GB2482188A

GB2482188A - Two dimensional film formation

Info

Publication number: GB2482188A
Application number: GB1012378.4A
Authority: GB
Inventors: Joost W M Frenken
Original assignee: Universiteit Leiden
Current assignee: Universiteit Leiden
Priority date: 2010-07-23
Filing date: 2010-07-23
Publication date: 2012-01-25
Also published as: GB201012378D0

Abstract

A method of forming a film of two-dimensional material comprises the step of depositing a two-dimensional material onto a stepped surface 12 of a metal substrate 10, the stepped surface including an array of substantially parallel steps 16, each step defining a terrace 18 and each step having a height of one atomic layer. Preferably the metallic substrate is a transition metal. Examples of two dimensional materials include graphene, graphane and hexagonal boron nitride. Also claimed is a metal substrate having a first surface for receiving a film and second recessed surface and a method of using a metallic substrate as a frame for forming a layer of two dimensional material.

Description

THIN FILM FORMATION

This invention relates to a method of forming a film of two-dimensional material, the use of a metal substrate in the formation of a film of two-dimensional material, a metal substrate, an assembly comprising a metal substrate defining a frame and a film of two-dimensional material suspended on the frame and a method of manufacturing thereof.

Reference herein to two-dimensional materials are intended to refer to atomically thin sheets of material, which are typically characterised as having strong in-plane bonds and weak inter-layer bonds coupling successive layers. These weak inter-layer bonds make it possible to form free-standing, two-dimensional films. Examples of two-dimensional materials include graphene, graphane and hexagonal boron nitride.

Two-dimensional materials have specific properties that render them compatible with a wide range of applications such as fast and flexible electronics, lasers, blo-sensors, atomically thin protective coatings, hydrogen storage and energy storage. For example, graphene demonstrates higher carrier mobility than conventional semiconductor materials, which can be exploited to improve the speed of electronics, such as, for example, microprocessors.

During the growth of a two-dimensional material, surface characteristics of the substrate on which the growth takes place play an important role in ensuring the formation of a high quality, two-dimensional film. However, perfect surface flatness is theoretically impossible and, in practice, it is time-consuming and costly to prepare surfaces having a theoretically minimal level of residual surface roughness. As a result, the surface intended for film deposition will typically include surface irregularities, which are likely to introduce imperfections into the structure of the two-dimensional film. Any such imperfections have a detrimental effect on the properties of the film.

The manufacture of a two-dimensional material also involves the lift-off of the two-dimensional material for incorporation into another device.

One such lift-off method involves the use of a support material such as, for example, polymethyl-methacrylate (PMMA). A layer of the support material is provided on the surface of the two-dimensional film and the metal substrate is etched away, or otherwise removed, leaving a bi-layer of two-dimensional material and the support material. The

I

bi-layer may then be positioned at its desired location and the support material may be dissolved, or otherwise removed, using a developer fluid for example.

The use of a support material as a lift-off aid however introduces an additional processing step into the overall manufacturing process, which adds to the complexity and cost of manufacture of a two-dimensional material.

According to a first aspect of the invention, there is provided a method of forming a film of two-dimensional material comprising the step of depositing a two-dimensional material onto a stepped surface of a metal substrate, the stepped surface including an array of substantially parallel steps, each step defining a terrace and each step having a height of one atomic layer.

In practice, it is impossible to prepare a metal surface that is perfectly flat and parallel to a specific crystal plane and difficult to prepare a metal surface that even approaches perfect flatness. As a result, it is common for metal surfaces to include atomic steps separated by a height difference of one atomic plane. Although the crystal structure of metals introduces a sideways shift of atomic lattices between different planes, a two-dimensional material tends to overgrow these atomic steps without introducing any error in its atomic weave.

However, when two steps oriented in different directions meet on the surface of the metal substrate, the two steps combine to form a corner structure. Due to its two-dimensional nature, the formation of a film of a two-dimensional material over this corner structure has been found to introduce a local point defect in the structure of the two-dimensional material.

The inclusion of an array of substantially parallel steps allows the surface characteristics of the metal substrate to be controlled. Although the stepped surface of the metal substrate may not define perfectly flat terraces, the terraces including height deviations from perfect flatness, the presence of the array of substantially parallel steps results in minor repositioning and reorientation of the respective step to accommodate the height deviations. Films of two-dimensional material have been found to be capable of overgrowing these repositioned and reoriented steps without introducing any local point defects.

The use of a metal substrate having an array of substantially parallel steps therefore minimises the formation of local point defects in a film of two-dimensional material and thereby ensures uniformfty throughout the film. This enhances the mechanical and electronic properties of the film. It also allows for the formation of high quality, defect- free films of two-dimensional material in a manner that may be readily translated to large-scale, large area production.

Such a method omits the need to prepare a metal surface that approaches perfect flatness and is parallel to a crystal plane, which can be a time-consuming and costly process.

Preferably the metal substrate has substantially the same lattice constant as the two dimensional material in a surface plane of the terrace of each step.

The metal substrate preferably has the same crystal symmetry as the two-dimensional material in a surface plane of the terrace of each step.

Selecting the metal substrate so that the crystal symmetry and/or the lattice constant in the surface plane of each terrace is substantially the same as that of the two-dimensional material not only minimises the residual stress in the resultant film, but also ensures that film islands formed at different locations on the stepped surface readily combine to form a uniform film of the two dimensional material.

In embodiments of the invention, the stepped surface of the metal substrate may be a vicinal surface.

By virtue of being miscut at small angles relative to a low-index crystallographic plane, each terrace of the vicinal surface has the same atomic arrangement and lattice constant as the low-index crystallographic plane. As such, deposition of the two-dimensional material on the vicinal surface results in a high quality, defect-free film.

The metal substrate may be formed from a transition metal or may be formed from a metallic composition or alloy including at least one transition metal. In such embodiments, the or each transition metal may be selected from a group including copper, nickel, cobalt, ruthenium or rhodium.

Transition metals have been found to have crystal symmetries and lattice constants that closely match the corresponding characteristics of some two-dimensional materials. For example, the crystal structure and lattice constant of the (111) crystallographic plane of nickel, copper and cobalt are very similar to the corresponding characteristics of graphene, graphane and hexagonal boron-nitride.

The method may further include the step of introducing a precursor gas into the vicinity of the stepped surface of the metal substrate. Such a precursor gas preferably includes atoms of carbon, boron and/or nitrogen, The precursor gas may be ethylene, benzene and/or borazine.

The formation of a film of two-dimensional material via the decomposition of precursor gas molecules adsorbed on the stepped surface of the metal substrate is compatible with large-scale, large area production.

In other embodiments, the metal substrate may include dissolved atoms of carbon. In such embodiments, the method may further include the step of heating the metal substrate followed by the step of cooling the metal substrate so as to segregate the dissolved atoms of carbon on the stepped surface of the metal substrate.

Graphene may be formed via the step of re-accumulating the dissolved carbon atoms on the stepped surface of the metal substrate, either alone or in combination with the decomposition of carbon-containing precursor gas molecules.

According to a second aspect of the invention there is provided a film of two-dimensional material manufactured in accordance with the method of the first aspect of the invention.

According to a third aspect of the invention, there is provided a use of a metal substrate in the formation of a film of two-dimensional material, wherein the metal substrate includes a stepped surface, the stepped surface including an array of substantially parallel steps, each step defining a terrace and each step having a height of one atomic layer.

According to a fourth aspect of the invention, there is provided a metal substrate having first and second surfaces located on opposite sides of the metal substrate, the first surface defining a deposition surface to receive a film of material and the second surface including one or more recesses.

The provision of one or more recesses in the second surface of the metal substrate not only allows the first surface of the metal substrate to provide an intact platform for the formation of a two-dimensional material, but also allows the second surface of the metal substrate to be readily machined at a later stage to define a frame structure. For example, following the formation of a two-dimensional material on the first surface, the second surface of the metal substrate may be uniformly machined using, for example, chemical or electrochemical machining to remove any remaining thickness separating the recess and the first surface.

Additionally the arrangement and shape of the or each recess may be designed, for example, to assist the incorporation of the film of two-dimensional material into another device or to fit the existing structure of a specific apparatus.

The first surface may define a stepped deposition surface including an array of substantially parallel steps, each step defining a terrace and each step having a height of one atomic layer.

In the two preceding aspects of the invention, the metal substrate may include dissolved atoms of carbon.

According to a fifth aspect of the invention, there is provided an assembly comprising a metal substrate defining a frame and a film of two-dimensional material suspended on the frame.

The robustness of a frame defined by a metal substrate reduces the risk of damaging the film during transport and manipulation of the film of two-dimensional material.

Additionally, if the film is to be incorporated into a specific apparatus, the frame may act as a package to facilitate correct positioning of the film of two-dimensional material within the specific apparatus. As an example, the shape of the frame may be modified to fit the existing structure of the apparatus.

In the three preceding aspects of the invention, the metal substrate may be formed from a transition metal or may be formed from a metallic composition or alloy including at least one transition metal. The or each transition metal is preferably selected from a group including copper, nickel, cobalt, ruthenium or rhodium.

According to a sixth aspect of the invention, there is provided a method of fabricating an assembly according to the fifth aspect of the invention comprising the steps of forming a film of two-dimensional material on a first surface of a metal substrate; and machining at least one specific area on a second, opposed, surface of the metal substrate so as to form a frame.

Preferably the method includes the step of machining the or each specific area to a predetermined thickness prior or subsequent to the formation of the film of two-dimensional material; and machining the specific area subsequent to the formation of the film of two-dimensional material so as to remove the or each specific area.

The or each specific area of the second surface of the metal substrate may be patterned using one or more of a variety of machining processes such as, for example, spark erosion, conventional machining, chemical machining/etching and electrochemical machining. Employing such machining processes reduces the overall manufacturing time by enabling removal of the bulk of the unwanted volume of the metal substrate in a single processing step.

The step of machining the or each specific area prior to the formation of the film of two-dimensional material removes the bulk of the unwanted volume prior to the formation of the film and thereby shortens the time required to remove the remainder of the specific area subsequent to the formation of the film. It therefore minimises the exposure of the film of two-dimensional material to other forms of processing and thereby minimises the risk of damage to the film of two-dimensional material.

The method may further include the step of coating the second surface of the metal substrate with a protective layer of material to define the specific area prior to the machining of the or each specific area to a predetermined thickness.

Such coating of the second surface may involve the use of, for example, photoresist, oxide layers or other similar materials to protect selected areas of the second surface during the machining step in order to define a frame having a specific shape.

The film of two-dimensional material may be formed on the first surface of the metal substrate in accordance with the method of forming a film of two-dimensional material according to any of the embodiments of the first aspect of the invention, wherein the first surface of the metal substrate is a stepped surface.

Preferred embodiments of the invention will now be described, by way of non-limiting exam pies.

Figure 1 shows a metal substrate according to an embodiment of the invention; Figure 2 shows a honey comb lattice of graphene including a periodic arrangement of six-member carbon ring structures; Figure 3 is a simplified illustration depicting the formation of graphene on a surface having the same crystal symmetry as Ni(1 11); Figure 4 illustrates a two-step process to form an assembly comprising a nickel substrate defining a frame and a graphene film suspended on the frame; Figure 5 illustrates the adsorption of precursor gas molecules on a stepped surface of a nickel substrate; Figure 6 shows a graphene film formed on the stepped surface of the nickel substrate of Figure 5 following decomposition of the adsorbed precursor gas molecules; and Figure 7 shows a honey comb lattice of graphene having six-member, five-member and seven-member carbon ring structures.

A metal substrate according to an embodiment of the invention is shown in Figure 1.

The metal substrate is formed from nickel and includes first and second surfaces 12,14.

In other embodiments, the metal substrate may be formed from another type of metal.

For example, the substrate may be formed from another transition metal selected from the group including copper, cobalt, rhodium or ruthenium or may be formed from a metallic composition or alloy including at least one transition metal.

The first surface 12 of the nickel substrate 10 is a stepped surface including an array of substantially parallel steps 16, each of these steps 16 defining a terrace 18 and having a height of one atomic layer. To prepare such a surface 12, a bulk Ni crystal is mechanically and/or chemically cut to form a vicinal surface that is miscut relative to the Ni (111) crystallographic plane 20.

By virtue of being prepared in this manner, the atomic arrangement and lattice constant of the nickel substrate 10 in the surface plane of each terrace 18 of the stepped surface 12 are the same as the Ni (111) crystallographic plane 20.

The purpose of preparing the first surface 12 of the nickel substrate 10 in this manner is to define a deposition surface for the formation of a high-quality, defect-free graphene film. As seen in Figure 2, graphene is a two-dimensional material consisting of sp2-bonded carbon atoms 22 in a six member carbon ring geometry arranged to form a honeycomb crystal lattice 24. The (111) crystallographic plane of the nickel substrate is therefore suitable as a platform for the formation of graphene because it has the same crystal symmetry as graphene and a lattice constant that closely matches that of graphene, as illustrated by the arrangement of carbon and nickel atoms 22,26 shown in Figure 3.

It should be noted that Figure 3 is a simplified illustration to depict the similar crystal characteristics of Ni(111) and graphene. Although the carbon atoms 22 have been placed at the fcc and hcp sites of the nickel substrate in Figure 3, the carbon atoms 22 may also be located at the on-top sites of the nickel substrate during the formation of the graphene film. This however has minimal effect on the formation of a graphene film having a honeycomb crystal lattice of the preferred six member carbon ring geometry.

It is also envisaged that in further embodiments, the first surface 12 of the nickel substrate 10 may be a vicinal surface miscut relative to a different low-index crystallographic plane.

The selection of the metal substrate and crystallographic plane is dependent on the crystal symmetry and lattice constant of the desired film of two-dimensional material.

The second surface 14 of the nickel substrate 10 is machined using a two-step process so as to define a nickel frame 28, as shown in Figure 4, where the graphene film 30 is suspended on the frame. A first step of the two-step process is carried out prior to the formation of the graphene film 26 and is outlined as follows.

The second surface 14 of the nickel substrate 10 is coated with a protective layer of material prior to the formation of the graphene film 30 on the first surface 12 of the nickel substrate 10. This second surface 14 is located on the opposite side of the nickel substrate 10 to the first surface 12. The purpose of coating the second surface 14 is to protect selected areas 32 of the second surface 14. This may be achieved using, for example, photoresist and oxide layers.

The unprotected areas of the second surface 14, which define a specific area 34 of the second surface 14, are machined to a predetermined thickness 36 to define one or more recesses 38, using chemical or ion beam machining for example, such that the first surface 12 of the nickel substrate 10 remains intact. After this machining step is completed, the protective layer of material is removed. The protective layer of material may be removed by using methods such as the use of plasma ashing to remove photoresist masks.

Alternatively the coating step may be omitted from the first step of the two-step process and such a first step may involve the machining of the specific area to a predetermined thickness using other machining methods such as conventional machining and spark erosion.

The machining of the second surface 14 is followed by the formation of the graphene film 30 on the stepped surface 12 of the nickel substrate 10 under ultra-high vacuum conditions ranging from io mbar to i0 mbar so as to ensure the purity of the film 30 and thereby improve the mechanical and electronic properties of the resultant graphene film 30.

The graphene film is formed via the decomposition of a carbon-containing precursor gas onto the first surface 12 of the nickel substrate 10. At this stage the carbon-containing precursor gas, such as ethylene or benzene, is introduced into the vicinity of the stepped surface 12 of the nickel substrate 10, which leads to the adsorption of precursor gas molecules 40 on the stepped surface 12, as shown in Figure 5. The precursor gas may, for example, be introduced via a gas injection needle connected to a source containing the precursor gas. The nickel substrate 10 is kept at a high temperature, which in this instance is 1000K, to induce or enhance decomposition of the adsorbed precursor gas molecules 40 so as to accelerate the formation of the graphene film. In addition, heating the nickel substrate 10 to a high temperature may be used to order the film of two-dimensional material into a completely regular structure. Referring to Figure 6, the adsorbed precursor gas molecules therefore decompose to form carbon atoms 22, which remain on the stepped surface 12 of the nickel substrate 10, and volatile components, which are pumped away by the vacuum pumping system.

This is followed by the cooling of the nickel substrate 10 to reduce its temperature. This is because at high temperatures, the decomposition of the carbon-containing precursor gas molecules adsorbed onto the stepped surface 12 of the nickel substrate 10 may lead to the dissolution of carbon components of the decomposed precursor gas molecules into the nickel substrate 10.

Cooling of the nickel substrate 10 decreases the maximum solubility of the carbon components, which leads to the re-accumulation of some of the dissolved carbon components 22 at the surface by segregating out of the nickel substrate 10. By controlling the cooling of the nickel substrate 10, the reaccumulation of the dissolved carbon atoms 22 is incorporated into the formation of the graphene film without affecting the uniformity of the graphene film.

It is envisaged that in embodiments of the invention, the nickel substrate may include dissolved atoms of carbon. As such, the graphene film may be formed solely via the re-accumulation of the dissolved atoms of carbon on the stepped surface of the nickel substrate.

In other embodiments, the pressure and temperature parameters of the film formation process may vary depending on the film of two-dimensional material to be manufactured and/or its desired characteristics.

The use of a stepped surface including an array of substantially parallel steps provides a suitable platform for the formation of a high quality, defect-free graphene film. In practice, the stepped surface 12 shown in Figure 1 may include height deviations from perfect flatness. However, the substantially parallel arrangement of the steps 16 minimises the introduction of new steps on the stepped surface. Instead, the existing substantially parallel steps 16 may be slightly repositioned and reoriented to accommodate these height deviations, which results in each substantially parallel step 16 taking the form of a meander. Graphene film has been found to be capable of overgrowing such meandering steps without introducing any local point defects into its structure.

Omitting the array of substantially parallel steps would lead to the formation of irregular features, such as corners, which may cause local point defects in the graphene film.

In graphene, these local point defects may be in the form of other carbon arrangements such as, for example, five-member and seven-member carbon ring structures 42,44, as shown in Figure 7. The presence of these local point defects would result in the graphene film retaining the shape of the nickel substrate even after transfer or lift-off from the nickel substrate, and would lead to non-flatness and wrinkles in the film. Such local point defects would also act as scattering centres for charge carriers, and thereby degrade the electronic properties of the graphene film.

The provision of the array of substantially parallel steps therefore allows the graphene film to retain the desired six-member carbon ring geometry throughout its structure, as shown in Figure 2, which enhances the mechanical and electronic properties of the graphene film.

Additionally this approach removes the need to prepare a surface for film formation, where the surface approaches perfect flatness and is parallel to a preferred crystal plane, which can be a time-consuming and costly process.

The use of a stepped surface with terraces having, in a surface plane of each terrace, a similar crystal symmetry and lattice constant to graphene reduces the residual stress in the resultant graphene film. The similar lattice constants of Ni (111) and graphene also ensures that any graphene islands formed on the stepped surface will readily combine to form a uniform graphene film. This overcomes a potential problem associated with the spontaneous growth characteristics of precursor gas decomposition, which may lead to the formation of graphene islands at different locations on the stepped surface of the nickel substrate.

The area of the stepped surface may be varied to accommodate the formation of different sizes of the graphene film without introducing local point defects. Consequently it is straightforward to scale the production of graphene upwards so as to improve its cost-effectiveness of the manufacturing process.

In other embodiments, it is envisaged that the process of forming the graphene layer may be repeated to form additional graphene layers so as to define a graphene film having a height of multiple atomic layers.

It is intended that the formation of a film of two-dimensional material having a height of multiple atomic layers falls within the scope of the claimed invention.

The second step of the two-step process to define the nickel frame is carried out following the formation of the graphene film and is outlined as follows.

In Figure 4, after the graphene film 30 is formed on the first surface 12 of the nickel substrate 10, the second surface 14 of the nickel substrate 10 undergoes a uniform etch 46 via, for example, chemical or electrochemical machining until the remaining thickness of the specific area 34 is reduced to zero. This leads to the formation of an assembly 48 comprising a nickel substrate 10 defining a frame 28 and a graphene film 30 suspended on the frame 28. The frame 28 provides a robust support structure, which may be used to manipulate and transport the graphene film 30 and thereby avoid direct contact with the graphene film 30. The remainder of the nickel substrate 10 defining the frame 28 may be etched away completely using, for example, chemical machining or mechanical cutting or tearing at a later stage to completely lift-off the graphene film 30, if desired.

When defining the frame 28, the first step of the two-step process may be carried out following, instead of before, the formation of the graphene film 30.

The reuse of the nickel substrate as a medium for manipulating and transporting the graphene film simplifies the overall manufacturing process. This is because it omits the use of other materials such as PMMA, which would otherwise introduce additional processing steps into the manufacture of the graphene film. In addition, the nickel substrate may be patterned and/or machined so as to allow the formation of different shapes of the frame. The shape of the frame may be designed, for example, to assist the incorporation of the graphene film into another device or to fit the existing structure of a specific apparatus.

It is envisaged that in other embodiments the nickel substrate may have a first surface with a surface profile that is different to the stepped surface profile of the first embodiment of the invention. The first surface of the nickel substrate may be shaped to have a specific surface profile so as to impart specific surface characteristics to the resultant graphene film. For example, the first surface of the nickel substrate may have an inclined or wavy surface profile.

It is also envisaged that in other embodiments, the second surface of the nickel substrate may be kept intact by omitting the two-step process to define a nickel frame. As such, the graphene film is formed on the first surface of the nickel substrate, where the nickel substrate remains intact.

In further embodiments, another two-dimensional material, instead of graphene, may be formed on the first surface of the nickel substrate, the two-dimensional material having an atomic arrangement that results in local point defects when overgrowing irregular structures such as corners. Examples of such two-dimensional materials include graphane and boron-nitride.

Claims

CLAIMS1. A method of forming a film of two-dimensional material comprising the step of depositing a two-dimensional material onto a stepped surface of a metal substrate, the stepped surface including an array of substantially parallel steps, each step defining a terrace and each step having a height of one atomic layer.
2. A method according to Claim I wherein the metal substrate has substantially the same lattice constant as the two-dimensional material in a surface plane of the terrace of each step.
3. A method according to any preceding claim wherein the metal substrate has the same crystal symmetry as the two-dimensional material in a surface plane of the terrace of each step.
4. A method according to any preceding claim wherein the stepped surface of the metal substrate is a vicinal surface.
5. A method according to any preceding claim wherein the metal substrate is formed from a transition metal.
6. A method according to any of Claims 1 to 4 wherein the metal substrate is formed from a metallic composition or alloy including at least one transition metal.
7. A method according to Claim 5 or Claim 6 wherein the or each transition metal is selected from 2 group including copper, nickel, cobalt, ruthenium or rhodium.
8. A method according to any preceding claim including the step of introducing a precursor gas into the vicinity of the stepped surface of the metal substrate.
9. A method according to Claim 8 wherein the precursor gas includes atoms of carbon, boron and/or nitrogen.
10. A method according to Claim 9 wherein the precursor gas includes ethylene, benzene and/or borazine.
11. A method according to any preceding claim wherein the metal substrate includes dissolved atoms of carbon.
12. A method according to Claim 11 further including the step of heating the metal substrate followed by the step of cooling the metal substrate so as to segregate the dissolved atoms of carbon on the stepped surface of the metal substrate,
13. A film of two-dimensional material manufactured in accordance with the method of any of the preceding claims.
14. Use of a metal substrate in the formation of a film of two-dimensional material, wherein the metal substrate includes a surface having an array of substantially parallel steps, each step defining a terrace and each step having a height of one atomic layer.
15. Use of a metal substrate according to Claim 14 wherein the metal substrate is formed from a transition metal.
16. Use of a metal substrate according to Claim 14 wherein the metal substrate is formed from a metallic composition or alloy including at least one transition metal.
17. Use of a metal substrate according to Claim 15 or Claim 16 wherein the or each transition metal is selected from a group including copper, nickel, cobalt, ruthenium or rhodium
18. Use of a metal substrate according to any of Claims 14 to 17 wherein the metal substrate includes dissolved atoms of carbon.
19. A metal substrate having first and second surfaces located on opposite sides of the metal substrate, the first surface defining a deposition surface to receive a film of material and the second surface including one or more recesses.
20. A metal substrate according to Claim 19 wherein the first surface defines a stepped deposition surface including an array of substantially parallel steps, each step defining a terrace and each step having a height of one atomic layer.
21. A metal substrate according to Claim 19 or Claim 20 wherein the metal substrate is formed from a transition metal.
22. A metal substrate according to Claim 19 or Claim 20 wherein the metal substrate is formed from a metallic composition or alloy including at least one transition metal.
23. A metal substrate according to Claim 21 or Claim 22 wherein the or each transition metal is selected from a group including copper, nickel, cobalt, ruthenium or rhodium.
24. A metal substrate according to any of Claims 19 to 23 wherein the metal substrate includes dissolved atoms of carbon.
25. An assembly comprising a metal substrate defining a frame and a film of two-dimensional material suspended on the frame.
26. An assembly according to Claim 25 wherein the metal substrate is formed from a transition metal.
27. An assembly according to Claim 25 wherein the metal substrate is formed from a metallic composition or alloy including at least one transition metal.
28. An assembly according to Claim 26 or 27 wherein the or each transition metal is selected from a group including copper, nickel, cobalt, ruthenium or rhodium.
29. A method of fabricating an assembly according to any of Claims 25 to 28 comprising the steps of: a. forming a film of two-dimensional material on a first surface of a metal substrate; and b. machining at least one specific area on a second, opposed, surface of the metal substrate so as to form a frame.
30. A method according to Claim 29 including the step of machining the or each specific area to a predetermined thickness prior or subsequent to the formation of the film of two-dimensional material; and machining the or each specific area subsequent to the formation of the film of two-dimensional material so as to remove the specific area.
31. A method according to Claim 30 further including the step of coating the second surface of the metal substrate with a protective layer of material to define the specific area prior to the machining of the or each specific area to a predetermined thickness.
32. A method according to any of Claims 29 to 31 wherein the film of two-dimensional material is formed on the first surface of the metal substrate in accordance with the method of any of Claims I to 10; and the first surface of the metal substrate is a stepped surface.