WO2008116968A1

WO2008116968A1 - Capacitively readable marking with conductor structure of carbon nanotubes, method of making thereof and use.

Info

Publication number: WO2008116968A1
Application number: PCT/FI2008/050122
Authority: WO
Inventors: Heikki SEPPÄ; Panu Helistö; Esko Kauppinen
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2007-03-23
Filing date: 2008-03-18
Publication date: 2008-10-02
Also published as: FI20075190A0; FI20075190L

Abstract

The present invention relates to a marking capacitively readable by an AC field and a method for forming such a marking and a novel use of carbon nanotubes. The marking comprises electrically lossy conductor areas (14, 15) arranged onto a substrate (10) in an information-encoding geometry, whereby the conductor areas contain carbon nanotubes. The invention makes it possible to produce imperceptible markings in a novel way by means of carbon nanotubes.

Description

Capacitively readable marking with conductor structure of carbon nanotubes, method of making thereof and use

The present invention relates to conductive structures formed on a substrate, especially capacitively readable electrically conductive markings according to the preamble of claim 1. The invention further relates to a novel method of manufacturing a conductor structure and a novel use.

The development of printed electronics and new marking technology applications has brought about a development of electrically conductive material compositions to be applied on various substrates. The most common of these and a strongly developing branch is the area of conductor structures based on conductive polymers.

In many cases an electric coding is needed on printed surfaces. The issue is about combining visual information (print) with electric information (coding). It is currently known that by placing a very thin polymer surface on paper or plastic it is possible to create a surface that can be read by impedance measurement. There are also capacitive reading techniques based on a high-frequency signal, whereby only the AC conductivity of the surface is significant.

The problem with known markings and more commonly in electrical conductors, especially in printed conductors, is that the structures produced with them can be seen with the eye or their readability from the surface is bad, whereby the size of the marking must be increased. Another disadvantage is their relatively high price.

In recent years, the promising electric conductivity properties of carbon nanotubes have also been noted in applications of printed electronics.

Fan et al., Journal of Materials Science, Sept. 2005, discloses an electrically conductive ink containing multi-wall carbon nanotubes (MWCNT). With the disclosed structure it is possible to produce a DC-conductive ink surface on, for example, paper.

Kordas et al., Small 2006, 2, No 8-9, 1021 — 1025, discloses another ink containing multi-wall carbon nanotubes that is suitable for inkjet printing. The produced DC-conductive surface is suitable for, e.g., printed electronics, EMC protection and sensors. WO 2007/015710, WO 2004/052559, WO 03/099709, US 2007/0065977 and US 2006/078286 disclose conductive structures, such as a antennae for an RFID tag and printed circuits formed by means of nanotubes. The aim of these publications is to produce lossless conductors with as good a DC conductivity as possible, but they are not suitable for the above-mentioned marking applications due to their weak capacitive readability.

The aim of the invention is to eliminate the above-mentioned problems and to produce a novel kind of conductor structure, especially a capacitively readable marking.

The aim of the invention is further to produce a novel method for manufacturing such a marking.

The invention is based on the observation that carbon nanotubes allow forming of a layer, invisible or at least weakly visible to the naked eye (transparency > 90 %), on a substrate, the layer, however, being sufficiently conductive for distinguishing the layer capacitively with high frequencies, especially when capacitively measuring the real part of the impedance. Thus the invention utilizes in a novel way the special property of carbon nanotubes of no strong interaction with light while being nearly ideal as a conductor.

At its most common the invention thus makes it possible to produce totally novel, essentially invisible conductor layers, conductors, markings and other conductor structures by means of carbon nanotubes.

A marking according to the invention comprises lossy conductor areas arranged on the substrate in an information-encoding geometry, the areas comprising carbon nanotubes. Such conductor areas can be capacitively recognized on the substrate by means of a high-frequency AC field for reading the information content of the marking.

Preferably the carbon nanotubes are on the substrate in a lossy conductor structure in which the nanotube structure is invisible to the naked eye, but it however produces a considerable AC conductivity on the surface.

By an "invisible" or "weakly visible" structure we mean in this context primarily a structure that does not perceptibly attenuate visible light. Especially the intensity attenuation caused by the structure is preferably in the magnitude of less than 10 %, which have experienced to be well achievable in connection with a conductive structure enhanced by nanotubes. The reflection effects caused by the structure should also be small or at least similar with the surface of the substrate, which can also be achieved with nanotubes. Most preferably the conductor material composition is totally transparent to visible light or at least translucent, whereby the substrate is visible under the conductor material pattern. It is to be noted that at its most common the invention also comprises coloured conductor structures that are, however, essentially the same in colour as the surface of the substrate, whereby the attenuation of the conductor pattern must be kept small at precisely the reflection wavelengths of the surface. In both cases the carbon nanotubes act in the structure as a component improving electrical conductivity (or producing it) but as essentially optically non-interacting component.

Especially preferably at least a main portion of the carbon nanotubes are single-walled due to their low interaction with light and good electrical conductivity.

Preferably the nanotubes are distributed in an optically clear medium, i.e. a medium with a low light attenuation (and preferably also having a low reflectivity) so that a sufficient amount of tunnel contacts are formed between the tubes via the medium or the material of the tubes is polarized and thereby it causes electrical conductivity for alternating voltage. Thus, carbon nanotubes improve the conductivity of the medium at at least high frequencies without essentially chancing the optical properties of the mixture.

In a method according to the invention for producing the conductor structure, carbon nanotubes are distributed on the substrate for forming a conductor structure that is electrically conductive and essentially invisible to the naked eye, most preferably for forming a marking that can be capacitively recognized.

More specifically, the marking according to the invention is characterized by what is disclosed in the characterizing part of claim 1. The method according to the invention is characterized by what is disclosed in the characterizing part of claim 20. The use according to the invention is characterized by what is disclosed in the characterizing part of claim 26. The conductor structure according to the invention is characterized by what is disclosed in the characterizing part of claim 30. Considerable advantages are achieved by means of the invention. Surprisingly, it can be used for producing a surface that is invisible, yet suitably conductive for at least marking applications. The costs of the marking material can, however, be kept low, as only a small number of nanotubes are needed by giving up the need for DC conductivity.

In addition to the single-wall nanotubes being optically invisible when in a structure of a suitable density, single tubes are also electromagnetically invisible, i.e. they are not easily connected to an electromagnetic field. They can not dissipate considerable amounts of power. Thus they are in all ways very suitable for invisible coding of information.

Manufacturing single-wall tubes is also less expensive than manufacturing multi-wall tubes.

The invention can be applied to packages, labels, receipts, tickets, periodicals and newspapers. It can be used for improving product safety, adding, for example, an origin marking to a product, manufacturing printed sensors, activating electronic services, enhancing the brand of a company or a product and so on.

According to one preferred embodiment the amount of carbon nanotubes in the structure is relatively low, i.e. sufficiently low to make the structure essentially DC-resistive, but still essentially AC-conductive. Thus, the amount of direct contacts between the tubes is low, but the density of the tubes is sufficient for forming a sufficient amount of tunnel connections that are conductive at higher frequencies. The amount and orientation of direct contacts of the tubes can also be used for effecting both DC and AC resistivity.

The AC conductivity required from the structure is, however, most preferably limited, i.e. the structure is lossy. According to one preferred embodiment the square resistance of the conductor structure on operation frequency (e.g. 10 MHz-2.4 GHz) is 10 kOhm-MOhm when capacitively measuring the real part of the impedance. As has been disclosed above, the square resistance is regulated by the concentration of the nanotubes and the mutual order, i.e. the amount of and contacts between nanotubes.

The proportion of single-wall tubes of the carbon nanotubes is typically at least 75 %, preferably at least 90 %. Most preferably the structure according to the invention forms on the substrate an AC conductive marking that can be capacitively read from the substrate. By a marking we mean conductive and non-conductive (or less conductive) areas positioned on the substrate, their shape and relative location on the substrate being designed to encode information. Such an invisible code can be read from the substrate by, for example, capacitively reading the real part of the impedance of the conductor areas. The code can be a binary bit code or it can be realized in some other way, depending on the geometry of the areas and the amount of the conductor levels of the conductor areas.

By an invisible structure and marking we mean a conductor pattern that is invisible or nearly indistinguishable to the naked eye in the wavelengths of visible light.

In the following the preferred embodiments of the invention are described in more detail and with reference to the appended drawing. In the drawing figures Ia and Ib show the principle picture of a conductive marking according to one preferred embodiment in cross-section and seen from above, figures 2a and 2b show the principle picture of a conductive marking according to a second preferred embodiment in cross-section and seen from above, figures 3 a and 3b show the principle picture of a conductive marking according to a third preferred embodiment in cross-section and seen from above, figures 4a and 4b show the principle picture of a conductive marking according to a fourth preferred embodiment in cross-section and seen from above,

A carbon nanotube is a one-crystal tube-like structure that can be single-walled or multi- walled. Irrespective of the amount of walls they are conductive in either conductive or semi- conductive sense. Single-wall nanotubes conduct electricity extremely well, but due to the space structure of electrons their kinetic resistance is high and thus the speed of light is very low. This means that for example the resonance frequency of a 300 nm tube is considerably lower than the speed of light. Because of this even a relatively large amount of separate nanotubes having a length between, e.g. 100 nm and 1 um, forms a transparent surface. Multi- wall tubes are also difficult to distinguish when on a surface in a low concentration. The present structure can be achieved by bonding the nanotubes directly to the substrate or via a medium. A solution in which the tubes are dispersed or dissolved in a conductive liquid medium, whereby two tunnel connections are formed between the two tubes in an electrical sense. Due to the tunnel connections the conductivity of alternating current is sufficient, but not too high, for reliable measurement of the real part of capacitive impedance at a close distance when the structure is produced as a strip having a width of at least 50 run on a substrate.

The medium can be, for example, a non-conductive or conductive polymer or ink, depending on the desired properties of the surface, hi order to achieve an invisible end result, the medium used is, naturally, transparent or the same colour as the surface of the substrate when dry.

In general, when producing invisible structures according to the invention, it is desirable to avoid excessive forming of tunnel contacts and especially stronger ohmic contacts than these, in order to keep the interaction of the wavelengths of visible light with the structure (dissipations) low and thereby to keep the visibility of the tubes low as well.

The present invention can be made invisible regardless of whether the produced structure is only AC conductive or both AC and DC conductive.

The marking carried out by means of a conductive structure can be capacitively read from the substrate. By this we mean either the measurement of the capacitance of the conductor areas of the marking or, more preferably, capacitive measuring of the loss (real part of the impedance) caused by the conductor areas. Considering the latest case it is most preferable to have the square resistance of the conductor structure in the range from 10 kOhm to 1 Mohm depending on the used read electrode structure. Thus the method does not require a very good electrical conductivity, which makes it possible to produce an invisible conductor much easier. The advantages of the carbon nanotube structure according to the invention are emphasized in this particular application. Additionally the measurement can be made with a large frequency (most preferably 10 MHz - 2.4 GHZ), whereby there is no need for DC conductivity. The high-frequency measurement of the real part of the impedance of the surface is in this context a remarkable application, as the increase of capacitance between two points is not a sufficient condition in all situation for recognizing the encoding. This is due to the fact that as the reading is capacitive, the variation of capacitance is naturally so large that encoding can not necessarily be distinguished.

The conductor structure according to the invention, especially the encoding marking made using it, is most preferably carried out as one of four main embodiments or as a combination of these. Figures 1-4 represent these embodiments and they are described in more detail hereinaafter. The substrate is marked with reference numbers 10, 20, 30 and 40, correspondingly, and it can, depending on the embodiment, comprise one or more layers (basic layer 11, 21, 31 and 41 + a possible coating layer or a number of coating layers).

With reference to figures Ia and Ib, according to the first embodiment on the well isolating substrate 10 is a marking layer 13 consisting essentially only of nanotubes 16. In the example of figure the substrate consists of a basic layer 10 and a coating 11 applied thereon, whereby at least the coating 11 is isolating. The nanotubes 16 are in contact with each other so that there is a tunnel connection between them. The orientation of the tubes 16 is either symmetrical (arbitrary order) or they have been partially oriented for improving electrical conductivity. The essential point about the structure, however, is that in order to achieve conductivity the amount of tubes 16 on the surface is sufficient and that there are sufficiently tunnel contacts between them. Otherwise the conductivity is not sufficient. In other words, in this embodiment the electrical conductivity of the surface is achieved only by means of nanotubes 16 and the measured real part of the impedance is determined on the basis of the mutual contacts of the nanotubes 16.

The nanotubes can, of course, be applied only on the desired areas of the substrate. Figure Ib shows a code consisting of a narrow (such as 50-150 um) rectangular strip 14 and a wide (such as 100-200 um) rectangular strip.

The marking can also be applied so that Hie nanotubes 16 are first brought to a larger area, e.g. the total area of the paper. Liquid can then later be added to some areas 14, 15 and the tunnel contacts on these areas 14, 15 can be increased by evaporating the liquid or its chemical reactions. On the other hand, it is possible to add contacts on certain areas also by means of a strong electric field. (An external field, especially an alternating current field, increases very much between the tubes and draws the tubes together.) Subsequent to the contact, Van der Waals forces keep the nanotubes in contact with each other. The increase of the amount of contacts leads to decrease of the real part of the impedance and thus to a recognizable code. Especially contacts realized by means of electrical field are preferred because this allows manufacture of an easily writable and reliably readable invisible marking. Writing realized by means of evaporation, chemical reaction or an electric field can also be applied to the embodiments shown in figures 2-4.

Figures 2a and 2b illustrate a code layer 23 laid on substrate 20, the substrate being conductive, dielectrically conductive or having a considerable amount of polarization losses. In this case it will be enough that the nanotubes 26 only orientate the electric field and no actual conductivity-causing tunnel resistance between the tubes 26 is not necessary. It is very useful if the tubes 26 can be oriented parallel (especially if the surface is polar and only contains polar losses) with the electric reading field (i.e. usually also parallel with the longitudinal axis rectangular area 24 or 25 of the code or other axis perpendicular to the reading direction). The structure leads to a recognizable real part in a serial impedance model with only large (such as over 10 MHz) frequencies. If the surface of the substrate 20 is conductive with a DC field as well, the whole structure is also typically conductive, because the electrons can tunnel between the nanotubes 26 and the substrate structure 20.

Figures 3a and 3b illustrate a structure in which the surface of the substrate 32 comprises a layer of electrically lossy (conductive or dielectrically lossy) material that is necessary in case the basic layer 32 does not other wise cause losses. Preferably the layer is thin and it is applied on the whole coded area 37, i.e. also on places where nanotubes 36 are not applied to. The advantage of a separate substrate material layer 32 is that it can be optimized either to be invisible or it can be white, whereby it is not distinguishable from e.g. white paper. The material can also be partially conductive (e.g. thin conductive polymer layer) or a dielectrically lossy material (e.g. pyroelectric or ferroelectric material). It can also be a mixture of conductive and polar material. A coding like this makes it possible to separately optimize the nanotube material 33 applied on the area of the bottom material and the encoding, whereby the encoding can be made, in addition to being invisible, also specially easy to read. In this method the amount of material can be minimized, as conductivity does not require a galvanic contact between the nanotubes 36. For the same reason the tubes 36 can be oriented strongly parallel for increasing the polarization effect.

Figures 4a and 4b show an encoding in which a polar or badly electrically conductive material (such as a conductive polymer) is mixed together with nanotubes 46 into a solvent for forming a nanotube-containing mixture. The mixture is applied as a layer 48 on a substrate 40 and allowed to dry. This embodiment allows producing an AC-conductive ink in which the amount of nanotubes is very small. The conductive polymer or the like conductor can be used in very small amounts for producing a sufficient conductivity, whereby the ink is preferred in this respect as well. The principle is to improve the properties of an invisible, badly conductive ink with a small amount of nanotubes 46. In this application it is also possible to use a large a amount of multi-wall tubes, if their clustering can be avoided. Because the ink also includes another conductive material, such as a polymer, it is possible to separate the nanoparticles from each other with these polymers.

Packages, for example, are often provided with a coating consisting of metal particles for increasing decorativity. These layers are not typically electrically conductive at low frequencies, as there is not a good electric contact between the metal particles. If a nanotube structure is applied below or on top of such layers, the AC conductivity will increase significantly. Especially, if the tubes are under a layer, a very inconspicuous marking can be produced.

The above disclosure makes it obvious that AC conductivity is important, especially at frequencies over 10 MHz. The construction can also be DC-conductive, even though this is not a requirement. If the surface is DC-conductive as well, it could possibly be used in an electric paper on in paper displays. An AC/DC conductive surface can also be used for removing electrical interferences or electrostatic charges.

In general, the structures according to the invention can be produced by means of ink containing a liquid medium, such as ink or polymer, and nanotubes dispersed therein. The main portion of the dispersed nanotubes consists most preferably of single- wall nanotubes, as has been disclosed in the above. The above-mentioned embodiments and the embodiments defined in the appended claims can be varied within the inventive idea described herein for producing in addition to information- encoding markings many other kinds of invisible conductor structures and products comprising such structures and for using them in various applications, such as marking applications, electronics applications and the like.

Claims

We claim:

1. A marking, capacitively readable by means of an AC field, the marking comprising electrically lossy conductor areas arranged in an information-encoding geometry on a substrate, characterized in that the conductor areas comprise nanotubes for forming a marking difficult to perceive with the naked eye.

2. A marking according to claim 1, characterized in that at least a main portion, typically at least 75 %, preferably at least 90 %, of the carbon nanotubes are single-walled.

3. A marking according to claim 1 or 2, characterized in that the carbon nanotubes are arranged so that they produce a considerable electric polarization on the conductor areas.

4. A marking according to any of the previous claims, characterized in that the square resistance at a frequency of over 10 MHz, preferably at a frequency chosen from the range of from 10 MHz to 2.4 GHz is 10 kOhm-1 MOhm, when capacitively measuring the real part of the impedance.

5. A marking according to any of the previous claims, characterized in that the conductor areas mainly contain nanotubes having a length from 100 nm to 1 um.

6. A marking according to any of the previous claims, characterized in that the nanotubes are directly bonded to the surface of the substrate on the conductor areas.

7. A marking according to any of claims 1-5, characterized in that the nanotubes are applied on conductor areas onto the surface of the substrate mixed with a medium, such as ink or a polymer, preferably with an inherentlyconductive medium, such as a conductive polymer.

8. A marking according claim 7, characterized in that the said mixture is such that nanotubes form tunnel connections with the medium.

9. A marking according to any of the previous claims, characterized in that the surface of the substrate is conductive and a considerable proportion of the tubes form tunnel connections with the substrate.

10. A marking according to any of the previous claims, characterized in that the substrate comprises a layer of electrically lossy material applied onto the surface on which the said conductor areas are located, whereby a considerable portion of the nanotubes form a tunnel connection with this material layer.

11. A marking according to any of claims 1-9, characterized in that the surface of the substrate is isolating and that a considerable amount of nanotubes are in contact with another nanotube so that a tunnel connection is formed between them.

12. A marking according to any of the previous claims, characterized in that the nanotubes are at least partially oriented so as to be parallel with each other for increasing polarization and AC conductivity in the layer.

13. A marking according to any of the previous claims, characterized in that the electrical conductivity of one or more conductor areas is increased by means of, for example, evaporation of liquid, chemical reactions or an electric field.

14. A marking according to any of the previous claims, characterized in that the coating containing metal particles has been applied under or over the layer of carbon nanotubes, whereby the carbon nanotube layer increases the electric conductivity of the coating comprising metal particles.

15. A marking according to any of the previous claims, characterized in that the conductor areas are arranged as a barcode-like formation comprising areas of different widths.

16. A marking according to any of the previous claims, characterized in that at least a portion of the carbon nanotubes are in ohmic contact with each other for producing DC conductivity as well.

17. A marking according to any of the previous claims, characterized in that the substrate comprises a bottom layer, such as a paper or cardboard sheet and a printing arranged onto the surface of the bottom layer.

18. A marking according to any of the previous claims, characterized in that the conductor areas are essentially transparent, the dampening of visible light being typically less than 10 %.

19. A marking according to any of the previous claims, characterized in that it is essentially non-conductive for direct current.

20. A method of producing a marking capacitively readable and difficultly perceivable by the naked eye by means of an AC field, in which method electrically lossy conductive material is applied onto a substrate in an information-encoding geometry, characterized in that a conductive material containing carbon nanotubes is used.

21. A method according to claim 20, characterized in that subsequent to application the mutual electrical contacting of the carbon nanotubes is improved or decreased or their orientation is changed on a portion of the area of the substrate.

22. A method according to claim 21, characterized in that contacting is improved or decreased by adding liquid to the substrate and evaporating it or by means of an external electric field.

23. A method according to any of claims 20-22, characterized in that the nanotubes are bonded directly to the surface of the substrate.

24. A method according to any of claims 20-23, characterized in that a marking according to any of claims 1-19 is formed.

25. A method according to any of claims 20-24, characterized in that the carbon nanotubes are printed onto a substrate when dispersed in a medium.

26. Use of carbon nanotubes for producing markings, which are capacitively readable, information-encoding and AC-conductive but electrically lossy and that do not essentially attenuate visible light.

27. The use according to claim 26 in applications of printed electronics.

28. The use according to any of claims 26-27, in which the main portion of the carbon nanotubes, typically at least 75%, preferably at least 90 %, are single-walled.

29. The use according to any of claims 26-28, in which the carbon nanotubes are mixed in a medium, such as ink or polymer, preferably a medium conductive in itself, such as a conductive polymer.

30. A conductor structure comprising

— a substrate, and — a capacitively readable lossy conductor material pattern in an information-encoding geometry applied onto the surface of the substrate, characterized by the combination that

- the conductor material pattern is formed by means of a material composition comprising carbon nanotubes, and - the conductor material pattern does not essentially attenuate visible light.