In recent years, two-dimensional materials have gained a great deal of attention from the scientific research and industrial production community as promising materials for the next-generation of ultrathin electronic and optoelectronic devices.^[1–3] Graphene, as the most famous member of two-dimensional materials, possesses extraordinary properties and is readily integrated in various applications.^[4,5] Compared with graphene, which is a semi-metal with no band gap by nature, transition metal dichalcogenide (TMD) with a substantial band gap has been investigated extensively. As a typical TMD material, MoS₂ has been found to have electronic applications because of its unique and exciting properties, such as the thickness-dependent band gap transition: from a direct band gap of 1.9 eV for a monolayer to an indirect one of 1.3 eV for bulk,^[6–8] excellent carrier mobility with a high on-off ratio,^[9,10] and valley-related physics.^[11,12] These attract a large number of researchers who dedicate themselves to exploring its unique performance and application research of the device from electronics to battery materials. Nevertheless, large-scale, layer-controlled and high-quality two-dimensional MoS₂ should be prepared for meeting its various application demands.

There are generally two kinds of preparation methods for two-dimensional MoS₂, including top-down methods, such as mechanical exfoliation,^[9] chemical exfoliation,^[13] and direct sonication in solvents,^[14] which separate the stacked thick layers (or even bulk materials) of MoS₂. However, the resulting MoS₂ are typically microscale with poor thickness uniformity, ranging from monolayer to tens of layers. To improve this, several bottom-up methods have been developed to directly synthesize large-area MoS₂ monolayers onto SiO₂/Si substrates, such as the sulphurisation of pre-deposited Mo^[15] or Mo oxide^[16] layers, the decomposition of thiomolybdates,^[17] and chemical vapor deposition (CVD) through a gas-phase reaction of MoO₃ and S.^[18] Among these, CVD is the most promising method to obtain high-quality MoS₂ films. The CVD-grown films usually have a variety of shapes, such as triangle, similar hexagon, truncated triangle, dendritic and polygonal shapes.^[19] The growths of different shapes would result in different crystal grain boundaries, and then affect the boundary energy in growth dynamics, even the quality of film. Thus, the nucleation mechanism and morphology evolution research would be the critical factors for fabricating the large area and high-quality MoS₂.

In this work, we report the directly CVD synthesis of MoS₂ flakes by using solid S and MoO₃ powders on sloping SiO₂/Si substrates. The nucleation mechanism and morphology evolution of MoS₂ grown by CVD are investigated. The results point out that the original nucleus is triangle. With the growth progress, the final shape of MoS₂ flakes could be determined by the growth speed of Mo termination and S termination sides under the S rich environment. The nucleation density also could be controlled by controlling the reactant concentration.

MoS₂ flakes were fabricated in a double heating zone furnace with a 50-mm-diameter quartz tube as shown in Fig. 1(a). A ceramic boat containing 200-mg sulfur powder (99.999%, Sigma-Aldrich) was placed in the first heating zone of the furnace, 10-mg MoO₃ powder (99.99%, Sigma-Aldrich) was placed in the second heating zone of the furnace, and 300-nm-thick SiO₂/Si substrate was placed slopingly downstream 6 cm away from the MoO₃ powder. The furnace was first evacuated to lower than 2 Pa using a vacuum pump and then purged 3 times with Ar. The MoO₃ powder was heated to 900 °C with a heating rate of 15 °C/min. In the fabricating process, the S and MoO₃ were set to achieve the highest temperature at the same time. Therefore, S started to heat to 190 °C with the heating rate of 20 °C/min when MoO₃ is 760 °C, and then maintained for 20 min with 130 sccm of Ar followed by natural cooling. The working pressure is 200 Pa.

Fig. 1.

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	Fig. 1. (color online) Schematic illustrations of (a) growth set up and (b) substrate sectioned into 5 parts.

The MoS₂ flakes were characterized by scanning electron microscopy (SEM) using the Nova NanoSEM 450 instrument, atomic force microscopy (AFM) recorded using a tapping mode in a Bruker Multimode 8 AFM system, and Raman spectroscopy and photoluminescence conducted using a JY Horiba Labram HR800 Evolution imaging confocal Raman microscope under an excitation wavelength of 532 nm.

Figure 1(b) shows the schematic illustration of the SiO₂/Si substrate sectioned into five parts according to the different reactant concentrations along the direction vertical to the gas flow. The corresponding SEM images of the five sections are shown in Fig. 2, which exhibits the evolution of MoS₂ flakes. There are no triangular MoS₂ flakes in section 1 at the bottom of the substrate as shown in Fig. 2(a), since the reactant concentration is too low for the further growing of the MoS₂ flakes. In section 2 as shown in Fig. 2(b), MoS₂ flakes with triangular shape^[20] randomly distributed on the substrate and with an average size of about 2 μm (inset of Fig. 2(b)), indicating that more reactants reach here and incorporate into the MoS₂ flakes. The triangular flake further develops into a similar hexagon in Fig. 2(c) (section 3) with the distance of the two opposite sides being about 2.5 μm (inset of Fig. 2(c)). In this section, the growth rates of crystal faces of S and Mo termination are close to conformity, indicating that the preferential growth of the crystal face is inhibited, which leads to the similar hexagon MoS₂ flakes. The nucleation density of section 3 does not change obviously compared with the scenario in section 2: it has only a little bit of increase in size and the morphology changes from triangular to a similar hexagon. In section 4 as shown in Figs. 2(d) and 2(e), a similar hexagon is converted into a truncated triangle with the size increasing from 4 μm to 14 μm, and a crystal face preferential growth mode appears again. In this section, the nucleation density increases with the increase of the reactant concentration. Meanwhile, some new triangles begin to grow on the same nucleation site at the center of the truncated triangles, which are all distributed randomly.^[21] The growth of a new layer occurs also at some non-nucleation points since there might be some defects. With the further increase of the reactant concentration, adjacent MoS₂ flakes connected to each other with few overlaps at the border (the color change does not appear obviously at the border in Fig. 2(f) (section 5)). The two-dimensional grain boundaries meet, merge and disappear, and even connect to a layer, which can be explained by the theory of multi two-dimensional nucleation growth discussed in the following part.

The AFM, Raman and PL spectra are used to evaluate the thickness and quality of the MoS₂ flakes. The height image obtained from a tapping mode AFM measurement is shown in Fig. 3(a). The height profile drawn at the edge as shown in Fig. 3(b) reveals a film thickness of ∼ 0.7 nm, which agrees well with one atomic layer of MoS₂ flakes.^[22]

Fig. 2.

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	Fig. 2. (color online) SEM images in five sections: panels (a), (b), (c), and (f) for sections of 1, 2, 3, and 5; panels (d) and (e) for section 4 from down to up.

Fig. 3.

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	Fig. 3. (color online) (a) AFM images of a MoS2 flakes on SiO2/Si substrate and (b) profile along the line in panel (a).

Figure 4(a) shows the spectra of MoS₂ flakes with different morphology and different regions marked as curves 1-5 (insets of Fig. 4(a)). Raman peaks corresponding to in-plane vibrations of Mo and S atoms ( mode) and out-of-plane vibration of the S atom (A_1g mode)^[23] are exhibited in curves 1–4 respectively, but do not emerge in curve 5, which indicates that the outside of the triangle is the substrate without the MoS₂ flakes. The differences between the two peaks (Δk) from curves 1–4 gradually increase. The Δk ∼ 20.4 cm⁻¹ of the small triangle (region 1) is close to synthesized monolayer MoS₂,^[8]but larger than that of exfoliated monolayer MoS₂ (Δk < 20 cm⁻¹),^[24] which is likely to be attributed to the defects that are introduced in the synthesis process. The increases of Δk in curves 2 and 3 regions might be caused by more defects introduced in the growing process, however, the Δk of region 4 increases to 23.2 cm⁻¹, which is corresponding to multilayer MoS₂. Besides, the full width at half-maximum (FWHM) of the peak in region 1 is ∼ 4.5 cm⁻¹,^[25] which further confirms the monolayer nature and high quality of the MoS₂ flakes. Figure 4(b) shows the Raman mapping of a truncated triangle with a new small triangle in the center by plotting the 2D spatial variation of the magnitude of the frequency difference between the A_1g and peaks to show the thickness uniformity. At the edge of the truncated triangle, most of the flakes have a Δk of < 21 cm^{− 1}, confirming a homogeneous monolayer. An obvious increase in the frequency difference can be observed in the center of the triangular domain, which indicates that some new layers grow on the original MoS₂ flakes. This confirms the SEM results in Fig. 2(e).

Fig. 4.

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	Fig. 4. (color online) (a) Raman spectra of different regions marked as 1–5 in the inset, the inset shows SEM images of the small triangle, the similar hexagon, the edge of truncated triangle, the center of truncated triangle and the outside of the triangle marked as 1–5 correspondingly, (b) Raman mapping of the magnitude of the frequency difference between A1g and .

Figure 5(a) shows the typical PL spectra of different regions marked as 1–5 in the insert of Fig. 4(a). The PL spectra are normalized by the Raman peak intensity to exclude external environmental effects such as laser power and local electric field factors. Pronounced luminescence emissions in 1–3 regions are observed at about 1.80 eV as well as a shoulder that can be seen at 1.96 eV as shown in the inset of Fig. 5(a), which can be correlated to the A and B exciton transition arising from the direct gap transition at the K point for the monolayer MoS₂.^[26] The PL intensities of 1–3 regions gradually decrease, which could be attributed to a large number of defects in the synthesis process. In contrast to the strong PL of monolayer MoS₂ flakes (region 1–3), the center of the truncated triangle with new layers (region 4) exhibits a weak peak at 1.75 eV, which corresponds to an indirect transition at lower energy. This distinguishing PL behavior of the multilayer MoS₂ flakes indicates that luminescence quantum efficiency is much higher in MoS₂ monolayer than in multilayer.^[27] This could also be observed in PL mapping in Fig. 5(b), in which the monolayer region is uniform and exhibits much stronger emission than the multilayer region.

Fig. 5.

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	Fig. 5. (color online) (a) PL spectra of different areas marked as 1–5 in the inset of Figure 4(a), (b) mapping image of the PL intensity of the truncated triangle.

The morphologies of the MoS₂ flakes evolve from triangle, similar hexagon to truncated triangle from sections 1 to 5 and the size of MoS₂ flakes increase gradually with the increase of the reactant concentration. In the growing process, the triangle MoS₂ nucleates first under any reactant concentration.^[19] This could be explained as the fact that the volatile MoO_{3 − x} species are initially deposited from the reduced MoO₃ by the sulfur vapor, and then MoO_{3 − x} species further react with sulfur vapor to form MoS₂ flakes,^[28] which leads to the ratio of Mo and S more than 1:2 at the beginning of the growing process. Therefore, the triangular Mo terminated nucleation is formed.^[18] The morphologies of MoS₂ flakes further evolve when the sizes of the triangles each reach 2 μm as shown in Fig. 2(c). The triangle evolves to a similar hexagon and then to a truncated triangle with the increase of the size. In this stage, the growth of MoS₂ is under the S rich environment, the Mo termination grows faster than S termination, leading to the change of the sides from Mo termination to S termination. Meanwhile, the MoS₂ morphology evolves from a Mo termination triangle to a similar hexagon with three sides of Mo termination and the other three equal sides of S termination, and then to truncated triangles with three long sides of S termination and three short sides of Mo termination as shown in Fig. 6.

Fig. 6.

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	Fig. 6. (color online) Schematic illustration of morphologies of MoS2 flakes in growth process.

In conclusion, we find that the nucleation of monolayer MoS₂ flake is a triangle. The final shape of MoS₂ flake is determined by the faster growth speed of Mo termination than that of S termination under the S rich environment.

Fig. 7.

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	Fig. 7. Schematic illustrations of the typical growth processes during the CVD growth of MoS2 films.

The nucleation density does not increase linearly with the increase of the reactant concentration in the growth process of the MoS₂ flakes, which is corresponding to the two-dimensional nucleation theory. The nucleation mechanism can be explained as follows: in the process of Gas-solid phase transition, Δ G = R T ln P 0 / P , where ΔG is the variation of free energy, R is the universal gas constant, T is the thermodynamic temperature, P is the supersaturated vapor pressure, and P₀ is the equilibrium vapor pressure. In order to make the phase transition occur spontaneously, that is, ΔG < 0, the P should be greater than P₀. Therefore, the supersaturated vapor pressure is the driving force for the phase transformation process. In section 1, the reactant concentration is low for MoS₂ to nucleate (P < P₀), so there are no nuclei formed as shown in Fig. 7(a). In section 2, the reactant concentration is higher (P > P₀) and steady nuclei are randomly distributed at the defect sites of the substrate. The growth can be understood from a thermodynamics view as shown in Fig. 7(b). The interface and surface free energy of nucleation in defects are lower than the surface free energy of the substrate, inducing MoS₂ species to only need to diffuse a comparatively short distance to coalesce into the nuclei already formed rather than initiating new ones,^[29] and then develops into MoS₂ flake. In this process, nucleation density remains nearly unchanged and only the size of MoS₂ flake increases with the increase of the reactant concentration. In section 4, the supersaturated vapor pressure is higher, and MoS₂ species form new nuclei at the defects and the original nucleation points before they can diffuse to the nuclei edge to form MoS₂ flakes as shown in Fig. 7(c). Therefore, the nucleation concentration increases and multilayer growth on original monolayer MoS₂ flakes emerges.

In this work, triangle, similar hexagon and truncated triangle MoS₂ flakes are synthesized on SiO₂/Si substrates by chemical vapor deposition. Our results reveal that the original nuclei are triangle in shape. The final shape and size of MoS₂ flake are determined by the faster growth speed of Mo termination than that of S termination under S rich environment. The nucleation density does not increase with the reactant concentration increasing linearly. Through reasonably controlling the nucleation density, crystal size and shape, high-quality monolayer MoS₂ flakes could be obtained. Our results are expected to be useful in further studying the MoS₂ growth and the scaled layer-controlled high-quality MoS₂ flakes for a wide range of applications.