Supporting Information

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1 Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, Supporting Information for Adv. Mater., DOI: /adma Stretchable Active Matrix Temperature Sensor Array of Polyaniline Nanofibers for Electronic Skin Soo Yeong Hong, Yong Hui Lee, Heun Park, Sang Woo Jin, Yu Ra Jeong, Junyeong Yun, Ilhwan You, Goangseup Zi, and Jeong Sook Ha*

2 Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, Supporting Information Stretchable Active Matrix Temperature Sensor Array of Polyaniline Nanofibers for Electronic Skin Soo Yeong Hong, Yong Hui Lee, Heun Park, Sang Woo Jin, Yu Ra Jeong, Junyeong Yun, Ilhwan You, Goangseup Zi, and Jeong Sook Ha* 1

3 Table S1. Surface and mechanical properties of PDMS, Ecoflex, and PET film. 2

4 Figure S1. (a) Schematic of the fabrication process for the stretchable AM temperature sensor array with an assembly of prepared layers. Red and blue dotted lines indicate via holes for liquid metal interconnection (red: drain line, blue: source line). SEM images of (b) SWCNTs and (c) polyaniline nanofibers. The inset shows the cross-sectional SEM image of the polyaniline film. 3

5 Ecoflex was spin-coated to form a bonding layer between the layers. The assembled substrate with Layer 2, Layer 3, and Layer 4 was completely annealed in an oven. To make liquid metal interconnections for source and drain lines of the AM, via holes were fabricated by piercing the assembled layer with a puncher. Layer 1 and the assembled substrate were attached together using the bonding layer. The entire substrate was cured completely in the oven. Then, the liquid metal Galinstan was injected into the embedded microchannels using a commercial syringe. Finally, the silver nanowire (Ag NW) sticker [1] was attached to form a better contact between the temperature sensor and the liquid metal interconnections as well as to protect the sensors from an external impact. Figure S1b and c show SEM images taken from the SWCNT channels of the TFT and the polyaniline nanofibers channels of the temperature sensor, respectively. The thickness of the polyaniline film is estimated to be 23 μm over the large area from the cross-sectional SEM image of Figure S1c. Fabrication of the stretchable substrate: Our stretchable substrate with a multilayered structure was fabricated by assembling four component layers, i.e., Layer 1, Layer 2, Layer 3, and Layer 4. The Ecoflex (Ecoflex 0030, Smooth-On) was poured in a steel mold with Fe wires (dia.~300 μm) protruding from the bottom surface, and then, it was cured in a dry oven at 65 C for 20 min. After detaching the 500 μm thick cured Ecoflex layer from the mold, microchannels were opened at the bottom of the layer (Layer 2 and Layer 3). 300 μm thick Layer 1 and Layer 4 were prepared by spin coating the Ecoflex at 600 rpm for 20 min. Next, on the unopened microchannel surface of Layer 2, Ecoflex was spin-coated at 2000 rpm for 20 s, and it was cured at 60 C for 1 min to form a bonding layer between Layer 2 and Layer 3. The assembled film of Layer 2 and Layer 3 was annealed in a dry oven. To form the source 4

6 line interconnection, via holes were fabricated by piercing the assembled film with a handmade puncher (hole diameter = 500 μm). After attaching it to Layer 4 using the bonding layer, the whole assembly was pierced for an electrical connection using the puncher. Above Layer 1 and Layer 4, 25 SWCNT TFTs and polyaniline temperature sensors on PET films were attached using the uncured Ecoflex, respectively. After assembling those 4 layers, the 1.6 mm thick stretchable substrate with embedded microchannels was successfully fabricated. The liquid metal Galinstan (Rotometals) was injected into the embedded microchannels using a commercial syringe. Finally, the Ag nanowire stickers were attached on the contacting area between the temperature sensors and liquid metal interconnections. Growth of SWCNTs: Randomly networked SWCNTs were grown on a SiO 2 substrate using ferritin catalysts. After drop coating of the diluted ferritin catalysts in deionized water with a volumetric mixing ratio of 1:2000 onto the SiO 2 substrate using a commercial syringe, the ferritin layer was covered with methanol for 1 min. Then, it was dried by blowing the N 2 gas and annealed at 900 C to form Fe 2 O 3 nanoparticles. Inside the CVD tube, the temperature was increased to 925 C under the continuous flow of 300 sccm H 2. Then, methane at 30 sccm, H 2 at 60 sccm, and Ar at 60 sccm were mixed with an ethanol bubbler, and the SWCNTs were grown at 925 C for 5 min at 1 atm. After the growth of the random networked SWCNTs, the CVD furnace was rapidly cooled down to an ambient condition under Ar flow of 80 sccm. Fabrication of the SWCNT TFTs: To form a bottom gate, Au/Ti (50/5 nm) was deposited on the PET film (100 μm) by e-beam evaporation. For the gate dielectric layer, poly(pyromelliticdianhydride-co-4,4 -oxydianiline) and amic acid (431176, electronic grade, Aldrich) were spin-coated onto the PET film at 500 rpm for 5 s and 4000 rpm for 1 min. The 5

7 randomly networked SWCNTs were transferred using the thermal release tape transfer method. After the CVD-grown SWCNTs were deposited, Au/Ti (50/5 nm) source-drain contacts were formed by e-beam evaporation. The SWCNT film was exposed to oxygen plasma of 20 sccm for 20 s at 150 W to confine the channel region by removing residual SWCNTs. Growth of Polyaniline nanofibers: Au (100 nm) film was deposited on the PET film (100 μm, 2 2 cm 2 ) by e-beam evaporation. The polyaniline film was grown on the deposited Au film by 300 cycles of electrochemical polymerization at a scan rate of 100 mv s -1 between 0 and 0.85 V. The polyaniline nanofibers could be grown with the working electrode (Au deposited PET film), counter electrode (Pt electrode), saturated calomel reference electrode (Ag/AgCl), and an aniline monomer. The 0.1 M aniline monomer was used with 0.5 M sulfuric acid (H 2 SO 4 ) (95.0%, Samchun Chemical). After the deposition of the polyaniline nanofiber film, the whole sample was dipped in a gold etchant (Gold Etchant TFA, Transene Co.) for 3 h and was carefully rinsed with deionized water (DI water) to remove the residual etchant. The adhesion of polyaniline nanofibers film on the PET film was confirmed in SEM images: No noticeable change in both the plane-view and cross-sectional SEM images after etching of Au is shown in Figure S2. Fabrication of the silver nanowire sticker: The PDMS film was spin-coated on a SiO 2 substrate at 1500 rpm for 30 s and half-cured at 65 C for 10 min. After cutting the PDMS film into 1 1 cm 2, the silver nanowire solution (1 wt% diluted in water, 65 nm average diameter, 10 μm average length, Ditto technologies) was dropped onto the half-cured PDMS film and dried at room temperature. Thus, the Ag NW sticker can be attached between the liquid metal interconnection and active devices for stable electrical contacts. 6

8 Characterization: The morphology of the SWCNTs and polyaniline nanofibers was obtained using SEM (Hitachi S-4800). The electrical performance of the SWNCT TFTs and temperature sensors was measured using HP 4140B under the ambient condition. Raman spectra were taken from the polyaniline nanofiber film under the wavelength of 532 nm and power of 0.5 mw (Horiba LabRam Aramis IR2). Optical images of the AM LED array and temperature sensors were obtained using a Canon EOS 7D camera. The infrared radiation thermometer (GM700, BENETECH) was used for measuring the temperature. 7

9 Figure S2. SEM images of polyaniline nanofibers (a) before and (b) after Au etching. The inset shows the cross-sectional SEM image of the polyaniline nanofibers film on the PET film. 8

10 Figure S3. Optical images of Ag nanowire sticker (a) before and (b) after peeling test with 3M scotch tape, respectively. (c) Optical image of 3M tape after peeling test. 9

11 Figure S4. (a) Transfer curve of a representative SWCNT TFT in the backplane at a drain voltage of -10 V. (b) Leakage current through the spin-coated 400 nm thick polyimide (PI) dielectric film with variation of the gate voltage (V G ). (c f) Statistical histograms of 25 TFTs showing distribution of (c) log of the current on/off ratio, (d) transconductance, (e) field effect mobility, and (f) threshold voltage at the drain voltage of -10 V. Channel length and width of the devices are 20 μm and 2 mm, respectively. The transfer curve (I DS V GS ) of a representative TFT measured at drain voltage of -10 V is presented in Figure S4a. It indicates the p-type characteristics but with a small trace of the off-state current caused by the metallic and small band gap semiconducting nanotubes in the 10

12 network that are not fully depleted by the gate bias. [5] Figure S4b clearly shows a good insulating property of the spin-coated 400 nm thick PI dielectric layer with a small leakage current of na at the gate voltage (V G ) between -20 and +20 V. The statistical histograms for the 25 TFTs are shown in Figure S4c f. The average value of the on/off current ratio is ~10 2. Moreover, the average transconductance (g m /W) is estimated to be 0.50 ± 0.3 μs/mm and the field effect mobility is 6.7 ± 4 cm 2 V -1 s -1, respectively. The average value for the threshold voltage is calculated to be -1.4 ± 0.3 V. The negative threshold voltage, indicating the enhancement mode behavior of the TFTs, is important for the operation of the backplane. [6] Here, the field effect mobility was calculated using the following equation for the parallel plate model. [7] I DS V G = g m = μc i (L W /L C )V DS (S1), where g m is the transconductance, C i the capacitance of the gate dielectric, L w and L c are the channel width and channel length, respectively. C i could be estimated using the dielectric constant and the thickness of the PI film that are [8] and 400 nm, respectively. 11

13 Figure S5. Variation in the normalized change of the drain current of the SWCNT TFT at temperatures between 25 and 45 C. Here, I DS and I DS0 are the drain currents at temperatures between 25 and 45 C, and at 25 C, respectively. 12

14 Figure S6. Schematic of stretchable AM temperature sensor array with embedded Galinstan interconnections (a) before and (b) after biaxial stretching of 30%. The total area and the area of a single active device are marked as pink and green dotted squares, respectively. Here, the total active device area can be calculated by 25 green dotted square. The table lists the fill factors of our stretchable device before and after the biaxial stretching by 30%. The fill factor of our stretchable substrate without any applied strain is 85.7%, and that under the 30% biaxial strain is 62.7%, which is much higher than that using the long serpentine interconnections previously reported by our group. [9]. 13

15 Figure S7. (a) Spatial distribution of the current on/off ratio obtained from the 5 5 SWCNT TFTs. (b) Transfer curves of a representative TFT (black dot) and those under bending with a bending radius of 14 mm (red triangle) and 30% biaxial stretching (blue diamond). (c) Spatial distribution of the normalized drain current of the 5 5 SWCNT TFTs under the applied strain of 30%. The spatial distribution of the current on/off ratio for the 5 5 SWCNT TFT AM backplane on the stretchable substrate is plotted in Figure S7a. The transfer curves of a representative TFT on the stretchable substrate measured with deformations of bending with a bending radius of 14 mm and biaxial stretching by 30% show no noticeable degradation in Figure S7b. This is attributed to the fabrication of the TFT array on our stretchable substrate. Figure S7c exhibits the normalized change of current, ΔI DS /I DS0, for the 5 5 array of the SWCNT TFTs upon stretching by 30%. Here, ΔI DS = I DS I DS0, where I DS0 and I DS are the currents before and after the application of strain, respectively. There appeared less than 4% change of drain current with the biaxial stretching of 30%. 14

16 Figure S8. (a) Optical image of AM μ-led array under biaxial stretching by 30%. (b) Positions of μ-leds for measuring the brightness under repeated deformation of bending and biaxial stretching by 30%. Change of normalized brightness (L/L 0 ) upon repeated cycles of (c) bending and (d) biaxial stretching by 30%. Here, L 0 and L are the brightness of μ-led array before and after deformation, respectively. P1, P2, and P3 correspond to yellow, green, and blue μ-leds, respectively. 15

17 Figure S9. Raman spectra of electrochemically polymerized polyaniline nanofiber film at 25 C (blue line) and the after heating cycle of 25 C 45 C 25 C (red line). The Raman peaks at 1160, 1480, 1558, and 1590 cm -1 are attributed to the C-H bending vibrations of quinoid rings, N-H bending vibrations, C-C stretching of the phenyl ring, and the C=C bending vibration of benzenoid rings, respectively. [10, 11] The Raman peaks of the polyaniline nanofibers exhibit almost no difference at 25 and 45 C. 16

18 Figures S10. (a) I-V curves of the temperature sensor at 0% strain in the temperature range of C. (b) Repeated cycles of temperature measurement between 25 and 45 C. (c) Response/recovery curve of the temperature sensor between 25 and 45 C. (d) Normalized current change of the sensor vs. inverse temperature ranging from 15 to 45 C. Here, ΔI = I-I 0, where I 0 and I are the currents at 25 C and at temperatures between 15 and 45 C, respectively. Figure S10a shows the I-V curves of temperature sensor at temperatures ranging from 15 to 45 C. At a fixed voltage of 1V, the current of the sensor increased from 1.15 μa at 15 C to 1.46 μa at 45 C, which is a clear indication of a negative temperature coefficient (NTC). [12] 17

19 Figure S10b indicates the stable performance of our sensor over repeated cycles of heating from 25 to 45 C, where the measured resistances correspond to those observed in Figure 4a. The response and recovery times are estimated to be 1.8 and 3.0 s, respectively (Figure S10c), comparable to those observed in the sensor of conducting polymer/cnt mixture. [13] It is known that the responsive behavior depends on many influencing factors such as measurement procedures, geometrical shape, cyclic repeatability, ambient temperature, and cooling rate of thermistors. [14] Here, Figure S10d shows that the normalized current change is defined as ΔI/I 0 = (I-I 0 )/I 0, where I and I 0 are the currents at temperature T from 288 (15 C) to 318 K (45 C) and at room temperature of 298 K, respectively. The current change exhibits linear dependence on inverse temperature, and the current sensitivity (S) is estimated to be 1.1 %/K (R 2 = 0.992) via the linear least squares fitting of the data. The mechanism of current change is most likely a standard temperature-resistance (current) dependence of the materials based on the temperature coefficient of resistance [12, 15-16] (current). 18

20 Figure S11. Normalized resistance change of the temperature sensor under various conditions: without encapsulation (dotted lines) and with encapsulation (solid lines). 19

21 Figure S12. ΔR/R 0 vs. temperature under biaxial strain up to 50%. 20

22 Figure S13. Optical image of measuring the temperature of the finger using an infrared radiation thermometer. 21

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