WO2009083720A1

WO2009083720A1 - Ionization sensing

Info

Publication number: WO2009083720A1
Application number: PCT/GB2008/004280
Authority: WO
Inventors: Stephen Richard Elliott; William Ireland Milne; Wenmiao Shu; Xiaozhi Wang
Original assignee: Cambridge Enterprise Limited
Priority date: 2007-12-31
Filing date: 2008-12-29
Publication date: 2009-07-09
Also published as: GB0725374D0

Abstract

An ionization sensor is provided, for use in sensing gaseous species, e.g. for security applications. The sensor has a substrate, a first electrode and a second electrode, both formed on the same substrate. Typically, both electrodes comprise a nanostructure such as a carbon nanotube. A potential difference applied between the electrodes allows the formation of a high electric field strength due to nanostructure shape effects, allowing ionization of the gaseous species at a characteristic electric field strength. Arrays of such sensors are also provided, to allow parallel sensing.

Description

IONIZATION SENSING

BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to the detection of analyte species by ionization sensing. The present invention provides a device and system for such sensing, and also methods for such sensing.

Related art

There is a pressing need for highly sensitive and/or selective devices to detect toxic or explosive gaseous or other species, e.g. for applications in defence and homeland security. In addition, gas sensors can be used in a wide variety of applications ranging from manufacturing processes, medical diagnosis, and safety alarms to environmental monitoring.

For many commercial applications, it is desirable to produce highly sensitive, selective, light-weight and low-power sensors .

Semiconducting oxide gas sensors are a commercially available technology which detect gas through an increase in electrical conductivity when reducing gases are absorbed on the sensor surface. Such sensors are in general sensitive, but lack selectivity and operate at elevated temperature, which is disadvantageous. Other types of gas sensors involving semiconducting materials including inorganic thin films, conducting polymers or organic semiconductors are also limited either due to lack of selectivity or to an extremely low conductivity, hence limiting the sensitivity. More importantly, any approach which involves the selective absorption of gas materials into sensing materials may inevitably suffer from irreversible binding of gas molecules, which may poison the sensor device and may cause deterioration of the detected signal.

Another approach to detecting gases with high sensitivity and selectivity is ion mobility spectrometry (IMS). This is capable of detecting and identifying very low concentrations of chemicals based upon the differential migration of gas-phase ions through a homogeneous electric field. However, IMS devices operate in a two-step process; first deionisation of gas molecules, then separation of the gas ions for detection, which are achieved using two separate instrumentations. In addition, operation under atmospheric pressure requires heating to above 100° C in order to remove ion clusters. All these contribute to increased complexity and energy consumption of the instrumentation, making it extremely challenging to construct light-weight, inexpensive, handheld devices. On the other hand, new types of gas sensors based on carbon nanotubes (CNTs) have been developed. It has been found [Collins P G et al (2000) "Extreme oxygen sensitivity of electronic properties of carbon nanotubes" Science 287 1801] that the electrical resistance of semiconducting single-walled carbon nanotubes (SWCNTs) can be changed when they are exposed to gaseous molecules and therefore served as semiconducting SWCNTs-based gas sensors. This type of sensor is highly sensitive, rapidly responsive and operates at room temperature and atmospheric pressure. However, this approach, based on electrical conductance changes of carbon nanotubes, may be limited by gas-diffusion kinetics, difficulty to identify gases with low adsorption energies, and a low capability to distinguish between gases or gas mixtures. In addition, the conductance of CNTs is highly sensitive to environmental conditions such as moisture, temperature and gas-flow rate.

Another gas-ionization sensor has been proposed by Modi et al [Modi et al, Nature, 424, 171-174, 2003, and US 2006/0251543] which is based on the electrical breakdown of gas molecules caused by the very high electric field at the tips of carbon nanotubes. In this disclosure, the cathode was aluminium and the anode was a vertically aligned multi-walled carbon nanotube (MWCNT) film on a SiO₂ substrate. The electrodes were separated by a glass spacer with a thickness of 150 μm. A high DC voltage was applied between the two electrodes generating a very high electric field surrounding the tips of carbon nanotubes, which ionize gas molecules in its close proximity. The threshold breakdown voltage and ionization current may be used to distinguish the gas species and concentrations. The sensors based on this mechanism were sensitive and selective, and were found to be unaffected by environmental changes.

A similar approach was taken by Hou et al 2007 [Hou Z., Wu J., Zhou W., Wei X., Xu D., Zhang Y and Cai B., "A MEMS-based ionization gas sensor using carbon nanotubes" IEEE Transactions on Electron Devices, Vol. 54, No. 6, June 2007, pp. 1545-1548] by forming a layer of carbon nanotubes on a first electrode formed on a substrate. The layer of carbon nanotubes was formed by screen printing a slurry of carbon nanotubes, thus providing a random array of carbon nanotubes. A second electrode was formed above and spaced apart from the carbon nanotubes, the spacing being via an electroplated nickel film.

SUMMARY OF THE INVENTION

The present inventors have realized, however, that the performance of the above sensors of Modi et al and Hou et al 2007 are limited by the fact that the two electrodes are separated by an air gap between two parallel substrates and the voltage required to break down gas molecules is highly dependent on the distance of the air gap between the two electrodes. The shorter is this distance, the lower the voltage that is required. However, as the gap is provided by a glass spacer or nickel spacer layer, respectively, sandwiched between the two electrodes, the gap distance is determined by the thickness of the spacer. For Modi et al, the gap distance is difficult to make extremely small. This resulted in a relatively high breakdown voltage (V_b) (for most gases V_b> 400 V). For Hou et al 2007, the gap distance depends on the reproducibility of the thickness of the nickel spacer layer. Therefore, the sensor performance is limited by difficulties in positioning the metal electrode and carbon-nanotube film in extremely close reproducible proximity. Additionally, the gap distance is susceptible to a variety of conditions of the spacer such as applied pressure and temperature. It may be difficult to position the two electrodes on separate substrates for parallel alignment with micrometer precision. Furthermore, the present inventors have found that the breakdown voltages of gases are also strongly dependent on the quality and uniformity of the carbon nanotubes such as density, diameter, shape, tip sizes, thickness and roughness of the carbon nanotube film. Such parameters are difficult to control using present carbon nanotube growth technologies. Variations in these parameters tends to compromise the sensitivity and selectivity of the sensor, limiting its ability to distinguish a particular gas molecule from a mixture.

Hou et al 2006 [Hou Z., Xu D., and Cai B. "Ionization gas sensing in a microelectrode system with carbon nanotubes"

Applied Physics Letters, 89, 213502, 2006] disclose a gas sensor chip in which hollow elongate slots of different width are etched into carbon nanotube layers, the slot width affecting the ionization breakdown voltages of different gaseous species. The carbon nanotube layer is deposited by screen printing, and thus provides a random array of carbon nanotubes. Subsequently, a layer of photoresist is formed on the carbon nanotube layer. Patterning of this layer by lithographic techniques allows the formation of a slot-shaped opening in the photoresist. The underlying carbon nanotube layer is then etched away, the etching forming a hollow slot shape in the carbon nanotube layer but also undercutting the photoresist, thereby forming a slot in the carbon nanotube layer or width greater than the width of the slot in the photoresist.

The present inventors have realised that a drawback with known sensors using carbon nanotubes is the difficulty of ensuring a uniform spacing between the electrodes. To some extent, this problem may be reduced if the carbon nanotubes are formed in the sensor with substantially no alignment, but this significantly reduces the effectiveness of the carbon nanotubes in providing electric field strength concentration. In the arrangement of Modi et al, since it is difficult to grow a population of carbon nanotubes to a uniform length, the tips of the carbon nanotubes will be at different spacings from the opposing metallic film electrode. Furthermore, it is very difficult to locate the metallic film electrode at a small but uniform distance from the tips of the carbon nanotubes, in order to enhance the electric field strength provided by a particular potential difference between the electrodes.

In a general aspect, therefore, the present inventors propose that the electrodes should be formed on the same substrate, in order that the spacing between them can be more accurately controlled.

Accordingly, in a first preferred aspect, the present invention provides an ionization sensor having a substrate, at least one first electrode and at least one second electrode, the first electrode comprising a nanostructure, there being a gap between the first electrode and the second electrode, control means for applying a potential difference between the first and second electrodes, wherein the first electrode and the second electrode are both formed on the substrate.

In a second preferred aspect, the present invention provides a method for sensing the presence of at least one analyte species using an ionization sensor having a substrate, at least one first electrode and at least one second electrode, the first electrode being formed of a nanostructure, there being a gap between the first electrode and the second electrode, wherein the first electrode and the second electrode are both formed on the substrate, the method including the step of applying a potential difference between the first and second electrodes to ionize said species.

In a third preferred aspect, the present invention provides an ionization sensing system including a sensor according to the first aspect, the system further including detecting means for detecting at least one characteristic of ionization of at least one analyte species.

In a fourth preferred aspect, the present invention provides a method for manufacturing an ionization sensor according to the first aspect, including the step of forming the first electrode and the second electrode on the substrate.

Further preferred aspects of the invention are set out in the claims .

Preferred and/or optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention, unless the context demands otherwise .

The term "nanostructure" as used herein, refers to a structure having at least one dimension of less than about 500 nm. Preferably, a nanostructure has at least one dimension of less than about 200 nm, less than about 100 nm, less than about 50 nm, less than about 20 nm or less than about 10 nm. Each of the three dimensions of the nanostructure may have a dimension of less than about 500 nin, less than about 200 nm, less than about 100 nm, less than about 50 nm, less than about 20 nm or less than about 10 nm. Alternatively, two of the three dimensions of the nanostructure may have a dimension of less than about 500 nm, less than about 200 nm, less than about 100 nm, less than about 50 nm, less than about 20 nm or less than about 10 nm.

Illustrative nanostructures useful in the present invention include, but are not limited to, a single or multi-walled nanotube, a nanowire, a nanodot, a quantum dot, a nanorod, a nanocrystal, a nanobud, a nanotetrapod, a nanotripod, a nanobipod, a nanoparticle, a nanosaw, a nanospring, a nanoribbon, a nanopyramid, a branched tetrapod or any other branched nanostructure, or any mixture thereof. The nanostructure can comprise organic materials, inorganic materials or a mixture thereof.

Preferably, the nanostructure is an elongate nanostructure. For example, the nanostructure may be a single-walled carbon nanotube or a multi-walled carbon nanotube, or a carbon nanofibre or nanorod.

The nanostructure may have a monocrystalline structure, a double-crystal structure, a polycrystalline structure, an amorphous structure, or a combination thereof. The nanostructure may comprise at least one of the following elements or compounds: Au, Ag, Pt, Pd, Ni, Co, Ti, Mo, W, Mn, Ir, Cr, Fe, C, Si, Ge, B, Sn, SiGe, SiC, SiSn, GeC, BN, InP, InN, InAs, InSb, GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, CdO, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, PbO, PbS, PbSe, PbTe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, InO, SnO, GeO, WO, TiO, FeO, MnO, CoO, NiO, CrO, VO, CuSn, CuF, CuCl, CuBr, CuI, AgF, AgCl, AgBr, AgI, CaCN₂, BeSiN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂, CuGeP₃, CuSi₂P3, Si₃N₄, Ge₃N₄, Al₂O₃, Al₂CO, In_x0_y, Sn_xOy, SiO_x, GeO_x, W_x0_y, Ti_x0_y, Fe_x0_y, Mn_x0_y, Co_xOy, Ni_xOy, Cr_xOy, V_x0_y, or MSiO₄, any alloys thereof, or any combination thereof, wherein x is an integer ranging from 1 to 5, y is an integer ranging from 1 to 5, and M is selected from Zn, Cr, Fe, Mn, Co, Ni, V, and Ti.

For example, the nanostructure may comprise C or Si.

The nanostructure can also comprise metallic or non-metallic alloys other than those listed above, a polymer, a conductive polymer, a ceramic material, or any combination thereof.

For example, the nanostructure may comprise a semiconductive material .

When a nanostructure comprises a semiconductive material, the semiconductive material may futher comprise a dopant. Dopants useful in the present invention include, but are not limited to: a p-type dopant, such as Li, B, Al, In, Mg, Zn, Cd, Hg, C, Si, an element from Group I of the periodic table, an element from Group II of the periodic table, an element from Group III of the periodic table or an element from Group IV of the periodic table; or an n-type dopant, such as, Si, Ge, Sn, S, Se, Te, P, As, Sb, Cl, or an element from group IV of the periodic table, an element from group V of the periodic table, an element from group VI of the periodic table or an element from group VII of the periodic table.

When the nanostructure is a nanotube, nanowire or nanoribbon, the nanotube, nanowire or nanoribbon can comprise a conductive or semiconductive material, such as an organic polymer, pentacene or a transition metal oxide.

The term "nanowire" is defined as any elongate material as described herein that includes at least one cross-sectional dimension less than 500 nm and has an aspect ratio of greater than 10 and is understood to include "whiskers" or

"nanowhiskers." The term "nanorod" refers to an elongate material as described herein which has an aspect ratio less than that of a nanowire.

Suitable nanostructures may be produced using any known methods, including, but not limited to, arc discharge, laser ablation, solution-based methods, vapor-phase methods or high- temperature substrate-based methods, such as those described in Greene et al., Angew. Chem. Int. Ed. £2:3031-3034 (2003), Baddour et al., Int. J. Chem. Reactor Eng. 3_, R3, (2005), and International Publication No. WO 02/017362.

Methods for making nanocrystals are described, for example, in Puntes et al., Science 291:2115-2117 (2001), U.S. Patent No. 6,306,736 to Alivastos et al., U.S. Patent No. 6,225,198 to Alivastos et al., U.S. Patent No. 5,505,928 to Alivastos et al., U.S. Patent No. 6,048,616 to Gallagher et al., and U.S. Patent No. 5,990,479 to Weiss et al., each of which is incorporated herein by reference in its entirety.

Methods for making nanowires are described, for example, in Gudiksen et al., J. Am. Chem. Soc. 122:8801-8802 (2000), Gudkisen et al. , Appl. Phys . Lett. 18_: 2214-2216 (2001), Gudiksen et al. , J. Phys. Chem. B 105: 4062-4064, Morales et al., Science 291:208-211 (1998), Duan et al. , Adv. Mater. ^2:298-302 (2000), Cui et al. , J. Phys. Chem. B 105:5213-5216 (2000), Puntes et al., Science 291:2115-2117 (2001), Peng et al., Nature. 404:59-61 (2000), U.S. Patent No. 6,306,736 to Alivastos et al., U.S. Patent No. 6,225,198 to Alivastos et al., U.S. Patent No. 6,036,774 to Lieber et al., U.S. Patent No. 5,897,945 to Lieber et al. and U.S. Patent No. 5,997,832 to Lieber et al., each of which is incorporated herein by reference in its entirety. Methods for making nanoparticles are described, for example, in Liu et al., J. Am. Chem. Soc. 12^3:4344 (2001), U.S. Patent No. 6,413,489 to Ying et al., U.S. Patent No. 6,136,156 to El-Shall et al., U.S. Patent No. 5,690,807 to Clark et al., each of which is incorporated herein by reference in its entirety.

Single-walled carbon nanotubes are rolled up graphene sheets. Their twist or chirality defines their optical and electrical properties. Single-walled carbon nanotubes useful in the present invention may have a diameter of about 0.1 nm or greater, preferably about 0.5 nm or greater, more preferably about 1.0 nm or greater. The diameter may be about 10 nm or less, more preferably about 3 nm or less or about 1.5 nm or less .

It is preferred that the nanostructure of the first electrode is aligned with respect to the substrate. For example, the nanostructure may be vertically aligned with respect to the substrate. Similarly, it is preferred that the second electrode is aligned with respect to the substrate. For example, the nanostructure of the second electrode may be vertically aligned with respect to the substrate. Where both the first and second electrode comprise at least one nanostructure, preferably these nanostructures are aligned with respect to each other. In the case where each nanostructure is an elongate nanostructure such as a nanotube (or other elongate nanostructure as set out above) , it is preferred that the elongate nanostructure has a proximal end and a distal end. Preferably, the proximal end of each elongate nanostructure is attached to the substrate. For example, the proximal end may be anchored at the substrate. Preferably the distal end of each nanostructure points substantially away from the substrate. The degree of alignment of the elongate nanostructures may be considered in terms of the distal end and the proximal end of each elongate nanostructure. A notional straight line may be drawn from the proximal end to the distal end of each elongate nanostructure. Preferably, when viewed in cross-section (or when viewed from one viewpoint) these lines are substantially aligned with each other. For example, at least 50% of these lines may be distributed within 45° of each other in a cross-sectional view. More preferably, at least 60%, at least 70%, or at least 80% may be distributed within 45° of each other. As another example, at least 50% (or at least 60%, at least 70%, or at least 80%) may be distributed within 40° of each other, more preferably within 35° of each other, within 30° of each other, within 25° of each other, within 20° of each other, within 15° of each other, or within 10° of each other. Such an analysis may typically be performed using an SEM image. The individual elongate nanostructures need not themselves be perfectly straight (although this is preferred) . Most preferably, the elongate nanostructures are vertically aligned with respect to the substrate. For example, the device may have an array of vertically aligned carbon nanotubes (VACNTs) .

The degree of alignment of the nanostructures may be affected by the method of manufacture of the nanostructures. For example, in the case where the nanostructures are carbon nanotubes, the growth process for the nanotubes may affect the alignment. Where carbon nanotubes are grown on the substrate using chemical vapour deposition (CVD) , for example, an electric field may be used to align the growing nanotubes, the alignment depending on the field profile. Where thermal CVD growth (i.e. no applied electric field gradient) is used, the alignment is less pronounced, or may even be random. However, where the nanotubes are grown closely packed together (e.g. using small catalyst dots on the substrate, the dots being closely packed) , then the Van der Waals forces between the growing nanotubes may result in suitable alignment of the nanotubes. This the case particularly for single walled carbon nanotubes. In the case of multiwalled carbon nanotubes, for example, a less dense packing of aligned nanotubes may be achieved by appropriately spacing the catalyst dots and applying an electric field gradient during growth as set out above . The lengths of the single-walled carbon nanotubes useful in the present invention may be about 0.01 μm or greater. The lengths may be about lOOμrn or less.

The single-walled carbon nanotubes may be commercially available or, alternatively, can be made by any known means including, but not limited to, a chemical vapor deposition process, a laser ablation process, an arc process, a fluid bed process or a gas-phase process using carbon monoxide. Processes for making single-walled carbon nanotubes include those disclosed, for example, in Liu et al., Science 280 : 1253- 1256 (1998); Bronikowski et al . , J. Vacuum Sci. Tech. A 1^:1800-1805 (2001); U.S. Patent No. 6,183,714; International Publication No. WO 00/26138; Dresselhaus et al., Carbon nanotubes, Topics of applied Physics 80, Springer (2001);

Lebedkin et al., Carbon £0: 417-423 (2000); and International Publication No. WO 00/17102, each of which is incorporated herein by reference in its entirety.

Multi-walled carbon nanotubes have multiple wall layers. A first type of multi-walled carbon nanotube has graphene sheets arranged in concentric cylinders, amounting to a small diameter single-walled carbon nanotube located coaxially within another, larger diameter single walled carbon nanotube. A second type of multi-walled carbon nanotube has a single graphene sheet rolled up to provide the multiple walls. Methods for growing arrays of multiwalled carbon nanotubes are disclosed in W. I. Milne et al., J. Mater. Chem. , 2004, 14, 933 - 943 "Carbon nanotubes as field emission sources", in K. B. K. Teo et al., 2003 Nanotechnology 14 204-211 "Plasma enhanced chemical vapour deposition carbon nanotubes/nanofibres—how uniform do they grow?" and in K. B. K. Teo et al., 2001 Applied Physics Letters 79 1534 "Uniform patterned growth of carbon nanotubes without surface carbon", the content of each of which is incorporated herein by reference in its entirety.

Multi-walled carbon nanotubes useful in the present invention may have a diameter of about 0.5 nm or greater, more preferably about 1.0 nm or greater, or 2 nm or greater, or 5 nm or greater. The diameter may be about 100 nm or less, more preferably about 50 nm or less, about 40 nm or less or about 30 nm or less .

The lengths of the multi-walled carbon nanotubes useful in the present invention may be about 0.01 μm or greater. The lengths may be about lOOμm or less.

Carbon nanobuds are essentially a combination of carbon nanotubes and fullerenes. One or more fullerenes are covalently bonded to the sidewall (typically outer sidewall) of a nanotube. Nanobuds can be further functionalized through known fullerene chemistry. Nanobuds may possess advantageous electrical and electronic properties, such as excellent field emission characteristics. Further detail relating to the properties and manufacture of carbon nanobuds is available from Nasibulin et al "A Novel Hybrid Carbon Nanomaterial" (2007) Nature Nanotechnology 2(3), 156-161, and WO2007/057501, the content of each of which is incorporated herein by reference in its entirety.

To enhance or optimize the performance, the nanostructure can be functionalized. Functionalization refers to the chemical or physical treatment of the nanostructure surface aimed at modifying and optimizing characteristics such as structure, orientation, bonding or conductivity. For example, the tip of the nanostructure may be functionalized with one or more metallic material, e.g. a metallic nanoparticle . This can provide lower ionization voltages in use of the sensor.

Preferably the analyte species is a gaseous analyte species. The preferred embodiments may be capable of detecting more than one type of gaseous analyte species. The sensor is thus preferably a gas ionization sensor.

The location of the first and second electrodes on the same substrate allows the gap between the electrodes to be precisely controlled. This is because the location of the first and second electrodes on the same substrate can be achieved using spatially precise techniques. It is preferred that the location of the first and/or second electrode with respect to the substrate is determined using a lithographic patterning technique. It is preferred that the nanostructures are grown in situ. This can provide more uniform alignment of nanostructures. Whether or not the nanostructures have been grown in situ can be determined, for example, by the skilled person using scanning electron microscopy. It is particularly preferred that the nanostructures are not patterned after growth of the nanostructures.

The first electrode may include two or more nanostructures.

These may be in electrical contact. Thus, during operation of the device, these may be at substantially the same electric potential .

It is preferred that the nanostructure is elongated in at least one dimension. This allows it to project from the substrate in order to provide a sharp tip or edge projected from the substrate. This allows the development of a high electric field strength at this region, with the . application of only a modest potential difference between the first and second electrodes .

The height of the nanostructure, or of the sharp tip or edge, from the substrate is preferably at least 0.5 μm. This height is more preferably at least 1 μm, at least 1.5 μm, or at least about 2 μm. This height is preferably at most 80 μm, at most

60 μm, at most 40 μm, at most 30 μm, at most 20 μm, at most 15 μm, at most 10 μm or at most about 8 μm. Where there is a plurality of nanostructures (e.g. in the first electrode and/or in the second electrode) it is preferred that the average height of the nanostructures falls within one or more of the above ranges.

One reason for ensuring that the nanostructure has a height in the ranges suggested above is to isolate (to an extent) the electric field at the tip or edge of the nanostructure from the effects of the substrate.

It is preferred that the second electrode also includes at least one nanostructure. The second electrode may have a height within one of the ranges set out above with respect to the first electrode. Advantageously, the height of the second electrode is substantially the same as the height of the first electrode .

The second electrode may include two or more nanostructures. These may be in electrical contact. Preferably they are in electrical contact at their proximal ends, e.g. only at their proximal ends. Thus, during operation of the device, these nanostructures may be at substantially the same electric potential .

It is possible for the first electrode and the second electrode to include substantially the same number of nanostructures. In that case, the first and second electrodes may be arranged substantially parallel to each other on the substrate. This allows a substantially uniform shortest distance between the electrodes. The advantage of this is that the electric field strength in the gap between the first and second electrodes may then be substantially uniform (apart from variations close to the electrodes due to, e.g., shape effects).

Alternatively, the first electrode may include fewer nanostructures than the second electrode. The first electrode may have only a single nanostructure, for example. It is still preferred to provide a uniform shortest distance between the first electrode and the second- electrode, even in this case. The second electrode may advantageously be shaped in order to achieve this. The second electrode may be formed in a curved shape on the substrate. For example, the first electrode may be located at or around a point which forms the centre of a circle, along an arc of which the second electrode is disposed. The second electrode may extend along 5° or more, 10° or more, 20° or more, 30° or more, 45° or more, 60° or more, 90° or more, 120° or more, 150° or more, 180° or more, 210° or more, 240° or more, 270° or more, 300° or more, 330° or more or 360° along the arc of the circle. There may be gaps in the second electrode of, e.g., 5° or more, 10° or more, 20° or more, 30° or more, or 45° or more. When in use, a potential difference applied to the first and second electrodes results in a mechanical force on the first and second electrodes due to electrostatic attraction. This can lead to deformation of the first and second electrodes. This can cause difficulties in ensuring that the first and second electrodes have durability. It may also cause difficulties in determining the electric field strength between the first and second electrodes, since the spacing of parts of the electrodes will depend on the voltage applied between the electrodes. Where the nanostructures are nanotubes, nanorods or nanofibres, the deformation may amount to bending of the nanostructures. In order to reduce at least partially the deformation experienced by the first electrode, the disposition of the second electrode in a circular arc around the first electrode can assist in balancing the forces on the first electrode. With this aim in mind, it is preferred that the second electrode extends all around the first electrode, optionally with gaps. These gaps may preferably be spaced angularly substantially equally along the second electrode.

The substrate may have one or more conductive leads or tracks formed on it. Preferably there is formed a first electrode conductive track and a second electrode conductive track, forming at least part of the control means.

Since the first electrode conductive lead should be in electrical contact with the nanostructure of the first electrode, but not in electrical contact with the second electrode, it is preferred that the first electrode conductive lead is electrically isolated from the second electrode. This may be by allowing the first electrode conductive lead to pass the second electrode via one of the gaps (or the only gap, if there is only one) in the second electrode. Alternatively, the first electrode conductive lead may be isolated from the second electrode by an insulating layer formed between them.

Preferably the gap between the first and second electrodes is a gap of at least 1 μm. More preferably, the gap is at least 2 μm, at least 4 μm, at least 6 μm, at least 8 μm or at least 10 μm. The gap may be 100 μm or less, 80 μm or less, 60 μm or less or 40 μm or less. Preferably the gap is at least equal to the height of the first electrode. More preferably the gap is at least equal to the height of the first electrode plus the height of the second electrode. Where, as discussed above, operation of the device causes deformation of the nanostructures, the gap is preferably spaced so that such deformation does not allow contact between the deformed electrodes .

The sensor may be one sensing element in an array of sensing elements. The array of sensing elements may be formed on the same substrate. Thus, each sensing element may include at least one first electrode and at least one second electrode, the at least one first electrode being formed of a nanostructure, there being a gap between the first electrode and the second electrode, control means for applying a potential difference between the first and second electrodes, wherein the first electrode and the second electrode are both formed on the substrate.

Each sensing element in the array may have its own independent control means. Alternatively, there may be provided common control means for two or more electrodes in two or more sensing elements. For example, either the first or the second electrode in two or more arrays may be maintained at a constant potential, e.g. 0 V.

The sensing elements may be arranged so that they are addressable. They may be addressed and/or switched in row- column form. The device may include suitable electronic components in order to achieve this. In particular, there may be provided electronic components in order to provide an active matrix of sensing elements, e.g. using thin film transistors.

The sensing elements may have different gap sizes between their first and second electrodes. This allows there to be provided a range of gap sizes. Thus, when the same potential difference is applied to the sensing elements, the different gap sizes provide the sensing elements with different electric field strengths. In this way, the array of sensing elements provides a corresponding array of electric field strengths. This can allow the device to analyse a gas sample in one step, since different gaseous species have different characteristic ionisation electric field strengths.

In the array of sensing elements, there may be two or more sensing elements provided with the same gap size between the first and second electrodes. This provides a degree of redundancy in the device, so that failure of one of the sensing elements can be accommodated without failure of the device.

In the array of sensing elements, there may be at least 5 groups of sensing elements (each group containing at least one sensing element) , the sensing elements in each group having different gap sizes to other groups. In order to provide an increased gradation of gap sizes, there may be at least 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100 or more such groups.

Preferably the sensing elements are spaced apart from each other in the array. This is typically in order to reduce the effect of each sensing element on the others. The nearest neighbour centre-to-centre spacing of the sensing elements is preferably at least 10 μm, more preferably at least 20 μm or at least 30 μm.

Preferably the sensing elements are provided so that the density of sensing elements is equivalent to at least 100 per cm². More preferably there are at least 1000 per cm², or at least 5000 per cm². It is not necessary that the sensing elements cover an area of at least 1 cm², but this may be advantageous .

Preferably, the sensing system has at least 20 sensing elements. More preferably, the sensing system has at least 100, at least 500, at least 1000, at least 5000 or at least 10,000 sensing elements.

Preferably the nanostructures are electrically conductive, in the sense of metallic conductivity. However, it is possible for the nanostructures to be semiconducting.

Preferably the nanostructures are aligned with each other so that elongate axes of each nanostructure are substantially parallel to each other. This assists in the uniformity of the gap between the first and second electrodes, in order to reduce a variation in this gap size with height from the substrate.

Preferably, in use, the sensor is operable at an applied voltage between the first electrode and the second electrode of 100 V or less. Preferably the voltage applied is 80 V or less, 60 V or less, 50 V or less, 40 V or less, 30 V or less, 20 V or less or 10 V or less. The advantage of using a relatively low voltage is that the system can be more easily made portable and mobile. For example, the system may be hand-held and/or battery operated. The sensor may be operated by applying a dc voltage between the first and second electrodes. This provides the simplest mode of operation. However, the sensor may be operated using pulsed voltages or ac voltage. One advantage of using non-dc voltage is that it may then be possible to reduce the dust or other contamination that arises from dc electrostatic effects. Furthermore, monitoring of the analysis signal may be easier (especially for small signals) where there is a frequency component for locking in.

The detecting means for detecting at least one characteristic of ionization of at least one gaseous species may be a voltage signal sensor or a current signal sensor, for example. It is possible to apply a voltage between the first and second electrode and measure the current between the first and second electrodes at that voltage. The current typically increases when ionization events occur between the first and second electrodes. Gaseous species ionize at characteristic electric field strengths. Thus, the identification of an increase in current at a particular electric field strength may assist in identifying the gaseous species. Thus, the detecting means may include a current meter. For example, the current meter may be capable of detecting a current in the nanoamp range. It is also possible to apply a current between the first and second electrode and measure the voltage required to achieve that current. This is based on the same ionization events as mentioned above.

Additional or alternative detecting means may be provided to detect other parameters. For example, an ionization event may affect the capacitance of the sensor. An ionization event may affect the force acting on one or more of the electrodes. This change of force may be detected by the detecting means. For example, the result may be vibration of one or more part of one or more of the electrodes.

Preferably the substrate includes an electrically conductive layer. This may be metallic or semiconducting, for example. One preferred material for the substrate is a silicon substrate. For example, silicon single crystal wafers provide a suitable flat surface for processing and for forming the first and second electrodes. It is preferred to use an electrically conductive layer in order to assist in the growth of suitable nanostructures . However, it is possible that the substrate is formed from an insulating material. For example, a quartz or glass substrate can be used as the substrate material .

The substrate may include an electrically insulating layer. This may be formed over the electrically conductive layer. The insulating layer assists in isolating the first and second electrodes from the electrically conductive substrate layer during use. In certain growth regimes (e.g. in dc plasma- assisted growth processes) , the electric field may need to penetrate through the substrate in order to achieve guided growth during the growth of the nanostructures . Furthermore, it is desirable to avoid charging of the substrate or of the growing nanostructures. This may restrict the thickness of the insulating layer. For example, the insulating layer may be at least 100 nm thick, more preferably at least 0.2 μm, at least 0.4 μm, at least 0.6 μm, at least 0.8 μm or at least 1 μm thick. The insulating layer may have a thickness of 5 μm or less, 4 μm or less, 3 μm or less or 2 μm or less.

The insulating layer may be formed of silicon oxide. This is particularly convenient where the electrically conductive layer is formed of silicon.

During manufacture of the sensor, there may be formed the first and/or second electrode conductive leads. These are preferably formed on the insulating layer. They may be deposited by any suitable film-forming process, e.g. sputtering. They may be patterned using a patterning process (e.g. photolithography, e- beam lithography) . The first and/or second electrode conductive leads are preferably metallic, e.g. Mo.

During manufacture of the sensor, there may further be deposited a catalyst. This is typically in order to promote the formation of the required nanostructures. The catalyst may be deposited as a layer on the first and/or second electrode conductive leads, and this may be before or after patterning of the first and/or second electrode conductive leads. The catalyst may also be metallic and may be deposited using any suitable film-forming process. The catalyst may subsequently be patterned using a patterning process (e.g. photolithography, e-beam lithography) but typically before growth of the nanostructures . The catalyst may be, for example, Fe and/or Ni. These are particularly suitable catalyst materials for the growth of multi-walled carbon nanotubes.

During use, the electric field strength in the gap between the first and second electrode is preferably at least 10⁵ Vm^"1. Preferably, the electric field strength in this location is at least 5 x 10⁵ Vm^"1, or at least 10⁶ Vm^"1. Close to the sharp edge or tip of the nanostructures, the local electric field strength may be up to ten times one of these values, due to shape effects causing electric field concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention, and further preferred and/or optional features that may be combined in any combination with any aspect of the invention, will now be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a schematic cross sectional view of a sensor device according to one embodiment of the invention.

Fig. 2 shows an SEM image of a sensor device according to an embodiment of the invention, having a parallel electrode structure .

Fig. 3 shows an SEM image of a sensor device according to an embodiment of the invention, having a semi-circular structure.

Fig. 4 shows an SEM image of a sensor device according to an embodiment of the invention, having a near-full circular structure .

Fig. 5 shows an SEM image of an array of sensing elements r according to an embodiment of the invention, each sensing element having a semi-circular structure.

Fig. 6 shows a real time ionization current curve at 50 V and the influence of ethanol vapour.

Fig. 7 shows a comparison of current signal of a parallel structure sensor and a circular structure sensor upon exposure to 5.3% ethanol vapour in nitrogen gas. Fig. 8 shows a comparison of current signals upon exposure to different gas vapours: methanol, acetone, ethanol, water vapour.

Fig. 9 shows a similar comparison to Fig. 8, except with a different current scale, for nitrogen, hexane, air, toluene and isopropyl alcohol (IPA) .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER PREFERRED AND/OR OPTIONAL FEATURES

The preferred embodiments of the present invention provide a new type of ionization sensor device which can provide highly sensitive, selective, low- power gas detection.

Referring to Fig. 1, the sensor device includes at least a pair of electrodes (i.e. cathode and anode electrodes), shown in Fig. 1 as the first electrode 12 and second electrode 14. Each electrode is formed from carbon nanotubes. As will be understood, conducting nanotubes, nanowires or other nanostructures, e.g. formed from other materials, may be used. The substrate 16 is a Si single crystal wafer. A SiO₂ insulating layer 18 is formed on the surface of the Si substrate. Conducting tracks 20, 22 are formed on the SiO₂ insulating layer. In this example, the conducting tracks 20, 22 are formed from Mo. They are patterned in order to ensure that they are insulated from each other. The electrodes 12, 14 are formed and aligned on the surface of their respective conducting track 20, 22. The electrodes 12, 14 are precisely spaced by a gap, with each electrode containing either a single nanotube or nanowire, or an array of nanotubes or nanowires. It will be understood that the orientation of the electrodes with respect to the substrate may be upright, lateral or any other suitable orientation. The electrodes are of substantially identical shape, size and length as they are synthesized under the same conditions. An external voltage source 24 is electrically connected between the electrodes, thereby generating a high electric field surrounding the sharp tips of the nanotubes. When gas molecules approach the close vicinity of the electrodes, the applied electric field triggers ionization or even chemical reactions (e.g. combustion, explosion) of the gas molecules. The ionization (and/or chemical reactions) can result in a change of current, electrical capacitance, or any other electrical and mechanical signals between the electrodes, which can be detected precisely by an external measuring device (not shown) .

This design has several major advantages. Each individual sensor can be formed to provide a highly uniform (i.e. reproducible) shape, as their size and position are typically individually defined by lithography. Using nanoscale- resolution lithography techniques (e.g. e-beam lithography), the spacing between the nanotubes on two electrodes can be precisely controlled on the nanometer scale. Therefore, only a very low voltage is required to electrically break down gas molecules, and hence this opens up the possibility for low- cost, low-weight and battery-powered devices. Further enhancement of sensitivity can be achieved by detection of the mechanical forces generated by the combustion or ionization of gases which may affect the spacing of the nanospaced electrodes .

The sensor device can be constructed in different geometries and shapes (see Figs. 2-4). This freedom of design allow the further enhancement of the sensitivity of detection. In addition, arrays of sensor devices (Fig. 5) can be achieved on the same substrate. This allows the production of a sensing system for more reliable and massively parallel sensing operations.

In an example process according to an embodiment of the invention, electron-beam lithography was used to define metal electrodes and catalyst areas on a Si wafer with a thermo-grown oxide layer. Patterned chips were put into a plasma-enhanced chemical vapour deposition (PECVD) system chamber with a base pressure of 10^"1 mbar and heated up for carbon nanotube growth. After reaching a temperature of 725 ⁰C, the nanotube synthesis was initiated immediately by introducing NH₃ and C₂H₂ gases in a ratio of 4 to 1 into the chamber and initiating the direct current glow discharge for a time of between 10 minutes to 1 hour. The glow discharge plasma was struck between the heated substrate holder and an anode situated above the cathode. After growth of the carbon nanotubes, the as-formed devices may be cleaned by a suitable cleaning process if necessary to remove amorphous carbon and other contaminations.

The embodiments of the invention provide new and improved methods and apparatus for detecting substantially any type of gas molecules with unprecedented sensitivity and selectivity. The sensor can be used for detecting traces of explosive substances for defence and security applications, and also for detecting flammable or toxic gases such as H₂, CO, methane, ethanol, propane, HF etc for conventional safety applications. Applications also include rapid analysis of specific gas molecules which are related to specific diseases. For example, an excessive amount of acetone present in human breath is linked to diabetes. It can be used for law enforcement applications, including precise detection of the trace level of alcohol or drugs in breath sample. The instrumentation of this type of sensor device can be light-weight and battery powered, which makes it possible to be employed as handheld sensor or standalone devices for applications in safety and security monitoring .

Fig. 6 shows a real time ionization current curve at 50 V and the influence of ethanol vapour for a sensor with the geometry- shown in Fig. 3. In this experiment, the sensor was placed in an evacuated chamber and a voltage of 50 V applied between the first and second electrodes. Ethanol vapour was introduced periodically. Also, out of phase with the ethanol introduction, nitrogen gas was introduced to purge the system. The effect of this gas introduction protocol is shown in the resulting ionization current curve in Fig. 6. Note the logarithmic current scale.

Fig. 7 shows a comparison of current signal of a parallel structure sensor (as shown in Fig. 2) and a circular structure sensor (as shown in Fig. 3) upon exposure to 5.3% ethanol vapour in nitrogen gas. It appears that the circular structure provides a lower signal. This is considered to be due to the fact that fewer ionization events take place in the vicinity of the tip of the single carbon nanotube of the first electrode in the circular structure compared with the many carbon nanotubes of the first electrode in the parallel structure.

Fig. 8 shows a comparison of current signals upon exposure to different gas vapours: methanol, acetone, ethanol, water vapour, following a similar experimental protocol to Fig. 7.

Fig. 9 shows a similar comparison to Fig. 8, except with a different current scale, for nitrogen, hexane, air, toluene and isopropyl alcohol (IPA).

Note that in both Figs. 8 and 9, the water vapour curve is included as a reference. It is clear that many vapours and gases including nitrogen, hexane, air, IPA and toluene induce very little current in the sensor below 100 V. This indicates that the threshold voltages, or turn-on voltages, are above 100 V in these embodiments.

The above embodiments of the present invention have been described by way of example. Modifications of these embodiments, further embodiments and modifications thereof will be apparent to the skilled person on reading this disclosure and as such are within the scope of the present invention.

Claims

1. An ionization sensor having a substrate, at least one first electrode and at least one second electrode, the first electrode comprising an elongate nanostructure, elongate in at least one dimension, there being a gap between the first electrode and the second electrode, control means for applying a potential difference between the first and second electrodes, wherein the first electrode and the second electrode are both formed on the substrate, and wherein the nanostructure has a proximal end attached to the substrate and a free distal end.

2. An ionization sensor according to claim 1 wherein the nanostructure is a nanotube, nanofibre or nanorod.

3. An ionization sensor according to claim 1 wherein the nanostructure is a multiwalled carbon nanotube.

4. An ionization sensor according to any one of claims 1 to 3 wherein the nanostructure has a diameter in the range 2 nm to

50 nm.

5. An ionization sensor according to any one of claims 1 to 4 wherein the height of the nanostructure from the substrate is at least 0.5 μm.

6. An ionization sensor according to any one of claims 1 to 5 wherein the height of the nanostructure from the substrate is at most 100 μm.

7. An ionization sensor according to any one of claims 1 to 6 wherein the second electrode includes at least one nanostructure .

8. An ionization sensor according to claim 7 wherein the second electrode includes two or more nanostructures .

9. An ionization sensor according to claim 7 or claim 8 wherein the nanostructure or nanostructures are each elongate nanostructures, elongate in at least one dimension, and wherein each nanostructure has a proximal end attached to the substrate and a free distal end.

10. An ionization sensor according to claim 9 wherein the nanostructure of the first electrode is aligned with the nanostructure or nanostructures of the second electrode.

11. An ionization sensor according to any one of claims 1 to 10 wherein the first electrode and the second electrode include substantially the same number of nanostructures.

12. An ionization sensor according to any one of claims 1 to 11 wherein the first and second electrodes are arranged substantially parallel to each other on the substrate.

13. An ionization sensor according any one of claims 1 to 10 wherein the first electrode includes fewer nanostructures than the second electrode.

14. An ionization sensor according to any one of claims 1 to 13 wherein a uniform shortest distance is provided between the first electrode and the second electrode, along the length of the second electrode.

15. An ionization sensor according to any one of claims 1 to 14 wherein the second electrode is formed in a curved shape on the substrate.

16. An ionization sensor according to claim 15 wherein the first electrode is located at or around a point which forms the centre of a circle, along an arc of which the second electrode is disposed.

17. An ionization sensor according to any one of claims 1 to 16 wherein the gap between the first and second electrodes is a gap of at least 1 μm.

18. An ionization sensor according to any one of claims 1 to 17 wherein the gap between the first and second electrodes is at least equal to the height of the first electrode.

19. An ionization sensing system including an array of sensing elements, each sensing element being according to any one of claims 1 to 18.

20. An ionization sensing system having an array of sensing elements formed on a substrate, each sensing element including at least one first electrode and at least one second electrode, the at least one first electrode being formed of a nanostructure, there being a gap between the first electrode and the second electrode, control means for applying a potential difference between the first and second electrodes, wherein the first electrode and the second electrode are both formed on the substrate, wherein the system has at least 1000 sensing elements.

21. An ionization sensing system according to claim 19 or claim 20 wherein the sensing elements have different gap sizes between their first and second electrodes, thereby providing a range of gap sizes in the array.

22. An ionization sensing system according to any one of claims 19 to 21 wherein the sensing elements are spaced apart from each other in the array so that the nearest neighbour centre-to-centre spacing of the sensing elements is at least 10 μm.

23. An ionization sensing system according to any one of claims 19 to 22 wherein the sensing elements are provided so that the density of sensing elements is equivalent to at least 1000 per cm².

24. A method for sensing the presence of at least one analyte species using an ionization sensor having a substrate, at least one first electrode and at least one second electrode, the first electrode comprising an elongate nanostructure, elongate in at least one dimension, there being a gap between the first electrode and the second electrode, wherein the first electrode and the second electrode are both formed on the substrate, and wherein the nanostructure has a proximal end attached to the substrate and a free distal end, the method including the step of applying a potential difference between the first and second electrodes to ionize said species.

25. A method according to claim 24 wherein the electric field strength in the gap between the first and second electrode is at least 10⁵ Vm^'1.

26. A method according to claim 24 wherein the electric field strength in the gap between the first and second electrode is at least 10⁶ Vm^"1.

27. A method for manufacturing an ionization sensor according to any one of claims 1 to 19, or for manufacturing an ionization sensing system according to any one of claims 20 to 23, including the step of forming the first electrode and the second electrode on the substrate by growing said nanostructure or nanostructures in situ.

28. A method according to claim 27 wherein the substrate includes an electrically conductive layer.

29. A method according to claim 28 wherein the substrate includes an electrically insulating layer formed over the electrically conductive layer.

30. A method according to claim 29 including the step of forming first and/or second electrode conductive leads on the insulating layer.

31. A method according to any one of claims 27 to 30 including the step of depositing a catalyst on the first and/or second electrode conductive leads, the catalyst being for promotion of growth of the nanostructure (s)

32. A method according to claim 29 including the step of patterning of the first and/or second electrode conductive leads and/or patterning the catalyst.