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Reduced graphene oxide nanoshells for flexible and stretchable conductors
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 095301 (http://iopscience.iop.org/0957-4484/27/9/095301) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 192.236.36.29 This content was downloaded on 04/02/2016 at 15:20
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 27 (2016) 095301 (7pp)
doi:10.1088/0957-4484/27/9/095301
Reduced graphene oxide nanoshells for flexible and stretchable conductors Wen-Shuai Jiang, Zhi-Bo Liu, Wei Xin, Xu-Dong Chen and Jian-Guo Tian The Key Laboratory of Weak Light Nonlinear Photonics, Ministry of Education, Teda Applied Physics School and School of Physics, Nankai University, Tianjin 300071, People’s Republic of China E-mail: [email protected] and [email protected] Received 13 December 2015 Accepted for publication 6 January 2016 Published 29 January 2016 Abstract
Graphene has been extensively investigated for its use in flexible electronics, especially graphene synthesized by chemical vapor deposition (CVD). To enhance the flexibility of CVD graphene, wrinkles are often introduced. However, reports on the flexibility of reduced graphene oxide (RGO) films are few, because of their weak conductivity and, in particular, poor flexibility. To improve the flexibility of RGO, reduced graphene oxide nanoshells are fabricated, which combine self-assembled polystyrene nanosphere arrays and high-temperature thermal annealing processes. The resulting RGO films with nanoshells present a better resistance stabilization after stretching and bending the devices than RGO without nanoshells. The sustainability and performance advances demonstrated here are promising for the adoption of flexible electronics in a wide variety of future applications. S Online supplementary data available from stacks.iop.org/NANO/27/095301/mmedia Keywords: reduced graphene oxide, flexible electronics, nanoshell, flexible, stretchable (Some figures may appear in colour only in the online journal) To enhance the flexibility of CVD graphene, a wrinkle is often introduced by the pre-strain substrate using the wavy patterns method for enhancing the flexibility of graphene devices, which employs a stress-relieving mechanism [7]. Moreover, Xie et al fabricated polyaniline/porous graphene electrodes via fabricating a sine wave Ni substrate and then growing graphene, which was used as a stretchable supercapacitor and had a better flexibility when the device was bent than stretched [8]. However, reports on the flexibility of reduced graphene oxide (RGO) films are few. RGO is obtained from graphene oxide (GO) by removing the oxygen functionalities, and has plenty of excellent properties, such as low cost, high throughput, chemical versatility, and scalability [9, 10]. It has been explored in a wide array of applications, including solar cells [11], sensors [12], organic electronic devices [13], supercapacitors [14], and transparent conductive electrodes [15–17]. Some efforts have been made to enhance the flexibility of RGO [18–23]. There are two main pathways to fabricating RGO-based flexibile electronic devices. One is inkjet printing, and some results are shown in table S1 of the
1. Introduction In recent years, there has been rapid development in flexible electronics. A variety of flexible and stretchable devices and systems have been investigated to meet the increasing demand for wearable and bio-implantable electronics. Structured samples combined with flexible bases have become a research trend in numerous studies, and have shown some remarkable progress. For example, by using a structure in which applied strain is released via the formation of wavy patterns after the deposition of metal electrodes on an elastomeric substrate, several prominent achievements have been made [1–3]. Jeong Sook Ha’s group has demonstrated some stretchable devices via stretchable arrays on a deformable polymer substrate with embedded liquid-metal interconnections [4–6]. Graphene has been extensively studied due to its excellent physical, mechanical, and electronic characteristics. Stretchable graphene electronic devices have been attempted and have achieved a great deal, especially in the case of graphene synthesized by chemical vapor deposition (CVD). 0957-4484/16/095301+07$33.00
1
© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 095301
W-S Jiang et al
supplementary material [18–21]. However, the flexible substrate cannot often tolerate high temperatures, and the maximum reduction is limited. The other pathway involves transferring graphene to a flexible substrate. Feng et al investigated the flexibility of an RGO composite (hydrazine hydrate) by transferring the micro-wrinkled RGO film onto an elastic polydimethylsiloxane (PDMS) substrate [22]. However, Eda et al investigated the flexibility of large-area ultrathin film of RGO by vacuum filtration and then transferring the film to flexible substrate, and found that hydrazine vapor alone was not sufficient to achieve maximum reduction, and annealing alone requires relatively high temperatures (>550 °C) [23]. Becerril et al have demonstrated that a hightemperature annealing (HTA) process for fabricating RGO is more sufficient than chemical reduction [24]. However, the flexibility of RGO films fabricated by HTA is poor and the corresponding reports are few. Here, we report a simple method for fabricating RGO nanoshells (RGONs) that combines self-assembled polystyrene (PS) nanosphere arrays and high-temperature thermal annealing processes. The self-assembled PS nanosphere array method, which is a low-cost, high-throughput, and efficient way of fabricating periodic nanostructures, has previously been used to fabricate large-area graphene nanomeshes [25– 27] and ultranarrow graphene nanoribbons [28]. The HTA is often used to achieve maximum reduction of GO. The resulting RGO film, which consists of nanoshells, exhibits greater resistance stability when subjected to stretching and bending than RGO films without nanoshells. A gold nanoshell plasmonic structure fabricated by self-assembling exhibited good stretchability [29]. Therefore, it can be expected that RGON films synthesized by self-assembly would also have good stretchability and flexibility owing to their three-dimensional (3D) structure. The improved performance of the fabricated RGON films suggests that they should be suitable for use in a wide variety of flexible electronic devices.
Figure 1. (a) Schematic illustrations of the RGON fabrication
process. First, a large-scale, closely packed monolayer of the PS nanospheres was formed on the precleaned SiO2 substrate through self-assembly. Then, an aqueous dispersion of GO was spin coated on the surface of the PS/SiO2 structure. Next, the GO film on the PS/SiO2 structure was thermally annealed at a high temperature, resulting in a RGON film. Finally, the entire RGON, which consisted of a monolayered array of nanoshells, was transferred onto a PDMS substrate. (b) Schematic illustration of the structure of the RGON film, which consisted of nanoshells.
into RGO; the nanoshell structures remained intact during the process. The PS nanospheres could be removed during the HTA process, as the polymer chain of PS degrades rapidly at temperatures greater than 360 °C. Thus, after the thermal reduction, we could obtain a RGO film with a periodic nanoshell structure on the SiO2 substrate; these nanoshells are labeled as RGONs. The RGON films obtained using the 500 nm and 1000 nm PS nanospheres are labeled RGON500 nm and RGON-1000 nm, respectively. The last step was to prepare an elastomeric substrate in order to test the stretchability and flexibility of the RGON films. A 2 mmthick transparent layer of PDMS was cured at 75 °C for 1 h. Then the PDMS/RGON/SiO2 structure was immersed into 5% KOH for 1 h to remove the underlying SiO2 substrate. The PDMS/RGON structure was cleaned five times in deionized water and then dried using nitrogen gas. In this manner, we could successfully fabricate an array of RGONs that faced upward on a PDMS substrate. The employed experimental strategy is based entirely on a low-cost, largescale self-assembly technique and is a simple, yet efficient, way of growing ordered RGON-based nanostructures over a large area. To demonstrate whether the amorphous carbon was generated after PS nanosphere degradation or not, scanning electron microscopy (SEM) and Fourier transform
2. Experiment The RGON fabrication process is depicted schematically in figure 1(a). A detailed description of the processes for fabricating the GO and polystyrene nanospheres used is given in the supplementary material (stacks.iop.org/NANO/27/ 095301/mmedia). The PS nanospheres used in this study had diameters of 500 nm and 1000 nm. A large-scale, closely packed monolayer of the PS nanospheres was fabricated on a precleaned SiO2 substrate through self-assembly. The GO used was prepared from natural graphite using a modified version of Hummer’s method [30] and was employed as the starting material in the fabrication of a high-quality graphene film. A dispersion of GO in water was spin coated onto the surface of the PS/SiO2 structure. Owing to the shadow effect, GO caps were formed on the PS spheres, resulting in nanoshells with PS cores. Subsequently, the GO film on the PS/ SiO2 structure was thermally annealed at a temperature of 800 °C. Through thermal reduction, the GO was converted 2
Nanotechnology 27 (2016) 095301
W-S Jiang et al
Figure 2. Morphology and structure of the RGON films. (a) Digital image, (b) optical microscopy image, and (c) SEM image of the closely packed monolayer of PS nanospheres with a diameter of 500 nm. (d) Digital image, (e) optical microscopy image, and (f) SEM image of RGON-500 nm. (g) Digital image, (h) optical microscopy image, and (i) SEM image of RGON-1000 nm. Scale bars in (c), (f), and (i) are 2 μm.
infrared spectroscopy (FTIR) are used. The results shown in figure S6 (supplementary material) indicate that no obvious amorphous carbon is observed. Figure 2 shows digital, optical microscopy, and SEM images of the PS, RGON-500 nm, and RGON-1000 nm arrays. Figure 2(a) shows two 2×2 cm2 SiO2 wafers with selfassembled PS nanospheres, exhibiting uniform, grating-like color dispersion, which indicates that the monolayered PS array was large in area and uniform in nature. The optical microscopy image in figure 2(b) and the SEM image in figure 2(c) confirm that a regularly arrayed PS nanosphere film was successfully fabricated through the above-mentioned steps. Surfaces on which monolayers of PS arrays have to be self-assembled must exhibit hydrophilicity. Therefore, the SiO2 substrates were treated with an O2 plasma prior to the formation of the selfassembled monolayers. The stronger the hydrophilic interaction, the larger the area of the PS monolayer, which is formed because of the van der Waals forces between the PS particles. After the spin coating of the GO layer on the PS/SiO2 structure and the thermal reduction of the layer, a large-area, homogeneous RGON film was formed. The structures of RGON-500 nm and RGON-1000 nm are shown in
figures 2(d) and (g). The optical microscopy images in figures 2(e) and (h) show that the periodicity of the RGONs corresponded to the configuration of the underlying PS nanosphere monolayer. The periodic gray color that can be seen in the SEM images in figures 2(f) and (i) shows clearly the morphologies of RGON-500 nm and RGON-1000 nm. We also characterized the as-grown RGON film through Raman spectroscopy (supplementary figure S1). It can be seen that, after thermal reduction, the Raman peak characteristic of PS disappears from the Raman spectrum of the RGONs, implying that all the PS nanospheres were removed during the high-temperature reduction process. The surface topography of the RGON film was characterized using atomic force microscopy (AFM), as shown in figure 3 and figure S2. The RGON film prepared by HTA exhibited a periodic structure consistent with the underlying PS nanosphere template. The gap between the nanoshells was approximately 500 nm for RGON-500 nm and 1000 nm for RGON-1000 nm. This is consistent with the results obtained from the SEM images mentioned above. Further, the heights of RGON-500 nm (72 nm) and RGON-1000 nm (186 nm) were far smaller than the heights of the PS microspheres, 3
Nanotechnology 27 (2016) 095301
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Figure 3. (a) AFM image and (b) 3D image of RGON-500 nm, and (c) its height profile along the white line in (a). (d) AFM image and (e) 3D
image of RGON-1000 nm film, and (f) its height profile along the white line in (e).
transparency, and flexibility. First, the RGO, RGON-500 nm, and RGON-1000 nm films were transferred from the SiO2 layer to the PDMS substrate, as shown in figures 4(a)–(c). On the surface, the RGON films did not exhibit any obvious differences from the RGO film despite their nanoshell structures. SEM images of RGON-500 nm and RGON-1000 nm on the PDMS substrates are shown in figures 4(d) and (e). It can be seen that hollow hemispheres were periodically arranged on their surfaces, indicating the existence of the RGONs and the absence of the PS nanospheres. The resulting RGON films consist of nanoshells with sheet resistance of 7.14 KΩ/, and 10.2 KΩ/, for RGON-500 and RGON1000, respectively. The sheet resistances were measured by the four-point probe method. They exhibit greater resistance stability when subjected to stretching and bending than RGO films without nanoshells. After the transfer from the SiO2 layer to the PDMS substrate, the sheet resistance of the RGO
which had diameters of 500 nm (151 nm) and 1000 nm (241 nm). This may be owing to the following three reasons: (1) the volume of the PS nanospheres decreased after the O2 plasma etching; (2) the AFM probe could not scan the bottoms of the nanoshell structures; and (3) during the hightemperature reduction to form the RGONs, the PS nanospheres might have collapsed partially, becoming soft, at temperatures higher than 125 °C, and then degraded at 280 °C. Furthermore, the AFM images of the GO layer on the spin-coated PS array shown in figure S3 confirm that the film was formed on the PS nanospheres.
3. Results and discussion To characterize the flexibility of the RGON films, we used PDMS as the substrate because of its high strength, 4
Nanotechnology 27 (2016) 095301
W-S Jiang et al
Figure 4. Digital images of (a) RGON-500 nm, (b) RGON-1000 nm, and (c) the RGO film on PDMS substrates. SEM images of (d) RGON-
500 nm and (e) RGON-1000 nm on PDMS substrates. (f) Sheet resistances of RGO, RGON-500 nm, and RGON-1000 nm before and after being transferred onto PDMS substrates. (g) Optical transmittances of RGO, RGON-500 nm, and RGON-1000 nm on PDMS substrates. Scale bars in (d) and (e) are 2 μm.
film increased from 9.34 KΩ/, to 591.9 KΩ/,, as shown in figure 4(f). The sheet resistance of RGO film on PDMS (591.9 KΩ/,) is much larger than that of RGON-500 nm (57.82 KΩ/,) and RGON-1000 nm (89.54 KΩ/,). This increase in the resistance after the transfer process is attributable to the bending nature of PDMS, as after being transferred, the RGO film broke (supplementary figure S4). In contrast, the 3D structure and large surface area of the RGON films effectively prevented them from breaking. We also measured the transmittances of the transferred films using an ultraviolet–visible spectrometer. The transmittance curves of the RGON films were similar to that of the RGO film. Compared with the transmittance and sheet resistance of RGO fabricated by different methods (supplementary table S2), the conductivity of RGO fabricated by HTA is relatively better than that fabricated by hydrazine. Owing to its 3D and periodical structure, it can be expected that RGON will exhibit excellent mechanical properties when used to make flexible and stretchable
electrodes. We evaluated the foldability of the RGO and RGON films on PDMS by measuring the changes in their resistances when bent with different radii (figure 5). The resistances of the RGON films showed little variation for bending radii of up to 40 mm and returned to their original values when the films were unbent. Notably, the resistances of the RGON films were less than ten times their original resistances for a bending radius of 30 mm, exhibiting good mechanical stability. In contrast, in the RGO film, which did not consist of nanoshells, the resistance increased by 80 times when the bending radius was 50 mm. Figure 5(b) shows an optical image of a completely bent RGO film; in this case, the resistance was too high to measure. In contrast, even when the bending radius was as high as 30 mm (figure 5(c)), the resistances of the RGON films did not increase by more than ten times. To determine the stretchability of the films, their resistances were measured under different uniaxial tensile strains, which ranged from 0 to 25%. As was noticed during the 5
Nanotechnology 27 (2016) 095301
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Figure 5. (a) Variations in the resistances of the RGO and RGON
films after being transferred onto a PDMS substrate and bent with different bending radii. (b), (c) The RGO and RGON films bent at the maximum bending radius, respectively.
Figure 6. (a) Variation in the resistances of the RGO and RGON
films on PDMS substrates when subjected to a uniaxial tensile strain. (b), (c) The RGO and RGON films when subjected to the maximum tensile stress.
folding tests, the RGON films exhibited much higher stretchability than the RGO film (figure 6). The RGON films could tolerate large strains (>25%) without exhibiting large cracks, while the as-prepared RGO sample cracked readily when subjected to even small strains (
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My IOPscience
Reduced graphene oxide nanoshells for flexible and stretchable conductors
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 095301 (http://iopscience.iop.org/0957-4484/27/9/095301) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 192.236.36.29 This content was downloaded on 04/02/2016 at 15:20
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 27 (2016) 095301 (7pp)
doi:10.1088/0957-4484/27/9/095301
Reduced graphene oxide nanoshells for flexible and stretchable conductors Wen-Shuai Jiang, Zhi-Bo Liu, Wei Xin, Xu-Dong Chen and Jian-Guo Tian The Key Laboratory of Weak Light Nonlinear Photonics, Ministry of Education, Teda Applied Physics School and School of Physics, Nankai University, Tianjin 300071, People’s Republic of China E-mail: [email protected] and [email protected] Received 13 December 2015 Accepted for publication 6 January 2016 Published 29 January 2016 Abstract
Graphene has been extensively investigated for its use in flexible electronics, especially graphene synthesized by chemical vapor deposition (CVD). To enhance the flexibility of CVD graphene, wrinkles are often introduced. However, reports on the flexibility of reduced graphene oxide (RGO) films are few, because of their weak conductivity and, in particular, poor flexibility. To improve the flexibility of RGO, reduced graphene oxide nanoshells are fabricated, which combine self-assembled polystyrene nanosphere arrays and high-temperature thermal annealing processes. The resulting RGO films with nanoshells present a better resistance stabilization after stretching and bending the devices than RGO without nanoshells. The sustainability and performance advances demonstrated here are promising for the adoption of flexible electronics in a wide variety of future applications. S Online supplementary data available from stacks.iop.org/NANO/27/095301/mmedia Keywords: reduced graphene oxide, flexible electronics, nanoshell, flexible, stretchable (Some figures may appear in colour only in the online journal) To enhance the flexibility of CVD graphene, a wrinkle is often introduced by the pre-strain substrate using the wavy patterns method for enhancing the flexibility of graphene devices, which employs a stress-relieving mechanism [7]. Moreover, Xie et al fabricated polyaniline/porous graphene electrodes via fabricating a sine wave Ni substrate and then growing graphene, which was used as a stretchable supercapacitor and had a better flexibility when the device was bent than stretched [8]. However, reports on the flexibility of reduced graphene oxide (RGO) films are few. RGO is obtained from graphene oxide (GO) by removing the oxygen functionalities, and has plenty of excellent properties, such as low cost, high throughput, chemical versatility, and scalability [9, 10]. It has been explored in a wide array of applications, including solar cells [11], sensors [12], organic electronic devices [13], supercapacitors [14], and transparent conductive electrodes [15–17]. Some efforts have been made to enhance the flexibility of RGO [18–23]. There are two main pathways to fabricating RGO-based flexibile electronic devices. One is inkjet printing, and some results are shown in table S1 of the
1. Introduction In recent years, there has been rapid development in flexible electronics. A variety of flexible and stretchable devices and systems have been investigated to meet the increasing demand for wearable and bio-implantable electronics. Structured samples combined with flexible bases have become a research trend in numerous studies, and have shown some remarkable progress. For example, by using a structure in which applied strain is released via the formation of wavy patterns after the deposition of metal electrodes on an elastomeric substrate, several prominent achievements have been made [1–3]. Jeong Sook Ha’s group has demonstrated some stretchable devices via stretchable arrays on a deformable polymer substrate with embedded liquid-metal interconnections [4–6]. Graphene has been extensively studied due to its excellent physical, mechanical, and electronic characteristics. Stretchable graphene electronic devices have been attempted and have achieved a great deal, especially in the case of graphene synthesized by chemical vapor deposition (CVD). 0957-4484/16/095301+07$33.00
1
© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 095301
W-S Jiang et al
supplementary material [18–21]. However, the flexible substrate cannot often tolerate high temperatures, and the maximum reduction is limited. The other pathway involves transferring graphene to a flexible substrate. Feng et al investigated the flexibility of an RGO composite (hydrazine hydrate) by transferring the micro-wrinkled RGO film onto an elastic polydimethylsiloxane (PDMS) substrate [22]. However, Eda et al investigated the flexibility of large-area ultrathin film of RGO by vacuum filtration and then transferring the film to flexible substrate, and found that hydrazine vapor alone was not sufficient to achieve maximum reduction, and annealing alone requires relatively high temperatures (>550 °C) [23]. Becerril et al have demonstrated that a hightemperature annealing (HTA) process for fabricating RGO is more sufficient than chemical reduction [24]. However, the flexibility of RGO films fabricated by HTA is poor and the corresponding reports are few. Here, we report a simple method for fabricating RGO nanoshells (RGONs) that combines self-assembled polystyrene (PS) nanosphere arrays and high-temperature thermal annealing processes. The self-assembled PS nanosphere array method, which is a low-cost, high-throughput, and efficient way of fabricating periodic nanostructures, has previously been used to fabricate large-area graphene nanomeshes [25– 27] and ultranarrow graphene nanoribbons [28]. The HTA is often used to achieve maximum reduction of GO. The resulting RGO film, which consists of nanoshells, exhibits greater resistance stability when subjected to stretching and bending than RGO films without nanoshells. A gold nanoshell plasmonic structure fabricated by self-assembling exhibited good stretchability [29]. Therefore, it can be expected that RGON films synthesized by self-assembly would also have good stretchability and flexibility owing to their three-dimensional (3D) structure. The improved performance of the fabricated RGON films suggests that they should be suitable for use in a wide variety of flexible electronic devices.
Figure 1. (a) Schematic illustrations of the RGON fabrication
process. First, a large-scale, closely packed monolayer of the PS nanospheres was formed on the precleaned SiO2 substrate through self-assembly. Then, an aqueous dispersion of GO was spin coated on the surface of the PS/SiO2 structure. Next, the GO film on the PS/SiO2 structure was thermally annealed at a high temperature, resulting in a RGON film. Finally, the entire RGON, which consisted of a monolayered array of nanoshells, was transferred onto a PDMS substrate. (b) Schematic illustration of the structure of the RGON film, which consisted of nanoshells.
into RGO; the nanoshell structures remained intact during the process. The PS nanospheres could be removed during the HTA process, as the polymer chain of PS degrades rapidly at temperatures greater than 360 °C. Thus, after the thermal reduction, we could obtain a RGO film with a periodic nanoshell structure on the SiO2 substrate; these nanoshells are labeled as RGONs. The RGON films obtained using the 500 nm and 1000 nm PS nanospheres are labeled RGON500 nm and RGON-1000 nm, respectively. The last step was to prepare an elastomeric substrate in order to test the stretchability and flexibility of the RGON films. A 2 mmthick transparent layer of PDMS was cured at 75 °C for 1 h. Then the PDMS/RGON/SiO2 structure was immersed into 5% KOH for 1 h to remove the underlying SiO2 substrate. The PDMS/RGON structure was cleaned five times in deionized water and then dried using nitrogen gas. In this manner, we could successfully fabricate an array of RGONs that faced upward on a PDMS substrate. The employed experimental strategy is based entirely on a low-cost, largescale self-assembly technique and is a simple, yet efficient, way of growing ordered RGON-based nanostructures over a large area. To demonstrate whether the amorphous carbon was generated after PS nanosphere degradation or not, scanning electron microscopy (SEM) and Fourier transform
2. Experiment The RGON fabrication process is depicted schematically in figure 1(a). A detailed description of the processes for fabricating the GO and polystyrene nanospheres used is given in the supplementary material (stacks.iop.org/NANO/27/ 095301/mmedia). The PS nanospheres used in this study had diameters of 500 nm and 1000 nm. A large-scale, closely packed monolayer of the PS nanospheres was fabricated on a precleaned SiO2 substrate through self-assembly. The GO used was prepared from natural graphite using a modified version of Hummer’s method [30] and was employed as the starting material in the fabrication of a high-quality graphene film. A dispersion of GO in water was spin coated onto the surface of the PS/SiO2 structure. Owing to the shadow effect, GO caps were formed on the PS spheres, resulting in nanoshells with PS cores. Subsequently, the GO film on the PS/ SiO2 structure was thermally annealed at a temperature of 800 °C. Through thermal reduction, the GO was converted 2
Nanotechnology 27 (2016) 095301
W-S Jiang et al
Figure 2. Morphology and structure of the RGON films. (a) Digital image, (b) optical microscopy image, and (c) SEM image of the closely packed monolayer of PS nanospheres with a diameter of 500 nm. (d) Digital image, (e) optical microscopy image, and (f) SEM image of RGON-500 nm. (g) Digital image, (h) optical microscopy image, and (i) SEM image of RGON-1000 nm. Scale bars in (c), (f), and (i) are 2 μm.
infrared spectroscopy (FTIR) are used. The results shown in figure S6 (supplementary material) indicate that no obvious amorphous carbon is observed. Figure 2 shows digital, optical microscopy, and SEM images of the PS, RGON-500 nm, and RGON-1000 nm arrays. Figure 2(a) shows two 2×2 cm2 SiO2 wafers with selfassembled PS nanospheres, exhibiting uniform, grating-like color dispersion, which indicates that the monolayered PS array was large in area and uniform in nature. The optical microscopy image in figure 2(b) and the SEM image in figure 2(c) confirm that a regularly arrayed PS nanosphere film was successfully fabricated through the above-mentioned steps. Surfaces on which monolayers of PS arrays have to be self-assembled must exhibit hydrophilicity. Therefore, the SiO2 substrates were treated with an O2 plasma prior to the formation of the selfassembled monolayers. The stronger the hydrophilic interaction, the larger the area of the PS monolayer, which is formed because of the van der Waals forces between the PS particles. After the spin coating of the GO layer on the PS/SiO2 structure and the thermal reduction of the layer, a large-area, homogeneous RGON film was formed. The structures of RGON-500 nm and RGON-1000 nm are shown in
figures 2(d) and (g). The optical microscopy images in figures 2(e) and (h) show that the periodicity of the RGONs corresponded to the configuration of the underlying PS nanosphere monolayer. The periodic gray color that can be seen in the SEM images in figures 2(f) and (i) shows clearly the morphologies of RGON-500 nm and RGON-1000 nm. We also characterized the as-grown RGON film through Raman spectroscopy (supplementary figure S1). It can be seen that, after thermal reduction, the Raman peak characteristic of PS disappears from the Raman spectrum of the RGONs, implying that all the PS nanospheres were removed during the high-temperature reduction process. The surface topography of the RGON film was characterized using atomic force microscopy (AFM), as shown in figure 3 and figure S2. The RGON film prepared by HTA exhibited a periodic structure consistent with the underlying PS nanosphere template. The gap between the nanoshells was approximately 500 nm for RGON-500 nm and 1000 nm for RGON-1000 nm. This is consistent with the results obtained from the SEM images mentioned above. Further, the heights of RGON-500 nm (72 nm) and RGON-1000 nm (186 nm) were far smaller than the heights of the PS microspheres, 3
Nanotechnology 27 (2016) 095301
W-S Jiang et al
Figure 3. (a) AFM image and (b) 3D image of RGON-500 nm, and (c) its height profile along the white line in (a). (d) AFM image and (e) 3D
image of RGON-1000 nm film, and (f) its height profile along the white line in (e).
transparency, and flexibility. First, the RGO, RGON-500 nm, and RGON-1000 nm films were transferred from the SiO2 layer to the PDMS substrate, as shown in figures 4(a)–(c). On the surface, the RGON films did not exhibit any obvious differences from the RGO film despite their nanoshell structures. SEM images of RGON-500 nm and RGON-1000 nm on the PDMS substrates are shown in figures 4(d) and (e). It can be seen that hollow hemispheres were periodically arranged on their surfaces, indicating the existence of the RGONs and the absence of the PS nanospheres. The resulting RGON films consist of nanoshells with sheet resistance of 7.14 KΩ/, and 10.2 KΩ/, for RGON-500 and RGON1000, respectively. The sheet resistances were measured by the four-point probe method. They exhibit greater resistance stability when subjected to stretching and bending than RGO films without nanoshells. After the transfer from the SiO2 layer to the PDMS substrate, the sheet resistance of the RGO
which had diameters of 500 nm (151 nm) and 1000 nm (241 nm). This may be owing to the following three reasons: (1) the volume of the PS nanospheres decreased after the O2 plasma etching; (2) the AFM probe could not scan the bottoms of the nanoshell structures; and (3) during the hightemperature reduction to form the RGONs, the PS nanospheres might have collapsed partially, becoming soft, at temperatures higher than 125 °C, and then degraded at 280 °C. Furthermore, the AFM images of the GO layer on the spin-coated PS array shown in figure S3 confirm that the film was formed on the PS nanospheres.
3. Results and discussion To characterize the flexibility of the RGON films, we used PDMS as the substrate because of its high strength, 4
Nanotechnology 27 (2016) 095301
W-S Jiang et al
Figure 4. Digital images of (a) RGON-500 nm, (b) RGON-1000 nm, and (c) the RGO film on PDMS substrates. SEM images of (d) RGON-
500 nm and (e) RGON-1000 nm on PDMS substrates. (f) Sheet resistances of RGO, RGON-500 nm, and RGON-1000 nm before and after being transferred onto PDMS substrates. (g) Optical transmittances of RGO, RGON-500 nm, and RGON-1000 nm on PDMS substrates. Scale bars in (d) and (e) are 2 μm.
film increased from 9.34 KΩ/, to 591.9 KΩ/,, as shown in figure 4(f). The sheet resistance of RGO film on PDMS (591.9 KΩ/,) is much larger than that of RGON-500 nm (57.82 KΩ/,) and RGON-1000 nm (89.54 KΩ/,). This increase in the resistance after the transfer process is attributable to the bending nature of PDMS, as after being transferred, the RGO film broke (supplementary figure S4). In contrast, the 3D structure and large surface area of the RGON films effectively prevented them from breaking. We also measured the transmittances of the transferred films using an ultraviolet–visible spectrometer. The transmittance curves of the RGON films were similar to that of the RGO film. Compared with the transmittance and sheet resistance of RGO fabricated by different methods (supplementary table S2), the conductivity of RGO fabricated by HTA is relatively better than that fabricated by hydrazine. Owing to its 3D and periodical structure, it can be expected that RGON will exhibit excellent mechanical properties when used to make flexible and stretchable
electrodes. We evaluated the foldability of the RGO and RGON films on PDMS by measuring the changes in their resistances when bent with different radii (figure 5). The resistances of the RGON films showed little variation for bending radii of up to 40 mm and returned to their original values when the films were unbent. Notably, the resistances of the RGON films were less than ten times their original resistances for a bending radius of 30 mm, exhibiting good mechanical stability. In contrast, in the RGO film, which did not consist of nanoshells, the resistance increased by 80 times when the bending radius was 50 mm. Figure 5(b) shows an optical image of a completely bent RGO film; in this case, the resistance was too high to measure. In contrast, even when the bending radius was as high as 30 mm (figure 5(c)), the resistances of the RGON films did not increase by more than ten times. To determine the stretchability of the films, their resistances were measured under different uniaxial tensile strains, which ranged from 0 to 25%. As was noticed during the 5
Nanotechnology 27 (2016) 095301
W-S Jiang et al
Figure 5. (a) Variations in the resistances of the RGO and RGON
films after being transferred onto a PDMS substrate and bent with different bending radii. (b), (c) The RGO and RGON films bent at the maximum bending radius, respectively.
Figure 6. (a) Variation in the resistances of the RGO and RGON
films on PDMS substrates when subjected to a uniaxial tensile strain. (b), (c) The RGO and RGON films when subjected to the maximum tensile stress.
folding tests, the RGON films exhibited much higher stretchability than the RGO film (figure 6). The RGON films could tolerate large strains (>25%) without exhibiting large cracks, while the as-prepared RGO sample cracked readily when subjected to even small strains (
