WO2021160874A1

WO2021160874A1 - Method for preparing a composite material with core-shell nanoparticles the shell of which is made from graphene

Info

Publication number: WO2021160874A1
Application number: PCT/EP2021/053586
Authority: WO
Inventors: Omar KASSEM; Mathilde Rieu; Jean-Paul Viricelle; Mohamed Saadaoui
Original assignee: Institut Mines Telecom
Priority date: 2020-02-14
Filing date: 2021-02-15
Publication date: 2021-08-19
Also published as: FR3107284A1; FR3107284B1

Abstract

The invention relates to a method for preparing a composite material comprising core-shell nanoparticles the core of which is made from a material which can absorb light and the shell of which is made from graphene, the method comprising the following steps: a) providing a substrate and an ink comprising nanoparticles which can absorb light, the nanoparticles being encapsulated in a polymer matrix; b) depositing the ink on a surface of the substrate, the surface or the ink comprising a carbon source selected from: • a polymer comprising an amide function and an aromatic function, • a hexose, • carbon nanoparticles, • self-assembled monolayers comprising aromatic rings, c) illuminating the surface of the ink-covered substrate with a pulsed light in order to bring the nanoparticles in the ink to a heating temperature greater than a dissociation temperature of the carbon in the carbon source, the carbon dissociated in this manner then being capable of diffusing or solubilising in the nanoparticles in the ink in order to form the composite material.

Description

PROCESS FOR THE PREPARATION OF A COMPOSITE MATERIAL WITH NANOPARTICLES OF THE CORE-SHELL TYPE OF WHICH THE SHELL IS IN

GRAPHENE

Technical field of the invention

The invention relates to a process for producing a composite material with nanoparticles of the core-shell type, the shell being made of graphene. It is in particular intended to be implemented on a flexible substrate.

This type of composite material can be used in many applications. It can thus be used in electrodes for lithium batteries, electrodes for triboelectric generators, sensitive elements of gas sensors, sensitive elements of temperature-humidity sensors, pressure sensors, transistors and in magneto. resistivity, etc.

The excellent physical properties of graphene make it an ideal constituent of the alloys used in this type of application where manufacturers seek to produce innovative electronic components that exhibit improved electronic performance.

The nanoparticles used in the process according to the invention consist of particles of generally nanometric size, that is to say whose size is between one nanometer and several hundred nanometers, or even beyond a micron.

Technical background

Numerous processes for the production of composite materials have been developed.

A process for developing carbon-based composite materials is described in the article Zhang et al., Tin-Nanoparticles Encapsulated in Elastic Hollow Carbon Spheres for High-Performance Anode Material in Lithium-Ion Battery, Advanced material, 2008 20, 1160-1165. The process aims to develop tin nanoparticles encapsulated in a carbon sphere. The first step is to make hollow spheres with two shells. A first shell is made of oxide tin and a second shell is carbon. If the first steps of the process are carried out at relatively low temperature, to obtain non-oxidized tin nanoparticles, it is necessary that the two-shell sphere is heated in a hydrothermal reactor at a temperature of 700 ° C for a period of time. at least four hours under a nitrogen atmosphere. This process does not make it possible to manufacture a composite material based on tin nanoparticles covered with graphene. In addition, such a method is not suitable for synthesizing the composite material on a flexible substrate and more generally any substrate made of a material whose properties are caused to degrade under the effect of high temperatures.

Wen et al., Binding Sn-based nanoparticles on graphene as the anode of rechargeable lithium-ion batteries, Journal of Materials Chemistry, 2012, 22, 3300 then proposed a method of binding tin nanoparticles on graphene. In this process, tin chloride and glucose are mixed in a solution based on graphene oxide, then the mixture obtained is brought to a temperature of 170 ° C for five hours, then brought to a higher temperature of 250 ° C to 900 ° C to reduce the nanoparticles of tin oxide thus obtained. However, the quality of the composite material ultimately obtained depends on the chemical oxidation of the tin nanoparticles. However, to reduce this chemical oxidation, this process requires carrying out a heat treatment at at least 550 ° C. Such a method is also not suitable for a synthesis of the composite material on a flexible substrate.

Luo et al., Graphene-confined Sn Nanosheets with Enhanced Lithium Storage Capability, Advanced Materials, 2012, 24, 3538-3543 have proposed a process for the synthesis of a composite material based on nanometric tin sheets coated with graphene sheets . The described process requires heat treatments at temperatures of about 500 ° C for two hours and then 1000 ° C for 5 minutes in order to reduce SnO2 @ C to Sn @ C. This method suffers from the same drawbacks as the aforementioned methods.

Wang et al., Sn / Graphene nanocomposite with 3D architecture for enhanced reversible lithium storage in lithium ion batteries, Journal of Material Chemistry, 2009, 19, 8378-8384 have proposed a process for manufacturing a tin-based composite material and low temperature graphene by gradually adding a reducing aqueous solution of NaBhU to a mixture of dispersed graphene oxide nanosheets, SnCl2.2H20 and citric acid. If this process is carried out well at low temperature, an additional structuring step must be implemented to improve the crystallinity of graphene. This step considerably increases the duration of the process since it requires thermal annealing at 200 ° C. for fifteen hours under an argon atmosphere.

Lin et al., Laser-induced porous graphene films from commercial polymers, Nature Communications, 5: 5714 doi: 10.1038 / ncomms6714 (2014) also proposed a method of transforming a polyimide film into a porous layer of induced graphene by laser. The process aims to synthesize graphene and not graphene-based composite materials. This process makes it possible to produce graphene at room temperature. However, the laser beam being by nature localized in space, in this case it has a diameter of about 120 miti, the duration of the process is particularly long and does not make it possible to treat relatively large surfaces such as that of an electrode.

More recently, Vishwakarma et al., Transfert free graphene growth on S1O2 substrate at 250 ° C, Scientific Reports 7, 43756; doi: 10.1038 / srep43756 (2017) have proposed a process for synthesizing thin layers of graphene on Sn on SiO 2 / Si substrates by pulsed laser ablation followed by annealing at a temperature of 250 ° C. If the temperature at which the process is carried out is low, the synthesis is carried out under a vacuum, in particular at a pressure of 5 x 10 ⁵ Pa. However, the use of vacuum in industrial processes is very restrictive since it requires specific pumping devices, ie sized for processes of this type, and generally very expensive.

The known methods of the prior art require either excessively high temperatures, or very long manufacturing times or even a very high vacuum. Moreover, not all of them are suitable for depositing on a flexible material.

There is therefore a need to improve the existing production processes.

Summary of the invention

To meet this need, the invention provides a process for producing a composite material comprising nanoparticles of the core-shell type, the core of which is made of a material capable of absorbing light and the shell is made of graphene. . The method according to the invention comprises the following steps: a) providing a substrate as well as an ink comprising nanoparticles capable of absorbing light, said nanoparticles being encapsulated in a polymer matrix; b) depositing the ink comprising the nanoparticles on a surface of the substrate, said surface or said ink comprising:

• either a polymer comprising an amide function and an aromatic function,

• either a hexose, for example fructose or glucose,

• or carbon nanoparticles,

• or self-assembled monolayers comprising aromatic rings, c) illuminate the surface of the substrate covered with ink with pulsed light to bring the nanoparticles included in the ink to a heating temperature greater than a carbon dissociation temperature included at the surface of the substrate, the carbon thus dissociated then being able to diffuse or to dissolve in the nanoparticles included in the ink to form said composite material.

Since nanoparticles are able to absorb light, they heat up when illuminated by pulsed light. The use of pulsed light allows nanoparticles to heat up to temperatures above 1000 ° C in less than a second. This heating of the nanoparticles has the consequence of in turn causing an increase in the temperature at the surface of the substrate at the interfaces with the nanoparticles included in the ink, which makes it possible to dissociate the carbon included in the surface of the substrate and subsequently to locally generate the diffusion or solubilization of the carbon atoms from the surface of the substrate in the nanoparticles, in particular at the periphery of the nanoparticles. It can also allow the state of the nanoparticles themselves to change during a melting process depending on the material the nanoparticles are made of.

The rise in the temperature of the surface of the substrate is localized and therefore only very locally degrades the surface of the substrate since it only takes place at the interfaces with the nanoparticles and therefore in the immediate vicinity of said nanoparticles. The method according to the invention is therefore particularly ingenious. since it is compatible with all types of substrate. This is because, unlike thermal annealing at high temperature, and even though the substrate is made of a material which degrades under the effect of temperature, only the material located at the interfaces with the nanoparticles is degraded. It should be noted that the capacity of the molecules of the material to dissociate, and subsequently the carbon molecules, when the temperature rises depends on the composition of this material. In fact, it comprises either a polymer comprising an amide function and an aromatic function, or a hexose, for example fructose or glucose, or carbon nanoparticles. These materials or substances are easily degradable at the temperatures at which the surface of the substrate located around the nanoparticles of the ink heats up.

The process by which carbon atoms fit into nanoparticles, i.e. diffusion or solubility, will depend on the material of which the nanoparticles are made and the ability of the carbon to diffuse or solubilize therein. What is important in the present invention is to graphitize the nanoparticles, that is to say to form graphene at the periphery of the nanoparticles so as to obtain a composite material comprising nanoparticles of the core-shell type whose core is made of the material. able to absorb light and the shell is made of graphene coming from the carbon of the surface of the substrate.

The phenomena described above occur "in cascade", that is to say one after the other and in less than a second of exposure to pulsed light, which makes the process of invention a particularly rapid process for forming a composite material comprising core-shell nanoparticles whose shell is made of graphene.

In addition, the encapsulation of the nanoparticles in a polymeric matrix makes it possible to avoid the aggregation of said nanoparticles, in particular during their printing on the surface of the substrate. The graphitization of the nanoparticles is then all the more effective when they are not aggregated since the pulsed light is then able to reach each nanoparticle and cause the said nanoparticle to heat up.

The encapsulation of the nanoparticles also makes it possible to significantly reduce the chemical oxidation of said nanoparticles. The chemical process of oxidation of the surface of a material is a natural phenomenon which tends to occur when said material is exposed to ambient air and / or under the effect of time. The encapsulation of the nanoparticles in a polymer matrix makes it possible to overcome this undesirable effect which, as has been seen in the sections relating to the prior art, can only be overcome by heating the material to a very high temperature, generally from 750 ° C or by using a high vacuum. In addition, reducing surface oxidation improves graphitization since surface oxidation reduces the light absorption capacity of nanoparticles and materials in general.

According to different characteristics of the invention which can be taken together or separately:

- the material capable of absorbing light is metallic or semiconductor,

- The material capable of absorbing light is based on tin (Sn);

the polymer comprising an amide function and an aromatic function is a polyimide;

- hexose is fructose or glucose;

- the self-assembled monolayers are made from hexadecane thiol (HDT), biphenyl-4-thiol (BPT), 1, 1 '-biphenyM -4-thiol (BPTC), tolanethioacetate (TT A), tolane disulfide (TDS), tolane methyl sulfide (TMS);

- during step b), the ink is deposited with a technique chosen from among printing, centrifugal coating, coating by dipping, drop deposition, Langmuir-Blodgett deposition or even casting in strip;

- in step b), the ink is deposited by inkjet printing;

- During step c), the pulsed light is generated by means of an excitation source emitting wide-band polychromatic electromagnetic radiation; said excitation source emits at wavelengths between 180 and 1200 nm; during step c), the pulsed light is generated by means of a laser source;

- during step c): o the amplitude of the pulsed light is obtained with a supply voltage of the excitation source of between 200 and 900 V, preferably around 360 V; o the frequency of illumination of the pulsed light is between 0.1 and 100 kHz, preferably around 10 kHz; o the number of pulses of the pulsed light is between 6 and 20, o the duration of a pulse is between 50 psec and 2 ms, preferably around 80 psec

- During inkjet printing, a distance between a printer ejection nozzle and the substrate is adjusted between 600 µm and 1.5 mm.

Presentation of figures

Other objects and characteristics of the invention will emerge more clearly in the following description, made with reference to the appended figures, in which:

- Figure 1 illustrates the steps of the process according to the invention,

- Figure 2 is a schematic sectional view of a nozzle of an inkjet printer,

- Figure 3 is a comparative figure illustrating the normalized emission spectrum of the xenon source and the absorbance of the graphene-based composite material @ Sn-NPs after inkjet printing with a printing bed maintained at 40 ° C,

- Figure 4 illustrates the emission spectrum of a xenon-based excitation source at 360 V and the relative light intensity absorbed by a thin layer based on tin nanoparticles after inkjet printing with a print bed maintained at 40 ° C,

- Figure 5 illustrates the power density emitted (in black) and absorbed (in light gray) during a cycle of 6 consecutive pulses, - Figure 6 illustrates the evolution of the temperature of the graphene composite material @ Sn-NPs during a cycle of 6 consecutive pulses,

- Figure 7 illustrates the evolution of the square resistance of tin lines deposited on Upilex® and treated by photonic annealing as a function of the temperature reached by the Sn lines,

- Figure 8 illustrates the evolution of the Raman signal of tin lines deposited on LIpilex® and treated by photonic annealing at temperatures of 1165 ° C (black), 1343 ° C (dark gray) and 1790 ° C (light gray),

- Figure 9 illustrates a transmission electron microscopy (TEM) image of tin nanoparticles deposited on Ilpilex®,

- Figure 10 illustrates the electrochemical characterizations of working electrodes whose active material is made from a composite material graphene @ Sn, graphite @ Sn and carbon @ Sn, the capacity of which is respectively illustrated in black, light gray and dark gray in figures 10b) and 10c),

Detailed description of the invention

The invention relates to a process for producing a composite material comprising core-shell nanoparticles, the core of which is made of a material capable of absorbing light and the shell is made of graphene.

With reference to FIG. 1, the method comprises the following steps: a) providing a substrate as well as an ink comprising nanoparticles capable of absorbing light, said nanoparticles being encapsulated in a polymer matrix; b) depositing the ink on a surface of the substrate, said surface or said ink comprising a carbon source chosen from:

• a polymer comprising an amide function and an aromatic function,

• a hexose,

• carbon nanoparticles,

• self-assembled monolayers comprising aromatic rings, c) illuminating the surface of the substrate covered with ink with pulsed light to bring the nanoparticles included in the ink to a heating temperature greater than a dissociation temperature of the carbon included in the carbon source, the carbon thus dissociated being then able to diffuse or to dissolve in the nanoparticles included in the ink to form said composite material.

In the following sections, we are first of all interested in the characteristics of the substrate and more particularly in the properties of the surface of the substrate on which the ink comprising the nanoparticles is deposited in the case where the carbon source belongs to the surface of the substrate. substrate.

In the first four embodiments which are described below, the surface of the substrate comprises the carbon source. As a result of this property, the surface of the substrate is therefore directly involved in the graphitization of the ink nanoparticles, the carbon atoms of said surface of the substrate being the precursor of graphene. Graphitization is the process by which a layer of graphene is formed on the surface of said nanoparticles. Indeed, the nanoparticles of the ink are initially, that is to say during steps a) and b), devoid of a graphene shell. It is during step c) of the process according to the invention that the formation of the graphene layer takes place as will be described in relation to the description relating to step c).

Any source of carbon is not necessarily suitable for carrying out the process according to the present invention. In fact, in the first four embodiments which are described below, the surface of the substrate comprises a material, a material or a chemical compound capable of degrading from a predetermined temperature which depends on the nature of this material. . With reference to the above-mentioned temperature, the term “degradation temperature” will be used. Incidentally, this means that below this temperature, this material is stable both structurally and chemically. It can probably be remembered that the degradation temperature is the temperature from which the components and / or the molecular structure of matter begin to deteriorate, generally by scission or breaking of bonds between atoms and / or groups of atoms. . The consequence of this degradation is a modification of the properties of this material starting from the aforementioned temperature. The method according to the invention exploits this property of the surface of the substrate in order to allow the carbon coming from said surface to dissociate and to diffuse or solubilize at the periphery, that is to say at the surface, of the nanoparticles. This property, like the previous one, is necessary but it is not sufficient.

Indeed, the inventors of the present invention have found that all the materials or compounds exhibiting the aforementioned properties do not allow the graphitization of the nanoparticles of the ink, but only a few. The following sections present the different materials and / or compounds that achieve this goal.

In a first embodiment of the present invention, the surface of the substrate comprises a polymer comprising an amide function and an aromatic function. The amide function corresponds to an organic compound having a nitrogen atom bonded to a carbonyl group, namely an R-C = 0 group, through its carbon atom. That being said, the nitrogen atom can be bonded to two or even three carbonyl groups, resulting in secondary amides - called imides - and tertiary amides. The aromatic function corresponds to a molecular structure whose atoms form planar cyclic structures. The number of aromatic "rings" is not limited in the polymer of the present invention.

The polymers of the aforementioned composition have a degradation temperature of a few hundred degrees, generally between 300 ° C and 500 ° C which depends on their proportion of aromatic rings. What matters is that the polymers of the aforementioned composition are capable of degrading at the heating temperature of the nanoparticles.

The polymers which comprise an amide function and an aromatic function have proved to be excellent candidates for the graphitization of nanoparticles in the context of the present invention. Indeed, when they are subjected to high temperatures, the polymers of this composition tend to dissociate at the level of the CN bonds of their amide function, that is to say the bonds between the nitrogen atom and carbonyl groups, thus releasing the carbon atoms necessary for graphitization. The presence of the aromatic function is important since it allows the polymer to maintain good dimensional stability at high temperature, that is to say a temperature above degradation temperature of the polymer without however preventing molecular cleavage at the CN bonds.

The polymers of the aforementioned composition are, for example, polyamides, such as aromatic or aromatic polyamides, semi-aromatic or polyphthalamides polyamides and polyarylamides which all comprise an aromatic function and an amide function. They are, for example, polyimides such as aromatic polyimides, semi-aromatic polyimides which all comprise an aromatic function and an imide function (secondary amide).

By way of illustration, the surface of the substrate can be made of UPILEX®. UPILEX® is a polyimide comprising an imide function and an aromatic compound which generally begins to degrade around 500 ° C. UPILEX® is a derivative of the 2,3,3 ', 4'-Biphenyl Tetracarboxylic Dianhydride (BPDA) monomer with the chemical formula C16H6O6. UPILEX® has the following chemical formula:

By way of illustration also, the substrate can be made of Kapton®. This polyimide is known to degrade from 400 ° C. Under these conditions, it would suffice to bring the nanoparticles to a temperature above 400 ° C. to allow the local dissociation of the molecules from the surface of the substrate. These are nonlimiting examples of polymers making it possible to implement the process of the present invention.

The substrate can be made entirely of the polymer comprising an amide function and an aromatic function. However, the substrate does not have to be made entirely from said polymer. What is important in the context of the present invention is that the surface of the substrate on which the ink is deposited during step b) comprises the polymer of the aforementioned composition.

According to a particular implementation of the invention, the substrate comprises a support on which said polymer is applied or fixed.

Preferably, said polymer can be provided in the form of a thin layer. Many polymers of the above composition are available under this form in trade. By way of example, mention may be made of IlPILEX®. In such a configuration, it is preferable that the support, whether rigid or flexible, has an appropriate thickness to mechanically support the polymer, for example a micrometric or millimeter thickness, or even more, depending on the envisaged application. The polymer can be fixed to the substrate so as to cover, at least in part, the surface on which the deposition of the ink is envisaged. In this regard, recourse may be had to methods known to those skilled in the art, for example gluing, heat sealing, ultrasonic welding, etc.

As a variant, said polymer can be provided / formulated in the form of an ink previously deposited on a support. In this case, it is possible either to use commercially available inks or to prepare an ink based on the polymer of the above composition as will be seen in the next sections. Similar to the configuration described above, it is also preferable to provide a support having suitable dimensions and shape to receive the ink. In this case, it will be possible to deposit the ink by various depositing methods known to those skilled in the art which will be explained in more detail below in connection with the description of step b) of the process.

For example, the support can be made of a saturated polyester-type polymer. Preferably, the saturated polyester is poly (ethylene terephthalate) (PET) or poly (ethylene naphthalate) (PEN). These widely used polymers can come in a wide variety of forms.

In a second embodiment of the present invention, the surface of the substrate comprises a hexose. A hexose is an organic substance belonging to the carbohydrate family whose carbon chain contains 6 carbon atoms. With the general formula C6H12O6, hexoses include a very large number of hydroxyl groups bonded to carbon atoms. When subjected to high temperatures, hexoses degrade by dissociating at their C-OH bonds. The molecular fragments thus formed constitute a precursor of graphene in the process according to the invention. That being said, under normal temperature and pressure conditions, ie ambient temperature and atmospheric pressure, the hexoses retain good stability over time. In this regard, let us first specify that the degradation temperature of hexoses is different from their melting point. As was specified previously, the degradation temperature corresponds to the temperature at which the molecular structure of these substances begins to deteriorate, which results in breaks in their molecular structure. The melting point corresponds, under given pressure conditions, to the temperature at which these substances pass from the solid state to the liquid state. In other words, the melting point provides an indication of the state of said substances at a given temperature and pressure and, concomitantly, testifies to the already significant changes which have taken place in their structure. The degradation temperature of hexoses is therefore generally lower than their melting point. Note that the duration of exposure to a given temperature also substantially influences the start of the degradation process.

For example, a suitable hexose for practicing the invention is glucose. Although the melting temperature of glucose is between about 146 ° C and 150 ° C, when subjected to temperatures between 110 ° C and 130 ° C, it already forms degradation byproducts. The more the temperature and the time of the heat treatment increase, the more the by-products are numerous and diversified. For example, see the article Woo et al., Préventive Nutrition and Food science, "Characteristics of the Thermal Degradation of Glucose and Maltose Solutions", 2015.

Like all hexoses, when the temperature rises well above its melting point, volatile compounds in small quantities are generated. The number and variety of volatile compounds generated increase as the temperature to which the substance is subjected increases. While such a phenomenon is very likely to occur in the process according to the invention, it is very limited because the contact surface between the substance and the nanoparticles is also very limited due to the size of said nanoparticles.

In addition, glucose, like most hexoses, is a substance which is chemically stable at atmospheric pressure over a relatively wide range of temperatures. This is not necessarily the case with citric acid, for example, which when used as a carbon source requires specific treatments. (Ex: lyophilization and generally freezing-drying processes) to control its degradation at very low temperature (temperature below 80 ° C).

In any event, as will be seen in more detail below, the temperature to which the nanoparticles are brought when they are exposed to pulsed light can be adjusted so as to take into account the temperature at which the substance decomposes. material from which the surface of the substrate is made but also that to which the process of solubilization or diffusion in the nanoparticles is effective.

Another hexose suitable for carrying out the process according to the invention is fructose. Fructose is preferred over glucose because as it breaks down, it releases more carbon atoms than glucose.

Unlike the first embodiment in which the substrate can be made entirely of the polymer, the hexose used in this embodiment of the invention is necessarily deposited on a support before it can be used as a deposition surface for the ink containing. nanoparticles. According to a particular implementation, whatever the substance envisaged, i.e. fructose, glucose, etc., the substrate is prepared according to the operating mode which follows. A mixture comprising water and hexose is prepared, then said mixture is deposited on a support, flexible or rigid, of appropriate thickness. The mixture thus deposited is allowed to dry until the water has completely evaporated. A substrate is thus obtained comprising the support and the hexose on the surface of the substrate intended to receive the ink comprising the nanoparticles.

In a third embodiment of the present invention, the surface of the substrate comprises carbon nanoparticles. In this case, it is the carbon nanoparticles which have the role of carbon precursor. The graphitization of the ink nanoparticles is slower than using the substrates according to the embodiments described above. However, it is made possible because the carbon present on the surface of the carbon nanoparticles diffuses very easily, which makes it possible to locally form the graphene shell of the ink nanoparticles.

According to a particular implementation of the invention, the carbon nanoparticles are deposited in the form of a thin layer by any method known in the art, for example by laser ablation of graphite, by creation of defects in carbon. diamond by means of an ion beam, by thermal decomposition of organic compounds, etc. In this regard, it should be noted that nanoparticles are not always pure since they may possibly include graphite in various proportions.

In a fourth embodiment of the present invention, the surface of the substrate comprises self-assembled monolayers (also known by the abbreviation SAM) comprising aromatic rings. Self-assembled monolayers are assemblages of molecules formed on the surfaces of a substrate or layer by adsorption. Adsorption, which should not be confused with absorption, is the physical process by which molecules attach themselves to a solid surface. In the context of this embodiment of the invention, these are molecules, but this phenomenon can also take place with atoms or ions.

In self-assembled monolayers of the aforementioned type, that is to say comprising aromatic rings, each molecule comprises a head, also called an attachment function, intended to be adsorbed on the substrate by intramolecular binding and a tail, also called a spacer, comprising aromatic rings. The end of the tail can be functionalized to change the surface energy (for example by grafting -OH bonds after an O2 plasma treatment). The use of aromatic rings in self-assembled monolayers provides greater surface covering power than self-assembled monolayers whose tails include alkyl rings due to their stronger intermolecular tt-p bonds.

Examples of self-assembled monolayers suitable for carrying out the present embodiment of the invention are based on hexadecane thiol (HDT), biphenyl-4-thiol (BPT), 1, 1 '-biphenyM -4-thiol (BPTC), tolanethioacetate (TTA), tolan disulfide (TDS), tolane methyl sulfide (TMS). Preferably, monolayers of the aforementioned type based on thiol groups will be used which, in addition to being the most studied in electronic applications, can be deposited by liquid or by gas by vacuum evaporation.

According to a particular implementation, the self-assembled monolayers only comprise aromatic rings. We are now interested in the characteristics of the ink comprising the nanoparticles in the case where the carbon source belongs to the ink, that is to say is included in said ink.

According to a fifth embodiment of the present invention, the ink comprising the nanoparticles also comprises one of the aforementioned materials. This is one of the materials targeted in the preceding embodiments, namely a polymer comprising an amide function and an aromatic function or a hexose or carbon nanoparticles or self-assembled monolayers comprising aromatic rings. In this connection, the term “ink” is understood to mean a fluid substance, preferably liquid or of low viscosity, intended to be deposited on the surface of the substrate. The ink has a specific composition which gives it properties allowing it to be used in the context of the present invention.

In the case where the ink comprising the nanoparticles also comprises a polymer of the aforementioned composition, one speaks of "polymeric" ink comprising at least one polymer comprising the amide function and the aromatic function. The ink can also include additives such as water, solvents, binders, dispersing agents or adhesion promoters. For example, if one plans to deposit ink on the surface of the substrate using the inkjet printing technique, additives may be used to improve the resolution of inkjet printing. Preferably, the polymer comprising the amide function and the aromatic function used in the ink can be in the form of a "varnish", which facilitates its deposition on the substrate. A varnish corresponding to such an implementation is, for example, ink PI-6643-001 from JNC.

In the event that the ink comprising the nanoparticles also comprises a hexose, it suffices to add to the ink comprising the nanoparticles a solution obtained by diluting the hexose in question, for example fructose or glucose, in water. The latter indeed have very good solubility in water, which facilitates their use in liquid form.

In the case where the ink comprising the nanoparticles also comprises the carbon nanoparticles, it is necessary to clearly distinguish the nanoparticles to be graphitized from the nanoparticles forming the carbon precursor. Carbon based or carbon / graphite blend inks are commercially available and can be used in combination with the ink comprising the nanoparticles to be graphitized. It is also possible to synthesize an ink comprising the two types of nanoparticles. In this case, reference may be made to the part of the description in relation to step a) of the method according to the invention describing the synthesis of the ink comprising the nanoparticles. Additives can be used.

In the case where the ink comprising the nanoparticles also includes the self-assembled monolayers of the above type, it suffices to add to the ink comprising the nanoparticles a solution comprising the self-assembled monolayers of the above type. In fact, most of the self-assembled monolayers of the aforementioned type are in the form of liquid substances. It is then possible to deposit the ink by liquid means by means of the deposition techniques which are presented in more detail in the remainder of this description.

Remember that it is quite possible, as specified in relation to the first embodiment described above, to provide the polymeric ink comprising the amide function and the aromatic function with a view to forming the surface of the substrate independently. ink comprising the nanoparticles to be graphitized. This is equally valid for all the other carbon precursors envisaged in the context of the present invention. In other words, SAMs could be delivered as ink, as could carbon nanoparticles or hexose.

In what follows, the various steps of the process according to the invention are described in more detail.

As seen in the introduction to this detailed description, the first step a) of the process consists not only in providing a substrate as described in the preceding sections but also in providing an ink comprising nanoparticles capable of absorbing light, nanoparticles being encapsulated in a polymer matrix.

It is possible to either use a commercially available ink or to manufacture the ink.

The synthesis of an ink comprising metallic nanoparticles is a process known from the prior art. In this case, it is necessary to provide a step of synthesis of the nanoparticles and a step of formulation of the ink. For such a setting In particular, reference may be made to patent application FR 3 041 649 A1 which describes a process for manufacturing an ink based on metallic nanoparticles.

According to the invention, the nanoparticles of the ink are made of a material capable of absorbing light. Advantageously, the nanoparticles exhibit very good, even excellent absorption of light. As will be seen below, the ability of nanoparticles to absorb light is a prerequisite for them to be able to heat up when subjected to a luminous flux and thus to allow the preparation of the composite material.

The ability of ink nanoparticles to absorb light depends on two factors. The first is the very nature of the material from which said nanoparticles are made. The second is in their composition and structure, especially on the surface.

Preferably the nanoparticles are made of a metal. The material for forming the nanoparticles can then preferably be based on gallium (Ga), indium (In), bismuth (Bi), platinum (Pt), iron (Fe), nickel (Ni ), lead (Pb), copper (Cu), titanium (Ti), manganese (Mg), vanadium (V), magnesium (Mg), aluminum (Al), silver (Ag) , gold (Au) or zinc (Zn). If the first metals mentioned have the common point of having good optical absorption, in addition to having low or even very low melting points, tin (Sn) has a much better absorption while having a relatively low melting temperature. (231.9 ° C). Thus, in a preferred embodiment, the material of which the nanoparticles are made, when they are metallic, is tin-based.

Alternatively, the nanoparticles are made from a semiconductor. In this case, the semiconductor material chosen to form the nanoparticles is preferably based on silicon (Si) or germanium (Ge). Silicon is a semiconductor material which, besides being very abundant, has very good optical absorption. In addition, its optical absorption can be increased by the insertion of dopants.

Alternatively, the nanoparticles can be made of an alloy, that is, a mixture of different materials. In any case, what matters is that the material is able to absorb light.

In addition, as mentioned previously, the nanoparticles are encapsulated in a polymeric matrix. Indeed, in order to allow a good absorption of light by the nanoparticles and ultimately synthesize a composite material as described above, it is necessary to prevent or at least limit the formation of an oxide layer on the surface of metallic or semiconductor materials when they are exposed to the atmosphere.

Remember that this native oxide layer, often called "surface contamination", which forms naturally on the surface of the aforementioned materials, creates structural defects on the surface layer of the material in contact with the atmosphere. These defects are, for example, impurities, gaps, disorder, etc. In any case, in the presence of such defects, nanoparticles would scatter more light than in their absence. That being said, in all cases, it is advisable to reduce the thickness of this native oxide layer in order to limit the diffusion of light, which is detrimental for good light absorption.

Step a) of synthesis of the ink used in the process according to the invention therefore makes it possible to limit or even avoid the formation of the oxide layer.

The "polymer matrix" is here a material based on one or more polymers making it possible to support and bind the nanoparticles. The encapsulation of the nanoparticles in the polymer matrix makes it possible to limit the contact of the nanoparticles with the atmosphere and therefore to limit or even prevent the process of formation of the oxide layer, as indicated above. In a preferred embodiment of the invention in which the nanoparticles are made of tin, the polymer matrix can be made of polyvinyl pyrrolidone (PVP). This polymer is advantageously a dispersing agent, which also makes it possible to avoid the agglomeration of several nanoparticles, in addition to forming a support and a binder for said nanoparticles.

According to a preferred implementation, the preliminary step of synthesizing the nanoparticles can be carried out under a controlled atmosphere. This makes it possible to further limit the process of formation of the native oxide layer at the time of encapsulation under a matrix of polymer (s). The term “controlled atmosphere” is understood to mean an atmosphere whose composition makes it possible to limit the process of formation of the oxide layer. For example, the atmosphere can be controlled in that it is poor in oxygen, relative to the level of oxygen typically contained in the air. This depletion can be carried out by a supply of neutral gas but by any other method known to those skilled in the art. For example, this step can be carried out in a glove box, the glove box typically allowing this type of operation to be carried out in a controlled atmosphere.

As seen in the introduction to this detailed description, the second step b) of the method according to the invention consists in depositing the ink comprising the nanoparticles on the surface of the substrate. As will be seen in detail below, the method used to deposit the ink is not limiting in the context of the present invention.

According to a preferred implementation, during step b), the deposition is carried out by printing. Printing makes it possible to deposit with a higher resolution than that conventionally obtained with other deposit techniques. It should be noted that printing can be used even if the surface of the substrate has been previously deposited by printing on a support, as previously described. However, care must be taken to ensure that the drying process is completed.

In the case of the inkjet printing process, the ink comprising the nanoparticles is preferably printed using an inkjet printer.

Referring to Figure 2, during inkjet printing, the distance di between an ejection nozzle 20 of the inkjet printer and the surface of the substrate 12 is advantageously between 250 μm and 1.5 mm, preferably between 500 µm and 800 µm. It can probably be specified at this stage that the substrate 10 preferably rests on a plate (not illustrated). That being said, what we are talking about here is the distance between the printer ejection nozzle and the surface 12 of the substrate facing towards the printer ejection nozzle 20. This operating parameter makes it possible to control the “time of flight” of the jet (s) and therefore influences the shape of the said jet (s). When the distance between the printer ejection nozzle and the platen is between 500 and 800 µm, the formation of jets in the form of drops is further promoted.

In this regard, for a waveform 22 of predetermined voltage, the ejection voltage of the nanoparticles also influences the shape of the jets. Preferably, the voltage applied for ejection of the nanoparticles (in ink form) is between 15 V and 19 V for a DMP-2800 printer, Dimatix. This makes it possible to generate ink drops of spherical shape and able to move in such a way. rectilinear between the printer ejection nozzle and the platen on which the substrate rests at the same speed. In addition, the drops do not break up during the journey between the ejection nozzle and the substrate.

Advantageously, the ink has a surface tension lower than the surface energy of the substrate. Thus, when the ink jets reach the surface of the substrate, they spread over said surface of the substrate. Anyway, at the end of step b), a stack is obtained comprising at least the substrate and a pattern comprising the ink comprising the nanoparticles deposited by the inkjet printer. If the surface of the substrate has been deposited on a support as described previously (bonding, heat sealing, ultrasound, inkjet printing, etc.), the additional layer formed in the stack must be taken into account, this one being necessarily positioned between the pattern formed by the ink and the support.

The printing of the ink comprising the nanoparticles can also be carried out by aerosol, heliography, screen printing or even flexography. These printing techniques are just as compatible for the implementation of step b) of the process according to the invention.

Preferably, at the end of step b), if the ink comprising the nanoparticles has been printed, a step of drying the ink is provided. However, it is not mandatory. Indeed, as will be described in more detail below, the pulsed light allows the ink to dry under the operating conditions required for the illumination step.

The deposition can also be carried out by other printing processes such as laser printing (“Laser Induced Forward Transfer”, LIFT, in English). Laser printing consists of irradiating, by means of a laser beam, a substrate comprising the material to be deposited so as to allow the transfer of said material from the substrate to a substrate on which the material is intended to be deposited with a micrometric resolution. The substrate comprising the material to be deposited is called the donor substrate while the substrate receiving the material after irradiation is called the recipient substrate. This technique makes it possible to carry out deposits with a resolution of the order of about ten micrometers to a few tens of micrometers. The deposition can also be carried out by centrifugal coating ("spin coating" in English), by dip coating ("dip coating" in English), by drop casting, by the deposition method by Langmuir-Blodgett (“Langmuir Blodgett Deposition” in English) or by tape casting (“doctor blading” in English).

Following step b) of depositing the ink comprising the nanoparticles on the surface of the substrate, the method according to the invention comprises a step c) consisting in illuminating the substrate with pulsed light to carry the nanoparticles included in the ink at a heating temperature greater than a dissociation temperature of the carbon comprised at the surface of the substrate. The nanoparticles targeted are the nanoparticles to be graphitized. This step is a photonic annealing.

In step c), the nanoparticles being exposed to pulsed light, they absorb the electromagnetic energy generated by the pulsed light, that is, they absorb the energy of photons. Indeed, as has already been mentioned previously, the nanoparticles are capable of absorbing light. This has the consequence of generating an increase in the temperature of the nanoparticles. Nanoparticles can store energy up to temperatures above 1000 ° C. This heating of the nanoparticles has the consequence of causing in turn an increase in the temperature of the surface of the substrate at the interfaces with said nanoparticles. This makes it possible to locally dissociate the molecules from the surface of the substrate, to "detach" the carbon from the surface of the substrate, and subsequently to locally generate the diffusion or the solubilization of the carbon atoms from the surface of the substrate in the nanoparticles, in particular at the periphery of the nanoparticles. It typically takes less than a second for the temperature of the nanoparticles to rise and for the above phenomena to occur subsequently.

In addition, raising the temperature of the nanoparticles can allow a change in state of the nanoparticles themselves during a melting process, which can facilitate the insertion or solubilization of carbon in the nanoparticle. This is for example the case when the nanoparticles are made of indium, whose melting point is around 156.7 ° C, or of gallium, whose melting point is around 29.8 ° C or else tin with a melting point of about 231.9 ° C. This fusion process is therefore photo-induced. In this regard, the amplitude, frequency and number of pulses are selected to ensure heating of the nanoparticles and thus allow the degradation of the molecules of the surface of the substrate located at the interface with the nanoparticles so that they release the carbon. that they contain, that is to say so as to dissociate the carbon from the surface of the substrate included at the interface between the nanoparticles and the surface of the substrate.

The consequence being to allow the diffusion or the solubilization of the carbon atoms coming from the substrate in the nanoparticles, in particular at the periphery of the nanoparticles. In practice, the aim is to bring the nanoparticles to a temperature sufficient to allow the degradation of the molecules on the surface of the substrate located at the interface with the nanoparticles and to allow the diffusion or the solubilization of the carbon atoms. In other words, the temperature reached by the nanoparticles is a control parameter that achieves the above advantages.

It should be remembered that the rise in the temperature of the surface of the substrate is localized and therefore only very locally degrades the surface, this process only taking place at the interfaces with the nanoparticles and therefore in the immediate vicinity of said nanoparticles.

The process by which the carbon is inserted into the nanoparticles, i.e. diffusion or solubilization, will depend on the material of which the nanoparticles are made and on the capacity of the carbon to diffuse or to solubilize therein. What is important in the present invention is to graphitize the nanoparticles, that is to say at least to form graphene at the periphery of the nanoparticles so as to obtain a composite material comprising nanoparticles of the core-shell type of which the core is made. of metallic or semiconductor material and the shell is made of graphene coming from the carbon of the surface of the substrate. A shell is thus formed having between four and five atomic layers of graphene.

Graphene is at least formed at the periphery of the nanoparticles. The nanoparticles can be made of a material for which an alloy is formed with the carbon coming from the substrate so that both graphene is formed at the periphery of the nanoparticles and the core of said nanoparticles is made of an alloy comprising carbon. This is the case, for example, when the ink contains as nanoparticles to graphitize silicon nanoparticles. The production process of the present invention makes it possible to obtain a composite material comprising core-shell nanoparticles whose core is made of an Si-C alloy and the shell is made of graphene. The material from which the heart is made may therefore not be completely pure.

According to a preferred embodiment of the present invention, during step c), the pulsed light is generated by means of an excitation source emitting wide-band polychromatic electromagnetic radiation. Preferably, the excitation source emits at wavelengths between 180 and 1200 nm. A source suitable for the implementation of step c) is therefore for example a discharge lamp comprising xenon, referred to as a xenon lamp in the following. Also preferably, two, or even several xenon lamps can be used. This type of source has the advantage of illuminating a very large area, which subsequently makes it possible to produce a larger quantity of the composite material, moreover in a very short time as has been seen previously.

Alternatively, the pulsed light source can also be a laser source. This solution is less attractive than the previous one since the laser can only scan a limited area. The use of the laser requires a scanning of the substrate line by line.

At this stage, we can undoubtedly specify that the dimensions of the nanoparticles can exceed one micron. In this configuration, the pulsed light source may preferentially emit at wavelengths located in the infrared range. This is because pulsed light photons need less energy to be absorbed by relatively large nanoparticles. However, when the particles have nanometric dimensions, the plasmon effect facilitates heating of said nanoparticles.

The parameters which control the characteristics of the pulsed light are the illumination frequency, the supply voltage (which has a direct impact on the pulse amplitude) and the number of pulses applied (or, which amounts to even a duration of application of the pulses, ie the total duration of annealing). All of these parameters ultimately contribute to the amount of energy deposited. The energy level to deposit will depend, each time, on the nature of the nanoparticles, but also on that of the surface of the substrate. It is this level of energy that will determine the temperature reached.

However, for the same amount of energy finally deposited, it is possible to provide different values for these parameters, depending on what is required.

Increasing the illumination frequency reduces annealing time, which helps limit sample cooling between each pulse and leads to more heat build-up. That being said, determining the optimum illumination frequency (s) results from a compromise between two phenomena. On the one hand, an illumination frequency that is too low will not increase the temperature of the nanoparticles, and therefore increase the temperature of the surface of the substrate. It will therefore be impossible to form the graphene shell of the nanoparticles and therefore to produce the composite material. On the other hand, too high an illumination frequency can damage the substrate.

Thus, the illumination frequency of the pulsed light may advantageously be between 0.1 and 100 kHz, preferably around 10 kHz. These illumination frequencies make it possible to overcome the aforementioned drawbacks and make it possible to raise the temperature of the nanoparticles sufficiently to allow the degradation of the molecules of the substrate located at the interface with the nanoparticles and the diffusion or the solubilization of the carbon atoms coming from the surface of the substrate in the nanoparticles. In addition, this allows minimum electrical resistance values to be achieved with less pulses applied, thereby reducing the total annealing time, and also extending the life of the lamps.

Increasing the supply voltage to the excitation source increases the energy deposited, and therefore the electromagnetic energy absorbed by the nanoparticles, without affecting the total annealing time. In other words, at constant illumination duration, increasing the supply voltage to the source allows the nanoparticles in the ink to reach higher temperatures. In fact, the supply voltage makes it possible to control the amplitude of the taps.

For example, it is possible to provide a supply voltage for the excitation source between 200 and 900 V, preferably around 360 V for a PulseForge 3200, Novacentrix device. The number of pulses may then be between 6 and 20. The duration of one pulse may be between 50 microseconds and 2 milliseconds, preferably be 80 microseconds.

Practical implementations of the present invention in which the carbon source is included in the substrate are described in relation to the first, second and third embodiments which follow.

First practical embodiment of the present invention: production of a composite material comprising nanoparticles of the core-shell type whose core is made of tin and the shell is made of graphene (graphene @ Sn-NPs) from a precursor made from tin nanoparticles printed on UPILEX®

Figure 1 illustrates schematically and synthetically the aforementioned production process and described in detail in the following.

Synthesis of the ink comprising the tin nanoparticles

The first step consists in synthesizing Sn nanoparticles very little oxidized at the surface by the polyol route. The synthesis is carried out by reduction of a tin salt in a polyol medium under a controlled atmosphere and at room temperature. Tin 2-ethylhexanoate (Sn [CH3 (CH2) 3CH (C2H5) C02] 2 - 92.5 - 100% Sigma-Aldrich) and sodium borohydride (NaBhU 99% Sigma-Aldrich) are used in this case. as a tin precursor and a reducing agent, respectively. The solvent used is diethylene glycol (DEG - 99% Sigma-Aldrich) which is systematically dried by 3A molecular sieve for three nights before each synthesis to make the solvent anhydrous.

Then, the polyvinyl pyrrolidone is used as an encapsulating agent to prevent the agglomeration of the tin nanoparticles during their dissolution and to avoid their excessive oxidation. In a glove box under nitrogen, 1 g of PVP (Mw = 1.3 million) and 2 g of NaBhU are dissolved in 100 mL of dried DEG. The mixture is then stirred in a 500 ml round-bottomed flask, the interior of which has been cleaned with a solution of aqua regia and then with ethanol in order to then be dried in the oven at 80 ° C. The mixture is kept under stirring for 3 hours to obtain a transparent and homogeneous solution. Then, 2 ml of tin 2-ethylhexanoate is injected into the solution with an injection rate of 4.5 ml / min. A solution is thus obtained with a black color which appears immediately after the injection of the tin salt, thus indicating the formation of the tin nanoparticles. The final solution obtained is stirred continuously for 60 minutes to give sufficient time to complete the reaction. For the recovery of the Sn nanoparticles in the solution, a centrifugation step is necessary. This step will make it possible to separate the nanoparticles from the DEG. Thus, the centrifugation is carried out with acetone and then with ethanol several times to recover the maximum number of nanoparticles from the solution (Thermo Scientific Heraeus Primo Centrifuge). During this step, the operations for emptying the supernatant recovered from centrifugation and those for washing with acetone or ethanol are carried out in the glove box in order to avoid any prolonged contact of the synthesized nanoparticles with air and therefore minimize their oxidation.

Observations by transmission electron microscopy show that the obtained tin nanoparticles are spherical, with an average size of 10.4 ± 3 nm. Characterizations by FTIR and by DRX show that the Sn nanoparticles are slightly oxidized on the surface but do not exhibit oxidation states by volume.

These nanoparticles are then dispersed in a mixture of solvents to formulate a new tin-based ink compatible with inkjet printing. Thus, 10% by mass of tin was dispersed in 60% by mass of ethylene glycol (EG) and 30% by mass of ethanol. The mixture is stirred under ultrasound to ensure a good suspension in the solvents. A filtration step (Minisart, pore size 5 µm) of the ink before it is transferred to the print cartridge eliminates the aggregates that are not solubilized by ultrasound.

The surface tension of the Sn ink of 33 mN / m is sufficiently lower than the surface energy of the plastic support used (Upilex-50S®) measured at 47 mN / m, thus allowing the ink to be spread over the substrate.

Printing the ink with the tin nanoparticles

To implement the printing of the ink on the substrate, the “standard” waveform given by the printer (DMP-2800, Dimatix) was used. The substrate used is Upilex®. Then, the ejection voltage of the ink comprising the tin nanoparticles was adjusted between 15 V and 19 V so as to generate spherical drops, with rectilinear trajectories, which move at the same speed and which do not break up. not during the nozzle-substrate path.

The printing conditions of the ink comprising the tin nanoparticles on the Upilex® are summarized in the following table:

[Table 1]

The films, i.e. thin layers, of tin are then printed on the Upilex®. Optionally, a drying step can be provided, but this is not mandatory, the drying can take place during step c) of photonic annealing.

Incidentally, although the ink deposition is carried out by inkjet printing in this exemplary embodiment, it can also be carried out by laser printing, centrifugal coating, dip coating, drop coating, spray coating, inkjet printing. Langmuir-Blodgett deposition method or band casting.

Photonic annealing

The system (PulseForge 3200, Novacentrix) integrates two Xenon lamps which produce photon emission in a pulsed manner over a wide spectrum of wavelengths ranging from 180 nm to 1200 nm.

The illumination frequency, the supply voltage, the number of pulses applied and the duration of the pulses are optimized in order to preserve the entire support and the shape of the printed pattern. In FIG. 3 is illustrated the emission spectrum (in black) of such an excitation source for an illumination frequency of 10 kHz, a supply voltage of 360 V. This voltage corresponds to the amplitude of the electric voltage applied to the terminals of the lamp. It is the same for all the draws. In gray is illustrated the relative intensity of light absorbed by a tin film when it is illuminated by means of said source. The intensity of the light absorbed is generally lower than the light emitted, in particular in the visible range. That being said, tin is well adept at absorbing light, which provides an indication of its ability to heat up when exposed to pulsed light.

As illustrated in FIG. 4, the emitted power per unit area density reaches 6000 W / m ² when a train of 6 pulses is applied, using the aforementioned operating parameters. Concomitantly, the surface density of power absorbed by the thin layer of tin can reach nearly 4000 W / m ² . For comparison, the power density received at the earth's surface is 1361 W / m ² . The surface density of the power absorbed is therefore very high.

In Figure 5 is shown the absorbance calculated as a percentage of the thin layer of tin (gray and continuous spectrum). Although the intensity of the light emitted (black and dotted spectrum) in the ultraviolet is relatively low, it can be seen that the absorbance of the thin tin film is maximum in the ultraviolet and the visible, where the emitted light intensity is high enough to be effectively absorbed by the thin layer of tin.

As illustrated in Figure 6, the absorption of light by the tin nanoparticles initiates heating of the sample up to several hundred degrees in a very short time, of the order of a millisecond. Thus, in 800 ps (or 0.8 ms) the thin layer of tin reaches a temperature of 1165 ° C. The annealing temperatures undergone by the tin samples are determined from a theoretical model based on the resolution of the heat equation and the calculation of the absorbed power density.

Where Q _a being the power density absorbed by the film

P and I are respectively the power and the intensity of the emitted light, E (x) is the energy emitted (measured by a bolometer), and Cp and k are the heat capacity and thermal conductivity.

Applying two cycles of 6 pulses each, with a frequency of 4 Hz between the two consecutive cycles. In each cycle comprising the 6 pulses, a supply voltage of 360 V and an illumination frequency of 10 kHz makes it possible to obtain a well-defined sample, having a square electrical resistance of the order of 170 W / sq, for a temperature of 1343 ° C.

A significant decrease in the square electrical resistance of the tin pattern is observed in Figure 7 as the energy deposited by the pulsed light increases and therefore the temperature of the thin film increases concomitantly. It should be noted that each point illustrated in FIG. 7 corresponds to a different sample, each sample having undergone a single photonic annealing.

This decrease in electrical resistance is due to graphitization by carbon atoms from Upilex®, which was confirmed using physicochemical characterizations by Raman (figure 8) and by TEM-HR (figures 9 and 10).

In Figure 8, peaks are observed at 1350 cm ^-1 , 1580 cm ^-1 , 2660 cm ^-1 , characteristic of carbonaceous materials (Malard et al., Phys Rep, 2009). The peak at 1350 cnr ¹ corresponds to a D band associated with disordered graphene while the peak at 1580 cnr ¹ corresponds to a G band associated with the E2g symmetry of the sp ² carbon.

Analysis of the data obtained by Raman characterization, in particular of the evolution of the ID / IG intensity ratio (ID corresponding to the peak at 1350 cm ¹ and IG corresponding to the peak at 1580 cm ¹ ) as a function of the square resistance, indicates that by having recourse to the process according to the invention, the graphitization begins at around 1300 ° C. due to the presence of a very slightly oxidized nano Sn catalyst at the surface.

FIG. 9 makes it possible to visualize the graphene covering said nanoparticles after the photonic annealing.

Battery applications

The functionality of the composite material comprising core-shell nanoparticles whose core is made of tin and the shell is made of graphene, such as previously described, was confirmed using said layer as the anode of a flexible lithium battery.

Electrochemical tests illustrated in FIG. 10 show that the initial discharge capacity is of the order of 2200 mAh.g ¹ , which is comparable to that found in the literature for tin / graphene composites.

Second practical embodiment of the present invention: development of a graphene @ Sn-NPs composite material from a precursor made of carbon nanoparticles printed on a carbon nanoparticle / glass substrate

An operating mode associated with this exemplary embodiment is described in the following.

1. Inkjet print the carbon nanoparticles on the glass substrate, for example a microscope slide

2. Leave to dry at 40 ° C on the inkjet printer bed

3. Inkjet print the ink based on Sn nanoparticles according to step b)

4. Leave to dry on the inkjet printer bed at 40 ° C

5. Apply the photonic annealing according to step c) under the same conditions as those used to graphitize the Upilex® using only the Sn-based ink

The carbon nanoparticles are supplied by Method Electronics, USA. They are provided in the form of an ink which is deposited and undergoes a drying step prior to step b). These preliminary steps correspond to steps 1. and 2. the above procedure. It is on the surface of the substrate thus obtained that step b) of the process is carried out.

Photonic annealing is carried out using the same system as previously described (PulseForge 3200, Novacentrix).

Although the deposition of the ink is carried out by inkjet printing in this exemplary embodiment, it can also be carried out by laser printing, centrifugal coating, dip coating, drop deposition, the Langmuir-Blodgett deposition method or band casting.

Third practical embodiment of the present invention: production of a graphene @ Sn-NPs composite material from a precursor made of tin nanoparticles printed on a fructose-based surface

1. Dissolve 5% wt. sugar (Fructose) in distilled water

2. Place a drop (100 mI_) by dropwise deposition on a glass substrate, for example a microscope slide, maintained at 40 ° C on a hot plate.

3. Apply a drop of the ink based on Sn nanoparticles (100 mI_) according to step b)

4. Dry the film obtained at 100 ° C on a hot plate.

5. Apply the photonic annealing according to step c) under the following conditions: duration of a pulse: 60 psec, illumination frequency: 2 KHz, supply voltage: 400 volts, number of pulses: 15, energy deposited : 6.7 J / cm ²

The dissolved fructose is deposited and undergoes a drying step prior to step b). These preliminary steps correspond to steps 1 and 2 of the above operating mode. It is on the surface of the substrate thus obtained that step b) of the process is carried out.

Although the deposition of the ink is carried out by drop deposition in this exemplary embodiment, it can also be carried out by inkjet printing, so-called LIFT laser printing, centrifugal coating, dip coating, the method of Langmuir-Blodgett deposit or band casting. In relation to the fourth exemplary embodiment, the following describes a practical implementation of the present invention in which the carbon source is included in the ink.

Fourth practical embodiment of the present invention: production of a graphene @ Sn-NPs composite material from a precursor made of a composite ink comprising both tin nanoparticles and fructose on a glass substrate

1. Disperse 10% wt. of Sn in a water / ethylene glycol / Tween20 ® mixture with a volume percentage 38/60/2

2. Add 5% wt. fructose

3. Shake to dissolve the sugar in the ink.

4. Inkjet print the ink on the glass substrate (microscope slide) according to step b) under the same conditions as those used to print the Sn ink

5. Dry at 40 ° C on the inkjet printer bed

6. Apply the photonic annealing according to step c) under the following conditions: duration of a pulse T: 60 psec, illumination frequency: 2 KHz, supply voltage: 400 volts, number of pulses: 15, energy deposited: 6.7 J / cm ²

In this case, the ink contains both the nanoparticles to be graphitized and the carbon source. The ink preparation steps correspond to steps 1., 2. and 3. of the above procedure. In step 4, therefore, both the carbon source and the nanoparticles to be graphitized are deposited.

Although the ink deposition is carried out by inkjet printing in this exemplary embodiment, it can also be carried out by laser printing, centrifugal coating, dip coating, drop coating, the deposition method. Langmuir-Blodgett or band casting.

Claims

1. Process for producing a composite material comprising nanoparticles of the core-shell type, the core of which is made of a material capable of absorbing light and the shell is made of graphene, the process comprising the following steps: a) providing a substrate as well as an ink comprising nanoparticles capable of absorbing light, said nanoparticles being encapsulated in a polymeric matrix; b) depositing the ink on a surface of the substrate, said surface or said ink comprising a carbon source chosen from:

• a polymer comprising an amide function and an aromatic function,

• a hexose,

• carbon nanoparticles,

• self-assembled monolayers comprising aromatic rings, c) illuminating the surface of the substrate covered with ink with pulsed light to bring the nanoparticles included in the ink to a heating temperature greater than a dissociation temperature of the carbon included in the carbon source, the carbon thus dissociated then being able to diffuse or to dissolve in the nanoparticles included in the ink to form said composite material.

2. The method of claim 1, wherein the material capable of absorbing light is metallic or semiconductor.

3. The method of claim 2, wherein the material capable of absorbing light is based on tin (Sn).

4. Method according to one of the preceding claims, wherein the polymer comprising an amide function and an aromatic function is a polyimide.

5. A method according to any one of claims 1 to 3, wherein the hexose is fructose or glucose.

6. Method according to any one of claims 1 to 3, wherein the self-assembled monolayers are made based on hexadecane thiol (HDT), biphenyl-4-thiol (BPT), 1, 1 '-biphenyH -4-thiol (BPTC), tolanethioacetate (TTA), tolan disulfide (TDS), tolane methyl sulfide (TMS).

7. A method according to any one of claims 1 to 5, wherein the mono-assembled layers comprise only aromatic rings.

8. Method according to any one of the preceding claims, wherein, during step b), the ink is deposited with a technique chosen from printing, centrifugal coating, dip coating, deposition. by drop, the Langmuir-Blodgett deposit or band casting.

9. A method according to any preceding claim, wherein in step b) the ink is deposited by inkjet printing.

10. The method of claim 9, wherein, during inkjet printing, a distance between an ejection nozzle of the printer and the substrate is adjusted between 600 µm and 1.5 mm.

11. A method according to any one of the preceding claims, wherein in step c) the pulsed light is generated by means of an excitation source emitting wide-band polychromatic electromagnetic radiation, said excitation source emitting at wavelengths between 180 and 1200 nm.

12. A method according to any one of claims 1 to 10, wherein in step c), the pulsed light is generated by means of a laser source.

13. A method according to any one of the preceding claims, wherein, in step c):

- the amplitude of the pulsed light is obtained with a supply voltage of the excitation source of between 200 and 900 V, preferably around 360 V;

- the frequency of illumination of the pulsed light is between 0.1 and 100 kHz, preferably around 10 kHz;

- the number of pulsed light pulses is between 6 and 20,

- the duration of a pulse is between 50 psec and 2 ms, preferably about 80 psec.