A method of preparing a diamond moire lattice

Abstract

The present application relates to the field of advanced material preparation technology, in particular to a method for preparing diamond moire lattice. The present application provides a method for preparing diamond moire lattice, non-diamond carbon forms linear moire fringe and hexagonal moire lattice under high temperature and high pressure. Unlike the moire superlattice generated by stacking two layers of 2D material at a twist angle, the diamond system moire lattice structure of the present application is an intermediate state in the process of synthesizing nanocrystalline diamond at high temperature and high pressure, and the twist angle area between the upper and lower crystal surfaces is generated by the huge internal stress to form the moire lattice structure. In addition, the present application studies the energy band structure of the above-mentioned hexagonal moire lattice by first principle calculation method, and finds that the diamond moire lattice has a significant trend to form a flat band, which is of great significance for constructing and studying three-dimensional moire lattice system in diamond.

Description

Technical Field

[0001] This invention relates to the field of advanced materials preparation technology, and specifically to a method for preparing diamond moiré lattices. Background Technology

[0002] The effective manipulation of crystal band structures and electronic states through the construction of artificial periodic structures has long been an important direction in condensed matter physics research, dating back to work on one-dimensional superlattices for semiconductors. A new research direction in condensed matter physics and materials science is two-dimensional moiré materials. Moiré superlattices (MSLs) are special cases of van der Waals stacked two-dimensional (2D) layered materials, generated by stacking two layers of 2D materials under certain lattice mismatch or twist angles. These MSL structures possess newly emerging global symmetry and periodicity. Due to periodic potential modulation, interlayer coupling, and the presence of strain, they can lead to novel physical properties different from those of the constituent 2D materials (e.g., the superconductivity of twisted graphene). In 2018, researchers from MIT reported in Nature 556 80 the discovery of superconductivity and related insulating states in the flat bands of “magic-angle” twisted bilayer graphene (t-BLG), prompting the exploration of two-dimensional moiré materials such as twisted bilayer graphene, monolayer graphene on bilayer graphene, transition metal dichalcogenide (TMD) homo-bilayer and hetero-bilayer.

[0003] Furthermore, in solid materials, the band structure determines the most fundamental electrical properties. If a material has a flat band structure, meaning that a large number of quantum states have similar kinetic energies, the system will exhibit a high density of electronic states. Therefore, flat bands become an excellent platform for realizing many novel quantum states. Currently, for two-dimensional materials, various methods have been developed to realize flat bands, such as applying external magnetic fields, constructing strain structures, and introducing rotation. The above research concerns two-dimensional moiré materials and their flat bands, but there is currently little research on three-dimensional moiré materials and their flat bands, requiring further development. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the purpose of this invention is to provide a method for preparing diamond moiré lattices. Non-diamond carbon forms linear moiré fringes and hexagonal moiré lattices under high temperature and high pressure conditions. The band structure of the hexagonal moiré lattice is studied by first-principles calculations, and it is found that diamond moiré lattices have a significant tendency to form flat bands. This is of great significance for constructing and studying three-dimensional moiré lattice systems in diamond.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] This invention provides a method for preparing diamond moiré lattices, wherein non-diamond carbon is subjected to a high temperature of 2400-2600K and a high pressure of 6-6.5GP for 100-500 hours to form linear moiré stripes and hexagonal moiré lattices.

[0007] Preferably, the non-diamond carbon is high-purity graphite.

[0008] More preferably, the purity of the non-diamond carbon is 99.999%.

[0009] Non-diamond carbon transforms into nanocrystalline diamond under high temperature and pressure (2400-2600 K). The three-dimensional diamond system with a moiré lattice structure is an intermediate state in the synthesis of nanocrystalline diamond, at which point the carbon atoms simultaneously possess sp orbitals. 2 Hybrid orbitals and sp 3 The hybrid orbitals indicate that this structure should be located at the graphite end of the graphite-diamond phase transition critical point. This region with a twisted angle between the upper and lower crystal planes is formed under harsh conditions of high temperature and pressure. Under immense internal stress, the carbon precursor phase transforms into the diamond phase. During the formation of a regular and uniform diamond bulk, due to the uneven distribution of temperature and stress, some diamond phases... <110> With the crystal orientation as the axis and the {110} plane as the interface, the structure is twisted to form a moiré lattice structure observed under a transmission electron microscope.

[0010] Compared with the prior art, the beneficial effects of the present invention are:

[0011] This invention provides a method for preparing diamond moiré lattices, in which non-diamond carbon forms linear moiré fringes and hexagonal moiré lattices under high temperature and high pressure conditions. The interplanar spacing of atoms in the linear moiré fringes (approximately...) is measured... ) and the spacing of the moiré fringes The calculated angles between the two sets of crystal planes in the linear moiré fringes are 6.8°, 7.4°, 8.7°, 10.2°, 11.9°, and 14.4°. By measuring the angle between the corresponding reciprocal vectors of the upper and lower crystal planes in the FFT image, the angles between the upper and lower crystal planes of the hexagonal moiré lattice are 8.98° and 11.54°. Then, this invention studies the band structure of the hexagonal moiré lattice using first-principles calculations. It was found that the band gap of the diamond moiré lattice is sharply reduced compared to the original diamond structure, even disappearing completely (0 eV). Furthermore, the band structure of the moiré lattice shows a clear trend towards flat bands near the Fermi level, and this trend strengthens as the rotation angle decreases from 11.54° to 8.98°. This invention provides a new approach for studying three-dimensional moiré lattice systems of diamond and offers a potential direction for constructing moiré lattices in three-dimensional materials to form flat bands. Attached Figure Description

[0012] Figure 1 These are typical microstructures of nanocrystalline diamond composite materials, where (ab) are bright-field images, the inset in a is an optical photograph of the nanocrystalline diamond sample used in this work, the inset in b is the selected area electron diffraction pattern of the sample; c shows various microstructures of diamond grains; d is a Wiener-filtered image of diamond polymorphs; e is a Wiener-filtered image of moiré fringes; f is a Wiener-filtered image of moiré fringes.

[0013] Figure 2 These are moiré fringe images, where a, c, e, and g are transmission electron microscope images of nanodiamond grains, and b, d, f, and h are Wiener filtered images of the corresponding nanodiamond grains within the line boxes.

[0014] Figure 3 These are moiré lattice images; where a and c are transmission electron microscope images of nanodiamond grains, and b and d are high-magnification electron microscope images of the corresponding nanodiamond grains within the line boxes.

[0015] Figure 4 It involves calculations based on the Mohr lattice model and electronic band structure, where a and c are based on... Figure 3 The orthogonal lattice models are constructed from the moiré lattice images in b and 3d; b and d are the band structure calculation results of the corresponding moiré lattice models. Detailed Implementation

[0016] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0017] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, and the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0018] Example 1: Preparation and Study of Diamond Moiré Lattice

[0019] 1. Preparation of nanocrystalline diamond composite materials

[0020] The non-diamond carbon used in the examples is high-purity graphite with a purity of 99.999%. The high-purity graphite is subjected to a high temperature of 2500K and a high pressure of 6GP for 300h to form a nano-polycrystalline diamond composite material. The nano-polycrystalline diamond composite material contains linear moiré stripes and a hexagonal moiré lattice similar to that of twisted bilayer graphene.

[0021] 2. Research on Nanocrystalline Diamond

[0022] Nanocrystalline diamond was analyzed using a transmission electron microscope (FEI Titan themis) with an accelerating voltage of 200 kV and an image resolution of 0.2 nm. The sample was thinned to a thickness of 60 nm using a FEI Helios UX before testing.

[0023] Figure 1 The typical microstructure of the nanocrystalline diamond composite material is shown, in which... Figure 1 a and b show tightly bound nanoparticles composed of numerous nanotwins. Figure 1 The optical photograph of the nanocrystalline diamond in the upper right corner shows the sample as a yellow, transparent block. Figure 1 b. The selected area electron diffraction pattern in the upper right corner shows obvious diffraction rings, proving that the sample is composed of nanocrystalline diamond; high-resolution image. Figure 1 c shows details of the nanoparticles, including nanotwins (TB), stacking faults (SF), diamond polytypes, moiré fringe, and moiré lattice; Figure 1 d shows the relationship with Figure 1 Wiener-filtered images of diamond polymorphs corresponding to the centerline box in c are used to identify diamond polymorph regions with a length of approximately 7 nm and a width of approximately 5 nm, in which a large number of non-3C polymorphs coexist with 3C diamonds. Figure 1 e and f are Wiener-filtered images of the moiré fringes corresponding to the line boxes, respectively. The two lines in the figures represent the crystal orientations corresponding to the first and second diffraction spots, respectively, forming the moiré fringes. Figure 1 The included angles between the two sets of crystal planes in the moiré fringes of e and f are 14.4° and 10.2°, respectively. Figure 1 The insets for e and f are Fast Fourier Transform (FFT) images that confirm the symmetry of the 6H and 10H diamond polymorphs. The superimposed atomic packing model of the 6H and 10H diamond polymorphs perfectly matches the observations from the high-resolution images. The 6H and 10H diamond polymorphs form a coherent interface with the adjacent 3C region, which is a characteristic of all polymorphs in composite materials.

[0024] 3. Study of Moiré fringes

[0025] Moiré fringes are produced by the interference of two sets of planes. In transmission electron microscopy, moiré fringes correspond to the interference between a pair of beams g1 and g2. If g1 is produced in the upper crystal and g2 is produced in the lower crystal, then each primary diffraction beam g1 in crystal 1 serves as the incident beam in the lower crystal, producing a pattern in crystal 2 around each primary diffraction spot of g1. This process is called secondary diffraction. Figure 1e and f show the results of the study on moiré fringes. Figure 2 a, c, e, and g are transmission electron microscope images of different regions of nanodiamond grains, while Figure 2 b, d, f, and h are Wiener-filtered images of the corresponding nanodiamond grains within the line boxes. The Fast Fourier Transform (FFT) image in the upper right corner indicates that the direction perpendicular to the paper is the diamond direction. <110> Crystal orientation. Moiré fringes are two sets of {111} planes along... <110> The two sets of {111} planes are formed by rotating the directions of each other, with the axis of rotation parallel to the {111} plane. The angle between the two sets of {111} planes is defined as θ. Figure 2 The lines in the graph form moiré fringes, and the angle between the lines is the angle θ between the crystal planes, which can be directly measured. Figure 2 The interplanar angles θ between crystal planes b, d, f, and h are 8.7°, 11.9°, 7.4°, and 6.8°, respectively. Besides directly measuring the angles, the interplanar spacing d and the moiré fringe spacing d' can also be used as indicators. m The angle θ between the crystal planes was calculated, and the specific data are shown in Table 1:

[0026] Table 1 Figure 1 and Figure 2 Calculated values ​​of the included angles of the moiré fringes

[0027]

[0028] The interference between the two sets of crystal planes forms moiré fringes, indicating that there are two layers of crystal planes exhibiting a certain degree of twist in the nanocrystalline diamond sample. Since the nanocrystalline diamond sample of this invention is synthesized under high temperature and high pressure (2400-2600 K), there are a considerable number of regions where the two layers of crystal planes exhibit twist (e.g., Figure 2 (As shown). The following section will introduce the moiré lattice formed by rotating two sets of crystal faces about an axis perpendicular to the crystal faces in diamond.

[0029] 4. Research on Moiré lattices

[0030] Besides the banded moiré patterns, other features also exist in the study of nanocrystalline diamond. Figure 1 Another type of moiré lattice was also observed. For example... Figure 3 As shown in Figure a, a moiré lattice region approximately 10 nm long and 10 nm wide is marked in the box. This moiré lattice is very similar to the real-space image of the moiré superlattice reported in Nano Lett. 21, 2832-2839 (2021). Figure 3 b is Figure 3 Wiener-filtered image of the Mohr lattice region in region a, from which the hexagonal pattern can be clearly distinguished, and... Figure 4 The pattern of the model constructed in a matches. Figure 3The fast Fourier transform image in the upper right corner clearly shows diffraction spots resembling twisted graphene, with the diffraction spots of the upper and lower crystal planes marked. The spots near the primary diffraction spots are moiré satellite spots. The moiré lattice structure is obtained by using diamond... <110> The crystal orientation is the axis, formed by the mutual twisting of two layers of {110} planes, with the twisted crystal planes perpendicular to the twist axis. The twist angle φ between the two layers of crystal planes is approximately 8.98°. Figure 3 As shown in Figure c, a moiré lattice region approximately 7 nm long and 7 nm wide is marked in the box. Figure 3 d is Figure 3 The Wiener-filtered image corresponding to the Mohr lattice region in c shows a hexagonal pattern that can be clearly distinguished from the image. Figure 4 The pattern of the model constructed in c matches. Figure 3 The fast Fourier transform image in the upper right corner clearly shows diffraction spots similar to twisted graphene, with diffraction spots of the upper and lower crystal planes marked. The spots near the first diffraction spot are moiré satellite spots, and the twist angle φ of the upper and lower crystal planes is about 11.54°.

[0031] 5. Study of the electronic structure of moiré lattices

[0032] (1) Model establishment

[0033] This study uses the Grain boundary model generation function in the Python Materials Genomics (pymatgen) materials analysis library to generate twisted grain boundaries, thus obtaining a twisted diamond model. The rotation axis is along the diamond

[110] direction, and the rotation plane is the (110) plane. The cutoff Σ value is set to 200 to obtain the rotation angle that satisfies the periodic boundary condition. Based on the obtained rotation angle, the corresponding twisted diamond model is generated, and the unit cell structure is generated using pymatgen and VASPKIT. Each unit cell contains approximately 400 to 1300 atoms.

[0034] (2) First Principles Calculation

[0035] This study uses CP2K software for first-principles calculations. First, the structure of the rotating diamond model at various angles is optimized. The plane wave cutoff energy is 300 Ry, and the plane wave cutoff energy of the reference mesh with a Gaussian cover of unit standard deviation is 60 Ry. Static self-consistent calculations utilize a self-consistent iterative process to handle molecular orbitals, with a convergence target accuracy of 10. -5 The LBFGS method is used in structural optimization, where the convergence criteria for maximum geometric change, maximum force component, root mean square geometric change, and root mean square force are all 10. -3After structural optimization, the density of states and band structure were calculated using CP2K. Static self-consistent calculations used the standard diagonalization method, the SMEAR method with the Fermi-Dirac distribution, an electron temperature of 300K, and the Broyden Mixing method. Band structure calculations were performed with similar parameter settings to the density of states calculations, but using k-point calculations with a 2×2×2 density. Band structure calculations were performed using the K-path provided by VASPKIT, SeeK-path, and Materials Studio.

[0036] (3) Calculation of Moiré lattice structure and electronic structure

[0037] Based on the above model establishment method and first-principles calculations, the... Figure 3 By analyzing the moiré lattice images in b and d, we obtained the orthorhombic lattice model of the moiré lattice and the calculated electronic structure results. Figure 4 a is based on Figure 3 The orthogonal lattice model constructed from the moiré lattice image in b has a lattice coordinate system in the lower right corner, where the a-axis is perpendicular to the plane of the paper; the front view is the view observed along the a-axis, and the right and bottom views are the views observed along the b-axis and c-axis of the lattice, respectively. The relative twist angle between the upper and lower crystal planes is 8.98°. Figure 4 b is based on Figure 3 The orthogonal lattice model constructed from the moiré lattice image in d has a lattice coordinate system in the lower right corner, where the a-axis is perpendicular to the plane of the paper; the front view is the view observed along the a-axis direction, and the right and bottom views are the views observed along the c-axis and b-axis directions of the lattice, respectively. The relative twist angle between the upper and lower crystal planes is 11.54°.

[0038] Figure 4 b and d represent the calculated band structure and density of states for the corresponding moiré lattice model. The figures clearly show a flat band structure; among them, the moiré lattice with a twist angle of 11.54° (such as...) Figure 4 As shown in d), band 1 is already quite flat, showing a clear trend towards flat banding, but band 2 above it still exhibits significant dispersion; when the twist angle decreases to 8.98° (as shown in d), Figure 4 (As shown in b) Band 2 exhibits relatively low dispersion, while both bands 1 and 2 are close to flat bands. This result suggests that further reducing the twist angle of the moiré lattice could potentially achieve flat bands similar to those formed by the moiré superlattice in twisted bilayer graphene. Figure 4As shown in b and d, the band structure within 0.2 eV around the Fermi level (0 eV) is flatter than that further away from the Fermi level, and the smaller the twist angle, the flatter the band. The band gap disappearance of the moiré lattice can be observed in the band structure calculations, which can be attributed to the synthesis conditions of the sample. The moiré lattice structure of the diamond system reported in this work is an intermediate state in the synthesis of nanocrystalline polycrystalline diamond under high temperature and high pressure (2400-2600 K). This region with a twist angle between the upper and lower crystal planes is generated under harsh conditions of high temperature and high pressure. Under the action of huge internal stress, the carbon precursor phase transforms into the diamond phase. During the formation of a regular and uniform diamond bulk, due to the uneven distribution of temperature and stress, some diamond phases transform into... <110> With the crystal orientation as the axis and the {110} plane as the interface, the atoms are twisted to form a moiré lattice structure observed under a transmission electron microscope. This structure serves as a transitional structure between graphite and diamond, where the carbon atoms simultaneously possess sp orbitals. 2 Hybrid orbitals and sp 3 The characteristics of the hybrid orbitals indicate that the structure should be located at the graphite end of the graphite-diamond phase transition critical point.

[0039] In summary, this invention observes linear moiré fringes and a hexagonal moiré lattice similar to that of twisted bilayer graphene in the nanocrystalline diamond prepared under high temperature and high pressure conditions. Through first-principles calculations, the band structure of the hexagonal moiré lattice was studied, revealing a significant tendency for the diamond moiré lattice to form flat bands. Compared to single-crystal diamond, the moiré diamond structure exhibits a narrower band gap and flatter band structure. This significant tendency towards flat bands contributes to increased electron localization, potentially enabling novel strongly correlated quantum states such as quantum Hall ferromagnetic states, fractional quantum Hall effects, quantum anomalous Hall effects, superconducting states, and Wigner crystals. The nanocrystalline diamond used in this invention is formed by the direct conversion of non-diamond carbon under high temperature and high pressure at 2400-2600 K. The Knoop hardness of this sample is 120-140 GPa, higher than the Knoop hardness of single-crystal diamond on the (100) and (001) planes (approximately 115 GPa). The mechanical properties of the nanocrystalline diamond prepared using this invention are significantly improved compared to those of traditional single-crystal diamond.

[0040] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for preparing a diamond moiré lattice, characterized in that, Non-diamond carbon is placed at a high temperature of 2400-2600 K and a high pressure of 6-6.5 GP for 100-500 h to directly form linear moiré fringes and hexagonal moiré lattices.

2. The method for preparing a diamond moiré lattice according to claim 1, characterized in that, The non-diamond carbon is high-purity graphite.

3. The method for preparing a diamond moiré lattice according to claim 2, characterized in that, The purity of the non-diamond carbon is 99.999%.

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