WO2014064057A2

WO2014064057A2 - Method for preparing a substantially clean monolayer of a two-dimensional material

Info

Publication number: WO2014064057A2
Application number: PCT/EP2013/071992
Authority: WO
Inventors: Jean-Nicolas LONGCHAMP; Conrad Escher; Hans-Werner Fink
Original assignee: Universität Zürich
Priority date: 2012-10-23
Filing date: 2013-10-21
Publication date: 2014-05-01
Also published as: WO2014064057A3

Abstract

A method for preparing a substantially clean monolayer of a two-dimensional material comprises the steps of: providing a first substrate (S1) having a first substrate surface (F1); growing a monolayer (G) of the two-dimensional material on the first substrate surface; applying a resin capping layer (R) on the first substrate surface so as to integrally cover the monolayer; etching away the first substrate to obtain a resin body having a resin body surface (FR) with the monolayer attached thereto; providing a second substrate (S2) having a second substrate surface (F2) being substantially congruent to the resin body surface; forming an assembly of the resin body and the second substrate by contacting the mutually congruent resin body surface and second substrate surface; and subjecting the assembly thus obtained to a thermal annealing process in ambient air so as to completely remove the resin body. By carrying out the thermal annealing process in the presence of a platinum metal (PM) contacting either the monolayer or the resin body, a substantially clean monolayer of the two-dimensional material on the second substrate surface is formed.

Description

Method for preparing a substantially clean monolayer of a two-dimensional material

Field of the Invention

The present invention generally relates to a method for preparing a substantially clean monolayer of a two-dimensional material, particularly of graphene. Moreover, the invention relates to a monolayer assembly comprising such a material.

Background of the Invention

The physical and electronic properties of graphene^1"2 depend to a large extent on its defect-free structure and on its cleanliness. Scattering of transport electrons at impurities is one of the major drawbacks in the use of graphene in electronic devices^3"5. In employing graphene as substrate in electron microscopy, the presence of residues is frustrating because these features are often of the same size as the object under study⁶. While the growth of defect free single-layer graphene by means of chemical vapor deposition (CVD) is nowadays routinely possible^7"8, easily accessible and reliable techniques to transfer graphene to different substrates in a clean manner are still lacking. The common technique for the transfer of the layers grown by means of CVD on a metallic substrate (usually Nickel or Copper) onto an arbitrary substrate is based on the use of a polymer layer, ordinarily polymethyl methacrylate (PMMA), spread or spin coated over graphene^{5, 9"} ¹⁰. The subsequent removal of the approximately hundred nanometer thick PMMA layer is a challenge and extensive efforts have been undertaken in the past few years to establish a reliable technique to retrieve the pristine graphene without PMMA residues^{3"5, 11"14}. Well-known chemical etchants for PMMA are acetone and chloroform¹⁵. Unfortunately, wet chemical treatment of the polymer lead to contaminated graphene layers with lots of residues left behind. Thermal annealing at temperature in the range of 300-400°C in vacuum ^{12, 15} or in an Ar/H₂ atmosphere¹⁵ appear to help the cleaning process. However, besides the fact that these techniques are not easily available, they do not lead to ultraclean freestanding graphene. Therefore, while freestanding clean graphene is essential for various applications, existing technologies for removing the polymer layer after the transfer of graphene to the desired substrate still leave significant contaminations behind. Accordingly, it is an object of the present invention to provide a simple and reliable method for preparing a substantially clean monolayer of graphene or of other two-dimensional materials.

Summary of the Invention

It was now found that the above object can be achieved by means of a method as defined in claim 1 .

Therefore, according to one aspect of the invention, there is provided a method for preparing a substantially clean monolayer of a two-dimensional material, the method comprising the steps of:

a) providing a first substrate composed of an etchable material and having a first substrate surface;

b) growing a monolayer of said two-dimensional material on said first substrate surface;

c) applying a resin capping layer on said first substrate surface so as to integrally cover said monolayer;

d) etching away said first substrate, thereby obtaining a resin body having a resin body surface with said monolayer attached thereto, said resin body surface being substantially congruent to said first substrate surface;

e) optionally subjecting said resin body to a washing step;

f) providing a second substrate having a second substrate surface, said second substrate surface being substantially congruent to said resin body surface;

g) forming an assembly of said resin body and said second substrate by con- tacting said mutually congruent resin body surface and second substrate surface, whereby said monolayer is sandwiched therebetween; h) subjecting said assembly to a thermal annealing process in ambient air at a temperature in the range of 175°C to 350°C so as to completely remove said resin body, thereby forming a substantially clean monolayer of said two- dimensional material on said second substrate surface;

said thermal annealing process h) being carried out in the presence of a platinum metal layer contacting either said monolayer or said resin body.

According to a further aspect of the invention, there is provided a monolayer assembly comprising a carrier substrate with a carrier substrate face onto which is attached a substantially clean monolayer of a two-dimensional material such as graphene.

Without being bound by theory, it appears that the above-defined method for preparing substantially clean freestanding two-dimensional materials utilizes the catalytic properties of platinum metals. This can be done with a platinum metal layer which is in direct contact either with the monolayer of two-dimensional material or with the resin body. Complete catalytic removal of polymer residues is achieved by annealing in air at moderate temperature between 175 and 350°C. Low-energy electron holography investigations prove that this method results in substantially clean freestanding graphene or other two-dimensional materials.

The cleanliness of the prepared freestanding graphene layers has been investigated by means of low-energy electron holography^16"17. Electrons with kinetic energy in the range of 50-1 OOeV are extremely sensitive to smallest amounts of contamination causing electrical field disturbances originating for instance from electrically non-conductive PMMA resin residues. Furthermore, low-energy electrons exhibit a scattering cross-section for atoms almost independent of their Z number, an advantage in comparison to transmission electron microscopy (TEM) in view of detecting the presence of possible hydrocarbon residues. In the present context, the term "substantially clean monolayer" shall be understood as a monolayer showing no hydrocarbon residues under detection with low-energy electron holography. The term "ultraclean monolayer" often used in the relevant technical literature will be taken as synonymous to "substantially clean monolayer" unless explicitly mentioned otherwise.

Advantageous embodiments of the invention are defined in the dependent claims and/or are described hereinbelow. The method may be used for a variety of two-dimensional materials including most notably graphene, boron nitride (BN), molybdenum di-sulfide (M0S₂) and tungsten di-sulfide (WS₂).

While the method will be generally described in relation to single monolayers of the two-dimensional material, it may also be applied to form arrangements with several monolayers stacked on top of each other or partially overlapping each other.

According to a preferred embodiment (claims 2 and 16), the two-dimensional material is graphene.

Several techniques of preparing monolayers of two-dimensional materials on a substrate are known in the art. In particular, such monolayers may be grown by means of chemical vapor deposition (CVD).

To be suitable for the present invention, the first substrate shall be composed of an etchable material, i.e. a material that can be etched away according to the above mentioned step d). Because the resin capping layer applied in step c) shall not be damaged or unsuitably altered by the etching step, it is necessary to adopt a sufficiently selective etching procedure, which in turn restricts the choice of etchable substrate materials. Therefore, according to an advantageous embodiment (claim 3), the first substrate is formed of copper, which has been shown to be a suitable material for graphene deposition and which, moreover, can readily be etched away, e.g. by contacting with an Fe2Cl3 solution (claim 4).

While various types of resins could be used for applying a cap onto a monolayer of a two-dimensional material such as graphene, poly(methylmethacrylate), generally abbreviated as PMMA, has been found to be particularly useful (claim 5). The thickness of the PMMA or other resin cap will generally be selected so as to provide sufficient support for handling while avoiding an excessive amount of material that will have to be removed. For example, the resin cap is applied by spin coating and has a thickness of 50 to 1000nm, preferably about 100 to 200nm. By definition, the term "platinum metal" comprises the elements Ru, Rh, Pd, Os, Ir and Pt. According to an advantageous embodiment, the platinum metal is selected from the group consisting of Pt, Pd and Rh (claims 6 and 19). Preferably the platinum metal is Pt or Pd (claims 7 and 20). In general, it is sufficient and appropriate to use thin layers of platinum metal, e.g. with a layer thickness of 5 to 100nm, particularly 10 to 50nm and preferably 15 to 25nm.

According to one variant, the contacting layer of a platinum metal is a coating present on the second substrate surface (claim 8) or on the carrier substrate face (claim 18) in case of the corresponding monolayer assembly. Therefore, the plat- inum metal contacting layer is directly adjacent to the monolayer of two- dimensional material. While the platinum metal layer is not in direct contact with the resin body surface but rather is separated therefrom by the monolayer of two-dimensional material sandwiched therebetween, it nonetheless promotes a clean and complete removal of the resin body, thus yielding a clean graphene monolayer attached to the second substrate. The second substrate shall be made of a material that is suitable for deposition of a thin layer of a platinum metal; moreover, it shall withstand a thermal annealing process according to the present invention, i.e. in ambient air at a temperature in the range of 175°C to 350°C. According to a preferred embodiment, the second substrate is formed of silicon nitride (claim 9; and claim 22 for the carrier substrate of a corresponding monolayer assembly). For many applications it is convenient to have a membrane-like thin second substrate, with a thickness of 30 to 100nm, preferably about 50nm.

According to another variant (claim 10), the contacting platinum metal layer is a coating applied on a bottom surface (SC) of a covering device (C) resting on top of said monolayer. In other words, the platinum metal layer is initially displaced from the monolayer of two-dimensional material due to the resin body arranged therebetween. As the resin material is progressively removed by thermal annealing, the resin body becomes progressively thinner until the platinum metal coated bottom surface of the covering device is in direct contact with the monolayer of two-dimensional material. Advantageously, the covering device is a platinum metal-coated silicon platelet (claim 1 1 ). The second substrate shall be made of a material that is suitable for bearing a monolayer of the selected two-dimensional material. Moreover, it shall withstand a thermal annealing process according to the present invention, i.e. in ambient air at a temperature in the range of 175°C to 350°C. According to a preferred embodiment, the second substrate is formed of silicon (claims 12 and claim 17 for the carrier substrate of a corresponding monolayer assembly). For many applications it is convenient to have a membrane-like thin second substrate, with a thickness of 30 to 100nm, preferably about 50nm.

For many applications, e.g. for various transmission mode measurements, it is desirable to produce essentially freestanding segments of the two-dimensional material. Therefore, according to an advantageous embodiment, the second substrate comprises a plurality of channels that are substantially perpendicular to the second substrate surface and form channel openings in said second sub- strate surface, with said channel openings having a transversal size in the range of 5nm to 5μηη (claims 13 and 21 ). It will be understood that for substantially circular channel openings the term "transversal size" corresponds to the channel diameter. For example, such channels may be drilled by means of a focused gal- Hum ion beam. In particular, the channels may have a transversal size between 250 and 1000nm.

According to the above defined method, the resin body surface formed in step d) is substantially congruent to the first substrate surface. Thereby, the term "sub- stantially congruent" shall mean that relevant surface portions of the first substrate and of the resin body having been formed in contact therewith have substantially identical shapes. With the prerequisite that the subsequent optional washing step e) does not cause any deformation or distortion of the resin body surface, it is necessary that the second substrate provided in the following step f) has a relevant portion of its second substrate surface being substantially congruent to the resin body surface. By "relevant portion" are meant any portions of the various surfaces that are ultimately intended to bear portions of graphene monolayer material. In particular, the congruence requirement does not apply to such "non-relevant" portions as side or rear faces of the various parts involved.

For example, the first substrate surface could have a convex shape, with the resin body surface then having a correspondingly concave shape and the second substrate again having a convex shape like the first substrate surface. According to a preferred embodiment, one starts with a substantially planar first substrate surface, meaning that the resin body surface and the second substrate surface are also substantially planar (claim 14).

Brief description of the drawings

The above mentioned and other features and objects of this invention and the manner of achieving them will become more apparent and this invention itself will be better understood by reference to the following description of various embod- iments of this invention taken in conjunction with the accompanying drawings, wherein are shown: a) a first substrate provided with a graphene monolayer and a resin cap, in a vertical section;

b) a resin body with a graphene monolayer attached thereto shortly before contacting the same with a platinum metal coated second substrate, in a vertical section; a flow chart of a first variant of preparation process of ultraclean freestanding graphene; a) an optical photography of a graphene-PMMA composite transferred onto a glass plate covered with gold and platinum before thermal annealing;

b) an optical photography of the same sample as in a. but after thermal annealing at 300°C for 30min; a light optical microscopy image of the sample shown in Fig. 3b); a) a scanning electron image of a hole of 1000nm in diameter milled in a SiN membrane coated with Pt;

b) a higher magnification image of the clustered Pt layer; a) a low-energy electron hologram of ultraclean freestanding graphene prepared with the method presented here;

b) for comparison, the result of the removal of the PMMA layer with acetone; a) a resin body with a graphene monolayer attached thereto shortly before contacting the same with a non-coated second substrate, in a vertical section;

b) the arrangement obtained in step a), after placing thereon a covering device having a platinum metal coated bottom surface; a flow chart of a second variant of preparation process of ultraclean freestanding graphene; Fig. 9 optical microscopy images of the subsequent deposition of two graphene sheets onto an oxidized Si-chip: left: a polymer graphene complex (PG1 ) has been placed with the PMMA capping layer facing up; middle: after catalytic removal of the PMMA the graphene layer (G1 ) rests on the Si-chip and subsequently a second polymer graphene complex (PG2) has been positioned onto the chip as to overlap (G1 ); right: After applying the platinum metal catalysis a second time, the two graphene layers (G1 and G2) are apparent as well as the region where they overlap (G1 +G2); the field of view in each image corresponds to about 3 mm;

Fig. 10 optical photographs of three overlapping graphene sheets subsequently deposited onto a 10x10 mm² oxidized Si-chip and catalytically cleaned (left: perspective photograph; middle: enlarged top view; right: same as middle image, but with solid traces outlining the various gra- phene sheets;

Fig. 1 1 a comparison between catalytically cleaned graphene (left) and the conventional method of exposing the PMMA layer to wet chemicals, acetone in this case (right), inspected by the transmission of low- energy electrons; both samples experienced the same thermal treatment prior to the insertion into the vacuum system; and Fig. 12 several examples of electron transmission through graphene transferred onto an array of holes of 2 micron in diameter milled in a titanium-gold covered SiN membrane; in these examples single, double and triple layers of graphene were detected.

Detailed description of the invention

It should be emphasized that Figs. 1 , 2, 7 and 8, which are intended to illustrate the method of the present invention, are not drawn to scale. In particular, the graphene monolayers, but also the platinum metal layers shown therein are drawn with greatly exaggerated thickness.

Variant 1

According to a first variant, the method for preparing a substantially clean monolayer of graphene starts by providing a first substrate (S1 ), for example a thin platelet of polycrystalline copper, having a first substrate surface (F1 ). A graphene monolayer (G) is then grown onto the upper surface, e.g. by a known CVD method. Subsequently a resin capping layer (R), e.g. a PMMA layer, is applied on the first substrate surface so as to integrally cover said graphene monolayer. The resulting composite part is shown in Fig. 1 a. Thereafter, the substrate is etched away, e.g. by Fe2Cl3 wet etching, as shown in Fig. 2(i), thereby obtaining a resin body having a resin body surface (FR) with said graphene monolayer attached thereto as shown in Fig. 2(ii). Thereafter, the resin body with attached graphene layer is subjected to a washing step, e.g. by rinsing with water four times, see Fig. 2(iii).

In a next step, a second substrate (S2), e.g. a thin membrane of silicon nitride, having a second substrate surface (F2) covered with a thin layer of a platinum metal is provided as shown in Fig. 1 b. As explained further above, the second substrate surface is substantially congruent to the resin body surface. In order to transfer the graphene layer onto the second substrate surface, an assembly of the resin body (R) and the second substrate (S2) is formed by contacting the two surfaces, whereby said graphene monolayer is sandwiched therebetween as shown in Fig. 1 b and Fig. 2(iv).

Finally, the assembly is subjected to a thermal annealing process in ambient air at a temperature in the range of 175°C to 350°C so as to completely remove the resin body, thereby forming a substantially clean monolayer of graphene on the platinum metal covered second substrate surface, as shown in Fig. 2(v).

In the arrangement shown in these figures, the second substrate is provided with a plurality of thin channels (C) that are substantially perpendicular to the second substrate surface (S2) and have channel openings in the second substrate surface. Accordingly, the portions of the graphene layer arranged across the channel openings are freestanding regions of graphene. Variant 2

According to a second variant, the method for preparing a substantially clean monolayer of graphene starts in the same manner as described above. First a composite part as shown in Fig. 1 a is formed, thereafter the substrate is etched away, e.g. by Fe₂Cl₃ wet etching, as shown in Fig. 2(i), thereby obtaining a resin body having a resin body surface (FR) with a graphene monolayer attached thereto as shown in Fig. 2(ii). Thereafter, the resin body with attached graphene layer is subjected to a washing step, e.g. by rinsing with water four times, see Fig. 2(iii). In a next step, a second substrate (S2), e.g. a thin membrane of silicon having an uncoated second substrate surface (F2) is provided as shown in Fig. 7a. Alternatively, the second substrate may have an inert coating such as Au/Ti on a silicon nitride substrate. As explained further above, the second substrate surface is substantially congruent to the resin body surface. In order to transfer the graphene layer onto the second substrate surface, an assembly of the resin body (R) and the second substrate (S2) is formed by contacting the two surfaces, whereby said graphene monolayer is sandwiched therebetween as shown in Fig. 7a and Fig. 8(i). Thereafter, as shown in Figs. 7b and 8(ii), a covering device (C) having a bottom surface (SC) coated with a thin layer of platinum metal is placed on top of the resin body (R).

Finally, the assembly is subjected to a thermal annealing process in ambient air at a temperature in the range of 175°C to 350°C so as to completely remove the resin body, thereby forming a substantially clean monolayer of graphene on the platinum metal covered second substrate surface, as shown in Fig. 8(iii), followed by removal of the covering device and leaving behind the second substrate bearing the clean graphene monolayer as shown in Fig. 8(iv).

Examples:

All the graphene layers used in this study were grown by conventional CVD method¹⁸ on a polycrystalline copper substrate. A PMMA capping (about 100 nanometer thick) is subsequently spin coated on top of graphene, see Fig. 2(i). The preparation of the clean freestanding graphene starts with the chemical wet- etching of the metallic substrate, as illustrated in Fig. 2(ii). After the complete removal of the underlying copper the remaining graphene-PMMA complex is rinsed four times with ultra-highly purified water for washing off the etching solution (Fig. 2(iii)).

According to the first variant, the third step of the preparation method consists of the placement of the graphene-PMMA composite on a metal (Pt, Pd or Rh) coat- ed 50nm thick silicon nitride membrane previously perforated by means of a focused gallium ion beam (Fig. 2 (iv)). After drying, the sample is placed onto a conventional laboratory heating plate for thermal annealing in air at a temperature in the range of 175 to 350°C (Fig. 2 (v)). Fig. 3 illustrates the degradation of the PMMA capping due to the presence of the catalytic metal while it remains present on a noble metal, i.e. gold in this case. On a 10x10 mm² microscopy glass plate, about half of its surface is coated with gold and the other half with platinum. A 2mm gap of uncoated S1O2 between the two metal layers was arranged in order to avoid cross diffusion of the metals. Fig. 3(a) displays the situation after the transfer of a graphene-PMMA composite onto the glass plate and across the gap between the gold and the platinum layers prior to thermal annealing. The PMMA is clearly visible on both metals. Fig. 3(b) shows the result after the annealing of the glass plate at 300°C for 30min. The PMMA was decomposed above the platinum layer, leaving clean graphene on the catalytic active metal. Similar results have been obtained for annealing at temperatures in the range 175-350°C, provided that the annealing time is properly adjusted. It varies between 6h for a temperature of 175°C and just 3min at 350°C. As evident from Fig. 3, the PMMA remains present on the gold surface and is apparently un-degraded. Fig. 4 shows an optical microscope image of the same sample as the one presented in Fig. 3 but at higher magnification. As mentioned above, after annealing the PMMA is degraded on the Pt surface while it remains intact on almost the entire S1O2 surface; a characteristic interference contrast originating from the presence of PMMA on the glass is clearly visible. An important aspect to note is that a region extending as far as several tens of microns from the Pt-metal coating out to the bare S1O2 was also cleaned by the thermal annealing process. It appears that the proximity of platinum is already sufficient to cause such catalytic reaction leading to the decomposition of PMMA. These findings clearly justify the hope that extended regions of clean freestanding graphene can be obtained by the method described above.

In order to investigate the cleanliness of such graphene samples on the nanometer-scale, we prepared free-standing graphene layers placed over holes between 250 and 1000nm in diameter milled by a focused gallium ion beam into a Pt coated SiN membrane. In Fig. 5(a) a SEM image of such a hole and in (b) a high magnification image of the substrate surface after sputter deposition of Pt are displayed. The thickness of the platinum layer amounts to 15nm. It is evident that the metal layer is not uniform but exhibits islands of about 50nm in size. It is believed that these nanometer-sized domains promote the catalytic activity of the platinum layer.

After the thermal annealing process, the prepared samples were directly transferred to our low-energy electron point source microscope. In this holographic setup inspired by the original idea of Gabor¹⁹ of in-line holography, a sharp (1 1 1 ) -oriented tungsten tip acts as source of a divergent beam of highly coherent elec- trons²⁰. The electron emitter can be brought as close as 200nm to the sample with the help of a 3-axis piezo-manipulator. Part of the electron wave impinging onto the sample is elastically scattered and represents the object wave, the un- scattered part of the wave represents the reference wave¹⁶. At a 68mm distant detector, the interference pattern between object- and the reference wave - the hologram - is recorded. The magnification in the image is given by the ratio of detector-tip-distance to sample-tip-distance. Fig. 6(a) displays an image of a freestanding ultraclean graphene layer covering a hole of 500nm in diameter recorded with our low-energy electron point source microscope at an electron kinetic energy of 61 eV and a total electron current of 50nA. Apparently, free- standing clean graphene is almost transparent even for low-energy electrons^{6, 21}. The presence of graphene can only be confirmed by the presence of individual absorbates, possibly from the gas phase, sticking to the monolayer. For comparison, Fig. 6(b) shows an image of freestanding graphene (70eV, 500nA) where the polymer layer was removed in the common manner by dissolving it with ace- tone. The resulting graphene layer is still polluted and almost in-transparent, even with a tenfold increased electron current, and the presence of PMMA residues is evident.

Similar results as the one presented in Fig. 6(a) were also obtained with Pd as catalyst. Attempts to use non-platinum-metals for the transfer and cleaning of graphene, as for instance gold, failed. Low-energy electron microscopy investi- gations revealed in these cases, either empty holes, where the graphene broke, or opaque holes, where the graphene was contaminated.

In summary, we have demonstrated that the use of platinum metals in the trans- fer of graphene leads to ultraclean freestanding layers. The decomposition process of the PMMA layer is of catalytic nature and is promoted by the presence of a platinum metal. Even in the vicinity of the platinum metal the polymer layer is removed revealing clean graphene on an arbitrary substrate or even freestanding. The degradation reaction occurs in the temperature range of 175-350°C and in air. This preparation method is easily accessible in every laboratory and does not require any special equipment. The access to clean graphene provides the possibility to use graphene as a substrate for electron microscopy, or in novel electronic devices, as molecular sieves or any other mesoscopic mechanical or electronic device employing ultraclean graphene as a pre-requisite.

According to the second variant, the third step of the preparation method consists of the placement of the graphene-PMMA composite onto the desired device. A platinum metal (Pt, Pd or Rh)-coated Si substrate is then placed on top of the device with the metal facing the PMMA layer. This sandwich structure, held together just by gravitational forces, is subsequently placed onto a conventional laboratory heating plate for thermal annealing in air at a temperature between 175 and 350 °C; i.e. well below the oxidation temperature of graphene in air.

In Figure 9, we show three low magnification optical microscopy images illustrat- ing the process described above. In this particular example, it has led to the interfacing of two partially overlapping graphene sheets onto an oxidized silicon chip. The oxide layer is just needed to provide the interference contrast for visualizing the individual graphene sheets. Of course, for applications aiming at mesoscopic integrated electronic devices, an oxide layer is indeed useful to guarantee electrical isolation of the graphene sheet for employing the two- dimensional electron gas in single- or multi-layer graphene. Figure 10 illustrates another example of interfacing graphene onto an oxidized Si-chip of 10x10 mm² square. Again, the 90nm thick oxide layer provides the interference contrast that allows to distinguish between single, double and triple layers once the process described above has been repeated three times in such a way that the three macroscopic graphene layer partially overlap.

In order to verify the cleanliness of graphene transferred and catalytically cleaned as described above, we used the same process and applied it to a titanium-gold covered SiN membrane rather than to an oxidized Si-chip. The milling of an array of 2 micron diameter holes into the membrane allows us to investigate the transmission of low-energy electrons through freestanding graphene and compare it to the conventional method of using wet etching to remove the polymer. As evident from Figure 1 1 , the close proximity of the platinum metal catalyzer resting on top of the PMMA while the sandwich structure is being an- nealed, leads to a much more effective removal of PMMA residues in comparison to dissolve and remove PMMA by acetone. In fact, the transmission of low- energy electron through the wet etched sample is hardly detectible if the same parameters of initial electron current and exposure time are chosen. In order to display the PMMA contamination, as shown in the right of Figure 10, twice the primary electron current and an exposure four times as long were needed to acquire the projection image.

Of course, the quality of the transferred graphene is given by the CVD process of growing graphene on the polycrystalline cupper substrate. Figure 12 shows that the commercially available samples we used for this study are not single layer graphene over large distances but contain also double as well as triple layers. However, these multilayers if subject to our catalytic process appear exceptionally clean as well. References

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Claims

A method for preparing a substantially clean monolayer of a two- dimensional material, the method comprising the steps of:

a) providing a first substrate (S1 ) having a first substrate surface (F1 ); b) growing a monolayer (G) of said two-dimensional material on said first substrate surface;

c) applying a resin capping layer (R) on said first substrate surface so as to integrally cover said monolayer;

d) etching away said first substrate, thereby obtaining a resin body having a resin body surface (FR) with said monolayer attached thereto, said resin body surface being substantially congruent to said first substrate surface;

e) optionally subjecting said resin body to a washing step;

f) providing a second substrate (S2) having a second substrate surface

(F2), said second substrate surface being substantially congruent to said resin body surface;

g) forming an assembly of said resin body and said second substrate by contacting said mutually congruent resin body surface and second substrate surface, whereby said monolayer is sandwiched therebetween; and

h) subjecting said assembly to a thermal annealing process in ambient air at a temperature in the range of 175°C to 350°C so as to completely remove said resin body, thereby forming a substantially clean monolayer of said two-dimensional material on said second substrate surface; characterized in that said thermal annealing process h) is carried out in the presence of a platinum metal (PM) layer contacting either said monolayer or said resin body.

The method according to claim 1 , wherein said two-dimensional material is graphene.

3. The method according to claim 1 or 2, wherein said first substrate is formed of copper.

4. The method according to claim 2 or 3, wherein said etching step d) is car- ried out by contacting said first substrate with an Fe₂Cl₃ solution.

5. The method according to one of claims 1 to 4, wherein said resin body is formed of poly(methylmethacrylate).

6. The method according to one of claims 1 to 5, wherein said platinum metal is selected from the group consisting of Pt, Pd and Rh.

7. The method according to claim 6, wherein said platinum metal is Pt or Pd.

8. The method according to one of claims 1 to 7, wherein said contacting platinum metal layer is a coating applied on said second substrate surface (F2).

9. The method according to claim 8, wherein said second substrate is formed of silicon nitride.

10. The method according to one of claims 1 to 7, wherein said contacting platinum metal layer is a coating applied on a bottom surface (SC) of a covering device (C) resting on top of said monolayer.

1 1 . The method according to claim 10, wherein said covering device is a platinum metal coated silicon platelet.

12. The method according to claim 10 or 1 1 , wherein said second substrate is formed of silicon.

13. The method according to one of claims 1 to 12, wherein said second substrate comprises a plurality of channels (C) that are substantially perpendicular to said second substrate surface and form channel openings in said second substrate surface, said channel openings having a transversal size in the range of 5nm to 5μηη.

14. The method according to one of claims 1 to 13, wherein said first substrate surface, said resin body surface and said second substrate surface are substantially planar.

15. A monolayer assembly, comprising a carrier substrate (S2) with a carrier substrate face (F2) onto which is attached a substantially clean monolayer of a two-dimensional material.

16. The monolayer assembly according to claim 15, wherein said two- dimensional material is graphene.

17. The monolayer assembly according to claim 15 or 16, wherein said carrier substrate is formed of silicon.

18. The monolayer assembly according to one of claims 15 to 17, wherein said carrier substrate face (F2) is coated with a layer of a platinum metal (PM).

19. The monolayer assembly according to claim 18, wherein said platinum metal is selected from the group consisting of Pt, Pd and Rh.

20. The monolayer assembly according to claim 19, wherein said platinum metal is Pt or Pd. 21 The monolayer assembly according to one of claims 15 to 20, wherein said carrier substrate comprises a plurality of channels (C) that are substantially perpendicular to said carrier substrate surface and form channel openings in said carrier substrate surface, said channels having a transversal size in the range of 5nm to 5μηη, at least one of said channel openings being covered by said monolayer.

The monolayer assembly according to one of claims 18 to 21 , wherein said carrier substrate is formed of silicon nitride.