CN111850509A - A method for preparing transition metal chalcogenide planar heterojunction by in-situ control method - Google Patents

Abstract

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)₆Mo₇O₂₄·4H₂O) and ammonium tungstate ((NH)₄)₁₀W₁₂O₄₁·xH₂O) 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.

Description

A method for preparing transition metal chalcogenide planar heterojunction by in-situ control method

technical field

The invention relates to a method for preparing a transition metal chalcogenide plane heterojunction by an in-situ control method, and belongs to the technical field of two-dimensional semiconductor materials.

Background technique

Two-dimensional layered transition metal chalcogenides (TMDs) are generally composed of two chalcogens and one metal element, namely MX ₂ , where M=molybdenum (Mo), tungsten (W); X=sulfur (S), selenium (Se), TMDs have received extensive attention due to their outstanding optical, electronic, and mechanical properties. Due to the relatively large band gap of TMDs, when the thickness is gradually reduced to a monolayer, its energy band structure transitions from an indirect band gap to a direct band gap, and the unique chiral optoelectronic properties generated by strong spin-orbit coupling make them provide Exciting opportunity to study new low-power digital electronic and optoelectronic devices. Furthermore, these _MX2 monolayers can be stacked/combined to create novel vertical or lateral heterostructures with unique geometric features and band structures, where planar heterojunctions can exhibit intrinsic pn junction properties such as rectification and The photovoltaic effect is expected to be applied to future micro-nano optoelectronic devices.

The current methods for preparing two-dimensional layered transition metal chalcogenide (TMDs) heterojunctions include: 1. Forming vertical heterojunctions by mechanical stacking. Fabrication of van der Waals _- ready MoS/ _WSe heterojunction devices on 280-nm-thick SiO _- coated Si substrates using co-lamination and mechanical transfer techniques, observing tunable diode-like current rectification and photovoltaic response across the pn interface . (Lee CH, Lee GH, Van Der Zande AM, et al. Atomically thin pn junctions with van der Waals heterointerfaces[J]. Nature Nanotechnology, 2014, 9(9): 676-681); 2. Chemical Vapor Deposition (CVD) "One step" method. The water-soluble solutions of ammonium molybdate tetrahydrate (NH ₄ ) ₆ Mo ₇ O ₂₄ 4H ₂ O and ammonium tungstate hydrate (NH ₄ ) ₁₀ W ₁₂ O ₄₁ .xH ₂ O were used as molybdenum source and tungsten source, through one step A high-quality WS ₂ /MoS ₂ in-plane heterostructure was formed by conventional atmospheric pressure CVD. (Chen K, Wan X, Xie W, et al. Lateral Built-In Potential of Monolayer MoS ₂ -WS ₂ In-Plane Heterostructures by a Shortcut Growth Strategy[J]. Advanced Materials, 2015, 27(41):6431); 3. CVD "two-step" growth method. MoSe ₂ is first grown by CVD method, and then the growing MoSe ₂ /SiO ₂ /Si is transferred to another CVD device, and WSe ₂ and MoSe ₂ are epitaxially grown along the edge and top surface of MoSe ₂ . Electronic and optoelectronic transport measurements across the _MoSe2 monolayer and _WSe2 / _MoSe2 bilayer show clear rectification properties and photovoltaic effects, indicating the formation of a pn heterojunction. However, during the transfer from the first CVD device to the second CVD device, the edges of MoSe ₂ may be passivated after exposure to ambient conditions (Gong Y, Lei S, Ye G, et al. Two-Step Growth of Two- Dimensional WSe ₂ /MoSe ₂ Heterostructures [J]. Nano Letters, 2015, 15(9):6135-6141).

Although there are more and more types and preparation methods of heterojunctions, most of them use solid sources as precursors. The amount of precursor required in the experiment is generally very small, which leads to a certain error when measuring the precursor with a balance, which hinders the precise control of the growth window of the heterojunction. Therefore, in order to precisely control the precursors, the inventors have previously explored a method for preparing two-dimensional layered transition metal chalcogenide heterojunctions using water-soluble precursors.

One is to grow WS ₂ /MoS ₂ lateral heterostructures using a two-step lateral epitaxial growth strategy. First, MoS ₂ nanosheets were synthesized on a clean SiO ₂ /Si substrate using conventional atmospheric pressure chemical vapor deposition (APCVD) and molybdenum trioxide (MoO ₃ ) powder as precursors. The samples were then quickly put into another furnace for the _second step of WS growth, the water-soluble ammonium tungstate hydrate (NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ O solution was put into a quartz boat, and then heated at 250 °C Heat for 1 hour to remove solvent. The SiO ₂ /Si substrate with MoS ₂ nanosheets was mounted on top of a quartz boat with MoS ₂ facing down to the ammonium tungstate to epitaxially grow WS ₂ . But switching from one CVD furnace to another increases the potential for contamination, sample passivation.

This leads to another preparation method. Water-soluble solutions as molybdenum source and tungsten source ammonium molybdate tetrahydrate (NH ₄ ) ₆ Mo ₇ O ₂₄ 4H ₂ O and ammonium tungstate hydrate (NH ₄ ) ₁₀ W ₁₂ O ₄₁ .xH ₂ O were put into In two separate quartz boats and heated at 250 °C for 1 h on a hot plate to remove the solvent, uniform thin layers of ammonium molybdate and ammonium tungstate were formed in the quartz as molybdenum and tungsten sources. Since a thin layer of these precursors adheres well to the bottom of the quartz boat, it is safe to place it face down. The cleaned _SiO2 /Si substrate was placed in a larger quartz boat with a close tilt with a quartz boat containing ammonium molybdate. The device was placed in the central heating zone of the quartz tube, firstly heated to 700 °C for ₁₀ min growth to form MoS2 nanosheets, then heated to 850 °C for 5 minutes, and kept at 850 °C to make WS2 grow along the epitaxy to form high _- temperature nanosheets. _Massive WS2/MoS2 in _- plane heterostructures. However, the molybdenum source and the tungsten source are placed in the same temperature zone. At 780°C, a large amount of the molybdenum source volatilizes, and a small amount of the tungsten source volatilizes at the same time, resulting in cross-contamination during the growth process, and it is easy to form alloys, so the process cannot be carried out very much. good control.

SUMMARY OF THE INVENTION

[technical problem]

In order to precisely control the precursors, the inventors have previously explored a method for preparing two-dimensional layered transition metal chalcogenide heterojunctions using water-soluble precursors. One of them is to grow WS ₂ /MoS ₂ lateral heterostructures using a two-step lateral epitaxial growth strategy, but this method requires switching from one CVD furnace to another, increasing the possibility of contamination and sample passivation. Another method is to place the water-soluble precursors of molybdenum source and tungsten source together for heating. First, the temperature is raised to 700 °C to grow MoS2 _nanosheets , and then the temperature is raised to 850 °C to make WS2 grow along epitaxy, forming high _- quality _WS2 / MoS ₂ in-plane heterostructure. However, because this method places the molybdenum source and the tungsten source in the same temperature zone, at 780 °C, a large amount of the molybdenum source volatilizes, and a small amount of the tungsten source volatilizes at the same time, resulting in cross-contamination during the growth process, and it is easy to form alloys, which cannot be Have good control over the process.

[Technical solutions]

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 ) ₆ Mo ₇ O ₂₄ ·4H ₂ O) and ammonium tungstate ( (NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ O) solution as the precursor, ammonium molybdate and ammonium tungstate are environmentally friendly, cheap, stable and easily soluble in water. A high-quality planar heterojunction is produced; and the resulting product is large in size, more widely used and mature, and can meet the needs of industrialization.

The invention provides a method for preparing a two-dimensional layered transition metal chalcogenide planar heterojunction, the method mainly comprising the following steps:

(1) processing the substrate;

(2) respectively preparing the precursor solutions of molybdenum source and tungsten source;

(3) heating and drying the precursor solutions of the molybdenum source and the tungsten source described in the step (2) as the molybdenum source and the tungsten source, putting the molybdenum source and the tungsten source into the tube furnace, and placing them in the tube furnace Good substrate processed in step (1), and sulfur or selenium source;

(4) Molybdenum source deposition: Pass the carrier gas into the tube furnace, when the tube furnace is heated to a temperature of 750-780°C, move the molybdenum source directly below the substrate, and the sulfur or selenium vapor is under the action of the carrier gas. volatilize to the molybdenum source to react, and deposit MoS ₂ or MoSe ₂ on the substrate;

(5) Deposition of the tungsten source: after the deposition of the molybdenum source in step (4), the temperature continues to rise, and after the temperature is raised to 850-880° C., the tungsten source is moved to just below the substrate on which the molybdenum source was deposited in step (4). Continue to deposit WS ₂ or WSe ₂ on the substrate;

(6) After the growth is completed, argon gas is introduced to flush away the unreacted molybdenum source and tungsten source and the temperature is accelerated. After natural cooling to room temperature, the sample is taken out to obtain a planar heterojunction.

In an embodiment of the present invention, the substrate in step (1) is Si or SiO ₂ .

In one embodiment of the present invention, the method for processing the substrate in step (1) is as follows: cutting SiO ₂ /Si as the substrate into a suitable size, and cleaning the cut substrate with an acetone solution The remaining impurities on the bottom surface were then ultrasonically cleaned with deionized water; then soaked in isopropanol for 0.5 h, taken out, rinsed with deionized water for several times, and finally dried with high pressure nitrogen on the surface of the remaining deionized water.

In an embodiment of the present invention, in step (2), (NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O is used as a raw material to prepare a precursor solution of molybdenum source.

In an embodiment of the present invention, in step (2), a precursor solution of a tungsten source is prepared by using (NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ O as a raw material.

In an embodiment of the present invention, the method for preparing the precursor solutions of the molybdenum source and the tungsten source in the step (2) is as follows: respectively preparing a concentration of 35-45 mg/mL, 10-20 mg/mL, 5-15 mg/mL mL of (NH ₄ ) ₁₀ W ₁₂ O ₄₁ , (NH ₄ ) ₆ Mo ₇ O ₂₄ .4H ₂ O, sodium chloride solution; then to the (NH ₄ ) ₁₀ W ₁₂ O ₄₁ .xH ₂ O solution was added NaCl The solution was used as the precursor solution of the tungsten source, and the NaCl solution was added to the (NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O solution as the precursor solution of the molybdenum source.

In one embodiment of the present invention, a NaCl solution is added to the (NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ O solution as the precursor solution of the tungsten source, wherein the added amount of the NaCl solution is by volume, which is the same as (NH 4 ) ₄ ) The dosage ratio of ₁₀ W ₁₂ O ₄₁ ·xH ₂ O solution is 1:(2～3).

In one embodiment of the present invention, a NaCl solution is added to the (NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O solution as the precursor solution of the molybdenum source, wherein the amount of the NaCl solution added by volume is the same as (NH _{4 )} ) The dosage ratio of ₆ Mo ₇ O ₂₄ ·4H ₂ O solution is 1:(2.5～3.5).

In an embodiment of the present invention, the method for heating and drying the precursor solutions of the molybdenum source and the tungsten source in step (3) is: heating on a heating table at 80-100° C. for 0.5-1 h to remove the solvent, This results in the formation of a uniform thin layer of ammonium molybdate and ammonium tungstate in the quartz.

In one embodiment of the present invention, in step (3), the precursor solutions of molybdenum source and tungsten source described in step (2) are put into two quartz boats, heated and dried as molybdenum source and tungsten source, Then put two quartz boats with tungsten source and molybdenum source on the left and right sides of another quartz boat with a pull ring; at the same time there is another quartz tube with both ends and the top unsealed, the inner two sides of the top of the quartz tube There are protrusions, so that the substrate described in step (1) can be placed on top of it; combine the quartz boat with the pull ring and the quartz tube; hook the pull ring of the quartz boat with a hooked quartz rod, and use an iron ring It is fixed on a hooked quartz rod, and the assembled device is placed in a tube furnace so that the substrate is in the second heating temperature zone of the tube furnace.

In an embodiment of the present invention, the sulfur or selenium source described in step (3) is placed in the first heating temperature zone of the tube furnace.

In one embodiment of the present invention, before performing step (4), the positions of the molybdenum source and the tungsten source for deposition should be determined. The specific method is as follows: control the position of the source in the quartz tube through an external magnet ring, and push the magnet first. The ring makes the tungsten source directly under the substrate, and the molybdenum source is downstream of the substrate at this time, mark the position of the magnet ring on the outer wall of the quartz tube with a marker (marked as the tungsten source mark); Push so that the molybdenum source is directly under the substrate, and the tungsten source is in the unheated area in the middle; at the same time, the position of the magnet ring at this time is also marked on the outer wall of the quartz tube (marked as the molybdenum source mark).

In one embodiment of the present invention, when depositing the molybdenum source in step (4), a carrier gas is first introduced into the tube furnace, and the outer magnet ring is moved so that the molybdenum source deviates from the substrate, so as to ensure that the substrate is not formed during the heating stage. Sample; when the tube furnace is heated to a temperature of 750-780 °C, the molybdenum source is pushed to the mark of the molybdenum source by moving the outer magnet ring, so that the molybdenum source is directly under the substrate, and the sulfur or selenium source evaporates to form sulfur or selenium The vapor, sulfur or selenium vapor is volatilized to the molybdenum source under the action of the carrier gas to react and deposit MoS ₂ or MoSe ₂ on the substrate.

In an embodiment of the present invention, the carrier gas in step (4) is a mixed gas of argon and hydrogen.

In an embodiment of the present invention, the time for depositing MoS ₂ or MoSe ₂ on the substrate in step (4) is 8-12 min.

In an embodiment of the present invention, when the tungsten source is deposited in step (5), after the deposition of the molybdenum source in step (4), the temperature is continued to rise, and after the temperature is raised to 850-880° C., the tungsten source is pushed by moving the outer magnet ring. To the molybdenum source mark, with the tungsten source directly below the substrate, continue to deposit WS2 or _{WSe2 on} the substrate.

In an embodiment of the present invention, when the temperature is raised to 810-840° C. in step (5), H ₂ needs to be fed into the edge of MoS ₂ or MoSe ₂ to better grow WS ₂ or WSe ₂ horizontally.

In an embodiment of the present invention, the time for depositing WS ₂ or WSe ₂ on the substrate in step (5) is 8-12 min.

The present invention provides a two-dimensional layered transition metal chalcogenide planar heterojunction prepared by the above method.

The present invention provides the application of the above two-dimensional layered transition metal chalcogenide planar heterojunction in optoelectronic devices.

[Beneficial effect]:

The two-step growth process will introduce other impurities and destroy the periodicity of the crystal domains, which will greatly reduce the performance of the device. Compared with the prior art, the present invention adopts the in-situ control of the aqueous solution precursor to prepare the two-dimensional layered transition metal chalcogenide planar heterojunction. The main advantages are:

The invention utilizes the in-situ control of the aqueous solution precursor, moves the molybdenum source and the tungsten source for deposition, does not need to manually perform secondary transfer stacking of two-dimensional materials, has good repeatability, the process is fast and efficient, and can continuously and controllably prepare high-quality and low-quality materials. Cost planar heterojunctions avoid cross-contamination issues.

Description of drawings

FIG. 1 is a diagram of an apparatus for preparing a MoSe ₂ /WSe ₂ planar heterojunction constructed in Example 1.

Figure 2 is the topography of the MoSe ₂ /WSe ₂ planar heterojunction prepared in Example 1, wherein (a) is the optical image of the MoSe ₂ /WSe ₂ planar heterojunction; (b) is black in (a) The dashed line marks the enlarged view of the sample.

3 is the Raman spectrum and PL spectrum of the MoSe ₂ /WSe ₂ planar heterojunction prepared in Example 1, wherein (a) is the Raman spectrum; (b) is the PL spectrum.

4 is a Raman mapping diagram of the MoSe ₂ /WSe ₂ planar heterojunction prepared in Example 1.

FIG. 5 is an optical image of the MoS ₂ /WS ₂ planar heterojunction prepared in Example 2. FIG.

6 is the Raman spectrum and PL spectrum of the MoSe ₂ /WSe ₂ planar heterojunction prepared in Example 2, wherein (a) is the Raman spectrum; (b) is the PL spectrum.

7 is an optical image of the MoS ₂ /WS ₂ planar heterojunction prepared in Comparative Example 1.

8 is an optical image of the MoS ₂ /WS ₂ planar heterojunction prepared in Comparative Example 2.

FIG. 9 is an optical image of the MoS ₂ /WS ₂ planar heterojunction prepared in Comparative Example 3. FIG.

Detailed ways

Based on the content contained in the claims, the present invention will be described in further detail below with reference to the embodiments and the accompanying drawings, but the embodiments of the present invention are not limited thereto.

[Example 1]

1. The solution of ammonium molybdate ((NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O) and ammonium tungstate ((NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ O) is used as the precursor, and the CVD method The process of fabricating large-area, low-cost, high-quality _MoSe2 / _WSe2 planar heterojunctions using in-situ controlled aqueous precursors:

(1) Processing the substrate: Use a diamond knife to cut the SiO ₂ as the substrate into a size (2.5*2.5cm) suitable for the diameter of the quartz boat, and try to keep the surface of the substrate as clean as possible when cutting. Put the cut substrate into a beaker, use acetone solution to clean the residual impurities on the surface of the substrate, and then use deionized water for ultrasonic cleaning; then soak in isopropyl alcohol for 0.5h, take it out and put it into a clean beaker, Rinse repeatedly with deionized water, and finally blow dry the substrate with high pressure nitrogen for use.

(2) Prepare (NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ O solution, (NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O solution and chlorine respectively with concentrations of 40 mg/mL, 15 mg/mL and 10 mg/mL sodium chloride (NaCl) solution; then take 0.5mL (NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O solution and 0.2 mL NaCl solution into a quartz boat (2cm*2cm*1.5cm), take 1mL (NH ₄ ) ₁₀ The W ₁₂ O ₄₁ ·xH ₂ O solution and 0.5 mL of NaCl solution were put into another quartz boat (2cm*2cm*1.5cm) to prepare the precursor solutions of molybdenum source and tungsten source.

(3) Put the two quartz boats in step (2) on the heating table, adjust the temperature to 100 ° C, and after heating for 30min, a uniform paste layer appears in the two quartz boats, which are used as the tungsten source and the molybdenum source. Place two quartz boats of tungsten source and molybdenum source on the left and right sides of a quartz boat with a pull ring (20cm*2.5cm*1.5cm); at the same time there is another quartz tube (20cm*3cm) that is not sealed at both ends and the top. *2.5cm), there are protrusions with a width of 1.5mm on both sides of the top of the quartz tube, and the substrate processed in step (1) can just be placed on top of it. Place the above-mentioned quartz boat with a pull ring into a quartz tube, hook the pull ring of the quartz boat with a hooked quartz rod, fix it on the hooked quartz rod with an iron ring, and fix it on the hooked quartz rod with an iron ring On the rod, hook the quartz rod to the pull ring of the quartz boat, and put the assembled device into the tube furnace, so that the substrate is in the second heating temperature zone. 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) Control the position of the molybdenum source and the tungsten source in the quartz tube through the external magnet ring, first push the magnet ring so that the tungsten source is directly below the substrate, and at this time the molybdenum source is downstream of the substrate, use a marker to place the quartz tube on the The outer wall marks the position of the magnet ring at this time (marked as the tungsten source mark); then push the magnet ring so that the molybdenum source is directly below the substrate, and the tungsten source is in the unheated area in the middle; at the same time, the outer wall of the quartz tube is also marked Show the position of the magnet ring at this time (marked as the molybdenum source mark).

Deposition of molybdenum source: 46 sccm of argon and 4 sccm of hydrogen are passed into the tube furnace as carrier gas, the selenium source is in the first temperature zone, and the temperature is raised to 250-300 ° C in 20 minutes and maintained for 30 minutes. The outer magnet ring was moved so that the molybdenum source was offset from the substrate to ensure that the surface at the substrate was clean and free of material formation during the ramp-up phase. When the temperature of the second heating temperature zone reaches 780°C within 25 minutes, pull the outer magnet of the quartz tube of the tube furnace to move the molybdenum source until the molybdenum source is directly below the substrate, and the tungsten source is in the middle unheated zone At this point, the selenium vapor is volatilized to the molybdenum source to react under the action of the carrier gas, so that the molybdenum source reacts, and MoSe ₂ is grown on the substrate; after 10 min, the growth of MoSe ₂ is completed.

(5) Deposition of tungsten source: after depositing molybdenum source, continue to heat up to 850°C within 5min. During the heating process, when the temperature reaches 810°C, adjust the flow of H to _30sccm ; when the temperature rises to 840°C, reduce H The flow rate of ₂ to 8sccm, while always maintaining 46sccm of argon; at the same time, pull the outer magnet of the quartz tube to make the tungsten source move, and move to just below the substrate on which the molybdenum source was deposited in step (4) (at the mark of the tungsten source). ), when the temperature reached 850°C, maintained for 10min to grow WSe ₂ .

(6) After the growth, the magnet ring outside the quartz tube of the tube furnace was pulled to move the two quartz boats to the downstream of the substrate, and 300 sccm of argon gas was introduced to flush away the unreacted source, and the temperature was accelerated at the same time. Cool to room temperature and remove the sample.

2. Morphology and structure characterization test:

The MoSe ₂ /WSe ₂ planar heterojunction prepared in this example was observed with an optical microscope. The test method was: adjust the magnification lens to observe the MoSe ₂ /WSe ₂ planar heterojunction under an optical microscope. After testing, the MoSe ₂ /WSe The optical image of the ₂ -plane heterojunction is shown in Figure 2, where (a) is the optical image of the MoSe ₂ /WSe ₂ planar heterojunction; (b) is the enlarged image of the sample marked by the black dotted line in (a). It can be seen from the figure that the outline of the heterojunction can be clearly distinguished in the MoSe ₂ /WSe ₂ planar heterojunction prepared in this example. The central region is MoSe ₂ , and the edge region is WSe ₂ .

Raman scanning is performed on the MoSe ₂ /WSe ₂ planar heterojunction prepared in this example. The test method is: select a point in the center region and edge region of the sample in Figure 2 (b) to perform Raman scanning, and the test results are obtained. Raman spectrogram of the MoSe ₂ /WSe ₂ planar heterojunction, it can be seen from the figure that the central region of the MoSe ₂ /WSe ₂ planar heterojunction prepared in this example is more obvious at 238cm ^-1 and 284cm ^-1 The characteristic peaks of , respectively correspond to the peaks of A _1g and E _2g of MoSe ₂ , and the wavenumber difference between the two is 46 cm ^-1 , indicating that the central region is a monolayer of MoSe ₂ . Similarly, it was found that two characteristic peaks appeared in the edge region at 250cm ^-1 and 260cm ^-1 , corresponding to E _2g and A _1g of WSe ₂ , respectively, indicating that the edge region was a single layer of WSe ₂ , as shown in Figure 3 ( a).

The fluorescence signal is collected for the MoSe ₂ /WSe ₂ planar heterojunction prepared in this example. The test method is: select a point in the center area and edge area of the sample in Figure 2 (b) to collect the fluorescence signal, and the MoSe is obtained after testing. The PL spectrum of the ₂ /WSe ₂ planar heterojunction, it can be seen from the figure that the central region sample of the MoSe ₂ /WSe ₂ planar heterojunction prepared in this example has a high-intensity and symmetrical characteristic peak at 795 nm , corresponding to the PL peak of MoSe ₂ with a corresponding band gap of 1.56 eV; while a characteristic peak with high intensity and symmetry appeared in the edge region at 760 nm, corresponding to the PL peak of WSe ₂ with a corresponding band gap of 1.63 eV. The above results show that both the as-prepared materials have direct band gaps, which further demonstrate their monolayer properties, as shown in Figure 3(b).

The Raman intensity surface scan was carried out on the MoSe ₂ /WSe ₂ planar heterojunction prepared in this example. The test method was as follows: the Raman intensity surface scan was made on the central area and the edge area of the sample, and the MoSe ₂ /WSe was obtained by testing. The Ramanmapping spectrum of the ₂ -plane heterojunction. It can be seen from the figure that the MoSe ₂ /WSe ₂ plane heterojunction prepared in this example can clearly distinguish the spatial distribution of the two materials, and the quality of the heterojunction prepared by CVD method. higher, as shown in Figure 4.

[Example 2]

1. Ammonium molybdate ((NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O) and ammonium tungstate ((NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ O solution are used as precursors, and the original A process for the preparation of large-area, low-cost, high _- quality MoS2/WS2 planar heterojunctions with position _- controlled aqueous precursors:

Steps (1) to (3) are the same as steps (1) to (3) of Example 1, wherein the selenium source is replaced with a sulfur source.

(4) Deposition molybdenum source: pass enough argon into the tube furnace to flush out the air in the tube. The sulfur source is in the first heating temperature zone. It is raised from room temperature to 180°C within 18 minutes and held for 32 minutes. The outer magnet ring is moved to make the molybdenum source deviate from the substrate. When the growth temperature of the molybdenum source is reached, it is pushed to the mark of the molybdenum source. The molybdenum source is placed directly under the substrate to ensure that the surface of the substrate is clean and no material is formed during the heating stage. When the temperature of the second heating temperature zone reaches 750°C within 25 minutes, pull the outer magnet of the quartz tube of the tube furnace to move the molybdenum source until the molybdenum source is directly below the substrate, and the tungsten source is in the middle unheated zone At the place where the sulfur vapor volatilizes under the action of the carrier gas to the molybdenum source to react, so that the molybdenum source reacts and MoS ₂ grows on the substrate, and 80 sccm of argon and 8 sccm of hydrogen are introduced as the carrier gas throughout the process; after 10 minutes, MoS ₂ has grown.

(5) Deposition of tungsten source: After depositing molybdenum source, continue to heat up to 850°C within 5min. During the heating process, when the temperature reaches 810°C, adjust the flow of H to _30sccm ; when the temperature rises to 830°C, reduce H The flow rate of ₂ to 5sccm to promote the growth of WS2 _; at the same time, 80sccm of argon gas is always maintained; at the same time, the external magnet of the quartz tube is pulled to make the tungsten source move and move to the bottom of the substrate on which the molybdenum source is deposited in step (4). , when the temperature reached 850 °C, WS ₂ was grown for 10 min.

(6) After the growth, the magnet ring outside the quartz tube of the tube furnace was pulled to move the two quartz boats to the downstream of the substrate, and 300 sccm of argon gas was introduced to flush away the unreacted source, and the temperature was accelerated at the same time. Cool to room temperature and remove the sample.

2. Morphology and structure characterization test:

The MoS ₂ /WS ₂ planar heterojunction prepared in this example was observed with an optical microscope. The test method was: adjust the magnification lens to observe the MoS ₂ /WS ₂ planar heterojunction under an optical microscope, and the MoS ₂ /WS Optical diagram of the ₂ -plane heterojunction. It can be seen from the figure that two materials can be clearly distinguished in the MoS ₂ /WS ₂ -plane heterojunction prepared in this example, the middle part is MoS ₂ , and the outer edge is WS ₂ , as shown in Figure 5.

Raman scanning is performed on the MoS ₂ /WS ₂ planar heterojunction prepared in this example. The test method is: select a point in the center area and the edge area of the sample in Figure 5 for Raman scanning, and obtain MoS ₂ / The Raman spectrum of the WS ₂ planar heterojunction, it can be seen from the figure that the central region of the MoS ₂ /WS ₂ planar heterojunction prepared in this example has obvious characteristic peaks at 384cm ^-1 and 403cm ^-1 , corresponding to the peaks of E _2g and A _1g of MoS ₂ , respectively, and the wavenumber difference between the two peak positions is 19 cm ⁻¹ , indicating that the prepared MoS ₂ is monolayer. There are two characteristic peaks at 350cm ^-1 and 419cm ^-1 in the edge region, which correspond to the 2LA(M) vibration mode and the A _1g vibration mode of WS ₂ , respectively, which are monolayer WS ₂ , as shown in (a) in Fig. 6 ).

The fluorescence signal is collected for the MoS ₂ /WS ₂ planar heterojunction prepared in this example. The testing method is as follows: select a point to collect the fluorescence signal in the central area and the edge area of the sample in Fig. 5 respectively, and obtain the PL spectrum after testing. It can be seen from the figure that the central region sample of the MoS ₂ /WS ₂ planar heterojunction prepared in this example has a characteristic peak with high intensity and symmetry at 660 nm, which corresponds to the PL peak of MoS ₂ and the corresponding energy is 1.8 eV, which proves that the prepared MoS ₂ is a monolayer; while a characteristic peak with high intensity and symmetry appears in the edge region at 625 nm, which corresponds to the PL peak of WS _2. The calculated band gap width is directly related to that of WS ₂ . The band gap is close to that of a single layer of WS ₂ , as shown in (b) in FIG. 6 .

To sum up, it can be seen from Example 1 and Example 2 that the present invention provides a method for preparing a two-dimensional layered transition metal chalcogenide planar heterojunction accurately and controllably in a large area; The secondary transfer stacking of materials has good repeatability, low cost, fast and efficient process, and simple preparation process.

[Comparative Example 1]

This comparative example deposited a mixture of molybdenum and tungsten sources together.

(1) Treatment of substrate: the same as step (1) in Example 1.

(2) Prepare (NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ O solution, (NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O solution and chlorine respectively with concentrations of 40 mg/mL, 15 mg/mL and 10 mg/mL Sodium chloride (NaCl) solution; then take 0.5 mL (NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O solution and 0.2 mL NaCl solution into the quartz boat, take 1 mL (NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ The O solution and 0.5 mL of NaCl solution were put into the same quartz boat to prepare a mixed precursor solution of molybdenum source and tungsten source.

(3) Put the quartz boat in step (2) on the heating table, adjust the temperature to 100° C., and after heating for 30 min, a uniform paste layer appears in both quartz boats. The substrate was placed directly above the quartz boat of the mixed source and the experimental setup was placed in the tube furnace so that it was in the second heating temperature zone. The sulfur source in the quartz boat is placed in the first heating temperature zone of the tube furnace.

(4) Deposition molybdenum source: pass enough argon into the tube furnace to flush out the air in the tube. The sulfur source is in the first temperature zone, rising from room temperature to 180°C within 18 minutes and holding for 32 minutes; when the temperature of the second heating temperature zone reaches 750°C within 25 minutes, the sulfur vapor volatilizes to the molybdenum source under the action of the carrier gas. Reaction, the molybdenum source reacts, and MoS ₂ is grown on the substrate, and 80 sccm of argon and 8 sccm of hydrogen are introduced as carrier gas throughout the process; MoS ₂ growth is completed after 10 min.

(5) Deposition of tungsten source: After depositing molybdenum source, continue to heat up to 850°C within 5min. During the heating process, when the temperature reaches 810°C, adjust the flow of H to _30sccm ; when the temperature rises to 830°C, reduce H ₂ to 5 sccm to promote the growth of WS ₂ ; at the same time, 80 sccm of argon was always maintained; when the temperature reached 850 °C, WS ₂ was maintained for 10 min to grow.

(6) After the growth, the magnet ring outside the quartz tube of the tube furnace was pulled to move the mixed source quartz boat to the downstream of the substrate, and 300 sccm of argon was introduced to wash away the unreacted source, and the temperature was accelerated at the same time. Cool to room temperature and remove the sample.

The MoS ₂ /WS ₂ planar heterojunction prepared in this comparative example was observed with an optical microscope. The test method was _: adjust the magnification lens to observe the MoS ₂ /WS ₂ planar heterojunction under an optical microscope. Optical image of the ₂ -plane heterojunction. It can be seen from the figure that the two materials cannot be distinguished in the MoS ₂ /WS ₂ -plane heterojunction prepared in this comparative example, the island density is large and the triangle island shape formed is defective, Figure 7.

[Comparative Example 2]

This comparative example utilizes a two-step process for deposition.

Steps (1) and (2) are the same as steps (1) and (2) in Example 1.

(3) The two quartz boats in step (2) are placed on the heating table, and the temperature is adjusted to 100° C. After heating for 30 minutes, a uniform paste layer will appear in the quartz boat, which is used as a molybdenum source and a tungsten source.

(4) Depositing the molybdenum source: place the substrate above the quartz boat of the molybdenum source, so that the substrate is just above the quartz boat containing the molybdenum source. The experimental setup 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 zone of the tube furnace. A sufficient amount of argon was passed through the tube furnace to flush the air out of the tube. The sulfur source is in the first temperature zone, rising from room temperature to 180°C within 18 minutes and holding for 17 minutes; when the temperature of the second heating temperature zone reaches 750°C within 25 minutes, the sulfur vapor volatilizes to the molybdenum source under the action of the carrier gas. Reaction, the molybdenum source is reacted, MoS ₂ is grown on the substrate, and kept for 10 min, the two temperature zones are heated at the same time, and 80 sccm of argon gas and 8 sccm of hydrogen gas are introduced as carrier gas in the whole process. After the growth, the magnet ring outside the quartz tube of the tube furnace was pulled to move the mixed source quartz boat to the downstream of the substrate, and 300 sccm of argon gas was introduced to wash away the unreacted source, and at the same time, the temperature was accelerated and cooled to room temperature naturally. , remove the sample.

(5) Deposition of tungsten source: the above-mentioned molybdenum source was replaced with a tungsten source, and the experimental apparatus was in the same position to carry out the growth of WS ₂ . Sufficient argon was passed through to flush the air out of the tube. The sulfur source is in the first temperature zone, rising from room temperature to 180°C within 18 minutes and holding for 17 minutes; the temperature of the second heating temperature zone reaches 850°C within 25 minutes and keeps it for 10 minutes, the two temperature zones are heated at the same time; 80sccm of argon is passed through the whole process. gas as the carrier gas. During the heating process, when the temperature reached 810 °C, the flow rate of _H2 was adjusted to 30 sccm; when the temperature rose to 830 °C, the flow rate of _H2 was reduced to ₅ sccm to the end of the growth to promote the growth of WS2. After the growth, the magnet ring outside the quartz tube of the tube furnace was pulled to move the mixed source quartz boat to the downstream of the substrate, and 300 sccm of argon gas was introduced to wash away the unreacted source, and at the same time, the temperature was accelerated and cooled to room temperature naturally. , remove the sample.

The MoS ₂ /WS ₂ planar heterojunction prepared in this comparative example was observed with an optical microscope. The test method was _: adjust the magnification lens to observe the MoS ₂ /WS ₂ planar heterojunction under an optical microscope. Optical diagram of the ₂ -plane heterojunction, it can be seen from the figure that the two materials cannot be clearly distinguished in the MoS ₂ /WS ₂ -plane heterojunction prepared in this comparative example, transferred from the first CVD device to the second CVD During the installation, there was cross _- contamination at the edges of MoS 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 below the substrate.

Steps (1) to (3) are the same as steps (1) to (3) in Example 1, wherein the positions of the molybdenum source and the tungsten source are not marked in step (3) in this comparative example.

(4) Deposition molybdenum source: pass enough argon into the tube furnace to flush out the air in the tube. The two temperature zones were heated at the same time, and 80 sccm of argon and 8 sccm of hydrogen were introduced as carrier gas throughout the process. The sulfur source is in the first temperature zone, rising from room temperature to 180°C within 18 minutes and held for 32 minutes; when the temperature of the second heating temperature zone reaches 750°C within 25 minutes, move the outer magnet ring to make the molybdenum source deviate from the substrate, and the molybdenum source is in the lining. On the left side of the bottom, the sulfur vapor is volatilized to the molybdenum source to react under the action of the carrier gas, so that the molybdenum source reacts, and MoS ₂ is grown on the substrate, and 80 sccm of argon and 8 sccm of hydrogen are introduced as the carrier gas throughout the process; 10min After the growth of MoS ₂ is completed.

(5) Deposition of tungsten source: After depositing molybdenum source, continue to heat up to 850°C within 5min. During the heating process, when the temperature reaches 810°C, adjust the flow of H to _30sccm ; when the temperature rises to 830°C, reduce H The flow rate of ₂ to 5sccm to promote the growth of WS2 _; at the same time, 80sccm of argon is always maintained; at the same time, the external magnet of the quartz tube is pulled to make the tungsten source move, so that it moves to the substrate on which the molybdenum source is deposited in step (4). On the left, when the temperature reached 850 °C, WS ₂ was grown for 10 min.

(6) After the growth, the magnet ring outside the quartz tube of the tube furnace was pulled to move the two quartz boats to the downstream of the substrate, and 300 sccm of argon gas was introduced to flush away the unreacted source, and the temperature was accelerated at the same time. Cool to room temperature and remove the sample.

The MoS ₂ /WS ₂ planar heterojunction prepared in this comparative example was observed with an optical microscope. The test method was _: adjust the magnification lens to observe the MoS ₂ /WS ₂ planar heterojunction under an optical microscope. ₂ -plane heterojunction optical diagram, it can be seen from the figure that the MoS ₂ /WS ₂ plane heterojunction prepared in this comparative example has multiple layers, as shown in Figure 9.

Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Anyone who is familiar with this technology can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, The protection scope of the present invention should be defined by the claims.

Claims (10)

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.

Images