Twist-Angle-Dependent Optoelectronics in a Few ... - ACS Publications

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Surfaces, Interfaces, and Applications

Twist-angle Dependent Optoelectronics in a fewlayer Transition-Metal Dichalcogenides Heterostructure Woosuk Choi, Imtisal Akhtar, Malik Abdul Rehman, Minwook Kim, Dongwoon Kang, Jongwan Jung, Yoon Myung, Jungcheol Kim, Hyeonsik Cheong, and Yongho Seo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15817 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Materials & Interfaces

Twist-angle Dependent Optoelectronics in a few-layer Transition-Metal Dichalcogenides Heterostructure Woosuk Choi1†, Imtisal Akhtar1†, Malik Abdul Rehman1†, Minwook Kim1, Dongwoon Kang1, Jongwan Jung1, Yoon Myung1, Jungcheol Kim3, Hyeonsik Cheong3, Yongho Seo1,2* 1Department

of Nanotechnology and Advanced Material Engineering, Sejong University, Seoul, 05006, Korea 2 Graphene Research Institute and HMC, Sejong University, Seoul, 05006, Korea 3

Department of Physics, Sogang University, Seoul 04107, Korea

†These

authors contributed equally to this work author E-mail: [email protected], Tel.:+82-2-3408-3689 *Corresponding

Abstract Lattice matching has been supposed to play an important role in the coupling between two materials in vertical heterostructure (HS). To investigate this role, we fabricated a heterojunction device with a few layers of p-type WSe2 and n-type MoSe2 with different crystal orientation angles. The crystal orientations of WSe2 and MoSe2 were estimated by using highresolution X-ray diffraction. Heterojunction devices were fabricated with twist angles of 0˚, 15˚ and 30˚. The I-V curve of the sample with the twist angle of 0° under the dark condition showed a diode-like behavior. The strong coupling due to lattice matching caused a wellestablished p-n junction. In cases of 15° and 30° samples, the van der Waals gap was built due to lattice mismatching, which resulted in the formation of a potential barrier. However, when the LED light of 365 nm (3.4 eV) was illuminated, excited electrons and holes were possible to jump beyond the potential barrier and the current flowed well in both forward and reverse directions. The effects of the twist angle were analyzed by spectral responsivity and external quantum efficiency, where it was found that the untwisted HS exhibited higher sensitivity under IR illumination while the twisting effect was not noticeable under UV illumination. From the photoluminescence and Raman spectroscopy, it was confirmed that the twisted HS showed a weak coupling due to the lattice mismatch. Key words: transition-metal dichalcogenides; heterostructure; twist angle; optoelectronics; a few-layer TMD; p-n junction;

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1. Introduction Recently, transition metal dichalcogenides (TMDs) having band gaps ranging from 1.1 to 1.9 eV have attracted considerable attention as promising two-dimensional (2D) layered materials, which can compensate for the lack of band gap in graphene. In particular, the TMDsbased p-n junction device stands as a key strategy for developing various kinds of applications, such as solar cells, photodetectors, light emitting diodes (LEDs) and photovoltaics.1-6 In order to improve the performance of those devices, diverse combinations of 2D hexagonal structure materials, such as graphene, hexagonal boron nitride (h-BN) and TMDs, have been suggested and studied. For instance, h-BN/graphene7, WSe2/graphene8, WSe2/MoS29, WS2/MoS210 and graphene/WSe2/MoS211 van der Waals (vdW) heterostructures (HSs) with vertical stacking, have been demonstrated. However, it has been difficult to match the lattices between two different 2D materials, in fabricating these HS devices. It was reported that weak vdW interaction between the 2D materials in HS fabricated by an artificial transfer induces an interlayer distance with mismatched lattices.12 A tunneling photocurrent depending on coupling strength was studied at the interface in MoS2/MoSe2 heterojunction devices by Luong, et al.13 In an effort to solve the lattice mismatch, many groups have fabricated HSs such as WS2-MoS214, WSe2/MoSe22, 15 and WTe2-MoTe216 with similar lattice parameters. Another approach has been attempted to match lattices by aligning the crystal orientation.17-18 In particular, the vertical HSs directly grown by chemical vapor deposition (CVD) have a strong coupling due to the perfect matching of the crystal directions between the respective materials, and exhibits higher performance than HSs fabricated by the transfer method.14 Heo et al. reported that the band structure in MoS2/WS2 monolayer stacks can be controlled from indirect- to direct-gap depending on the twist angle, and experimentally confirmed this via heteroepitaxial stacking growth.18 Unfortunately, the grown layers may have a lot of defects in general, the twist angle cannot be adjusted intentionally in the growth process,19-20 and the synthesis of the specific layer at specific location is not controlled. 21-22 To fabricate samples with specific twist angles, the crystal orientation of the single crystal must be determined. Transmission electron microscopy (TEM)18, 23-24 and second harmonic generation (SHG)25-26 were known as methods for determining the crystal orientation. TEM is capable of analyzing the crystal orientation and composition of various materials accurately. However, TEM analysis has a disadvantage as the sample cannot be inspected without


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destroying the device. The SHG method is adequate to estimate the crystal orientation for single layer materials. However, it is known that the resolution of the crystal orientation angle is low and second harmonic power for the multilayer is much lower than that for a single layer, which makes the analysis difficult.27-29 On the contrary, the high-resolution X-ray diffraction (HRXRD) method had advantages of simple measurement, high angle resolution (0.1˚ resolution), and non-destructive inspection. In this paper, we used HR-XRD to find the single crystal direction of p-type WSe2 and n-type MoSe2 and the fabricated few-layer HS devices with twist angles of 0˚, 15˚ and 30˚. By analyzing the fabricated devices, we investigated the effect of the twist angle and how it differs from HS composed of monolayers. Through this analysis, we have provided evidence of how important the lattice matching is in vertically coupled HS devices.

2. Experimental Section 2.1 Sample fabrication mechanism A special method was developed to fabricate HS devices with specific twist-angles. Starting from a single crystal flake (~1 mm, purchased from HQ Graphene), micro-mechanical exfoliation using Scotch tape was performed to obtain a few-layer WSe2 or MoSe2, instead of chemical intercalation.30-33 Scotch tape with exfoliated flakes was mounted on the HR-XRD to determine the crystal orientation. In the repeated exfoliation process, the Scotch tape was moved in translational motions only, without twisting to keep the single crystalline orientation for all flakes. After determining the crystal orientation, the Scotch tape was attached to a SiO2 (300 nm)/Si substrate to transfer a few-layers of WSe2 (or MoSe2) on it. The thin flakes of WSe2 or MoSe2 on the Si substrate were found under an optical microscope, and an atomic force microscope (AFM) was used to select smooth and clean flakes. Thereafter, we picked up the few-layer MoSe2 on a polypropylene carbonate (PPC) film via a so-called ‘pick-up’ method.34 The few-layer WSe2 on the other substrate was picked up in the same way after adjusting the twist angles (0˚, 15˚ and 30˚) on the same PPC with MoSe2. (For details, see Supporting data Fig. S1) The twist angle for a representative sample was confirmed by second harmonic generation, as shown in the Supporting Information Fig. S9. Afterwards, they were transferred onto the other Si substrate and it was placed in a chloroform solution for 20 minutes to remove the PPC film and rinsed with isopropanol. For electrical



characterization of an HS device, a polymethyl methacrylate was coated as a resist, and e-beam lithography was performed to draw electrode patterns. Finally, e-beam evaporation was used to deposit Pd(20 nm)/Au(40 nm) on WSe2 and Ti(20 nm)/Au(40 nm) on MoSe2. In order to reduce the contact resistance avoiding the Schottky barrier, Pd was deposited on the p-type WSe2 while Ti was deposited on the n-type MoSe2.35-36 Surface oxidation or contamination was not found in the fabrication, and absence of 1T phase was confirmed by x-ray photoelectron spectroscopy, as shown in Supporting Information Fig. S8.

2.2 Device measurements (HR-XRD, AFM, Raman, PL, I-V curve) High-resolution X-ray diffraction (PANalytical X'pert PRO) was used to confirm the crystal orientation of WSe2 and MoSe2. While 2θ was fixed at 37.8 ˚ corresponding to (103) plane, two other angles, Ψ and Φ, (defined in the Supporting data Fig. S2) were scanned independently. The scan size of Ψ was 10˚ to 80˚ and the step size was 3˚, and the scan size of Φ was 0˚ to 360˚ and step size was 1˚. Ψ was fixed first, Φ was rotated from 0˚ to 360˚, and Ψ was moved to the next step. For an accurate thickness measurement of the samples, commercial AFMs (XE-100 by Park systems Inc., n-Tracer by Nano Focus Inc.) were used in contact mode with a scan rate of 0.3 Hz. Photoluminescence (PL) was measured using a micro Raman system (Nanofinder 30) with an argon laser (532 nm, 0.3 mW) under ambient conditions. The Raman spectrum was measured using another micro Raman system (Renishaw, InVia systems) with a laser line of 514.5 nm (argon ion) under ambient conditions. For comparison, PL and Raman were measured at three different regions: WSe2 only, MoSe2 only, and the overlapped area in the heterostructure devices. In order to investigate electrical and optoelectronic properties, I-V curves and transfer curves were measured under different LED lights (365 nm at 36 mW/cm2, 530 nm at 30.5 mW/cm2, and 850 nm at 102.39 mW/cm2) illumination. All electrical measurements were performed at room temperature under vacuum (~ 10 mTorr) conditions.

3. Results and discussion 3.1 HR-XRD for crystal alignment To investigate the effect of the crystal twist angle, the crystal directions of the few-layers WSe2 and MoSe2 were determined before fabricating HS with the twist angles. In both cases


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of WSe2 and MoSe2, the angle 2θ = 37.8˚ corresponds to crystal plane (103) with a strong peak,37 and the incident beam and detector were fixed at this angle. The crystal flakes on the pieces of Scotch tape were measured, and the pole figures as functions of (Ψ, Φ) are shown in Figure 1(a). Six peaks appeared because WSe2 and MoSe2 form a hexagonal lattice structure. At a fixed Ψ=58˚ value with the highest peaks in the data, the intensity as a function of Φ is shown in Figure 1(b). The peaks in the line profile represent the crystal direction with the spacing of 60˚ between the peaks. As these results indicate sharp peaks, it was confirmed that the crystal orientation of flakes were aligned well. From these line profiles, the tilt angles of WSe2 and MoSe2 were estimated as 26.3˚ and 7.5˚, respectively. Based on these tilt angles, the vertical p-n junctions of WSe2 and MoSe2 with twist angles of 0˚, 15˚, and 30˚ were fabricated as illustrated in Fig. 1(c). The expected lattice structures of WSe2 and MoSe2 with twist angles 0˚, 15˚ and 30˚ as shown in Fig. 1(d).

Figure 1 High-resolution x-ray diffraction data of WSe2 and MoSe2 and schematics of WSe2/MoSe2 heterostructure p-n junction. (a) HR-XRD results of WSe2 and MoSe2 are plotted in the pole figure. (b) The line profile of the Φ scan is plotted at fixed Ψ with the highest intensity. (c) Schematic drawing of the WSe2-MoSe2 device. WSe2-MoSe2 heterostructure p-n junction on SiO2 substrate was illuminated by LED lights (365 nm, 560 nm, 850 nm) in a vacuum at room temperature. (d) Illustration of crystal orientation with different twist angles (0˚, 15˚ and 30˚).

As a preliminary test, field-effect transistors (FET) with WSe2 and MoSe2, separately were fabricated, and transfer curves and the optoelectronic characteristics were measured to check the carrier types. (Figure S3-4) These devices were made of a few layers of WSe2 and MoSe2,



which were confirmed by the Raman spectra. The I-V characteristics of WSe2 showed that the dark current was low. (Figure S3(c)). When the LED light was illuminated on the WSe2 device, the current increases remarkably as the photon energy was increased with short wavelength. The transfer curves depending on the gate voltage revealed that the current raised dramatically as the gate voltage became negative as shown in Fig. S3(d). This confirmed that WSe2 was a p-type semiconductor. On the other hand, when the gate voltage was less than -40 V, the current was reduced as the shorter wavelength LED was illuminated. This implies that the holes generated by the field effect were recombined with the electrons excited by high energy photon illumination. This unexpected behavior has been observed by other groups, which was explained by the dissociation of adsorbed molecules on the TMD surface.38 The on/off ratio depending on the gate voltage in our sample was 106, which is similar to the monolayer WSe2 based FET as reported previously.35 In the case of MoSe2 devices, Ti was selected as the contact metal that lowered the Schottky barrier, and it was confirmed that the current amounted to 74 nA, when 1 V was applied in a dark condition. (See Fig. S4) While the current was slightly increased from the dark state at 530 nm and 850 nm illuminations, it was increased noticeably only under the 365 nm light illumination (3.4 eV). (Fig. S4(c)) Because the few layer MoSe2 had an indirect band gap, electrons and holes were considerably generated under the high-energy illumination. The current was increased when the gate voltage was positive in the transfer curve, confirming that MoSe2 was an n-type semiconductor (Fig. S4(d)). Contrary to WSe2, the larger current under the 365 nm illumination was due to the increased number of electrons generated by the higher photon energy.

3.2 Microscopic image and I-V curve Figure 2(a) shows an optical micrograph of a typical heterostructure device, where MoSe2 is on the right, and WSe2 on the top side. In the middle of WSe2 and MoSe2, the light blue area was the vertically overlapped heterostructure, and the yellow areas were the electrodes at both ends. Figures 2(b-c) are images of the surface morphology of the heterostructure obtained by AFM. The thicknesses of WSe2 and MoSe2 were measured around 3 nm from the line profile shown in Fig. 2(d). All the HS devices were made of WSe2 and MoSe2 with this thickness corresponding to 4 layers, as the thickness of a single layer is 0.7 nm.


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Figure 2 Microscopic analysis of the WSe2/MoSe2 heterostructure p-n junction. (a) The optical micrograph of the WSe2/MoSe2 device. (b) 3-dimensional and (c) plane images of the AFM surface morphology of HS. (d) Line profiles of WSe2 and MoSe2 were plotted in red and blue lines, respectively

Figures 3(a, b) show the I-V curves of representative samples with different twist angles. The I-V curves in the dark state of the 0° sample showed a diode-like behavior of the p-n junction, and when the voltage was 1 V, the current was 0.62 nA as shown in Fig. 3(a), which was larger than the p-n junctions of the monolayer.11, 39 As a result, it was confirmed that the p-type WSe2 and the n-type MoSe2 were coupled to form a p-n junction with a built-in potential. (i) The band diagrams of the p-n junction without bias are shown in Fig. 3(c). A Kelvin probe measurement revealed that the work function for multilayer MoSe2 is lower than that for multilayer WSe2. (Supporting Information Fig. S7) As the Fermi level of p-type WSe2 is lower than that of n-type MoSe2, the energy bands for WSe2 is shifted upward due to the band bending. (ii) When the forward bias was applied, the potential barrier decreased, and the electrons of MoSe2 in the conduction band were easily transferred to WSe2. Also, the holes of WSe2 in the



valence band can be easily transferred to MoSe2 under the dark state. (iii) On the contrary, when a reveres bias was applied, a large potential barrier was formed, and the current was reduced. Under the LED illumination, the current of the 0° sample was increased in the forward direction more than that in the dark state as the electron and hole pairs are generated by the photon energy. In Fig 3(a), (ii) the larger photocurrents were induced under shorter wavelength (365 and 560 nm) illuminations. As the shorter wavelength photons have higher energy, the more electrons in the valence band can be excited to the conduction band. (iii) On the other hand, the potential barrier became higher in the reverse bias, and almost no current flowed in the dark condition. When it was illuminated by high-energy photons, the limited numbers of electrons and holes were generated near the junction, and a low current flowed through the potential barrier.

Figure 3 I-V curves of heterostructures with different twist angles ((a) 0˚, (b) 15˚) under LED illumination with different wavelengths: 365 nm, 560 nm, and 850 nm. Schematic energy band diagrams are shown for the (c) p-n junction HS with a staggered band alignment and (d) HS with a vdW gap at different bias voltages: (i) zero bias, (ii) forward bias, and (iii) reverse bias. Electrons (-) and holes (+) in the dark state were indicated as white symbols. Red symbols and dotted lines represent the electron and hole generation in the 850 nm LED illumination, while green in 530 nm and blue in 365 nm.

The I-V curves of the HSs with twist angles 15˚ and 30˚ shown in Figure 3(b) and S5


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exhibited a completely different behavior, compared to the 0° sample. In the case of two twisted samples under the dark condition, the photocurrent hardly flowed in the order of ~10 pA. (Fig. S5(c)) It was reported that the interlayer distance at the interface of HS depends on the twist angle.17 While the vdW HS has a high-symmetry-stacking with the strongest interlayer coupling strength for twist angle of 0° or 60°, HSs with the other angles have weak coupling strength due to mismatched atomic alignment. The coupling strength also can be changed by thermal annealing.13 We assume that the weak coupling expands the vdW gap with a potential barrier, and the p-n junction may not be formed normally. 12, 40 Thus, a potential barrier at the interface blocked the carriers under the dark condition, and the conductance was decreased as shown in Fig. 3(b). However, the photocurrent appeared noticeably only under UV (365 nm) illumination. Due to its high energy, the electrons and holes generated by a 365 nm photon can go beyond the potential barrier formed by the vdW gap as shown in Fig. 3(d).40 Therefore, the I-V curves under the UV illumination showed linear behavior according to the band diagram without band bending. In Figure S5(a), the current of the 30˚ sample was even lower than the 15˚ sample, due to the higher potential barrier being induced as the interlayer distance was increased.17



Figure 4 A comparison of (a-c) spectral responsivity and external quantum efficiency with different the twist angle under the LED illuminations ((a, d, g) 365 nm, (b, e, h) 560 nm, (c, f, i) 365 nm). The responsivity and EQE data in (d-f) were obtained at a fixed bias of 1 V, while EQE data in (g-i) were obtained at 0 V.

Spectral responsivity (R) and external quantum efficiency (EQE) were estimated to compare the twist angle effects with different wavelength illuminations. Spectral responsivity is defined as the photocurrent ratio to the intensity of the illuminated light. It can be expressed by:

R=

𝐼𝑝ℎ 𝑃0 × 𝑆

,

where Iph is the difference between the illuminated (Iillum) and dark (Idark) current i.e. (Iph = Iillum – Idark), P0 is the illumination power density, and S is the effective illuminated area. The spectral responsivity and external quantum efficiency vary depending on the wavelength of light, the intensity of light, bias voltage, and gate voltage (Vg). The intensity of light was fixed as described in the experimental section. R was calculated under bias 1 V and Vg = 0 V. Figure


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4(a) shows the responsivity in IR (850 nm) illumination depending on the twist angle. Four samples for each twist angle were prepared and measured individually, and the median values are plotted together. In particular, it had lower responsivity at 15˚ and 30˚, which implied that the junction was not well formed due to the lattice mismatching. According to Nayak et al. the lattice parameter and interlayer distance of the HS lattice vary depending on the twist angle in the range from 0˚ to 60˚ with the maximum distance at 30˚.17 In Heo et al.’s paper, the band structure changes from the direct bandgap to the indirect band gap as the interlayer distance increases.18 It was plausible that the twisted samples have lower spectral responsivities, but under UV (365 nm) illumination there was no consistent tendency except the experimental error, as the photons had high energy above the potential barrier. (Fig. 4(c)) The large experimental error was attributed to the delicacy in fabrication of the HS device. Responsivities of photovoltaic devices based on monolayer TMDs reported by others were in large variations from 0.1 mA/W to 880 A/W, in deed.41-43 EQE can be estimated from the responsivity simply by: EQE =

𝑅 × ℎ𝑐 𝑒λ

,

where h is the Planck’s constant, c the speed of light, e the charge of the electron, and λ the wavelength of the light illuminated. EQE under IR illumination was around a few percent, as the light has a low-energy, but the untwisted device (0˚) exhibited a remarkably higher value compared with the other twisted devices as shown in Fig. 4(d). EQE value of the untwisted device under visible illumination (530 nm) was around 100% as shown in Fig. 4(e), which is much higher than that reported by Lee et al.11 Indeed, EQE can be greater than 100%, especially under a bias, because the applied voltage provides carriers, and excitation of photons increases the conductivity of the junction. Also, the remarkable increment of EQE from IR to visible illumination is in good agreement with the paper.11 The data of EQE measured at zero bias voltage are shown in Fig. 4(g-i), where the angle dependency trend is similar to the EQE at 1 V bias, though the absolute values are much lower. Under UV illumination, the twist angle dependence of EQE was weakened due to the same reason for the responsivity as shown in Fig. 4(f).

3.3 Photoluminescence and Raman Spectroscopy To investigate the excitons in the heterojunctions with different twist angles between



MoSe2 and WSe2, we measured the PL at room temperature. In the Fig 5(a), two peaks (peak A at 1.5 eV and peak I at 1.7 eV) in the WSe2 spectrum indicate that the number of layers was around four.29, 44 The peak A representing the direct band gap was weakened compared with the monolayer, and peak I corresponding the indirect band gap was lowered compared with bulk state. These results were consistent with the number of layers obtained from AFM data. MoSe2 spectrum showed a peak near 1.6 eV, corresponding to the K-K direct band gap. This PL peak around 1.5–1.6 eV exhibits its maximum intensity for a single-layer MoSe2, and the few-layer MoSe2 show gradual decrement with the number of layers.45 In the PL spectrum of the MoSe2/WSe2 heterostructures, the peak for MoSe2 was clear, but WSe2 peaks were suppressed. The reason why the MoSe2 peak was much stronger than the WSe2 peaks is supposed to be due to the geometry having MoSe2 on the top surface.

Figure 5 Photoluminescence and Raman spectra of MoSe2/WSe2 vertical heterostructures. (a) PL


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spectra of individual few layers WSe2 (blue), MoSe2 (red) and HS (black). (b) PL spectra of HSs with different twist angles (0˚, 15˚ and 30˚). (c) Raman spectra of individual few layers WSe2 (blue), MoSe2 (red), and HS (black). (d) Raman spectra of HSs with different twist angles: 0˚(orange), 15˚(green) and 30˚(purple).

Figure 5 (b) shows the PL spectra for HSs depending on the twist angle. The weak peak near 1.68 eV observed in all angles is supposed to be red-shifted from peak I of WSe2, due to interlayer radiative recombination.23 Another red-shifted PL signal (1.51 eV) from peak A of the WSe2 was clear particularly in the 30˚ sample. In the 30˚ sample of which the interlayer distance was considered to be large, WSe2 and MoSe2 were less influenced by each other, and peak A of WSe2 was strong. A similar result for twist angle 29˚ was reported by Nayak et al.17 On the other hand, an interlayer exciton peak at 1.3 eV for monolayer based WSe2/MoSe2 HS was reported by others,17, 46-48 which was not found in our sample. The peak at 1.3 eV has a low intensity even in monolayer based HS, and it is only noticeable for the samples with the twist angles of 0˚ and 60˚.17 The spatially separated interlayer excitons were not expected in HS composed of a few layers, based on our experimental results. Raman spectroscopy was adopted to investigate the phonon oscillation of the lattice of the WSe2/MoSe2 heterostructures. The Raman shift in high frequency range from 200 cm-1 to 420 cm-1 was measured including the in-plane and out-of-plane modes of WSe2 and MoSe2. The Raman spectra of the WSe2, MoSe2, and MoSe2/WSe2 heterojunction are shown in Fig. 5(c). The intensities and positions of E12g (and A1g, 250.6 cm-1) and 2LA (261.1 cm-1) modes in the Raman spectrum of WSe2 correspond to four-layers, roughly.29, 49 This result was similar to the estimated number of layers from the thickness measured by AFM. The Raman spectra of MoSe2 including A1g mode (243.6 cm-1) and E12g mode (287.1 cm-1) also show a similar number of layers when referred to other papers.45 The Raman spectra of the WSe2/MoSe2 heterojunction was similar to that of MoSe2 with the biggest peak of A1g mode of MoSe2 because MoSe2 was in the upper position. In addition, the E12g and 2LA mode peaks of WSe2 at the bottom of the heterostructures were found. The phonon mode modification at HS may not significantly influence the Raman data, considering the thickness of the multilayer devices. In Figure 5(d), the Raman spectra of the heterostructures were compared according to the twist angle. Due to different intensities of the Raman spectrum for each sample, the average values of the five samples were calculated for each twist angle. It can be seen that the position and intensity of the A1g and E12g modes of MoSe2 in HS were almost the same as those in MoSe2 only. However, the intensity of the E12g and 2LA modes of WSe2 were changed. In



particular, both of the peaks were increased in the 30˚ sample. In order to analyze it statistically, the positions and intensities of the peaks in HSs were compared as shown in the Figure S6 with 5 samples for each twist angle. The positions of Raman modes of WSe2 and MoSe2 were similar to those in a monolayer based HS as shown Figure S6(b-c).17 As the variation of the Raman shifts of each mode for different samples were within 2 to 3 cm-1, it can be seen that no significant effect of the twist angle was observed.29, 49 However, when the intensities of the peaks were analyzed in detail, E12g and 2LA modes of WSe2 were strong, particularly in the 30˚ sample. (Figure S6(a)) A similar behavior confirming our result was found in the monolayer heterojunction, where the WSe2 E2g mode peak at 29˚ twist angle was especially high.17 As a result, it was confirmed that both Raman and PL signals of WSe2 were strong at the 30˚ sample, and these results can be attributed to the large vdW gap at the twist angle 30˚.

4. Conclusions We observed that a large-scale photocurrent was generated at the HSs composed of fewlayers TMDs as compared with that of monolayers. When the twist angle was 0˚, it was found that the p-n junction was well formed because the interlayer distance was short. In cases of 15˚ and 30˚, a potential barrier was induced due to the vdW gap, and electrons and holes excited by low energy photon illumination could not tunnel the barrier. However, when the UV was illuminated, the highly excited electrons and holes could jump beyond the potential barrier, and the bipolar current flowed in both forward and reverse biases. It was difficult to distinguish the different Raman and PL peak shifts depending on the twist angle, but it was confirmed that the intensities of WSe2 peaks below MoSe2 layer became clear at the 30˚ sample. This implies that the twisted orientation causes decoupling of the heterojunction. The importance of crystal orientation matching in such vertical heterojunction devices has been recognized, and lattice matching technique can be applied in optoelectronic devices to strengthen junction coupling. For the optoelectronic device application, scalability or high yield is a critical issue, and the chemical intercalation32-33 or CVD deposition50 can be applied instead of mechanical exfoliation for that purpose.

Supporting Information Sample fabrication process of WSe2-MoSe2 heterostructure p-n junction; HR-XRD data of


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disarranged crystalline flakes; Optoelectronic properties of FETs, I-V curve of heterostructure with different twist angles; Raman signatures of MoSe2/WSe2 vertical heterostructures; second harmonic generation analysis: X-ray photoelectron spectroscopy result; Kelvin probe force microscopy images;

ACKNOWLEDGMENT This research was supported by the Priority Research Centers Program (2010-0020207) and Basic Science Research Program (2017R1A2B4002379) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education. This work was also supported by the World Class 300 Project (S2561932) of the MOTIE, MSS(Korea).



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Figure 1 329x153mm (150 x 150 DPI)



Figure 2 256x191mm (150 x 150 DPI)


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Figure 3 335x185mm (150 x 150 DPI)



Figure 4 346x280mm (150 x 150 DPI)


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Figure 5 230x196mm (150 x 150 DPI)


Twist-Angle-Dependent Optoelectronics in a Few ... - ACS Publications

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