KR101038670B1 - Carbon Nano-tube-based Cathode Fabrication Method for Growing Carbon Nano tubes on a Metal Tip without any Buffer Layer - Google Patents

Abstract

The present invention grows carbon nanotubes directly without a separate buffer layer on the cobalt tip, thereby reducing contact resistance and reducing heat generation due to field emission, thereby improving adhesion of carbon nanotubes, thereby improving the electric field for a long time. The present invention relates to a method for producing a carbon nanotube anode, which has a long lifetime in the emission operation. According to an aspect of the present invention, there is provided a method of manufacturing a carbon nanotube anode, comprising: preparing a cobalt tip having a cobalt (Co) wire end in a conical shape and without a buffer layer on the cobalt tip. Directly growing carbon nanotubes.

Cobalt Tip, Carbon Nanotube, ICP-CVD, Electron Beam Source, X-ray

Description

Carbon Nano-tube-based Cathode Fabrication Method for Growing Carbon Nano tubes on a Metal Tip without any Buffer Layer}

The present invention relates to a method for manufacturing a carbon nanotube anode, and in particular, by growing carbon nanotubes directly without a buffer layer on the Co (cobalt) tip to improve the contact (contact) and reduce the heat generation (adhesion) of carbon nanotubes The present invention relates to a method for producing a carbon nanotube anode, which has a long lifespan in the field emission for a long time.

Due to the excellent electrical, mechanical, and chemical properties of carbon nanotubes, they are widely used as electron emission sources in X-ray tubes, fluorescent tubes, and flat panel displays. Conventionally, X-ray tube using a hot electron emission source such as tungsten filament has been widely used, but it is expensive to manufacture, operates at high temperature conditions, and consumes a lot of power, especially when applied to a fine focused x-ray electron source. It is difficult to miniaturize and has a short lifespan, and its utilization is being lowered.

 As a general electron emission source based on carbon nanotubes, a structure in which a buffer layer is formed on a metal tip such as W (tungsten), a catalyst material is formed, and carbon nanotubes are grown. In such a structure, due to the multi-layered structure, the contact resistance is large, which causes a decrease in voltage supply for electron emission. As a result, there is a problem in that heat is generated during long periods of high pressure operation, thereby degrading adhesiveness. Accordingly, there are vulnerabilities in which important characteristics such as threshold voltage, maximum emission current, reliability, and lifetime are degraded.

Accordingly, the present invention is to solve the above problems, an object of the present invention is to process Co (cobalt) having excellent physical properties as a catalyst for the electrical properties and carbon nanotube growth in the form of a fine tip, By growing carbon nanotubes directly without a buffer layer on the top, improving the contact resistance and reducing the heat generation to improve the adhesion of the carbon nanotubes to provide a method for producing a carbon nanotube anode to have a long life in the field emission for a long time .

First, to summarize the features of the present invention, according to one aspect of the present invention, a method for producing a carbon nanotube negative electrode, to produce a metal tip so that the end of the metal wire is conical shape and Growing carbon nanotubes directly on the metal tips.

The metal tip is characterized in that the cobalt tip.

The cobalt tip itself may act as a catalyst for the growth of carbon nanotubes in the nucleation process by utilizing the electrical conductivity of the cobalt tip.

The contact resistance characteristic of the structure having carbon nanotubes grown directly on the cobalt tip without a buffer layer may increase the adhesion between the carbon nanotubes and the Co tip by reducing operating voltage and heat generation.

Prior to the growing step, microfabrication, i.e., sharpening the metal wire in the form of a conical tip with KCl or NaCl solution for the fabrication of the metal tip, and the processed metal tip with NH ₃ plasma Surface treatment (dry etching).

In the dry etching, a nucleation process is performed only around the cobalt tip tip by using an electric field concentration around the cobalt tip tip without using a means for masking a portion other than around the cobalt tip tip. Accordingly, selective growth of the carbon nanotubes may be performed only around the cobalt tip end.

The metal wire may be 2 mm or less in diameter and the tip of the metal tip may be 300 nm or more in diameter.

The metal wire may be made of alloys thereof including Co, Ni, Fe, W, Invar, and the like.

The surface treatment may be performed for 10 to 20 minutes at a pressure of 300 to 500 mTorr, RF power of 300 to 450 W, and a DC bias of 100 to 200 V while flowing NH ₃ gas at 100 to 300 sccm.

The growing step is characterized in that for 10 to 20 minutes using a mixed gas of the content ratio of 200: 125 of NH ₃ and C ₂ H _{2 at} a pressure of 300 ~ 500mTorr and RF power 150 ~ 300W.

The nucleation process of the metal tip and the growth of carbon nanotubes are characterized in that the ICP-CVD (inductively coupled plasma-chemical vaor deposition) reactor.

In this manner, the carbon nanotube cathode may have a reduction in emission current of 25% or less from the initial value after driving for 50 hours or more at the emission current of 10 mA.

As described above, according to the carbon nanotube cathode according to the present invention, the Co (cobalt) tip having excellent electrical properties can be directly added to the cobalt tip without a buffer layer so that it can act as a catalyst for carbon nanotube growth. By growing, the contact resistance can be improved to obtain the maximum emission current even at a low operating voltage, thereby reducing heat generation and increasing the adhesion of the carbon nanotubes, thereby ensuring high reliability and long life.

In order to fully understand the present invention, the operational advantages of the present invention, and the objects achieved by the practice of the present invention, reference should be made to the accompanying drawings which illustrate preferred embodiments of the present invention and the contents described in the accompanying drawings.

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

1 is a flowchart illustrating a method of manufacturing a carbon nanotube negative electrode according to an embodiment of the present invention.

Carbon nanotube anode according to an embodiment of the present invention, by making a cobalt fine tip so that the end of the cobalt wire basically conical shape, is made by growing carbon nanotubes directly without a buffer layer on the cobalt tip.

To this end, first, a cobalt wire having a diameter of 2 mm or less is prepared and a cobalt fine tip having a tip of 300 nm or more in diameter, for example, about 1.0 mm, has a conical shape by a predetermined electrolysis method (S110). Here, the electrolysis is a sharp etching by wet etching (etching) the cobalt wire with the KCl solution, and NaCl solution may be used as the etching solution (S120).

In addition, the use of the cobalt tip is to have a good electrical conductivity characteristics and to act as a catalyst to facilitate the growth of carbon nanotubes. Hereinafter, a method of manufacturing a carbon nanotube anode by growing carbon nanotubes on a cobalt tip is described, but is not limited thereto. Nickel (Ni) and iron, which may act as a catalyst and have excellent electrical conductivity instead of the cobalt tip, will be described. (Fe), tungsten (W), etc., or Invar (nickel, iron, carbon, etc.) and these alloys containing these alloys also apply.

The finely processed cobalt tip is transferred to an ICP-CVD (Inductively Coupled Plasma-Chemical Vapor Deposition) reactor and surface treatment is first performed by dry etching (S130). For example, dry surface treatment uses NH ₃ plasma as the nucleation process of carbon nanotubes. This surface treatment is NH ₃ plasma for 10 to 20 minutes under pressure 300 ~ 500mTorr, RF power 300 ~ 450W, DC bias 100 ~ 200V while flowing NH ₃ gas at 100 ~ 300sccm in high vacuum state as shown in [Table 1] In the state.

Next, carbon nanotubes (CNT) are grown on the cobalt tip surface-treated as described above in an ICP-CVD reactor. NH ₃ at pressures of 300 to 500 mTorr and 150 to 300 W And growth is performed for 10 to 20 minutes using a mixed gas having a content ratio (mass ratio) of 200: 125 of C ₂ H ₂ (S140).

TABLE 1

Nucleation Process Growth process NH ₃ 100-300 sccm,
Pressure 300 ~ 500mTorr,
RF power 300 ~ 450W,
DC bias 100-200 V
10 to 20 minutes NH ₃ : C ₂ H ₂ 200: 125,
Pressure 300 ~ 500mTorr,
RF power 150 ~ 300W,
10 to 20 minutes

The carbon nanotubes grown in this way are hexagonal crystal lattice consisting of six carbons connected to each other to form a tubular shape, and the diameter of the tube is only several tens to tens of nanometers (nm), which is called carbon nanotubes. Since the length of carbon nanotubes can be grown from tens of nanometers to tens of micrometers (mm), the β value (field enhancement factor), which determines the electron emission ability, is more than 1000, which is very large compared to other materials. It has a structure that emits electrons much easier than the method of extracting electrons by hot electron emission from tungsten filament. Since it uses quantum field emission method, relatively less power is consumed in extracting the same amount of electrons as compared to the hot electron emission method.

As described above, the microstructure physical properties and electrical properties of the carbon nanotubes in which the carbon nanotubes were grown on the cobalt tip were measured and analyzed (S150). In particular, Field Emission Scanning Electron Microscopy (FESEM) (see FIG. 2), High-Resolution Transmission Electron Microscopy (HRTEM) (see FIG. 3), Raman Spectrum (see FIG. 4), current-voltage characteristics, 30 hours or more Stability characteristics during the measurement were analyzed. The measurement of the current-voltage characteristics is performed by using an Induim Tin Oxide (ITO) substrate as an anode and having a cobalt tip in Cu as a cathode, and then fixing a predetermined vacuum at a distance of 250 mm between the two poles. Conditions, for example, pressure 8 × 10 ⁻⁸ Torr.

In addition, according to a widely used method, carbon nanotubes are coated with a buffer layer such as TiN on a metal such as a W (tungsten) tip, and a catalyst material such as cobalt (Co) is applied thereon. After coating, the carbon nanotubes may be grown by injecting a carbon nanotube source gas such as C ₂ H ₂ under conditions similar to the above CVD atmosphere. Such a carbon nanotube cathode structure CNT / Co / TiN / W-tip is generally well known, hereinafter referred to as CNT-A, and the buffer layer as described above according to an embodiment of the present invention manufactured in the same manner as in FIG. The structure CNT / Co, in which the carbon nanotubes are grown, will be referred to as CNT-B, and the existing structure (CNT-A) will be described by comparing the structure (CNT-B) of the present invention.

2 shows a FESEM image (a) of the structure CNT-A of the conventional carbon nanotube anode and a FESEM image (b) of the structure CNT-B of the carbon nanotube anode of the present invention.

As shown in FIG. 2, the average diameters and lengths of the prepared carbon nanotubes were compared in two areas (Area-1 of 6.3 mm and Area-2 of 8.2 mm), respectively, in Area-1 of the CNT-A structure. 32 nm and 1.73 mm and 33-2 and 1.85 mm, respectively, in the area-2, relatively uniform, and 34 nm and 1.93 mm in the area-1 in the CNT-B structure, respectively, and 27 nm in the area-2, respectively. And 0.74 mm were found to have poor uniformity. In the CNT-A structure, the cobalt layer formed on the TiN buffer layer is 8 nm or less, and dry etching is easily performed, and thus, the carbon nanotube growth is uniformly distributed.

However, in the CNT-B structure, an electric field is concentrated around the tip of the cobalt tip during the nucleation process (S130), and thus dry etching is performed by ion particles in the plasma state concentrated around the tip of the cobalt tip, thereby nucleating only around the tip of the cobalt tip. The process can be done. Accordingly, the carbon nanotubes may be selectively grown in the carbon nanotube growth process (S140) only at the nucleation site formed around the cobalt tip by the nucleation process (S130). In the conventional CNT-A structure, in order to selectively grow carbon nanotubes at the tip of the tip, a shadow mask for selectively depositing catalytic metal (Co) at the tip of the tip (W-tip) shadow mask). For example, as shown in FIG. 7, in the catalytic metal deposition process, the bottom of the tip (W-tip) is covered with a shadow mask, and the catalyst metal (Co) is deposited only at the tip of the tip (W-tip). After the carbon nanotubes are grown on the catalytic metal (Co). As such, when carbon nanotubes are directly grown on the cobalt tip as in the present invention, carbon nanotubes may be selectively grown only around the tip of the cobalt tip even without using a shadow mask.

Figure 3 shows the HRTEM image (a) of the structure CNT-A of the conventional carbon nanotube cathode and the HRTEM image of the structure CNT-B of the carbon nanotube anode of the present invention. At the bottom right of each image, a local enlarged image of the image is inserted.

As shown in FIG. 3, the multilayered crystal structure is shown in both structures, and in the CNT-A structure, the graphite layer is smaller than in the CNT-B structure, whereas the graphite layer is sharper than in the CNT-B structure. Appeared to achieve. This is because the CNT-A structure has less amorphous carbon particles in the carbon nanotubes, and thus shows relatively better crystallinity.

Figure 4 shows the Raman spectrum (a) of the structure CNT-A of the conventional carbon nanotube anode and the Raman spectrum (b) of the structure CNT-B of the carbon nanotube anode of the present invention.

Due to the crystallization of graphite in the vicinity of 1580cm ^-1 of Raman spectrum appears, the G-peak, due to the external carbonaceous impurities D-peak of the Raman spectrum appears in the vicinity of 1350cm ^-1. When carbon nanotubes are grown under the same conditions, the intensity ratio (I _D / I _G ) of D-peak and G-peak represents a difference in crystallinity of carbon nanotubes due to a change in the amount of crystallized carbon. Although it is known that the catalyst greatly influences the crystallinity of the carbon nanotubes in the carbon nanotube nucleation process, as shown in FIG. 4, the I _D / I _G in the CNT-A is slightly smaller than in the CNT-B, No difference was found. When estimated from the FESEM image as shown in FIG. 3, the small appearance of the CNT-A structure is because the CNT-A structure is relatively easily etched during the catalytic etching process using NH ₃ plasma. Thus, although the catalyst etch in the CNT-B structure where the thick cobalt tip itself is used as a catalyst is not completely uniform and has an isolated structure, relatively uniform nucleation of the isolated island structure at the cobalt tip end. It can be made.

5 shows the current-voltage characteristics of the structure CNT-A of the conventional carbon nanotube cathode and the current-voltage characteristics of the structure CNT-A of the carbon nanotube anode of the present invention.

Electron emission at the carbon nanotube cathode is obtained through field emission, for example tunneling, showing linearity as shown in the Fowler-Nordheim relational graph inserted in FIG. . In other words, β values appeared as 1509 in CNT-A and 1539 in CNT-B structure, respectively, without significant difference in both structures.

In addition, the operating voltages required to produce a 10mA uniform emission current were around 920V in CNT-A and 780V in CNT-B structure. At an operating voltage of 1000 V, 20 mA was seen in CNT-A, but a significantly larger value of 134 mA in the CNT-B structure. These results indicate that the electron emission characteristics of the carbon nanotubes are greatly influenced by the contact between the substrate and the carbon nanotubes as well as the properties of the carbon nanotubes themselves. This is because in CNT-A, a buffer layer forms a layered layer and a separate catalyst layer is formed on the W-tip, so that the contact resistance in such a multilayer structure reduces the voltage supply required for electron emission and thus a wider surface. This is because the contact is weakened and the operating voltage is greatly increased. On the other hand, in CNT-B, the cobalt tip and the carbon nanotubes are in direct contact with each other, and there is no multilayer structure contact as described above. In addition, C-Co eutectic structures can occur in carbon nanotube growth and thus have very low contact resistance, enabling efficient electron emission at low voltages.

6 is a FESEM image and graph for comparing the stability characteristics of the structure CNT-A of the conventional carbon nanotube anode and the structure CNT-B of the carbon nanotube anode of the present invention.

Carbon nanotube cathode-based electron emitters affected by contact resistance, etc., are not only concerned with the field emission characteristics of the electron emitters during long-term operation, The stability may vary. FIG. 6 is a result obtained by applying a constant voltage to the carbon nanotube cathodes of both structures and measuring the change in emission current with time. A voltage generating a uniform emission current of 10 mA for at least 30 hours, for example 920 V for the CNT-A structure and 780 V for the CNT-B structure, was applied and measured at 30 minute intervals. After 30 hours, the emission current decreases to 78% for the CNT-A structure and less than 25% for the CNT-B structure. As can be seen from the FESEM image of FIG. 6, in the CNT-A structure, most of the carbon nanotubes formed at the end of most tips are removed after 30 hours of application of the corresponding voltage, and a change in tip shape is observed. It is analyzed that carbon nanotubes melted due to the heat generated by the contact resistance of the multilayer structure at the tip of a high voltage applied during field emission. In particular, after 33 hours in the CNT-A structure, the current measurement is no longer possible due to the frequent arc discharge. In contrast, in the CNT-B structure, the current is generated while the reduction of the emission current to the initial value is less than 25% even after more than 50 hours after applying the corresponding voltage, and the carbon nanotube is partially maintained at the cobalt tip off. It was confirmed.

As described above, in the carbon nanotube negative electrode (CNT-B) according to the present invention, the Co (cobalt) tip itself having excellent electrical properties may have a diameter of about 1.0 mm so as to act as a catalyst for carbon nanotube growth. By growing carbon nanotubes directly at the ends of conical cobalt tips, the contact resistance can be improved and maintained for more than 30 hours with little change in maximum emission current at low operating voltages, thereby reducing heat generation and increasing the adhesion of carbon nanotubes. High reliability and long life can be ensured. The carbon nanotube cathode (CNT-B) according to the present invention may be applied to an X-ray system as an excellent and stable electron emission source.

As described above, optimal embodiments have been disclosed in the drawings and the specification. Although specific terms have been used herein, they are used only for the purpose of describing the present invention and are not intended to limit the scope of the invention as defined in the claims or the claims. Therefore, those skilled in the art will understand that various modifications and equivalent other embodiments are possible from this. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

1 is a flowchart illustrating a method of manufacturing a carbon nanotube negative electrode according to an embodiment of the present invention.

Figure 2 is a FESEM image for comparing the diameter and the length of the carbon nanotubes in the structure (a) of the conventional carbon nanotube anode (b) of the carbon nanotube anode of the present invention.

Figure 3 is an HRTEM image for comparing the crystal state of the carbon nanotubes in the structure (a) of the conventional carbon nanotube anode and the structure (b) of the carbon nanotube anode of the present invention.

4 is a Raman spectrum for comparing the intensity ratio of D-peak and G-peak of carbon nanotubes in the structure (a) of the conventional carbon nanotube anode (b) and the structure (b) of the carbon nanotube anode of the present invention. .

5 is a graph for comparing the current-voltage characteristics in the structure of the conventional carbon nanotube anode and the structure of the carbon nanotube anode of the present invention.

6 is a FESEM image and graph for comparing the stability characteristics of the structure of the conventional carbon nanotube anode and the structure of the carbon nanotube anode of the present invention.

7 is a view for explaining a method of using a shadow mask during the deposition of catalytic metal for the production of a conventional carbon nanotube anode.

Claims (11)

Fabricating the metal tip by wet etching such that the end of the metal wire has a conical shape; Surface treating the metal tip by dry etching; And Growing carbon nanotubes directly on the metal tips; The nucleation process is performed only around the tip of the metal tip by using an electric field concentration around the tip of the metal tip having a constant electrical conductivity when surface treatment by the dry etching, When the carbon nanotubes are grown, selective growth of the carbon nanotubes is performed only around the tip of the metal tip according to the site by the nucleation process. And the metal tip itself acts as a catalyst for the growth of carbon nanotubes. The method of claim 1, wherein the metal tip is a cobalt tip. delete The method of claim 1, wherein the contact resistance characteristic of the structure having the carbon nanotubes directly grown on the metal tip increases the adhesion by reducing the operating voltage and heat generation. delete delete The method of manufacturing a carbon nanotube anode according to claim 1, wherein the carbon nanotube anode is made in an ICP-CVD reactor. The method of claim 1, wherein the metal wire has a diameter of 2 mm or less and the tip of the metal tip has a diameter of 300 nm or more. The method of claim 1, wherein the metal wire is made of an alloy including Co, Ni, Fe, W, or Invar. Carbon nanotube anode prepared using the method of claim 1. The carbon nanotube anode of claim 10, wherein a reduction of the emission current from an initial value after driving for at least 50 hours at a discharge current of 10 mA of the metal tip on which the carbon nanotubes are grown is 25% or less.

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