Potassium tungsten bronze nanosheet and preparation method thereof

Abstract

The application discloses a potassium tungsten bronze nanosheet and a preparation method thereof. A mixed powder containing a tungsten source is placed in a porcelain boat, then a fluorine crystal mica substrate is placed above the mixed powder containing the tungsten source, the distance between the mixed powder containing the tungsten source and the fluorine crystal mica substrate is controlled to be greater than 4 mm, then the porcelain boat is placed in a heating furnace, and a chemical vapor deposition reaction is carried out under a protective atmosphere, so that the potassium tungsten bronze nanosheet with a single crystal structure is obtained on the surface of the fluorine crystal mica substrate. The application solves the problem that the structure phase and the electrical characteristics are difficult to control by using a traditional method, has the advantages of low synthesis temperature, high product quality, strong phase selectivity and the like, and can be widely applied to the fields of micro-nano electronic devices and functional materials.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of two-dimensional transition metal oxide material preparation, and particularly relates to a potassium tungsten bronze nanosheet and a preparation method thereof. BACKGROUND

[0002] Two-dimensional materials have great potential in electronic devices, optoelectronic devices and energy applications due to their unique layered structure and tunable electronic, optical and magnetic properties. Potassium tungsten bronze (K x WO3) as a typical alkali tungsten bronze compound has excellent properties such as metal-semiconductor transition, superconductivity and electrochromism, and its structure phase is closely related to the doping concentration of potassium ions.

[0003] However, the research on K x WO3 two-dimensional nanosheet in the prior art is mainly limited to powder, nanowire or nanorod morphology, and it is difficult to accurately control the structure phase. This is mainly limited by the following factors: the high-temperature synthesis condition (about 1473℃) of WO3 leads to a complex process; the source of potassium ions and its diffusion distribution are difficult to control; and it is difficult to obtain high-quality single-crystal two-dimensional nanosheet at a lower temperature.

[0004] Therefore, it is urgent to propose a new method capable of realizing high-quality and phase-controllable growth of potassium tungsten bronze two-dimensional nanosheet at a lower temperature. SUMMARY

[0005] In view of the deficiencies of the prior art, a first purpose of the present application is to provide a preparation method of potassium tungsten bronze nanosheet.

[0006] A second purpose of the present application is to provide potassium tungsten bronze nanosheet prepared by the above preparation method.

[0007] In order to achieve the above purposes, the present application adopts the following technical solutions:

[0008] The preparation method of the potassium tungsten bronze nanosheet comprises the following steps: placing mixed powder containing a tungsten source in a porcelain boat, then placing a fluorine crystal mica substrate above the mixed powder containing the tungsten source, and controlling the distance between the mixed powder containing the tungsten source and the fluorine crystal mica substrate to be greater than 4mm, then placing it in a heating furnace, and performing a chemical vapor deposition reaction in a protective atmosphere to obtain a single-crystal structure potassium tungsten bronze nanosheet on the surface of the fluorine crystal mica substrate.

[0009] During the chemical vapor deposition reaction, the temperature of the mixed powder containing the tungsten source is 650-850℃.

[0010] The mixed powder containing the tungsten source is composed of WO3 powder, NaCl and amorphous carbon powder, or the mixed powder containing the tungsten source is composed of WO3 powder and NaCl.

[0011] The method of the present application uses NaCl, WO3 powder or WO3 powder, NaCl and amorphous carbon powder as precursors, and fluorine crystal mica substrate is placed on the precursor powder, when heated to 650-850℃, the precursor pyrolysis in situ generates WO 3-x , and the fluorine crystal mica substrate generates high-activity K + , because the potassium source and the tungsten source are very close in space, and K + is continuously generated in situ in the growth cavity, which provides optimal conditions for the embedding of potassium ions into the WO 3-x lattice being formed, potassium vapor uniformly diffuses to the growth substrate surface under the action of the carrier gas, and embeds into the interstitial sites of the tungsten-oxygen octahedral structure with atomic level precision, forming a stable K x WO3 compound, this "bottom-up" in-situ doping method avoids the problem of uneven potassium ion diffusion in the later stage of the traditional method, and ensures the uniformity of doping and the high quality of the sample.

[0012] Experiments found that the addition of NaCl powder is the key to obtaining nanosheet morphology, during the vapor deposition process, the temperature of the mixed powder containing tungsten source is controlled to be 650-850℃, at this temperature, the NaCl powder will volatilize (the melting point of NaCl is 801℃, the vapor pressure has increased significantly at 650℃), the volatilized NaCl vapor will react with the fluorine crystal mica substrate, the interlayer of fluorine crystal mica (chemical formula KAl2(Si3Al)O 10 (OH)2) contains K + ions. Na + in NaCl may undergo ion exchange reaction with K + in the interlayer of mica to generate volatile KCl. The chemical reaction may be: NaCl (g) + K + (from mica) → KCl (g) + Na + (may be embedded in mica), this process changes the non-volatile K + , which is originally "locked" in the mica lattice, into gaseous KCl molecules, promoting its free migration on the mica surface, high precursor concentration and high substrate temperature provide sufficient energy and raw materials for surface reaction and atomic migration, the atoms adsorbed on the mica surface have high enough mobility to find the position with the lowest energy, thereby tending to form a thermodynamically stable, two-dimensional planar structure (nanosheet) matching the substrate lattice, this two-dimensional epitaxial growth mode is the key to obtaining single-crystal nanosheets.

[0013] It is found that the chlorine (Cl) in NaCl can produce a "surface passivation" effect. For example, when NaCl reacts with WO3, chlorine-containing tungsten oxide compounds are produced. During the deposition process, the Cl atoms or ions in these chlorine-containing gas-phase species (especially those with W-Cl bonds) tend to preferentially adsorb on the K x specific crystal faces of the WO3 crystal. This selective adsorption significantly reduces the surface energy of these crystal faces and greatly hinders atomic deposition in this direction (i.e., vertical growth is inhibited). The lateral growth rate of the crystal is much higher than the vertical growth rate, resulting in the evolution of thermodynamically stable single-crystal nanosheets.

[0014] In addition, in the present application, the temperature of the mixed powder containing tungsten sources needs to be effectively controlled, because the process of reducing WO3 by carbon and the volatilization process of NaCl are both extremely sensitive to temperature. If the temperature is too high, the reaction rates will increase exponentially, causing the precursors to react rapidly and run out, resulting in insufficient continuous supply of precursors, which will stop the deposition process, leading to smaller nanosheet size, lower density, or even failure to form a continuous film. In addition, a very high concentration of gas-phase precursors will be produced at too high a temperature. When the partial pressure exceeds the saturated vapor pressure, homogeneous nucleation will occur, causing molecules in the gas phase to combine directly in the gas flow to form tiny tungsten oxide or potassium tungsten bronze particles. These particles will deposit in the form of powder on the substrate, forming rough, irregular polycrystalline particles or floccules instead of flat single-crystal nanosheets.

[0015] Preferably, when the mixed powder containing tungsten sources is composed of WO3 powder, NaCl, and amorphous carbon powder, the molar ratio of WO3 powder, NaCl, and amorphous carbon powder is 30-50:0.5-3:1-5.

[0016] In actual operation, the WO3 powder, NaCl, and amorphous carbon powder are combined and ground according to the designed molar ratio to obtain the mixed powder containing tungsten sources.

[0017] When the mixed powder contains WO3 powder, NaCl, and amorphous carbon powder, the amorphous carbon cooperates with K + in the reaction process to effectively control the value of x, thereby accurately "tailoring" the final crystal structure. In the presence of amorphous carbon, part of the K + is fixed in the K2CO3 crystal, resulting in a decrease in the effective K + concentration, and the formation of rectangular two-dimensional K 0.2~ 0.3 WO3 single-crystal nanosheets exhibit metallic properties. It is found that when amorphous carbon is present in the mixed powder, the amorphous carbon acts as a reducing agent to reduce WO3 to low-valence tungsten oxides with strong volatility. The chlorine-containing species (such as KCl and WO2Cl2) produced by NaCl will strongly and selectively adsorb on the Kx The selective adsorption will significantly reduce the surface energy of the adsorbed crystal plane, and greatly inhibit the deposition of atoms in these crystal directions, thus precisely "carving" out regular rectangular two-dimensional nanosheets with straight edges and sharp corners, like a "mold".

[0018] The molar ratio of WO3 powder, NaCl and amorphous carbon powder is controlled within the above range, and the final performance is optimal. When the amount of carbon powder is too small, the reducing power of WO3 is insufficient, resulting in insufficient concentration of tungsten source in the gas phase, and the growth of the product is limited. The product obtained on the mica substrate has low density and small size, and may even not be continuously covered. In addition, the morphology will also change. If the reducing power is insufficient, the morphology control effect of Cl (forming regular rectangles) will be weakened, and the product will tend to form irregular polygons. If the amount of carbon powder is too large, WO3 will be reduced too much. The extremely strong reducing environment may reduce WO3 to tungsten dioxide or even tungsten metal powder. These products are not easy to volatilize and will remain in the source area in solid form or be deposited on the substrate in the form of particles, resulting in impure product phase. If the amount of NaCl is too small, there is not enough NaCl vapor to react with mica and "carry out" K + in the form of KCl, resulting in a decrease in the potassium source participating in the reaction. The transport of tungsten is insufficient, and there is a lack of Cl source, making it difficult to form volatile tungsten oxychloride compounds, and the transport of tungsten mainly depends on the reduced oxides, which is less efficient. The overall result is that the concentration of the precursor before and after is low, resulting in sparse products on the substrate and small nanosheet size. If the amount of NaCl is too large, it will also cause the gas phase to become supersaturated, greatly increasing the probability of homogeneous nucleation in the gas flow, resulting in the generation of a large amount of powder-like by-products and deposition on the substrate. In addition, the extremely high Cl coverage may excessively inhibit the growth of all crystal planes, resulting in growth stagnation or the formation of small and thick particles rather than large and thin nanosheets.

[0019] In addition, it is found that only amorphous carbon powder can obtain rectangular two-dimensional K 0.2~0.3 WO3 single crystal nanosheets. If the amorphous carbon powder is replaced by graphite powder, the product obtained is almost the same as when the mixed powder of NaCl and WO3 is used as the reaction source, because the layered structure of graphite is composed of strong sp² hybridization covalent bonds and interlayer van der Waals forces, which is very stable. The flaky structure of graphite makes its specific surface area much smaller than that of amorphous carbon, and the contact area with the reactants is small. In addition, the reduction reaction of graphite requires a higher temperature to destroy its stable structure and start the reaction, while amorphous carbon powder has a large specific surface area and a rich microporous structure. There are a large number of unsaturated carbon atoms (dangling bonds) and structural defects in the disordered structure, which have high site energy and are extremely active and easily participate in the reduction reaction, providing a large contact area for the reactants and making the chemical reaction more likely to occur.

[0020] Preferably, when the mixed powder containing tungsten source is composed of WO3 powder and NaCl, the molar ratio of WO3 powder to NaCl is 30-50:0.5-3.

[0021] It is found in the experiment that, when there is no amorphous carbon, the interaction between NaCl vapor or molten NaCl and the surface of WO3 destroys the surface structure of WO3, thus reducing the activation energy of the decomposition of WO3, making WO3 decompose at a lower temperature or at a faster rate. Meanwhile, NaCl can react with WO3 or the substoichiometric oxide produced by the decomposition to form a very volatile tungsten oxychloride compound. However, when there is no carbon powder, the adsorption of Cl ions is reduced, the anisotropic growth inhibition is reduced, and the growth of the crystal is more dominated by its intrinsic anisotropy, thus showing more irregular polygons close to its natural growth habit.

[0022] In actual operation, the mixed powder containing tungsten source is placed in a porcelain boat, which is then placed in the center heating zone of a chemical vapor deposition furnace. A protective atmosphere is introduced into the chemical vapor deposition furnace, and the air in the tube is removed.

[0023] It is found in the experiment that, when the reaction is carried out in a protective atmosphere, the protective atmosphere acts as a protective gas and a carrier gas at the same time, reducing the influence of external environmental gases and being more conducive to the formation and growth of nanosheets.

[0024] Preferably, the fluorine crystal mica substrate has a thickness of 0.5-1.0 mm.

[0025] It is found in the experiment that the thickness of mica and the K + concentration (effective K + concentration participating in the reaction) are related. In the experiment, the K + concentration of the mica is 0.5-1.0 mm. + The reaction between NaCl vapor and the surface of mica (Na + + K + (mica) → K + (gas phase) + Na + (mica)) consumes the K + on the surface, thus forming a concentration gradient inside the mica, which is the driving force for the diffusion of K + from the inside to the surface. Therefore, only a mica sheet with a moderate thickness can provide a high and stable K + effective concentration. At the beginning of the reaction, it can maintain a high K + concentration on the surface, and the K +High flux and stability, which is conducive to the growth of large size, high quality nanosheet growth, if the mica sheet is too thin, in the early reaction, the surface K + will be quickly consumed, while the internal replenishment speed may not keep up with the consumption speed, leading to the rapid decline of surface K + concentration, which will make the growth process terminate prematurely due to "lack of food" in the later stage, however, if the thickness is too thick, the potassium supply is excessive and the growth is excessive. The thick mica provides almost unlimited K + source. High K + flux may cause the deposition rate to be too fast, and the nanosheet may not only grow laterally, but also grow longitudinally (i.e. thickening), leading to the formation of thick and irregular sheet structure, and even changing from two-dimensional growth to three-dimensional island growth, losing the characteristics of ultra-thin nanosheet.

[0026] In a preferred embodiment, the distance between the mixed powder containing tungsten source and the fluorine crystal mica substrate is 5-8 mm. It is found through experiments that the performance is optimal when the distance between the mixed powder containing tungsten source and the fluorine crystal mica substrate is controlled within this range. Firstly, sufficient concentration is provided, and the distance is close enough to ensure that a higher precursor flux reaches the substrate, providing sufficient raw materials for two-dimensional epitaxial growth. Secondly, homogeneous nucleation can be avoided. This distance provides high concentration and leaves a sufficient diffusion path for precursor molecules to reach the substrate surface in an orderly manner for heterogeneous nucleation and epitaxial growth, rather than randomly gathering in the gas flow. Thirdly, the temperature is uniform. This distance ensures that the source and the substrate are both in the flattest constant temperature zone of the furnace, and the temperature field is uniform and stable. If the distance is too close, the supersaturation of the precursor is too high, leading to rapid and uneven growth, and it is easy to cause the gas phase supersaturation to exceed the critical value of homogeneous nucleation, thereby forming small crystal nuclei of tungsten oxide or KWO spontaneously in the gas flow before reaching the substrate, ultimately forming a collection of polycrystalline particles, and clean and flat single-crystal nanosheets cannot be obtained. If the distance is too far, the precursor concentration is too diluted, leading to insufficient growth driving force, and only sparse and small nanocrystals or no deposition on the mica sheet, and the nanosheet cannot grow large, and the K doping is insufficient.

[0027] In a preferred embodiment, a groove is provided on the upper edge of the side wall of the porcelain boat, and the mixed powder containing tungsten source is placed in the porcelain boat, and then the fluorine crystal mica substrate is inverted and placed on the groove.

[0028] In the present application, by inverting the fluorine crystal mica substrate on the groove of the porcelain boat, a local, quasi-closed micro-reaction cavity is formed. This cavity greatly limits the diffusion path of the precursor vapor, which has two advantages: firstly, very high local concentration. The volatile tungsten species (W x O Y, WO2Cl2, etc.) and potassium species (KCl) are confined in this small cavity and cannot escape quickly. This leads to very high vapor pressure and local concentration of precursors in the cavity. Second, controlled atmosphere. The slow flow rate of carrier gas in this cavity forms a relatively stagnant atmosphere, which further facilitates the accumulation of precursors and reduces the exchange with the external environment.

[0029] In a preferred embodiment, the temperature of the fluoromica mica substrate is 400-500 °C. It is found that the fluoromica mica substrate has the optimal surface mobility when the temperature is in this range. At this temperature, the adsorbed atoms have enough energy to diffuse on the surface, move to the correct position in the lattice, and incorporate into the existing crystal nucleus edge, rather than forming new crystal nuclei, which promotes the lateral growth of the crystal and facilitates the formation of large-area, single-crystal nanosheets. If the substrate temperature is too high, the gas-phase precursors (such as KCl, WO2Cl2, etc.) and potassium species (KCl) are confined in this small cavity and cannot escape quickly. This leads to very high vapor pressure and local concentration of precursors in the cavity. Second, controlled atmosphere. The slow flow rate of carrier gas in this cavity forms a relatively stagnant atmosphere, which further facilitates the accumulation of precursors and reduces the exchange with the external environment. x O Y ) molecules gain too much energy and decompose directly or re-evaporate from the surface before they can react on the surface. The deposition rate drops sharply, causing the nanosheet growth to slow down or even stop, resulting in sparse and small-sized final products. In addition, it can also exacerbate the K + The extraction process from the mica body causes the mica structure to be destroyed, thus failing to provide a good template for epitaxial growth, and if the temperature is too low, the atomic mobility is insufficient, the growth is limited by kinetics, the atoms adsorbed on the substrate surface do not have enough energy to diffuse over long distances, and cannot find the lowest-energy lattice position, resulting in a large number of crystal nuclei forming randomly, but each crystal nucleus does not grow much. The final product is a polycrystalline thin film composed of countless small, randomly oriented nanocrystals, rather than a large-area single-crystal nanosheet.

[0030] In a preferred embodiment, the time of chemical vapor deposition is 20-40 min. Controlling the time of chemical vapor deposition within this range can obtain large-size, high-crystallinity nanosheets, and if the time is too short, only sparse, small nanosheets can be formed on the mica substrate, and if the time is too long, the growth enters an unnecessary overgrowth stage, destroying the two-dimensional morphology and crystal quality. On the one hand, the ultra-thin two-dimensional nanosheet gradually becomes a thick three-dimensional sheet crystal, or even stacks into multiple layers, on the other hand, it is also possible to perform secondary nucleation on the top of the already grown nanosheet or in the blank area between the nanosheets, forming new small grains with different orientations, resulting in a rough surface, and the final product may be a polycrystalline thin film rather than a single-crystal array.

[0031] In actual operation, after the reaction is completed, the heating is stopped, and the substrate is naturally cooled to room temperature under the protection of gas flow, thereby obtaining potassium tungsten bronze (K x WO3) two-dimensional nanosheets of different structural phases on the substrate.

[0032] Preferably, when the mixed powder containing tungsten source is composed of WO3 powder, NaCl and amorphous carbon powder, the chemical structural formula of the potassium tungsten bronze nanosheet is K x WO3, wherein x is 0.2-0.3;

[0033] When the mixed powder containing tungsten source is composed of WO3 powder and NaCl, the chemical structural formula of the potassium tungsten bronze nanosheet is K x WO3, wherein x is 0.31-0.35.

[0034] The application also provides the potassium tungsten bronze nanosheet prepared by the preparation method.

[0035] The potassium tungsten bronze nanosheet is of single crystal structure, and the thickness of the potassium tungsten bronze nanosheet is 3-30 nm.

[0036] Preferably, the structural formula of the potassium tungsten bronze nanosheet is K x WO3, wherein x is 0.2-0.3, and the potassium tungsten bronze nanosheet is rectangular and has metallicity.

[0037] or

[0038] The structural formula of the potassium tungsten bronze nanosheet is K x WO3, wherein x is 0.31-0.35, and the potassium tungsten bronze nanosheet is polygonal and has semiconductivity.

[0039] In the application, when there is amorphous carbon powder, part of K + is fixed in K2CO3 crystal, resulting in a low effective K + concentration, and rectangular two-dimensional K 0.2~0.3 WO3 single crystal nanosheet is generated, wherein the aspect ratio is about 10:1, the single crystal nanosheet under the structural formula exhibits metallicity, and the room temperature conductivity reaches 3.0×10 5 S / m, and exhibits metal-semiconductor phase transition. Since the proportion of K + ions is low, the average distance between electrons is far, which makes the effect of Coulomb repulsion more significant, and localization is more likely to occur. Therefore, the surface exhibits excellent metallicity at room temperature, but as the temperature decreases, the electron thermal energy decreases, the wave function of the electron changes from an extended state to a localized state, thereby causing the conductivity to disappear, and the material gradually changes into a semiconductor, and even into an insulator when the temperature continues to decrease.

[0040] Without the carbon source, polygonal K 0.31~0.35 WO3 nanosheet is obtained, which has semiconductivity at room temperature, and the field effect transistor switching ratio can reach 10 5 -10 6 .

[0041] Principles and Advantages

[0042] The method of the present application uses NaCl, WO3 powder or WO3 powder, NaCl and amorphous carbon powder as precursors, and fluorine crystal mica substrate is placed on the top of the precursor powder. When heated to 650-850℃, the precursor pyrolysis generates in-situ high reactivity WO 3-x , and the fluorine crystal mica substrate generates high activity K + . Since the potassium source is very close to the tungsten source in space, and K + is generated in-situ and continuously in the growth cavity, which provides optimal conditions for the embedding of potassium ions into the WO 3-x lattice being formed. Potassium vapor uniformly diffuses to the growth substrate surface under the action of the carrier gas, and embeds into the interstitial sites of the tungsten-oxygen octahedral structure with atomic level precision, forming a stable K x WO3 compound. This "bottom-up" in-situ doping method avoids the problem of uneven late-stage diffusion of potassium ions in traditional methods, ensuring the uniformity of doping and the high quality of the sample.

[0043] The specific mechanism of action can be divided into the following aspects:

[0044] Low-temperature reduction mechanism: traditional WO3 reduction requires extremely high temperature (~1473℃) to overcome the reaction energy barrier. The present application uses NaCl and amorphous carbon mixed with WO3 powder as precursors, which will pyrolyze in-situ to generate high reactivity WO 3-x during heating, significantly reducing the activation energy required for the reduction reaction, thereby successfully reducing WO3 to a sub-stoichiometric tungsten oxide (WO 3-x ) skeleton that can be used to form tungsten bronze at a lower temperature.

[0045] In-situ and uniform doping mechanism: the other product of the above reaction is high activity K + from the fluorine crystal mica substrate. Since the potassium source is very close to the tungsten source in space, and K + is generated in-situ and continuously in the growth cavity, which provides optimal conditions for the embedding of potassium ions into the WO 3-x lattice being formed. Potassium vapor uniformly diffuses to the growth substrate surface under the action of the carrier gas, and embeds into the interstitial sites of the tungsten-oxygen octahedral structure with atomic level precision, forming a stable K x WO3 compound. This "bottom-up" in-situ doping method avoids the problem of uneven late-stage diffusion of potassium ions in traditional methods, ensuring the uniformity of doping and the high quality of the sample.

[0046] Controllable structure phase mechanism: the structure phase of potassium tungsten bronze is directly determined by the concentration of embedded potassium ions (x value). The present application precisely controls the non-crystalline carbon in the reaction process by precisely controlling the non-crystalline carbon in the reaction process + , realizes effective control of x value, and realizes precise "cutting" of the final crystal structure. Under the condition of non-crystalline carbon, part of K + is fixed in K2CO3 crystal, resulting in a decrease in the effective K + concentration, generating rectangular two-dimensional K 0.3 WO3 single crystal nanosheet, showing metallic properties; in the absence of non-crystalline carbon, K + concentration is higher, generating polygonal two-dimensional K 0.33 WO3 single crystal nanosheet, showing semiconductor properties.

[0047] By optimizing the combination of the above parameters, K x WO3 two-dimensional nanosheet with a specific structure can be targetedly grown.

[0048] Compared with the prior art, the method provided by the present application has the following significant advantages and positive effects:

[0049] Significant reduction in growth temperature: the synthesis temperature of K x WO3 is reduced from ~1473℃ of the traditional method to 650℃, greatly reducing the requirements for equipment and energy consumption, reducing the volatilization, pollution and defects caused by high temperature, and making the process safer, more economical and easier to implement.

[0050] Realize precise control of structure phase: for the first time, through a simple CVD method, selective growth of different crystal structure phases of K x WO3 two-dimensional material is realized. This provides a reliable solution to the long-standing phase control problem in the research of this material, and lays a material foundation for the research of its phase-dependent properties (such as metal-insulator transition, electrochromic performance) and application development.

[0051] High material quality and excellent morphology: the method can obtain single-crystal two-dimensional nanosheet with smooth surface, perfect crystallinity, uniform thickness (nanoscale), and large lateral size (hundreds of microns). Such high-quality materials are ideal choices for constructing high-performance micro / nano electronic and optoelectronic devices.

[0052] Simple process, good repeatability and strong scalability: fluorine crystal mica substrate is used to provide potassium source and substrate function at the same time, simplifying the design of the source and the configuration of the reaction chamber, avoiding the instability problem caused by multi-source co-evaporation, and greatly improving the repeatability and reliability of the process. The principle has the possibility of being extended to other alkali metal tungsten bronze systems, providing a new idea for the preparation of similar materials. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 The two-dimensional K x Schematic diagram of controllable growth of WO3 nanosheets.

[0054] Figure 2 The two-dimensional K x Typical morphology optical picture of WO3 nanosheets; wherein Figure 2 a in the formula is K 0.33 Optical picture of WO3 nanosheets, Figure 2 b in the formula is K 0.3 Optical picture of WO3.

[0055] Figure 3 The two-dimensional K 0.33 TEM and SAED characterization results of WO3 nanosheets; wherein Figure 3 a in the formula is K 0.33 Low-magnification TEM planar morphology picture of WO3 nanosheets, Figure 3 b in the formula is high-angle annular dark field scanning TEM (HAADF-STEM) picture, Figure 3 c in the formula is two-dimensional K 0.33 SAED picture of WO3 nanosheets.

[0056] Figure 4 The two-dimensional K 0.3 TEM and SAED characterization results of WO3 nanosheets; wherein Figure 4 a in the formula is K 0.3 Low-magnification TEM planar morphology picture of WO3 nanosheets, Figure 4 b in the formula is high-angle annular dark field scanning TEM (HAADF-STEM) picture, Figure 4 c in the formula is two-dimensional K 0.3 SAED picture of WO3 nanosheets.

[0057] Figure 5 K 0.3 WO3 nanosheets prepared in Example 1 and K 0.33 Electrical property test curve of WO3 nanosheets, wherein Figure 5 a in the formula is K 0.33 Output characteristic curve of a field effect transistor (FET) device made of WO3 nanosheets at room temperature when the gate voltage is modulated from -60 V to 60 V; Figure 5 b in the formula is K 0.33 Transfer characteristic curve of a field effect transistor (FET) device made of WO3 nanosheets, wherein Figure 5 c in the formula is K0.3 The output characteristic curve of a field effect transistor (FET) device made of the WO3 nanosheets at room temperature when the gate voltage is modulated from -60 V to 60 V; Figure 5 d in the formula is K 0.3 The transfer characteristic curve of a field effect transistor (FET) device made of the WO3 nanosheets.

[0058] Figure 6 The morphology of the product prepared in Comparative Example 1.

[0059] Figure 7 The substrate after the reaction in Comparative Example 2. DETAILED DESCRIPTION

[0060] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to one skilled in the art that the present application can be practiced without the specific details set forth in this description. In other instances, well-known methods have not been described in detail in order to avoid obscuring the present application.

[0061] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for describing particular embodiments only and is not intended to be limiting of the application.

[0062] Example 1

[0063] WO3 powder, NaCl, amorphous carbon were mixed in a molar ratio of 35:1:3.5 and ground to obtain a mixed powder. The mixed powder was placed in a porcelain boat, the upper edge of the side wall of the porcelain boat was provided with a groove, after the mixed powder was placed in the porcelain boat, a fluorine crystal mica substrate was inverted and placed on the groove, and the distance between the mixed powder and the fluorine crystal mica substrate (thickness of 0.6 mm) was controlled to be 5 mm. Then the porcelain boat was placed in the center heating zone of a CVD furnace, Ar was introduced as a carrier gas, the temperature of the mixed powder was controlled to be 650℃, at this time the temperature of the fluorine crystal mica substrate was 500℃, and chemical vapor deposition was carried out for 30 min, and then the porcelain boat was cooled, thereby obtaining deposited K x WO3 nanosheets.

[0064] Example 2:

[0065] The WO3 powder, NaCl are mixed in a molar ratio of 35:1 and ground to obtain a mixed powder, the mixed powder is placed in a ceramic boat, a groove is provided on the upper edge of the side wall of the ceramic boat, after the mixed powder is placed in the ceramic boat, a fluorine crystal mica substrate is inverted on the groove, and the distance between the mixed powder and the fluorine crystal mica substrate (thickness of 0.6 mm) is controlled to be 5 mm, then the ceramic boat is placed in the center heating zone of a CVD furnace, Ar is introduced as a carrier gas, the temperature of the mixed powder is controlled to be 650 DEG C, at this time the temperature of the fluorine crystal mica substrate is 500 DEG C, and chemical vapor deposition is carried out for 30 min, and then the ceramic boat is cooled, thereby obtaining the deposited K x WO3 nanosheet.

[0066] Comparative Example 1

[0067] The other conditions are the same as in Example 1, except that KCl particles are mixed with WO3 powder, NaCl and amorphous carbon to obtain a mixed powder, and then a SiO2 / Si substrate is used; and the product is obtained.

[0068] Comparative Example 2

[0069] The other conditions are the same as in Example 1, except that NaCl is not added.

[0070] Comparative Example 3

[0071] The other conditions are the same as in Example 1, except that amorphous carbon is replaced by graphite powder.

[0072] Performance test and characterization

[0073] First, the chemical composition of the samples of Example 1 and Example 2 is analyzed by X-ray energy dispersive spectroscopy (EDS), and the K / W atomic ratio is calculated to be about 0.30 and 0.33 respectively, and further combined with X-ray diffraction (XRD) characterization, the results of the two are consistent, indicating that the nanosheet obtained in Example 1 and Example 2 is a single-crystal hexagonal phase potassium tungsten bronze, and the chemical formula is K 0.3 WO3 and K 0.33 WO3.

[0074] Figure 2 The two-dimensional K x WO3 nanosheet morphology diagram obtained in Example 1 and Example 2 under the condition of amorphous carbon and without amorphous carbon assistance is shown. Figure 2 The optical picture of K 0.33 WO3 obtained in Example 2 without amorphous carbon assistance is shown in a, from which it can be clearly seen that the nanosheet morphology is polygonal, and the longest side length of the polygon is up to hundreds of microns, and the thickness of the nanosheet is 3-20 nm. Figure 2 The K 0.3Optical image of WO3, showing regular rectangular nanosheet morphology, from which K 0.3 The width of the WO3 nanosheet is about 15 μm, the length is about 120 μm, and the thickness of the nanosheet is 6-30 nm.

[0075] Figure 3 Two-dimensional K 0.33 TEM and SAED characterization results of the WO3 nanosheet; wherein Figure 3 a in the formula is K 0.33 Low-magnification TEM planar morphology image of the WO3 nanosheet, showing a clear two-dimensional surface morphology of the nanosheet, Figure 3 b in the formula is a high-angle annular dark-field scanning TEM (HAADF-STEM) image, showing clear and bright lattice fringes, further indicating that the prepared K 0.33 The WO3 nanosheet has a very high crystalline quality, and the measured interlattice spacing is 0.38 nm, which matches the interlattice spacing of the hexagonal K x The (002) crystal face of the WO3 perfectly matches, Figure 3 c in the formula is two-dimensional K 0.33 SAED image of the WO3 nanosheet, showing only one set of clear and sharp diffraction spots, which can be well indexed to the (002) crystal face, proving the single-crystal structure thereof.

[0076] Figure 4 Two-dimensional K 0.3 TEM and SAED characterization results of the WO3 nanosheet; wherein Figure 4 a in the formula is K 0.3 Low-magnification TEM planar morphology image of the WO3 nanosheet, from which it can be seen that the K 0.3 The WO3 nanosheet exhibits complete rectangular morphology and uniform color, indicating that the K 0.3 The WO3 nanosheet has a uniform thickness and a relatively high crystalline quality. Figure 4 b in the formula is a high-angle annular dark-field scanning TEM (HAADF-STEM) image, showing clear and bright lattice fringes, further indicating that the prepared K 0.3 The WO3 nanosheet has a very high crystalline quality. The measured interlattice spacings of two groups of crystal faces are 0.37 and 0.38 nm, respectively, which match the interlattice spacings of the hexagonal K 0.3 The interlattice spacings of the (110) and (002) crystal faces of the WO3 match; Figure 4 c in the formula is two-dimensional K 0.3 SAED image of the WO3 nanosheet, showing only one set of clear and sharp diffraction spots, which can be well indexed to the (110) and (002) crystal faces, proving the single-crystal structure thereof.

[0077] Figure 5 K 0.3 WO3nanosheets and K 0.33 Electrical property test curve of WO3nanosheets.

[0078] Regarding Figure 5 In the test, first, K x WO3nanosheets on mica substrate were transferred to SiO2 / Si substrate by wet transfer method, then micro-nano processing technology was used to define electrode pattern, electron beam exposure technology, and finally metal deposition process was used to obtain K x WO3nanosheets based field effect transistor (FET) device, and then electronic transmission performance test was carried out in vacuum environment by using probe station and semiconductor analyzer.

[0079] Figure 5 a-b in the figure shows the electronic transmission property of field effect transistor (FET) device made of single K 0.33 WO3nanosheets without carbon assistance. In the figure Figure 5 a is the output characteristic curve of the device prepared in example 2 at room temperature when the gate voltage is modulated from -60 V to 60 V, the linear I DS -V DS curve shows that the metal electrode forms good ohmic contact with K 0.33 WO3nanosheets. The on-current of the device increases with the increase of back gate bias, which shows good regulation effect with gate voltage, and the inset is the optical microscope picture of the device.

[0080] Figure 5 b is the transfer characteristic curve of the device, and it can be seen that the current gradually increases with the increase of voltage, which shows that the current is sensitive to the back gate regulation, presents n-type semiconductor conduction behavior, and I ON / I OFF is about 10 6 , and the sample device presents semiconductor conduction behavior.

[0081] In the figure Figure 5 c is the output characteristic curve of the field effect transistor (FET) device made of K 0.3 WO3nanosheets at room temperature when the gate voltage is modulated from -60 V to 60 V; when V BG is changed from -60 V to 60 V, I DS has little change, and the linear I DS -V DS curve shows that the metal electrode forms ideal ohmic contact with K 0.3 WO3nanosheets. The inset is the optical microscope picture of the device.

[0082] Figure 5 d is K 0.3 Transfer characteristic curves of a field-effect transistor (FET) device fabricated from WO3 nanosheets. Based on K 0.30 Current versus back gate voltage V in WO3 nanosheet devices BG Insensitivity (from -60 to +60 V) indicates the conductive behavior of the metal.

[0083] Figure 6 As shown, when KCl source is used as K + When the source was obtained, a number of nanorod-shaped materials with a length of about 10 μm were finally deposited on the SiO2 / Si substrate. This is because K in KCl is difficult to detach and dop into the WO3 structure (the binding energy of K is ~294 eV), and alkali metal salts are difficult to use as dopants under normal conditions.

[0084] Figure 7 As can be seen, almost no products are visible on the mica substrate because without the addition of NaCl, the evaporation melting point of WO3 cannot be reached at this temperature, and a chemical reaction cannot occur.

[0085] Furthermore, testing showed that the product obtained in Comparative Example 3 was consistent with that in Example 2.

[0086] The above-described embodiments are merely one example of the implementation of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this invention should be determined by the appended claims.

Claims

1. A method for preparing potassium tungsten bronze nanosheets, characterized in that: The mixed powder containing a tungsten source is placed in a porcelain boat, then a fluorine crystal mica substrate is placed above the mixed powder containing a tungsten source, and the distance between the mixed powder containing a tungsten source and the fluorine crystal mica substrate is controlled to be greater than 4 mm, then the porcelain boat is placed in a heating furnace, and a chemical vapor deposition reaction is carried out under a protective atmosphere to obtain potassium tungsten bronze nanosheets with a single crystal structure on the surface of the fluorine crystal mica substrate; During the chemical vapor deposition reaction, the temperature of the mixed powder containing a tungsten source is 650-850℃; The mixed powder containing a tungsten source is composed of WO3 powder, NaCl and amorphous carbon powder, or the mixed powder containing a tungsten source is composed of WO3 powder and NaCl; When the mixed powder containing a tungsten source is composed of WO3 powder, NaCl and amorphous carbon powder, the molar ratio of WO3 powder, NaCl and amorphous carbon powder is 30-50:0.5-3:1-5; When the mixed powder containing a tungsten source is composed of WO3 powder and NaCl, the molar ratio of WO3 powder and NaCl is 30-50:0.5-3; The thickness of the fluorine crystal mica substrate is 0.5-1mm; The distance between the mixed powder containing a tungsten source and the fluorine crystal mica substrate is 5-8mm.

2. The method for preparing potassium tungsten bronze nanosheets according to claim 1, characterized in that: The upper edge of the side wall of the porcelain boat is provided with a groove, and after the mixed powder containing a tungsten source is placed in the porcelain boat, the fluorine crystal mica substrate is inverted and placed on the groove.

3. The method for preparing potassium tungsten bronze nanosheets according to claim 1, characterized in that: The temperature of the fluorine crystal mica substrate is 400-500℃.

4. The method for preparing potassium tungsten bronze nanosheets according to claim 1, characterized in that: The time of the chemical vapor deposition reaction is 20-40min.

5. The method for preparing potassium tungsten bronze nanosheets according to claim 1, characterized in that: When the mixed powder containing a tungsten source is composed of WO3 powder, NaCl, and amorphous carbon powder, the chemical structural formula of the potassium tungsten bronze nanosheet is K x WO3, wherein x is 0.2-0.3, When the mixed powder containing tungsten source is composed of WO3powder, NaCl, the chemical structural formula of the potassium tungsten bronze nanosheet is K x WO3, wherein x is 0.31~0.

35.

6. The potassium tungsten bronze nanosheet prepared by the method of any one of claims 1-5, characterized in that: The potassium tungsten bronze nanosheets have a single crystal structure, and the thickness of the potassium tungsten bronze nanosheets is 3-30nm.

7. The potassium tungsten bronze nanosheets prepared by the method of claim 6, wherein: The potassium tungsten bronze nanosheet has a structural formula of K x WO3, wherein x is 0.2-0.3, and the potassium tungsten bronze nanosheet is rectangular and has metallic properties; or The potassium tungsten bronze nanosheet has a structural formula of K x WO3, wherein x is 0.31-0.35, and the potassium tungsten bronze nanosheet is polygonal and has semiconductor properties.

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