TECHNICAL FIELD

[0001] The present application relates to deposition and patterning of graphene oxide. In particular, the invention relates to deposition and patterning of graphene oxide using irradiation.

BACKGROUND

[0002] Graphene oxide (GO) is a promising material for production of graphene-based electronic components. In contrast to bulk graphite and graphene the GO is solvent dispersible, insulating and light-brown in color. It is quite easy to process and compatible with printing technologies. The insulating GO can be reduced to form chemically modified graphene, the so- called reduced graphene oxide (rGO), which exhibits unique electrical and chemical properties. There are several conventional methods of GO reduction including chemical treatment by reducing agent (hydrazine, etc.), thermal annealing to high temperatures (>500°C), and laser treatment. One promising technique is photo-thermal reduction of GO upon exposure to a pulsed xenon light at ambient conditions. Photonic flash reduction has attracted attention in the last years, which shows interest in processing of GO. Methods of photonic reduction known in the art describe changes in properties of GO after reduction. These properties include thickness, color, chemical reactivity, surface topography, thermal stability etc.

SUMMARY

[0003] According to a first aspect of the present invention, a method is disclosed. The method comprises: providing a substrate; depositing a Graphene Oxide (GO) film on the substrate; exposing at least one part of the GO film to photonic irradiation for reducing said at least one part of the GO film, thereby producing a pattern of at least one reduced GO (rGO) area in the GO film; and selectively depositing polar material onto the GO film according to said pattern of at least one rGO area.

[0004] The method according to this aspect may be, for example, a method for patterning Graphene Oxide, or a method for controlled patterning of Graphene Oxide, or a method for patterned deposition of Graphene Oxide. By photonic irradiation is meant electromagnetic irradiation with any spectrum including visible, ultraviolet and infrared wavelengths. Hereinafter "polar material" is defined as a complex material containing polar solvents with relatively high surface tension, and "non-polar material" refers to complex materials with relatively low surface tension. The interaction between polar materials is more energetically favorable than the interaction of polar materials with non-polar materials. "Hydrophilic material" is defined as a solid substance with relatively high surface free energy, i.e. a solid polar material that naturally has an affinity for water. "Hydrophobic material" is defined as a solid non-polar substance with relatively low surface free energy, which naturally repels water. In other words, for the purposes of this application, the terms "hydrophobic" and "hydrophilic" are used to describe non-liquid materials based on their interaction with water. The terms "polar" and "non-polar", on the other hand, may refer to both solid and liquid materials (and materials in any other state).

[0005] According to an embodiment of the present invention, depositing a GO film on the substrate may comprise depositing a GO film that has a thickness of less than 200 nanometers on the substrate.

[0006] According to an embodiment, exposing at least one part of the GO film to photonic irradiation comprises exposing at least one part of the GO film to irradiation with a wavelength spectrum between 200 nanometers and 900 nanometers.

[0007] According to an embodiment, photonic irradiation with a wavelength spectrum between 200 nanometers to 900 nanometers is produced by a xenon flash lamp.

[0008] For the purposes of this specification, a flash lamp refers to any lamp that may produce photonic irradiation for very short (10 ^~5 seconds) or quite long (up to 10 minutes) periods of time, or both.

[0009] According to an embodiment, exposing at least one part of the GO film to photonic irradiation is performed for a period of time between 1.Ox 10 ^-5 seconds and 10 minutes. [0010] According to an embodiment, exposing at least part of the GO film to photonic irradiation is performed in a series of pulses. Each of these pulses can be between 10 microseconds and 1 millisecond long.

[0011] According to an embodiment, selectively depositing polar material comprises printing of polar GO ink according to said pattern of at least one rGO area. Due to the differences in surface energies, interaction of polar ink with hydrophilic material is more energetically favorable than its interaction with hydrophobic material.

[0012] According to an embodiment, printing of polar GO ink comprises printing of polar GO ink that has a thickness of more than 200 nanometers.

[0013] According to an embodiment, depositing a GO film comprises depositing a GO film by spray coating on the substrate. Optionally, spin-coating, printing and film transfer may be used to deposit the GO film.

[0014] According to an embodiment, a substrate is a flexible plastic substrate.

[0015] According to an alternative embodiment, a substrate is a rigid glass substrate.

[0016] According to an embodiment, the method further comprises: doping the deposited GO film with a surface agent.

[0017] Doping the GO film with the above surface agents can provide increased contrast in wettability in the resulting structure.

[0018] According to an embodiment, selectively depositing polar material onto the GO film according to said pattern of at least one rGO area comprises selectively depositing polar material onto at least one area of the GO film that is outside of the pattern of at least one rGO area.

[0019] In other words, according to this embodiment, the polar material is selectively deposited onto at least one unreduced area of the GO film. According to an embodiment, exposing at least one part of the GO film to photonic irradiation comprises exposing at least one part of the GO film to photonic irradiation using a mask. According to an embodiment, the mask may be a photomask.

[0020] According to an embodiment, the method also comprises: exposing the GO film with the polar material to photonic irradiation, thereby producing a pattern of at least one hydrophobic conductive structure on the insulating rGO film. [0021] In other words, according to this embodiment, exposure to irradiation occurs at least twice - after depositing a Graphene Oxide film on the substrate, and then after selectively depositing polar material.

[0022] According to an embodiment, exposing at least one part of the GO film with the polar material to photonic irradiation comprises drying the GO film with the polar material and treating the same by a photonic flash that has a wavelength spectrum between 200 nanometers to 900 nanometers and is produced by a xenon flash lamp.

[0023] According to an embodiment, one or more conductive electrodes are deposited on the substrate, wherein the GO film is deposited on the substrate with the deposited one or more conductive electrodes. In other words, one or more conductive electrodes are deposited on the substrate prior to the deposition of a GO film.

[0024] According to an embodiment, exposing at least one part of the GO film to photonic irradiation comprises exposing at least one part of the GO film to photonic irradiation from a source that is positioned, at a predetermined distance, on the side of the substrate on which the GO film was deposited. In other words, if the deposition takes place on the top of the substrate, the structure is irradiated from above - i.e. from a source that is positioned, at a predetermined distance, on the top side of the structure.

[0025] According to an embodiment, exposing at least one part of the GO film to photonic irradiation comprises exposing at least one part of the GO film to photonic irradiation from a source that is positioned, at a predetermined distance, on the side of the substrate opposite to the side on which the GO film was deposited. In other words, if the deposition takes place on the top of the substrate, the structure is irradiated from below - i.e. from a source that is positioned, at a predetermined distance, on the bottom side of the structure.

[0026] According to an aspect of the present invention, a method is disclosed. The method comprises: providing a substrate; depositing a GO film on the substrate; selectively depositing a pattern of ultraviolet (UV) curable ink on the deposited GO film; and exposing the GO film with the UV curable ink to photonic irradiation for reducing areas of the GO film outside of the selectively deposited pattern of UV curable ink, and for curing the UV curable ink.

[0027] The method may be, for example, a method for maskless patterning of GO, or a method of maskless deposition of patterned GO. The pattern of UV curable ink may shield the area of the GO film on which it was deposited from irradiation, thereby blocking the reduction of these areas.

[0028] According to an embodiment, selectively depositing a pattern of UV curable ink on the deposited GO film comprises inkjet printing of UV curable ink on the deposited GO film. Optionally, any of a variety of deposition methods can be used in this embodiment including, but not limited to, dispensing, slot die coating, spray coating, soft lithography, micromolding in capillaries, screen printing, offset printing, reverse offset printing, gravure printing, flexography, aerosol jet printing, and the like.

[0029] According to an embodiment, the photonic irradiation is produced by a xenon flash lamp.

[0030] According to an embodiment, the GO film deposited on a substrate has a thickness of more than 200 nanometers on the substrate.

[0031] According to an embodiment, depositing a GO film comprises depositing a GO film by spray coating on the substrate. Optionally, spin-coating, printing and film transfer may be used to deposit the GO film.

[0032] According to an embodiment, selectively depositing a pattern of UV curable ink comprises selectively depositing a pattern of polymer UV ink.

[0033] According to an embodiment, at least one of the following polymer materials, without limitation to any of the materials, is used in selective deposition of a pattern of polymer UV ink: acrylic, polyurethane, polysiloxane, and epoxy resins.

[0034] According to an aspect of the present invention, a device is disclosed. The device comprises a reactor and a flash lamp, and the device is configured to: hold a substrate inside the reactor; deposit a Graphene Oxide (GO) film on the substrate; expose at least one part of the GO film to photonic irradiation by the flash lamp for reducing said at least one part of the GO film, thereby producing a pattern of at least one reduced GO (rGO) structure and at least one unreduced GO structure on the substrate; and selectively deposit polar material on the at least one unreduced GO structure on the substrate according to said pattern. The device may further be configured to hold a mask between the flash lamp and the substrate, where the mask is configured to shield at least a portion of the substrate from the flash lamp.

[0035] The flash lamp of the reactor is capable of producing photonic irradiation for various periods of time from 10 ^~5 seconds to 10 minutes. [0036] According to an aspect of the present invention, a device is disclosed. The device comprises a reactor and a flash lamp, and the device is configured to: hold a substrate inside the reactor; deposit a Graphene Oxide (GO) film on the substrate; expose at least one part of the GO film to photonic irradiation by the flash lamp for reducing said at least one part of the GO film, thereby producing a pattern of at least one reduced GO (rGO) structure and at least one unreduced GO structure on the substrate; and selectively deposit polar material on the at least one unreduced GO structure on the substrate according to said pattern.

[0037] According to an aspect of the present invention, an apparatus is disclosed. The apparatus comprises at least one processor; at least one memory coupled to the at least one processor, the at least one memory comprising program code instructions which, when executed by the at least one processor, cause the apparatus to perform the methods according to any of the above embodiments.

[0038] According to an aspect of the present invention, an apparatus is disclosed. The apparatus comprises means to: hold a substrate inside the reactor; deposit a Graphene Oxide (GO) film on the substrate; expose at least one part of the GO film to photonic irradiation by the flash lamp for reducing said at least one part of the GO film, thereby producing a pattern of at least one reduced GO (rGO) structure and at least one unreduced GO structure on the substrate; and selectively deposit polar material on the at least one unreduced GO structure on the substrate according to said pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] For a more complete understanding of example embodiments of the present invention, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:

[0040] FIGURE la is a graph of the static Water Contact Angle (WCA) and Resistance against irradiation time for a thin GO film;

[0041] FIGURE lb is a graph of resistance against irradiation time;

[0042] FIGURE 2 shows a method according to an embodiment of the present invention; and

[0043] FIGURE 3 shows a method according to another aspect of the invention. DETAILED DESCRIPTON OF THE EMBODIMENTS

[0044] Exemplary embodiments of the present invention and its potential advantages are understood by referring to Figures 1 through 3 of the drawings.

[0045] Some mechanisms underlying embodiments of the present invention can be better understood with reference to Figs, la and lb. The reduced Graphene Oxide (rGO) is hardly solvent dispersible and thus difficult to utilize in printing and coating technologies in contrast to the Graphene Oxide (GO). Therefore, rGO patterning at a small scale remains technically challenging.

[0046] Wettability and electrical properties of rGO change dramatically as the degree of photonic flash reduction increases. This becomes evident from Fig. la which is a graph showing GO dependencies of the static Water Contact Angle (WCA) in degrees and Resistance in MOhms against irradiation time in seconds. The WCA is an indicator of wettability, the bigger the angle the more hydrophobic the material is. The irradiation time is an indicator of reduction of GO, the longer the material is exposed the more energy is delivered and thus the more reduced it becomes until the GO layer under irradiation is fully reduced. The resistance is used as an indicator of electrical properties.

[0047] A wettability contrast of approximately 35 degrees can be achieved within 60 seconds of irradiation, as demonstrated by the dependency pointing to the left scale. The ·' hydrophilic property of Graphene Oxide is one of its intrinsic properties, while the hydrophobic properly is achieved by GO reduction. The wettability contrast can be increased even further by doping the ultrathin GO layer with surface agents because they increase the surface free energy of GO, which in turn reduces the WCA. The plot pointing to the right scale shows an increase in resistance for thin films of GO.

[0048] Fig. lb is a graph of resistance in MOhms against irradiation time for GO films of a different thickness, which gives a more detailed look at the conductivity/resistance correlation shown in Fig. la. Before reduction by irradiation, GO is an insulator. The conductivity of thick GO films increases significantly with increase of irradiation time, making rGO reduced from thick films a conductor. However, the conductivity of thin and ultrathin GO coatings actually decreases as the depth of reduction increases.

[0049] Most likely the ultrathin GO films, less than 200 nm in thickness, increase resistance due to suppressing ionic channels of charge transfer across the sample during reduction and degradation of inter-particle contacts caused by explosive degassing processes leading to exfoliated and disorderly packed rGO sheets with weaker π-π interaction While the conductivity of individual rGO sheets may be increased compared to GO as deposited, the overall conductivity decreases because of the loss of density and disordering. On the other hand, in case of thick (>200 nanometers) and very thick films (>1 micrometer) this process is compensated by higher density of packed GO sheets so that the particle-to-particle contacts are still maintained during reduction. Thus, thicker GO films show a decrease in resistance with irradiation time.

[0050] Based on these observations, it appears to be possible to create both hydrophilic and hydrophobic areas on a single surface simultaneously. Furthermore, anisotropic wetting of GO ink (or other polar ink) on the patterned GO film can be utilized during e.g. printing in order to fabricate fine conductive rGO domains (or other structures). A variety of suitable polar inks can also be used for controlled printing on GO-rGO coating. Polar ink or material refers to a complex material containing polar solvents with relatively high surface energy, while non-polar material refers to complex materials with relatively low surface energy. The interaction between polar materials is more energetically favorable than the interaction of polar materials with non- polar materials. Hydrophilic material is a solid polar material that naturally has an affinity for water, while hydrophobic material is then a solid non-polar substance with relatively low surface free energy, which naturally repels water.

[0051] Fig. 2 shows an exemplary embodiment of the present invention. The wettability contrast of ultrathin GO films and difference in electrical properties of relatively thin and thick GO films are used in this method of GO patterning by a combination of xenon flash reduction and printing technique. A substrate 201 is provided. The substrate may be e.g. a flexible plastic substrate or a rigid glass substrate. A GO film 202 is then deposited onto the substrate. For example it may be deposited by spray coating, spin-coating, printing and film transfer. The thickness of the GO film may vary between 5 nanometers and 200 nanometers. Next, at least one part of the film is exposed to photonic irradiation. A mask may be applied to the film, for example a photomask, prior to the photonic irradiation. Alternatively, a mask may be used at a distance from the substrate during the irradiation. A xenon flash lamp may be used to irradiate the film for a predetermined amount of time. The wavelength of the photonic irradiation may be between 200 nm and 900 nm. The xenon flash lamp or another source of irradiation may be positioned above or below the substrate, or in any other suitable position at a predetermined distance. The irradiation reduces said at least one part of the GO film, thereby producing a pattern of at least one rGO area 203 in the GO film. If a mask is used, areas of the GO film that are not protected by the mask are reduced. Unreduced GO is indicated by 204. After this, polar material 205 is deposited onto the GO film according to said pattern of at least one rGO area. In the exemplary embodiment shown on Fig. 2 the polar material is deposited onto the areas of the unreduced GO 204 which is hydrophilic. The polar material may be a thick layer of GO ink that is selectively deposited onto the unreduced GO 204. The selective deposition can be, for example, inkjet printing. The polar material "overfilling" during selective deposition is prevented in this method because the hydrophobic rGO regions stop undesirable diffusion. Some alignment may be required during the selective deposition. A result is a pattern that is a combination of hydrophilic polar material and thin hydrophobic reduced GO on the substrate. In an additional optional step, the structure can then be dried and exposed to photonic irradiation again, e.g. by the same flash lamp. A mask is not necessarily used during the second irradiation. In case thick GO ink was used as polar material, the resulting structure is fully hydrophobic because of the second irradiation and comprises a pattern of conductive rGO 206 on the insulating (dielectric) thin rGO. Such resulting structure is nearly transparent.

[0052] These patterned rGO and GO structures can be used for interconnects or sensing applications including humidity, temperature and capacitive touch sensing. In addition, the electrical properties of rGO and functionalized rGO can be very sensitive to different analytes such as NO, N0 ₂ , NH ₃ , H ₂ , alcohols and other organic vapors, thus making rGO a promising platform for highly sensitive gas sensors. Generally, a film of GO as deposited has a sheet resistance about 10 GQ/a (GigaOhms per square) to 100 G /o at ambient humidity depending on film thickness. The thin rGO layers exhibit approximately 100-200 GQ/a. The thick rGO layers approach about 10 kii/a of sheet resistance, which is 3-4 orders of magnitude lower than the thin rGO.

[0053] This method is quite easy to scale up and use in a wide variety of applications, does not involve any chemicals which result in hazardous waste, and can be compatible with flexible substrates and Roll-to-Roll manufacture. In an embodiment, doping of the deposited GO film with surface agents is possible to improve wettability contrast. [0054] Fig. 3 shows an exemplary method according to one aspect of the present invention. As a general remark regarding underlying principles of this aspect of the invention, a flash lamp (e.g. a xenon flash lamp) has an emission spectrum ranging approximately from 200 to 900 nm which coincides with the absorption spectrum of GO films. These films mainly absorb the light in Ultraviolet (UV) region with an absorption peak at approximately 231 nm. On the other hand, parts GO film can be effectively blocked from unwanted xenon flash light by thin plastic or polymer films, deposited e.g. in form of UV curable ink. This can prevent reduction of the GO in certain areas, allowing reduction in the remaining areas,

[0055] With reference to Fig. 3, a maskless GO-rGO patterning method is shown. The method includes providing a substrate 301 , for example a rigid glass substrate or a flexible substrate. A GO film 302 is then deposited step on the substrate. After this, a selective deposition of a pattern of insulating UV curable ink 303 onto the deposited GO film is performed. The insulating UV curable ink may comprise polymers such as acrylic, polyurethane, polysiloxane and/or epoxy resins. An exemplary deposition technique is inkjet printing. A number of printing methods, for example, but not limited to, dispensing, slot die coating, spray coating, soft lithography, micromolding in capillaries, screen printing, offset printing, reverse offset printing, gravure printing, flexography, aerosol jet printing can be used for deposition of the ink. After depositing of UV curable ink, the GO film with the ink is exposed to photonic irradiation for reducing areas of the GO film outside of the selectively deposited pattern of UV curable ink, and for curing the UV curable ink. A xenon flash lamp or a UV lamp may be used to irradiate the film for a predetermined amount of time. The source of irradiation may be positioned above or below the substrate, or in any other suitable position at a predetermined distance. The UV curable ink can be cured simultaneously with the reduction of areas of the GO film. This results in a structure with a pattern of conductive rGO 304 on the insulating GO film which is located in the areas under the cured insulator 305.

[0056] A particular benefit of this approach is that use of a photomask is not necessary.

[0057] According to an aspect of the present invention, a device is disclosed. The device may comprise a reactor in which a substrate is provided, as well as a flash lamp, and be configured to hold a substrate inside the reactor; deposit a Graphene Oxide (GO) film on the substrate; expose at least one part of the GO film to photonic irradiation by the flash lamp for reducing said at least one part of the GO film, thereby producing a pattern of at least one reduced GO (rGO) structure and at least one unreduced GO structure on the substrate; and selectively deposit polar material on the at least one unreduced GO structure on the substrate according to said pattern. It may further be configured to create conditions in the reactor suitable for the processes described above.

[0058] According to an aspect of the present invention, a device is disclosed. The device may comprise a reactor in which a substrate is provided, as well as a flash lamp, and be configured to hold a substrate inside the reactor; deposit a GO film on the substrate; selectively deposit a pattern of ultraviolet (UV) curable ink on the deposited GO film; and expose the GO film with the UV curable ink to photonic irradiation for reducing areas of the GO film outside of the selectively deposited pattern of UV curable ink, and for curing the UV curable ink. It may further be configured to create conditions in the reactor suitable for the processes described above.

[0059] The flash lamp may comprise a xenon flash lamp which may have a linear tube design and an emission spectrum of between about 200 nanometers and about 900 nanometers. The device may also include a power supply, a discharge module, and a controller configured to control pulse energy, pulse frequency, pulse duration and irradiation time of the flash lamp. The pulse frequency range may be in between approx. 1 Hertz to approx. 300 Hertz. The pulse duration range may be from approx. 10 microseconds to approx. 5 milliseconds. The power range may vary in between 10 Watts and 3000 Watts. The device may further include a mask disposed between the flash lamp and the substrate, where the mask is configured to shield at least a portion of the substrate from the flash lamp.

[0060] Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein is compatibility with flexible substrates and Roll-to-Roll manufacturing. Another technical effect of one or more of the example embodiments disclosed herein is clean production of patterned GO-rGO structures without the output of chemical waste. Another technical effect of one or more of the example embodiments disclosed herein is that Xenon flash method can be easily coupled to mass production in a printing method for graphene and graphene derivatives. The wettability and conductivity can be tailored in situ for the described processes.

[0061] An apparatus in accordance with the invention includes at least one processor in communication with a memory or memories. The processor is configured to store, control, add and/or read information from the memory. The memory may comprise one or more computer programs which can be executed by the processor. The processor may also be configured to control the functioning of the apparatus. The processor may be configured to control other elements of the apparatus by effecting control signaling. The processor may, for example, be embodied as various means including circuitry, at least one processing core, one or more microprocessors with accompanying digital signal processor(s), one or more processor(s) without an accompanying digital signal processor, one or more coprocessors, one or more multi- core processors, one or more controllers, processing circuitry, one or more computers, various other processing elements including integrated circuits such as, for example, an application specific integrated circuit (ASIC), or field programmable gate array (FPGA), or some combination thereof. Signals sent and received by the processor may include any number of different wireline or wireless networking techniques.

[0062] The memory can include, for example, volatile memory, non-volatile memory, and/or the like. For example, volatile memory may include Random Access Memory (RAM), including dynamic and/or static RAM, on-chip or off-chip cache memory, and/or the like. Nonvolatile memory, which may be embedded and/or removable, may include, for example, readonly memory, flash memory, magnetic storage devices, for example, hard disks, floppy disk drives, magnetic tape, etc., optical disc drives and/or media, non-volatile random access memory ( NVRAM), and/or the like.

[0063] If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined.

[0064] Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims.

[0065] It is also noted herein that while the above describes example embodiments of the invention, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims.