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CVD Graphene Transfer from Copper Substrate to Polymer
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 4 (2017) 11476–11479
www.materialstoday.com/proceedings
RJCAM 2016
CVD Graphene Transfer from Copper Substrate to Polymer Ilya A. Kostogruda,*, Evgeniy V. Boykoa,b, Dmitry V. Smovzha a
Institute of Thermophysics SB RAS, prosp. Lavrentyev 1, 630090 Novosibirsk, Russia b Novosibirsk State University, Pirogov Str. 2, 630090 Novosibirsk, Russia
Abstract This work compares the methods of CVD-graphene transfer by hot pressing on a PET/EVA polymer and spin coating with dissolved PMMA polymer. Two variants of copper removal are considered: mechanical cleavage and chemical etching. The effect of these methods on transmittance and electrical resistance of graphene is investigated. It is shown that copper mechanical cleavage results in an increase in electrical resistance of graphene, which is associated with formation of additional defects in graphene. When PET/EVA polymer hot pressing is used, graphene is deformed due to the difference in thermal expansion coefficients. This also results in an increase in resistance of graphene. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2016 Russia-Japan conference Advanced Materials: Synthesis, Processing and Properties of Nanostructures. Keywords: transfer, graphene, CVD, polymer.
1. Introduction Due to such properties as high electro- and thermal conductivity, chemical stability, mechanical strength, graphene has become a promising material for many applications: opto- and nanoelectronics, composite materials, gas sensors, transparent electrodes, etc. [1]. When speaking about the properties of graphene, an ideal object is meant; it is one layer of carbon atoms without any defects in hexagonal lattice. In practice this lattice type can contain defects in the form of cells different from hexagons [2]. The sizes of individual graphene planes vary from several nanometers to several millimeters [1], and on their edges, there are the dangling bonds. These bonds are
* Corresponding author. Tel.:+7-961-216-39-26. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2016 Russia-Japan conference Advanced Materials: Synthesis, Processing and Properties of Nanostructures.
Ilya A. Kostogrud et al. / Materials Today: Proceedings 4 (2017) 11476–11479
11477
passivated by different elements, for example, by hydrogen atoms. All these aspects influence graphene properties, such as conductivity, transparency, stability, etc. Moreover, there is a concept of “few-layered” graphene [3, 4]. Currently, to form the transparent electrodes, indium-tin oxide (ITO) is used. In connection with mass production of transparent film electrodes for liquid crystalline displays (LCDs) and touch screens, the demand for indium increases together with its price. High conductivity and transmittance of graphene make it a good candidate to replace ITO electrodes. It is most promising to use graphene for making the flexible transparent conductors. The most perspective method to obtain large-area graphene films is chemical vapor deposition (CVD) on metal substrate, in particular, on copper [1, 5]. During the CVD process, the nucleation sites are formed on the copper surface, and graphene isles extend in all directions from these sites. Then they increase, reach the borders of each other, and cover all the surface of copper. The borders between the neighboring graphene isles are the defects. They can be deposited on each other. Irregularities, defects and copper grain boundaries transform to the graphene relief. Ideally it is necessary to obtain a continuous graphene surface without “joins” or isle boundaries. In practice, it is necessary to use a maximally smooth copper surface, tend to scattered location of nucleation sites, and perform synthesis until full coverage of the substrate so that there are no a gap between the graphene isle boundaries. The number of layers is the parameter under discussion in application to the transparent conductors. One layer of graphene absorbs 2.3% of normal incident light [6]. Maximal transmittance should be achieved for transparent conductors. On the contrary, a few layered structure will be more stable and reliable. The breaks and defects in one layer can be overlapped and compensated by the neighboring layers. This will provide less electrical resistance of the material. Copper is a conductor and it is not transparent in the visible range of optical radiation, so it is difficult to study the electrical and optical properties of graphene on copper substrate. Therefore, it is necessary to transfer graphene on a non-conductive transparent substrate. Graphene is bounded with the copper substrate by van der Waals interaction, and this is a relatively weak link. It is possible to select the materials with high adhesion to graphene, to overcome this interaction. Suitable candidates for the role of the substrate for graphene in application to the transparent conductors are different polymers. They can combine such properties as flexibility and high transmittance in the visible range of radiation, and they have different degrees of adhesion. For this purpose, various transfer methods are used [7, 8]. Graphene adhesion to polymer and copper plays the important role at transfer. To form a molecular bond, heterogeneous bodies should be approximated to the distance of molecular force action (several angstroms). The better contact over the entire surface requires, at least, one body in the form of liquid. The polymer can be converted from the solid to the liquid state by means melting or by using a solvent. The aim of this work is investigation of various methods of CVD-graphene transfer to the polymer. To study the influence of the transfer procedure, electrical resistance and transmittance of the samples were measured. 2. Experimental Section In this paper we used few layer graphene (3-5 layers), prepared by the CVD method on copper. This technique was described in detail in [9]. Graphene was synthesized on the copper foil (Alfa Aesar 99.8%) under the atmospheric pressure and following parameters: synthesis temperature of 1070⁰C, synthesis time of 10 minutes, and composition of gas mixture Ar (89) + H2 (20) + CH4 (0.22) sccm. The quality of graphene was estimated by using the method of Raman spectroscopy. Graphene was transferred using two types of polymers. In the first case, the copper substrate with graphene was placed between two layers of polyethylene terephthalate polymer (PET) coated with the adhesive composition of ethylene vinyl acetate (EVA). Then, this stack was placed into a hot press machine and stayed under the influence of pressure and temperature of 180⁰C for 10 minutes. The resulting stack was cooled to the room temperature, and the polymer was mechanically cleaved (Graphene/PET/EVA-mech). Since adhesion between the polymer and graphene is higher than between graphene and copper, it is transferred to the polymer. Copper was also removed by dissolving in a 40% solution of nitric acid, and then the sample was washed in distilled water (Graphene/PET/EVA-chem). In the second case, the polymethylmethacrylate (PMMA) polymer was dissolved in acetone and applied to copper coated with graphene by spin coating. Under the normal conditions, acetone evaporates and a PMMA film is formed. Time of complete evaporation depends on the film thickness. During polymerization and acetone
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Ilya A. Kostogrud et al. / Materials Today: Proceedings 4 (2017) 11476–11479
evaporation, the film volume changes together with the linear sizes. Due to this, the mechanical stress is formed, and the film bounces from the copper substrate. This effect can be minimized by reducing the thickness of polymer. Limiting the polymer thickness is determined by the strength and method of further processing. In our case, the PMMA film bounced from the graphene/copper substrate in about 40 minutes. In this case, there was no graphene transfer or it was partial. This is evidenced by the lack of conductivity on the resulting sample. In 10 minutes, the polymer firmed partially, and copper was etched in 40% nitric acid solution. After etching in acid solution, the film was washed in distilled water; at that time, it solidified completely (Graphene/PMMA-chem). Graphene remained on the polymer. This is demonstrated by sample conductivity. At mechanical separation of PMMA film from the copper substrate, its deformation or disruption occurs. The methods of graphene transfer are illustrated schematically in Figure 1.
Fig. 1. The stages of graphene transfer from copper to polymer: (a) hot pressing of PET-EVA polymer and copper substrate coated by graphene; (b) spin-coating of the PMMA film on copper substrate coated by graphene; (c) copper substrate removal by mechanical splitting; (d) chemical etching of copper in nitric acid.
Transmittance and electrical resistance of samples were measured. To characterize thin film conductivity, the value of sheet resistance (RSq), independent on the linear size of the region and expressed in Ω*square-1 (Ω sq-1) units, was used [10]. In our case to measure resistance, the films of 1*1 cm2 with two copper contacts on the edges were used. Film transmittance was measured in the visible spectrum on the wavelengths from 380 to 730 nm by the DFC-458 C spectrograph. In this paper, the average value of transmittance in the measured wavelengths range is indicated. Transmittance has an error of about 10% related to inhomogeneity of polymer film thickness. 3. Results and Discussion The results are presented in Table 1. Table 1. Transmittance and electrical resistance of samples. RSq (kΩ sq-1)
Sample
Transmittance (%)
PET/EVA
87,4
-
PMMA
92,4
-
Graphene/PET/EVA-chem
71,7
2.6
Graphene/PMMA-chem
65,6
1.2
Graphene/PET/EVA-mech
58,4
21
In the case of graphene mechanical transfer on PET/EVA, sample resistance was 21kΩ sq-1. In the case of copper chemical etching from PET, it was 2.6 kΩ sq-1. Conductivity increase can be explained by graphene deformation at mechanical transfer [11], see Fig. 2. While graphene was transferred on PMMA, the resistance was 1.2 kΩ sq-1, and this is smaller as compared with the value obtained for hot pressing transfer on PET/EVA. The coefficients of linear
Ilya A. Kostogrud et al. / Materials Today: Proceedings 4 (2017) 11476–11479
11479
thermal expansion of copper and EVA differ by an order of magnitude, 16.6*10-6 ⁰C-1 and 180*10-6 ⁰C-1, respectively. For the graphite plane, the linear thermal expansion coefficient is negative until the temperature of 427⁰C and equals -1.2*10-6 ⁰C-1. This is the reason why mechanical strains and deformations appear on the contact of these materials at heating and cooling. At hot pressing, the sample is heated to 180⁰C and then it is cooled to the room temperature. The formed mechanical strains damage the graphene film, thus increasing its resistance.
Copper
Polymer Fig. 2. Graphene deformation.
Transmittance in the visible spectrum for the PMMA film used without graphene is 92,4%, and for PET/EVA, it is 87,4%. Transmittance of graphene+PMMA composite was 65,6%. For graphene transferred on PET/EVA with mechanical removal of copper, transmittance was 58,4% and for the graphene+PET/EVA sample with chemical etching of copper, it was 71,7%. A significant decrease in transmittance in case of mechanical removal of copper from PET/EVA related to graphene deformation. In the case of dissolved PMMA deposition, formation of air bubbles in the resulting film was observed; this decreased film transmittance. To prevent this, the PMMA solution should be degassed before deposition. 4. Conclusions In this paper, the methods of CVD-graphene transfer through hot pressing on PET/EVA polymer and deposition of dissolved PMMA are compared; two options for copper removing are considered. The most promising method is transfer by deposition of dissolved PMMA with subsequent chemical etching of copper. Graphene is less damaged, and resulting resistance of the sample is lower. Degassing the PMMA solution allows enhancement of transmittance of graphene+PMMA composite. Although the transfer method with chemical etching of copper results in lower electrical resistance of samples, the practical application of graphene requires the development of transfer methods with preservation of the copper substrate for the repeated use. Acknowledgements The reported study was funded by RFBR, according to the research project No. 16-32-00230 mol_а. References [1] K.S Novoselov, V.I. Fal’ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, Nature 490 (2012) 192–200. [2] A. Hashimoto, K. Suenaga, A. Gloter, K. Urita, S. Iijima, Nature 430 (2004) 870–873. [3] A.K.Geim, K.S.Novoselov, Nat. Mater. 6 (2007) 183–191. [4] C. N. R. Rao, A. K. Sood, Rakesh Voggu, K. S. Subrahmanyam, J. Phys. Chem. Lett. 1 (2010) 572–580. [5] R. Muñoz, C. Gómez-Aleixandre, Chem. Vap. Depos. 19 (2013) 297–322. [6] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, A. K. Geim, Science 320 (2008) 1308 (1– 7). [7] J.Kang, D. Shin, S. Baea, B.H. Hong, Nanoscale 4 (2012) 5527–5537. [8] G. J.M. Fechine, I.Martin-Fernandez, G. Yiapanis, R. Bentini, E.S. Kulkarni, R. V. Bof de Oliveira, X. Hu, I. Yarovsky, A. H. Castro Neto, B. Ozyilmaz, Carbon 83 (2015) 224–231. [9] I. A. Kostogrud, K. V. Trusov, D.V. Smovzh, Advanced Materials Interfaces 3 (2016) 1500823 (1–6). [10] Y. Chen, X.-L. Gong, J.-G. Gai, Advanced Science 3 (2016) 1500343 (1-15). [11] C. Chen, C. Hsieh, Thin Solid Films 570 (2014) 595–598.
ScienceDirect Materials Today: Proceedings 4 (2017) 11476–11479
www.materialstoday.com/proceedings
RJCAM 2016
CVD Graphene Transfer from Copper Substrate to Polymer Ilya A. Kostogruda,*, Evgeniy V. Boykoa,b, Dmitry V. Smovzha a
Institute of Thermophysics SB RAS, prosp. Lavrentyev 1, 630090 Novosibirsk, Russia b Novosibirsk State University, Pirogov Str. 2, 630090 Novosibirsk, Russia
Abstract This work compares the methods of CVD-graphene transfer by hot pressing on a PET/EVA polymer and spin coating with dissolved PMMA polymer. Two variants of copper removal are considered: mechanical cleavage and chemical etching. The effect of these methods on transmittance and electrical resistance of graphene is investigated. It is shown that copper mechanical cleavage results in an increase in electrical resistance of graphene, which is associated with formation of additional defects in graphene. When PET/EVA polymer hot pressing is used, graphene is deformed due to the difference in thermal expansion coefficients. This also results in an increase in resistance of graphene. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2016 Russia-Japan conference Advanced Materials: Synthesis, Processing and Properties of Nanostructures. Keywords: transfer, graphene, CVD, polymer.
1. Introduction Due to such properties as high electro- and thermal conductivity, chemical stability, mechanical strength, graphene has become a promising material for many applications: opto- and nanoelectronics, composite materials, gas sensors, transparent electrodes, etc. [1]. When speaking about the properties of graphene, an ideal object is meant; it is one layer of carbon atoms without any defects in hexagonal lattice. In practice this lattice type can contain defects in the form of cells different from hexagons [2]. The sizes of individual graphene planes vary from several nanometers to several millimeters [1], and on their edges, there are the dangling bonds. These bonds are
* Corresponding author. Tel.:+7-961-216-39-26. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2016 Russia-Japan conference Advanced Materials: Synthesis, Processing and Properties of Nanostructures.
Ilya A. Kostogrud et al. / Materials Today: Proceedings 4 (2017) 11476–11479
11477
passivated by different elements, for example, by hydrogen atoms. All these aspects influence graphene properties, such as conductivity, transparency, stability, etc. Moreover, there is a concept of “few-layered” graphene [3, 4]. Currently, to form the transparent electrodes, indium-tin oxide (ITO) is used. In connection with mass production of transparent film electrodes for liquid crystalline displays (LCDs) and touch screens, the demand for indium increases together with its price. High conductivity and transmittance of graphene make it a good candidate to replace ITO electrodes. It is most promising to use graphene for making the flexible transparent conductors. The most perspective method to obtain large-area graphene films is chemical vapor deposition (CVD) on metal substrate, in particular, on copper [1, 5]. During the CVD process, the nucleation sites are formed on the copper surface, and graphene isles extend in all directions from these sites. Then they increase, reach the borders of each other, and cover all the surface of copper. The borders between the neighboring graphene isles are the defects. They can be deposited on each other. Irregularities, defects and copper grain boundaries transform to the graphene relief. Ideally it is necessary to obtain a continuous graphene surface without “joins” or isle boundaries. In practice, it is necessary to use a maximally smooth copper surface, tend to scattered location of nucleation sites, and perform synthesis until full coverage of the substrate so that there are no a gap between the graphene isle boundaries. The number of layers is the parameter under discussion in application to the transparent conductors. One layer of graphene absorbs 2.3% of normal incident light [6]. Maximal transmittance should be achieved for transparent conductors. On the contrary, a few layered structure will be more stable and reliable. The breaks and defects in one layer can be overlapped and compensated by the neighboring layers. This will provide less electrical resistance of the material. Copper is a conductor and it is not transparent in the visible range of optical radiation, so it is difficult to study the electrical and optical properties of graphene on copper substrate. Therefore, it is necessary to transfer graphene on a non-conductive transparent substrate. Graphene is bounded with the copper substrate by van der Waals interaction, and this is a relatively weak link. It is possible to select the materials with high adhesion to graphene, to overcome this interaction. Suitable candidates for the role of the substrate for graphene in application to the transparent conductors are different polymers. They can combine such properties as flexibility and high transmittance in the visible range of radiation, and they have different degrees of adhesion. For this purpose, various transfer methods are used [7, 8]. Graphene adhesion to polymer and copper plays the important role at transfer. To form a molecular bond, heterogeneous bodies should be approximated to the distance of molecular force action (several angstroms). The better contact over the entire surface requires, at least, one body in the form of liquid. The polymer can be converted from the solid to the liquid state by means melting or by using a solvent. The aim of this work is investigation of various methods of CVD-graphene transfer to the polymer. To study the influence of the transfer procedure, electrical resistance and transmittance of the samples were measured. 2. Experimental Section In this paper we used few layer graphene (3-5 layers), prepared by the CVD method on copper. This technique was described in detail in [9]. Graphene was synthesized on the copper foil (Alfa Aesar 99.8%) under the atmospheric pressure and following parameters: synthesis temperature of 1070⁰C, synthesis time of 10 minutes, and composition of gas mixture Ar (89) + H2 (20) + CH4 (0.22) sccm. The quality of graphene was estimated by using the method of Raman spectroscopy. Graphene was transferred using two types of polymers. In the first case, the copper substrate with graphene was placed between two layers of polyethylene terephthalate polymer (PET) coated with the adhesive composition of ethylene vinyl acetate (EVA). Then, this stack was placed into a hot press machine and stayed under the influence of pressure and temperature of 180⁰C for 10 minutes. The resulting stack was cooled to the room temperature, and the polymer was mechanically cleaved (Graphene/PET/EVA-mech). Since adhesion between the polymer and graphene is higher than between graphene and copper, it is transferred to the polymer. Copper was also removed by dissolving in a 40% solution of nitric acid, and then the sample was washed in distilled water (Graphene/PET/EVA-chem). In the second case, the polymethylmethacrylate (PMMA) polymer was dissolved in acetone and applied to copper coated with graphene by spin coating. Under the normal conditions, acetone evaporates and a PMMA film is formed. Time of complete evaporation depends on the film thickness. During polymerization and acetone
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Ilya A. Kostogrud et al. / Materials Today: Proceedings 4 (2017) 11476–11479
evaporation, the film volume changes together with the linear sizes. Due to this, the mechanical stress is formed, and the film bounces from the copper substrate. This effect can be minimized by reducing the thickness of polymer. Limiting the polymer thickness is determined by the strength and method of further processing. In our case, the PMMA film bounced from the graphene/copper substrate in about 40 minutes. In this case, there was no graphene transfer or it was partial. This is evidenced by the lack of conductivity on the resulting sample. In 10 minutes, the polymer firmed partially, and copper was etched in 40% nitric acid solution. After etching in acid solution, the film was washed in distilled water; at that time, it solidified completely (Graphene/PMMA-chem). Graphene remained on the polymer. This is demonstrated by sample conductivity. At mechanical separation of PMMA film from the copper substrate, its deformation or disruption occurs. The methods of graphene transfer are illustrated schematically in Figure 1.
Fig. 1. The stages of graphene transfer from copper to polymer: (a) hot pressing of PET-EVA polymer and copper substrate coated by graphene; (b) spin-coating of the PMMA film on copper substrate coated by graphene; (c) copper substrate removal by mechanical splitting; (d) chemical etching of copper in nitric acid.
Transmittance and electrical resistance of samples were measured. To characterize thin film conductivity, the value of sheet resistance (RSq), independent on the linear size of the region and expressed in Ω*square-1 (Ω sq-1) units, was used [10]. In our case to measure resistance, the films of 1*1 cm2 with two copper contacts on the edges were used. Film transmittance was measured in the visible spectrum on the wavelengths from 380 to 730 nm by the DFC-458 C spectrograph. In this paper, the average value of transmittance in the measured wavelengths range is indicated. Transmittance has an error of about 10% related to inhomogeneity of polymer film thickness. 3. Results and Discussion The results are presented in Table 1. Table 1. Transmittance and electrical resistance of samples. RSq (kΩ sq-1)
Sample
Transmittance (%)
PET/EVA
87,4
-
PMMA
92,4
-
Graphene/PET/EVA-chem
71,7
2.6
Graphene/PMMA-chem
65,6
1.2
Graphene/PET/EVA-mech
58,4
21
In the case of graphene mechanical transfer on PET/EVA, sample resistance was 21kΩ sq-1. In the case of copper chemical etching from PET, it was 2.6 kΩ sq-1. Conductivity increase can be explained by graphene deformation at mechanical transfer [11], see Fig. 2. While graphene was transferred on PMMA, the resistance was 1.2 kΩ sq-1, and this is smaller as compared with the value obtained for hot pressing transfer on PET/EVA. The coefficients of linear
Ilya A. Kostogrud et al. / Materials Today: Proceedings 4 (2017) 11476–11479
11479
thermal expansion of copper and EVA differ by an order of magnitude, 16.6*10-6 ⁰C-1 and 180*10-6 ⁰C-1, respectively. For the graphite plane, the linear thermal expansion coefficient is negative until the temperature of 427⁰C and equals -1.2*10-6 ⁰C-1. This is the reason why mechanical strains and deformations appear on the contact of these materials at heating and cooling. At hot pressing, the sample is heated to 180⁰C and then it is cooled to the room temperature. The formed mechanical strains damage the graphene film, thus increasing its resistance.
Copper
Polymer Fig. 2. Graphene deformation.
Transmittance in the visible spectrum for the PMMA film used without graphene is 92,4%, and for PET/EVA, it is 87,4%. Transmittance of graphene+PMMA composite was 65,6%. For graphene transferred on PET/EVA with mechanical removal of copper, transmittance was 58,4% and for the graphene+PET/EVA sample with chemical etching of copper, it was 71,7%. A significant decrease in transmittance in case of mechanical removal of copper from PET/EVA related to graphene deformation. In the case of dissolved PMMA deposition, formation of air bubbles in the resulting film was observed; this decreased film transmittance. To prevent this, the PMMA solution should be degassed before deposition. 4. Conclusions In this paper, the methods of CVD-graphene transfer through hot pressing on PET/EVA polymer and deposition of dissolved PMMA are compared; two options for copper removing are considered. The most promising method is transfer by deposition of dissolved PMMA with subsequent chemical etching of copper. Graphene is less damaged, and resulting resistance of the sample is lower. Degassing the PMMA solution allows enhancement of transmittance of graphene+PMMA composite. Although the transfer method with chemical etching of copper results in lower electrical resistance of samples, the practical application of graphene requires the development of transfer methods with preservation of the copper substrate for the repeated use. Acknowledgements The reported study was funded by RFBR, according to the research project No. 16-32-00230 mol_а. References [1] K.S Novoselov, V.I. Fal’ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, Nature 490 (2012) 192–200. [2] A. Hashimoto, K. Suenaga, A. Gloter, K. Urita, S. Iijima, Nature 430 (2004) 870–873. [3] A.K.Geim, K.S.Novoselov, Nat. Mater. 6 (2007) 183–191. [4] C. N. R. Rao, A. K. Sood, Rakesh Voggu, K. S. Subrahmanyam, J. Phys. Chem. Lett. 1 (2010) 572–580. [5] R. Muñoz, C. Gómez-Aleixandre, Chem. Vap. Depos. 19 (2013) 297–322. [6] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, A. K. Geim, Science 320 (2008) 1308 (1– 7). [7] J.Kang, D. Shin, S. Baea, B.H. Hong, Nanoscale 4 (2012) 5527–5537. [8] G. J.M. Fechine, I.Martin-Fernandez, G. Yiapanis, R. Bentini, E.S. Kulkarni, R. V. Bof de Oliveira, X. Hu, I. Yarovsky, A. H. Castro Neto, B. Ozyilmaz, Carbon 83 (2015) 224–231. [9] I. A. Kostogrud, K. V. Trusov, D.V. Smovzh, Advanced Materials Interfaces 3 (2016) 1500823 (1–6). [10] Y. Chen, X.-L. Gong, J.-G. Gai, Advanced Science 3 (2016) 1500343 (1-15). [11] C. Chen, C. Hsieh, Thin Solid Films 570 (2014) 595–598.