Atomic-layered molybdenum disulfide (MoS2), a new two-dimensional (2D) material, has gained intense attention due to its outstanding electrical and optical properties ^[1–5]. When bulk MoS2 is reduced to monolayer sheet, an indirect-to-direct bandgap transition occurs, leading to extremely high quantum efficiency for light emission ^[6,7]. Compared with graphene, MoS2 has a visible bandgap to allow the field-effect transistors based on this material to be effectively switching off. Moreover, the carrier mobility is impressively high, up to 400cm2V−1s−1 in theory ^[8] and 45cm2V−1s−1 demonstrated in experiments ^[2] for monolayer MoS2. In order to fulfill the demands for device applications, substantial efforts have been devoted into synthesize atomically thin MoS2 film, including micromechanical exfoliation ^[4,5,9,10], liquid exfoliation ^[11–13], and physical vapor deposition ^[14]. However, MoS2 films are often limited to their micrometer size, which hinders large-scale device applications. Hence, it is critical to develop practical methods to grow large-area MoS2 thin film aiming for device fabrications.

Similar to the growth of large-area graphene film, chemical vapor deposition (CVD) has been successfully applied to synthesize large-area MoS2 film. Zhan et al. ^[15] reported the production of large-area few-layer MoS2 film through sulfiding the Mo/SiO2 substrate but the thin films possess poor quality. Alternatively, the thermal decomposition of (NH4)2MoS4 ^[16] in the presence of mixed gas could also induce large-area MoS2 thin film; however, the pre-deposition of precursors by dip-coating lacks the control of thickness. Lee et al. ^[17] chose MoO3 powder as precursor for CVD growth of MoS2 thin films by seeding the substrate with reduced graphene oxides. The resulting product may have contamination of the seeds. Recently, Yu et al. ^[18] reported a self-limiting CVD approach to produce large-area MoS2 thin films on various substrates but a very low carrier mobility (0.003–0.03cm2V−1s−1) was achieved.

In this work, we report a seedless and scalable growth method using ambient pressure CVD to produce high quality few-layer MoS2 thin film. Compared with previous reports which used low-pressure CVD to grow MoS2 ^[18,19], the present method has simpler operation procedures and better repeatabilities. The as-grown MoS2 on SiO2/Si is suitable for following device fabrication without transfer process which could induce defects and impurities. The scalable growth method can be further adopted to more complicated photonic structures such as waveguides and gratings, affording compatability with integrated photonic circuits.

Figure 1(a) schematically shows the experimental setup to synthesize MoS2 thin film on SiO2 substrate. The sulfur powder was located at upstream of the CVD system. The MoO3 powder was placed in a quartz boat in the furnace and the SiO2 substrate was next to the MoO3 source. During the growth, sulfur and MoO3 were heated to 100°C and 650°C, respectively. The carrying gas argon was at a flow rate of 150 sccm and the system was kept at ambient pressure. As illustrated in Fig. 1(a), MoO3 is reduced by sulfur vapor to form MoO3−x species ^[17,20], which will diffuse to SiO2 substrate and eventually lead to nucleation and growth of MoS2 thin film.

Figures 1(b)–1(d) show scanning electron microscope (SEM) images of MoS2 films with different morphologies. The growth times are 5, 12, and 30 min for Figs. 1(b), 1(c), and 1(d), respectively, with the same growth temperature of 650°C. It is found that these samples represent different growth stages. At the early stage of growth, the triangle-shaped MoS2 crystals randomly distribute on SiO2 substrate, as shown in Fig. 1(b). Upon prolonging the growth time to 12 min, smaller grains turn into larger ones by adsorbing atoms, as indicated in Fig. 1(c). It should be noted that the crystals tend to interconnect rather than overlap with each other when they grow larger. When the growth time is prolonged to 30 min, full coverage of MoS2 film was achieved due to the coalescence of large crystals, as shown in Fig. 1(d). The uniformity is verified by the small difference in the contrast over the whole film. The white color lines mainly correspond to gaps among crystals. When MoS2 film is fully covered on the substrate, incoming MoS2 species will nucleate to form thicker triangle crystals (as indicated by the red arrows), whereas a small portion of them will aggregate to form curved nanoparticles or quasi-one-dimensional nanowires.

Figure 1(e) shows the representative atomic force microscopy (AFM) topography of MoS2 film on SiO2 substrate. It reveals a few triangular domains interconnecting with each other. The homogeneous color contrast indicates good uniformity. The line profiles indicate thickness of about 1.32 and 1.96 nm, corresponding to bilayer ^[18] and trilayer MoS2 film ^[21]. The few-layer MoS2 thin film grown on SiO2 substrate can be completely peeled off and transferred to other arbitrary substrates. Figure 1(f) shows a centimeter-size MoS2 film which was transferred onto polydimethylsiloxane (PDMS) film with good continuity.

Spectroscopic characterizations were carried out to evaluate the quality of MoS2 film. Figure 2(a) shows Raman spectra of the as-grown MoS2 film measured at the excitation of 514 nm, which reveals two characteristic peaks. The peak is around 380cm−1 attributed to the in-plane E2g mode resulting from opposite vibration of two S atoms with respect to the Mo atom. The peak is around 400cm−1 assigned to the A1g mode which is associated with the out-of-plane vibration of only S atoms in the opposite directions ^[22,23]. The frequency difference (Δ) between E2g and A1g increases with the thickness, which can be used to identify the number of layer in atomic layered MoS2 ^[16,20,24]. In this context Δ is calculated to be ∼21.5 and 22.9cm−1, corresponding to bilayer and trilayer MoS2 film, respectively ^[21,22]. Figure 2(b) shows the photoluminescence (PL) spectrum (with 514 nm laser excitation) which also consists of two pronounced emission peaks at around 625 and 670 nm. The strong light emission corresponds to the direct excitonic transitions at the Brillouin zone K point ^[20]. The appearance of these two emission peaks is in consistent with two excitonic absorption bands around 620 and 670 nm shown in the UV−visible spectrum [Fig. 2(c)].

Transmission electron microscopy (TEM) is also employed to characterize the quality and crystal structure of transferred MoS2 film. Figure 3(a) shows the microstructure of a piece of MoS2 film folded on TEM grid. The high-resolution TEM (HRTEM) image clearly resolves the lattice of few-layer MoS2 film. The selected area electron diffraction (SAED) pattern in the inset of Fig. 3(b) contains multi-group 6-fold symmetry spots, which indicates that the measured zone of MoS2 film contains several grains possessing different crystal orientations. Meanwhile, the X-ray photoelectron spectroscopy (XPS) results shown in Fig. 3(c) reveal the binding energies of Mo-3d3/2 and Mo-3d5/2 at 232.5 and 229.3 eV, respectively. Two peaks located at 163.2 and 162.0 eV are attributed to S-2p1/2 and S-2p3/2, respectively. These binding energy results are in good consistent with previous reports ^[25,26], suggesting the successful growth of layered MoS2 film. The X-ray diffraction (XRD) profile for few-layer MoS2 is shown in Fig. 3(d). The diffraction peaks at 14.4° and 33.1° are attributed to the (002) and (100) crystal planes, respectively ^[6,16].

To investigate the electrical performance of our sample, we fabricated back gate field-effect transistors using few-layer MoS2 directly grown on SiO2/Si substrate. The typical transfer Id versus Vg curve of MoS2 transistor device is shown in Fig. 4(a). We can extract the device on-off ratio up to 105, which is sufficiently high for transistor device based on few-layer MoS2 considering that back gate structure was used. Furthermore, the calculated field effect carrier mobility turns out to be 3.4cm2V−1s−1 via μ=dIDdVgLWdε1VD, where ε is the dielectric constant of SiO2; L and W are the channel length and channel width, respectively. It is worth noting that the room temperature carrier mobility of our device is comparable to that based on monolayer MoS2 with the similar configuration, which are usually in the range of 0.1–10cm2V−1s−1 ^[27,28]. Figure 4(b) shows output characteristics of the few-layer MoS2 device. It is seen that the drain current is dependent on the drain–source voltage at various back-gate voltages, which implies good gate tunability of our device.

In summary, we have successfully developed a facile method to grow large-area MoS2 on SiO2/Si substrate via ambient pressure CVD. The material characterization results show that our MoS2 films are mainly two to three atomic layers with good uniformity and high quality. The few-layer MoS2 thin film show good electrical properties, i.e., the mobility of our MoS2 transistor is comparable to that of transistors based on monolayer MoS2 flakes. Our work provides a route toward scalable growth of high-quality few-layer MoS2 film and the MoS2 thin films are ideal materials for electronics and optoelectronic devices.

The MoS2 films were synthesized on SiO2/Si substrates (SiO2 thickness, 300 nm) via CVD technique. The substrates were cleaned in acetone and isopropanol successively. Prior to the growth, argon gas (99.999%) was used to clean the growth chamber at room temperature. High-purity MoO3 (99.5%, Alfa Aesar) and sulfur powder (99.5%, Sinopharm Chemical Reagent, Shanghai Co., Ltd.) were kept in two separate quartz boats in the furnace with the substrates placed downstream of the CVD system. For the growth of MoS2, the furnace was heated up to 650°C and maintained for 30 min, while the argon gas was purging with a flow rate of 150 sccm at ambient pressure. Afterward, the furnace was cooled down naturally to room temperature.

Raman and PL spectra were collected by micro-Raman system (Horiba Jobin Yvon, HR800). The excitation wavelength is 514 nm, and the laser spot is focused to <2μm with 100× objective lens. UV-visible absorption spectra were obtained by UV-visible-IR spectrometer (PerkinElmer, Lambda750 system). Chemical composition and crystal orientation were studied using XPS (KRATOS Analytical, KRATOS AXIS Ultra DLD) and XRD (PANPANalytical B.V.P, Ruki). Surface morphology of the samples was examined by SEM (FEI, Quanta 200 FEG) and AFM (Bruker, Dimension Icon). TEM (FEI, Tecnai G2 F20) was used to investigate the microstructures.

The transistor devices were directly fabricated on MoS2 films grown on SiO2/Si substrates. The highly doped n-type silicon of SiO2/Si substrate serves as the back-gate electrode. The device fabrication includes UV lithography to define the device pattern and electron-beam evaporation to deposit source and drain electrodes (100 nm Au on top of 5 nm Ti).

The measurements were performed by probe station (Cascade M150) equipped with a semiconductor property analyzer (Keithley 2400) at room temperature in ambient conditions.