JP2007242253A - Sharpened carbon nanotube and electron source using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To acquire a distinct observation image by enhancing the field intensity of the electron source of an electron microscope. <P>SOLUTION: This electron microscope has an electron emitting negative electrode having a carbon nanotube, and an extraction device to cause field emission of an electron, and uses the carbon nanotube having an acute angle portion in an almost conic form at its tip part. A manufacturing method having a process to heat-treat the carbon nanotube having the acute angle portion by leaving it so as to stand still on the temperature side lower and higher than a phase transition temperature is used. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a carbon nanotube with a sharpened tip shape. The present invention also relates to the application of the carbon nanotube to an electron source.

  Japanese Patent Application Laid-Open No. 2004-079223 (Patent Document 1) discloses an electron source using carbon nanotubes, and describes that an electron beam with high luminance and a narrow energy width can be obtained. Although many studies are currently being conducted on electron sources, the correlation between the beam pattern and the electron emission site of the carbon nanotube is still unclear. In particular, it is said that the dimensions of the carbon nanotubes are quantized in the radial direction, and the size of the electron emission site is also greatly discussed in academic societies from the viewpoint of the uncertainty principle. This makes it difficult to design an electron gun on electron optics when the characteristics of an electron beam are used as an electron source of an electron microscope.

JP 2004-079223 A

  In the electron microscope, it is necessary to improve the electric field strength of the electron source in order to make the observation image clear, but there is a limit to increasing the voltage. Accordingly, an object of the present invention is to provide an electron source that suppresses a voltage increase and improves the electric field strength.

  The present invention for solving the above-mentioned object is an electron microscope having an electron emission cathode having carbon nanotubes and an extraction device for field emission of electrons, and using carbon nanotubes having a substantially conical acute angle at the tip. It is in. In addition, a manufacturing method including a step of heat-treating the carbon nanotube having an acute angle portion by standing at a lower temperature side and a higher temperature side than the phase transition temperature is used.

  A feature of the present invention for solving the above-described problems resides in a carbon nanotube electron source in which an acute angle portion is provided at the tip of the carbon nanotube. The acute angle portion has a substantially conical shape such as a pencil shape. By adopting the above-described configuration, it is possible to improve the electric field strength of the electron gun even when the applied voltage is constant.

The opening angle of the pencil-shaped tip is preferably 120 degrees or less. The electric field strength of the electron source is
E (electric field intensity) = V (applied voltage) / R (radius of curvature of the electron source tip). Therefore, by setting the opening angle to about 120 °, the radius of curvature of the tip is about ½, and a double electric field strength can be obtained.

  The opening angle is the angle formed when the solid angle at the tip of the sharpened carbon nanotube is projected onto a plane.

  Another feature of the present invention for solving the above-described problem is a method for producing carbon nanotubes, which is a temperature lower than 550 ° C. to 620 ° C. and higher than the phase transition temperature (temperature at which the carbon nanotubes become amorphous; 660 ° C.). The heat treatment at 700 ° C. to 1200 ° C. is performed at least once with the carbon nanotubes standing still.

  The time for performing the heat treatment step is preferably 0.8 to 4 hours at a low temperature and 0.8 to 2 hours at a high temperature.

  According to the electron source having the above configuration, it is possible to sharpen an image of an electron microscope. In addition, it is possible to reduce the applied voltage when acquiring images of the same degree and improve the durability of the electron microscope.

  Hereinafter, details of the present invention will be described.

(Production method of carbon nanotube)
Typical methods for producing carbon nanotubes include an arc discharge method, a laser ablation method, and a vapor phase (CVD) method.

  In the arc discharge method, two graphite rods having a diameter of 10 mm and a length of 100 mm were prepared, and the end faces of each other were attached. The two graphite rods were placed so that their central axes were aligned with each other, and a gap of about 1-2 mm was formed after contact. In this state, a DC voltage may be applied to cause arc discharge, but there is a possibility that gas components in the atmosphere are mixed to contaminate the carbon nanotube. Therefore, the inside of the sample chamber in which the graphite rod was installed was evacuated using a rotary pump to remove the gas component. Thereafter, an inert gas such as He or Ar was introduced to adjust the pressure to almost atmospheric pressure. The arc discharge was performed by DC arc discharge with a current in the range of 150 to 200 amperes and a voltage in the range of 10 to 20 volts. When the arc flew from the cathode side graphite rod to the anode side graphite rod, the graphite was volatilized from the end surface of the anode side graphite rod and deposited on the end surface of the cathode side graphite rod.

  In order to continuously deposit the carbon nanotubes on the end face of the graphite rod on the cathode side, the gap was adjusted while feeding the graphite rod so as not to change the gap.

  Further, the gap was constantly adjusted while operating within the range of the current and voltage.

  When arc discharge concentrates in one place, carbon nanotubes may be deposited as if horns grew from the end face. This is because horn formation is likely to cause electrical leakage due to contact between the anode rod and the cathode rod.

  Further, when a carbon nanotube is produced under the same production conditions using a graphite rod with a hole formed in the center of the graphite rod and filled with boron or carbon boride, a boron-doped carbon nanotube can be produced. The manufacturing conditions at that time do not change. The boron content can be adjusted by the amount of filling, and is preferably 0.5 to 5 mol% from the hardness of the carbon nanotube and the electron emission characteristics when used in an electron microscope.

  Next, a method for producing carbon nanotubes by CVD will be described. We manufactured using the thermal CVD method. An apparatus configuration for performing the thermal CVD method is shown in FIG. The raw material of the carbon nanotube is a liquid organic solvent such as toluene or methanol. First, these organic solvents are vaporized. As a vaporization method, there are a method in which an organic solvent is sent to a reactor core tube by a syringe and volatilized in the reactor core tube, or a method in which the solvent is previously vaporized and the flow rate is adjusted with a flow meter and a valve. Ar was used as a carrier gas for diluting the evaporated solvent. The vaporized solvent is grown on carbon nanotubes using a catalyst on a substrate or in a vacuum reaction tube. As the catalyst, iron acetate, ferrocene, nickel acetate, nickelocene, cobalt acetate and the like can be used. When growing on a substrate, the catalyst is used alone or in combination, and the solution is applied to a silicon substrate. In the case of vapor phase growth inside the furnace core tube, it was dissolved in a raw material organic solvent and vaporized at the same time as the solvent.

  The temperature of the furnace was set in the range of 600 to 1200 ° C., and carbon nanotubes were produced. When grown on the substrate, carbon nanotubes were formed on the substrate, and when vapor grown, they were formed along the furnace wall of the vacuum tube. Further, since it was grown in the vapor phase and discharged together with the gas, it was trapped and taken out later.

When a carbon compound containing nitrogen such as melamine or pyrazine was dissolved in the liquid organic solvent and used, nitrogen-doped carbon nanotubes were generated. In that case, Ar and ammonia were mixed and used as the carrier gas. By setting the mixing ratio of Ar and ammonia in the range of 1: 1 to 20: 1, the amount of nitrogen in the carbon nanotube could be controlled to 1 to 5 atomic%. When controlling the nitrogen amount to 2 to 4 atomic%, the ratio of Ar to ammonia gas was set to about 10: 1. The nitrogen content in the carbon nanotubes increased as the ammonia gas blending ratio increased. The furnace temperature is desirably 800 to 1000 ° C. The nitrogen content increased as the temperature was lowered.

Although the gas flow rate must be changed depending on the diameter of the reaction tube, the total gas flow rate per unit area is preferably in the range of 1.0 to 100 ml / cm ² / min. In the present embodiment, a quartz tube having an inner diameter of 60 mm was used and the gas flow rate was 20 ml / min.

(Manufacturing method of carbon nanotube with sharp corner)
Hereinafter, the manufacturing method of the carbon nanotube which has an acute angle part at the front-end | tip of this invention is demonstrated.

  First, the carbon nanotube for heat treatment of the present invention was prepared according to the above production method. As the carbon nanotube, a carbon nanotube (A) made of only carbon, a carbon nanotube (B) containing boron in carbon, and a carbon nanotube (C) containing nitrogen in carbon were prepared. Carbon nanotubes (A) and (B) were prepared by arc discharge. Moreover, the carbon nanotube (C) was created by the CVD method. These carbon nanotubes had a substantially hemispherical tip shape.

Next, the above-mentioned carbon nanotubes were left in a vacuum atmosphere at a pressure of 1 × 10 ⁻³ Pa or less in a vacuum heat treatment apparatus to perform heat treatment.

  The heat treatment on the low temperature side was performed at 600 ° C. for 1 hour, and then the apparatus temperature was increased over 10 ° C./minute, held at 1050 ° C. on the high temperature side for 1 hour, and then allowed to cool. As a result of the heat treatment, the carbon nanotube having a thickness of about 25 nm and a tip opening angle of about 130 ° C. has a tip opening angle of about 10 degrees with almost no change in thickness. FIG. 2 shows the carbon nanotubes before and after the heat treatment.

When the temperature of the carbon nanotube is increased in a vacuum atmosphere higher than 1 × 10 ⁻³ Pa, the carbon nanotube becomes amorphous at about 660 ° C. (phase transition temperature) or lower, and the semi-amorphous carbonaceous material becomes higher than the phase transition temperature. Graphitized. By leaving the carbon nanotubes during the heat treatment, carbon nanotubes having different tip shapes from those before the heat treatment were obtained.

  In the heat treatment, the low temperature side is preferably set in the range of 550 to 620 ° C. This is because amorphization does not proceed quickly unless the temperature is 550 ° C. or higher. Moreover, when it becomes 620 degreeC or more, it will become too close to a phase transition temperature and amorphous-ized and graphitization will advance simultaneously.

  Moreover, it is good to make a high temperature side into the range of 700 to 1200 degreeC. This is because carbon nanotube formation does not proceed when the temperature is 700 ° C. or lower. Further, when the temperature is 1200 ° C. or higher, there is a possibility of phase transition from carbon nanotubes to graphite. In particular, when the temperature is higher than 2000 ° C., the graphitization temperature is reached, and the cylindrical structure of the carbon nanotubes is destroyed, and there is a risk that the flat graphite sheets are split and broken one by one.

  The holding time of the heat treatment is preferably 0.1 to 8 hours on the low temperature side and 0.1 to 2 hours on the high temperature side. This is because the low temperature side is not sufficiently amorphized at 0.1 hours or less, and amorphization proceeds too much at 4 hours or more, and the shape of the carbon nanotubes may disappear. When the high temperature side is 0.1 hours or less, it is not sufficiently graphitized and does not change even when heated for 2 hours or more.

  When the heat treatment was performed with stirring, the carbon nanotubes were cut but the tip was not sharpened.

  The phase transition temperature is a first order transition and a phase transformation. Since the phase transition is a thermal activation process, the reaction does not proceed quickly without sufficient heat input. The second order transition such as the magnetic transformation temperature is aimed at the fact that the reaction proceeds at once when the transition temperature is reached. It is generally known to follow Arrhenius' law where the activation energy increases exponentially. In addition, it is considered that transformation can be started at a relatively low temperature in atomic defects such as vacancies, a collection of the defects, and a strained portion where carbon forms a five-membered ring.

  Transformation requires energy to rearrange geometric atoms. If there is a defect, less energy is required for rearrangement, but if there is no defect, the rearrangement energy is higher. Therefore, the phase transition temperature of the carbon nanotube has a range of 660 ° C. ± 40 ° C. Setting the heat treatment temperature within this range should be avoided because different reactions may occur in each part of the carbon nanotube.

  Next, as shown in FIG. 3, heat treatment was performed by repeating the above steps twice. The heat treatment temperature and time are the same as in Example 1. As a result, the tip shape became an acute angle. On the other hand, some carbon nanotubes were cut by multiple heat treatments, and short carbon nanotubes were obtained as a whole.

  Amorphization of the carbon nanotube starts centering on the defect portion. If a defect portion exists in the carbon nanotube, tearing or the like may occur from the portion. By repeating the heat treatment twice, carbon nanotubes having a length of 10 μm or more may become amorphous at 2-3 locations, and may be half or less in length.

Next, the influence of the heat treatment on the length of the carbon nanotube was examined. The time for maintaining the heat treatment temperature on the low temperature side in the process of Example 2 was changed, and the heat treatment was repeated twice, and the change in the length of the carbon nanotube after the heat treatment was measured. Check about 500 pieces for each sample by hand and round off the decimal point to 1μm, 2μm, 3μm, 4μm, 5μm, 6
The numbers were classified into μm, 7 μm, 8 μm, 9 μm, 10 μm or more.

  The result is shown in FIG. Before the heat treatment, many were 10 μm or more, but the heat treatment increased those less than 10 μm. When held at 600 ° C. for 3 hours, a larger number of 2 μm length was obtained compared to 1 hour. Further, as a result of holding at 600 ° C. for 6 hours, substantially the same number of carbon nanotubes of 1 to 4 μm were obtained.

  Therefore, when a long carbon nanotube is required, it is preferable to make the amorphization time short. Specifically, it is preferably 0.5 hours or less. In addition, when carbon nanotubes having different lengths are required, it is preferable that the length is 5 hours or longer.

(Electron source using sharpened carbon nanotubes)
An electron source was prepared using the carbon nanotubes prepared in Example 1 above. The carbon nanotube used was made of carbon, had a diameter of about 25 nm, a length of about 5 μm, and an opening angle of an acute angle portion at the tip of about 10 °.

  FIG. 5 shows a schematic configuration diagram of a carbon nanotube electron source for an electron microscope of the present invention. The electron source shown in FIG. 1 includes a single carbon nanotube having an acute angle portion at the tip, a cathode composed of a conductive base material supporting the carbon nanotube, an extraction device that emits electrons in an electric field, and an acceleration device that accelerates electrons. It is an example comprised from.

  An electron source operation experiment was conducted using an electron source having an acute angle at the tip. On the other hand, as a comparative example, an electron source similar to the above was prepared except that a normal carbon nanotube having a circular arc at the tip was used, and the operation experiment was similarly performed.

  The apex of the tip of the carbon nanotube before the heat treatment may not be the rotational center position of the diameter of the carbon nanotube (see FIG. 6). However, the apex of the sharpened carbon nanotube tip after the heat treatment is on the extension line (center position) of the shaft. As a result, the electron source to which the carbon nanotube of the present invention is applied does not shift the beam position, and the adjustment of the beam axis is easy. In addition, the beam is emitted straight and there is an effect that there is little noise.

  Furthermore, since the tip is sharpened, the current density increases in proportion to the voltage, and when normal carbon nanotubes are used, field emission cannot be achieved unless 2 kV (threshold voltage) is applied. Field emission was possible at a voltage of 0.5 kV. As a result, an electron source / electron microscope with low power consumption and low power consumption should be provided. In addition, if the voltage is the same, the current density and the electric field strength increase, so that the acquired image becomes bright and a high-definition electron microscope can be provided.

  In addition, as a technique for improving the resolution by converging the electron beam of an electron microscope, it has been studied to employ magnetic field superposition. Unlike the dissipation of the electron beam used in the conventional electromagnetic lens, this method condenses the electron beam in order to increase the beam current in the magnetic field superposition. When the electron source of the present invention is employed, an effect of being easily superposed because of the small diameter of the tip is expected.

(AFM using sharpened carbon nanotubes)
In recent years, a cantilever for a scanning probe microscope using carbon nanotubes has been studied. In particular, it is described that a fine unevenness of a sample surface is detected using an atomic force microscope (AFM) or the like by utilizing the thin tip of a carbon nanotube (Japanese Patent Laid-Open No. 2000-249712, etc.). FIG. 7 shows the device configuration of the cantilever part.

  The results of using a probe using a carbon nanotube having a sharp corner like a pencil at the tip of the present invention and a probe using a carbon nanotube having a tip cut by applying a conventional pulse voltage are shown below. Compared. Both used carbon nanotubes with a diameter of about 25 nm.

  For evaluation of resolution, Veeco DI5000 was used. The contact mode was used to measure the shape of the surface by contacting the surface of the silicon semiconductor while resonating the cantilever. The spring constant of the cantilever used at that time was 1.8 N / m 2.

  As a result, the carbon nanotube probe tip having a sharp corner like a pencil shape at the tip realized a resolution of about 1.5 nm which is the tip diameter of the sharp corner. On the other hand, with the carbon nanotubes cut by applying a pulse voltage to the tip, a resolution of about 25 nm corresponding to the diameter of the carbon nanotubes was obtained.

  In addition, since the tip of the carbon nanotube of the present invention has an apex on the rotation center axis of the carbon nanotube, it is possible to obtain an image with little distortion. It was found that the tip shape of the carbon nanotube probe was directly linked to the image resolution as it was, and sharpening of the carbon nanotube was effective in improving the resolution.

Schematic of a CVD apparatus (carbon nanotube manufacturing apparatus) that manufactures carbon nanotubes. The tip of a conventional carbon nanotube before heat treatment. The front-end | tip part of the carbon nanotube of this invention after heat processing. FIG. 6 is a diagram illustrating a heat treatment process of Example 2. The figure which shows the dependence to the heat treatment process of carbon nanotube length. The schematic block diagram of the carbon nanotube electron source for electron microscopes. Carbon nanotube with the apex of the tip deviating from the axis of rotation. (A) is the structure schematic of the cantilever part for scanning probe microscopes, (b) is an enlarged view of the cantilever probe part.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1a ... Furnace (heater), 1b ... Reaction tube, 1c ... Valve, 1d ... Flow meter, 1e ... Liquid nitrogen trap for product recovery, 1f ... Exhaust gas trap for neutralization of ammonia gas, 5a ... Carbon nanotube, 5b ... Electrode 5c ... electrode support, 5d ... extraction electrode, 5e ... acceleration electrode, 5f ... extraction electrode power supply, 5g ... acceleration electrode power supply, 5h ... field emission cathode, 7a ... cantilever, 7b ... base, 7c ... holder, 7d ... Carbon nanotube (probe).

Claims (10)

An electron microscope having an electron emission cathode having carbon nanotubes and an extraction device for field emission of electrons,
2. The electron microscope according to claim 1, wherein the carbon nanotube has a substantially conical shape at a tip portion on the side from which electrons are emitted, and the tip portion has a closed structure.   2. The electron microscope according to claim 1, wherein the substantially conical shape at the tip of the carbon nanotube has an opening angle of 120 ° or less. The electron microscope according to claim 1 or 2,
The diameter of the said carbon nanotube is 30 nm or less, The electron microscope characterized by the above-mentioned. The electron microscope according to any one of claims 1 to 3,
The electron microscope characterized in that the carbon nanotube contains 0.5 to 5.0 mol% boron or nitrogen. The electron microscope according to any one of claims 1 to 4,
An electron microscope characterized in that the apex of the substantially conical shape of the carbon nanotube is on the rotation center axis of the carbon nanotube. A method for producing a carbon nanotube having an acute angle portion at a tip,
A method of producing a carbon nanotube, comprising: standing still at 550 to 620 ° C. on the low temperature side, and then heat-treating the carbon nanotube at 700 to 1200 ° C. on the high temperature side. It is a manufacturing method of the carbon nanotube according to claim 6,
The method for producing carbon nanotubes, wherein the low temperature side heat treatment step is 0.1 to 8 hours, and the high temperature side heat treatment time is 0.1 to 2 hours. A method for producing a carbon nanotube according to claim 6 or 7,
A method for producing carbon nanotubes, comprising repeating the heat treatment steps on the low temperature side and the high temperature side. A method for producing a carbon nanotube according to any one of claims 6 to 8, comprising:
The method for producing carbon nanotubes, wherein the carbon nanotubes subjected to the heat treatment contain boron or nitrogen. A scanning probe microscope having a probe and a cantilever that senses a change in the position of the probe, wherein the probe is a carbon nanotube manufactured by the manufacturing method according to claim 6. Scanning probe microscope.

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