Method for transferring and stacking two-dimensional materials using composite pdms hemispheres

Abstract

The application provides a method for transferring and stacking two-dimensional materials by using composite PDMS hemispheres. The method provided by the application realizes accurate contact of two-dimensional materials in a small area, accurate picking of target materials from materials in a pile and edge materials of a substrate, and transfer of the materials and stacking of heterojunctions, so that no other materials affect the final device. The method provided by the application can also realize accurate removal of unwanted materials around the target materials, and is expected to further develop low-cost device micro-nano processing.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor materials technology, specifically relating to a method for transferring and stacking two-dimensional materials using composite PDMS hemispheres. Background Technology

[0002] Two-dimensional materials (such as graphene, transition metal sulfides, black phosphorus, and boron nitride) and their heterostructures (structures that precisely stack or integrate different two-dimensional materials) offer novel material platforms and solutions for addressing bottlenecks in fields such as electronic information technology, energy, environment, and medicine, thanks to their unique properties including atomic-level thickness, tunable electronic band structures, abundant surface / interface effects, excellent mechanical flexibility, and optical transparency. Currently, methods for transferring two-dimensional materials and stacking heterostructures are mainly divided into dry transfer and wet transfer. Dry transfer can achieve precise transfer of two-dimensional materials with weak adhesion to the substrate, while wet transfer can achieve transfer of two-dimensional materials with strong adhesion to the substrate, but cannot achieve precise transfer. However, both dry and wet transfer methods can only achieve transfer by contacting two-dimensional materials over a large area, not by contacting them over a small area.

[0003] Material obtained through mechanical stripping is always present in sheets, and there may be other unwanted materials around the target material, or the target material may be located at the edge of the substrate. This can significantly affect heterojunction stacking, subsequent exposure, coating, and even device performance. Currently, commonly used transfer methods achieve transfer by contacting two-dimensional materials over a large area, making it impossible to achieve precise transfer operations by contacting two-dimensional materials with a small area to remove unwanted materials or pick up the target material separately.

[0004] Therefore, there is an urgent need in the field for a method to achieve precise transfer and stacking of two-dimensional materials by contacting two-dimensional materials with a small area. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a method for accurately transferring and stacking two-dimensional materials by contacting two-dimensional materials with a small area.

[0006] In a first aspect, the present invention provides a method for transferring two-dimensional materials, the method comprising the following steps:

[0007] 1) A composite PDMS hemisphere is prepared on a planar carrier, such as a glass slide, using a polydimethylsiloxane (PDMS) prepolymer and a curing agent. The composite PDMS hemisphere includes a PDMS hemisphere base and a PDMS protrusion, and the PDMS protrusion forms the apex of the PDMS hemisphere base.

[0008] 2) A polyvinyl alcohol (PVA) solution is used to form a PVA film on the surface of the composite PDMS hemisphere that completely covers the PDMS protrusions but does not completely cover the composite PDMS hemisphere;

[0009] 3) Align the PDMS protrusions of the composite PDMS hemisphere on the planar carrier with the two-dimensional material on the substrate, and use a transfer platform to press the planar carrier down to a distance where the PVA film covering the PDMS protrusions can contact the two-dimensional material under thermal expansion. Heat the composite PDMS hemisphere to a first temperature and maintain it at the first temperature for a first time period so that the two-dimensional material is bonded to the PVA film.

[0010] 4) The temperature of the composite PDMS hemisphere is lowered to a second temperature, and the composite PDMS hemisphere is raised to transfer the two-dimensional material to the planar carrier;

[0011] 5) Transfer the composite PDMS hemisphere with the two-dimensional material attached to the target position on the substrate and heat it to the third temperature, then lift the composite PDMS hemisphere so that at least a portion of the PVA film on the PDMS protrusion and the two-dimensional material fall off onto the substrate together;

[0012] 6) Remove at least a portion of the PVA film to achieve the transfer of the two-dimensional material.

[0013] In a second aspect, the present invention provides a method for transferring and stacking two-dimensional materials, the method comprising the following steps:

[0014] 1) A composite PDMS hemisphere is prepared on a planar carrier, such as a glass slide, using a polydimethylsiloxane (PDMS) prepolymer and a curing agent. The composite PDMS hemisphere includes a PDMS hemisphere base and a PDMS protrusion, and the PDMS protrusion forms the apex of the PDMS hemisphere base.

[0015] 2) A polyvinyl alcohol (PVA) solution is used to form a PVA film on the surface of the composite PDMS hemisphere that completely covers the PDMS protrusions but does not completely cover the composite PDMS hemisphere;

[0016] 3) Align the PDMS protrusion of the composite PDMS hemisphere on the planar carrier with the two-dimensional material on the substrate, and use a transfer platform to press the planar carrier down to a distance where the outermost structural element of the composite PDMS hemisphere can contact the two-dimensional material under thermal expansion. Heat the composite PDMS hemisphere to a first temperature and maintain it at the first temperature for a first time period, so that the two-dimensional material is bonded to the outermost structural element of the composite PDMS hemisphere.

[0017] 4) The temperature of the composite PDMS hemisphere is lowered to a second temperature, and the composite PDMS hemisphere is raised to transfer the two-dimensional material to the planar carrier;

[0018] 5) Optionally, repeat steps 3) through 4) to obtain a first stack of two-dimensional materials;

[0019] 6) Transfer the composite PDMS hemisphere, which is bonded with the two-dimensional material or the first stack, to a target position on the substrate and heat it to a third temperature. Raise the composite PDMS hemisphere so that at least a portion of the PVA film on the PDMS protrusion and the two-dimensional material or the first stack fall off onto the substrate together.

[0020] 7) Remove at least a portion of the PVA film to obtain a two-dimensional material stack as the target product.

[0021] In summary, this invention provides a method for precisely transferring and stacking two-dimensional materials by contacting them with a small area. This method offers the following advantages: it enables precise transfer and stacking of two-dimensional materials by contacting them with a small area, allowing for the accurate pickup of target materials from clustered materials and substrate edge materials, and the transfer and stacking of heterojunctions, ensuring that no other materials influence the final device. Furthermore, this method can also precisely remove unwanted materials around the target material, potentially leading to the development of low-cost micro / nano fabrication processes. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the specific embodiments of the present invention, the accompanying drawings in the specific embodiments will be briefly described below.

[0023] Figure 1 (a) and Figure 1 (b) shows the composite PDMS hemisphere prepared in Example 1; Figure 1 (c) shows a PVA solution dropped onto a composite PDMS hemisphere in Example 1.

[0024] Figure 2 The diagram shows a process flow diagram of stacking hBN and graphene using the method of the present invention in Example 1.

[0025] Figure 3 (a) shows the optical images of hBN before, during and after pickup in Example 1 from left to right. The green part of the pickup image is the part where PVA contacts the substrate. Figure 3 (b) shows the optical images of graphene before, during and after pickup in Example 1 from left to right. The green part of the pickup image is the part where PVA contacts the substrate. Figure 3(c) shows, from left to right, images of PVA in contact with the substrate at 140°C, PVA detaching from the silicon wafer along with hBN and graphene heterojunctions, and the final silicon wafer. Figure 3 The three sub-images in (d) show, from left to right, optical images of hBN before stacking, graphene before stacking, hBN and graphene heterojunction in Example 1.

[0026] Figure 4 (a) and Figure 4 (b) Shows the PDMS hemispheres prepared in Comparative Example 8; Figure 4 (c) shows PVA solution dropped onto a PDMS hemisphere in Comparative Example 8.

[0027] Figure 5 An optical image showing a PVA film about to contact a two-dimensional material in an example of the present invention is shown.

[0028] Figure 6 An optical image is shown illustrating how minute vibrations of the platform during the transfer of two-dimensional material cause the two-dimensional material to curl (marked by a red circle) in an example of the present invention. Detailed Implementation

[0029] The present invention will be described in detail below. It should be understood that the following description is merely illustrative and is not intended to limit the scope of the invention; the scope of protection of the invention is defined by the appended claims. Furthermore, those skilled in the art will understand that modifications can be made to the technical solutions of the present invention without departing from its spirit and intent. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the subject matter pertains. Before a detailed description of the invention, the following definitions are provided to better understand it.

[0031] In the context of this invention, many embodiments use the expressions "comprising," "including," or "basically / mainly composed of." The expressions "comprising," "including," or "basically / mainly composed of" should be understood as open-ended expressions, indicating that they include not only the elements, components, parts, and method steps specifically listed after the expression, but also other elements, components, parts, and method steps. Additionally, in this document, the expressions "comprising," "including," or "basically / mainly composed of" may also be understood as closed-ended expressions in certain circumstances, indicating that they only include the elements, components, parts, and method steps specifically listed after the expression, and do not include any other elements, components, parts, or method steps. In this case, the expression is equivalent to the expression "composed of."

[0032] It should be noted that, unless explicitly stated in the context, all numerical values ​​or ranges mentioned in this article are defined by the term "about". In this article, for a given numerical value, the term "about" means ±5% of that value, such as ±4%, ±3%, ±2%, or ±1%. For a range of numerical values, the term "about" means ±5% of the upper or lower limit of that range, such as ±4%, ±3%, ±2%, or ±1%.

[0033] In this paper, ordinal numbers such as "first," "second," and "third" are sometimes used to modify or limit elements, components, parts, method steps, solutions, solvents, temperatures, systems, etc. It should be noted that in this paper, these expressions are only used to distinguish the elements, components, parts, method steps, solutions, solvents, temperatures, systems, etc. that they modify or limit, and are not intended to limit their order or importance.

[0034] As previously stated, the present invention aims to provide a method for accurately transferring stacked two-dimensional materials by contacting two-dimensional materials with a small area.

[0035] Therefore, in a first aspect, the present invention provides a method for transferring two-dimensional materials, the method comprising the following steps:

[0036] 1) A composite PDMS hemisphere is prepared on a planar carrier, such as a glass slide, using a polydimethylsiloxane (PDMS) prepolymer and a curing agent. The composite PDMS hemisphere includes a PDMS hemisphere base and a PDMS protrusion, and the PDMS protrusion forms the apex of the PDMS hemisphere base.

[0037] 2) A polyvinyl alcohol (PVA) solution is used to form a PVA film on the surface of the composite PDMS hemisphere that completely covers the PDMS protrusions but does not completely cover the composite PDMS hemisphere;

[0038] 3) Align the PDMS protrusions of the composite PDMS hemisphere on the planar carrier with the two-dimensional material on the substrate, and use a transfer platform to press the planar carrier down to a distance where the PVA film covering the PDMS protrusions can contact the two-dimensional material under thermal expansion. Heat the composite PDMS hemisphere to a first temperature and maintain it at the first temperature for a first time period so that the two-dimensional material is bonded to the PVA film.

[0039] 4) The temperature of the composite PDMS hemisphere is lowered to a second temperature, and the composite PDMS hemisphere is raised to transfer the two-dimensional material to the planar carrier;

[0040] 5) Transfer the composite PDMS hemisphere with the two-dimensional material attached to the target position on the substrate and heat it to the third temperature, then lift the composite PDMS hemisphere so that at least a portion of the PVA film on the PDMS protrusion and the two-dimensional material fall off onto the substrate together;

[0041] 6) Remove at least a portion of the PVA film to achieve the transfer of the two-dimensional material.

[0042] In a second aspect, the present invention provides a method for transferring and stacking two-dimensional materials, the method comprising the following steps:

[0043] 1) A composite PDMS hemisphere is prepared on a planar carrier, such as a glass slide, using a polydimethylsiloxane (PDMS) prepolymer and a curing agent. The composite PDMS hemisphere includes a PDMS hemisphere base and a PDMS protrusion, and the PDMS protrusion forms the apex of the PDMS hemisphere base.

[0044] 2) A polyvinyl alcohol (PVA) solution is used to form a PVA film on the surface of the composite PDMS hemisphere that completely covers the PDMS protrusions but does not completely cover the composite PDMS hemisphere;

[0045] 3) Align the PDMS protrusion of the composite PDMS hemisphere on the planar carrier with the two-dimensional material on the substrate, and use a transfer platform to press the planar carrier down to a distance where the outermost structural element of the composite PDMS hemisphere can contact the two-dimensional material under thermal expansion. Heat the composite PDMS hemisphere to a first temperature and maintain it at the first temperature for a first time period, so that the two-dimensional material is bonded to the outermost structural element of the composite PDMS hemisphere.

[0046] 4) The temperature of the composite PDMS hemisphere is lowered to a second temperature, and the composite PDMS hemisphere is raised to transfer the two-dimensional material to the planar carrier;

[0047] 5) Optionally, repeat steps 3) through 4) to obtain a first stack of two-dimensional materials;

[0048] 6) Transfer the composite PDMS hemisphere, which is bonded with the two-dimensional material or the first stack, to a target position on the substrate and heat it to a third temperature. Raise the composite PDMS hemisphere so that at least a portion of the PVA film on the PDMS protrusion and the two-dimensional material or the first stack fall off onto the substrate together.

[0049] 7) Remove at least a portion of the PVA film to obtain a two-dimensional material stack as the target product.

[0050] The following section provides a detailed introduction to the methods of the first and second aspects:

[0051] The inventors discovered through testing that when the PVA film completely covers the surface of the composite PDMS hemisphere, the PVA film wrinkles after the composite PDMS hemisphere undergoes thermal expansion and contraction. This affects imaging and can lead to uneven stress, sometimes preventing the successful adhesion of two-dimensional materials. In one embodiment, the projection of the PVA film onto the composite PDMS hemisphere covers at least 4 / 25 of the projected area of ​​the composite PDMS hemisphere. In a specific embodiment, the projection of the PVA film onto the composite PDMS hemisphere covers 4 / 25, 8 / 25, 12 / 25, 16 / 25, 20 / 25, 24 / 25, and any value between these values. In a preferred embodiment, the projection of the PVA film onto the composite PDMS hemisphere covers 4 / 25 to 9 / 25 of the projected area of ​​the composite PDMS hemisphere. In one specific embodiment, the projection of the PVA film onto the composite PDMS hemisphere covers 4 / 25, 5 / 25, 6 / 25, 7 / 25, 8 / 25, 9 / 25, and any value between thereof of the projected area of ​​the composite PDMS hemisphere. In another embodiment, the PVA film covers only the PDMS protrusions of the composite PDMS hemisphere.

[0052] In one embodiment, in step 2), the PVA solution is placed on the surface of the composite PDMS hemisphere and baked at 75°C-85°C for 15-20 min. In a specific embodiment, the PVA solution baking temperature can be 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, or any value between these values. In a preferred embodiment, the PVA solution baking temperature is 80°C. In a specific embodiment, the PVA solution baking time can be 15 min, 16 min, 17 min, 18 min, 19 min, 20 min, or any value between these values. In a preferred embodiment, the PVA solution baking time is 20 min. The inventors found through multiple tests that when the baking temperature is below the above range, the PVA solution cannot be completely dried on the composite PDMS hemisphere, resulting in unsuccessful adhesion of the two-dimensional material. Small PVA droplets can be clearly observed under a microscope, and these droplets move during the pressing process using a transfer platform. When the baking temperature is above the above range, the PVA film becomes too brittle. When the baking time is shorter than the above range, the PVA solution cannot be dried on the composite PDMS hemisphere and cannot be used for the transfer and stacking of two-dimensional materials. When the PVA droplets are poked with tweezers, the flow of the PVA droplets can be clearly observed with the naked eye.

[0053] In step 3) of the method in the first aspect, regarding "the distance at which the PVA film covering the PDMS protrusions contacts the two-dimensional material under thermal expansion," this can be determined in practice by observing in real time whether an aperture appears around the two-dimensional material under a microscope. In a specific example, the planar carrier is pressed down using a transfer platform until an aperture just appears around the two-dimensional material. Figure 5 At this point, the distance between the PVA film and the two-dimensional material on the substrate is "the distance at which the PVA film covering the PDMS protrusions can contact the two-dimensional material under thermal expansion."

[0054] In step 3) of the second aspect of the method, the distance at which the outermost structural element of the composite PDMS hemisphere can contact the two-dimensional material under thermal expansion can also be determined by using the aperture around the two-dimensional material, which will not be elaborated here.

[0055] In step 3), the contact area between the PDMS protrusion and the two-dimensional material on the substrate can be observed in real time, and the contact area between the PDMS protrusion and the two-dimensional material on the substrate is maintained within a certain range so that the composite PDMS hemisphere does not contact non-target two-dimensional material and only the PDMS protrusion contacts the target two-dimensional material. In one embodiment, the projection of the PDMS protrusion on the substrate is completely within the two-dimensional material. In a preferred embodiment, the projection of the PDMS protrusion on the composite PDMS hemisphere covers 1 / 25 of the projected area of ​​the composite PDMS hemisphere.

[0056] In one specific example, minute vibrations of the transfer platform caused significant changes in the contact area and position between the PDMS protrusions and the two-dimensional material graphene, resulting in uneven stress on the two-dimensional material and consequently, the curling of the two-dimensional material. Figure 6 ), cracks or wrinkles.

[0057] In step 3) of the methods of the first and second aspects, the composite PDMS hemisphere is thermally expanded by heating to bring the PVA film into contact with the two-dimensional material or the first two-dimensional material into contact with the second two-dimensional material. Therefore, the composite PDMS hemisphere cannot be rapidly heated to prevent it from damaging the two-dimensional material on the substrate. In one embodiment, the composite PDMS hemisphere is heated to the first temperature at a heating rate not exceeding 10°C / min. In a specific embodiment, the heating rate may be 10°C / min, 9°C / min, 8°C / min, 7°C / min, or a lower value.

[0058] In one embodiment, the first temperature is 75℃-85℃, and the first time period is 8-10 min. In a specific embodiment, the first temperature can be 75℃, 76℃, 77℃, 78℃, 79℃, 80℃, 81℃, 82℃, 83℃, 84℃, 85℃, or any value between these values, and the first time period can be 8 min, 8.5 min, 9 min, 9.5 min, 10 min, or any value between these values. In a preferred embodiment, the first temperature is 80℃, and the first time period is 10 min. The inventors found through multiple tests that: when the first temperature is below the above range, the success rate of transferring two-dimensional materials is low or unstable; when the first temperature is above the above range, the PVA film becomes brittle and cannot adhere to the two-dimensional material; and when the first time period is short, the success rate of transferring two-dimensional materials is unstable.

[0059] In one embodiment, the second temperature is 20°C-45°C. In a specific embodiment, the second temperature can be 20°C, 22°C, 24°C, 26°C, 28°C, 30°C, 32°C, 34°C, 36°C, 38°C, 40°C, 42°C, 45°C, or any value between these values. The inventors found through numerous tests that when the second temperature exceeds the above range, the success rate of transferring two-dimensional materials is extremely low.

[0060] In one embodiment, the third temperature is 130°C-140°C. In a specific embodiment, the third temperature can be 130°C, 131°C, 132°C, 133°C, 134°C, 135°C, 136°C, 137°C, 138°C, 139°C, 140°C, or any value between these values. In a preferred embodiment, the third temperature is 140°C. The inventors have found through numerous tests that when the third temperature is below the above range, the PVA film cannot detach from the substrate.

[0061] In one embodiment, a portion of the PVA film on the PDMS protrusion covers a single two-dimensional material or a stack of two-dimensional materials, contacts the substrate, and detaches together onto the substrate, while a portion of the PVA film remains on the surface of the PDMS protrusion. In a preferred embodiment, the complete PVA film on the PDMS protrusion covers a single two-dimensional material or a stack of two-dimensional materials, contacts the substrate, and detaches together onto the substrate. Those skilled in the art will understand that since the PDMS substrate does not contact the two-dimensional material during the transfer and stacking process, a PVA film still remains on its surface.

[0062] The following section provides a further introduction to the second aspect of the method:

[0063] In one implementation, in step 6), the target location has a last two-dimensional material; step 6) includes: pressing the planar carrier down using a transfer platform to a distance from which the outermost structural element bonded to the composite PDMS hemisphere, under thermal expansion, can contact the last two-dimensional material; heating the composite PDMS hemisphere to a third temperature to allow the first stack and the last two-dimensional material to bond together to form a second stack of two-dimensional materials; and causing at least a portion of the PVA film and the second stack to detach together onto the substrate. The distance from which the outermost structural element bonded to the composite PDMS hemisphere, under thermal expansion, can contact the last two-dimensional material can also be determined using the aperture around the two-dimensional material, which will not be elaborated here. Those skilled in the art will understand that without step 5), the method can obtain a stack of two two-dimensional materials; with one step 5), the method can obtain a stack of three two-dimensional materials; with two steps 5), the method can obtain a stack of four two-dimensional materials; and so on.

[0064] In another embodiment, in step 6), the target location is clean; step 6) includes: pressing the planar carrier down using a transfer platform to a distance from which the outermost structural element bonded to the composite PDMS hemisphere, under thermal expansion, can contact the substrate; then heating the PDMS hemisphere to the third temperature, wherein heating to the third temperature causes at least a portion of the PVA film and the first stack to detach together onto the substrate, at which point the first stack is the stack of all the two-dimensional materials to be stacked. Here, "the distance from which the outermost structural element bonded to the composite PDMS hemisphere, under thermal expansion, can contact the substrate" refers to the distance from which the outermost structural element bonded to the composite PDMS hemisphere contacts the substrate during the process of heating the composite PDMS hemisphere to the third temperature.

[0065] In one embodiment of the method of the first or second aspect, removing the at least partial PVA film comprises: immersing a substrate carrying the at least partial PVA film and the two-dimensional material or the stack of the two-dimensional material in deionized water for a period of time to remove the at least partial PVA film. In a preferred embodiment, the temperature of the deionized water is 35°C-45°C. In a specific embodiment, the temperature of the deionized water can be 35°C, 36°C, 37°C, 38°C, 39°C, 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, and any value between therewith. In a more preferred embodiment, the temperature of the deionized water is 40°C. In a specific embodiment, the immersion is performed for at least 3 minutes.

[0066] The method of the first or second aspect of the present invention does not have any particular limitations on the type, area, etc. of the two-dimensional material. In one embodiment, the two-dimensional material is one or more selected from boron nitride, graphene, transition metal sulfides, chromium oxychloride, and black phosphorus. In a preferred embodiment, the transition metal sulfide is molybdenum disulfide and / or palladium diselenide.

[0067] In one embodiment of the second aspect of the method, the two-dimensional materials constituting the first stack can be the same or different; if the two-dimensional materials are different, the stack can also be referred to as a heterojunction. In another embodiment, the two-dimensional materials constituting the second stack can be the same or different.

[0068] Example

[0069] The present invention and its technical effects will be clearly and completely described below with reference to embodiments and accompanying drawings, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0070] Example 1

[0071] This embodiment utilizes the stacking method of the present invention to prepare a heterojunction of hBN and graphene, specifically including the following steps:

[0072] 1. Preparation of composite PDMS hemispheres

[0073] First, the PDMS prepolymer silicone elastomer base (DOWSIL 184 Silicone ElastomerBase, Dow) and the curing agent silicone elastomer curing agent (SYLGARD 184 Silicone Elastomer Curing Agent, Dow) were mixed at a volume ratio of 10:1 and stirred thoroughly. The prepared solution was placed in a vacuum chamber to allow air bubbles to be automatically expelled, preventing them from affecting subsequent microscope imaging. After the air bubbles disappeared, the solution was heated at 60°C for 30 minutes to thicken it. Then, the solution was dropped onto a glass slide and heated at 125°C for 3 minutes to completely solidify it, resulting in the self-made PDMS hemispherical base.

[0074] Then, using a pipette, 20 µL of the above solution was dropped onto the base of the PDMS hemispherical structure to form a very small droplet. The solution was then heated at 125°C for 3 minutes to allow it to completely solidify, resulting in a composite PDMS hemispherical structure with a base diameter of approximately 5 mm and a protrusion diameter of approximately 1 mm. Figure 1 (a) and Figure 1 (b)).

[0075] 2. Preparation of PVA solution

[0076] Add 0.6 g of polyvinyl alcohol (PVA) to a solution of 20 mL of deionized water and 20 mL of anhydrous ethanol, and stir thoroughly at 60 °C for 2 h to obtain a PVA solution.

[0077] 3. PVA solution film formation on composite PDMS hemispheres

[0078] Use a dropper to drop the PVA solution onto the composite PDMS hemisphere. Figure 1 (c) and bake at 80°C for 20 min to allow PVA to form a film directly on the composite PDMS hemispheres. Since PVA is insoluble in ethanol, the PVA film can be avoided from being too thick after the ethanol evaporates.

[0079] 4. Transfer stacked heterojunction

[0080] A diagram illustrating this step is shown below. Figure 2 As shown:

[0081] ① First, place the glass slide containing the composite PDMS hemisphere / PVA film on the transfer platform, locate the hexagonal boron nitride (hBN) using an optical microscope and align it with the composite PDMS hemisphere, especially the center of its PDMS protrusions.

[0082] ② Control the glass slide to slowly press down using a transfer platform. Just before contacting the hBN (when a halo appears near the hBN), slowly heat it to 80°C. Use thermal expansion and contraction instead of manual pressing to ensure uniform force. Maintain this temperature at 80°C for 10 minutes, then stop heating. During heating, the PDMS will continuously expand; therefore, the glass slide must be slowly raised to adjust the contact area between the PVA film and the hBN within the required range to avoid the PVA film contacting other materials besides the target hBN and to prevent damage to the target hBN.

[0083] ③ During the cooling process, PDMS will continue to shrink. Therefore, it is necessary to slowly press down the glass slide to maintain the contact area between the PVA film and hBN within the required range. When the temperature drops to room temperature, quickly lift the glass slide upward. The hBN will adhere to the PVA film and rise together with the composite PDMS hemisphere.

[0084] ④ Locate the graphene using an optical microscope and align it with the composite PDMS hemisphere, especially the center of its PDMS protrusions. Using a transfer platform, slowly press the glass slide down until it is about to contact the graphene (a halo is observed near the graphene). Slowly heat the slide to 80°C, using thermal expansion and contraction instead of manual pressing to ensure uniform force. Maintain this temperature at 80°C for 10 minutes, then stop heating. During heating, the PDMS will continuously expand; therefore, the contact area between the hBN and graphene must be adjusted within the required range by slowly raising the glass slide to avoid the PVA film contacting other materials besides the target graphene and to prevent damage to the target graphene.

[0085] ⑤ During the cooling process, PDMS will continue to shrink, so the contact area between hBN and graphene needs to be maintained within the required range by slowly pressing down the glass slide; when the temperature drops to room temperature, the glass slide is quickly lifted upwards. Since the van der Waals bonding force between hBN and graphene is greater than the van der Waals bonding force between graphene and silicon oxide substrate, graphene will adhere to hBN and rise together with the composite PDMS hemisphere.

[0086] ⑥ Locate the target position using an optical microscope and align it with the composite PDMS hemisphere, especially the center of its PDMS protrusions. Control the glass slide to slowly press down to a certain distance from the substrate using a transfer platform. Slowly heat the composite PDMS hemisphere to allow it to expand thermally, so that the heterojunction of hBN and graphene adheres to the clean silicon wafer substrate. Since the substrate is clean and free of impurities, it is not necessary to control the contact area until it is heated to 140°C. Then lift the glass slide, and the PVA film and heterojunction sample that have come into contact with the substrate will fall off onto the substrate together.

[0087] ⑦ After cooling to room temperature, immerse the silicon wafer containing the PVA film and the heterojunction sample in deionized water at 40°C for 3 minutes to remove the PVA film. Blow dry the surface liquid to obtain the heterojunction sample of hBN and graphene. Figure 3 The optical images of hBN and graphene before, during, and after pickup are shown, revealing that the method provided by this invention can precisely transfer two-dimensional materials without affecting other materials near the target two-dimensional material.

[0088] Figure 2 The flowchart is only for understanding the concept of the present invention. The relative sizes of the composite PDMS hemisphere, PVA film and two-dimensional material are not intended to limit the scope of the present invention. Those skilled in the art will understand that in actual operation, the two-dimensional material is much smaller than the composite PDMS hemisphere and PVA film.

[0089] Comparative Example 1

[0090] This embodiment compares the effects of different baking temperatures on the transfer of two-dimensional materials when PVA solution is formed on the surface of a composite PDMS hemispherical film. The baking temperatures used in this embodiment include 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, and 90℃, and specifically include the following steps:

[0091] 1. Prepare multiple composite PDMS hemispheres using the same steps as in Example 1.

[0092] 2. Prepare a PVA solution using the same steps as in Example 1.

[0093] 3. Using a dropper, PVA solution was dropped onto a composite PDMS hemisphere, and baked at 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, and 90℃ for 20 min each, allowing PVA to form a film directly on the composite PDMS hemisphere. The results showed that baking at 60℃, 65℃, and 70℃ failed to completely dry the PVA solution on the composite PDMS hemisphere, and small PVA droplets were clearly visible under a microscope. Baking at 75℃, 80℃, and 85℃ successfully dried the PVA solution on the composite PDMS hemisphere, forming a film. Although baking at 90℃ dried the PVA solution on the composite PDMS hemisphere, the PVA film was brittle.

[0094] 4. Using the same steps as in Example 1, the hBN / graphene heterojunction was transferred and stacked. The results showed that: baking the PVA solution at 60°C, 65°C, and 70°C failed to completely dry it on the composite PDMS hemisphere, resulting in the inability to successfully bond the two-dimensional material and ultimately the failure to transfer and stack the heterojunction. Small PVA droplets could be clearly observed under a microscope, and the PVA droplets would move during the pressing process using the transfer platform. Baking the PVA solution at 75°C, 80°C, and 85°C allowed the PVA solution to dry completely on the composite PDMS hemisphere and form a film, successfully transferring and stacking the hBN / graphene heterojunction. The film obtained by baking the PVA solution at 90°C was brittle, resulting in the inability to successfully bond the two-dimensional material and ultimately the failure to transfer and stack the heterojunction.

[0095] Comparative Example 2

[0096] This embodiment compares the effect of different baking times for PVA solution film formation on the surface of a composite PDMS hemispherical surface on the transfer of two-dimensional materials. The baking times used in this embodiment include 5 min, 8 min, 10 min, 15 min, 18 min, and 20 min, and specifically include the following steps:

[0097] 1. Prepare multiple composite PDMS hemispheres using the same steps as in Example 1.

[0098] 2. Prepare a PVA solution using the same steps as in Example 1.

[0099] 3. Using a dropper, PVA solution was dropped onto a composite PDMS hemisphere and baked at 80℃ for 5 min, 8 min, 10 min, 15 min, 18 min, and 20 min, respectively, to allow PVA to form a film directly on the composite PDMS hemisphere. The results showed that the PVA solution could not be dried on the composite PDMS hemisphere after baking for 5 min, 8 min, and 10 min, and the flow of the PVA droplet could be clearly observed with the naked eye when poked with tweezers; the PVA solution successfully dried and formed a film on the composite PDMS hemisphere after baking for 15 min, 18 min, and 20 min.

[0100] 4. Using the same steps as in Example 1, the hBN / graphene heterojunction was transferred and stacked. The results showed that the PVA solution could not be dried on the composite PDMS hemisphere after baking for 5 min, 8 min, and 10 min, which meant it could not be used for the transfer and stacking of two-dimensional materials. The PVA solution could be dried and formed a film on the composite PDMS hemisphere after baking for 15 min, 18 min, and 20 min, and the hBN / graphene heterojunction was successfully transferred and stacked.

[0101] Comparative Example 3

[0102] This embodiment compares the effects of different first temperatures (i.e., the temperature at which the PVA film is bonded to the two-dimensional material and the temperature at which the two-dimensional materials are bonded together) on the transfer of the two-dimensional material. The first temperatures used in this embodiment include 50°C, 60°C, 70°C, 72°C, 75°C, 80°C, 85°C, and 90°C, and specifically includes the following steps:

[0103] 1. Prepare multiple composite PDMS hemispheres using the same steps as in Example 1.

[0104] 2. Prepare a PVA solution using the same steps as in Example 1.

[0105] 3. Use a dropper to drop the PVA solution onto the composite PDMS hemisphere and bake at 80°C for 20 min to allow PVA to form a film directly on the composite PDMS hemisphere.

[0106] 4. The same steps as in Example 1 were used to transfer and stack the hBN / graphene heterojunction, wherein multiple experiments were conducted using multiple composite PDMS hemispheres for each first temperature. The results showed that: when the first temperature was 50°C or 60°C, the success rate of transferring the two-dimensional material was extremely low; when the first temperature was 70°C or 72°C, the success rate of transferring the two-dimensional material was unstable, and occasionally the two-dimensional material could not be adhered; when the first temperature was 75°C, 80°C, or 85°C, the success rate of transferring and stacking the two-dimensional material was high and stable; when the first temperature was 90°C, the PVA film became brittle and could not adhere to the two-dimensional material, thus making it impossible to transfer the stacked two-dimensional material.

[0107] Comparative Example 4

[0108] This embodiment compares the effects of different first time periods (i.e., the time period during which the PVA film and the two-dimensional material are bound together at the first temperature, and the time period during which the two-dimensional materials are bound together at the first temperature) on the transfer of two-dimensional materials. The first time periods used in this embodiment include 5 min, 6 min, 7 min, 8 min, 9 min, and 10 min, and specifically include the following steps:

[0109] 1. Prepare multiple composite PDMS hemispheres using the same steps as in Example 1.

[0110] 2. Prepare a PVA solution using the same steps as in Example 1.

[0111] 3. Use a dropper to drop the PVA solution onto the composite PDMS hemisphere and bake at 80°C for 20 min to allow PVA to form a film directly on the composite PDMS hemisphere.

[0112] 4. The same steps as in Example 1 were used to transfer and stack the hBN / graphene heterojunction, wherein multiple experiments were conducted using multiple composite PDMS hemispheres for each first time period. The results showed that when the first time period was 5 min, 6 min, or 7 min, the success rate of transferring the two-dimensional material was unstable, and occasionally the two-dimensional material could not be adhered; when the first time period was 8 min, 9 min, or 10 min, the success rate of transferring and stacking the two-dimensional material was higher and more stable.

[0113] Comparative Example 5

[0114] This embodiment compares the effects of different second temperatures (i.e., the temperature at which the two-dimensional material is transferred) on the transfer of the two-dimensional material. The second temperatures used in this embodiment include 20°C, 30°C, 40°C, 45°C, 50°C, 55°C, and 60°C. The specific steps include:

[0115] 1. Prepare multiple composite PDMS hemispheres using the same steps as in Example 1.

[0116] 2. Prepare a PVA solution using the same steps as in Example 1.

[0117] 3. Use a dropper to drop the PVA solution onto the composite PDMS hemisphere and bake at 80°C for 20 min to allow PVA to form a film directly on the composite PDMS hemisphere.

[0118] 4. The same steps as in Example 1 were used to transfer and stack the hBN / graphene heterojunction, wherein multiple experiments were conducted using multiple composite PDMS hemispheres for each second temperature. The results showed that the success rate of transferring the two-dimensional material was extremely low when the second temperature was 50°C, 55°C, or 60°C; the success rate of transferring and stacking the two-dimensional material was higher and more stable when the second temperature was 20°C, 30°C, 40°C, or 45°C.

[0119] Comparative Example 6

[0120] This embodiment compares the effects of different third temperatures (i.e., the temperature at which the PVA film detaches) on the transfer of two-dimensional materials. The third temperatures used in this embodiment include 120°C, 125°C, 130°C, 135°C, and 140°C, and specifically include the following steps:

[0121] 1. Prepare multiple composite PDMS hemispheres using the same steps as in Example 1.

[0122] 2. Prepare a PVA solution using the same steps as in Example 1.

[0123] 3. Use a dropper to drop the PVA solution onto the composite PDMS hemisphere and bake at 80°C for 20 min to allow PVA to form a film directly on the composite PDMS hemisphere.

[0124] 4. The same steps as in Example 1 were used to transfer and stack the hBN / graphene heterojunction, wherein multiple experiments were conducted using multiple composite PDMS hemispheres for each third temperature. The results showed that when the third temperature was 120°C or 125°C, the PVA film could not detach from the substrate; when the third temperature was 130°C, 135°C, or 140°C, the success rate of transferring and stacking the two-dimensional material was high and stable.

[0125] Comparative Example 7

[0126] This embodiment compares the effects of different PVA film coverage areas on the transfer of two-dimensional materials, specifically including the following steps:

[0127] 1. Prepare two composite PDMS hemispheres (composite PDMS hemisphere A and composite PDMS hemisphere B) using the same steps as in Example 1.

[0128] 2. Prepare a PVA solution using the same steps as in Example 1.

[0129] 3. Using a dropper, PVA solution was dropped onto the composite PDMS hemispheres and baked at 80℃ for 20 min, allowing PVA to form a film directly on the composite PDMS hemispheres. Specifically, the surface of composite PDMS hemisphere A was completely covered by the PVA film, while only the raised portion of composite PDMS hemisphere B was covered by the PVA film. Results showed that wrinkles appeared on the PVA film surface of composite PDMS hemisphere A; the raised portion of composite PDMS hemisphere B successfully formed a film.

[0130] 4. Using the same steps as in Example 1, the hBN / graphene heterojunction was transferred and stacked. The results showed that the composite PDMS hemisphere A could not transfer two-dimensional materials and affected the imaging effect; the composite PDMS hemisphere B successfully achieved the transfer and stacking of two-dimensional materials.

[0131] Comparative Example 8

[0132] This embodiment prepares PDMS hemispheres with a different structure from the composite PDMS hemispheres of the present invention to stack hBN and graphene, specifically including the following steps:

[0133] 1. A PDMS hemisphere was prepared using the same steps as in Example 1, but unlike Example 1, this PDMS hemisphere did not have protrusions. Figure 4 (a) and Figure 4 (b)).

[0134] 2. Prepare a PVA solution using the same steps as in Example 1.

[0135] 3. Use a dropper to drop the PVA solution onto the PDMS hemisphere. Figure 4 (c) and baked at 80°C for 20 min to allow PVA to form a film directly on the PDMS hemisphere, which does not completely cover the surface of the PDMS hemisphere.

[0136] 4. Transfer stacked heterojunction

[0137] ① First, place the glass slide containing PDMS / PVA on the transfer platform, locate the hexagonal boron nitride (hBN) using an optical microscope and align it with the center vertex of the PDMS hemisphere. Other two-dimensional materials also exist near hBN.

[0138] ② Control the slide to slowly press down using a transfer platform. When it is about to contact hBN (a halo is observed near hBN), slowly heat it to 80°C. Use thermal expansion and contraction instead of manual pressing to ensure uniform force. Maintain the temperature at 80°C for 10 minutes, then stop heating.

[0139] ③ When the temperature drops to room temperature, lift the glass slide upwards. hBN will adhere to the PVA film and rise together with the PDMS hemisphere. However, at this time, non-target two-dimensional materials from the substrate are still adhered to the PVA film. Therefore, the PDMS hemisphere prepared in this embodiment cannot achieve the purpose of accurately transferring stacked two-dimensional materials by contacting the two-dimensional materials with a small area.

[0140] The method for transferring and stacking two-dimensional materials using composite PDMS hemisphericals provided by the present invention has been described in detail above. Specific embodiments have been used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for transferring two-dimensional materials, the method comprising the following steps: 1) A composite PDMS hemisphere is prepared on a planar carrier using polydimethylsiloxane PDMS prepolymer and curing agent. The composite PDMS hemisphere includes a PDMS hemisphere base and a PDMS protrusion, and the PDMS protrusion constitutes the apex of the PDMS hemisphere base. 2) A PVA film is formed on the surface of the composite PDMS hemisphere using a polyvinyl alcohol (PVA) solution, which completely covers the PDMS protrusions but does not completely cover the composite PDMS hemisphere; 3) Align the PDMS protrusions of the composite PDMS hemisphere on the planar carrier with the two-dimensional material on the substrate, and use a transfer platform to press the planar carrier down to a distance where the PVA film covering the PDMS protrusions can contact the two-dimensional material under thermal expansion. Heat the composite PDMS hemisphere to a first temperature and maintain it at the first temperature for a first time period so that the two-dimensional material is bonded to the PVA film. 4) The temperature of the composite PDMS hemisphere is lowered to a second temperature, and the composite PDMS hemisphere is raised to transfer the two-dimensional material to the planar carrier; 5) Transfer the composite PDMS hemisphere with the two-dimensional material attached to the target position on the substrate and heat it to the third temperature, then lift the composite PDMS hemisphere so that at least a portion of the PVA film on the PDMS protrusion and the two-dimensional material fall off onto the substrate together; 6) Remove at least a portion of the PVA film to achieve the transfer of the two-dimensional material.

2. The method according to claim 1, wherein, The planar carrier is a glass slide.

3. The method according to claim 1, wherein, In step 5), the complete PVA film on the PDMS protrusion and the two-dimensional material are detached from the substrate.

4. The method according to claim 1, wherein, The projection of the PVA film onto the composite PDMS hemisphere covers at least 4 / 25 of the projected area of ​​the composite PDMS hemisphere.

5. The method according to claim 4, wherein, The projection of the PVA film onto the composite PDMS hemisphere covers 4 / 25 to 9 / 25 of the projected area of ​​the composite PDMS hemisphere.

6. The method according to claim 1, wherein, In step 2), the PVA solution is placed on the surface of the composite PDMS hemisphere and baked at 75℃-85℃ for 15-20 min.

7. The method according to claim 6, wherein, In step 2), the PVA solution is placed on the surface of the composite PDMS hemisphere and baked at 80°C for 20 min.

8. The method according to claim 1, wherein, In step 3), the projection of the PDMS protrusion on the substrate is entirely within the two-dimensional material.

9. The method according to claim 8, wherein, In step 3), the projection of the PDMS protrusion onto the composite PDMS hemisphere covers 1 / 25 of the projected area of ​​the composite PDMS hemisphere.

10. The method according to claim 1, wherein, The composite PDMS hemisphere is heated to the first temperature at a heating rate not exceeding 10°C / min.

11. The method according to claim 1, wherein, The first temperature is 75℃-85℃, and the first time period is 8-10 min.

12. The method according to claim 11, wherein, The first temperature is 80℃, and the first time period is 10 minutes.

13. The method according to claim 1, wherein, The second temperature is 20℃-45℃.

14. The method according to claim 1, wherein, The third temperature is 130℃-140℃.

15. The method according to claim 14, wherein, The third temperature is 140°C.

16. The method according to claim 1, wherein, Removing the at least partial PVA film includes immersing a substrate containing the at least partial PVA film and the two-dimensional material in deionized water for a period of time to remove the at least partial PVA film.

17. The method according to claim 16, wherein, The temperature of the deionized water is 35℃-45℃.

18. The method according to claim 17, wherein, The temperature of the deionized water is 40°C.

19. The method of claim 16, wherein, The soaking process shall last for at least 3 minutes.

20. The method according to any one of claims 1-19, wherein, The two-dimensional material is one or more of boron nitride, graphene, transition metal sulfides, chromium oxychloride, and black phosphorus.

21. The method according to claim 20, wherein, The transition metal sulfide is molybdenum disulfide and / or palladium diselenide.

22. A method for transferring and stacking two-dimensional materials, the method comprising the following steps: 1) A composite PDMS hemisphere is prepared on a planar carrier using polydimethylsiloxane PDMS prepolymer and curing agent. The composite PDMS hemisphere includes a PDMS hemisphere base and a PDMS protrusion, and the PDMS protrusion constitutes the apex of the PDMS hemisphere base. 2) A PVA film is formed on the surface of the composite PDMS hemisphere using a polyvinyl alcohol (PVA) solution, which completely covers the PDMS protrusions but does not completely cover the composite PDMS hemisphere; 3) Align the PDMS protrusion of the composite PDMS hemisphere on the planar carrier with the two-dimensional material on the substrate, and use a transfer platform to press the planar carrier down to a distance where the outermost structural element of the composite PDMS hemisphere can contact the two-dimensional material under thermal expansion. Heat the composite PDMS hemisphere to a first temperature and maintain it at the first temperature for a first time period, so that the two-dimensional material is bonded to the outermost structural element of the composite PDMS hemisphere. 4) The temperature of the composite PDMS hemisphere is lowered to a second temperature, and the composite PDMS hemisphere is raised to transfer the two-dimensional material to the planar carrier; 5) Repeat steps 3) to 4) to obtain the first stack of two-dimensional materials; 6) Transfer the composite PDMS hemisphere with the first stack to the target position on the substrate and heat it to the third temperature, then lift the composite PDMS hemisphere so that at least a portion of the PVA film on the PDMS protrusion and the first stack fall off onto the substrate together; 7) Remove at least a portion of the PVA film to obtain a two-dimensional material stack as the target product.

23. The method according to claim 22, wherein, The planar carrier is a glass slide.

24. The method according to claim 22, wherein, The complete PVA film on the PDMS protrusion and the first stacked body detached together and fell onto the substrate.

25. The method according to claim 22, wherein, The projection of the PVA film onto the composite PDMS hemisphere covers at least 4 / 25 of the projected area of ​​the composite PDMS hemisphere.

26. The method of claim 25, wherein, The projection of the PVA film onto the composite PDMS hemisphere covers 4 / 25 to 9 / 25 of the projected area of ​​the composite PDMS hemisphere.

27. The method according to claim 22, wherein, In step 2), the PVA solution is placed on the surface of the composite PDMS hemisphere and baked at 75℃-85℃ for 15-20 min.

28. The method according to claim 27, wherein, In step 2), the PVA solution is placed on the surface of the composite PDMS hemisphere and baked at 80°C for 20 min.

29. The method according to claim 22, wherein, In step 3), the projection of the PDMS protrusion on the substrate is entirely within the two-dimensional material.

30. The method according to claim 29, wherein, In step 3), the projection of the PDMS protrusion onto the composite PDMS hemisphere covers 1 / 25 of the projected area of ​​the composite PDMS hemisphere.

31. The method according to claim 22, wherein, The composite PDMS hemisphere is heated to the first temperature at a heating rate not exceeding 10°C / min.

32. The method according to claim 22, wherein, The first temperature is 75℃-85℃, and the first time period is 8-10 min.

33. The method according to claim 32, wherein, The first temperature is 80℃, and the first time period is 10 minutes.

34. The method according to claim 22, wherein, The second temperature is 20℃-45℃.

35. The method according to claim 22, wherein, The third temperature is 130℃-140℃.

36. The method according to claim 35, wherein, The third temperature is 140°C.

37. The method according to claim 22, wherein, In step 6), the target location has the last two-dimensional material; step 6) includes: using a transfer platform to press the planar carrier down to a distance that allows the outermost structural element bonded to the composite PDMS hemisphere to contact the last two-dimensional material under thermal expansion, heating the composite PDMS hemisphere to a third temperature so that the first stack and the last two-dimensional material bond to form a second stack of two-dimensional materials, and causing at least a portion of the PVA film and the second stack to detach together onto the substrate.

38. The method according to claim 22, wherein, Removing the at least partial PVA film includes immersing a substrate containing the at least partial PVA film and the two-dimensional material stack in deionized water for a period of time to remove the at least partial PVA film.

39. The method according to claim 38, wherein, The temperature of the deionized water is 35℃-45℃.

40. The method according to claim 39, wherein, The temperature of the deionized water is 40°C.

41. The method according to claim 38, wherein, The soaking process shall last for at least 3 minutes.

42. The method according to any one of claims 22-41, wherein, The two-dimensional material is one or more of boron nitride, graphene, transition metal sulfides, chromium oxychloride, and black phosphorus.

43. The method according to claim 42, wherein, The transition metal sulfide is molybdenum disulfide and / or palladium diselenide.

44. The method according to any one of claims 22-41, wherein, The two-dimensional materials constituting the first stack may be the same or different.

45. The method of claim 37, wherein, The two-dimensional materials constituting the second stack may be the same or different.

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