Highly Transparent, Flexible Conductors and Heaters Based on Metal

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Surfaces, Interfaces, and Applications

Highly Transparent, Flexible Conductors and Heaters Based on Metal Nanomesh Structures Manufactured Using an All Water-Based Solution Process Sung Min Lee, Seungwoo Oh, and Suk Tai Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17415 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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ACS Applied Materials & Interfaces

Highly Transparent, Flexible Conductors and Heaters Based on Metal Nanomesh Structures Manufactured Using an All Water-Based Solution Process

Sung Min Lee, Seungwoo Oh, Suk Tai Chang* School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea E-mail: [email protected]

KEYWORDS: metal nanomesh, solution process, transparent electrodes, flexible electrodes, transparent heaters

ACS Paragon Plus Environment

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ABSTRACT: Metal mesh is a promising material for flexible transparent conducting electrodes due to its outstanding physical and electrical properties. The excellent control of the sheet resistance and transmittance provided by the metal mesh electrodes is a great advantage for microelectronic applications. Thus, over the past decade, many studies have been performed in order to realize high-performance metal mesh; however, the lack of cost-effective fabrication processes and the weak adhesion between the metal mesh and substrate have hindered its widespread adoption for flexible optoelectronic applications. In this study, a new approach for fabricating robust silver mesh without using hazardous organic solvents is achieved by combining colloidal deposition and silver enhancement steps. The adhesion of the metal mesh was greatly improved by introducing an intermediate adhesion layer. Various patterns relevant to optoelectronic applications were fabricated with a minimum feature size of 700 nm, resulting in high optical transmittance of 97.7% and also high conductivity (71.6 Ω sq-1) of the metal mesh. In addition, we demonstrated an effective transparent heater using the silver mesh with excellent exothermic behavior, which heated up to 245 °C with 7 V applied voltage.


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Introduction Transparent conducting electrodes have played a key role in the development of various optoelectronic devices, such as solar cells, touch panels, smart windows, sensors, light-emitting diodes (LEDs), and transparent heaters.1 Over the past two decades, indium tin oxide (ITO) has been the most popular transparent electrode due to its outstanding transmittance and electrical conductivity. However, as interest in flexible next-generation optoelectronic devices is increasing, the brittleness2 and price volatility3 of ITO has motivated the search for new materials as flexible transparent conducting electrodes (FTCEs). In order to develop substitutes for ITO, much effort has been devoted to fabricating FTCE materials based on carbon nanomaterials such as graphene4-6 and carbon nanotubes (CNTs).7-9 Both of these materials have major advantages of excellent flexibility and high electrical conductivity, which originate from their unique lattice structure.10 However, most graphene is synthesized by chemical vapor deposition (CVD), which is a highly energy intensive process, inhibiting low-cost fabrication of graphene-based electrodes.11 Although cost-effective mass production of chemically exfoliated graphene sheets can be realized by solution processing, their relatively low electrical conductivity due to defective lattice structures limits their use for highly efficient FTCEs.12,13 In the case of CNTs, most production methods include energy intensive processes, including arc-discharge, laser ablation, and CVD.14 Furthermore, CNTs are synthesized as a multi-dispersed mixture of metallic and semiconducting CNTs, and subsequent complex purification processes are required to separate metallic CNTs from semi-conducting CNTs or singled-wall CNTs from multi-walled CNTs.15,16 Therefore, reliable low-cost synthesis processes need to be developed before graphene and CNTs can be efficiently applied for FTCE applications.


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Nanostructured metal thin films are another promising FTCE material; for example, random metal nanowire networks and regular patterned metal mesh have recently been highlighted. The excellent electrical conductivity and flexibility of these structures makes them suitable for highperformance flexible optoelectronic devices.17 In particular, metal nanowires can be easily prepared in the form of a liquid dispersion of highly electrically conductive metals, such as silver or copper nanowires, enabling mass production.18-20 However, producing FTCEs by coating the metal nanowires requires an additional procedure, such as pressurization at high temperature to reduce the film roughness and enhance contact between individual nanowires.21 In addition, increasing the adhesion strength between the metal nanowires and substrates remains a challenge; otherwise metal nanowire films easily delaminate from the substrates.22,23 In contrast, metal mesh has recently been demonstrated as a more promising candidate for FTCEs as its transmittance and sheet resistance can be easily controlled by adjusting the screen opening ratio, while maintaining outstanding electrical conductivity and flexibility.24 However, the widespread adoption of metal mesh for FTCE applications has suffered setbacks, including the lack of cost-effective fabrication processes and weak adhesion between the metal mesh and substrate. Generally, the fabrication of metal mesh electrodes is followed by physical vapor deposition (PVD), which requires expensive vacuum systems.25-27 Furthermore, due to the top-down process of PVD, a lot of metal is wasted during deposition. Therefore, many studies have investigated electrohydrodynamic printing (EHD) of metal mesh, which enables low-cost fabrication of well-defined metal mesh structures.28,29 Nevertheless, EHD also has limitations related to the patterning accuracy and printing speed, limiting reliable fabrication of FTCEs.30-32 In addition, the inherent weak adhesion between the metal and substrate hinders reproducible production of metal mesh electrodes and subsequent processing. Although some studies have reported enhanced adhesion of metal mesh


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using a substrate-embedded structure, both cost-effective fabrication and improved adhesion have rarely been achieved using the same technique.33,34 In this study, we developed an all water-based solution process for fabricating high-resolution silver nanomesh with strong adhesion to appropriate substrates. We measured the transmittance and sheet resistance of the mesh in order to evaluate its suitability for FTCE applications. In addition to regular mesh patterns, we also fabricated various patterns, including hexagonal mesh, micro-perforated plate, concentric circles, and random-network mesh. Such fine silver meshes were realized on rigid or plastic substrates by enhancing silver ions on the surface of gold nanoparticles (Au NPs), which were selectively deposited over 3-aminopropyltriethoxysilane (APTES) patterns. Although this strategy has been known as an effective technique for coordinating metal growth,35-37 it had not been applied for fabricating highly transparent metal mesh electrodes due to its difficulty of selective colloidal seeds deposition.38 However, we achieved highly selective deposition of Au nanoparticles by using UV-ozone etching for the formation of

deposition-inducing region (APTES patterns) and deposition-blocking layer

(hydroxylated region). The adhesion strength of the silver mesh was evaluated using peel-off tests. We also evaluated the flexibility of the mesh by performing cyclic bending tests at a 5 mm bending radius. In order to demonstrate the feasibility of our silver mesh, we fabricated and characterized a transparent heater. We expect that these findings can contribute to the development of highperformance FTCE structures based on silver nanomesh.


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Experimental Section Patterning APTES on substrates: Plain glass slides (76 mm × 26 mm; Paul Marienfeld GmbH & Co) cleaned by piranha solution and polyester film (V7610, SKC) cleaned by air plasma (PDC32G, Harrick Plasma) for 30 min were used as substrates. In order to prepare the APTES layer on the substrates, clean substrates were immersed in a 1:1 mixture of deionized (DI) water and ethanol with 2% APTES (440140, Sigma Aldrich) for 1 h. Then, the substrates were washed in DI water and dried in a flow of nitrogen gas. The APTES-coated substrates were annealed at 120 °C on a hotplate for 30 min in order to complete the silanization reaction. After annealing, unreacted residues were removed by sonication (Branson 5800) in DI water for 30 min and dried in a nitrogen gas flow. The APTES-coated substrates were exposed to UV light (custom UV chamber, Atech Co., Ltd) for 26 min through a quartz chrome mask to obtain the APTES-patterned substrates. Preparation of gold nanoparticles solution: 400 mL of DI water was heated on a hotplate at 130 °C with stirring at 600 rpm. When the DI water started to boil, 138 µL of 30 wt.% HAuCl4 solution (484385, Sigma Aldrich) was added. After mixing well, 40 mL of 62 mM sodium citrate tribasic dihydrate (S4641, Sigma Aldrich) solution was added. The color of the solution changed from yellow to colorless, black, purple, and then red. After the color of the solution turned to deep wine-red, the solution was heated and stirred for 30 min for further growth of Au NPs. Then, the solution was cooled while stirring at 600 rpm at 25 °C. The solution was highly concentrated using a centrifuge (Supra 22k, Hanil Science Medical) with a filter (Amicon ultra 15 ml 100K, Merck) at 2000 G for 20 min. Then, the concentrated solution was quantified using UV-visible spectroscopy (V-670, Jasco) and diluted with the filtered solution to obtain the desired concentration.


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Fabrication of silver patterns: Deposition of Au NPs on the APTES pattern was conducted using the microcontact dip-coating (MCDC) method. The Au NP solution was injected between the APTES-patterned substrate (glass substrate or polyester film) and air plasma-treated glass slide (cover plate) with a 10 μm spacer. The deposition of Au NPs proceeded for 10 min, then the cover plate was carefully separated from the Au-NP-deposited substrate in a bath of DI water. The substrate was then washed in DI water and dried in a flow of nitrogen gas. Then, a 1:1 mixture of silver enhancer solution A (S5020, Sigma Aldrich) and B (S5145, Sigma Aldrich) was prepared and diluted with DI water to the desired concentration. Then, the enhancer solution was injected between the substrate and cover plate with a 25 μm spacer. Silver enhancement proceeded for 5 min and the cover plate was carefully removed from the silver-enhanced substrate in a bath of DI water. The obtained silver-patterned substrate was washed in DI water and dried in nitrogen gas. Then, the silver-patterns on glass substrate and polyester film were annealed on a hotplate at 300 °C for 20 min and at 120 °C for 90 min, respectively. Characterization of metal mesh: Images of the metal mesh were obtained using optical microscopy (Olympus BX-51), FE-SEM (Carl Zeiss SIGMA), and an IR thermal camera (M8, Wuhan Guide Infrared Co., Ltd). Topographic analyses were carried out using AFM (NX-10 complete AFM, Park Systems) in non-contact mode. The sheet resistance and linear resistance were measured using a Keithley 2602A with a four-point probe system (MSA001, MS Tech) and a two-probe system (237-ALG-2, Keithley), respectively. The optical transmittance was measured using a UV-vis-NIR spectrophotometer (V-670, Jasco). A motorized stage (AL1-1515-3S, Micro Motion Technology, Valley Center) was used for bending tests.


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Results and Discussion Details of the fabrication process for the metal nanomesh are illustrated in Figure 1. The costeffective fabrication of silver mesh with high adhesion strength was achieved by combining a colloidal Au NP deposition technique with silver enhancement. In order to induce deposition of colloidal Au NPs and enhance the adhesion strength of the silver mesh, APTES was used as it has high affinity with silver and gold due to its amine group. Self-assembled monolayers of APTES were deposited on the substrate and high-resolution patterns were produced using UV-ozone exposure through a quartz-chrome photomask.39 Then, the microcontact dip-coating (MCDC) method was used to minimize material consumption during the Au NP deposition and silver enhancement steps. A glass slide (cover plate) cleaned using air plasma was fixed on the APTESpatterned substrate with a spacer to fix the gap between them. Then, a Au NP dispersion was injected between the substrate and cover plate. The Au NP dispersion spontaneously spread over the substrate due to the enhanced surface energy of the air-plasma-treated cover plate. While the contact between the dispersion and substrate was maintained, Au NPs were selectively deposited on the pre-patterned APTES layer. After Au NPs deposition, silver shells were formed during subsequent silver enhancement, creating conducting pathways. In order to reduce the contact resistance between the silver shells, annealing was performed, resulting in a highly conductive silver mesh. Compared to conventional dip coating, which is often used for colloidal deposition of NPs and requires milliliter-scale volume of solution, our MCDC method consumes only microliters of solution during deposition. In addition, all processes are vacuum-free and carried out under ambient conditions, enabling low-cost fabrication of silver mesh. The basic principles of silver mesh fabrication are illustrated in Figure 2. First, selective deposition of Au NPs is driven by electrostatic interaction between the APTES-patterned substrate


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and the Au NPs. As shown in Figure 2a, UV-ozone patterning implants negatively charged hydroxyl group on the substrate, which etch the APTES layer. Therefore, Au NPs which are stabilized by negatively charged citrate ions are prevented from depositing on the hydroxylated area by electrostatic repulsion, while Au NPs are adsorbed onto the positively charged APTES layer by electrostatic attraction.40-43 The corresponding atomic force microscopy (AFM) and inset field-emission scanning electron microscopy (FE-SEM) image in Figure 2c shows well-defined Au NP patterns on the APTES-patterned substrate. However, it is difficult to form conducting films only by colloidal deposition of Au NPs. As the Au NPs are deposited, the positive surface charge of the APTES-patterned substrate is neutralized by pre-deposited Au NPs on the APTES layer, reducing the adsorption rate of the Au NPs. Therefore, the second step of silver enhancement was required to form a conducting pathway. As shown in Figure 2b, the silver enhancement solution contained silver ions as the silver source and hydroquinone as the reducing agent. The reduction of silver by hydroquinone is greatly enhanced by adsorption of silver ions on surface of gold.44 Therefore, when the silver enhancement solution was injected between the Au-NP-covered substrate and cover plate, silver ions were reduced on the surface of the Au NPs, rather than forming nuclei in solution, resulting in the growth of silver shells on Au NP cores.45 The silver shells connected to create a conducting pathway; the well-defined silver line pattern composed of these shells is shown in the AFM and inset FE-SEM images in Figure 2d. A variety of essential micropatterns for microelectronic applications can be easily fabricated using our method, such as the square mesh, honeycomb, micro-perforated plate, concentric circles, and random network patterns shown in Figure 3. In this study, silver patterns were fabricated using a 0.04 wt.% Au NP dispersion, 10 min deposition time, 5 min silver enhancement time, and annealing at 300 °C for 20 min (unless otherwise noted). High-resolution silver mesh with a 700


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nm linewidth and 50 μm spacing was obtained. The thickness of the silver mesh was 107 nm, where the well-defined edge of the silver mesh can be seen in the topographic image in Figure 3f. In addition, complex patterns can be fabricated, including a random mesh pattern with a wide range of linewidths and spacings from hundreds of nanometers to micrometer scale. Hence, our fabrication method can fabricate both high-resolution regular mesh patterns and also random structures by more simple process than previously reported works.46,47 In particular, the excellent patterning ability of our method, regardless of pattern complexity, has great potential for eliminating the haze which has hindered wide-scale adoption of metal mesh for display applications.24 In order to confirm the reliability of silver mesh fabrication, the controllability of our fabrication method was investigated by varying the Au NP concentration (CGNP), enhancement solution concentration (CAg.E), enhancement time (tAg.E,), and annealing temperature (Ta). The sheet resistance and transmittance of silver meshes with a 1.8 μm linewidth and 50 μm spacing were measured using a 4-point probe station and UV-visible-NIR spectroscopy. For characterization, CGNP is varied from 0.01 wt.% to 0.04 wt.% and CAg.E is varied form 40% to 100%. The effect of varying CGNP and CAg.E on the sheet resistance and transmittance of the silver mesh is summarized in Figure 4 and S1. The effect of tAg.E on these parameters is shown in Figure S1c. For short enhancement times ( 220 for transparent electrode.60 In addition, the mechanical properties of the silver mesh were characterized, as shown in Figure 7. Firstly, the bendability was tested using a silver mesh with a 50 μm linewidth and 50 μm spacing, deposited on a polyester film. Figure 7a and 7b show characteristic bending curves with a bending radius of 5–2 mm by tensile and compressive stress. Figure 7a shows a well-defined characteristic curve. The sheet resistance gradually increased with tensile loading, while it decreased with compressive loading. The bending stability of the silver mesh was confirmed by applying 1000 bending cycles; only a negligible increase in sheet resistance was observed. Secondly, the adhesion strength was tested using a 2 cm × 2 cm silver mesh with 2.6 μm linewidth and 50 μm spacing, based on test method B of the ASTM D3359 standard adhesion test procedure (Figure 7c). Instead of analyzing the delamination ratio as described in the test method, we measured the change in the linear resistance between contact electrode pads before and after a peel-off test, which is closely related to delamination of silver mesh. We used this modified method to estimate damage from delamination as exact measurement of the delamination ratio over the


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entire mesh area is difficult, while resistance changes are easily measured. In order to measure linear resistance, copper tape and silver paste were used as contact electrode pads on both sides of the silver mesh. Figure 7c shows ultra-high adhesion strength between the silver mesh and the substrate without the aid of any physical embedding structure. After the first cycle of the peel-off test, an increase in resistance of only 0.13% was observed at 2.5 N cm-1, and 0.9% at 7.1 N cm-1, indicating negligible damage during peel-off tests. Furthermore, considering that the resistance of silver mesh can be more dramatically increased by a short in the conducting pathway compared to that of non-patterned conducting silver film, it is clear that we demonstrated a great improvement in the adhesion strength using our fabrication method. Considering the adhesion strength classification provided by the D3359 standard, we classified our material as 4B–5B, corresponding to the strongest adhesion grade (5% delamination for 4B and 0% delamination for 5B). In addition, even after multiple peel-off tests, the silver mesh maintained good conductivity. Such outstanding adhesion of the silver mash originated from covalent anchoring between the silver mesh and substrate via the APTES layer. As APTES is strongly bound to the substrate by siloxane bonds, while the amine groups in APTES covalently bond with silver atoms.61,62 Hence, the APTES layer acts as an adhesion layer and increases the adhesion strength between the silver mesh and substrate. The introduction of a self-assembled monolayer of APTES achieved improved adhesion of the metal mesh without any costly deposition processes or embedded structures. Hence, we demonstrated cost-effective fabrication of silver mesh with high adhesion to both rigid and flexible substrates, which is significant for its widespread use for various industrial applications of both hard and soft electronics. In order to confirm the feasibility of our method for practical usage, we used our silver mesh to prepare and characterize a transparent heater. We obtained silver mesh with superior exothermic


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properties, where its performance was compared with previously reported devices, as shown in Table S1. Silver mesh with 2.6 μm linewidth and 50 μm spacing was deposited on a 2.5 cm × 2.5 cm glass substrate. Copper tape and silver paste were used as contact electrode pads, which were connected to a DC power supply. The temperature profile was obtained using an infrared (IR) camera. The voltage-dependent temperature profile of the transparent heater is shown in Figure 8a. It can be seen that the temperature could be easily controlled by adjusting the applied voltage, where the maximum temperature reached 245 °C at 7 V (Note that the maximum measurable temperature of the IR camera is 250 °C). We also measured the time-dependent temperature profile over 2–7 V. As shown in Figure 8b, a rapid response and saturation curve were observed. In addition, the temperature was uniformly distributed over the patterned area (white dashed line in Figure 8c). In order to confirm the heating ability, a water evaporation test was performed. Most of the water droplets were removed within 35 s of the power being applied (Figure 8 d–f and Movie S1). In order to simulate practical applications such as demisting and snow removal from automobile windows or eyeglasses, our transparent heater was exposed to a rain-like condition, constant spraying with water (Movie S2). Even under a continuous water spray, water droplets immediately evaporated from substrate. The performance of our transparent heater was suitable for practical applications.

Conclusions Here, we proposed a new approach for fabricating silver mesh by combining a colloidal deposition technique with silver enhancement. High-resolution silver mesh was successfully fabricated with a minimum feature size of 700 nm. The resistance and transmittance of the mesh was easily controlled by adjusting the deposition parameters and pattern geometry. An outstanding combination of transmittance and sheet resistance was achieved by a mesh with a 2.6 μm linewidth


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and 50 μm spacing (RS = 9.06 Ω sq-1 at T= 85.48%, FoM = 255). In addition, the adhesion strength of the silver mesh was greatly improved by introducing APTES as an adhesive layer, without any physical embedding structures. In addition, the application of silver mesh on a flexible polymeric substrate was successfully achieved. A transparent heater based on our silver mesh exhibited excellent exothermic properties, heating up to 245 °C at 7 V. We realized a new approach for costeffective fabrication of high-resolution silver mesh using an all-water-based solution process which is environmentally friendly. Thus, we expect that our work will greatly contribute to the wide-spread adoption of metal mesh in next-generation optoelectronic devices, and the sustainable development of fabrication technologies.


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Figure 1. Schematic illustrations of the all-water-based metal mesh fabrication process using MCDC. (a) and (b) APTES layer etched by UV/ozone. (c) Au NP dispersion injected between APTES-patterned substrate and cover plate. (d)-(e) Silver shells grown on the deposited Au NPs by silver enhancement. (f) Silver nanomesh annealed on a hotplate.


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Figure 2. Schematic illustrations of the mechanisms of (a) Au NP deposition and (b) silver enhancement. Topographic AFM images of the metal mesh (c) before and (d) after silver enhancement. Coating conditions: Au NPs deposition with 10 μL cm-2 of 0.04wt.% Au NP dispersion for 10 min. Silver was enhanced with 25 μL cm-2 of enhancement solution.


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Figure 3. Optical images of various patterns and a topographic image of the mesh. (a) 700 nm linewidth square mesh with 50 μm spacing. (b) 1.6 μm linewidth hexagonal mesh with 60 μm side length. (c) Micro-perforated plate with 45 μm diameter. (d) Concentric circles. (e) Random network mesh with varying linewidths. (f) Topographic image of metal mesh with 1.8 μm linewidth and 50 μm spacing. Coating conditions: Au NP deposition for (a)–(f) 10 μL cm-2 of 0.04 wt.% dispersion for 10 min and silver enhancement with 25 μL cm-2 solution; annealing at a) 120 °C for 90 min and (b)–(f) 300 °C for 20 min.


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Figure 4. Sheet resistance and transmittance as function of (a) Au NP concentration (CGNP) and (b) enhancement solution concentration (CAg.E). Topographic images with different (c)–(e) CGNP and (f)–(h) CAg.E values. Coating conditions (except in the case of varied parameters): Au NP deposition with 10 μL cm-2 of 0.04 wt.% Au NP dispersion for 10 min, silver enhancement with 25 μL cm-2 solution, and annealing at 300 °C for 20 min.


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Figure 5. (a) Sheet resistance and transmittance as a function of annealing temperature. (b) Average grain diameter and sheet resistance versus annealing temperature. Topographic and FESEM images of metal mesh (c, e) before and (d, f) after annealing. Coating conditions: 2.5 cm × 2.5 cm silver mesh with 2.6 µm line width and 50 µm spacing prepared by Au NP deposition with 10 μL cm-2 of 0.04 wt.% dispersion for 10 min, and silver enhancement with a 25 μL cm-2 solution.


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Figure 6. Transmittance at 550 nm versus sheet resistance for recently reported transparent conducting electrodes, including our samples, Ag mesh, Au nanowires, Au mesh, Au nanofiber, Cu mesh, Pt mesh, and AgNi mesh.


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Figure 7. Characterization of the mechanical properties of the metal mesh. (a) Stress-dependent sheet resistance profiles under tensile and compressive loading. (b) Bending stability test for 1000 cycles of tensile loading with a 5 mm bending radius. (c) Resistance measured during the standard adhesion test method B in D3359.


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Figure 8. Demonstration of transparent heater performance. (a) Temperature versus DC voltage. (b) Time-dependent temperature profile for 2, 3, 5, and 7 V. (c) IR image of heater showing the uniform temperature distribution at 5 V. (d)–(f) Water evaporation test with 5 V operating voltage.


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ASSOCIATED CONTENT Supporting Information Sheet resistance and thickness profile (Figure S1), the change in sheet resistance and transmittance with respect to annealing condition (Figure S2), AFM images and FE-SEM images at different annealing temperature for saturation time. (Figure S3), XRD spectrum of Ag-Au alloy film before annealing and after annealing (Figure S4), summary of recently reported transparent heater based on various systems (Tables S1), water evaporation test for transparent heater (Movie S1), and transparent heater exposed by rain-like condition (Movie S2). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2016R1A2B4012992).


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Table of Contents


35

Highly Transparent, Flexible Conductors and Heaters Based on Metal

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