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Fabrication of enhanced silver nanowire films via self-assembled gold nanoparticles without post-treatment
Materials Letters 236 (2019) 218–221
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Fabrication of enhanced silver nanowire films via self-assembled gold nanoparticles without post-treatment Qi Su a, Tao Xue b, Yibo Zhang a, Kuibo Lan a, Qiang Zou a,⇑ a b
School of Microelectronics, Tianjin University, Tianjin 300072, PR China Analysis and Testing Center, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 4 August 2018 Received in revised form 15 October 2018 Accepted 15 October 2018 Available online 16 October 2018 Keywords: Transparent electrode Silver nanowire Gold nanoparticle Microstructure Thin films
a b s t r a c t Transparent Silver nanowire (AgNW) electrodes have been widely explored in many applications as a promising alternative to relatively expensive indium tin oxide (ITO). For these applications, high contact resistance at the junctions of AgNWs is a main problem to be solved. In this letter, gold nanoparticles (AuNPs)-self-assembled method has been demonstrated to improve the optoelectronic performance of AgNW films. The sheet resistance of this novel AgNW film is measured as low as 13.2 X/sq, and the transmittance is high to 86.1%. Self-assembled AuNPs can enrich the percolation paths between AgNWs with negligible loss of optical properties, thereby realizing the excellent optoelectronic performance of AgNW films. Moreover, no extra post-treatment procedures are needed for the fabrication process. All these advantages indicate this method has great potentials for achieving high performance transparent electrodes in emerging optoelectronic devices. Ó 2018 Published by Elsevier B.V.
1. Introduction Transparent conductive electrodes (TCE) have been widely applied in a large ranges of optoelectronic devices such as solar cells, organic light-emitting diodes and touch panels [1]. ITO is the most common material used for TCE, which, however, is limited in large-scale applications because of the scarcity of indium and expensive vacuum deposition process. Therefore, other candidates for TCE, such as carbon nanotubes (CNTs), graphene, conducting polymers, and metallic nanostructures, have been investigated [2]. Among these materials, AgNW transparent films have promising optoelectronic performances. Moreover, largescale solution processes can be conveniently applied to fabricate AgNW films, which are efficient and cost-effective compared with ITO [3]. However, fabrication of AgNW with low sheet resistance at a high optical transmittance is still a challenge. A series of posttreatment methods, including thermal annealing, mechanical pressing, plasmonic welding, joule heating, conducting-material joining, ion beam irradiation, etc., have been explored to improve the optoelectronic properties of AgNW electrodes [3,4–7]. But these post-treatment methods are complicated and need specific equipments [8]. As a result, it is highly challenging to develop a simple as well as effective way to enhance the optoelectronic
⇑ Corresponding author. E-mail address: [email protected] (Q. Zou). https://doi.org/10.1016/j.matlet.2018.10.099 0167-577X/Ó 2018 Published by Elsevier B.V.
performance without extra post-treatment procedures. In this study, we try to resolve this problem by exploring a simple fabrication method without post-treatment, which is based on selfassembled AuNPs, to strengthen the optoelectronic performance of AgNW films. A gold nanoparticles-self-assembled (GNSA) AgNW film, with both a low sheet resistance (13.2 O/sq) and a high transmittance (86.1% at 550 nm), was fabricated on glass substrates. The excellent optoelectronic performance of GNSA AgNW film accounts for its potential application as a transparent conductor.
2. Experimental procedure Water solution of AuNPs (diameter: <10 nm, concentration: 0.2 g/mL) and ethanol solution of AgNWs (diameter: 90 nm, concentration: 20 g/mL) were prepared for the fabrication. AuNPs and AgNWs solution were mixed at a ratio of 50:1 for 30 min, then the mixture was centrifuged at 6000 rpm for 10 s. After removing the supernatant AuNPs, ethanol was added into the mixture until the concentration of AuNPs-decorated AgNWs solution was 13.3 mg/mL. Then AgNW film was fabricated by rod-coating of 10 lL AgNW solution on glass substrate (15 mm 40 mm). By repeating the coating process, AgNW films with different optoelectronic performances were realized. The sheet resistance was measured by four-point probe measurement (FOUR PROBES TECH, model RTS-8) and scanning electron microscopy (SEM) images were taken on a NanoSEM 430 electron microscope. The transmit-
Q. Su et al. / Materials Letters 236 (2019) 218–221
tance spectrum were taken by an ultraviolet-visible spectrophotometer (METACH, UV-5100). Finite element method (FEM) simulations were performed using Comsol Multiphysics. 3. Results and discussion Parts of AgNW surface are attached with AuNPs after adding AuNPs solution into pristine AgNW solution. The schematic morphologies of both pristine AgNW and AuNPs-decorated AgNW solution are shown in Fig. 1a,b, respectively. The SEM image of pristine AgNW are shown in Fig. 1e,f, indicating a smooth surface. However, the AuNPs-decorated AgNWs show a rough surface in Fig. 1g,h. To verify that the AgNWs are decorated with AuNPs, Energy Dispersive X-Ray (EDX) mappings of Fig. 1g are illustrated in Fig. 1i,j. After each rod-coating process, the thin liquid (20 lm) films of ethanol, which is covering entire substrate, quickly break up into a large amount of small slices. These small slices of ethanol are more likely aggregate at nanowire junctions and nanogaps that acting as a certain kind of capillaries [1], as illustrated in Fig. 1c. Parts of the AuNPs decorated on the nanowire surface are gradually released into ethanol, while the moving range of AuNPs depends on the size of ethanol droplet. Most of the AuNPs in the ethanol droplet tend to move towards the center of droplet due to the surface tension gradient and this phenomenon is described as Marangoni effect [9]. After ethanol evaporation process completing, most of the AuNPs in the droplet are absorbed at junction and nanogaps, as illustrated in Fig. 1d and corresponding SEM image (Fig. 1k). The further evidence that AuNPs are self-assembled at the junctions and nanogaps are shown in Fig. 1l and its broader view Fig. S1. The two lines of AuNPs are parallel to each other and the blank space between them is the trace of AgNW, which is shifted to other
219
place due to mechanical force in rod-coating process. It can be seen that most of AuNPs are aggregated in the region of two lines and few AuNPs are absorbed on the other substrate, indicating that AuNPs are self-assembled by capillary force. To further understand the effect of self-assembled AuNPs on reducing contact resistance of AgNW junctions, FEM simulations on AgNW junctions are carried out. As illustrated in Fig. 2a, there is only a small point acting as current path, thus suffering from high contact resistance at the junctions of AgNWs. By contrast, when there are some self-assembled AuNPs at the junctions, the number of current paths increases and the distribution of potentials becomes more uniform. Therefore, the contact resistance is reduced to a lower value. The sheet resistances of the GNSA and pristine AgNW film with varying times are shown in Fig. 2c. Compared with the infinite sheet resistance of pristine AgNW, the sheet resistance of GNSA AgNW film is only 433.3 X/sq after the first time of coating. The sheet resistance of pristine AgNW film reduces to 39.4 X/sq after ten coating times. On the other hand, the sheet resistance of GNSA AgNW film decreases to 38.4 X/sq with only five coating times. As a consequence, it can be concluded that AuNPs at the junctions lead to lower resistances for the excellent electrical performance of GNSA AgNW films. The sheet resistance and transmittance of GNSA AgNW film are also illustrated in Fig. 2d. At 550 nm wavelength, the resistances are measured as 73.5, 38.4, 21.3, 13.2 X/sq, corresponding to 94.1, 89.7, 88.8 and 86.1% transmittance, respectively. GNSA AgNW film without post-treatment shows excellent optoelectronic performance compared to most of new TCE, including graphene-AgNW hybrid [2], polymer-soldered AgNW [3], graphene-wrapped AgNW [4], high force pressed AgNW [8], CNTs [10], graphene [11] and CuNWs [12], as illustrated in Fig. 2e. The AuNPs make little difference on
Fig. 1. Schematic of fabrication procedures and corresponding SEM images of AgNW film. (a,b,c,d) Schematic of fabrication procedures, the insert of (d) is the schematic of entire AgNW film. SEM image of (e,f) pristine AgNW film, (g,h) AuNPs-decorated AgNWs. (i,j) EDX mapping of (g). (k) SEM image of self-assembled AuNPs on AgNW film and (l) the trace of self-assembled AuNPs.
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Q. Su et al. / Materials Letters 236 (2019) 218–221
Fig. 2. FEM simulations of potential distribution at the AgNW junctions and experiments of Optoelectronic performance of GNSA AgNW film. FEM simulations of (a) pristine AgNW junction and (b) GNSA junction. (c) Experimental data of sheet resistances of pristine AgNW film and GNSA AgNW film as a function of the coating time. (d) Experimental data of transmittance spectra of GNSA AgNW film with four different sheet resistances. (e) Comparison of the Optoelectronic performance of the GNSA AgNW film (our work) with various TCE published in the literature and (f) pristine AgNW film.
the transmittance of the GNSA AgNW film because most of AuNPs are located at the junctions and nanogaps and few AuNPs are left on the substrate. However, the AuNPs located at the junctions decrease contact resistance greatly. Therefore, the high optoelectronic performance of GNSA AgNW film attributes to the lower contact resistance at a high transmittance. Further evidence is presented in Fig. 2f, the only difference between GNSA and pristine AgNW film is whether there are AuNPs. Fig. 3a presents the circuit diagram that transparent conductor is connected to a power source of 1.8 V and Light Emitting Diode
(LED). As shown in Fig. 3b, the sheet resistance and transmittance of the pristine AgNW film are 13.4 X/sq and 39.3%, respectively. The transmittance of GNSA AgNW film is, however, more than twice as much as that of pristine AgNW film with the similar sheet resistance (13.7 X/sq). Beside standard optoelectronic applications, AgNW films also are targeted for space applications. Therefore, not only optoelectronic performance but also irradiation stability of AgNW film are of great importance [13–15]. And these can be further explored in future researches.
Q. Su et al. / Materials Letters 236 (2019) 218–221
221
Fig. 3. (a) Circuit diagram including AgNW transparent conductor. (b) Pristine AgNW film and (c) GNSA AgNW film as transparent conductor.
4. Conclusions
References
To sum up, we have reported that AuNPs were self-assembled at the AgNW junctions and nanogaps by simple rod-coating procedures. The pristine AgNW film suffers from relatively high sheet resistance due to loose contact of junctions. Self-assembled AuNPs at the junctions increase the contact area of AgNWs, thus reducing the contact resistance. A much more convenient fabrication method is explored without any post-treatment. The excellent optoelectronic performance of the GNSA AgNW film make it great potentials as a transparent conductor in many optoelectronics applications.
[1] Y. Liu, J. Zhang, H. Gao, Y. Wang, Q. Liu, S. Huang, C.F. Guo, Z. Ren, Nano Lett. 17 (2017) 1090–1096. [2] M.-S. Lee, K. Lee, S.-Y. Kim, H. Lee, J. Park, K.-H. Choi, H.-K. Kim, D.-G. Kim, D.-Y. Lee, S. Nam, J.-U. Park, Nano Lett. 13 (2013) 2814–2821. [3] X. Liang, T. Zhao, P. Zhu, Y. Hu, R. Sun, C.-P. Wong, ACS Appl. Mater. Inter. 9 (2017) 40857–40867. [4] R. Chen, S.R. Das, C. Jeong, M.R. Khan, D.B. Janes, M.A. Alam, Adv. Funct. Mater. 23 (2013) 5150–5158. [5] B. Bari, S. Honey, M. Morgan, I. Ahmad, R. Khan, A. Muhammad, K. Alamgir, S. Naseem, M. Malik, Curr. Appl. Phys. 15 (2015) 642–647. [6] H. Shehla, A. Ishaq, A. Awais, F.T. Thema, I. Javed, D. Wang, K. Rauf, K. Nasir, N. Shahzad, M. Maaza, Curr. Nanosci. 12 (2016) 774–780. [7] S. Honey, S. Khan, J. Iqbal, M. Madhuku, I. Ahmad, S. Naseem, M. Malik, Curr. Nanosci. 11 (2015) 792–796. [8] S. De, T.M. Higgins, P.E. Lyons, E.M. Doherty, P.N. Nirmalraj, W.J. Blau, J.J. Boland, J.N. Coleman, ACS Nano 3 (2009) 1767–1774. [9] X. Xu, J. Luo, Appl. Phys. Lett. 91 (2007) 124102. [10] R.V. Salvatierra, C.E. Cava, L.S. Roman, A.J.G. Zarbin, Adv. Funct. Mater. 23 (2013) 1490–1499. [11] Y. Yang, J. Dai, Y. Li, M. Jia, X. Huang, Carbon 115 (2017) 285–292. [12] A.R. Rathmell, B.J. Wiley, Adv. Mater. 23 (2011) 4798–4803. [13] M. Maaza, O. Nemraoui, A.C. Beye, C. Sella, T. Derry, Sol. Energ. Mat. Sol. C 90 (2006) 111–119. [14] C.B. Mtshali, L. Kotsedi, B.D. Ngom, C.L. Ndlangamandla, O.M. Ndwandwe, M. Maaza, Nucl. Instrum. Meth. B 296 (2013) 22–25. [15] N. Thovhogi, C. Mtshali, L. Kotsedi, T. Doyle, M. Maaza, Mater. Today 2 (2015) 4053–4059.
Acknowledgment This work was supported by the Project Technologically Beneficial to People of Qiangdao (China) (17-3-3-90-nsh) and Science and Technology Support Key Project of Tianjin (China) (16YFZCSY00850). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2018.10.099.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Fabrication of enhanced silver nanowire films via self-assembled gold nanoparticles without post-treatment Qi Su a, Tao Xue b, Yibo Zhang a, Kuibo Lan a, Qiang Zou a,⇑ a b
School of Microelectronics, Tianjin University, Tianjin 300072, PR China Analysis and Testing Center, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 4 August 2018 Received in revised form 15 October 2018 Accepted 15 October 2018 Available online 16 October 2018 Keywords: Transparent electrode Silver nanowire Gold nanoparticle Microstructure Thin films
a b s t r a c t Transparent Silver nanowire (AgNW) electrodes have been widely explored in many applications as a promising alternative to relatively expensive indium tin oxide (ITO). For these applications, high contact resistance at the junctions of AgNWs is a main problem to be solved. In this letter, gold nanoparticles (AuNPs)-self-assembled method has been demonstrated to improve the optoelectronic performance of AgNW films. The sheet resistance of this novel AgNW film is measured as low as 13.2 X/sq, and the transmittance is high to 86.1%. Self-assembled AuNPs can enrich the percolation paths between AgNWs with negligible loss of optical properties, thereby realizing the excellent optoelectronic performance of AgNW films. Moreover, no extra post-treatment procedures are needed for the fabrication process. All these advantages indicate this method has great potentials for achieving high performance transparent electrodes in emerging optoelectronic devices. Ó 2018 Published by Elsevier B.V.
1. Introduction Transparent conductive electrodes (TCE) have been widely applied in a large ranges of optoelectronic devices such as solar cells, organic light-emitting diodes and touch panels [1]. ITO is the most common material used for TCE, which, however, is limited in large-scale applications because of the scarcity of indium and expensive vacuum deposition process. Therefore, other candidates for TCE, such as carbon nanotubes (CNTs), graphene, conducting polymers, and metallic nanostructures, have been investigated [2]. Among these materials, AgNW transparent films have promising optoelectronic performances. Moreover, largescale solution processes can be conveniently applied to fabricate AgNW films, which are efficient and cost-effective compared with ITO [3]. However, fabrication of AgNW with low sheet resistance at a high optical transmittance is still a challenge. A series of posttreatment methods, including thermal annealing, mechanical pressing, plasmonic welding, joule heating, conducting-material joining, ion beam irradiation, etc., have been explored to improve the optoelectronic properties of AgNW electrodes [3,4–7]. But these post-treatment methods are complicated and need specific equipments [8]. As a result, it is highly challenging to develop a simple as well as effective way to enhance the optoelectronic
⇑ Corresponding author. E-mail address: [email protected] (Q. Zou). https://doi.org/10.1016/j.matlet.2018.10.099 0167-577X/Ó 2018 Published by Elsevier B.V.
performance without extra post-treatment procedures. In this study, we try to resolve this problem by exploring a simple fabrication method without post-treatment, which is based on selfassembled AuNPs, to strengthen the optoelectronic performance of AgNW films. A gold nanoparticles-self-assembled (GNSA) AgNW film, with both a low sheet resistance (13.2 O/sq) and a high transmittance (86.1% at 550 nm), was fabricated on glass substrates. The excellent optoelectronic performance of GNSA AgNW film accounts for its potential application as a transparent conductor.
2. Experimental procedure Water solution of AuNPs (diameter: <10 nm, concentration: 0.2 g/mL) and ethanol solution of AgNWs (diameter: 90 nm, concentration: 20 g/mL) were prepared for the fabrication. AuNPs and AgNWs solution were mixed at a ratio of 50:1 for 30 min, then the mixture was centrifuged at 6000 rpm for 10 s. After removing the supernatant AuNPs, ethanol was added into the mixture until the concentration of AuNPs-decorated AgNWs solution was 13.3 mg/mL. Then AgNW film was fabricated by rod-coating of 10 lL AgNW solution on glass substrate (15 mm 40 mm). By repeating the coating process, AgNW films with different optoelectronic performances were realized. The sheet resistance was measured by four-point probe measurement (FOUR PROBES TECH, model RTS-8) and scanning electron microscopy (SEM) images were taken on a NanoSEM 430 electron microscope. The transmit-
Q. Su et al. / Materials Letters 236 (2019) 218–221
tance spectrum were taken by an ultraviolet-visible spectrophotometer (METACH, UV-5100). Finite element method (FEM) simulations were performed using Comsol Multiphysics. 3. Results and discussion Parts of AgNW surface are attached with AuNPs after adding AuNPs solution into pristine AgNW solution. The schematic morphologies of both pristine AgNW and AuNPs-decorated AgNW solution are shown in Fig. 1a,b, respectively. The SEM image of pristine AgNW are shown in Fig. 1e,f, indicating a smooth surface. However, the AuNPs-decorated AgNWs show a rough surface in Fig. 1g,h. To verify that the AgNWs are decorated with AuNPs, Energy Dispersive X-Ray (EDX) mappings of Fig. 1g are illustrated in Fig. 1i,j. After each rod-coating process, the thin liquid (20 lm) films of ethanol, which is covering entire substrate, quickly break up into a large amount of small slices. These small slices of ethanol are more likely aggregate at nanowire junctions and nanogaps that acting as a certain kind of capillaries [1], as illustrated in Fig. 1c. Parts of the AuNPs decorated on the nanowire surface are gradually released into ethanol, while the moving range of AuNPs depends on the size of ethanol droplet. Most of the AuNPs in the ethanol droplet tend to move towards the center of droplet due to the surface tension gradient and this phenomenon is described as Marangoni effect [9]. After ethanol evaporation process completing, most of the AuNPs in the droplet are absorbed at junction and nanogaps, as illustrated in Fig. 1d and corresponding SEM image (Fig. 1k). The further evidence that AuNPs are self-assembled at the junctions and nanogaps are shown in Fig. 1l and its broader view Fig. S1. The two lines of AuNPs are parallel to each other and the blank space between them is the trace of AgNW, which is shifted to other
219
place due to mechanical force in rod-coating process. It can be seen that most of AuNPs are aggregated in the region of two lines and few AuNPs are absorbed on the other substrate, indicating that AuNPs are self-assembled by capillary force. To further understand the effect of self-assembled AuNPs on reducing contact resistance of AgNW junctions, FEM simulations on AgNW junctions are carried out. As illustrated in Fig. 2a, there is only a small point acting as current path, thus suffering from high contact resistance at the junctions of AgNWs. By contrast, when there are some self-assembled AuNPs at the junctions, the number of current paths increases and the distribution of potentials becomes more uniform. Therefore, the contact resistance is reduced to a lower value. The sheet resistances of the GNSA and pristine AgNW film with varying times are shown in Fig. 2c. Compared with the infinite sheet resistance of pristine AgNW, the sheet resistance of GNSA AgNW film is only 433.3 X/sq after the first time of coating. The sheet resistance of pristine AgNW film reduces to 39.4 X/sq after ten coating times. On the other hand, the sheet resistance of GNSA AgNW film decreases to 38.4 X/sq with only five coating times. As a consequence, it can be concluded that AuNPs at the junctions lead to lower resistances for the excellent electrical performance of GNSA AgNW films. The sheet resistance and transmittance of GNSA AgNW film are also illustrated in Fig. 2d. At 550 nm wavelength, the resistances are measured as 73.5, 38.4, 21.3, 13.2 X/sq, corresponding to 94.1, 89.7, 88.8 and 86.1% transmittance, respectively. GNSA AgNW film without post-treatment shows excellent optoelectronic performance compared to most of new TCE, including graphene-AgNW hybrid [2], polymer-soldered AgNW [3], graphene-wrapped AgNW [4], high force pressed AgNW [8], CNTs [10], graphene [11] and CuNWs [12], as illustrated in Fig. 2e. The AuNPs make little difference on
Fig. 1. Schematic of fabrication procedures and corresponding SEM images of AgNW film. (a,b,c,d) Schematic of fabrication procedures, the insert of (d) is the schematic of entire AgNW film. SEM image of (e,f) pristine AgNW film, (g,h) AuNPs-decorated AgNWs. (i,j) EDX mapping of (g). (k) SEM image of self-assembled AuNPs on AgNW film and (l) the trace of self-assembled AuNPs.
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Q. Su et al. / Materials Letters 236 (2019) 218–221
Fig. 2. FEM simulations of potential distribution at the AgNW junctions and experiments of Optoelectronic performance of GNSA AgNW film. FEM simulations of (a) pristine AgNW junction and (b) GNSA junction. (c) Experimental data of sheet resistances of pristine AgNW film and GNSA AgNW film as a function of the coating time. (d) Experimental data of transmittance spectra of GNSA AgNW film with four different sheet resistances. (e) Comparison of the Optoelectronic performance of the GNSA AgNW film (our work) with various TCE published in the literature and (f) pristine AgNW film.
the transmittance of the GNSA AgNW film because most of AuNPs are located at the junctions and nanogaps and few AuNPs are left on the substrate. However, the AuNPs located at the junctions decrease contact resistance greatly. Therefore, the high optoelectronic performance of GNSA AgNW film attributes to the lower contact resistance at a high transmittance. Further evidence is presented in Fig. 2f, the only difference between GNSA and pristine AgNW film is whether there are AuNPs. Fig. 3a presents the circuit diagram that transparent conductor is connected to a power source of 1.8 V and Light Emitting Diode
(LED). As shown in Fig. 3b, the sheet resistance and transmittance of the pristine AgNW film are 13.4 X/sq and 39.3%, respectively. The transmittance of GNSA AgNW film is, however, more than twice as much as that of pristine AgNW film with the similar sheet resistance (13.7 X/sq). Beside standard optoelectronic applications, AgNW films also are targeted for space applications. Therefore, not only optoelectronic performance but also irradiation stability of AgNW film are of great importance [13–15]. And these can be further explored in future researches.
Q. Su et al. / Materials Letters 236 (2019) 218–221
221
Fig. 3. (a) Circuit diagram including AgNW transparent conductor. (b) Pristine AgNW film and (c) GNSA AgNW film as transparent conductor.
4. Conclusions
References
To sum up, we have reported that AuNPs were self-assembled at the AgNW junctions and nanogaps by simple rod-coating procedures. The pristine AgNW film suffers from relatively high sheet resistance due to loose contact of junctions. Self-assembled AuNPs at the junctions increase the contact area of AgNWs, thus reducing the contact resistance. A much more convenient fabrication method is explored without any post-treatment. The excellent optoelectronic performance of the GNSA AgNW film make it great potentials as a transparent conductor in many optoelectronics applications.
[1] Y. Liu, J. Zhang, H. Gao, Y. Wang, Q. Liu, S. Huang, C.F. Guo, Z. Ren, Nano Lett. 17 (2017) 1090–1096. [2] M.-S. Lee, K. Lee, S.-Y. Kim, H. Lee, J. Park, K.-H. Choi, H.-K. Kim, D.-G. Kim, D.-Y. Lee, S. Nam, J.-U. Park, Nano Lett. 13 (2013) 2814–2821. [3] X. Liang, T. Zhao, P. Zhu, Y. Hu, R. Sun, C.-P. Wong, ACS Appl. Mater. Inter. 9 (2017) 40857–40867. [4] R. Chen, S.R. Das, C. Jeong, M.R. Khan, D.B. Janes, M.A. Alam, Adv. Funct. Mater. 23 (2013) 5150–5158. [5] B. Bari, S. Honey, M. Morgan, I. Ahmad, R. Khan, A. Muhammad, K. Alamgir, S. Naseem, M. Malik, Curr. Appl. Phys. 15 (2015) 642–647. [6] H. Shehla, A. Ishaq, A. Awais, F.T. Thema, I. Javed, D. Wang, K. Rauf, K. Nasir, N. Shahzad, M. Maaza, Curr. Nanosci. 12 (2016) 774–780. [7] S. Honey, S. Khan, J. Iqbal, M. Madhuku, I. Ahmad, S. Naseem, M. Malik, Curr. Nanosci. 11 (2015) 792–796. [8] S. De, T.M. Higgins, P.E. Lyons, E.M. Doherty, P.N. Nirmalraj, W.J. Blau, J.J. Boland, J.N. Coleman, ACS Nano 3 (2009) 1767–1774. [9] X. Xu, J. Luo, Appl. Phys. Lett. 91 (2007) 124102. [10] R.V. Salvatierra, C.E. Cava, L.S. Roman, A.J.G. Zarbin, Adv. Funct. Mater. 23 (2013) 1490–1499. [11] Y. Yang, J. Dai, Y. Li, M. Jia, X. Huang, Carbon 115 (2017) 285–292. [12] A.R. Rathmell, B.J. Wiley, Adv. Mater. 23 (2011) 4798–4803. [13] M. Maaza, O. Nemraoui, A.C. Beye, C. Sella, T. Derry, Sol. Energ. Mat. Sol. C 90 (2006) 111–119. [14] C.B. Mtshali, L. Kotsedi, B.D. Ngom, C.L. Ndlangamandla, O.M. Ndwandwe, M. Maaza, Nucl. Instrum. Meth. B 296 (2013) 22–25. [15] N. Thovhogi, C. Mtshali, L. Kotsedi, T. Doyle, M. Maaza, Mater. Today 2 (2015) 4053–4059.
Acknowledgment This work was supported by the Project Technologically Beneficial to People of Qiangdao (China) (17-3-3-90-nsh) and Science and Technology Support Key Project of Tianjin (China) (16YFZCSY00850). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2018.10.099.