Interlayer Transition and Infrared Photodetection in Atomically Thin

Mar 7, 2016 - We demonstrate the type-II staggered band alignment in MoTe2/MoS2 van der Waals (vdW) heterostructures and an interlayer optical transit...
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Interlayer Transition and Infrared Photodetection in Atomically Thin Type-II MoTe2/MoS2 van der Waals Heterostructures

Kenan Zhang,† Tianning Zhang,† Guanghui Cheng,⊥ Tianxin Li,† Shuxia Wang,† Wei Wei,† Xiaohao Zhou,† Weiwei Yu,† Yan Sun,† Peng Wang,† Dong Zhang,‡ Changgan Zeng,⊥ Xingjun Wang,† Weida Hu,† Hong Jin Fan,§ Guozhen Shen,*,‡ Xin Chen,*,† Xiangfeng Duan,*,∥ Kai Chang,‡,¶ and Ning Dai†,¶ †

National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China ‡ State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China § Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ∥ Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States ⊥ Hefei National Laboratory for Physical Sciences at the Microscale (HFNL) and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ¶ Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: We demonstrate the type-II staggered band alignment in MoTe2/MoS2 van der Waals (vdW) heterostructures and an interlayer optical transition at ∼1.55 μm. The photoinduced charge separation between the MoTe2/ MoS2 vdW heterostructure is verified by Kelvin probe force microscopy (KPFM) under illumination, density function theory (DFT) simulations and photoluminescence (PL) spectroscopy. Photoelectrical measurements of MoTe2/ MoS2 vdW heterostructures show a distinct photocurrent response in the infrared regime (1550 nm). The creation of type-II vdW heterostructures with strong interlayer coupling could improve our fundamental understanding of the essential physics behind vdW heterostructures and help the design of next-generation infrared optoelectronics. KEYWORDS: van der Waals heterostructure, MoS2, MoTe2, interlayer transition, type-II band alignment electronic and optoelectronic devices.8,9,12,13 The type-II staggered band alignment is capable of generating interlayer optical excitation because the conduction band minimum and valence band maximum belong to two independent components possessing different work functions, and thus can modulate the interlayer transition energy and induce the charge spatial separation.5,8,13−15 Recently, atomically thin vdW heterostructures have sparked intensive interest in the

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he creation of van der Waals (vdW) heterostructures from atomically thin two-dimensional (2D) layered materials (e.g., graphene or transition-metal dichalcogenides)1−4 can allow flexible integration of highly distinct materials at the atomic scale, thus offering a versatile platform for diverse electronic and optoelectronic devices (e.g., transistors, photodetectors, light emitter diodes and solar cells).5−10 However, most vdW heterostructures reported to date display a spectral response that is limited by the intrinsic band gap of the constituting materials.11−13 Artificial semiconductor heterostructures with type-II staggered band alignments can offer unique opportunity for the design of advanced © XXXX American Chemical Society

Received: February 7, 2016 Accepted: March 7, 2016

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DOI: 10.1021/acsnano.6b00980 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano community for next-generation optoelectronic devices.13,14,16−21 The interlayer band alignment can be largely tuned by stacking various 2D materials via either mechanical transfer or direct CVD growth without the limit of lattice matching.14,22 Existing MX2/MX2 (M = Mo, W; X = S, Se, Te) heterostructures have shown unique photoelectric responses in the visible wavelength due to intrinsic band gap of the isolated monolayers.13,19,23−25 Most of these vdW heterostructures possess the type-II band alignment with an interband transition ranging from infrared to visible wavelengths. Nonetheless, typeII vdW heterostructures are insufficiently explored to date for an interlayer transition excitation in the infrared region, which is important for optical communication, environmental monitoring, medical diagnostics and even military applications.26 Here, we demonstrate a type-II MoTe2 /MoS2 vdW heterostructures that show interlayer optical transition for the creation of an infrared (e.g., at ∼1.55 μm) photodetector beyond the limits of the intrinsic band gap of the constituting material. The monolayer (ML) MoS2 typically exhibits a direct band gap of ∼1.80−1.90 eV and the monolayer (ML) MoTe2 exhibits a direct band gap of ∼1.05 eV.27−34 The interlayer coupling interaction between MoTe2 and MoS2 results in a type-II staggered band alignment,35 which can be confirmed with a low-frequency vibration mode and a photoluminescence (PL) change in the MoTe2/MoS2 heterostructures. Systematic studies using Kelvin probe force microscopy (KPFM), electrical transport measurements, and the first principle calculations demonstrate that the type-II staggered band alignment is responsible for an interlayer infrared excitation in MoTe2/MoS2 vdW heterostructures. Photoelectrical measurements of MoTe2/MoS2 vdW heterostructures show a distinct photocurrent response in the infrared region (1550 nm). The creation of type-II vdW heterostructures provides a new opportunity to manipulate optoelectronics of semiconductor 2D heterostructures and design next-generation infrared optoelectronics.

Figure 1. Interlayer infrared excitation in the MoTe2/MoS2 vdW heterostructures. (a) Schematic diagram of a MoTe2/MoS2 vdW heterostructure device under infrared light excitation. (b) Schematic illustrations of type-II interband excitation processes in MoTe2/MoS2 vdW heterostructures. The value of interband gap (Δ) is obtained by theoretical calculation. (c) Cross-sectional view of the atomic structures and the calculated bonding charge densities of MoTe2/MoS2 heterostructures. The orange, yellow, and blue spheres represent Te, S, and Mo, respectively. The red/ green and blue/green represent the distributions of negative and positive charges, respectively.

Methods).36 The layer thickness of MoTe2 and MoS2 materials was verified by atomic force microscopy (AFM) and Raman spectroscopy (see Figure S2, Supporting Information). Raman spectroscopy is sensitive to phonon behaviors or the interlayer interaction in layered 2D atomic crystals.37,38 Comparing the Raman spectra of the MoTe2/MoS2 vdW heterostructures with an isolated MoS2 or MoTe2 monolayer (Figure 2b−d), we note that the vibrational modes of MoS2 (A1g, E12g) soften by ∼1.5−2 cm−1, while those of MoTe2 soften by ∼1.0 cm−1 after the MoTe2 monolayer was placed on the MoS2 monolayer (Figure S2). The mode softening indicates the interlayer coupling between MoTe2 and MoS2 monolayers. The effects of adsorbates and residuals could be eliminated when the MoTe2/MoS2 heterostructures were annealed for the formation of vdW heterostructures in a pure hydrogen environment (atmosphere pressure) at 280 °C for 3 h (see Figure S3, Supporting Information).23,39 The shear and layer breathing modes of 2D layered materials represent the relative motions of the planes themselves and mostly appear in the low-frequency region.40 In the lowfrequency Raman spectra shown in Figure 2e, a vibrational mode at ∼28.7 cm−1 is observed in MoTe2/MoS2 heterostructure, which might be due to the interlayer interaction between ML-MoS2 and ML-MoTe2. It is noted that the vibrational mode at ∼22.0 cm−1 is different from that of the bulk and pure multilayer MoTe2 when the MoTe2 multilayer was transferred on the top of MoS2 monolayer (Figure 2f and Figure S2f). All these suggest that the changed vibrational modes are related with the interlayer interaction in the 2D layered and stacked heterostructures.40 In situ Raman and photoluminescence (PL) spectroscopy and mapping are performed to further verify the interlayer coupling effects in the MoTe2/MoS2 heterostructure (Figure 3a). Both the Raman intensity mapping using the MoTe2 E12g mode at 235 cm−1 (Figure 3b) and PL spatial mapping of MoS2

RESULTS AND DISCUSSION Panels a and b of Figure 1 show the schematic illustration of the device layout and the interlayer excitation process, respectively. Although with an intrinsic response cutoff wavelength of 700 nm for ML-MoS2 and 1200 nm for ML-MoTe2, the interlayer gap is about 0.66 eV (∼1880 nm) from the valence band maximum of ML-MoTe2 to the conduction band minimum of ML-MoS2. Therefore, such a type-II heterostructure device can allow a transition from valence band of ML-MoTe2 to conduction band of ML-MoS2 under infrared light with the wavelength longer than 1200 nm. To probe the distribution of electrons and holes in the atomically thin MoTe2/MoS2 heterostructure, we have conducted density-function theory (DFT) calculations (Figure 1c). It is found that the electrons are localized in the ML-MoS2, while holes are localized in the ML-MoTe2, clearly indicating the charge separation in the MoTe2/MoS2 heterostructure. However, in both cases of individual bilayer MoS2 and bilayer MoTe2, the positive and negative charges are not localized in each monolayer, but completely mixed within the whole bilayers with no clear charge separation (see Figure S1, Supporting Information). Figure 2a shows an optical micrograph of the MoTe2/MoS2 vdW heterostructure by transferring a mechanically exfoliated monolayer of MoTe2 onto a monolayer of MoS2 crystals grown on SiO2/Si substrate using an alignment transfer process (see B

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Figure 2. Crystalline vibrational analysis of the MoTe2/MoS2 vdW heterostructures. (a) Optical micrograph of MoTe2/MoS2 vdW heterostructures on 285 nm SiO2/Si substrate by transferring exfoliated MoTe2 monolayer on MoS2 monolayer. The scale bar is 5 μm. Raman spectra (b) of the CVD-grown monolayer MoS2, the exfoliated monolayer MoTe2, and the as-transferred MoTe2/MoS2 vdW heterostructures, (c) of the MoS2 monolayer and MoTe2/MoS2 vdW heterostructures before and after annealing in a pure hydrogen environment at 280 °C for 3 h, and (d) of zooming panel c in the range 380−410 cm−1 (i.e., the region within dotted line). (e and f) The low-frequency Raman spectra of the layered MoS2, MoTe2, and MoTe2/MoS2 vdW heterostructure.

To further verify the charge transfer or separation in the MoTe2/MoS2 type-II heterostructure, we utilized KPFM to quantitatively analyze the changes in surface potential or work function at the nanometre scale.42−45 Figure 4a shows schematically the KPFM measurement of the MoTe2/MoS2 vdW heterostructure (Figure 4b). The in situ AFM topography and KPFM surface potential images of the heterojunction (region I indicated in Figure 4b) in the dark and under illumination are shown in Figure 4c−f. AFM height profiles verify that the thickness of ML-MoTe2 on ML-MoS2 is ∼0.85 nm, which is similar to the value reported elsewhere.46,47 The surface potential difference (SPD) of the MoTe2/MoS2 heterostructure is determined to be about 110 mV in the dark. Under illumination (the excitation laser wavelength is 680 nm), the SPD increases to about 130 mV. This significant change of SPD between dark and illumination conditions can be ascribed to an obvious optoelectric effect occurring in the MoTe2/MoS2 heterostructure under light illumination.

emission at 1.86 eV (Figure 3c) show clearly the heterojunction formation over the MoTe2/MoS2 overlapping area. The PL emission from MoS2 was strongly quenched in the MoTe2/ MoS2 heterostructure regions (dark regions in Figure 3c). Figure 3d,e shows the corresponding PL spectra for MoTe2/ MoS2 heterostructures, ML-MoS2 and ML-MoTe2 layers. The PL quenching for both MoS2 and MoTe2 monolayers is observed in the MoTe2/MoS2 vdW heterostructures, which implies that the charge separation in the heterostructure region occurs.8,19,41 To exclude the influence of dielectric environment, the inverse MoS2/MoTe2 heterostructure was also characterized; it is observed that both Raman and PL spectra of inverse MoS2/MoTe2 heterostructures (see Figure S4, Supporting Information) are similar to those of the positive MoTe2/MoS2 heterostructures. Hence, the Raman and PL data (Figures 2 and 3) are in good agreement with the DFT calculation results, strongly indicating type-II MoTe2/MoS2 band alignment and charge separation at the interface. C

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Figure 3. (a) An optical image of another MoTe2/MoS2 vdW heterostructure. The scale bar is 5 μm. (b) Intensity map of the MoTe2-E12g Raman mode of sample in panel a. (c) PL intensity map of the emission at about 1.86 eV (MoS2). (d) PL spectra in the range of 1.70−2.15 eV for the three regions indicated in panel c: monolayer MoS2 (I), monolayer MoTe2/monolayer MoS2 vdW heterostructures (II), and multilayer MoTe2/monolayer MoS2 heterostructures (III). (e) PL spectra in the range of 0.78−1.55 eV of monolayer MoTe2 (α), monolayer MoTe2/monolayer MoS2 vdW heterostructures (β), and multilayer MoTe2 (γ).

The energy band diagrams of the MoTe2/MoS2 heterostructure in the dark and light conditions are sketched in Figure 4g. The MoTe2/MoS2 heterostructure forms a type-II band alignment and their Fermi levels align in equilibrium condition (in the dark). The work function of MoS2 (ΦMoS2) is smaller than that of MoTe2 (ΦMoTe2); therefore, an apparent SPD (about 110 mV by KPFM) is produced. Under illumination, the photoinduced electron−hole pairs are generated mainly in MLMoTe2 because of its smaller band gap energy. Subsequently, the type-II band alignment of the MoTe2/MoS2 heterostructure will lead to electron flow into the MoS2 side, causing separated holes and electrons residing in two different monolayers (which has been predicted also by the above calculations in Figure 1c). As a result, the quasi-Fermi levels of these two materials shift in opposite directions. In other words, the work function difference of MoS2 and MoTe2 widens under illumination, corresponding to increase in SPD. Moreover, we have preformed the in situ AFM and KPFM measurements on a more asymmetric heterostucture (region II where the right side of MoS2 is confined between MoTe2 on both sides, see Figure S5, Supporting Information). In the dark, the SPD of the MoTe2/MoS2 heterostructure displays an equal value in two sides (∼104 mV). However, under light condition, the increment of SPD is evidently different on two sides (about

Figure 4. Charge transport in MoTe2/MoS2 vdW heterostructures. (a) Schematic diagram of KPFM measurement of the MoTe2/MoS2 vdW heterostructures. (b) An optical micrograph of MoTe2/MoS2 vdW heterostructures. The scale bar is 5 μm. (c and d) The synchronous measurements for surface morphology (i.e., AFM) and surface potential images (i.e., KPFM) in the region I in panel b in the dark. The scale bar is 500 nm. (e and f) The synchronous measurements for AFM and KPFM images under laser illumination (680 nm). Line scanned profiles in all measurements are also executed here. The scale bar is 500 nm. (g) The energy band diagrams for MoTe2/MoS2 vdW heterostructures in the dark and light, respectively. Φ, EF, EVAC, ECB, and EVB are work function, Fermi levels, the energy position of vacuum, conduction band, and valence band, respectively.

21 vs 61 mV, see Figure S5c,e). This can be explained by the MoS2 on the right of the MoTe2/MoS2 heterostructure, which is closely adjacent to a large piece of MoTe2. As a result, there are more electrons injected from the MoTe2 into MoS2 which further lowers the work function of MoS2 (higher quasi-Fermi level position). D

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Figure 5. Interlayer infrared response in the MoTe2/MoS2 vdW heterostructures. (a) An optical micrograph of fabricated MoTe2/MoS2 vdW heterostructure device. The scale bar is 10 μm. (b) Experimental Ids−Vds curves in the dark (black line) and under infrared light illumination (1550 and 2000 nm). The inset shows the photovoltaic effect of the fabricated device under 1550 nm light illumination. near the sulfur powder. The second zone was heated to 680 °C and kept at that temperature for 5 min under an argon atmosphere. Device Preparation. The MoTe2/MoS2 vdW heterostructures are fabricated using a transfer technique. First, the pregrown MoS2 monolayers by chemical vapor deposition are selected to serve as the bottom layer. Next, PMMA/PVA double layers are spun on a clean surface. On top of the film, we mechanically exfoliated MoTe2 monolayer crystal by repeatedly peeling. Using adherent tape to support the film, we affixed it onto the manipulator under the optical microscope with the MoTe2 monolayer facing down. In the following step, the MoTe2 monolayer is covered to the target MoS2 monolayer precisely. Then, the organic film was removed after rinsing in acetone overnight. Finally, metallic contacts were fabricated by standard electron beam lithography, thermal deposition of gold/chromium (50 nm/10 nm), and lift-off processes. Raman and PL Measurements. Raman and PL measurements were performed using a Nanofinder 30 (TII Tokyo Instruments, Inc.). Raman and PL spectra were measured at room temperature with a 532 nm excitation laser (2 mW). Raman spectra were calibrated by the Raman shift of single crystal silicon at 520.4 cm−1. Raman measurements were conducted using an 1800 g/mm grating to disperse the signal and generate a spectral resolution of less than 1 cm−1 (a 300 g/mm grating was used for PL measurements). Lowfrequency Raman measurements were performed using Tri-Vista Triple Raman Spectroscopy (PI) with a liquid-nitrogen cooled charge coupled device detector (CCD). The excitation laser wavelength was 514.5 nm (2 mW). KPFM Measurements. The KPFM measurements were performed using Veeco/DI Multimode Scanning Probe Microscopy with Nanoscope IV controller. A 680 nm laser was used during the KPFM analysis. Optoelectronic Measurement. The electrical measurements were performed using Lake Shore TTPX probe station and Keithley 4200 semiconductor characterization system. The excitation laser wavelength was 637 nm (power, 10 μW; size, 800 nm), 1550 nm (power, 0.5 mW; size, 2 μm), and 2000 nm (power, 0.5 mW; size, 2 μm).

The above discussions clearly demonstrate the formation of type-II band alignment in the MoTe2/MoS2 monolayer heterostructure. The strong interlayer coupling in such heterostructure can readily enable us to design an infrared photodetector device with wider photoelectrical response by exploiting interlayer excitation in MoTe2/MoS2 vdW heterostructures (Figure 5a). To probe the infrared sensitivity of the device, we have measured the photocurrent from the MoTe2/ MoS2 vdW heterostructures device illuminated under an infrared laser with a wavelength at 1550 or 2000 nm. Comparing the output characteristics (Ids−Vds) measured under dark and infared illumination at different wavelengths, it is evident that a clear photoresponse is observed at 1550 nm, while essentially there is no response at 2000 nm (Figure 5b and Figures S7−S9). However, for the individual device made of a pure MoS2 or MoTe2 monolayer, no photoresponse was observed at 1550 nm, although there were obvious responses at 637 nm (Figures S6 and S8). These photoelectric measurements clearly demonstrate that interlayer transition can allow infrared excitation and infrared detection as determined by the band offset in the type-II heterostructures, instead of the intrinsic band gap limit of the constituting materials.8,13,14,35

CONCLUSION We have demonstrated, for the first time, the interlayer infrared excitation in the type-II MoTe2/MoS2 vdW monolayer heterostructures. The interlayer interactions between MoTe2 and MoS2 monolayer lead to the formation of type-II heterostructures, which is verified with a low-frequency Raman vibration mode. The type-II band alignment and interlayer charge separation in the MoTe2/MoS2 vdW heterostructures are confirmed with KPFM. Finally, infrared (1550 nm) photoresponse between the two stacked monolayers has been observed. While the dynamics and sensitivity of infrared photoresponse in the vdW heterostructures need more exploration, we believe that our studies will be valuable for better understanding of the interlayer coupling and for fabrication flexible and transparent infrared optoelectronic devices.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00980. Additional experimental and computational results (PDF)

METHODS CVD Synthesis of MoS2 Monolayers. A two-zone CVD horizontal quartz tube furnace is used to grow MoS2 monolayers. Sulfur powder (Sigma-Aldrich) was placed in the fist zone with a temperature of 250 °C. The substrates (285 nm-SiO2/Si) were put on top of MoO3 powder (Sigma-Aldrich), and placed in the second zone

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. E

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Notes

The authors declare no competing financial interest.

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