A method of preparing a two-dimensional material heterojunction

Abstract

The application provides a method for preparing a two-dimensional material heterojunction, which comprises the following steps: (1) subjecting a target two-dimensional material to mechanical cleavage in a vacuum device; (2) subjecting a bottom layer material to pretreatment in the vacuum device; and (3) transferring the target two-dimensional material to the pretreated bottom layer material in the vacuum device; wherein the vacuum degree of the vacuum device is less than 1*10 ‑7 Pa. The method of the application can form various two-dimensional material heterojunctions with high quality interfaces, such as no bubbles and impurities, and highly unstable interfaces, such as easily oxidized interfaces. The technical solution of the application can solve the problems of the existing two-dimensional material heterojunction preparation technology, such as the unsatisfactory interface quality and the single type. The technical solution of the application can realize the preparation of various two-dimensional material heterojunctions with highly clean, easily oxidized interfaces and complex interfaces.

Description

Technical Field

[0001] This invention belongs to the field of materials science. Specifically, this invention relates to a method for preparing two-dimensional material heterostructures. Background Technology

[0002] Two-dimensional materials and their heterostructures are key elements of novel quantum devices and are expected to lead the next generation of industrial technology revolution. Methods for preparing two-dimensional materials are mainly divided into two categories: bottom-up methods, represented by molecular beam epitaxy and chemical vapor deposition, and top-down methods, represented by mechanical exfoliation. Bottom-up methods can only prepare specific two-dimensional materials on specific substrates, and the prepared two-dimensional materials usually have a definite orientation. This greatly limits the types of two-dimensional heterostructures that can be obtained. Top-down methods are usually carried out in the atmosphere or in a glove box, and their cleanliness is far inferior to ultra-high vacuum technology. Therefore, this method is only suitable for very stable two-dimensional materials, such as graphene. To date, although theorists have predicted many exotic heterostructures, only a very small number have been realized.

[0003] CN 107055467 B discloses a vacuum transfer device for preparing two-dimensional van der Waals heterostructures. However, this patent only describes the transfer process being completed under vacuum (the pretreatment of the material and substrate is not completed under vacuum), and the vacuum level is low, only 10. -5 Pa. This method can only prevent the formation of bubbles at the interface, but it cannot obtain novel heterojunctions. The main reason is that the heterojunction has already been oxidized or adsorbed with a large number of impurities before being transferred into the vacuum chamber.

[0004] Therefore, a universal heterojunction fabrication technique is needed to obtain arbitrary two-dimensional material heterojunctions. Summary of the Invention

[0005] To address the limitations of the aforementioned two-dimensional material heterojunction fabrication techniques, the present invention aims to provide a method for preparing two-dimensional material heterojunctions. This method is universally applicable; by performing pretreatment and transfer processes on the target material and underlying material under ultra-high vacuum conditions, highly clean two-dimensional material heterojunctions with various easily oxidized and atmospherically unstable interfaces can be obtained.

[0006] The above-mentioned objective of the present invention is achieved through the following technical solution.

[0007] This invention provides a method for preparing two-dimensional material heterostructures, comprising the following steps:

[0008] (1) Mechanically cleaving the target two-dimensional material in a vacuum device;

[0009] (2) The bottom material is pretreated in the vacuum device;

[0010] (3) The target two-dimensional material is transferred onto the pretreated substrate material in the vacuum device;

[0011] The vacuum level of the vacuum device is less than 1×10⁻⁶. -7 Pa.

[0012] Preferably, in the method of the present invention, the vacuum degree of the vacuum device is less than or equal to 5 × 10⁻⁶. -8 Pa.

[0013] The inventors of this application unexpectedly discovered that highly clean two-dimensional material heterostructures with various easily oxidized and atmospherically unstable interfaces can be obtained by pretreating the underlying material and mechanically cleaving the target two-dimensional material in a high vacuum environment, rather than by performing these operations in the atmosphere or glove box.

[0014] The surface treatment of the underlying material is primarily aimed at obtaining an atomically flat surface. This increases the contact area during subsequent exfoliation, improving sample transfer efficiency. Furthermore, it allows for the acquisition of material surfaces that are unstable or nonexistent under conventional atmospheric, glove box, or rough vacuum conditions, providing novel heterojunction interfaces and broadening the types of two-dimensional material heterojunctions. Specific implementation methods include various ultra-high vacuum sample processing and preparation techniques, such as molecular beam epitaxy, annealing, ion sputtering, and plasma treatment. The transfer of the target two-dimensional material refers to obtaining two-dimensional material heterojunctions in an ultra-high vacuum environment using dry transfer techniques through pressing and separation.

[0015] Preferably, in the method of the present invention, the pretreatment includes heat treatment, surface sputtering, cleaning and etching, or surface thin film growth.

[0016] Preferably, in the method of the present invention, the vacuum device includes a vacuum chamber, a first driving module, a heatable sample holder, a second driving module, a compressible sample holder, and an expandable functional component. The first driving module is disposed on the vacuum chamber and connected to the heatable sample holder via a driving rod; the second driving module is disposed on the vacuum chamber at a position opposite to the first driving module and connected to the compressible sample holder via a driving rod; wherein the first driving module can drive the heatable sample holder to move up and down and rotate at any angle within a plane, and the second driving module can drive the compressible sample holder to move up and down and rotate at any angle within a plane, so that the target two-dimensional material on the heatable sample holder and the underlying material on the compressible sample holder adhere to and separate from each other. The expandable functional component includes an evaporation source furnace, an ion gun, and a radio frequency plasma source to pre-treat the underlying material, and the expandable functional component is electrically connected to the vacuum chamber.

[0017] Preferably, in the method of the present invention, the heatable sample holder is provided with tungsten wire to heat the underlying material when the target two-dimensional material and the underlying material are attached to each other.

[0018] Preferably, in the method of the present invention, the compressible sample holder is provided with a spring and a scale to determine the pressure applied when transferring the target two-dimensional material onto the pretreated substrate material.

[0019] Preferably, in the method of the present invention, both the heatable sample holder and the compressible sample holder are provided with sample trays to be adapted to the sample trays that carry the target two-dimensional material and the underlying material.

[0020] Preferably, in the method of the present invention, the vacuum device further includes a sample introduction unit connected to the vacuum cavity for transferring the sample to the heatable sample holder or compressible sample holder, and a measurement unit for in-situ characterization of the prepared two-dimensional material heterostructure.

[0021] In a specific embodiment of the present invention, the extended functional components include, but are not limited to, an evaporation source furnace, an ion gun, and a radio frequency plasma source. Their main purpose is for surface treatment of the underlying material, such as thin film deposition, surface etching, and surface heat treatment. The extended functional components are located diagonally below the vacuum cavity.

[0022] In a specific embodiment of the present invention, both the first driving module and the second driving module can move vertically and rotate in-plane, which can precisely control the relative rotation angle of the two two-dimensional materials in the heterojunction.

[0023] In a specific embodiment of the present invention, the heatable sample holder includes a high-purity tungsten wire, which is heated by a DC source to heat the underlying material.

[0024] In a specific embodiment of the present invention, the compressible sample holder is equipped with a vertical spring and a scale for determining the magnitude of pressure during transfer.

[0025] In a specific embodiment of the present invention, the heatable sample holder and the compressible sample holder are used in conjunction with corresponding sample holders, and the target two-dimensional material (or the parent single crystal) and the underlying material are fixed on the sample holder.

[0026] In a specific embodiment of the present invention, the universal two-dimensional material heterostructure fabrication technology relies on the vertical stacking and separation of the heatable sample holder and the compressible sample holder. These are controlled by the first driving module and the second driving module. Utilizing the difference in bonding forces between the materials on different surfaces of the sample holder, the target two-dimensional material is transferred to the underlying material, thereby forming a high-quality heterostructure.

[0027] In a specific embodiment of the present invention, the device further includes corresponding sample introduction and measurement units, both of which are connected to the main cavity. The sample introduction unit is mainly used to transfer the sample holder to the heatable sample holder or the compressible sample holder. The measurement cavity is mainly used to perform in-situ characterization of the prepared two-dimensional material heterostructure in a vacuum, and the characterization methods include, but are not limited to, microscopic observation, probe scanning, and energy dispersive spectroscopy.

[0028] The present invention has the following beneficial effects:

[0029] The method of this invention can form various universal two-dimensional material heterojunctions. These heterojunctions not only possess high-quality interfaces, such as being free of bubbles and impurities, but can also be highly unstable, such as easily oxidized interfaces. These unstable interfaces are difficult to prepare in atmospheric, glove box, and rough vacuum environments. Implementing the technical solution of this invention can solve the problems of insufficient interface quality and limited variety in existing two-dimensional material heterojunction preparation technologies. Implementing the technical solution of this invention can achieve the preparation of various highly clean two-dimensional material heterojunctions with easily oxidized interfaces and complex interfaces.

[0030] Because the method of this invention can achieve the fabrication of these high-quality, universal two-dimensional material heterostructures, more interfacial physical phenomena and effects can be explored. Furthermore, through in-situ characterization in an ultra-high vacuum environment, the intrinsic properties of the materials are more easily revealed by researchers. Attached Figure Description

[0031] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

[0032] Figure 1 These are a set of typical two-dimensional material heterostructures obtained by the method of the present invention;

[0033] Figure 2 This is another typical set of two-dimensional material heterostructures obtained by the method of the present invention;

[0034] Figure 3a for Figure 1 Raman spectra of monolayer graphene prepared in [the process];

[0035] Figure 3b for Figure 1 Raman spectra of monolayer molybdenum disulfide prepared in [the preparation process];

[0036] Figure 4a for Figure 2 Raman spectrum of molybdenum disulfide on Au film located on silicon wafer surface;

[0037] Figure 4b for Figure 2 Raman spectrum of tungsten diselenide on Au thin film located on silicon wafer surface;

[0038] Figure 5a It is a light microscope image of a molybdenum disulfide / Au film heterojunction prepared in a high-vacuum environment;

[0039] Figure 5b It is a light microscope image of a molybdenum disulfide / Au film heterojunction prepared in a low-vacuum environment;

[0040] Figure 6 It is a schematic diagram of the overall device of the present invention;

[0041] Figure 7 It is a schematic diagram of the appearance of the heatable sample holder and the compressible sample holder in the device of the present invention;

[0042] Figure 8 It is a schematic diagram of the structure of the sample holder supporting the sample tray in the device of the present invention;

[0043] Among them, reference numerals:

[0044] 1 - Vacuum chamber;

[0045] 2 - First drive module;

[0046] 3 - Heatable sample holder;

[0047] 4 - Second drive module;

[0048] 5 - Compressible sample holder;

[0049] 6 - Extended function component;

[0050] 71 - Tungsten wire;

[0051] 72 - Sample tray groove;

[0052] 81 - Scale;

[0053] 82 - Spring;

[0054] 83 - Sample tray groove. Detailed implementation manners

[0055] The present invention will be further described in detail below in conjunction with the specific implementation manners. The examples given are only for clarifying the present invention, rather than limiting the scope of the present invention. The present invention will show a series of two-dimensional material heterojunctions obtained through the present invention through the following examples.

[0056] Example 1

[0057] The vacuum device of the present invention [[ID=6​​The terms "front," "back," "left," "right," "up," and "down" used in the following description are based on the orientation shown in the accompanying drawings. The vacuum apparatus used in the method of this invention includes a vacuum chamber 1, a first drive module 2, a second drive module 4, a heatable sample holder 3, a compressible sample holder 5, and an expandable functional component 6. The first drive module 2 and the second drive module 4 are mounted on the vacuum chamber 1, i.e., the fabrication of two-dimensional material heterostructures is achieved under the ultra-high vacuum environment confined by the vacuum chamber 1. Specifically, the first drive module 2 is located above the vacuum chamber 1 and connected to the heatable sample holder 3; the second drive module 4 is located below the vacuum chamber 1 and connected to the compressible sample holder 5. The first drive module 2 and the second drive module 4 are concentric in the vertical direction. The expandable functional component 6 is located diagonally below the vacuum chamber 1, and includes, but is not limited to, an evaporation source furnace, an ion gun, and a radio frequency plasma source.

[0059] The specific structures of the heatable sample holder 3 and the compressible sample holder 5 are as follows: Figure 7 As shown, the heatable sample holder 3 includes a tungsten wire 71 and a sample tray 72. The compressible sample holder 5 includes a scale 81, a spring 82, and a sample tray 83. Figure 8 A typical sample holder is shown for use with sample trays 72 and 83.

[0060] The vacuum chamber 1 is also connected to a sample introduction unit and a measurement unit on its left and right sides. The sample introduction unit allows the sample holder to be transferred to sample trays 72 and 83 for further heterojunction preparation. The measurement unit allows the prepared heterojunction sample to be transferred into its interior for in-situ vacuum characterization.

[0061] In this embodiment, the first drive module 2 drives the heatable sample holder 3 to move vertically, and can also rotate at any angle in the horizontal plane. The second drive module 4 drives the compressible sample holder 5 to move vertically, and can also rotate at any angle in the horizontal plane.

[0062] In this embodiment, to achieve the fabrication of a universal two-dimensional material heterojunction, the underlying material is fixed on a sample holder, which is then transferred to the sample tray 83 of the compressible sample holder 5; the target two-dimensional material (or the parent single crystal) is fixed on another sample holder, which is then transferred to the sample tray 72 of the heatable sample holder 3. By controlling the first driving module 2 and the second driving module 4, the two sample holders continuously approach each other in the vertical direction, eventually bringing the two sample holders into contact. Due to the different bonding forces of the samples on the upper and lower surfaces of the two sample holders, the two-dimensional material (or the parent single crystal) will partially transfer to the underlying material; the next step is for the two sample holders to separate in the vertical direction, completing the heterojunction fabrication.

[0063] In some preferred embodiments, the above transfer process can be heated, specifically by the tungsten filament 71 of the heatable sample holder 3 generating heat when a direct current is applied, and the heat can radiate onto the sample holder.

[0064] In some preferred embodiments, the above transfer process can control the docking pressure of the two sample holders. Specifically, the compressible sample holder 5 is equipped with a spring 82, the amount of compression of which reflects the magnitude of the pressure, as indicated by a scale 81.

[0065] In some preferred embodiments, to better reflect the universality of two-dimensional material heterostructure preparation, i.e., various types of interfaces can be obtained, the surface treatment methods of the underlying material are very diverse. The underlying material is used for the preparation of the above-mentioned two-dimensional material heterostructure after surface treatment. These surface treatment methods can be (i) heat treatment; (ii) surface sputtering, cleaning and etching, which is mainly achieved through the extended functional component 6 connected to the vacuum chamber 1, i.e., the extended functional component 6 is selected as an ion gun, radio frequency plasma source, etc.; (iii) surface thin film growth, which is also achieved through the extended functional component 6 connected to the vacuum chamber 1, i.e., the extended functional component 6 is selected as various evaporation sources, including but not limited to thermal evaporation sources, electron beam evaporation sources, high-temperature pyrolysis sources. In particular, for (ii) and (iii), the vertical height of the heatable sample holder 3 can be controlled by the first driving module 2 so that the ion gun, radio frequency plasma source and various evaporation sources can be directly aligned with the center of the sample tray 72.

[0066] Example 2

[0067] Heterojunction of Si(111) and BSCCO

[0068] In this embodiment, the vacuum degree of the vacuum device of the present invention is controlled to be 5×10⁻⁶. -8 Pa. The underlying material—the Si(111) substrate—is first transferred to the sample holder 72 of the heatable sample holder 3. By applying a high voltage of about 1 kV between the tungsten wire 71 and the sample holder, the thermionic electrons released when the tungsten wire 71 heats up will bombard the sample holder under the action of the electric field, thereby causing the sample holder to reach a very high temperature (>1000℃). The Si(111) substrate is heated from 700℃ to about 1250℃ at a very fast rate, held for about 15 seconds, and then quickly cooled back to 700℃. After repeating this several times, a highly clean (7×7) reconstruction is formed on the surface of Si(111). After the Si(111) cools to room temperature, it is transferred to the sample holder 83 of the compressible sample holder 5. The BSCCO single crystal is mechanically cleaved in this device to obtain BSCCO flakes, which are then transferred to the sample holder 72 of the heatable sample holder 3. Finally, by controlling the vertical bonding and separation of the two sample holders, the transfer of the target two-dimensional material to the Si(111) substrate is completed, forming a BSCCO / Si(111) heterojunction. (See...) Figure 1 .

[0069] As can be seen from this embodiment, the Si(111) substrate can be reconstructed (7×7) by surface heat treatment in an ultra-high vacuum environment. This surface reconstruction can only be stable in an ultra-high vacuum environment. Existing technologies cannot achieve a two-dimensional heterojunction between the Si(111) substrate and the target two-dimensional material.

[0070] Example 3

[0071] Heterojunction of metal thin film with molybdenum disulfide and tungsten diselenide

[0072] In this embodiment, the vacuum degree of the vacuum device of the present invention is controlled to be 5×10⁻⁶. -8 Pa. First, the underlying material—a silicon wafer—is transferred to the sample tray 72 of the heatable sample holder 3. Surface deposition of the silicon wafer is achieved through the extended functional component 6. Specifically, the extended functional component 6 is selected from various evaporation sources, and the evaporation source is directly aligned with the center of the sample tray 72. Depending on the type of metal film to be deposited, an evaporation source within the corresponding temperature range is selected, as detailed in Table 1. The silicon wafer is kept at room temperature, and the evaporation source heats the metal to be evaporated, forming a stable beam that deposits onto the substrate surface to form a film. The film deposition time is controlled at approximately 10 minutes, and the film thickness is controlled at approximately 10 nm. After completion, the evaporation source is slowly cooled to room temperature. Next, the silicon wafer is transferred to the sample tray 83 of the compressible sample holder 5. Molybdenum disulfide and tungsten diselenide single crystals are mechanically cleaved in this device to obtain corresponding thin films, which are then transferred to the sample tray 72 of the heatable sample holder 3. Finally, by controlling the vertical bonding and separation of the two sample holders, the transfer of the target two-dimensional material to the metal thin film on the silicon wafer is completed, forming the heterojunction to be prepared. (See...) Figure 2 .

[0073] Table 1. Deposition conditions of various metal thin films in this embodiment.

[0074]

[0075] This embodiment demonstrates that many metal thin films are easily oxidized in atmospheric, glove box, or rough vacuum environments. For example, Fe will rapidly turn into Fe oxide on its surface in air or a glove box, making it impossible to construct heterojunctions of elemental Fe films and two-dimensional materials using conventional methods. This example demonstrates how heterojunctions of elemental Fe films and two-dimensional materials can be obtained.

[0076] Example 4

[0077] Heterojunctions of oxides with graphene, molybdenum disulfide, BSCCO and FeSe

[0078] In this embodiment, the vacuum degree of the vacuum device of the present invention is controlled to be 5×10⁻⁶. -8 Pa. The underlying material—the oxide substrate—is first transferred to the sample tray 72 of the heatable sample holder 3. By applying a high voltage of approximately 1 kV between the tungsten filament 71 and the sample tray, the thermionic electrons released when the tungsten filament 71 heats up bombard the sample tray under the influence of the electric field, thereby raising the sample tray to a very high temperature (>1000℃). After the substrate is held at such a high temperature for a period of time and then allowed to cool naturally, atomically smooth and sharp steps can be formed on its surface. For example, an alumina substrate is heated at 1200℃ for about 2 hours. The heating conditions vary slightly depending on the type of oxide substrate; see Table 2 for details. After the substrate cools to room temperature, it is transferred to the sample tray 83 of the compressible sample holder 5. Graphite, molybdenum disulfide, BSCCO, and FeSe single crystals are mechanically cleaved in this device to obtain corresponding flakes, which are then transferred to the sample tray 72 of the heatable sample holder 3. Finally, by controlling the vertical bonding and separation of the two sample holders, the transfer of these two-dimensional materials to the oxide substrate is completed, forming a two-dimensional material / oxide heterojunction. (See...) Figure 1 .

[0079] Table 2 Heating conditions for various oxide substrates in this embodiment

[0080]

[0081] As can be seen from this example, although various oxides are relatively stable, they will still adsorb a large number of impurities and gas molecules in the air. These impurities and gas molecules will be bound at the interface of the heterojunction, affecting the quality of the heterojunction. High-quality heterojunctions can be obtained through the method of this invention.

[0082] Comparative Example 1

[0083] The molybdenum disulfide / Au thin film heterojunction was prepared using the preparation method of Example 3, except that the vacuum level of Example 3 was adjusted to a low vacuum environment (approximately 1 Pa).

[0084] In this comparative example, the apparatus of the present invention was used in a vacuum environment (~10) -8 Au thin films were deposited on the surface of silicon wafers under low vacuum (~1 Pa) and low vacuum (~1 Pa) conditions. After being left to stand for one day, the Au thin films were used to prepare molybdenum disulfide / Au thin film heterojunctions. Figure 5a and Figure 5b The differences between the heterojunctions prepared under these two conditions are shown: the sample obtained by the method in Example 3 of this invention has a large area of ​​monolayer molybdenum disulfide, high overall coverage, and a large effective area of ​​heterojunction; while the sample obtained under low vacuum environment has low overall coverage and a very limited monolayer molybdenum disulfide area.

[0085] Comparative Example 2

[0086] FeSe / SrTiO3 heterojunctions were prepared using the method described in Example 4, while the control experiment was conducted in a glove box environment using an SrTiO3 substrate. This substrate was not heated at high temperatures in this apparatus; it was only subjected to ultrasonic cleaning with anhydrous ethanol at a power of 100W for 10 minutes.

[0087] The SrTiO3 substrate that has only undergone the above cleaning process has a very rough surface, with a vertical height undulation of 5 nm within a 10-micrometer lateral range. The resulting heterojunction interface is not smooth enough, and the FeSe flakes are very fragmented. In contrast, the sample prepared using the method of the present invention exhibits atomically smooth steps on the SrTiO3 substrate, with the FeSe flakes adhering tightly to it and maintaining a complete shape.

Claims

1. A method for preparing a two-dimensional material heterostructure, comprising the following steps: (1) Mechanically cleaving the target two-dimensional material in a vacuum device; (2) The bottom material is pretreated in the vacuum device; (3) The target two-dimensional material is transferred onto the pretreated substrate material in the vacuum device; in, The vacuum level of the vacuum device is less than or equal to 5 × 10⁻⁶. -8 Pa; The underlying material is a Si(111) substrate, a silicon wafer, or an oxide substrate; When the underlying material is a Si(111) substrate or an oxide substrate, the pretreatment is performed by heat treatment; When the underlying material is a silicon wafer, the pretreatment is performed by surface thin film growth; The vacuum device includes a vacuum chamber, a first drive module, a heatable sample holder, a second drive module, a compressible sample holder, and expandable functional components; The first driving module is disposed on the vacuum chamber and connected to the heatable sample holder via a driving rod; the second driving module is disposed on the vacuum chamber at a position opposite to the first driving module and connected to the compressible sample holder via a driving rod; wherein the first driving module can drive the heatable sample holder to move up and down and rotate at any angle in the plane, and the second driving module can drive the compressible sample holder to move up and down and rotate at any angle in the plane so that the target two-dimensional material on the heatable sample holder and the bottom material on the compressible sample holder can adhere to and separate from each other. The extended functional components include an evaporation source furnace, an ion gun, and a radio frequency plasma source to pre-treat the underlying material, and the extended functional components are electrically connected to the vacuum chamber.

2. The method according to claim 1, wherein, The heatable sample holder is equipped with tungsten wires to heat the underlying material when the target two-dimensional material and the underlying material are attached to each other.

3. The method according to claim 1, wherein, The compressible sample holder is equipped with a spring and a scale to determine the pressure applied when transferring the target two-dimensional material onto the pretreated substrate.

4. The method according to claim 1, wherein, Both the heatable sample holder and the compressible sample holder are equipped with sample trays to fit the sample trays that support the target two-dimensional material and the underlying material.

5. The method according to claim 1, wherein, The vacuum device also includes a sample introduction unit connected to the vacuum chamber for transferring samples to the heatable or compressible sample holder, and a measurement unit for in-situ characterization of the prepared two-dimensional material heterostructure.

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