JP2011198923A - Method of manufacturing circuit board, the circuit board, and method of manufacturing the circuit board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To fine a silver film after baking, and to improve the working efficiency.SOLUTION: While a droplet-like ink 2 discharged from an inkjet head 1 is flying, or immediately after the droplet-like ink 2 is impacted on a substrate 3, laser beams 4 are applied to the droplet-like ink 2 and particles adjacent to metal nanoparticles and/or metal oxide nanoparticles within the droplet-like ink 2 are combined, and the droplet-like ink includes metal nanoparticles and/or metal oxide nanoparticles in a different composition. By enlarging the partial metal nanoparticles or metal oxide nanoparticles, metal nanoparticles and/or metal oxide nanoparticles in a minute diameter originally included and enlarged metal nanoparticles or metal oxide nanoparticles are mixed in ink printed on the substrate 3.

Description

  The present invention relates to a method for manufacturing a circuit board and the like, in which fine droplets of ink are ejected from an inkjet head, and droplets are landed on a required location on a substrate in accordance with a device pattern.

  In recent years, electronic components such as notebook PCs and smartphones have been required to be lighter, thinner, and smaller as electronic devices become smaller, thinner, lighter, and more functional. In particular, in order to realize a low-carbon society, it is also desired that the manufacturing process in the industry be changed to a form with less environmental load. From this point of view, technology that applies printing technology to the manufacture of electronics is attracting attention, and a new field of printed electronics is flourishing. Unlike the process of repeating film formation and etching based on the conventional photolithography technology, the printed electronics is composed of a process of applying a necessary amount of material to a necessary place. For this reason, there is little waste of material and energy consumption is also low.

  There are various printing technologies such as flexographic printing, gravure printing, and screen printing. Among them, the inkjet method can design and manufacture circuits on demand, and patterns can be created without removing materials. Because it can be formed, it is especially expected as an environmentally friendly technology.

  In the ink jet method, ink in the form of minute droplets is ejected from an ink jet head, and droplets are landed on a necessary place on a substrate in accordance with a device pattern. Ink is a liquid containing functional materials. When forming conductor wiring, silver complex ink (ink complexed with silver and dissolved in solvent) or silver nanoparticle ink (silver from several nm to several tens of nm) Ink dispersed in a solvent is generally used. The droplet-like ink ejected from the nozzle of the head flies through the space between the head and the substrate facing it, and landed on the substrate. The landed droplets flow on the substrate according to the interfacial energy between the substrate and the ink, and spread. A film-like pattern made of a functional material is obtained by evaporating the ink solvent. In the case of metal nanoparticle inks including silver, silver particles are sintered by further heat treatment, and a wiring pattern having conductivity close to a bulk can be formed.

  There are two reasons why extremely small particles of several to several tens of nm are used for the silver nanoparticle ink. One is to obtain the dispersion stability of the particles in the ink. Metals have a density as high as several to several tens of times that of solvents, so they tend to settle due to gravity and produce precipitates. Therefore, by reducing the particle size, a stable dispersion state can be realized by the effect of the viscous resistance of the solvent and the Brownian motion. Another is to lower the firing temperature. It is known that the melting point of the metal particles decreases as the metal particles become finer, and the particle size particularly drops rapidly from about 10 nm. For example, the melting point of silver is 961 ° C. in the bulk state, but it is considered to drop to about a few hundred degrees at a few dozen nm, and a silver circuit can be easily formed on a resin substrate or the like. In addition, in the ink using metal nanoparticles, it coats with the organic film so that metal nanoparticles may not sinter in ink.

  The conventional ink jet method has a problem that a fine pattern cannot be drawn because the droplet 21 that has landed on the substrate 3 spreads greatly as shown in FIG. As a countermeasure against this, a technique has been adopted in which the wetting and spreading of the ink is suppressed by applying a liquid repellent treatment to the substrate surface, but the uneven wetting and spreading of the droplets called the bulge effect and the film thickness called the coffee stain phenomenon. Since nonuniformity is likely to occur, the limit of miniaturization is about 30 to 50 μm for the wiring width.

  In addition, since silver nanoparticle ink contains a large amount of organic substances such as solvents and dispersants, a large number of voids (bubble cavities) are generated in the fired silver film, and the electrical resistance is orders of magnitude higher than that of the bulk. The problem of getting high in the difference was easy to occur.

  Furthermore, there is a problem that work efficiency is low. In order to obtain a practical low resistance wiring, it is necessary to have a certain thickness of the wiring. However, in the metal nanoparticle ink, the metal content in the ink is at most about 10% by volume, and this ink is on the substrate. Therefore, a thickness of only about a submicron can be obtained only by landing one droplet. For this reason, in order to obtain a thick wiring, it was necessary to repeatedly apply the coating.

International Publication No. 2009/076023 JP 2006-239899 A JP 2004-288517 A

  An object of the present invention is to provide an ink jet method and an ink jet apparatus capable of achieving miniaturization of metal or metal oxide film and improvement of workability in forming a wiring pattern on a substrate by the ink jet method.

  According to one embodiment of the present invention, a laser beam is ejected from an inkjet head and droplets of ink containing metal nanoparticles and / or metal oxide nanoparticles having different compositions are flying or immediately after landing on a substrate. Irradiate the droplet-shaped ink, and select the wavelength of the laser beam as the wavelength included in the absorption band caused by the surface plasmon of the metal nanoparticles and / or metal oxide nanoparticles in the droplet-shaped ink. , A metal obtained by enlarging some of the metal nanoparticles or metal oxide nanoparticles in the droplet-like ink and enlarging the ink printed on the substrate with metal nanoparticles or metal oxide nanoparticles having a small diameter An ink jet method is provided in which nano particles or metal oxide nanoparticles are mixed.

  According to another aspect of the present invention, a laser that is ejected from an ink jet head and includes droplets of ink containing metal nanoparticles and / or metal oxide nanoparticles having different compositions is in flight or immediately after landing on a substrate. Light is applied to the droplet ink, and the wavelength of the laser beam is selected as the wavelength included in the absorption band caused by the surface plasmon of the metal nanoparticles and / or metal oxide nanoparticles in the droplet ink. Then, a part of the metal nanoparticles or metal oxide nanoparticles in the droplet-shaped ink is enlarged, and the ink printed on the substrate is enlarged with the metal nanoparticles or metal oxide nanoparticles having a small diameter. Ink jet method characterized in that mixed metal nanoparticles or metal oxide nanoparticles are mixed, wherein the metal nanoparticles and / or metal oxide nanoparticles have a particle size of 100 nm or less Furthermore inkjet method is provided, wherein.

  Further, according to another aspect of the present invention, liquid droplets ejected from an inkjet head and containing metal nanoparticles and / or metal oxide nanoparticles having different compositions are flying or just after landing on a substrate. In this case, the wavelength of the laser beam is included in the absorption band caused by the surface plasmon of the metal nanoparticles and / or metal oxide nanoparticles in the droplet ink. In order to enlarge some of the metal nanoparticles or metal oxide nanoparticles in the droplet-shaped ink, the ink printed on the substrate is changed to a metal nanoparticle or metal oxide nanoparticle with a small diameter. Provided is a substrate provided with a circuit by an ink jet method, characterized in that enlarged metal nanoparticles or metal oxide nanoparticles are mixed.

  Further, according to another aspect of the present invention, liquid droplets ejected from an inkjet head and containing metal nanoparticles and / or metal oxide nanoparticles having different compositions are flying or just after landing on a substrate. In this case, the wavelength of the laser beam is included in the absorption band caused by the surface plasmon of the metal nanoparticles and / or metal oxide nanoparticles in the droplet ink. In order to enlarge some of the metal nanoparticles or metal oxide nanoparticles in the droplet-shaped ink, the ink printed on the substrate is changed to a metal nanoparticle or metal oxide nanoparticle with a small diameter. Provided is an electronic component and / or an electronic device using a substrate manufactured by an ink jet method, characterized in that enlarged metal nanoparticles or metal oxide nanoparticles are mixed.

  In addition, according to another aspect of the present invention, the ink jet head has an ink jet head for landing liquid droplet ink on a substrate, and the liquid ink ejected from the ink jet head is flying or landing on the substrate. An ink jet apparatus comprising: a laser that irradiates a droplet-shaped ink with laser light immediately after being formed, wherein the ink includes metal nanoparticles and / or metal oxide nanoparticles having different compositions Provided.

  In addition, according to another aspect of the present invention, the ink jet head has an ink jet head for landing liquid droplet ink on a substrate, and the liquid ink ejected from the ink jet head is flying or landing on the substrate. An ink jet apparatus characterized by having a laser for irradiating a drop-shaped ink with laser light immediately after being formed, wherein the ink contains metal nanoparticles and / or metal oxide nanoparticles having different compositions The wavelength of the laser beam is a wavelength included in an absorption band caused by a surface plasmon of the metal nanoparticles and / or metal oxide nanoparticles in the ink. The

  According to the present invention, a low resistance and fine wiring pattern can be obtained, and a thick film wiring can be easily formed.

FIG. 1A is a conceptual diagram of the present invention. FIG. 1B is an enlarged view of a droplet. It is a graph which shows the absorptance with respect to the laser wavelength for every size of a metal nanoparticle. It is a graph which shows the absorptance with respect to the laser wavelength for every kind of metal nanoparticle. It is a conceptual diagram of a prior art.

  Next, an embodiment of the present invention will be described with reference to the drawings. The light applied to the droplet-shaped ink is not limited to laser light, but here, description will be made using laser light.

  FIG. 1 schematically illustrates the present invention.

  In the present embodiment, as shown in FIG. 1, the inkjet head 1 is arranged so that the droplet-like ink 2 is landed on the substrate 3 on which the wiring is formed. A laser 5 that irradiates a laser beam 4 immediately after the droplet-like ink 2 containing the metal nanoparticles or metal oxide nanoparticles ejected from the ink jet head 1 has landed on the substrate is disposed. Is done. FIG. 1B is a conceptual diagram of the ink 2. In FIG. 1B, reference numeral 221 indicates first metal nanoparticles, and reference numeral 223 indicates second metal nanoparticles. In addition, although the laser 5 was used as the light source below, the light source is not limited to the laser 5.

  In addition, although it is also possible for the metal nanoparticles to be conductive metal oxide nanoparticles, in the following description, in order to avoid complication, the metal nanoparticles or metal oxide nanoparticles are simply referred to as metal nanoparticles. Sometimes called particles. Furthermore, as a combination of metal nanoparticles and / or metal oxide nanoparticles having different compositions, a combination of metal nanoparticles having different compositions, a combination of metal nanoparticles and metal oxide nanoparticles, or metal oxide nanoparticles having different compositions. There are combinations.

  According to one aspect of the present invention, as shown in FIG. 1A, which is a conceptual diagram, a laser beam is emitted immediately after the droplet-like ink 2 ejected from the inkjet head 1 is flying or immediately after landing on the substrate 3. 4 is irradiated. At this time, the wavelength of the laser beam 4 is selected as the wavelength included in the absorption band caused by the surface plasmon of the metal nanoparticles, and the metal nanoparticles in the droplet-like ink 2 are combined with the adjacent particles. In this way, some of the metal nanoparticles in the ink are enlarged to make the ink printed on the substrate a mixture of fine metal nanoparticles and enlarged metal nanoparticles. To do.

  In the above description, “mixed” may include particles in which metal nanoparticles are bonded in addition to enlarged metal nanoparticles and non- enlarged metal nanoparticles.

  The significance is described next.

  Free electrons collectively vibrate inside the metal, which is called plasmon. In general, plasmons do not interact with light, but there are special plasmons that interact with light on the metal surface, which are called surface plasmons. The interaction means that the light is absorbed, and is a phenomenon that becomes remarkable in a specific wavelength band. This is shown in FIG. FIGS. 2 and 3 show how the peak of the wavelength of light absorbed by the metal nanoparticles changes depending on the size (particle size) of the metal nanoparticles (Au or the like). In FIG. 2, the wavelength dependence of the optical absorptance when the size of the Au nanoparticles is 2 nm, 12 nm, and 50 nm, respectively, is indicated by a one-dot chain line, a solid line, and a dotted line. In FIG. 2, the wavelength of light at the absorption peak is the wavelength of the surface plasmon of the corresponding metal nanoparticle. FIG. 1B shows metal nanoparticles 221 and 223 having different compositions in the ink 2. The vibration frequency of surface plasmons varies depending on the type of metal nanoparticles. This is shown in FIG. 3 for the absorption bands of gold and silver nanoparticles.

  Therefore, when the ink droplets are irradiated with light that interacts with the surface plasmons of the metal nanoparticles, only the metal nanoparticles having a specific size and specific composition in the ink can be selectively heated. Only the metal nanoparticles having the composition can easily be enlarged.

  Further, since the vibration frequency of the surface plasmon varies depending on the particle diameter, the interaction with the laser beam becomes weaker as the particles become larger, and it becomes difficult to be heated. Thereby, even if it irradiates with a laser beam excessively, a metal nanoparticle does not enlarge excessively.

  The wavelength of light that is not absorbed by the ink solvent is preferably selected as an embodiment of the present invention. Thereby, rapid evaporation of the solvent can be prevented. For this reason, fluctuations in the flying trajectory of the droplet-like ink due to the evaporation of the solvent can be kept low, and the accuracy of the landing position can be prevented from being lowered.

  As an embodiment of the present invention, it is preferable to selectively irradiate a specific liquid droplet in synchronization with the timing at which the ink is ejected. The significance of this is as follows.

  In order to reliably irradiate the droplet-shaped ink with laser light, it is preferable to make the beam diameter larger than the droplet. However, in this case, if the intensity of light is large, all the metal nanoparticles contained in one droplet may be enlarged. Therefore, for example, irradiation is performed for each droplet, such as irradiating the first droplet and not irradiating the second droplet.

  The ink that has landed on the substrate is mixed with the ink that adjoins (that is, landed immediately before) due to its fluidity, and as a result, the metal nanoparticles having different particle diameters can be mixed.

  Further, the embodiment will be supplemented. When light that interacts with the surface plasmon of one metal nanoparticle is irradiated, only the metal nanoparticle in the ink can be selectively heated as described above. Moreover, since the peak wavelength of absorption shifts to a longer wavelength as the metal nanoparticles become enlarged, the interaction with the laser beam gradually weakens and becomes difficult to be heated. Thereby, even if it irradiates with a laser beam excessively, a metal nanoparticle does not enlarge excessively. In addition, the other metal nanoparticles have a small absorption at the wavelength of the laser beam, so this hardly becomes a core and enlarges (however, the start of enlargement is from one metal nanoparticle, As a result, the other metal nanoparticles are included as a result of aggregation and fusion with adjacent metal nanoparticles.) Therefore, by simply irradiating the droplets with laser light, it is possible to obtain a state in which metal nanoparticles having a small diameter originally contained and enlarged metal nanoparticles are mixed. Moreover, the size distribution of the particles contained in the ink after irradiation with the laser light can be easily controlled by changing the ratio of the respective metal nanoparticles.

  The above means can be used in combination.

  According to the present embodiment, when the ink landed on the substrate and the substrate are heated and baked, the enlarged metal nanoparticles are present, so that a low-resistance wiring can be easily obtained. Further, the presence of fine metal nanoparticles that are not irradiated with laser light does not impair the property of “can be fired at a low temperature”. In addition, since particles with a small diameter enter and accumulate in the gaps between the enlarged particles, the ratio of organic substances is small and voids are difficult to be formed. This effect also makes it easy to obtain low resistance wiring.

  Furthermore, since the enlargement of the metal nanoparticles does not occur in the ink jet head, the dispersibility of the metal nanoparticles in the ink does not occur. Therefore, the ink is not clogged in the ink jet head.

  Furthermore, since the solvent evaporates to some extent when irradiated with light, the fluidity of the ink decreases. As a result, wetting and spreading after landing on the substrate is reduced, so that fine wiring can be obtained, and even if droplets of ink are overprinted, the coffee stain phenomenon is less likely to occur, so the thick film is easily uniform. Can be formed.

  The “ink” applied in the present invention is exemplified below.

(1) An ink in which nanoparticles are dispersed in a solvent, in which two or more kinds of particles having different compositions are mixed is desirable.

(2) The nanoparticles preferably have an average particle size of 100 nm or less. If it is larger than this, the nanoparticles will settle in the ink, causing clogging in the ink head. In addition, since the melting point is high, the sintering temperature must be increased, which is not suitable because it causes thermal damage to the substrate.

(3) The nanoparticle is a metal and / or alloy containing at least one of gold, silver, copper, palladium, platinum, tin, bismuth, cobalt, chromium, iron, nickel, indium, titanium, and zinc, or a metal oxide The thing containing is preferable.

(4) The nanoparticles are preferably individually coated with an organic film.

(5) The solvent is a hydrocarbon solvent such as hexane, toluene, decane, or tetradecane, an unsaturated aliphatic alcohol such as oleyl alcohol, or a branched chain such as 2-octyldodecanol, isocetyl alcohol, or isotridecyl alcohol. Those containing an aliphatic alcohol or an ester solvent such as isopropyl myristate, isostearyl palmitate and ethyl oleate are preferred.

An ink containing Ag nanoparticles and Au nanoparticles was printed on a polyimide resin substrate by an inkjet method. Au nanoparticles having an average particle diameter of 4 nm and Ag nanoparticles having an average particle diameter of 6 nm were used. Ag nanoparticles and Au nanoparticles are mixed at a weight ratio of 1: 1. The inkjet head used a piezo drive type and ejected approximately 6 pl of droplet-like ink at a cycle of 200 Hz. The ink-jet head is separated from the substrate surface by about 1 mm, and the laser beam is irradiated from the lateral direction almost immediately before the droplets land on the substrate. In order to selectively heat the Au particles among the nanoparticles, the second harmonic (wavelength 532 nm, 35 mJ / pulse) of the Nd-YAG laser was used as the laser. In order to selectively heat the Ag particles, the second harmonic of the titanium sapphire laser may be irradiated,
Absorption bands due to surface plasmons are on the longer wavelength side of Au, and the absorption band peak shifts to the longer wavelength side as the nanoparticles become larger. preferable. The substrate was not heated during printing.

  When the ink landed on the substrate was observed with TEM, large particles of several tens of nm were found. As a result of investigation, this could be classified into those in which many fine particles were aggregated and those in which they were dissolved into one particle. On the other hand, such particles were not found in the ink printed by the ink jet method without irradiating the laser beam. From this, it was confirmed that aggregation and fusion (enlargement) of the metal nanoparticles in the ink proceeded by laser irradiation.

  When this ink was printed on a substrate by an ink jet method and heated in the atmosphere at 220 ° C. for 15 minutes and sintered, a circuit pattern having a metallic luster was obtained. The electrical resistivity was about 12 μΩ · cm, and a high-quality thin film about 6 to 7 times that of bulk Ag of 1.59 μΩ · cm or bulk Au of 2.21 μΩ · cm could be obtained.

  An ink containing Ag nanoparticles and Au nanoparticles was printed on a polyimide resin substrate by an inkjet method. Au particles having an average particle diameter of 4 nm and Ag particles having an average particle diameter of 6 nm were used. Ag nanoparticles and Au nanoparticles are mixed at a weight ratio of 9: 1. The inkjet head used a piezo drive type and ejected approximately 6 pl of droplet-like ink at a cycle of 200 Hz. The laser light is irradiated from the lateral direction almost immediately after the droplets land on the substrate. In order to selectively heat the Au particles among the nanoparticles, the second harmonic of the Nd-YAG laser (wavelength 532 nm, 25 mJ / pulse) was used. In order to selectively heat the Ag particles, the second harmonic of the titanium sapphire laser may be irradiated, but for the same reason as in Example 1, it is more preferable to enlarge the Au particles by this treatment. The substrate was not heated during printing.

  When this ink was printed on a substrate by an ink jet method and heated in the atmosphere at 240 ° C. for 25 minutes and sintered, a circuit pattern having a metallic luster was obtained. This electrical resistivity was about 7μΩ · cm, and a good quality thin film about 4 times that of 1.59μΩ · cm for bulk Ag or 2.21μΩ · cm for bulk Au could be obtained.

  An ink containing Ag nanoparticles and Cu nanoparticles was printed on a liquid crystal polymer substrate by an inkjet method. Ag particles having an average particle diameter of 3 nm and Cu particles having an average particle diameter of 8 nm were used. Ag nanoparticles and Cu nanoparticles are mixed in a weight ratio of 9: 1. The ink jet head was an electrostatic suction type, and about 0.5 pl of droplet-like ink was ejected at a cycle of 2 kHz. The laser light is irradiated from substantially the lateral direction immediately before the droplets land on the substrate. In order to selectively heat Cu particles among the nanoparticles, an argon ion laser (wavelength: 514 nm, 1 W) was used as the laser. In order to selectively heat the Ag particles, the second harmonic of the titanium sapphire laser may be irradiated. However, since Cu is easily oxidized, it is not easy to sinter compared to Ag nanoparticles. For this reason, Cu particles are preferred for enlargement. The substrate was not heated during printing.

  When this ink was printed on a substrate by an ink jet method and heated at 250 ° C. for 30 minutes and sintered, a circuit pattern having a metallic luster was obtained. This electrical resistivity was about 10μΩ · cm, and a high-quality thin film about 6 times that of bulk Ag of 1.59μΩ · cm or bulk Cu of 1.68μΩ · cm could be obtained.

  On a glass substrate, ink containing Au nanoparticles and Cu nanoparticles was printed by an inkjet method. Au particles having an average particle diameter of 4 nm and Cu particles having an average particle diameter of 8 nm were used. Au nanoparticles and Cu nanoparticles are mixed in a weight ratio of 20:80. The ink jet head was a piezo type, and about 4 pl of liquid droplet ink was ejected at a cycle of 50 Hz. The laser light is irradiated from substantially the lateral direction immediately before the droplets land on the substrate. A dye laser (wavelength 560 nm, 10 mJ / pulse) was used as a laser to selectively heat Cu particles among the nanoparticles. The Cu particles are preferably enlarged by this treatment for the same reason as in Example 3. The substrate was not heated during printing.

When this ink is printed on the substrate by the ink jet method and heated at 350 ° C for 30 minutes in air at 350 ° C for 30 minutes and heated to 96% N ₂ + 4% H ₂ , a glossy circuit pattern is obtained. It was. The electrical resistivity was about 10 μΩ · cm, and a high-quality thin film about 6 times that of bulk Ag of 1.59 μΩ · cm or bulk Cu of 1.67 μΩ · cm could be obtained.

  As an embodiment, the metal nanoparticles may be indirectly heated by irradiating the substrate with laser light.

  Furthermore, it is not essential to heat the substrate.

  The laser light irradiation position and its effect will be described.

  When irradiating a laser beam before the droplet-shaped ink is flying (that is, immediately before being ejected from an inkjet nozzle and landing on the substrate), it can be irradiated under the same conditions without being affected by the surface state of the substrate. There are advantages. Further, when laser light is irradiated immediately after the droplet-shaped ink has landed on the substrate, the light transmitted through the ink is reflected on the surface of the substrate and irradiated again on the ink, so that there is an advantage that efficiency is good.

DESCRIPTION OF SYMBOLS 1 Inkjet head 2 Ink droplet 21 in flight The droplet-shaped ink 23 which landed on the board | substrate in a prior art The droplet-shaped ink 221 landed on the board | substrate in this invention 1st metal nanoparticle 223 2nd metal nanoparticle 3 Substrate 4 Laser light 5 Laser

Claims (6)

  The droplet-shaped ink that is ejected from the ink jet head and containing droplets of metal nanoparticles and / or metal oxide nanoparticles having different compositions is flying or immediately after landing on the substrate. At this time, the wavelength of the laser beam is selected as the wavelength included in the absorption band caused by the surface plasmon of the metal nanoparticles and / or the metal oxide nanoparticles in the droplet-like ink, and one of the droplet-like inks is selected. A part of the metal nanoparticles or metal oxide nanoparticles enlarged, and the ink printed on the substrate with the metal nanoparticles or metal oxide nanoparticles enlarged and the metal nanoparticles or metal oxide nanoparticles enlarged An ink jet method characterized in that a mixed state is formed.   The inkjet method according to claim 1, wherein the metal nanoparticles and / or metal oxide nanoparticles having different compositions have a particle size of 100 nm or less.   A substrate provided with a circuit manufactured by the method according to claim 1.   An electronic component and / or an electronic device using the substrate according to claim 3.   An inkjet head for landing droplet-shaped ink on a substrate, and the droplet-shaped ink is irradiated with laser light while the droplet-shaped ink ejected from the inkjet head is flying or immediately after landing on the substrate. An ink jet apparatus, wherein the ink contains metal nanoparticles and / or metal oxide nanoparticles having different compositions.   6. The ink jet apparatus according to claim 5, wherein the wavelength of the laser light is a wavelength included in an absorption band caused by surface plasmons of metal nanoparticles and / or metal oxide nanoparticles in the ink.

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