A LEGO-type van der Waals heterojunction and a preparation method thereof

Abstract

The present invention discloses a Lego-type van der Waals heterojunction and a preparation method thereof, wherein microdots and arrays prepared by PDMS and PPC are used to transfer and stack large-area two-dimensional van der Waals materials on various types of substrates. The preparation method of the present invention mainly includes three parts: the first part is the preparation of microdots and arrays thereof; the second part is the van der Waals transfer system; and the third part is the transfer step and control conditions. The present invention transfers and stacks two-dimensional van der Waals materials on various types of substrates by using microdots or arrays thereof, and almost no adhesive is left in the process, and the degumming step can be omitted. It is a method for quickly realizing the clean transfer of two-dimensional materials, which greatly simplifies the preparation process and avoids the influence of degumming on the structure and performance of the Lego-type van der Waals heterojunction, and is expected to meet the needs of large-scale industrialization.

Description

Technical Field

[0001] The present invention belongs to the technical field of micro-nanostructure and material processing, and specifically relates to a Lego-type van der Waals heterojunction and a preparation method thereof. Background Art

[0002] The electrical properties of silicon-based semiconductor materials significantly decline at extreme sizes due to quantum confinement effects. Researching new materials is an opportunity and challenge in the post-Moore era. Two-dimensional van der Waals materials such as graphene and transition metal sulfides can stack van der Waals heterojunctions without considering lattice matching, and can be used in the preparation of various electronic devices. According to current literature reports, the dry transfer method for preparing heterojunctions has a low success rate because the polymer glue is easily broken, and the subsequent degumming is required. It is not easy to remove the degumming completely, and it is easy to cause structural damage and introduce external contamination during degumming, which ultimately affects the performance of the device. As a result, the existing dry transfer method for preparing heterojunctions is complicated to operate, time-consuming, and has a long cycle. The yield is not high and it is not easy to prepare on a large scale industrially. Summary of the invention

[0003] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a Lego-type van der Waals heterojunction and a preparation method thereof. The present invention transfers and stacks two-dimensional van der Waals materials on various types of substrates through micro-dots or their arrays. In this process, almost no adhesive residue remains, and the degumming step can be omitted. It is a method for quickly realizing the clean transfer of two-dimensional materials, which greatly simplifies the preparation process and avoids the influence of degumming on the structure and performance of the Lego-type van der Waals heterojunction, and is expected to meet large-scale industrial needs.

[0004] The technical solution adopted by the present invention is as follows:

[0005] A method for preparing a Lego-type van der Waals heterojunction comprises the following steps:

[0006] The micro-dot unit is heated to a first preset temperature, and the micro-dot unit is moved to pick up the stacked two-dimensional material to form a Lego-style van der Waals heterojunction. The micro-dot unit is then heated to a second preset temperature, and then the micro-dot unit is removed from the Lego-style van der Waals heterojunction. The Lego-style van der Waals heterojunction is prepared;

[0007] The micro-dot unit includes a supporting substrate and a pickup portion disposed on the supporting substrate, the pickup portion includes a protrusion disposed on the supporting substrate and a film coated on the outside of the protrusion, and the diameter of the top of the protrusion is less than 30 μm;

[0008] When the micro-dot unit is heated to a first preset temperature, the adhesive film can bond the two-dimensional material together, and when the micro-dot unit is heated to a second preset temperature, the adhesive film can separate from the two-dimensional material.

[0009] Preferably, a plurality of said micro-dot units in an array form are used to pick up the stacked two-dimensional material;

[0010] A plurality of micro-dot units in an array form are arranged on a support to form a micro-dot array;

[0011] The support body is a rigid support body or an elastic support body, and the shape of the support body is a plane or a curved surface.

[0012] Preferably, the duty cycle of the micro-dot array is 20%-80%.

[0013] Preferably, a heating wire for heating the micro-dot units is provided in the support body.

[0014] Preferably, the protruding portion of the pickup portion of the micro-dot unit comprises a PDMS sheet and a PDMS dot, the PDMS sheet is arranged on the surface of the supporting substrate, the PDMS dot is arranged on the PDMS sheet, and the adhesive film cover is arranged outside the overall structure of the PDMS sheet and the PDMS dot and is bonded to the surface of the supporting substrate;

[0015] The PDMS sheet is a sheet-like structure obtained after the curing reaction of polydimethylsiloxane, and the PDMS dot is a convex structure obtained after the curing reaction of polydimethylsiloxane.

[0016] Preferably, the adhesive film is a PPC film, and the PPC film is bonded to the surface of the supporting substrate by a double-sided adhesive tape;

[0017] The first temperature is 20-40°C, and the second temperature is 70-120°C.

[0018] Preferably, the shape of the protrusion of the picking-up portion of the micro-dot unit is spherical, conical, truncated cone or cylindrical.

[0019] Preferably, when the micro-dot unit is moved to pick up the stacked two-dimensional materials to form a Lego-style van der Waals heterojunction, the first layer of two-dimensional material in the two-dimensional material used to form the Lego-style van der Waals heterojunction is picked up by adhesive film bonding, and then the other two-dimensional materials except the last layer of two-dimensional material are stacked together in turn through the van der Waals force between the two-dimensional materials, and then the two-dimensional material picked up by the micro-dot unit is placed on the last layer of two-dimensional material and adsorbed together by van der Waals force, and then the micro-dot unit is heated to a second preset temperature, and then the micro-dot unit is removed from the Lego-style van der Waals heterojunction, and the preparation of the Lego-style van der Waals heterojunction is completed.

[0020] Preferably, when removing the microdot unit from the Lego-type van der Waals heterojunction, the microdot unit is first moved a preset distance in a direction parallel to the surface of the two-dimensional material at a speed not greater than 1 μm / s to separate the adhesive film from the surface of the two-dimensional material, and then the microdot unit is removed.

[0021] The present invention also provides a Lego-type van der Waals heterojunction, which is prepared by the preparation method of the present invention as described above.

[0022] The present invention has the following beneficial effects:

[0023] In the preparation method of the Lego-type van der Waals heterojunction of the present invention, the contact area between the adhesive film and the surface of the two-dimensional material can be made smaller through the protrusions with the vertex diameter less than 30 μm, which can reduce unnecessary contact and accurately pick up the desired piece of two-dimensional material without picking up the materials around the two-dimensional material; at the same time, it is not easy to leave residual glue, thus eliminating the subsequent glue removal steps and the pollution caused by the glue removal and the damage to the heterojunction structure and performance.

[0024] Furthermore, the present invention organizes micro-dot units into a micro-dot array for picking up stacked two-dimensional materials, so that a larger area of ​​Lego-style van der Waals heterojunction can be processed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 (a) is a schematic diagram of a form of multi-level micro dots in an embodiment of the present invention; FIG. 1 (b) is a schematic diagram of another form of multi-level micro dots in an embodiment of the present invention;

[0026] FIG2(a) is a schematic diagram of a micro-dot array in one form according to an embodiment of the present invention; FIG2(b) is a schematic diagram of a micro-dot array in another form according to an embodiment of the present invention; FIG2(c) is a schematic diagram of a micro-dot array in another form according to an embodiment of the present invention;

[0027] FIG3(a) is a schematic diagram of the support structure of the micro-dot array shown in FIG2(a); FIG3(b) is a schematic diagram of the support structure of the micro-dot array shown in FIG2(b); FIG3(c) is a schematic diagram of the support structure of the micro-dot array shown in FIG2(c);

[0028] FIG4 (a) is a schematic diagram of the preparation process of the micro-dot unit of the embodiment of the present invention; FIG4 (b) is a photograph of the micro-dot unit prepared in the embodiment of the present invention;

[0029] Figure 5 It is a schematic diagram of a transfer platform of the method for preparing a Lego-type van der Waals heterojunction of the present invention;

[0030] Figure 6 It is a schematic diagram of the preparation process of the van der Waals heterojunction of the method for preparing the Lego-type van der Waals heterojunction of the present invention;

[0031] Figure 7 It is a light microscope image of a BN-FGT-BN-FGT van der Waals heterojunction prepared by the method for preparing a Lego-type van der Waals heterojunction of the present invention;

[0032] Figure 8 It is a light microscope image of a BN-graphene-BN van der Waals heterojunction prepared by the method for preparing a Lego-type van der Waals heterojunction of the present invention;

[0033] Fig. 9 It is a light microscope image of a BN-TBG-BN van der Waals heterojunction prepared by the method for preparing a Lego-type van der Waals heterojunction of the present invention.

[0034] In the figure, 1-micro-dot unit, 101-support substrate, 102-double-sided tape, 103-PPC film, 104-PDMS sheet, 105-PDMS dots, 2-micro-dot array, 201-support body, 202-heating wire, 3-upper moving platform, 401-lower moving platform, 402-heating device, 403-sample, 6-microscope system. DETAILED DESCRIPTION

[0035] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0036] The preparation method of the Lego-type van der Waals heterojunction of the present invention includes three parts: the first part is the preparation of the micro-dot unit 1 and its array (i.e., the micro-dot array 2), the second part is the van der Waals transfer system, and the third part is the transfer steps and control conditions.

[0037] The first part is the preparation of the micro-dot unit 1 and the micro-dot array 2, which includes the following steps.

[0038] Step 1, preparing a PDMS (polydimethylsiloxane) mixed liquid and a PDMS sheet 104;

[0039] Step 2, preparing a PPC (poly(methyl ethylene carbonate)) solution and a PPC film 103;

[0040] Step 3, preparing a micro-dot unit 1 by using the PDMS mixed liquid obtained in step 1 and step 2, the PDMS sheet 104 and the PPC film 103;

[0041] Step 4: Prepare the micro-dot unit 1 obtained in step 3 into a micro-dot array 2.

[0042] The specific detailed process of the above scheme of the present invention is as follows:

[0043] In step 1, the silica gel and the curing agent are mixed in a mass ratio of 10:1, and then placed in a vacuum drying oven for degassing at room temperature for 6 hours to obtain a PDMS mixed liquid. After degassing, the PDMS mixed liquid is slowly poured into a glass petri dish, shaken and spread to make the liquid thickness 1mm, and then placed in a vacuum drying oven for curing at 80°C for 6 hours to obtain a PDMS sheet 104.

[0044] In step 2, PPC solid particles and anisole solution were weighed in a mass ratio of 1.765:10 and placed in a small reagent bottle. Then, the mixture was heated on a hot plate at 80°C and shaken every 1 hour. The mixture was dissolved evenly in about 24 hours. A PPC solution with a mass fraction of 15% was obtained. The PPC solution was spin-coated on a silicon wafer (6000 r / min, 2 min) on a spunbond to obtain a PPC film 103.

[0045] In step 3, the preparation process of the micro-dot unit 1 is shown in FIG4 (a). The specific structure of the micro-dot unit 1 can be found in Figure 1a and Figure 1b The micro-dot unit 1 includes a supporting substrate 101, a PPC film 103, a PDMS sheet 104 and a PDMS dot 105. The PDMS sheet 104 is arranged on the surface of the supporting substrate 101, and the PDMS dot 105 is arranged on the PDMS sheet 104. The PPC film 103 is covered on the outside of the overall structure of the PDMS sheet 104 and the PDMS dot 105 and is bonded to the surface of the supporting substrate 101 through a double-sided adhesive 102. Taking the third-level micro-dot (see Figure 1 (a)) as an example, the preparation process of the micro-dot unit 1 includes the following steps: Step I, cut a 4mm×4mm PDMS sheet 104 with a blade and place it on one end of a clean supporting substrate 101 (such as a glass slide). Step II, use a wire with a diameter of 1mm, dip a drop of the mixed PDMS solution, drop it on the PDMS plate 104, and cure it on a hot plate at 130°C for 5 minutes to form a dome. Step III, dip the mixed PDMS solution with a wire of 200 microns in diameter, drop it on the cured PDMS dome, and cure it for 5 minutes to form a double dome. Step IV, dip the mixed PDMS solution with a wire of 20 microns in diameter, drop it on the cured PDMS double dome, and cure it for 5 minutes to form a triple dome, wherein the diameter of the top dome is 30 microns, and the finished product is shown in FIG4 (b). The same method can be used to prepare multi-level micro dots (i.e., PDMS dots 105) of the required size, as shown in FIG1 (a) for three dots and FIG1 (b) for five dots. PDMS dots 105 of other shapes and sizes, such as conical, truncated cone or cylindrical, can be prepared by pouring PDMS solution into a template for curing. Finally, the PPC film 103 spin-coated on the silicon wafer is transferred to the PDMS dot 105 with tape, or the PPC solution is directly spin-coated on the PDMS dot 105.

[0046] In step 4, the micro-dot array 2 is as shown in Figure 2 (a) to Figure 2 (c), and the micro-dot units 1 are prepared into an array according to the requirements and placed on substrates of different shapes. The micro-dot units 1 in step 3 can be directly placed on various types of supports 201 with tweezers and arranged according to the requirements. For microstructures, micro-dot units 1 of various sizes and shapes can be prepared using PDMS by nanoimprinting. As shown in Figures 3 (a) to 3 (c), the micro-dot units (PPC / PDMS / support substrate) and the supporting substrate (i.e., support body 201) of the array, the material of the support body 201 can be a rigid transparent material (such as glass, plastic) or a flexible transparent material with a certain elasticity (such as a material with an elastic modulus E between 0.05MPa-206GPa). A heating wire can be embedded in the support body 201, and the micro-dot array 2 can be heated by a DC source to pick up a large area of ​​material at a higher temperature, and then the material can be transferred to another substrate at a higher temperature without leaving PPC residue.

[0047] The second part is the van der Waals transfer system. Figure 5 As shown, the transfer platform is placed in the glove box and consists of three parts: a microscope system 6, a high-precision transfer platform (upper moving platform 3 and lower moving platform 401) and a heating device 402. The microscope system 6 includes a microscope, a CCD camera and a computer for easy observation. The upper moving platform 3 and the lower moving platform 401 can move in three directions of XYZ. The upper moving platform 3 is used to fix the micro-dot unit 1. The lower moving platform 401 can also move in the three directions of XYZ and rotate in the XY plane to fix the sample 403. The heating device 402 is placed on the lower moving platform 401 to heat the sample.

[0048] The third part is the transfer steps and control conditions: the prepared micro-dot unit 1 is fixed upside down on the transfer platform, and the sample is glued to the bottom. The samples here are pre-selected samples, namely two-dimensional material A, two-dimensional material B, two-dimensional material C and two-dimensional material D. The stacking process of the van der Waals heterojunction is shown in the attached figure. Figure 6As shown, the following steps are included: Step a, the PPC film 103 has good viscosity at 20-40°C, and the viscosity is relatively good at 40°C. Therefore, in the experiment, the temperature is limited to 40°C to heat the PPC film 103, and the dome of the micro-dot unit 1 is aligned with the selected material A, and the micro-dot moves up and down in Z. The time for the micro-dot to contact the silicon wafer is about 0.5 seconds, and the two-dimensional material A is quickly picked up. Step b, the micro-dot unit 1 carries the two-dimensional material A and aligns it with the selected two-dimensional material B. The two-dimensional material B is adjusted by rotation. The dome of the micro-dot unit 1 contacts the silicon wafer, and the two-dimensional material B is picked up as quickly as possible, so that the dome of the micro-dot unit 1 has an AB structure. Next, the micro-dot unit 1 carries the AB structure and aligns it with the selected two-dimensional material C. The two-dimensional material C is adjusted by rotation. The dome of the micro-dot unit 1 contacts the silicon wafer, and the two-dimensional material C is picked up as quickly as possible, so that the dome of the micro-dot unit 1 has an ABC structure. Finally, the viscosity of the PPC film 103 is weak at 70°C. When the temperature is raised to the range of 70-120°C, the micro-dot unit 1 with the ABC structure is aligned with the selected two-dimensional material D. The two-dimensional material D is adjusted at a good angle by rotation. The dome of the micro-dot unit 1 contacts the silicon wafer. The micro-dot unit 1 is moved very slowly (no more than 1 μm / s) in the X direction, the PPC film 103 is peeled off from one side, the ABC structure is placed on the silicon wafer, the heterojunction is separated from the PPC film 103, and then the micro-dot unit 1 is moved in the Z direction, so that there is an ABCD heterojunction on the silicon wafer.

[0049] According to the above scheme, if a micro-dot array 2 is used, a large-area Lego-style van der Waals heterojunction can be prepared, and the array can be used for picking up materials on flexible and irregularly shaped substrates. For the micro-dot array, its duty cycle is 20%-80%.

[0050] The technical solution of the present invention uses a transfer platform to pick up stacked two-dimensional materials at different temperatures, which can quickly prepare a two-dimensional van der Waals heterojunction with a clean surface, and micro-dot arrays of different shapes and sizes can be used to transfer two-dimensional materials over a large area. Micro-dot arrays of different shapes and sizes can be used to transfer two-dimensional materials supported by various substrates, including rigid substrates and flexible substrates, flat substrates and curved substrates.

[0051] The PDMS dots 105 may be composed of a single or multiple levels and may be spherical, conical, cylindrical, etc. The size of the dots 105 may range from tens of nanometers to several millimeters and the aspect ratio may range from 10:1 to 1:10.

[0052] Example 1

[0053] The present invention is specifically described below by preparing a BN-Fe3GeTe2-BN-Fe3GeT2 van der Waals heterojunction.

[0054] Step 1: Prepare PDMS mixed liquid and PDMS sheet 104

[0055] The silica gel and the curing agent are mixed in a mass ratio of 10:1, and then placed in a vacuum drying oven for degassing at room temperature for 6 hours to obtain a PDMS mixed liquid. After degassing, the PDMS mixed liquid is slowly poured into a glass petri dish, shaken and spread to make the liquid thickness 1 mm, and placed in a vacuum drying oven for curing at 80°C for 6 hours to obtain a PDMS sheet 104.

[0056] Step 2: Preparation of PPC solution and PPC film 103

[0057] Weigh 1.765 g of PPC solid particles and dissolve them in a small reagent bottle with 10 ml of anisole. Then, heat on a hot plate at 80°C and shake once every 1 hour to dissolve evenly for 24 hours. Obtain a PPC solution with a mass fraction of 15%. Spin coat PPC (6000 r / min, 2 min) on a silicon wafer on a spunbond to obtain a PPC film 103.

[0058] Step 3: Prepare micro dot unit 1

[0059] First, use a blade to cut a 4mm×4mm PDMS sheet 104 and place it on one end of a clean glass slide. Next, use a 1mm diameter iron wire to dip a drop of the mixed PDMS solution, drop it on the PDMS, and cure it on a hot plate at 130°C for 5 minutes to form a dome. Then, use a 200-micron diameter iron wire to dip the mixed PDMS solution, add it dropwise on the cured PDMS dome, and also cure it for 5 minutes to form a double dome. Next, use a 20-micron diameter iron wire to dip the mixed PDMS solution, add it dropwise on the cured PDMS double dome, and also cure it for 5 minutes to form a triple dome, in which the diameter of the top dome is 30 microns. Finally, use tape to transfer the PPC film 103 spin-coated on the silicon wafer to the triple dome.

[0060] Step 4: Preparation of van der Waals heterojunction

[0061] The prepared micro-dot unit 1 is fixed upside down on the transfer platform, and the sample is glued to the bottom. The sample here is a sample selected in advance, generally two silicon wafers, namely BN sample and FGT sample. The stacking process of the van der Waals heterojunction is shown in the attached figure. Figure 6As shown. First, PPC has strong viscosity at 40°C. When the temperature rises to 40°C, the dome of micro-dot unit 1 is aligned with the selected BN. Micro-dot unit 1 moves up and down in Z direction. The dome of micro-dot unit 1 contacts the silicon wafer for 0.5 seconds, and BN is quickly picked up. Then, micro-dot unit 1 is aligned with the selected FGT with BN, and the FGT is adjusted by rotation. The micro-dot contacts the silicon wafer, and the FGT is quickly picked up. In this way, there is BN-FGT on micro-dot unit 1. Next, micro-dot unit 1 is aligned with the selected BN with BN-FGT, and the BN is adjusted by rotation. The micro-dot contacts the silicon wafer, and BN is quickly picked up. In this way, there is BN-FGT-BN on micro-dot unit 1. Finally, the PPC film 103 has weak viscosity at 100°C. The temperature is raised to 100°C. The micro-dot unit 1 carries BN-FGT-BN and is aligned with the selected FGT. The FGT is adjusted at a good angle by rotation. The micro-dot unit 1 contacts the silicon wafer and very slowly moves the upper micro-dot in the X direction. The PPC film 103 is peeled off from one side, and the BN-FGT-BN is placed on the silicon wafer. The heterojunction is separated from the PPC film, and then the micro-dot unit 1 moves upward in the Z direction, so that there is a BN-FGT-BN-FGT heterojunction on the silicon wafer. Figure 7 This is the optical microscope image of the BN-FGT-BN-FGT heterojunction prepared in this example. From top to bottom, they are the top layer of hBN, the second layer of FGT, the third layer of hBN, and the fourth layer of FGT. Figure 7 The BN-FGT-BN-FGT heterojunction prepared by the present invention is clearly shown above, and its morphology is clear and the surface is clean.

[0062] Example 2

[0063] The present invention is specifically described below by preparing a BN-graphene-BN van der Waals heterojunction.

[0064] Step 1: Prepare PDMS mixed liquid and PDMS sheet 104

[0065] The silica gel and the curing agent are mixed in a mass ratio of 10:1, and then placed in a vacuum drying oven for degassing at room temperature for 6 hours to obtain a PDMS mixed liquid. After degassing, the PDMS mixed liquid is slowly poured into a glass petri dish, shaken and spread to make the liquid thickness 1 mm, and placed in a vacuum drying oven for curing at 80°C for 6 hours to obtain a PDMS sheet 104.

[0066] Step 2: Preparation of PPC solution and PPC film 103

[0067] Weigh 1.765 g of PPC solid particles and dissolve them in a small reagent bottle with 10 ml of anisole. Then, heat on a hot plate at 80°C and shake once every 1 hour to dissolve evenly for 24 hours. Obtain a PPC solution with a mass fraction of 15%. Spin coat PPC (6000 r / min, 2 min) on a silicon wafer on a spunbond to obtain a PPC film 103.

[0068] Step 3: Prepare micro dot unit 1

[0069] First, use a blade to cut a 4mm×4mm PDMS sheet 104 and place it on one end of a clean glass slide. Next, use a 1mm diameter iron wire to dip a drop of the mixed PDMS solution, drop it on the PDMS, and cure it on a hot plate at 130°C for 5 minutes to form a dome. Then, use a 200-micron diameter iron wire to dip the mixed PDMS solution, add it dropwise on the cured PDMS dome, and also cure it for 5 minutes to form a double dome. Next, use a 20-micron diameter iron wire to dip the mixed PDMS solution, add it dropwise on the cured PDMS double dome, and also cure it for 5 minutes to form a triple dome, in which the diameter of the top dome is 30 microns. Finally, use tape to transfer the PPC film 103 spin-coated on the silicon wafer to the triple dome.

[0070] Step 4: Preparation of van der Waals heterojunction

[0071] The prepared micro-dot unit 1 is fixed upside down on the transfer platform, and the sample is glued to the bottom. The sample here is a sample selected in advance, usually two silicon wafers, namely BN sample and graphene sample. First, PPC has strong viscosity at 40°C. The temperature is raised to 40°C, and the dome of the micro-dot unit 1 is aligned with the selected BN. The micro-dot unit 1 is moved up and down in the Z direction. The time for the micro-dot to contact the silicon wafer is 0.5 seconds, and the BN is quickly picked up. Then, the micro-dot unit 1 is aligned with the selected graphene with BN, and the graphene is adjusted by rotating the angle. The micro-dot unit 1 contacts the silicon wafer and the graphene is quickly picked up, so that there is BN-graphene on the micro-dot unit 1. Finally, the PPC film 103 has weak viscosity at 100°C. The temperature is raised to 100°C, and the microdot unit 1 with BN-graphene is aligned with the selected BN. The BN is adjusted at a good angle by rotation. The microdot unit 1 contacts the silicon wafer and very slowly moves the upper microdot in the X direction. The PPC film 103 is peeled off from one side, and the BN-graphene is placed on the silicon wafer with BN. The heterojunction is separated from the PPC, and then the microdot unit 1 is moved in the Z direction, so that there is a BN-graphene-BN heterojunction on the silicon wafer. Figure 8This is a light microscope image of the BN-graphene-BN heterojunction prepared in this example, from top to bottom, the topmost layer is hBN, the second layer is graphene, and the third layer is hBN. The image clearly shows that the BN-graphene-BNT heterojunction prepared by the present invention has a clear morphology and a clean surface.

[0072] Example 3

[0073] The present invention is specifically described below by preparing a BN-TBG (twisted bilayer graphene)-BN van der Waals heterojunction.

[0074] Step 1: Prepare PDMS mixed liquid and PDMS sheet 104

[0075] The silica gel and the curing agent are mixed in a mass ratio of 10:1, and then placed in a vacuum drying oven for degassing at room temperature for 6 hours to obtain a PDMS mixed liquid. After degassing, the PDMS mixed liquid is slowly poured into a glass petri dish, shaken and spread to make the liquid thickness 1 mm, and placed in a vacuum drying oven for curing at 80°C for 6 hours to obtain a PDMS sheet 104.

[0076] Step 2: Preparation of PPC solution and PPC film 203

[0077] Weigh 1.765 g of PPC solid particles and dissolve them in a small reagent bottle with 10 ml of anisole. Then, heat on a hot plate at 80°C and shake once every 1 hour to dissolve evenly for 24 hours. Obtain a PPC solution with a mass fraction of 15%. Spin coat PPC (6000 r / min, 2 min) on a silicon wafer on a spunbond to obtain a PPC film 103.

[0078] Step 3: Prepare micro dot unit 1

[0079] First, use a blade to cut a 4mm×4mm PDMS sheet 104 and place it on one end of a clean glass slide. Next, use a 1mm diameter iron wire to dip a drop of the mixed PDMS solution, drop it on the PDMS, and cure it on a hot plate at 130°C for 5 minutes to form a dome. Then, use a 200-micron diameter iron wire to dip the mixed PDMS solution, add it dropwise on the cured PDMS dome, and also cure it for 5 minutes to form a double dome. Next, use a 20-micron diameter iron wire to dip the mixed PDMS solution, add it dropwise on the cured PDMS double dome, and also cure it for 5 minutes to form a triple dome, in which the diameter of the top dome is 30 microns. Finally, use tape to transfer the PPC film 103 spin-coated on the silicon wafer to the triple dome.

[0080] Step 4: Preparation of van der Waals heterojunction

[0081] The prepared micro-dot unit 1 is fixed upside down on the transfer platform, and the sample is glued to the bottom. Here, the sample is selected in advance, generally two silicon wafers, namely BN sample and graphene sample. First, PPC has strong viscosity at 40°C. When the temperature is raised to 40°C, the micro-dot unit 1 is aligned with the selected BN, and the micro-dot moves up and down in Z. The time for the micro-dot to contact the silicon wafer is 0.5 seconds, and the BN is quickly picked up. Then, the micro-dot unit 1 with BN is aligned with the selected graphene. The graphene is adjusted by rotating the angle so that the edge of the BN contacts the center of the graphene. Half of the graphene is quickly picked up, and the single-layer graphene flakes are sliced, so that there is BN-graphene on the micro-dot unit 1, and the remaining half of the graphene remains on the silicon wafer. Next, the lower platform rotates 1.3 degrees, and the micro-dot unit 1 with BN-graphene on the top is aligned with the remaining half of the graphene, and the graphene is quickly picked up, so that there is BN-TBG on the micro-dot unit 1. Then, finally, PPC has a weaker viscosity at 100°C, the temperature is raised to 100°C, the micro-dot unit 1 with BN-TBG is aligned with the selected BN, the BN is adjusted at a good angle by rotation, the micro-dot unit 1 contacts the silicon wafer, and the micro-dot unit 1 is very slowly moved in the X direction, the PPC film 103 is peeled off from one side, and the BN-TBG is placed on the silicon wafer with BN, the heterojunction is separated from the PPC, and then the micro-dot unit 1 is moved in the Z direction, so that there is a BN-TBG-BN heterojunction on the silicon wafer. Fig. 9 This is a light microscope image of the BN-TBG-BN heterojunction prepared in this example, from top to bottom, the topmost layer of hBN, the second and third layers of twisted graphene, and the fourth layer of hBN. The image clearly shows that the BN-TBG-BNT heterojunction prepared by the present invention has a clear morphology and a clean surface.

[0082] The present invention places micro dots made of PDMS (polydimethylsiloxane) and polymer glue PPC (polymethyl ethylene carbonate) at one end of a glass slide, and then fixes it in a three-dimensional clamping block. The transfer platform is used to pick up stacked two-dimensional materials at different temperatures, and a two-dimensional van der Waals heterojunction with a clean surface can be quickly prepared. The micro dots of the present invention have a small contact area with the substrate, which reduces unnecessary contact, can accurately pick up the desired piece of material, and will not pick up the material around the material; it is not easy to leave residual glue, and the subsequent glue removal step is omitted. The PPC of the micro dots of the method is not easy to break and can be used multiple times, eliminating the operation of frequently replacing the micro dots on the glass slide, improving the success rate of building a heterojunction, and reducing the time cost of building a heterojunction, which is expected to achieve industrial production.

Claims

1. A method for preparing a Lego-type van der Waals heterojunction, characterized in that: The process includes: The micro-dot unit is heated to a first preset temperature, and the micro-dot unit (1) is moved to pick up the stacked two-dimensional material to form a Lego-type van der Waals heterojunction. The micro-dot unit (1) is then heated to a second preset temperature, and then the micro-dot unit (1) is moved away from the Lego-type van der Waals heterojunction. The Lego-type van der Waals heterojunction is prepared; The micro-dot unit (1) comprises a supporting substrate (101) and a pick-up portion arranged on the supporting substrate (101), the pick-up portion comprising a protrusion arranged on the supporting substrate (101) and a film covering the outside of the protrusion, the diameter of the top of the protrusion being less than 30 μm; When the micro-dot unit (1) is heated to a first preset temperature, the adhesive film can bond the two-dimensional material together, and when the micro-dot unit (1) is heated to a second preset temperature, the adhesive film can separate from the two-dimensional material. The raised portion of the pick-up portion of the micro-dot unit (1) comprises a PDMS sheet (104) and a PDMS dot (105); the PDMS sheet (104) is arranged on the surface of the supporting substrate (101); the PDMS dot (105) is arranged on the PDMS sheet (104); and the adhesive film cover is arranged outside the overall structure of the PDMS sheet (104) and the PDMS dot (105) and is bonded to the surface of the supporting substrate (101); The PDMS sheet (104) is a sheet-like structure obtained after a curing reaction of polydimethylsiloxane, and the PDMS dot (105) is a convex structure obtained after a curing reaction of polydimethylsiloxane; When the micro-dot unit (1) is moved to pick up the stacked two-dimensional materials to form a Lego-type van der Waals heterojunction, the first layer of two-dimensional material in the two-dimensional material used to form the Lego-type van der Waals heterojunction is picked up by adhesive film bonding, and then the other two-dimensional materials except the last layer of two-dimensional material are stacked together in sequence through the van der Waals force between the two-dimensional materials, and then the two-dimensional material picked up by the micro-dot unit (1) is placed on the last layer of two-dimensional material and adsorbed together through the van der Waals force, and then the micro-dot unit (1) is heated to a second preset temperature, and then the micro-dot unit (1) is removed from the Lego-type van der Waals heterojunction, and the Lego-type van der Waals heterojunction is prepared.

2. The method for preparing a Lego-type van der Waals heterojunction according to claim 1, characterized in that: Using a plurality of the micro-dot units (1) in an array form to pick up stacked two-dimensional materials; A plurality of micro-dot units (1) in an array form are arranged on a support body (201) to form a micro-dot array (2); The support body (201) is a rigid support body or an elastic support body, and the shape of the support body (201) is a plane or a curved surface.

3. The method for preparing a Lego-type van der Waals heterojunction according to claim 2, characterized in that: The duty cycle of the micro-dot array (2) is 20%-80%.

4. The method for preparing a Lego-type van der Waals heterojunction according to claim 2, characterized in that: A heating wire (202) for heating the micro-dot unit (1) is provided in the support body (201).

5. The method for preparing a Lego-type van der Waals heterojunction according to claim 1, characterized in that: The adhesive film is a PPC film (103), and the PPC film (103) is bonded to the surface of the supporting substrate (101) via a double-sided adhesive (102); The first preset temperature is 20-40°C, and the second preset temperature is 70-120°C.

6. The method for preparing a Lego-type van der Waals heterojunction according to claim 1, characterized in that: The shape of the protrusion of the picking-up portion of the micro-dot unit (1) is spherical, conical, truncated cone or cylindrical.

7. The method for preparing a Lego-type van der Waals heterojunction according to claim 1, characterized in that: When removing the micro-dot unit (1) from the Lego-type van der Waals heterojunction, the micro-dot unit (1) is first moved a preset distance in a direction parallel to the surface of the two-dimensional material at a speed not greater than 1 μm / s, so that the adhesive film is separated from the surface of the two-dimensional material, and then the micro-dot unit (1) is removed.

8. A Lego-type van der Waals heterojunction, characterized in that: The Lego-type van der Waals heterojunction is prepared by the preparation method described in any one of claims 1-7.

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