Research Article www.acsami.org
Silver Nanowire Transparent Conductive Films with High Uniformity Fabricated via a Dynamic Heating Method Yonggao Jia, Chao Chen, Dan Jia, Shuxin Li, Shulin Ji,* and Changhui Ye* Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Technology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China
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S Supporting Information *
ABSTRACT: The uniformity of the sheet resistance of transparent conductive films is one of the most important quality factors for touch panel applications. However, the uniformity of silver nanowire transparent conductive films is far inferior to that of indium-doped tin oxide (ITO). Herein, we report a dynamic heating method using infrared light to achieve silver nanowire transparent conductive films with high uniformity. This method can overcome the coffee ring effect during the drying process and suppress the aggregation of silver nanowires in the film. A nonuniformity factor of the sheet resistance of the as-prepared silver nanowire transparent conductive films could be as low as 6.7% at an average sheet resistance of 35 Ω/sq and a light transmittance of 95% (at 550 nm), comparable to that of high-quality ITO film in the market. In addition, a mechanical study shows that the sheet resistance of the films has little change after 5000 bending cycles, and the film could be used in touch panels for human−machine interactive input. The highly uniform and mechanically stable silver nanowire transparent conductive films meet the requirement for many significant applications and could play a key role in the display market in a near future. KEYWORDS: silver nanowires, transparent conductive films, nonuniformity factor, dynamic heating, mass transport, evaporation control
1. INTRODUCTION Flexible transparent conducting films (TCFs) are the key part to a wide range of applications including flexible touch screens,1 solar cells,2−4 sensors,5 supercapacitors,6 light-emitting diodes,7 transparent heaters,8 and printable electronics.9,10 Nowadays, the main TCFs in the market are indium-doped tin oxide (ITO) coated on flexible substrates; however, the drawbacks of ITO, such as the scarcity of indium, the high temperature deposition process, and the fragile nature, dramatically limit the large-scale application in next-generation flexible electronic devices.11 To address these drawbacks, several new TCFs including metal grids,12 metal nanowires,13−16 conductive polymers,17 carbon nanotubes,18,19 and graphene20−23 have been proposed. Among these candidates, carbon nanotubes and graphene are limited by their low conductivity and the difficulty in large-scale production. The main problems of conductive polymers are a low conductivity and a high visible light absorption. It is necessary to dramatically enhance the process yield of products and resolve the visual sense problem for application of metal grids. Silver nanowires (AgNWs) have attracted great attention due to the excellent conductivity, the high transparency, and the flexibility of AgNW TCFs.24−27 Liu et al.28 have fabricated AgNW films with a sheet resistance of 40 Ω/sq and a visible light transmittance of 97.3% (550 nm). Jin et al.29 have obtained a sheet resistance of 32 Ω/sq and a visible light transmittance of 93% through using a CaAlg layer between © 2016 American Chemical Society
AgNWs and polyethylene terephthalate (PET) substrate. The electrical conductivity and light transmittance performance of these AgNW TCFs are comparable to or even better than those of ITO films; however, another determining factor for the implement of AgNW TCFs in the field of touch panels, that is, the uniformity of the sheet resistance, has to be seriously addressed before this new material could really compete for market share. For touch panel applications, one should keep the variation of the sheet resistance as small as possible (generally, a standard deviation of the sheet resistance should be lower than 10%) to precisely locate the touching coordinate by scanning the voltage drops along x and y directions. Although some papers reported the simultaneous achievement of a small sheet resistance and a high visible light transmittance, a deviation of the sheet resistance away from the average value is as high as 30% or more. Much effort has been devoted to improving the uniformity of AgNW TCFs, for example, by mechanical pressing30 or by coating with PEDOT:PSS31,32 or graphene.33,34 Mechanical pressing could achieve good smoothness and low sheet resistance; however, the sheet resistance uniformity still did not satisfy the requirement for high-end applications. Additional PEDOT:PSS32 or graphene34 layers on Received: January 14, 2016 Accepted: March 30, 2016 Published: March 30, 2016 9865
DOI: 10.1021/acsami.6b00500 ACS Appl. Mater. Interfaces 2016, 8, 9865−9871
Research Article
ACS Applied Materials & Interfaces
Figure 1. AgNW TCFs deposited (a, d) under natural drying, (b, e) under static heating, and (c, f) under dynamic heating. The distance between the heating light source and the film is 40 cm, and the moving speed of the substrate in the dynamic heating is 5 cm/s. process was repeated to required layers as film property was qualified. In a static heating fashion, the films were kept below the light source with a certain spacing, whereas in a dynamic heating fashion, the film was moved under the light source with a certain velocity. 2.4. Characterization. The morphology of the films was characterized with a scanning electron microscope (SU-8020) at an accelerating voltage of 10 kV. Transmission electron microscopy analysis was carried out by JEM-2010. Temperature of the films under different heating conditions was measured by an infrared thermometer (Fluke). Sheet resistance of AgNW TCFs was measured by a standard four-point probe method (RST-9, Four-Probe Tech.). Optical transmittance spectra were obtained on a UV−vis−NIR spectrometer (Shimadzu SolidSpec-3600).
AgNW films could also increase the conductivity, and the sheet resistance uniformity improved dramatically; however, these methods involved spin coating or other complicated timeconsuming processes, making them difficult for large-scale production. Despite the previous work reported in the literature in improving the uniformity of AgNW TCFs, it is still a formidable challenge to reduce the standard deviation of the sheet resistance below 10% that corresponds to the medium quality of ITO films. In this paper, we present a dynamic heating method to fabricate AgNW TCFs with high uniformity. The standard deviation of the sheet resistance of the as-coated AgNW TCFs is as low as 6.7%. We also show that the highly uniform TCFs could be used in touch panels for human− machine interactive input.
3. RESULTS AND DISCUSSION In a normal coating process, the deposition of AgNWs on a substrate is subjected to the influence of mass transport under a concentration gradient, the well-known “coffee ring” effect.36 During the drying of the film, aggregation of AgNWs will take place at the periphery of the droplet where the solution evaporates fast due to temperature variation, and then inhomogeneous film will form under surface tension caused by different evaporation rate on the film surface.37−39 As shown in Figure 1, panels a and d, in a natural drying process, the evaporation rate is slow, and the “coffee ring” effect causes the random aggregation of AgNWs on the surface of PET substrate. To suppress the “coffee ring” effect, one could decrease the mass transport by speeding up the evaporation rate of the solution with an external heating input.40,41 We found that the heating condition played a key role on depositing uniform AgNW TCFs. We employed an infrared light as an external heating source. Two heating fashions have been compared in obtaining uniform AgNW TCFs, namely, a static heating and a dynamic heating, where the former refers to heating of the film under a static condition below the infrared light source, and the latter heating with the horizontal moving of the film. In Figure 1, panels b and e, in the static heating fashion, the distribution of AgNWs in the film is more uniform than that of the natural drying process; however, there is still aggregation of AgNWs in the film. In Figure 1, panels c and f, we can see that in a dynamic heating the uniformity of AgNW TCFs is much better than that in a static heating on large observation scale of square millimeter. Actually, AgNWs with same size distribution as shown in Figure S1 were used in the mentioned experiments,
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Silver nitrate (AgNO3) (≥99.8%, Shanghai Qiangshun Chemical Reagent Co, Ltd.), ethylene glycol (EG) (Sinopharm Chemical Reagent Co., Ltd.), polyvinylpyrrolidone (PVP) (average Mw of 55 000) (Sinopharm Chemical Reagent Co., Ltd.), PVP (average Mw of 360 000) (Sigma-Aldrich), isopropyl alcohol (≥99.7%, Sinopharm Chemical Reagent Co., Ltd.), and hydroxypropyl methyl cellulose (HPMC, Sigma-Aldrich) were used as received without further purification. Polyethylene terephthalate (PET, Toray) was used as substrate for TCFs. 2.2. Synthesis of AgNWs. The AgNWs were synthesized by the reduction of AgNO3 in the presence of PVP in EG as reported in our previous work.35 After reacting for 130 min, the solution was cooled down to room temperature, and the product was cleaned with ethanol, centrifuged, and dispersed in ethanol for further use. 2.3. Film Coating. The nanowire suspension was diluted to 0.1, 0.2, 0.3, 0.6, and 1.2 mg/mL, respectively, with isopropyl alcohol (12.5 vol %), DI water (12.5 vol %), and alcohol (75 vol %). Then, HPMC (0.05 wt %) was added into the diluted suspensions. PET substrate was fixed on a glass pane with 3M scotch tape. Scheme S1 shows the rod coating processes. First, a guide rail was applied to control the motion trail of glass rod and meanwhile the thickness of wet film. Second, the suspension was dropped on the substrate in arrays to ensure the uniform distribution of silver nanowires on every zone. Third, the suspension was scraped longitudinally forward and backward and transversely forward and backward along and guide rail to make the distribution of silver nanowires more uniform. Fourth, the infrared light atop the wet film was turned on to dry the film, after which a coating layer was finished by turning off the light. The whole 9866
DOI: 10.1021/acsami.6b00500 ACS Appl. Mater. Interfaces 2016, 8, 9865−9871
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Distribution of sheet resistance of AgNW TCFs with a dynamic heating. (b) Histogram of the measured sheet resistance. (c) NUF of AgNW TCFs with different heating processes.
but different heating modes led to different spatial distribution of AgNWs on the substrate. It is the dynamic heating that introduces randomly and evenly distribution of AgNWs on the substrate. Of course it is ideal to coat AgNWs with narrow size distribution randomly and evenly on the substrate for optimum uniformity. We define a nonuniformity factor (NUF) to evaluate the standard deviation of the sheet resistance of the films away from the average value as n
NUF =
2 1 ∑i = 1 (R i − R̅ ) n R̅ 2
Figure 3. Optical transmittance of AgNW TCFs measured at nine different sites.
where n is the number of measurements on the film of different sites, and Ri and R̅ are the measured resistance and the average resistance of all the measurements, respectively. The smaller the NUF, the more uniform the film. The distribution of the sheet resistance on a film with dynamic heating is shown in Figure 2, panel a. The film has been divided into 64 parts of the same size. The sheet resistance of each part was measured, and the data were marked therein. The NUF of the sheet resistance is calculated as 6.7% as shown in Figure 2, panels b and c, much lower than that of the film with a static heating (20%) and a natural drying (23%). The uniformity of the film with dynamic heating is comparable to that of the high-end ITO (5%−10%) and even better than that of a commercial ITO film (11.2%). In addition to the uniformity of the sheet resistance, we have also measured the uniformity of the light transparence of AgNW TCFs on nine different sites of the same film. Similarly, we calculated the NUF as 0.77% (Figure 3). Table 1 compares the sheet resistance, the transmittance at 550 nm, and NUF of AgNW TCFs reported by different researchers and of a commercial ITO film. The dynamic heating produced AgNW TCFs with better uniformity than those under static heating. We have measured the heating uniformity of these two fashions on the substrate. Figure 4 shows the temperature of PET substrates at nine different sites. We can see that the temperature is not uniform on the substrate under the static heating. When the substrate
moves under the infrared heating source in a dynamic heating process, the temperature on the substrate is much more uniform. The reason for the difference lies in that the infrared heating source itself does not heat uniformly because of the arrangement of the infrared tubes, making the middle zone heating more efficiently than the margin. However, every part of the film accepts almost the same quantity of heat when the film moves with a certain speed from one side to the other side of the heating field, which makes the temperature of the substrate more uniform. From Figure 4, it could be observed that the temperature of the substrate under dynamic heating is on one hand more uniform, but on another hand lower than that under static heating. Is it possible to monotonously decrease NUF by increasing the heating power? We have studied the effect of the height of the heating source above the film as well as the moving speed of the substrate on the uniformity of AgNW TCFs. As shown in Figure 5, panels a and b, when the height of the heating source above the film is smaller or larger than 40 cm, and the moving speed of the substrate is faster or slower than 5 cm/s, NUF increases under both conditions. It means that lowest NUF exists only under optimal conditions. To explore the reason, we have tested the temperature distribution on PET substrates under different heights and speeds (Figure 9867
DOI: 10.1021/acsami.6b00500 ACS Appl. Mater. Interfaces 2016, 8, 9865−9871
Research Article
ACS Applied Materials & Interfaces Table 1. Key Properties of AgNW TCFs and a Commercial ITO Film materials
Rs (Ω/square)
transmittance (%)
NUF of the sheet resistance (%)
ref
method
AgNWs/PEDOT:PSS AgNWs/PVA AgNWs AgNWs ITO
17 0.67 35 21 55
90.7 85 95.5 93 85
11.8 6 6.7 7.9 11.2
32 34 our work our work NITTO DENKO CO.,LTD
spin-coating spin-coating/pressing rod-coating rod-coating CVD
similar film NUF of 7.3% was obtained. However, if we reduced the moving speed below the threshold for temperature uniformity to 2.0 cm/s (Figure S2b), though light power was diminished to maintain the average temperature of about 35 °C, the film NUF increased from 6.7% to 13.5%. It is easily understood the importance of temperature uniformity. As to heating temperature, when it is low, the evaporation of the solution is slow, and mass transport dominated “coffee ring” effect impairs the uniformity of films as demonstrated before. However, when the heating temperature reaches above a certain value, surface tension fluctuations during fast evaporation by using mixed solvents will lead to inhomogeneous film.42 Another question one may wonder is which contributes most to the lowest NUF, the dynamic heating or the rod coating? As shown in Figure S3, though the same rod coating was applied, heating by oven or hot plate gave bigger film NUF than that by dynamic heating. The reason lies in that dynamic heating is an in situ drying method and has high heating and cooling rates. The heating we did by oven had a time delay due to the bringing of film from coating table to the oven. The heating we did by hot plate had slow heating and cooling so we kept the hot plate turning on for multilayer coating, which disturbed the distribution state of AgNWs in wet film before rod-coating. Therefore, dynamic heating is vital for small NUF of AgNW TCFs, and rod coating helps to demonstrate the beneficial effect of dynamic heating in guaranteeing the even distribution
Figure 4. Temperature of PET substrates with a static heating and a dynamic heating.
5c,d). By increasing the moving speed steadily to 5 cm/s, the substrate temperature becomes more and more even; further speed increment only leads to temperature decrement. Positioning the substrate under different heights corresponds to different values but similar distribution of substrate temperature. It indicates that optimal heating temperature and temperature uniformity are the key roles to get uniform film. If these two factors are fixed, the NUF will be similar and not changed by other conditions. As shown in Figure S2a, by diminishing both the height and the light power, heating temperature and temperature uniformity were maintained so
Figure 5. Variation of the sheet resistance NUF of AgNW TCFs with (a) the height of the heating source above the film by fixing the moving speed of the substrate at 5.0 cm/s, and (b) the moving speed of the substrate by fixing the height of the heating source above the film at 40 cm; temperature distribution on the film of different (c) heights and (d) speeds. 9868
DOI: 10.1021/acsami.6b00500 ACS Appl. Mater. Interfaces 2016, 8, 9865−9871
Research Article
ACS Applied Materials & Interfaces of AgNWs in wet film. The dynamic heating will not work if the prerequisite could not be guaranteed. For example, wet film made by spin coating suffered from gradient AgNW distribution from center to the edge and had a much higher NUF of 101.1% after dynamic heating. When considering the sheet resistance uniformity, one has to simultaneously consider the optical transmittance and the sheet resistance because these parameters generally counteract with each other. Besides the heating method mentioned before, the concentration of AgNW ink will also affect these parameters through its function in tuning the ink viscosity.30 We have investigated the effect of the mass density of AgNWs in the suspension on these parameters by fixing the using amount of AgNWs and changing the mass density and therefore the coating layers. From Figure 6, it is obvious that by reducing the
increases by further reducing the mass density. It is easily understood that high mass density leads to viscous AgNW suspension that is not suitable for uniform film coating. As a result, AgNWs are not evenly distributed on the film, which is detrimental for the percolation of AgNW network, and large NUF could be anticipated. By reducing the mass density through multilayer coating, sheet resistance and its uniformity could be improved by evenly distributed AgNWs on the film, but enhanced light scattering among these evenly distributed AgNWs will lead to the decrease of optical transmittance of the film. When the mass density is fairly small, wet coating of the dilute AgNW suspension could be disturbed by airflow, vibration, and so on;30 therefore, it is difficult to further distribute AgNWs more evenly and the NUF increases again. Then we may wonder how to coat a film of a certain sheet resistance and an optical transmittance with minimum NUF. It is recommended to choose an optimal mass density first to tune the suspension viscosity for uniform film coating and then repeat the multilayer coating until the optical parameters is on the edge of satisfaction. As shown in Figures S4 and S5, for same coating layers, 0.2 mg/mL leads to the minimum NUF and is chosen to be the mass density for repeated coating until the transmittance is nearly 95%. Finally, Figure 7 shows that AgNW TCFs have a good flexibility with little change of the sheet resistance after bending for 5000 cycles with a bending radius of 1.5 cm. The maximum change of sheet resistance of AgNW TCFs after bending is under 8%. Under the same bending condition, the sheet resistance of a commercial ITO film increased for several orders (Figure 7b). Figure 7, panel c shows the application of our uniform AgNW TCFs in a touch panel for exact writing input (please refer to the video in the Supporting Information). With a nonuniform film, the writing could not be correctly displayed on the computer screen because the voltage scanning could not be properly carried out. The homemade touch panel has been frequently used for more than one year and is working well. The excellent mechanical stability and antioxidation ability of the film could be partially ascribed to the function of HPMC, which has been added to the AgNW suspension. When coating
Figure 6. Optical transmittance, sheet resistance, and NUF of the sheet resistance for AgNW TCFs with different mass density of AgNWs.
mass density of AgNWs, the sheet resistance decreases monotonously, and the transmittance first keeps nearly unchanged and then decreases rapidly. As to the NUF, it decreases quickly to a minimum value of 6.7% at a mass density of 0.2 mg/mL where an optical transmittance of 95.5% and a sheet resistance of 35 Ω/sq have been achieved, and then it
Figure 7. Bending test of AgNW TCFs and commercial ITO films. (a) The bending test process with the radius of the film as 1.5 cm. (b) The resistance variation of the films during the process of the bending test. (c) Demonstration for exact writing input using the uniform AgNW TCFs. 9869
DOI: 10.1021/acsami.6b00500 ACS Appl. Mater. Interfaces 2016, 8, 9865−9871
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films, this polymer can act as cross-linker among AgNWs and between AgNW and flexible substrates like PET.43 When the film is exposed in air, the polymer can protect AgNWs from oxidation and acid corrosion due to its strong adsorption on metal surface and its donation ability of electrons.44 The aging test shown in Figure S6 supports the idea. One question is still open regarding the stability of AgNW TCFs with a high load of power.45 Therefore, future work toward real-world applications of AgNW TCFs has to cover the full understanding of the possible heat-driven or electric fielddriven migration of Ag ions causing the breakdown of the performance.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00500. One scheme picturing the coating process, six additional figures demonstrating the microstructure of AgNWs, the distribution of AgNWs in films by different coating and heating methods, the electrical and optical properties of films and air stability of uniform films, and two videos elaborating on application results of uniform and nonuniform films in touch panels (PDF) Nonuniform TCF panel (AVI) Uniform TCF panel (AVI)
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REFERENCES
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4. CONCLUSIONS In summary, we have developed a dynamic heating method to fabricate AgNW TCFs with high uniformity (an NUF of 6.7%), which is comparable to that of high-quality ITO products (the NUF of 5−10%). It has been demonstrated in this work that the drying fashion plays a pivotal role on determining the distribution of AgNWs in the film and the uniformity of the sheet resistance of the TCFs. The effective dynamic heating method through heating the AgNW film under an infrared light source by moving the film at a certain speed is compatible with the industrial roll-to-roll printing process of TCFs and is quite instructive for commercialization of AgNW TCFs. With the highly uniform AgNW TCFs, writing input could be properly displayed on a computer screen, whereas for the less uniform ones, the display is worse. Moreover, AgNW TCFs in this work also exhibit excellent stability against mechanical bending with the sheet resistance changing below 8% after bending for 5000 cycles. The ability to fabricate AgNW TCFs with high uniformity and good mechanical stability makes it practical to utilize this material for next generation flexible electronics.
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Research Article
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos. 11274308, 21401202, 51502295, and 51501182) and the CAS/SAFEA International Partnership Program for Creative Research Teams. 9870
DOI: 10.1021/acsami.6b00500 ACS Appl. Mater. Interfaces 2016, 8, 9865−9871
Research Article
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DOI: 10.1021/acsami.6b00500 ACS Appl. Mater. Interfaces 2016, 8, 9865−9871
