Valley-Selective Linear Dichroism in Layered Tin Sulfide - ACS

Aug 14, 2018 - Valley-related physics, optics, and electronics have been extensively investigated in transition metal dichalcogenides with a honeycomb...
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Valley-selective Linear Dichroism in Layered Tin Sulfide Fengnian Xia, Chen Chen, Xiaolong Chen, Yuchuan Shao, Bingchen Deng, Qiushi Guo, and Chao Ma ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00850 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Valley-selective Linear Dichroism in Layered Tin Sulfide Chen Chen1, Xiaolong Chen1, Yuchuan Shao1,2, Bingchen Deng1, Qiushi Guo1, Chao Ma1 and Fengnian Xia1* 1

Department of Electrical Engineering, Yale University, New Haven, Connecticut 06511, USA

*To whom correspondence should be addressed: [email protected]

Abstract Valley-related physics, optics and electronics have been extensively investigated in transition metal dichalcogenides (TMDs) with honeycomb lattice such as molybdenum disulfide (MoS2). Their photoemission properties exhibit a circular dichroism due to energy degenerate but inequivalent valleys located at K and K′ in momentum space. Recently the linear dichroism was observed in few-layer black phosphorus with highly anisotropic in-plane optical properties. Here, we report a novel valley-selective linear dichroism of the photoluminescence in layered thin-film Tin Sulfide (SnS). We identified two PL emission peaks, exhibiting an orthogonal linear dichroism, arising from two inequivalent valleys in energy band. The photon emission from Γ-X valley is completely x-polarized, while in contrast, the photon emission from Γ-Y valley is purely y-polarized. This unique valley-selective linear dichroism in SnS is further confirmed by the band edge absorption measurements along Γ-X and Γ-Y directions, respectively. Our observations of valley-selective linear dichroism provide the direct experimental evidence of the previously predicted highly anisotropic electronic band structure of SnS and its unique valleyrelated optical transition rule, which offers opportunities for its applications in optoelectronic and valleytronic devices. Keywords Tin Sulfide; Valley-Selective; Linear Dichroism; Photoluminescence; Absorption Spectrum;

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Introduction The rise of two-dimensional materials provides an extraordinary platform for investigating many novel physics phenomena. For example, valley-related optical selective rule1 was demonstrated in semiconducting monolayer transition metal dichalcogenides (TMDs) with two energy degenerate but inequivalent valleys (K and K’), such as MoS22-4 and tungsten diselenide (WSe2).5 In these materials, the K(K’) valley can be pumped with σ+(σ-) photons followed by a photon emission of the same helicity, subjected to the optical selective rule.1 Since its demonstration in monolayer MoS2,2-4 valley has been intensively investigated as an emerging degree of freedom and is referred as pseudospin in analogy to the electron spin.6-7 This valleyrelated optical selectivity also leads to the observation of valley Hall effect, providing great opportunity for the realization of valleytronic device.8-9 In contrast to the extensive studies on the circular dichroism in TMDs with honeycomb lattice, recently the linear dichroism was demonstrated in black phosphorus with a low lattice symmetry.10-13 However, black phosphorus only possesses one-fold linear dichroism property, which limits its potential for the exploration of the valley-related degree of freedom.14-15 On the other hand, in monolayer16-17 and bulk18 group-IV monochalcogenides, theorists predict that two pair of valleys along Γ-X and Γ-Y directions can be selectively pumped with linearly polarized light with different polarization, respectively. Also, in bulk germanium selenide (GeSe), linear polarization-resolved absorption spectra were reported.18 However, in terms of photoluminescence, only linear dichroism19 from Γ valley was reported in bulk GeS previously, due to the fact that the energy gap at Γ point of the Brillouin zone is much smaller.18 Therefore, optical transition and PL emission are much more efficient along Γ point, leading to the disappearance of the valley-selective linear dichroism in PL spectra. Other than GeS and black phosphorus, strong optical anisotropy and linear dichroism have also been reported in other materials with reduced lattice symmetry such as transition metal dichalcogenides (e.g. ReS220, ReSe221 and their alloys22), transition metal trichalcogenides (e.g. ZrS323 and TiS324-25) and transition metal pentatellurides (e.g. ZrTe526). However, in these materials, the linear dichroism is not related to valley-selectivity. Here, we report the valley-selective linear dichroism of the photoluminescence in layered thinfilm Tin Sulfide (SnS) and uncover its unique valley-related optical transition rules. SnS belongs to the family of group-IV monochalcogenides, where strong in-plane anisotropy of optical and 2 ACS Paragon Plus Environment

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electrical properties has previously been observed due to its low crystalline symmetry.27-29 In this work, we observe two emission peaks in its PL spectrum, arising from the nearly degenerate but inequivalent valleys along Γ-X and Γ-Y directions, respectively.18 These two emission peaks show an orthogonal linear dichroism, subject to the corresponding optical transition rules in these two valleys. The photon emission from Γ-X valley is purely x-polarized while the photon emission from Γ-Y valley is purely y-polarized. Since the energy gaps from these two pair of valleys are similar, the PL emission from both valleys can be observed, leading to the emerging valley-selective linear dichroism. This unique valley-selective linear dichroism is further confirmed by the polarization-dependent optical absorption spectra. The absorption band edge redshifts when polarization direction is tuned from x-direction to y-direction, consistent with the observations in PL spectrum. Our comprehensive experimental study of valley-selective linear dichroism in SnS, using PL, optical absorption and Raman spectroscopy offers new possibility of manipulating valley degree of freedom in materials with reduced lattice symmetry. Our two-fold linear dichroism demonstration here is different from the circular dichroism observed in honeycomb lattice materials such as MoS2 and simple one-fold linear dichroism in materials with reduced lattice symmetry such as TiS3 and ReS2. Main Context  The SnS has a layered orthorhombic crystal structure, subjective to the  space group, similar

to that of black phosphorus and other group-IV monochalcogenides (Figure 1a).27-29 The armchair and zigzag chains of SnS are defined as x and y direction, respectively, and z direction is defined as the direction perpendicular to the layers. Due to its highly asymmetric crystal structure, SnS exhibits strong in-plane anisotropic physical properties.30-31 The layered SnS samples for optical characterization were first deposited onto 285nm SiO2/Si substrate using standard exfoliation approach.32-33 Figure. 1b shows the optical microscope image of a typical SnS flake used for optical characterization. The thickness of this particular SnS flake is 109 nmthick, determined by the Atomic Force Microscopy (AFM) measurement, as shown in the inset of Figure. 1b. We performed optical characterization on samples of different thickness ranging from 50 to 500 nm, which show similar results and can be considered as bulk SnS. The SnS flake was further characterized by Raman scattering spectroscopy (See Methods). Three prominent Raman modes were resolved, corresponding to the B3g, Ag1 and Ag2 mode, as reported 3 ACS Paragon Plus Environment

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previously.34 All these Raman modes show the typical Lorentz shape and they are positioned at 161 cm-1, 191 cm-1 and 217 cm-1 (Figure 1c), respectively. The polarization dependence of Raman spectrum is a convenient way to determine the crystal orientation.35-36 We measured the angular resolved Raman spectrum by rotating the sample with respect to the laser polarization (Figure S1a). The inset of Figure. 1c shows the angular resolved Raman intensity of Ag1 mode, where 0 degree is determined as the x-direction of the SnS crystal. The rotation angle θ is referred to the intersect angle between the laser polarization direction and x direction. The B3g mode intensity shows a good agreement with the dependence of   2 + , while the intensity of two Ag modes were well fitted with the dependence of   + (Figure S1b), where a and b are fitting parameters. This observation agrees well with previous Raman studies in SnS.37-38 The distinct behavior of B3g mode and the Ag modes can be attributed to the different Raman tensors governing them.37-38 With angular-resolved Raman scattering measurements, we can determine the armchair (x) and zigzag (y) direction of the SnS flake, which set a reference for the crystal orientation for the following polarization-resolved PL and absorption measurements. We then performed the photoluminescence (PL) measurement to investigate the dichroism in SnS. The PL spectrum was measured with the micro-PL setup with an excitation photon energy of 2.33 eV and excitation power of 500 µW. Since the PL emission was significantly quenched at room temperature39, all the PL measurements were performed under 77K. (See Methods). Two photon emission peaks were resolved from the PL spectrum, locating at 1.160 eV and 1.204 eV (Figure 2a). Here, we use IY and IX to denote the intensity of these two peaks, respectively. To exclude the possible PL emission from the substrate, we measured emission spectrum directly from the SiO2/Si substrate. As expected, a much weaker emission peak was observed at 1.130 eV, which can be attributed to the indirect band gap of the silicon.40 Since the excitation light will be significantly attenuated by the SnS flake (~100 nm), the emission from the SiO2/Si substrate can be ignored when the SnS PL is measured. The PL intensity depends linearly with the excitation laser power (Figure S2). According to the theoretical calculation results reported previously, SnS exhibits two valleys with close energy gaps along Γ-X and Γ-Y directions in momentum space.18, 41

If the energies of the conduction band minimum (CBM) and the valence band maximum

(VBM) of the Γ-X valley and Γ-Y valley are defined as  ( = , ;  = , ), respectively, previous calculations indicate  − "# <  − " < # − "# ,42 as illustrated in Figure. 2b. Since previous first-principle calculation has already provided a comprehensive understanding of 4 ACS Paragon Plus Environment

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the electronic band structure of SnS, here we show a schematic diagram in reference to those works.18, 41 Hence, the IX (1.204 eV) peak probably arises from the band edge transition between # and "# and IY (1.160 eV) PL peak is due to the band edge transition between  and " . Such assignments are further confirmed by the polarization-resolved PL and absorption spectrum measurement, which will be discussed below. To further reveal the anisotropic properties of the band edge emissions, we performed PL measurements under different laser excitation polarization. In this measurement, the polarization direction of the laser was tuned while there is no polarization control in the detection path. Figure. 2c shows the PL spectrum where θ represents the intersect angle between the excitation laser polarization and the x-direction of SnS. We fitted the measured PL spectrum with Gaussian Formula and extracted the intensity of IX (1.204 eV) and IY (1.160 eV) emission peaks. The extracted PL intensity of IX and IY peaks were correspondingly plotted in Figure. 2d and Figure. 2e, and they showed the   - and   - dependent shapes respectively. To highlight the excitation anisotropy, the angular resolved PL intensity is plotted from 0.8 to 1. For band edge emission along Γ-X valley, IX reached its maximum with excitation polarization parallel to x direction. In contrast, IY from Γ-Y valley reached its maximum when excitation polarization is along the y- direction. Our observation suggests that for each valley in the momentum space, the emission dominates when the excitation polarization is parallel to its corresponding crystal orientation in the real space. We further calculated the degree of polarization for anisotropic emissions from Γ-X and Γ-Y valleys, which is defined as &# − '# ' − & % = # ; % = & + '# ' + & #

respectively. Here ( ( = ), *;  = , ) represents the emission from  valley under -polarized excitation. The emission from X (Y) valley indicates a degree of polarization % # =8.5% (% =3.4%). Despite of the low degree of polarization, the distinct excitation anisotropic behavior of the two PL peaks was a result of the valley-related optical selective rules. According to the symmetry analysis based on group theory, the interband transition probability for both valleys under different excitation polarization has been calculated.18 Under linearly polarized excitation, the electrons in Γ-X valley can be only excited with x-polarized photons while the Γ-Y valley can be only pumped with y-polarized excitation. However, due to the off-resonant excitation 5 ACS Paragon Plus Environment

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photon energy (2.33 eV) in our PL measurements, the valley electrons selected by corresponding excitation polarization acquire significantly higher energy in the conduction band. Though the valley-related optical selection rule is still preserved, the excited electron is easily relaxed to the other valley.43 Hence the intervalley scattering of the excited electrons is significantly enhanced, which leads to suppressed degree of polarization. The slight difference between % # and % can be attribute to the detailed characteristics of the different band structures of Γ-X and Γ-Y valley, which may lead to the different rate of intervalley scattering and relaxation of the excited electrons. To further clarify the optical transition rules in SnS, we also performed PL measurements under different detection polarization (See Methods). The optical setup for angular-resolved PL measurement is illustrated in Figure. S3. The excitation polarization was kept parallel to the ydirection of SnS while PL signal of different emission polarization was detected. Figure. 3a shows the PL spectrum under different detection polarization. Here, θ represents the angle between the detection polarization and x-direction. We observed a novel valley-selective linear dichroism phenomenon for the emission peaks, as illustrated in Figure. 3b. The emission from ΓX valley reached the highest intensity under x-polarized detection while vanished under ypolarized detection, indicating a completely x-polarized emission from Γ-X valley. In contrast, the emission from Γ-Y valley was completely y-polarized. Both emissions from Γ-X valley and Γ-Y valley emerged under the detection polarization = 45° . We extracted the angular resolved PL intensity of both peaks, which was plotted in the polar coordinate (Figure 3c). The emission from Γ-X valley and Γ-Y valley show the pattern of   and   , respectively. The results demonstrate the perfect and orthogonal linear dichroism in Γ-X and Γ-Y valleys of SnS in photoluminescence. Compared with black phosphorus where single-fold linear dichroism is observed, our results represent the first observation of the valley-selective linear dichroism in photoluminescence arising from its inequivalent valleys in energy band of SnS. Meanwhile, the PL emissions from both valleys have different peak positions and are well resolved and perfectly polarized, which may be useful for future valleytronic applications. The optical transition rule of the PL emission is subject to interband transitional probability of 

the electron-hole pair, which is determined by ./0( .12 .03 4. .16-17 Here 0( and 03 represent the wave function of the initial and final states of the transition electron, respectively, while 12 ( = 6 ACS Paragon Plus Environment

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), *) is defined as the momentum operator where n is the direction of photon polarization. The optical transition is allowed only if the moment matrix elements /0( .12 .03 4 have non-vanishing values16-17. Meanwhile, we define the initial state and the final state in Γ-X and Γ-Y valleys as 5

5

0( and 03 (6 = ), *) respectively. For the valley in Γ-X direction, group theory analysis gives a vanishing moment matrix element along the y-direction /0(& .1' .03& 4 = 0.18 Hence, the photon transition along y-direction in Γ-X valley is forbidden. In contrast, electron transition is allowed along the x-direction in Γ-X valley due to the non-vanishing value of /0(& .1& .03& 4.18 The strongly x-polarized photon emission from Γ-X valley in SnS is similar to that observed in black phosphorus.10, 44 However, different from black phosphorus, there is a nearly energy degenerate valley along Γ-Y direction in the energy band of SnS, where group theory analysis gives non'

'

'

'

vanishing value of /0( .1' .03 4 and vanishing value of /0( .1& .03 4.18 Hence, y-polarized photon transition is allowed while x-polarized photon transition is forbidden, leading to a completely ypolarized PL emission from the Γ-Y valley. Previously, the linear dichroism has also been studied in another group-IV monochalcogenides, Germanium Sulfide (GeS),19 of which the band structure also shows two valleys along Γ-X and Γ-Y direction with close energy gaps.18 However, only single-fold linear dichroism was observed in GeS. This is because the band gap at Γ point in GeS is much smaller than that at Γ-X and Γ-Y valleys,27 hence more preferred for optical transition. As a result, the linear dichroism of GeS is determined by the optical transition rule at Γ point, lack of freedom for valley manipulation. In contrast, the band gap of Γ-X and Γ-Y valley is significantly smaller than that at Γ point in SnS,41 making it an extraordinary platform for investigating valley-related linear dichroism and corresponding optical transition rule. Furthermore, we tuned excitation polarization to the x-direction and repeated the above PL measurements under different detection polarization (see Figure 3d). Regardless of the excitation polarization, the emission from both Γ-X and Γ-Y valley preserved its anisotropic properties, which remained x-polarized and y-polarized, respectively. This is because the photon emission arises from the band edge transition in individual valleys and is only subject to the optical transition rules in corresponding valleys. Hence the linear dichroism of photon emissions from both valleys is robust against the excitation photon energy and polarization.

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At last, we performed the linear polarization-resolved absorption spectrum measurement on SnS flakes (See Method).45 In this experiment, the SnS flakes were exfoliated directly onto the gold film deposited by thermal evaporation on SiO2/Si substrate to achieve a strong reflection of the broadband white light (Figure S4). The reflection spectrum of both SnS/gold film and gold film was measured at 77 K respectively, illuminated by an incident beam with tunable polarization in the x-y plane, which is defined as 8 and 89 . Then the absorption spectrum of the SnS flakes was estimated as 1 − 8/89 . This configuration has been widely used to study the absorption spectrum of the black phosphorus in previous study.10, 14 Here, we improved the configuration by using the gold film in substitution of the SiO2/Si substrate to exclude the possible absorption and transmission by SiO2/Si considering the nearly perfect reflection in measured wavelength region.46 Figure. 4a shows the anisotropic absorption spectrum under the illumination of x- and y-polarizations (solid line), together with previous PL spectrum (dots). We extracted the band gap of SnS from the onset of absorption spectrum. The band gap along Γ-X (Y) direction was 1.173 (1.147) eV. These values agreed quite well with the values 1.204 (1.160) eV along Γ-X (Y) direction obtained from the PL measurements. The consistence between the absorption and PL results further confirmed that the two PL peaks originate from the unique band edge transitions along Γ-X and Γ-Y directions. The angular resolved absorption spectra under different illumination polarization are plotted in Figure. S5. The absorption edge of SnS shows a clear redshift when the incident light polarization was rotated from x- to y-direction. We further extracted the band gap of SnS along different crystal directions from the absorption spectrum measurement, as shown in Figure. 4b. The extracted angular resolved band gap was well fitted with the function of a  + b , where a and b are fitting parameters. Here, a=1.148 eV represents the energy gap along Γ-Y direction and b=0.030 eV represents the energy difference between Γ-X and Γ-Y valley, consistent with the value directly extracted from the absorption spectra. We further measured the linearly-polarized absorption spectrum of SnS under 298K (Figure S6). The absorption edge exhibits an overall redshift compared with the low temperature results, a typical semiconductor property47 indicating that our observations are band gap transitions. Moreover, the redshift of the absorption edge along y-direction compared with that along x-direction is similar to that observed at 77 K. This observation confirmed that the band gap along Γ-X direction is larger than that along Γ-Y direction, consistent with our previous measurement. More importantly, it provided strong evidence for the optical selective rule since 8 ACS Paragon Plus Environment

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the absorption of x-polarized photon was forbidden in the valley along Γ-Y direction.18 In that case, the Γ-Y valley can be pumped only with y-polarized photon, demonstrated by the redshift of the absorption edge under y-polarized illumination.

Conclusion In conclusion, we reported the experimental observation of the novel valley-selective linear dichroism of photoluminescence in layered thin-film SnS. Two photon emission peaks, arising from Γ-X and Γ-Y valley transitions respectively, are purely linearly polarized along x- and ydirections, respectively, regardless of the excitation laser polarization. This valley-selective linear dichroism and the highly anisotropic band structure of SnS were further elaborated by angular-resolved optical absorption spectroscopy. Our observations reveal the unique optical transition rules in a low-symmetry group IV monochalcogenide SnS, further establishing it as a promising material for valley-related applications. Note added: We became aware of a related work on the valley contrast optical properties of SnS during the manuscript preparation process48.

Method Optical Measurements: The Raman scattering spectroscopy was performed with Horiba LabRAM HR Evolution Raman Microscope with excitation photon energy of 2.33eV. The polarization dependent Raman measurements were performed by rotating the sample with respect to the laser polarization. The polarization-resolved PL and optical absorption measurements were performed in a homemade system, as shown in Figure. S2. A half-wave plate was used to tune the polarization of the excitation laser, which was focused onto the sample with a 40x microscope objective. The backscattering emission signal was collected with the same objective, whose polarization was analyzed by a combination of broadband half-wave plate and a linear polarizer. The signal was detected by an Andor Sharmock SR750 Spectrometer equipped with an iDus 420 series CCD camera. The sample was mounted in Janis ST-500 Microscopy Cryostat for low temperature measurements. For PL measurements, we used a 532-nm solid state

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laser as the excitation source. For optical absorption measurements, we used the stabilized Tungsten-Halogen light source to generate a broadband white light.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website

Author Information Corresponding Author *E-mail: [email protected] Author Contribution C.C and X.C contribute equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Present Address 2

(Y. Shao) Department of Applied Physics Science, University of North Carolina at Chapel Hill,

Chapel Hill, North Carolina 27599

Acknowledgement We acknowledge the partial financial support from the Office of Naval Research Young Investigator Program (N00014-15-1-2733). We thank Professor Antonio Castro Neto and Dr. Aleksandr Rodin of National University of Singapore for reading our manuscript and providing 10 ACS Paragon Plus Environment

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helpful comments. We also thank Zishan Wu of Yale University for assistance on Raman measurements.

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(19) Ho, C.-H.; Li, J.-X. Polarized Band-Edge Emission and Dichroic Optical Behavior in Thin Multilayer GeS. Adv. Opt. Mater. 2017, 5, 1600814. (20) Aslan, O. B.; Chenet, D. A.; van der Zande, A. M.; Hone, J. C.; Heinz, T. F. Linearly Polarized Excitons in Single- and Few-Layer ReS2 Crystals. ACS Photonics 2016, 3, 96-101. (21) Arora, A.; Noky, J.; Drüppel, M.; Jariwala, B.; Deilmann, T.; Schneider, R.; Schmidt, R.; Del PozoZamudio, O.; Stiehm, T.; Bhattacharya, A.; Krüger, P.; Michaelis de Vasconcellos, S.; Rohlfing, M.; Bratschitsch, R. Highly Anisotropic in-Plane Excitons in Atomically Thin and Bulklike 1T′-ReSe2. Nano Letters 2017, 17, 3202-3207. (22) Wen, W.; Yiming, Z.; Xuelu, L.; Hung‐Pin, H.; Zhen, F.; Yanfeng, C.; Xinsheng, W.; Mei, Z.; Kuan‐Hung, L.; Fei‐Sheng, H.; Yi‐Ping, W.; Ying‐Sheng, H.; Ching‐Hwa, H.; Ping‐Heng, T.; Chuanhong, J.; Liming, X. Anisotropic Spectroscopy and Electrical Properties of 2D ReS2(1–x)Se2x Alloys with Distorted 1T Structure. Small 2017, 13, 1603788. (23) Pant, A.; Torun, E.; Chen, B.; Bhat, S.; Fan, X.; Wu, K.; Wright, D. P.; Peeters, F. M.; Soignard, E.; Sahin, H. Strong dichroic emission in the pseudo one dimensional material ZrS 3. Nanoscale 2016, 8, 16259-16265. (24) Island, J. O.; Biele, R.; Barawi, M.; Clamagirand, J. M.; Ares, J. R.; Sánchez, C.; van der Zant, H. S. J.; Ferrer, I. J.; D’Agosta, R.; Castellanos-Gomez, A. Titanium trisulfide (TiS3): a 2D semiconductor with quasi-1D optical and electronic properties. Scientific Reports 2016, 6, 22214. (25) Papadopoulos, N.; Frisenda, R.; Biele, R.; Flores, E.; Ares, J.-R.; Carlos, S.; van der Zant, H. S.; Ferrer, I. J.; D'Agosta, R.; Castellanos-Gomez, A. Large birefringence and linear dichroism in TiS 3 nanosheets. Nanoscale 2018. (26) Qiu, G.; Du, Y.; Charnas, A.; Zhou, H.; Jin, S.; Luo, Z.; Zemlyanov, D. Y.; Xu, X.; Cheng, G. J.; Ye, P. D. Observation of Optical and Electrical In-Plane Anisotropy in High-Mobility Few-Layer ZrTe5. Nano Letters 2016, 16, 7364-7369. (27) Tan, D.; Lim, H. E.; Wang, F.; Mohamed, N. B.; Mouri, S.; Zhang, W.; Miyauchi, Y.; Ohfuchi, M.; Matsuda, K. Anisotropic optical and electronic properties of two-dimensional layered germanium sulfide. Nano Res. 2017, 10, 546-555. (28) Xu, X.; Song, Q.; Wang, H.; Li, P.; Zhang, K.; Wang, Y.; Yuan, K.; Yang, Z.; Ye, Y.; Dai, L. In-Plane Anisotropies of Polarized Raman Response and Electrical Conductivity in Layered Tin Selenide. ACS Appl. Mater. Interfaces 2017, 9, 12601-12607. (29) Li, L.; Gong, P.; Wang, W.; Deng, B.; Pi, L.; Yu, J.; Zhou, X.; Shi, X.; Li, H.; Zhai, T. Strong In-Plane Anisotropies of Optical and Electrical Response in Layered Dimetal Chalcogenide. ACS Nano 2017, 11, 10264-10272. (30) Tian, Z.; Guo, C.; Zhao, M.; Li, R.; Xue, J. Two-Dimensional SnS: A Phosphorene Analogue with Strong In-Plane Electronic Anisotropy. ACS Nano 2017, 11, 2219-2226. (31) Zhang, Z.; Yang, J.; Zhang, K.; Chen, S.; Mei, F.; Shen, G. Anisotropic photoresponse of layered 2D SnS-based near infrared photodetectors. J. Mater. Chem. C 2017, 5, 11288-11293. (32) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. (33) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451-10453. (34) Chandrasekhar, H. R.; Humphreys, R. G.; Zwick, U.; Cardona, M. Infrared and Raman spectra of the IV-VI compounds SnS and SnSe. Phys. Rev. B 1977, 15, 2177-2183. (35) Wu, J.; Mao, N.; Xie, L.; Xu, H.; Zhang, J. Identifying the Crystalline Orientation of Black Phosphorus Using Angle-Resolved Polarized Raman Spectroscopy. Angew. Chem. 2015, 127, 2396-2399. (36) Wolverson, D.; Crampin, S.; Kazemi, A. S.; Ilie, A.; Bending, S. J. Raman Spectra of Monolayer, FewLayer, and Bulk ReSe2: An Anisotropic Layered Semiconductor. ACS Nano 2014, 8, 11154-11164. 12 ACS Paragon Plus Environment

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(37) Li, M.; Wu, Y.; Li, T.; Chen, Y.; Ding, H.; Lin, Y.; Pan, N.; Wang, X. Revealing anisotropy and thickness dependence of Raman spectra for SnS flakes. RSC Adv. 2017, 7, 48759-48765. (38) Xia, J.; Li, X.-Z.; Huang, X.; Mao, N.; Zhu, D.-D.; Wang, L.; Xu, H.; Meng, X.-M. Physical vapor deposition synthesis of two-dimensional orthorhombic SnS flakes with strong angle/temperaturedependent Raman responses. Nanoscale 2016, 8, 2063-2070. (39) Plechinger, G.; Schrettenbrunner, F. X.; Eroms, J.; Weiss, D.; Schüller, C.; Korn, T. Low‐temperature photoluminescence of oxide‐covered single‐layer MoS2. Phys. Status Solidi Rapid Res. Lett. 2012, 6, 126128. (40) Alex, V.; Finkbeiner, S.; Weber, J. Temperature dependence of the indirect energy gap in crystalline silicon. J. Appl. Phys. 1996, 79, 6943-6946. (41) ACS Applied Materials & InterfacesMalone, B. D.; Kaxiras, E. Quasiparticle band structures and interface physics of SnS and GeS. Phys. Rev. B 2013, 87, 245312. (42) Gomes, L. C.; Carvalho, A. Phosphorene analogues: Isoelectronic two-dimensional group-IV monochalcogenides with orthorhombic structure. Phys. Rev. B 2015, 92, 085406. (43) Kioseoglou, G.; Hanbicki, A. T.; Currie, M.; Friedman, A. L.; Gunlycke, D.; Jonker, B. T. Valley polarization and intervalley scattering in monolayer MoS2. Appl. Phys. Lett. 2012, 101, 221907. (44) Wang, X.; Jones, A. M.; Seyler, K. L.; Tran, V.; Jia, Y.; Zhao, H.; Wang, H.; Yang, L.; Xu, X.; Xia, F. Highly anisotropic and robust excitons in monolayer black phosphorus. Nat. Nanotechnol. 2015, 10, 517. (45) Lambros, A. P.; Geraleas, D.; Economou, N. A. Optical absorption edge in SnS. J. Phys. Chem. Solids 1974, 35, 537-541. (46) Loebich, O. The optical properties of gold. Gold Bulletin 1972, 5, 2-10. (47) Varshni, Y. P. Temperature dependence of the energy gap in semiconductors. Physica 1967, 34, 149-154. (48) Lin, S.; Carvalho, A.; Yan, S.; Li, R.; Kim, S.; Rodin, A.; Carvalho, L.; Chan, E. M.; Wang, X.; Castro Neto, A. H.; Yao, J. Accessing valley degree of freedom in bulk Tin(II) sulfide at room temperature. Nat. Commun. 2018, 9, 1455.

Figure Caption Figure 1. a) Schematic illustration of the crystal structure of layered SnS. (left panel: front view of the x-z plane; right panel: top view of the x-y plane) b) Optical micrograph of a typical SnS flake on 285nm SiO2/Si. Insert: The atomic force microscopy scan performed on the SnS flake. Its thickness is determined to be 109nm. c) Raman scattering spectra of the SnS sample. Three prominent Raman peaks are observed at 161 cm-1 (B3g mode), 191 cm-1 (Ag1 mode), and 217 cm1

(Ag2 mode). The Lorentz fitting curves of these modes are shown by blue, green and yellow

lines, respectively. Inset: Angular resolved Raman intensity of Ag1 mode. The 0 degree is determined as the x-direction of the SnS crystal. The excitation photon energy for Raman measurements is 2.33 eV. Figure 2. a) Photoluminescence (PL) spectra from the SnS sample (grey) and SiO2/Si substrate (blue). The measurements here and below were performed with excitation photon energy of 2.33 13 ACS Paragon Plus Environment

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eV under 77 Kelvin. b) Band structure schematics of layered SnS. There are two valleys with close energy gaps in Γ-X (yellow) and Γ-Y (blue) direction. c) PL spectra under different excitation light polarization directions θ, with respect to the x-direction. d) Angular resolved PL intensity of the 1.204 eV (yellow) and 1.160 eV (blue) emission peaks. The fitting curves for PL emissions from Γ-X and Γ-Y valley are indicated by yellow and blue lines, respectively. Figure 3. a) The emission polarization-resolved PL spectra of the SnS sample. The excitation polarization was kept along the Y direction. The 1.204 eV and 1.160 eV peaks are polarized along X- and Y- directions, respectively. b) Schematic illustration of the optical selective rule of the band edge emission. c) The polarization angle-resolved PL intensity under Y-polarized laser excitation. d) The polarization angle-resolved PL intensity under X-polarized laser excitation. The emission properties of both peaks are independent of the excitation laser polarization. Figure 4. a) Anisotropic absorption spectra of SnS and its comparison with PL spectra. b) Angular resolved energy gap substracted from polarization dependent absorption spectra.

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For Table of Contents Use Only Valley-selective Linear Dichroism in Layered Tin Sulfide Chen Chen1, Xiaolong Chen1, Yuchuan Shao1,2, Bingchen Deng1, Qiushi Guo1, Chao Ma1 and Fengnian Xia1* 1

Department of Electrical Engineering, Yale University, New Haven, Connecticut 06511, USA

*To whom correspondence should be addressed: [email protected]

Valley-selective linear dichroism of photoluminescence in layered Tin Sulfide are observed. The PL emissions, arising from optical transitions in Γ-X and Γ-Y valley of band structure, are completely x-polarized and y-polarized, respectively. The valley-selective linear dichroism is further demonstrated by linearly-polarized absorption spectrum. Our study offers new opportunity of manipulating valley degree of freedom in materials with reduced lattice symmetry.

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