Gas sensor

Abstract

This invention provides a gas sensor with fast response and high sensitivity to oxygen. The gas sensor (100) of this invention comprises: a substrate (10), a first pad electrode (12A) and a second pad electrode (12B), a nanowire (14) made of a specific metal, and an oxide layer (16) made of a high-resistivity semiconductor oxide of a metal of a different type than the metal constituting the nanowire (14). The first pad electrode (12A) and the second pad electrode (12B) are formed on the substrate (10). The nanowire (14) connects the first pad electrode (12A) and the second pad electrode (12B) and is formed on the substrate (10). The oxide layer (16) is formed in contact with the nanowire (14). This contact between the nanowire (14) and the oxide layer (16) achieves high-speed response and high sensitivity to oxygen.

Description

Technical Field

[0001] This invention relates to a gas sensor that combines nanowires made of a specific metal with an oxide made of a high-resistivity semiconductor. Background Technology

[0002] Currently, various gas sensors are used in appliances, vehicles, ships, aircraft, and other applications that maintain daily life. For example, in automobiles, it is necessary to control the air-fuel mixture ratio supplied to the engine to improve combustion efficiency and ensure exhaust emissions meet environmental standards. In this control system, an oxygen sensor using a solid electrolyte (e.g., zirconium oxide), as described in Patent Document 1, is used. Furthermore, resistive oxygen sensors using oxide semiconductors, as described in Patent Document 2, are known as oxygen sensors.

[0003] Furthermore, Non-Patent Document 1 describes a gas sensor in which a first pad electrode and a second pad electrode are formed on a glass substrate, and platinum nanowires are connected to these pad electrodes. In this gas sensor, hydrogen gas is detected based on the change in resistance detected between the first pad electrode and the second pad electrode while a constant voltage is applied between them.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2015-137998;

[0007] Patent Document 2: Japanese Patent Application Publication No. 2003-149189.

[0008] Non-patent literature

[0009] Non-patent document 1: F. Yang et al., The Surface Scattering-Based Detection of Hydrogen in Air Using a Platinum Nanowire, Nano Lett. 12 (2012) 2924-2930 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] Oxygen sensors using solid electrolytes, as described in Patent Document 1, have the following problems: they require a reference gas, are not sufficiently sensitive to oxygen, and have a slow response time (around tens of seconds). Furthermore, resistive oxygen sensors using oxide semiconductors, as described in Patent Document 2, also have the following issues: while they can achieve a response time of less than 10 seconds at very high operating temperatures such as 900°C and 600°C, at relatively low operating temperatures of around 300°C, the response time is over several hundred seconds, making them slow and hardly meeting industrial needs.

[0012] Furthermore, while the gas sensor using platinum nanowires described in Non-Patent Document 1 can detect hydrogen, it does not describe the detection of oxygen. In other words, sensor characteristics such as response speed and sensitivity to oxygen have not been verified.

[0013] Therefore, in view of the above problems, the object of the present invention is to provide a gas sensor with fast response speed and high sensitivity to oxygen.

[0014] Solution for solving the problem

[0015] To address the aforementioned problems, the inventors conducted in-depth research and made the following discoveries. The inventors conceived of a gas sensor comprising nanowires made of a specific metal such as platinum (Pt) and an oxide layer made of a specific high-resistivity semiconductor. By arranging the nanowires in contact with the oxide layer made of the high-resistivity semiconductor, a second conduction path is formed in addition to the first conduction path through which charge carriers (oxygen vacancies, electrons, etc.) pass in the nanowires. This second conduction path is the path through which charge carriers (oxygen vacancies, electrons, etc.) are injected from the nanowires into the oxide layer, move within the oxide layer, and return to the nanowires. The inventors discovered that by having these two conduction paths, a significant effect of fast response speed and high sensitivity to oxygen can be obtained.

[0016] The main structure of the present invention, based on the above findings, is as follows.

[0017] [1] A gas sensor having:

[0018] A substrate having an insulating surface;

[0019] First pad electrode and second pad electrode are formed on the insulating surface of the substrate.

[0020] Nanowires connecting the first and second pad electrodes and formed on or above the insulating surface of the substrate, comprising one or more selected from platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os), iridium (Ir), and alloys thereof; and

[0021] An oxide layer, in contact with the nanowire, is composed of a high-resistivity semiconductor oxide of a metal of a different kind than the metal constituting the nanowire, and the oxide layer satisfies at least one or both of the following: (I) located between the insulating surface of the substrate and the nanowire, in contact with the lower surface of the nanowire; (II) located on the nanowire, in contact with the upper surface of the nanowire.

[0022] A current is flowed between the first pad electrode and the second pad electrode, and gas is detected based on the change in the electrical signal detected between the first pad electrode and the second pad electrode.

[0023] [2] According to the gas sensor described in [1] above, wherein,

[0024] The nanowires have a linewidth of 5 nm or more, and are less than 150 nm, preferably less than 50 nm, and more preferably less than 30 nm.

[0025] [3] According to the gas sensor described in [1] or [2] above, wherein,

[0026] The thickness of the nanowire is greater than 1 nm and less than 20 nm, preferably less than 10 nm.

[0027] [4] The gas sensor according to any one of [1] to [3] above, wherein,

[0028] The length of the nanowire is 80 nm or more, preferably 400 nm or more, and less than 1 m.

[0029] [5] The gas sensor according to any one of [1] to [4] above, wherein,

[0030] The high-resistivity semiconductor constituting the oxide layer is selected from one or more of cerium oxide, tin oxide, zirconium oxide, zinc oxide, tungsten oxide, iron oxide, nickel oxide, cerium-zirconium oxide, titanium oxide, cobalt oxide, niobium oxide, tantalum oxide, rhodium oxide, and hafnium oxide.

[0031] [6] The gas sensor according to any one of [1] to [5] above, wherein,

[0032] The thickness of the oxide layer is 5 nm or more, preferably less than 100 nm.

[0033] [7] The gas sensor according to any one of [1] to [6] above, wherein,

[0034] The nanowires are in ohmic contact with the oxide layer.

[0035] [8] The gas sensor according to any one of [1] to [7] above, wherein,

[0036] The substrate is any one of a glass substrate, an alumina substrate, a zirconia substrate, and a silicon substrate on which a silicon oxide film is formed.

[0037] [9] The gas sensor according to any one of [1] to [8] above, wherein,

[0038] The first pad electrode and the second pad electrode are made of the same type of metal as the nanowire.

[0039]

[10] The gas sensor according to any one of [1] to [9] above, wherein,

[0040] The gas is oxygen.

[0041] Invention Effects

[0042] The gas sensor of this invention has a fast response speed and high sensitivity to oxygen. Attached Figure Description

[0043] Figure 1A This is a schematic perspective view of a gas sensor 100 according to one embodiment of the present invention.

[0044] Figure 1B This is a schematic top view of the gas sensor 100.

[0045] Figure 1C yes Figure 1B Section II.

[0046] Figure 1D yes Figure 1B Sectional view II-II.

[0047] Figure 2 Figures (A) to (E) illustrate the manufacturing process of a gas sensor 100 according to one embodiment of the present invention.

[0048] Figure 3 These are SEM images of the upper surface of a gas sensor with the nanowires having various linewidth W and length L values.

[0049] Figure 4This is a graph showing the change of current I over time in Experiment Example 1.

[0050] Figure 5 This is a graph showing the change of current I over time in Experiment Example 2.

[0051] Figure 6 This is a graph showing the change of current I over time in Experiment Example 3.

[0052] Figure 7A This is a graph showing the change of current I over time in Experiment Example 4.

[0053] Figure 7B This is a graph showing the change of current I over time in Experiment Example 4.

[0054] Figure 8A This is a graph showing the change of current I over time in Experiment Example 5.

[0055] Figure 8B This is a graph showing the change of current I over time in Experiment Example 5.

[0056] Figure 9 This is a graph showing the change of current I over time in Experiment Example 6.

[0057] Figure 10 This is a graph showing the change of current I over time in Experiment Example 7.

[0058] Figure 11 This is a graph showing the change of current I over time in Experiment Example 8.

[0059] Figure 12 This is a graph showing the change of current I over time in Experiment Example 9.

[0060] Figure 13 (A) and (B) are graphs showing the change of current I over time in Experiment Example 10.

[0061] Figure 14 This is a graph showing the change of current I over time in Experiment Example 11. Detailed Implementation

[0062] (Gas sensor)

[0063] Reference Figure 1A , Figure 1B , Figure 1C and Figure 1DA gas sensor 100 according to one embodiment of the present invention includes a substrate 10, a first pad electrode 12A and a second pad electrode 12B, a nanowire 14 made of a specific metal, and an oxide layer 16 made of a high-resistivity semiconductor. The first pad electrode 12A and the second pad electrode 12B are formed on or above the substrate 10. The nanowire 14 connects the first pad electrode 12A and the second pad electrode 12B and is formed on or above the substrate 10. The oxide layer 16 is formed in contact with the nanowire 14. Detailed effects will be described later, but in this embodiment, high-speed response and high sensitivity to oxygen are achieved through this contact between the nanowire 14 and the oxide layer 16. Furthermore, in this specification, "A formed on or above B" means that A is formed directly on B or indirectly on B with other layers in between.

[0064] [Gas detection agencies]

[0065] In the gas sensor 100, current flows between a first pad electrode 12A and a second pad electrode 12B, and gas is detected based on changes in the electrical signal detected between the first pad electrode 12A and the second pad electrode 12B. For example, as Figure 1B As shown, a power supply 18 and an ammeter 20 are connected in series between the first pad electrode 12A and the second pad electrode 12B, and a voltmeter 22 is connected in parallel with them. In this case, when a constant voltage is applied between the first pad electrode 12A and the second pad electrode 12B through the power supply 18, the ammeter 20 detects the change in current between the first pad electrode and the second pad electrode, and gas is detected based on the detected change in current. Alternatively, when a constant current flows between the first pad electrode 12A and the second pad electrode 12B, gas can be detected based on the change in voltage detected between the first pad electrode 12A and the second pad electrode 12B. Furthermore, instead of the aforementioned change in current or voltage, gas can also be detected based on the change in resistance detected between the first pad electrode 12A and the second pad electrode 12B. That is, the aforementioned "electrical signal" refers to current, voltage, or resistance. In this embodiment, since the circuit of the gas sensor can be constructed using only two terminals, a gas sensor can be built without adding wiring or circuitry. In this embodiment, there are no particular restrictions on the type of gas detected, but oxygen is preferred to be detected.

[0066] [Substrate]

[0067] The substrate 10 supports the first pad electrode 12A and the second pad electrode 12B, as well as the nanowire 14 and the oxide layer 16, which serve as the gas detection unit. There are no particular limitations on the substrate 10 as long as it has an insulating surface; for example, any substrate can be used, such as a glass substrate, an alumina substrate, a zirconia substrate, or a silicon substrate with a silicon oxide film formed on its surface. There are no particular limitations on the shape and size of the substrate 10; however, when using a substrate with a rectangular main surface shape, the dimensions can be, for example, in the range of length: 10–300 mm × width: 10–300 mm × thickness: 0.3–1.2 mm.

[0068] [First pad electrode and second pad electrode]

[0069] The first pad electrode 12A and the second pad electrode 12B supply current to the nanowire 14 and are a pair of electrodes necessary for detecting changes in electrical signals corresponding to changes in gas concentration. The shape and size of the first pad electrode 12A and the second pad electrode 12B are not particularly limited as long as they are formed on or above the insulating surface of the substrate 10. However, if the shape of the main surface is rectangular, the dimensions can be, for example, in the range of length: 30–500 μm × width: 30–500 μm × thickness: 10–500 nm. The metal constituting the first pad electrode 12A and the second pad electrode 12B is not particularly limited and can be any metal, but from the perspective of process simplicity, a metal of the same type as the nanowire described later is preferred.

[0070] [Nanometer wires]

[0071] Nanowire 14 connects the first pad electrode 12A and the second pad electrode 12B and is formed on the insulating surface of the substrate 10, serving as one of the components constituting the gas detection section. Importantly, nanowire 14 is composed of one or more selected from platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os), iridium (Ir), and alloys thereof, preferably selected from platinum (Pt), palladium (Pd), and alloys thereof, and most preferably selected from platinum (Pt) and alloys thereof. The elements used in nanowire 14 are platinum group elements and function as catalysts. As catalysts, platinum group elements have the effects of activating gases, promoting gas adsorption and chemical reactions between oxides and gases, increasing the amount of adsorbed oxygen, and increasing oxygen ion concentration. Through these catalytic effects, the sensitivity of the gas response is improved.

[0072] The shape of nanowire 14 is not particularly limited, such as Figure 1D As shown, the preferred cross-sectional shape of the nanowire 14 perpendicular to the extension direction is rectangular.

[0073] The linewidth W of the nanowire 14 is not particularly limited as long as it is on the order of nm (i.e., less than 1 μm). However, from the perspective of more fully realizing high-speed response and high sensitivity to oxygen, it is preferable to be less than 150 nm, more preferably less than 50 nm, and even more preferably less than 30 nm. Furthermore, from the perspective of high-speed response and high sensitivity, the narrower the linewidth W of the nanowire 14, the better; therefore, there is no particular limitation on its lower limit. However, due to process constraints, it is preferable that the linewidth W of the nanowire 14 is 5 nm or more.

[0074] There is no particular limitation on the thickness of the nanowire 14, but from the perspective of more fully realizing high-speed response and high sensitivity to oxygen, a thickness of 20 nm or less is preferred, and more preferably 10 nm or less. Furthermore, from the perspective of high-speed response and high sensitivity, the thinner the nanowire 14, the better; therefore, there is no particular limitation on its lower limit. However, from the perspective of process constraints and the conductivity of the nanowire, a thickness of 1 nm or more is preferred.

[0075] There are no particular restrictions on the cross-sectional area of ​​the nanowire 14 perpendicular to the extension direction, but from the perspective of more fully realizing high-speed response and high sensitivity to oxygen, 3000 nm is preferred. 2 Below, 1500nm is preferred. 2 The following further optimizes the 1000nm process. 2 The following further optimizes the 600nm process. 2 The following further optimizes the 500nm process. 2 The following further optimizes the 300nm process. 2 From the perspective of high-speed response and high sensitivity, the smaller the cross-sectional area of ​​the nanowire 14 perpendicular to the extension direction, the better; therefore, there is no particular limitation on its lower limit. However, due to process constraints, a cross-sectional area of ​​the nanowire 14 perpendicular to the extension direction of 15 nm is preferred. 2 above.

[0076] There is no particular limitation on the length L of the nanowire 14, but from the perspective of achieving a more efficient high-speed response and high sensitivity to oxygen, a length of 80 nm or more is preferred, and more preferably 400 nm or more. From the perspective of high-speed response and high sensitivity, the longer the length L of the nanowire 14, the better; therefore, there is no particular upper limit. However, due to process constraints, it is preferable that the length L of the nanowire 14 is less than 1 m.

[0077] [Oxide layer]

[0078] The oxide layer 16 is made of a high-resistivity semiconductor and is in contact with the nanowire 14. In this embodiment, high-speed response and high sensitivity to oxygen are achieved through this contact between the nanowire 14 and the oxide layer 16. As for the contact method between the nanowire 14 and the oxide layer 16, it is important that the oxide layer 16 at least satisfies one or both of the following: (I) located between the insulating surface of the substrate 10 and the nanowire 14, in contact with the lower surface of the nanowire 14; (II) located on the nanowire 14, in contact with the upper surface of the nanowire 14.

[0079] like Figure 1A As shown, this embodiment relates to the above-described method (I), where the oxide layer 16 is located on the insulating surface of the substrate 10 (on: above), and the first pad electrode 12A, the second pad electrode 12B, and the nanowire 14 are located on the oxide layer 16 (on: above). However, the present invention is not limited to this and may also relate to the above-described method (II), for example, where the first pad electrode 12A, the second pad electrode 12B, and the nanowire 14 are located on the insulating surface of the substrate 10 (on: above), and the oxide layer 16 is formed on the nanowire 14 (on: above). The above-described methods (I) and (II) also include cases where the nanowire 14 is embedded in the oxide layer 16, and cases where the oxide layer 16 contacts not only the upper and lower surfaces of the nanowire 14 but also the side surfaces.

[0080] The high-resistivity semiconductor constituting the oxide layer 16 can be any oxide semiconductor with a resistivity of 1 Ω·m or more and 100,000 Ω·m or less at 300°C. For example, it is preferably selected from one or more of cerium oxide (CeO2), tin oxide (SnO2), zirconium oxide (ZrO2), zinc oxide (ZnO), tungsten oxide (WO3), iron oxide (Fe2O3), nickel oxide (NiO), cerium-zirconium oxide (CeZrO4), titanium oxide (TiO2), cobalt oxide (Co3O4), niobium oxide (Nb2O5), tantalum oxide (Ta2O5), rhodium oxide (Rh2O3), and hafnium oxide (HfO2), with particular preference given to one or more of cerium oxide, tin oxide, and zirconium oxide. Furthermore, the aforementioned oxide semiconductor is not limited to oxides with stoichiometric compositions; it can also be oxides deviating from stoichiometric compositions, and may include oxides with different oxidation numbers. Oxygen is adsorbed on the surface of the oxide semiconductor constituting the oxide layer 16. The adsorbed oxygen removes electrons from the oxide semiconductor, becoming O. - O 2- Oxygen ions. Due to oxygen ions O - O 2-The presence of a negative charge leads to surface depletion and decreased conductivity in the case of n-type semiconductors, while increasing carrier concentration and conductivity in the case of p-type semiconductors. When adsorbed oxygen ions react chemically with the gas, the oxygen ions are removed, and the surface charge generated by the oxygen ions disappears. This causes a change in the conductivity of the oxide semiconductor. This change in conductivity is a change in conductivity flowing parallel to the nanowires and is correlated with the detection of the gas response.

[0081] Importantly, the high-resistivity semiconductor (oxide semiconductor) constituting the oxide layer 16 is an oxide of a metal of a different type than the metal constituting the nanowire 14. In the adsorption, generation, and elimination of oxygen / oxygen ions, as well as the generation and elimination of oxygen vacancies, activation energies exist in each process, and these activation energies depend on the oxide semiconductor. For example, as in the Pt / PtO2 combination, when the oxide semiconductor is a metal oxide constituting the nanowire 14, the activation energy is high, resulting in a slower response time and recovery time, and lower sensitivity for the gas sensor. Conversely, when the oxide semiconductor is an oxide of a metal of a different type than the metal constituting the nanowire 14, the activation energy is low, therefore, the response time and recovery time of the gas sensor are faster, and the sensitivity is higher.

[0082] There is no particular limitation on the thickness of the oxide layer 16, but from the perspective of achieving a more sufficient high-speed response and high sensitivity to oxygen, it is preferable to be 5 nm or more. For the oxide layer 16 located in position (I) above, there is no particular limitation on its upper limit of thickness, but since the effect of the present invention will saturate even if the thickness is too thick, from the perspective of cost and processing time, the thickness of the oxide layer 16 is preferably 100 nm or less. For the oxide layer 16 located in position (II) above, there is no particular limitation on its upper limit of thickness, but when the thickness is too thick, the air permeability deteriorates, resulting in insufficient gas supply to the nanowires 14; therefore, the thickness of the oxide layer 16 is preferably 100 nm or less.

[0083] [Contact between nanowires and oxide layer]

[0084] As described above, in this embodiment, the contact between the nanowire 14 and the oxide layer 16 is crucial; specifically, an ohmic contact is formed. Thus, in addition to the first conduction path through which charge carriers (oxygen vacancies, electrons, etc.) pass in the nanowire 14, a second conduction path is formed. This second conduction path is the path through which charge carriers (oxygen vacancies, electrons, etc.) are injected from the nanowire 14 into the oxide layer 16, move within the oxide layer 16, and return to the nanowire 14. By having these two conduction paths, a significant effect of fast response speed and high sensitivity to oxygen can be achieved.

[0085] Although not intended to limit the invention, the inventors believe that the mechanism for achieving this effect is as follows.

[0086] In oxides, oxygen vacancies (oxygen vacancies) arise depending on the oxygen concentration; the lower the oxygen concentration, the higher the oxygen vacancy density. These oxygen vacancy sites function as donor sites that trap and release electrons. Electrons released from these oxygen vacancies are either supplied to conductors as charge carriers for band conduction, or they can hop between oxygen vacancy sites.

[0087] The metals (Pt, Pd, etc.) constituting the nanowires, through catalysis, help lower the activation energy for oxygen vacancies, making them readily generated. This, in turn, lowers the activation energy for electron donation and acceptance (charge injection) at the metal-oxide interface, reducing the height of the Schottky barrier between the metal and oxide. Furthermore, oxygen vacancies increase the carrier density near the interface, thus facilitating a reduction in the width of the Schottky barrier to below tens of nanometers. The reduction in barrier height due to catalysis and the increase in carrier density due to oxygen vacancies both lower the resistance based on the Schottky barrier between the metal and oxide, enabling ohmic contacts. At the interface where an ohmic contact is formed, the resistance is almost constant regardless of the electric field at the interface, and electron donation and acceptance at the metal / oxide interface are difficult to hinder.

[0088] Atoms of metals (such as Pt and Pd) in contact with oxides tend to diffuse more readily into the oxide during metal vapor deposition or annealing at several hundred °C, thus enabling atomic doping. When metal atoms are doped into oxides in this manner, the catalytic effect becomes extremely high compared to the host metal. In metal-doped oxides, compared to undoped oxides, the high catalytic effect facilitates the formation of oxygen vacancies, resulting in a lower potential barrier height and a smaller barrier width due to the increased carrier concentration. Because of these effects, metal atom doping into oxides contributes to improved gas sensor response due to the ease of oxygen vacancies, the formation of ohmic contacts and a reduction in ohmic resistance, as well as increased electron conductivity in the oxide.

[0089] Through an ohmic contact at the metal / oxide interface, electrons injected from the metal into the oxide flow through the oxide via band conduction or hopping conduction and can return to the nanowire at the ohmic contact. When a voltage is applied across both the metal nanowire and the nanowire in contact with the oxide, electron conduction occurs within the metal nanowire. In addition to electron conduction within the wire, there is also electron conduction through a conduction path (bypass circuit) where charges injected from the metal into the oxide are conducted and return to the metal. Because the current flowing through the bypass circuit across the oxide is strongly dependent on the oxygen defect concentration, the gas sensor's response is highly sensitive.

[0090] Because the contact resistance in an ohmic contact is independent of the electric field, the electron injection frequency between the metal and oxide is independent of the voltage applied to the line terminals. This results in the present invention, for example, a gas sensor response can be obtained as effectively as with higher applied voltages, even at applied voltages of tens of mV.

[0091] Reports indicate that oxidation of metals on metal surfaces increases surface scattering during metal conduction, and the resulting change in conductivity due to this surface scattering is used to determine the response of gas sensors. At the interface where the metal, which introduces oxygen vacancies into the oxide, contacts the oxide, reduction plays a role. This reduction is related to the reduction of the oxidized metal surface. At the metal interface in contact with the oxide, the metal is less prone to oxidation due to reduction, thus reducing surface scattering during metal conduction within the metal wire. Furthermore, the formation process of the ohmic contact clearly shows that surface scattering at the metal / oxide interface depends on the oxygen vacancy concentration, which changes the barrier height and barrier width of the Schottky interface. In gas sensors utilizing variations in the conductive surface scattering frequency of metal nanowires, surface scattering is caused by oxidation of the metal monomer surface. In contrast, the gas sensor of this invention, as described above, causes surface scattering due to oxygen vacancies in the oxide, thus the conductive surface scattering mechanism is different.

[0092] [Effect]

[0093] As described above, the gas sensor 100 according to this embodiment can achieve the effects of fast response speed (i.e., short response time and short recovery time) and high sensitivity to oxygen. Furthermore, the gas sensor 100 of this embodiment can detect oxygen even at low operating temperatures (e.g., around 60–300°C). Moreover, since the gas sensor 100 of this embodiment can detect oxygen even at low oxygen concentrations, it also has the effect of detecting a wide range of oxygen concentrations.

[0094] [Manufacturing method of oxygen sensor]

[0095] Reference Figure 2 Sections (A) to (E) describe a preferred method for manufacturing the gas sensor 100 of this embodiment.

[0096] Reference Figure 2(A) First, an oxide layer 16 is formed on a substrate 10. Specifically, a precursor solution of the oxide constituting the oxide layer 16 is prepared, the precursor solution is applied to the substrate 10, and then the substrate 10 is heat-treated. For example, in the case of forming a cerium oxide (CeO2) layer, cerium acetate hydrate Ce(CH3CO2)3·xH2O can be dissolved in propionic acid to prepare a cerium oxide (CeO2) precursor solution of a specified concentration.

[0097] The following are examples of precursor substances used to obtain the various oxides. More than one of these precursor substances can be used.

[0098] Cerium oxide (CeO2): cerium acetate hydrate Ce(CH3CO2)3·xH2O; cerium nitrate hexahydrate Ce[NO3]3·6H2O; cerium acetylacetone hydrate Ce[C5H7O2]3·xH2O

[0099] Tin oxide (SnO2): anhydrous tin chloride (II) SnCl2; tin acetylacetonate (II) C 10 H 14 O4Sn;tert-Butyltin(IV)Sn[OC(CH3)3]4

[0100] Zirconium oxide (ZrO2): Zirconium butoxide (IV)Zr(OC4H9)4; Zirconium isopropoxide (IV) isopropanol complex Zr(OCH(CH3)2)4·(CH3)2CHOH; Zirconium oxynitrate (IV) hydrate ZrO(NO3)2·xH2O

[0101] Zinc oxide (ZnO): Zinc nitrate hexahydrate Zn[NO3]2·6H2O; Zinc acetylacetonate (II)C 10 H 14 O4Zn; Zinc acetate dihydrate Zn[CH3COO]2·2H2O

[0102] Tungsten oxide (WO3): Tungsten chloride (VI)WCl6; Tungsten ethoxide (VI)C 12 H 30 O6W

[0103] Iron oxide (Fe₂O₃): ferric nitrate nonhydrate Fe[NO₃]₃·9H₂O; ferric acetate (II) Fe[CO₂CH₃]₂; ferric acetylacetone (III) Fe[C₅H₇O₂]₃

[0104] Nickel oxide (NiO): Nickel acetylacetonate (II) Ni(C5H7O2)2; Nickel acetate (II) tetrahydrate Ni[OCOCH3]2·4H2O; Nickel nitrate (II) hexahydrate Ni[NO3]2·6H2O

[0105] Cerium-zirconium oxide (CeZrO4): cerium acetate hydrate Ce(CH3CO2)3·xH2O; zirconium butoxide (IV)Zr(OC4H9)4

[0106] Titanium oxide (TiO2): Titanium isopropoxide (IV)Ti[OCH(CH3)2]4; Titanium butoxide (IV)Ti(OCH2CH2CH2CH3)4

[0107] Cobalt oxide (Co3O4): Cobalt nitrate hexahydrate Co[NO3]2·6H2O; Cobalt(II) acetylacetonate Co(C5H7O2)2; Cobalt(II) acetate [CH3CO2]2Co; Cobalt(II) chloride CoCl2

[0108] Niobium oxide (Nb₂O₅): Niobium chloride (V)NbCl₅; Niobium ethanol (V)Nb(OCH₂CH₃)₅; Niobium n-butanol C 20 H 45 NbO5

[0109] Tantalum oxide (Ta₂O₅): Tantalum ethoxide (V)Ta(OC₂H₅)₅; Tantalum tetraethoxy-2,4-glutaric acid (V)C 13 H 27 O6Ta; Tantalum chloride (V)TaCl5

[0110] Rhodium oxide (Rh2O3): Rhodium(II) acetate dimer Rh2(OOCCH3)4; Rhodium(III) nitrate hydrate N3O9Rh·xH2O; Rhodium(III) acetylacetone [CH3COCHCOCH3]3Rh

[0111] Hafnium oxide (HfO2): Hafnium chloride (IV)HfCl4; Hafnium 2,4-glutaric acid (IV)C 20 H 28 O8Hf;Hafnium(IV)nitrateHf(NO3)4

[0112] Furthermore, as a solvent, one or more selected from propionic acid, 2-methoxyethanol (CH3OCH2CH2OH), and 1-butanol (C4H9OH) can be used. The concentration of the precursor solution is not particularly limited, but is preferably around 0.05 to 0.5 mol / kg depending on the desired film thickness. The coating method is not particularly limited, but spin coating is preferred. In this case, the rotation speed can be appropriately determined according to the desired thickness, for example, it can be in the range of 1000 to 4000 rpm. For the oxide layer 16, both conductivity and permeability are required. To obtain high conductivity, conductive pathways need to be formed; if grain boundaries are present, it is difficult to obtain a response due to carrier trapping at the boundaries. On the other hand, in order to achieve the increase or decrease of oxygen vacancies in the oxide near the nanowires relative to the gas environment, a porous structure with good permeability is preferred. From the perspective of obtaining these two functions, heat treatment conditions are preferred, which are heat treatment held at an ambient temperature in the range of 250 to 800°C in air for 0.5 to 2 hours. Alternatively, the thickness of the oxide layer 16 can be adjusted by combining multiple coatings with heat treatment.

[0113] Next, as Figure 2 As shown in (B), a photoresist film 30 is formed on the oxide layer 16. The photoresist film 30 can be formed by applying a photoresist composition for electron beam exposure onto the oxide layer 16 and drying it. There are no particular limitations on the application method, but spin coating is preferred. The thickness of the photoresist film 30 only needs to be appropriately set to be thicker than the desired thickness of the nanowires 14 (in the case where the nanowires 14 are formed together with the pad electrodes 12A and 12B, thicker than both of them).

[0114] Next, as Figure 2 As shown in (C), the resist film 30 is developed to form a mask pattern 32 of a predetermined shape. The mask pattern 32 is fabricated by exposing and developing the resist film 30 using electron beam lithography. The shape of the mask pattern 32 can be appropriately set according to the linewidth W and length L of the nanowires 14 to be formed. Figure 2 As shown in (C) to (E), when the nanowire 14 is formed together with the pad electrodes 12A and 12B, the shape of the mask pattern 32 can be determined according to the dimensions of the nanowire 14 and the pad electrodes 12A and 12B to be formed. The oxide layer 16 is exposed from the portion of the mask pattern 32 where the resist film has been removed.

[0115] Next, as Figure 2As shown in (D), the metal film 34 is formed, for example, by electron beam evaporation or sputtering. In this case, a first portion 34A of the metal film is formed on the mask pattern 32, and a second portion 34B of the metal film is formed on the oxide layer 16 exposed from the portion of the mask pattern 32 where the resist film has been removed. Preferably, the metal film 34 includes a Ti layer with a thickness of approximately 1-5 nm, and a layer formed thereon consisting of a metal (e.g., Pt) constituting the nanowires 14 and the pad electrodes 12A, 12B. The Ti layer also functions as an adhesive layer, bonding the layer consisting of the metal constituting the nanowires 14 and the pad electrodes 12A, 12B to the oxide layer 16. Alternatively, a Cr layer can be used instead of the Ti layer as the adhesive layer.

[0116] Next, as Figure 2 As shown in (E), the mask pattern 32 is peeled off, and nanowires 14 and pad electrodes 12A, 12B are formed on the oxide layer 16 by a peeling process that removes the first portion 34A of the metal film formed thereon. In this example, as Figure 2 As shown in (C) to (E), the nanowire 14 is formed together with the pad electrodes 12A and 12B. However, it is also possible to form only the nanowire 14 in the above process, and then form the pad electrodes 12A and 12B separately, for example, using a general photolithography method. Furthermore, although the Ti or Cr layer is formed as an adhesive layer for bonding the nanowire 14 to the oxide layer 16, during the evaporation process of the metal constituting the nanowire 14, Ti or Cr diffuses into the metal layer, and most of the Ti or Cr layer disappears. Between the oxide layer 16 and the nanowire 14, Ti or Cr remains as islands in some locations, but in most locations, the oxide layer 16 and the nanowire 14 are in direct contact, forming an ohmic contact.

[0117] After the above processes, a gas sensor 100 can be manufactured. Additionally, according to... Figure 2 In the gas sensor 100 fabricated by (A) to (E), the contact method of the nanowire 14 / oxide layer 16 is the method described in (I) above. This is achieved by reversing the formation order of the nanowire 14 and the oxide layer 16 (i.e., changing the order of their formation). Figure 2 The process of (A) and Figure 2 By reversing the order of processes (B) to (E), a gas sensor with the contact method of nanowire 14 / oxide layer 16 described in (II) above can be fabricated. However, in this case, oxide layer 16 is also formed on pad electrodes 12A and 12B. Therefore, the oxide layer 16 on pad electrodes 12A and 12B needs to be removed, for example, by argon sputtering, reactive ion etching, physical scraping, etc., so that electrical contact can be obtained from pad electrodes 12A and 12B.

[0118] Example

[0119] [Experimental Example 1]

[0120] <Fabrication of Gas Sensors>

[0121] To fabricate a gas sensor, follow these steps: First, prepare a Si substrate (length: 15mm × width: 15mm × thickness: 0.5mm) with a surface layer of approximately 1μm composed of SiO2.

[0122] Next, cerium acetate hydrate Ce(CH3CO2)3·xH2O (99.9% manufactured by Sigma-Aldrich) was dissolved in propionic acid (manufactured by Wako Chemical Co., Ltd.) to prepare a 0.4 mol / kg cerium oxide (CeO2) precursor solution. After mixing, the precursor solution was stirred at 1000 rpm for 30 minutes on a heated plate (110°C) to obtain a clear and homogeneous solution. Then, the solution was passed through a 0.2 μm PTFE needle filter and stored at 4°C.

[0123] The precursor solution was applied to the substrate using a spin-coating method at 4000 rpm, and then heat-treated at 250°C for 1 hour using a rapid thermal annealing (RTA) apparatus to form a cerium oxide (CeO2) layer (thickness: 28 nm) on the substrate. Furthermore, the resistivity of cerium oxide (CeO2) at 300°C is 200 Ω·m.

[0124] Subsequently, Pt nanowires were formed on the CeO2 layer using an electron beam lithography (EBL) apparatus (Elionix ELS-7500EX). Specifically, an electron beam resist ZEP-520A was applied to the CeO2 layer using spin coating, and then a mask pattern of a specified shape was drawn using the EBL apparatus. A Ti layer (thickness: 3 nm) and a Pt layer (thickness: 10 nm) were then formed using electron beam evaporation. Following a stripping process to remove the mask pattern, Pt nanowires (linewidth W: 8 nm, line thickness: 10 nm, line length L: 416 nm, rectangular cross-sectional shape perpendicular to the line) were formed on the CeO2 layer. Furthermore, although the Ti layer was formed as an adhesive layer to bond the Pt nanowires to the CeO2 layer, during the Pt layer evaporation process, Ti diffused within the Pt layer, and most of the Ti layer disappeared. Between the CeO2 layer and the Pt nanowires, Ti remains as islands in some locations, but in most locations, the CeO2 layer is in direct contact with the Pt nanowires, forming an ohmic contact. That is, the CeO2 layer is located between the substrate and the Pt nanowires, in contact with the lower surface of the Pt nanowires.

[0125] Next, using conventional photolithography, a first pad electrode and a second pad electrode (150μm × 150μm) were fabricated, consisting of a Ti layer (thickness: 5nm) and a Pt layer (thickness: 40nm) on top of it. This resulted in the fabrication of a gas sensor composed of a CeO2 layer and Pt nanowires.

[0126] <Oxygen Detection Test>

[0127] The aforementioned gas sensor was placed in a variable oxygen pressure measuring chamber. Using a semiconductor parameter analyzer system (Keysight Technologies B1500A), with a constant voltage (V = 1.0V) applied between the first and second pad electrodes, oxygen detection experiments were conducted based on the change in current I detected between the first and second pad electrodes, and the characteristics of various sensors were evaluated. The operating temperature T was set to 300℃ (573K). When oxygen was ON (open), the oxygen pressure was increased from 10... -3 When the vacuum pressure changes to 5 Pa and the oxygen pressure is turned off, the oxygen pressure is restored from 5 Pa to 10 Pa. -3 Pa (vacuum). That is, oxygen pressure P. O2 Set to 5 Pa. Additionally, no carrier gas is introduced into the measuring chamber; only oxygen is introduced. The oxygen is ON (open) at 0 seconds and OFF (closed) at 300 seconds. Figure 4 It is a graph showing how the current I changes over time.

[0128] Here, the parameters related to sensor characteristics are defined as follows.

[0129] I: Current value at a certain moment

[0130] I0: Initial current value

[0131] I 01 Initial current value (current value when oxygen is ON)

[0132] I 02 Initial current value (current value when oxygen is OFF)

[0133] I s : Saturation current value

[0134] I s1 Saturation current value (after oxygen is ON)

[0135] I s2 Saturation current value (after oxygen is turned off)

[0136] ΔI1=I 01 -I s1

[0137] ΔI2=I02 -I s2

[0138] Sensitivity S(%) = (ΔI1 / I) 01 )×100

[0139] Response time t res :

[0140] Current value I from initial current value I 01 Time required to change only 90% of ΔI1

[0141] Recovery time t rec :

[0142] Current value I from initial current value I 02 Time required to change only 90% of ΔI2

[0143] In addition, if the current value is not saturated even after 300 seconds after the oxygen is turned ON or OFF, the current value at the moment 300 seconds after the oxygen is turned ON or OFF is considered the saturated current value.

[0144] In this experimental example, the response time t res The recovery time is 21 seconds, and the recovery time is t. rec It has a response time of 20 seconds, enabling a high-speed response to oxygen. Furthermore, its sensitivity S is 2.4%, achieving high sensitivity to oxygen.

[0145] [Experimental Example 2]

[0146] The linewidth W of the Pt nanowires was set to five levels: 8 nm, 23 nm, 50 nm, 74 nm, and 124 nm (refer to...). Figure 3 (Left side), except that, a gas sensor was fabricated under the same conditions as in Experimental Example 1. Then, under the same conditions as in Experimental Example 1 (applied voltage V = 1.0V, operating temperature T = 300℃ (573K), oxygen pressure P), O2 An oxygen detection test was conducted at 5 Pa. The oxygen level was ON (open) at 0 seconds and OFF (closed) at 300 seconds. Figure 5 This is a graph showing the change of current I over time in each level. Furthermore, the sensor performance in each level is shown in Table 1.

[0147] [Table 1]

[0148]

[0149] Table 1 clearly shows that the narrower the linewidth W, the faster the response to oxygen. On the other hand, the sensitivity S is less dependent on the linewidth W.

[0150] [Experiment Example 3]

[0151] The length L of the Pt nanowires was set to 416 nm and five levels: 980 nm, 288 nm, 191 nm, and 93 nm (refer to...). Figure 3 (Right side), except that, a gas sensor was fabricated under the same conditions as in Experimental Example 1. Then, under the same conditions as in Experimental Example 1 (applied voltage V = 1.0V, operating temperature T = 300℃ (573K), oxygen pressure P), O2 An oxygen detection test was conducted at 5 Pa. The oxygen level was ON (open) at 0 seconds and OFF (closed) at 300 seconds. Figure 6 This is a graph showing the change of current I over time in each level. Furthermore, the sensor performance in each level is shown in Table 2.

[0152] [Table 2]

[0153]

[0154] As can be clearly seen from Table 2, the longer the line length L, the faster the oxygen response and the higher the sensitivity.

[0155] [Experiment Example 4]

[0156] A gas sensor was fabricated under the same conditions as in Experimental Example 1. Then, the applied voltage V was set to various levels (0.01V, 0.05V, 0.1V, 0.5V, 1.0V, 2.0V, 2.5V, 3.0V), except that the conditions were the same as in Experimental Example 1 (operating temperature T = 300℃ (573K), oxygen pressure P...). O2 An oxygen detection test was conducted at 5 Pa. The oxygen was turned ON after 100 seconds and OFF after 500 seconds. Figure 7A and Figure 7B This is a graph showing the change of current I over time at each level. Furthermore, the sensitivity S at each level is also recorded in the graph.

[0157] Depend on Figure 7A and Figure 7B It is evident that the response time and recovery time are independent of the applied voltage V. Furthermore, the sensitivity S is also independent of the applied voltage V, remaining almost constant between 1.42% and 1.56%. Moreover, a response can be observed even when the applied voltage V is 10mV. Therefore, with noise reduction measures, the gas sensor in this experimental example can operate sufficiently even at low applied voltages of 10mV. This contributes to the realization of low-power gas sensors.

[0158] [Experiment Example 5]

[0159] A gas sensor was fabricated under the same conditions as in Experimental Example 1. Then, the operating temperature T was set to various levels (303K, 333K, 363K, 393K, 423K, 453K, 543K, 573K), and all other conditions were the same as in Experimental Example 1 (applied voltage V = 1.0V, oxygen pressure P). O2 An oxygen detection test was conducted at 5 Pa. The oxygen level was ON (on) at 0 seconds and OFF (off) at 300 seconds. Additionally, the operating temperature T was adjusted by setting the temperature of a micro-heater located on the back of the substrate. Figure 8A and Figure 8B This is a graph showing the change of current I over time in each level. Furthermore, the sensor performance in each level is shown in Table 3.

[0160] [Table 3]

[0161]

[0162] Table 3 clearly shows that the higher the operating temperature T, the faster the response and the higher the sensitivity to oxygen. Furthermore, a response can be obtained at an operating temperature of 303K, which is close to room temperature, indicating that there is no need to increase the sensor temperature during operation. Therefore, the gas sensor of this invention can achieve low-power operation without the need for a heater.

[0163] [Experiment Example 6]

[0164] A gas sensor was fabricated under the same conditions as in Experimental Example 1. Then, the oxygen pressure P was... O2 Various levels (10Pa, 5Pa, 3Pa, 0.1Pa) were set, and oxygen detection tests were conducted under the same conditions as in Experimental Example 1 (applied voltage V = 1.0V, operating temperature T = 300℃ (573K)). Oxygen was ON (open) at 0 seconds and OFF (closed) at 300 seconds. Figure 9 This is a graph showing the change of current I over time in each level. Furthermore, the sensor performance in each level is shown in Table 4.

[0165] [Table 4]

[0166]

[0167] Table 4 clearly shows that the sensitivity S depends on oxygen pressure; the higher the oxygen pressure, the greater the sensitivity. On the other hand, the response time and recovery time are less dependent on oxygen pressure. Furthermore, it is evident that a response can be obtained even at an oxygen pressure of 0.1 Pa, indicating a wide range of detectable oxygen concentrations.

[0168] [Experiment Example 7]

[0169] The Pt nanowires were set to thicknesses of 3 nm, 6 nm, and 10 nm. Otherwise, the gas sensors were fabricated under the same conditions as in Experimental Example 1. Then, under the same conditions as in Experimental Example 1 (applied voltage V = 1.0 V, operating temperature T = 300 °C (573 K), oxygen pressure P...),... O2 =5Pa) to conduct oxygen detection test. Oxygen is ON (on) at 0 seconds, and then OFF / ON (off / on) is switched every 100 seconds thereafter. Figure 10 This is a graph showing the change of current I over time at each level. According to... Figure 10 The calculated sensitivity S is 3.8% for a thickness of 3 nm, 2.5% for a thickness of 6 nm, and 3.6% for a thickness of 10 nm. This indicates that the sensitivity S is not highly dependent on the Pt thickness. Since the conductivity of Pt nanowires cannot be achieved when the thickness is less than 1 nm, a thickness of 1 nm or more is preferred.

[0170] [Experiment Example 8]

[0171] The CeO2 layer thickness was set to two levels: 28 nm and 56 nm. Otherwise, the gas sensor was fabricated under the same conditions as in Experimental Example 1. Then, under the same conditions as in Experimental Example 1 (applied voltage V = 1.0 V, operating temperature T = 300 °C (573 K), oxygen pressure P...), the gas sensor was... O2 =5Pa) and oxygen detection test are performed simultaneously. Oxygen is ON (on) at 0 seconds, and then OFF / ON (off / on) is switched every 100 seconds thereafter. Figure 11 This is a graph showing the change of current I over time at each level. (From...) Figure 11 It can be seen that the response time and recovery time are independent of the CeO2 layer thickness. Furthermore, according to... Figure 11 The calculated sensitivity S is 2.5% in both levels. This indicates that the sensitivity S is independent of the CeO2 layer thickness.

[0172] [Experiment Example 9]

[0173] The formation order of the CeO2 layer and Pt nanowires was reversed, but the gas sensor was fabricated under the same conditions as in Experimental Example 1. Specifically, first, as in Experimental Example 1, a Si substrate with a surface layer composed of SiO2 was prepared. Next, Pt nanowires were formed on the SiO2 layer using an electron beam lithography (EBL) apparatus (Elionix ELS-7500EX). Specifically, an electron beam resist ZEP-520A was applied to the SiO2 layer using spin coating, and then a mask pattern of a predetermined shape was drawn using the EBL apparatus. Then, a Ti layer (thickness: 3 nm) and a Pt layer (thickness: 10 nm) were formed thereon using electron beam evaporation. Finally, a stripping process was performed to remove the mask pattern, and Pt nanowires (linewidth W: 8 nm, line thickness: 10 nm, line length L: 416 nm, cross-sectional shape perpendicular to the line: rectangular) were formed on the SiO2 layer. In addition, the Ti layer is formed as an adhesive layer to bond Pt nanowires to the SiO2 layer.

[0174] Next, using conventional photolithography, a first pad electrode and a second pad electrode (150μm×150μm) consisting of a Ti layer (thickness: 5nm) and a Pt layer (thickness: 40nm) on top of it were fabricated on the SiO2 layer.

[0175] Next, a precursor solution was applied using a spin-coating method at 4000 rpm to cover the SiO2 layer, Pt nanowires, and the first and second pad electrodes on the substrate. Then, a rapid thermal annealing (RTA) process was used to heat-treat the substrate at 400°C for 1 hour, thereby forming a cerium oxide (CeO2) layer (thickness: 28 nm) to cover the SiO2 layer, Pt nanowires, and the first and second pad electrodes. In this case, the Pt nanowires are embedded in the CeO2 layer. That is, the CeO2 layer is located on the Pt nanowires and contacts the upper surface of the Pt nanowires. Furthermore, the CeO2 layer also contacts the sides of the Pt nanowires. An ohmic contact is formed between the CeO2 layer and the Pt nanowires.

[0176] Finally, the CeO2 layer on the first and second pad electrodes was removed by physical scraping, thereby enabling electrical contact between the first and second pad electrodes. This resulted in the fabrication of a gas sensor composed of a CeO2 layer and Pt nanowires.

[0177] Then, under the same conditions as in Experimental Example 1 (applied voltage V = 1.0V, operating temperature T = 300℃ (573K), oxygen pressure P), O2 =5Pa) to conduct oxygen detection test. Oxygen is ON (on) at 0 seconds, and then OFF / ON (off / on) is switched every 100 seconds thereafter. Figure 12 This is a graph showing the change of current I over time. In this experimental example, the response time t...res The recovery time is 25 seconds, and the recovery time is t. rec It has a response time of over 68 seconds, enabling a high-speed response to oxygen. Furthermore, its sensitivity S is 1.5%, achieving high sensitivity to oxygen.

[0178] [Experiment Example 10]

[0179] The oxide layer was replaced with either a SnO2 layer or a ZrO2 layer, and the gas sensor was fabricated under the same conditions as in Experimental Example 1. Then, the sensor was tested under the same conditions as in Experimental Example 1 (applied voltage V = 1.0V, operating temperature T = 300℃ (573K), oxygen pressure P...). O2 =5Pa) to conduct oxygen detection tests.

[0180] In the case of a SnO2 layer, oxygen is ON (on) after 100 seconds, and then OFF / ON (off / on) is switched approximately every 100 seconds thereafter. Figure 13 The graph (A) shows the change of current I over time. Response time t res The recovery time is 12 seconds, t. rec It has a response time of 8 seconds, enabling a high-speed response to oxygen. Furthermore, its sensitivity S is 2.3%, achieving high sensitivity to oxygen.

[0181] In the case of a ZrO2 layer, oxygen is ON at 0 seconds and OFF at 600 seconds. Figure 13 (B) is a graph showing the change of current I over time. Response time t res It has a 12-second response time, enabling a high-speed oxygen response, but the recovery time t is... rec It can last for more than 2 hours. In addition, the sensitivity S is 97%, which enables high sensitivity to oxygen.

[0182] Furthermore, the formation of the SnO2 layer is carried out according to the following steps. Tin(II) acetylacetone is applied to... 10 H 14 O4Sn (99.9% manufactured by Sigma-Aldrich) was dissolved in propionic acid (manufactured by Wako Chemical Co., Ltd.) to prepare a 0.4 mol / kg tin oxide (SnO2) precursor solution. After mixing, the precursor solution was stirred at 1000 rpm for 30 minutes on a heated plate (110°C) to obtain a clear and homogeneous solution. The solution was then passed through a 0.2 μm PTFE needle filter and stored at 4°C.

[0183] The precursor solution was applied to the substrate using a spin-coating method at 3000 rpm, and then heat-treated at 300°C for 1 hour using a rapid thermal annealing (RTA) apparatus to form a tin oxide (SnO2) layer (thickness: 30 nm) on the substrate. Furthermore, the resistivity of tin oxide (SnO2) at 300°C is 10 Ω·m.

[0184] Furthermore, the ZrO2 layer was formed according to the following steps: A zirconium oxide (ZrO2) layer (thickness: 30 nm) was formed on the substrate by sputtering using a sputtering target made of ZrO2 in an Ar environment. Additionally, the resistivity of zirconium oxide (ZrO2) at 300 °C is 80000 Ω·m.

[0185] [Experimental Example 11]

[0186] A gas sensor was fabricated under the same conditions as in Experimental Example 1. Then, a voltage V was applied. d The voltage was set to 0.1V. Nitrogen gas (flow rate: 1000 sccm) was circulated in the measuring chamber as the carrier gas. Otherwise, the oxygen detection test was conducted under the same conditions as in Experiment 1 (operating temperature T = 300℃ (573K)). The oxygen was ON at 0 seconds and OFF at 600 seconds. The oxygen flow rate was set to 50 sccm, and the oxygen concentration was 5%. Figure 14 This is a graph showing the change of current I over time. This experimental example demonstrates that oxygen can be detected even in the presence of a carrier gas.

[0187] Production utilization potential

[0188] The gas sensor of the present invention has a fast response speed and high sensitivity to oxygen, and therefore has the potential to be applied to oxygen sensors, breathing sensors, etc. in internal combustion engines.

[0189] Explanation of reference numerals in the attached figures

[0190] 100: Gas sensor;

[0191] 10: Substrate;

[0192] 12A: First pad electrode;

[0193] 12B: Second pad electrode;

[0194] 14: Nanowires;

[0195] 16: Oxide layer;

[0196] 18: Power supply;

[0197] 20: Ammeter;

[0198] 22: Voltmeter;

[0199] 30: Resist film;

[0200] 32: Mask pattern;

[0201] 34: Metal film;

[0202] 34A: The first part of the metal film;

[0203] 34B: The second part of the metal film.

Claims

1. A gas sensor, comprising: A substrate having an insulating surface; First pad electrode and second pad electrode are formed on the insulating surface of the substrate; Nanowires, which connect the first pad electrode and the second pad electrode and are formed on the insulating surface of the substrate, are composed of one or more selected from platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os), iridium (Ir), and alloys thereof; and An oxide layer, which is in contact with the nanowire, is made of a high-resistivity semiconductor of an oxide of a metal of a different kind than the metal constituting the nanowire, and the oxide layer satisfies at least one or both of the following: (I) located between the insulating surface of the substrate and the nanowire, in contact with the lower surface of the nanowire; (II) located on the nanowire, in contact with the upper surface of the nanowire. When current flows between the first pad electrode and the second pad electrode, and oxygen is detected based on changes in the electrical signal detected between the first pad electrode and the second pad electrode, a first conduction path and a second conduction path are formed. The first conduction path is the path through which charge carriers pass in the nanowire, and the second conduction path is the path through which charge carriers are injected from the nanowire into the oxide layer, move in the oxide layer, and return to the nanowire.

2. The gas sensor according to claim 1, wherein, The nanowires have a linewidth of 5 nm or more and 150 nm or less.

3. The gas sensor according to claim 1, wherein, The thickness of the nanowire is greater than 1 nm and less than 20 nm.

4. The gas sensor according to claim 1, wherein, The length of the nanowire is 80 nm or more.

5. The gas sensor according to claim 1, wherein, The high-resistivity semiconductor constituting the oxide layer is selected from one or more of cerium oxide, tin oxide, zirconium oxide, zinc oxide, tungsten oxide, iron oxide, nickel oxide, cerium-zirconium oxide, titanium oxide, cobalt oxide, niobium oxide, tantalum oxide, rhodium oxide, and hafnium oxide.

6. The gas sensor according to claim 1, wherein, The thickness of the oxide layer is 5 nm or more.

7. The gas sensor according to claim 1, wherein, The nanowires are in ohmic contact with the oxide layer.

8. The gas sensor according to claim 1, wherein, The substrate is any one of a glass substrate, an alumina substrate, a zirconia substrate, and a silicon substrate on which a silicon oxide film is formed.

9. The gas sensor according to claim 1, wherein, The first pad electrode and the second pad electrode are made of the same type of metal as the nanowire.

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