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Carbon Nanotube and Graphene Hybrid Thin Film for Transparent Electrodes and Field Effect Transistors Sung Ho Kim, Wooseok Song, Min Wook Jung, Min-A Kang, Kiwoong Kim, Sung-Jin Chang, Sun Sook Lee, Jongsun Lim, Jinha Hwang, Sung Myung,* and Ki-Seok An Low dimensional sp2 carbon materials including single-walled carbon nanotubes (swCNTs) and graphene have recently received a great deal of attention for potential uses in transparent and flexible nanoelectronics due to their remarkable mechanical, electrical, thermal, and optical properties.[1–5] Recently, many studies have focused on hybrid nanostructures that include graphene and functional materials, which complement deficiencies of graphene such as challenges in functionalization of the graphene surface or the zero-band gap of graphene.[6–8] Graphene-based hybrid nanostructures have attracted enormous interest in particular for two applications: transparent and flexible electrodes (TEs) and field effect transistors (FETs). To date, indium tin oxide (ITO) is most commonly used as a TE in solar cells, touch panels, and organic light emitting diode panels. In order to replace ITO films because of sustainability and price concerns, alternative materials with properties including a sheet resistance of ∼100 Ω/sq and optical transparency of ∼90% are required. Considering the opacity (2.3 ± 0.1%) and sheet resistance (∼1 kΩ/sq) of monolayer graphene grown by chemical vapor deposition (CVD), approximately 4-layer graphene film may be a good candidate for TEs.[9] Previous approaches to such films have utilized layer-by-layer stacking techniques that require complicated and time consuming procedures to fabricate low-resistance graphene TEs. In contrast, the optical transmittance and sheet resistance of swCNT-graphene hybrid films can be manipulated by a simple coating with swCNT solution.[10] In other studies, reduced graphene oxide and CNT hybrid nanostructures were prepared by adopting a low temperature solution process for application to TEs.[11] Although these approaches showed excellent processability of graphene-based hybrid thin films, significant

S. H. Kim,[+] Dr. W. Song,[+] M. W. Jung, M.-A. Kang, K. Kim, Dr. S. S. Lee, Dr. J. Lim, Dr. S. Myung, Dr. K.-S. An Thin Film Materials Research Group Korea Research Institute of Chemical Technology (KRICT) Yuseong Post Office Box 107 Daejeon 305-600, Republic of Korea E-mail: [email protected] Dr. S.-J. Chang Department of Chemistry Chung-Ang University 84 Heukseok-ro, Dongjak-gu, Seoul 156-756, Korea Prof. J. Hwang Department of Materials Science and Engineering Hongik University Seoul 121-791, Republic of Korea [+]These authors contributed equally to this work.

DOI: 10.1002/adma.201400463

Adv. Mater. 2014, DOI: 10.1002/adma.201400463

improvements in the electrical or optical properties would be necessary for their use in high performance graphene-based TEs and FETs. Despite the superior electrical properties of graphene such as ambipolar conductance, ballistic transport, and extremely high carrier mobility at room temperature,[4,6] their semimetallic nature has also been a major hindrance to the development of graphene-based FETs. Meanwhile, the electronic structures of swCNTs, described as a cylindrically rolled graphene sheets, are determined by their diameter and chirality.[12] When swCNTs are synthesized, they contain a mixture of approximately one-third metallic and two-thirds semi-conducting nanotubes. Thus, we anticipate improved performance of swCNT/graphene hybrid film-based FETs in terms of on-off current ratio. This result is supported by the fact that the on-off ratio of random network swCNT-based FETs containing a mixture of metallic and semiconducting nanotubes was greater than that of graphene-based FETs.[13] Herein, we demonstrate a facile methodology for the formation of swCNT/graphene hybrid films for use in high-performance TEs and FETs. In order to decrease the contact resistance between graphene and swCNTs, swCNTs were first spin-coated onto Cu foil, and graphene films were subsequently synthesized on the swCNT/ Cu foil using thermal CVD (TCVD) to prepare hybrid films. Furthermore, we also demonstrate that the high performance swCNT/graphene hybrid layer with swCNT network film had an orientation that could be controlled by adjustment of the spincoating speed. The formation of hybrid films was investigated by resonant Raman spectroscopy, and the electrical transport properties of the hybrid films were evaluated for nanoelectronic applications. Graphene hybrid film was synthesized by the method that graphene was grown using TCVD on the Cu foil coated with swCNT, as depicted in Figure 1. swCNTs were first dispersed in 1,2-dichlorobenzene with ultrasonic vibration to prepare a swCNT suspension. The swCNT solution was spin-coated onto the UV-treated Cu foil at 500, 1500, and 3000 rpm. After spincoating, the swCNT/Cu was annealed at 150 °C for 1 min to remove 1,2-dichlorobenzene. The Cu foil coated with swCNTs was heated from room temperature to 1050 °C inside the TCVD reactor while introducing H2 (6 sccm) and Ar (150 sccm). After the TCVD had been heated to the target temperature, methane (CH4, 10 sccm) was then introduced as a carbon feedstock along with H2 gas for 20 min to synthesize graphene. The graphene/swCNT hybrid layer on Cu foil was then cooled to room temperature. After the synthesis of graphene/swCNT thin film, the graphene-based hybrid material was transferred onto SiO2 (300 nm)/Si(100) by a transfer method similar to a previous work.[26] The graphene films were patterned using Al patterns as masks for graphene channels and oxygen plasma etching.

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graphene synthesis. These images confirm that uniformly distributed and aligned swCNTs from the spin-coating process were combined with the graphene layer and that the swCNTs remained stable during the CVD process. After the CVD process for graphene growth, the presence of graphene wrinkles, Cu grain boundaries and swCNTs coated on the flat Cu surface were observed. Figure 2e exhibits TEM image of the graphene-based hybrid layer, revealing that this is an evidence for the existence of continuously connected swCNTs and graphene. Figure 3a–c exhibits the aligned angle distribution obtained from an analysis of SEM images of swCNTs coated on Cu foil with various spin-coating speeds. These results indicate that the alignment and density of swCNTs can be tuned by adjusting the spin-coating speed (Supporting information Figure 1. Schematic diagram of the fabrication of hybrid thin film based on carbon nanoFigure S1). We anticipated that the aligntubes and graphene. a) Spin-coating of swCNTs on the polished Cu foil. b) Graphene synthesis ment and density control of swCNTs could on Cu foil coated with swCNTs. c) Patterning of the graphene layer on the initial substrate. affect the overall electrical properties of the d) Polymer-assisted transfer of the hybrid layer to a flexible substrate. e) Fabrication of graphene-based hybrid layer, a supposielectrodes on the graphene-based hybrid layer. tion that is supported by previous results including the spin-assisted alignment and density of swCNTs.[14] By controlling the direction of swCNTs Finally, graphene-based devices were fabricated by deposition of source and drain electrodes (10 nm Cr/70 nm Au) through a in a graphene sheet, we prepared two different types of hybrid shadow mask without the need of any lithographic processing. devices. Figure 3d and e shows electrical resistance histograms of graphene-based hybrid devices coated with horizontally Structural characteristics of the bare Cu foil and swCNTaligned (type i) and longitudinally aligned (type ii) swCNTs, coated Cu foil before and after graphene growth were analyzed in which the average resistance was approximately 2.41 and by scanning electron microscopy (SEM), atomic force micros1.39 kΩ, respectively. This reduction of electrical resistance in copy (AFM), and transmission electron microscopy (TEM), as type ii graphene-based hybrid devices was presumably caused shown in Figure 2a–e. As shown in Figure 2b, swCNTs were by an increase in the number of swCNTs, which help charge uniformly coated without aggregation, and swCNTs had a specarriers to flow through the grain boundaries of the graphene cific orientation according to the spin-coating direction. When sheet and avoid defect scattering at graphene boundaries. 0.2 mg/mL swCNT solution was spin-coated at a spin rate of 3000 rpm for 30 sec, the density of deposited swCNTs on Cu Raman spectroscopy was used for characterization of grafoil was approximately 15 /µm2. Figure 2c and d shows SEM phene, swCNTs, and the graphene/swCNT hybrid layer. Figure 4 shows Raman spectra of swCNTs, graphene, and and AFM images of the graphene-based hybrid layer after

Figure 2. SEM images of a) bare Cu foil, swCNT-coated Cu foil b) before and c) after graphene growth. d) AFM images, height profile and e) TEM images corresponding to c).

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COMMUNICATION Figure 3. Histogram showing the aligned angle distribution of swCNTs spin-coated onto Cu foil at a) 500, b) 1500, and c) 3000 rpm, respectively. Electrical resistance histograms of graphene-based hybrid devices coated with d) horizontally aligned swCNTs and e) longitudinally aligned swCNTs.

swCNTs-graphene hybrid thin film, in which representative swCNTs and graphene fingerprints, with a radial breathing mode (RBM), D-band, G-band, and 2D-band are apparent.[15,16] The RBM of resonant Raman spectra was recorded at an excitation wavelength of 514 nm. The swCNTs were confirmed by the presence of a RBM, which originated from the phonon vibration mode of carbon atoms in the radial direction.[16] The assignment of (n, m) indices for swCNTs was carried out by curve fitting each RBM peak with a Lorentzian function. From the deconvoluted RBM peaks, the diameters of the swCNTs could be determined according to the equation: w (cm−1) = 223.5/d (nm) + 12.5,[17] where w and d denote the wavenumber and diameter, respectively. Diameter-assigned peaks were labelled using (n, m) indices based on the extended tight-binding (ETB) Kataura plot.[18] As shown in Figure 4a, (16, 6)S, (14, 6)S, (12, 7)S, and (13, 5)S tubes were observed. The

absence of a D-band reflects the formation of highly crystalline swCNTs, as shown in Figure 4b. In addition, the RBM peaks were also observed by Raman mapping (100∼300 cm−1) with an excitation wavelength of 473 nm. And the Raman spectra with an excitation wavelength of 632.8 nm were recorded at 25 different regions of the sample (supporting information Figure S3). The G+ and G−-bands at 1591 and 1567 cm−1 originate from displacement of carbon atoms along the tube axis and circumferential directions,[16] respectively. For graphene, no RBM peaks, a G-band with a single Lorentzian feature, and a strong 2D-band were observed, as shown in Figure 4c and d. The 2D-band originated from an intervalley double resonance Raman process involving an electron and two iTO phonons at the K point.[15] Figure 4e and f shows the RBM and overview spectra of the swCNT-graphene hybrid film, which indicate the coexistence of swCNTs and graphene. In a previous report,

Figure 4. Raman spectra from excitation at 514 nm. a, c, e) RBM and b, d, f) overview spectra of swCNTs, graphene, and swCNT-graphene hybrid nanostructures. The structural (n, m) indices and transition energies were extracted from the ETB Kataura plot.

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Figure 5. a) Schematic illustration and SEM image of electrochemically gate-tuned swCNT-graphene hybrid FETs. b) Transfer (IDS−VG) at VDS = 0.1 V and (c) output characteristics (IDS−VDS) of graphene, swCNTs-graphene hybrid film, and swCNT network film. Plots of d) the on-off ratio and the onstate current and (e) field effect mobility and Dirac voltage for graphene, swCNT-graphene hybrid film, and swCNT network film. Plots of (f) optical transmittance at 550 nm and (g) sheet resistance of graphene, swCNTs, swCNT-graphene hybrid films, and swCNTs coated on transferred graphene. h) A photograph of swCNT-graphene hybrid films on a PET substrate.

the 2D-band of a swCNT/graphene hybrid nanostructure consisted of two components.[19] However, our results suggest that the 2D-band of swCNT-graphene hybrid film exhibits a single Lorentzian feature. This discrepancy can be understood by the fact that the 2D-band position and lineshape are determined by the electronic structure and chirality of swCNTs. In our study, semiconducting tubes were predominantly observed at an excitation wavelength of 514 nm, whereas metallic tubes were detected in the previous report. These results are supported by the absence of a Breit-Wigner-Fano lineshape at approximately 1500 cm−1.[20] Next, graphene-based hybrid FETs were prepared for the analysis of electrical transport of swCNT-graphene hybrid films, as depicted in Figure 5a. Cr (10 nm)/Au (70 nm) were used as the source/drain electrodes, and 1-buty-3-methylimidazolium (BmimPF6) was used as the ionic liquid.[21,22] The transfer curve (IDS-VG) at a VDS = 0.1 V of graphene exhibited a charge-neutral Dirac point (VDirac) at a positive gate voltage (VG) and asymmetric hole and electron conduction, as shown in Figure 5b. This result can be explained by an unintentional p-type doping effect from residual water molecules on the graphene surface.[23] The on-off ratio (Ion/Ioff) and the on-state current (Ion) of devices based on swCNTs-graphene hybrid film both increased slightly. This could be due to participation of swCNTs as an additional conducting path after the formation of the hybrid films. Furthermore, after the formation of swCNTs-graphene hybrid film, VDirac shifted toward positive voltages due to heavy p-type doping. The transfer curve of swCNTs reveals unipolar p-type behavior. The on-off ratio increased slightly whereas the on-state current decreased. Based on the transfer curves, we calculated the field effect mobilities (µ) of graphene, swCNTsgraphene and swCNTs FETs corresponding to 341.7 ± 259.4, 4

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394.46 ± 176.27, and 58.78 ± 36.17 cm2/V·s, respectively. From the output characteristics of graphene, swCNTs-graphene, and swCNTs FETs, it was found that the electrical conductivity dramatically increased after the formation of swCNTs-graphene, as seen in Figure 5c. The Ion/Ioff, Ion, µ, and VDirac were summarized in Figure 5d, e and Table S1 in supporting information. Figure 5f and g shows the optical transmittance at 550 nm and the sheet resistance of graphene, swCNTs, and the swCNTgraphene hybrid film. In addition, graphene film spin-coated with swCNTs was also prepared to compare with the hybrid film. Monolayer graphene was transferred onto the polyethylene terephthalate (PET) film, after which swCNTs were coated onto the transferred graphene/PET film. The optical transmittances of graphene, swCNTs, swCNT-graphene hybrid film, and swCNTs coated on the transferred graphene were observed to be 97.0, 97.1, 96.4 and 96.2%, respectively. These results suggest that the synthesis of monolayer graphene was achieved since the opacity of graphene is 2.3 ± 0.1%.[9] After formation of swCNT-graphene hybrid films, the optical transmittance was greater than that of bilayer graphene films (∼95.4%). The sheet resistances of graphene, swCNTs, swCNT-graphene hybrid films, and graphene coated with swCNTs were 1000, 1200, 300, and 1100 Ω/sq, respectively. Interestingly, the sheet resistances of swCNTs-graphene hybrid films decreased dramatically compared with graphene and graphene spin-coated with swCNTs, and was comparable to that of previous hybrid materials, including AuCl3 doped graphene films (150 Ω/sq at 87%) and Ag nanowire/graphene films (800 Ω/sq at 96.5%).[24,25] Importantly, the sheet resistance of swCNTs-graphene hybrid films was significantly lower than that of swCNTs coated on transferred graphene, which presumably originates from the low contact resistance between graphene and swCNTs in the hybrid

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Experimental Section Preparation of swCNTs and swCNT-coated Cu foils: Purified swCNTs (0.02 g, Carbon Nano-material Technology Co, Ltd.) were dispersed in 100 mLof 1,2-dichlorobenzene with ultrasonic vibration for 20 min to prepare a swCNT suspension. 100 µm thick Cu foil was mechanically polished with metal polish paste followed by irradiation with ultraviolet (UV) light to prepare a hydrophilic surface. The swCNT solution (0.6 ml) was spin-coated onto the UV-treated Cu foil at 500, 1500, 3000 rpm for 30 sec. After spin-coating, the swCNT/Cu was annealed at 150 °C for 1 min to remove 1,2-dichlorobenzene. Synthesis and Transfer of Graphene Sheets: Graphene film was grown on 100-µm thick Cu foil (from Alfa Aesar) coated with swCNT by TCVD. Copper foil was inserted in a quartz tube and heated to 1050 °C under ambient pressure with flowing H2 and Ar. After flowing reaction gas mixtures (CH4 : H2 : Ar = 100 : 6 : 150 sccm) for ∼20 min at a pressure of 1.9 Torr for 45 min, the sample was cooled to room temperature. PMMA was spin-coated on hybrid graphene layer, and Cu foil was removed by the copper etchant (dilute FeCl3 solution in deionized water). After rinsing the graphene surface with deionized water, the PMMA-coated graphene was transferred to various substrates, such as SiO2 (300 nm)/ Si(100) and PET by a transfer method similar to a previous work.[26] Finally, PMMA was carefully removed by acetone. Preparation and Measurement of Graphene Devices: For the graphene device fabrication, Al pattern were first deposited onto the graphene/ swCNTs films on SiO2 (300nm)/Si(100) and subsequent oxygen plasma etching was carried out. After removing Al patterns by Al etchant, the graphene/swCNTs channels were formed. Finally, electrochemically gated graphene/swCNTs-based FETs were fabricated by employing 1-butyl-3-methylimidazolium (BmimPF6) as an ionic liquid and the deposition of source and drain electrodes (10 nm Cr/70 nm Au) through a shadow mask. Here, the length and width of graphenebased devices are 100 and 200 µm, respectively. A Keithley-4200 semiconductor parameter analyzer was used for measurement and data collection.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Adv. Mater. 2014, DOI: 10.1002/adma.201400463

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film. Figure 5h shows a photograph of the swCNTs-graphene hybrid film on PET (size = 2 × 2 cm2). It should be noted that large-area, uniform, and high-performance swCNT-graphene hybrid films have been fabricated for applications in TEs. Significantly, our approach may pave the way toward the fabrication of semiconducting swCNT-graphene hybrid FETs with high on/off ratios and superb electrical conductivity for highperformance flexible devices. Graphene-based hybrid sheets combined with swCNTs could be applied to the fabrication of flexible and transparent electrodes on PET film, due to the high conductivity and flexibility of graphene sheets with swCNTs. In summary, swCNTs and graphene were combined to provide high-performance TEs and FETs. The density and alignment of pre-coated swCNTs could be controlled by adjusting the spin-coating speed, a crucial factor for achieving high-performance TEs. Based on the transfer characteristics for hybrid film-based FETs, we achieved improved Ion/Ioff and on-state current with respect to pristine graphene. Notably, the hybrid film possessed a sheet resistance of 300 Ω/sq with 96.4% transparency, which was comparable to that of hybrid materials in previous reports.

This research was supported by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea (2013K000162) and by a grant (2011-0031636) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT and Future Planning, Korea. Received: January 28, 2014 Revised: March 14, 2014 Published online:

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Carbon nanotube and graphene hybrid thin film for transparent electrodes and field effect transistors.

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