BACKGROUND The present invention relates to methods of transferring two-dimensional materials onto substrates.

Typically, two-dimensional materials are supported by solid and hard materials. The optimal transfer of chemically grown two-dimensional materials is extremely important because it can affect the quality of the material by contamination of the material, adding stress or causing crystalline damage.

The most common method for transferring two-dimensional materials is by coating them temporarily with a polymer, which prevents crack formation/migration and folding of its surface. However, this polymer-assisted procedure results in the surface of the two-dimensional material being contaminated by polymer residue. Also, the application of different chemicals in the transfer process can alter the resulting quality of the two-dimensional material. Therefore, in the recent years, novel polymer-free transfer methods have been developed most of which, use physical supports of a micrometre length size.

Manipulating two-dimensional materials, such as graphene, directly on the surface of an aqueous solution is desirable because the surface of such a solution is ultimately flat, clean and two-dimensional. Therefore, aqueous solutions represent an ideal substrate for two- dimensional materials. However, using an aqueous solution as a substrate for supporting two- dimensional materials presents its own challenges due to the breaking and folding of two- dimensional materials when floating on an aqueous solution. The vibration of the surface of the aqueous solution can induce cracks, thus damaging the lattice of said two-dimensional materials. It is, therefore, desirable to have an improved dynamic lateral support for polymer-free transfer of two-dimensional materials.

It has surprisingly been found that a flexible and dynamic lipid-based scaffold is able to laterally clamp two-dimensional materials at their edges. The development of a scaffold prevents such two-dimensional materials from i) breaking apart, ii) vibrating and iii) moving around. This immobilization by so-called Langmuir-Blodgett films have been found to provide a new strategy not only for the transfer of two-dimensional materials, but also for the study of two- dimensional materials on the surface of aqueous solutions.

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 method of transferring a two-dimensional material to a substrate.

According to a first aspect of the invention, there is provided a method of transferring a two- dimensional material to a substrate comprising:

(i) providing a two-dimensional material on a support;

(ii) positioning the supported two-dimensional material on the surface of an aqueous solution within a container, wherein the support is in contact with the surface of the aqueous solution;

(iii) laterally clamping the supported two-dimensional material using a molecular clamp;

(iv) removing the support by chemical means; and

(v) transferring the two-dimensional material onto a substrate.

Advantageously, the molecular clamp is selected from a lipid monolayer, cluster of molecules or macroscopic beads. Preferably, the molecular clamp is a lipid monolayer, optionally wherein the lipid is selected from the group consisting of 1,2-dipalmitoyl-sr?-glycero-3-phosphocholine, 1 ,2-dilauroyl-sn- glycero-3-phosphocholine, 1,2-dimyristoyl-sn-glycero-3-phosphocholine or 1 ,2-distearoyl-sn- glycero-3-phosphocholine. Conveniently, in step (iv) the support is removed by an etchant present in the aqueous solution.

Advantageously, the etchant is selected from ammonium persulfate, iron (III) chloride, nitric acid, ammonium hydroxide/hydrogen peroxide, sulfuric acid/hydrogen peroxide, hydrofluoric acid, ethylenediamine pyrocatechol, aqua regia, tetramethylannmonium hydroxide, potassium hydroxide, potassium cyanide, hydrogen peroxide/sodium bisulphate, potassium nitrate/hydrochloric acid, sodium persulfate or iron (III) nitrate, preferably wherein the etchant is ammonium persulfate.

The choice of etchant is dependent on the material supporting the two-dimensional material. Preferably when copper is used as the support, then an aqueous solution of ammonium persulfate may be selected as the etchant.

Conveniently, the substrate is selected from silicon wafer, a TEM quantifoil grid, TEM nanochips, silicon nitride chips or glass chips.

Advantageously, the two-dimensional material support is a catalytic metal, preferably wherein the metal is selected from copper, nickel, platinum, gold, palladium, iridium, ruthenium, cobalt, rhodium, rhenium or iron, more preferably wherein the metal is copper. Preferably, the two-dimensional material is selected from the group consisting of graphene, borophene, germanene, silicene, stanene, molybdenum disulphide, boron nitride, tungsten diselenide, tungsten disulphide, fluorographene and phosphorene.

Conveniently, the two-dimensional material is graphene.

Advantageously, the two-dimensional material is graphene and the support is comprised of copper.

In an embodiment, the method further comprises the step of replacing the aqueous solution with pure water after the support has been removed so as to remove contaminants from the solution and the surface of the two-dimensional material.

Advantageously, the substrate is placed at the bottom of the container before positioning the supported two-dimensional material on the surface of an aqueous solution within the container.

Preferably, the two-dimensional material is transferred onto the substrate by removing the aqueous solution from the container and lowering the two-dimensional material onto the surface of the substrate.

In another embodiment, the two-dimensional material is transferred onto the substrate by lowering the substrate onto the upper surface of the two-dimensional material and retracting the substrate so as to result in the two-dimensional material adhering to the substrate and being removed from the surface of the aqueous solution. Advantageously, the transferred two-dimensional material on the substrate is rinsed with pure water so as to remove contaminants from the two-dimensional material.

Conveniently, the surface of the substrate which comes into contact with the two-dimensional material is coated with a lipid monolayer, optionally wherein the lipid is selected from the group consisting of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1 ,2-dilauroyl-sn-glycero-3- phosphocholine, 1 ,2-dimyristoyl-s/ glycero-3-phosphocholine or 1 ,2-distearoyl-sn-glycero-3- phosphocholine.

Preferably the substrate is coated with a lipid monolayer by a process comprising the steps of:

(i) submerging the substrate in water contained within a vessel;

(ii) preparing a lipid monolayer on the surface of the water;

(iii) retracting the substrate from the trough and passing it through the lipid monolayer; and

(iv) transferring the lipid monolayer onto the surface of the substrate.

According to another aspect of the invention, there is provided a two-dimensional material on a substrate obtainable according to the methods detailed above.

As used herein, the term "two-dimensional material" refers to a material comprised of a single layer of atoms as well as to a plurality of such layers having a thickness of less than about 100 nanometres. Examples of such two-dimensional materials are graphene, borophene, germanene, silicene, stanene, molybdenum disulphide, boron nitride, tungsten diselenide, tungsten disulphide, fluorographene and phosphorene.

As used herein, the term "graphene" refers to a molecule in which a plurality of carbon atoms (e.g., in the form of f ive-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, a graphene may be viewed as a single layer of carbon atoms that are covalently bound to each other (most typically sp ² 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 nanometres. 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 nanometres.

As used herein, the term "clamp" in reference to molecular clamping of two-dimensional materials, means that the two-dimensional material sheet is mechanically pushed and held in place at its edges by the presence of the clamping molecules. There is no covalent bonding between the clamping molecules and the two-dimensional material.

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

Figure 1 provides a schematic of an embodiment of the method of the invention.

Figure 2 provides a schematic of another embodiment of the method of the invention.

Figure 3 shows optical images of the movement of copper foil supports on the surface of an aqueous solution which are laterally clamped at varying surface pressures.

Figure 4 provides optical images of graphene having been transferred onto a silicon wafer with thermally oxidized surface of 285 nm thickness, using the method of the invention at various lipid surface pressures.

Figure 5 shows fluorescent quenching microscopy images of graphene transferred with different surface pressures of lipids and the corresponding images after processing.

Figure 6(a) shows different surface pressure of lipids as function of the etching time of the copper foil.

Figure 6(b) shows IR-ERS spectra of each graphene transfer with the different compression of lipids.

Figure 6(c) shows rupture index of the different surface pressure of the lipids.

Figure 6(d) shows raman spectra of graphene transferred to SiC>2/Si substrate with different surface pressures of lipids.

DETAILED DESCRIPTION OF THE INVENTION Lipid monolayers spread on aqueous solution interfaces always present a finite surface tension due to lipid/air interactions. As described in A. Girard-Egrot et al., Nanobiotechnology of Biomimetic Membranes, 2007, K. Klopfer ef a/., J. Colloid Interface Sci., 1996, 182, 220- 229 and S. Baoukina ef a/., Langmuir: the ACS journal of surfaces and colloids, 2007, 23, 12617-12623, lipids may be deposited on the surface of an aqueous solution of a Langmuir trough and compressed, causing the lipid layer to pass through different characteristic phases, such as the gaseous state (G), the liquid expanded state (LE), the liquid condensed state (LC) and the solid state (S). By plotting the surface pressure as function of the area compressed on the trough, the compression isotherms exhibit plateaus characteristic for different lateral packing phases.

If an object, for example a two-dimensional material, is initially introduced at the surface of an aqueous solution and lipids are subsequently deposited onto this aqueous surface, as the intermolecular distance between the lipids decreases, the force exerted by the monolayer film of lipids will also be transmitted to the two-dimensional material, inducing pressure on its edges. Therefore, the dynamic pressure of the lipid layer can keep the domains of two- dimensional material together and prevent expansion/formation of cracks. According to the method of the invention, an atomically thin material, for example a two- dimensional material, is positioned at the liquid/air interface of an aqueous solution and is subsequently caged at the edges.

This method may be applied to any two-dimensional material. For example, the two- dimensional material may be selected from the group comprising graphene, borophene, germanene, silicene, stanene, molybdenum disulphide, boron nitride, tungsten diselenide, tungsten disulphide, fluorographene and phosphorene. Preferably, the two-dimensional material is graphene. The two-dimensional material is provided on a support. For example, the support may be a catalytic metal, such as a metal selected from the group comprising copper, nickel, platinum, gold, palladium, iridium, ruthenium, cobalt, rhodium, rhenium or iron. Preferably, the support is a copper support. Any suitable lipid which forms a monolayer on the surface of an aqueous solution may be used for laterally clamping the supported two-dimensional material. For example, 1 ,2-dilauroyl-sn- glycero-3-phosphocholine, 1 ,2-dimyristoyl-sr?-glycero-3-phosphocholine or 1 ,2-distearoyl-sn- glycero-3-phosphocholine may be used. Preferably, 1 ,2-dipalmitoyl-s/7-glycero-3- phosphocholine (DPPC) lipids are used.

Additionally, the lateral clamps material is not limited solely to the use of lipid molecules. Any other molecules, clusters of molecules or even macroscopic objects, such as small and light beads that float on the surface of the aqueous solution, may be used.

The resulting two-dimensional material may be used in studying the molecular insights of two- dimensional material-lipid interfaces for bio-sensing applications.

EXAMPLES

Example 1 1 ,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipids were dissolved in an organic solution of chloroform/methanol (3:1 ) and deposited on the air/liquid interface of an aqueous solution containing ammonium persulfate (APS) within a Langmuir-Blodgett trough. The monolayer was compressed until a surface pressure of 60 mN/m and the surface pressure- area compression isotherm of the system was determined. The isotherm obtained was used to reproduce the same surface pressure conditions on the surface of an aqueous solution contained within petri-dishes.

Lipid monolayers prepared with different surface pressures (0-50 mN/m) were adsorbed on the air/liquid interface of aqueous solutions containing APS at a concentration of 0.5 M contained within petri-dishes immediately after placing graphene supported on copper on the surface of the aqueous solution (see Figure 1a and 1b and Figure 2a and 2b).

The copper foil was instantly surrounded by the lipids which for high surface pressures (≥ 5 mN/m) prevented any noticeable movement during all the etching procedure, this may be observed in Figure 3. It was determined that for surface pressures≥ 20 mN/m, the lipid film is in a LC state, characterized for being a well-organized and compact layer that consequently holds the graphene in place through the process.

The effect of the lipids on stabilizing the edges of graphene during etching was monitored. Small vibrations on the surface of the etchant forced the copper foils in the petri-dishes with low surface pressures (< 10 mN/m) to move during etching. For higher surface pressures, however, the foils were fixed in position throughout the etching procedure. Movement of the copper foil induced cracks and folding of the supported graphene, as observed for surface pressures of 0 and 5mN/m. For surface pressures > 10 mN/m the foil remained substantially immobilised during the etching procedure. Once the copper foil was etched away, the graphene was ready to be transferred onto the final substrate (see Figures 1a to 1c and Figures 2a and 2b). Two different approaches were used for the transfer. In the air transfer approach, a Si/Si02 substrate was lowered from above onto the graphene and gently contacted with the surface of the graphene (see Figure 2c). The graphene was then transferred onto the substrate, and immediately after this fishing step, APS contaminants remaining on the surface of graphene were rinsed with a continuous flow of ultrapure water.

In the aqueous transfer approach, however, the APS was replaced by ultrapure water with a continuous flow of water in and APS out (see Figure 1c). The surface of the water was then lowered to gently place graphene onto the substrate, prepositioned in the petri-dish (see Figure 1d)

Figure 4 shows the optical microscope images of the several graphene samples with different surface pressures transferred by air transfer to Si/Si0 ₂ substrate. Clearly, the sample without any lipid lateral support suffers from large cracks (see Figure 4a, 0 mN/m). Indeed, the low amplitude vibrations of the surface of the etchant and the transfer could be responsible for the resulting damage. The graphene sheets transferred in the presence of the lipid monolayer at the edges, however, contain less cracks and surface damage, showing the advantage of the lateral clamp support. Some wrinkles are visible in the samples transferred with surface pressures > 30 mN/m (see Figure 4c and 4d). Such wrinkles are generally aligned parallel and perpendicular to the edges of the graphene and can be attributed to the lateral pressure due to the surrounding lipids.

Infrared External Reflection Spectroscopy (IR-ERS) measurements were acquired to confirm whether lipids interact with the basal plane of graphene after the transfer to Si/Si02 substrate (see Figure 6b). Surprisingly, no significant absorption bands characteristic for the stretching vibrations of the lipid acyl chains were presented. This suggests that for higher surface pressures, where a delay on the etching time was observed, the interaction between the lipid molecules underneath graphene and the water is stronger than the lipids-graphene interaction, allowing the molecules to remain at the interface. Also, for terms of comparison, the infrared spectra of well-organized lipid monolayer transferred on a Si/Si0 ₂ substrate by Langmuir-Blodgett technique (surface pressure of 30mN/m) with a graphene sheet above was measured. The results show an intense absorbance on the symmetric and asymmetric methylene vibrations of the lipid acyl chains at around 2910cm ^-1 and 2845cm ^-1 , confirming the assumption that the lipids remained at the interface. Due to the use of an aqueous solution as a support, the basal plane of the graphene is not contaminated allowing the fabrication of clean devices directly on the surface of an aqueous solution. Therefore, we also developed a novel polymer-free transfer, preventing polymeric contamination and folding of graphene. With this non-covalent concept, graphene can be manipulated from the edges leading additionally to an optimal transfer to arbitrary substrates.

Figure 5 shows the fluorescence quenching microscopy images of the several graphene samples with different surface pressures transferred to a SiCVSi substrate. Clearly, the sample without any lipid lateral support suffers from large cracks (Figure 1 , 0 mN/m). Indeed, the low amplitude vibrations of the surface of the etchant and the transfer could be responsible for such damages. The graphene sheets transferred in the presence of the lipid monolayer, however, are continuous to a large extent presenting less cracks and damage at the surface with increasing surface pressures, showing the advantage of the lateral clamp support. Therefore, small cracks are formed due to presumably some vibration of the solution during transfer. This is confirmed by the rupture index calculated for graphene transferred with different lipid surface pressures, as shown in Figure 6(c).

Figure 6(a) shows different surface pressure of lipids as function of the etching time of the copper foil. Figure 6(b) shows IR-ERS spectra of each graphene transfer with the different compression of lipids.

Figure 6(c) shows rupture index of the different surface pressure of the lipids. Figure 6(d) shows raman spectra of graphene transferred to Si02/Si substrate with different surface pressures of lipids.

The required time for etching the copper (t _e t _C h) was in average different for the distinct petri- dishes. The estimated f _ec /?as the function of surface pressure (Figure 6a) was plotted: as the etching is a gradual process, the time span was defined as that being between the moment some parts of the foil started to be transparent up to the moment the foil was not visible anymore, as the etching time ranges. Generally, increasing the surface pressure increases tech. Indeed, internal pressure in the lipid monolayer most probably drives lipids towards the foil/etchant interface; migrated lipids can form a layer at the interface which delays the etching. The delay depends on the population of the migrated lipid molecules which itself is governed by the surface pressure. It was noticed that the petri-dish with the highest surface pressure (50 mN/m) does not follow this trend. Again, in line with what was mentioned above, this result suggests that a considerable amount of the lipids collapsed reducing the surface pressure at the interface and consequently underneath the foil.

The quality of the graphene transferred with different lipid surface pressures was analysed by Raman spectroscopy (Figure 6d). Each sample showed identical results, observing a sharp and intensive 2D peak (-2680 cm ^-1 ) characteristic for a single layer graphene, a very weak D peak at -1350 cm ^-1 and a pointed G peak (-1590 cm ^-1 ), confirming the excellent quality of graphene.