Highly Bendable and Durable Transparent Electromagnetic

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Functional Inorganic Materials and Devices

A Highly Bendable and Durable Transparent Electromagnetic Interference Shielding Film Prepared by Wet Sintering of Silver Nanowires Dong Gyu Kim, Jong Han Choi, Duck-Kyun Choi, and Sang Woo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07054 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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ACS Applied Materials & Interfaces

A Highly Bendable and Durable Transparent Electromagnetic Interference Shielding Film Prepared by Wet Sintering of Silver Nanowires

Dong Gyu Kim,†,§ Jong Han Choi,†,∥ Duck-Kyun Choi,§ and Sang Woo Kim*†,‡

†

Clean Energy Research Center, Korea Institute of Science and Technology (KIST), Seoul

02792, Republic of Korea ‡

Division of Energy Environment Technology, KIST school, University of Science and

Technology (UST), Seoul 02792, Republic of Korea §

Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic

of Korea ∥Department

of Chemical and Biological Engineering, Korea University, Seoul, 02841,

Republic of Korea

ABSTRACT: Electromagnetic (EM) wave emissions from wearable or flexible smart display devices can cause product malfunction and have a detrimental effect on human health. Therefore, EM shielding strategies are becoming increasingly necessary. Consequently, herein, we prepared a transparent acrylic polymer-coated/reduced graphene oxide/silver nanowire (Ag NW) (A/RGO/SANW) EM interference (EMI) shielding film via liquid-to-vapor pressure-assisted wet sintering. The film exhibited enhanced Ag NW network formation and antireflection (AR) effects. The wet-sintered Ag NW shielding film had a threshold radius of curvature (ROC) of 1

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0.31 mm at a film thickness of 100 µm, demonstrating its high flexibility, while the conventional indium tin oxide (ITO) shielding film had a threshold ROC of ~5 mm. The EMI shielding effectiveness (SE) of the A/RGO/SANW multilayer film was approximately twice that of the ITO film at a similar relative transmittance (84%–85%). The optical relative reflectance of the Ag NW layer was reduced due to the AR effect, and the visible light transmittance was considerably improved owing to the different refractive indices in the multilayer shielding film. Because the acrylic coating layer had a high contact angle, the multilayer film exhibited high temperature and humidity durability with little change in the SE over 500 h at 85 °C and 85% relative humidity. The multilayer film comprising wetsintered Ag NW exhibited high flexibility and humidity durability, high shielding performance (more than 24 dB at a relative transmittance of 85% or more), and high mass productivity, making it highly applicable for use as a transparent shielding material for future flexible devices.

KEYWORDS: transparent electromagnetic shielding, flexible electromagnetic interference shielding film, silver nanowire, sintering, acrylic polymer, graphene oxide

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1. INTRODUCTION

The demand for flexible components for use in wearable devices and highly portable foldable displays has significantly increased in recent years.1–5 However, these components tend to be relatively small, indicating that electromagnetic interference (EMI) generated by each independent component can impact product function and shorten the operational lifetime.6,7 In addition, previous studies have shown that the EM waves radiated from certain devices can have harmful direct or indirect effects on human health.8–10 Accordingly, the need for appropriate shielding to suppress the harmful EM waves generated by certain electronic devices is increasing.11–13

Transparent EMI shielding films for flexible devices must exhibit high EMI shielding effectiveness (SE), high visible-light transmittance, high flexibility, ultrathin, lightweight, and excellent durability.12–14 However, EMI shielding materials that satisfy all these criteria have not yet been developed. The performance of an EMI shielding material strongly depends on its electrical conductivity, permeability, thickness, and the EM frequency of interest.15–18 Above all, to develop a superior flexible transparent shielding film, the material must have improved electrical conductivity, which is an intrinsic material characteristic, while maintaining a high degree of visible light transmittance.

To address these issues, various transparent EMI shielding materials, including transparent conducting oxides (TCOs),19,20 metal meshes,21,22 and graphene,23–24 have been investigated. Recently, hybrid EMI shielding films, which combine a metal mesh with graphene, have been studied.26,27 Indium tin oxide (ITO) is the most widely used TCO material owing to its high 3



transmittance and electrical conductivity. However, ITO is unsuitable for use as a transparent flexible material28,29 because it is brittle and fractures easily. Moreover, its threshold radius of curvature (ROC) is greater than 7 mm,30 which is defined as the ROC at which a 10% change in the electrical resistance is observed. Al-doped ZnO, developed as an alternative to ITO, exhibits relatively high flexibility and high optical relative transmittance (84.1%); however, it suffers from a poor shielding performance of 6.5 dB at frequencies of 0.3–1.5 GHz,20 which limits its use as a transparent shielding material. For higher EMI shielding and optical performance, metal mesh films have been studied.21,22 Metal meshes exhibit high optical transmittance and EMI shielding performance. However, they have disadvantages in that they are costly (owing to their complicated preparation) and have poor repetitive fatigue bending characteristics.31 To address the problems associated with their complicated manufacturing process, a metal mesh film fabricated by ink jet printing has been developed.21 However, its EMI SE is only 20 dB (8–12 GHz) at a relative transmittance of 88.2% and it is poorly flexible. Monolayer graphene is a promising alternative material for fabricating transparent EMI shielding films owing to its excellent optical and electronic properties. However, despite its high relative transmittance of 97%, it exhibits a low SE of 2.27 dB (2.2–7 GHz) owing to its ultralow thickness and the presence of defects.23 Multilayer graphene, which shows improved EMI shielding performance, exhibits a high SE of 19.14 dB (18–26.5 GHz) at a relative transmittance of 80.5%.25 However, its preparation is too complicated for it to be produced on a large scale.

To overcome these drawbacks, metal NWs have been reported to be promising nextgeneration transparent and flexible EMI shielding materials. Among them, silver nanowires (Ag NWs) are regarded as the most promising candidate owing to their excellent 4


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transmittance, electrical conductivity, and mechanical flexibility.32–34 Furthermore, Ag NWs can be wet coated onto flexible substrates with continuous equipment using solutions dispersed as inks, thus enabling low-cost and high-volume production.35 Owing to these properties, Ag NWs have been studied as a highly bendable transparent EMI shielding material.36 However, despite these advantages, Ag NWs have several issues that need to be addressed if they are to be used as a shielding material. First, in the polyol synthesis process, the insulating capping agent used to promote anisotropic growth surrounds the AgNWs,37 which indicates that the electrically conducting network structure is not sufficiently formed and the contact conductance between the NWs is poor. Furthermore, Ag NWs weakly bind to substrates, are susceptible to oxidation, and reflect light, causing scattering.38

Recently, a number of methods have been proposed to enhance the electrical conductivity of Ag NWs by strengthening their network structures. These include high-temperature sintering,39–41 mechanical pressing,42,43 and light-induced welding.44,45 Another approach is to combine Ag NWs with conductive polymers to inhibit their light scattering and improve their bonding with flexible substrates.46 However, the former methods have the limitations that high temperature and/or high pressure are required, both of which are drawbacks to largescale production and can damage the polymeric components of the material. Furthermore, the light-induced welding of Ag NWs requires the usage of expensive optical equipment due to sophisticated precision control and high energy output for nanowire sintering. The use of the latter conductive polymer and Ag NWs exhibits a problem that the conductive polymer is easily deteriorated and that the conductivity of the polymer is lowered.

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Herein, we report on the development of a transparent multilayer EMI shielding film comprising Ag NWs arranged in a network that is reinforced by a wet sintering process assisted by liquid-to-vapor pressure (LVP). The EMI shielding performance of the multilayer film was evaluated in the frequency range 0.3–3.0 GHz, which corresponded to a quasimicrowave frequency band widely used in communication systems, such as television and radio transmissions, channel broadcasting, mobile phones, and local area networks.

2. EXPERIMENTAL SECTION 2.1. Materials An isopropyl alcohol (IPA) suspension (004c) containing Ag NWs (0.5 wt%) was purchased from N & B, Korea. The diameters and lengths of the Ag NWs in the IPA suspension were in the ranges 35–45 nm and 5–15 µm, respectively. A polyethylene terephthalate (PET) film (RX160) with a thickness of 100 µm was purchased from HSfinetech, Korea. A graphene oxide (GO) suspension (GO-A400) was purchased from Grapheneall, Korea. The GO suspension contained 0.6 wt% GO dispersed in distilled water, and the GO flakes had a single-layer thickness of 80% and a size of 0.5–5 µm. An aqueous acrylic emulsion comprising 30% solids (emulsion size = ~100 nm) was used for top coating the Ag NW films. 2.2. Fabrication of transparent EMI Ag NW films Figure 1a shows a schematic of the wet sintering process used to prepare the Ag NWs proposed herein. The pristine Ag NWs were wet coated onto the PET film using an automatic film applicator (PI-1210; Sangyo, Japan). The Ag NW films were then dried on a hot plate at 80 °C for 10 min and further dried in a constant-temperature drying chamber at 60 °C for 1 h. 6


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Figure 1. Schematic of the fabrication of silver (Ag) nanowire (NW) electromagnetic interference (EMI) shielding films: (a) overall fabrication process, (b) wet sintering process using hot rolling system, and (c) an A/RGO/SANW EMI shielding multilayer film obtained from the process.

To develop an Ag NW film with a strong network structure, the pristine Ag NW films were sprayed or coated with water or a water-based GO suspension, after which the Ag NWs were wet sintered with the aid of LVP using a roll-to-roll system at temperatures of 40 °C–140 °C and a speed of 3.2 cm/s. The sintered Ag NW (SANW) film and the GO/Ag NW (GO/SANW) film were dried in a constant-temperature drying chamber at 60 °C for 1 h. The RGO/SANW film was obtained by reduction from the GO/SANW film with hydrazine vapor and then drying.48 The acrylic microemulsion was spin coated onto the SANW and GO/SANW films at a 7



rotation speed of 1,000 rpm for 1 min. The acrylic polymer-coated SANW (A/SANW) film and the acrylic polymer-coated GO/SANW (A/GO/SANW) film were dried on a hot plate at 80 °C for 10 min. For complete curing, the films were dried in a constant-temperature drying chamber at 60 °C for 1 h. Figure 1b shows the photographs of the wet sintering process used to prepare the Ag NW EMI shielding film. Figure 1c shows the Ag NW EMI shielding film fabricated by the wet sintering process.

2.3. Characterizations of EMI shielding films The images of the Ag NW films were obtained using field-emission scanning electron microscopy (FESEM; Nova SEM; FEI, USA). The surface roughness of the Ag NW films was measured using atomic force microscopy (AFM; XE-100; Park Systems; Korea). The sheet resistances of the Ag NW films were measured using the four-point probe method (NCP-TFP; Mitsubishi Chemical, Japan). The optical properties of the Ag NW films were measured using a UV spectrophotometer (CM5; Scinco, Korea) in the wavelength range 350– 750 nm. Bending tests were performed using the collapsing radius test.49 The resistance changes of the Ag NW films according to the ROCs were measured using a bending tester (see Supplementary Figure S2). Silver paste was coated onto both ends of the Ag NW films for electrode formation. A DC power supply source (GPD-3303s; GW Instek, Taiwan) was connected to both sides of the Ag NW film electrodes, and the electrical current changes according to the ROC were calculated as the resistance changes of the bent state. Note that when a voltage is applied to an Ag NW film, heat is generated via joule heating.50 Therefore, voltage should be regulated so that the temperature of the Ag NW film remains below 40 °C, thereby ensuring that the resistance is not affected by the temperature. The constant temperature and humidity tests of the Ag NW films were performed by a testing agency 8


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(KOPTRI, Korea). The electrical properties of the acrylic-layer-coated Ag NW films cannot be measured using the four-point probe equipment. Therefore, electrodes were formed at both ends of Ag NW films and the resistance changes were measured using a DC power supply source. Infrared (IR) images of the Ag NW films were obtained using an IR camera (T620; FLIR, USA). The EMI SE was measured using an RF vector network analyzer (VNA 8753ES; Agilent, USA) in the frequency range 0.03–3 GHz. A coaxial sample holder was used with a shielding effect measurement kit (S-39D-D4935, Keycom, Japan) designed according to the ASTM D4935-10 standard. For VNA calibration, SOLT calibration was performed using a well-defined short, open and load (50 Ω characteristic impedance). The size of the circular film sample for shielding effect measurement was 49 mm outside diameter. The EM transmittance (T) and reflectance (R) were calculated according to Equation 1 using the measured S-parameters of the two-port network system, which include the reflection coefficient ( or ) and transmission coefficient ( or ). When the total output of the incident EM radiation is 1, the EM absorbance A can be obtained from equation (2) by subtracting R and T (see Supplementary Figure S1). The total EMI SE (SE ) of a solid T

material with no apertures can be expressed as the sum of the three loss factors: effective reflection (SE ), effective absorption (SE ), and multiple reflections (SE R

Equation 3–6.51,52 Since SE

A

MR

MR

), as shown in

can be ignored when SE is greater than 10 dB,53,54 SE can A

T

be approximated as the sum of SE and SE . When an incident wave passes through a shield, A

R

the SE at the interface between mediums 1 (free space) and 2 (shield) is related to the R

difference in the characteristic impedances between the media, which is logarithmically proportional to the ratio of the impedance of the incident wave (ZW) and the impedance of the

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shield (ZS), as presented in Equation 4. SE and SE are obtained using Equations 4 and 5, R

A

respectively, which can be applied to a plane wave that approaches the shield at normal incidence. Finally, SE can be calculated by Equation 6, which is the sum of Equation 4 and T

Equation 5 that are given below.

| |2 = R, | |2 = T

(1)

A=1−R−T

(2)

SE = SE + SE + SE T

R

SE = 20 log R

A

| |

| |

= SE + SE (dB)

MR

R

A

= −10 log (1 − R) (dB)

(3)

(4)

SE = −10 log (dB)

(5)

A

SE = −10 log(1 − R) − 10 log (dB)

(6)

T

3. RESULTS AND DISCUSSION 3.1. Fabrication of EMI shielding films Figures 2a–2f show the SEM and AFM images of the network structures of the Ag NW EMI shielding films. As shown in Figure 2a, the Ag NWs become electrically connected as the NWs stack; however, the contact resistance can be increased by ohmic contact owing to the weak network between the NWs. Figure 2e shows the surface roughness of the Ag NW films, which serves as a criterion to assess the electrical network between the NWs. The Ra roughness value of the pristine Ag NWs was measured to be 23.5 nm. Figure 2b shows the SEM image of the SANW layer wet sintered at 120 °C, indicating that the stacked NWs are 10


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fully networked by LVP-assisted wet sintering, thereby increasing the electrical contact between the NWs. When comparing the surface roughnesses in Figures 2e and 2f, it can be observed that the Ra roughness value decreases from 23.5 to 14.3 nm owing to the sintering effect. Figure 2c shows an SEM image of GO (0.5 wt%) coated onto SANW. The Ra roughness value is significantly reduced to 5.3 nm by the uniform coating of GO onto the sintered Ag NW film. Figures 2d and 2h show the SEM and AFM images of the acrylic polymer coated onto the GO/SANW film. The Ra roughness value increased to 13.1 nm because of the covering of the GO surface with 40−50-nm acrylic nanoparticles. The hydrophobic acrylic coating layer prevents the Ag NWs from being oxidized by moisture and oxygen in the external environment.

Figure 2. Scanning electron microscopy (SEM; 15° tilted) and atomic force microscopy (AFM) images of the Ag NW EMI shielding films fabricated via wet sintering process: (a and 11



e) pristine Ag NW/polyethylene terephthalate (PET) film, (b and f) SANW/PET film, (c and g) GO/SANW/PET film, and (d and h) A/GO/SANW/PET film. Ra indicates the mean surface roughness, calculated on 5 x 5 µm2 area. (a–d) scale bar = 100 nm and (e–h) scale bar = 1 µm.

3.2. Electrical and optical properties Figure 3a shows the change in the sheet resistance of the Ag NW films with the roll temperature during both wet and dry sintering processes. In the dry sintering process, the sheet resistance shows little change up to 120 °C, after which it gradually decreases with increasing temperature. The sheet resistance is reduced by ~10.5% from 38.9 to 34.8 Ω/□ at 140 °C. This result is consistent with those of the previous reports wherein the sintering of Ag NWs occurred in the temperature range 140 °C–180 °C.39 However, such a high-temperature sintering method is unsuitable for its application to flexible substrates comprising polymer materials having a low melting point of 280 °C or less.

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Figure 3. (a) Variation in sheet resistances of Ag NW films fabricated by dry and wet sintering processes and (b) sheet resistance vs. relative transmittance (at 550 nm) compared with Ag NWs (in this work) and previously reported Ag NWs prepared via sintering.

In the wet sintering process, the sheet resistance drastically decreases from 38.9 to 15.6 Ω/□ in the roll temperature range 20 °C–120 °C. The first step decrease in the sheet resistance is 18.3% between 40 and 80 °C. The second step decrease in the sheet resistance is a significant 13



reduction of 55.0% in the temperature range 80 °C–100 °C, where a phase transition from liquid to vapor occurs, unlike the case of the dry sintering process. This result indicates that the NWs are softened and sintered with the aid of LVP. The last step decrease is a slight reduction up to 59.6% in the temperature range 100 °C–120 °C, followed by a resistance increase as the temperature rises to 140 °C. Compared to the dry sintering process, the reduction in the sheet resistance in the wet sintering process is much greater, indicating that sintering is complete at a much lower temperature of 100 °C. The GO/SANW film shows almost the same sheet resistance decrease up to 120 °C but shows no change in the sheet resistance at higher temperatures. This is attributed to the fact that as GO is coated onto the SANW/PET substrate, the adhesion of the NWs to the substrate is increased so that the NWs do not peel off from the substrate under a relatively high vapor pressure at 140 °C. Figure 3b compares the sheet resistances of pristine Ag NWs, the Ag NWs prepared by wet sintering, and the Ag NWs produced using previously reported methods, including hightemperature sintering,40 mechanical pressing,42 and light-induced welding,44,45. The sheet resistances of the A/GO/SANW and A/RGO/SANW films prepared via wet sintering slightly increase as the relative transmittance increases, whereas the sheet resistances of the pristine Ag NWs and Ag NWs from previous studies tend to significantly increase at a relative transmittance that was greater than 75% (except that prepared via light-induced welding). Compared with the films prepared using the previously reported methods other than lightinduced welding, the films prepared via wet sintering are superior in terms of sheet resistance to relative transmittance. However, the wet sintering method is superior to the light-induced welding method in terms of commercialization and mass production because it may be applied to roll-to-roll processing. 14


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Figure 4a shows a plot of relative transmittance vs. the amount of GO coated onto SANW. For typical coating methods, the relative transmittance tends to decrease as the concentration of the solution increases. However, as shown in Figure 4a, the relative transmittance improves as the GO content increases. When the 0.5 wt% GO-coated SANW film was coated with the acrylic polymer, no change was observed in the relative transmittance. Thus, the acrylic polymer has high transparency and no effect on overall transmittance. However, the relative transmittance of the A/RGO (0.5 wt%)/SANW film was slightly lowered by reducing the GO layer in the A/GO (0.5 wt%)/SANW film.

Figure 4. (a) Relative transmittance values of Ag NW films with GO and acrylic polymer coatings at 550 nm, (b) Relative transmittance and reflectance values of Ag NW films, (c–g) SEM images of GO-coated Ag NW EMI shielding films with different GO contents: (c) 0%, 15



(d) 0.05%, (e) 0.2%, and (f and g) 0.5%. (h) SEM image of the A/GO (0.5 wt%)/SANW EMI shielding film. □ and ○ represent the relative transmittance and relative reflectance of pristine Ag NWs, respectively, in (b).

Figure 4b shows a plot of the relative transmittance and reflectance values of the films vs. GO concentration. The results show that the relative transmittance improves as the GO concentration increases. The relative transmittance improves because light loss is reduced with the commensurate decrease in relative reflectance. The relative reflectance gradually decreases depending on the GO concentration, which is confirmed by the SEM images in Figures 4c–4f. The relative transmittance of the 0.05 wt% GO-coated Ag NWs increases by 3% from that of pristine Ag NWs (79.6%) to 82.6%. As shown in Figure 4d, the fraction of the Ag NWs not covered by GO is more than 40%. In the case of the 0.2 wt% GO-covered NWs shown in Figure 4e, the relative transmittance is increased to 84.5% and the SEM image shows that the uncovered area of the sample is less than 5%. The relative transmittance of the 0.5 wt% GO-coated Ag NWs is further increased to 86.7%. The SEM images in Figures 4f, 4g, and 4f confirm that the GO and acrylic coating layers cover all the Ag NW surfaces in this sample. To suppress reflectivity, the refractive indexes of the component layers should be controlled such that the light reflected from the Ag NWs and that from the overcoat layer interfere with each other. Thus, the larger the GO coverage, which has a refractive index different from that of the Ag NWs, the higher the increase in transmittance. In addition, reflectance is also influenced by the surface roughness and the incident angle of the light. The SEM images in Figures 2d, 2h, and 4f show that the surface of the acrylic coating layer comprises 16


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nanoparticles and is therefore not smooth. Thus, reflectance is further reduced by the acrylic coating layer. 3.3. EMI shielding performance Figures 5a–5c show the reflection loss, absorption loss, and total EMI SE of the Ag NW EMI shielding films fabricated using each method.

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Figure 5. (a) Reflection loss, (b) absorption loss, (c) total EMI shielding effectiveness (SE) of Ag NW EMI shielding films, and (d) comparison of total EMI SE vs. optical transmittance (at 550 nm) for Ag NW films and other EMI shielding films.

In this case of a good conductor, SE and SE are approximately given by equations 7 and 8, R

A

respectively.53,54

SE ∝ 10 log " R

σ# % µ# $

(7)

SE ∝ t'σ( µ( f

(8)

A

SE

R

at the interface between two media is related to the difference in characteristic

impedances between the incident wave and the surface of the shield. Since the relative permeability µr of nonmagnetic silver is 1, SE is logarithmically proportional to the relative R

electrical conductivity σr, as shown in equation 7. SE

A

is directly proportional to the

thickness of the shield and proportional to the square root of σr, as shown in equation 8. Thus, the total EMI SE of the Ag NW film can be improved by decreasing the ohmic resistance between the NWs. For RGO or CNT containing nanocomposites51,54 that are not good conductors, the EMI shielding effectiveness should be taken into account magnetic permeability and dielectric permittivity. Figure 5a shows that the SE values of the pristine Ag NWs and A/GO/SANW before and R

after the wet sintering process increase by 2.1 times from 4.1 to 8.6 dB (at 1 GHz) at the same 18


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optical transmittance (90.3%). For GO/SANW, A/SANW, and A/GO/SANW with sintered Ag NWs coated with acrylic polymer and GO, the SE values of the coatings show little R

dependence on frequency at 0.1–3 GHz. Figure 5b shows that the SE value of SANW A

increases by approximately 1.5 times from 9.4 to 13.9 dB as compared to that of the pristine Ag NWs at the same optical transmittance (90.3%). In addition, with improved optical transmittance, the SE value for A/GO/SANW (or A/SANW) increases by approximately 1.6 A

times from 9.4 to 15.5 dB as compared to that of the pristine Ag NWs. Electrical conductivity can be calculated as shown in equation 9:

* = ,,

(9)

+

where * is the electrical conductivity (S/m), Rs is the sheet resistance (Ω/□), and t is the thickness of the sample (m). The increase in SE for the pristine Ag NWs and SANW is due A

to the decrease in sheet resistance from 39.0 to 17.3 Ω/□ and the decrease in the thickness of the Ag NWs from 115.2 to 64.1 nm by wet sintering and σ increasing significantly by 3.6 times from 3.5 × 105 to 12.6 × 105 S/m. Furthermore, as shown in Figures 4a and 4b, the optical relative reflectance increases from 60% to 80% upon wet sintering when compared to those of the pristine Ag NWs and SANW. This result is consistent with the tendency of good conductors to exhibit increased EM reflectance and decreased EM absorbance when the electrical conductivity is improved, as depicted in Supplementary Figure S1a and S1b. Figure 5c shows that the SE values of SANW, A/RGO/SANW, and A/GO/SANW increase by T

approximately 1.5, 1.7, and 1.8 times upon wet sintering as compared to that of the pristine Ag NWs at the same optical transmittance. In particular, as shown in Supplementary Figure 19



S3, the SET value of A/GO/SANW shows little change with frequency even at 8–18 GHz, indicating that the transparent film is applicable as a material for shielding devices at a wide range of radio frequencies.

Figure 5d and Table 1 shows the total EMI SE values of various shielding materials reported in the literature and those of the Ag NW EMI shielding films prepared herein. The total EMI SE values of the Ag NW films prepared herein are much higher than those of the other shielding materials at the same transmittance (except for that of the graphene/metal mesh with almost the same EMI SE value).

3.4. Flexibility and durability Figure 6a shows the changes in the resistance changes of Ag NW EMI shielding films with optical relative transmittance and ROC. For the pristine Ag NWs, ∆R/R0 increases as the ROC decreases and the optical relative transmittance increases. The optical relative transmittance of the Ag NWs decreases as the surface density of the NWs increases and the networking property is enhanced. Thus, as the NW density increases and the friction between the NWs increases, the stresses exerted on the NWs are dispersed and the threshold ROC is reduced. Consequently, Ag NWs with enhanced networking property have high flexibility; however, their optical transmittance is decreased.

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Table 1. Comparison of the total electromagnetic interference (EMI), shielding effectiveness (SE), and optical transmittance of silver (Ag) nanowire (NW) EMI shielding materials at different optical transmittance values

EMI shielding materials

Frequency (GHz)

SET (dB)

Al-doped ZnO

0.3−1.5

6.5

Relative transmittance (%) 84.2

Metal mesh

8−12

20.0

88.2

21

Triangular ring metal mesh

12−18

17−21

95

22

Monolayer graphene

2.2−7

2.3

97

23

RGO

0.5−8.5

6.4

62

24

Multi-layer graphene

18–26.5

19.1

80.5

25

Graphene/metal-mesh

12−18

14.1

90.5

26

PMMA-graphene/metal-mesh

12−18

23.6−28.9

91

27

Graphene/metal-mesh/dielectric

26.5

34.5

85

56

Ag NWs

8−12

12.5−16.0

80

34

PES/Ag NWs/PET

8−12

16

85

36

CA/Ag NW/polyurethane

8−12

20.7

92

14

ITO

0.5−3

14.0

84.1

In this study

Pristine Ag NWs

0.5−3

13.5

90.3

In this study

SANW

0.5−3

20.4

90.3

In this study

A/GO/SANW

0.5−3

24.1

90.3

In this study

A/GO/SANW

0.5−3

28.0

85.2

In this study

A/GO/SANW

0.5−3

30.1

80.3

In this study

A/RGO/SANW*

0.5−3, 8−18

23.9

84.9

In this study

A/RGO/SANW

0.5−3

24.1

82.5

In this study

Ref. 20

*: SET at 8-18 GHz is shown in Supplementary Figure S3.

21



Figure 6. (a and b) Resistance changes (∆R/R0) of the Ag NW EMI shielding films with radius of curvature (ROC) and optical relative transmittance (at 550 nm), (c, d, f, g, i, and j) infrared (IR) thermographs, and (e, h, and k) SEM images of each Ag NW film (90.4% at 550 nm; applied voltage = 1.5 V) before and after bending at ROC = 1.35 mm. (c–e) Pristine Ag NW film, (f–h) SANW film, and (i–k) A/GO/SANW film.

The SANW film with sintered NWs has flexibility higher than that of the pristine Ag NW film at the same optical transmittance, as shown in Figure 6b. This result is also evident in Table 2, showing that the threshold ROC decreases from 2.1 to 1.2 mm. As shown in Figure 6b and Table 2, for A/GO/SANW with enhanced adhesion to substrates with acrylic polymer and GO coatings, the threshold ROC is dramatically decreased to 0.4 mm. These results demonstrate that the decrease in the critical threshold ROC value at the same optical transmittance is due to the effect of increased thickness or the adhesion force and not the 22


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surface density of the Ag NWs. The thickness of the acrylic polymer overcoat layer was measured to be 100–150 nm, indicating that this layer was considerably thin. Therefore, the effect of the thickness of the overcoat layer on decreasing the threshold ROC is negligible.30 Consequently, the threshold ROC reduction is attributed to an increase in adhesion between the substrate and the Ag NWs by the overlayer coating.

Table 2. Threshold radius of curvatures (ROCs) and relative transmittances of the Ag NW EMI shielding films

2.1

Relative transmittance (%) 90.4

1.7

84.5

1.4

76.5

SANW

1.2

90.2

GO/SANW

1.2

90.4

A/SANW

1

90.3

0.4

90.4

0.42

84.2

0.31

78.9

0.44

83.7

Threshold ROC (mm)

Ag NW EMI shielding film

Pristine Ag NWs

A/GO/SANW

A/RGO/SANW

The threshold ROC reduction was confirmed by the IR and SEM images that depict the image during the time at which electricity is applied after bending and restretching the film. Figures 6c and 6d show the thermal image and heat distribution, respectively, of the pristine 23



Ag NW film after bending at an of ROC of 1.35 mm. The temperature of the bent area in the bent and restretched film was measured to be 23.3 °C (Figure 6d), whereas that of the rest of the film was measured to be below 26.5 °C (Figure 6c). Figure 6e shows that the cause of the increased resistance is that the NWs are stressed upon exceeding the threshold ROC and are fractured at the bending edge. As shown in Figure 6f, the surface temperature is ~5 °C higher than that of the pristine Ag NW film at the same optical transmittance (90.4%). This result demonstrates that the resistance of the Ag NWs is reduced by wet sintering. Joule heating is a process wherein the energy of an electric current is converted into heat as it flows through a resisting material. Therefore, when the resistance is decreased, the heating property is improved. Figure 6g shows the surface temperature of the SANW film. The temperature of the bent area was measured to be 35.5 °C, while that of the rest of the film was measured to be 35.2 °C and 34.8 °C. Figure 6h shows that the NWs were not fractured at the ROC same as that of the pristine Ag NWs (1.35 mm). This result demonstrates that the flexibility of Ag NWs is improved by wet sintering. Figure 6j shows the IR image of the A/GO/SANW film bent at an ROC of 1.35 mm. The surface temperature of the bent area of the A/GO/SANW film was measured to be 37.3 °C, while that of the rest of the film was 35.6 °C. The resistance is not changed even at the same ROC, and the A/GO/SANW film has the same temperature distribution throughout. Thus, the flexibility of Ag NW EMI SE films is dramatically improved by the NW sintering effect and enhanced adhesion to the substrate by the acrylic polymer and GO coatings. Figure 7a shows the results of the taping tests performed with 3M scotch tape, which were used to assess the adhesion between the substrate and the coating layers. The resistance of the 24


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pristine Ag NWs increases by ~19% for the 1-N taping test, whereas that for the 5-N taping test increases by 65% for 5 N. Beyond 10 N, the resistance converges to infinity. For the SANW film, a 10% increase in resistance is observed in the 10-N taping test. This result shows that wet sintering improves adhesion to the substrate. The GO/SANW, A/SANW, and A/GO/SANW films showed no change in the resistance, even in the 10-N taping test, because the adhesion to the substrate is improved considerably by the acrylic polymer and GO coatings.

Figure 7. (a) Resistance changes (∆R/R0) of the Ag NW EMI shielding films (T = 90.3%; at 550 nm) after taping test with number of cycles (N). (b) Constant temperature and humidity (85 °C/85% RH) test results. (c) Contact angles of Ag NW EMI shielding films: (c1) Pristine Ag NWs, (c2) SANW, (c3) GO/SANW, (c4) RGO/SANW, (c5) A/SANW, and (c6) A/GO/SANW.

25



Figure 7b shows the changes in resistance with time for each Ag NW EMI shielding film under the same humidity and temperature conditions (85 °C/85% RH). The resistance of the pristine Ag NWs slightly decreases until 72 h and then increases by 50% at 120 h, followed by an increase to 410% at 500 h. The decrease in the resistance until 48 h is thought to be due to the effect of humidity, which reduces the contact resistance by softening the PVP, an impurity remaining in the NW.42 An interesting result is that the resistance of GO/SANW, which is expected to exhibit low water permeability, becomes almost infinite after 24 h. Conversely, the resistance of the A/GO/SANW film remains unchanged after 500 h. Furthermore, the resistance of the A/SANW film only exhibited a slight change. Figure 7c shows the water contact angles of the films, which reflect their hydrophilicities. The contact angles of the pristine Ag NW, SANW, GO/SANW, RGO/SANW, A/SANW, and A/GO/SANW films are 33.5°, 47.86°, 59.6°, 77.6°, 86.1°, and 86.3° respectively. The pristine Ag NWs have a hydrophilic PVP coating and a low contact angle because moisture can easily penetrate into the void space between the NWs. In the case of the SANW film, hydrophilic PVP is partially removed upon wet sintering and the contact angle is improved by 42%. Unlike hydrophilic GO,55 RGO is hydrophobic. Thus, GO absorbs moisture under constant humidity conditions and promotes Ag NW oxidation over 500 h. Conversely, the hydrophobic acrylic polymer coating on A/SANW and A/RGO/SANW effectively prevents Ag NW oxidation. Figure 8 shows the SE value of each Ag NW film under constant humidity conditions. Similar to the changes in resistance, the SE value of the SANW film decreases with time: the SE value reduces by 28% after 360 h and by 68% after 500 h. For the GO/SANW film, Ag NW oxidation is promoted by GO and the SE value decreases by 100% after 120 h. 26


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Conversely, the SE values of A/SANW and A/RGO/SANW remain nearly unchanged, even after 500 h, which is consistent with the results of the 85 °C/85% RH tests shown in Figure 7b.

Figure 8. Total SE change (∆SET/SET0) of the Ag NW EMI shielding films after constant temperature and humidity (85 °C/85% RH) tests.

27



4. CONCLUSIONS Herein, an LVP-assisted wet sintering of Ag NWs for a highly bendable, durable, and transparent EMI shielding film was presented. Using this process, the sintering temperature can be lowered to strengthen network formation between the NWs, thereby effectively reducing their contact resistance. Consequently, the electrical conductivity of the Ag NW EMI shielding film was significantly improved compared to that of pristine Ag NWs and the SE value was considerably improved from 13.5 to 24.1 dB at a transmittance of 90.3%. The threshold ROC of the Ag NW EMI shielding film was significantly reduced from 2.1 to 0.31 mm owing to the increased adhesion to the substrate imparted by the acrylic polymer and GO (or RGO) multilayer coatings. In addition, the AR effect of A/GO/SANW (or A/RGO/SANW) multilayers with different refractive indexes significantly reduced the reflectance of the Ag NW film, improving the transmittance by 7%. Owing to the water repellency of the acrylic polymer coating, the EMI SE value of the Ag NW film remained almost unchanged for 500 h under 85 °C/85% RH conditions. The Ag NW EMI SE film fabricated via LVP-assisted wet sintering can be mass produced by the roll-to-roll process and has high durability and flexibility. In addition, the Ag NW EMI shielding film has an excellent SE value and high transmittance, making it highly applicable to devices, including flexible displays, flexible touchscreen panels, and wearable devices.

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ACKNOWLEDGMENTS This material is based upon work performed by the Clean Energy Research Center, supported through the Korea Institute of Science and Technology under Award Grant No. 2E28350.

ASSOCIATED CONTENT Supporting Information Figures S1, S2, and S3 provided in the Supporting Information document (.pdf file).

List of Supplementary Figures

Supplementary Figure S1. EM reflectance, absorbance, and transmittance of Ag NW EMI shielding films (T = 90.3 %; at 550 nm). Supplementary Figure S2. Image of the radius of curvature (ROC) bending tester; a schematic of ROC. Supplementary Figure S3. Total EMI SE of A/RGO/SANW EMI shielding film at 8–18 GHz.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +82-2-958-5526. Fax: +82-2-958-5219

29



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417x154mm (300 x 300 DPI)



Figure 1. Schematic of the fabrication of silver (Ag) nanowire (NW) electromagnetic interference (EMI) shielding films: (a) overall fabrication process, (b) wet sintering process using hot rolling system, and (c) an A/RGO/SANW EMI shielding multilayer film obtained from the process. 80x42mm (300 x 300 DPI)


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Figure 2. Scanning electron microscopy (SEM; 15° tilted) and atomic force microscopy (AFM) images of the Ag NW EMI shielding films fabricated via wet sintering process: (a and e) pristine Ag NW/polyethylene terephthalate (PET) film, (b and f) SANW/PET film, (c and g) GO/SANW/PET film, and (d and h) A/GO/SANW/PET film. Ra indicates the mean surface roughness, calculated on 5 x 5 µm2 area. (a–d) scale bar = 100 nm and (e–h) scale bar = 1 µm. 87x50mm (300 x 300 DPI)



Figure 3. (a) Variation in sheet resistances of Ag NW films fabricated by dry and wet sintering processes and (b) sheet resistance vs. relative transmittance (at 550 nm) compared with Ag NWs (in this work) and previously reported Ag NWs prepared via sintering. 192x251mm (300 x 300 DPI)


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Figure 4. (a) Relative transmittance values of Ag NW films with GO and acrylic polymer coatings at 550 nm, (b) Relative transmittance and reflectance values of Ag NW films, (c–g) SEM images of GO-coated Ag NW EMI shielding films with different GO contents: (c) 0%, (d) 0.05%, (e) 0.2%, and (f and g) 0.5%. (h) SEM image of the A/GO (0.5 wt%)/SAWSANW EMI shielding film. □ and ○ represent the relative transmittance and relative reflectance of pristine Ag NWs, respectively, in (b). 103x71mm (300 x 300 DPI)



Figure 5. (a) Reflection loss, (b) absorption loss, (c) total EMI shielding effectiveness (SE) of Ag NW EMI shielding films, and (d) comparison of total EMI SE vs. optical transmittance (at 550 nm) for Ag NW films and other EMI shielding films. 109x81mm (300 x 300 DPI)


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Figure 6. (a and b) Resistance changes (∆R/R0) of the Ag NW EMI shielding films with radius of curvature (ROC) and optical relative transmittance (at 550 nm), (c, d, f, g, i, and j) infrared (IR) thermographs, and (e, h, and k) SEM images of each Ag NW film (90.4% at 550 nm; applied voltage = 1.5 V) before and after bending at ROC = 1.35 mm. (c–e) Pristine Ag NW film, (f–h) SANW film, and (i–k) A/GO/SANW film. 83x45mm (300 x 300 DPI)



Figure 7. (a) Resistance changes (∆R/R0) of the Ag NW EMI shielding films (T = 90.3%; at 550 nm) after taping test with number of cycles (N). (b) Constant temperature and humidity (85 °C/85% RH) test results. (c) Contact angles of Ag NW EMI shielding films: (c1) Pristine Ag NWs, (c2) SANW, (c3) GO/SANW, (c4) RGO/SANW, (c5) A/SANW, and (c6) A/GO/SANW. 89x44mm (300 x 300 DPI)


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Figure 8. Total SE change (∆SET/SET0) of the Ag NW EMI shielding films after constant temperature and humidity (85 °C/85% RH) tests. 106x74mm (300 x 300 DPI)


Highly Bendable and Durable Transparent Electromagnetic

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