WO2016162885A1

WO2016162885A1 - Process for transferring graphene oxide monolayer sheets on substrates

Info

Publication number: WO2016162885A1
Application number: PCT/IN2016/050009
Authority: WO
Inventors: Syed Salahuddin MAJOR; Venkatadivakar BOTCHA; Gulbagh SINGH; Pavan Kumar NARAYANAM; Raman S SRINIVASA
Original assignee: Indian Institute Of Technology Bombay
Priority date: 2015-04-09
Filing date: 2016-01-13
Publication date: 2016-10-13

Abstract

Provided is a process for transferring graphene oxide (GO) monolayer sheets on substrates. Also provide is a simple, fast, energy efficient and economical process for forming uniformly distributed, well-defined, flat and adherent graphene oxide monolayer sheets on the substrates.

Description

PROCESS FOR TRANSFERRING GRAPHENE OXIDE

MONOLAYER SHEETS ON SUBSTRATES

FIELD OF INVENTION

[0001] The present invention relates to a process for transferring graphene oxide (GO) monolayer sheets on substrates. The present invention relates to a simple, fast, energy efficient and economical process for forming uniformly distributed, well-defined, flat and adherent graphene oxide monolayer sheets on substrates. The present application claims the benefit of and priority to Indian Patent Application No. 1486/MUM/2015, filed April 9, 2015, the entire contents of which are incorporated into this application.

BACKGROUND OF THE INVENTION

[0002] Graphene is a two-dimensional planar sheet of carbon atoms in a honeycomb lattice. Graphene has attracted great interest in recent years owing to its unique physical, chemical, mechanical and electrical properties. Several technological improvements in graphene materials results in development of modern electronic devices such as sensors, memory devices and other nanoscale electronic devices

[0003] Graphene oxide is one of the first commercially available graphene materials used as precursor to make reduced graphene oxide, considered to be an alternative to graphene and its composites in a cost effective way. Graphene oxide is prepared by oxidation of graphite powder and can be dispersed in water. Graphene oxide consists of a single atomic carbon network with phenol hydroxyl and epoxide group on basal plane and carboxylic acid group at the edges. The oxygen functionalization renders graphene oxide hydrophilic and easily dispersible in water. Owing to presence of hydrophilic and ionizable carboxylic acid groups at edge makes graphene oxide to behave as amphiphilic. Due to the amphiphilic nature, graphene oxide can form a stable single layer colloidal dispersion in liquids such as water.

[0004] The graphene oxide layers can be assembled in air-water interface and deposited on to solid substrates with continuously tunable density of sheets over a large area. Solution processable graphene oxide can be used to fabricate paper-like films with excellent mechanical and electrical properties for application in nano and microelectronic devices .

[0005] In order to utilize the numerous excellent properties of graphene, depositing graphene to a particular substrate becomes an important factor. Several techniques have been reported for the transfer of graphene oxide dispersed in various solvents on to solid substrates, such as spin coating or spray coating as explained in WO2013040636A1 and in "Versatile carbon hybrid films composed of vertical carbon nanotubes grown on mechanically compliant graphene films", Adv. Mater. 22 (2010) 1247, by D. H. Lee, J. E. Kim et al. Transfer of graphene oxide monolayers onto substrate is also performed by drop casting as explained in "Photoconductivity of bulk-film-based graphene sheets ", Small 5 (2009) 1682 by X. Lv, Y. Huang et al., by dip coating as explained in "Transparent, conductive graphene electrodes for dye -sensitized solar cells", Nano Lett. 8 (2008) 323 by X. Wang, L. Zhi et al. , by knife -blading and vacuum spraying as explained in WO2013040636A1, and by electrophoretic deposition as explained in "Large-area chemically modified graphene films: electrophoretic deposition and characterization by soft X- ray absorption spectroscopy" by V. Lee, L. Whittaker et al. The use of the aforementioned deposition methods has usually been found to result in the formation of multilayers aggregates, overlapped sheets and GO sheets with fold or wrinkles.

[0006] Other methods to obtain high quality graphene layer(s) include mechanical exfoliation using a scotch tape as explained in US20100126660A1 and US7071258B 1, stamping method as explained in US20090200707A1, epitaxial growth as explained in US 7619257 B2, thermal decomposition of SiC, and chemical vapor deposition (CVD) as explained in US8388924B2. Mechanical exfoliation and stamping are simple and low cost methods to obtain graphene films on substrates. However, these techniques have limitations of non -uniformity in lateral distribution as well as the number of layers. Further, these are not amenable to large scale production of graphene films. Epitaxial graphene has also been grown by the thermal decomposition of single crystalline SiC in vacuum at high temperatures, at which Si is removed, leaving a layer of graphene. However, single crystal SiC, which is used as a precursor, is very expensive and large sized graphene sheets cannot be easily prepared by this method. In another approach, graphene films are grown on catalyst layers such as Ni, Cu etc., by using thermal chemical vapor deposition (CVD). Though CVD is advantageous for obtaining large-sized graphene sheets, the graphene films thus prepared consist of a mixture of monolayers and multilayers and the graphene sheets are not easily separable/ transferable from the metallic catalyst layer to the surface of other substrates, such as Si and Si/Si0₂, which facilitate device fabrication. The aforesaid methods often yield agglomerates of graphene oxide and overlapping sheets with folds and wrinkles. These methods are also expensive and obtaining monolayer graphene sheets of reproducible thickness is difficult.

[0007] Langmuir-Blodgett (LB) technique is another method available for deposition of graphene oxide monolayers onto a substrate to yield graphene oxide monolayers, wherein the transfer is carried out by the application of surface pressure, compression of monolayers at the air-water interface. However, this technique involves multiple mechanical parts and multiple steps which consumes time and cost. [0008] Therefore, there is a need for an alternative method for transfer of graphene oxide monolayers onto a substrate which is simple, economical, fast and energy efficient and which provides good control on the morphology of the transferred graphene oxide monolayers. Also, there is a need for an alternative method which can control the surface distribution of graphene oxide in the transferred monolayers so that it can be used for device application directly.

SUMMARY

[0009] Accordingly the embodiments The present invention provides a process for transferring graphene oxide (GO) monolayer sheets on substrates. The process comprises:

a) providing a receptacle with a subphase;

b) dipping and suspending a substrate in the subphase; c) dispersing a graphene oxide solution on the subphase at the air-liquid interface; and

d) transferring graphene oxide on the substrate by draining the subphase at a predetermined speed to allow formation of graphene oxide monolayer sheet on the substrate

OBJECTS OF THE INVENTION

[0010] An object of the present invention is to provide a process for transferring graphene oxide (GO) monolayer sheets on substrates which is simple, cost effective, fast and energy efficient.

[0011] Another object of the present invention is to provide an environmental friendly process for transfer of graphene oxide monolayer sheets from the air- liquid interface on to suitable substrates.

[0012] Yet another object of the present invention is to achieve uniformly distributed, well-defined, flat and adherent graphene oxide monolayer sheets on substrates. [0013] Yet another object of the present invention is to provide a process which does not require complex equipments and is suitable for scale up.

BRIEF DESCRIPTION OF FIGURES

[0014] The foregoing summary, as well as the following detailed description of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, there are shown in the drawings embodiments which are presently preferred and considered illustrative. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown therein. In the drawings:

[0015] FIG. 1 is a schematic diagram for the deposition of graphene oxide monolayer sheets by the process of the present invention, (a) Ultra pure De-ionized water as a subphase in a receptacle, (b) Substrate dipped in subphase, (c) Spreading of GO solution and (d) Transfer of GO monolayer sheets by draining of subphase.

[0016] FIG. 2(a-c) is Atomic Force Microscopy (AFM) image of graphene oxide monolayer sheets transferred on Si substrate at different subphase pH (as indicated in the figure), surface pressure of 7 mN/m and meniscus speed of 1 mm/min. The height profiles of the sheets are shown as insets.

[0017] FIG. 2(d-f) is Atomic Force Microscopy image of graphene oxide monolayer sheets transferred on Si0₂/Si substrate at different subphase pH (as indicated in the figure), surface pressure of 7 mN/m and meniscus speed of 1 mm/min. The height profiles of the sheets are shown as insets.

[0018] FIG. 3(a-c) is Atomic Force Microscopy image of graphene oxide monolayer sheets transferred on Si substrate with different surface pressures (as indicated in the figure), subphase pH of 5.5 and meniscus speed of 1 mm/min. The height profiles of the sheets are shown as insets.

[0019] FIG. 3(d-f) is Atomic Force Microscopy image of graphene oxide monolayer sheets transferred on Si0₂/Si substrate at different surface pressures (as indicated in the figure), subphase pH of 5.5 and meniscus speed of 1 mm/min. The height profiles of the sheets are shown as insets.

[0020] FIG. 4(a-b) is Atomic Force Microscopy image of graphene oxide monolayer sheets transferred on Si substrate at different meniscus speeds (as indicated in the figure), subphase pH of 5.5 and surface pressure of 7 mN/m. The height profiles of the sheets are shown as insets.

[0021] FIG. 4(c-d) is Atomic Force Microscopy image of graphene oxide monolayer sheets transferred on Si0₂/Si substrate at different meniscus speeds (as indicated in the figure), subphase pH of 5.5 and surface pressure of 7 mN/m. The height profiles of the sheets are shown as insets.

[0022] FIG. 5(a) is Atomic Force Microscopy image of reduced graphene oxide sheets on Si0₂/Si surfaces. Graphene oxide monolayers were transferred at subphase pH of 5.5, surface pressure of 7 mN/m and meniscus speed of 1 mm/min and reduced subsequently by hydrazine treatment followed by heat treatment. The height profiles of the sheets are shown as insets.

[0023] FIG. 5(b) is Atomic Force Microscopy image of reduced graphene oxide monolayer sheets on Si surfaces. Graphene oxide monolayers were transferred at subphase pH of 5.5, surface pressure of 7 mN/m and meniscus speed of 1 mm/min and reduced subsequently by hydrazine treatment followed by heat treatment. The height profiles of the sheets are shown as insets.

[0024] FIG. 6 is Scanning Electron Microscopy (SEM) images [at different magnifications (a) 50X and (b) 500X] of graphene oxide monolayer sheets transferred on Si0₂/Si substrate at subphase pH of 5.5, surface pressure of 7 mN/m and meniscus speed of 1 mm/min.

[0025] FIG. 7 shows Raman Spectra of (a) as-transferred graphene oxide monolayer sheets and (b) reduced graphene oxide monolayer sheets. Graphene oxide monolayer sheets were transferred at subphase pH of 5.5, surface pressure of 7 mN/m and meniscus speed of 1 mm/min and reduced subsequently by hydrazine treatment followed by heat treatment.

[0026] FIG. 8(a) shows the I_DS-V_DS curves of graphene oxide and reduced graphene oxide monolayers. A typical SEM image of two probe contact structure is shown as inset in Fig. 8(a); FIG. 8 (b) shows the transfer characteristics of bottom-gated Field Effect Transistor fabricated with reduced graphene oxide monolayer. The schematic of bottom gated FET is shown as inset in FIG. 8(b). Graphene oxide monolayer sheets were transferred at subphase pH of 5.5, surface pressure of 7 mN/m and meniscus speed of 1 mm/min and reduced subsequently by hydrazine treatment followed by heat treatment.

DETAILED DESCRIPTION OF INVENTION

[0027] In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges.

[0028] As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.

[0029] The terms "preferred" and "preferably" refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

[0030] When the term "about" is used in describing a value or an endpoint of a range, the disclosure should be understood to include both the specific value and end-point referred to.

[0031] As used herein, the terms "comprising" "including," "having," "containing," "involving," and the like are to be understood to be open-ended, i.e. to mean including but not limited to. [0032] Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described. All publications and other references mentioned herein are incorporated by reference in their entirety. Numeric ranges are inclusive of the numbers defining the range.

[0033] The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.

[0034] The term "graphene oxide" used herein refers to an oxide prepared by oxidizing graphite. Graphene oxide (GO) is an electrically insulating material composed of a single graphene carbon sheet with oxygen functional groups bonded perpendicular to the graphene basal- plane.

[0035] Graphene oxide can be reduced to form "reduced graphene oxide" by the removal of the oxygen function groups and recovery of the aromatic double -bonded carbon. Reduced graphene oxide can have similar shapes and physical properties when compared with graphene.

[0036] The term "meniscus speed" used herein is synonymous to the term "draining speed" and refers to the rate at which graphene oxide monolayers are deposited onto a substrate at air-liquid interface along the vertical direction.

[0037] The term "subphase" has the same meaning as the same technical term is commonly used and understood in the literature relating to methods for forming a monolayer film on an aqueous medium, or "phase", such as the Langmuir-Blodgett method, and refers to a liquid. Also, it is the subphase from which graphene oxide monolayers are transferred to the substrates.

[0038] The terms "transfer", "transferring", "transferred" are synonymous to the terms "deposit", "depositing" and "deposited" respectively.

[0039] The present invention provides a process for transferring graphene oxide (GO) monolayer sheets on substrates. The process comprises:

a) providing a receptacle with a subphase;

d) transferring graphene oxide on the substrate by draining the subphase at a predetermined speed to allow formation of graphene oxide monolayer sheet on the substrate.

[0040] The process of the present invention consumes negligible energy and is suitable for scale up.

[0041] The process of the present invention is a process for the transfer of graphene oxide (GO) monolayers sheets on substrates, which may be suitable for subsequent fabrication of novel nano -electronic devices. The process involves spreading of GO suspension in an organic solvent over aqueous subphase, contained in a receptacle of any shape and size. A pre-cleaned substrate is dipped into the subphase and mounted/suspended vertically. Controlled draining of the subphase from the receptacle leads to the transfer of GO monolayer sheets onto the substrate. In the present invention, the relative motion between the fluid/liquid level and the substrate is achieved by draining the subphase, thereby lowering the monolayer at the air-liquid interface with respect to the substrate, along the vertical direction. [0042] The transfer of graphene oxide (GO) monolayer sheets on the substrates is effected by the combination of the relative differences between interactions of GO-substrate and GO-subphase, dispersibility of GO in subphase, substrate pre -treatment. This has resulted in a simple, cost effective and scalable process. The process of the present invention allows the transfer of graphene oxide (GO) monolayer sheets on the substrates without the application of surface pressure by an external source.

[0043] The process variables for the transfer of GO sheets, such as, subphase pH, surface pressure (which depends on the concentration and volume of graphene oxide solution for a given area of the receptacle), concentration of GO and draining speed have been optimized on different types of substrates to achieve control over the morphology, surface density and uniformity of the transferred GO monolayer sheets. The subphase conditions, surface pressure and meniscus speed were optimized to obtain uniformly distributed and non -overlapping graphene oxide monolayers having thickness of 1.0+0.2 nm.

[0044] Surface morphology studies of the GO monolayer sheets have been carried out to understand the influence of process parameters. The GO monolayer sheets have been reduced to form reduced graphene oxide (RGO) monolayer sheets which exhibit morphological stability and electrical properties (assessed in back gated FET geometry) comparable to those formed by vertical LB transfer, making them suitable for nanoscale device fabrication.

[0045] In an embodiment of the present invention, the graphene oxide solution comprises dispersing an exfoliated graphene oxide in a solvent. The absorbance of GO solution was standardized to maintain the requisite concentration of the dispersing solution.

[0046] The solvent is selected from the group consisting of an organic, a non-organic, an aqueous or a combination thereof. Non-limiting examples of the organic solvent include n-methylpyrrolidone, ethylene glycol, glycerin, dimethylpyrrolidone, acetone, tetrahrdrofuran, acetonitrile, dimethylformamide and alcohol. Particularly, the solvent used in the present invention is a mixture of water and alcohol.

[0047] The exfoliated graphene oxide may be prepared by

Hummers-Offeman' s method.

[0048] The graphene oxide solution after dispersion is allowed to equilibrate prior to draining the subphase, during which the solvent is allowed to evaporate.

[0049] In an embodiment of the present invention, the receptacle has a drain tube with a stop cork to control the rate/speed of drainage of the subphase.

[0050] The subphase of the present invention is either water, distilled water, double distilled water, micro -filtered water, ultra-filtered water, de-ionized water, ultra-filtered and de-ionized water, nuclease free water or a combination thereof. The pH of the subphase is in the range of 3.5 to 6.5 and the surface pressure of subphase ranges from 0.5 mN/m to 12 mN/m.

[0051] The level of the subphase in the receptacle may be controlled by draining the subphase at desired rate/speed, which controls the linear speed of the air-liquid interface along the vertical direction, which is referred to as the "meniscus speed". The subphase is drained at a speed (i.e. draining speed/meniscus speed) of 0.5 mm/min to 10 mm/min.

[0052] In an embodiment of the present invention, the substrate is pre-treated with an organic solvent and made hydrophilic. Said substrate is selected from the selected from the group consisting of Si/Si0₂, undoped silicon doped or n-type doped silicon, p-type doped silicon, quartz, glass, CaF₂, mica, sapphire, nickel (Ni), titanium (Ti), iron (Fe), cobalt (Co), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), iridium (Ir), platinum (Pt) or a combination thereof.

[0053] The graphene oxide monolayer sheets transferred on the substrate are uniformly distributed, well- defined, flat and adherent.

[0054] In an embodiment of the present invention, a plurality of graphene oxide monolayer may be deposited onto the substrate.

[0055] The transferred graphene oxide monolayer sheets on the substrate may be subsequently reduced to form reduced graphene oxide (RGO) monolayers.

[0056] In another aspect of the present invention, there is provided an electronic device comprising the transferred graphene oxide monolayer sheet on the substrate.

[0057] The following examples are provided to better illustrate the claimed invention and are not to be interpreted in any way as limiting the scope of the invention. All specific materials and methods described below, in whole or in part, fall within the scope of the invention. These specific compositions, materials, and methods are not intended to limit the invention, but merely to illustrate specific embodiments falling within the scope of the invention. One skilled in the art may develop equivalent materials, and methods without the exercise of inventive capacity and without departing from the scope of the invention. It will be understood that many variations can be made in the procedures herein described while still remaining within the bounds of the invention. It is the intention of the inventors that such variations are included within the scope of the invention.

EXAMPLES

PREPARATION OF GRAPHENE OXIDE SOLUTION

[0058] Graphene oxide was chemically exfoliated from natural graphite powder (Bay Carbon, SP-1 graphite) by a modified Hummers and Offeman's method. A mixture of graphite powder (Bay carbon, SP-1) and NaN0₃ were stirred in an ice bath, to which sulfuric acid (H₂S0₄), followed by potassium permanganate (KMn0₄ - 3g) were added. The mixture was stirred at 35+5°C for approximately 1 hour, which was diluted with ultra filtered Millipore water and the temperature was raised to 90+5 °C. This mixture was further diluted slowly with ultra filtered Millipore water at this temperature, followed by addition of Hydrogen peroxide (H₂0₂). The warm solution was filtered using a Teflon membrane filter of 0.45 μιη porosity and washed with warm water. The filter-cake was then dispersed in water and gently shaken, followed by low powered sonication. This graphene oxide suspension in water was centrifuged several times to remove un- exfoliated aggregates and larger particles.

PREPARATION OF SUBSTRATE

[0059] The Si0₂/Si substrate was prepared by growing -100 nm Si0₂ by thermal oxidation of Si (100) n+ type substrates at 1100°C. The resistivity of Si subjected to thermal oxidation was in the range of 0.001 - 0.005 Ω-cm. The thickness of Si and Si0₂/Si substrates were 0.2 mm.

The Si and Si0₂/Si substrate prior to deposition of graphene oxide monolayers were cleaned as follows:

a) Ultrasonic cleaning with Milli-Q water, acetone and isopropyl alcohol for 2-3 minutes followed by rinsing with Milli-Q water; b) cleaning with NH₄OH:H₂0₂:H₂0 (1 :1 :2) solution at a 80°C for 20 minutes to remove organic contaminants present on the surface of the substrates; and

c) final rinsing with Milli-Q water.

LANGMUIR-BLODGETT (LB) DEPOSITION

[0060] LB monolayer depositions were carried out by a KSV-3000 instrument. The exfoliated GO sheets dispersed in the mixture of water: methanol was transferred onto solid substrates by LB deposition procedure. In this process, ultra-filtered and de-ionized water (Millipore, 18.2 ΜΩ-cm) was used as subphase and the pH of the subphase was maintained at 5.5+0.2, using dilute HCl/NaHC0₃. For LB deposition experiments, 20 ml of GO solution was spread slowly on the subphase, and the monolayer was allowed to stabilize for ~ 60 min before compression. The monolayer compression was carried out at a constant barrier speed of 5 mm/min. During the compression, the GO monolayer exhibits gas phase behaviour at larger areas and with increasing compression (decrease of area), a sharp increase in surface pressure takes place below certain values of area. This is followed by a nearly constant slope isotherm, resembling features associated with the liquid condensed region seen in pressure-area isotherms of Langmuir monolayers of amphiphilic molecules. The sharp increase in surface pressure is associated with the repulsive edge interaction between GO sheets due to the presence of ionizable carboxylic and phenolic hydroxy groups. Silicon and silicon covered with a thermally grown -100 nm thick Si0₂ layer were used as substrates. Silicon and Si0₂/Si substrates were cleaned with organic solvents and made hydrophilic by RCA-1 treatment (NH₄OH:H₂0₂:H₂0 - 1 :1:2). The substrates were finally rinsed ultrasonically in de-ionized water prior to LB deposition. The compressed GO monolayers were transferred onto silicon and Si0₂/Si at a target pressure 10 mN/m and a constant lifting speed of 3 mm/min.

PROCESS OF THE PRESENT INVENTION FOR THE TRANSFER OF GRAPHENE OXIDE MONOLAYER SHEETS ON THE SUBSTRATE

[0061] The transfer of graphene oxide monolayer sheets was carried out with a simple setup described below and shown in FIG. 1. This setup consists of a glass/teflon receptacle with a drain tube, with a stop cork, used to control the rate of draining the contents of the receptacle. A glass rod across the rim of the beaker is used to vertically suspend the substrate into the subphase. The temperature of the subphase is controlled by external heating / cooling. The receptacle is cleaned well with soap solution and rinsed with Milli-Q water. The receptacle is then filled with Milli-Q water as subphase. The level of the subphase in the receptacle can be controlled by draining the subphase at a desired rate, which controls the linear speed of the air-water interface along the vertical direction.

[0062] The flowchart for the deposition of GO monolayer sheets is shown in FIG. 1. For the deposition of GO monolayer sheets, ultra-filtered and de-ionized water (Millipore, 18.2 ΜΩ-cm) was used as subphase. The subphase pH was controlled and maintained by using dilute HCl/NaHC0₃. The substrates were cleaned with organic solvents and made hydrophilic by RCA-1 treatment (NH₄OH:H₂0₂:H₂0 - 1:1:2). The substrates were finally rinsed ultrasonically in de-ionized water prior to deposition. Silicon and silicon covered with a thermally grown -100 nm thick Si0₂ layer were used as substrates. A dispersion of GO sheets in water: methanol mixture (1 :5) is used as the spreading solution, whose concentration was maintained by monitoring the absorbance of the GO dispersion. The GO spread at the air-water interface at a typical rate of -200 μΙ7ηιίη. After spreading the GO solution on the subphase, it was allowed to equilibrate for - 30 min, during which, methanol was allowed to evaporate. The transfer of GO sheets on Si and Si0₂/Si substrates were carried out in the subphase pH range of 3.5 to 6.5 and surface pressure range of 0.5 - 12 mN/m, as measured with a Wilhelmy plate. Typically, 0.5 - 5 ml spreading solution was required to achieve the surface pressure in the above range. Subsequently, the GO sheets at the air-water interface were transferred on to the substrates by draining the water, leading to meniscus speeds in the range of 0.5-10 mm/min. [0063] The transferred GO sheets were reduced with hydrazine vapors followed by annealing in vacuum (~10^~5 mbar) at 80°C for 30 mm and at 400 °C in argon ambient for 6 hrs respectively, to obtain RGO monolayers.

INSTRUMENTATION

[0064] The surface morphology of GO as well as RGO was studied by a Digital Instrument Veeco-Nanoscope IV Multimode scanning probe microscope in tapping mode. Micro-Raman spectroscopy was carried out using a Horiba Jobin Yvon HR800 confocal Raman microprobe equipped with a 514 nm Ar⁺ laser. Scanning electron microscopy (SEM) measurements were carried out in an electron beam lithography system (Raith 150-Two) operated at 10 kV. Electrical characterisation of GO and RGO based FETs were performed using a semiconductor characterization system (4200-SCS semiconductor characterization system by Keithley instruments).

ATOMIC FORCE MICROSCOPY (AFM) ANALYSIS

[0065] The AFM images of GO sheets transferred at subphase pH of 5.5 on Si and Si0₂/Si substrates, surface pressure of 7 mN/m and meniscus speed of 1 mm/min are shown in FIG. 2(a) and (d), respectively. Uniformly distributed, closely spaced, morphologically flat and adherent GO sheets are transferred on both substrates, without any curled-up edges. The size of the GO sheets was found to be in the range of 15 - 30 μηι in most of the cases and the thickness of the sheets is ~ 1 nm, which is the typical thickness reported for GO monolayers. This confirms the monolayer character of GO sheets transferred on to solid substrates by this method. The effect of subphase pH on the transfer behaviour of GO sheets on Si and Si0₂/Si substrates has been studied at lower and higher subphase pH. The AFM images of GO sheets transferred on Si and Si0₂/Si substrates at subphase pH of 3.5 and 6.5 are included in FIG 2, along with the images of the sheets transferred at subphase pH of 5.5 (which were discussed above). GO sheets transferred on Si at subphase pH of 3.5 are found to be well defined but display smaller surface density. However, at higher subphase pH ~ 6.5, the transferred GO sheets on both substrates show substantial overlap, compared to those transferred in the subphase pH range of 3.5 - 5.5. The substantial overlap of GO sheets at subphase pH ~ 6.5 is attributed to higher wettability and hydrophilicity of the GO sheets under these pH conditions, possibly aided by the formation of a water lubricating layer, which may facilitate overlapping of GO sheets.

[0066] In order to study the effect of surface pressure, GO sheets have been transferred on Si and Si0₂/Si substrates at a subphase pH of 5.5 and meniscus speed of 1 mm/min. It was shown in FIG. 2 that uniformly distributed GO sheets are transferred on Si and Si0₂/Si, at a surface pressure of 7 mN/m. Typical AFM images of GO sheets transferred on Si at surface pressures of 0.5 mN/m, 7 mN/m and 12 mN/m are shown in FIGs. 3(a), 3(b) and 3(c), respectively, and the images of those transferred on Si0₂/Si are shown in FIGs. 3(d), 3(e) and 3(f), respectively. The density of non-overlapping GO sheets is found to increase gradually with increase in surface pressure up ~ 7 mN/m, above which, a tendency of GO sheets to overlap is seen, as shown for a typical case of a surface pressure of 12 mN/m (Fig. 3(c) and 3(f)), in which overlapping GO sheets are seen. These observations demonstrate that the process of the present invention allows the transfer of GO sheets onto solid substrates with a control on the surface density of the sheets, which can be achieved by varying the surface pressure, which in turn, can be simply controlled by suitably choosing the volume of spreading solution of a certain concentration, for a given receptacle.

[0067] The effect of meniscus speed on the transfer of GO sheets on Si and Si0₂/Si has been studied at subphase pH of 5.5 and surface pressure of 7 niN/m. It was shown above in FIGs. 2(a) and 2(d) that well defined, morphologically flat and uniformly distributed GO sheets are transferred on Si and Si0₂/Si at a meniscus speed of 1 mm/min. FIG. 4 shows AFM images of GO sheets at different meniscus speeds of 3 mm/min and 10 5 mm/min on both substrates. It is found that at meniscus speeds in the range of 1-3 mm/min, uniformly distributed GO monolayer sheets are transferred on both substrates. However, at a higher meniscus speeds -10 mm/min, no GO sheets are transferred on Si substrate whereas on Si0₂/Si substrate, the surface density of GO sheets is found to decrease.

0 [0068] FIG. 5 shows the AFM images of graphene oxide monolayers transferred by the method of present invention on both Si and Si0₂/Si substrates, subsequent to the reduction process (reduced graphene oxide monolayers). The reduced graphene oxide monolayers on both substrates are seen be closely-spaced sheets of 10-30 μιη in size, lying flat5 on the substrate surface. A comparison of FIG. 5(a) and 5(b) suggests that there is no significant change in the overall surface morphology of the sheets after the reduction process. The AFM height profiles at the edges of both graphene oxide monolayers and reduced graphene oxide monolayers show a thickness of ~ 1 nm, confirming the monolayer character of the0 sheets, both before and after reduction. These features are similar to those observed for graphene oxide monolayers deposited by the conventional LB technique.

EFFECT OF LIFTING SPEED OF LANGMUIR-BLODGETT

METHOD AND THE MENICUS SPEED OF THE PRESENT

5 INVENTION

[0069] The effect of lifting speed and meniscus speed on morphology of GO monolayer sheets has been shown in below tables on both Si and Si0₂/Si substrates.

Lifting speed/

Langmuir- B lodgett Present Invention Meniscus speed GO sheets with rough and crumpled Uniform deposition, well defined, flat

0.5-1 mm/min morphology as well as multilayers and adherent GO sheets were

and graphitic particles transferred

Uniform deposition, we// defined, Uniform deposition, well defined, flat

3 mm/min flat and adherent GO sheets were and adherent GO sheets were

transferred transferred

GO sheets with crumpled

> 10 mm/min morphology as well as some No GO sheets were transferred

multilayers

Table 1: Morphology of GO monolayer sheets on Si substrate:

Lifting speed/

Langmuir-Blodgett Present Invention Meniscus speed

Uniform deposition, well defined, flat Uniform deposition, well defined,

0.5-1 mm/min

and adherent GO sheets flat and adherent GO sheets

Uniform deposition, well defined, flat Uniform deposition, well defined,

3 mm/min

and adherent GO sheets flat and adherent GO sheets

Uniform deposition, well defined,

Uniform deposition, well defined, flat

flat and adherent GO sheets were

> 10 mm/min and adherent GO sheets were

transferred but density of sheets is transferred but density of sheets is low

low

Table 2: Morphology of GO monolayer sheets on Si0₂/Si substrate:

5 SCANNING ELECTRON MICROSCOPY (SEM) ANALYSIS

[0070] Typical SEM images of the transferred GO monolayer sheets on Si0₂/Si substrate are shown in FIG. 6 at different magnifications. The

SEM images show on a large scale, that well defined uniformly distributed,

non-overlapping GO sheets are transferred under optimized conditions (at

10 subphase pH -5.5, surface pressure of ~7 mN/m and meniscus speed ~1

mm/min).

RAMAN SPECTRA ANALYSIS

[0071] Typical Raman spectra of as deposited graphene oxide 15 monolayers and reduced graphene oxide monolayers obtained by method of

present invention are shown in FIG. 7. The spectra show two prominent peaks, which are associated with well known G- and D-modes of graphite. As shown in FIG.7, the G-mode appears at ~ 1600 cm^"1 for graphene oxide monolayers and shifts to ~ 1590 cm^"1 for reduced graphene oxide monolayers. The red shift of G-mode is clearly indicative of the reduction of graphene oxide monolayers, as its peak values in the range of 1580-1588

-1 9

cm^" have been attributed to sp bonded carbon in graphene. These Raman features are similar to those observed for graphene oxide monolayers deposited by the conventional LB technique.

MEASUREMENT OF ELECTRICAL CONDUCTIVITY

[0072] FIG. 8(a) shows the typical I-V characteristics of graphene oxide monolayers and reduced graphene oxide monolayers, measured in bottom gated field effect transistor (FET) geometry. A typical SEM image of the two probe contact structure is shown as inset. The graphene oxide monolayers show nearly insulating behavior with conductivity in the range of (10 ⁶ - 10^"5) S/cm. In contrast, the conductivity increases to (2 - 10) S/cm for the reduced graphene oxide monolayers. The conductivity values of reduced graphene oxide monolayers are comparable to the usually reported values for reduced graphene oxide monolayers obtained by LB technique, followed by solid state reduction.

[0073] FIG. 8(b) shows the typical transfer characteristics of reduced graphene oxide monolayers with gate voltage in the range of - 40 to +40 V and recorded at a V_dS of 1 V in air. The schematic of a bottom gated FET is shown as an inset in FIG 8(b). The transfer characteristics show that reduced graphene oxide monolayers exhibit ambipolar behavior as the charge neutrality point is observed on the positive side of the gate voltage. The effective carrier mobility was calculated using μ = (L/WCGVDS) (AIDS /AVG), where L and W are the channel length and width, CG is the gate oxide capacitance, VDS is the source-drain voltage, IDS is the source-drain current, and VG is the gate voltage. The linear regions of the transfer characteristics were used to obtain the slope AI_DS /AV_G and estimate the mobilities. For devices using reduced graphene oxide monolayers, the hole and electron mobilities are found to be in the range of (0.05 - 2.5) cm²/V-s and (0.05 - 0.5) cm²/V-s, respectively. These results indicate that the charge carriers in reduced graphene oxide monolayers are dominantly holes. The values of field effect mobilities observed in the present work are comparable to those reported for reduced graphene oxide monolayers obtained by hydrazine treatment followed by heat treatment at temperatures in the range of 400°C to 1000°C.

[0074] The method for deposition of graphene oxide monolayers of the present invention is an alternative to LB deposition for the transfer of graphene oxide monolayers from the air-water interface on to suitable substrates. This technique is simple, low cost, fast, energy efficient and scalable. The behavior of graphene oxide monolayers at the air-water interface is highly sensitive to subphase conditions as well as the nature of interaction between graphene oxide monolayers and is seen to influence the transfer of graphene oxide monolayers on to the substrates. Though the loading of the spreading solution and the rate at which the liquid phase is drained are sufficient to obtain the requisite surface density, morphology, and adhesion of the graphene oxide monolayers on the substrates, independent measurements of surface pressures and meniscus speed were carried out to enable comparison with the process parameters in the conventional LB technique. Thus, an optimized window of operation in the present method is at subphase pH of 5.0 - 6.5, surface pressure of 5 - 7 mN/m and meniscus speed of 1 - 3 mm/min for achieving the transfer of well defined, morphologically flat and uniformly distributed graphene oxide monolayers on both Si and Si0₂ substrates. These morphological futures may be compared to those obtained by LB technique namely, subphase pH of 5.0 - 6.5, target pressure of 10 - 15 mN/m and lifting speed of 3 - 5 mm/min. The monolayer character of transferred graphene oxide monolayers has been established by height profiles of AFM images. The method of deposition of graphene oxide the present invention is also seen to offer control of surface density of non -overlapping graphene oxide monolayers on the substrates, which hitherto, has been the unique feature of conventional LB technique. The graphene oxide monolayers have been reduced to form reduced graphene oxide monolayers which exhibit morphological stability during the reduction process. The suitability of the reduced graphene oxide monolayers thus obtained for device applications have been established by fabricating back gated FETs. The transfer characteristics show that the electrical parameters of reduced graphene oxide and the device are comparable to those fabricated with graphene oxide monolayers transferred by LB technique, thereby establishing that the new process is an alternative to the vertical LB process for transferring device quality graphene oxide monolayers on suitable substrates, especially Si0₂/Si.

[0075] In summary, the method for deposition of graphene oxide monolayers is simple, economical, scalable and energy efficient way of transferring graphene oxide monolayers onto a substrate. The subphase conditions, surface pressure and meniscus speed were optimized to obtain uniformly distributed and non -overlapping graphene oxide monolayers having thickness of 1.0+0.2 nm

Claims

STATEMENT OF CLAIMS

We claim:

1) A process for transferring graphene oxide monolayer sheet on a substrate comprising:

a) providing a receptacle with a subphase;

b) dipping and suspending a substrate in the subphase;

c) dispersing a graphene oxide solution on the subphase at the air-liquid interface; and

2) The process as claimed in claim 1, wherein the graphene oxide solution comprises dispersing an exfoliated graphene oxide in a solvent.

3) The process as claimed in claim 2, wherein the solvent is selected from the group consisting of an organic, a non-organic, an aqueous or a combination thereof.

4) The process as claimed in claims 2 and 3, wherein the solvent is an organic solvent selected from the group consisting of n- methylpyrrolidone, ethylene glycol, glycerin, dimethylpyrrolidone, acetone, tetrahrdrofuran, acetonitrile, dimethylformamide and alcohol.

5) The process as claimed in claims 2 and 3, wherein the solvent is a mixture of water and alcohol.

6) The process as claimed in claim 1, wherein the graphene oxide solution after dispersion is allowed to equilibrate prior to draining the subphase. 7) The process as claimed in claim 1, wherein the receptacle has a drain tube with a stop cork to control the speed of drainage of the subphase.

8) The process as claimed in claim 1, wherein the subphase is selected from the group consisting of water, distilled water, double distilled water, micro -filtered water, ultra -filtered water, de-ionized water, ultra-filtered and nuclease free water or a combination thereof.

9) The process as claimed in claim 1, wherein the pH of the subphase is in the range of 3.5 to 6.5.

10) The process as claimed in claim 1, wherein the surface pressure of subphase ranges from 0.5 mN/m to 12 mN/m.

11) The process as claimed in claim 1, wherein the subphase is drained at a speed of 0.5 mm/min to 10 mm/min.

12) The process as claimed in claim 1, wherein the substrate is pre- treated with an organic solvent and made hydrophilic.

13) The process as claimed in claims 1 and 12, wherein the substrate is selected from the selected from the group consisting of Si/Si0₂, undoped silicon doped or n-type doped silicon, p-type doped silicon, quartz, glass, CaF₂, mica, sapphire, nickel (Ni), titanium (Ti), iron (Fe), cobalt (Co), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), iridium (Ir), platinum (Pt) or a combination thereof.

14) The process as claimed in claim 1, wherein the graphene oxide monolayer sheets are uniformly distributed on the substrate.

15) The process as claimed in claim 1, wherein a plurality of graphene oxide monolayer is deposited onto the substrate.

16) The process as claimed in claim 1, wherein the process further comprises reducing the transferred graphene oxide monolayer on the substrate. 17) An electronic device comprising the transferred graphene oxide monolayer sheet on the substrate as claimed in any of the preceding claims.