Unconventional stoichiometric ratio compound, preparation method thereof and semiconductor material

Abstract

The invention discloses an unconventional stoichiometric ratio compound, a preparation method thereof and a semiconductor material. The chemical formula of the compound with the unconventional stoichiometric ratio is NaCl2, the compound is prepared on a nanometer material substrate with positive potential by taking a NaCl solution as a raw material under the conditions of normal temperature and normal pressure, and the semiconductor material has a wide band gap under the conditions of normal temperature and normal pressure and can be applied to multiple fields of high-frequency devices, photoelectric devices, quantum materials and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to a compound and a preparation method thereof, and a semiconductor material, in particular to a compound with a non-conventional stoichiometric ratio and a preparation method thereof, and a semiconductor material. BACKGROUND

[0002] In the search for new wide-bandgap semiconductors with unusual properties, people are limited by the traditional framework of stoichiometry, which greatly limits people's thinking. In the early 19th century, John Dalton proposed to represent the proportion of different elements (anions and cations) in a compound by atomic ratio, and named it stoichiometry. According to the law, if the valence of atoms A and B is m and n respectively, then the expression of the stable compound composed of A and B atoms should be A n B m This law well explains the formation rule of compounds, and further guides the design and synthesis of materials. For example, the stable compound composed of sodium and chlorine elements can only be sodium chloride (NaCl) crystal with a 1:1 stoichiometric ratio.

[0003] The team of inventors has previously prepared two-dimensional crystals of unconventional stoichiometries, Na2Cl, Na3Cl, CaCl, under ambient conditions (Shi, G. et al. Two-dimensional Na-Cl crystals of unconventional stoichiometries on graphene surface from dilute solution at ambient conditions. Nature Chemistry 10, 776-779 (2018); Zhang, L. et al. Novel 2D CaCl crystals with metallicity, room-temperature ferromagnetism, heterojunction, piezoelectricity-like property and monovalent calcium ions. National science review 8, nwaa274 (2021)). This growth process benefits from the hydrated cation-pi / hydrated cation-metal interactions in the salt solution. The differences of these crystals from conventional stoichiometry crystals provide new insights for people to re-understand the bonding mechanism of chemical bonds and the corresponding electronic configuration. Benefiting from this strange crystal and electronic structure, these two-dimensional unconventional stoichiometry crystals exhibit some unexpected electrical, magnetic, optical and mechanical properties. However, the strong interface effect and the short-range interaction between the graphene substrate and the crystal limit the growth of the material in three-dimensional space. The synthesis of three-dimensional unconventional stoichiometry crystals under ambient conditions still needs breakthrough technological innovation.

[0004] In recent years, researchers have synthesized a series of unconventional stoichiometric ratio compounds under high pressure conditions, including Na-Cl system crystals, tellurium hydrides (H4Te, H5Te2 and HTe3), and bulk Na2He phase, etc. For example, unlike the traditional 1:1 stoichiometric ratio of NaCl salt crystals, Zhang et al. synthesized a series of unconventional stoichiometric ratio Na-Cl system crystals under high pressure, covering Na3Cl (>77 GPa high pressure stable), Na2Cl (>100 GPa high pressure stable), Na3Cl2 (>120 GPa high pressure stable), NaCl3 (>20 GPa high pressure stable) and NaCl7 crystals (>142 GPa high pressure stable) (Zhang, W. et al. Unexpected stable stoichiometries of sodium chlorides. Science 342, 1502-1505 (2013)). In 2016, Zhong et al. designed three unconventional stoichiometric ratio crystals that can exist stably under high pressure environment above 140 GPa, including H4Te, H5Te2 and HTe3, among which H4Te is the compound with the highest hydrogen content in chalcogen hydrides (Zhong, X. et al. Tellurium hydrides at high pressures: High-temperature superconductors. Physical review letters 116, 057002 (2016)). In 2017, Dong et al. synthesized a thermodynamically stable compound Na2He with fluorite type structure under 113 GPa high pressure environment through theoretical guidance experiments, which broke the previous phenomenon that only helium clathrates were formed by van der Waals forces (Dong, X. et al. A stable compound of helium and sodium at high pressure. Nature Chemistry 9, 440-445 (2017)).

[0005] If three-dimensional unconventional stoichiometric ratio crystals can be synthesized under normal temperature and pressure conditions, we will have greater freedom and ability to control materials to have more complex electronic and optical properties. The properties of the aforementioned published three-dimensional unconventional stoichiometric ratio crystal materials synthesized under high pressure conditions, such as near room temperature superconductivity, transparent inorganic electronic compounds and high energy density characteristics, etc., will also be realized under room temperature conditions. It should be noted that the formation and maintenance of the stability of these compounds under normal temperature and pressure conditions has been a long-term challenge.

[0006] To solve the above problems, the application provides a non-stoichiometric compound, a preparation method of the compound and a semiconductor material, the compound can be prepared under normal temperature and pressure and grow into a bulk crystal in three-dimensional space. SUMMARY

[0007] The application aims to provide a non-stoichiometric compound, a preparation method of the compound and a semiconductor material. The compound of the application overcomes the problems of instability under normal temperature and pressure, and inability to grow in three dimensions, has a wide band gap, and can be widely used in the field of semiconductors.

[0008] To solve the above problems, the first object of the application is to provide a non-stoichiometric compound, the chemical formula of the compound is NaCl2.

[0009] Preferably, the crystal structure of the compound belongs to a monoclinic system and has a space group C2 / m symmetry.

[0010] Preferably, the structure of the compound is stable under normal temperature and pressure.

[0011] Preferably, the unit cell parameters of the compound are α = γ = 90.00°, β = 116.89°.

[0012] Preferably, the unit cell parameters of the compound are α = γ = 90.00°, β = 116.89°.

[0013] Preferably, the crystal structure of the compound comprises an atomic pair layer composed of Cl and an ionic layer composed of Na-Cl, wherein the atomic pair layer and the ionic layer are arranged in a vertical direction.

[0014] Preferably, each Cl atom in the ionic layer is connected to 5 Na atoms; each Na atom in the ionic layer is connected to 6 Cl atoms, wherein 5 Cl atoms are from the ionic layer and 1 Cl atom is from the atomic pair layer.

[0015] Preferably, the Na atoms in the ionic layer form ionic bonds with the Cl atoms in the ionic layer; the Na atoms in the ionic layer form ionic bonds with the Cl atoms in the atomic pair layer; and a pair of Cl atoms in the atomic pair layer form a covalent bond.

[0016] Preferably, the atomic pair layer and the ionic layer grow into a bulk crystal layer by layer in a vertical direction.

[0017] Another object of the present application is to provide a method for preparing the compound of the non-stoichiometric ratio, the crystal structure of which comprises atomic pair layers of Cl and ionic layers of Na-Cl, which are grown layer by layer in the longitudinal direction to form a bulk crystal.

[0018] Preferably, the compound is grown on a nanomaterial substrate with a positive potential.

[0019] Preferably, the nanomaterial substrate with a positive potential is selected from a graphene film with a positive potential.

[0020] Preferably, the nanomaterial substrate with a positive potential is a polyethyleneimine-modified graphene oxide film.

[0021] Preferably, the compound is grown on a nanomaterial substrate at normal temperature and pressure.

[0022] Preferably, the polyethyleneimine-modified graphene oxide film is soaked in an unsaturated NaCl solution, and after cleaning and drying treatment of the film, the NaCl2 compound is formed on the film.

[0023] Preferably, the concentration of the NaCl solution is 1-4 M (mol / L), and the soaking time is 8-16 hours.

[0024] Preferably, the soaking is carried out at normal temperature and pressure.

[0025] Preferably, the compound can be separated from the substrate by mechanical exfoliation.

[0026] Another object of the present application is to provide a semiconductor material comprising NaCl2 as described above.

[0027] Preferably, the semiconductor material has a band gap of 4.22-4.27 eV.

[0028] The principle that NaCl2 can realize three-dimensional growth on a nanomaterial substrate with a positive potential in the application belongs to a special strategy of inducing layer-by-layer crystal growth. Taking a polyethyleneimine modified graphene oxide film as an example, due to the existence of hydration ion-pi interaction and electrostatic interaction, alkali metal cations and halogen anions can be enriched at the interface of a salt solution-polyethyleneimine modified graphene oxide film. When the concentration of a NaCl salt solution increases, alkali metal cations and halogen anions are massively enriched on the surface of the polyethyleneimine modified graphene oxide film, and even can cover the surface of the film. According to the knowledge of statistical physics, under thermal disturbance, these massively enriched substances have a great possibility to reach order, and then form a thin layer of NaCl ionic crystal through ionic bond interaction. At the same time, the high cationic property of polyethyleneimine can attract halogen anions to be enriched at the NaCl ionic layer, so as to make the whole system tend to be electrically neutral, and the enriched halogen anions can be bonded in a covalent layer of halogen atom pairs in a covalent bond mode. The above layer-by-layer crystallization process leads to the stable growth of three-dimensional bulk NaCl2 compounds on the nanomaterial substrate with a positive potential.

[0029] The application has the following advantages: the NaCl2 of the application is a compound with a non-conventional stoichiometric ratio, which can exist stably at normal temperature and pressure and can grow in three-dimensional space. The preparation method of the NaCl2 compound of the application can be carried out at normal temperature and pressure, and the preparation method is simple and low in cost. The semiconductor material has a wide band gap and has a broad application prospect in the field of semiconductors. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a convex hull graph of a Na-Cl system;

[0031] Figure 2 is a phonon spectrum graph of the NaCl2 crystal of the application;

[0032] Figure 3 is an evolution graph of temperature and energy with time at 100K and 300K temperatures of the NaCl2 crystal of the application obtained by AIMD;

[0033] Figure 4 is an initial structure and a final structure at 100K and 300K temperatures of the NaCl2 crystal of the application obtained by AIMD;

[0034] Figure 5 is a bulk crystal structure graph of NaCl2 on a polyethyleneimine modified graphene oxide film of the application;

[0035] Figure 6 is a two-dimensional projection graph of an electron localization function of the NaCl2 crystal of the application in the a-c plane;

[0036] Figure 7Interlayer Cl of NaCl2 crystal of the application obtained by SSAdNPD analysis 2 Front view (left) and side view (right) of 2c-2e bonding analysis of NaCl2 crystal of the application;

[0037] Figure 8 Interlayer Cl of NaCl2 crystal of the application obtained by NBO analysis 2 Bonding of NaCl2 crystal of the application;

[0038] Figure 9 Band structure, total density of states and projected density of states of NaCl2 crystal of the application obtained based on HSE06 functional;

[0039] Figure 10 Band-decomposed charge density map at the highest point of valence band (left) and at the lowest point of conduction band (right) of NaCl2 crystal of the application;

[0040] Figure 11 Optical absorption spectrum of NaCl2 crystal of the application obtained based on HSE06 functional;

[0041] Figure 12 High-angle annular dark-field scanning transmission (HADDF-STEM) image of Na-Cl@p-GO film of the application (the upper right corner of the inserted image is the selected area of Na and Cl element ratio);

[0042] Figure 13 High-resolution transmission electron microscopy image of Na-Cl@p-GO film of the application (upper left image), its magnified image (upper right image) and its electron diffraction pattern (inserted image in the upper right image), and the inverse fast Fourier transform image after removing the background in the red rectangular area (lower image) of the application;

[0043] Figure 14 Theoretical prediction of interlayer Na atomic distance of NaCl2 crystal of the application in a-b plane;

[0044] Figure 15 Preparation flow chart of cryo-EM sample;

[0045] Figure 16 Transmission electron microscopy selected area electron diffraction image of (601) crystal plane (left image) and (301) crystal plane (right image) of NaCl2 crystal of the application;

[0046] Figure 17 Theoretical prediction of (601) crystal plane (left image) and (301) crystal plane (right image) of NaCl2 crystal of the application;

[0047] Figure 18 Ultraviolet-visible-infrared spectrum of NaCl2 crystal of the application;

[0048] Figure 19 Steady-state photoluminescence spectra of NaCl crystal, p-GO film, and NaCl2@p-GO film of the present invention;

[0049] Figure 20 This is a graph showing the temperature-dependent resistivity change of the NaCl2@p-GO film of the present invention. Detailed Implementation

[0050] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0051] In one embodiment, the chemical formula of the compound having an unconventional stoichiometric ratio is NaCl2.

[0052] The theoretical calculations for the structural stability of NaCl2 compounds are as follows:

[0053] To predict crystal structures, the inverse material design software package IM, based on a multi-objective global optimization algorithm, was used. 2 The ODE package (open source) performs structure search on NaCl2 crystals. A series of lowest-energy structures are predicted using the Differential Evolution (DE) algorithm. During the structure search, the total energy is set as the objective function, and this objective is achieved through sorting, selection, and generation of offspring in each DE step. Thirty DE iterations are selected, with 30 structures as the population. In each selection operation, 60% of the optimal solutions are retained, while the remaining 40% are supplemented by randomly generated structures. The lowest-energy steady-state NaCl2 structure is obtained through structure search calculations. The calculation results are as follows: Figure 1 As shown, the position of NaCl2 on the Na-Cl phase diagram indicates that bulk NaCl2 does not readily decompose spontaneously into bulk NaCl ionic crystals and Cl2 gas. The phonon spectrum of bulk NaCl2 was calculated using the PHONOPY program, as shown below. Figure 2 As shown, the phonon spectrum of bulk NaCl2 has no imaginary frequencies, indicating that bulk NaCl2 is kinetically stable. First-principles molecular dynamics (AIMD) simulations were used to verify the thermodynamic stability of bulk NaCl2. The supercell size used for bulk NaCl2 in the AIMD simulation was [size missing]. A single Γ-point was used to sample k points in the Brillouin zone. The AIMD simulation temperatures were set to 100 K and 300 K, respectively. AIMD results (e.g.) Figure 3 ,Figure 4 As shown in the figure, the structure of bulk NaCl2 crystals remains stable at temperatures of 100K and 300K, proving that bulk NaCl2 is thermodynamically stable.

[0054] The formation energy of NaCl2 can be calculated using the following formula.

[0055]

[0056] Calculations show that the formation energy of NaCl2 is -1.24 eV / atom. The fact that the formation energy is less than zero indicates that NaCl2 is energy stable.

[0057] The configuration of NaCl2 compounds is calculated as follows:

[0058] The crystal structure of NaCl₂ was calculated using the Vienna Ab initioSimulation Package (VASP) software based on density functional theory (DFT). The Projected Augmented Wave (PAW) method was used to describe the interaction between the ionic core and electrons. The generalized gradient approximation (GGA) based on the Perdew-Bueke-Ernzerhof (PBE) was used as the exchange-correlation functional for structure optimization and electronic structure property studies. The plane wave cutoff energy was set to 600 eV. The k-point value based on the Monkhorst-Pack grid was 12 × 12 × 5. The convergence criteria for structure optimization and electronic self-consistent iteration were the total energy of the system and the convergence of the Hellmann-Feynman force to below 10, respectively. -6 eV and like Figure 5 As shown, bulk NaCl2 crystals are formed by two NaCl ion layers connected by Cl atom pairs along the c-direction and arranged periodically. Within each NaCl ion layer, each Cl atom (labeled Cl...) 1 It is bonded to five Na atoms, and each Na atom is bonded to six Cl atoms, which contain five NaCl ion layers. 1 An atom is a Cl atom in a pair of Cl atoms bonded to a NaCl ion layer (called Cl...). 2 The NaCl2 crystal structure is an unprecedented configuration among reported Na-Cl crystals.

[0059] The lattice parameters of NaCl2 obtained from theoretical calculations using the DFT method are as follows: α=γ=90.00°, β=116.89°, space group is C2 / m symmetry (No.12).

[0060] The electronic structure of NaCl2 is calculated as follows:

[0061] Electron Localization Function (ELF) analysis of Cl in NaCl2 crystals 1 and Cl 2 The electronic structure properties. For example... Figure 6 As shown, Cl in the NaCl ion layer 1 The surrounding electrons exhibit strong locality, which indicates that Na and Cl... 1 Ionic bonds were formed between them. Conversely, Cl... 2 Electrons in the atom-pair layer occupy a pair of Cl atoms 2 The middle region of the atom, which indicates that Cl 2 Covalent bonds formed between atoms. The charge state of the NaCl2 crystal structure was analyzed using the Bader charge analysis code developed by Henkelman's group. Bader charge analysis results show that each Na atom lost 0.84 electrons, while each Cl atom lost... 1 and Cl 2 The atoms gained 0.76 and 0.08 electrons respectively, in Na and Cl. 1 Cl 2 Ionic bonds are formed between Na and Cl. 2 The weak charge transfer between them helps stabilize the NaCl ion layer and Cl in the NaCl2 structure. 2 Atom pairs. The chemical bond properties of the NaCl2 structure were analyzed using the SSAdNDP and natural bond orbital (NBO) methods, such as... Figure 7 , Figure 8 As shown. NBO analysis results revealed Cl 2 sp formed between atomic pairs 3 The orbitals are similar in nature to the covalent bonds in the Cl2 molecule. The above calculations prove that bulk NaCl2 crystals are formed by ionic and covalent bonds, a characteristic that ensures the electroneutrality of the entire crystal and the stability of NaCl2 crystals with unconventional stoichiometry.

[0062] The band structure of NaCl2 is calculated as follows:

[0063] The band structure, electronic density of states, band decomposition charge density, and optical absorption spectrum of NaCl2 were calculated using the HSE06 hybrid functional, which consists of 25% Hartree-Fock exchange-correlation functionals and 75% PBE exchange-correlation functionals. The band gap of bulk NaCl2 calculated based on the HSE06 functional was 4.220 eV.

[0064] Analysis of the electronic structure properties of NaCl2 and Cl- through band structure and projected density of states. 1 and Cl 2 Orbital contributions from atoms. For example... Figure 9 As shown, NaCl2 crystal has a direct band gap of 4.22 eV, with both its valence band maximum and conduction band minimum located at the high-symmetry Z point. This band gap value is much larger than that of the currently popular wide-bandgap semiconductors 4H-SiC (3.26 eV) and GaN (3.39 eV), and is closer to β-Ga2O3 (4.90 eV). The projected density of states of NaCl2 crystal indicates that both the valence band maximum and conduction band minimum are contributed by the s and p orbitals of Cl atoms. The band decomposition charge density results (…) Figure 10 This indicates that the highest point of the valence band is mainly composed of Cl. 1 sp 3 Orbital contribution, while the conduction band minimum is mainly due to Cl 2 sp 3 Orbital contributions, which further provide Cl 2 Evidence of bonding between atoms.

[0065] NaCl2 possesses unique optical properties due to its crystal structure. The light absorption spectrum of NaCl2 crystal along the a, b, and c directions is shown below. Figure 11 As shown, NaCl2 crystals exhibit optical isotropy in the a and b directions. The first absorption peak begins at the band edge at 4.22 eV and reaches its peak value at 4.62 eV. However, NaCl2 crystals exhibit optical anisotropy between the a(b) and c directions. The first absorption peak along the c direction reaches a peak value of 4.70 eV. Notably, the absorptivity of NaCl2 crystals along the c direction is much greater than that along the a and b directions. This optical anisotropy is due to the structural anisotropy of NaCl2 crystals, indicating that NaCl2 crystals are potential materials for achieving significant mechanical and optical properties through manipulation.

[0066] According to one embodiment, NaCl2 can be prepared by immersing a polyethyleneimine-modified graphene oxide membrane (p-GO membrane) in an unsaturated NaCl solution for 8-16 hours, and then rinsing the membrane with deionized water for a few seconds to remove free Na. + Cl - Ions. The membrane is then dried under vacuum at 60-70℃ for 12 hours to form NaCl2 on the membrane.

[0067] The polyethyleneimine-modified graphene oxide membrane (p-GO membrane) can be prepared by the following method: A graphene oxide (GO) suspension is prepared from natural graphite powder using a modified Hummer method. The GO suspension is diluted to 2 mg / mL, and 300 μL of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC, CAS: 1892-57-5) is added to 30 mL of the 2 mg / mL GO suspension. The mixed suspension is stirred on a magnetic stirrer for more than 1 h to ensure complete reaction. Then, 0.06 g of polyethyleneimine (PEI, CAS: 9002-98-6) is added to the completely reacted suspension, and the mixture is stirred continuously for more than 3 h to modify the GO membrane with PEI. Under the modification of PEI, the zeta potential of the GO suspension is adjusted from negative to positive, resulting in the p-GO membrane. Next, the mixed suspension was placed in a dialysis tube for dialysis until the conductivity of the water in the beaker dropped below 1. During dialysis, the water in the beaker should be changed frequently to prepare a PEI-modified GO suspension (p-GO suspension). The p-GO suspension was then used to prepare a polyethyleneimine-modified graphene oxide membrane (p-GO membrane) by a drop-feed method. The p-GO membrane was then dried under vacuum at 60-70℃ for 12 hours.

[0068] NaCl2 is formed on the p-GO membrane according to the above embodiments, and the NaCl2 can be separated from the membrane in the following manner:

[0069] Step 1: Pretreatment. Before peeling, examine the graphene oxide film under a microscope to determine the location and distribution of crystals. Step 2: Wetting. Gently wet the graphene oxide film with ethanol. This step aims to reduce the interaction between the crystals and graphene oxide through the surface tension of the liquid. Step 3: Ultrasonic Treatment. Place the wetted sample in ethanol and treat it in an ultrasonic bath for several minutes. Ultrasound helps relax the interface between the crystals and graphene oxide, making the crystals easier to peel off. Step 4: Mechanical Peeling. Gently scrape away the crystals under a microscope using fine tweezers or a sharp needle. Try to maintain the integrity of the graphene oxide film. Transparent tape can be used to assist peeling; gently apply the tape to the area to be peeled and then slowly pull it off. Step 5: Cleaning. After peeling, immerse the graphene oxide film in ethanol and gently agitate to remove any attached crystals or other impurities. Repeat the cleaning process several times until all unwanted crystals are completely removed. Step 6: Drying. Allow the NaCl2 crystals to air dry naturally in a dust-free environment or dry them using nitrogen. Step 7: Inspection and Storage. Inspect the treated NaCl2 crystals using a microscope and store them in a dry, clean environment to avoid contamination.

[0070] According to one embodiment, a semiconductor material comprising NaCl2. Under normal temperature and pressure conditions, the semiconductor material has a bandgap of 4.22-4.27 eV. This semiconductor material exhibits a wide bandgap and can serve as a substitute for wide-bandgap semiconductor materials such as GaN and 4H-SiC, and can be applied in semiconductor fields such as optoelectronic devices, high-frequency devices, and quantum materials.

[0071] In the following description, implementation methods are illustrated in more detail with reference to embodiments, and implementation effects are illustrated in more detail with reference to experimental examples. However, these embodiments and experimental examples are not to be construed as limiting the scope of this disclosure.

[0072] Example 1

[0073] A GO suspension was prepared from natural graphite powder using a modified Hummer method. The GO suspension was diluted to 2 mg / mL, and 300 μL of EDC was added to 30 mL of the 2 mg / mL GO suspension. The mixture was stirred on a magnetic stirrer for 1 h to ensure complete reaction. Then, 0.06 g of PEI was added to the reacted suspension, and the mixture was stirred continuously for 3 h to modify the GO membrane with PEI. Under PEI modification, the zeta potential of the GO suspension was adjusted from negative (-33 mV) to positive (+36 mV), resulting in a p-GO membrane. Next, the mixed suspension was dialyzed in a dialysis tube until the conductivity of the water in the beaker dropped below 1. During dialysis, the water in the beaker was frequently changed to prepare the p-GO suspension. Individual p-GO membranes were prepared from the p-GO suspension using a dropwise method and dried under vacuum at 70 °C for 12 h. The p-GO membrane was immersed in a 1M unsaturated NaCl solution for 8 hours under normal temperature and pressure. The p-GO membrane was then rinsed with deionized water for a few seconds to remove free Na+ from the membrane surface. + Cl - Ions. The membrane was dried under vacuum at 70°C for 12 hours to obtain dried NaCl2 on the p-GO membrane.

[0074] Example 2

[0075] Unlike Example 1, the p-GO membrane was immersed in a 3M unsaturated NaCl solution for 8 hours under normal temperature and pressure conditions. After washing and drying, dry NaCl2 was obtained on the p-GO membrane.

[0076] Example 3

[0077] Unlike Example 1, the p-GO membrane was immersed in a 4M unsaturated NaCl solution for 8 hours under normal temperature and pressure conditions. After washing and drying, dry NaCl2 was obtained on the p-GO membrane.

[0078] Example 4

[0079] Unlike Example 1, the p-GO membrane was immersed in a 1M unsaturated NaCl solution for 12 hours under normal temperature and pressure conditions. After washing and drying, dry NaCl2 was obtained on the p-GO membrane.

[0080] Example 5

[0081] Unlike Example 1, the p-GO membrane was immersed in a 3M unsaturated NaCl solution for 12 hours under normal temperature and pressure conditions. After washing and drying, dry NaCl2 was obtained on the p-GO membrane.

[0082] Example 6

[0083] Unlike Example 1, the p-GO membrane was immersed in a 4M unsaturated NaCl solution for 12 hours under normal temperature and pressure conditions. After washing and drying, dry NaCl2 was obtained on the p-GO membrane.

[0084] Example 7

[0085] Unlike Example 1, the p-GO membrane was immersed in a 1M unsaturated NaCl solution for 16 hours under normal temperature and pressure conditions. After washing and drying, dry NaCl2 was obtained on the p-GO membrane.

[0086] Example 8

[0087] Unlike Example 1, the p-GO membrane was immersed in a 3M unsaturated NaCl solution for 16 hours under normal temperature and pressure conditions. After washing and drying, dry NaCl2 was obtained on the p-GO membrane.

[0088] Example 9

[0089] Unlike Example 1, the p-GO membrane was immersed in a 4M unsaturated NaCl solution for 16 hours under normal temperature and pressure conditions. After washing and drying, dry NaCl2 was obtained on the p-GO membrane.

[0090] Example 10

[0091] Dry NaCl2 was prepared on the p-GO membrane according to Examples 1-9. The NaCl2 can be separated from the membrane as follows: Step 1: Pretreatment. Before starting the peeling, the graphene oxide membrane is examined under a microscope to determine the location and distribution of crystals. Step 2: Wetting treatment. The graphene oxide membrane is gently wetted with ethanol. The purpose of this step is to reduce the interaction between the crystals and graphene oxide by utilizing the surface tension of the liquid. Step 3: Ultrasonic treatment. The wetted sample is placed in ethanol and treated in an ultrasonic bath for several minutes. Ultrasonic waves help relax the interface between the crystals and graphene oxide, making the crystals easier to peel off. Step 4: Mechanical peeling. The crystals are gently scraped off under a microscope using fine tweezers or a sharp needle. The integrity of the graphene oxide membrane should be maintained as much as possible. Transparent tape can be used to assist peeling; the tape is gently applied to the area to be peeled off and then slowly pulled up. Step 5: Cleaning. After peeling, the graphene oxide membrane is immersed in ethanol and gently shaken to remove any attached crystals or other impurities. Repeat the cleaning several times until all unwanted crystals are completely removed. Step Six: Drying. Allow the NaCl2 crystals to air dry naturally or use nitrogen to dry them in a dust-free environment. Step Seven: Inspection and Storage. Inspect the treated NaCl2 crystals using a microscope and store them in a dry, clean environment to avoid contamination.

[0092] Experimental Example 1

[0093] Energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy

[0094] The Na-Cl@p-GO films obtained in the above examples were analyzed using a JEOL JEM-F200(HR) TEM equipped with a JEOL 4k×4k CMOS camera at 200 kV using high-angle annular dark-field scanning projection (HAADF-STEM) atomic imaging technology and energy-dispersive X-ray spectroscopy (EDS). The results are as follows: Figure 12 As shown, Na and Cl elements are uniformly distributed on the p-GO film, with no obvious elemental segregation or phase separation observed, and the Na:Cl atomic ratio is approximately 1:2. Further X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Fisher ESCALAB Xi+ instrument. The XPS instrument parameters were set as follows: Al KAlpha source gun, ion energy of 3000 eV, current of 2.73 μA, grating size of 2.5 mm, and spot size of 650 μm. The XPS results confirmed that the Na:Cl atomic ratio is mainly concentrated around 0.5.

[0095] Experiment Example 2

[0096] Transmission electron microscope images

[0097] The Na-Cl@p-GO film obtained in the above examples was analyzed at room temperature using a FEI Talos F200C transmission electron microscope (TEM) operating at 200 kV. Conventional (non-low temperature) high-resolution transmission electron microscope images of the Na-Cl@p-GO film were obtained, allowing observation of the microstructure and atomic structure of the NaCl2 crystals grown on the p-GO film. Clear regions of the NaCl2 lattice were observed on the p-GO film. Figure 13 (See the top left image). The area marked by the red rectangle in the image has been further enlarged. Figure 13 (See the upper right image). The magnified electron diffraction pattern of the NaCl2 crystal region is shown below. Figure 13 As shown in the upper right corner of the upper right image, a clearer lattice image was obtained through Fourier transform and inverse fast Fourier transform to remove the background. Figure 13 (See the image below). The inverse fast Fourier transform image shows that the interlayer spacing of Na atoms in the NaCl2 crystal is approximately... Compared with the theoretically predicted interatomic spacing of Na Good fit ( Figure 14 (Where yellow represents Na and green represents Cl). Therefore, the experimentally measured lattice parameters are: These experimental results are consistent with the theoretical prediction of a bulk NaCl2 phase with C2 / m symmetry, proving that this bulk phase can be synthesized under room temperature conditions.

[0098] Experimental Example 3

[0099] Cryo-electron microscopy imaging

[0100] Because electron beams at room temperature cause severe damage to samples, we cannot quantitatively observe and verify the three-dimensional properties of crystals using tilting and multiple-exposure imaging methods. Therefore, it is necessary to use cryo-electron microscopy (cryo-EM) to rapidly freeze and preserve the NaCl2 crystal structure, achieving high-resolution three-dimensional crystal visualization. To this end, an ultrathin pGO film was prepared to perform three-dimensional crystal structure imaging under these low-damage and in-situ conditions. 2 μL of a 0.02 mg / mL dialyzed pGO suspension was uniformly dispersed on a porous carbon TEM mesh and deposited to prepare an ultrathin pGO film (thickness <10 nm, size >2000 μm). 2 The TEM grid consisted of orthogonally arranged circular holes with a diameter of ~1.2 μm, and the center-to-center distance between adjacent holes was ~2.5 μm. The TEM grid was heated in a vacuum environment at 343 K for two hours to dry it into a film. The ultrathin pGO film was immersed in a 3M NaCl solution for 12 hours to allow NaCl2 crystals to crystallize on the pGO film at room temperature. The sample in a humid state was rapidly frozen using a FEI Vitrobot, ensuring 100% humidity and room temperature throughout the process. After quickly blotting the solution from the sample (NaCl2@pGO film) surface with filter paper, the NaCl2@pGO film was rapidly frozen in liquid ethane. Subsequently, the frozen sample was transferred to liquid nitrogen for storage. Under liquid nitrogen conditions, the NaCl2@pGO film was placed on a Gatan 626cryo-TEM support, and the louvers were securely closed to ensure internal environmental isolation and prevent temperature changes, allowing for cryo-transmission electron microscopy analysis. During the transfer of the NaCl2@pGO film to the TEM column, it is crucial to maintain the film temperature below -170°C to preserve its pristine state and ensure accurate high-resolution imaging. Using a FEI F200C transmission electron microscope with a FEI CETA 4k×4k CMOS camera, cryo-electron microscopy images of the NaCl2 crystals on the pGO film were observed and captured in situ at -180°C, under low electron dose conditions of 200kV and 80kV. Selected area electron diffraction (SAED) images were obtained through a selected area aperture of approximately 350nm, with exposure times set between 0.5 and 2s, and chamber settings of 80kV (chamber 1) and 200kV (chamber 2). A schematic diagram of cryo-electron microscopy sample preparation is shown below.Figure 15 As shown. Using the above method, crystal planes with higher indices, such as (601) (as shown). Figure 16 As shown in the left figure), and when the sample was tilted at a small angle of 5°, the (301) crystal plane of NaCl2 was successfully observed (as shown in the left figure). Figure 16 As shown in the right figure, this electron diffraction pattern is similar to the theoretically predicted pattern. Figure 17 The results are very consistent, proving that bulk unconventional stoichiometric NaCl2 crystals did indeed grow on the pGO film.

[0101] Experiment Example 4

[0102] UV-Vis absorption spectrum and steady-state photoluminescence spectrum

[0103] The NaCl2@p-GO film was optically characterized using an Agilent CARY 5000 UV-Vis-NIR spectrometer. Figure 18 As shown, the UV-Vis-IR absorption spectrum of the bulk NaCl2 crystal and the Tauss fitting results together indicate that the optical bandgap of the NaCl2 crystal is 4.27 eV, which is very close to the theoretical value (4.22 eV). The NaCl crystal, pGO film, and NaCl2@p-GO film were tested using a steady-state / transient fluorescence spectrometer FLS1000 in a vacuum environment at 300 K. Figure 19 As shown, the steady-state photoluminescence (PL) spectroscopy results of the NaCl2@p-GO film reveal a significant PL peak at 2.97 eV (418 nm), which is smaller than the theoretically calculated band gap (4.22 eV) based on the HSE06 functional. The PL peak at 418 nm is likely due to charge transfer between the NaCl2 crystal and the p-GO film. When the sample temperature is lowered to 4 K, the PL peak intensity of the NaCl2@pGO film only increases slightly and fully recovers upon heating to 300 K, indicating that the optical properties of the NaCl2@p-GO film are highly stable across the entire temperature range of 4–300 K. These results suggest the potential application of bulk NaCl2 in far-ultraviolet photodetectors.

[0104] Experimental Example 5

[0105] Resistance changes with temperature

[0106] The transport properties of the NaCl2@p-GO film were tested using a PPMS-9 instrument with a standard four-probe method. Temperature-dependent resistivity results for bulk NaCl2 crystals ( Figure 20 The results show that the surface resistivity of the thin-film sample decreases with increasing temperature, exhibiting the electrical properties of a semiconductor. It should be noted that the surface resistivity of the p-GO film exceeds 100 MΩ. This result highlights the unique semiconductor inherent properties of bulk NaCl2 crystals.

Claims

1. A compound with unconventional stoichiometry, characterized in that, The chemical formula of the compound is NaCl2.

2. The compound according to claim 1, characterized in that, The compound has a monoclinic crystal structure and exhibits space group C2 / m symmetry.

3. The compound according to claim 1 or 2, characterized in that, The structure of the compound remains stable under normal temperature and pressure conditions.

4. The compound according to any one of claims 1-3, characterized in that, The cell parameters of the compound are: α=γ=90.00°, β=116.89°.

5. The compound according to claim 4, characterized in that, The cell parameters of the compound are: α=γ=90.00°, β=116.89°.

6. The compound according to any one of claims 1-5, characterized in that, The crystal structure of the compound comprises an atom-pair layer composed of Cl and an ionic layer composed of Na-Cl, wherein the atom-pair layer and the ionic layer are arranged longitudinally at intervals.

7. The compound according to claim 6, characterized in that, Each Cl atom in the ion layer is connected to 5 Na atoms; each Na atom in the ion layer is connected to 6 Cl atoms, wherein 5 Cl atoms come from the ion layer and 1 Cl atom comes from the atom pair layer.

8. The compound according to claim 6 or 7, characterized in that, Ionic bonds are formed between Na atoms in the ion layer and Cl atoms in the ion layer; ionic bonds are formed between Na atoms in the ion layer and Cl atoms in the atom pair layer. In the atom pair layer, a covalent bond is formed between a pair of Cl atoms.

9. The compound according to any one of claims 6-8, characterized in that, The atom-pair layer and the ion layer are grown layer by layer in the longitudinal direction to form a bulk crystal.

10. A method for preparing a compound according to any one of claims 1-9, characterized in that, The crystal structure of the compound comprises an atom-pair layer composed of Cl and an ionic layer composed of Na-Cl, wherein the atom-pair layer and the ionic layer are grown layer by layer in the longitudinal direction to form a bulk crystal.

11. The preparation method according to claim 10, characterized in that, The compound is grown on a nanomaterial substrate with a positive potential.

12. The preparation method according to claim 11, characterized in that, The positively charged nanomaterial substrate is selected from a positively charged graphene film.

13. The preparation method according to claim 11, characterized in that, The nanomaterial substrate with a positive potential is a polyethyleneimine-modified graphene oxide film.

14. The preparation method according to any one of claims 10-13, characterized in that, The compound was grown on a nanomaterial substrate under ambient temperature and pressure conditions.

15. The preparation method according to claim 13, characterized in that, The polyethyleneimine-modified graphene oxide membrane was immersed in an unsaturated NaCl solution, and after cleaning and drying, the NaCl2 compound was formed on the membrane.

16. The preparation method according to claim 15, characterized in that, The NaCl solution concentration is 1-4 M (mol / L), and the soaking time is 8-16 hours.

17. The preparation method according to claim 15 or 16, characterized in that, The soaking was carried out under normal temperature and pressure conditions.

18. The preparation method according to any one of claims 10-17, characterized in that, The compound can be separated from the substrate by mechanical exfoliation.

19. A semiconductor material, characterized in that, The semiconductor material comprises the compound as described in any one of claims 1-9.

20. The semiconductor material as claimed in claim 19, characterized in that, The semiconductor material has a band gap of 4.22-4.27 eV.

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