nanomaterials Article

Copper Micro-Labyrinth with Graphene Skin: New Transparent Flexible Electrodes with Ultimate Low Sheet Resistivity and Superior Stability Hak Ki Yu Department of Materials Science and Engineering and Department of Energy Systems Research, Ajou University, Suwon 16499, Korea; [email protected]; Tel.: +82-31-219-1680 Academic Editor: Ho Won Jang Received: 15 July 2016; Accepted: 31 August 2016; Published: 1 September 2016

Abstract: We have developed self-assembled copper (Cu) micro-labyrinth (ML) with graphene skin for transparent flexible electrodes of optoelectronic devices. The Cu ML is simply formed by heating a thin Cu film with a 100-nm thickness on a SiO2 /Si substrate at 950 ◦ C under hydrogen ambient to block the oxidation. Moreover, the Cu ML can have graphene skin at the surface by inserting carbo-hydroxyl molecules (Cx Hy ) during heating due to the catalytic decomposition of C–H bonds on the Cu surface. The Cu ML with graphene skin (Cu ML-G) has superior sheet resistivity below 5 Ω/sq and mechanical flexibility without cracks at the bending radius of 0.1 cm. Although the transmittance of Cu ML-G is a little lower (70%~80%) than that of conventional metallic nanowires electrodes (such as Ag, ~90% at the visible wavelength), it has good thermal stability in conductivity without any damage at 200 ◦ C due to a micro-sized pattern and graphene skin which prohibits the surface migration of Cu atoms. Keywords: Cu micro-labyrinth; transparent flexible electrode; graphene

1. Introduction Recently, due to the development of organic materials and nanotechnology, the research for flexible functionality of optoelectronic devices are attracting great interests [1–3]. Among several issues for the development of ultimate flexible optoelectronic devices, the transparent flexible electrodes (TFE) which determine the efficiency of the devices is one of the most important research topics [4–10]. Although many materials for the TFE have been studied, such as metallic nanowire connection (Au, Ag, and Cu) [4,5], carbon based materials such as carbon nanotube, graphene, and conducting polymers [6–8], and conducting oxide nanostructures (Sn doped In2 O3 : Indium-Tix-Oxide (ITO) and doped ZnO) [9,10], each material has demerits for the TFE applications. The metallic nanowire meshes are not suitable for harsh environmental conditions (high temperature and humidity). Because the melting point of nano-sized materials is lower than that of bulk status [11,12], high temperature operation even at 100 ◦ C causes severe migration and disconnection of metallic nanowires, resulting in low conductivity. The carbon-based nano-structures also have an inherent limitation in conductivity (carbon nanotubes: 100~1000 Ω/sq, graphene: 100~10,000 Ω/sq, and PEDOT-PSS: 100~1000 Ω/sq) compared with metallic nanowire (10~100 Ω/sq). Moreover, the conducting oxide nanostructures also have demerits in the limited conductivity itself and weaknesses in acid processes (decomposed easily even with small concentration of H+ ) [13,14]. Meanwhile, we have focused on the composite materials using the above TFE materials. The greatest weakness of the metal nanowire mesh is low thermal stability, but it has the most superior conductivity. On the other hand, the carbon-based materials have good thermal stability, but they have low

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conductivity, relatively. To increase To theincrease thermal the stability of metal nanowire, thenanowire, atomic carbon layer have low conductivity, relatively. thermal stability of metal the atomic called graphene can be used by wrapping the metal surfaces. carbon layer called graphene can be used by wrapping the metal surfaces. As As shown shown in in Figure Figure 1, 1, thin thin Cu Cu film film (about (about 100 100 nm nm on on amorphous amorphous substrate) substrate) can can be be easily easily ◦ C at the reduction ambient, agglomerated and partially evaporated at a temperature higher than 900 agglomerated and partially evaporated at a temperature higher than 900 °C resulting resulting in in aa micro-labyrinth micro-labyrinth (ML) (ML) pattern. pattern. The Cu ML can have graphene skin at the the surface surface by by inserting inserting carbo-hydroxyl carbo-hydroxyl molecules molecules (CH (CH44 in this experiment) during heating due to to the the catalytic catalytic decomposition decomposition of C–H C–H bonds bonds on on the the Cu Cu surface. surface. The The micro-sized micro-sized diameter diameter of of the the Cu Cu labyrinth labyrinth can can have have aa higher higher melting melting point than nano-sized connection, although the transmittance can be degraded. Moreover, Moreover, the the graphene graphene skin skin can can block block the the surface surface migration migration of of Cu Cu atoms atoms at at the the high high temperature temperature annealing, resulting in good thermal stability [15]. The graphene skin also support superior mechanical annealing, resulting in good thermal stability [15]. The graphene skin also support superior strength when the Cuwhen ML used as extreme circumstances (for the flexibility, theflexibility, Cu ML with mechanical strength the Cu ML usedbending as extreme bending circumstances (for the the graphene skingraphene (Cu ML-G) beML-G) transferred polydimethylsiloxane (PDMS) support and the SiO2 Cu ML with skincan (Cu can bevia transferred via polydimethylsiloxane (PDMS) support etching shown in Figure 1). in Figure 1). and theprocess SiO2 etching process shown

Figure 1. 1. Schematics experimental idea: Heating of Cu (100 (100 nm, on SiO 2 covered Si) makes Figure Schematicsofofthe the experimental idea: Heating of film Cu film nm, on SiO2 covered Si) a micro-labyrinth pattern. The pattern can have graphene skin at the surface by inserting carbomakes a micro-labyrinth pattern. The pattern can have graphene skin at the surface by inserting hydroxyl molecules (CH4 (CH in this experiment). By using polydimethylsiloxane (PDMS) support and carbo-hydroxyl molecules 4 in this experiment). By using polydimethylsiloxane (PDMS) support SiO2SiO etching technology, the Cu labyrinth pattern with graphene skin cancan be be transferred and used as and etching technology, the Cu labyrinth pattern with graphene skin transferred and used 2 flexible electrodes. as flexible electrodes.

2. Results 2. Results and and Discussion Discussion Figure 22 shows shows optical optical microscope microscope images images (each (each inset inset is magnified magnified scanning scanning electron electron microscope microscope Figure (SEM) images) images) of of Cu Cu film filmon onSiO SiO22/Si /Si after heating for 10 min with with H H22 (20 sccm) and and CH CH44 (6 sccm) (SEM) ◦ C950 ambient(total (totalvapor vapor pressure mTorr) (a) ◦1000 900 °C. the Because the ambient pressure 460460 mTorr) at (a)at1000 C, (b)°C, 950(b) and°C (c) and 900 ◦(c) C. Because diffusion diffusion width of Cu atoms during heating depends more strongly on the diffusivity determined by width of Cu atoms during heating depends more strongly on the diffusivity determined by temperature temperature (Arrhenius than[16], heating time [16], the severe change inwas morphology was shown (Arrhenius relation) than relation) heating time the severe change in morphology shown among those among those temperatures. The ideal labyrinth which should behas connected has the temperatures. The ideal labyrinth pattern, which pattern, should be connected but the leastbut coverage onleast the ◦ coverage on the substrate, was acquired by heating at 950 °C. The contents of Cu can be modulated substrate, was acquired by heating at 950 C. The contents of Cu can be modulated by using different by using different thicknesses of Cu film. we (below use thin100 Cu nm), film (below 100 nm), the Cu and area thicknesses of Cu film. If we use thin CuIffilm much of the Cu much area isofopened is opened and the Cu pattern is after disconnected heating. theifother hand, if we Cuthan film the Cu pattern is disconnected heating. after On the other On hand, we use thick Cuuse filmthick (more (more than 100take nm),more it will take time to have an optimized labyrinth during 100 nm), it will time to more have an optimized labyrinth pattern duringpattern annealing. Theannealing. existence The existence graphene on the surface shown Raman by measuring (Figure 2d). of graphene onofthe Cu surface wasCu shown by was measuring spectraRaman (Figurespectra 2d). The graphene ◦ The graphene grown on the Cu area (heated at 1000 °C) shows the properties of monolayer graphene, grown on the Cu area (heated at 1000 C) shows the properties of monolayer graphene, namely large namely large 2D/G with[17]. low The D lines [17]. The graphene showssmall a relatively 2D/G line ratio withline lowratio D lines spectrum of spectrum graphene of shows a relatively 2D/G small ratio 2D/Gincreased ratio withDincreased D lines byheating reducing heating temperature to 900 °C. The strong D lineto is with lines by reducing temperature to 900 ◦ C. The strong D line is related 2 2 bonded carbon sheets by forming an island growth of related to broken hexagonal symmetry of sp broken hexagonal symmetry of sp bonded carbon sheets by forming an island growth of graphene graphene to low of carbon at reduced[18]. temperature [18].theHowever, for the due to low due solubility of solubility carbon at reduced temperature However, for sample heated at sample 950 ◦ C, heated at and 950 2D/G °C, theratio D line and and 2D/Git can ratioreasonably is weak, and it can reasonably be concluded that the the D line is weak, be concluded that the monolayer of graphene monolayer ofCu graphene is covered on ML. is covered on Cu ML.

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Figure 2. Optical microscope images(each (each inset is is magnified microscope (SEM) Figure 2. 2. Optical magnifiedscanning scanningelectron electron microscope (SEM) Figure Opticalmicroscope microscopeimages images (each inset inset is magnified scanning electron microscope (SEM) images) of Cu film on SiO 2/Si after heating for 10 min with H2 (20 sccm) and CH4 (6 sccm) ambient images) of of CuCu film onon SiO after heating for 10 min with H (20 sccm) and CH4 (6 sccm) ambient 2 /Si images) film SiO 2/Si after heating for 10 min with H22(20 sccm) and CH4 (6 sccm) ambient (total vapor pressure 460 mTorr) at (a) 1000 ◦°C, (b) 950 °C, Raman spectra of ◦ C; and ◦ (d)(d) (total vapor pressure C; (b) and(c) (c)900 900°C, Ramanspectra spectra (total vapor pressure460 460mTorr) mTorr)atat(a) (a)1000 1000 °C, (b) 950 950 °C, and (c) 900 °C,C;(d) Raman of of graphene covered copper micro-labyrinth (Cu ML) with respect to heating temperature. graphene covered copper to heating heatingtemperature. temperature. graphene covered coppermicro-labyrinth micro-labyrinth(Cu (CuML) ML) with with respect respect to

Figure 3. X-ray diffraction of the Cu(111) film (100-nm thickness) on the C-plane sapphire; (a) theta̅ 0} and sapphire 2theta 3.scan with log scaleofand azimuthal scan with fixed θ along Cu {22 Figure diffraction the Cu(111) film (100-nm thickness) on thethe C-plane thetaFigure 3. X-ray X-ray diffraction of (b) the Cu(111) film (100-nm thickness) on thesapphire; C-plane(a)sapphire; {33̅00}scan direction. Both exhibit hexagonal symmetry peaks spaced by 60°, which an epitaxial {22̅has 2theta with log scale and (b) azimuthal scan with fixed θ along the Cu 0} and sapphire (a) theta-2theta scan with log scale and (b) azimuthal scan with fixed θ along the Cu {220} and ̅ 0]exhibit ̅ 00]C−sapphire relationship: (111)[11 . (c)peaks SEM spaced images by of Cu on C-sapphire after ̅ 00} direction. Cu ∥ (0001)[11 {33 Both hexagonal symmetry 60°,film which has an epitaxial sapphire {3300} direction. Both exhibit hexagonal symmetry peaks spaced by 60◦ , which has an ̅ 0]Cu ̅ 00]CH heating for 10 min with H2∥(20 sccm) and 4 (6 sccm) ambient (total vapor pressure 460 mTorr) at a relationship: (111)[11 (0001)[11 . (c) SEM images of Cu film on C-sapphire after epitaxial relationship: (111)[110]Cu k (0001C−sapphire )[1100]C−sapphire ; (c) SEM images of Cu film on C-sapphire temperature 900~1000 heating for 10of min with H°C. 2 (20 sccm) and CH4 (6 sccm) ambient (total vapor pressure 460 mTorr) at a after heating for 10 min with H2 (20 sccm) and CH4 (6 sccm) ambient (total vapor pressure 460 mTorr) temperature of 900~1000 °C.◦ at a The temperature of 900~1000 C. on substrate type that has a different crystal structure and surfaceCu ML pattern depends

freeThe energy. For pattern the comparison, donetype the same experiments oncrystal the C-plane sapphire (0006) Cu ML depends we on have substrate that has a different structure and surfaceThe Cu ML pattern depends on substrate type that has a different crystal structure and free energy. For the comparison, we have done the same experiments on the C-plane sapphire (0006)

surface-free energy. For the comparison, we have done the same experiments on the C-plane sapphire

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(0006) substrate. Thefilm Cu film grown C-plane sapphireshows shows(111) (111) preferred preferred epitaxial substrate. The Cu (100 (100 nm)nm) grown on on C-plane sapphire epitaxial orientation—shown in Figure 3a. The atomic distance between Cu (111) surface atoms is 0.255 nm, orientation—shown in Figure 3a. The atomic distance between Cu (111) surface atoms is 0.255 nm, The atomic atomic lattice lattice mismatch mismatch with with 8.6% 8.6% while the O–O atomic distance in C-plane sapphire is 0.279 nm. The allows for pseudo-epitaxial pseudo-epitaxial growth growth of Cu (111) plane on C-plane sapphire. The 6-fold symmetry of the Cu (111) plane shown in Figure 3b can be understood as a twin formation formation of 3-fold Face Centered Cubic (FCC) Cu due to pseudo-epitaxial growth. [19,20]. Figure 3c shows SEM images of the Cu film after heating for 10 min with H22 (20 (20 sccm) sccm) and and CH CH44(6(6sccm) sccm)ambient ambient(total (totalvapor vaporpressure pressure460 460mTorr) mTorr) ◦ C, 950 ◦ C, and 1000 ◦ C. The tendency for the disconnection of the Cu film and island formation at 900 °C, 950 °C, and 1000 °C. The tendency for the disconnection of the Cu film and island formation the heating heating temperature temperature is is similar, similar, but but the the residual residual pattern pattern of of Cu Cu is is totally totally different. different. by increasing the The basic form of retained retained Cu film film is is aa triangular-fractal triangular-fractal pattern, pattern, resulting resulting in hexagonal hexagonal pattern which substrate is related to the 6-fold symmetry symmetry measured measured in in Figure Figure3b. 3b. In In this this work, work,we wehave haveused usedSiO SiO22/Si /Si substrate for the easy transfer of the Cu pattern by etching a SiO2 layer. The sheet and transmittance of CuofML skin (Cuskin ML-G) forML-G) the application sheetresistivity resistivity and transmittance Cuwith MLgraphene with graphene (Cu for the as transparent electrodes electrodes are shownare inshown Figurein4.Figure The experimental resultsresults were compared with application as transparent 4. The experimental were compared alternative TFE such as Mono-layer graphene, Ag nanowires, and ITO nano-branches (insets show with alternative TFE such as Mono-layer graphene, Ag nanowires, and ITO nano-branches (insets the synthesized alternative materials). Compared with these materials, Cu ML-G show the synthesized alternative materials). Compared withalternative these alternative materials, Cushows ML-Ga lower transmittance (moderate(moderate about 75%) due 75%) to a micro-sized metal pattern; however, shows low shows a lower transmittance about due to a micro-sized metal pattern;ithowever, it sheet resistivity, even below 3 Ω/sq. Moreover, it shows superior mechanical and thermal stability, shows low sheet resistivity, even below 3 Ω/sq. Moreover, it shows superior mechanical and thermal which is which important for the application to the harsh operation conditioncondition of optoelectronic devices stability, is important for the application to the harsh operation of optoelectronic (see Figure 5). The mechanical stability was confirmed by carrying out a bending test (Figure and devices (see Figure 5). The mechanical stability was confirmed by carrying out a bending test 5a) (Figure compared the results those ofthose conventional ITO film onfilm polyethylene terephthalate (PET), 5a) and compared the against results against of conventional ITO on polyethylene terephthalate Sigma), Aldrich-product number 639281). The relative sheet resistivity of theof ITO after (PET Sigma Aldrich‐product number 639281). The relative sheet resistivity thefilm ITOdegraded film degraded −1 curvature, -1 bending at 0.5 cm whereas the electrodes made with Cu ML-G were stable until bending after bending at 0.5 cm curvature, whereas the electrodes made with Cu ML-G were stable until 1 . The origin at 10 cm−at of origin these phenomena is the brittleness of the oxideofmaterials. ITO film on bending 10 cm-1. The of these phenomena is the brittleness the oxideInmaterials. In PET, ITO a severe mechanical crack lines block the lines electron movement, resulting in an abrupt increase sheet film on PET, a severe mechanical crack block the electron movement, resulting in aninabrupt resistivity, the metal has a flexibility due to Moreover, theductility. grapheneMoreover, which covers increase inwhereas sheet resistivity, whereas the metal hasductility. a flexibility due to the Cu ML itself has covers excellent characteristics, includingcharacteristics, a high tensile strength (~130 GPa) and graphene which Cumechanical ML itself has excellent mechanical including a high tensile elastic modulus (~1 TPa) [21]. modulus (~1 TPa) [21]. strength (~130 GPa) and elastic

Figure 4. The Figure 4. The sheet sheet resistivity resistivity and and transmittance transmittance of of Cu Cu ML ML with with graphene graphene skin skin (Cu (Cu ML-G) ML-G) for for the the application as transparent electrodes with alternative TFE materials such as mono-layer graphene, Ag application as transparent electrodes with alternative TFE materials such as mono-layer graphene, nanowires, and indium-tin-oxide (ITO) nano-branches (insets show the SEM images of ITO nanoAg nanowires, and indium-tin-oxide (ITO) nano-branches (insets show the SEM images of ITO branches and Ag for for the the monolayer graphene, thetheRaman optical nano-branches and nanowires; Ag nanowires; monolayer graphene, Ramanspectrum spectrum with with optical microscope to aa SiO SiO2-covered microscope image image after after transferring transferring to -covered Si Si substrate). substrate). 2

The thermal stability of Cu ML-G is also superior compared with Ag nanowires and ITO nanobranches (Figure 5b). The sheet resistivity of the Ag nanowire gradually increased with respect to

[22]. When the heat is supplied to ITO surface, water molecules can be evaporated and release the captured electrons, thereby recovering the conductivity of the ITO nano-branches. The sight increase in resistivity of ITO nano-branches after long-term annealing can be understood as a loss of metallic indium, which was located in the nano-branch junction. Compared with Ag nanowires and ITO nano-branches, the Cu ML-G shows stable sheet resistivity. Because the pattern size is micro-range, Nanomaterials 2016, 6, 161 there is no significant drop in the melting point of the Cu metal. Moreover, the graphene skin on the5 of 7 Cu surface hinders the movement of Cu atoms for agglomeration, resulting in superior thermal stability.

Figure 5. (a) Sheet resistivitychange changeofofITO ITOfilm/polyethylene film/polyethylene terephthalate (PET) and CuCu ML-G with Figure 5. (a) Sheet resistivity terephthalate (PET) and ML-G with respect to the various curvatures. (b) Change of sheet resistivity with respect to annealing time for respect to the various curvatures; (b) Change of sheet resistivity with respect to annealing time for the the Cu ML-G and alternative transparent flexible electrodes (TFE) materials such as ITO nanoCu ML-G and alternative transparent flexible electrodes (TFE) materials such as ITO nano-branches branches and Ag nanowires at 40% humidity and 200 °C temperature conditions. and Ag nanowires at 40% humidity and 200 ◦ C temperature conditions.

3. Materials and Methods

The thermal stability of Cu ML-G is also superior compared with Ag nanowires and ITO Cu film(Figure preparation: Thermal SiO2 (200 nm) covered Si (100) and C-planeincreased sapphire was as to nano-branches 5b). The sheet resistivity of the Ag nanowire gradually withused respect a starting substrate. After cleaning sequentially with acetone, isopropyl alcohol, and de-ionized annealing time at 200 ◦ C and 40% humidity, due to the severe disconnection of the junction by Ag atom water, Cu films were deposited by electron beam deposition using a high purity Cu source from migration. The migration and agglomeration of Ag nanowire can be carried out at this low temperature Sigma-Aldrich (item number: 254177, Cu beads, 2–8 mm, 99.9995% trace metals basis). The Cu films because the nano-sized metals have a lower melting point than its bulk status. The ITO nano-branches were grown to a 100-nm thickness at a rate of 0.03 nm/sec. The growth chamber pressure was have relatively at stable even atdeposition, long-termand thermal annealing. decrease in the sheet maintained aboutresistivity, 10−6 Torr during the substrate wasThe heldinitial at room temperature. resistivity of ITO nano-branches can be understood as the evaporation of water (H O) molecules Cu micro-labyrinth with graphene skin: The Cu film/sapphire substrate was2 loaded into a on the quartz ITO surface, which were adsorbed at the normal ambient. The adsorbed water molecules on ITO tube CVD chamber. (1) The pressure in the quartz chamber was pumped down to 3 mTorr surface to capturepump. electrons, ITO lessinserted conductive Whenatthe usingtend a mechanical (2) Amaking 40-sccmthe flow of nano-branches hydrogen gas was to the[22]. chamber 950heat is supplied toThe ITOCu surface, water molecules be temperature evaporated 900~1000 and release electrons, mTorr. (3) films were heated to the can target °C. the (4) Acaptured 6-sccm flow of methane gas with 20 sccm of hydrogen was introduced into the chamber for 10 min (total pressure: thereby recovering the conductivity of the ITO nano-branches. The sight increase in resistivity of ITO 460 mTorr) for thelong-term graphene growth on Cu after graphene growth the furnace waswhich cooledwas nano-branches after annealing cansurface; be understood as a loss of metallic indium, downintothe thenano-branch room temperature underCompared a continuous 20-sccm flow of hydrogen to block oxidation of the Cu. Cu located junction. with Ag nanowires and ITO nano-branches, Transfer of Cu ML-G: (PDMS) elastomer (Sylgard 184, Dow Corning) was prepared by a mixing ML-G shows stable sheet resistivity. Because the pattern size is micro-range, there is no significant base andmelting curing agent 10:1Cu weight ratio and curing 70 °C forskin >2 h on or at temperature for the drop in the pointinofathe metal. Moreover, theat graphene theroom Cu surface hinders >12 h. Then, the SiO2 layer was removed by buffer oxide etch (BOE) for 5 min. movement of Cu atoms for agglomeration, resulting in superior thermal stability. Preparation of alternative TFEs: We had prepared the alternative TFE materials based on the previous results [9]. 3. Materialsexperimental and Methods

Cu film preparation: Thermal SiO2 (200 nm) covered Si (100) and C-plane sapphire was used as a starting substrate. After cleaning sequentially with acetone, isopropyl alcohol, and de-ionized water, Cu films were deposited by electron beam deposition using a high purity Cu source from Sigma-Aldrich (item number: 254177, Cu beads, 2–8 mm, 99.9995% trace metals basis). The Cu films were grown to a 100-nm thickness at a rate of 0.03 nm/s. The growth chamber pressure was maintained at about 10−6 Torr during deposition, and the substrate was held at room temperature. Cu micro-labyrinth with graphene skin: The Cu film/sapphire substrate was loaded into a quartz tube CVD chamber. (1) The pressure in the quartz chamber was pumped down to 3 mTorr using a mechanical pump; (2) A 40-sccm flow of hydrogen gas was inserted to the chamber at 950 mTorr; (3) The Cu films were heated to the target temperature 900~1000 ◦ C; (4) A 6-sccm flow of methane gas with 20 sccm of hydrogen was introduced into the chamber for 10 min (total pressure: 460 mTorr) for the graphene growth on Cu surface; after graphene growth the furnace was cooled down to the room temperature under a continuous 20-sccm flow of hydrogen to block oxidation of Cu. Transfer of Cu ML-G: (PDMS) elastomer (Sylgard 184, Dow Corning) was prepared by a mixing base and curing agent in a 10:1 weight ratio and curing at 70 ◦ C for >2 h or at room temperature for >12 h. Then, the SiO2 layer was removed by buffer oxide etch (BOE) for 5 min.

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Preparation of alternative TFEs: We had prepared the alternative TFE materials based on the previous experimental results [9]. 4. Conclusions In this study, graphene-skinned Cu ML were fabricated for the application of TFE by simple heating of Cu film on SiO2 /Si at the ambient of H2 and CH4 . The Cu ML-G can be transferred by PDMS support and SiO2 etching process. The Cu ML-G has superior sheet resistivity below 5 Ω/sq and mechanical flexibility, preventing any cracks up to a 0.1-cm bending radius. Although the transmittance of Cu ML-G is a little lower (70%~80%) than that of commercial metallic nanowires electrodes, it has superior thermal stability in conductance up to 200 ◦ C due to a micro-sized pattern and graphene skin, which prohibits the surface migration of Cu atoms at high temperature. We believe that the Cu ML-G can be used very soon as suitable electrode systems for flexible optoelectronic devices under severe operation circumstances. Acknowledgments: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1C1B1009030). Author Contributions: Hak Ki Yu designed and performed experiments individually. Hak Ki Yu analyzed the data and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript: Cu ML Cu ML-G ITO TFE

Copper Micro-Labyrinth Copper Micro-Labyrinth with Graphene skin Indium-Tix-Oxide Transparent Flexible Electrodes

References 1. 2.

3.

4.

5. 6. 7. 8.

9.

Liu, D.; Kelly, T.L. Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nat. Photonics 2014, 8, 133–138. [CrossRef] Han, T.-H.; Lee, Y.; Choi, M.-R.; Woo, S.-H.; Bae, S.-H.; Hong, B.H.; Ahn, J.-H.; Lee, T.-W. Extremely efficient flexible organic light-emitting diodes with modified graphene anode. Nat. Photonics 2012, 6, 105–110. [CrossRef] Mates, J.E.; Bayer, I.S.; Palumbo, J.M.; Carroll, P.J.; Megaridis, C.M. Extremely stretchable and conductive water-repellent coatings for low-cost ultra-flexible electronics. Nat. Commun. 2015, 6, 8874. [CrossRef] [PubMed] Liang, J.; Li, L.; Chen, D.; Hajagos, T.; Ren, Z.; Chou, S.-Y.; Hu, W.; Pei, Q. Intrinsically stretchable and transparent thin-film transistors based on printable silver nanowires, carbon nanotubes and an elastomeric dielectric. Nat. Commun. 2015, 6, 7647. [CrossRef] [PubMed] Guo, H.; Lin, N.; Chen, Y.; Wang, Z.; Xie, Q.; Zheng, T.; Gao, N.; Li, S.; Kang, J.; Cai, D.; et al. Copper Nanowires as Fully Transparent Conductive Electrodes. Sci. Rep. 2013, 3, 670–692. [CrossRef] [PubMed] Park, H.; Chang, S.; Zhou, X.; Kong, J.; Palacios, T.; Gradecak, S. Flexible Graphene Electrode-Based Organic Photovoltaics with Record-High Efficiency. Nano Lett. 2014, 14, 5148–5154. [CrossRef] [PubMed] Huang, S.; Zhao, C.; Pan, W.; Cui, Y.; Wu, H. Direct Writing of Half-Meter Long CNT Based Fiber for Flexible Electronics. Nano Lett. 2015, 15, 1609–1614. [CrossRef] [PubMed] Vosgueritchian, M.; Lipomi, D.J.; Bao, Z. Highly Conductive and Transparent PEDOT: PSS Films with a Fluorosurfactant for Stretchable and Flexible Transparent Electrodes. Adv. Funct. Mater. 2012, 22, 421–428. [CrossRef] Yu, H.K.; Kim, S.; Koo, B.; Jung, G.H.; Lee, B.; Ham, J.; Lee, J.-L. Nano-branched transparent conducting oxides: Beyond the brittleness limit for flexible electrode applications. Nanoscale 2012, 4, 6831–6834. [CrossRef] [PubMed]

Nanomaterials 2016, 6, 161

10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20. 21. 22.

7 of 7

Susrutha, B.; Giribabu, L.; Singh, S.P. Recent advances in flexible perovskite solar cells. Chem. Commun. 2015, 51, 14696–14707. [CrossRef] [PubMed] Allen, G.L.; Bayles, R.A.; Gile, W.W.; Jesser, W.A. Small, Particle Melting of Pure Metals. Thin Solid Films 1986, 144, 297–308. [CrossRef] Yu, H.K.; Dong, W.J.; Jung, G.H.; Lee, J.-L. Three-dimensional nanobranched indium-tin-oxide anode for organic solar cells. ACS Nano 2011, 5, 8026–8032. [CrossRef] [PubMed] Huang, C.J.; Su, Y.K.; Wu, S.L. The effect of solvent on the etching of ITO electrode. Mater. Chem. Phys. 2004, 84, 146–150. [CrossRef] Maki, H.; Ikoma, T.; Sakaguchi, I.; Ohashi, N.; Haneda, H.; Tanaka, J.; Ichinose, N. Control of surface morphology of ZnO (000-1) by hydrochloric acid etching. Thin Solid Films 2002, 411, 91–95. [CrossRef] Lee, J.; Lee, A.; Yu, H.K. Graphene protected Ag nanowires: Blocking of surface migration for thermally stable and wide-range-wavelength transparent flexible electrodes. RSC Adv. 2016. revision submitted. Porter, D.A.; Easterling, K.E.; Sherif, M.Y. Phase Transformation in Metals and Alloys; CRC Press, Taylor & Francis: Boca Raton, FL, USA, 2009. Yu, H.K.; Balasubramanian, K.; Kim, K.; Lee, J.-L.; Maiti, M.; Ropers, C.; Krieg, J.; Kern, K.; Wodtke, A.M. Chemical vapor deposition of graphene grown on “peeled-off” epitaxial Cu(111) foil: A simple approach to improved properties. ACS Nano 2014, 8, 8636–8643. [CrossRef] [PubMed] Mattevi, C.; Kim, H.; Chhowalla, M. A review of chemical vapor deposition of graphene on copper. J. Mater. Chem. 2011, 21, 3324–3334. [CrossRef] Yu, H.K.; Baik, J.M.; Lee, J.-L. Design of interfacial layer to block chemical reaction for epitaxial ZnO growth on Si substrate. Cryst. Growth Des. 2011, 11, 2438–2443. [CrossRef] Yu, H.K.; Baik, J.M.; Lee, J.-L. Self-connected and habitually tilted piezoelectric nanorod array. ACS Nano 2011, 5, 8828–8833. [CrossRef] [PubMed] Lee, C.; Wei, X.; Kysar, J.W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385–388. [CrossRef] [PubMed] Lee, J.-H. Gas sensors using hierarchical and hollow oxide nanostructures: Overview. Sens. Actuators B 2009, 140, 319–336. [CrossRef] © 2016 by the author; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).

Copper Micro-Labyrinth with Graphene Skin: New Transparent Flexible Electrodes with Ultimate Low Sheet Resistivity and Superior Stability.

We have developed self-assembled copper (Cu) micro-labyrinth (ML) with graphene skin for transparent flexible electrodes of optoelectronic devices. Th...
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