Highly Stretchable and Transparent Electromagnetic Interference

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Highly Stretchable and Transparent Electromagnetic Interference Shielding Film Based on Silver Nanowire Percolation Network for Wearable Electronics Applications Jinwook Jung, Habeom Lee, Inho Ha, Hyunmin Cho, Kyun Kyu Kim, Jinhyeong Kwon, Phillip Won, Sukjoon Hong, and Seung Hwan Ko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14626 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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ACS Applied Materials & Interfaces

Highly Stretchable and Transparent Electromagnetic Interference Shielding Film Based on Silver Nanowire Percolation Network for Wearable Electronics Applications Jinwook Jung,△ Habeom Lee,△ Inho Ha,△ Hyunmin Cho,△ Kyun Kyu Kim,△ Jinhyeong Kwon,△ Phillip Won, △ Sukjoon Hong,‡ Seung Hwan Ko*,△,⊥ △Applied

Nano and Thermal Science Lab, Department of Mechanical Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

‡

Department of Mechanical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do, 15588, Korea

⊥

Institute of Advanced Machinery and Design (SNU-IAMD), Seoul National University, Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

[*]

To whom correspondence should be addressed. Prof. Seung Hwan Ko ([email protected])

Keywords: Electromagnetic interference shielding, transparent, stretchable, silver nanowire percolation network, wearable electronics

ACS Paragon Plus Environment


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Abstract Future electronics are expected to develop into wearable forms and an adequate stretchability is required for the forthcoming wearable electronics considering various motions occurring in human body. Along with stretchability, transparency can increase both the functionality and esthetic features in future wearable electronics. In this paper, we demonstrate, for the first time, a highly stretchable and transparent electromagnetic interference (EMI) shielding layer for wearable electronic applications with silver nanowire percolation network on elastic PDMS substrate. The proposed stretchable and transparent electromagnetic interference shielding layer shows a high electromagnetic wave shielding effectiveness even at a high tensile strain condition. It is expected for the silver nanowire percolation network based electromagnetic interference shielding layer to be beyond the conventional electromagnetic interference shielding materials and to broaden its application range to various fields that require optical transparency or non-planar surface environment such as biological system, human skin, and wearable electronics.


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1. Introduction Recent advances in electronics have brought huge conveniences to humanity, and it has become impossible to separate our lives from various electronic devices such as television, radio, and mobile electronics. Naturally, our daily life is flooded with electromagnetic (EM) waves inevitably emitted by those electronic devices.1-4 However, as the use of these electronic devices happens more frequently, potential effects of EM radiation to human body have emerged as important problems. It is also reported that the long-term exposure of human body to EM waves can increase the risk of various health problems, including cancers.5-7 Also, in terms of packaging of electronic devices, EM waves emitted from the electronic devices may interfere with other delicate devices and cause malfunctions811

.

Typically, metals in various forms (metal sheet/film, metal foam, metal coating) are applied as a conventional electromagnetic interference (EMI) shielding material. Recently, nanomaterials such as conducting polymers, carbon nanotubes and graphene have received much attentions as EM interference (EMI) shielding materials,2, 10, 12-20 in order to meet the newly emerging requirements including flexibility, stretchability, and transparency for the new class of applications in wearable electronics. Carbon based nanomaterials and conducting polymers however fail to satisfy these requirements at the same time. Metals are still the best material for the efficient EMI shielding even though they are not inherently transparent nor stretchable. And most of the carbon based nanomaterials in EMI shielding researches have focused on the traditional aspects such as simply increasing the shielding effectiveness against density of the materials,8, 21-23 without considering the requirements of the future electronics. In recent years, though a few researches about transparent24-25 or stretchable EMI shielding materials26-27 were reported, there are no materials satisfying both stretchability and transparency at the same time, despite the fact that an EMI shielding material with those two functionalities at once is highly favorable in next-generation wearable electronic applications. In this research, for the first time, we demonstrate a highly stretchable and transparent electromagnetic interference shielding (STEMIS) layer for wearable electronic applications with silver nanowire (Ag NW) percolation networks on elastic PDMS substrates. The proposed STEMIS



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layer shows high EMI shielding effectiveness of 31.7 dB even at the 50 % strain condition and it maintains its EMI shielding performance under repeated stretching cycle which is one of the inevitable operating conditions for wearable electronics. The examples of shielding the EM wave emitted from wireless power transfer system further prove its effectiveness in protecting valuable wearable devices such as a pacemaker from external electromagnetic wave hazards28-29 to retain the functionalities.

2. Results and Discussions The schematic image illustrated in Figure 1a shows the proposed STEMIS layer which has high transparency with high electromagnetic shielding properties even under stretching condition. The STEMIS film is composed of a transparent thin film of Ag NW percolation network on the stretchable and transparent PDMS substrate. When the electromagnetic wave is incident, the small part of EM wave is transmitted while the most of the EM wave is reflected or absorbed. In this process, the electrical conductivity of the EM shielding material is known to be the most important factor in determining the EM wave shielding characteristics30-32. Since the STEMIS film is based on a metallic film with high electrical conductivity, it can effectively shield the EM waves accordingly. Furthermore, due to the unique characteristics of metallic nanowire percolation network, the STEMIS film possess both high metallic conductivity and new functionality such as transparency and stretchability. Figure 1b shows the image of the fabricated STEMIS film. The high transparency of the STEMIS layer is due to the nature of the metal percolation network structure33-35 with large voids as shown in the SEM image in Figure 1c. When forming a transparent and conductive layer with Ag NWs, one of the most important things is the aspect ratio of the NWs. Generally, the longer the length, the lower the minimum density required to form a percolation network (percolation threshold), and the higher electrical conductivity can be achieved with smaller amount of NWs.36-37 It is also known that the larger the length of the components of percolation network, the more mechanically stable under mechanical deformation35. In this study, Ag NWs having about 100 µm length are synthesized


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by the modified polyol process and the resultant percolation network based on these NWs ensures high transparency and stretchability. Concentration of Ag NW is one of the key control factors to adjust the transparency and electrical conductivity in percolation network electrodes. As shown in Figure 2a, STEMIS layers having Ag NW area densities of 166, 333, 499 and 666 mg/m2 exhibit optical transmittance of 93.8, 92.4, 85.2 and 76.8 % at 550 nm respectively. The area density of Ag NW also decides the electrical conductivity of the STEMIS layer and as shown in Figure 2b, the measured sheet resistances are 15, 4.4, 3.1 and 2.0 ohm/sq at area densities of 166, 333, 499 and 666 mg/m2, respectively. These dependencies of optical transmittance and electrical conductivity on Ag NW density are similar to other reports on Ag NW percolation network based transparent and stretchable conductors. The electro-mechanical stability of STEMIS layer under various strain condition is analyzed by repeated strain test and the experimental setup is shown in Figure 3a. In order to secure the firm adhesion between PDMS substrate and two glass holders, plasma treatment is applied on the glass to activate -OH group. After the STEMIS film is fixed between two glass holders, the copper tapes are connected to both side which are located on the glass holders, and thus only the freestanding region of STEMIS film is selectively stretched during the stretching test. The resistance change under various strain derived by motorized stage is measured up to 50% strain for 5 cycles. The Ag NW percolation network films maintain their electrical conductivity with small resistance change even at large strain condition. It is because, although a few parts of the Ag NW network are disconnected and isolated, the most parts of the Ag NWs network keep the connection and rearrange to preserve the percolation network.38-39 More specifically in all Ag NW density conditions, after more than one stretching cycles, the resistance at the unstrained condition is higher than the respective initial resistance. This phenomenon can be derived from the partial disconnection of Ag NWs from percolation network during the first stretching procedure. However, after a few cycles the electrical conductance is fully recovered during a stretching/releasing cycle and the resistance at unstrained condition remains unchanged in consecutive cycles, which is also analogous to previous reports38. It is also noted that



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the degree of the resistance change is small as the concentration of the Ag NW gets high since it has more alternative conducting path when stretched and less number of isolated percolation network is generated. The long term stability under repeated cyclic stretching is evaluated with 1,000 times cyclic stretching test and the result is presented in Figure 3c. The sheet resistance of the electrode slightly increases as the stretching cycle increases, but eventually converges to a constant value. The experimental setup for characterization of the EMI shielding effectiveness of fabricated STEMIS film is presented in Figure 4a. The two terminals of the N5230A network analyzer are connected to the X band waveguide through the APC-7 cable. The EMI shielding effectiveness is defined using the ratio of the incident power (Pi) to the transmission power (Pt) as shown in the following equation 18, 40.

= 10 log = 10 |

|

(1)

, where represents the power transmitted from port i to port j. Generally, it is known that the EMI shielding effectiveness required for the practical applications such as mobile phones and laptop computers is about 20 dB9, 26-27, 41. The EMI shielding effectiveness of STEMIS film at 10 GHz is about 20 dB even at the lowest Ag NW density of 166 mg/m2 and as the area density of the Ag NW increases, the EMI shielding effectiveness also increases as represented in Figure 4b. From the equation 1, EMI shielding effectiveness at 20 dB and 30 dB can be interpreted as 99 % and 99.9 % of the incident EM wave is blocked by the film, respectively. In the same context, the Figure 4c explains that the EM shielding efficiency of the STEMIS film is larger than 99% at all NW area density conditions. In order to evaluate the role of Ag NW and PDMS substrate on the EMI shielding effectiveness, EMI shielding effectiveness of Ag NW on various substrates is plotted in Figure S1, and from the results it is confirmed that the electrical conductivity of the Ag NW layer has larger influence on the EMI shielding effectiveness rather than the substrates. Thus, the thin transparent metallic layer of Ag NW percolation network is the core material for effective EMI shielding. Theoretically, the Ag NW


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percolation network shows a good-conductor approximation (σ/ωε0 >>0) behavior and the EMI shielding effectiveness can be successfully approximated as a following equation15, 42-43 (2)

%

-

-

%

-

-

-

SE = 10log "# $&' )*+ℎ . − *+ . 0 + 23&' )+45ℎ . + +45 . 0 + 2 )*+ℎ . + (

(

-

*+ . 067

(2)

, where σ is conductivity, ω is the angular frequency, ε0 is permittivity of vacuum, µ (=µ0µr) is permeability, d is thickness, and δ (=82/:;

SE = 20log 1 + ?( = −20 logABC D + 20 log BC + @

>(

(3)

, where Z0 (=376.7 Ω) is the wave impedance of free space. This equation well fits with EMI shielding effectiveness of STEMIS film as plotted in Figure 4d. Generally, the total shielding effectiveness is the sum of EMI shielding effectiveness by three mechanisms; reflection (R), absorption (A) and multiple reflection while multiple reflection can be ignored since the active layer is much thinner than skin depth. Thus the total shielding effectiveness is sum of shielding effectiveness by reflection (SER) and by absorption (SEA) and the SER and SEA can be calculated from the following equations.8, 40

Total = H + I = 10 log

H = 10 JH = 10log J|

|

(4)


(5)


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JH

I = 10

J| | | |

= 10log

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(6)

In order to evaluate the dominant shielding mechanism of STEMIS film, SER and SEA at 10 GHz for various Ag NW density conditions are plotted in the Figure 4e. It should be noted that SER tends to saturate at some level even if the Ag NW density increases further. This tendency is similar to the results of previous reports on the effect of concentration or the thickness of EMI shielding materials to the final EMI shielding effectiveness.26, 44-45. In addition, since the SEA is larger than the SER at all Ag NW densities, it can be inferred that the dominant EM shielding mechanism is absorption rather than reflection, which is in accord to the reports for other EM shielding materials.8, 46 However, according to the power balance of transmission, reflection, and absorption versus incident power which is shown in Figure 4f, the actual amount of power shielded by reflection is larger than that due to absorption. For example, when an EM wave is incident to an EMI shielding material having 10 dB of SER and 20 dB of SEA, firstly, the 90 % of power is reflected (10 dB of SER) and then the 99% of the non-reflected power (20 dB of SEA, 9.9 % of incident power) is absorbed by the EMI shielding material. And the EMI shielding material can be said to have a total shielding effectiveness of 30 dB (99.9%). Relatively large portion of absorption in the low Ag NW density film (166 mg/m2) can be explained in the similar way. Since the amount of initial reflection is small for such low density film, the absorbable power increases accordingly, leading to higher power balance of absorption. Up to this point, we have studied the EMI shielding effectiveness of the STEMIS film without considering the mechanical strain. However, maintaining EMI shielding effectiveness under mechanical strain condition is very important for a stretchable EMI shielding materials to effectively protect the next generation wearable electronics from EMI. Therefore, in order to measure EMI shielding effectiveness with various strain condition, the X band waveguide is installed between the moving stage and a STEMIS film is placed on the waveguide as shown in Figure 5a. EMI shielding effectiveness of the STEMIS film is measured under various strain condition (up to 50%, Figure 5b) with different Ag NW area densities. When the Ag NW density of the STEMIS is higher than 333 mg/m2, the EMI shielding effectiveness is maintained at 20 dB or larger even at a large strain of 50%.


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In order to evaluate the relationship between EMI shielding effectiveness and sheet resistance under various stretching state, EMI shielding effectiveness with various strain is plotted with sheet resistance in Figure 5c. For this, sheet resistance is calculated as described in supporting information Figure S2. EMI shielding effectiveness decreases at all Ag NW density conditions with increase of sheet resistance, which is induced by stretching. However, it can be seen from the slope variations that the degree of degradation of EMI shielding effectiveness due to the increase of sheet resistance is slowed down with the decrease of Ag NW density. Considering the upshift of EMI shielding effectiveness appears at low Ag NW density, this phenomenon can be derived from the generation of isolated networks as illustrated in Figure 5d. Although the isolated network has negligible influence to the measured resistance changes as it is disconnected with the conducting path, it can effectively shield the incoming EM wave since they are still highly conductive metal network. As a proof of concept for the proposed STEMI shielding material in various applications, a demonstration of successful EMI shielding for a wireless power transmission system is shown in Figure 6a. The function generator applies a square function voltage of 2.95 MHz to the wireless power transmission coil where the receiver coil receives the energy and turn on LED (Figure 6b). When the STEMIS film is inserted between two wireless power transmission coils (Figure 6c), the EM energy transmission is efficiently blocked by STEMIS film and the LED is turned off. This simple demonstration shows the potential application of STEMIS film for efficient EMI shielding in wearable electronics components.

3. Conclusion In summary, we present a highly stretchable, transparent and efficient EMI shielding materials for the first time by integrating highly transparent and stretchable Ag NW percolation network onto the PDMS substrates. In this structure, thanks to high conductivity and mechanical properties of Ag NW percolation network, high EMI shielding effectiveness even at high strain can be obtained with a moderate transparency. We investigated EMI shielding mechanism of the STEMIS film and from the results it is confirmed that the EMI shielding is dominated by the highly conductive thin Ag NW



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layers. Also, despite that the measured SEA is larger than the SER, the actual amount of EM wave shielded by reflection is larger than that by absorption. Furthermore, by measuring in-situ EMI shielding effectiveness with various strain condition, we proved that STEMIS film has high EMI shielding properties under large strain thus can be used to protect wearable electronics which have to endure high strain condition. This highly stretchable and transparent metal nanowire based EMI shielding film is expected to overcome the limitation of the conventional EMI shielding material by providing extra freedom in stretchability and transparency to expand the application area further in soft or non-planar surfaces such as biological system and human skin.

4. Experimental Long Ag NW synthesis: Long Ag nanowires (100 µm in length and 100 nm in diameter) were synthesized by modified polyol process using one-pot process, where all reagents are mixed into a triangular flask at once. In this method, 50 mL of ethylene glycol (EG), 0.4 g of polyvinylpyrrolidone (Mw≈ 360,000) and 0.5 g of silver nitrate (AgNO3) were sequentially dissolved using a magnetic stirrer. The stirrer was carefully removed from the mixture solution once all chemicals were thoroughly dissolved. Then, 800 µl of as-prepared CuCl2·2H2O (3.3 mM) solution in EG was rapidly injected into the mixture and stirred mildly. Lastly, the mixture solution was immersed in a preheated silicone oil bath at 130 °C. The growth of Ag nanowires in the mixture was proceeded at the elevated temperature for 3 h. When the growth was finished, the resultant solution was cleaned using acetone and ethanol to remove the chemical residues along with centrifugation of 3000 rpm for 10 min. This cleaning process was securely repeated three to four times. The resultant Ag nanowires were redispersed in ethanol (EtOH) for the use. Ag NW transfer on PDMS substrate by vacuum filtration: Ag NW percolation network layer was transferred on the PDMS substrate by vacuum filtration & transfer method.45, 47-48 First of all, a PDMS substrate was prepared by mixing PDMS solution and curing agent (Sylgard 184, Dow corning) in a ratio of 10:1, followed by curing at 60 °C for 3 hours. For the filtration of Ag NW solution, an all


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glass filtration system with 47mm diameter membranes is used. Nylon filter and PTFE filter were sequentially placed on the sintered glass disc placed on a glass flask. From this point, a vacuum pump connected to the flask is kept operating and a glass funnel was tightly assembled on the layered filter with a metal clip. Subsequently, a sufficient amount of ethanol (100 to 150 ml) was poured into the glass funnel and before the ethanol solution passes through the filters, Ag NW solution was added to the ethanol. Because the filtration rate of the solution was sufficiently slow, this filtration process took a few minutes. After the filtration process was complete, the clip and funnel are removed from the glass filtration kit and a PDMS substrate treated with plasma treatment was placed on the PTFE filter where the Ag NW network was formed. After a few minutes, the PTFE filter was removed from the PDMS substrate and the Ag NW network remained on the PDMS substrate. The resultant Ag NW network / PDMS composite was directly used in this study without any additional post treatment such as thermal annealing and laser sintering processes. Characterizations : Optical transmittance of STEMIS film at 400~800 nm wavelength range was measured by Jasco V-770 UV-Vis-NIR spectroscopy using same thickness of PDMS as reference film. Resistance change during stretching condition was measured by anchoring STEMIS film onto the glass pieces. For this process, the cleaned slide glasses were treated by plasma for activate –OH group. Then, slide glasses are fixed at a distance of 1 cm. The STEMIS films are attached on the glasses for about 1 hour and made electrical connection with copper tape and silver paste. The slide glasses were mounted on the motorized stage and the resistance was measured upto 50% strain, and the measurement was performed for a total of 5 cycles. Electromagentic interference shielding properties were measured by N5230A network analyzer which connected to two X band waveguide and calibrated with Transmission-Reflection-Load (TRL) technique at both ends of the waveguide. The measured frequency range was 8.2~12.4 GHz and intermediate frequency (IF) bandwidth of 1 kHz was set to increase the accuracy of the measurements. Specimens are cut into slightly larger in dimension as compared to window of waveguide (22.86 × 10.16 mm2) and mounted between two waveguide followed by aligned and tightly fixed to avoid any



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leakage paths. In order to measure the electromagnetic shielding characteristics in the stretched state, a moving stage was constructed and one waveguide was fixed on the floor. The sample (70 × 25 mm2) was placed on it and strain was applied. The opposite waveguide was covered and the alignment was adjusted using the alignment pin.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant (2017R1A2B3005706, NRF-2016R1A5A1938472).

Supporting Information: -

EMI shielding effectiveness of Ag NW on various substrates; the sheet resistance calculation model for strain condition STEMI film (PDF)

-

A video clip of effectively shielding wireless power transfer (MOV)


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Highly Stretchable and Transparent Electromagnetic Interference

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