Ferroelectric monolayer vanadium trichloride film and preparation method thereof

Abstract

The application relates to a ferroelectric monolayer vanadium trichloride film and a preparation method thereof. The method comprises the following steps: evaporating and depositing VCl3 powder on the surface of a NbSe2 substrate under an ultrahigh vacuum atmosphere, and performing annealing treatment, so that the ferroelectric monolayer vanadium trichloride film is obtained. The sample preparation strategy of molecular beam epitaxy under an ultrahigh vacuum environment is used, high-purity VCl3 powder is evaporated and deposited on a NbSe2 substrate, an atomically flat monolayer VCl3 sample is obtained through annealing treatment, in-situ scanning tunneling microscope technology and scanning tunneling spectrum technology are combined, and it is determined that the monolayer VCl3 is a semiconductor material with ferroelectricity. The method provides a new method and idea for introducing ferroelectricity in the family of transition metal trichlorides, has high application potential and scientific research value, meanwhile, the successful introduction of ferroelectricity in the system also provides a new platform for carrying out multi-state regulation and research.

Description

Technical Field

[0001] This application relates to the field of nanomaterials technology, and in particular to a ferroelectric monolayer vanadium trichloride thin film and its preparation method. Background Technology

[0002] For modern device applications, spontaneous polarization in solid materials plays a crucial role. With the development of two-dimensional van der Waals materials, materials exhibiting spontaneous polarization have been gradually discovered. Among them, transition metal trihalides (MX3, where M represents a transition metal and X represents Cl, Br, and I, hereinafter referred to as TMT) are one of the most widely studied magnetic materials, containing a rich array of magnetic ground states. For example, monolayers of CrI3 and CrBr3 exhibit out-of-plane ferromagnetism, and monolayer CrCl3 exhibits XY ferromagnetism. Vanadium-based trihalides possess even richer quantum states, such as Mott insulating states, strong magnetic anisotropy, and spin-lattice coupling effects. Recent magnetic and optical experiments have revealed that bulk VI3 is a ferromagnetic semiconductor with a band gap of 0.6 eV. Recent magnetic circular dichroism experiments have shown that this ferromagnetism is retained even at the monolayer limit. Furthermore, theoretical calculations indicate that monolayers of VCl3 and VBr3 may also exhibit ferromagnetism or antiferromagnetism.

[0003] Since most TMT materials possess a certain band gap, they satisfy an important prerequisite for ferroelectricity. Therefore, multiple theoretical predictions indicate that ferroelectricity can be achieved in monolayer TMTs through different material manipulation methods. The core of this approach is to introduce additional variables to break the spatial inversion symmetry, thereby achieving polarization. On the other hand, experimental studies on artificially breaking crystal symmetry to achieve ferroelectricity in van der Waals materials have also been reported. For example, in transition metal chalcogenides or hexagonal boron nitride systems, reversible out-of-plane ferroelectricity can be achieved by controlling the van der Waals stacking order between two layers. Therefore, the field considers introducing ferroelectricity into magnetic TMT materials to be highly feasible and has broad application and research value. However, how to control the fabrication of ferroelectric monolayer TMT materials currently lacks a feasible scheme. Therefore, the preparation strategy for ferroelectric VCl3 materials has high application potential and scientific research value. Summary of the Invention

[0004] This application provides a ferroelectric monolayer vanadium trichloride thin film and its preparation method to solve the problem of difficulty in achieving ferroelectricity in TMT materials in related technologies.

[0005] In a first aspect, this application provides a method for preparing a ferroelectric monolayer vanadium trichloride thin film, comprising the following steps:

[0006] Under an ultra-high vacuum atmosphere, VCl3 powder is evaporated and deposited on the surface of an NbSe2 substrate, followed by annealing to obtain a ferroelectric monolayer vanadium trichloride thin film.

[0007] In some embodiments, the annealing temperature is 120–170°C and the annealing time is 60–240 min.

[0008] In some embodiments, the vacuum level under the ultra-high vacuum atmosphere is ≤1×10⁻⁶. -5 Torr.

[0009] In some embodiments, the NbSe2 substrate is a 2H phase NbSe2 crystal.

[0010] In some embodiments, the temperature of the NbSe2 substrate is maintained at room temperature during VCl3 powder deposition.

[0011] In some embodiments, "evaporating and depositing VCl3 powder on the surface of an NbSe2 substrate" specifically includes:

[0012] VCl3 powder is placed in an evaporation source, and the evaporation source is heated to degas the VCl3 powder;

[0013] After degassing is completed, the evaporation source is heated to sublimate the VCl3 powder and deposit it onto the surface of the NbSe2 substrate.

[0014] In some embodiments, the deposition rate of VCl3 powder is 1 molecular layer / hour.

[0015] In some embodiments, the following steps are included before "evaporating and depositing VCl3 powder on the surface of the NbSe2 substrate":

[0016] Cleavage of NbSe2 substrate under ultra-high vacuum atmosphere.

[0017] In some embodiments, the following steps are included before "cleaving the NbSe2 substrate":

[0018] The NbSe2 substrate was heat-treated in an ultra-high vacuum atmosphere at a temperature of 300–400 °C for 6–12 hours.

[0019] Secondly, this application also provides a ferroelectric monolayer vanadium trichloride thin film, which is prepared by any of the methods described above.

[0020] The beneficial effects of the technical solution provided in this application include:

[0021] This application utilizes a sample preparation strategy based on molecular beam epitaxy under ultra-high vacuum conditions to evaporate and deposit high-purity VCl3 powder onto an NbSe2 substrate. Annealing then yields an atomically flat monolayer VCl3 sample. Due to charge transfer between the substrate and the sample, the spatial inversion symmetry of VCl3 is broken, thereby inducing ferroelectricity in the sample. This provides an effective, simple, and controllable strategy for preparing ferroelectric VCl3 materials. This method offers a new approach and concept for introducing ferroelectricity into the transition metal trihalomethane family, possessing high application potential and research value. Furthermore, the successful introduction of ferroelectricity into this system provides a new platform for multi-state manipulation and research. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 Scanning tunneling microscope images of a monolayer vanadium trichloride thin film provided in the embodiments of this application and Comparative Example 1 on an NbSe2 substrate, wherein, Figure 1 (a) Figure 1 (b) Figure 1 (c) and Figure 1 (d) represent the surface morphology of the samples obtained at annealing temperatures of 120℃, 150℃, 170℃ and 200℃, respectively.

[0024] Figure 2 This is a scanning tunneling microscope image of a monolayer vanadium trichloride thin film on an NbSe2 substrate provided in Embodiment 1 of this application, wherein... Figure 2 (a) is a diagram showing the atomic-level structure characteristics of VCl3 inside a single domain under a relatively high scanning bias voltage; Figure 2 (b) is a diagram of the atomic-level structure characteristics of VCl3 inside a single domain under a lower scanning bias voltage; Figure 2 (c) is the scanning tunneling spectrum of a single-layer vanadium trichloride thin film;

[0025] Figure 3 This is a diagram showing the ferroelectric polarization results of a single-layer vanadium trichloride thin film provided in Example 1 of this application, wherein... Figure 3 (a) is a scanning tunneling microscopy image of a multidomain region of a single-layer vanadium trichloride thin film; Figure 3 (b) Spatially resolved scanning tunneling spectrum of a single-layer vanadium trichloride thin film; Figure 3 (c) is a schematic diagram of the ferroelectric polarization-induced band shift of a single-layer vanadium trichloride thin film;

[0026] Figure 4 This is a characterization image of the ferroelectroforming reversal of a single-layer vanadium trichloride thin film provided in Example 1 of this application, wherein... Figure 4 (a) shows the morphology of a single-layer vanadium trichloride thin film at a scanning voltage of 1V. Figure 4 (b) shows the morphology of a single-layer vanadium trichloride film at a scanning voltage of 2V;

[0027] Figure 5 The figures shown are characterization diagrams of the morphology, structure, and electronic properties of a monolayer vanadium trichloride film on graphene provided in Comparative Example 2 of this application. Figure 5 (a) is a scanning tunneling microstructure image of a monolayer of vanadium trichloride on graphene; Figure 5 (b) is an atomically resolved image of a single layer of vanadium trichloride on graphene; Figure 5 (c) is the spatially resolved scanning tunneling spectrum of a single layer of vanadium trichloride on graphene. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0029] In a first aspect, embodiments of this application provide a method for preparing a ferroelectric monolayer vanadium trichloride thin film, comprising the following steps:

[0030] Under an ultra-high vacuum atmosphere, VCl3 powder is evaporated and deposited on the surface of an NbSe2 substrate, followed by annealing to obtain a ferroelectric monolayer vanadium trichloride thin film.

[0031] This application utilizes a sample preparation strategy based on molecular beam epitaxy under ultra-high vacuum conditions to evaporate and deposit high-purity VCl3 powder onto an NbSe2 substrate. Annealing then yields an atomically flat monolayer VCl3 sample. Due to charge transfer between the substrate and the sample, the spatial inversion symmetry of VCl3 is broken, thereby inducing ferroelectricity in the sample. This provides an effective, simple, and controllable strategy for preparing ferroelectric VCl3 materials. This method offers a new approach and concept for introducing ferroelectricity into the transition metal trihalomethane family, possessing high application potential and research value. Furthermore, the successful introduction of ferroelectricity into this system provides a new platform for multi-state manipulation and research.

[0032] It should be noted that ferroelectric materials are materials with spontaneous polarization, which can be reversed by applying an external electric field.

[0033] Specifically, the preparation and characterization of the ferroelectric monolayer vanadium trichloride thin film provided in this application can be performed using a combined system of ultra-high vacuum molecular beam epitaxy and ultra-low temperature scanning tunneling microscopy.

[0034] In some embodiments, the annealing temperature is 120–170°C and the annealing time is 60–240 min.

[0035] The quality of thin film crystallization and ferroelectricity is closely related to the annealing temperature and annealing time. When the annealing time is constant, too low an annealing temperature will cause vanadium trichloride to mix with certain amorphous morphologies; too high an annealing temperature will cause vanadium trichloride to decompose into vanadium dichloride. Therefore, it is necessary to control the annealing temperature between 120 and 170°C and the annealing time between 60 and 240 min.

[0036] Furthermore, the preferred annealing temperature is 150°C and the annealing time is 60 min.

[0037] In some embodiments, the vacuum level under an ultra-high vacuum atmosphere is ≤1×10⁻⁶. -9 Torr.

[0038] In some embodiments, the NbSe2 substrate is a 2H phase NbSe2 crystal.

[0039] Preferably, the NbSe2 crystal is a bulk crystal.

[0040] In some embodiments, the temperature of the NbSe2 substrate is maintained at room temperature during VCl3 powder deposition.

[0041] Compared to depositing samples while maintaining the substrate at the annealing temperature, samples deposited at room temperature and then annealed have a lower defect density.

[0042] In some embodiments, "evaporating and depositing VCl3 powder on the surface of an NbSe2 substrate" specifically includes:

[0043] VCl3 powder is placed in an evaporation source, and the evaporation source is heated to degas the VCl3 powder;

[0044] After degassing is completed, the evaporation source is heated to sublimate the VCl3 powder and deposit it onto the surface of the NbSe2 substrate.

[0045] Because VCl3 powder is hygroscopic and corrosive, the VCl3 powder initially loaded into the vacuum chamber will deliquesce and absorb a small amount of water. Therefore, the powder needs to be pre-degassed before deposition.

[0046] Furthermore, "placing VCl3 powder in an evaporation source and heating the evaporation source to degas the VCl3 powder" specifically includes:

[0047] VCl3 powder was placed in an evaporation source, and the temperature of the evaporation source was gradually increased to 260℃. During the heating process, the VCl3 powder was kept under a vacuum of ≤1×10⁻⁶. -5 In a Torr atmosphere, after the temperature rises to 260℃, it is kept at that temperature for 2 hours to degas the VCl3 powder.

[0048] Preferably, the evaporation source is a DC heating evaporation source.

[0049] Furthermore, "after degassing is completed, the evaporation source is continued to be heated to sublimate the VCl3 powder and deposit it onto the surface of the NbSe2 substrate" specifically includes:

[0050] The evaporation source was heated to 250°C at a rate of 10°C / min until the vacuum level stabilized at 2×10⁻⁶. -9 When the Torr is below a certain value, the VCl3 molecular beam begins to deposit onto the surface of the NbSe2 substrate, and the deposition takes 1 hour.

[0051] In some embodiments, the deposition rate of VCl3 powder is 1 molecular layer / hour.

[0052] By controlling the deposition rate of VCl3 powder at 1 molecular layer / hour, it is ensured that a single-layer two-dimensional VCl3 film is obtained in the subsequent growth.

[0053] In some embodiments, the following steps are included before "evaporating and depositing VCl3 powder on the surface of the NbSe2 substrate":

[0054] Cleavage of NbSe2 substrate under ultra-high vacuum atmosphere.

[0055] Cleavage operations are performed on NbSe2 substrates to obtain atomically flat surfaces.

[0056] In some embodiments, the following steps are included before "cleaving the NbSe2 substrate":

[0057] The NbSe2 substrate was heat-treated in an ultra-high vacuum atmosphere at a temperature of 300–400 °C for 6–12 h.

[0058] The NbSe2 substrate is heat-treated to remove gas and obtain clean crystals to avoid contaminating the subsequently grown thin films.

[0059] Furthermore, the preferred heat treatment temperature is 300℃ and the heat treatment time is 12h.

[0060] Secondly, embodiments of this application also provide a ferroelectric monolayer vanadium trichloride thin film, which is prepared using any of the methods described above.

[0061] The present application will be further described below through specific embodiments.

[0062] Raw material and instrument specifications:

[0063] VCl3 powder: purchased from Thermo Fisher Scientific, purity 99%.

[0064] Bulk NbSe2 substrate: purchased from Shanghai Spectrum Precision Instruments Technology Co., Ltd.

[0065] Conductive silver paste: EPO-TEK H20E.

[0066] Ultra-low temperature scanning tunneling microscope: Unishoku-1300.

[0067] K-cell type evaporation source: laboratory design and installation.

[0068] DC heated sample stage: Unishoku-1300.

[0069] Example 1

[0070] This embodiment provides a method for preparing a ferroelectric monolayer vanadium trichloride thin film, comprising the following steps:

[0071] Step 101: Preparation before cleavage of bulk NbSe2 substrate

[0072] Mix components A and B of EPO-TEK H20E in a 1:1 ratio and stir thoroughly. Apply a small amount of the mixed conductive silver paste to the copper sheet of the sample holder using a copper wire. Then, use Teflon tweezers to place the NbSe2 crystal onto the silver-coated copper sheet. Place the sample holder in a preheated oven at 120°C and bake for 1 hour. After 1 hour, turn off the oven and allow the sample holder to cool naturally before removing it. Prepare the silver paste again according to the aforementioned ratio and apply a small amount to a cleavage copper rod (10mm long, approximately 2mm in diameter). Then, attach the copper rod to the area of ​​the NbSe2 crystal to be cleaved. Place the sample holder back in the oven at 120°C and bake for one hour.

[0073] Step 102: Vacuum cleavage of bulk NbSe2 substrate

[0074] After baking in step 101, allow the sample holder to cool naturally, then transfer it into the vacuum chamber of the molecular beam epitaxy (MBE) apparatus and heat it to approximately 300°C overnight. Once the sample holder has cooled to room temperature, gently push the copper rod on the sample holder to peel off the surface layer of the NbSe2 crystal, resulting in a clean and smooth NbSe2 surface.

[0075] Step 103: Growth of a monolayer ferroelectric VCl3 thin film:

[0076] (1) Load VCl3 powder into the evaporation source crucible, then quickly install it into the growth chamber, and promptly evacuate the chamber to a rough vacuum. Then gradually increase the temperature of the evaporation source to 260℃, while maintaining the chamber vacuum at no higher than 1×10⁻⁶. -5 Torr, after rising to 260℃, maintain this temperature for 2 hours to complete pre-degassing.

[0077] (2) Next, the temperature of the evaporation source is increased to 250°C at a rate of 10°C / min, and the vacuum degree is stabilized at 2×10⁻⁶. - 9 When the temperature is below the specified range (Tr), the freshly cleaved NbSe2 substrate is aligned directly facing the evaporation source, and the evaporation source baffle is opened, allowing VCl3 to begin depositing on the NbSe2 substrate. After one hour, the baffle is closed, and the evaporation source temperature is lowered to complete the growth. Subsequently, the NbSe2 substrate is heated to 150°C using a DC heating stage for annealing for one hour, yielding a monolayer ferroelectric VCl3 film.

[0078] Example 2

[0079] This embodiment provides a method for preparing a ferroelectric monolayer vanadium trichloride thin film, which is basically the same as that in Example 1, except that in step (2) of step 103, the annealing temperature in this embodiment is 120°C.

[0080] Example 3

[0081] This embodiment provides a method for preparing a ferroelectric monolayer vanadium trichloride thin film, which is basically the same as that in Example 1, except that in step (2) of step 103, the annealing temperature in this embodiment is 170°C.

[0082] Comparative Example 1

[0083] This comparative example provides a method for preparing a single-layer vanadium dichloride thin film, which is basically the same as that in Example 1, except that in step (2) of step 103, the annealing temperature of this comparative example is 200°C.

[0084] Comparative Example 2

[0085] This comparative example provides a method for preparing a single-layer vanadium trichloride thin film, which is basically the same as that in Example 1, except that the substrate in this comparative example is graphene.

[0086] Performance Characterization

[0087] The properties of the vanadium trichloride thin films prepared in the above embodiments and comparative examples were characterized, and the following results were obtained:

[0088] (1) Basic physical property characterization

[0089] Please see Figure 1 The images provided in the examples are scanning tunneling microscope images of the monolayer vanadium trichloride thin film and Comparative Example 1 on an NbSe2 substrate.

[0090] in, Figure 1 (a) A monolayer vanadium trichloride sample annealed at 120 °C, containing a small amount of amorphous phase (VCl). x It coexists with ferroelectric vanadium trichloride samples; Figure 1 (b) is a monolayer vanadium trichloride sample annealed at 150 °C, at which a uniform and clean ferroelectric phase monolayer vanadium trichloride can be obtained. Figure 1 (c) is a monolayer vanadium trichloride sample annealed at 170℃. Due to the high substrate temperature, some chlorine was removed from the sample, resulting in a small amount of vanadium dichloride phase present in it. Figure 1 (a) to (c) show that the ferroelectric monolayer vanadium trichloride still dominates in this temperature range, but substrate temperatures that are too high or too low will reduce the purity of the ferroelectric monolayer vanadium trichloride, with an annealing temperature of 150°C being optimal.

[0091] Figure 1 (d) is an image of the monolayer vanadium chloride sample prepared in Comparative Example 1. After annealing at 200°C, vanadium trichloride decomposes upon heating, and some chlorine is released to form a pure vanadium dichloride film.

[0092] Please see Figure 2 The image shows an atomic resolution scanning tunneling microscope image and scanning tunneling spectrum of the monolayer vanadium trichloride thin film prepared in Example 1 on an NbSe2 substrate.

[0093] Figure 2 (a) and Figure 2 (b) shows the atomic-level structural characteristics of VCl3 within a single domain, where... Figure 2 (a) is obtained with a higher scanning bias voltage (1.5V), characterized by ellipsoidal bright spots arranged in a hexagonal lattice, with the distance between the bright spots being approximately 0.6 nm; Figure 2 (b) is obtained at a lower scan bias voltage (-0.38V), in which the Cl atoms on the surface exhibit trimerization characteristics, with three adjacent Cl atoms arranged together in a boomerang shape.

[0094] Figure 2 (c) is the scanning tunneling spectrum of a single-layer vanadium trichloride thin film. As can be seen from the figure, the single-layer VCl3 film exhibits semiconductor properties with a band gap of 2.04 electron volts. Its Fermi level is close to the conduction band region, indicating that the material is n-type electron doped.

[0095] (2) Ferroelectricity characterization

[0096] Please see Figure 3The figure shows the ferroelectric polarization results of the monolayer vanadium trichloride thin film prepared in Example 1.

[0097] in, Figure 3 (a) is a scanning tunneling microscopy image of a single-layer vanadium trichloride thin film in a multi-domain region. The internal height of the same domain varies, characterized by a lower height near the domain wall-A and a higher height near the domain wall-B.

[0098] Figure 3 (b) shows the spatially resolved scanning tunneling spectrum of a single-layer vanadium trichloride thin film. The figure shows that the spectral characteristic peaks in the region near domain wall A shift upwards, and vice versa. This phenomenon is a typical characteristic of ferroelectric materials.

[0099] Figure 3 (c) is the phenomenon of ferroelectric polarization-induced band shift (i.e.) Figure 3 (b) is a schematic diagram of the phenomenon shown, where domain wall region A carries a negative charge and domain wall region B carries a positive charge. Spontaneous polarization within a single domain results in net positive and negative charges at the domain walls at its two ends, respectively, thereby generating an effective electric field within the domain, causing energy shift in the energy band near the domain walls.

[0100] Please see Figure 4 The image shows the ferroelectroforming reversal characterization of the monolayer vanadium trichloride thin film prepared in Example 1 under scanning voltage induction.

[0101] in, Figure 4 (a) shows the morphology of a single-layer vanadium trichloride thin film at a scanning voltage of 1V. Figure 4 (b) shows the morphology of a single-layer vanadium trichloride thin film at a scanning voltage of 2V. From the figure, it can be seen that 4(a) and... Figure 4 The spatial distribution of ferroelectric domains in (b) shows some differences, with 4(a) having only 3 ferroelectric domains in different directions, while Figure 4 In (b), the number of ferroelectric domains increases to four; simultaneously... Figure 4 The areas of the three ferroelectric domains in (a) are also... Figure 4 There is a significant difference in (b). This indicates that when the sample is scanned with a larger scanning voltage (2V), there is a certain effective electric field between the scanning tip and the sample, which in turn induces the flipping of ferroelectric domains in VCl3.

[0102] In conclusion, Figure 3 This demonstrates that the monolayer vanadium trichloride thin film prepared in Example 1 possesses spontaneous polarization characteristics. Figure 4 This demonstrates that the monolayer vanadium trichloride thin film prepared in Example 1 has polarization reversal characteristics induced by an external electric field, thus illustrating that Example 1 of this application provides a ferroelectric monolayer vanadium trichloride thin film.

[0103] See Figure 5 As shown, it is a characterization diagram of the morphology, structure and electronic properties of the monolayer vanadium trichloride film on graphene prepared in Comparative Example 2.

[0104] in, Figure 5 (a) and (b) show the phase morphology and atomic structure of monolayer vanadium trichloride prepared on graphene substrate. Compared with monolayer vanadium trichloride on NbSe2 substrate, vanadium trichloride on graphene does not have obvious domains and domain walls, and there is no significant distortion in atomic structure. Figure 5 (c) shows the spatially resolved electronic structure characterization of the monolayer vanadium trichloride on graphene, indicating the absence of ferroelectric polarization-induced band shifts or band bending. In summary, the NbSe2 substrate plays an essential role in the preparation of the ferroelectric monolayer vanadium trichloride in these embodiments.

[0105] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing a ferroelectric monolayer vanadium trichloride thin film, characterized in that, Includes the following steps: Under an ultra-high vacuum atmosphere, VCl3 powder is evaporated and deposited on the surface of an NbSe2 substrate, followed by annealing to obtain a ferroelectric monolayer vanadium trichloride thin film. "Evaporating and depositing VCl3 powder onto the surface of an NbSe2 substrate" specifically includes: VCl3 powder is placed in an evaporation source, and the evaporation source is heated to degas the VCl3 powder; After degassing is completed, the evaporation source is heated to sublimate the VCl3 powder and deposit it onto the surface of the NbSe2 substrate; The vacuum level under the ultra-high vacuum atmosphere is ≤1×10⁻⁶. -5 Torr; The annealing temperature is 120~170 ℃, and the annealing time is 60~240 min.

2. The method for preparing a ferroelectric monolayer vanadium trichloride thin film as described in claim 1, characterized in that, The NbSe2 substrate is a 2H phase NbSe2 crystal.

3. The method for preparing a ferroelectric monolayer vanadium trichloride thin film as described in claim 1, characterized in that, During VCl3 powder deposition, the temperature of the NbSe2 substrate was maintained at room temperature.

4. The method for preparing a ferroelectric monolayer vanadium trichloride thin film as described in claim 1, characterized in that, The deposition rate of VCl3 powder is 1 molecular layer / hour.

5. The method for preparing a ferroelectric monolayer vanadium trichloride thin film as described in claim 1, characterized in that, The following steps are included before "evaporating and depositing VCl3 powder on the surface of the NbSe2 substrate": Cleavage of NbSe2 substrate under ultra-high vacuum atmosphere.

6. The method for preparing a ferroelectric monolayer vanadium trichloride thin film as described in claim 5, characterized in that, The following steps are included before "cleaving the NbSe2 substrate": The NbSe2 substrate was heat-treated in an ultra-high vacuum atmosphere at a temperature of 300-400 °C for 6-12 h.

7. A ferroelectric single-layer vanadium trichloride thin film, characterized in that, It is prepared by the method described in any one of claims 1-6.

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