Scalable Production of a Few-Layer MoS2/WS2 Vertical Heterojunction Array and Its Application for Photodetectors Yunzhou Xue,†,‡,∥ Yupeng Zhang,‡,∥ Yan Liu,†,∥ Hongtao Liu,§ Jingchao Song,‡ Joice Sophia,† Jingying Liu,‡ Zaiquan Xu,‡ Qingyang Xu,† Ziyu Wang,‡ Jialu Zheng,‡ Yunqi Liu,§ Shaojuan Li,*,† and Qiaoliang Bao*,†,‡ †
Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, People’s Republic of China ‡ Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia § Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *
ABSTRACT: Vertical heterojunctions of two two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention recently. A variety of heterojunctions can be constructed by stacking different TMDs to form fundamental building blocks in different optoelectronic devices such as photodetectors, solar cells, and light-emitting diodes. However, these applications are significantly hampered by the challenges of large-scale production of van der Waals stacks of atomically thin materials. Here, we demonstrate scalable production of periodic patterns of few-layer WS2, MoS2, and their vertical heterojunction arrays by a thermal reduction sulfurization process. In this method, a two-step chemical vapor deposition approach was developed to effectively prevent the phase mixing of TMDs in an unpredicted manner, thus affording a well-defined interface between WS2 and MoS2 in the vertical dimension. As a result, large-scale, periodic arrays of few-layer WS2, MoS2, and their vertical heterojunctions can be produced with desired size and density. Photodetectors based on the as-produced MoS2/WS2 vertical heterojunction arrays were fabricated, and a high photoresponsivity of 2.3 A·W−1 at an excitation wavelength of 450 nm was demonstrated. Flexible photodetector devices using MoS2/WS2 heterojunction arrays were also demonstrated with reasonable signal/noise ratio. The approach in this work is also applicable to other TMD materials and can open up the possibilities of producing a variety of vertical van der Waals heterojunctions in a large scale toward optoelectronic applications. KEYWORDS: molybdenum disulfide, tungsten disulfide, vertical heterojunction, photodetector, flexible device photovoltaic cells, 10 and flexible field-effect transistors (FETs).11 Vertical integration of layered materials formed by stacking up two 2D materials offers new opportunities to design novel electronic and photonic devices, which present a rich collection of physics and electronics. The combination of MoS2 and WS2 thin layers with other 2D materials has been employed in fabricating many kinds of 2D heterojunctions.12−15 Vertical FETs fabricated by stacking graphene and MoS2 have shown a
T
he zero band gap in graphene and its limitation for digital electronics have stimulated the efforts to explore post-graphene two-dimensional (2D) semiconducting materials. Recently, atomically thin layered transition metal dichalcogenides (TMDs) such as MoS2 and WS2 have received increasing attention due to their 2D structure with sizable band gap in the visible to near-infrared spectral range.1,2 These materials have many excellent characteristics, such as wonderful flexibility,3 moderate carrier mobility,4 and layer-dependent electronic and optical properties,5,6 making them the most promising candidates for future optoelectronics. Moreover, TMD materials show intriguing potential for widespread applications such as photodetectors,7,8 light-emitting diodes,9 © 2015 American Chemical Society
Received: September 5, 2015 Accepted: December 8, 2015 Published: December 8, 2015 573
DOI: 10.1021/acsnano.5b05596 ACS Nano 2016, 10, 573−580
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Figure 1. Schematic diagram of MoS2/WS2 vertical heterojunction preparation and corresponding devices fabrication. (a) SiO2/Si substrate; (b) photoresist was exposed to form square holes on the SiO2/Si substrate; (c) WO3 square arrays (5 nm thick) were deposited on the SiO2/Si substrate; (d) WO3 square arrays (5 nm) subjected to reduced sulfurization to form WS2; (e) photoresist was exposed to form rectangle holes on part of square WS2 on the SiO2/Si substrate; (f) 2 nm Mo rectangle arrays were deposited on part of the square WS2 on the SiO2/Si substrate; (g) 2 nm Mo rectangle arrays subjected to reduced sulfurization to form MoS2; (h) photoresist was exposed to form pair holes on part of the square WS2 and rectangle MoS2, respectively; (i) devices were fabricated by depositing Au electrodes on WS2 and MoS2 heterojunction arrays.
Figure 2. (a−c) Optical microscope images of WS2, MoS2, and their vertical heterojunction arrays, respectively. (d−f) Similar to that of (a−c) but having high-resolution optical microscope images. (g−i) SEM images of WS2, MoS2, and their vertical heterojunction, respectively.
high current density of 5000 A·cm−2 and on−off ratio >1000.13 A graphene and MoS2 vertical heterojunction was used to realize phototransistors with ultrahigh photoresponsivity greater than 10 7 A·W −1 .14 WS 2 was used in the vertical heterojunction stacked with graphene, as well, and its internal quantum and maximum external quantum efficiency could reach up to 85 and 55%, respectively.15 Besides, MoS2 and WS2
were also combined with h-BN to explore their own intrinsic properties.16,17 As the first step to realize these applications, TMDs must be synthesized in large scale with desired architectures, thus the controllable and scalable production is of great importance for their applications. Until now, mechanical exfoliation, 18 chemical vapor deposition (CVD),19−21 and solution-phase processes22 have been 574
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Figure 3. (a) AFM image of MoS2/WS2 vertical heterojunction. The data were collected from the intersection between WS2 and MoS2 sheets, as indicated in the black box marked in Figure 2f. (b) Zoom-in AFM image of the WS2 region indicated by the white box in (a). (c) Zoom-in AFM image of the MoS2 region indicated by the black box in (a). (d−f) XPS spectra of the MoS2/WS2 vertical heterojunction, WS2, and MoS2, respectively. The insets of (e,f) show higher-resolution spectra of W, S, and Mo peaks. The small peak at ∼236 eV (marked by black arrow) suggests that a small amount of Mo atoms was oxidized to MoO3.
680 °C for 2 h under ambient pressure. This approach can effectively prevent the mixture of MoS2 with WS2 in a conventional CVD reaction, thus affording a well-defined interface between WS2 and MoS2 in the vertical dimension. Using aligned photolithography, source and drain electrodes can be deposited on WS2 and MoS2, respectively, to form a photodiode. Figure 2a−i shows the typical optical microscopy and scanning electron microscopy (SEM) images of square WS2 sheets, rectangle MoS2 sheets, and vertical MoS2/WS2 heterojunction arrays, which were uniformly patterned on SiO2/Si substrates. The size of WS2 sheets and MoS2 sheets were designed to be 50 × 50 μm2 and 25 × 60 μm2, respectively. From the high-resolution optical microscopy (Figure 2d−f) and SEM images (Figure 2g−i), we can see that the WS2 and MoS2 sheets are well-defined with very uniform contrast, irrespective of a single WS2 or MoS2 sheet or vertical MoS2/WS2 heterojunction, which is attributed to the homogeneous thickness of WS2 and MoS2 sheets. Furthermore, optical microscopy and SEM images of Figure 2f,i indicate that the overlap part of the WS2 sheet and MoS2 sheet are wellseparated, where the MoS2 sheet stacks on the top of the WS2 sheet. The reason that the overlap part did not mix together to form a lateral hybrid film of WS2−MoS2 is due to the two-step growth process we have employed. The WS2 crystals were formed during the reduction sulfurization process of WO3 at higher temperature (800 °C) in the first step, whereas the MoS2 crystals were formed during the reduction sulfurization process of Mo at lower temperature (680 °C) in the second step. Consequently, unlike the conventional CVD approach,21 the infusion of Mo into WO3 to form a lateral WS2−MoS2 mixture is effectively prevented. Atomic force microscopy (AFM) was further used to probe the detailed surface morphology and the thicknesses of WS2, MoS2 sheets, and their vertical heterojunctions. Figure 3a shows the AFM topography of the intersection of WS2 and MoS2 sheets, as indicated in the black box marked in Figure 2f. The
developed to produce TMD films, single crystal flakes, and solution sheet platelets. The mechanical exfoliation plus aligned transfer is the first and most widely used method for studying these van der Waals heterostructures, but the successful stacking is more by chance with low yield and low efficiency. Certain efforts have been made to produce TMD heterostructures by CVD,21 but normally lateral rather than vertical structures were produced as the routine methods cannot prevent the phase mixing in the vertical dimension. It is noteworthy that none of these approaches are suitable to produce a MoS2/WS2 heterojunction in a large scale. Hence, scalable production of TMD heterojunctions with wellcontrolled architectures is still a great challenge and difficult to realize by means of existing exfoliation or growth processes. In the present work, we report a new controllable way to produce a few-layer MoS2/WS2 vertical heterojunction in a large scale via sulfurization of patterned WO3 and Mo sheets with an ambient pressure thermal reduction process. The coverage, size, and shape of WS2 and MoS2 sheets can be controlled as desired, and their thicknesses were determined by the thicknesses of the predeposited WO3 and Mo sheets. Moreover, photodetector device arrays can be fabricated based on those MoS2/WS2 vertical heterojunctions and revealed the reliable photoresponsivity characteristics both on rigid and flexible substrates.
RESULTS AND DISCUSSION As schematically shown in Figure 1, the square WO3 thin sheet arrays were deposited onto the SiO2/Si substrates by thermal evaporation, followed by reduction sulfurization at 800 °C for 1 h under ambient pressure to form atomic thin WS2 sheets. With the help of a patterned photoresist rectangle, Mo sheets were deposited on part of the above obtained WS2 sheets by electron beam evaporation. Finally, in order to obtain the desired vertical MoS2/WS2 heterojunctions, the stacked Mo/WS2 sheets were kept in the furnace for reduction sulfurization at 575
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Figure 4. (a) Raman spectra of WS2, MoS2, and their vertical heterojunction. (b) Optical microscope images of the MoS2/WS2 vertical heterojunction device. (c,d) Raman maps of integrated intensities of peaks at 351 and 382 cm−1. Data were collected from the black box in panel b, with a scanning area of 80 × 80 μm2. (e) Single PL spectra of MoS2, WS2, and their vertical structure. (f) PL mapping of MoS2/WS2 vertical heterojunction, collected from the white box in panel b, with a scanning area of 21 × 21 μm2.
atoms are not oxidized.27 Figure 3f displays the characteristic S 2p3/2, S 2p1/2, Mo 3d5/2, and 3d3/2 peaks of MoS2 at 162.7, 164.0, 229.9, and 233.1 eV, respectively, which are consistent with the previous reports for MoS2.28 Additionally, the stoichiometric ratio for S and Mo almost reached 2:1. Similarly, there is no prominent peak between 168 and 170 eV in the XPS spectrum of MoS2 (Figure 3f), which also proves the absence of oxidized sulfur atoms. However, a small peak located at ∼236 eV (marked by black arrow) corresponding to the Mo6+ 3d3/2 orbital was observed in the spectrum of Mo, suggesting that a small amount of Mo atoms was oxidized to MoO3.28 To evaluate the vibrational properties and the thicknesses of our WS2, MoS2, and their heterojunctions, Raman spectroscopy with the laser excitation wavelength at 532 nm was performed. Figure 4a shows the Raman spectra for the WS2, MoS2, and their heterojunction. As reported before, there are four Ramanactive modes (1E2g, A1g, E1g, and 2E2g) for the bulk WS2 and MoS2 crystals.29 However, generally only 1E2g and A1g modes were observable in Raman spectroscopy, due to the selection rules for scattering geometry (E1g) or limited rejection of the Rayleigh scattering radiation (2E2g).30 The 1E2g is an in-plane optical mode, corresponding to the vibrational motion of Mo and S atoms in the x−y plane, whereas the A1g mode is an outof-plane vibration, corresponding to the vibrational motion of two S atoms along the z-axis of the unit cell.28 With the increase of the number of layers, the frequency of the 1E2g and A1g modes decreases and increases, respectively. This behavior is associated with the stronger dielectric screening of long-range Coulomb interactions between the effective charges in thicker samples. The contrary variation of the frequencies of these two Raman modes versus layer thickness renders the difference in their frequencies (Δf), which is a particularly effective thickness indicator.31 For our MoS2, the measured Δf is 23.2 cm−1 between 1E2g and A1g peaks, which indicates that the thickness of the rectangle MoS2 sheets is two to three atomic layers,28 in good agreement with previous AFM results. In the case of WS2, the 1E2g and A1g modes appear at 351 and 417 cm−1,
AFM height profile indicates that the thickness of MoS2/WS2 heterojunction is ∼6.1 nm. To determine the thicknesses of WS2 and MoS2 sheets, we have also measured the pure WS2 and MoS2 regions, respectively. The white and black boxes in Figure 3a correspond to WS2 and MoS2, respectively, and the corresponding zoom-in AFM images are shown in Figure 3b,c. The thicknesses of WS2 and MoS2 are measured to be ∼3.9 and ∼2.1 nm, suggesting five layers of WS2 and three layers of MoS2. X-ray photoelectron spectroscopy (XPS) was performed to analyze the bonding characteristics as well as the chemical composition of the MoS2/WS2 heterojunction. To obtain the corresponding XPS data, we used large-area MoS2, WS2, and MoS2/WS2 heterojunction films as the samples, prepared at the same conditions as with the MoS2/WS2 heterojunction arrays mentioned above. The corresponding results are shown in Figure 3d−f. For the convenience of analysis, the standard C 1s peak of 284.8 eV was used as a reference for correcting any effects of charge accumulation on the samples. As we expected, the XPS spectrum (Figure 3d) clearly resolves W, S, and Mo element peaks in the range of 0−250 eV. The obtained binding energies of W 4f, Mo 3d, and S 2p orbitals were located at 33.3, 229.9, and 162.7 eV, respectively, which are in good agreement with previously reported positions for WS2 and MoS2.23−25 To analyze the binding energies of WS2 and MoS2 in detail, core level measurements were conducted, as shown in Figure 3e,f. The W peaks shown in the left inset in Figure 3e are located at 33.3, 35.5, and 39.0 eV, corresponding to W 4f7/2, W 4f5/2, and W 5p3/2, respectively.26 The energy of these peaks indicates a W valence of +4 according to previous reports.26 Moreover, it is noteworthy that the peak at ∼37.9 eV corresponding to the WO3 orbital is absent, indicating that almost all of the deposited WO3 has been converted to WS2.27 The right inset of Figure 3e shows the S peak located at 162.1 and 163.3 eV, which corresponds to S 2 p1/2 and S 2p3/2 orbitals of divalent sulfide ions in WS2. Analogously, there is no obvious peak observed between 168 and 170 eV, indicating that the sulfur 576
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Figure 5. (a,d) TEM images of WS2 and MoS2, respectively. The insets in panels a and d show the folded edges of the films with 4−5 and 3 atomic layers, respectively. (b,e) HRTEM images of WS2 and MoS2, respectively. (c,f) SAED patterns of WS2 and MoS2, respectively.
that at 675 nm suggests that the synthesized MoS2 sheets are a few atomic layers.35 For the MoS2/WS2 heterojunction, the dominant peak blue shifts to 623 nm due to the band bending and exerts an influence on each other.19 Moreover, the PL intensity of the MoS2/WS2 heterojunction is much lower than that of the individual WS2 and MoS2 sheets. PL mapping of peak intensity of the MoS2/WS2 heterojunction is shown in Figure 4f, from which we can observe that the light emission of our WS2 and MoS2 sheets are uniform in each subregion. Similar to the observation in PL spectra, the intensity of the overlapping region is still lower than the non-overlapping regions, clearly demonstrating the effective charge carrier transfer between WS2 and MoS2 sheets. The WS2 and MoS2 samples were transferred to copper grids to investigate the microstructure, thickness, as well as film quality using high-resolution transmission electron microscopy (HRTEM). Figure 5a,d shows the typical low-magnification TEM images showing the morphology of the atomically thin WS2 and MoS2 films. Some wrinkles observed on the edge of the film were caused by film folding during the transfer process. The folded edges in the inset of Figure 5a,d reveal that the WS2 and MoS2 are 4−5 and 3 layers, respectively, consistent with previous AFM, Raman, and PL measurements. The HRTEM images in Figure 5b,e show that the samples exhibit a fairly high crystalline quality. The selected area electron diffraction (SAED) patterns of WS2 and MoS2 in Figure 5c,f reveal diffraction points arranged in multiple hexagons, indicating that both of them are polycrystalline films.36 Moreover, HRTEM images (Figure S3a−c) of the folded edge of the heterostructure film clearly show three layers of MoS2 plus five layers of WS2, with slightly different contrast and a clear interface. The numbers of layers agree well with the AFM results; that is, the thicknesses of WS2 and MoS2 are ∼3.9 nm (five layers) and ∼2.1 nm (three layers), respectively. In order to further evaluate the sample quality in terms of optoelectrical properties, FET arrays based MoS 2 /WS 2 heterojunctions were fabricated on the SiO2/Si substrate with gold as the source/drain electrodes and heavily doped Si as the
respectively, indicating that the layer number of the square of the WS2 sheet is around 4−5, consistent with earlier reports.32 However, in the case of the MoS2/WS2 heterojunction, the positions of 1E2g and A1g phonon modes do not change with respect to those of individual sheets. This suggests that WS2 and MoS2 do not affect each other’s long-range Coulomb interactions between the effective charges.33 Raman mapping images of WS2, MoS2 (Figure S2a,b), and MoS2/WS2 heterojunction (Figure 4c,d, with collected data from the black box in Figure 4b) sheets were also acquired by their peak intensities. It is generally observed that both the overlap and non-overlap regions are uniform. However, the Raman intensity of the overlapping region is much lower than that of the non-overlapping region. In the case of WS2 (Figure 4d), the top-layer MoS2 may prevent effective collection of the Raman scattering signal from WS2. However, it is interesting to find that the Raman signal from MoS2 is also weakened when it stays on top of WS2 (Figure 4c), indicating that the underneath WS2 still can affect the Raman intensity of the top MoS2. As reported before, WS2 and MoS2 are expected to undergo a transition from an indirect band gap semiconductor in multilayer structures to a direct band gap semiconductor in monolayer structures. Hence, photoluminescence (PL) can be employed as an alternative tool to identify the band gap for both of them. Few-layer WS2 and MoS2 also give PL signatures, and the peak positions exhibit a gradual but distinct red shift with increasing number of layers. In order to study the PL of patterned few-layer WS2 and MoS2 sheets, a green light laser at the wavelength of 532 nm was used to excite a single spectrum at specific locations as well as scan point-by-point over the heterojunction. Figure 4e shows the PL spectra of WS2, MoS2, and their heterojunction. In WS2, the peak position is located at 628 nm, corresponding to a 4−5 atomic layer film.34 In MoS2, two peaks at 625 nm (B1 excitation, originating from the valence band splitting caused by strong spin−orbit coupling) and 675 nm (A1 excitation, derived from direct band gap transition) are observed (Figure 4e and Figure S2e). The fact that the peak intensity located at 625 nm is much stronger than 577
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Figure 6. (a) MoS2/WS2 vertical heterojunction device arrays on the SiO2/Si substrate. (b) Schematic of a MoS2/WS2 vertical heterojunction phototransistor. (c) Current−voltage characteristics of a transistor based on the MoS2/WS2 vertical heterojunction, measured without illumination. The inset in (c) shows the band alignment for few-layer WS2 and MoS2 flakes. (d) Time-dependent photocurrent of the MoS2/ WS2 vertical heterostructure with different incident power on the SiO2/Si substrate. (e) Dependence of photocurrent and photoresponsivity on incident light power (at 405 nm); the green and black dots correspond to original data. (f) Time dependence of photocurrent based on the WS2/MoS2 vertical heterostructure during the laser switching on/off under the positive source−drain voltage Vsd from 1 to 4 V. (g) Photo image of the MoS2/WS2 vertical heterojunction device arrays on the PDMS substrate. (h) Optical microscope image of the single MoS2/WS2 vertical heterojunction device on the PDMS substrate. (i) Time-dependent photocurrent of the MoS2/WS2 vertical heterojunction with different incident power on the PMDS substrate.
increasing power. This may correlate to the decrease of unoccupied states in the conduction bands of MoS2 and WS2 as the power intensity increases.37 In addition, the trap states in these two 2D materials or between them and the SiO2 substrate may also contribute to a nonlinear trend, similar to the situation of the single-layer MoS2 phototransistor.7 The highest photoresponsivity of the MoS2/WS2 heterojunction can reach up to 2.3 A·W−1. In addition, a time-dependent photocurrent at different source−drain voltage is shown in Figure 6f. With the increase in Vsd, the photocurrent can be significantly increased. The photocurrent can be switched ON and OFF quickly and repetitively, while the light source is turned on and off at different source−drain voltages, suggesting a very good stability and repeatability. Moreover, an interesting phenomenon is that the MoS2/WS2 heterojunction can also perform as a self-driven photodetector without applying a source−drain bias, as shown in Figure S5. The photocurrent can change repetitively and rapidly with light is turned on and off, which is due to the wellmatched band structure in the MoS2/WS2 heterojunction (see inset of Figure 6c). The built-in electric field at the interface of the MoS2/WS2 heterojunction could effectively separate and extract the photogenerated carriers.38 More importantly, the as-produced 2D heterojunctions can be transferred onto soft substrates to fabricate flexible or wearable devices in a large scale. Figure 6g shows an array of MoS2/WS2 heterojunction devices on the bending PDMS substrate. The optical image of a single two-probe heterojunction device is shown in Figure 6h, in which two Au
back gate. Figure 6a shows the optical image of MoS2/WS2 FET arrays, which actually spreads out over an area of 25 mm2. Figure 6b shows the schematic of a phototransistor device based on the MoS2/WS2 vertical heterojunction. Figure 6c shows the I−V characteristics of MoS2/WS2 heterojunction and presents obvious rectifying behavior, which is due to the formation of a junction between these two 2D materials.18 As shown in the inset of Figure 6c, electrons will migrate from WS2 to MoS2 to keep balance between electron potential when they form the vertical heterojunction, leading to a built-in potential at the interface. With the reduction in the built-in potential, electrons can easily shift from WS2 to MoS2 when a positive bias is applied, which results in the ON state. On the other hand, when a negative bias is applied, the barrier height increases and electrons cannot easily shift from MoS2 to WS2, resulting in the much smaller reverse current, as shown in the I−V curve in Figure 6c. To investigate the photoresponse of the heterojunction FET devices, a series of photoelectric measurements were performed using a blue laser (450 nm) at room temperature. Figure 6d shows the device response to pulse light at different optical pumping power, from which we observed that the device can be effectively switched ON and OFF while the light source is turned on and off. The amplitude of the electrical signal is modulated by different light powers. Figure 6e shows the photoresponsivity and photocurrent as a function of the light power. It is found that the photocurrent increases nonlinearly, whereas the photoresponsivity decreases exponentially with 578
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AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions ∥
Y.X., Y.Z., and Y.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We acknowledge the support from the National Natural Science Foundation of China (Nos. 51222208, 51290273, and 91433107), the youth 973 program (2015CB932700), China Postdoctoral Science Foundation (2014M550303, 2014M551654), the Jiangsu Province Postdoctoral Science Foundation (No. 1301020A), the Natural Science Foundation of Jiangsu Province (No. BK20130328), the Doctoral Fund of Ministry of Education of China (Grant No. 20123201120026), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Collaborative Innovation Center of Suzhou Nano Science and Technology, ARC DECRA (DE120101569), and Discovery Project (DP140101501). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).
CONCLUSION In summary, patterned WS2, MoS2, and their vertical heterojunction arrays were produced by a thermal reduction sulfurization process. We can controllably produce periodic arrays of the WS2, MoS2, and their vertical heterojunction sheet, where the size, density, and geometry of 2D sheets can be adjusted as desired. Moreover, a two-step approach was developed to effectively prevent the mixture of MoS2 with WS2, thus affording a well-defined interface between WS2 and MoS2 in the vertical dimension. The photoelectric measurements confirm that the MoS2/WS2 vertical heterojunction exhibits photoresponsivity of 2.3 A·W−1. Due to the built-in electric field at the interface of the heterojunction, it can be used for a self-driven photodetector without applying a source− drain bias. Flexible MoS2/WS2 heterojunction device arrays were also fabricated, and reasonable photodetection performance was demonstrated. The approach demonstrated here is also transplantable for making a variety of TMD heterojunctions and may open up new possibilities toward optoelectronic applications.
REFERENCES (1) Late, D. J.; Liu, B.; Matte, H. S. S. R.; Dravid, V. P.; Rao, C. N. R. Hysteresis in Single-Layer MoS2 Field Effect Transistors. ACS Nano 2012, 6, 5635−5641. (2) Xu, Z.; Zhang, Y.; Lin, S.; Zheng, C.; Zhong, Y.; Xia, X.; Li, Z.; Sophia, P. J.; Fuhrer, M. S.; Cheng, Y.; et al. Synthesis and Transfer of Large-Area Monolayer WS2 Crystals: Toward the Recyclable Use of Sapphire Substrates. ACS Nano 2015, 9, 6178−6187. (3) Pu, J.; Yomogida, Y.; Liu, K.-K.; Li, L. J.; Iwasa, Y.; Takenobu, T. Highly Flexible MoS2 Thin-Film Transistors with Ion Gel Dielectrics. Nano Lett. 2012, 12, 4013−4017. (4) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (5) Mak, K. F.; He, K.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in Monolayer MoS2. Nat. Mater. 2013, 12, 207−211. (6) Zeng, H.; Dai, J.; Yao, W.; Xiao, D.; Cui, X. Valley Polarization in MoS2 Monolayers by Optical Pumping. Nat. Nanotechnol. 2012, 7, 490−493. (7) Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497−501. (8) Chang, Y.-H.; Han, Y.; Pu, J.; Chang, J. K.; Hsu, W. T.; Huang, J. K.; Hsu, C. L.; Chiu, M. H.; Takenobu, T.; Li, H.; et al. Monolayer MoSe2 Grown by Chemical Vapor Deposition for Fast Photodetection. ACS Nano 2014, 8, 8582−8590. (9) Britnell, L.; Ribeiro, R. M.; Eckmann, A.; Jalil, R.; Belle, B. D.; Mishchenko, A.; Kim, Y. J.; Gorbachev, R. V.; Georgiou, T.; Morozov, S. V. Strong Light Matter Interactions in Heterostructures of Atomically Thin Films. Science 2013, 340, 1311−1314. (10) Bernardi, M.; Palummo, M.; Grossman, J. C. Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using Two-Dimensional Monolayer Materials. Nano Lett. 2013, 13, 3664− 3670. (11) Kim, S.; Konar, A.; Hwang, W.-S.; Lee, J. H.; Lee, J.; Yang, J.; Jung, C.; Kim, H.; Yoo, J. B.; Choi, J. Y. High-Mobility and Low-Power Thin-Film Transistors Based on Multilayer MoS2 Crystals. Nat. Commun. 2012, 3, 1011.
METHODS A two-step chemical vapor deposition approach was developed to produce MoS2/WS2 heterojunctions in a large scale. In step one, the photoresist was first spin-coated on the SiO2/Si substrate and exposed to form the array of periodic square holes. Subsequently, 5 nm WO3 was thermally deposited onto the corresponding square holes. Then the SiO2/Si substrate with WO3 was placed downstream and sulfur powder upstream of the quartz tube. Both of them were placed out of the heating area of the furnace with a distance of 5 cm; Ar was introduced as the carrier gas (flow rate = 250 sccm). After the furnace temperature reached 750 °C, the sulfur powder was heated to 180 °C using the heating belt. When the furnace temperature reached 800 °C, the SiO2/Si substrates with WO3 were moved into the center of the furnace and maintained for 1 h to form WS2. In step two, the photoresist was spin-coated on the SiO2/Si substrate with a square WS2 array and exposed again to form the periodic rectangle hole array on part of the WS2 sheets. Two nanometer Mo was deposited by electron beam evaporation to form the periodic rectangle Mo arrays. Similar to the process of producing WS2 mentioned above, the furnace temperature was kept at 680 °C and the growth time was 2 h. The devices were fabricated by a photolithography technique, and a typical device fabrication process involves UV lithography to define the device pattern and electron beam evaporation to deposit the source and drain electrodes (i.e., 60 nm Au on top of 5 nm Ti).
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05596. Additional figures (PDF) 579
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ACS Nano
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DOI: 10.1021/acsnano.5b05596 ACS Nano 2016, 10, 573−580
