KR100927191B1 - Method for Improving Field Emission of Carbon Nanotubes - Google Patents

Abstract

The present invention relates to a method for improving the field emission characteristics of carbon nanotubes, and more particularly, to improve the field emission uniformity and device life through an electric homogenization treatment that reduces the length difference between carbon nanotubes in an oxidizing gas atmosphere. The present invention relates to a method for improving field emission characteristics of nanotubes.

The electrical homogenization treatment of carbon nanotubes according to the present invention has a phenomenon that the tip ends are damaged and broken by electron emission when long carbon nanotubes are heated to a high temperature of about 2000 K due to Joule heat by intensive electron emission. To increase the number of carbon nanotube emitters that operate.

In this case, by using oxygen and ozone, which are representative oxidizing gases, field emission uniformity and lifetime can be effectively improved in a short time. In particular, ozone, which is more oxidative, is characterized by a shorter treatment time than oxygen.

In the case of the homogenization treatment using the oxidizing gas in the constant current mode, the homogenization treatment time can be made very short because the Joule heat is made constant and released.

Description

Improving Field Emission of Carbon Nanotubes {Improvement Of Field Emission Characteristic Of Cabon Nanotubes By Oxidative Electrical Conditioning}

The present invention relates to a method for improving the field emission characteristics of carbon nanotubes, and more particularly, to improve the field emission uniformity and device life through an electric homogenization treatment that reduces the length difference between carbon nanotubes in an oxidizing gas atmosphere. The present invention relates to a method for improving field emission characteristics of nanotubes.

The present invention can be applied to improve the field emission of all carbon nanotube devices manufactured in various forms.

Conventional methods of manufacturing field emission devices using carbon nanotubes include a method of forming a catalyst on a substrate and then vertically growing the carbon nanotubes by chemical vapor deposition, and using carbon nanotubes manufactured by various synthetic methods. After mixing with liquid solvent, spin coater, spray, ink-jet method, viscous polymer resin, solvent, filler, and other additives are mixed with carbon nanotubes. There is a method of screen printing after making a paste.

Through several processes, field emission devices can be made in 2-, 3-, and 4-pole structures, and cathodes and anodes are packaged with a certain insulating space. An anode fluorescent film and a spacer are mounted for application to a display.

Carbon nanotube field emission flat panel display devices, such as FED and BLU, which are currently being studied, are eco-friendly devices that do not contain mercury at all, and because they emit light evenly across the substrate, they are thin and can greatly reduce the number of optical components.

In addition, despite the many advantages of local dimming, which can drastically improve the power consumption and contrast ratio of existing LCDs, and impulse driving that can eliminate afterimages in a fast video, the technology is still completed. There are many difficulties to overcome until commercialization.

Among them, the problem of uniformly emitting electrons in a large area and maintaining a long lifespan is the most difficult and must be solved in terms of commercialization.

When releasing the required amount of electrons in a certain area of the specimen, the field emission lifetime and uniformity of the specimen will depend on the number of emitters actually participating in the electron emission. However, the specimens produced by screen printing have large carbon nanotubes, so only the long ones operate during field emission, and only a few carbon nanotubes emit electrons, resulting in uneven emission and lifetime of emitters. There is a problem of shortening.

In addition, when manufacturing carbon nanotube emitters by screen printing, there are many carbon nanotube emitters in one dot, which have different lengths as mentioned above. When the same electric field is applied to carbon nanotubes of different lengths, the highest electric field is applied at the tip of the longest carbon nanotube.

At this time, the magnitude (E) of the electric field applied to the tip end of the carbon nanotube corresponds to the magnitude of the external electric field (E ₀ ) multiplied by the electric field reinforcement factor (β). That is, E = βE ₀ . The field reinforcement factor β is determined by the shape of the carbon nanotubes, that is, the length l and the radius r, and the relationship is given by β = l / r.

If the length of the carbon nanotubes are different from each other, the electric field (E) applied to the tip of the carbon nanotube tips having different lengths is different depending on the applied external electric field (E ₀ ), so that the field emission characteristics between the carbon nanotubes are different.

As a result, only a small number of long carbon nanotubes emit electrons intensively, and most of the remaining short carbon nanotubes do not participate in field emission. Shorter carbon nanotubes do not contribute to electron emission because the electric field is shielded by the long carbon nanotubes.

As a result, electrons are emitted from only a few carbon nanotubes, resulting in poor field emission uniformity and short lifetime. In addition, even if carbon nanotube emitters have a similar length, it is difficult to expect effective field emission by canceling electric fields from each other when formed at high density.

In order to secure the required field emission uniformity and lifetime, there is a need for a technique for optimizing the carbon nanotube paste and emitter manufacturing process to make the length between the carbon nanotube emitters as uniform as possible.

However, in the method of manufacturing an emitter using screen printing, it is fundamentally impossible to make all carbon nanotube emitters have the same length.

In order to solve the above problems, an object of the present invention is to emit the electrons uniformly in a large area and to improve the lifetime by uniformly distributing the carbon nanotubes in the oxidizing gas atmosphere by electric field emission. The present invention provides a method for improving field emission of carbon nanotubes.

In order to solve the above problems, the method for improving the electric field emission of carbon nanotubes according to the present invention is characterized in that the electric homogenization treatment of carbon nanotubes in an oxidizing gas atmosphere.

Here, the oxidizing gas may be anything as long as it is oxidizing, and is preferably oxygen (O ₂ ) or ozone (O ₃ ).

The electric homogenization process may be performed in a constant voltage mode or a constant current mode.

When performed in the constant voltage mode, the electrical stabilization treatment is performed under a constant pressure, followed by injection of an oxidizing gas, and the homogenization treatment is performed until the time taken to decrease to 1/4 of the initial emission current density. .

Here, the constant pressure is preferably a pressure of 10 ^-7 torr or less.

In the case where the constant current mode is performed, the oxidizing gas is injected while maintaining a constant reference current, and the homogenization treatment is performed until the voltage rises to a predetermined reference voltage.

Here, the constant reference current preferably maintains a reference current of 28 uA corresponding to a current density of 8 mA / cm ² .

In addition, the reference voltage is preferably 600V (2V / um electric field).

In addition, the injected oxidizing gas is preferably injected in the range of 5X10 ^-5 torr to 5 X 10 ^-4 torr.

When an electric field is applied to carbon nanotubes of different lengths, only long carbon nanotubes that emit a strong electric field emit electrons, and the carbon nanotubes that emit electrons can be heated to a considerable temperature by Joule heat. .

Here, the carbon nanotubes heated by the electron emission by the field emission in the oxidizing gas atmosphere according to the present invention is attacked by the oxidizing gas, the oxidizing gas reacts with the carbon nanotubes in the field emission and burns from the tip end or is defective. By cutting the carbon nanotubes in the weak body part, only the length of the long carbon nanotubes is selectively reduced to uniform the length distribution of the carbon nanotubes, thereby producing an excellent effect of improving the uniformity and lifetime of the light emission.

Hereinafter, specific embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a schematic diagram of electron emission of carbon nanotubes.

Referring to FIG. 1, carbon nanotubes are heated to a considerably high temperature by Joule heat during electron emission, which is a known fact.

Longer carbon nanotubes in a given electric field emit more electrons and thus generate more heat, resulting in faster reaction rates with oxidizing gases.

Therefore, the electric homogenization treatment in the oxidizing gas atmosphere shortens the length of carbon nanotubes selectively by short oxidizing by selectively oxidizing the long carbon nanotubes, thereby greatly reducing the length difference between the carbon nanotubes and increasing the number of carbon nanotube emitters participating in the field emission. do.

Figure 2 shows the carbon nanotubes arranged vertically in stages according to a preferred embodiment of the present invention.

More specifically, (a) of FIG. 2 illustrates a carbon nanotube film formed by screen printing and then fired, and (b) illustrates a vertically arranged carbon nanotube by protruding in a paste with an adhesive tape. (C) shows carbon nanotubes arranged vertically by a strong electric field at high vacuum, and (d) shows carbon nanotubes arranged vertically after an electric homogenization treatment with an oxidizing gas (oxygen O ₂ ) according to the present invention. The tube is shown.

Referring to FIG. 2, carbon nanotubes are not covered at all when they are baked after screen printing as shown in (a), but after removing the surface layer of the paste with an adhesive tape, as shown in (b) Ruins appear outward and stand vertically.

Although several carbon nanotubes stand long on the surface, most of the carbon nanotubes are entangled with each other and can be seen attached to the bottom. If you look at the surface of the long carbon nanotubes that are exposed, the particles appear to be messy. The attached particles are presumably polymers or fillers.

The carbon nanotube emitter treated with tape was subjected to a long field emission by applying a strong electric field under high vacuum to obtain an image of (c). When a strong electric field is applied to the carbon nanotubes, the bent and folded carbon nanotubes are straightened. As the carbon nanotubes are straightened, the relative length distribution becomes larger. As the carbon nanotubes were heated by the field emission in a high vacuum, impurities on the surface were decomposed to clean the surface of the carbon nanotubes.

Due to the large length difference between the carbon nanotubes, the specimen of (c) shows uneven field emission. Therefore, in order to reduce the length of long carbon nanotubes, the field emission was performed in an oxidizing atmosphere, which is shown in (d).

As can be seen here, the long carbon nanotubes disappeared and their length distribution was similar.

Figure 3 is a graph showing the distribution of the length of the carbon nanotubes before and after the electric homogenization treatment in an oxidizing atmosphere according to a preferred embodiment of the present invention.

More specifically, Figure 3 (a) is the length distribution of the carbon nanotubes after removing the surface layer of the paste with an adhesive tape, (b) is the length distribution of the carbon nanotubes electrically treated by a strong electric field at high vacuum. (c) is the length distribution of carbon nanotubes after the electric homogenization treatment in an oxidizing atmosphere (using oxygen O ₂ ) according to the present invention.

Referring to FIG. 3, the average length was 6.7 μm before the homogenization treatment as shown in (b), but decreased to 3.8 μm after the electric homogenization treatment as in (c), and the standard deviation decreased from 3.47 to 2.48.

This shows that the electric homogenization treatment using oxidizing gas has a great effect of shortening the length of the carbon nanotube emitter in a short time. As the length difference between the carbon nanotubes is reduced, more carbon nanotubes participate in electron emission under the applied electric field, thereby improving the field emission uniformity and reducing the amount of current emitted per carbon nanotube, thereby reducing the lifetime of the emitter. It will be greatly improved.

The electric homogenization treatment in oxidizing gas atmosphere is focused on electron emission in long carbon nanotube emitters, which increases the reactivity with oxidizing gases by Joule heat and shortens their length in a short time to make the overall difference in emitter length. The luminance uniformity and lifespan characteristics are improved by reducing

On the other hand, in order to homogenize the carbon nanotubes at a faster time, the results of experiments using ozone that is more reactive than oxygen are as follows.

Figure 4 is a graph comparing the electrical homogenization treatment rate of oxygen and ozone as oxidizing gas according to a preferred embodiment of the present invention in a constant voltage mode.

Measurements were performed in constant voltage mode at frequency 100 Hz, duty 1/1000, and substrate spacing 500 μm. After the electrical stabilization treatment at a pressure of 10 ^-7 torr or less, oxygen and ozone were injected up to 5 × 10 ^-5 torr.

Here, the pressure and the amount of the oxidizing gas (oxygen or ozone) to be injected can be selectively adjusted according to the environment or conditions.

In this experiment, the electrical homogenization treatment time was taken as the time taken for the initial emission current density to decrease to 1/4. Accordingly, as shown in FIG. 4, the specimen, which took about 7 minutes for the electric homogenization treatment using oxygen, had a discharge current density reduced to 1/4 of its initial value in 2 minutes in an ozone atmosphere.

This shows that ozone has a treatment effect about three times faster than oxygen.

Generally, ozone generation methods include ozone generation using a UV lamp having a UV-C wavelength and ozone generation by corona discharge of oxygen. In the case of the present invention, ozone was generated using an ozone generator using corona discharge to generate stronger ozone, and then injected into the chamber through a leak valve, and the degree of vacuum was confirmed through a vacuum gauge.

The gas atmosphere of the ozone homogenization process was injected with ozone until 5 X 10 ^-4 torr. For reference, the oxidizing gas rapidly damages the carbon nanotubes which are emitting electrons at ~ 10 ^-7 torr.

Ozone is an isotope of oxygen, and the bond of the third atom in the three oxygen atoms is weak and easily separated and becomes the generator oxygen (nascent oxygen).

The generator oxygen is very unstable in chemical bonds and easily oxidizes other substances, and the oxidative decomposition effect has 6 times stronger efficacy than chlorine. Chemically unstable ozone is easily decomposed by electrons moving at high speed after field emission, and it is assumed that the generated generator oxygen aggressively attacks carbon nanotubes, thereby exhibiting an effect of faster electric homogenization than oxygen.

However, ozone also showed a current recovery after electric homogenization, like oxygen.

Figure 5 is a graph measuring the electrical homogenization treatment rate of the oxidizing gas ozone according to a preferred embodiment of the present invention in a constant current mode.

Field emission characteristics were measured at a frequency of 500 Hz, duty 1/60, and substrate spacing of 300 μm, and 8 mA emission current (8 mA / cm ² ) per unit area was applied to shorten the homogenization time according to the repeated ozone treatment. It was performed while maintaining. The behavior of the voltage to emit a reference current of 28 μA, corresponding to a current density of 8 mA / cm ² , was investigated.

In high vacuum, ozone was injected to 5x10 ^-4 torr after the voltage was stabilized while maintaining the reference current of 28 μA for a long time, and the voltage rapidly increased. Since the emission current is larger in the constant current mode than in the constant voltage mode, the homogenization time is shortened because the oxidation rate of carbon nanotubes by ozone is faster in the constant current mode.

Here, the constant current mode is to check the voltage change over time while maintaining a constant current, and the length is shortened by the damage of the carbon nanotubes and eventually the emission current is reduced, the constant current mode is a program to make the current constant This automatically raises the voltage to maintain the same current.

The ozone homogenization treatment stopped ozone injection when the voltage increased to 600 V (2 V / μm electric field) in a constant current mode of 28 uA (8 mA per unit area) to generate excessive current in the carbon nanotube emitter.

Here, the reference current, the amount of ozone injected and the voltage that is the reference for the electrical homogenization time may be arbitrarily selected according to the experiment or the conditions.

As soon as the ozone injection was stopped, the voltage dropped and recovered. Oxygen, which is known to degrade the field emission characteristics of carbon nanotubes, is desorbed from the surface of carbon nanotubes at high vacuum after ozone injection stops, and thus the electron emission characteristics are improved. The ozone homogenization treatment time was about 20 minutes and oxygen desorption took 2 hours.

Figure 6a is a graph measuring the field emission I-V characteristics of the ozone treatment according to a preferred embodiment of the present invention, Figure 6b shows the F-N curve obtained from the I-V graph of Figure 6a.

Referring to FIG. 6A, first I-V characteristics of the specimens treated with the adhesive tape were measured. The carbon nanotubes were subjected to electrical stabilization in high vacuum step by step so that the carbon nanotubes were fully upright and stable in operation, and then I-V characteristics were measured again in high vacuum.

In the same specimen again, the current was fixed at 28 μA in constant current mode, ozone was injected to 5 × 10 ⁻⁴ torr, and ozone homogenization was performed until the voltage increased by 600 V. The final IV characteristics were measured after stopping the ozone injection and allowing sufficient voltage recovery at high vacuum.

After the adhesive tape treatment, the threshold electric field of the specimen was 6.5 V / μm, which was significantly increased to 8 V / μm after the electric homogenization treatment. Joule heating by field emission breaks the vulnerable part of carbon nanotubes, removes long carbon nanotubes and removes adsorbents on the surface of carbon nanotubes, which help electron emission. The threshold electric field increases to about 10 V / μm after ozone treatment, and the overall driving voltage is greatly increased.

In addition, as can be seen from the FN curve of FIG. 6B, the field reinforcement factor (β) of each graph was calculated, and the field reinforcement factors after the adhesive tape treatment and the electric homogenization treatment were reduced to 1037 and 1014, while the ozone homogenization was slightly reduced. After treatment, it dropped significantly to 802. It can be seen indirectly that ozone treatment significantly reduced the length of carbon nanotubes.

FIG. 7 illustrates observing a light emitting image while changing power before and after performing an ozone homogenization treatment according to a preferred embodiment of the present invention.

7 is an enlarged light emission image before and after 0.3W processing.

The measurement at this time used a square pulse wave of frequency 500 Hz, duty 1/60, gap 300 μm. In Fig. 7, the voltage and current at each power, and the uniformity of light emission a-du are calculated and shown in Table.

7 shows that the voltage increases and the current decreases at the same power after the homogenization treatment, compared to before the ozone homogenization treatment, and the a-dus after the ozone homogenization treatment are all higher than before the homogenization treatment, increasing from as little as 2.7% to as much as 6.8%. Able to know.

In order to specifically compare the light emission uniformity improvement by the ozone homogenization treatment, the 0.3 W images of FIGS. 7D and 7H are enlarged in FIG. 8.

Referring to FIG. 8, most of the dots emit light even before ozone treatment, but when the dots marked as rectangles are compared with each other, the dots that were weak by the ozone treatment become stronger, and the dots which are strong The light emission of the dots was weakened, and the brightness difference between the dots was reduced. It is believed that the uniformity of luminescence is improved as ozone treatment reduces the carbon nanotube length difference between dots.

FIG. 9 graphically illustrates a-du of FIG. 7.

Here, the a-du (average-dot uniformity) is a field emission emission image of a dot patterned carbon nanotube emitter specimen using a CCD mounted in a chamber, and then 256 points on each dot. A value calculated by assigning luminance of gray scale to calculate luminance uniformity between dots.

The a-du is defined by the following equation.

a-du = {1- (σ / m)} × 100 (%)

Here, m and sigma represent average values and standard deviations of luminance at which several dots are seen, and are usually measured for 10x10, that is, 100 or more dots. Referring to FIG. 9, a-du according to ozone treatment is shown. Shows an average increase of about 3.9% in total power.

By adopting the pixel concept of the actual display device, a-pu was calculated using Equation 1 below assuming that 20 dots are included in one pixel.

Here, the a-pu (average-pixel uniformity) means a value calculated by calculating uniformity between pixels, and may be calculated by Equation 1 below.

Ozone treatment increased a-pu about 1.5% at low power, about 0.5% at high power, and about 0.9% on average. 95%, which is required for the commercialization of display devices, meets this criterion at power of 0.3 W or more before ozone treatment, but meets this criterion at about 0.2 W when ozone treatment.

 10 is a graph comparing the lifespan of the specimens subjected to the electric homogenization treatment only and the ozone homogenization treatment according to the preferred embodiment of the present invention.

It is a graph comparing the lifespan of specimens subjected to electrical homogenization and ozone homogenization. The lifetime is defined as the time taken for the emission current to decrease to 1/2 of the initial value. Square pulse waves with vacuum degree of ~ 10 ^-8 torr, frequency 500 Hz, duty 1/60, and gap 300 μm were used for the experiment. (The lifetime of the specimen is fixed at a voltage with an initial reference current of 7 μA (current density of 2 mA / cm ² ) and then 3.5 μA (current density of 1 mA / cm ² ) with current equal to 1/2 of the initial current in constant voltage mode. It is a measure of the time until it falls to.)

It was confirmed that the lifespan of 45 hours before the ozone homogenization treatment increased about four times to 180 hours after the ozone homogenization treatment. It is estimated that the ozone homogenization treatment can remove the long carbon nanotubes and increase the number of carbon nanotubes that are actually operated, thereby reducing the amount of current burdened by individual carbon nanotubes, and thus enduring the same conditions longer. The lifetime of the display device measured here is measured in much harsher conditions than the actual driving conditions so that it can be tested in a short time.

Although the detailed description of the present invention described above has been described with reference to the preferred embodiment of the present invention, the protection scope of the present invention is not limited to the above embodiment, and those skilled in the art will appreciate It will be understood that various modifications and changes can be made in the present invention without departing from the spirit and scope of the invention.

1 is a schematic diagram of electron emission of carbon nanotubes.

Figure 2 shows the carbon nanotubes arranged vertically in stages according to a preferred embodiment of the present invention.

Figure 3 is a graph showing the distribution of the length of the carbon nanotubes before and after the electric homogenization treatment in an oxidizing atmosphere according to a preferred embodiment of the present invention.

Figure 4 is a graph comparing the electrical homogenization treatment rate of oxygen and ozone as oxidizing gas according to a preferred embodiment of the present invention in a constant voltage mode.

Figure 5 is a graph measuring the electrical homogenization treatment rate of the oxidizing gas ozone according to a preferred embodiment of the present invention in a constant current mode.

Figure 6a is a graph measuring the field emission I-V characteristics of the ozone treatment according to a preferred embodiment of the present invention, Figure 6b shows the F-N curve obtained from the I-V graph of Figure 6a.

FIG. 7 illustrates observing a light emitting image while changing power before and after performing an ozone homogenization treatment according to a preferred embodiment of the present invention.

7 is an enlarged light emission image before and after 0.3W processing.

FIG. 9 graphically illustrates a-du of FIG. 7.

10 is a graph comparing the lifespan of the specimens subjected to the electric homogenization treatment only and the ozone homogenization treatment according to the preferred embodiment of the present invention.

Claims (9)

By applying an electric field in an atmosphere in which ozone (O ₃ ) is introduced into a chamber in a high vacuum state, electron emission characteristics of carbon nanotubes are uniformed by uniformizing the length difference of carbon nanotubes having different lengths by intensive electron emission of carbon nanotubes. Method for improving the field emission characteristics of carbon nanotubes, characterized in that the uniformity. The method according to claim 1, Method for improving the field emission characteristics of the carbon nanotubes, After maintaining a constant current in the high vacuum chamber, and the voltage is stabilized under the constant current, Ozone (O ₃ ) is injected into the chamber to remove the tip tip of the non-uniform carbon nanotubes by intensive electron emission of the carbon nanotubes, thereby equalizing the length difference of the carbon nanotube emitters, thereby emitting electrons between the carbon nanotubes. A method for improving field emission characteristics of carbon nanotubes, characterized in that the characteristics are uniform. The method according to claim 1, Method for improving the field emission characteristics of the carbon nanotubes, In a state in which a constant pressure is maintained by continuously adding ozone (O ₃ ) to a high vacuum chamber, By applying a voltage to remove the tip end of the carbon nanotubes by intensive electron emission of the carbon nanotubes to equalize the length difference of the carbon nanotube emitters, thereby uniformizing the electron emission characteristics between the carbon nanotubes Method for improving field emission characteristics of carbon nanotubes. delete delete delete delete delete delete

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