In-Plane Optical Anisotropy and Linear Dichroism in Low-Symmetry

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In-Plane Optical Anisotropy and Linear Dichroism in Low-Symmetry Layered TlSe Shengxue Yang, Chunguang Hu, Minghui Wu, Wanfu Shen, Sefaattin Tongay, Kedi Wu, Bin Wei, Zhaoyang Sun, Chengbao Jiang, Li Huang, and Zhongchang Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05162 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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In-Plane Optical Anisotropy and Linear Dichroism in Low-Symmetry Layered TlSe Shengxue Yang,†,# Chunguang Hu,‡, # Minghui Wu,§, # Wanfu Shen,‡, # Sefaattin Tongay,∥ Kedi Wu, ∥ Bin Wei, ⊥ Zhaoyang Sun,‡ Chengbao Jiang,*,† Li Huang,*,§ Zhongchang Wang*,⊥ †

School of Materials Science and Engineering, Beihang University, Beijing 100191, P. R. China

‡

State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Weijin Road, CN-300072 Tianjin, P. R. China

§

Department of Physics, Southern University of Science and Technology, Shenzhen 518005, P. R. China

∥

School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287, United States ⊥

Department of Quantum and Energy Materials, International Iberian Nanotechnology Laboratory (INL), Av. Mestre José Veiga s/n, Braga 4715-330, Portugal

ABSTRACT: In-plane anisotropy of layered materials adds another dimension to their applications, opening up avenues in diverse angle-resolved devices. However, to fulfill a strong inherent in-plane anisotropy in layered materials still poses a significant challenge as it often requires a low-symmetry nature of layered materials. Here, we report the fabrication of a member of layered semiconducting AIIIBVI compounds TlSe which possesses a low-symmetry tetragonal structure, and investigate its anisotropic light-matter interactions. We first identify the

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in-plane Raman-intensity anisotropy of thin-layer TlSe, offering unambiguous evidence that the anisotropy is sensitive to crystalline orientation. Further in-situ azimuth-dependent reflectance difference microscopy enables to directly evaluate in-plane optical anisotropy of layered TlSe and we demonstrate that the TlSe shows a linear dichroism under polarized absorption spectra arising from in-plane anisotropic optical property. As a direct result of the linear dichroism, we successfully fabricate TlSe devices for polarization-sensitive photodetection. The discovery of layered TlSe with a strong in-plane anisotropy not only facilitates its applications in linear dichroic photodetection, but opens up more possibilities for other functional device applications.

KEYWORDS: anisotropy, linear dichroism, low-symmetry layered material, TlSe, polarizationsensitive photodetector

Low-symmetry layered materials show unique in-plane anisotropic optical, electrical, and thermal properties, which expand their potentials for designing diverse polarization-resolved devices, including polarization-sensitive photodetectors, integrated polarization-controllers, and linearly-polarized ultrafast lasers.1-3 To date, the anisotropic carrier mobility and thermal conductivity of low-symmetry layered materials have been observed.4 However, the investigation of anisotropic layered materials remains scarce and the research on their in-plane anisotropy-dependent properties are still at the initial stage. Developing low-symmetry layered materials and investigating their in-plane anisotropy for anisotropy-related applications are timely and of great scientific significance. Linear dichroism (LD), which is defined as the differential absorbance of linearly polarized light parallel or vertical to an orientation axis of a sample,5 is being actively pursued for its


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diverse applications, e.g. polarization-sensitive broadband photodetector, infrared polarizer, near-field imaging, filter, and the detection of molecular orientation and structure.5-8 As a form of electromagnetic spectroscopy, the measurement of LD is based upon light-matter interaction, which typically requires samples to have a periodic structural anisotropy.9 Layered materials may serve as a potential anisotropic member for probing the LD in that they often show a lowsymmetry crystal structure and hence are sensitive to linearly polarized light. Among the layered materials, AIIIBVI compounds possess combined merits of group III (A = Ga, In, Tl) and VI elements (B = S, Se), e.g. good stability, high mobility, efficient photoresponse, and large nonlinear effect.10-13 As a narrow-gap semiconductor, binary AIIIBVI compound TlSe has a low-symmetry tetragonal structure,14 and therefore it is expected to exhibit unique in-plane anisotropy. However, previous study on TlSe has mainly been concentrated on the visible/infrared detection and the band structure of its bulk.14-17 For example, the responsivity of an infrared detector based on a p-type TlSe single-crystal chip can reach 6 × 105 V/W.15 The band gap of bulk TlSe for indirect transition has been estimated to be 0.73 eV according to the absorption and transmission measurements,16 and its electronic structure could be modulated by uniaxial pressure and temperature.17 Nevertheless, to the best of our knowledge, the layered nature and related properties of TlSe have not yet been investigated thoroughly, particularly, it remains unclear the in-plane anisotropy and its resulting linear dichroism of this low-symmetry layered material. Here, we report a member of group III-VI semiconductors, the low-symmetry layered TlSe, which possesses in-plane anisotropy and linear dichroism. Density functional theory (DFT) calculations reveal that its band structures and effective mass are highly anisotropic, and direct observation by in-plane Raman-intensity anisotropy provides evidence to strong sensitivity of


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TlSe to crystalline orientation. Upon using an imaging tool, the azimuth-dependent reflectance difference microscopy (ADRDM) technique, we identify the crystalline orientation and quantitatively evaluate in-plane optical anisotropy of layered TlSe, and find importantly an explicit linear dichroism of TlSe, which directly reflects its anisotropic nature. We also find that polarization-sensitive photodetectors based on layered TlSe show anisotropic photoresponse with a dichroic ratio of 2.65. These findings demonstrate that the layered TlSe can serve as a category of anisotropic semiconductor, which holds substantial promise in optical, optoelectronic, and electronic applications. Results Bulk TlSe single crystals were prepared by the Bridgman method using a three-zone tube furnace, where high-purity Tl and Se sealed in quartz ampoules were used as starting materials (see Methods).14 X-ray diffraction (XRD) of the synthesized TlSe single crystals reveals a few sharp peaks in the XRD pattern (Figure S1a in Supporting Information), consolidating the high quality of the obtained sample. All diffraction peaks can be readily indexed to the tetragonal unit cell (JCPDS No. 22-1476, a = b = 8.0166 Å, c = 6.969 Å, α = β = γ = 90°, I4/mcm 140) without any impurity phases. Two strong peaks at 31.2° ((220) peak) and 65.5° ((440) peak) can be observed, suggesting that the TlSe crystals prefer to grow along the [110] axis. Bonding states of a TlSe crystal are identified from X-ray photoelectron spectroscopy (XPS), as shown in Figure S1b-d, in which the predominant signal of Tl and Se is clearly visible. Figure S1b shows the high-resolution spectra of Tl 4f and Se 3d peaks. Two strong peaks at around 118.1 and 122.6 eV are assigned to Tl 4f7/2 and Tl 4f5/2, respectively (Figure S1c). In addition, two other peaks located at 53.4 and 54.2 eV of Se 3d spectra represent the binding energy of Se 3d5/2 and Se 3d3/2, respectively (Figure S1d).


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Bulk TlSe crystallizes in a body-centered-tetragonal (bct) structure with a space group of 18

I4/mcm ( D4h ) and holds 8 formula units in the unit cell (Figure S2). Monolayer TlSe possesses the 2-fold (C2) rotational symmetry, the lattice of which is rectangular with two different lattice parameters.18 Monovalent Tl+ and trivalent Tl3+ ions are considered to coexist within the TlSe compound, leading to its mixed valency. The formula of binary chain-like TlSe is therefore more precisely denoted as Tl+[Tl3+(Se2-)2].19 As shown in Figure 1a,b, the tetragonal structure of TlSe is built up from TlSe2 chains, in which each Tl3+ ion is coordinated by four Se2- ions and each Se2- ion is connected to two Tl3+ ions, while Tl+ ions are localized between negatively charged chains and these chains are hold together by means of weak ionic interchain forces.20 This leads to easy cleavage of TlSe crystals along the tetragonal axis. We simulate the atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) image using bulk atomic models (Figure 1c), in which the c-axis and a-axis of TlSe crystal are marked, supporting the crystal orientation of TlSe. The zigzag and armchair directions correspond to the c-axis and a-axis, respectively, which might give rise to in-plane anisotropy in layered TlSe. To further identify microstructures of the TlSe sample, we perform transmission electron microscopy (TEM) and scanning TEM (STEM) analyses of a thin area of the mechanically exfoliated specimen (Figure S3a). Analysis of a selected-area diffraction pattern (SADP) shows that the flakes grow along (110) plane (Figure S3b), and further high-resolution TEM (HRTEM) image (Figure S3c) and its fast Fourier transform (FFT) image (Figure S3d) confirm this in-plane orientation. The growth orientation of TlSe crystal is hence along (110) plane rather than (001) plane, and the TlSe (110) monolayer possesses only 2-fold (C2) rotational symmetry in contrast to the 4-fold (C4) rotational symmetry for the (001). To address the relative stability of (110) and


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(001) planes, we also conduct density functional theory (DFT) calculations based on the projector-augmented wave (PAW) method. We find that the calculated energy of TlSe (110) monolayer is about 0.11 eV per formula lower than that of (001) surface, in consistence with our HRTEM and STEM results. No surface reconstruction has been observed on the (110) surface during our DFT relaxation process, despite that small surface relaxation has been found. To extract atomic information, we show in Figure 1d,e HAADF and bright-field (BF) STEM images of the TlSe flake viewed from [110] axis. Since image density of an atomic column in a HAADF STEM mode is roughly proportional to Z1.7 (Z: atomic number),21 the contrast in Figure 1d and Figure S4a is brighter for heavier Tl atom (Z=81) and darker for the lighter Se atom (Z=34), revealing the plainly visible TlSe chains and the sharp tetragonal lattice fringe, indicative of the single-crystalline nature. Particularly, one can clearly identify the crystal orientation of TlSe flake, i.e. zigzag ([001] axis) and armchair ([1 1 0] axis) directions with an intersection angle of 90°, as also confirmed in the BF STEM image (Figure 1e and Figure S4b). To identify chemically individual atomic column in bulk, we perform atomic-resolution electron energy loss spectroscopy (EELS) analysis of the TlSe in a selectively analyzed region along [110] direction. Figure 1f shows the magnified HAADF STEM image of this analyzed region overlaid with the atomic model and Figure 1g‒i shows the corresponding EELS mapping of Tl, Se and their combination. As shown in Figure 1g‒i, the spectrum images of both the Tl and Se correspond correctly to the HAADF image, providing further support to the obtained crystal structure of TlSe at the atomic scale.22 Figure S5a shows the band structure of bulk TlSe calculated by the hybrid functional theory (HSE06). The band gap of bulk TlSe is estimated as 0.55 eV, which fits well with the experimental result (0.73 eV).16 The bulk TlSe is an indirect band gap semiconductor with


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valence band maximum (VBM) located at a special G point and conduction band minimum (CBM) at a general point along O-C line. Importantly, both the band structure and effective mass for bulk TlSe are highly anisotropic. Figure S5b shows the calculated partial density of states (PDOS) of bulk TlSe, where one can note that the VBM is dominated by hybridization of 4p orbitals of Se and 6s orbitals of Tl, and that anisotropy of band structure originates from p orbitals. Figure S6a illustrates the first Brillouin zone where high-symmetry points are marked as Γ, M, N, H, and F. The effective mass at point P and S' shows a strong anisotropic nature: it is 0.64 me along [001] while 0.35 me along [1 1 0] at the point P and S'. As shown in Figure S6b, the Fermi surface around VBM (~0.2 eV) takes an ellipsoid shape implying the anisotropic feature of effective mass as well. For the CBM, the effective mass is 0.18 me along [001] direction, while it is 0.24 me along [1 1 0] direction. We also plot Fermi surface around CBM (~0.2 eV), as shown in Figure S6c, where it differs largely from sphere shape, indicating that the band around CBM exhibits strong non-parabolic characteristics. The TlSe flakes were exfoliated onto a Si wafer with a 300 nm SiO2 layer. A typical optical image of few-layer TlSe flakes is shown in Figure S7a, and Figure S7b shows an atomic force microscopy (AFM) image of a typical TlSe flake, where its height is about 18 nm (Figure S7c). Figure S8a presents the unpolarized Raman spectrum of the TlSe flake with a thickness of 18 nm, where six active Raman modes are observed at 28, 37, 92, 140, 159, and 203 cm-1. The Raman intensity of bulk TlSe obtained from DFT calculations is shown in Figure S8b, where symmetry of the observed active Raman modes is also labeled. According to the group theory and the phonon dispersion results (Figure S9 and S10), TlSe has twenty-four phonon modes and the corresponding frequency values and atom moving directions of these modes at Gamma point are listed in Figure S11. Among these phonon modes, twelve modes are Raman active, indicating


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that most of them can be experimentally detected. Based upon the theoretical calculations, in the experimental Raman spectrum, there are three Eg modes (28, 37 and 140 cm-1), two B2g modes (92 and 203 cm-1), and one A1g mode (159 cm-1). The rest of active Raman modes that are not observed in the experiments may be possible to have weak electron-phonon interactions or low frequency. To address the thickness-dependent behaviors, we also compare the Raman spectra of the TlSe samples with different thicknesses, as shown in Supporting Information Figure S12. The Raman spectra are almost the same for the samples with thicknesses of 2.6 and 18 nm, indicating that the Raman behavior does not change significantly with layer thickness, at least for the samples of more than 5-layer thickness. Raman modes can provide rich information about crystal structure and phonon vibration.23 Owing to the low-symmetry tetragonal structure, layered TlSe is supposed to exhibit in-plane Raman-intensity anisotropy. To probe in-plane anisotropic optical properties, we first carried out angle-resolved polarized Raman spectroscopy (APARS) on the 18-nm-thick TlSe flake under both the parallel and cross backscattering configurations at room temperature. The incident laser is vertically polarized using a linear polarizer, while vertically or horizontally polarized Raman signals (denoted as parallel or cross configuration) can be selectively detected.24 Figure 2a,b presents the typical polarized Raman spectra of TlSe flake under the parallel and cross configurations, where one can notice that all the six active modes are detected in these two polarization configurations, and their peak positions are in line with those in the unpolarized Raman spectrum (Figure S8b). The intensity of these active Raman modes alters under parallel and cross configurations, showing a clear dependence on the polarization angle. The polar plots of angle-resolved polarized Raman intensity for six active Raman modes under two configurations are summarized in Figure 2c, including three Eg modes, two B2g modes


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and one A1g mode. The active Raman modes exhibit varying degree of anisotropy. In the parallel configuration, Eg modes at 28, 37 and 140 cm-1 show 4-leaf patterns with a variation period of 90°, and the maximum Raman intensities are toward 45° and 135° (225° and 315°). B2g modes (92 and 203 cm-1) exhibit a different two-fold anisotropy with maximum intensity at 0° (180°). Similarly, A1g mode also has a 2-leaf pattern with a period of 180°, but intensity maximizes along y-direction. Furthermore, in the cross configuration, Eg modes still present a four-fold anisotropy, but the maximum intensities change to 0° and 90° (180° and 270°). Interestingly, both the B2g and A1g modes show 4-leaf patterns and the intensity of B2g mode approaches a maximum at angles of 45° and 135°. The A1g mode also has the strongest intensity along 45° and 135°. These results imply that the intensity of Raman modes either maximizes or minimizes when the polarization direction of the laser is the same as the orientation of the crystal. Such interesting phenomena in TlSe indicate that APARS could be used to identify the crystalline orientation and that the Raman intensity anisotropy strongly depends on the phonon symmetry. To gain an in-depth insight into Raman intensity anisotropy, we apply the Raman tensor to fit the polarized Raman intensity. From Eqs. (8) to (13) in the Supporting Information, one can notice that the polar plots depend not only on the configuration but also on the phase difference of Raman tensor. We simulated the polarized Raman intensity based on the Raman tensor, and the possible patterns of polar plots are shown in Figure S13. The calculated curves match well with the experimental ones (Figure 2c). The polar plots of Eg modes are 4-leaf patterns, and the pattern for the cross configuration seems to be rotated by 45° in comparison to that in the parallel configuration. In the parallel configuration, the polar plots of A1g and B2g modes are 2-leaf patterns, while they are changed to 4-leaf patterns under cross configuration. According to Eq.


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(8) in the Supporting Information, the

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|a| |a| value determines the direction of the main axis: if |b| |b|

>1, the main axis is along the y-direction. Figure S14 shows the optical transition selection rules along the two directions of TlSe crystalline orientation. We further investigate the in-plane optical anisotropy of layered TlSe using a simple azimuth-dependent reflectance difference microscopy (ADRDM) technique. Figure S15 shows schematic diagram of the ADRDM measurement. ADRDM measures the normalized reflectance difference (∆R) between two arbitrary orthogonal directions in the surface plane (x, y) of sample at perpendicular incidence:25,26 Rx − R y ∆R =2 = 2N , R Rx + R y

(1)

where x and y are the coordinate system of measurement setup.24 When varying the relative directions (θ) of linearly incident polarized light, the dimensionless value N(θ) presents a periodic change, which can be fitted by:

N (θ ) =

Rzz − Rac cos 2(θ − θ 0 ) , Rzz + Rac

(2)

where Rzz and Rac are the reflectance along zigzag and armchair direction of the TlSe orientation, respectively, ߠ denotes the azimuthal angles of the incident light, and ߠ଴ denotes the zigzag orientation of sample. Hence, the optical eigenaxes of the anisotropic sample are unambiguously determined by simply plotting the N(θ) as a function of the incident azimuthal angle θ, from which the maximum and minimum RD signals are assigned along with the high and low reflectance axes of sample, respectively. ADRDM technique can allow to effectively identify the


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crystalline orientations and quantitatively evaluate in-plane optical anisotropy of low-symmetry layered materials. Figure 3a,b shows typical ADRDM results of an ultrathin TlSe flake. A TlSe flake was first mechanically exfoliated onto an isotropic SiO2/Si (100) wafer under an optical microscope (Figure 3a), and N(θ) was recorded when the incident polarization rotated from 0° to 360° with a step size of 15°. From the polar plots of amplitude shifted ADRDM results shown in Figure 3b, one can see that the value of N(θ) maximizes at 90° (270°) while minimizes at 0° (180°). Two specific axes of TlSe orientation located at 0° and 90° are identified accordingly. As an imaging tool, ADRDM offers an in-situ visualization way to directly evaluate the in-plane optical anisotropy of low-symmetry layered materials. Figure 3c presents ADRDM images taken at various angles (0° ~ 165°), where variation in intensity of N(θ) is reflected by different color. The N(θ) intensity of the TlSe flake reaches a maximum (dark red) and minimum (dark purple) at the two specific axes, while that of isotropic background always keeps zero (yellow). We also applied ADRDM to investigate the broadband in-plane optical anisotropy of TlSe ranging from visible to near infrared regime. To eliminate optical interference effect of the transparent SiO2 layer, we exfoliated the TlSe thin flakes on a sapphire substrate (Al2O3 (001) substrate). The backside of Al2O3 substrate is artificially roughed. The inset in Figure 3d shows the optical image of a typical TlSe flake on the Al2O3 (001) substrate. Figure 3d shows the RD spectrum measured at θ0 = 0°, which represents a direct observation of in-plane optical anisotropy of TlSe in a wide range. Interestingly, a derivative peak around the E1 (~2.02 eV) and E1+∆1 (~2.25eV) transitions and a broad peak at E2 (~1.57 eV) transition are directly observed, indicating that in-plane optical anisotropy is strong under the excitation around these two


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wavelengths. The so-called three-phase model (air/TlSe/Al2O3) provides a simple way to approximate and understand the features of RD spectrum by27-29

∆R ∆r 4π d ≈ 2 Re( ) ≈ A(ω )∆ε s '' R r λ

(3)

''( β ) ''(α ) ''( β ) where ∆ε1'' = ε T''(lαSe) − ε TlSe , ε TlSe and ε TlSe are the principal values of imaginary part of TlSe film,

d is the thickness of TlSe film, and λ is the excitation wavelength. Here, we neglect the absorption of sapphire substrate and A(ω ) = 1/ (1 − ε b (ω )) , where ε b (ω ) denotes the dielectric function of Al2O3. Apparently, ∆R / R is proportional to the anisotropy of imaginary part of dielectric function, relating to the optical extinction. Thus, we attribute the RD peaks to the energy transitions between conduction and valence band structures, which will be discussed later. The spectroscopic characterization not only confirms the strong in-plane optical anisotropy of TlSe in a broadband range but also provides valuable information about the anisotropic nature of the optical extinction of TlSe flake. To verify the linear dichroism of layered TlSe, we performed polarization-dependent absorption spectra (PDAS) on a typical TlSe thin flake mechanically exfoliated onto a quartz substrate (see inset of Figure 4a for optical image of the TlSe flake). The PDAS measurements were conducted in photon energy ranging from 1.25 to 2.5 eV. Anisotropic absorption is observed by changing the polarization angle from 0° to 90° (i.e. c-axis to a-axis), where the absorption is severer along the zigzag direction (c-axis) than the armchair direction (a-axis) in a wide energy range (Figure 4a). Figure 4b shows the calculated absorption spectra of bulk TlSe based on the single-particle approximation (DFT (VASP)) and Bethe-Salpeter Equation, including the many-body electron-hole interaction (DFT+BSE). In comparison to the DFT, the


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BSE method can usually provide more spectrum features at low-energy range, indicating that the electron-hole interaction plays an important role in the optical absorption process of TlSe. According to the calculated absorption (Figure 4b), the imaginary part of the dielectric function, which represents the amount of light absorption in the zigzag direction, is larger than that in the armchair direction under the photon energy range from 1.2 to 2.5 eV, giving rise to anisotropic absorption in layered TlSe. In view of the fact that only electron-photon interaction is involved in the absorption process, PDAS can be used as a reliable alternative to identify the crystalline orientation. To shed light on the anisotropic optical absorption and linear dichroism along two directions of TlSe orientation, we performed calculations of electron-photon interactions, as shown in Table S1, S2. Figure 4c shows the detailed optical selection rules of some critical K points near the CBM and VBM, in which the critical point types are labeled, such as M0, M1, M2 and M3. Some M1 or M2 type critical points along G-Γ and S′-Γ line have the jumping energy of about 2.0 eV, which corresponds to the absorption summit of ~633 nm. Around the CBM, the jumping energy is about 1.44 eV, representing the summit of ~860 nm. By taking a typical transition process at 2.0 eV as an example, one can note that electron transition occurs between Δ1 and Δ2, and the maximum absorption occurs when light is polarized along the zigzag (c-axis) direction, which is consistent with the derivative feature of RD spectrum in Figure 3d. In terms of the calculated optical selection rules, the absorption along zigzag direction is preferred in comparison to that along armchair direction in energy range from 1.2 to 2.5 eV, which is consistent with the experimental results. Linear dichroism (LD) of TlSe induced by its anisotropic nature enables to design polarization-sensitive photodetectors. To test this scenario, we fabricate two-terminal devices for


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polarization-sensitive photodetection of multilayer TlSe by using red light of 633 nm as illumination source (Figure 5a). The polarization of incident light can be controlled via a GlanTaylor polarizer and a half-wave plate (HWP), and the change in photocurrent as a function of polarization angle is obtained by rotating the HWP by every 5° while keeping the light power constant.30 In this way, the real step size of the incident polarization change is 10°. Prior to the polarization photoresponse measurements, we examined photocurrent (Iph) as a function of drain voltage (Vd) for a typical TlSe device of ~20 nm (Figure S16) under 633 nm wavelength light with different light intensity. From Figure 5b, one can see that the photocurrent turns strong as the light intensity increases. By fitting the data in Figure 5b, we conclude that the photocurrent is linearly proportional to light intensity, satisfying the power law Iph ~ Pα, where P is light intensity and α is empirical value. As shown in Figure S17, Iph exhibits an intensity dependence of ~0.97, indicating a highly efficient generation of carriers transferred from photons.31 Photoresponsivity R λ of the device can be estimated by the equation

Rλ =

I ph PS

,

(4)

where the light intensity P is 100 mW cm-2 and the effective illumination area S is about 9 µm2. The calculated R λ is ~1.48 A/W, which is higher than those of the graphene photodetectors and the single-layer MoS2 phototransistors.32,33 Figure 5c shows 2D color map of anisotropic photocurrent, where one can see that the photocurrent with incident light polarized at 0° and 180° is much larger than that of incident polarization at 90° and 270°. This means that the linear dichroic photocurrent is efficiently generated in the TlSe device. The evolution of photocurrent as a function of polarization angle φ


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at Vd = 1 V is plotted in the polar coordination, showing a 2-leaf pattern (Figure 5d). Based upon the polarization-sensitive photoresponse measurement, we calculate the dichroic ratio Rdph using the following expression

Rdph =

I ph , I ⊥ph

(5)

where I ph is photocurrent (Iph) parallel to the orientation of polarizer and I ⊥ph is the Iph perpendicular to the polarizer. The calculated Rdph at 633 nm illumination is ~2.65. From the absorption spectrum shown in Figure 4a, the absorption anisotropy appears in a wide range of photon energy, implying that TlSe photodetector can fulfill polarization photodetection ranging from visible to near infrared region. These findings offer strong support to the anisotropic absorption and resulting polarization sensitive photodetection via linear dichroism based on TlSe, further consolidating the anisotropic nature of TlSe crystal. Conclusions Seeking the layered materials in AIIIBVI compounds with in-plane anisotropy and linear dichroism might open up avenues in fulfilling many diverse applications. We demonstrate that low-symmetry layered TlSe shows an anisotropic nature in crystal structure, highly anisotropic band structures and effective mass, and a strong sensitivity to crystalline orientation. By using the ADRDM imaging technique, which allows to probe the anisotropic N(θ) intensity as a function of polarized angles, we offer unambiguous evidence to in-plane optical anisotropy of layered TlSe. We further show that the layered TlSe possesses explicit linear dichroism under the PDAS and find an anisotropic photoresponse up to 2.65 through designing polarization-sensitive


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photodetectors based on TlSe. This work not only discovers a layered semiconductor with strong in-plane anisotropy, but provides insightful guidelines in designing angle-resolved devices in terms of low-symmetry layered materials for potential optical, optoelectronic and electronic applications. Methods Synthesis and micromechnial exfoliation of TlSe single crystal. TlSe single crystals were prepared by the Bridgman technique using a three-zone tube furnace. High-purity Tl (99.999%) and Se (99.999%) were used as starting materials and sealed in quartz ampoules. The ampoule was first held in furnace at 500 oC for 24 h, and then moved to a low-temperature zone of the furnace. Afterwards, high-quality TlSe single crystal was obtained. X-ray diffraction patterns (XRD) were collected with a D/MAX-2500 X-ray diffractometer using Cu-Kα radiation source, and 2θ was chosen in the range of 10° ~ 80°. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Fisher Escalab 250Xi system using Al-Kα X-ray radiation source (hv = 1486.6 eV). The source of 150 W, a pass energy of 20 eV, and dwell time of 300 ms were employed to measure high-resolution spectra. All the high-resolution spectra were calibrated by C 1s peak at 284.8 eV. The TlSe flakes were micromechanically exfoliated from a bulk TlSe single crystal onto 300 nm or 285 nm SiO2/Si (100) subtrates and Al2O3 (001) substrates for different measurements. The flakes were roughly observed by optical microscope (Olympus BX53). Thickness of TlSe flakes were then confirmed by atomic force microscopy (Bruker Dimension Icon Scanning Probe Microscope). Raman characterization. Raman and angle-resolved polarized Raman experiments were performed using a Horiba Jobin-Yvon HR800 system with a 100× objective and 1800 lines/mm


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grating. The laser wavelength was 633 nm, and the laser spot size was approximately 1 µm. The laser power was 100 µW for 633 nm laser, and the spectral resolution was ~1 cm-1. For the angle-resolved polarized Raman spectra (APARS) measurements, the incident polarization was fixed to be vertical by using a polarizer in the beam path, and the polarization directions of scattering light were set to be parallel or perpendicular to the incident polarization. The samples were placed on a rotation stage and clockwise rotated by 360° with a step size of 10°. The polar plots of polarized Raman intensity were obtained by fitting the spectral peaks with the Lorentzian/Gaussian function. Absorption characterization. Absorption and angle-resolved absorption spectra (APAS) were characterized by Jasco MSV-5200 microscopic spectrophotometer. The range of absorption spectrum was 1.25-2.5 eV, and the sample collection area was ~20 × 20 µm. For APAS measurement, the incident polarization was realized by rotating the half-wave plate with a step size of 15°. Azimuth-dependent reflectance difference microscopy (ADRDM). The schematic diagram of the ADRDM experiment was shown in Figure S15. A deuterium combined with a halogen lamp and a monochromator were used as light source unit to select excitation wavelength. A linear polarizer and a liquid crystal variable retarder (LCVR) were used to modulate the polarization state of incident light. We rotated the polarizer and LCVR consistently in the azimuth-resolved measurements and the instrumental coordinate was decided by the fast and slow axes of polarizer. An objective with 5× magnification was used to focus light onto the sample. The reflected light was collected by the same objective and routed through LCVR and polarizer,


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followed by a CCD detector. The RD signal was solved by light intensity on the CCD detector. More details on the setup was described in our previous report.24 TEM characterization. Specimens for transmission electron microscopy (TEM) and scanning TEM (STEM) observations were prepared by first mechanically exfoliating the TlSe flakes onto a 300 nm SiO2/Si wafer, followed by spin-coating a layer of polymethyl methacrylate (PMMA) onto the wafer at 4000 rpm for 40 s. Subsequently, the PMMA/TlSe film was peeled off by etching SiO2 layer in 5 M NaOH solution, and the film was then transferred onto the TEM grid. Eventually, the PMMA was removed using acetone. The flake orientation and microstructures were characterized by TEM, HRTEM, and selected area diffraction pattern (SADP) using JEOL JEM-2100 electron microscope operated at an accelerating voltage of 200 kV. The HAADF and BF images were observed with a FEI Titan ChemiSTEM equipped with a probe corrector, which offers an opportunity to probe structures with sub-Å resolution. A HAADF detector with an inner semi-angle of more than 60 mrad was utilized, and the EELS was recorded using a Gatan Enfina system equipped on the STEM. Image simulations were performed using the WinHREM program (HREM Research Inc.) based on the multislice method. Device fabrication and measurements. The photodetection devices based on the TlSe flakes were fabricated by standard electron-beam lithography (EBL), and Ni/Au was deposited as contact electrodes by electron-beam evaporation (EBE). The photoresponse measurements were carried out using a semiconductor parameter analyzer (Agilent B2902A) in a probe station at room temperature under ambient conditions. Polarization dependence of the photoresponse was performed by rotating the directions of light polarization using a Glan-Taylor polarizer and a half-wave plate, while keeping light power constant throughout the measurements.


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Density Functional Theory Calculations. The band structure of TlSe was calculated by density foundational theory (DFT) based on projector augmented-wave (PAW) method implemented in VASP.34-36 For the exchange-correlation approximation, the Perdew, Wang local density approximation (LDA) was used. Since LDA tends to underestimate the band gap, we also performed hybrid functional theory HSE06 calculations. The plane wave cutoff energy was set to be 400 eV and k-point mesh was 11 × 11 × 11. The phonon dispersion was calculated through the code Phonopy by the frozon phonon method. A very large supercell containing 512 atoms was used in the phonon calculations.37 For the Raman intensity calculation, the second order perturbation method implemented in Quantum Espresso was used.38 VASP and Yambo were used to carry out the calculations of optical absorption spectra.39 More than 200 bands (162 empty states) were used in the whole optical calculations. For Yambo Bethe-Salpeter method calculation, eight valence and eight conductance bands were considered for electron-hole pairs and k-point mesh was set to be 9 × 9 × 9. For Quantum Espresso and Yambo calculations, plane wave within cut-off of 60 Ry is used. For all of calculations, spin-orbital coupling (SOC) effect has been included. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD pattern, XPS spectra, crystal structure, TEM and HRTEM images, HAADF and BF STEM images, band structure, PDOS, first Brillouin zone, Fermi surface around VBM and CBM, phonon dispersion, atom moving directions of phonon modes at Gamma point, HSE06 obtained band structure, calculated Raman intensity and unpolarized Raman spectrum, optical selection


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rules of TlSe; optical image of few-layer TlSe flakes on a Si wafer with a 300 nm SiO2 layer, corresponding AFM image and cross-section scan; character table of D4h and its subgroups; simulated possible patterns of polar plots for anisotropic Raman intensities; schematic diagram of the ADRDM measurement; optical image of a typical TlSe two-terminal device; experimental data fitting of photocurrent curves as changing of light intensities (PDF) AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] *Email: [email protected] Author Contributions #

S.Y., C.H., M.W. and W.S. contributed equally to this work.

ACKNOWLEDGMENT S.Y. is supported by the National Natural Science Foundation of China (NSFC) under Grant No. 51602014

and

the

Fundamental

Research

Funds

for

the

Central

Universities

(50100002017101022). C.J. is supported by the NSFC under Grant No. 51331001 and the Key Natural Science Foundation of Beijing (Grant No. 2151002). C.H. is supported by the NSFC (Grant No. 61008028) and the National key research and development program (2017YFF0107003). L.H. acknowledges supports by the NSFC (Grant No. 11774142) and Shenzhen Peacock Plan Team under Grant No. KQTD2016022619565991. Z.W. acknowledges supports by the NSFC under Grant Nos. 51728202 and 11332013.


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REFERENCES (1)

Yang, Y.; Liu, S. C.; Yang, W.; Li, Z.; Wang, Y.; Wang, X.; Zhang, S.; Zhang, Y.; Long, M.; Zhang, G.; Xue, D. J.; Hu, J. S.; Wan, L. J. Air-Stable In-Plane Anisotropic GeSe2 for Highly Polarization-Sensitive Photodetection in Short Wave Region. J. Am. Chem. Soc. 2018, 140, 4150–4156.

(2)

Yang, H.; Jussila, H.; Autere, A.; Komsa, H.; Ye, G.; Chen, X.; Hasan, T.; Sun, Z. Optical Waveplates Based on Birefringence of Anisotropic Two-Dimensional Layered Materials. ACS Photonics 2017, 4, 3023–3030.

(3)

Li, D.; Jussila, H.; Karvonen, L.; Ye, G.; Lipsanen, H.; Chen, X.; Sun, Z. Polarization and Thickness Dependent Absorption Properties of Black Phosphorus: New Saturable Absorber for Ultrafast Pulse Generation. Sci. Rep. 2015, 5, 15899.

(4)

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.

(5)

Yuan, H.; Liu, X.; Afshinmanesh, F.; Li, W.; Xu, G.; Sun, J.; Lian, B.; Curto, A. G.; Ye, G.; Hikita, Y.; Shen, Z.; Zhang, S. C.; Chen, X.; Brongersma, M.; Hwang, H. Y.; Cui, Y. Polarization-Sensitive Broadband Photodetector Using a Black Phosphorus Vertical P–N Junction. Nat. Nanotech. 2015, 10, 707−713.

(6)

Hicks, M. R.; Rodger, A.; Lin, Y.; Jones, N. C.; Hoffmann, S. V.; Dafforn, T. R. Rapid Injection Linear Dichroism for Studying the Kinetics of Biological Processes. Anal. Chem. 2012, 84, 6561−6566.


21

ACS Nano 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

(7)

Page 22 of 34

Wang, X.; Li, Y.; Huang, L.; Jiang, X. W.; Jiang, L.; Dong, H.; Wei, Z.; Li, J.; Hu, W. Short-Wave Near-Infrared Linear Dichroism of Two-Dimensional Germanium Selenide. J. Am. Chem. Soc. 2017, 139, 14976–14982.

(8)

Qiao, J.; Kong, X.; Hu, Z. X.; Yang, F.; Ji, W. High-Mobility Transport Anisotropy and Linear Dichroism in Few-Layer Black Phosphorus. Nat. Commun. 2015, 5, 4475.

(9)

Zhang, E.; Wang, P.; Li, Z.; Wang, H.; Song, C.; Huang, C.; Chen, Z. G.; Yang, L.; Zhang, K.; Lu, S.; Wang, W.; Liu, S.; Fang, H.; Zhou, X.; Yan, H.; Zou, J.; Wan, X.; Zhou, P.; Hu, W.; Xiu, F. Tunable Ambipolar Polarization-Sensitive Photodetectors Based on HighAnisotropy ReSe2 Nanosheets. ACS Nano 2016, 10, 8067−8077.

(10) Bandurin, D. A.; Tyurnina, A. V.; Yu, G. L.; Mishchenko, A.; Zólyomi, V.; Morozov, S. V.; Kumar, R. K.; Gorbachev, R. V.; Kudrynskyi, Z. R.; Pezzini, S.; Kovalyuk, Z. D.; Zeitler, U.; Novoselov, K. S.; Patanè, 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. Nanotech. 2016, 12, 223. (11) Lei, S.; Wen, F.; Ge, L.; Najmaei, S.; George, A.; Gong, Y.; Gao, W.; Jin, Z.; Li, B.; Lou, J.; Kono, J.; Vajtai, R.; Ajayan, P.; Halas, N. J. An Atomically Layered InSe Avalanche Photodetector. Nano Lett. 2015, 15, 3048−3055. (12) 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 FewLayer GaSe Sheets. Angew. Chem. 2015, 127, 1201–1205.


22

Page 23 of 34 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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(13) Yang, S.; Li, Y.; Wang, X.; Huo, N.; Xia, J. B.; Li, S. S.; Li, J. High Performance FewLayer GaS Photodetector and Its Unique Photo-Response in Different Gas Environments. Nanoscale 2014, 6, 2582. (14) Hussein, S. A.; Nagatn, A. T.; Mohamed, M. Characteristic of the Photoconductivity of Thallium Monoselenide. Cryst. Res. Technol. 1989, 24, 685–692. (15) Nayar, P. S. A New Far Infrared Detector. Infrared Phys. 1974, 14, 31–36. (16) Picker, P. B.; Tiller, H. D. Optical Energy Gap in TlSe. Phys. Stat. Sol. 1968, 29, 153. (17) Gashimzade, F. M.; Orudzhev, G. S. Effect of Pressure and Temperature on the Band Structure of TlSe. Phys. Stat. Sol. (b) 1981, 106, K67. (18) Niu, C.; Buhl, P. M.; Bihlmayer, G.; Wortmann, D.; Blügel, S.; Mokrousov, Y. TwoDimensional Topological Crystalline Insulator and Topological Phase Transition in TlSe and TlS Monolayers. Nano Lett. 2015, 15, 6071−6075. (19) Demishev, G. B., Kabalkina, S. S.; Kolobyanina, T. N. X-Ray Studies of Thallium Chalcogenides TlS and TlSe up to 37 GPa. Phys. Stat. Sol. (a) 1988, 108, 89. (20) Aliev, A. M.; Nizametdinova, M. A.; Steinshreiber, V. Y. Lattice Dynamics and Specific Heat of Thallium Monoselenide. Phys. Stat. Sol. (b) 1981, 107, K151. (21) Sun, R.; Wang, Z.; Saito, M.; Shibata, N.; Ikuhara, Y. Atomistic Mechanisms of Nonstoichiometry-Induced Twin Boundary Structural Transformation in Titanium Dioxide. Nat. Commun. 2015, 6, 7120.


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(22) Wang, Z.; Saito, M.; McKenna, K. P.; Gu, L.; Tsukimoto, S.; Shluger, A. L.; Ikuhara, Y. Atom-Resolved Imaging of Ordered Defect Superstructures at Individual Grain Boundaries. Nature 2011, 479, 380−383. (23) Li, L.; Wang, W.; Gong, P.; Zhu, X.; Deng, B.; Shi, X.; Gao, G.; Li, H.; Zhai, T. 2D GeP: An Unexploited Low-Symmetry Semiconductor with Strong In-Plane Anisotropy. Adv. Mater. 2018, 30, 1706771. (24) Yang, S.; Yang, Y.; Wu, M.; Hu, C.; Shen, W.; Gong, Y.; Huang, L.; Jiang, C.; Zhang, Y.; Ajayan, P. M. Highly In-Plane Optical and Electrical Anisotropy of 2D Germanium Arsenide. Adv. Funct. Mater. 2018, 28, 1707379. (25) Shen, W.; Hu, C.; Li, S.; Hu, X. Using High Numerical Aperture Objective Lens in MicroReflectance Difference Spectrometer. Appl. Surf. Sci. 2017, 421, 535–541. (26) Shen, W.; Hu, C.; Tao, J.; Liu, J.; Fan, S.; Wei, Y.; An, C.; Chen, J.; Wu, S.; Li, Y.; Liu, J.; Zhang, D.; Sun, L.; Hua, X. Resolving the Optical Anisotropy of Low-Symmetry 2D Materials. Nanoscale 2018, 10, 8329–8337. (27) Wassermeier, M.; Behrend, J.; Zettler, J. T.; Stahrenberg, K.; Ploog, K. H. In-Situ Spectroscopic Ellipsometry and Reflectance Difference Spectroscopy of GaAs (001) Surface Reconstructions. Appl. Surf. Sci. 1996, 107, 48–52. (28) Martin, D. S.; Weightman, P. Reflection Anisotropy Spectroscopy: A New Probe of Metal Surfaces. Surf. Interface Anal. 2001, 31, 915–926.


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(29) Chiarotti, G.; Chiaradia, P.; Arciprete, F.; Goletti, C. Sum Rules in Surface Differential Reflectivity and Reflectance Anisotropy Spectroscopies. Appl. Surf. Sci. 2001, 175–176, 777–782. (30) Liu, F.; Zheng, S.; He, X.; Chaturvedi, A.; He, J.; Chow, W. L.; Mion, T. R.; Wang, X.; Zhou, J.; Fu, Q.; Fan, H. J.; Tay, B. K.; Song, L.; He, R. H.; Kloc, C.; Ajayan, P. M.; Liu, Z. Highly Sensitive Detection of Polarized Light Using Anisotropic 2D ReS2. Adv. Funct. Mater. 2016, 26, 1169–1177. (31) Yang, S.; Wu, M.; Wang, H.; Cai, H.; Huang, L.; Jiang, C.; Tongay, S. Ultrathin Ternary Semiconductor TlGaSe2 Phototransistors with Broad-Spectral Response. 2D Mater. 2017, 4, 035021. (32) Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74–80. (33) Mueller, T.; Xia, F.; Avouris, P. Graphene Photodetectors for High-Speed Optical Communications. Nat. Photonics 2010, 4, 297–300. (34) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (35) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244–13249. (36) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953.


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(37) Togo, A.; Tanaka, I. First Principles Phonon Calculations in Materials Science. Scripta Mater. 2015, 108, 1–5. (38) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I. QUANTUM ESPRESSO: A Modular and OpenSource Software Project for Quantum Simulations of Materials. J. Phys. Condens. Mat. 2009, 21, 395502. (39) Marini, A.; Hogan, C.; Grüning, M.; Varsano, D. Yambo: An ab initio Tool for Excited State Calculations. Comput. Phys. Commun. 2009, 180, 1392–1403.


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Figure 1. Atomic-scale analysis of the TlSe. (a,b) Atomic model of TlSe viewed from [001] (a) and [110] (b) directions. (c) Simulated HAADF STEM image viewed from [110] direction. (d,e) Atomic-resolution HAADF (d) and BF STEM (e) images viewed from [110] direction. (f) Magnified HAADF STEM image of an analyzed region overlaid with the atomic model observed from [110] direction. In the atomic model, the Tl and Se atoms are denoted in yellow and red, respectively. (g‒i) EELS mapping of Tl (g), Se (h) and their combination (i).

Figure 2. ARPRS of a TlSe flake under two configurations. Typical polarized Raman spectra measured under parallel (a) and cross (b) configurations. (c) Polar plots and fitting of Raman active modes. The blue dots and the gray lines correspond to the experimental values and the fitting curves of polarized Raman intensity. The excitation wavelength is 633 nm.

Figure 3. Crystalline orientation identification of ultrathin TlSe flakes by the ADRDM. (a) Optical image of a TlSe flake on an isotropic SiO2/Si (100) wafer. The scale bar is 20 µm. (b) Polar plots of amplitude shifted ADRDM results as a function of polarization angles. An amplitude offset value of 0.08 is added to compensate the negative RD values in the polar plot. The black dots are the experimental values, while the red line is the cosine fitted curve. (c) ADRDM images taken at different angles from 0° to 165° with a step size of 15°. The excitation wavelength is 600 nm. (d) RD spectrum of a typical TlSe flake on an Al2O3 (001) substrate. Insert shows the optical image of the sample and the scale bar is 10 µm.


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Figure 4. In-plane optical absorption anisotropy and optical selection rules in layered TlSe. (a) PDAS of a TlSe flake measured with photon energy of 1.25-2.5 eV. Inset is the optical image of a typical TlSe flake on a quartz substrate. 0° and 90° correspond to c- and a-axis directions of TlSe crystalline orientation, respectively. (b) The calculated absorption as a function of laser photon energy along two directions of TlSe crystalline orientation. (c) HSE06 obtained band structure of TlSe. The red lines denote the possible c-axis polarized light jump, while the green lines denote the jump of a-axis polarized light.

Figure 5. Anisotropic photoresponse of the TlSe device. (a) 3D schematic diagram of the TlSe photodetection device. The linearly polarization of incident laser was controlled by a half-wave plate. (b) Photocurrent as a function of Vd for a typical TlSe device under 633 nm wavelength light with different light intensities. (c) 2D colormap of anisotropic photocurrent. X-axis and Yaxis denote the drain voltage Vd and the polarization angle φ, respectively. The photocurrent anisotropy can be reflected by the change of color. (d) The polar plots of the photocurrent as a function of φ at Vd = 1V (from 0° to 360° with a step size of 10°). The gray dots are the experimental values, while the blue line is the fitted curve. The wavelength of incident laser was 633 nm, and the laser power was 100 mW cm-2.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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In-Plane Optical Anisotropy and Linear Dichroism in Low-Symmetry

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