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Novel transparent conductor with enhanced conductivity: hybrid of silver nanowires and dual-doped graphene
Accepted Manuscript Title: Novel Transparent Conductor with Enhanced Conductivity: Hybrid of Silver Nanowires and Dual-Doped Graphene Author: Hiesang Sohn Yun Sung Woo Weonho Shin Dong-Jin Yun Taek Lee Felix Sunjoo Kim Jinyoung Hwang PII: DOI: Reference:
S0169-4332(17)31156-X http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.129 APSUSC 35810
To appear in:
APSUSC
Received date: Revised date: Accepted date:
31-12-2016 3-4-2017 17-4-2017
Please cite this article as: H. Sohn, Y.S. Woo, W. Shin, D.-J. Yun, T. Lee, F.S. Kim, J. Hwang, Novel Transparent Conductor with Enhanced Conductivity: Hybrid of Silver Nanowires and Dual-Doped Graphene, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.04.129 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Novel Transparent Conductor with Enhanced Conductivity: Hybrid of Silver Nanowires and Dual-Doped Graphene
Hwange,*
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Hiesang Sohna,b*, Yun Sung Woob, Weonho Shinb, Dong-Jin Yunc, Taek Leea,*, Felix Sunjoo Kimd,*, and Jinyoung
Department of Chemial Engineering, Kwangwoon University, Seoul 01897, Korea
b
Inorganic Materials Laboratory, Samsung Advanced Institute of Technology, Suwon 443-803, Korea
Platform Technology Laboratory, Samsung Advanced Institute of Technology, Suwon 443-803, Korea
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c
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a
School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 06974, Korea
e
School of Electronics and Information Engineering, Korea Aerospace University, Goyang-si 10540, Korea
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d
*Corresponding author. E-mail address: [email protected] (H. Sohn), [email protected] (F. S. Kim), [email protected]
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(T. Lee), [email protected] (J. Hwang)
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Abstract
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We present hybrid transparent conducting films based on silver nanowires (Ag NWs) and doped graphene through
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novel dual co-doping method by applying various dopants (HNO3 or Au for p-doping and N2H4 for n-doping) on top and bottom sides of graphene. We systematically investigated the effect of dual-doping on their surface as well as electrical and optical properties of graphene and Ag NW/graphene hybrid films through the combination study with various dopant types (p/p, p/n, n/p, and n/n). We found that the p/p-type dual-doped (p-type dopant: HNO3) graphene and its hybrid formation with Ag NWs appeared to be the most effective in enhancing the electrical properties of conductor (doped graphene with ∆R/R0=84 % and Ag NW/doped graphene hybrid with ∆R/R0=62 %), demonstrating doped monolayer graphene with high optical transmittance (TT=97.4%) and sheet resistance (Rs=188 ohm/sq.). We also note that dual-doping improved such electrical properties without any significant debilitation of optical transparency of conductors (doped graphene with ∆TT=0.1% and Ag NW/doped graphene hybrid with ∆TT=0.4%). In addition, the enhanced conductivity of p-type dual-doped graphene allows a hybrid system to form co-percolating network in which Ag NWs can form a secondary conductive path at grain boundaries of polycrystalline graphene.
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Highlights 1) Ag nanowire-doped graphene hybrid prepared by novel top and bottom co-doping method
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2) Improved doping efficiency (conductivity enhancement, ∆R/R0=84 %) of graphene based on optimized ptype (HNO3) dual co-doping
3) Enhanced conductivity of doped graphene without debilitation of optical transmittance (∆TT=0.1%)
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4) The lowest Rs (188 ohm/sq.) of monolayer graphene at high optical transmittance (97.4 %)
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5) Effective resistance reduction of Ag NW on dual doped-graphene through co-percolating conduction
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Keywords: graphene; dual-doping; Ag nanowire-graphene hybrid; electrical conductivity; flexible transparent conductive film
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1. Introduction
There have been burgeoning demands for deformable transparent conductive electrodes (TCEs) for the
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application in next-generation displays and wearable electronics [1-3]. Currently, conductive oxides (e.g., indium tin
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oxide; ITO) have been widely employed in various electronic gadgets, including smart phones, flat-panel televisions, solar cells, and solid-state lighting devices because of their high optical transparency and conductivity [1-3].
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However, despite the excellent electrical and optical properties of ITO, it has a limited application in flexible TCEs since ITO is mechanically brittle and is easily cracked under bending [2,3]. Both price of its source element (indium) and production cost under high temperature and vacuum processing are also expensive for large-scale deployment. As a result, there has been a continuous and increasing demand to replace ITO with novel transparent conducting materials [1-3].
In this context, networks of silver nanowires (Ag NWs) have been considered as a cost-effective and sustainable alternative to ITO for stretchable and flexible electrodes, since Ag NW-based conductors are mechanically durable and possess higher optical transparency than ITO at equivalent sheet resistance (Rs) [2,3]. However, Ag NW networks suffer from disadvantages limiting their application in commercial devices. For instance, inherent junction resistance associated with charge transport between randomly oriented nanowires reduces the electrical conductivity [2]. Although this may be overcome by welding at overlapping junctions, such temperature
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treatment could damage substrates and underlying components [2,3]. Ag NW networks have a high contact resistance between network and active materials [2]. To address the problems of Ag NW networks, it has been a highlight to introduce Ag NWs with a
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secondary conductive nanostructure to form a hybrid for forging inter-nanowire connection and enhancing performance [4-18]. That is, the hybrids of Ag NWs and conductive nanostructures have been developed with a combination of different nanomaterials, including carbon nanotubes, conductive polymers, metal oxide, and
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graphene [4-17]. Especially, hybridization of Ag NWs with a two-dimensional graphene appears to be promising to
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address various issues. Hybrid structure shows enhanced conductivity with excellent optical properties [10-13,15,16]. Together at junctions and to substrates, Ag NWs are tightly held and adhered to the substrate, thereby hindering
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fracturing of Ag NWs under tension [13-16]. It shows high mechanical flexibility and stretchability after hybrid formation, allowing its potential use for high-performance flexible electronics. For instance, Liang et al. reported a hybrid of Ag NW and graphene prepared by solution based process of Ag NW and graphene, which exhibited high
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optical transmittance (TT) (88 %) and low Rs (22 Ω/sq.) [10]. UV light-emitting diodes based on Ag NW and graphene hybrids were demonstrated by Seo et al [11]. In another work, Kholmanov et al. presented a conductor
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with good electrical and optical properties (TT: 90 %, Rs: 64 Ω/sq.) built on a hybrid of Ag NW and chemical vapor
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deposition (CVD)-grown graphene through a modified graphene transfer approach [12]. Lee et al. also reported a high performance stretchable TCE using a hybrid of Ag NW and CVD graphene [13]. Chen et al. presented highly
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conductive free standing paper with graphene/Ag NW hybrid [14]. Recently, Park et al. presented soft transparent electrodes for an inorganic light-emitting diode fitted on a soft eye contact lens with an Ag NW and graphene hybrid [15]. All the above-mentioned studies indicate the potential of Ag NW and graphene hybrid systems towards various types of transparent electrodes, exhibiting superior electrical and optical properties to ITO [16]. To further boost the conductivity of Ag NW/graphene hybrid films, it is necessary to enhance the electrical property of either Ag NW or graphene, and reduce the contact resistance between them. However, it is difficult to enhance the electrical property of Ag NW without compromising the optical property of the hybrid because of high diffused light scattering of Ag NW [2,4,5]. Alternatively, several studies have been conducted to enhance the electrical properties of the hybrids through graphene modification. Choi et al. reported an optimized chemical doping process of the hybrid of Ag NW and graphene [17]. Lee et al. demonstrated the improved conductivity of the hybrid films by applying a non-uniform electric field on graphene [18].
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In this work, we achieved the conductivity enhancement of Ag NW/graphene hybrid films by implementing dual-doped graphene where the top and bottom sides of graphene were doped with various chemical dopants (p-type dopant: HNO3 or Au; n-type dopant: N2H4). By applying the simple dual co-doping approach of graphene, the work
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function of graphene can be effectively engineered, resulting in the conductivity enhancement of graphene and the contact resistance reduction between graphene and Ag NWs. Different from the previous bottom-doping approach based on single graphene etchant (e.g., benzimidazole) [19], our novel doping process composed of bottom and top
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co-doping has versatility in terms of chemical dopant types for bottom doping. We investigated the effects of doping
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on the materials through the systematic analyses of their surface, electrical, and optical properties. We found that, in the case of HNO3 p-doing and N2H4 n-doping, the p/p-type dual-doping is the most effective to enhance the
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conductivity, compared with other combinations (n/p, p/n, and n/n). We also observed that the p/p-type dual-doping can further improve the conductivity of Ag NW/graphene hybrid system. The enhanced conductivity of graphene by the dual doping reduces contact resistance between Ag NWs and graphene, leading to the formation of the co-
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percolating network where Ag NWs can be an additional conductive path at the grain boundary of the polycrystalline graphene. Note that Ag NW/dual-doped graphene hybrids with enhanced conductivity do not
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2. Experimental
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exhibits any significant compromise of optical transmittance.
2.1. Materials
We used polycrystalline graphene (4" Monolayer Graphene on Cu foil) purchased from Graphene square (Korea), which was grown by CVD on a Cu substrate. Ag NWs with an average length of 20 µm ± 5 µm and an average diameter of 30 nm ± 5 nm were obtained from Aiden Co. (Korea) and used without further purification. Other reagents were obtained from Sigma-Aldrich, unless otherwise specified.
2.2. Preparation of Conducting Films Conducting Films of Doped Graphene: Schematic procedures and structures of doped graphenes and hybrid conducting films are shown in Scheme 1. Briefly, as-received graphene on Cu foil was covered by a layer of poly(methyl methacrylate) (PMMA) by spin-coating and subjected to removal of its copper substrate by a Cu
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etchant (FeCl3). Subsequently, Cu substrate-etched free-standing graphene/PMMA bilayers were floated on deionized water to remove/dissolve remnant Cu etchant (FeCl3) under the graphene. For bottom doping, as-rinsed graphene/PMMA bilayers were re-floated on a doping solution of various p- or n- type dopants for 5-10 min. A
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dilute aqueous solution of HNO3 or AuCl3 was used for p-doping, and a hydrazine (N2H4) solution (1 M; 60 °C) was employed for n-doping. After bottom-doping, doped graphene/PMMA bilayers were transferred onto a polyethylene terephthalate (PET) substrate, rinsed with acetone for removal of the PMMA supporting layer, and subjected to top-
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doping. We applied a conventional vapor-phase doping process by putting graphene under HNO3 vapor for 3 min
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(p-doping) or N2H4 vapor for 10 min (n-doping). Some samples were top-side doped by applying a p-type doping solution of AuCl3 (0.1-1 M) dissolved in nitromethane through spin-coating onto the graphene. We also prepared
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various dual-doped graphene samples with a combination of doping systems (p-/p-, p-/n-, n-/p-, and n-/n-types) to study the dopant-type effects.
Hybrid Conducting Films of Ag NW and doped Graphene: We prepared the hybrid conducting film of the
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Ag NW network and graphene by a wire-bar coating. A solution of Ag NW (0.15 wt.% of Ag NWs in a coating solution) was spread onto the graphene by the Meyer rod (RDS 3; 30 mm/sec). The Rs of the Ag NW network can
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be regulated by controlling its wet thickness after solution coating. After deposition of Ag NWs on graphene, the
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2.3. Characterizations
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films were annealed at 100 °C for 5 min in the oven to completely remove the solvent.
The morphology and surface properties of the samples were characterized through various imaging and spectroscopic tools. The surface morphology of hybrid films was imaged by a scanning electron microscope (SEM; NOVA NanoSEM 630) at an accelerating voltage of 5 kV. The chemical bonds of the samples were analyzed via Xray photoelectron spectroscopy (XPS; Escalab 250) with an Al Kα (1486.6 eV) source. The Raman spectra were obtained with a Renishaw Raman Spectrometer under backscattering geometry. Spectra were averaged over 20 accumulations, and the laser power (633 nm) was kept at 2.5 mW. The optical transmittance of the films including the substrate (PET) was recorded on a haze meter (NDH 5000). The Rs was measured by an in-line four-point probe surface resistivity meter (R-CHEK, RC2175). The Rs reported in this work is an average of at least five points for each sample.
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3. Results and discussion 3.1. Transparent Conductive Films of Dual-doped Graphene and Hybrid
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Hybrid transparent conducting films demonstrated in this work were prepared by bottom-doping and topdoping of graphene followed by hybridization with Ag NWs, as illustrated in Scheme 1. The aim of our novel dual co-doping process was to enable high-performance transparent conducting films without significant debilitation of
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optical properties. First, CVD-grown graphene on a copper substrate was covered by PMMA, which acted as a
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carrier film and a protecting layer during bottom-doping (a→b). Cu substrate was then etched by Cu-etchant (FeCl3) (b→c). Note that, although FeCl3 has been shown to act as a p-type dopant for graphene [20], we have thoroughly
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washed the Cu-etchant (FeCl3) on graphene with a copious amount of water after the etching process. The surface investigation with XPS confirmed that the graphene is free from FeCl3 (Figure S2). The PMMA/graphene was subject to bottom-doping by placing the bilayer on a doping solution with a controlled concentration (c→d),
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followed by transferring onto a plastic substrate (PET) (d→e) and removal of PMMA (e→f). Unlike bottom-doping, top-doping was performed through a vapor treatment to minimize any damage of the graphene (f→g). Simple bar-
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coating resulted in a formation of the well-controlled Ag NW network onto the graphene (g→h). The as-formed Ag
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PET substrate).
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NW/dual-doped graphene hybrid films had a low Rs (188 ohm/sq.) and high transmittance (97.4%, 90.5%, including
(Scheme 1)
Fig. 1 displays the SEM images of the pristine and hybrid films obtained at different preparation stages and a digital photo image of a hybrid transparent electrode built on a plastic substrate (PET). Fig. 1a is an image of pristine graphene, while Fig. 1b and c show doped graphene. Graphene samples before hybridization show a smooth surface without any noticeable features. After single and dual-doping, it does not show any significant change in the graphene morphology. A hybrid of Ag NW and doped graphene shown in Fig. 1d shows a clear network of Ag NWs formed on the graphene. All insets included in SEM pictures are the corresponding digital images of the conducting films built on graphene, doped graphene, and their hybrid with Ag NWs. The optical images show almost similar optical transparency, indicating that the conductor is not significantly affected by chemical doping or hybridization
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with Ag NWs. Fig. 1e displays the digital image of the hybrid conducting film of Ag NWs and dual-doped graphene constructed on plastic substrate (PET), exhibiting good flexibility and high optical transmittance.
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(Fig. 1)
Fig. 2 shows the spectroscopic information of the pristine and doped graphene samples at various stages of p-type doping with nitric acid. In the XPS experiment, the nitrogen 1s signal-which was not present in the pristine
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graphene-was detected after doping. We deconvoluted the N 1s signals in the doped graphene samples through
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Gaussian fitting, and found three peaks. After bottom-doping, the peaks were observed at 406.2, 401.2, and 399.3 eV, corresponding to NOx, N-H (or pyrrolic), and C-N (pyridinic) bondings, respectively. It is believed that the
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dopant is chemically influencing the graphene, because the N 1s peak not only occurred for NOx bonding, but also for the other chemical bondings (such as pyrrolic and pyridinic bondings) [21,22]. This observation indicates that HNO3 chemically dopes the graphene with NO2 as well as N atoms [21,22]. The samples dual-doped with HNO3 for
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both bottom and top sides showed same peaks at 406.2, 401.1, and 399.2 eV. The doping effect was stronger after
each state.
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dual-doping compared to the single-doped state, based on the comparison of the ratio of CN signal to NOx signal in
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Raman analysis further confirmed the dual-doping effects of graphene by displaying peak shifts in the spectra (Fig. 2b). The G peak and the 2D peak shifted after bottom-doping and dual-doping, clearly showing that
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HNO3-doped graphene films have blue shift for both G and 2D peaks. D-band peaks were not observed at any stage of doping, indicating that HNO3 treatment was not destructive to the chemical bonds of graphene. The peak position of G band shifts upon doping [23,24]. An upward shift in the G band position is due to the phonon stiffening effect by charge extraction of p-type doping [23]. The G band (1592 cm−1) in the pristine graphene shifted by 3 cm−1 from 1592 to 1595 cm−1 after HNO3 bottom-doping. The G band in pristine graphene shifted by 11 cm−1 from 1592 to 1603 cm−1 after dual-doping. Such an additional peak shift of the Raman peak in the dual-doped graphene comes from an augmented doping effect by top- and bottom-side dual-doping [23,24]. The Raman shift in the 2D peak from 2634 to 2657 cm−1 is attributed to the doping of nitrogen atoms, consistent with the XPS result [24].
(Fig. 2)
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3.2. Electro-optical Properties of Conducting Films Fig. 3 displays the electrical and optical properties of as-formed doped graphene at different doping conditions. Electrical properties of doped graphene were measured for its Rs, while the optical properties were
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measured for the optical transmittance of graphene on a PET substrate. We investigated the electrical properties of various conductors by comparing their doping efficiencies (conductivity enhancement, ∆Rs/Rs,0), which was defined as the ratio of the change of sheet resistance after doping (∆Rs) to the original resistance (Rs,0). When the graphene
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was single-doped with HNO3, the Rs first dropped from 1.17 kΩ/sq. in the pristine state to 456 Ω/sq. in the doping
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concentration of 1 M (Fig. 3a). The Rs was steady at ~460 Ω/sq. up to 5 M, and then increased to 29 kΩ/sq. under 10 M of HNO3. Dual-doping with identical dopants (i.e., HNO3) for both top and bottom doping resulted in much lower
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Rs of 189 Ω/sq. under optimal conditions. We also tried asymmetric doping of graphene, with HNO3 as the bottom dopant and gold (Au) as the top dopant. The Rs of asymmetrically dual-doped graphene (HNO3/Au)-where the lowest Rs was achieved at 1 M AuCl3 doping-was slightly larger than that of the symmetric dual-doping (i.e. doping
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both sides with HNO3). Such an asymmetrical dual-doping of organic/inorganic dopants suggests that our system can be further extended to different dual-doping systems by judicious selection of various dopant combinations [25].
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Note that, we also compared the effect of multiple single side-doping with that of dual-doping on the electrical
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property of graphene with same dopant (HNO3) (Figure S3). As shown in Figure S3, the doping efficiency (conductivity enhancement) of graphene is not significantly improved by multiple times single-side doping. In
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addition, the doping efficiency of dual-doping (84 %) exceeds that of single-side doping (max. 65.7 %) prepared at all different treatments.
The optical transmittance (including PET substrate) of the bottom-doped graphene peaked to be 90.8 % at 0.5 M solution doping, and was higher than 90.5 % throughout the whole range studied, except for 10 M doping (Fig. 3b). In the case of graphene samples with dual symmetric doping, the highest was 90.6 % at 1 M solution doping. The samples with asymmetric doping had transmittances of 90.4-90.6 %. Note that the optimum doping condition was obtained at 1 M HNO3, where the lowest Rs was achieved while maintaining a relatively high transmittance. Asymmetrically dual-doped graphene (HNO3/Au) retained its high optical transmittance (90.4 %), which is a little bit lower than that of HNO3 dual-doped graphene.
(Fig. 3)
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Fig. 4 compares the Rs and optical transmittance of doped graphene prepared by various combinations of ptype (HNO3) and n-type (N2H4) doping systems. Interestingly, p-type doping generally resulted in lower Rs, whereas
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the resistance increased after n-type doping (Fig. 4a). Compared to pristine graphene with a typical Rs of 0.8-1.1 kΩ/sq., the resistance values after bottom-doping changed to 0.35-0.39 kΩ/sq. and 5.6-6.7 kΩ/sq. for p-type and ntype dopants, respectively. Dual-doping exhibited a resistance change according to the type of additional doping. It
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shows that p/p-doped graphene has the lowest Rs compared with the other systems, indicating the effectiveness of
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p/p-type dual-doping. A significant reduction in Rs after p-type doping was also observed when the graphene was treated by bottom n-type doping followed by top p-type doping. The sample with n/p-type dopants for bottom/top
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doping had an Rs of 1.19 kΩ/sq., which is almost less than a fifth of the resistance (6.7 kΩ/sq.) of n-type bottomdoped graphene. On the other hand, n-top-doping after p-bottom-doping increased the resistance from 0.35 kΩ/sq. to 1.16 kΩ/sq. The optical transmittance of the dual-doped graphene samples was in the range of 90.4-90.7 %. It was
with the pristine and single-doped graphenes.
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noticeable that the homogeneously dual-doped graphene samples showed almost identical transmittance compared
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The high efficiency of p/p-type dual-doping of graphene with HNO3 can be attributed to the effectively
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designed bandgap engineering. It is well known that band structure engineering of graphene is an effective way to modify the electronic properties of graphene, because pristine graphene has zero bandgap [26-29]. Typically, the
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band structure of graphene can be engineered through various dopings. In this context, the doping of graphene plays an important role in increasing carrier density as well as in changing the electronic structure. Accordingly, we designed the doping of graphene with optimal band structure with a p-type dopant. As shown in a comparative doping test based on HNO3 and N2H4 (Fig. 4), p-type dual-doping was the most effective way to enhance the conductivity of the graphene films, because the work function of graphene could be shifted to the p-type side, thereby enhancing the conductivity of the graphene [26-29]. The doping of graphene is effective not only because of the zero band gap but also the Dirac cone. The low density of states close to the Fermi level of graphene can be very efficiently filled/emptied by dopants.
(Fig. 4)
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Fig. 5 compares the Rs and optical transmittance of Ag NW/graphene hybrid films obtained by p/p-type dual-doped or un-doped graphene. After applying the same amount of Ag NWs above the percolation limit of their network on both graphene films, the Rs of the pristine graphene and the p/p-type dual-doped graphene decreased
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from 0.86 kΩ/sq. to 0.78 kΩ/sq. and from 0.32 kΩ/sq. to 0.29 kΩ/sq., respectively. The Rs value of Ag NW network on the un-doped graphene is estimated based on the equation for the parallel combination of two resistors (1/Rs,undoped-Gr + 1/Rs,AgNW = 1/Rs,hybrid, where Rs,undoped-Gr=0.86 kΩ/sq. and Rs,hybrid=0.78 kΩ/sq.), and the calculated Rs
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of Ag NW network (Rs,AgNW) on the pristine graphene hybrid film is 8.39 kΩ/sq. However, the Rs of the p/p-type
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dual-doped graphene hybrid film fabricated with the same amount of the Ag NWs on the pristine graphene is less than Rs of the parallel combination of p/p-doped graphene (Rs,doped-Gr) and the Ag NW network (Rs,AgNW). The Rs of
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the p/p-type doped graphene and Ag NW hybrid film is 0.29 kΩ/sq., while the Rs of the parallel combination of the p/p-doped graphene and Ag NW network is 0.31 kΩ/sq. (1/Rs,doped-Gr + 1/Rs,AgNW = 1/Rs,hybrid, where Rs,doped-Gr=0.32 kΩ/sq. and Rs,AgNW =8.39 kΩ/sq.). Obviously, the Ag NWs do not affect the doping state of graphene as presented in
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the Raman spectra of the Ag NW/p-type dual-doped graphene, which show no significant shift (Fig. S1). Instead, we believe that the contact resistance between the graphene and the Ag NWs (RC,Gr-Ag in Fig. 5c) decreases because of
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the enhanced conductivity of the p/p-doped graphene, resulting in further reduction of the Rs of the doped-graphene
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and Ag NW hybrid film through the co-percolating conduction [30]. In the co-percolating network, Ag NW can be a bypass of electron transport at the grain boundaries of the CVD grown polycrystalline graphene, as shown in Fig. 5c
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[30]. In addition, consistent with the results in the dual-doped graphene (Fig. 4a), the hybrid film of Ag NW and p/ptype dual-doped graphene exhibits the lowest Rs compared to other types of dopant combinations (p/p, p/n, and n/p) in hybrid films (Fig. S2), indicating the retained effectiveness of p/p-type dual-doping in hybrid system [12,19,2729]. Fig. 5b displays the optical transmittance of conductive films prepared at doped/un-doped and hybrid conditions, exhibiting good retention of the optical properties of p-type dual-doped hybrid films, comparable to hybrid of Ag NW and bare graphene. In addition, although the optical transmittance of a hybrid conductor slightly decreases (less than 0.5%) after Ag NW hybridization, we believe the hybrid conductor of graphene/Ag NW is still valid approach because additional dose of Ag NWs can further decrease the resistance of conductor without significant decrease of optical transmittance [13,15].
(Fig. 5)
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It is hard to directly compare the results of individual work prepared and tested by various conditions. Nevertheless, it is still meaningful to see where we stand in terms of electrical and optical parameters. Our
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conducting electrodes provide with comparable conductivity to the best records of previously reported similar works while out-perform in optical properties (See Table S1 in Supplementary Information) [11,12,17,31-35]. Although some previous works show superior conductance of graphene to that of our dual-doped graphene, our approach
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exhibits the lowest Rs (188 ohm/sq.) under the condition of high transmittance (97.4 %; 90.6 % including PET
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substrate), and higher doping efficiency (~84 %) at monolayer-level graphene. The hybrids of doped graphene and Ag NWs also show enhanced conducting properties and high optical transparency compared with reported results,
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indicating that our method is a valid approach to enhancing conductivity without compromising optical transmittance.
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4. Conclusions
In this work, we have demonstrated Ag NW/dual-doped graphene hybrids with enhanced conductivity,
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where the top and bottom sides of the graphene were doped with p-type dopants (HNO3) to engineer effective work
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function of the dual-doped graphene. In addition, such p-type dual-doped graphene and its hybrids were found as the most effective to enhance conductivity based on systematic and comparative investigation of the doping effect on
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the graphene and graphene/Ag NW hybrid films. Compared with the current state-of-the-art in Ag NW and graphene hybrids, this work made several breakthroughs, summarized as follows.
1.
Novel dual co-doping method to improve doping efficiency (conductivity enhancement): Our novel dualdoping method based on top and bottom co-doping of graphene was quite effective to enhance the doping efficiency of monolayer graphene based conductor (Rs: 188 ohm/sq.) by 84 % compared to pristine graphene, without debilitation of its original optical transmittance (dual-doped graphene: 97.4 %, pristine graphene: 97.5 %).
2.
Optimized dual co-doping system: Under the varied combinations of dopant types for dual-doping (p/p, p/n, n/p, and n/n) with HNO3 for p-doping and N2H4 for n-doping, we found that the p/p-type dual-doping
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among other combinations was the most effective in improving the electrical property of graphene and its hybrid with Ag NWs. 3.
Enhanced conductivity of Ag NW networks on doped-graphene hybrids: The hybrid conductor of Ag NWs
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on dual-doped graphene exhibits improved electrical conductivity (62 % reduction of sheet resistance) and somewhat retained optical transparency (Ag NW/dual-doped graphene hybrid: 90.1 % including substrate) compared with Ag NW/native graphene hybrid (90.3 % including substrate) because of further resistance
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reduction through the co-percolating conduction.
Current result indicates that our novel hybrid conductor of Ag NW/dual-doped graphene perform comparably to
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most of the reported works in conductivity, and out-perform in optical properties. We believe that our facile doping and hybridization approach can open a new direction to high-performance transparent conducting electrodes for
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flexible displays, solar cells, and wearable electronic devices fitted on a curved substrate.
Acknowledgements
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H. S acknowledge that present research has been conducted by the research grant of Kwangwoon
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University in 2017. F.S.K acknowledges the support of National Research Foundation of Korea (NRF) program by
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the Korea government Ministry of Science, ICT, and Future Planning (NRF-2014M3A7B4051749).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/xxx.
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Figure Captions
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Scheme 1. Schematic illustration of the experimental procedure and structures of dual-doped graphene and silver nanowire (Ag NW)/dual-doped graphene hybrid conducting films. (a) chemical vapor deposition (CVD)-grown
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graphene on Cu substrate; (b) poly(methyl methacrylate) (PMMA)/graphene bilayer on Cu substrate; (c) PMMA/graphene bilayer; (d) PMMA/graphene bilayer with bottom doping; (e) PMMA/graphene bilayer with
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bottom doping on substrate; (f) graphene with bottom doping on substrate; (g) graphene on substrate with top and
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bottom dual-doping; (h) Ag NW/dual-doped graphene hybrid conducting film.
Fig. 1. Scanning electronic microscopy (SEM) and digital photo-images of samples at different doping stages: (a)
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Pristine graphene; (b) bottom-doped graphene; (c) top/bottom dual-doped graphene; and (d) Ag NW/dual-doped
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graphene hybrid. Insets show the corresponding optical images with a scale bar of 3 cm. (e) Digital image of the as-
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formed hybrid conductor film on a plastic substrate. The dashed box indicates the hybrid conducting film.
Fig. 2. (a) X-ray photoelectron spectroscopy (N 1s) and (b) Raman spectra of pristine graphene and doped graphene samples at different doping conditions.
Fig. 3. (a) Sheet resistance; and (b) optical transmittance of doped graphene as a function of dopant type (HNO3 and Au) and concentration.
Fig. 4. (a) Sheet resistance; and (b) transmittance of graphene under various combinations of doping types and states (HNO3 for p-doping and N2H4 for n-doping).
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Fig. 5. (a) Sheet resistance; (b) transmittance of Ag NW/graphene hybrid film formed with pristine (red) and p/ptype dual-doped (blue) graphene; and (c) Schematic illustration of Ag NW/polycrystalline graphene hybrid co-
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percolating system.
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Scheme 1. Schematic illustration of the experimental procedure and structures of dual-doped graphene and silver nanowire (Ag NW)/dual-doped graphene hybrid conducting films. (a) chemical vapor deposition (CVD)-grown graphene on Cu substrate; (b) poly(methyl methacrylate) (PMMA)/graphene bilayer on Cu substrate; (c)
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PMMA/graphene bilayer; (d) PMMA/graphene bilayer with bottom doping; (e) PMMA/graphene bilayer with
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bottom doping on substrate; (f) graphene with bottom doping on substrate; (g) graphene on substrate with top and
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bottom dual-doping; (h) Ag NW/dual-doped graphene hybrid conducting film.
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Fig. 1. Scanning electronic microscopy (SEM) and digital photo-images of samples at different doping stages: (a) Pristine graphene; (b) bottom-doped graphene; (c) top/bottom dual-doped graphene; and (d) Ag NW/dual-doped
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graphene hybrid. Insets show the corresponding optical images with a scale bar of 3 cm. (e) Digital image of the as-
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formed hybrid conductor film on a plastic substrate. The dashed box indicates the hybrid conducting film.
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Fig. 2. (a) X-ray photoelectron spectroscopy (N 1s) and (b) Raman spectra of pristine graphene and doped graphene samples at different doping conditions.
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Fig. 3. (a) Sheet resistance; and (b) optical transmittance of doped graphene as a function of dopant type (HNO3 and
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Au) and concentration.
Fig. 4. (a) Sheet resistance; and (b) transmittance of graphene under various combinations of doping types and states (HNO3 for p-doping and N2H4 for n-doping).
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Fig. 5. (a) Sheet resistance; (b) transmittance of Ag NW/graphene hybrid film formed with pristine (red) and p/ptype dual-doped (blue) graphene; and (c) Schematic illustration of Ag NW/polycrystalline graphene hybrid copercolating system.
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S0169-4332(17)31156-X http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.129 APSUSC 35810
To appear in:
APSUSC
Received date: Revised date: Accepted date:
31-12-2016 3-4-2017 17-4-2017
Please cite this article as: H. Sohn, Y.S. Woo, W. Shin, D.-J. Yun, T. Lee, F.S. Kim, J. Hwang, Novel Transparent Conductor with Enhanced Conductivity: Hybrid of Silver Nanowires and Dual-Doped Graphene, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.04.129 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Novel Transparent Conductor with Enhanced Conductivity: Hybrid of Silver Nanowires and Dual-Doped Graphene
Hwange,*
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Hiesang Sohna,b*, Yun Sung Woob, Weonho Shinb, Dong-Jin Yunc, Taek Leea,*, Felix Sunjoo Kimd,*, and Jinyoung
Department of Chemial Engineering, Kwangwoon University, Seoul 01897, Korea
b
Inorganic Materials Laboratory, Samsung Advanced Institute of Technology, Suwon 443-803, Korea
Platform Technology Laboratory, Samsung Advanced Institute of Technology, Suwon 443-803, Korea
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c
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a
School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 06974, Korea
e
School of Electronics and Information Engineering, Korea Aerospace University, Goyang-si 10540, Korea
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d
*Corresponding author. E-mail address: [email protected] (H. Sohn), [email protected] (F. S. Kim), [email protected]
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(T. Lee), [email protected] (J. Hwang)
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Abstract
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We present hybrid transparent conducting films based on silver nanowires (Ag NWs) and doped graphene through
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novel dual co-doping method by applying various dopants (HNO3 or Au for p-doping and N2H4 for n-doping) on top and bottom sides of graphene. We systematically investigated the effect of dual-doping on their surface as well as electrical and optical properties of graphene and Ag NW/graphene hybrid films through the combination study with various dopant types (p/p, p/n, n/p, and n/n). We found that the p/p-type dual-doped (p-type dopant: HNO3) graphene and its hybrid formation with Ag NWs appeared to be the most effective in enhancing the electrical properties of conductor (doped graphene with ∆R/R0=84 % and Ag NW/doped graphene hybrid with ∆R/R0=62 %), demonstrating doped monolayer graphene with high optical transmittance (TT=97.4%) and sheet resistance (Rs=188 ohm/sq.). We also note that dual-doping improved such electrical properties without any significant debilitation of optical transparency of conductors (doped graphene with ∆TT=0.1% and Ag NW/doped graphene hybrid with ∆TT=0.4%). In addition, the enhanced conductivity of p-type dual-doped graphene allows a hybrid system to form co-percolating network in which Ag NWs can form a secondary conductive path at grain boundaries of polycrystalline graphene.
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Highlights 1) Ag nanowire-doped graphene hybrid prepared by novel top and bottom co-doping method
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2) Improved doping efficiency (conductivity enhancement, ∆R/R0=84 %) of graphene based on optimized ptype (HNO3) dual co-doping
3) Enhanced conductivity of doped graphene without debilitation of optical transmittance (∆TT=0.1%)
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4) The lowest Rs (188 ohm/sq.) of monolayer graphene at high optical transmittance (97.4 %)
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5) Effective resistance reduction of Ag NW on dual doped-graphene through co-percolating conduction
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Keywords: graphene; dual-doping; Ag nanowire-graphene hybrid; electrical conductivity; flexible transparent conductive film
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1. Introduction
There have been burgeoning demands for deformable transparent conductive electrodes (TCEs) for the
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application in next-generation displays and wearable electronics [1-3]. Currently, conductive oxides (e.g., indium tin
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oxide; ITO) have been widely employed in various electronic gadgets, including smart phones, flat-panel televisions, solar cells, and solid-state lighting devices because of their high optical transparency and conductivity [1-3].
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However, despite the excellent electrical and optical properties of ITO, it has a limited application in flexible TCEs since ITO is mechanically brittle and is easily cracked under bending [2,3]. Both price of its source element (indium) and production cost under high temperature and vacuum processing are also expensive for large-scale deployment. As a result, there has been a continuous and increasing demand to replace ITO with novel transparent conducting materials [1-3].
In this context, networks of silver nanowires (Ag NWs) have been considered as a cost-effective and sustainable alternative to ITO for stretchable and flexible electrodes, since Ag NW-based conductors are mechanically durable and possess higher optical transparency than ITO at equivalent sheet resistance (Rs) [2,3]. However, Ag NW networks suffer from disadvantages limiting their application in commercial devices. For instance, inherent junction resistance associated with charge transport between randomly oriented nanowires reduces the electrical conductivity [2]. Although this may be overcome by welding at overlapping junctions, such temperature
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treatment could damage substrates and underlying components [2,3]. Ag NW networks have a high contact resistance between network and active materials [2]. To address the problems of Ag NW networks, it has been a highlight to introduce Ag NWs with a
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secondary conductive nanostructure to form a hybrid for forging inter-nanowire connection and enhancing performance [4-18]. That is, the hybrids of Ag NWs and conductive nanostructures have been developed with a combination of different nanomaterials, including carbon nanotubes, conductive polymers, metal oxide, and
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graphene [4-17]. Especially, hybridization of Ag NWs with a two-dimensional graphene appears to be promising to
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address various issues. Hybrid structure shows enhanced conductivity with excellent optical properties [10-13,15,16]. Together at junctions and to substrates, Ag NWs are tightly held and adhered to the substrate, thereby hindering
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fracturing of Ag NWs under tension [13-16]. It shows high mechanical flexibility and stretchability after hybrid formation, allowing its potential use for high-performance flexible electronics. For instance, Liang et al. reported a hybrid of Ag NW and graphene prepared by solution based process of Ag NW and graphene, which exhibited high
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optical transmittance (TT) (88 %) and low Rs (22 Ω/sq.) [10]. UV light-emitting diodes based on Ag NW and graphene hybrids were demonstrated by Seo et al [11]. In another work, Kholmanov et al. presented a conductor
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with good electrical and optical properties (TT: 90 %, Rs: 64 Ω/sq.) built on a hybrid of Ag NW and chemical vapor
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deposition (CVD)-grown graphene through a modified graphene transfer approach [12]. Lee et al. also reported a high performance stretchable TCE using a hybrid of Ag NW and CVD graphene [13]. Chen et al. presented highly
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conductive free standing paper with graphene/Ag NW hybrid [14]. Recently, Park et al. presented soft transparent electrodes for an inorganic light-emitting diode fitted on a soft eye contact lens with an Ag NW and graphene hybrid [15]. All the above-mentioned studies indicate the potential of Ag NW and graphene hybrid systems towards various types of transparent electrodes, exhibiting superior electrical and optical properties to ITO [16]. To further boost the conductivity of Ag NW/graphene hybrid films, it is necessary to enhance the electrical property of either Ag NW or graphene, and reduce the contact resistance between them. However, it is difficult to enhance the electrical property of Ag NW without compromising the optical property of the hybrid because of high diffused light scattering of Ag NW [2,4,5]. Alternatively, several studies have been conducted to enhance the electrical properties of the hybrids through graphene modification. Choi et al. reported an optimized chemical doping process of the hybrid of Ag NW and graphene [17]. Lee et al. demonstrated the improved conductivity of the hybrid films by applying a non-uniform electric field on graphene [18].
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In this work, we achieved the conductivity enhancement of Ag NW/graphene hybrid films by implementing dual-doped graphene where the top and bottom sides of graphene were doped with various chemical dopants (p-type dopant: HNO3 or Au; n-type dopant: N2H4). By applying the simple dual co-doping approach of graphene, the work
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function of graphene can be effectively engineered, resulting in the conductivity enhancement of graphene and the contact resistance reduction between graphene and Ag NWs. Different from the previous bottom-doping approach based on single graphene etchant (e.g., benzimidazole) [19], our novel doping process composed of bottom and top
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co-doping has versatility in terms of chemical dopant types for bottom doping. We investigated the effects of doping
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on the materials through the systematic analyses of their surface, electrical, and optical properties. We found that, in the case of HNO3 p-doing and N2H4 n-doping, the p/p-type dual-doping is the most effective to enhance the
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conductivity, compared with other combinations (n/p, p/n, and n/n). We also observed that the p/p-type dual-doping can further improve the conductivity of Ag NW/graphene hybrid system. The enhanced conductivity of graphene by the dual doping reduces contact resistance between Ag NWs and graphene, leading to the formation of the co-
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percolating network where Ag NWs can be an additional conductive path at the grain boundary of the polycrystalline graphene. Note that Ag NW/dual-doped graphene hybrids with enhanced conductivity do not
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2. Experimental
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exhibits any significant compromise of optical transmittance.
2.1. Materials
We used polycrystalline graphene (4" Monolayer Graphene on Cu foil) purchased from Graphene square (Korea), which was grown by CVD on a Cu substrate. Ag NWs with an average length of 20 µm ± 5 µm and an average diameter of 30 nm ± 5 nm were obtained from Aiden Co. (Korea) and used without further purification. Other reagents were obtained from Sigma-Aldrich, unless otherwise specified.
2.2. Preparation of Conducting Films Conducting Films of Doped Graphene: Schematic procedures and structures of doped graphenes and hybrid conducting films are shown in Scheme 1. Briefly, as-received graphene on Cu foil was covered by a layer of poly(methyl methacrylate) (PMMA) by spin-coating and subjected to removal of its copper substrate by a Cu
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etchant (FeCl3). Subsequently, Cu substrate-etched free-standing graphene/PMMA bilayers were floated on deionized water to remove/dissolve remnant Cu etchant (FeCl3) under the graphene. For bottom doping, as-rinsed graphene/PMMA bilayers were re-floated on a doping solution of various p- or n- type dopants for 5-10 min. A
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dilute aqueous solution of HNO3 or AuCl3 was used for p-doping, and a hydrazine (N2H4) solution (1 M; 60 °C) was employed for n-doping. After bottom-doping, doped graphene/PMMA bilayers were transferred onto a polyethylene terephthalate (PET) substrate, rinsed with acetone for removal of the PMMA supporting layer, and subjected to top-
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doping. We applied a conventional vapor-phase doping process by putting graphene under HNO3 vapor for 3 min
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(p-doping) or N2H4 vapor for 10 min (n-doping). Some samples were top-side doped by applying a p-type doping solution of AuCl3 (0.1-1 M) dissolved in nitromethane through spin-coating onto the graphene. We also prepared
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various dual-doped graphene samples with a combination of doping systems (p-/p-, p-/n-, n-/p-, and n-/n-types) to study the dopant-type effects.
Hybrid Conducting Films of Ag NW and doped Graphene: We prepared the hybrid conducting film of the
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Ag NW network and graphene by a wire-bar coating. A solution of Ag NW (0.15 wt.% of Ag NWs in a coating solution) was spread onto the graphene by the Meyer rod (RDS 3; 30 mm/sec). The Rs of the Ag NW network can
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be regulated by controlling its wet thickness after solution coating. After deposition of Ag NWs on graphene, the
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2.3. Characterizations
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films were annealed at 100 °C for 5 min in the oven to completely remove the solvent.
The morphology and surface properties of the samples were characterized through various imaging and spectroscopic tools. The surface morphology of hybrid films was imaged by a scanning electron microscope (SEM; NOVA NanoSEM 630) at an accelerating voltage of 5 kV. The chemical bonds of the samples were analyzed via Xray photoelectron spectroscopy (XPS; Escalab 250) with an Al Kα (1486.6 eV) source. The Raman spectra were obtained with a Renishaw Raman Spectrometer under backscattering geometry. Spectra were averaged over 20 accumulations, and the laser power (633 nm) was kept at 2.5 mW. The optical transmittance of the films including the substrate (PET) was recorded on a haze meter (NDH 5000). The Rs was measured by an in-line four-point probe surface resistivity meter (R-CHEK, RC2175). The Rs reported in this work is an average of at least five points for each sample.
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3. Results and discussion 3.1. Transparent Conductive Films of Dual-doped Graphene and Hybrid
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Hybrid transparent conducting films demonstrated in this work were prepared by bottom-doping and topdoping of graphene followed by hybridization with Ag NWs, as illustrated in Scheme 1. The aim of our novel dual co-doping process was to enable high-performance transparent conducting films without significant debilitation of
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optical properties. First, CVD-grown graphene on a copper substrate was covered by PMMA, which acted as a
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carrier film and a protecting layer during bottom-doping (a→b). Cu substrate was then etched by Cu-etchant (FeCl3) (b→c). Note that, although FeCl3 has been shown to act as a p-type dopant for graphene [20], we have thoroughly
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washed the Cu-etchant (FeCl3) on graphene with a copious amount of water after the etching process. The surface investigation with XPS confirmed that the graphene is free from FeCl3 (Figure S2). The PMMA/graphene was subject to bottom-doping by placing the bilayer on a doping solution with a controlled concentration (c→d),
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followed by transferring onto a plastic substrate (PET) (d→e) and removal of PMMA (e→f). Unlike bottom-doping, top-doping was performed through a vapor treatment to minimize any damage of the graphene (f→g). Simple bar-
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coating resulted in a formation of the well-controlled Ag NW network onto the graphene (g→h). The as-formed Ag
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PET substrate).
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NW/dual-doped graphene hybrid films had a low Rs (188 ohm/sq.) and high transmittance (97.4%, 90.5%, including
(Scheme 1)
Fig. 1 displays the SEM images of the pristine and hybrid films obtained at different preparation stages and a digital photo image of a hybrid transparent electrode built on a plastic substrate (PET). Fig. 1a is an image of pristine graphene, while Fig. 1b and c show doped graphene. Graphene samples before hybridization show a smooth surface without any noticeable features. After single and dual-doping, it does not show any significant change in the graphene morphology. A hybrid of Ag NW and doped graphene shown in Fig. 1d shows a clear network of Ag NWs formed on the graphene. All insets included in SEM pictures are the corresponding digital images of the conducting films built on graphene, doped graphene, and their hybrid with Ag NWs. The optical images show almost similar optical transparency, indicating that the conductor is not significantly affected by chemical doping or hybridization
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with Ag NWs. Fig. 1e displays the digital image of the hybrid conducting film of Ag NWs and dual-doped graphene constructed on plastic substrate (PET), exhibiting good flexibility and high optical transmittance.
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(Fig. 1)
Fig. 2 shows the spectroscopic information of the pristine and doped graphene samples at various stages of p-type doping with nitric acid. In the XPS experiment, the nitrogen 1s signal-which was not present in the pristine
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graphene-was detected after doping. We deconvoluted the N 1s signals in the doped graphene samples through
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Gaussian fitting, and found three peaks. After bottom-doping, the peaks were observed at 406.2, 401.2, and 399.3 eV, corresponding to NOx, N-H (or pyrrolic), and C-N (pyridinic) bondings, respectively. It is believed that the
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dopant is chemically influencing the graphene, because the N 1s peak not only occurred for NOx bonding, but also for the other chemical bondings (such as pyrrolic and pyridinic bondings) [21,22]. This observation indicates that HNO3 chemically dopes the graphene with NO2 as well as N atoms [21,22]. The samples dual-doped with HNO3 for
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both bottom and top sides showed same peaks at 406.2, 401.1, and 399.2 eV. The doping effect was stronger after
each state.
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dual-doping compared to the single-doped state, based on the comparison of the ratio of CN signal to NOx signal in
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Raman analysis further confirmed the dual-doping effects of graphene by displaying peak shifts in the spectra (Fig. 2b). The G peak and the 2D peak shifted after bottom-doping and dual-doping, clearly showing that
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HNO3-doped graphene films have blue shift for both G and 2D peaks. D-band peaks were not observed at any stage of doping, indicating that HNO3 treatment was not destructive to the chemical bonds of graphene. The peak position of G band shifts upon doping [23,24]. An upward shift in the G band position is due to the phonon stiffening effect by charge extraction of p-type doping [23]. The G band (1592 cm−1) in the pristine graphene shifted by 3 cm−1 from 1592 to 1595 cm−1 after HNO3 bottom-doping. The G band in pristine graphene shifted by 11 cm−1 from 1592 to 1603 cm−1 after dual-doping. Such an additional peak shift of the Raman peak in the dual-doped graphene comes from an augmented doping effect by top- and bottom-side dual-doping [23,24]. The Raman shift in the 2D peak from 2634 to 2657 cm−1 is attributed to the doping of nitrogen atoms, consistent with the XPS result [24].
(Fig. 2)
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3.2. Electro-optical Properties of Conducting Films Fig. 3 displays the electrical and optical properties of as-formed doped graphene at different doping conditions. Electrical properties of doped graphene were measured for its Rs, while the optical properties were
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measured for the optical transmittance of graphene on a PET substrate. We investigated the electrical properties of various conductors by comparing their doping efficiencies (conductivity enhancement, ∆Rs/Rs,0), which was defined as the ratio of the change of sheet resistance after doping (∆Rs) to the original resistance (Rs,0). When the graphene
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was single-doped with HNO3, the Rs first dropped from 1.17 kΩ/sq. in the pristine state to 456 Ω/sq. in the doping
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concentration of 1 M (Fig. 3a). The Rs was steady at ~460 Ω/sq. up to 5 M, and then increased to 29 kΩ/sq. under 10 M of HNO3. Dual-doping with identical dopants (i.e., HNO3) for both top and bottom doping resulted in much lower
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Rs of 189 Ω/sq. under optimal conditions. We also tried asymmetric doping of graphene, with HNO3 as the bottom dopant and gold (Au) as the top dopant. The Rs of asymmetrically dual-doped graphene (HNO3/Au)-where the lowest Rs was achieved at 1 M AuCl3 doping-was slightly larger than that of the symmetric dual-doping (i.e. doping
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both sides with HNO3). Such an asymmetrical dual-doping of organic/inorganic dopants suggests that our system can be further extended to different dual-doping systems by judicious selection of various dopant combinations [25].
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Note that, we also compared the effect of multiple single side-doping with that of dual-doping on the electrical
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property of graphene with same dopant (HNO3) (Figure S3). As shown in Figure S3, the doping efficiency (conductivity enhancement) of graphene is not significantly improved by multiple times single-side doping. In
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addition, the doping efficiency of dual-doping (84 %) exceeds that of single-side doping (max. 65.7 %) prepared at all different treatments.
The optical transmittance (including PET substrate) of the bottom-doped graphene peaked to be 90.8 % at 0.5 M solution doping, and was higher than 90.5 % throughout the whole range studied, except for 10 M doping (Fig. 3b). In the case of graphene samples with dual symmetric doping, the highest was 90.6 % at 1 M solution doping. The samples with asymmetric doping had transmittances of 90.4-90.6 %. Note that the optimum doping condition was obtained at 1 M HNO3, where the lowest Rs was achieved while maintaining a relatively high transmittance. Asymmetrically dual-doped graphene (HNO3/Au) retained its high optical transmittance (90.4 %), which is a little bit lower than that of HNO3 dual-doped graphene.
(Fig. 3)
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Fig. 4 compares the Rs and optical transmittance of doped graphene prepared by various combinations of ptype (HNO3) and n-type (N2H4) doping systems. Interestingly, p-type doping generally resulted in lower Rs, whereas
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the resistance increased after n-type doping (Fig. 4a). Compared to pristine graphene with a typical Rs of 0.8-1.1 kΩ/sq., the resistance values after bottom-doping changed to 0.35-0.39 kΩ/sq. and 5.6-6.7 kΩ/sq. for p-type and ntype dopants, respectively. Dual-doping exhibited a resistance change according to the type of additional doping. It
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shows that p/p-doped graphene has the lowest Rs compared with the other systems, indicating the effectiveness of
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p/p-type dual-doping. A significant reduction in Rs after p-type doping was also observed when the graphene was treated by bottom n-type doping followed by top p-type doping. The sample with n/p-type dopants for bottom/top
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doping had an Rs of 1.19 kΩ/sq., which is almost less than a fifth of the resistance (6.7 kΩ/sq.) of n-type bottomdoped graphene. On the other hand, n-top-doping after p-bottom-doping increased the resistance from 0.35 kΩ/sq. to 1.16 kΩ/sq. The optical transmittance of the dual-doped graphene samples was in the range of 90.4-90.7 %. It was
with the pristine and single-doped graphenes.
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noticeable that the homogeneously dual-doped graphene samples showed almost identical transmittance compared
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The high efficiency of p/p-type dual-doping of graphene with HNO3 can be attributed to the effectively
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designed bandgap engineering. It is well known that band structure engineering of graphene is an effective way to modify the electronic properties of graphene, because pristine graphene has zero bandgap [26-29]. Typically, the
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band structure of graphene can be engineered through various dopings. In this context, the doping of graphene plays an important role in increasing carrier density as well as in changing the electronic structure. Accordingly, we designed the doping of graphene with optimal band structure with a p-type dopant. As shown in a comparative doping test based on HNO3 and N2H4 (Fig. 4), p-type dual-doping was the most effective way to enhance the conductivity of the graphene films, because the work function of graphene could be shifted to the p-type side, thereby enhancing the conductivity of the graphene [26-29]. The doping of graphene is effective not only because of the zero band gap but also the Dirac cone. The low density of states close to the Fermi level of graphene can be very efficiently filled/emptied by dopants.
(Fig. 4)
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Fig. 5 compares the Rs and optical transmittance of Ag NW/graphene hybrid films obtained by p/p-type dual-doped or un-doped graphene. After applying the same amount of Ag NWs above the percolation limit of their network on both graphene films, the Rs of the pristine graphene and the p/p-type dual-doped graphene decreased
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from 0.86 kΩ/sq. to 0.78 kΩ/sq. and from 0.32 kΩ/sq. to 0.29 kΩ/sq., respectively. The Rs value of Ag NW network on the un-doped graphene is estimated based on the equation for the parallel combination of two resistors (1/Rs,undoped-Gr + 1/Rs,AgNW = 1/Rs,hybrid, where Rs,undoped-Gr=0.86 kΩ/sq. and Rs,hybrid=0.78 kΩ/sq.), and the calculated Rs
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of Ag NW network (Rs,AgNW) on the pristine graphene hybrid film is 8.39 kΩ/sq. However, the Rs of the p/p-type
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dual-doped graphene hybrid film fabricated with the same amount of the Ag NWs on the pristine graphene is less than Rs of the parallel combination of p/p-doped graphene (Rs,doped-Gr) and the Ag NW network (Rs,AgNW). The Rs of
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the p/p-type doped graphene and Ag NW hybrid film is 0.29 kΩ/sq., while the Rs of the parallel combination of the p/p-doped graphene and Ag NW network is 0.31 kΩ/sq. (1/Rs,doped-Gr + 1/Rs,AgNW = 1/Rs,hybrid, where Rs,doped-Gr=0.32 kΩ/sq. and Rs,AgNW =8.39 kΩ/sq.). Obviously, the Ag NWs do not affect the doping state of graphene as presented in
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the Raman spectra of the Ag NW/p-type dual-doped graphene, which show no significant shift (Fig. S1). Instead, we believe that the contact resistance between the graphene and the Ag NWs (RC,Gr-Ag in Fig. 5c) decreases because of
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the enhanced conductivity of the p/p-doped graphene, resulting in further reduction of the Rs of the doped-graphene
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and Ag NW hybrid film through the co-percolating conduction [30]. In the co-percolating network, Ag NW can be a bypass of electron transport at the grain boundaries of the CVD grown polycrystalline graphene, as shown in Fig. 5c
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[30]. In addition, consistent with the results in the dual-doped graphene (Fig. 4a), the hybrid film of Ag NW and p/ptype dual-doped graphene exhibits the lowest Rs compared to other types of dopant combinations (p/p, p/n, and n/p) in hybrid films (Fig. S2), indicating the retained effectiveness of p/p-type dual-doping in hybrid system [12,19,2729]. Fig. 5b displays the optical transmittance of conductive films prepared at doped/un-doped and hybrid conditions, exhibiting good retention of the optical properties of p-type dual-doped hybrid films, comparable to hybrid of Ag NW and bare graphene. In addition, although the optical transmittance of a hybrid conductor slightly decreases (less than 0.5%) after Ag NW hybridization, we believe the hybrid conductor of graphene/Ag NW is still valid approach because additional dose of Ag NWs can further decrease the resistance of conductor without significant decrease of optical transmittance [13,15].
(Fig. 5)
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It is hard to directly compare the results of individual work prepared and tested by various conditions. Nevertheless, it is still meaningful to see where we stand in terms of electrical and optical parameters. Our
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conducting electrodes provide with comparable conductivity to the best records of previously reported similar works while out-perform in optical properties (See Table S1 in Supplementary Information) [11,12,17,31-35]. Although some previous works show superior conductance of graphene to that of our dual-doped graphene, our approach
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exhibits the lowest Rs (188 ohm/sq.) under the condition of high transmittance (97.4 %; 90.6 % including PET
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substrate), and higher doping efficiency (~84 %) at monolayer-level graphene. The hybrids of doped graphene and Ag NWs also show enhanced conducting properties and high optical transparency compared with reported results,
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indicating that our method is a valid approach to enhancing conductivity without compromising optical transmittance.
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4. Conclusions
In this work, we have demonstrated Ag NW/dual-doped graphene hybrids with enhanced conductivity,
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where the top and bottom sides of the graphene were doped with p-type dopants (HNO3) to engineer effective work
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function of the dual-doped graphene. In addition, such p-type dual-doped graphene and its hybrids were found as the most effective to enhance conductivity based on systematic and comparative investigation of the doping effect on
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the graphene and graphene/Ag NW hybrid films. Compared with the current state-of-the-art in Ag NW and graphene hybrids, this work made several breakthroughs, summarized as follows.
1.
Novel dual co-doping method to improve doping efficiency (conductivity enhancement): Our novel dualdoping method based on top and bottom co-doping of graphene was quite effective to enhance the doping efficiency of monolayer graphene based conductor (Rs: 188 ohm/sq.) by 84 % compared to pristine graphene, without debilitation of its original optical transmittance (dual-doped graphene: 97.4 %, pristine graphene: 97.5 %).
2.
Optimized dual co-doping system: Under the varied combinations of dopant types for dual-doping (p/p, p/n, n/p, and n/n) with HNO3 for p-doping and N2H4 for n-doping, we found that the p/p-type dual-doping
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among other combinations was the most effective in improving the electrical property of graphene and its hybrid with Ag NWs. 3.
Enhanced conductivity of Ag NW networks on doped-graphene hybrids: The hybrid conductor of Ag NWs
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on dual-doped graphene exhibits improved electrical conductivity (62 % reduction of sheet resistance) and somewhat retained optical transparency (Ag NW/dual-doped graphene hybrid: 90.1 % including substrate) compared with Ag NW/native graphene hybrid (90.3 % including substrate) because of further resistance
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reduction through the co-percolating conduction.
Current result indicates that our novel hybrid conductor of Ag NW/dual-doped graphene perform comparably to
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most of the reported works in conductivity, and out-perform in optical properties. We believe that our facile doping and hybridization approach can open a new direction to high-performance transparent conducting electrodes for
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flexible displays, solar cells, and wearable electronic devices fitted on a curved substrate.
Acknowledgements
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H. S acknowledge that present research has been conducted by the research grant of Kwangwoon
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University in 2017. F.S.K acknowledges the support of National Research Foundation of Korea (NRF) program by
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the Korea government Ministry of Science, ICT, and Future Planning (NRF-2014M3A7B4051749).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/xxx.
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Figure Captions
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Scheme 1. Schematic illustration of the experimental procedure and structures of dual-doped graphene and silver nanowire (Ag NW)/dual-doped graphene hybrid conducting films. (a) chemical vapor deposition (CVD)-grown
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graphene on Cu substrate; (b) poly(methyl methacrylate) (PMMA)/graphene bilayer on Cu substrate; (c) PMMA/graphene bilayer; (d) PMMA/graphene bilayer with bottom doping; (e) PMMA/graphene bilayer with
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bottom doping on substrate; (f) graphene with bottom doping on substrate; (g) graphene on substrate with top and
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bottom dual-doping; (h) Ag NW/dual-doped graphene hybrid conducting film.
Fig. 1. Scanning electronic microscopy (SEM) and digital photo-images of samples at different doping stages: (a)
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Pristine graphene; (b) bottom-doped graphene; (c) top/bottom dual-doped graphene; and (d) Ag NW/dual-doped
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graphene hybrid. Insets show the corresponding optical images with a scale bar of 3 cm. (e) Digital image of the as-
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formed hybrid conductor film on a plastic substrate. The dashed box indicates the hybrid conducting film.
Fig. 2. (a) X-ray photoelectron spectroscopy (N 1s) and (b) Raman spectra of pristine graphene and doped graphene samples at different doping conditions.
Fig. 3. (a) Sheet resistance; and (b) optical transmittance of doped graphene as a function of dopant type (HNO3 and Au) and concentration.
Fig. 4. (a) Sheet resistance; and (b) transmittance of graphene under various combinations of doping types and states (HNO3 for p-doping and N2H4 for n-doping).
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Fig. 5. (a) Sheet resistance; (b) transmittance of Ag NW/graphene hybrid film formed with pristine (red) and p/ptype dual-doped (blue) graphene; and (c) Schematic illustration of Ag NW/polycrystalline graphene hybrid co-
an
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percolating system.
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Scheme 1. Schematic illustration of the experimental procedure and structures of dual-doped graphene and silver nanowire (Ag NW)/dual-doped graphene hybrid conducting films. (a) chemical vapor deposition (CVD)-grown graphene on Cu substrate; (b) poly(methyl methacrylate) (PMMA)/graphene bilayer on Cu substrate; (c)
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PMMA/graphene bilayer; (d) PMMA/graphene bilayer with bottom doping; (e) PMMA/graphene bilayer with
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bottom doping on substrate; (f) graphene with bottom doping on substrate; (g) graphene on substrate with top and
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bottom dual-doping; (h) Ag NW/dual-doped graphene hybrid conducting film.
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Fig. 1. Scanning electronic microscopy (SEM) and digital photo-images of samples at different doping stages: (a) Pristine graphene; (b) bottom-doped graphene; (c) top/bottom dual-doped graphene; and (d) Ag NW/dual-doped
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graphene hybrid. Insets show the corresponding optical images with a scale bar of 3 cm. (e) Digital image of the as-
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d
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an
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formed hybrid conductor film on a plastic substrate. The dashed box indicates the hybrid conducting film.
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Fig. 2. (a) X-ray photoelectron spectroscopy (N 1s) and (b) Raman spectra of pristine graphene and doped graphene samples at different doping conditions.
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Fig. 3. (a) Sheet resistance; and (b) optical transmittance of doped graphene as a function of dopant type (HNO3 and
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Au) and concentration.
Fig. 4. (a) Sheet resistance; and (b) transmittance of graphene under various combinations of doping types and states (HNO3 for p-doping and N2H4 for n-doping).
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Fig. 5. (a) Sheet resistance; (b) transmittance of Ag NW/graphene hybrid film formed with pristine (red) and p/ptype dual-doped (blue) graphene; and (c) Schematic illustration of Ag NW/polycrystalline graphene hybrid copercolating system.
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