JP4825968B2 - Carbon nanotube sensor and manufacturing method thereof - Google Patents

Description

  The present invention relates to a carbon nanotube sensor (hereinafter referred to as a CNT sensor) that detects a measurement gas that is inexpensive, responds at high speed, has high detection accuracy, and is easy to manufacture, and a manufacturing method thereof.

  In recent years, a gas sensor that detects a gas using a carbon nanotube (hereinafter referred to as CNT) and a CNT sensor have attracted attention because of their potential. In this CNT sensor, when gas molecules are adsorbed on the semiconductor CNT, charge transfer occurs between them, and the electrical characteristics (conductance, capacitance) of the semiconductor CNT change. Therefore, this phenomenon is used to detect gas. However, at present, there are only a limited number of gases that are actually useful as sensors, and the response speed is not sufficient.

  With regard to the structure of such a CNT sensor, a CNT sensor in which a large number of semiconductor CNTs are grown on the sensor electrode by a CVD method between a pair of electrodes under an atmosphere in which a magnetic field is applied, or a large number of semiconductor CNTs generated in advance are used as a solvent. There has been proposed a CNT sensor or the like that is randomly dispersed by being dispersed between the electrodes, applied between the electrodes, and dried (see Patent Document 1). However, in both sensors, since nano-sized semiconductor CNTs cannot be freely manipulated, they are directly grown on electrodes or applied.

  The present inventor has conventionally applied DEPIM (Dielectrophoretic Impedance Measurement) to polarize micro-sized micro objects such as bacteria by applying an unequal electric field and collect the micro objects by dielectrophoretic force. (Patent Document 2), this DEPIM method can be used to integrate CNTs between microelectrodes easily and accurately while measuring impedance with a monitoring device. It has been found that the same apparatus used for monitoring can be used for gas detection as it is using the produced CNT sensor.

Therefore, the present inventor used a CNT sensor manufactured by the DEPIM method as a high-voltage electric device that has been relatively unreliable and has not been fully elucidated, such as a gas insulated switchgear (hereinafter referred to as GIS). ) And was used as a gas sensor for detecting the cracked gas of sulfur hexafluoride (SF ₆ ) gas, and succeeded in measuring cracked gas at a ppb level that cannot be measured by conventional gas checkers (special Application 2004-21531).

  However, research on CNT sensors has just begun. For example, it has been known that a potential barrier is formed at the junction between a metal and a semiconductor, but recently, it has been reported that ambient oxygen adsorbs and affects the potential barrier of a semiconductor CNT. (See Non-Patent Document 1). However, no further detailed report has been made at present. In other words, the CNT sensor has started macro research, but the problems for practical use of the CNT sensor remain unsolved.

  By the way, the metal work function is related to the level of the potential barrier. The work function of a metal is the minimum value of the energy required to extract electrons at the Fermi level in the metal from the surface into the vacuum, but when the atoms and molecules adsorb on the surface and have a charge, an electric dipole is induced. This is thought to be because the height of the potential barrier changes when electrons are extracted from the surface into the vacuum, and the work function changes (see Non-Patent Document 2). This is considered to be behind the report of Non-Patent Document 1.

  And, for example, when the above-mentioned oxygen, halogen atoms, etc. having a large electronegativity are adsorbed on the metal surface, these adsorbents are negatively charged, and the emitted electrons receive a repulsive force on the surface, so that the work function increases. . Conversely, when alkali metal or alkaline earth metal atoms having a small ionization potential are adsorbed, the adsorbed atoms are positively charged, the potential barrier is lowered, and the work function is decreased.

  In addition, when the measurement of a work function is needed, it has conventionally measured using an electron beam, an ultraviolet-ray, and an ion beam (for example, refer patent document 3).

  However, the value of the work function of CNT is said to be about 4 eV to 5 eV because CNT is difficult to control the material and has many uncertain factors. However, it is possible to increase the level of control by inserting other foreign substances. For example, the work function of a single-walled carbon nanotube tube bundle when Cs is intercalated is 4.6 eV to 4.9 eV (see, for example, Patent Document 4).

JP 2003-227808 A Japanese Patent Laid-Open No. 2003-224 JP 2001-50916 A Japanese Patent Laid-Open No. 2004-534661 V. Derycke, R. Martel, J. Appenzeller, Ph. Avouris, “Controlling doping and carrier injection in carbon nanotube tarnsister”, Appl. Phys. Lett., 80, 2002, p. 2773-p. 2775 Yoshimasa Murata, Katsumi Yagi, and Takeo Hattori, Advanced Electronics Series I-19 “Physical Properties of Solid Surface and Interface”, Baifukan, March 20, 1999, p. 80

  As described above, the CNT sensor has been studied macroscopically as in Patent Document 1, but the problems in practical use of the CNT sensor remain unsolved. In this regard, the CNT sensor based on the DEPIM method of Patent Document 2 proposed by the inventor has drastically opened the possibility of a gas sensor, but problems such as further high-speed response and high accuracy of the CNT sensor remain unsolved. is there. And when the practical subject with respect to such a CNT sensor is solved, it is thought that a CNT sensor occupies a fixed position as a gas sensor.

  Accordingly, an object of the present invention is to provide a carbon nanotube sensor that is highly sensitive and responds to measurement gas at high speed, is inexpensive, and is easy to manufacture.

  Another object of the present invention is to provide a manufacturing method for manufacturing a carbon nanotube sensor that is highly sensitive, responds to measurement gas at high speed, is inexpensive, and is easy to manufacture.

The present invention relates to a semiconductor carbon nanotube material is electrically joined with end cross-linked with a pair of aluminum electrodes, measuring a change in resistance between the aluminum electrode based on the adsorption of any gas measurement of the NO _x or SF ₆ decomposition gas around The carbon nanotube sensor detects the gas concentration, and is characterized in that a Schottky junction is formed between the semiconductor carbon nanotube material and the aluminum electrode.

  According to the carbon nanotube sensor of the present invention, it is possible to respond to the measurement gas with high sensitivity and high speed, and it is inexpensive and easy to manufacture.

First embodiment of the present invention, a semiconductor carbon nanotube material is electrically joined with end cross-linked with a pair of aluminum electrodes, between the aluminum electrodes based on the adsorption of any gas measurement of the NO _x or SF ₆ decomposition gas around A carbon nanotube sensor that detects a gas concentration by measuring a change in resistance of a carbon nanotube sensor, wherein a Schottky junction is formed between a semiconductor carbon nanotube material and an aluminum electrode. Since the junction between the aluminum electrodes is a Schottky junction, a rapid response is obtained when the gas measurement is started, the measurement can be performed in a short time, and the sensitivity is increased. The aluminum electrode showed a sensitivity and response speed of 100 times more chromium electrode, the SF ₆ decomposition gas generated in the NO _x and GIS in question in the exhaust gas or the like in a short time by an inexpensive gas sensor can be accurately measured.

The second form of the present invention is a form subordinate to the first form, and the measurement gas induces charge transfer based on the properties of the aluminum between the measurement gas and the aluminum electrode by adsorption to the Schottky junction. This is a carbon nanotube sensor that changes the height of the Schottky barrier and produces a predetermined reverse bias resistance. The height of the Schottky barrier depends on the properties of the measurement gas and aluminum , such as work function, oxide film, etc. Can be changed, a resistance of a predetermined reverse bias can be generated, a rapid response can be made, and the measurement can be performed in a short time.

  A third form of the present invention is a form subordinate to the second form, and is a carbon nanotube sensor in which the height of the Schottky barrier varies depending on the measurement gas concentration, and the height of the Schottky barrier is Since it changes depending on the measurement gas concentration, the measurement becomes easy and the accuracy becomes high.

A fourth form of the present invention is a form subordinate to the first to third forms, wherein the semiconductor carbon nanotube material is a p-type semiconductor, and the work function of aluminum is in a gas atmosphere containing a measurement gas. This is a carbon nanotube sensor having a large work function. In the combination of a p-type semiconductor and an aluminum electrode, a Schottky barrier is formed, a rapid response can be made when gas measurement is started, and measurement can be performed in a short time.

A fifth form of the present invention is a form subordinate to the first to third forms, wherein the semiconductor carbon nanotube material is an n-type semiconductor, and the work function of aluminum is in a gas atmosphere containing a measurement gas. This is a carbon nanotube sensor having a small work function. In the combination of an n-type semiconductor and an aluminum electrode, a Schottky barrier is formed, a rapid response can be made when gas measurement is started, and measurement can be performed in a short time.

A sixth form of the present invention is a form dependent on the first to fifth forms, and is a carbon nanotube sensor composed of an integrated body in which a semiconductor carbon nanotube material is bridged between aluminum electrodes. The nanomaterial cross-linked structure responds at high speed and has high detection accuracy. It can be used at room temperature, has high sensitivity, has a fast response, is easy to manufacture and is inexpensive, can easily obtain an electrical output, and can be used repeatedly.

A seventh form of the present invention is a form subordinate to the sixth form, wherein the aluminum electrode has an electric field concentration edge for generating an unequal electric field when an AC voltage is applied, and the integrated body is formed by dielectrophoresis. This is a formed carbon nanotube sensor, and it can be manufactured inexpensively because it is electrodynamically operated by dielectrophoresis with an electric field concentration edge on the electrode, and the carbon nanomaterial is basically oriented in the electric field direction. Responds at high speed, and the detection accuracy is high.

An eighth aspect of the present invention is a form dependent on the seventh aspect, wherein the aluminum electrode is a thin film electrode provided on an insulating substrate, and an electric field concentration edge is formed on each of the electrodes. This is a carbon nanotube sensor that is the edge of the protruding portion, and can be made into a small and thin electrode, which is easy to manufacture.

In the ninth embodiment of the present invention, the work function of the p-type semiconductor carbon nanotube material and aluminum is measured in a measurement gas atmosphere of either NO _x or SF ₆ decomposition gas , and the work function of the p-type semiconductor carbon nanotube material is select larger than the work function of aluminum, aluminum is a method of manufacturing the carbon nanotube sensors bridging the p-type semiconductor carbon nanotube material as a pair of electrodes, one of the measurement gas and the aluminum of the NO _x or SF ₆ decomposition gas, Depending on the characteristics of the p-type semiconductor carbon nanotube material, the height of the Schottky barrier can be changed, a rapid response can be made, and the measurement can be performed in a short time.

In the tenth aspect of the present invention, the work function of an n-type semiconductor carbon nanotube material and aluminum is measured in a measurement gas atmosphere of either NO _x or SF ₆ decomposition gas , and the work function of the n-type semiconductor carbon nanotube material is A method of manufacturing a carbon nanotube sensor in which an n-type semiconductor carbon nanotube material is cross-linked using aluminum as a pair of electrodes and having a work function smaller than that of aluminum, a measurement gas of either NO _x or SF ₆ decomposition gas , aluminum, Depending on the characteristics of the n-type semiconductor carbon nanotube material, the height of the Schottky barrier can be changed, a rapid response can be made, and the measurement can be performed in a short time.

Example 1
Hereinafter, a CNT sensor, a gas detection device, and a manufacturing method of the CNT sensor of Example 1 of the present invention will be described. In this specification, nano- and micron-sized long carbon nanotubes are suitable as sensor materials, so they are referred to as CNTs as a whole, but these CNTs are carbon nanohorns, carbon nano-onions, carbon nanofibers, etc. Including those with a structure. In the first embodiment, the case where the measurement gas is NO ₂ gas, the metal electrode is aluminum, and the CNT material is a single-walled nanotube (SWCNT) will be described. However, the present invention is not limited to this.

  FIG. 1A is an explanatory diagram of a carbon nanotube sensor in Example 1 of the present invention, FIG. 1B is an explanatory diagram of a main part of FIG. 1A, and FIG. An SEM photograph of the integrated carbon nanotube material, FIG. 2B is a simulation diagram of the electric field distribution in FIG. 2A, and FIG. 3 is a diagram of a gas measuring device for detecting gas by mounting the carbon nanotube sensor in Example 1 of the present invention. It is a block diagram.

1 (a) and 1 (b), 1 is a chip-like CNT sensor for detecting NO _x gas such as NO ₂ and NO gas, SF ₆ decomposition gas, etc., 1a and 1b are castle wall type electrodes, combs A pair of electrodes made of aluminum or the like having a shape such as a tooth-shaped electrode, 2 is an integrated semiconductor carbon nanotube material (hereinafter referred to as CNT material) composed of single-walled nanotubes (SWCNT), etc., and 3a and 3b can perform dielectrophoresis An electric field concentration edge such as a bent edge (hereinafter referred to as an edge) that generates an unequal electric field, 4 is an insulating substrate, and 5a and 5b are connection terminals of the electrodes 1a and 1b. The CNT material 2 is the detection element in Example 1. Reference numeral 6 denotes a joint where the bridging ends of the CNT material 2 are electrically joined to the electrodes 1a and 1b. The junction 6 of the first embodiment is a Schottky junction.

Since the present invention has a close relationship among the three elements of the measurement gas, the CNT material 2, and the metal electrode, this will be briefly described first. Relationship of the measurement gas and the metal electrodes, the former induces charge transfer adsorbed on the latter, it is possible to change the work function phi _M metal at the junction, thereby changing the Schottky barrier height phi _B As long as it has a relationship capable of generating a resistance of a predetermined reverse bias. Further, the relationship between the CNT material 2 and the measurement gas is that when the former comes into contact with the latter, the sensor (detection element) body causes a resistance change (or impedance change) depending on the gas concentration, and a Schottky barrier height φ _B The semiconductor CNT can be easily controlled, in other words, the semiconductor CNT having an S value (= ∂φ _B / ∂φ _M ) as close to 1 as possible. As described above, the CNT sensor according to the first embodiment of the present invention is determined from the mutual relationship between a plurality of elements such as the measurement gas, the work function of the metal of the electrodes 1a and 1b, and the work function of the CNT material 2.

First, the CNT material 2 in Example 1 will be described. As described above, the CNT material 2 is a CNT such as a single-walled nanotube (SWCNT) or a multi-walled nanotube (MWCNT). CNT is basically a carbon atom having various long shapes such as a cylindrical shape, but is a generic name including a long shape such as a closed rugby ball, and is a nanometer (10 ^{− 9} m) means a substance having a structure of scale size (diameter). Note that “nano” is a name when focusing on the structural unit of the material, and even if a plurality of these aggregate to form an aggregate and become a micron scale (10 ⁻⁶ m), the CNT That's it. In addition, although the main constituent element is carbon, it is natural that CNT materials include elements into which elements other than carbon are inserted for the purpose of controlling characteristics.

  When the CNT material 2 is accumulated on the electrodes 1a and 1b by dielectrophoresis (DEPIM method), the CNT material 2 is once mixed in a solvent such as ethanol, and an AC voltage is applied to the electrodes 1a and 1b in the suspension. It accumulates by dielectrophoresis between the electric field concentration edges 3a and 3b (see FIGS. 1B and 2B) where the electric field strength becomes the largest among the unequal electric fields generated thereby. This manufacturing method by this dielectrophoresis will be described in detail later. After accumulation, the solvent is evaporated and adsorbed on the insulating substrate 4 in a crosslinked state (see the photograph in FIG. 2A).

The CNT material 2 used in Example 1 has properties as a p-type semiconductor. It is produced by an arc discharge method with an average diameter of 1 nm (15 nm as a bundle), a length of 1 μm to 4 μm, and a purity of 50%. It is known that the CNT material 2 exhibiting semiconductor characteristics includes a p-type in which carriers are holes and an n-type in which electrons are carriers. Which type is selected can be controlled by the structure of the CNT material and the doping of other elements. For example, an n-type semiconductor can be formed by doping K, Rb, or the like. The CNT material 2 may be produced by any production method such as a CVD method or a thermal decomposition method. Further, when the CNT material 2 can be grown directly on the electrodes 1a and 1b by these methods, integration by dielectrophoresis is not necessarily required. The CNT sensor 1 uses the fact that a measurement gas such as NO ₂ gas is adsorbed on the surface of such an assembly of the CNT material 2 to exchange electrons and appear as a resistance change between the electrodes 1a and 1b. It is.

  Next, the electrodes 1a and 1b will be described. The castle wall type electrode shown in FIG. 1 has a large number of rectangular protrusions formed at intervals of one pitch (for example, 50 μm to 100 μm) on the opposite sides of the electrodes 1a and 1b. They are arranged one pitch apart from each other, for example, 5 μm to 10 μm apart. Edge portions of the protruding portions of the electrodes 1a and 1b are electric field concentration edges 3a and 3b, and an electric field is particularly concentrated between the electric field concentration edges 3a and 3b (see FIG. 2B). Many shapes, such as a comb-tooth shape and a saw-tooth shape, can be used without being limited to a rectangular shape. In addition, a comb-teeth-shaped electrode has a narrow gap (for example, 5 μm to 10 μm width) in which a pair of electrodes formed with teeth (for example, 30 μm to 100 μm width) is inserted and combined in a groove in a nested manner. ), And an unequal electric field is formed mainly in the groove between the edges in the thickness direction, whereby a large number of CNT materials 2 are accumulated.

  The electrodes 1a and 1b of Example 1 are aluminum thin film electrodes, which are formed by plating, vapor deposition, sputtering or the like on an insulating substrate 4 such as glass, plastic, or silicon oxide, and etching by photolithography or the like. The thickness of the thin film is desirably about 50 nm to 200 nm.

As can be seen from the above , the material of the electrodes 1a and 1b is not limited to aluminum in theory . Any work gas φ _M may be used as long as the measurement gas, for example, NO _x is adsorbed and the work function φ _M changes at the Schottky junction. In the case of the p-type CNT material 2, the work function φ _S is It satisfies the _{M <phi S} need be chosen. Moreover, when using the dielectrophoresis (DEPIM method) as a material for the electrodes 1a and 1b, a metal having a small ionization tendency is desirable so that electrolysis does not occur when an AC voltage is applied. However, aluminum is suitable as the material for the electrodes 1a and 1b.

Next, the CNT sensor of Example 1 and its detection principle will be described. FIG. 4A is an explanatory diagram of the work function of the metal and carbon nanotube of the CNT sensor in Example 1 of the present invention, FIG. 4B is an explanatory diagram of the main part of the equivalent circuit of the CNT sensor of FIG. (A) is explanatory drawing of the Schottky barrier before the contact of the metal of the CNT sensor and carbon nanotube in Example 1 of this invention, FIG.5 (b) is after the contact of the metal of the CNT sensor and carbon nanotube of (a). FIG. 6 is an exploded view illustrating the response of the CNT sensor to NO _{2 according} to the first embodiment of the present invention.

Example CNT material 2 1 is a p-type semiconductor, among metals such as shown in the table of FIG. 4 (a), the inherently small metal work function phi _M is (aluminum) is selected, also CNT material 2 Since the work function φ _{S is} also selected to be larger than φ _M , a Schottky junction having a rectifying function is easily formed between the semiconductor CNT material 2 aggregate and the metal electrodes 1a and 1b. Is done. When the electrode is aluminum, φ _M <φ _S is satisfied if it is a normal CNT. As shown in the equivalent circuit of the CNT sensor in FIG. 4B, _assuming that the resistance of the Schottky junction is R _j1 , R _j2 and the resistance of the CNT material 2 is R _NT , the CNT sensor 1 in the equivalent circuit of the CNT sensor. The resistance R can be expressed as R = R _j1 + R _NT + R _j2 .

_Assuming that a condition for forming a Schottky junction between the electrodes 1a and 1b and the CNT material 2, that is, φ _M <φ _S is satisfied, one of R _j1 and R _j2 is always reversed in the equivalent circuit. Bias, the other is a forward bias resistor. If R _j1 is a reverse bias and R _j2 is a forward bias, R _j2 << R _j1 , R _{NT and} R≈R _j1 + R _NT can be expressed. This R _j1 is a factor that increases the response speed of the present invention.

Next, the Schottky barrier of the CNT sensor will be described based on FIGS. 5 (a) and 5 (b). FIG. 5A shows a state before the metal and semiconductor CNT material 2 are in contact. φ _M is the work function of the metal, and φ _S is the work function of the CNT material 2. Note that the work function is energy required for releasing internal electrons into the vacuum, and is given by the difference between the vacuum level VL and the Fermi level E _f . The energy from the bottom E _c of the conduction band of the CNT material 2 to the vacuum level VL is expressed as an electron affinity χ _s , and the energy gap E _g is E _g = E _c −, where E _v is the energy at the top of the valence band. the E _v. Further, the difference between the top E _c of the valence band surface from the vacuum level VL is energy E _S.

  Since the Fermi level on the CNT material 2 side is lower than the Fermi level on the metal side, when the metal and the CNT material 2 are brought into contact, electrons move from the metal side to the CNT material 2 side, and the valence band of the p-type semiconductor Holes are neutralized. Then, electrons remain on the surface of the CNT material 2 and are negatively charged, and electrons are insufficient on the metal surface and become positively charged. For this reason, as shown in FIG. 5B, the energy band of the CNT material 2 is curved and inclined toward the metal side, and a Schottky barrier against holes is formed.

In this case diffusion potential _{V d} is given by eV _d = φ _{S -φ M,} height phi _B of the Schottky barrier to the diffusion of the holes when viewed from the metal side becomes _E S _{-φ M.} Since E _S = χ _s + E _{g as} can be seen from FIG. 5A, the Schottky barrier height φ _B is represented by φ _B = χ _s + E _g −φ _M. On the other hand, in the n-type semiconductor, the height of the Schottky barrier with respect to electrons is φ _B = φ _M −χ _s .

Accordingly, the height φ _B of the Schottky barrier changes due to this influence when the measurement gas, for example, NO _x is adsorbed on the metal of the junction 6 and electrons move from the NO _x to change the work function φ _M. Is. At this time, the degree of change depends on the gas concentration, and is affected by the presence and properties of an oxide film formed on the surface of the metal as in the case of a diode having a MOS structure. For example, the work function of Al is 4.28 eV, and the work function of Ti is 4.33 eV, which is a fairly close value, but Ti is a titanium oxide film or the like or is exposed to NO _x gas or the like. However, the Schottky barrier height φ _B does not change greatly. Therefore, no reverse bias resistance is generated. Thus for control of phi _B must consider the effect of the nature of the metal.

  Next, the relationship between the sensor body (CNT) of the CNT sensor and the measurement gas that is the measurement target will be described. As shown in the equivalent circuit of FIG. 4B, a predetermined DC voltage is applied to measure the current value, and the resistance between the electrodes 1a and 1b is calculated. Then, the resistance measured in this way is used to obtain the gas concentration using calibration data. In the DEPIM method, dielectrophoresis is performed while measuring and monitoring impedance (conductance, capacitance). If this conductance measuring device is used, the device for dielectrophoresis can be used for gas detection as it is. This will be described later.

Now, the response of the CNT material 2 varies depending on whether the measurement gas is an oxidizing or reducing gas. For example, the resistance of reducible NH ₃ increases rapidly, whereas the resistance decreases with an oxidizing gas such as NO ₂ . This is because the CNT material 2 of Example 1 is a p-type semiconductor. That is, when reducing NH ₃ molecules are adsorbed to the CNT material 2, electrons move from the NH ₃ molecules to the CNT material 2, and the hole density decreases near the surface of the CNT material 2, thereby increasing the resistance. On the other hand, when oxidizing NO ₂ molecules are adsorbed, electrons move from the CNT material 2 to NO ₂ , the hole density increases, and the resistance decreases. And if the CNT material 2 is an n-type semiconductor, it will show the reverse tendency with respect to measurement gas.

  Thus, if the p-type and n-type of the CNT material 2 are properly used, the gas concentration can be detected by increasing or decreasing the measured resistance of the oxidizing gas and the reducing gas. In the present invention, in order to further constitute the CNT sensor, the metal electrode and the CNT material 2 are selected from the point of work function with respect to the oxidizing gas and the reducing gas, and these measurement gases are adsorbed to the joint 6. Sometimes changing the Schottky barrier allows very rapid concentration measurements.

Therefore, hereinafter, a change in the resistance R when a CNT sensor in which a CNT material is cross-linked to an aluminum electrode is exposed to NO ₂ gas will be described. As described above, _{R≈R j1} + R _NT . According to the upper graph of FIG. 6, the resistance R _{j1 serving as} a reverse bias increases very rapidly immediately after contact with the NO ₂ gas because the Schottky barrier is high. is doing. In contrast, as shown in FIG. 6 the middle graph, the resistance _{R NT} of CNT material 2 itself is in a state not in contact with the NO ₂ gas is _{R NTO,} gradually from contact with the NO ₂ gas is decreasing. Electrons move from the CNT material 2 to NO ₂ and the response is slow, but the resistance is lowered because the hole density is increased. Therefore, as shown in the lower graph of FIG. 6, the resistance R of the entire CNT sensor rises sharply from the inherent resistance value _RNTO of the CNT material 2 when exposed to NO ₂ gas, and the resistance R _j1 is maximum. It can be seen that this point has a tendency to gradually decrease with the maximum value as a point.

In this regard, in the conventional CNT sensor 1, the relationship (work function) between the metal electrode and the CNT material 2 in the measurement gas atmosphere is not considered, but R _j1 , R _j2 << R _NT With CNTs connected, only _RNT was measured as shown in the middle of FIG. Therefore, the response speed of the CNT sensor 1 is basically determined only by the adsorption speed of the measurement gas with respect to the CNT material 2, and there is a limit to improving the response.

However, in the present invention, by using a Schottky junction, responsiveness can be remarkably improved by R _{j1 of R≈R j1} + R _NT . This improvement in response speed due to R _j1 is obtained by controlling the height of the Schottky barrier by using the metal type and properties of the electrode as important factors in addition to the CNT material 2 and the measurement gas. The response speed is improved from a completely new viewpoint.

In the first embodiment, the gas concentration is measured by either R _j1 or R _NT . However, by collecting the calibration data, only R _{j1 at the} Schottky junction is used, which is extremely fast and short. A CNT sensor that measures the gas concentration over time is also feasible, and the present invention includes this.

  As described above, in the method of manufacturing the CNT sensor of the present invention, in the case of a p-type semiconductor CNT material, the work function of the p-type semiconductor CNT material is measured in an atmosphere of a measurement gas. The p-type semiconductor CNT material may be cross-linked by selecting a material larger than the work function and using the metal as a pair of electrodes. The n-type semiconductor CNT material may be cross-linked using the metal as a pair of electrodes by measuring and selecting a work function of the n-type semiconductor CNT material smaller than that of the metal. It is possible to provide a CNT sensor in which the height of the Schottky barrier barrier is easily changed depending on the characteristics of the measurement gas, the metal, and the CNT material.

Subsequently, a gas detection apparatus using the CNT sensor of Example 1 for measuring a measurement gas such as NO ₂ gas will be described with reference to FIG. In FIG. 3, reference numeral 7 denotes a gas measuring device that is equipped with the CNT sensor 1 and detects a measuring gas such as NO ₂ gas. The gas measuring device 7 of Example 1 is not only used at the time of measurement, but can be used as it is for a CNT material migration device that dielectrophores the CNT material 2 when the CNT sensor 1 is manufactured.

  11 is a power supply unit that applies an AC voltage for measurement between the electrodes 1a and 1b, 12 is a measurement unit that can measure the impedance between the electrodes 1a and 1b, and 13 is composed of a microprocessor or the like. A calculation control unit that reads and functions and controls at least the power supply unit 11 and the measurement unit 12 and performs calculation, 14 is a display unit, 15 is a memory unit that stores programs and data, and 15a is calibration data of resistance change of the measurement gas. Is a calibration data section, and 16 is a timer section. The power supply unit 11 is a DC or AC power supply, and the AC power supply is controlled by the arithmetic control unit 13. In the first embodiment, the amplitude can be adjusted between 1 kHz and 10 MHz for an amplitude of 0.5 V to 10 V in the case of a DC voltage, and further in the case of an AC. In the first embodiment, a sine wave is applied as an AC voltage, but it means a voltage such as a triangular wave or a square wave that changes the direction of flow at a substantially constant period, and the average value of the currents on both the positive and negative sides is equal. is there.

  The measuring unit 12 is provided with a resistance for current detection of about 1 kΩ, and is inserted in series in the voltage application circuit shown in FIG. 3, applying a sinusoidal high-frequency voltage with a frequency of 100 kHz and an amplitude of 0.5 V to obtain a conductance. The gas concentration is obtained by measuring. As will be described below, this gas detection device can be used in common with the CNT material migration device when measuring with an AC voltage. When a DC voltage is used, only the magnitude of the current is measured by a current detection resistor, the resistance between the electrodes 1a and 1b is calculated, and the gas concentration is calculated from the calibration data in the calibration data section 15a.

  Now, a CNT material migration apparatus for producing the CNT sensor 1 of Example 1 will be described. FIG. 7 is a configuration diagram of the CNT material migration apparatus according to the first embodiment of the present invention. In FIG. 7, reference numeral 21 denotes a power source unit for dielectrophoresis for applying an alternating voltage to generate dielectrophoresis between the electrodes 1a and 1b, 22 denotes a measurement unit capable of measuring the impedance between the electrodes 1a and 1b, 23 Is composed of a microprocessor and the like, functions by reading programs and data, controls at least the power supply unit 21 and the measurement unit 22 and performs calculations, 24 is a display unit for displaying on a display, 25 is a program and A memory unit for storing data, 25a is a data unit storing the amount of accumulation and time, and 26 is a time measuring unit. In order to suppress electrolysis of the CNT suspension and electrode material during dielectrophoresis, the power supply unit 21 is preferably an AC power supply. The control configuration described above of the CNT material migration device is basically the same as that of the gas measurement device 7, and the gas measurement device 7 is shared as a gas measurement / dielectric migration control device 7a.

  Next, reference numeral 27 denotes a migration chamber for introducing a suspension solvent in which the CNT material 2 is suspended in a solvent such as ethanol for the purpose of dielectrophoresis. 28 is a container that stores suspended suspension solvent, 29 is a pump that sends the suspension solvent to the electrophoresis chamber 27, and 30 is a container that applies ultrasonic vibration provided to suspend the CNT material 2 in the solvent. The stirrers 31 and 32 are electromagnetic valves.

  A process for producing the CNT sensor 1 using this CNT material migration apparatus will be described. The thin film electrodes 1a and 1b are formed on the insulating substrate 4 and the CNT material 2 prepared in advance in, for example, ethanol having a concentration of about 1 μg / ml in the container 28, for example, an average diameter of 1 nm prepared by an arc discharge method ( A single-walled CNT (with a purity of 50%) having an average bundle diameter of 15 nm and a length of 1 μm to 4 μm is poured. The calculation control unit 23 operates the stirring device 30 for about 60 minutes to disperse the CNT material 2. In this state, the data section opens the solenoid valves 31 and 32 and operates the pump 29 to send the suspension into the electrophoresis chamber 27 having a volume of about 15 μl at 0.5 ml / min.

  Next, a high frequency sine wave high frequency voltage of 100 kHz and amplitude 4 V is applied between the electrodes 1a and 1b, and dielectrophoresis is started by the generated unequal electric field. The timing unit 26 starts counting from this timing. Along with the time measurement by the time measuring unit 26, the current is measured by the measurement unit 22. The arithmetic control unit 23 reads out a predetermined current value or conductance corresponding to the planned accumulation amount of the CNT sensor 1 from the data unit 25a, stops the power source unit 21 when it counts out, stops the pump 29, and after the water falls, the electromagnetic valve 31 and 32 are closed. Air is circulated through the electrophoresis chamber 27 at room temperature to evaporate ethanol in a relatively short time. After drying, the CNT sensor 1 in which the CNT material 2 is accumulated and crosslinked is taken out. By managing the time for dielectrophoresis and the conductance of the CNT sensor in this way, the amount of CNT material 2 accumulated can be controlled, and the highly sensitive CNT sensor 1 can be easily manufactured.

By the way, the dielectrophoretic force F _DEP can be expressed by complex number expression as F _DEP = 2πε _m · a ³ · Re [K] ▽ E ² . Where ε _{m is} the dielectric constant of the suspension, a is the radius of the CNT material when approximated by a sphere, Re [K] is a parameter depending on the complex dielectric constant of the micro object and the suspension, and E is the electric field strength. is there. This Re [K] changes positively and negatively with the frequency f of the electric field used for dielectrophoresis as a parameter. A positive dielectrophoretic force works in a specific frequency range, for example, 10 kHz to 1 MHz, and a negative dielectrophoretic force works in other cases. Therefore, it is necessary to accumulate the CNT material 2 by selecting the frequency and applying the maximum positive dielectrophoretic force F _DEP .

  Although some CNT materials are spherical like fullerene, the CNT material 2 of Example 1 of the present invention is a nano-sized long and fibrous material. However, according to experiments, long and fibrous objects can be operated in exactly the same way as spheres, and the CNT material migration device uses a positive dielectrophoretic force to move a polarized object to a region where the electric field is maximum. Can be made. The frequency may be determined experimentally. Note that such frequency and voltage amplitude are set for each CNT material 2, and the relationship between the dielectrophoresis time and the integrated amount is stored in the data portion 25a.

  As described above, the CNT sensor 1 of Example 1 can easily integrate the CNT material 2 on the microelectrode using the electrophoretic dielectrophoresis and easily form a bridge between the electrodes 1a and 1b. The CNT sensor 1 can be easily manufactured at low cost.

Now, the experimental results of the CNT sensor of Example 1 described above will be described. FIG. 8 shows the types of metal electrodes of the CNT sensor and the response in NO ₂ in Example 1 of the present invention. FIG. 9 shows changes in the NO ₂ gas concentration of the Al and Cr electrodes of the CNT sensor in Example 1 of the present invention. FIG. 10 is an explanatory diagram of the resistance change rate and gas concentration change in NO ₂ of the Al and Cr electrodes of the CNT sensor in Example 1 of the present invention, and FIG. 11 is the CNT sensor in Example 1 of the present invention. of Al, which is a response diagram of the conductance change in SF ₆ decomposition gas of Cr electrode.

FIG. 8 shows a case in which a pair of electrodes is made of Ti, Cr, Pd, and Al, exposed to 1 ppm of NO ₂ gas, the resistance change ΔR is measured, and normalized by the initial resistance R ₀ . When the electrodes of the CNT sensor are Ti, Cr, and Pd, the resistance change ΔR / R ₀ gradually decreases when exposed to NO ₂ gas for 540 seconds. Thereafter, when N ₂ gas was introduced, the resistance change ΔR / R ₀ gradually increased. Thereafter, when ultraviolet rays were irradiated, ΔR / R ₀ quickly returned to the initial value.

However, when the electrode of the CNT sensor is Al, ΔR / R ₀ rapidly increases when exposed to NO ₂ gas for 540 seconds. The reverse is true for Ti, Cr, and Pd. Thereafter, it gradually decreased by the action of the CNT material 2, and when the N ₂ gas was introduced, the resistance change ΔR / R ₀ gradually increased. Thereafter, when ultraviolet rays are irradiated, ΔR / R ₀ quickly returns to the initial value. Thus, when the electrode is Al, the Schottky barrier can be easily controlled, and the gas concentration can be measured at high speed.

FIG. 9 shows the response when the NO ₂ gas concentration is changed with respect to the Al electrode and the Cr electrode. The increase rate of the resistance change ΔR / R ₀ immediately after exposure of the Al electrode to NO ₂ gas is the concentration. It is smaller in proportion to The resistance change rate and gas concentration change of the Al electrode and Cr electrode are as shown in FIG. Al electrode and Cr electrode are also almost proportional to gas concentration change, but the resistance change rate of Al electrode shows a value nearly 10 times (especially around 0.01 ppm) of Cr electrode, and the response speed of CNT sensor Can be seen to greatly improve

In other experimental examples of the CNT sensor, the Al electrode exhibits a sensitivity and response speed 100 times greater than that of the Cr electrode with respect to SF ₆ decomposition gas (oxidizing gas). FIG. 11 shows the amount of change ΔR / R ₀ of the CNT sensor resistance after generation of SF ₆ decomposition gas for the Al electrode placed in the pipe connected to the GIS and the two Cr electrodes placed in the pipe and in the tank, respectively. Show. Conventionally, SF ₆ cracked gas has been designed to be detected in the GIS tank, but the fact that this is inevitable is also seen from the measurement results of ΔR / R ₀ of the two Cr electrodes (in the pipe and in the tank) in FIG. it is obvious. That is, even if it is intended to detect the SF ₆ decomposition gas with the Cr electrode in the pipe, the diffused SF ₆ decomposition gas cannot be detected at a significant level.

However, in the case of an Al electrode, SF ₆ cracked gas diffused from the GIS tank after generation of SF ₆ cracked gas can be quantitatively detected even when it is arranged in a pipe. Compared with a Cr electrode placed at the same position, According to the data, a sensitivity difference of 100 times or more (detectable up to 0.01 ppb level) is shown. Moreover, this response speed is extremely high, and if this CNT sensor is used, there is no need to install a gas sensor in the tank for GIS equipment diagnosis as in the past, and as a powerful SF ₆ decomposition gas sensor, and GIS equipment diagnosis It can be an epoch-making diagnostic gas sensor.

As described above, according to the CNT sensor of Example 1 of the present invention and the manufacturing method thereof, it can respond to the measurement gas with high sensitivity and high speed, and can be manufactured at low cost. Further, it is possible to detect gas such as NO _x gas in exhaust gas and GIS SF ₆ decomposition gas, which has been difficult in the past, at low cost, with high sensitivity and at high speed.

  The present invention can be applied to a CNT sensor that detects a measurement gas that is inexpensive, responds at high speed, has high detection accuracy, and is easy to manufacture.

(A) Explanatory drawing of the carbon nanotube sensor in Example 1 of this invention, (b) Explanatory drawing of the principal part of (a) (A) SEM photograph of carbon nanotube material integrated between electrodes by dielectrophoresis in the present invention, (b) Simulation diagram of electric field distribution of (a) 1 is a configuration diagram of a gas measuring device that detects a gas by mounting a carbon nanotube sensor in Embodiment 1 of the present invention. (A) Explanatory drawing of the work function of the metal of a CNT sensor and carbon nanotube in Example 1 of this invention, (b) Essential part explanatory drawing of the equivalent circuit of the CNT sensor of (a) (A) Explanatory drawing of the Schottky barrier before the contact of the metal of the CNT sensor and the carbon nanotube in Example 1 of the present invention, (b) of the Schottky barrier after the contact of the metal of the CNT sensor and the carbon nanotube of (a) Illustration Exploded view illustrating the response of the CNT sensor to NO ₂ in Example 1 of the present invention 1 is a configuration diagram of a carbon nanotube material migration apparatus in Example 1 of the present invention. CNT sensor metal electrode type and NO ₂ response diagram in Example 1 of the present invention FIG. 3 is a response diagram according to a change in the NO ₂ gas concentration of the Al and Cr electrodes of the CNT sensor in Example 1 of the present invention Illustration of the resistance change rate and change in gas concentration in in NO ₂ in the CNT sensors Al, Cr electrode in Example 1 of the present invention Response diagram of resistance change in SF ₆ decomposition gas of Al and Cr electrodes of CNT sensor in Example 1 of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 CNT sensor 1a, 1b Electrode 2 Carbon nanotube material 3a, 3b Electric field concentration edge 4 Insulating substrate 5a, 5b Detection terminal 6 Junction part 7 Gas measuring device 7a Gas measurement / dielectrophoresis control device 11, 21 Power supply part 12, 22 Measurement unit 13, 23 Operation control unit 14, 24 Display unit 15, 25 Memory unit 15a Calibration data unit 16, 26 Timekeeping unit 25a Data unit 27 Electrophoresis chamber 28 Container 29 Pump 30 Stirrer 31, 32 Solenoid valve

Claims (10)

A semiconductor carbon nanotube material is electrically bonded to a pair of aluminum electrodes at a bridging end, and a resistance change between the aluminum electrodes is measured based on adsorption of a measurement gas of either NO _x or SF ₆ decomposition gas in the surrounding gas. A carbon nanotube sensor for detecting a concentration, wherein a Schottky junction is formed between the semiconductor carbon nanotube material and the aluminum electrode. The measurement gas induces charge transfer based on the properties of the aluminum with the aluminum electrode by adsorption to the Schottky junction, thereby changing the height of the Schottky barrier, and having a predetermined reverse bias. 2. The carbon nanotube sensor according to claim 1, wherein resistance is generated. The carbon nanotube sensor according to claim 2, wherein the height of the Schottky barrier varies depending on the concentration of the measurement gas. 4. The carbon according to claim 1, wherein the semiconductor carbon nanotube material is a p-type semiconductor and has a work function larger than that of aluminum in a gas atmosphere containing the measurement gas. Nanotube sensor. The carbon according to any one of claims 1 to 3, wherein the semiconductor carbon nanotube material is an n-type semiconductor and has a work function smaller than that of aluminum in a gas atmosphere containing the measurement gas. Nanotube sensor. The carbon nanotube sensor according to any one of claims 1 to 5, wherein the semiconductor carbon nanotube material is composed of an aggregate cross-linked between the aluminum electrodes. The carbon nanotube sensor according to claim 6, wherein the aluminum electrode has an electric field concentration edge for generating an unequal electric field when an AC voltage is applied, and the integrated body is formed by dielectrophoresis. 8. The carbon nanotube according to claim 7, wherein the aluminum electrode is a thin film electrode provided on an insulating substrate, and the edge for electric field concentration is an edge of a protruding portion formed on each of the electrodes. Sensor. The work function of the p-type semiconductor carbon nanotube material and aluminum is measured under a measurement gas atmosphere of either NO _x or SF ₆ decomposition gas , and the work function of the p-type semiconductor carbon nanotube material is larger than that of aluminum. A method of manufacturing a carbon nanotube sensor, comprising: selecting and cross-linking the p-type semiconductor carbon nanotube material using aluminum as a pair of electrodes. The work function of the n-type semiconductor carbon nanotube material and aluminum is measured under a measurement gas atmosphere of either NO _x or SF ₆ decomposition gas , and the work function of the n-type semiconductor carbon nanotube material is smaller than the work function of aluminum. A method of manufacturing a carbon nanotube sensor, comprising: selecting and cross-linking the n-type semiconductor carbon nanotube material using aluminum as a pair of electrodes.