Method for preparing a suspended two-dimensional material film and suspended two-dimensional material film

Abstract

The application discloses a preparation method of a suspended two-dimensional material film and the suspended two-dimensional material film. The preparation method comprises the following steps: providing a substrate, wherein the substrate has a first surface and a second surface opposite to each other; forming a sacrificial layer film on the first surface of the substrate; forming a through hole on the second surface of the substrate, and the through hole exposes the sacrificial layer film; obtaining a two-dimensional material film, transferring the two-dimensional material film to the sacrificial layer film, and the two-dimensional material film covers the through hole; etching the sacrificial layer film from the second surface of the substrate until the two-dimensional material film is exposed, and a suspended two-dimensional material film is formed. The preparation method of the suspended two-dimensional material film and the suspended two-dimensional material film have the advantages that the two-dimensional material film has low transfer difficulty, the light source has little influence on the test, and the test means is effectively widened.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of preparation of two-dimensional semiconductor optoelectronic devices, in particular to a method for preparing a suspended two-dimensional material film and the suspended two-dimensional material film. BACKGROUND

[0002] In recent years, transition metal dichalcogenides (TMDCs) are a general term for a large class of two-dimensional layered materials, with a chemical formula of MX2, wherein M is a transition metal element (including Mo, W, Re, etc.), and X is a chalcogen element (S, Se, Te, etc.), and three atomic layers form a sandwich structure of X-M-X with the M atomic layer as the center layer. The TMDCs layers are stacked together by weak van der Waals forces between layers, so that single-layer or multi-layer TMDCs materials can be obtained on a bulk material by mechanical exfoliation. With the decrease of the number of layers, the exciton binding energy and phonon strength increase significantly, and when the number of layers becomes a single layer, the energy band structure also changes from an indirect band gap to a direct band gap. By artificially stacking single-layer TMDCs materials, homogeneous or heterogeneous structures can be prepared to obtain interlayer excitons, adjustable type II energy band structures and other special properties, which have broad application prospects in the fields of optics, electronics, superconductivity, etc., and also make them an ideal platform for studying quantum effects, spin Hall effect, light-matter interaction, etc.

[0003] Through investigation, the current related researches are mostly to transfer the TMDCs and other two-dimensional materials to a substrate, but the influence of the substrate will not only reduce the signal intensity of the TMDCs and other two-dimensional materials, but also cause special phenomena such as interlayer exciton to be difficult to be observed. In order to eliminate the negative influence of the substrate, some researchers also put forward a suspension scheme of the TMDCs and other two-dimensional materials. The current suspension scheme of the TMDCs and other two-dimensional materials is only to form a stepped height difference structure on the substrate by a deep hole or a particle, and then transfer the TMDCs and other two-dimensional materials to the surface of the structure to obtain a suspended TMDCs and other two-dimensional material region. This kind of suspension scheme of the TMDCs and other two-dimensional materials has obvious defects. Since the thickness of the substrate is generally in the order of millimeters, it is difficult to drill a through hole in the substrate when making a deep hole. A large amount of light source penetrates the TMDCs and other two-dimensional material single-layer sample with only three atomic layers into the deep hole, and is reflected by the hole wall and the hole bottom in the hole to act on the TMDCs and other two-dimensional material sample again and penetrate the sample to be collected by the equipment together. Not only will it affect the experimental results, but also will cause a sharp drop in the signal-to-noise ratio. In addition, this kind of scheme can only test the reflection results (reflection spectrum, photoluminescence, etc.) of the sample, and cannot complete the transmission related signal test (such as transmission spectrum, etc.). Even if the etching scheme is optimized, the diameter of the round hole is limited by the size of the single-layer TMDCs and other two-dimensional material sample (generally only a few tens of microns). The excessive hole diameter will also increase the difficulty of transferring the TMDCs and other two-dimensional material thin film with atomic thickness. At the same time, the depth and diameter of the round hole will cause the time cost of etching the round hole to increase exponentially. Therefore, it is a great challenge to design a new suspension scheme of the TMDCs and other two-dimensional materials and successfully prepare a TMDCs and other two-dimensional material thin film and its homogeneous / heterogeneous structure which are truly suspended in a strict sense.

[0004] The information disclosed in this section is only intended to increase the understanding of the overall background of the present application and should not be considered as acknowledging or implying in any form that the information constitutes prior art known to those of ordinary skill in the art. SUMMARY

[0005] The purpose of the present application is to provide a preparation method of a suspended two-dimensional material thin film and a suspended two-dimensional material thin film, which has the advantages of low difficulty in transferring the two-dimensional material thin film, small influence of light source during testing, and effective widening of testing means.

[0006] To achieve the above object, the embodiment of the present application provides a preparation method of a suspended two-dimensional material film, comprising: providing a substrate, the substrate having opposite first and second surfaces; forming a sacrificial layer film on the first surface of the substrate; forming a through hole on the second surface of the substrate, the through hole exposing the sacrificial layer film; obtaining a two-dimensional material film, transferring the two-dimensional material film to the sacrificial layer film, and the two-dimensional material film covering the through hole; etching the sacrificial layer film from the second surface of the substrate until the two-dimensional material film is exposed, to form a suspended two-dimensional material film.

[0007] In one or more embodiments of the present application, the obtaining of the two-dimensional material film, the transferring of the two-dimensional material film to the sacrificial layer film, and the covering of the two-dimensional material film to the through hole, comprises: obtaining a single two-dimensional material film single layer or multiple layers, and transferring the single two-dimensional material film single layer or multiple layers to the surface of the sacrificial layer film in a directional manner, and covering the through hole; or obtaining a single two-dimensional material film single layer, and transferring the two-dimensional material film single layer to the surface of the sacrificial layer film multiple times to obtain a homogeneous structure, wherein each layer of the two-dimensional material film single layer covers the through hole; or obtaining multiple two-dimensional material film single layers, and transferring the multiple two-dimensional material film single layers to the surface of the sacrificial layer film in sequence to obtain a heterogeneous structure, wherein each layer of the two-dimensional material film single layer covers the through hole.

[0008] In one or more embodiments of the present application, the second harmonic signal of each layer of the two-dimensional material film single layer is tested, and in the process of transferring the two-dimensional material film single layer, the stacking angle between each layer of the two-dimensional material film single layer is controlled according to the second harmonic signal result, to obtain a homogeneous structure or a heterogeneous structure with different angles.

[0009] In one or more embodiments of the present application, the fluorescence emission spectrum of the two-dimensional material film is tested, and the number of layers of the two-dimensional material film single layer is accurately determined according to the dependence of the fluorescence emission spectrum of each layer of the two-dimensional material film single layer on the number of layers.

[0010] In one or more embodiments of the present application, the method for obtaining the two-dimensional material film includes mechanical exfoliation and CVD growth, and the method for transferring the two-dimensional material film includes dry transfer and wet transfer.

[0011] In one or more embodiments of the present application, forming a through hole on the second surface of the substrate comprises: forming a protective film layer on the second surface of the substrate; forming a hole on the protective film layer by using electron beam lithography; performing anisotropic etching on the substrate in the hole by using a chemical etching method to form a through hole exposing the sacrificial layer film; wherein the radial cross-sectional dimension of the through hole gradually increases in the direction away from the sacrificial layer film.

[0012] In one or more embodiments of the present application, the two-dimensional material film comprises: a two-dimensional transition metal chalcogenide film, a graphene film, a perovskite film, and a quantum well film; the two-dimensional transition metal chalcogenide film comprises: a WS2 film, a WSe2 film, a MoS2 film, and a MoSe2 film.

[0013] In one or more embodiments of the present application, the material constituting the substrate has anisotropy, and the substrate comprises a silicon substrate, a silicon oxide substrate, and an aluminum silicon salt substrate.

[0014] In one or more embodiments of the present application, the sacrificial layer film comprises a silicon nitride film, an aluminum oxide film, an aluminum nitride film, and a gold film.

[0015] Embodiments of the present application provide a method for preparing a suspended two-dimensional material film, comprising: providing a substrate, the substrate having opposite first and second surfaces; forming a sacrificial layer film on the first surface of the substrate; forming a through hole on the second surface of the substrate, the through hole exposing the sacrificial layer film; forming a micro-nano structure penetrating the sacrificial layer film on the sacrificial layer film exposed by the through hole; obtaining a two-dimensional material film, transferring the two-dimensional material film to the sacrificial layer film, and the two-dimensional material film covering the through hole; using the sacrificial layer film as a mask, forming the micro-nano structure on the two-dimensional material film covering the through hole; continuing to etch the sacrificial layer film from the second surface of the substrate until the two-dimensional material film is exposed, forming a suspended two-dimensional material film with a micro-nano structure.

[0016] In one or more embodiments of the present application, forming the micro-nano structure on the two-dimensional material film comprises: simultaneously etching the sacrificial layer film with a micro-nano structure in the through hole and the two-dimensional material film covering the surface thereof from the second surface of the substrate, the etching process forming the micro-nano structure on the two-dimensional material film while thinning the sacrificial layer film.

[0017] In one or more embodiments of the present application, plasma-reactive ion etching is used to etch the two-dimensional material film covering the through hole using the sacrificial layer film in the through hole as a mask to form a micro-nano structure on the two-dimensional material film.

[0018] In one or more embodiments of the present application, the method for forming micro-nano structures on the sacrificial layer film includes focused ion beam microscopy and electron beam lithography technology.

[0019] In one or more embodiments of the present application, the method for obtaining the two-dimensional material film, transferring the two-dimensional material film onto the sacrificial layer film, and covering the through-hole arrangement with the two-dimensional material film includes: obtaining a single two-dimensional material film single layer or multiple layers, and transferring the single two-dimensional material film single layer or multiple layers onto the surface of the sacrificial layer film in a directional manner to cover the through-hole arrangement; or obtaining a single two-dimensional material film single layer, and transferring the single two-dimensional material film single layer onto the surface of the sacrificial layer film multiple times to obtain a homogenous structure, wherein each layer of the single two-dimensional material film single layer covers the through-hole arrangement; or obtaining multiple two-dimensional material film single layers, and transferring the multiple two-dimensional material film single layers onto the surface of the sacrificial layer film in a sequential manner to obtain a heterogeneous structure, wherein each layer of the two-dimensional material film single layer covers the through-hole arrangement.

[0020] In one or more embodiments of the present application, the second harmonic signal of each layer of the two-dimensional material film single layer is tested, and the stacking angle between each layer of the two-dimensional material film single layer is controlled during the transfer of the two-dimensional material film single layer stack according to the second harmonic signal results, to obtain a homogenous structure or a heterogeneous structure with different angles.

[0021] In one or more embodiments of the present application, the fluorescence emission spectrum of the two-dimensional material film is tested, and the number of layers of the two-dimensional material film is accurately determined according to the dependence of the fluorescence emission spectrum of each layer of the two-dimensional material film single layer on the number of layers.

[0022] In one or more embodiments of the present application, the method for obtaining the two-dimensional material film includes mechanical exfoliation and CVD growth, and the method for transferring the two-dimensional material film includes dry transfer and wet transfer.

[0023] In one or more embodiments of the present application, the method for forming a through-hole on the second surface of the substrate includes: forming a protective film layer on the second surface of the substrate; forming a hole on the protective film layer using electron beam lithography; and performing anisotropic etching on the substrate in the hole using chemical etching to form a through-hole that exposes the sacrificial layer film; wherein the radial cross-sectional dimension of the through-hole gradually increases in a direction away from the sacrificial layer film.

[0024] In one or more embodiments of the present application, the two-dimensional material film includes a two-dimensional transition metal chalcogenide film, a graphene film, a perovskite film, and a quantum well film; the two-dimensional transition metal chalcogenide film includes a WS2 film, a WSe2 film, a MoS2 film, and a MoSe2 film.

[0025] In one or more embodiments of the present application, the material constituting the substrate has anisotropy, and the substrate includes a silicon substrate, a silicon oxide substrate, and an aluminum silicon salt substrate.

[0026] In one or more embodiments of the present application, the sacrificial layer film includes a silicon nitride film, an aluminum oxide film, an aluminum nitride film, and a gold film.

[0027] Embodiments of the present application provide a suspended two-dimensional material film prepared by the above-mentioned method for preparing a suspended two-dimensional material film.

[0028] Compared with the prior art, the method for preparing a suspended two-dimensional material film according to the embodiments of the present application has the advantages of low transfer difficulty of the two-dimensional material film, small influence of a light source on testing, and effective widening of testing means, by transferring the two-dimensional material film single layer, multiple layers, heterostructures, or homostructures to the surface of the suspended sacrificial layer film, instead of directly transferring to the through hole of the substrate, and by the bearing of the sacrificial layer film.

[0029] The method for preparing a suspended two-dimensional material film according to the embodiments of the present application can match two-dimensional material films of different sizes by controlling the size of the suspended sacrificial layer film, and can use the suspended sacrificial layer film as a mask plate to make photonic crystal structures or other micro-nano structures on the suspended sacrificial layer film by using a focused ion beam scanning electron microscope and other methods, and to prepare two-dimensional photonic crystal structures or other micro-nano structures of the suspended two-dimensional material film by using inductively coupled plasma-reactive ion etching.

[0030] The method for preparing a suspended two-dimensional material film according to the embodiments of the present application can greatly reduce the transfer difficulty, expand the sample testing means, and prepare photonic crystal structures or other micro-nano structure patterns on the two-dimensional material film by using the sacrificial layer as a mask plate, thereby helping many two-dimensional materials such as transition metal chalcogenides and graphene to go from the laboratory to practical applications.

[0031] The method for preparing a suspended two-dimensional material film according to the embodiments of the present application can be widely used in the field of two-dimensional material optoelectronic device preparation. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1is a process flow diagram of the method for manufacturing a suspended two-dimensional material thin film according to the first embodiment of the present application;

[0033] Figure 2 is a process step schematic diagram of the method for manufacturing a suspended two-dimensional material thin film according to the first embodiment of the present application;

[0034] Figure 3 is an optical micrograph of the suspended sacrificial layer thin film (SiN thin film) used in the method for manufacturing a suspended two-dimensional material thin film in Example 1.

[0035] Figure 4 is a fluorescence emission spectrum of mechanically exfoliated monolayer WS2 and WSe2 (various two-dimensional material thin film monolayers) used in the method for manufacturing a suspended two-dimensional material thin film in Example 1 under 532 nm laser excitation.

[0036] Figure 5 is an optical micrograph of mechanically exfoliated monolayer WSe2 (a, dotted area) and WS2 (b, dotted area) (various two-dimensional material thin film monolayers) used in the method for manufacturing a suspended two-dimensional material thin film in Example 1, and its second harmonic signal (dot) and fitting result (solid line) under 1064 nm laser excitation.

[0037] Figure 6 is an optical micrograph of mechanically exfoliated monolayer WSe2 (vertically extending dotted area) and WS2 (horizontally extending dotted area) (various two-dimensional material thin film monolayers) used in the method for manufacturing a suspended two-dimensional material thin film in Example 1 after being sequentially transferred onto the suspended sacrificial layer thin film (SiN thin film), and the lattice rotation angle between WS2 and WSe2 monolayers is controlled by the second harmonic signal.

[0038] Figure 7 is an optical micrograph of the WS2 / WSe2 heterostructure on the suspended sacrificial layer thin film (SiN thin film) used in the method for manufacturing a suspended two-dimensional material thin film in Example 1 after the sacrificial layer thin film (SiN thin film) is etched away from the back by hydrogen fluoride.

[0039] Figure 8 is a process flow diagram of the method for manufacturing a suspended two-dimensional material thin film according to the second embodiment of the present application;

[0040] Figure 9 is a process step schematic diagram of the method for manufacturing a suspended two-dimensional material thin film according to the second embodiment of the present application;

[0041] Figure 10 is a structure diagram of a suspended two-dimensional material thin film according to the second embodiment of the present application.

[0042] Figure 11An optical micrograph of a WS2 / WSe2 heterostructure etched from the back using plasma-reactive ion etching on a suspended sacrificial layer film (SiN film) used in the method of preparing a suspended two-dimensional material film in Example 2.

[0043] Figure 12 An optical micrograph of a suspended sacrificial layer film (SiN film) used in the method of preparing a suspended two-dimensional material film in Example 3. DETAILED DESCRIPTION

[0044] The specific embodiments of the present application are described in detail below with reference to the accompanying drawings, but the scope of protection of the present application is not limited by the specific embodiments.

[0045] Unless otherwise clearly indicated, throughout the specification and claims, the term "comprise" or variations such as "comprises" or "comprising" will be understood to imply the inclusion of a stated element or group of elements but not the exclusion of any other element or group of elements.

[0046] As described in the background, the direct formation of a two-dimensional material film such as TMDCs on a substrate not only reduces the signal strength of the two-dimensional material such as TMDCs, but also makes it difficult to observe special phenomena such as interlayer excitons. In the prior art, in order to solve this problem, a suspension scheme of two-dimensional materials such as TMDCs is used. For example, a stepped height difference structure is formed on the substrate by a deep hole or a particle, and then the two-dimensional material such as TMDCs is transferred to the surface of the structure to obtain a suspended two-dimensional material such as TMDCs region. However, such a suspension scheme of two-dimensional materials such as TMDCs has obvious defects.

[0047] Since the thickness of the substrate is generally in the order of millimeters, it is difficult to etch the substrate to form a through hole when making a deep hole. A large amount of light source penetrates the TMDCs two-dimensional material single-layer sample with only three atomic layers into the deep hole, and is reflected by the hole wall and the hole bottom in the hole, and then acts on the TMDCs two-dimensional material sample again, and is collected together with the equipment by penetrating the sample. Not only will it affect the experimental results, but it will also cause a sharp drop in signal-to-noise ratio; in addition, such a scheme can only test the reflection results (reflection spectrum, photoluminescence, etc.) of the sample, and cannot complete the transmission-related signal test (such as transmission spectrum, etc.), even if the etching scheme is optimized, the substrate is forcibly etched, the diameter of the circular hole is limited by the size of the single-layer TMDCs two-dimensional material sample (generally only a few tens of microns), and the aperture of the circular hole will also increase the difficulty of transferring the TMDCs two-dimensional material film with an atomic thickness, and the depth and aperture of the circular hole will both cause the time cost of etching the circular hole to increase exponentially.

[0048] Based on this, the application provides a preparation method of a suspended two-dimensional material film and the suspended two-dimensional material film, first, a suspended sacrificial layer film is made, a two-dimensional material film is directionally transferred to the suspended sacrificial layer film, then the suspended sacrificial layer film is etched from the back, and a suspended two-dimensional material film is formed. The application sets a sacrificial layer film first to bear the transfer of the two-dimensional material film, then removes the sacrificial layer film according to the position where the two-dimensional material film needs to be suspended, and completes the preparation of the suspended two-dimensional material film. At the same time, the suspended sacrificial layer film can also be used as a mask plate to make photonic crystal structures or other micro-nano structures on the suspended sacrificial layer film by using a focused ion beam scanning electron microscope and other methods, and to prepare two-dimensional photonic crystal structures or other micro-nano structures of the suspended two-dimensional material film by using inductively coupled plasma-reactive ion etching, which has the advantages of low difficulty in transferring the two-dimensional material film, small influence of light source on testing, and effective widening of testing means.

[0049] As shown in Figure 1 The preparation method of the suspended two-dimensional material film according to the first embodiment of the application comprises the following steps: s1, providing a substrate, the substrate has opposite first and second surfaces; s2, forming a sacrificial layer film on the first surface of the substrate; s3, forming a through hole on the second surface of the substrate, the through hole exposes the sacrificial layer film; s4, obtaining a two-dimensional material film, transferring the two-dimensional material film to the sacrificial layer film, and the two-dimensional material film covers the through hole; s5, etching the sacrificial layer film from the second surface of the substrate until the two-dimensional material film is exposed, and a suspended two-dimensional material film is formed.

[0050] Specifically, step s3 comprises the following steps: forming a protective film layer on the second surface of the substrate; forming a hole on the protective film layer by using electron beam lithography; and performing anisotropic etching on the substrate in the hole by using chemical etching, to form a through hole that exposes the sacrificial layer film, and the radial cross-sectional size of the through hole gradually increases in the direction away from the sacrificial layer film.

[0051] In step s4, the single two-dimensional material film single layer or multiple layers can be obtained by mechanical exfoliation or CVD growth or other methods; the single two-dimensional material film single layer or multiple layers can be transferred to the surface of the sacrificial layer film in a directional manner by dry transfer technology or wet transfer technology or other transfer technology, and cover the through hole arrangement. Alternatively, the single two-dimensional material film single layer can be obtained by mechanical exfoliation or CVD growth or other methods, and the single two-dimensional material film single layer can be transferred to the surface of the sacrificial layer film multiple times by dry transfer technology or wet transfer technology or other transfer technology to obtain a homogeneous structure, wherein each two-dimensional material film single layer covers the through hole arrangement. Alternatively, multiple two-dimensional material film single layers can be obtained by mechanical exfoliation or CVD growth or other methods, and the multiple two-dimensional material film single layers can be transferred to the surface of the sacrificial layer film in sequence by dry transfer technology or wet transfer technology or other transfer technology to obtain a heterogeneous structure, wherein each two-dimensional material film single layer covers the through hole arrangement. The rotation angle between the layers in the homogeneous structure or the heterogeneous structure of the two-dimensional material film can be accurately controlled during the transfer process. Before forming the homogeneous structure or the heterogeneous structure, the second harmonic signal of each two-dimensional material film single layer can be tested, and during the transfer of the two-dimensional material film single layer stack, the stacking angle between each two-dimensional material film single layer can be controlled according to the second harmonic signal result to obtain a homogeneous structure or a heterogeneous structure with different angles.

[0052] In the present embodiment, the two-dimensional material film can be a single layer, multiple layers, a homogeneous structure or a heterogeneous structure, and the number of layers of the stacked homogeneous or heterogeneous structure can be any layer. The number of layers of the two-dimensional material film can be accurately determined by testing the fluorescence emission spectrum of the two-dimensional material film according to the dependence of the fluorescence emission spectrum of each two-dimensional material film single layer on the number of layers. The two-dimensional material film can include two-dimensional transition metal chalcogenide film, graphene film, perovskite film, quantum well film, etc.; wherein the two-dimensional transition metal chalcogenide film can include WS2 film, WSe2 film, MoS2 film, MoSe2 film, etc. The material constituting the substrate has anisotropy, and the substrate can include a silicon substrate, a silicon oxide substrate, an aluminum silicon salt mine substrate, etc. When the substrate is selected from a silicon substrate, the etching of the silicon substrate can use potassium hydroxide, sodium hydroxide or hydrogen chloride, etc. The sacrificial layer film can include silicon nitride film, aluminum oxide film, aluminum nitride film, gold film, etc. When the sacrificial layer film is selected from silicon nitride film, the etching of the silicon nitride film can use hydrofluoric acid. The protective film can include PMMA film (polymethyl methacrylate film).

[0053] It can be understood that the size and thickness of the substrate and the sacrificial layer film can be changed, and the size of the suspended sacrificial layer film can also be changed to adapt to the transfer of two-dimensional materials of different sizes and different types.

[0054] It can be understood that, due to the possibility of seed in the CVD-grown two-dimensional material film such as TMDCs, and the inevitable presence of large multi-layer or bulk material in the mechanically exfoliated TMDCs, the mutual stacking or occupation of the bulk in the transfer process of preparing multi-layer homo- or hetero-structures will lead to transfer failure. Therefore, in the scheme of the present application, the bulk can be removed under an optical microscope using adhesive tape.

[0055] Reference Figure 2 As shown, first, a low-stress sacrificial layer film 20 is prepared on a substrate 10. A protective film is spin-coated on the substrate 10 side of the substrate / sacrificial layer film, a hole is made on the protective film using electron beam lithography, according to the anisotropic characteristics of the substrate 10, the side where the substrate is located is chemically etched, and part of the substrate 10 is etched away to form a through hole 11, and the sacrificial layer film 20 is not etched, thereby obtaining a suspended sacrificial layer film.

[0056] Secondly, a single-layer or multi-layer two-dimensional material film 30 is obtained by mechanically exfoliating from the bulk material of the two-dimensional material film, and is transferred to a PDMS film. Under an optical microscope, the two-dimensional material film single-layer or multi-layer is directionally transferred to the surface of the suspended sacrificial layer film using a dry transfer technology of two-dimensional material, or the two-dimensional material film single-layer is sequentially transferred to the surface of the suspended sacrificial layer film to obtain a homo- or hetero-structure.

[0057] Finally, the substrate / sacrificial layer film piece with the two-dimensional material film 30 is suspended and placed on a hydrofluoric acid solution, and is left to stand at room temperature. Every 5-10 minutes, the microscope is observed to ensure that the sacrificial layer film is completely etched away, forming a suspended two-dimensional material film single-layer, multi-layer, homo- or hetero-structure.

[0058] Among them, the size of the suspended sacrificial layer film can be controlled by the size of the opening on the protective film on the back of the substrate and the thickness of the substrate, to adapt to the suspension preparation of materials of different sizes, different types and different application scenarios. The dry transfer technology can change the type of PDMS film, the heating temperature and time, whether to pre-treat the bulk of the two-dimensional material film, etc. In addition, the dry transfer technology can be replaced by wet transfer and other transfer schemes, and the PDMS film can be replaced by PMMA, PC (polycarbonate) film, etc. The etching process of the sacrificial layer film, the concentration of the etching solution, the etching time, the reaction temperature and other parameters can be adjusted according to the thickness of the sacrificial layer film, etc.

[0059] The first embodiment of the present application is described in detail through a specific embodiment, which is helpful to understand the specific technical scheme of the first embodiment of the present application.

[0060] Example 1:

[0061] A 20 nm thick SiN film was grown on a Si wafer with a size of 5 mm*5 mm and a thickness of 200 μm, and an optical micrograph of the SiN film is shown in FIG. 1, where a is a front view, b is a back view focused on the Si, and c is a back view focused on the interface between the Si and the SiN. Figure 3

[0062] A 300 nm thick PMMA was spin-coated on the Si side of the SiN / Si, a 280 μm*280 μm hole was made on the PMMA by electron beam lithography, the Si was etched from the back by potassium hydroxide, and a four-prism-shaped hole with a top size of 20 μm*20 μm was formed, i.e., the size of the SiN overhanging film was 20 μm*20 μm and 20 nm.

[0063] Single-layer WS2 and WSe2 were mechanically exfoliated from bulk WS2 and WSe2 materials and transferred to a PMDS film, the second harmonic signal of the single-layer WS2 and WSe2 was tested under 1064 nm laser excitation, and fitting was performed to reduce measurement error. Referring to FIG. 2, Figures 4 to 5 Figure 4 is a fluorescence emission spectrum of mechanically exfoliated WS2 and WSe2 under 532 nm laser excitation (613.5 nm for WS2 and 744.5 nm for WSe2); Figure 5 is an optical micrograph of mechanically exfoliated single-layer WSe2 (a, dotted area) and WS2 (b, dotted area), and the second harmonic signal (point) and fitting result (solid line) thereof under 1064 nm laser excitation.

[0064] Under an optical microscope, single-layer WS2 and WSe2 were sequentially transferred to the overhanging SiN by dry transfer technology, to ensure that the single-layer WS2 completely covers the overhanging SiN, and the lattice rotation angle between the single-layer WS2 and WSe2 was controlled according to the second harmonic signal; the transfer temperature was controlled to be 60-100 degrees Celsius, and the heat preservation time during the transfer process was 5-20 minutes. Referring to FIG. 3, Figure 6 Figure 6 is an optical micrograph of single-layer WSe2 (vertically extending dotted area) and WS2 (horizontally extending dotted area) sequentially transferred to the overhanging SiN, and the lattice rotation angle between the single-layer WS2 and WSe2 was controlled by the second harmonic signal, where the rotation angle of the two layers in a is 30 degrees, and the rotation angle of the two layers in b is 10 degrees.

[0065] ​​​Two SiN / Si wafers, one with a WS2 / WSe2 heterostructure and the other without, were suspended in hydrofluoric acid solution. The SiN / Si wafer without the WS2 / WSe2 heterostructure served as a control group. The wafers were left to stand at room temperature, and observed under a microscope every 5-10 minutes. When complete etching of SiN was observed in the SiN / Si wafer without the WS2 / WSe2 heterostructure, it indicated that the suspended WS2 / WSe2 heterostructure had been successfully fabricated. Figure 7 As shown, Figure 7 These are optical micrographs of the WS2 / WSe2 heterostructure on suspended SiN after the SiN has been etched away from the back by hydrogen fluoride. In a, the two layers are rotated at an angle of 30 degrees, and in b, the two layers are rotated at an angle of 10 degrees.

[0066] like Figure 8 As shown, a method for preparing a suspended two-dimensional material thin film according to a second embodiment of the present invention includes: s10, providing a substrate having opposing first and second surfaces; s20, forming a sacrificial layer thin film on the first surface of the substrate; s30, forming vias on the second surface of the substrate, the vias exposing the sacrificial layer thin film; s40, forming a micro / nano structure penetrating the sacrificial layer thin film on the sacrificial layer thin film exposed by the vias; s50, obtaining a two-dimensional material thin film, transferring the two-dimensional material thin film onto the sacrificial layer thin film, and the two-dimensional material thin film covering the vias; s60, using the sacrificial layer thin film as a mask, forming a micro / nano structure on the two-dimensional material thin film covering the vias; s70, etching the sacrificial layer thin film from the second surface of the substrate until the two-dimensional material thin film is exposed, forming a suspended two-dimensional material thin film.

[0067] Specifically, step s30 includes: forming a protective film layer on the second surface of the substrate; forming a hole on the protective film layer using electron beam lithography; and anisotropically etching the substrate inside the hole using chemical etching to form a through hole exposing the sacrificial layer film, wherein the radial cross-sectional dimension of the through hole gradually increases in the direction away from the sacrificial layer film.

[0068] In step s40, a micro / nano structure can be formed on the sacrificial layer film using focused ion beam microscopy or electron beam lithography.

[0069] In step s50, the single two-dimensional material film single layer or multiple layers can be obtained by mechanical exfoliation or CVD growth or other methods; the single two-dimensional material film single layer or multiple layers can be transferred to the surface of the sacrificial layer film by dry transfer technology or wet transfer technology or other transfer technology, and cover the through-hole arrangement. Alternatively, the single two-dimensional material film single layer can be obtained by mechanical exfoliation or CVD growth or other methods, and the single two-dimensional material film single layer can be transferred to the surface of the sacrificial layer film multiple times by dry transfer technology or wet transfer technology or other transfer technology to obtain a homogeneous structure, wherein each two-dimensional material film single layer covers the through-hole arrangement. Alternatively, multiple two-dimensional material film single layers can be obtained by mechanical exfoliation or CVD growth or other methods, and the multiple two-dimensional material film single layers can be transferred to the surface of the sacrificial layer film in sequence by dry transfer technology or wet transfer technology or other transfer technology to obtain a heterogeneous structure, wherein each two-dimensional material film single layer covers the through-hole arrangement. The rotation angle between the layers in the homogeneous structure or the heterogeneous structure of the two-dimensional material film can be accurately controlled during the transfer process. Before forming the homogeneous structure or the heterogeneous structure, the second harmonic signal of each two-dimensional material film single layer can be tested, and during the transfer of the two-dimensional material film single layer stack, the stacking angle between each two-dimensional material film single layer can be controlled according to the second harmonic signal result to obtain a homogeneous structure or a heterogeneous structure with different angles.

[0070] In step s60, the plasma-reactive ion etching is used, and the sacrificial layer film in the through-hole is used as a mask plate to etch the sacrificial layer film with micro-nano structures in the through-hole and the two-dimensional material film covering the surface of the sacrificial layer film from the second surface of the substrate. During the etching process, the micro-nano structure is formed on the two-dimensional material film, and the sacrificial layer film is thinned.

[0071] In the embodiment, the two-dimensional material film can be a single layer, multiple layers, a homogenous structure or a heterogeneous structure, and the number of stacked homogenous or heterogeneous structures can be any layer. The number of layers of the two-dimensional material film can be accurately determined by testing the fluorescence emission spectrum of the two-dimensional material film, and according to the dependence of the fluorescence emission spectrum of each layer of the two-dimensional material film single layer on the number of layers. The two-dimensional material film can include a two-dimensional transition metal dichalcogenide film, a graphene film, a perovskite film, a quantum well film, and the like; wherein the two-dimensional transition metal dichalcogenide film can include a WS2 film, a WSe2 film, a MoS2 film, a MoSe2 film, and the like. The material constituting the substrate has anisotropy, and the substrate can include a silicon substrate, a silicon oxide substrate, an aluminum silicon salt mine substrate, and the like. When the substrate is selected from a silicon substrate, the etching of the silicon substrate can use potassium hydroxide, sodium hydroxide or hydrogen chloride, etc. The sacrificial layer film can include a silicon nitride film, an aluminum oxide film, an aluminum nitride film, a gold film, and the like. When the sacrificial layer film is selected from a silicon nitride film, the etching of the silicon nitride film can use hydrofluoric acid. The protective film can include a PMMA film.

[0072] It can be understood that the size and thickness of the substrate and the sacrificial layer film can be changed, and the size of the suspended sacrificial layer film can also be changed to adapt to the transfer of two-dimensional materials of different sizes and different types.

[0073] It can be understood that since the two-dimensional material film grown by CVD, such as TMDCs, can have seeds, and the mechanically exfoliated TMDCs also inevitably have large multi-layer or bulk materials, during the transfer process of preparing a multi-layer homogenous or heterogeneous structure, the mutual stacking or occupation of the bulk will cause the transfer to fail. Therefore, in the scheme of the present application, the bulk can be removed under an optical microscope using an adhesive tape.

[0074] Reference Figure 9 As shown, first, a low-stress sacrificial layer film 20 is prepared on a substrate 10. A protective film is spin-coated on the substrate 10 side of the substrate / sacrificial layer film, a hole is made on the protective film using electron beam lithography, according to the anisotropic characteristics of the substrate 10, the side where the substrate 10 is located is chemically etched, and part of the substrate 10 is etched to form a through hole 11, and the sacrificial layer film 20 is not etched, thereby obtaining a suspended sacrificial layer film.

[0075] Secondly, a photonic crystal air hole structure or other micro-nano structure A is processed on the suspended sacrificial layer film 20 using a focused ion beam microscope. The suspended sacrificial layer film 20 can also be used as a mask plate in the subsequent process.

[0076] Then, the single-layer or multi-layer two-dimensional material film 30 is obtained by mechanical exfoliation from the bulk material of the two-dimensional material film, and is transferred to the PDMS film. Under an optical microscope, the two-dimensional material film is transferred to the surface of the suspended sacrificial layer film using a dry transfer technique of the two-dimensional material, or the two-dimensional material film is sequentially transferred to the surface of the suspended sacrificial layer film to obtain a homogeneous structure or a heterogeneous structure.

[0077] Then, the two-dimensional material film above the through hole 11 is etched from the back of the substrate 10 (the side away from the sacrificial layer film) using the photonic crystal pore structure or other micro-nano structure A processed by the above steps, using the sacrificial layer film 20 in the through hole 11 as a mask plate. The two-dimensional material film 30 is left with the required photonic crystal structure or micro-nano structure B. While the two-dimensional material film 30 in the through hole 11 is etched, the sacrificial layer film 20 is also gradually thinned.

[0078] Finally, the substrate / sacrificial layer film piece with the two-dimensional material film 30 is suspended over the hydrofluoric acid solution and left at room temperature. Every 5-10 minutes, the microscope is observed to ensure that the sacrificial layer film is completely etched, forming a suspended two-dimensional material film single-layer, multi-layer, homogeneous or heterogeneous structure.

[0079] The size of the suspended sacrificial layer film can be controlled by the size of the opening in the protective film on the back of the substrate and the thickness of the substrate, to adapt to the suspension preparation of materials of different sizes, different types and different application scenarios. The dry transfer technique can be changed according to the actual transfer situation, such as the type of PDMS film, the heating temperature and time, whether the two-dimensional material film is pre-processed, etc. In addition, the dry transfer technique can be replaced by wet transfer or other transfer schemes, and the PDMS film can be replaced by PMMA, PC or other films. The etching process of the sacrificial layer film, the concentration of the etching solution, the etching time, the reaction temperature and other parameters can be adjusted according to the thickness of the sacrificial layer film.

[0080] Figure 10 The structure diagram of the suspended two-dimensional material film of the second embodiment of the present application, which includes a substrate 10, a sacrificial layer film 20 formed on the first surface of the substrate 10, and a two-dimensional material film 30 formed on the surface of the sacrificial layer film 20. The substrate 10 has a through hole 11 that penetrates the substrate 10 in the thickness direction. The sacrificial layer film 20 has another through hole that penetrates the substrate 10 in the thickness direction, and the through hole is coaxial with and corresponds to the through hole 11 on the substrate 10. The two-dimensional material film 30 covers the through hole and the through hole 11, and the two-dimensional material film 30 has a micro-nano structure pattern B, such as a photonic crystal structure.

[0081] The second embodiment of this application will be described in detail below through a specific example, so as to facilitate understanding of the specific technical solution of the second embodiment of this application.

[0082] Example 2:

[0083] A 20 nm thick SiN thin film was grown on a Si wafer with dimensions of 5 mm * 5 mm and a thickness of 200 μm. Optical micrographs of the SiN thin film are shown below. Figure 3 As shown, a is a front view, b is a back view focused on Si, and c is a back view focused on the interface between Si and SiN.

[0084] A 300nm thick PMMA layer is spin-coated on the Si side of the SiN / Si. A 280μm*280μm hole is fabricated on the PMMA using electron beam lithography. The Si is then etched from the back using potassium hydroxide to form a flat-topped quadrangular pyramidal hole with a size of 20μm*20μm on the upper layer. In other words, the size of the SiN suspended film is 20μm*20μm, 20nm.

[0085] The desired structure was fabricated on a SiN suspended film using a focused ion beam microscope. In this embodiment, a photonic crystal pore structure was fabricated with a period of 680 nm and a diameter of 400 nm. The pore was a through hole.

[0086] Monolayers of WS2 and WSe2 were obtained by mechanical peeling from bulk WS2 and WSe2 materials and transferred to PMDS. The second harmonic signals of the monolayer WS2 and WSe2 were tested under 1064nm laser excitation and fitted to reduce measurement error.

[0087] Under an optical microscope, monolayer WS2 and WSe2 are sequentially transferred onto suspended SiN using a dry transfer technique, ensuring that the monolayer WS2 completely covers the suspended SiN. The lattice rotation angle between the WS2 and WSe2 monolayers is controlled according to the second harmonic signal. The transfer temperature is controlled at 60-100 degrees Celsius, and the holding time during the transfer process is 5-20 minutes.

[0088] SiN / Si wafers with and without WS2 / WSe2 heterostructures were simultaneously placed in a plasma-reactive ion etching (PRI) apparatus. Etching was performed from the back side of the SiN / Si wafers, controlling the etching conditions and time to etch through the heterostructure above the pores of the SiN photonic crystal while preserving a portion of the SiN thin film. (Reference) Figure 11 As shown, Figure 11 These are optical micrographs of the suspended SiN WS2 / WSe2 heterostructure etched from the back of the wafer using plasma-reactive ion etching. In a, the two layers are rotated at an angle of 30 degrees, and in b, the two layers are rotated at an angle of 10 degrees.

[0089] Two SiN / Si pieces with WS2 / WSe2 heterostructure and without WS2 / WSe2 heterostructure are placed on a hydrofluoric acid solution, wherein the SiN / Si without WS2 / WSe2 heterostructure is used as a control group, and is placed under a microscope every 5-10 minutes at room temperature, and when the SiN of the SiN / Si piece without WS2 / WSe2 heterostructure is completely etched, it is indicated that the suspended WS2 / WSe2 heterostructure photonic crystal has been successfully prepared.

[0090] Example 3:

[0091] The difference between this example and the above-mentioned examples 1 and 2 is that the size of the suspended SiN film is 465 μm*465 μm. Figure 12 Figure 12 It is an optical micrograph of the suspended SiN used in the preparation method of the suspended two-dimensional material film; wherein a is a front view, b is a back view focused on Si, and c is a back view focused on the interface between Si and SiN.

[0092] Example 4:

[0093] The difference between this example and the above-mentioned examples 1 and 2 is that the WS2 / WSe2 heterostructure is replaced by a single-layer WS2.

[0094] Example 5:

[0095] The difference between this example and the above-mentioned examples 1 and 2 is that the WS2 / WSe2 heterostructure is replaced by a multi-layer WS2.

[0096] Example 6:

[0097] The difference between this example and the above-mentioned example 2 is that the photonic crystal pore structure on the suspended SiN film is prepared by electron beam lithography and plasma-reactive ion etching technology.

[0098] Compared with the prior art, the suspended two-dimensional material film preparation method according to the embodiment of the present application has the advantages of low difficulty in transferring the two-dimensional material film, small influence of light source on testing, and effective widening of testing means, by transferring the single-layer, multi-layer, heterostructure or homogenous structure of the two-dimensional material film to the surface of the suspended sacrificial layer film, which is easier to transfer due to the bearing of the sacrificial layer film compared with being directly transferred to the through hole of the substrate.

[0099] ​The preparation method of the suspended two-dimensional material film according to the embodiment of the present application can match two-dimensional material films of different sizes by controlling the size of the suspended sacrificial layer film, and can also use the suspended sacrificial layer film as a mask to make photonic crystal structures or other micro-nano structures on the suspended sacrificial layer film by using various methods such as a focused ion beam scanning electron microscope, and use inductively coupled plasma-reactive ion etching to prepare two-dimensional photonic crystal structures or other micro-nano structures of the suspended two-dimensional material film.

[0100] The preparation method of the suspended two-dimensional material film according to the embodiment of the present application can greatly reduce the transfer difficulty, expand sample testing means, and can use the sacrificial layer as a mask to prepare photonic crystal structures or other micro-nano structure patterns on the two-dimensional material film, thereby helping transition metal sulfide compounds, graphene and many other two-dimensional materials to transition from the laboratory to practical applications.

[0101] The preparation method of the suspended two-dimensional material film according to the embodiment of the present application can be widely used in the field of two-dimensional material optoelectronic device preparation.

[0102] The foregoing description of specific exemplary embodiments of the present application is intended to be illustrative only and is not intended to limit the present application to the precise forms described. Many modifications and variations are possible in light of the above teachings without departing from the spirit or essential characteristics of the present application. The particular illustrative embodiments were chosen and described in order to explain the principles of the present application and its practical application to thereby enable others skilled in the art to best utilize the present application and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the present application be defined by the claims and their equivalents.

Claims

1. A method for preparing a suspended two-dimensional material thin film, characterized in that, include: A substrate is provided, the substrate having opposing first and second surfaces; A sacrificial layer film is formed on the first surface of the substrate; A via is formed on the second surface of the substrate, the via exposing the sacrificial layer film; A two-dimensional material film is obtained, and the two-dimensional material film is transferred onto the sacrificial layer film, wherein the two-dimensional material film covers the through hole. The sacrificial layer film is etched from the second surface of the substrate until the two-dimensional material film is exposed, forming a suspended two-dimensional material film.

2. The method for preparing a suspended two-dimensional material thin film as described in claim 1, characterized in that, The method of obtaining a two-dimensional material film, transferring the two-dimensional material film onto the sacrificial layer film, and having the two-dimensional material film cover the through-hole configuration includes: Obtain one or more layers of a single two-dimensional material film, directionally transfer them to the surface of the sacrificial layer film, and cover the through-holes; or, A single layer of a two-dimensional material film is obtained, and the single layer of the two-dimensional material film is transferred multiple times to the surface of the sacrificial layer film to obtain a homogeneous structure, wherein each layer of the two-dimensional material film covers the through-hole; or... Multiple two-dimensional material film monolayers are obtained, and the multiple two-dimensional material film monolayers are sequentially transferred to the surface of the sacrificial layer film to obtain a heterogeneous structure, wherein each two-dimensional material film monolayer covers the through hole.

3. The method for preparing a suspended two-dimensional material thin film as described in claim 2, characterized in that, By testing the second harmonic signal of each layer of the two-dimensional material film, the stacking angle between each layer of the two-dimensional material film is controlled according to the result of the second harmonic signal during the transfer and stacking process, so as to obtain homogeneous or heterogeneous structures stacked at different angles.

4. The method for preparing a suspended two-dimensional material thin film as described in claim 1, characterized in that, The methods for obtaining the two-dimensional material film include mechanical exfoliation and CVD growth, and the methods for transferring the two-dimensional material film include dry transfer and wet transfer.

5. The method for preparing a suspended two-dimensional material thin film as described in claim 1, characterized in that, Forming a through-hole on the second surface of the substrate includes: A protective film layer is formed on the second surface of the substrate; Holes are formed on the protective film layer using electron beam lithography; The substrate inside the hole is anisotropically etched using chemical etching to form a through hole that exposes the sacrificial layer film; The radial cross-sectional dimension of the through hole gradually increases in the direction away from the sacrificial layer film.

6. The method for preparing a suspended two-dimensional material thin film as described in claim 1, characterized in that, The two-dimensional material thin films include: two-dimensional transition metal chalcogenide thin films, graphene thin films, perovskite thin films, and quantum well thin films; The two-dimensional transition metal chalcogenide thin film includes: WS2 thin film, WSe2 thin film, MoS2 thin film, MoSe2 thin film; and / or, The material constituting the substrate is anisotropic, and the substrate includes a silicon substrate, a silicon oxide substrate, an aluminosilicate substrate; and / or, The sacrificial layer film includes silicon nitride film, aluminum oxide film, aluminum nitride film, and gold film.

7. A method for preparing a suspended two-dimensional material thin film, characterized in that, include: A substrate is provided, the substrate having opposing first and second surfaces; A sacrificial layer film is formed on the first surface of the substrate; A via is formed on the second surface of the substrate, the via exposing the sacrificial layer film; A micro / nano structure is formed on the sacrificial layer film exposed by the vias, penetrating the sacrificial layer film; A two-dimensional material film is obtained, and the two-dimensional material film is transferred onto the sacrificial layer film, wherein the two-dimensional material film covers the through hole. The sacrificial layer film is used as a mask to form the micro / nano structure on a two-dimensional material film covering the through-hole; The sacrificial layer film continues to be etched from the second surface of the substrate until the two-dimensional material film is exposed, forming a suspended two-dimensional material film with a micro-nano structure.

8. The method for preparing a suspended two-dimensional material thin film as described in claim 7, characterized in that, Forming the micro / nano structure on the two-dimensional material film includes: The sacrificial layer film with micro-nano structures and the two-dimensional material film covering the surface of the via are simultaneously etched from the second surface of the substrate. During the etching process, the micro-nano structures are formed on the two-dimensional material film, while the sacrificial layer film is thinned.

9. The method for preparing a suspended two-dimensional material thin film as described in claim 7, characterized in that, Methods for forming micro- and nanostructures on the sacrificial layer film include focused ion beam microscopy and electron beam lithography.

10. A suspended two-dimensional material thin film, characterized in that, It is prepared by the method of preparing suspended two-dimensional material thin film according to any one of claims 1-6, or by the method of preparing suspended two-dimensional material thin film according to any one of claims 7-9.

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