Method for efficiently obtaining cross-sectional characteristic crystal orientation of two-dimensional material

Abstract

The present application relates to the field of transmission electron microscopy characterization of two-dimensional material structure, and particularly to a method for efficiently obtaining cross-section characteristic crystal orientation of two-dimensional material, which is suitable for two-dimensional van der Waals layered material and two-dimensional non-layered material. A focused ion beam microscope (FIB) is used to sample in the direction perpendicular to (or parallel to) the long straight edge of the two-dimensional material edge, and the two-dimensional material sampling direction is parallel to the half-moon-shaped carrier net string length direction for welding, so as to prepare a cross-section sample capable of reflecting atomic layer stacking sequence; the half-moon-shaped carrier net string length direction is placed perpendicular to the TEM sample rod axial direction, and then small-angle tilting is performed to quickly realize sample crystallographic orientation. The present application lays a foundation for efficiently analyzing crystallographic information such as crystal structure, stacking defects and surface reconstruction of the material by observing the cross-section sample of the two-dimensional material.

Description

Technical fields:

[0001] This invention relates to the field of transmission electron microscopy characterization of two-dimensional material structures, specifically a method for efficiently obtaining the characteristic crystal orientation of two-dimensional material cross sections, applicable to two-dimensional van der Waals layered materials and two-dimensional non-layered materials. Background technology:

[0002] The planar observation method of transmission electron microscopy (TEM) for analyzing the structure of two-dimensional materials is only applicable to systems with few atomic layers or known material systems (such as graphene, hexagonal boron nitride, and molybdenum disulfide). For multi-atom-layer two-dimensional material systems, especially those with unknown phases (such as In₂Se₃, MoSi₂N₄, and MnBi₂Te₄), structural analysis requires the use of cross-sectional observation to determine the out-of-plane stacking order of the atomic layers. However, the crystallographic orientations that reflect the atomic layer stacking of two-dimensional materials are specific. Obtaining these characteristic orientations within a limited cross-sectional area is crucial for analyzing the structure of two-dimensional materials. Focused ion beam microscopy (FIB) is a commonly used instrument for preparing TEM cross-sectional samples. The crystallographic orientation of the cross-sectional sample in the TEM involves the cutting and sampling direction of the FIB relative to the two-dimensional material and the placement of the cross-sectional sample on the TEM sample holder. Arbitrary FIB sampling and TEM sample mounting can easily lead to a significant deviation of certain low-index crystal orientations from the electron beam direction, easily exceeding the tilt angle range of the sample holder, or requiring a long time to correct the zone axis. Summary of the Invention:

[0003] The purpose of this invention is to provide a method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section. This method utilizes a fibrillated metallographic sampler (FIB) to cut and sample the two-dimensional material perpendicular to (or parallel to) its long straight edge. The sampling direction of the two-dimensional material is then welded and thinned parallel to the chord length of a crescent-shaped mesh, thereby preparing a low-index cross-sectional sample that reflects the atomic layer stacking sequence. Simultaneously, the chord length of the crescent-shaped mesh is placed perpendicular to the axial direction of the TEM sample rod, allowing for rapid acquisition of specific crystallographic orientations of the two-dimensional material within the TEM through small-angle tilting. This provides information on the atomic arrangement and stacking sequence within and between the two-dimensional material layers, offering guidance for the study of material properties.

[0004] The technical solution of this invention is:

[0005] A method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section includes the following steps:

[0006] (1) Two-dimensional material samples are transferred onto a substrate using mechanical exfoliation or polymethyl methacrylate-assisted method;

[0007] (2) Place the substrate sample into a focused ion beam microscope, select a regular two-dimensional material sample with a long straight edge according to the morphological characteristics, and take samples perpendicular to or parallel to the long straight edge of the sample. Use a micro-nano processing robot to transfer the cut sample to a crescent-shaped carrier for a focused ion beam microscope, and weld and thin the sample with the sampling direction parallel to the chord length of the crescent-shaped carrier.

[0008] (3) The chord length of the crescent-shaped carrier mesh is perpendicular to the axis of the transmission electron microscope sample rod, that is, the out-of-plane direction of the two-dimensional material is parallel to the axis of the transmission electron microscope sample rod.

[0009] (4) The sample rod is installed in a transmission electron microscope, and the characteristic crystal orientation that reflects the stacking order and surface reconstruction information of the two-dimensional material is obtained by tilting the sample at a small angle.

[0010] The method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross section, in step (1), includes two-dimensional materials including van der Waals layered materials or non-layered materials.

[0011] In the method for efficiently obtaining the crystal orientation of cross-section features of two-dimensional materials, in step (1), the two-dimensional material is a two-dimensional material with six-fold or three-fold symmetry, or a two-dimensional material with other symmetries.

[0012] In the method for efficiently obtaining the crystal orientation of cross-section features of two-dimensional materials, in step (1), the sampling direction is accurately determined by electron diffraction or high-resolution atomic imaging of the two-dimensional material planar sample, and the sampling direction is perpendicular to or parallel to the long straight edge.

[0013] In the method for efficiently obtaining the crystal orientation of a two-dimensional material cross section, in step (1), the two-dimensional material has at least one long straight edge.

[0014] In the method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross section, in step (1), the tilt angle of the transmission electron microscope sample is -5° < α < 5° and -5° < β < 5°.

[0015] In the method for efficiently obtaining the crystal orientation of cross-section features of two-dimensional materials, in step (1), a two-dimensional material transfer platform is used to cover the surface of the unstable two-dimensional material with sheet graphite or hexagonal boron nitride.

[0016] In the method for efficiently obtaining the cross-sectional crystal orientation of two-dimensional materials, in step (1), the two-dimensional material transfer substrate is a silicon wafer, a quartz wafer, or a metal wafer.

[0017] The design principle of this invention is:

[0018] Two-dimensional materials are a class of crystalline materials. Two-dimensional materials in thermodynamic equilibrium have regular shapes, and their edges are mostly stable low-index crystal planes. For example, common edge structures in graphene are "zigzag" and "armchair" shapes, with the former being more stable than the latter; the stable edge structure of MoS2 is also "zigzag." Stable edge structures have relatively large lengths, so the long straight edges in graphene and MoS2 correspond to their stable "zigzag" edges. Therefore, FIB sampling perpendicular to the "zigzag" edges can reveal the stacking pattern (AB or ABC stacking) between layers of multilayer graphene and MoS2. However, two-dimensional materials that deviate from equilibrium during growth or have complex atomic structures have long straight "armchair" edges, thus requiring FIB sampling parallel to these edges.

[0019] The principle applies not only to two-dimensional materials with six or three-dimensional symmetries, but also to two-dimensional materials with other symmetries (such as orthorhombic black phosphorus); the principle applies not only to van der Waals layered two-dimensional materials, but also to non-layered two-dimensional materials.

[0020] Furthermore, when the TEM sample rod is tilted along the β-axis (perpendicular to the sample rod axis), the sample position shifts significantly and easily moves out of the field of view. However, when tilted along the α-axis (sample rod axis), the sample shift is smaller. Placing the two-dimensional material with its out-of-plane direction parallel to the sample rod axis minimizes the tilt angle along the β-axis. Combined with a reasonable cross-sectional sample sampling direction, this ensures that both the α and β axes are within a small tilt angle range. This allows for rapid alignment of specific zone axes, improving crystal orientation efficiency and laying the foundation for the analysis of two-dimensional material structures.

[0021] The advantages and beneficial effects of this invention are:

[0022] (1) The present invention can effectively obtain low index cross-sectional samples that reflect the atomic layer stacking order of two-dimensional materials by performing FIB cutting and sampling in a specific direction. At the same time, TEM sample loading in a specific direction can ensure that the specific low index crystal orientation is close to the electron beam direction. Then, crystal orientation can be achieved by tilting the sample at a small angle. This method can eliminate the blindness of searching for the zone axis and improve the crystal orientation efficiency of two-dimensional material cross-sectional samples in TEM.

[0023] (2) The present invention utilizes a two-dimensional material transfer platform to cover two-dimensional materials with sheet graphite or hexagonal boron nitride, which can effectively prevent damage to the sample during sputtering deposition of a metal protective layer. Attached image description:

[0024] Figure 1 : Schematic diagram of the operation process of the method of the present invention.

[0025] Figure 2Image a shows In2Se3 mechanically exfoliated onto a silicon wafer, observed under an optical microscope. Image b shows In2Se3 covered with a graphite protective layer. Image c is a schematic diagram of the FIB cutting sampling direction, where the dashed line represents the long straight edge of the sample, and the short line is the FIB sampling position perpendicular to the dashed line. Image d is a high-angle annular dark-field image (HAADF) of the In2Se3 cross-section sample.

[0026] Figure 3 HAADF image of a Bi2Te3 cross-section sample observed according to the method of the present invention.

[0027] Figure 4 HAADF image of FeIn2Se4 cross-section sample observed according to the method of the present invention.

[0028] Figure 5 HAADF image of a two-dimensional Mo2C cross-section sample observed according to the method of the present invention. Detailed implementation method:

[0029] like Figure 1 As shown, in the specific implementation process, the operation flow of the method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section according to the present invention is as follows:

[0030] Two-dimensional material samples are transferred to a silicon wafer coated with 300 nm thick SiO2 using mechanical exfoliation or a polymethyl methacrylate (PMMA)-assisted method. If necessary, a two-dimensional material transfer platform is used to cover the two-dimensional material with regular shapes (e.g., triangles, hexagons, etc., with at least one long straight edge) with sheet graphite or hexagonal boron nitride. The edge type of the two-dimensional material with regular shapes on the microgrid can be determined based on the planar sample using electron diffraction or high-resolution imaging. The silicon wafer sample is placed in a FIB (Film Embedding Infrared) chamber. Two-dimensional material samples with regular shapes and long straight edges are selected based on morphological characteristics. A metal (Pt or W) protective layer with a thickness of ~100 nm is deposited on the two-dimensional material samples with regular shapes and long straight edges. Subsequently, sampling is performed perpendicular to (or parallel to) the long straight edge of the sample. A micro / nano processing robot is used to transfer the cut sample to a crescent-shaped copper grid in the FIB and weld it. During welding, the sample sampling direction is parallel to the chord direction of the crescent-shaped copper grid. The thickness of the cross-sectional sample is then reduced to 50–100 nm. The chordal direction of the crescent-shaped copper mesh is perpendicular to the axis of the TEM sample rod. The sample rod is then inserted into the TEM, and characteristic crystal orientations that reflect information such as the stacking order and surface reconstruction of the two-dimensional material are quickly obtained by tilting the sample at a small angle (-5° < α, β < 5°, where a negative angle indicates counterclockwise rotation of the sample rod).

[0031] The TEM sample holder typically achieves biaxial tilting of the sample along two orthogonal axes: the α-axis and the β-axis. The α-axis represents the major axis of the TEM sample holder, and the β-axis represents the direction at the front end of the TEM sample holder, perpendicular to the electron beam and the major axis. α-axis tilting is achieved by rotating the TEM sample holder axially via the goniometer stage of the transmission electron microscope, while β-axis tilting requires a built-in biaxial tilting mechanism that drives the front end structure to rotate along the β-axis.

[0032] The method of the present invention will be further described in detail below with reference to embodiments and accompanying drawings. The described embodiments are only some embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0033] Example 1

[0034] like Figure 2 As shown, firstly, In₂Se₃ with a thickness of ~180 nm was transferred onto a silicon wafer using mechanical exfoliation. An optical microscope was used to locate the sample with a long straight edge, and a two-dimensional material transfer platform was used to transfer sheet graphite with a thickness of ~70 nm onto the selected sample. Using a material transfer array (FIB), samples were cut perpendicular to the long straight edge of the sample and welded to a crescent-shaped copper mesh, which was then thinned to a cross-sectional sample thickness of ~50 nm. During welding, the sample sampling direction was parallel to the chord direction of the crescent-shaped copper mesh. Subsequently, the chord direction of the crescent-shaped copper mesh was placed perpendicular to the TEM sample rod axis and the sample was mounted. Finally, in the TEM, the sample rod was rotated 3.08° along the α-axis and 1.10° along the β-axis, and the atomic structure of the In₂Se₃ cross-section sample was observed using HAADF imaging. Figure 2 ), and obtained rhombohedral phase In2Se3 from Crystal orientation observation of…ABCABC… interlayer atomic stacking information.

[0035] Example 2

[0036] First, a 2D Bi₂Te₃ sample with a thickness of ~50 nm was transferred onto a silicon wafer using mechanical exfoliation. The sample with the long straight edge was located using an optical microscope. A Pt protective layer with a thickness of ~100 nm was deposited on the Bi₂Te₃ sample using FIB (Film-Injection Brush), and the sample was cut perpendicular to the long straight edge. The removed sample was then soldered onto a crescent-shaped copper mesh and thinned to a cross-sectional sample thickness of ~50 nm, ensuring the sample sampling direction was parallel to the chord length of the crescent-shaped copper mesh during soldering. Subsequently, the chord length of the crescent-shaped copper mesh was placed perpendicular to the TEM sample rod axis and the sample was mounted. Finally, the sample rod was rotated 0.42° along the α-axis and 1.10° along the β-axis in the TEM, and the atomic structure of the Bi₂Te₃ cross-sectional sample was observed using HAADF imaging. Figure 3 ), and obtained rhombohedral phase Bi2Te3 from Crystal orientation observation of…ABCABC… interlayer stacking information.

[0037] Example 3

[0038] First, a 30 nm thick two-dimensional FeIn₂Se₄ sample was transferred onto a silicon wafer using mechanical exfoliation. The sample with the long straight edge was located using an optical microscope. Using fibrillation-intercalation (FIB), an amorphous carbon layer of approximately 50 nm thick and a Pt protective layer of approximately 100 nm thick were sequentially deposited on the FeIn₂Se₄ sample. Then, the sample was cut perpendicular to the long straight edge and sampled. The removed sample was then welded to a crescent-shaped copper mesh and thinned to a cross-sectional sample thickness of approximately 50 nm. During welding, the sample sampling direction was parallel to the chord direction of the crescent-shaped copper mesh. The crescent-shaped copper mesh was then positioned perpendicular to the axis of the TEM sample holder and mounted. Finally, in the TEM, the sample holder was rotated -2.23° along the α-axis and -3.35° along the β-axis, and the atomic structure of the FeIn₂Se₄ cross-sectional sample was observed using HAADF imaging. Figure 4 (), obtained rhombohedral phase FeIn2Se4 from Crystal orientation observation of…ABCABC… interlayer stacking information.

[0039] Example 4

[0040] A CVD-grown, ~10 nm thick two-dimensional non-layered Mo2C sample was transferred onto a silicon wafer using PMMA-assisted transfer. The sample with the long straight edge was located using an optical microscope. A ~100 nm thick Pt protective layer was sputtered onto the Mo2C sample using FIB. Subsequently, the sample was cut and sampled perpendicular to the long straight edge using FIB. The removed sample was then soldered onto a crescent-shaped copper mesh and thinned to a cross-sectional sample thickness of ~50 nm. During soldering, the sample sampling direction was parallel to the chord direction of the crescent-shaped copper mesh. The chord direction of the crescent-shaped copper mesh was then aligned perpendicular to the TEM sample rod axis and the sample was mounted. Finally, the sample rod was rotated 0.76° along the α-axis and -1.59° along the β-axis in the TEM, and the atomic structure of the Mo2C cross-section sample was observed using HAADF imaging. Figure 5 The stacking sequence of Mo atomic layers …ABAB… and the stacking sequence …ABCABC…, stacking faults and surface reconstruction information of orthorhombic non-van der Waals layered Mo2C observed from the

[010] crystal orientation were obtained.

[0041] The results show that this invention utilizes focused ion beam microscopy (FIB) to sample along the long straight edge of a two-dimensional material, thereby preparing a cross-sectional sample reflecting the atomic layer stacking order. The out-of-plane direction of the two-dimensional material on the cross-sectional sample is placed parallel to the axis of the transmission electron microscope (TEM) sample rod, and then a small-angle tilt is performed to quickly achieve crystallographic orientation of the sample. This invention lays the foundation for efficiently resolving crystallographic information such as the crystal structure, stacking defects, and surface reconstruction of materials by observing cross-sectional samples of two-dimensional materials.

Claims

1. A method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section, characterized in that, Includes the following steps: (1) Two-dimensional material samples are transferred to the substrate by mechanical peeling or polymethyl methacrylate-assisted method; the tilt angle of the transmission electron microscope sample is -5° < α < 5° and -5° < β < 5°. When the TEM sample rod is tilted perpendicular to the β axis, the sample position will be greatly shifted and it is easy to move out of the field of view. However, when the sample rod is tilted to the α axis, the sample shift is smaller. Placing the two-dimensional material out-of-plane direction parallel to the axial direction of the sample rod can minimize the tilt angle of the β axis. Combined with a reasonable cross-sectional sample sampling direction, it is ensured that both the α and β axes are within the small tilt angle range, and the specific zone axis is quickly corrected. (2) The substrate sample is placed in a focused ion beam microscope. According to the morphological characteristics, a regular two-dimensional material sample with a long straight edge is selected. The sample is taken perpendicular to or parallel to the long straight edge of the sample. The cut sample is transferred to a crescent-shaped support for the focused ion beam microscope using a micro-nano processing robot. The sample sampling direction is parallel to the chord length direction of the crescent-shaped support for welding and thinning, thereby preparing a low-index cross-sectional sample that can reflect the atomic layer stacking sequence. At the same time, the chord length direction of the crescent-shaped support is placed perpendicular to the TEM sample rod axis so that the specific crystallographic orientation of the two-dimensional material can be quickly obtained in the TEM by small-angle tilting, and the information on the atomic arrangement and stacking sequence of the two-dimensional material layer is obtained. (3) The chord length of the crescent-shaped carrier mesh is perpendicular to the axis of the transmission electron microscope sample rod, that is, the out-of-plane direction of the two-dimensional material is parallel to the axis of the transmission electron microscope sample rod. (4) The sample rod is installed in a transmission electron microscope, and the characteristic crystal orientation that reflects the stacking order and surface reconstruction information of the two-dimensional material is obtained by tilting the sample at a small angle.

2. The method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section according to claim 1, characterized in that, In step (1), the two-dimensional material includes van der Waals layered materials or non-layered materials.

3. The method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section according to claim 1, characterized in that, In step (1), the two-dimensional material is a two-dimensional material with six-fold or three-fold symmetry, or a two-dimensional material with other symmetries.

4. The method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section according to claim 1, characterized in that, In step (1), the sampling direction is accurately determined by electron diffraction or high-resolution atomic imaging of the two-dimensional material planar sample, and the sampling direction is perpendicular to or parallel to the long straight edge.

5. The method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section according to claim 1, characterized in that, In step (1), the two-dimensional material has at least one long straight edge.

6. The method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section according to claim 1, characterized in that, In step (1), a two-dimensional material transfer platform is used to cover the surface of an unstable two-dimensional material with sheet graphite or hexagonal boron nitride.

7. The method for efficiently obtaining the characteristic crystal orientation of a two-dimensional material cross-section according to claim 1, characterized in that, In step (1), the two-dimensional material transfer substrate is a silicon wafer, a quartz wafer, or a metal wafer.

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