Method for preparing transition metal chalcogenide planar heterojunction by in-situ control method
阅读说明：本技术 一种原位控制法制备过渡金属硫属化物平面异质结的方法 (Method for preparing transition metal chalcogenide planar heterojunction by in-situ control method ) 是由 万茜 顾嫣芸 陈琨 于 2020-07-28 设计创作，主要内容包括：本发明公开了一种原位控制法制备过渡金属硫属化物平面异质结的方法,属于二维半导体材料技术领域。本发明利用目前公认的最有效生长的化学气相沉积法制备过渡金属硫属化物平面异质结,相比现有的两步法生长,本发明使用钼酸铵((NH<Sub>4</Sub>)<Sub>6</Sub>Mo<Sub>7</Sub>O<Sub>24</Sub>·4H<Sub>2</Sub>O)与钨酸铵((NH<Sub>4</Sub>)<Sub>10</Sub>W<Sub>12</Sub>O<Sub>41</Sub>·xH<Sub>2</Sub>O)的溶液来作为前驱体,并且通过移动钼源与钨源于衬底的正下方进行沉积,本发明无需人工进行二维材料的二次转移堆叠,重复性好,过程快速高效,连续可控地制备了高质量低成本的平面异质结,避免了交叉污染的问题；并且水溶性前驱体溶液同时具备环保、便宜、稳定和易溶于水等优点。(The invention discloses a method for preparing a transition metal chalcogenide planar heterojunction by an in-situ control method, and belongs to the technical field of two-dimensional semiconductor materials. The present invention utilizes the presently recognized most efficient growth of chemical vapor deposition to prepare transition metal chalcogenide planar heterojunctions, using ammonium molybdate ((NH) () as compared to the existing two-step growth) 4 ) 6 Mo 7 O 24 ·4H 2 O) and ammonium tungstate ((NH) 4 ) 10 W 12 O 41 ·xH 2 O) solution is used as a precursor, and the molybdenum source and the tungsten source are moved to deposit under the substrate, so that the method does not need manual secondary transfer stacking of two-dimensional materials, has good repeatability, is quick and efficient in process, and continuously and controllably prepares the high-quality and low-cost planar heterojunctionThe problem of cross contamination is avoided; and the water-soluble precursor solution has the advantages of environmental protection, low price, stability, water solubility and the like.)
1. A method for preparing a two-dimensional layered transition metal chalcogenide planar heterojunction, characterized by essentially comprising the steps of:
(1) processing the substrate;
(2) respectively preparing precursor solutions of a molybdenum source and a tungsten source;
(3) heating and baking the precursor solutions of the molybdenum source and the tungsten source in the step (2) to be used as a molybdenum source and a tungsten source, putting the molybdenum source and the tungsten source into a tube furnace, and placing the substrate processed in the step (1) and a sulfur source or a selenium source in the tube furnace;
(4) depositing a molybdenum source: introducing carrier gas into the tube furnace, moving the molybdenum source to the position right below the substrate when the tube furnace is heated to the temperature of 750-780 ℃, volatilizing sulfur or selenium steam to the molybdenum source under the action of the carrier gas to react, and depositing MoS on the substrate2Or MoSe2；
(5) Depositing a tungsten source: after the molybdenum source deposition in the step (4) is finished, continuously heating, moving the tungsten source to the position right below the substrate deposited with the molybdenum source in the step (4) after the temperature is raised to 850-880 ℃, and continuously depositing WS on the substrate2Or WSe2；
(6) And after the growth is finished, introducing argon to flush away unreacted molybdenum source and tungsten source and accelerate the temperature reduction, and taking out a sample after the sample is naturally cooled to room temperature to obtain the planar heterojunction.
2. The method according to claim 1, wherein the substrate in step (1) is Si or SiO2。
3. The method of claim 1, wherein step (2) is performed with (NH)4)6Mo7O24·4H2Preparing a precursor solution of a molybdenum source by taking O as a raw material; with (NH)4)10W12O41·xH2And preparing a precursor solution of the tungsten source by using O as a raw material.
4. The method according to claim 1, wherein in step (3), the precursor solutions of the molybdenum source and the tungsten source in step (2) are respectively placed into two quartz boats, heated and baked to be used as the molybdenum source and the tungsten source, and then the two quartz boats provided with the tungsten source and the molybdenum source are respectively placed on the left side and the right side of the other quartz boat with a pull ring; simultaneously, the quartz tube is provided with the other quartz tube with two ends and an unsealed top, and bulges are arranged on two sides in the top of the quartz tube, so that the substrate in the step (1) can be placed on the top of the quartz tube; combining a quartz boat with a pull ring and a quartz tube; hooking a pull ring of the quartz boat by using a hooked quartz rod, fixing the hooked quartz rod on an iron ring, and putting the hooked device into the tube furnace to enable the substrate to be in a second heating temperature area of the tube furnace; and (4) placing the sulfur or selenium source in the first heating temperature zone of the tube furnace.
5. The method of claim 1, wherein the molybdenum source is deposited in step (4) by introducing a carrier gas into the tube furnace and deflecting the molybdenum source away from the substrate by moving the outer magnet ring; heating the tube furnace to 750-780 ℃, and moving an outer magnet ring to enable a molybdenum source to be positioned under the substrate so as to deposit MoS on the substrate2Or MoSe2。
6. The method according to claim 1 or 5, wherein the carrier gas in step (4) is a mixed gas of argon and hydrogen, and MoS is deposited on the substrate2Or MoSe2The time of (2) is 8-12 min.
7. The method of claim 1, wherein during the deposition of the tungsten source in the step (5), after the deposition of the molybdenum source in the step (4) is finished, the temperature is continuously raised to 850-880 ℃, the outer magnet ring is moved to enable the tungsten source to be positioned right below the substrate, and the WS continues to be deposited on the substrate2Or WSe2。
8. The method according to claim 1 or 7, wherein in the step (5), when the temperature is raised to 810-840 ℃, H needs to be introduced2Activated MoS2Or MoSe2Of the edge of (a).
9. The two-dimensional layered transition metal chalcogenide planar heterojunction prepared by the method according to any one of claims 1 to 8.
10. Use of a two-dimensional layered transition metal chalcogenide planar heterojunction as claimed in claim 9 in an optoelectronic device.
Technical Field
The invention relates to a method for preparing a transition metal chalcogenide planar heterojunction by an in-situ control method, and belongs to the technical field of two-dimensional semiconductor materials.
Background
Two-dimensional layered transition metal chalcogenides (TMDs) are generally composed of two chalcogens and one metal element, i.e., MX2Wherein M ═ molybdenum (Mo), tungsten (W); x ═ sulfur (S), selenium (Se), TMDs have received wide attention for their excellent optical, electronic and mechanical properties. TMDs, due to their relatively large band gap, when gradually reduced in thickness to a single layer, have a band structure that transitions from an indirect band gap to a direct band gap, and the unique chiral optoelectronic properties resulting from strong spin-orbit coupling, provide exciting opportunities to study new types of low-power digital electronic and optoelectronic devices. Furthermore, these MXs2The monolayer can be stacked/combined to create a novel vertical or transverse heterostructure with unique geometrical characteristics and energy band structure, wherein the planar heterojunction can show intrinsic p-n junction characteristics, such as rectification characteristics and photovoltaic effect, and is expected to be applied to future micro-nano optoelectronic devices.
Current methods of preparing two-dimensional layered transition metal chalcogenide (TMDs) heterojunctions include: 1. the vertical heterojunction is formed by mechanical stacking. SiO at 280nm thickness using co-lamination and mechanical transfer techniques2MoS fabrication of van der Waals stacks on coated Si substrates2/WSe2Heterojunction devices, tunable diode-like current rectification and photovoltaic response across the p-n interface are observed. (Lee C-H, Lee G-H, Van Der Zande A M, et al]Nature Nanotechnology,2014,9(9): 676-; 2. chemical Vapor Deposition (CVD) "one-step" processes. Mixing the water solution with ammonium molybdate tetrahydrate (NH)4)6Mo7O24·4H2O and ammonium tungstate hydrate (NH)4)10W12O41·xH2O as molybdenum and tungsten source for forming high quality WS by one-step atmospheric CVD2/MoS2An in-plane heterostructure. (Chen K, Wan X, Xie W, et al. Lateral build-In Potential of Monolayer MoS2-WS2In-Plane Heterostructures by a Shortcut Growth Strategy[J]Advanced Materials,2015,27(41): 6431); CVD "two-step" growth method. MoSe2First by CVD method and then by growingMedium of MoSe2/SiO2the/Si is transferred into another CVD apparatus along with the MoSe2Edge and top surface epitaxial growth of WSe2、MoSe2. Cross MoSe2Single layer and WSe2/MoSe2The electron and photoelectric transmission measurements of the bilayer show clear rectifying properties and photovoltaic effects, indicating the formation of a p-n heterojunction. While being transferred from the first CVD apparatus to the second CVD apparatus, MoSe2May be passivated after exposure to ambient conditions (Gong Y, Lei S, Ye G, et al. Two-Step Growth of Two-Dimensional WSe2/MoSe2Heterostructures[J].Nano Letters,2015,15(9):6135-6141)。
Although the variety and preparation method of the heterojunction is increasing, most of them use a solid source as a precursor. The amount of the precursor required in the experiment is generally small, which causes a certain error in measuring the precursor by a balance, and the accurate control of the heterojunction growth window is hindered. Therefore, in order to precisely control the precursors, the inventors previously explored a preparation method for preparing a two-dimensional layered transition metal chalcogenide heterojunction using a water-soluble precursor.
One method for growing WS by adopting two-step lateral epitaxial growth strategy2/MoS2A lateral heterostructure. First, conventional Atmospheric Pressure Chemical Vapor Deposition (APCVD) and molybdenum trioxide (MoO) were used3) Powder as precursor in clean SiO2Synthesis of MoS on a/Si substrate2Nanosheets. The sample is then quickly placed in another furnace for a second WS step2Growing water-soluble ammonium tungstate hydrate (NH)4)10W12O41·xH2The O solution was placed in a quartz boat and then heated at 250 ℃ for 1 hour to remove the solvent. Will carry MoS2SiO of nanosheet2the/Si substrate is arranged on the top of the quartz boat, MoS2Downward facing ammonium tungstate to epitaxially grow WS2. However, the transition from one CVD furnace to another increases the likelihood of contamination and sample passivation.
Thus leading to another preparation method. Molybdenum tetrahydrate serving as water-soluble solution of molybdenum source and tungsten sourceAmmonium salt (NH)4)6Mo7O24·4H2O and ammonium tungstate hydrate (NH)4)10W12O41·xH2O was placed in two separate quartz boats and heated on a hot plate at 250 deg.C for 1h to remove the solvent, thereby forming a uniform thin layer of ammonium molybdate and ammonium tungstate in the quartz as a molybdenum source and a tungsten source. Since a thin layer of these precursors adheres well to the bottom of the quartz boat, it is safe to face it down. SiO to be cleaned2the/Si substrates were closely tilted with a quartz boat containing ammonium molybdate, placed in a larger quartz boat. The device is placed in a central heating temperature zone of a quartz tube, and is firstly heated to 700 ℃ to grow for 10min to form MoS2Nanosheets, then warmed to 850 ℃ for 5 minutes and held at 850 ℃ to allow WS2Forming high quality WS along the epitaxial growth2/MoS2An in-plane heterostructure. However, the molybdenum source and the tungsten source are placed in the same temperature region, and at 780 ℃, the molybdenum source is volatilized in a large amount, and a small amount of the tungsten source is volatilized at the same time, so that cross contamination occurs in the growth process, and the alloy is easy to generate, so that the process cannot be well controlled.
Disclosure of Invention
[ problem ] to
In order to precisely control the precursors, the inventors previously explored a preparation method for preparing a two-dimensional layered transition metal chalcogenide heterojunction by using water-soluble precursors. One of them is to adopt two-step lateral epitaxial growth strategy to grow WS2/MoS2Lateral heterostructures, but this approach requires a switch from one CVD furnace to another, increasing the likelihood of contamination, sample passivation. Another method comprises heating the water-soluble precursors of molybdenum source and tungsten source together, heating to 700 deg.C, and growing MoS2Nanosheets and then heating to 850 ℃ to WS2Forming high quality WS along the epitaxial growth2/MoS2An in-plane heterostructure. However, because the molybdenum source and the tungsten source are placed in the same temperature region, the molybdenum source is volatilized in a large amount at 780 ℃, and a small amount of the tungsten source is volatilized at the same time, so that cross contamination occurs in the growth process, and the molybdenum source and the tungsten source are easy to generateAlloy, the process cannot be well controlled.
[ solution ]
In view of the above problems, the present invention provides a method for preparing a two-dimensional layered transition metal chalcogenide planar heterojunction using ammonium molybdate ((NH)4)6Mo7O24·4H2O) and ammonium tungstate ((NH)4)10W12O41·xH2O) solution is used as a precursor, and ammonium molybdate and ammonium tungstate have the advantages of environmental protection, low price, stability, water solubility and the like; the in-situ control aqueous solution precursor is adopted to prepare a high-quality planar heterojunction; and the obtained product has large size, is more widely and mature in application and can meet the industrial requirement.
The invention provides a method for preparing a two-dimensional layered transition metal chalcogenide planar heterojunction, which mainly comprises the following steps:
(1) processing the substrate;
(2) respectively preparing precursor solutions of a molybdenum source and a tungsten source;
(3) heating and baking the precursor solutions of the molybdenum source and the tungsten source in the step (2) to be used as a molybdenum source and a tungsten source, putting the molybdenum source and the tungsten source into a tube furnace, and placing the substrate processed in the step (1) and a sulfur or selenium source in the tube furnace;
(4) depositing a molybdenum source: introducing carrier gas into the tube furnace, moving the molybdenum source to the position right below the substrate when the tube furnace is heated to the temperature of 750-780 ℃, volatilizing sulfur or selenium steam to the molybdenum source under the action of the carrier gas to react, and depositing MoS on the substrate2Or MoSe2；
(5) Depositing a tungsten source: after the molybdenum source deposition in the step (4) is finished, continuously heating, moving the tungsten source to the position right below the substrate deposited with the molybdenum source in the step (4) after the temperature is raised to 850-880 ℃, and continuously depositing WS on the substrate2Or WSe2；
(6) And after the growth is finished, introducing argon to flush away unreacted molybdenum source and tungsten source and accelerate the temperature reduction, and taking out a sample after the sample is naturally cooled to room temperature to obtain the planar heterojunction.
In one embodiment of the present invention, the substrate in step (1) is Si or SiO2。
In one embodiment of the present invention, the method for processing a substrate in step (1) is: SiO to be used as a substrate2Cutting the substrate into proper size, cleaning the residual impurities on the surface of the substrate by using an acetone solution, and then ultrasonically cleaning the substrate by using deionized water; and then putting the mixture into isopropanol to be soaked for 0.5h, taking the mixture out, washing the mixture for multiple times by using deionized water, and finally blowing the residual deionized water on the surface by using high-pressure nitrogen.
In one embodiment of the present invention, the step (2) is performed with (NH)4)6Mo7O24·4H2And preparing a precursor solution of the molybdenum source by using O as a raw material.
In one embodiment of the present invention, the step (2) is performed with (NH)4)10W12O41·xH2And preparing a precursor solution of the tungsten source by using O as a raw material.
In one embodiment of the present invention, the method for preparing the precursor solution of the molybdenum source and the tungsten source in the step (2) is: respectively preparing (NH) with the concentration of 35-45 mg/mL, 10-20 mg/mL and 5-15 mg/mL4)10W12O41、(NH4)6Mo7O24·4H2O, sodium chloride solution; then to (NH)4)10W12O41·xH2Adding NaCl solution as precursor solution of tungsten source into O solution to obtain (NH)4)6Mo7O24·4H2And adding a NaCl solution into the O solution to serve as a precursor solution of the molybdenum source.
In one embodiment of the invention, the reaction is carried out in (NH)4)10W12O41·xH2Adding NaCl solution serving as precursor solution of the tungsten source into the O solution, wherein the addition amount of the NaCl solution is calculated according to the volume, and (NH)4)10W12O41·xH2The dosage ratio of the O solution is 1 (2-3).
In one embodiment of the invention, the reaction is carried out in (NH)4)6Mo7O24·4H2Adding NaCl solution into the O solution as a precursor solution of the molybdenum source, wherein the adding amount of the NaCl solution is calculated by volume, and (NH)4)6Mo7O24·4H2The dosage ratio of the O solution is 1 (2.5-3.5).
In one embodiment of the present invention, the method for heating and baking the precursor solutions of the molybdenum source and the tungsten source in step (3) is as follows: and heating the quartz blank on a heating table at 80-100 ℃ for 0.5-1 h to remove the solvent, thereby forming a uniform ammonium molybdate and ammonium tungstate thin layer in the quartz.
In one embodiment of the invention, in the step (3), the precursor solutions of the molybdenum source and the tungsten source in the step (2) are placed into two quartz boats, heated and baked to be used as the molybdenum source and the tungsten source, and then the two quartz boats provided with the tungsten source and the molybdenum source are respectively placed at the left side and the right side of the other quartz boat with the pull ring; simultaneously, the quartz tube is provided with the other quartz tube with two ends and an unsealed top, and bulges are arranged on two sides in the top of the quartz tube, so that the substrate in the step (1) can be placed on the top of the quartz tube; combining a quartz boat with a pull ring and a quartz tube; the pull ring of the quartz boat is hooked by a hooked quartz rod, the hooked quartz rod is fixed on the hooked quartz rod by an iron ring, and the hooked device is put into a tube furnace, so that the substrate is in a second heating temperature area of the tube furnace.
In one embodiment of the present invention, the sulfur or selenium source of step (3) is placed in the first heating temperature zone of a tube furnace.
In an embodiment of the present invention, before performing step (4), positions of the molybdenum source and the tungsten source during deposition are determined, and the specific method is as follows: the position of a source in the quartz tube is controlled by an external magnet ring, the magnet ring is pushed firstly to enable a tungsten source to be right below a substrate, the molybdenum source is positioned at the downstream of the substrate, and the position of the magnet ring (marked as tungsten source mark) at the moment is marked on the outer wall of the quartz tube by a marking pen; then the magnet ring is pushed to enable the molybdenum source to be positioned under the substrate, and the tungsten source is positioned in the middle unheated zone; at the same time, the position of the magnet ring at this time is marked on the outer wall of the quartz tube (marked as molybdenum source).
In one embodiment of the present invention, step (4) deposits a molybdenum sourceWhen in use, carrier gas is firstly introduced into the tube furnace, and the outer magnet ring is moved to enable the molybdenum source to deviate from the substrate so as to ensure that no sample is formed at the substrate in the temperature rise stage; when the tube furnace is heated to the temperature of 750-780 ℃, the molybdenum source is pushed to the molybdenum source mark position by moving the external magnet ring, so that the molybdenum source is positioned under the substrate, the sulfur or selenium source is evaporated to form sulfur or selenium steam, the sulfur or selenium steam is volatilized to the molybdenum source position under the action of the carrier gas to react, and MoS is deposited on the substrate2Or MoSe2。
In one embodiment of the present invention, the carrier gas in the step (4) is a mixed gas of argon and hydrogen.
In one embodiment of the present invention, the step (4) of depositing MoS on the substrate2Or MoSe2The time of (2) is 8-12 min.
In an embodiment of the invention, when the tungsten source is deposited in the step (5), after the molybdenum source is deposited in the step (4), the temperature is continuously increased to 850-880 ℃, the tungsten source is pushed to the molybdenum source mark by moving the outer magnet ring, so that the tungsten source is positioned under the substrate, and the WS is continuously deposited on the substrate2Or WSe2。
In one embodiment of the present invention, when the temperature in step (5) is increased to 810-840 ℃, H needs to be introduced2Activated MoS2Or MoSe2To better grow WS horizontally2Or WSe2。
In one embodiment of the present invention, WS is deposited on the substrate in step (5)2Or WSe2The time of (2) is 8-12 min.
The invention provides a two-dimensional layered transition metal chalcogenide planar heterojunction prepared by the method.
The invention provides application of the two-dimensional layered transition metal chalcogenide planar heterojunction in an optoelectronic device.
[ advantageous effects ]:
other impurities are introduced during the two-step growth process and the periodicity of the crystal domains is destroyed, which greatly reduces the performance of the device. Compared with the prior art, the method for preparing the two-dimensional layered transition metal chalcogenide planar heterojunction by adopting the in-situ control aqueous solution precursor has the main advantages that:
according to the invention, the in-situ control of the aqueous solution precursor is utilized, the deposition is carried out by moving the molybdenum source and the tungsten source, the manual secondary transfer stacking of two-dimensional materials is not needed, the repeatability is good, the process is rapid and efficient, the high-quality and low-cost planar heterojunction is continuously and controllably prepared, and the problem of cross contamination is avoided.
Drawings
FIG. 1 is the prepared MoSe set forth in example 12/WSe2Device diagram of planar heterojunction.
FIG. 2 shows MoSe prepared in example 12/WSe2Topography of planar heterojunctions, wherein (a) is MoSe2/WSe2An optical diagram of a planar heterojunction; (b) an enlarged view of the sample is marked by the black dashed line in (a).
FIG. 3 shows MoSe prepared in example 12/WSe2A Raman spectrum and a PL spectrum of the planar heterojunction, wherein (a) is the Raman spectrum; (b) is a PL spectrum.
FIG. 4 shows MoSe prepared in example 12/WSe2Raman mapping of planar heterojunctions.
FIG. 5 is the MoS prepared in example 22/WS2Optical diagram of a planar heterojunction.
FIG. 6 shows MoSe prepared in example 22/WSe2A Raman spectrum and a PL spectrum of the planar heterojunction, wherein (a) is the Raman spectrum; (b) is a PL spectrum.
FIG. 7 is a MoS prepared in comparative example 12/WS2Optical diagram of a planar heterojunction.
FIG. 8 is a MoS prepared in comparative example 22/WS2Optical diagram of a planar heterojunction.
FIG. 9 is a MoS prepared in comparative example 32/WS2Optical diagram of a planar heterojunction.
Detailed Description
The invention is described in further detail below with reference to examples and figures, but the embodiments of the invention are not limited thereto, as exemplified by the contents contained in the claims.
[ example 1 ]
1. With ammonium molybdate ((NH)4)6Mo7O24·4H2O) and ammonium tungstate ((NH)4)10W12O41·xH2O) solution is used as a precursor, and the large-area, low-cost and high-quality MoSe is prepared by the CVD method by adopting the in-situ control of the aqueous solution precursor2/WSe2Process of planar heterojunction:
(1) processing the substrate: SiO to be used as a substrate by a diamond knife2Cut into the size (2.5 x 2.5cm) suitable for the caliber of the quartz boat, and keep the surface of the substrate clean as much as possible when cutting. Putting the cut substrate into a beaker, cleaning residual impurities on the surface of the substrate by using an acetone solution, and then carrying out ultrasonic cleaning by using deionized water; and then putting the substrate into isopropanol, soaking for 0.5h, taking out the substrate, putting the substrate into a clean beaker, repeatedly washing the substrate with deionized water, and finally blowing the substrate dry with high-pressure nitrogen for later use.
(2) Respectively preparing (NH) with the concentration of 40mg/mL, 15mg/mL and 10mg/mL4)10W12O41·xH2O solution, (NH)4)6Mo7O24·4H2O solution and sodium chloride (NaCl) solution; then 0.5mL (NH) was taken4)6Mo7O24·4H2Placing the O solution and 0.2mL NaCl solution into quartz boat (2cm x 1.5cm), and collecting 1mL (NH)4)10W12O41·xH2The O solution and 0.5mL NaCl solution were placed in another quartz boat (2 cm. times.2 cm. times.1.5 cm) to prepare a precursor solution to the molybdenum source and the tungsten source.
(3) And (3) placing the two quartz boats in the step (2) on a heating table, adjusting the temperature to 100 ℃, and heating for 30min to ensure that uniform pasty layers appear in the two quartz boats and serve as a tungsten source and a molybdenum source. Placing two quartz boats of a tungsten source and a molybdenum source on the left side and the right side of one quartz boat (20cm x 2.5cm x 1.5cm) with pull rings; and the other quartz tube (20cm x 3cm x 2.5cm) with two ends and an unsealed top is arranged at the same time, two sides in the top of the quartz tube are provided with protrusions with the width of 1.5mm, and the substrate processed in the step (1) can be just placed on the top of the quartz tube. The quartz boat with the pull ring is placed in a quartz tube, the pull ring of the quartz boat is hooked by a hooked quartz rod, the quartz boat is fixed on the hooked quartz rod by an iron ring and fixed on the hooked quartz rod by the iron ring, the pull ring of the quartz boat is hooked by the quartz rod, and the well-built device is placed in a tube furnace, so that the substrate is in a second heating temperature area. And the selenium source in the quartz boat is placed in the first heating temperature zone of the tube furnace. The device is shown in figure 1.
(4) The positions of a molybdenum source and a tungsten source in a quartz tube are controlled by an external magnet ring, the magnet ring is pushed firstly to enable the tungsten source to be right below a substrate, the molybdenum source is positioned at the downstream of the substrate, and the position of the magnet ring at the moment (marked as a tungsten source mark) is marked on the outer wall of the quartz tube by a marking pen; then the magnet ring is pushed to enable the molybdenum source to be positioned under the substrate, and the tungsten source is positioned in the middle unheated zone; at the same time, the position of the magnet ring at this time is marked on the outer wall of the quartz tube (marked as molybdenum source).
Depositing a molybdenum source: introducing argon gas of 46sccm and hydrogen gas of 4sccm into the tube furnace as carrier gas, wherein the selenium source is in the first temperature zone, and the temperature is raised to 300 ℃ within 20min and is kept for 30 min. The outer magnet ring is moved so that the molybdenum source is deflected away from the substrate to ensure that the surface of the substrate is clean and no material is formed during the temperature ramp. When the temperature of the second heating temperature zone reaches 780 ℃ within 25min, pulling a magnet outside a quartz tube of the tube furnace to move the molybdenum source to a position right below the substrate, wherein the tungsten source is positioned at the middle unheated zone, and the selenium steam is volatilized to the molybdenum source under the action of the carrier gas to react so as to react the molybdenum source and grow MoSe on the substrate2(ii) a MoSe after 10min2And (5) finishing the growth.
(5) Depositing a tungsten source: after the molybdenum source deposition is finished, the temperature is continuously increased to 850 ℃ within 5min, and H is adjusted when the temperature reaches 810 ℃ in the temperature increasing process2The flow rate of (2) is up to 30 sccm; when the temperature rises to 840 ℃, H is reduced2The flow rate of the argon gas is 8sccm, and meanwhile, the argon gas is always kept at 46 sccm; at the same time, pulling a magnet outside the quartz tube to move the tungsten source to a position right below the substrate (tungsten source mark position) deposited with the molybdenum source in the step (4)) When the temperature reaches 850 ℃, maintaining for 10min to grow the WSe2。
(6) After the growth is finished, pulling a magnet ring outside a quartz tube of the tube furnace to enable the two quartz boats to move to the downstream of the substrate, introducing argon of 300sccm to wash away unreacted sources, and meanwhile, cooling the quartz boats at an accelerated speed until the quartz boats are naturally cooled to room temperature, and taking out a sample.
2. And (3) appearance and structure characterization testing:
MoSe prepared in this example2/WSe2The optical microscope observation is carried out on the planar heterojunction, and the test method comprises the following steps: MoSe observed under an optical microscope by adjusting a microscope2/WSe2Planar heterojunction, MoSe obtained by testing2/WSe2A planar heterojunction optical diagram as shown in FIG. 2, wherein (a) is MoSe2/WSe2An optical diagram of a planar heterojunction; (b) an enlarged view of the sample is marked by the black dashed line in (a). As can be seen from the figure, the MoSe prepared in this example2/WSe2The profile of the heterojunction can be clearly resolved in a planar heterojunction. Wherein the central region is MoSe2The edge area is WSe2。
MoSe prepared in this example2/WSe2The planar heterojunction is subjected to Raman scanning, and the testing method comprises the following steps: respectively selecting a point in the central region and the edge region of the sample (b) in FIG. 2 to perform Raman scanning, and testing to obtain MoSe2/WSe2Planar heterojunction Raman spectrogram, from which it can be seen that the MoSe prepared in this example2/WSe2The central region of the planar heterojunction is 238cm-1And 284cm-1Has obvious characteristic peaks corresponding to MoSe respectively2A of (A)1gAnd E2gAnd the difference in the wave number of the two is 46cm-1MoSe with a single layer in the center region2. Similarly, the edge area was found to be 250cm-1And 260cm-1Two characteristic peaks appear, corresponding to WSe respectively2E of (A)2gAnd A1gWSe with single layer edge region2As in (a) of fig. 3.
MoSe prepared in this example2/WSe2The plane heterojunction collects fluorescence signals, and the testing method comprises the following steps: selecting a point in the central region and the edge region of the sample of (b) in FIG. 2 to collect fluorescence signals, and testing to obtain MoSe2/WSe2PL Spectroscopy of planar heterojunction, from which it can be seen that MoSe prepared in this example2/WSe2The central region sample of the planar heterojunction shows a characteristic peak with high intensity and symmetry at 795nm, corresponding to MoSe2The corresponding band gap is 1.56 eV; while the edge region shows a characteristic peak with high intensity and symmetry at 760nm, which corresponds to WSe2The corresponding band gap is 1.63 eV. The above results show that both materials produced have a direct bandgap, and further demonstrate their monolayer properties, as shown in fig. 3 (b).
MoSe prepared in this example2/WSe2The plane heterojunction is subjected to Raman intensity surface scanning, and the testing method comprises the following steps: surface scanning of Raman intensity is carried out on the central area and the edge area of the sample, and MoSe is obtained through testing2/WSe2Planar heterojunction Raman spectrogram, as can be seen from the graph, the MoSe prepared by the embodiment2/WSe2The spatial distribution of the two materials can be clearly distinguished by the planar heterojunction, and the heterojunction prepared by the CVD method has higher quality, as shown in figure 4.
[ example 2 ]
1. With ammonium molybdate ((NH)4)6Mo7O24·4H2O) and ammonium tungstate ((NH)4)10W12O41·xH2Using O solution as precursor, and preparing large-area, low-cost and high-quality MoS by using CVD method and in-situ control of aqueous solution precursor2/WS2Process of planar heterojunction:
steps (1) to (3) were the same as steps (1) to (3) of example 1, except that the selenium source was replaced with a sulfur source.
(4) Depositing a molybdenum source: sufficient argon was passed into the tube furnace to flush the air from the tube. The sulfur source is in the first heating temperature zone, the temperature is raised from room temperature to 180 ℃ within 18min and kept for 32min, the outer magnet ring is moved to make the molybdenum source deviate from the substrate and arrive at the first heating temperature zoneAnd pushing the molybdenum source to the molybdenum source mark position when the molybdenum source grows at the temperature, so that the molybdenum source is positioned under the substrate, and the surface of the substrate is ensured to be clean at the temperature rise stage without forming materials. When the temperature of the second heating temperature zone reaches 750 ℃ within 25min, pulling a magnet outside a quartz tube of the tube furnace to move the molybdenum source to a position right below the substrate, wherein the tungsten source is positioned at the middle unheated zone, volatilizing sulfur vapor to the molybdenum source under the action of carrier gas to react so as to react the molybdenum source, and growing MoS on the substrate2Argon gas of 80sccm and hydrogen gas of 8sccm are introduced as carrier gas in the whole process; MoS after 10min2And (5) finishing the growth.
(5) Depositing a tungsten source: after the molybdenum source deposition is finished, the temperature is continuously increased to 850 ℃ within 5min, and H is adjusted when the temperature reaches 810 ℃ in the temperature increasing process2The flow rate of (2) is up to 30 sccm; when the temperature rises to 830 ℃, H is reduced2To 5sccm to facilitate WS2Growing of (3); simultaneously, argon gas of 80sccm is always kept; pulling a magnet outside the quartz tube to move the tungsten source to the position below the substrate deposited with the molybdenum source in the step (4), and maintaining the growth WS for 10min after the temperature reaches 850 DEG C2。
(6) After the growth is finished, pulling a magnet ring outside a quartz tube of the tube furnace to enable the two quartz boats to move to the downstream of the substrate, introducing argon of 300sccm to wash away unreacted sources, and meanwhile, cooling the quartz boats at an accelerated speed until the quartz boats are naturally cooled to room temperature, and taking out a sample.
2. And (3) appearance and structure characterization testing:
MoS prepared in this example2/WS2The optical microscope observation is carried out on the planar heterojunction, and the test method comprises the following steps: MoS observed under an optical microscope by adjusting a microscope2/WS2Planar heterojunction, MoS obtained by testing2/WS2Planar heterojunction optical diagram, from which it can be seen that the MoS prepared in this example2/WS2The two materials can be clearly distinguished in the planar heterojunction, and the middle part of the planar heterojunction is MoS2The outer edge is WS2As in fig. 5.
MoS prepared in this example2/WS2The planar heterojunction is subjected to Raman scanning, and the testing method comprises the following steps: respectively selecting a point from the central area and the edge area of the sample of the figure 5 to perform Raman scanning, and obtaining MoS through testing2/WS2Planar heterojunction Raman spectrogram, as can be seen from the graph, the MoS prepared by the embodiment2/WS2The central region of the planar heterojunction is 384cm-1And 403cm-1Has obvious characteristic peaks corresponding to MoS2E of (A)2gAnd A1gThe wave number difference between the two peak positions is 19cm-1Showing the MoS prepared2Is single-layered. The edge regions are 350cm in length-1And 419cm-1Two characteristic peaks appear, corresponding to WS respectively22LA (M) vibration mode and A1gVibration mode of single-layer WS2As in (a) of fig. 6.
MoS prepared in this example2/WS2The plane heterojunction collects fluorescence signals, and the testing method comprises the following steps: as can be seen from the PL spectrum obtained by testing a point selected for collecting fluorescence signals in the central region and the edge region of the sample of FIG. 5, the MoS prepared in this example2/WS2The central area sample of the plane heterojunction has a characteristic peak with high intensity and symmetry at 660nm, which corresponds to MoS2The corresponding energy was 1.8eV, demonstrating that the MoS prepared2Is single-layered; while the edge region shows a characteristic peak with high intensity and symmetry at 625nm, corresponding to WS2Calculated corresponding band gap width and WS2Is close to the direct band gap of a single layer WS2As shown in fig. 6 (b).
In conclusion, it can be seen from the embodiments 1 and 2 that the invention provides a method for preparing a two-dimensional layered transition metal chalcogenide planar heterojunction in a precise, controllable and large-area manner; the method has the advantages of no need of manual secondary transfer and stacking of two-dimensional materials, good repeatability, low cost, rapid and efficient process and simple preparation process.
Comparative example 1
This comparative example deposited by mixing together a molybdenum source and a tungsten source.
(1) Processing the substrate: same as in step (1) in example 1.
(2) Respectively preparing (NH) with the concentration of 40mg/mL, 15mg/mL and 10mg/mL4)10W12O41·xH2O solution, (NH)4)6Mo7O24·4H2O solution and sodium chloride (NaCl) solution; then 0.5mL (NH) was taken4)6Mo7O24·4H2Placing O solution and 0.2mL NaCl solution into quartz boat, taking 1mL (NH)4)10W12O41·xH2And putting the O solution and the 0.5mL NaCl solution into the same quartz boat to prepare the mixed precursor solution of the molybdenum source and the tungsten source.
(3) And (3) placing the quartz boats in the step (2) on a heating table, adjusting the temperature to 100 ℃, and heating for 30min to ensure that uniform pasty layers appear in the two quartz boats. The substrate was placed directly above the quartz boat of the mixing source and the experimental setup was placed in a tube furnace so that it was in the second heating temperature zone. And the sulfur source in the quartz boat is placed in the first heating temperature zone of the tube furnace.
(4) Depositing a molybdenum source: sufficient argon was passed into the tube furnace to flush the air from the tube. The sulfur source is in the first temperature zone, and is heated from room temperature to 180 ℃ within 18min and is kept for 32 min; when the temperature of the second heating temperature zone reaches 750 ℃ within 25min, the sulfur vapor is volatilized to the molybdenum source under the action of the carrier gas to react, so that the molybdenum source reacts to grow MoS on the substrate2Argon gas of 80sccm and hydrogen gas of 8sccm are introduced as carrier gas in the whole process; MoS after 10min2And (5) finishing the growth.
(5) Depositing a tungsten source: after the molybdenum source deposition is finished, the temperature is continuously increased to 850 ℃ within 5min, and H is adjusted when the temperature reaches 810 ℃ in the temperature increasing process2The flow rate of (2) is up to 30 sccm; when the temperature rises to 830 ℃, H is reduced2To 5sccm to facilitate WS2Growing of (3); simultaneously, argon gas of 80sccm is always kept; after the temperature reaches 850 ℃, the growth WS is maintained for 10min2。
(6) After the growth is finished, pulling a magnet ring outside a quartz tube of the tube furnace to move the mixed source quartz boat to the downstream of the substrate, introducing argon of 300sccm to wash away unreacted sources, and simultaneously, cooling the mixed source quartz boat at an accelerated speed until the mixed source quartz boat is naturally cooled to room temperature, and taking out a sample.
MoS prepared for this comparative example2/WS2The optical microscope observation is carried out on the planar heterojunction, and the test method comprises the following steps: MoS observed under an optical microscope by adjusting a microscope2/WS2Planar heterojunction, MoS obtained by testing2/WS2Optical diagram of planar heterojunction, from which it can be seen that MoS prepared by the present comparative example2/WS2The two materials cannot be distinguished in the planar heterojunction, the island density is high and the formed triangular island has defects in shape, as shown in fig. 7.
Comparative example 2
This comparative example was deposited using a two-step process.
Steps (1) and (2) are the same as steps (1) and (2) in example 1.
(3) And (3) placing the two quartz boats in the step (2) on a heating table, adjusting the temperature to 100 ℃, and heating for 30min to enable uniform pasty layers to appear in the quartz boats to serve as a molybdenum source and a tungsten source.
(4) Depositing a molybdenum source: and placing the substrate above the quartz boat with the molybdenum source, so that the substrate is just above the quartz boat with the molybdenum source. The experimental set-up was placed in a tube furnace so that it was in the second heating temperature zone. The sulfur source in the quartz boat was placed in the first heating temperature zone of the tube furnace. Sufficient argon was passed into the tube furnace to flush the air from the tube. The sulfur source is in the first temperature zone, and is heated from room temperature to 180 ℃ within 18min and is kept for 17 min; when the temperature of the second heating temperature zone reaches 750 ℃ within 25min, the sulfur vapor is volatilized to the molybdenum source under the action of the carrier gas to react, so that the molybdenum source reacts to grow MoS on the substrate2Keeping the temperature for 10min, heating the two temperature zones simultaneously, and introducing 80sccm of argon and 8sccm of hydrogen as carrier gases in the whole process. After the growth is finished, pulling a magnet ring outside a quartz tube of the tube furnace to move the mixed source quartz boat to the downstream of the substrate, introducing argon of 300sccm to wash away unreacted sources, and simultaneously, cooling the mixed source quartz boat at an accelerated speed until the mixed source quartz boat is naturally cooled to room temperature, and taking out a sample.
(5) Depositing a tungsten source: changing the molybdenum source into tungsten source, placing the experimental device at the same position, and performing WS2The growth of (2). Sufficient argon was introduced to flush the air from the tube. The sulfur source is in the first temperature zone, and is heated from room temperature to 180 ℃ within 18min and is kept for 17 min; the temperature of the second heating temperature zone reaches 850 ℃ within 25min and is kept for 10min, and the two temperature zones are heated simultaneously; argon gas of 80sccm is introduced as a carrier gas throughout the process. Regulating H when the temperature reaches 810 ℃ in the temperature rising process2The flow rate of (2) is up to 30 sccm; when the temperature rises to 830 ℃, H is reduced2The flow rate of (2) to 5sccm to the end of growth to promote WS2The growth of (2). After the growth is finished, pulling a magnet ring outside a quartz tube of the tube furnace to move the mixed source quartz boat to the downstream of the substrate, introducing argon of 300sccm to wash away unreacted sources, and simultaneously, cooling the mixed source quartz boat at an accelerated speed until the mixed source quartz boat is naturally cooled to room temperature, and taking out a sample.
MoS prepared for this comparative example2/WS2The optical microscope observation is carried out on the planar heterojunction, and the test method comprises the following steps: MoS observed under an optical microscope by adjusting a microscope2/WS2Planar heterojunction, MoS obtained by testing2/WS2Optical diagram of planar heterojunction, from which it can be seen that MoS prepared by the present comparative example2/WS2MoS during the transfer from the first CVD apparatus to the second CVD apparatus in which the two materials cannot be clearly distinguished in the planar heterojunction2The edges of (a) present cross-contamination after exposure to ambient conditions, as shown in figure 8.
Comparative example 3
This comparative example did not place the molybdenum and tungsten sources directly under the substrate.
Steps (1) to (3) are the same as steps (1) to (3) in example 1, wherein step (3) in this comparative example does not mark the positions of the molybdenum source and the tungsten source.
(4) Depositing a molybdenum source: sufficient argon was passed into the tube furnace to flush the air from the tube. The two temperature zones are heated simultaneously, and argon gas of 80sccm and hydrogen gas of 8sccm are introduced as carrier gas in the whole process. The sulfur source is in the first temperature zone, and is heated from room temperature to 180 ℃ within 18min and is kept for 32 min; when the temperature of the second heating temperature zone reaches 750 ℃ within 25min, the outer magnet ring is moved to make the molybdenum source deviate from the liningThe molybdenum source is arranged at the left side of the substrate, the sulfur vapor is volatilized to the molybdenum source under the action of the carrier gas to react, so that the molybdenum source reacts, and MoS grows on the substrate2Argon gas of 80sccm and hydrogen gas of 8sccm are introduced as carrier gas in the whole process; MoS after 10min2And (5) finishing the growth.
(5) Depositing a tungsten source: after the molybdenum source deposition is finished, the temperature is continuously increased to 850 ℃ within 5min, and H is adjusted when the temperature reaches 810 ℃ in the temperature increasing process2The flow rate of (2) is up to 30 sccm; when the temperature rises to 830 ℃, H is reduced2To 5sccm to facilitate WS2Growing of (3); simultaneously, argon gas of 80sccm is always kept; pulling a magnet outside the quartz tube to move the tungsten source to the left side of the substrate deposited with the molybdenum source in the step (4), and maintaining the growth WS for 10min after the temperature reaches 850 DEG C2。
(6) After the growth is finished, pulling a magnet ring outside a quartz tube of the tube furnace to enable the two quartz boats to move to the downstream of the substrate, introducing argon of 300sccm to wash away unreacted sources, and meanwhile, cooling the quartz boats at an accelerated speed until the quartz boats are naturally cooled to room temperature, and taking out a sample.
MoS prepared for this comparative example2/WS2The optical microscope observation is carried out on the planar heterojunction, and the test method comprises the following steps: MoS observed under an optical microscope by adjusting a microscope2/WS2Planar heterojunction, MoS obtained by testing2/WS2Optical diagram of planar heterojunction, from which it can be seen that MoS prepared by the present comparative example2/WS2There are multiple layers of planar heterojunctions, as in fig. 9.
Although the present invention has been described with reference to the preferred embodiments, it should be understood that various changes and modifications can be made therein by those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.