KR101760604B1 - Method for synthesizing horizontal ITO nanowires - Google Patents

Abstract

The present invention relates to a method of manufacturing a horizontally grown indium-tin-oxide (ITO) nanowire, and a method of manufacturing a horizontally aligned ITO nanowire according to the present invention comprises: 1) 2) it can be manufactured at a lower process temperature than the conventional method using a metal catalyst, and 3) the hydrogen reduction environment is used; therefore, the conventional carbothermal reduction effect of carbon on the ITO nanowire fabricated by the manufacturing method using the reducing environment.

Description

TECHNICAL FIELD [0001] The present invention relates to a method for producing a horizontally aligned ITO nanowire,

The present invention relates to a method of manufacturing a horizontally grown indium-tin-oxide (ITO) nanowire, wherein the horizontal ITO nanowire produced in the present invention has excellent electrical and optical characteristics and excellent gas probe sensitivity.

The present invention relates to a horizontal ITO nanowire having excellent electrical and optical properties as well as gas probe sensitivity, and a method for producing the same, by using an epitaxial indium-tin-oxide (ITO) .

Currently, ITO is a key material for making optical and electronic devices such as displays, batteries, and semiconductors. In particular, it is a transparent semiconductor having a wide band gap of 3.5 to 4.3 eV as an n-type semiconductor in the manufacturing step. In addition, it has high electrical conductivity and high transparency in the visible region, and thus can be used as various optical and electrical devices. It is also an extremely useful material as an enlarged reflector for infrared reflective coatings and vertical cavity surface emitting laser (VISEL) lasers. Because of its high temperature stability, it can also be used as a variety of sensors that can operate at temperatures above 1400 ° C. In particular, it exhibits excellent performance as a gas sensor having excellent sensitivity.

In recent years, ITO has been manufactured in the form of 1-dimensional nanomaterials (nanowires, nano-rods, nano-belts, nanoribbons, nanotubes) field effect transistors) have been attempted. Among them, ITO nanowires can be easily manufactured, and are attracting attention because of their high specific surface area, excellent crystallinity, and easy control of diameter and length. Typically, thermal chemical vapor deposition (CVD), sputtering, evaporation, and vapor transport methods are used.

Thermal CVD method or vapor transport method usually uses a method in which ITO powder or a powder containing In and Sn is evaporated at a high temperature to convert to a vapor form, and then the ITO nanowire is grown on the substrate surface using Ar carrier gas. In this case, metal nanocrystals such as Au are deposited on the surface of the substrate in advance to utilize Vapor-Liquid-Solid (VLS) growth mechanism. However, since Au nanocrystals are attached to the tip of ITO nanowire, it acts as an impurity when it is manufactured as an electronic device, and a post-treatment process is necessary to remove it.

In general, the thermal CVD method used for manufacturing ITO nanowires uses a carbothermal reduction method in which In ₂ O ₃ , SnO ₂ and graphite powder are mixed at a certain ratio as raw materials. Graphite is used to supply the carbon used as a reduction agent. However, the content of carbon in ITO is increased. In addition, it has been pointed out that the role of carbon, which is still inevitably contained, has not been precisely clarified and it is a disadvantage of improving the characteristics of the device.

Most of the ITO nanowires through the VLS growth mechanism reported to date have a vertical arrangement perpendicular to the substrate. Au nanocrystals as catalysts. In addition, a high-temperature process of 900 DEG C or more is essential. Vertical growth Since ITO nanowires are grown to a density of 1 μm or more, they have a disadvantage that they must be dispersed after being precipitated in acetone, ethanol or the like in order to be applied to FETs and sensors using a single ITO nanowire.

In order to solve the above problems, the inventors of the present invention have proposed a method of manufacturing a horizontal array ITO nanowire which can prevent side effects due to carbon inflow by applying a hydrogen reduction environment instead of a carbothermal reduction environment during a thermal CVD process, And when it is used in an optical element, it shows excellent optical characteristics, and the present invention has been completed.

Japanese Patent Application Laid-Open No. 10-2014-0022328

An object of the present invention is to provide a method of manufacturing a horizontally aligned ITO (Indium-Tin-Oxide) nanowire by thermal CVD in a hydrogen reduction environment.

It is another object of the present invention to provide a horizontally aligned ITO nanowire fabricated by the above manufacturing method.

It is still another object of the present invention to provide an optical element including the horizontally arranged ITO nanowires.

It is another object of the present invention to provide an electronic device comprising the horizontally aligned ITO nanowires.

It is another object of the present invention to provide a gas probe sensor comprising the horizontally aligned ITO nanowires.

In order to achieve the above object,

The present invention relates to a method of manufacturing an ITO (Indium-Tin-Oxide) powder;

The step of placing the ITO powder of the step 1 in a ceramic boat (step 2);

Placing the substrate on the ceramic boat (step 3);

Placing the ceramic boat and the substrate in a tubular electric furnace and maintaining a vacuum (step 4); And

A step of setting the process temperature of the tubular electric furnace at 500-900 캜 and introducing Ar-containing gas (step 5);

The method comprising the steps of:

In addition, the present invention provides a horizontally aligned ITO nanowire manufactured by the above manufacturing method.

Further, the present invention provides an optical element including the horizontally arranged ITO nanowires.

The present invention also provides an electronic device including the horizontally arranged ITO nanowires.

Further, the present invention provides a gas probe sensor comprising the horizontally arranged ITO nanowires.

A method of manufacturing a horizontally aligned ITO nanowire according to the present invention includes:

1) Since the metal catalyst is not used, the cost is reduced, the metal catalyst impurity remains, the ITO nanowire has excellent physical properties,

2) it can be manufactured at a lower process temperature than the conventional method using a metal catalyst,

3) Since the hydrogen reduction environment is used, there is an effect of blocking the carbon inflow to the ITO nanowires manufactured by the conventional manufacturing method using the carbothermal reduction environment.

1 is a schematic diagram of a thermal CVD method for producing horizontal ITO nanowires in a hydrogen reduction environment compatible with the present invention.
2 is an image obtained by FE-SEM of a horizontal ITO nanowire manufactured by holding at 700 ° C for 2 hours. (a) is an image enlarged 300 times, and (b) is an image enlarged 1000 times.

3 is an EDX mapping image of a single ITO nanowire portion in the specimen of FIG. (a) is an FE-SEM image. (b) shows the atomic distribution of In. (c) is the atomic distribution of Sn.
FIG. 4 shows the results of chemical composition analysis by EDX spectroscopy of a single ITO nanowire in the sample of FIG. 2. FIG. In the round tip region, spontaneously formed Sn nanocrystals are dominant, and the ITO stem has an atomic ratio of In and Sn of about 9: 1.

FIG. 5 is an FE-SEM image for identifying the generation mechanism of a single ITO nanowire in the specimen of FIG. The initial Sn nanocrystals (FIG. 5 (a)) are initially formed and then the ITO stem portion is formed and grown. (Figs. 5 (b) and 5 (c))
FIG. 6 is a graph of XRD analysis of horizontal ITO nanowires manufactured by holding at 700 ° C. for 2 hours.
7 is a θ-rocking curve obtained by analyzing the crystal quality of a horizontal ITO nanowire manufactured by holding at 700 ° C. for 2 hours. ITO (222) and ITO (400) Bragg peaks, respectively.
8 is an image obtained by observing the atomic structure of a horizontal ITO nanowire manufactured by holding at 700 DEG C for 2 hours with a high-resolution TEM.
FIG. 9 is a Raman spectroscopy curve obtained by analyzing the optical properties of a horizontal ITO nanowire manufactured by holding at 700 ° C. for 2 hours.

Unless defined otherwise, all technical terms used in the present invention have the following definitions and are consistent with the meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In addition, preferred methods or samples are described in this specification, but similar or equivalent ones are also included in the scope of the present invention. The contents of all publications referred to herein are incorporated herein by reference.

The term "about" is used herein to refer to a reference quantity, a level, a value, a number, a frequency, a percent, a dimension, a size, a quantity, a weight, or a length of 30, 25, 20, 25, 10, 9, 8, 7, Level, value, number, frequency, percent, dimension, size, quantity, weight or length of a variable, such as 4, 3, 2 or 1%.

The term "at%" means an element percentage.

The term "wt%" means weight percent.

Throughout this specification, the words "comprises" and "comprising ", unless the context requires otherwise, include the steps or components, or groups of steps or elements, Steps, or groups of elements are not excluded.

Hereinafter, the present invention will be described in detail.

The present invention relates to a method of manufacturing an ITO (Indium-Tin-Oxide) powder;

The step of placing the ITO powder of the step 1 in a ceramic boat (step 2);

Placing the substrate on the ceramic boat (step 3);

Placing the ceramic boat and the substrate in a tubular electric furnace and maintaining a vacuum (step 4); And

A step of setting the process temperature of the tubular electric furnace at 500-900 캜 and introducing Ar-containing gas (step 5);

The method comprising the steps of:

In the manufacturing method according to the present invention, horizontal ITO nanowires can be manufactured by the thermal CVD method under the hydrogen reduction environment of the ITO powder using the equipment as shown in FIG.

In the manufacturing method according to the present invention, the step 1 is a step of preparing an ITO powder.

Specifically, the ITO powder may be a mixed powder of In ₂ O ₃ powder and SnO ₂ powder, a mixed powder of In ₂ O ₃ powder and SnO powder, a mixed powder of In ₂ O ₃ powder and Sn powder, ₂ O ₃ powder and SnO ₂ powder are preferably used. The fraction of the Sn element is adjustable to 1-20 at%, preferably 8-12 at%, particularly preferably 10 at%. If the Sn content exceeds 20 at%, the structural contribution of SnO ₂ is much higher than that of ITO. The ITO powder is put into a bottle and shaken evenly.

In the manufacturing method according to the present invention, the step 2 is a step of putting the ITO powder into a ceramic boat.

The ITO powder must be transferred to a ceramic boat that can withstand high temperatures. At this time, materials such as alumina, zirconia, quartz, and sapphire can be used for the ceramic boat. Boats should be 10-20 mm wide, 50-100 mm long and 5-20 mm high. In the present invention, an alumina boat having a width of 10 mm, a length of 60 mm and a height of 15 mm was used.

The amount of the ITO powder is 0.001-1 g. Since the ITO powder is expensive, a proper amount to participate in the reaction should be selected. If the amount of ITO powder exceeds 1 g, ITO powder not participating in the reaction is wasted. In the present invention, 0.1 g of ITO powder is used as an example.

In the manufacturing method according to the present invention, the step 3 is a step of disposing the substrate on the ceramic boat.

The ITO nanowires can be epitaxially grown on a substrate, with the surface of the substrate facing the ITO powder contained in the boat. A substrate may be used a substrate consisting of ZnO, Si, Ge, GaN, InP, Sapphire, InP, GaAs, GaP, Si 3 N 4, SiO 2, SiC, Ga 2 O 3, an element of glass or the like. In the present invention, a horizontal ITO nanowire is manufactured using Sapphire (0001) substrate as an example.

In the manufacturing method according to the present invention, the step 4 is a step of placing the ceramic boat and the substrate in a tubular electric furnace and maintaining a vacuum. Here, it is preferable that the ceramic boat and the substrate are positioned in the middle of the tubular electric furnace.

In the conventional thermal CVD method under the carbothermal reduction environment, the position of the substrate is located at a distance of about 1-15 cm from the ceramic boat, while in the present invention, the position of the substrate is arranged on the ceramic boat to produce the horizontal ITO nanowire. Placing the substrate away from the ceramic boat lowers the density of the horizontal nanowires and makes it difficult to produce horizontal ITO nanowires of lengths greater than 10 μm. It is preferable that the degree of vacuum is maintained at about 5 × 10 ^-2 to 1 × 10 ^-6 torr. In the present invention, horizontal ITO nanowires were fabricated by setting to 5 × 10 ^-3 torr, for example.

In the manufacturing method according to the present invention, step 5 is a step of setting the process temperature of the tubular electric furnace and introducing Ar-containing gas.

The process temperature for preparing the horizontal ITO nanowire is 500-900 ° C. Preferably 650 to 750 ° C, and particularly preferably 700 ° C. If the process temperature is less than 500 ° C., there may be a problem that the ITO powder does not evaporate in a vapor form. If the process temperature is higher than 900 ° C., Sn, which serves as a self-crao, can not be deposited on the substrate surface, .

The carrier gas for preparing the horizontal ITO nanowire may be a mixed gas of hydrogen and argon to provide a hydrogen reduction environment. The content of the hydrogen gas may be 0.2 to 5 wt%, preferably 3.5 to 4.5 wt% %, Particularly preferably 4% by weight. If the content of the hydrogen gas is less than 0.2 wt%, the reducing action of the ITO powder may be very limited. If the content of the hydrogen gas is more than 5 wt%, the hydrogen may be explosively reacted with the residual oxygen.

The flow rate of the gas may be in the range of 5-1000 sccm (standard cubic centimeter per minute), but it is preferably carried out while maintaining the flow rate at 300-800 sccm. In addition, the deposition pressure can be maintained at a constant value in the range of 1 to 50 torr.

The method of manufacturing the horizontally aligned ITO nanowire according to the present invention is as follows: 1) since the metal catalyst is not used, the cost is reduced, and the metal catalyst impurities are not left, and the physical properties of the ITO nanowire are excellent; 2) 3) Because hydrogen reduction environment is used, the problem of carbon injection into the ITO nanowires manufactured by the conventional manufacturing method using the carbothermal reduction environment is fundamentally cut off .

The present invention also provides a horizontally aligned ITO nanowire fabricated by the above manufacturing method. The horizontally aligned ITO nanowires are characterized by having a length of 20-500 μm and a diameter of 20-500 nm.

Further, the present invention provides an optical element including the horizontally arranged ITO nanowires.

The present invention also provides an electronic device including the horizontally arranged ITO nanowires.

Further, the present invention provides a gas probe sensor comprising the horizontally arranged ITO nanowires.

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the following examples are illustrative of the present invention, and the present invention is not limited by the following examples.

< Example  1> Horizontal ITO Nanowire  Produce

Horizontal ITO nanowire samples were fabricated on a sapphire (0001) substrate under a hydrogen reduction environment using thermal CVD.

Specifically, the tubular electric furnace of Fig. 1 was used. The tubular electric furnace is a 3-zone alumina tube with a diameter of 60 mm and a length of 500 mm. The vacuum state (5 × 10 ^-3 torr) was maintained until the process temperature was reached.

The process temperature was maintained at 650-700 ° C. After the process temperature was reached, a carrier gas was injected to produce ITO vapor. H ₂ (4%) + Ar (purity 99.99%) mixture Gas was used, and the flow rate was set to 500 sccm (standard cubic centimeter per minute). The process pressure was maintained at 10 torr when manufacturing horizontal ITO nanowires.

The process retention time was set to 2 hours, and after 2 hours, the flow of the mixed gas was shut off and the reactor was cooled to room temperature under vacuum.

< Experimental Example  1> Horizontal ITO Nanowire  Sympathy

The horizontal ITO nanowires prepared in Example 1 are as shown in Fig.

2 is an FE-SEM image of the horizontal ITO nanowire prepared in Example 1. FIG. 2 (a) is a 300-magnification FE-SEM image and (b) is a 1000-magnification FE-SEM image.

As shown in FIG. 2, ITO nanowires grown long in the horizontal direction of the substrate can be identified. The shape of the horizontal ITO nanowire was found to be divided into a tip portion of a cogged shape and a stem portion extending beneath it. The length was between approximately 20 and 200 μm, and the length of 40 μm was the most abundant.

< Experimental Example  2> Horizontal ITO Nanowire  Elemental distribution analysis

In order to understand the details of the growth mechanism of the horizontal ITO nanowires, atomic distributions of single ITO nanowires were analyzed through FE-SEM and energy dispersive x-ray analysis (EDX) mapping.

3 shows an image obtained by measuring a composition distribution map of a single nanowire. 3 (a) is an FE-SEM image, and FIGS. 3 (b) and 3 (c) are mapping images showing the distribution of elements of In and Sn. In-Lα and Sn-Lα characteristic lines were measured.

As shown in FIG. 3, as indicated by the red arc of the image, the round tip portion has a distribution of Sn, and the stem portion includes both In and Sn. It was confirmed that a small amount of SnO ₂ was added to the In ₂ O ₃ as the In shade of the stem portion was supposed to be higher than the Sn shade.

FIG. 4 is a result of quantitative analysis of elemental distributions of In and Sn. EDX spectroscopy, which was measured at the tip of the round shape, at the neck portion, which is the connecting portion of the tip and the stem, and at the stem portion, was shown.

As shown in FIG. 4, the characteristic curves of Al and O are prominent, which is attributed to the sapphire element used as the substrate. In addition, element content was measured from the area of In-Lα and Sn-Lα characteristic curves of spectroscopy. In the round shape tip part, the content of Sn was ~ 95 at% and the content of In was ~ 5 at%. In addition, the content of oxygen was almost not observed in the tip. Unlike conventional ITO nanowires using Au nanocrystals, horizontal ITO nanowires grow in the present invention without using a metal nanocrystal catalyst. This is because the self-catalytic Sn Nanocrystals act as catalysts, indicating that nanowires have grown through the Vapor-Liquid-Solid (VLS) growth mechanism. The neck part, which is the interface between the round shape tip and the stem, is almost equal to the composition of the ITO powder used as the raw material since the content of In is ~ 91 at% and the content of Sn is ~ 9%. The elemental distribution in the stem part showed a tendency that the content of Sn was slightly lowered to ~ 92 at%, and the content of Sn was ~ 8 at%. This is because the Sn exists in the round tip part, and thus the amount of Sn tends to decrease as it moves away from the reaction site tip. But overall it is similar to the ITO element content.

3 and 4, it was found that the growth of the horizontal ITO nanowires can be performed by a self-catalytic VLS growth mechanism using Sn nanocrystals as catalysts. In the hydrogen reduction environment according to the present invention, vapor is easily formed in the form of In ⁺ and In ^{3 +} ions in the ITO powder. In addition, the vapor of Sn ^{2 +} and Sn ^{4 +} ions can be easily formed, and the ionized Sn vapor exists at an unbalanced state at the manufacturing process temperature, and when combined with the substrate, it forms a stable Sn nanocrystal do.

FIG. 5 is an FE-SEM image showing an initial self-catalytic VLS growth mechanism. 5 (a) shows the Sn nanocrystals initially formed. 5 (b) shows the initial stage in which the ITO stem is formed and grows from the round tip, and then the stem continues to grow as shown in Fig. 5 (c).

< Experimental Example  3> Horizontal ITO Nanowire XRD  analysis

X-ray diffraction (XRD) experiments were conducted to analyze the crystallographic properties of the horizontal ITO nanowires.

Specifically, the horizontal ITO nanowire samples prepared in Example 1 were subjected to XRD analysis, and the results are shown in FIG.

Fig. 6 is a graph of XRD analysis of the horizontal ITO nanowire target prepared in Example 1. Fig.

As shown in FIG. 6, a sapphire (006) Bragg peak was observed at Q _z = 2.902 Å ^-1 . C-cut sapphire (0001) was used as a substrate in the example of the present invention. Several Bragg peaks corresponding to ITO nanowires were observed. Peaks were measured at Q _z = 2.1508 Å ^-1 , Q _z = 2.4832 Å ^-1 , Q _z = 4.3 Å ^-1 , and Q _z = 4.9654 Å ^-1 . This corresponds to (222), (400), (444), and (800) of the ITO crystal, respectively. ITO having a BCC structure is a crystal plane having (222) and (400) high surface density. Especially, ITO thin films grown on sapphire (0001) substrates grow first with (222) planes and form epitaxial relationships. This is because the crystal plane (222) plane in which lattice mismatch between sapphire substrate and ITO can be minimized. In addition, the peak of In (101) (Q _z = 2.3096 Å ^-1 ) and the peak of Sn (220) (Q _z = 2.743 Å ^-1 ) were observed to be weak due to the round shape tip.

In order to analyze the crystal quality of the horizontal ITO nanowire, the θrocking curve was measured at the ITO (222) and ITO (400) Bragg peaks, and the results are shown in FIG. The full width at half maximum (FWHM) of the rocking curve corresponds to the mosaicity indicating the degree to which the crystal plane deviates from the vertical direction of the substrate. The FWHM of the ITO (222) plane is 0.0947 degrees and the FWHM of the ITO (400) plane is 0.0454 degrees. This indicates that the quality of the horizontal ITO nanowire produced in the present invention is very good. FWHM = 0.005 ° for Sapphire (0006) plane and FWHM = 0.1 ° for thin film with good quality.

< Experimental Example  4> Horizontal ITO Nanowire TEM  analysis

High - resolution transmission electron microscopy (TEM) analysis was performed to analyze the ultrafine atomic structure of horizontal ITO nanowires. Figure 8 shows a TEM image of a single horizontal ITO nanowire.

8 is a TEM analysis of the horizontal ITO nanowire samples prepared in Example 1. FIG.

As shown in Fig. 8, the ITO nanowires can be classified into a round shape tip, a stem as a stem, and a neck portion connecting a tip and a stem. The round shape tip shows the atomic distribution corresponding to tetragonal Sn nanocrystals, mainly Sn single crystals. Neck and stem parts also exhibit perfectly regular atomic structures, which indicate that they have grown to ITO and high quality single crystals.

< Experimental Example  5> Horizontal ITO Nanowire  Optical characterization

FIG. 9 shows the optical characteristic analysis results of the horizontal ITO nanowires prepared in Example 1. FIG.

Specifically, the optical characteristics were determined by micro-Raman spectroscopy analysis. The spectrum of the light emitted by the 514 nm Ar gas laser was measured using a CCD (charge coupled device) detector. The spectrometer has a resolution of 1800 gr / mm. Spectrum was measured at 100-1000 cm ^-1 . The exposure time was analyzed as 2 seconds.

9 is a micro-Raman spectroscopy analysis graph of the horizontal ITO nanowire samples prepared in Example 1. FIG.

As shown in FIG. 9, peaks were observed at wave number = 133, 307, 367, 496 and 630 cm ^-1 . Typically 9: ITO having a composition ratio of 1 is very similar to the Raman spectroscopy results also In ₂ O ₃ single crystal since it has a crystallographic atomic structure substantially similar to the In ₂ O _3. Therefore, it was confirmed that the spectrum of a typical ITO nanowire is a single crystal ITO having a BCC structure represented by 4Ag + 4Eg + 14Tg + 5Au + 5Eu + 16Tu. Raman spectra of ITO single crystals with rhombohedral structure are observed at 164, 180, 220, 272, 387, 504, 593 and 644 cm ^-1 , respectively.

In particular, the strongest peak observed in the low frequency region of 133 cm ^-1 is observed geotinde the BCC structure of the single crystal ITO vibration mode, FWHM of the peak value is 2.83 cm ^- indicates that the quality of the single crystal to ^one.

Claims (13)

Preparing an indium-tin-oxide (ITO) powder (step 1);
The step of placing the ITO powder of the step 1 in a ceramic boat (step 2);
Placing the substrate on the ceramic boat (step 3);
Placing the ceramic boat and the substrate in a tubular electric furnace and maintaining a vacuum (step 4); And
(Step 5) of setting the process temperature of the tubular electric furnace at 500-900 DEG C and introducing hydrogen-containing Ar gas,
Wherein the content of hydrogen (H ₂ ) gas in the hydrogen-containing Ar gas in step 5 is 0.2-5 wt%.
The method according to claim 1,
Wherein the ITO powder is a mixed powder of In ₂ O ₃ powder and SnO ₂ powder, a mixed powder of In ₂ O ₃ powder and SnO powder, or a mixed powder of In ₂ O ₃ powder and Sn powder.
3. The method of claim 2,
Wherein the percentage of Sn element relative to In element in the ITO powder is 1-20 at%.
The method of claim 3,
Wherein the percentage of Sn element relative to In element in the ITO powder is 8-12 at%.
The method according to claim 1,
The substrate ZnO, Si, Ge, GaN, InP, Sapphire, InP, GaAs, GaP, Si 3 N 4, SiO 2, SiC, Ga 2 O 3 and consisting of a one element selected from the group consisting of glass Lt; / RTI &gt; substrate.
The method according to claim 1,
And the vacuum of step 4 is 5 x 10 ^-2 to 1 x 10 ^-6 torr.
The method according to claim 1,
Wherein the process temperature of step 5 is 650-750 占 폚.
delete The method of claim 1,
Wherein the gas flow rate in step 5 is 5-1000 sccm (standard cubic centimeter per minute).
A horizontally aligned ITO nanowire fabricated by the method of claim 1.
An optical element comprising the horizontally arranged ITO nanowires of claim 10.
An electronic device comprising the horizontally aligned ITO nanowires of claim 10.
A gas probe sensor comprising the horizontally aligned ITO nanowires of claim 10.

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