nonlinearity of CVD-Grown MoS2 Monolayer as Probed by WavelengthDependent Second-Harmonic Generation. Phys. Rev. B 2014, 90, 121409. 43.

David, S. N.; Zhai, Y.; van der Zande, A. M.; O'Brien, K.; Huang, P. Y.; Chenet, D. A.;

Hone, J. C.; Zhang, X.; Yin, X. B. Rapid, All-Optical Crystal Orientation Imaging of TwoDimensional Transition Metal Dichalcogenide Monolayers. Appl. Phys. Lett. 2015, 107, 111902.


21

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44.

Page 22 of 23

Hsu, W. T.; Zhao, Z. A.; Li, L. J.; Chen, C. H.; Chiu, M. H.; Chang, P. S.; Chou, Y. C.;

Chang, W. H. Second Harmonic Generation from Artificially Stacked Transition Metal Dichalcogenide Twisted Bilayers. ACS Nano 2014, 8, 2951-2958. 45.

Cheng, J.; Jiang, T.; Ji, Q.; Zhang, Y.; Li, Z.; Shan, Y.; Zhang, Y.; Gong, X.; Liu, W.;

Wu, S. Kinetic Nature of Grain Boundary Formation in As-Grown MoS2 Monolayers. Adv. Mater. 2015, 27, 4069-4074. 46.

Zhang, Q.; Ha, S. T.; Liu, X.; Sum, T. C.; Xiong, Q. Room-Temperature Near-Infrared

High‑Q Perovskite Whispering-Gallery Planar Nanolasers. Nano Lett. 2014, 14, 5995-6001. 47.

Yu, Y.; Yu, Y.; Huang, L.; Peng, H.; Xiong, L.; Cao, L. Giant Gating Tunability of

Optical Refractive Index in Transition Metal Dichalcogenide Monolayers. Nano Lett. 2017, 17, 3613-3618. 48.

Wei, C.; Liu, S. Y.; Zou, C. L.; Liu, Y.; Yao, J.; Zhao, Y. S. Controlled Self-Assembly of

Organic Composite Microdisks for Efficient Output Coupling of Whispering-Gallery-Mode Lasers. J. Am. Chem. Soc. 2015, 137, 62-65. 49.

Liu, X.; Zhang, Q.; Chong, W. K.; Yip, J. N.; Wen, X.; Li, Z.; Wei, F.; Yu, G.; Xiong,

Q.; Sum, T. C. Cooperative Enhancement of Second-Harmonic Generation from a Single CdS Nanobelt-Hybrid Plasmonic Structure. ACS Nano 2015, 9, 5018-5026. 50.

Lu, F. F.; Li, T.; Hu, X. P.; Cheng, Q. Q.; Zhu, S. N.; Zhu, Y. Y. Efficient Second-

Harmonic Generation in Nonlinear Plasmonic Waveguide. Opt. Lett. 2011, 36, 3371-3373. 51.

Oulton, R. F.; Sorger, V. J.; Genov, D. A.; Pile, D. F. P.; Zhang, X. A Hybrid Plasmonic

Waveguide for Subwavelength Confinement and Long-Range Propagation. Nat. Photonics 2008, 2, 496-500.


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