Two-dimensional ultrathin m2az4 crystals and preparation method thereof

Abstract

The present application relates to the field of ultra-thin A-Z single-side passivated M2AZ4 crystal material and chemical vapor deposition (CVD) preparation thereof, in particular to a kind of two-dimensional ultra-thin M2AZ4 crystal and preparation method thereof.In the two-dimensional ultra-thin M2AZ4 crystal, it contains seven atomic layers of Z-A-Z-M-Z-M-Z.Au foil is used as a growth substrate, a thin film containing transition metal M is first deposited on the surface of the Au foil, pre-storage of M source in the Au foil substrate is realized by annealing treatment, Z element is introduced, and M2AZ4 crystal with uniform thickness is grown by reacting with the stored M element precipitated on the surface of the Au foil at high temperature not higher than the melting point of Au, and the Au foil substrate is etched and transferred to any target substrate.The present application has the characteristics of simple preparation process, out-of-plane mirror symmetry breaking structure of the material, uniform thickness, adjustable lateral size, etc.

Description

Technical Field

[0001] This invention relates to the field of two-dimensional ultrathin M2AZ4 crystal materials and their preparation by chemical vapor deposition (CVD), specifically to a type of two-dimensional ultrathin M2AZ4 (M is a transition metal element, A is a group IV element, and Z is a group V element) crystal and its preparation method, which is suitable for the preparation of high-quality, uniform-thickness, ultrathin M2AZ4 crystals. Background Technology

[0002] In recent years, two-dimensional MA2Z4 material systems (such as MoSi2N4) have exhibited a semiconductor bandgap of 1.94 eV, a theoretical carrier mobility more than four times higher than that of MoS2, ultra-high strength of up to 66 GPa, and excellent environmental stability due to their unique seven-atom-layer stacked structure (N-Si-N-Mo-N-Si-N). The "element passivation-structure reconstruction" synthesis strategy established in this study provides a new and universally applicable approach for the preparation of two-dimensional materials, significantly expanding the boundaries of their physical properties and applications.

[0003] Due to breakthroughs in experimental synthesis, researchers have systematically conducted theoretical explorations of two-dimensional MA2Z4 and its derivatives. Density functional theory (DFT) calculations show that this material family has a sandwich structure consisting of seven atomic layers (AZMZAZ), and its chemical composition can be directionally controlled: the M-site atom is an early transition metal (Mo, W, V, Nb, Ti, Zr, Hf, or Cr), the A-site atom is Si or Ge, and the Z-site atom is N, P, or As. Through precise design of element combinations and stacking order, this system exhibits rich quantum properties: for example, α1-MoSi2N4 exhibits strong exciton-phonon coupling quantum behavior and possesses spin polarization and plasmon resonance characteristics; α1-TaSi2N4 and NbSi2N4 increase the superconducting transition temperature to 24.6 K and 30.4 K respectively by passivating TaN2 and NbN2 with Si-N bilayers; the band structure of β2-SrGa2Se4 and SrGa2Te4 exhibits cross-inversion at the Γ point and the nontrivial invariant Z2=1, indicating that they are topological insulators; due to the energy splitting of the spin-up (↑) and spin-down (↓) bands, δ4-VSi2P4 exhibits ferromagnetism; and α1-VSi2N achieves a continuous phase transition from valley half-semiconductor (VHSC) to quantum anomalous Hall state (VQAH) under strain, proving the dynamic tunability of its topological state. These properties enable MA2Z4 materials to demonstrate revolutionary application potential in fields such as spintronic devices, field-effect transistors, and ultrasensitive gas sensors.

[0004] However, the development of this material system currently faces serious bottlenecks, with experimental synthesis lagging far behind theoretical predictions. To date, only centimeter-scale preparations of MoSi₂N₄ and WSi₂N₄ have been achieved. Existing preparation methods struggle to achieve high-quality, uniform thickness, and controllable size growth of M₂AZ₄ materials. As a derivative of the MA₂Z₄ system, the novel structural material M₂AZ₄ exhibits rich quantum behaviors (such as the spin Hall effect and second-order nonlinear optical effects) due to its out-of-plane mirror symmetry-broken structure, showing significant potential in electronics, optoelectronics, catalysis, optics, and topological devices. Therefore, exploring novel M₂AZ₄ materials is of great significance for expanding the family of two-dimensional materials and for researching their corresponding properties and applications. Summary of the Invention

[0005] The purpose of this invention is to provide a type of two-dimensional ultrathin M2AZ4 crystal and its preparation method. Each layer of the two-dimensional ultrathin M2AZ4 crystal consists of seven atomic layers (ZAZMZMZ), forming a vertically asymmetric structure. Its preparation process is simple, the thickness is uniform, the size is easy to control, and the material structure is unique. It has created a variety of MA2Z4 derivatives, expanded the material system, and enriched the physical properties.

[0006] The technical solution of this invention is: A method for preparing a type of two-dimensional ultrathin M2AZ4 crystal uses a highly catalytically active Au foil as a growth substrate. First, a thin film containing a transition metal M is deposited on the surface of the Au foil. The MA source or MZ source is pre-stored in the Au foil substrate by annealing in an environment with an A source or Z source. Then, Z element is introduced and reacted with the stored M element precipitated on the surface of the Au foil in a low-concentration A environment at a high temperature not higher than the melting point of Au to grow a uniform thickness M2AZ4 crystal. Subsequently, the Au foil substrate is etched to transfer it to any target substrate. Among them, M represents transition metal elements, A represents group 4 main elements, and Z represents group 5 main elements.

[0007] The method for preparing the two-dimensional ultrathin M2AZ4 crystal introduces A and Z elements by using a precursor containing A and Z. The precursor is a solid, powder, liquid or gas that volatilizes or decomposes to release A or Z at high temperature.

[0008] The method for preparing the two-dimensional ultrathin M2AZ4 crystal, wherein M is a transition metal element, including but not limited to molybdenum, tungsten, titanium, zirconium, hafnium, vanadium, niobium, tantalum or chromium; A is a group IV element, including but not limited to silicon or germanium, wherein silicon precursors include but not limited to elemental silicon, quartz or silane, and germanium precursors include but not limited to elemental germanium or germane; Z is a group V element, including but not limited to nitrogen, phosphorus or arsenic, wherein nitrogen precursors include but not limited to ammonia or nitrogen, phosphorus precursors include but not limited to white phosphorus or red phosphorus, and arsenic precursors include but not limited to elemental arsenic.

[0009] The method for preparing two-dimensional ultrathin M2AZ4 crystals allows the crystal size to be controlled by changing the deposition thickness of the transition metal M film or the concentration of the A source, ultimately achieving the growth of high-quality, large-size two-dimensional ultrathin crystals.

[0010] The method for preparing the two-dimensional ultrathin M2AZ4 crystal uses a flat Au foil with a purity greater than 99.9 wt% and a thickness of 10 μm to 50 μm. A transition metal M thin film is physically vapor-deposited on the Au foil to obtain a transition metal / gold bimetallic growth substrate. The physical vapor deposition method includes magnetron sputtering or thermal evaporation. The deposition conditions are: deposition rate of 0.01 to 1 nm / s, and the thickness of the transition metal M layer on the Au foil surface is 0.1 to 200 nm. The target materials used include, but are not limited to, pure transition metal targets, alloy targets of transition metal and gold, and alloy targets of transition metal and Group IV or V elements.

[0011] The method for preparing the two-dimensional ultrathin M2AZ4 crystal involves growing a uniform thickness M2AZ4 crystal by chemical vapor deposition at a temperature of 800℃ to 1050℃ and a growth time of 10 minutes to 240 minutes. By extending the growth time, the size of the M2AZ4 crystal is increased. The carrier gas is hydrogen or a mixture of hydrogen and an inert gas.

[0012] The method for preparing the two-dimensional ultrathin M2AZ4 crystal involves uniformly coating a polymer protective layer on the surface of the M2AZ4 crystal before transfer to protect it and prevent damage during subsequent processing. The Au foil substrate is removed by wet chemical etching, and the resulting protective layer / M2AZ4 crystal composite film is transferred to the target substrate. The protective layer is then removed by dissolving it with an organic solvent.

[0013] The preparation method of the two-dimensional ultrathin M2AZ4 crystal uses a polymer that is one or a mixture of two or more of polymethyl methacrylate, polyethylene, polystyrene, and polypropylene. The organic solvent used to remove the polymer protective layer is one or a mixture of two or more of ketones, chlorinated hydrocarbons, halogenated hydrocarbons, and aromatic hydrocarbons.

[0014] The method for preparing the two-dimensional ultrathin M2AZ4 crystal involves etching an Au foil substrate with a gold etching solution. The gold etching solution is composed of potassium iodide and iodine aqueous solution, sodium thiosulfate aqueous solution, potassium bromide and bromine aqueous solution, ferric chloride aqueous solution, hydrogen peroxide and sulfuric acid aqueous solution.

[0015] The two-dimensional ultrathin M2AZ4 crystal prepared by the above method has a structure consisting of seven atomic layers ZAZMZMZ, with out-of-plane mirror symmetry broken and a thickness of no more than 2 nm.

[0016] The design concept of this invention is: Structurally, the two-dimensional ultrathin M2AZ4 crystal can be viewed as a two-dimensional material with a single-sided passivated structure, composed of monolayers and bilayers of Group IV and Group V transition metal compounds, specifically M2Z3. Group V transition metal compounds are a class of non-layered materials. By introducing Group IV element A to form an AZ monolayer, the surface dangling bonds of the monolayer M2Z3 transition metal compound are passivated, changing the island-like growth caused by surface energy constraint to layered growth. This results in a novel type of two-dimensional ultrathin M2AZ4 material composed of seven atomic layers: ZAZMZMZ, exhibiting a vertically asymmetric structure.

[0017] Highly catalytically active Au foil substrate: Au substrate has higher catalytic activity than Cu substrate, and its lattice mismatch rate is only 0.201%, which is beneficial to the growth of M2AZ4 crystals.

[0018] Pre-storage of M source: By employing physical vapor deposition (PVD) technology, a thin film containing transition metal M is deposited on the surface of Au foil, and annealing is performed in an environment with A source or Z source to achieve pre-storage of MA source or MZ source in Au foil substrate, which greatly improves the growth rate of M2AZ4 crystal.

[0019] A third element, Z or A, is introduced and reacted with the stored element that has precipitated onto the surface of the Au foil at a reaction temperature not higher than the melting point of gold.

[0020] The concentration of the A source should be controlled: the concentration of the A source should be controlled during this growth process to achieve efficient and rapid growth of two-dimensional ultrathin M2AZ4 crystals.

[0021] This method combines physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques, enabling the growth of high-quality, uniformly thick, and dimensionally controllable M2AZ4 crystals.

[0022] The advantages and beneficial effects of this invention are: This invention proposes a class of two-dimensional ultrathin M2AZ4 crystal materials, wherein M is a transition metal element, including but not limited to molybdenum, tungsten, titanium, zirconium, hafnium, vanadium, niobium, tantalum, or chromium; A is a Group IV element, including but not limited to silicon or germanium; and Z is a Group V element, including but not limited to nitrogen, phosphorus, or arsenic. Depending on the composition, it exhibits different physical properties, such as the spin Hall effect and second-order nonlinear optical effects.

[0023] 2. The method for preparing a type of two-dimensional ultrathin M2AZ4 crystal proposed in this invention employs chemical vapor deposition, which can be carried out under normal pressure. It is convenient to operate and easy to control, and can efficiently and rapidly obtain two-dimensional ultrathin M2AZ4 crystals. The crystal size depends on the thickness of the transition metal M film and the concentration of the A source. The preparation process is simple, the growth rate is fast, and it is suitable for the preparation of high-quality new structure crystals.

[0024] 3. In the preparation process of this invention, the transition metal M source can be pre-stored inside the Au foil together with the A source after the annealing process, without having to cross the catalytic substrate to reach the Au foil surface for growth, which greatly improves the growth rate of M2AZ4 crystal.

[0025] 4. The present invention can also grow and prepare a single layer of M2AZ4 crystals on the back side of coated Au foil.

[0026] 5. The two-dimensional ultrathin M2AZ4 crystal prepared by this invention is of great significance for creating other novel two-dimensional MA2Z4 system derivatives. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the experimental setup for growing two-dimensional ultrathin M2AZ4 crystals using the CVD method. In the diagram, 1-gas inlet; 2-metal substrate; 21-Au foil; 22-transition metal thin film; 3-heating furnace; 4-quartz tube; 5-gas outlet; Figure 2 A photograph of a 10 μm Mo2SiN4 crystal grown by CVD on a Mo / Au substrate; Figure 3 A photograph of a Mo2SiN4 thin film grown on a Mo / Au substrate by CVD transferred onto a 1 square centimeter SiO2 / Si substrate. Figure 4 The image shows a high-resolution scanning transmission electron micrograph of a single-layer Mo2SiN4 crystal grown by CVD, which demonstrates that the sample has high crystal quality. Figure 5 XPS spectra of monolayer Mo2SiN4 films grown by CVD are shown. Figure a shows the high-resolution spectra of N 1s and Mo 3p, figure b shows the high-resolution spectra of Mo 3d, and figure c shows the high-resolution spectra of Si 2p. Figure 6 A model of the monolayer Mo2SiN4 crystal structure obtained by CVD growth; Figure 7 Results for the Optical Second Harmonic Generation (SHG) test system; Figure 8To characterize the electrical properties of monolayer Mo2SiN4 crystals grown by CVD, Figure a shows the temperature-dependent resistance curves of the monolayer Mo2SiN4 crystal transferred to SiO2 / Si, where the horizontal axis T represents temperature (K) and the vertical axis R represents ohmic resistance (Ohm). The curves in the figure indicate that the Mo2SiN4 crystal is a half-metallic material. Figure b shows the IV test curves of the monolayer Mo2SiN4 crystal transferred to SiO2 / Si, where the horizontal axis V... d Represents voltage (V), with the vertical axis I. d The current (A) is represented by the curve, and the slope K of the linear segment in the figure represents the resistance of the material. Detailed Implementation In its specific implementation, the method for preparing two-dimensional ultrathin M2AZ4 crystals of this invention uses a highly catalytically active Au foil as the growth substrate. Physical vapor deposition (PVD) is employed to deposit a thin film containing transition metal M on the Au foil surface. Annealing in an environment with either an A or Z source achieves pre-storage of the MA or MZ source in the Au foil substrate. Then, a third element, Z or A, is introduced and reacts with the stored element precipitated on the Au foil surface at a reaction temperature not exceeding the melting point of gold. The concentration of the A source is controlled during this growth process, thereby achieving efficient and rapid growth of the two-dimensional ultrathin M2AZ4 crystal. Subsequently, etching of the Au foil substrate transfers the M2AZ4 thin film to any target substrate.

[0028] By using precursors containing A and Z, which are solids, powders, liquids, or gases that volatilize or decompose to release A or Z at high temperatures, the introduction of A and Z elements can be more precisely controlled in terms of amount and rate, thus contributing to the uniform growth of M2AZ4 crystals. Furthermore, by selecting appropriate precursor morphologies, different experimental conditions and equipment requirements can be accommodated, improving the flexibility and applicability of the preparation method.

[0029] A flat Au foil with a purity greater than 99.9 wt% and a thickness of 10 μm to 50 μm is used. A transition metal M thin film is deposited on the Au foil by physical vapor deposition to obtain a transition metal / gold bimetallic growth substrate. The physical vapor deposition method includes magnetron sputtering or thermal evaporation. The deposition conditions are: deposition rate 0.01 to 1 nm / s, and the thickness of the transition metal M layer on the Au foil surface is 0.1 to 200 nm. The target materials used include, but are not limited to, pure transition metal targets, alloy targets of transition metal and gold, and alloy targets of transition metal and Group IV or V elements.

[0030] Uniform thickness M2AZ4 crystals were grown using chemical vapor deposition at temperatures ranging from 800℃ to 1050℃ for growth times ranging from 10 minutes to 240 minutes. Extending the growth time increased the size of the M2AZ4 crystals. Hydrogen or a mixture of hydrogen and an inert gas was used as the carrier gas. These parameters are crucial to the growth quality and efficiency of the M2AZ4 crystals. Hydrogen, as a carrier gas, provides a reducing environment, which is conducive to crystal formation. The temperature range ensures that the reaction proceeds below the melting point of gold while providing sufficient energy to promote crystal growth. The growth time range allows for adjustment based on specific circumstances to obtain optimal crystal quality and size.

[0031] Before transfer, a polymer protective layer is uniformly coated on the surface of the M2AZ4 crystal to protect it and prevent damage during subsequent processing. The Au foil substrate is removed by wet chemical etching, which is a simple and effective method suitable for processing large-area samples. The resulting protective layer / M2AZ4 crystal composite film is transferred to the target substrate, and the protective layer is removed by dissolving it with an organic solvent. This step does not cause additional damage to the M2AZ4 crystal.

[0032] The polymer used is one or a mixture of two or more of polymethyl methacrylate, polyethylene, polystyrene, and polypropylene. The organic solvent used to remove the protective layer of the polymer is one or a mixture of two or more of ketones, chlorinated hydrocarbons, halogenated hydrocarbons, and aromatic hydrocarbons.

[0033] The Au foil substrate was etched using a gold etching solution, which consisted of potassium iodide and iodine aqueous solution, sodium thiosulfate aqueous solution, potassium bromide and bromine aqueous solution, ferric chloride aqueous solution, and hydrogen peroxide and sulfuric acid aqueous solution. The two-dimensional ultrathin M2AZ4 crystal prepared by this invention has a structure consisting of seven atomic layers ZAZMZMZ, with out-of-plane mirror symmetry broken and a thickness not exceeding 2nm.

[0034] The CVD growth of two-dimensional ultrathin M2AZ4 crystals, taking the two-dimensional transition metal silicon-nitrogen ternary compound Mo2SiN4 as an example, involves the following specific steps: A transition metal M thin film is physically vapor-deposited on Au foil to obtain a transition metal / gold bimetallic layer growth substrate. A silicon wafer or quartz wafer is placed above or in front of the growth substrate as a silicon source, and ammonia is used as a nitrogen source. The composite substrate is annealed at high temperature and in a carrier gas atmosphere to achieve pre-storage of transition metals M and Si in the Au foil substrate. Subsequently, N element is introduced. At a reaction temperature not exceeding the melting point of gold, the concentration of Si source is controlled to allow M and Si atoms that have diffused uniformly to the catalytically active Au foil substrate surface to react with the introduced N atoms and grow a two-dimensional ultrathin Mo2SiN4 crystal.

[0035] The growth substrate used is a transition metal M thin film / gold bimetallic layer. The thickness of the transition metal M layer deposited by physical vapor deposition is 0.1–200 nm, preferably 0.1–2 nm, and the deposition rate is 0.01–1 nm / s, preferably 0.01–0.03 nm / s. The thickness of the Au foil used is 10 μm–50 μm, preferably 10 μm–20 μm, and the purity is 99.9 wt%–99.999 wt%, preferably 99.999 wt%. The nitrogen source used in the CVD growth process is ammonia or nitrogen, and the silicon source is silicon wafer, quartz wafer, or silane. When growing other two-dimensional M2AZ4 crystals, the germanium source is germanium wafer or germanane, the phosphorus source is elemental phosphorus, and the arsenic source is elemental arsenic. The carrier gas used for CVD growth is hydrogen or a mixture of hydrogen and an inert gas (carrier gas flow rate of 20 mL / min to 1000 mL / min). The CVD growth temperature is 800℃ to 1050℃, preferably 940℃ to 1040℃; the growth time is 30 min to 240 min, preferably 60 min to 180 min; the cooling rate after the reaction is 10℃ / min to 600℃ / min, preferably 200℃ / min to 600℃ / min.

[0036] (2) Coating of protective layer: A layer of polymer is uniformly coated on the surface of two-dimensional ultrathin M2AZ4 crystal to prevent it from being damaged during subsequent processing; these polymers are one or more of polymethyl methacrylate, polyethylene, polystyrene and polypropylene.

[0037] (3) Dissolution of Au foil substrate: The Au foil substrate is removed by dissolving it with gold etching solution to obtain a protective layer / two-dimensional ultrathin M2AZ4 composite film; the dissolving solution for removing the Au foil substrate is potassium iodide and iodine aqueous solution, sodium thiosulfate aqueous solution, potassium bromide and bromine aqueous solution, ferric chloride aqueous solution, hydrogen peroxide and sulfuric acid aqueous solution, etc., and the molar concentration of the dissolving solution is 0.1 mol / L to 5 mol / L.

[0038] (4) Removal of the protective layer: The obtained protective layer / two-dimensional ultrathin M2AZ4 composite film is placed on the target substrate, and the protective layer covering the surface of the two-dimensional M2AZ4 crystal is dissolved and removed with an organic solvent. The organic solvent used to remove the polymer protective layer is one or more of the following: acetone, ethyl lactate, dichloroethane, trichloroethylene, chloroform, etc., ketones, chlorinated hydrocarbons, halogenated hydrocarbons, aromatic hydrocarbons, etc.

[0039] The two-dimensional ultrathin M2AZ4 crystal obtained by this invention can have a size of up to 18 μm. The film size depends on the thickness of the transition metal M film and the concentration of the A source in physical vapor deposition. The prepared MA2Z4 crystal has a uniform composition and is strictly controlled to be a single-layer crystal with a thickness of about 1 nm, exhibiting high crystal quality.

[0040] The present invention will now be described in further detail with reference to embodiments and accompanying drawings. Example 1

[0041] First, such as Figure 1 As shown, this invention uses a horizontal reactor to grow two-dimensional ultrathin Mo2SiN4 crystals. The horizontal reactor has a gas inlet 1 and a gas outlet 5 at each end. A molybdenum / gold bimetallic substrate foil (molybdenum obtained by physical vapor deposition on Au foil, with a thickness of 0.4 nm and a molybdenum target purity of 99.95 wt%; Au foil 0.5 cm × 0.5 cm × 10 μm, with a purity of 99.999 wt%) is placed in the central high-temperature region of the horizontal reactor (furnace tube diameter 22 mm, reaction zone length 20 mm). Annealing is performed by heating to 1040 °C in a hydrogen atmosphere and holding for 0.5 hours (hydrogen flow rate during heating is 200 mL / min, heating rate is 20 °C / min). Then, a mixture of ammonia and hydrogen gas (gas flow rates are 7 mL / min for ammonia and 200 mL / min for hydrogen) is introduced to begin the growth of two-dimensional Mo2SiN4. The growth time is 1 hour. After growth, the gas is discharged at 500 °C. Rapid cooling at a rate of ℃ / min yielded Mo2SiN4 crystals with a thickness of approximately 1 nm on the Au foil surface.

[0042] Then, a polymethyl methacrylate (PMMA) ethyl lactate solution (PMMA accounting for 4 wt%) was dropped onto the surface of an Au foil on which Mo2SiN4 crystals were grown. A PMMA film with a thickness of 200 nm was formed by spin coating at 5000 rpm. After baking at 130 °C for 10 minutes, the film was placed in an aqueous solution of 1 mol / L potassium iodide and 0.01 mol / L iodine and reacted at 25 °C for 60 minutes to dissolve the Au foil substrate, thus transferring the PMMA / Mo2SiN4 film onto a SiO2 / Si substrate. The PMMA was then dissolved with acetone at 55 °C, finally achieving the successful transfer of Mo2SiN4 crystals.

[0043] The composition, crystal structure, morphology and thickness of Mo2SiN4 crystals were characterized using optical microscopy, transmission electron microscopy, atomic force microscopy and optical second harmonic generation (SHG) testing system. The results showed that the obtained monolayer Mo2SiN4 crystal with trigonal structure was triangular, with an average size of 7 μm and a thickness of about 1 nm. It exhibited strict two-dimensional ultrathin film, high crystal quality and half-metal properties. Example 2

[0044] First, such as Figure 1As shown, this invention uses a horizontal reactor to grow two-dimensional ultrathin Mo2SiN4 crystals. The horizontal reactor has a gas inlet 1 and a gas outlet 5 at each end. A molybdenum / gold bimetallic substrate foil (molybdenum obtained by physical vapor deposition on Au foil, with a thickness of 0.4 nm and a molybdenum target purity of 99.95 wt%; Au foil 0.5 cm × 0.5 cm × 10 μm, with a purity of 99.999 wt%) is placed in the central high-temperature region of the horizontal reactor (furnace tube diameter 22 mm, reaction zone length 20 mm). Annealing is performed by heating to 1040 °C in a hydrogen atmosphere and holding for 0.5 hours (hydrogen flow rate 200 mL / min, heating rate 20 °C / min). Then, a mixture of ammonia and hydrogen is introduced (ammonia flow rate 7 mL / min, hydrogen flow rate 200 mL / min), and two-dimensional Mo2SiN4 growth begins. The growth time is 3 hours. After growth, the solution is heated to 500 °C. Rapid cooling at a rate of ℃ / min yielded Mo2SiN4 crystals with a thickness of approximately 1 nm on the Au foil surface.

[0045] Then, a polymethyl methacrylate (PMMA) ethyl lactate solution (PMMA accounting for 4 wt%) was dropped onto the surface of an Au foil on which Mo2SiN4 crystals were grown. A PMMA film with a thickness of 200 nm was formed by spin coating at 5000 rpm. After baking at 130 °C for 10 minutes, the film was placed in an aqueous solution of 1 mol / L potassium iodide and 0.01 mol / L iodine and reacted at 25 °C for 60 minutes to dissolve the Au foil substrate, thus transferring the PMMA / Mo2SiN4 film onto a SiO2 / Si substrate. The PMMA was then dissolved with acetone at 55 °C, finally achieving the successful transfer of Mo2SiN4 crystals.

[0046] The composition, crystal structure, morphology and thickness of Mo2SiN4 crystal were characterized using optical microscopy, transmission electron microscopy, atomic force microscopy and optical second harmonic generation (SHG) testing system. The results showed that the obtained monolayer Mo2SiN4 crystal with trigonal structure was trapezoidal, with an average size of 12 μm and a thickness of about 1 nm. It exhibited strict two-dimensional ultrathin film, high crystal quality and half-metal properties. Example 3

[0047] First, such as Figure 1As shown, this invention uses a horizontal reactor to grow two-dimensional ultrathin Mo2SiN4 crystals. The horizontal reactor has a gas inlet 1 and a gas outlet 5 at each end. A molybdenum / gold bimetallic substrate foil (molybdenum obtained by physical vapor deposition on Au foil, with a thickness of 0.4 nm and a molybdenum target purity of 99.95 wt%; Au foil 0.5 cm × 0.5 cm × 10 μm, with a purity of 99.999 wt%) is placed in the central high-temperature region of the horizontal reactor (furnace tube diameter 22 mm, reaction zone length 20 mm). Annealing is performed by heating to 1040 °C in a hydrogen atmosphere and holding for 0.5 hours (hydrogen flow rate 200 mL / min, heating rate 20 °C / min). Then, a mixture of ammonia and hydrogen is introduced (ammonia flow rate 4 mL / min, hydrogen flow rate 200 mL / min), and the growth of two-dimensional Mo2SiN4 begins. The growth time is 1 hour. After growth, the solution is heated to 500 °C. Rapid cooling at a rate of ℃ / min yielded Mo2SiN4 crystals with a thickness of approximately 1 nm on the Au foil surface.

[0048] Then, a polymethyl methacrylate (PMMA) ethyl lactate solution (PMMA accounting for 4 wt%) was dropped onto the surface of an Au foil on which Mo2SiN4 crystals were grown. A PMMA film with a thickness of 200 nm was formed by spin coating at 5000 rpm. After baking at 130 °C for 10 minutes, the film was placed in an aqueous solution of 1 mol / L potassium iodide and 0.01 mol / L iodine and reacted at 25 °C for 60 minutes to dissolve the Au foil substrate, thus transferring the PMMA / Mo2SiN4 film onto a SiO2 / Si substrate. The PMMA was then dissolved with acetone at 55 °C, finally achieving the successful transfer of Mo2SiN4 crystals.

[0049] The composition, crystal structure, morphology and thickness of Mo2SiN4 crystal were characterized using optical microscopy, transmission electron microscopy, atomic force microscopy and optical second harmonic generation (SHG) testing system. The results showed that the obtained monolayer Mo2SiN4 crystal with trigonal structure was triangular, with an average size of 5 μm and a thickness of about 1 nm. It exhibited strict two-dimensional ultrathin film, high crystal quality and half-metal properties.

[0050] like Figure 1 The diagram shows an experimental setup for growing high-quality two-dimensional ultrathin M2AZ4 thin films using the CVD method of this invention. Taking the two-dimensional transition metal silicon-nitrogen ternary compound M2SiN4 as an example, it mainly includes a gas inlet 1, a metal substrate 2 (Au foil 21, transition metal thin film 22), a heating furnace 3, a quartz tube 4, and a gas outlet 5. The carrier gas enters the quartz tube 4 of the heating furnace 3 from the gas inlet 1 and exits from the gas outlet 5. The metal substrate 2 is composed of Au foil 21 and an upper transition metal thin film 22.

[0051] like Figure 2 The image shows Mo2SiN4 crystals obtained by CVD on a Mo / Au substrate. The image shows that the material has uniform and complete crystal domains on its surface.

[0052] like Figure 3 As shown, the two-dimensional ultrathin Mo2SiN4 crystals obtained by CVD on Mo / Au substrates can be completely transferred to SiO2 / Si and PET substrates. The optical contrast of the transferred samples is basically consistent, indicating that the Mo2SiN4 crystals have uniform thickness.

[0053] like Figure 4 As shown, transmission electron microscopy characterization results indicate that the Mo2SiN4 crystals prepared by this CVD method have high crystal quality.

[0054] like Figure 5 As shown, X-ray photoelectron spectroscopy (XPS) results indicate that Mo 3p at approximately 395 eV... 3 / 2 The peaks indicate the presence of Mo-N bonds. According to Mo 3d... 3 / 2 and Mo 3d 5 / 2 The peak positions indicate that all molybdenum species in Mo2SiN4 are in a single valence state, suggesting high crystal quality. Furthermore, the N 1s peak can be decomposed into two peaks at approximately 397.1 eV and 398.2 eV, corresponding to Mo-N and Si-N bonds, respectively. The Si 2p peak at approximately 102.0 eV... 3 / 2 The peaks confirmed the presence of Si-N bonds, and no Si-O bond peaks (approximately 103.8 eV) were observed.

[0055] like Figure 6 The crystal structure model of Mo2SiN4 is shown below.

[0056] like Figure 7 As shown, the optical second harmonic generation (SHG) test system results indicate that the Mo2SiN4 crystal has high crystal quality.

[0057] like Figure 8 As shown in Figure a, the two-dimensional ultrathin Mo2SiN4 crystal obtained by the CVD method exhibits half-metallic properties. Figure b shows the two-dimensional ultrathin Mo2SiN4 crystal obtained by the CVD method with a resistivity of 22.1774 Ω·m.

[0058] The above results demonstrate that this invention uses Au foil as a growth substrate and employs physical vapor deposition (PVD) technology to deposit a thin film containing transition metal M on the Au foil surface. By annealing in an environment with either an A or Z source, the MA or MZ source is pre-stored in the Au foil substrate. Then, a third element, Z or A, is introduced, reacting with the stored element precipitated on the Au foil surface at a temperature not exceeding the melting point of gold. The concentration of the A source is controlled during this growth process, thereby achieving efficient and rapid growth of two-dimensional ultrathin Mo2SiN4 crystals. The Mo2SiN4 crystals can be transferred cleanly and without damage to any target substrate by etching away the low-melting-point metal layer. This invention features a simple preparation process, efficient and rapid preparation flow, easily controllable product thickness and size, and a unique material structure. Furthermore, the two-dimensional layered Mo2SiN4 crystals obtained by this combined PVD and CVD preparation method are strictly controlled to be a single layer with a uniform structure, high crystallinity, excellent environmental, chemical, thermal stability, and mechanical properties, as well as a unique asymmetric structure. This efficient and rapid preparation method makes it possible to industrialize high-quality two-dimensional ultrathin Mo2SiN4 crystals in fields such as electronic devices, optoelectronic devices, valley electronics, energy storage, catalysis, and sensors.

Claims

1. A method for preparing a type of two-dimensional ultrathin M2AZ4 crystal, characterized in that, Using a highly catalytically active Au foil as the growth substrate, a thin film containing transition metal M is first deposited on the surface of the Au foil. The MA source or MZ source is pre-stored in the Au foil substrate by annealing in an environment with an A source or Z source. Then, Z element is introduced and reacted with the stored M element precipitated on the surface of the Au foil at a high temperature not higher than the melting point of Au and in a low-concentration A environment to grow a uniform thickness M2AZ4 crystal. Subsequently, the Au foil substrate is etched to transfer it to any target substrate. Among them, M represents transition metal elements, A represents group 4 main elements, and Z represents group 5 main elements.

2. The method for preparing two-dimensional ultrathin M2AZ4 crystal according to claim 1, characterized in that, A and Z elements are introduced by using precursors containing A and Z. The precursors are solids, powders, liquids or gases that volatilize or decompose to release A or Z at high temperatures.

3. The method for preparing two-dimensional ultrathin M2AZ4 crystal according to claim 2, characterized in that, M includes molybdenum, tungsten, titanium, zirconium, hafnium, vanadium, niobium, tantalum, or chromium; A includes silicon or germanium, with silicon precursors including elemental silicon, quartz, or silane, and germanium precursors including elemental germanium or germanane; Z includes nitrogen, phosphorus, or arsenic, with nitrogen precursors including ammonia or nitrogen gas, phosphorus precursors including white phosphorus or red phosphorus, and arsenic precursors including elemental arsenic.

4. The method for preparing a two-dimensional ultrathin M2AZ4 crystal according to claim 1, characterized in that, The size of the two-dimensional M2AZ4 crystal can be controlled by changing the deposition thickness of the transition metal M film or the concentration of the A source.

5. The method for preparing a two-dimensional ultrathin M2AZ4 crystal according to claim 1, characterized in that, A smooth Au foil with a purity greater than 99.9 wt% and a thickness of 10 μm to 50 μm is used. A transition metal M thin film is deposited on the Au foil by physical vapor deposition to obtain a transition metal / gold bimetallic growth substrate. The physical vapor deposition method includes magnetron sputtering or thermal evaporation. The deposition conditions are: deposition rate 0.01 to 1 nm / s, and the thickness of the transition metal M layer on the Au foil surface is 0.1 to 200 nm. The target materials used include pure transition metal targets, alloy targets of transition metal and gold, and alloy targets of transition metal and Group IV or V elements.

6. The method for preparing a two-dimensional ultrathin M2AZ4 crystal according to claim 1, characterized in that, M2AZ4 crystals of uniform thickness were grown by chemical vapor deposition at temperatures ranging from 800℃ to 1050℃ and growth times ranging from 10 minutes to 240 minutes. The size of the M2AZ4 crystals was increased by extending the growth time. The carrier gas was hydrogen or a mixture of hydrogen and an inert gas.

7. The method for preparing a two-dimensional ultrathin M2AZ4 crystal according to claim 1, characterized in that, Before transfer, a polymer protective layer is uniformly coated on the surface of the M2AZ4 crystal to protect it and prevent damage during subsequent processing. The Au foil substrate is removed by wet chemical etching, and the resulting protective layer / M2AZ4 crystal composite film is transferred to the target substrate. The protective layer is then removed by dissolving it with an organic solvent.

8. The method for preparing a two-dimensional ultrathin M2AZ4 crystal according to claim 7, characterized in that, The polymer used is one or a mixture of two or more of polymethyl methacrylate, polyethylene, polystyrene, and polypropylene. The organic solvent used to remove the protective layer of the polymer is one or a mixture of two or more of ketones, chlorinated hydrocarbons, halogenated hydrocarbons, and aromatic hydrocarbons.

9. The method for preparing a two-dimensional ultrathin M2AZ4 crystal according to claim 7, characterized in that, The Au foil substrate was etched using a gold etching solution, which consisted of potassium iodide and iodine aqueous solution, sodium thiosulfate aqueous solution, potassium bromide and bromine aqueous solution, ferric chloride aqueous solution, hydrogen peroxide and sulfuric acid aqueous solution.

10. A two-dimensional ultrathin M2AZ4 crystal prepared by the method of claim 1, characterized in that, The M2AZ4 crystal structure consists of seven atomic layers: ZAZMZMZ, with out-of-plane mirror symmetry broken and a thickness not exceeding 2 nm.

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