Universal Transfer and Stacking of Chemical Vapor Deposition Grown

May 9, 2016 - Chemical vapor deposition (CVD) has shown great potential in synthesizing various high-quality two-dimensional (2D) transition metal dic...
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Universal Transfer and Stacking of Chemical Vapor Deposition Grown Two-Dimensional Atomic Layers with Water-Soluble Polymer Mediator Zhixing Lu, Lifei Sun, Guanchen Xu, Jingying Zheng, Qi Zhang, Jingyi Wang, and Liying Jiao* Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Chemical vapor deposition (CVD) has shown great potential in synthesizing various high-quality two-dimensional (2D) transition metal dichalcogenides (TMDCs). However, the nondestruction transfer of these CVD-grown 2D TMDCs at a high yield remains a key challenge for applying these emerging materials in various aspects. To address this challenge, we designed a water-soluble transfer mediator consisting of two polymers, polyvinylpyrrolidone (PVP) and poly(vinyl alcohol) (PVA), which can form strong interactions with CVD-grown 2D TMDCs for the nondestruction transfer of these materials. With this mediator, we realized the physical transfer of CVD-grown MoS2 flakes and several other 2D TMDCs, including 2D alloys and heterostructures to a wide range of substrates at a high yield of >90% with well-retained properties as evidenced by various microscopic, spectroscopic, and electrical measurements. Field-effect transistors (FETs) made on thustransferred CVD-grown MoS2 monolayers exhibited obviously higher mobility than those transferred by chemical method. We also constructed several artificial 2D crystals showing very strong interlayer coupling by the multiple transfer of CVD-grown 2D TMDCs monolayers with this approach. This transfer approach will make versatile CVD-grown 2D materials and their artificial stacks with pristine qualities easily accessible for both fundamental studies and practical applications. KEYWORDS: transfer, two-dimensional, transition metal dichalcogenides, molybdenum disulfide, water-soluble polymer

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the transfer process, can degrade the properties of these inorganic TMDCs as suggested by the decreased carrier mobility in the transferred MoS2 (3−11.4 cm2 V−1 s−1 for asgrown MoS2 monolayers9,10,14 and 0.02−0.7 cm2 V−1 s−1 for the PMMA-transferred samples12,15). So physical transfer approaches without chemical etchants are more desirable for transferring these TMDCs. Physical transfer of mechanically exfoliated 2D TMDCs has been achieved with PDMS stamps16 or by the penetration of water into the sample−substrate interface,17 but the strong sample−substrate interactions introduced by the high-temperature growth process may hinder the applications of these approaches in transferring CVD-grown 2D TMDCs. Recently, transfer of CVD-grown MoS2 films based on physical delamination from substrate assisted by surface-energy18 or ultrasonic bubbling19 has been reported, but the transferred MoS2 showed very low mobility of 0.03−0.04 cm2 V−1 s−1. Therefore, the nondestruction transfer of these

wo-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted considerable attention for their great potential in the fields of nanoelectronics and optoelectronics.1−4 Single-layer molybdenum disulfide (MoS2) with a direct band gap5 is a promising semiconducting material that goes beyond graphene for the next generation of electronics, especially flexible electronics.6−8 Great efforts have been made on the controlled synthesis of high quality 2D MoS2 and other 2D TMDCs by chemical vapor deposition (CVD), whereas the high growth temperature of 600−900 °C is only compatible with a few inorganic substrates, which greatly limited the applications of these materials.9,10 Therefore, similar to graphene, the reliable transfer of the as-grown 2D TMDCs to a wide range of substrates, including plastics, is an essential step for their practical applications in various aspects. Currently, the transfer of CVD-grown TMDCs simply follows the approach derived from graphene, carbon nanotubes, and other low-dimensional materials, which is based on chemically detaching samples from the growth substrates using poly(methyl methacrylate) (PMMA) thin film as a mediator.11−13 However, the etchants, such as KOH and HF solutions used in © XXXX American Chemical Society

Received: February 6, 2016 Accepted: May 9, 2016

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DOI: 10.1021/acsnano.6b00961 ACS Nano XXXX, XXX, XXX−XXX

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design of this mediator is the key to the successful transfer of CVD-grown 2D TMDCs. The mediator consists of two watersoluble polymers, polyvinylpyrrolidone (PVP) and poly(vinyl alcohol) (PVA). The bottom PVP film serves as the adhesive layer owing to its strong adhesion and good wetting capability which ensures conformal contacts to 2D TMDCs. To further improve the wettability of the bottom layer, we added the monomer N-vinylpyrrolidone (NVP) with similar surface energy to that of TMDCs (40 and 45 mJ/m2 for MoS2 and WS2, respectively)20 into the PVP solution. Meanwhile, the top PVA film is used as the supporting layer to reinforce the PVP film which lacks flexibility as result of the more rigid chain segments. The combination of these two polymers provides strong mediator−sample interaction and sufficient strength to the mediator, both of which are critical for the successful peeling of CVD-grown 2D TMDCs from the growth substrates. The transfer process is very simple and repeatable. Taking the transfer of CVD-grown MoS2 flakes as an example, PVP mixed with NVP in ethanol solution was first spin-coated onto MoS2 flakes supported on SiO2/Si substrates and baked to remove the solvents. Then, PVA aqueous solution was spincoated on the top of the first polymer layer, and these two water-soluble polymer integrated into a solid film after baking which can firmly carry the MoS2 flakes. Next, the bilayer polymer mediator together with MoS2 atomic layers can be easily peeled off from the substrate by tweezers or a roller as the result of the strong PVP−sample interactions. Afterward, the polymer film carrying the MoS2 samples can be transferred to various substrates. Finally, the polymer mediator can be easily dissolved in water, which is environmental friendly and effectively avoids the degradation of 2D MoS2 by chemical etching and doping to retain the pristine properties of MoS2. Furthermore, the concentration of PVP solution was adjustable to provide enough adhesive force for 2D materials with varied thickness (See details in Methods). The transfer results of CVD-grown MoS2 were evaluated with various microscopic, spectroscopic, and electrical characterizations. No obvious changes in morphologies or relative locations of the MoS2 flakes were observed in the optical image of the transferred sample (Figure 1b,c). After the transfer, no cracks or wrinkles were found in the MoS2 flakes, and the height and surface smoothness of the as-grown MoS2 flakes were preserved as shown in the atomic force microscope (AFM) images and section profile analysis (insets of Figure 1b,c, Figure S2). The yield of this transfer method was >90% by calculating the percentage of the perfectly transferred flakes in all as-grown MoS2 flakes (Figure S3). Next, we utilized transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) to evaluate the crystallinity of transferred MoS2 flakes in details. Figure 1d displayed the TEM image of a MoS2 monolayer flake transferred to holey carbon TEM grid with our transfer approach, showing very clean nature of the transferred flake. The collected SAED patterns at multiple locations of the transferred MoS2 monolayer were shown in Figure 1e,f. All the patterns we acquired displayed a hexagonal symmetry of 2H-MoS2 with the same orientations, indicating no degradation of the crystalline quality of the CVDgrown MoS2 was introduced by the transfer process. After confirming that our transfer approach can retain the morphology and structure of the as-grown MoS2 flakes, we turned to characterize the spectroscopic properties of the transferred MoS2 with Raman and photoluminescence (PL) spectroscopy. The Raman spectra taken on the CVD-grown

CVD-grown 2D TMDCs at a high yield remains a challenge for their applications in high-performance field effect devices. To address this challenge, we develop a physical transfer approach specifically for CVD-grown 2D TMDCs which well reserves their intrinsic properties by using a rationally designed watersoluble polymer film as the mediator.

RESULTS AND DISCUSSION This physical transfer approach is based on the mechanical peeling of the CVD-grown 2D TMDCs from the growth substrates with a bilayer water-soluble polymer thin film as the mediator, which is schematically illustrated in Figure 1a. The

Figure 1. Transfer of CVD-grown MoS2. (a) Schematic for the transfer of CVD grown MoS2 using the water-soluble bilayer polymer. (b and c) Optical and AFM images (insets) of CVD grown MoS2 before and after transfer, respectively. Insets scale bar: 2 μm. (d) TEM image of a transferred MoS2 flake on holey carbon grid. Inset: High-resolution TEM image of the flake, scale bar: 2 nm. (e and f) SAED patterns taken on the area of MoS2 flake marked with 1 and 2 in (d). B

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Figure 2. Characterizations of the transferred MoS2 monolayers. (a and b) Raman mapping images of CVD grown MoS2 before and after transfer, respectively. (c and d) Typical Raman and PL spectra of MoS2 before (black line) and after (red line) transfer, respectively. (e) Ids− Vgs curves for a FET fabricated on PVP + PVA transferred MoS2 flake probed at various biases (from bottom to top: 1, 10, 100, 500, and 1000 mV) in vacuum. Inset: optical image of the device. (f) Distributions of mobility and on/off current ratios of FETs made on MoS2 flakes transferred with PVP + PVA (red square) and PMMA (blue square) for comparison.

MoS2 showed two Raman peaks in the range of 300−500 cm−1 under 532 nm excitation, which precisely corresponded to the E12g and A1g modes of single-layer (1L) MoS2. The in-plane E12g mode upshifted by ∼1.7 cm−1 possibly due to the release of sample−substrate interaction during the transfer process,18,21 while the out-of-plane A1g mode remained unchanged (Figure 2c), indicating that no doping by polymer residue was induced to the transferred MoS2.22 Raman intensity mapping images of a MoS2 flake before and after transfer (Figure 2a,b) showed identical contrast and the Raman intensity was uniform across the transferred flake, further confirming the nondestruction transfer of the MoS2 flake. The PL spectra acquired on the same flake displayed identical band gap of ∼1.80 eV for before and after transfer (Figure 2d), indicating that the band structure of the MoS2 was not affected by the transfer process. To investigate the electrical performance of the transferred MoS2 flakes, we fabricated back-gated field effect transistors (FETs) by electron beam lithography (EBL) and electron beam deposition. The transfer characteristics of a transferred MoS2 flake measured at room temperature in vacuum (∼10−6 mbar)

behaved as an n-type semiconductor with an on/off current ratio of ∼107 and the estimated mobility of this device was ∼15.4 cm2 V−1 s−1 (Figure 2e and Figure S4), which was comparable to as-grown MoS2 with similar device configurations.9,10,14 We also compared the performance of the FETs made on MoS2 transferred by our approach with those of transferred by PMMA, and the PVP + PVA transferred MoS2 exhibited obviously higher mobility (∼9.0 cm2 V−1 s−1 on average) than the PMMA-transferred ones (∼3.0 cm2 V−1 s−1 on average) (Figure 2f), suggesting that our water-soluble mediator highly protected the intrinsic properties of CVDgrown MoS2. Besides transferring CVD-grown MoS2 flakes from SiO2/Si substrate, this method can be applied for MoS2 flakes and continuous films grown on sapphire substrate as well (Figure S5). We further applied this approach to transfer other 2D TMDCs, such as WS2 monolayers,23 MoS2/WS2 alloys, and MoS2/WS2 vertical heterostructures,24 and all these materials can be reliably transferred (Figure S6a,b and Figure 3a−d). In addition, 2D atomic crystals with limited chemical stability like C

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individual monolayers as indicated by the PL mapping images (Figure 4b,f) due to lack of band renormalization between layers after transfer.29 After annealing in vacuum at 300 °C for 60 min, the PL intensities in all overlapped regions decreased drastically (Figure 4c,g and Figures S8−S10). For example, the PL intensity from WS2 in the overlapped region decreased by >250 times (Figure 4d), and the PL intensity from MoxWyS2 decreased by >150 times (Figure 4h). The reduced photoluminescence in the heterostructures was originated from the strong interlayer coupling and efficient interfacial charge transfer, which was related to the cleanliness of interface.30 Moreover, the quenching of PL in the overlapped region was much more dramatic than previously reported MoS2/WS2 heterostructures constructed by PMMA-mediated transfer (∼60 times)30 and direct growth (∼80 times),24,31 which indicated the cleaner interface of the heterostructures stacked by our approach. Therefore, this nondestruction transfer mediator holds the potential for building high quality functional 2D structures to further extend the family of 2D atomic crystals.

CONCLUSION In summary, we designed a bilayer mediator consisting of two water-soluble polymers, polyvinylpyrrolidone (PVP) and poly(vinyl alcohol) (PVA), specifically for the nondestruction transfer and stacking of CVD-grown 2D TMDCs. The strong mediator−sample interactions and the water solubility of the mediator enabled the physical transfer of CVD-grown MoS2 flakes and other 2D TMDCs to a large variety of substrates at high yield, which is significant to retain their intrinsic properties for various applications. We also constructed high quality artificial 2D crystals with very clean interface and strong interlayer coupling by layer-by-layer stacking with the watersoluble mediator. This nondestruction and universal transfer approach is highly desirable for extending the applications of CVD-grown 2D TMDCs to various aspects and opens up an alternative way for building 2D hybrid structures, which will make diverse 2D structures accessible for both fundamental studies and practical applications.

Figure 3. Transfer of different CVD-grown 2D materials onto various substrates. (a and b) Optical images of CVD-grown 1L MoxWyS2 before and after transfer, respectively. (c and d) Optical images of CVD-grown MoS2/WS2 heteorostructures before and after transfer, respectively. (e and f) Transfer of CVD-grown MoS2 flakes onto Au/Si substrate and PMMA thin film, respectively.

METHODS MoTe2 can also survive the transfer process (Figure S6c,d). Therefore, this approach is universal for the nondestruction transfer of various CVD-grown 2D TMDCs, including their alloys and heterostructures. Furthermore, this approach is tolerant with diverse target substrates as long as they are not water-soluble despite of the surface properties, including metals (Au), hydrophobic substrates (HOPG), plastics (PET), and even polymer thin films (PMMA) (Figure 3e,f and Figure S7), which enables the transfer of CVD-grown 2D TMDCs onto flexible substrates and even onto chips with integrated circuits for building complicated flexible electronic devices. Since this transfer approach is faithful and universal, we then utilized the water-soluble mediator to stack homo- and heterovertical bilayers with the monolayers of 2D atomic crystals as building blocks for preparing functional artificial 2D materials, which has been confirmed as an effective way for enriching the functionalities of the 2D TMDCs.25−28 For example, we built the vertical heterostructures of WS2/MoS2, MoxWyS2/MoS2, and MoS2/MoS2 bilayers by the sequential transfer as shown in Figure 4a,e and Figures S8−S10. The stacking orientations between the two layers can be well-controlled with an aligner under optical microscope. The as-stacked structures behaved as

Synthesis of 2D TMDCs. The growth precursor of Mo oxide (MoOx) was prepared by electrochemical anodization.32 To synthesize MoS2 flakes, a piece of 300 nm SiO2/Si substrate covered with the Mo oxide precursors was placed at the center of CVD furnace of a standard 1-in. quartz tube furnace (Figure S1). A ceramic boat with sulfur powder was placed at the upstream side of the tube furnace. After the system was purged with Ar for 20 min, the furnace was heated to 800 °C at a rate of 20 °C min−1 with 100 sccm Ar. When the temperature of furnace reached 650 °C, sulfur was heated by a heating belt with an individual temperature controller at ∼180 °C. And then the furnace was cooled naturally after staying at 800 °C for another 10 min. The heating belt for sulfur was removed when the furnace was cooled to 400 °C. The above condition was used for the growth of monolayer MoS2 flakes with length of 10−50 μm. WS2 grown on sapphire was prepared by CVD according to the procedure described in ref 23. For synthesis of MoxWyS2, the growth precursor of MoxWyO3 was produced by hydrothermal method.33 The growth of MoxWyS2 was performed in the same CVD system as the growth of MoS2. After the system was purged with Ar for 20 min, the furnace was heated to 850 °C at a rate of 20 °C min−1 with 150 sccm Ar. When the temperature of furnace reached 820 °C, sulfur was heated at ∼180 °C by the same heating belt as mentioned above. And then the furnace was cooled naturally after staying at 850 °C for another 10 min. The heating belt for sulfur was removed when the furnace was cooled to 400 °C. MoS2/ D

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Figure 4. Stacking vertical heterobilayers with the monolayers of 2D atomic crystals. (a and e) Schematic, optical, and Raman mapping images of WS2/MoS2 and MoxWyS2/MoS2 bilayers built by the sequential transfer, respectively. Scale bars of Raman mapping images: 10, 5 μm. (b and c) PL mapping images of as-stacked WS2/MoS2 bilayers before and after annealing, respectively. Scale bars: 10 μm. (d) PL spectra of the as-stacked WS2/MoS2 heterostructures after annealing. (f and g) PL mapping images of as-stacked MoxWyS2/MoS2 bilayers before and after annealing, respectively. Scale bars: 5 μm. (h) PL spectra of the as-stacked MoxWyS2/MoS2 heterostructures after annealing. Device Fabrication and Measurement. The transferred MoS2 devices were fabricated using electron-beam lithography followed by electron-beam deposition of metal thin films. Ti/Au (5/50 nm) were used as the source and drain electrodes. The obtained FETs were measured with probe station under high vacuum (∼10−6 mbar) at room temperature using Agilent B1500 A. The carrier mobility of FETs was estimated using the equation below.34

WS2 vertical heterostructures were grown on 300 nm SiO2/Si substrate by CVD process that our group previously reported.24 Transfer of CVD-Grown MoS2 Flakes. First, 0.75 g of PVP (Alfa Aesar, average MW 58 000), 1.5 mL of NVP (J&K, 99.5%), and 0.75 mL of H2O were dissolved in ethanol to prepare 10 mL of solution. A few drops of this solution were spin-coated on the SiO2/Si substrate with MoS2 monolayer flakes at 2500 rpm for 1 min, followed by baking at 70 °C for 1 min to remove the solvents. To enhance the strength of the polymer mediator, 9 wt % PVA (Alfa Aesar, 98−99% hydrolyzed, high molecular weight) aqueous solution was spin-coated on the top of PVP film, and these two water-soluble polymer integrated into a solid film after baking in the same conditions as above. The thickness of the polymer mediator was ∼1.2 μm measured by AFM (Figure S11). Then, about 1 mm wide polymer strips were scratched by a blade at the edges of the substrate, and the polymer mediator carrying MoS2 flakes was peeled off slowly from the growth substrate by tweezers or a roller. Next, the polymer mediator was attached to the target substrate through the electrostatic force. Finally, the polymer mediator was removed by soaking in deionized water at 70−80 °C for ∼20 min, and the MoS2 monolayer flakes were left on the target substrate. For transferring multilayer (more than 10 layers) materials or TMDCs grown on sapphire substrates, the mass of PVP was increased to 1.5 g to provide sufficient adhesive force. Characterizations. The optical images were captured with Olympus BX 51M microscope. AFM images were taken with Bruker Dimension Icon in tapping mode. Raman and PL spectra were collected with Horiba-Jobin-Yvon Raman system under 532 nm laser excitation focused through a 100× objective lens with a power of 1 mW. Raman mapping images were performed with a step of 0.8 μm. The Si peak at 520.7 cm−1 was used for calibration in the data analysis. TEM images and SEAD patterns were acquired with JEOL 2100 at 200 kV.

μ=

dI L × ds W × (ε0εr /d) × Vds dVbg

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00961. CVD growth setup; more characterizations on the transferred MoS2 and stacked 2D materials; transfer of other 2D materials; transfer of CVD-grown MoS2 onto different substrates; characterizations of the polymer mediator (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS This work is supported by NSFC (Nos. 21322303, 51372134, and 21573125), National Program for Thousand Young Talents of China, Tsinghua University Initiative Scientific Research Program, and Tsinghua-Foxconn Nanotechnology Research Center Research Program. We acknowledge Prof. Yanfeng Zhang and Jianping Shi for providing CVD-grown WS2 samples. We also acknowledge Dake Hu for providing MoS2 continuous film grown on sapphire substrate. REFERENCES (1) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699−712. (2) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (3) Fiori, G.; Bonaccorso, F.; Iannaccone, G.; Palacios, T.; Neumaier, D.; Seabaugh, A.; Banerjee, S. K.; Colombo, L. Electronics Based on Two-Dimensional Materials. Nat. Nanotechnol. 2014, 9, 768−779. (4) Huang, X.; Zeng, Z.; Zhang, H. Metal Dichalcogenide Nanosheets: Preparation, Properties and Applications. Chem. Soc. Rev. 2013, 42, 1934−1946. (5) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (6) Lee, G. H.; Yu, Y.-J.; Cui, X.; Petrone, N.; Lee, C.-H.; Choi, M. S.; Lee, D. Y.; Lee, C.; Yoo, W. J.; Watanabe, K.; Taniguchi, T.; Nuckolls, C.; Kim, P.; Hone, J. Flexible and Transparent MoS2 FieldEffect Transistors on Hexagonal Boron Nitride-Graphene Heterostructures. ACS Nano 2013, 7, 7931−7936. (7) Chang, H. Y.; Yang, S.; Lee, J.; Tao, L.; Hwang, W. S.; Jena, D.; Lu, N.; Akinwande, D. High-Performance, Highly Bendable MoS2 Transistors with High-K Dielectrics for Flexible Low-Power Systems. ACS Nano 2013, 7, 5446−5452. (8) 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. (9) van der Zande, A. M.; Huang, P. Y.; Chenet, D. A.; Berkelbach, T. C.; You, Y.; Lee, G.-H.; Heinz, T. F.; Reichman, D. R.; Muller, D. A.; Hone, J. C. Grains and Grain Boundaries in Highly Crystalline Monolayer Molybdenum Disulphide. Nat. Mater. 2013, 12, 554−561. (10) Najmaei, S.; Liu, Z.; Zhou, W.; Zou, X.; Shi, G.; Lei, S.; Yakobson, B. I.; Idrobo, J.-C.; Ajayan, P. M.; Lou, J. Vapour Phase Growth and Grain Boundary Structure of Molybdenum Disulphide Atomic Layers. Nat. Mater. 2013, 12, 754−759. (11) Jiao, L.; Fan, B.; Xian, X.; Wu, Z.; Zhang, J.; Liu, Z. Creation of Nanostructures with Poly(methyl Methacrylate)-Mediated Nanotransfer Printing. J. Am. Chem. Soc. 2008, 130, 12612−12613. (12) Lee, Y.-H.; Zhang, X.-Q.; Zhang, W.; Chang, M.-T.; Lin, C.-T.; Chang, K.-D.; Yu, Y.-C.; Wang, J. T.-W.; Chang, C.-S.; Li, L.-J.; Lin, T.W. Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24, 2320−2325. (13) Lin, Y.-C.; Zhang, W.; Huang, J.-K.; Liu, K.-K.; Lee, Y.-H.; Liang, C.-T.; Chu, C.-W.; Li, L.-J. Wafer-Scale MoS2 Thin Layers Prepared by MoO3 Sulfurization. Nanoscale 2012, 4, 6637−6641. (14) Han, G. H.; Kybert, N. J.; Naylor, C. H.; Lee, B. S.; Ping, J.; Park, J. H.; Kang, J.; Lee, S. Y.; Lee, Y. H.; Agarwal, R.; Johnson, A. T. C. Seeded Growth of Highly Crystalline Molybdenum Disulphide Monolayers at Controlled Locations. Nat. Commun. 2015, 6, 6128. (15) Wang, X.; Feng, H.; Wu, Y.; Jiao, L. Controlled Synthesis of Highly Crystalline MoS2 Flakes by Chemical Vapor Deposition. J. Am. Chem. Soc. 2013, 135, 5304−5307. (16) Castellanos-Gomez, A.; Buscema, M.; Molenaar, R.; Singh, V.; Janssen, L.; van der Zant, H. S. J.; Steele, G. A. Deterministic Transfer of Two-Dimensional Materials by All-Dry Viscoelastic Stamping. 2D Mater. 2014, 1, 011002. F

DOI: 10.1021/acsnano.6b00961 ACS Nano XXXX, XXX, XXX−XXX