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Silver nanowire-based flexible transparent composite film for curvature measurement Wei Xu, Lu Zhong, Feng Xu, Wenfeng Shen, Weijie Song, and Shulei Chou ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00620 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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ACS Applied Nano Materials

Silver Nanowire-based Flexible Transparent Composite Film for Curvature Measurements Wei Xu,† Lu Zhong,† Feng Xu,† Wenfeng Shen,† Weijie Song*,†,‡, and Shulei Chou*,§ †

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences,

Ningbo, 315201, China ‡

Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou,

213164, China §

Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong,

NSW 2522 Australia Keywords: bending sensor, curvature measurement, silver nanowire, percolation, flexible conductive film

Abstract: Transparent, flexible and conductive epoxy resin film embedding with silver nanowires is fabricated through a simple process. The effect of silver nanowire areal mass density to the electrical and optical properties of films is investigated. It is found that resistance of transparent composite film will change regularly during the bending process. The influence mechanism of bending behavior to electrical property is explored. Based on this mechanism, the curvature in the bending process can be acquired immediately by measuring the resistance of the

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composite film. High elastic modulus (800 MPa) of the composite film is benefit for distinguishing the factor of resistance-change caused by bending or stretching with low force. Additionally, high stability of the bending detector is demonstrated with 500 times repeated bending and a six-months durability test.

1. INTRODUCTION The measurement of curvature is universally applicable in both industry and daily life. For example, the micro-processing technology involves measuring small curvature in all kinds of bending workpieces.1 In the field of optical processing, curvature is a key parameter in determining the quality of optical components.2-4 With today’s increasing enthusiasm for flexible electronic devices5-7 and smart wearable devices,8-10 there are still some challenges concerning the real-time monitoring of bending,11 such as bending a flexible display or dynamics of the human elbow. Conventional methods for measuring curvature are mainly divided into two categories, i.e. contact and non-contact. Spherometers and templates are the two most commonly used contact methods,12-14 and these methods may easily cause damage to the surfaces. Two types

of

non-contact

measurement

method,

i.e.

autostigmatic

measurement

and

interferometers,15-18 require expensive stable workstations because a slight vibration during measuring can seriously impact the testing results. These factors make conventional curvature measuring methods impractical for wide applications practically, especially for flexible electronic and smart wearable devices. One of the most important functions of smart wearable devices and medical health monitor is transforming the signals of dynamic processes into electrical ones.19-21 Most commonly used part for curvature measurements in flexible electronic devices is strain sensor, which was fabricated with conductive materials and elastotic substrates. Carbon nanotube (CNT) was one of the


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conductive materials for flexible and transparent capacitive / resistive strain sensors. Cai et al. fabricated CNT-based capacitive strain sensors which can detect strains up to 300% with excellent durability.22 Rahimi et al. made the unidirectional strain senor via selective laser pyrolization of thermoset polymers to CNT.23 Nie et al. embedded multiwalled carbon nanotubes (MWCNTs) meshes in polydimethylsiloxane (PDMS) films to form strain sensor with transparency up to 87% and a gauge factor of 1140 at a small strain of 8.75%.24 Self-powered sensors based on piezoelectrically enhanced nanocomposite micropillar array of polyvinylidene fluoride-trifluoroethylene / barium titanate and MWCNT were also designed in recent years.25 Silver nanowire (AgNW) was another alternative materials for strain sensor. Ho et al. fabricated the microfluidic E-skin sensor through a lamination process involving two AgNW-embedded rubbery microfluidic channels arranged in a crisscross fashion.26 Yao et al. developed highly stretchable multifunctional sensors with screen printed AgNW and Ecoflex elastomer, which can detect strain (up to 50%), pressure (up to ~1.2 MPa) and finger touch with high sensitivity.27 In addition, the combination of flexibility and optical transparency is essential for smart electronics and display devices. That is why it is necessary for the bending sensor to be integrated with the optoelectronic device, which allows direct observation through it. Herein, we provide a new method of measuring the curvature for object bending behavior. A thin polymer composite film embedding with AgNW random networks was fabricated through a convenient process. Composite film resistance changes regularly with the curvature in the bending process. Therefore, we can know the curvature immediately in the bending process by measuring the composite film resistance. Compared with most reported sensors based on AgNW and elastomer, our sensors possess as high elastic modulus as 800 MPa, which is much higher than many elastomers. This effect was benefit for distinguishing the factor of resistance-change


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caused by bending or stretching with low force. Furthermore, such transparent conductive film possesses a high optical transmittance of 81.0% at 550 nm, which can be directly integrated with the flexible screen. In addition, the performance loss due to the bending behavior is much less compared to the stretching behavior. 2. RESULTS AND DISCUSSION 2.1. Preparation and Characterization of AgNWs and AgNW/resin Composite Film Silver nanowires were synthesized using a modified polyol method, which is described in Experiment Section in detail. The morphology of as-synthesized AgNWs was characterized using scanning electron microscopy (SEM), as shown in Figure1a. There are no other morphologies of silver nanocrystals except for nanowires. The XRD pattern of uniform silver nanowires is shown in Figure 1b. The five strongest peaks can be observed at 38.2°, 44.4°, 64.5°, 77.5° and 81.6°, which can be indexed to the (111), (200), (220), (311) and (222) crystalline planes of the face-centered-cubic silver crystal according to the silver JCPDS Card No. 04-0783. The distributions of diameter and length of as-synthesized AgNWs are shown in Figure 1c and d. The silver nanowires present a relatively high length-diameter ratio of exceeding 1100 with an average diameter of 98 nm and average length of 110 µm.


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Figure 1. (a) SEM image of purified silver nanowires. (b) XRD pattern of silver nanowires. (c) histogram of diameter size distribution. (d) histogram of length size distribution. Figure 2 schematically illustrates the fabrication process of AgNW/resin composite film. The areal mass density (AMD) of AgNW has an important effect on the sheet resistance of conductive films due to the contacts between AgNWs. As deposition volume of AgNW dispersion was constant, the areal mass density of the AgNW was controlled simply by adjusting the concentration of silver nanowires dispersion in our work. Figure 3a-h reveals SEM images of AgNW networks with different densities in the range of 23.0 to 185.0 mg m-2, which were calculated using Image J software (developed by Wayne Rasband). More connections formed with increasing AMD of AgNWs. Table 1 tabulates and provides in detail the areal mass density of a series of samples and their corresponding sheet resistance. As the areal mass density increases, the AgNWs network sheet resistance gradually decreases. When the AMD of AgNW are 7.2 mg m-2, 69.2 mg m-2 and 204.5 mg m-2, the corresponding sheet resistance values of networks are 3630.0 Ω sq-1, 31.1 Ω sq-1 and 6.7 Ω sq-1, respectively.

Figure 2. Schematic of the fabrication of AgNW/resin composite film. Table 1. Electrical properties of samples from different areal mass densities

Samples

AMD (mg m-2)

Sheet resistance (Ω sq-1)

1#

7.2

3630.0

2#

11.0

806.5


5


3#

23.0

156.4

4#

41.5

68.2

5#

56.4

42.2

6#

69.2

31.1

7#

87.6

24.1

8#

109.1

16.2

9#

158.8

9.5

10#

185.0

7.6

11#

204.5

6.7

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The optical transmission spectra of samples with different AMD of AgNW are given in Figure 3i. Since there were more AgNWs per unit area in the samples, the AgNW-uncovered space of composite film became narrower, which led to reducing of the transparency of light passing through the composite film. As shown in Figure 3i, when the AMD is 87.6 mg m-2 with sheet resistance of 24.1 Ω sq-1, the transmittance of composite films is 85.0% at 550 nm. As the density of AgNW raised to 185.0 mg m-2 with sheet resistance of 7.6 Ω sq-1, the transmittance at 550 nm of composite films is reduced to 81.0%.


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Figure 3. SEM images (a-h) and transmittance (i) of AgNW/resin film with different AgNW AMD. (j) Sheet resistance of AgNW/resin film with various AgNW AMD. Inset: fit of the experimental data to the percolation equation. 2.2. Percolation property of AgNW Networks Percolation theory has provided innovative insights into understanding the electrical behavior of Ag nanowire networks.26-28 Ideally, the network electrical conductivity σ can be expressed as following equation

∝

(1)

where n is the network density, nc is the network density with the percolation threshold, and m is the percolation exponent. As demonstrated by Monte Carlo simulations, for an infinite 2D


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systems of percolating objects, the percolation exponent m approximately equals to 4/3. According to the investigation made by Li and Zhang:29

= 5.63726 ± 0.00002

(2)

the length of the wire, , is most likely to be a determining factor with regard to the critical density in the 2D stick system. Additionally, the light scattered by the wires and the network transparency seems to be influenced by both the diameter and length of nanowires.30 Thus, it is reasonable to conclude that the influence of nanowire dimensions on AgNW network properties is significant. Clearly, based on Equation 1, it seems appropriate to deduce that the network electrical resistance R should follow this dependence:

∝

(3)

Here, is the electrical resistivity of an individual AgNW. Bid et al. formulated and confirmed experimentally a relation between the resistivity of a nanowire and its diameter.31 In accordance with their finding, the electrical resistivity of an individual AgNW can be expressed as follows:

= +

!" #

%$

(4)


8

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with being the resistivity of bulk silver, & the nanowire diameter and ' the bulk mean free path (( ≈ 0.5). Furthermore, on the basis of Equation 1, 2, 3 and 4, an expression of the network resistance depending on nanowire diameter, length and network density can be written as follows:

= ) *+& *+& = ) , +

!" - .*+& #

%$

67

/.0/12345 #$% 8 9

%$

:

(5)

In Equation 5, ; is the mass density of bulk silver and AMD is the areal mass density of silver in the sample. Though unknown, C is the same order of magnitude for all types of Ag nanowires. Clearly, C can vary slightly and should at least be dependent upon the network geometry that was kept unchanged for all experiments of our research. Furthermore, it depends on the nanowires dimensions distribution, which varies across different nanowire types and different qualities of AgNW crystalline. This model remains valid only when the contact resistance between adjacent nanowires is kept to a minimum. Figure 3j illustrates the curve fitted in accordance with Equation 5, and a logarithmic plot of data fitted linearly well with coefficient of determination of 0.999 in the inset. As evidenced by Figure 3j, the experimental values of the critical areal mass density AMDc is 4.876 mg m-2, which agrees with aforementioned theoretical value (4.326 mg m-2). The conductivity exponent m was 1.358, which approaches the value (1.33) repeatedly reported in the 2D NW network system. 2.3. Electrical Property During Bending Figure 4a demonstrates the changes of resistance of AgNW resin film with different curvature κ. The negative number of κ is used to represent the inward curvature. When |κ| = 0 ~ 0.06 mm-1,


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the resistance changes little for both inward and outward curvature. When curvature |κ| > 0.06 mm-1, the resistance for outward curvature film begin to increase remarkably, while the resistance for inward curvature film begin to decrease slowly and then eventually stabilizes. For instance, while the κ = 0.07 mm-1, the R/R0 is 1.04. As the κ changes from 0.10 mm-1 to 0.13 mm1

, the value of R/R0 increases from 1.60 to 2.15. As evidenced by Figure 4c, the trend of the film

resistance is similar with different areal mass densities of AgNWs. Based on the bending characteristics of AgNW/resin film, a bending sensor can be readily prepared. Using a 25.0 mm × 25.0 mm composite film outward in this work, we can reliably measure a radius of 7.0 mm ~ 14.0 mm. Resistance-time curve is shown in Figure S2. The film was bent in 0.2 s, and was released after holding for 15 s. As shown in Figure S2a, the resistance of the film with initial of 18.0 Ω increased to 20.5 Ω during the first 2 s, which was 98.6% of final steady resistance. But the recovery of the film after the force released was a little bit slow.

Figure 4. (a) Changes of sheet resistance of AgNW/resin film with AMD = 87.6 mg m-2 in concave and convex bending conditions. (b) Changes of resistance of AgNW/resin film with different surface AgNW densities. (c) Resistance of AgNW/resin film versus radius of curvature.


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The predicted mechanism of bending behavior is shown in the inset of Figure 4a. When AgNW/resin film is bent, the contact points between adjacent nanowires changes accordingly. When the film is bent outwards, the contact points embedded in the AgNWs surface are inclined to be separated from each other due to the binding effect of the resin substrate. As the contact area decreases gradually, the contact resistance increases accordingly. The relationship holds true until the contact point becomes separated completely from the AgNWs and thereafter the effective contact point becomes invalid. Meanwhile, contact resistance is enhanced significantly, and there is a remarkable increase in the resistance of the whole film. Conversely, when the AgNW/resin film is bent inwards, the AgNWs surfaces are pressed against each other, forming a closer contact between points. The contact resistance is thus reduced. Furthermore, a small number of adjacent nanowires that exist in the original AgNW network will begin to make contact, leading to a smaller resistance. Because only a few invalid contact points remain in the original AgNW network, the resistance can no longer be reduced when contact points is saturated. As the junction resistance is the main factor of the composite film, the changes of resistance of AgNW/resin film with different AgNW densities, as shown in Figure 4b, are almost same may because the probability of reduction of contact points is similar. 2.4. Repeated Bending Property and Durability Test Multiple bending experiments were conducted with this composite film through a lab-made multiple bending test device, as shown in Figure 5. Clearly, it seems that the reversible contact of the adjacent nanowires not only lead to the extreme level of flexibility, but also contribute to the exceptional durability, as revealed by multiple bending cycles with more than 500 times tests (Figure 5a). Relative to the AgNW/PET film, the AgNW/resin film showed little degradation in electrical properties. The morphology of the composite film is shown in Figure S1. After 1000


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cyclic tests, the morphology of the film is similar with initial film. But after 2000 cyclic tests, some silver nanowires were cracked or bent, which led to the increasing of resistance.

Figure 5. Repeated bending property and durability test. (a) Electrical resistances of AgNW/PET and AgNW/resin films with different bending cycles. (b) Curvature-dependent electrical resistance of AgNW/resin film after different bending times. (c) Durability test of the AgNW/resin film after six months. (d) Radius of curvature versus resistance change R/R0 for different sizes of AgNW/resin-film bending sensors. As shown in Figure 5b, the detection accuracy of curvature was not compromised by multiple bending of the AgNW/resin film. According to the measurement curve, we derive the value of a, b and c in Equation 6:


12

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B

? = @ + A'B + C

(6)

The R-κ curve of either 200 or 500 times bending tests basically coincides with the initial curve. The average error of the radius measurement is 2.6% and 2.8%, respectively. As shown in Figure 5c, the test curves are consistent after six months, with average error of the radius measurement of 2.4%. Due to the impact of ambient air on composite film, the resistance of the film without bending becomes slightly higher. Meanwhile, due to the resin film hardening, the R/R0 becomes higher than that of the initial state of the bending, and the R/R0-κ curve is increasing in a general trend at slightly slower rates. And we also considered the radius of curvature (r) to R/R0 characteristic of different sizes as shown in Figure 5d. According to the experimental data (Figure 5d), we derive the value of a, b and c in Equation 6. Table 2 lists the specific values of these parameters. Table 2. The values of a, b and c for different sizes of AgNW/resin-film bending sensors Samples

a

b

c

25.0 mm × 25.0 7.59 mm

3.03

-0.95

20.0 mm × 20.0 7.54 mm

4.00

-0.91

10.0 mm × 10.0 7.38 mm

4.02

-0.93

2.5. Application in Curvature Measurement and Human-motion Detection The applications of the AgNW/resin film in curvature measurement and bending detection were also demonstrated. As shown in Figure 6a, the external diameter of cylinder with different


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sizes can be readily detected by using the 25.0 mm × 25.0 mm composite film. According to the values given in Table 2, we can get radius of cylinder by substituting R/R0 value into following equation:

B

? = 7.59 3.03'B 0.95

(7).

We measured the external diameters of the 5 ml, 10 ml and 25 ml cylinders at 13.20 mm, 16.08 mm and 23.06 mm, respectively. Compared to the values (13.12 mm, 16.03 mm and 22.43 mm) measured by micrometer caliper, our measurement has a high degree of precision. Additionally, such curvature detector can be used in a flexible device to detect certain bending behaviors. For instance, this kind of sensor can be placed on gloves. As demonstrated in Figure 6b, the bending sensor can be easily assembled on gloves after AgNW/resin films are attached with two copper electrodes. The smart gloves can accurately detect the degree of bending in fingers in real time. In addition, compared to the strain sensor with stretchable polymers such as PDMS, our bending sensor can accurately measure the radius of the object bending while strain sensor cannot correctly distinguish the electrical signal whether caused by stretching behavior or bending behavior of the stretchable device.


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Figure 6. Application in curvature measurement and human-motion detection using the bending sensors. (a) Measurement of radius of cylinders in different sizes. (b) Detection of bending behaviors of finger joints. 3. CONCLUSIONS In summary, we present a novel curvature measurement sensor for the first time by embedding highly transparent AgNW percolation network into the resin film. In this structure, thanks to the nearly unstretchable substrate (as shown in Figure S3) with high elastic modulus of 800 MPa, the change of resistance whether caused by bending or stretching with low force can be distinguished. Repeated bending test and durability test revealed that curvature measurement sensors of AgNW/resin films are quite stable after six months. The optical and electrical properties of AgNW network in different surface densities were investigated elaborately. We also discussed the mechanism of changes in contact resistance of AgNW network during bending. On top of that, AgNW/resin films were used to prepare thin-film curvature sensors with properties such as flexibility and high transmittance. These curvature sensors have potentials to be integrated with other electronic devices for virtual reality, robotics and health care. 4. EXPERIMENTAL SECTION 4.1. Materials. Poly(vinylpyrrolidone) (PVP) (MW = 360,000) was obtained from SigmaAldrich. Ethylene glycol (EG), silver nitrate (AgNO3) (AR ≥ 99.8%), acetone, ethanol and sodium chloride (NaCl) were all purchased from Shanghai Aladdin industrial Co., Ltd. All reagents were used as received without further purification. 4.2. Synthesis of AgNWs. The AgNWs in our study are synthesized using the modified polyol method.32 First of all, 0.5 g PVP (MW = 360,000) was dissolved into 50 mL EG, and heated to 130∘C for 1 h. The 50 mL AgNO3 solution (0.059 M in EG) was added drop wise into the three-


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necked round-bottomed flask equipped with a thermocontroller and a magnetic bar. Then 50 µL NaCl solution (0.1 M in EG) was injected into the flask and stirred for 2 min. The reaction continued for 180 min, and then was cooled to ambient temperature. Centrifugation was carried out to remove impurities and separate AgNWs after adding acetone. Finally, the AgNWs were suspended in ethanol at various concentrations for future use. 4.3. Fabrication of the AgNW/resin Film. Deposition was performed by spin coating a solution of AgNW dispersed in isopropanol on 5 cm × 5 cm Corning glass (C1737-S111) substrates. The samples were deposited from nanowire solutions of various concentrations but with a constant deposition volume of 1 ml. To achieve good surface coverage and minimize waste, we deposited the solution onto the rotating substrate (1500 rpm) in pulses (about 1 drop per second), and in two stages of 0.5 ml each with a 30 second pause between them, allowing the surface to dry. All the samples were annealed at 200 °C for 20 min to reach their minimum resistance, and their sheet resistance was then measured with four points probe Lucas Labo, pro4. Epoxy resin was prepared using Araldite 2020 (Huntsman) by mixing the “base fluid A” and the “curing agent B” with a ratio of 10:3. After air bubbles disappeared, the liquid mixture was dropped onto the AgNW/glass substrate then thermally cured at 40 °C for 6 hours to form cross-linked and solid resin. Then the cured resin matrix together with all AgNWs was peeled off from the substrate, which is shown in Figure 2. 4.4. Characterization of AgNW and Composite Film. All Scanning Electron Microscopy (SEM) images were obtained on an environmental FEI Quanta 250 FEG SEM. The network density of each sample was calculated from SEM images taken at different locations on each deposited sample. The software Image J was used to calculate the areal fractional coverage


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which corresponds to the percentage of substrate surface covered by nanowires from which the areal mass density was deduced. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: *****. SEM images of the composite film; Resistance-time curve of the composite film during bending; Stress-strain and resistance-strain curves AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. ORCID Wei Xu: 0000-0002-4953-4121 Wenfeng Shen: 0000-0003-3370-9801 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank the financial support from the program for Ningbo Municipal Science and Technology Innovative Research Team (2016B10005), National Natural Science Foundation of China (61774160) and Ningbo Natural Science Foundation (2017A610026, 2017A610021).


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TOC A new method for measuring the curvature and a new type sensor for monitoring of object bending behavior is developed based on the AgNW/resin polymer composite film. We preliminary explore the influence mechanism of bending behavior to electrical property. Its potential applications in portable radius measuring instrument and human-motion detection are demonstrated.


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Silver nanowire-based flexible transparent ... - ACS Publications

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