KR102453020B1 - Layered compounds and nanosheets containing Zinc and Sulfur, and electrical devices using the same - Google Patents

Abstract

An object of the present invention is to provide a layered compound containing zinc and sulfur, a nanosheet that can be made through the same, and an electrical device including the materials. In the present invention to achieve the above object, in the present invention [Formula 1] M _2-x Zn _y S _z (M is at least one type of group 1 element, 0<x<2.0, 1.5≤y≤2.25, 2.75≤z It is possible to provide a layered structure compound represented by ≤3.25).

Description

Layered compounds and nanosheets containing zinc and sulfur, and electrical devices using the same

The present invention relates to a layered compound and nanosheet containing zinc and sulfur, and an electric device using the same, and more particularly, to a layered compound and nanosheet containing zinc and sulfur containing an alkali metal and having various electrical properties And it relates to an electric device using the same.

Layered compounds connected to interlayers through van der Waals bonds can exhibit various properties, and two-dimensional (2D) nanosheets with a thickness of several nanometers to hundreds of nanometers are manufactured by separating them by physical or chemical methods. Research on this is active.

In particular, low-dimensional materials such as nanosheets are expected to have innovative new functions that existing bulk materials do not have, and have great potential as next-generation future materials to replace existing materials.

However, layered structure compounds having a two-dimensional crystal structure have been limited to materials such as graphite or transition metal chalcogen compounds so far, and there is a problem in that they do not develop into materials of various compositions.

On the other hand, zinc sulfide is a compound semiconductor material, and because it has various basic properties and unique optical and chemical applicability, it can be used in light emitting diodes, electroluminescence, infrared windows, sensors, Although it is being used in lasers, biomaterials, and bio devices, there is no known well-known ternary zinc sulfide having a layered structure so far.

The ternary zinc sulfide compound having a layered structure can be applied more diversified than the existing zinc sulfide compound having a different crystal structure, and can also be applied to new areas that have not been applied before.

An object of the present invention is to provide a layered compound containing sulfur and zinc, a nanosheet that can be made through the same, and an electric device including the materials.

In the present invention to achieve the above object, in the present invention [Formula 1] M _2-x Zn _y S _z (M is at least one type of group 1 element, 0<x<2.0, 1.5≤y≤2.25, 2.75≤z It is possible to provide a layered structure compound represented by ≤3.25).

In addition, in the present invention, the [Formula 1] M _2-x Zn _y S _z (M is at least one type of group 1 element, 0<x<2.0, 1.5≤y≤2.25, 2.75≤z≤3.25) It is possible to provide a nanosheet made by a physical or chemical exfoliation method, including a composition to be used.

In addition, in the present invention, it is possible to provide an electric device including the layered structure compound or nanosheet as described above.

In addition, the electric device may be a memristor.

The layered structure compounds and nanosheets that can be provided through the present invention may have various electrical properties, thereby enabling the development of new electrical devices.

1 is a conceptual diagram of a layered structure compound and a nanosheet made according to the present invention.
2 is a graph showing the XRD diffraction pattern for the layered structure compound according to the present invention.
Figure 3 shows the STEM (Scanning Transmission Electron Microscope) analysis results for the layered structure compound according to the present invention.
4 shows a scanning electron microscope (SEM) image of the layered structure compound according to the present invention and a transmission electron microscope (TEM) image of the nanosheet.
5 shows an atomic force microscopy (AFM) image of the nanosheet according to the present invention and a line profile thereof.
6 is an XRD analysis result of the layered structure compound according to the present invention.
7 shows the results of STEM (Scanning Transmission Electron Microscope) analysis of the layered structure compound according to the present invention.
8 shows a scanning transmission electron microscope (STEM) analysis result and an SEM image of the layered structure compound according to the present invention.
9 is an XRD analysis result of the layered structure compound according to the present invention.
10 is a ferroelectric property evaluation result through PFM (Piezoresponse Force Microscopy) of the nanosheet according to the present invention.

Hereinafter, the configuration and operation of the embodiment of the present invention will be described with reference to the accompanying drawings. In the following description of the present invention, if it is determined that a detailed description of a related well-known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. In addition, when a part 'includes' a certain component, this means that other components may be further included, rather than excluding other components, unless otherwise stated.

The layered structure compound or nanosheet according to the present invention may be represented by the following formula (1).

[Formula 1] M _2-x Zn _y S _z

(M is at least one type of group 1 element, 0<x<2.0, 1.5≤y≤2.25, 2.75≤z≤3.25)

In general, ZnS has a Wurzite or Zinc Blende crystal structure, and a layered structure cannot appear, so it is impossible to make a nanosheet by exfoliating it.

In order to overcome this, the inventors placed an additive element _between Zn _y S _z layers by adding a Group 1 element (hereinafter referred to as “additive element”) to Zn _y S _{z .} was able to make The additive elements positioned between these Zn _y S _z layers weakly couple the Zn _y S _z layers through van der Waals bonds, so the plane on which these additional elements are positioned forms a cleaved plane that is easily split along this plane. .

Accordingly, the layered compound according to the present invention can be easily exfoliated through either or both physical or chemical methods into a Zn _y S _z layer along this cleavage plane. This exfoliation becomes easier as the additive element is removed is done Therefore, Zn _y S _z nanosheets can be easily prepared from such a layered compound by a physical or chemical exfoliation method, where some additional elements may remain in the Zn _y S _z nanosheets.

If the additive element is continuously removed, the distance between the Zn _y S _z layers in the compound gradually widens, weakening the bonding force between the layers, and eventually the bonding between the layers disappears, which may result in cracks between the layers. Therefore, in the layered structure of the layered compound described in the present invention, the bonding force between the Zn _y S _z layers is eliminated as well as when the repeated two-dimensional Zn _y S _z layers are bonded to each other through van der Waals bonds by an additive element. This includes the case where the distance between the layers becomes wider and cracks appear. As the additive element is removed and the bond between the layers is weakened, peeling of the nanosheet can be made more easily.

The nanosheet made by exfoliating from such a layered compound may be a Zn _y S _z single layer, but may be several hundred nm thick because it may be made by overlapping a plurality of layers. In general, nanosheets can exhibit anisotropy according to a two-dimensional shape only when the thickness is less than a certain level in relation to the width in the transverse direction. is preferably 0.1 or less. Since the width of the nanosheet made through the present invention can be 5 μm or more, the thickness of the nanosheet is preferably 500 nm or less. Here, some additional elements may remain in the Zn _y S _z nanosheet.

As such, the nanosheet according to the present invention refers to a sheet that is peeled off by a physical or chemical method from a layered compound, and includes a case in which the Zn _y S _z layer is not only a single layer but also a case made of a plurality of layers.

A conceptual diagram of an example of such a layered compound and nanosheet is shown in FIG. 1 , in which an additive element 11 is located between the Zn _y S _z layer 10 of M ₂ Zn _y S _z and the Zn _y S _z layer ( 10) shows that the bond is maintained between the layers, where as the additive element 11 is removed, it becomes M _2-x Zn _y S _z , and the bond between the Zn _y S _z layers 10 is weakened, and when it is physically or chemically peeled off Finally, it is shown that the Zn _y S _z nanosheets 20 are made. The nanosheets made in this way may still contain some of the additive elements 11 .

The layered structure compound according to the present invention can be made through a layered structure compound that can be synthesized using various Group 1 elements in addition to the conventionally known layered structure compounds. Theoretically, Group 1 elements such as Na, K, Cs A layered structure compound representing the chemical formula of M ₂ Zn _y S _z is possible. Therefore, it can be said that the layered compound of M _2-x Zn _y S _z that can be made through this also falls within the scope of the present invention.

The actual inventors derived a layered material having a new composition and crystal structure through calculation, and as a result, a new layered structure of Na ₂ Zn ₂ S ₃ could be synthesized.

The layered structure compound and nanosheets can be made with the layered structure compound and nanosheets through it without completely removing the additional elements from the layered structure compound. be able to

Even when the additive element M is removed, the surface on which some additional elements remain remains a cleaved surface on which physical exfoliation occurs easily, or the bonding force between the Zn _y S _z layers is removed, _causing cracks as the distance between the layers increases. The _z layer maintains a two-dimensional layer, so that the compound has a layered structure.

On the other hand, in the M ₂ Zn _y S _z layered structure compound, as the additive element M is partially removed, the bonding force between the Zn _y S _z layers is weakened, so that the Zn _y S _z layer can be easily peeled off. Therefore, x may be 0.1≤x≤1.2 so that exfoliation is easy and there is no layer structure collapse or crystal structure change due to excessive removal of additional elements. Here, the crystal structure of the layered compound may have a space group of C2/c. Nanosheets peeled from the layered structure compound having such a range of x are also equally x 0.1≤x≤1.2, and the crystal structure may have a space group of C2/c.

In addition, the additive element remaining in the M _2-x Zn _y S _z layered compound or nanosheet may be in the range where x is 0.3≤x≤1.0 based on the above-mentioned [Formula 1].

In the layered structure compound in which M, which is an additive element, is partially removed and a certain amount remains, M, which is an additive element, remaining between the layers can move, thereby exhibiting various electrical properties. Therefore, it may be preferable that more than a certain fraction of the additional elements are removed from the M _2-x Zn _y S _z compound and some remain. For this purpose, the range of x may be in the range of 0.3≤x≤1.0.

In addition, the additive element remaining in the M _2-x Zn _y S _z layered compound or nanosheet may be in a range where x is 1.2<x based on the above-mentioned [Formula 1].

As described above, as the additive element is removed, the crystal structure of the layered compound may change. When the additive element is removed and x exceeds 1.2, the Zn _y S _z layer may change to the wurtzite structure represented by ZnS. can Even in this case, the surface on which some additional elements remain remains a cleaved surface where physical exfoliation occurs easily, or the bonding force between the Zn _y S _z layers is removed and cracks appear as the distance between the layers widens. Accordingly, the Zn _y S _z layer is still a two-dimensional layer , and the compound has a layered structure.

The M _2-x Zn _y S _z layered compound or nanosheet may be, in particular, M _2-x Zn _y S _z where M is Na, and the starting compound before some Na is removed may be Na ₂ Zn ₂ S ₃ .

As described above, the inventors predicted a compound with a stable structure through calculation to make _a Zn _y S _z compound with _a layered structure _. could

A strong acid such as nitric acid or hydrochloric acid can be used to remove the additive element. As the additive element is removed through such a strong acid, hydrogen ions contained in the strong acid are combined into the compound to provide a layered structure compound containing hydrogen and a nanosheet through it. be able to

The layered structure compound or nanosheet containing hydrogen ions may be expressed by the following [Formula 2].

[Formula 2] M _2-x H _a Zn _y S _z

(M is at least one type of group 1 element, 0<x<2.0, 1.5≤y≤2.25, 2.75≤z≤3.25)

Here, hydrogen ions are added below the amount of the additive element removed by replacing the additive element.

The above-described layered compound and nanosheet exhibit various properties as a result of analysis, and these properties will be described below. The layered structure compounds and nanosheets described herein include both cases with and without additional elements.

In addition, in XRD measurement using CuKα ray, the layered compound and the nanosheet may have a space group of C2/c.

The layered structure compound or nanosheet as described above can exhibit various electrical properties due to its inherent layered structure and residual additive elements.

First, the layered compound or nanosheet according to the present invention exhibits ferroelectric-like properties.

The ferroelectric characteristic is generally a characteristic of an oxide having an asymmetric structure such as a perovskite structure BaTiO ₃ , and the ferroelectric characteristic appears according to a change in the position of Ba located at the center.

However, the layered compound or nanosheet according to the present invention does not have such asymmetric structure, but nevertheless exhibits ferroelectric-like properties. The reason why it exhibits ferroelectric-like properties even though it is not an asymmetric structure is thought to be that the positions of the remaining additive elements move according to the external electric field.

Through the ferroelectric-like properties of the layered structure compound or nanosheet according to the present invention, it can be applied to various electrical devices.

In addition, the layered compound or nanosheet according to the present invention exhibits resistance switching properties.

If a material has resistance switching characteristics, the current does not increase linearly according to the voltage applied to the material, but when an initial voltage is applied, the material maintains a high resistance state and the increase in current is insignificant and reaches a certain critical point. As it changes to a low-resistance state, the current rapidly increases.

This resistance switching characteristic is a characteristic generally found in oxides. Recently, the development of memory devices such as memristors capable of storing information such as flash memories using these characteristics has been active, and the layered structure compound of the present invention and the Nanosheets can be actively used in the development of memory devices such as memristors by utilizing resistance switching characteristics.

[Example]

1) Layered Na ₂ Zn ₂ S ₃ Synthesis

Na, Zn, and S were weighed in a molar ratio of 2:2:3, mixed, and put into an alumina crucible. Thereafter, it was put in a quartz tube and double sealed to block outside air. This process was carried out in a glove box in an argon atmosphere. After that, in the box furnace, the temperature was raised to 450°C, which is the temperature at which Na, Zn, and S can all exist as liquids, maintained for 72 hours, then raised to 950°C again, maintained for 50 hours, cooled to 600°C for 10 hours, and again 120 hours was maintained to obtain a Na ₂ Zn ₂ S ₃ sample.

2) Removal of Na

Na was removed from the layered Na ₂ Zn ₂ S ₃ sample by reacting with time in a mixed solution of CuCl ₂ in methanol. The element ratio of Zn also changed with the removal of Na, and the results are shown in the table below.

sample name reaction time element ratio sample A - Na:Zn:S = 2:2:3 sample B 10 minutes Na:Zn:S = 1.2:1.9:3.0 sample C 20 minutes Na:Zn:S = 0.6:1.7:3.0

3) Nanosheeting process

For the samples prepared as shown in Table 1, after washing and drying with methanol, peeled nanosheets were prepared using a tape.

The analysis results for the layered structure compound and the nanosheet obtained through the above-described examples will be described in detail below.

2 is a graph showing a pattern according to XRD analysis, an XRD pattern (Na ₂ Zn ₂ S ₃ _ref) of Na ₂ Zn ₂ S ₃ having a C2/c space group expected through calculation (Na 2 Zn 2 S 3 _ref) and The XRD pattern of the synthesized sample A is shown.

The actually synthesized sample A matches the calculated XRD pattern of Na ₂ Zn ₂ S ₃ , so it can be seen that the prepared sample A is Na ₂ Zn ₂ S ₃ having a monoclinic C2/c space group. The peaks occurring at 9.56° and 13.52° in FIG. 2 are peaks corresponding to the (110) and (200) planes of Na ₂ Zn ₂ S ₃ , respectively.

3 is a STEM (Scanning Transmission Electron Microscope) image and diffraction pattern for the zone of sample A. Through this, the structure of the C2/c space group could be confirmed at the atomic level.

4 shows a scanning electron microscope (SEM) image of sample A, an SEM image of sample B, and a transmission electron microscope (TEM) image of a nanosheet peeled from sample B. FIG.

Sample B is a layered compound in which Na is partially removed from Sample A, and it can be expressed as Na _1.2 Zn _1.9 S ₃ , and it can be seen from the SEM image that the layered structure is maintained even when Na is partially removed. In addition, it was found that the nanosheet can also be easily peeled off from Sample B, which has such a layered structure.

5 shows an atomic force microscopy (AFM) image of a nanosheet exfoliated from sample B and a line-profile accordingly. Nanosheets with a thickness of 5 to 15 nm could be easily obtained through tape peeling.

6 is a graph showing the XRD pattern, showing the ZnS reference data (ZnS) of the wurzite crystal structure, the XRD pattern of the sample A, and the XRD pattern of the sample B. As shown in FIG.

In the XRD patterns of samples A and B, it was found that the peaks at 26.9°, 28.5°, 30.5°, 47.6°, and 51.8°, which are five major peaks in wurzite-structured ZnS, did not appear.

7 shows an STEM image and a Fast-Fourier Transformation (FFT) Electron Diffraction (ED) pattern corresponding to the [010] zone of Sample B. It was confirmed that Na was partially removed at the Na position between the layers.

8 shows a STEM image, a Selected Area Electron Diffraction (SAED) pattern, and an SEM image of Sample C. FIG.

By STEM, it was confirmed that the arrangement was identical to that of wurzite ZnS at the atomic level, indicating that sample C had a wurzite crystal structure. Although the crystal structure was changed, it still showed a layered structure as seen in the SEM image.

9 is a graph showing the XRD pattern, showing the ZnS reference data (ZnS) of the wurzite crystal structure, the XRD pattern of the sample A, and the XRD pattern of the sample C. As shown in FIG.

In Sample A, peaks at 26.9°, 28.5°, 30.5°, 47.6°, and 51.8°, which are the five main peaks of wurtzite-structured ZnS, did not appear, but in Sample C, it was found that peaks appear at these angles. .

10 is a PFM (Piezoresponse Force Microscopy) measurement result of the nanosheet peeled from the sample B. In the graph, a coercive voltage was formed near -2V and 2V, and it was found that Sample B had ferroelectric-like properties.

Claims (24)

A layered structure compound represented by the following formula (1).
[Formula 1] M _2-x Zn _y S _z
(M is at least one type of group 1 element, 0.1≤x<2.0, 1.5≤y≤2.25, 2.75≤z≤3.25) The method of claim 1,
Wherein x is 0.1≤x≤1.2, the layered structure compound. The method of claim 1,
Wherein x is 0.3≤x≤1.0, the layered structure compound. The method of claim 1,
Wherein M is Na, a layered structure compound. The method of claim 1,
The layered structure compound further comprises H, the layered structure compound. The method of claim 1,
Wherein x is 1.2<x, the layered structure compound. 6. The method according to any one of claims 1 to 5,
The crystal structure of the layered structure compound represents a space group of C2/c, a layered structure compound. 7. The method of claim 6,
The layered compound is wurzite (Wurzite) representing the crystal structure, the layered compound. 7. The method according to any one of claims 1 to 6,
The layered structure compound exhibits a ferroelectric-like property, a layered structure compound. 7. The method according to any one of claims 1 to 6,
The layered structure compound exhibits resistance switching properties, a layered structure compound. A nanosheet comprising a composition represented by the following Chemical Formula 1 and made by a physical or chemical peeling method.
[Formula 1] M _2-x Zn _y S _z
(M is at least one type of group 1 element, 0.1≤x<2.0, 1.5≤y≤2.25, 2.75≤z≤3.25) 12. The method of claim 11,
Wherein x is 0.1≤x≤1.2, nanosheets. 12. The method of claim 11,
Wherein x is 0.3≤x≤1.0, the nanosheet. 12. The method of claim 11,
Wherein M is Na, nanosheets. 12. The method of claim 11,
The composition further comprises H, nanosheets. 12. The method of claim 11,
Wherein x is 1.2<x, nanosheet. 16. The method according to any one of claims 11 to 15,
The crystal structure of the composition represents a C2/c space group, nanosheets. 17. The method of claim 16,
The composition represents a wurzite (Wurzite) crystal structure, nanosheets. 17. The method according to any one of claims 11 to 16,
wherein the composition exhibits ferroelectric-like properties. 17. The method according to any one of claims 11 to 16,
The composition exhibits resistance switching properties, nanosheets. 17. The method according to any one of claims 11 to 16,
The thickness of the nanosheet is 500 nm or less, nanosheet. An electrical device comprising the layered compound according to claim 1 . An electrical device comprising the nanosheet according to claim 11 . 24. The electrical device according to any one of claims 22 to 23, wherein the electrical device is a memristor.

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