A Highly Sensitive Capacitive-type Strain Sensor Using Wrinkled

Aug 2, 2018 - Capacitive-type strain sensors are excellent candidates for practical applications due to their great linearity and low hysteresis; howe...
0 downloads 0 Views 1MB Size
Subscriber access provided by UNIV OF DURHAM

Communication

A Highly Sensitive Capacitive-type Strain Sensor using Wrinkled Ultra-thin Gold Films Roda Nur, Naoji Matsuhisa, Zhi Jiang, Md Osman Goni Nayeem, Tomoyuki Yokota, and Takao Someya Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02088 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

1

A Highly Sensitive Capacitive-type Strain Sensor

2

using Wrinkled Ultra-thin Gold Films

3

Roda Nur1, Naoji Matsuhisa1, Zhi Jiang1,2, Md Osman Goni Nayeem1, Tomoyuki Yokota1, and

4

Takao Someya1,2,3

5

1

6

Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

7

2

Thin-Film Device Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

8

3

Center for Emergent Matter Science (CEMS), RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198,

9

Japan

Department of Electrical Engineering and Information Systems, The University of Tokyo, 7-3-1

10

11

12

13

14

15

ACS Paragon Plus Environment

1

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

16

ABSTRACT:

17

Soft strain sensors are needed for a variety of applications including human motion and health

18

monitoring, soft robotics, and human/machine interactions. Capacitive-type strain sensors are

19

excellent candidates for practical applications due to their great linearity and low hysteresis;

20

however, a big limitation of this sensor is its inherent property of low sensitivity when it comes

21

to detecting various levels of applied strain. This limitation is due to the structural properties of

22

the parallel plate capacitor structure during applied stretching operations. According to this

23

model, at best the maximum gauge factor (sensitivity) that can be achieved is 1. Here, we report

24

the highest gauge factor ever achieved in capacitive-type strain sensors utilizing an ultra-thin

25

wrinkled gold film electrode. Our strain sensor achieved a gauge factor slightly above 3 and

26

exhibited high linearity with negligible hysteresis over a maximum applied strain of 140%. We

27

further demonstrated this highly sensitive strain sensor in a wearable application. This work

28

opens up the possibility of engineering even higher sensitivity in capacitive-type strain sensors

29

for practical and reliable wearable applications.

30

KEYWORDS: strain sensor, gold, wrinkled film, capacitor, stretchable electronics

31 32 33 34 35 36 37

ACS Paragon Plus Environment

2

Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

38

Recently, there has been great interest towards developing soft sensors for a variety of

39

applications including wearable sensors1, electronic skin2,3, soft robotics4, and manufacturing

40

processes5. For these types of applications, it is critical that these sensors be lightweight, highly

41

conformable, soft, and mechanically durable for long-term use. Strain sensors, in particular, are

42

important for monitoring a variety of activities ranging from motion capture6-9 to health-care

43

applications10. Strain sensors act as transducers, which convert an applied mechanical

44

deformation of strain into an electronic quantity such as a change in resistance, capacitance,

45

current, or voltage depending on the type of sensing mechanism utilized by the sensor.

46

Possessing higher sensitivity in strain sensors is an important feature, since this benefit allows

47

for the clear distinction between large and subtle motions. A strain sensor with higher sensitivity

48

would be capable of assigning values to small and large motions with extreme value differences.

49

Out of the previously mentioned sensing mechanisms for strain sensors, piezoelectric-type and

50

resistive-type sensing are capable of achieving very large gauge factors (high sensitivity).

51

Piezoelectric-type strain sensors rely on a strain induced change in the Schottky barrier height at

52

the metal-semiconductor interface. These sensors are typically composed of piezoelectric-based

53

nanowire networks and have demonstrated very large GFs over 3,00011. Resistive-type strain

54

sensors are capable of achieving high sensitivities through employing strain dependent material

55

designs such as micro-cracking propagation12 and conductive percolation networks13.

56

When it comes to the use of strain sensors in practical applications, characteristics such as a

57

linear strain response, high stretchability, and low hysteresis are needed. Although resistive-type

58

strain sensors offer outstanding GFs and can be made highly stretchable, most of these sensors

59

tend to display a nonlinear strain response, hysteresis, and larger overshooting behavior in

60

response to dynamic strains, which can be challenging when calibrating for practical use.

ACS Paragon Plus Environment

3

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

61

Capacitive-type strain sensors meet these requirements for practical applications; however, they

62

suffer from the limitation of low sensitivity where the theoretical best gauge factor that can be

63

achieved is 1. This limitation arises from the parallel-plate capacitor structure. Under the

64

application of uniaxial strain to this structure, it produces a simultaneous change to the area and

65

dielectric thickness, and as a result the capacitance has a linear relationship with applied strain.

66

Capacitive-type strain sensors utilizing conductive networks of CNTs as a stretchable electrode

67

and a silicone-based elastomer as a stretchable dielectric layer have achieved GFs of 1 at 100%14

68

and 150%15 strain. Recently, an approach of combining a conductive textile electrode and a

69

silicone-based dielectric resulted in producing a GF of 1.23 over 100% applied strain16.

70

However, to further increase the value of capacitive-type strain sensors in practical applications,

71

it is crucial to have higher sensitivity to distinguish between a variety of different motions.

72

Here, we present a structure for designing highly sensitive capacitive-type strain sensors,

73

which is a wrinkled capacitor structure. Our strategy to achieve higher sensitivity was to

74

introduce an additional degree of freedom to the parallel plate capacitor structure through an out-

75

of-plane deformation via spontaneous wrinkling. This structure was generated by transferring

76

ultra-thin gold film electrodes to the top and bottom of a pre-stretched adhesive dielectric layer.

77

This sensor achieved a gauge factor of 3.05 with a high stretchability up to 140% strain with

78

negligible hysteresis and high linearity. Its mechanical durability was also tested by performing

79

1,000 stretching cycles where it displayed a stable performance.

80

Although Au films are inherently brittle materials, which fracture under 1% applied strain17,18,

81

they can be made into stretchable conductors by applying a pre-strain to the substrate prior to a

82

metal deposition19. This metal deposition pre-straining technique can provide strain

83

accommodation up 30%20; however, for realizing highly stretchable applications with Au film

ACS Paragon Plus Environment

4

Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

84

conductors another strategy is needed. Thin metal films as stretchable conductors offer high and

85

stable conductivities during applied strain21 when compared to nanomaterials22 and nanowire

86

network-based materials23. Realizing highly stretchable thin metal film conductors on an ultra-

87

thin PET foil up to 275% strain has been previously reported where it demonstrated a reliable

88

electrical and mechanically durable performance21. Therefore, we selected Au as our electrode to

89

benefit from its high and stable conductivity with applied strain, and parylene as a supporting

90

insulating substrate layer for its high flexibility and thickness scalability. The fabrication process

91

for developing the wrinkled ultra-thin Au film strain sensor can be found in the Methods section

92

and the assembly and structure in figure 1a. This wrinkled structure was formed from the

93

spontaneous wrinkling of the intrinsically non-stretchable parylene/Au film on top of a soft

94

dielectric layer. This compact 3d structure allows for uniaxial strain accommodation and enables

95

the electrode film to become highly stretchable through transitioning between a flat and

96

compressed wrinkled state upon stretch/release cycling. Figures 1b,c show the parylene/Au film

97

in its “relaxed” compressed state and figures 1d,e show its “planar-like” stretched state.

98

The strain response of this sensor under loading and unloading can be found in figure 1f

99

where the maximum applied strain was 140%. This strain sensor exhibited high linearity with an

100

average R2 = 0.98 and it also showed minimal hysteresis. For a strain sensor with a GF of 1 that

101

is subjected to an applied strain of 140%, we expect to obtain a relative change in capacitance of

102

140%. However, with this wrinkled capacitor structure we obtained a value of 428% for the

103

same applied strain. During the application of strain to our wrinkled capacitor, it experiences a

104

structural transformation from a highly compressed wrinkled small area into a more elongated

105

planar structure where the deviation in the capacitance value from the theoretical GF limit

106

becomes larger with increasing strain. In figure 1g, we compare our strain sensor’s GF as a

ACS Paragon Plus Environment

5

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 24

107

function of stretchability with previously reported capacitive-type strain sensors. Again, the

108

theoretical GF limit for capacitive-type strain sensors is 1; however, our ultra-thin wrinkled

109

capacitor significantly exceeds this limitation for sensitivity. As a result, this sensor has more

110

capacity to measure and distinguish motions over a linear region without requiring a large

111

amount of applied strain.

112

We then investigated the performance of the 500 nm film strain sensor through characterizing

113

its electrical and mechanical properties. This sensor’s dynamic behavior was tested to analyze its

114

transient response to various applied strain levels. We applied a step and hold input for a time

115

duration of 20 seconds at the following strain levels: 50%, 80%, 100%, and 120%, which can be

116

seen in figure 2a. The ramping rate was adjusted to provide a good time overlap of the different

117

strain levels, so the 50%, 80%, 100%, and 120% rate was 200 mm/sec, 400 mm/sec, 800

118

mm/sec, and 800 mm/sec respectively. In response to the constant static loading, this sensor

119

showed a less than 1% drift error for all tested strain levels, and in addition it showed no

120

overshooting behavior, which is a common issue seen in resistive-type strain sensors. When we

121

performed the same experiment with a thicker parylene layer of 1 µm, we found there was

122

significant drifting present during the static loading period, which worsened at higher strain

123

levels. In general, the 500 nm parylene film showed excellent reliability and stability for time

124

response measurements. Another important property of strain sensors is its mechanical durability

125

to withstand long-term stretch and release cycling. This sensor’s durability was tested by

126

performing 1, 000 stretch/release cycles at 30% and 50% strain. The results can be found in

127

figure 2b for 50% strain and figure S1 for 30% strain, where we compare our sensor’s

128

performance against the expected results for the theoretical best GF value of 1. For the 50%

129

strain, we observed a high initial value of ~120%, which corresponds to a ~2.4 amplification. A

ACS Paragon Plus Environment

6

Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

130

comparison between the 500 nm and 1 µm parylene thick films for 50% strain cycling can be

131

found in figure S2. During the stretch/release cycling, there is a trend of a gradual decline in the

132

capacitance. This decline mostly likely arises from the viscous drifting behavior of the VHB

133

adhesive elastomer and/or the possible fatigue of the metal electrode film although the cycling of

134

the metal electrode displayed stable resistance over the same number of cycles21.

135

Reliable sensing under the influence of environmental effects such as humidity and

136

temperature are important aspects to consider for practical applications. The moisture present

137

from a humid environment can directly influence the conductivity of an electrode6,35. For

138

resistive-type strain sensors, an additional encapsulation layer can help to protect the electrode

139

layer; however, this additional layer may cause some degradation in its performance and can also

140

increase the overall thickness of the sensor. We tested the effects of humidity to our sensor under

141

an unstrained (0%) and strained (30% and 50%) state where the humidity of its environment was

142

gradually increased (figure 2c). We found that the influence of moisture did not degrade the

143

performance of this sensor. We further performed a water contact angle measurement on the

144

wrinkled electrode (figure S3, S4) and found its structure provided a hydrophobic surface with

145

an average water contact angle of 117.9˚ for 0% strain, 110.4˚ for 30% strain, and 110.6˚ for

146

50% strain. In contrast, we found the unwrinkled electrode film had a hydrophilic surface with

147

an average water contact angle of 82.6˚. A possible reason for the stability of this sensor under

148

humid conditions may come from the hydrophobic nature of the wrinkled electrode, which can

149

help to prevent a large wettability of moisture on the electrode’s surface. We also tested the

150

influence of temperature on the strain sensor’s performance over the range of 20˚C to 50˚C. In

151

general, a change in temperature causes an expansion/contraction in materials. A problem that

152

arises for strain sensors is that a thermal contraction or expansion can be detected as a change in

ACS Paragon Plus Environment

7

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

153

strain. From figure 2d, we tested the unstrained (0%) and strained (30%) capacitance as the

154

temperature was gradually increased. We found this strain sensor to be stable up to 35˚C;

155

however, there was a gradual increase in capacitance as the temperature was further increased to

156

50˚C. The net relative capacitance change over the entire temperature range was a 2.7% increase

157

for 0% strain and a 5.4% increase for 30% strain.

158

Next, we investigated the influence of the electrode’s film thickness on the gauge factor. Under

159

the strain range of 0% to 100%, we found a declining trend in the GF of the strain sensor starting

160

from the 500 nm film down to the 4.5 µm film in figure 2e. In figure 2f, we compared the strain

161

response characteristics of the sensors over the same applied strain range. The absolute

162

capacitance values for these sensors can be found in figure S5. With the 4.5 µm thick parylene

163

films, the GF already falls below 1, which corresponds to a much lower sensitivity. The linearity

164

dependence on the electrode film thickness for the various strain sensors are summarized in table

165

S1 where a decline in the R2 value begins to occur with the 3.5 µm sensor. For thicker parylene

166

films starting at 3.5 µm, the wrinkled deformation becomes much larger and results in an

167

extreme deformation of the soft dielectric layer where parts of the wrinkled electrode are

168

completely delaminated (figure S6). Due to the non-ideal wrinkled capacitor structure of thicker

169

electrode films, the GF of the strain sensor results in a much lower value. In addition, we

170

attempted to further scale the parylene film to 250 nm; however, we found this film to fracture

171

easily in the wrinkled state and resulted in the electrode becoming electrically disconnected.

172

Therefore, the optimum region of this wrinkled capacitive-type strain sensor utilizing this

173

structure and materials is between 500 nm and 2.5 µm for the electrode film thickness.

174

In mechanics, the buckling or wrinkling of materials is regarded to be a failure mode, since the

175

onset of this state corresponds to a transition into an unstable regime due to the presence of

ACS Paragon Plus Environment

8

Page 9 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

176

compressive forces or stresses that exceeds the critical value for stability28. However, in

177

Stretchable Electronics, the wrinkling of materials or devices is a strain engineering design

178

technique to embed strain accommodation into non-stretchable materials29. When it comes to the

179

spontaneous formation of wrinkles to relieve stress, there is an energy balance tradeoff between

180

the stiff upper film which prefers a longer wavelength and the underlying soft substrate that

181

prefers a shorter wavelength30. For large compressions of thin rigid films on compliant

182

substrates, the up-down symmetry of the wrinkled pattern is broken, and several bifurcations are

183

introduced leading to the generation of complex multi-periodic wrinkles31. This trend of

184

increasing complexity in the spontaneous wrinkling of the 500 nm electrode film as a function of

185

pre-straining can be seen in figure S7. We found that larger pre-strains resulted in generating a

186

smaller compressed initial length of the strain sensor (figure S8) and also produced an increase in

187

both its stretchability and gauge factor (figure S9). For the strain sensors with electrode film

188

thicknesses of 500 nm, 1.4 µm, and 2.5 µm, we measured its average cross-sectional wrinkling

189

amplitude at 0% strain in figure 3b. As expected, there is a declining trend in the amplitude as

190

the electrode film thickness is scaled down. Interestingly for our strain sensor, we found that

191

thinner film electrodes with lower wrinkling parameters offered an improvement to the

192

sensitivity. To investigate this trend further, we measured the average dielectric thickness at 0%

193

and 100% strain for the same sensors in figure 3c. Here, we found that by reducing the electrode

194

film thickness, it resulted in producing a thinner dielectric with increasing applied strain. In

195

particular, the 500 nm film displayed a larger net change in the dielectric thickness when

196

compared to the thicker micron-sized films (figure S10).

197

Capacitive-type strain sensors utilizing a parallel plate structure undergo a geometrical change

198

during an applied strain. Under the conditions of uniaxial strain, the length of the capacitor

ACS Paragon Plus Environment

9

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

199

experiences an elongation, whereas the width and dielectric thickness of the capacitor

200

experiences a contraction. Equation 1 shows the formula for the capacitance of a non-stretched

201

capacitor where εo is the permittivity constant, εr is the dielectric constant, and lo, ωo, do are the

202

initial length, width, and thickness of the capacitor.

Co 

203

εo εr lo ωo

(1)

do

204

Upon applying uniaxial strain, equation 2 shows the change in capacitance due to elongation and

205

contraction effects27. If we select materials for the electrode and dielectric that have the same

206

Poisson’s ratio, we can eliminate the effects of contraction and obtain a linear relationship

207

between capacitance and strain.

C 

208

εo εr 1+ε lo 1-νelectrode ωo 1-νdielectric do

 1+ε Co

(2)

209

When it comes to determining the gauge factor (or sensitivity), we simply can use the normalized

210

change in capacitance over the applied strain range in equation 3. Again, assuming we have the

211

linear relationship from equation 2, we will find that at best we can achieve a gauge factor (GF)

212

of 1.

213

GF=

∆C Co

ε

=1

(3)

214

In contrast to the parallel plate structure, our wrinkled capacitor has an additional degree of

215

freedom of deformation along the axis that is perpendicular to the longitudinal strain direction.

216

This out-of-plane strain accommodation allows for two additional factors for enhancing the

217

change in capacitance, which is the suppression of the width contraction and the free contraction

218

of the dielectric layer (figure 3a). The application of uniaxial strain to this accordion-like

ACS Paragon Plus Environment

10

Page 11 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

219

structure causes a structural transformation of planarization from its initial compressed compact

220

structure. During this transformation, an unfolding process of the electrode layer occurs, which

221

we found to help inhibit the effects of the lateral strain contraction of the capacitor. Over the

222

strain range of 0% to 100%, we measured the average change in width with respect to strain for

223

the strain sensors in figure 3d. Here, we found that the micron-sized thick electrodes provided

224

resistance in suppressing the Poisson effect by maintaining a constant width. Conversely, the 500

225

nm electrode became susceptible to width contraction at around 80% strain; however, it still

226

maintains 95% of its original width at 100% strain. This result shows that this out-of-plane

227

structural transformation helps to preserve the area increase of the capacitor by suppressing the

228

loss of area due to the lateral contraction in the width.

229

Another factor for the enhancement in the capacitance change is the free contraction of the

230

dielectric layer with increasing strain. As previously mentioned and in figure 3c, we found that

231

scaling down the electrode film thickness resulted in producing a thinner dielectric. For the

232

wrinkled capacitor structure, both the top and bottom layers of the dielectric are under

233

compressive forces due to the wrinkled formation of the electrodes. In general, the scaling down

234

in film thickness helps to enhance its flexibility and reduces its weight32. Here, thinner electrode

235

films help to impose lesser compressive forces on the dielectric layer. As previously mentioned,

236

the 500 nm strain sensor’s dielectric layer is able to contract more in comparison to the thicker

237

sensors.

238

For the parallel-plate model, the design approach is to cancel the width contraction with the

239

dielectric contraction. However, for our proposed wrinkled capacitor model, we keep the width

240

constant to ensure a larger area change and also to benefit from the effects of a thinner dielectric.

241

The modified strained capacitance formula for the wrinkled capacitor can be found in equation 4.

ACS Paragon Plus Environment

11

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

242

C 1+ε

C  1-ν o

dielectric 

Page 12 of 24

(4)

243

One important parameter for an enhanced capacitance is the Poisson’s ratio of the dielectric layer.

244

From our analysis of varying the electrode film thickness, we found that the thickness of the

245

electrode directly influences the Poisson’s ratio of the dielectric material. Assuming that the

246

width is constant with strain, we calculated the Poisson’s ratio of the various strain sensors based

247

on the measured average strained dielectric values. We obtained the “effective” Poisson’s ratio

248

values in table 1. As a reference, this sensor’s dielectric layer is 3M’s VHB tape, which has a

249

Poisson’s ratio of ~0.49. Due to the compressive forces required to form wrinkling of stiff films

250

on compliant substrates, there is a declining trend in the “effective” Poisson’s ratio of the

251

dielectric layer due to the electrode’s increased film thickness. A Poisson’s ratio of 0.5 represents

252

a free contraction and declining values represents a lower degree of contraction, which was

253

observed from the dielectric thickness measurements. Figures 3e-g shows a comparison of our

254

wrinkled capacitor model and the parallel-plate model (GF = 1 condition) along with the

255

experimental results of the strain sensors. The wrinkled capacitor model displays a better fit to

256

the experimental results compared to the parallel-plate model which predicts the same

257

performance independent of the electrode’s film thickness. For the 2.5 µm case in figure 3g, the

258

wrinkled capacitor model slightly overestimates the strained capacitance. This most likely arises

259

from this electrode film having much larger surface deformations, so the average measured

260

dielectric thickness may not be as accurate. Another issue is the deviation of the wrinkled

261

capacitor model at 100% strain. As this model is non-linear at high strains, perhaps there are

262

other non-idealities that begin to occur around this point that help to extend the linearity of the

263

experimental strain sensor’s performance. A future direction would be the investigation of a

264

correction term to account for the extended linearity at larger strains. In summary, we conclude

ACS Paragon Plus Environment

12

Page 13 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

265

that the major factors involved for the increased sensitivity in thinner film electrodes arises from

266

the presence lesser compressive forces exerted on the soft dielectric layer enabling a larger

267

dielectric thickness reduction and the contraction minimization on the capacitor’s width due to

268

the structural out-of-plane deformation.

269

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

Thickness

Effective Poisson’s Ratio of the Dielectric

500 nm

0.50

1.4 µm

0.43

2.5 µm

0.40

Electrode Film

270 271

To demonstrate the sensitivity of this strain sensor, we selected a wearable application where

272

the motion of bending a finger was tested. The strain sensor was placed onto the joint of an index

273

finger and its angle sensitivity from gradual bending was measured in figure 4. The finger

274

bending poses consisted of four angles starting at a relaxed pose at 0˚ and then it is bended to

275

angles of 45˚, 90˚, and 120˚. For the 500 nm strain sensor, the results can be seen in figure 4c

276

where the corresponding relative change in capacitance values were 0%, ~33%, ~103%, and

277

~142% for the respective bending angles. In order to highlight the enhanced sensitivity of this

278

sensor, we performed the same demonstration, but with a strain sensor of a lower sensitivity (the

279

3.5 µm sensor) where its results can be seen in figure 4d. For the same bending poses, the lower

280

GF sensor displayed a relative change in capacitance values of 0%, ~4%, ~12%, and ~16%. The

281

range of values detected by the higher GF sensor was ~142% while the lower GF sensor showed

282

a range of ~16% for the same motion detected. From comparing the performance of these

ACS Paragon Plus Environment

13

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

283

sensors, it is clear to see that the higher GF sensor is capable of providing more contrast between

284

the different degrees of motion detected, which is a valuable feature for applications requiring a

285

higher resolution in motion detection. Although we have demonstrated a single sensor in this

286

work, a future direction would be the development of an array structure where a more detailed

287

strain distribution could be obtained. The technique of Silicon Nanoribbon (SiNR) Electronics33

288

offers a flexible and customizable design process that is capable of designing array structures

289

with location specific strain accommodation for sensing applications34. In comparison to the

290

SiNR approach, our design is better suited for large area sensing applications where the strain

291

distribution is more uniform. Our technique also offers a simple fabrication process and a

292

reliable high sensitivity for strain detection.

293

In summary, we presented a structure that generated an increase in sensitivity to capacitive-

294

type strain sensors via a wrinkled out-of-plane deformation. Our strain sensor exceeded the

295

theoretical GF limit of 1 by a factor of 3 times. We found that by scaling the film thickness of

296

our electrode into the nanometer-scale allowed for this increase in sensitivity. When it comes to

297

the mechanical properties of this strain sensor, it exhibited a highly linear behavior with minimal

298

hysteresis and no overshooting. Under long-term cycling, we found that this strain sensor was

299

stable and showed minor degradation in its capacitance value. Lastly, this strain sensor was

300

demonstrated in a wearable application to show the functionality of its enhanced sensitivity. Our

301

work demonstrates the possibility of achieving higher sensitivity in capacitive-type strain sensors

302

and opens up new possibilities for further enhancement of its sensitivity for practical and reliable

303

wearable applications.

304

Methods.

ACS Paragon Plus Environment

14

Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

305

Fabrication.

306

Firstly, a polyimide (PI) substrate (UBE Polyimide Film UPILEX 125 µm)was pre-treated with a

307

fluoropolymer coating (Novec 1700/7100 3M Company) in order to provide an easy

308

delamination of the electrode in the future transferring step. Next, a thin 500 nm film of parylene

309

(diX-SR Daisan Kasei Company) was deposited by a chemical vapor deposition (CVD) process.

310

It was then followed by a thermal evaporation of a 50 nm thick Au film layer using a patterned

311

shadow mask with the dimensions of 2 cm in length and 1 cm in width. Since Au has good

312

adhesion to parylene films, we did not need to include an additional metal adhesion layer. This

313

parylene/Au film was then transferred to a PDMS sheet (Elecom’s Liquid Crystal Display

314

Protection Film) to reverse the film order. Next, 3M’s VHB (4910) adhesive elastomer with a

315

500 µm thickness served as a stretchable dielectric layer. This elastomer was firstly pre-

316

stretched, and then the top and bottom electrodes were transferred sequentially. Upon relaxing

317

the elastomer, both the top and bottom electrodes spontaneously formed compressive wrinkled

318

films.

319

Characterization.

320

Shimadzu’s Autograph AG-X stretcher was used to provide an applied loading, and the

321

capacitance measurements were obtained from Agilent’s 4284A Precision LCR meter and

322

Keysight E4980AL Precision LCR meter which was connected to a computer for data

323

acquisition. For the temperature measurements, the strain sensor was placed in near contact to a

324

hotplate (As One’s RSH-1DN) and enclosed in an insulating Al foil environment. The humidity

325

measurements were performed in the same insulating environment as the temperature set-up, but

326

an ultrasonic humidifier purchased from Prismate (model number: NPM-1200) was used to

ACS Paragon Plus Environment

15

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

327

provide humidity. Both temperature and humidity were measured using a temperature/humidity

328

sensor purchased from As One (product number: 1-5459-01).

329

FIGURES

330

ACS Paragon Plus Environment

16

Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

331 332 333 334 335 336 337

Nano Letters

Figure 1 | Ultra-thin 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.

338

339 340 341 342 343 344 345 346

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 1,000 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.

347 348

ACS Paragon Plus Environment

17

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

349 350 351 352 353 354 355 356 357

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) 500 nm strain sensor fitted with both models. (f) 1.4 µm strain sensor fitted with both models. (g) 2.5 µm strain sensor fitted with both models.

358 359 360

ACS Paragon Plus Environment

18

Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

361 362 363 364 365 366 367 368

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) 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) 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˚.

369 370 371

ASSOCIATED CONTENT

372

Supporting Information.

373

Experimental details and characterization. (PDF)

374 375

AUTHOR INFORMATION

376

Corresponding Author

ACS Paragon Plus Environment

19

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

377

[email protected]

378

Author Contributions

379

R.N., N.M., and T.S. conceived the project. R.N. designed and performed the experiments,

380

fabrication, and characterization. Z.J. helped with the measurements. R.N. and M.N. performed

381

the demonstration. R.N. and N.M. analyzed the mechanism. R.N., N.M., Z.J., and T.Y. prepared

382

the manuscript. R.N. wrote the manuscript with comments from all the authors.

383

Notes

384

The authors declare no competing financial interest.

385

ACKNOWLEDGMENT

386

This work was supported by the Japan Science and Technology Agency (JST) program SICORP.

387

R.N. is supported by the Japanese Government (MEXT) Scholarship. Z.J. is supported by the

388

JRA program at RIKEN and the SEUT RA program at the University of Tokyo. We thank Dr. D.

389

Ordinario for his assistance with the manuscript formatting, R. Shidachi for his assistance with

390

the sensor photography, and H. Jin for technical support from the University of Tokyo.

391 392 393 394 395 396 397

REFERENCES (1) Kim, J.; Kim, M.; Lee, M-S.; Kim, K.; Ji, S.; Kim, Y-T.; Park, J.; Na, K.; Bae, K-H.; Kim, H.K.; Bien, F.; Lee, C.Y.; Park, J-U. Nat. Commun. 2017, 8, 14997. (2) Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai, T. Proc. Natl Acad. Sci. USA 2004, 101, 9966-9970.

398

(3) Kim, D-H.; Lu, N.; Ma, R.; Kim, Y-S.; Kim, R-H.; Wang, S.; Wu, J.; Won, S.M.; Tao,

399

H.; Islam, A.; Yu, K.J.; Kim, T-I.; Chowdhury, R.; Ying, M.; Xu, L.; Li, M.; Chung, H-

ACS Paragon Plus Environment

20

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

400

J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y-W.; Omenetto, F.G.; Huang, Y.;

401

Coleman, T.; Rogers, J.A. Science 2011, 333, 838-843.

402 403 404 405 406 407 408 409

(4) Bartlett, N.W; Lyau, V.; Raiford, W.A.; Holland, D.; Gafford, J.B.; Ellis, T.D.; Walsh, C.J. J. Med. Devices 2015, 9, 030918. (5) Someya, T.; Kato, Y.; Sekitani, T.; Iba, S.; Noguchi, Y.; Murase, Y.; Kawaguchi, H.; Sakurai, T. Proc. Natl Acad. Sci. USA 2005, 102, 12321-12325. (6) Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; Izadi-Najafabadi, A.; Futaba, D.N.; Hata, K. Nat. Nanotechnol. 2011, 6, 296-301. (7) Wang, Y.; Wang, L.; Yang, T.; Li, X.; Zang, X.; Zhu, M.; Wang, K.; Wu, D.; Zhu, H. Adv. Funct. Mater. 2014, 24, 4666-4670.

410

(8) Roh, E.; Hwang, B-U.; Kim, D.; Kim, B-Y.; Lee, N-E. ACS Nano 2015, 9, 6252-6261.

411

(9) Wu, X.; Han, Y.; Zhang, X.; Lu, C. ACS Appl. Mater. Interfaces 2016, 8, 9936-9945.

412

(10) Wu, H.; Liu, Q.; Du, W.; Li, C.; Shi, G. ACS Appl. Mater. Interfaces 2018, 10, 3895-

413 414 415 416 417 418 419 420 421

3901. (11) Wu, J.M.; Chen, C.Y; Zhang, Y.; Chen, K.H.; Yang, Y.; Hu, Y.; He, J.H.; Wang, Z.L. ACS Nano 2012, 6, 4369-4374. (12) Kang, D.; Pikhitsa, P.V.; Choi, Y.W.; Lee, C.; Shin, S.S.; Piao, L.; Park, B.; Suh, K-Y.; Kim, T-I.; Choi, M. Nature 2014, 516, 222-226. (13) Cai, Y.; Shen, J.; Ge, G.; Zhang, Y.; Jin, W.; Huang, W.; Shao, J.; Yang, J.; Dong, X. ACS Nano 2018, 12 56-62. (14) Cohen, D.J.; Mitra, D.; Peterson, K.; Maharbiz, M.M. Nano Letters 2012, 12, 18211825.

ACS Paragon Plus Environment

21

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

422 423 424 425

Page 22 of 24

(15) Shin, U-H.; Jeong, D-W.; Park, S-M.; Kim, S-H.; Lee, H.W.; Kim, J-M. Carbon 2014, 80, 396-404. (16) Atalay, A.; Sanchez, V.; Atalay, O.; Vogt, D.M.; Haufe, F.; Wood, R.J.; Walsh, C.J. Adv. Mater. Technol. 2017, 1700136.

426

(17) Neugebauer, C.A. Journal of Applied Physics 1960. 31, 1096-1101.

427

(18) Lacour, S.P.; Wagner, S.; Huang, Z.; Suo, Z. Applied Physics Letters 2003, 82, 2404-

428 429 430

2406. (19) Lacour, S.P; Jones, J.; Wagner, S.; Li, T.; Suo, Z. Proceedings of the IEEE 2005, 93, 1459-1467.

431

(20) Adrega, T.; Lacour, S.P. J. Micromech. Microeng. 2010, 20, 055025.

432

(21) Drack, M.; Graz, I.; Sekitani, T.; Someya, T.; Kaltenbrunner, M.; Bauer, S. Adv. Mater.

433 434 435 436 437 438

2015, 27, 34-40. (22) Kim, Y.; Zhu, J.; Yeom, B.; Di Prima, M.; Su, X.; Kim, J-G.; Yoo, S.J.; Uher, C.; Kotov, N.A. Nature 2013, 500, 59-63. (23) Chun, K-Y.; Oh, Y.; Rho, J.; Ahn, J-H.; Kim, Y-J.; Choi, H.R.; Baik, S. Nat. Nanotechnol. 2010, 5, 853-857. (24) Lipomi, D.J.; Vosgueritchian, M.; Tee, B.C.K.; Hellstrom, S.L.; Lee, J.A.; Fox, C.H.;

439

Bao, Z. Nat. Nanotechnol. 2011, 6, 788-792.

440

(25) Yao, S.; Zhu, Y. Nanoscale 2014, 6, 2345-2352.

441

(26) Atalay, O.; Atalay, A.; Gafford, J.; Wang, H.; Wood, R.; Walsh, C.; Adv. Mater.

442

Technol. 2017, 2, 1700081.

443

(27) Amjadi, K.; Kyung, K.; Park, I.; Sitti, M. Adv. Funct. Mater. 2016, 26, 1678-1698.

444

(28) Gere, J.M. Mechanics of Materials, 6th ed.; Thomson-Brooks/Cole: Pacific Grove, 2004.

ACS Paragon Plus Environment

22

Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

445 446

Nano Letters

(29) Kim, D-H.; Ahn, J-H.; Choi, W.M.; Kim, H-S.; Kim, T-H.; Song, J.; Huang, Y.Y.; Liu, Z.; Lu, C.; Rogers, J.A. Science 2008, 320, 507-511.

447

(30) Khang, D-Y.; Rogers, J.A.; Lee, H.H. Adv. Funct. Mater. 2009, 19, 1526-1536.

448

(31) Brau, F.; Vandeparre, H.; Sabbah, A.; Poulard, C.; Boudaoud, A.; Damman, P. Nat.

449

Phys. 2011, 7, 56-60.

450

(32) Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.;

451

Drack, M.; Schwodiauer, R.; Graz, I.; Bauer-Gogonea, S.; Bauer, S.; Someya, T. Nature

452

2013, 499, 458-463.

453

(33) Kim, D-H.; Song, J.; Choi, W.M.; Kim, H-S.; Kim, R-H.; Liu, Z.; Huang, Y.Y.; Hwang,

454

K-C.; Zhang, Y-W.; Rogers, J.A. Proc. Natl Acad. Sci. USA 2008, 105, 18675-18680.

455

(34) Kim, J.; Lee, M.; Shim, H.J.; Ghaffari, R.; Cho, H.R.; Son, D.; Jung, Y.H.; Soh, M.;

456

Choi, C.; Jung, S.; Chu, K.; Jeon, D.; Lee, S-T.; Kim, J.H.; Choi, S.H.; Hyeon, T.; Kim,

457

D-H. Nat. Commun. 2014, 5, 5747.

458

(35) Hong, S.K.; Yang, S.; Cho, S.J.; Jeon, H.; Lim, G. Sensors 2018, 18, 1171.

459 460 461 462 463 464 465 466 467

ACS Paragon Plus Environment

23

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

468 469 470

Table of Contents Graphic

471 472

ACS Paragon Plus Environment

24