WO2017211992A1

WO2017211992A1 - Method for the transfer of graphene

Info

Publication number: WO2017211992A1
Application number: PCT/EP2017/064037
Authority: WO
Inventors: Liubov BELYAEVA; Grégory F. Schneider
Original assignee: Universiteit Leiden
Priority date: 2016-06-10
Filing date: 2017-06-08
Publication date: 2017-12-14
Also published as: GB201610167D0

Abstract

The present invention relates to a method of transferring a graphene film to a substrate comprising providing a graphene film grown on a support and positioning the supported graphene film at the interface of a biphasic mixture, wherein the biphasic mixture comprises an aqueous phase and an organic phase, and wherein the phases are immiscible, wherein the aqueous phase further comprises an etchant which completely etches the graphene support from the graphene film. After the etching procedure has taken place, the biphasic mixture is cooled to a temperature such that the organic phase solidifies but the aqueous phase remains a liquid, wherein the graphene film sticks to the solidified organic phase. The solidified organic phase is then separated from the aqueous phase and the graphene of the solidified phase is applied to a substrate. The solidified organic phase is then removed by melting, subliming or evaporating the organic phase.

Description

METHOD FOR THE TRANSFER OF GRAPHENE

The present invention relates to a method for transferring a graphene sheet to a substrate. Graphene is known for its outstanding mechanical, optical and electrical properties. Graphene films, which have been synthesised by chemical vapor deposition (CVD), have been shown to have uniform structure and be of high quality.

Copper is one of the many metals which is used to catalyse the growth of graphene. However, the problem with graphene grown on copper is that it tends to crack due to copper oxidation, particularly in water. Therefore, the transfer of graphene films to substrates of interest has been a particular challenge. For years now, long chain polymers have been used in transfer methods to prevent cracking and to preserve the two- dimensional nature of graphene (Suk et al. "Transfer of CVD-grown monolayer graphene onto arbitrary substrates.," ACS Nano, vol. 5, no.9, pp. 6916-24, Sep 2011 ).

Unfortunately, because of their macromolecular structures, the use of polymers in the transfer of graphene is problematic because they stick and irreversibly modify the inherent chemical and physical properties of graphene (Pirckle et al. "The effect of chemical residues on the physical and electrical properties of chemical vapor deposited graphene transferred to Si0₂, Appl. Phys. Lett., vol. 99, no. 12, p. 122108, 201 1).

Instead of using a polymer, existing polymer-free techniques suggest special frames and holders to keep the integrity of the floating graphene while the water level is carefully lowered down to transfer the graphene onto a substrate (Lin et al "A direct and polymer- free method for transferring graphene grown by chemical vapor deposition to any substrate" ASC Nano no.2, pp. 1784-1791 , 2014). In one example, graphene was transferred to TEM quantifoil grids without using any polymer. The TEM grid was placed on graphene on copper floating in the etchant and a drop of isopropanol was cast on top putting the grid into a direct contact with graphene. However, this technique cannot be used for other substrates that are heavier or have featured morphology (Algara-Siller ef al, "Square ice in graphene nanocapillaries.," Nature, vol. 519, no. 7544, pp. 443-5, Mar. 2015). It is therefore desirable to have a polymer-free method for the transfer of graphene. The Applicant has surprisingly found a technique for transferring grown graphene samples onto various arbitrary substrates without using a polymer. The method of the present invention is based on the use of a biphasic mixture as an alternative to polymers. While polymers are known to protect graphene from folding during the transfer of graphene, the Applicant has found that organic solvents (such as cyclohexane) can operate similarly but without major contamination or wrinkle formation of the graphene.

This improved technique is called interfacial caging and uses a mixture of an organic solvent and an aqueous solution where graphene floats (is "caged") between the two phases. It is important that the two liquids are immiscible and that the organic solvent has a lower density and higher melting point with respect to the aqueous phase. This enables using this interfacial caging technique for graphene transfer. The biphasic mixture serves as a soft and flexible stabilizing medium to prevent the graphene from cracking and results in a flatter graphene surface.

The present invention will now be described, by way of example, with reference to the accompanying figures in which:

Figure 1 comprises five photographs showing interfacial caging during the transfer of graphene to TEM grids. Figure 2 is a schematic showing the biphasic transfer methods.

Figure 3 shows optical images showing the characterization of continuality and quality of graphene transferred by the cyclohexane-assisted caging method, the PMMA-assisted method, the top-fishing method and hexane-assisted method. Figure 3a shows an optical image of graphene transferred by top-fishing method; Figure 3b shows an optical image of graphene transferred by the PMMA-assisted method; Figure 3c shows an optical image of graphene transferred by the hexane-assisted method; Figure 3d shows an optical image of graphene transferred by the biphasic method following the second approach; Figure 3e shows a Raman spectra of graphene samples transferred to silicon wafers by cyclohexane-assisted, PMMA-assisted and top-fishing methods; and Figure 3f shows a SEM image of graphene transferred to quantifoil TEM grids by the biphasic method.

Figure 4 show atomic force microscopy (AFM) images and height profiles of graphene samples transferred to silicon wafer by (a) biphasic method (b) PMMA-assisted method (c) top-fishing method; and (d) hexane-assisted transfer method. Figure 5 shows diffraction patterns of graphene taken at the beginning of exposure to electron beam and then after 14 and 15 minutes, respectively.

Figure 6 shows the electrolyte gate voltage (Vref) dependent sheet conductance (G) of polymer-free graphene at biphasic interface (black) and epoxy substrate (grey).

According to the present invention, there is provided a method of transferring a graphene film to a substrate comprising:

(i) providing a graphene film grown on a support;

(ii) positioning the supported graphene film at the interface of a biphasic mixture, wherein the biphasic mixture comprises an aqueous phase and an organic phase, and wherein the phases are immiscible, wherein the aqueous phase further comprises an etchant which completely etches the graphene support from the graphene film;

(iii) after etching has taken place, cooling the biphasic mixture to a temperature such that the organic phase solidifies but the aqueous phase remains a liquid, wherein the graphene film sticks to the solidified organic phase,

(iv) separating the solidified organic phase from the aqueous phase;

(v) applying the graphene of the solidified phase to a substrate; and

(vi) removing the solidified organic phase by melting, subliming or evaporating the organic phase.

In a further aspect of the present invention, there is also provided a method of transferring a graphene film to a substrate comprising:

(i) providing a graphene film grown on a support and placing it on a substrate;

(ii) positioning the supported graphene film substrate at the interface of a biphasic mixture, wherein the biphasic mixture comprises an aqueous phase and an organic phase, and wherein the phases are immiscible, wherein the aqueous phase further comprises an etchant which completely etches the graphene support from the graphene film;

(iv) removing the solidified organic phase by melting, subliming or evaporating the organic phase. The above methods used to perform the interfacial caging can also be described as "biphasic caging" methods. The biphasic mixture comprises at least one aqueous phase and at least one organic phase. What is key for the method is that the phases are immiscible and that the organic phase has a plastic crystal phase of above 0 °C. Preferably, the aqueous phase is water and comprises at least one etchant.

In accordance with the present invention, the organic phase can be selected from a solvent which is less dense than water and is immiscible with water. Preferably, the organic solvent has a melting point between -5°C and 20°C. More preferably, the organic solvent is selected from cyclohexane, cyclooctane, cyclodecane, cyclononane, p-xylene, 2- methyl-2-propanethiol, dimethyl butanol, hexanoic acid, 2,2-dimethyl 3-pentanol, 4- ethylaniline, η,η-dimethylaniline, 2,3-dimethylaniline, aminooctane, 2,3,3-trimethyl 2- pentanol, 2,2-dimethylpropanal, cyclohexanecarbonitrile, 2,5-dimethyl- 2,4-hexadiene, cis-2-methylcyclohexanol, 2,5-dimethylaniline, 2,6-dimethylaniline, cyclohexanol, 2,3,3- trimethyl-2-butanol, 3,4-dimethylaniline, cyclooctanol, p-methylbenzylcyanide, 4- propylphenol, or combinations thereof. In a preferred embodiment, the organic phase is cyclohexane.

In a preferred embodiment, the biphasic mixture comprises water as the aqueous phase and cyclohexane as the organic phase. Water and cyclohexane are immiscible (solubility of cyclohexane in water is 0.006% at 25°C, solubility of water in cyclohexane is 0.01% at 20°C). In the temperature window of -87°C<T<7°C cyclohexane forms a plastic crystal phase, whereas water is liquid at 0°C. Biphasic transfer can be carried out at temperatures between 0°C and 7°C in which cyclohexane is plastic crystal and water is a liquid.

As described above, when the graphene film grown on a support and is inserted into the biphasic mixture, an etchant is used to completely etch away the support from the graphene which leaves the graphene floating ("caged") in between the two phases. The etchant must be capable of etching the support (typically the support is a catalytic metal such as copper) and is selected from, but not limited to, ammonium persulfate, iron(lll) chloride, nitric acid, ammonium hydroxide/hydrogen peroxide, sulfuric acid/hydrogen peroxide, hydrogen peroxide/sodium bisulfate, potassium nitrate/hydrochloric acid, sodium persulfate or iron(lll) nitrate. The etchant is insoluble in the organic phase which minimises the interchange of matter between the two phases. Preferably, the etchant is ammonium persulfate. Once inserted into the biphasic mixture, the sample floats directly at the interface exposing graphene to the cyclohexane phase and exposing copper to the etchant solution. When the copper is completely etched, graphene remains floating between the two phases. The two phases apply pressure from each side of the graphene acting as a firm but flexible shell which limits perturbations and conforms the surface of the graphene.

In one aspect according to the present invention, the transfer of graphene is executed by cooling the mixture down after etching (see Figure 2). The organic phase has a higher freezing point than the aqueous phase. The biphasic mixture is cooled down to a temperature such that the organic phase solidifies, but the aqueous phase remains liquid.

In a preferred embodiment, the organic phase is cyclohexane and the aqueous phase is a solution of ammonium persulfate. After completion of the etching step, the mixture is cooled down to 2°C. At this temperature, cyclohexane solidifies but the solution of ammonium persulfate remains liquid. The graphene sticks to the surface of the solidified cyclohexane. Due to the soft and flexible jelly-like texture of high-temperature solid phase of cyclohexane this step does not induce the formation of cracks in graphene. The solidified layer comprising cyclohexane with graphene on it can be easily transferred and placed on a substrate (see Figure 2a). To avoid residues of ammonium persulfate on graphene, cyclohexane is then rinsed with cold water (2°C). In order to remove the solidified cyclohexane, the cyclohexane is left to vaporize (cyclohexane is very volatile, it normally sublimates in 15-90 min depending on the volume).

In a second approach, the substrate is placed directly on a copper foil covered with graphene (Figure 2b). During etching, due to the interfacial tension between water and cyclohexane, both graphene and the substrate can float at the interface. After solidification, cyclohexane is removed by melting and subsequently washing with water. The fact that graphene was in direct contact with the substrate from the very start of the transfer prevents the formation of ammonium persulfate residues between the graphene and the substrate. The contaminations from the other side of graphene can be removed by rinsing the sample with water.

Having a different affinity to water, copper with graphene and the substrate (for example, a silicon wafer) tend to repel from each other. This problem was overcome by wrapping copper foil around the substrate. The substrate which is suitable for the present invention is selected from a silicon wafer, a TEM quantifoil grid, TEM nanochips, silicon nitride chips or glass chips. Substrates made from other types of materials such as polymers and metals can also be used. The skilled person would be aware of further suitable materials from which the substrate can be made. The graphene support is a catalytic metal, preferably wherein the metal is copper, nickel, platinum, gold, palladium, iridium, ruthenium, cobalt, rhodium, rhenium or iron. In a preferred embodiment, the monolayer graphene films were grown on a 25 pm thick copper foil in a cold wall chemical vapor deposition system (as described in the "Handbook of chemical vapor deposition (CVD)", H. O. Pierson).

Figure 2 shows schematics of the biphasic transfer a) the first approach. Graphene is inserted in the biphasic solution (1 ), the copper is etched (2), the solution is cooled down to 2°C until the cyclohexane phase solidifies (3), and the cyclohexane phase with graphene stuck on it is transferred onto a substrate (4). After that the sample is kept at 2°C until the cyclohexane evaporates (sublimates) b) second approach. Graphene is inserted in the biphasic solution together with a wafer on top of it (1 ), the copper is etched (2), the solution is cooled down to 2°C until the cyclohexane phase solidifies (3), and the cyclohexane phase with graphene/wafer stuck on it is removed from the solution (4). After that the sample is rinsed with water to remove the residues of cyclohexane and APS.

To prove the necessity of having the layer of solid cyclohexane for the transfer, we also performed three "control" transfers. In the first control transfer the first approach was reproduced and liquid cyclohexane, i.e. at room temperature was used and the graphene was "fished out" directly with a silicon wafer. As a result, no graphene was found on the wafer. In the second control experiment the second approach was reproduced also skipping the freezing step. Again, no graphene was transferred to the substrate, which, therefore, indicates the essential role of the solidification of cyclohexane for transferring graphene. For the third control experiment the second approach transfer method was used without any cyclohexane layer, i.e. simply placed a silicon wafer on top of graphene grown on copper floating on the surface of etchant. No graphene was found on the wafer after the transfer demonstrating the necessity of the supporting upper layer.

The soft gel-like structure of the plastic solid phase of cyclohexane conforms the surface of graphene preventing mechanical damaging from occurring. The same effect has also been observed using hexamethyldisilane, cyclohexanol and combinations thereof. This is because these particular solvents form a plastic crystal phase. Solvents such as these, and in particular cyclohexane, are also highly volatile and this is an additional advantage when reducing the contamination of graphene. Furthermore, the transfer process is completely driven by manipulating the physical properties of cyclohexane and doesn't require multiple tools. To evaluate the feasibility and relevance of the present invention, a comparison was made with the most commonly used polymer- transfer method, so called polymethyl methacrylate (P MA)-assisted transfer and with the potentially most "clean" method, described herein as "top-fishing", where graphene is simply picked up by a wafer while floating on the surface of the etchant. PMMA protects and conforms the surface of graphene and therefore allows the transfer of large continuous areas of graphene. However, this method has the disadvantage of inevitable contamination of graphene with the polymer. In contrast, the top-fishing transfer method results in cleaner, but discontinuous graphene samples with multiple irregularities (folding's, wrinkles, cracks etc.). The samples, however, have very different topological features. Graphene transferred with the biphasic caging clearly has lower density of wrinkles, and the wrinkles themselves are shorter and narrower as opposed to the other three samples (Figure 3a). Image of PMMA- transferred graphene has multiple closed white lines all over the sample (Figure 3b). Those could be interpreted as wrinkles or as polymer residues segregated on the grain boundaries of graphene. Top-fished graphene, as expected, exhibits repetitive pattern of bigger, compare to PMMA- and cyclohexane-transferred graphene, parallel wrinkles (white lines with the length of few micrometers and height up to 10 nm, see Figure 3c). This is in a good agreement with the optical images. The surface of the hexane-transferred graphene is covered with wrinkles too, but of smaller size with respect to the top-fished sample (Figure 3d).

Among all the different methods of transfer, samples transferred by PMMA-assisted and biphasic technique showed similar continuality and least amounts of cracks; graphene transferred to the silicon wafer through the first approach of biphasic method, however, has traces of residual ammonium persulfate (Figure 3c) which will require a further washing step. Graphene samples transferred with PMMA and second biphasic approach look similar and don't have obvious imperfections (Figure 3d and b). Graphene transferred by top-fishing method is clearly less uniform and has multiple wrinkles, which is clearer in the high magnification image. Those wrinkles originate from the moment when graphene floating on the etchant surface meets the substrate that doesn't conform the graphene surface. The samples transferred with hexane resemble the top-fished samples - during the scooping of the graphene from the biphasic mixture it broke into smaller pieces and became more wrinkled.

Samples transferred to silicon wafers by three different methods show very similar Raman signatures with the peaks typical for a monolayer graphene sample (Figure 3e), e). Sharp 2D peak, that fits one Lorentz function indicates monolayer graphene (see A. C. Ferrari et al, "Raman spectroscopy as a versatile tool for studying the properties of graphene.," Nat. Nanotechnol., vol. 8, no. 4, pp 235-46, Apr 2013). Negligibly small D peaks indirectly evidence that almost no defects in graphene structure were generated during the transfer processes (again see A. C. Ferrari ef al.,).

Figures 3f and g show the Scanning Electron Microscopy (SEM) images of the samples transferred in a biphasic method on quantifoil TEM grids. Full coverage has been achieved in large scale. Free standing graphene membranes are free from wrinkles, tearings and visible contaminations (see Figure 3f).

SEM of the grids with graphene transferred by the biphasic method performed according to the present invention showed close to full coverage of the grid with graphene. However, inevitably there are areas with cracks and holes, which are seen as uncovered holes in quantifoil film. These defect areas provide a contrast with graphene-covered holes which is necessary for visualization and confirmation of presence of graphene on the grid. In general, no clear distinction between the biphasic transfer and PMMA-assisted method can be made from SEM images. The possible reason for that could be that nonconductive contaminating species can't be visualized with this technique.

Of the experiments performed, most importantly were the Atomic Force Microscopy (AFM) experiments to see if the methods of the present invention resulted in residues, cracks, wrinkles or folding in the graphene sheet. A typical AFM image of graphene transferred to a silicon wafer by PMMA-assisted method has multiple features that correspond to wrinkles, PMMA residues and dust particles (see T. Hallam et al., "Strain, Bubbles, Dirt, and Folds: A Study of Graphene Polymer Assisted-Transfer,", Adv. Mater. Interfaces, vol. 1 , no. 6, p. n/a-n/a, Sep 2014).

The samples, however, have very different topological features. Graphene transferred with the biphasic caging clearly has lower density of wrinkles, and the wrinkles themselves are shorter and narrower as opposed to the other three samples (Figure 4a). Image of PMMA-transferred graphene has multiple closed white lines all over the sample (Figure 4b). Those could be interpreted as wrinkles or as polymer residues segregated on the grain boundaries of graphene. Top-finished graphene, as expected, exhibits repetitive pattern of bigger, compare to PMMA- and cyclohexane-transferred graphene, parallel wrinkles (white lines with the length of few micrometers and height up to 10 nm, see Figure 4c). This is in a good agreement with the optical images. The surface of the hexane- transferred graphene is covered with wrinkles too, but of smaller size with respect to the top-fished sample (Figure 4d).

In conclusion, AFM studies showed better uniformity of graphene transferred by the biphasic method of the present invention. This confirms the advantage of transversal biphasic caging for reducing vibrations and corrugations of graphene surface during the transfer process In addition to that, it's clear that having a top liquid layer on the graphene improves the fishing process (less wrinkles on the sample transferred by hexane-assisted method), but in terms of integrity and smoothness of the surface cannot compete with the polymer-assisted method and the biphasic method reported in this work.

A transmission electron microscopy (TEM) study of graphene transferred to quantifoil grids also showed that the graphene didn't have folding's and wrinkles. The sample exhibited almost no change in diffraction patterns taken over 15 minutes, which indicates, that no noticeable contamination took place on the graphene surface (Figure 5).

The biphasic caging with three principally different transfer methods were compared and summarized the results shown in Table 1. In PMMA-assisted method graphene is supported by a polymer, which allows for controllable further handling and keeps the integrity of graphene. The polymer conforms graphene surface and prevents it from formation of large wrinkles. However, we observed that the polymer doesn't prevent the graphene from formation of multiple smaller wrinkles. Another major drawback of PMMA- based technique is polymer residues. Opposed to it, the top-fishing and hexane-assisted methods result in cleaner, but cracked and wrinkled graphene. Table 1. Analysis of graphene samples transferred with biphasic caging, PMMA-assisted method and top-fishing method

partial partial full coverage of coverage of full coverage of coverage of

Continuality

the wafer the wafer the wafer the wafer

Density of

low high low high

cracks

2-15 nm high,

2-3 nm high, 2-15 nm high, up >15 nm high,

Size of wrinkles up to 10 pm

0.5-2μηπ long to 10 pm long >10 pm long long

Density of

low medium high high

wrinkles

In the biphasic caging graphene, on the one hand, is softly supported from its both sides, which minimizes such irregularities as wrinkles, crumples and folding's. On the other hand, cyclohexane, contrarily to PMMA, is a smaller molecule without conjugated electron system, i.e. not prone to ττ-π stacking on graphene surface, which together with its high volatility make cyclohexane very easy to remove. The method allows easy handling without subjecting graphene to harsh treatments. Big areas of graphene can be transferred without inducing defects and big cracks, which was confirmed by Raman spectroscopy, optical, Atomic Force and Scanning Electron microscopy.

In conclusion, the PMMA-assisted method graphene is supported by a polymer, which allows for controlled further handling and keeps the integrity of graphene. The polymer conforms graphene surface and prevents it from formation of large wrinkles. However, the polymer doesn't prevent the graphene from formation of multiple smaller wrinkles. Another major drawback of PMMA-based technique is polymer residues. The top-fishing method results in cleaner, but cracked and wrinkled graphene.

In the biphasic caging methods according to the present invention, the graphene is softly supported from its both sides, which minimizes such irregularities as wrinkles, crumples and folding's. Furthermore, using cyclohexane as the organic phase, contrarily to PMMA, is a smaller molecule without conjugated electron system, i.e. not prone to T T stacking on graphene surface, which together with its high volatility make cyclohexane very easy to remove. The method allows easy handling without subjecting graphene to harsh treatments. Big areas of graphene can be transferred without inducing defects and big cracks, which was confirmed by Raman spectroscopy, optical, Atomic Force and Scanning Electron microscopy. Biphasic electrolyte-gated graphene field-effect transistor

The electrical properties of the free-floating graphene were tested and the results were as follows. While graphene was floating at the organic/water interface, copper electrodes (25 pm Cu) were protected by using PMMA against the etchant, leaving the upper surface for electrical contact. As a control, we also manufactured a monolayer graphene on epoxy as in W. Fu, C. Nef, a Tarasov, M. Wipf, R. Stoop, O. Knopfmacher, M. Weiss, M. Calame, and C. Schonenberger, "High mobility graphene ion-sensitive field-effect transistors by noncovalent functionalization.," Nanoscale, vol. 5, no. 24, pp. 12104-10, Dec. 2013.

The graphene device at biphasic interface exhibits a significantly higher average carrier mobility (-3470 cm2/Vs, ~1940 cm2/Vs for hole and -5000 cm2/Vs for electron, see Figure ) compared to -1505 cm2/Vs (-940 cm2/Vs for hole and -2070 cm2/Vs for electron) on epoxy substrate. This indicates that the biphasic configuration helps to retain the electrical properties of graphene as the electrical performance of the graphene device is significantly improved. This can be due to the fact that this process reliably results in a graphene layer that has clean surfaces on both side free of polymer contamination. This last point is important because resist residues can prevent surface functionalization and suppress sensing. Such a graphene sheet at biphasic interface is, therefore, ideal for sensing applications, especially when a flexible and high-performance graphene devices are needed. We would also like to note here that depending on the quality of the CVD graphene, the floating graphene devices tend to break if the CVD graphene contains too much defects.

Figure 6. The electrolyte gate voltage (Vref) dependent sheet conductance (G) of polymer- free graphene at biphasic interface (black) and epoxy substrate (red). The gate voltage of the charge neutrality point VCNP 0.16 mV compared to -0.2 mV on epoxy substrate. Depending on the quality of the CVD graphene, the floating graphene devices tend to break if the CVD graphene contains too many defects.

Claims

1. A method of transferring a graphene film to a substrate comprising:

(i) providing a graphene film grown on a support;

(iv) separating the solidified organic phase from the aqueous phase;

(v) applying the graphene of the solidified phase to a substrate; and

2. A method of transferring a graphene film to a substrate comprising:

(i) providing a graphene film grown on a support and placing it on a substrate;

(iv) removing the solidified organic phase by melting, subliming or evaporating the organic phase.

3. The method according to Claim 1 or Claim 2, wherein the aqueous phase is water.

4. The method according to any preceding claim, wherein the organic phase is selected from cyclohexane, cyclooctane, cyclodecane, cyclononane, p-xylene, 2-methyl- 2-propanethiol, dimethyl butanol, hexanoic acid, 2,2-dimethyl 3-pentanol, 4- ethylaniline, η,η-dimethylaniline, 2,3-dimethylaniline, aminooctane, 2,3,3-trimethyl 2-pentanol, 2,2- dimethylpropanal, cyclohexanecarbonitrile, 2,5-dimethyi- 2,4-hexadiene, cis-2- methylcyclohexanol, 2,5-dimethylaniline, 2,6-dimethylaniline, cyclohexanol, 2,3,3- trimethyl-2-butanol, 3,4-dimethylaniline, cyclooctanol, p-methylbenzylcyanide, 4- propylphenol or combinations thereof.

5. The method according to any one of Claim 1 or Claim 2, wherein the organic phase is cyclohexane.

6. The method according to any preceding claim, wherein the etchant is selected from ammonium persulfate, iron(lll) chloride, nitric acid, ammonium hydroxide/hydrogen peroxide, sulfuric acid/hydrogen peroxide, hydrogen peroxide/sodium bisulfate, potassium nitrate/hydrochloric acid, sodium persulfate or iron(lll) nitrate.

7. The method according to any one of Claims 1 to 5, wherein the etchant is ammonium persulfate.

8. The method according to Claim 2, wherein the supported graphene film is held in position with the substrate by wrapping extra support material around the supported graphene film and substrate.

9. The method according to any preceding claim, wherein the substrate is selected from silicon wafer, a TEM quantifoil grid, TEM nanochips, silicon nitride chips or glass chips.

10. The method according to any preceding claim, wherein the graphene support is a catalytic metal preferably wherein the metal is selected from copper nickel, platinum, gold, palladium, iridium, ruthenium, cobalt, rhodium, rhenium or iron.

1 1. The method according to Claim 8 wherein the metal is copper.

12. The method according to any preceding claim, wherein the method further comprises a step of washing any contaminants from the graphene film using water.

13. A graphene film substrate obtainable according to any one of the preceding claims. 4. An in-situ sensing application comprising a graphene film substrate according to Claim 13.