WO2013038130A1

WO2013038130A1 - Method for producing graphene

Info

Publication number: WO2013038130A1
Application number: PCT/GB2012/000719
Authority: WO
Inventors: David Andrew EVANS
Original assignee: Aberystwyth University
Priority date: 2011-09-14
Filing date: 2012-09-14
Publication date: 2013-03-21
Also published as: GB2511434A; GB201404539D0; GB201115865D0

Abstract

A method for producing graphene comprises:- (i) providing a layer of catalyst on a diamond (111) surface, (ii) heating the diamond and/or catalyst layer with a heat source until a desired thickness of graphene is produced on the exposed surface of the catalyst.

Description

METHOD FOR PRODUCING GRAPHENE

The present application relates to compositions comprising graphene and methods for producing graphene.

Graphene is formed of single atomic planes of carbon with a structure similar to a single plane of graphite. Graphene's unique electronic, spintronic and optical properties has made it the focus of much attention since first demonstrated in material exfoliated from crystalline graphite (Novoselov, 2004). The combination of hig electrical conduction, optical transparency and spin transport properties (Tombros e( al, 2007) makes it attractive for many , applications from photovoltaic cells to spin valves. Despite intense activity, there remain challenges in producing high quality material in large areas, both free-standing and on a range of substrates.

High quality graphene can be produced on a variety of substrates, and this has facilitated the observation of fundamentally new phenomena (Bostwick, 2010; Lizzit ei al, 2010). The most widely used substrates are single crystal metals such as rhodium and iridium (Wintterlin, 2009). However, not only are they unsuitable for large-scale commercialization, they are fundamentally problematic for devices that make use of the surface transport properties due to charge transport through the conductive substrates (Hofmann & Wells, 2009).

Semiconducting substrates, in particular SiC, offer a suitable alternative since they also enable high quality grapliene growth (Geim, 2009). Additionally, the bulk doping can be selected such that the graphene transport properties dominate a device/measurement. However, there are many fundamental challenges to the industrialization of this process (Bostwick, 2009, Dimitrakopoulos et al, 2010). These include the high temperatures and the complex chemistry that can lead to the formation of competing carbon-rich stable surface phases.

Carbon, 49, 1006- 1012, (201 1 ). relates to the production of multilayer graphene gron by precipitation upon cooling of nickel on diamond. The document discloses that multilayer graphene is grown by precipitation upon cooling of a thin nickel film deposited by e-beam evaporation on single crystal diamond (001 ) orientated substrates. The authors claim that a nickel layer on (00 I ) diamond extracts carbon at high temperatures and releases it only when cooled to room temperature. In addition, the authors were not able to determine the onset temperature. In the disclosure, there is no strong evidence that high quality graphene can be grown and there is no evidence for the crystallinity of the graphene or the nickel (no diffraction or photoelectron methods). The disclosure does not disclose the steps needed in order to control the formation of one, two or more layers of graphene. Furthermore, there is no disclosure of measurement of the orientation of the graphene planes to the diamond surface.

There is, therefore, a need for improved and alternative compositions comprising graphene and methods of producing graphene.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method for producing graphene. the method comprising:-

(i) providing a layer of catalyst on a diamond (1 1 1) surface,

(ii) heating the diamond and/or catalyst layer with a heat source until a desired thickness of graphene is produced on the exposed surface of the catalyst.

Surprisingly, it has been found that the methods of the present invention can be used to produce graphene of a very high quality and large area af temperatures which are realistic for industrial scale use.

This is particularly the case in view of previous investigations into the metal catalysed graphitization of diamond (Fyfe, 2001) where it was concluded that it would be unlikely that the graphite formed from the graphitization of a (H I) diamond would be uniform. Rather, based upon the experiments performed, which were aimed at studying the activation energy of various surfaces, the graphitic layer would be expected to exhibit large eruptive features widel distributed across the surface. In direct contrast to the results expected by Fyfe w»j_> ^lt P"»GI H ϋΐ^γαιωυ nave r

been able to produce very high quality graphene using a diamond (111) surface and a layer of catalyst, with the methods used relying on the diamond and/or catalyst layer being heated until a desired thickness of graphene is produced. By controlling the process in this way. a layer of high quality and large area graphene can be produced.

Preferably, the formation of graphene on the exposed surface of the catalyst is monitored and tlie heat source is removed when a desired thickness of graphene has been produced. Preferably, the formation of graphene ou the exposed surface of the catalyst is monitored in situ.

The formation of graphene on the exposed surface of the catalyst can be monitored in situ using techniques involving the use of light (including, for example, lasers and infrared), x-rays, electrons, ions photoeleclrons and scanned probes.

Preferably, tlie formation of graphene on the exposed surface of the catalyst is monitored in situ using one or more of x-ray, optical or electron based techniques.

Preferably, the formation of graphene on the exposed surface of the catalyst is monitored in situ by one or more of X-ray Photoelectron Spectroscopy (XPS), REal-time Electron Spectroscopy (REES), Low Energy Electron Diffraction (LEED), Scanning Tunneling Microscopy (STM) and Angle-Resolved Photo-Electron Spectroscopy (ARPES).

Preferably, the diamond and/or catalyst layer is heated for a period of time which has been calculated to produce a defined thickness of graphene, preferably based upon calibration experiments. For example, it will be within the expertise of a skilled person to conduct a number of calibration experiments to calculate tlie length of heating required to produce a particular thickness of graphene.

Preferably, the catalyst is a transition metal catalayst. Preferably, the catalyst is selected from iron, cobalt and nickel. Further preferably, the catalyst is selected from iron and cobalt. Most preferably, the catalyst is iron.

Preferably, the catalyst is provided at a thickness of at least about one monolayer, preferably at least about Inm, preferably between about I nm and about 15nm, preferably between about Inm and about 5 nra, preferably between about ! nm and about 3 nm.

The layer of catalyst can be deposited on the diamond surface at a temperature as high as the reaction temperature. For example, in some embodiments, the catalyst can be deposited at a temperature of between about -200°C and about 700°C. In some embodiments, the layer of catalyst is deposited on the diamond surface at a temperature of between about 10°C and about 30^CC, preferably between about 15°C and about 25°C, most preferably at about 2 TC.

Preferably, the layer of catalyst is deposited on the diamond surface in a vacuum.

Preferably, the diamond and/or catalyst layer is heated at a graphitisation temperature at which the formation of graphene occurs, preferably at a temperature of between about 500°C and about 750°C, preferably between about 600°C and about 700°C, preferably at least about 675°C.

Preferably, the diamond and/or catalyst layer is heated at a graphitization temperature until a desired thickness of graphene is formed on the exposed surface of the catalyst. For example, in some embodiments, the diamond and/or catalyst layer is heated at a graphitization temperature for between about 500s and about 900s, preferably between about 600s and about 800s, preferably between about 700s and about 800s, preferably about 750s.

Preferably, the diamond and/or catalyst layer is heated to a graphitization temperature by increasing the temperature at a rate which allows the formation of graphene on the exposed surface of the catalyst to be monitored in sifu.

Preferably, the diamond and/or catalyst layer is heated to a graphitization temperature by increasing the temperature at a rate of between about 0.5°C s^"1 and about 10 °C s^"'. preferably, between about 1°C s^'1 and about 5 °C s^'1, for example between about 1°C s^'1 and about 2 °C s^'' .

Preferably, once the desired thickness of graphene has been produced, the, heat source is removed and the diamond/catalyst graphene layer sample is allowed to cool, preferably to a temperature as low as about 30°C, preferably to as low as about 20°C.

Preferably, the diamond and/ r catalyst layer is heated in a vacuum environment.

Preferably, the desired thickness of graphene is one layer of graphene. Alternatively, the desired thickness of graphene may be multiple layers of graphene, for example at least about 2, 3, 4, 5, 6, 7, 8, 9. 10 or more layers of graphene.

According to another aspect of the present invention, there is provided graphene produced by a method as described herein.

According to a further aspect of the present invention, there is provided a layer of graphene produced by a method as described herein.

Another aspect of the present invention relates to a diamond having a (1 1 1) suz-face, wherein the (111) surface is provided with a layer of catalyst for catalysing the formation of a layer of graphene thereon.

^ Preferably, the catalyst is a transition metal catalyst.

Preferably, the catalyst is selected from iron, cobalt and nickel. Further preferably, the catalyst is selected from iron and cobalt. Most preferably, the catalyst is iron.

Preferably, the catalyst is provided at a thickness of at least about one monolayer, preferably at least about lnm, preferably between about Inm and about I Snm, preferably between about lnm and about 5nm, preferably between about l nm and about 3nm, most preferably about 2nm. A further aspect of the invention relates to a composition comprising graphene, metal and diamond wherein the metal is provided on a (1 1 1 ) surface of the diamond.

Preferably, the metal is provided between the graphene and the diamond. Preferably, the metal is a transition metal preferably selected from iron, cobalt and nickel. More pi eferabiy, the metal metal is selected from iron and cobalt. Most preferably, the metal is iron.

Preferably, the metal is provided at a thickness of at least about one monolayer, preferably at least about I nm, preferably between about inm and about 15nm, preferably between about I m and about 5nm, preferably between about I nm and about 3nm, most preferably about 2nm.

A further aspect of the invention relates to a composition comprising graphene, iron and diamond.

Preferably, the iron is provided between the graphene and the diamond. Preferably, the iron is provided on a (1 1 1 ) surface of the diamond.

Preferably, the iron is provided at a thickness of at least about one monolayer, preferably at least about I nm. preferably between about Inm and about 15nm, preferably between about Inm and about Snm, preferably between about Inm and about 3nm, most preferably about 2nm.

Example embodiments of the present invention will now be described with reference to the accompanying figures, in which:-

Figure 1 shows (a) Cl s core levels of diamond (1 1 1 ) before and after graphitisation (hv = 1254 eV). The markers represent snapshot spectra recorded in 4 s and the solid lines are fitted curves to these data, (b) and (c) LEED patterns at 85 eV and 105 eV for the clean diamond (1 1 1) surface, (d) Cls and Fe 2p3/2 electron emission intensity variation extracted from real-time XPS data recorded in parallel during a temperature cycle between 20°C and 750°C. (e) and (f) LEED patterns at 85 eV and 105 eV for the epitaxial graphene layer; and Figure 2 shows (a) Schematic representation oi^: the atomic structure for SiC + Fe before annealing (the Fe layer is shown as a bcc lattice), (b) SiC + Fe following annealing. The reaction products are FeSi_x (shown as FeSi in the figure) and graphene. (c) Schematic representation of the atomic stiucture for diamond + Fe. The bcc Fe lattice matches well with the diamond (111) surface, (d) Diamond + Fe following annealing. The catalysed graphitization results in a graphene film on the surface that is lattice-matched to both the bcc Fe and to the underlying diamond (1 I I).

Figure 3 shows schematics of devices using a conipcsiton of the invention comprising a semiconductor-metal-graphene structure using the efficient electron conduction through the structure. The semiconductor (C) is a carbon-containing crystal, the metal (B) is a transition metal catalyst and the top graphene layer (A) obtains its carbon atoms from the semiconductor using the process of the invention. In pannel (a) an optoelectronic device is shown where light (thick arrow) penetrates through the transparent layers to the carbon-containing semiconductor where it is absorbed and turned into charges (electrons, holes, excitons) (thin arrows) that are transported efficiently through the structure to be collected at the graphene and at the semiconductor. In pannel (b), a switch transistor is shown where charge (diin arrows) is injected effeciveiy into the graphene channel by the first stiucture, transmitted / modulated by the graphene channel and collected by the second structure.

Figure 4 shows, in the left pannel a side-view schematic of a composition of the invention produced by the method of the invention. The diamond (bottom) is matched to the iron (middle) and graphene (top). In the right pannel, experimentally-measured diffraction patterns arc shown for each component in the structure. Remarkably, these show that the top- view hexagonal atomic structures are the same size and the same orientation for each layer in the composition.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to methods for producing graphene. The methods used in the invention and delailed examples of the invention are set out below.

Within this specification embodiments have been described in a way which enables a clear and concise specification to be written, but it is intended and will be appreciated that embodiments may be variously combined or separated without parting from the invention.

Within this specification, the term ''about" means plus or minus 20%, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2%.

Within this specification, the term "graphitization temperature" means a temperature at which graphene is formed on the exposed surface of the catalyst layer. The exact temperature, or temperature range, may depend upon the reaction condition parameters but can be determined by a skilled person through in situ monitoring of the exposed surface of the - catalyst or via calibration experiments performed utilizing the same reaction condition parameters. By way of example, the graphitization temperature may be between about 500°C and about 750°C, preferably between about 60T C and about 700^oC, preferably at least about 675°C.

It will be appreciated that reference to monitoring the formation of graphene on die exposed surface of the catalyst "<7? situ" means that the formation of graphene is monitored as it occurs, In this way, it is possible to continuously monitor the formation of graphene and the amount of iron and carbon during heating. This is in contrast to methods wherein a substrate and/ov catalyst layer is heated in stages with the heat source being periodically removed and the formation of graphitic carbon thus being periodically stopped so that the amount of graphitic carbon produced can be periodically recorded.

Within this specification, it will be appreciated that the "exposed surface of the catalyst" means the surface of the catalyst which is not in contact with the diamond surface.

Within this specification, "a diamond (1 1 1 ) surface" means that specific crystallographic plane that is defined according to convention by the indices 1 , 1 and 1. This surface can be prepared by cleaving and/or polishing followed by chemical cleaning. It is also possible to anneal the surface in a vacuum and to expose the surface to gases. High quality surfaces of different leremtiiatioii can be prepared in these ways.

Graphene is formed from single atomic planes of carbon with a structure similar to a single plane of graphite. However, it has material properties that are very different from graphite. For example, it is transparent and an excellent conductor of electrons. One of the main technological challenges is to economically produce large area graphene of high structural quality. Current methods include exfoliation from graphite, growth from gases on metal crystals and decomposition of carbon containing solids such as fullerenes and silicon carbide. The present invention provides a new method based on the conversion of carbon atoms in a diamond (1 11) surface to graphene.

In particular, the present invention relates to the growth of graphene on diamond using a transition metal catalyst, For example, epitaxial graphene on diamond (1 11 ) surfaces using Fe, Co or Ni.

In accordance with the invention, the methods described herein provide for epitaxial graphene fabrication on diamond (1 1 1 ) surfaces using iron. The procedure is provided in detail below. An iron thin film is grown in vacuum on the diamond surface and this is then controHably heated until a chemical reaction is initiated at a known temperature. This reaction removes carbon from the diamond, transports it through the iron and deposits it as a high quality graphene layer on top of the iron. The growth may be stopped when there is one layer of carbon on the surface, but can be continued to grow further graphene layers as desired. The process can be controlled to give single or many layers of graphene. The work described herein provides the temperature for initiation of this reaction and the thickness of the iron film/layer. It also proves that the graphene is of high structural quality, that it lies on the surface of the iron and that it is made up of non-diamond, sp2-bonded carbon.

As described herein, the catalytic conversion of sp3 carbon to sp2 carbon on a diamond (11 1) surface using a transition metal (Fe) catalyst has been developed to provide a controlled, reduced temperature method for graphene growth. The approach is to fabricate metal and graphene films on the oriented crystalline substrate in a clean vacuum environment with programmable temperature cycling. X-ray Photoelectron Spectroscopy (XPS) was employed to study the core levels of the substrate and overlayers at each stage of the experiment, although if will be appreciated that other techniques can be used. In addition to traditional scanned-mode XPS, REal-time Electron Spectroscopy (REES) in snapshot mode (typically 4 s) has been applied in-situ during each processing stage. Complementary Low Energy Electron Diffraction (LEED), Scanning Tunneling Microscopy (STM), Photoelectron Microscopy (PEEM) and Angle-Resolved Photo-Electron Spectroscopy (ARPES) measurements were also performed.

Experiments ere also performed using SiC for comparison. Whilst the growth process on diamond is truly catalysed, for SiC a stable product (FeSi_x) is produced by the graphite formation and thus' the process self-terminates. In both cases, the graphitisation can be precisely controlled by temperature. Surprisingly, the growth of graphene on the diamond (1 11) surface produced graphene of a very high quality and in large amounts.

An Fe layer was grown on the diamond (1 11) surface and subsequently annealed causing a graphitic carbon layer to form on the surface. Snapshot spectra (recorded in 4s) at the start and end of the experiment are shown in Fig. 1(a) along with fitted curves generated using symmetric (diamond) and asymmetric (C_gr.,) lineshapes. Furthermore, the Cls and Fe2p core levels were continuously monitored throughout. During the growth of the Fe film, the intensity variation of both core levels was consistent with a laminar growth mode. As the Fe overlayers thickness exceeded around 5nm, the substrate Cls emission was reduced below the detection limit A piogramraed temperature ramp was then initialed. In the illustrated case (Fig. 1(d)), there was little variation in the Cls peak intensity during the initial stages of annealing. Up to 500°C, the relative Fe2p peak intensity was also largely unchanged indicating that there was no significant clustering of this Fe film with temperature. There was, however, a small increase in intensity in the Fe2p intensity above around 500°C that could indicate some restructuring of the film. At around 675°C, there was a rapid increase in the Cls peak intensity and a corresponding decrease in the Fe2p intensity, indicating the formation of a carbon surface layer. Analysis of the, lineshape (Fig. 1(a)) confirmed this layer to be graphitic. Graphitizatton continued until the temperature ramp was reversed. There was little further change in the peak intensities during cooling. This method allows graphitic L I carbon of selected thickness to be grown with sub-monolayer precision by control of the temperature ramp.

The graphitisation process was dependent on the temperature ramp. In the illustrated case, the onset, appeared to be at around 675°C₄ however at slower heating rates, there was evidence that the onset could be significantly lower. At very slow rates, it could be as low as about 500°C.

* LEED measurements were performed both on the clean diamond (1 1 1 ) surface prior to Fe deposition' (Fig. 1 (b) and (c)) and on the graphitized sample after annealing (Fig. 1 (e) and (0). Due to the close lattice match of graphene and the diamond (i l l) surface, the magnitude of the tecipiocal space unit cell captured in LEED images was unchanged following graphitization. However, the intensity variation of the diffracted beams with electron energy (IV characteristics) depended on the crystal symmetry and out-of-plane lattice parameters and hence should be markedly different for diamond (1 1 1) and graphitic carbon. For example, at 65 eV and 105 eV, the two sets of symmetry-equivalent first order beams from diamond (1 1 1 ) displayed maxima (Fig. 1(b) and (c) respectively) whereas LEED measurements of the same surface after graphitization and under similar conditions (Fig. 1(e) and (f)) revealed no such maxima, but a very similar in-piane unit cell, and the same in-plane orientation. This proves mat the surface carbon layer was indeed graphitic.

The resulting structures formed using this new approach for graphene growth on diamond and for comparison SiC are shown schematically in Fig. 2. For SiC (Fig. 2(a) and (b) the graphene layer formed on top of a siiicide layer that is shown simplistically as FeSi. For diamond, (Fig. 2(c) and (d))_s the Fe film is shown as bcc ( M l ) which is well lattice- matched to both diamond and graphene. However there is evidence of small amounts of Fe-C bonding at the interfaces and within the Fe layer.

Surprisingly, in contrast to the reaction on SiC, the Fe does not form an intermediate product on diamond as illustrated in Figures 2(c) and (d). In this case, the Fe catalyses the conversion from s -' to sp² carbon, depositing the latter as a graphene/graphite film on top of the Fe layer. The excellent epitaxial match between all three components in this structure (Figure 2(d) and Figure 4) offers the potential for very high quality and large area graphene production at industrially realistic temperatures.

The use of the present invention to produce graphene of controlled, thickness opens up a wealth of new possibilities; to make use of graphene's electrical conductivity in patterned tracks on such surfaces using lithographic metallic patterns, and to facilitate, for example, the construction of new two-element spin devices which exploit graphene's spin transport, Fe's spin injection and diamond's spin storage properties. The reduced temperature growth of graphene-on-semiconductor structures afforded by this solid-state method brings many of the proposed uses of graphene into the reach of industrial processing.

It was surprising to discover that an iron film as thin as 2 nm could be grown on diamond and on silicon carbide without becoming rough and clustered. This uniform film is an essential precursor to successful growth of graphene over a wide area. For diamond(l 1 1 ),' there is an additional bonus in that experimental evidence shows that the iron layer is made up of atoms in a regular crystal that matches very closely with the structure of the diamond substrate. This crystal matching is also transferred to the graphene layer to provide a three- component crystal structure that involves a semiconductor, metal and graphene through which electrons can move without being affected b imperfections in the crystal. The structure is thus very valuable for devices that involve the transport of electrons. The structure is also valuable as an optical / opto-ejectronic / photonic structure since the whole object is optically- transparent (if the metal layer is thin enough) while being electrically conducting.

Examples of possible uses of the composition according to the invention are:

1. a switch / transistor where the semiconductor and metal element selects the electrons that are then transported widtout loss along the graphene. The electrons can be selected and modulated to carry information and are then collected at a second semiconductor-metal- graphene structure.

2. a device for conversion of energy from light vo electricity (optimised at the UV and x-ray part of the spectrum by the band-gap of the diamond). This could be a sensor, a light detector or a photovoltaic cell. Light (meaning electromagnetic radiation of visible, ultraviolet and x-ray wavelengths) passes through the graphene and the metal and is absorbed by the diamond generating charges that can be collected and transported to provide a current or voltage.

These are illustrated schematically in Figure 3.

In Figure 4, the particular case of diamond(l l l )-Fe-graphene is shown where the atoms are in registry across the layers. This surprisingly .high quality structure is confirmed by electron diffraction on each of the layer surfaces. ^'Hie hexagonal structure of each layer matches the others in size and orientation and shows that this matching is presevered across the whole of the crystal surface. This means that the graphejie grows in the same way in a sheet at all points on the diamond surface and hence the size of the graphene sheet is defined by the size of the starting diamond surface. Single crystals of areas cm x cm are common for diamond. For silicon carbide 10s cm x 10s cm are common.

Comparison between compositions of the invention wherein metal is provided on different diamond surfaces.

Table 1 below summarises differences between compositions according to the invention wherein iron is provided on the the (11 1) diamond surface or the (001 ) diamond surface. According to the invention, the diamond surface is treated with an iron layer and heated to form a layer of graphene/graphiie.

The comparison demonstrates that:^■

(a) For the ( 1 1 1) surface, the crystal match is perfect for all three materials in the composition as shown in Figure 4. Diffraction has been measured using LEED for the diamond, the iron and the graphene for this surface (Figure 4).

(b) The crystal match is less perfect for the (001) surface - the match is good in only one direction on the surface. LEED diffraction for the clean (001) diamond surface could be observed, but it has not been possible to observe LEED for the iron layer or the graphene layer in the composition. (c) The ousel reaction temperature for the production of graphene on diamond provided with an iron coating is lower (better) for the (I I I) surface compared to onset reaction temperature for the production of graphene on diamond provided with an iron coating on a (001 ) surface.

(d) Real-time measurements have now shown that graphene forms from carbon that is extracted from the diamond in the presence of the catalyst film at high temperature. This is in contrast to the claims made in the art prior to the date of the invention.

Table 1

It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications are covered by the appended claims.

The content of all references cited herein are incoiporated herein by reference in their entirety.

References

Bostwick A. el al., Physical Review Letters 103, (2009). Bostwick, A. et al , Science 328, 99-] 002 (20) 0).

Dimitiakopoulos, C. et al., Journal of Vacuum Science and Technology' B 28, 985-992 (2010).

Fyfe, D.J., PhD Thesis, University of Wales Aberystwyth (2001). Gcim, A.K., Science 324, J 530- 1534 (2009). .

Hofmann. P. & Wells, J.W., Journal of Physics: Condensed Matter 21, 013003 (2009). Lizzil, S. el al.. Nature Physics 6, 345-348 (2010). Novoselov, .S. et al Science 306 666-669 (2004).

Tombros, N,„ Jozsa, C, Popinciuc, M., Jonkman, H.T. & van Wees, B.J., Nature 448, 571-574 (2007).

W tterlin, J & Bocquet, M.-L, Surface Science 603, 1841 -1852 (2009).

Claims

1. A method for producing graphene, the method comprising:-

(i) providing a layer of catalyst on a diamond (1 1 1 ) surface,

(ii) heating the diamocd and/or. catalyst layer iih a heat source until a desired thickness of graphene is produced on the exposed surface of the catalyst.

2. A method according to claim 1 , wherein the formation of graphene on the exposed ^' surface of the catalyst is monitored and the heat source is removed when a desired thickness of graphene has been produced.

3. A method according to claim , 1 or 2, wherein the formation of graphene on the exposed surface of the catalyst is monitored in situ.

4. A method according to any preceding claim, wherein the formation of graphene on the exposed surface of the catalyst is monitored in situ by one or more of x-ray, optical or election based techniques, for example selected from X-ray Phofoelectron Spectroscopy (XPS), REal-time Electron Spectioscopy (REES), Low Energy Electron Diffraction (LEED), Scanning Tunneling Microscopy (STM), Photoelectron Microscopy (PEEM) and Angle- Resolved Photo-Electron Spectroscopy (ARPES).

5. A method according to any preceding claim, wherein the diamond and/or catalyst layer is heated for a period of time winch has been calculated to produce a defined thickness of graphene.

6. A method according to any preceding claim, wherein the catalyst is a transition metal catalayst.

7. A method according to claim 6, wherein the catalyst is selected from iron, cobalt and nickel.

8. A method according to claim 6 or 7, wherein the catalyst is selected from iron and cobalt, s 5 8

9. A method according to any preceding claim, wherein the catalyst is provided at a thickness of at least about one monolayer.

!O. A method according to any preceding claim, wherein the diamond and/or catalyst layer is heated at a graphftisation temperature at which the formation of graphene occurs.

1 1. A method according to claim 10, wherein the diamond , and/or catalyst layer is heated at a graphitizatiom temperature until a desired thickness of graphene is formed on the exposed surface of the catalyst.

12. A method according to claim 10 or 11, wherein the diamond and/or catalyst layer is heated to a graphitization temperature by increasing the temperature at a rate which allows the formation of graphene on the exposed surface of the catalys to be monitored in sifu.

13. . A method according to claim 12, wherein the diamond and/or catalyst layer is heated to a graphitization temperature by increasing the temperature at a rate of between about 0.5"C s^'1 and about 10°C s ^l.

14. A method according to any preceding claim, wherein once the desired thickness of graphene has been produced,, the heat source is removed and the diamond/catalyst graphene layer sample is allowed to cool.

15. A method according to any preceding claim, wherein the diamond and/or catalyst layer is heated in a vacuum environment. f

16. A method according to any preceding claim, wherein the desired thickness of graphene is at least one layer of graphene.

17. Graphene produced by a method according to any preceding claim.

18. A layer of graphene produced by a method according to any of claims 1 to 16.

19. A diamond having a (111) surface, wherein tiie (1 1 1) surface is provided with a layer of catalyst for catalysing tine formation of a layer of graphene thereon.

20. A diamond according to claim 19, wherein the catalyst is a transition metal catalayst.

21. A diamond according to claim 20, wherein the catalyst is selected from iron, cobalt and nickel.

22. A composition comprising graphene, a metal and diamond wherein the metal is pro ided on a ( 111 ) surface of the diamond .

23. A composition according to claim 22, wherein the metal is provided- between the graphene and the diamond.

24. A composition according to claim 22 or 23, wherein the metal is a transition metal.

25. A composition according to any one of claims 22 to 24, wherein the metal is selected from iron, cobalt and nickel.

26. A composition according to any one of claims 22 to 25, wherein the metal is provided at a thickness of at least about one monolayer, optionally at least about I nm, optionally between about I nm and about I Snm, optionally between about I nm and about Sntn, optionally between about Inm and about 3mn, optionally about 2nm.

27. A composition comprising graphene, iron and diamond.

28. A composition according to claim 27, wherein the iron is provided between the graphene and the diamond.

29. A composition according to claim 27 or 28, wherein the iron is provided on a (111) surface of the diamond.

30. A composition according to any one of claims 27 to 29, wherein the iron is provided at a thickness of at least about one monolayer optionally a least about I nm, optionally between about . Inm and about 15nm, optionally between about I nm and about 5nm, optionally between about Inm and about 3nm, optionally about 2nm.