Patent Application
20120244358
The present invention relates generally to graphene transfer and more specifically to dry graphene transfer from metal foils.
Interest in organic electronics comes from the low cost of plastics, and the relative ease of organic compounds processing with most successful application in organic light-emitting devices (OLEDs), thin-film transistors (TFTs) and thin film organic photovoltaic cells. See, e.g., S. R. Forrest, The path to ubiquitous and low-cost organic electronic appliances on plastic, Nature, 428, 911-918 (2004); C. D. Sheraw et al., Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates, Appl. Phys. Lett., 80(6), 1088-1090 (2002); D. R. Hines et al., Nanotransfer printing of organic and carbon nanotube thin-film transistors on plastic substrates, Appl. Phys. Lett., 86, 163101 (2005); and H. Hoppe et al., Organic solar cells: an overview, J. Mater. Res., 19(7), 1924-1945 (2004), the entire contents of each are incorporated herein by reference. Flexible electronic devices typically rely on the conducting properties of small molecules, conductive polymers and biological materials. However, their inherent low carrier mobilities (<1 cm2V−1s−1), low electrical conductivity (σ˜10−6 S cm−1) and low charge carrier velocity (10 cm s−1) represent serious limitation and underlines the need of perfect transparent conductor that will possess both the flexibility of organics and higher carrier mobilities. Graphene has emerged as this long sought conductor due to its zero-band gap, extremely high electron mobilities of 10,000-70,000 cm2V−1s−1, and low absorption (2.3%) in the visible spectrum. See, e.g., W. Warta et al., Ultrapure, high mobility organic photoconductors, Appl. Phys. A, 36, 163-170 (1985); P. E. Burrows et al., Electroluminescence from trap-limited current transport in vacuum deposited organic light emitting devices, Appl. Phys. Lett., 64, 2285-2287 (1994); F. Schwierz, Graphene transistors, Nature Nanotechnol., 5, 487-496 (2010); and F. Bonaccorso et al., Graphene photonics and optoelectronics, Nature photonics, 4, 611-622 (2010), the entire contents of each are incorporated herein by reference. Successful graphene incorporation into plastics shows promise in the production of flexible touch screens, displays, smart windows as well as a viable replacement for ITO technology. See, e.g., J. K. Wassei et al., Graphene, a promising transparent conductor, Materials today, 13(3), 52-59 (2010); L. G. D. Arco et al., Continuous, highly flexible, and transparent graphene films by chemical vapor deposition for organic photovoltaics, ACS Nano, 4(5), 2865-2873 (2010); and P. Matyba et al., Graphene and mobile ions: the key to all-plastic, solution processed light-emitting devices, ACS Nano, 4(2), 637-642 (2010), the entire contents of each are incorporated herein by reference. However, a truly manufacturable process in any of the above mentioned applications requires controlled, uniform graphene growth and precise graphene placement on top of organic surfaces, along with the development of cost-effective techniques for organic device fabrication.
High-quality graphene is typically produced via thermal graphitization of SiC, or chemical vapor deposition (CVD) on metals substrates, with the latter showing the most promise for lower cost and scalability to large areas. See, e.g., X. Li et al., Large-area synthesis of high-quality and uniform graphene films on copper foils, Science, 324, 1312-1314 (2009) and S. Bae et al., Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nature Nanotechnol., 5, 574-578 (2010), the entire contents of each is incorporated herein by reference. For device fabrication, graphene has to be transferred to semiconductor or plastic substrates. Typically, the graphene transfer procedure involves using a polymer mediator (PMMA or PDMS) to keep the graphene film intact and prevent folding while the Cu foil is chemically etched. The latter requires strong acids such as HNO3, which often produces hydrogen bubbles and damages the graphene. See, e.g., W. Regan et al., A direct transfer of large-area graphene, Appl. Phys. Lett., 96, 113102 (2010) and K. S. Kim et al., Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature, 457, 706-710 (2009), the entire contents of each is incorporated herein by reference. Aqueous iron chloride (FeCl3) has been explored as a mild alternative to acid etching, but the metal foil etching step is a hazardous and time consuming process that can take up to twelve hours to complete. See, e.g., V. P. Verma et al., Large-area graphene on polymer film for flexible and transparent anode in field emission device, Appl. Phys. Lett., 96, 203108 (2010), the entire contents of which are incorporated herein by reference. Extensive soaking of the graphene/PMMA stack in deionized water is needed for the removal of the etchant's residuals. This step may also take hours. After the metal foil is dissolved, the graphene/PMMA stack is placed onto an arbitrary surface and the polymer mediator is dissolved in acetone to yield “pristine” graphene on semiconductor or polymer substrate. The detrimental effect of the polymer solvent residues in graphene has been recognized and include introduction of variety of defects into the graphene layer. Thus, alternative approaches to the use of polymer substrate as a mediator mainly involving thermal release tape have been explored. See, e.g., J. D. Caldwell et al., Technique for the dry transfer of epitaxial graphene onto arbitrary substrates, ACS Nano, 4(2), 1108-1114 (2010); Y. Lee et al., Wafer-scale synthesis and transfer of graphene films, Nano Lett., 10, 490-493 (2010); and L. Song et al., Transfer printing of graphene using gold film, ACS Nano, 3(6), 1353-1356 (2009), the entire contents of each are incorporated herein by reference. As an example, the protocol used by Bae et al. involves placing tape/graphene/Cu stack between two rollers at 0.2 MPa pressure, then the Cu foil is chemically etched. The Gr/tape stack is washed with diionized water and placed on target substrate. Gr/tape/substrate is exposed to mild heat (90-120° C.) to remove tape residues. While the protocol for transfer varies between research groups, the metal foils are always chemically etched, which is time consuming, the graphene film is altered by the exposure to chemicals, the metal substrates are destroyed and hazardous chemical waste is generated—none of which is desirable for a large scale device production.
The aforementioned problems are overcome in the present invention which provides a method for dry graphene transfer comprising growing graphene on a growth substrate, chemically modifying a transfer substrate to enhance its adhesion to graphene, contacting the graphene on the growth substrate with the transfer substrate and transfer printing; and separating the transfer substrate with attached graphene from the growth substrate. The growth substrate may be copper foil. The transfer substrate may be a polymer, such as polystyrene or polyethylene, or an inorganic substrate. Also disclosed is the related composite material made by this process.
These and other features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following detailed description, appended claims, and accompanying drawings.
The present invention is directed towards a method of dry graphene transfer from a growth substrate (for example but not limited to copper foil, silicon carbide, boron nitride, etc.) to a transfer substrate, which may a polymer (for example but not limited to oxygen containing polymers, nitrogen containing polymers, esters, etc.) or an inorganic substrate (for example but not limited to oxides, nitrides, metals, silicon, and silicon oxide). In a preferred embodiment, the transfer substrate is polystyrene or polyethylene. One preferred embodiment relies on the differential adhesion between the growth substrate and the transfer substrate against the graphene layer. Thus, when the graphene-transfer substrate cohesive strength is higher than that of the graphene-growth substrate, a successful print is observed. This straightforward approach eliminates damage to the graphene associated with wet chemical transfer techniques, reduces the transfer time and preserves the metal foil substrates.
The method of dry graphene transfer relies on three distinct steps as shown in
In the second step, the TFPA treated polymer surface was placed in contact with graphene covered Cu foil and printed, under heat and pressure, in a NX 2000 Nano Imprinter. In the final step, the polymer substrate with attached graphene was separated from the metal foil. To ensure successful transfer, the adhesion on two interfaces polymer/TFPA and TFPA/graphene has to be higher than the adhesion between graphene and Cu. Each step is described in detail in the following sections.
Graphene was grown on Cu foils following procedure outlined by X. Li et al., Large-area synthesis of high-quality and uniform graphene films on copper foils, Science, 324, 1312-1314 (2009). The developed dry graphene transfer process relies on the differential adhesion between graphene/growth substrate and the receiving substrate. If the bond strength between graphene and the receiving substrate is high enough compared to the van der Waals graphene/metal foil interactions, graphene can be transferred to any substrate. Thus, the key for graphene transfer is the polymer surfaces preparation prior transfer print. The polymers of choice are polystyrene (PS) and polyethylene (PE) because of simplicity of their chemical structure containing only carbon and hydrogen and in the same time providing us with insight of the how important are the aromatic domains of polystyrene vs. the aliphatic polymer chains of polyethylene. Polystyrene was purchased from Tekra, while the ultra-high molecular weight polyethylene was purchased from Goodfellow. Also, it should be taken into account that polystyrene is an amorphous polymer with randomly oriented domains, whereas ultra-high molecular weight polyethylene has crystalline domains that are embedded into the amorphous matrix.
Polystyrene and polyethylene are chemically inert and so their surfaces must be chemically modified prior to TFPA molecule attachment. Plasma-based functionalization was chosen because it enables the incorporation of a variety of chemically reactive moieties (E. H. Lock et al., Surface composition, chemistry and structure of polystyrene modified by electron-beam-generated plasma, Langmuir, 26(11), 8857-8868 (2010), the entire contents of which is incorporated herein by reference). In this case electron beam-generated plasma was used to introduce oxygen functional groups such as hydroxyls, carboxyls, and carbonyls. The plasma can produce OH, COOH, a combination of OH and COOH, or NH2 groups on the surface.
The electron beam was produced by applying a −2 kV pulse to a linear hollow cathode for a selected pulse width and duty factor. The emergent beam passed through a slot in a grounded anode and was then terminated at a second grounded anode located further downstream. The electron beam volume between the two anodes defines the ionization source volume, with the dimensions set by the slot size (1×25 cm2) and the anode-to-anode length (40 cm). Beam spreading from collisions with the background gas was suppressed by a co-axial magnetic field (150 Gauss) produced by a set of external coils. Because the beam is collimated, few high energy electrons strike the surface of the material. The system vacuum was maintained by a 250 l/s turbo pump, with a base pressure of 5×10−6 Torr. The operating pressure was achieved by introducing CO2 (purity >99.995%) through the mass flow controllers and throttling the pumping speed using a manual gate valve. The samples were placed on a 10.2 cm diameter stage located at 2.5 cm from the nominal edge of the electron beam. The stage was held at ground potential and room temperature. Polymers were treated at a pressure of 100 mTorr, treatment time of 1 minute and duty factor of 10%.
After CO2 plasma high oxygen incorporation was detected. In the case of polystyrene and polyethylene the amounts were 17 at. % and 15 at. % respectively (
A closer look in the C1s region (Tables II, III) reveals that multiple types of oxygen functionalities were incorporated into the polymer structure for both polymers including hydroxyls (C—O), carbonyls (C═O), carboxyls (COOH) and carbonates (CO3) in different ratios. Whereas the concentration of hydroxyls is the highest (4.62 at. % for PS; 5.27 at. % for PE) for both polymers, the presence of other oxygen functionalities was polymer specific. For polystyrene, carbonyls and carbonates had approximately equal concentrations of 2 at. % and carboxyl concentration is 1 at. %. For polyethylene, carbonyl (3.13 at. %) and carboxyls (3.53 at. %) concentrations were comparable. The samples also contained 1.5 at. % of carbonate. After plasma treatment due to substitution into the benzene rings the π-π* feature characteristic for benzene rings organization on the surface is reduced by a factor of three.
The azide molecule used in this study TFPA-NH2 is soluble in methanol, and can thus be used with a large variety of organic substrates. It was synthesized as follows. All starting materials were of reagent grade and used without further purification. N-succinimidyl-4-azidotetrafluorobenzoate was synthesized from a previously published procedure [ref]. 1H-NMR was performed on a Brüker ADVANCE 300 spectrometer. N-ethylamino-4-azidotetrafluorobenzoate (1): To a 250 mL round bottom was added ethylenediamine (6.36 g, 105.8 mmol) and 100 mL acetonitrile. The mixture was cooled to 0° C. and (N-succinimidyl-4-azidotetrafluorobenzoate (2.00 g, 6.02 mmol) in 25 mL acetonitrile was added dropwise over 20 min. The reaction was stirred at 0° C. for an additional 1 h and the white precipitate that had formed filtered using a Buchner funnel. To the filtrate was added 100 mL chloroform and washed with water (3×50 mL) and dried over magnesium sulfate. The solvent was removed in vacuo and the solid was vacuum dried to yield 1 (1.25 g, 75%) as a yellow solid. 1H-NMR (300 MHz, CDCl3): δ 6.64 (br, 1H), 3.45 (m, 2H), 2.92 (m, 2H). 13C-NMR (75 MHz, CDCl3): δ 168.61, 160.25, 25.63.
For successful graphene transfer, there should be a uniform monolayer of TFPA-NH2 that is properly oriented. That is the NH2-end group should be attached to the polymer and the azide end group is attached to graphene. This is ensured since the azide end group of TFPA-NH2 molecule is only activated via heat (T>140° C.) or UV exposure and thus will not react with the plasma activated polymer surface during deposition.
To explore bond strength requirements at the TPFA/polymer interface, two different attachment protocols were developed. In the first approach, the TFPA molecule was attached to the plasma functionalized polymer surfaces via its amine end group to carboxyl and hydroxyl groups on the basis of hydrogen bond (
Elemental X-ray photoelectron spectroscopy (XPS) analysis of TFPA coated polystyrene and polyethylene using both attachment schemes reveals nitrogen and fluorine incorporation in all cases. However, the level of incorporation is different—in the case of polystyrene both PS H and PS R samples before print contain approximately 4 at. % of nitrogen and 4 at. % fluorine (Table I). In the case of polyethylene PE H contains 3 at. % of nitrogen and 2.9 at. % of fluorine, PER contains 2 at. % of nitrogen and 1.5 at. % of fluorine (Table I). These lower numbers suggest lower TFPA coverage on polyethylene in comparison with polystyrene. Proper interpretation of the group distribution on the polymer surface after PFPA deposition is difficult because different chemical groups e.g. C—O, C═N, C—CF; C═O, C—F and COOH, O═C═N (see Tables II, III) overlap in the spectrum.
The TFPA coated polymer substrates were brought into contact with a graphene covered Cu foil at a pressure of 3.4 MPa and heated to 150° C. for 30 minutes. The azide-end of the TFPA molecule was activated and a covalent azide bond between the graphene and the TFPA was formed. It should be noted that these transfer print conditions were not optimized. The same results might be achieved with pressure much lower than 3.4 MPa. Also the TFPA-NH2 molecule could be UV activated. As a control, unmodified PS and polyethylene PE substrates (hereon referred to as PS ref and PE Ref) were also printed against graphene on Cu foil.
After printing, the polymer substrates were separated from the Cu foil.
To evaluate the quality of the transferred graphene onto polystyrene and polyethylene substrates Raman spectroscopy and Microraman imaging were performed. Raman spectra were acquired using a Renishaw In Via Raman microscope with a constant power of 20 mW of power, and exposure time of 20 seconds. A Horiba, LabRAM ARAMIS Confocal Raman microscope equipped with 532 nm laser was used for the microraman maps. The microscope was calibrated to the 520.7 cm−1 peak of a silicon (111) wafer to within ±2 cm−1 prior to each measurement. All measurements used an exposure time of 10 seconds and were averaged over two exposures. The spot size of the laser when focused on the sample surface is approximately 1 μm. The DuoScan feature of the ARAMIS microscope was used to raster the laser spot over a 2 μm square. A general bandwidth of 100 cm−1 for polystyrene centered at the peak maximum with the baseline removed was used for integration. Peak selection and integration was preformed with LabSpec 5 version 5.55.10 software provided by Horiba Scientific. The Cu foil residuals were analyzed as well. The electrical properties of both polymers before and after transfer were evaluated via sheet resistance measurements.
After graphene transfer print to polystyrene and polyethylene as shown in
In some areas graphene was transferred to polystyrene and polyethylene references. After graphene print to PS ref the total amount of oxygen was unchanged, however closer look in the C1s region revealed that the concentration of π-π* feature characteristic for benzene rings present on the surface was increased from 3 at. % to ca. 5 at. % (Table II). This could only occur due to graphene deposition. When graphene was printed onto polyethylene reference the small amount of oxygen present onto the polymer surface was completely undetectable (Table III). Raman spectra (
A more careful investigation of the three significant regions in graphene identification G and 2D showing ordered graphitic structure and D region showing defects in the graphene structure (
While the Raman spectra shown in
Samples were electrically characterized using a standard 4-probe station with 1 mm spaced probe tips.
Understanding the graphene transfer to polystyrene and polyethylene references lies in analyzing the effects of the chosen printing conditions (150° C. and 500 psi) on the polymer structure. In the case of polystyrene, transfer was not uniform. Indeed there were large areas where no graphene was detected even though the printing occurs well above its glass transition temperature (100° C.) allowing for increased chain mobility. In the case of polyethylene, the transfer could be affected by the melted crystalline domains in the polymer (peak melting temperature of 137° C.) (S. M. Kurtz, The UHMW PE Handbook: ultra-high molecular weight polyethylene in total joint replacement, Elsevier Inc. San Diego Calif. (2004), the entire contents of which is incorporated herein by reference).
The experimental evidence suggests successful graphene transfer to both homopolymers—polyethylene and polystyrene. However, the sheet resistance measurements indicate that the coverage among different samples, and different polymers (PS vs. PE) is different. Successful graphene transfer may rely on the creation of uniform PFPA monolayer allowing for proper azide-groups presentation toward the graphene sheet before printing. The print to untreated polymers is governed by their microstructure and chemical composition.
The transfer printing results presented above are a clear indication that high quality monolayer graphene can be directly printed to a device appropriate substrate providing that the adhesion between the graphene/device substrate is higher than the growth/substrate adhesion. The developed method generates no chemical waste, allows the metal foils to be reused and causes no changes in the graphene electrical properties. The observed difference in the sheet resistance is linked to the graphene coverage. For a complete graphene coverage (PS H and PS R) very low sheet resistance were measured. Hydrogen bond on TFPA/polymer interface provided sufficient adhesion for graphene transfer. The synthesized azide molecule could be applied for coating of a wide range of organics thus extending the range of possible substrates to which graphene can be transferred. Similarly, other growth foils could be used as well.
The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” are not to be construed as limiting the element to the singular.
This application claims the benefit of U.S. Provisional Application 61/466,248 filed on Mar. 22, 2011 by Evgeniya H. Lock et al., entitled “Dry Graphene Transfer to Polymers,” the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61466248 | Mar 2011 | US |
