Preparation method of 3R phase multilayer ternary compound MA2Z4 material

Abstract

The application relates to the field of two-dimensional layered ternary compounds and chemical vapor deposition (CVD) preparation thereof, in particular to a preparation method of 3R phase multilayer ternary compound MA2Z4 material. A liquid Cu-Au alloy is used to coat a transition metal M to form a growth substrate, A elements and Z elements are introduced at a temperature higher than the melting point of the alloy, a 3R phase multilayer MA2Z4 ternary compound crystal is grown through CVD, and the electrochemical bubbling method is used to transfer the crystal to any substrate. The application has the characteristics of simple preparation process, easy control of product composition, thickness and size, and obtainment of 3R phase crystals with complete ABC stacking, and the SHG behavior of the product is enhanced with the increase of the layer number, which proves the expandable strong polarization characteristics, lays a foundation for the research and application of two-dimensional MA2Z4 crystals in the fields of nonlinear optics, piezoelectricity, valley spin, sliding ferroelectricity, bulk photovoltaic and the like.

Description

Technical fields:

[0001] This invention relates to the field of two-dimensional layered ternary compounds and their preparation by chemical vapor deposition (CVD), specifically a method for preparing a 3R phase multilayer ternary compound material MA2Z4. Background technology:

[0002] Since the discovery of graphene in 2004, its atomic-level thickness and unique structure have demonstrated excellent optical and electrical properties, showing great promise for applications in electronic / optoelectronic devices. Subsequently, the family of two-dimensional materials has been continuously enriched, with transition metal chalcogenides (TMDs) representing two-dimensional semiconductors injecting fresh vitality into the post-Moore's Law integrated circuit industry. To meet development needs, various methods such as doping and crystal phase control have been developed to regulate the performance of two-dimensional semiconductors. Different stacking configurations lead to different interlayer coupling effects and band hybridization modes, resulting in unique effects on property regulation. For example, unlike the 2H phase, the odd and even layers of the 3R phase compounds in the ABC stacking of TMDs are non-centrosymmetric, resulting in strong polarization, which has unique applications in nonlinear optics, piezoelectricity, and slip ferroelectricity. However, the 3R phase of most layered materials is metastable and currently only grows on non-metallic substrates such as silicon wafers and mica. Crystal quality is difficult to control, and due to energy constraints, it is difficult to obtain samples with completely 3R stacking. Therefore, selecting a suitable substrate to prepare stable and homogeneous 3R-phase two-dimensional layered materials is even more challenging. Furthermore, current stacking research is limited to monocomponent graphene and bicomponent TMDs, while further increasing the number of components will bring new degrees of freedom to performance regulation. Therefore, how to select a suitable substrate to prepare fully 3R-stacked multicomponent two-dimensional layered materials is of great significance for scientific research and application development.

[0003] Chinese invention patent publication CN113718227A proposes a class of two-dimensional layered ternary compounds and their preparation method. The molecular formula of the two-dimensional layered ternary compound is MX2Y4 (equivalent to MA2Z4 in this invention, where A corresponds to X and Z corresponds to Y), where M is a transition metal element, X is a Group 4 element, and Y is a Group 5 element. Each layer consists of 7 atomic layers: YXYMYXY, and the layers are bonded together by van der Waals forces. The emerging MA2Z4 material not only possesses ultra-high mechanical strength and high theoretical carrier mobility but also excellent stability. Numerous theoretical calculations show that MA2Z4 also exhibits excellent piezoelectricity and high thermal conductivity, indicating broad application prospects. However, compared to graphene and binary materials, the ternary MA2Z4 crystal structure and chemical composition are more complex, making controlled preparation more difficult. Currently, only centimeter-scale monolayer MoSi2N4 films and micron-scale monolayer WSi2N4 domains can be prepared using Cu / Mo or Cu / W solid substrates. By increasing the nitrogen source concentration, few-layer MoSi2N4 and WSi2N4 domains with random stacking of less than 5 μm were obtained, exhibiting disordered stacking structures. This is because the ordered lattice of the solid substrate presents a high diffusion barrier to the underlying metal, making source supply difficult and thus posing a challenge to the fabrication of multilayer MA2Z4 samples.

[0004] Both Cu and Au possess excellent catalytic activity and have been applied in the growth of graphene, boron nitride, and TMDs, respectively. However, there are currently no reports of using an alloy of the two to grow two-dimensional materials. Phase diagrams show that they are infinitely miscible in any ratio, with a minimum melting point of 900℃. The liquid substrate exhibits a lower interaction potential, making it easier to induce rotational alignment of multilayer crystals. Therefore, designing a highly catalytically active liquid Cu-Au alloy substrate holds promise for achieving controlled growth of ordered stacked multilayer MA2Z4 crystals, laying the foundation for subsequent research on high-performance nonlinear optical responses, piezoelectricity, slip ferroelectricity, and other physical properties and applications. Summary of the Invention:

[0005] The purpose of this invention is to provide a method for preparing a 3R-phase multilayer ternary compound, MA2Z4. A liquid Cu-Au alloy is used as the growth substrate. This substrate not only promotes the cleavage and recombination of molecules or groups containing elements such as A and Z due to its high catalytic activity, but also facilitates the continuous and stable diffusion of M atoms in the bottom layer due to its high degree of disorder, thereby growing an energy-dominant 3R-phase multilayer MA2Z4 crystal. The 3R-phase multilayer MA2Z4 crystal grown on the Cu-Au alloy substrate can be transferred to any target substrate via an electrochemical bubbling method, and the substrate can be reused.

[0006] The technical solution of this invention is:

[0007] A method for preparing a 3R-phase multilayer ternary compound MA2Z4 material, using liquid Cu-Au alloy-coated transition metal foil as a substrate, and growing the 3R-phase multilayer ternary compound by chemical vapor deposition, includes the following steps:

[0008] Step 1: Coat Cu foil, foamed Cu or Cu powder, Au foil, foamed Au or Au powder sequentially onto transition metal M foil, then place them in a quartz boat, and heat them in a tube furnace to above the melting point of copper under a hydrogen atmosphere to fully melt and uniformly alloy them, thus obtaining a liquid Cu-Au alloy / transition metal composite substrate.

[0009] Step 2: Lower the temperature, introduce a carrier gas to introduce elements A and Z, and use the transition metal diffused from the bottom layer of transition metal foil to provide the M source for the growth of 3R phase multilayer MA2Z4 crystals; wherein M is a transition metal element, including but not limited to Mo, W, Ti, Zr, Hf, V, Nb or Ta; A is a Group 4 element, including but not limited to Si or Ge; and Z is a Group 5 element, including but not limited to N, P or As.

[0010] Step 3: After growth is complete, the quartz boat along with the growth substrate is quickly pushed to the low temperature zone, hydrogen and other growth carrier gases are turned off and argon gas is introduced for rinsing, to obtain 3R phase multilayer MA2Z4 crystals on Cu-Au / M substrate.

[0011] Step 4: Spin-coat the sample surface with a polymethyl methacrylate lactate solution and dry it. Then, use an electrochemical bubbling method to transfer the 3R phase multilayer MA2Z4 crystals onto any substrate.

[0012] In the preparation method of the 3R phase multilayer ternary compound MA2Z4 material, in step 1, the precursors for forming the Cu-Au alloy are Cu foil, Cu foam or Cu powder, and Au foil, Au foam or Au powder; wherein, when Cu foil and Au foil are used, their thicknesses are 12.5-100 μm and 10-100 μm, respectively; when Cu foam and Au foam are used, their thicknesses are 50-500 μm and 50-500 μm, respectively; when Cu powder and Au powder are used, the Cu powder mesh size is 100-400 mesh, and the Au powder particle diameter is 0.8-1.5 μm.

[0013] In the preparation method of the 3R phase multilayer ternary compound MA2Z4 material, in step 1, the thickness of the transition metal M foil is 50-500 μm.

[0014] In the preparation method of the 3R phase multilayer ternary compound MA2Z4 material, in step 1, hydrogen is used as a carrier gas and needs to be introduced throughout the process and kept at a constant flow rate of 50-1000 sccm, preferably 200-500 sccm.

[0015] In the preparation method of the 3R phase multilayer ternary compound MA2Z4 material, in step 1, the temperature of the tube furnace is raised to above the melting point of copper to fully melt and uniformly alloy the two materials. The temperature range is 1085℃~1200℃, and the holding time is 1~10 minutes.

[0016] In the preparation method of the 3R phase multilayer ternary compound MA2Z4 material, in step 2, after uniform alloying, the temperature is reduced to 900℃~1060℃, which needs to be higher than the melting point of the alloy. The specific temperature depends on the Cu-Au atomic ratio.

[0017] In the preparation method of the 3R phase multilayer ternary compound MA2Z4 material, in step 2, the type and flow rate of the carrier gas for introducing the Z element depend on the element. Specifically: when introducing the N element, NH3 needs to be introduced, and the NH3 gas flow rate is 2-10 sccm, preferably 3-5 sccm; when introducing the P element, PH3 needs to be introduced, and the gas flow rate is 2-20 sccm; when introducing the As element, AsH3 needs to be introduced, and the gas flow rate is 2-20 sccm.

[0018] In the preparation method of the 3R phase multilayer ternary compound MA2Z4 material, in step 2, the growth time of the 3R phase multilayer MA2Z4 crystal is 1 to 24 hours.

[0019] In the preparation method of the 3R phase multilayer ternary compound MA2Z4 material, in step 4, the electrolyte used in the electrochemical bubbling method is a 1 mol / L NaOH aqueous solution, and a DC power supply with a constant current of 0.1-0.2 mA is used.

[0020] The method for preparing the 3R phase multilayer ternary compound MA2Z4 material is described above. In the 3R phase multilayer ternary compound MA2Z4 material, each layer consists of 7 atomic layers ZAZMZAZ, and the layers are bonded together by van der Waals forces. The number of layers is two or more, and the stacking method is ABC.

[0021] The design principle of this invention is:

[0022] This invention proposes a liquid Cu-Au alloy as a growth substrate. Both Cu and Au possess excellent catalytic activity and are widely used in the growth of graphene, boron nitride, and TMDs, respectively. However, there are currently no reports of using an alloy of these two materials to grow two-dimensional materials. According to the Cu-Au phase diagram, the two are infinitely miscible in any ratio, with a minimum melting point of 900℃. Therefore, temperatures lower than the melting points of pure copper and pure gold can be designed to prepare the liquid alloy substrate. The liquid alloy substrate has two advantages: firstly, its high degree of disorder and high catalytic activity facilitate a continuous supply of the source, leading to the layer-by-layer growth of multilayer crystals; secondly, its lower surface interaction potential makes it easier to promote rotational alignment between the layers of the multilayer crystal. Theoretical calculations show that the 3R phase MA2Z4 is more energy-dominant, therefore, high-quality multilayer MA2Z4 crystals with complete 3R stacking are more easily obtained on the Cu-Au alloy liquid substrate. Therefore, the method of the present invention can not only achieve the growth of two-dimensional materials on liquid substrates at relatively low temperatures, thereby saving energy, but also obtain multilayer 3R phase crystals with complete ABC stacking, injecting fresh blood into the development of nonlinear optics, piezoelectricity, valley spin, slip ferroelectricity and other fields.

[0023] The advantages and beneficial effects of this invention are:

[0024] 1. This invention uses chemical vapor deposition to prepare a 3R phase multilayer ternary compound MA2Z4 material, which is based on a liquid Cu-Au alloy substrate at a relatively low temperature. It can be transferred non-destructively to any substrate by electrochemical bubbling. The substrate can be reused. Compared with existing methods for preparing MA2Z4, the method of this invention is more cost-effective.

[0025] 2. In the 3R phase multilayer ternary compound MA2Z4 material of the present invention, the M element can be various elements such as Mo and W, which has a certain degree of universality.

[0026] 3. The 3R phase multilayer ternary compound MA2Z4 material of the present invention is prepared by chemical vapor deposition. All crystals grown on the substrate are ABC stacked and strictly controlled to be 3R phase.

[0027] 4. The 3R phase multilayer ternary compound MA2Z4 material obtained by the method of the present invention has a second harmonic response (SHG) signal that increases with the number of layers, indicating that it has non-centrosymmetric characteristics and strong polarization performance. Moreover, this polarization characteristic can be accumulated and extended with the number of layers, and has potential application value in nonlinear optics, piezoelectricity, slip ferroelectricity and other fields.

[0028] 5. The 3R phase multilayer ternary compound MA2Z4 material of this invention has the advantages of simple preparation process, easy control of product composition, thickness and size, and the ability to obtain 3R phase crystals with complete ABC stacking. Moreover, the enhanced SHG behavior of its product with increasing number of layers confirms its scalable strong polarization characteristics, laying the foundation for the research and application of two-dimensional MA2Z4 crystals in nonlinear optics, piezoelectricity, valley spin, slip ferroelectricity, bulk photovoltaics and other fields. Attached image description:

[0029] Figure 1 This invention presents a schematic diagram of the chemical vapor deposition process for preparing the 3R phase multilayer ternary compound MA2Z4 material.

[0030] Figure 2 Optical microscope (a), scanning electron microscope (b), and atomic force microscope (c) images of 3R phase multilayer WSi2N4 crystals prepared by CVD on liquid Cu-Au alloy substrate; and thickness distribution curve (d) of 3R phase multilayer WSi2N4 crystals, where the horizontal axis Distance is the distance to the surface (μm) and the vertical axis Relative height is the relative thickness (nm).

[0031] Figure 3 Raman spectra of 3R-phase multilayer WSi₂N₄ crystals prepared by CVD on a liquid Cu-Au alloy substrate, depending on the number of layers. In the figure, the horizontal axis represents the Raman shift (cm). -1 The vertical axis, Normalized Raman Intensity, represents the intensity of the silicon peak (520.7 cm⁻¹). -1 Normalized relative intensity (arb.units).

[0032] Figure 4 The following are the layer-dependent second harmonic (SHG) response spectra of 3R-phase multilayer WSi2N4 crystals prepared by CVD on a liquid Cu-Au alloy substrate: (a) SHG Wavelength (nm) represents the wavelength of the optical second harmonic, and Intensity (arb.units) represents the relative intensity count; and (b) linear relationship curve of SHG signal intensity of 3R-phase multilayer WSi2N4 crystals with the number of layers, where layer number (n) is the horizontal axis and square root of intensity (arb.units) is the vertical axis.

[0033] Figure 5Light micrograph (a) of 3R phase multilayer MoSi2N4 crystal prepared by CVD on a liquid Cu-Au alloy substrate; and Raman spectrum of 3R phase multilayer MoSi2N4 crystal (b). The horizontal axis Raman shift represents the Raman shift (cm). -1 The vertical axis Intensity represents the relative intensity count (arb.units). Detailed implementation method:

[0034] In the specific implementation process, the present invention first coats Cu foil (or foamed Cu, Cu powder, etc.) and Au foil (or foamed Au, Au powder, etc.) sequentially onto a transition metal M foil (M is Mo, W, Ti, Zr, Hf, V, Nb, Ta, etc.), then places it in a quartz boat, and under a hydrogen atmosphere, heats it in a tube furnace to above the melting point of copper to fully melt and uniformly alloy the two materials, obtaining a liquid Cu-Au alloy / transition metal composite substrate; then lowers the temperature and introduces a carrier gas containing A and Z elements to provide A to the growth system. Simultaneously, the transition metal diffused from the bottommost transition metal foil provides the M source for the growth of 3R phase multilayer MA2Z4 crystals. After growth, the quartz boat along with the growth substrate is quickly pushed to the low-temperature region, hydrogen and ammonia are turned off, and argon is introduced for rinsing to obtain 3R phase multilayer MA2Z4 crystals on a Cu-Au / M substrate. A polymethyl methacrylate (PMMA) lactic acid ester solution is spin-coated onto the sample surface and dried. The 3R phase multilayer MA2Z4 crystals are then transferred to any substrate using an electrochemical bubbling method.

[0035] like Figure 1 As shown, the chemical vapor deposition method for preparing the 3R phase multilayer ternary compound MA2Z4 material proposed in this invention includes the following specific steps:

[0036] (1) Cu foil (or foamed Cu, Cu powder, etc.) and Au foil (or foamed Au, Au powder, etc.) are sequentially coated onto transition metal M foil (M is a transition metal element of IVB-VIB group such as Mo, W, Ti, Zr, Hf, V, Nb, Ta, etc.) to form a Cu foil / Au foil / M foil stacked coating structure with continuously adjustable size and thickness. Then, it is placed in a quartz boat and heated to above the melting point of copper in a tube furnace under a hydrogen atmosphere so that Cu and Au are fully melted and uniformly alloyed to obtain a liquid Cu-Au alloy / transition metal M foil composite substrate with continuously adjustable composition.

[0037] (2) Lower the temperature and introduce carrier gas to provide A and Z sources to the growth system; at the same time, the transition metal diffused from the bottom layer of transition metal foil provides M source, and the vapor phase growth of 3R phase multilayer MA2Z4 crystal is carried out from bottom to top.

[0038] (3) After growth is completed, the quartz boat along with the growth substrate is quickly pushed to the low temperature zone, all growth carrier gases such as hydrogen are turned off and argon is introduced for rinsing, and 3R phase multilayer MA2Z4 crystals on Cu-Au / M substrate are obtained.

[0039] (4) Spin-coat the sample surface with a polymethyl methacrylate (PMMA) ethyl lactate solution and dry it. Use the electrochemical bubbling method to transfer the 3R phase multilayer MA2Z4 crystal onto any substrate and perform relevant morphology, structure and optical characterization.

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

[0041] Example 1

[0042] like Figure 1 As shown, this embodiment uses a liquid Cu-Au alloy substrate to prepare 3R phase multilayer WSi2N4, including the following steps:

[0043] (1) First, cut Cu foil, Au foil and W foil into rectangles of 5mm×7mm. The thickness of Cu foil is 12.5μm, the thickness of Au foil is 10μm and the thickness of W foil is 100μm. They are ultrasonically cleaned in acetone, deionized water and ethanol for ten minutes to remove organic matter from the surface of the metal foil. Then, Cu foil and Au foil are wrapped on W foil in sequence and placed in a quartz boat. Under a hydrogen atmosphere of 200sccm, the temperature is raised to 1090℃ in a tube furnace and held for 1min to fully melt and uniformly alloy the two to obtain a liquid Cu-Au alloy / W foil composite substrate.

[0044] (2) Continue to use 200 sccm of hydrogen as the carrier gas, lower the temperature to 950℃, introduce 2.8 sccm of ammonia, and grow for 1.5 hours to obtain 3R phase multilayer WSi2N4 crystals.

[0045] (3) After growth, the quartz boat and the growth substrate are quickly pushed to the low temperature zone, the hydrogen and ammonia are turned off and argon is introduced for rinsing, and 3R phase multilayer WSi2N4 crystals on Cu-Au / W substrate are obtained.

[0046] (4) A polymethyl methacrylate (PMMA) ethyl lactate solution was spin-coated onto the surface of a WSi2N4 / Cu-Au / W substrate and dried. The 3R phase multilayer WSi2N4 crystals were then transferred to an arbitrary substrate using an electrochemical bubbling method, and their morphology, structure, and optical characteristics were characterized. The PMMA used had a mass fraction of 4%. The PMMA was uniformly spin-coated at 2000 rpm for 60 s, followed by drying and curing at 60℃, 120℃, and 180℃ for 20 min each, resulting in a PMMA / WSi2N4 / Cu-Au / W composite substrate. The PMMA film thickness was 140–200 nm, the 3R phase WSi2N4 crystal thickness was 1.09–40 nm, the lateral dimension was 5–30 μm, and the Cu-Au / W metal foil composite substrate thickness was approximately 122.5 μm. A PMMA / WSi₂N₄ / Cu-Au / W composite substrate was used as the cathode in a 1 mol / L NaOH aqueous solution electrolytic cell. The 3R phase WSi₂N₄ crystals were peeled off from the substrate using an electrochemical hydrogen generation bubbling method. A constant current power supply of 0.1–0.2 mA was used.

[0047] The morphology of the 3R phase WSi2N4 crystal was characterized using optical microscopy, scanning electron microscopy, and atomic force microscopy. The quality of the 3R phase WSi2N4 crystal was characterized using Raman spectroscopy. The non-centrosymmetric structure of the 3R phase WSi2N4 crystal was characterized using SHG. The results show that the prepared 3R phase WSi2N4 has a complete structure, high quality, and strong polarization properties.

[0048] Example 2

[0049] like Figure 1 As shown, this embodiment uses a liquid Cu-Au alloy substrate to prepare 3R phase multilayer WSi2N4, including the following steps:

[0050] (1) First, cut Cu foil, Au foil and W foil into rectangles of 5mm×7mm. The thickness of Cu foil is 50μm, the thickness of Au foil is 10μm and the thickness of W foil is 100μm. They are ultrasonically cleaned in acetone, deionized water and ethanol for ten minutes to remove organic matter from the surface of the metal foil. Then, Au foil and Cu foil are wrapped on W foil in sequence and placed in a quartz boat. Under a hydrogen atmosphere of 200sccm, the temperature is raised to 1090℃ in a tube furnace and held for 1min to fully melt and uniformly alloy the two to obtain liquid Cu-Au alloy / W foil composite substrate.

[0051] (2) Continue to use 200 sccm of hydrogen as the carrier gas, lower the temperature to 1030℃, introduce 2.8 sccm of ammonia, and grow for 3 hours to obtain 3R phase multilayer WSi2N4 crystal.

[0052] (3) After growth, the quartz boat and the growth substrate are quickly pushed to the low temperature zone, the hydrogen and ammonia are turned off and argon is introduced for rinsing, and 3R phase multilayer WSi2N4 crystals on Cu-Au / W substrate are obtained.

[0053] (4) A polymethyl methacrylate (PMMA) ethyl lactate solution was spin-coated onto the surface of a WSi2N4 / Cu-Au / W substrate and dried. The 3R phase multilayer WSi2N4 crystals were then transferred to an arbitrary substrate using an electrochemical bubbling method, and their morphology, structure, and optical characteristics were characterized. The PMMA used had a mass fraction of 4%. The PMMA was uniformly spin-coated at 2000 rpm for 60 s, followed by drying and curing at 60℃, 120℃, and 180℃ for 20 min each, resulting in a PMMA / WSi2N4 / Cu-Au / W composite substrate. The PMMA film thickness was 140–200 nm, the 3R phase WSi2N4 crystal thickness was 1.09–60 nm, the lateral dimension was 5–20 μm, and the Cu-Au / W metal foil composite substrate thickness was approximately 160 μm. A PMMA / WSi₂N₄ / Cu-Au / W composite substrate was used as the cathode in a 1 mol / L NaOH aqueous solution electrolytic cell. The 3R phase WSi₂N₄ crystals were peeled off from the substrate using an electrochemical hydrogen generation bubbling method. A constant current power supply of 0.1–0.2 mA was used.

[0054] The morphology of the 3R phase WSi2N4 crystal was characterized using optical microscopy, scanning electron microscopy, and atomic force microscopy. The quality of the 3R phase WSi2N4 crystal was characterized using Raman spectroscopy. The non-centrosymmetric structure of the 3R phase WSi2N4 crystal was characterized using SHG. The results show that the prepared 3R phase WSi2N4 has a complete structure, high quality, and strong polarization properties.

[0055] Example 3

[0056] like Figure 1 As shown, this embodiment uses a liquid Cu-Au alloy substrate to prepare 3R phase multilayer WSi2N4, including the following steps:

[0057] (1) First, cut Cu foil, Au foil and W foil into rectangles of 5mm×7mm. The thickness of Cu foil is 12.5μm, the thickness of Au foil is 50μm and the thickness of W foil is 100μm. They are ultrasonically cleaned in acetone, deionized water and ethanol for ten minutes to remove organic matter from the surface of the metal foil. Then, Au foil and Cu foil are wrapped on W foil in sequence and placed in a quartz boat. Under a hydrogen atmosphere of 200sccm, the temperature is raised to 1090℃ in a tube furnace and held for 1min to fully melt and uniformly alloy the two to obtain liquid Cu-Au alloy / W foil composite substrate.

[0058] (2) Continue to use 200 sccm of hydrogen as the carrier gas, lower the temperature to 1020℃, introduce 2.8 sccm of ammonia, and grow for 3 hours to obtain 3R phase multilayer WSi2N4 crystal.

[0059] (3) After growth, the quartz boat and the growth substrate are quickly pushed to the low temperature zone, the hydrogen and ammonia are turned off and argon is introduced for rinsing, and 3R phase multilayer WSi2N4 crystals on Cu-Au / W substrate are obtained.

[0060] (4) A polymethyl methacrylate (PMMA) ethyl lactate solution was spin-coated onto the surface of a WSi2N4 / Cu-Au / W substrate and dried. The 3R phase multilayer WSi2N4 crystals were then transferred to an arbitrary substrate using an electrochemical bubbling method, and their morphology, structure, and optical characteristics were characterized. The PMMA used had a mass fraction of 4%. The PMMA was uniformly spin-coated at 2000 rpm for 60 s, followed by drying and curing at 60℃, 120℃, and 180℃ for 20 min each, resulting in a PMMA / WSi2N4 / Cu-Au / W composite substrate. The PMMA film thickness was 140–200 nm, the 3R phase WSi2N4 crystal thickness was 1.09–20 nm, the lateral dimension was 20–50 μm, and the Cu-Au / W metal foil composite substrate thickness was approximately 162.5 μm. A PMMA / WSi₂N₄ / Cu-Au / W composite substrate was used as the cathode in a 1 mol / L NaOH aqueous solution electrolytic cell. The 3R phase WSi₂N₄ crystals were peeled off from the substrate using an electrochemical hydrogen generation bubbling method. A constant current power supply of 0.1–0.2 mA was used.

[0061] The morphology of the 3R phase WSi2N4 crystal was characterized using optical microscopy, scanning electron microscopy, and atomic force microscopy. The quality of the 3R phase WSi2N4 crystal was characterized using Raman spectroscopy. The non-centrosymmetric structure of the 3R phase WSi2N4 crystal was characterized using SHG. The results show that the prepared 3R phase WSi2N4 has a complete structure, high quality, and strong polarization properties.

[0062] Example 4

[0063] like Figure 1 As shown, this embodiment uses a liquid Cu-Au alloy substrate to prepare 3R phase multilayer MoSi2N4, including the following steps:

[0064] (1) First, cut Cu foil, Au foil and Mo foil into rectangles of 5mm×7mm. The thickness of Cu foil is 12.5μm, the thickness of Au foil is 10μm and the thickness of Mo foil is 100μm. They are ultrasonically cleaned in acetone, deionized water and ethanol for ten minutes to remove organic matter from the surface of the metal foil. Then, Cu foil and Au foil are wrapped on Mo foil in sequence and placed in a quartz boat. Under a hydrogen atmosphere of 200sccm, the temperature is raised to 1090℃ in a tube furnace and held for 1min to fully melt and uniformly alloy the two to obtain liquid Cu-Au alloy / Mo foil composite substrate.

[0065] (2) Continue to use 200 sccm of hydrogen as the carrier gas, lower the temperature to 950℃, introduce 2.8 sccm of ammonia, and grow for 1.5 hours to obtain 3R phase multilayer MoSi2N4 crystal.

[0066] (3) After growth, the quartz boat and the growth substrate were quickly pushed to the low temperature zone, the hydrogen and ammonia were turned off and argon was introduced for rinsing, and 3R phase multilayer MoSi2N4 crystals on Cu-Au / Mo substrate were obtained.

[0067] (4) A polymethyl methacrylate (PMMA) ethyl lactate solution was spin-coated onto the surface of a MoSi₂N₄ / Cu-Au / Mo substrate and dried. The 3R phase multilayer MoSi₂N₄ crystals were then transferred to an arbitrary substrate using an electrochemical bubbling method, and their morphology, structure, and optical characteristics were characterized. The PMMA used had a mass fraction of 4%. The PMMA was uniformly spin-coated at 2000 rpm for 60 s, followed by drying and curing at 60℃, 120℃, and 180℃ for 20 min each, resulting in a PMMA / MoSi₂N₄ / Cu-Au / Mo composite substrate. The PMMA film thickness was 140–200 nm, the 3R phase MoSi₂N₄ crystal thickness was 1.07–40 nm, and the Cu-Au / Mo metal foil composite substrate thickness was approximately 122.5 μm. A PMMA / MoSi₂N₄ / Cu-Au / Mo composite substrate was used as the cathode in a 1 mol / L NaOH aqueous solution electrolyzer. The 3R phase MoSi₂N₄ crystals were exfoliated from the substrate using an electrochemical hydrogen generation bubbling method. A constant current power supply of 0.1–0.2 mA was used.

[0068] The morphology of the 3R phase MoSi2N4 crystal was characterized using optical microscopy, scanning electron microscopy, and atomic force microscopy. The quality of the 3R phase MoSi2N4 crystal was characterized using Raman spectroscopy. The non-centrosymmetric structure of the 3R phase MoSi2N4 crystal was characterized using SHG. The results show that the prepared 3R phase MoSi2N4 has a complete structure, high quality, and strong polarization properties.

[0069] like Figure 2 As shown, the 3R phase multilayer WSi2N4 crystal obtained in Example 2 exhibits a pyramid shape with sharp and clear edges, and an average single-layer thickness of approximately 1.09 nm.

[0070] like Figure 3 As shown, the Raman spectrum peaks of the 3R phase multilayer WSi2N4 crystal obtained in Example 2 are sharp and increase synchronously with the increase of the number of layers, indicating that the obtained crystal has high quality.

[0071] like Figure 4 As shown, the 3R phase multilayer WSi2N4 crystal obtained in Example 2 has a significant second harmonic (SHG) response, and its SHG signal intensity increases quadratically with the increase of the number of layers, indicating that its non-centrosymmetric structure brings strong polarization characteristics.

[0072] like Figure 5 As shown, Example 2 yields a 3R phase multilayer MoSi2N4 crystal.

[0073] The above results demonstrate that this invention utilizes a liquid Cu-Au alloy as the growth substrate. Its high degree of disorder and high catalytic activity facilitate a continuous supply of the source, enabling the layer-by-layer growth of multilayer crystals. Furthermore, the smaller surface interaction potential in the liquid state makes it easier to align the layers of the multilayer crystal to the energy-dominant 3R stacking state, thus obtaining stable, fully 3R phase MA2Z4 crystals. The Cu-Au alloy used has a lower melting point than pure metals, ensuring liquid growth at temperatures below the melting points of Cu (1084℃) and Au (1064℃), which is significant for energy conservation. This invention overcomes the previous difficulty in obtaining large-domain, fully ABC-stacked 3R phase MA2Z4 crystals. The enhanced SHG behavior with increasing layer number, resulting from its unique stacking configuration, confirms its scalable and powerful polarization characteristics, laying the foundation for scientific research on novel ternary materials in fields such as nonlinear optics, piezoelectricity, and slip ferroelectricity.

Claims

1. A method for preparing a 3R phase multilayer ternary compound MA2Z4 material, characterized in that, A 3R-phase multilayer ternary compound was grown by chemical vapor deposition using a liquid Cu-Au alloy-coated transition metal foil as a substrate. The method includes the following steps: Step 1: Coat Cu foil, foamed Cu or Cu powder, Au foil, foamed Au or Au powder sequentially onto transition metal M foil, then place them in a quartz boat, and heat them in a tube furnace to above the melting point of copper under a hydrogen atmosphere to fully melt and uniformly alloy them, thus obtaining a liquid Cu-Au alloy / transition metal composite substrate. Step 2: Lower the temperature, introduce a carrier gas to introduce elements A and Z, and use the transition metal diffused from the bottom layer of transition metal foil to provide the M source for the growth of 3R phase multilayer MA2Z4 crystals; wherein M is a transition metal element, including but not limited to Mo, W, Ti, Zr, Hf, V, Nb or Ta; A is a Group 4 element, including but not limited to Si or Ge; and Z is a Group 5 element, including but not limited to N, P or As. Step 3: After growth is complete, the quartz boat along with the growth substrate is quickly pushed to the low temperature zone, hydrogen and other growth carrier gases are turned off and argon gas is introduced for rinsing, to obtain 3R phase multilayer MA2Z4 crystals on Cu-Au / M substrate. Step 4: Spin-coat the sample surface with a polymethyl methacrylate lactate solution and dry it. Then, use an electrochemical bubbling method to transfer the 3R phase multilayer MA2Z4 crystals onto any substrate.

2. The preparation method of the 3R phase multilayer ternary compound MA2Z4 material according to claim 1, characterized in that, In step 1, the precursors for forming the Cu-Au alloy are Cu foil, Cu foam or Cu powder, and Au foil, Au foam or Au powder. When Cu foil and Au foil are used, their thicknesses are 12.5–100 μm and 10–100 μm, respectively. When Cu foam and Au foam are used, their thicknesses are 50–500 μm and 50–500 μm, respectively. When Cu powder and Au powder are used, the Cu powder has a mesh size of 100–400 mesh and the Au powder has a particle diameter of 0.8–1.5 μm.

3. The method for preparing the 3R phase multilayer ternary compound MA2Z4 material according to claim 1, characterized in that, In step 1, the thickness of the transition metal M foil is 50–500 μm.

4. The method for preparing the 3R phase multilayer ternary compound MA2Z4 material according to claim 1, characterized in that, In step 1, hydrogen is used as the carrier gas and must be introduced throughout the process with a constant flow rate of 50–1000 sccm.

5. The method for preparing the 3R phase multilayer ternary compound MA2Z4 material according to claim 1, characterized in that, In step 1, the temperature of the tube furnace is increased to above the melting point of copper to fully melt and uniformly alloy the two materials. The temperature range is 1085℃~1200℃, and the holding time is 1~10 minutes.

6. The method for preparing the 3R phase multilayer ternary compound MA2Z4 material according to claim 1, characterized in that, In step 2, after uniform alloying, the temperature is lowered to 900℃~1060℃, which must be higher than the alloy melting point. The specific temperature depends on the Cu-Au atomic ratio.

7. The method for preparing the 3R phase multilayer ternary compound MA2Z4 material according to claim 1, characterized in that, In step 2, the type and flow rate of the carrier gas for introducing element Z depend on the element. Specifically: for introducing element N, NH3 needs to be introduced with a flow rate of 2-10 sccm; for introducing element P, PH3 needs to be introduced with a flow rate of 2-20 sccm; and for introducing element As, AsH3 needs to be introduced with a flow rate of 2-20 sccm.

8. The method for preparing the 3R phase multilayer ternary compound MA2Z4 material according to claim 1, characterized in that, In step 2, the growth time of the 3R phase multilayer MA2Z4 crystal is 1 to 24 hours.

9. The method for preparing the 3R phase multilayer ternary compound MA2Z4 material according to claim 1, characterized in that, In step 4, the electrolyte used in the electrochemical bubbling method is a 1 mol / L NaOH aqueous solution, powered by a DC power supply with a constant current of 0.1–0.2 mA.

10. The method for preparing the 3R phase multilayer ternary compound MA2Z4 material according to any one of claims 1 to 9, characterized in that, In the 3R phase multilayer ternary compound MA2Z4 material, each layer consists of 7 atomic layers: ZAZMZAZ. The layers are bonded together by van der Waals forces. There are two or more layers, and the stacking method is ABC.

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