In-plane in-phase heterojunction of two-dimensional material and atomic-scale construction and characterization method of in-plane in-phase heterojunction

Abstract

The invention discloses an in-plane in-phase heterojunction of a two-dimensional material and an atomic-scale construction and characterization method of the in-plane in-phase heterojunction. The method comprises the following steps: step 1, preparing a single-layer 1T-NbSe2 / 1T-VSe2 in-phase heterojunction on a graphene substrate by adopting a molecular beam epitaxy two-step growth method; step 2, performing scanning tunneling microscope (STM) characterization on the morphology of the heterojunction material in a room temperature environment; 3, transferring the sample to a low-temperature scanning tunneling microscope through an ultrahigh vacuum transfer device, and carrying out atomic-scale morphology characterization and electronic state characterization; the composition, the structure and the interface quality of the heterojunction are accurately controlled, the high-quality preparation of the NbSe2 / VSe2 in-plane in-phase heterojunction of the 1T phase is realized, and a basis is provided for subsequent material research and device application.

Description

Technical Field

[0001] The present invention belongs to the field of nanomaterials and application technologies, and in particular relates to a two-dimensional material in-plane homogeneous heterojunction and an atomic-level construction and characterization method thereof. Background Art

[0002] In-plane heterojunction of two-dimensional materials is an important research direction in the field of materials science in recent years. It refers to a structure formed by splicing two or more different (or the same elements but different properties) two-dimensional materials together in the same plane. This structure has shown unique performance and broad application prospects in the fields of electronics, optics, thermal science, etc.

[0003] The in-plane heterojunction of two-dimensional materials combines different two-dimensional materials in the same plane to form a heterostructure, thereby combining the advantages of each material to produce new physical properties and application potential. Its advantages include: the two-dimensional material itself has an atomic or molecular thickness, which makes the in-plane heterojunction also extremely thin, which is conducive to the miniaturization and integration of devices; some two-dimensional materials have a wide band gap and high mobility, and these characteristics are retained in the in-plane heterojunction, which is conducive to improving the performance of the device; through the coupling between heterogeneous materials, the in-plane heterojunction can show richer physical properties than a single two-dimensional material, such as the regulation of electrical, optical, magnetic and other properties.

[0004] Although in-plane heterojunctions of two-dimensional materials have broad application prospects, there are still some problems to be solved: how to accurately control the composition, structure and interface quality of the heterojunction is the key to achieving high-performance in-plane heterojunctions. At present, the preparation technology still needs further optimization and improvement; reducing the defects of the splicing interface of two-dimensional heterogeneous materials and realizing more diverse physical properties and application potential is also an urgent problem to be solved. Summary of the invention

[0005] The technical problem to be solved by the present invention is: to provide a two-dimensional material in-plane homogeneous heterojunction and its atomic-level construction and characterization method, so as to accurately control the composition, structure and interface quality of the heterojunction and reduce the defects of the splicing interface of the two-dimensional heterogeneous material.

[0006] The technical solution of the present invention is:

[0007] A two-dimensional material in-plane homogeneous heterojunction and an atomic-level construction and characterization method thereof, the method comprising:

[0008] Step 1: Prepare a single-layer 1T-NbSe2 / 1T-VSe2 heterojunction on a graphene substrate using a two-step molecular beam epitaxy growth method;

[0009] Step 2, characterizing the morphology of the heterojunction material by scanning tunneling microscopy at room temperature;

[0010] Step 3: Transfer the sample to a low-temperature scanning tunneling microscope through an ultra-high vacuum transfer device for atomic-level morphology and electronic state characterization.

[0011] The graphene substrate is obtained by cleaving highly oriented pyrolytic graphite in an ultra-high vacuum environment and then annealing at 600° C. to 900° C. for 1 hour.

[0012] The graphene substrate is made of 99.999% pure silicon carbide material at 600°C.

[0013] The obtained structure is annealed at 900°C for 1 hour and then annealed at 1300°C to 1400°C for 1 hour.

[0014] The method for preparing a single-layer 1T-NbSe2 / 1T-VSe2 homogeneous heterojunction on a graphene substrate by a two-step molecular beam epitaxy growth method comprises:

[0015] Step 1.1, on the surface of the graphene substrate, selenium atoms are deposited on the surface of the graphene substrate through a thermal resistance evaporation source, and at the same time, niobium atoms are deposited on the surface of the graphene substrate through an electron beam thermal evaporation source; the graphene substrate is maintained at 580° C. to 600° C., so that the selenium atoms and the niobium atoms are fully combined with each other, the growth time lasts for 40 to 60 minutes, and the deposition rate is 0.4 layers / hour, and finally a two-dimensional ordered single layer 1T-NbSe2 is formed on the surface of the graphene substrate;

[0016] Step 1.2: On the surface of the graphene substrate on which the two-dimensional ordered single-layer 1T-NbSe2 is grown, selenium atoms are deposited on the graphene substrate or the single-layer 1T-NbSe2 surface by a thermal resistance evaporation source, and at the same time, vanadium atoms are deposited on the graphene substrate or the single-layer 1T-NbSe2 surface by an electron beam thermal evaporation source, while the graphene substrate and the single-layer 1T-NbSe2 are kept at 230°C.

[0017] ~260℃, so that the selenium atoms and vanadium atoms are fully combined with each other, the growth time lasts 40 to 60 minutes, the deposition rate is 0.4 layer / hour, and finally a two-dimensional ordered single-layer 1T-NbSe2 / 1T-VSe2 heterojunction is formed on the surface of the graphene substrate.

[0018] The deposition ratio of Se to Nb is greater than 10:1.

[0019] The deposition ratio of Se to V is greater than 20:1.

[0020] Beneficial effects of the present invention:

[0021] The two-dimensional material in-plane isomorphic heterojunction and the atomic-level construction and characterization method of the present invention are reasonably conceived and can grow a new type of 1T-NbSe2 / 1T-VSe2 in-plane heterojunction; in particular, the growth parameters of different two-dimensional materials are strictly controlled by the molecular beam epitaxy method to achieve precise control of the composition, structure and interface quality of the heterojunction, so it has important value in the study of strong electron correlation characteristics.

[0022] The main advantage of the present invention is that it effectively reduces the defects of the splicing interface of two-dimensional heterogeneous materials, and realizes more diverse physical properties and application potential. The grown in-plane homogeneous heterojunction is of high quality, and its atomic structure and electronic structure can be characterized by scanning tunneling microscopy. The present invention realizes the high-quality preparation of 1T phase NbSe2 / VSe2 in-plane homogeneous heterojunction, which provides a basis for subsequent material research and device applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the preparation process of a single-layer 1T-NbSe2 in the two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method of the present invention;

[0024] Figure 2 A scanning tunneling microscope image of a single-layer 1T-NbSe2 two-dimensional material prepared on the surface of graphene in the two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method of the present invention at room temperature;

[0025] Figure 3 It is a schematic diagram of the process effect of growing a two-dimensional ordered single layer 1T-NbSe2 and continuing to grow 1T-VSe2 on the surface of a graphene substrate in the two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method of the present invention to achieve the preparation of a 1T-NbSe2 / 1T-VSe2 in-plane heterojunction;

[0026] Figure 4 This is a scanning tunneling microscope image at room temperature after preparing a 1T-NbSe2 / 1T-VSe2 in-plane heterojunction in the two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method of the present invention;

[0027] Figure 5 The scanning tunneling microscope image at a low temperature of -269°C is a two-dimensional material in-plane heterojunction and a method for its atomic-level construction and characterization after the 1T-NbSe2 / 1T-VSe2 in-plane heterojunction is prepared in the present invention, and the charge density wave superperiods of the single-layer 1T-NbSe2 and 1T-VSe2 are marked, as well as the period display diagram after fast Fourier transformation;

[0028] Figure 6After preparing the 1T-NbSe2 / 1T-VSe2 in-plane heterojunction in the two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method of the present invention, a scanning tunneling microscope is used at a low temperature of -269°C to perform differential electrical line spectrum profile imaging across the heterojunction interface. DETAILED DESCRIPTION

[0029] This embodiment provides a two-dimensional material in-plane homogeneous heterojunction and an atomic-level construction and characterization method thereof, which mainly includes the following steps:

[0030] (1) A single-layer 1T-NbSe2 / 1T-VSe2 heterojunction was prepared on a graphene substrate using a two-step molecular beam epitaxy growth method;

[0031] Among them, the molecular beam epitaxy (MBE) in the above step (1) is a newly developed epitaxial film-making method and also a special vacuum coating process. Its core lies in generating vapor by heating under ultra-high vacuum conditions and collimating it into a molecular beam or atomic beam through a small hole, and then directly spraying it onto a single crystal substrate at an appropriate temperature, so as to prepare high-quality crystalline films by growing films layer by layer.

[0032] The basic principle of MBE is to heat the elements that make up the film into directional molecular beams or atomic beams in their respective molecular beam furnaces under ultra-high vacuum conditions, and then irradiate them onto the heated substrate. Through adsorption, decomposition, migration, nucleation, growth and other processes, the atoms enter the lattice position to complete epitaxial growth. The entire growth process is carried out under ultra-high vacuum conditions, avoiding the influence of impurities, so that high-purity films can be prepared. MBE equipment is a complex system, mainly including vacuum system, evaporation source furnace, sample holder auxiliary system, beam source furnace and real-time monitoring system. Among them, the vacuum system provides a higher vacuum environment for growth, the evaporation source furnace is used to heat and generate vapor and collimate it into molecular beams or atomic beams, the sample holder auxiliary system is responsible for the multi-dimensional movement and temperature control of the sample in the chamber, and the beam source furnace is a container for placing or heating the source material. There are two main types of molecular beam evaporation sources currently widely used. One is the Knudsen diffusion (K-Cell) evaporation source, whose key components include crucibles, resistance heaters, cooling systems, temperature measurement systems and controllable baffles. The K-Cell evaporation source is mainly suitable for materials with lower melting points, and its operating temperature range is usually between 120°C and 1030°C. Another type is the electron beam (E-Beam) heating evaporation source, which generates hot electrons through a heated filament. These electrons are accelerated by high voltage and focused on the target material to achieve heating. The electron beam evaporation source can reach higher temperatures than traditional heating technology and is suitable for materials with higher melting points.

[0033] (2) The morphology of the heterojunction material is characterized by scanning tunneling microscopy at room temperature.

[0034] (3) The samples are transferred to a low-temperature scanning tunneling microscope through an ultra-high vacuum transfer device for atomic-level morphology and electronic state characterization.

[0035] In the above step (1), a high-quality single-layer 1T-NbSe2 / 1T-VSe2 heterojunction is prepared on the surface of multilayer graphene. The high-quality graphene substrate can be obtained by mechanically cleaving purchased highly oriented pyrolytic graphite (highly oriented pyrolytic graphite is a new type of graphite that has been treated with high temperature and has performance close to that of single-crystal graphite) in an ultra-clean environment, and annealing at 600°C to 1000°C (lower than 600°C, the degassing is not clean, and higher than 1000°C may cause graphene defects, and the best implementation value in this embodiment is 800°C) for 1 hour, or by annealing silicon carbide with a purity of 99.999% at 600°C to 1000°C for 1 hour and then at 1350°C to 1400°C (the best implementation value in this embodiment is 1370°C) for 1 hour.

[0036] The schematic diagram of the preparation process of the single-layer 1T-NbSe2 of the present invention is as follows: Figure 1 The left part of the figure shows that niobium atoms and selenium atoms are deposited on the surface of graphene in the present invention, while the substrate is heated and maintained at a growth temperature of 590° C. The right part of the figure shows the single-layer 1T-NbSe2 islands that are finally grown on the graphene surface.

[0037] The specific process of the above step (1) is as follows:

[0038] (1.1) Obtain high-quality graphene substrate in an ultra-high vacuum chamber;

[0039] (1.2) On the surface of a high-quality graphene substrate, selenium atoms are deposited on the surface of the graphene substrate by a thermal resistance evaporation source. At the same time, niobium atoms are deposited on the surface of the graphene substrate by an electron beam thermal evaporation source. At the same time, the graphene substrate is maintained at 580°C to 600°C, so that the selenium atoms and niobium atoms are fully combined with each other. The deposition ratio of Se to Nb is greater than 10:1, ensuring a Se-rich environment. The growth time lasts for 40 to 60 minutes, and the deposition rate is 0.4 layer / hour. Finally, a two-dimensional ordered single layer 1T-NbSe2 is formed on the surface of the graphene substrate.

[0040] (1.3) On the surface of a graphene substrate on which a two-dimensional ordered single layer of 1T-NbSe2 is grown, selenium atoms are deposited on the graphene substrate or the single layer of 1T-NbSe2 by a thermal resistance evaporation source, and at the same time, vanadium atoms are deposited on the graphene substrate or the single layer of 1T-NbSe2 by an electron beam thermal evaporation source, while the graphene substrate and the single layer of 1T-NbSe2 are maintained at 230°C.

[0041] ~260℃, so that the selenium atoms and vanadium atoms are fully combined with each other, the growth time lasts 40 to 60 minutes, the deposition rate is 0.4 layer / hour, and finally a two-dimensional ordered single-layer 1T-NbSe2 / 1T-VSe2 heterojunction is formed on the surface of the graphene substrate.

[0042] In the above step (2), the morphology of the heterojunction material is characterized by scanning tunneling microscopy at room temperature, such as Figure 2 As shown in the room temperature scanning tunneling microscope image, the single layer 1T-NbSe2 has a hexagonal island structure on the graphene substrate.

[0043] like Figure 3 As shown, Se atoms and V atoms were deposited on the sample with 1T-NbSe2 islands grown by MBE co-deposition, and the substrate temperature was kept at 250°C. The Se atom deposition rate was about 0.001 layer / min, and the deposition ratio of Se to V was greater than 20:1, ensuring the growth of 1T-VSe2 in a Se-rich environment, and finally forming a two-dimensional ordered single-layer 1T-NbSe2 / 1T-VSe2 heterojunction on the surface of the graphene substrate.

[0044] In the above step (2), the morphology of the heterojunction material is characterized by scanning tunneling microscopy at room temperature, such as Figure 4 As shown in the room temperature scanning tunneling microscope image, there is a circle of single-layer heterogeneous material around the single-layer NbSe2 in the shape of a regular hexagonal island.

[0045] In the above step (3), the sample is transferred to a low-temperature scanning tunneling microscope through an ultra-high vacuum transfer device for atomic-level morphology characterization, such as Figure 5 As shown in the figure, the sample is transferred to the cryogenic STM by an ultra-high vacuum transfer device for fine atomic-level morphology and STS characterization. Figure 5 As shown in the upper row, a large-scale STM morphology characterization was performed at the interface of the heterojunction, and two distinct charge density wave (CDW) superperiodic patterns were found on both sides of the interface. The structure of the interface area was magnified and measured, and a small-scale atomic-resolution STM image was obtained. From the atomic-resolution STM morphology map ( Figure 5 (Bottom row) The characteristic (√13×√13) CDW superlattice of 1T-NbSe2 and the characteristic (√3×√7) CDW superlattice of 1T-VSe2 were seen, thus identifying the two heterogeneous materials of the prepared in-plane heterojunction.

[0046] In the above step (3), the sample is transferred to a low-temperature scanning tunneling microscope through an ultra-high vacuum transfer device for electronic state characterization, such as Figure 6 As shown, the 1T-NbSe2 / 1T-VSe2 interface is further characterized ( Figure 6 The electronic structure of the sample (upper left panel) was determined by scanning tunnelling microscopy (STM) in constant-height mode (dI / dV) differential conductivity spectroscopy to measure the electronic density of states (DOS) at different energies. Figure 6 The lower left panel is along Figure 6 The solid line dI / dV line spectrum in the upper left panel that crosses the heterojunction boundary shows the change of DOS near the Fermi level with atomic precision. Unlike 1T-VSe2, 1T-NbSe2 is strongly affected by the interface effect, so its band gap gradually decreases near the boundary, while it still maintains the Mott insulating state inside. A smooth transition of the band gap is observed near the interface between metallic 1T-VSe2 and Mott insulator 1T-NbSe2, which can be attributed to the proximity effect of the two different CDW materials. Figure 6 The right column more intuitively shows the differential conductance diagrams of the intrinsic part of the in-plane heterojunction and the band gap change at the interface.

[0047] The invention has a reasonable conception and can grow a high-quality single-layer 1T-NbSe2 / 1T-VSe2 isophase heterojunction with a heterojunction interface that transitions from a Mott insulating state to a metallic state.

Claims

1. A two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method, characterized in that: The method comprises: Step 1: Prepare a single-layer 1T-NbSe2 / 1T-VSe2 heterojunction on a graphene substrate using a two-step molecular beam epitaxy growth method; Step 2, characterizing the morphology of the heterojunction material by scanning tunneling microscopy at room temperature; Step 3: Transfer the sample to a low-temperature scanning tunneling microscope through an ultra-high vacuum transfer device for atomic-level morphology and electronic state characterization.

2. The two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method according to claim 1, characterized in that: The graphene substrate is obtained by cleaving highly oriented pyrolytic graphite in an ultra-high vacuum environment and then annealing at 600° C. to 900° C. for 1 hour.

3. The two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method according to claim 1, characterized in that: The graphene substrate is obtained by annealing a silicon carbide material with a purity of 99.999% at 600° C. to 900° C. for 1 hour and then annealing at 1300° C. to 1400° C. for 1 hour.

4. The two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method according to claim 1, characterized in that: The method for preparing a single-layer 1T-NbSe2 / 1T-VSe2 homogeneous heterojunction on a graphene substrate by a two-step molecular beam epitaxy growth method comprises: Step 1.1, on the surface of the graphene substrate, selenium atoms are deposited on the surface of the graphene substrate through a thermal resistance evaporation source, and at the same time, niobium atoms are deposited on the surface of the graphene substrate through an electron beam thermal evaporation source; the graphene substrate is maintained at 580° C. to 600° C., so that the selenium atoms and the niobium atoms are fully combined with each other, the growth time lasts for 40 to 60 minutes, and the deposition rate is 0.4 layers / hour, and finally a two-dimensional ordered single layer 1T-NbSe2 is formed on the surface of the graphene substrate; Step 1.2, on the surface of a graphene substrate with a two-dimensional ordered single-layer 1T-NbSe2 grown, selenium atoms are deposited on the surface of the graphene substrate or the single-layer 1T-NbSe2 by a thermal resistance evaporation source, and at the same time, vanadium atoms are deposited on the surface of the graphene substrate or the single-layer 1T-NbSe2 by an electron beam thermal evaporation source. At the same time, the graphene substrate and the single-layer 1T-NbSe2 are maintained at 230° C. to 260° C., so that the selenium atoms and the vanadium atoms are fully combined with each other. The growth time lasts for 40 to 60 minutes, and the deposition rate is 0.4 layer / hour, and finally a two-dimensional ordered single-layer 1T-NbSe2 / 1T-VSe2 isophase heterojunction is formed on the surface of the graphene substrate.

5. The two-dimensional material in-plane heterojunction and the atomic-level construction and characterization method thereof according to claim 4, characterized in that: The deposition ratio of Se to Nb is greater than 10:

1.

6. The two-dimensional material in-plane heterojunction and its atomic-level construction and characterization method according to claim 4, characterized in that: The deposition ratio of Se to V is greater than 20:1.

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