CN115231610A

CN115231610A - Two-dimensional nanosheet and preparation method thereof

Info

Publication number: CN115231610A
Application number: CN202210934672.4A
Authority: CN
Inventors: 耿凤霞; 嵇锦鹏; 蒋坤
Original assignee: Suzhou University
Current assignee: Suzhou University
Priority date: 2022-08-04
Filing date: 2022-08-04
Publication date: 2022-10-25
Anticipated expiration: 2042-08-04
Also published as: CN115231610B

Abstract

The invention discloses a preparation method of a two-dimensional nanosheet, which comprises the steps of carrying out calendaring treatment on non-van der Waals layered crystal particles, and then carrying out mechanical stripping to obtain the two-dimensional nanosheet. Mechanical peeling, commonly referred to as scotch tape, is possible with tape (sometimes assisted by an interposer) only when the inter-layer interaction of the bulk material is dominated by weak van der waals forces; however, many functional materials with a layered stacked crystal structure have significant electron density overlap between layers, constitute a non-van der waals structure, and cannot be peeled off directly with an adhesive tape. The invention successfully obtains few-layer or even single-layer structures of various non-van der Waals layered crystal materials for the first time by using the rolling pretreatment and combining mechanical stripping, and a novel physical phenomenon can be observed in the stripped two-dimensional crystal.

Description

Two-dimensional nanosheet and preparation method thereof

Technical Field

The invention belongs to the nanotechnology, and particularly relates to a two-dimensional nanosheet and a preparation method thereof.

Background

Two-dimensional materials having a thickness of a few atomic layers or even a single atomic layer have attracted considerable interest in recent years. Limiting the thickness to sub-nanometer scale imparts a number of new and different dimensional-related physical properties and applications to the material (Novoseov, K.S.; geim, A.K.; morozov, S.V.; jiang, D.; zhang, Y.; dubonos, S.V.; grigiova, I.V.; firsov, A.A. Electric Field Effect in atomic Thin Carbon Films.Science 2004, 306, 666−669. Tan, C.; Cao, X.; Wu, X.; He, Q.; Yang, J.; Zhang, X.; Chen, J.; Zhao, W.; Han, S.; Nam, G.-H.; Sindoro, M.; Zhang, H. Recent Advances in Ultrathin Two-Dimensional Nanomaterials. Chem. Rev.2017, 117, 6225−6331.Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutierrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; Johnston-Halperin, E.; Kuno, m.; Plashnitsa, V. V.; Robinson, R. D.; Ruoff, R. S.; Salahuddin, S.; Shan, J.; Shi, L.; Spencer, M. G.; Terrones, M.; et al. Progress, Challenges, and Opportunities in Two Dimensional Materials beyond Graphene. ACS Nano2013, 7, 2898−2926.Zhou, J.; Lin, J.; Huang, X.; Zhou, Y.; Chen, Y.; Xia, J.; Wang, H.; Xie, Y.; Yu, H.; Lei, J.; Wu, D.; Liu, F.; Fu, Q.; Zeng, Q.; Hsu, C.-H.; Yang, C.; Lu, L.; Yu, T.; Shen, Z.; Lin, H.; Yakobson, B.; Liu, Q.; Suenaga, K.; Liu, G.; Liu, Z. A Library of Atomically Thin Metal Chalcogenides. Nature 2018, 556, 355-361.). Mechanical stripping, also commonly referred to as scotch tape processes, is considered the best way to obtain high quality two-dimensional materials and retain their intrinsic structure and properties because no chemical reactions are involved in the process. Introduction of auxiliary intermediaries (e.g., au, al) ₂ O ₃ Etc.) can be used to enhance the adhesion of the substrate to the target crystal and increase the contact area, thereby peeling off the large-sized nanosheets. But do notThe method needs to select different medium materials according to different stripping objects, especially needs to remove the medium materials through a complicated method after stripping, and the intrinsic properties of the two-dimensional material can be greatly limited by the introduced impurities. Furthermore, mechanical stripping with tape (sometimes with the aid of an interposer) is only possible when the interlayer interactions in the bulk material are dominated by weak van der Waals (vdW) forces (Deng, Y.; yu, Y.; song, Y.; zhang, J.; wang, N.; sun, Z.; yi, Y.; wu, S.; zhu, J.; wang, J.; chen X.; zhang, Y, gate-Tunable Ron-Temperature ferromagnetic in Two-Dimensional Fe.; and the like ₃ GeTe ₂ . Nature 2018, 563, 94-99). After the first successful mechanical exfoliation of graphene, single-layer structures of layered materials such as hexagonal boron nitride (h-BN), transition Metal Dihalides (TMDs), metal Organic Frameworks (MOFs), black Phosphorus (BP), etc. have also been reported in succession. It is noteworthy that there are many functional materials with a layered stacked crystal structure, but there is significant overlap in electron density between layers. For example, some metal oxides may be viewed as a stack of rigid layers, where each layer consists of metal-oxygen polyhedrons connected by edges or corners and extending in a two-dimensional manner. Adjacent layers in these structures typically have metallic or electrostatic attractive forces between them, called non-van der waals structures (non-vdW), whose interlayer interactions are significantly higher than van der waals layered structures. These materials cannot be mechanically peeled directly into a single or few layers due to the strong electronic coupling between adjacent layers. The mechanical exfoliation of thin layers of such non-van der waals materials is interesting and important from the point of view of structure-property relationships and potential applications of new two-dimensional analogs. Therefore, it is necessary to develop a method for mechanically peeling off non-van der waals layered structures.

Disclosure of Invention

The process of the invention comprises a simple calendering pretreatment followed by mechanical peeling using scotch tape to obtain a thin layer; successfully strip a variety of materials including metals (Bi, sb), semiconducting metal oxides and chalcogenides (SnO, V) ₂ O ₅ 、Bi ₂ O ₂ Se) and superconducting compound (KV) ₃ Sb ₅ ). Electron density theory calculations verify that there is strong electron coupling between the structural layers and that the exfoliation energy is typically several times higher than that of graphite, which naturally makes conventional exfoliation of these materials difficult or even impossible.

The invention adopts the following technical scheme:

a preparation method of a two-dimensional nanosheet comprises the steps of calendaring crystal particles, and then mechanically stripping the crystal particles to obtain the two-dimensional nanosheet; preferably, the crystal particles are flatly laid and then subjected to calendaring treatment, and then the two-dimensional nanosheet is obtained through mechanical stripping.

In the present invention, the crystal particles are non-van der Waals layered structure crystal particles such as metal particles, semiconductor metal oxide particles, chalcogenide particles, superconducting compound particles, and the like, and the crystal particles are, by way of example, metals (Bi, sb), semiconductor metal oxides, and chalcogenides (SnO, V) ₂ O ₅ 、Bi ₂ O ₂ Se) and superconducting compound (KV) ₃ Sb ₅ ) Crystal particles; preferably, the particle size of the crystal particles is in the order of micrometers to millimeters, such as 1 μm to 5mm, preferably 1 to 500 μm, and further, 10 to 200 μm.

In the present invention, the rolling treatment is carried out by using a roll or bar, and preferably, the rolling load is 0.5 to 10Kg and the speed is 10 to 300mm/min. For the crystals with van der waals structures, such as graphite, two-dimensional nanosheets can be obtained through conventional mechanical stripping, but for the crystal particles with non-van der waals structures and strong interlayer interaction, the two-dimensional nanosheets cannot be obtained through conventional mechanical stripping, the invention creatively provides that the two-dimensional nanosheets are obtained through conventional mechanical stripping after calendering, and the thickness of the two-dimensional nanosheets is 0.1-50 nm, particularly 0.1-30 nm, particularly 0.3-10 nm, and more importantly 0.5-5 nm.

According to the method, the mechanical stripping is adhesive tape stripping, the adhesive tape is used for adhering particles, the particles are folded and pressed and then torn, the thin-layer material can be adhered to the adhesive tape, the method is a conventional technology, the graphite stripping is carried out by the prior art to obtain graphene, but the non-van der Waals layered structure crystals can not be thinned or nanoscale flakes can not be obtained by directly adopting the adhesive tape stripping, and single-layer or few-layer two-dimensional nanosheets can not be obtained.

In the present invention, the rolling is a unidirectional rolling, which is conventionally understood, for example, a roller presses the crystal particles to travel in a unidirectional direction, not to travel back against the crystal particles.

As an example, crystal particles are flatly laid on a bottom plate of an electric rolling roller device, and then are rolled in a single wheel in a single direction to obtain rolled particles; and then sticking the calendered particles by using a transparent adhesive tape, and then stripping to obtain a thin layer, namely the two-dimensional nanosheet of the product of the invention, so that the two-dimensional nanosheet assembly material, such as a superconducting material, an optical material, an electrode material, a heat conduction material, a conductive material and the like, can be prepared.

The invention successfully obtains few layers or even single-layer structures of several materials for the first time by using calendering pretreatment and then stripping transparent adhesive tapes, and can observe exciting new phenomena in stripped two-dimensional materials. The metallic antimony becomes a semiconductor with a band gap of 2.01 eV; the light absorption range of the semiconductor SnO can be adjusted from an Infrared Region (IR) to an ultraviolet region, and the band gap is adjusted from 0.60 eV of a bulk body to 3.65 eV of a single layer. Further, KV ₃ Sb ₅ Thin sheets are a very promising two-dimensional superconducting material. The novel results of the present invention thus provide a general approach to mechanical cleavage of non-van der waals layered materials and provide a variety of novel 2D materials.

Drawings

FIG. 1 (a) is a graph showing the peeling energy (meV A) for the non-Van der Waals layered material investigated in the present invention ^-2 ) (ii) a The values for graphite are also listed for reference. The exfoliation energy of non-van der Waals layered structures is typically 1.3-3.6 times that of graphite (which is in the vdW force range). According to the reference (Nano lett. 2018, 182759-2765), the peeling energy is calculated as the difference in ground state energy between the bulk material (per atomic layer) and the single bias layer. (b) The calculated contour plot of the charge density difference of the representative structure of the material of the invention shows strong interlayer coupling; the red and blue isosurfaces represent electron accumulation and depletion, respectively. (c) These materials are shown as single or few layers for the corresponding AFM images.

Fig. 2 is a theoretical stability prediction for exfoliated monolayer SnO. (a) side and top views of SnO. (b) A calculated contour map of charge density difference of the SnO crystal structure shows that the interlayer electron density is reduced and the interlayer interaction is attenuated; the red and blue regions represent electron accumulation and depletion, respectively. (c) harmonic phonon dispersion spectrum of single-layer SnO. (d) The temperature (upper left corner) and total energy (lower left corner) changes with trajectory time, obtained from molecular dynamics simulations of monolayer SnO at 300 and 600K; right panel: corresponding snapshot at the end of the molecular dynamics simulation. (e) O on SnO layer was calculated using a density functional calculated climbing elastic band (CI-NEB) method ₂ The minimum energy path for dissociation. Illustration is shown: representative CI-NEB configurations along the pathway include an initial state for stable adsorption of molecular oxygen, a transition state, and a final state for free atomic oxygen. Calculated O ₂ The dissociation barrier was 0.58 eV, indicating that SnO flakes were oriented to O under normal conditions ₂ The attack has good stability.

Fig. 3 is a representation of the crystal structure before and after calendering. An XRD pattern of SnO crystal and a Raman spectrum. The SnO crystals after the rolling treatment are expressed as M-SnO. SEM images of SnO crystals (c) before the rolling treatment and (d) after the rolling treatment. The inset in each figure shows a schematic view of the stacking plane. (e) measuring lateral force on SnO by AFM. Inserting a drawing: schematic experimental diagram. (f) STEM images of the cross-sections of SnO and M-SnO crystals.

Fig. 4 is a characterization of SnO flakes of different thicknesses. (a) Typical optical microscope images of a exfoliated SnO layer on a glass substrate. (b-f) representative AFM images of SnO flakes having a thickness of 1-5 layers. (g) high resolution TEM images of SnO flakes. (h) Comparing the raman spectra of single-layer SnO with bulk SnO and with raman spectra after exposure to air for 3 months and additional heating to 200 ℃ in air; laser wavelength: 532 And (5) nm.

FIG. 5 is a Raman spectrum of (a) SnO flakes; illustration is shown: an optical image of the nanoplatelets. (b) A. The _1g And (c) E _g Is at 15X 15 μm ² Raman intensity mapping plot collected over area with step length of 500And (5) nm. The results show that the thickness uniformity is high and the local bonding structure is good.

FIG. 6 is a Si/SiO ₂ AFM images of SnO pieces on substrates before and after exposure to ambient air for 3 months and after additional heating in air at 200 ℃.

Fig. 7 compares the physical properties of single-layer SnO flakes with SnO blocks. (a) Ribbon structure, (b) by drawing (Ahv) ^1/2 The band gap was experimentally estimated as a function of hv, and the experimental and fitted values are plotted as solid and dashed lines, respectively. The band gap modulation covers the entire spectral range from infrared to ultraviolet. (c) energy band structure of antimony and (d) experimental estimation of band gap. When thinned to a single layer, the metallic bulk material is converted to a semiconductor with a band gap of 2.01 eV. (e) The band gap regulation range from bulk to single-layer SnO and Sb is compared with other reported two-dimensional semiconductors.

Fig. 8 shows (a) optical transmittance and (b) absorbance of SnO nanosheets analyzed by ultraviolet-visible (UV-Vis) absorption. Collecting spectra on a single thin slice deposited on a transparent quartz plate substrate by using an ultraviolet-visible spectrometer, wherein the radius of a laser spot is 2 mu m; inset (a): optical photographs of single flake and bulk SnO crystals. The arrows indicate the increase in thickness. (c) graph of band gap as a function of thickness.

Fig. 9 shows the characteristics of the Bi material. (a-b) representative SEM images and (c) X-ray diffraction patterns. Bismuth powder was purchased from alatin industries ltd. The lateral dimension of the crystallites is about 50 μm.

Fig. 10 shows the characteristics of Sb material. (a-b) representative SEM images and (c) X-ray diffraction patterns. Sb powders were purchased from Allantin industries, inc. The lateral size of the crystallites is about 60-80 μm.

FIG. 11 is V ₂ O ₅ The nature of the material. (a-b) representative SEM images at different magnifications and (c) X-ray diffraction patterns. Vanadium pentoxide powder was purchased from alatin industries ltd. The lateral dimension of the crystallites is about 150 μm.

FIG. 12 shows Bi ₂ O ₂ Characterization of Se material. (a-b) representative SEM images at different magnifications and (c) X-ray diffraction patterns. Bi ₂ O ₂ Se powder was purchased from Aladdin industries, inc. The lateral dimension of the crystallites is about 50 μm.

FIG. 13 shows a single crystal KV ₃ Sb ₅ The nature of the material. (a-b) representative SEM images at different magnifications. (c) an X-ray diffraction pattern.

FIG. 14 shows (a) Si/SiO ₂ AFM images of Sb nanosheets on a substrate; from left to right: freshly peeled flakes, flakes after exposure to air for 3 months, and flakes after additional heating in air at 200 ℃ for 10 minutes, and (b) corresponding raman spectra. No significant change in flake morphology, thickness or spectral characteristics was observed, indicating good O resistance ₂ And (4) stability.

FIG. 15 shows (a) Si/SiO ₂ AFM images of Bi nanosheets on the substrate; from left to right: freshly peeled flakes, flakes after exposure to air for 3 months, and flakes after additional heating in air at 150 ℃ and 200 ℃ for 10 minutes, and (b) corresponding raman spectra. At the temperature of 150 ℃, no obvious change of the shape, thickness or spectral characteristics of the nano-sheet is observed, which shows that the Bi nano-sheet has good O resistance ₂ And (4) stability.

FIG. 16 shows (a) Si/SiO ₂ Bi on the substrate ₂ O ₂ AFM images of Se plates; from left to right: freshly peeled flakes, flakes after exposure to air for 3 months, and flakes after heating in air at 200 ℃ for 10 minutes, and (b) corresponding raman spectra. No significant change in flake morphology, thickness or spectral characteristics was observed, indicating that the Bi nanosheets have good O resistance ₂ And (4) stability. .

FIG. 17 is a graph of KV less than 2 nm thick (one or two layers) ₃ Sb ₅ Representative AFM images of (a).

FIG. 18 is an optical microscopic representative image (taken in transmission mode) of a direct taped-off SnO sheet on a glass slide substrate, showing that these layered crystals are still thick and opaque.

Detailed Description

The exfoliation energy of non-van der Waals layered structures is typically several times higher than that of graphite (FIG. 1 a), whichNaturally, the exfoliation of these materials becomes difficult. The method of the invention comprises a simple calendering pretreatment, followed by mechanical stripping using transparent tape to obtain a thin layer, successful in stripping a wide range of materials, including metals (Bi, sb), semiconducting metal oxides and chalcogenides (SnO, V) ₂ O ₅ 、Bi ₂ O ₂ Se) and superconducting compound (KV) ₃ Sb ₅ ) The electron density calculation verifies that strong electron coupling exists between the structural layers (figures 1 b-c). The present invention, using calendering pretreatment followed by scotch tape stripping, successfully achieves for the first time a few-layer or even single-layer structure of a variety of materials and enables exciting new phenomena to be observed in the stripped 2D material. The metal antimony becomes a semiconductor with a band gap of 2.01 eV; the light absorption range of the semiconductor SnO can be adjusted from an Infrared Region (IR) to an ultraviolet region, and the band gap is adjusted from 0.60 eV of a bulk body to 3.65 eV of a single layer. In addition, thin KV ₃ Sb ₅ Is a potential two-dimensional superconductor. The novel results of the present invention thus provide a general approach to mechanical delamination of non-van der waals layered materials and provide a variety of new 2D materials.

SnCl ₂ •2H ₂ O (AR, 98%) and NaOH (AR, 96%) were purchased from the national pharmaceutical group Chemicals, inc. V ₂ O ₅ Crystals of (metal-based, 99.99%), bi (metal-based, 99.99%) and Sb (metal-based, 99.99%) were purchased from alatin industries ltd (shanghai, china). Bi ₂ O ₂ Se (metal based, 99.99%) was purchased from Nanjing Migramidge science and technology, inc. (Co., ltd.). All reagents were not purified. Scotch tape was purchased from texas megana new energy limited. The heat release tape (single-sided, heat release temperature 120 ℃) was purchased from Jiangsu Xiancheng nano material science and technology Co., ltd. Polydimethylsiloxane films (0.5 mm thick) were purchased from lotyang altrmer commercial ltd. Si/SiO ₂ Substrate (SiO) ₂ Thickness: 300 nm) was provided by mircobian technologies, inc.

And (5) testing the material. The crystal morphology of the samples was studied using a powder X-ray diffraction system (XRD, X' Pert-Pro MPD) equipped with a Cu/K α 1 target (λ =1.5418 a)A bulk structure. The sample morphology was studied by scanning electron microscopy (SEM, hitachi SU 8010). The surface chemistry of the samples was tested under ultra-high vacuum using the Al/Ka target of X-ray photoelectron spectroscopy (XPS) on Escalab 250Xi X-ray photoelectron spectroscopy (Thermo Fisher Scientific Inc.), with a standard deviation of the binding energy of 0.1 eV. Atomic force microscopy (AFM; bruker Instruments Dimension) was used to characterize the lateral dimensions and thickness of flakes on a silicon substrate. A high resolution transmission electron microscope (HR-TEM) was performed using a FEI Tecnai G2F 20S-TWIN TMP equipped with a field emission gun operating at an accelerating voltage of 200 kV. The mechanically stripped flakes were transferred directly onto copper grids, atomic images were collected using a Themis aberration corrected scanning transmission electron microscope (STEM, HF 5000), and samples were prepared by Focused Ion Beam (FIB), where high current gallium ion beam was used to strip surface atoms to complete micro-nano surface topography processing. The ultraviolet-visible near infrared transmission spectrum of the peeled-off sheet on a transparent quartz slide was recorded at 20/30 PV, with a differential spectrophotometer at ambient temperature (Craic Technologies Inc.), from which band gap values were calculated using a Tauc diagram, measuring a wavelength range of 350-2000 nm, and Raman spectra were collected at 532 nm laser excitation (power: 1 mW) with a spot radius of 2 μm using a confocal Raman spectrometer (WITec Alpha 300R) equipped with a UHTS 300 spectrometer (600 lines per mm of grating) and a CCD detector (DU 401A-BV-352). Focusing the laser beam and collecting the Raman signal using a 100 Xobjective lens, transferring the lift-off foil to Si/SiO ₂ On the substrate.

Synthesis example

And preparing SnO crystal particles. In a typical procedure, 0.02 mol (4.50 g) of SnCl are added with stirring ₂ •2H ₂ O was dissolved in 70 mL of ultrapure water, and NaOH was then added until the pH of the mixture reached 9. After stirring for a further 30 minutes, the mixture was transferred to a 100 mL teflon lined autoclave, sealed and heated at 150 ℃ for 15 hours. And then allowed to cool to room temperature. The product was collected by centrifuging the mixture, washed alternately 3 times with distilled water and anhydrous ethanol, and then dried overnight in vacuo to give blue-black SnO crystals.

Adopts a fluxing agent method to prepare the single crystal KV ₃ Sb ₅ It is expressed as KSb ₂ The alloy is grown by fluxing agent. K. V, sb elements and KSb ₂ The precursors were sealed in a tantalum crucible and then in a high vacuum quartz ampoule at a molar ratio of 1. The ampoule is heated to 1273K for 20 hours and then cooled to 773K. The single crystal having a lateral dimension of about 1000 μm and a silvery luster was separated from the flux by centrifugation.

The present invention is prepared by first calendering the crystalline particles and then mechanically peeling the particles through transparent tape, using prior art all dry techniques to transfer to the target substrate. By way of example, the invention consists in rolling the crystal particles flat on a surface, i.e. applying a shear force on the crystal, the rolling being in one direction only (not back and forth), resulting in continuous and unidirectional calendering, with a roller or bar load of 0.5 to 10kg and a speed of 10 to 500mm/min. Then according to the conventional method: peeling the rolled crystal particles by using a transparent adhesive tape to obtain a thin layer (namely the product two-dimensional nanosheet of the invention), separating the thin layer from the transparent adhesive tape by using a heat release adhesive tape, transferring the thin layer to a polydimethylsiloxane film through heating release, further vertically pressing the adhesive tape on a target substrate, and transferring the peeled thin layer to various substrates, such as glass, silicon or silicon/silicon dioxide; the transfer step using a heat release tape and/or a polydimethylsiloxane film may also be omitted. In the invention, the specific tiling method is a conventional technology, is not limited, and does not influence the realization of the technical effect of the invention.

Example one

Clamping the crystal particles between two pieces of paper, and then pressing a plastic rod on the surface by hand to roll in a single direction to obtain calendered particles; again using a conventional classical tape stripping method, namely: the transparent adhesive tape is adhered with the rolled particles, and the rolled particles are folded in half and torn off after being pressed to form peeling, so that a thin-layer material, namely the two-dimensional nanosheet of the invention, is obtained. Then baked at 100 c for 5 seconds (to help maintain the transverse dimension of the sheet), then the thin layer was separated from the clear tape using a thermal release tape, and then released at 150 c as required for testing, transferring the sheet to a different substrate.

The crystal particles are respectively metal Bi, metal Sb, semiconductor metal oxides SnO and V ₂ O ₅ 、Bi ₂ O ₂ Se, superconductive compound KV ₃ Sb ₅ 。

Example two characterization of properties of SnO two-dimensional nanoplatelets.

The crystal structure of SnO belongs to P4/nmm space group, has a tetragonal unit cell structure, and Sn and O atoms are Sn _1/2 −O−Sn _1/2 Sequential edge [001]The crystal directions are alternately arranged to form a layered sequence (fig. 2 a). Each O atom is coordinated to four surface metal Sn atoms to form Sn ₄ An O tetrahedron. Therefore, the lone pair electrons composed of the Sn 5s orbitals are directed to the interlayer spacing, and thus a strong dipole-dipole interaction exists between the adjacent SnO layers. The differential charge density plot of the structure shows high electron density between the layers, indicating that there is a strong interlayer interaction (fig. 2 b). Due to having 48.4 meV A ^-2 The strong binding interaction between adjacent layers of high peel energy (quantified by the difference in ground state energy between the bulk material and the monolayer, as in the prior art), does not allow the creation of a monolayer of flakes by existing mechanical peeling. The present invention strips a single-layer SnO flake from SnO crystal particles, the nanosheet having an unusual metal capped structure in which two atomic layers of Sn sandwich an atomic layer of O. The crystallographic thickness of the SnO monolayer was 0.38 nm. The calculated single-layer SnO phonon spectrum shows that all phonon branches of the entire Brillouin region are positive and that no virtual frequencies are present (fig. 2 c), indicating the structural stability of the 2D SnO flakes in the ground state. The stability of such single-layer nanoplates to environmental or oxidative environments and temperatures was further investigated, as this is crucial for basic research and technical applications. Theoretical estimation of the oxidation barrier by the climbing elastic band method using DFT calculation shows that SnO flakes are stable and resistant to O ₂ Attack (fig. 2 e). Molecular dynamics simulations at 300 and 600K temperatures showed no significant structural dissociation (fig. 2 d), indicating that SnO exfoliation flakes have a higher thermal stability. These findings strongly suggest that a single layer of 2D SnO is stable once separated from the bulk crystal stack.

The invention prepares SnO crystal particles through hydrothermal reaction of stannous chloride dihydrate and sodium hydroxide (see synthesis example for detailed information). The X-ray diffraction pattern (XRD) of the crystals after hydrothermal reaction shows diffraction peaks indexed to tetragonal unit cells by lattice parameters: a = b =3.8016 (4) a, c =4.8441 (5) a (fig. 3 a), indicating the formation of pure-phase SnO under these conditions. X-ray photoelectron spectroscopy (XPS) confirmed the presence of Sn and O elements with Sn/O ratios close to 1. The high resolution Sn 3d spectrum shows two significant peaks at 486.1 and 494.5 eV, corresponding to Sn, respectively ²⁺ Sn 3d of _5/2 And Sn 3d _3/2 A core energy level. The valence state also coincides with the blue-black color of the crystal (see FIG. 3a for an inset). Raman spectrum of the original crystal shows E _g At 112 cm ^-1 A is _1g Peak at 210 cm ^-1 Here, this is characteristic of SnO structure (fig. 3 b). Scanning Electron Microscopy (SEM) showed that SnO crystals were 100 μm in lateral dimension and approximately 10 μm thick (FIG. 3 c). The sheet-like shape may be associated with an inherently anisotropic layer structure, the thickness of which is associated with the stacking of the sheets. A simple calendering treatment of these crystals can cause a change in the SnO crystal structure, known as M-SnO. Morphological characterization of the SEM showed planar sliding (fig. 3 d), with a new diffraction peak observed in the low angle region of the XRD pattern, indicating a slight increase in the interlayer spacing from 4.844 a to 4.949 a (fig. 3 a). Figure 3e is an in-situ AFM. Figure 3f Scanning Transmission Electron Microscopy (STEM) shows repulsion between adjacent layers and increases the repeat distance in the stacking direction. No voids were observed at the Sn sites. The in-plane Sn-Sn distance varies slightly from 2.684A to 2.715A, which is consistent with the XRD results. In-plane Raman vibration mode E after calendering treatment _g Is significantly reduced (fig. 3 b).

In the method disclosed by the invention, after the calendering treatment, the crystal can be peeled and layered into a single SnO sheet by using a conventional transparent adhesive tape method; optical microscopy images of exfoliated SnO flakes transferred onto transparent glass substrates showed high transparency (fig. 4 a). The number of layers was verified by AFM as shown in FIGS. 4 b-f. The minimum thickness of a two-dimensional SnO sheet measured by AFM is 0.8nm, based on the crystal structure of the two-dimensional SnO sheet, the thickness of a SnO single layer is 0.38 nmConsidering the 0.1-0.6nm interface "dead layer" that is typically present between the release sheet and the substrate, the measured 0.8nm height cannot correspond to two or more layers, but should correspond to a single layer of SnO. Layers of different thickness were measured, specifically 1.3, 1.8, 2.4 and 2.9 nm, with the thickness increasing in steps of about 0.5 nm, as shown in the inset of fig. 4c, the ideal step size is the repeat distance of crystallization (0.48 nm) of adjacent layers, matching exactly the experimentally measured step size value, so that the lamellae corresponded to

layers

2, 3, 4 and 5. The size of the stripping sheet is micron-sized, the size of the single-layer sheet is 2-6 mu m, the size of the 5-layer sheet is increased to about 15 mu m (figure 4 b-i), and the requirements of basic research and application on inherent material characteristics and certain devices can be met. High-resolution Transmission Electron Microscopy (TEM) showed crossed lattice fringes with a spacing of 2.7 a, corresponding to the lattice fringe spacing of the (110) planes (fig. 4 g). Careful examination of the images revealed no significant defects, indicating that excessive metal vacancy defects were not produced during the calendering process. In Raman spectrum, in-plane vibration mode E _g The intensity of (A) decreases rapidly with decreasing sample thickness and is hardly detectable for a single layer (FIG. 4 h), A _1g The peak appears red shifted (-4 cm) ^-1 ) As the number of layers decreases, the vibration modes in the layered structure soften. On a single sheet E _g And A _1g The raman intensity mapping of the modes further confirms the thickness and the high uniformity of the local bonding structure (fig. 5). Notably, no SnO was observed even after exposure of the samples to air for more than 3 months and additional heating to 200 deg.C ₂ Which confirms the structural stability of the exfoliated SnO flakes under ambient conditions. AFM studies confirmed that the morphology and thickness of the flakes remained intact (fig. 6).

After the bulk SnO crystals are exfoliated into flakes, their physical properties are significantly changed. Theoretical predictions indicate that the band gap increases significantly from 0.64 eV for bulk SnO to 3.95 eV for a single layer (fig. 7 a). Experimentally, the absorption spectrum of SnO flakes shows the shape characteristics of semiconductors with sharp absorption edges (fig. 8), consistent with theoretical estimates. As the thickness of the nanosheets decreases, the bandgap of the SnO flake increases (fig. 7 b), the bandgap modulation spans the entire spectral range from infrared to ultraviolet, and this very wide absorption window is the largest absorption window of 2D semiconductors reported to date (fig. 7 e).

The appropriate sample position is selected based on the contrast difference between the substrate and the laminate using an optical microscope attachment on the instrument. The original bulk crystals appear black due to opacity compared to the high transparency peeled sheet. Thinner flakes exhibit higher transparency. Based on (alpha hv) ^1/2 Tauc diagram with hv, whereinαWhich represents the light absorption coefficient of the light-emitting diode,his the constant of the planck constant of,νis the incident photon frequency, the optical band gap value is extracted by extrapolating the fitted line to the intercept (α = 0).

Example three physical properties of other material sheets obtained by the present invention.

The invention is applicable to a variety of materials, including metals (Bi, sb), semiconducting metal oxides and chalcogenides (SnO, V) ₂ O ₅ 、Bi ₂ O ₂ Se) and superconducting compound (KV) ₃ Sb ₅ ) (ii) a The interlayer interactions and corresponding AFM images are shown in fig. 1b and c. In particular, the metal antimony (Sb) is a three-dimensional pseudo laminar crystal and belongs toR3-mh space group, with triangular and hexagonal lattices. The crystal can be regarded as an abcabcabc stack layer with a curved honeycomb arrangement of antimony atoms. It is noteworthy that the minimum interlayer spacing is only 0.23 nm, which means that the interlayer interactions are mainly chemical (structure and density plot in fig. 1 b), which makes direct mechanical exfoliation difficult to achieve. By using the method of the present invention, a 1.2 nm thick monolayer of Sb can be obtained. Characterization details of antimony and other exemplary materials are given in fig. 9-13. From these materials (including Bi, sb and Bi) ₂ O ₂ Se), even when heated additionally in air, exhibits stability against oxidation for a long period of time (fig. 14 to 16). Importantly, metallic antimony was converted to wide bandgap semiconductor (2.01 eV) when thinned to a monolayer 1.2 nm thick (figure 7c, d). The invention achieves a broad modulation of the thickness-related physical properties that may be associated with strong electron coupling in the interlayer region. In non-van der Waals structures, as the number of layers decreases, it is strongThe unique thickness-property relationships observed in these materials further highlight the importance of extending 2D flakes into non-van der waals materials, as strong interlayer interactions can result in subtle changes in the lattice structure that affect the physical properties of the exfoliated flakes in addition to size effects.

KV ₃ Sb ₅ Is a member of the recently discovered quasi-two-dimensional Kagome metal family, of the general formula AV ₃ Sb ₅ (a: K, rb, cs). The material belongs to the P6/mmm space group, and the layers are connected through chemical bonds between A and V. The Kagome lattice of transition metal atoms is considered as an excitation platform for studying a series of electron-associated phenomena, including charge density waves, extraordinary Hall effects, and superconductivity, and produces surprising results. Compared to bulk materials, the 2D structure has several advantages: the two-dimensional geometry will enhance quantum fluctuation and correlation and may also facilitate charge modulation by carrier doping, all of which may alter the superconductivity and charge density waves. For example, for CsV ₃ Sb ₅ For flakes of thickness 60 nm, the superconducting transition temperature Tc increases from about 2.5K in bulk to 4.28K, whereas the opposite behavior is observed when the sample is further thinned to 4.8 nm, i.e. the Tc decreases to 0.76K. The transition temperature of the charge density wave changes in a reverse trend with the thickness. One recent work reported hole doping by natural oxidation of the Cs layer by simply exposing it to air for a few minutes, taking advantage of the reactivity of the surface a layer. For lamellae with a thickness of less than 82 nm,T _c a significant jump to about 4.7K. KV ₃ Sb ₅ The Tc of the bulk is low, 0.93K, since valence electrons on Cs are easily lost, K-related materials may be a better platform to more effectively regulate carrier concentration. Thin KV ₃ Sb ₅ Abnormal Hall conductivity of crystal (thickness about 105 nm) is as high as 15507 ohm ^-1 cm ^-1 . Regarding peeling of such materials, it is reported that CsV cannot be peeled off using a conventional, conventional scotch tape method ₃ Sb ₅ The crystal is thinned to the nanometer scale (below 100 nm), which is attributed to the chemical interaction between Sb and Cs layers; k is smaller in size than Cs, which means KV ₃ Sb ₅ In the binding phaseThe stronger the interaction, making it more difficult to peel. Unexpectedly, KV of 2-5 nm thickness was obtained using calendering in combination with conventional tape stripping of the present invention ₃ Sb ₅ Flakes, corresponding to 2 to 5 layers (fig. 1c and 17). Even when exposed to air for at least 10 minutes at ambient conditions, the 5.4 nm flakes have a fairly smooth surface, KV ₃ Sb ₅ Successful exfoliation to a single layer or few layers will provide new opportunities for studying unconventional superconductivity and interaction with charge density waves in a two-dimensional Kagome lattice.

Application examples

Flatly paving the crystal particles on a bottom plate of an electric rolling roller (HZ-2403), and performing single-wheel rolling at a speed of 200 mm/min to obtain rolled particles; and sticking the calendered particles by using a transparent adhesive tape, folding and pressing the particles, and tearing the particles to form peeling, so as to obtain a thin-layer material, namely the two-dimensional nanosheet of the invention. The crystal particles are respectively metal Bi, metal Sb, semiconductor metal oxides SnO and V ₂ O ₅ 、Bi ₂ O ₂ Se, superconductive compound KV ₃ Sb ₅ The obtained two-dimensional nano-sheet is similar to the embodiment, comprises a single layer or few layers of peeling sheets, and the transverse dimension can reach 15 μm. The method not only has universality for various crystal particles, but also can be used for preparing by industrial equipment, thereby providing a foundation for industrialization.

In summary, the present invention proposes a general solution for the mechanical exfoliation of various crystalline structures with non-van der Waals type interlayer forces, including metals (Bi, sb), semiconducting metal oxides and chalcogenides (SnO, V) ₂ O ₅ 、Bi ₂ O ₂ Se) and superconducting compound (KV) ₃ Sb ₅ ). Mechanical peeling was successfully achieved by simple calendering. New 2D sheets from non-van der waals structures show significantly good physical properties that are distinct from crystalline bulk; the band gap can be adjusted from 0.60 Ev (IR) for bulk SnO to 3.65 Ev (UV) for single layer; the metal-semiconductor (2.01 eV bandgap) transition occurs when bulk Sb transitions to a monolayer. Single and few layers KV achieved in this work ₃ Sb ₅ Are exciting products of 2D superconductors.The invention provides a method for mechanically peeling a non-van der waals layered structure into a high-quality 2D analogue for the first time, and opens a door for easily preparing a new material family with potential application.

Comparative example

The conventional tape stripping method is adopted: and (3) directly sticking SnO crystal particles (not rolled) by using transparent adhesive tapes, folding and pressing the SnO crystal particles, and tearing the SnO crystal particles to form peeling, so as to obtain a peeled product. Fig. 18 is an example of an optical microscope image of tape-peeled SnO pieces on a glass slide substrate taken in transmission mode, and it can be seen that these layered crystals are still thick and opaque, with a thickness in the micrometer range, indicating that conventional tape peeling cannot result in two-dimensional sheets, and even slices with a thickness in the nanometer range.

Metals (Bi, sb), V ₂ O ₅ 、Bi ₂ O ₂ Se, superconductive compound (KV) ₃ Sb ₅ ) Sheets with a thickness of less than 0.2 μm cannot be obtained with conventional classical tape stripping.

Two-dimensional (2D) materials at single layer thicknesses have many new properties and thickness dependencies. Mechanical exfoliation of a layered structure is the most effective method to obtain ultrathin flakes, but this method is limited to materials where the interlayer interactions are controlled by weak van der waals forces, and is not applicable to materials that are not van der waals structures. The invention discloses a general method for mechanically stripping non-van der Waals structures for the first time so as to obtain various novel two-dimensional materials comprising metals (Bi and Sb), semiconductor metal oxides and sulfur compounds (SnO and V) ₂ O ₅ 、Bi ₂ O ₂ Se) and superconducting compound (KV) ₃ Sb ₅ ). The process of the present invention involves calendering the stock and then mechanically peeling the slipped structure using typical scotch tape processes to yield a stable single or multi-layer material with exciting new physical properties. For example, band gaps of metals and semiconductors are modulated in a wide range according to the number of layers (0 to 2.01 eV Sb, 0.60 eV (IR) to 3.65 eV (UV) SnO). Also obtains several layers of KV ₃ Sb ₅ The material is an exciting material for researching the unconventional superconductivity. The new direct mechanical stripping non-van der Waals laminar material of the inventionThe method greatly broadens the usability of the 2D material to explore its unique physical properties and practical applications.

Claims

1. A preparation method of a two-dimensional nanosheet is characterized in that crystal particles are subjected to calendaring treatment and then are mechanically stripped to obtain the two-dimensional nanosheet.

2. A method of making two-dimensional nanoplatelets as in claim 1 wherein the crystalline particles are non-van der waals layered crystalline particles.

3. A method of preparing two-dimensional nanoplatelets according to claim 2 wherein the non-van der waals layered crystal particles are metal particles, metal oxide semiconductor particles, metal sulfide semiconductor particles, superconducting compound particles, or the like.

4. A method of making two-dimensional nanoplatelets according to claim 1 wherein the crystalline particles have a particle size in the range of micrometers to millimeters.

5. A method of making two-dimensional nanoplatelets according to claim 4 wherein the crystalline particles have a particle size of 1 μm to 5mm.

6. The preparation method of a two-dimensional nanosheet according to claim 1, wherein the crystalline particles are tiled and then subjected to a calendering process, followed by mechanical exfoliation to obtain the two-dimensional nanosheet.

7. The preparation method of the two-dimensional nano-sheet according to claim 1, wherein a roller or a bar is used for calendering; mechanical stripping is tape stripping.

8. Two-dimensional nanoplatelets prepared according to the method of preparing two-dimensional nanoplatelets of claim 1.

9. Two-dimensional nanoplatelets according to claim 8 wherein the thickness of the two-dimensional nanoplatelets is between 0.1nm and 50nm.

10. Use of the two-dimensional nanoplatelets of claim 1 in the preparation of a two-dimensional nanoplatelet assembly material.