Large-Scale Diffusion Barriers from CVD Grown Graphene

Transcription

1 Large-Scale Diffusion Barriers from CVD Grown Graphene Christian Wirtz, Nina C. Berner, and Georg S. Duesberg * Graphene has been long thought of as a perfect barrier material due to its impermeability to all gases as well as mechanical and chemical durability. Moreover, graphene layers are transparent and conductive, significantly widening the field of potential applications beyond simple barrier coatings. However, it is very challenging to realize such barriers on a macroscopic scale as immaculate large area films are not available. In this work, a highly effective oxygen gas barrier made from multiple layers of chemical vapor deposited graphene is presented. The individual graphene layers are stacked using a modified polymer-assisted transfer method, avoiding polymer residue yielding an oxygen-tight arrangement. A stack of three layers of graphene transmitted 6.9 cm 3 m 2 d 1 of O 2 which corresponds to cm 3 cm/cm2 s (cm Hg) when normalized to thickness and pressure. This is several orders of magnitude better than any macroscale graphene coating reported to date and performs on a level that can compete with most modern coatings while being much thinner and conductive. 1. Introduction Since its discovery in 2004 graphene has made great impact on the scientific community. [1 ] Its exceptional mechanical and electrical properties make it an ideal candidate for many applications, ranging from microelectronics to polymer composite reinforcement. [2 ] The 2D carbon sheet was also postulated to be the ultimate barrier material, showing no permeability to any atoms or molecules. [3 ] This property has potential applications as oxygen and moisture barrier in food packaging, window seals, and microelectronics packaging. [4 ] In particular, electronic devices like flexible displays require transparent and conducting barrier layers, both properties that graphene can fulfill. [5 ] Furthermore, molecular sieving and DNA sequencing using graphene membranes with defined mesoscale pores has been demonstrated; however, any scalable realization of these technologies will primarily require a perfect barrier. [6 ] Also MEMS devices such as pressure sensors have been reported, all of which rely on airtight graphene layers. [7 ] C. Wirtz, Prof. G. S. Duesberg School of Chemistry Trinity College Dublin 2, Ireland duesberg@tcd.ie C. Wirtz, Dr. N. C. Berner, Prof. G. S. Duesberg CRANN and AMBER institute Trinity College Dublin 2, Ireland DOI: /admi Graphene s impermeability to gases, including helium, was experimentally confirmed by Bunch et al. in [3a ] In their work, they covered cavities of µm with exfoliated graphene and found it to be so impermeable that they were unable to put an actual number to its barrier performance due to instrumental constraints. To our knowledge, no experiment on a large, cm 2, scale where most barriers will ultimately be used has been demonstrated with any remotely similar success. The lack of reports outlining barrier properties commensurate with those published by Bunch et al. [3a ] on large areas conveys that this is not as easy and straightforward as it may seem. This is mainly due to the fact that macroscopic samples of perfect graphene without defects are required for barriers. These are impossible to obtain by mechanically exfoliating graphene as this technique only yields microscale samples. [8 ] Though there are plenty of reports of liquid or chemically exfoliated graphene flakes blended with polymers to improve the barrier properties of said polymers, reports presenting effective graphene coatings are rare. [9 ] This can be attributed to the circumstance that chemical vapor deposition (CVD) produced graphene has to be utilized for coatings on such a scale. However, this graphene possesses grain boundaries and is not defect-free. Moreover, CVD graphene is grown on a catalyst from which it has to be transferred to the substrate, potentially introducing cracks, holes, and bubbles. [10 ] In the most notable work using CVD grown graphene as coating, Kim et al. covered 13.8 cm 2 of poly(1-methylsilyl- 1-propyne) (PTMSP) film with five graphene layers using the conventional polymer-assisted transfer method. [11 ] This resulted in a reduction of oxygen permeability from 730 to 29 barrer (1 barrer = cm3 cm/cm2 s (cm Hg)) with respect to pristine PTMSP. This improvement may appear impressive, but one has to keep in mind that PTMSP is a highly permeable polymer, pronouncing the barrier effect of graphene, a point emphasized by the authors themselves. [12 ] In the work presented here, we investigated the barrier properties of CVD grown graphene stacks transferred onto polyethylene terephthalate (PET), a polymer with inherently good barrier properties and commonly used in packaging. [13 ] The graphene was purpose-grown using a process designed to yield graphene with few defects and holes at grain boundaries. Large area samples of 5 cm 2 on 150 µm thick PET substrates were fabricated using the conventional and a modified polymerassisted transfer method, the latter avoiding polymer residues (1 of 5)

2 Figure 1. a) Schematic of a diffusion barrier made from a graphene coating on PET. b) Oxygen diffusion through PET membranes with no graphene (blue crosses), an applied two-layer stack using the modifi ed transfer method (red circles), and a three-layer stack using the modifi ed transfer method (black squares). between layers and yielding an oxygen-tight arrangement. The effect of the transfer process as well as the number of layers necessary to build an effective oxygen barrier was investigated. 2. Results The diffusion of oxygen gas through a membrane of 5 cm 2 separating chambers with oxygen and nitrogen, respectively, was measured as described in the Supporting Information. An image of the macroscopic sample holder is shown in Figure S2 (Supporting Information). The most notable measurements are shown in Figure 1 b. Measuring started when the chambers had equilibrated to an apparent flow of 10 cm 3 m 2 d 1 but as this still contains residual oxygen from within the film and from the atmosphere as introduced during loading it does not reflect the real permeability. Equilibration of the sample can take between a few hours and several days. The PET substrate on its own showed oxygen permeability of 13.9 cm 3 m 2 d 1, corresponding to barrer which is close to the literature value. We did not observe any enhancement of the barrier properties when only a single layer of graphene was applied on top of the PET (see Figure 1 b) and Table 1 ). This is most likely due to imperfections in the graphene and not surprising as it is well known that transferred graphene has holes and cracks as well as the inherent grain boundaries, all of which allow for gas transport through the layer. [10b ] Even though we used our purpose-grown graphene with as little defects as possible, we could not reach an airtight gas barrier with a single layer CVD graphene. Table 1. Measured values of oxygen transport through 150 µm PET covered with graphene. The conventional transfer method which allows for polymer residue between graphene layers is strongly inferior to our modifi ed method. No. of graphene layers on 150 µm PET Conventional transfer [permeability cm 3 m 2 d 1 ] Modifi ed transfer [permeability cm 3 m 2 d 1 ] Adding an additional graphene layer, effectively creating a stack, should improve the barrier properties as statistically it should cover most holes and cracks of the first layer and vice versa. This can be achieved in two ways as seen in Figure 2 : Either a second layer is applied onto the first one already on the final substrate by standard polymer-assisted transfer (Figure 2 d) or it can be done in a modified transfer where a graphene PMMA film is directly transferred onto another piece of graphene on copper (rather than the final substrate) which is subsequently etched and allows for transfer of the now twolayer stack (Figure 2 a c,e). The main difference between the two methods is that the standard transfer results in polymer residue between layers while the modified transfer does not. [14 ] To ensure good adhesion and conformity in both cases the PMMA is always reflowed above its glass transition temperature ( 150 C) after each layer is added. The conventional transfer method did not yield any improvement with application of a second layer whereas the modified method lowered the oxygen permeability of the stack from 13.9 to 7.8 cm 3 m 2 d 1 which is a significant improvement in its barrier properties. We attribute this result to interlayer gas diffusion through one hole/crack, in between sheets to a second hole/crack as indicated in Figure 3. This is made possible by polymer residue between layers, likely suppressing tight stacking. It is a well-known problem that polymer residues exist when graphene is transferred with PMMA and thus these residues will be trapped as they were stacked with the conventional transfer method as depicted in Figure 2 d. [14a ] These residues prevent close contact between the layers, allowing molecules to be mobile between them and hence opening up a quick diffusion path through the whole stack. Further investigation of this result showed that a stack of three graphene layers, produced with the modified transfer method, reduced oxygen permeability even further to 4.6 cm 3 m 2 d 1. An image of such a stack of three graphene layers on PET produced by the modified method is shown in Figure 4a. One can see that the film is immaculate and transparent over its full size. The scanning electron microscope (SEM) image in Figure 4 b on a SiO 2 substrate shows that the film has very few features on the microscale, implying uniformity. We note that this method can be scaled up and optimized for much larger sample sizes (2 of 5)

3 Figure 2. Graphene stack production: a) Graphene is grown on copper by CVD and b) coated by polymer for transfer. c) After dissolving the copper the graphene is transferred onto another piece of graphene on copper and the process is repeated until the desired number of layers has been transferred. d) The conventional transfer with polymer residue between layers works by building the stack layer-by-layer on the final substrate. e) The modified transfer without polymer residue between layers works by building the stack under one polymer layer and by transferring to the final substrate at the end. In order to further investigate the effect of tight stacking CVD graphene, we inspected the stacks with scanning Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy is capable of examining the interlayer coupling of two adjacent graphene layers.[15] In Figure 2, the averaged Raman spectra of 5000 spectra taken over a µm area of a single sheet and five layers of graphene transferred onto Si/ SiO2 is shown. One can observe a significant decrease of the 2D peak with respect to the G peak upon stacking. This implies interaction between the layers which requires close proximity of the layers and would be impossible with significant amounts of polymer residue between the layers. XPS on a sample transferred to Si/SiO2 confirms that the film is extremely thin. The decrease in the peaks characteristic to SiO2 with respect to an uncoated sample leads to a thickness of nm for a five-layer film, slightly thicker than minimum thickness of 1.75 nm expected for this number of layers (see the Supporting Information). 3. Discussion In order to properly evaluate the graphene s barrier properties, its contribution has to be extracted so it can be compared without the substrate s contribution. For the standard transfer this is not possible as it gave no noticeable improvement, in agreement with the findings of Kim et al.[11] Doing so for our three-layer stack from modified transfer, its graphene contribution to the barrier turns out to be barrer (see the Supporting Information for calculations). This is comparable to many modern packing materials as it performs slightly better than SiOx and is on track for improving thin, conducting barriers to the point where they become useful for oxygen-sensitive devices like OLEDs.[16] At the same time it is much thinner than many other coatings. To compare our findings to the value from Kim et al.[11] the substrate contribution has to be subtracted and only the graphene compared. Their film thickness is 100 µm (private communication). This means that their graphene itself has a permeability of barrer which is on par with liquid crystalline polymers.[17] Using our similar yet different method of film production we produced barriers with a graphene contribution of a factor of almost 5000 lower than this best result presented so far. We tentatively assign the tremendous improvement over reports of similarly large scale to our modified transfer method which avoids the mere possibility of polymer residue between layers. These findings imply that the diffusion path in between graphene layers with polymer residues present is dominated by the polymer and hence very fast. When the graphene layers are in Figure 3. Schematic representation of the proposed mechanism of diffusion through the different stacks: a) In a conventionally transferred stack polymer residue creates space between the graphene layers to allow for gas diffusion. b) In the modified transfer method polymer residue is absent and the graphene layers are in closer contact. Molecules can still enter through holes and cracks but diffusion between the sheets is greatly inhibited. (3 of 5)

4 Figure 4. a) Photograph of a three-layer fi lm of graphene on 150 µm PET. b) SEM of a fi ve-layer fi lm of graphene on SiO 2 showing very good uniformity with only a small amount of PMMA residue on the surface. c) Raman map of the intensity ratio of the G and D peak of CVD grown graphene over an area of µm. d) Comparison of the average Raman spectra of a single layer (black) and a fi ve-layer stack of graphene (red). The single layer has a 2D/G ratio of 1.1 whereas a fi ve-layer stack of the same material shows a signifi cant reduction in the 2D peak with respect to the G peak. This implies that the layers are interacting with each other rather than just sitting independently in a stacked formation. Such interaction requires close contact, supporting our contaminant-free transfer method. intimate contact this diffusion is highly suppressed. While it is known that graphite can easily get intercalated with a number of elements, little is known about the diffusion speeds of gases parallel to the graphite planes. Furthermore, we propose that at any hole-like features there are dangling bonds that may bond to the graphene layer above them once they are released from the copper substrate underneath. This would lead to further blocking of diffusion paths. The exact details and mechanism of what is happening remain a subject of further studies and discussion. Even though we report a tremendous improvement in the fabrications of large-scale graphene barriers it is unlikely that they will be able to compete with commercial barriers for high volume and low budget applications. While the graphene stacks inherent barrier properties are comparable to SiO x or AlO x, the production of thicker barriers by stacking graphene layers would be increasingly more expensive whereas oxides are more easily scaled. [18 ] However, its combination of conductivity, flexibility, and transparency gives graphene barriers a potential place in flexible electro-optical applications like OLEDs where neither metals nor oxides can be used. [5b ] A high biocompatibility gives further potential for use as flexible in vivo barrier around implants where the high cost is not as important and makes way for practicality. [19 ] To investigate the viability of using our film in applications, we measured sheet resistance with four-point measurements on the three-layer film which showed a sheet resistance of 536 Ω sq 1. Any transparency measurements were skewed as the 150 C heating step caused the PET to become cloudy. As this effect varies from sample to sample it was not possible to obtain accurate% transmission results. However, as each graphene layer is almost exclusively monolayer [20 ] one can expect a decrease in transparency of slightly over 2.3% per layer. [21 ] In summary, highly impermeable, conducting, transparent, ultra-thin graphene barriers have been produced on a large scale. This is a very important step toward future flexible gas barriers which are required in many aspects of modern appliances. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was funded by SFI SFI-Pica: PI_10/IN.1/I3030. The authors thank the advanced microscopy laboratory (AML) for the use of their SEM facilities. Received: February 12, 2015 Revised: June 11, 2015 Published online: [1] A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183. [2] a) A. K. Geim, Science 2009, 324, 1530 ; b) S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen, R. S. Ruoff, Nature 2006, 442, 282 ; c) S. Kim, J. Nah, I. Jo, D. Shahrjerdi, L. Colombo, Z. Yao, E. Tutuc, (4 of 5)

5 S. K. Banerjee, Appl. Phys. Lett. 2009, 94, ; d) C. Yim, N. McEvoy, G. S. Duesberg, Appl. Phys. Lett. 2013, 103, [3] a) J. S. Bunch, S. S. Verbridge, J. S. Alden, A. M. van der Zande, J. M. Parpia, H. G. Craighead, P. L. McEuen, Nano Lett. 2008, 8, 2458 ; b) O. Leenaerts, B. Partoens, F. M. Peeters, Appl. Phys. Lett. 2008, 93, [4] a) T. V. Duncan, J. Colloid Interface Sci. 2011, 363, 1 ; b) B. M. Yoo, H. J. Shin, H. W. Yoon, H. B. Park, J. Appl. Polym. Sci. 2014, 131, [5] a) T. H. Han, Y. Lee, M. R. Choi, S. H. Woo, S. H. Bae, B. H. Hong, J. H. Ahn, T. W. Lee, Nat. Photonics 2012, 6, 105 ; b) J. Meyer, P. R. Kidambi, B. C. Bayer, C. Weijtens, A. Kuhn, A. Centeno, A. Pesquera, A. Zurutuza, J. Robertson, S. Hofmann, Sci. Rep. 2014, 4, [6] a) S. Garaj, W. Hubbard, A. Reina, J. Kong, D. Branton, J. A. Golovchenko, Nature 2010, 467, 190 ; b) B. M. Venkatesan, R. Bashir, Nat. Nanotechnol. 2011, 6, 615 ; c) H. L. Du, J. Y. Li, J. Zhang, G. Su, X. Y. Li, Y. L. Zhao, J. Phys. Chem. C 2011, 115, ; d) S. P. Koenig, L. D. Wang, J. Pellegrino, J. S. Bunch, Nat. Nanotechnol. 2012, 7, 728. [7] Y. Xu, Z. D. Guo, H. B. Chen, Y. Yuan, J. C. Lou, X. Lin, H. Y. Gao, H. S. Chen, B. Yu, Appl. Phys. Lett. 2011, 99, [8] K. S. Novoselov, V. I. Fal ko, L. Colombo, P. R. Gellert, M. G. Schwab, K. Kim, Nature 2012, 490, 192. [9] a) H. D. Huang, P. G. Ren, J. Chen, W. Q. Zhang, X. Ji, Z. M. Li, J. Membr. Sci. 2012, 409, 156 ; b) H. Kim, Y. Miura, C. W. Macosko, Chem. Mater. 2010, 22, 3441 ; c) Y.-H. Yang, L. Bolling, M. A. Priolo, J. C. Grunlan, Adv. Mater. 2013, 25, 493. [10] a) X. Li, C. W. Magnuson, A. Venugopal, R. M. Tromp, J. B. Hannon, E. M. Vogel, L. Colombo, R. S. Ruoff, J. Am. Chem. Soc. 2011, 133, 2816 ; b) T. Hallam, N. C. Berner, C. Yim, G. S. Duesberg, Adv. Mater. Interfaces 2014, 1, [11] H. W. Kim, H. W. Yoon, S.-M. Yoon, B. M. Yoo, B. K. Ahn, Y. H. Cho, H. J. Shin, H. Yang, U. Paik, S. Kwon, J.-Y. Choi, H. B. Park, Science 2013, 342, 91. [12] T. Masuda, E. Isobe, T. Higashimura, K. Takada, J. Am. Chem. Soc. 1983, 105, [13] V. Siracusa, Int. J. Polym. Sci. 2012, 2012, 11. [14] a) N. Peltekis, S. Kumar, N. McEvoy, K. Lee, A. Weidlich, G. S. Duesberg, Carbon 2012, 50, 395 ; b) Y. Wang, S. W. Tong, X. F. Xu, B. Ozyilmaz, K. P. Loh, Adv. Mater. 2011, 23, [15] A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, A. K. Geim, Phys. Rev. Lett. 2006, 97, [16] a) J. S. Lewis, M. S. Weaver, IEEE J. Sel. Top. Quantum Electron. 2004, 10, 45 ; b) N. Inagaki, S. Tasaka, H. Hiramatsu, J. Appl. Polym. Sci. 1999, 71, [17] J. S. Chiou, D. R. Paul, J. Polym. Sci., Part B: Polym. Phys. 1987, 25, [18] a) H. Chatham, Surf. Coat. Technol. 1996, 78, 1 ; b) G. L. Graff, R. E. Williford, P. E. Burrows, J. Appl. Phys. 2004, 96, [19] M. C. Duch, G. R. S. Budinger, Y. T. Liang, S. Soberanes, D. Urich, S. E. Chiarella, L. A. Campochiaro, A. Gonzalez, N. S. Chandel, M. C. Hersam, G. M. Mutlu, Nano Lett. 2011, 11, [20] C. Wirtz, K. Lee, T. Hallam, G. S. Duesberg, Chem. Phys. Lett. 2014, 595, 192. [21] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, A. K. Geim, Science 2008, 320, (5 of 5)