Synthesis of Monolayer MoSe2 with Controlled Nucleation via Reverse-Flow Chemical Vapor Deposition

Two-dimensional (2D) layered semiconductor materials, such as transition metal dichalcogenides (TMDCs), have attracted considerable interests because of their intriguing optical and electronic properties. Controlled growth of TMDC crystals with large grain size and atomically smooth surface is indeed desirable but remains challenging due to excessive nucleation. Here, we have synthesized high-quality monolayer, bilayer MoSe2 triangular crystals, and continuous thin films with controlled nucleation density via reverse-flow chemical vapor deposition (CVD). High crystallinity and good saturated absorption performance of MoSe2 have been systematically investigated and carefully demonstrated. Optimized nucleation and uniform morphology could be achieved via fine-tuning reverse-flow switching time, growth time and temperature, with corresponding growth kinetics proposed. Our work opens up a new approach for controllable synthesis of monolayer TMDC crystals with high yield and reliability, which promote surface/interface engineering of 2D semiconductors towards van der Waals heterostructure device applications.


Introduction
Two-dimensional (2D) layered transition metal dichalcogenides (TMDCs) (MX 2 : M = W, Mo. X = S, Se, Te) [1,2] have triggered significant attention for their atomically thin geometry and novel band structures, which have shown promising applications in the fields of electronics, optoelectronics, valleytronics, as well as energy conversion and storage [3][4][5][6]. Most semiconducting TMDCs host a bandgap from visible to near-infrared (NIR) range, exhibiting a transition from indirect bandgap to direct bandgap with thickness reduced from multilayer to monolayer due to quantum confined effect [7,8]. Among various TMDCs, MoSe 2 has a tunable bandgap in the range of 1.1-1.5 eV by the thickness [9,10], making it one of the most desirable candidates in photovoltaic single-junction solar cells and photoelectrochemical cells [11]. MoSe 2 not only shows great potential for the application in field-effect transistors (FETs) and electronic logic devices but also provides an ideal platform in optoelectronics with its narrower bandgap, larger spin-splitting energy and higher optical absorbance than MoS 2 [12][13][14][15][16].

Synthesis of MoSe 2
MoSe 2 was grown in a home-modified two-temperature zone furnace CVD system with a 2-inch quartz tube (see Figure 1a). Both sides of the quartz tube are equipped with a gas inlet and outlet. The direction of gas flow can be switched by turning on gas valves 1 and 4 and turning off gas valves 2 and 3 or vice versa. The growth process is divided into two continual growth stages as shown in Figure 1b. A reverse-flow from zone 2 to zone 1 is applied during the temperature ramping and stabilization in stage I before growth, while a forward flow from zone 1 to zone 2 is applied for the epitaxial growth in stage II. MoO 3 (15 mg, Aladdin, 99.95%) and selenium (Se) powders (300 mg, Aladdin, 99.95%) were used as precursors. MoO 3 was placed in a corundum boat at the center of heating zone 2 with 300 nm SiO 2 /Si substrate faced down on top of the boat, while Se powders were placed in another corundum boat at the center of heating zone 1. The distance between the two boats was 20 cm. Firstly 300 sccm Argon (Ar) gas was introduced for 10 min before growth in order to flush the gas line and remove oxygen, then the furnace was heated to the desired growth temperature at a ramping rate of 25 • C per minute from room temperature (RT) with a reverse-flow of 45 sccm Ar. After that, a mixture of Ar/H 2 (45 sccm/5 sccm) was used as the carrier gas with a forward-flow and the reaction process lasted for 10-15 min. The flow direction and rate did not change during the growth Nanomaterials 2020, 10, 75 3 of 10 until the furnace cooled down to RT naturally. The diagram of the growth temperature program is shown in Figure 1c.
Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 10 growth until the furnace cooled down to RT naturally. The diagram of the growth temperature program is shown in Figure 1c.

Characterization of MoSe2
The surface morphology of MoSe2 was systematically examined by employing an optical microscope (OM, Nikon Inc., Tokyo, Japan), atomic force microscope (AFM, Bruker Inc., Beerlika, MA, USA), and scanning electron microscope (SEM, Hitachi Inc., Tokyo, Japan). Raman spectra and photoluminescence (PL) measurements were performed with a 532 nm laser (Renishaw Inc., Wotton underedge, UK). Crystalline stoichiometry was analyzed using an X-ray photoemission spectroscopy (XPS, Thermo Fisher Scientific Inc., Waltham, MA, USA) and X-ray diffraction instrument (XRD, Bruker Inc.). High-resolution transmission electron microscopy (HRTEM, JEOL Inc., Tokyo, Japan) operated at 200 kV was carried out to demonstrate the ideal crystallinity and atomic structure of MoSe2. Open-aperture Z-scan measurements were performed using a homemade mode-locked Yb fiber laser operating at 1064 nm with a pulse width of 15 ps and a repetition rate of 40 MHz.

Results and Discussions
The synthesis of atomically thin TMDC crystals is delicate and sensitive to growth conditions. In a typical CVD growth process with a single forward gas flow direction, the uncontrolled nucleation hinders the reliable formation of large-size crystals and irregular morphology and poor uniformity surface easily appeared (see Figure S1a,b). A reverse-flow strategy from the substrate to the source could prevent the unintended supply of the chemical vapor source to eliminate uncontrolled nucleation. However, if the reverse-flow is introduced immediately after the end of the growth, the sudden flux change may cause crystal flakes self-decompose into fragments (see Figure S1c). Therefore, exploring the appropriate time to switch the reverse-flow is crucial for ensuring the controllable growth of homogeneous monolayer MoSe2. If the reverse-flow is switched to forwardflow before reaching the designed temperature (~30 min), it leads to inhomogeneous morphology and apparent roughness on the substrate with a high nucleation density ~10,600 mm −2 , as shown in Figure 2a. As shown in Figure 2b, if the reverse-flow is prematurely switched to forward-flow before

Characterization of MoSe 2
The surface morphology of MoSe 2 was systematically examined by employing an optical microscope (OM, Nikon Inc., Tokyo, Japan), atomic force microscope (AFM, Bruker Inc., Beerlika, MA, USA), and scanning electron microscope (SEM, Hitachi Inc., Tokyo, Japan). Raman spectra and photoluminescence (PL) measurements were performed with a 532 nm laser (Renishaw Inc., Wotton underedge, UK). Crystalline stoichiometry was analyzed using an X-ray photoemission spectroscopy (XPS, Thermo Fisher Scientific Inc., Waltham, MA, USA) and X-ray diffraction instrument (XRD, Bruker Inc.). High-resolution transmission electron microscopy (HRTEM, JEOL Inc., Tokyo, Japan) operated at 200 kV was carried out to demonstrate the ideal crystallinity and atomic structure of MoSe 2 . Open-aperture Z-scan measurements were performed using a homemade mode-locked Yb fiber laser operating at 1064 nm with a pulse width of 15 ps and a repetition rate of 40 MHz.

Results and Discussions
The synthesis of atomically thin TMDC crystals is delicate and sensitive to growth conditions. In a typical CVD growth process with a single forward gas flow direction, the uncontrolled nucleation hinders the reliable formation of large-size crystals and irregular morphology and poor uniformity surface easily appeared (see Figure S1a,b). A reverse-flow strategy from the substrate to the source could prevent the unintended supply of the chemical vapor source to eliminate uncontrolled nucleation. However, if the reverse-flow is introduced immediately after the end of the growth, the sudden flux change may cause crystal flakes self-decompose into fragments (see Figure S1c). Therefore, exploring the appropriate time to switch the reverse-flow is crucial for ensuring the controllable growth of homogeneous monolayer MoSe 2 . If the reverse-flow is switched to forward-flow before reaching the designed temperature (~30 min), it leads to inhomogeneous morphology and apparent roughness on the substrate with a high nucleation density~10,600 mm −2 , as shown in Figure 2a. As shown in Figure 2b, if the reverse-flow is prematurely switched to forward-flow before reaching the desired temperature (~10 min), the source vapor will quickly reach the downstream substrate and react, forming a rough multilayer and monolayer flakes with jagged edges coexisted. If the reverse-flow is switched~5 min before the setpoint, the multilayer MoSe 2 flakes will further decrease with a nucleation density~8000 mm −2 (see Figure 2c). The reverse-flow was introduced during the temperature ramping stage and the optimum switching time was precisely regulated to be~1 min before reaching the desired growth temperature, and then forward-flow was applied until the end of the growth. By effectively suppressing excessive nucleation and self-decomposition, monolayer MoSe 2 flakes with uniform morphology and atomically smooth surface have been successfully synthesized with a lower nucleation density~5700 mm −2 (Figure 2d). The nucleation density can be controlled by fine-tuning the reverse-flow switching time before growth, with the statistical results shown in Figure 2i. As the gas flow rate increased, the monolayer flakes merged with each other to form a continuous film. The nucleation density is well controlled by optimizing the experimental parameters such as reverse-flow switching time, growth time and temperature. Figure 3a shows a typical continuous and uniform MoSe2 film synthesized on the SiO2/Si substrate with the reverse-flow method (growth temperature: ~760 °C, mixed vapor concentration: 80 sccm Ar/5 sccm H2, growth time: 10 min). Monolayer MoSe2 became dominant during the CVD growth with the introduction of H2 [30]. Similarly, triangular monolayer MoSe2 flakes up to 110 μm and bilayer MoSe2 flakes can be controllably grown under the same temperature (760 °C) and a mixture of Ar/H2 (45 sccm/5 sccm) for 10-15 min (Figure 3b and Figure S2a), revealing thermodynamically stable edge termination and the threefold symmetry of the unit cell. Most bilayer MoSe2 flakes exhibit an AA stacking structure (the relative rotation angle of two vertically stacked triangles θ = 0°), while a few AB stacking bilayer (the relative rotation angle θ = 60°), hexagonal bilayer and trilayer flakes can also be observed, which may The growth time and temperature were also demonstrated to play key roles in the nucleation process and corresponding growth kinetics. Firstly, we changed growth time and fixed other parameters, when growing 5 min, small-size MoSe 2 flakes (2-3 µm) were observed ( Figure 2e) with a high nucleation density~11700 mm −2 . By increasing the growth time to 10 min, homogeneous monolayer MoSe 2 flakes could be synthesized with a moderate nucleation density ∼5700 mm −2 (Figure 2d), among which the largest one is up to 110 µm. Bilayer MoSe 2 crystals appeared when the growth time increasing to 15 Nanomaterials 2020, 10, 75 5 of 10 min and the nucleation density was reduced to~5000 mm −2 (Figure 2f). The statistical variation of nucleation density with growth time was shown in Figure S1d. The precursors volatilize as the growth time increases, fewer precursors may lead to lower supersaturation, which therefore results in lower nucleation density. Then, we changed the growth temperature and fixed other parameters. At 720 • C, small-size MoSe 2 flakes (~5 µm) occurred with a nucleation density of~6900 mm −2 (Figure 2g). By increasing the temperature to 760 • C, homogeneous monolayer MoSe 2 flakes could be synthesized with a moderate nucleation density~5700 mm −2 (Figure 2d). While at 810 • C, large-size monolayer and small-size multilayer flakes coexisted with a reduced nucleation density of~3700 mm −2 (Figure 2h). The statistical variation of nucleation density with temperature was shown in Figure 2j. In general, the longer growth time usually leads to a lower nucleation density, while lower growth temperature typically results in a higher nucleation density. It is consistent with the nucleation model of the vapor phase deposition, that the nucleation probability is proportional to the supersaturation and inversely proportional to the temperature [29].
The nucleation density is well controlled by optimizing the experimental parameters such as reverse-flow switching time, growth time and temperature. Figure 3a shows a typical continuous and uniform MoSe 2 film synthesized on the SiO 2 /Si substrate with the reverse-flow method (growth temperature:~760 • C, mixed vapor concentration: 80 sccm Ar/5 sccm H 2 , growth time: 10 min). Monolayer MoSe 2 became dominant during the CVD growth with the introduction of H 2 [30]. Similarly, triangular monolayer MoSe 2 flakes up to 110 µm and bilayer MoSe 2 flakes can be controllably grown under the same temperature (760 • C) and a mixture of Ar/H 2 (45 sccm/5 sccm) for 10-15 min (Figure 3b and Figure S2a), revealing thermodynamically stable edge termination and the threefold symmetry of the unit cell. Most bilayer MoSe 2 flakes exhibit an AA stacking structure (the relative rotation angle of two vertically stacked triangles θ = 0 • ), while a few AB stacking bilayer (the relative rotation angle θ = 60 • ), hexagonal bilayer and trilayer flakes can also be observed, which may indicate that AA stacking order is more favorable under lower growth temperature. Figure S2b,c shows SEM images of monolayer and bilayer MoSe 2 triangular flakes with clear surface and boundary, confirming smoothness and high uniformity of the sample. The height of MoSe 2 film, monolayer, and bilayer crystals were measured~0.7 nm,~0.65 nm, and~0.8 nm respectively by AFM (Figure 3c,d and Figure S2d), which is comparable to exfoliated samples [12] and our previous works [22,23].
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 10 peaks for monolayer and bilayer MoSe2 are located at 287 cm −1 and 285 cm −1 , respectively. The A1g peak is blue-shifted due to increasing interlayer coupling, and E peak is red-shifted may result from long-range coulomb interlayer interactions from monolayer to bilayer, the peak spacing between A1g and E mode decreases as layer number increases from monolayer (47 cm −1 ) to bilayer (44 cm −1 ), which is consistent with previous results [9,10,20,31]. The monolayer MoSe2 occurs a prominent emission peak at ~807 nm (A-exciton) in the PL spectrum, which is much higher than the intensity of bilayer MoSe2 whose red-shifted peak at ~ 818 nm by more than 10 times (Figure 3f), indicating that the transition from a direct bandgap of monolayer MoSe2 at 1.54 eV to an indirect bandgap of bilayer MoSe2 at 1.51 eV, distinctly different from the bulk bandgap of ~1.1 eV [9,10]. Raman intensity maps ~240 cm −1 (A1g mode) of the monolayer MoSe2 films and flakes reveal a particular uniform color contrast, verifying the highly uniform surface morphology and crystalline quality cover across the substrate (Figure 3g-h). The optical images, Raman (A1g mode) and PL peak (807 nm) intensity maps of AA stacking bilayer MoSe2 have shown a strong contrast ( Figure S3a-c) and reveal the distinct boundary between the monolayer and bilayer regions. The optical images and Raman peak (A1g and E mode) intensity maps of a hexagonal MoSe2 flake are also shown ( Figure  S3d-f), reflecting that the second layer triangles begin to grow from the same nucleation site at the center of the first layer flakes. We have further carried out XPS measurements to identify the stoichiometry and elemental composition of monolayer MoSe2. The binding energies of Mo 3d3/2 and Mo 3d5/2 are ~232.4 eV and ~229.3 eV (Figure 4a), and the binding energies of Se 3d5/2 and Se 3d3/2 are ~54.9 eV and ~55.8 eV (Figure 4b) respectively, in good agreement with previously reported values [10,20]. The atomic ratio of Mo and Se calculated from the XPS spectra is 1:1.92, which is very close to the stoichiometry of The crystallinity and optical properties of monolayer and bilayer MoSe 2 flakes were evaluated by employing Raman and PL measurements (Figure 3e-f). There are two characteristic peaks of Raman spectra of MoSe 2 , which correspond to the out-of-plane A 1g and in-plane E 1 2g vibration mode. The A 1g peaks for monolayer and bilayer MoSe 2 are located at 240 cm −1 and 241 cm −1 , and the E 1 2g peaks for monolayer and bilayer MoSe 2 are located at 287 cm −1 and 285 cm −1 , respectively. The A 1g peak is blue-shifted due to increasing interlayer coupling, and E 1 2g peak is red-shifted may result from long-range coulomb interlayer interactions from monolayer to bilayer, the peak spacing between A 1g and E 1 2g mode decreases as layer number increases from monolayer (47 cm −1 ) to bilayer (44 cm −1 ), which is consistent with previous results [9,10,20,31]. The monolayer MoSe 2 occurs a prominent emission peak at~807 nm (A-exciton) in the PL spectrum, which is much higher than the intensity of bilayer MoSe 2 whose red-shifted peak at~818 nm by more than 10 times (Figure 3f), indicating that the transition from a direct bandgap of monolayer MoSe 2 at 1.54 eV to an indirect bandgap of bilayer MoSe 2 at 1.51 eV, distinctly different from the bulk bandgap of~1.1 eV [9,10]. Raman intensity maps~240 cm −1 (A 1g mode) of the monolayer MoSe 2 films and flakes reveal a particular uniform color contrast, verifying the highly uniform surface morphology and crystalline quality cover across the substrate (Figure 3g-h). The optical images, Raman (A 1g mode) and PL peak (807 nm) intensity maps of AA stacking bilayer MoSe 2 have shown a strong contrast ( Figure S3a-c) and reveal the distinct boundary between the monolayer and bilayer regions. The optical images and Raman peak (A 1g and E 1 2g mode) intensity maps of a hexagonal MoSe 2 flake are also shown ( Figure S3d-f), reflecting that the second layer triangles begin to grow from the same nucleation site at the center of the first layer flakes.
We have further carried out XPS measurements to identify the stoichiometry and elemental composition of monolayer MoSe 2 . The binding energies of Mo 3d 3/2 and Mo 3d 5/2 are~232.4 eV and 229.3 eV (Figure 4a), and the binding energies of Se 3d 5/2 and Se 3d 3/2 are~54.9 eV and~55.8 eV (Figure 4b) respectively, in good agreement with previously reported values [10,20]. The atomic ratio of Mo and Se calculated from the XPS spectra is 1:1.92, which is very close to the stoichiometry of MoSe 2 . The crystalline stoichiometry was further confirmed by an XRD spectrum as shown in Figure 4c. The monolayer MoSe 2 film was transferred to a TEM grid with the aid of polymethyl methacrylate (PMMA) solution. Figure 4d,e display bright-field TEM images of continuous MoSe 2 film and triangle MoSe 2 flakes, respectively. The high crystallinity of MoSe 2 was further demonstrated by a high-resolution TEM (HRTEM) image and the fast Fourier transformation (FFT) pattern in the inset (Figure 4f). The FFT pattern clearly reveals as-transferred MoSe 2 film is a single crystal with a hexagonal lattice structure. The lattice spacing measured from the atomically resolved TEM image is 0.28 nm, corresponding to the (1010) plane [9,10,20]. The optical modulator plays an important role in the pulsed laser generation system. MoSe 2 has been previously studied as broadband optical modulator materials for pulsed fiber laser systems due to their strong light-matter interaction [14−16]. The nonlinear optical absorption of CVD-grown MoSe 2 and exfoliated samples is usually weakened by their random morphology, uncontrollable size and non-uniform thickness. Large-size monolayer MoSe 2 flakes ( Figure S4a) and uniform continuous film ( Figure S4b) can also be grown on the sapphire substrate with good light transmittance [10,22,23,32,33]. Open-aperture Z-scan measurements were performed using a homemade mode-locked Yb fiber laser operating at 1064 nm with a pulse width of 15 ps and a tunable repetition rate of 40 MHz [34]. The experimental setup is shown in Figure 5a and we further investigated the nonlinear optical absorption properties of as-grown MoSe 2 films (Figure 5b). The laser beam was split into two parts: one part was measured by a power meter, and the other was focused by a lens with a focal length of 75 mm onto the samples with the focused beam waist estimated to be~80 µm and Rayleigh length of 2.0 mm. The transmitted laser was focused by a lens with a focal length of 100 mm and measured by another power meter. The sample was fixed to a stepper motor, and its movement was controlled by Nanomaterials 2020, 10, 75 7 of 10 a computer program. The nonlinear saturable absorption parameters can be obtained by fitting the Z-scan curves. The fitting formula of the open aperture Z-scan curves could be described as [35]: where T is normalized transmittance, and ∆R, I S , I 0 and Z 0 are the modulation depth, saturation fluence, incident pulse fluence and Rayleigh length of the incident laser beam, respectively. According to the fitting curve (Figure 5c), the saturation strength I S was fitted to be 110 MW/cm 2 , corresponding to the modulation depth ∆R of around 38.2%, indicating the desired nonlinearity. The large modulation depth and low saturation strength show a good saturated absorption performance, indicating that the MoSe 2 film has great potential as an excellent saturable absorber in a pulsed laser generation system.  The optical modulator plays an important role in the pulsed laser generation system. MoSe2 has been previously studied as broadband optical modulator materials for pulsed fiber laser systems due to their strong light-matter interaction [14−16]. The nonlinear optical absorption of CVD-grown MoSe2 and exfoliated samples is usually weakened by their random morphology, uncontrollable size and non-uniform thickness. Large-size monolayer MoSe2 flakes ( Figure S4a) and uniform continuous film ( Figure S4b) can also be grown on the sapphire substrate with good light transmittance [10,22,23,32,33]. Open-aperture Z-scan measurements were performed using a homemade modelocked Yb fiber laser operating at 1064 nm with a pulse width of 15 ps and a tunable repetition rate of 40 MHz [34]. The experimental setup is shown in Figure 5a and we further investigated the nonlinear optical absorption properties of as-grown MoSe2 films (Figure 5b). The laser beam was split into two parts: one part was measured by a power meter, and the other was focused by a lens with a focal length of 75 mm onto the samples with the focused beam waist estimated to be ~80 μm and Rayleigh length of ~2.0 mm. The transmitted laser was focused by a lens with a focal length of 100 mm and measured by another power meter. The sample was fixed to a stepper motor, and its movement was controlled by a computer program. The nonlinear saturable absorption parameters can be obtained by fitting the Z-scan curves. The fitting formula of the open aperture Z-scan curves could be described as [35]: indicating that the MoSe2 film has great potential as an excellent saturable absorber in a pulsed laser generation system.

Conclusions
In conclusion, we have developed a general and reliable reverse-flow CVD strategy for controllable synthesis of high-quality monolayer MoSe2 crystals up to 110 μm, as well as uniform films. The excessive nucleation density was suppressed, while high crystallinity and uniform morphology of MoSe2 were carefully demonstrated. The optimal growth was achieved via precisely controlling reverse-flow switching time, growth time and temperature. The modulation depth and saturation strength of MoSe2 on sapphire substrates were measured to be 38.2% and 110 MW·cm 2 respectively, the desired nonlinearity enables it as an ideal optical modulator material for pulsed laser and photodetectors. This work paves a new way for the controllable growth of 2D TMDCs and facilitates the development of electronic and optoelectronic applications.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Optical images of MoSe2 flakes and variation of nucleation density with growth time. Figure S2: Optical, SEM and AFM images of MoSe2 flakes. Figure S3: The optical image, Raman and PL intensity map of triangular and hexagonal MoSe2 flakes. Figure S4: The optical images, Raman spectrum and intensity map of monolayer MoSe2. All authors have read and agreed to the published version of the manuscript.

Conclusions
In conclusion, we have developed a general and reliable reverse-flow CVD strategy for controllable synthesis of high-quality monolayer MoSe 2 crystals up to 110 µm, as well as uniform films. The excessive nucleation density was suppressed, while high crystallinity and uniform morphology of MoSe 2 were carefully demonstrated. The optimal growth was achieved via precisely controlling reverse-flow switching time, growth time and temperature. The modulation depth and saturation strength of MoSe 2 on sapphire substrates were measured to be 38.2% and 110 MW·cm 2 respectively, the desired nonlinearity enables it as an ideal optical modulator material for pulsed laser and photodetectors. This work paves a new way for the controllable growth of 2D TMDCs and facilitates the development of electronic and optoelectronic applications.
Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/10/1/75/s1, Figure S1: Optical images of MoSe2 flakes and variation of nucleation density with growth time. Figure S2: Optical, SEM and AFM images of MoSe2 flakes. Figure S3: The optical image, Raman and PL intensity map of triangular and hexagonal MoSe2 flakes. Figure S4: The optical images, Raman spectrum and intensity map of monolayer MoSe 2 . All authors have read and agreed to the published version of the manuscript.