Method for preparing single-layer chromium triiodide thin film through Van der Waals heteroepitaxy

Abstract

The invention relates to the technical field of nano materials, and particularly discloses a method for heteroepitaxy of a large-area single-layer chromium triiodide magnetic semiconductor film on a Van der Waals substrate HOPG, which comprises the following steps: taking an A-level HOPG (0001) surface as a substrate, carrying out mechanical cleavage on the A-level HOPG (0001) surface, detecting the surface quality after cleavage by a scanning tunneling microscope, and carrying out ultrahigh vacuum annealing and degassing. Testing a scanning tunneling microscope morphology graph in a constant current mode before and after growth; and epitaxially growing a single-layer CrI3 film in a room-temperature high-iodine atmosphere in a cavity with background vacuum superior to 3 * 10 <-10 > mbar. Through the molecular beam epitaxy technology, the obtained CrI3 sample has high crystal quality and low surface roughness and has great application prospects in the fields of two-dimensional spin electronic devices, light-operated magnetic devices, quantum topological elements and the like, and the method is high in repeatability and has certain economic value.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of nanomaterials, and particularly relates to a method for preparing a van der Waals heteroepitaxial monolayer chromium triiodide film. BACKGROUND

[0002] For advanced magnetic quantum devices, how to controllably obtain large-size monolayer magnetic two-dimensional semiconductor is crucial. A wide range of monolayer CrI3 has two-dimensional layered ferromagnetism and adjustable spin state at room temperature, and thus has very wide potential applications. In the field of two-dimensional spin electronic devices, monolayer CrI3 can be used to construct tunneling magnetoresistance devices; in the field of light-controlled magnetic devices, monolayer CrI3 can be used for light-controlled memory and spin optoelectronic devices; in the field of quantum topological elements, monolayer CrI3 can be used to construct Majorana quasi-particle platforms to realize topological quantum computing.

[0003] Currently, monolayer CrI3 is mainly obtained through a “top-down” method, i.e. mechanical exfoliation. The size and number of layers of monolayer CrI3 obtained by this method are uncontrollable, and it is difficult to accurately obtain large-size and target-number-of-layers CrI3 samples. Moreover, after dry transfer, monolayer CrI3 inevitably has organic glue residues on the surface. It is extremely difficult to completely remove these organic clusters, and CrI3 itself is extremely sensitive to the atmosphere. Under a microscope, the surface of the bulk CrI3 after cleavage rapidly degrades, and monolayer CrI3 will decompose and disappear within 10 seconds in the atmosphere. Therefore, methods such as soaking, ultrasonic treatment and rinsing of monolayer CrI3 after dry transfer need to be carried out in an inert protective atmosphere such as nitrogen or argon. Moreover, when some organic solvents are used to treat monolayer CrI3, the solvents themselves will erode the surface of the monolayer CrI3 sample, and new organic impurities will be introduced after cleaning, thereby affecting the reliability and stability of monolayer CrI3 devices.

[0004] In order to overcome the above problems, molecular beam epitaxy technology is used as a new method for preparing monolayer CrI3. High-quality monolayer CrI3 samples can be obtained on Au(111), but there may be charge transfer between the metal substrate and CrI3, and the binding force between them is much greater than that of van der Waals material epitaxy CrI3. When the next step of manufacturing a device is performed, the soft metal single crystal surface may be damaged during the transfer of the epitaxied CrI3, thereby greatly increasing the manufacturing cost. The use of a van der Waals substrate can solve the above problems, but the inertness of the van der Waals substrate brings new challenges, such as low quality of the epitaxied CrI3 sample and easy decomposition into CrI2.

[0005] Therefore, a new process method is needed to controllably grow high-quality monolayer CrI3 material on a van der Waals substrate, thereby promoting the development of device and industrialization of monolayer CrI3. SUMMARY

[0006] The application provides a method for preparing a van der Waals hetero-epitaxial monolayer chromium triiodide film, so as to solve the problem of controllable acquisition of large-size high-quality monolayer CrI3 film on a van der Waals substrate in the prior art, and make the monolayer CrI3 film have high crystal quality and low surface roughness.

[0007] A method for preparing a van der Waals hetero-epitaxial monolayer chromium triiodide film, comprising the following steps:

[0008] S1, selecting an A-grade HOPG (0001) substrate, mechanically cleaving the substrate by using a tape, and then sending the substrate into an ultrahigh vacuum system to be degassed at 530 DEG C to a vacuum degree of 2.0x10 -9 mbar;

[0009] S2, performing scanning tunneling microscope and low-energy electron diffraction characterization on the cleaved HOPG substrate, and confirming that an atomically flat surface is obtained;

[0010] S3, loading 4N-purity chromium powder as a chromium source by using a K-cell source, degassing at 1200 DEG C to a vacuum degree of 1.5x10 - 9 mbar; loading 5N-purity CrI3 powder as an iodine source by using a K-cell source, and degassing at 300 DEG C;

[0011] S4, under the condition that the base vacuum degree of a cavity is 3x10 -10 mbar, controlling the temperature of the chromium source to be 1100 DEG C, the temperature of the CrI3 source to be 280 DEG C, co-evaporating on the surface of the HOPG substrate at room temperature for 10 min, and the working vacuum degree being 8.0x10 -9 mbar;

[0012] S5, after the chromium source is turned off, annealing at 125 DEG C for 1 h under the condition that the iodine atmosphere is 8.0x10 -9 mbar and the temperature is raised at a rate of 5 DEG C / min, and then naturally cooling the iodine atmosphere to 50 DEG C after the annealing is completed, and then turning off the iodine source;

[0013] S6, performing scanning tunneling microscope characterization to confirm that a monolayer CrI3 film is obtained.

[0014] Preferably, the HOPG substrate in step S1 has a size of 5mmx5mmx1mm, is fixed by using a molybdenum back plate and a 0.3mm tantalum foil before being cleaved, and the tantalum foil is fixed on the surface of the molybdenum back plate which is polished by using 500-mesh sandpaper and ultrasonically cleaned by using isopropanol-anhydrous ethanol by using a spot welding machine.

[0015] Preferably, the tape mechanical cleaving in step S1 uses a transparent pressure-sensitive tape, and the cleaved substrate is transferred to a low-temperature scanning tunneling microscope system by using a fast sample introduction chamber, and the base vacuum of the low-temperature scanning tunneling microscope system is less than or equal to 6.1x10 -11 mbar.

[0016] Preferably, the step S2 is carried out at a low temperature of 78K, and the constant current mode scanning tunneling microscope is used to characterize the roughness of the substrate surface

[0017] Preferably, the evaporation rate of the chromium source in the step S4 is The K-cell source is realized by a large crucible.

[0018] Preferably, the confirmation standard of the single-layer CrI3 film in the step S6 includes:

[0019] (a) the surface step height

[0020] (b) the surface roughness

[0021] (c) the atomic resolution image is consistent with the CrI3 lattice structure;

[0022] (d) the scanning tunneling spectrum shows a characteristic tunneling resistance.

[0023] Preferably, the growth process is completed in a molecular beam epitaxy system, and the background vacuum degree of the cavity is better than 3x10 - 10 mbar.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] (1) By cleaving the substrate and ultra-high vacuum degassing, the substrate surface impurity pollution is effectively avoided, and a good substrate foundation is provided for high-quality heteroepitaxy;

[0026] (2) The chromium evaporation source is degassed at a high temperature of 1200℃, which effectively removes the existence of chromium oxide and other impurities in the evaporation source, and obtains a high-purity chromium plating beam, which provides favorable conditions for epitaxial growth;

[0027] (3) After growth, a high-iodine atmosphere cooling strategy is adopted, which effectively avoids the decomposition of the single-layer CrI3 film on the inert substrate surface of HOPG, and a large range of high-quality single-layer CrI3 samples are observed by scanning tunneling microscope, which provides a new method for controllable preparation of single-layer CrI3 film, and the obtained sample has great application prospect in two-dimensional magnetic quantum devices. The single-layer CrI3 film prepared by this method has the characteristics of good repeatability, no pollution, large sample size, etc., and has certain economic value. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The preparation flowchart of the present application is shown in the figure;

[0029] Figure 2 The substrate real object diagram after the growth of the sample of the present application is shown in the figure;

[0030] Figure 3 Scanning tunneling microscope (STM) large size topography of the thin film before and after growth of the present application;

[0031] Figure 4 Thin film mesa height map and surface relief distribution map of the present application;

[0032] Figure 5 Atomic resolution map of the thin film under liquid helium (LHe) of the present application;

[0033] Figure 6 Scanning tunneling spectroscopy (STS) map of the thin film of the present application;

[0034] Figure 7 Scanning tunneling microscope topography of a single CrI3 defect in different filled and empty states under atomic resolution of the thin film of the present application. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0036] As shown in Figures 1 to 7 :

[0037] Embodiment: A method for growing a van der Waals heteroepitaxial monolayer chromium triiodide thin film, comprising:

[0038] S1, selecting a (0001) surface HOPG (highly oriented pyrolytic graphite) substrate for epitaxial thin film, the substrate is a Nanjing Muke Nanometer Technology Co., Ltd. specification 5mmx5mmx1mm A grade highly oriented graphite. The molybdenum backplate and 0.3mm thick tantalum foil are pretreated, the surface of the backplate is polished with 500 mesh sandpaper to remove burrs and contaminants, the molybdenum backplate and tantalum foil are sequentially placed in isopropyl alcohol and anhydrous ethanol, and the backplate and tantalum foil are blown by a high-pressure air gun with 5N high-purity nitrogen, to obtain a clean HOPG substrate;

[0039] S2, using a spot welder to spot weld the 0.3mm tantalum foil and the molybdenum backplate, fixing the HOPG substrate on the metal molybdenum backplate, checking that the HOPG substrate is fixed tightly without shaking, and obtaining the sample shown in Figure 2

[0040] ​S3, cleave the HOPG substrate with the Scotch Magic Tape, spread the tape on the surface of the HOPG substrate, gently scrape the surface of the substrate with a carbon fiber flat tweezers to remove the bubbles between the tape and the substrate, and gently tear off the tape. The multi-layer (0001) surface of the HOPG substrate is mechanically exfoliated to expose a clean and fresh surface. The exfoliated HOPG substrate is quickly vacuumed through the rapid sample chamber, and when the vacuum reaches 5x10 -8 mbar, the sample is transferred to the ultra-high vacuum low-temperature scanning tunneling microscope system, and the background vacuum of the scanning cavity is 5.0x10 -11 mbar;

[0041] S4, cool the HOPG substrate to 78K in the scanning tunneling microscope, and scan the surface of the exfoliated substrate in the constant current mode to determine the flat and clean (0001) exfoliated surface of the HOPG substrate. The scanning result is shown in Figure 3 a, the step edge and the mesa are sharp, and there is no impurity cluster. The surface roughness is 0.1 nm, and the surface fluctuation distribution is shown in Figure 3 b;

[0042] S5, transfer the HOPG substrate to the preparation cavity, degas at 530°C, and the background vacuum is 2x10 -10 mbar. The degassing ends when the vacuum is 2x10 -9 mbar;

[0043] S6, when the substrate in the preparation cavity is cooled to room temperature, the vacuum is restored to the order of 10 -10 mbar;

[0044] S7, transfer the HOPG substrate in situ to the growth cavity, and the cavity background vacuum is 3x10 -10 mbar. The evaporation rate of the metal chromium source is controlled at This time, the evaporation source temperature is maintained at 1100°C, the cavity vacuum is 1.5x10 -9 mbar, the CrI3 source is heated to 280°C, and when the vacuum is maintained at 8x10 -8 mbar, the shutter is opened and the HOPG substrate is grown at room temperature for 10 min;

[0045] S8, after the growth is completed, the chromium source shutter is closed, the HOPG substrate is heated to 125°C at a rate of 5°C / min, and the iodine atmosphere is 8x10 - 8 mbar for annealing for 60 min;

[0046] S9, after the annealing is completed, the heating is turned off, the high iodine atmosphere is unchanged at 8x10 -8 mbar, and when the temperature drops to 50°C, the iodine source shutter is closed;

[0047] S10, the sample after growth is placed into a scanning tunneling microscope cavity and cooled to 78K for real-space imaging. The scanning tunneling microscope is observed in constant current mode Figure 3 c shows the CrI3 film, the surface roughness The surface relief distribution is shown in Figure 3 d, indicating that a high-quality CrI3 film is obtained. The surface step height is shown in Figure 4 b The step measurement position is referred to Figure 4 a green line part, Figure 4 a surface relief distribution is shown in Figure 4 b, which shows that the obtained CrI3 film sample is a single layer. The atomic resolution image under liquid helium conditions is shown in Figure 5 a, considering that the test lattice is similar in size to the CrI3 lattice, the fast Fourier transform is shown in Figure 5 b, and it is judged that the sample is CrI3. Figure 6 The scanning tunneling spectrum in the above-mentioned Figure 7 reveals that the single-layer CrI3 film has a large tunneling resistance.

[0048] Embodiments of the present application are given for example and description, although embodiments of the present application have been shown and described above, it is understood that the above-mentioned embodiments are exemplary and cannot be understood as limiting the present application, and changes, modifications, replacements and modifications of the above-mentioned embodiments by ordinary skilled in the art within the scope of the present application should be included in the protection scope of the present application.

Claims

1. A method for forming a van der Waals heteroepitaxial monolayer chromium triiodide thin film, characterized in that: The following steps are involved: S1, select A-grade HOPG (0001) substrate, the tape is mechanically cleaved and sent into the ultra-high vacuum system at 530℃ for degassing to a vacuum degree of 2.0×10 -9 mbar; S2. Scanning tunneling microscopy and low-energy electron diffraction characterization are performed on the cleaved HOPG substrate to confirm that an atomically flat surface is obtained; S3, using K-cell source loaded with 4N purity chromium powder as chromium source, degassing at 1200℃ to a vacuum degree of 1.5×10 -9 mbar; a K-cell source was used to load 5N purity CrI3 powder as the iodine source and degassed at 300℃; S4, when the background vacuum of the cavity is 3×10 -10 Under the condition of mbar, the temperature of chromium source was controlled at 1100℃ and CrI3 source at 280℃, and they were co-evaporated on the surface of HOPG substrate at room temperature for 10 min. The working vacuum was 8.0×10 -9 mbar; S5, after turning off the chromium source, at 8.0×10 -9 In an iodine atmosphere, heat the mixture to 125°C at a rate of 5°C / min and anneal for 1 hour. After annealing, maintain the iodine atmosphere and naturally cool it to 50°C, then turn off the iodine source. S6. Scanning tunneling microscopy was used to confirm that a single-layer CrI3 film was obtained.

2. The method for forming a van der Waals heteroepitaxial monolayer chromium triiodide thin film according to claim 1, wherein: In step S1, the HOPG substrate has a size of 5 mm × 5 mm × 1 mm and is fixed by a molybdenum back plate and a 0.3 mm tantalum foil before cleavage. The tantalum foil is fixed to the surface of the molybdenum back plate polished with 500-grit sandpaper and ultrasonically cleaned with isopropyl alcohol and anhydrous ethanol by a spot welder.

3. The method for forming a van der Waals heteroepitaxial monolayer chromium triiodide thin film according to claim 1, wherein: In step S1, the tape is mechanically cleaved using a transparent pressure-sensitive tape, and after cleavage, it is transferred to a rapid sampling chamber with a background vacuum of ≤6.1×10 -11 mbar's cryogenic scanning tunneling microscope system.

4. The method for forming a van der Waals heteroepitaxial monolayer chromium triiodide thin film according to claim 1, wherein: In step S2, constant current scanning tunneling microscope characterization is performed at a low temperature of 78K to confirm the surface roughness of the substrate.

5. The method for forming a van der Waals heteroepitaxial monolayer chromium triiodide thin film according to claim 1, wherein: The chromium source evaporation rate in step S4 is This is achieved through a large crucible of the K-cell source.

6. The method for forming a van der Waals heteroepitaxial monolayer chromium triiodide thin film according to claim 1, wherein: The confirmation criteria for the single-layer CrI3 film in step S6 include: (a) Surface step height (b) Surface roughness (c) The atomically resolved image is consistent with the CrI3 lattice structure; (d) Scanning tunneling spectrum showing characteristic tunnel resistance.

7. The method for growing a single-layer chromium triiodide thin film using van der Waals heteroepitaxial growth as claimed in any one of claims 1 to 6, characterized in that: The growth process is completed in a molecular beam epitaxy system, and the background vacuum of the cavity is better than 3×10 -10 mbar.

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