Method for obtaining vitreous graphene on substrates, product that can be obtained using the method, use thereof and sensor comprising the product

Abstract

The present invention relates to a method for obtaining vitreous graphene on substrates. The method is defined by the simultaneous in-situ growth and transfer of vitreous graphene on substrates using a laser. The method promotes the graphitisation of a carbon source located between two substrates, which results in vitreous graphene synthesis. Vitreous graphene patterns are successfully generated on an internal surface of one of the substrates. The resulting product is used to manufacture gas sensors.

Description

[0001] PROCESS FOR OBTAINING VITREOUS GRAPHENE ON SUBSTRATES, PRODUCT OBTAINABLE BY THE PROCESS, USE OF THE SAME AND SENSOR THAT COMPRISES THE PRODUCT

[0002] DESCRIPTION

[0003] FIELD OF INVENTION

[0004] The present invention relates to a process for producing glassy graphene on substrates. The process, using a laser, achieves in situ growth of graphene and the transfer of glassy graphene between substrates. The process promotes the graphitization of an initial carbon source located between two substrates, resulting in the synthesis of glassy graphene on one of them. The resulting product is used in the manufacture of gas sensors.

[0005] STATE OF THE ART

[0006] Current methods for laser-induced graphene growth involve direct laser exposure to a polymer substrate. These methods often present several limitations, such as the presence of gaseous environments and the difficulty in obtaining complex growth configurations, resulting in a decrease in graphene quality, which in turn limits potential applications.

[0007] International patent WO2019 / 038558 A1 relates to a method for forming a 3D graphene material adhered to the surface of a substrate, comprising: providing a carbon source on the surface of the substrate and exposing at least a part of the carbon source and / or at least a part of the substrate, to a laser, thereby converting at least a part of the carbon source into a 3D graphene material adhered to the surface of the substrate.

[0008] Patent document CN110426429 describes a two-dimensional glassy graphene-based chemical sensor array comprising a substrate layer, a sensor material layer, and an electrode layer, arranged successively from bottom to top. The sensor material layer is made of a glassy graphene material; the glassy graphene is prepared using a polymer-assisted deposition method.

[0009] International patent application WO2023 / 118866 A1 relates to a method of manufacturing a carbon nanostructure, in the form of a carbon foam material. The method comprises the steps of: (a) using a first laser beam to irradiate an encapsulated or subsurface region of a carbon precursor material below the surface of the material, to create carbon foam in that subsurface region and an amorphous material above the carbon foam, and then (b) using a second laser beam to remove or ablate the amorphous material above the carbon foam.

[0010] It is of great interest to develop a process for obtaining laser-induced graphene and transferring glassy graphene. This process provides better control over the reaction conditions and is more precise, in order to obtain a product useful for gas sensors.

[0011] DESCRIPTION OF THE INVENTION

[0012] The present description relates to a process for obtaining graphene and glassy graphene on substrates. The process begins with the coating of one side of a substrate with a layer of a carbon source. This layer is placed in a sandwich-type arrangement between two substrates. The sandwich structure is exposed to laser radiation. The transparency of the substrate, which is greater than the wavelength of the laser, allows the carbon source layer to reach it, initiating its transformation into graphene. The gases resulting from this interaction cool rapidly upon coming into contact with the lower substrate. This surface, the lower substrate, which is not directly exposed to the laser, cools rapidly, leading to the formation of a glassy carbon structure. This process allows the simultaneous synthesis and transfer of graphene and glassy graphene and does not require any type of cleaning, such as exfoliation or chemical cleaning.

[0013] The sandwich structure ensures that the lower substrate is not directly exposed to laser radiation, as is the upper substrate. This differential exposure is a critical aspect of the method. The heat generated by the interaction of the laser with the upper substrate is rapidly transferred through the lower substrate.

[0014] The process includes a crucial cooling step to prevent the complete crystallization of the carbon structures on the lower substrate, contributing to the formation of a material, glassy graphene, which has properties somewhere between graphene and glassy carbon.

[0015] Therefore, the present invention relates to a process for obtaining graphene on a first substrate and glassy graphene on a second substrate comprising the steps of: a) applying a layer of a carbon source on the first substrate that is transparent to laser light; b) placing the face of the first substrate coated by the carbon source in step a) in contact with the uncoated second substrate to obtain a sandwich-type structure, where the first substrate is positioned on the second substrate and the face of the first substrate coated with the carbon source remains inside; c) irradiating laser light on the first substrate and through the sandwich structure obtained in step b), to obtain graphene on the inner face of the first substrate and glassy graphene on the inner face of the second substrate; d) cooling the sandwich-type structure after step c) and / or at the same time as step c).

[0016] With the process of the invention, graphene is formed on the first substrate and graphene is transferred to the second substrate, obtaining glassy graphene on this second substrate.

[0017] The term "glassy graphene" refers to a carbon compound that exists in an intermediate state between glassy carbon and graphene. Glassy graphene has high crystallinity; its characteristics are a blend of characteristics of glassy carbon and graphene. Glassy carbon is characterized by carbon atoms forming isotropic connections, resulting in a glassy state without long-range ordering, crystalline domains, or grain boundaries. Graphene has high crystalline quality and a glassy carbon morphology.

[0018] The process promotes the graphitization of the carbon source in the sandwich structure, formed by the first and second substrates, resulting in the synthesis of graphene on the first substrate and glassy graphene on the second substrate.

[0019] Therefore, another aspect of the invention is a sandwich structure of two substrates obtainable according to the process described above where the first substrate has graphene on the inner face and the second substrate has glassy graphene on the inner face.

[0020] Another aspect of the invention is a substrate that has glassy graphene on one side, obtainable by the process of the invention.

[0021] The process is applied to fabricate gas sensors, which showed improved sensing performance compared to known substrates with a direct laser-induced graphene (LIG) layer.

[0022] Thus, other aspects of the invention are the use of a substrate having glassy graphene on one side obtainable by the process of the first aspect in a gas sensor, and the gas sensor including the second substrate having glassy graphene on one side obtainable by the process of the first aspect of the invention. The relationship between the structure of the glassy graphene and its gas detection properties improves the performance of the gas detection sensor.

[0023] FIGURES

[0024] Figure 1 shows a schematic of the process of the invention.

[0025] Figure 2 shows a field emission scanning electron microscope (SEM) image of the surface morphology of graphene (letters “a” to “e”) obtained on the first top silicon substrate in the sandwich structure. Figure 2(f) shows a cross-sectional image of the top silicon substrate. Figure 2(g) shows the glassy graphene obtained on the second bottom silicon substrate. Figure 2(h) shows a cross-sectional image of the glassy graphene obtained on the second bottom silicon substrate.

[0026] Figures 3A and 3B show the responses of the graphene obtained on the first upper substrate and of the glassy graphene (GV) obtained on the second substrate to NO2 ppb (3A) and NO2 ppm (3B) concentrations. In both graphs 3A and 3B, the blue line shows the response of the glassy graphene obtained on the second lower substrate, measured at 45°C; the red line shows the response of the glassy graphene obtained on the second lower substrate, measured at room temperature; the green line shows the response of the graphene obtained on the first upper substrate, measured at 45°C; the purple line shows the response of the graphene obtained on the first upper substrate, measured at room temperature, and the black line shows a reference graphene obtained by laser at room temperature.

[0027] Figures 4A-4D show the detection of NO2 (range 5 to 100 ppm, horizontal axis) at 45 °C and ambient temperature for:

[0028] 4A) Glassy graphene obtained on the second lower substrate and measurement performed at 45°C 4B) Glassy graphene obtained on the second lower substrate and measurement performed at room temperature 4C) Graphene obtained on the first upper substrate and measurement performed at 45°C on the upper slide and 4D) Graphene obtained on the first upper substrate and measurement performed at room temperature. In all graphs 4A to 4D the resistance is shown as a blue line, and on the left vertical axis and the gas concentration with a red line and on the right vertical axis.

[0029] Figures 5A-5D show the detection of NO2 (range 50 to 1000 ppb, horizontal axis) at 45 °C and ambient temperature for:

[0030] 5A) Glassy graphene obtained on the second lower substrate and measurement performed at 45°C 5B) Glassy graphene obtained on the second lower substrate and measurement performed at room temperature 5C) Graphene obtained on the first upper substrate and measurement performed at 45°C on the upper slide and 5D) Graphene obtained on the first upper substrate and measurement performed at room temperature. In all graphs 5A to 5D the resistance is shown as a blue line, and on the left vertical axis and the gas concentration with a red line and on the right vertical axis.

[0031] DETAILED DESCRIPTION OF THE INVENTION

[0032] Figure 1 shows a diagram of the first aspect of the invention which relates to a process for obtaining graphene (1) on a first substrate (2) and glassy graphene (3) on a second substrate (4) comprising the steps of: a) applying a layer of a carbon source (5) on the first substrate (2) which is transparent to laser light (6); b) placing the face of the first substrate (2) coated by the carbon source (5) in step a) in contact with the uncoated second substrate (4) to obtain a sandwich structure, where the first substrate (2) is positioned on the second substrate (4) and the face of the first substrate (2) coated with the carbon source (5) remains inside; c) irradiating laser light (6) on the first substrate (2) and through the sandwich structure obtained in step b), to obtain graphene (1) on the inner face of the first substrate and glassy graphene (3) on the inner face of the second substrate;d) cooling the sandwich structure after step c) and / or at the same time as step c).;

[0033] In a preferred embodiment, the carbon source (5) is a polymer, preferably polyimide, more preferably polyimide dissolved in N-methyl-2-pyrrolidone (NMP). In a preferred embodiment, the polymer is decorated with metal nanoparticles or metal oxide nanoparticles.

[0034] Particularly, the first substrate (2) is silicon. In a preferred embodiment, the second uncoated substrate (4) is selected from: silicon, alumina, polyimide, micromachined substrate and glass, particularly it is silicon.

[0035] In particular, step a) is carried out by spin coating and curing. Preferably, spin coating is carried out at a speed in the range of 200 rpm to 400 rpm for a time in the range of 10 seconds to 60 seconds. Preferably, curing is carried out at a temperature in the range of 100 °C to 200 °C. More preferably, curing is carried out in a time between 1 minute and 20 minutes. Preferably, the thickness of the layer in step a) is less than 1000 nm, more preferably between 300 nm and 1000 nm.

[0036] In particular, the laser light (6) is a CO2 laser light.

[0037] Preferably, the cooling step is carried out in a cooling system (7) comprising a copper layer (7.1) on which the sandwich structure is deposited, the copper layer (7.1) being on an aluminium heat sink (7.2), the aluminium heat sink (7.2) conducting heat through a system of heat pipes (7.3) to a finned heat exchanger (7.4) where air flows. The cooling system (7) is used to enhance rapid heat dissipation from the second lower substrate (4).

[0038] As stated, another aspect of the invention is the gas sensor that includes the second substrate (4) that has on one side vitreous graphene obtainable by the process of the first aspect of the invention. Preferably, the gas sensor comprises a substrate with electrodes, gold or platinum electrodes being more preferred. More preferably, the gas sensor comprises a heater. Preferably, the second substrate (4) is micromachined. of the invention

[0039] A polyimide resin solution was prepared following the manufacturer's instructions (5 g of P84 dissolved in 15 ml of NMP), and the solution was degassed in a vacuum chamber for 1 h. In the next step, 1 ml of the resin solution was poured onto the center of a silicon substrate. It was rotated at a speed of 300 rpm for 30 seconds; until a silicon substrate with an evenly distributed layer of polyimide with a constant thickness was obtained. The coated silicon substrate was placed on a hot plate, and the resin was cured at 160 °C for 10 minutes. The obtained polyimide resin layer was uniform and had an average thickness of 700 nm. The cured resin adhered firmly to the substrate, and no cracks or defects were observed on the surface.

[0040] In the experiment, a CO2 laser (Synyard 48-01 with an 80 mm Flyer scanning head and a beam diameter of 116 pm) was used. The optimal laser flux was calculated by simulating the laser flux and taking into account the absorbance of the silicon substrate (25 W, speed 80 mm / s and a resolution of 400 dots per inch (DPI) for a 1 mm * 1 mm rectangle (1 mm 2 )). The coated silicon substrate was then placed on a second, uncoated silicon substrate, forming a sandwich structure. Laser light was directed through the top (coated) silicon wafer, reaching the polymer layer and initiating laser pyrolysis of the polymer. The polymer layer on the surface of the top silicon substrate replicated the pattern on the bottom, uncoated silicon substrate, partially transferring the pattern to the lower substrate as well.

[0041] Material characterization

[0042] Field emission scanning electron microscopy (FEM) analysis provided insight into the morphology and structure of glassy graphene, particularly its thickness and porous characteristics. Cross-sectional images of the polymer layer indicated that the 0.7 µm P84 layer expanded by a factor of 10 on the first top substrate, and a glassy graphene pattern with a thickness of 300 to 700 nm (wavy structure) was transferred to the second bottom substrate. Field emission SEM images also revealed a porous structure in the glassy graphene, with interconnected voids and pores distributed across its surface; these 3D porous structures contribute to the increased surface area of ​​the glassy graphene. Glassy graphene possesses a microstructure that lies somewhere between glassy carbon and graphene, exhibiting a significant degree of crystallinity with a twisted lattice.Glassy carbon is characterized by carbon atoms forming isotropic connections, resulting in a glassy state with no long-distance order, crystalline domains, and grain boundaries. Glassy graphene has both high crystalline quality and the morphology of glassy carbon. In some areas of the samples, the glassy graphene appears as a wavy, mirror-like surface (Figure 2 gh), compact, connected, and crack-free.

[0043] In addition, the obtained graphene were analyzed by X-ray photoelectron spectroscopy (XPS). The results obtained from the analysis give the following results, the graphene on the inner face of the first substrate, presented a high carbon purity of 97.83%, with oxygen and nitrogen levels of 1.55% and 0.63%, respectively; while the glassy graphene, on the inner face of the second substrate, obtained presented a carbon purity of 87.53%, with higher concentrations of oxygen (8.90%) and nitrogen (3.57%), which contributes to its greater chemical reactivity and gas detection capabilities. Characterization of gas detection

[0044] The NO2 gas detection performance of glassy graphene was evaluated at concentrations of 50 ppb to 100 ppm in a Teflon sealed chamber with a volume of 35 cm 3at room temperature and 45 °C. The detection chamber was connected to a gas mixing and delivery system using calibrated gas cylinders and pure, dry air (Air Premier purity: 99.999%) as carriers. The resistance of the different sensors was then monitored using a multimeter (HP 34972A, Agilent), and changes in resistance were recorded as different gas concentrations were applied. To reduce system power consumption and operate under more realistic experimental conditions, the total flow rate was set to a low rate (100 mL / min) using a set of mass flow controllers (Bronkhorst High-Tech BV) and solenoid valves. The sensors were stabilized in dry air for 30 minutes before being exposed to a given gas concentration for 10 minutes.Responses at various concentrations were recorded by applying successive dilutions of the gas and defining the sensor response as AR / R0 expressed as a percentage, where AR corresponds to the resistance changes recorded during 10 minutes of exposure to the gas, while R0 is given by the sensor resistance in air (or baseline).

[0045] The gas sensing response of the fabricated gas sensors based on glassy graphene was recorded by varying the NO2 gas concentration. The sensors showed a significant response, characterized by a maximum decrease of 38.5% in resistance for a concentration of 100 ppm at 45°C for the glassy graphene formed on the second substrate and 6% at 45°C for the graphene on the first substrate. Glassy graphene outperformed graphene in sensitivity, selectivity, and response time. This superior performance can be attributed to the unique properties of glassy graphene, including its high crystallinity and twisted lattice planes. The stability of the gas sensing response was confirmed over 5 measurement cycles, showing no drift or degradation. The repeatability of the measurements was also demonstrated with consistent results obtained across multiple tests.

Claims

CLAIMS 1. A process for obtaining graphene (1) on a first substrate (2) and glassy graphene (3) on a second substrate (4), comprising the steps of: a) applying a layer of a carbon source (5) on the first substrate (2) that is transparent to the laser light (6); b) placing the face of the first substrate (2) coated by the carbon source (5) in step a) in contact with the uncoated second substrate (4) to obtain a sandwich-type structure, where the first substrate (2) is positioned on the second substrate (4) and the face of the first substrate (2) coated with the carbon source (5) remains inside; c) irradiating laser light (6) on the first substrate (2) and through the sandwich structure obtained in step b), to obtain graphene (1) on the inner face of the first substrate and glassy graphene (3) on the inner face of the second substrate; d) cooling the sandwich-type structure after step c) and / or at the same time as step c).

2. The process according to claim 1, characterized in that the material of the first substrate (2) is silicon and the material of the second substrate (4) is selected from: silicon, alumina, polyimide and glass.

3. The process according to any one of claims 1 to 2, characterized in that the carbon source (5) is decorated with metal nanoparticles or metal oxide nanoparticles.

4. The process according to any of claims 1 to 3, characterized in that the carbon source (5) is polyimide.

5. The process according to any of claims 1 to 4, characterized in that step a) is carried out by spin coating and curing.

6. The process according to claim 5, characterized in that the spin coating is performed at a speed in a range between 200 rpm and 400 rpm for a time in a range between 10 seconds and 60 seconds.

7. The process according to any of claims 5 to 6, characterized in that the curing is carried out at a temperature in a range between 100 °C and 200 °C.

8. The process according to any one of claims 1 to 7, characterized in that the laser light (6) is a CO2 laser light.

9. The process according to any one of claims 1 to 8, characterized in that the thickness of the layer in step a) is between 300 nm and 1000 nm.

10. A sandwich-type structure of two substrates obtainable by the process defined in any of claims 1 to 9, characterized in that the first upper substrate (2) has graphene on the inner face and the second lower substrate (4) has glassy graphene on the inner face.

11. A substrate characterized in that it has glassy graphene on one side, obtainable by the process of claim 1 to 9.

12. Use of the substrate defined in claim 11 in a gas sensor.

13. Gas sensor characterized in that it comprises the substrate defined in claim 11.

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