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Defects, a challenge for graphene in flexible electronics
Solid State Communications 229 (2016) 49–52
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Fast-track Communication
Defects, a challenge for graphene in flexible electronics A.J.M. Giesbers a,n, P.C.P. Bouten a, J.F.M. Cillessen a, L. van der Tempel a, J.H. Klootwijk a, A. Pesquera b, A. Centeno b, A. Zurutuza b, A.R. Balkenende a a b
Photonic Materials and Devices, Philips Research, High Tech Campus 4, 5656 AE Eindhoven, the Netherlands Graphenea, S.A. A75022608 Tolosa Hiribidea 76, 20018 Donostia-San Sebastian, Spain
art ic l e i nf o
a b s t r a c t
Article history: Received 14 August 2015 Received in revised form 2 January 2016 Accepted 4 January 2016 Available online 8 January 2016
In this work we present the effect of defects in graphene on its potential for application in flexible electronics. We visualize defects at the grain boundaries, transfer defects and local atomic defects. We show that these defects are currently determining the gas barrier properties of graphene. Under strain up to only 2% we show that these defects lead to cracks in the graphene thereby deteriorating its conductivity. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Graphene Flexible Gas barrier Defects Water vapor transmission rate Stress Bending
1. Introduction
2. Defects in graphene
Graphene is a material of many superlatives such as high mobility [1], superior thermal conductivity [2], large maximum allowable strain [3] and extremely low permeability [4]. These properties lead to visions of many potential applications. However, some major challenges must be tackled to enable industrial application. One of the most significant issues in large area graphene are defects. We will quantify the effect of these defects for flexible graphene electrodes and graphene based gas barriers. Further, a method to visualize the defects is presented. We will show that the structural defect density can locally (um2-scale) be as high as 1010 cm 2 in good quality transferred CVD graphene. The defects strongly reduce the applicable strain in flexible applications from the theoretical 20% strain to about 1.2% strain. Also the barrier properties of graphene are severely reduced: the water vapor transmission rate is 3 10 2 g/m2/day at 50% humidity and 20 °C, more than 4 orders below the intrinsic potential.
To visualize the structural defects in graphene a CVD grown graphene layer [5] is covered by a thin layer of gold (Au) (25 nm) and transferred (top down) to a Si/SiO2 wafer. This leads to a sample consisting of a Si/SiO2 substrate followed by a thin Au layer covered by a single graphene top layer. Exposing this sample to a potassium iodide (KI) solution etches pinholes in the Au layer through those defects in the graphene layer that are large enough for the KI-etchant to pass, whereas the rest of the Au is protected by the graphene crystal lattice. The etch time was typically 2 min at a temperature of 20 °C with an etch speed of vetch ¼10 nm/min. The pin holes in the thin Au layer result in a large contrast in scanning electron micrographs making this a powerful tool to visualize very small structural defects. Fig. 1 presents typical results after etching showing three distinct types of etch pits. In total 3 different samples were analyzed with this technique with a total area of 3 cm2. Agglomerates of etch pits like indicated in area I in Fig. 1 are observed over the entire sample. Typically in dimensions of an area of a few tens of um2 in an area of a few thousand um2 (Fig. 1 is a typical example). The density of etch pits in these areas can be as high as 1010 cm 2 with etch pit sizes ranging from 40 nm to a few 100 nm. The sample is further covered by a network of etch pits (area II in Fig. 1) with pit sizes similar to the ones in the agglomerates. Typically the etch pit lines enclose unaffected areas of 5–10 mm in diameter. Finally, we observe large etch pits, up to
n
Corresponding author. E-mail addresses: [email protected] (A.J.M. Giesbers), [email protected] (A.R. Balkenende). http://dx.doi.org/10.1016/j.ssc.2016.01.002 0038-1098/& 2016 Elsevier Ltd. All rights reserved.
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several micrometers in size, scattered over the sample like the one indicated in area III in Fig. 1. These last ones are so large that they must be ascribed to areas where graphene is missing e.g. due to incomplete graphene growth or transfer defects. The smaller etch pits, of about 40 nm in size, likely originate from point defects and are ascribed to atomic defects, considering etch rate (10 nm/min.) and time (2 min). The atomic pinhole size [6] of about 3 Å in case of one missing atom is in theory large enough for water ( 2 Å) and the etchant molecules ( 2 Å) to pass, whereas the carbonhexagon in graphene with a diameter of 2.1 Å is impermeable. At this point however we cannot rule out that the expected atomic defect grew larger during e.g. etching of the Cu-substrate in FeCl3 [7] or due to the transfer itself where additional bonds could be broken at the weaker defect sites [8] resulting in nanopores at these sites. The irregular shape of the slightly larger etch pits suggest groups of atomic defects where the crystal lattice is no longer a perfect honeycomb [6] or larger defects where locally multiple atoms are missing. Accordingly, the lines forming a network of etch pits are ascribed to defects at the grain boundary lines. A high concentration of atomic defects is expected here due to a mismatch in the crystallographic orientation or alignment of the graphene lattice in the various grains [9]. This attribution is also in agreement with the typical grain size of 5–10 mm [9,10].
Fig. 1. Scanning electron micrographs of a graphene covered gold layer after etching in a KI solution (t¼ 2 min, T ¼20 °C, vetch ¼ 10 nm/min). Clearly visible are etch pits at the graphene grain boundaries, atomic defects and holes.
3. Defects and stress In the next section we will discuss the effect of these defects on the strength of graphene. Graphene is considered an ideal transparent conductor in flexible applications due to its large elasticity. Its mechanical stiffness is 1 TPa, and the intrinsic breaking strength is 130 GPa, at 20% strain [3]. However, our measurements on large area graphene show that the electrical resistance already increases irreversibly at a strain of 1.2%. To measure this, CVD grown graphene was transferred to polyethylene naphthalate (PEN)-foil. The foil is 200 mm thick and has a RMS roughness of 5 nm. We use two types of samples, one with a 25 nm Au interlayer between the Au and the graphene to visualize the defects in a similar way as shown above, and one with the graphene directly on top of the PEN foil. The presence of the gold interlayer has no notable effect on the applied strain. The samples are controllably bent from flat down to 5 mm bending radius, while graphene's electrical resistance is monitored (inset Fig. 2a). In this way we can apply up to 2% strain, either tensile or compressive by bending upwards or downwards. For comparison we use the same PEN foil with a 50 nm ITO layer. At roughly 1% strain the ITO is known to form cracks perpendicular to the bending direction [11]. This leads to an abrupt and strong increase in the resistance of the layer as is evident from Fig. 2a. Reversing the strain will push the ITO at the cracks back into contact, almost restoring the resistance. In the case of graphene the resistance strongly increases for strains over 1.2% and does not return to its initial resistance value after relaxation, implying that the change is irreversible. Interestingly, the resistance perpendicular to the bending direction only increased by roughly half of that in the parallel direction, whereas at the start the material was isotropic. The maximum strain observed for large area CVD graphene is much lower than reported previously for a single graphene flake (420%) [3]. The probable cause for this is elucidated by the experiments with the gold interlayer and subsequent etching, showing the critical role of defects. Fig. 2a–b shows optical transmission micrographs visualizing the defect structure for three different strain levels. At 0% strain we observe the grain boundary structure as discussed above. When increasing the strain (Fig. 2c and d) the etching pattern changes from random grain boundaries to parallel etch lines. This is also clearly visualized in the SEM picture in Fig. 2e. Evidently, during bending crack lines are initiated 1 ε = 1.8 %
a
ε= 0 %
d
e
b
2
ε = 2.2 %
20 μm ε = 0.8 % 10 μm
f 20 μm
c
ε = 1.6 %
20 μm
10 μm
10 μm
Fig. 2. (Color Online) (a) Resistance change as a function of the applied strain for ITO (solid black) and graphene (dashed red). The initial resistance of ITO ¼57 Ω and Graphene¼ 1.43 kΩ. Inset: Photo of the automated bending setup, in which the resistance is measured while bending the graphene-on-PEN samples to a 5 mm radius. (b)– (d) Optical transmission images of grapheneþ Au etched in a KI solution after various applied strains. (e) Scanning electron micrograph of the etch pits in the gold after etching through bend graphene (1.6% strain). (f) Optical transmission image of a Au þgraphene samples etched in KI-solution after two way bending. The markers on the side show the sequence of bending and the applied strain. (g) Scanning electron micrograph of the same sample.
A.J.M. Giesbers et al. / Solid State Communications 229 (2016) 49–52
barrier film in high end applications. A concrete example is the use of graphene as a water vapor barrier in OLEDs [4,12]. The organic materials and the low work function cathode metals (barium) in OLEDs are very sensitive to water and oxygen [13,14]. For successful application of flexible OLEDs, where polymeric substrates are used, good encapsulation of the devices is essential [15,16]. Usually, an expensive stack of organic and inorganic layers is used to create the required low permeability, reaching water vapor transmission rates (WVTR) of 10 6 g/m2/day. To measure the WVTR of large area graphene samples we use a standard calcium test [16,17]. This test method is based on the oxidation of a thin calcium film (o100 nm). The WVTR is determined by measuring the oxidation rate of the calcium film by means of its transparency. Initially, the calcium forms a highly reflective metallic film. In the presence of water calcium oxidizes into an increasingly transparent calcium oxide / hydroxide layer. The optical transmission through the (partly reacted) calcium film is therefore, after calibration, a measure for the amount of reacted calcium and the associated amount of water penetrated through the barrier layer. Following the transmission as a function of time while exposing the samples to 50% relative humidity at a temperature of 20 °C directly leads to the water vapor transmission rate through the barrier encapsulating the calcium under these conditions. The samples used for this test consist of a PEN-foil substrate with an SU-8 resist planarization layer. On top of these foils
perpendicular to the strain direction at the defects already present in graphene. These defects can be the structural defects as illustrated above but cracks might also initiate at other types of defects like add-atoms or sp3-bonds. By increasing the strain the cracks propagate across the sample which explains the increase in resistance of the graphene/PEN samples during bending. The cracks increase the path length, thus increasing the resistance. The orientation of the cracks accounts for the measured resistance anisotropy. The apparent absence of the initial grain boundary etch pit lines in the optical images in Fig. 2 is explained by the smaller defect dimensions and thus slower etch rates, thereby making the etch pits smaller than the optical resolution. When we consecutively apply strain in the perpendicular direction (see Fig. 2f and g) crack lines perpendicular to the first cracks are formed. The crack density is similar but the length of the cracks now typically extends only between two of the cracks from the first bend, which act as stress release points.
4. Barrier properties and defects Another graphene property that is dominated by defects is gas permeability, which is of importance in gas barrier layers for e.g. organic solar cells, OLEDs, food and medical packaging. Single graphene flakes were shown to be impermeable to helium [4] and therefore graphene is expected to be a promising candidate as a
Graphene hBN
-1
Ca
Ca
PEN foil
H = 50%, T = 20oC
2
Barrier
WVTR (g/m /day)
10 Ca
51
-2
10
-3
10
0 hours
4mm 32 hours
4mm
0
1
2 3 # layers
4
25 hours
4mm
50 hours
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Fig. 3. (Color Online) (a) Schematic cross-section of the test samples showing the PEN-foil substrate (blue) with a SU8 planarization layer (light blue), the graphene layer (black), the calcium patches (orange) and the sealing barrier (green). (b) (0 h, top left) Top view of the actual sample prior to testing showing the PEN foil outline (gray) completely covered with graphene taped on a glass substrate at the corners for easy handling and nine calcium pads (dark gray). The black square on the left side serves as alignment marker and black standard for the automated measurement setup. The other four images show the calcium pads below a single layer of graphene after various exposure times, where the calcium pads were illuminated from the back side. (c) Water vapor transmission rates as a function of the number of layers for both graphene and hexagonal boron nitride (hBN). (d) Microscope image of partly oxidized calcium after 75 h of water exposure (RH ¼50%, T¼ 20 °C) underneath a 4-layer graphene barrier. The gray areas are the reflective metallic calcium and the dark areas are the transparent calcium oxide areas.
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graphene is transferred by Graphenea following a standard transfer procedure [18]. The calcium films are subsequently deposited on top of the graphene in a water and oxygen free glove box. The calcium deposition is followed by the deposition of a barrier layer with a WVTR of at least 10 6 g/m2/day, which is checked by control samples on a glass substrate deposited in the same run. Permeability is then determined by the graphene layer as long as the barrier properties of graphene exceed those of PENfoil (10 1 g/m2/day). Fig. 3a shows a schematic cross-section of the sample. Nine 4 4 mm2 patches of calcium are deposited on a 25 25 mm2 PEN foil which is completely covered with graphene. For each of the nine patches the WVTR is determined in the calcium test and averaged to a single value. Every 3 hours the transmission was measured to follow the reacted amount of calcium in time (Fig. 3b–d). The transition from metallic calcium at 0 h to transparent calcium oxide at 50 h is evident and leads to the WVTR values in Fig. 3c. A single layer barrier has a WVTR¼ 3 10 2 g/m2/ day, which improves to WVTR ¼5 10 3 g/m2/day for 4 layers of graphene, comparable to a single inorganic oxide or nitride coating [19]. Similar experiments on hexagonal boronnitride (hBN, Graphene Supermarket) clearly show its current lower quality compared to graphene. The microstructure of the oxidized Ca is similar to the structure observed for Au-etching (Fig. 1). As the processing order is different, this stresses our opinion that the defects that are visualized do not originate from the processing, but are intrinsic tot he graphene layer. As can be seen in Fig. 3d the grain boundary and local defects play a role in the WVTR but more important for barrier properties are the larger holes and defect agglomerates. Interestingly, the microscopic images taken during the calcium test after partial oxidation of samples where we transfered 1, 2 and 4 layers show a similar defect pattern. Apparently, water is able to reach all defects in the underlying layers and not only penetrates through overlapping defects in the case of multiple layers. This indicates that water intercallates between two stacked graphene layers, leading to in-plane transport of water to the defects in the bottom layer. The increased pathlength of water tot he bottom layer improves the WVTR of the multilayer stacks only to a limited extend. The similar defect pattern in the reacted Ca at the same time indicates that the transport of intercalated water in between two layers is not the limiting factor. This is in agreement with theoretically and experimentally findings [20–22]. Adding more layers will improve the WVTR but it will not help to cover and mask defects in the layer below.
5. Conclusion In conclusion, we visualized the structural defect density in a state of the art CVD graphene. We showed that these defects are
located at the grain boundaries and in micrometer sized areas of defect agglomerates in which the defect density can locally be as high as 1010 cm 2. These defects are currently limiting graphene properties such as applicable strain and gas permeability.
Acknowledgments We like to acknowledge P. Klaassen for technical support with the WVTR experiments. The research leading to these results has received funding from the European Union Seventh Framework Program under grant agreement No. 604391 Graphene Flagship.
References [1] K.I. Bolotin, K.J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim, H.L. Stormer, Solid State Commun. 146 (2008) 351–355. [2] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Nano Lett. 8 (2008) 902–907. [3] C. Lee, X. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385. [4] 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. 8 (2008) 2458. [5] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S.K. Banerjee, L. Colombo, R.S. Ruoff, Science 324 (2009) 1312. [6] D.J. Appelhans, L.D. Carr, M.T. Lusk, New J. Phys. 12 (2010) 125006. [7] Y. Fan, K. He, H. Tan, S. Speller, J.H. Warner, Chem. Mater. 26 (2014) 4984. [8] G.-H. Lee, R.C. Cooper, S.J. An, S. Lee, A. van der Zande, N. Petrone, A. G. Hammerberg, C. Lee, B. Crawford, W. Oliver, J.W. Kysar, J. Hone, Science 340 (2013) 1073–1076. [9] P.Y. Huang, C.S. Ruiz-Vargas, A.M. van der Zande, W.S. Whitney, M.P. Levendorf, J.W. Kevek, S. Garg, J.S. Alden, C.J. Hustedt, Y. Zhu, J. Park, P.L. McEuen, D.A. Muller, Nature 469 (2011) 389. [10] Z. Fei, A.S. Rodin, W. Gannett, S. Dai, W. Regan, M. Wagner, M.K. Liu, A. S. McLeod, G. Dominguez, M. Thiemens, A.H. Castro Neto, F. Keilmann, A. Zettl, R. Hillenbrand, M.M. Fogler, D.N. Basov, Nat. Nanotechnol. 8 (2013) 821. [11] P.C.P. Bouten, P.J. Slikkerveer, Y. Leterrier, Mechanics of ITO on Plastic Substrates for Flexible, In: G.P. Crawford (Ed.), Displays, in Flexible Flat Panel Displays, John Wiley & Sons, Ltd, Chichester, UK, http://dx.doi.org/10.1002/ 0470870508.ch6. [12] Y. Su, V.G. Kravets, S.L. Wong, J. Waters, A.K. Geim, R.R. Nair, Nat. Commun. 5 (2014) 4843. [13] R. Grover, R. Srivastava, O. Rana, D.S. Mehta, M.N. Kamalasanan, J. Encap. Adsorpt. Sci. 1 (2011) 23. [14] M. Stössel, J. Staudigel, F. Steuber, J. Simmerer, A. Winnacker, Appl. Phys. A 68 (1999) 387. [15] Y.C. Han, C. Jang, K.J. Kim, K.C. Choi, K.-H. Jung, B.-S. Bae, Org. Electron. 12 (2011) 609. [16] W. Keuning, P. Van de Weijer, H. Lifka, W.M.M. Kessels, M. Creatore, M. J. Vac. Sci. Technol. A 30 (2012) 01A131. [17] G. Nisato P.C.P. Bouten P.J. Slikkerveer W.D. Bennet G.L. Graff N. Rutherford L. Wiese. Asia Disp./IDW’01 Proc. 2001 p1435. [18] X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R.D. Piner, L. Colombo, R. S. Ruoff, Nano Lett. 9 (2009) 4359. [19] T.N. Chen, D.S. Wuu, C.C. Chiang, Y.P. Chen, R.H. Horng, J. Electrochem. Soc. 153 (2006) F244. [20] K. Falk, F. Sedlmeier, L. Joly, R.R. Netz, L. Bocquet, Nano Lett. 10 (2010) 4067. [21] G. Cicero, J.C. Grossman, E. Schwegler, F. Gygi, G. Galli, J. Am. Chem. Soc. 130 (2008) 1871. [22] D.-E. Lee, G. Ahn, S. Ryu, J. Am. Chem. Soc. 136 (2014) 6634.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Fast-track Communication
Defects, a challenge for graphene in flexible electronics A.J.M. Giesbers a,n, P.C.P. Bouten a, J.F.M. Cillessen a, L. van der Tempel a, J.H. Klootwijk a, A. Pesquera b, A. Centeno b, A. Zurutuza b, A.R. Balkenende a a b
Photonic Materials and Devices, Philips Research, High Tech Campus 4, 5656 AE Eindhoven, the Netherlands Graphenea, S.A. A75022608 Tolosa Hiribidea 76, 20018 Donostia-San Sebastian, Spain
art ic l e i nf o
a b s t r a c t
Article history: Received 14 August 2015 Received in revised form 2 January 2016 Accepted 4 January 2016 Available online 8 January 2016
In this work we present the effect of defects in graphene on its potential for application in flexible electronics. We visualize defects at the grain boundaries, transfer defects and local atomic defects. We show that these defects are currently determining the gas barrier properties of graphene. Under strain up to only 2% we show that these defects lead to cracks in the graphene thereby deteriorating its conductivity. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Graphene Flexible Gas barrier Defects Water vapor transmission rate Stress Bending
1. Introduction
2. Defects in graphene
Graphene is a material of many superlatives such as high mobility [1], superior thermal conductivity [2], large maximum allowable strain [3] and extremely low permeability [4]. These properties lead to visions of many potential applications. However, some major challenges must be tackled to enable industrial application. One of the most significant issues in large area graphene are defects. We will quantify the effect of these defects for flexible graphene electrodes and graphene based gas barriers. Further, a method to visualize the defects is presented. We will show that the structural defect density can locally (um2-scale) be as high as 1010 cm 2 in good quality transferred CVD graphene. The defects strongly reduce the applicable strain in flexible applications from the theoretical 20% strain to about 1.2% strain. Also the barrier properties of graphene are severely reduced: the water vapor transmission rate is 3 10 2 g/m2/day at 50% humidity and 20 °C, more than 4 orders below the intrinsic potential.
To visualize the structural defects in graphene a CVD grown graphene layer [5] is covered by a thin layer of gold (Au) (25 nm) and transferred (top down) to a Si/SiO2 wafer. This leads to a sample consisting of a Si/SiO2 substrate followed by a thin Au layer covered by a single graphene top layer. Exposing this sample to a potassium iodide (KI) solution etches pinholes in the Au layer through those defects in the graphene layer that are large enough for the KI-etchant to pass, whereas the rest of the Au is protected by the graphene crystal lattice. The etch time was typically 2 min at a temperature of 20 °C with an etch speed of vetch ¼10 nm/min. The pin holes in the thin Au layer result in a large contrast in scanning electron micrographs making this a powerful tool to visualize very small structural defects. Fig. 1 presents typical results after etching showing three distinct types of etch pits. In total 3 different samples were analyzed with this technique with a total area of 3 cm2. Agglomerates of etch pits like indicated in area I in Fig. 1 are observed over the entire sample. Typically in dimensions of an area of a few tens of um2 in an area of a few thousand um2 (Fig. 1 is a typical example). The density of etch pits in these areas can be as high as 1010 cm 2 with etch pit sizes ranging from 40 nm to a few 100 nm. The sample is further covered by a network of etch pits (area II in Fig. 1) with pit sizes similar to the ones in the agglomerates. Typically the etch pit lines enclose unaffected areas of 5–10 mm in diameter. Finally, we observe large etch pits, up to
n
Corresponding author. E-mail addresses: [email protected] (A.J.M. Giesbers), [email protected] (A.R. Balkenende). http://dx.doi.org/10.1016/j.ssc.2016.01.002 0038-1098/& 2016 Elsevier Ltd. All rights reserved.
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several micrometers in size, scattered over the sample like the one indicated in area III in Fig. 1. These last ones are so large that they must be ascribed to areas where graphene is missing e.g. due to incomplete graphene growth or transfer defects. The smaller etch pits, of about 40 nm in size, likely originate from point defects and are ascribed to atomic defects, considering etch rate (10 nm/min.) and time (2 min). The atomic pinhole size [6] of about 3 Å in case of one missing atom is in theory large enough for water ( 2 Å) and the etchant molecules ( 2 Å) to pass, whereas the carbonhexagon in graphene with a diameter of 2.1 Å is impermeable. At this point however we cannot rule out that the expected atomic defect grew larger during e.g. etching of the Cu-substrate in FeCl3 [7] or due to the transfer itself where additional bonds could be broken at the weaker defect sites [8] resulting in nanopores at these sites. The irregular shape of the slightly larger etch pits suggest groups of atomic defects where the crystal lattice is no longer a perfect honeycomb [6] or larger defects where locally multiple atoms are missing. Accordingly, the lines forming a network of etch pits are ascribed to defects at the grain boundary lines. A high concentration of atomic defects is expected here due to a mismatch in the crystallographic orientation or alignment of the graphene lattice in the various grains [9]. This attribution is also in agreement with the typical grain size of 5–10 mm [9,10].
Fig. 1. Scanning electron micrographs of a graphene covered gold layer after etching in a KI solution (t¼ 2 min, T ¼20 °C, vetch ¼ 10 nm/min). Clearly visible are etch pits at the graphene grain boundaries, atomic defects and holes.
3. Defects and stress In the next section we will discuss the effect of these defects on the strength of graphene. Graphene is considered an ideal transparent conductor in flexible applications due to its large elasticity. Its mechanical stiffness is 1 TPa, and the intrinsic breaking strength is 130 GPa, at 20% strain [3]. However, our measurements on large area graphene show that the electrical resistance already increases irreversibly at a strain of 1.2%. To measure this, CVD grown graphene was transferred to polyethylene naphthalate (PEN)-foil. The foil is 200 mm thick and has a RMS roughness of 5 nm. We use two types of samples, one with a 25 nm Au interlayer between the Au and the graphene to visualize the defects in a similar way as shown above, and one with the graphene directly on top of the PEN foil. The presence of the gold interlayer has no notable effect on the applied strain. The samples are controllably bent from flat down to 5 mm bending radius, while graphene's electrical resistance is monitored (inset Fig. 2a). In this way we can apply up to 2% strain, either tensile or compressive by bending upwards or downwards. For comparison we use the same PEN foil with a 50 nm ITO layer. At roughly 1% strain the ITO is known to form cracks perpendicular to the bending direction [11]. This leads to an abrupt and strong increase in the resistance of the layer as is evident from Fig. 2a. Reversing the strain will push the ITO at the cracks back into contact, almost restoring the resistance. In the case of graphene the resistance strongly increases for strains over 1.2% and does not return to its initial resistance value after relaxation, implying that the change is irreversible. Interestingly, the resistance perpendicular to the bending direction only increased by roughly half of that in the parallel direction, whereas at the start the material was isotropic. The maximum strain observed for large area CVD graphene is much lower than reported previously for a single graphene flake (420%) [3]. The probable cause for this is elucidated by the experiments with the gold interlayer and subsequent etching, showing the critical role of defects. Fig. 2a–b shows optical transmission micrographs visualizing the defect structure for three different strain levels. At 0% strain we observe the grain boundary structure as discussed above. When increasing the strain (Fig. 2c and d) the etching pattern changes from random grain boundaries to parallel etch lines. This is also clearly visualized in the SEM picture in Fig. 2e. Evidently, during bending crack lines are initiated 1 ε = 1.8 %
a
ε= 0 %
d
e
b
2
ε = 2.2 %
20 μm ε = 0.8 % 10 μm
f 20 μm
c
ε = 1.6 %
20 μm
10 μm
10 μm
Fig. 2. (Color Online) (a) Resistance change as a function of the applied strain for ITO (solid black) and graphene (dashed red). The initial resistance of ITO ¼57 Ω and Graphene¼ 1.43 kΩ. Inset: Photo of the automated bending setup, in which the resistance is measured while bending the graphene-on-PEN samples to a 5 mm radius. (b)– (d) Optical transmission images of grapheneþ Au etched in a KI solution after various applied strains. (e) Scanning electron micrograph of the etch pits in the gold after etching through bend graphene (1.6% strain). (f) Optical transmission image of a Au þgraphene samples etched in KI-solution after two way bending. The markers on the side show the sequence of bending and the applied strain. (g) Scanning electron micrograph of the same sample.
A.J.M. Giesbers et al. / Solid State Communications 229 (2016) 49–52
barrier film in high end applications. A concrete example is the use of graphene as a water vapor barrier in OLEDs [4,12]. The organic materials and the low work function cathode metals (barium) in OLEDs are very sensitive to water and oxygen [13,14]. For successful application of flexible OLEDs, where polymeric substrates are used, good encapsulation of the devices is essential [15,16]. Usually, an expensive stack of organic and inorganic layers is used to create the required low permeability, reaching water vapor transmission rates (WVTR) of 10 6 g/m2/day. To measure the WVTR of large area graphene samples we use a standard calcium test [16,17]. This test method is based on the oxidation of a thin calcium film (o100 nm). The WVTR is determined by measuring the oxidation rate of the calcium film by means of its transparency. Initially, the calcium forms a highly reflective metallic film. In the presence of water calcium oxidizes into an increasingly transparent calcium oxide / hydroxide layer. The optical transmission through the (partly reacted) calcium film is therefore, after calibration, a measure for the amount of reacted calcium and the associated amount of water penetrated through the barrier layer. Following the transmission as a function of time while exposing the samples to 50% relative humidity at a temperature of 20 °C directly leads to the water vapor transmission rate through the barrier encapsulating the calcium under these conditions. The samples used for this test consist of a PEN-foil substrate with an SU-8 resist planarization layer. On top of these foils
perpendicular to the strain direction at the defects already present in graphene. These defects can be the structural defects as illustrated above but cracks might also initiate at other types of defects like add-atoms or sp3-bonds. By increasing the strain the cracks propagate across the sample which explains the increase in resistance of the graphene/PEN samples during bending. The cracks increase the path length, thus increasing the resistance. The orientation of the cracks accounts for the measured resistance anisotropy. The apparent absence of the initial grain boundary etch pit lines in the optical images in Fig. 2 is explained by the smaller defect dimensions and thus slower etch rates, thereby making the etch pits smaller than the optical resolution. When we consecutively apply strain in the perpendicular direction (see Fig. 2f and g) crack lines perpendicular to the first cracks are formed. The crack density is similar but the length of the cracks now typically extends only between two of the cracks from the first bend, which act as stress release points.
4. Barrier properties and defects Another graphene property that is dominated by defects is gas permeability, which is of importance in gas barrier layers for e.g. organic solar cells, OLEDs, food and medical packaging. Single graphene flakes were shown to be impermeable to helium [4] and therefore graphene is expected to be a promising candidate as a
Graphene hBN
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H = 50%, T = 20oC
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51
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4mm 32 hours
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Fig. 3. (Color Online) (a) Schematic cross-section of the test samples showing the PEN-foil substrate (blue) with a SU8 planarization layer (light blue), the graphene layer (black), the calcium patches (orange) and the sealing barrier (green). (b) (0 h, top left) Top view of the actual sample prior to testing showing the PEN foil outline (gray) completely covered with graphene taped on a glass substrate at the corners for easy handling and nine calcium pads (dark gray). The black square on the left side serves as alignment marker and black standard for the automated measurement setup. The other four images show the calcium pads below a single layer of graphene after various exposure times, where the calcium pads were illuminated from the back side. (c) Water vapor transmission rates as a function of the number of layers for both graphene and hexagonal boron nitride (hBN). (d) Microscope image of partly oxidized calcium after 75 h of water exposure (RH ¼50%, T¼ 20 °C) underneath a 4-layer graphene barrier. The gray areas are the reflective metallic calcium and the dark areas are the transparent calcium oxide areas.
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graphene is transferred by Graphenea following a standard transfer procedure [18]. The calcium films are subsequently deposited on top of the graphene in a water and oxygen free glove box. The calcium deposition is followed by the deposition of a barrier layer with a WVTR of at least 10 6 g/m2/day, which is checked by control samples on a glass substrate deposited in the same run. Permeability is then determined by the graphene layer as long as the barrier properties of graphene exceed those of PENfoil (10 1 g/m2/day). Fig. 3a shows a schematic cross-section of the sample. Nine 4 4 mm2 patches of calcium are deposited on a 25 25 mm2 PEN foil which is completely covered with graphene. For each of the nine patches the WVTR is determined in the calcium test and averaged to a single value. Every 3 hours the transmission was measured to follow the reacted amount of calcium in time (Fig. 3b–d). The transition from metallic calcium at 0 h to transparent calcium oxide at 50 h is evident and leads to the WVTR values in Fig. 3c. A single layer barrier has a WVTR¼ 3 10 2 g/m2/ day, which improves to WVTR ¼5 10 3 g/m2/day for 4 layers of graphene, comparable to a single inorganic oxide or nitride coating [19]. Similar experiments on hexagonal boronnitride (hBN, Graphene Supermarket) clearly show its current lower quality compared to graphene. The microstructure of the oxidized Ca is similar to the structure observed for Au-etching (Fig. 1). As the processing order is different, this stresses our opinion that the defects that are visualized do not originate from the processing, but are intrinsic tot he graphene layer. As can be seen in Fig. 3d the grain boundary and local defects play a role in the WVTR but more important for barrier properties are the larger holes and defect agglomerates. Interestingly, the microscopic images taken during the calcium test after partial oxidation of samples where we transfered 1, 2 and 4 layers show a similar defect pattern. Apparently, water is able to reach all defects in the underlying layers and not only penetrates through overlapping defects in the case of multiple layers. This indicates that water intercallates between two stacked graphene layers, leading to in-plane transport of water to the defects in the bottom layer. The increased pathlength of water tot he bottom layer improves the WVTR of the multilayer stacks only to a limited extend. The similar defect pattern in the reacted Ca at the same time indicates that the transport of intercalated water in between two layers is not the limiting factor. This is in agreement with theoretically and experimentally findings [20–22]. Adding more layers will improve the WVTR but it will not help to cover and mask defects in the layer below.
5. Conclusion In conclusion, we visualized the structural defect density in a state of the art CVD graphene. We showed that these defects are
located at the grain boundaries and in micrometer sized areas of defect agglomerates in which the defect density can locally be as high as 1010 cm 2. These defects are currently limiting graphene properties such as applicable strain and gas permeability.
Acknowledgments We like to acknowledge P. Klaassen for technical support with the WVTR experiments. The research leading to these results has received funding from the European Union Seventh Framework Program under grant agreement No. 604391 Graphene Flagship.
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