Transparent conductive reduced graphene oxide thin ...

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Nov 13, 2014 - ... SUN ZhiPei, ZHOU YueLiang, JIN KuiJuan and YANG GuoZhen ... *Corresponding author (WANG Can, email: [email protected]; JIN ...
Transparent conductive reduced graphene oxide thin films produced by spray coating SHI HongFei, WANG Can, SUN ZhiPei, ZHOU YueLiang, JIN KuiJuan and YANG GuoZhen Citation: SCIENCE CHINA Physics, Mechanics & Astronomy 58, 014202 (2015 ); doi: 10.1007/s11433-014-5614-y View online: http://engine.scichina.com/doi/10.1007/s11433-014-5614-y View Table of Contents:http://engine.scichina.com/publisher/scp/journal/SCPMA/58/1 Published by the Science China Press

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SCIENCE CHINA Physics, Mechanics & Astronomy • Article •

January 2015 Vol. 58 No. 1: 014202 doi: 10.1007/s11433-014-5614-y

Transparent conductive reduced graphene oxide thin films produced by spray coating† SHI HongFei1, WANG Can1*, SUN ZhiPei2, ZHOU YueLiang1, JIN KuiJuan1,3* & YANG GuoZhen1,3 1

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China; 2 Department of Micro- and Nanosciences, Aalto University, PO Box 13500, FI-00076 Aalto, Finland; 3 Collaborative Innovation Center of Quantum Matter, Beijing 100190, China Received September 21, 2014; accepted October 10, 2014; published online November 13, 2014

Reduced graphene oxide thin films were fabricated on quartz by spray coating method using a stable dispersion of reduced graphene oxide in N,N-Dimethylformamide. The dispersion was produced by chemical reduction of graphene oxide, and the film thickness was controlled with the amount of spray volume. AFM measurements revealed that the thin films have near-atomically flat surface. The chemical and structural parameters of the samples were analyzed by Raman and XPS studies. It was found that the thin films show electrical conductivity with good optical transparency in the visible to near infrared region. The sheet resistance of the films can be significantly reduced by annealing in vacuum and reach 58 k with a light transmittance of 68.69% at 550 nm. The conductive transparent properties of the reduced graphene oxide thin films would be useful to develop flexible electronics. reduced graphene oxide, transparent conductive film, spray coating PACS number(s): 72.80.-r, 78.20.Ci, 81.05.Uw, 73.61.Ph, 8.66.Qn Citation:

Shi H F, Wang C, Sun Z P, et al. Transparent conductive reduced graphene oxide thin films produced by spray coating. Sci China-Phys Mech Astron, 2015, 58: 014202, doi: 10.1007/s11433-014-5614-y

Graphene has attracted much attention due to its unique electron transport and optical properties. This one-atomthick material has a ‘minimum’ conductivity of ~4 e2/h even when the carrier concentration tends to be zero [1]. Mobility as high as 106 cm V1 s1 has been observed in suspended pristine graphene [2]. The sheet resistance of an intrinsic single layer graphene is about 6 k/□ [1]. Although the value is not low enough, graphene can be staked and doped to achieve a lower sheet resistance value. The optical transmittance of a suspended single layer graphene can reach 97.7% in a quite broad band from 300 to 2500 nm. Five layers of graphene have a transmittance of 88.5%. Assum*Corresponding author (WANG Can, email: [email protected]; JIN KuiJuan, email: [email protected]) †Contributed by JIN KuiJun (Associate Editor) © Science China Press and Springer-Verlag Berlin Heidelberg 2014

ing carrier concentration ~3.4×1012 cm2, mobility ~2×104 cm2 V1 s1, the theoretically predicated sheet resistance is only 16.7 Ω/□ [2]. The level of doping concentration and mobility is attainable in a laboratory, making graphene a promising candidate for indium tin oxide as transparent conductive films. Graphene oxide (GO) can be viewed as a functionalized analogue of graphene [3]. The oxygen-containing functional groups, both on the basal plane and at the edges, can be removed by chemical or thermal reduction [4]. The reduced graphene oxide (RGO) thus possesses many characteristics of graphene, e.g., transparent, conductive [5], photo-electronic [6], and field effect [7]. Once viewed only as a precursor for graphene, RGO is now attracting chemists’ and physicists’ attention for its own properties [8–10]. Electrical phys.scichina.com

link.springer.com

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Shi H F, et al.

Sci China-Phys Mech Astron

and photonic properties of RGO can be modified by changing the cover rate of functional groups on the carbon basal plane [5,11,12]. This makes RGO a novel platform for optoelectronic devices. For example, slightly reduced GO possesses an enhanced blue photoluminescence [13]. Solution processed RGO thin films have been proved to be a promising candidate for nanoscale photodetectors and photoelectronic switches [14]. Nonlinear optical properties of RGO thin films are tunable from saturable absorption to two photo absorption for broadband (from UV to near-infrared) [15]. RGO thin films have also been used in nonvolatile memories [16], super-capacitors [17], solar cells [18], and so on. There are several methods for producing RGO films from RGO solution: spin-coating [19], spray-coating [20], ink jet printing [21], membrane-filtration [22], and Langmuir-Blodgett method [23]. The spray-coating method we used is simple and easily controllable. In this study, we have produced ultra-stable RGO dispersion and have made flat RGO thin films with spray-coating method. Chemical and structural characterizations, as well as the effect of annealing, have been investigated on the films.

1 Experiment We started from GO produced by the modified Hummers method [24,25]. 3 mg of GO powder was dispersed in 1 mL de-ionized water by mild sonication. After adding 9 mL N,N-Dimethylformamide (DMF) and 34 L phenylhydrazine, the light-brown suspension was put in a 80°C water bath, and stirred for 12 h. The resulting RGO/DMF suspension, mainly containing single layer RGO sheets, was homogeneous and ultra-stable with no aggregation over months (shown in Figure 1(a)). The ultra-stability of the dispersion is probably due to the fact that the phenyl group of phenylhydrazine is covalently bonded with the RGO sheets, causing a steric effect that separates the sheets away from each other [26]. Further diluted RGO/DMF suspension was sprayed onto 190°C preheated quartz substrates by an airbrush [20] (as shown in Figure 1(b)). The solvents were vaporized rapidly and the remaining RGO sheets formed a uniform thin film on the quartz substrate. To further remove the solvents, all thin film samples were dried in a vacuum oven at 150°C for 12 h after spray-coating. The thickness of the RGO thin film was controlled by varying the spay volume of the RGO/DMF solution. To investigate the possible annealing effects on optical and electrical properties of RGO thin-film, two samples with thickness of ~14 nm were annealed under high vacuum (103 Pa) at 400°C and 800°C respectively for 3 h to further remove the bonded phenylhydrazine and reduce the RGO thin-films. In this paper, we will mainly focus on these three samples (without annealing, 400°C annealed and 800°C annealed). In addition, for comparison, a reference GO thin film sample without phenylhydrazine was also prepared by drop casting the primary

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Figure 1 setup.

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(Color online) (a) RGO and GO suspension; (b) spray-coating

GO/DMF solution on a 50°C preheated quartz substrate. All samples (including RGO thin films and GO reference films) were characterized by UV-vis spectrum (SpectraPro500i), Raman spectroscopy (JY-T64000), X-ray photoelectron spectroscopy (XPS, ESCALAB 250) and AFM (Asylum Research MFP-3D) to study their fundamental physical and chemical properties. Conductivity of the films was measured with a standard van der Pauw configuration in ambient condition.

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Results and discussion

Thickness of the films can be roughly controlled by spray volume. To determine the relation between the thickness and spray volume, three samples with different spray volume (50, 100 and 150 L) have been produced. The AFM images on the three samples are shown in Figure 2. For the 50 L sample, only a small fraction of the substrate surface is covered by RGO flakes, and the flakes rarely overlay on each other, so it is easy to calculate the cover rate. The obtained cover rate for the 50 L sample is about 30.1%. Therefore, the spray volume needed for a full single layer is estimated to be about 167 L. This is verified by the 150 L sample which is almost completely covered by RGO flakes. The typical RGO samples used in this study have around 12 layers by using 2 mL solution. The surface roughness of these 12 layers samples are about 1.9 nm indicating a reasonable flat film. Because the RGO flakes are spray deposited on the substrate layer by layer, the thickness of the film is well controllable and the surface is adequately flat. Moreover, from the AFM image of the 50 L sample, the obtained thickness for a single layer RGO flake before annealing is about 1.2 nm. This value is much larger than that of pristine graphene (i.e., 0.3 nm [27]), but comparable to that reported for single-layer RGO materials (e.g., 1.3 nm in ref. [26] and 1.2 nm in ref. [28]). The increased thickness of RGO is due to the existence of residual covalently bonded reducing agent [26]. After annealing at 400°C in a vacuum condition, the thickness is reduced to 0.94 nm. It is most likely because of the partly removal of residual covalently

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Figure 2

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AFM image of 50 (a), 100 (b) and 150 (c) L sprayed samples. The cross section of the red line marked in the 50 L sample is shown in (d).

bonded phenylhydrazine on RGO. The 50 L sample was not annealed at 800°C for AFM test. Raman spectra of the RGO samples are shown in Figure 3, in comparison to GO and graphite. The most prominent feature in the graphite spectrum is the G peak around 1581 cm1 corresponding to the in-plane vibration of the graphite lattice [29,30]. In GO, this peak broadens and blue-shifts to form a broad band at 1592.7 cm1. This blue shift has been explained by the amorphous model of GO [31]. In that model, there are areas of sp2 carbon clusters with an alternating pattern of single-double carbon bonds [3]. The blue shift remains in the RGO samples indicating that this kind of structure has not been changed by the chemical reduction [13]. Another feature of the GO’s spectrum is the D peak around 1355 cm1 that originates from a breathing mode forbidden in perfect graphite and activated by defects [29,30]. Average size of the sp2 clusters mentioned above or in-plane correlation length La is related to the ratio of intensity of D band and G band (ID/IG): La=44 Å/(ID/IG) [32]. The in-plane correlation lengths derived from our Raman spectra of RGO samples are shown in Table 1. La decreases slightly with annealing, probably because of the appearance of smaller sp2 regions with further reduction during annealing [13]. Another possible reason for the shorter La is the loss of carbon atoms during annealing, which leaves behind defects on the basal-plane of the RGO sheets [33]. The 2D peaks around 2700 cm1, however, are not obvious. Figure 4 shows the XPS measurements of our RGO thin films and the GO sample. Using the method described in ref. [34], the C/O ratio of our RGO sample without annealing is estimated about 3.0, which indicates a reasonably good re-

duction of our RGO sample. The C/O ratio of the GO sample is 2.3, implying that the GO is highly oxidized. After annealing, the C/O ratio increases slightly to 3.1 and 3.4 for the samples annealed at 400°C and 800°C, respectively, and this suggests the oxygen content in RGO can be further decreased by vacuum annealing. Figure 5 illustrates the linear absorbance spectra of the RGO samples. The giant peak at 273 nm originating from -* absorption [11] almost doesn’t shift during annealing. The absorbance at infrared region increases and becomes more flat with increasing the annealing temperature. It is known that pristine graphene has a large universal absorption at the visible and infrared regions [2,26]. The spectrum

Figure 3 Raman spectra of the RGO samples with different annealing conditions. Spectra of GO and graphite have also been shown for comparison.

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Figure 4 C1s peaks in the XPS spectra of the RGO samples with different annealing conditions. C—C peak is assigned at 284.7 eV. Sub-peaks with chemical shifts 1.5, 2.5 and 4.0 eV were assigned to C—OH, C==O and O==C—OH respectively. The C—N sub-peak only exist in RGO samples. C:O atom ratio is also shown at the left side of each curve.

Figure 5 Linear absorption spectra for the RGO samples with different annealing conditions. The -* peak is marked. Table 1 Basic information of the RGO samples with different annealing conditions obtained in our investigation RGO without RGO annealed at RGO annealed at annealing 400°C for 3 h 800°C for 3 h ID/IG 1.06 1.07 1.20 La (Å) 41.7 41.2 36.7 C:O rate 3.0 3.1 3.4 Transmittance at 79.6 72.1 68.7 550 nm (%) Sheet resistance (kΩ/□) 3140 997 58

of the RGO sample annealed in vacuum at high temperature is close to that of pristine graphene. That is to say, the band structure of a high-temperature annealed RGO sample is similar to that of pristine graphene. All the three samples

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have good transparency in a broad band, from visible to near infrared. The transmittance listed in Table 1 for the films is obtained from the absorbance at 550 nm. Moreover, the measured sheet resistance for the films is also listed in Table 1. It can be seen that the as-synthesized RGO film doesn’t have good conductivity, but the sheet resistance can be significantly reduced by annealing in vacuum. Upon annealing in vacuum at 800°C for 3 h, the sheet resistance of the RGO film decreases by almost two orders of magnitude to be about 58 k. To explain the observed behavior, a reported model for amorphous structure of GO is used [3]. In this model, oxygen-containing groups in GO tend to aggregate amorphously, forming sp3 matrix in which isolated sp2 islands (size~3 nm) are buried in. After reduction, the sp2 clusters typically do not change but sp2 domains with smaller size can form. This eventually causes percolation of the insulating sp3 matrix, leading to a dramatic increase of electric conductivity [5]. In our experiment, GO film is insulating as expected. However, our as-synthesized (only 150°C vacuum dried) RGO films is conductive, though the sheet resistance is high, indicating the small sp2 domains have already percolated the sp3 matrix. The conductance increases further with annealing temperature, suggesting that the sp2 clusters in RGO is further connected by more sp2 domains formed during reducing. XPS and Raman results suggest a certain degree of reduction during annealing. Although the degree of reduction is moderate, its impact on conductivity can be significant, because concentration of the sp2 sites in RGO is not far from the percolation threshold. A small increase in C:O ratio may cause a sharp increase in conductivity [5]. Another possible reason for the decrease of resistance with annealing is that the residual reduction agents are further evaporated by vacuum heating which is supported by AFM results, resulting in better conductive contacts between RGO flakes.

3

Conclusion

We have fabricated transparent conductive film on quartz using a spray coating method. Thicknesses of the films can be controlled by the amount of spray volume. The films have good transparency in a broad band (from visible to near infrared). The sheet resistance of the films drops by two orders of magnitude after annealing in high vacuum at 800°C. Both thermal reduction of oxygen-containing groups and thermal detaching of residual reagent are the possible reasons for the increase in electric conductivity with increasing annealing temperature. This work was supported by the National Key Basic Research Program of China (Grant No. 2013CBA01703), the National Natural Science Foundation of China (Grant No. 11174355), Teknologiateollisuus TT-100, the

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European Union’s Seventh Framework Programme (Grant No. 631610), and Aalto University (Finland). 1 2 3 4 5

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