Phase Identification and Strong Second Harmonic Generation in Pure

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Phase Identification and Strong Second Harmonic Generation in Pure #-InSe and Its Alloys Qiaoyan Hao, Huan Yi, Huimin Su, Bin Wei, Zhuo Wang, Zhezhu Lao, Yang Chai, Zhongchang Wang, Chuanhong Jin, Junfeng Dai, and Wenjing Zhang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00487 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 9, 2019

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Phase Identification and Strong Second Harmonic Generation in Pure ɛ-InSe and Its Alloys Qiaoyan Hao,§ Huan Yi,§ Huimin Su,¦ Bin Wei,ǀ Zhuo Wang,§ Zhezhu Lao,ǁ Yang Chai,ǂ Zhongchang Wang,ǀ Chuanhong Jin,ǁ Junfeng Dai,¦ Wenjing Zhang§,*

§International

Collaborative Laboratory of 2D Materials for Optoelectronics Science and

Technology, Shenzhen University, Shenzhen 518060, P. R. China ¦Department

of Physics, Southern University of Science and Technology, Shenzhen

518055, P. R. China ǀInternational

Iberian Nanotechnology Laboratory, Av Mestre Jose Veiga, P-4715330

Braga, Portugal ǁState

Key Laboratory of Silicon Materials, School of Materials Science and

Engineering, Zhejiang University, Hangzhou 310027, P. R. China ǂDepartment

of Applied Physics, Hong Kong Polytechnic University, Hong Kong

999077, P. R. China

ACS Paragon Plus Environment

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ABSTRACT: Two-dimensional material indium selenide (InSe) has offered a new platform for fundamental research in virtue of its emerging fascinating properties. Unlike 2H-phase transition-metal dichalcogenides (TMDs), ɛ phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers, and predicted to have a strong second harmonic generation (SHG) effect. In this work, we find that the asprepared pure InSe, alloyed InSe1-xTex and InSe1-xSx (x=0.1 and 0.2) are ɛ phase structures, and exhibit excellent SHG performance from few-layer to bulk-like dimension. This high SHG efficiency is attributed to the non-centrosymmetric crystal structure of the

ɛ-InSe system, which has been clearly verified by aberration-corrected scanning transmission electron microscopy (STEM) images. The experimental results show that the SHG intensities from multi-layer pure ɛ-InSe and alloyed InSe0.9Te0.1 and InSe1-xSx (x=0.1 and 0.2) are around 1-2 orders of magnitude higher than that of the monolayer TMD systems, and even superior to that of GaSe with the same thickness. The estimated nonlinear susceptibility χ(2) of ɛ-InSe is larger than that of ɛ-GaSe and monolayer TMDs. Our study provides first-hand information about the phase identification of ɛ-InSe, and


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indicates an excellent candidate for nonlinear optical (NLO) applications as well as the possibility of engineering SHG response by alloying.

KEYWORDS: phase, indium selenide, second harmonic generation, alloy, nonlinear Two-dimensional (2D) layered materials, particularly the family of 2H-phase transitionmetal dichalcogenides (TMDs), have attracted great attentions due to their novel electronic and optical properties and potential applications in photodetectors, light emission devices, photovoltaic devices and so forth.1-3 Recently, second harmonic generation (SHG) has been extensively studied among 2D TMDs such as MoS2, MoSe2 and WS2.4-8 It has been employed as a non-destructive and sensitive method to quickly probe the crystal symmetry, crystallographic orientation, grain boundary and local strain vector.9-11 Furthermore, studying SHG will extend their optoelectronic applications into the nonlinear regimes, such as frequency converters, optical spectroscopies and image processing.8, 12-13 Despite of the potential applications, the frequency conversion efficiency of 2H-phase TMDs family is constrained by the atomic thickness, as the SHG intensity is attenuated with thickness in the odd number of layers and almost disappears


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in the even number of layers.14-15 The pursuit of appropriate 2D crystals without the thickness limitation is therefore crucial in addressing the challenges for 2D materials applied to nonlinear optics devices. Emergent candidates include layered metalmonochalcogenide Ⅲ-Ⅵ semiconductor, for instance gallium selenium (GaSe), the bulk form of which has been proved to be important second-order nonlinear material.16-18 For instance, Hao et al. reported the layer-dependent nonlinear properties of SHG in fewlayer ɛ-GaSe sheets, demonstrating the intrinsic SHG signal can be maintained in the bilayer GaSe.19 Later, Yu et al. observed non-resonant SHG signal in monolayer GaSe, which was the strongest among all the reported 2D atomic crystals.20 InSe, which is structurally similar to GaSe, has provided another fertile ground for exploring nonlinear light-matter interactions and optical applications in virtue of its tunable band gap with thickness and emerging fascinating properties.21-22 However, there are three basic polytypes including β-InSe, ɛ-InSe and γ-InSe, which differ in the symmetry and structure of the crystal lattice, and present totally different nonlinear optical properties, thus it is important to determine the phase structure for the studied InSe.23 As schematically shown in Figure 1a, unlike γ-phase with ABC stacking


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order, both β- and ε-InSe belong to the 2H phase with AB stacking mode. The β-InSe with even layers is centrosymmetric with the inversion center at the hollow site of hexagons, while the ɛ-InSe possesses non-centrosymmetric structure ranging from monolayer to bulk. Recently, a surge of attention has been focused on the experimental and theoretical studies of SHG from InSe. Slablab et al. found that amorphous InSe thin films fabricated by using thermal evaporation exhibited strong SHG effect.24 Leisgang et

al. employed polarization-resolved SHG to determine the crystal axes in encapsulated single-layer InSe.25 Dani et al. observed the transition of SHG effect from 2D crystals towards 3D crystals of the γ-InSe.26 Subsequently, Liu et al. discovered strong SHG signal arising from monolayer InSe, indicating its potential in the optoelectronic field.27 Zhai et al. reported SHG effect from the chemical vapor deposition (CVD)-grown InSe nanoflakes.28 Nevertheless, there is no explicit crystalline phase determination for the ɛInSe, and the SHG response properties remain unexplored, while theoretical calculation has predicted that the ɛ-InSe can present efficient nonlinear optical effect due to configuration of In-Se building block and interlayer interactions of In-Se compound.29-30 On the other hand, it has been reported that the SHG is more efficient in the alloyed


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TMDs, and the SHG intensity can be tuned by modifying the stoichiometry.31-32 Hence, it would be highly desirable to reveal the atomic resolution structure of ɛ-InSe and investigate the nonlinear optical properties benefiting from its intrinsic noncentrosymmetric structure. Here, the crystalline phase and excellent nonlinear optical effect in ɛ-InSe and its ternary alloys for both the even and odd layers, were demonstrated by aberrationcorrected STEM and SHG response properties, respectively. The polarization-resolved, layer number and excitation wavelength dependent SHG intensities of the pure and alloyed ɛ-InSe are studied. We find that the SHG intensities of the pure and alloyed ɛInSe are around 1-2 orders of magnitude higher than that of monolayer WS2 at the excitation wavelength of 800 nm, and better than that of corresponding ɛ-GaSe at the excitation wavelength ranging from 800 nm to 1280 nm. The current work provides solid information about the phase identification of ɛ-InSe and demonstrates the strategy of tuning SHG intensity by chemical doping.

RESULTS AND DISCUSSION


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The single crystal bulk InSe was synthesized by modified Bridgman method as described in experimental section. The multi-layer InSe flakes mechanically exfoliated from bulk samples were characterized by photoluminescence (PL), Raman and scanning transmission electron microscopy (STEM). Figure 1b shows a typical X-ray diffraction (XRD) pattern of the as-synthesized pure InSe, which matches with the standard data file PDF 34-1431 and suggests high crystalline purity.33-34 The cell parameters were indexed to be a = b = 0.401 nm and c = 1.666 nm, respectively, indicating that the stacking period is composed of two atomic layers. Thus the assynthesized InSe features 2H phase, which should either be β- or ɛ-phase. The pure crystalline phase of typical S- and Te-alloyed samples is demonstrated by XRD patterns as exhibited in Figure S1. They are 2H phase, which is the same as the pure InSe. PL spectrum of the as-synthesized InSe displays a significant peak at 998 nm as shown in Figure 1c, indicating an optical band gap of 1.24 eV. Raman spectrum in Figure 1d reveals three dominant peaks at 114.4, 176.5 and 226.2 cm-1, which correspond to the A1 1g, E1 2g and A2 1g vibration modes of InSe, respectively.34 The weak intensity peak at around 193.0 cm-1 is attributed to E2 2g vibration mode, indicating the ɛ-phase


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of InSe according to the previous study.35-36

Figure 1. Schematic view of InSe three typical phases and characterization of the assynthesized ɛ-InSe crystal. (a) Side view (up) and top view (down) for InSe three typical phases: β-InSe, ɛ-InSe and γ-InSe. Orange and blue spheres represent selenium and indium atoms, respectively. (b) The XRD pattern of the as-synthesized ɛ-InSe. (c) PL spectrum of ɛ-InSe in bulk form. (d) Raman spectrum of ɛ-InSe in bulk form excited by 532 nm laser. High angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) images, coupled with imaging simulation, were further employed to identify the as-synthesized InSe crystal phase. Figure 2b shows an experimental HAADF-STEM


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image of InSe nanosheet observed along the [001] zone axis, which is confirmed by the selected area electron diffraction pattern (SAED) in Figure 2c. It is obvious that there are atoms at the central site of each smallest hexagon, which matches with the stacking mode of ɛ-phase as illustrated in Figure 1a and suggests the broken inversion symmetry in the as-synthesized crystal. Figure 2f shows the intensity line profile extracted from the red dashed line marked in Figure 2b, from which three atomic column intensities can be separated and labeled as X1 (red dashed circle), X2 (orange dashed circle) and X3 (blue dashed circle), respectively. As demonstrated in supporting information (Table S1 and Figure S2), the strongest atomic columns of X1 site must be occupied by the superposition of both In and Se atoms, while the stronger atomic columns of X3 site are expected to be occupied by the single In atoms and the weakest X2 site occupied by the single Se atoms as listed in Table S1. To verify the ɛ phase of the InSe nanosheet, the HAADF-STEM image (see Figure S2) was taken from the thicker sample, which agrees with the simulation result. The crystal structure and the chemical composition of the typical ternary alloy InSe0.8Te0.2 were also characterized by HAADF-STEM image and energy-dispersive X-ray spectrometry (EDS) in Figure S3.


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The experimental STEM image of InSe0.8Te0.2 is similar to that of pure ɛ-InSe, indicating the alloyed InSe samples are ɛ-phase. The EDS mapping confirms that the sample contains three elements: In, Se and Te, and shows the elemental distribution is quite uniform. Figure S4 exhibits the STEM of the typical S-alloyed sample InSe0.8S0.2, which is also consistent with the structure of ε-phase. Thus, the S- and Te-substituted doping of InSe may not change the phase structure, and the lattice structures of alloyed InSe could present non-centrosymmetric. As ε-InSe has the same lattice parameters as β-InSe (a = b = 0.405 nm, c = 1.693 nm), with a hexagonal unit cell and belongs to the symmorphic D1 3h group,37 the crystal structure of ε-InSe was further calculated by employing density functional (DFT) calculations, as shown in Figure S5, in which the thickness of monolayer is ~0.85 nm. For better accuracy, relaxations were performed including the van der Waals (vdW) interaction using optB88-vdW functionals.38 Figure 2e shows the SAED pattern taken along the [010] zone axis, which is compatible with the pattern of the β or ε phase. The lattice parameter measured from experimental results (Figure S5) is also accordance with the calculation (a = b = 0.406 nm) and the above-mentioned XRD result. Figure 2d


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shows the high-resolution HAADF-STEM image projected along [010] zone axis. The stacking mode of atoms corresponds to ε-InSe, with a stacking sequence ABAB..., and the thickness of monolayer measured here is ~0.83 nm, which matches with the calculated results well. As mentioned above, the as-synthesized pure InSe can be furtherly identified to be ε phase.

Figure 2. Z-contrast imaging and intensity analysis of ɛ-InSe. (a) The high-angle annular dark-field image of ɛ-InSe nanosheet. (b) The HAADF-STEM image projected along [001] zone axis taken from the marginal square area in (a). (c) The corresponding SAED pattern of (b). (d) The HAADF-STEM image projected along [010] zone axis. (e) The corresponding SAED pattern of (d). (f) Picture above is the experimental intensity


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profile taken from the red dashed line marked in (b), and below provides the simulated intensity profile along the [001] zone axis of 5 layer ɛ-InSe. The SHG effect provides an effective and non-destructive way to characterize the crystal symmetry, and can further confirm the phase structure. The dependence of the SHG intensity on laser excitation power was measured (Figure 3d) at λex = 1020 nm with spot size ~3.0 μm. An emission peak centered at 510 nm is observed, which is exactly at the half wavelength of the excitation laser, and shows a quadratic dependence on the excitation power with an exponent of 1.89, apparently indicating the second-order nonlinear process.39 Moreover, the SHG intensities of ε-InSe with different layer numbers were measured under excitation of λex = 1020 nm. As shown in Figure 3e, it’s obvious that SHG intensity is significantly enhanced with increasing the thickness from 3 layer to 9 layer under the identical experimental conditions. The powerlaw fitting of the SHG intensity gives an exponent of 2.10 (Figure 3f) when the layer number is above 5 layer, indicating an approximately quadratic dependence on the layer number, which is attributed to that the sample thickness is much less than the coherence length.40-41 On the other hand, when the ε-InSe flakes are thinner than 5


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layer, the power-law fitting of the SHG intensity yields an exponent of 3.54, showing a rough cubic dependence on the layer number, which is attributed to the change of nonlinear coefficient of thinner samples (below 5 layer), and similar experimental results have been observed in ε-GaSe.19 Importantly, the above experimental results demonstrate that SHG can be generated in ε-InSe with an arbitrary number of layer, distinguishing it from the 2H-phase TMDs, which are SHG-active only in their oddlayered form.14, 42-43 The absence of oscillatory SHG between the odd and even number of layers confirms that all of the as-synthesized InSe with different layer numbers are of non-centrosymmetric structure and absent of the inversion center, which is consistent with the ε-phase structure. Polarization-resolved SHG measurement was performed for pure and alloyed InSe samples, where the SHG intensity is plotted as a function of the sample rotation angle. As shown in the Figure 3c, the parallel SHG emission leads to a defined six-petal pattern, indicating three-fold rotational symmetry of the multilayer InSe with the 2H phase structure. The polar patterns for S- and Te-alloyed InSe flakes are of the same as pure InSe (see Figure S6), suggesting that the alloyed InSe samples are 2H phase structures. The SHG intensity mapping images are uniform for pure InSe


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samples within the same layer (Figure S7), suggesting that the lattice symmetry and phase structure of samples are quite uniform.

Figure 3. SHG properties of few-layer pure ɛ-InSe. (a) Schematic of SHG process in ɛInSe. (b) Optical image of an exfoliated ε-InSe nanosheet with different number of layers. The inset shows the height profiles measured by AFM along the white lines of different layer numbers. (c) The polarization-resolved SHG intensity of ε-InSe as the function of the excitation laser polarization against the crystalline lattice direction. (d) Normalized SHG intensity as a function of laser excitation power. The inset shows its log-log scale plot. The SHG intensity used the integrated counts of the SHG spectra in the spectral window from 505 nm to 515 nm. (e) The SHG spectra from the ε-InSe nanosheet in (b) under 1020 nm excitation wavelength. (f) The SHG intensity as a function of layer number. To evaluate the nonlinear efficiency of ε-InSe and its alloys, the excitation laser wavelength dependence of the SHG intensities for ε-GaSe and ε-InSe was measured,


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since ε-GaSe has been proved to be a promising nonlinear crystal, and its nonlinear coefficient is the highest in the reported 2D materials.20 Figure 4a displays the normalized SHG intensities of ε-InSe and ε-GaSe flakes with the same thickness under excitation of λex = 1000 nm at the same experimental conditions. It is found that the SHG intensity of ε-InSe is twice as strong as that of ε-GaSe. In addition, the SHG peak intensities for ε-InSe are stronger than those of ε-GaSe among the whole excitation wavelength range from 800 nm to 1280 nm (Figure 4b), suggesting that the nonlinear efficiency of ε-InSe is better than that of ε-GaSe, which has been reported to have the strongest SHG intensity among all the 2D materials. As a representative member of TMDs family, monolayer WS2 was chosen as well to compare with ε-InSe under excitation wavelength of 800 nm. The normalized SHG intensity of ε-InSe is around 1-2 orders of magnitude larger than that from monolayer WS2. As shown in Figure 4c, at the same experimental conditions, the thickness dependence of SHG intensities for pure εInSe, ε-GaSe, Te-alloyed InSe0.9Te0.1, S-alloyed InSe0.9S0.1 and InSe0.8S0.2 are measured. We can find that the SHG intensities of ε-GaSe in the measurement thickness range are the weakest, and the SHG intensity of ε-InSe can be modulated by


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alloys, where the SHG intensities of Te-alloyed InSe0.9Te0.1 are the strongest, and the SHG intensities of S-alloyed InSe0.9S0.1 and InSe0.8S0.2 are generally weaker than that of pure ε-InSe samples. In addition, it is found that the SHG intensities vibrate with thickness, i.e., don’t present monotonic increase from 6 layer to 100 layer. This vibration is due to the interference in the multilayer system composed of vacuum, the ε-InSe nanosheet, the 300 nm SiO2 and Si substrate, which was illustrated in previous study.17 According to the results, stronger and broader SHG response can be expected by optimizing the composition of alloys, which was demonstrated in the TMDs system.31 Our work provides a very promising system for SHG effect, which may motivate theoretical research on the role of alloying and expand the potential application in nonlinear optics.


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Figure 4. Comparison of the SHG intensities of ɛ-InSe, ɛ-GaSe and monolayer WS2. (a) SHG spectra from multi-layer (~20 nm) ɛ-InSe and multi-layer (~20 nm) ɛ-GaSe flakes under 1000 nm excitation wavelength. (b) The comparison of SHG intensities between

ɛ-InSe (red columns) and ɛ-GaSe (blue columns) under different excitation wavelengths. The SHG intensity of monolayer WS2 (green column) at λex=800 nm was also measured as a reference. (c) Layer-dependent SHG from five typical samples ranging from 6 layer to 100 layer under excitation wavelength at 1000 nm. The error bars are ± 6.5 % for the experiments. All the SHG intensities are normalized by laser power and integration time. To quantitatively estimate the SHG efficiency of ε-InSe, the second-order nonlinear coefficient χ(2) of ε-InSe, WS2 and GaSe was further calculated (Table S2), which gives 1.3×10-11, 2.0×10-12 and 1.1×10-11 m V-1, respectively, under excitation wavelength of 800 nm. ɛ-InSe exhibits the largest nonlinear susceptibility χ(2) among the three material systems, which is 6 times over WS2 and larger than GaSe, respectively. In addition, χ(2) of ε-InSe is 3 times over GaSe under excitation wavelength of 900 nm. Note that the SHG signal of the above three samples were measured under the same conditions for each excitation wavelength, thus the relative comparison of χ(2) is not influenced by the complex factors during the experiment and calculation process. CONCLUSION


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In summary, the crystal structures of the as-prepared pure InSe, alloyed InSe0.8S0.2 and InSe0.8Te0.2 were clearly demonstrated by HAADF-STEM. We found that the pure InSe and its alloys are ɛ-phase structure and exhibit excellent SHG performance from fewlayer to bulk-like dimension. Impressively, the optimal SHG intensities from multi-layer pure ɛ-InSe and alloys are around 1-2 orders of magnitude higher than that from monolayer WS2 at the excitation wavelength of 800 nm, and even superior to that of ɛGaSe with the same thickness at the excitation wavelength ranging from 800 nm to 1280 nm. Moreover, the modulation of SHG intensity by alloying can serve as a general strategy for other materials to develop NLO crystals over a broad spectral range with high nonlinear efficiency.

ASSOCIATED CONTENT

SUPPORTING INFORMATION Supporting Information: Experimental section, additional information on analysis of the three atomic columns as exhibited in Figure 2b and Figure 2f, the simulation results of HAADF-STEM, HAADF-STEM of N (>20) layer ɛ-InSe and its alloys, the polarization-


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resolved SHG intensity of alloys, SHG mapping, the calculation process of the χ(2). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] ORCID iD Wenjing Zhang: 0000-0001-6931-900X Author Contributions §Q.

Y. Hao and H. Yi contributed equally to this work.

Notes The authors declare no conflict of interest.

ACKNOWLEDGMENT This work at Shenzhen University was supported by the National Natural Science Foundation of China (Grant Nos. 51472164 and 61805159), the 1000 Talents Program for Young Scientists of China, Shenzhen Peacock Plan (Grant No.


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KQTD2016053112042971), the Educational Commission of Guangdong Province (Grant Nos. 2015KGJHZ006 and 2016KCXTD006), the Science and Technology Planning Project of Guangdong Province (Grant No. 2016B050501005), Natural Science Foundation of SZU (Grant No. 000050). Z. Wang is supported by the National Natural Science Foundation of China (Grant No. 61805159). H. M. Su is supported by the National Natural Science Foundation of China (Grant No. 11604139). Z. C. Wang is supported by the National Natural Science Foundation of China (Grant No. 51728202).

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MoS2 with Broken Inversion Symmetry: A Promising Ultrathin Nonlinear Optical Device.

Adv. Mater. 2017, 29, 1701486. (8) Autere, A.; Jussila, H.; Dai, Y.; Wang, Y.; Lipsanen, H.; Sun, Z. Nonlinear Optics with 2D Layered Materials. Adv. Mater. 2018, 30, 1705963. (9) 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. (10) 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. (11) Liang, J.; Zhang, J.; Li, Z. Z.; Hong, H.; Wang, J. H.; Zhang, Z. H.; Zhou, X.; Qiao, R. X.; Xu, J. Y.; Gao, P.; Liu, Z. R.; Liu, Z. F.; Sun, Z. P.; Meng, S; Liu, K. H.; Yu, D. P. Monitoring Local Strain Vector in Atomic-Layered MoSe2 by Second-Harmonic Generation. Nano lett. 2017, 17, 7539-7543.


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(12) Hu, L.; Wei, D. S.; Huang, X. R. Second Harmonic Generation Property of Monolayer TMDCs and Its Potential Application in Producing Terahertz Radiation. J. Chem. Phys. 2017, 147, 244701. (13) Sun, Z.; Martinez, A.; Wang, F. Optical Modulators with 2D Layered Materials. Nat.

Photonics 2016, 10, 227-238. (14) 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. (15) Li, D. W.; Xiong, W.; Jiang, L. J.; Xiao, Z. Y.; Golgir, H. R.; Wang, M. M.; Huang, X.; Zhou, Y. S.; Lin, Z.; Song, J. F.; Ducharme, S.; Jiang, L.; Silvain, J. F.; Lu, Y. F. Multimodal Nonlinear Optical Imaging of MoS2 and MoS2‑Based Van der Waals Heterostructures.

ACS nano 2016, 10, 3766-3775. (16) Bringuier, E.; Bourdon, A.; Piccioli, N.; Chevy, A. Optical Second-Harmonic Generation in Lossy Media: Application to GaSe and InSe. Phys. Rev. B 1994, 49, 1697116982. (17) Tang, Y. H.; Mandal, K. C.; McGuire, J. A.; Lai, C. W. Layer- and Frequency-


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Dependent Second Harmonic Generation in Reflection from GaSe Atomic Crystals. Phys.

Rev. B 2016, 94, 125302. (18) Hu, L.; Huang, X.; Wei, D. Layer-Independent and Layer-Dependent Nonlinear Optical Properties of Two-Dimensional GaX (X = S, Se, Te) Nanosheets. Phys. Chem.

Chem. Phys. 2017, 19, 11131-11141. (19) Jie, W. J.; Chen, X.; Li, D.; Xie, L.; Hui, Y. Y.; Lau, S. P.; Cui, X. D.; Hao, J. H. LayerDependent Nonlinear Optical Properties and Stability of Non-Centrosymmetric Modification in Few-Layer GaSe Sheets. Angew. Chem. Int. Ed. 2015, 54, 1185-1189. (20) Zhou, X.; Cheng, J. X.; Zhou, Y. B.; 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, 7994-7997. (21) Bandurin, D. A.; Tyurnina, A. V.; Yu, G. L.; Mishchenko, A.; Zolyomi, V.; Morozov, S. V.; Kumar, R. K.; Gorbachev, R. V.; Kudrynskyi, Z. R.; Pezzini, S.; Kovalyuk, Z. D.; Zeitler, U.; Novoselov, K. S.; Patane, A.; Eaves, L.; Grigorieva, I. V.; Fal'ko, V. I.; Geim, A. K.; Cao, Y. High Electron Mobility, Quantum Hall Effect and Anomalous Optical Response in Atomically Thin InSe. Nat. Nanotechnol. 2017, 12, 223-227.


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(22) Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R.; Mohite, A. D.; Ajayan, P. M. Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe. ACS

Nano 2014, 8, 1263-1272. (23) Yang, Z. B.; Jie, W. J.; Mak, C. H.; Lin, S. H.; Lin, H. H.; Yang, X. F.; Yan, F.; Lau, S. P.; Hao, J. H. Wafer-Scale Synthesis of High-Quality Semiconducting TwoDimensional Layered InSe with Broadband Photoresponse. ACS nano 2017, 11, 42254236. (24) Koskinen, K.; Slablab, A.; Divya, S.; Czaplicki, R.; Chervinskii, S.; Kailasnath, M.; Radhakrishnan, P.; Kauranen, M. Bulk Second-Harmonic Generation from Thermally Evaporated Indium Selenide Thin Films. Opt. Lett. 2017, 42, 1076-1079. (25) Leisgang, N.; Roch, J. G.; Froehlicher, G.; Hamer, M.; Terry, D.; Gorbachev, R.; Warburton, R. J. Optical Second Harmonic Generation in Encapsulated Single-Layer InSe. AIP Adv. 2018, 8, 105120. (26) Deckoff-Jones, S.; Zhang, J. J.; Petoukhoff, C. E.; Man, M. K. L.; Lei, S. D.; Vajtai, R.; Ajayan, P. M.; Talbayev, D.; Madeo, J.; Dani, K. M. Observing the Interplay Between


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Surface and Bulk Optical Nonlinearities in Thin van der Waals Crystals. Sci. Rep. 2016,

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2017, 4, 38-44. (32) Lucking, M. C.; Beach, K.; Terrones, H. Large Second Harmonic Generation in Alloyed TMDs and Boron Nitride Nanostructures. Sci. Rep. 2018, 8, 10118. (33) Mudd, G. W.; Svatek, S. A.; Ren, T.; Patane, A.; Makarovsky, O.; Eaves, L.; Beton, P. H.; Kovalyuk, Z. D.; Lashkarev, G. V.; Kudrynskyi, Z. R.; Dmitriev, A. I. Tuning the Bandgap of Exfoliated InSe Nanosheets by Quantum Confinement. Adv. Mater. 2013, 25, 5714-5718. (34) Tamalampudi, S. R.; Lu, Y.; Kumar, R. U.; Sankar, R.; Liao, C.; Moorthy, B. K.; Chen, C.; Chou, F.; Chen, Y. High Performance and Bendable Few-Layered InSe Photodetectors with Broad Spectral Response. Nano lett. 2014, 14, 2800-2806. (35) Jandl, S.; Carlone, C. Raman Spectrum of Crystalline InSe. Solid State Commun. 1978, 25, 5-8. (36) Carlone, C.; Jandl, S. Second Order Raman Spectrum and Phase Transition in InSe.

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(38) Sun, Y.; Luo, S.; Zhao, X. G.; Biswas, K.; Li, S. L.; Zhang, L. InSe: a Two-Dimensional Material with Strong Interlayer Coupling. Nanoscale 2018, 10, 7991-7998. (39) Shen, Y. R. The Principles of Nonlinear Optics, Wiley-Interscience, New York 2003. (40) Heflin, J. R.; Figura, C.; Marciu, D.; Liu, Y.; Claus, R. O. Thickness Dependence of Second-Harmonic Generation in Thin Films Fabricated from Ionically Self-Assembled Monolayers. Appl. Phys. Lett. 1999, 74, 495-497. (41) Heflin, J. R.; Guzy, M. T.; Neyman, P. J.; Gaskins, K. J.; Brands, C.; Wang, Z. Y.; Gibson, H. W.; Davis, R. M.; Van Cott, K. E. Efficient, Thermally Stable, Second Order Nonlinear Optical Response in Organic Hybrid Covalent/Ionic Self-Assembled Films.

Langmuir 2006, 22, 5723-5727. (42) Janisch, C.; Wang, Y. X.; Ma, D.; Mehta, N.; Elias, A. L.; Perea-Lopez, N.; Terrones, M.; Crespi, V.; Liu, Z. W. Extraordinary Second Harmonic Generation in Tungsten Disulfide Monolayers. Sci. Rep. 2014, 4, 5530. (43) 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.


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TOC Graphic:


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Figure 1. Schematic view of InSe three typical phases and characterization of the as-synthesized ɛ-InSe crystal. (a) Side view (up) and top view (down) for InSe three typical phases: β-InSe, ɛ-InSe and γ-InSe. Orange and blue spheres represent selenium and indium atoms, respectively. (b) The XRD pattern of the assynthesized ɛ-InSe. (c) PL spectrum of ɛ-InSe in bulk form. (d) Raman spectrum of ɛ-InSe in bulk form excited by 532 nm laser. 149x101mm (300 x 300 DPI)


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Figure 2. Z-contrast imaging and intensity analysis of ɛ-InSe. (a) The high-angle annular dark-field image of ɛ-InSe nanosheet. (b) The HAADF-STEM image projected along [001] zone axis taken from the marginal square area in (a). (c) The corresponding SAED pattern of (b). (d) The HAADF-STEM image projected along [010] zone axis. (e) The corresponding SAED pattern of (d). (f) Picture above is the experimental intensity profile taken from the red dashed line marked in (b), and below provides the simulated intensity profile along the [001] zone axis of 5 layer ɛ-InSe. 150x94mm (300 x 300 DPI)


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Figure 3. SHG properties of few-layer pure ɛ-InSe. (a) Schematic of SHG process in ɛ-InSe. (b) Optical image of an exfoliated ε-InSe nanosheet with different number of layers. The inset shows the height profiles measured by AFM along the white lines of different layer numbers. (c) The polarization-resolved SHG intensity of ε-InSe as the function of the excitation laser polarization against the crystalline lattice direction. (d) Normalized SHG intensity as a function of laser excitation power. The inset shows its log-log scale plot. The SHG intensity used the integrated counts of the SHG spectra in the spectral window from 505 nm to 515 nm. (e) The SHG spectra from the ε-InSe nanosheet in (b) under 1020 nm excitation wavelength. (f) The SHG intensity as a function of layer number. 150x80mm (300 x 300 DPI)


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Figure 4. Comparison of the SHG intensities of ɛ-InSe, ɛ-GaSe and monolayer WS2. (a) SHG spectra from multi-layer (~20 nm) ɛ-InSe and multi-layer (~20 nm) ɛ-GaSe flakes under 1000 nm excitation wavelength. (b) The comparison of SHG intensities between ɛ-InSe (red columns) and ɛ-GaSe (blue columns) under different excitation wavelengths. The SHG intensity of monolayer WS2 (green column) at λex=800 nm was also measured as a reference. (c) Layer-dependent SHG from five typical samples ranging from 6 layer to 100 layer under excitation wavelength at 1000 nm. The error bars are ± 6.5 % for the experiments. All the SHG intensities are normalized by laser power and integration time. 149x41mm (300 x 300 DPI)


Phase Identification and Strong Second Harmonic Generation in Pure

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