carbon nanotube films

Accepted Manuscript Transparent and Conductive Hybrid Graphene/Carbon Nanotube Films Alexandra L. Gorkina, Alexey P. Tsapenko, Evgenia P. Gilshteyn, Tatiana S. Koltsova, Tatiana V. Larionova, Alexander Talyzin, Anton S. Anisimov, Ilya V. Anoshkin, Esko I. Kauppinen, Oleg V. Tolochko, Albert G. Nasibulin PII:

S0008-6223(16)30035-5

DOI:

10.1016/j.carbon.2016.01.035

Reference:

CARBON 10666

To appear in:

Carbon

Received Date: 29 December 2015 Accepted Date: 11 January 2016

Please cite this article as: A.L. Gorkina, A.P. Tsapenko, E.P. Gilshteyn, T.S. Koltsova, T.V. Larionova, A. Talyzin, A.S. Anisimov, I.V. Anoshkin, E.I. Kauppinen, O.V. Tolochko, A.G. Nasibulin, Transparent and Conductive Hybrid Graphene/Carbon Nanotube Films, Carbon (2016), doi: 10.1016/ j.carbon.2016.01.035. 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.

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Transparent and Conductive Hybrid Graphene/Carbon Nanotube Films

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Alexandra L. Gorkinaa, Alexey P. Tsapenkoa, Evgenia P. Gilshteyna, Tatiana S. Koltsovab, Tatiana V. Larionovab, Alexander Talyzinc, Anton S. Anisimovd, Ilya V. Anoshkine, Esko I. Kauppinene, Oleg V. Tolochkob, Albert G. Nasibulina,e* a

Skolkovo Institute of Science and Technology, Nobel str. 3, 143026, Moscow, Russia Peter the Great St.Petersburg Polytechnic University, Polytechnicheskaya 29, 195251 St. Petersburg, Russia c Umeå University, SE-901 87 Umeå, Sweden d Canatu Ltd., Konalankuja 5, 00390, Helsinki, Finland e Aalto University, Department of Applied Physics, 00076 Aalto, Finland

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Abstract

Carbon nanomaterials (carbon nanotubes (CNTs) and graphene) are promising materials for optoelectronic applications, including flexible transparent and conductive films (TCFs) due to their extraordinary electrical, optical and mechanical properties. However, the performance of CNT- or graphene-only TCFs still needs to be improved. One way to enhance the optoelectrical properties of TCFs is to hybridize CNTs and graphene. This

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approach leads to creation of a novel material that exhibits better properties than its individual constituents. In this work, the novel hybrid CNT-graphene nanomaterial was fabricated by graphene oxide deposition on top of CNT films. The graphene oxide was then reduced by thermal annealing at ambient atmosphere or in H2 atmosphere. At the

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final step the CNT-graphene hybrids were chemically doped using gold(III) chloride. As a result, we show that the hybrids demonstrate excellent optoelectrical performance with

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the sheet resistance as low as 73 Ω/ at 90% transmittance.

* Corresponding author: +7 916 69 03 812; E-mail: [email protected] (Albert Nasibulin)

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1. Introduction Transparent conductive films (TCFs) are widely used in electronic devices, such as organic light emitting diodes (OLEDs), liquid crystal displays (LCDs), touch screens and

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solar cells [1]. The demand for these devices has been growing steadily over the last

years; for example, the number of touch screen shipments is expected to reach 2.8 billion units in 2016 [2]. Nowadays the dominant materials for TCFs are doped metal oxides, most commonly indium tin oxide (In2O3:Sn, ITO), which provides excellent electrical

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conductivity, while being optically transparent. Metal oxides, however, have several drawbacks, including high refractive index and haze, spectrally non-uniform optical

transmission and restricted chemical robustness. Moreover, they are scarce and require

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expensive preparation methods such as vacuum sputtering or pulsed laser deposition. The use of the metal oxides in future flexible and wearable devices is impeded due to their limitations in mechanical properties.

The aforementioned disadvantages of ITO triggered numerous efforts in discovering

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alternative materials for TCFs. According to Displaybank, non-ITO TCFs will make up to 34% of transparent conductive film market in 2017. Materials such as conductive polymers [3], carbon nanomaterials [4-6], metal nanogrids [7], silver nanowires (AgNWs) [8] and copper nanowires (CuNWs) [9] were explored as potential

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replacements for ITO.

Carbon nanotubes (CNTs) and graphene are particularly promising as potential ITO

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replacements in TCFs due to their unique properties. They have neutral color (in contrast to the yellowish color of ITO) [6], can be easily transferred to flexible substrates such as polyethylene terephthalate (PET) [10] and retain their excellent conductivity even after repeated bending [11]. However, CNT- and graphene-based TCFs have to demonstrate optoelectrical performance that matches or is superior to that of ITO-based TCFs. This result has not been achieved yet and further work on improving nanocarbon-based TCFs is required.

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One approach to improve the performance of CNT- and graphene-based TCFs is to create hybrid materials, which take advantage of the synergistic effects between CNTs and graphene [12-15]. Since the conductivity of the CNT films is limited by the junction resistance (resistance between individual CNTs), it is expected that the electrical

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performance can be significantly improved by placing graphene on the surface of CNTs due to the appearance of large area π-stacking interaction between the carbon

nanomaterials. Several methods for fabrication of such hybrids were reported. Solutionbased methods usually include utilization of ultrasonication of graphene or graphene

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oxide (GO) and CNTs and, if required, reduction of GO to reduced graphene oxide (rGO) [16-18]. The latter can be achieved by various methods, which are thoroughly reviewed

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elsewhere [19].

Another approach to create the CNT-graphene hybrids was realized by chemical vapor deposition (CVD) method [20-22]. Hybrid films fabricated by sequential self-assembly were also reported [23-26]. Apart from the in situ growth of CNTs on graphene, CVDgrown materials can be combined after the synthesis process [27, 28]. CVD allows to

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control the morphology and properties of the hybrids, while solution-based methods do not require complex processing and can be utilized for commercially available materials.

The goal of this work is to elaborate a simple method appropriate for high-volume

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production of highly conductive transparent films. We report a novel, scalable and commercially feasible process for the fabrication of hybrid CNT-graphene TCFs using

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aerosol-synthesized CNTs and commercially available graphene oxide solutions. First, a SWNT film is deposited onto a substrate, then a thin layer of graphene oxide is deposited on top of the SWNT film via spray deposition. The next step is the reduction of GO, followed by chemical doping of the hybrid with AuCl3. This method yields transparent and conductive films with state-of-the-art sheet resistance value of 73 Ω/ at 90% transmittance (at 550 nm).

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2. Experimental

2.1. Materials

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Aqueous dispersion of graphene oxide with a concentration of 0.1 mg/ml was prepared

using graphite oxides synthesized by a modified Hummers’ method and purchased from Akkolab, Moscow. According to the information provided by the manufacturer, the GO

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has the following elemental composition: 58.0 ± 1.0% C, 1.5 ± 0.5% H, 39.0 ± 1.0% O.

Randomly oriented SWNT films with high purity were synthesized by the aerosol

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(floating catalyst) CVD method described elsewhere [29, 30]. Briefly, the method is based on ferrocene vapor thermal decomposition in the atmosphere of CO at the temperature of 880 ˚C. The as-synthesized SWNTs were collected by passing the flow through microporous filters downstream of the reactor. The supplied SWNT films were deposited on nitrocellulose filters. The measured optical transmittances (at 550 nm) of the

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films, depending on their thickness, were 95, 90, 85, and 80%.

Gold(III) chloride trihydrate (>99.9% metal basis) and acetonitrile (99%, cat. II) were purchased from Sigma-Aldrich.

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2.2. Fabrication of SWNT/rGO hybrids

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SWNT films were transferred from the nitrocellulose filters to quartz substrates using the dry transfer technique described by Kaskela et al. in [9]. A spray-coating technique (schematically illustrated in Fig. S1) was used to achieve the deposition of graphene oxide flakes onto quartz substrates or carbon nanotube films. An airbrush (JAS 1142 with the nozzle diameter of 0.3 mm and a JAS 1202 pump) was used to apply water dispersion of GO sheets onto heated SWNT films or quartz substrates. The inlet pressure difference was set to 1 bar. The distance between the tip of the nozzle and the substrate was maintained at 10 cm. As the air valve on the airbrush was triggered, the GO dispersion was atomized into small droplets, which were carried by air onto the preheated substrate.

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Since graphene oxide is an insulator, its reduction is required. Two methods were applied here to perform the reduction of graphene oxide: 1) thermal annealing at ambient atmosphere by heating GO on a hot plate (further referred to as ambient atmosphere

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reduction), 2) high-temperature annealing in hydrogen atmosphere (further referred to as H2 reduction). For the ambient atmosphere reduction samples were annealed at 300°C for 8 minutes under normal atmosphere using an IKA C-MAG HP 7 hot plate. The samples were then dried in air for 20 minutes before further characterization. For H2 reduction, the

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samples were kept in 90% Ar, 10% H2 atmosphere at 1000°C or 750°C. The gas flow

was set to 135 cm3/min and 15 cm3/min for Ar and H2, respectively. The annealing time

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was varied from 5 to 32 minutes.

The final step for the fabrication of SWNT/rGO hybrids was chemical doping with gold(III) chloride. AuCl3 was dissolved in acetonitrile at the concentration of 15 mmol/L. A 20 µL droplet of solution was then cast onto the prepared SWNT-rGO hybrid and dried

2.3. Characterization

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in air for 10 minutes.

Sheet resistance, RS, of the samples was measured using a linear four-point probe (with

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the distance of 1 mm between the electrodes) connected to a current source and a digital voltmeter (Jandel RM3000 Test Unit). The optical absorbance, A, measurements were

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performed using an Ocean Optics QE Pro spectrometer with a halogen light source (Ocean Optics DH-2000) and a Perkin-Elmer LAMBDA 750 UV/Vis/NIR spectrophotometer. Spectrum from a clean quartz substrate was used to eliminate the substrate contribution. The measurements were performed at a wavelength of λ = 550 nm, at which the human eyes exhibit the highest sensitivity [31]. To characterize the resistance that a given film would have at the transmittance of T = 90% a figure of merit, K = 1/(RS A), or equivalent resistance, RE = 1/(K log10(10/9)), can be used [32]. In this work, various transparent conductive films were compared using the equivalent resistance values.

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Contact angle, θ, was used to determine the degree of GO reduction: graphene is a hydrophobic material [33, 34], while graphene oxide is hydrophilic one. Therefore, the contact angle naturally increases with GO reduction. The contact angle was measured by

solid and the drop profile at the three-phase point.

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placing a 20 µL droplet atop the sample surface and measuring the angle between the

Scanning electron microscopy (SEM) was used to characterize the morphology of the

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samples. SEM images were acquired using 110 FEI Quanta 3D FEG and FEI Helios 650 Nanolab microscopes. Transmission electron microscopy (TEM) using a FEI Tecnai G2

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F20 microscope was used to investigate the structure and geometry of graphene oxide flakes. The microscope was operated at the accelerating voltage of 80 kV. TEM samples were deposited on TEM grids by either spray-coating or dip-coating. X-ray photoelectron spectroscopy (XPS) was used to study the elemental composition of the samples.

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3. Results and discussion

3.1. Materials characterization

The structure of graphene oxide was characterized using TEM (Fig. 1) and SEM (Fig.

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S2). It has been confirmed that GO sheets are stacked, and these stacks can consist of one to twenty layers. Absorbance spectroscopy, SEM and TEM were used to characterize the

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structure of CNT films. The diameter of the nanotubes was determined from the absorbance spectrum. The M11 transition occurred at the wavelength of λ = 950 nm, which corresponds to the transition energy of E = 1.3 eV, the S22 transition occurred at λ = 1360 nm (E = 0.91 eV), and the S11 transition – at λ = 2480 nm (E = 0.5 eV). These transition energies correspond to the tube diameter of approximately 2.1 nm. This value was confirmed by the direct measurements of SWNT diameters using TEM. Since absorbance is a linear function of the SWNT film thickness [9], the thickness of the film, h, can be calculated as h = 417 × Aλ = 550 nm [35]. For a film with T = 90%, the calculated

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thickness is 19 nm. The films demonstrated sheet resistance of 368±51 Ω/ at the transmittance of 88±0.7 %.

The structure of the SWNT-rGO hybrids was characterized by SEM (Fig. 1, d, and Fig.

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S3) and TEM (Fig. 1, c). The electron diffraction pattern (inset in Fig. 1, c) shows five hexagonal patterns and, therefore, confirms that graphene layers are overlapped. SEM

imaging of the scratched hybrid sample reveals that the rGO sheets form a thin, uniform

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layer on top of the SWNT films.

Fig. 1. Materials characterization: (a) TEM micrograph of a SWNT film, (b) TEM micrograph of graphene oxide film, (c) TEM micrograph of SWNT/rGO hybrid with an electron diffraction pattern, taken from the rGO area and showing mismatch in the graphene layers, (d) SEM micrograph of scratched surface of an SWNT/rGO hybrid.

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3.2. Spray deposition of GO

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Fig. 2. Materials characterization: (a) Raman spectra of a SWNT film, GO film and SWNT/GO hybrid; (b) UVvis-NIR absorbance spectra of a pristine SWNT film (black), an undoped hybrid SWNT-rGO film (red), pristine GO film (green) and rGO film (orange). The hybrid film demonstrated higher absorbance than SWNT film at shorter wavelengths of 180-1800 nm, while the S11 peak is significantly less pronounced for the hybrid film.

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For the selection of an optimal temperature of the substrate, graphene oxide was deposited on conductive aluminum substrates and the resulting samples were analyzed using SEM. The distance between the airbrush and the substrate, as well as the feed rate, were kept constant. At lower temperatures, such as 100 and 125°C, the droplets are

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agglomerating and the resulting droplet size is more than 100 µm. This leads to highly non-uniform coverage of the substrate with GO sheets. When the substrate is heated to

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250°C and higher, the water evaporates before the droplet reaches the substrate. Consequently, GO sheets, that tend to stay within the droplet, become crumpled, and form 3D structures on the substrate surface. This is unacceptable for the CNT-graphene hybrids for TCFs, since for better performance the graphene sheets have to form a flat layer on top of CNTs.

One more phenomenon can be noticed at temperatures of 200 to 250°C: so-called “coffee rings”, ringlike stains along the droplets’ contact lines, start forming. This was explained by higher liquid evaporation rate at the droplet’s edge [36]. Formation of coffee rings

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leads to non-uniform distribution of GO flakes on the substrate’s surface and is, therefore, detrimental to the material’s conductivity. Considering all of the above, substrate temperatures of 150 to 200°C were considered optimal for graphene oxide spray deposition. In this range, the GO sheets are distributed uniformly on the substrate surface.

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Further experiments were carried out with the substrates heated to 200°C.

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Fig. 3. The effect of substrate temperature on the morphology of spray-deposited graphene oxide films: SEM micrographs of GO on aluminum. Low substrate temperatures (100-125°C) correspond to non-uniform coverage of the substrate, high substrate temperatures (250-300°C) correspond to crumpling of GO sheets. Temperatures of 150-200°C are considered optimal for GO deposition.

3.3. Thermal reduction of GO

Two methods for GO reduction were evaluated in this study: ambient atmosphere reduction and H2 reduction. Even though full reduction of GO cannot be achieved by heating in the ambient atmosphere, this method is low-cost, fast and does not require special equipment or use of hazardous chemicals. During ambient atmosphere reduction,

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partially oxidized carbon is transformed into CO2 and fully reduced graphene [37]. The following parameters were identified to be important for the thermal reduction at ambient conditions: 1) temperature, 2) treatment time. These two parameters have opposite effect on the thermal reduction of GO: the higher the temperature, the lower is the treatment

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time required for maximum achievable reduction. A series of experiments was therefore performed at a fixed temperature of 200°C with treatment times varied from 0.5 to 32 minutes (see supplementary material). The contact angle, θ, and the transmittance

change, ∆T, were used to evaluate the degree of reduction of the GO films. The contact

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angle increases until the treatment time of 8 minutes is reached, and starts decreasing

after 8 minutes. This behavior likely indicates that at 200°C, oxidation starts to progress

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at annealing times longer than 8 min. The maximum contact angle achieved during the thermal annealing at ambient atmosphere is 58°, which is significantly lower than the anticipated contact angle for water on graphene of 90−100° [38]. The temporal evolution of the transmittance change shows that at t = 8 min transmittance stops decreasing. Therefore, the reduction time of 8 min was considered to be optimal for the thermal

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reduction at the ambient atmospheric treatment at 200°C.

The mean sheet resistance of hybrid SWNT/rGO films obtained by ambient atmosphere reduction is lower than that of SWNT-only films (302 Ω/ for hybrids and 368 Ω/ for SWNT films), their equivalent resistance is higher due to the decrease in transmittance. It

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can be concluded that the degree of reduction (and the resulting increase in conductivity) achieved by ambient atmosphere reduction is not enough to account for the decrease in

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optical transmittance. The performance of these films was further improved by chemical doping.

Thermal annealing in the H2 atmosphere was performed for samples with various SWNT and GO film thicknesses. Similarly to ambient atmosphere reduction, the quality of hybrid materials depends on the annealing time and annealing temperature. An XPS study was performed to compare the reduction quality for the samples reduced in H2 and at ambient atmosphere. Survey O1s and C1s XPS spectra are presented in Fig. 4. C1s spectra show that the SWNT-GO sample contains the biggest amount of oxygen-

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containing carbon bonds. The amount of oxygen containing bonds is significantly lower in the sample that was reduced at ambient atmosphere. The sample that reduced in H2 has the lowest amount of carbon-oxygen bonds, which suggests that this method is more effective at removing oxygen-containing functional groups and reducing graphene oxide.

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The results of the XPS studies are in agreement with the analysis of optoelectrical

performance of the samples. The samples produced by annealing in H2 atmosphere

demonstrate lower equivalent resistance: their mean sheet resistance is RE = 264 Ω/ ,

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while the samples reduced in air had mean value of RE = 485 Ω/ .

The structure of the samples was studied using SEM and AFM. An AFM micrograph and

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a height profile of a SWNT-rGO hybrid obtained by reducing GO in H2 is presented in

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Fig. S4.

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Fig. 4. XPS spectra of SWNT-GO and SWNT-rGO hybrids. (a) Comparison of XPS survey spectra of SWNT-GO, SWNT-rGO (reduced in ambient atmosphere) and SWNT-rGO (reduced in H2) hybrids, (b) O1s XPS spectra of SWNT-GO, SWNT-rGO (reduced in ambient atmosphere) and SWNT-rGO (reduced in H2) hybrids, (c) C1s XPS spectra of SWNT-GO and SWNT-rGO hybrids. The sample that was annealed in H2 atmosphere has the lowest amount of carbon-oxygen bonds, which suggests that this method is more effective at removing oxygen-containing functional groups and reducing graphene oxide.

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3.4. Chemical doping of SWNT/rGO hybrids

Chemical doping with AuCl3 was performed for all hybrid samples. The results of doping in terms of optoelectrical performance of the samples are presented in Fig. 5. Both sheet resistance and transmittance of the samples decrease significantly after the AuCl3 doping. Despite the decrease in the transmittance, an equivalent resistance of the doped samples is significantly lower: median equivalent resistance for all samples before doping is 567

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Ω/ , after doping – 108 Ω/ , which constitutes a five-fold decrease in the equivalent resistance.

The results of chemical doping are different for the samples that reduced in H2 and for

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those that were reduced in ambient atmosphere (Fig. 5). Even though XPS studies show that H2 reduction was more effective at reducing GO, the samples produced by this

method demonstrate higher equivalent resistance after AuCl3 doping (Fig. 5, c). The

mean RE of the samples produced by annealing in H2 and AuCl3 doping is 138 Ω/ , while

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the mean RE of the samples produced by ambient atmosphere reduction and AuCl3

doping is 98 Ω/ . This effect can be attributed to the fact that during H2 reduction some

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of the structural defects are being repaired. The resulting SWNT/rGO hybrids have better optoelectrical performance, but are significantly less susceptible to chemical doping.

The best optoelectrical performance achieved in this work is 73 Ω/ at 90% transmittance. This sample was acquired by ambient atmosphere reduction and subsequent AuCl3 doping. This film demonstrates significantly improved optoelectrical

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performance as opposed to SWNT-only film (440 Ω/ at 90% transmittance), AuCl3doped rGO film (12.2 kΩ/ at 90% transmittance) or AuCl3-doped SWNT film (127 Ω/ at 90% transmittance). Compared with optoelectrical performance of CNT/graphene hybrids reported in literature (see Fig. 6), the samples produced in this work also show

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superior performance: Bittolo Bon and coworkers achieved sheet resistance of 409 Ω/ at

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90% transmittance [17], Kholmanov et al. — 291 Ω/ [27], Tung et al. — 344 Ω/ [16].

The optoelectrical performance of the fabricated SWNT/rGO hybrids this work is already better than the performance of ITO on PET substrates [1] and satisfies most of the requirements for device electrodes [7].

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Fig. 5. Effect of AuCl3 doping on SWNT/rGO hybrids: (a) Median equivalent resistance before doping is 567 Ω/□, after doping – 108 Ω/□, which constitutes a five-fold decrease in equivalent resistance, (b) AuCl3 doping is significantly more effective for the samples that were reduced in ambient conditions: these samples demonstrate lower equivalent resistance after doping than the samples that were reduced in hydrogen, (c) hybrid SWNT-rGO films demonstrate significantly lower resistance after AuCl3 doping, (d) comparison of equivalent resistance of SWNT-only films and SWNT-rGO hybrids before and after doping with AuCl3.

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Fig. 6. Comparison of optoelectrical performance of CNT/graphene hybrids reported in literature and in present work. Dashed lines indicated theoretical relationship between transmittance and sheet resistance and can be used to calculate equivalent sheet resistance at 90% T.

The optoelectrical performance of SWNT/rGO hybrids obtained by the method introduced in this work can be further improved. One way to increase conductivity without sacrificing transparency of the film is using monolayer GO (or graphene) flakes.

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Another route is to choose a GO deposition method that allows for uniform distribution of GO flakes on SWNT film, e.g. Langmuir-Blodgett assembly. Moreover, using GO

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with large flakes might yield films with better conductivity.

4. Conclusions

A novel method for the fabrication of hybrid SWNT-rGO transparent conductive films has been introduced. The method involves dry deposition of an SWNT film onto a substrate, spray deposition of GO, its subsequent thermal reduction and, finally, chemical doping with AuCl3. Two methods for thermal reduction of graphene oxide were evaluated: ambient atmosphere reduction and H2 reduction. It has been shown that H2

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reduction is a more efficient method for GO reduction. However, chemical doping has more influence on the samples that were reduced at ambient atmosphere. This process yields hybrid films with the state-of-the-art performance: sheet resistance as low as 73

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Ω/ at 90% transmittance.

Fabricated films satisfy most of the requirements for applications as TCFs in various devices. Moreover, the SWNT-rGO hybrids show better performance than reduced

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graphene oxide or than rGO-CNT hybrids reported previously.

The method introduced in this study has several advantages over the other methods for

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fabrication of CNT/graphene hybrids. Firstly, it can be used with commercially available SWNT films and GO dispersions. Moreover, large area films can be fabricated by spray deposition, which is important for potential scale-up for industrial applications. Optoelectrical characteristics of the hybrid films can be fine-tuned by adjusting the individual thicknesses and morphologies of SWNT and GO films.

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Acknowledgements

We thank Dr. Alexey A. Goryunkov from Lomonosov Moscow State University for providing expertise, advice and laboratory facilities for the experiments. This research

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was supported by the Ministry of Education and Science of Russian Federation under the

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grant agreement no. 1425320 (Project DOI: RFMEFI58114X0006).

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