Suspension, colloid or reticular structure composed of liquid metal droplets combined with graphene-based particles, respective inks, transparent stretchable conductors, and processes for obtaining them

Abstract

A suspension or colloid consisting of liquid metal droplets combined with graphene-based particles, wherein the liquid metal is gallium or a gallium alloy, and the graphene-based particles are selected from graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or combinations thereof. Each ink, transparent stretchable conductor, and their acquisition processes; also, a conductor obtained by applying a coating of the suspension, colloid, network, or ink according to any of the previous claims onto a substrate and laser sintering the coating, in particular the conductor is an electrode or a circuit trace or a circuit. Uses include electrodes for optoelectronic devices, piezoresistive composites sensitive to pressure or strain, pressure or strain sensors, temperature sensors, electroluminescence devices, photovoltaic devices, memory devices, or energy storage devices.

Description

Technical Field

[0001] The present disclosure relates to materials, methods, and processes for the synthesis, deposition, and laser processing of liquid metal nanoparticles coated with graphene oxide for application to stretchable electronics, displays, and optoelectronic devices such as stretchable and flexible optoelectronic devices, stretchable and flexible energy storage devices, sensors, and memory devices.

Background Art

[0002] Next-generation electronic devices, including optoelectronics such as displays and optoelectronic devices such as photovoltaic devices, are desired to be thin, bendable, and stretchable. This allows existing surfaces to be converted into active surfaces that incorporate energy and display information. For example, the roof of a house or the window or dashboard of a car, or the surface of a fiber can be transformed into a smart surface with functions. Stretchable transparent conductive films (TCFs) are expected to be applied to new fields of optoelectronic devices such as stretchable thin-film displays, e-skins for robots, and interactive e-textiles. However, unlike general flexible TCFs, the fabrication of stretchable transparent conductors (STCs) remains a major challenge.

Summary of the Invention

Problems to be Solved by the Invention

[0003] Recent efforts regarding the fabrication of transparent TCFs have focused on the use of ion conductors or highly aspect-ratio conductive fillers such as silver nanowires (AgNWs) or carbon nanotubes (CNTs). Ion conductors have high stretchability but very low electrical conductivity and lack long-term stability due to water evaporation.

[0004] Transparent conductors based on high aspect ratio conductors such as AgNWs have been studied by several groups over the past few years. High aspect ratio conductors can penetrate even with a low metal ratio, enabling the formation of a conductive thin film with large voids. Although promising, AgNWs are very expensive, their film formation is difficult, their adhesion to the substrate is low, and they suffer from poor contact at wire-wire junctions. These problems are obstacles to scalable manufacturing and also affect performance against mechanical strain. Usually, the resistance to strain and strain cycles is limited. This is related to the brittleness of the nanowire junctions and their high contact resistance.

[0005] Currently, liquid metals (LMs) such as eutectic gallium indium (EGaIn) are being accepted as a major option in stretchable electronics because they combine high conductivity, extremely high stretchability, excellent cycle performance, self-healing properties, and a low gauge factor (GF). Therefore, the development of a transparent conductor based on EGaIn LM is highly desired. However, EGaIn is inherently reflective, and even when applied as an ultra-thin film, a non-transmissive ultra-thin film (<3 nm) oxide shell is rapidly formed.

[0006] These facts are disclosed to explain the technical problems addressed by the present disclosure.

Means for Solving the Problems

[0007] In one embodiment, materials and methods for the low-cost scalable production of a stretchable transparent conductor based on specially designed liquid metal nanodroplets are shown. This is achieved through surface modification of liquid metal droplets using graphene oxide sheets, or engineering composites where liquid metal droplets are bound to high aspect ratio carbon-based sheets such as graphene oxide (GO), followed by post-laser assisted sintering. By designing the synthesis technique and varying the amount of graphene oxide incorporated, the film-forming technique, and the parameters of the post-stage sintering, various composite materials with different transparency, conductivity, mechanical, and chemical elasticity can be obtained. It is also designed to be applicable by various coating methods such as spray coating, roll coating, and thin film coating by blade coating. Furthermore, a technique has been developed to adjust various parameters of the film after film formation by laser processing. This includes the conductivity of the film, the density of the 3D permeable network, and transparency.

[0008] In one embodiment, an infrared fiber laser is used to sinter the ink to form a highly conductive and stretchable transparent film. Laser sintering typically turns non-conductive or very low-conductivity electrodes (e.g., on the order of MΩ / Cm 2 into highly conductive electrodes (in the range of Ω / cm 2 ).

[0009] The same does not occur when the film of non-functionalized LM nanodroplets is irradiated with a laser, and the film does not become transparent or stretchable. This is because the graphene oxide layer changes the surface properties of the LM nanodroplets in terms of the existing amount of gallium oxide and the mechanical, thermal, and chemical stability of these particles.

[0010] In fact, the application of EGaIn micro- and nanomaterials to soft electronics has been demonstrated. EGaIn nanoparticles (NPs) are aggregates of a gallium oxide shell and a liquid core and have been reported as laser-sensitive materials for generating conductive patterns on soft substrates such as PDMS (polydimethylsiloxane). The laser ruptures the nanometer Ga2O3 semiconductor shell around the EGaIn particles, forming conductive EGaIn micropaths [7, 8]. However, achieving electrical transparency is a unique feature that occurs only through surface modification of the EGaIn nanoparticles. Furthermore, liquid metal droplets in previous studies are very sensitive to pH and rapidly aggregate into large spheres in strongly acidic or basic solutions, limiting many of their applications such as energy storage or sensor electrodes.

[0011] Here, graphene oxide (GO) is used for surface modification, but the overall concept can be extended to other materials that can bind to gallium oxide by galvanic substitution or surface charge. This material is coated on a substrate as a thin film and sintered with a laser.

[0012] By varying laser parameters such as power and speed, the thin film can be sintered or ablated to adjust transparency and conductivity. In another embodiment, a CO2 laser was used to fabricate a semiconductor composite that can be used as a programmable memristor by applying a current and a pressure-sensitive film whose electrical resistance changes when mechanical pressure is applied.

[0013] To fabricate a transparent conductor, it has been demonstrated that EGaIn nanoparticles can be laser-assisted self-organized into a 3D permeable network, resulting in the formation of a 3D porous microstructure that enables light transmission. Surface modification of liquid metals is generally achieved by adding GO sheets to a solution containing micro- or nano-droplets of liquid metals. A trace amount of GO (0.001 - 0.1 wt%) is sufficient for surface modification. The addition of GO brings two radical effects to ink synthesis and the formation of transparent conductors. First, when GO sheets are added, liquid metal droplets pre-assemble into a network of aggregates wrapped by large sheets of graphene oxide. As a result, a high-concentration ink precipitates, which can be collected and coated onto a substrate. LM nanoparticles without graphene need to be applied by spraying. Second, upon laser sintering, the GO-EGaIn nanocomposite self-assembles to form a porous 3D structure, which is not observed in LM nanoparticles without graphene (Figs. 5 and 7). The formation of such a three-dimensional structure was impossible with conventional lithography techniques but has become possible through simple laser-assisted self-organization thanks to the GO sheets.

[0014] Note that even without laser sintering, by adjusting the size of the GO sheets, the coating film can be made slightly transparent, but there is a limit to the transparency, and moreover, the sample has no conductivity or very low conductivity. Laser sintering significantly improves transparency and conductivity through various mechanisms. This includes partial reduction of graphene oxide, thinning of the graphene oxide sheets, aggregation and sintering of liquid metal particles, etc. Aggregation by laser significantly improves conductivity and transparency. The conductivity is improved by six orders of magnitude from megaohms to several ohms. This essentially means that non-conductive samples become conductive. Transparency is improved by the reduction of the occupied surface area and occupied volume as small particles aggregate into large aggregates. In both cases, the graphene oxide sheets function as a guide for the binding of liquid metal droplets. Therefore, their shape, size, and concentration play important roles in obtaining a transparent conductor.

[0015] Depending on the type of laser and the applied power, various scenarios can occur and the properties of the film can be adjusted. Although laser sintering was shown here, this can be extended to other sintering techniques such as thermal sintering or photothermal sintering.

[0016] This disclosure is the first of its kind regarding materials and methods for obtaining transparent conductors. This includes new ink formulations and synthesis techniques that include low and high concentration GO-EGaIn inks that can self-assemble into clusters for 3D permeable network formation. By adjusting the type and output of the laser, partially sintered (kiloohm conductivity range), fully sintered (ohm range), and composites by ablation can be obtained. This enables the fabrication of transparent or translucent, flexible or stretchable electrodes, sensors, memristors, and energy storage devices.

[0017] Compared to the materials and methods for stretchable transparent conductors shown in the prior art literature (e.g., nanotubes, silver nanowires), the disclosed invention enables significant improvements in both conductivity and stretchability (more than 6 times improvement compared to the highest records).

[0018] EGaIn particles decorated with graphene can potentially combine the advantages of graphene, namely high surface area, excellent mechanical and chemical resistance, with the excellent electromechanical properties of liquid metals, such as high electrical conductivity. Furthermore, the solid-liquid interface between graphene and liquid metal can facilitate charge transfer within the composite material.

[0019] Furthermore, it is also disclosed that a chemically stable thin film can be developed by coating GO on EGaIn nanoparticles. Thereby, EGaIn nanoparticles can be used in energy storage devices. The stability of EGaInNPs is known to depend on an ultrathin (about 0.5 - 3 nm) Ga2O3 shell. When exposed to highly alkaline electrolytes used in batteries and supercapacitors, EGaInNPs lose their oxide shell and coalesce into larger LM droplets, thereby losing their structure and surface area. Therefore, previous attempts to use EGaIn as an electrode for supercapacitors have been limited to using liquid metal in bulk form [see References 11 - 13]. Although these EGaIn-based SCs have been shown to be stretchable, the areal capacitance of these devices remains on the order of 10 - 30 mF / cm 2 and does not reach typical values for SC energy storage applications. Stabilizing thin films from liquid metal nanodroplets in an alkaline solution is the key to improving energy storage capacity.

[0020] Also disclosed herein are techniques for film formation and patterning of circuits based on this material. Stretchable transparent conductors (STCs) with an unprecedented combination of about 1400% stretchability and about 2X10^6 S / m conductivity have been demonstrated. Unlike AgNW-based STCs that require complex synthesis and deposition processes, STCs are formed in a few minutes. All manufacturing processes, including ink synthesis, coating, and laser sintering, are achieved using low-cost, readily available equipment and scalable processes. Furthermore, laser-assisted fabrication of large electrodes, stretchable displays, and sensing devices with complex shapes and micrometer features that can be fabricated using simultaneous laser reduction, patterning, and ablation of thin films coated with GO@EGaIn (graphene oxide - EGaIn) composites is disclosed.

[0021] Overall, this technique serves as a versatile method for scalable production of laser-reduced GO@EGaIn (graphene oxide - EGaIn) electrodes with micron-sized features in seconds, useful for rapid prototyping. Different from previous graphene and GO film-forming methods such as CVD (Chemical Vapor Deposition) and spin coating, simple coating techniques used in this study, such as spray coating, thin film coating, and direct drawing, can deposit large-area conductors. Some of these coating techniques are performed manually, but the electrodes obtained after laser treatment show acceptable reproducibility in terms of electrical resistance, and the surface roughness is in the micrometer range. Therefore, this material composition and the developed manufacturing method represent a step towards scalable and low-cost production of graphene-based large-area electrodes, transparent stretchable conductors, energy storage electrodes, sensing devices, etc.

[0022] A suspension or colloid composed of liquid metal droplets combined with graphene-based particles is disclosed, wherein the liquid metal is gallium or a gallium alloy, and the graphene-based particles are selected from graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or combinations thereof.

[0023] In one embodiment, the liquid metal is gallium or a gallium alloy, and the graphene-based particles are selected from the list of graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or combinations thereof [i.e., a network combination of filaments].

[0024] A suspension, colloid, or network structure in which the liquid metal droplets are coated with graphene-based particles is also disclosed.

[0025] In an embodiment, the weight ratio of the graphene-based particles to the liquid metal droplets is 0.0001 - 0.5%, preferably 0.001 - 0.1%.

[0026] Also disclosed is an ink containing a concentrated reticular structure according to any one of claims 2 to 4 of the claims, obtained by separating the reticular structure from a colloid or suspension according to claim 1 of the claims.

[0027] In one embodiment, the ink is obtained by the following steps, namely, suspending graphene-based particles in a first medium to obtain a first suspension; mixing a liquid metal with the first suspension to obtain a mixture of a reticular structure of the liquid metal droplets and a reticular structure of the graphene-based particles; and separating a concentrated reticular structure of the liquid metal droplets and the graphene-based particles from the mixture, or suspending liquid metal droplets in a second medium to obtain a second suspension; mixing the graphene-based particles with the second suspension to obtain a mixture of a reticular structure of the liquid metal droplets and the graphene-based particles; and separating a concentrated reticular structure of the liquid metal droplets and the graphene-based particles from the mixture.

[0028] In an embodiment, the ink is obtained by suspending the graphene-based particles in the first medium to obtain the first suspension, suspending the liquid metal droplets in the second medium to obtain the second suspension, mixing the suspensions, and separating a concentrated reticular structure of the liquid metal droplets and the graphene-based particles from the mixture, wherein the first medium and the second medium are miscible [the first medium and the second medium can be regarded as co-solvents].

[0029] In an embodiment, the separation is performed by precipitation, centrifugation, and / or filtration.

[0030] In an embodiment, the first medium is water or an aqueous solvent, and in particular both the first medium and the second medium are water or an aqueous solvent.

[0031] In an embodiment, the second medium is ethanol or an alcohol-based solvent.

[0032] Also disclosed is a printable ink further comprising a binder for improving ink adhesion and / or viscosity, particularly for nozzle extrusion or screen printing.

[0033] Also disclosed is a conductor obtainable by applying a coating of a suspension, colloid, network, or ink according to any of the disclosed embodiments onto a substrate and laser sintering the coating, particularly wherein the conductor is an electrode or a circuit trace or a circuit.

[0034] In an embodiment, the conductor is transparent or translucent.

[0035] In an embodiment, the conductor is flexible or stretchable.

[0036] In an embodiment, the coating is performed by spraying, rod coating, slot die, inkjet printing, aerosol jet printing, or blade coating.

[0037] In an embodiment, the conductor comprises a conductive pattern obtained by laser patterning or lithography.

[0038] In one embodiment, the gallium alloy is an alloy of gallium-indium or gallium-indium-tin or eutectic gallium-indium.

[0039] Also disclosed is a process for obtaining a suspension or colloid, the process comprising the step of combining liquid metal droplets with graphene-based particles, wherein the liquid metal is gallium or a gallium alloy and the graphene-based particles are selected from the list of graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or combinations thereof.

[0040] A process for obtaining a network structure of liquid metal droplets combined with graphene-based particles, which includes combining liquid metal droplets with graphene-based particles, wherein the liquid metal is gallium or a gallium alloy, and the graphene-based particles are selected from the list of graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or combinations thereof, is also disclosed.

[0041] In one embodiment, the process for obtaining a suspension, colloid, or network structure is obtained by coating the liquid metal droplets with the graphene-based particles.

[0042] In an embodiment, the first medium is water or an aqueous solvent, particularly both the first medium and the second medium are water or an aqueous solvent, and particularly the pH of the aqueous solution containing graphene-based particles is 1 to 6, preferably 2 to 3.5.

[0043] In an embodiment, the second medium is ethanol or an alcohol-based solvent, and particularly the liquid metal is 0.5 to 10% (w / w) of ethanol or an alcohol-based solvent.

[0044] In an embodiment, the laser is a fiber laser having a wavelength in the range from UV to IR.

[0045] In an embodiment, the conductor comprises a conductive pattern obtainable by laser patterning or lithography.

[0046] A process for recovering liquid metal particles from a suspension is also disclosed by adding a liquid suspension containing particles having a zeta potential opposite to that of the liquid metal to promote the binding between the liquid metal and the added particles.

[0047] Also disclosed is an apparatus comprising a suspension, colloid or reticular structure according to any of the disclosed embodiments, an ink according to any of the disclosed embodiments, or a conductor according to any of the disclosed embodiments.

[0048] In an embodiment, the apparatus is a photoelectric device, a piezoresistive composite sensitive to pressure or strain, a pressure or strain sensor, a temperature sensor, an electroluminescence device, a photovoltaic device, a memory device, or an electrode for an energy storage device.

[0049] The following figures provide preferred embodiments for explaining the present disclosure and should not be regarded as limiting the scope of the present invention.

Brief Description of the Drawings

[0050]

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Embodiments for Carrying Out the Invention

[0051] The present invention relates to a colloid composed of liquid metal droplets coated with graphene-based particles, where the liquid metal is gallium or a gallium alloy, and the graphene-based particles are selected from the list of graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or combinations thereof.

[0052] In one embodiment, a graphene oxide (GO) - liquid metal (LM) (hereinafter referred to as GO@LM) composite or for a transparent conductor was prepared. This composite is also called a GO-coated EGaIn LM droplet or a GO-coated LM droplet.

[0053] In one approach, GO suspension in water, bulk EGaIn, and acetic acid were used to prepare the GO@LM (GO-coated LM droplet) composite. It should be noted that upon mixing, GO@LM (also referred to here as GO@EGaIn) may convert to rGO@LM (reduced graphene oxide-coated liquid metal) depending on the solution and its pH. Referring to Figure 1A), the rGO@EGaIn colloidal solution was obtained by sonicating 1 g of EGaIn in 20 ml of 0.4% (wt) aqueous graphene oxide solution (Graphenea) for approximately 20 minutes and then adding 1 ml of acetic acid (0.1 M) in between. The weight ratio of GO / EGaIn was only 0.08. This solution was mixed at 2000 rpm for 3 minutes using a planetary mixer (Thinky ARE-250). As a result, a suspension of partially reduced GO-coated EGaIn particles was obtained (Figure 1B). Next, this solution was spray-coated onto a substrate (glass or polymethyl methacrylate (PMMA)) using a spray gun. Measuring the sheet resistance of the spray-coated sample before laser reduction, it was approximately (30 ± 5) kΩ / sq, which was almost three orders of magnitude lower than that of the spray-coated EGaIn particles before laser reduction. Finally, the spray-coated sample was further reduced using a 20 W MOPA laser (JPT).

[0054] Here, the term rGO@EGaIn refers to reduced graphene-decorated EGaIn particles before laser processing. It should be noted that during sonication, GO is partially reduced. To distinguish, the material after laser reduction was denoted as LrGO@EGaIn.

[0055] To evaluate the influence of laser parameters (pulse width, power, speed) on the LrGO@EGaIn film, an rGO@EGaIn film spray-coated on glass was fabricated, and a MOPA laser was used thereon at 3 mm each 2Forty squares were laser-reduced. Each set of four squares is a repetition of the same parameters, as shown in Figure 2's B-i). Referring to Figure 2's A-i) and Figure 2's A-ii), the role of the laser pulse width was first evaluated. The laser output and speed were fixed at 85% and 50 mm / s respectively, and the pulse width was varied according to the pulse width / frequency dependency table provided by the laser manufacturer, with the frequency decreasing gradually between 350 kHz and 40 kHz. In this way, the pulse width was increased up to 6 - 250 nanoseconds. Conductivity was not measured in films processed with a pulse width of 30 nanoseconds (pulse width) or more. The sample with a 9 ns pulse width was able to achieve the best trade-off between conductivity and transparency. Also, from Figure 2's A-i) and Figure 2's B-i), it can be noticed that despite the samples being spray-coated by hand, the final registered electrical resistance after laser area reduction shows reproducible results.

[0056] With the pulse width fixed at 9 ns, the frequency at 240 kHz, and the laser speed at 50 mm / s, the laser output was evaluated. The laser output was varied. The sheet resistance increased with the applied fluence, which was due to excessive delamination of the material. Nevertheless, opaque and translucent conductive squares were obtained. Therefore, 70% output was selected for the next sample. It should be noted that the aim of this work was to reach the best trade-off between transparency and conductivity.

[0057] Finally, as shown in Figure 2's C), the laser speed was varied between 10 mm / s and 100 mm / s. As shown in Figure 2's C), when the laser speed was varied between 10 mm / s and 100 mm / s, a low laser speed was sufficient and a sheet resistance of 1 kΩ / sq or less was obtained when transparency was not important. Figure 2's D) shows the dependency of the transmittance on the laser power. At 100% output, the optical transparency reaches 50% or more. It should be noted that the increase in sheet resistance does not mean that the actual electrical conductivity of the material is also decreasing. Instead, it only indicates that material thinning is occurring, resulting in a higher sheet resistance.

[0058] To better understand the crystal phase of the synthesized composite material, X-ray diffraction (XRD) analysis was performed on the as-sprayed films in the range of 2θ angles from about 7° to about 45° with or without laser treatment. In this way, three types of samples were analyzed. One is the sample without laser irradiation, which is called the pristine sample, and the other two are the samples treated with 50% and 75% laser power, corresponding to the opaque sample and the translucent sample, respectively. The results are shown in Fig. 3A). In the XRD pattern of the initial sample, a sharp peak of GO related to the (0 0 1) crystal plane was clearly observed at 2θ = 9.89°. Also, in this sample, a broad peak related to the (1 1 1) plane of β-Ga2O3 was seen at 2θ = 34.16°.

[0059] In the laser-processed samples, more peaks related to the crystalline materials are visible, especially in the sample processed with 50% laser power (opaque film). Due to the laser treatment, all graphene oxides were changed to reduced graphene oxides. Accordingly, the peak at 2θ = 9.89° completely disappeared, and a broad peak with lower intensity compared to GO related to the (0 0 2) plane of rGO appeared at 2θ = 25.47°.

[0060] In the diffraction pattern with 50% laser power, peaks of monoclinic β-Ga2O3 (JCPDS No.01-087-1901.) were detected at 2θ values. Peaks of monoclinic β-Ga2O3 (JCPDS No.01-087-1901.) at 18.88, 30.1, 30.4, 31.7, 33.4, 35.1, 37.5, 38.4° were detected, corresponding to the (-2 0 1), (4 0 0), (-4 0 1), (-2 0 2), (-1 1 1), (1 1 1), (4 0 1), (-3 1 1) planes, respectively, and peaks of cubic indium (JCPDS No.00-005-0642) were also detected at 2θ. Peaks of cubic indium (JCPDS No.00-005-0642) at 33°, 36.4°, 39.2° corresponding to (1 0 1), (0 0 2), (1 1 0) were seen.

[0061] When the laser output was increased to 75%, the layer became thinner, and most of the peaks that were clearly observed in the previous samples disappeared. In this diffraction pattern, only one main plane of monoclinic β-Ga2O3 (1 1 1) was detected at 2θ = 35.1°, and two main planes of cubic indium (1 0 1) and (1 1 0) were detected at 2θ = 33° and 39.2°, respectively.

[0062] Surprisingly, the MOPA laser induces a local high temperature in the sprayed film and further induces the crystallinity of Ga2O3. Previously, it was reported that laser-sintered EGaIn remained amorphous, while the crystallinity of Ga2O3 in sprayed EGaIn nanoparticles was induced only by thermal sintering at 300 °C or higher [see Reference 9].

[0063] In the embodiment, Raman spectroscopy was also used to further investigate the effect of laser processing. Fig. 3B) shows a general profile of the obtained Raman spectra of the samples investigated. Figs. 3C) - E) show the plots of I 2D / I G and I D / I G and the full width at half maximum of the D band of the spectra of the samples investigated. From Fig. 3C), it can be seen that the I 2D / I G ratio reaches a maximum at a laser output of about 60%, and the decrease in GO increases with the laser output. When the laser output exceeds this value, the I 2D / I G ratio begins to decrease due to the influence of the increase in disorder (more defects) that tends to increase the intensity ratio. Note that the 60% output is the threshold at which the sample begins to show transparency. At a laser output of about 80% or more, the I 2D / I GThe ratio reaches an almost constant value. This result, when combined with the data shown in Figures 3D) and 3E), seems to indicate that there is no significant difference in the degree of decrease between GO and disorder at laser outputs of about 80% or more. It should be noted that the laser output mentioned here is merely an indicator and is relative to the type of laser. By changing the type of laser or the lens and other characteristics, the output of the laser will change. For example, when using a lower-output or higher-output laser, the required parameters will also change, so it is necessary to adjust these parameters according to the laser used.

[0064] Hereinafter, a graphene liquid metal network structure for a stretchable transparent conductor will be described.

[0065] In one embodiment, the materials and methods mentioned above were used to fabricate a flexible transparent conductor. Here, a new and novel formulation, synthesis, deposition, and laser processing process that enables a highly stretchable and highly conductive transparent conductor is disclosed. This new synthesis method is designed to intentionally not become unstable in the solvent, enabling the precipitation of the GO-EGaIn network structure. In contrast to the low-concentration solution for spray coating, this new GO-EGaIn nanocomposite network structure can be recovered as a high-concentration substance that can be coated by general thin-film coating techniques.

[0066] Generally, an important issue in the process of generating EGaIn particles is to overcome their high surface tension to combine adjacent droplets. To overcome this, it is necessary to form a physical barrier on the surface of the droplets. In most works, this is done by the gallium oxide layer that naturally forms on the surface of EGaIn droplets. In an aqueous medium with a pH of 3 - 11, the passivating oxide is stable [see Reference 10]. However, simply sonicating liquid metal in water results in an unstable colloidal suspension, and the suspension usually precipitates within a few tens of minutes. When the medium is changed from water to ethanol, the formed EGaIn particles remained floating for several weeks. This high colloidal stability is thought to be due to the formation of a graphite carbon coating during sonication. In addition to gallium oxide, these stabilizing outer shells prevent the coalescence of droplets, subsequent growth, and the formation of large precipitates of LMP at the bottom of the container. Furthermore, by increasing the sonication time, the average particle size can be reduced. However, increasing the sonication time may selectively remove gallium and promote the phase transition from liquid to solid, potentially forming indium-enriched nanoparticles. These dispersions can be used in the manufacture of electronics using spray or inkjet printing methods [see Reference 1], but a high ratio of ethanol / EGaIn is required during sonication, which makes ink deposition, especially for generating a uniform film, more difficult.

[0067] In one embodiment, a graphene liquid metal network is synthesized. The motivation is to have a high-concentration ink of particles generated by sonication, and this ink can be applied using a rod coating / thin film applicator or other similar techniques. The printing time is significantly reduced compared to a typical low-concentration LMPs suspension. The synthesis method is formulated to avoid the recombination of EGaIn particles into larger droplets while clustering into a network. The formulation is designed to produce an ink filled with spherical EGaIn particles with a liquid core that reduces the thickness of the gallium oxide shell. The gallium oxide shell is a high aspect ratio sheet that binds to the liquid metal particles.

[0068] These sheets can be cut into smaller sheets, but it is generally preferred to maintain the large sheets to form the transparent conductor. This is because these large sheets determine how the liquid metal droplets self-assemble into a permeable network that leaves holes allowing light transmission. That is, by surface potential or galvanic replacement, as the liquid metal particles bind to these GO sheets, the size of the high aspect ratio sheets can be designed so that after deposition of the film from this ink, they do not completely cover the entire surface of the substrate and leave some holes for light transmission.

[0069] Therefore, EGaIn was dispersed in ethanol using it as a medium, and the ultrasonic treatment time was shortened to a few minutes. For example, in one embodiment, the ultrasonic treatment time was only 10 minutes, while conventional studies had an ultrasonic treatment time of 1 hour or more. In this embodiment, 0.4 g of EGaIn in 20 g of ethanol was ultrasonically treated on an ice bath to avoid the formation of indium-enriched particles and the growth of gallium oxide shells. Using ethanol as the medium reduces particle re-aggregation more than water. However, it may cause particle coating with carbon. To recover the particles, a co-solvent system of ethanol and GO aqueous dispersion was formulated to explore the interaction between GO sheets and LMPs. Encapsulation of EGaIn liquid metal nanoparticles (LMNP) using graphene oxide (GO) [References 2, 10] and reduced GO (rGO) [References 11, 12] has been used to change the surface properties of LMNP. When water is used as the solvent, adding GO is sufficient to form a conformal encapsulation layer on the surface of the dispersed liquid metal droplets. However, such encapsulation does not occur in ethanol [see Reference 3]. To overcome this drawback, a co-solvent system of ethanol and GO aqueous dispersion (pH 2.2 - 2.5) was formulated. When mixed with ethanol, the pH of the co-solvent increases. As a result, the acid-treated co-solvent exposes the bare metal of the LMP by dissolving the gallium oxide shell [References 11, 12], enabling the interaction between the GO sheet and the LMP. This process is governed by the positive zeta potential of the LMNP and the negatively charged GO sheet. GO tends to bind to the surface of LMPs to balance the charge. Aggregation of LMPs occurs due to the electrostatic interaction between multiple LMPs and large GO sheets. As a result, small clusters of LMPs are formed on the high aspect ratio graphene oxide sheets. The LMPs act as fixed points for the exposed GO clusters on the surface. Surface LMNP attached to a GO sheet can interact with the surface of another GO sheet without nearby LMNP, resulting in the formation of a larger GO-LMNP network that finally precipitates by gravity.Using the optimized relationship between GO and LMP, the deposition of LMP and graphene is balanced, which means that GO has a specific amount to establish the deposition mechanism. When GO becomes excessive, the LMP adhering to each GO sheet significantly decreases, and there are fewer anchor points for other LMP-depleted clusters to bind to, resulting in process degradation. Furthermore, excessive GO increases the viscosity of the co-solvent and degrades the deposition process. Conversely, if the amount of GO is too small, there are not enough clusters on the medium to promote interactions (the clusters cannot be confined to each other). However, the amount of GO required to collect all the dispersed particles depends on the surface area of the particles. For example, if the same amount of LM is sonicated for 2 hours instead of 10 minutes, the amount of GO required to induce precipitation should be seven times higher.

[0070] Compared with the previously disclosed prior art, the GO / EGaIn ratio decreased to approximately one-fiftieth (0.08% - 0.0015). The synthesis process and solution were also changed. In one case, 20 g of ethanol, 50 mg of graphene oxide (GO), and 0.4 g of EGaIn were sonicated at 90% power for 10 minutes to prepare GO-EGaIn ink. Then, 100 - 150 mg of GO (4 mg per 1 ml) was dropped (3 drops) into the solution and manually mixed. As a result, the particles precipitate at the bottom of the flask. The particles settle due to electrostatic adsorption on their surface. In this case, by removing the excess ethanol from the upper part of the flask, a high-concentration ink can be separated. This has advantages over low-viscosity inks using liquid metal EGaIn droplets that should be applied using spray coating or the like. The combination of acidic pH in the GO dispersion and the use of ethanol play an important role in avoiding the oxidation of Ga and GO coating. In a similar process, other carbon products such as graphene quantum dots, carbon particles, and tubes can also be used instead of GO.

[0071] To fabricate a stretchable transparent conductor, the concentrated GO-EGaIn network is applied onto a transparent elastic polymer such as styrene-isoprene block copolymer or PDMS using a thin film applicator or rod.

[0072] Note that the amount of GO plays an important role in changing the properties of the composite material both before and after laser processing. GO improves the mechanical and chemical properties of the nanoparticles. However, excessive GO reduces the conductivity of the sample.

[0073] After drying, an infrared wavelength MOPA (Master Oscillator Power Amplifier) laser was used to "activate" the ink. A rectangle (20x50 mm) was hatched at 20% output with a line spacing of 0.01 mm. The resistance value of the sample after laser sintering is about 4 Ω. This is more than a 100-fold improvement compared to the previous formulations disclosed in this patent that had an electrical resistance of 500 Ω or more. The "activation" of the laser causes further reduction and thinning of the GO, as well as the removal of gallium oxide that occurs as self-organization of liquid metal droplets around the GO sheets, resulting in conductivity and improved transparency. Figure 4 shows an overview of the process up to synthesis, precipitation, and laser processing. After laser sintering, the ink self-assembles into a permeable network of EGaIn.

[0074] Note that when laser sintering is performed on a film of LM nanoparticles not functionalized with GO, the film does not become transparent.

[0075] In one embodiment, Figure 5 is SEM imaging comparing laser processing of liquid metal nanoparticles with no graphene oxide decoration (top) and with graphene oxide decoration (bottom). As can be seen, without GO, the resulting device covers the surface and blocks light transmission.

[0076] In one embodiment, FIG. 6 shows a schematic diagram of the GO@EGaIn network structure after vapor deposition and after laser processing. As can be seen, the partially reduced GO sheets function as guidelines for attaching the EGaIn droplets. When the laser scans the electrodes, these EGaIn droplets coalesce into conductive lines and become smaller in volume compared to the sample before laser processing. Therefore, empty spots remain that allow light transmission. If there is no GO, the same does not occur.

[0077] In one embodiment, FIG. 6B shows optical images of three electrodes with an increased GO amount (I - III) and increased transparency.

[0078] In one embodiment, FIG. 7 shows a film obtained by laser sintering EGAIN droplets without GO, which is non - transparent (lower part), and a film of GO - EGaIn deposited on a transparent substrate (middle part). Depending on the GO concentration and film thickness, the result is a slightly transparent and non - conductive or very low - conductivity range (kilo - ohm to mega - ohm). The upper image is of the same film after laser processing, which is transparent and highly conductive (<10Ω).

[0079] In one embodiment, FIG. 8 shows the optical transmittance of a laser - sintered sample (about 55 - 70% transmittance) and another electrode laser - patterned into a honeycomb structure (about 90% transmittance). Laser patterning can increase the transmittance by selectively removing material from the film. However, this is not necessary for most applications.

[0080] In an embodiment, FIG. 9 shows an example of a conductive semi - transparent device and the electro - mechanical property evaluation of the sample. This sample maintains high conductivity even under large mechanical strain. As can be seen, the transparent conductor withstands a strain of 1300%, and its electrical resistance remains below 10Ω even at a strain of 150%. FIG. 10 shows 5000 repetitions of a 100% strain cycle. The sample maintains stable behavior even under these harsh conditions.

[0081] In an embodiment, FIG. 11 shows an example of a stretchable electroluminescence device in which both sides of the conductor are GO-EGaIn ink. In this electroluminescence device, the intermediate layer includes a composite of an elastic material and electroluminescence powder. The upper layer and the lower layer are composed of the same GO-EGaIn ink. In this configuration, the electroluminescence device functions without adding a dielectric layer with a generally high dielectric constant, which is common in electroluminescence elements. Therefore, the manufacturing becomes simpler. Furthermore, electroluminescence light can be seen equally from both sides of the electroluminescence element.

[0082] In an embodiment, FIG. 12 shows an electroluminescence element under extreme strain conditions.

[0083] In an embodiment, FIG. 13 shows a multi-pixel electroluminescence element composed of conductive row electrodes and column electrodes fabricated with a GO-EGaIn network structure and laser sintered and laser patterned using the previously disclosed technique.

[0084] In one embodiment, FIGS. 14 and 15 show the light transmittance, electrical resistance, and strain tolerance of a transparent conductor fabricated by this method, compared to the prior art, which shows a clear and significant improvement over the state of the art. According to the present invention, the conductivity and maximum strain tolerance of the transparent conductor can be significantly improved. Compared to a silver nanowire-based transparent conductor, the liquid metal provides better strain tolerance due to its fluidity. However, heretofore, it has been impossible to manufacture a liquid metal-based transparent conductor based on the self-assembly of liquid metal droplets guided by a graphene oxide sheet to which the liquid metal droplets are bonded.

[0085] The graphene oxide sheet functions as a guideline for the liquid metal droplets. Since the liquid metal droplets selectively bond to these sheets, they do not spread over the entire surface, allowing for slight light transmission. By laser sintering, these particles are further bonded into a more compact shape, creating more space for increased light transmittance.

[0086] Liquid metal droplets without GO coating also stain when touched. GO@EGaIn also improved the mechanical resistance to particle rupture and the smearing behavior.

[0087] Next, a graphene liquid metal electrode for energy storage is disclosed.

[0088] The use of EGaIn as an electrode in batteries and energy storage devices is promising due to its mechanical deformability, high electrical conductivity, dendrite-free operation, and self-healing property of LM.

[0089] In one embodiment, by using EGaIn nanoparticles (NPs) coated with GO, high-performance EGaIn-based SCs and batteries can be formed. The thin-film electrodes fabricated by this composite have extremely high chemical stability compared to EGaIn NPs without GO. This makes it possible for the first time to use EGaIn NPs as stable energy storage electrodes even in the presence of strongly acidic or strongly alkaline electrolytes. Furthermore, a simple, rapid, low-cost, and scalable manufacturing technique based on single-step laser processing of GO@EGaIn is disclosed. This enables the fabrication of conductive interdigitated patterns from pre-coated films in a few minutes. We use an infrared laser (IR) with a wavelength of 1064 nm, which is easy to use, to convert GO@EGaIn into a reduced graphene oxide (herein referred to as LrGO@EGaIn) nanocomposite. At high power, this laser can simultaneously pattern the film into the desired shape by full ablation or partial ablation.

[0090] Referring to Fig. 16A, first, 1 g of bulk EGaIn was sonicated in 20 ml of an aqueous solution-based GO solution to synthesize GO-coated EGaIn nanodroplets. It is known from the literature that the stability of EGaIn nanodroplets depends on an ultrathin (<3 nm) Ga2O3 shell (Fig. 16B-i). By fiber laser irradiation, the Ga2O3 shell can be removed, and the conductivity can be significantly improved (Fig. 16B-ii). Unfortunately, neither EGaIn NPs nor the laser-sintered LM scaffold is stable when exposed to a highly alkaline solution. According to the optical images in Figs. 16B-i and ii, a few seconds after dropping 6 M KOH aqueous solution, the LM droplets / scaffolds are exposed from the surface and aggregated into bulk LM, so that the background of the coated glass can be seen. In contrast, the EGaIn composite film encapsulated with GO remains intact after being exposed to the same 6 M KOH aqueous solution (Figs. 16B-iii, iv). By this approach, EGaIn nanodroplets can be utilized as active electrodes for redox reactions. Figs. 16C-i and ii schematically show the role of the GO sheet in protecting EGaIn from chemical corrosion.

[0091] In one embodiment, a simple manual spray coating gun was used to form a GO@EGaIn thin film on various substrates on a hot plate. Then, an IR MOPA laser (1064 nm) was used to perform both patterning of the desired shape by complete ablation of the coating material from the film (e.g., formation of an interdigital structure) and further reduction of the GO layer to reduced graphene oxide (rGO). In the case of an energy storage electrode, the surface area and energy storage capacity can be improved by laser treatment (Fig. 16D), but the device can function even without laser reduction. In a special case of energy storage, laser reduction can be similarly performed with other wavelengths and other types of lasers such as a CO2 laser. The laser reduction further decreased the sheet resistance to about 30 KΩ / □~1 KΩ / □.

[0092] In one embodiment, referring to FIG. 16E, the soft-matter supercapacitor can be fabricated in minutes from a pre-coated film by simultaneously performing laser reduction and patterning (ablation). The film itself contains highly stretchable Ag-EGaIn-SIS (styrene-isoprene block copolymer)

[16] as the first current collector (CC), followed by a carbon black-SIS (CB-SIS) film as the second CC. Both are coated by a thin film applicator, and the binder-free rGO@EGaIn nanocomposite is coated by spray coating. FIG. 16F shows an example of a thin film SC fabricated by this technique, demonstrating its ability to be stretched, bent, twisted, and rolled up.

[0093] In an embodiment, cyclic voltammetry (CV) was used to study the electrochemical behavior of two-electrode symmetric rGO@EGaIn SCs in the presence of 6M KOH hydrogel electrolyte. Referring to FIG. 17A, the CV curves of rGO@EGaIn are described at scan rates of 10 - 200 mV s -1 in a potential window of 0 - 2V. The redox peaks in the CV plot of rGO@EGaIn / / rGO@EGaIn SC are related to the Faradaic redox reaction, indicating the pseudocapacitive behavior of the SC. Furthermore, the overall shape of the CV graph changes little with increasing scan rate. This indicates that increasing the scan rate improves the mass transport and reversibility of these electrodes. FIG. 17B shows the galvanostatic charge-discharge (GCD) plots of the SC at various current densities and in a potential range of 0 - 2V. Consistent with the CV plot, the non-linear shape of the GCD plot indicates the Faradaic behavior of the rGO@EGaIn electrode. Furthermore, the low IR drop (about 0.17V) indicates the low internal resistance of the rGO@EGaIn electrode. The inset in FIG. 17B shows the variation of areal capacitances at different current densities, with 1.2 F / cm 2 at a charge-discharge current of 300 μA / cm 2 and 85 mF / cm 2 at 3 mA / cm 2is in the range. As expected, decreasing the current density increases the areal-specific capacitance. It should be noted that all of these results are based on the SC with an electrode thickness of 3 μm. Figure 17C shows the areal-specific capacitance of various electrode thicknesses from approximately 0.5 μm to ~15 μm at a discharge current density of 1 mA / cm 2 As the electrode thickness increases from approximately 0.5 μm to ~3 μm, it is observed that the areal-specific capacitance increases quasi-linearly (when the thickness increases by about 6 times, the areal-specific capacitance increases by about 6 times). However, beyond this range, the areal capacitance of the approximately 15-μm-thick electrode is 1.6 times that of the 3-μm-thick electrode.

[0094] Figure 17D shows the ratio of the areal capacitance to the initial capacitance as a function of the number of cycles. The capacitor shows cycle stability and retains approximately 98.4% of the initial capacitance even after 1000 charge-discharge cycles at 3 mA / cm 2 The inset shows a 4-cycle zoom window at the start of the measurement (3 hours later) and at the end of the measurement (32 hours later).

[0095] Figure 17E shows the behavior of the SC when a 30% mechanical strain is applied. Figure 17F schematically shows the electrochemical reactions during charge and discharge at each electrode. Since both electrodes are made of the same composite material, there is initially no potential difference between the two electrodes. Therefore, the redox reaction is non-spontaneous and is activated by the supplied electrical energy to initiate the reaction. Therefore, immediately after the manufacture of the SC, a one-time activation step is performed to form metal ions necessary to generate a potential difference between the electrodes. During the charging process, KOH is converted into K + ions and OH - ions and moves towards the electrode with the opposite charge (pH about 14). In the oxidation-reduction reaction, at the anode, Ga is converted into GaO3 3- ions during charging (oxidation), and simultaneously at the cathode, GaO3 3- ions are converted into Ga 2+It is converted into ions (reduced). The reverse process occurs during discharge. According to the Pourbaix Diagram, the reaction of the following formula occurs.

Number

Number

[0096] In an embodiment, FIG. 18 shows an electrochemical cycle process.

[0097] FIG. 18 shows, A) a diagram of a supercapacitor obtained during charging and discharging (the marked points represent sampling positions). B) is an SEM image at different stages as shown in the above figure. The inserted image) is a BSE image at the same position of the SEM.

[0098] In one embodiment, FIG. 18 shows that the results seem to show that indium is moving from the EGaIn droplets during the charging process. The reason for the plateau at 0.5 volts is related to the removal of the protective layer of gallium oxide (Ga2O3) in some of the particles, which allows contact between the electrolyte and the liquid metal free of oxides, resulting in the formation of gallium ions (GaO3 3- and Ga 2+ ) and indium oxide (In2O3) particles. The disappearance of the Ga2O3 layer at 0.5 V is consistent with previous studies [Reference 14] that investigated the switchable surface activity of liquid metals.

[0099] If charging continues further, more gallium ions are released and indium oxide is formed as a secondary product. Comparing the scanning electron microscope images of the anode and cathode, it can be seen that the indium particles formed at the anode are cubic, while those at the cathode are spherical. This is because the amount of oxygen on the two electrodes is different. As the number of cycles increases, the particle size of the alloy bulk particles decreases, indicating that more gallium ions are released to participate in the redox process. Also, although a part of Ga2O3 is removed during the electrochemical cycle, it is interesting that GO can protect the EGaIn NPs from the composite over 1000 cycles of electrochemical cycles.

[0100] In an embodiment, FIG. 19 shows an integrated patch composed of a dipole antenna for remote energy harvesting, which is coupled to an RF to DC (P1110B RF) board for converting energy into a constant DC voltage and a GO@EGaIn supercapacitor for energy. This circuit is on a soft and stretchable thin-film SIS elastomer substrate. It should be noted that all circuit components, namely the antenna, electrical interconnections, and supercapacitor, were rapidly patterned from the same coating film. First, the supercapacitor was charged using the transmitting antenna, and then the LED was lit using the energy stored in the SC.

[0101] Also disclosed are a memristor and a pressure sensor based on a liquid metal composite coated with GO. The liquid metal composite coated with GO also exhibits the behavior of a memristor. Here, such behavior is shown for the first time through the fabrication of a device consisting of a partially laser-sintered composite of GO-EGaIn, as depicted in FIG. 19. Here, a CO2 laser was used to partially sinter the composite material. This step enables the creation of a composite with a memory element and a sensing element. On the other hand, the top electrode shown in FIG. 19 was fully sintered using an IR laser to create a highly conductive electrode that is connected to the memristor composite material.

[0102] In an embodiment, FIG. 20 shows the operation of the memristor of the device.

[0103] In addition to the memristor device, this composite is a pressure-sensitive film, and its conductivity improves when pressure is applied.

[0104] Therefore, by combining this composite material with two types of lasers and changing the laser output, a complex sensor or memory device can be fabricated.

[0105] In an embodiment, FIG. 22 shows a transparent pressure sensor fabricated by laser processing and patterning of a GO-EGaIn network structure. This film is applied onto the screen of a mobile phone to create a transparent pressure sensor that can take into account the pressure applied by the user as an input for a game or other program.

[0106] As used herein at all times, the term "comprising" is intended to indicate the presence of the recited features, integers, steps, components, but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0107] The present disclosure should not be limited in any sense to the described embodiments, and those skilled in the art will foresee many possibilities for its modification. The above-described embodiments can be combined.

[0108] The following claims further define specific embodiments of the present disclosure.

[0109] [References] 1. Liu, S. et al., Laser Sintering of Liquid Metal Nanoparticles for Scalable Manufacturing of Soft and Flexible Electronics, ACS Appl. Mater. Interfaces 10, 28232 - 28241 (2018). 2. Liu, S., Reed, S. N., Higgins, M. J., Titus, M. S., and Kramer - Bottiglio, R., Oxide rupture - induced conductivity in liquid metal nanoparticles by laser and thermal sintering, Nanoscale (2019) doi:10.1039 / c9nr03903a. 3. Creighton, M. A., Yuen, M. C., Morris, N. J., and Tabor, C. E., Graphene - based encapsulation of liquid metal particles., Nanoscale 12, 23995 - 24005 (2020). 4. Wang, Y., Wang, S., Chang, H., and Rao, W., Galvanic Replacement of Liquid Metal / Reduced Graphene Oxide Frameworks., Adv. Mater. Interfaces 7, 2000626 (2020). 5. Saborio, M. G., et al., Liquid Metal Droplet and Graphene Co-Fillers for Electrically Conductive Flexible Composites., Small (2020) doi:10.1002 / smll.201903753. 6. Duan, M., et al., EGaIn Fiber Enabled Highly Flexible Supercapacitors., ACS Omega 6, 24444-24449 (2021). 7. So, J.-H. and Koo H.-J., Study on the Electrochemical Characteristics of a EGaIn Liquid Metal Electrode for Supercapacitor Applications., Trans. Korean Hydrog. new energy Soc. 27, 176-181 (2016). 8. Kim, M. G., et al., All-Soft Supercapacitors Based on Liquid Metal Electrodes with Integrated Functionalized Carbon Nanotubes., ACS Nano 14, (2020). 9. Hynes Navasingh, R. J., et al. Graphene-based nano metal matrix composites: A review. in Nanocarbon and its Composites: Preparation, Properties and Applications. (2018). doi:10.1016 / B978-0-08-102509-3.00006-7. 10. Alotaibi, F., Tung, T. T., Nine, M. J., Coghlan, C. J., and Losic, D. Silver Nanowires with Pristine Graphene Oxidation Barriers for Stable and High Performance Transparent Conductive Films. ACS Appl. Nano Mater. (2018) doi:10.1021 / acsanm.8b00255. 11. Abbasi, R., et al. Photolithography-enabled direct patterning of liquid metals. J. Mater. Chem. C 8, 7805-7811 (2020). 12. Kim, M. gu, Brown, D. K., and Brand, O. Nanofabrication for all-soft and high-density electronic devices based on liquid metal. Nat. Commun. 11, 1-11 (2020). 13. Tavakoli, M., Paisana, H., De Almeida, A. T., and Majidi, C. Liquid metal fusion with conductive inks and pastes. (2020). 14. Khan, M. R., Eaker, C. B., Bowden, E. F., and Dickey, M. D. Giant and switchable surface activity of liquid metal via surface oxidation. Proc. Natl. Acad. Sci. U. S. A. 111, 14047-14051 (2014).

Claims

1. A suspension or colloid comprising liquid metal droplets bound to graphene-based particles, wherein the liquid metal is gallium or a gallium alloy, and the graphene-based particles are selected from graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or a combination thereof.

2. A network structure of liquid metal droplets bonded to graphene-based particles, wherein the liquid metal is gallium or a gallium alloy, and the graphene-based particles are selected from graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or a combination thereof.

3. A suspension, colloid, or reticular structure according to claim 1 or 2, wherein the liquid metal droplets are coated with graphene-based particles.

4. A suspension, colloid, or reticular structure according to claim 1 or 2, wherein the weight ratio of the graphene-based particles to the liquid metal droplets is 0.0001 to 0.5%, preferably 0.001 to 0.1%.

5. An ink comprising the concentrated network structure described in claim 2, obtained by separating the network structure from the colloid or suspension described in claim 1.

6. The ink according to claim 5, wherein the following steps are performed: A method obtainable by the steps of: suspending graphene-based particles in a first medium to obtain a first suspension; mixing a liquid metal with the first suspension to obtain a mixture of the liquid metal droplets and the reticular structure of the graphene-based particles; and separating the concentrated liquid metal droplets and the reticular structure of the graphene-based particles from the mixture, or An ink obtainable by the steps of: suspending liquid metal droplets in a second medium to obtain a second suspension; mixing graphene-based particles with the second suspension to obtain a mixture of the liquid metal droplets and graphene-based particles in a network structure; and separating the concentrated network structure of the liquid metal droplets and graphene-based particles from the mixture.

7. An ink according to claim 5, wherein the ink can be obtained by the steps of: suspending the graphene-based particles in a first medium to obtain a first suspension; suspending the liquid metal droplets in a second medium to obtain a second suspension; mixing the suspensions; and separating the concentrated network structure of the liquid metal droplets and the graphene-based particles from the mixture, wherein the first medium and the second medium are miscible.

8. An ink according to any one of claims 5 to 7, wherein ink separation is performed by sedimentation, centrifugation, and / or filtration.

9. The ink according to claim 6 or 7, wherein the first medium is water or an aqueous solvent, and in particular both the first medium and the second medium are water or an aqueous solvent.

10. The ink according to claim 6 or 7, wherein the second medium is ethanol or an alcohol-based solvent.

11. A printable ink according to any one of claims 5 to 7, further comprising a binder for improving ink adhesion and / or viscosity, particularly for improving ink adhesion and / or viscosity for nozzle extrusion or screen printing.

12. A conductor obtainable by the steps of applying a coating of a suspension, colloid, network structure, or ink according to any one of claims 1, 2, 5 to 7 onto a substrate, and laser sintering the coating, the conductor being particularly an electrode, circuit trace, or circuit.

13. A conductor according to claim 12, wherein the conductor is transparent or translucent.

14. The conductor according to claim 12, wherein the conductor is flexible or stretchable.

15. A conductor according to claim 12, wherein the coating is performed by spraying, rod coating, slot die, inkjet printing, aerosol jet printing, or blade coating.

16. A conductor according to claim 12, wherein the conductor comprises a conductive pattern obtained by laser patterning or lithography.

17. A process for obtaining a suspension or colloid, comprising the step of binding liquid metal droplets with graphene-based particles, wherein the liquid metal is gallium or a gallium alloy, and the graphene-based particles are selected from graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or a combination thereof.

18. A process for obtaining a network structure of liquid metal droplets bonded to graphene-based particles, comprising the step of bonding liquid metal droplets to graphene-based particles, wherein the liquid metal is gallium or a gallium alloy, and the graphene-based particles are selected from graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, carbon nanotubes, or a combination thereof.

19. A process for obtaining a suspension, colloid, or network structure according to claim 17 or 18, the process comprising coating the liquid metal droplets with graphene-based particles.

20. A process for obtaining an ink comprising the concentrated network structure described in claim 18, the process comprising separating the network structure from the colloid or the suspension.

21. A process for obtaining the ink according to any one of claims 5 to 7, The ink is obtained by the steps of: suspending the graphene-based particles in the first medium to obtain the first suspension; mixing the liquid metal with the first suspension to obtain a mixture of the liquid metal droplets and the graphene-based particles; and separating the concentrated network structure of the liquid metal droplets and graphene-based particles from the mixture. The ink is obtained by the steps of: suspending the liquid metal droplets in the second medium to obtain the second suspension; mixing the graphene-based particles with the second suspension to obtain a mixture of the liquid metal droplets and the graphene-based particles in a network structure; and separating the concentrated network structure of the liquid metal droplets and the graphene-based particles from the mixture. The ink is obtained by the steps of: suspending the graphene-based particles in the first medium to obtain the first suspension; suspending the liquid metal droplets in the second medium to obtain the second suspension; mixing the suspensions; and separating the concentrated network structure of the liquid metal droplets and graphene-based particles from the mixture. A process in which the first medium and the second medium are miscible.

22. A process for obtaining the ink according to claim 20, comprising the steps of sedimentation, centrifugation, and / or filtration.

23. A process for obtaining the ink according to claim 21, wherein the first medium is water or an aqueous solvent, and in particular both the first medium and the second medium are water or an aqueous solvent, and in particular the pH of the aqueous solution containing the graphene particles is 1 to 6, preferably 2 to 3.

5.

24. A process for obtaining the ink according to claim 21, wherein the second medium is ethanol or an alcohol-based solvent, and in particular the liquid metal is 0.5 to 10% (w / w) of the ethanol or alcohol-based solvent.

25. A process for obtaining a transparent or translucent conductor, comprising the steps of applying a coating of a suspension, colloid, network structure, or ink according to any one of claims 1, 2, 5 to 7 onto a substrate, and laser sintering the coating, wherein the conductor is an electrode, a circuit trace, or a circuit.

26. The process according to claim 25, wherein the coating is performed by spraying, rod coating, slot die coating, inkjet printing, aerosol jet printing, or blade coating.

27. The process according to claim 25, wherein the laser is a fiber laser having a wavelength in the range of UV to IR.

28. A process according to claim 25, comprising the step of obtaining a conductive pattern by laser patterning or lithography.

29. The process according to claim 25, wherein the gallium alloy is a gallium-indium alloy, a gallium-indium-tin alloy, or a eutectic gallium-indium alloy.

30. A process according to claim 17 or 18, wherein the weight ratio of the graphene-based particles to the liquid metal droplets is 0.0001 to 0.5%, preferably 0.001 to 0.1%.

31. A process for recovering liquid metal particles from a suspension by adding a liquid suspension containing particles with a zeta potential opposite to that of the liquid metal, in order to promote bonding between the liquid metal and the added particles.

32. An apparatus comprising a suspension, colloid or reticular structure according to claim 1 or 2, an ink according to any one of claims 5 to 7, or a conductor according to claim 12.

33. The apparatus according to claim 32, wherein the apparatus is a photoelectronic device, a pressure or strain-sensitive piezoresistive composite, a pressure or strain sensor, a temperature sensor, an electroluminescent device, a photovoltaic device, a memory device, or an electrode for an energy storage device.

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