Preparation method and application of monolayer molybdenum disulfide and tungsten disulfide

Abstract

The application relates to a preparation method and application of monolayer MoS2 and WS2 and belongs to the field of inorganic two-dimensional materials. The steps are as follows: a substrate is pretreated and used as a CVD growth substrate; a precursor KMS2 is weighed and placed at the bottom of a container; the pretreated surface of the substrate is covered on the precursor; the container with the precursor and the substrate is placed in a quartz tube, the quartz tube is installed in a tube furnace, and the tube furnace is sealed and installed; vacuum is drawn in the quartz tube and argon is introduced, and other gases are completely removed; the tube furnace is heated to a set temperature and then is kept warm; after the reaction in the quartz tube is completed, the substrate is taken out when the temperature is naturally cooled to room temperature, and the preparation is completed. The size of the prepared monolayer MoS2 and WS2 can reach more than 300 microns, and the material surface is free of obvious defects; the technical problem solved by the application is that a large number of nucleation cores generated due to high steam pressure in the process of preparing monolayer MoS2 and WS2 by using traditional MoO3 / WO3 and sulfur powder as raw materials is overcome.

Description

Technical Field

[0001] This invention belongs to the field of inorganic two-dimensional materials, specifically relating to a method for preparing monolayer MoS2 and WS2 and their applications. Background Technology

[0002] Ultrathin two-dimensional transition metal chalcogenides (TMDs) have broad application prospects in future electronic and optoelectronic devices due to their unique layered structure, tunable band gap, and excellent optical, electrical, and magnetic properties.

[0003] Molybdenum disulfide (MoS2) and tungsten disulfide (WS2), as two-dimensional transition metal chalcogenides, have been extensively studied. MoS2 and WS2 are layered materials exhibiting rich polymorphic structures. Based on the different filling states of the 4d orbitals of Mo / W atoms, MoS2 can be divided into a semiconductor phase (2H phase) and a metallic phase (1T phase). The band gap of 2H MoS2 changes from 1.2 eV to 1.9 eV with decreasing layer number, thus few-layer MoS2 exhibits significant optical properties. The properties of monolayer MoS2 and WS2 can be modified through defect modulation, phase modulation, and stress modulation, leading to their wide applications in field-effect transistors, sensors, optoelectronic devices, and hydrogen evolution catalysis.

[0004] Currently, the main methods for preparing monolayer MoS2 and WS2 include mechanical exfoliation, liquid phase exfoliation, metal precursor conversion, physical vapor deposition (PVD), and chemical vapor deposition (CVD). Mechanical exfoliation and liquid phase exfoliation can yield high-quality monolayer MoS2 and WS2, but these methods are inefficient. While metal precursor conversion can prepare thin, large-size MoS2 and WS2 films, the resulting MoS2 and WS2 films contain numerous grain boundaries and defects, leading to poor film uniformity and consequently, poor crystal quality. PVD samples are generally too small, only 30-50 micrometers in size, while CVD results typically only reach a maximum size of 100-200 micrometers.

[0005] When precursors are introduced in gaseous form, it facilitates the uniform mixing of reactive materials. Simultaneously, the high speed and strong migration ability of atoms and molecules at high temperatures promote domain growth. Therefore, CVD is considered the most effective method for preparing large-size, high-quality, large-domain, and controllable-layer-number two-dimensional materials. However, in the traditional TMDs preparation process, the high vapor pressure of the metal oxide source and sulfur powder at high temperatures leads to the formation of numerous nucleation cores on the substrate, hindering the growth of monolayer crystals. In the CVD process, under sulfur-rich conditions, additional sulfur vapor is provided to replace the oxygen in the MoO3 precursor. Due to the competition between Mo-O and Mo-S bonds in the reaction chamber, a small number of residual O atoms replace the S atoms in the generated molybdenum disulfide sheets. Calculations show that these O atoms are 1.99 eV more stable than the corresponding 2S and generally tend to desorb into the gas phase, leaving vacancies at the S sites, i.e., S vacancies. In addition, the volatilization temperatures of the molybdenum source and the sulfur source are very different, so it is difficult to ensure that the ratio of molybdenum source to sulfur source in the reaction region is 1:2, which also provides conditions for the generation of sulfur vacancies, thus causing defects in the monolayer MoS2.

[0006] Therefore, this invention designs a growth method for large-size, low-lattice-defect monolayer MoS2 and WS2, which can obtain low-lattice-defect monolayer MoS2 and WS2 with sizes greater than 300 micrometers. Summary of the Invention

[0007] The technical problem to be solved:

[0008] To overcome the shortcomings of existing technologies, this invention provides a method for preparing large-sized, low-lattice-defect monolayer MoS2 and WS2. Using KMS2 (M = Mo, W) as a precursor, and applying it to a CVD deposition method, the prepared monolayer MoS2 and WS2 can achieve sizes exceeding 300 micrometers, with no obvious defects on the material surface. The technical problem solved by this invention is to overcome the problem of generating a large number of nucleation cores due to high vapor pressure during the traditional preparation of monolayer MoS2 and WS2 using MoO3 / WO3 and sulfur powder as raw materials.

[0009] The technical solution of this invention is: a method for preparing monolayer MoS2 and WS2, the specific steps of which are as follows:

[0010] Step 1: Pre-treat the substrate and use it as the CVD growth substrate;

[0011] Step 2: Weigh the precursor KMS2 and place it at the bottom of the container, then cover the precursor with the pretreated substrate.

[0012] Step 3: Place the container containing the precursor and substrate inside the quartz tube, then install the quartz tube inside the tube furnace, and finally seal the tube furnace.

[0013] Step 4: Evacuate the quartz tube and introduce argon gas to remove other gases.

[0014] Step 5: Heat the tubular furnace to the set temperature, and then maintain the temperature.

[0015] Step 6: After the reaction in the quartz tube is complete, allow it to cool naturally to room temperature, then turn off the argon gas and remove the substrate to complete the preparation.

[0016] A further technical solution of the present invention is: in step one, the substrate is a mica sheet; the pretreatment method is to cut it open with a scalpel, and the exposed clean surface is used as the growth surface.

[0017] A further technical solution of the present invention is: in step two, M in the precursor KMS2 is Mo and W, and the container is an Al2O3 ceramic boat.

[0018] A further technical solution of the present invention is: in step three, the container is placed in the central area of ​​the quartz tube; both ends of the tube furnace are sealed by furnace plugs and flange components are installed.

[0019] A further technical solution of the present invention is as follows: the method for removing other gases in step four is to turn on the vacuum pump, evacuate the quartz tube to 5-10 Pa, and then introduce argon gas into the quartz tube at a flow rate of 500 sccm until the gas pressure inside the quartz tube reaches atmospheric pressure. This process is repeated three times to completely remove other gases from the quartz tube.

[0020] A further technical solution of the present invention is: in step five, the heating rate of the tube furnace is 40℃·min. -1 Heat to 650-900℃ and hold for 5-12 minutes.

[0021] A further technical solution of the present invention is: in step five, the argon flow rate during monolayer MoS2 growth is 50 sccm.

[0022] An apparatus for preparing monolayer MoS2 and WS2 includes a tube furnace, a vacuum pump, and an argon tank. The vacuum pump and the argon tank are connected to the tube furnace via pipes for regulating the vacuum environment and argon environment inside the tube furnace.

[0023] The tubular furnace has a heating zone arranged circumferentially, and a quartz tube is installed inside it as a heating reaction chamber; both ends of the tubular furnace are sealed by furnace plugs.

[0024] An application of monolayer MoS2 as a catalyst.

[0025] An application of a single-layer WS2 as a field-effect transistor material.

[0026] Beneficial effects

[0027] The beneficial effects of this invention are as follows:

[0028] 1. In the precursor KMS2 (M = Mo, W) of the present invention, the ratio of Mo / W and S is 1:2, which optimizes the problem that the ratio of molybdenum source and sulfur source on the substrate is difficult to control in traditional growth. The monolayer MoS2 and WS2 prepared by using KMS2 (M = Mo, W) as precursor have no obvious defects on the substrate.

[0029] 2. This invention introduces an alkali metal cation K into the precursor. + This process helps reduce the vapor pressure of the precursor at high temperatures, minimizing nucleation and promoting the growth of monolayer MoS2 and WS2. KMoS2 first undergoes dehydration at 150℃ and structural transformation at 400℃. Within the MoS2 growth temperature range (650–900℃), all K, Mo, and S elements in KMoS2 simultaneously evaporate into the gas phase, forming K-Mo-S vapor with very low partial pressure and a slight excess of sulfur. Due to the very small space between the precursor and the substrate, the K-Mo-S vapor source immediately condenses on the nearby substrate surface and provides raw materials for the growth of MoS2 sheets under the catalysis of K. The significant weight loss of traditional CVD precursor MoO3 and PVD precursor MoS2 between 700℃ and 800℃ indicates that the high vapor pressure during the CVD process leads to a fast nucleation rate and small crystallization domain for MoS2. In contrast, the lower vapor pressure of KMoS2 enables lower density MoS2 nuclei, and the in-situ generated K promoter significantly increases the growth rate, resulting in large-sized MoS2 crystals. Furthermore, the required Mo:S ratio in K-Mo-S vapor can form defect-free MoS2, thereby avoiding the problems of insufficient sulfidation in MoO3-based CVD and sulfur deficiency in MoS2-based PVD processes.

[0030] 3. This invention provides a simpler preparation method for large-size monolayers, capable of preparing monolayers of MoS2 and WS2 with sizes exceeding 300 micrometers, and reducing lattice defects; the method of this invention is easy to control and has good repeatability. Attached Figure Description

[0031] Figure 1 A schematic diagram of the experimental setup for growing large-sized monolayer MoS2 using KMoS2 as a raw material;

[0032] Figure 2 This diagram illustrates the temperature and airflow settings during the CVD growth process of large-sized monolayer MoS2 using KMoS2 as the raw material.

[0033] Figure 3 X-ray diffraction pattern of the precursor KMoS2;

[0034] Figure 4The image shows the morphology of the precursor KMoS2 under a scanning electron microscope and its energy-dispersive X-ray spectroscopy (EDS) image.

[0035] Figure 5 Optical photographs of large-size, low-lattice-defect monolayer MoS2 grown using KMoS2 as a raw material;

[0036] Figure 6 To obtain the Raman spectrum of large-size, low-lattice-defect monolayer MoS2 grown using KMoS2 as raw material;

[0037] Figure 7 To obtain the photoluminescence spectrum of large-size, low-lattice-defect monolayer MoS2 grown using KMoS2 as raw material;

[0038] Figure 8 Atomic force microscopy results of large-size, low-lattice-defect monolayer MoS2 grown using KMoS2 as raw material;

[0039] Figure 9 A double aberration-corrected transmission electron microscope image of large-sized, low-lattice-defect monolayer MoS2 grown using KMoS2 as raw material;

[0040] Figure 10 X-ray photoelectron spectroscopy (XPS) image of large-sized, low-lattice-defect monolayer MoS2 grown using KMoS2 as a raw material.

[0041] Figure 11 The X-ray diffraction pattern of the precursor KWS2;

[0042] Figure 12 Optical photographs of large-size, low-lattice-defect monolayer WS2 grown using KWS2 as a raw material;

[0043] Figure 13 To obtain the Raman spectrum of large-size, low-lattice-defect monolayer WS2 grown using KWS2 as raw material;

[0044] Figure 14 The photoluminescence spectrum of large-size, low-lattice-defect monolayer WS2 was obtained by using KWS2 as raw material.

[0045] Explanation of reference numerals in the attached diagram: 1. Furnace plug, 2. Heating zone. Detailed Implementation

[0046] The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention.

[0047] Example 1:

[0048] A method for preparing large-size MoS2 and WS2 with low lattice defects includes the following steps:

[0049] Embodiment 1 of this invention provides a method for preparing large-size, low-lattice-defect MoS2. The experiment on the large-size, low-lattice-defect monolayer MoS2 two-dimensional material was carried out in a vacuum tube furnace, and the experimental setup is as follows: Figure 1 As shown in the diagram, the temperature and gas flow settings during the growth of large-size, low-lattice-defect monolayer MoS2 two-dimensional materials are as follows: Figure 2 As shown.

[0050] Specifically, the steps include the following:

[0051] 1) The substrate used for sample preparation was the fresh side of a mica sheet cut with a scalpel. Oxygen plasma treatment was used to remove organic contaminants from the fresh side of the mica sheet.

[0052] 2) Weigh 70 mg of the precursor KMoS2 and spread it evenly on the bottom of an Al2O3 ceramic boat. Cover the KMoS2 with the fresh side of the mica sheet facing the precursor. Use the confinement method to make it easier for the volatilized MoS2 molecules to deposit on the mica sheet, thus promoting MoS2 growth.

[0053] 3) Place the Al2O3 ceramic boat with the precursor and mica sheets in the middle of a quartz tube with a diameter of 50 mm;

[0054] 4) Place furnace plugs at both ends of the temperature zone and assemble the flange components;

[0055] 5) Turn on the mechanical pump and evacuate the quartz tube to 5-10 Pa. Then, send high-purity argon gas into the quartz tube at a flow rate of 500 sccm until the gas pressure inside the quartz tube reaches atmospheric pressure. Repeat this process three times to remove all other gases from the quartz tube.

[0056] 6) Open the exhaust port of the tube furnace, adjust the flow rate of high-purity argon to 50 sccm as the carrier gas to promote the reaction until the heating is finished, and turn off the argon gas after the furnace body cools down to room temperature.

[0057] 7) At 40℃·min -1 The heating rate rapidly increased from room temperature to 750℃;

[0058] 8) Keep warm at 750℃ for 7 minutes;

[0059] 9) After the reaction is complete and the substrate is allowed to cool naturally to room temperature, the growth substrate is removed to complete the preparation, and a monolayer MoS2 with a size greater than 800 micrometers can be obtained.

[0060] Example 2:

[0061] A method for preparing large-size MoS2 and WS2 with low lattice defects includes the following steps:

[0062] To verify the universality of our invention, Embodiment 2 of this invention provides a method for preparing large-size low-lattice-defect WS2. The experiment on the large-size low-lattice-defect monolayer WS2 material was carried out in a vacuum tube furnace, and the experimental setup is as follows: Figure 1 As shown in the diagram, the temperature and gas flow settings during the growth of a large-size, low-lattice-defect monolayer WS2 two-dimensional material are as follows: Figure 2 As shown.

[0063] Specifically, the steps include the following:

[0064] 1) The substrate used for sample preparation was the fresh side of a mica sheet cut with a scalpel. Oxygen plasma treatment was used to remove organic contaminants from the fresh side of the mica sheet.

[0065] 2) Weigh 70 mg of precursor KWS2 and spread it evenly on the bottom of an Al2O3 ceramic boat. Cover the KWS2 with the fresh side of a mica sheet, using a confinement method to make it easier for volatile WS2 molecules to deposit on the mica sheet, thus promoting WS2 growth.

[0066] 3) Place the Al2O3 ceramic boat with the precursor and mica sheets in the middle of a quartz tube with a diameter of 50 mm;

[0067] 4) Place furnace plugs at both ends of the temperature zone and assemble the flange components;

[0068] 5) Turn on the mechanical pump and evacuate the quartz tube to 5-10 Pa. Then, send high-purity argon gas into the quartz tube at a flow rate of 500 sccm until the gas pressure inside the quartz tube reaches atmospheric pressure. Repeat this process three times to remove all other gases from the quartz tube.

[0069] 6) Open the exhaust port of the tubular furnace and adjust the flow rate of high-purity argon to 50 sccm as the carrier gas to promote the reaction.

[0070] Continue heating until the furnace body cools to room temperature, then turn off the argon gas.

[0071] 10) At 40℃·min -1 The heating rate rapidly increased from room temperature to 750℃;

[0072] 11) Keep warm at 750℃ for 7 minutes;

[0073] 12) After the reaction is complete and the substrate is naturally cooled to room temperature, the growth substrate is removed to complete the preparation, and a monolayer WS2 with a size greater than 350 micrometers can be obtained.

[0074] Verification of the effect of the example:

[0075] To examine the basic properties of monolayer MoS2 and WS2 obtained by growing using KMS2 (M = Mo, W) as a precursor, we conducted a series of analytical tests on the precursor KMS2 (M = Mo, W) and the MoS2 and WS2 prepared in Examples 1 and 2.

[0076] The purity and phase state of the precursor were characterized using XRD. The XRD pattern of the precursor KMoS2 is shown below. Figure 3 As shown, the peaks (001), (002), and (004) indicate that the crystallinity is high and the precursor is a pure phase.

[0077] The morphology and size of the precursor KMoS2 material were characterized using scanning electron microscopy. Figure 4 The scanning electron microscope images shown indicate that the individual single crystals are lamellar with a lateral dimension of approximately 5-100 μm. The EDS images show that potassium is uniformly distributed in the crystals.

[0078] The morphology and size of the grown monolayer MoS2 were characterized using optical microscopy. Optical images of large-size monolayer MoS2 are shown below. Figure 5 As shown, the monolayer MoS2 prepared using KMoS2 as a precursor is a standard triangle, with a maximum size of 770 micrometers, which is much larger than the size of monolayer MoS2 reported in most literature.

[0079] Raman spectroscopy was used to further characterize the composition and number of layers of the grown material. Figure 6 This is the spectrum obtained by Raman characterization of the prepared MoS2. We found two characteristic peaks A in the Raman spectrum. 1 2g and E 1g Furthermore, the distance between the two characteristic peaks was found to be ΔK = 19.48 cm. -1 This proves that our prepared sample had few defects and was a single layer. The Raman laser wavelength was 532 nm.

[0080] Figure 7 This is the photoluminescence spectrum of the prepared MoS2. From the photoluminescence spectrum, we found a characteristic peak at 770 nm, which also indicates that the prepared MoS2 is a monolayer.

[0081] Figure 8 The figure shows the atomic force microscopy characterization results of monolayer MoS2 in Example 1. It can be seen from the figure that the thickness of the epitaxial layer is greater than 0.6 nm, which is consistent with the conclusion from Raman spectroscopy and photoluminescence spectroscopy that the large-size low-lattice-defect MoS2 prepared using KMoS2 as a precursor is monolayer.

[0082] Figure 9This is a double aberration-corrected transmission electron microscope (TEM) image of a large-sized monolayer MoS2 grown using KMoS2 as the raw material. The TEM image reveals the local atomic arrangement of the prepared MoS2; the brighter dots represent Mo atoms, and the darker dots represent S atoms. The Mo and S atoms in the image are arranged regularly and periodically, without any obvious defects. This demonstrates that the large-sized monolayer MoS2 grown using KMoS2 as the raw material has a defect-free basal surface and is of high quality.

[0083] Figure 10 The image shows an X-ray photoelectron spectroscopy (XPS) pattern. The binding energies of Mo and S elements measured by X-ray photoelectron spectroscopy on the obtained sample are consistent with those in the 2H phase of MoS2, and the estimated atomic ratio of Mo:S is 1:2, further indicating the high purity of the obtained MoS2. This is consistent with the double aberration-corrected transmission electron microscopy image.

[0084] The purity and phase state of the precursor were characterized using XRD. The XRD pattern of the precursor KWS2 is shown below. Figure 11 As shown, the sharp peaks (001), (002), (003), (004), (005), and (006) indicate that the crystallinity is high and the precursor is a pure phase.

[0085] The morphology and size of the grown monolayer WS2 were characterized using optical microscopy. Optical images of large-size monolayer WS2 are shown below. Figure 12 As shown, the monolayer WS2 prepared using KWS2 as a precursor is a standard triangle, with a maximum size of 350 micrometers.

[0086] Figure 13 This is the spectrum obtained by Raman characterization of the prepared WS2. Spectra were observed at approximately 350 and 417 cm⁻¹. -1 The two characteristic Raman peaks at the location belong to the E of WS2. 1 2g and A 1g The mode is a single layer. The laser wavelength for Raman spectroscopy is 532 nm.

[0087] Figure 14 The photoluminescence spectrum of a large-size, low-lattice-defect monolayer WS2 was obtained by growing KWS2 as a raw material. A strong PL peak at approximately 625 nm was observed on the obtained sheet, further indicating that the sheet is a monolayer.

[0088] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A method for preparing monolayer MoS2 and WS2, characterized in that... The specific steps are as follows: Step 1: Pre-treat the substrate and use it as the CVD growth substrate; Step 2: Weigh the precursor KMS2 and place it at the bottom of the container. Cover the precursor with the pretreated substrate. M in the precursor KMS2 is Mo and W. Step 3: Place the container containing the precursor and substrate inside the quartz tube, then install the quartz tube inside the tube furnace, and finally seal the tube furnace. Step 4: Evacuate the quartz tube and introduce argon gas to remove other gases. Step 5: Heat the tube furnace to the set temperature and then hold it at that temperature; the heating rate of the tube furnace is 40℃·min. -1 Heat to 650-900℃ and hold for 5-12 minutes; Step 6: After the reaction in the quartz tube is complete, allow it to cool naturally to room temperature, then turn off the argon gas and remove the substrate to complete the preparation.

2. The method for preparing monolayer MoS2 and WS2 according to claim 1, characterized in that: In step one, the substrate is a mica sheet; the pretreatment method is to cut it open with a scalpel, and the exposed clean surface is used as the growth surface.

3. The method for preparing monolayer MoS2 and WS2 according to claim 1, characterized in that: In step two, the container is an Al2O3 ceramic boat.

4. The method for preparing monolayer MoS2 and WS2 according to claim 1, characterized in that: In step three, the container is placed in the central area of ​​the quartz tube; both ends of the tube furnace are sealed with furnace plugs and are equipped with flange components.

5. The method for preparing monolayer MoS2 and WS2 according to claim 1, characterized in that: The method for removing other gases in step four is as follows: turn on the vacuum pump, evacuate the quartz tube to 5-10 Pa, and then introduce argon gas into the quartz tube at a flow rate of 500 sccm until the gas pressure inside the quartz tube reaches atmospheric pressure. Repeat this process three times to completely remove other gases from the quartz tube.

6. The method for preparing monolayer MoS2 and WS2 according to claim 5, characterized in that: In step five, the argon flow rate during monolayer MoS2 growth is 50 sccm.

Images