EP3325958A1 - Sensor element for a chemical sensor and chemical sensor - Google Patents

Sensor element for a chemical sensor and chemical sensor

Info

Publication number
EP3325958A1
EP3325958A1 EP15898415.3A EP15898415A EP3325958A1 EP 3325958 A1 EP3325958 A1 EP 3325958A1 EP 15898415 A EP15898415 A EP 15898415A EP 3325958 A1 EP3325958 A1 EP 3325958A1
Authority
EP
European Patent Office
Prior art keywords
layer
sensor element
conducting layer
sensing layer
sensing
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP15898415.3A
Other languages
German (de)
French (fr)
Other versions
EP3325958A4 (en
Inventor
Yue Chau Garen KWAN
Ging Meng Ng
Liping Sharon CHIA
Rui Quan GOH
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Robert Bosch GmbH
Original Assignee
Robert Bosch GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Robert Bosch GmbH filed Critical Robert Bosch GmbH
Publication of EP3325958A1 publication Critical patent/EP3325958A1/en
Publication of EP3325958A4 publication Critical patent/EP3325958A4/en
Withdrawn legal-status Critical Current

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Classifications

    • 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
    • G01N27/126—Composition of the body, e.g. the composition of its sensitive layer comprising organic polymers

Definitions

  • the present invention relates to a sensor element for a chemical sensor and a chemical sensor.
  • TRANSISTORS “ (L. Xian, J. Ya-Dong, X. Guang-Zhong, D. Xiao-Song, T. Hui- Ling, Y. Jian-Fei, F. Song-Qi; in Apperceiving Computing and Intelligence Analysis (ICACIA), 2010, International Conference, pp. 470-473) describes the detection of different concentrations of ethanol in nitrogen gas using a single thin film of copper phthalocyanine as sensing material.
  • the present invention provides a sensor element for a chemical sensor with the features of claim 1 and a chemical sensor with the features of claim 10.
  • the present invention provides a sensor element/a chemical sensor that has a good sensitivity even at temperatures below 80°C. Thus, it is not necessary to provide any heating equipment for the operation of the sensor element/chemical sensor, as it is for instance often necessary for a conventional metal oxide sensor device. Furthermore, as it is not necessary to heat any part of the sensor element/chemical sensor during its operation, its power consumption is reduced compared to the prior art.
  • the conducting layer (comprising the at least one second material different from the at least one first material of the sensing layer) reduces the overall resistance of the sensor element/chemical sensor during its operation, while keeping its advantageous sensitivity (selectivity) to the at least one target analyte.
  • an electrical signal detected between the first electrode and the second electrode is enhanced.
  • This enhancement of the electrical signal (which can be in the miliiampere range, for instance) facilitates the processing of the sensor element/chemical sensor by current ASIC implementations.
  • a variety of different sensing materials with a good sensitivity for the at least one target analyte can be used as the at least one first material within the sensing layer.
  • the addition of the conducting layer (comprising the at least one second material different from the at least one first material of the sensing layer) hardly restricts the variety of sensing materials useable for the sensing layer.
  • the conducting layer comprises the at least one first material of the sensing layer in addition to the at least one second material.
  • the sensing layer comprises an inorganic metal oxide and/or a metal phthalocyanine as the at least one first material.
  • Inorganic metal oxide (MOx) sensing materials have good operation currents and a high sensitivity for at least one target analyte.
  • the addition of the conducting layer (comprising for instance reduced graphene oxide or graphene as the at least one second material) further reduces the operation temperature of the sensor element/chemical sensor comprising an inorganic metal oxide as sensing material.
  • sensing materials comprising metal phthalocyanine/organic metal phthalocyanine have become attractive due to their high selectivity and low operational temperatures, which reduce their power consumption during operation. Additionally, sensing materials comprising metal phthalocyanine/organic metal phthalocyanine can be tailored for use with different target analytes by modifying the functional side chains and/or metal center. Furthermore, by adding the conducting layer (comprising the at least one second material different from metal phthalocyanine/organic metal phthalocyanine) the output current is enhanced even though the metal phthalocyanine/organic metal phthalocyanine is used as the at least one first material.
  • the sensing layer comprises the metal phthalocyanine as the at least one first material and the conducting layer comprises the metal phthalocyanine doped with a dopant as the at least one second material.
  • the conducting layer comprises a conductor and/or a semiconductor as the at least one second material.
  • the conducting layer comprises tetraf I u ro-tetra cya no-quinodimethane, tetrathianaphthacene, zinc oxide, copper oxide, molybdenum trioxide and/or reduced graphene oxide as the at least one second material.
  • the materials mentioned in the preceding phrase do not have to be used within the sensing layer, they do not influence the selectivity of the sensor element/chemical sensor for the at least one target analyte.
  • the sensing layer comprises zinc phthalocyanine as the at least one first material and the conducting layer comprises reduced graphene oxide as the at least one second material.
  • the sensing layer comprises zinc phthalocyanine as the at least one first material and the conducting layer comprises the zinc phthalocyanine doped with molybdenum trioxide as the at least one second material.
  • the sensor element may also comprise an evaluator designed for detecting at least one physical value of the conducting layer and for determining and outputting an information about a presence and/or the amount of the at least one target analyte dependent on the at least one physical value of the conducting layer.
  • the information determined by the evaluator can be used for a variety of different applications, for instance in consumer applications.
  • Fig. 1 a to 1 d a first embodiment of the inventive sensor element, wherein
  • Fig. 1a and 1b show schematic drawings of the sensor element without and with its at least one target analyte, and wherein Fig. 1c and 1d show different changes in its Fermi level due to the presence of the at least one target analyte;
  • Fig. 3a to 3d different examples of measurements with other embodiments of the inventive sensor element.
  • Fig. 1 a to 1 d refer to a first embodiment of the inventive sensor element, wherein Fig. 1a and 1b show schematic drawings of the sensor element without and with its at least one target analyte, and wherein Fig. 1c and 1d show different changes in its Fermi level due to the presence of the at least one target analyte.
  • the sensor element 10 shown in Fig. 1a and 1 b is designed to be used in a chemical sensor (not shown).
  • the use of the sensor element 10 is not restricted to a certain type of chemical sensors.
  • the chemical sensor with the sensor element 10 can be a gas sensor, especially a solid state gas sensor.
  • the sensor element 10 may be used in a variety of different applications, such as consumer applications, particularly for consumer electronics.
  • the sensor element 10 may even be used for applications, for which the use of conventional analytic systems such as gas chromatography and optical detection methods are expensive and difficult to scale down.
  • a variety of different target analytes 12 may be detected by an operation of the sensor element 10.
  • the sensor element 10 has a substrate ⁇ not shown) with a first electrode 14a and a second electrode 14b.
  • the first electrode 14a and the second electrode 14b are designed for a voltage application between the first electrode 14a and the second electrode 14b.
  • a first conducting line 16a may be linked to the first electrode 14a, while a second conducing line 16b is electrically connected with the second electrode 4b. It is thus possible to apply a (changing) voltage V between the first electrode 14a and the second electrode 14b.
  • the sensor element 10 has a sensing layer 18.
  • the sensing layer 18 comprises at least a first material.
  • the at least one first material of the sensing layer 18 is not in the form of corns or granulates. Instead, the at least one first material is deposited as a layer-wise deposit.
  • the sensing layer 18 fills a (first) layer- wise volume (completely).
  • the sensing layer 18 is a layer without any hole or opening.
  • the sensing layer 18 may be deposited using any chemical or supplementation-evaporation technique.
  • the at least one first material of the sensing layer 18 is a material to which the at least one target analyte 12 may attach.
  • the at least one target analyte 12 may also be absorbed by the at least one first material or form a binding with the at least one first material.
  • the at least one first material of the sensing layer 18 may comprise functional side chains and/or receptors specifically binding the at least one target analyte 12.
  • an electric field E within the sensing layer 18 is changed/created dependent on an amount of the at least one target analyte 12
  • the sensor element 10 may be designed for at least one target analyte 12 chosen from a variety of different target analytes 12, a lot of different materials may be used as the at least one first material of the sensing layer 18.
  • the sensing layer 18 can be made either organic or inorganic or a combination of both.
  • the sensing layer 18 may comprise an inorganic metal oxide (MOx) and/or a metal phthalocyanine/organic metal phthalocyanine (mPc) as the at least one first material. Special examples for the at least one first material of the sensing layer 18 are given below.
  • the sensor element 10 also comprises a conducting layer 20.
  • the conducting layer 20, which comprises at least a second material different from the at least one first material of the sensing layer 18, provides an electrical connection between the first electrode 14a and the second electrode 14b.
  • the conducting layer 20 may spread between the first electrode 14a and the second electrode 14b.
  • the conducting layer 20 may also be suspended between the first electrode 14a and the second electrode 14b.
  • a current l 0 or l s may flow through the conducting layer 20 between the first electrode 14a and the second electrode 14b.
  • the conducting layer 20 is covered at least partially by the sensing layer 18.
  • the at least one second material of the conducting layer 20 is also not in the form of corns or granulates.
  • the at least one second material is deposited as a layer-wise deposit in a way that the conducting layer 20 fills a (second) layer-wise volume (completely).
  • the conducting layer 20 is a layer without any hole or opening.
  • any chemical or supplementation- evaporation technique may be performed.
  • the conducting layer 20 may comprise a conductor and/or a (doped) semiconductor as the at least one second material.
  • the conducting layer 20 consists either of a conductor (with a variable carrier concentration) or a semiconductor. In all these cases any change of the electric field E within the sensing layer 18, which covers the conducting layer 20 at least partially, increases or decreases a concentration of the electrons or holes within the conducting layer 20.
  • the at least one second material of the conducting layer 20 could be either organic, inorganic or a combination of both. Special examples of the at least one second material of the conducting layer 20 are given below.
  • Fig. 1a shows the sensor element 10 in the absence of its at least one target analyte 12.
  • Fig. 1 b shows the sensor element 10 in a situation when its at least one target analyte 12 is present in a surrounding of the sensing layer 18.
  • the at least one target analyte 12 is attached/absorbed/bound at least partially on and/or within the sensing layer 18 and the electric field E within the sensing layer 18 is changed/created dependent on the amount of the at least one target analyte 12 attached/absorbed/bound on and/or within the sensing layer 18.
  • Fig. 1 b shows the example of the formation of a dipole 22 schematically.
  • the change/creation of the electric field E within the sensing layer 18 results in a change of an electric field E or a Fermi level within the conducting layer 20.
  • the electric field E or the Fermi level is changed within the conducting layer 20 in case that the electric field E within the sensing layer 18 is changed/created.
  • the change of the electric field E or the Fermi level within the conducting layer 20 depends on the amount of the at least one target analyte 12
  • Fig. 1c and 1d show different changes in the Fermi level of the at least one second material of the conducting layer 20 due to the presence of the at least one target analyte 12. Shown are always a conduction band 24 and a valence band 26 of the conducting layer 20. According to the general understanding, the change of the electric field E (which is caused for instance by the creation of at least one dipole 22 between the at least one target analyte 12 and the at least one first material of the sensing layer 18) results in a shift 28 of the Fermi level (from a former Fermi level F f to a new Fermi level F n ). In the example of Fig.
  • the at least one second material of the conducting layer 20 is a conductor and therefore a shift 28 of the Fermi level into a region where the "conductor has more electrons" occurs, resulting in a reduction of resistance.
  • the at least one second material of the conducting layer 20 is a semiconductor and therefore a shift 28 of the Fermi level into a region where "more electrons are allowed to enter traps in the forbidden band" occurs, resulting in an increase of the "hole concentration" and a reduction of resistance.
  • a decrease of the resistance of the conducting layer 20 occurs in the presence of the at least one target analyte 12 .
  • This decrease of the resistance of the conducting layer 20 can be measured by a measurement of the current flow through the conducting layer 20, for instance.
  • the operation of the sensor element 10 shown in Fig. 1a and 1 b therefore does not require any measurement of a conductance, resistance, capacitance or potential of the sensing layer 18. Instead, a presence and/or an amount/concentration of the at least one target analyte 12 within the surrounding of the sensing layer 18 is detectable by a change of at least one physical value of the conducting layer 20.
  • a base line current l 0 is flowing through the conducting layer 20 in the absence of the at least one target analyte 12 in the surrounding of the sensing layer 18.
  • the presence of the at least one target analyte 12 within the surrounding of the sensing layer 18 and its interaction with the sensing layer 18 causes that a sensing current l s (different from the base line current l 0 ) flows through the conducting layer 20.
  • the current flow through the conducting layer 20 is changed due to the presence of the at least one target analyte 12 in the surrounding of the sensing layer 18, wherein the sensing current l s is normally dependent on the amount of the at least one target analyte 12
  • the sensing current l s also depends on the amount/concentration of the at least one target analyte 12 present in the surrounding of the sensing layer 18. It is therefore easy to detect the presence of the at least one target analyte 12 in the surrounding of the sensing layer 18 and/or to measure the
  • the sensor element 10 or the chemical sensor with the sensor element 10 may also comprise an evaluator (not shown).
  • the evaluator is designed for detecting the at least one physical value of the conducting layer 20 (for instance by measuring the current l 0 or l s ) and for determining and outputting an information about a presence and/or the amount/concentration of the at least one target analyte 12 dependent on the at least one physical value of the conducting layer 20.
  • An Evaluator fulfilling these requirements is easy to produce and does not require much volume. A lot of different materials can be used as the at least one second material of the conducting layer 20.
  • the conducting layer 20 may comprise tetrafluro-tetracyano- quinodimethane, tetrathianaphthacene, zinc oxide, copper oxide, molybdenum trioxide and/or reduced graphene oxide as the at least one second material.
  • the conducting layer 20 may comprise the at least one first material of the sensing layer 18 in addition to the at least one second material.
  • the at least one second material may be at least one dopant added to the at least one first material.
  • the addition of the conducting layer 20 facilitates the detection of the presence and/or the amount/concentration of the at least one target analyte 12. It is therefore possible to use a sensing material (as the at least one first material) within the sensing layer 18, even though the electric properties of this sensing material may be disadvantageous for a conventional (chemical) sensor.
  • a sensing material as the at least one first material
  • mPc metal phthaiocyanine
  • this obstacle is overcome by the addition of the conducting layer 20. This results in an overcome of the conventional limits to the read-out accuracy of conventional sensors.
  • All the examples for the at least one second material mentioned here provide an improvement of the current of the sensor element 10 by several orders of magnitude while retaining a low operating temperature (generally below 80°C) and the same selectivity for the at least one target analyte 12 as in the absence of the at least one conducting layer 20.
  • the materials mentioned here provide a high flexibility in material selection for specific use-case to be addressed by integration of different sensing materials (as the at least one first material) within the sensing layer 8.
  • Fig. 2a to 2c show different examples of measurements with other embodiments of the inventive sensor element.
  • the sensing layer 18 comprises a metal phthalocyanine (mPc) as the at least one first material (as sensing material).
  • the sensing layer 18 has a high thermal stability.
  • the use of a metal phthalocyanine as the at least one first material makes it easy to provide a molecular design of the at least one first material (as sensing material) that ensures a good interaction of the at least one first material with the target analyte 12. It is therefore ensured that the sensing layer 18 fulfills its role after an introduction of the target analyte 12.
  • the sensing layer 18 comprises zinc
  • phthalocyanine (ZnPc) as the at least one first material and/or the conducting layer 20 comprises reduced graphene oxide (rGO) as the at least one second material.
  • the sensing layer 18 consists of zinc phthalocyanine (ZnPc) as the at least one first material and/or the conducting layer 20 consists of reduced graphene oxide (rGO) as the at least one second material.
  • Nitrogen dioxide (N0 2 ) was used as the at least one target analyte 12 for the measurements shown in Fig. 2a to 2c.
  • the layer thicknesses of the sensing layer 18 and the conducting layer 20 differ in the examples of Fig. 2a to 2c.
  • the examples of Fig. 2a and 2b both have the sensing layer 18 as well as the conducting layer 20, wherein the layer thickness of the sensing layer 18 is 1 nm and the layer thickness of the conducting layer 20 is 5 nm in the example of Fig. 2a and the layer thickness of the sensing layer 18 is 5 nm and the layer thickness of the conducting layer 20 is 5 nm in the example of Fig. 2b.
  • the sensor element 10 has only the sensing layer 18 with a layer thickness of 60 nm, but no conducting layer 20.
  • the abscissae are timelines t (in seconds) and the ordi nates denote a current I (in milliampere or nanoampere) and a resistance R (in kiloohm).
  • Different concentrations c of nitrogen dioxide (N0 2 ) as the at least one target analyte 12 are given in ppb (parts per billion).
  • a sensitivity to nitrogen dioxide of inventive sensor elements 10 is tested by using 5 nm (nanometer) of reduced graphene oxide (rGO) as the conducting layer 20.
  • the thickness of the sensing layer 18 of zinc phthalocyanine (ZnPc) was chosen to be either 1 nm (nanometer) (example of Fig. 2a) or 5 nm (nanometer) (example of Fig. 2b).
  • ZnPc zinc phthalocyanine
  • the target analyte 12 is bound to the sensing layer 18 in both cases. This causes a charge transfer and a formation of at least on dipole 22.
  • This dipole formation creates a different electric field E that permeates the conducting layer 20, changing a carrier concentration within the conducting layer 20 and hence a change of its resistance R.
  • This change of resistance R is in the examples of Fig. 2a and 2b easily detectable by measuring the current I (in the milliampere range).
  • a sensing layer 18 with a layer thickness of 60 nm (nanometer) in a device without the conducting layer 60 resulted only in a current I in the nanoampere range.
  • a comparable sample with a layer thickness of 5 nm of zinc phthalocyanine but without the conducting layer 60 had current levels that were too low to be measured.
  • (dR/dt , is the rate of change of the resistance R before the addition of nitrogen dioxide (N0 2 ) as the at least one target analyte 12
  • (dR/dt) a is the rate of change of the resistance R after the addition of nitrogen dioxide (N0 2 ) as the at least one target analyte 12.
  • Table 1 gives the values of the sensitivity S for all the examples of Fig. 2a to 2c:
  • the sensitivity S is the most suited for the detection of low concentrations c of nitrogen dioxide (N0 2 ) (as the at least one target analyte 12) when a sensor element 10 with 1 nm of zinc phthalocyanine (ZnPc) together with 5 nm of reduced graphene oxide (rGO) is used.
  • N0 2 nitrogen dioxide
  • ZnPc zinc phthalocyanine
  • rGO reduced graphene oxide
  • Fig. 3a to 3d show different examples of measurements with another
  • the conducting layer 20 consists of the at least one first material of the sensing layer 18 doped with the at least one second material.
  • the sensing layer 18 comprises a metal phthalocyanine as the at least one first material and the conducting layer comprises the metal phthalocyanine doped with the dopant as the at least one second material.
  • the conductivity of such a conduction layer 20 changes due to the change/creation of the electric field E within the sensing layer 18.
  • the sensing layer 18 comprises zinc phthalocyanine (ZnPc) as the at least one first material and the conducting layer 20 comprises the zinc phthalocyanine (ZnPc) doped with molybdenum trioxide ( o0 3 ) as the at least one second material.
  • ZnPc zinc phthalocyanine
  • the conducting layer 20 consists of zinc phthalocyanine doped with molybdenum trioxide (Mo0 3 ) (as the only second material).
  • Mo0 3 molybdenum trioxide
  • the sensor element 10 has a sensing layer 18 of zinc phthalocyanine (ZnPc) with a layer thickness of 1 nm (nanometer) and a conducting layer 20 of zinc phthalocyanine (ZnPc) doped with 5 % molybdenum trioxide (Mo0 3 ) with a layer thickness of 5 nm (nanometer).
  • ZnPc zinc phthalocyanine
  • Mo0 3 molybdenum trioxide
  • the sensor element 10 has a sensing layer 18 of zinc phthalocyanine (ZnPc) with a layer thickness of 1 nm (nanometer) and a conducting layer 20 of zinc phthalocyanine (ZnPc) doped with 5 % molybdenum trioxide (Mo0 3 ) with a layer thickness of 10 nm (nanometer).
  • ZnPc zinc phthalocyanine
  • Mo0 3 molybdenum trioxide
  • the zinc phthalocyanine sensing layer 18 has a resistance R in the order of giga-ohms and can be observed to change the resistance R of the conducting layer 20 in response to interaction with nitrogen dioxide (N0 2 ) as the at least one target analyte 12. (These responses are also attributed to dipole formation and/or a Fermi level change.)
  • N0 2 nitrogen dioxide
  • these responses are also attributed to dipole formation and/or a Fermi level change.
  • a thickness of the conducting layer 20 is increased from 5 nm to 10 nm, an increase in the current I and in the sensitivity S is observed (with little to no effect on the selectivity).
  • the architecture of the sensor element 10 can be modified to form a layered stack. Because thin layers of the at least one first material of the sensing layer 18 and the at least one second material are often permeable to most gases, it is also possible to form a sensor element 10 with a layered stack comprising a sensing layer 18, a conducting layer 20, a sensing layer 18 (and) a conducting layer 20 (and so on). For instance, thin layers of zinc phthalocyanine (ZnPc) and reduced graphene oxide (rGO) are slightly permeable to most gases.
  • ZnPc zinc phthalocyanine
  • rGO reduced graphene oxide
  • a sensor element 10 with a layered stack comprising at least a reduced graphene oxide layer, a zinc phthalocyanine layer, a reduced graphene oxide layer (and) a zinc phthalocyanine layer (and so on). Accordingly, it is also possible to form a sensor element 10 with a layered stack comprising at least a zinc phthalocyanine layer doped with molybdenum trioxide, a zinc phthalocyanine layer, a zinc phthalocyanine layer doped with molybdenum trioxide (and) a zinc phthalocyanine layer (and so on). Such a sensor element 10 may yield even higher sensitivities S.

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Abstract

The present invention provides a sensor element (10) for a chemical sensor with a substrate with a first electrode (14a) and a second electrode (14b) designed for a voltage application and a sensing layer (18) comprising at least a first material, wherein, in case that at least one target analyte (12) is present, an electric field (E) within the sensing layer (18) is changed, wherein the sensor element (10) also comprises a conducting layer (20) comprising at least a second material, wherein the conducting layer (20) provides an electrical connection between the first electrode (14a) and the second electrode (14b) and is covered at least partially by the sensing layer (18), and wherein an electric field (E) or a Fermi level is changed within the conducting layer (20) in case that the electric field (E) within the sensing layer (18) is changed.

Description

Description
Title
The present invention relates to a sensor element for a chemical sensor and a chemical sensor.
Prior art
The use of metal phthalocyanine (mPc) as a sensing material to detect at least one target anaiyte is known in the prior art. For instance, publication„ ETHANOL GAS SENSORS BASED ON COPPER PHTHALOCYA INE THIN-FILM
TRANSISTORS " (L. Xian, J. Ya-Dong, X. Guang-Zhong, D. Xiao-Song, T. Hui- Ling, Y. Jian-Fei, F. Song-Qi; in Apperceiving Computing and Intelligence Analysis (ICACIA), 2010, International Conference, pp. 470-473) describes the detection of different concentrations of ethanol in nitrogen gas using a single thin film of copper phthalocyanine as sensing material.
Disclosure of the invention
The present invention provides a sensor element for a chemical sensor with the features of claim 1 and a chemical sensor with the features of claim 10.
Advantages of the present invention
The present invention provides a sensor element/a chemical sensor that has a good sensitivity even at temperatures below 80°C. Thus, it is not necessary to provide any heating equipment for the operation of the sensor element/chemical sensor, as it is for instance often necessary for a conventional metal oxide sensor device. Furthermore, as it is not necessary to heat any part of the sensor element/chemical sensor during its operation, its power consumption is reduced compared to the prior art.
Moreover, the conducting layer (comprising the at least one second material different from the at least one first material of the sensing layer) reduces the overall resistance of the sensor element/chemical sensor during its operation, while keeping its advantageous sensitivity (selectivity) to the at least one target analyte. Thus, an electrical signal detected between the first electrode and the second electrode is enhanced. This enhancement of the electrical signal (which can be in the miliiampere range, for instance) facilitates the processing of the sensor element/chemical sensor by current ASIC implementations.
A variety of different sensing materials with a good sensitivity for the at least one target analyte can be used as the at least one first material within the sensing layer. The addition of the conducting layer (comprising the at least one second material different from the at least one first material of the sensing layer) hardly restricts the variety of sensing materials useable for the sensing layer.
In an advantageous embodiment of the sensor element the conducting layer comprises the at least one first material of the sensing layer in addition to the at least one second material. Thus, even a small amount of the at least one second material within the conducting layer ensures the above-mentioned advantages.
Preferably, the sensing layer comprises an inorganic metal oxide and/or a metal phthalocyanine as the at least one first material. Inorganic metal oxide (MOx) sensing materials have good operation currents and a high sensitivity for at least one target analyte. The addition of the conducting layer (comprising for instance reduced graphene oxide or graphene as the at least one second material) further reduces the operation temperature of the sensor element/chemical sensor comprising an inorganic metal oxide as sensing material. Metal
phthalocyanine/organic metal phthalocyanine sensing materials have become attractive due to their high selectivity and low operational temperatures, which reduce their power consumption during operation. Additionally, sensing materials comprising metal phthalocyanine/organic metal phthalocyanine can be tailored for use with different target analytes by modifying the functional side chains and/or metal center. Furthermore, by adding the conducting layer (comprising the at least one second material different from metal phthalocyanine/organic metal phthalocyanine) the output current is enhanced even though the metal phthalocyanine/organic metal phthalocyanine is used as the at least one first material.
In a special embodiment the sensing layer comprises the metal phthalocyanine as the at least one first material and the conducting layer comprises the metal phthalocyanine doped with a dopant as the at least one second material. Thus, the above-mentioned advantages can be realized in one embodiment.
In another advantageous embodiment the conducting layer comprises a conductor and/or a semiconductor as the at least one second material. For instance, the conducting layer comprises tetraf I u ro-tetra cya no-quinodimethane, tetrathianaphthacene, zinc oxide, copper oxide, molybdenum trioxide and/or reduced graphene oxide as the at least one second material. As the materials mentioned in the preceding phrase do not have to be used within the sensing layer, they do not influence the selectivity of the sensor element/chemical sensor for the at least one target analyte.
In a preferred embodiment the sensing layer comprises zinc phthalocyanine as the at least one first material and the conducting layer comprises reduced graphene oxide as the at least one second material. In another preferred embodiment the sensing layer comprises zinc phthalocyanine as the at least one first material and the conducting layer comprises the zinc phthalocyanine doped with molybdenum trioxide as the at least one second material. The advantages of both embodiments are explained in more details below.
Additionally, the sensor element may also comprise an evaluator designed for detecting at least one physical value of the conducting layer and for determining and outputting an information about a presence and/or the amount of the at least one target analyte dependent on the at least one physical value of the conducting layer. The information determined by the evaluator can be used for a variety of different applications, for instance in consumer applications.
The above-mentioned advantages can also be achieved by a chemical sensor with such a sensor element. Brief description of the drawings
More features and advantages of the present invention will be explained in more details with reference to the drawings, which show:
Fig. 1 a to 1 d a first embodiment of the inventive sensor element, wherein
Fig. 1a and 1b show schematic drawings of the sensor element without and with its at least one target analyte, and wherein Fig. 1c and 1d show different changes in its Fermi level due to the presence of the at least one target analyte;
Fig. 2a to 2c different examples of measurements with other embodiments of the inventive sensor element; and
Fig. 3a to 3d different examples of measurements with other embodiments of the inventive sensor element.
Embodiments of the invention
Fig. 1 a to 1 d refer to a first embodiment of the inventive sensor element, wherein Fig. 1a and 1b show schematic drawings of the sensor element without and with its at least one target analyte, and wherein Fig. 1c and 1d show different changes in its Fermi level due to the presence of the at least one target analyte.
The sensor element 10 shown in Fig. 1a and 1 b is designed to be used in a chemical sensor (not shown). The use of the sensor element 10 is not restricted to a certain type of chemical sensors. For instance, the chemical sensor with the sensor element 10 can be a gas sensor, especially a solid state gas sensor. The sensor element 10 may be used in a variety of different applications, such as consumer applications, particularly for consumer electronics. The sensor element 10 may even be used for applications, for which the use of conventional analytic systems such as gas chromatography and optical detection methods are expensive and difficult to scale down. Thus, a variety of different target analytes 12 may be detected by an operation of the sensor element 10. The sensor element 10 has a substrate {not shown) with a first electrode 14a and a second electrode 14b. The first electrode 14a and the second electrode 14b are designed for a voltage application between the first electrode 14a and the second electrode 14b. For instance, a first conducting line 16a may be linked to the first electrode 14a, while a second conducing line 16b is electrically connected with the second electrode 4b. It is thus possible to apply a (changing) voltage V between the first electrode 14a and the second electrode 14b.
The sensor element 10 has a sensing layer 18. The sensing layer 18 comprises at least a first material. The at least one first material of the sensing layer 18 is not in the form of corns or granulates. Instead, the at least one first material is deposited as a layer-wise deposit. Thus, the sensing layer 18 fills a (first) layer- wise volume (completely). Preferably, the sensing layer 18 is a layer without any hole or opening. The sensing layer 18 may be deposited using any chemical or supplementation-evaporation technique.
The at least one first material of the sensing layer 18 is a material to which the at least one target analyte 12 may attach. The at least one target analyte 12 may also be absorbed by the at least one first material or form a binding with the at least one first material. Specially, the at least one first material of the sensing layer 18 may comprise functional side chains and/or receptors specifically binding the at least one target analyte 12. Furthermore, in case that the at least one target analyte 12 is attached/absorbed/bound on and/or within the sensing layer 18, an electric field E within the sensing layer 18 is changed/created dependent on an amount of the at least one target analyte 12
attached/absorbed/bound on and/or within the sensing layer 18. As the sensor element 10 may be designed for at least one target analyte 12 chosen from a variety of different target analytes 12, a lot of different materials may be used as the at least one first material of the sensing layer 18. The sensing layer 18 can be made either organic or inorganic or a combination of both. For instance, the sensing layer 18 may comprise an inorganic metal oxide (MOx) and/or a metal phthalocyanine/organic metal phthalocyanine (mPc) as the at least one first material. Special examples for the at least one first material of the sensing layer 18 are given below. The sensor element 10 also comprises a conducting layer 20. The conducting layer 20, which comprises at least a second material different from the at least one first material of the sensing layer 18, provides an electrical connection between the first electrode 14a and the second electrode 14b. For instance, the conducting layer 20 may spread between the first electrode 14a and the second electrode 14b. However, the conducting layer 20 may also be suspended between the first electrode 14a and the second electrode 14b. Thus, a current l0 or ls may flow through the conducting layer 20 between the first electrode 14a and the second electrode 14b. Moreover, the conducting layer 20 is covered at least partially by the sensing layer 18. The at least one second material of the conducting layer 20 is also not in the form of corns or granulates. Instead, the at least one second material is deposited as a layer-wise deposit in a way that the conducting layer 20 fills a (second) layer-wise volume (completely). In a preferred embodiment, the conducting layer 20 is a layer without any hole or opening. For the deposition of the conducting layer 20 any chemical or supplementation- evaporation technique may be performed.
Any material with a sufficient conductivity can be used as the at least one second material of the conducting layer 20. Thus, the conducting layer 20 may comprise a conductor and/or a (doped) semiconductor as the at least one second material. In an advantageous embodiment, the conducting layer 20 consists either of a conductor (with a variable carrier concentration) or a semiconductor. In all these cases any change of the electric field E within the sensing layer 18, which covers the conducting layer 20 at least partially, increases or decreases a concentration of the electrons or holes within the conducting layer 20. The at least one second material of the conducting layer 20 could be either organic, inorganic or a combination of both. Special examples of the at least one second material of the conducting layer 20 are given below.
Fig. 1a shows the sensor element 10 in the absence of its at least one target analyte 12. In opposite to Fig. 1 a, Fig. 1 b shows the sensor element 10 in a situation when its at least one target analyte 12 is present in a surrounding of the sensing layer 18. In the presence of the at least one target analyte 12, the at least one target analyte 12 is attached/absorbed/bound at least partially on and/or within the sensing layer 18 and the electric field E within the sensing layer 18 is changed/created dependent on the amount of the at least one target analyte 12 attached/absorbed/bound on and/or within the sensing layer 18. (Fig. 1 b shows the example of the formation of a dipole 22 schematically.)
The change/creation of the electric field E within the sensing layer 18 results in a change of an electric field E or a Fermi level within the conducting layer 20. Thus, the electric field E or the Fermi level is changed within the conducting layer 20 in case that the electric field E within the sensing layer 18 is changed/created. The change of the electric field E or the Fermi level within the conducting layer 20 depends on the amount of the at least one target analyte 12
attached/absorbed/bound on and/or within the sensing layer 18.
Fig. 1c and 1d show different changes in the Fermi level of the at least one second material of the conducting layer 20 due to the presence of the at least one target analyte 12. Shown are always a conduction band 24 and a valence band 26 of the conducting layer 20. According to the general understanding, the change of the electric field E (which is caused for instance by the creation of at least one dipole 22 between the at least one target analyte 12 and the at least one first material of the sensing layer 18) results in a shift 28 of the Fermi level (from a former Fermi level Ff to a new Fermi level Fn). In the example of Fig. 1c, the at least one second material of the conducting layer 20 is a conductor and therefore a shift 28 of the Fermi level into a region where the "conductor has more electrons" occurs, resulting in a reduction of resistance. In the example of Fig. 1d, the at least one second material of the conducting layer 20 is a semiconductor and therefore a shift 28 of the Fermi level into a region where "more electrons are allowed to enter traps in the forbidden band" occurs, resulting in an increase of the "hole concentration" and a reduction of resistance.
Thus, in the presence of the at least one target analyte 12 a decrease of the resistance of the conducting layer 20 occurs. This decrease of the resistance of the conducting layer 20 can be measured by a measurement of the current flow through the conducting layer 20, for instance. The operation of the sensor element 10 shown in Fig. 1a and 1 b therefore does not require any measurement of a conductance, resistance, capacitance or potential of the sensing layer 18. Instead, a presence and/or an amount/concentration of the at least one target analyte 12 within the surrounding of the sensing layer 18 is detectable by a change of at least one physical value of the conducting layer 20. It is therefore sufficient to detect for instance a change in a conductance, resistance, capacitance or potential of the conducting layer 20 for determining a presence and/or the amount/concentration of the at least one target analyte 12 within the surrounding of the sensing layer 18.
In the example of Fig. 1a, a base line current l0 is flowing through the conducting layer 20 in the absence of the at least one target analyte 12 in the surrounding of the sensing layer 18. As can be seen by a comparison of Fig. 1a and 1b, the presence of the at least one target analyte 12 within the surrounding of the sensing layer 18 and its interaction with the sensing layer 18 causes that a sensing current ls (different from the base line current l0) flows through the conducting layer 20. In other words, the current flow through the conducting layer 20 is changed due to the presence of the at least one target analyte 12 in the surrounding of the sensing layer 18, wherein the sensing current ls is normally dependent on the amount of the at least one target analyte 12
attached/absorbed/bound on and/or within the sensing layer 18.
As the amount of the at least one target analyte 12 attached/absorbed/bound on and/or within the sensing layer 18 normally depends on the amount/concentration of the at least one target analyte 12 present in the surrounding of the sensing layer 18, the sensing current ls also depends on the amount/concentration of the at least one target analyte 12 present in the surrounding of the sensing layer 18. It is therefore easy to detect the presence of the at least one target analyte 12 in the surrounding of the sensing layer 18 and/or to measure the
amount/concentration of the at least one target analyte 12 present in the surrounding of the sensing layer 18 by measuring the change of the current flow through the conducting layer 20.
The sensor element 10 or the chemical sensor with the sensor element 10 may also comprise an evaluator (not shown). Preferably, the evaluator is designed for detecting the at least one physical value of the conducting layer 20 (for instance by measuring the current l0 or ls) and for determining and outputting an information about a presence and/or the amount/concentration of the at least one target analyte 12 dependent on the at least one physical value of the conducting layer 20. An Evaluator fulfilling these requirements is easy to produce and does not require much volume. A lot of different materials can be used as the at least one second material of the conducting layer 20. Specially, standard micro fabrication materials such as (doped) polysilicon, polysiloxane, graphene and/or conducting graphene oxide may be used as the at least one second material of the conducting layer 20. Furthermore, the conducting layer 20 may comprise tetrafluro-tetracyano- quinodimethane, tetrathianaphthacene, zinc oxide, copper oxide, molybdenum trioxide and/or reduced graphene oxide as the at least one second material. Also, the conducting layer 20 may comprise the at least one first material of the sensing layer 18 in addition to the at least one second material. Especially, the at least one second material may be at least one dopant added to the at least one first material.
For all the examples of the materials of the conducting layer 20 given in the preceding paragraph, it is possible to provide an easily detectable change in the at least one physical value of the conducting layer 20. Thus, the addition of the conducting layer 20 facilitates the detection of the presence and/or the amount/concentration of the at least one target analyte 12. It is therefore possible to use a sensing material (as the at least one first material) within the sensing layer 18, even though the electric properties of this sensing material may be disadvantageous for a conventional (chemical) sensor. For instance, while conventional sensors using metal phthaiocyanine (mPc) generally exhibit a low output current (e.g. in the range of picoampere to nanoampere), this obstacle is overcome by the addition of the conducting layer 20. This results in an overcome of the conventional limits to the read-out accuracy of conventional sensors.
All the examples for the at least one second material mentioned here provide an improvement of the current of the sensor element 10 by several orders of magnitude while retaining a low operating temperature (generally below 80°C) and the same selectivity for the at least one target analyte 12 as in the absence of the at least one conducting layer 20. Moreover, the materials mentioned here provide a high flexibility in material selection for specific use-case to be addressed by integration of different sensing materials (as the at least one first material) within the sensing layer 8. Fig. 2a to 2c show different examples of measurements with other embodiments of the inventive sensor element.
In the examples of Fig. 2a to 2c, the sensing layer 18 comprises a metal phthalocyanine (mPc) as the at least one first material (as sensing material). Thus, the sensing layer 18 has a high thermal stability. Furthermore, the use of a metal phthalocyanine as the at least one first material makes it easy to provide a molecular design of the at least one first material (as sensing material) that ensures a good interaction of the at least one first material with the target analyte 12. It is therefore ensured that the sensing layer 18 fulfills its role after an introduction of the target analyte 12.
In the examples of Fig. 2a to 2c, the sensing layer 18 comprises zinc
phthalocyanine (ZnPc) as the at least one first material and/or the conducting layer 20 comprises reduced graphene oxide (rGO) as the at least one second material. Specially, in the examples of Fig. 2a to 2c, the sensing layer 18 consists of zinc phthalocyanine (ZnPc) as the at least one first material and/or the conducting layer 20 consists of reduced graphene oxide (rGO) as the at least one second material. Nitrogen dioxide (N02) was used as the at least one target analyte 12 for the measurements shown in Fig. 2a to 2c.
The layer thicknesses of the sensing layer 18 and the conducting layer 20 differ in the examples of Fig. 2a to 2c. The examples of Fig. 2a and 2b both have the sensing layer 18 as well as the conducting layer 20, wherein the layer thickness of the sensing layer 18 is 1 nm and the layer thickness of the conducting layer 20 is 5 nm in the example of Fig. 2a and the layer thickness of the sensing layer 18 is 5 nm and the layer thickness of the conducting layer 20 is 5 nm in the example of Fig. 2b. In the example of Fig. 2c, the sensor element 10 has only the sensing layer 18 with a layer thickness of 60 nm, but no conducting layer 20.
In the coordinate systems of Fig. 2a to 2c the abscissae are timelines t (in seconds) and the ordi nates denote a current I (in milliampere or nanoampere) and a resistance R (in kiloohm). Different concentrations c of nitrogen dioxide (N02) as the at least one target analyte 12 are given in ppb (parts per billion). In the examples of Fig. 2a and 2b, a sensitivity to nitrogen dioxide of inventive sensor elements 10 is tested by using 5 nm (nanometer) of reduced graphene oxide (rGO) as the conducting layer 20. The thickness of the sensing layer 18 of zinc phthalocyanine (ZnPc) was chosen to be either 1 nm (nanometer) (example of Fig. 2a) or 5 nm (nanometer) (example of Fig. 2b). Upon interaction with the target analyte 12, the target analyte 12 is bound to the sensing layer 18 in both cases. This causes a charge transfer and a formation of at least on dipole 22. This dipole formation creates a different electric field E that permeates the conducting layer 20, changing a carrier concentration within the conducting layer 20 and hence a change of its resistance R. This change of resistance R is in the examples of Fig. 2a and 2b easily detectable by measuring the current I (in the milliampere range).
Compared to the examples of Fig. 2a and 2b, the use of a sensing layer 18 with a layer thickness of 60 nm (nanometer) in a device without the conducting layer 60 (example of Fig. 2c) resulted only in a current I in the nanoampere range. (A comparable sample with a layer thickness of 5 nm of zinc phthalocyanine but without the conducting layer 60 had current levels that were too low to be measured.)
The sensitivity S of all the examples of Fig. 2a to 2c to nitrogen dioxide (N02) is characterized according to equation (E 1 ):
Wherein (dR/dt , is the rate of change of the resistance R before the addition of nitrogen dioxide (N02) as the at least one target analyte 12 and (dR/dt)a is the rate of change of the resistance R after the addition of nitrogen dioxide (N02) as the at least one target analyte 12.
Table 1 gives the values of the sensitivity S for all the examples of Fig. 2a to 2c:
It thus becomes obvious that the sensitivity S is the most suited for the detection of low concentrations c of nitrogen dioxide (N02) (as the at least one target analyte 12) when a sensor element 10 with 1 nm of zinc phthalocyanine (ZnPc) together with 5 nm of reduced graphene oxide (rGO) is used.
Fig. 3a to 3d show different examples of measurements with another
embodiments of the inventive sensor element.
In the examples of Fig. 3a to 3d, the conducting layer 20 consists of the at least one first material of the sensing layer 18 doped with the at least one second material. For instance, the sensing layer 18 comprises a metal phthalocyanine as the at least one first material and the conducting layer comprises the metal phthalocyanine doped with the dopant as the at least one second material. Thus, it is possible to reduce the resistivity of the at least one conducting layer 20 by adding the at least one second material as the at least one dopant. As explained above, the conductivity of such a conduction layer 20 changes due to the change/creation of the electric field E within the sensing layer 18.
For instance, the sensing layer 18 comprises zinc phthalocyanine (ZnPc) as the at least one first material and the conducting layer 20 comprises the zinc phthalocyanine (ZnPc) doped with molybdenum trioxide ( o03) as the at least one second material. In the examples of Fig. 3a to 3d, zinc phthalocyanine (ZnPc) is used as the (only) first material (sensing material) of the sensing layer 18 and the conducting layer 20 consists of zinc phthalocyanine doped with molybdenum trioxide (Mo03) (as the only second material). In the example of Fig. 3a and 3b, the sensor element 10 has a sensing layer 18 of zinc phthalocyanine (ZnPc) with a layer thickness of 1 nm (nanometer) and a conducting layer 20 of zinc phthalocyanine (ZnPc) doped with 5 % molybdenum trioxide (Mo03) with a layer thickness of 5 nm (nanometer). In the example of Fig. 3c and 3d, the sensor element 10 has a sensing layer 18 of zinc phthalocyanine (ZnPc) with a layer thickness of 1 nm (nanometer) and a conducting layer 20 of zinc phthalocyanine (ZnPc) doped with 5 % molybdenum trioxide (Mo03) with a layer thickness of 10 nm (nanometer).
In the coordinate systems of Fig. 3a and 3c the abscissae are timelines t (in seconds) and the ordinates denote a current I (in microampere) and
concentrations c of nitrogen dioxide (N02) given in ppb (parts per billion). In the coordinate systems of Fig. 3b and 3d the abscissae denote the concentrations c of nitrogen dioxide (N02) (in parts per billion) and the ordinates denote the sensitivity S according to equation (E 1 ).
In both prototypes, the zinc phthalocyanine sensing layer 18 has a resistance R in the order of giga-ohms and can be observed to change the resistance R of the conducting layer 20 in response to interaction with nitrogen dioxide (N02) as the at least one target analyte 12. (These responses are also attributed to dipole formation and/or a Fermi level change.) When a thickness of the conducting layer 20 is increased from 5 nm to 10 nm, an increase in the current I and in the sensitivity S is observed (with little to no effect on the selectivity).
For all the examples given above, the architecture of the sensor element 10 can be modified to form a layered stack. Because thin layers of the at least one first material of the sensing layer 18 and the at least one second material are often permeable to most gases, it is also possible to form a sensor element 10 with a layered stack comprising a sensing layer 18, a conducting layer 20, a sensing layer 18 (and) a conducting layer 20 (and so on). For instance, thin layers of zinc phthalocyanine (ZnPc) and reduced graphene oxide (rGO) are slightly permeable to most gases. It is therefore possible to form a sensor element 10 with a layered stack comprising at least a reduced graphene oxide layer, a zinc phthalocyanine layer, a reduced graphene oxide layer (and) a zinc phthalocyanine layer (and so on). Accordingly, it is also possible to form a sensor element 10 with a layered stack comprising at least a zinc phthalocyanine layer doped with molybdenum trioxide, a zinc phthalocyanine layer, a zinc phthalocyanine layer doped with molybdenum trioxide (and) a zinc phthalocyanine layer (and so on). Such a sensor element 10 may yield even higher sensitivities S.
All the advantages mentioned above are also provided by a chemical sensor with such a sensor element 10.

Claims

Claims
1. Sensor element (10) for a chemical sensor with: a substrate with a first electrode (14a) and a second electrode (14b), wherein the first electrode (14a) and the second electrode (14b) are designed for a voltage application between the first electrode (14a) and the second electrode (14b); and a sensing layer (18) comprising at least a first material, wherein, in case that at least one target analyte (12) is present in a surrounding of the sensing layer (18), an electric field (E) within the sensing layer (18) is changed dependent on an amount of the at least one target analyte (12) attached on and/or within the sensing layer (18); characterized in that the sensor element (10) also comprises a conducting layer (20) comprising at least a second material different from the at least one first material of the sensing layer (18), wherein the conducting layer (20) provides an electrical connection between the first electrode (14a) and the second electrode (14b) and is covered at least partially by the sensing layer (18), and wherein an electric field (E) or a Fermi level is changed within the conducting layer (20) in case that the electric field (E) within the sensing layer (18) is changed.
2. Sensor element (10) according to claim 1 , wherein the conducting layer (20) comprises the at least one first material of the sensing layer (18) in addition to the at least one second material.
3. Sensor element (10) according to claim 1 or 2, wherein the sensing layer (18) comprises an inorganic metal oxide and/or a metal phthalocyanine as the at least one first material. Sensor element (10) according to claim 3, wherein the sensing layer (18) comprises the metal phthalocyanine as the at least one first material and the conducting layer (20) comprises the metal phthalocyanine doped with a dopant as the at least one second material.
Sensor element (10) according to any of the preceding claims, wherein the conducting layer (20) comprises a conductor and/or a semiconductor as the at least one second material.
Sensor element (10) according to claim 5, wherein the conducting layer (20) comprises tetrafluro-tetracyano-quinodimethane,
tetrathianaphthacene, zinc oxide, copper oxide, molybdenum trioxide and/or reduced graphene oxide as the at least one second material.
Sensor element (10) according to claim 6, wherein the sensing layer (18) comprises zinc phthalocyanine as the at least one first material and the conducting layer (20) comprises reduced graphene oxide as the at least one second material.
Sensor element (10) according to claim 6, wherein the sensing layer (18) comprises zinc phthalocyanine as the at least one first material and the conducting layer (20) comprises the zinc phthalocyanine doped with molybdenum trioxide as the at least one second material.
Sensor element (10) according to any of the preceding claims, wherein the sensor element (10) also comprises an evaluator designed for detecting at least one physical value (I, R) of the conducting layer (20) and for determining and outputting an information about a presence and/or the amount (c) of the at least one target analyte (12) dependent on the at least one physical value (I, R) of the conducting layer (20).
Chemical sensor with a sensor element (10) according to any of the preceding claims.
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