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WO2025224439A1 - Capteur électrochimique à base de graphène - Google Patents

Capteur électrochimique à base de graphène

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Publication number
WO2025224439A1
WO2025224439A1 PCT/GB2025/050857 GB2025050857W WO2025224439A1 WO 2025224439 A1 WO2025224439 A1 WO 2025224439A1 GB 2025050857 W GB2025050857 W GB 2025050857W WO 2025224439 A1 WO2025224439 A1 WO 2025224439A1
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WIPO (PCT)
Prior art keywords
zinc oxide
electrochemical sensor
graphene
layer
sensor according
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Pending
Application number
PCT/GB2025/050857
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English (en)
Inventor
Ivor GUINEY
Sebastian Dixon
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Paragraf Ltd
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Paragraf Ltd
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Publication of WO2025224439A1 publication Critical patent/WO2025224439A1/fr
Pending legal-status Critical Current
Anticipated expiration legal-status Critical

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
    • G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
    • G01N27/125—Composition of the body, e.g. the composition of its sensitive layer
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
    • G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
    • G01N27/129—Diode type sensors, e.g. gas sensitive Schottky diodes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/22—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance
    • G01N27/227—Sensors changing capacitance upon adsorption or absorption of fluid components, e.g. electrolyte-insulator-semiconductor sensors, MOS capacitors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/414—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/414—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS
    • G01N27/4141—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS specially adapted for gases
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/414—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS
    • G01N27/4146—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS involving nanosized elements, e.g. nanotubes, nanowires
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/22—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance
    • G01N27/221—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance by investigating the dielectric properties
    • G01N2027/222—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating capacitance by investigating the dielectric properties for analysing gases

Definitions

  • 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.
  • 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.
  • Nanoscale Adv., 2024, 6, 11 describes critical aspects when using nanomaterials as sensing substrates for the application in chemiresistive gas sensors.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • a first aspect of the present invention provides an electrochemical sensor for the detection of an analyte in a sample, the sensor comprising:
  • 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:
  • 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.
  • 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.
  • the sensor is a gas sensor for detecting the presence of an analyte in a gaseous sample.
  • 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.
  • the non-metallic surface may in some embodiments be a semiconducting surface, and in other preferred embodiments an insulating surface.
  • 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.
  • YSZ yttria-stabilised zirconia
  • CaF2 AIN
  • sapphire aluminium oxide
  • silicon oxide silicon nitride
  • 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.
  • the substrate may consist of one such material (sapphire being one preferred example).
  • the substrate comprises, or consists of, a first layer, which provides the non-metallic growth surface, on a “substrate support” layer.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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).
  • 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.
  • 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).
  • transfer processes are generally not suitable for large scale manufacture (such as on silicon based substrates in fabrication plants), and are not economical.
  • 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.
  • 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.
  • 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.
  • the method involves forming graphene by thermal CVD such that decomposition is a result of heating the carbon-containing precursor.
  • the temperature of the growth surface during CVD i.e. wafer temperature
  • the temperature of the growth surface during CVD 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 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.
  • the temperature setting input to some CVD reactors 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®).
  • the CVD reaction chamber comprises a close-coupled showerhead having a plurality, or an array, of precursor entry points.
  • 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.
  • 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.
  • 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.
  • an external coolant for example water
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the most common carbon-containing precursor in the art for graphene growth is methane (CF ).
  • 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.
  • the precursor does not comprise a heteroatom, such that the precursor consists of carbon and hydrogen.
  • the carbon-containing precursor is a hydrocarbon, preferably an alkane.
  • the organic compound comprise at least two methyl groups (-CH3).
  • 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.
  • the sensor further comprises a barrier layer on the graphene layer structure.
  • the barrier layer preferably has a thickness of less than 10 nm, and in some preferred embodiments less than 5 nm.
  • 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.
  • a conformal (i.e. continuous) layer of h-BN 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.
  • 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.
  • 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).
  • the zinc oxide is formed on the barrier layer.
  • 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.
  • 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).
  • 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.
  • 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).
  • the uniform thickness provides a uniform separation of the sensing surface from the graphene layer structure improving device performance.
  • 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.
  • 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).
  • 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.
  • 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).
  • H2S hydrogen sulfide
  • m-plane that is, (10-10).
  • zinc oxide adopts a hexagonal crystal structure.
  • 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.
  • 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.
  • the crystallographic orientation of the crystalline growth surface is selected to direct the orientation of the zinc oxide layer.
  • 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.
  • the growth surface of the substrate is also hexagonal (10-10) or cubic (100).
  • hexagonal (10-10) or cubic (100) may be provided by the same materials described above.
  • 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.
  • the substrate may be Si(100)/YSZ(100) or Si(111 )/Sc 2 O 3 (111 ).
  • 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).
  • MOCVD high temperature growth
  • 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%.
  • step (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.
  • the substrate/wafer may be kept under an atmosphere of nitrogen or argon.
  • 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.
  • the forming step (d) is performed in the steps of:
  • 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 H 2 O) for the formation of the zinc oxide, especially in the absence of a barrier layer prior to zinc oxide layer formation.
  • oxygen or an oxygen containing species
  • Suitable precursors are well-known to those skilled in the art.
  • zinc oxide may be formed using a dialkyl zinc precursor, preferably dimethyl zinc.
  • 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.
  • the zinc oxide layer may also be formed by evaporation or sputtering techniques.
  • 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.
  • 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.
  • 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.
  • each metal contact is preferably provided in direct contact with the graphene layer structure.
  • 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.
  • the contacts may be provided entirely on the upper surface of the graphene layer structure.
  • 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.
  • the layers and contacts of the sensor may be patterned by conventional techniques during manufacture, such as by photolithography and/or using masks.
  • the senor preferably further comprises a dielectric coating layer which may be etched to expose a window through to the zinc oxide sample-receiving surface.
  • 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.
  • 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.
  • 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.
  • the electrochemical sensor further comprises a third metal contact arranged under the graphene and within the substrate as a gate electrode.
  • the electrochemical sensor further comprises a heater for regenerating the sample-receiving surface, preferably a resistive heater within the substrate.
  • a metal layer such as 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.
  • 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.
  • each of the plurality of electrochemical sensors are generally identical, with multiple sensor readings allowing for an improvement in accuracy of detection.
  • the device further comprises a “reference” which is equivalent to the electrochemical sensor, but which does not comprise the zinc oxide layer.
  • 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.
  • 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.
  • 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.
  • 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).
  • 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.
  • Figure 1 is a schematic cross-section of a sensor according to an exemplary embodiment of the present invention.
  • FIG. 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 )).
  • 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.
  • 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.
  • 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).
  • 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.

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Abstract

L'invention concerne un capteur électrochimique conçu pour la détection d'un analyte dans un échantillon, le capteur comprenant : (a) un substrat pourvu d'une surface de croissance cristalline ; (b) une structure de couche de graphène sur la surface de croissance ; (c) éventuellement une couche barrière sur la structure de couche de graphène ; (d) une couche d'oxyde de zinc conforme ou sensiblement conforme sur la structure de couche de graphène ou, lorsqu'elle est présente, sur la couche barrière, la couche d'oxyde de zinc fournissant une surface de réception d'échantillon à orientation cristallographique sensible à l'analyte, la couche d'oxyde de zinc ayant une épaisseur maximale inférieure à 200 nm ; et (e) au moins des premier et second contacts métalliques agencés pour permettre l'observation d'un changement des propriétés électriques de la structure de couche de graphène.
PCT/GB2025/050857 2024-04-22 2025-04-22 Capteur électrochimique à base de graphène Pending WO2025224439A1 (fr)

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