Claims:1. A method for patterning of graphene on a substrate, the method comprising:
forming a graphene layer on the substrate;
depositing a metal layer at desired sections on the graphene layer;
coating a photoresist on the graphene layer and the metal layer deposited at desired sections on the graphene layer; and stripping the photoresist thereby removing the graphene layer from the metal deposited sections.

2. The method as claimed in claim 1, wherein the metal is selected from a group comprising Gold, Palladium, Copper, Nickel and Platinum.

3. The method as claimed in claim 1, wherein the substrate is a SiO2/Si substrate.

4. The method as claimed in claim 1, wherein the metal layer is deposited by on the graphene layer, by a technique selected from a group comprising of electron beam evaporation, sputtering, thermal evaporation and electroplating followed by patterning the metal using lithography.

5. The method as claimed in claim 1, wherein the metal layer deposited on the graphene layer has a thickness ranging from about 30nm to 250nm, preferably less than 50 nm.

6. The method as claimed in claim 1, wherein the photoresist is selected from a group comprising positive photoresist and negative photoresist, wherein the positive photoresist is a mixture containing phenol formaldehyde resin with diazonaphthoquinone; and negative photoresist is selected from a group comprising poly(thio-ene), methyl methacrylate, poly(methyl methacrylate).

7. The method as claimed in claim 1, wherein the coating of the photoresist is carried out by a technique selected from a group comprising drop casting and spin coating.

8. The method as claimed in claim 1, wherein the coating of photoresist on the metal is carried out by spin coating under revolution ranging from about 100 to 6000 rpm for a duration ranging from about 10 seconds to 3 minutes.

9. The method as claimed in claim 1, wherein upon coating the photoresist, the substrate is heated to a temperature ranging from about 60 ºC to 150 °C for a duration ranging from about 10 to 30 minutes, followed by cooling to a temperature ranging from about 20 °C to 40 ºC.

10. The method as claimed in claim 1, wherein the stripping of photoresist is carried out in a solvent selected from a group comprising acetone, isopropanol, methanol and ethanol by a technique selected from a group comprising sonication and peeling

11. The method as claimed in claim 1, wherein the graphene is a multilayer graphene having layers ranging from about 3 to 8.

12. The method as claimed in claim 1, wherein the graphene is single layer graphene.

13. The method as claimed in claim 1, wherein adhesion of the graphene with the metal is stronger than that with the substrate.

14. The method as claimed in claim1, wherein adhesion of the graphene with the photoresist is weaker than that with the substrate.

15. The method as claimed in claim 1, wherein the graphene is deposited on the substrate using Chemical vapor deposition (CVD) process.

16. The method as claimed in claim 1, wherein the graphene is deposited in the form of reduced graphene oxide dispersion using a method selected from a group comprising spin coating, spray coating, and screen printing.

17. The method as claimed in claim 1, wherein the graphene is deposited in the form of graphene oxide dispersion using a method selected from a group comprising spin coating, spray coating, and screen printing; followed by reduction process selected from a group comprising of thermal, electrical joule heating and chemical methods.
, Description:TECHNICAL FIELD
The present disclosure generally relates to a field of material science. The present disclosure particularly relates to patterning of graphene on a substrate.

BACKGROUND OF THE DISCLOSURE
Graphene has significant potential for industrial applications as an electrode, flexible electrical circuits and as a capacitor. Many of these applications requires patterning of graphene into predefined shapes. Traditionally patterning has been performed after synthesis by covering the desired patterns with lithographically generated masks and destructively removing the expressing graphene with oxygen plasma/plasma etching, ultra-violet ozone, reactive ion etching, stamping using poly dimethyl siloxane (PDMS) or by directly destroying excess graphene through direct writing using a laser or ion beam.

Each of these methods, however, suffers from a mixture of disadvantages such as lacking scalability, fabrication speed or restrictions on the target substrate onto which the graphene can be transferred. Further, these techniques can decrease the quality of graphene and leave rough graphene edges. For instance, PDMS stamp transfer method might lead to imprinting on the undesired area in case of small dimensions and also with aging on the hard pressing it to the source and the target. In case of plasma etching there is burning of graphene which leads to raised and rough edges on patterning and it is an expensive process. In case of metal-chemical etch removal, it is limited to usage of shadow masks and can remove one layer at a time only.

Furthermore, the material deposition and lift off steps for defining graphene patterns have been shown to affect the doping of the graphene film and the graphene electrode-interface, thus deteriorating the performance of the fabricated devices.

Thus, there is a long felt need to develop a method for patterning of graphene that overcomes the above noted limitations

SUMMARY OF THE DISCLOSURE
The present disclosure relates to a method for patterning of graphene on a substrate utilizing the differences in adhesion of various materials to graphene, the method comprises- forming a graphene layer on the substrate; depositing a metal layer at desired sections on the graphene layer; coating a photoresist on the graphene layer and the metal layer deposited at desired sections on the graphene layer; and stripping the photoresist thereby removing the graphene layer from the metal deposited sections.

An object of the present disclosure is to provide low cost, simple and effective method for patterning of graphene which is devoid of any negative effect on the graphene that is patterned on a substrate.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
For the purpose that the disclosure may be easily perceived and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure where:

FIGURE 1 illustrates optical confocal microscopic images of metal- a) palladium; b) gold, deposited through shadow mask by e-beam evaporation on SiO2/p-Si substrates; c) patterned graphene on SiO2/Si substrates.

FIGURE 2 illustrates Raman spectra of graphene on SiO2/p-Si before and after patterning and bare substrate at the graphene removed area.

FIGURE 3 schematic representation of method of patterning of graphene on a substrate according to the present disclosure.

FIGURE 4 illustrates Atomic force microscopy (AFM) images of patterned graphene obtained by plasma etching (a) and by the method of the present invention (b).

DETAILED DESCRIPTION
The present disclosure relates to a method for patterning of graphene on a substrate.

In an embodiment of the present disclosure, the method for patterning of graphene comprises-
forming a graphene layer on the substrate;
depositing a metal layer at desired sections on the graphene layer;
coating a photoresist on the graphene layer and the metal layer deposited at desired sections on the graphene layers; and
stripping the photoresist thereby removing the graphene layer from the metal deposited sections.

In an embodiment of the present disclosure, the substrate is SiO2/p-Si substrate.

In an embodiment of the present disclosure, the graphene is formed on the substrate by technique including but not limited to chemical vapor deposition.

In an embodiment of the present disclosure, the graphene is deposited on the substrate in the form of reduced oxide dispersion using technique selected from a group comprising spin coating, spray coating and screen printing.

In another embodiment of the present disclosure, the graphene is deposited on the substrate in the form of graphene oxide dispersion using technique selected from a group comprising spin coating, spray coating and screen printing, followed by reduction process selected from a group comprising of thermal, electrical joule heating and chemical methods.

In an exemplary embodiment of the present disclosure, the graphene is formed on the substrate by CVD method on metal foil and then transferred by Poly (methyl methacrylate) (PMMA) assisted wet transfer on to the target substrate such as SiO2/p-Si substrate. The transfer process comprises: a) spin coating a layer of PMMA; b) chemical etching of copper in FeCl3; c) scooping of graphene/PMMA on to the substrate; and d) PMMA removal with acetone.

In an embodiment of the present disclosure, the graphene formed on the substrate is a single layer graphene.

In another embodiment of the present disclosure, the graphene formed on the substrate is multilayer graphene having about 3 to 8 layers.

In another embodiment of the present disclosure, the graphene formed on the substrate is multilayer graphene having about 3 layers, about 4 layers, about 5 layers, about 6 layers, about 7 layers or about 8 layers.

In an embodiment of the present disclosure, the metal layer is deposited through a shadow mask having a smallest feature size of about 100micron to enable the deposition of the metal on the desired section of the graphene layer.

In an embodiment of the present disclosure, the metal layer is deposited by a technique selected from electron beam evaporation, sputtering, thermal evaporation and electroplating.

In an embodiment of the present disclosure, the metal is selected from a group comprising gold, palladium, copper, nickel and platinum.

In an embodiment of the present disclosure, the metal layer deposited on the graphene layer has a thickness ranging from about 30nm to 250nm.

In another embodiment of the present disclosure, the metal layer deposited on the graphene layer has a thickness of about 30nm, about 40nm, about 50nm, about 60nm, about 70nm, about 80nm, about 90nm, about 100nm, about 120nm, about 130nm, about 140nm, about 150nm, about 160nm, about 170nm, about 180nm, about 190nm, about 200nm, about 210nm, about 220nm, about 230nm, about 240nm or about 250nm.

In an exemplary embodiment of the present disclosure, about 50nm of metal is deposited on the substrate through shadow mask having smallest feature size of about 100microns in one direction, by electron beam evaporation under high vacuum condition.

In an embodiment of the present disclosure, the photoresist is selected from a group comprising positive photoresist and negative photoresist, wherein the positive photoresist is a mixture containing phenol formaldehyde resin with diazonaphthoquinone; and wherein the negative photoresist is selected from a group comprising poly(thio-ene), methyl methacrylate, poly (methyl methacrylate).

In an embodiment of the present disclosure, the photoresist is coated on to the graphene layer and the metal layer deposited on the substrate by a technique selected from a group comprising drop casting and spin coating.

In an embodiment of the present disclosure, the photoresist is coated on to the graphene layer and the metal layer deposited on the substrate by spin coating under revolution ranging from about 100rpm to 6000rpm for a duration ranging from about 10 seconds to 180 seconds.

In another embodiment of the present disclosure, the photoresist is coated on to the graphene layer and the metal layer deposited on the substrate by spin coating under revolution ranging from about 100rpm, about 500rpm, about 1000rpm, about 1500rpm, about 2000rpm, about 2500rpm, about 3000rpm, about 3500rpm, about 4000rpm, about 4500rpm, about 5000rpm, about 5500rpm or about 6000rpm for a duration of about 10seconds, about 20seconds, about 40seconds, about 60seconds, about 80seconds, about 100seconds, about 120seconds, about 140seconds, about 160seconds or about 180seconds.

In an embodiment of the present disclosure, upon coating the photoresist, the substrate is heated to a temperature ranging from about 60ºC to 150ºC for a duration ranging from about 10minutes to 30minutes, followed by cooling to a temperature ranging from about 20ºC to 40ºC.

In an embodiment of the present disclosure, upon coating the photoresist, the substrate is heated to a temperature of about 60ºC, about 70ºC, about 80ºC, about 90ºC, about 100ºC, about 110ºC, about 120ºC, about 130ºC, about 140ºC or about 150ºC for a duration of about 10minutes, about 12minutes, about 14minutes, about 16minutes, about 18mintues, about 20minutes, about 22minutes, about 24minutes, about 26minutes, about 28minutes or about 30minutes, followed by cooling to a temperature of about 20ºC, about 22ºC, about 24ºC, about 26ºC, about 28ºC, about 30ºC, about 32ºC, about 34ºC, about 36ºC, about 38ºC or about 40ºC.

In an embodiment of the present disclosure, the stripping of the photoresist from the substrate is carried out in a solvent selected from a group comprising acetone, isopropanol, methanol and ethanol, by a technique selected from a group comprising sonication and peeling.

In an embodiment of the present disclosure, the stripping of the photoresist from the substrate is carried out at a temperature ranging from about 20ºC to 40ºC.

In another embodiment of the present disclosure, the stripping of the photoresist from the substrate is carried out at temperature of about 20ºC, about 22ºC, about 24ºC, about 26ºC, about 28ºC, about 30ºC, about 32ºC, about 34ºC, about 36ºC, about 38ºC or about 40ºC.

In an exemplary embodiment of the present disclosure, positive photoresist (S-1813) was spin coated at about 4000rpm for about 1minute and the substrate was subjected to heating to a temperature ranging from about 60ºC to 150ºC under atmospheric air for a duration ranging from about 10minutes to 30minutes, followed by cooling to a temperature ranging from about 20ºC to 40ºC, followed by stripping the photoresist.

In an embodiment of the present disclosure, the stripping of the photoresist removes graphene only at those metal covering site along with metal itself, leaving only bare graphene on other areas, thereby causing patterning of graphene. This is caused due to: a) adhesion of graphene with metal is stronger than that with SiO2; and b) adhesion of graphene with photoresist is weaker than that with SiO2.

In an embodiment, of the present disclosure metal and the photoresist are in flush contact with graphene on the substrate. Upon stripping of both the metal and the photoresist by mechanical means the difference in adhesion energy of the metal and the photoresist with graphene in comparison with the underlying silica layer leads to selective removal of graphene at the metal site only, not underneath the polymer.

In an embodiment of the present disclosure, the advantage of the method is that it is simple and cost effective when compared to existing complicated and expensive plasma etching process. The plasma etching process leads to raised and rougher edges of graphene which tends to give rise to larger trap states, an undesired property for device applications. The patterned graphene obtained by the method of the present disclosure is devoid of raised edges of graphene which is confirmed with atomic force microscope imaging technique (illustrated in figure 4).

In an embodiment of the present disclosure, the mechanical shear removal employed during stripping the photoresist at once causes removal of the multiple layers graphene attached to metal. However, in the conventional process of metal-chemical etching, the metal can remove one layer of graphene underneath, implying that the process needs to be repeated several times with perfect alignments with the previous pattern which is a tedious process in case of multilayer graphene film.

In an embodiment of the present disclosure, figure 1 illustrates optical confocal microscopic images of metal such as palladium(a) and gold (b) deposited through shadow mask by electron beam evaporation on SiO2/p-Si substrate and c) illustrates the patterned graphene on SiO2/p-Si substrate

In an embodiment of the present disclosure, figure 2 illustrates Raman spectra analysis of graphene on SiO2/p-Si substrate before and after patterning and bare substrate at the graphene removed area. The Raman analysis shows that the graphene has a taller G peak than the 2Dpeak, depicting that the graphene on the substrate is multilayer in nature.

In an embodiment of the present disclosure, figure 4 illustrates atomic force microscopic images of the patterned graphene obtained by plasma etching (a) and of the patterned graphene obtained by the method of the present invention (b). The figure 4 explicitly demonstrates that the patterned graphene obtained by the method of the present invention has smooth edges when compared to the patterned graphene obtained by plasma etching, which has rough edges (In the figure 4, the scale bars are of 2 microns and the height of the step is about 0.5nm to 0.7nm).

Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon the description provided. The embodiments provide various features and advantageous details thereof in the description. Description of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments. The examples provided herein are intended merely to facilitate an understanding of ways in which the embodiments provided may be practiced and to further enable those of skilled in the art to practice the embodiments provided. Accordingly, the following examples should not be construed as limiting the scope of the embodiments.

EXAMPLES

EXAMPLE 1: Forming graphene on a substrate
The graphene was grown by CVD method on copper foil, wherein a mixture of methane and hydrogen gas is passed through a low pressure CVD system containing the copper substrate which acts as a catalyst for the pyrolysis process to form carbon and their assembly into graphene lattice on the metal surface. The temperature for growth of graphene is maintained at about 1000ºC and at a pressure of about 10-2 mbar before the passage of precursor gas. After the formation of graphene, the temperature is cooled down to room temperature and the graphene on top of copper foil is transferred by PMMA assisted wet transfer on to target SiO2/p-Si substrate.
The transfer process is carried out by- a) spin coating a layer of PMMA; b) chemical etching of copper in FeCl3; c) scooping of the graphene/PMMA on to the substrate; and d) PMMA removal was done with acetone.

EXAMPLE 2: Patterning of Graphene
About 50nm of gold was deposited on graphene layered substrate through a shadow mask (with smallest feature size of about 100microns in one direction) by electron beam evaporation under high vacuum condition, wherein the graphene layered substrate and the gold are placed inside vacuum chamber under a pressure of about 10-6 mbar. Then an electron beam of about 4kV to 5kV/30mA to 100mA is focused onto the gold which energies it to get sublimated and get deposited on the graphene layered substrate through the shadow mask.
Thereafter, a layer of positive photoresist (S-1813) was spin coated at about 4000rpm for about 1 minute, followed by heating the substrate at a temperature of about 99ºC under atmospheric air for about 10minutes and cooling the substrate to a temperature of about 20ºC to 40ºC. Further, the photoresist stripping was carried out under acetone at a temperature of about 20ºC to 40ºC. The stripping lead to the removal of graphene only at those metal covering site along with metal itself leaving only bare graphene on other areas. This is due to: a) adhesion of graphene with metal is stronger than that with SiO2 and b) adhesion of graphene with photoresist is weaker than that with SiO2.