Graphene-based electrochemical sensor

Abstract

There is provided an electrochemical sensor for the detection of an analyte in a sample, the sensor comprising: (a) a substrate having a crystalline growth surface; (b) a graphene layer structure on the growth surface; (c) optionally a barrier layer on the graphene layer structure; (d) a conformal or substantially conformal zinc oxide layer on the graphene layer structure or, when present, on the barrier layer, the zinc oxide layer providing a sample-receiving surface having a crystallographic orientation sensitive to the analyte, the zinc oxide layer having a maximum thickness of less than 200 nm; and (e) at least first and second metal contacts arranged to allow observation of a change in the electrical properties of the graphene layer structure.

Description

[0001] Graphene-based electrochemical sensor

[0002] The present invention relates to an electrochemical sensor comprising a graphene layer structure, and a method for the manufacture of an electrochemical sensor. More particularly, the electrochemical sensor comprises a zinc oxide layer on the graphene layer structure, the zinc oxide layer providing a sample-receiving surface. The graphene layer structure acts as a transducer converting the response of the zinc oxide layer upon interaction with the analyte to be detected into measurable electrical signals.

[0003] Graphene is a well-known two-dimensional material that it being utilised in industry to transform and enhance electronic devices. One important category of electronic devices is sensors whose sensitivity amongst other properties can be drastically improved by the unique electronic properties associated with the two-dimensional nature of graphene. Sensors have become ever more important in society, for example, in order to monitor the quality and / or the presence of contaminants in the environment or food, and in medical devices for diagnostics.

[0004] Zhang et al. “A review on two-dimensional materials for chemiresistive- and FET-type gas sensors” Phys. Chem. Chem. Phys., 2021 , 23, 15420 reviews the recent development of 2D material-based chemiresistive- and FET-type gas sensors, including the sensing mechanisms, chemical structures and synthesis methods of 2D materials and their sensing properties.

[0005] Recum and Hirsch “Graphene-based chemiresistive gas sensors” Nanoscale Adv., 2024, 6, 11 describes critical aspects when using nanomaterials as sensing substrates for the application in chemiresistive gas sensors.

[0006] Salvatore Gianluca Leonardi “Two-Dimensional Zinc Oxide Nanostructures for Gas Sensor Applications” Chemosensors, 2017, 5, 17 summarizes most of the research articles focused on the investigation of 2D ZnO structures including nanosheets, nanowalls, nanoflakes, nanoplates, nanodisks, and hierarchically assembled nanostructures as a sensitive material for conductometric gas sensors.

[0007] Franco et al. “A review on chemiresistive ZnO gas sensors” Sensors and Actuators Reports, 4, 2022, 100100 surveys recent advances on different types of chemiresistive ZnO-based gas sensors, focusing on how the morphology and structure of these materials influence on the sensor response.

[0008] WO 2016 / 100049 A1 discloses a chemically-sensitive field effect transistor, the chemically-sensitive field effect transistor comprises a CMOS structure comprising a conductive source and a conductive drain, a channel and an analyte-sensitive dielectric layer. The channel is composed of a onedimensional transistor material or a two-dimensional transistor material. US 2017 / 0102358 A1 discloses chemically-sensitive FETs comprising a conductive source, a conductive drain, and a channel composed of a one-dimensional (1 D) or two-dimensional (2D) transistor material, which channel extends from the source to the drain and is fabricated using semiconductor fabrication techniques on top of a wafer.

[0009] CN 105092646 B discloses a reduced graphene oxide I metal oxide composite film gas sensor comprising a sensitive device, wherein a nanoscale metal oxide particle film is provided on the sensitive device, and pores exist between metal oxide particles; a reduced graphene oxide film is provided on the nanoscale metal oxide particle film; and at the contact interface between the reduced graphene oxide film and the nanoscale metal oxide particle film, functional groups of the reduced graphene oxide film are bonded to metal oxide particles of the nanoscale metal oxide particle film.

[0010] US 9291613 B2 relates to nanostructured sensor systems for measurement analytes, for example by measurement of variations of electrical properties of nanostructure elements in response to an analyte, such as biomolecule, organic and inorganic species, including environmentally and medically relevant volatiles and gases, such as NO, NO2, CO2, NH3, H2, CO and the like.

[0011] US 2021 / 0123878 A1 discloses a chemi-capacitive sensor which includes a lower electrode including a conductor, an insulation part formed on the lower electrode and including an insulator, an upper electrode disposed on the insulation part and including a first electrode and a second electrode spaced apart from the first electrode, and a detection part disposed on the first electrode, the second electrode, and the insulation part between the first electrode and the second electrode and including at least one selected from the group consisting of a carbon nanomaterial and a metal-oxide-coated carbon nanomaterial.

[0012] KR 20230010376 A discloses a gas detection system comprising a sensor which may include a substrate, a graphene sheet positioned on an upper portion of the substrate, and metal oxide nanoparticles doped on the graphene sheet.

[0013] There is however still a need for further improved sensors, including those which can incorporate and maintain high quality graphene (i.e. pristine graphene) so as to be able to benefit from the exceptional properties afforded by this two-dimensional material. As identified in Nanoscale Adv., 2024, 6, 11 , the term graphene may be used across the literature as a placeholder for all types of carbon-based layered crystals, including pristine graphene, but also defective structures as in laser-induced graphene, graphene oxide (GO) or reduced graphene oxide (rGO), but these have entirely different electrical, mechanical and chemical properties. The present invention aims to overcome, or at least reduce, the aforementioned problems in the prior art so as to provide high quality graphene-based sensors and methods for their manufacture, or to at least provide a commercially viable alternative thereto.

[0014] Thus, a first aspect of the present invention provides an electrochemical sensor for the detection of an analyte in a sample, the sensor comprising:

[0015] (a) a substrate having a crystalline growth surface;

[0016] (b) a graphene layer structure on the growth surface;

[0017] (c) optionally a barrier layer on the graphene layer structure;

[0018] (d) a conformal or substantially conformal zinc oxide layer on the graphene layer structure or, when present, on the barrier layer, the zinc oxide layer providing a sample-receiving surface having a crystallographic orientation sensitive to the analyte, the zinc oxide layer having a maximum thickness of less than 200 nm; and

[0019] (e) at least first and second metal contacts arranged to allow observation of a change in the electrical properties of the graphene layer structure.

[0020] A second aspect of the present invention also provides a method for the manufacture of an electrochemical sensor for the detection of analyte in a sample, the method comprising:

[0021] (a) providing a substrate having a crystalline growth surface;

[0022] (b) forming a graphene layer structure on the growth surface in an MOCVD reactor;

[0023] (c) optionally forming a barrier layer on the graphene layer structure;

[0024] (d) forming a conformal or substantially conformal zinc oxide layer on the graphene layer structure or, when present, on the barrier layer, the zinc oxide layer providing a sample-receiving surface having a crystallographic orientation sensitive to the analyte, the zinc oxide layer having a thickness of less than 200 nm; and

[0025] (e) forming at least first and second metal contacts in an arrangement to allow observation of a change in the electrical properties of the graphene layer structure.

[0026] The present disclosure will now be described further. In the following passages, different aspects / embodiments of the disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. As such, it will be appreciated that the method of the second aspect described herein may manufacture the electrochemical sensor of the first aspect. Equally, the electrochemical sensor may be obtained by the method of manufacture.

[0027] The present invention relates to an electrochemical sensor. An electrochemical sensor is intended to refer to an electronic chemical sensor in which detection of the presence of an analyte (or confirmation of its absence) in a sample is determined by observing the electrical properties of the graphene layer structure. As such, an electrochemical sensor may refer to, for example, a chemiresistive sensor, which is typically a two electrode sensor which measures the change in conductivity / resistivity of the graphene layer structure, or a FET-type sensor, which typically is a three electrode sensor which may measure further properties, such as electric current, threshold voltage and swing rate. Due to the linear band structure around the Dirac point in pristine graphene, the conductance of graphene is very sensitive to electronic changes in its immediate environment. In a particularly preferred embodiment, the sensor is a gas sensor for detecting the presence of an analyte in a gaseous sample.

[0028] The sensor comprises a substrate having a crystalline growth surface. The crystalline surface is generally a non-metallic growth surface, and in accordance with the method of manufacture, involves formation of the graphene layer structure directly onto the growth surface. As such, the non-metallic surface may in some embodiments be a semiconducting surface, and in other preferred embodiments an insulating surface.

[0029] Preferably, the crystalline growth surface of the substrate is selected from the group consisting of yttria-stabilised zirconia (YSZ), CaF2, AIN, sapphire (aluminium oxide), silicon oxide, silicon nitride, or a rare-earth oxide, more preferably YSZ, sapphire, or a rare-earth oxide. Such materials are also particularly suitable for forming by MOCVD, particularly on a silicon support and / or the rare-earth oxide, and as described further herein, may also be formed in-situ before formation of the graphene layer structure.

[0030] In some embodiments, the substrate may consist of one such material (sapphire being one preferred example). In some preferred embodiments, the substrate comprises, or consists of, a first layer, which provides the non-metallic growth surface, on a “substrate support” layer. Preferably, the substrate support layer comprises silicon. A silicon support layer, includes a “pure” silicon wafer (essentially consisting of silicon, doped or undoped) or what may be referred to as a CMOS wafer which includes additional associated circuitry. In one specific example of such circuitry, as described further herein, the support may comprise a resistive heater. A resistive heater embedded within the substrate may, for example, be formed of metals such as titanium, gold and / or platinum.

[0031] First layers comprising or consisting of rare earth oxides (which preferably include and are selected from yttrium, erbium and / or scandium oxides) are particularly preferably provided on a silicon substrate support. Scandium oxide, for example, is particularly preferred as a growth surface for the direct formation of high quality graphene thereon. Such substrates are preferred for devices such as gas sensors. The thickness of the substrate support layer is generally much thicker than the thickness of the first layer thereon. Typically, the substrate support layer has a thickness of 250 pm to 1 .5 mm, for example from 400 pm to 1 mm. On the other hand, the thickness of the first layer of such a substrate is substantially thinner and may be formed on the substrate support by epitaxy such as molecular beam epitaxy (MBE) or high temperature sputtering. Preferably, the thickness is at least 2 nm, preferably at least 5 nm and / or less than 500 nm, preferably less than 100 nm. Suitable ranges for the thickness of the first layer are preferably 5 nm to 100 nm, preferably 10 to 50 nm.

[0032] The sensor comprises a graphene layer structure on the growth surface. The method comprises directly forming the graphene layer structure on a growth surface of the substrate by CVD, and more particularly, in an MOCVD reactor. That is, the graphene layer structure of the sensor may therefore be described as a CVD-grown graphene layer structure grown directly on the growth surface).

[0033] Forming the graphene layer structure directly on the substrate avoids steps such as physical transfer which can otherwise introduce impurities and / or defects which does not allow for the conformal growth for high quality zinc oxide thereon.

[0034] For example, direct formation avoids using transfer polymers which are difficult to remove. A person skilled in the art can readily ascertain whether the graphene layer structure is one that has been grown directly on the substrate by CVD. This may be determined using conventional techniques in the art such as atomic force microscopy (AFM) and energy dispersive X-ray (EDX) spectroscopy. The graphene layer structure is devoid of copper contamination and devoid of organic polymer residues by virtue of the complete absence of contacting these materials with the graphene in the process (graphene being commonly grown by CVD indirectly on a sacrificial catalytic metal substrate - typically copper or other metals such as nickel - before transfer to a non-metallic surface). Furthermore, such transfer processes are generally not suitable for large scale manufacture (such as on silicon based substrates in fabrication plants), and are not economical.

[0035] Forming may be considered synonymous with synthesising, manufacturing, producing, depositing and growing. Graphene is a very well-known two-dimensional material referring to an allotrope of carbon comprising a single layer of carbon atoms in a hexagonal lattice. Graphene, as used herein, refers to a graphene layer structure, preferably having from 1 to 10 monolayers of graphene. Accordingly, the sensor may, and preferably does, comprise a monolayer of graphene, though other embodiments may nevertheless comprise multilayer graphene. Few layer (e.g. 2 or 3 layer graphene) or monolayer graphene is particularly preferred in view of its unique electronic properties which make it especially suitable for sensing applications.

[0036] The graphene layer structure may also be ‘Stranski-Krastanov’-type graphene, that is, a monolayer topped with additional multilayer grains or islands of carbon across the surface. Such extra material may help with the nucleation of the zinc oxide layer that is formed thereon.

[0037] CVD refers generally to a range of chemical vapour deposition techniques, each of which involve deposition to produce thin film materials such as two-dimensional crystalline materials like graphene, optionally under vacuum / reduced pressure. Volatile precursors, those in the gas phase or suspended in a gas, are decomposed to liberate the necessary species to form the desired material, carbon in the case of graphene.

[0038] CVD as described herein is intended to refer to thermal CVD such that the formation of graphene from the decomposition of a carbon-containing precursor is the result of the thermal decomposition of said carbon-containing precursor. A CVD layer formed directly on a surface can be distinguished from one transferred, either due to impurities or other defects such as cracks and wrinkles.

[0039] The method involves forming graphene by thermal CVD such that decomposition is a result of heating the carbon-containing precursor. Preferably, the temperature of the growth surface during CVD (i.e. wafer temperature) is from 700°C to 1 ,350°C, preferably from 800°C to 1 ,250°C, more preferably from 1 ,000°C to 1 ,250°C. The inventors have found that such temperatures are particularly effective for providing graphene growth directly on the materials described herein by CVD. Preferably, the CVD reaction chamber used in the method disclosed herein is a cold-walled reaction chamber wherein a heater coupled to the substrate is the only source of heat to the chamber.

[0040] As will be appreciated by those skilled in the art, the temperature setting input to some CVD reactors (i.e. set temperature) will generally be greater than the actual wafer temperature (such as with MOCVD reactors available from Aixtron®). The set temperature may be 100°C (or more) greater than the wafer temperature, for example from 1 ,300°C to 1 ,400°C. The wafer temperature may be measured using conventional techniques, for example using an optical probe. Other apparatuses may have a temperature feedback control whereby the reactor achieves the same wafer temperature as the input temperature (such as with high rotation rate MOCVD reactors available from Veeco®).

[0041] In a particularly preferred embodiment, the CVD reaction chamber comprises a close-coupled showerhead having a plurality, or an array, of precursor entry points. Such CVD apparatus comprising a close-coupled showerhead may be known for use in MOCVD processes. Accordingly, the method may alternatively be said to be performed using an MOCVD reactor comprising a close-coupled showerhead. In either case, the showerhead is preferably configured to provide a minimum separation of less than 100 mm, more preferably less than 25 mm, even more preferably less than 10 mm, between the surface of the substrate and the plurality of precursor entry points. As will be appreciated, by a constant separation it is meant that the minimum separation between the surface of the substrate and each precursor entry point is substantially the same. The minimum separation refers to the smallest separation between a precursor entry point and the substrate surface. Accordingly, such an embodiment involves a “vertical” arrangement whereby the plane containing the precursor entry points is substantially parallel to the plane of the substrate surface (i.e. the growth surface). The precursor entry points into the reaction chamber are preferably cooled. The inlets, or when used, the showerhead, are preferably actively cooled by an external coolant, for example water, so as to maintain a relatively cool temperature of the precursor entry points such that the temperature of the precursor as it passes through the plurality of precursor entry points and into the reaction chamber is less than 100°C, preferably less than 50°C. For the avoidance of doubt, the addition of precursor at a temperature above ambient does not constitute heating the chamber, since it would be a drain on the temperature in the chamber and is responsible in part for establishing a temperature gradient in the chamber.

[0042] Preferably, a combination of a sufficiently small separation between the substrate surface and the plurality of precursor entry points and the cooling of the precursor entry points, coupled with the heating of the substrate to with a decomposition range of the precursor, generates a sufficiently steep thermal gradient extending from the substrate surface to the precursor entry points to allow graphene formation on the substrate surface. As disclosed in WO 2017 / 029470, very steep thermal gradients may be used to facilitate the formation of high-quality and uniform two-dimensional material layers directly on non-metallic substrates, preferably across the entire surface of the substrate. The substrate may have a diameter of at least 5 cm (2 inches), at least 15 cm (6 inches) or at least 30 cm (12 inches). Particularly suitable apparatus for the method described herein include an Aixtron® Close-Coupled Showerhead® reactor and a Veeco® TurboDisk reactor.

[0043] Consequently, in a particularly preferred embodiment wherein the method of the present invention involves using a method as disclosed in WO 2017 / 029470, forming a graphene layer structure directly on a substrate by CVD comprises: providing the growth substrate on a heated susceptor in a close-coupled reaction chamber, the close-coupled reaction chamber having a plurality of cooled inlets arranged so that, in use, the inlets are distributed across the growth surface and have constant separation from the substrate; cooling the inlets to less than 100°C (i.e. so as to ensure that the precursor is cool as it enters the reaction chamber); introducing a carbon-containing precursor in a gas phase and / or suspended in a gas through the inlets and into the close-coupled reaction chamber; and heating the susceptor to achieve a growth surface temperature of at least 50°C in excess of a decomposition temperature of the precursor, to provide a thermal gradient between the substrate surface and inlets that is sufficiently steep to allow the formation of graphene from carbon released from the decomposed precursor; wherein the constant separation is less than 100 mm, preferably less than 25 mm, even more preferably less than 10 mm. The rotation rate of the heated susceptor in a close-coupled reaction chamber is typically less than 300 rpm, or even less than 200 rpm. In another particularly preferred embodiment wherein the method involves using a method as disclosed in WO 2019 / 138231 , forming a graphene layer structure directly on a substrate by CVD comprises: providing the growth substrate on a heated susceptor in a reaction chamber, the reaction chamber having a plurality of inlets arranged so that, in use, the inlets are distributed across the growth surface and have constant separation from the substrate; rotating the heated susceptor at a rotation rate of at least 600 rpm, preferably up to 3000 rpm; introducing a carbon-containing precursor in a gas phase and / or suspended in a gas through the inlets and into the reaction chamber; and heating the susceptor to achieve a growth surface temperature of at least 50°C in excess of a decomposition temperature of the precursor; wherein the constant separation is at least 12 cm, preferably up to 20 cm.

[0044] The most common carbon-containing precursor in the art for graphene growth is methane (CF ). The inventors have found that it is preferable that the carbon-containing precursor used to form graphene is an organic compound, that is, a chemical compound, or molecule, that contains a carbon-hydrogen covalent bond, which comprises two or more carbon atoms. The carbon-containing precursor is preferably a C3-C10 organic compound consisting of carbon and hydrogen and, optionally, oxygen, nitrogen, fluorine, chlorine and / or bromine, even more preferably a C6-C9 organic compound. In a preferred embodiment, the precursor does not comprise a heteroatom, such that the precursor consists of carbon and hydrogen. In other words, preferably the carbon-containing precursor is a hydrocarbon, preferably an alkane. It is also preferable that the organic compound comprise at least two methyl groups (-CH3).

[0045] The high quality “pristine” graphene produced by such CVD methods provides for a sensor which may benefit from the unique electronic properties of the graphene, whilst synergistically working with the zinc oxide layer as both a sensing layer and a protective layer.

[0046] In some embodiments, the sensor further comprises a barrier layer on the graphene layer structure. Where present, the barrier layer preferably has a thickness of less than 10 nm, and in some preferred embodiments less than 5 nm. Preferably, the barrier layer is an inorganic nitride, preferably boron, aluminium, gallium or silicon nitride. Hexagonal boron nitride is one example of a particularly preferred barrier layer and, like graphene, is a known two-dimensional material perse. The barrier layer can help to encapsulate the graphene layer structure to aid in protection from atmospheric contamination, and indeed critically the analyte during use. Due to the exceptional sensitivity of pristine graphene to its environment, encapsulation of the surface of the graphene, and ideally its edges, provides for a more reliable and robust device. A conformal (i.e. continuous) layer of h-BN, for example, may have a thickness in the range of from 5 to 10 nm. Thinner layers are preferred in order to reduce any detrimental effect of screening the electric field from the graphene, though this depends on the dielectric constant of the barrier material. The presence of a barrier layer allows for a substantially conformal zinc oxide layer where full encapsulation of the graphene layer structure is not achieved by the zinc oxide layer. Furthermore, unlike the charge neutral mono-elemental graphene, the barrier layer may assist in zinc oxide nucleation and growth of the preferred crystallographic orientation (for example, due to presence of an electric dipole at the surface of a hetero-elemental material), similar to that provided by the crystallographic orientation of the growth surface of the substrate.

[0047] The sensor further comprises a conformal or substantially conformal zinc oxide layer on the graphene layer structure, the zinc oxide layer providing a sample-receiving surface. The zinc oxide layer advantageously has a thickness of less than 200 nm (i.e. the zinc oxide layer is a thin film obtained by the epitaxial growth described herein in which the crystallographic orientation of the metal oxide layer is directed by the orientation of the substrate). As will be appreciated, where the optional barrier layer is present, the zinc oxide is formed on the barrier layer.

[0048] The method described herein allows for the formation of a relatively thin zinc oxide layer. The zinc oxide layer may also be referred to herein as a sensing layer since the surface of the zinc oxide is exposed and in use, contacts a sample (which is generally a liquid or gaseous sample). The samplereceiving surface of the zinc oxide layer has a crystallographic orientation that is selected to be selective to particular analytes of interest. Upon interaction of the zinc oxide surface with relevant analyte to be detected, the change in electronic environment induces a change in the electronic properties in the underlying graphene layer structure (in what may be referred to as charge coupling). A relatively thin layer of less than 200 nm allows for the greater electronic interaction between the sensing layer and the graphene transducer, improving sensitivity of the device. Such sensors may be used for quantitative determination of the analyte concentration in the sample. Preferably, the zinc oxide layer has a minimum thickness of at least 3 nm, preferably at least 5 nm (particularly so as to provide a conformal layer) and preferably the maximum thickness is less than 100 nm. By way of example, the minimum and maximum thicknesses of the zinc oxide layer may be in the range of from 20 to 40 nm. It is particularly preferred that the zinc oxide layer has a substantially uniform thickness (i.e. planar surface). Without wishing to be bound by theory, the uniform thickness provides a uniform separation of the sensing surface from the graphene layer structure improving device performance. In other embodiments described herein, the zinc oxide layer may be directionally etched to expose the desired crystallographic orientation (i.e. crystal plane). Such etching provides a substantially ordered textured surface (e.g. regular pyramids), with a textured surface advantageously increasing the surface area of the sample-receiving surface. However, etching can expose other crystal planes which can be problematic, though these can be coated with a dielectric material using photolithography (e.g. using a mask to first protect the desired plane before dielectric deposition and stripping the mask). In one preferred embodiment of the sensor of the present invention, the electrochemical sensor is for the detection of acetone and the crystallographic orientation of the sample-receiving surface provided by the zinc oxide is c-plane, that is, (0001 ). Zinc oxide is known to have an absorption energy in the (0001 ) orientation that matches that for acetone. This means that it can be used to highly selectively detect the presence of, ketones, particularly acetone, in a gas sample. In another preferred embodiment, the electrochemical sensor is for the detection of hydrogen sulfide (H2S) and the crystallographic orientation of the sample-receiving surface provided by the zinc oxide is m-plane, that is, (10-10). As is known in the art, zinc oxide adopts a hexagonal crystal structure. In order to function selectively, it is then also important that the zinc oxide layer has a good crystallographic consistency and is not amorphous. Generally a majority of the surface of the zinc oxide layer will adopt a single crystallographic orientation as described herein, more particularly at least about 90%, preferably at least about 95%. More preferably, the entire sample-receiving surface of the zinc oxide layer adopts the crystallographic orientation sensitive to the analyte. The degree to which the surface adopts a particular orientation may be readily measured by conventional techniques in the art, including by X- ray diffraction. The zinc oxide layer may nevertheless be polycrystalline whilst the sample-receiving surface still provides substantially a single crystal plane.

[0049] The inventors have found that they can grow a zinc oxide sensing layer with the desired crystallographic orientation on (or over) graphene. This has a benefit that through careful selection of the substrate, for example, they can perform remote epitaxy to improve the zinc oxide crystal structure. This means that they can grow a thinner layer than might otherwise be expected. The thinner the zinc oxide, the more sensitive the final device. It is, however, critical that the zinc oxide is conformal to protect the graphene during use, although this can be aided with the growth of an intervening barrier layer as described herein. The inventors have found that manufacture of the device has been made possible through the use of CVD-grown graphene, grown directly onto the substrate surface since the absence of defects and contamination that is inevitably present in other graphene would otherwise lead to defects in the zinc oxide layer, particularly in view of the high temperatures required for epitaxial growth.

[0050] In preferred embodiments, the crystallographic orientation of the crystalline growth surface is selected to direct the orientation of the zinc oxide layer. For example, where a uniform zinc oxide layer is desired with a c-plane surface, such as for acetone sensing, it is preferred that the growth surface of the underlying substrate is also hexagonal (0001 ) (such as with sapphire or AIN) or cubic (111 ) (such as with YSZ, CaF2 or rare-earth oxides such as SC2O3). The cubic (111 ) orientation has a three-fold rotational symmetry which promotes growth along the six-fold rotationally symmetric c-plane by remote (or van der Waals) epitaxy. Where a uniform zinc oxide layer is desired with an m-plane surface, preferably the growth surface of the substrate is also hexagonal (10-10) or cubic (100). These alternative orientations may be provided by the same materials described above. Whilst YSZ wafers may be commercially available with the desired crystallographic orientation, these may alternatively be provided by epitaxial growth on a silicon wafer of the same crystallographic orientation, the growth surface generally being single crystalline. By way of example, the substrate may be Si(100) / YSZ(100) or Si(111 ) / Sc2O3(111 ).

[0051] The sensor of the present invention therefore provides a selective device with a high sensitivity (through a thin but conformal layer of zinc oxide and a pristine graphene layer structure) together with fast response times (resulting from the high electronic mobility afforded by graphene). It is critical that the graphene layer structure is protected from the atmosphere and the inventors have found that this can be achieved especially through high temperature growth (e.g. MOCVD) of the protecting layers (i.e. the barrier and metal oxide layers).

[0052] In some embodiments, the electrochemical sensor does not comprise the barrier layer and it is then preferred that the zinc oxide layer is conformal. In other embodiments, the electrochemical sensor does comprise the barrier layer and preferably the zinc oxide layer covers at least 95% of a surface area of the barrier layer, more preferably at least 98%.

[0053] In respect of the method of manufacture, together with step (b) of forming the graphene layer structure, it is preferred that one or both of forming steps (c) and (d), are each performed in an MOCVD reactor, and which may more preferably be formed in-situ in a single MOCVD reactor. It is also preferred that the graphene is maintained under a substantially inert (i.e. oxygen and moisture free) environment between formation and deposition of the further layers thereon. For example, the substrate / wafer may be kept under an atmosphere of nitrogen or argon.

[0054] It is an advantage of the present invention that a conformal, or substantially conformal, zinc oxide layer may be formed without damage to the graphene by high temperature deposition techniques, and so as to provide the desired crystal quality of the zinc oxide needed for the sensing applications. It is generally preferred that the forming step (d) is performed at a temperature above 700°C, and in some embodiments above 900°C and / or up to 1 ,200°C, preferably up to 1 ,100°C. The barrier layer, when present, may be formed at a temperature of from 25°C to 400°C.

[0055] In one preferred embodiment, the forming step (d) is performed in the steps of:

[0056] (d’) depositing a metal zinc layer; and

[0057] (d”) oxidising the metal zinc layer; and optionally repeating steps (d’) and (d”) one or more times.

[0058] Such a method allows for the deposition of a metal zinc layer before introduction of oxygen, or an oxygen containing species (e.g. oxygen or H2O) for the formation of the zinc oxide, especially in the absence of a barrier layer prior to zinc oxide layer formation. Suitable precursors are well-known to those skilled in the art. For example, zinc oxide may be formed using a dialkyl zinc precursor, preferably dimethyl zinc. Following deposition of the metal zinc layer, repeating steps (d’) and (d”) may be carried out step-wise in an ALD process, or alternatively, zinc and oxygen precursors may be introduced into the reaction chamber simultaneously in an MOCVD process. ALD is particularly preferred.

[0059] The zinc oxide layer may also be formed by evaporation or sputtering techniques. In another preferred embodiment, the forming step (d) is performed by evaporation of zinc oxide onto the surface of the graphene layer structure, and then annealing the zinc oxide to form the zinc oxide layer (for example by a rapid thermal anneal). Annealing of the zinc oxide may be performed at temperatures above about 400°C, and may be formed below 900°C. Since evaporation is not a reactive process (i.e. the process involves evaporation of zinc oxide from a zinc oxide target), there is also less risk of graphene damage by such methods.

[0060] The sensor further comprises at least first and second metal contacts arranged to allow observation of a change in the electrical properties of the graphene layer structure. As will be appreciated, the contacts are arranged such that the zinc oxide layer lies therebetween. Metal electrical contacts may be deposited by any conventional technique in the art, and may be formed of one or more metals such as chromium, titanium, aluminium, nickel, platinum and / or gold.

[0061] The metal contacts are preferably provided in direct contact with the graphene layer structure. For example, each metal contact may be provided in contact with an edge of the graphene layer structure, and may further extend onto the adjacent upper surface of the graphene layer structure. Alternatively, the contacts may be provided entirely on the upper surface of the graphene layer structure. In other embodiments, the contacts are not in direct contact, but may be separated by a thin dielectric layer whilst still allowing for current flow (e.g. separated by the thin barrier layer). It is also generally preferred that the first and second metal contacts do not contact the zinc oxide layer.

[0062] The layers and contacts of the sensor may be patterned by conventional techniques during manufacture, such as by photolithography and / or using masks.

[0063] In some embodiments, the sensor preferably further comprises a dielectric coating layer which may be etched to expose a window through to the zinc oxide sample-receiving surface. Such a dielectric layer may be required where the metal contacts do not contact the zinc oxide layer and serves to fully encapsulate the graphene layer structure, together with the contacts, the zinc oxide layer, and barrier layer if present.

[0064] In other preferred embodiments, the zinc oxide is deposited after the dielectric layer, through a window thereof so as to be deposited on the graphene layer structure (or barrier layer). That is, the method may comprise masking and patterning the graphene layer structure (optionally simultaneously with the barrier layer if present) into given device area by removing the graphene where it is not required (i.e. patterning the graphene) and then depositing a layer of dielectric material. This may be deposited conformally across the wafer / substrate which then requires further patterning, or directly through a mask. The deposition of the dielectric material allows for the full encapsulation of the underlying graphene and its edges. The dielectric material is not particularly limited and any conventional dielectric material in the art may be used, typically a metal oxide, such as aluminium oxide or hafnium oxide. Typically, the thickness of such layer may be at least 20 nm, such as from 50 nm to 200 nm. Such a thickness typically also provides a preferred separation between the contacts and the zinc oxide in the final device of an equivalent distance. The metal contacts may then be deposited, preferably in direct contact with the graphene layer structure. That is, the dielectric layer may be patterned by photolithography if required (e.g. if deposited across the wafer) to expose a portion of the graphene for deposition of the contacts. The dielectric layer is also patterned to provide a window (or opening) between the positions of the first and second contacts for the zinc oxide layer which is then deposited and patterned.

[0065] In some preferred embodiments, the electrochemical sensor further comprises a third metal contact arranged under the graphene and within the substrate as a gate electrode. In a generally applicable and preferred embodiment, the electrochemical sensor further comprises a heater for regenerating the sample-receiving surface, preferably a resistive heater within the substrate. As described above, a metal layer such a platinum may provide a suitable layer embedded within the substrate whereby resistive heating may be used to heat the zinc oxide layer to a sufficient temperature to desorb the analyte.

[0066] In a further aspect, the present invention provides a device comprising one or more electrochemical sensors as described herein, each of the electrochemical sensors formed on a common substrate. As such, each of the plurality of electrochemical sensors are generally identical, with multiple sensor readings allowing for an improvement in accuracy of detection. In some preferred embodiments, the device further comprises a “reference” which is equivalent to the electrochemical sensor, but which does not comprise the zinc oxide layer. Thus the reference is an unfunctionalised graphene sensor. Subsequent analysis and quantification of the detected analyte may then be performed based on a difference in the measurement(s) from the electrochemical sensor(s) and the reference.

[0067] A further aspect of the present invention provides a use of the electrochemical sensor described herein, in which the sample-receiving surface has a c-plane (0001 ) crystallographic orientation, for the detection of acetone in a breath sample. It is well-known that human breath samples comprise acetone as a result of normal metabolic processes. Patients with diabetes are known to have a higher concentration of acetone in their breath, with the severity of the disease correlating with the concentration of acetone, with the concentration typically ranging from 0.9 to 1 .8 ppm. The sensor of the present invention with a selective zinc oxide layer can be used to detect acetone in the breath sample of a patient and may therefore be used to help diagnose diabetes. Detection of acetone as a volatile organic compound is nevertheless commercially important and relevant for safety in many industrial applications for environmental monitoring and the electrochemical sensor is suitable for use in such applications.

[0068] In a further aspect, the present invention provides a use of the electrochemical sensor described herein, in which the sample-receiving surface has an m-plane (10-10) crystallographic orientation, for the detection of hydrogen sulfide in a gas stream of a fuel cell. Hydrogen sulfide is known to be poisonous to fuel cells, as well as being a noxious gas that is a common byproduct of the decomposition of organic matter, or of industrial processes (such as natural gas extraction and metal refining). As such, early detection of hydrogen sulfide at low levels is required for environmental airquality monitoring, and when used in fuel cells can help to extend the cell lifetime by avoiding contact of the contaminated gas with the cell.

[0069] The present invention will now be described further with reference to the following non-limiting Figure, in which:

[0070] Figure 1 is a schematic cross-section of a sensor according to an exemplary embodiment of the present invention.

[0071] Figure 1 illustrates an exemplary electrochemical sensor 100. The sensor 100 comprises a substrate formed of a silicon support 105 and a layer of metal oxide 1 10 thereon (e.g. YSZ or scandium oxide). The silicon support may be a commercially available silicon wafer, the surface 105’ of which has the desired crystallographic orientation (e.g. (1 1 1 )). The metal oxide layer 1 10 may be epitaxially grown on the silicon support 105 providing a surface 1 10’ having a corresponding crystallographic orientation (e.g. (1 1 1 )).

[0072] The sensor 100 further comprises a graphene monolayer 1 15 on the surface 1 10’ of the substrate, the graphene monolayer having grown in an MOCVD reaction chamber by a method in accordance with WO 2017 / 029470. The sensor further comprises a uniform thickness zinc oxide layer 120 grown by remote (van der Waals) epitaxy at high temperature (such as by MOCVD) to provide a samplereceiving surface 120’ having a crystallographic orientation which is directed by the crystallographic orientation of surface 1 10’. Accordingly, the sample-receiving surface may have a c-plane (0001 ) crystallographic orientation. The zinc oxide layer is conformal (i.e. continuous) across the graphene monolayer 1 15 and subsequently patterned away from the edges of the graphene monolayer 1 15. The exemplary sensor 100 does not comprise a barrier layer. First and second metal contacts 125a, 125b are provided in direct contact with opposite edges of the graphene monolayer 115 with the zinc oxide layer 120 arranged therebetween, each contact extending onto the adjacent upper surface of the graphene monolayer. Sensor 100 further comprises a patterned dielectric layer 130 which ensures complete encapsulation of the graphene monolayer 115, coating the first and second metal contacts 125a, 125b, as well as edges and adjacent portions of the upper surface of the zinc oxide layer 120.

[0073] As used herein, the singular form of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of “consisting essentially of” (intended to mean that specific further components can be present provided they do not materially affect the essential characteristic of the described feature) and “consisting of” (intended to mean that no other feature may be included such that if the components were expressed as percentages by their proportions, these would add up to 100%, whilst accounting for any unavoidable impurities), unless the context clearly dictates otherwise.

[0074] It will be understood that, although the terms “first”, “second”, etc. may be used herein to describe, for example, various elements, layers and / or portions, the elements, layers and / or portions should not be limited by these terms. These terms are only used to distinguish one element, layer or portion from another, or a further, element, layer or portion. It will be understood that the term “on” is intended to mean “directly on” such that there are no intervening layers between one material being said to be “on” another material. Spatially relative terms, such as “under”, “below”, “beneath”, “lower”, “over”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s). It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device as described herein is turned over, elements described as “under” or “below” other elements or features would then be oriented “over” or "above" the other elements or features. Thus, the example term "under" can encompass both an orientation of over and under. The device may be otherwise oriented and the spatially relative descriptors used herein interpreted accordingly.

[0075] Numerical lower and upper limits of features described herein may preferably be combined to provide a closed range.

[0076] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents. For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.

Claims

Claims:1 . An electrochemical sensor for the detection of an analyte in a sample, the sensor comprising:(a) a substrate having a crystalline growth surface;(b) a graphene layer structure on the growth surface;(c) optionally a barrier layer on the graphene layer structure;(d) a conformal or substantially conformal zinc oxide layer on the graphene layer structure or, when present, on the barrier layer, the zinc oxide layer providing a sample-receiving surface having a crystallographic orientation sensitive to the analyte, the zinc oxide layer having a maximum thickness of less than 200 nm; and(e) at least first and second metal contacts arranged to allow observation of a change in the electrical properties of the graphene layer structure.

2. The electrochemical sensor according to claim 1 , wherein the crystalline growth surface of the substrate is selected from the group consisting of YSZ, CaF2, AIN, sapphire, silicon oxide, silicon nitride, or a rare-earth oxide, preferably YSZ, sapphire, or a rare-earth oxide.

3. The electrochemical sensor according to claim 2, wherein the substrate comprises a silicon support and a first layer thereon which provides the crystalline growth surface, preferably wherein the first layer is YSZ or a rare-earth oxide.

4. The electrochemical sensor according to any preceding claim, wherein the electrochemical sensor is for the detection of acetone and the crystallographic orientation of the sample-receiving surface is c-plane (0001 ).

5. The electrochemical sensor according to claim 4, wherein the crystalline growth surface has a crystallographic orientation which is hexagonal (0001 ) or cubic (111 ).

6. The electrochemical sensor according to any one of claims 1 to 3, wherein the electrochemical sensor is for the detection of hydrogen sulfide and the crystallographic orientation of the sample-receiving surface is m-plane (10-10).

7. The electrochemical sensor according to claim 6, wherein the crystalline growth surface has a crystallographic orientation which is hexagonal (10-10) or cubic (100).

8. The electrochemical sensor according to any preceding claim, which does not comprise the barrier layer and wherein the zinc oxide layer is conformal.

9. The electrochemical sensor according to any preceding claim, which does comprise the barrier layer and wherein the zinc oxide layer covers at least 95% of a surface area of the barrier layer.

10. The electrochemical sensor according to any preceding claim, wherein the zinc oxide layer has a minimum thickness of at least 3 nm, preferably at least 5 nm, and preferably a maximum thickness of less than 100 nm.11 . The electrochemical sensor according to any preceding claim, wherein the barrier layer has a thickness of less than 10 nm, preferably less than 5 nm.

12. The electrochemical sensor according to any preceding claim, wherein the barrier layer is an inorganic nitride, preferably boron, aluminium, gallium or silicon nitride.

13. The electrochemical sensor according to any preceding claim, further comprising a heater for regenerating the sample-receiving surface, preferably a resistive heater within the substrate.

14. The electrochemical sensor according to any preceding claim, wherein the first and second metal contacts are in direct contact on the graphene layer structure.

15. The electrochemical sensor according to any preceding claim, wherein the first and second metal contacts do not contact the zinc oxide layer.

16. The electrochemical sensor according to any preceding claim, further comprising a third metal contact arranged under the graphene and within the substrate as a gate electrode.

17. A method for the manufacture of an electrochemical sensor for the detection of analyte in a sample, the method comprising:(a) providing a substrate having a crystalline growth surface;(b) forming a graphene layer structure on the growth surface in an MOCVD reactor;(c) optionally forming a barrier layer on the graphene layer structure;(d) forming a conformal or substantially conformal zinc oxide layer on the graphene layer structure or, when present, on the barrier layer, the zinc oxide layer providing a sample-receiving surface having a crystallographic orientation sensitive to the analyte, the zinc oxide layer having a thickness of less than 200 nm; and(e) forming at least first and second metal contacts in an arrangement to allow observation of a change in the electrical properties of the graphene layer structure.

18. The method according to claim 17, wherein one or both of forming steps (c) and (d), are each performed in an MOCVD reactor, preferably in-situ in a single MOCVD reactor.

19. The method according to claim 17 or claim 18, wherein the forming step (d) is performed in the steps of:(d’) depositing a metal zinc layer; and(d”) oxidising the metal zinc layer; and optionally repeating steps (d’) and (d”) one or more times.

20. The method according to any of claims 17 to 19, wherein the forming step (d) is performed at a temperature above 700°C, preferably above 900°C.21 . The method according to claim 17, wherein the forming step (d) is performed by evaporation of zinc oxide onto the surface of the graphene layer structure, and then annealing the zinc oxide to form the zinc oxide layer.

22. The method according to any one of claims 17 to 21 , wherein forming step (d) further comprises directionally etching the zinc oxide layer to expose the sample-receiving surface having the crystallographic orientation.

23. Use of the electrochemical sensor according to claim 4 for the detection of acetone in a breath sample.

24. Use of the electrochemical sensor according to claim 6 for the detection of hydrogen sulfide in a gas stream of a fuel cell.

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