Low-temperature growth of layered molybdenum disulphide with controlled clusters

Layered molybdenum disulphide was grown at a low-temperature of 350 °C using chemical vapour deposition by elaborately controlling the cluster size. The molybdenum disulphide grown under various sulphur-reaction-gas to molybdenum-precursor partial-pressure ratios were examined. Using spectroscopy and microscopy, the effect of the cluster size on the layered growth was investigated in terms of the morphology, grain size, and impurity incorporation. Triangular single-crystal domains were grown at an optimized sulphur-reaction-gas to molybdenum-precursor partial-pressure ratio. Furthermore, it is proved that the nucleation sites on the silicon-dioxide substrate were related with the grain size. A polycrystalline monolayer with the 100-nm grain size was grown on a nucleation site confined substrate by high-vacuum annealing. In addition, a field-effect transistor was fabricated with a MoS2 monolayer and exhibited a mobility and on/off ratio of 0.15 cm2 V−1 s−1 and 105, respectively.

Diverse research has shown that graphene is a promising candidate for analogues of conventional electronic devices [1][2][3] . Although it possesses the extraordinary properties of a high electron mobility, elasticity, heat conductivity, and flexibility, graphene is not suitable for transistor and photonic devices owing to the lack of a bandgap (0 eV for pristine graphene). Molybdenum disulphide (MoS 2 ), a layered structural material which coheres by the covalent bonding of one molybdenum atom between two sulphur atoms and interlayer van der Waals forces, has emerged as a new two-dimensional (2D) material owing to its tuneable band gap [from an indirect bandgap of 1.2 eV (bulk) to a direct bandgap of 1.8 eV (monolayer)] 4 and ambient stability 5 .
The fabrication of a MoS 2 monolayer was first attempted by a micromechanical exfoliation method similar to the approach used for the fabrication of graphene, and the possibility of using MoS 2 as a channel material for a field-effect transistor (FET) was verified 6,7 . Recent achievements 8 in obtaining high-performance FET devices using MoS 2 as a channel material with a dielectric screening method 9,10 gave rise to a number of synthetic processes such as micromechanical [11][12][13][14] and chemical exfoiliation 15,16 , lithiation 17 , thermolysis 18,19 , and two-step thermal evaporation 20 . Subsequently, the sulphurization of a pre-deposited Mo 21,22 was developed, and it was shown that sulphurization is a somewhat suitable method for the synthesis of large-area MoS 2 . However, MoS 2 fabricated by the sulphurization of a pre-deposited Mo exhibit non-uniformity and a low field-effect mobility 21 compared to exfoliated samples, and they occasionally grow perpendicular to the substrate 22 because of ineffective incorporation of sulphur into the pre-deposited Mo. Chemical vapour deposition (CVD) is a well-known method for growing large-area MoS 2 . Lee et al. (ref. 23) demonstrated that CVD using molybdenum oxisulphides (MoO 3−x ) reduced from molybdenum trioxide (MoO 3 ) and sulphur powder is a highly effective method for growing MoS 2 atomic layers on a dielectric substrate. Studies 24-32 with a similar method have demonstrated the effective growth of large-area 24 , high-quality MoS 2 with a larger grain size [25][26][27] and control of the number of layers 31 .
However, to the best of our knowledge, a feasible method for growing a MoS 2 at low-temperatures of below 400 °C has not yet been reported, as it still requires the sulphurization of MoO 3−x at high temperatures ranging from 650 to 850 °C. The 2D materials are suitable materials for next-generation electronic devices such as Scientific RepoRts | 6:21854 | DOI: 10.1038/srep21854 flexible, stretchable, and wearable devices. This devices normally fabricated based on plastic substrate. However, the melting temperature of most plastic substrates (PET, PEN, PI, etc.) are lower than 400 °C that makes impossible to use high-temperature approaches for direct growth. The conventional method to fabricate the flexible devices is using transfer of high-temperature grown 2D materials to plastic substrates 33 . This transfer method does not guarantee the productivity and reproducibility compared to direct growth owing to structure deformation (cracks and wrinkles) 34,35 and remained polymer residues 36 . Thus, low-temperature growth method can open  the cost-and time-effective method for fabrication of flexible devices. Notwithstanding the few works which  demonstrated the possible methods for low-temperature growth [37][38][39] , the electrical performance of as-grown MoS 2 were not reported but only the processes and characterization of samples were presented. Typically, higher temperatures facilitate the growth of a high-quality film owing to the small number of nuclei, the long diffusion length on the surface, and the effective desorption of volatile substances. However, at lower temperatures, the growth of high-quality films is challenging, especially for monolayer growth owing to the small critical radius for nucleation and the short diffusion length on the surface. Herein, we report a direct one-step low-temperature CVD process for the growth of high-quality layered MoS 2 with control of the cluster size and the nucleation sites using Mo(CO) 6 and hydrogen sulphide (H 2 S) as the precursor and reaction gas, respectively. Spectroscopic and microscopic analyses demonstrate that differently structured (3D or 2D) MoS 2 are formed by changing the S-reaction-gas (P SR ) to Mo-precursor (P MoP ) partial-pressure ratio (P SR /P MoP ), and monolayer islands of MoS 2 with a grain size of 100 nm were grown on a nucleation-site-confined silicon dioxide (SiO 2 ) substrate with an optimized P SR /P MoP . In addition, the electrical performance of back-gate FET device using monolayer MoS 2 was examined.

Results and Discussion
It is known that a CVD process using Mo(CO) 6 tend to create large aggregates 40 , Mo-based 3D structured films 41,42 and films containing considerable amounts of carbides or oxides, such as Mo 2 C or MoOC, depending on the deposition conditions 43 . Although many disadvantages are caused by the carbonyl (CO) ligand radiating from the central Mo atom, lower decomposition temperature ( Supplementary Fig. S1) make Mo(CO) 6 a suitable precursor for low-temperature growth. To achieve the 2D growth of layered MoS 2 at 350 °C, we developed a novel method that control the cluster size by feeding precise amount of Mo precursor and the nucleation sites on the SiO 2 substrate by high-vacuum annealing. Although previous studies have verified that large amounts of carrier gas (Ar or H 2 ) can facilitate decarbonylation 38 , the use of carrier gases is excluded in our experiment because large amounts of carrier gas eventually increase the absolute amount of precursor vapour. In order to examine our strategic approach, the experiment was carried out under various P SR /P MoP . The P MoP was precisely controlled using a chiller-heater unit connected to a precursor canister ( Supplementary Fig. S2a). Growth was carried out using a showerhead-type reactor to assist in the creation of a uniform flow 32 ( Supplementary Fig. S2b). Before growth, the SiO 2 substrate was pre-cleaned using acetone, isopropyl alcohol (IPA), and deionized (DI) water to prevent nucleation near dust particles 26 . Subsequently, the substrate was loaded into a load-lock chamber for several seconds to prevent any surface contamination under ambient conditions and transported to the main reactor followed by growth for a specific time at a substrate temperature of 350 °C at various Mo(CO) 6 sublimation temperatures (0 to 80 °C) and H 2 S flow rates (10 to 100 sccm). In our preliminary experiment conducted with a lower P SR /P MoP , structural changes and impurities incorporation ( Supplementary Fig. S3) in MoS 2 depending on the cluster size were observed and revealed that the partial-pressure ratio is the key parameter for 2D growth ( Supplementary Fig. S3a). Figure 1a shows atomic force microscopy (AFM) images of various samples grown at different values of P SR /P MoP . At a lower P SR /P MoP (case 1 and 2), irregular 3D islands with small grain sizes were grown. As P SR /P MoP (case 3) increases, the morphology was changed to a mixed structure which was consisted of irregular 3D islands and 2D triangular islands. At a much higher P SR /P MoP , the structure completely changed to 2D triangular islands with larger grain sizes (case 4). The fact that 3D structural MoS 2 formation at lower P SR /P MoP compare to higher P SR /P MoP may affected by cluster formation in gas phase. At a lower P SR /P MoP , a larger amount of Mo(CO) 6 vapour sublimes and larger-size MoS 2 clusters are formed by the gas-phase reactions. Consequently, the formed clusters were adsorbed onto the surface, and 3D MoS 2 islands were grown (Fig. 1b). At a higher P SR / P MoP , quasi-2D MoS 2 islands are grown on the surface by desorbing volatile by-products and transformed into monolayer MoS 2 by surface diffusion (Fig. 1c). The Raman spectroscopy results of the grown MoS 2 are in agreement with the corresponding atomic structure measurement results (Fig. 1d). The difference between two Raman modes (∆k) resulting from in-plane vibration (E 1 2g ) and out-of-plane vibration (A 1g ) was measured 21.7 cm −1 for MoS 2 grown at a lower P SR /P MoP (cases 1-3) owing to the coincidence of monolayer and bilayer MoS 2 (Fig. 1d), and it further decreased to 18.8 cm −1 at a higher P SR /P MoP (case 4) with a decrease in the full width at half maximum (FWHM) of the E 1 2g mode (Fig. 1e) and an increase in the photoluminescence (Fig. 1f). The FWHM of the PL spectra at higher P SR /P MoP (case 4, Fig. 1f) measured as 26.3 nm, in comparable with high-temperature grown MoS 2 27 . These results indicate that 2D structural MoS 2 could be grown at under the higher P SR /P MoP condition. The formation of 2D islands is elucidated by a theoretical consideration of the chemical potential and surface energy. Schweiger et al. (ref. 44) revealed that the type of edge termination (Mo-or S-edge) and the coverage by sulphur atoms of the monolayer MoS 2 cluster were affected by the chemical potential of sulphur and the relationship with the corresponding parameters such as the ratio of S to Mo (Supplementary Fig. S4). Under strongly sulphiding conditions (high H 2 S partial pressure), the lower chemical potential of sulphur causes 100% coverage of the Mo edge (or S edge) by 100% sulphur to have the lowest surface energy. Under these conditions, the layer atoms are more strongly attracted to the substrate than to themselves, thereby facilitating 2D growth. The S-to-Mo ratios of 1.37, 1.99, 1.95, and 2.27 were measured for MoS 2 grown from lower to higher values of P SR /P MoP from X-ray photoelectron spectroscopic (XPS) analyses (Fig. 1g,h). These observations explain the structural changes and demonstrate that control of cluster size and strongly sulphiding conditions are a crucial factor for the layered growth of a MoS 2 at lower temperatures.
Scientific RepoRts | 6:21854 | DOI: 10.1038/srep21854 The grain size of polycrystalline 2D materials is the most important characteristic for determining its physical and electrical properties 45 . At lower temperatures, the grain size of 2D materials is much smaller than those at higher temperatures owing to the small diffusion length on the surface. We observed single-crystal monolayer MoS 2 domains grown at various values of P SR /P MoP by AFM (Supplementary Fig. S5). However, no grain sizes greater than 50 nm were observed under even strongly sulphiding conditions (P SR /P MoP = 594). This experiment reveals the existence of a grain-size limit at 350 °C. To overcome this limitation due to the short diffusion length on the surface, nucleation sites were artificially manipulated by annealing the substrate in high-vacuum. To examine the effect of nucleation-site manipulation on the grain size, we grew monolayer MoS 2 on three different substrates: piranha (H 2 SO 4 :H 2 O 2 = 3:1)-treated, bare, and high-vacuum annealed SiO 2 substrates, as seen in Fig. 2. A larger number of triangular MoS 2 islands with the smaller grain size was created on the piranha-treated substrate  ( Fig. 2a), whereas a smaller number of islands with the larger grain size was created on the vacuum-annealed substrate (Fig. 2c) compared to that of the bare SiO 2 substrate (Fig. 2b). It is known that the hydroxylated or hydrogen-passivated dangling bonds of amorphous SiO 2 provide many reactive surface sites compared to an unsaturated surface 46,47 . In contrast, the high-vacuum annealing treatment dissociates the hydrogen-passivated dangling-bond entities 47,48 . To clarify the nucleation and growth mechanism on the different substrates, the AFM images obtained at different growth times demonstrate that the MoS 2 nuclei occupy every preferred nucleation site during the early phase of growth and then attach to the edges of as-grown monolayer islands, and no more nucleation was observed during growth ( Supplementary Fig. S6). The monolayer MoS 2 islands were grown up to 100 nm on the nucleation-site-confined substrate. For the growth of MoS 2 with larger grain sizes at lower temperatures, it is crucial to manipulate the affinity of the nuclei and the substrate; thus, the grain-size limitation can be overcome. The effect of substrate temperature on grain size was also examined (Supplementary Fig. S7). The grain size and FWHM of E 1 2g mode of grown MoS 2 were decreased by decreasing temperature owing to short diffusion length on the surface.
The number of a MoS 2 layer has been conventionally controlled by modulating the thickness of the pre-deposited Mo 21 , the surface energy 31 , or the supersaturation 38 . The grown MoS 2 using our method exhibit the characteristic of layered growth (the detailed growth process is shown in Supplementary Fig. S8) without changing other parameters. Different surface colours were observed for different numbers of layers in Fig. 3a, in which highly uniform large-area MoS 2 were grown on 1 × 1 cm 2 SiO 2 substrates and grown at the wafer scale up to 3" in size ( Supplementary Fig. S9), as confirmed by an ellipsometry mapping analysis. We also used Raman spectroscopy and photoluminescence measurements to confirm the thickness of the as-grown MoS 2 . The Raman spectrum of each sample exhibits red and blue shifts of the E 1 2g and A 1g , respectively, as the number of layers increases (Fig. 3b). The ∆k values were measured to be 18.8, 22.6, 23.6, 24.5, and 25 cm −1 (Fig. 3c) from monolayer to pentalayer 49,50 . The normalized intensity was increased for thicker MoS 2 owing to optical interference effect on SiO 2 / Si 49 . When the substrate temperature decreased to 250 °C, the bilayer islands were grown on uncovered monolayer MoS 2 owing to short diffusion length ( Supplementary Fig. S7). The two dominant absorption peaks (near 670 and 620 nm) correspond to two direct excitonic transitions (A1 and B1, respectively) which were observed from the photoluminescence measurements. The intensity of A1 direct excitonic transition was decreased and shifted to the red with increasing number of layer ( Supplementary Fig. S8i), in agreement with previous reports 4,5 . Our cluster-size control method provides a feasible way for the layered growth of MoS 2 at the wafer scale and open the effective way for the photoelectric device applications without transfer process.
The atomic structure of an as-grown monolayer MoS 2 was evaluated by aberration-corrected scanning transmission electron microscopy (Cs-STEM) high-angle annular dark-field (HAADF) imaging. Figure 4a shows a low-magnification STEM-HAADF image of a MoS 2 monolayer transferred onto a carbon grid by a conventional wet-etching method. The white region represents the overlapping MoS 2 monolayer during transfer, and the grey region indicates a polycrystalline MoS 2 monolayer. The approximate domain size is 100 nm and is in agreement with our previous observations using AFM (Supplementary Fig. S6b). The high-magnification HAADF image of the selected area shows the atomic structure of the grain boundary by two triangular domains (Fig. 4b). The fast Fourier transform (FFT) patterns in the inset of Fig. 4b indicate the hexagonal structures of the two single-crystal MoS 2 domains with a 31° tilt angle. From the image reconstructed by smoothing and Fourier filtering (Fig. 4c), a uniform single-crystal MoS 2 domain was observed, and the merge to create a grain boundary (indicated by the dashed white line in Fig. 4c), thereby forming a polycrystalline MoS 2 monolayer. Moreover, the samples grown at a higher P SR /P MoP exhibit better quality compared to those grown at a lower P SR /P MoP (Supplementary Fig. S10). This microscopic observation reveals that a highly uniform and large-grain MoS 2 polycrystalline monolayer was grown even at 350 °C. Furthermore, the domain structure and grain boundary closely resemble MoS 2 grown at higher temperature. The low-temperature grown monolayer MoS 2 was used to fabricate a back-gate FET to examine the electrical performance. The device was fabricated with a MoS 2 monolayer without patterning and has a channel length and width of 5 and 10 μ m, respectively (inset in Fig. 4d). The MoS 2 monolayer was not treated after growth, and measurements were obtained at room temperature under ambient conditions. The FET device exhibits conventional n-type semiconductor behaviour with a mobility of 0.15 cm 2 V −1 s −1 (Fig. 4d). The maximum on/ off ratio was 10 5 in the gate-voltage range of − 150 to 150 V with a 5-V source-drain bias voltage that was ten times lower than high-temperature grown MoS 2 by using CVD [24][25][26][27] and exfoliated MoS 2 (1-10 cm 2 V −1 s −1 ).

Conclusion
In conclusion, we developed a novel method for the layered growth of large area and high-quality MoS 2 compare to other low-temperature method at a low-temperature of 350 °C using Mo(CO) 6 by controlling the cluster size and nucleation sites. Furthermore, we first demonstrate the potential use of low-temperature grown MoS 2 as practical FET device. A structural transition from 3D clusters to 2D monolayers by changing P SR /P MoP and controlling the grain size with confined nucleation sites were demonstrated. These two parameters are key factors for the low-temperature growth of the layered MoS 2 . The low-temperature growth of 2D materials represented by graphene and transition-metal dichalcogenides is crucial for the application of next-generation flexible and wearable devices. Thus, our results suggest novel approaches for the preparation of 2D materials under lower temperature conditions.

Methods
Growth process. Layered MoS 2 was grown by a showerhead-type reactor using Mo(CO) 6 (≥ 99.9%, Sigma Aldrich, CAS number 13939-06-5) as a precursor. Highly doped (< 0.005 Ω ·cm) p-type Si with a 300-nm-thick SiO 2 layer was used as the substrate. The substrates were pre-cleaned and placed onto a silicon carbide (SiC)coated susceptor in a load-lock chamber within a short period to prevent any contamination in the ambient environment. The heating block in the CVD reactor was pre-heated to 350 °C before growth. The susceptor with the substrate was transferred to the reactor, and the substrate temperature was increased over a period of 10 min in an Ar flow having a purity of 99.999%. The growth was carried out using only sublimed precursor vapour with a high-purity H 2 S flow for growth times at a constant pressure of 0.5 Torr. The substrates were transferred to the load-lock chamber after growth and cooled down for 1 h with 100 sccm Ar flow ( Supplementary Fig. S2a). The treatment after growth was not carried out with any known method (such as Ar and H 2 S annealing at a high temperature). All analyses and characterization were performed using as-grown samples. AFM measurement. The morphology, grain size, and nucleation and growth processes were evaluated using AFM (XE-150, Park Systems). For better quality, an image was measured using a super sharp silicon tip with a radius of curvature of < 5 nm (SSS-NCHR, NANOSENSORS). A soft X-ray ionizer module was applied to prevent electrostatic charge during measurement. The image was taken over a 1 or 2 μ m 2 area with a 512 × 512 pixel resolution and a measurement speed of 0.5 Hz. The images were resized to 750 nm 2 .
Spectroscopy. Raman spectroscopy measurements were carried out using a DXR Raman Microscope (Thermo Scientific). A laser with an excitation wavelength of 532 nm, a spot size of 0.7 μ m, and a power of 8 mW was used. The approximate spectral resolution is 0.5 cm −1 , and the 520.8 cm −1 Si peak was used for normalization. Photoluminescence  HAADF-STEM. HAADF-STEM images were taken using Cs-STEM (Titan cubed G2 60-300, FEI) operated at 300 kV with a 50-100 pA screen current and a 19.3 mrad convergence angle. The images were further smoothed and Fourier filtered to improve the contrast.
Electrical performance measurement. The back-gate FET device was fabricated by using electron-beam evaporation to deposit Ti/Au (5/50 nm) electrodes directly onto an as-grown MoS 2 monolayer. The electrode shapes were patterned using electron-beam lithography of a PMMA (950 C4, MicroChem) layer and developed with diluted MIBK (MIBK:IPA = 1:1, MicroChem) solution. The lift-off process was conducted by immersion into dichloromethane (DCM) and IPA and drying with high-purity N 2 (99.999%). The electrical performance of the device was measured at room temperature under ambient conditions using an in-house four-probe station with a precision semiconductor parameter analyser (4156A, Hewlett-Packard). The device was not annealed.