Preparation method of corner two-dimensional material

Abstract

The invention belongs to the technical field of two-dimensional materials, and particularly relates to a preparation method of a corner heterojunction. According to the preparation method, when the two-dimensional material sheet is prepared, based on the characteristics of smooth surface and high viscosity of the hydrophilic polymer layer, a large-area two-dimensional material can be stripped from the silicon substrate with the hydrophilic polymer film layer, the stripping process is clean, and the two-dimensional material is not damaged; when the corner heterojunction is prepared, an intermediate two-dimensional material or an active interface does not contact any polymer in the whole transfer process by adjusting the shape and size of a transfer medium based on a Van der Waals force pickup transfer method, so that the pollution of a heterogeneous interface is greatly reduced; the efficient and fast preparation of the Van der Waals heterostructure without polymer residues and bubbles on the interface is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of two-dimensional materials, and relates to a method for preparing a corner two-dimensional material, and specifically to a method for preparing a corner heterojunction. Background Art

[0002] The use of interlayer van der Waals forces to orderly stack different two-dimensional materials to prepare two-dimensional material homojunctions or heterojunctions has important research significance in the field of two-dimensional materials. Two-dimensional material van der Waals heterojunctions or homojunctions have very important and extensive applications in new optoelectronic devices, catalysis, energy and other fields. Recent studies have found that by changing the relative rotation angle between adjacent two-dimensional materials in a heterojunction, a series of novel physical properties can be obtained, such as unconventional superconducting properties, molar excitons, tunneling conductance, etc. The regulation of the rotation angle provides a new way to regulate two-dimensional material heterojunctions or homojunctions, which is of great significance for expanding the potential application of two-dimensional materials in the field of micro-nano optoelectronics.

[0003] At present, the common methods for preparing corner two-dimensional materials mainly include chemical vapor deposition and mechanical stripping and pick-up transfer. Chemical vapor deposition (for example, see reference 1, reference 2) requires the use of a complex chemical vapor deposition system to finely control multiple parameters such as temperature, air pressure, precursors, etc. during the growth process. Therefore, the preparation of corner two-dimensional materials by chemical vapor deposition has problems such as complex process, low yield, many product defects, and poor controllability. Mechanical stripping and pick-up transfer (for example, see reference 3, reference 4) is currently the most commonly used method for preparing corner two-dimensional materials. The two-dimensional material used in this method is stripped from the crystal, with a more complete structure and better properties. However, this method still has problems such as low yield, small material size, difficulty in picking up, and complex preparation process. Therefore, it is of great scientific significance and practical value to develop a simple and well-controllable method for preparing corner two-dimensional materials.

[0004] References:

[0005] Reference 1: CN113186595A

[0006] Reference 2: CN113666418A

[0007] Cited literature 3: CR Dean, AF Young, I Meric, et al. Boron nitride substrates for high-quality graphene electronics [J]. Nature Nanotechnology, 2010, 5: 722-726.

[0008] Cited literature 4: L Wang, I Meric, PY Huang, et al. One-dimensional electrical contact to a two-dimensional material [J]. Science, 2013, 342: 614-617. Summary of the invention

[0009] Problem that the invention aims to solve

[0010] In order to solve the deficiencies in the prior art, the purpose of the present invention is to provide a method for preparing corner two-dimensional materials to solve the problems of low yield, small material size, difficult picking, complex preparation process, and easy impurity contamination in the current mechanical peeling and pick-up transfer method.

[0011] Solutions for solving problems

[0012] It has been found that the above technical problems can be solved by implementing the following technical solutions:

[0013] [1]. A method for preparing a corner heterojunction, comprising the following steps:

[0014] 1) Pretreatment of silicon substrate: coating a solution containing a hydrophilic polymer on the surface of the silicon substrate to form a hydrophilic polymer film layer;

[0015] 2) Preparation of substrate samples: mechanically peeling a two-dimensional material onto the hydrophilic polymer film layer of the silicon substrate to form a substrate sample having a two-dimensional material flake, wherein the two-dimensional material comprises a first two-dimensional material and a second two-dimensional material to respectively form a substrate sample 1 having the first two-dimensional material and a substrate sample 2 having the second two-dimensional material, wherein the first two-dimensional material and the second two-dimensional material are different at least in chemical composition;

[0016] 3) a first picking-up and transferring step: picking up and transferring the first two-dimensional material sheet from the substrate sample 1 to a transfer medium to form a first two-dimensional material layer on the transfer medium;

[0017] 4) a second picking and transferring step: using a transfer medium on which the first two-dimensional material layer is formed, and utilizing the interaction force between different two-dimensional materials to pick up and transfer the second two-dimensional material sheet from the substrate sample 2 to the first two-dimensional material layer of the transfer medium, so as to form a second two-dimensional material layer, and the contact area between the second two-dimensional material layer and the first two-dimensional material layer is smaller than the total area of ​​the second two-dimensional material layer;

[0018] 5) a step of segmenting and transferring: segmenting and transferring the portion of the second two-dimensional material layer on the transfer medium that is not in contact with the first two-dimensional material layer to another substrate sample 3 having the first two-dimensional material sheet, thereby forming a second two-dimensional material layer A retained on the transfer medium and a second two-dimensional material layer B on the substrate sample 3;

[0019] 6) A step of forming a corner heterojunction: laminating the second two-dimensional material layer A and the second two-dimensional material layer B at a certain angle to form a corner heterojunction with a four-layer structure;

[0020] Wherein, the interaction force between the first two-dimensional material and the second two-dimensional material is stronger than the interaction force between the two-dimensional material and the non-two-dimensional material, and the non-two-dimensional material includes a hydrophilic polymer film layer or a transfer medium;

[0021] Steps 3) to 6) are all performed in the transfer system.

[0022] [2]. The preparation method according to [1], wherein in step 1),

[0023] The hydrophilic polymer is a polymer having multiple hydroxyl groups;

[0024] The content of the hydrophilic polymer in the solution containing the hydrophilic polymer is 2-4 wt %.

[0025] [3]. The preparation method according to [1] or [2], wherein in step 2),

[0026] The first two-dimensional material is boron nitride;

[0027] The second two-dimensional material is any one of graphene, black phosphorus, transition metal chalcogenides, transition metal carbides, transition metal nitrides and transition metal oxides.

[0028] [4]. The preparation method according to any one of [1] to [3], wherein in step 3),

[0029] The transfer medium is a polymer material, preferably a PDMS / PPC composite film or a PDMS / PC composite film;

[0030] The transfer medium is substantially hemispherical in shape.

[0031] [5]. The preparation method according to any one of [1] to [4], wherein in step 4),

[0032] The contact area between the second two-dimensional material layer and the first two-dimensional material layer accounts for 20-60% of the total area of ​​the second two-dimensional material layer.

[0033] [6]. The preparation method according to any one of [1] to [5], wherein in step 5),

[0034] The areas of the second two-dimensional material layer A and the second two-dimensional material layer B are the same or different;

[0035] The sum of the areas of the second two-dimensional material layer A and the second two-dimensional material layer B is equal to the area of ​​the second two-dimensional material layer.

[0036] [7]. The preparation method according to any one of [1] to [6], wherein

[0037] Water is involved in the first pick-up and transfer step, the second pick-up and transfer step, and the corner heterojunction formation step.

[0038] [8] The preparation method according to any one of [1] to [7], wherein

[0039] The first picking-up and transferring step, the second picking-up and transferring step, and the corner heterojunction forming step further include one or more of washing and drying steps.

[0040] [9]. The preparation method according to [8], wherein:

[0041] The washing is performed using a washing liquid; the washing liquid is water; and / or,

[0042] The drying is performed using a dry gas; the dry gas is dry air or dry nitrogen.

[0043]

[10] . The preparation method according to any one of [1] to [9], wherein

[0044] The transfer system includes a substrate stage, a transfer bracket, an optical microscope and a water supply component; wherein the substrate stage is connected to a heating system and is assisted by a vacuum chuck to fix the substrate sample, and the transfer bracket is used to clamp a glass slide adhered with a transfer medium.

[0045] Effects of the Invention

[0046] Through the implementation of the above technical solution, the beneficial effects of the present invention are:

[0047] Compared with the traditional mechanical stripping method, the preparation method provided by the present invention, when preparing a two-dimensional material slice, coats a hydrophilic polymer film on a silicon substrate as an auxiliary layer, and utilizes the characteristics of the hydrophilic polymer layer's smooth surface and strong adhesion to achieve a large area of ​​two-dimensional material stripped on a silicon substrate with a hydrophilic polymer film layer. The stripping efficiency is high, and the stripping process is clean without damaging the two-dimensional material.

[0048] Compared with the traditional pick-up transfer method, the preparation method provided by the present invention, when preparing the van der Waals heterostructure, is based on a transfer method based on van der Waals force picking (for example, a transfer method based on PDMS / PC or PDMS / PPC), and by adjusting the shape and size of the transfer medium, the intermediate two-dimensional material or the active interface will not contact any polymer during the entire transfer process, thereby greatly reducing the contamination of the heterogeneous interface and achieving efficient and fast preparation of van der Waals heterostructures without polymer residue and bubble on the interface.

[0049] The preparation method provided by the present invention has the advantages of simple process, low energy consumption, environmental friendliness, good stability, strong controllability, mild experimental conditions, etc. It can realize the rapid and large-scale preparation of corner two-dimensional materials, and overcomes the problems of low yield, small area, and difficulty in picking up in the traditional mechanical stripping method, as well as the complex process, many product defects, and poor controllability in the chemical vapor deposition method. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 Schematic diagram of the preparation of the PVA solution in Example 1 of the present invention;

[0051] Figure 2 This is a real picture of the silicon wafer after spin coating with PVA solution in Example 1 of the present invention;

[0052] Figure 3 This is an optical image of the two-dimensional material sheet prepared in Example 1 of the present invention;

[0053] Figure 4 This is a physical picture of the transfer medium prepared in Example 1 of the present invention;

[0054] Figure 5 This is a physical diagram of the transfer system constructed in Example 1 of the present invention;

[0055] Figure 6 Schematic diagram of the preparation process of the corner heterojunction in Example 1 of the present invention.

[0056] Description of Reference Numerals

[0057] Figure 5 In the figure, 1 is a substrate stage; 2 is a transfer bracket; 3 is an optical microscope; 4 is a water supply component; and 5 is a water supply auxiliary component. DETAILED DESCRIPTION

[0058] Various exemplary embodiments, features and aspects of the present invention will be described in detail below. The word "exemplary" used here means "used as an example, embodiment or illustrative". Any embodiment described here as "exemplary" is not necessarily interpreted as being superior or better than other embodiments.

[0059] In addition, in order to better illustrate the present invention, numerous specific details are provided in the following specific embodiments. It should be understood by those skilled in the art that the present invention can be implemented without certain specific details. In other examples, methods, means, equipment and steps well known to those skilled in the art are not described in detail in order to highlight the subject matter of the present invention.

[0060] Unless otherwise stated, the units used in this specification are all international standard units, and the numerical values ​​and numerical ranges appearing in the present invention should be understood to include the inevitable systematic errors in industrial production.

[0061] In this specification, the word "may" means both performing a certain process and not performing a certain process.

[0062] In this specification, the references to "some specific / preferred embodiments", "other specific / preferred embodiments", "embodiments", etc., mean that the specific elements (e.g., features, structures, properties and / or characteristics) described in connection with the embodiments are included in at least one embodiment described herein, and may or may not exist in other embodiments. In addition, it should be understood that the elements may be combined in various embodiments in any suitable manner.

[0063] In this specification, the numerical range expressed using "a numerical value A to a numerical value B" means a range including the endpoints numerical values ​​A and B.

[0064] In the present specification, a numerical range expressed using "above" or "below" means a numerical range including the number.

[0065] In the present specification, the use of "optional" or "optional / optionally" indicates that certain substances, components, execution steps, application conditions and other factors are used or not used.

[0066] In this specification, unless otherwise specified, the "normal temperature" or "room temperature" used generally refers to the temperature of 23±2°C.

[0067] In this specification, the unit names used are all international standard unit names, and unless otherwise stated, the "%" used indicates weight or mass percentage.

[0068] In this specification, "comprising", "having", "including" or "containing" may mean inclusive or open-ended, not excluding additional, uncited elements or method steps. At the same time, "comprising", "having", "including" or "containing" may also mean closed, excluding additional, uncited elements or method steps.

[0069] In this specification, "about" is used to define the numerical range and parameters of the present invention. All numerical values ​​are approximate, and the specific relevant numerical values ​​are presented as accurately as possible. Unless otherwise expressly stated, it should be understood that all ranges, quantities, values ​​and percentages used in the present invention are modified by "about". Here, "about" usually means that the actual value is within ±5%, ±3%, ±1% or ±0.5% of a particular value or range.

[0070] In this specification, "first", "second", "the first" or "the second" may modify various components regardless of order and / or importance, but these expressions do not limit the corresponding components, and the above expressions are only configured for the purpose of distinguishing an element from other elements. For example, a first two-dimensional material and a second two-dimensional material represent different two-dimensional materials, although both are two-dimensional materials. Without departing from the scope of the present invention, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element.

[0071] The present invention provides a method for preparing a corner heterojunction, which comprises the following steps:

[0072] 1) Pretreatment of silicon substrate: coating a solution containing a hydrophilic polymer on the surface of the silicon substrate to form a hydrophilic polymer film layer;

[0073] 2) Preparation of substrate samples: mechanically peeling a two-dimensional material onto the hydrophilic polymer film layer of the silicon substrate to form a substrate sample having a two-dimensional material flake, wherein the two-dimensional material comprises a first two-dimensional material and a second two-dimensional material to respectively form a substrate sample 1 having the first two-dimensional material and a substrate sample 2 having the second two-dimensional material, wherein the first two-dimensional material and the second two-dimensional material are different at least in chemical composition;

[0074] 3) a first picking-up and transferring step: picking up and transferring the first two-dimensional material sheet from the substrate sample 1 to a transfer medium to form a first two-dimensional material layer on the transfer medium;

[0075] 4) a second picking and transferring step: using a transfer medium on which the first two-dimensional material layer is formed, and utilizing the interaction force between different two-dimensional materials to pick up and transfer the second two-dimensional material sheet from the substrate sample 2 to the first two-dimensional material layer of the transfer medium, so as to form a second two-dimensional material layer, and the contact area between the second two-dimensional material layer and the first two-dimensional material layer is smaller than the total area of ​​the second two-dimensional material layer;

[0076] 5) a step of segmenting and transferring: segmenting and transferring the portion of the second two-dimensional material layer on the transfer medium that is not in contact with the first two-dimensional material layer to another substrate sample 3 having the first two-dimensional material sheet, thereby forming a second two-dimensional material layer A retained on the transfer medium and a second two-dimensional material layer B on the substrate sample 3;

[0077] 6) A step of forming a corner heterojunction: laminating the second two-dimensional material layer A and the second two-dimensional material layer B at a certain angle to form a corner heterojunction with a four-layer structure;

[0078] Wherein, the interaction force between the first two-dimensional material and the second two-dimensional material is stronger than the interaction force between the two-dimensional material and the non-two-dimensional material, and the non-two-dimensional material includes a hydrophilic polymer film layer or a transfer medium;

[0079] Steps 3) to 6) are all performed in the transfer system.

[0080] Step 1): Pretreatment of silicon substrate

[0081] The main purpose of pretreatment of the silicon substrate is to make the surface of the silicon substrate smoother and more sticky, which is beneficial to the enhancement of the load and binding force of the two-dimensional material, and then to the peeling off of the two-dimensional material with large area and uniform thickness.

[0082] In the present invention, the specific operation steps of pre-treating the silicon substrate are as follows: coating a solution containing a hydrophilic polymer on the surface of the silicon substrate to form a hydrophilic polymer film layer.

[0083] In a specific embodiment of the present invention, the hydrophilic polymer is a polymer having multiple hydroxyl groups, for example, polyvinyl alcohol (PVA) and the like.

[0084] In a specific embodiment of the present invention, the content of the hydrophilic polymer in the solution containing the hydrophilic polymer is 2-4wt%, for example, it can be 2wt%, 2.2wt%, 2.5wt%, 2.8wt%, 3wt%, 3.2wt%, 3.5wt%, 3.8wt%, 4wt% and the like.

[0085] Specifically, the present invention does not specifically limit the solvent in the solution containing the hydrophilic polymer, and can be any feasible polar solvent in the art, preferably water. In some specific embodiments, there is no specific limitation on the type of water used in the present invention, and can be one or more of purified water, distilled water, deionized water and water for injection.

[0086] The present invention does not specifically limit the coating method of the solution containing the hydrophilic polymer, and can adopt coating methods commonly known in the art, such as brush coating, spray coating, dip coating, spin coating, casting, curtain coating, extrusion coating, etc., preferably spin coating. In some specific embodiments, the spin coating speed is 6000-10000r / min, preferably 7000-9000r / min, for example, it can be 6000r / min, 6500r / min, 7000r / min, 7500r / min, 8000r / min, 8500r / min, 9000r / min, 9500r / min, 10000r / min, etc.

[0087] In some specific embodiments, after coating the solution containing the hydrophilic polymer, the resulting hydrophilic polymer film may be subjected to an independent drying step to form a hydrophilic polymer film layer. In the present invention, there is no particular limitation on the drying method, and a drying method commonly known in the art may be used, such as natural drying, wind drying, heating drying, oven drying, etc.

[0088] In some other specific embodiments, step 1) can be performed in an atmospheric environment or in a gas atmosphere. From the viewpoint of reducing costs, it is preferably performed in an atmospheric environment.

[0089] Step 2): Preparation of substrate samples

[0090] The two-dimensional material is mechanically peeled off onto the hydrophilic polymer film layer of the silicon substrate to form a substrate sample having a two-dimensional material flake.

[0091] In the present invention, there is no particular limitation on the type of two-dimensional material, and it can be selected according to actual needs. In some embodiments, the two-dimensional material includes hexagonal boron nitride (h-BN), graphene, black phosphorus (BP), transition metal chalcogenides, transition metal carbides (such as Mo 2 C), transition metal nitrides (such as MoN 2 ), transition metal oxides, etc. These materials may be used alone or in combination of two or more.

[0092] In some embodiments, the atomic composition of transition metal dichalcogenides (TMDs) is MX 2 , wherein M represents a transition metal element, and X represents a chalcogen element (S, Se, Te); in some specific embodiments, the transition metal chalcogenide comprises molybdenum disulfide (MoS 2 ), tungsten disulfide (WS 2 ), niobium disulfide (NbS2 ), tantalum disulfide (TaS 2 ), titanium disulfide (TiS 2 ), zirconium disulfide (ZrS 2 ), rhenium disulfide (ReS 2 ), Molybdenum diselenide (MoSe 2 ), tungsten diselenide (WSe 2 ), niobium diselenide (NbSe 2 ), nickel diselenide (NiSe 2 ), zirconium diselenide (ZrSe 2 ), rhenium diselenide (ReSe 2 ), molybdenum ditelluride (MoTe 2 ), tungsten ditelluride (WTe 2 )wait.

[0093] In some specific embodiments, the transition metal oxide comprises molybdenum trioxide (MoO 3 ), tungsten trioxide (WO 3 ), tantalum trioxide (TaO 3 ), titanium dioxide (TiO 2 ), manganese dioxide (MnO 2 ), ruthenium dioxide (RuO 2 )wait.

[0094] In the present invention, the specific operation steps of the mechanical peeling are as follows: a single crystal block of the two-dimensional material is adhered to a tape, and the tapes adhered with the single crystal block are adhered to each other to obtain a tape adhered with the two-dimensional material.

[0095] In some specific embodiments, the tape preferably includes 3M tape or electronic grade tape (referred to as blue film tape). The present invention has no special requirements on the way the single crystal block of the two-dimensional material is adhered to the tape, and any adhesion method known in the art can be used.

[0096] In some specific embodiments, the bonding is preferably performed multiple times. The present invention bonds the tape with the single crystal block adhered thereto, the thickness of the single crystal block is continuously reduced, and the coverage on the tape is continuously increased, thereby peeling the single crystal block of the two-dimensional material into a two-dimensional thin sheet.

[0097] Further, when the two-dimensional material substantially completely covers the tape, the tape adhered with the two-dimensional material is pressed onto the hydrophilic polymer film layer of the silicon substrate, and the tape is peeled off to form a substrate sample having a thin sheet of the two-dimensional material.

[0098] In some preferred embodiments, the step of heating the silicon substrate is further included before peeling off the tape. In some specific embodiments, the temperature of the heating treatment is 60-100°C, preferably 70-90°C, and more preferably 75-80°C; the time of the heating treatment is preferably 5s-100s, and more preferably 10s-60s. The present invention preferably determines a suitable heating time according to the heating temperature to prevent the tape from melting. The present invention heats the substrate sample with a two-dimensional material flake, thereby increasing the adsorption force of the hydrophilic polymer film on the two-dimensional material, weakening the interaction force between the two-dimensional material and the tape, thereby facilitating the subsequent separation of the two-dimensional material from the tape, and increasing the yield and area of ​​two-dimensional material peeling. In the present invention, the heating treatment is preferably performed on a heating table.

[0099] In some specific embodiments, the two-dimensional material includes a first two-dimensional material and a second two-dimensional material to respectively form a substrate sample 1 having a first two-dimensional material and a substrate sample 2 having a second two-dimensional material, and the first two-dimensional material and the second two-dimensional material are different at least in chemical composition.

[0100] It should be noted that the number of substrate samples having the first two-dimensional material and the number of substrate samples having the second two-dimensional material prepared by the present invention can be the same or different, and can be one or more. For example, the number of substrate samples having the first two-dimensional material can be two, and the number of substrate samples having the second two-dimensional material can be one. Specifically, the preparation can be carried out according to the actual required amount.

[0101] In some specific embodiments, the first two-dimensional material is boron nitride.

[0102] In some specific embodiments, the second two-dimensional material is any one of graphene, black phosphorus, transition metal chalcogenides, transition metal carbides, transition metal nitrides and transition metal oxides.

[0103] Further, the prepared substrate sample is placed under an optical microscope to observe the two-dimensional material attached to the hydrophilic polymer film layer, and the two-dimensional material with a suitable size and thickness is selected to proceed to the subsequent steps.

[0104] Step 3): First pick-up transfer steps

[0105] The purpose of the first pick-up transfer is mainly to pick up and transfer the first two-dimensional material sheet from the substrate sample 1 to the transfer medium to form a first two-dimensional material layer on the transfer medium, that is, transfer medium / first two-dimensional material layer.

[0106] The current pick-up transfer method in the prior art is a fully dry transfer method based on PDMS (which can be called a PDMS peeling transfer method or a PDMS-assisted transfer method). This method is based on the viscoelasticity of the PDMS film and directly peels the two-dimensional material onto the PDMS film. PDMS is a hydrophobic silicone material with uniform thickness and high transparency. Small pieces of PDMS film are usually called PDMS stamps. The viscoelastic PDMS stamp can be used as a transfer medium to realize the free assembly of heterogeneous structures of different two-dimensional materials.

[0107] The inventors found during the experiment that it is difficult to pick up different two-dimensional materials to stack multi-layer van der Waals heterostructures using only PDMS. In some specific embodiments, the transfer medium is a polymer material, preferably a PDMS / PPC composite film or a PDMS / PC composite film. By using PDMS and a high molecular polymer PC or PPC to form a composite transfer medium PDMS / PC or PDMS / PPC, it is possible to effectively pick up different two-dimensional materials to stack multi-layer van der Waals heterostructures.

[0108] Furthermore, in order to complete the preparation of high-quality heterostructures while ensuring the cleanliness of the interface of the two-dimensional material, the present invention processes the shape of the transfer medium to reduce the pickup contact area, which is conducive to suppressing bubbles at the heterogeneous interface and can effectively suppress interface contaminants. In some specific embodiments, the shape of the transfer medium is substantially hemispherical. By solidifying the transfer medium into a transfer stamp with a lens shape to reduce the contact area, the interface bubbles are easily removed during the transfer process, and the preparation of a van der Waals heterostructure with no interface bubbles is achieved.

[0109] In some specific embodiments, water is involved in the first picking-up and transferring step.

[0110] In some specific embodiments, the first picking and transferring step is performed in a transfer system.

[0111] Step 4): Second pick-up transfer steps

[0112] Using a transfer medium that forms the first two-dimensional material layer, and utilizing the interaction force between different two-dimensional materials, a second two-dimensional material sheet is picked up and transferred from the substrate sample 2 to the first two-dimensional material layer of the transfer medium to form a second two-dimensional material layer, and the contact area between the second two-dimensional material layer and the first two-dimensional material layer is smaller than the total area of ​​the second two-dimensional material layer.

[0113] In the present invention, the portion of the second two-dimensional material layer that is not in contact with the first two-dimensional material layer may or may not be in contact with the surface of the transfer medium.

[0114] In some specific embodiments, the contact area between the second two-dimensional material layer and the first two-dimensional material layer accounts for 20-60% of the total area of ​​the second two-dimensional material layer, preferably 30-50%, for example, it can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, etc.

[0115] In some specific embodiments, water is involved in the second picking and transferring step.

[0116] In some specific embodiments, the second picking and transferring step is performed in a transfer system.

[0117] Step 5): Split transfer steps

[0118] The portion of the second two-dimensional material layer on the transfer medium that is not in contact with the first two-dimensional material layer is transferred to another substrate sample 3 having a first two-dimensional material sheet by segmentation, thereby forming a second two-dimensional material layer A retained on the transfer medium and a second two-dimensional material layer B on the substrate sample 3.

[0119] In some specific embodiments, the areas of the second two-dimensional material layer A and the second two-dimensional material layer B are the same or different.

[0120] In some specific embodiments, the sum of the areas of the second two-dimensional material layer A and the second two-dimensional material layer B is equal to the area of ​​the second two-dimensional material layer.

[0121] In some specific embodiments, the split transfer step is performed in a transfer system.

[0122] Step 6): Steps for forming corner heterojunction

[0123] The second two-dimensional material layer A and the second two-dimensional material layer B are bonded at a certain angle to form a corner heterojunction with a four-layer structure, namely, first two-dimensional material layer / second two-dimensional material layer A / second two-dimensional material layer B / first two-dimensional material layer.

[0124] In some specific embodiments, water is involved in the step of forming the corner heterojunction.

[0125] In some specific embodiments, the step of forming a corner heterojunction is performed in a transfer system.

[0126] In some specific embodiments, such as Figure 5As shown, the transfer system includes a substrate stage 1, a transfer bracket 2, an optical microscope 3, a water supply component 4 and a water supply auxiliary component 5. Among them, the substrate stage is connected to the heating system and the substrate sample is fixed with the assistance of a vacuum chuck, which can not only adsorb the silicon substrate used for the transfer sample, but also heat the silicon substrate to a set temperature; the displacement platform where the substrate stage is located can translate in two axes in the XY direction, and can not only move on the same horizontal plane, but also rotate on the same horizontal plane. By controlling the displacement platform, the position and angle of the two-dimensional material on the substrate sample can be changed. The transfer bracket is used to clamp the slide with the transfer medium, and the positioning mark can ensure that the transfer medium is in the same position when fixed multiple times; the displacement platform where the transfer bracket is located can move in three axes of XYZ. By controlling the displacement platform, the horizontal position and height of the transfer medium can be controlled, so as to pick up and transfer. The optical microscope is used to observe the whole process of picking up and transferring the substrate sample to confirm the position of the transferred sample and whether it is picked up successfully. The water supply component is used to provide water to dissolve the hydrophilic polymer adsorbed on the substrate so as to transfer the two-dimensional material from the substrate sample to the transfer medium. The water supply component is fixed on the displacement stage by a clamp and moves with the displacement stage. The water supply auxiliary component is used to assist the water supply component in accurately controlling the amount of water injection. The water supply auxiliary component and the water supply component are fixed on the same displacement stage and at the same height. When water injection is required, the water supply auxiliary component can be slowly rotated to push the water supply component. Since the displacement of the water supply auxiliary component is very small, it can avoid excessive water injection and affecting the unpicked area.

[0127] In some specific embodiments, the transfer system may further include an image acquisition device and a processor, the image acquisition device being used to acquire an observation picture observed through an optical microscope, the processor being used to receive the observation picture for image recognition, and judging whether the transfer sample on the transfer bracket is in close contact with the substrate sample on the substrate stage through the result of image recognition.

[0128] In addition, the transfer system of the present invention, in addition to the various components described above, may also include, without limitation, various auxiliary components for realizing the functions of the corresponding components, such as pipelines, power control components, automatic control units, etc.

[0129] It is worth noting that when the corner heterojunction (also known as van der Waals heterostructure) contains two-dimensional materials that are sensitive to the atmosphere, moisture, etc., the above-mentioned transfer system needs to be placed in a glove box filled with inert gas (such as high-purity argon) to minimize the impact of oxygen and water on the structure and properties of the two-dimensional material.

[0130] (Other steps)

[0131] In some specific embodiments, the first pick-up and transfer step, the second pick-up and transfer step, and the corner heterojunction formation step further include one or more of washing and drying steps.

[0132] As for washing, the washing is performed using a washing liquid. The washing liquid is used to wash away a small amount of residual hydrophilic polymer substances or other impurities on the surface of the transfer medium with the two-dimensional material layer attached thereto from the two-dimensional material layer and the surface of the transfer medium, thereby obtaining a purer two-dimensional material layer. When washing, the temperature can be controlled at 5 to 30° C., preferably 15 to 25° C.; the time can be controlled at 1 min to 1.5 h, preferably 20 min to 1 h.

[0133] The present invention does not specifically limit the washing liquid, and it can be any feasible washing liquid in the art. Specifically, the washing liquid is water. By using the washing liquid to wash the transfer medium with the two-dimensional material layer attached, the hydrophilic polymer substance or other impurities remaining on the two-dimensional material layer and the surface of the transfer medium can be effectively removed.

[0134] The present invention does not specifically limit the washing method, and any feasible washing method in the art may be used. Preferably, the washing is performed by immersion.

[0135] As for drying, the drying is performed by using a drying gas. The present invention can remove water from the transfer medium with the two-dimensional material layer attached by drying. Preferably, the water is removed from the transfer medium with the two-dimensional material layer attached after washing. When the water content in the transfer medium with the two-dimensional material layer attached is low enough, the drying step can also be omitted.

[0136] Specifically, the drying gas is dry air or dry nitrogen. In some specific embodiments, the temperature of the drying gas is 0-30°C, for example, 5°C, 10°C, 15°C, 20°C, 25°C, etc.; the drying time is 0.5min-2h, for example, 5min, 10min, 15min, 20min, 30min, 40min, 50min, 1h, 1.5h, etc.

[0137] In some specific embodiments, the step of forming the corner heterojunction further includes the step of transferring the corner heterojunction to a silicon wafer.

[0138] In the present invention, the transfer medium in the sample to be transferred loses its viscosity by heating, so that the corner heterojunction sample is separated from the transfer medium and transferred to a clean silicon wafer. In some specific embodiments, different temperatures and heating times are required according to different viscosities of different transfer media.

[0139] Through the preparation method of the present invention, it can be ensured that the preparation of the corner two-dimensional material of the present invention is carried out efficiently and orderly, and the prepared corner two-dimensional material has a large area and high cleanliness.

[0140] Example

[0141] The embodiments of the present invention will be described in detail below in conjunction with the examples, but those skilled in the art will appreciate that the following examples are only used to illustrate the present invention and should not be considered to limit the scope of the present invention. If no specific conditions are specified in the examples, they are carried out according to normal conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0142] Embodiment 1:

[0143] A method for preparing a corner heterojunction (boron nitride / graphene / graphene / boron nitride), comprising the following steps:

[0144] 1) Preparation of PVA solution:

[0145] like Figure 1 As shown, at room temperature, a certain mass of polyvinyl alcohol (PVA) powder was weighed and slowly added into water under magnetic stirring to prepare a PVA solution with a mass fraction of 3%.

[0146] 2) Pretreatment of silicon substrate:

[0147] Use a coating machine to spin coat the PVA solution on the cleaned silicon wafer at a speed of 8000 r / min to form a PVA film layer. Figure 2 As shown, there are obvious spin coating marks on the surface of the silicon wafer after spin coating, and the part of the silicon wafer surface covered with the PVA film changes color under the influence of the film, becoming more blue than the original purple.

[0148] 3) Preparation of substrate samples:

[0149] The blue film tape and the boron nitride crystal are adhered to each other to obtain a tape with boron nitride attached thereto; the tape with boron nitride attached thereto is attached to a silicon wafer so that the boron nitride contacts the PVA film layer, and pressed for a period of time. The silicon wafer is placed on a heating table at 80°C and heated for 10 seconds. After cooling, the tape is slowly removed to obtain a substrate sample with a boron nitride flake.

[0150] The blue film tape and the graphene crystal are adhered to each other to obtain a tape with graphene attached thereto; the tape with graphene attached thereto is attached to a silicon wafer so that the graphene contacts the PVA film layer, and pressed for a period of time. The silicon wafer is placed on a heating table at 80°C and heated for 10 seconds. After cooling, the tape is slowly torn off to obtain a substrate sample with graphene flakes.

[0151] The silicon wafer with the two-dimensional material flake attached is placed under an optical microscope for observation, such as Figure 3 As shown, the two-dimensional material flakes obtained by this embodiment have a large area (most of the sizes are within the range of 20 to 100 μm), and the thickness is highly uniform, which is convenient for subsequent picking up, transfer and other steps. It should be noted that different two-dimensional materials have different requirements for size and thickness. In this embodiment, the thickness of the graphene flakes obtained is generally single-layer, double-layer or a few layers thick, while the thickness of the boron nitride flakes is several layers thick; after finding the two-dimensional materials that meet the requirements, record their positions so that they can be easily found when picking up later.

[0152] 4) Preparation of PDMS / PPC transfer medium:

[0153] On a transparent glass slide, a protrusion with a shape close to a hemisphere is first made with PDMS. Before the protrusion solidifies, the glass slide can be inverted to increase the thickness of the protrusion. Then, a layer of polypropylene carbonate (PPC) film is coated on the protrusion to obtain a PDMS / PPC transfer medium.

[0154] 5) Construction of transfer system:

[0155] like Figure 5 As shown, the transfer system used in the present invention is mainly composed of the following five parts:

[0156] 1 is a substrate stage (which can be translated in two axes in X and Y directions): the substrate stage is connected to a heating system and is assisted by a vacuum chuck to fix the substrate sample. It can not only adsorb the silicon substrate used to transfer the sample, but also heat the silicon substrate to a set temperature; the displacement platform on which the substrate stage is located can not only move on the same horizontal plane, but also rotate on the same horizontal plane. By controlling the displacement platform, the position and angle of the two-dimensional material on the substrate sample can be changed.

[0157] 2 is a transfer bracket (or a slide moving table, which can move in three axes of XYZ): the transfer bracket is used to clamp the slide with the transfer medium. The positioning mark can ensure that the transfer medium is in the same position when fixed multiple times; by controlling the displacement platform where the transfer bracket is located, the horizontal position and height of the transfer medium can be controlled, so as to pick up and transfer it.

[0158] 3 is an optical microscope: The optical microscope is used to observe the entire process of substrate sample picking and transfer to confirm the position of the transferred sample and whether the pick-up is successful.

[0159] 4 is a water supply component (such as a syringe): the water supply component is used to provide water to dissolve the PVA adsorbed on the substrate so as to transfer the two-dimensional material from the substrate sample to the transfer medium. The water supply component is fixed on the displacement stage by a clamp and moves with the displacement stage.

[0160] 5 is a water supply auxiliary component (such as a fine screw): the water supply auxiliary component and the water supply component are fixed on the same displacement platform and at the same height; when water needs to be injected, the water supply auxiliary component can be slowly rotated to push the water supply component. Since the displacement of the water supply auxiliary component is very small, it is possible to avoid excessive water injection and affecting the unpicked area.

[0161] 6) Pick up and transfer:

[0162] Since two-dimensional materials such as graphene and boron nitride are not sensitive to components in the air, the picking and transfer process is completed entirely in the atmosphere; for some samples that are sensitive to air, it can be completed in a glove box.

[0163] The steps of picking up and transferring in this embodiment are performed in the transfer system, and the specific operations are as follows:

[0164] First, place the substrate sample 1 with the boron nitride flake on the substrate stage and turn on the vacuum pump to suck it. After finding the recorded boron nitride flake using an optical microscope, install the glass slide on the transfer bracket so that the transfer medium PDMS / PPC protrusion faces downward. Use an optical microscope to find the thickest center position of the transfer medium (PDMS / PPC), and slowly lower the PDMS / PPC so that the boron nitride (h-BN) flake on the substrate 1 below is in full contact with the part of the transfer medium close to the center, as shown in the figure. Figure 6 As shown in a; the specific operation of the picking step is: adjust the displacement stage where the syringe is located so that the needle of the syringe is placed in the gap between the transfer medium and the silicon substrate, after confirming the position of the needle tip through an optical microscope, slowly turn the fine screw until water is observed to cover the surrounding area of ​​the contact area between the transfer medium and the silicon substrate, slowly lift the transfer medium, allow the water to gradually penetrate under the transfer medium, dissolve the PVA film on the surface of the silicon substrate, and pick up the h-BN from the substrate 1, thereby forming PDMS / PPC / h-BN; in order to further remove the PVA film that may remain on the surface of the h-BN and the transfer medium, the PDMS / PPC / h-BN can also be washed and dried. The specific operation of the washing and drying steps is: soak the PDMS / PPC / h-BN in deionized water for washing for 40 minutes, and then put it into a nitrogen drying oven for drying.

[0165] Then, the substrate 1 is replaced with the substrate sample 2 with graphene flakes, and with the help of an optical microscope, the PDMS / PPC / h-BN on the transfer holder is precisely aligned with the graphene flakes on the lower substrate 2, so that the h-BN is in contact with only a part of the graphene, and the rest of the graphene is in direct contact with the transfer medium, e.g. Figure 6As shown in b; the above-mentioned picking operation is repeated, and the stronger van der Waals force between h-BN and graphene causes the graphene to detach from the substrate 2 and be picked up by h-BN, thereby forming PDMS / PPC / h-BN / graphene, and the above-mentioned washing and drying operations are repeated.

[0166] Subsequently, the substrate 2 was replaced with a substrate sample 3 having a boron nitride sheet, and the graphene portion of the PDMS / PPC / h-BN / graphene that was not in contact with the h-BN was fully contacted with the h-BN on the underlying substrate 3, as shown in FIG. Figure 6 As shown in Figure 3, since the thickness of the h-BN on the transfer medium and the h-BN on the silicon substrate are both greater than the thickness of the picked-up graphene, under the strong interaction force between the two-dimensional materials, the graphene will be divided into two parts, namely graphene A and graphene B, where graphene A contacts the h-BN on the transfer medium and graphene B contacts the h-BN on the silicon substrate, as shown in Figure 3. Figure 6 d; keep in contact for a while, because the attraction between h-BN and graphene is greater than the attraction between the transfer medium and graphene, when the transfer medium is lifted, graphene A and the h-BN on the transfer medium will be lifted together with the transfer medium (i.e. PDMS / PPC / h-BN / graphene A), and graphene B and the h-BN on the silicon substrate will remain on the silicon substrate 3 below (i.e. silicon wafer / PVA / h-BN / graphene B), as shown in FIG. Figure 6 As shown in e.

[0167] Finally, with the help of an optical microscope, the substrate stage was rotated to rotate graphene B at a certain angle relative to graphene A, and the PDMS / PPC / h-BN / graphene A on the transfer holder was aligned with the silicon wafer / PVA / h-BN / graphene B on the substrate stage, and the PDMS / PPC / h-BN / graphene A was slowly lowered to make graphene A fully contact with graphene B, as shown in FIG. Figure 6 As shown in Figure 5, the above-mentioned picking operation is repeated to pick up the corner heterojunction as a whole (i.e., PDMS / PPC / h-BN / graphene A / graphene B / h-BN), wash and dry it, transfer it to a clean silicon wafer, and heat it to 110°C to make the PPC film on the transfer medium lose its viscosity, thereby dropping the corner heterojunction on the silicon wafer to obtain a double-layer graphene encapsulated by h-BN (i.e., h-BN / graphene A / graphene B / h-BN); since the angle of graphene A on the transfer medium does not change, the angle of rotation of graphene B is the angle of rotation of the corner two-dimensional material. For ease of understanding, please refer to Figure 6 g and Figure 6 h, they are Figure 6 e and Figure 6 f corresponds to the top view.

[0168] It is worth noting that in the traditional preparation of heterojunction samples, organic solvents are needed to remove the residual polymer materials of the transfer medium in the heterojunction samples, but this method is difficult to completely remove the residual polymer materials of the transfer medium, and at the same time introduces the problem of organic molecules; the present invention adopts a heating treatment method to ensure the efficient removal of the residual polymer materials of the transfer medium and improve the cleanliness of the sample. In addition, in the preparation method of the present invention, only the outermost layer of boron nitride contacts PPC during the picking and transfer process, and the internal graphene (or other two-dimensional materials) is wrapped by boron nitride from top to bottom, which effectively avoids the pollution problem during the transfer process.

[0169] It should be noted that, although the technical solution of the present invention is introduced with specific examples, those skilled in the art will appreciate that the present invention should not be limited thereto.

[0170] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or technical improvements in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing a corner heterojunction, characterized in that: The following steps are involved: 1) Pretreatment of silicon substrate: coating a solution containing a hydrophilic polymer on the surface of the silicon substrate to form a hydrophilic polymer film layer; 2) Preparation of substrate samples: mechanically peeling a two-dimensional material onto the hydrophilic polymer film layer of the silicon substrate to form a substrate sample having a two-dimensional material flake, wherein the two-dimensional material comprises a first two-dimensional material and a second two-dimensional material to respectively form a substrate sample 1 having the first two-dimensional material and a substrate sample 2 having the second two-dimensional material, wherein the first two-dimensional material and the second two-dimensional material are different at least in chemical composition; 3) a first picking-up and transferring step: picking up and transferring the first two-dimensional material sheet from the substrate sample 1 to a transfer medium to form a first two-dimensional material layer on the transfer medium; 4) a second picking and transferring step: using a transfer medium on which the first two-dimensional material layer is formed, and utilizing the interaction force between different two-dimensional materials to pick up and transfer the second two-dimensional material sheet from the substrate sample 2 to the first two-dimensional material layer of the transfer medium, so as to form a second two-dimensional material layer, and the contact area between the second two-dimensional material layer and the first two-dimensional material layer is smaller than the total area of ​​the second two-dimensional material layer; 5) a step of segmenting and transferring: segmenting and transferring the portion of the second two-dimensional material layer on the transfer medium that is not in contact with the first two-dimensional material layer to another substrate sample 3 having the first two-dimensional material sheet, thereby forming a second two-dimensional material layer A retained on the transfer medium and a second two-dimensional material layer B on the substrate sample 3; 6) A step of forming a corner heterojunction: laminating the second two-dimensional material layer A and the second two-dimensional material layer B at a certain angle to form a corner heterojunction with a four-layer structure; Wherein, the interaction force between the first two-dimensional material and the second two-dimensional material is stronger than the interaction force between the two-dimensional material and the non-two-dimensional material, and the non-two-dimensional material includes a hydrophilic polymer film layer or a transfer medium; Steps 3) to 6) are all performed in the transfer system.

2. The preparation method according to claim 1, characterized in that: In step 1), The hydrophilic polymer is a polymer having multiple hydroxyl groups; The content of the hydrophilic polymer in the solution containing the hydrophilic polymer is 2-4 wt %.

3. The preparation method according to claim 1 or 2, characterized in that: In step 2), The first two-dimensional material is boron nitride; The second two-dimensional material is any one of graphene, black phosphorus, transition metal chalcogenides, transition metal carbides, transition metal nitrides and transition metal oxides.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step 3), The transfer medium is a polymer material, preferably a PDMS / PPC composite film or a PDMS / PC composite film; The transfer medium is substantially hemispherical in shape.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In step 4), The contact area between the second two-dimensional material layer and the first two-dimensional material layer accounts for 20-60% of the total area of ​​the second two-dimensional material layer.

6. The preparation method according to any one of claims 1 to 5, characterized in that: In step 5), The areas of the second two-dimensional material layer A and the second two-dimensional material layer B are the same or different; The sum of the areas of the second two-dimensional material layer A and the second two-dimensional material layer B is equal to the area of ​​the second two-dimensional material layer.

7. The preparation method according to any one of claims 1 to 6, characterized in that: Water is involved in the first pick-up and transfer step, the second pick-up and transfer step, and the corner heterojunction formation step.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The first picking-up and transferring step, the second picking-up and transferring step, and the corner heterojunction forming step further include one or more of washing and drying steps.

9. The preparation method according to claim 8, characterized in that: The washing is performed using a washing liquid; the washing liquid is water; and / or, The drying is performed using a dry gas; the dry gas is dry air or dry nitrogen.

10. The preparation method according to any one of claims 1 to 9, characterized in that: The transfer system includes a substrate stage, a transfer bracket, an optical microscope and a water supply component; wherein the substrate stage is connected to a heating system and is assisted by a vacuum chuck to fix the substrate sample, and the transfer bracket is used to clamp a glass slide adhered with a transfer medium.

Images