WO2006121349A1

WO2006121349A1 - Hydrogen sensors and fabrication methods

Info

Publication number: WO2006121349A1
Application number: PCT/NZ2006/000101
Authority: WO
Inventors: Simon Anthony Brown; Andreas Lassesson; Joris Van Lith
Original assignee: Nano Cluster Devices Limited
Priority date: 2005-05-09
Filing date: 2006-05-09
Publication date: 2006-11-16

Abstract

A hydrogen sensor device is disclosed composed of a non-contiguous film(s) of nanoscale clusters between two or more contacts on a substrate. The non-contiguous film responds to the presence of hydrogen gas by a change in electrical resistivity between the contacts due to conduction according to one of ohmic conduction within the film between the two or more contacts; tunnelling conduction within the film between the two or more contacts; or a combination of ohmic and tunnelling conduction within the film between the two or more contacts. A method of preparing such a sensor is also disclosed.

Description

HYDROGEN SENSORS AND FABRICATION METHODS

FIELD OF THE INVENTION The present invention relates to methods of preparing cluster films and the fluid sensors formed by such methods. More particularly but not exclusively the invention relates to a method of preparing such structures by the assembly of conducting nanoparticles.

BACKGROUND Hydrogen holds the potential of becoming a clean reliable and affordable way to store and transport energy. However as the explosive limit of hydrogen in air is only 4% the uptake of such an energy source has considerable dangers. Further it is an odorless gas, thus we have to rely solely on hydrogen sensors to warn us of any leakage. For this cheap, small, fast, reliable, maintenance free and sensitive hydrogen leak detectors will be necessary in the future in large quantities.

At present good hydrogen sensors are necessary for: process gas monitoring, detection of hydrogen build up in lead acid storage cells, leak detection in petrochemical applications, detection of impending transformer failure in electric power plants, monitoring hydrogen build up in radioactive waste tanks and for many other applications.

Currently the most promising hydrogen sensors are thin film sensors that use a multitude of operating principles. However, existing hydrogen sensors suffer from a multitude of limitations, like large response time, low sensitivity, short life time, high cost. In this patent an invention is presented which could improve on some, if not on all of these limitations. At the very least it will provide a viable alternative to existing hydrogen sensors.

Several previous hydrogen sensors have been described in the literature based on the known increase in resistance of a material (such as Pd) on absorption of hydrogen [I]. Palladium is special compared to other metals in that it can absorb large volumes of hydrogen. The mechanical and electrical properties of Palladium depend on the amount of hydrogen absorbed and thus Palladium can be used as a transduction material for hydrogen sensors. Other gases cannot diffuse through the Palladium crystal, which makes Palladium a highly specific transduction material. [1]

Both the mechanical and electrical properties of Palladium as a function of hydrogen pressure are determined by the hydrogen concentration inside the crystal (x), which is a function of the external hydrogen pressure (Pm)- With increasing hydrogen pressure the hybrid palladium hydrogen system moves from the α phase through the two phase region and finally it reaches the β phase. This is illustrated by the isotherm in figure 9: in region I the palladium is said to be in the α phase; region II is called the two-phase region or miscibility gap; in region III the palladium is said to be in the β phase.

The lattice constant of the palladium crystal in the β phase is 3.5% larger than that of the α phase. The change happens over a relatively small pressure range, which also means that the mechanical and electrical properties of Pd are here most sensitive to the P_H2- The resistance of bulk Pd increases approximately linearly with the hydrogen concentration to approximately 1.8 times the base resistance for x = 0.8 at 25⁰C [I]. The specific pressure at which the phase change happens is strongly temperature dependent, but at 2O⁰C it is around 2x10³ Pa [I].

Because of the mechanical relaxation processes related to a sudden change in lattice constant, in bulk Pd the pressure- hydrogen concentration isotherm will show a hysteresis. Since the mechanical relaxation process in clusters is different from that of bulk palladium, possibly the presence of the hysteresis is less pronounced. Numerous cycles of absorption and desorption can cause significant deformation of Pd thin films.

Substituting Pd by Pd alloys (especially in the palladium silver series) can greatly improve the mechanical stability of the thin film. [1] Palladium clusters with a diameter less than 5 nm show a different pressure concentration isotherm than bulk palladium [2]. This is because for these diameters the ratio of surface to bulk atoms is large. The smaller the cluster diameter, the less pronounced the phase change region. The difference in the properties of nanoclusters to bulk materials may enable devices which have novel properties.

Poisoning of the palladium surface is caused by physisorbed and chemisorbed molecules. Physisorbed molecules like H₂O, O₂, CO and CH₂ can be replaced by hydrogen by subjecting the surface to a high hydrogen pressure. Chemisorbed molecules like H₂S and SO₂ are very difficult to remove. All these molecules prevent the hydrogen from diffusing into the bulk, therefore they increase the response time of the hydrogen concentration in the bulk to a change in hydrogen pressure from the outside. Some molecules even reduce the response and thus pose a real problem for thin film Pd sensors.

US patent application 2003/0079999 discloses a hydrogen sensor which achieves a decrease in resistance on absorption of hydrogen into the sensor material. In that case, the prior art relies on an array of wires which are incomplete (i.e. where at least some of the wires have gaps). There the methodology of the invention relied on electrodeposition to form the wires on surface steps on a graphite substrate and the wires had to be lifted off the graphite substrate in an epoxy matrix.

US patent application 2005/6849911 describes a hydrogen sensor and/or switch from a number of columns of Pd-Ag alloy nanoparticles fabricated using electrodeposition. US patent applications 2005/0155858, 2004/0261500 and 2004/0070006 describe variations on that theme.

In addition to wires as sensors, others have investigated films. Barr [3] looked at the conductance-temperature characteristic of discontinuous palladium films by electron beam evaporation of atomic vapour onto cooled rock salt substrate. The model that Barr uses to describe his data assumes that the electrical conduction is due to quantum mechanical tunnelling. The model assumes two competing effects on exposure to hydrogen. First surface absorption of hydrogen by palladium decreases the Fermi levels of the palladium islands and thus increases the effective barrier height between two islands, decreasing the conductance. Second, a hydrogen induced expansion of the lattice constant decreases the barrier width and hence increases the conductance. A further series of papers from Binghamton University [4,5,6] discuss the effects of hydrogen absorption on the electrical conduction of discontinuous palladium films on glass substrates within the model of Barr [3].

Dankert [7] proposed fabrication of hydrogen sensors comprising palladium islands on a sapphire substrate with a surface coverage just below the percolation threshold. Expansion of these islands upon exposure to hydrogen would then increase the surface coverage above the percolation threshold, causing the film to change from non- conducting to conducting. For the fabrication of the islands Dankert uses Joule heating of a 10 nm sputtered palladium film (i.e. the heating is done using a current through the film). It appears however, unlikely that it is possible to fabricate a discontinuous film below the percolation threshold using this method. The initial resistance of Dankert' s films is only 1 kΩ suggesting strongly that the conductance is ohmic and that current paths already exist in a conducting film. In a attempt to overcome concerns about slow response times Xu[8] made hydrogen sensors using discontinuous Palladium films grown by thermal evaporation of Pd onto a self-assembled siloxane layer which is intended to reduce adhesion of the Pd to the substrate.

In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art. OBJECT OF THE INVENTION

It is an object of the invention to provide a method of preparing fluid sensors, including hydrogen sensors which ameliorate at least some of the abovementioned disadvantages, or which at least provide the public with a useful alternative. An alternative object is to provide an alternative method for the preparation of nanoscale fluid sensors particularly hydrogen sensors, using metal clusters.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a hydrogen sensor device comprising a non-contiguous film(s) of nanoscale metal or semiconductor clusters running substantially between two or more contacts and responsive to the presence of hydrogen gas, and a detector in communication with the contacts such that the contacts and the non-contiguous film(s) form a circuit.

Preferably the response of the device to the presence of hydrogen gas is a change in electrical resistivity between the two or more contacts, where the conduction is due to one of the following processes: ohmic conduction within the film between the two or more contacts; tunnelling conduction within the film between the two or more contacts; a combination of ohmic and tunnelling conduction within the film between the two or more contacts.

Preferably the non-contiguous film(s) comprises a series of networks of metal or semiconductor clusters on an insulating or semiconducting substrate.

Preferably the clusters are of palladium or an alloy thereof. Preferably when an alloy, the clusters comprise palladium mixed with one or more of silver, rhodium, ruthenium, yttrium and/or nickel. Alternatively the clusters are of yttrium, or palladium coated yttrium. Preferably at least in the region between the contacts, and intermediate between the contacts and the non-contiguous film(s), there is an intermediate film of nanoparticles, which are inert to or have a lower responsivity to the presence of hydrogen gas than the plurality of clusters making up the non-contiguous film(s).

Preferably the intermediate film is discontinuous. Preferably the intermediate film is metallic and the principal mode of conduction between the plurality of particles making up the intermediate film is tunnelling conduction.

In one embodiment, in the presence of hydrogen gas, one or more gap(s) in the noncontiguous film(s) close to form at least one continuous link between the contacts thereby decreasing the resistivity of the circuit due to ohmic conduction. Preferably, in the absence of hydrogen gas, the gap(s) in the film open thereby increasing the resistivity of the circuit. In an alternative embodiment, in the presence of hydrogen gas, the width of the gap(s) in the non-contiguous film(s) will decrease thereby decreasing the resistivity of the circuit due to tunnelling conduction. Preferably, in the absence of hydrogen gas, the width of the gap(s) in the film may increase thereby increasing the resistivity of the circuit.

In an alternative embodiment, in the presence of hydrogen gas, the width of at least some of the gap(s) in the non-contiguous film(s) decrease thereby decreasing the resistivity of the circuit due to tunnelling conduction, and additionally one or more gap(s) in the non-contiguous film(s) close to form at least one continuous link between the contacts thereby decreasing the resistivity of the circuit due to the formation of an ohmic conduction path.

In any of the embodiments in one form the number of discontinuous links is sufficiently large that tunnelling conduction dominates the conductive properties of the film. Preferably tunnelling conduction is substantially the sole means of conduction between the contacts upon exposure to hydrogen gas. Preferably the non-contiguous film(s), or the non-contiguous film(s) together with the intermediate film, comprises a network of clusters whose surface coverage in the region between the contacts is close to the percolation threshold in two dimensions. Preferably the non-contiguous film(s), or the non-contiguous film(s) together with the intermediate film, comprise a network of particles whose volume filling fraction is close to the percolation threshold in three dimensions.

Preferably the average diameter of the clusters is between 0.3nm and l,000nm. More preferably the average diameter of the clusters is between 0.3nm and lOOnm.

Alternatively or additionally the average diameter of the clusters is between 0.3 and 5nm

Preferably using clusters of less than 5nm allows modification or optimisation of the amount of hydrogen gas absorbed.

Preferably the clusters have been formed by inert gas aggregation. Preferably the method of creation of the vapour which aggregates to form the clusters is by thermal evaporation, sputtering, magnetron sputtering (either AC or DC), arc discharge, electron beam heating or laser irradiation.

Preferably the substrate is selected from one of the group consisting of silicon, silicon nitride, silicon oxide, aluminium oxide, indium tin oxide, germanium, gallium arsenide or another III-V semiconductor, quartz, or glass.

Preferably the intermediate film is composed of nanoparticles selected from one of the group consisting of Au, Ag, Cu, Sb, Bi or Pb.

Preferably the nanoparticles of the intermediate film have diameters less than 20nm. Preferably the nanoparticles of the intermediate film have diameters less than 5nm. Preferably the clusters of the non- contiguous film exist as loosely formed aggregates of more than one cluster, or as coalesced masses or islands of clusters.

Preferably the mode of conduction between the coalesced masses or islands of clusters of the intermediate film is entirely tunnelling conduction.

Preferably the aggregates of more than one cluster or the coalesced masses or islands of clusters are separated by gaps smaller than 5nm.

Preferably a further layer of inert material is present between the non-contiguous film(s) and the intermediate film reducing interaction between the particles of these films. Preferably the inert material is an insulating oxide, such as Al₂O₃.

Preferably the device includes a layer or modifying material between the substrate and the non-contiguous film(s) effective to dictate the cluster film structure, such as a polymer or organic compound eg siloxane.

Preferably the device further includes a resist or other organic compound or an oxide or other insulating layer on top of the clusters of the non-contiguous film(s). Preferably the resist is capable of stabilising the morphology and properties of the film(s) or of protecting the film(s) from damage or of preventing exposure of the film to one or more fluids.

Preferably the device further includes an outer barrier selected from one of —

- a protective outer coating permeable to hydrogen gas but substantially impermeable to one or more species which may contaminate the non-contiguous fϊlm(s), or

- an activated carbon outer barrier is provided to prevent species contaminating the non-contiguous film while allowing the passage of hydrogen gas.

In one embodiment of the device there are a plurality of films between the two or more contacts, preferably arranged in a Wheatstone Bridge configuration. Preferably in the device there are two contacts which are separated by a distance smaller than 200 microns; more preferably separated by a distance less than 1 OOOnm; even more preferably separated by a distance less than 200nm.

Preferably the geometry of the contacts has been optimised by percolation theory calculations to result in non-contiguous film(s) at a surface coverage of particles on the substrate of less than the percolation threshold for a macroscopic contact separation.

In one embodiment some of the particles of the non-contiguous film(s) are insulated from other particles of the non-contiguous film.

In one embodiment the detector may additionally or alternatively measures the impedance of the film(s) between the two or more contacts.

In one embodiment the detector may be selected from one of the group of a galvanometer, an ohmmeter, a potentiostat, a lock-in amplifier and a multimeter.

In one embodiment more than one may be used in a measurement circuit.

In one embodiment a multiplicity of measurements may be made at more than one frequency.

Preferably the device may be incorporated within a circuit comprising a power source and a means of monitoring the current flowing through the circuit and/or a means of monitoring the voltage substantially across the one or more pairs of contacts.

Preferably the presence of hydrogen gas triggers an alarm.

Preferably the device includes a heater element to control the temperature of the film(s), preferably the heater element is used to tune the responsiveness of the non-contiguous film(s) to hydrogen gas. In one embodiment the non-contiguous film is a palladium alloy, and the temperature of operation and/or the alloy composition have been selected in order that the maximal or desired sensitivity of the sensor will occur for the hydrogen pressure range of interest.

According to a further aspect of the invention there is provided hydrogen gas sensing apparatus comprising at least hydrogen gas sensing devices having two or more electrical contacts and one or more non-contiguous film(s) of nanoscale metal or semiconductor clusters running substantially between the two or more contacts, connected in a circuit with a power source and a means for monitoring the current flowing through the circuit and / or a means for of monitoring the voltage substantially across the one or more pairs of contacts.

Preferably the clusters have an average diameter between 0.3 nm and 1,000 nm. More preferably the clusters have an average diameter between 0.5 nm and lOOnm. Additionally or alternatively the clusters have an average diameter between 0.3 nm and 5nm.

Preferably the clusters are of palladium, or an alloy or palladium or yttrium or palladium coated yttrium.

Preferably there are two contacts separated by a distance less than 1000 nm.

Preferably the apparatus further includes an alarm which is triggered when hydrogen gas is present. Preferably the one or more hydrogen sensing devices is/are a device disclosed above.

Preferably the apparatus includes a heater element to control the temperature of the noncontiguous film(s). Preferably at least the hydrogen sensing device(s) are at least partially surrounded by a membrane capable of selectively transmitting gases. Or wherein the hydrogen sensing device is contained within a gas permeable housing, or within a housing having at least a gas permeable window.

Preferably the apparatus includes contaminant removing species or means selected from one or more of:

- a quantity of activated carbon capable of removing oxygen, water and/or other contaminants;

- a thin layer of oxidation inhibitor on the non-contiguous film(s); - A thin layer of hydrogen permeable polymer on the non-contiguous fϊlm(s).

Preferably the apparatus includes or is in communication with other sensors capable of detecting fluids other than hydrogen, e.g. in combination with a H₂S or SO₂ sensor.

According to a further aspect of the invention there is provided a method of preparing a nanoscale hydrogen sensor device comprising the steps of:

- providing a substrate having two or more electrical contacts;

- deposition of a plurality of nanoscale metal or semiconducting clusters onto the substrate at least in the region between the contacts, to achieving a non-contiguous film(s) responsive to the presence of hydrogen gas, running substantially between the contacts,

- providing a detector in communication with the contacts such that in the presence of hydrogen gas of the film(s) is/are detected. Preferably the step of depositing the plurality of particles between the contacts to achieve the non-continuous film(s) comprises one or both the steps of:

■ monitoring the conduction between the contacts and ceasing deposition at or near to the onset of tunnelling or ohmic conduction, and/or

■ modifying the substrate surface, or taking advantage of pre-existing topographical features, which will encourage the deposited clusters to form the film(s).

Preferably there is the further step of providing a discontinuous metal film of a metal or semiconductor which is inert or has a lower responsivity to the presence of hydrogen gas than the non-contiguous film(s), on the substrate prior to the deposition of the plurality of clusters, and the step of deposition of a plurality of clusters on the substrate comprises depositing the clusters at least partially on the discontinuous metal film in at least the region between the contacts.

Preferably the plurality of clusters are of palladium or an alloy thereof, or yttrium, or palladium coated yttrium. Preferably the alloy is palladium mixed with one or more of silver, rhodium, ruthenium, yttrium and/or nickel.

Preferably the clusters have an average diameter between 0.3nm and lOOOnm. More preferably the clusters have an average diameter between 0.5nm and lOOnm. Alternatively or additionally the clusters have an average diameter between 0.3nm and 5nm.

Preferably the metal clusters are prepared by inert gas aggregation. Preferably the method of creation of the vapour which aggregates to form the clusters is by thermal evaporation, sputtering, magnetron sputtering (either AC or DC), arc discharge, electron beam heating or laser irradiation. Preferably in the step of preparation of the metal clusters and/or control of cluster source parameters and/or use of a subsequent step of cluster size selection (but prior to the cluster deposition) allows control of the cluster size and thereby allows control of the characteristics of the sensor device including the mode of conduction which will result upon exposure to hydrogen gas.

Preferably there is a further step of stabilizing the morphology of the non-contiguous film by application of an overlayer.

Preferably the metal film is prepared from particles of Au, Ag, Cu, Sb, Bi or Pb.

Preferably the particles of the metal film have diameters smaller than 20nm. More preferably the particles of the metal film have diameters smaller than 5nm.

Preferably the particles of the metal film are separated by gaps with dimensions smaller than 5nm.

Preferably the particles of the metal film has been formed by inert gas aggregation and are deposited by uniform deposition onto the substrate resulting in the aggregation of the particles into coalesced masses or island(s) on the substrate surface.

Preferably there may be a pre-step of deposition of a thin film resulting in modification of the interaction between the clusters and the substrate. Preferably the properties of the film can be used to tailor the sensor characteristics. Preferably the thin film consists of a siloxane, or a polymer layer.

Preferably prior to the deposition of the metal clusters there is a step of substrate surface modification to increase or decrease the surface roughness. Preferably the roughness of the substrate is used to tailor the cluster film structure after deposition. Preferable the roughness of the substrate can be used to tailor the sensor characteristics. Preferably the substrate roughness is modified using reactive ion etching or wet chemical etching procedures.

Preferably after the deposition of the metal clusters there is a step of applying a resist or other organic compound or an oxide or other insulating layer on top of the clusters effective in stabilizing the morphology and properties of the non-contiguous film(s).

Preferably the resist or organic compound is PMMA, photoresist or SU8, and the insulating layer is SiO_x, SiN or Al_xO_y.

Preferably there is a further step of

- applying a protective coating over the non-contiguous film(s) permeable to hydrogen gas but substantially impermeable to species which may contaminate the film(s) and/or - an activated carbon barrier is over the film(s) to prevent species contaminating the film while allowing the passage of hydrogen gas.

Preferably the clusters of the non-contiguous film(s) is/are of palladium or an alloy thereof, or yttrium, or palladium coated yttrium.

Preferably the clusters of non-contiguous film(s) is/are a palladium alloy of palladium mixed with one or more of silver, rhodium, ruthenium, yttrium and/or nickel.

Preferably the method includes a step prior to the deposition of the particles of applying a resist or other organic compound or an oxide or other insulating layer to the substrate and then processing using lithography to define a region or regions where the metal clusters once deported will form a continuous or discontinuous web between the contacts, and another region where the metal clusters will be insulated from the conducting network. According to a further aspect of the invention there is provided a nanoscale hydrogen gas sensing device prepared according to the above method.

According to a further aspect of the invention there is provided a hydrogen gas sensor device comprising an non-contiguous film of nanoscale metal or - semiconducting clusters running substantially between two contacts on an insulating or semiconducting substrate, the non-contiguous film being responsive to hydrogen gas, an intermediate discontinuous metal film between the non-contiguous film and the substrate, wherein the metal clusters of the non-contiguous film are between 0.3 and lOOOnm in diameter, and wherein the mode of conduction within the intermediate film is tunnelling conduction, whilst the mode of conduction of the non-contiguous film upon exposure to hydrogen gas is one of: ohmic conduction within the film between the two or more contacts; tunnelling conduction within the film between the two or more contacts; - a combination of ohmic and tunnelling conduction within the film between the two or more contacts.

Preferably the intermediate film is of coalesced gold nanoparticles.

Preferably the clusters are of palladium or an alloy thereof, are of size between 5 - lOOnm.

According to a further aspect of the invention there is provided a method of assessing the hydrogen gas content in an environment including contacting the device of as above and/or apparatus as above with the environment and reading or otherwise assessing the output of the detector.

According to a further aspect of the invention there is provided a hydrogen sensor device substantially as herein described with reference to one or more of the Figures and/or Examples. According to a further aspect of the invention there is provided Hydrogen sensing apparatus substantially as herein described with reference to one or more of the Figures and/or Examples.

According to a further aspect of the invention there is provided a method of preparing a nanoscale hydrogen gas sensor device substantially as herein described with reference to one or more of the Figures and/or Examples.

As used herein the term "and/or" means "and" or "or", or both..

As used herein "(s) following a noun means the plural and/or singular forms of the noun.

The term "comprising" as used in this specification means "consisting at least in part of, that is to say when interpreting independent paragraphs including that term, the features prefaced by that term in each paragraph will need to be present but other features can also be present.

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7).

To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting DEFINITIONS

"Nanoscale" as used herein has the following meaning - having one or more dimensions in the range 0.1 to 1000 nanometres.

"Nanoparticle" as used herein has the following meaning - a particle with dimensions in the range 0.1 to 1000 nanometres, which includes atomic clusters formed by inert gas aggregation or otherwise.

"Nanowire" as used herein has the following meaning - a pathway formed by the assembly nanoparticles which is electrically conducting (via ohmic conduction or tunnelling conduction, for example). "Contact" as used herein has the following meaning - an area on a substrate, usually but not exclusively comprising an evaporated metal layer, whose purpose is to provide an electrical connection between the nanowire or cluster deposited film and an external circuit or any other electronic device. A single contact has a width, w; in respect of two contacts they are separated by a separation L. "Atomic Cluster" or "Cluster" as used herein has the following meaning - a nanoscale aggregate of atoms formed by any gas aggregation or one of a number of other techniques [9]. The term applies to a wide range of dimensions but here we are particularly concerned with clusters of diameter in the range 0.1 nm to lOOOnm, and typically comprising between 2 and 10⁷ atoms. "Substrate" as used herein has the following meaning - a material comprising one or more layers which is used as the structural foundation for the fabrication of the device.

The substrate may be modified by the deposition of electrical contacts, by doping or by lithographic processes intended to cause the formation of surface texturing.

"Conduction" as used herein has the following meaning - electrical conduction which includes ohmic conduction and tunnelling conduction. The conduction may be highly temperature dependent as might be expected for tunnelling devices or a semiconducting nanowire as well as only moderately temperature dependent as might be expected for metallic conduction.

"Tunneling conduction" is conduction by quantum mechanical transport of electrons through a classically insulating barrier between two classically conducting materials or particles. "Ohmic conduction" as used herein describes all other forms of conduction, excluding tunnelling conduction.

"Non-contiguous film" means a film made up of individual units which may be part of a connected network. It is not restricted to a single linear form but may be direct or indirect. It may also have side branches or other structures associated with it. The individual units may or may not be fully coalesced and the film has at least a limited number of critical pathways. The film may be homogeneous or inhomogeneous. The film may be either non-conducting or conducting (either by ohmic or tunnelling conduction) in the absence of the fluid to be detected (hydrogen gas). The film may be a percolating film close to the percolation threshold, with coverage either above or below the percolation threshold..

"Percolation Theory" as used herein has the following definition- theory relating to the formation of connected structures of randomly occupied sites, where there is a regular lattice of sites (site percolation) or not (continuum percolation). "Percolation Theory" includes all variations on this theory which allow calculations of important parameters such as the conductivity of a network of sites, probability of percolation and correlation length. It also includes variations on this theory which focus on bonds connecting the site (bond percolation). "Percolation Threshold" as used herein has the following definition- The least occupancy of the available sites in Percolation Theory at which a connected structure of sites exists which spans one of the dimensions of the system and / or allows electrical conduction across the system.

"Fluid" as used herein has the following definition - a liquid or gaseous substance, material, element, compound or chemical. BRIEF DESCRIPTION OF THE FIGURES

The invention is further described with reference to the accompanying figures:

Figure 1. Current as a function of time for Pd clusters deposited (at 0.2 A/s) on a single pair of contacts separated by 100 μm. Figure 2. Resistance as a function of time after end of deposition for a Pd cluster film close to the percolation threshold.

Figure 3. A typical cluster yield from the Pd source as a function of diameter for two different source settings.

Figure 4. Resistance as a function of time for the first exposure of three Pd cluster films, with different initial resistances, to hydrogen.

Figure 5. Resistance as a function of time for exposure of a Pd cluster film sensor to hydrogen illustrating the short response time of the sensor.

Figure 6 Resistance as a function of time for exposure of a thick Pd cluster film to hydrogen illustrating the long response time of the sensor.

Figure 7. Relative change in resistance as a function of hydrogen pressure for two Pd cluster film sensors with different initial resistances. Both sensors were made using 6nm diameter clusters.

Figure 8. Hydrogen pressure as a function of relative resistance change for a Pd cluster film sensor (R₀ = 1MΩ).

Figure 9. Schematic relationship between external hydrogen pressure and hydrogen concentration in bulk Pd.

Figure 10. Scanning Electron Microscopy image of a Pd cluster film, coverage -0.67, exposed to hydrogen.

Figure 11. Scanning Electron Microscopy image of a Pd cluster film, coverage -0.77, exposed to hydrogen. Figure 12. Resistance of Pd cluster films after hydrogen exposure as a function of resistance before hydrogen exposure.

Figure 13. Scanning electron microscopy image of a gold island film with a 2 nm effective thickness.

Figure 14. Sensor characteristic, resistance versus hydrogen pressure, for different temperatures.

Figure 15. Schematic representation percolating type sensors.

Figure 16. Schematic representation percolating tunnelling type sensors.

Figure 17. Schematic representation tunnelling type sensors.

Figure 18. Resistance of Au islands on SiO₂ as a function of effective Au thickness.

Figure 19. Resistance as a function of temperature of an Au island film.

Figure 20. Arrhenius type plot with the data of temperature of an Au island film.

Figure 21. The current (and thus inversely, resistance) between two contacts (separated by lOOμm) versus time during deposition of Pd nanoclusters on bare SiO₂ (full line) and Au island covered SiO₂ (dashed line).

Figure 22. Resistance of Pd cluster films with Au islands after hydrogen exposure as a function of resistance before hydrogen exposure.

Figure 23. Resistance between two contacts (separated by lOOμm) versus temperature for a Pd cluster film on top of SiO₂, for a sensor where the resistance increased after the initial hydrogen load. Figure 24. Arrhenius type plot with the data of figure 23.

Figure 25. Resistance as a function of time for the first exposure of two Pd cluster films, with Au islands, to hydrogen.

Figure 26 The response of a Pd cluster sensor (without Au-islands) to repeated hydrogen loads.

Figure 27 The response of a Pd cluster sensor (with Au-islands) to repeated hydrogen loads.

Figure 28. Stability sensor response of Pd cluster films, for films with and without a polymer cover layer.

Figure 29. Voltage current characteristics of Pd nano-particle films with coverage below the percolation threshold.

Figure 30 Resistance between two contacts (separated by lOOμm) versus temperature for a Pd cluster film on top of SiO₂, for a sensor where the resistance dropped after the initial hydrogen load.

Figure 31 Photograph of a multiple contact sensor containing three individual sensors.

The scale is in millimetres.

Figure 32 Relative change in resistance as a function of hydrogen pressure for a Pd cluster film sensor made using clusters with 3.5nm diameter.

Figure 33 Relative change in resistance as a function of hydrogen concentration for a pure Pd cluster film sensor shown in comparison to the expected response of Pd-Ag and Pd-Ni alloy cluster film sensors. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of fabricating percolating films of nanoclusters which act as fluid sensors. One preferred form of the invention is a hydrogen sensor.

As discussed in more detail below there are three preferred embodiments. All three embodiments are nanoscale embodiments relying on the use of nanoscale metal clusters, preferably formed by inert gas aggregation, to form a non-contiguous film or film(s) between the electrical contacts of the sensing device. Upon exposure to the fluid to be sensed (which is typically hydrogen gas) these films will respond in certain ways (dependent upon their arrangement, configuration or mode of preparation) giving rise to detection of the fluid.

The three preferred embodiments are referred to as:

1. percolating sensors (for example, Figure 15)

2. percolating - tunnelling sensors (for example, Figure 16) 3. tunnelling sensors (for example Figure 17).

Note however that combinations of these primary embodiments may be realised in practice, and in particular that a tunnelling sensor may develop percolating/ohmic conduction paths on exposure to hydrogen. However, by control of the device we can maximise or minimise the contribution of either type of conduction to the way in which the device works.

Percolating films of clusters have previously been described in the context of the formation of nanowires [9]. The purpose of this document was to provide a method for the preparation of nanowires which were automatically connected to a pair of contacts so as to remove the laborious manipulation of nanoscale building blocks which is a common obstacle to the commercial development of nanoscale devices. As the basis of the present work, it is noted that close to the percolation threshold a percolating film is comprised of both a very limited number of interconnected pathways (described above as nanowires) and a large number of clusters or isolated pathways which are disconnected from either the contacts, or the nanowires, or both. Figure 15 shows a schematic illustration of a connected pathway (solid line) and several noncontiguous pathways in a percolating film. It is further noted that conduction between isolated pathways or isolated particles may occur by tunnelling conduction.

It is further recognised that the prior art describes several materials, of which palladium and its alloys are the best known and most useful examples, which are able to absorb significant amounts of hydrogen. On absorbing hydrogen the crystal lattice of these materials is known to expand significantly, causing a macroscopic increase in the volume of the material. Yttrium and palladium coated yttrium are examples of further materials which expand upon absorbing hydrogen.

Our novel sensing devices and novel method of preparing these devices rely on the use of nanoscale metal clusters and our ability to control these clusters to form sensors of desired functionality and configuration. Furthermore the combination of cluster deposition technology with lithography and semiconductor device technology is highly advantageous in that is potentially enables ready mass production of the devices. A particular feature of at least preferred embodiments of our technology is the dominance in electrical transport through the devices of tunnelling conduction, which in combination with the change in tunnel barrier width on absorption of hydrogen results in extremely sensitive sensors. Sensors have been achieved whose resistance changes by an order of magnitude on exposure to hydrogen.

The advantages of at least preferred embodiments of our technology (compared with many competing technologies) include that: - The sensors are formed using only simple and straightforward techniques, i.e. cluster deposition and relatively low resolution lithography.

The nanocluster transducer is automatically connected to electrical contacts. In-situ monitoring of the electrical current during deposition of the clusters allows the deposition to be controlled to allow formation of a sensor with the desired conducting/ sensing properties.

No manipulation of the clusters is required to form the sensor. The sensors have an unusually high degree of sensitivity and selectivity.

The sensors have a linear response over a substantial part of the hydrogen pressure range of interest. - The sensors work in vacuum and at ambient pressures below atmospheric pressures.

The sensors consume little or no electrical power in their ready state (i.e. in the absence of hydrogen).

It is demonstrated in the examples below that the controlled conjunction of a percolating film of Pd clusters close to the percolation threshold and the increase in the diameter of a nanoscale Pd cluster on absorption of hydrogen, leads to a significant increase in the conductivity of the film. This increase in conductivity is manifested as a very significant drop in resistance, which can be measured via the contacts provided. The magnitude of the resistance change is such that the percolating film provides a great degree of sensitivity, with a typical figure of merit ΔG/Go~4OO%, for 4% hydrogen under atmospheric pressure. Here ΔG is the change in conductance with respect to the conductance of the sensor in the absence of hydrogen G₀. The increase in conductivity of the film may occur because of : i) the connection of previously isolated pathways and clusters to a main conducting path through the percolating film, or ii) the creation of a main conducting path through the percolating film, or iii) a decrease in the size of the gaps between the clusters and a consequent onset of, or increase in, a tunnelling current, or iv) a combination of i-iii above, as discussed in more detail below..

Pd-based sensor design considerations and selectivity

The expansion of the lattice of Pd on absorption of hydrogen is known [1] to result from a phase change in the crystal structure (from the α phase to the β phase). This phase transition is manifested as a plateau in a plot of ambient hydrogen pressure versus hydrogen content of the Pd lattice (see Figure 9). It has been reported that the position of this plateau (i.e. the ambient hydrogen pressure at which a large change in hydrogen content occurs) is dramatically shifted by changes in temperature. The position of this plateau, and the curvature of the isotherm around it, effectively govern the amount of lattice expansion and therefore the increase in diameter of the clusters, and consequently the hydrogen pressure at which the sensor is most sensitive. This principle is demonstrated by the sensor characteristics obtained at different temperatures shown in Figure 14 (for a percolating tunnelling sensor). Note that the sensor characteristics shown in Figure 14 are similar to the characteristics shown in figure 8, but they are not identical because they are from sensors prepared under slightly different conditions. A preferred embodiment of the invention is therefore a hydrogen sensor whose sensitivity is tuned via a change of temperature, and which includes a heater and temperature sensor provided to allow control of the temperature.

Similarly, it has been shown [2] that the size of a cluster governs the shape of the pressure - hydrogen content isotherm and that, particularly for cluster sizes below ~5nm, the amount of hydrogen absorption is significantly modified. A preferred embodiment of the invention is therefore a hydrogen sensor whose sensitivity is tuned via a change of cluster size.

As noted above, hydrogen absorption by various Pd alloys has been studied extensively in the literature. Pd/Ag and Pd/Ni alloys are particular examples, but many alloys of Pd will show similar effects. [1] It has been reported that such alloys may have advantageous mechanical properties, and that for example, crumpling of thin Pd sheets on repeated exposure to hydrogen can be avoided in Pd/Ag sheets. [I]. A preferred embodiment of the invention is therefore a hydrogen sensor comprising nanoparticles of such alloy materials. Due to differences in the absorption of hydrogen by alloyed materials, alloying can clearly also be used to control the sensitivity and selectivity of the hydrogen sensor. This is illustrated in Figure 33 where the relative change in resistance of a sensor prepared with pure Pd clusters is shown in comparison with the expected response of Pdo_.95Ago_.os and Pdo_.95Nio_.o₅ alloys (the subscripts denote atomic fractions). The alloy isotherms were calculated from the pure Pd isotherm taking into consideration the different hydrogen solubility of the alloys. The Ag-alloy has significantly lower hydrogen solubility than pure Pd. Sensors prepared with Ag-alloy clusters would thus be able to sense much lower hydrogen concentrations than pure Pd sensors. The Ni-alloy, with its lower solubility, could on the other hand be used in environments with higher hydrogen concentrations. Clearly there is a trade off- higher sensitivity in one range of hydrogen pressures comes at the cost of reduced sensitivity in another range. Note that ultimately the highest sensitivities are observed for pressures corresponding to the "miscibility gap" i.e. the pressure range where the sensor material is undergoing a phase change.

The expansion of the lattice of Pd on absorption of hydrogen, as described above, is a highly selective process, since other gases cannot be absorbed at interstitial sites in the same way. Hydrogen sensors formed from this material are therefore highly selective. The resistance of Pd and its alloys to poisoning by many other gaseous species (such as SO₂ and CH₄) is also highly advantageous in the formation of sensors.

Several cluster films between contacts with fixed spacing can be connected in series. In this way the absolute change in resistance of the cluster film series is increased compared to a single cluster film. Similarly, the contacts can be interdigited to increase the contact area between the cluster film and the contacts. Depending on the readout system that will be used any of these two or a combination can be used to improve fabrication tolerances or signal to noise ratio.

Deposition of the Clusters Formation of atomic clustersjs a simple process whereby metal vapour is evaporated into a flowing inert gas stream which causes the condensation of the metal vapour into small particles. This process is known as inert gas aggregation (IGA). The metal vapour is often produced by thermal evaporation, but clusters could equally well be formed using cluster sources of any other design, a non-exclusive list of which includes sources which achieve a metal vapour by sputtering, magnetron sputtering (either AC or DC), arc discharge, electron beam heating or laser irradiation, (see e.g. the sources described in the review [10]).The particles are carried through a nozzle by the inert gas stream so that a molecular beam is formed. Particles from the beam can be deposited onto a suitable substrate.

The basic design of a cluster deposition system is described in Ref [H]. Our own deposition system is described in Ref [12], the contents of which are hereby incorporated by way of reference. It consists of a cluster source and a series of differentially pumped chambers that allow ionisation, size selection, acceleration and focussing of clusters before they are finally deposited on a substrate. In fact, while such an elaborate system is desirable, it is not essential, and we have formed devices in relatively poor vacuums without ionisation, size selection, acceleration or focussing.

For production of hydrogen sensors we prefer a magnetron sputtering source which produces ionised Pd clusters which can readily be mass selected. The Pd clusters are formed in a narrow size range (typically we prefer a mean size l-15nm) and the method is particularly suitable for further mass selection [12] if higher control of the particle size distribution is desired (e.g. a size range 3±0.1nm has been achieved [12]). Figure 3 shows two size distributions created using different source conditions. Sensors have been fabricated with clusters of various sizes; we have found that Pd clusters with diameters >6nm give the highest sensitivities for the present substrates and other favoured experimental parameters (compare for examples Figures 32 and 7, for clusters of 3.5nm and 6nm diameter respectively). The molecular beam of clusters formed has sufficiently large diameter to ensure a relatively homogeneous beam spot and to enable a number of devices to be formed simultaneously if desired.

Our previous patent specification [9] has described cluster formation and deposition techniques that we have used. Results and examples are included by reference.

As discussed previously we have three principal device types which are within the scope of the invention. Our method of making the devices, using metal clusters, and particularly clusters prepared according to our techniques (preferably inert gas aggregation) and using our film forming techniques (as discussed above) allows us to select which of the following devices we prepare. As noted above, a preferred embodiment of the invention is one in which electrical transport through the device is by tunnelling conduction, as the modulation of the size of the tunnel gap on absorption of hydrogen results in a dramatic change in electrical resistance.

A). Percolating cluster assembled nanodevices and the related method

A previous patent specification [9] describes in detail the theory of percolation, and the results and examples are included by reference. In this section we consider the case of the range of initial (i.e. prior to exposure to the fluid to be detected) coverages (p) where the majority of the conduction may be via ohmic conduction (although some contribution from tunnelling conduction is not excluded). This will typically be the case for p≥p_c (where p_c is the coverage corresponding to the percolation threshold: p_c ~ 0.67 in typical continuous 2D films [9]) which is illustrated schematically in Figure 15.

Percolation theory predicts that the variation of conductivity of a percolating film in the range of coverages p>p_c is expected to follow the power law relationship σ(p) ∞ (p -pj' where the exponent t is typically -1.3. The expansion of clusters due to absorption of hydrogen will cause an increase in cluster radius and therefore an increase in surface coverage, p. Due to the form of this relationship, a small change in cluster radius will cause a change in p proportional to the radius squared, and therefore a large increase in (p -p_c) which will lead to a dramatic change in conductivity.

It should be noted that films with coverages both just above and just below the percolation threshold are potentially useful as sensors. For p<p_c , and in the absence of tunnelling conduction (which is considered in the next section), the sensor will be open circuit prior to exposure to hydrogen, and will become conducting on exposure to hydrogen, resulting in a very large change of resistance. Such films may be advantageous for alarm sensors or when minimising the current flowing is necessary in applications where low power consumption is advantageous. For p>p_c the sensor will be conducting prior to exposure to hydrogen, but will become more conducting on exposure to hydrogen.

The description, in [9] of a methodology for reducing the coverage at the percolation threshold may be advantageous in this application. In [9] it was shown that p_c can be shifted from ~60% to ~20% simply by adjusting the contact spacing, and this effect can therefore be used to control the connectivity of the cluster film (or alternatively the number of structures near to the main conduction path but unconnected from it) which comprises the sensor, and thereby the sensitivity.

B). Percolating / tunnelling cluster assembled nanodevices and the related method In this section we consider the case of the range of initial (i.e. prior to exposure to the fluid to be detected) coverages where the majority of the conduction may be via tunnelling conduction (although some contribution from ohmic conduction is not excluded). This will typically be the case for p<p_c i.e. for coverages below the percolation threshold, since insufficient material is provided to allow an ohmic conduction path. In this case, we assume that the spacing between clusters is not sufficiently large so as to eliminate conduction altogether i.e. p>p_t, where p_t is the minimum coverage at which tunnelling conduction can be observed. Note that although the conduction is dominated by tunnelling conduction in this case, the film is still a percolating film in that it consists of a random array of filled sites, albeit that the surfaces coverage is insufficient to allow the formation of a fully-connected pathway. This embodiment of the invention is illustrated schematically in Figure 16.

For p_t^p^_c, and in the absence of any ohmic conduction path, the sensor will have a very small tunnelling current prior to exposure to hydrogen. On exposure to hydrogen an increase in tunnelling current may occur because the expansion of the clusters (due to absorption of hydrogen) decreases the size of the tunnelling gap between the clusters. Since the tunnelling current depends exponentially on the gap size, the measured change in resistance may be very large. Such films may be advantageous for alarm sensors or when minimising the current flowing is necessary in applications where low power consumption is advantageous.

Furthermore it is noted that the scope of the invention includes sensors which include regions in which conduction is by tunnelling, and other regions in which conduction is ohmic. These parts may be arrayed serially, but in the preferred embodiment illustrated in Figure 15 the sensor consists of a limited number of (percolating) paths in which conduction is ohmic while there exist one or more other paths which are discontinuous and in which at least part of the conduction is by tunnelling. In some cases the conduction through the tunnelling paths may exceed or be comparable to the conduction through the ohmic path. In other words in such sensors the conduction upon exposure to the fluid will be contributed to by both ohmic and tunnelling conduction.

We emphasise that the separation of the tunnelling (p<p_c) and ohmic (p>p_c) conduction cases is for illustrative purposes only, and that there may be examples where tunnelling and ohmic conduction may coexist. This is particularly likely to be the case near to the percolation threshold. For example, if the expansion of the clusters shown in Figure 16 is sufficiently great many of the clusters will make contact with each other and a strongly connected pathway such as that illustrated in Figure 15 may be achieved.

C) Devices dominated by tunnelling conduction

In a further preferred embodiment, the substrate surface between the contacts can be covered with small gold islands by thermal evaporation of a very thin gold layer prior to deposition of the clusters. Due to the surface energies of the materials, Au prefers to form islands on both SiO_x and SiN surfaces, rather than a homogeneous thin film. If the coverage of the initial layer of islands is sufficient (but not so high as to cause ohmic conduction) when a voltage is applied between the contacts, there will be a small tunnelling current through the gold islands which can be measured. Then, when clusters are deposited between the contacts, the tunnelling conductance increases exponentially with time (Figure 21). In this embodiment of the invention (illustrated schematically in Figure 17), the initial layer of islands (smaller, darker in Figure 17) are inert and do not respond to hydrogen, while the clusters (larger, lighter in Figure 17) expand on exposure to hydrogen and thereby close, or partially close, the tunnelling gaps.

In this embodiment, the coverage of the surface by both the Au islands and the Pd clusters can be controlled so as to achieve conduction which is dominated by tunnelling, for relatively sparse coverages. Alternatively, as the coverage of either component is increased, a greater number of clusters and islands are in contact and may form percolating paths which conduct at least partially, or possibly substantially, by ohmic conduction (see B above).

Further Sensor Design Considerations

Cluster versus atomic deposition Although the preferred method of formation of percolating, percolating-tunnelling, and tunnelling hydrogen sensors is the cluster deposition method described extensively herein, such films can be achieved by other methods, most notably the deposition of atomic vapour which aggregates to form clusters on the substrate. Sensors formed by this and other methods are included within the scope of the claims.

Diffusion/Temperature

One requirement for percolating film forming technology [9] is that when clusters land on the insulating surface between the electrical contacts they do not move significantly. This will almost always be the case for relatively large clusters (greater than about 10 nanometres in diameter), even at room temperature. For smaller clusters, or in the rare cases where even large clusters diffuse across the insulating surface of interest, the sample can be cooled down prior to deposition to eliminate surface diffusion. In the case of Pd clusters deposited to form hydrogen sensors we have observed a limited degree of diffusion resulting in aggregates of clusters on the surface of the film. The discussion of percolation and/or tunnelling between clusters above is intended to included the possibility that the percolating objects comprising aggregates of clusters and /or that the tunnelling occurs between aggregates of clusters, as well as fully coalesced particles. Figures 10 and 11 show that these aggregates may be relatively loosely bound, although it should be noted that these images are obtained after exposure to hydrogen and that the aggregates may be formed as a result of this exposure.

It is further noted that in the case of a cluster which expands on exposure to hydrogen, the level of anchoring of the cluster to the substrate is of significance. If clusters are strongly anchored to the surface, their mobility will be limited, possibly reducing coalescence with neighbouring clusters and limiting the amount of restructuring of the film during initial exposure to hydrogen. However the clusters ability to expand will be hindered, and the sensitivity of the sensor may be reduced. Improved anchoring can be achieved by, for example, increasing the substrate or cluster temperature during deposition, annealing post deposition, or increasing the kinetic energy of the incident clusters. Conversely, more weakly anchored clusters should provide higher sensitivity due to a greater ability to expand, but a larger initial restructuring of the film.

Surface Quality

The interaction between the nanoparticles can be modified by changing the physical properties of the surface prior to deposition of the nanoparticles. For example, the surface roughness can be modified by wet-chemical etching steps, or reactive ion etching. Therefore a preferred embodiment of the invention is a hydrogen sensor whose performance is optimized by modifying the surface roughness so as to minimise or at least alter the binding of the clusters to the surface and hence the sensitivity and / or the drift characteristics of the sensors.

The interaction between the nanoparticles can also be controlled by the choice of the surface material on which the particles are deposited. This can be realized by depositing a thin film of a specific material on top of the substrate. Non-excluding examples of materials that could have useful properties are polymers, Silica and siloxane. A preferred embodiment of the invention is therefore a hydrogen sensor whose performance is optimized by deposition of a thin film prior to deposition of the nanoparticles. We note that the restructuring of the films observed after the completion of cluster deposition (either in the presence or absence of hydrogen) may cause an increase in the size of the clusters observed on the surface, and consequent change in the surface coverage. Sensors comprising such restructured cluster deposited films are included in the scope of this application. The temperature considerations discussed above may be used to control the change in cluster diameter.

Coatings Stability is a key requirement for application of the hydrogen sensors. Because of the small size of the nanoparticles, there are many processes which can lead to long term instability or irreversible response of the sensor film to consecutive exposures to hydrogen. Examples of such processes are hydrogen induced dislocation of the particles, migration of the particles over the surface and hydrogen induced or intrinsic coalescence of particles. One method for the alleviation of such problems is the coating of the sensor surface with a material which inhibits motion of the nanoparticles. Figure 28 shows the stability of the response of two comparable sensors with and without a photoresist overlayer. Application of the overlayer significantly improves the stability of the sensor response. A preferred embodiment of the invention is therefore a hydrogen sensor whose stability is improved by application of an overlayer after the deposition of the nanoparticles. The overlayer may be of many materials including for example inorganic, insulating films such as SiO_x, or Al₂O₃, but we have focussed on the use of polymeric materials such as photoresist, SU8 and PMMA which can be applied by depositing a drop on the surface or by spin coating.

Membranes

Poisoning of the surface by molecules abundant in ambient atmosphere (H₂O, O₂, CO, CH₂, H₂S and SO₂) can reduce the magnitude and increase the time of the response of the hydrogen sensor. In order to prevent this, the sensor can be covered with a protective layer which is permeable for hydrogen, but not for these pollutants. The literature contains many concepts and designs for membranes which are permeable to hydrogen, and not to other gases which can be useful here. These include, but are not limited to, films or thin sheets of: pure palladium, Pd-Ag/PSS or palladium alloys; silicon; polydimethylsiloxane; ion-transport cermet membrane composed of, for example, 60vol% BaCeO. SY(X₂O₃ and 40vol% nickel powder; tantalum; Inconel 600; silica; alumina; iron; chitin/chitosan; steel; Al-killed low carbon enamel-grade steel; polyethers; CsHSO₄ composites; and vanadium alloy foils such as VTi5 and VCuI -10.

Membranes which are permeable to hydrogen could be used to enhance the effectiveness of the sensors described herein, by allowing hydrogen to reach the sensor, but preventing oxidization of the palladium clusters, or preventing poisoning of the sensor by other gases such as H₂S or SO₂ which would not be able to penetrate the membrane.

The simplest use of such a membrane would be as a simple covering of the sensor. However a multitude of alternatives can be conceived, including:

• a canister built from permeable material in which the sensor is situated;

• a canister with a window of permeable material;

• a canister with a window of a robust porous material covered with a very thin membrane;

• a thin layer of oxidation inhibitor deposited directly on the cluster film and • a thin layer of hydrogen permeable polymer deposited directly on the cluster film.

A thin layer deposited on top of the cluster film may reduce the response time of the sensor. A small canister and a thin membrane would allow faster equilibration of the hydrogen pressure. In addition, the canister could contain material intended to remove oxygen or a desiccant, as well as a variety of other sensors for other gases.

INDUSTRIAL APPLICABILITY

The basic sensor of the invention comprise a film of clusters deposited between a pair of contacts. A preferred embodiment (illustrated in Figure 31) comprises several films of particles deposited between several pairs of contacts. The sensors are fabricated on

Si wafer substrates, which enable many devices to be produced simultaneously and then diced up and packaged in such a way that they can simply be inserted into a socket on (for example) a printed circuit board within a hand held electronic control module.

The percolating cluster films described in this application are intended primarily to be used as hydrogen sensors, however the methodology and devices described here could be used for a variety of alternative sensors (for both liquid and gaseous analytes).

In one form, four percolated films are arranged into a basic Wheatstone bridge. In order to do this, the contact geometry of the contacts needs to be adjusted to accommodate 4 pairs of contacts, but all four percolating films can be deposited at the same time. After the deposition, two opposing films are covered with a hydrogen impermeable layer. The resistance of these films will not depend on the outside hydrogen pressure; however, this resistance will still depend on the temperature. When no hydrogen is present, this bridge will remain balanced independent of the temperature. This is ideal for alarm sensors, which should not give false positives under changing temperature conditions.

An additional method of preventing a response of the sensor to a change in ambient temperature is to provide a temperature stabilised system consisting of a temperature sensor, a heater, and feedback electronics.

EXAMPLES

The invention is further illustrated by the following examples:

Sensors fabricated on a silicon / SiN / SiOx substrate

The hydrogen sensors discussed in this section consist of a palladium cluster film in between two contact pads, typically supported on a silicon substrate or silicon substrate coated with a thin layer of SiN or SiO_x. A later section describes sensors fabricated on substrates which have an additional film of Au islands. Sample preparation

The substrate for the contacts is a flat, square silicon piece cut out of a standard 0.55 mm thick wafer with a 200 nm silicon-oxide layer on top to provide a non-conducting and very flat surface. The first step in the process is to clean the substrates through a standard rinsing procedure using acetone, then methanol and finally IPA.

Contact pads are formed on the substrate by thermal evaporation of 5 nm NiCr (80% Ni and 20% Cr) followed by 50 nm of Au through a shadow mask covering the area where no metal should be deposited. Thus two contacts, 3 mm wide and separated about 100 μm, are created. Au is chosen as it is readily available and NiCr is necessary to improve the adhesion to the silicon-oxide. This simple contact geometry and fabrication procedure is adequate for the present sensors, but we note that many more sophisticated lithographic procedures can be used to produce contacts of smaller dimensions, or of other geometries.

Finally the wafer is diced into 1 cm² samples. To prevent degradation of the surface of the sample during this procedure, a thin PMMA polymer layer is spun on top. This layer is removed after the dicing using a standard cleaning procedure. Note that in commercial production it is expected that the contacts would be scaled down to allow the substrate to be diced into ~ 1 mm² pieces.

Cluster deposition

The general cluster deposition method is already described in [9, 12]. The clusters are prepared using a magnetron sputter cluster source, which consists of a 300mm long, liquid nitrogen or water cooled tube with an inner diameter of 95mm. A Pd sputter target is installed at the front of a water cooled head mounted on a translatable arm inside the tube. A voltage of around -300V is applied to the target while a grounded stainless steel shield at a distance of 0.5mm from the target acts as cathode.

As argon gas is introduced into the source, plasma consisting of free electrons and argon ions forms in front of the target. The positive argon ions are accelerated towards the target and sputter it upon impact. The sputtered Pd material is cooled by the argon gas in the source and condenses into clusters. The clusters are ejected from the source towards the sensor substrate through a nozzle. The cluster size can be varied between 1-15nm, for example, by changing the distance between the sputter target and the nozzle, by changing the argon pressure in the source, by introducing helium together with the argon or by changing the temperature of the source. This is illustrated in Figure 3 where two cluster size distributions are plotted for two different mixtures of argon and helium in the source. Generally smaller clusters are produced when helium is added.

Since in some embodiments it is important that the surface coverage is controlled, in situ measurements of the cluster film resistivity are performed during deposition. Current as a function of time is shown in Figure 1 for a typical deposition run for an applied voltage of 240 mV across the two contacts. The resistivity of the film decreases very fast close to the percolation threshold. When the deposition is turned off after 75 seconds (in the example of Figure 1), the current continues to increase due to restructuring of the cluster film. The restructuring is thought to be due to coalescence and relocation of the clusters and it continues for some time until it stabilizes. This is further illustrated in Figure 2 where the resistance of the sample is shown as a function of time after the deposition (t=0). In this example a sensor is realised with coverage close to the percolation threshold.

By comparing the measured deposited mass on a thickness monitor at the percolation threshold with that predicted by percolation theory, the average cluster size deposited on this particular sample can be estimated to be around 3.5 - 4 nm. This estimate is consistent with the time of flight mass spectrometry data depicted in Figure 3. We can deposit clusters of a variety of materials with diameters up to 15 nm with our current experimental setup. All sensors described in this section were prepared using clusters with diameters of 6nm (see Figure 3) unless otherwise stated. Sensor response to the first exposure to hydrogen

After deposition and stabilization of the Pd cluster film it is subjected to a hydrogen load of 3 Torr for a few minutes. This first exposure to hydrogen results in an irreversible change to the film structure. The change in resistance of three cluster films upon hydrogen exposure is shown in Figure 4. Depending on the starting resistance, which is proportional to the coverage of the film, the film resistance after exposure to hydrogen either increases or decreases by many orders of magnitude, unless the initial resistance is within a limited range of initial resistances.

The change of resistance upon hydrogen exposure is due to the efficient hydrogen absorption of Pd and its effect on the clusters in the film. As the clusters absorb hydrogen they expand and come into contact with neighbouring clusters. This will increase the number of conduction paths between the contact pads resulting in a lower resistance. The cluster-cluster contact may, however, also lead to an irreversible coalescence of clusters. Coalescence may either result in formation of large islands with gaps in-between, and hence a breaking of conduction paths for coverage below p_c or it might result in a more solid set of connections between the coalesced clusters and therefore a network with very low resistance. Neither of these effects are reversible and it is hence desirable to balance them out by depositing films with coverage between these two regimes, see Figure 26, where the resistance change upon hydrogen exposure is due to a reversible cluster expansion that is not followed by coalescence.

Figures 10 and 11 depict SEM pictures of cluster films at two different coverages. Both films have been exposed to hydrogen and the coverage after deposition has been estimated to be 0.67 and 0.77 respectively, i.e. slightly below and above p_c. The resolution is not high enough to see individual clusters, or to make out continuous paths between the contacts, but it can be seen that the clusters aggregate into 'islands' with typical diameters of 20 to 40 nm and it is possible to recognise larger irregular structures with dimensions at least up to several 100 nm in the film with higher coverage. The larger structures are less prominent on the film produced with a lower coverage. This is in good agreement with the scenario described above for the first hydrogen exposure. It should also be mentioned that the resistance increased by four orders of magnitude for the low coverage film while it decreased by two orders of magnitude for the film with higher coverage upon the first hydrogen exposure.

Temperature dependence

The temperature dependence of tunnelling conductance is in a simplified form given by:

G ~ exp(-E_c / kT) or G ~ (1/T).exp(-E_c / kT)

With G the conductance, E₀ the Coulomb charging energy, k the Boltzmann constant and T the temperature in K.

Figures 23 and 24 show the temperature dependence of the resistance of a typical sensor (for which the resistance had previously increased after the initial hydrogen exposure). The cluster size was around 3.5nm for this particular sensor, see Figure 3 for size distribution. Figure 24, which is an Arrhenius plot of the data shown in Figure 23, shows that the Coulomb charging energy for this particular sensor was determined to be 0.007 eV (the full line is a linear fit to the data), which is consistent with theoretical estimations for clusters of 3.5 nm diameter. Figure 24 therefore shows that the temperature dependence of the sensors described herein is consistent with tunnelling. Also characteristic for tunnelling conduction is a non-linear dependence of the current through the film on the voltage over the film. Figure 29 illustrates precisely such a nonlinear dependence. It can be therefore be concluded the sensors described in this section are percolating-tunnelling type sensors i.e. they exhibit elements, such as the dependence of film properties on surface coverage, characteristic of percolation and other characteristics, such as resistance temperature behaviour, characteristic of tunnelling. The structure of the sensor may be visualised schematically as in figure 16. Figure 30 shows the temperature dependence of a cluster layer for which the resistance decreased strongly after the initial hydrogen exposure, resulting in a device with low resistance and a response to hydrogen far smaller than typical sensors with high resistances. The resistance of the films shown in Figure 30 increases with increasing temperature, which indicates that the main conduction mechanism is not tunnelling conduction, i.e. this sensor exhibit conduction behaviour typical of most metallic materials. Such films are much less sensitive than films of which the resistance increases after the initial hydrogen exposure. The examples presented below all relate to cluster films for which the resistance increased after the initial hydrogen exposure and which exhibit tunnelling conduction.

Response time

The response of a similar film can be seen in Figure 5 for two hydrogen exposures at two different pressures. The response time can be estimated, from the exponential edges on the response waveforms, to be around ten seconds which is considerably faster than the response of a very thick Pd cluster film, with coverage many time larger than the percolation threshold, as shown in Figure 6. The increase in sensor resistance for the latter film (to be contrasted with the decrease in sensor resistance in Figure 5) is however due to the increased resistivity of bulk Pd and not due to the expansion and contraction of clusters, i.e. the response is dominated by metallic conduction and this is not a percolating tunnelling type sensor. The response in Figure 6 is similar to commercial Pd based hydrogen sensors of the prior art based on thin films of Pd. The comparison between the cluster sensor and the thick film sensor also shows that the sensitivity differs by almost an order of magnitude between these types of sensors. The cluster film sensors are hence more sensitive and respond significantly faster to hydrogen exposure.

Reversible sensor response

The result from multiple hydrogen exposures of a reversible cluster film can be seen in Figure 26. The conductance during hydrogen exposure is clearly dependent on the hydrogen pressure and the conductance returns to the approximate starting value upon purging of the hydrogen. The shifts in the baseline (zero hydrogen) are due to this film being one which was deposited shortly before the measurements; the baseline stabilises after many cycles.

Typical sensor response characteristics (i.e. change in resistance as a function of pressure) are shown in Figure 8. It is instructive to compare this with the Palladium pressure-concentration isotherm in Figure 9. Clearly the sensor response characteristic (Figure 8) is similar to the isotherm (Figure 9), which illustrates that the response is largely due to the expansion of the clusters on absorption of hydrogen, and also that knowledge of the isotherms for Pd and its alloys (i.e. isotherms at different temperatures) can be used to design sensors e.g. to gain maximum sensitivity by selecting a temperature and alloy concentration such that the most rapid change in resistance occurs in the pressure range of most interest for the particular desired application, or to minimise hysteresis and restructuring effects that may occur in the miscibility/ phase change regime (II in Figure 9) by selecting temperature and alloy concentration such that the sensor operates in region I (in Figure 9) in the pressure range of most interest for the particular desired application. In Figure 14 it is illustrated how the operating temperature can be used to shift the miscibility gap and hence control the hydrogen pressure at which the highest sensitivity is achieved. Figure 33 illustrates the same effect achieved by changing the alloy concentration in the clusters from pure Pd to either Pd₀₉5Ag₀₀₅ or Pdo.₉sNio os-

As noted above (see Figure 4), after the first hydrogen load the resulting film resistance for initially very similar films may differ by many orders of magnitude. This can be seen in figure 12 where films with initial resistances within a narrow range result in large variations of final resistances upon the first hydrogen load while still functioning as reversible hydrogen sensors (closed circles). This is evidence that films close to the percolation threshold are required to achieve reversible sensors. It is noted that initial resistances for films with coverages significantly different from the percolation threshold have irreversible changes under the first hydrogen load, and in general less desirable sensor properties.

Figure 7 shows the resistance, normalized to the resistance in the absence of hydrogen (R₀), as a function of partial hydrogen pressure for two films with different R₀ values.

Note that the relative change is virtually independent of the base resistance R₀. This allows us to infer that as long as the coverage is not too large allowing bulk properties to dominate or too low with no conduction paths between the electrodes even in the presence of hydrogen, the cluster films may function as hydrogen sensors with similar relative response.

A common problem with sensors is that the fabrication reproducibility of the sensors is thus that individual calibration is necessary. This may be very costly. From Figure 7 it can be seen that the relative change of the resistance as a function of P_H2 is very similar for base resistances of 1 MΩ to 70 GΩ. This very large tolerance of the sensor response to base resistance makes it possible that individual calibration of the sensors might not be necessary.

Polymer cover layer In order to improve stability of the sensor response a polymer overlayer was deposited on top of the clusters. In Figure 28 the sensor response to 5% H₂ has been obtained using a standard characterization procedure. On average there was a period of 7 days in between each consecutive measurement. The sensor response Ri_og has been defined as:

Clearly it can be seen that for the sensors of this particular example the polymer overlayer significantly stabilizes the sensor response. Sensors fabricated on substrates covered with gold islands

Sample preparation

The sample preparation procedure is identical to that of the sensor on the unmodified silicon surface. The only difference is that the substrate is covered with a gold island film after the gold contacts have been realized and before the samples are diced.

The substrate surface between the contacts is covered with small gold islands by thermal evaporation of a very thin gold layer, which rather than forming a homogeneous thin film prefer to form a series of isolated islands as shown in Figure 13 which is a scanning electron micrograph picture of a gold island film with mean thickness 2nm. A small tunnelling current can be measured through the gold islands. In Figure 18 the resistance of a gold island film is shown as a function of effective gold layer thickness. The exponential change in resistance with increasing mean gold layer thickness is due to a steady increase in mean island size and consequent decrease in mean island separation (tunnelling conductance is expected to vary exponentially with the width of the tunnel barrier).

In Figure 19 the temperature dependence of the resistance of a gold island film (before cluster deposition) is shown to be consistent with tunnelling. The Coulomb charging energy for this particular gold island film was determined to be 0.014 eV (linear fit in an Arrhenius plot, Figure 20).

Cluster deposition The procedure for cluster deposition is identical to that of the sensor on the unmodified silicon surface. In this case however, when clusters are deposited between the contacts, the tunnelling resistance reduces exponentially with time (dashed line in Figure 21). This allows in situ control of the cluster surface coverage for coverages well below the percolation threshold. For the sensor treated in this example, the surface coverage is less than the percolation threshold, contrary to the sensor fabricated on an unmodified silicon substrate, as treated in the previous example, where the surface coverage was around the percolation threshold.

For samples without gold islands, the onset curve is much sharper (solid line in Figure 21 - see also Figure 1) since there is only a very small surface coverage region in which the tunnelling current can be measured, and as such control of the surface coverage is much more critical for samples without gold islands, than for samples with gold islands.

For samples without Au islands irreversible restructuring of the cluster film upon the first hydrogen exposure makes the precise coverage even more critical. Thus, using a substrate with small gold islands relaxes the tolerances for the coverage greatly and allows greater control in achieving the desired coverage.

Sensor response to the first exposure to hydrogen

In Figure 25 the resistance as a function of time for the first hydrogen load is depicted for two Au island sensors. In figure 22 the resistances of a series of sensors before and after the first hydrogen load are depicted. The open circles represent sensors that respond irreversibly to exposure to hydrogen and the closed circles refer to sensors that respond reversibly. Clearly the initial sensor resistance after cluster deposition is less critical for obtaining reversible sensors for sensors fabricated on a silicon substrate covered with gold islands, than for sensors fabricated on an unmodified silicon substrate

(compare with Figure 12).

Temperature dependence

Sensors fabricated using gold islands display a qualitatively similar temperature dependence of the resistance as sensors fabricated on an unmodified silicon substrate (see Figure 23). Hence tunnelling is confirmed as the main conduction mechanism. Sensor response

The response of sensors (e.g. resistance versus time plots) fabricated with gold islands (Figure 27) is comparable to sensors fabricated on samples without gold islands (compare with Figure 26).

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

F.A. Lewis, The palladium hydrogen system, academic press, 1967 A. Pundt, M. Suleiman, C. Bahtz, M.T. Reetz, R. Kirchheim, N.M. Jisrawi, Hydrogen and Pd-clusters, Materials Science and Engineering B 108, 19-23 (2004). A. Barr, Thin Solid Films, 41, 1977. H.H. Xiao, 36^th Annual Technical Conference Proceedings 1993. Fan Wu and J.E. Morris, Thin Solid Films, 246 (1994), pp. 17-23 J.E. Morris, An. Kiesow, Min Hong, Fan Wu, Int. j. Electronics, 81 (1996), pp. 441-447 O.Dankert and A. Pundt, Applied Phys. Letters, 81, 1618 (2002). Xu, T., Zach, M.P., Xiao, Z.L., Rosenmann, D., WeIp, U., Kwok, W.K., Crabtree, G. W., Applied Physics Letters, 86, (2005), p 203104 International Patent Application number PCT/NZ02/00160; NZ Patent Application number 51367, "Nanoscale Electronic Devices and Fabrication Methods". W. de Heer, Rev. Mod. Phys. 65, 611 (1993) L M. Goldby et al, Rev. Sci. Inst. 68, 3327 (1997). R. Reichel, J. G. Partridge, A. D. F. Dunbar, S. A. Brown, ' Construction and Application of a UHV Compatible Cluster Deposition System', Accepted for publication in the Journal of Nanoparticle Research (2006).

Claims

WHAT WE CLAIM IS:

1. A hydrogen sensor device comprising a non-contiguous film(s) of nanoscale metal or semiconductor clusters running substantially between two or more contacts and responsive to the presence of hydrogen gas to be detected, and a detector in communication with the contacts such that the contacts and the noncontiguous film(s) form a circuit.

2. A device as claimed in claim 1 wherein the response of the device to the presence of hydrogen gas is a change in electrical resistivity between the two or more contacts, where the conduction is due to one of the following processes:

- ohmic conduction within the film between the two or more contacts;

- tunnelling conduction within the film between the two or more contacts;

- a combination of ohmic and tunnelling conduction within the film between the two or more contacts.

3. A device as claimed in claim 1 or 2 wherein the non-contiguous film(s) comprises a series of networks of metal or semiconductor clusters on an insulating or semiconducting substrate.

4. A device as claimed in any one of the preceding claims wherein the clusters are of palladium or an alloy thereof.

5. A device as claimed in any one of claims 1 to 3 wherein the clusters are of yttrium, or palladium coated yttrium.

6. A device as claimed in claim 4 wherein the clusters are of a palladium alloy comprising palladium mixed with one or more of silver, rhodium, ruthenium, yttrium and/or nickel.

7. A device as claimed in any one of the preceding claims wherein at least in the region between the contacts, and intermediate between the contacts and the noncontiguous film(s), there is an intermediate film of nanoparticles, which are inert to or have a lower responsivity to the presence of hydrogen gas than the plurality of clusters making up the non-contiguous film(s).

8. A device as claimed in claim 7 wherein the intermediate film is discontinuous.

9. A device as claimed in claim 8 wherein the intermediate film is metallic and the principal mode of conduction between the plurality of particles making up the intermediate film is tunnelling conduction.

10. A device as claimed in any one of the preceding claims in which, in the presence of hydrogen gas, one or more gap(s) in the non-contiguous fihn(s) close to form at least one continuous link between the contacts thereby decreasing the resistivity of the circuit due to ohmic conduction.

11. A device as claimed in claim 10 wherein in the absence of hydrogen gas, the gap(s) in the film open thereby increasing the resistivity of the circuit.

12. A device as claimed in any one of claims 1 to 9 in which, in the presence of hydrogen gas, the width of the gap(s) in the non-contiguous film(s) will decrease thereby decreasing the resistivity of the circuit due to tunnelling conduction.

13. A device as claimed in claim 12 wherein, in the absence of hydrogen gas, the width of the gap(s) in the film may increase thereby increasing the resistivity of the circuit.

14. A device as claimed in any one of claims 1 to 9 wherein, in the presence of hydrogen gas, the width of at least some of the gap(s) in the non-contiguous film(s) decrease thereby decreasing the resistivity of the circuit due to tunnelling conduction, and additionally one or more gap(s) in the non-contiguous film(s) close to form at least one continuous link between the contacts thereby decreasing the resistivity of the circuit due to the formation of an ohmic conduction path.

15. A device as claimed in claims 12 to 14 wherein the number of discontinuous links is sufficiently large that tunnelling conduction dominates the conductive properties of the film.

16. A device as claimed in claim 15 wherein tunnelling conduction is substantially the sole means of conduction between the contacts upon exposure to hydrogen gas.

17. A device as claimed in any one of the preceding claims wherein the noncontiguous film(s), or the non-contiguous film(s) together with the intermediate film, comprises a network of clusters whose surface coverage in the region between the contacts is close to the percolation threshold in two dimensions.

5

18. A device as claimed in claim 17 wherein the non-contiguous film(s), or the noncontiguous film(s) together with the intermediate film, comprise a network of particles whose volume filling fraction is close to the percolation threshold in three dimensions.

10

19. A device as claimed in any one of the preceding claims wherein the average diameter of the clusters is between 0.3nm and l,000nm.

20. A device as claimed in claim 19 wherein the average diameter of the clusters is 15 between 0.3nm and lOOnm.

21. A device as claimed in claim 19 wherein the average diameter of the clusters is between 0.3 and 5nm .

20 22. A device as claimed in any one of the preceding claims wherein the clusters have been formed by inert gas aggregation.

25

23. A device as claimed in any one of claims 3 to 22 wherein the substrate is selected from one of the group consisting of silicon, silicon nitride, silicon oxide, aluminium oxide, indium tin oxide, germanium, gallium arsenide or another III- V semiconductor, quartz, or glass.

5

24. A device as claimed in any one of claims 4 to 23 wherein the intermediate film is composed of nanoparticles selected from one of the group consisting of Au, Ag, Cu, Sb, Bi or Pb.

10 25. A device as claimed in claim 24 wherein the nanoparticles of the intermediate film have diameters less than 20nm.

26. A device as claimed in claim 25 wherein the nanoparticles of the intermediate film have diameters less than 5nm.

15

27. A device as claimed in claims 9 to 26 wherein the clusters of the non-contiguous film exist as loosely formed aggregates of more than one cluster, or as coalesced masses or islands of clusters.

20 28. A device as claimed in claim 27 wherein the mode of conduction between the coalesced masses or islands of clusters of the intermediate film is entirely tunnelling conduction.

25

29. A device as claimed in claims 27 or 28 wherein the aggregates of more than one cluster or the coalesced masses or islands of clusters are separated by gaps smaller than 5nm.

5

30. A device as claimed in any one of claims 4 to 29 wherein a further layer of inert material is present between the non-contiguous film(s) and the intermediate fihn reducing interaction between the particles of these films.

10 31. A device as claimed in claim 30 wherein the inert material is an insulating oxide.

32. A device as claimed in any one of claims 3 to 31 wherein the device includes a layer or modifying material between the substrate and the non-contiguous film(s) effective to dictate the cluster film structure.

15

33. A device as claimed in any one of the preceding claims further including a resist or other organic compound or an oxide or other insulating layer on top of the clusters of the non-contiguous film(s).

20 34. A device as claimed in any one of the preceding claims wherein the device further includes an outer barrier selected from one of - a protective outer coating permeable to hydrogen gas but substantially impermeable to one or more species which may contaminate the noncontiguous film(s), or

25 an activated carbon outer barrier is provided to prevent species contaminating the non-contiguous film while allowing the passage of hydrogen gas.

5 35. A device as claimed in any one of the preceding claims wherein there are a plurality of films between the two or more contacts.

36. A device as claimed in claim 35 wherein the plurality of films is arranged in a Wheatstone Bridge configuration.

10

37. A device as claimed in any one of the preceding claims wherein there are two contacts which are separated by a distance smaller than 200 microns.

38. A device as claimed in claim 37 wherein the contacts are separated by a distance 15 less than lOOOnm.

39. A device as claimed in claim 38 wherein the contacts are separated by a distance less than 200nm.

20 40. A device as claimed in any one of the preceding claims wherein the geometry of the contacts has been optimised by percolation theory calculations to result in non-contiguous film(s) at a surface coverage of particles on the substrate of less than the percolation threshold for a macroscopic contact separation.

25

41. A device as claimed in any one of the preceding claims wherein some of the particles of the non-contiguous film(s) are insulated from other particles of the non-contiguous film.

42. A device as claimed in any one of the preceding claims wherein the detector additionally or alternatively measures the impedance of the fihn(s) between the two or more contacts.

43. A device as claimed in any one of the preceding claims wherein the detector is selected from one of the group of a galvanometer, an ohmmeter, a potentiostat, a lock-in amplifier and a multimeter.

44. A device as claimed in any one of the preceding claims wherein more than one detector is used in a measurement circuit.

45. A device as claimed in any one of the preceding claims wherein a multiplicity of measurements may be made at more than one frequency.

46. A device as claimed in any one of the preceding claims wherein the device is incoiporated within a circuit comprising a power source and a means of monitoring the current flowing through the circuit and/or a means of monitoring the voltage substantially across the one or more pairs of contacts.

47. A device as claimed in any one of the preceding claims wherein the presence of hydrogen gas triggers an alarm.

48. A device as claimed in any one of the preceding claims wherein the device includes a heater element to control the temperature of the fϊlm(s).

49. A device as claimed in claim 48 wherein the heater element is used to tune the responsiveness of the non-contiguous film(s) to hydrogen gas.

50. A device as claimed in claim 48 or 49 wherein the non-contiguous film is a palladium alloy, and the temperature of operation and/or the alloy composition have been selected in order that the maximal or desired sensitivity of the sensor will occur for the hydrogen pressure range of interest.

51. Hydrogen gas sensing apparatus comprising at least hydrogen gas sensing devices having two or more electrical contacts and one or more non-contiguous film(s) of nanoscale metal or semiconductor clusters running substantially between the two or more contacts, connected in a circuit with a power source and a means for monitoring the current flowing through the circuit and / or a means for of monitoring the voltage substantially across the one or more pairs of contacts.

52. Apparatus as claimed in claim 51 wherein the clusters have an average diameter between 0.3 nm and 1,000 nm.

53. Apparatus as claimed in claim 52 wherein the clusters have an average diameter 5 between 0.5 nm and 1 OOnni.

54. Apparatus as claimed in claim 52 wherein the clusters have an average diameter between 0.3 nm and 5nm.

10 55. Apparatus as claimed in claim 54 wherein the clusters have been formed by inert gas aggregation.

56. Apparatus as claimed in any one of claims 51 to 55 wherein the clusters are of palladium, or an alloy or palladium or yttrium or palladium coated yttrium.

15

57. Apparatus as claimed in any one of claims 51 to 56 wherein there are two contacts separated by a distance less than 1000 nm.

58. Apparatus as claimed in any one of claims 51 to 57 further including an alarm 20 which is triggered when hydrogen gas is present.

59. Apparatus as claimed in any one of claims 51 to 58 wherein the one or more hydrogen sensing devices is/are a device claimed in any one of claims 1 to 4.

25

60. Apparatus as claimed in any one of claims 51 to 59 including a heater element to control the temperature of the non-contiguous film(s).

61. Apparatus as claimed in any one of claims 51 to 60 wherein at least the 5 hydrogen sensing device(s) are at least partially surrounded by a membrane capable of selectively transmitting gases. Or wherein the hydrogen sensing device is contained within a gas permeable housing, or within a housing having at least a gas permeable window.

10 62. Apparatus as claimed in any one of claims 51 to 61 including contaminant removing species or means selected from one or more of: a quantity of activated carbon capable of removing oxygen, water and/or other contaminants; a thin layer of oxidation inhibitor on the non-contiguous film(s); 15 - A thin layer of hydrogen permeable polymer on the non-contiguous film(s).

63. Apparatus as claimed in any one of claims 51 to 62 further including or in communication with other sensors capable of detecting fluids other than

20 hydrogen.

64. A method of preparing a nanoscale hydrogen sensor device comprising the steps of:

- providing a substrate having two or more electrical contacts; 25 - deposition of a plurality of nanoscale metal or semiconducting clusters onto the substrate at least in the region between the contacts, to achieving a non-contiguous fihn(s) responsive to the presence of hydrogen gas, running substantially between the contacts,

- providing a detector in communication with the contacts such that in the presence of hydrogen gas the response of the film(s) is/are detected.

65. A method as claimed in claim 64 wherein the step of depositing the plurality of particles between the contacts to achieve the non-continuous fibn(s) comprises one or both the steps of:

^■ monitoring the conduction between the contacts and ceasing deposition at or near to the onset of tunnelling or ohmic conduction, and/or ^■ modifying the substrate surface, or taking advantage of preexisting topographical features, which will encourage the deposited clusters to form the film(s).

66. A method as claimed in claim 65 wherein there is the further step of providing a discontinuous metal film of a metal or semiconductor which is inert or has a lower responsivity to the presence of hydrogen gas than the non-contiguous film(s), on the substrate prior to the deposition of the plurality of clusters, and the step of deposition of a plurality of clusters on the substrate comprises depositing the clusters at least partially on the discontinuous metal film in at least the region between the contacts.

67. A method as claimed in any one of the preceding claims wherein the plurality of 5 clusters are of palladium or an alloy thereof, or yttrium, or palladium coated yttrium.

68. A method as claimed in claim 67 wherein the alloy is palladium mixed with one or more of silver, rhodium, ruthenium, yttrium and/or nickel.

10

69. A method as claimed in any one of claims 64 to 68 wherein the clusters have an average diameter between 0.3nm and lOOOnm.

70. A method as claimed in claim 69 wherein the clusters have an average diameter 15 between 0.5nm and lOOnm.

71. A method as claimed in claim 69 wherein the clusters have an average diameter between 0.3nm and 5nm.

20 72. A method as claimed in any one of claims 67 to 71 wherein the metal clusters are prepared by inert gas aggregation.

25

73. A method as claimed in claim 72 wherein in the step of preparation of the metal clusters and/or control of cluster source parameters and/or use of a subsequent step of cluster size selection (but prior to the cluster deposition) allows control of the cluster size and thereby allows control of the characteristics of the sensor

5 device including the mode of conduction which will result upon exposure to hydrogen gas.

74. A method as claimed in any one of claims 64 to 73 wherein there is a further step of stabilizing the morphology of the non-contiguous film by application of

10 an overlayer.

75. A method as claimed in any one of claims 66 to 74 wherein the metal film is prepared from particles of Au₅ Ag, Cu, Sb, Bi or Pb.

15 76. A method as claimed in claim 75 wherein the particles of the metal film have diameters smaller than 20nm.

77. A method as claimed in claim 76 wherein the particles of the metal film have diameters smaller than 5nm.

20

78. A method as claimed in one of claims 75 to 77 wherein the particles of the metal film are separated by gaps with dimensions smaller than 5nm.

25

79. A method as claimed in claim 78 wherein the particles of the metal film has been formed by inert gas aggregation and are deposited by uniform deposition onto the substrate resulting in the aggregation of the particles into coalesced masses or island(s) on the substrate surface.

5

80. A method as claimed in any of claims 64 to 79 wherein there is a pre-step of deposition of a thin film resulting in modification of the interaction between the clusters and the substrate.

10 81. A method as claimed in any one of claims 64 to 80 wherein prior to the deposition of the metal clusters there is a step of substrate surface modification to increase or decrease the surface roughness.

82. A method as claimed in any one of claims 64 to 81 wherein after the deposition 15 of the metal clusters there is a step of applying a resist or other organic compound or an oxide or other insulating layer on top of the clusters effective in stabilizing the morphology and properties of the non-contiguous film(s).

83. A method as claimed in any one of claims 64 to 82 wherein there is a further 20 step of applying a protective coating over the non-contiguous film(s) permeable to hydrogen gas but substantially impermeable to species which may contaminate the film(s) and/or

25 an activated carbon barrier is over the film(s) to prevent species contaminating the film while allowing the passage of hydrogen gas.

84. A device as claimed in any one of the preceding claims wherein the clusters of the non-contiguous film(s) is/are of palladium or an alloy thereof, or yttrium, or palladium coated yttrium.

85. A device as claimed in claim 84 wherein the clusters of non-contiguous film(s) is/are a palladium alloy of palladium mixed with one or more of silver, rhodium, ruthenium, yttrium and/or nickel.

86. A method as claimed in any one of claims 64 to 85 further including a step prior to the deposition of the particles of applying a resist or other organic compound or an oxide or other insulating layer to the substrate and then processing using lithography to define a region or regions where the metal clusters once deported will form a continuous or discontinuous web between the contacts, and another region where the metal clusters will be insulated from the conducting network.

87. A nanoscale hydrogen gas sensing device prepared according to the method of any one of claims 64 to 86.

88. A hydrogen sensor device comprising an non-contiguous film of nanoscale metal or semiconducting clusters running substantially between two contacts on an insulating or semiconducting substrate, the non-contiguous film being responsive to hydrogen gas, an intermediate discontinuous metal film between the non-contiguous film and the substrate, wherein the metal clusters of the noncontiguous film are between 03 and lOOOnm in diameter, and wherein the mode of conduction within the intermediate film is tunnelling conduction, whilst the 5 mode of conduction of the non-contiguous film upon exposure to hydrogen gas is one of: ohmic conduction within the film between the two or more contacts; tunnelling conduction within the film between the two or more contacts; a combination of ohmic and tunnelling conduction within the film 10 between the two or more contacts.

89. A device as claimed in claim 88 wherein the intermediate film is of coalesced ^• gold nanoparticles.

15 90. A device as claimed in claimed 89 wherein the clusters are of palladium or an alloy thereof, are of size between 1 - 20nm.

91. A method of assessing the hydrogen gas content in an environment including contacting the device of claim 1 and/or apparatus of claim 51 with the

20 environment and reading or otherwise assessing the output of the detector.

92. A hydrogen gas sensor device substantially as herein described with reference to one or more of the Figures and/or Examples.

25

93. Hydrogen sensing apparatus substantially as herein described with reference to one or more of the Figures and/or Examples.

94. A method of preparing a nanoscale hydrogen gas sensor device substantially as herein described with reference to one or more of the Figures and/or

Examples.