Chemical Vapor Deposition of High‐Quality Large‐Sized MoS2 Crystals on Silicon Dioxide Substrates

Large‐sized MoS2 crystals can be grown on SiO2/Si substrates via a two‐stage chemical vapor deposition method. The maximum size of MoS2 crystals can be up to about 305 μm. The growth method can be used to grow other transition metal dichalcogenide crystals and lateral heterojunctions. The electron mobility of the MoS2 crystals can reach ≈30 cm2 V−1 s−1, which is comparable to those of exfoliated flakes.

Recently Chen. et al. studied the role of oxygen on the growth of MoS 2 , and obtained large-sized crystals by a low-pressure CVD method. [ 31 ] However the introduction of oxygen into reaction system is dangerous, and is not an essential prerequisite to the growth of large-sized crystals. Here, we found that, by controlling the growth process under ambient pressure using a two-stage CVD method, the nucleation density of MoS 2 can be signifi cantly reduced, thus also forming large-sized crystals. Unlike expensive sapphire, the direct growth of MoS 2 crystals on low-cost SiO 2 /Si substrates is more compatible with current Si processing techniques for fabrication of electronic devices. The as-made MoS 2 grains are monolayer crystals. Their maximum size can reach up to ≈305 µm, comparable to that of previous reports. [27][28][29][30] Raman spectroscopy, transmission electron microscopy (TEM) and fi eld effect transistor (FET) measurements indicate that these crystals have excellent crystallinity and electronic properties. The electron mobility can reach about 30 cm 2 V −1 s −1 with an on/off ratio above 10 6 . The growth method can also be used to grow other TMD crystals such as MoSe 2 and WS 2 . We further use the method for epitaxial growth of lateral MoS 2 /WS 2 heterojunctions. The atomically sharp in-plane junctions have excellent current rectifi cation behavior, which is important for potential applications in electronics and optoelectronics.
The CVD process was performed under ambient pressure and the detailed growth procedures are described in Figure 1 a and Figure S1 in the Supporting Information. According to previous reports, [ 32,33 ] to realize the growth of large-sized 2D materials, it is important to decrease the nucleation density and increase the growth rate of the nuclei. To achieve that, our strategy is to compartmentalize the growth process: separating the induction stage from the growth stage. The induction stage is needed to isolate the growth substrate before the targeted high temperature and equilibrium evaporation rate is reached, since the nucleation and growth can occur during heating stage ( Figure S2, Supporting Information), resulting in the formation This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Post-graphene, there is intense interest in transition metal dichalcogenide (TMD) owing to its unique properties of large spin-orbit coupling and a bandgap, which offer new possibilities in electronics and valleytronics. [1][2][3] MoS 2 is one of the most widely studied TMDs. It is a layered 2D material in which the transition metal Mo atoms are sandwiched between two planes of S atoms. [ 4,5 ] Bulk MoS 2 crystals have an indirect bandgap of ≈1.29 eV, however its monolayer exhibits a direct bandgap of ≈1.8 eV. [ 6 ] Monolayer MoS 2 gives rise to strong photo-and electro-luminescence due to the direct bandgap. [ 7,8 ] According to previous reports, [ 9,10 ] the room temperature mobility of MoS 2 can reach ≈410 cm 2 V −1 s −1 with a high on/off ratio of 10 8 . The excellent optical and electrical properties render MoS 2 an attractive candidate for applications as transistor, photodetectors, photovoltaic cells, piezoelectricity, and spintronic devices. [9][10][11][12][13][14] To date, many efforts have been developed to prepare monolayer MoS 2 , including micromechanical exfoliation, chemical of a high density of smaller crystals. Briefl y, MoS 2 crystals were grown on SiO 2 /Si substrates with sulfur (S) and molybdenum trioxide (MoO 3 ) as the precursors using a modifi ed CVD system (Figure 1 a). MoO 3 powder (about 1.0 mg) was placed on a quartz slide, which were located in the heating zone center of the furnace. A smaller quartz test tube, containing 0.8 g of S, was located upstream, and the open end exposed to the center of the furnace. Unlike the widely used method in which substrates are put face-down above the MoO 3 source, our SiO 2 /Si substrate was put at the downstream side (the left picture in Figure 1 a). During the induction phase, the furnace temperature was raised to 850 °C and 200 sccm Ar was introduced in a direction fl owing away from the substrate to prevent any unintentional nucleation and growth of MoS 2 crystals. When the targeted growth temperature and equilibrium vapor pressure in the growth zone was reached, the SiO 2 /Si substrate was rapidly introduced into the growth zone where MoO 3 sources were located by using a homemade setup. Meanwhile, the direction of gas fl ow was reversed and fl ow rate set to 20 sccm to allow reactants to fl ow to the substrate (the right picture in Figure 1 a). The growth time was about 10 min ( Figure S3, Supporting Information). Compared with the general one-stage growth process, the physical segregation of the CVD process into induction and growth stages allows the substrate to be exposed to the targeted high temperature and vapor pressure quickly, thus avoiding undesired nucleation during the ramp up period ( Figure S4, Supporting Information). Figure 1 b show the optical image of the as-grown MoS 2 crystals. For comparison, we also show the optical image of MoS 2 crystals grown by a one-stage method ( Figure S5, Supporting Information). Due to the optical contrast, it is straightforward to identify MoS 2 domains from the SiO 2 substrate. Similar to previous reports, [27][28][29][30] adjacent MoS 2 crystals have coalesced to form a fi lm. The crystal size of MoS 2 crystals ranges from several tens to hundreds of micrometers. Discrete smaller crystals show a regular triangular morphology, while larger crystals easily form twin crystals with smaller crystals (Figure 1 c). An enlarged image of MoS 2 crystals, shown in Figure 1 d, displays a uniform color contrast on the SiO 2 /Si substrate, indicating that the crystals are of uniform thickness. Figure 1 e shows a size histogram of MoS 2 crystals observed using optical microscopy. The majority of the MoS 2 crystals are one order of magnitude in area than those produced using one-stage method.
To identify the number of layers for our MoS 2 sample, the edges of crystals are measured using atomic force microscopy (AFM). Figure 1 f,g are typical tapping mode AFM images of a MoS 2 crystal. The sharper, straighter edge may indicate the formation of molybdenum zigzag (Mo-zz) edge structure. [ 29 ] The homogeneity of fi lm thickness is evidenced by color homogeneity. Height profi les across MoS 2 edge samples (Figure 1 g) show that thickness of our sample is about 0.95 nm, corresponding to monolayer MoS 2 .
MoS 2 crystals were further characterized by using TEM, selected area electron diffraction (SAED), scanning transmission electron microscope (STEM). These techniques provide important information about the structure and quality of MoS 2 crystals as detailed below. After the MoS 2 crystals were transferred to a copper grid, the layer count on the edge of the image (   Figure 2 c displays one set of hexagonal symmetrical patterns, indicating the hexagonal lattice structure of MoS 2 crystals. [ 34 ] The atomic structure of MoS 2 crystals was studied by annular dark fi eld (ADF) imaging (Figure 2 d). The corresponding atomic model is shown in Figure 2 e. Because the signal intensity in the STEM-ADF image is directly related to the average atomic number (Z), STEM-ADF image can thus be used to visualize the spatial distribution of Mo ans S due to their different image contrast levels. [ 35 ] The sharp atomic images indicate that our samples have a high crystalline quality, in accordance with previous reports.
X-ray photoelectron spectroscopy (XPS) was used to examine the elemental composition and bonding of MoS 2 samples. Only four elements (Mo, S, O, and Si) are observed in the spectra ( Figure S6, Supporting Information), confi rming that MoS 2 was directly synthesized on SiO 2 /Si substrates. The Mo 3 d and S 2 p peaks provide important information about the stoichiometry and bonding of the MoS 2 crystals (Figure 2 f,g). The Mo 3 d 3/2 and 3 d 5/2 peaks are located at ≈230.0 and ≈233.2 eV, respectively, while the S 2 p 1/2 and S 2 p 3/2 peaks are located at ≈164.0 and ≈162.9 eV, respectively. These peak positions are consistent with the reported values for 2H-MoS 2 crystals. [ 34 ] The positions of the Mo peaks indicate the reduction of Mo from Mo 6+ (MoO 3 ) to Mo 4+ (MoS 2 ). The Mo/S ratio obtained from Mo 3 d and S 2 p XPS is about 1:1.97, suggesting that the CVD MoS 2 fi lm is stoichiometric with some S vacancies, [ 36 ] which were reported as the dominant point defect in CVD-grown MoS 2 . [ 37 ] Raman and photoluminescence (PL) microscopy are powerful methods for the characterization of crystal quality and bandgap in TMD materials. Typical monolayer MoS 2 crystals were characterized with Raman and PL using a laser wavelength of 532 nm. Figure 3 a shows the Raman spectrum of the MoS 2 sample. The monolayer sheet exhibits two characteristic Raman bands at 400.2 and 383.4 cm −1 , corresponding to the A 1g and E 1 2g modes of monolayer MoS 2 crystals, [ 28,34 ] and their full-width-half-maximum (FWHM) values are about 6.8 and 3.8 cm −1 , respectively. The PL spectrum (Figure 3 b) shows highly distinct photoluminescence peaks at ≈623 and 673 nm, corresponding to the A1 and B1 direct excitonic transitions of MoS 2 monolayer, respectively. [ 8,28 ] To probe the microscale structure of the crystal, we also conducted Raman and PL mapping centered at ≈400.1 cm −1 (the A 1g mode), ≈383.4 cm −1 (the E 1 2g mode) and ≈673 nm (the PL mode), as shown in Figure 3 c-e. The uniform color intensity observed suggests that the MoS 2 crystal is uniform in thickness.
To assess the generality of the method for growing other TMDs crystals, we also tried to synthesize MoSe 2 and WS 2 crystals using a similar strategy. MoSe 2 and WS 2 were grown using MoO 3 , Se and WO 3 , S powders as the source precursors respectively. The difference is that a small quantity of H 2 (1.5 sccm) is required to enhance the selenization reaction of  MoO 3 during the growth of MoSe 2 crystals. The introduction of H 2 also changes the relative edge free energy of Se edges and Mo edges, thus forming hexagonal crystals under suitable conditions. [ 38,39 ] Nevertheless, we have obtained large-sized MoSe 2 and WS 2 crystals on SiO 2 /Si substrates. (Figure 3 f,g). Figure 3 h shows the Raman sprectra of these MoSe 2 and WS 2 crystals. The A 1g and E 2g modes of MoSe 2 single-layer are located at ≈239.7 cm −1 (A 1g ), 286.2 cm −1 (E 2g ) respectively, while the A 1g and E 2g modes of WS 2 single-layer are located at 418.8 and 352. 3 cm −1 respectively. [ 40,41 ] The PL spectra (inset) shows the characteristic emission peaks corresponding to the emission of MoSe 2 (≈794 nm) and WS 2 (≈632 nm) monolayer. [ 42,43 ] These results indicate that these crystals are monolayer crystals with perfect optical properties.
To investigate the electronic quality of the CVD-grown MoS 2 crystals, we measured the electrical transport properties.  Figure 4 c. The I DS value increases monotonically with increasing V G , which is indicative of n-type semiconducting behavior. The fi eld-effect mobility of this MoS 2 FET was estimated to be ≈28 cm 2 V −1 s −1 with an on/off rario above 10 6 . The mobilities of all the 20 devices we measured are in the range of 1−30 cm 2 V −1 s −1 , comparable to prevous reports. [28][29][30] The mobility could be improved by high-k top gate dielectrics and interface engineering. [ 10,31,44 ] Beyond the growth of single crystals, we have also realized the growth of WS 2 crystals along the edges of MoS 2 crystals, and formed MoS 2 /WS 2 lateral heterojunctions by our method ( Figure 5 a and Figure S7, Supporting Information). Observation under STEM indicates that the lateral interface is atomically sharp (Figure 5 b), without extensive (WMo)S 2 alloying region. [45][46][47][48] The chemical modulation cross the lateral heterostructure is confi rmed by elemental mapping using electron energy-loss spectroscopy (EELS) imaging ( Figure 5 c−e). The Raman and PL mapping of the characteristic peaks and peaks of WS 2 and MoS 2 also revealed the structural modulation between MoS 2 and WS 2 ( Figure 5 f,g and Figure S8, Supporting Information). The lateral stitching of MoS 2 monolayer and WS 2 monolayer has formed an in-plane heterojunction. The electrical  transport across the interface of monolayer MoS 2 /WS 2 in-plane heterojunctions was measured ( Figure S9, Supporting Information). The forward bias current is higher than the reverse current, suggesting reasonably good rectifi cation across this inplane heterojunction.
In summary, we have successfully realized the growth of large-sized, high-quality MoS 2 crystals. The nucleation density of crystals can be decreased by separating the induction stage from the growth stage, and the maximum size of MoS 2 crystals can reach about 305 µm. Electrical transport measurements indicate that the MoS 2 crystals have electron mobility up to about 30 cm 2 V −1 s −1 , comparable to those of exfoliated fl akes and CVD synthetic crystals. The growth method can also be used to grow other TMD crystals such as MoSe 2 and WS 2 , suggesting the universality of of the method. In addition, we have also demonstrated the lateral

Experimental Section
Preparation of MoS 2 Crystals : MoS 2 crystals were grown on dielectric substrates by using a modifi ed ambient pressure CVD method. A little MoO 3 powder (about 1.0 mg) was placed on growth substrate which was introduced into the heating zone center of the 2 in. furnace. A smaller quartz tube with one end sealed containing 0.8 g of sulfur powder was located upstream, and the open end extended to the center of the furnace. The SiO 2 /Si growth substrate was put at the downstream side. The furnace temperature was raised to 850 °C and 200 sccm Ar was introduced in a direction fl owing away from the substrate. The SiO 2 / Si substrate was moved and made close to MoO 3 sources. Meanwhile, the direction of fl owing gas was chaged and the gas fl ow at 20 sccm was controlled . After stabilizing the system for 10 min, the furnace was cooled to room temperature.
Characterization : Optical images were obtained using a Nikon ECLIPSE LV100D microscopy. AFM images were performed using a Bruker Dimension FastScan Atomic Force Microscope in the tapping mode. Raman spectra were recorded at room temperature using a WITec Raman Microscope with laser excitation at 532 nm. TEM was performed with FEI Titan transmission electron microscope operated at 80 kV. STEM imaging and EELS analysis were performed on an aberrationcorrected Nion UltraSTEM-100 operating at 60 kV. XPS analysis was carried out on an Omicron EAC2000-125 analyzer. Base pressure during analysis was 10 −9 Torr. An Al Kα monochromatized radiation (hν = 1486.6 eV) was employed as the X-ray source.
Device and Electrical Measurements : Triangular MoS 2 crystals were etched into ribbons by electron beam lithography (EBL) and oxygen plasma. FETs were fabricated on SiO 2 /Si wafers with Ti/Au (5/50 nm) as source-drain electrodes and the doped silicon substrate as the back gate. The FET characteristics were measured in N 2 at room temperature. A Keithley 4200SC semiconductor parameter analyzer was used to measure the electrical characteristics of the devices.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.