Bottom-up water-based solution synthesis for a large MoS2 atomic layer for thin-film transistor applications

A bottom-up water-based solution-process method was developed for atomic layered MoS2 with a one-step annealing process and no sulfurization. The chosen MoS2 precursor is water soluble and was carefully formulated to obtain good coating properties on a silicon substrate. The coated precursor was annealed in a furnace one time to crystallize it. This method can obtain a large and uniform atomic layer of 2D MoS2 with 2H lattice structure. The number of atomic layers (4–7) was controlled through the precursor concentrations and showed good uniformity, which was confirmed by STEM and AFM. Four types of thin-film transistors (TFTs) were prepared from the solution-processed MoS2 on Al2O3 and SiO2 dielectric with either thermal evaporated Al or printed Ag source and drain electrodes. The best result shows an improved mobility of 8.5 cm2 V−1 s−1 and a reasonable on–off ratio of about 1.0 × 105 with solid output saturation.


INTRODUCTION
Transitional metal dichalcogenide (TMDC) has a hexagonal structure with one layer of IV, V, and VI transitional atom (M) between two layers of chalcogen atoms (X). M-X combinations result in strong covalent bonds, while the adjacent layers have Van der Waals bonds, allowing for separation between layers 1 . TMDCs have a band gap that is tunable from 1 eV (bulk) to 2 eV (monolayer) and are promising for transistors, optoelectronic devices, and future electronic devices 2 . The most actively studied TMDC substance is molybdenum disulfide (MoS 2 ).
There have been many methods to make two-dimensional (2D) TMDC materials that can be categorized into top-down and bottom-up. The top-down methods start from bulk MoS 2 crystal form as the raw material such as mechanical exfoliation, chemical exfoliation, and ultra-sonication based on the weak interlayer Van der Waals force. Though simple, these methods have such drawbacks like small scale, random shape, and hard-controlled thickness, thus limiting their practical applications. Comparably, bottom-up methods that have high quality, controllable layer number and scale can be classified into chemical vapor deposition (CVD) or metal oxide (MO) CVD [3][4][5] and solution-processed routes.
CVD is the most commonly studied way to synthesize MoS 2 on a large scale. However, this method requires a sulfurization process where sulfur powder is placed in a furnace to facilitate the embedding of sulfur atoms in the 2D lattice structures. The toxic materials used in this method are a weakness when manufacturing.
Solution processes for 2D materials are developed due to the advantages of a simpler method, high throughput, large size, and low cost with an environmentally friendly process. Among those processes, ammonium tetrathiomolybdate ((NH 4 ) 2 MoS 4 ) precursor was used to make a MoS 2 thin film by dip coating 6 . However, this method needed a two-step thermolysis with an expensive sapphire substrate and was not totally applicable to large-area preparation. Another approach was to use a mixed solvent system to obtain MoS 2 layers by a solution process [7][8][9] , where a two-step annealing process and sulfurization or pulsed laser annealing were also required.
Our group has focused on solution processes that do not require sulfurization. A unique solution process was reported using (NH 4 ) 2 MoS 4 precursor without CVD 10 . The first key point was making the 2D MoS 2 precursor solution by developing a new sulfur-rich solution and making thin film coatings with good wetting properties. The second point was growing MoS 2 atomic layers without sulfurization in a one-step annealing process to obtain a 2H MoS 2 structure 10 . Most recently, direct patterning of MoS 2 layer was also developed without sulfurization 11 .
In this study, a deionized (DI) water-based solution process was developed for MoS 2 thin film by bottom-up thermolysis method for the first time. This method was relatively easy to handle, safe, and environmentally friendly because water was used as one of the main solvents. A new MoS 2 formulation was prepared with the addition of sulfur to the ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O) precursor. The MoS 2 film was also completed without sulfurization and with a single step of annealing, as additional sulfur vapor did not need to be delivered for the sulfurization process because of its sulfurrich solution. This method for synthesizing MoS 2 distinguished our work from others [6][7][8][9] .
This synthesized MoS 2 layer was used as a semiconductor in thin-film transistors (TFTs) with Al or Ag source and drain (S/D) electrodes instead of Au/Ti system 10,12 . Applying printed electrodes for the contacts on solution-processed MoS 2 is interesting because of the possibility of large-area electronics with low costs and an environmentally friendly printing process. Electrohydrodynamic (EHD) jet printing is a good candidate for printing Ag paste regardless of the materials' viscosity 13 . We fabricated spin-coated MoS 2 TFTs with a high yield, with either thermally evaporated Al or EHD jet-printed Ag S/D electrodes, along with SiO 2 or Al 2 O 3 gate dielectrics to provide four types of TFTs, as shown in Fig. 1. Eventually, the TFT group with the combination of high-k Al 2 O 3 and Al shows improved performance in electron mobility and in on-to-off current ratio. Our work suggests that this methodology can be applied to multifarious devices and has the potential for scalability in 2D materials.

Development of a new precursor solution
To prepare solution-processed MoS 2 , previous studies used (NH 4 ) 2 MoS 4 as a precursor and faced difficulties in coating uniformity due to the surface tension of ionic thiomolybdate (MoS 4 2− ) clusters 7 . In general, sulfur-flux (sulfurization) processes are necessary because sulfur is lost during annealing, even though the sulfur is dissolved in the precursor formulation. In this work, (NH 4 ) 6 Mo 7 O 24 ·4H 2 O as Mo source was employed. DI water was chosen as a solvent for preparing the Mo precursor solution due to it being easily found, inexpensive, non-toxic, and environmentally friendly. The S source was important in making our work dissimilar from CVD methods as we combined the S source solution with the Mo solution. CVD methods instead places the S powder inside the furnace during high-temperature annealing. The advantage of using all precursors in the liquid phase is to prevent the uncontrollable flow of powder in the furnace.
While (NH 4 ) 6 Mo 7 O 24 ·4H 2 O is dissolved well in water, sulfur could not be dissolved in numerous solvents such as water or alcohol and only dissolves in small amounts in amine derivatives. Even though CS 2 is a good solvent for sulfur, CS 2 solution does not show good wetting on a SiO 2 substrate. As CS 2 has a low boiling point of 46.3°C, it constantly evaporates at room temperature, and it is not possible to use the solvent alone. CS 2 can be mixed well with ethanolamine to dissolve sulfur to prepare S source. Therefore, a sulfur-containing MoS 2 precursor solution was formulated with CS 2 , ethanolamine, and DI water. The solvents used in the solution are some of the major factors in the coating condition. Therefore, the ratio of DI water as a solvent in the Mo's precursor and ethanolamine as a solvent of sulfur greatly affects the coating state on the wafer substrate.
After several formulation studies, Mo precursor and sulfur solutions were prepared at ratios of 3:1-1:3 to check the wetting and coating conditions. All solutions were mixed well and showed good stability. However, the coating states of the different solution mixtures on the silicon wafer were different (Supplementary Fig. 1a). If the ratio of DI water-based solution is high, pin holes and wrinkling occur, resulting in an uneven coating layer. This is unfit for spin-coating since the sulfur and CS 2 do not dissolve in DI water at all.
The most uniform thin film occurred at a ratio of 1:2. We measured the contact angle on the SiO 2 substrate after ultraviolet (UV)/O 3 treatment for 30 min, and it seemed that stable coating was possible due to the good wetting characteristics at a contact angle of 4.0°( Supplementary Fig. 1b). Based on this, the final solution was prepared with a Mo:S atomic ratio of 1:2.
Two precursor solutions were prepared with and without CS 2 to find out how the CS 2 affects the coating conditions of the sulfur solution. Supplementary Fig. 1c shows the state of the solution before and after adding CS 2 to the mixture of sulfur and ethanolamine. If the ethanolamine is added with sulfur, it starts to melt very slowly and gradually turns green, and there is no visible sediment, but the solution turns deep green. When adding CS 2 in the mixture, it turns transparent yellow within seconds.
These two solutions were mixed with a DI water-based (NH 4 ) 6 Mo 7 O 24 ·4H 2 O solution and spin-coated on SiO 2 . The layer without CS 2 was highly patterned due to uncovered part of the surface ( Supplementary Fig. 1d). With the addition of CS 2 , a clear coating of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O precursor solution could be obtained ( Supplementary Fig. 1e). After the precursor solution was spin-coated on a silicon wafer, the films were soft baked at 150°C for 30 min and thermally annealed at 500-1000°C in a furnace for 1 h. Sulfurization in CVD was not necessary in this method. Figure 2a shows the Raman spectra for different annealing temperatures. Two main peaks were observed at the positions of the E 1 2g and A 1g modes, which represent the out-of-plane and inplane vibration energies, which are indicators of the layers in the thin atomic films. The Raman spectra are very similar to that of MoS 2 from (NH 4 ) 2 MoS 4 precursor 14,15 . Peaks were observed at 382.7 and 407.8 cm −1 for the E 1 2g and A 1g modes, respectively. As the growth temperature increased, the position of peak E 1 2g was blue-shift while the position of peak A 1g showed a red-shift. As a result, the frequency differences between the two peaks were 22, 24.3, and 25.1 cm −1 for annealing temperatures of 700, 900, and 1000°C, respectively showing thicker layer with increasing growth temperature. This may be due to the different growth mechanisms of MoS 2 crystal at different temperatures 16 .

Characterization of MoS 2 thin films
Competition between the surface energy and interfacial energy results in structural changes from vertical growth and horizontal growth 16 . As solid precursors were decomposed below 800°C, the MoS 2 layers grew layer by layer vertically and its structural dimension progressively transitioned to grow horizontally. At above 800°C, the MoS 2 nano-crystals started to precipitate with the average grain size of crystals growing, thus forming honeycomb MoS 2 . The peaks started to appear weakest at 500°C and increased with the annealing temperature. The intensity of the peaks increased gradually, and the strongest intensity occurred at 1000°C. After annealing at 1100°C, the spincoated MoS 2 precursor seemed to evaporate, and no peak was found in the Raman spectra. Therefore, controlling the annealing temperature is important to obtain crystalline MoS 2 layers during the annealing process. Figure 2b shows the X-ray diffractometry (XRD) spectra of MoS 2 crystals with respect to the annealing temperatures. No peak was observed until 900°C. A weak peak at 2θ = 14.2°begins to appear at 900°C, and there is a sharp increase at 1000°C. Interestingly, more peaks appeared at 2θ = 44.1°, 59.5°, and 29.2°at 1000°C. These peaks correspond to the (002), (004), (006), and (008) crystal planes of MoS 2 , respectively 17 . The optimal annealing temperature for the DI water-based precursor solution is 1000°C, and the greatest crystallinity appeared at this temperature. Several large peaks come from the silicon wafer, which were confirmed by the XRD results of MoS 2 , SiO 2 , and MoS 2 single crystals. All XRD peaks of MoS 2 crystallized at 1000°C were well synthesized and matched well with the peak positions of a MoS 2 single crystal. Supplementary Fig. 2a, b show the X-ray photoelectron spectroscopy (XPS) results of MoS 2 annealed at 1000°C. The binding energy of the peaks at 232.2, 229.0, 226.2, 162.9, and 161.7 eV correspond to Mo 3d 3/2 , Mo 3d 5/2 , S 2S, S 2p 1/2 , and S 2p 3/2 , respectively. The binding energy corresponding to Mo-S crystal was assigned to 3d 5/2 in Mo and 2p 3/2 in the S peak 18 . There are two types of lattice structures of MoS 2 : a 2H structure, in which the S atom is on the same line as Mo, and a 1T structure, in which the S atom is crossed around Mo. This structure changes the characteristics of the MoS 2 . The 2H structure indicates semiconductor characteristics, but the 1T structure is quasi-metallic 1 . Due to the differences in the atomic structure, the binding energy is also different. The corresponding peaks confirmed that a 2H structure of MoS 2 was obtained from the DI water-based (NH 4 ) 6 Mo 7 O 24 ·4H 2 O precursor 18,19 .
The atomic ratio of Mo and S was confirmed by XPS results (Supplementary Table 1). Although the 1:2 ratio is ideal, the atomic ratio was actually 1:2.1-1:2.3 due to the characteristics of the synthesis. A higher precursor concentration results in a more ideal Mo-to-S ratio, and there might be more chance for each component to crystallize during the reaction. MoS 2 was synthesized without significant deviation of the atomic ratio, which confirms the reliability of the system for MoS 2 synthesis.
The Raman spectra were analyzed with precursor concentrations of 0.005 and 0.025 M, as shown in Fig. 2c. The peak positions in both E 1 2g and A 1g modes depend on the number of layers of MoS 2 . Therefore, the number of layers can be counted using the frequency difference (Δκ) between E 1 2g and A 1g peaks. Comparing the peak positions with the spectrum obtained from other works [20][21][22] , we predict that the spin-coated films are bi-layer (Δκ = 22.8 cm −1 ) and a few-layer (Δκ = 25.0 cm −1 ) structure for MoS 2 prepared from 0.005 and 0.025 M, respectively.
The photoluminescence (PL) spectra of MoS 2 thin films were obtained from various precursor concentrations using a 532-nm excitation laser, as shown in Fig. 2d. The peaks around 620 and 670 nm confirm the 2H structure of the synthesized MoS 2 . Furthermore, as the number of layers decreases, the PL peak becomes stronger as the indirect band gap is converted to a direct band gap 19 . Therefore, as the concentration of the solution decreases, the number of layers of the synthesized MoS 2 decreases, resulting in strong PL peaks.
The morphology of several MoS 2 samples was observed using an optical microscope and atomic force microscopic (AFM) measurement after transferring the MoS 2 films to silicon wafers. Figure 3 shows MoS 2 layers that were spin-coated from three concentrations with the same spin conditions. It is clear that different concentrations can create visually distinct surfaces. The lowest concentration of 0.0065 M was unable to fully cover the substrate because the layer was too thin (Fig. 3a). Formulations with ≥0.0125 M resulted in full coverage because of the higher amount of solute. Nonetheless, 0.0125 M was the optimal concentration for spin coating, which can be seen in Fig. 3b. The resulting film has no holes, in contrast compared to the case of 0.0065 M and the free clusters that occurred with 0.025 M (Fig. 3c).
For smaller-scale observations, AFM images were captured from two 1 × 1-μm 2 MoS 2 samples made with 0.0125 and 0.025 M. A smoother surface was observed based on the root mean square roughness (R q ) for the film prepared from 0.0125 M, which was about 0.18 nm, while for the film from higher concentration had R q of around 0.20 nm (Fig. 3d, g). In Fig. 3e, h,   the MoS 2 thickness was evaluated. As shown in Fig. 3f, i, the height profiles measured along the chosen lines of the two MoS 2 films show that there were four and seven layers for 0.0125 and 0.025 M, respectively. These results were confirmed by Raman and high-resolution transmission electron microscopic (TEM) analyses. The thickness increased as the precursor concentration increased. The concentration of 0.0125 M was chosen to make an active layer for TFTs due to the thinner and smoother surface.
High annular dark-field (ADF) images were analyzed using scanning TEM (STEM) to obtain the detailed microstructures of MoS 2 . We identified the number of layers from a cross-sectional TEM image. To do this, a carbon coating was placed on top of the MoS 2 , and platinum was deposited to prepare it for focused ion beam analysis. Figure 4a shows the plan-view STEM image of large-scale MoS 2 layer. The plan view shows the grain boundaries between the multiple crystals that are formed during the annealing of the precursor solution. The inset of Fig. 4a is the selected area electron diffraction (SAED) pattern from inner to outer, corresponding to (100), (110), (200), (210), and (300) crystal planes. The sharp and bright diffraction rings of SAED implying the improvement of crystallinity after annealing at 1000°C.
In Fig. 4b, the dark-field STEM image at high resolution confirms the arrangement of atoms in a penta-layered structure in the honeycomb field image. The image confirms that there is a 2H structure of MoS 2 , which matches with the Raman spectra and XPS results 1 . Figure 4c shows low-resolution cross-sectional STEM image obtained with 0.0125 M. Figure 4d shows a high-resolution STEM image where the MoS 2 layer is divided into four layers. This was also confirmed by the AFM thickness measurement. Figure 4e shows high-resolution cross-sectional STEM images of MoS 2 obtained at 0.025 M. The white line is a part of a large connection of Mo atom where MoS 2 consists of about seven layers, which was confirmed by AFM.
Because the coating from DI water-based MoS 2 precursor had very good conditions, the synthesis was attempted on a larger substrate of 5 × 5 cm 2 to verify the possibility of large-scale application. Figure 5a shows a picture of a transferred MoS 2 thin film on a new SiO 2 substrate after being synthesized with a large size of 5 × 5 cm 2 . The thin film of MoS 2 was large with good uniformity. This proves the possibility of using the bottom-up synthesis methods for large-size MoS 2 atomic layer formation with good uniformity.
To investigate the film transparency, MoS 2 films with different precursor concentrations were transferred on glass substrates, as shown in Fig. 5b. A lower concentration resulted in more transparency in the visible region because of the smaller number of layers, as shown in Fig. 5c. The transmission at 550 nm was 46.2, 59.7, 80.1, and 85.5% for concentrations of 0.005, 0.025, 0.0125, and 0.005 M, respectively. The transparency peaks are related to the band structure of MoS 2 films, and there are some red-shifts in the range above 600 nm, which are related to the increase of PL spectra 7,18,23    make rather simple and consistent coatings, but the coating is a little thicker and provides more atomic layers in films.

Electrical characteristics of MoS 2 thin-film transistors
TFTs were fabricated with the DI water-based synthesized MoS 2 as a semiconducting layer to evaluate the electrical properties. To form the S/D electrodes on the MoS 2 layer, Al was used instead of Ti/Au unlike in previous publications 10,12 , because the Fermi level of Al, as a cheap material, is close to the conduction band of MoS 2 6 . EHD jet-printed Ag was applied to MoS 2 TFTs on two gate dielectrics because of the similar energy level of Ag and Ti. The detailed EHD jet printing of Ag paste jetting is described in Supplementary Note 1.
The TFTs were prepared depending on the insulator layer and deposition methods with the same channel length and width of 140 and 1500 μm, respectively, as shown in Fig. 6a. This resulted in 4 groups of 15 devices each. In our experiments, our water-soluble MoS 2 precursor using a spin-coating method with high successful patterning electrodes (100% for thermal evaporated Al, 90% for EHD jet printed Ag) did show that the device manufacture yield was very good. We can thus predict that the homogenous and large-scale MoS 2 based on our water-soluble MoS 2 would facilitate a high manufacture yield of devices. Figure 6 shows the transfer and output characteristic curves of spin-coated MoS 2 TFTs. Figure 6b, c show the transfer properties for device groups A and B with SiO 2 dielectric. V GS was varied from −20 to 100 V and V DS was kept at 10 V for the SiO 2 gate insulator, while V GS of −5 to 40 V and V DS of 1 V were applied for the Al 2 O 3  insulator (Fig. 6d, e) due to the different dielectric materials with different breakdown voltages.
The inset of each transfer curve figure shows a representative top-view microscope image for each typical device group. All devices express pronounced n-type MoS 2 behavior. The fieldeffect mobility (μ eff ) of TFTs was extracted from the linear region using the conventional equation at a fixed V DS (1 V for Al 2 O 3 and 10 V for SiO 2 cases). The peak mobility values of MoS 2 devices are 0.011, 0.12, 2.6, and 8.5 cm 2 V −1 s −1 for groups A-D, respectively, as summarized in Table 1.
It is very interesting that remarkable improvement was observed in the performance of TFTs as the gate dielectric changed from SiO 2 to Al 2 O 3 , and there was reasonable improvement when changing S/D electrodes from printed Ag to evaporated Al. The mobility differences are over 70 times after switching gate dielectric from low-k SiO 2 to high-k dielectric Al 2 O 3 . The on-to-off current ratio ranges also improved as 10 4 -10 5 for Al 2 O 3 dielectric and <10 4 for SiO 2 .
All MoS 2 TFTs exhibit both linear and saturation regions (Fig. 6f-i). The output curves at several gate voltages after sweeping the drain voltage confirm the typical behavior of MoS 2 . It is clear that a better Ohmic contact property obtained in low V DS output curves in the case of Al S/D electrodes. As shown in Fig. 6f, h, the drain-source current of devices is linear at low drain-source   voltages and saturates as V DS increases in the low gate-voltage range. There is less saturation at higher gate voltages for the case of printed Ag. For the cases of Al S/D electrodes in Fig. 6g, i, even at low or high gate voltage, output curves of devices show good contact and are linear at low V DS , but there is more solid saturation at high V DS . Two reasons could explain the results derived from the TFT characterization. First, the carrier mobility of MoS 2 TFTs with printed Ag electrodes is quite low compared to TFTs with thermalevaporated Al. This is probably due to poor charge injection from Ag electrodes to the MoS 2 layer, even though equivalent low Schottky barrier heights for electrons could be derived from both metals. There is poor charge diffusion from the top layer of Ag to the underlying layer and channel because of the less dense texture and porous structure of Ag layer, as observed in SEM and AFM ( Supplementary Fig. 3). Less contact and conduction problems between printed S/D and MoS 2 are expected because of any residues that formed at the relatively low sintering temperature of 150°C. Although the TFTs with the EHD jetprinted Ag S/D shows limited performance, the printing technique makes devices based on MoS 2 or even other 2D materials possible with a low-cost process.
Second, the MoS 2 TFT with high-k Al 2 O 3 (k = 7) exhibits outstanding electrical characteristics compared with TFTs based on low-k SiO 2 (k = 3.9). These improved properties can be explained by the screening effect from the high-k dielectric, which can damp Coulomb scattering from charged impurities. In addition, enhanced performance of the MoS 2 TFT is also attributed to the smooth surface at the MoS 2 /Al 2 O 3 interface after transferring, as observed in AFM. Much improved properties can be achieved for solution-processed MoS 2 from dielectric engineering.
It is worth noting that our water-based MoS 2 transistors without a post-annealing step or encapsulation exhibited linear mobility up to 8.5 cm 2 V −1 s −1 that is really improved compared to our similar approach but different solution base 10 and much higher than that of some other solution process 24,25 . This is comparable with those of previous publications proposed by other solution approach on sapphire 9,26 and seeded growth method 27 , which needed complex process and chemical treatments with additional sulfur. This result was even higher than that of CVD-grown monolayer MoS 2 with ink-jet printed Ag as the S/D 12 , multi-step CVD-grown MoS 2 with expensive Au electrodes 28 and some based on CVD 29,30 . Supplementary Table 2 shows details about mobility and current ratio of our devices compared with those previously reported in the literature. Our MoS 2 TFT system may face many obstacles for high performance because of the solution process, such as water or oxygen absorption, intrinsic structural defects (point defects and grain boundaries), and interfacial issues among semiconductor and gate dielectric and S/D. Nevertheless, we believe that our method will facilitate major developments for 2D MoS 2 materials in low-cost printed electronic applications.

DISCUSSION
In this study, a new DI water-based formulation for MoS 2 was successfully developed using a precursor of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O as Mo source and sulfur solution as S source. The DI water-based precursor solution was spin-coated and be converted to MoS 2 thin film by one-step annealing without CVD sulfurization. The synthesized MoS 2 layer could be fabricated with large size over 4", good uniformity, and transparency in a 2H MoS 2 structure, which was confirmed by analytical methods. The synthesized MoS 2 had a smooth and homogeneous surface when being spincoated from a solution concentration of ≥0.0125 M. The resulting MoS 2 layers were 4 and 7 with 0.0125 and 0.025 M precursor solution, respectively. Therefore, the atomic layers of MoS 2 can be controlled by the concentration of the DI water-based precursor.
Four TFT groups were prepared with MoS 2 semiconductor, Al, and EHD jet-printed Ag S/D electrodes on different dielectrics. The solution-processed MoS 2 TFT on Al 2 O 3 gate dielectric with Al S/D electrodes shows improved electrical properties, such as a mobility of 8.5 cm 2 V −1 s −1 and an on-off ratio of about 1.0 × 10 5 . Solid saturation with good contact can also be obtained from all MoS 2 TFTs in output characteristics. The simple and environment friendly water-based solution method could open the new pathway for low-cost preparation of other TMDC thin films in nextgeneration electronics.

Materials and solution preparation
In this work, precursor solution for spin-coating MoS 2 with several concentrations (0.005-0.05 M) were prepared from Mo and S precursor solution source. Mo solution and S solution were dissolved separately using different relevant solvents, which were 5 mL DI water (for Mo) and 10 mL ethanolamine accompanied with carbon disulfide (for S). CS 2 with a small amount of 1 mL played an important role to help dissolve well S powder in ethanolamine and to make S-rich solution as well. Then two source solutions were mixed together with the ratio of 1 Mo:2 S (wt%) to get final precursor solution.
To obtain uniform thin films, the solution should sufficiently wet the SiO 2 /Si substrate. Thus, the substrate was treated with UV/O 3 for 30 min to improve the wettability. The MoS 2 precursor solution was spin-coated at 3000 rpm for 30 s and then pre-baked at 150°C for 30 min. Subsequently, the films were heated in a quartz tube furnace at elevated temperatures ranging from 500 to 1000°C at a ramp rate of 15°C min −1 for 1 h to obtain MoS 2 films. The growth of MoS 2 inside furnace was under low vacuum (~10 -2 Torr) after removing air.

Fabrication of MoS 2 thin-film transistors
To investigate the translation of the synthesized MoS 2 to electronic applications, MoS 2 TFTs with bottom-gate and top-contact structure were fabricated on 300-nm-thick SiO 2 or 80-nm Al 2 O 3 as the gate insulator. The (NH 4 ) 6 Mo 7 O 24 ·4H 2 O precursor layer was created by a spin-coating technique with a 0.0125 M precursor solution after UV/O 3 treatment to improve wetting. The precursor was annealed at 1000°C of tubular furnace to obtain a crystalline MoS 2 thin-film layer. The S/D electrodes of the TFT were fabricated by depositing 100-nm-thick Al using thermal evaporation with shadow masks or EHD jet-printed Ag for comparison.
Silver paste was modified by mixing 100 parts of the original Ag paste (4000 pcs, AD-V7-108) with 1 part of Silveray solvent and 3 parts of propylene glycol methyl ether acetate (PGMEA, Sigma Aldrich) based on our recent publication 13 . As an alternative dielectric, 80 nm of Al 2 O 3 was deposited on a silicon wafer using atomic layer deposition. As-grown MoS 2 layers were transferred onto Al 2 O 3 /Si substrate to fabricate TFTs using a conventional wetting method with the support of polymethyl methacrylate (PMMA) 10 .
Next, silver paste was printed in Taylor cone jet mode on both as-grown and transferred MoS 2 films using an EHD jet printer, followed by drying for 30 min at 150°C on a hot plate to form the S/D electrodes of the TFTs. More detail about EHD jet printing is addressed in Supplementary Note. The procedure of the TFT fabrication and a cross-sectional view of devices are shown in Figs. 1 and 6a.

Characterization of MoS 2 thin films and devices
The synthesized MoS 2 thin films were analyzed by Raman spectroscopy, XRD, UV-Vis spectroscopy, AFM (Nano expert II EM4SYS), and XPS. Atomicresolution ADF-STEM (JOEL-ARM 200F) was used to obtain atomic images and observe the microstructure with a spherical aberration corrector (CEOS GmbH) at 200 K. The semi-angles of the ADF detector were changed from 90 to 200 mrad with a scan rate of 6 μm pixel −1 and 512 × 512 pixels per frame.
TEM samples were prepared using a transfer technique on the synthesized MoS 2 with PMMA on a Cu mesh coated with a grid of lacey carbon. For the preparation of transparent samples, MoS 2 was transferred to a glass substrate using a conventional PMMA method. A parameter analyzer (Keithley 4200) was used to characterize the electrical properties of the MoS 2 TFT in a dark room at room temperature.