Two-dimensional material cross section TEM sample with clean surface and preparation method of two-dimensional material cross section TEM sample

Abstract

The invention discloses a two-dimensional material cross section TEM sample with a clean surface and a preparation method thereof. The TEM sample comprises a protective layer which uses a two-dimensional transfer metal film as FIB spraying and milling thinning, and after FIB thinning, in-situ stripping is carried out on the protective layer on the surface of a two-dimensional material under the driving action of a piezoelectric controller by using a chemical etching needle point in a TEM to obtain the TEM sample. The sample can be applied to in-situ TEM tribology or electricity research related to a surface interface. The preparation method has high universality and can be suitable for various two-dimensional materials such as graphene, molybdenum disulfide and boron nitride.

Description

Technical Field

[0001] The invention relates to a two-dimensional material cross-section TEM sample with a clean surface and a preparation method thereof, belonging to the technical field of TEM samples. Background Art

[0002] Two-dimensional materials, such as graphene and molybdenum disulfide, show great potential in the field of superlubricity due to their unique physical and chemical properties. The atomic-level thickness and high specific surface area of ​​these materials give them significant advantages in reducing friction and wear, which is crucial to improving the energy efficiency and life of mechanical systems. Although two-dimensional materials exhibit excellent lubrication properties at the macroscopic level, their superlubrication mechanisms at the microscopic and even atomic scales are still not fully understood, which limits their further application in micro-nano electromechanical systems. In situ transmission electron microscopy (TEM) technology plays an irreplaceable role in this research field. It can provide real-time, high-resolution observations to reveal the atomic-level dynamics of materials under actual friction conditions, thereby providing a key scientific basis for the design and manufacture of more efficient and reliable micro-nano systems.

[0003] However, in situ TEM friction research places extremely strict requirements on the preparation of two-dimensional material samples, especially the cross-sectional preparation of samples. The thickness of the sample must be precisely controlled at the nanometer level to ensure that the electron beam can penetrate and provide clear imaging. In addition, in order to achieve direct observation of the friction interface, the friction interface of the sample needs to be exposed very cleanly without any contaminants or damage. In the process of preparing TEM samples using traditional focused ion beam (FIB) technology, the method of spraying a protective layer is usually used to protect the sample from damage by high-energy ion beams. However, the spraying process itself may introduce new damage, which is a problem that cannot be ignored for two-dimensional materials that are extremely sensitive to damage. In addition, due to the strong bonding force between the protective layer and the sample, it becomes extremely difficult to remove these protective layers to expose a clean friction interface.

[0004] Therefore, there is an urgent need to develop a new preparation method in this field that can avoid the damage of high-energy ion beams to two-dimensional materials and effectively expose a clean friction interface after sample thinning to facilitate precise in situ TEM research. Summary of the invention

[0005] Purpose of the invention: The first purpose of the present invention is to provide a cross-sectional TEM sample of a two-dimensional material with a clean surface. The second purpose of the present invention is to provide a method for preparing a cross-sectional TEM sample of a two-dimensional material with a clean surface.

[0006] Technical solution: The present invention discloses a method for preparing a cross-sectional TEM sample of a two-dimensional material with a clean surface. The method comprises using a two-dimensional transferred metal film as a FIB spray-plated protective layer. After completing the preparation of the cross-sectional TEM sample of the two-dimensional material by FIB, the metal protective layer on the surface of the two-dimensional material is in-situ peeled off by using a chemically etched needle tip in the TEM under the driving action of a piezoelectric controller.

[0007] Furthermore, the two-dimensional material is graphene, molybdenum disulfide, boron nitride, indium selenide or MXene material, etc.

[0008] Furthermore, the metal film is an Au film, a W film, an Ag film or a Cu film.

[0009] Furthermore, the method for preparing a TEM sample of a two-dimensional material cross section with a clean surface of the present invention comprises the following steps:

[0010] (1) Sputtering a nanometer-thick metal layer on polydimethylsiloxane (PDMS) using magnetron sputtering;

[0011] (2) Using a two-dimensional material transfer platform, the metal layer sputtered on the PDMS is transferred to the surface of the two-dimensional material under the assistance of pressure, thereby obtaining a metal film on the surface of the two-dimensional material;

[0012] (3) Using FIB to prepare cross-sectional TEM samples of two-dimensional materials with metal films and transfer them to FIB copper grids;

[0013] (4) In the TEM, the metal thin film on the surface of the two-dimensional material is peeled off using a metal needle tip prepared by chemical etching to expose a clean friction interface of the two-dimensional material and obtain a TEM cross-section sample of the two-dimensional material with a clean surface.

[0014] Furthermore, in step (1), in step (1), the thickness of the metal film is 20-50 nm.

[0015] Furthermore, in step (2), the pressure is 10-30 MPa, and the metal film is transferred from PDMS to the surface of the two-dimensional material.

[0016] Furthermore, in step (3), when the metal film on the surface of the two-dimensional material is prepared by FIB, the thinning parameters are:

[0017] (1) Ion beam voltage 20-30kV, ion beam current 0.8-0.6nA; upper and lower angle compensation ±0.5°-1°, processing to a thickness within 1um;

[0018] (2) Ion beam voltage 16-20 kV, ion beam current 0.2-0.4 nA; upper and lower angle compensation ±0.5°-1°, processing to a thickness within 500 nm;

[0019] (3) The ion beam voltage is 16-20 kV, and the ion beam current is about 0.2 nA; the upper and lower angle compensation is 0°, and multiple ion milling holes are punched in the direction perpendicular to the thinned sample. The hole diameter is controlled within 1 um, and the depth is controlled within the range of penetrating the metal layer without damaging the two-dimensional material;

[0020] (4) Ion beam voltage 8-10kV, ion beam current 0.1nA-80pA; upper and lower angle compensation 0.5°-1° processing to a thickness within 100nm.

[0021] Furthermore, in step (4), the metal needle tip prepared by chemical etching is a W needle tip with a diameter of less than 200 nm, and the etching solution for chemical etching is a 0.2-0.5 mol / L NaOH solution.

[0022] Furthermore, in step (4), during peeling, with the aid of an in-situ mechanoelectric coupling sample rod, the metal needle tip is moved by a piezoelectric controller to contact the metal protective layer on the surface of the two-dimensional material for in-situ peeling.

[0023] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0024] (1) The present invention can successfully prepare TEM cross-sectional samples with almost no damage to the top two-dimensional material by using a two-dimensionally transferred metal film as a FIB spray-plated protective layer. By using a metal needle tip to in-situ peel off the metal protective layer in the TEM, a two-dimensional material cross-sectional TEM sample with a clean surface can be obtained.

[0025] (2) The method of obtaining a cross-sectional TEM sample of a two-dimensional material with a clean surface by using a two-dimensional transferred metal film as a FIB sprayed protective layer in the present invention is universal for a variety of two-dimensional materials.

[0026] (3) The cross-sectional TEM samples of two-dimensional materials with clean surfaces prepared by the present invention are suitable for various in-situ TEM experiments related to surface and interface research, such as in-situ electrical property research of ohmic contact of two-dimensional material-metal interface, in-situ TEM tribology research of two-dimensional materials, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the pressure-assisted two-dimensional transfer of Au thin film as a protective layer for two-dimensional materials;

[0028] Figure 2 TEM image of a cross-section sample of MoS2 on a conductive silicon substrate with an Au protective layer prepared by FIB;

[0029] Figure 3 TEM image of a MoS2 cross-section sample with a clean surface after in-situ peeling of the Au protective layer;

[0030] Figure 4 MoS2 cross-sectional TEM samples prepared under different FIB thinning parameter designs;

[0031] Figure 5 TEM images of MoS2 cross section with and without two-dimensional physically transferred Au protection. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0033] The following will clearly and completely describe the technical solutions of various embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0034] The present invention relates to a method for preparing a two-dimensional material cross-section TEM sample with a clean surface and a two-dimensional material cross-section TEM sample with a clean surface, which comprises: sputtering a nanometer-thick Au film on PDMS by magnetron sputtering, and then transferring the Au film to the surface of the two-dimensional material using a two-dimensional transfer platform under the auxiliary action of pressure, and then playing a protective role in the process of preparing the two-dimensional material TEM cross-section sample by FIB. The prepared cross-section TEM sample is subjected to in-situ stripping in the TEM to remove the surface Au protective layer, so that a two-dimensional material TEM cross-section sample with a clean surface and a complete lattice can be obtained.

[0035] Example 1

[0036] 1. Use a scalpel to cut a 1 mm x 1 mm piece of polydimethylsiloxane (PDMS) and transfer it to the front end of the glass slide.

[0037] 2. Use magnetron sputtering (ISC150T) to sputter a layer of Au film with a thickness of about 20 nm on the surface of PDMS. The sputtering power is 20 W and the sputtering time is 180 s.

[0038] 3. Using a two-dimensional material transfer platform, the Au film on the PDMS was transferred to the surface of the polycrystalline MoS2 film on the conductive silicon substrate under pressure-assisted action, as a protective layer for FIB thinning of MoS2. Figure 1 shown.

[0039] 4. Use a focused ion beam (FIB, Helios 5CX) to spray a 10um×10um×2um Pt protective layer on the two-dimensional physically transferred Au-protected MoS2 surface.

[0040] 5. Use a focused ion beam to extract the MoS2 loaded with Au film and transfer it to the FIB grid for milling and thinning.

[0041] Since two-dimensional materials are extremely sensitive to damage, when using FIB for milling thinning, it is necessary to design the thinning parameters in a gradient based on reducing the ion beam voltage. The thinning parameters used in this example are:

[0042] (1) Ion beam voltage 30kV, ion beam current 0.79nA; upper and lower angle compensation ±0.8°, processing to a thickness of about 1um;

[0043] (2) Ion beam voltage 16 kV, ion beam current 0.2 nA; upper and lower angle compensation ± 0.8°, processing to a thickness of about 500 nm;

[0044] (3) Ion beam voltage 16 kV, ion beam current 0.2 nA; upper and lower angle compensation 0°, multiple ion milling holes are punched in the direction perpendicular to the thinned sample, the hole size is controlled within 1 um, and the depth is controlled within the range of penetrating the Au protective layer without damaging the two-dimensional material;

[0045] (4) Ion beam voltage 8 kV, ion beam current 80 pA; upper and lower angle compensation ± 0.8° processing to a thickness of about 100 nm; the obtained conductive silicon substrate MoS2 cross-section sample with Au protective layer is as follows Figure 2 shown.

[0046] 6. A W needle tip with a tip diameter of about 200 nm was prepared by chemical etching, and the etching solution was 0.2 mol / L NaOH solution.

[0047] 7. The prepared W needle tip and the cut two-dimensional material cross-section TEM sample were loaded on the moving end and the fixed end of the TEM in-situ electrical sample holder (Nanofactory STM-TEM holder) respectively, and then placed in the TEM (Tecnai G220).

[0048] 8. Move the W needle tip through the piezoelectric controller (Mains 220V) to contact the metal protective layer on the surface of the two-dimensional material at the fixed end and perform in-situ peeling to expose the clean MoS2 surface and obtain a cross-sectional TEM of the two-dimensional material with a clean surface. The TEM image of the MoS2 cross-sectional sample with a clean surface after in-situ peeling of the Au protective layer is shown in Figure 1. Figure 3 shown.

[0049] Example 2

[0050] This example is used as a control group, and the experimental process is the same as that of Example 1, except that a constant ion beam voltage of 30 kV is used for thinning. This highlights the quality of the two-dimensional material sample obtained by the FIB thinning parameter design in the present invention, and highlights the importance of the FIB thinning parameter design.

[0051] 1. Use a scalpel to cut a 1 mm x 1 mm piece of polydimethylsiloxane (PDMS) and transfer it to the front end of the glass slide.

[0052] 2. Use magnetron sputtering (ISC150T) to sputter a layer of Au film with a thickness of about 20 nm on the surface of PDMS. The sputtering power is 20 W and the sputtering time is 180 s.

[0053] 3. Use a two-dimensional material transfer platform to transfer the Au film on PDMS to the surface of the polycrystalline MoS2 film on the conductive silicon substrate under pressure-assisted action, as a protective layer for FIB thinning of MoS2.

[0054] 4. Use a focused ion beam (FIB, Helios 5CX) to spray a 10um×10um×2um Pt protective layer on the two-dimensional physically transferred Au-protected MoS2 surface.

[0055] 5. Use a focused ion beam to extract the MoS2 loaded with Au film and transfer it to the FIB grid for milling and thinning.

[0056] During FIB thinning, a constant 30kV ion beam voltage is used, and the current decreases from 0.79nA to 41pA. The specific thinning parameters are:

[0057] (1) Ion beam voltage 30kV, ion beam current 0.79nA; upper and lower angle compensation ±0.8°, processing to a thickness of about 1um;

[0058] (2) Ion beam voltage 30 kV, ion beam current 0.2 nA; upper and lower angle compensation ±0.8°, processing to a thickness of about 500 nm;

[0059] (3) Ion beam voltage 30 kV, ion beam current 0.2 nA; upper and lower angle compensation 0°, multiple ion milling holes are punched in the direction perpendicular to the thinned sample, the hole size is controlled within 1 um, and the depth is controlled within the range of penetrating the protective layer without damaging the two-dimensional material;

[0060] (4) Ion beam voltage 30 kV, ion beam current 80 pA; upper and lower angle compensation ±0.8° processing to a thickness of about 100 nm.

[0061] 6. A W needle tip with a tip diameter of about 200 nm was prepared by chemical etching, and the etching solution was 0.2 mol / L NaOH solution.

[0062] 7. Load the prepared W needle tip and the cut two-dimensional material cross-section TEM sample on the moving end and fixed end of the TEM in-situ electrical sample holder (Nanofactory STM-TEM holder), respectively, and then put it into the TEM (Tecnai G2 20).

[0063] 8. Move the W needle tip to contact the metal protective layer on the surface of the two-dimensional material at the fixed end through the piezoelectric controller (Mains 220V) and perform in-situ peeling to expose the clean MoS2 surface and obtain the cross-sectional TEM sample of the two-dimensional material as shown in Figure 4 As shown in a.

[0064] like Figure 4 As shown in Figure a, a MoS2 cross-sectional TEM sample obtained by constant 30kV ion beam voltage thinning. When the ion beam current is reduced to 0.2nA for thinning, it can be clearly found that the MoS2 sample produces obvious bulging phenomenon, which causes unevenness in the thickness direction of the sample. When the ion beam current is further reduced to 80pA for thinning in the later stage, these bulging areas will produce voids, affecting the sample quality. And after Example 1 in Figure b adopts a gradient to reduce the ion beam voltage design, when the ion beam current is reduced to 0.2nA for thinning, the sample does not produce obvious bulging phenomenon, and further thinning in the later stage does not produce voids, effectively improving the sample quality.

[0065] Example 3

[0066] This example is used as a control group, and the experimental process is the same as that of Example 1, except that two-dimensional physical transfer Au is not used as a protective layer, highlighting the importance of two-dimensional physical transfer Au as a protective layer in the present invention for obtaining cross-sectional TEM samples without damaging the surface of two-dimensional materials.

[0067] 1. Use a focused ion beam (FIB, Helios 5CX) to directly spray a 10um×10um×2um Pt protective layer on the MoS2 surface.

[0068] 2. Use a focused ion beam to extract MoS2 and transfer it to a FIB grid for milling and thinning.

[0069] The thinning parameters used in this example are the same as those in Example 1, specifically:

[0070] (1) Ion beam voltage 30kV, ion beam current 0.79nA; upper and lower angle compensation ±0.8°, processing to a thickness of about 1um;

[0071] (2) Ion beam voltage 16 kV, ion beam current 0.2 nA; upper and lower angle compensation ± 0.8°, processing to a thickness of about 500 nm;

[0072] (3) Ion beam voltage 16 kV, ion beam current 0.2 nA; upper and lower angle compensation 0°, multiple ion milling holes are punched in the direction perpendicular to the thinned sample, the hole size is controlled within 1 um, and the depth is controlled within the range that does not damage the two-dimensional material;

[0073] (4) Ion beam voltage 8kV, ion beam current 80pA; upper and lower angle compensation ±0.8° processing to a thickness of about 100nm. Get the TEM of the two-dimensional material cross section, such as Figure 5 As shown in a.

[0074] like Figure 5 As shown, Figure a is a TEM characterization image of the MoS2 cross-section sample obtained in this example. Pt is directly sprayed on the MoS2 surface as a protective layer using FIB, but the Pt spraying will also cause surface damage to sensitive two-dimensional materials, forming a damage layer about 30nm thick, and it is impossible to obtain a MoS2 surface with a clear surface lattice, affecting the surface quality of the MoS2 cross-section TEM sample. Figure b is a TEM characterization image of the MoS2 cross-section sample in Example 1 with the addition of two-dimensional physically transferred Au for protection. A clear lattice can be observed on the MoS2 surface under Au protection, and no obvious amorphous damage layer is produced. This shows that the two-dimensional physical transfer of Au as a protective layer can effectively avoid damage to the two-dimensional material during FIB spraying of Pt and milling thinning.

Claims

1. A method for preparing a TEM sample of a two-dimensional material cross section with a clean surface, characterized in that: The preparation method includes using a two-dimensional transferred metal film as a FIB sprayed protective layer, completing the preparation of a two-dimensional material cross-section TEM sample by FIB, and then using a chemically etched needle tip in the TEM to in-situ peel off the metal protective layer on the surface of the two-dimensional material under the drive of a piezoelectric controller.

2. The method for preparing a TEM sample of a two-dimensional material cross section with a clean surface according to claim 1, characterized in that: The two-dimensional material is graphene, molybdenum disulfide, boron nitride, indium selenide or MXene material.

3. The method for preparing a TEM sample of a two-dimensional material cross section with a clean surface according to claim 1, characterized in that: The metal film is an Au film, a W film, an Ag film or a Cu film.

4. The method for preparing a TEM sample of a two-dimensional material cross section with a clean surface according to claim 1, characterized in that: The following steps are involved: (1) sputtering a metal layer with a nanometer thickness on polydimethylsiloxane by magnetron sputtering; (2) Using a two-dimensional material transfer platform, the metal layer sputtered on the PDMS is transferred to the surface of the two-dimensional material under the assistance of pressure, thereby obtaining a metal film on the surface of the two-dimensional material; (3) Using FIB to prepare cross-sectional TEM samples of two-dimensional materials with metal films and transfer them to FIB copper grids; (4) In the TEM, the metal thin film on the surface of the two-dimensional material is peeled off using a metal needle tip prepared by chemical etching to expose a clean friction interface of the two-dimensional material and obtain a TEM cross-section sample of the two-dimensional material with a clean surface.

5. The method for preparing a TEM sample of a two-dimensional material cross section with a clean surface according to claim 4, characterized in that: In step (1), the thickness of the metal film is 20-50 nm.

6. The method for preparing a TEM sample of a two-dimensional material cross section with a clean surface according to claim 4, characterized in that: In step (2), the pressure is 10-30 MPa, and the metal film is transferred from PDMS to the surface of the two-dimensional material.

7. The method for preparing a TEM sample of a two-dimensional material cross section with a clean surface according to claim 4, characterized in that: In step (3), when the metal film on the surface of the two-dimensional material is prepared by FIB, the thinning parameters are: (1) Ion beam voltage 20-30 kV, ion beam current 0.8-0.6 nA; upper and lower angle compensation 0.5°-1°, processing to a thickness within 1 um; (2) Ion beam voltage 16-20 kV, ion beam current 0.2-0.4 nA; upper and lower angle compensation 0.5°-1°, processing to a thickness within 500 nm; (3) The ion beam voltage is 16-20 kV, and the ion beam current is about 0.2 nA; the upper and lower angle compensation is 0°, and multiple ion milling holes are punched in the direction perpendicular to the thinned sample. The hole diameter is controlled within 1 um, and the depth is controlled within the range of penetrating the metal layer without damaging the two-dimensional material; (4) Ion beam voltage 8-10kV, ion beam current 0.1nA-80pA; upper and lower angle compensation 0.5°-1° processing to a thickness within 100nm.

8. The method for preparing a TEM sample of a two-dimensional material cross section with a clean surface according to claim 4, characterized in that: In step (4), the metal needle tip prepared by chemical etching is a W needle tip with a diameter of less than 200 nm, and the etching solution for chemical etching is a 0.2-0.5 mol / L NaOH solution.

9. The method for preparing a TEM sample of a two-dimensional material cross section with a clean surface according to claim 4, characterized in that: In step (4), during peeling, with the aid of an in-situ mechanoelectric coupling sample rod, a piezoelectric controller is used to move the metal needle tip to contact the metal protective layer on the surface of the two-dimensional material for in-situ peeling.

10. A two-dimensional material cross-sectional TEM sample with a clean surface obtained by the preparation method according to any one of claims 1 to 9.

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