WO2017211996A1

WO2017211996A1 - 2-dimensional nano-gap break-junction electrode structure, method of fabrication thereof; and biomolecule detection apparatus comprising said electrode structure

Info

Publication number: WO2017211996A1
Application number: PCT/EP2017/064043
Authority: WO
Inventors: Grégory F SCHNEIDER; Amedeo BELLUNATO; Jan VAN RUITENBEEK
Original assignee: Universiteit Leiden
Priority date: 2016-06-10
Filing date: 2017-06-08
Publication date: 2017-12-14
Also published as: GB201610183D0

Abstract

The present invention relates to a structure comprising at least one pair of opposing electrodes comprising a two-dimensional electrically conductive material layer wherein the two electrodes are positioned adjacent to one another so that the planes of the two-dimensional electrically conductive material are twisted relative to one another so as to result in a single-atomic point of contact between the two electrodes, and wherein the distance between the two electrodes at the point of contact is from about 0 nm to about 500 nm.

Description

2-DIMENSIONAL NANO-GAP BREAK-JUNCTION ELECTRODE STRUCTURE, METHOD OF FABRICATION THEREOF; AND BIOMOLECULE DETECTION

APPARATUS COMPRISING SAID ELECTRODE STRUCTURE

BACKGROUND The present invention relates to structures and methods of making such structures and their use as platforms for biomolecule detection/sequencing.

The mono-atomic thickness of two-dimensional materials is comparable with the spacing between bases/monomers composing biomolecules/biopolymers. Hence, such materials may potentially provide adequate resolution for biopolymer sequencing, such as DNA strand sequencing.

Two graphene electrodes positioned very close to each other is a model system to achieve biomolecule/biopolymer sequencing. An electrical current tunnelling between the electrodes depends on the molecule travelling through the gap.

Fabrication of nanogap devices employing conventional nanofabrication techniques requires extremely high levels of resolution and alignment accuracy. The complexity of fabrication through such methods is one of the reasons why such systems have not yet been realised.

The present invention addresses the problems currently faced in the production of structures for biomolecule detection/sequencing.

The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

The present invention provides a structure and method for preparing such structures and also provides for their use in biomolecule detection and sequencing.

According to a first aspect of the invention, there is provided a structure comprising at least one pair of opposing electrodes comprising a two-dimensional electrically conductive material layer wherein the two electrodes are positioned adjacent to one another so that the planes of the two-dimensional electrically conductive material are twisted relative to one another so as to result in a single-atomic point of contact between the two electrodes, and wherein the distance between the two electrodes at the point of contact is from about 0 nm to about 500 nm. Preferably, the distance between the two electrodes at the single-atomic point of contact is about 0 nm and the two electrodes are in direct contact. Conveniently, the distance between the two electrodes at the single-atomic point of contact is greater than 0 nm but less than about 200 nm, such as from about 0.3 nm to about 200 nm, for example from about 0.3 nm to about 100 nm; and preferably from about 0.3 nm to about 50 nm, creating a nano-sized aperture between the single-atomic point of contact. Advantageously, the nano-sized aperture is the size of an atom. For example, the distance between the single-atomic point of contact between the two electrodes may be about the same size as the diameter of an atom, such as any atom in the periodic table, for example carbon, which has a Van der Waals radius of around 0.170 nm, i.e. a diameter of 0.340 nm. The skilled person will be aware of the various definitions available for the diameter of atoms of the periodic table.

Preferably, the edge of the two-dimensional electrically conductive material has been chemically modified, and contact is made through the chemically modified edges. Conveniently, the edge of the two-dimensional material has been chemically modified using organic chemistry tools such as plasma chemistry and solution chemistry, optionally, wherein the chemically modified edge comprises oxygen, nitrogen and hydrogen groups.

Advantageously, the two-dimensional electrically conductive material is supported by a substrate layer along its entire length up until its edge leaving the edge of the two-dimensional electrically conductive material exposed.

Preferably, the two-dimensional electrically conductive material is sandwiched, embedded or deposited between two substrate support layers along its entire length up until its edge leaving the edge of the two-dimensional electrically conductive material exposed.

Conveniently, the support substrate material is an epoxy resin, silicon/silicon dioxide (Si/Si0₂), silicon carbide (SiC), silicon nitride (SiN), sapphire, methacrylate polymer or thiol-ene polymer. Advantageously, the thickness of the two-dimensional electrically conductive material layer is from about 0.3 nm to about 1000 nm, such as from about 0.3 nm to about 500 nm, for example from about 0.3 nm to about 50 nm, preferably the thickness is about 0.3 nm. Preferably, the thickness of the support substrate is from about 20 nm to about 1 mm, such as from about 20 nm to about 1000 nm, for example from about 20 nm to about 500 nm, preferably from about 20 nm to about 100 nm and most preferably about 20 nm. Conveniently, the distance between the single-atomic point of contact of the two electrodes can be adjusted mechanically and/or electromechanically or otherwise.

Advantageously, the two-dimensional electrically conductive material is selected from the group consisting of graphene, borophene, germanene, silicene, stanene, molybdenum disulphide, boron nitride, tungsten diselenide, tungsten disulphide, fluorographene, phosphorene and thin films of polythiophenes, conducting polymers and metals, such as gold.

Conveniently, the two-dimensional electrically conductive material is crystalline, preferably wherein the two-dimensional material is crystalline along its entire length to its edge.

Preferably, the two-dimensional electrically conductive material layer electrodes are connected to an electrical circuit.

According to another aspect of the invention, there is provided an apparatus for biomolecule detection and sequencing, wherein the apparatus comprises a structure as detailed above.

Preferably, the apparatus is selected from a sensor, local probe microscope or biomolecule sequencer. According to a further aspect of the invention, there is provided the use of a structure or an apparatus as detailed above for biomolecule detection, and sequencing.

According to another aspect of the invention, there is provided a method of fabricating a structure as detailed above.

Preferably, the method comprises the steps of:

providing at least one pair of opposing electrodes comprising a two-dimensional electrically conductive material layer;

positioning the two electrodes adjacent to one another; and

twisting the planes of the two-dimensional electrically conductive material relative to one another so as to result in a single-atomic point of contact between the two electrodes. Conveniently, the method also comprises a step of supporting the two-dimensional electrically conductive material layer on a substrate layer so as to support the two-dimensional electrically conductive material layer along its entire length up until its edge leaving the edge of the two- dimensional electrically conductive material exposed.

Advantageously, the method also comprises sandwiching, embedding or depositing the two- dimensional electrically conductive material layer between two substrate support layers along its entire length up until its edge leaving the edge of the two-dimensional electrically conductive material exposed.

Preferably, the method further comprises the step of connecting the electrodes to an electrical circuit.

Conveniently, the method further comprises a step of chemically functionalising the edges of the two-dimensional electrically conductive material layers.

As used herein, the term "two-dimensional material" refers to a thin film material having a thickness of less than about 100 nanometers, such as a thin film of gold having a thickness of less than about 100 nanometers. Indeed, other metals may be used for the formation of thin two-dimensional films along with thin films of polythiophenes and conducting polymers. Furthermore, the term "two-dimensional material" also encompasses materials comprised of a single layer of atoms as well as to a plurality of such layers having a thickness of less than about 100 nanometers. Examples of such materials are graphene, borophene, germanene, silicene, stanene and phosphorene, boron nitride, molybdenum disulphide, tungsten diselenide, tungsten disulphide and fluorographene.

As used herein, the term "graphene" refers to a molecule in which a plurality of carbon atoms (e.g., in the form of five-membered rings, six-membered rings, and/or seven-membered rings) are covalently bound to each other to form a (typically sheet-like) polycyclic aromatic molecule. Consequently, and at least from one perspective, graphene may be viewed as a single layer of carbon atoms that are covalently bound to each other (most typically sp² bonded). It should be noted that under the scope of this definition, the term "graphene" also includes molecules in which several (e.g., two, three, four, five to ten, one to twenty, one to fifty, or one to hundred) single layers of carbon atoms are stacked on top of each other to a maximum thickness of about 100 nanometers. Consequently, the term "graphene" as used herein refers to a single layer of aromatic polycyclic carbon as well as to a plurality of such layers having a thickness of less than about 100 nanometers. Where it is stated that the two-dimensional electrically conductive material layer is supported by a substrate layer, this includes where the two-dimensional layer is positioned on the surface of a substrate layer so as to be supported on only one side and also includes where the two- dimensional layer is embedded/sandwiched between two substrate layers so as to be supported on both sides.

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

Figure 1 provides two schematic views of a structure according to the present invention comprising a pair of opposing electrodes comprising a two-dimensional electrically conductive material layer wherein the two electrodes are positioned adjacent to one another so that the planes of the two-dimensional electrically conductive material are twisted relative to one another so as to result in a single-atomic point of contact between the two electrodes. The top image illustrates such a structure where the two-dimensional electrically conductive material layers are unsupported and the bottom image illustrates such a structure where the two-dimensional electrically conductive material layers are supported. Figure 2 details the concept of the reciprocal twisting of two two-dimensional electrodes.

Figure 3 is a schematic view of a "mechanically controllable break junction" method of arriving at a supported two-dimensional electrically conductive material. Figures 4(a) and 4(b) are scanning electron microscope (SEM) micrographs of a graphene layer deposited on the surface of a silicon substrate, wherein the graphene has been broken in two using the "mechanically controllable break junctions" method.

Figure 5 is a schematic view of a "shadow mask ion etching" method of arriving at a supported two-dimensional electrically conductive material.

Figure 6(a) is an optical micrograph of PMMA assisted deposited graphene across the two halves of a Si/Si02 substrate before etching. Figure 6(b) is an optical micrograph taken after PMMA removal. Figure 7(a) shows an example of the alignment stage for the deposition of graphene, and Figure 7(b) shows a modified scanning-tunnelling microscope for positioning two graphene edges into tunnelling distance. Figures 8 and 9 detail an AFM analysis, performed in tapping mode, of the edge of graphene sheet on top of an oxidized silicon substrate after breaking with the mechanically controllable break junction method. Figure 8 shows the topography while Figure 9 shows the phase image for the same area. Figures 10 and 11 detail observations for the edge of a graphene sheet on top of an oxidized silicon substrate after etching by the shadow mask ion etching method, using scanning electron microscopy, atomic force microscopy and Raman spectroscopy.

Figure 12 is a schematic view of an "ultramicrotomy" method to arrive at a two-dimensional electrically conductive film embedded between two polymeric layers.

Figure 13 is a detailed representation of a graphene ribbon embedded within two polymeric layers. Figure 14(a) is an optical micrograph of a thiolated polymeric block embedding a graphene layer.

Figure 14(b) is an optical micrograph of a block interface embedding graphene after microtome sectioning.

Figure 15 shows the passivation of the edge of a graphene layer which has been electrochemically functionalised with nitrodiazobenzene ( O2C6H4N2).

Figure 16 details the current analysis between two graphene electrodes at 300K, as obtained by the mechanically controllable break junctions method.

Figure 17 details the current analysis between two graphene electrodes at 4K, as obtained by the mechanically controllable break junctions method. Figure 18 details the current analysis between two graphene electrodes at 300K, as obtained by the shadow mask ion etching method. DETAILED DESCRIPTION OF THE INVENTION

The invention provides a structure comprising at least one pair of opposing electrodes comprising a two-dimensional electrically conductive material layer wherein the two electrodes are positioned adjacent to one another so that the planes of the two-dimensional electrically conductive material are twisted relative to one another so as to result in a single-atomic point of contact between the atomically thin edges of the opposing electrodes.

Figure 1 is an illustration of a structure according to the present invention. The upper image of Figure 1 details two electrically conductive two-dimensional material layers that are positioned adjacent to one another. The layers have been twisted relative to one another so as to result in a single-atomic point of contact between the two layers. The two-dimensional layers may be comprised of any suitable material. Examples of such two-dimensional materials are graphene, borophene, germanene, silicene, stanene, molybdenum disulphide, tungsten diselenide, boron nitride, tungsten disulphide, fluorographene, phosphorene and thin films of polythiophenes, conducting polymers and metals, such as gold. Preferably, the two- dimensional material is graphene.

The structure comprises a single-atomic point of contact between the edges of the two two- dimensional layers. The distance between the single-atomic point of contact may be from about 0 nm to about 500 nm, such as from about 0 nm to about 200 nm, for example from about 0 nm to about 100 nm, preferably from about 0 nm to about 50 nm, more preferably the distance is about 0 nm. In an embodiment, the single atomic contact point can be adjusted mechanically and/or electromechanically or otherwise.

Indeed, the distance between the single-atomic point of contact may be equivalent to about the size of an atom. For example, the distance between the single-atomic point of contact between the two electrodes may be about the same size as the diameter of any atom in the periodic table, for example carbon, which has a Van der Waals radius of around 0. 70 nm, i.e. a diameter of 0.340 nm.

The electrode two-dimensional layers may be incorporated into an electronic circuit with the electronic current passing across the single-atomic contact point. This single-atomic contact point within the structure may act as a nanofluidic channel for the migration of, for example, biomolecules in the final setup. In the embodiment where the structure is used for biomolecule detection, and sequencing, when a single strand of a biopolymer passes across the single-atomic contact point, the electrical current passing across the contact point will fluctuate depending on the particular monomer unit passing across the single-atomic contact point of the two atomically thin electrode edges at any given time. Biopolymers such as single-stranded DNA may be passed between the two electrically conductive two-dimensional material contacts through the single- atomic contact point. Thereby the biopolymer will modulate the tunnel current between the electrode edges, depending on the identity of the monomer in the proximity of the contacts. Adjusting the tilt angle of the two planes permits the fine-tuning of the effective width of the tunnelling current path on the atomic scale, i.e. the number of atoms at the edge involved in the tunnelling process. The mechanical adjustment of the distance also allows for enough space for molecule to pass between the contact points of the two electrodes whilst maintaining a large enough current passing across the contact points for detection. The distance between the contact points on the edges of the two-dimensional material layers can be adjusted by using piezo-electric actuators.

In a preferred embodiment, the electrically conductive two-dimensional material is crystalline all the way up to the edges.

Preferably, the edges of the electrically conductive two-dimensional material layers are functionalized by suitable edge chemical modification in order to optimize the speed of translocation relative to the read out bandwidth, and in order to exploit the advantages of recognition tunnelling.

The lower image of Figure 1 details a structure according to another embodiment of the present invention. In this embodiment, the electrically conductive two-dimensional material is supported upon a suitable substrate which supports the two-dimensional layer along its entire length up until its edges wherein the atomically sharp edges of the two-dimensional electrically conductive material film are exposed.

The exposed edges allow for the electrically conductive two-dimensional layers to be positioned adjacent to one another and for a single-atomic point of contact to exist by the twisting of the two layers relative to one another.

Such structures are useful in many analytical procedures including, but not limited to, i) the atomic characterisation of molecules between the single-atomic contacts through tunnelling spectroscopic methods, ii) studying non-confined media such as gas, liquids and solids, iii) detecting molecules in motion, iv) sequencing (bio)polymers, v) characterising polymers, and vi) studying the electrical transport through single (organic and/or bioorganic) molecules. Unlike known structures wherein the electrically conductive two-dimensional materials are superimposed, parallel or facing one another, the structures of the present invention achieve fine resolution during the above-mentioned methods due to the fact that tunnelling takes place between the edges of the two electrically conductive two-dimensional materials, which are twisted relative to one another so as to arrive at a single-atomic point of contact between the two layers.

The concept of the reciprocal twisting of two two-dimensional electrodes is detailed in Figure 2. In Figure 2(a) two layers of two-dimensional conductive material are placed with their edges facing each other. A single-atomic point of contact is developed by twisting one layer with respect the other, creating a single point intersection between two atomic lines as shown in Figure 2(b). The distance between the two points of contact can be tuned in order to modulate the tunnelling current intensity flowing across the two layers, thus developing a gap as shown in Figure 2(c). METHODS FOR PREPARING SUPPORTED ELECTRICALLY CONDUCTIVE TWO-DIMENSIONAL LAYERS

"Mechanically controllable break junctions" method

Figure 3 is a detailed schematic of a method ("Mechanically controllable break junctions" method) of preparing a two-dimensional electrically conductive material film having atomically sharp supported edges, which is supported along its entire length up until its edges and, wherein the atomically sharp edges of the two-dimensional electrically conductive material film are exposed. In this figure, the electrically conductive two-dimensional material is graphene which has been deposited on the surface of a support. However, any other electrically conductive two-dimensional material, as detailed above, may be used. For example, the electrically conductive two-dimensional material may comprise a material selected from graphene, borophene, germanene, silicene, stanene, molybdenum disulphide, boron nitride, tungsten diselenide, tungsten disulphide, fluorographene, phosphorene and thin films of polythiophenes, conducting polymers and metals, such as gold and combinations thereof. Furthermore, the two-dimensional material may have been grown directly on the substrate through thermal, chemical or other means. The support substrate in this example is crystalline silicon with a silicon oxide surface layer. However, the silicon substrate may be replaced by any other suitable substrate that can be made to break with an atomically sharp break edge. For example, the support may be comprised of a material selected from the list consisting of sapphire, glass, silicon/silicon dioxide (Si/Si0₂), silicon carbide (SiC) and silicon nitride (SiN).

After the electrically conductive two-dimensional material has been deposited on the surface of the support or has been grown on top of the support, the support is broken into at least two pieces by applying a force in the centre of the opposing face of the support to the face having the two-dimensional material deposited on it, and applying force to the edges of the face of the support having the two-dimensional material deposited on it.

In an embodiment, the surface of the support may be scored so as to aid the fracturing of the support into two pieces. Preferably, the scoring may be made by microfabrication.

The application of this force results in the support breaking with an atomically sharp edge and also results in the two-dimensional layer deposited on the surface breaking or tearing to form atomically sharp edges. If the breaking of the supported two-dimensional layer is performed in a vacuum, this would lead to the formation of radicals at the break edge and ultimately to the rearrangement of the structure of the edge of the two-dimensional layer. Alternatively, the chemistry of the edge of the two dimensional material may be modified by breaking the supported two-dimensional material in a suitable environment, e.g., gaseous or liquid environment, wherein the environments contain selected compounds which react with the formed radicals thus functionalising the edges.

For example, the supported two-dimensional material may be broken in a purified water environment, which would lead to the broken edges of the two-dimensional material being functionalised by O, H or OH groups.

The edges of the two-dimensional material may be functionalised with other groups by preparing the broken edges of the two-dimensional material in different environments, such as creating the broken edge under different organic/inorganic liquids. The skilled person is aware of numerous reactions which can be performed to functionalise the edges. Crystalline silicon has naturally occurring "cleavage planes." When fractured, the crystalline silicon has a tendency to cleave or fracture along these well-defined crystal planes. By identifying and choosing a preferred geometric shape, cleavage can be promoted along a preferred plane, thus producing an atomically sharp edge.

Figures 4(a) and 4(b) are scanning electron microscope (SEM) micrographs of a graphene layer deposited on the surface of a silicon substrate, wherein the graphene has been broken in two as described above in the "mechanically controllable break junctions" method. In this embodiment the graphene layer is deposited by poly(methyl methacrylate) (PMMA) assisted deposition or alternatively grown on top of a substrate, such as in case of SiC substrates, and is subsequently broken into two parts in a controlled fashion.

Indeed, crystalline substrates such as Si/Si02 are preferred since they allow an atomically precise rupturing along the crystalline direction of the Si crystals. Alternative substrates such as quartz can be adopted. It is envisaged that non-crystalline substrates can be adopted where a precise breaking edge can be provided.

The breaking of the substrate produces a tear in the graphene producing two substrate halves covered with graphene up to the edges of the cracking line.

In the SEM image of Figure 4(a) the darker region (upper region) is the area covered by graphene, while the lighter region (the lower region) represents the surface of the Si0₂. Figure 4(b) provides a magnification of the area covered by graphene up to the edge. The lighter areas are ruptures on the graphene surface through which S1O2 is exposed.

"Shadow mask ion etching" method

Figure 5 is a detailed schematic according to another method ("Shadow mask ion etching" method) of preparing a two-dimensional electrically conductive material film having atomically sharp supported edges, which is supported along its entire length up until its edges and, wherein the atomically sharp edges of the two-dimensional electrically conductive material film are exposed. In this figure, the substrate layer is crystalline silicon with a silicon oxide surface. However, as detailed above, any other suitable substrate that can be made to break with an atomically sharp break edge may be used. The silicon substrate is broken into at least two pieces in a similar manner as described above by applying a force in the centre of one of the faces of the support and applying force to the opposing face of the support. This results in the crystalline silicon support fracturing along a crystal plane, thus producing an atomically sharp edge.

The graphene layer, which is primarily supported on a polymer stamp, is deposited onto the two halves of the fractured crystalline support and, preferably, a gap is left between the two halves of the silicon support. Graphene is used as the electrically conductive two-dimensional material in this embodiment. However, any other electrically conductive two-dimensional material may be used in place of graphene as detailed above.

The resulting structure is then exposed to a dry etching procedure. The dry etching procedure may comprise the use of use of a plasma comprising fluorocarbons, oxygen, chlorine, boron trichloride, hydrogen, air, methane, ammonia or a mixture thereof. Preferably, the etching procedure uses an oxygen plasma.

Through using a reactive plasma for the dry etching procedure, the edge of the two- dimensional material being chemically functionalised, with the functional group introduced being dependent upon the plasma used. For example, using an O2 plasma for the dry etching procedure would result in the edge of the two-dimensional material layer being functionalised with oxygen groups, whereas using an NH₃ plasma would result in the edges being functionalised with nitrogen containing moieties.

The structure is exposed to the dry etching procedure from the side comprising the silicon substrate. The fracture edge of the silicon substrate serves as an atomically sharp shadow mask, which protects the graphene up until its edge. The exposed graphene which is not covered by the silicon substrate is etched away during the dry etching procedure. After which, the polymer stamp may be removed leaving the graphene on the surface of the silicon support and being supported along its entire length up until its edges. In an embodiment, the polymer stamp is removed by dissolving in acetone.

Alternatively, the substrates are supported from the other side. The polymer stamp brings graphene in contact with the two halves of the substrate and the polymer stamp is removed to complete the transfer. The resulting suspended graphene is etched away by a similar process as described above through a hole in the support. Figure 6(a) is an optical micrograph of PMMA assisted deposited graphene across the two halves of a Si/Si02 substrate before etching whereas Figure 6(b) is an optical micrograph taken after PMMA removal. In this case the substrate is preferably a crystalline material such as Si/Si0₂ which allows atomically precise crack propagation. As above, non-crystalline materials can be employed where a precise cracking procedure can be obtained. In this embodiment, the substrate is broken before the deposition of graphene, subsequently the two halves of the substrate are precisely aligned by a mechanical alignment stage designed for this purpose, which may be seen in Figure 7(a). The distance between the two halves of the substrate can vary between a few nano-meters to a few hundred micro-meters. Afterwards, a two-dimensional material, such as graphene, is deposited by PMMA (or another protective polymer) assisted deposition on top of the two halves of the substrate. The graphene is suspended over the gap between the two halves of the substrate. Subsequently the suspended part of graphene is etched (e.g. by dry etching) and, finally, the PMMA is dissolved in acetone (or ethyl acetate in the case of using cellulose acetate butyl (CAB) as the substrate).

This process uncovers the graphene that extends up to the edges of the two halves of the substrate, while the suspended portion has been etched. In fact, the etching is selective and etches only the suspended portion as long as the rest of the graphene is sandwiched between the PMMA (or other polymeric protection, e.g. CAB) and the substrate.

Finally, the two sides of the substrate are precisely carried in proximity by a second alignment system which can reduce the distance between the electrodes down to the spacing required to allow the instauration of a tunnelling current. This system may be seen in Figure 7(a), which shows an example of the alignment stage for the deposition of graphene. The two halves of the substrate are installed on a tray which regulates the distance between the substrate parts by means of a microstep manipulator (in this case the stage can manipulate the distance to within an order of pm). Figure 7(b) shows a modified scanning-tunnelling microscope, which may also be used for positioning two graphene edges into tunnelling distance as detailed above.

Figure 8(a) is an AFM image, performed in tapping mode, of the edge of graphene sheet on top of an oxidized silicon substrate after breaking with the mechanically controllable break junction method. For most of the edge the graphene extends all the way to the edge of the support, to within the resolution of about 1 nm of the AFM images. Here, a part of the edge was selected that has a defect such that the contrast to the silicon oxide substrate is visible in the top right part neighbouring the edge. The contrast is more clearly visible in the phase image of Figure 9. The step height along the line drawn in Figure 8(a) is shown in Figure 8(b). The height distribution in a rectangular strip around the line in Figure 8(a) is shown in Figure 8(c). The height profile suggests that the step is a single layer of graphene.

Figures 10 and 1 1 provide further information regarding the characterization of a graphene layer prepared via the shadow mask ion etching method detailed above.

Figure 10(c) is a scanning electronic microscopy (SEM) of a graphene layer extending to the edge of the S1O2 support. The uniformity of the grey scale in this image indicates the homogeneity of the graphene sheet along the entire of the support.

Figures 10(a) and 10(b) are two Raman spectra taken on the surface of the graphene layer and at the edge, respectively. These figures show that only graphene is observed and they also illustrate the quality of the graphene layer and the fact that the same quality is preserved at the edge.

Figure 11 shows an atomic force microscopy (AFM) image of the surface of the graphene layer prepared by the shadow mask ion etching method. The edge of the graphene and substrate is seen as the dark band on the left. The inset is a line trace taken over the edge of a small defect of missing graphene (shown as a white line at the lower left in the main image), which agrees with the expected height profile of a graphene sheet on top of Si0₂. Nevertheless, in the surrounding regions the graphene extends uniformly up to the edge. "Ultramicrotomy" method

Figure 12 is a detailed schematic of another method ("Ultramicrotomy" method) of preparing a two-dimensional electrically conductive material film having atomically sharp supported edges, which is supported along its entire length up until its edges and, wherein the atomically sharp edges of the two-dimensional electrically conductive material film are exposed.

As shown in Figure 12, graphene that has been grown on a copper support is deposited on the surface of an epoxy resin layer which has been stained with a staining agent selected from Rhodamine B, pyrene derivatives, oxazine derivatives, acridine derivatives, fluorescein and any other fluorophores that fluoresce in the visible spectrum, preferably, Rhodamine B is used as the staining agent. The graphene film is placed so as to be in contact with the surface of the epoxy resin layer so that the copper support is on top of the graphene. Therefore, the graphene film is sandwiched between the epoxy resin layer and the copper support. In this example, graphene is the electrically conductive two-dimensional material referred to above, although graphene may be substituted for any other electrically conductive two-dimensional material as detailed herein.

Although the graphene used in this particular example has been grown on copper, graphene, or indeed any other electrically conductive two-dimensional material grown on any other suitable support, such as nickel, ruthenium and iridium may be used. Additionally, graphene from any other sources, such as exfoliated graphite or chemically reduced graphene.

A wire is then deposited on the surface of the copper support and is fixed to the support using a metallic paint, preferably a silver paint, or through the use of soft metals, such as indium or through the deposition of metallic leads through a mask. The wire is present so that the electrically conductive two-dimensional material (graphene) layer may be connected to an external circuit as required.

The structure is then subjected to an etching procedure to remove the copper support via the use of an etchant solution. Suitable etchants include, but are not limited to, ammonium persulfate, ferric chloride, hydrofluoric acid, ethyienediamine pyrocatechol, aqua regia, tetramethylammonium hydroxide, potassium hydroxide, potassium cyanide and hydrogen peroxide. Preferably, ammonium persulfate is used as the etchant. After which, a second epoxy resin layer is deposited on top of the electrically conductive two-dimensional material layer thus sandwiching the electrically conductive two-dimensional material layer between the two polymeric layers. This second layer is not stained with Rhodamine B. Therefore, as one polymeric layer is stained and the other is not, this allows easy visualisation of the interface between the two layers, thus allowing identification of the position at which the electrically conductive two-dimensional material lies between the two epoxy layers.

After the electrically conductive two-dimensional material layer has been sandwiched between these epoxy resin layers, the resulting block is sliced at a perpendicular angle to the basal plane of the electrically conductive two-dimensional material layer using ultramicrotomy methods discussed herein to arrive at a polymer sheet comprising an electrically conductive two-dimensional material layer extending from the upper surface to the lower surface of the polymer sheet. This may be better visualised in Figure 13, which shows a thin epoxy resin layer, of around 20 to 1000nm, comprising a graphene ribbon, which is the electrically conductive two-dimensional material in this example, extending from the upper to the lower surface of the epoxy resin layer. In the ultramicrotomy method, the graphene, which is the two-dimensional material in this embodiment, is sandwiched between two polymeric layers (e.g. epoxy resin or thiolated polymer layers). The conductive two-dimensional material, such as graphene, may be grown on a sacrificial substrate, e.g. Cu and deposited on an uncured polymer with the two- dimensional material in contact with the polymer and exposing the sacrificial substrate. The polymer is then cured and the sacrificial substrate is etched in an etching solution, such as ammonium persulfate or iron chloride where the sacrificial substrate is Cu. A second layer of polymer is subsequently applied. Alternatively, the first layer of polymer can be firstly cured and then the two-dimensional material can be deposited, for example, by PM A assisted transfer (or another protective polymer such as CAB), as in the case of graphene. Afterwards the PMMA may be removed by, for example, dissolution in acetone. Subsequently a second layer of polymer is applied. One of the polymeric layers can be modified by the addition of a fluorescent dye in order to highlight the contrast between the two layers simplifying the localization of the graphene film in the sample.

The two-dimensional conductive material, such as graphene, is electrically connected by, among the others, Ag paste, wire bonding, lithography or evaporation of the electrode(s), before the application of the second layer of polymer.

The two-dimensional material sandwiched between the two polymeric layers, referred to as the block, is subsequently sectioned by microtomy.

The microtomy produces thin slabs of polymer (from a few to a few hundred nm) embedding a graphene layer whose edges are exposed. It is these edges that may be used in the present invention to arrive at a single-atomic point of contact. Alternatively, two blocks can be sectioned by microtomy and employed to develop the tunnelling device. Indeed, the microtome sections the surface of the block exposing the edge of the two-dimensional material such as graphene, which can be employed as the tunnelling electrode. The electrical connection developed before the application of the second layer of polymer of the block conveniently fits the intended purpose.

Figure 14(a) is an optical micrograph of a thiolated polymeric block embedding a graphene layer. In the present case none of the layers are modified by a fluorescent dye. The wire is connected to graphene by Ag paste.

Figure 14(b) is an optical micrograph of a block interface embedding graphene after microtome sectioning. The fluorescent dye induces the contrast between the lower and the upper layers. The centre line represents the interface between the two layers where the graphene is embedded.

As with the "mechanically controllable break junctions" method described above, the chemistry of the edge of the two-dimensional material prepared via the microtomy method may be modified by slicing the embedded two-dimensional material onto the surface of an aqueous solution containing selected compounds which react with the edge of the two-dimensional layer thus functionalising the edges.

For example, the supported two-dimensional material may be sliced into a purified water environment, which would lead to the broken edges of the two-dimensional material being functionalised by O, H or OH groups. Alternatively, the edge of the two dimensional material may be electrochemically modified with various chemical groups, such as, for example, nitrodiazobenzene. Figure 15 shows the passivation of the edge of a graphene layer which has been electrochemically functionalised with nitrodiazobenzene ( O2C6H4N2). In this embodiment, a graphene layer was embedded between two thiol-functionalised polymer layers and was wired as previously described. The edge of the embedded graphene layer was placed in HCI (0.1 M) aqueous solution containing nitrodiazobenzene at a concentration of 5 mM. The edge of the graphene was used as the working electrode, whereas Ag/AgCI was used as the counter electrode.

50mV/s sweeps were performed in order to promote the radicalization of the diazocompound into ( O2C6H4*) which readily binds to the exposed edge of the graphene. The nitro groups passivate the graphene edge reducing the exchange current with the redox probe, which can be seen in the graph provided in Figure 15. The sample was tested in 5mM K4(Fe(ll)(CN)6) before and after the functionalization step. The edge was tested by cyclovoltammetry (CV) at 50mV/s.

In a further embodiment, the edges of the two-dimensional material may be modified using a plasma as with the "shadow mask ion etching" method. Indeed, it is envisaged that the above methods may be combined/mixed to arrive at a two-dimensional electrically conductive material film having atomically sharp supported edges. For example, after breaking a support and a two-dimensional material layer deposited on its surface, any residual overhanging fragments of the two-dimensional material layer may be removed by dry etching, e.g. through the use of a plasma as detailed above, or by electrochemical methods exploiting the concentration of the voltage drop at the point of contact between two electrodes prepared by one of the methods described above.

These methods described above all arrive at an electrically conductive two-dimensional material film having atomically sharp supported edges which is supported along its entire length up until its edges, wherein the atomically sharp edges of the electrically conductive two- dimensional material are exposed. This allows for the exposed edges of the electrically conductive two-dimensional material to be chemically modified. For example, the edges of the electrically conductive material may be chemically modified to comprise oxygen, hydrogen or nitrogen groups.

Functionalisation of the edge of the electrically conductive two-dimensional material allows the creation of suitable binding sites for enhanced selectivity and, therefore, such functionalised electrically conductive two-dimensional materials are suitable for use in structures according to the invention for biomolecule detection and sequencing.

CURRENT ANALYSIS BETWEEN TWO GRAPHENE ELECTRODES

Graphene was deposited on a Si/Si02 wafer by PMMA polymer assisted transfer as described above in relation to "mechanically controllable break junctions" method.

After transferring the graphene layer, the PMMA was removed by immersion in acetone and the two extremities of the graphene layers were connected electrically by Ag paint. The supported graphene layer was loaded inside a cryostat and the wafer was broken by applying force at the middle of the wafer. The two parts of the wafer were put back in proximity and set apart again. The current across the graphene layer was measured at 300K during each step in this procedure, the results of which may be seen in Figure 16. This procedure was repeated on a second sample at 4K, the results of which may be seen in Figure 17. After breaking, the graphene assumes strongly non-linear current-voltage characteristics.

The results detailed in Figure 16 and 17 show that it is possible to break the graphene and apply a current across the two halves of the layer by putting them back into proximity with one another. In a separate test run, graphene was deposited on top of two Si/Si02 substrates that were separated from each other by fracture, according to the shadow mask ion etching method. The two twisted graphene edges were interfaced at a nanometric distance as depicted in Figure 1. Figure 18(a) details the tunnelling curve across two graphene electrodes prepared according to the "shadow mask ion etching" method detailed herein. The symmetricity of the sigmoidal curve with respect to the potential ( bias) confirms the tunnelling is developed across two electrodes of identical composition, thus two graphene films. The relationship between the nanogap distance and the tunnelling current is detailed in Figure 18(b). The exponential decay in function of the distance confirms the instauration of a tunnelling regime across the electrodes.

Claims

A structure comprising at least one pair of opposing electrodes comprising a two- dimensional electrically conductive material layer wherein the two electrodes are positioned adjacent to one another so that the planes of the two-dimensional electrically conductive material are twisted relative to one another so as to result in a single-atomic point of contact between the two electrodes, and wherein the distance between the two electrodes at the point of contact is from about 0 nm to about 500 nm.

The structure according to Claim 1 , wherein the distance between the two electrodes at the single-atomic point of contact is about 0 nm and the two electrodes are in direct contact.

The structure according to Claim 1 , wherein the distance between the two electrodes at the single-atomic point of contact is greater than 0 nm but less than about 200 nm, such as from about 0.3 nm to about 200 nm, for example from about 0.3 nm to about 100 nm; and preferably from about 0.3 nm to about 50 nm, creating a nano-sized aperture between the single-atomic point of contact.

The structure according to Claim 3, wherein the nano-sized aperture is the size of an atom.

The structure according to any preceding claim, wherein the edge of the two- dimensional electrically conductive material has been chemically modified, and contact is made through the chemically modified edges.

The structure according to Claim 5, wherein the edge of the two-dimensional material has been chemically modified using organic chemistry tools such as plasma chemistry and solution chemistry, optionally, wherein the chemically modified edge comprises oxygen, nitrogen and hydrogen groups.

The structure according to any preceding claim, wherein the two-dimensional electrically conductive material is supported by a substrate layer along its entire length up until its edge leaving the edge of the two-dimensional electrically conductive material exposed.

The structure according to any preceding claim, wherein the two-dimensional electrically conductive material is sandwiched, embedded or deposited between two substrate support layers along its entire length up until its edge leaving the edge of the two-dimensional electrically conductive material exposed.

The structure according to Claim 7 or Claim 8, wherein the support substrate material is an epoxy resin, silicon/silicon dioxide (Si/SiC^), silicon carbide (SiC), silicon nitride (SiN), sapphire, methacrylate polymer or thiol-ene polymer.

The structure according to any preceding claim, wherein the thickness of the two- dimensional electrically conductive material layer is from about 0.3 nm to about 1000 nm, such as from about 0.3 nm to about 500 nm, for example from about 0.3 nm to about 50 nm, preferably the thickness is about 0.3 nm.

The structure according to any of Claims 7 to 10, wherein the thickness of the support substrate is from about 20 nm to about 1 mm, such as from about 20 nm to about 1000 nm, for example from about 20 nm to about 500 nm, preferably from about 20 nm to about 100 nm and most preferably about 20 nm.

The structure according to any preceding claim, wherein the distance between the single-atomic point of contact of the two electrodes can be adjusted mechanically and/or electromechanically or otherwise.

The structure according to any preceding claim, wherein the two-dimensional electrically conductive material is selected from the group consisting of graphene, borophene, germanene, silicene, stanene, molybdenum disulphide, boron nitride, tungsten diselenide, tungsten disulphide, fluorographene, phosphorene and thin films of polythiophenes, conducting polymers and metals, such as gold.

The structure according to Claim 13, wherein the two-dimensional electrically conductive material is crystalline, preferably wherein the two-dimensional material is crystalline along its entire length to its edge.

15. The structure according to any preceding claims, wherein the two-dimensional electrically conductive material layer electrodes are connected to an electrical circuit.

16. An apparatus for biomolecule detection and sequencing, wherein the apparatus comprises a structure according to any of Claims 1 to 15. 7. An apparatus according to Claim 16, wherein the apparatus is selected from a sensor, local probe microscope or biomolecule sequencer. The use of a structure according to any of Claims 1 to 15 or an apparatus according to Claim 16 or Claim 17 for biomolecule detection, and sequencing.

A method of fabricating a structure according to any of Claims 1 to 15. The method according to Claim 19 comprising the steps of:

positioning the two electrodes adjacent to one another; and

twisting the planes of the two-dimensional electrically conductive material relative to one another so as to result in a single-atomic point of contact between the two electrodes.

The method according to Claim 20, wherein the method also comprises a step of supporting the two-dimensional electrically conductive material layer on a substrate layer so as to support the two-dimensional electrically conductive material layer along its entire length up until its edge leaving the edge of the two-dimensional electrically conductive material exposed.

The method according to Claim 20 or Claim 21 , wherein the method also comprises sandwiching, embedding or depositing the two-dimensional electrically conductive material layer between two substrate support layers along its entire length up until its edge leaving the edge of the two-dimensional electrically conductive material exposed.

The method according to any of Claims 20 to 22, further comprising the step of connecting the electrodes to an electrical circuit.

The method according to any of Claims 20 to 23, wherein the method further comprises a step of chemically functionalising the edges of the two-dimensional electrically conductive material layers.