CN114853361A

CN114853361A - Application of water bath ultrasound in layer-by-layer self-assembly of graphene metamaterial thin film

Info

Publication number: CN114853361A
Application number: CN202210391268.7A
Authority: CN
Inventors: 姚涛; 陈甦; 王浩程
Original assignee: Innofocus Photonics Technology Pty Ltd
Current assignee: Innofocus Photonics Technology Pty Ltd
Priority date: 2022-04-14
Filing date: 2022-04-14
Publication date: 2022-08-05

Abstract

The invention provides application of water bath ultrasound in layer-by-layer self-assembly of a graphene metamaterial thin film. The invention utilizes ultrasonic water bath to prevent GO nano-sheets from accumulating in the solution and on the surface of the substrate, and in addition, after every 3-10 coating cycles, the GO solution is subjected to ultrasonic treatment by a high-power ultrasonic rod. This allows good control of the thickness of the layers and the flatness of the surface. This allows good control of the thickness of the layers and the flatness of the surface. Therefore, the quality of the coated graphene metamaterial film can be greatly improved, and the graphene metamaterial film is suitable for wide application, especially optical application. The prepared graphene metamaterial thin film can be further partially or completely reduced so as to convert a GO layer into a graphene (reduced graphene oxide) layer for different applications.

Description

Application of water bath ultrasound in layer-by-layer self-assembly of graphene metamaterial thin film

Technical Field

The invention belongs to the field of graphene film preparation, and particularly relates to application of water bath ultrasound in layer-by-layer self-assembly of a graphene metamaterial film.

Background

Graphene is a carbon material in the form of a two-dimensional monolayer consisting of a single layer of sp2 bonded carbon atoms arranged in a hexagonal honeycomb structure. Due to the unique electronic, chemical and of grapheneMechanical properties, graphene-based films have been widely used. Graphene is expected to provide various functions in various fields. In particular, their unique electron transfer behavior and excellent optical properties are very attractive for optoelectronic and energy conversion applications. However, the ultra-thin nature of single-layer graphene over a broad band of wavelengths

And low light absorption (2.3%) limits its ability to provide adequate light modulation, thereby limiting the performance of optical applications. Metamaterials comprising alternating graphene and dielectric layers are artificially structured materials aimed at obtaining extremely high optical responses. The graphene metamaterial with the layered artificial structure can enhance light modulation; therefore, theoretical studies indicate that these materials can be used for energy harvesting, light emitting devices, all optical communication devices, and spintronic devices. However, due to the imprecise control and complex transfer processes of conventional mechanical lift-off and deposition methods, the fabrication of graphene-based metamaterials remains a significant challenge, and experimental demonstration is limited to a few examples. Graphene multilayer metamaterials have recently been synthesized separately by Chemical Vapor Deposition (CVD), Atomic Layer Deposition (ALD) and assisted by graphene layer transfer processes. The quality of the metamaterial is sensitive to the deposition conditions and the transfer process becomes difficult to control as the number of layers increases. Inaccurate transfer and deposition processes, as well as the difficulty of aligning the different layers, hinder the fabrication of graphene-based metamaterials. Thus, the number of periods of the graphene multilayer metamaterial remains below six, which limits the optical modulation and its further functionalization. Furthermore, complex deposition and layer transfer processes are not feasible for real-life device applications.

Heretofore, a solution-based thin film synthesis technique has been developed to produce graphene metamaterials composed of alternating single-layer Graphene Oxide (GO)/graphene and dielectric layers (e.g., PDDA) without the need for a transfer step. The single-step process produces metamaterials on different substrates having arbitrary surfaces, shapes and sizes. The mass of the metamaterial is independent of the number of layers and surface area up to 100 layers. The surface roughness of the entire metamaterial thickness can be kept at 2nm, which is comparable to the surface roughness obtained by the most advanced vacuum deposition techniques. In addition, laser-mediated photoreduction can convert GO into graphene, and effectively reduce the band gap of the graphene-based metamaterial by removing oxygen functional groups. Thus, the optical response can be tunable, which is attractive for producing functional photonic devices. The effective complex refractive index and the photoconductivity of the multilayer graphene-based metamaterial were measured by a spectroscopic ellipsometer. The calculated photoconductivity of the photoreduction graphene is almost the same as that of the CVD graphene. In addition, the solution phase method enables the application of the graphene-based metamaterial in a water environment. To take advantage of the laser processability and water resistance of metamaterials, it has been demonstrated to create 18nm water-immersed ultrathin planar lenses for microfluidic biophotonic devices.

However, solution-based methods result in the agglomeration of GO nano-flakes on the substrate surface due to the interaction of GO with PDDA in solution. In the washing process, redundant GO or PDDA cannot be completely removed, and the opposite surface charge characteristics enable GO and PDDA to mutually attract to form large clusters, so that the smoothness of the graphene metamaterial film is seriously damaged, and the optical performance is influenced. Meanwhile, the thickness of each layer cannot be accurately controlled. The larger the number of layers, the more pronounced the effect. Therefore, there is a need to develop new coating techniques to avoid agglomeration of GO nanoflakes, thereby improving the smoothness of the metamaterial coatings and accurately controlling the thickness of each layer.

Disclosure of Invention

The invention aims to provide application of water bath ultrasound in layer-by-layer self-assembly of a graphene metamaterial thin film.

The invention provides application of water bath ultrasound in layer-by-layer self-assembly of a graphene metamaterial film,

and alternately immersing the substrate into a solution of a positively charged polymer and a graphene oxide solution in an ultrasonic bath environment, and alternately depositing a polymer dielectric layer and a graphene oxide layer through electrostatic self-assembly.

Preferably, when the surface of the substrate is positively charged, the substrate is sequentially and alternately immersed into a graphene oxide solution and a positively charged polymer solution, and a graphene oxide layer and a polymer dielectric layer are sequentially deposited on the surface of the substrate;

and when the surface of the substrate is negatively charged, the substrate is sequentially and alternately immersed into a positively charged polymer solution and a graphene oxide solution, and a polymer dielectric layer and a graphene oxide layer are sequentially deposited on the surface of the substrate.

Preferably, the positively charged polymer comprises one or more of polyethyleneimine, polydiallyldimethylammonium chloride, poly [2- (N, N-dimethylamino) ethyl methacrylate ], chitosan and chlorophyll.

Preferably, the mass concentration of the solution of the positively charged polymer is 1-5%; the concentration of the graphene oxide solution is 1-10 mg/mL.

Preferably, the frequency of the ultrasonic bath is 20-400 kHz, and the time of the ultrasonic bath is 10-60 s.

Preferably, the solution of the positively charged polymer and the graphene oxide solution are subjected to ultrasonic treatment every 2-10 deposition cycles.

Preferably, the ultrasonic treatment frequency of the solution of the positively charged polymer and the graphene oxide solution is 20-400 kHz; the ultrasonic treatment time is 450-600 s.

Preferably, after the polymer dielectric layers and the graphene oxide layers which are alternately laminated are obtained through deposition, the graphene oxide layers are fully or partially reduced, and the graphene metamaterial thin film is obtained.

Preferably, the graphene oxide layer is reduced in whole or in part by one or more of thermal reduction, photoreduction and chemical reduction.

Preferably, after the polymer dielectric layer or the graphene oxide layer is deposited each time, the deposited substrate is cleaned and dried, and then the subsequent deposition step is performed.

The invention provides application of water bath ultrasound in layer-by-layer self-assembly of a graphene metamaterial film. The invention utilizes ultrasonic water bath to prevent GO nano-sheets from accumulating in the solution and on the surface of the substrate, and in addition, after every 3-10 coating cycles, the GO solution is subjected to ultrasonic treatment by a high-power ultrasonic rod. This allows good control of the thickness of the layers and the flatness of the surface. This allows good control of the thickness of the layers and the flatness of the surface. Therefore, the quality of the coated graphene metamaterial film can be greatly improved, and the graphene metamaterial film is suitable for wide application, especially optical application. The prepared graphene metamaterial thin film can be further partially or completely reduced so as to convert a GO layer into a graphene (reduced graphene oxide) layer for different applications.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.

FIG. 1 is a schematic diagram of an ultrasonic water bath assisted layer-by-layer graphene metamaterial coating process of the present invention;

FIG. 2 is an optical microscope image of graphene metamaterial coatings of varying thickness of the present invention with and without an ultrasonic water bath;

FIG. 3 is a processed 2D optical profilometer image of different locations on a 120nm (30 layer) thick ultrasonic water bath assisted graphene metamaterial coating in accordance with the present invention;

FIG. 4 is a 3D optical profilometer image after processing at different locations on a 120nm (30 layer) thick ultrasonic waterbath assisted graphene metamaterial coating in accordance with the present invention;

FIG. 5 is an Atomic Force Microscope (AFM) image of a substrate coated with a single layer of PDDA (a), a substrate coated with a single layer of GO (b), and a substrate coated with a 5-layer graphene metamaterial thin film in accordance with the present invention;

FIG. 6 is a Raman spectrum of a GO solution and a coated graphene metamaterial thin film on a glass substrate in accordance with the present invention;

FIG. 7 is a Scanning Electron Microscope (SEM) image of an optical fiber without and with a conformally coated graphene metamaterial thin film in accordance with the present invention;

FIG. 8 is an SEM image of silicon square (a) and slot (b) waveguides coated with graphene metamaterial thin films of the present invention;

FIG. 9 is a SEM image of a conformal coating on a silicon nanostructure array with a graphene metamaterial film of the present invention (a) and a nanostructure array without (b) and (c) and (d) graphene metamaterial film coatings;

FIG. 10 is an optical photograph of a glass substrate with a 1 μm thick graphene metamaterial coating in accordance with the present invention; the silver areas show areas reduced by laser light;

fig. 11 is a raman spectrum of the graphene metamaterial thin film and the reduced graphene metamaterial thin film according to the present invention.

Detailed Description

The invention provides application of water bath ultrasound in layer-by-layer self-assembly of a graphene metamaterial thin film, which is characterized in that,

In the present invention, if the surface of the substrate is negatively charged, the substrate is first immersed in a polymer solution for a certain period of time, and a layer of positively charged material is deposited on the negatively charged surface under the conditions of an ultrasonic bath to form a positively charged surface.

The substrate with the deposited positively charged surface is then removed, the substrate rinsed in Deionized (DI) water, and then dried with compressed air or nitrogen.

Then the dried substrate is immersed into graphene oxide solution, under the condition of ultrasonic bath, the graphene oxide layer with negative electricity is deposited on the surface with positive electricity,

the substrate with the deposited graphene oxide layer is then removed, the substrate is rinsed in Deionized (DI) water and then dried with compressed air or nitrogen to obtain a polymer-GO layer for one deposition cycle.

Repeating the steps to obtain more layers until the required layers or thickness is reached.

If the surface of the substrate is positively charged, firstly immersing the substrate into a graphene oxide solution for a certain time, and depositing a graphene oxide layer on the positively charged surface of the substrate under the condition of ultrasonic bath;

the substrate with the deposited graphene oxide layer is then removed, the substrate is rinsed in Deionized (DI) water, and the substrate is then dried with compressed air or nitrogen.

The dried substrate is then immersed in a positively charged polymer solution for a period of time to deposit a layer of positively charged material on the surface of the graphene oxide layer under ultrasonic bath conditions to form a positively charged surface.

The substrate with the deposited positively charged surface is then removed, the substrate rinsed in Deionized (DI) water, and then dried with compressed air or nitrogen to obtain a polymer-GO layer for one deposition cycle.

During the above deposition process, the negatively charged graphene oxide layer provides a negatively charged surface upon which another layer of positively charged material may subsequently be deposited. Alternating layers of positively and negatively charged materials are bonded to each other by electrostatic interactions. The coated graphene metamaterial thin films consist of alternating GO and polymer layers.

Between each deposition step, any unattached material (e.g., unattached polymer or unattached graphene oxide) can be removed by washing the sample.

Once the desired number of layers is achieved, the graphene metamaterial film can be dried. Sample drying may be performed with compressed air or nitrogen. The GO layer in the film can be converted, in whole or in part, to a graphene layer by a reduction process.

In the invention, the frequency of the ultrasound in the process of depositing the graphene oxide layer is preferably 20-400 kHz, more preferably 50-350 kHz, such as 20kHz,50 kHz, 100kHz, 150kHz, 200kHz, 250kHz, 300kHz, 350kHz, 400kHz, and is preferably a range value with any value as an upper limit or a lower limit; the time of the ultrasonic treatment, i.e., the deposition time, is preferably 10 to 60s, more preferably 20 to 50s, such as 10s, 20s, 30s, 40s, 50s, 60s, and is preferably a range value with any of the above values as the upper limit or the lower limit.

The frequency of ultrasound in the polymer deposition process is preferably 20-400 kHz, more preferably 50-350 kHz, such as 20kHz,50 kHz, 100kHz, 150kHz, 200kHz, 250kHz, 300kHz, 350kHz, 400kHz, and is preferably a range value with any value of the above mentioned values as an upper limit or a lower limit, and the time of ultrasound, i.e. the deposition time is preferably 10-60 s, more preferably 20-50 s, such as 10s, 20s, 30s, 40s, 50s, 60s, and is preferably a range value with any value of the above mentioned values as an upper limit or a lower limit.

In the invention, the solution of the positively charged polymer and the graphene oxide solution are subjected to ultrasonic treatment every 2-10 deposition cycles to ensure the quality of the solution, and the thickness and the surface flatness of each layer are further controlled, preferably, the solution of the positively charged polymer and the graphene oxide solution are subjected to ultrasonic treatment every 3-8 deposition cycles, and more preferably, the solution of the positively charged polymer and the graphene oxide solution are subjected to ultrasonic treatment every 5-6 cycles.

In the present invention, the frequency of the ultrasonic treatment is preferably 20 to 400kHz, more preferably 50 to 350kHz, such as 20kHz,50 kHz, 100kHz, 150kHz, 200kHz, 250kHz, 300kHz, 350kHz, 400kHz, preferably a range value in which any of the above values is an upper limit or a lower limit; the time of the ultrasonic treatment is preferably 450-600 s, more preferably 500-550 s, such as 450s, 460s, 470s, 480s, 490s, 500s, 510s, 520s, 530s, 540s, 550s, 560s, 570s, 580s, 590s, 600s, and preferably ranges with any of the above values as an upper limit or a lower limit.

Preparation of GO solution

Graphite may be oxidized using conventional methods to produce graphite oxide. In some embodiments, an oxidation process may be employed, such as the Hummers process (journal of the American chemical society,1958,80(6),1339) or a modified Hummers process (ACS nano,2010,4(8), 4806). The graphite oxide is exfoliated to produce graphene oxide sheets. Exfoliation of the graphite oxide can be performed using exfoliation techniques and conditions known in the art. The graphene oxide may be suspended in a solvent and exfoliated in the solvent under conditions sufficient to cause separation of the graphene oxide sheets, thereby forming a graphene oxide solution. The graphene oxide solution includes isolated graphene oxide sheets suspended in a solvent. The isolated graphene oxide sheets may be in the form of a single layer or several layers.

The graphite oxide in solution can be mechanically exfoliated to produce graphene oxide sheets, which are then dispersed in a solvent. Mechanical stripping can be achieved using sonication. As will be appreciated by those skilled in the art, sonication involves the application of acoustic energy to agitate the graphite oxide and ultimately cause disruption of the graphene oxide lattice layers in the graphite material. The destruction of the lattice layer results in separation of graphene oxide sheets. Known sonication methods and conditions that may be used to exfoliate graphite oxide may be used. The sonication can be performed using a sonicator or an ultrasonic bath. The graphite oxide may be sonicated at a frequency in the range of about 20kHz to about 400kHz, with a frequency of about 20kHz being preferred. Sonication can be carried out for a period of seconds to hours. The time period may vary depending on, for example, the amount of graphite oxide to be exfoliated and the frequency of sonication. In one set of embodiments, the graphite oxide may be sonicated for a period of time ranging from about 5 minutes to several hours, preferably from about 20 minutes to about 1 hour, more preferably about 30 minutes.

After exfoliation of the graphite oxide in solution, a graphene oxide solution is formed. The graphene oxide solution may include graphene oxide in the form of a single layer and/or several layers. The few-layer form may include 2 to 10 graphene oxide flakes.

In the invention, the concentration of the graphene oxide solution is preferably 1-10 mg/mL, more preferably 3-8 mg/mL, and most preferably 5-6 mg/mL.

Positively charged polymers

In the present invention, the positively charged material may be a positively charged polymer such as Polyethyleneimine (PEI), polydiallyldimethylammonium chloride (PDDA), poly [2- (N, N-dimethylamino) ethyl methacrylate ] (PDMAEMA), and chitosan, or a positively charged polymer. Chlorophyll and the like. In one preference, the positively charged material is a positively charged polymer.

The mass concentration of the positively charged polymer solution is preferably 1-5%, more preferably 2-4%, and most preferably 2-3%; the solvent used in the solution of the positively charged polymer is preferably water.

Substrate material

In the present invention, the graphene metamaterial may be coated on any substrate having different materials, areas and shapes. Suitable substrates preferably include the following materials:

semiconductor and crystalline materials: silicon, GaAs, GaN, Al ₂ O ₃ 、SiO ₂ Fused quartz, diamond, lithium niobate;

plastics and polymer materials: PVC, PI, PET, PU, glass fiber sheet;

an optical fiber;

metal: aluminum, steel, iron, copper, brass, gold, silver, tin;

and (3) ceramic.

In the present invention, in order to ensure high quality of the graphene metamaterial thin film, the substrate may be pretreated to provide a clean surface for the coating process.

The pretreatment process comprises the following steps:

firstly, cleaning a base material by using ultrasonic waves in acetone for 15-20 min, then cleaning the base material by using ultrasonic waves in ethanol for 15-20 min, then cleaning the base material by using ultrasonic waves in deionized water for 15-20 min, and finally taking out and drying the cleaned base material in air.

In the present invention, the substrate may be any shape, or a substrate having a prefabricated nanostructure, such as a nanopillar array, a nanopore array, or a 3D nanostructure. The method of the present invention can be applied to substrates of any shape or pre-fabricated structure.

Reduction of graphene oxide

The process of the present invention includes the step of subjecting the prepared graphene metamaterial membrane to a reduction process to reduce at least one oxygen-containing functional group present in the graphene oxide membrane.

The reduction process may reduce one or more oxygen-containing functional groups present in one or more graphene oxide sheets within the graphene metamaterial film. In some embodiments, the reduction process reduces at least one oxygen-containing functional group in the plurality of graphene oxide sheets.

The reduction of the oxygen-containing functional group removes the functional group from the graphene oxide sheet and results in the formation of a graphene (reduced graphene oxide) sheet.

After the reduction process, a reduced graphene metamaterial film is produced. The reduced graphene metamaterial membrane includes at least one sheet of graphene (reduced graphene oxide) and may include a plurality of reduced graphene oxide sheets. When at least one oxygen-containing functional group in the graphene oxide sheet is reduced and removed, a graphene (reduced graphene oxide) sheet is formed.

It will be understood by those skilled in the art that all graphene oxide sheets in the reduced graphene metamaterial membrane are not essential to the present invention. However, the method of the present invention provides that at least one graphene oxide sheet in the membrane is reduced.

In some embodiments, a portion of the graphene oxide sheets in the graphene metamaterial membrane are reduced. In such embodiments, the resulting film comprises a mixture of graphene oxide sheets and graphene (reduced graphene oxide) sheets. The resulting film may thus be a partially reduced metamaterial film. However, such partially reduced films are still considered reduced graphene metamaterial films according to the present invention.

In some embodiments, each graphene oxide sheet in the graphene metamaterial membrane is reduced.

One skilled in the art will appreciate that the reduction process conditions can be adjusted to vary the amount of oxygen-containing functional groups that are reduced, thereby varying the degree or extent of reduction. As explained further below, variations in the degree of reduction may allow the sieving characteristics of the reduced graphene metamaterial membrane to be tailored.

In some embodiments, the methods of the present disclosure can selectively reduce oxygen-containing functional groups located in the pores or in the interlayer spaces of the graphene metamaterial membrane. Selectivity may be possible because the type of oxygen-containing functional groups present in the pores may be different from that in the interlayer space. For example, a porous graphene oxide membrane as described herein may comprise hydroxyl and epoxy functional groups attached to the basal planes of the graphene oxide sheets, which extend into the interlayer spaces between the graphene oxide sheets. Meanwhile, carbonyl and carboxyl functional groups may be attached to defect edges of graphene oxide sheets, and thus such functional groups may be present in pores of the graphene oxide sheets.

The reduction process described herein is capable of distinguishing between different types of oxygen-containing functional groups, and thus the process of the present invention is capable of selectively reducing different oxygen-containing functional groups located at different positions in the graphene oxide layer.

Reducing the oxygen-containing functional group according to the method of the present invention results in removing the oxygen-containing functional group and sp3 carbon atoms from the graphene oxide sheet and forming more hydrophobic graphene domains.

The reduction of the graphene metamaterial membrane can be performed under suitable conditions. In one set of embodiments, the reduction process can involve thermal, chemical, or photo-reduction of the oxygen-containing functional groups, as well as mixtures of these reduction processes.

Thermal reduction of the graphene metamaterial membrane may involve heating the membrane to a suitable temperature to reduce at least one oxygen-containing functional group. In one set of embodiments, the graphene metamaterial film may be heated to a temperature of up to 1100 ℃. In one set of embodiments, the graphene metamaterial film may be heated at a temperature in a range of about 80 ℃ to about 1100 ℃. During thermal reduction, bonds connecting oxygen-containing functional groups to graphene basal planes are cleaved, resulting in the generation of H ₂ O、CO ₂ And CO gas. The temperature is preferably not more than 1100 ℃, because above this temperature, C — C bonds in the graphene network are broken, thereby introducing defects in the graphene-based plane. The heat energy applied can be varied to vary the degree of reduction. In some embodiments, the thermal reduction may be performed in a substantially oxygen-free environment (e.g., in a vacuum or inert atmosphere) to reduce the undesirable incorporation of base graphene sheetsIncidence of defects or damage.

The chemical reduction of the graphene metamaterial membrane may be performed by means of a chemical agent or other chemical process that removes at least one oxygen-containing functional group. In one form of the method of the invention, the graphene metamaterial membrane is reduced by treating the graphene oxide membrane with a chemical agent. The chemical agent may be selected from those conventionally used in the art. Such chemical agents include, but are not limited to, hydrazine, alcohols, sodium borohydride, acetic acid and acetic acid, alkali metal hydroxides (e.g., sodium hydroxide and potassium hydroxide), metal powders (iron or aluminum powders), ammonia, hexylamine, sulfur containing compounds (NaHSO) ₃ 、Na ₂ SO ₃ 、Na ₂ S ₂ O ₄ 、Na ₂ S ₂ O ₃ 、Na ₂ S·9H ₂ O、SOCl ₂ And SO ₂ ) Hydroxylamine hydrochloride, urea, lysozyme, vitamin C, N-methyl-2-pyrrolidone, poly (norepinephrine), Bovine Serum Albumin (BSA), TiO ₂ Nanoparticles, oxides of manganese and bacterial respiration.

Photoreduction of graphene metamaterial films can involve illuminating the film with radiation in a suitable light source or beam. The irradiation can cause a thermal (i.e., photothermal) or chemical (i.e., photochemical) effect that reduces at least one oxygen-containing functional group present in the porous graphene oxide film. Photoreduction includes photoreduction of oxygen-containing functional groups by irradiating the graphene metamaterial film with a light beam or radiation.

Photothermal reduction may involve irradiating a graphene metamaterial thin film with light or radiation and generating localized heat in the thin film. The amount of heat generated after irradiation depends on the source of the light or radiation and the thermal properties of the graphene oxide film. Parameters such as the wavelength and/or intensity of the light source and the illumination time (i.e., duration) can affect the degree of reduction. In a preferred mode, the photothermal reduction is carried out in a substantially oxygen-free environment, such as in a vacuum or in an inert atmosphere, such as a nitrogen or argon atmosphere. In photothermal reduction, the light or radiation may include different forms of electromagnetic radiation, including photoradiation.

Photothermal reduction may be performed using light or radiation of any suitable wavelength. Suitable wavelengths may range from the ultraviolet range (about 10nm) to the infrared range (about 100 μm).

In some embodiments, the CO ₂ A suitable wavelength of the laser light may be from about 248nm to 10.6 μm.

Photothermal reduction may be performed using any suitable type of light source or radiation source. Suitable light or radiation sources preferably have sufficient power to generate the least amount of heat. In some embodiments, a suitable light source or radiation source has sufficient power to heat the graphene metamaterial membrane to a temperature of at least about 200 ℃ during the reduction process. Some examples of light sources that may be used to facilitate photothermal reduction include, but are not limited to, UV lamps, focused sunlight, and flashlights.

Photothermal reduction of a porous graphene oxide film may involve irradiating the graphene metamaterial film with a beam of light or radiation having sufficient irradiance to generate a minimum amount of heat. The appropriate spot size may be selected based on the radiation power of the light or radiation source (i.e., the light or radiation source provided) to provide sufficient radiant flux (power), i.e., sufficient "irradiance", at watts per square meter (W/m) of surface per unit area ² ) Is a unit. Thus, the higher the source power, the larger the surface area treated. For femtosecond lasers, the selected average power may be in the range of 1 to 1000 microwatts (μ W) for Continuous Wave (CW) lasers, and the selected average power may be in the range of 10 to several hundred milliwatts (mW), preferably in the range of 10 to 100 mW. For a UV lamp or other light source, the selected power output may be in the range of 100 to 1000 watts, for example a power output of about 100W. The light source may include pulsed sources (including pulsed lasers and camera flash lamps) and continuous wave sources (including sunlight, ultraviolet lamps and laser diodes).

In some embodiments, photothermal reduction may advantageously allow for control of the removal of oxygen functional groups by adjusting the power of the light source or radiation. Different powers may be used to generate different temperatures. Conversely, because different oxygen-containing functional groups may have different binding energies, the different oxygen-containing functional groups may dissociate at different temperatures, thereby allowing selective removal of particular oxygen-containing functional groups.

Photochemical reduction uses shaped light pulses or radiation to control the chemical reactions that occur during the reduction of the porous graphene oxide film. Thus, light or radiation can facilitate chemical reduction of one or more oxygen-containing functional groups in the graphene metamaterial film. Selective removal of oxygen-containing functional groups can be facilitated by the use of shaped light pulses or radiation.

The shaped pulses, e.g. shaped light pulses, may be provided by a suitable light source or radiation source. In some embodiments, the shaping pulses may be provided from a femtosecond laser. Any suitable femtosecond laser may be used. Furthermore, any suitable spot size may be used. The spot size depends on the laser power, the average power of the laser depends on the repetition rate of the laser pulses (several tens of μ W for 1kHz, several mW for 80 MHz).

In some embodiments, the selective reduction of oxygen-containing functional groups can be controlled by varying the pulse shape. In such embodiments, the pulse shape may be iteratively updated by a feedback loop that takes input from an in situ monitoring method, such as raman spectroscopy or Fourier Transform Infrared (FTIR) spectroscopy.

Photoreduction by irradiation with a beam or radiation may be advantageous because it provides the ability to precisely control the reduction process. In some embodiments, the higher the power of the light beam, the higher the proportion of oxygen-containing functional groups in the graphene oxide film that are reduced.

Reducing oxygen-containing functional groups in graphene metamaterial thin films using photoreduction techniques may be advantageous because the type and coverage of oxygen-containing functional groups in the thin films can be controlled by adjusting parameters (e.g., wavelength, power, exposure time). ) The light source of (1). Therefore, the surface characteristics of the graphene metamaterial thin film can be easily controlled to adapt to different applications.

For example, when light or short wavelength radiation is used for photo-reduction, the power may be reduced due to the higher photon energy. In addition, for a given wavelength and power of light or radiation, increasing the exposure time can increase the number of oxygen-containing functional groups that are removed, thereby increasing the degree of reduction of the graphene oxide film. Furthermore, for shaped light pulses, the repetition rate, pulse width, and pulse shape may also affect the extent of reduction.

For a given radiation source, the working power range can be determined by the scanning power. The lower power threshold (i.e. the lower threshold) of the light beam can be determined by observing the transmission change under an optical microscope. The upper power threshold of the beam (i.e., the ablation/combustion threshold) can be determined by visual observation when ablation occurs. This can be done by using a microscope. The upper and lower thresholds may specify a power operating range in which photoreduction may be performed. By controlling the power of the beam within an operating range, selective oxygen-containing functional group removal can be achieved.

Photoreduction also provides the ability to locally reduce oxygen-containing functional groups in selected regions of the graphene metamaterial membrane. Thus, it is possible to form patterned films comprising selected regions of graphene oxide and graphene (reduced graphene oxide) for particular applications.

For example, patterning with a photoreduction process may be achieved by laser patterning or light irradiation, which may be achieved by a mask. The mask may cover a defined area of the graphene metamaterial thin film and may help to direct or control how light or radiation reaches that area of the thin film. This in turn may help control how the oxygen-containing functional groups are reduced in particular regions of the membrane. In this way, a reduced graphene metamaterial film having local regions of different degrees of reduction may be formed.

The graphene metamaterial and the reduced graphene metamaterial thin film prepared according to the invention can be used for various applications. Particularly in renewable energy collection such as photovoltaic and photothermal applications where high light absorption is required.

The graphene metamaterial thin films produced by the method of the present invention can advantageously be very thin, having a thickness in the nanometer range. In some embodiments, the thickness of the graphene metamaterial film may be in a range from about 30nm to about 3 μm. The thickness of the graphene metamaterial thin film can be easily varied to suit a particular application.

The invention will now be described with reference to the following examples. It should be understood, however, that these examples are provided for the purpose of illustrating the present invention and they in no way limit the scope of the present invention.

Preparing a graphene oxide solution:

natural graphite powder (SP-1, Bay Carbon) (20g) was placed in concentrated H at 80 deg.C ₂ SO ₄ (30mL)、K ₂ S ₂ O ₈ (10g) And P ₂ O ₅ (10g) In solution. The resulting dark blue mixture was thermally separated and cooled to room temperature over 6 hours. The mixture was then carefully diluted with distilled water, filtered, and washed on the filter until the pH of the rinse water became neutral. The product was dried in air at ambient temperature overnight. This graphite peroxide was then oxidized by Hummers' method. Graphite oxide powder (20g) was placed in cold (0 ℃ C.) concentrated H ₂ SO ₄ (460 mL). Gradually adding KMnO under stirring and cooling ₄ (60g) So that the temperature of the mixture could not reach 20 ℃. The mixture was then stirred at 35 ℃ for 2 hours and distilled water (920mL) was added. A large amount of distilled water (2.8L) and 30% H was added over 15 minutes ₂ O ₂ The reaction was stopped by the solution (50mL) after which the color of the mixture turned bright yellow. The mixture was filtered and washed with 1:10HCl solution (5L) to remove metal ions. The graphite oxide product was suspended in distilled water to give a viscous brown 2% dispersion which was dialyzed to completely remove the metal ions and acid. The synthesized graphite oxide was suspended in water to obtain a brown dispersion, which was dialyzed to completely remove residual salts and acids. Ultrapure Milli-Q water was used for all experiments. The purified graphite oxide suspension was then dispersed in water to produce a 0.05 wt% dispersion. Graphite oxide was exfoliated to GO by sonicating the dispersion for 30 minutes using a Brandson digital sonifier (S450D, 20kHz,500W, 30% amplitude). The brown dispersion obtained was then centrifuged at 3,000 r.p.m. for 30 minutes. Any un-exfoliated graphite oxide (typically present in very small amounts) was removed using an Eppendorf 5702 centrifuge with a rotor radius of 14 cm.

Preparation of PDDA solution and GO solution

A PDDA solution with a mass concentration of 2% was prepared and sonicated using a digital sonicator for 7 minutes 30 seconds with an amplitude of 10%.

A GO solution of 5mg/mL was prepared and sonicated using a digital sonicator for 8 min 30 sec with an amplitude of 15%.

Preparation of graphene metamaterial coating

Putting GO and PDDA solution in a beaker into an ultrasonic bath (temperature is 22-24℃)

The cleaned glass substrate (25 mm. times.25 mm. times.1 mm) was immersed in the PDDA solution for 30 seconds and then taken out.

Washing with deionized water;

drying the substrate with compressed air;

the PDDA coated glass substrate was immersed in the GO solution for 30 seconds and then removed.

Washing with deionized water;

drying the substrate with compressed air;

repeating the process for N times to obtain an N-layer graphene metamaterial film;

the solution was sonicated every 5 cycles according to the recipe shown in the table to maintain the quality of the solution.

Characterization of coated graphene metamaterial films

To examine the quality of the coated graphene metamaterial films and compare the presence or absence of ultrasonic water bath assistance, the quality was first examined using an optical microscope. Since the graphene metamaterial has a strong optical response, a small variation in thickness in an optical microscope image shows a non-uniform optical transmission. A set of optical microscope images for comparing the presence or absence of the ultrasonic water bath effect is shown in fig. 2, where the number of layers of the graphene metamaterial thin film is increased from 5 to 30 layers with a step size of 5. It can be seen that one ultrasonic bath showed uniform light transmission from 5 layers to 30 layers. In contrast, in the case of the one without the ultrasonic water bath, particularly when the number of layers is more than 15, the cluster of the surface increases, and the surface roughness increases as the number of layers increases.

The surface roughness of the coated graphene metamaterial was then characterized using an optical profiler capable of accurately measuring surface profiles over large areas with nanometer precision. To measure the surface roughness over a large area, we looked for 9 points on the coating surface to see the average surface roughness. The two-dimensional contour plot is shown in fig. 3, where it can be seen that the surface roughness at different locations remains within 20nm, confirming the high quality of the graphene metamaterial thin films prepared using the ultrasonic bath-assisted method. Further, a 3D surface map is shown in fig. 4 to visualize surface roughness.

We further performed an Atomic Force Microscope (AFM) to measure the exact thickness of each layer, and the results are shown in fig. 5. The PDDA monolayer (fig. 5(a)) and the GO monolayer (fig. 5(b)) were 2nm and 1nm thick, respectively, confirming the coating with a monolayer of GO in each iteration. The total thickness of each layer (single-layer PDDA + single-layer GO) is about 3-4 nm. An atomic force microscope of the five-layer structure is shown in FIG. 5(c), where it can be seen that the thickness of each layer is equal. The actual measurement thicknesses of the graphene metamaterial thin films with different layers are shown in the following table:

TABLE 1 actually measured thickness of graphene metamaterial thin films with different number of layers

Number of layers	5	10	15	20	25	30
							Thickness (nm)	～20	～40	～60	～80	～100	～120

In Table 1, "-20" indicates a thickness of about 20nm, and so on.

Raman spectroscopy was used to confirm the material composition of the graphene metamaterial coating, as shown in fig. 6. The marked D peak (at 1380 cm) can be seen in the figure ^-1 Left and right) and G peak (at 1600 cm) ^-1 Left and right) of graphene-based materials, the coating of GO material was confirmed.

Finally, the same method was applied to optical fiber coatings to demonstrate the ability of conformal coatings. Scanning Electron Microscope (SEM) images of bare optical fiber and optical fiber with graphene metamaterial coating are shown in fig. 7(a) and (b), respectively. As we can see, the graphene metamaterial thin film conformally covered the surface of the optical fiber, confirming the ability to conformally coat.

Furthermore, we show a coating of graphene metamaterial thin films on different silicon waveguide structures, with dimensions of only a few microns. SEM images of the square and slot waveguides are shown in fig. 8(a) and (b), respectively. It can be seen that the graphene metamaterial thin film is conformally coated on the waveguide, and the gap in the slot waveguide is only 150 nm.

In addition, the method is also applied to coating the graphene metamaterial thin film on the nanostructure array. A schematic diagram of the coating process is shown in fig. 9 (a). SEM images of the nanostructure array without graphene metamaterial coating are shown in fig. 9 (b). SEM images of the coated nanostructure array are shown in fig. 9(c) and (d). It can be seen that the thin film conformally covers each of the nanopillars in the array.

To demonstrate the reduction of graphene metamaterial thin films, we used a laser to reduce the thin films. The optical photograph is shown in fig. 10, wherein the dark part is a glass substrate coated with a graphene metamaterial thin film 1 μm thick, and the silver square is a reduced area. The raman spectra of the prepared graphene metamaterial and the reduced graphene metamaterial thin film are shown in fig. 11, wherein the change of the ratio of the D peak to the G peak can be seen.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims

1. The application of water bath ultrasound in the layer-by-layer self-assembly of the graphene metamaterial thin film is characterized in that,

2. The use according to claim 1, wherein when the substrate surface is positively charged, the substrate is sequentially and alternately immersed into a graphene oxide solution and a positively charged polymer solution, and a graphene oxide layer and a polymer dielectric layer are sequentially deposited on the substrate surface;

3. Use according to claim 1, wherein the positively charged polymer comprises one or more of polyethyleneimine, polydiallyldimethylammonium chloride, poly [2- (N, N-dimethylamino) ethyl methacrylate ], chitosan and chlorophyll.

4. The use according to claim 1, wherein the mass concentration of the solution of the positively charged polymer is 1-5%; the concentration of the graphene oxide solution is 1-10 mg/mL.

5. The use according to claim 1, wherein the frequency of the ultrasonic bath is 20 to 400kHz and the time of the ultrasonic bath is 10 to 60 s.

6. The use according to claim 1, wherein the solution of positively charged polymer and graphene oxide solution are sonicated every 2 to 10 deposition cycles.

7. The use according to claim 6, wherein the frequency of the ultrasonic treatment of the solution of the positively charged polymer and the graphene oxide solution is 20 to 400 kHz; the ultrasonic treatment time is 450-600 s.

8. The application of claim 1, wherein after the polymer dielectric layers and the graphene oxide layers which are alternately laminated are deposited, the graphene oxide layers are fully or partially reduced to obtain the graphene metamaterial thin film.

9. Use according to claim 8, wherein the graphene oxide layer is reduced wholly or partially by one or more of thermal reduction, photoreduction and chemical reduction.

10. The use according to any one of claims 1 to 9, wherein each time the polymer dielectric layer or graphene oxide layer is deposited, the deposited substrate is washed and dried before the subsequent deposition steps.