- Supporting Information -

Transcription

1 - Supporting Information - Highly Sensitive Piezocapacitive Sensor for Detecting Static and Dynamic Pressure Using Ion-Gel Thin Films and Conductive Elastomeric Composites Sun Geun Yoon, Byoung Joon Park, and Suk Tai Chang* School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 06974, Republic of Korea stchang@cau.ac.kr S-1

2 Figure S1. (a) Bump profile of sandpaper (grit #120)-molded CPC. Variables h1, h2, and w in graph are left-side height, right-side height, and width of a microscale bump structure, respectively. (b) An optical cross-sectional image of sandpaper (grit #220)-molded neat PDMS. For optical observation, the CPC layer was fabricated with neat PDMS without CNTs. (c) Number average bump size distributions of sandpaper molded-cpc with different grit numbers. The height of each bump is obtained from an average value of h1 and h2 presented in (a). Average height and width displayed in (c) are obtained from surface profiles of the CPC layer measured with profilometer. (d) Sandpaper and sandpaper-molded CPC surface profiles (grit #220) measured with profilometer. S-2

3 Figure S2. Average ground capacitance of I-CPC pressure sensor as a function of grit number of sandpaper molds. Each data point in the graph was obtained from an average of four values from repeated measurements. Figure S3. Relative capacitance as a function of applied pressure for I-CPC pressure sensors with flat CPC layers containing different CNT concentrations. S-3

4 Figure S4. (a) Schematic of a CPC bump structure under compression. Dashed square box shows a change of the bump height from h0 to h0-d by the vertical pressure. (b-d) Photographs of compression test of sandpaper-molded CPC for estimating compressive strain (εcomp) under different CNT concentrations. Each image of applied pressure state was obtained by a digital camera at a distance of 30 cm from the object. Length of H (1 cm) denoted in (c) is used as a criterion for estimating length of h, or the height between plate and pressure tip of the force gauge, in (d). Schematic in (c) depicts the utilized structure for measuring the compressive strain of the CPC layer. (e) Experimental results and their fitting profiles of compressive strain as a function of applied pressure for the CPC layers containing different CNT concentrations. Fitting parameters in each fitted line are presented in Table S1. (f, g) Theoretically calculated contact area between the ion gel and the CPC layer (Acontact) and between the ion gel and CNTs at the CPC layer (ACNT) as a function of applied pressure. S-4

5 Table S1. Fitting equation of compressive strain (εcomp) and fitting parameters in each CNT concentration. Equation εε cccccccc = aa bb cc pp p: applied vertical pressure a, b, and c: fitting parameters CNT content (wt%) a b c R Supporting Information S1. Theoretical calculation of contact area variations between the CPC layer and the ion gel. From Figure S1c, it was confirmed that the shape of bumps in CPC layer is roughly an ovalshaped hemispherical dome. Therefore, we assumed that the irregular bump in CPC layer was a symmetrical oval-shaped dome structure, as shown in Figure S4a. By using this assumption, the contact area variations between CPC layer and ion gel could be estimated. As shown in Figure S4a, the height and diameter (width) of the bump were denoted as h0 and 2R, respectively. After applying pressure, the bump structure was compressed by transferred force from the ion-gel film and the height of the bump was reduced as h0-d. This induced the contact area variation from Acontact, 0 to Acontact, as exhibited in Figure S4a. Therefore: AA cccccccccccccc = ππ RR2 h 0 2 (h 0 2 (h 0 dd) 2 ) (1) In addition, compressive strain (εcomp) of the bump structure could be expressed as follows: εε cccccccc = (h 0 dd) h 0 h 0 (2) By combining equation (1) and (2), Acontact could be rearranged as follows: AA cccccccccccccc = ππrr 2 (1 1 + εε cccccccc 2 ) (3) The compressive strain (εcomp) of the CPC layer could be investigated from analyzing the height S-5

6 (h) of the compressed CPC in each pressure state photographed (Figure S4b, S4c, and S4d), thereby creating compression profiles as a function of pressure under different CNTs concentrations as shown in Figure S4e. Because of the effective compressive modulus of the bump structures on the CPC layer, S1 the compressive strain profiles of each CPC layer showed asymptotic curve shapes. Therefore, the experimentally measured compressive strains at each pressure were fitted with asymptotic curves; parameters of the fitting equation are presented in Table S1. By utilizing this equation, the contact area (Acontact) changes between the ion gel and the CPC layer could be estimated as shown in Figure S4f. The probability of contact formation between ion gel and CNTs of the CPC layer was dependent on the CNT concentrations in the CPC layers. Consequently, from the equation of Acontact, the contact area with CNTs at the interface between ion gel and CPC layer (ACNT) could be estimated as follows: S2,S3 AA CCCCCC = AA cccccccccccccc VV ff 2ππ 2 = VV 2 ff RR 2 4ππ (1 1 + εε cccccccc 2 ) (4) where Vf is the volume fraction of CNTs in the CPC layer. The volume fractions of 0.5, 1, and 1.5 wt% CNT concentrations in the CPC layer were 0.06, 0.114, and 0.162, respectively. From equation (4), ACNT as a function of pressure was introduced, as shown in Figure S4g. The area profiles presented in Figure S4f and S4g were calculated based on one oval-shaped hemispherical bump structure. The average height (h0) and width (2R) of grit #220 in Figure S1c were applied to calculate Acontact and ACNT in Figure S4f and S4g. S-6

7 Figure S5. Capacitance changes as a function of applied pressure under different IL concentrations. Figure S6. Typical time-dependent capacitance changes of I-CPC pressure sensors with different IL concentrations under the applied pressure of 2 kpa. The CPC layer of the pressure sensors was prepared with grit #220 sandpaper mold and 1 wt% CNTs. S-7

8 Table S2. Comparison of active materials, structure, minimum detection threshold, operating voltage, and pressure sensitivity of capacitive-type pressure sensors in the literature. Active materials Structure Minimum detection Operating voltage [V] Sensitivity [kpa 1 ] (Range) Reference Conductive textile and fluorosilicone Parallel plate capacitor with air gap 10 mg N/A 0.91 (<0.5 kpa) 0.53 (0.5-2 kpa) [16] CNT microyarn Fiber-array capacitor with ecoflex dielectric 0.38 Pa N/A (<0.1 kpa) (>10 kpa) [17] Fe-Mg and Poly(glycerol sebacate) Parallel plate capacitor with regular micro-array dielectric 5 mg N/A 0.76 (<2 kpa) 0.11 (2 10 kpa) [18] Au and PS bead Ecoflex and CNT/Ecoflex nanocompos ite ITO and PDMS Parallel plate capacitor with micropatterned electrode and PS bead dielectric Parallel plate capacitor with porous ecoflex dielectric Parallel plate capacitor with porous PDMS dielectric 17.5 Pa [19] 0.16 Pa N/A (<5 kpa) (>30 kpa) [20] <2.42 Pa (<1 kp) [21] ITO/PET and PDMS Parallel plate capacitor with porous PDMS dielectric 1 Pa ( kpa) 0.01 ( kpa) [22] 9.55 (0 0.2 kpa) Grit # (0.2 2 kpa) Ion gel and CNT/PDMS nanocompo sites Parallel plate capacitor with ion gel electrodes and microbumped dielectric <5 Pa Grit # (2 8 kpa) 4.07 (0 0.2 kpa) 1.08 (0.2 2 kpa) This work 0.45 (2 8 kpa) S-8

9 Table S3. Comparison of minimum detection threshold, operating voltage, and pressure sensitivity of piezoresistive- and transistor-type pressure sensors in the literature. Active materials Transduction mechanism Minimu m detection Operating voltage [V] Sensitivity [kpa 1 ] (Range) Reference Graphene, Polyurethane Piezoresistive 9 Pa (<2 kpa) 0.03 (2-10 kpa) [5] CNT/PDMS nanocomposite Piezoresistive 0.2 Pa (<0.5 kpa) [6] Polypyrrole Piezoresistive 1 Pa N/A 133 (< 30 Pa) [7] CNT/PDMS reverse micelle structure Piezoresistive 0.25 kpa N/A N/A [8] PU-dispersed PEDOT:PSS 1 Piezoresistive N/A ( Pa) 4.88 ( kpa) [9] Gold-deposited PDMS micro-bump Piezoresistive N/A (0-70 Pa) 1.38 ( kpa) [10] Carbon black-silicone rubber nanocomposite Piezoresistive N/A N/A 13.8 ( kpa) [11] Graphene film Piezoresistive 9 Pa (9-560 Pa) [12] Gold-deposited PDMS micro-pillar and polyaniline Piezoresistive <15 Pa 1V 2.0 ( kpa) 0.87 ( kpa) [13] Ge/Si NW and pressure sensitive rubber Transistor N/A 5 (V GS ) 11.5 µs kpa -1 [24] Pentacene and P(VDF-TrEE) 2 Transistor N/A 5 (V GS ) N/A [25] PiI2T-Si semiconductor and PDMS microstructure array 3 Transistor N/A 200 (VDS and VGS) 8.2 (<8 kpa) [26] S-9

10 Graphene, ion gel Transistor N/A 2 (V GS ) 0.12 (< 40 kpa) [27] Organic semiconductor and suspended gate Transistor <0.5 Pa 60 (V DS and V GS ) 192 (0-5 kpa) [28] 1 PU: Polyurethane, PEDOT:PSS: Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate 2 P(VDF-TrFE): Poly(vinylidenefluoride-co-trifluoroethylene) 3 Pil2T-Si: Polyisoindigobithiophene-siloxane Figure S7. Schematic of experimental setup for lateral pressure movement detection. Samples A and B shown in the image (dashed square) correspond to samples shown in Figure 5c and 5g, respectively. Pressure-sensing area is cm 2. The pressure tip moves only along the middle line of the sensing area. Applied pressure is modulated by counterweights above the pressure tip. To ensure constant force application, weight exerted on the sensor was monitored during the lateral pressure movement. S-10

11 Figure S8. Capacitance changes as a function of pressure position under different moving distances of the pressure load. S-11

12 Figure S9. (a) Schematic of pressure movement monitoring with a stacked sensor structure. The lateral pressure movement is performed with the experimental setup shown in Figure S7. The top-positioned sensor and the bottom-positioned sensor are I-CPC pressure sensors with asymmetric and symmetric configurations, as shown in Figure 1b and 1c, respectively. Channels 1 and 2 are connected to the top- and bottom-positioned sensors, respectively. (b) Capacitance changes and the load positions as a function of time for reciprocal movements between positions 1 and 2 with 15 g counterweight. (c) Capacitance changes and the load positions as a function of time for reciprocal movements between positions 1 and 3 with 35 g counterweight. Moving speed of the pressure in both tests is mm s -1. S-12

13 Figure S10. (a) Capacitance change of I-CPC pressure sensor as a function of time for detecting finger positions. (b) Photographs of two-channel-connected I-CPC pressure sensor and corresponding finger position shown in the first profile (dashed square) of (a). S-13

14 References [S1] Tee, B. C. K.; Chortos, A.; Dunn, R. R.; Schwartz, G.; Eason, E.; Bao, Z. Tunable Flexible Pressure Sensors Using Microstructured Elastomer Geometries for Intuitive Electronics. Adv. Funct. Mater. 2014, 24, [S2] Park, J.; Lee, Y.; Hong, J.; Ha, M.; Jung, Y.-D.; Lim, H.; Kim, S. Y.; Ko, H. Giant Tunneling Piezoresistance of Composite Elastomers with Interlocked Microdome Arrays for Ultrasensitive and Multimodal Electronic Skins. ACS Nano 2014, 8, [S3] Pan, N. Analyrical Characterization of the Anisotropy and Local Heterogeneity of Short Fiber Composites: Fiber Fraction as a Variable. J. Compos. Mater. 1994, 28, S-14