Universal Substrate-Trapping Strategy To Grow Strictly Monolayer

Jun 26, 2017 - Monolayer transition metal dichalcogenides (TMDs) possess great potential in the electronic and optoelectronic devices on account of th...
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Universal Substrate-Trapping Strategy To Grow Strictly Monolayer Transition Metal Dichalcogenides Crystals Min Ju,†,§ Xiaoyang Liang,†,§ Jinxin Liu,†,§ Lu Zhou,† Zheng Liu,† Rafael G. Mendes,‡ Mark H. Rümmeli,‡ and Lei Fu*,† †

College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China IFW Dresden, P.O. Box 270116, 01069 Dresden, Germany

‡

S Supporting Information *

ABSTRACT: Monolayer transition metal dichalcogenides (TMDs) possess great potential in the electronic and optoelectronic devices on account of their unique electronic structure as well as outstanding characteristics. However, presented growth approaches are hardly to avoid multilayers and the root cause of this thermodynamic growth process lies on the overflowing transition metal sources. Here, a novel substrate-trapping strategy (STS) is developed to achieve monolayer TMDs crystals over the whole substrate surface. A designed substrate with appropriate reaction activity to fix the extra precursors is the key, for which the dominance of the dynamics will be established, thus leading to strict self-limited monolayer growth behavior. The high-quality nature of the synthesized monolayer MoS2 crystals is also clarified by transmission electron microscopy characterizations and field-effect transistors performance. Excellent tolerance to variations in growth parameters of STS is therefore exhibited and, moreover, it is also verified in achieving strictly monolayer WS2 crystals, thus demonstrating the universality of this approach. The facile strategy opens up a new avenue in growth of monolayer TMDs and may facilitate their industrial applications.

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adopted to obtain monolayer TMDs. In addition, modifying the surface adsorption of growth substrates19−21 or monolayer crystals,22 which coupled with controlling the growth time20,21 or other key parameters,22 can also remit the thermodynamic effect during the growth. However, all of these factors complicate the growth kinetics of TMDs and contribute to the inevitable formation of the multilayers.23 The core issue lies in that there never exists an effective strategy for fundamentally suppressing the extra precursors. Here, as demonstrated by the growth of MoS2, we first develop a substrate-trapping strategy (STS) to trap the overflowing Mo precursors into the substrate and therefore greatly suppress nonuniform diffusion and nucleation. Sodalime glass with its main components of SiO2, Na2O, and CaO that exhibit appropriate trapping ability with the extra precursors (MoO3) is employed as the substrate. Therefore, the growth of MoS2 could be converted into a kinetically controlled process and monolayer MoS2 crystals with almost 100% ratio on the whole substrate was easily achieved. In addition, such a strategy shows excellent tolerance to the variations of the growth conditions. The high-quality nature of them is clarified by transmission electron microscopy (TEM) characterizations and field-effect transistors (FETs) perform-

onolayer transition metal dichalcogenides (TMDs) have been proved to possess a unique electronic structure, on account of the two-dimensional (2D) quantum confinement effect at the strictly atomic-thickness level.1,2 The outstanding characteristics, such as field effect properties,3,4 direct band gap1,2 as well as large spin-valley coupling,5,6 make them the promising candidates in the construction of next-generation electronic, photoelectric, and valleytronic devices.7−9 Therefore, it is of great importance to fabricate high-quality TMDs crystals with strictly atomic thickness. However, the present chemical vapor deposition (CVD) approaches, which offer the greatest potential to yield TMDs single crystals, are always hard to avoid multilayer flakes.10−12 The lack of a significant breakthrough in layer-number-control aspect of these CVD systems lies on their inherent problems. Generally, transition metal sources (e.g., oxides) were provided excessively during a long-time volatilization process, where the quantity was about 105 times larger than the demands of covering the whole substrate with monolayer crystals, as well as no significant interaction is involved between currently used substrates and precursors.10−13 Excess precursor would be aggregated and diffusing nonuniformly on the solid substrate surface with low diffusion rate, thus making mass transport process the ratelimiting step14 and then forming thermodynamically stable multilayers.15 Greatly reducing the partial pressure of precursors continuously and steadily16,17 or accelerating the mass transport process via introducing low pressure18 has been © 2017 American Chemical Society

Received: May 14, 2017 Revised: June 23, 2017 Published: June 26, 2017 6095

DOI: 10.1021/acs.chemmater.7b01984 Chem. Mater. 2017, 29, 6095−6103

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Figure 1. Characterization of monolayer MoS2 crystals obtained via STS process. (a) A typical low-magnification OM image of the as-prepared monolayer MoS2 crystals, where the region is marked in (b). (b) The photograph of the obtained sample. (c−e) SEM images of MoS2 crystals at three randomly selected areas marked in (b). (f) The typical Raman and PL spectra of STS-synthesized monolayer MoS2 crystals. (g) The statistical analysis of the Raman peak difference as well as PL peak position of ∼100 single crystals in (a).

Figure 2. Characterizations of the STS-synthesized high-quality monolayer MoS2 single crystal. (a) The OM image of the individual monolayer MoS2 single crystal. (b, c) The corresponding Raman and PL mapping of the single crystal with uniform Raman intensity at 385 cm−1 as well as uniform PL intensity at 684 nm, respectively. (d) AFM image of an individual MoS2 single crystal after being transferred on the Si/SiO2 substrate, and the inset of (d) is the corresponding height profile line at the edge. (e) The typical TEM image of the MoS2 crystal at low magnification. (f) The SAED pattern of the crystal taken at the area marked in (e). (g) A typical result of the edge analysis based on TEM to confirm the monolayer nature of the STS-synthesized MoS2. (h) The FFT filtered atomic image based on HRTEM.

1a, which presents a homogeneous contrast and uniform distribution of the crystals over the whole region. Meanwhile, the OM images of the MoS2 crystals transferred onto 300 nm Si/SiO2 with more obvious optical contrast are displayed in Figure S3 to further demonstrate the strict monolayer character of the STS MoS2 samples. A photograph of the growth substrate is displayed in Figure 1b, where the marked region on the center of the trapping substrate is related to Figure 1a. To evaluate the universality of the crystal thickness and distribution, SEM images of three randomly selected areas marked in Figure 1b are displayed in Figure 1c−e. Almost consistent homogeneous contrast and distribution is presented, thus proving a homogeneous contrast of crystals over the whole region as well as an improved uniformity of the crystals distribution. It is believed to be induced by the fast migration rates of active sources on the liquid substrate surface, compared

ance. Furthermore, the STS growth of strictly monolayer WS2 crystals over the whole substrate is also achieved successfully, which demonstrates the universality of this approach. Thus, we believe that the proposed precursor-trapping concept could also be extended to synthesize other 2D materials, thus promoting and accelerating the practical industrial applications of them.

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RESULTS AND DISCUSSION The STS growth process was performed at an atmosphericpressure CVD growth process, which utilizes sufficient MoO3 and S powders as precursors, and the schematic illustration of it is displayed in Figure S1. A trapping layer (Figure S2) was employed as the growth substrate while a Fe foil with good wettability toward it was introduced as the support substrate. A typical optical micrograph (OM) image at low magnification of the as-prepared monolayer MoS2 crystals is displayed in Figure 6096

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Figure 3. Schematic illustration of the STS growth process and the XPS study of the obtained samples to clarify the trapping mechanism. (a) Schematic diagram of the STS growth process. (b, c) XPS spectra of the STS-synthesized sample for the binding energies of Mo 3d taken at the surface and the bulk, respectively. (d) XPS composition profiles of elements along the surface normal direction on the substrate after growth (the relative composition of each element is represented by a special color for clarity). (e) XPS depth profiles of Mo composition on the samples obtained under various amounts of MoO3 supply.

to the flakes on the Si/SiO2 substrate.13 To demonstrate the strictly atomic thickness of STS-fabricated MoS2 crystals, Raman and photoluminescence (PL) spectra were carried out to characterize the typical features of them (Figure 1f). A Raman shift difference of 19.4 cm−1 between the A1g peak (located at 404.2 cm−1) and the E2g peak (located at 384.8 cm−1) (Figure S4) indicates the monolayer nature of MoS2.24 The characterized PL emission peak of MoS2 is centered at 683 nm. A red shift of ∼10 nm of the PL peak position is presented,10 and it is thought to be caused by the substrate effect comparing to the PL spectrum of the transferred crystal (Figure S5). Furthermore, a statistical analysis of the Raman peak difference as well as PL peak position of ∼100 single crystals in Figure 1a was carried out to confirm the uniformity (Figure 1g). Here, the standard deviations of the peak differences and the peak positions are only 1.31 cm−1 and 2.06 nm, respectively, and the corresponding average values are 18.74 cm−1 and 683.74 nm, respectively. The statistics further indicate a uniform thickness over the selected region, and more detailed data collected from other randomly selected areas are displayed in Figure S6 to explore the reproducibility. In addition, a Raman peak difference mapping and a PL peak position mapping of the whole substrate with uniform contrast are presented in Figure S7, where a grid in these mapping images is represented by a single (representative) spectrum collected from the MoS2 crystal in the corresponding area. A typical OM image of an STS-synthesized monolayer MoS2 crystal is displayed in Figure 2a, which exhibits its intrinsic triangle shape. The corresponding Raman mapping of the E2g mode (∼385 cm−1), which was recorded via utilizing a 532 nm laser, as well as the PL mapping at ∼684 nm is exhibited in Figure 2b,c, respectively. Both of them present a uniform intensity over the whole triangular crystal, which indicates that the MoS2 single crystal was synthesized successfully with a

uniform crystallinity via the STS process. In addition, an atomic force microscope (AFM) was also utilized to examine the thickness and morphology of the obtained MoS2 crystal after being transferred onto the Si/SiO2 substrate (Figure 2d). The corresponding height profile presented inside indicates the atomic-thickness nature of the crystal, on account of that the value (∼0.74 nm) obtained at the flake edge is in agreement with the reported values from previous CVD monolayer MoS2 crystals11 or exfoliated sample.24 Furthermore, a larger-scale crystal with an arc-edge-triangle morphology, which slightly differed from the small-sized MoS2 crystals (Figure 2a), is presented in the AFM image. Here, the morphology difference, which was also presented in a previous study,25 may be caused by the distinct growth behavior of 2D materials in the STS growth process (Figure S8). Meanwhile, TEM was also utilized to characterize the microstructure of the MoS2 crystals obtained via the STS process. Figure 2e is a typical image with uniform contrast of the monolayer sample after being transferred onto the Cu grid at low magnification. Here, dashed lines and solid lines indicate the folded edges and pristine edges, respectively, where arrows indicate the fold direction. TEM-based selected area electron diffraction (SAED) was also conducted, and the obtained result is presented in Figure 2f. The SAED pattern taken at the region marked in Figure 2e only shows one set of diffraction spots, which demonstrates the single crystallinity nature of the STS-synthesized monolayer MoS2 crystals. Figure 2g exhibits a typical result of the edge analysis based on TEM, which was carried out on the STS MoS2 crystals. Only one atomic layer is observed in this high-resolution image as well as well-organized atomic arrangement on the edge, which indicates a high crystallinity of the crystal. A typical TEM image of the folded edge of the STS-fabricated MoS2 crystal is displayed in Figure S9 to further confirm the monolayer nature of the MoS2 crystal. High-resolution TEM (HRTEM) was 6097

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colors are introduced to represent the relative composition of them. Here, an obvious linear concentration gradient of Mo composition inside of the trapping layer is exhibited, which is believed to be derived from the Fick’s first law

utilized to further reveal the atomic crystal structure of the crystals, and a typical FFT filtered image is presented in Figure 2h, where the corresponding nonfiltered image is displayed in Figure S10. Hexagonal packing of S and Mo atoms with wellordered atomic arrangement, which are marked as blue and green circles tentatively, is presented as well as no obvious defect or heteroatom is observed. Here, the lattice distances are measured to be about ∼0.160 nm, corresponding to the acknowledged value of the (110) plane (marked as red lines), and ∼0.275 nm, corresponding to that of the (100) plane (marked as yellow lines), respectively.13 In addition, the values are also in good agreement with the standard PDF card (JPCDS card No. 37-1492). We have also measured the lattice distances based on the SAED patterns in Figure 2f to confirm the results, and the values are ∼0.165 nm, assigned to the (110) plane, and ∼0.278 nm, assigned to the (100) plane, respectively. It indicates that these values agreed well with the lattice distances measured in the HRTEM image. Meanwhile, the measured value of the lattice constant is ∼0.315 nm, which is in line with the previous report.13 All of these further characterizations demonstrate the atomic-thickness nature and great crystallinity of the MoS2 crystals grown via the STS process. Generally, comparing with the demands of covering the whole substrate with monolayer MoS2, the supplied precursors are usually ∼105 times larger in quantity.10−13 Meanwhile, no obvious chemical interaction is encouraged between Mo precursors and the classical insert substrate (such as Si/SiO2 and sapphire) in this catalyst-needless growth process. Redundant Mo precursors would be aggregated and diffused nonuniformly on the substrate surface with a low diffusion rate, and these growth conditions make the mass transport process become the rate-limiting step, which further leads to the formation of thermodynamic product.14 Multilayers are proved to be thermodynamically stable,15 and therefore, the generating of them is inescapable on the common solid insert substrates.10−12 Thus, further increasing the supply of oxides can lead to the growth of multilayered MoS2 flakes.26 In contrast, we contribute the growth of high-quality strictly monolayer MoS2 crystals on a liquid substrate to the overflowtrapping characteristic of it, and the basic idea is schematically shown in Figure 3a. In the STS growth process, excess Mo oxides were dissolved into the liquids on account of the high chemical activity between them27 and the resultant is believed to be MoO42−, which is stable in the molten condition.28 As demonstrated by X-ray photoelectron spectroscopy (XPS) depth analysis of the substrate after growth, the direct evidence of the formation of MoO42− in the bulk is exhibited in Figure 3c, where the two peaks centered at 232.4 and 234.8 eV are assigned to the binding energy of Mo 3d5/2 and 3d3/2 of Na2MoO4, respectively.29 Meanwhile, a distinct difference in the Mo-containing component is presented in the surface, where the characterized peaks (226.1 eV of S 2s and 228.9 eV, 232.1 eV of Mo 3d5/2 and 3d3/2) are assigned to MoS2 (Figure 3b).30 Here, The C 1s peak is fixed at 284.8 eV. Meanwhile, another stable component assigned to MoSi231 is also observed inside of the substrate, where the peaks are centered at 227.4 and 230.5 eV (Figure 3c).27 In addition, the corresponding XPS spectrum of S 2p of MoS2 is exhibited in Figure S11, where the results agreed well with the previous work.30 The XPS composition profile of elements along the surface normal direction is presented in Figure 3d, where various specific

Fg = −Dg Δcg /Δxg

which can be rewritten as a linear equation cg = −xg Fg /Dg + csr

where Fg is the diffusion flux rates in the substrate, Dg is the corresponding diffusion coefficient, csr is the initial concentration on the substrate surface, and cg and xg are the concentration and depth. Therefore, the Mo sources flux moves from the substrate surface (high concentration region) to the substrate bulk (low concentration region) via crossing the concentration gradient. It indicates that the liquid trapping layer possesses a perfect solubility toward Mo precursors, which proved a reasonable applicability of it in the STS process. In addition, an appropriate Mo concentration is therefore guaranteed on the substrate surface all along the STS growth process on account of the stable concentration gradient. XPS depth analysis of the trapping substrate under various quantities of Mo supply was also performed, and the profiles of the Mocomposition variation are exhibited in Figure 3e. When the amount of the supplied MoO3 powders was 15 mg, the resultant curves could be described as cg = −0.088 × xg + 1.078

When the amount of the supplied MoO3 powders was 5 mg, the resultant curves could be described as cg = −0.034 × xg + 0.322

The corresponding Pearson’s ratios reached 0.99 and 0.98, respectively. The fitting curves presented distinct values of Fg/ Dg, which indicates a different diffusion rate of Mo on glass bulk when given different amounts of MoO3 precursors. It was induced by an enlarged concentration gradient or, in other words, by supplying a larger amount of precursors. Meanwhile, about 3 times enlargement of csr was presented, which was similar to the variation of MoO3 supplies. It indicates a highly correlated linear relationship between Mo concentration and the depth (proportional to the etching times), thus confirming that the dissolution−diffusion process highly accords with the Fick’s first law. Therefore, the basic idea of our proposed substrate-trapping mechanism could be demonstrated. No obvious peak assigned to Mo is presented on the XPS study of the samples without using MoO3 (Figure S12). Furthermore, X-ray diffraction (XRD) is also employed to explore the STS samples and, unexpectedly, several visible peaks assigned to crystallized molybdate and MoSi2 are observed in this amorphous system (Figure S13), which further demonstrates the Mo-trapping characteristic of the employed liquid trapping layer. As overflowing Mo precursors could be dissolved into the trapping substrate, the partial pressure of them is therefore greatly reduced. Here, the flux of consumed active precursors at the surface, marked as Fsr, can be given by Fsr = KsrCsr

where Ksr is the reaction constant (assuming first-order rate kinetics) at the surface.14 Meanwhile, according to the Daltons 6098

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Figure 4. Typical high-magnification OM images of STS MoS2 crystals grown with MoO3 precursors of varying supplied quantities, where (a) for 6 mg, (b) for 11 mg, (c) for 17 mg, (d) for 22 mg, (e) for 31 mg, (f) for 33 mg, and (g) for 40 mg. (h) The statistical distribution of the layer number and the size of the MoS2 crystal synthesized with MoO3 precursors of varying supplied quantities.

Figure 5. Electrical properties of STS-synthesized monolayer MoS2 crystals after being transferred to the Si/SiO2 substrate. (a) Schematic diagram of the FET device. (b) Ids−Vg characteristic curve of a typical FET device and the inset of (b) is the SEM image of the device as well as the corresponding Ids−Vg curve in normal scale. (c) Ids−Vds characteristic curves of the same FET device at Vg with various values. (d) The statistics of the mobility and the on/off ratio of the obtained FET devices.

law of partial pressures, Csr can be rewritten as Psr/P0, where Psr is the partial pressure of the active precursors at the surface and P0 is chamber pressure or atmospheric pressure in the STS system. The equation is then rewritten as

converting the growth process into the kinetic-dominant growth process, thus achieving the strictly monolayer MoS2 crystals (kinetic product) over the whole substrate surface.15 Concerning the Mo-trapping ability of the substrate, the maximum tolerance of it toward overflowing Mo precursors is explored to further present the substrate-trapping mechanism in a quantificational way. Here, the high-magnification OM images of the synthesized MoS2 crystals (Figure 4a−g) and the corresponding statistical results of the controlled experiments (Figure 4h) indicate a sudden change of MoS2 layer number after supplying MoO3 with the amount exceeding the substrate’s trapping limitation for the MoO3. When the supplied amount of the oxide precursors is under the limitation, the diffusion rate of Mo in the bulk of the substrate accords well with the Fick’s first law and the concentration of active Mo species could be maintained in an appropriated value, thus making the growth process follow the substrate-trapping mechanism. In terms of supplying precursors exceeding the limitation, the diffusion rate cannot catch up with the masstransport rate and the trapping ability of the substrate toward partial active species would be suppressed, which leads to an accumulation of them on the substrate surface. The surface concentration of Mo species will increase along with the growth time. Therefore, the surface reaction rate is accelerated and the mass transport rate is reduced, thus leading to the growth of multilayers. Meanwhile, when under the trapping limitation of

Fsr = KsrPsr /P0

Therefore, the reduction of Psr leads to the decrease of the surface-reaction rates. In addition, the flux of the mass transport process Fmt (the diffusion process for active species getting to the substrate surface from gas flow) is given as Fmt = hmt (Cgf − Csr ) = hmt (Pgf − Psr )/P0

where the hmt is the coefficient of mass transport process, and Cgf and Pgf are the concentration and partial pressures of sources in the gas flow.14 Mathematically, the reduction of Psr can also lead to an increase of mass-transport rates. The correlation of the experimental results with simulations based on the equations regarding the diffusion of Mo on the gas flow is displayed in Figure S14. Meanwhile, it is believed that the liquid trapping substrate also possesses the capability to accelerate the mass-migration rates on the surface.25 A significant increase of Fmt and decrease of Fsr arise in the STS system compared to the thermodynamic-dominant growth process on a classical solid substrate (Fsr > Fmt). The overall effect of introducing the liquid trapping layer is making the surface reaction the rate-limiting step (Fmt > Fsr) and further 6099

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Figure 6. The universality and fault-tolerance property toward various growth parameters of the STS growth process. (a) OM image of as-obtained monolayer WS2 crystals at low magnification. (b, c) Typical Raman and PL spectra of the STS-synthesized monolayer WS2 crystal. Layer distribution determined by RGB color analysis of MoS2 flakes (d−f) obtained in the STS process as well as (g−i) directly growth on Si/SiO2 substrate under various amounts of MoO3 supply. The inset exhibits the corresponding OM images.

based on CVD growth MoS2 crystals10−13 or exfoliated flakes.9 The high quality nature of the STS-synthesized monolayer MoS2 crystals can therefore be demonstrated by electrical transport measurements. We believe that the electrical transport properties of the STS-grown crystals can be further improved via optimizing interface engineering as well as device structure.4 The universality of the STS process is also explored via applying it to the growth process of WS2 with WO3 as the overflowing precursors. The result is displayed in Figure 6a, where the obtained monolayer crystals with homogeneous contrast and uniform distribution over the whole region are presented in the low-magnification OM image. The typical Raman and PL spectra were carried out to characterize the monolayer features of the WS2 crystals. Here, a difference of ∼65 cm−1 between the 2LA(M) peak and the A1g peak of the Raman spectrum (Figure 6b) indicates the monolayer nature of WS2.32 The characterized emission peak of the PL spectrum (Figure 6c) is centered at 623 nm, which agrees well with that of monolayer WS2 synthesized in previous work.33 The XPS depth analysis of the substrate after STS growth of WS2 is displayed in Figure S15 to demonstrate that the growth of WS2 on the liquid trapping layer also follows the substrate-trapping mechanism. Thus, the universality of the proposed STS growth process is demonstrated reasonably. In addition, the fault-tolerance property toward various growth parameters of the STS was also clarified via carrying out a series controlled experiments. Here, the influence of the quantity of overflowing Mo precursors was also discussed to reveal the trapping capacity of the liquid substrate in the STS process accompanied by results on Si/SiO2 as contrast. A series of images, which were achieved via analyzing the green channel

the substrate, increasing the Mo active species can also induce an obvious enlargement of the single crystal, while slightly exceeding the limitation will greatly diminish the crystal size owing to the formation of multilayers. Therefore, the presented quantificational analysis further demonstrates the substratetrapping mechanism and strongly explores the STS growth process as well. To evaluate the electrical properties of STS-grown MoS2 single crystals, back-gated FETs were fabricated on the 300 nm SiO2/Si substrates, which were based on transferred monolayer MoS2 crystals. Figure 5a displays the schematic diagram of the FET device with 15 nm Cr/50 nm Au as source and drain electrodes, which were defined by electron beam lithography (EBL) and then evaporated by thermal depositions. Figure 5b displays the I−V characteristic curve of a typical FET device with a channel length of ∼8 μm and a channel width of ∼3 μm. The exhibited Ids−Vg curve of the STS-synthesized monolayer MoS2 crystal indicates the typical n-type conduction, where the field-effect mobility of the device is estimated to be about 12 cm2 V−1 s−1 and the on/off current ratio is calculated to be about 108. The corresponding I−V characteristics in normal scale are displayed inside of Figure 5b (low right) as well as the scanning electron microscope (SEM) image of the fabricated FET device (upper left). The Ids−Vds characteristic curves of the same FET device at Vg with various values are presented in Figure 5c, where the linear and symmetric curves indicate an Ohmic-like contact is formed at the source and drain electrodes. In addition, the transport characteristics of 15 STS-grown MoS2 FETs are also analyzed (Figure 5d), and the resulting data indicates that the mobility and the on/off ratio of these devices are in the range of 3−15 cm2 V−1 s−1 and 106− 108, respectively, where the values are comparable to the FETs 6100

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∼700 °C as well as the low-temperature furnace was moved to heat the S powders at 100 °C. Meanwhile, a flow rate of 7.5 sccm H2 was introduced to the chamber for 5 min to initiate MoS2 growth. After growth, both furnaces were then moved to the original position to prevent subsequent reaction and the glass was allowed to cool down to room temperature in a pure Ar atmosphere. In terms of WS2, the growth process was performed at 850 °C and WO3 powders were employed as precursors. The amounts of WO3 and the S powders were 0.002 and 0.1 g, respectively. Other experimental operations were similar to that of MoS2 growth. The MoO3 powders with a purity of 99.9999 wt % and the S powders with a purity of 99.95 wt % were purchased from Alfa Aesar China (Tianjin) Co. Ltd. In the condition of WS2, the growth process was performed at 850 °C and WO3 powders were employed as precursors. The WO3 powders with a purity of 99.8 wt % were purchased from Alfa Aesar China (Shanghai) Co. Ltd. Meanwhile, the CVD growth process of MoS2 performed on Si/SiO2 is the same as that on glass except for the substrate. Transfer Process. MoS2 on the growth substrate was transferred onto a Si/SiO2 substrate using the following method. First, a thin layer of poly(methyl methacrylate) (PMMA) was spin-cast on the as-grown MoS2 side of the substrate at 2500 rpm for a minute. The PMMA coated samples were baked at 150 °C for 10 min, and they were then wet etched using 1% HF for 2−3 h, resulting in MoS2/PMMA films floating in the etchant. These films were then collected manually and first placed in ultrapure water for 10 min. Finally, they were collected onto the 300 nm SiO2/Si substrates and PMMA was removed by acetone. In terms of TEM samples, the 300 nm SiO2 substrates were replaced by the Cu grids and other operations were similar. Characterization. Optical images were taken with an optical microscope (Olympus DX51), and the AFM image was recorded in an NT-MDT system via semicontact mode. Raman and PL spectroscopy with an excitation wavelength of 532 nm was performed via a laser micro-Raman spectrometer (Renishaw in Via). The PL spectra were collected at room temperature. The TEM images were taken with an aberration-corrected, high-resolution TEM (AC-HRTEM, FEI Titan3) operating at 80 kV. The MoS2 samples were directly transferred onto copper grids. XPS was performed on an ESCALAB 250Xi using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV). SEM images were obtained by an SEM (Zeiss Merlin Compact). All the FET measurements were performed at room temperature in air, and the current (I)−voltage (V) data were collected in a probe station under ambient conditions using an Agilent 4155C. MoS2 FET Fabrication. MoS2 crystals were directly transferred onto 300 nm SiO2/Si substrates via the PMMA-assisted transfer method. This was followed by acetone rinsing to remove the PMMA residues. FETs were fabricated via a standard EBL procedure. The electrodes (15 nm Cr/50 nm Au) were deposited by thermal evaporation. The lift-off process was performed with acetone, followed by isopropyl alcohol cleaning.

contrast of MoS2 crystals on substrate, are displayed in Figure 6 d−i to present their layer-number distribution. It can be found that the layer thicknesses of all the STS MoS2 crystals are limited to strictly monolayer while the size enlarged obviously following the increase of Mo sources at the same time. It further demonstrates the overflowing-trapping characteristic of liquid substrate and the as-caused kinetic-dominant growth mechanism of the STS growth process. In addition, a morphology variation accompanied by the size enlargement of the crystals, which was also presented in previous study25 and thought to be caused by distinct growth behaviors of 2D materials in the STS growth process, is observed. On the other hand, persistently increased layer number and nonuniform thickness distribution of MoS2 flakes on Si/SiO2 along with the increasing of oxides are observed, which indicates a thermodynamic-dominant growth mechanism. The STS growth process presents excellent tolerance to the variation (6 times) of overflowing Mo precursors, thus proving the proposed substrate-trapping mechanism. Furthermore, under various CVD parameters besides increasing the MoO3 supplies, such as S supplies, growth temperature, growth time as well as cooling rate, the STS growth processes are keeping to the selflimited growth behaviors all the while (Figure S16 and Table S1). Excellent tolerance to variations in growth parameters of STS is therefore presented on account of its intrinsic kineticdominant growth mechanism.

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CONCLUSIONS In conclusion, the employment of a liquid trapping layer as the substrate enables trapping the overflowing Mo sources via forming stable resultant MoO42− in the molten condition. The decreasing of precursors’ partial pressure and accelerating their diffusion rates on the liquid surface make the MoS2 growth path a kinetically controlled process, which allows us to achieve strictly monolayer crystals over the whole substrate. Meanwhile, the high-quality nature of the monolayer MoS2 crystals is clarified by TEM characterizations and FET performance. The proposed STS can also be applied to the growth of WS2 crystals with strictly atomic thickness, thus demonstrating its universality. Furthermore, the intrinsic kinetic-dominant growth mechanism of it enables the excellent tolerance to variations in growth parameters. We believe that our proposed facile STS approach opens up a new avenue in growth of strictly monolayer TMDs and will facilitate their industrial applications.

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METHODS

ASSOCIATED CONTENT

S Supporting Information *

Synthesis of Strictly 2D TMDs by STS. STS growth processes of TMDs are carried out as follows. Commercial soda-lime glass supported by an Fe foil was utilized as the trapping substrate in the experiment. Before growth, the substrates were cut into small pieces (1 cm × 1 cm), and then they were ultrasonicated and rinsed with acetone, alcohol, and ultrapure water prior to being dried under a nitrogen stream. The amount of MoO3 powders was 0.015 g and that of S powders was 0.03 g. At the early stage of the growth process, the substrate was placed in a quartz tube at the high-temperature heating zone of the furnace as well as the MoO3 and S powders were placed at the upstream of this tube, where MoO3 powders were placed slightly away from the high-temperature zone and S powders were slightly away from the low-temperature zone to prevent heating. During the growth process, the substrate was first heated to 1000 °C in an Ar atmosphere with a gas flow rate of 150 sccm for 10 min in order to form a liquid flat glass plane. After a procedural cooling process for 20 min, the growth process was performed at 750 °C and the furnace was then moved toward the upstream side to heat the MoO3 powders at

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01984. Schematic illustration of the STS growth process; component analysis of the soda-lime glass; OM images of the STS monolayer MoS2 crystal; the fitting curve of Raman spectrum; PL spectra of the monolayer MoS2 crystals; the reproducibility of the STS growth process; Raman peak difference and PL peak position mapping; illustration of the morphology transformation of MoS2 flakes; TEM images; XPS spectrum of S 2p of MoS2; XPS analysis of the substrate; correlation of the experimental results with simulations based on the equations regarding the diffusion of Mo on the gas flow; XPS depth analysis of the soda-lime glass after STS 6101

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growth of WS2; detailed growth parameters for testing the tolerance of STS growth process (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mark H. Rümmeli: 0000-0002-4448-1569 Lei Fu: 0000-0003-1356-4422 Author Contributions §

M.J., X.L., and J.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the Natural Science Foundation of China (Grants 21473124, 21673161). We thank Hao Huang and Prof. Lei Liao for their assistance with FET measurements and Dr. Yong Liu for his assistance with XPS depth analysis.

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DOI: 10.1021/acs.chemmater.7b01984 Chem. Mater. 2017, 29, 6095−6103

Article

Chemistry of Materials Gutierrez, H. R.; Pradhan, N. R.; Balicas, L.; Mallouk, T. E.; LopezUrias, F.; Terrones, H.; Terrones, M. Controlled Synthesis and Transfer of large-Area WS2 Sheets: from Single Layer to Few Layers. ACS Nano 2013, 7, 5235−5242. (33) Okada, M.; Sawazaki, T.; Watanabe, K.; Taniguch, T.; Hibino, H.; Shinohara, H.; Kitaura, R. Direct Chemical Vapor Deposition Growth of WS2 Atomic Layers on Hexagonal Boron Nitride. ACS Nano 2014, 8, 8273−8277.

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DOI: 10.1021/acs.chemmater.7b01984 Chem. Mater. 2017, 29, 6095−6103