FR2779826A1 - DETECTOR OF OZONE, NITROGEN DIOXIDE OR ANY OTHER POLLUTANT GAS AND USE THEREOF - Google Patents

Abstract

The invention concerns a detector in the form of a field-effect transistor (13) arranged in a chamber (14). The transistor (13) electroattractive part is formed by a macromolecular material layer (4). The latter is exposed to a mixture containing at least one polluting gas, such as O3 and NO2, which causes it to be doped. A measuring device (10) comprises means for measuring the intensity of the current (IDS) which circulates between the transistor drain electrode (5) and source electrode (6) at least at one given time after the detector has been initialised, said at least polluting gas presence causing the layer (4) material to be doped depending on the concentration in polluting gas, the current (IDS)intensity and the intensity gradient being related to the concentration in polluting gas.

Description

DETECTOR OF OZONE, NITROGEN DIOXIDE OR ANY OTHER

POLLUTANT GAS AND ITS USE

  The present invention relates to a detector capable of detecting and measuring the quantity of ozone or other polluting species including NO2 present in a gas. The invention is intended both for industrial uses

  and environmental control only in the laboratory area.

  The object of the invention is to facilitate this detection and this measurement.

  The ecological pressures in modern societies and the risks posed by various pollutions, lead to measuring the presence of polluting gases. Among these, ozone and nitrogen dioxide (NO2) are particularly revealing gases. An essential characteristic of these gases is their high reactivity as oxidizing agents. The same effects are however expected with reducing gases. During pollution episodes (heavy traffic, or factory opening hours), the ozone level in cities, due to an excess of NO which reacts with 03, may be lower than the ozone level in rural areas. The relative decrease in ozone levels in city centers then leads to an increase in the NO2 level. For this reason, the severity of pollution can, for example, be expressed by the sum of the concentrations of ozone and

NO2.

  In human dwellings, ozone and NO2 (among others) appear as gases likely to be harmful to humans. Their presence in too high a concentration causes troubles. The detection of the polluting gas content is therefore an important parameter for knowing the air quality. In another application, it is sought to measure the redox character of the combustion products which emanate from boilers. We even plan to regulate the carburetion of these boilers, the optimal fuel / oxidizer mixture leading to the weakest character.

redox of the atmosphere.

  As much the excess of ozone in the habitable layers is harmful, as much its deficit in the stratospheric layers is also harmful: indeed, ozone exerts a filtration of the ultraviolet radiations there. Her

  presence prevents warming of the atmosphere.

  In practice, there is a need to measure the presence of ozone over a very large concentration range. Thus, it is necessary to be able to titrate ozone from a fraction of a part per billion (ppb) up to several

  tens of parts per million. The same is true for NO2.

  The ozone and NO2 titration devices currently available on the market essentially use the ultraviolet absorption properties of these molecules. They have the disadvantage of being expensive,

and difficult to transport.

  Portable devices of the electrochemical or chemical type are known for this purpose. The following assay methods are known, for example: calorimetric analysis, redox titration, coulometric analysis and the use of indicator tubes containing indigo. Indicator badges

  ozone are also marketed.

  It should also be noted that the dosage of ozone poses specific problems since ozone is thermally unstable and

photochemically.

  The NO2 detection and dosing devices are based on

  details, on the previous principles.

  We know by the article "Electrical and Optical Properties of Phthalocyanine Films", due to Messrs H. LAURS and G. HEILAND, published in the review ELECTRONICS AND OPTICS, Thin Solid Film, vol. 149 (1987),

  pages 129 to 142, a description of the electrical and optical properties of the

  phthalocyanine (Pc) as well as metallophthalocyanine (PcM). The presentation of the properties of phthalocyanine in this article is made in a context where a metallophthalocyanine is used as doped molecular insulating material. The behavior of this material is influenced by the content of the gas in which it bathes. In fact, the described physical effects related to phthalocyanine are of the disposable type. In fact, this article describes the use of transistors in which the sensor, for a measurement of oxidizing gas content, is only obtained by depositing lithium on the

phthalocyanine layer.

  The present invention does not require deposition of a strongly metal

  reducing agent on the metallophthalocyanine layer.

  The principle of the invention is to constitute a field effect transistor whose conduction channel is created in a molecular material, either a metallophthalocyanine, or a similar derivative. The measurement circuits include means for measuring the current which passes through this transistor between the source and the drain. Electrical measurements are made in

  continuous, through the use of a voltage ramp, pulse or alternating.

  It was found in the invention, that the source-drain current was linked to the more or less strong presence of ozone or NO2. Also, in the invention, there will be retained in particular a protocol in which the measurement is carried out after a short duration (after exposure

  polluting gas) after initializing or resetting the sensor.

  The subject of the invention is therefore a detector and sensor for ozone, NOZ and various polluting gases, characterized in that it comprises a field effect transistor, the electroactive part of which consists of a layer of metallophthalocyanine subjected to the influence of a gas containing ozone or NO2, this transistor being connected to a device for measuring the current flowing

  in this transistor, in particular after an initialization of this detector.

  The invention will be more fully detailed on reading the

  description which follows and examination of the figures which accompany it. These

  are given for information only and in no way limit the invention. The figures show: Figure 1: a molecular transistor, shown diagrammatically, usable in the detector of the invention. The geometry of this transistor does not require the use of techniques related to microelectronics; Figure 2: a schematic sectional representation of an embodiment of a detector according to the invention derived from the techniques of microlithography; Figure 3: a schematic representation of a preferred improvement in the use of the detector of the invention; Figures 4a and 4b: representations of embodiments of the source and drain regions of a detector transistor according to the invention in microlithographic technique; Figure 5: a schematic representation of a preferred method for measuring the rate of polluting gas; Figure 6: a time diagram for measuring an ozone detection signal by a molecular transistor described in Figure 2; Figure 7: a time diagram for measuring a detection signal

  NO2 by the molecular transistor described in Figure 2.

  Figures 1 and 2 show ozone detectors produced in the form of a transistor according to the invention. The transistor of FIG. 2 requires the use of microlithography, not that of FIG. 1. FIG. 1 shows a bipolar transistor comprising on a metal grid a layer of polysilicon (nSi) surmounted by an insulating layer of SiO2. The SiO2 layer is surmounted by a layer of Si3N4, itself surmounted by a layer of metallophthalocyanine. Two drain and source electrodes placed at the end of the conduction channel formed by this last layer constitute the rest of the transistor. The principle of the measurement consists in measuring a source drain current of this transistor for a polarity Vgs of the gate, then

  that a gas to be detected is in contact with the metallophthalocyanine layer.

  The detector of FIG. 2 comprises a transistor formed, in this example, on a substrate 1. The substrate 1 is intended to constitute the gate of the transistor. The substrate 1 is a highly doped silicon crystal. In this example, this silicon substrate is doped P ++ by injection, diffusion or implantation of boron atoms. It is surmounted by a layer of silicon oxide 2. A layer 3 of Si3N4 deposited on layer 2 of silicon oxide constitutes a second layer of gate dielectric. A layer 4 deposited above layer 3 is made of a molecular material. Current can flow between a source 5 and a drain 6 of the transistor in layer 4. Instead of using a metallophthalocyanine as the semiconductor material of layer 4 of such a field effect transistor, related derivatives can be used such as porphyrins, azaporphyrins, porphyrazines, etc. The source 5 and drain 6 electrodes are connected by connections 7 and 8 respectively to the terminals of a

  power supply 9 and a current measurement device 10.

  The power supply 9 can be continuous, pulsed or even alternative. The gate electrode 1 is connected via a connection 11 to a power supply 12 which makes it possible to vary the gate source voltage. The transistor thus formed is

  sensitive to the action of ozone and NO2.

  Layer 2 can be obtained by oxidation of the upper layer of substrate 1. It can also be obtained by deposition. The thickness of layer 2 is for example of the order of 100 nanometers. The nitride layer 3 can be obtained by an SiH4 / NH3 plasma or by chemical deposition

  in vapor phase. Si3N4 can also be deposited by spraying.

  The thickness of layer 3 is of the order of 100 nanometers. Layer 4 of metallophthalocyanine is obtained by vacuum deposition of a sublimed metallophthalocyanine. This can be chosen from nickel, zinc, copper, lead, iron, manganese, cobalt or other phthalocyanines, or even the non-metallated derivative. The thickness of layer 4, in one example, is of the order of 50 nanometers. The deposition rate is between a few tenths of a nanometer at

a few nanometers per second.

  It was found in the invention that the response time of the detector was reduced if the layer 4 was of the amorphous or quasi-amorphous type instead

to be polycrystalline.

  A number of PcM unsubstituted metallophthalocyanines (M = H2, Cu, Ni, Fe, etc.) are commercially available at low cost. Their purity is however poor and sublimation by entrainment with nitrogen under partial vacuum (7 torr) was carried out before using this material to manufacture the field effect transistor. It has been discovered that the sensor properties of metallophthalocyanines depend on multiple parameters. Three of them are: - the ability of the molecular unit to reduce and then oxidize, - the propensity of the central metal to form adducts with various small molecules (02, CO, EtOH, H2O ... ), - the morphology of the thin film used: polycrystallinity, grain size,

  type of defects, amorphous or quasi-amorphous character.

  An important parameter concerns the amorphous or polycrystalline nature of the thin layer. Polycrystalline materials lead to kinetics which are generally much slower for adsorption and

  gas desorption; the response times of the sensors are therefore longer.

  We then tried to use mixtures of phthalonitrile and phthalonitrile substituted by fluorine heteroatoms in order to favor the formation of thin amorphous (or quasi-amorphous) layers of materials

  molecular. Fluorine can be replaced by any other group.

  The procedure in an example was, to synthesize zinc phthalocyanine, to constitute a mixture of 0.96 g phthalonitrile (Mw: 128.13; 7.5 mmol) / 1.23 g 4,5-difluorophthalonitrile (Mw: 164.13; 7.5 mmol) / 1.96 g Zn / 18 ml chloronaphthalene. The theoretical proportions of each product can be calculated by assuming equal reactivity

  for both types of phthalonitriles.

  Mass spectrometry (chemical ionization; NH3) made it possible to verify that the expected products were well synthesized. It has been verified that sublimation does not alter the relative proportions of the various constituents

of (F2) xPcZn.

  The ultraviolet, visible and infrared absorption spectra show significant differences between the two types of phthalocyanine (pure and mixed PcZn). PcZn: 344.5 nm; 604 nm; 668.5 nm (F2) xPcZn: 339 nm; 599 nm; 662 nm IR spectra were made on the two previous materials. In particular, characteristic lines of the C-F link have been noted between

1,050 and 1,400 cm-.

  X-ray diffraction images were taken on thin films deposited under vacuum of PcZn and (F2) xPcZn. The size of the crystallites could be deduced from the width at mid-height of the diffraction lines (taking into account the natural width of the lines). It has thus been found that PcZn leads to a crystallite size of approximately 100 nm, which is in agreement with electron microscopy measurements. In the case of (F2) xPcZn, having

  several isomers and several products leads to a material almost

  amorphous. We could later see the importance of this observation on

the reactivity of the transistor.

  The measuring device according to the invention comprises a circuit for measuring the intensity of the current which passes through the transistor associated with means 13 for measuring the current at a given instant after initialization of the detector. In practice, the circuit and the electrical means will be produced by a microcontroller which will carry out an analog digital conversion of the value of the measured current, which will include a clock, counters, as well as a prerecorded program for delivering a measurement signal to

  regular or irregular interval.

  If necessary, the microcontroller will be perfected and capable of processing the evolution of the measurements over time. These techniques of

  signal processing are of known type.

  Figure 3 shows a schematic representation of an improved use of the detector of the invention. In it, a transistor 13, conforming to that of FIG. 2, or of FIG. 1, is placed inside a box 14. The layer of phthalocyanine forms the upper face of the transistor 13, in direct contact with the atmosphere. The box 14 is covered with a removable cover 15 made of a material such as Duralumin (known alloy of aluminum, copper and magnesium) of stainless steel

  or other strongly reducing alloys or similar materials.

  One of the measurement principles is as follows. The hood 15 is closed

  above the speaker for a few minutes. This has the effect of destroying all or part of the ozone contained in the air underlying the cover 15. At the end of this initialization, the cover is opened and the experiment begins. At the end of a given duration, for example of the order of 1 to 2 minutes after

  the opening, the current is measured by one of the methods described.

  The dosing of ozone, NO2 or other polluting gases can also be done by alternating an air flow free of pollutants with an air flow

responsible for these.

  In order to increase the sensitivity and to widen the range of products detected from the detector formed, a light source 16 has been associated with the box 14 intended to illuminate the thin layer of phthalocyanine constituting the detector during its use. The lamp 16 provides

  approximately an illumination corresponding to 100 milliwatt per cm2.

  It causes an increase in sensitivity. Similarly, a heating resistor 17 can make it possible to bring the detector to a given temperature, for example of the order of 100 C. Preferably, the heating resistor 17 consists of a resistor etched and electrically insulated on the rear face of the substrate 1 in silicon. However, other geometries already known can be used for the heating resistance (radiator). The use of a platinum resistor allows the temperature to be measured simultaneously. The results may be available on a conventional interface. We choose the

  detector loads as a function of the ozone concentration to be measured.

  The particularity of the sensor of the invention is that it has a reactivity to the presence of ozone or other polluting gases in the measured atmosphere. The phenomenon which occurs is a phenomenon of partial oxidation of metallophthalocyanine which has the effect of doping it and thus constituting a more or less conductive layer, more or less doped according to the concentration of ozone or other oxidizing species. N-type doping can be envisaged with reducing gases. The phenomenon evolves over time. What is remarkable, according to the invention, is that the slope of the growth is linked to the concentration of polluting gas. Figures 4a and 4b show preferred embodiments of the drain and source electrodes 5 and 6. The electrodes 5 and 6 can be interdigitated, Figure 4a. In one example, each electrode comprises sixteen fingers whose length is of the order of 632 micrometers. Consequently, the width of the conduction channel (W) is of the order of 20,000 micrometers. The conduction channel length is equal to the interelectrode distance which in one example is 10 micrometers. Under these conditions, the W / L ratio

  of the transistor formed is of the order of 2000.

  Figure 4b shows another embodiment in which the width 18 of a single conduction channel produced between two elongate electrodes 5 and 6 is 3000 micrometers. The length 19 of the channel was produced for values of 10, 20, 50 or 100 micrometers. This makes it possible by reducing the size to increase the measured currents. The solutions for producing Figures 4a and 4b relate to microlithographic type manufacturing. The device shown in Figure 1 is more conventional and requires only

  the use of a cover (wire, tape or mask).

  A method of using the device of the invention can be proposed.

  According to FIG. 5, two identical detectors in accordance with the invention receive a gas through an inlet orifice, the ozone content of which must be

  measured. One of the detectors further comprises a filtering device 20.

  In one example, the device includes a Duralumin filter causing

  a very rapid reduction of the ozone present in the injected atmosphere.

  In contrast, the other orifice allows gas to be injected as it is available directly into the detector. At the output, a comparator 21 normalizes the

  results delivered by the two detectors.

  The measurements are generally carried out by an impulse method: the source-drain current is measured after application of a drainsource voltage in a time window (application time: from zero to a few hundred milliseconds) at VGS = VDS (VGS: grid voltage- source, VDS: drain-source voltage). The IDS drain-source current is measured when it appears after a time that can vary from zero to a few hundred

  milliseconds for the devices described.

  Figure 6 finally shows the evolution 22 (solid line curve) of the drain-source IDS current which may be close to a few nanoamps when a layer of PcNi is subjected to an alternative concentration 23 (dashed curve) of 275 ppb d 'ozone in the air. The nickel phthalocyanine layer (300A) is alternately subjected to 275 ppb of 03 in ambient air filtered over activated carbon and to rest periods without air circulation (polarization: Vs = OV, VD = VG = -3V ). Similarly, Figure 7 shows the variation of the drain-source current when the thin layer of molecular material is exposed to a pulse concentration of 100 ppb of NOz in synthetic air. The nickel phthalocyanine layer (300A) is alternately subjected to 100 ppb of NO2 in synthetic air (polarization: Vs = 5V, VD = VG = 0) and to air

1 5 synthetic.

  In both cases, a prior calibration then allows, by a measurement at a given date close to initialization, to quickly know

the desired gas rate.

Claims (8)

  1 - Ozone, nitrogen dioxide or any other polluting gas detector, characterized in that it comprises a field effect transistor (1-6), the electroactive part of which (4) consists of a layer of metallophthalocyanine subjected to the influence (19) of a gas containing ozone, nitrogen dioxide or any other polluting gas, the source and drain electrodes of this transistor being connected to a device (11, 17) for measure of   current flowing through this transistor at a given time.   2 - Detector according to claim 1, characterized in that the transistor comprises - a conductive substrate serving as a gate of the transistor, - above the substrate, a layer of silicon oxide surmounted by a layer of nitride to serve as gate insulator, and above the gate insulator, a layer of metallophthalocyanine ensuring conduction 3 - Detector according to claim 2 characterized in that the substrate   is a highly doped silicon crystal.   4 - Detector according to one of claims 2 to 3, characterized in that   that the drain and source electrodes of the channel are produced thanks to the use of a mask.   - Detector according to one of claims 2 to 4, characterized in that   that drain and source electrodes of the channel are made by microlithography.   6 - Detector according to one of claims 2 to 5, characterized by at   at least one of the following characteristics: - the metallophthalocyanine is a copper metallophthalocyanine,   zinc, lead, iron, cobalt or nickel or even a non-derivative   metallized, - the metallophthalocyanine layer measures approximately a few tens of nm, - the nitride layer has a thickness of approximately 100 nm or less,   - The silicon oxide layer has a thickness of the order of 100 nm.   7 - Detector according to claims 2 and 6, characterized in that it   has a heat source, preferably in the form of a resistance   of platinum engraved and isolated under the substrate.   8 - Detector according to one of claims 1 to 7 characterized in that   that it is made in a box covered with a cover made of a material   allowing the decomposition of ozone in particular Duralumin.   9 - Detector according to one of claims 1 to 8 characterized in that   that it includes a light source to subject the transistor to a flux   photonics during detection and measurement.   - Detector according to one of claims 1 to 9 characterized in that   that the metallophthalocyanine layer is amorphous or almost amorphous and is obtained by vacuum sublimation, the amorphous nature being obtained thanks to the presence of substituted isomers of phthalocyanines, heteroatoms of   fluorine which can be used as a substituent.   1il - Detector according to one of claims 1 to 10 modified in that   that the metallophthalocyanine layers be replaced by derivatives   related: porphyrin, porphyrazine, azaporphyrins.   12 - Use of a detector according to one of claims 1 to 11,   characterized in that it allows the detection of other gases with redox properties different from those of ozone, in particular NO2, SO2, H2S, C12, Br2.