Ultra-high aspect ratio copper-nanowire-based hybrid transparent ...
Ultra-high aspect ratio copper-nanowire-based hybrid transparent conductive electrodes with PEDOT:PSS and reduced Graphene Oxide exhibiting reduced surface roughness and improved stability Zhaozhao Zhu*†, Trent Mankowski*†, Kaushik Balakrishnan†, Ali Sehpar Shikoh‡, Farid Touati‡, Mohieddine A. Benammar‡, Masud Mansuripur†, and Charles M. Falco† *E-mail: [email protected] *E-mail: [email protected] †
College of Optical Sciences, The University of Arizona, Tucson, Arizona, USA ‡
Department of Electrical Engineering, Qatar University, Doha, Qatar
Supplementary information
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Copper nanowire aspect ratio calculation The CuNWs were synthesized and deposited onto glass substrates using the method described in the experiment section. Scanning Electron Microscope (SEM) images of these samples reveal the dimensions of the synthesized nanowires. A number of CuNWs were randomly selected from these SEM images, and their lengths and diameters were measured and displayed as histograms. From these measurements, the average length of the nanowires is found to be 75 μm, while their diameter is 45 nm, corresponding to an average aspect ratio of ~ 1600.
(a)
(c)
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Figure 1. (a) and (b) SEM images of the synthesized CuNWs obtained with the second fabrication method described in the experiment section. The lower two graphs are histograms of (c) length and (d) diameter, obtained from randomly selected CuNWs. The average length is 75 μm while the average diameter is 45 nm, corresponding to an average aspect-ratio of ~1600.
Deformation of CuNWs under long plasma treatment To remove the organic coatings on the nanowires and to fuse the nanowires at wire-wire junctions, we subjected the deposited CuNWs to plasma treatment (as an alternative to thermal annealing in a forming gas atmosphere). When the plasma treatment duration was less than 2 minutes, we observed a significant improvement of the film’s conductivity (from non-conductive to a sheet-resistance in the range 20-100 Ω/square). However, when the plasma treatment lasted for over 3 minutes, the films became non-conductive. The SEM image in Fig.2 explains this irreversible damage to the CuNW thin film. Even though the overall CuNW distribution is preserved after the 3minute plasma treatment, the nanowires are seen to have become deformed and disconnected from the network, resulting in a non-conductive film.
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Figure 2. SEM image of CuNWs on a glass substrate after 3 minutes of continuous plasma treatment. The nanowires are seen to have become deformed and disconnected from the network.
Completeness of the CuNW transfer to the PEDOT:PSS layer In the fabrication process of PEDOT:PSS/CuNW hybrid electrodes, we flip the CuNW film onto the PEDOT:PSS film and apply pressure with a shop press. To examine the completeness of the transfer process, we applied pressure only to one half of the glass substrate on which CuNWs were deposited. Figure 3 shows an optical microscope image of the glass substrate taken after the transfer process. A well-defined boundary is seen to exist in this image, which represents the boundary of the region to which pressure was applied. It is seen clearly that the bottom part, to which pressure was applied, has no CuNWs left, indicating that indeed ~100% of the nanowires have been transferred to the PEDOT:PSS layer.
Figure 3. Optical microscope image of CuNWs on a glass substrate after their partial transfer to a PEDOT:PSS film. The bottom half of the image, where pressure was applied, indicates the complete transfer of CuNWs to the PEDOT:PSS layer, while the top half shows the existence of a residual CuNW network on the glass substrate.
Silver nanowires and an alternative method of copper nanowire synthesis and deposition Commercial silver nanowires purchased from Sigma-Aldrich had lengths ranging from 20 µm to 50 µm, and diameters from 120 nm to 150 nm. Our own silver nanowires (AgNWs) were synthesized via a polyol process similar to that reported by Sun et al.[1] Our synthesized silver nanowires had average length of 32 µm and average diameter of 120 nm. The optical transmittance versus sheet-resistance of our various transparent conductive electrodes made with AgNWs are reported later in this section.
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Our copper nanowires were synthesized (alternate route) in a one-step solution process following the procedure reported by Rathmell et al .[2] Briefly, in a three-neck flask, 20 mL of NaOH solution (15 M) was heated up to 65 °C. Copper (II) nitrate trihydrate (0.2 M, Sigma Aldrich) was added to the mixture along with 200 µL of ethylenediamine (60.10 MW, Sigma Aldrich) and stirred at 700 rpm for 5 minutes before the addition of 15 µL of hydrazine (35 wt%, Sigma Aldrich). The reaction was continued for 20 minutes. The resulting nanowire aggregates were removed from the solution, and were subsequently dispersed (with vigorous stirring) in a polyvinylpyrrolidone (10,000 MW, Sigma Aldrich) solution. After half an hour, the solution was centrifuged (5000 rpm, 10 minutes) and decanted several times to remove the by-products and excess particulates. The resulting aggregates were then dispersed in methanol for storage and deposition on substrates using the methods discussed below. The as-synthesized and purified CuNW films were characterized using optical and electron microscopy. The results of our morphological evaluations are shown in Figs. 4(a)-4(c). The CuNW suspensions remain well dispersed in chloroform. The nanowires were found to be between 30 µm and 50 µm long, with diameters ranging from 120 nm to 500 nm.
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(b)
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Figure 4. (a) Optical image of synthesized copper nanowires via our alternate route. (b) SEM image. (c) Close-up SEM image showing wire diameters. (d) Homogeneous suspension of synthesized copper nanowires in a glass vial.
Using spray-coating under Argon flow, we deposited percolating films of copper nanowires with optical transmittances ranging from 65 % to 88% (at λ = 550 nm). However, the as-deposited films did not have any measurable electrical conductivity at this point. After thermal annealing in a reducing atmosphere (5% Hydrogen, 95% Nitrogen) at 180 °C, the measured sheet resistance reached values as low as 5 Ω/square. Spin-coated copper nanowire films were created using a number of coating cycles, and the measured optical transmittances ranged from 50 % to 82%, with sheet resistances in the range of 2.5 – 700 Ω/square after plasma treatment in low-pressure air for one minute. Drop casting copper nanowires in chloroform proved to be an effective method of deposition, achieving percolating layers with sheet resistances in the range of 5 -15 Ω/square (after plasma annealing), and 50 %– 60% optical transmittance (at λ = 550 nm), although drop-casting as a method of thin-film fabrication cannot be relied upon for consistent reproducibility. Vacuum filtration and transfer technique was also employed to create percolating films, but was discarded due to the excessive amount of waste generated. Figure 5 shows the measured values of optical transmittance versus sheet resistance for all reported materials and experimental methods in the present work. Also shown for comparison are the characteristics of the conventional ITO transparent conductive electrodes.
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Figure 5. Transmittance versus sheet resistance for all reported materials and experimental methods in this work. Also shown for comparison are the characteristics of the conventional ITO transparent conductive electrode (×).
References (1) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y., Crystalline Silver Nanowires by Soft Solution Processing. Nano Letters 2002, 2, 165–168. (2) Rathmell, A.R.; Bergin, S.M.; Hua, Y.L.; Li, Z.Y., The Growth Mechanism of Copper Nanowires and Their Properties in Flexible, Transparent Conducting Films. Advanced Materials 2010, 22, no.32, 3558-3563.
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College of Optical Sciences, The University of Arizona, Tucson, Arizona, USA ‡
Department of Electrical Engineering, Qatar University, Doha, Qatar
Supplementary information
1
Copper nanowire aspect ratio calculation The CuNWs were synthesized and deposited onto glass substrates using the method described in the experiment section. Scanning Electron Microscope (SEM) images of these samples reveal the dimensions of the synthesized nanowires. A number of CuNWs were randomly selected from these SEM images, and their lengths and diameters were measured and displayed as histograms. From these measurements, the average length of the nanowires is found to be 75 μm, while their diameter is 45 nm, corresponding to an average aspect ratio of ~ 1600.
(a)
(c)
(b)
(d)
Figure 1. (a) and (b) SEM images of the synthesized CuNWs obtained with the second fabrication method described in the experiment section. The lower two graphs are histograms of (c) length and (d) diameter, obtained from randomly selected CuNWs. The average length is 75 μm while the average diameter is 45 nm, corresponding to an average aspect-ratio of ~1600.
Deformation of CuNWs under long plasma treatment To remove the organic coatings on the nanowires and to fuse the nanowires at wire-wire junctions, we subjected the deposited CuNWs to plasma treatment (as an alternative to thermal annealing in a forming gas atmosphere). When the plasma treatment duration was less than 2 minutes, we observed a significant improvement of the film’s conductivity (from non-conductive to a sheet-resistance in the range 20-100 Ω/square). However, when the plasma treatment lasted for over 3 minutes, the films became non-conductive. The SEM image in Fig.2 explains this irreversible damage to the CuNW thin film. Even though the overall CuNW distribution is preserved after the 3minute plasma treatment, the nanowires are seen to have become deformed and disconnected from the network, resulting in a non-conductive film.
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Figure 2. SEM image of CuNWs on a glass substrate after 3 minutes of continuous plasma treatment. The nanowires are seen to have become deformed and disconnected from the network.
Completeness of the CuNW transfer to the PEDOT:PSS layer In the fabrication process of PEDOT:PSS/CuNW hybrid electrodes, we flip the CuNW film onto the PEDOT:PSS film and apply pressure with a shop press. To examine the completeness of the transfer process, we applied pressure only to one half of the glass substrate on which CuNWs were deposited. Figure 3 shows an optical microscope image of the glass substrate taken after the transfer process. A well-defined boundary is seen to exist in this image, which represents the boundary of the region to which pressure was applied. It is seen clearly that the bottom part, to which pressure was applied, has no CuNWs left, indicating that indeed ~100% of the nanowires have been transferred to the PEDOT:PSS layer.
Figure 3. Optical microscope image of CuNWs on a glass substrate after their partial transfer to a PEDOT:PSS film. The bottom half of the image, where pressure was applied, indicates the complete transfer of CuNWs to the PEDOT:PSS layer, while the top half shows the existence of a residual CuNW network on the glass substrate.
Silver nanowires and an alternative method of copper nanowire synthesis and deposition Commercial silver nanowires purchased from Sigma-Aldrich had lengths ranging from 20 µm to 50 µm, and diameters from 120 nm to 150 nm. Our own silver nanowires (AgNWs) were synthesized via a polyol process similar to that reported by Sun et al.[1] Our synthesized silver nanowires had average length of 32 µm and average diameter of 120 nm. The optical transmittance versus sheet-resistance of our various transparent conductive electrodes made with AgNWs are reported later in this section.
3
Our copper nanowires were synthesized (alternate route) in a one-step solution process following the procedure reported by Rathmell et al .[2] Briefly, in a three-neck flask, 20 mL of NaOH solution (15 M) was heated up to 65 °C. Copper (II) nitrate trihydrate (0.2 M, Sigma Aldrich) was added to the mixture along with 200 µL of ethylenediamine (60.10 MW, Sigma Aldrich) and stirred at 700 rpm for 5 minutes before the addition of 15 µL of hydrazine (35 wt%, Sigma Aldrich). The reaction was continued for 20 minutes. The resulting nanowire aggregates were removed from the solution, and were subsequently dispersed (with vigorous stirring) in a polyvinylpyrrolidone (10,000 MW, Sigma Aldrich) solution. After half an hour, the solution was centrifuged (5000 rpm, 10 minutes) and decanted several times to remove the by-products and excess particulates. The resulting aggregates were then dispersed in methanol for storage and deposition on substrates using the methods discussed below. The as-synthesized and purified CuNW films were characterized using optical and electron microscopy. The results of our morphological evaluations are shown in Figs. 4(a)-4(c). The CuNW suspensions remain well dispersed in chloroform. The nanowires were found to be between 30 µm and 50 µm long, with diameters ranging from 120 nm to 500 nm.
(a)
(b)
(c)
(d)
Figure 4. (a) Optical image of synthesized copper nanowires via our alternate route. (b) SEM image. (c) Close-up SEM image showing wire diameters. (d) Homogeneous suspension of synthesized copper nanowires in a glass vial.
Using spray-coating under Argon flow, we deposited percolating films of copper nanowires with optical transmittances ranging from 65 % to 88% (at λ = 550 nm). However, the as-deposited films did not have any measurable electrical conductivity at this point. After thermal annealing in a reducing atmosphere (5% Hydrogen, 95% Nitrogen) at 180 °C, the measured sheet resistance reached values as low as 5 Ω/square. Spin-coated copper nanowire films were created using a number of coating cycles, and the measured optical transmittances ranged from 50 % to 82%, with sheet resistances in the range of 2.5 – 700 Ω/square after plasma treatment in low-pressure air for one minute. Drop casting copper nanowires in chloroform proved to be an effective method of deposition, achieving percolating layers with sheet resistances in the range of 5 -15 Ω/square (after plasma annealing), and 50 %– 60% optical transmittance (at λ = 550 nm), although drop-casting as a method of thin-film fabrication cannot be relied upon for consistent reproducibility. Vacuum filtration and transfer technique was also employed to create percolating films, but was discarded due to the excessive amount of waste generated. Figure 5 shows the measured values of optical transmittance versus sheet resistance for all reported materials and experimental methods in the present work. Also shown for comparison are the characteristics of the conventional ITO transparent conductive electrodes.
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Figure 5. Transmittance versus sheet resistance for all reported materials and experimental methods in this work. Also shown for comparison are the characteristics of the conventional ITO transparent conductive electrode (×).
References (1) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y., Crystalline Silver Nanowires by Soft Solution Processing. Nano Letters 2002, 2, 165–168. (2) Rathmell, A.R.; Bergin, S.M.; Hua, Y.L.; Li, Z.Y., The Growth Mechanism of Copper Nanowires and Their Properties in Flexible, Transparent Conducting Films. Advanced Materials 2010, 22, no.32, 3558-3563.
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