An ultra-thin two-dimensional transition metal nitride single crystal structure, its preparation method and applications

Abstract

The present invention discloses a method for preparing and applying a single crystal structure of ultrathin two-dimensional transition metal nitride, belonging to the technical field of nanomaterials. The method mainly includes the following steps: preparing transition metal salt solutions with different concentrations; performing surface pretreatment on a sapphire substrate to make its surface hydrophilic; using a spin coater to spin coat the transition metal salt solution onto the surface of the sapphire substrate; and carrying out a chemical vapor deposition reaction in a temperature-programmed tube furnace to prepare a single crystal of ultrathin two-dimensional transition metal nitride. By adjusting the growth temperature and the concentration of the precursor, single crystals of ultrathin two-dimensional transition metal nitride with different phases can be controllably prepared. This preparation method of directly melting transition metal salts by adjusting temperature and atmospheric pressure is simple and feasible, and can be extended to the controllable preparation of other transition metal nitrides, laying a good foundation for the controllable preparation and application research of subsequent nitride thin film materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanomaterials, and specifically relates to an ultra-thin two-dimensional transition metal nitride single crystal structure and a preparation method and application thereof. Background Art

[0002] Layered two-dimensional materials, with their unique electronic structure and large specific surface area, are a hot topic in microelectronics, physics and chemistry. In recent years, two-dimensional transition metal nitrides have great electrochemical activity and high conductivity, and have potential applications in electrical conversion and storage; in industry, they are used in surface coatings of wear-resistant and anti-oxidation materials due to their high hardness coefficient; because transition metal nitrides have the properties of precious metals, they have been widely used in heterogeneous catalytic reactions, such as water gas shift, reverse water gas shift, synthetic ammonia, hydrocarbon reforming, etc. Two-dimensional layered materials can provide very high two-dimensional electronic conductivity, and the transition metals exposed on the surface of the sheets play a vital role in chemical reactions. Therefore, the performance of two-dimensional transition metal nitrides in energy storage, industry, catalysis, etc. has been greatly improved. On the other hand, two-dimensional transition metal nitride single crystals can provide an ideal model structure for basic research.

[0003] At present, there are several methods for preparing transition metal nitride films: (1) magnetron sputtering, which results in a rough surface and small grains; (2) single crystal surface nitridation, such as ammoniation on Mo(100), which results in molybdenum nitride that is not a traditional two-dimensional material; (3) atomic layer deposition, such as using WF6 in ammonia to obtain tungsten nitride films; (4) chemical vapor deposition, which uses Cu-Mo bimetallic ammoniation to obtain molybdenum nitride. So far, there are still challenges in the controllable preparation of ultra-thin, regular, two-dimensional transition metal nitride single crystals. Therefore, how to controllably prepare two-dimensional transition metal nitrides has become an important issue that researchers in this field need to solve. Summary of the invention

[0004] In view of the above technical problems, the present invention overcomes the shortcomings of the prior art and provides a method for preparing ultra-thin two-dimensional nitride single crystals. Taking the preparation of ultra-thin two-dimensional molybdenum nitride single crystals as an example, by adopting chemical vapor deposition, sodium molybdate is used as a precursor, sapphire is used as a substrate, sodium molybdate can form a liquid-solid interface with sapphire when it reaches the melting point, and ammonia is used as a nitrogen source. Ultra-thin two-dimensional molybdenum nitride single crystals grown epitaxially can be obtained on the sapphire surface. By regulating the growth temperature and the concentration of the precursor, the controllable preparation of two-dimensional molybdenum nitride with different physical phases can be achieved. This method of nitridation using molybdate as a precursor is simple and easy, and the structure is easy to control. It can be extended to other transition metal nitrides, such as tungsten nitride, vanadium nitride, chromium nitride, etc., which lays a good foundation for the preparation and application research of nitride films.

[0005] The technical solution of the present invention is as follows:

[0006] On the one hand, the present invention provides a method for preparing an ultrathin two-dimensional transition metal nitride single crystal structure, and the method comprises the following steps:

[0007] (1) Prepare a precursor solution for standby;

[0008] (2) Perform surface pretreatment on the single crystal oxide substrate to remove surface impurities and obtain a hydrophilic surface;

[0009] (3) Spin-coat the precursor solution on the hydrophilic surface of the substrate by using a spin coater, and dry the substrate by using a heating device;

[0010] (4) Place the substrate spin-coated with the precursor solution into a temperature-programmed tube furnace, use an inert gas as the carrier gas, heat the substrate to 950-1200 °C, after the temperature reaches the set temperature, introduce ammonia or nitrogen, and keep warm; after the reaction ends, close ammonia or nitrogen, and cool down to room temperature in argon to obtain an ultrathin two-transition metal nitride single crystal structure.

[0011] Further, in the step (1), preparing the precursor solution is to dissolve the precursor in deionized water.

[0012] Further, in the step (1), the concentration of the precursor solution is 75-300 mg / mL, preferably 75 mg / mL, 150 mg / mL, 300 mg / mL; the precursor includes sodium molybdate, sodium tungstate, sodium chromate, sodium vanadate, etc.

[0013] Further, the single crystal oxide substrate in the step (2) includes sapphire and strontium titanate.

[0014] Further, the sapphire is single crystal Al2O3(0001); the strontium titanate includes niobium-doped strontium titanate and iron-doped strontium titanate.

[0015] Further, the method for surface pretreatment of the single crystal oxide substrate in the step (2) includes: respectively performing multiple cyclic cleaning on the substrate surface by using ethanol and deionized water, and continuing to process the cleaned substrate in an oxygen plasma cleaner for 15-60 minutes, the output power of the oxygen plasma cleaner is 18 W, and the oxygen or air flow rate introduced into the oxygen plasma cleaner is 20-50 sccm, and a hydrophilic substrate surface can be obtained.

[0016] Further, the heating device in the step (2) includes an infrared heating lamp.

[0017] Further, in the spin coating process in step (3), the process and parameters are as follows: 0.2 mL of the precursor solution is aspirated and dropped onto the surface of a substrate with a size of 10*10 mm, and then spin coating is carried out at 3000 - 5000 rpm for 30 - 60 seconds; the drying temperature is 100 - 150 °C.

[0018] Further, the programmed temperature tube furnace in step (4) is at atmospheric pressure.

[0019] Further, the programmed temperature rate in step (4) is 10 - 15 °C / min.

[0020] Further, the inert gas in step (4) includes argon.

[0021] Further, the flow rate of the carrier gas in step (4) is 100 - 200 sccm.

[0022] Further, the flow rate of ammonia in step (4) is 10 sccm.

[0023] Further, the heat preservation time in step (4) is 2 - 6 hours.

[0024] Further, in the present invention, by setting different growth temperatures in step (4) and different concentrations of the precursor in step (1), ultrathin two-dimensional transition metal nitride single crystal structures with different phases are obtained.

[0025] On the other hand, the present invention provides an ultrathin two-dimensional transition metal nitride single crystal structure, and the ultrathin two-dimensional transition metal nitride single crystal structure is prepared by the above method. The ultrathin two-dimensional transition metal nitride single crystal structure nucleates and grows at the liquid-solid interface between the molten precursor and the substrate to form a single crystal; the ultrathin two-dimensional transition metal nitride single crystal structure includes an ultrathin two-dimensional molybdenum nitride single crystal structure, an ultrathin two-dimensional tungsten nitride single crystal structure, an ultrathin two-dimensional chromium nitride single crystal structure, and an ultrathin two-dimensional vanadium nitride single crystal structure.

[0026] Further, for the preparation of the ultrathin two-dimensional molybdenum nitride single crystal structure, when the growth temperature is 850 - 950 °C, a δ-phase epitaxial molybdenum nitride can be formed at low coverage, and an epitaxial transition metal nitride single crystal structure coexisting with δ-phase and γ-phase can be formed at high coverage; when the growth temperature is 1100 - 1200 °C, a γ-phase in-plane single crystal can be formed.

[0027] In the above preparation method, taking the preparation of ultrathin two-dimensional molybdenum nitride single crystal as an example, in step (3), sodium molybdate is uniformly covered on the surface of sapphire. In step (4), in an argon atmosphere, when the temperature rises to 687 °C, sodium molybdate forms a molten state and forms a liquid-solid interface with the substrate sapphire. When the reaction temperature is 950 °C or 1200 °C, ammonia is introduced at a flow rate of 10 sccm, and the ultrathin two-dimensional molybdenum nitride nucleates and grows at the liquid-solid interface formed by the molten sodium molybdate and sapphire. After the reaction ends, the surface of the sapphire is covered with triangles with a lateral size of about 300 - 500 nm, a thickness of 10 - 50 nm and consistent orientation, as well as a small amount of hexagonal structures. By changing its precursor, different types of ultrathin two-dimensional nitrides can be obtained, such as preparing ultrathin two-dimensional tungsten nitride, vanadium nitride and chromium nitride using sodium tungstate, sodium vanadate and sodium chromate.

[0028] Meanwhile, the present invention applies the ultrathin two-dimensional transition metal nitride single crystal structure prepared above as a model catalyst in the field of heterogeneous catalysis.

[0029] Beneficial effects

[0030] 1. The present invention provides a preparation method for a novel ultrathin two-dimensional transition metal nitride single crystal structure. The method uses a single crystal oxide as a substrate, places the single crystal oxide substrate spin-coated with a transition metal precursor into a temperature-programmed tube furnace, and uses chemical vapor deposition reaction to prepare the ultrathin two-dimensional transition metal nitride single crystal structure. In this preparation process, for the first time, a transition metal precursor such as sodium molybdate is directly used as a metal source to controllably prepare the ultrathin two-dimensional transition metal nitride single crystal structure.

[0031] 2. By controlling the growth temperature and the concentration of the precursor in the preparation method of the present invention, the quality and phase structure of the ultrathin two-dimensional transition metal nitride single crystal structure can be controlled; when the concentration of the precursor sodium molybdate is 75 mg / mL and the reaction temperature is 950 °C, an in-plane single crystal of molybdenum nitride with regular morphology, a thickness of 10 - 50 nm and a phase structure of δ-phase can be prepared; when the concentration of the precursor sodium molybdate is increased to 150 / 300 mg / mL and the reaction temperature is 950 °C, an in-plane single crystal of molybdenum nitride with a coexisting phase structure of δ-phase and γ-phase can be prepared; when the reaction temperature is increased to 1200 °C and the concentration of sodium molybdate is 75 / 150 / 300 mg / mL, in-plane single crystals of molybdenum nitride with a phase structure of γ-phase under different coverage degrees can be prepared; the obtained ultrathin two-dimensional transition metal nitride single crystal structure (such as ultrathin two-dimensional molybdenum nitride) can be applied to the fields of catalysis and electrocatalysis.

[0032] 3. The present invention uses a transition metal salt as a precursor and utilizes the liquid-solid interface formed by the molten state and the single crystal substrate to prepare a growth method for an ultrathin two-dimensional transition metal nitride single crystal structure. This method is simple and easy to implement, and the structure is easy to control. The growth of ultrathin two-dimensional molybdenum nitride single crystals can be extended to other transition metal nitrides and other single crystal oxide substrates, laying a good foundation for the subsequent preparation and application research of nitride films. Description of the Drawings

[0033] Figure 1 Scanning electron microscope image (a) and atomic force microscope image (b) of the ultrathin two-dimensional molybdenum nitride prepared in Example 1 at a reaction temperature of 950 °C, a sodium molybdate solution concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours.

[0034] Figure 2 Elemental distribution of the ultrathin two-dimensional molybdenum nitride prepared in Example 1 at a reaction temperature of 950 °C, a sodium molybdate solution concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours; distribution of elements (a) Mo and (b) N in the triangular structure; distribution of elements (c) Mo and (d) N in the hexagonal structure.

[0035] Figure 3 X-ray photoelectron spectroscopy of the ultrathin two-dimensional molybdenum nitride prepared in Example 1 at a reaction temperature of 950 °C, a sodium molybdate solution concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours, (a) N 1s + Mo 3p, (b) Mo 3d.

[0036] Figure 4 X-ray diffraction pattern of the ultrathin two-dimensional molybdenum nitride prepared in Example 1 at a reaction temperature of 950 °C, a sodium molybdate solution concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours.

[0037] Figure 5 Atomic force microscope images of the ultrathin two-dimensional molybdenum nitride prepared in Example 2 at a reaction temperature of 950 °C, different sodium molybdate solution concentrations, a sapphire substrate, and a reaction time of 2 hours, (a) sodium molybdate solution concentration of 150 mg / mL, (b) sodium molybdate solution concentration of 300 mg / mL.

[0038] Figure 6 X-ray diffraction patterns of the ultrathin two-dimensional molybdenum nitride prepared in Example 2 at a reaction temperature of 950 °C, sodium molybdate solution concentrations of 150 mg / mL and 300 mg / mL respectively, a sapphire substrate, and a reaction time of 2 hours.

[0039] Figure 7Scanning electron microscope image (a) and atomic force microscope image (b) of the ultrathin two-dimensional molybdenum nitride prepared in Example 3 at a reaction temperature of 1200 °C, a sodium molybdate solution concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours.

[0040] Figure 8 Scanning electron microscope image (a) and atomic force microscope image (b) of the ultrathin two-dimensional molybdenum nitride prepared in Example 3 at a reaction temperature of 1200 °C, a sodium molybdate solution concentration of 150 mg / mL, a sapphire substrate, and a reaction time of 2 hours.

[0041] Figure 9 Scanning electron microscope image (a) and atomic force microscope image (b) of the ultrathin two-dimensional molybdenum nitride prepared in Example 3 at a reaction temperature of 1200 °C, a sodium molybdate solution concentration of 300 mg / mL, a sapphire substrate, and a reaction time of 2 hours.

[0042] Figure 10 X-ray photoelectron spectroscopy of the ultrathin two-dimensional molybdenum nitride prepared in Example 3 at a reaction temperature of 1200 °C, a sodium molybdate solution concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours, (a) N 1s + Mo 3p, (b) Mo 3d.

[0043] Figure 11 X-ray diffraction pattern of the ultrathin two-dimensional molybdenum nitride prepared in Example 3 at a reaction temperature of 1200 °C, sodium molybdate solution concentrations of 75 mg / mL, 150 mg / mL, and 300 mg / mL, a sapphire substrate, and a reaction time of 2 hours.

[0044] Figure 12 Scanning electron microscope image of the ultrathin two-dimensional molybdenum nitride prepared in Example 4 at a reaction temperature of 950 °C, a sodium molybdate solution concentration of 75 mg / mL, a niobium-doped strontium titanate substrate, and a reaction time of 2 hours, (a) triangular structures appear on the surface of strontium titanate, (b) region of triangular structures with consistent orientation.

[0045] Figure 13 X-ray photoelectron spectroscopy of the ultrathin two-dimensional molybdenum nitride prepared in Example 4 at a reaction temperature of 950 °C, a sodium molybdate solution concentration of 75 mg / mL, a niobium-doped strontium titanate substrate, and a reaction time of 2 hours, (a) N 1s + Mo 3p, (b) Mo 3d, (c) comparison of Na 1s before and after water washing

[0046] Figure 14Scanning electron microscope images of the ultrathin two-dimensional nitrides prepared in Example 5 with sodium tungstate, sodium vanadate, and sodium chromate solutions at a reaction temperature of 950 °C, a concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours; (a)-(b): tungsten nitride; (c)-(d): chromium nitride; (e)-(f): vanadium nitride.

[0047] Figure 15 X-ray diffraction pattern of the ultrathin two-dimensional nitrides prepared in Example 5 with sodium tungstate, sodium vanadate, and sodium chromate solutions at a reaction temperature of 950 °C, a concentration of 75 mg / mL for each solution, a sapphire substrate, and a reaction time of 2 hours.

[0048] Figure 16 Scanning electron microscope images of the ultrathin two-dimensional molybdenum nitride prepared in Comparative Example 1 at a low temperature of 750 °C, with an ammonium molybdate solution concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours.

[0049] Figure 17 X-ray photoelectron spectroscopy of the ultrathin two-dimensional molybdenum nitride prepared in Comparative Example 1 at a low temperature of 750 °C, with an ammonium molybdate solution concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours, (a) N1s + Mo 3p, (b) Mo 3d.

[0050] Figure 18 X-ray diffraction pattern of the ultrathin two-dimensional molybdenum nitride prepared in Comparative Example 1 at a low temperature of 750 °C, with an ammonium molybdate solution concentration of 75 mg / mL, a sapphire substrate, and a reaction time of 2 hours. Detailed implementation manners

[0051] The present invention will be further described below through examples. Professionals in the field can easily understand the advantages and effects of the present invention from the content disclosed in the specification. The following examples are only for more detailed and specific descriptions and should not be construed as limiting the present invention in any way.

[0052] Example 1

[0053] Preparation of δ-phase ultrathin two-dimensional molybdenum nitride with a reaction temperature of 950 °C, an ammonium molybdate solution concentration of 75 mg / mL, and a sapphire substrate

[0054] (1) Preparation of 75 mg / mL ammonium molybdate solution

[0055] Weigh 3.000 g of ammonium molybdate particles using an analytical balance and place them in a clean beaker. Measure 40 mL of deionized water using a measuring cylinder and pour it into the beaker. Oscillate the solution using an ultrasonic cleaner to fully dissolve the ammonium molybdate. After dissolution, store the solution using a volumetric flask to obtain an ammonium molybdate solution with a concentration of 75 mg / mL.

[0056] (2) Pretreatment of Sapphire Substrate

[0057] The sapphire substrate was cleaned three times in a cycle with absolute ethanol and deionized water, and then purged with a nitrogen gas gun. Then the sapphire substrate was processed in an oxygen plasma cleaner with an output power of 18 W, a processing time of 30 minutes, and an oxygen flow rate of 50 sccm.

[0058] (3) Spin-Coating of Sodium Molybdate Solution

[0059] The sapphire substrate after hydrophilic treatment should be spin-coated as soon as possible. Use a syringe to suck 0.2 mL of 75 mg / mL sodium molybdate solution and drop it onto the surface of a 10*10 mm sapphire, and then spin-coat for 30 seconds at 3000 rpm. The spin-coated sapphire substrate was dried with an infrared heating lamp, and the heating temperature was about 100 °C.

[0060] (4) Preparation of Ultrathin Two-Dimensional Molybdenum Nitride Single Crystal

[0061] The sapphire substrate spin-coated with sodium molybdate solution was placed in a temperature-programmed tube furnace. Using high-purity argon as the carrier gas, the substrate was heated to 950 °C. After the temperature reached the set temperature, ammonia gas was introduced with an ammonia flow rate of 10 sccm and held for 2 hours. After the reaction was completed, the ammonia gas was turned off and cooled to room temperature in argon to obtain an ultrathin two-dimensional molybdenum nitride single crystal. Figure 1 For the characterization of the prepared ultrathin two-dimensional molybdenum nitride using a scanning electron microscope and an atomic force microscope. The sapphire surface was mainly covered with regular structures such as regular triangles and hexagons, with a size of about 300 - 500 nm( Figure 1 a). The triangular and hexagonal structures had a consistent orientation( Figure 1 b). The atomic force microscope showed that the thickness of the above-prepared two-dimensional molybdenum nitride was about 50 nm. Figure 2 For the energy spectrum element distribution of the two regular structures of triangles and hexagons, it was shown that both structures were composed of molybdenum and nitrogen elements.

[0062] Figure 3 For the X-ray energy spectrum diagram of the ultrathin two-dimensional structure covering the sapphire surface, it was shown that this structure was molybdenum nitride, and after calculation, the atomic ratio of molybdenum to nitrogen in this structure was 1:1. Figure 4 For the X-ray diffraction characterization of this structure, the results showed that molybdenum nitride coexisted with δ(002) and δ(004), and it was an in-plane single crystal.

[0063] Example 2

[0064] The reaction temperature was 950 °C, the concentration of the sodium molybdate solution was 150 mg / mL or 300 mg / mL, the substrate was sapphire, and the preparation of ultrathin two-dimensional molybdenum nitride

[0065] (1) Preparation of 150 mg / mL and 300 mg / mL sodium molybdate solutions

[0066] Weigh 6.000 and 12.000 g of sodium molybdate particles using an analytical balance and place them in clean beakers respectively. Measure 40 mL of deionized water with a measuring cylinder and pour it into the beakers. Use an ultrasonic cleaner to fully dissolve the sodium molybdate, and after dissolution, store it in volumetric flasks to obtain sodium molybdate solutions with concentrations of 150 mg / mL and 300 mg / mL respectively.

[0067] (2) Pretreatment of sapphire substrate

[0068] Clean the sapphire substrate three times in a cycle with absolute ethanol and deionized water, and blow it with a nitrogen gas gun. Then place the sapphire substrate in an oxygen plasma cleaner for treatment. The set output power is 18 W, the treatment time is 30 minutes, and the oxygen flow rate is set to 50 sccm.

[0069] (3) Spin-coating of sodium molybdate solution

[0070] The sapphire substrate should be spin-coated as soon as possible after hydrophilic treatment. Use a syringe to suck 0.2 mL of 150 mg / mL or 300 mg / mL sodium molybdate solution and drop it onto the surface of a 10*10 mm sapphire, and then perform spin-coating at 3000 rpm for 30 seconds. After spin-coating, dry the sapphire substrate with an infrared heating lamp, and the heating temperature is about 100 °C.

[0071] (4) Preparation of ultrathin two-dimensional molybdenum nitride single crystals

[0072] Place the sapphire substrate spin-coated with sodium molybdate solution into a temperature-programmed tube furnace. Use high-purity argon as the carrier gas, heat the substrate to 950 °C, and after the temperature reaches the set temperature, introduce ammonia gas and keep it warm for 2 hours. After the reaction is completed, turn off the ammonia gas and cool it to room temperature in argon to obtain ultrathin two-dimensional molybdenum nitride single crystals. Figure 5 For the characterization of the prepared ultrathin two-dimensional molybdenum nitride using a scanning electron microscope. Using a 150 mg / mL sodium molybdate solution as the precursor, the sapphire surface is covered with triangular structures, and hexagonal structures begin to appear above the triangles ( Figure 5 a). As the concentration of the precursor increases, when the concentration of sodium molybdate is 300 mg / mL, the hexagonal structures of the triangular structures also increase ( Figure 5 b). Figure 6 For the X-ray diffraction characterization of molybdenum nitride prepared with 150 mg / mL and 300 mg / mL sodium molybdate solutions, the results show that molybdenum nitride coexists in two phases of δ-phase and γ-phase, and the main existing forms are δ(002), δ(004) and γ(111)γ(222), all of which belong to in-plane single crystals.

[0073] Example 3

[0074] The reaction temperature is 1200 °C, the concentration of sodium molybdate solution is 75 mg / mL, 150 mg / mL or 300 mg / mL, the substrate is sapphire, and the preparation of γ-phase ultrathin two-dimensional molybdenum nitride

[0075] (1) Preparation of 75 mg / mL, 150 mg / mL and 300 mg / mL sodium molybdate solutions

[0076] Weigh 3.000 g, 6.000 g and 12.000 g of sodium molybdate particles respectively using an analytical balance and put them into a clean beaker. Measure 40 mL of deionized water with a measuring cylinder and pour it into the beaker. Use an ultrasonic cleaner to fully dissolve the sodium molybdate, and then store it in a volumetric flask after dissolution to obtain sodium molybdate solutions with concentrations of 75 mg / mL, 150 mg / mL and 300 mg / mL respectively.

[0077] (2) Pretreatment of sapphire substrate

[0078] The sapphire substrate is cleaned three times in a cycle with absolute ethanol and deionized water and then purged with a nitrogen air gun. Then the sapphire substrate is processed in an oxygen plasma cleaner with an output power of 18 W, a processing time of 30 minutes, and an oxygen flow rate of 50 sccm.

[0079] (3) Spin coating of sodium molybdate solution

[0080] The sapphire substrate after hydrophilic treatment should be spin-coated as soon as possible. Use a syringe to suck 0.2 mL of sodium molybdate solution with a concentration of 75 mg / mL, 150 mg / mL or 300 mg / mL respectively and drop it onto the surface of a 10*10 mm sapphire, and then spin-coat for 30 seconds at 3000 rpm. The spin-coated sapphire substrate is dried with an infrared heating lamp, and the heating temperature is about 100 °C.

[0081] (4) Preparation of ultrathin two-dimensional molybdenum nitride single crystal

[0082] Put the sapphire substrate spin-coated with sodium molybdate solution into a temperature-programmed tube furnace, use high-purity argon as the carrier gas, heat the substrate to 1200 °C, and after the temperature reaches the set temperature, introduce ammonia gas and keep it warm for 2 hours. After the reaction is completed, turn off the ammonia gas and cool it to room temperature in argon to obtain an ultrathin two-dimensional molybdenum nitride single crystal. Figure 7 Characterization of the prepared ultrathin two-dimensional molybdenum nitride with a sodium molybdate concentration of 75 mg / mL by scanning electron microscopy and atomic force microscopy. The sapphire surface shows a low coverage rate and regular triangular structures( Figure 7 a), and the triangular structures show consistent orientations( Figure 7b), The lateral dimension of the triangular structure is approximately 300 - 500 nm, and atomic force microscopy shows that the thickness of the prepared two-dimensional molybdenum nitride is approximately 50 nm. Figure 8 Characterization of the prepared ultrathin two-dimensional molybdenum nitride using scanning electron microscopy and atomic force microscopy with a sodium molybdate concentration of 150 mg / mL. The sapphire surface shows a relatively high coverage rate, with regular triangular and hexagonal structures ( Figure 8 a), The triangular and hexagonal structures show a consistent orientation ( Figure 8 b), The lateral dimension of the triangular structure is approximately 300 - 500 nm, and atomic force microscopy shows that the thickness of the prepared two-dimensional molybdenum nitride is approximately 50 nm. Figure 9 shows the characterization of the prepared ultrathin two-dimensional molybdenum nitride using scanning electron microscopy and atomic force microscopy with a sodium molybdate concentration of 300 mg / mL. The sapphire surface shows a high coverage rate, with regular triangular and hexagonal structures (Figure 9a), and the triangular structure shows a consistent orientation ( Figure 9 b), The lateral dimension of the triangular structure is approximately 300 - 500 nm, and atomic force microscopy shows that the thickness of the prepared two-dimensional molybdenum nitride is approximately 10 nm.

[0083] Figure 10 X-ray photoelectron spectroscopy of the sapphire surface covered with an ultrathin two-dimensional structure shows that the structure is molybdenum nitride, and after calculation, the atomic ratio of molybdenum to nitrogen in this structure is 1:1. Figure 11 X-ray diffraction characterization of ultrathin two-dimensional molybdenum nitride prepared with three different concentrations of sodium molybdate at 1200 °C shows that it is γ-phase molybdenum nitride, and its main forms are γ(111) and γ(222), which are in-plane single crystals.

[0084] Example 4

[0085] The reaction temperature is 950 °C, the concentration of the sodium molybdate solution is 75 mg / mL, the substrate is niobium-doped strontium titanate, and the preparation of ultrathin two-dimensional molybdenum nitride

[0086] (1) Preparation of a 75 mg / mL sodium molybdate solution

[0087] Weigh 3.000 g of sodium molybdate particles using an analytical balance and place them in a clean beaker. Measure 40 mL of deionized water using a measuring cylinder and pour it into the beaker. Use an ultrasonic cleaner to fully dissolve the sodium molybdate, and then store it in a volumetric flask after dissolution to obtain a sodium molybdate solution with a concentration of 75 mg / mL.

[0088] (2) Pretreatment of the sapphire substrate

[0089] The Nb-doped strontium titanate substrate was cleaned three times with anhydrous ethanol and deionized water, and purged with a nitrogen air gun. The Nb-doped strontium titanate substrate was then treated in an oxygen plasma cleaning machine with an output power of 18 W, a treatment time of 30 minutes, and an oxygen flow rate of 50 sccm.

[0090] (3) Spin coating of sodium molybdate solution

[0091] The Nb-doped strontium titanate substrate should be spin-coated as soon as possible after hydrophilic treatment. Use a syringe to draw 0.2mL of 75 mg / mL sodium molybdate solution and drop it on the surface of the Nb-doped strontium titanate with a size of 10*10mm, and then spin-coat it at 3000rpm for 30 seconds. After spin-coating, use an infrared heating lamp to dry the sapphire on the Nb-doped strontium titanate substrate, and the heating temperature is about 100℃.

[0092] (4) Preparation of ultra-thin two-dimensional MoN single crystals

[0093] The Nb-doped strontium titanate substrate with spin-coated sodium molybdate solution was placed in a programmed temperature tube furnace, and the substrate was heated to 950°C using high-purity argon as a carrier gas. After the temperature reached the set temperature, ammonia was introduced and kept warm for 2 hours. After the reaction was completed, the ammonia was turned off and the temperature was cooled to room temperature in argon to obtain an ultra-thin two-dimensional molybdenum nitride single crystal. Figure 12 The ultrathin two-dimensional molybdenum nitride prepared by scanning electron microscopy at a sodium molybdate concentration of 75 mg / mL. The surface of Nb-doped strontium titanate presents a regular triangular structure ( Figure 12 a), the triangular structure shows consistent orientation ( Figure 12 b), the lateral size of the triangular structure is about 300-500nm. Figure 13 X-ray electron spectrum of the ultra-thin two-dimensional structure covered on the sapphire surface. (13a) N 1s + Mo 3p, (13b) Mo 3d, (13c) Na 1s comparison before and after water washing, indicating that the structure is molybdenum nitride. After calculation, the atomic ratio of molybdenum to nitrogen in the structure is 1:1.

[0094] Example 5

[0095] The reaction temperature is 950℃, the concentration of sodium tungstate, sodium chromate and sodium vanadate solution is 75mg / mL, the substrate is sapphire, and the preparation of ultra-thin two-dimensional nitride

[0096] (1) Preparation of 75 mg / mL sodium tungstate, sodium chromate, and sodium vanadate solutions

[0097] Weigh 3.000 g of sodium tungstate, sodium chromate, and sodium vanadate particles separately using an analytical balance and place them in a clean beaker. Measure 40 mL of deionized water using a graduated cylinder and pour it into the beaker. Use an ultrasonic cleaner to fully dissolve the sodium molybdate. After dissolution, store it using a volumetric flask to obtain sodium tungstate solution, sodium chromate solution, and sodium vanadate solution with a concentration of 75 mg / mL respectively.

[0098] (2) Pretreatment of sapphire substrate

[0099] Clean the sapphire substrate three times in a cycle using absolute ethanol and deionized water, and blow it with a nitrogen gas gun. Then, place the sapphire substrate in an oxygen plasma cleaner for treatment. Set the output power to 18 W, the treatment time to 30 minutes, and the oxygen flow rate to 50 sccm.

[0100] (3) Spin-coating of sodium tungstate, sodium chromate, and sodium molybdate solutions

[0101] The sapphire substrate after hydrophilic treatment should be spin-coated as soon as possible. Use a syringe to separately aspirate 0.2 mL of 75 mg / mL sodium tungstate, sodium chromate, or sodium vanadate solution and drop them onto the surface of a 10*10 mm sapphire respectively. Then, perform spin-coating at 3000 rpm for 30 seconds. The spin-coated sapphire substrate is dried using an infrared heating lamp, and the heating temperature is about 100 °C.

[0102] (4) Preparation of ultrathin two-dimensional nitride single crystals

[0103] Place the sapphire substrate spin-coated with sodium tungstate, sodium chromate, or sodium vanadate solution into a programmable temperature tube furnace. Use high-purity argon as the carrier gas to heat the substrate to 950 °C. After the temperature reaches the set temperature, introduce ammonia gas and keep it warm for 2 hours. After the reaction is completed, turn off the ammonia gas and cool it to room temperature in argon to obtain ultrathin two-dimensional tungsten nitride single crystals. Figure 14 Characterization of ultrathin two-dimensional nitrides prepared using a scanning electron microscope with a concentration of 75 mg / mL for sodium tungstate, sodium chromate, and sodium vanadate. Regular triangular structures appear on the sapphire surface. Among them, Figures 14(a)-(b) are WN, (c)-(d) are CrN, and (e)-(f) are VN.

[0104] Figure 15 X-ray diffraction pattern of the sapphire surface covered with an ultrathin two-dimensional structure. The results show that WN, CrN, and VN are all single crystals.

[0105] Comparative Example 1

[0106] No molybdenum nitride single crystals can be obtained with inappropriate precursor solutions

[0107] (1) Preparation of 75 mg / mL ammonium molybdate solution

[0108] Weigh 3.000 g of ammonium molybdate particles using an analytical balance and place them in a clean beaker. Measure 40 mL of deionized water using a measuring cylinder and pour it into the beaker. Use an ultrasonic cleaner to fully dissolve the ammonium molybdate. After dissolution, store it in a volumetric flask to obtain an ammonium molybdate solution with a concentration of 75 mg / mL.

[0109] (2) Pretreatment of sapphire substrate

[0110] Clean the sapphire substrate three times in a cycle using anhydrous ethanol and deionized water, and blow it with a nitrogen gas gun. Then place the sapphire substrate in an oxygen plasma cleaner for treatment. Set the output power to 18 W, the treatment time to 30 minutes, and the oxygen flow rate to 50 sccm.

[0111] (3) Spin-coating of ammonium molybdate solution

[0112] The sapphire substrate after hydrophilic treatment should be spin-coated as soon as possible. Use a syringe to suck 0.2 mL of 75 mg / mL ammonium molybdate solution and drop it onto the surface of a 10*10 mm sapphire, and then perform spin-coating at 3000 rpm for 30 seconds. The spin-coated sapphire substrate is dried using an infrared heating lamp, and the heating temperature is about 100 °C.

[0113] (4) Preparation of ultrathin two-dimensional molybdenum nitride single crystal

[0114] Place the sapphire substrate spin-coated with ammonium molybdate solution into a temperature-programmed tube furnace. Use high-purity argon as the carrier gas, heat the substrate to 750 °C. After the temperature reaches the set temperature, introduce ammonia gas and keep it warm for 2 hours. After the reaction is completed, turn off the ammonia gas and cool it to room temperature in argon to obtain a two-dimensional molybdenum nitride single crystal. Figure 16 Characterize the prepared two-dimensional molybdenum nitride using a scanning electron microscope with an ammonium molybdate concentration of 75 mg / mL. The sapphire surface presents various structures such as regular quadrilaterals and triangles, and there is no consistent orientation effect in the structure. The lateral size of the triangular structure is about 300 - 500 nm. Figure 17 This is the X-ray photoelectron spectroscopy diagram of the ultrathin two-dimensional structure covering the sapphire surface, indicating that this structure is molybdenum nitride. After calculation, the atomic ratio of molybdenum to nitrogen in this structure is 2:1, and there is a thick oxide layer covering the surface. Figure 18 This is the X-ray diffraction characterization of this structure, and the results show that molybdenum nitride is a polycrystalline structure.

[0115] In summary, the present invention uses a transition metal salt as a precursor, simply controls the growth temperature and the concentration of the precursor, and obtains an ultrathin two-dimensional transition metal nitride single crystal structure that cannot be obtained by the prior art. This method is simple, easy to implement and control, has a wide application range, can be extended to the controllable preparation of other nitrides, and lays a good foundation for the subsequent controllable preparation and application research of nitride thin films.

[0116] The above embodiments are provided only to describe the present invention and are not intended to limit the scope of the present invention. The scope of the present invention is defined by the appended claims. All equivalent substitutions and modifications made without departing from the main idea and principle of the present invention shall be covered within the scope of the present invention.

Claims

1. A method for preparing an ultra-thin two-dimensional transition metal nitride single crystal structure, characterized in that: It includes the following steps: (1) Prepare a precursor solution with a concentration of 75 - 300 mg / mL for standby; (2) Perform surface pretreatment on the single-crystal oxide substrate to remove surface impurities and obtain a hydrophilic surface; (3) Spin-coat the precursor solution on the hydrophilic surface of the substrate using a spin coater, and dry the substrate using a heating device; (4) Place the substrate spin-coated with the precursor solution into a programmed temperature tube furnace. Use an inert gas as the carrier gas, heat the substrate to 950 - 1200 °C. After the temperature reaches the set temperature, introduce ammonia or nitrogen and keep it warm. After the reaction ends, close the ammonia or nitrogen, and cool it to room temperature in argon to obtain an ultra-thin two-dimensional transition metal nitride single crystal structure; In the step (1), the precursor is sodium molybdate, sodium tungstate, sodium chromate, or sodium vanadate; In the step (2), the single-crystal oxide substrate is a sapphire substrate or a strontium titanate substrate.

2. The preparation method according to claim 1, characterized in that: The sapphire substrate is a single-crystal Al2O3(0001) substrate; the strontium titanate substrate includes a niobium-doped strontium titanate substrate and an iron-doped strontium titanate substrate.

3. The preparation method according to claim 1, wherein: The method for surface pretreatment of the single-crystal oxide substrate in the step (2) includes: repeatedly cleaning the substrate surface with ethanol and deionized water respectively, and then treating the substrate in an oxygen plasma cleaner for 15 - 60 minutes. The oxygen or air flow rate introduced into the oxygen plasma cleaner is 20 - 50 sccm.

4. The preparation method according to claim 1, wherein: In the step (3), the spin-coating parameters are 3000 - 5000 rpm, and the spin-coating time is 30 - 60 s; the drying temperature is 100 - 150 °C; in the step (4), the heating rate is 10 - 15 °C per minute; the argon flow rate is 100 - 200 sccm, the ammonia flow rate is 5 - 10 sccm, and the heat preservation time is 2 - 6 hours; the inert gas in the step (4) includes argon.

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