FR2974021A1 - PROCESS FOR THE PREPARATION OF METALLIC PARTICLES - Google Patents

Abstract

This method for the preparation of metal particles consists in: depositing a metal layer (2) on a substrate (1); irradiating (4) the metal layer with a laser; a blade (3) of transparent or quasi-transparent material at the wavelength of the laser being interposed between the metal layer (2) and the laser source.

Description

FIELD OF THE INVENTION The invention relates primarily to a process for the preparation of microparticles and metal nanoparticles by laser irradiation of a metal layer.

The present invention is of particular interest in all fields in which metal particles can be used, including, but not limited to, catalysis, electronics, or health.

PRIOR STATE OF THE TECHNIQUE

The preparation of microparticles (10-7 to 10-4 m) and metal nanoparticles (<10-7 m) has been the subject of numerous studies which have made it possible to develop various methods of synthesis, in particular by chemical means (vapor phase). liquid phase, solid or mixed medium), physicochemical (physical vapor phase deposition), physical, mechanical (wear or high energy grinding). Indeed, the optimization of the synthesis conditions makes it possible to obtain the narrowest particle size distribution possible, but also to favor a precise particle shape. The control of these parameters presents a major stake, the properties of the particles of the same metal being able to vary according to their size and their form. For example, magnetic rod-shaped particles do not necessarily have the same properties as particles of the same but smaller metal, and in the form of spheres.

Among the most common metal particle synthesis methods, there may be mentioned the chemical vapor phase, liquid phase, solid medium or mixed medium.

With regard to physicochemical synthesis, methods using a thermal plasma, changes of state such as evaporation and condensation, or physical vapor deposition are among the most used.

SUMMARY OF THE INVENTION

The Applicant has developed a method for preparing metal particles physico-chemically, by laser irradiation of a thin metal layer. This method particularly relates to laser ablation which consists in irradiating a target of a given material with a focused laser beam.

More specifically, the present invention relates to a process for the preparation of metal particles, consisting in: depositing a metal layer on a substrate; irradiating the metal layer with a laser; a blade of material transparent to the wavelength of the laser being interposed between the metal layer and the laser source.

By "transparent material" or "material transparent to the wavelength of the laser" is meant a transparent or quasi-transparent material at the wavelength of the laser, that is to say a material which, advantageously , does not absorb or very little, the incident energy of the laser, usually less than 20%.

The laser used is advantageously of the excimer or exciplex type. It is therefore mainly an ultraviolet laser. However, in a particular embodiment, the laser may be a CO2 laser in the infra-red domain.

Advantageously, the excimer laser used in the context of the present invention may be chosen from the group comprising excimer lasers F2 (157 nm), ArF (193 nm), KrF (248 nm), XeBr (282 nm), XeC1 (308 nm), XeF (351 nm), CaF2 (193 nm), KrC1 (222 nm), or Cl2 (259 nm).

The photochemical process of the invention is a photo-ablation process. It involves in particular: the absorption of the incident energy by the metal layer; thermal propagation within the metal layer; the breaking of the chemical bonds within the metal layer; and the desorption of the metal particles, the particles are detached from the substrate. In view of the increase in heat caused by the laser irradiation to which it is subjected, the metal layer melts and may possibly in some cases vaporize. Thus, the temperature at the surface of the metal layer may be greater than the melting point of the metal.

Moreover, as already pointed out, the laser irradiation makes it possible to break the metallic bonds within the metal layer. During the laser pulse, the heat propagates in the metal layer, and the blade of transparent material promotes the bursting of the metal layer. Indeed, in the absence of a blade of transparent material, the laser irradiation causes the swelling of the metal layer, the metal is lifted from the substrate and ejected uncontrollably. Nevertheless, in both cases, small metal droplets can be formed before being ejected from the substrate by desorption. However, in the process according to the present invention, the droplets are then transformed into micro- or nano-particles after cooling, and thus hardening, on the transparent material blade. After ejection of the metal from the surface of the substrate, the particles are redeposited on the blade of transparent material. They can then be harvested from said transparent material blade, in particular by immersing the transparent material blade in an ultrasound bath.

In view of the phenomena used in the context of the process according to the present invention, and in particular desorption, the nature of the metal particles obtained can be influenced according to the metaplastic interface but also according to the nature and thickness of the deposited metal layer. on the substrate.

Advantageously, the substrate is made of a thermal insulating material at least on the part on which the metal layer is deposited. It is even more advantageously: - plastic (ethylene polynaphthalate (PEN), or polyimide such as KAPTON®); Glass; - polycarbonate; polytetrafluoroethylene (PTFE, Teflori); acrylate polymers; steel covered with a layer of plastic at least 50 nm thick. Since the substrate is preferably made of plastic, its melting point is generally less than 300 ° C.

During irradiation, the incident energy is absorbed superficially by the metal layer. The duration and the intensity of the laser pulse are therefore adapted according to the substrate, the nature and the thickness of the metal layer.

Preferably, in the process according to the present invention, the metal layer is deposited on the substrate by the techniques of the prior art and, more particularly, by physical vapor deposition (PVD) (according to the acronym "Physical vapor deposition"), by chemical vapor deposition (CVD) (or the term "chemical vapor deposition"), or by printing.

In the context of the present invention, the metal layer which is irradiated is advantageously made of a thermally conductive material, preferably of: a pure metal or a mixture of metals, advantageously a pure metal chosen from the group comprising gold, copper, nickel, palladium, and silver; an alloy; superimposed metal layers; or a metal oxide, advantageously AgO, ZnO or ITO, and mixtures thereof.

ITO (Indium Tin Oxide), or indium-tin oxide, is a mixture of indium oxide (In2O3) and tin oxide (SnO2).

In a particular embodiment, the metal layer may be: a metal selected from the group consisting of gold, copper, nickel, palladium, and silver; or in - a metal oxide, advantageously AgO, ZnO or ITO.

In general, the metal layer has, before irradiation, a thickness advantageously between 10 and 200 nm, and more particularly in the case of a laser whose wavelength is 248 nm or 308 nm.

In the process according to the invention, the metal layer is covered with a blade of transparent material, advantageously glass, quartz, borosilicate or plastic such as PEN.

In a preferred embodiment, the blade of transparent material is quartz. In a more particular case, the face of the blade of transparent material facing the metal layer may be covered with an anti-adherent coating, preferably a fluorinated coating. Thus, the adhesion between the blade of transparent material and the substrate during irradiation is limited.

In a particular embodiment, the non-adherent face of the transparent material blade may comprise fluorinated SAMs (self-assembling monolayers or self-assembled monolayers), for example fluorotris [3- (trifluoromethyl) phenyl). ] silane grafted with oxygen plasma, but also a fluorinated deposit obtained by plasma under SF6 gas or by spin coating ("spin coating") of a fluorinated layer such as Teflon®.

The blade of transparent material rests advantageously on the metal layer. In other words, the blade of transparent material is advantageously in contact with the metal layer. However, in another embodiment, the transparent material blade and the metal layer may be spaced a distance of at most 1 mm.

In order to have a good mechanical strength, the blade of transparent material advantageously has a thickness greater than or equal to 0.5 mm. Even more advantageously, it has a thickness of 1 mm.

As already said, the blade of transparent material is advantageously made of quartz. It therefore has a relatively low UV absorption coefficient, unlike a conventional glass slide.

In the process according to the present invention, the metal layer is irradiated with a laser whose fluence is advantageously between 10 and 1000 mJ / cm 2, and more preferably between 10 and 200 mJ / cm 2. The fluence of the laser corresponds to the surface density of energy. Depending on the power or the fluence, but also depending on the irradiation time, particles whose size may vary from 10 to 1000 nm can thus be prepared, advantageously from 10 to 500 nm.

In addition, the duration of firing lasers, is generally of the order of 20 ns. However, it is possible to multiply the laser shots in the same area so as to reach the fluence implemented in the method according to the present invention. In general, when the thickness of the metal layer is less than 200 nm, a single excimer laser shot may be sufficient.

In general, the particles obtained according to the method of the present invention may be in spherical form or in the form of sheets or plates. As a result, particle size refers primarily to the particle diameter or the dimension in the plane of the sheets or plates constituting the metal particles.

Indeed, in general, three types of metal particle shapes can be obtained, in particular for a laser whose wavelength is 308 nm: when the laser fluence is greater than or equal to 90 mJ / cm 2 and / or when the thickness of the metal layer is less than or equal to 60 nm, droplets are formed. After depositing the droplets on the transparent material blade and curing, globally spherical particles are obtained, their size being between 10 and 60 nm; when the fluence of the laser is between 50 and 90 mJ / cm 2 and / when the thickness of the metal layer is between 10 and 60 nm, metallic particles are in the form of sheets are obtained, their size in the 20 plane being between 100 and 500 nm. In this case, the thickness of the sheets obtained is substantially equal to that of the metal layer; when the laser fluence is less than 50 mJ / cm 2 and / or when the thickness of the metal layer is greater than 60 nm, metal particles in the form of plates are obtained, their dimension in the plane being greater than 1 25 minutes. The thickness of the plates is substantially equal to that of the metal layer.

The shape of the particles also depends on the spacing between the transparent material blade and the metal layer. The smaller this spacing, the more spherical the particles are. Moreover and as already said, the shape also depends on the thickness of the metal layer.

Those skilled in the art will be able to adjust all the parameters according to the nature of the metal layer but also according to the size and the shape of the particles which it wishes to prepare. Indeed, the values given above for a laser having a wavelength of 308 nm, may be lower in the case of a laser having a lower wavelength.

Advantageously, the particles obtained by implementing the process according to the present invention have a spherical shape.

In a particular embodiment, when the fluence of the laser is less than 70 5 mJ / cm 2, platelets or platelets having an average dimension in the plane of 500 nm are obtained.

In another particular embodiment, when the laser fluence is greater than 100 mJ / cm 2, for a metal layer having a thickness of 50 nm, spherical shaped particles having an average size of 50 nm are obtained.

In a particular embodiment of the present invention, prior to irradiation, the substrate coated with the metal layer, and the blade of transparent material are immersed in a liquid. Advantageously, the liquid in which the substrate coated with the metal layer, and the blade of transparent material are immersed has a thermal conductivity greater than that of the air to absorb energy and / or a different optical index the blade of transparent material to focus or defocus the incident beam. This thus makes it possible to modify the incident energy of the laser, but also to control the shape and size of the particles.

DESCRIPTION OF THE FIGURES The invention and the advantages thereof will be apparent from the following figures and examples given in a nonlimiting manner. FIG. 1 illustrates the prior art and shows the irradiation of a metal layer in the absence of a transparent material blade, thereby highlighting the swelling of the metal layer. FIG. 2 illustrates the irradiation of a metal layer by means of a laser, according to the process of the invention, in which a blade of transparent material is deposited on the metal layer.

EXAMPLE OF CARRYING OUT THE INVENTION A gold metal layer of 30 nm thickness is deposited by vacuum evaporation on a plastic substrate 125 μm thick in PEN (polyethylene naphthalate). A 1 mm thick quartz glass slide is then deposited on the plastic metal stack thus produced, which is insulated with a UV excimer laser (308 nm). The total duration of the irradiation is 20 ns. The quartz glass blade prevents swelling of the metal layer, causing breaks in the bonds and the ejection of fine metal droplets.

After the metal layer has exploded, the expelled gold particles are recovered on the quartz plate. These gold particles have an average diameter of 30 nm.

As illustrated in FIG. 1, in the absence of a transparent material blade (prior art), the metal layer 2 is deformed under the effect of the heat produced by the laser irradiation 4. In effect, the metal layer 2 and Since substrate 1 has different expansion coefficients, a bimetallic effect causes swelling of the metal layer. In this process of photo ablation, the swelling of the metal layer is accompanied by the ejection of the metal present in the irradiated zone.

In the context of the present invention (FIG. 2), the blade made of transparent material 3 prevents the swelling of the metal layer 2 while promoting its bursting. During laser irradiation 4, the metal is torn from the substrate 1 and ejected in the form of small droplets. The transparent material blade makes it possible to collect these droplets of metal which, after cooling, form micro or nanoparticles. 10 30

Claims (14)

REVENDICATIONS1. Process for the preparation of metal particles, consisting of: depositing a metal layer (2) on a substrate (1); irradiating (4) the metal layer with a laser; a blade (3) of transparent or quasi-transparent material at the wavelength of the laser being interposed between the metal layer (2) and the laser source. 2. Process for the preparation of metal particles according to claim 1, characterized in that the laser is an excimer laser. 3. Process for the preparation of metal particles according to one of the preceding claims, characterized in that the blade of transparent material is in contact with the metal layer. 4. Method for the preparation of metal particles according to claim 1, characterized in that the blade of transparent material and the metal layer are spaced at a distance of 1 mm at most. 20 5. Process for the preparation of metal particles according to one of the preceding claims, characterized in that the blade of transparent material is glass, quartz, borosilicate, or plastic. 6. Process for the preparation of metal particles according to one of the preceding claims, characterized in that the fluence of the laser is between 10 and 1000 mJ / cm 2. 7. Process for the preparation of metal particles according to one of the preceding claims, characterized in that the metal layer is: a metal selected from the group consisting of gold, copper, nickel, palladium, and money; or in - a metal oxide, advantageously AgO, ZnO or ITO. 8. Process for the preparation of metal particles according to one of the preceding claims, characterized in that the metal layer is deposited by physical vapor deposition (PVD), by chemical vapor deposition (CVD), or by printing. .15 9. Process for the preparation of metal particles according to one of the preceding claims, characterized in that the metal layer has a thickness of between 10 and 200 nm when the laser has a wavelength of 248 nm or 308 nm. 10. Process for the preparation of metal particles according to one of the preceding claims, characterized in that the face of the blade of transparent material facing the metal layer is covered with a fluorinated coating. 10 11. Process for the preparation of metal particles according to one of the preceding claims, characterized in that the substrate is a material selected from the group comprising: - plastics, such as ethylene polynaphthalate or polyimide; 15 - the glass; polycarbonates; polytetrafluoroethylene; acrylate polymers; - steel covered with a layer of plastic at least 50 nm thick. 20 12. Process for the preparation of metal particles according to one of the preceding claims, characterized in that the particle size is between 10 and 500 nm. 25 13. Process for the preparation of metal particles according to one of the preceding claims, characterized in that before irradiation, the substrate covered with the metal layer, and the blade of transparent material are immersed in a liquid. 30 14. Process for the preparation of metal particles according to claim 13, characterized in that the liquid has a thermal conductivity greater than that of the air and / or an optical index different from that of the blade made of transparent material.