Strong In-Plane Anisotropies of Optical and Electrical Response in

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Strong In-Plane Anisotropies of Optical and Electrical Response in Layered Dimetal Chalcogenide Liang Li,†,# Penglai Gong,‡,# Weike Wang,§ Bei Deng,‡ Lejing Pi,† Jing Yu,† Xing Zhou,† Xingqiang Shi,‡ Huiqiao Li,† and Tianyou Zhai*,†,∥ Downloaded via UNIV OF WINNIPEG on June 24, 2018 at 15:18:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

State Key Laboratory of Material Processing and Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, P.R. China ‡ Department of Physics, Southern University of Science and Technology, Shenzhen 518055, P.R. China § Key Laboratory of Low-dimensional Quantum Structures and Quantum Control of Ministry of Education, Synergetic Innovation Center for Quantum Effects and Applications, College of Physics and Information Science, Hunan Normal University, Changsha 410081, P.R. China ∥ Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, Tianjin University, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P.R. China S Supporting Information *

ABSTRACT: An interesting in-plane anisotropic layered dimetal chalcogenide Ta2NiS5 is introduced, and the optical and electrical properties with respect to its in-plane anisotropy are systematically studied. The Raman vibration modes have been identified by Raman spectra measurements combined with calculations of phonon-related properties. Importantly, the Ta2NiS5 flakes exhibit strong anisotropic Raman response under the angle-resolved polarized Raman spectroscopy measurements. We found that Raman intensities of the Ag mode not only depend on rotation angle but are also related to the sample thickness. In contrast, the infrared absorption with light polarized along the a axis direction is always larger than that in the c axis direction regardless of thickness under the polarization-resolved infrared spectroscopy measurements. Remarkably, the first-principles calculations combined with angle-resolved conductance measurements indicate strong anisotropic conductivity of Ta2NiS5. Our results not only prove Ta2NiS5 is a promising in-plane anisotropic 2D material but also provide an interesting platform for future functionalized electronic devices. KEYWORDS: anisotropy, Ta2NiS5, 2D material, polarized Raman spectra, electrical anisotropy

T

unique puckered orthorhombic crystal structure with armchair and zigzag atomic arrangement along x and y directions, respectively,9,21−23 thus leading to anisotropic band structure with anisotropic optical, electrical, and thermal properties.21 So far, the anisotropic carrier mobility,8 in-plane anisotropic thermal conductivity,24 polarization-resolved photoluminescence,11 polarization-resolved absorption,8,13 and polarization-

wo-dimensional (2D) layered materials such as graphene and metal chalcogenides have attracted tremendous attention owing to their highly potential applications in next-generation electronics and optoelectronics.1−7 In contrast to the widely studied graphene and MoS2, which have high in-plane symmetry, recently, a new category of 2D materials has emerged, such as black phosphorus (BP),8−13 Re dichalcogenides,14−17 Ge and Sn monochalcogenides,18 and transition metal trichalcogenides19,20 that possess low-symmetrical crystal structures and in-plane anisotropic physical properties. Take BP for example; it holds a © 2017 American Chemical Society

Received: July 11, 2017 Accepted: September 13, 2017 Published: September 13, 2017 10264

DOI: 10.1021/acsnano.7b04860 ACS Nano 2017, 11, 10264−10272

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Cite This: ACS Nano 2017, 11, 10264-10272

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ACS Nano resolved Raman12,13,25 have been observed. Due to interest in BP, other low-symmetrical 2D materials including ReS2,26,27 ReSe2,28 TiS3,19 GaTe,29,30 SnS,31 and WTe232 with respect to the in-plane anisotropic electronic transport properties and light−matter interactions have also been extensively studied. Inplane anisotropic materials, which give us another degree of freedom for tuning the electrical and optical properties, thus expand the range of opportunities for designing interesting semiconductor devices and exploring unique applications.33 For example, a polarization-sensitive mid-infrared photodetector with high gain was obtained in BP transistors,34 and the polarization-resolved Raman enhancement of the CuPc molecule probe was achieved using BP and ReS2 as surfaceenhanced Raman scattering (SERS) substrates.35 Terahertz nanodetectors with selective, controllable plasma wave, bolometric and thermoelectric response were designed based on BP using its inherent electrical and thermal in-plane anisotropy,36 and digital inverter was successfully devised based on anisotropic ReS2 field-effect transistors (FETs).15 Despite this rapid progress, the research on anisotropic 2D materials is generally still in its early stage. Discovering new materials and exploring their in-plane anisotropy-dependent properties and applications is thus not only scientifically significant but also technologically needed. Here, we introduce dimetal chalcogenide Ta2NiS5 as an interesting member to the in-plane anisotropic 2D material family. In contrast to the previously studied BP, SnSe, ReS2, and TiS3 that only have one or two elements, the Ta2NiS5 system consists of three elements, which can bring more degrees of freedom for tuning physical properties just by stoichiometric variation. Raman spectra measurements combined with calculations of phonon-related properties have been used to identify the Raman vibration modes. The angleresolved polarized Raman spectroscopy (ARPRS) shows the Ta2NiS5 has strong in-plane anisotropy, and the anisotropic Raman scattering is thickness-dependent. Interestingly, polarization-resolved infrared spectroscopy (PRIRS) always shows absorption with light polarized along the a axis direction that is higher than that along the c axis direction regardless of thickness. Moreover, a remarkable in-plane anisotropic conductance is predicted in Ta2NiS5 using first-principles calculations. Besides, the angle-resolved conductance (ARC) measurement shows the conductance along a axis is 1.41 times larger than that along c axis at 293 K. Our work not only gives an interesting in-plane anisotropic ternary layered material of Ta2NiS5 but also provides fundamental information with respect to its anisotropy, which may be helpful for designing functionalized devices.

Figure 1. Crystal structure of Ta2NiS5. (a) Perspective side view of few-layer Ta2NiS5, (b) top view of monolayer Ta2NiS5. (c) Inverse fast Fourier transform (i-FFT) HRTEM image and (d) enlarged image of the Ta2NiS5 (010) surface.

exhibits a sharp lattice fringe, in parallel with the clear fast Fourier transformation (FFT) image (Figure S1, Supporting Information), indicating the single-crystalline nature. From the enlarged HRTEM image of Figure 1d, we can identify the lattice constants: a = 3.4 Å and c = 15.1 Å, which are very close to the literature’s results.38 The energy-dispersive X-ray spectroscopy (EDS) spectrum of a Ta2NiS5 flake shown in Figure S2a shows strong signals of Ta, Ni, and S and agrees well with the stoichiometry of Ta2NiS5. The corresponding elemental mapping shown in Figure S2c−e clearly reveals the uniform distribution of Ta, Ni, and S in the Ta2NiS5 flake. Moreover, the X-ray photoelectron spectroscopy (XPS) was used to examine the elemental binding energies of the Ta2NiS5 crystal sample, as shown in Figure S3. Two peaks located at 23.9 and 25.9 eV of Ta 4f spectra in Figure S3a can be assigned to the Ta 4f2/7 and Ta 4f2/5, respectively.39 The peak around 854 eV of the Ni 2p spectra in Figure S3b can be identified as Ni 2p2/3.39 In addition, two main peaks located at 161.1 and 162.4 eV of the S 2p spectra in Figure S3c are ascribed to S2− of Ta2NiS5. The Ta2NiS5 flakes were exfoliated onto a silicon wafer with a 300 nm oxidation layer. The atomic force microscope (AFM) images of three typical flakes are shown in Figure 2a. From the top to bottom of Figure 2a, one can see that the absolute height of the corresponding sections of Figure 2a is 5.2, 10.9, and 69.9 nm, corresponding to 8 layers (L), 17, and 111 L, respectively (a single layer is 0.63 nm). Figure 2b shows the Raman spectra of these three flakes. It can be seen that the shapes of all the Raman spectra with different thicknesses are similar, except the peak intensities are different. The thickness-dependent light absorption capacity,40 optical interference,41,42 and band structure with layer thickness43 may play roles in the variation of Raman intensities of layer materials. In our case, there is little change in band structure with thickness of Ta2NiS5 (see Figure 5), the variation of Raman intensities mainly comes from the first two factors. Four peaks located at 39.5, 64.4, 125.5, and 147.5 cm−1 are observed in the detection range of Figure 2b. The crystal structure of bulk Ta2NiS5 is characterized by the space group Cmcm (D17 2h), which contains 32 atoms in the orthorhombic crystal unit cell and 16 atoms in the primitive

RESULTS AND DISCUSSION Dimetal chalcogenide Ta2NiS5 has an interesting van der Waals layered structure.37,38 Each layer is three atoms thick; the middle sheets of Ni and Ta atoms are tetrahedrally and octahedrally coordinated by top and bottom sheets of S atoms to form NiS4 and TaS6 units, respectively (Figure 1a). Along the crystalline a axis, the NiS4 and TaS6 units are arranged in linear chains, whereas along the c axis, these units formed zigzag chains, and each NiS4 unit is separated by two TaS6 units (Figure 1b). Due to the different chain structures along the a axis and c axis, considerable in-plane anisotropic physical properties could be expected in Ta2NiS5. The high-resolution transmission electron microscopy (HRTEM) image of the (010) surface of an exfoliated sample presented in Figure 1c 10265


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Figure 3. (a) Raman spectra of the Ta2NiS5 flake at temperatures ranging from 100 to 293 K. The dashed lines are plotted for clarity. (b) Temperature dependence of the peak positions of B2g, 2Ag, and 3 Ag, from bottom to top. The lines are the corresponding fitted lines. Figure 2. (a) AFM images of Ta2NiS5 flakes with different thicknesses. All scale bars: 5 μm. (b) Raman spectra of corresponding Ta2NiS5 flakes with different thicknesses. (c) Schematic diagrams of atomic displacement for modes 1Ag, B2g, 2 Ag, and 3Ag.

with increasing temperature. The peak positions as a function of temperature were fitted using a linear equation: ω(T) = ω0 + χT, where ω0 is the peak position of B2g, 2Ag, and 3Ag modes at 0 K and χ is the first-order temperature coefficient of the corresponding Raman modes, which also describes the slope of the fitted line. The fitted values of χ for B2g, 2Ag, and 3Ag modes are found to be −0.00717, − 0.00958 and −0.01533 cm−1 K−1, respectively. It can be seen that the three peak positions show different temperature dependences, in which the 3Ag mode is most sensitive to the temperature. Previous works have revealed that variation of the Raman modes with temperature mostly arises from the thermal anharmonicity, which includes the thermal expansion and volume contribution.44,49,50 The change in Raman frequency can be determined by44,51

cell, respectively. Thus, there are 16 × 3 vibrational modes at the Γ point in Brillouin zone (BZ). The irreducible representations of the optical phonons in bulk Ta2NiS5 at the Γ point include 1Ag, B2g, 2Ag, and 3Ag, where all the modes are Raman-active. One B2g mode and three Ag modes are predicted to be observable when the incident laser is perpendicular to the (010) surface, according to the current study. The phonon modes for these irreducible representations with the Ramanactive modes are displayed in Figure 2c. The force vectors correspond to twisting motions for modes 1Ag and B2g, stretching motions for 2Ag and 3Ag, respectively. Meanwhile, the calculated frequencies are in agreement with our experimental results shown in Table 1.

Δω ≡ (χT + χV )ΔT = =

Table 1. Experimental and Calculated Frequencies of the Detected Phonon Modes 1

experiment (cm−1) calculation (cm−1)

Ag

B2g

39.5 40.3

64.4 60.5

2

Ag

125.5 128.4

⎛ dω ⎞ ⎛ dω ⎞ ⎜ ⎟ ΔT + ⎜ ⎟ ΔT ⎝ dT ⎠V ⎝ dV ⎠T

⎛ dω ⎞ ⎛ dω ⎞ ⎛ dω ⎞ ⎜ ⎟ ΔT + ⎜ ⎟ ⎜ ⎟ ΔT ⎝ dT ⎠V ⎝ dV ⎠T ⎝ dT ⎠ P

(1)

where χ = χT + χV, in which χT is self-energy shift due to the phonon modes coupling and χV is the volume change arises from thermal expansion.44,52 As presented in Table S1, the temperature coefficient χ of the Ta2NiS5 flake is comparable to that of graphene44 and MoS245,53 and is smaller than BP,46 SnS,47 and SnSe.48 It was reported that the first-order temperature coefficient of the Raman modes in layered materials is related to the van der Waals interaction between each layer.54,55 In BP, SnS, and SnSe, the strong van der Waals interaction between individual layers leads to the large χ,48 whereas in graphene and MoS2, the van der Waals interaction is weak, leading to small χ. In our case, the Ta2NiS5 is stacked by weak van der Waals interaction,37,56 thus, the χ is relatively small. ARPRS measurements were first performed on the 10.9 nm thick flake at room temperature. We defined the a axis and c axis directions as the x and z, respectively. The incident light was polarized along the horizontal direction (Figure S4). A linear polarizer was placed in front of the detector, and only the Raman signals polarized parallel or perpendicular to the incident light polarization (denoted as parallel or perpendicular polarization configuration, respectively) were collected. Figure 4a,b presents Raman spectra of the flake evolving with angles in

3

Ag

147.5 148.1

Temperature-dependent Raman spectroscopy is an efficient way to study the thermal conductivity, thermal expansion, and the atomic bonds of layered materials.44−48 In this work, we performed the Raman characterization on the 10.9 nm thick Ta2NiS5 flake at temperatures ranging from 100 to 293 K, as shown in Figure 3a. Two features can be observed: one is that the intensity of all the Raman peaks decreases with increasing temperature; the other is all the Raman peaks are red-shifted and softened when temperature increases from 100 to 293 K. The first phenomenon results from the enhancement of anharmonic coupling of two modes at higher temperature,48 as such coupling will make the modes share energy and decrease the vibrational excitations. The later one occurs mainly due to the thermal anharmonicity,44,48 which will be discussed in the following. Figure 3b shows the typical plots for the temperature dependences of peak positions of B2g, 2Ag, and 3Ag modes. One can see that the three modes behave nearly linearly 10266


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ACS Nano ⎧⎛ ⎞2 | a| I(A g ,||) ∝ |c|2 ⎨⎜sin 2 θ + cos φca cos2 θ ⎟ ⎠ |c | ⎩⎝ +

⎛ | a| ⎞2 ⎫ ⎜ sin φca cos2 θ ⎟ ⎬ ⎝ |c | ⎠⎭

(4)

I(B2g ,||) ∝ |e|2 sin 2 2θ

(5)

I(B2g , ⊥) ∝ |e|2 cos2 2θ

(6)

where ∥ and ⊥ represent parallel and perpendicular polarizations, respectively, and φac = φa − φc is the phase difference. From Figure 4c−e and Figure S5c, it can be seen that the calculated curves fitted well with the experimental data (the fitted values of |a|/|c| and φac are shown in Table 2). It is Table 2. Fitted Values of |a|/|c| and φac for the Detected Ag Modes of Ta2NiS5 Flakes with Different Thickness 2

Ag

3

Ag

Figure 4. ARPRS of a 10.9 nm Ta2NiS5 flake under the parallel polarization configuration. (a) Typical polarized Raman spectra. (b) False-color plot of polarized Raman intensity. Polar plot and fitting of (c) B2g, (d) 2Ag, and (e) 3Ag Raman modes.

6.8 nm

10.9 nm

85 nm

198 nm

0.80 41 0.68 23

1.30 84 1.50 45

1.15 41 0.92 28

1.25 49 0.85 26

interesting to note that when the incident light polarization is parallel to the a axis, I(Ag,∥) gets the maximum value, whereas I(B2g,∥) achieves the minimum intensity of 0 (Figure 3c−e); when the incident light polarization is parallel or perpendicular to the a axis direction, I(B2g,⊥) achieves the maximum value. Next, we only discuss the Ag mode under parallel configuration. It should be noted that the 1Ag has a 180° variation period, whereas the 2Ag mode has a 90° variation period with two different local maxima at θ = 0 and 90°; such difference is due to the different values of a and c in the Raman tensor. The above results indicated that with the crystalline orientation (a or c direction) along the polarization direction of the light, the intensity of the Raman modes reaches a local maximum or minimum value, so the ARPRS technique can be used to identify the crystalline orientation of Ta2NiS5. However, the ARPRS alone cannot ascertain the specific direction of a axis and c axis of Ta2NiS5; combining with other techniques such as polarized absorption13 or ARC12 is necessary to confirm the specific direction. To further study the in-plane anisotropic optical properties, we carried out ARPRS (under parallel configuration) and PRIRS measurements on Ta2NiS5 with different thickness, as shown in Table 3. The top of Table 3 shows the optical micrograph of the exfoliated flakes; it is interesting to see that the obtained flakes always have a long stripe shape with two sharp parallel edges. We can see that the B2g mode does not show clear thickness dependence (the B2g mode of Ta2NiS5 flakes with different thickness shows obvious 90° variation period with four-lobed shape). However, the anisotropic Raman scattering of the Ag modes shows obvious thickness dependence. The 2Ag shows maximum intensity at 90 and 270° and secondary maxima intensity at 0 and 180° for the 6.8 nm flake, whereas for 10.9 (Figure 4d), there were 85 and 198 nm flakes. The phenomenon is just the opposite; the 3Ag shows maximum intensity at 0 and 180° for the 10.9 nm (Figure 4e) flake, but are 90 and 270° for others. From eq 4, we can see that |a|/|c| > 1 or |a|/|c| < 1 determines whether the main axis is along the a axis or c axis direction, and φac is positively related

parallel configuration. It can be seen that the B2g mode vanishes when the rotation angle is 0, 90, or 180°, whereas 2Ag and 3Ag modes have the strongest intensity at the rotation angle of 0 or 180° (the intensity of 1Ag is weak and is not discussed here). Under the perpendicular configuration shown in Figure S5a,b, the intensity of the B2g mode approaches a maximum when the rotation angle reaches 0, 90, or 180°, whereas the 2Ag and 3Ag modes become nearly undetectable. These observations clearly reveal that the polarized Raman spectra are dependent on crystalline orientation. The intensity of observed Raman vibration modes can be expressed as57 I ∝ |e iRes|2

|a|/|c| φac |c|/|a| φac

(2)

where ei and es are the unit polarization vectors of incident and scattered lasers, respectively, and R is the Raman tensor. The unit polarization vector is ei = (cos θ, 0, sin θ), where θ is the angle between incident light polarization and a axis direction of the Ta2NiS5. Similarly, under the parallel configuration, es can be written as es = (cos θ, 0, sin θ), while in the perpendicular configuration, it can be written as es = (−sin θ, 0, cos θ). For an absorptive material, the Raman tensor elements are complex values, with real and imaginary parts.25 Thus, the Raman tensor of Ag and B2g can be expressed as58 ⎛|a|eiφa ⎛ 0 0 |e|eiφe ⎞ 0 0 ⎞ ⎜ ⎟ ⎜ ⎟ i φ R(A g) = ⎜ 0 |b|e b 0 ⎟ R(B2g ) = ⎜ 0 0 0 ⎟ ⎜ iφ ⎟ ⎜ ⎟ ⎝|e|e e 0 0 ⎠ ⎝ 0 0 |c|eiφc ⎠ (3)

where φa, φb, φc, and φe are the corresponding phases of the Raman tensor elements.13,25 Then, the Raman scattering intensities of different modes can further be expressed as 10267


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Table 3. Polar Plots of Fitted Peak Intensities of Raman Modes under Parallel Polarization Configuration and PRIRS of Ta2NiS5 Flakes with Different Thickness

Figure 5. Band structures of (a) bulk and (b) monolayer Ta2NiS5. Fermi level has been set to be zero. The top of the valence band and bottom of the conduction band are highlighted in color. The effective mass near the valence band maximum and conduction band minimum is also displayed.

to the strength of the secondary maximum on Ag. The fitted values of |a|/|c| and φac are shown in Table 2, and these values are obviously thickness-dependent. Previous studies have revealed that polarization-dependent interference effect takes great responsibility for the thickness-dependent anisotropic Raman scattering in BP,13,59 and this effect may also play a role in Ta2NiS5. Generally, Raman scattering is a complicated process, which involves both electron−photon and electron− phonon interactions.13,25,60 In contrast, the optical absorption is

relatively simple, which involves only electron−photon interaction, and has been proven to be a reliable method for crystalline orientation identification for BP.13 Here, we measured the PRIRS with light polarized along x or y direction, as shown in the bottom of Table 3. For all flakes, the absorption edges are at approximately 1600 cm−1, which means Ta2NiS5 has a robust band gap of ∼0.2 eV with thickness independence, consistent with our calculations (see Figure 5). More importantly, the absorptions with light polarized along x (a 10268


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ACS Nano axis) direction are always larger than z (c axis) direction regardless of thickness, showing a sharp contrast of anisotropic Raman scattering. Thus, we can give a short conclusion that the ARPRS combined with PRIRS can be used to identify the specific crystalline orientation of Ta2NiS5. In order to investigate the electronic properties with respect to its in-plane anisotropy of Ta2NiS5, the first-principles calculations were used to investigate the electronic structure. The first BZ of the primitive cell for the bulk Ta2NiS5 and its projected surface BZ with high-symmetric k points are shown in Figure S6. Our calculated results show that the optimized lattice constants of bulk Ta2NiS5 are 3.42, 12.28, and 15.18 Å for a, b, and c, respectively, which are in good agreement with the experimental values (a = 3.42, b = 12.15, and c = 15.10 Å, respectively).38 For the monolayer, the lattice constants (a = 3.40 and c = 14.91 Å) slightly shrink, relative to the bulk. The electronic band structures of bulk and monolayer Ta2NiS5, together with the fitted effective masses, are then calculated. We predict that bulk Ta2NiS5 is a semiconductor with a direct band gap of 0.36 eV at the Γ point (Figure 5a), consistent with the experimental value of 0.13 eV.37 The band structure displays highly anisotropic dispersions near the valence band maximum and conduction band minimum. A fit of these bands using the nearly free electron model yields effective carrier masses along the Γ−X direction (a direction), which are small and similar, namely, 0.34 m0 for electrons and 0.39 m0 for holes, whereas these along Z−Γ (c direction) are slightly larger, having the respective values of 3.63 m0 and 0.79 m0. Larger values are obtained along the Γ−Y direction (b direction, i.e., the interlayer vertical direction), where the carrier effective masses are 4.38 m0 and 6.87 m0, respectively (Table 4). In the

band structure of the monolayer shows that it is still a semiconductor, with a slightly larger direct band gap of 0.39 eV at the Γ point (Figure 5b). The highly in-plane anisotropic effective masses remain in monolayer systems (Table 4). For the Γ−X direction in the monolayer, the carrier effective masses are 0.31 m0 (hole) and 0.27 m0 (electron), slightly lower than those in the bulk, along the same direction. Remarkably, the effective mass in the Γ−Y direction for the electrons is 2.36 m0, 1.5 times smaller than its bulk value of 3.63 m0. In contrast, the effective mass for the holes is 0.84 m0, very close to its bulk value of 0.79 m0. The similarity between the band structures and the anisotropic effective masses of bulk and monolayer Ta2NiS5 indicates that few-layer Ta2NiS5 is likely to be a 2D direct-band-gap semiconductor with high anisotropic mobility. We then return to the 10.9 nm flake; the ARC measurement was carried out to investigate the electrical anisotropy. Twelve electrodes with a width of 0.9 μm spaced at an angle of 30° were fabricated as shown in Figure 6a. Each pair of electrodes’ channel length is 8 μm. Here, we defined the direction along the a axis as the 0° reference. The I−V measurements were taken between each pair of diagonal contacts at different temperatures. Figure S7 shows the typical I−V curves at 293 K; one can clearly see that the device shows good Ohmic contact, and the electric transport is angular-dependent. The obtained DC conductivity at different temperatures shown in Figure 6b clearly shows angular dependence. The conductivity in an anisotropic material at a certain angle θ can be described by the equation8,61 σθ = σx sin2 θ + σz cos2 θ, where σx and σz refer to the conductivity along the a axis and c axis directions, respectively. The resulting calculated conductance fits well with the measurement data (Figure 6b). Figure 6c shows the calculated conductivity ratio (σa/σc) at different temperatures; one can see that the conductivity ratio decreases nearly linearly from 1.78 to 1.41 with temperatures ranging from 80 to 293 K, which is comparable to black phosphorus8 and ZrTe5.61 The temperature-dependent anisotropic electrical conductivity may come from the different temperature dependence of the effective masses of electrons along different directions.18 It should be noted that the fringing current will spread the total current, and thus the anisotropic conductivity is always underestimated.18,61 Like BP,12 with the anisotropic Raman scattering in Figure 4 and anisotropic conductance in Figure 6, the crystalline orientation of Ta2NiS5 can also been identified.

Table 4. Effective Mass of Carriers in Monolayer and Bulk Ta2NiS5a carrier type

layers

ma*/m0

e

1 bulk 1 bulk

0.31 0.34 0.27 0.39

h

mb*/m0 4.38 6.87

mc*/m0 2.36 3.64 0.84 0.79

Carrier types “e” and “h” denote “electron” and “hole”, respectively. ma*, mb*, and mc* are carrier effective masses for directions along a, b and c axes, respectively. a

monolayer, its crystal cell has the same orthorhombic structure with primitive cell. The 2D rectangular BZ is the projected surface (i.e., the Z̅ Γ̅X̅ plane), as shown in Figure S6 (left). The

Figure 6. (a) Schematic view of device structure with 12 electrodes spaced at 30° apart. (b) Angle-dependent DC conductance at different temperatures. (c) Conductance ratio along a and c axis directions as a function of temperature. Insets: Optical image of the 10.9 nm Ta2NiS5 flake device. The channel length and width of each pair of electrodes are 8 and 0.9 μm, respectively. 10269


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code.62,63 The PBE64 functional with ultrasoft65 projector-augmented wave (PAW) potential66 is used to describe the electronic exchange and correlation. The cutoff energy for wave function (charge density) is 47 (387) Ry. The convergence thresholds for electronic energy and phonon perturbation potential were set to be 10−10 and 10−8 Ry. Electron-Related Property Calculations. First-principles calculations were performed using the PAW method66 with a plane-wave basis set as implemented in the Vienna ab initio simulation package.67 In optimizing the geometry of the bulk Ta2NiS5, van der Waals interactions were considered in the form of the optB88-vdW functional.68 The energy cutoff for the plane-wave basis was set to 400 eV for all the calculations. The k-mesh of 7 × 7 × 2 and 13 × 3 in geometric relaxation and electronic structure calculations were adopted to sample the 3D and 2D first Brillouin zones, respectively. We carry out the band structures calculations with the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06).69

CONCLUSION In summary, we have synthesized bulk Ta2NiS5 single crystals via the chemical vaport transport method and mechanically exfoliated it down to few-layer flakes. Characterization such as TEM and EDS confirmed the single-crystalline structure and composition of Ta2NiS5. The Raman spectra combined with the calculations of phonon-related properties reveal different phonon dispersion modes at Γ points. ARPRS show strong thickness-dependent in-plane anisotropy Raman response, whereas PRIRS always shows higher absorption with light polarized along the a axis rather than the c axis, independent of thickness. These results indicate that combination of ARPRS with PRIRS can be used to identify the specific crystalline orientation of Ta2NiS5 and can be expanded to other in-plane anisotropic 2D materials. Moreover, the first-principles calculations reveal a remarkable in-plane anisotropic electrical transport in Ta2NiS5; the angle-resolved electrical transport measurements show a maximum conductivity along the a axis direction and a minimum conductivity along the c axis direction, with the conductivity ratio of 1.41 at 293 K. This work provides an interesting in-plane anisotropic ternary layered material Ta2NiS5 and fundamental information with respect to its anisotropic, which may beneficial for future functionalized device design.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04860. Crystal structure, EDS results, EDS mapping, and XPS results of Ta2NiS5; optical microscopy images of exfoliated Ta2NiS5 flakes on SiO2; polarized Raman spectra under the cross-polarization configuration; first BZ of the primitive cell for Ta2NiS5; typical I−V curves (PDF)

METHODS Synthesis and Mechanical Exfoliation of Ta2NiS5 Single Crystals. High-quality Ta2NiS5 single crystals were synthesized by chemical vapor transport technique using iodine as transporting agent. One gram of starting materials of high-purity Ta, Ni, and S powders (99.9%, Alfa) in atomic ratios of 2:1:5 was placed inside a sealed quartz tube with a vacuum level of about 10−5 Torr. The tube was slowly heated to 850 °C on one end, while the other end was kept at 800 °C. After 10 days of growth, shiny crystals with plate-like shape were obtained on the cold end. The Ta2NiS5 flakes were isolated from corresponding bulk crystals and transferred to a Si substrate with a 300 nm thick SiO2 layer using the scotch-tape-based mechanical exfoliation method. An optical microscope (Olympus, BX51) was used to examine the flakes. The thickness of the Ta2NiS5 flakes was precisely determined by AFM (Bruker Dimension FastScan). Characterization. The transmission electron microscopy, highresolution transmission electron microscopy image, high-angle annular dark-field image, and energy-dispersive X-ray spectroscopy (EDS) mapping were obtained on a JEOL 2100F system. X-ray photoelectron spectroscopy was conducted using a Thermo Fisher VGESCALAB 250 instrument. The Raman and polarized Raman experiments were performed on a confocal microscope spectrometer (WITec Alpha300 Raman) with 100× microscope objective and 532 nm laser, and the power was kept at 0.5 mW in order to avoid sample damage and laserinduced heating. For polarized Raman specifically, the incident laser beam was polarized, with the scattered light with polarization that was either parallel or perpendicular to the incident polarization. The sample was rotated 360° with a step of 10°. Infrared spectroscopy was taken by using a Bruker Optics Fourier transfer infrared spectrometer (Vertex 70) integrated with an infrared microscope system (Hyperion TM 2000). An infrared polarizer was used to achieve polarized light. Device Fabrication and Angle-Resolved Electrical Conductivity Measurements. The star-shaped 12 electrode device was fabricated by standard electron-beam lithography (FEI Quanta 650 SEM and Raith Elphy Plus) and deposited Cr/Au(10 nm/40 nm) as contact electrodes using thermal evaporation (Nexdep, Angstrom Engineering). The electrical characterizations were performed in a probe station (CRX-6.5K, Lake Shore) equipped with a semiconductor analyzer system (4200SCS, Keithley). Phonon-Related Property Calculations. The phonon-related properties at the Γ point were calculated using density functional perturbation theory,62 which is implemented in the Quantum Espresso

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Huiqiao Li: 0000-0001-8114-2542 Tianyou Zhai: 0000-0003-0985-4806 Author Contributions #

L.L. and P.G. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 91622117, 51472097, and 51727809), the National Key Research and Development Program of “Strategic Advanced Electronic Materials” (Grant No. 2016YFB0401100), the National Basic Research Foundation of China (Grant No. 2015CB932600), the Project funded by China Postdoctoral Science Foundation (Grant Nos. 2017M610474 and 2015LH0018), and the Fundamental Research Funds for the Central University (Grant Nos. 2015CB932600 and 2017KFKJXX007). REFERENCES (1) Koppens, F.; Mueller, T.; Avouris, P.; Ferrari, A.; Vitiello, M.; Polini, M. Photodetectors Based on Graphene, Other Two-Dimensional Materials and Hybrid Systems. Nat. Nanotechnol. 2014, 9, 780− 793. (2) Chhowalla, M.; Jena, D.; Zhang, H. Two-Dimensional Semiconductors for Transistors. Nat. Rev. Mater. 2016, 1, 16052. (3) Duan, X.; Wang, C.; Pan, A.; Yu, R.; Duan, X. Two-Dimensional Transition Metal Dichalcogenides as Atomically Thin Semiconductors: Opportunities and Challenges. Chem. Soc. Rev. 2015, 44, 8859−8876. (4) Li, L.; Wang, W.; Chai, Y.; Li, H.; Tian, M.; Zhai, T. Few-Layered PtS2 Phototransistor on h-BN with High Gain. Adv. Funct. Mater. 2017, 27, 1701011. 10270


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