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A Sandwiched/Cracked Flexible Film for MultiThermal Monitoring and Switching Devices Yanlong Tai, Tao Chen, and Gilles Lubineau ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05467 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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ACS Applied Materials & Interfaces

A Sandwiched/Cracked Flexible Film for MultiThermal Monitoring and Switching Devices Yanlong Tai1,2, Tao Chen2*, Gilles Lubineau1*

1

King Abdullah University of Science and Technology (KAUST), Physical Science and

Engineering Division (PSE), COHMAS Laboratory, Thuwal 23955-6900, Saudi Arabia 2

Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 1219

Zhongguan West Road, Ningbo 315201, China

KEYWORDS Electronic skin, temperature sensing, thermal switches, tunable cracked microstructures, flexible monitoring or switching devices, piezoresistive behavior.

ACS Paragon Plus Environment

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ABSTRACT

Polydimethylsiloxane (PDMS)-based flexible films have substantiated advantages in various sensing applications. Here, we demonstrate the highly-sensitive and programmable thermalsensing capability (thermal index, B, up to 126 × 103 K) of flexible films with tunable sandwiched microstructures (PDMS/cracked single-walled carbon nanotube (SWCNT) film/PDMS) when a thermal stimulus is applied. We found that this excellent performance results from the following features of the film’s structural and material design: (1) the sandwiched structure allows the film to switch from a three-dimensional to a twodimensional in-plane deformation (2) the stiffness of the SWCNT film is decreased by introducing micro cracks that make deformation easy and that promote the macroscopic piezoresistive behavior of SWCNT crack islands and the microscopic piezoresistive behavior of SWCNT bundles. The PDMS layer is characterized by a high coefficient of thermal expansion (α = 310 × 10-6 K-1) and low stiffness (~ 2 MPa) that allow for greater flexibility and higher temperature sensitivity. We determined the efficacy of our sandwiched, cracked, flexible films in monitoring and switching flexible devices when subjected to various stimuli, including thermal conduction, thermal radiation, and light radiation.

1. Introduction

Flexible bilayer films using carbon nanoparticles (e.g., carbon nanotubes, graphene platelets) deposited on a polydimethylsiloxane (PDMS) substrate have demonstrated usefulness in various applications1, 2 for sensing pressure,3 light,4 temperature,5 humidity,6 and gas.7 This is mainly because these bilayer structures take full advantage of all material properties: the high thermal sensitivity and the low stiffness of the PDMS,8, 9 as well as the strong expansion


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(humidity) or contraction (thermal) response to environmental stimuli and the excellent piezoresistive performance of the carbon-nanomaterial layer.10, 11

The sensing behavior of such layered films is the combined result of the shape-changing behavior (expansion or contraction) of each layer.12, 13 As a mechanical-pressure or strain sensor, the carbon-nanomaterial layer has a much higher Young’s modulus than that of the PDMS layer.14 Therefore, an ultrathin or cracked carbon film can be applied to improve the film’s sensitivity with the aims to decrease its stiffness and to enhance its piezoresistive effect.15, 16 Bilayer films exhibit greater sensing capability, even to the activities of insects (ants, bees, snails, etc.) easily,17-19 compared with silicon-based films.20 In sensing humidity, the hygroscopic expansion of the carbon layer plays a crucial role in the final sensitivity, despite its deformation is restricted by the PDMS layer. Therefore, an ultrathin carbon film modified with a strong moisture-absorption polymer material can be used.21 This improves the gap distance between the carbon nanoparticles when stimulated by humidity and results in a signal increase of several orders of magnitude. However, when used as thermal, light, or electrical sensors, the bilayer films will generate three-dimensional (3D) deformation because of the opposite CTE of each layer (negative CTE in the carbon layer and positive CTE in PDMS layer).4, 22, 23 The bilayer film is then prone to bending, which reduces its performance. This may explain why little research has been reported on high-performing thermal-sensing materials besides that on thermal actuators or stretchable thermal resistors.24, 25

Inspired by possibilities of layered materials, we demonstrate here how to take advantage of sandwiched flexible films with modulated cracked microstructures to create highly-sensitive, programmable, and flexible thermal-sensing device.


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First, a sandwiched, flexible film with cracked microstructures (SCFFs) is designed and fabricated to investigate the sensing mechanism and to demonstrate the excellent capabilities in thermal sensing. The excellent performance of our films can be explained by the design of the sandwiched structure, the macroscopic piezoresistive behavior of cracked SWCNT islands (CSIs), and the microscopic piezoresistive behavior of SWCNT bundles. All these mechanisms work together to enhance the macroscopic properties of the SCFFs (Figure 1a).

Second, because of the microstructure-dependent thermal-sensing performance of SCFFs, we developed a series of SCFFs with tailored microstructures (diameters of the CSIs) based on the classic mechanical-buckling theory. We evaluated their programmable sensitivities to thermal stimulus via the thermal index (B). We then compared the performance of our films to available thermal-sensing materials to show the great value of SCFFs.

Third, to further verify the thermal-sensing capabilities of SCFFs, we tested a series of practical applications using flexible devices for monitoring and switching, including a wearable temperature patch (monitor, thermal conduction); a heat-regulated circuit (100 °C, switch and monitor, thermal radiation); and a light-regulated circuit (100W, switch and monitor, light radiation).


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Figure

1.

Highly-sensitive,

thermal-sensing

flexible

films:

design,

preparation,

phenomenology, and performance. a) The film design. b) A typical SCFF sample (Scale bar is 1 cm). c) AFM image of the cracked microstructure. d) A SEM image of the cross section (Scale bar is 20 µm). e) Electrical impedance spectroscopies of SCFF at different temperatures from 1 kHz to 2 MHz; the inset is an equivalent-circuit model produced through an impedance-curve-fitting program based on MATLAB software. f) Deformation of SCFFs induced by a horizontal stress load with different magnifications to demonstrate the


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piezoresistive behaviors of CSIs and SWCNT bundles; the dotted red lines show the conductive networks of the SWCNTs on the PDMS substrate; the scale bars are 50 µm, 20 µm, 1 µm. g) Summary of the thermal-sensing performance of nanomaterial/PDMS flexible films to reveal the influence of the microstructure.

2. Results and Discussion 2.1. The Thermal-Sensing Mechanism of SCFFs The sensing behavior of carbon nanomaterial/PDMS films comes from the combined result of the shape-changing behavior (expansion or contraction) of each layer. Under thermal stimuli, carbon nanomaterial/PDMS films generate 3D deformation because of the opposite CTE of each layer (negative CTE in carbon and positive CTE in PDMS). Specifically, as the temperature increases, the carbon layer contracts while the PDMS layer expands. Thus, this film bends to the side of the carbon layer. This is the typical mechanism of thermal actuators.26-28 Only the deformation of the carbon layer in the horizontal direction participates in the sensing behavior.

To attain high thermal sensitivity, we adopted a sandwiched film (PDMS/SWCNT/PDMS) design to guarantee that the deformation of the films remained in the plane. A discontinuous carbon film with a cracked microstructure was used such that it could freely follow the shapechanging behavior of PDMS. The dimensional change of the sandwiched film induced by thermal variation is described in Equation 1:29

ε i ≈ ∆T α i

(1) ，

where εi is the expansion (top and bottom of the PDMS layer) due to temperature variation, αi is the CTE for the PDMS, and △T is the change in temperature. We note that the contribution of the cracked carbon layer can be neglected due to its cracked microstructure. More details


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can be seen Figure 1a and Figure S1. Figure 1b shows a typical SCFF sample. Respective AFM and SEM images describe the morphology in detail, as seen in Figure 1c and 1d.

Figure 1e demonstrates the change in impedance of the SCFFs at different temperatures via electrical impedance spectroscopy (EIS). It can be found that there is an increasing value for both the real/imaginary parts of the impedance from 104-105 ohms (25 °C) to above 109 ohms (65 °C), indicating the efficacy of our strategy for sensing increases in temperature. As expected, this temperature-sensing capability is closely related to changes in the cracked conductive microstructure with increased temperature, i.e., the cracks between the fragments open and the fragments stretch as the temperature increases.

To clarify the nature of this piezoresistivity, the variation in microstructure of the SCFFs when strained were characterized, as seen in Figure 1f. The crack openings between the fragments and the stretching of fragment themselves can be seen clearly. These were defined as macroscopic and microscopic piezoresistive behavior of the CSIs, respectively. Both deformations trigger a change in resistance of SWCNT-bundle network. In fact, the macroscopic variation in resistance is the combined results, influenced by a cumulative effect of above two mechanisms. Meanwhile, an equivalent-circuit model was produced through an impedance-curve-fitting program based on MATLAB software (inset of Figure 1e). We assumed a circuit made of a series resistance (r, regarded as a microscopic piezoresistive effect) and a parallel resistance and capacitor (Rp, Cp, regarded as a macroscopic piezoresistive effect).30

To further evaluate the contributions these two piezoresistive behaviors make, we tested the responses of silver nanoparticle (Ag NPs)/PDMS film, cracked SWCNT/PDMS film,


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uncracked SWCNT/PDMS film, and patterned SWCNT/PDMS film to temperature changes, as shown Figure 1g. It can be seen that only minimal increases in temperature resulted in an exponential increase in resistance for Ag-nanoparticle film, which results from the piezoresistive behavior of inter-Ag-particle contacts (the diameter of each particle is around 2-10 nm). As for the uncracked SWCNT/PDMS films, its resistance stably increased following an increased temperature. This increase in resistance probably resulted from the total microscopic piezoresistivity of SWCNT bundles. These results indicate that the role of macroscopic piezoresistive behaviors related to cracks was more crucial than that of microscopic piezoresistive behaviors in resistance responses. As for the cracked SWCNT/PDMS film, it showed a high sensitivity over a wide range of temperatures, which was benefited from both piezoresistive behaviors mentioned above. The resistance of the patterned SWCNT/PDMS film decreased with increasing temperature because the influence from the piezoresistive effect discussed above was so low that the thermal resistance effect of the semiconductor material (SWCNT) was more significant.31


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Figure 2. Preparation and characterization of wrinkled flexible films (WFFs) and cracked flexible films (CFFs) with different homogeneous microstructures. a) Illustration of the preparation process and mechanism, and a finite-element method modeling WFFs with different drying temperatures to show the amplitude and distribution of stresses in SWCNT/PDMS films. The stress value on the scale bar was calculated using the values in Table S1. b) SEM images of WFFs produced with different dosages of SWCNT ink (WFF-1: 2 µl/cm2; WFF-2: 4 µl/cm2; WFF-3: 6 µl/cm2); all scale bars are 50 µm. c) SEM images of CFFs fabricated from the corresponding WFFs in (b); all scale bars are 50 µm. d) The relationship between the dosage of SWCNT ink, the diameter of the CSIs, and the sheet resistance of WFFs and CFFs. The PDMS layer was 200 ± 10 µm thick.


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2.2. Preparation of SCFFs with Tailored Microstructures From the description above, we know that SCFF has a strong thermal-sensing capability, which is not only a result of its sandwich structure, but also strongly related to the micromorphology of the SWCNT layer. We aimed to regulate this cracked microstructure to control the thermal-sensing performance by design.

According to previous reports, SWCNT/PDMS films can easily buckle when subjected to the equibiaxial compressive stresses that result from the difference in CTE between the two layers (PDMS: 310 × 10-6 K-1 and SWCNT: -1.5 × 10-6 K-1).32, 33 The amplitude of generated stress can be evaluated via a proportional relationship of the wavelength of mechanical buckling as indicated in Equation 2:34

λ = 2Π h (

Ef 3 Es

)1/3

(2)

E f is the Young’s modulus of nanomaterial coating (SWCNT, thickness: h). Es is the

Young’s modulus of the substrate (PDMS). The wavelength (λ) can be tuned by adapting the thickness of the SWCNT layer and of course, determines the subsequent cracking pattern. We, therefore, sought to control the morphology of the cracked pattern by regulating the thickness of the SWCNT layer through the preparation process shown in Figure 2a.


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Figure 2b shows the strong relationship between the post-buckling morphology and the thickness of the SWCNT film. In particular, when the dosage of SWCNT ink on the PDMS film is 2 µl/cm2, 4 µl/cm2, and 6 µl/cm2, the wavelength increases from 0.8 µm, to 5.6 µm, to 16.7 µm, respectively. These results are consistent with Equation 2.

After simply stretching the SWCNT/PDMS film with our hands along the X-/Y- direction, the SWCNT film started to crack, generating a homogenous microstructure. Results in Figure 2c shows that the diameter of the crack fragment (CSIs) increased from 2.8 µm, to 14.6 µm, to 22.3 µm, proportional to the original mechanical-buckling wavelength increase from 0.8 µm, to 5.6 µm, to 16.7 µm, respectively, as shown in Figure 2d. The variation in sheet resistance of the films were also summarized in Figure 2d, which changes from 51.2 kohm/sq, to 22.3 kohm/sq, to 6.8 kohm/sq as the cracks are introduced, compared with the original value of 0.82 kohm/sq, 0.3 kohm/sq, 0.02 kohm/sq, respectively. Note that the higher sheet resistance will lead to a higher contact resistance as well as a higher macroscopic resistance.


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Figure 3. Characterization of the thermal-sensing performance of SCFFs. a) Illustration of the test method. b) The relationship between resistance of SCFF (1 cm × 2 cm) and temperature; the inset is the X-log(Y) curves. c) Relationship between ln(R) and 1000/T; the inset presents the thermal index (B), which was generated through the thermal-index value of SCFFs. d) A summary of the recent literature with the key parameters for thermal-sensing materials. e) The relationship between the breaking temperature and the number of compressive cycles for SCFF-2; the inset demonstrated a durability test of a 4 cm × 0.8 cm SCFF-2 (a compressed load: 0 N -0.08 N; a frequency: 1 Hz). A polyethylene terephthalate (PET) film (thickness: 125 µm) was used as the supporting layer because of the low modulus of SCFFs. f) Resistance-response curves of SCFF-2 to different temperatures (30 °C, 50 °C, 70 °C) to show its rapid response time. g) Real-time thermal monitoring and switching performance of SCFF-2 with an enlarged plot in the green area. The green arrows with


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gradient colors present the corresponding relationships between resistance and temperature. h) A wearable temperature patch for the monitoring of human body temperature. All data is the average value of three samples.

2.3. Thermal-Sensing Performance of SCFFs Accordingly, we further confirmed the predictions about the relationship between the cracked microstructure and the thermal-sensing performance by monitoring variations in both resistance and temperature, as shown in Figure 3a.

Figure 3b shows that, with an increasing temperature, the resistance of SCFFs changes by multiple orders of magnitude. The breaking temperature was 49.1 °C for SCFF-1, 62.2 °C for SCFF-2, and 78.9 °C for SCFF-3, respectively. In practical applications, the working temperature was defined as 90% of their breakage temperature, 44.1 °C, 55.98 °C, and 71.01 °C, respectively, after calculation.

Accordingly, thermal index (B) defined by Equation 3 was used to further characterize the great thermal sensitivity of SCFFs:35

ln( R ) = ln( R0 ) +

B T

(3)

where R0 is the resistance at T = ∞, R is the resistance at temperature (T).

From Figure 3c, a linear relationship between ln(R) and 1/T can be observed for all SCFFs. Note the fitted slopes is B, and the results are presented in the inset: 37.4 × 103 K for SCFF-1, 23.8 × 103 K for SCFF-2, and 16.8 × 103 K for SCFF-3, respectively. These values can be as


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high as 112~126 × 103 K in the original temperature variation range of 24.3 °C to 28.6 °C. The tunable performance with SCFF-2 is much better than that of stretchable graphene/PDMS films (0.9 ~ 2.0 ×103 K, around 11.9 ~ 26.4 times higher with peaks up to 60.5 ~ 134 times higher)25, 35. This performance is also better than the 2-5 ×103 K observed for typical transition metal oxides (around 4.8 ~ 11.8 times higher with peaks up to 24.2 ~ 60.5 times higher).36, 37 More details are presented in Figure 3d and Table S2.

Laminated films get usually damage during service life by delamination that corresponds to a progressive separation between the functional layers. This is especially critical for PDMS based films due to its high surface energy.38 We therefore performed a compressive durability test. We used a polyethylene terephthalate (PET) film (thickness = 125 µm) as a supporting layer due to the low modulus of SCFFs. Results in Figure 3e demonstrate that SCFF (4 cm × 0.8 cm) present a no-noticeable variation in resistance amplitude after 10,000 cycles, indicating that the film has adequate mechanical durability with a negligible drift, as well as a stable breaking temperature.

The typical thermal-response tests of SCFF-2 was presented in Figure 3f through three different temperatures (30 °C, 50 °C, 70 °C, respectively). The breaking temperature is 62.2 °C and room temperature is 24.3 °C. The results intuitively show that SCFF-2 is efficient as a thermal meter not only within the range of its working temperature, but also beyond the breaking temperature as a thermal switch. In addition, we also observed the following rapid response times: 0.28 s for 70 °C, 0.44 s for 50 °C, and 0.77 s for 30 °C.

Figure 3g shows further evidence of the temperature monitoring and switching capabilities of SCFF. Our results show that a SCFF has measured the changes of temperature within 90% of


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its breaking temperature accurately. Additionally, in the range of 25-36 °C, SCFF-2 can sense changes in temperature as small as 0.12 °C and can be used as thermometer. We developed a wearable temperature patch and confirmed its excellent performance (Figure 3h).

Accordingly, we also investigated the thermal-response behaviors of SCFFs compared with other configurations, including light and thermal radiation, using the testing method shown in Figure 4a and 4b. Results in Figure 4c show the performance of SCFF-2 when subjected to light radiation. We found that the LED works on and off. It responded by turning a 100 W light on and off at the distance between the light and SCFF sample (0.8 cm × 4 cm) of 10 cm. The role played by SCFF in the circuit was like a light switch. On the other hand, when this distance increased from 10 cm to 20 cm, 30 cm, 40 cm, and 50 cm, the intensity of the light reduced. The role played by SCFF in the circuit was then like a light monitor. Figure 4d presents a similar phenomenon of SCFF-2 responding to a thermal rod, which can work as a thermal switch or thermal monitor. However, we found that the working distance of light radiation is much longer than that of thermal radiation. More details are presented in Figure S2.


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Figure 4. Applications of SCFFs as flexible monitoring and switching devices. a) The configuration and b) illustration of the test method. c) The light monitor (green arrow) with different distances between the light (100 W) and SCFF-2, from 10 cm to 50 cm, and the light switch (red arrow: light on or off) with the distance of 10 cm. c) The thermal monitor (green arrow) with different distances between the thermal rod (100 °C) and the SCFF, from 0.3 cm to 2 cm. d) The light switch (red arrow: light on or off) with the critical distance at around 0.3 cm. All data values are the average of three measurements.

3. Conclusion In conclusion, we fabricated and characterized flexible films with a tunable cracked microstructure of sandwiched PDMS/SWCNT/PDMS. These films demonstrated highlysensitive and programmable thermal-sensing capabilities when subjected to a thermal stimulus (thermal index, B, up to 126 × 103 K). This excellent result was confirmed to be


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closely related to the sandwiched structure and physical performance of the materials we used as film layers. These features work together to enhance the macroscopic sensing properties of SCFFs. The efficiency of the developed SCFFs was also successfully confirmed in flexible devices for switching and monitoring different sensing applications, including thermal conduction (a wearable temperature patch); light radiation (a lightbulb, 100 W); and thermal radiation (a thermal rod, 100 °C). We also will do the further research to explore various practical applications of SCFFs as flexible thermal-monitoring and switching devices, especially for electrical skin.

4. Experimental Section

Materials: We purchased SWCNTs from Cheap Tubes, Inc. with a length of 5-30 µm, an outer diameter of 1-2 nm, 2.56 wt. % COOH groups and over 95 wt. % purity. We prepared the ink (1 mg/mL) in the laboratory according to our previous methods,39 and an image of a typical SWCNT ink can be seen in Figure S3. We purchased PDMS (Sylgard® 184) from Dow Corning Inc. We purchased 125-µm PET films from Teonex® Inc. We used Deionized (DI) water in all experimental processes.

Preparation of SCFFs: The prepared SWCNT ink above was dropped onto a plasmapretreated PDMS film (thickness = 200 µm, the base to curing agent = 10:1, cured at 70 °C for 6 h,) through a Thermo Scientific Finnpipette (0.2 - 2 µl) to control a concentration around 2 µl/cm2. Then, we dried these films in an oven at 100 °C for 1 h, accordingly,

cooling to room temperature. Another mixture of PDMS with the same weight as the previous that of PDMS to keep the same thickness of the bottom and upper PDMS layer was dropped onto the prepared SWCNT/PDMS film (SWCNT side) homogeneously. After removing the air bubbles in a vacuum environment, we dried the sandwiched film (PDMS/SWCNT/PDMS) in an oven at 100 °C around 1 h. Then,


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we simply stretched this film along the X and Y directions with our hands 20 times with an elongation around 20 % each time, controlled via a ruler. The SWCNT layer started to crack and generated a homogeneous microstructure, which can be seen in Figure 2c. The thickness of the SWCNT layer is a very significant factor to the internal stress of the sandwiched structure, as does the morphology of both the mechanical buckling and the cracked microstructure of the SWCNT layer. Therefore, we controlled the concentration of the SWCNT ink on the PDMS layer with the Thermo Scientific Finnpipette mentioned above to adjust the thickness of the SWCNT layer after drying. Typical flexible films with tunable cracked microstructures of sandwiched PDMS/SWCNT/PDMS can be seen in Figure 1b. The silver nanoparticle/PDMS film was produced via the same process, which was then dried at room temperature for 3 h. The “S” configuration of the SWCNT on PDMS film (thickness = 200 µm) was fabricated through a template and then was dried for 3 h at room temperature.

Characterization and Measurements: We examined the microstructures of the prepared WFFs and CFFs through an atomic force microscope (AFM, Veeco, Dimension 3100) and a scanning electron microscope (SEM, Quanta 600, FEI Company).

We performed the cyclic mechanical tests on a PC-controlled universal test machine (Instron 5944 with a 5-N load cell) with a PC-recordable multimeter (Agilent 34401A), and the sheetresistance test through a 4-point probe system (Pro4-440N, Lucas Labs). We monitored the variation in impedance of SCFF-2 at different temperatures by a LCR meter (Agilent E4980A). We controlled the temperature with a hotplate (CB162, Stuart Company), and monitored it using a PC-recordable thermometer (TC-08, Pico). The default environmental


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conditions were controlled at a relative humidity (RH) of 41.3% and a temperature of 24.3 °C. An illustration of the setups for the temperature sensing tests is given in Figure 3a.

ASSOCIATED CONTENT Supporting Information. Supporting information is available free of charge on the ACS Publications website at http://pubs.acs.org. The different thermal deformations of WFF and SCFF, relationship between the distance (h) and the temperature of SCFFs, basic properties of SWCNT and some common materials, a summary of the key parameters of SCFFs with variation in temperature. Correspondence and requests for materials should be addressed to T.C or G.L.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Prof. Gilles Lubineau); [email protected] (Prof. Tao Chen) Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT


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We express gratitude to King Abdullah University of Science and Technology (KAUST) baseline research funding, the Natural Science Foundation of China (51573203, 21404111 and 51503216), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (QYZDB-SSW-SLH036), Ningbo Science and Technology Bureau (2013B10040, and 2014B82010), the National Basic Research Program of China (2011CB605602) for financial support.

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