Transition metal dichalcogenides (TMD), MX2 (M = Mo, W; X = S, Se, Te), have attracted considerable attention for their great potential in the fields of catalysis, nanotribology, microelectronics, lithium batteries, hydrogen storage, medical and optoelectronics.1–12 MoS2 nano-materials have been known in the form of nested fullerene-like nanodots and one-dimensional nanotubes.1–4, 13–17 Stimulated by the discovery of two-dimensional graphene monolayer and its rich physical phenomenon, inorganic graphene analogues such as layered MoS2, where the Mo layer is sandwiched between two sulfur layers by covalent forces, have created great interest in the past few years. Recently, Radisavljevic et al. have demonstrated that the transistors fabricated with the exfoliated MoS2 monolayer18, 19 exhibit high on-off current ratio and good electrical performance, which may be used in future electronic circuits requiring low stand-by power. The strong emission inherited from the direct gap structure of monolayer MoS2 also promises the applications in optoelectronics.20–22
Substantial efforts have been devoted to prepare thin-layer MoS2, including scotch tape based micromechanical exfoliation,18–24 intercalation assisted exfoliation,25–27 liquid exfoliation,28 physical vapor deposition,29, 30 hydrothermal synthesis,31 thermolysis of single precursor containing Mo and S.32, 33 The lateral size of the MoS2 films synthesized by the aforementioned methods is often on the order of several micrometers; however, the synthesis of large-size MoS2 thin layers is still a challenge. Chemical vapor deposition (CVD) has been one of the most practical methods for synthesizing large-area graphene34–36 and graphene analogues such as boron nitride and BCN nanosheets.37, 38 The sulfurization of MoO3 using the CVD method has been adopted to synthesize MoS2 materials; however, the reaction normally leads to MoS2 nanoparticles or nanorod structures during the synthesis.39, 40 To our best knowledge, synthesis of large-area, monolayer MoS2 films on amorphous SiO2 substrates using a CVD method has not yet been reported. In this contribution, CVD is adopted to synthesize MoS2 layer directly on SiO2/Si substrates using MoO3 and S powders as the reactants. The growth of MoS2 is very sensitive to the substrate treatment prior to the growth. The use of graphene-like molecules for the substrate treatment, such as reduced graphene oxide (rGO), perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), promotes the layer growth of MoS2. Large-area MoS2 layers can be directly obtained on amorphous SiO2 surfaces without the need to use highly crystalline metal substrates or an ultrahigh vacuum environment, which is in clear contrast to the reported epitaxial growth of MoS2 nano-islands on crystalline Au(111) surfaces in ultrahigh vacuum.29 Spectroscopic, microscopic and electrical measurements suggest that the synthetic process leads to the growth of monolayer, bilayer and few-layer MoS2 sheets. These MoS2 films are highly crystalline and their size is up to several millimeters.
Figure 1a schematically illustrates our experimental set-up. The MoO3 powder (0.4 g) was placed in a ceramic boat and the SiO2/Si substrate was faced down and mounted on the top of boat. A separate ceramic boat with sulfur powder (0.8 g) was placed next to the MoO3 powder. Prior to the growth, a droplet of aqueous reduced graphene oxide (rGO), PTAS or PTCDA solution, was spun on the substrate surface followed by drying at 50 °C. During the synthesis of MoS2 sheets, the reaction chamber was heated to 650 °C in a nitrogen environment. At such a high temperature, MoO3 powder was reduced by the sulfur vapor to form volatile suboxide MoO3–x.39 These suboxide compounds diffused to the substrate and further reacted with sulfur vapor to grow MoS2 films. Figure 1b displays the OM of the MoS2 sheets obtained on the SiO2/Si substrate pretreated with an rGO solution and inset shows that a white dot is present at the center of a star-shaped MoS2 sheet, where these dots seem to act as the seeds for growing MoS2 layers. More images are shown in Supporting Information Figure S1 to evidence the observation. The star-shaped MoS2 can be merged to form a continuous MoS2 film (with a lateral size up to 2 mm) as shown in the upper area of Figure 1b, where the seed density is higher. In Figure 1c, smooth surface morphology of MoS2 sheets is observed with atomic force microscope (AFM), suggesting that a layer structure of MoS2 is formed. The cross-sectional height in Figure 1d reveals that the thickness of the MoS2 film is ∼0.72 nm, which corresponds to a monolayer MoS2 sheet based on previous reports for a monolayer MoS2 on a Si/SiO2 substrate.26 In addition to monolayer MoS2, we also occasionally find bilayer, trilayer and other thicker layers. Supporting Information Figure S2 provides AFM images and cross-sectional profiles of the thicker MoS2 films. Supporting Information Figure S3 shows the optical microscopy (OM) images of the layered MoS2 grown respectively with the PTAS and PTCDA pre-treatments, where the MoS2 layer growth is initiated from the PTAS or PTCDA molecular aggregates. Similar to the role of rGO, the PTAS or PTCDA molecular aggregates act as the seeds for growing MoS2 thin layers. Experimentally, the MoS2 layer growth initiated by rGO is more homogenous in layer thickness. Hence, the subsequent discussions are mainly based on rGO initiated MoS2 thin layers.
To explore the Raman and PL dependency on MoS2 layer thickness, we identify an area with MoS2 monolayer, bilayer and trilayer films. Figure 2a and 2b respectively shows the mapping constructed by plotting the integrated MoS2 Raman peak intensity (360 ∼ 420 cm−1) and the PL peak intensity (650 ∼ 700 nm) in confocal measurements. The thickness distribution seems to correlate well to the contrast in OM image (Figure 2c). The MoS2 monolayer sheet exhibits two Raman characteristic bands at 403.8 and 385.8 cm−1 with the full-width-half-maximum (FWHM) values of 6.6 and 3.5 cm−1, corresponding to the A1g and E2g modes respectively. Note that the peak frequency difference between A1g and E2g modes (Δ) can be used to identify the layer number of MoS2. The value of Δ (18 cm−1) in Figure 2d evidences the existence of monolayer MoS2.24, 41 The inset in Figure 2d shows that the Δ value increases with the layer number of MoS2, where the layer number is confirmed by AFM thickness (Supporting Information Figure S2). These results agree well with the observation for exfoliated MoS2 layer.23 In Figure 2e, the PL spectrum shows two pronounced emission peaks at 627 and 677 nm24 and these emissions are known as the A1 and B1 direct excitonic transitions.44 The emission intensity (normalized by the Raman scattering at ∼482 nm) obviously decreases with the layer number. This can be reasoned by the fact that the optical bandgap transforms from indirect to direct one when the dimension of MoS2 is reduced from a bulk form to a monolayer sheet.19 The X-ray photoelectron spectroscopy (XPS) scans for the monolayer MoS2 sample confirm the chemical bonding states of the MoS2 layers (Supporting Information Figure S4). These binding energies are all consistent with the reported values for MoS2 crystal.32, 43
Figure 3a shows the transmission electron microscopy (TEM) image for the monolayer MoS2. The high resolution TEM image (Figure 3b) and the corresponding selected area electron diffraction (SAED) pattern with  zone axis (inset of Figure 3b) reveal the hexagonal lattice structure with the lattice spacing of 0.27 and 0.16 nm assigned to the (100) and (110) planes. The distinct SAED pattern suggests that the crystalline domain of the MoS2 layer is at least 160 nm in lateral size (SAED aperture size ∼160 nm in our measurement). Figure 3c displays the TEM image for the selected grain boundary area as indicated by the inset AFM, where the junction between two MoS2 domains is clearly seen. The in-plane X-ray diffraction (XRD) profile for the MoS2 monolayer synthesized by the CVD method is shown in Figure 3d and the diffraction peaks at 32.4 and 58 degree are attributed to the (100) and (110) crystal planes respectively. Meanwhile, the stoichiometry of the MoS2 film has been separately confirmed with XPS (S/Mo ratio ∼ 2.08) and transmission electron microscopy energy dispersive X-ray spectroscopy (TEM-EDX) as shown in Supporting Information Figure S5.
To evaluate the electrical performance of the MoS2 sheets, we fabricate bottom-gated transistors on SiO2/Si using conventional photolithography. The bottom-gate transistors were fabricated by evaporating Au electrodes directly on top of the MoS2 layer. Figure 4 demonstrates the transfer curve (drain current Id vs. gate voltage Vg) for the device prepared from a MoS2 monolayer. Inset shows the top view OM of the device. The on-off current ratio is approximately 1 × 104. The field-effect mobility of holes was extracted based on the slope ΔId/ΔVg fitted to the linear regime of the transfer curves using the equation μ = (L/WCoxVd)(ΔId/ΔVg), where L, W and Cox are the channel length, width and the gate capacitance.44 The effective field effect mobility for the MoS2 device can be up to 0.02 cm2/(V-s) in ambient, in agreement with previous reports.18, 41, 45, 46 We note that the valley point of the transfer curve is at -84 V and the FET shows the typical n-type behavior, which is consistent with other reports.18, 47 Although the device exhibits a reasonably high on/off current ratio, there is still room to improve the carrier mobility. The relatively lower carrier mobility than the mechanically exfoliated MoS2 is likely limited by the structural defects, such as the grain boundary observed by TEM (Figure 3c).
As revealed in Figure 2b, the star-shaped MoS2 layers were grown from center seeds, which suggest that the nucleation was a crucial step. The spin-casting of rGO solution before CVD growth introduced some tiny rGO flakes on the substrate surfaces, which experimentally enhanced the growth of MoS2 layers. Supporting Information Figure S6 and Table S1 shows that the morphology of the synthesized MoS2 film is significantly affected by surface treatments. Without treating the substrate surface with rGO solution, only MoS2 particles were found on the substrate. Other control experiments where the substrates separately cast with a graphene oxide (GO), hydrazine or KCl solution show that no MoS2 layers but only sparsely distributed MoS2 nano-particles are observed on substrates. Compared with more ordered aromatic structures of the graphene-like molecules including rGO, PTAS, and PTCDA, the GO is with randomly distributed defects and dangling bonds, which might be one of the reasons not being able to initiate layer growth. Although the GO may be thermally reduced to rGO48 at the MoS2 growth temperature (650 °C), the formation of MoS2 seeds should involve many other factors such as the reaction between MoO3 and S, the attachment of MoO3–x vapors onto GO, the conversion of MoO3-x to MoS2, and the morphology of the MoS2 seeds formed on substrates. These reactions may occur during the temperature ramping period. It is likely that the MoS2 seed morphology formed on GO prefers particle growth rather than layer growth. It is noted that our experimental results only allow us to conclude that the rGO treatment helps to form the MoS2 seeds which prefers and promotes the layer growth of MoS2. The morphology and structure of the seeds, requiring intense research efforts, are currently under investigation in our group. Meanwhile, we observe that both MoS2 and WS2, two typical transition metal dichalcogenides (TMD), exhibits similar layer growth behavior on the substrates pre-treated with graphene-like molecules (Supporting Information Figure S7). The growth of MoS2 and WS2 layers is highly reproducible with our experimental conditions. A similar enhancement is expected to be observed in other transition-metal-disulfide TMD family materials.
In conclusion, large-area MoS2 films are directly synthesized on SiO2/Si substrates with chemical vapor deposition. It is noteworthy that the growth of MoS2 is not unique to SiO2 substrates and it is also observed on other insulating substrates such as sapphire. The as-synthesized films are consisted of monolayer, bilayer and other few-layer MoS2. Chemical configurations, including stoichiometry and valence states of MoS2 layers are confirmed with XPS. Raman spectra and PL performance of the monolayer MoS2 are presented. TEM and SAED demonstrate that the monolayer MoS2 exhibits six-fold symmetry hexagonal lattice and high crystallinity. The electric measurement for the bottom-gate transistor shows a N-type semiconductor behaviour and the on-off current ratio is approximately 1 × 104. The seeding approach can be further used to grow other transition metal dichalcogenides.