Facile Synthesis of Graphene on Cu nanowires via Low Temperature

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Facile Synthesis of Graphene on Cu nanowires via Low Temperature Thermal CVD for the Transparent Conductive Electrode Hahnjoo Yoon, Dong Su Shin, Taek Gon Kim, Dohyun Kim, and Jinsub Park ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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ACS Sustainable Chemistry & Engineering

Facile Synthesis of Graphene on Cu nanowires via Low Temperature Thermal CVD for the Transparent Conductive Electrode Hahnjoo Yoona§, Dong Su Shina§, Taek Gon Kima, Dohyun Kima, Jinsub Parka,b,* a

Department of Electronics and Computer Engineering, Hanyang University, 222, Wangsimni-

ro, Seongdong-gu, Seoul 04763, South Korea b

Department of Electronic Engineering, Hanyang University, 222, Wangsimni-ro, Seongdong-

gu, Seoul 04763, South Korea *Corresponding author e-mail : [email protected]

ABSTRACT: We report on the application of graphene (Gr)/copper (Cu) shell/core nanowires (NWs) as a transparent conducting electrode (TCE) for optoelectronic devices. First, we optimize the synthesis conditions of Cu NWs by control of the agent material, which shows excellent TCE properties. Synthesized Cu NWs have high transmittance (T) and low sheet resistance (Rs) which are 88.92 % at 450 nm and 17.95 Ω/sq respectively, comparable to those of ITO. However, the Cu NWs long-term stability problem of oxidation that occurs even at room temperature is an impediment for their application. To suggest a possible solution of the oxidization issue,

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graphene is deposited on the surface of Cu NWs by using thermal chemical vapor deposition (CVD) at low temperature. The formation of graphene coated Cu NWs was performed at low temperature (600 °C) to avoid surface damages on Cu NWs with systematically optimized conditions. The graphene coated Cu NWs have not only high transmittance and conductivity (88.85 % at 450 nm and 36.91 Ω/sq), but also tremendous long-term stability that prevents oxidation of Cu NWs. Finally, we demonstrated the improvement of light extraction in GaNbased light emitting diodes (LEDs) using our suggested Gr/Cu NWs film as a TCE. Electroluminescence (EL) of LED IV, with Gr/Cu NWs-based TCE at a current injection of 100 mA, were dramatically improved (x2.63) by its current spreading effect, and efficiency droop phenomenon was slightly slackened due to reduce series resistance of the LEDs. Our experimental results clearly show that the Gr/Cu NWs can be used as an TCE to improve the optoelectronic performance of LEDs and they can replace with the ITO without the degradation of electrical and optical properties.

Keyword: Graphene, Cu nanowires, CVD, TCE, Light emitting diode

INTRODUCTION Over the last two-decades, research for flexible and stretchable transparent displays using flexible optoelectronic devices, wearable devices, and touch screens has been widely conducted1. Indium tin oxide (ITO) has been used in transparent devices as a standard transparent conducting electrode (TCE) in various research and industrial fields. Generally, two major electrical and optical parameters of sheet resistance (Rs) and optical transmittance (%) respectively, must be


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considered to satisfy the required property for being used as a TCE. ITO has been acted as a keyplayer as an excellent TCE due to its high transmittance (≈ 90 %) in visible wavelength and low sheet resistance (≈ 10 Ω □ -1)2. However, the scarcity of indium reserves increased costs gradually, and the critical weakness of ITO in mechanical strength and strong absorption in the UV region revealed its limitations to be a factor in flexible and optoelectronic devices3. Hence, it is necessary to make much endeavor to find new alternative materials to replace with ITO. In order to match it with sustainable requirements, carbon nanotubes, graphene, metal nanowires, and metal oxides were suggested as TCE materials in the past decade4,5. Among the suggested material systems, copper (Cu) nanowires (NWs) are a promising material as a flexible and transparent electrode due to the fact that it is 1,000 times cheaper than indium6. Also, Cu NWs have good transmittance in visible wavelength and UV range as well3. Although the silver (Ag) NWs can be a powerful competitor among of metal nanowires, it is rather more expensive than Cu NWs due to its rarity. Furthermore, in application of Ag NWs to TCE in optoelectronic devices, it has low transmittance in the range of UV wavelength and undergoes severe thermal damages during the annealing process conducted to minimize contact resistance of Ag TCE7. Unfortunately, Cu NWs also have a critical problem for use as a TCE because they have a longterm stability problem due to the oxidation that occurs even at room temperature, which makes it difficult for practical use in reliable optoelectronic devices8. To enhance the time dependent stability of Cu NWs, many efforts have been made to address this issue by the coating of passivation layers using Zn, Sn, In, Ni, Co, Ag, Au, Pt, and aluminum-doped zinc oxide / aluminum oxide etc. onto the surface of Cu NW9,10. However, these protection layers tend to cause a rough surface morphology which interrupts light transmission properties of optoelectronic devices6,11. Recently, some groups have made efforts to prevent of oxidation of


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Cu NWs by formation of graphene on Cu NWs using plasma-enhanced chemical vapor deposition (PECVD) methods and CVD using solid and liquid carbon sources12-14. Graphene is composed of a single layer of carbon atoms arranged in a two-dimensional (2D) honeycomb lattice and has received significant attention retaining its useful characteristics such as excellent electrical, thermal and mechanical properties15. In addition, graphene was found to be oxygen prohibitive, and several studies have been published to demonstrate the passivation properties of graphene on protecting metal including Cu, Fe, and Ni from oxidation8. In order to reduce the damage of Cu NWs during formation of graphene, a low temperature process is an essential requirement because the melting points of 30 – 50 nm diameter Cu NWs is reduced to 400 – 600 °C from that of bulk Cu materials by the size effects16. Recently, Ahn et al. reported the PECVD method to form the graphene6 to take advantage of the decreased growth temperature as low as 380 – 500 ℃, but the PECVD process can cause collateral damage on a graphene surface because the surface is disrupted by the incidence of high-energy particles and radiation depending on the magnitude of the RF voltage in a plasma environment

17,18

. In addition, the

another deposition method of graphene on Cu nanosilks via low pressure chemical vapor deposition (LPCVD) by controlling position of a sample in a quartz required the cautious movement of magnetic slider to obtain the high quality graphene to minimize the thermal damages of nanosilks19. In this study, we successfully demonstrate the graphene coated Cu NWs (Gr/Cu) shell/core hybrid structure using thermal CVD at low temperature by controlling of flow rate of used gases and reaction temperature. After optimization of Gr/Cu NWs, the shell-core structured hybrid networks formed by vacuum filtration method and CVD process using in-situ formation on the


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Cu NWs formed on GaN based light-emitting diodes (LEDs) as a TCE. The dramatic current spreading effect of Gr/Cu NWs film contributes to an enhancement in the light extraction efficiency of GaN based LEDs.

EXPERIMENTAL DETAILS Synthesis of Cu NWs and fabrication of Cu NW TCEs In order to synthesize Cu NWs, synthesis materials (copper chloride dihydrate (CuCl2·2H2O), glucose, and hexadecylamine (HDA)) and products for cleaning (hexane, isopropyl alcohol (IPA), and acetic acid) were obtained from Sigma-Aldrich Korea. The details of the synthesis of Cu NWs was described in the other report20. 15 mM copper chloride and 60 mM HDA were added to 20 ml of deionized water, and then sonicated for 30 min until a light blue emulsion was formed. 30 mM glucose was added to the above solution and the mixed solution was stirred for 1 hr at 55 °C on a hot plate, then the solution was transferred to a glass bottle and tightly capped with paraffin tape and placed in a pre-heated oven at 102 °C for 6 hr. The obtained brown solution of Cu NWs was washed with hexane, IPA, and acetic acid several times. And then, Cu NWs were deposited on a target substrate by vacuum filtration method with a membrane filter (a pore size of 1 µm and 25 mm in diameter) purchased from ADVANTEC (Japan).

Formation of Graphene-Cu shell/core NWs using thermal CVD The prepared Cu NWs were deposited on p-GaN surface of LEDs, and loaded into the CVD chamber. For graphene deposition on Cu NWs, a low temperature process is essential to prevent thermal damages of Cu NWs. Thus, graphene was synthesized by using thermal CVD at low temperature (600 °C) with ambient pressure of 4.3 × 10-3 Torr. Optimized reaction gas mixture


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composed of methane, hydrogen, and argon (CH4:H2:Ar = 20:30:20 sccm) was injected into the chamber. Methane was injected only at the growth stage at 600 °C, other gas conditions fixed. After graphene growth (10 min) was finished, the chamber was immediately cooled to room temperature with continual injection of carrier gases.

Fabrication of blue LEDs with different TCEs To make patterned Cu NWs electrodes for LED III, Cu NWs were deposited on p-GaN in GaNbased LED epi structure by the vacuum filtration method. Then, a photolithograph process was performed to form traditional rectangle-shaped LED chips (size = 410×470 µm2). An unnecessary area of Cu NWs on p-GaN was selectively removed through a wet etching process with diluted nickel etchant (nickel etchant:D.I. water = 1:2) for 1 min and 20 sec. ITO of LED II as a top TCE material was deposited by a sputter, and rapid thermal anneal (RTA) treatment at 600 °C for 1 min and wet-etching process by liquid crystal display etchants (LCE-12) instead of the nickel etchant was followed step by step. On the patterned Cu NWs, to fabricate LED IV, graphene was additionally formed on Cu NWs by CVD at 600 °C with specific gas condition. The mesa etching process for LED I, LED II, LED III, and LED IV was done by inductively coupled plasma (ICP) to remove p-GaN layer till open the n-GaN in certain area. Finally, Cr/Au (30 nm / 300 nm) electrodes were simultaneously deposited on p-GaN and n-GaN by an e-beam evaporator after a photolithography process. Characterization The morphology of the prepared copper nanowires was examined using a scanning electron microscopy (SEM (S-4800), Hitachi). For the study of atomic and molecular structure of Cu NWs, X-ray diffraction (XRD) patterns were recorded using an X-ray diffractometer


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(miniFlex600, Rigaku). The transmittance and sheet resistances of Cu NW films were measured using a UV-visible spectrometer (Lambda 650s) and a four-point probe with a source meter (2611A, Keithley). For characterization of deposited graphene, a Raman spectrometer (NRS3100) was used to analyze the number of layers and quality of the deposited graphene. Furthermore, the atomic composition and the binding state of graphene was investigated by Xray photoelectron spectroscopy (VG ESCALAB 220i), and transmission electron microscopy (JEM-2100F) was performed to analyze core-shell structured Cu NWs.

RESULT AND DISSCUSSION To adjust the morphology of Cu NWs, a suitable amount of HDA, one of the materials for the synthesis, plays a key role in controlling diameter and length of Cu NWs as reported in our previous paper21. After successful synthesis of Cu NWs by solution-based wet chemical process, the average diameter, length, and aspect ratio of Cu NWs from randomly selected 50 wires in SEM images were 55 ± 2.0 nm, 35 ± 2.2 µm, and 636.3, respectively (Fig. S1). The Cu NWs solution was washed with IPA and hexane (IPA:hexane = 1:2) several times, and treated with acetic acid for 10 sec to remove residual organics formed during chemical reactions. And then, the dilute suspension of Cu NWs in mixture solution (IPA, hexane, and acetic acid) was filtered onto a membrane filter paper. Cu NWs on the filter paper were transferred onto a target substrate by physical pressure with a metal rod. Fig. 1a is a photograph of synthesized Cu NWs suspension in D.I. water that shows the reddish-brown color, which is well agreed with previously reported results20. In order to use metal nanowires as a TCE, the formation of Cu NW network plays a key role to guarantee the high conductivity and transparency properties. Formation of a dense network and intersection points of Cu NWs can be observed by FE-SEM as


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shown in Fig. 1b and c. The intersection points of Cu NWs should be welded to minimize contact resistance between Cu NWs22. To investigate crystal structure of Cu NWs, the phase compositions of the Cu NWs were characterized by XRD, as shown in Fig. 2. The XRD pattern showed the face centered cubic (FCC) structure of copper nanowires with diffraction peaks located at 2θ = 43.36 and 50.54 ° ascribed to (1 1 1) and (2 0 0) plane of Cu crystal, respectively, which are characteristic peaks of Cu that well matched previously reported data (JCPDS 040836). The broad diffraction peak at 30 to 40 ° corresponds to the glass substrate. Other diffraction peaks related with CuO, Cu2O, and oxide layers were not observed, which confirmed the purity of the metallic Cu NWs.

Figure 1. (a) A photographic image of synthesized Cu NWs in D.I. water. (b) SEM image of a dense web of Cu NWs formed by vacuum filtration method on SiO2. (c) Intersection points of Cu NWs.


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Cu (111)

Intensity (a.u.)

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Glass Cu (200)

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40

50

60

70

80

2θ (degree)

Figure 2. XRD pattern of Cu NWs on glass substrate. Before depositing graphene, a thermal stability experiment of prepared Cu NWs was systematically performed to decide the optimized temperature for the formation of graphene and to minimize thermal damages on Cu NWs during CVD process. The result of morphology changes in different temperatures from 500 to 1000 °C was investigated by SEM as shown in Fig. S2. Small particles have a lower melting point than bulk material, which is well known as the size-dependent melting point depression10,23. As expected, we observed a lot of fragments and damage of Cu NWs at high temperature at over 800 °C. At 700 °C, Cu NWs were partially melted toward fragmented wires. At 600 and 500 °C, there were no critical damages on wires except for the Cu NWs with diameter below 50 nm. In other words, the temperatures at 600 and 500 °C do not critically hamper the conductivity property. During the annealing process, wirewire junction resistance can be minimized due to formation of welded wires (Fig. S3). Hence, thermal CVD process should be conducted below 600 °C to minimize the damages of Cu NWs. In order to synthesize the graphene on the surface of Cu NWs, suitable CVD parameters must be considered, such as temperature, gas flow rate, and pressure. Generally, to form high-quality


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graphene, the CVD process should be conducted at 800 – 1000 °C. For reaching the adequate temperature range or as close as possible to the values, we focused on material properties rather than CVD itself. By increasing the diameter of Cu NWs, it is possible for these to endure the high temperature process. Thus, we selected Cu NWs of 55 nm in diameter to improve thermal stability.

Figure 3. (a) A standard CVD system diagram, (b) CVD process cycle for graphene growth: A = annealing stage (40 min) with H2/Ar = 30/20 sccm, B = growth stage (10 min) with H2/Ar/CH4 = 30/20/20 sccm, and C = cooling stage (180 min) with H2/Ar = 30/20 sccm, (c) A schematic diagram of graphene growth mechanism on Cu NW.

Figure 3a shows a standard thermal CVD system. Cu NWs transferred onto p-GaN surface top-layer of epi-structure of GaN-based LED were placed in the vacuum chamber (4.3 × 10-3 Torr) of CVD, and heated to 600 °C with a H2 (30 sccm)/Ar (20 sccm) mixture flowing to prevent oxidation. After attaining to 600 °C, CH4 (20 sccm) was additionally injected to the


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chamber for 10 min. As depicted in Fig. 3b, methane (CH4) is injected only at the growth stage (B) with a H2 (30 sccm)/Ar (20 sccm) mixture flowing continually. After the growth stage ended, the chamber was cooled to room temperature immediately. Figure 3c is the growth mechanism of graphene on Cu NWs. With hydrogen (H2) and argon (Ar) as carrier gases, methane (CH4) as a precursor is flowed into the chamber and diffuses through the boundary layer from main stream and reach the surface of Cu NWs. And then, CH4 undergoes adsorption on Cu NW surfaces and catalytic dehydrogenation to form active carbon species. Consequently, graphene lattice is formed on the surface of the catalyst during diffuse step. Inactive species like hydrogen are desorbed from the surface and form molecular hydrogen for diffusion out of the surface through boundary layer24.

Figure 4. (a) SEM and (b) TEM images of graphene-Cu core/shell NWs. (c-d) HR-TEM images of middle and edge of NWs. Inset is line profile intensity of selective line on edge of the NWs. (e-g) EDS mapping analysis of graphene-Cu core/shell NWs.


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To evaluate the structure and chemical composition of Gr/Cu shell/core NWs formed by the thermal CVD process at optimized temperature (600 °C), we investigated SEM, transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) mapping analysis, as shown in Fig. 4. The as-grown graphene/Cu NWs show the slight melting at the crosslink regions of nanowires, which is related to the effect of 600℃ annealing during the formation of graphene as shown in SEM image of Fig. 4a. From the high resolution (HR) - TEM images (Fig. 4b), we definitely observed multi-layer (thickness = 1.5 – 6 nm) graphene coated along surface of Cu NWs and confirmed it as shell-core structured NWs (Gr/Cu NWs). Moreover, Fig. 4c displays middle of Gr/Cu NW and lattice structure in specific area (inset). The lattice distance was 0.211 nm, corresponding to the (111) plane of Cu. In addition, pure graphene layers can be observed at edge of Gr/Cu NWs shown in Fig. 4d, and an intensity profile along the red pixel line was measured as shown in inset of Fig. 4d. The lattice constant of the graphene is 0.263 nm that is quite close to the value of the zigzag orientation of the graphene and well matched theoretical value25,26. Furthermore, to analyze chemical composition of Gr/Cu shell/core NWs, EDS mapping was performed and its result shows in Fig. 4e-g using copper (Cu) and carbon (C) elements for Gr/Cu shell/core NWs. The red dots (Cu) were clearly covered by yellow dots (C) and that revealed structure of the shell/core NWs. From the TEM images, graphene was successfully synthesized on surface of Cu NWs as a passivation layer, and these results shows that graphene can be formed by normal CVD (600 °C) without plasma and other carbon sources (liquid and solid) aids to reduce threshold energy for the synthesis.


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Figure 5. (a-c) Raman spectra of Gr/Cu NWs with different CVD temperatures (from 300 to 1000 °C). (d) I2D/IG and ID/IG ratioes of Gr/Cu NWs with different Gr formation temperature. For the formation of Gr/Cu NWs, graphene was systematically grown on Cu NWs in the temperature range from 300 to 1000 °C. To analyze graphene quality and layers, Gr/Cu NWs film was investigated by Raman spectroscopy at an excitation wavelength of 532 nm. In the Raman spectra shown in Fig. 5a-c, the typical main peaks which is D, G, and 2D band can be observed at 1360, 1592, and 2673 cm-1 respectively. D band indicates structural disorder and defect of graphene, and 2D band represents the double-resonant Raman scattering. In Fig. 5d, the intensity ratio of D and G band (ID/IG) was 0.96 at 1000 °C, and the ratio of 2D and G band (I2D/IG) showed 0.34. A high ID/IG ratio meant strong defects and structural disorder of graphene


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film, and we observed that typical high temperatures (800 to 1000 °C) have higher ratio values than the value at low temperatures, below 800 °C. Due to high temperature process, Cu NWs continually undergo morphology distortion during process of CVD, well known as Rayleigh instability16, so defects and structural disorder become more abundant by increasing CVD temperature. The low ratio of 2D and G indicates that multiple-layer graphene (I2D/IG < 1.25) is formed on the surface of Cu NWs27.

Figure 6. (a)(c) Raman spectra and (b)(d) XPS C 1s spectra of Graphene-Cu shell/core NWs with different methane gas flows/duration: CH4 (20 sccm/20 min) and (20 sccm/10 min) respectively. Injection rate and duration of methane (CH4) in CVD system are very important to form highquality graphene. Therefore, we used Raman spectroscopy and X-ray photoelectron spectroscopy


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(XPS) to analyze the quality and number of layers of graphene depending on the specific flow rate of mixture gases (CH4/H2/Ar = 20/30/20 sccm) with different duration of CH4 injection. Figure 6 displays Raman and XPS (C 1s) spectra for Gr/Cu NWs. The Raman spectra shows the state of defect, disorder, and layer numbers of graphene grown with increasing of CH4 injection time in Fig. 6a and b. When Cu NWs were exposed at 600 °C for 20 min, the wires incurred more damage than one with 10 min exposure, so the ID/IG ratio was higher as increasing the injection duration. These results were also proved in XPS data. In Fig. 6c, we injected CH4 (20 sccm) in growth stage for 20 min and confirmed sp2/sp3 hybridization ratio by XPS. C 1s spectrum of graphene contains five main peaks at 284.45, 285.25, 286.25, 287.60, and 288.28 eV corresponding to C=C (sp2), C-C (sp3), C-O, C=O, and O-C=O respectively. The sp2 peak at 284.45 eV indicates carbon atoms are well arranged in a two-dimensional graphite-like honeycomb lattice18. On the other hand, the sp3 peak centered at 285.25 eV is a type of amorphous carbon, so called as diamond like carbon. As sp3 carbon ratio is increased, it leads carbon type toward low conductivity in material property. The other three peaks such as C-O, C=O, and O-C=O indicate epoxy, carbonyl, carboxylate functional groups that graphene lattice with defects have chemical bonds with oxygen. In another case of CH4 (20 sccm) for 10 min (Fig. 6d), it similarly displays main five peaks at 284.45, 285.25, 286.25, 287.69, and 288.62 eV corresponding to C=C (sp2), C-C (sp3), C-O, C=O, and O-C=O respectively. However, sp2 ratio decreased when compared with previous XPS data (20 min). In further detail, Table S1 shows carbon chemical composition ratio with different methane injection time. Injection of methane for 10 min showed a high C=C (sp2) ratio which was 74.99 % compared with the 20 min one (63.74 %). The results meant that the case of methane injection for 10 min showed more abundant in ratio of well-arranged graphite like honeycomb lattice. In addition, we conducted


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reproductive test for 10 min methane injection case, and the XPS spectra is shown in Fig. S4. We achieved a similar ratio of chemical composition between C=C (sp2) and C-C (sp3) when comparing it with previous data, and successfully synthesized high-quality graphene layers on Cu NWs. Moreover, graphene layers play a crucial role in protecting Cu NWs from oxidation. For long-term stability from the oxidation, Gr-Cu shell/core NWs had been exposed in the air on a hot plate at 80 °C/60 hr. Figure 7 shows the sheet resistance (Rs) normalized with respect to the initial resistance (R0) as a function of the time at 80 °C. Initial sheet resistance of Cu NWs and Gr/Cu NWs were 8.27 and 72.88 Ω/sq respectively. However, after exposure to air, those values were 102.71 and 56.48 Ω/sq respectively. The sheet resistance of the bare Cu NWs reaches to the value which is about thirteen times higher than the initial resistance after 60 hr. On the other hand, graphene-Cu shell/core NWs maintains the initial sheet resistance. In order to investigate the thermal-humidity stability of Cu NWs and Gr/Cu NWs, the thermo-hygrostat for reliability test (85℃/85% RH) was conducted as shown Fig. 7b. The Rs of the Cu NWs is about 1200 times increased comparing to R0 in 18hr. After 36hr, the Cu NWs show the insulator property. In contrary to that, Gr/Cu NWs show the ~800 times increased sheet resistance value after 2 weeks. The results of thermos-hygrostat for reliability test shows the clear enhanced thermal oxidation stability of Gr/Cu NWs from high temperature and humidity. Furthermore, for another stability test, we measured XRD spectroscopy of Cu NWs and Gr/Cu NWs which are exposed to air on hot plate in more harsh temperature (200 °C) than previous one, as shown in Fig. S5. In Fig. S5a, Cu diffraction peaks were located at 43.56 and 50.72 ° ascribed to plane of (111) and (200) respectively. After the annealing process in air at 200 °C, the Cu oxide peak of CuO and Cu2O was observed at 36.76 and 40.5 °. On the other hand, CuO and Cu2O peak for Gr/Cu NWs XRD spectra was not found even after the harsh annealing process in Fig. S5b.


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Other XRD peaks at 37.78 and 41.96 ° are relevant to sapphire material, and broad diffraction peak occurred at 25 to 30 ° region is related with graphene. Overall, these results clearly show the ability of graphene as a barrier to conserve Cu NWs from surface oxidation.

Figure 7. (a) Long-term stability on Hot plate (80 ℃) and (b) thermo-hygrostat(85 ℃ / 85 % RH) test of graphene coated Cu NWs, comparing with bare Cu NWs.

Figure 8 exhibits the transmittance and sheet resistance of Cu NWs and Gr/Cu NWs with a different amount of Cu NWs solution, and ITO. As mentioned, sheet resistance and optical transmittance (T) must be considered to satisfy the required property for being used as a TCE. Fig. 8a shows optical images of different top materials formed on a sapphire substrate with pure Cu NWs and Gr/Cu NWs electrode. We additionally placed bare sapphire and 200 nm-thick ITO coated sapphire for a clear explanation. As the concentration decreased of Cu NWs, the color of Cu NWs was a bit faded to become high transparent consequently. Figure 8b is optical transmittance (%) spectra of Cu NWs and Gr/Cu NWs in different amount of Cu NWs, and ITO on sapphire substrate. The corresponding transmittance spectra is measured mainly wavelengths


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of visible range from 300 to 800 nm. The Cu NWs and Gr/Cu NWs films show high transparency in both visible and UV regions, so this property proves that these are suitable materials for display and optoelectronic devices. In detail, at 450 nm, the transmittance percentage of 20, 30, and 40 µL Cu NW solutions was 88.92, 88.25, and 82.95 % respectively. In case of 20, 30, and 40 µL Gr/Cu NWs film, the transmittance percentage was 90.91, 88.85, and 86.88 respectively. From those values, the transmittance was slightly increased after the formation of Gr by CVD process. The sharp slopes of Cu NWs and Gr/Cu NWs spectra at the range of 540 to 600 nm can be observed due to the localized surface plasmon resonance (LSPR) 28

. The two concentrations (20 and 30 µL) show high transparency which is close to 90 %. The

transmittance versus sheet resistance of the samples with different concentration of Cu NWs and Gr/Cu NWs is shown in Fig. 8c. The sheet resistance and transmittance of 30 µL of Cu NWs solution is 13.11 Ω/sq and 88.25 % respectively, whereas 30 µL of Gr/Cu NWs shows 36.91 Ω/sq and 88.85 %. The slight increase of resistance after formation of graphene on Cu NWs compared to bare Cu NWs is related to the small amount surface damage of Cu NWs annealed at 600 ℃, which made the deterioration of the conductivity of Cu-Gr NWs. From the data, whole Gr/Cu NWs samples shows higher transmittance than Cu NWs’ one, whereas conductivity was decreased compared to Cu NWs. As a reference sample, 200 nm-thickness ITO/sapphire shows optical and electrical properties of 81.47 % at 450 nm and 47.80 Ω/sq and very low transmittance heading to zero in UV wavelength range.


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Figure 8. (a) Photographic images of Cu NWs, Gr/Cu NWs, and ITO on sapphire. (b) Wavelength-dependent transmittance in the different amount of Cu NWs and Gr/Cu NWs comparing with 200 nm-thickness ITO. (c) The sheet resistance (Ω/sq) and transmittance (%) of Cu NWs, Gr/Cu NWs, and ITO.

Figure 9. Schematic diagrams of LEDs device structure with different TCEs with p&n pad electrodes (Cr/Au): (a) LED I–reference, (b) LED II-ITO, (c) LED III-Cu NWs, and (d) LED IVGr/Cu NWs.


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Finally, we adopted Gr coated Cu NWs as a transparent electrode to GaN-based LEDs for an enhancement of light extraction. Schematic diagrams of traditional rectangular-shaped LED (chip size = 410 × 470 µm2) structure with different top-TCE materials are shown in Fig. 9a-d. The four different types of LEDs, Reference, ITO, Cu NWs, and Gr/Cu NWs, were systematically compared, and these LEDs were named LED I, LED II, LED III, and LED IV respectively. After transfer of Cu NWs by filtration method onto p-GaN/MQW/n-GaN LED epistructure, graphene was formed by thermal CVD, and then Cr/Au (30 nm/300 nm) was deposited by an e-beam evaporator on p- and n-GaN as contact metal pads. For analysis of the characterization

of

each

LED,

current–voltage

(I-V)

characteristic

curves

and

electroluminescence (EL) for four different samples were measured as shown in Fig. 10. From the I-V curves (Fig. 10a), turn-on voltages of LED I, LED II, LED III, and LED IV were 4.5, 5.5, 5.4, and 4.6 V respectively. The turn-on voltage of LED IV which is close to LED I was attributed to the improvement by annealing to minimize contact resistance between p-GaN and Cu NWs during CVD process. In case of LED I, Fig. 10b, light emission occurred just around ppad electrode by current crowding effects at 0.1 mA current injection into the metal pad electrode. However, LEDs adopted current spreading layers such as LED II, LED III, and LED IV showed that light was brightly emitted on entire surface area of p-GaN in the chips. By the current spreading effect, current can be injected all area of p-GaN, so it consequently overcomes or minimizes current crowding effect resulting from high resistivity of p-GaN. Figure 10c shows EL peak intensity versus current at 100 mA of LED I, LED II, LED III, and LED IV. At 100 mA, EL intensities of LED II, LED III, and LED IV were dramatically enhanced by 2.08, 2.38, and 2.63 times respectively when comparing it with LED I. As increasing current injection, all types of the LEDs exhibit efficiency droop phenomenon, deviation from the linear proportional


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relationship between EL intensity and inject current as increasing current injection, as shown in Fig. 10d. At 60 to 100 mA, LED II and LED III showed similar tendency in droop percentage, LED II (20.0 and 16.08 %)/LED III (20.5 and 13.6 %). However, efficiency droop for LED IV was slackened at the region of 60 to 100 mA, and it can be distinguishable with ITO and Cu NWs properties. The efficiency droop phenomenon of LED IV was alleviated because series resistance of Gr/Cu NWs/p-GaN was minimized after CVD annealing process, and this was attributed to reduced heating effect due to relative low series resistance and the current spreading property of Gr/Cu NWs29. The evidence of the reduced heating effect can be found to drop back into Figure 10c. The EL peak positions of LED I to IV taken 100 mA current injection were 448.21, 445.74, 446.09, and 445.38 nm respectively. The EL positions of LED II, III, and IV was blue-shifted by 2.5, 2.1, and 2.8 nm respectively from the peak location of LED I. The results are well matched that as the temperature of LEDs with higher current injection is increased, the EL peak is red-shifted due to bandgap shrinkage30-31. LED IV adopted Gr/Cu NWs TCE showed the most blue-shifted sample, so it possessed the lowest temperature of the chips. LED III and LED IV from 20 to 100 mA showed higher EL intensities than LED II due to high transmittance (T) and low sheet resistance (Ω/sq) of the materials.


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Figure 10. Electrical and optical performance of LEDs with different TCEs. a) I-V characteristic curves, (b) Light spreading images of four different LEDs at injection current of 0.1 mA, (c) Electroluminescence (EL) spectra with 100 mA current injections, and (d) EL peak intensity in different current injections.

CONCLUSIONS We successfully synthesized Gr-coated Cu shell/core NWs by thermal CVD at a low temperature (600 °C) with optimized formation conditions. Multi-layer graphenes as passivation layers to prevent Cu NW from oxidation displayed a remarkable oxidation barrier from stability test at 80 °C for 60 hr. Also, the transparent conducting electrode of Gr/Cu NWs showed high abilities in optical and electrical properties (sheet resistance and transmittance of 36.91 Ω/sq and 88.85 %,


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respectively) comparable to ITO’s one. Blue LEDs with Gr/Cu NWs as a TCE showed 2.63 times higher EL intensity than the value of reference LEDs. Furthermore, Gr/Cu NW adopted LEDs show the reduced efficiency droop phenomenon compared to ones with Cu NWs and ITO because minimization of series resistances between materials. From our results, Gr/Cu NWs clearly showed potentiality as one of the next generation TCEs to replace ITO with them.


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ASSOCIATED CONTENT Supporting Information Additional data and results (Histograms of diameter and length of Cu NWs, SEM images, XPS spectra and XRD patterns)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions §

Co-first authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF-2015R1A1A1A05027848, NRF-2018R1D1A1B07048382 ).


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For Table of Contents Use Only

Synopsis Formation of graphene on Cu NWs via low-temperature thermal CVD and application of them to GaN-based LEDs as transparent electrode.


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Figure 11. (a) A photographic image of synthesized Cu NWs in D.I. water. (b) SEM image of a dense web of Cu NWs formed by vacuum filtration method on SiO2. (c) Intersection points of Cu NWs. 168.6 x 49.4 mm (150 x 150 DPI)


31


Cu (111)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 51

Glass Cu (200)

30

40

50 60 2θ (degree)

70

80

Figure 12. XRD pattern of Cu NWs on glass substrate. 120 x 84.9 mm (150 x 150 DPI)


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Figure 13. (a) A standard CVD system diagram, (b) CVD process cycle for graphene growth: A = annealing stage (40 min) with H2/Ar = 30/20 sccm, B = growth stage (10 min) with H2/Ar/CH4 = 30/20/20 sccm, and C = cooling stage (180 min) with H2/Ar = 30/20 sccm, (c) A schematic diagram of graphene growth mechanism on Cu NW. 128.4 x 67.9 mm (150 x 150 DPI)


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Figure 14. (a) SEM and (b) TEM images of graphene-Cu core/shell NWs. (c-d) HR-TEM images of middle and edge of NWs. Inset is line profile intensity of selective line on edge of the NWs. (e-g) EDS mapping analysis of graphene-Cu core/shell NWs. 90.8 x 51.7 mm (150 x 150 DPI)


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Figure 15. (a-c) Raman spectra of Gr/Cu NWs with different CVD temperatures (from 300 to 1000 °C). (d) I2D/IG and ID/IG ratioes of Gr/Cu NWs with different Gr formation temperature. 138.9 x 100.6 mm (150 x 150 DPI)


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Figure 16. (a)(c) Raman spectra and (b)(d) XPS C 1s spectra of Graphene-Cu shell/core NWs with different methane gas flows/duration: CH4 (20 sccm/20 min) and (20 sccm/10 min) respectively. 160.5 x 111.6 mm (150 x 150 DPI)


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Figure 17. (a) Long-term stability on Hot plate (80 ℃) and (b) thermo-hygrostat(85 ℃ / 85 % RH) test of graphene coated Cu NWs, comparing with bare Cu NWs. 121.2 x 47.6 mm (150 x 150 DPI)


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Figure 18. (a) Photographic images of Cu NWs, Gr/Cu NWs, and ITO on sapphire. (b) Wavelength-dependent transmittance in the different amount of Cu NWs and Gr/Cu NWs comparing with 200 nm-thickness ITO. (c) The sheet resistance (Ω/sq) and transmittance (%) of Cu NWs, Gr/Cu NWs, and ITO. 203.3 x 52.5 mm (150 x 150 DPI)


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Figure 19. Schematic diagrams of LEDs device structure with different TCEs with p&n pad electrodes (Cr/Au): (a) LED I–reference, (b) LED II-ITO, (c) LED III-Cu NWs, and (d) LED IVGr/Cu NWs. 87.4 x 45.5 mm (150 x 150 DPI)


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Figure 20. Electrical and optical performance of LEDs with different TCEs. a) I-V characteristic curves, (b) Light spreading images of four different LEDs at injection current of 0.1 mA, (c) Electroluminescence (EL) spectra with 100 mA current injections, and (d) EL peak intensity in different current injections. 128 x 84.4 mm (150 x 150 DPI)


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94.5 x 38.8 mm (150 x 150 DPI)


41


Figure 1. (a) A photographic image of synthesized Cu NWs in D.I. water. (b) SEM image of a dense web of Cu NWs formed by vacuum filtration method on SiO_2. (c) Intersection points of Cu NWs 246x73mm (120 x 120 DPI)


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Figure 2. XRD pattern of Cu NWs on glass substrate. 144x110mm (120 x 120 DPI)



Figure 3. a) A standard CVD system diagram, (b) CVD process cycle for graphene growth: A = annealing stage (40 min) with H2/Ar = 30/20 sccm, B = growth stage (10 min) with H2/Ar/CH4 = 30/20/20 sccm, and C = cooling stage (180 min) with H2/Ar = 30/20 sccm, (c) A schematic diagram of graphene growth mechanism on Cu NW. 262x139mm (96 x 96 DPI)


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Figure 4. (a) SEM and (b) TEM image of graphene-Cu core/shell NWs. (c-d) HR-TEM images of middle and edge of NWs. Inset is line profile intensity of selective line on edge of the NWs. (e-g) EDS mapping analysis of graphene-Cu core/shell NWs. 240x136mm (96 x 96 DPI)



Figure 5. (a-c) Raman spectra of Gr/Cu NWs with different CVD temperatures (from 300 to 1000 °C). (d) I2D/IG and ID/IG ratioes of Gr/Cu NWs with different Gr formation temperature. 261x188mm (96 x 96 DPI)


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Figure 6. (a)(c) Raman spectra and (b)(d) XPS C 1s spectra of Graphene-Cu shell/core NWs with different methane gas flows/duration: CH_4 (20 sccm/20 min) and (20 sccm/10 min) respectively. 199x137mm (120 x 120 DPI)



Figure 7. (a) Long-term stability on Hot plate (80 ℃) and (b) thermo-hygrostat(85℃ / 85 % RH) test of graphene coated Cu NWs, comparing with bare Cu NWs. 320x125mm (96 x 96 DPI)


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Figure 8. (a) Photographic images of Cu NWs, Gr/Cu NWs, and ITO on sapphire. (b) Wavelength-dependent transmittance in the different amount of Cu NWs and Gr/Cu NWs comparing with 200 nm-thickness ITO. (c) The sheet resistance (Ω/sq) and transmittance (%) of Cu NWs, Gr/Cu NWs, and ITO. 285x74mm (120 x 120 DPI)



Figure 9. Schematic diagrams of LEDs device structure with different TCEs with p&n pad electrodes (Cr/Au): (a) LED I–reference, (b) LED II-ITO, (c) LED III-Cu NWs, and (d) LED IV-Gr/Cu NWs 208x104mm (120 x 120 DPI)


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Figure 10. Electrical and optical performance of LEDs with different TCEs. a) I-V characteristic curves, (b) Light spreading images of four different LEDs at injection current of 0.1 mA, (c) Electroluminescence (EL) spectra with 100 mA current injections, and (d) EL peak intensity in different current injections. 175x121mm (120 x 120 DPI)


Facile Synthesis of Graphene on Cu nanowires via Low Temperature

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