WO2011046775A1

WO2011046775A1 - Producing transparent conductive films from graphene

Info

Publication number: WO2011046775A1
Application number: PCT/US2010/051488
Authority: WO
Inventors: Rodney S. Ruoff; Li Xuesong
Original assignee: Board Of Regents, The University Of Texas System
Priority date: 2009-10-13
Filing date: 2010-10-05
Publication date: 2011-04-21

Abstract

A process for forming transparent conducting films from graphene. Graphene is grown on a thin copper foil to form a graphene/copper stack. A polymer film is coated on both sides of the graphene/copper stack. The copper is then etched away by an aqueous solution of iron nitrate and the graphene films are left attached to the polymer films. After washing the graphene/polymer stacks with distilled water and placed on a target substrate and dried, a liquid polymer solution is dropped on the cured polymer layer thus at least partially dissolving the coated polymer. After the liquid polymer solution is solidified, it is dissolved away by acetone thereby leaving the graphene film on the target substrate. The graphene film may then be transferred on top of other graphene films on a flexible substrate thereby forming a transparent conducting film with multi-layers of graphene films which exhibit high conductance and transparency.

Description

PRODUCING TRANSPARENT CONDUCTIVE FILMS FROM GRAPHENE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to the following commonly owned co-pending U.S. Patent Application:

Provisional Patent Application Serial No. 61/251,018, "Producing Transparent Conductive Films From Graphene," filed October 13, 2009, and claims the benefit of its earlier filing date under 35 U.S.C. § 119(e).

TECHNICAL FIELD

[0002] The present invention relates to transparent conductive films, and more particularly to producing transparent conductive films from graphene.

BACKGROUND

[0003] Transparent conductive films have a high conductivity (for example, a resistivity of 1x10^" Ωαη or less) and a high transmittance in the visible region and thus are utilized as the electrodes of solar cells, liquid crystal display elements and other various light-receiving elements. Transparent conductive films are typically made up of a layer of transparent conducting oxide, generally in the form of indium tin oxide (ITO), because of its good electrical properties and ease of fabrication. However, these thin films are usually fragile and such problems as lattice mismatch and stress-strain constraints lead to restrictions in possible uses for transparent conductive films. Further, ITO has been shown to degrade with time when subject to mechanical stresses.

[0004] Graphene is a material that is a one-atom-thick planar sheet of sp -bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The carbon-carbon bond length in graphene is approximately 1.42 A. Graphene is the basic structural element of all other graphite materials including graphite, carbon nanotubes and fullerenes. Graphene has high electrical conductivity and high optical transparency making it a candidate for transparent conducting electrodes, required for such applications such as liquid crystal displays, photovoltaic cells, etc. Further, graphene exhibits high mechanical strength and flexibility which are advantageous in comparison to ITO, which is brittle. [0005] Transparent conducting films have been made from dispersons of chemically reduced graphene oxide or exfoliated graphite. However, the resulting transparent conducting films have high electrical resistance and low optical transmittance because of the discontinuity and non- uniformity of the films. If, however, a process could be developed to make transparent conducting films from graphene that takes advantage of the properties of graphene without the discontinuity or non-uniformity of the films, then an improved transparent conducting film could be produced with higher conductivity and transparency.

[0006] Therefore, there is a need in the art for producing transparent conducting films from graphene that results in transparent conducting films with high uniformity, high conductivity and transparency.

BRIEF SUMMARY

[0007] In one embodiment of the present invention, a method for producing transparent conductive films from graphene comprises growing graphene on a foil of a first element forming a stack of graphene/first element. The method further comprises coating both sides of the stack of graphene/first element with a polymer film. Additionally, the method comprises etching the first element of the stack of graphene/first element by an aqueous solution where graphene films are left attached to the polymer film thereby forming a stack of graphene/polymer. Further, the method comprises washing the graphene/polymer stack with distilled water. In addition, the method comprises placing the washed stack of graphene/polymer on a target substrate and dried. Furthermore, the method comprises dropping a liquid polymer solution on the dried stack of graphene/polymer on the target substrate thereby at least partially dissolving the polymer film of the stack of graphene/polymer. Additionally, the method comprises dissolving the liquid polymer solution after the liquid polymer solution has solidified thereby leaving a graphene film on the target substrate. In addition, the method comprises transferring the graphene film onto a flexible substrate.

[0008] The foregoing has outlined rather generally the features and technical advantages of one or more embodiments of the present invention in order that the detailed description of the present invention that follows may be better understood. Additional features and advantages of the present invention will be described hereinafter which may form the subject of the claims of the present invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0009] A better understanding of the present invention can be obtained when the following detailed description is considered in conjunction with the following drawings, in which:

[0010] Figure 1 is a flowchart of a method for producing transparent conductive films from graphene in accordance with an embodiment of the present invention; and

[0011] Figures 2A-2H depict schematic sectional views illustrating the steps of the process for forming transparent conductive films from grapheme described in Figure 1 in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0012] The present invention comprises a process for forming transparent conducting films from graphene. In one embodiment of the present invention, graphene is grown by chemical vapor deposition from a hydrocarbon (e.g., methane) on a thin copper foil thereby forming a graphene/copper stack. A polymer film (e.g., polymethyl-methacrylate (PMMA)) is coated on both sides of the graphene/copper stack. The copper is then etched away by an aqueous solution of iron nitrate and the graphene films are left attached to the polymer films. After washing the graphene/polymer stacks with distilled water and placed on a target substrate and dried, a liquid polymer solution (e.g., PMMA) is dropped on the cured polymer layer thus at least partially dissolving the coated polymer. After the liquid polymer solution is solidified, it is dissolved away by acetone thereby leaving the graphene film on the target substrate. The graphene film may then be transferred on top of other graphene films on a flexible substrate thereby forming a transparent conducting film with multi-layers of graphene films which exhibit high conductance and transparency.

[0013] As stated in the Background section, transparent conducting films have been made from dispersons of chemically reduced graphene oxide or exfoliated graphite. However, the resulting transparent conducting films have high electrical resistance and low optical transmittance because of the discontinuity and non-uniformity of the films. If, however, a process could be developed to make transparent conducting films from graphene that takes advantage of the properties of graphene without the discontinuity or non-uniformity of the films, then an improved transparent conducting film could be produced with higher conductivity and transparency. Therefore, there is a need in the art for producing transparent conducting films from graphene that results in transparent conducting films with high uniformity, high conductivity and transparency.

[0014] A process for producing transparent conducting films from graphene that results in transparent conducting films with high conductivity and transparency is discussed below in connection with Figures 1, 2A-2H. Figure 1 is a flowchart of a method for producing transparent conductive films from grapheme. Figures 2A-2H depict schematic sectional views illustrating the process of producing transparent conductive films from grapheme described in Figure 1. [0015] Referring to Figure 1, Figure 1 is a flowchart of a method 100 for forming transparent conductive films from graphene in accordance with an embodiment of the present invention. Figure 1 will be discussed in conjunction with Figures 2A-2H, which depict schematic sectional views illustrating the steps of the process for forming transparent conductive films from grapheme described in Figure 1 in accordance with an embodiment of the present invention.

[0016] Referring to Figure 1, in conjunction with Figures 2A-2H, in step 101, graphene is grown from methane on a foil of copper by chemical vapor deposition, thereby forming a stack of graphene 201 and copper 202 as illustrated in Figure 2A. In one embodiment, the size of the foil of copper is about 1 centimer wide, about 4 centimeters long and about 0.025 millimeters thick. While the description herein discusses growing graphene on a foil of copper, it is noted that other elements may be used to grow graphene. For example, other hydrocarbon precursors may be used in replace of methane, such as acetylene, propylene, ethylene, benzene, toluene and derivatives thereof. Furthermore, the size of the graphene is limited only by the size of the copper foil, whose dimensions can be quite large, such as 50 centimeters wide, 2 meters long and 50 micrometers thick.

[0017] In step 102, a polymer film, such as polymethyl-methacrylate (PMMA) or polyethylene terephthalate, is coated on both sides 203 A, 203B of graphene/copper stack 201/202 as illustrated in Figure 2B. In one embodiment, the thickness of the polymer film ranges from tens of nanometers to a few micrometers. While the illustrations of Figures 2A-2H depict PMMA as the polymer film, any type of polymer may be used if it can be coated on the graphene/copper stack to form a film, it is inert to the etchant, it can be re-dissolved and re-cured, and it can be dissolved away in the last step.

[0018] In step 103, copper 202 of graphene/copper stack 201/202 is etched by an aqueous solution of iron nitrate (Fe(N0₃)₃) where the graphene films 201 are left attached to polymer films 203A, 203B as illustrated in Figure 2C thereby forming a stack of graphene/polymer 201/203A; 201/203B.

[0019] In step 104, the graphene/polymer stacks 201/203 A; 201/203B are washed with distilled water. In step 105, the washed graphene/polymer stacks 201/203A; 201/203B are placed on a target substrate 204 (e.g., glass) and dried as illustrated in Figure 2D. It is noted that polymer film 203B is not depicted in Figure 2D for ease of illustration.

[0020] In step 106, an appropriate amount of the polymer film (e.g., PMMA) 206 is dropped on the cured polymer layer 203 A (e.g., PMMA) to cover the whole illustrated in Figure 2E, thereby partially or fully dissolving the coated polymer film 203 A as illustrated in Figure 2F. This step may be referred to herein as the re-dissolving step.

[0021] In step 107, after the liquid polymer (e.g., PMMA) 206 is solidified (this step may be referred to herein as the re-curing step), as illustrated in Figure 2F, it is dissolved away with acetone thereby leaving a graphene film 201 on target substrate 204 as illustrated in Figure 2G. By implementing the second coating and curing of the polymer film (e.g., PMMA), as discussed in connection with steps 106, 107, the underlying graphene 201 is mechanically relaxed leading to a better contact with substrate 204 and allowing efficient transfer of graphene films 201 onto a flexible substrate, as discussed below, with few cracks.

[0022] In step 108, the process outlined above in steps 101-107 can be repeated to form subsequent graphene films 201 on target substrates 204.

[0023] In step 109, graphene films 201 are transferred onto a flexible substrate 207 on top of another thereby forming multi-layer graphene films 208 as illustrated in Figure 2H. Since graphene is highly transparent and has excellent electrical conductivity, the graphene films 201 that are transferred can be used as transparent conducting films. That is, graphene films 201 transferred onto flexible substrate 207 can be used as a flexible transparent conducting film. By stacking layers of graphene films 201 on top of one another on flexible substrate 207 to form multi-layers of graphene 208, the sheet resistance of the graphene transparent conducting film can be decreased since the conductance of multi-layer stacked graphene films 208 may not simply be a superposition of the conductance of each layer. The cracks that are present in one film may be bridged by its neighboring films thereby increasing the conductivity.

[0024] In some implementations, method 100 may include other and/or additional steps that, for clarity, are not depicted. Further, in some implementations, method 100 may be executed in a different order presented and that the order presented in the discussion of Figures 1 and 2A-2H is illustrative. Additionally, in some implementations, certain steps in method 100 may be executed in a substantially simultaneous manner or may be omitted.

[0025] Although the method is described in connection with several embodiments, it is not intended to be limited to the specific forms set forth herein, but on the contrary, it is intended to cover such alternatives, modifications and equivalents, as can be reasonably included within the spirit and scope of the invention as defined by the appended claims.

Claims

CLAIMS:

1. A method for producing transparent conductive films from graphene, the method comprising:

growing graphene on a foil of a first element forming a stack of graphene/first element; coating both sides of said stack of graphene/first element with a polymer film;

etching said first element of said stack of graphene/first element by an aqueous solution where graphene films are left attached to said polymer film thereby forming a stack of graphene/polymer;

washing said stack of graphene/polymer with distilled water;

placing said washed stack of graphene/polymer on a target substrate and dried;

dropping a liquid polymer solution on said dried stack of graphene/polymer on said target substrate thereby at least partially dissolving said polymer film of said stack of graphene/polymer;

dissolving said liquid polymer solution after said liquid polymer solution has solidified thereby leaving a graphene film on said target substrate; and

transferring said graphene film onto a flexible substrate.

2. The method as recited in claim 1, wherein said first element comprises copper.

3. The method as recited in claim 2, wherein said graphene is grown from methane on said copper foil by chemical vapor deposition.

4. The method as recited in claim 3, wherein said copper foil is about 1 centimeter wide, about 4 centimeters long and about 0.025 millimeters thick.

5. The method as recited in claim 3, wherein said copper foil is between 1 centimeter and 50 centimeters wide, between 4 centimeters and 2 meters long, and between 0.025 millimeters and 50 micrometers thick.

6. The method as recited in claim 2, wherein said graphene is grown from a hydrocarbon on said copper foil by chemical vapor deposition.

7. The method as recited in claim 6, wherein said hydrocarbon comprises one of the following: acetylene, propylene, ethylene, benzene, toluene and derivatives thereof.

8. The method as recited in claim 6, wherein said copper foil is between 1 centimeter and 50 centimeters wide, between 4 centimeters and 2 meters long, and between 0.025 millimeters and 50 micrometers thick.

9. The method as recited in claim 1, wherein said polymer film comprises polymethylmethacrylate.

10. The method as recited in claim 1, wherein said polymer film comprises polyethylene terephthalate.

11. The method as recited in claim 1 further comprising:

etching said first element of said stack of graphene/first element by said aqueous solution of iron nitrate.

12. The method as recited in claim 1, wherein said target substrate comprises glass.

13. The method as recited in claim 1, wherein said liquid polymer solution comprises polymethyl-methacrylate .

14. The method as recited in claim 1, wherein said liquid polymer solution comprises polyethylene terephthalate.

15. The method as recited in claim 1 further comprising:

dissolving said liquid polymer solution after said liquid polymer solution has solidified by acetone.

16. The method as recited in claim 1 further comprising:

transferring said graphene film on top of another graphene film on said flexible substrate thereby forming multi-layer graphene films.

17. The method as recited in claim 1, wherein a thickness of said polymer film ranges from tens of nanometers to a few micrometers.