A Highly Sensitive Capacitive-type Strain Sensor Using Wrinkled

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Letter Cite This: Nano Lett. 2018, 18, 5610−5617

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A Highly Sensitive Capacitive-type Strain Sensor Using Wrinkled Ultrathin Gold Films Roda Nur,† Naoji Matsuhisa,† Zhi Jiang,†,‡ Md Osman Goni Nayeem,† Tomoyuki Yokota,† and Takao Someya*,†,‡,§ †

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Department of Electrical Engineering and Information Systems, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Thin-Film Device Laboratory and §Center for Emergent Matter Science (CEMS), RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *

ABSTRACT: Soft strain sensors are needed for a variety of applications including human motion and health monitoring, soft robotics, and human/machine interactions. Capacitivetype strain sensors are excellent candidates for practical applications due to their great linearity and low hysteresis; however, a big limitation of this sensor is its inherent property of low sensitivity when it comes to detecting various levels of applied strain. This limitation is due to the structural properties of the parallel plate capacitor structure during applied stretching operations. According to this model, at best the maximum gauge factor (sensitivity) that can be achieved is 1. Here, we report the highest gauge factor ever achieved in capacitive-type strain sensors utilizing an ultrathin wrinkled gold film electrode. Our strain sensor achieved a gauge factor slightly above 3 and exhibited high linearity with negligible hysteresis over a maximum applied strain of 140%. We further demonstrated this highly sensitive strain sensor in a wearable application. This work opens up the possibility of engineering even higher sensitivity in capacitive-type strain sensors for practical and reliable wearable applications. KEYWORDS: Strain sensor, gold, wrinkled film, capacitor, stretchable electronics

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large GFs over 3000.11 Resistive-type strain sensors are capable of achieving high sensitivities through employing strain dependent material designs such as microcracking propagation12 and conductive percolation networks.13 When it comes to the use of strain sensors in practical applications, characteristics such as a linear strain response, high stretchability, and low hysteresis are needed. Although resistive-type strain sensors offer outstanding GFs and can be made highly stretchable, most of these sensors tend to display a nonlinear strain response, hysteresis, and larger overshooting behavior in response to dynamic strains, which can be challenging when calibrating for practical use. Capacitive-type strain sensors meet these requirements for practical applications; however, they suffer from the limitation of low sensitivity where the theoretical best gauge factor that can be achieved is 1. This limitation arises from the parallel-plate capacitor structure. Under the application of uniaxial strain to this structure, it produces a simultaneous change to the area and dielectric thickness, and as a result the capacitance has a linear relationship with applied strain. Capacitive-type strain sensors

ecently, there has been great interest toward developing soft sensors for a variety of applications including wearable sensors,1 electronic skin,2,3 soft robotics,4 and manufacturing processes.5 For these types of applications, it is critical that these sensors be lightweight, highly conformable, soft, and mechanically durable for long-term use. Strain sensors in particular are important for monitoring a variety of activities ranging from motion capture6−9 to health-care applications.10 Strain sensors act as transducers, which convert an applied mechanical deformation of strain into an electronic quantity such as a change in resistance, capacitance, current, or voltage depending on the type of sensing mechanism utilized by the sensor. Possessing higher sensitivity in strain sensors is an important feature, because this benefit allows for the clear distinction between large and subtle motions. A strain sensor with higher sensitivity would be capable of assigning values to small and large motions with extreme value differences. Out of the previously mentioned sensing mechanisms for strain sensors, piezoelectric-type and resistive-type sensing are capable of achieving very large gauge factors (high sensitivity). Piezoelectric-type strain sensors rely on a strain induced change in the Schottky barrier height at the metal−semiconductor interface. These sensors are typically composed of piezoelectric-based nanowire networks and have demonstrated very © 2018 American Chemical Society

Received: May 22, 2018 Revised: July 24, 2018 Published: August 2, 2018 5610

DOI: 10.1021/acs.nanolett.8b02088 Nano Lett. 2018, 18, 5610−5617

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Figure 1. Ultrathin wrinkled Au film strain sensor. (a) Schematic of the assembly and structure of the Au film strain sensor. (b,d) Photographs of the strain sensor in its relaxed compressed state at 0% strain and in its stretched state at 140% strain. Scale bar, 1 cm. (c,e) Corresponding optical micrographs of the wrinkled Au film electrode at 0% and 140% strain. Scale bar, 100 μm. (f) Capacitive strain response during loading (black) and unloading (red) up to 140% strain. (g) A comparison of the gauge factor as a function of stretchability with other reported capacitive-type strain sensors.

140% strain with negligible hysteresis and high linearity. Its mechanical durability was also tested by performing 1000 stretching cycles where it displayed a stable performance. Although Au films are inherently brittle materials, which fracture under 1% applied strain,17,18 they can be made into stretchable conductors by applying a prestrain to the substrate prior to a metal deposition.19 This metal deposition prestraining technique can provide strain accommodation up 30%;20 however, for realizing highly stretchable applications with Au film conductors another strategy is needed. Thin metal films as stretchable conductors offer high and stable conductivities during applied strain21 when compared to nanomaterials22 and nanowire network-based materials.23 Realizing highly stretchable thin metal film conductors on an ultrathin PET foil up to 275% strain has been previously reported where it demonstrated a reliable electrical and mechanically durable performance.21 Therefore, we selected Au as our electrode to benefit from its high and stable

utilizing conductive networks of CNTs as a stretchable electrode and a silicone-based elastomer as a stretchable dielectric layer have achieved GFs of 1 at 100%14 and 150%15 strain. Recently, an approach of combining a conductive textile electrode and a silicone-based dielectric resulted in producing a GF of 1.23 over 100% applied strain.16 However, to further increase the value of capacitive-type strain sensors in practical applications, it is crucial to have higher sensitivity to distinguish between a variety of different motions. Here, we present a structure for designing highly sensitive capacitive-type strain sensors, which is a wrinkled capacitor structure. Our strategy to achieve higher sensitivity was to introduce an additional degree of freedom to the parallel plate capacitor structure through an out-of-plane deformation via spontaneous wrinkling. This structure was generated by transferring ultrathin gold film electrodes to the top and bottom of a prestretched adhesive dielectric layer. This sensor achieved a gauge factor of 3.05 with a high stretchability up to 5611

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Figure 2. Electrical/mechanical strain testing, environmental effects, and gauge factor tuning. (a) Time response of a step and hold test at various applied strain levels. (b) Mechanical testing of cyclic durability for 1000 stretch/release cycles at 50% strain. (c) Strain sensor performance under the conditions of humidity at 0%, 30%, and 50% strain. (d) Strain sensor performance under the condition of temperature at 0% and 30% strain. (e) Tuning the gauge factor of the strain sensor through varying the electrode film thickness. (f) Capacitive strain response of the strain sensors over 100% applied strain.

We then investigated the performance of the 500 nm film strain sensor through characterizing its electrical and mechanical properties. This sensor’s dynamic behavior was tested to analyze its transient response to various applied strain levels. We applied a step and hold input for a time duration of 20 s at the following strain levels: 50%, 80%, 100%, and 120%, which can be seen in Figure 2a. The ramping rate was adjusted to provide a good time overlap of the different strain levels, so the 50%, 80%, 100%, and 120% rate was 200, 400, 800, and 800 mm/sec, respectively. In response to the constant static loading, this sensor showed a less than 1% drift error for all tested strain levels, and in addition it showed no overshooting behavior, which is a common issue seen in resistive-type strain sensors. When we performed the same experiment with a thicker parylene layer of 1 μm, we found there was significant drifting present during the static loading period, which worsened at higher strain levels. In general, the 500 nm parylene film showed excellent reliability and stability for time response measurements. Another important property of strain sensors is its mechanical durability to withstand long-term stretch and release cycling. This sensor’s durability was tested by performing 1000 stretch/release cycles at 30% and 50% strain. The results can be found in Figure 2b for 50% strain and Figure S1 for 30% strain, where we compare our sensor’s performance against the expected results for the theoretical best GF value of 1. For the 50% strain, we observed a high initial value of ∼120%, which corresponds to an ∼2.4 amplification. A comparison between the 500 nm and 1 μm parylene thick films for 50% strain cycling can be found in Figure S2. During the stretch/release cycling, there is a trend of a gradual decline in the capacitance. This decline most likely arises from the viscous drifting behavior of the VHB adhesive elastomer and/or the possible fatigue of the metal electrode film although the cycling of the metal electrode displayed stable resistance over the same number of cycles.21

conductivity with applied strain, and parylene as a supporting insulating substrate layer for its high flexibility and thickness scalability. The fabrication process for developing the wrinkled ultrathin Au film strain sensor can be found in Methods and the assembly and structure in Figure 1a. This wrinkled structure was formed from the spontaneous wrinkling of the intrinsically nonstretchable parylene/Au film on top of a soft dielectric layer. This compact three-dimensional structure allows for uniaxial strain accommodation and enables the electrode film to become highly stretchable through transitioning between a flat and compressed wrinkled state upon stretch/release cycling. Figure 1b,c shows the parylene/Au film in its “relaxed” compressed state and Figure 1d,e shows its “planar-like” stretched state. The strain response of this sensor under loading and unloading can be found in Figure 1f where the maximum applied strain was 140%. This strain sensor exhibited high linearity with an average R2 = 0.98 and it also showed minimal hysteresis. For a strain sensor with a GF of 1 that is subjected to an applied strain of 140%, we expect to obtain a relative change in capacitance of 140%. However, with this wrinkled capacitor structure we obtained a value of 428% for the same applied strain. During the application of strain to our wrinkled capacitor, it experiences a structural transformation from a highly compressed wrinkled small area into a more elongated planar structure where the deviation in the capacitance value from the theoretical GF limit becomes larger with increasing strain. In Figure 1g, we compare our strain sensor’s GF as a function of stretchability with previously reported capacitivetype strain sensors.14−16,24−26 Again, the theoretical GF limit for capacitive-type strain sensors is 1; however, our ultrathin wrinkled capacitor significantly exceeds this limitation for sensitivity. As a result, this sensor has more capacity to measure and distinguish motions over a linear region without requiring a large amount of applied strain. 5612

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presence of compressive forces or stresses that exceeds the critical value for stability.28 However, in stretchable electronics the wrinkling of materials or devices is a strain engineering design technique to embed strain accommodation into nonstretchable materials.29 When it comes to the spontaneous formation of wrinkles to relieve stress, there is an energy balance trade-off between the stiff upper film which prefers a longer wavelength and the underlying soft substrate that prefers a shorter wavelength.30 For large compressions of thin rigid films on compliant substrates, the up−down symmetry of the wrinkled pattern is broken, and several bifurcations are introduced leading to the generation of complex multiperiodic wrinkles.31 This trend of increasing complexity in the spontaneous wrinkling of the 500 nm electrode film as a function of prestraining can be seen in Figure S7. We found that larger prestrains resulted in generating a smaller compressed initial length of the strain sensor (Figure S8) and also produced an increase in both its stretchability and gauge factor (Figure S9). For the strain sensors with electrode film thicknesses of 500 nm, 1.4 μm, and 2.5 μm, we measured its average cross-sectional wrinkling amplitude at 0% strain in Figure 3b. As expected, there is a declining trend in the amplitude as the electrode film thickness is scaled down. Interestingly for our strain sensor, we found that thinner film electrodes with lower wrinkling parameters offered an improvement to the sensitivity. To investigate this trend further, we measured the average dielectric thickness at 0% and 100% strain for the same sensors in Figure 3c. Here, we found that by reducing the electrode film thickness, it resulted in producing a thinner dielectric with increasing applied strain. In particular, the 500 nm film displayed a larger net change in the dielectric thickness when compared to the thicker micron-sized films (figure S10). Capacitive-type strain sensors utilizing a parallel plate structure undergo a geometrical change during an applied strain. Under the conditions of uniaxial strain, the length of the capacitor experiences an elongation, whereas the width and dielectric thickness of the capacitor experiences a contraction. Equation 1 shows the formula for the capacitance of a nonstretched capacitor where εo is the permittivity constant, εr is the dielectric constant, and lo, ωo, do are the initial length, width, and thickness of the capacitor.

Reliable sensing under the influence of environmental effects such as humidity and temperature are important aspects to consider for practical applications. The moisture present from a humid environment can directly influence the conductivity of an electrode.6,35 For resistive-type strain sensors, an additional encapsulation layer can help to protect the electrode layer; however, this additional layer may cause some degradation in its performance and can also increase the overall thickness of the sensor. We tested the effects of humidity to our sensor under an unstrained (0%) and strained (30% and 50%) state where the humidity of its environment was gradually increased (Figure 2c). We found that the influence of moisture did not degrade the performance of this sensor. We further performed a water contact angle measurement on the wrinkled electrode (figure S3, S4) and found its structure provided a hydrophobic surface with an average water contact angle of 117.9° for 0% strain, 110.4° for 30% strain, and 110.6° for 50% strain. In contrast, we found the unwrinkled electrode film had a hydrophilic surface with an average water contact angle of 82.6°. A possible reason for the stability of this sensor under humid conditions may come from the hydrophobic nature of the wrinkled electrode, which can help to prevent a large wettability of moisture on the electrode’s surface. We also tested the influence of temperature on the strain sensor’s performance over the range of 20 °C to 50 °C. In general, a change in temperature causes an expansion/contraction in materials. A problem that arises for strain sensors is that a thermal contraction or expansion can be detected as a change in strain. From Figure 2d, we tested the unstrained (0%) and strained (30%) capacitance as the temperature was gradually increased. We found this strain sensor to be stable up to 35 °C; however, there was a gradual increase in capacitance as the temperature was further increased to 50 °C. The net relative capacitance change over the entire temperature range was a 2.7% increase for 0% strain and a 5.4% increase for 30% strain. Next, we investigated the influence of the electrode’s film thickness on the gauge factor. Under the strain range of 0− 100%, we found a declining trend in the GF of the strain sensor starting from the 500 nm film down to the 4.5 μm film in Figure 2e. In Figure 2f, we compared the strain response characteristics of the sensors over the same applied strain range. The absolute capacitance values for these sensors can be found in Figure S5. With the 4.5 μm thick parylene films, the GF already falls below 1, which corresponds to a much lower sensitivity. The linearity dependence on the electrode film thickness for the various strain sensors is summarized in Table S1 where a decline in the R2 value begins to occur with the 3.5 μm sensor. For thicker parylene films starting at 3.5 μm, the wrinkled deformation becomes much larger and results in an extreme deformation of the soft dielectric layer where parts of the wrinkled electrode are completely delaminated (Figure S6). Because of the nonideal wrinkled capacitor structure of thicker electrode films, the GF of the strain sensor results in a much lower value. In addition, we attempted to further scale the parylene film to 250 nm; however, we found this film to fracture easily in the wrinkled state and resulted in the electrode becoming electrically disconnected. Therefore, the optimum region of this wrinkled capacitive-type strain sensor utilizing this structure and materials is between 500 nm and 2.5 μm for the electrode film thickness. In mechanics, the buckling or wrinkling of materials is regarded to be a failure mode, because the onset of this state corresponds to a transition into an unstable regime due to the

εoεrlo ωo do

Co =

(1)

Upon applying uniaxial strain, eq 2 shows the change in capacitance due to elongation and contraction effects.27 If we select materials for the electrode and dielectric that have the same Poisson’s ratio, we can eliminate the effects of contraction and obtain a linear relationship between capacitance and strain. C=

εoεr[1 + ε]lo[1 − νelectrodeε]ωo = [1 + ε]Co [1 − νdielectricε]do

(2)

When it comes to determining the gauge factor (or sensitivity), we simply can use the normalized change in capacitance over the applied strain range in eq 3. Again, assuming we have the linear relationship from eq 2, we will find that at best we can achieve a gauge factor (GF) of 1. GF = 5613

ΔC Co

ε

=1

(3) DOI: 10.1021/acs.nanolett.8b02088 Nano Lett. 2018, 18, 5610−5617

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Figure 3. Mechanism for higher sensitivity. (a) Schematic overview of the structural transformation of the wrinkled capacitor structure under the application of uniaxial strain. (b) Average cross-sectional wrinkle amplitude of the strain sensors at 0% strain. (c) Average dielectric thickness as a function of strain for the various strain sensors. (d) Average width of the capacitors as a function of strain. Model 1 is the wrinkled capacitor model. Model 2 is the parallel-plate model. (e) A 500 nm strain sensor fitted with both models. (f) A 1.4 μm strain sensor fitted with both models. (g) A 2.5 μm strain sensor fitted with both models.

In contrast to the parallel plate structure, our wrinkled capacitor has an additional degree of freedom of deformation along the axis that is perpendicular to the longitudinal strain direction. This out-of-plane strain accommodation allows for two additional factors for enhancing the change in capacitance, which is the suppression of the width contraction and the free contraction of the dielectric layer (Figure 3a). The application of uniaxial strain to this accordion-like structure causes a structural transformation of planarization from its initial compressed compact structure. During this transformation, an unfolding process of the electrode layer occurs, which we found to help inhibit the effects of the lateral strain contraction of the capacitor. Over the strain range of 0−100%, we measured the average change in width with respect to strain for the strain sensors in Figure 3d. Here, we found that the micron-sized thick electrodes provided resistance in suppressing the Poisson effect by maintaining a constant width. Conversely, the 500 nm electrode became susceptible to width contraction at around 80% strain; however, it still maintains 95% of its original width at 100% strain. This result shows that this out-of-plane structural transformation helps to preserve the area increase of the capacitor by suppressing the loss of area due to the lateral contraction in the width. Another factor for the enhancement in the capacitance change is the free contraction of the dielectric layer with increasing strain. As previously mentioned and in Figure 3c, we found that scaling down the electrode film thickness resulted in producing a thinner dielectric. For the wrinkled capacitor

structure, both the top and bottom layers of the dielectric are under compressive forces due to the wrinkled formation of the electrodes. In general, the scaling down in film thickness helps to enhance its flexibility and reduces its weight.32 Here, thinner electrode films help to impose lesser compressive forces on the dielectric layer. As previously mentioned, the 500 nm strain sensor’s dielectric layer is able to contract more in comparison to the thicker sensors. For the parallel-plate model, the design approach is to cancel the width contraction with the dielectric contraction. However, for our proposed wrinkled capacitor model, we keep the width constant to ensure a larger area change and also to benefit from the effects of a thinner dielectric. The modified strained capacitance formula for the wrinkled capacitor can be found in eq 4. C=

Co[1 + ε] [1 − νdielectricε]

(4)

One important parameter for an enhanced capacitance is the Poisson’s ratio of the dielectric layer. From our analysis of varying the electrode film thickness, we found that the thickness of the electrode directly influences the Poisson’s ratio of the dielectric material. Assuming that the width is constant with strain, we calculated the Poisson’s ratio of the various strain sensors based on the measured average strained dielectric values. We obtained the “effective” Poisson’s ratio values in Table 1. As a reference, this sensor’s dielectric layer is 3M’s VHB tape, which has a Poisson’s ratio of ∼0.49. Because 5614

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and the contraction minimization on the capacitor’s width due to the structural out-of-plane deformation. To demonstrate the sensitivity of this strain sensor, we selected a wearable application where the motion of bending a finger was tested. The strain sensor was placed onto the joint of an index finger and its angle sensitivity from gradual bending was measured in Figure 4. The finger bending poses consisted of four angles starting at a relaxed pose at 0° and then it is bended to angles of 45°, 90°, and 120°. For the 500 nm strain sensor, the results can be seen in Figure 4c where the corresponding relative change in capacitance values were 0%, ∼33%, ∼103%, and ∼142% for the respective bending angles. In order to highlight the enhanced sensitivity of this sensor, we performed the same demonstration but with a strain sensor of a lower sensitivity (the 3.5 μm sensor) where its results can be seen in Figure 4d. For the same bending poses, the lower GF sensor displayed a relative change in capacitance values of 0%, ∼4%, ∼12%, and ∼16%. The range of values detected by the higher GF sensor was ∼142% while the lower GF sensor showed a range of ∼16% for the same motion detected. By comparing the performance of these sensors, it is clear to see that the higher GF sensor is capable of providing more contrast between the different degrees of motion detected, which is a valuable feature for applications requiring a higher resolution in motion detection. Although we have demonstrated a single sensor in this work, a future direction would be the development of an array structure where a more detailed strain distribution could be obtained. The technique of Silicon Nanoribbon (SiNR) Electronics33 offers a flexible and customizable design process that is capable of designing array structures with location specific strain accommodation for sensing applications.34 In comparison to the SiNR approach, our design is better suited for large area sensing applications where the strain distribution is more uniform. Our technique also offers a simple fabrication process and a reliable high sensitivity for strain detection.

Table 1. Effective Poisson’s Ratios of the Dielectric Layer electrode film thickness

effective Poisson’s ratio of the dielectric

500 nm 1.4 μm 2.5 μm

0.50 0.43 0.40

of the compressive forces required to form wrinkling of stiff films on compliant substrates, there is a declining trend in the “effective” Poisson’s ratio of the dielectric layer due to the electrode’s increased film thickness. A Poisson’s ratio of 0.5 represents a free contraction and declining values represents a lower degree of contraction, which was observed from the dielectric thickness measurements. Figure 3e−g show a comparison of our wrinkled capacitor model and the parallelplate model (GF = 1 condition) along with the experimental results of the strain sensors. The wrinkled capacitor model displays a better fit to the experimental results compared to the parallel-plate model which predicts the same performance independent of the electrode’s film thickness. For the 2.5 μm case in Figure 3g, the wrinkled capacitor model slightly overestimates the strained capacitance. This most likely arises from this electrode film having much larger surface deformations, so the average measured dielectric thickness may not be as accurate. Another issue is the deviation of the wrinkled capacitor model at 100% strain. As this model is nonlinear at high strains, perhaps there are other nonidealities that begin to occur around this point that help to extend the linearity of the experimental strain sensor’s performance. A future direction would be the investigation of a correction term to account for the extended linearity at larger strains. In summary, we conclude that the major factors involved for the increased sensitivity in thinner film electrodes arise from the presence lesser compressive forces exerted on the soft dielectric layer enabling a larger dielectric thickness reduction

Figure 4. Demonstration of motion detection with the Au film strain sensor. For a wearable application, the strain sensor is mounted onto the joint of an index finger. (a) The strain sensor is in a relaxed pose which corresponds to a bending angle of 0°. (b) The strain sensor is in a bent pose whose angle is referenced from the relaxed pose. (c) A 500 nm strain sensor. The relative change in capacitance from the gradual finger bending and relaxing over four bending angles of 0°, 45°, 90°, and 120°. (d) A 3.5 μm strain sensor. The relative change in capacitance from the gradual finger bending and relaxing over four bending angles of 0°, 45°, 90°, and 120°. 5615

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Nano Letters In summary, we presented a structure that generated an increase in sensitivity to capacitive-type strain sensors via a wrinkled out-of-plane deformation. Our strain sensor exceeded the theoretical GF limit of 1 by a factor of 3 times. We found that by scaling the film thickness of our electrode into the nanometer-scale allowed for this increase in sensitivity. When it comes to the mechanical properties of this strain sensor, it exhibited a highly linear behavior with minimal hysteresis and no overshooting. Under long-term cycling, we found that this strain sensor was stable and showed minor degradation in its capacitance value. Lastly, this strain sensor was demonstrated in a wearable application to show the functionality of its enhanced sensitivity. Our work demonstrates the possibility of achieving higher sensitivity in capacitive-type strain sensors and opens up new possibilities for further enhancement of its sensitivity for practical and reliable wearable applications. Methods. Fabrication. First, a polyimide (PI) substrate (UBE Polyimide Film UPILEX 125 μm) was pretreated with a fluoropolymer coating (Novec 1700/7100 3 M Company) in order to provide an easy delamination of the electrode in the future transferring step. Next, a thin 500 nm film of parylene (diX-SR Daisan Kasei Company) was deposited by a chemical vapor deposition (CVD) process. It was then followed by a thermal evaporation of a 50 nm thick Au film layer using a patterned shadow mask with the dimensions of 2 cm in length and 1 cm in width. Because Au has good adhesion to parylene films, we did not need to include an additional metal adhesion layer. This parylene/Au film was then transferred to a PDMS sheet (Elecom’s Liquid Crystal Display Protection Film) to reverse the film order. Next, 3M’s VHB (4910) adhesive elastomer with a 500 μm thickness served as a stretchable dielectric layer. This elastomer was first prestretched, and then the top and bottom electrodes were transferred sequentially. Upon relaxing the elastomer, both the top and bottom electrodes spontaneously formed compressive wrinkled films. Characterization. Shimadzu’s Autograph AG-X stretcher was used to provide an applied loading, and the capacitance measurements were obtained from Agilent’s 4284A Precision LCR meter and Keysight E4980AL Precision LCR meter which was connected to a computer for data acquisition. For the temperature measurements, the strain sensor was placed in near contact to a hot plate (As One’s RSH-1DN) and enclosed in an insulating Al foil environment. The humidity measurements were performed in the same insulating environment as the temperature setup but an ultrasonic humidifier purchased from Prismate (model number NPM-1200) was used to provide humidity. Both temperature and humidity were measured using a temperature/humidity sensor purchased from As One (product number 1-5459-01).

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Takao Someya: 0000-0003-3051-1138 Author Contributions

R.N., N.M., and T.S. conceived the project. R.N. designed and performed the experiments, fabrication, and characterization. Z.J. helped with the measurements. R.N. and M.N. performed the demonstration. R.N. and N.M. analyzed the mechanism. R.N., N.M., Z.J., and T.Y. prepared the manuscript. R.N. wrote the manuscript with comments from all the authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Science and Technology Agency (JST) program SICORP. R.N. is supported by the Japanese Government (MEXT) Scholarship. Z.J. is supported by the JRA program at RIKEN and the SEUT RA program at the University of Tokyo. We thank Dr. D. Ordinario for his assistance with the manuscript formatting, R. Shidachi for his assistance with the sensor photography, and H. Jin for technical support from the University of Tokyo.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b02088. Experimental details and characterization (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tomoyuki Yokota: 0000-0003-1546-8864 5616

DOI: 10.1021/acs.nanolett.8b02088 Nano Lett. 2018, 18, 5610−5617

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DOI: 10.1021/acs.nanolett.8b02088 Nano Lett. 2018, 18, 5610−5617