WO2015149116A1

WO2015149116A1 - Graphene process and product

Info

Publication number: WO2015149116A1
Application number: PCT/AU2015/000198
Authority: WO
Inventors: Shailesh Kumar; Kostyantyn Ostrikov; Timothy Anthony VAN DER LAAN
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2014-04-04
Filing date: 2015-04-02
Publication date: 2015-10-08

Abstract

A method of preparing a free or transferrable graphene sheet comprising pre-treating the surface of a polycrystalline copper substrate with a Hydrogen-containing plasma to enhance the relative proportion of (101) and (111) facets relative to (100) facets; depositing graphene onto the substrate by contacting the surface of the substrate with a plasma comprising a carbon source gas for a deposition period; and decoupling graphene from the substrate by exposing the graphene and substrate to a polar liquid to instantly decouple the graphene from the substrate to provide a free or transferrable graphene sheet. Also, a graphene structure comprising a plurality of stacked planar carbon layers, wherein at least one layer of the structure comprises integrated horizontal and vertical nanosheet portions. The graphene structure comprises a plurality of interconnected microwells.

Description

Title: Graphene Process and Product

Field of the Invention

[001 ] The invention relates to a process for the preparation of graphene that is deposited by low temperature plasma enhanced chemical vapour deposition onto a substrate in a manner that enables facile exfoliation of the graphite from the substrate. The invention also relates to graphene microwell structures preparable by such a process.

Background Art

[002] Graphene exhibits unique electronic, optical, chemical and mechanical properties'¹¹. Because of its extremely high electron mobility (electrons move through graphene about 100 times faster than silicon), very low absorption in the visible spectrum and relative flexibility and elasticity (compared to inorganics such as indium tin oxide), supported horizontal graphene as an active functional material has been revolutionising the fields of flexible, transparent and ultra-light nano-devices, from optoelectronics¹²¹ to sensors'³'⁴¹.

[003] Although graphene is normally a flat, sheet like substance, it also has the ability to be deposited onto substrates in a way that allows for a degree of vertical orientation. This allows for the creation of controlled graphene microstructures, which are potentially useful in electron emission, bio-recognition and drug/gene/protein delivery applications among others. Graphene in a vertical orientation offers substantially enhanced functionality'⁵'⁶¹ compared to horizontally oriented graphene. This is due to enabled vertical in-plane ultra-fast charge transport through the accessible basal planes which also provide a relatively high density of low contact-resistance sites for synergistically adsorbing and/or immobilising a range of quantum dots'⁷¹, chemical and bio-specific molecules'⁸¹, for example.

[004] Graphene has been produced by several methods. However, the widespread applications of graphenes, and in particular vertical graphenes, in nano-devices are somewhat limited due to several issues which need to be addressed. The mass production of graphene, which would be required for widespread commercial use, has been targeted by a few general processes, including:

• mechanical grinding of graphite and dispersion in solution followed by self assembly.

• thermal graphitisation of SiC.

• chemical vapour deposition (CVD) onto metal substrates .

[005] Of these three methods, CVD onto metal substrates is the most promising, as high quality films of graphene can be deposited at elevated temperatures'⁶'¹²¹. [006] However, CVD onto substrates has some underlying drawbacks - one in particular being that once the graphene has been deposited onto the substrate, it generally needs to be removed or transferred to a different substrate for use. Typically, for instance, the graphene must be transferred to a semiconductor or plastic substrate for subsequent device fabrication. Often, the removal of graphene films is via wet chemistry. Wet chemistry¹⁹¹ should be avoided as far as possible in the fabrication of high quality vertical graphene films, since wet chemistry results in defect-associated¹¹⁰¹ loss of in-plane charge carrier transport. Loss of in-plane charge carrier transport can compromise the promising properties required for efficient field-emission, ultra-fast sensing and nano-electronics based devices'²'⁶'¹¹¹.

[007] An example of a wet chemical process is that disclosed in US 20130187097. In that case, the graphene is deposited at about 500°C or below using inductively coupled plasma-CVD onto a metal substrate or a metal catalyst layer residing on a non-metal substrate. After deposition, the graphene is separated from the deposition substrate by the etching of the metal layer, such as by acid or by FeCI₃ solution. However, it has been reported that graphene in such etching processes can be damaged by bubbles of evolved hydrogen. Such damage results in atomic scale flaws, which are sufficient to disrupt electron transfer. Wet chemical processes also use hazardous chemicals, the residuals of which need to be removed before use.

[008] Non wet chemical means are available, for instance, US 20120244358 describes a transfer process in which graphene is first deposited onto a conventional growth substrate, such as a copper substrate. The deposited graphene is then contacted with a transfer substrate, such as a polymer or inorganic substrate with an enhanced adhesion to graphene. This results in transfer printing of the graphene onto the transfer substrate. This avoids etching, but requires functionalised polymers as intermediates and also may require subsequent organic solvent wash out of the transfer layer.

[009] Another drawback with CVD is that it consumes large amounts of energy and is not particularly cost-effective for scalable fabrication. It is also a demanding process to achieve controlled graphene layer thickness and a high density of reactive open graphene edges in the vertical graphene networks at a relatively low-temperature in a cost and energy-efficient manner. Also, it is extremely difficult to simultaneously achieve both a scalable structural integrity amongst the few vertically-stacked graphene layers in a vertical graphene film and a desirable level of optical transparency. A structurally-integrated graphene structure is required to minimise the conductance loss at junction interfaces and to ensure sufficient mechanical stability and also to provide functionalities upon large deformation for flexible and lightweight optoelectronics and other devices'⁴¹. While high quality of vertical graphene networks can be deposited only on certain catalytic solid substrates'⁶¹ in CVD process, as mentioned it is challenging to decouple the graphene films from the substrate and transfer them onto a device platform without using time-consuming, expensive and hazardous chemical process'¹³'¹⁴¹ to eliminate the structural defects in the vertical graphene networks and the platform and also the loss of the substrate during the process, is critical. [0010] Thus, the need exists in the art for a process of preparing graphene which can provide graphene in an easily isolated, low defect form.

[001 1] There is also a need to produce new forms of vertical graphene that exhibit desirable controllable electronic, optical and mechanical properties.

[0012] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

[0013] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

Summary

[0014] According to a first aspect the invention provides a method of preparing a free or transferrable graphene sheet comprising the steps of:

a) pre-treating the surface of a polycrystalline copper substrate with a Hydrogen-containing plasma to enhance the relative proportion of (101 ) and (111 ) facets relative to (100) facets;

b) depositing graphene onto the substrate by contacting the surface of the substrate with a plasma comprising a carbon source gas for a deposition period;

c) decoupling graphene from the substrate by exposing the graphene and substrate to a polar liquid to instantly decouple the graphene from the substrate to provide a free or transferrable graphene sheet.

[0015] Preferably the proportion of (101 ) and (111 ) facets, in combination, relative to (100) facets is increased to at least 4%., and more preferably the proportion of (101 ) and (111 ) facets, in combination, relative to (100) facets is increased to at least 15%.

[0016] Preferably, pre-treating the surface passivates the substrate surface.

[0017] The pre-treating is carried out for a period sufficient to hydrophilise the surface of the substrate, as determined by reduction in a water contact angle of the substrate. The water contact angle of the substrate after pre-treating is for preference less than 6° more preferably even less than 3°.

[0018] It is preferred that during pre-treating the substrate has a surface temperature maintained solely through direct plasma heating, and it is also preferable during pre-treating that the substrate has a surface temperature maintained in the range 90-200°C.

[0019] Preferably, the Hydrogen containing plasma is derived from Hydrogen gas alone or from a mixture of Argon and Hydrogen. In the case where the Hydrogen containing plasma is derived from a mixture of Argon and Hydrogen the Hydrogen-containing plasma is preferably derived from a 1 : 10 mixture of Argon and Hydrogen gases. It is also preferred that the mixture of Argon and Hydrogen gases is fed at 0.8 to 10 Pa.

[0020] Preferably the pre-treating is carried out for at least 10 seconds and more preferably pre-treating is carried out for at least 30 seconds.

[0021] Preferably the Hydrogen-containing plasma is produced by RF power of about 100-800W.

[0022] Preferably during depositing graphene, the substrate has a surface temperature maintained solely by direct plasma heating, and it is also preferable during depositing graphene, that the substrate has a surface temperature maintained in the range 200-300°C.

[0023] Preferably the carbon source gas comprises a gas selected from the group consisting of methane, acetylene, ethylene, and fatty acid monomers. Most preferably the carbon source gas comprises methane.

[0024] Preferably the deposition period is 2-10 minutes.

[0025] Preferably the carbon source gas is fed at 1.0 to 5.0 10 Pa.

[0026] Preferably during the deposition the plasma is produced by RF power of about 650-900W.

[0027] Preferably the pre-treating and deposition are carried out in a continuous single process for production of transferrable graphene.

[0028] Preferably the polar liquid is water.

[0029] According to another aspect the invention provides a free or transferrable graphene sheet prepared by the method of the present invention.

[0030] According to another aspect the invention provides a graphene structure comprising a plurality of stacked planar carbon layers, wherein at least one layer of the structure comprises integrated horizontal and vertical nanosheet portions.

[0031] Preferably a portion of at least one stacked carbon layer forms a first vertical nanosheet portion, and wherein the first vertical nanosheet portion is supported by at least one second vertical nanosheet portion.

[0032] Preferably a portion of at least one stacked carbon layer forms a first vertical nanosheet portion, and wherein the first vertical nanosheet portion is supported by at least one second vertical nanosheet portion and wherein the first and second vertical nanosheet portions in combination with a plurality of adjacent vertical nanosheet portions form a graphene microwell.

[0033] Preferably the vertical nanosheet portion is connected to a horizontal nanosheet portion without an intervening junction.

[0034] Preferably a plurality of adjacent vertical nanosheet portions form a petal-like vertical graphene (PVG) structure in an X-Y plane, thereby to define a T-junction microwell in a X-,Y- and Z- plane.

[0035] Preferably the size of the microwells in X-Y plane is 1-5μιη and more preferably the microwells in the X-Y plane is 1 -2μιη.

[0036] Preferably the graphene structure comprises a plurality of microwells, and more preferably it comprises a plurality of interconnected microwells, wherein adjacent microwells have a common portion of a vertical petal-like ridge. More preferably the plurality of microwells defines a substantially regular honeycomb structure.

Brief Description of the drawings:

[0037] Figure 1 : Illustrates a number of facets of the plasma-enabled wet-chemical-/binder-free, highly integrated 3-D vertical graphene network of the present invention:

a) Growth process of graphene microwells). Electron backscattered diffraction (EBSD) mapped images show the plane orientation in Z-direction of a polycrystalline surface of Copper substrate (foil) dominated by the (100) plane. After H₂+Ar plasma treatment, there was an increase in the (101 ) and (111 ) phases and also in crystal size even at a foil temperature of <190°C. After adding methane (CH₄), the structurally-integrated horizontal graphene (HG) and vertical graphene (VG) layers were fabricated at a foil temperature of <270°C.

b) Digital images show that the graphene microwell film is instantly decoupled from the substrate (copper foil) upon dipping in either water or de-ionized water at room temperature .

c) The graphene microwell film after transferring onto a polyethylene (PET) sheet shows -76% transparency at wavelength of 550 nm.

d) Digital images show the 10th bending cycle of the graphene microwell/PET films from flat to a minimum radius of curvature of 1 mm.

e) Scanning Electron Microscopy (SEM) images of graphene microwell films after the 10th bending cycle. Top view shows a highly integrated, petal-like vertical graphene network which has a high density of graphitic edges -0.4 km/cm².

(f) SEM images of graphene microwell films after the 10^th bending cycle. Bottom view shows the horizontal graphene integrated at the bottom of vertical graphene networks. Unless mentioned, scale bar is 25 μιη. [0038] Figure 2: Integrated structural properties of graphene microwells (a-f) and the decoupling mechanism (g).

a) A SEM image that shows that the petal-like vertical graphene layers are structurally connected to each other to form the micron/sub-micron size cup-like features/containers in a graphene microwell film, b-d) A low-resolution transmission electron microscopy (TEM) image (b) shows the edges of vertical graphene layers and a high-resolution image (c) reveals that the vertical sheets are as thin as 4 graphene layers and their planar spacing is >0.361 nm which indicates a weak inter-planar interaction. These planes terminate one after other (shown by a white arrow mark) and create highly reactive open edge planes. The selected area electron diffraction pattern of (d) also confirms a few graphene layers in the vertical sheets.

e-f) The images reveal that petal-like vertical graphene layers are the native curled parts of two adjacent horizontal graphene layers which eventually coalesce to each other (e) and vertically grow higher in the direction of electric field which is created by the plasma sheath. The petal-like vertical graphene layers are thin (e) and thick (f) as a result of low and high methane concentration, respectively during the plasma deposition process.

g) The schematic here shows the integrated petal-like vertical graphene structure in a graphene microwell film. When the copper foil is dipped into water, the H₂0 molecules intercalate and diffuse between the hydrophobic graphene microwell film and hydrophilic copper surface and decouple the film from the foil. Due to gravity, the copper foil sinks and the film is decoupled (exfoliated) from the substrate and thus floats freely in the water. The white arrow mark shows the terminated graphene edge plane in the schematic of graphene microwell.

i-h) Raman spectra (i) reveals that there is almost negligible change in spectrum of graphene microwells before and after the transfer process, see top two spectra. The bottom spectra of 2(i) show a very weak 2D-peak in the absence of integral-connection (h). Unless mentioned, scale bar is 50 nm.

[0039] Figure 3: Micro-Raman spectra of graphene microwell films on a twice used copper foil, (a) after 1 st deposition process, (b) after 2nd deposition process the graphene microwell being transferred in between the deposition processes.

[0040] Figure 4: Schematic shows the plasma-substrate configuration inside the plasma CVD system for growth of graphene microwell films.

[0041] Figure 5: SEM of graphene microwell structure showing an individual salt crystal stored in the microwell.

[0042] Figure 6: Comparison of the performance of two types of graphene structures useful in nano- devices. c) The sheet resistance (a) after 10th bending cycle from flat to 1 mm radius of curvature shows that graphene microwell (GMW) recovers 81 % of its conductance while the petal-like vertical graphene (PVG) one could gain only 42%. The significant recovery of sheet resistance of graphene microwell is due to integrated connectivity among the vertically-stacked graphene layers as well as between the vertical and horizontal graphene layers (b). c) A gas-sensing device made from the two different films show that the sensitivity of GMW for N0₂ gas at room temperature is almost one order of magnitude higher than for PVG films.

[0043] Figure 7: A schematic that shows how a graphene microwell film would be integrated into a gas- sensing platform device.

[0044] Figure 8: A photograph illustrating the wettability of the substrate before and after plasma activation

Detailed Description

[0045] The present invention provides a plasma-enabled modification of nanoscale crystallographic features of a copper substrate surface at a substantially low temperature, 190°C or below, which not only activates the surface for deposition of a highly networked graphene film but also facilitates the instant decoupling and detachment of the film from the copper substrate when it is dipped in water (de-ionized or common tap water) at room temperature (22°C).

[0046] In the present invention, the process is carried out at low substrate temperatures, for example, 90-200°C, more usually around 190°C. These relatively low plasma deposition temperatures reflect the fact that the substrate is not heated by the plasma alone, that is, there is no external heating of the substrate, since it is not required. The use of such relatively low temperatures means the process is relatively mild and amenable to industrial use.

[0047] The schematic in Figure 1 a shows a one step, cost and energy-efficient growth process for producing high quality graphene films that can be readily released from the deposition substrate. In the present invention, the crystal orientation of polycrystalline copper surface, dominated by the presence of more than 50%, and more usually more than 90% (100) plane facets is effectively modified by its exposure to a hydrogen containing plasma, such as H₂+Ar plasma for 90 seconds, although shorter periods, such as 30 seconds or even 10 seconds can be effective. As a result of this modification, the higher facets of graphene, (101 ) and (111 ), emerged on the surface of the substrate. (See, Figure 1 a, in which the EBSD images in the figure shows z-plane of the foils). Typically, after around 10 seconds, around 4-5% (4.5%) of the (100) polycrystalline copper surface is converted to (101 ) and (111 ) facets. Longer pre-treatment times will lead to a higher level of conversion of the (100) facets to the (101 ) and (111 ) facets, for example, up to 15% conversion to (101 ) and (111 ) facets in total has been observed and conversions of even higher than 50% (101 ) and (111 ) are possible. In the context of the present invention, instantaneous release of the graphene film is achieved with as low as 4.5% of (101 ) and (111 ) facets, so there is no practical benefit, in terms of release, in aiming for significantly higher proportions of the (101 ) and (111 ) facets. [0048] In addition to the pre-treatment causing recrystallization of the substrate surface, it is also believed that it leads to a degree of passivation on the surface of the recrystallised material. The passivation takes place simultaneously with the recrystallization, or immediately afterwards. Without wishing to be bound by theory, it is thought that this combination of changes in the substrate ultimately results in the ability of subsequently deposited graphene to be instantly released upon contact with ionic liquids. Normally, at the low deposition temperatures used in the present invention, little or no passivation of (100) facets would be expected. However, with the increased proportions of (101 ) and (111 ) facets present, the mixed (100), (101 ), (111 ) copper surface surprisingly passivates at much lower temperatures (as low as 120-130°C)

[0049] The pre-treatment plasma can be generated from hydrogen alone, but it is best to use an portion of an inert carrier gas, which will have a higher molecular weight, such as Argon, since the collision of larger molecules with the surface is believed to facilitates the changes at the surface. Thus, the combination of hydrogen and an inert gas such as Argon is ideal. A 1 : 10 mixture of Ar to Hydrogen is ideal, but the ratio can be higher, for example, 1 :30 Ar:H₂. Using Hydrogen alone, the plasma will take around 30 seconds to heat the substrate and change the surface. Using 1 : 10 Argon:Hydrogen will lead to a similar result in around 10 seconds.

[0050] The graphene is then deposited onto the copper in accordance with other known processes, in particular, graphene is applied to the modified substrate by plasma enhanced CVD, such as for example, the deposition of graphene from an CH₄+H₂+Ar plasma mixture. Other carbon source gases besides methane can be used for the deposition of graphene in the present invention, for instance, ethylene or monomer fatty acid can be used.

[0051] It is preferred if the deposition onto the substrate takes place directly following on from substrate pre treatment, for instance, in the same chamber immediately after pre-treatment. In this way, the pre- treatment and deposition stages are contiguous, and the entire process can be conducted continuously.

[0052] As before, the deposition process is carried out without heating of the substrate, with the substrate being heated by the plasma only. Due to the slightly different nature of the deposition plasma, and the power required (650-900W) , the substrate temperature is a little higher, typically in the range of 200-300°C, however, those skilled in the art will appreciate that this is a very low temperature for graphene deposition.

[0053] The gases for the deposition plasma are fed at above 1.0 Pa (it is difficult to ignite the plasma below that pressure) and up to around 12 Pa, with a range of 1 -5Pa being more usual.

[0054] It was found that the amount of carbon deposited could influence the structure of graphene, from a traditional flat graphene sheet to the novel graphene microwells disclosed and claimed herein. [0055] Using a methane:argon:hydrogen ratio of 0.2: 1 : 10 to 0.8: 1 : 10, the graphene was deposited as horizontal films. However, using a methane:argon:hydrogen ratio of 1 : 1 : 10 or above produced graphene having the novel microwell structure of the present invention.

[0056] Thus, using a partial pressure of below 1 Pa of carbon gas produced a flat graphene sheet, whereas 1.0 or above produced graphene having a microwell structure. The best quality flat sheet graphene was found to be deposited at a methane:argon:hydrogen ratio of around 0.2: 1 : 10 to 0.3: 1 : 10

[0057] After the deposition process on the activated substrate is complete, the copper foil and deposited graphene are submerged in a polar liquid such as water, preferably deionised water, whereupon the graphene instantly decouples from the foil and is detached so that it floats freely on the water (Figure 1 b). The decoupled graphene is transparent (76% transparency at an optical wavelength of 550 nm (Figure 1 c)).

[0058] The term "instant" is to be considered in its normal context, that is, upon contact with the polar liquid, there is no delay observable to the naked eye in the lift off of the film - it is envisaged that the decoupling in polar fluid could be carried out as part of a continuous graphene production process.

[0059] A number of polar liquids have been tested, such as water, ethanol and acetone, and all were found to work well. The decupling process as tested in water is also very robust in relation to the conditions under which it will work. For instance, the presence of contaminants, we found to have no observable effect. The decoupling works well in deionised water, tap water and even water which has been deliberately dosed with up to 0.7 M of sodium chloride. The presence of calcium and lithium chloride also have no observable effect upon the decoupling. Although in use it would be preferred to use tap water, this being the best balance between cost and purity, it is not essential to use tap water or high quality water.

[0060] The instant decoupling was also found to be independent of temperature. The results in water at 4°C, at room temperature and at higher temperatures all exhibited instant decoupling. In boiling water, the film decoupled from the substrate well, but some agglomeration of the film was observed due to mechanical agitation of the boiling water.

[0061] The free graphene film can then be transferred to any other desired sheet material . Graphene could be deposited onto polymers, paper, silicon, inorganics or many other substrates. For example, the released graphene can be transferred onto a polyethylene (PET) sheet or mesoporous alumina discs.

[0062] In terms of the mechanism, but without wishing to be bound by theory, it appears that the surface energy of the copper substrate is strongly influenced by the exposure to the hydrogen containing (H₂+Ar) plasma which modifies its surface from a polycrystalline predominantly (100) surface to a passivated surface having (101 ) and (111 ) facets. This structural change is thought to effect the wetting properties. [0063] As a result of the plasma pre-treatment step of the present invention, the wettability of the copper surface is effectively changed from slightly hydrophobic (water contact angle ~ 70°) to hydrophilic (-6°) when it is exposed to a hydrogen containing (H₂+Ar) plasma. Thus the plasma pre-treatment step is considered to be a hydrophilising step.

[0064] As mentioned, it is believed that the relatively high wettability of the pre-treated copper foil originates from the change in its nanoscale crystallographic features (Figure 1 a) which occur during the hydrogen plasma treatment. This theory was confirmed by the inventors who deposited graphene films onto copper foil just after a few seconds of H₂+Ar plasma pre-treatment and compared this with films deposited onto copper foil with no pre-treatment. In these cases, the observed change in wettability of copper foil was very small and the film did not readily decouple from the copper surface when it was submerged in water. The change in nanoscale crystallographic features on copper foil in these processes is relatively small due to significant carbon species impingement and graphene film growth on the surface.

[0065] Whilst the copper substrate becomes hydrophilic in nature, the deposited graphene film remains highly hydrophobic (contact angle -120°) (Figure 2g) which thus facilitates its decoupling. That is, the copper surface interacts strongly with water after the deposition process, while the bottom layer of the graphene film only interacts weakly with water. As the substrate is dipped into water at room temperature, due to reverse interaction strength, water molecules are likely to intercalate and quickly diffuse between the film and copper surface (Figure 2g). This will substantially reduce the weak graphene-copper bond by the screening effect and decouple the graphene film from the copper substrate.

[0066] Once exfoliated (decoupled), graphene films can then be readily transferred from the water onto a secondary substrate, ready for assembly into a device, sensor, or other end use applications.

[0067] Using the method of the present invention, a range of chemically diverse secondary substrates (paper, polymer, glass, ceramics) are suitable. The method of the present invention avoids the use of wet chemistry or a polymeric transfer material.

[0068] The decoupling and transfer process of the present invention does not require either a coating of carrier material or solvent-rinsing which could damage the graphene structure.

[0069] The copper foil can be reused many times to produce transferable graphene films of similar quality with only minor treatment.

[0070] The process of the present invention is also highly energy efficient relative to alternative processes. [0071] The present invention uses only 21 % of the energy of conventional thermal CVD processes and only 46% of the energy of reported plasma enhanced CVD processes.

[0072] These calculations do not include the energy used in running the rotary pump for the extra time in the thermal CVD and previously reported PECVD processes, so the actual efficiencies of the present process would actually be better than the values given above.

[0073] Thus, the present invention provides an approach for preparing, decoupling and transferring the deposited graphene films which is instant, gentle, relatively cost-effective and environmentally friendly in terms of energy and material efficiency.

[0074] Additionally, it was found that the pre-treatment process of the present invention allows for modification of the subsequent deposited graphene, and in particular, facilitates the formation of some novel graphene architectures.

[0075] The present invention also provides a scalable fabrication of a high quality highly integrated 3-D vertical graphene networks in form of graphene "microwells" on copper Cu foil in a simple, low temperature plasma based process. The networks comprise high structural integration among a few vertically-stacked graphene layers and also between the horizontally and vertically orientated graphene layers without use of any passive and massive wet-chemicals or binders. The graphene microwell films are not only optically transparent (76% at a wavelength of 550 nm), but also exhibit mechanical and structural integrity that remains nearly unaltered after repeated deformations. The plasma process which enables the simultaneous growth and integration of a continuous horizontal graphite film and vertical petal like graphene layer in 3-5 minutes at temperature <270°C, is at least 50% more energy efficient than previously reported plasma CVD methods.

[0076] The highly integrated 3-D graphene networks may be further described as comprising a plurality of horizontally stacked planar carbon layers, wherein at least one upper carbon layer is "curled up" to form a vertical nanosheet, and wherein the adjacent vertical nanosheets form a petal-like vertical graphenes (PVG) structure, thereby to define a microwell.

[0077] By "curled up" one means that the upper layer (s) protrudes upwards or could be puckered to form a vertical nanosheet.

[0078] The present application also allows for control over certain morphologies such as (integrated) nano-connectivity and thickness of the vertical graphene layer, properties which together provide a high density of long reactive open graphene edge planes (>0.4 km/cm²) in the vertical orientation.

[0079] In the present invention in which CH₄ is mixed with H₂ and Ar, horizontal graphene layers integrated with petal-like vertical graphene layers are deposited on the foil surface (Figure 1 e,f). In the absence of the pre-treatment step, lower structural quality deposited petal-like vertical graphene (PVG) films (Figure 2h) are obtained. That is, the direct and immediate exposure of methane plasma to the copper does not modify the copper surface. The higher index facets of copper are considered to stimulate the nucleation and enhance the growth rate of graphene on copper foil. The present method thus provides an ultrafast growth of 3-D networks of dispersed vertical graphene onto a continuous horizontal graphene layer film after CH₄ exposure. The substrate temperature during the growth of graphene microwells is low, preferably below 300°C, more preferably below 280°C. As explained above, the process of the present invention is much more energy-efficient than previously reported processes for the graphene deposition.

[0080] The films produced by this method comprise of integrated networks of horizontally and vertically orientated graphene layers that form during a low power, low temperature gas-plasma process using a carbon source plasma such as a obtained from a CH₄+H₂ ⁺Ar gas mixture, onto polycrystalline copper foils. No external heating is required and the copper surface temperature, preferably in the range of 180- 270°C, is achieved as a result of only plasma heating.

[0081] The plasma-assisted activation of the copper foil not only facilitates exfoliation, decoupling of the graphene, but also imparts high quality morphology to the deposited films.

[0082] The highly integrated, inter-connected vertical graphene layers of the present invention together form micron/sub-micron cup-like features, which are potential containers for isolating biochemical and/or chemical species such as cells, proteins, organic solutes and the like.

[0083] The thickness of the petal-like vertical graphene nanosheets of the present invention is controlled by the concentration of H₂ with respect to CH₄ (in terms of partial pressure) in the gas mixture of the plasma deposition process. The Ar in the gas mixture also played an important role for the growth of the graphene microwell. At high Ar concentration (>20%) in gas mixture, a continuous layer of horizontal graphene is compromised and the growth of petal like vertical graphene is preferred. This is possibly due to the damages at the initially deposited horizontal graphene layer film caused by excessive Ar ions bombardment, which creates a number of defect sites and the vertical graphene nanosheets eventually grow from those sites. With the increased partial pressure of CH₄ in terms of concentration in the gas mixture, the impingement rate of carbon species on the substrate surface increases which leads to a fast growth rate. However, it also increases the number of sp³-defects in the graphene film. Therefore, the interplay between the impingement rate of carbon species and the growth and etching rates of graphene film is crucial to control of the thickness and quality of graphene microwells.

[0084] The vertical 3-D networks of the present invention are quite unique in that they provide graphene sheets which are self supported in two dimensions, yet do not have any junctions in transitioning from a horizontal plane. There is a continuous path for electrons from the horizontal to the vertical and it is seen that these graphene wells have high reactivity at the top edges of the well. [0085] The vertical 3-D networks produced by the present invention remain robust during decoupling and after the transfer to chemically diverse targeted surfaces. Their high mechanical strength is attributed to their structural integrity, that is, the vertical petal-like nanosheets are simultaneously interconnected to the horizontal graphene film. Any shear force exerted on the film during the subsequent decoupling transfer process is likely to be distributed among the vertical inter-connections and the tendency to scroll up is thus suppressed even in the absence of any material coating. Further, the copper substrate can again be re- used for deposition of similar quality of graphene microwells.

[0086] The structurally robust integrated vertical 3-D can thus be readily transferred to a range of chemically diverse targeted substrates (paper, polymer, glass, etc.) without use of wet chemistry or polymeric transfer materials and retained their properties after bending several times at 1 mm radius of curvature.

[0087] These structurally robust integrated vertical 3-D networks can be used in the electronics field as they have been shown by the inventors to have excellent electrical properties, such as sheet resistance comparable to transferred horizontal graphene films produced in plasma and thermal CVD processes at elevated temperatures. Furthermore the inventors have shown that even after deformation (bending) excellent sheet resistance is sustained. Usually, the sheet resistance of a horizontal graphene films produced in plasma CVD system is high (> 1 KOhm/cm²) due to a large number of defective sites which trap the charge carriers. It appears that the structures of the present invention significantly reduce charge carrier trapping sites in both horizontal and vertical graphene layers which in turn reduces overall sheet resistance, providing higher vertical and horizontal in-plane carrier mobilities.

[0088] The transferred films can also be used as sensors, particularly gas sensors, such as for nitrogen dioxide (N0₂) gas sensing. N0₂ is a hazardous gas in the environment and continued or frequent exposure may cause increased incidences of acute respiratory illness. Therefore, its detection at trace levels is important. The graphene microwell films, integrated into real nano-devices, provide high performance sensors for gas detection.

[0089] In summary, a scalable production of a highly integrated 3-D vertical graphene network in the form of graphene microwells on copper foils has been demonstrated in a simple, low temperature plasma-based process. The networks comprise high structural integration among vertically-stacked graphene layers and the horizontally orientated layers of graphene without use of any passive and massive wet-chemicals or binders. The plasma process also enables the facile decoupling of the high- quality film from the copper substrate when it is exposed to water at room temperature. This leads to a relatively cost-effective and environment-friendly transfer process. These graphene microwell films are not only optically transparent (76% at a wavelength of 550 nm), but also exhibit mechanical and structural integrity that is robust enough to retain its reactivity, as well as physical and electric properties after repeated deformation. The production process is at least 50% more energy efficient than previously reported plasma CVD methods. These films exhibit excellent sensing properties in dry environments for hazardous N0₂ gas. These findings are promising for the applications of graphene microwells in a range of high performance flexible, transparent and ultra-light nano-devices.

Experimental Details:

Surface Pre-treatment and Graphene Deposition:

[0090] Polycrystalline copper foils (dominated by (100) plane) of 0.125 mm thickness were used as substrates. Changing the thickness of the substrate was not observed to result any difference in the resultant graphene film or the ease with that could lift off. The foils were cleaned by gentle washing using ethanol to remove any surface contamination and loaded into an inductively coupled plasma CVD reactor. After achieving a high-vacuum environment (1 x 10^~4 Pa), the foil was exposed to a plasma discharge in a mixture of Ar and H₂ gases (ratio 1 :3) for 2 min and then CH₄ gas was loaded with Ar and H₂ gases for 3 minutes for deposition of a graphene microwell film.

[0091] Polycrystalline copper foils (thickness 0.125 mm, annealed, 99.8% purity) were purchased from the Advent Research Materials Ltd UK. A single piece of copper foil (3.0 x 3.0 cm²) was mounted on a stage. The stage was then loaded into the plasma-enhanced CVD chamber and elevated to allow the foil to be in direct contact with the plasma during the process (as shown in Figure 4).

[0092] After achieving a high vacuum (~ 1 x 10^~4 Pa), Ar+H₂ gas mixture (ratio 1 :3) was fed into the chamber to generate the plasma at RF power and chamber pressure of 400 W and 2.5 Pa, respectively. After 90 sec of the treatment, CH₄ was injected into the chamber and the RF power was increased to 680 W, while the chamber pressure was maintained at 2.5 Pa.

[0093] Measurement of the substrate's wettability and hence hydrophilicity was made by measuring the water contact angle of the substrate, both before and after activation. The contact angle for the hydrophilic surface changes with time. This is because of the air-pockets between the water and the surface. As the air-pockets escape with time (within few minutes), the contact angle decreases rapidly due to wettability properties of the surface. However, the contact angle does not change much on the hydrophobic surface due to the surface properties.

[0094] Therefore, the contact angle is measured after 5 minutes after a water droplet is placed on the copper surface. A very high resolution camera is used to take the photo of a water-drop on copper foil and the Image J software is used to measure the contact angle. Figure 8 represents a typical photograph.

[0095] A clean chamber was required in order to obtain graphene films that could be easily transferred or have highly structured graphene microwell films. Graphene Transfer:

[0096] After the deposition process on the activated substrate is completed, the copper foil and deposited graphene are submerged in water, preferably deionised water at ambient temperature (22°C), upon which the graphene instantly decoupled from the foil and lifted-off onto the water surface (Figure lb). The copper foil sank.

[0097] The isolated graphene film resting on the water surface could then be transferred to any other desired material simply by sliding the substrate below the suspended graphene and lifting the graphene from the water. For example, in this way, the released graphene could be transferred onto a polyethylene (PET) sheet. The graphene exhibited almost 76% transparency at an optical wavelength of 550 nm (Figure 1 c).

[0098] The copper foil was re-used for several sequential experiments. Minor cleaning was required between each experiment. For example, after loading the foil into the plasma CVD chamber, the foil was first treated with hydrogen plasma at 200 W for 5 minutes to etch away any carbon left over on the foil during the transfer process. Figure 3 shows the micro-Raman spectra collected from the graphene films after the use of the foil for two deposition processes, which shows a small fraction of change in peak intensity ratio and indicates almost similar quality of deposited graphene.

Graphene Microwell structural analyses:

[0099] Microscopy and spectroscopy: The graphene microwell films were characterised by SEM (Zeiss Auriga field emission SEM, operated at 5 kV) equipped with EBSD (NordlysNano EBSD detector, Oxford Instruments) and TEM (JEOL 3000F, operated at 300 kV). Micro-Raman spectra were collected at room temperature using a Renishaw in Via confocal Raman spectroscope (with a laser source 514 nm).

[00100] The structural quality and integrity of graphene microwell films were analysed and confirmed by collecting micro-Raman spectra (Figure 2i) from the film before (spectrum 2) and after the transfer process (spectrum 3). The graphitic G-peak at 1579 cm^"1 is associated with the doubly degenerated phonon mode at the Brillouin zone centre. However, the 2D-peak at 2702 cm^"1 corresponding to second- order phonon scattering at the zone boundary with a broad full width half maximum (FWHM) of 41 cm^"1 suggests the misoriented out-of-plane stackings in few graphene layers'²²¹. This also indicates the weaker interaction between stacked layers and larger interplanar distance than in graphite, as also shown by TEM image (Figure 2c). The intensity ratio (/_2D /fc ) of -0.9 further confirms that the graphene microwell material is composed of a few graphene layers. There is also a disorder D-peak observed at 1352 cm^"1 in the spectra which is most likely due to the presence of highly dense open graphitic edges and possibly few structural defects in the graphene layers.

[00101] An angle-viewed scanning electron microscopy (SEM) image, Figure 2a, shows the structural integration among the vertical petal-like graphene layers in a graphene microwell film on a PET sheet after the transfer process. These inter-connected graphene layers together form micron/sub-micron cuplike features, which are potential containers for isolating biochemical and/or chemical species such as cells, proteins, etc. A high-resolution SEM image (Figure 2b) shows that these graphene layers become thinner and almost transparent towards the top edges. Further, the transmission electron microscopy (TEM) analyses reveal that the top of graphene microwell consist of 3-4 layers of atomic carbon layers with an interlayer distance of -0.361 nm which is larger than that in graphite (Figure 2c). This suggests weaker-bonding existing among the layers and individual graphene layer terminating one after another, creating thinner petal-like sheets as well as the open graphene edge planes (as indicated by arrows in Figure 2b,c). Thus, the produced graphene microwells consist of a high density of open planes (>0.4 km/cm²) and, depending on their zig-zag and/or armchair¹¹⁷¹ configuration, they may possess localised higher density of states near Fermi-level¹¹²'¹⁷¹ and induce higher surface activity compared to a graphene basal plane. The electron diffraction pattern (Figure 2d) also confirms the presence of a few graphene layers in the graphene microwell film.

[00102] Further analyses were performed to reveal the structural integration between the horizontal and vertical graphene layers in the graphene microwell film. Figures 2e,f show that the two neighbouring graphene layers parallel to the substrate surface are curled up to form a vertical nanosheet. This suggests that horizontal thin graphene layers are first deposited on the Cu substrate due to the catalytic effect and the top few graphene layers on the horizontal graphene layer film curl up possibly due to the plasma-assisted field effect. After detaching from the native thin layers, the graphene layers grow up in a vertical direction in the presence of continuous delivery of carbon species coalesce with neighbouring curled graphene layers and form petal-like vertical nanosheets (as shown in schematic in Figure 2g). Further, these petal-like vertical graphene sheets coalesce with neighbouring nanosheets and form a network. Thus, the vertical stacking of graphene layers is the integral part of horizontally-stacked graphene layers. Such integration provides not only very reliable mechanical stability but also allows efficient electron kinetics from vertical to horizontal graphene layers where the resistive path is ideally from the single graphene layer rather than their interfaces.

[00103] The thickness of the petal-like vertical graphene nanosheets is controlled by the concentration of H₂ with respect to CH₄ (in terms of partial pressure) in the gas mixture. As two graphene layers coalesce with each other, the nanosheets in the vertical growth direction (due to the localised directional electric field of the plasma sheath) become thinner (consisting of 3-4 atomic carbon layers) as they grow further in the presence of high partial pressure of H₂ (concentration was >70%) in gas mixture during the deposition process (Figure 2e). This could be attributed to dominant fast etching and saturation of the outer graphene layer in the presence of hydrogen species. As the CH₄ concentration is increased (i.e. reduced H₂ pressure), the impingement of carbon species increases rapidly which not only causes a higher growth rate but also thicker nanosheets (Figure 2f). However, the Ar in the gas mixture also plays an important role for the growth of the graphene microwell. The growth of petal-like vertical graphene was observed without any continuous graphene film at its bottom (Figure 2h) under conditions of high Ar concentration (>20%) in the gas mixture. This is believed to be due to the damage at the initially deposited horizontal graphene layer film caused by excessive Ar ion bombardment, which creates a number of defect sites and the vertical graphene nanosheets eventually grow from those sites'⁶¹. With the increased partial pressure of CH₄ in terms of concentration in the gas mixture, the impingement rate of carbon species on the substrate surface increases which leads to a fast growth rate. However, it also increases the number of sp³-defects in the graphene film. Therefore, the interplay between the impingement rate of carbon species, the growth and etching rates of graphene film is crucial to control of the thickness and quality of the graphene microwells.

[00104] The vertical 3-D networks remained robust during decoupling and after the transfer to chemically diverse surfaces. This is attributed to its structural integrity where vertical petal-like nanosheets are a integral part of the horizontal graphene film and are simultaneously interconnected to each other. Any shear force exerted on the film during the decoupling and transfer process is likely to be distributed among the vertical inter-connections and a tendency to scroll up is easily avoided even in the absence of the carrier material coating. Further, the copper substrate could again be used for deposition of similar- quality graphene microwells.

[00105] After the transfer process, the intensity ratio and peak position of all peaks did not change noticeably. This suggests that water did not diffuse in the graphene microwell film during the transfer process and the structural quality and integrity of the film remained almost unaltered. Figure 2i also shows a Raman spectrum collected from petal-like vertical graphene film which was not integrated with horizontal graphene sheets (spectrum 1 ).

[00106] The graphene microwell structure is shown in Figure 5. In addition, Figure 5 shows individual salt crystals which can be stored in the cup-like structures which are formed by the petal-like vertical graphene, integrated together (dotted circle).

[00107] To demonstrate the applicability of cup-like structures formed by the integrated petal-like vertical graphene layers, a small amount of phosphate buffered saline (PBS) solution was dropped onto the graphene microwells and allowed to dry. PBS solution was chosen as it is a buffer solution commonly used in biological and biochemical research. After the drop had dried, it was observed that the salt crystals were formed in individual cup-like containers and petal-like vertical graphene layers separate them. Thus, the graphene microwell films could be used as micro-containers to store, contain and/or separate biological cells, clusters of proteins, chemicals, etc.

Electrical transport properties:

[00108] The films were transferred onto polyethylene (PET) sheets to measure electrical transport data. The electrical resistance data for the films were carefully taken at different bending curvatures using two- probe methods. Figure 6a shows the electrical resistance of graphene microwell electrodes compared to a non-integrated one (at a film transparency of 76%) from flat to bent at a radius of curvature of 1 mm after 10 cycles. The sheet resistance of graphene microwells (-350 Ohm/cm²) is relatively lower than any transferred horizontal graphene films produced in plasma CVD and comparable to horizontal graphene produced in thermal CVD processes at an elevated temperature (1000°C). In comparison, petal-like vertical graphene films show a relatively higher resistance of 1900 Ohm/cm². Upon bending the films (see Figure 1d), the sheet resistance of the films increased up to 410 and 2300 Ohm/cm² at a bending radius of 1 mm for graphene microwells and petal-like vertical graphenes, respectively. Usually, the sheet resistance of a horizontal graphene films produced in plasma CVD systems is >1 KOhm/cm² due to abundance of defect sites which trap the charge carriers. When the vertical graphene nanosheets are deposited at the defect sites in the process, the overall sheet resistance is likely to increase due to the junction tunnelling effects. In the case of the present invention, the vertical petal-like nanosheets are highly integrated and constitute the bending parts of horizontal graphene layers. Therefore, this integration almost eliminates the charge carrier trapping sites in both horizontal and vertical graphene layers and reduces overall sheet resistance. This suggests that the graphene microwell films have high vertical and horizontal in-plane carrier mobilities.

[00109] The presence of a reasonable number of defect sites on the horizontal graphene layers at the bottom of graphene microwells which still contribute to the film resistance (350 Ohm/cm²) cannot be neglected. While the graphene microwells and petal-like vertical graphene films recovered almost 81 % and 42% conductivity, respectively, after the bent position, there was only <5% deviation in the film resistance value from 1st to 2nd cycle of bending and <0.1 % to 10th cycle (data is not shown here). Thus, graphene microwell films without any external binding materials show a great potential as a functional electrode material for flexible and transparent nano-devices.

Gas-sensing device (dry environment):

[001 10] For assessing their potential as sensors the films were transferred onto mesoporous alumina discs as required for the assembly into a real gas sensing device. All sensitivities of the films were measured in terms of their charge conductance at room temperature using the experimental method reported before¹²⁴¹. It was found that the graphene microwell and petal-like vertical graphene films showed a detectable response to the gas down to about 5 ppm (Figure 6c). When the flow of N0₂ is turned off, the conductance of film slowly decreases. The conductivity of the sensors increased in response to 5 minutes exposure to 50 ppm concentrations of N0₂. However, the sensitivities of the graphene microwell films were observed almost one order higher than petal-like vertical graphene films. This also suggests that the defects in the graphene microwells are minimal with a fast charge transport between the two integrated vertical graphene layers and along the horizontal and vertical graphene layers (Figure 6b), and are uninfluenced by the presence of small amount of adsorbed molecules. However, in petal-like vertical graphene films, the charge carriers are trapped at the junctions and/or defective sites and the presence of ad-molecules has little effect on their transport properties which results in low sensitivity. Thus, the graphene microwell films are high performing functional materials for nano-devices for detecting gases in dry environments. [001 1 1] Gas-sensing measurements were performed using the device shown in Figure 7. Two separate sensing platforms were fabricated by transferring the graphene microwell and petal-like vertical graphene films onto porous anodic alumina templates (pore diameter 200 nm) and attaching two electrodes (6.0 mm apart) on each platform. A chemiresistor platform was prepared by cutting a sample of the size of 5 x 3 mm² and attaching it to a filament holder using silver epoxy. The platform was then inserted into the sensing device which had a volume of- 60 cm³. Dry nitrogen was used as a buffer gas. A mixture of 50 ppm N0₂ in nitrogen (Spectra Seal, BOC Limited) was further mixed with pure dry nitrogen buffer gas to obtain N0₂ concentration between 50 and 5 ppm. The electrical resistivity measurements were performed using a Keithley picoammeter (model 6487) and current limit was set at 10 nA to minimise/avoid Joule heating of the sample.

Energy efficiency:

[001 12] The process of the present invention is also highly energy efficient relative to alternative processes. The energy consumption for various graphene processes was calculated for

• the plasma enhanced CVD (PECVD) process of the present invention;

• a conventional thermal CVD (TCVD) process for growth of graphene; and

• previously reported PECVD process for growth of vertical graphene nanosheets.

[001 13] The calculations are based on the assumptions that all parts and components are same for PECVD and TCVD processes, except that an extra RF power source and a turbomolecular pump are used in the PECVD process and a thermal furnace is used in TCVD process.

[001 14] Conventional TCVD process takes almost 20 minutes for the deposition of a high quality of graphene film. Previously reported PECVD processes'⁵'¹²¹ use a similar turbomolecular pump. It is also assumed that the processes required 20 minutes extra time to run the pump necessary to cool down the substrate after its temperature reached at least 800°C due to use of external heating source ^{[ 2]}.

[001 15] Energy consumption in the PECVD process of the present invention for growth of graphene (total time 6.5 minutes):

RF power= 600 W (for 1.5 minutes for H₂+Ar treatment using 400 W) + 2100 W (for 3 minutes for CH₄+H₂+Ar treatment using 700 W) = 2700 W.

Turbomolecular pump= 4320 W (for 2 minutes before plasma process to reach maximum rotation speed at a voltage of 240 V and at an average current of 9A) + 3240 W (for 4.5 minutes at a voltage of 240 V and current 3.0 A; during this plasma process period turbomolecular pump has maximum rotation and uses less current) =7560 W.

Total energy consumption = 10260 W.

[001 16] Energy consumption in conventional thermal CVD process (total time 20 minutes): Thermal furnace power= 50000 W (for 20 minutes to heat the furnace from room temperature to at least 900°C and to deposit the graphene film; commonly used thermal furnace requires 2.5 kW power source) Total energy consumption = 50000 W.

[001 17] Energy consumption in previously reported PECVD processes for growth of vertical graphene nanosheets (total time 24 minutes):

Microwave power= 2000 W (for 2 minutes for the growth using 1000 W^{[ 2]}).

Heating system= 1400 W (for 7 minutes; to heat up the substrate up to 800°C within 5 minutes and to keep the temperature for 2 minutes during the growth; it is assumed that heating unit withdraw 10 A current at 20 V).

Turbomolecular pump= 4320 W (for 2 minutes before plasma process; to reach maximum rotation speed at a voltage of 240 V and at an average current of 9A) + 14400 W (for 20 minutes at a voltage of 240 V and current 3.0 A; during this plasma process period turbomolecular pump has maximum rotation and uses less current) = 18720 W

Total energy consumption = 22120 W

[001 18] Thus, the present invention uses only 79% of the energy of conventional thermal CVD processes and only 53% of the energy of reported PECVD processes.

[001 19] These calculations do not include the energy used in running the rotary pump for the extra time in the TCVD and previously reported PECVD processes, so the actual efficiencies of the present process would actually be better than the values given above.

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Claims

1. A method of preparing a free or transferrable graphene sheet comprising the steps of:

a) pre-treating the surface of a polycrystalline copper substrate with a Hydrogen-containing plasma to enhance the relative proportion of (101 ) and (111 ) facets relative to (100) facets; b) depositing graphene onto the substrate by contacting the surface of the substrate with a plasma comprising a carbon source gas for a deposition period;

2. A method according to any one of the preceding claims wherein the proportion of (101 ) and (111 ) facets, in combination, relative to (100) facets is increased to at least 4%.

3. A method according to any one of the preceding claims wherein the proportion of (101 ) and (111 ) facets, in combination, relative to (100) facets is increased to at least 15%.

4. A method according to any one of the preceding claims wherein pre-treating the surface

passivates the substrate surface.

5. A method according to any one of the preceding claims wherein during pre-treating the substrate has a surface temperature maintained solely through direct plasma heating.

6. A method according to any one of the preceding claims wherein during pre-treating the substrate has a surface temperature maintained in the range 90-200°C.

7. A method according to any one of the preceding claims wherein the Hydrogen containing plasma is derived from Hydrogen gas alone or from a mixture of Argon and Hydrogen.

8. A method according to any one of the preceding claims wherein the Hydrogen containing plasma is derived from a mixture of Argon and Hydrogen.

9. A method according to any one of the preceding claims wherein the Hydrogen-containing plasma is derived from a 1 : 10 mixture of Argon and Hydrogen gases.

10. A method according to claim 9 where the mixture of Argon and Hydrogen gases is fed at 0.8 to 10 Pa.

11. A method according to any one of the preceding claims wherein the pre-treating is carried out for at least 10 seconds.

12. A method according to any one of the preceding claims wherein the pre-treating is carried out for at least 30 seconds.

13. A method according to any one of the preceding claims wherein the Hydrogen-containing plasma is produced by RF power of about 100-800W.

14. A method according to any one of the preceding claims wherein during depositing graphene, the substrate has a surface temperature maintained solely by direct plasma heating.

15. A method according to any one of the preceding claims wherein during depositing graphene, the substrate has a surface temperature maintained in the range 200-300°C.

16. A method according to any one of the preceding claims wherein the carbon source gas

comprises a gas selected from the group consisting of methane, acetylene, ethylene, and fatty acid monomers.

17. A method according to any one of the preceding claims wherein the carbon source gas

comprises methane.

18. A method according to any one of the preceding claims wherein he deposition period is 2-10 minutes.

19. A method according to any one of the preceding claims where the carbon source gas is fed at 1.0 to 5.0 10 Pa.

20. A method according to any one of the preceding claims wherein during the deposition the plasma is produced by RF power of about 650-900W.

21. A method according to any one of the preceding claims wherein the pre-treating and deposition are carried out in a continuous single process for production of transferrable graphene.

22. A method according to any one of the preceding claims wherein the polar liquid is water

23. A free or transferrable graphene sheet prepared by the method of any one of the preceding claims.

24. A graphene structure comprising a plurality of stacked planar carbon layers, wherein at least one layer of the structure comprises integrated horizontal and vertical nanosheet portions.

25. A graphene structure according to claim 24 wherein a portion of at least one stacked carbon layer forms a first vertical nanosheet portion, and wherein the first vertical nanosheet portion is supported by at least one second vertical nanosheet portion

26. A graphene structure according to claim 24 or 25 wherein a portion of at least one stacked carbon layer forms a first vertical nanosheet portion, and wherein the first vertical nanosheet portion is supported by at least one second vertical nanosheet portion and wherein the first and second vertical nanosheet portions in combination with a plurality of adjacent vertical nanosheet portions form a graphene microwell.

27. A graphene structure according to any one of claims 24 to 26 wherein the vertical nanosheet portion is connected to a horizontal nanosheet portion without an intervening junction

28. A graphene structure according to any one of claims 24 to 27 wherein a plurality of adjacent vertical nanosheet portions form a petal-like vertical graphene (PVG) structure in an X-Y plane, thereby to define a T-junction microwell in a X-,Y- and Z- plane.

29. A graphene structure according to claim 28 wherein the size of the microwells in X-Y plane is 1- 5μιη.

30. A graphene structure according to claim 28 or 29 wherein size of the microwells in the X-Y plane is 1-2μιη.

31. A graphene structure according to any one of claims 24 to 30 comprising a plurality of

microwells.

32. A graphene structure according to claim 31 comprising a plurality of interconnected microwells, wherein adjacent microwells have a common portion of a vertical petal-like ridge.

33. A graphene structure according to claim 31or 32 wherein the plurality of microwells defines a substantially regular honeycomb structure.