WO2012087247A2

WO2012087247A2 - An array smell sensor based on the measurement of the junction impedance of nanowires with different metals

Info

Publication number: WO2012087247A2
Application number: PCT/SI2011/000078
Authority: WO
Inventors: Aljaž DRNOVŠEK; D. Dragan Mihailovic
Original assignee: Institut "Jožef Stefan"; Center Odličnosti Nanoznanosti In Nanotehnologije - Co
Priority date: 2010-12-22
Filing date: 2011-12-20
Publication date: 2012-06-28
Also published as: WO2012087247A3; SI23582A

Abstract

The invention describes a multi-element array recognitive sensor, i.e. "e-nose", based on the detection of analytes (1) within nanowire/metal junctions. The contact impedance between a nanowire (6) and a metal electrode (4) changes when different molecules are adsorbed in the region of contact between the metal electrodes (4) and the nanowire (6). The impedance change of each element is different when different metal electrodes (9-12) are used, which forms the basis for a multi-element sensor made with different materials, each of which giving a different response. The recognitive sensing properties are obtained by analyzing - using appropriate software the response of the entire array and comparing it with the reference response for different analytes (1).

Description

An array smell sensor based on the measurement of the junction impedance of nanowires with different metals.

The object of the present invention is a recognitive array smell sensor, i.e. electronic nose, which works by detecting the change of impedance of an array of nanowire- metal junctions. Different elements in the array are made by placing nanowires in contact with a different metal for each case. When capillary condensation of analytes occurs in the nanowire-metal junction, the impedance of the junction changes. The adsorption properties of different analytes are different on different metal surfaces, causing each element to have a different response to any particular analyte, which forms the basis for a multi-element array sensor made with different contact metal materials, each of which has a different response to any particular analyte. The recognitive sensing properties are obtained by analyzing - using appropriate software - the response of the entire array and comparing it with the reference response for different analytes.

Background of the invention

Current state of the art chemical sensors employ the use of surface acoustic wave (SAW) detectors, polymer nano-composite detectors, quartz microbalance or metal oxide sensor (MOS) devices and decorated or functionalized nanowires (NWs) or nanotubes (NTs). Larger devices use mass spectroscopy to determine molecular mass of the analyte. For recognitive detection, array sensors are necessary, such as exist in the case of polymer nanocomposite sensors. The polymer nanocomposite sensors rely on the diffusion of analyte molecules into the bulk polymer which changes the resistance of each element, while typically carbon black is used as a conducting filler to increase the conductivity of the sensor.

One possible approach to improving sensitivity is to use nanowires or nanotubes as sensing elements. The high surface-to-volume ratios associated with these

nanostructures make their electrical properties more sensitive to species adsorbed on their surfaces and in the contacts between nanowires. The actual sensing mechanisms may be very diverse. Penner and co-workers (E.C. Walter, F. Faview and R. . Penner, Anal.Chem. vol. 74, p.1546 (2002); F. Favier, E.C. Walter, M.P. Zach, T. Benter and R.M. Penner, Science vol. 293, p.2227 (2001)) fabricated a hydrogen sensor using Pd nanowires supported on the surface of a polymeric thin film. Each nanowire contained many break junctions along their length. The gap between them changed as hydrogen gas was adsorbed into the Pd crystal lattice, and the resistance of such nanowires exhibited a strong dependence on the gas concentration. In other experiments Cui and co-workers have modified the surfaces of semiconductor nanowires and implemented them as highly sensitive, real-time sensors for pH and biological species (Y. Cui, Q. Wei, H. Park and CM. Lieber, Science vol. 293, p.1289 (2001)). The mechanism were described in terms of the change in surface charge caused by protonation and deprotonation. More recently Law and co-workers fabricated a room-temperature photochemical NO2 sensor based on individual single crystalline oxide nanowires and nanoribons (M. Law, H. King, F. Kim, B. Messer and P. Yang, Angew. Chem.lnt.Ed. vol. 41 , p.2405 (2002)), where nitrogen dioxide acts as an electron-trapping adsorbate on Sn0₂ surfaces, and can be monitored by measuring the electrical conductance of the material.

To achieve recognitive detection of gases and vapors one needs to construct an array of different sensors, which have different response to different analytes, so it is important to investigate new types of sensor materials. Li and coworkers have demonstrated that chemisorbed species can also be detected as changes in conductance on gold metal nanowires (C.Z.Li et al, Appl. Phys. Lett. vol. 76, p.1333 (2000)). Early work on single wall carbon nanotubes (SWCNTs) (J.Kong, et al., Science vol. 87, p.622 (2000), P.G. Collins et al, Science, vol. 287, p.1801 , (2000), Bekyanova et al., J.Phys.Chem. B 108, 19717 (2004), T.Someya et al., Nano Lett. 3, 877, (2003), J.Li et al., Nano Lett. vol. 3,p. 929 (2003). S. Snow, F.K. Perkins

E.H.Houser, Science, vol. 307, p.1942 (2005)) has shown than the conductance changes in response to the presence of molecular adsorbates on the surface of SWCNTs. Different detection schemes have been used, such as detection of resistance, field-effect transistor configurations (W.U.Wang et al., PNAS vol. 102, p.3208 (2005), A.Star et al., Nano Letters vol. 3, p.459 (2003), ibid., Nano Letters vol. 3, p.1421 (2003) 3 M. Law, H. King, F. Kim, B. Messer and P. Yang, Angew.

Chem.lnt.Ed. vol. 41 , p. 2405 (2002)). Snow and Perkins (E.S.Snow and F.K.Perkins, Nano Letters, vol. 5, p.2414 (2005), Mark C. Lonergan et al. Chem Mater, 8, 2298 (1996), Frederic Favier et. Al. Science, vol. 293, p.2227 (2001), Xingjiu Huang et.al. Nanotechnology vol. 15, p.1284 (2004), Y. S. Kim et.al. Sensors and Actuators B vol. 108, p.285 (2005)) have combined measurements of conductance and capacitance to extract intrinsic properties of the adsorbed species. Detailed vapor sensor

measurements were reported recently on Li2Mo₆Se₆ nanowire films(X.Qi and

F.E.Osterloh, J.Am. Chem. Soc. vol. 127, p.7666 (2005), X.Qi et al., Annal.Chem. vol. 78, p.1306 (2006)). There the resistivity was found to increase upon exposure to different analytes such as hexane, THF, ethanol and DMSO. The resistivity increase is proposed to be due to the change in inter-bundle contacts caused by the

condensation of analyte molecules between the nanowire bundles, effectively reducing the hopping (tunneling) between individual bundles. Unfortunately,

is sensitive to oxygen, which limits its usefulness as a practical sensor.

An alternative material with the desired properties is Mo₆Sg_-xlx, (MoSI) (D. Vrbanic et al, Nanotechnology vol. 15, p.635 (2004)), with similar electronic properties as

Li2Mo6Se6, but is stable in air up to 200 °C and is chemically inert. It is conducting (B.Bercic et al., Applied Phys. Lett. vol. 88, p. 173103 (2006)) and can be made in different diameter bundles by adjusting dispersion (Mihailovic, Prog. Mat. Sci. vol. 54, p. 309, (2009)) and growth conditions (Dvorsek et al., J. Appl Phys., vol. 102, p.

1 14308 (2007)). It has been shown to be a suitable sensor material for two-contact resistance sensing (B.Bercic et al., Applied Phys. Lett. 88, 173103 (2006), M.Devetak et al., Chem. Mater, vol. 20, p.1773-177 (2008)) in the form of networks.

The drawback of current sensors is the relatively limited selectivity, slow response times limited by the diffusion of the analyte, problems with reproducibility, small operating range in terms of concentration, saturation and unsuitability for mass production or sensitivity to air. The problem solved by the present invention relates to the invention of an array sensor with multiple elements providing a recognitive response, scaleable architecture, ease of manufacture, small energy consumption and high and specific responsivity to many different analytes. Description of the invention

The description is supported by the figures, presenting:

Figure 1. A schematic drawing of an individual sensing element comprising of a nanowire 6 bridging the gap 8 between electrodes 4.

Figure 2. A schematic diagram of a sensor circuit element comprising a set of electrodes 4 and nanowires 6 bridging the gap 8 between the electrodes 4.

Figure 3. A schematic figure showing the multi-element sensor.

Figure 4. The normalised response, i.e. change in resistance of an individual sensor to different analytes 1 at different concentrations measured in ppm

Figure 5. The response of four sensor elements using Ni 9, Pd 10, Pt 11 and Ti electrodes 12 bridged by nanowires 6 in each case to acetone, 2-butanol, methanol and water, respectively.

The problem of recognition of different analytes 1 present either individually or simultaneously in the vapour phase is solved by a multi-element array sensor, in which each element has a different response to each of the analytes 1 present and where an analyte 1 is recognised by measurement and analysis of the response of the entire sensor array, in which each sensor circuit consists of a set of interdigital electrodes 4, separated by a small gap 8. Nanowires 6 are deposited over the electrodes 4, bridging them to form an electrical contact. The contact region between the nanowire 6 and metal electrode 4 in each element, which is typically a line of contact points - defined as the capillary condensation region 5, hereinafter CCR 5 and presented by Figure 1 - when populated by analyte 1 molecules changes the impedance of the circuit.

The nanowire 6 can be a Mo6S9_-xl_x, bundle, where 3 < x < 6, or other inorganic or organic nanowire bundle, nanotube bundle or polymer bundle or rope. The nanowires 6 may be composed of thinner polymers or molecular wires ( Ws) in disordered form or may be crystalline, all henceforth described as nanowires 6.

The electrodes are made with different conducting materials. Each element consists of two contact electrodes 4 bridged by a nanowire 6. The electrode 4 material can be metals such as, but not limited to Ti, Pd, Ni, Au, Mo, Ag, Pt or any other conducting material such as: indium-tin-oxide InSnO (ITO), carbon-based materials, conducting polymer, synthetic metals, conducting composites, doped semiconductors and organic materials, or other conducting material such as, but not limited to, carbon black, deposited by ink-jet printing, screen printing, evaporation, sputtering, electroplating or other method. The sensitivity of each element to the different analytes 1 is different, permitting selective detection of analytes 1. Contact leads 2 are secured with contact paste 3 in order to obtain good electrical contact with electrode 4. The electrodes 4 are deposited on a substrate 7, such as an oxidized silicon wafer, plastic, glass or alumina.

The number of analyte 1 molecules in the CCR 5 of each element is related to the ambient analyte 1 vapor pressure. The change of impedance of each element is related to the number of analyte 1 molecules in the CCR 5. The impedance is thus directly related to the analyte 1 vapor pressure.

The sensing properties of each sensing element differ due to any one, or any combination of properties listed (but not limited to those listed): adsorption and desorption coefficients (either physisorption or chemisorption) for the analyte 1 molecules on the electrode 4 material itself or the nanowires 6, different roughnesses of the electrode 4 or the nanowire 6, different work functions of the electrodes 4 and/or the nanowire 6, different surface tension of analytes 1 on the electrode 4 or the nanowires 6.

A typical sensor uses 4 to 32 or more elements, each made from a different

metal/nanowire combination.

The electrodes 4 may be deposited on the substrate 7 by electroplating, sputtering, evaporation, screen-printing, ink-jet printing in combination with photolithography, laser lithography, electron-beam lithography etc. To obtain different metal electrodes 4 or change their characteristics , existing electrodes may be overcoated by

electroplating, atomic layer deposition or other method. The nanowires 6 may be deposited individually over each junction, or the entire surface may be covered by a sparse mesh of nanowires 6.

In another embodiment, the contacts of different metals are deposited by evaporation onto a silicon substrate 7 through a mask, and the nanowires 6 are deposited from solution by drop casting or by spin casting.

The gap 8 between the sensor electrodes 4 in Figure 2 can be bridged by a nanotube or multiple nanotubes, a nanowire or nanowires 6, a mat or network of nanotubes or nanowires 6, bundles of nanotubes or nanowires 6 which makes sparse electrical contacts with the electrodes. The material can be MoSI molecular nanowire 6 bundles, nanowires 6, nanotubes of different kinds, provided they have a metallic or

semimetallic conductivity.

In an alternative embodiment, the nanowire 6 material is rubbed over the electrodes 4, providing mechanical deposition of a thin nanowire 6 film bridging the gap 8 over the electrodes.

In an alternative embodiment, the nanowires 6 or nanotubes are deposited onto the gap 8 region bridging electrodes 4 by use of dielectrophoresis, to attract them to the region of the contacts.

In a further embodiment, the nanowires 6 are sprayed onto the electrodes 4 and across the gap 8 with an airbrush or with an ultrasonic spray system.

In another embodiment, the response of each sensor within an array is modified by introducing different molecular layers into the tunneling junction for example by coating the nanowires 6 with a surfactant before deposition on the contact. This way the number of different elements can be significantly increased, increasing the recognitive abilities of the array sensor

The properties of the CCR 5 may be modified by adjusting the electrode roughness, altering the sensitivity. The sensor acts as a multi-element resistor array. The resistance of each element changes in response to the presence of analyte 1 molecules. The response of each element 9-12 may be different because each element uses different metallic electrode 4 to the nanowire 6. Different metals have different surface adsorption and desorption characteristics, which means that different analytes 1 may accumulate differently in the metal-nanowire junctions. Different metals also have different work functions, which results in different electron transfer characteristics between the nanowire 6 and the metal electrode 4 through the analyte 1.

In repeated operation, the chemisensor may need to be regenerated by removal of analyte 1. This can be done by heating the sensor in inert gas, vacuum or an active gas. The heating can be done by passing a current, either continuous or pulsed through the sensor itself, or by resistor in proximity with the device, or by optical means, such as a laser or flash.

Specific advantages the sensor has:

1. Recognitive response is obtained by choosing different materials for the electrodes 4 and/or nanowires 6, giving a very large number of possible sensor elements, each with a different response.

2. A wider range of response than percolation sensors.

3. Easily scalable architecture based on the use of different metal electrodes 4.

4. Ease of manufacture with multiple sensors.

5. Array sensors can be made very small.

6. Extremely low power dissipation.

7. Regeneration may performed by heating the substrate 7 or passing a larger current through the device causing evaporation of molecules in the junction region.

8. The response is resistive and can be easily recorded and analysed with standard measurement techniques.

9. The device geometry is very flexible, the metallic electrode 4 may be deposited on different substrates 7 and by different techniques. 10. The deposition of the metallic electrode 4 can be easily adapted to large- volume production (including screen printing of paste, inkjet printing, evaporation, sputtering, electro-chemical deposition.

Example

The following example illustrates the invention without limiting it thereto.

An array sensor as in Figure 3 is constructed using an array of four geometrically identical interdigital electrodes 4 deposited onto an oxidized silicon substrate 7, with a gap 8 of 2 micrometers between the electrodes 4. A single element is shown in Figure 2. Each electrode 4 is made from a different metal, which are deposited on the Si/Si- oxide substrate 7 by sputtering and patterned using electron beam lithography.

M06S3I6 nanowire 6 bundles of different diameters are deposited across each of the electrodes 4 by dielectrophoresis in such a way that they form a contact with both electrodes 4 as shown in Figure 3. The sensor array is placed into a suitable micro- cell into which analytes 1 are introduced with a nitrogen carrier gas. The resistance change due to the presence of an analyte 1 is measured using a standard high impedance multimeter. The resistance change is different for different analytes 1 as shown in for the case of an Au electrode deposited over Ti metal and follows the characteristic sensitivity curve predicted by M.Devetak et al., (Chem. Mater, vol. 20, p.1773-177 (2008)), which shows the response as a function of concentration in ppm for a number of different analytes. Exposure of a 4-element sensor constructed from Ni 9, Pd 10, Pt 11 and Ti 12 to water, methanol, 2-butanol and acetone vapours show an increase in resistance which depends on the concentration of the analyte 1 vapour and which is different for each analyte/electrode metal combination as shown in Figure 5. The resulting response of multiple sensor elements is analysed using appropriate algorithms, such as PCA or neural network programs.

According to the invention, the array smell sensor based on the measurement of the junction resistance between nanowires 6 and different metals is composed of multiple elements, each of which detects the presence of an analyte or analytes 1 in the capillary condensation region 5 between the nanowire or nanowires 6 and the conducting electrode 4, where each element is made from a different material. A sensor, whose selectivity to different analytes 1 is based on the choice of the conducting electrode material, such as Ti, Ni, Zn, Au, etc. or conducting material such as InSnO, conducting polymer or contact paste 3, preferably carbon contact paste. Its sensing properties arise from the change in impedance of the electrical contact between the nanowire 6, and the electrode 4 due to the presence of analyte 1 molecules in the capillary condensation region 5 between the electrodes 4 and a nanowire 6 placed on top of the electrodes 4.

The multi-element array sensor is composed of the said sensors with electrodes 4 made of different conducting materials 9-12.

The electrodes 4 of the sensor are made by screen printing technology, ink-jet technology, sputtering, lithography or atomic layer deposition (ALD) or any other means. The electrodes 4 are bridged by a nanowire 6, nanotube or any objects or composite object which form a close contact with electrode 4 material, leaving sufficient space for analyte 1 molecules in the CCR 5. The electrodes 4 or nanowires 6 can be covered by surfactant molecules or by atomic layer deposition, or other method with the aim of altering the properties of the CCR 5 .

A multi-element array sensor according is characterized by the fact that each sensor element responds differently to an analyte 1.

A recognitive smell sensor array according to the invention provides recognition because each element of the array can respond differently to any particular analyte 1 , resulting in a fingerprint signature for each analyte 1 and can detect the presence of a particular analyte 1 amongst a number of analytes 1 present simultaneously.

Claims

1. The array smell sensor based on the measurement of the junction resistance between nanowires (6) and different metals, characterized by being composed of multiple elements, each of which detects the presence of an analyte or analytes (1) in the capillary condensation region (5), abbreviated CCR (5), between the nanowire or nanowires (6) and the conducting electrode (4), where each element is made from a different material.

2. A sensor according to claim 1 , whose selectivity to different analytes (1) is based on the choice of the conducting electrode (4) material, preferably Ti, Ni, Zn, Au, etc. or conducting material, preferably InSnO, conducting polymer or contact paste (3).

3. A sensor according to claims 1 and 2, in which the sensing properties arise from the change in impedance of the electrical contact between the nanowire (6), and the electrode (4) due to the presence of analyte (1) molecules in the capillary

condensation region (5) between the electrodes (4) and a nanowire (6) placed on top of the electrodes.

4. A multi-element array sensor which is composed of sensors according to claims 1 , 2 and 3 with electrodes (4) made of different conducting materials.

5. A sensor according to claims 1- 4 above where the electrodes (4) are made by screen printing technology, ink-jet technology, sputtering, electroplating, lithography or atomic layer deposition (ALD) or any other means.

6. A sensor according to claims 1- 5 where the electrodes (4) are bridged by a

nanowire (6), nanotube or any objects or composite object which form a close contact with electrode (4) material, leaving sufficient space for analyte (1) molecules in the CCR (5).

7. A sensor according to claims 1-5 where the electrodes (4) or nanowires (6) are covered by surfactant molecules or by atomic layer deposition, or other method with the aim of altering the properties of the CCR (5).

8. A multi-element array sensor according to claims 1-7, characterized by the fact that each element responds differently to an analyte (1).

9. A recognitive smell sensor array which provides recognition because each element of the array can respond differently to any particular analyte (1), resulting in a fingerprint signature for each analyte (1).

10. A sensor according to claims 1-9 which can detect the presence of a particular analyte (1) amongst a number of analytes (1) present simultaneously.