Synthesis of Large-Area Tungsten Disulfide Films on Pre-Reduced

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Synthesis of Large-Area Tungsten Disulfide Films on Pre-Reduced Tungsten Suboxides Substrates Soo Ho Choi, Stephen Boandoh, Young Ho Lee, Joo Song Lee, Ji-Hoon Park, Soo Min Kim, Woochul Yang, and Ki Kang Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12151 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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ACS Applied Materials & Interfaces

Synthesis of Large-Area Tungsten Disulfide Films on Pre-Reduced Tungsten Suboxides Substrates Soo Ho Choi †,Ұ, Stephen Boandoh‡,Ұ, Young Ho Lee‡, Joo Song Lee┴, Ji-Hoon Park§,#, Soo Min Kim┴*, Woochul Yang†*, and Ki Kang Kim‡*

†

Department of Physics, Dongguk University-Seoul, 04620, Republic of Korea

‡

Department of Energy and Materials Engineering, Dongguk University-Seoul, 04620, Republic

of Korea ┴Institute

of Advanced Composite Materials, Korea Institute of Science and Technology (KIST),

Chudong-ro 92, Bongdong-eup, Wanju-gun, Jeonbuk, 55324, Republic of Korea §

Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon, 16419,

Republic of Korea #

Department of Physics, Sungkyunkwan University (SKKU), Suwon, 16419, Republic of Korea

*

Corresponding author E-mail: [email protected], [email protected], [email protected]

KEYWORDS

ACS Paragon Plus Environment

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transition metal dichalcogenides; tungsten disulfide; chemical vapor deposition; hydrazine; reduction

ABSTRACT We report a facile method for the synthesis of large-area tungsten disulfide (WS2) film by means of chemical vapor deposition (CVD). To promote WS2 film growth, the precursor solution, which includes pre-reduced tungsten suboxides, is prepared by using hydrazine as the strong reduction agent and spin-coated onto the growth substrate. Growth is then carried out in a CVD chamber vaporized with dimethyl disulfide (DMDS) as the sulfur precursor. While only WS2 flakes are grown with un-reduced tungsten precursor under hydrogen atmosphere, WS2 film is readily attained on pre-reduced tungsten suboxide substrates without the need for further reduction by hydrogen, which is noted to induce discontinuity of grown film. The result presents the coverage of WS2 to be proportional to the amount of reduced tungsten suboxides, which is revealed by X-ray photoelectron spectroscopy. Furthermore, it is found that the multilayer WS2 flakes grow along the grain boundary, which allows analysis of the grain size of WS2 film by optical microscopy image only. WS2 field effect transistors are fabricated by conventional photolithography show an average electron mobility of 0.4 cm2V-1s-1 and a high on/off ratio of 106 at room temperature.


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INTRODUCTION

Transition metal dichalcogenides (TMDCs) composed of two different elements: transition metals sandwiched by two chalcogens to form MX2 (M: transition metals (Mo, W, etc.); X: chalcogens (S, Se, and Te)), have a layered structure, which is maintained by weak Van der Waals interactions between the layers.1 The metallicity varies from metal to semiconductor, and ranges from 0-2 eV, depending on the physical structure, such as 1T, 1T’, and 2H.2, 3 Their exotic physical and chemical properties, such as high carrier mobility, tightly bound exciton, strong spin-orbit coupling, and various band offsets, not only catalyze studies of the intrinsic properties of TMDCs, but also facilitate new functional platforms for valleytronics, electrophoto-catalysis, thinnest diodes, sensors, batteries, and biomedicine.4-9 Specifically, monolayer 2H-semiconducting TMDCs (s-TMDCs) are direct band gap semiconductors, whereas thick sTMDCs are typically indirect band gap semiconductors.7 The direct nature of the band gaps of monolayer s-TMDCs makes them strong candidate materials for high efficiency optoelectronics and solar cells, as well as flexible and transparent electronics.10-12 Among the 2H-s-TMDCs, tungsten disulfide (WS2) shows high photoluminescence (PL) quantum yield (~10-3) at room temperature, which is two orders higher than that of molybdenum disulfide (MoS2).13 This makes it one of the strongest candidate materials for light-emitting devices, solar cells, and photodetectors. However, most of the applications are demonstrated with WS2 flakes prepared by mechanical exfoliation, limiting large-area applications. Therefore, ways to obtain large-area WS2 remains an issue that needs extensive investigation. Two representative methods of atomic layer deposition (ALD) and chemical vapor deposition (CVD) have been utilized to achieve large-area WS2 film.14, 15 ALD produces a large-area WS2


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film with controllable thickness, but the quality of the WS2 film is poor. On the other hand, the CVD method is widely utilized for the growth of WS2 film with relatively high quality compared to the ALD method. Large-area WS2 film has been successfully synthesized on gold foil by means of CVD, and transferred onto target substrate.16, 17 However, the use of expensive gold substrates makes the process economically unfeasible. Alternatively, insulating substrates such as silicon dioxide (SiO2), sapphire (Al2O3), and hexagonal boron nitride (h-BN), and seeding promoter (i.e., perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) and graphite oxide)-coated insulating substrate can be used.18-21 However, only few papers have reported the growth of WS2 films, rather than the growth of WS2 flakes.21-25 This might be attributed to the uncontrollable growth kinetics. Typically, solid phase precursors have been used, such as tungsten trioxide and sulfur powders. To increase the flux of precursors on the growth substrate, they were placed underneath the growth substrate (tungsten trioxide powder) and at the upstream of the growth substrate in the furnace (sulfur powder), respectively.21 In this case, controlling the evaporation of these precursors is very challenging, because the saturation vapor pressure of the solid phase precursors exponentially depends on the temperature, as well as being strongly dependent on the amount of precursors. Therefore, special care for the conditions is required, such as in weighing precursors, and the control of temperature at the precursor-mounted zone in CVD. Otherwise, the reproducibility of the growth of WS2 is very low. This is one of drawbacks for the commercialization of the WS2 film. Another issue is the evaporation of tungsten trioxide. Compared to the low melting point of molybdenum trioxide (795 oC), tungsten trioxide is thermally stable, with a melting point of around 1,400 oC. Therefore, to evaporate the tungsten trioxide efficiently, a reduction process is required to generate the volatile WO3-x. To promote this reduction, supplement of hydrogen is necessary during the growth of WS2. But the etching


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of WS2 by hydrogen occurs simultaneously, hindering the WS2 film growth. Recently, the synthesis of wafer-scale polycrystalline transition metal sulfides (MoS2 and WS2) film on insulating substrate has been reported, using controllable precursors, such as diethyl disulfide (liquid phase), and tungsten carbonyl (solid phase).26 However, the growth of WS2 film took a day, and the quality of WS2 film (i.e. the flake size was an order of ten micrometers in length) was low. Therefore, the development of alternative strategies for the growth of WS2 film is highly necessary. Herein, we report the facile synthesis of large-scale WS2 film by means of CVD. The prereduced tungsten suboxides are prepared by adding hydrazine (N2H4) as a strong reducing agent to an aqueous sodium tungstate (Na2WO4) solution. Prior to growth, the amount of tungsten precursor is well-controlled by varying the concentration of Na2WO4 solution, followed by spincoating onto the growth substrate. Growth is then proceeded with a liquid phase of dimethyl disulfide ((CH3)2S2, DMDS) as the sulfur precursor. The amount of sulfur precursor is controlled by a bubbling system. From the results, it is apparent that, pre-reduction of the tungsten oxide by hydrazine prior to growth is a crucial step in the synthesis of continuous WS2 film relative to the simultaneous reduction of the tungsten oxide during growth with hydrogen, which mainly induces growth of discontinuous domains. Furthermore, the domain size of the WS2 film is on the order of a hundred micrometer, proving that WS2 film is of high quality. It is also found that bilayer and multilayer WS2 flakes grow near the grain boundary, which is confirmed by highresolution transmission electron microscopy (HR-TEM). Finally, we demonstrate that the WS2 field effect transistor (FET) can be fabricated by conventional photo-lithography for large-area electronic applications.


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RESULTS AND DISCUSSION

Figure 1a shows a schematic of the synthesis of the WS2 film. Sodium tungstate (Na2WO4) as tungsten precursor is dissolved in distilled water. To reduce the tungsten oxide ions (WO42-), hydrazine as a reducing agent is added to the precursor solution. At the same time, the growth substrate (SiO2/Si) is kept in 0.5 M NaOH solution for 30 mins to make a hydrophilic surface, which increases the wettability of tungsten precursors. The prepared tungsten precursor solution is uniformly spun onto the NaOH-treated SiO2/Si substrate. Growth of WS2 film is then carried out at 850 oC under Ar and DMDS atmosphere. The liquid phase of DMDS is supplied using a bubbling system (see the experimental section for the detailed growth conditions). DMDS decomposes into methanethiol (CH3SH), ethane (CH2=CH2), carbon disulfide (CS2), and a sufficient amount of hydrogen sulfide (H2S) at elevated temperature, allowing the use of sulfur precursors.27 Figure 1b shows the photographs of the bare SiO2/Si substrate and as-grown WS2 film on SiO2/Si substrate. The color of substrate changes from blue to blue-green after the growth of WS2 film, with some areas assuming the dark green coloration. Figure 1c shows the optical image of the as-grown WS2 film. The growth substrate is fully covered by WS2, proving that a complete WS2 film is grown by our new method. The three different color contrasts observed in the optical image as indicated by arrows, are monolayer, few layer, and multilayer regions. This is attributed to the Fresnel effect for different thickness of WS2 on SiO2/Si substrate.28 It is noted that the as-grown WS2 film is easily detached whenever it is dipped in DI water due to the dissolution of the leftover precursors between SiO2 and WS2 film, which is similar in observation with the previous report (see Supporting Information Figure S1).29 Therefore, a transfer of WS2 is performed to prevent the damage of WS2 film during further process.


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To estimate the thickness variation in WS2 film, it is characterized by atomic force microscopy (AFM), Raman spectroscopy, and PL. Figure 2a is a representative optical image of the WS2 film after transfer onto SiO2/Si substrate by a conventional Poly(methyl methacrylate) (PMMA) method.30 The background is covered by monolayer WS2 film, and multilayer islands are clearly observed. To measure the thickness of WS2, the selected region indicated by the red-dashed box in Figure 2a is analyzed by AFM. Figure 2b is an AFM image corresponding to the red-dashed box. 2D characteristic wrinkles with a high density are observed in the monolayer region, whereas the density of wrinkles is very low at the thicker region (i.e. the 3L WS2 region). This is attributed to the wrinkles being easily formed at the thinner region during the transfer process. Figure 2c shows the height profiles along the white-dotted lines (I, II, and III) in Figure 2b. The height profile for line I (top panel in Figure 2b) indicates that the thickness of WS2 is around 0.65 nm, confirming that the background of WS2 film is indeed monolayer. The middle and bottom panels in Figure 2c show the height profiles along lines II and III. The thickness of the additional layers on top of the monolayer are around 0.78 and 1.75 nm, respectively, indicating that additional layers grow on top of the monolayer (1L) WS2 to form the bilayer (2L) and triple layer (3L) WS2, respectively. In addition, thicker WS2 islands are frequently observed along the grain boundaries (GBs), which might be attributed to accumulation of precursors when WS2 domains are merged (see Supporting Information Figure S2). The details for GBs are discussed later. The optical properties of WS2 film depending on its thickness are characterized by Raman spectroscopy and PL. Figure 2d and e are the Raman mapping for the intensity of E2g phonon, and PL mapping image for the intensity of A exciton, respectively, in the red-dashed box in Figure 2a.15 Generally, the intensity of E2g phonon increases with the increase of the number of layers.31 The intensity of E2g becomes strong as a function of the thickness of WS2 from


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monolayer to triple layer regions, corroborating with the AFM analysis in Figure 2b and c. In contrast with the change of Raman intensity, the intensity of A exciton in PL spectra decreases with increasing thickness of WS2, due to the change of band gap nature from direct to indirect band gap transition, caused by increasing the interlayer coupling between the pz orbital of sulfur atom and the d orbital of tungsten atom.1 The intensity of A exciton in Figure 2e becomes strong from triple layer region to monolayer region. The change of the intensities of A exciton and E2g phonon in both mapping images is well-matched with the different optical contrasts in the optical image as shown in Figure 2a, indicating that the different thicknesses of WS2 can be estimated from the optical image only. Figure 2f and g present the representative Raman and PL spectra from mono-, bi-, and triple layers. The frequency difference (∆ω) between E2g and A1g in the Raman spectra is another indicator for estimating the thickness of WS2.31 The ∆ω at the monolayer region is around 65 cm-1, which is in good agreement with the previous work for monolayer WS2. The values for the regions of bi- and triple layers are 68 and 69 cm-1, respectively, which are very well in agreement with previously reported values.31 On the other hand, the intensity of A exciton dramatically decreases with increasing thickness of WS2 as shown in Figure 2g, which is consistent with previous observations.32 The physical crystalline structure of WS2 film is further characterized by TEM. Figure 3a shows a low magnification TEM image of WS2 film transferred to the TEM grid. The black lines consisting of multi-layer WS2 flakes are clearly evident, which are indicated by yellow-dotted lines. Figure 3b depicts further characterization of the quality of monolayer WS2 region by HRTEM. A well-defined atomic arrangement is clearly observed, indicating that WS2 is of a high crystallinity at this local area. The inset of Figure 3b shows a fast Fourier transformation (FFT) image from the whole HR-TEM image. Only a single set of six hexagonal dots is obtained,


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indicating the WS2 film is indeed 2H structure. The d-spacings of the (11-20) and (11-10) planes are measured to be 0.16 and 0.27 nm respectively, which is in good agreement with the reported values for 2H-WS2.17 In order to understand the lattice orientation in the WS2 film, the selected area electron diffraction (SAED) patterns measurements were performed at three different regions divided by multi-layer WS2 flake lines, as indicated by the yellow-dotted lines in Figure 3a. Each region is indexed by Roman numerals from I to VI, respectively. Figure 3c shows the SAED patterns corresponding to each Roman numeral. Each image shows one set of six hexagonal dots, as guided by yellow-dashed lines. Three pairs of regions, namely, (I, II), (III, IV), and (V, VI) are well-aligned with each pair having a specific direction, meaning that the lattice orientations are identical within each pair. This proves that the two regions in the pair are that of a single grain. Furthermore, Figure 3d shows the SAED pattern from the region between I and III wherein only two sets of six hexagonal dots are present, indicating that only two grains exist at those regions. This implies GBs exist in close proximity to the yellow-dotted lines. In addition, it can be deduced that the multilayer WS2 flakes observed near the GB in the optical image are probably epitaxially grown along the different lattice directions. As a consequence, the multilayer WS2 flake lines observed in the optical image are grown in close proximity to GBs, and the WS2 grain inside the GBs is a single crystal, and allows the grain structure of the WS2 film to be visualized in an optical image. In order to investigate the effect of hydrazine on WS2 film growth, the growth of WS2 is conducted in two different ways: 1) on Na2WO4-coated substrate under hydrogen and argon atmosphere, and 2) on pre-reduced tungsten oxide-coated substrate under only Ar atmosphere. Figure 4a-c shows the optical images of as-grown WS2 flakes on Na2WO4-coated substrate as a function of the hydrogen flow rate. With increasing hydrogen flow rate, both WS2 flake size and


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the coverage decrease. These effects might be attributed to the promotion of the reduction of tungsten oxide, and the etching of WS2 by hydrogen. Hydrogen reduces the tungsten oxide to form the volatile tungsten suboxides (WO3 + xH2 → WO3-X + xH2O), resulting in the increase of the amount of tungsten precursors at earlier growth stage.21 With the tungsten suboxides being readily volatile, growth is prematurely terminated due to the rapid evaporation of the tungsten source, eventually terminating the supply of tungsten for the growth process. This leads to the increase of nucleation sites and growth of multilayer WS2 flakes. Hydrogen also acts as an etchant for WS2 (WS2 + 2H2 → W + 2H2S).33 While the etching effect might be negligible at earlier growth stage due to the large amount of tungsten precursor supply, it will be dominant at later growth stage due to the depletion of tungsten precursors. This is apparent as the size of WS2 flakes decrease as a function of the H2 flow rate. Hence, it is conceivable that growth of continuous WS2 film is hindered by use of only hydrogen as the reductant. For the second growth approach, synthesis with substrate coated with pre-reduced tungsten suboxides with hydrazine as the reductant in 0.02 M Na2WO4 solution is investigated. To study the effect of hydrazine, the substrates were analyzed by X-ray photoelectron spectroscopy (XPS) after thermal annealing under argon atmosphere at the growth temperature. Figures 4d-f display the XPS W 4f core level spectra of the annealed substrates, which are coated with Na2WO4 solution without any hydrazine (W/O), and mixed with 0.47 M, and 1.42 M hydrazine precursor solutions, respectively. The core level spectra are deconvoluted into four representative peaks, which are assigned to W6+ 4f5/2 (~35.8 eV), W6+ 4f7/2 (~37.9 eV) from WO42- ions (indicated by red-dashed lines), and W5+ 4f5/2 (~35.2 eV) and W5+ 4f7/2 (~37.4 eV) from reduced tungsten suboxides (indicated by blue-dashed lines), respectively. The intensities of W6+ 4f5/2 (~35.8 eV) and W6+ 4f7/2 (~37.9 eV) peaks for W/O hydrazine sample are strong, whereas those of W5+ 4f5/2


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(~35.2 eV) and W5+ 4f7/2 (~37.4 eV) peaks are weak (Figure 4d). These weak intensities might be attributed to the limited reduction or decomposition of WO42- ions by thermal annealing. This effectively means limited suboxides can only be generated via thermal energy, which inevitably results in growth of only WS2 flakes rather than the intended film. In contrast, the intensities of W5+ 4f5/2 (~35.2 eV) and W5+ 4f7/2 (~37.4 eV) peaks gradually increases with the concentration of hydrazine, indicating sufficient amount of suboxides are generated prior to synthesis. These results demonstrate the role of hydrazine as an effective reducing agent for WO42- ions. To correlate WS2 film growth with the addition of hydrazine, WS2 is grown on each substrate without hydrogen supply. Figure 4g-i are the optical microscopy images of the as-grown WS2 on W/O, 0.47, and 1.42 M hydrazine substrates, respectively. While WS2 does not fully cover the substrate with W/O and 0.47 M hydrazine, a complete WS2 film is achieved on 1.42 M hydrazine substrate. The domain size of the WS2 film is observed to be greater than 200 µm in lateral size. Figure 4j shows the WS2 coverage and the areal ratio (AR) of W5+ ions to W5+ + W6+ ions in W 4f core level spectra (Figure 4d-f) as a function of the concentration of hydrazine. Both the coverage and AR gradually increases with increasing concentration of hydrazine, implying that the reduced tungsten suboxides are a key ingredient for the WS2 film growth. It is noted that even though a high concentration of sodium tungstate is used, the full coverage of WS2 film could not easily be achieved without hydrazine reduction (see Supporting Information Figure S3). Figure 4k and l show the expected WS2 growth models with the aforementioned two approaches: hydrogen and hydrazine reductions, respectively. The volatile tungsten suboxides produced by hydrogen reduction diffuse onto the surface, forming nuclei which then which then reacts with H2S stemming from the decomposition of DMDS to form WS2 domains. In addition to aiding in the reduction of the oxides (WO42-), hydrogen simultaneously etches the WS2 flakes.


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The etching effect (WS2 + 2H2 → W + 2H2S) becomes dominant at later growth stage due to the depletion of tungsten precursors, which is attributed to the consumption of limited tungsten precursors. Therefore, the number of WS2 nucleation sites increases while the flake size decreases with the increase in hydrogen supply, as shown in Figure 4k. On the other hand, prereduced tungsten suboxides by hydrazine reduction directly participate in growth of WS2 without etching, ensuring the attainment of full-coverage WS2 film. As a consequence, the pre-reduction of tungsten oxide by hydrazine is an indispensable process in order to achieve growth of WS2 film. It is noted that our facile strategy can be further extended to other insulating substrate such as sapphire substrate (see Supporting Information Figure S4). To examine the electrical properties of the WS2 film, WS2 FET devices are fabricated by a conventional photolithography technique. Figure 5a shows a schematic of the FET device with an optical microscopy image (inset). The typical channel length (L) and width (W) are defined to be 30 µm and 40 µm, respectively. A highly p-doped Si substrate and 300 nm oxide layer are used as a back-gate electrode and a dielectric layer, respectively. As the source and drain electrodes, 50 nm thick gold electrodes are deposited onto 5 nm thick chromium adhesion layer. All the electrical measurements are conducted at room temperature. Figure 5b is a representative output characteristics with different gate bias (VGS = 0 – 60 V) with 20 V step. A nonlinear characteristic drain-source current (IDS) curve is clearly observed, indicating the formation of large Schottky barrier between the electrodes and WS2 channel layer. This might be attributed to the large difference between the work function of the metal electrode, and the electron affinity of WS2 or Fermi level pinning by sulfur vacancies (see Supporting Information Figure S5).34,35 For the extraction of the FET performance, the IDS is measured as a function of gate voltage (VGS) at VDS = 5 V. Figure 5c show a representative transfer characteristics curve. The current flow is


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clearly observed at positive VGS, indicating that the WS2 FET device exhibits n-type semiconductor characteristics, in good agreement with previous reports.17 The mobility is calculated by the field effect transistor model; IDS = µFE·C·W·(1/L)·(VGS-VT)·VDS, where C, W, L, and VT are the field effect mobility, gate capacitance, channel width, channel length, and threshold voltage, respectively. The calculated electron mobility of this WS2 FET device achieves 0.63 cm2V-1s-1 with on/off ratio of ~ 106. Figure 5d shows the distributions of the mobilities and on/off ratios of several WS2 FETs. The average value of the electron mobility is ~ 0.4 cm2V-1s-1 with on/off ratios of ~ 105. Table 1 lists the reported mobility values, device structures, and fabrication methods from the previous reports. The values for electron mobility and on/off ratio are very wide, which might be attributed to the fabrication methods or the quality of WS2 samples after transfer.36 We believe that the use of an optimized fabrication process will further improve the values.

CONCLUSION We have successfully synthesized large-scale WS2 film on pre-reduced tungsten suboxides substrate with high reproducibility. The reduced tungsten oxide is prepared by using a strong reducing agent of hydrazine. While WS2 flakes are only grown on tungsten oxide substrate under hydrogen atmosphere, WS2 film is readily attained on pre-reduced tungsten suboxides substrate under only argon atmosphere. This revealed that the coverage of WS2 strongly corresponds to the amount of reduced tungsten oxides, which can be controlled by the amount of hydrazine. Furthermore, it is found that multilayer WS2 flakes are preferentially grown along the grain boundary, allowing the size of the WS2 domain in the WS2 film to be visualized by optical microscopy only. A grain size is achieved of as large as several hundred micrometers. We


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believe that our unprecedented strategy not only helps investigation of the growth mechanism, but also accelerates the commercialization of s-TMDCs for electronic and optoelectronic applications.

EXPERIMENTAL SECTION Preparation of the tungsten liquid precursor 0.02 M Sodium tungstate dihydrate (Na2WO4·H2O, 99.995%, Sigma-Aldrich) as the tungsten precursor was prepared in DI water. To reduce the tungsten oxides in solution, solutions with varying amount of hydrazine as the reducing agent were prepared. To prepare the 0.47 M and 1.42 M hydrazine solutions in 0.02 M Na2WO4 aqueous solution, 100 and 300 µl of 15.6 M hydrazine hydrate was added to 3 ml of 0.02 M Na2WO4 aqueous solution, respectively. The solution was kept for 30 min for the pre-reduction of tungsten oxides. Substrate preparation 2 cm × 2 cm SiO2/Si substrate was cleaned by acetone, isopropyl alcohol. The organic solvent was removed by N2 blow. Cleaned SiO2/Si substrate was kept in 0.5 M NaOH solution to make a hydrophilic surface. After 30 min, the substrate was rinsed with DI water. Water drops were flushed off by N2 blow, and the substrate was dried in an oven (~ 80 oC) for 5 min. The prepared tungsten precursor solution was spun onto the substrate at 2,500 rpm for 1 min. The growth of WS2 film A two-inch home-made one-zone furnace system was used for the synthesis of the WS2 film. The system was connected to high-purity (99.9999%) argon, hydrogen, and dimethyl disulfide ((CH3)2S2, >99%, Sigma-Aldrich) bubbler. Na2WO4 precursor coated substrate was positioned


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on the center of a quartz plate (5 cm × 5 cm). The quartz plate was loaded at the center of the 2inch quartz tube. The quartz tube was then purged by 500 sccm of argon gas for 15 min. The temperature of the quartz tube was elevated to 850 oC for 15 min (~50 oC/min) with 350 sccm of argon flow. During growth, 2 sccm of dimethyl disulfide was supplied with a bubbling system for 30 min, to grow WS2 film.37 After growth, the quartz tube was cooled down to room temperature in 30 mins. Characterization methods In order to characterize WS2 film with the various instruments, WS2 film was transferred with a conventional PMMA method, which is well described elsewhere.14, 20, 25 Briefly, PMMA was spun onto as-grown WS2 film/SiO2/Si substrate, followed by detaching the PMMA/WS2 film from SiO2/Si substrate on 2 M KOH solution. The floated PMMA/WS2 film on 2 M KOH solution was transferred onto fresh aqueous solution several times to remove the residual sodium ions (see Supporting Information Figure S5). The resultant film was transferred onto the target substrates such as SiO2/Si and TEM grid and kept in a dry oven to remove the residual water. Finally, PMMA layer was removed by acetone and thermal annealing at 350 oC for 3 hours under Ar/H2 atmosphere. The morphology and thickness of the grown WS2 film were analyzed via optical microscopy (Nikon LV-IM, Nikon) and atomic force microscopy (N8-NEOS, Bruker) via the non-contact mode. Micro-Raman and PL (XperRam100, Nanobase) measurements were performed to confirm the crystallinity and thickness of the grown WS2 film. 532 nm wavelength laser was used for the Raman and PL measurement. The pre-reduction of tungsten oxides was analyzed via X-ray photoelectron spectroscopy (K-alpha, Thermo Fisher Scientific). For analysis of the atomic structure and orientations of the WS2 film, transmission electron microscopy (Techni, FEI)


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was used, after general PMMA transfer of the WS2 film onto the 200 mesh Cu TEM grid with 1.2 µm diameter holes (658-200 CU, Ted Pella, Inc.). Fabrication and measurement of the WS2 FETs Standard photolithography was performed for the fabrication of the WS2 FETs. The WS2 film was transferred to the SiO2/Si substrate (300 nm thick oxide layer on highly p-doped Si substrate, resistivity: < 0.005 Ωcm). After the detachment of the growth substrate, the PMMA/WS2 layer was rinsed on DI water for 10 min to avoid the contamination from Na ions on the bottom of WS2 film. The transferred WS2 film, excluding the channel region, was etched by reactive ion etching system (RIE, All for system Co.), with 10 sccm of SF6 for 10 sec. Source and drain electrodes (Cr/Au, 5/50 nm) were deposited via thermal evaporation. A probe station (MS-tech) and SMU analyzer (Keithley 4200) were used for the I-V measurement of WS2 FETs. I-V measurement was performed under vacuum condition (~10-6 Torr) after thermal annealing under nitrogen atmosphere at 200 oC for 5 hours in order to evaporate residual water and enhance the contact between the active channel and metal electrode.


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Figures:

Figure 1. WS2 film growth. (a) Schematic illustration of the WS2 growth procedure. (b) Photography image of the bare SiO2/Si substrate and as-grown WS2 film. (c) Optical image of the as-grown WS2 film. White arrows indicate monolayer, few layer, and multilayer WS2 regions.


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Figure 2. Optical property of the transferred WS2 film. (a) Optical image of the transferred WS2 film. White arrows point to the monolayer (1L), bilayer (2L), and triple layer (3L) WS2 regions. (b) AFM image of the red-dashed box in (a). (c) Height profiles along lines I, II, and III, respectively, in (b). (d) and (e) Raman and PL mapping images for the intensities of 2LA(M) and A exciton, respectively, corresponding to (b). (f) and (g) Representative Raman and PL spectra of 1L, 2L, and 3L WS2, respectively.


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Figure 3. Grain structure of the WS2 film. (a) TEM image of the WS2 film on the TEM grid. Yellow-dotted lines indicate multilayer WS2 lines. (b) HR-TEM image of the WS2 film. The inset shows the FFT image extracted from the whole image. (c) SAED patterns of each region marked by the Roman numerals in (a). Yellow dashed line guides the orientation of the six hexagonal dots. (d) SAED pattern at the grain boundary between regions I and III. The tilt angle between them is 23.6°.


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Figure 4. Effect of hydrazine on the growth of WS2. (a)–(c) Optical images of the as-grown WS2 flakes on the Na2WO4-coated substrate as a function of the hydrogen flow rate: without (W/O), 5, and 10 sccm, respectively. (d)–(f) XPS W 4f core level spectra of the annealed growth substrates, which were coated with W/O, 0.47, and 1.42 M hydrazine precursor solutions, respectively. Four representative peaks are assigned to W6+ 4f5/2 (~35.8 eV), W6+ 4f7/2 (~37.9 eV) from WO42- ions (red-dashed lines), W5+ 4f5/2 (~35.2 eV), and W5+ 4f7/2 (~37.4 eV) from reduced tungsten suboxides (blue-dashed lines), respectively. (g)–(i) Optical images of the as-grown WS2 on the (d)–(f) substrates, respectively. (j) WS2 coverage and the areal ratio (AR) of W5+ ions to W5+ + W6+ ions in W 4f core level spectra as a function of hydrazine concentration. (k) and (l)


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Schematics of the two expected WS2 growth models with hydrogen and hydrazine reductions, respectively.


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Figure 5. Electrical characteristics of the WS2 film. (a) Schematic of the WS2 field effect transistor (FET). The inset shows an optical image of a representative device. The scale bar is 200 µm. (b) Output characteristics with different gate bias. (c) Representative transfer characteristic of the WS2 FET at VDS = 5 V. (d) Extracted mobilities and on/off ratios of the WS2 FETs.


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Table 1. Reported mobility values, device structure, and fabrication method from this work and the previous reports. Mobility

On/off ratio

Structure

Temperature

Fabrication method

Reference number

0.04-0.63

104-106

Back-gate

RT

Photolithography

This work

0.28, 0.46

102

Liquid-gate

RT

e-beam lithography

15

0.01

105

Back-gate

RT

e-beam lithography

29

0.005-0.01

10

Top-gate

RT

e-beam lithography

38

20.4

108

Back-gate

RT

e-beam lithography

17

1.0-2.0

106-107

Back-gate

RT

e-beam lithography

16

10.9-14.3

106

Top-gate

RT

Photolithography

26


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ASSOCIATED CONTENT Supporting Information Supporting information Figure S1-S5. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author. *E-mail: [email protected], [email protected], [email protected]

Author Contributions. Ұ

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015R1C1A1A02037083). W. Y. acknowledges the support from the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (No. 2015R1D1A1A01058991 and No. 2016R1A6A1A03012877). S. M. K.


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acknowledges support from the Korea Institute of Science and Technology (KIST) Institutional Program. J. H. P. acknowledges the support from the Institute for Basic Science (IBS-R011-D1).

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38. Orofeo, C. M.; Suzuki, S.; Sekine, Y.; Hibino, H. Scalable Synthesis of Layer-controlled WS2 and MoS2 Sheets by Sulfurization of Thin Metal Films. Appl. Phys. Lett. 2014, 105, 083112.


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Table of Contents Graphic and Synopsis


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Synthesis of Large-Area Tungsten Disulfide Films on Pre-Reduced

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