METHOD FOR TREATING GRAPHENE SHEETS FOR LARGE-SCALE TRANSFER USING FREE-FLOAT METHOD

Abstract

A method for transferring a graphene sheet from a copper substrate to a functional substrate includes forming the graphene sheet on the copper substrate using chemical vapor deposition, and irradiating the graphene sheet disposed on the copper substrate with a plurality of xenon ions using broad beam irradiation to form a prepared graphene sheet. The prepared graphene sheet is resistant to forming unintentional defects induced during transfer of the prepared graphene sheet to the functional substrate. The method further includes removing the copper substrate from the prepared graphene sheet using an etchant bath, floating the prepared graphene sheet in a floating bath, submerging the functional substrate in the floating bath, and decreasing a fluid level of the floating bath to lower the prepared graphene sheet onto the functional substrate.

Claims

1. A method for transferring a graphene sheet from a copper substrate to a functional substrate comprising: forming the graphene sheet on the copper substrate using chemical vapor deposition; irradiating the graphene sheet formed on the copper substrate with a plurality of xenon ions using broad beam irradiation to form a prepared graphene sheet removing the copper substrate from the prepared graphene sheet using an etchant bath; floating the prepared graphene sheet in a floating bath; submerging the functional substrate in the floating bath; and decreasing a fluid level of the floating bath to lower the prepared graphene sheet onto the functional substrate.

2. The method of claim 1, wherein the graphene sheet comprises an area of 1 cm.sup.2 or larger.

3. The method of claim 1, wherein the broad beam irradiation is collimated.

4. The method of claim 1, wherein the plurality of xenon ions is applied at a voltage in a range of about 100 V to about 1500 V.

5. The method of claim 1, wherein the plurality of xenon ions is applied at a voltage in a range of about 250 V to about 750 V.

6. The method of claim 1, wherein the plurality of xenon ions is applied at a voltage of about 500 V.

7. The method of claim 1, further comprising heating the graphene sheet formed on the copper substrate to a temperature ranging from about 50° C. to about 100° C.

8. The method of claim 1, further comprising heating the graphene sheet disposed on the copper substrate to a temperature of about 80° C.

9. The method of claim 1, wherein the plurality of xenon ions is provided at a flux of about 6.24×10.sup.11 Xe.sup.+/cm.sup.2/s to about 6.24×10.sup.14 Xe.sup.+/cm.sup.2/s.

10. The method of claim 1, wherein the plurality of xenon ions is provided at a flux of about 6.24×10.sup.12 Xe.sup.+/cm.sup.2/s to about 6.24×10.sup.13 Xe.sup.+/cm.sup.2/s.

11. The method of claim 1, wherein the plurality of xenon ions is provided at a flux of about 3.75×10.sup.13 Xe.sup.+/cm.sup.2/s.

12. The method of claim 1, wherein the graphene sheet formed on the copper substrate is irradiated with the plurality of xenon ions for a contact time resulting in a total fluence of about 6.24×10.sup.12 Xe.sup.+/cm.sup.2 to about 2.5×10.sup.13 Xe.sup.+/cm.sup.2.

13. The method of claim 1, wherein the graphene sheet formed on the copper substrate is irradiated with the plurality of xenon ions for a contact time resulting in a total fluence of about 1.25×10.sup.13 Xe.sup.+/cm.sup.2.

14. A method for transferring a graphene sheet from a copper substrate to a functional substrate comprising: forming the graphene sheet on the copper substrate using chemical vapor deposition; irradiating the graphene sheet formed on the copper substrate with a plurality of neon ions using broad beam irradiation to form a prepared graphene sheet removing the copper substrate from the prepared graphene sheet using an etchant bath; floating the prepared graphene sheet in a floating bath; submerging the functional substrate in the floating bath; and decreasing a fluid level of the floating bath to lower the prepared graphene sheet onto the functional substrate.

15. The method of claim 14, further comprising heating the graphene sheet formed on the copper substrate to a temperature of about 50° C. to about 100° C.

16. The method of claim 14, wherein the graphene sheet formed on the copper substrate is irradiated with the plurality of neon ions for a contact time resulting in a total fluence of about 6.24×10.sup.12 ions/cm.sup.2 to about 7.5×10.sup.13 ions/cm.sup.2.

17. The method of claim 14, wherein the graphene sheet formed on the copper substrate is irradiated with the plurality of neon ions for a contact time resulting in a total fluence of up to 2×10.sup.14 ions/cm.sup.2.

18. A method for transferring a graphene sheet from a growth substrate to a functional substrate comprising: forming the graphene sheet on the growth substrate; irradiating the graphene sheet formed on the growth substrate with a plurality of ions to form a prepared graphene sheet removing the growth substrate from the prepared graphene sheet using an etchant bath; floating the prepared graphene sheet in a floating bath; submerging the functional substrate in the floating bath; and decreasing a fluid level of the floating bath to lower the prepared graphene sheet onto the functional substrate.

19. The method of claim 18, wherein the graphene sheet comprises an area of 1 cm.sup.2 or larger.

20. The method of claim 18, wherein the growth substrate is a copper substrate.

21. The method of claim 18, wherein the growth substrate is a nickel substrate.

22. The method of claim 20, wherein the graphene sheet is formed on the copper substrate using chemical vapor deposition.

23. The method of claim 21, wherein the graphene sheet is formed on the nickel substrate using chemical vapor deposition.

24. The method of claim 18, wherein the plurality of ions comprises noble gas ions.

25. The method of claim 24, wherein the noble gas ions comprise xenon ions.

26. The method of claim 24, wherein the noble gas ions comprise neon ions.

27. The method of claim 24, wherein the noble gas ions comprise argon ions.

28. The method of claim 18, wherein the plurality of ions is applied to the graphene sheet formed on the growth substrate using broad beam irradiation.

29. The method of claim 28, wherein the broad beam irradiation is collimated.

30. The method of claim 18, wherein the plurality of ions is applied to the graphene sheet formed on the growth substrate at a voltage of about 100 V to about 1500 V.

31. The method of claim 18, wherein the plurality of ions is applied at a flux of about 1 nA/mm.sup.2 to about 1000 nA/mm.sup.2.

32. The method of claim 18, wherein the plurality of ions is applied at a flux of about 10 nA/mm.sup.2 to about 100 nA/mm.sup.2

33. The method of claim 18, wherein the plurality of ions is applied at a flux of about 40 nA/mm.sup.2 to about 80 nA/mm.sup.2.

34. The method of claim 18, wherein the plurality of ions is applied at a flux of about 60 nA/mm.sup.2.

35. The method of claim 18, wherein the graphene sheet formed on the growth substrate is irradiated with the plurality of ions for a contact time resulting in a total fluence of about 10 nAs/mm.sup.2 to about 120 nAs/mm.sup.2.

36. The method of claim 18, wherein the graphene sheet formed on the growth substrate is irradiated with the plurality of ions for a contact time resulting in a total fluence of about 10 nAs/mm.sup.2 to about 40 nAs/mm.sup.2.

37. The method of claim 18, wherein the graphene sheet formed on the growth substrate is irradiated with the plurality of ions for a contact time resulting in a total fluence of about 20 nAs/mm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1A is a schematic, perspective view of a growth substrate used in the formation of a graphene sheet according to an embodiment of the present invention.

[0042] FIG. 1B is a schematic, perspective view of the graphene sheet formed on the growth substrate of FIG. 1A.

[0043] FIG. 2 is a schematic view of a transfer preparation apparatus to prepare the graphene sheet of FIG. 1B for free-float transfer.

[0044] FIG. 3A is a schematic, perspective view of an etching step of the growth substrate from the prepared graphene sheet of FIG. 2 using a free-float transfer method.

[0045] FIG. 3B is a schematic, perspective view of a transfer step of the prepared graphene sheet of FIG. 2 to a functional substrate using the free-float transfer method.

[0046] FIG. 4 shows a large-scale graphene sheet prepared using the transfer preparation apparatus of FIG. 2 after removal of the growth substrate.

[0047] FIG. 5 shows the large-scale graphene sheet of FIG. 4 after transfer to a functional substrate using the free-float transfer method.

[0048] FIG. 6 is a scanning electron microscope (SEM) micrograph of a graphene sheet transferred to a functional substrate using the free-float transfer method.

[0049] FIG. 7 is a detailed view of the SEM micrograph of FIG. 6.

DETAILED DESCRIPTION

[0050] Some embodiments provide a system and method for treating graphene sheet that has been grown on a growth substrate before the growth substrate is removed and the graphene sheet transferred to a functional substrate using the free-float transfer method. The treatment provides a pristine (e.g., substantially residual/contaminant-free) graphene sheet having little to no unintended defects, which is capable of being transferred from the growth substrate with reduced risk of failure (e.g., little risk of tearing, cracking, or forming other undesirable defects) in transferring the sheet to a functional substrate during the free-float transfer method. In some embodiments, the graphene sheet is modified, and thus prepared for transfer, through an application of energy to the graphene sheet while it is disposed on the growth substrate. The energetic application may be in the form of a broad beam ion source configured to irradiate the graphene sheet with ions (e.g., group 18 element ions) such that the graphene sheet is prepared for reliable, large-scale transfer while disposed on the growth substrate. Thus, some of the systems and methods described herein eliminate the need of secondary coating materials (e.g., polymers) to aid in the transfer of the graphene sheet to the functional substrate, thus eliminating the risk of lowering the quality of the graphene sheet through contaminants introduced by the use of secondary coating materials. Accordingly, the transfer preparation method of some of the embodiments allows for the reliable transfer of high quality graphene sheets on a large-scale (i.e., 1 cm.sup.2 or larger) using the free-float transfer method.

[0051] FIGS. 1A-1B illustrate a method for growing a large-scale graphene or graphene-based sheet onto a growth substrate according to some embodiments. FIG. 1A shows a first step of preparing a growth substrate 10 for use in the production of a graphene sheet. The growth substrate 10 may be any growth substrate appropriate for the production of graphene. For example, in some embodiments, the growth substrate 10 is a metal catalyst, such as copper or nickel. As shown in FIG. 1A, the growth substrate 10 is a copper substrate, which is prepared by cleaning the surface with a solvent and annealing the substrate 10 at a high temperature.

[0052] After preparation of the growth substrate 10, graphene is grown on both the upper and bottom surface of the growth substrate 10, which may be accomplished through chemical vapor deposition (CVD) by exposing the growth substrate 10 to gaseous reactants until graphene is formed. The CVD process results in graphene sheets being synthesized on both a bottom surface of the growth substrate 10 and an upper surface of the growth substrate 10. As shown in FIG. 1B, the graphene sheet synthesized on the bottom surface is removed, while the graphene sheet 10 synthesized on the upper surface is utilized for transfer to a functional substrate. After growth, the graphene sheet 10 may have carbonaceous material on its surface which, in some cases, may be the result of the growth of the graphene sheet 10 on the copper substrate. The carbonaceous material may be a material such as amorphous carbon, one or more hydrocarbons, oxygen-containing carbon compounds, nitrogen-containing carbon compounds, or combinations thereof. In the embodiment shown in FIG. 1B, the graphene sheet 10 is a large-scale sheet having a cross-sectional area in the planar direction of at least 1 cm.sup.2 or greater.

[0053] Once the graphene sheet 20 has been deposited onto the upper surface of the growth substrate 10, the graphene sheet 20 may then be transferred to a substrate for a desired application. As shown in FIG. 2, before the graphene sheet 20 is removed from the growth substrate 10, the graphene sheet 20 is prepared for transfer using a transfer preparation apparatus 100. The transfer preparation apparatus 100 is configured to impart energy to the graphene sheet 20 and growth substrate 10 structure. For example, the transfer preparation apparatus 100 may be configured to impart ion irradiation to the graphene sheet 20 and growth substrate 10. As shown in FIG. 2, the transfer preparation apparatus 100 may be an ion source configured to supply a plurality of ions 50 to the graphene sheet 20.

[0054] In certain embodiments, the transfer preparation apparatus 100 may be configured to provide broad beam ion irradiation to the graphene sheet 20 and the growth substrate 10. The broad beam ion source may be collimated or substantially collimated (e.g., five degrees from normal). The plurality of ions 50 may comprise of ions that are singly charged or multiply charged. In some embodiments, the plurality of ions 50 may be noble gas ions, such as ions of an element from Group 18 of the periodic table. In some embodiments, the plurality of ions 50 may be organic ions or organometallic ions. The organic or organometallic ions may have an aromatic component. In addition, the molecular mass of the organic or organometallic ions may range from 75 to 200 or 90 to 200. In some embodiments, the plurality of ions 50 may comprise Ne.sup.+ ions, Ar.sup.+ ions, tropylium ions, and/or ferrocenium ions. In certain embodiments, the plurality of ions 50 comprises Xe.sup.+ ions.

[0055] The ion source may be configured to supply the plurality of ions 50 at a voltage in a range of about 100 V to about 1500 V. In some embodiments, the plurality of ions 50 may be applied at a voltage in a range of about 250 V to about 750 V. In certain embodiments, the plurality of ions 50 (e.g., Xe.sup.+ ions) may be applied at a voltage of about 500 V.

[0056] During the transfer preparation process, the graphene sheet 20 and the growth substrate 30 may be heated to a temperature ranging from about 50° C. to about 100° C. In some embodiments, the graphene sheet 20 and the growth substrate 30 may be heated to a temperature of about 80° C. In other embodiments, the graphene sheet 20 and the growth substrate 30 may be kept at room temperature. In addition, the graphene sheet 20 and the growth substrate 30 may be exposed to a pressure of less than 5×10.sup.−7 Torr. In some embodiments, the graphene sheet 20 and the growth substrate 30 may be exposed to a pressure ranging from 1×10.sup.−7 Torr to 5×10.sup.−6 Torr. In some embodiments, this process may be set to occur over several hours or overnight.

[0057] The ion source may be configured to provide the plurality of ions 50 at a flux of about 1 nA/mm.sup.2 (6.24×10.sup.11 ions/cm.sup.2/s) to about 1000 nA/mm.sup.2 (6.24×10.sup.14 ions/cm.sup.2/s). In some embodiments, the plurality of ions 50 is provided at a flux of about 10 nA/mm.sup.2 (6.24×10.sup.12 ions/cm.sup.2/s) to about 100 nA/mm.sup.2 (6.24×10.sup.13 ions/cm.sup.2/s) In certain embodiments, the plurality of ions 50 is provided at a flux of about 40 nA/mm.sup.2 (2.5×10.sup.13 ions/cm.sup.2/s) to about 80 nA/mm.sup.2 (5.0×10.sup.13 ions/cm.sup.2/s). In certain embodiments, the plurality of ions 50 is provided at a flux of about 60 nA/mm.sup.2 (3.75×10.sup.13 ions/cm.sup.2/s). In embodiments where the plurality of ions 50 comprises Xe.sup.+ ions, the plurality of ions 50 may be provided at a flux of about 6.24×10.sup.11 Xe.sup.+/cm.sup.2/s to about 6.24×10.sup.14 Xe.sup.+/cm.sup.2/s. In other embodiments, the plurality of ions 50 comprises Xe.sup.+ ions provided at a flux of about 6.24×10.sup.12 Xe.sup.+/cm.sup.2/s to about 6.24×10.sup.13 Xe.sup.+/cm.sup.2/s. In other embodiments, the plurality of ions 50 comprises Xe.sup.+ ions provided at a flux of about 3.75×10.sup.13 Xe.sup.+/cm.sup.2/s.

[0058] The graphene sheet 20 and the growth substrate 30 may be exposed to the ion source for a contact time resulting in a total fluence of about 10 nAs/mm.sup.2 (6.24×10.sup.12 ions/cm.sup.2) to about 40 nAs/mm.sup.2 (2.5×10.sup.13 ions/cm.sup.2). In certain embodiments, the graphene sheet 20 and the growth substrate 30 are exposed for under a second such that the total fluence is 20 nAs/mm.sup.2 (1.25×10.sup.13 ions/cm.sup.2). In embodiments where the plurality of ions comprises Xe.sup.+ ions, the graphene sheet 20 and the growth substrate 30 may be exposed for a contact time that results in a total fluence of about 10 nAs/mm.sup.2 to about 40 nAs/mm.sup.2 (or about 6.24×10.sup.12 Xe.sup.+/cm.sup.2 to about 2.5×10.sup.13 Xe.sup.+/cm.sup.2). In certain embodiments where the plurality of ions 50 comprises Xe.sup.+ ions, the total exposure time results in a total fluence of about 1.25×10.sup.13 Xe.sup.+/cm.sup.2. The upper limit of total fluence for the transfer preparation process may increase as the atomic number of the plurality of ions 50 decreases. In some embodiments, the upper limit of the total fluence may be about 120 nAs/mm.sup.2. In other embodiments, the upper limit of the total fluence may be about 500 nAs/mm.sup.2. In some embodiments, the upper limit of the total fluence may be about 1000 nAs/mm.sup.2. For example, in embodiments where the plurality of ions comprises Ne.sup.+ ions, the graphene sheet 20 and the growth substrate 30 may be exposed for a contact time that results in a total fluence of about 10 nAs/mm.sup.2 (6.24×10.sup.12 ions/cm.sup.2) to about 120 nAs/mm.sup.2(7.5×10.sup.13 ions/cm.sup.2/s). In some embodiments, the graphene sheet 20 and the growth substrate 30 may be exposed to a plurality of neon ions for a contact time that results in a total fluence of about about 10 nAs/mm.sup.2 to about 500 nAs/mm.sup.2. In other embodiments, the graphene sheet 20 and the growth substrate 30 may be exposed to a plurality of neon ions for a contact time that results in a total fluence of about about 10 nAs/mm.sup.2 to about 1000 nAs/mm.sup.2. In yet other embodiments, the graphene sheet 20 and the growth substrate 30 may be exposed to a plurality of neon ions for a contact time that results in a total fluence of up to 2×10.sup.14 ions/cm.sup.2.

[0059] After the above treatment, the graphene sheet 20 and the growth substrate 30 may be exposed to about 1 atm of N.sub.2 as a final step in the process before transferring of the graphene sheet 20 to the functional substrate. The result of the preparation process is, in effect, a “toughened” graphene sheet 20 that may be reliably transferred to a functional substrate using the unsupported free-float transfer method while being resistant to forming or inducing unintentional defects (tears, cracks, wrinkles, unintentionally-created pores) in the graphene sheet 20 during the free-float transfer process. The treatment thus provides a toughened graphene sheet 20 that is capable of providing a high coverage area (e.g., 99% or more of the functional substrate is covered by the graphene sheet) over the functional substrate and a clean surface for effective use of other treatment processes (e.g., perforating processes). While not being restricted to any particular theory for the mechanism that prepares or toughens the graphene sheet 20 for transfer, the toughening may be facilitated by the presence of the carbonaceous material and the interaction between the graphene sheet 20 and the copper growth substrate 10 interface. The ion beam irradiation may provide sufficient energy to the carbonaceous material to reform the graphene sheet 20 while on the copper substrate 10 to a pristine layer due to the sputtering of the carbon atoms present in and/or on the surface of the graphene sheet 20.

[0060] Once the graphene sheet 20 has been prepared using the transfer preparation apparatus 100, the graphene sheet 20 and the growth substrate 10 composite is placed in an etchant bath 30, as shown in FIG. 3A. The etchant bath 30 allows the growth substrate 10 to be etched away such that a clean graphene sheet 20 remains. The etchant bath 30 may be any appropriate etchant capable of etching the growth substrate 10 from the graphene sheet 20. For example, for copper-based growth substrates, the etchant bath 30 may include iron chloride, iron nitrate, and/or ammonium persulfate. In some embodiments, the graphene sheet 20 and the growth substrate 10 composite may be placed in a second etchant bath 30, which may include the same or a different etchant, to further aid in the complete etching of the growth substrate 10 from the graphene sheet 20.

[0061] As shown in FIG. 3B, the etchant bath 30 is then gradually removed and replaced with a floating bath 35 that may serve as a floating mechanism to transfer the graphene sheet 20 to a functional substrate 40. The floating bath 35 may be a water-based solution, such as water (e.g., deionized water) or a mixture of water and a solvent (e.g., isopropyl alcohol). For example, in some embodiments, the etchant bath 30 may be removed by the gradual introduction of deionized water, which may then be additionally introduced as a mixture of deionized water and isopropyl alcohol. As the graphene sheet 20 floats in the floating bath 35, the functional substrate 40 may be introduced below a bottom surface of the graphene sheet 20, as shown in FIG. 3B. In some embodiments, a floating frame (not shown) may be disposed around the graphene sheet 20 during this process to provide stability to the graphene sheet 20 as it floats in the solution and then applied to the functional substrate 40. The floating bath 35 is then gradually removed such that the fluid level decreases to lower the graphene sheet 20 onto the substrate 40. One or more additional graphene sheets 20 that have been prepared for transfer using the transfer preparation apparatus 100 may be stacked onto the functional substrate 40 as needed using the free-float transfer method.

[0062] FIGS. 4 and 5 show images of a graphene sheet that was prepared for transfer by an embodiment of a transfer preparation apparatus configured to supply collimated broad beam ion irradiation using Xe.sup.+ ions. FIG. 4 shows a prepared graphene sheet after removal of the copper growth substrate by chemical etching. The prepared graphene sheet shown in FIG. 4 is large-scale sheet having dimensions approximately 9 cm by 14 cm (or about 126 cm.sup.2 extended planar area). The black circular markings shown in FIG. 4 delineate the boundaries of the graphene sheet.

[0063] FIG. 5 shows a prepared graphene sheet like that shown in FIG. 4 after it has been transferred to a functional substrate (a polymer membrane substrate in the embodiment shown in FIG. 5). Like FIG. 4, the prepared graphene sheet is a large-scale sheet having dimensions approximately 9 cm by 14 cm. As shown in FIG. 5, the graphene sheet and functional substrate composite shows a graphene sheet that is free of visible, unintentional defects. While some defects may occur along the edges due to collisions with the walls of the etchant bath tank while the sheet was free-floating, the prepared graphene sheet does not show any visible defects (e.g., visible tears, crack, or wrinkles) within the main body of the sheet even after the free-float and lowering of the graphene sheet onto the functional substrate without the use of secondary polymer support materials. This indicates that the preparation process of the graphene sheet using the transfer preparation apparatus results in a graphene sheet that is toughened to be resistant to unintentional defects that may arise during the free-float transfer process.

[0064] FIGS. 6 and 7 show SEM images of a prepared graphene sheet that was prepared for transfer by an embodiment of a transfer preparation apparatus configured to supply collimated broad beam ion irradiation using Xe.sup.+ ions. After preparation, the prepared graphene sheet was transferred to a functional substrate in the form of a track-etched polymer substrate having a plurality of pores using the free-float transfer method as described above. In the embodiment shown in the figures, the plurality of pores has a nominal pore size ranging from 350 nm to 450 nm. The total field of view shown in FIG. 6 is approximately 0.036 mm.sup.2 (about 225 μm×160 μm), while FIG. 7 shows a detailed area of the top-left quadrant of the graphene sheet shown in FIG. 6.

[0065] The pores present in the polymer substrate that are covered by the prepared graphene sheet are shown as medium gray in FIGS. 6 and 7. Pores that are uncovered due to unintentional defects present in the prepared graphene sheet due to the transfer process are shown in black. As shown in FIGS. 6 and 7, greater than 99% of the substrate pores are covered by the prepared graphene sheet indicating high coverage area of the prepared graphene sheet over the polymer substrate.

[0066] Some embodiments have been described in detail with particular reference to preferred embodiments thereof, but it will be understood by those skilled in the art that variations and modifications may be effected within the spirit and scope of the claims.