Supporting Information

Transcription

1 Supporting Information Fully Stretchable Optoelectronic Sensors Based on Colloidal Quantum Dots for Sensing Photoplethysmographic Signals Tae-Ho Kim*, Chang-Seok Lee, Sangwon Kim, Jaehyun Hur, Sangmin Lee, Keun Wook Shin, Young-Zoon Yoon, Moon Kee Choi, Jiwoong Yang, Dae-Hyeong Kim, Taeghwan Hyeon, Seongjun Park*, Sungwoo Hwang* 1

2 1. Supplementary Note 1.1. Theoretical Calculation of the Amplitude as a Function of Strain Assuming that a wrinkled substrate follows the sine curve (A(ε)sin{α(ε)t}), the amplitude of curve (A(ε)) at a given strain (ε) can be calculated by the following equation for a fixed arc length (L) of one period in a sine curve: r(t) = (t, A(ε)sin {α(ε)t}) (1) r (t) = (1, A(ε)α(ε)cos {α(ε)t}) (2) λ(ε) 0 λ(ε) 0 L = r (t) r (t)dt = (1 + A(ε) 2 α(ε) 2 cos 2 {α(ε)t})dt (3) λ(ε) = λ o (1 + ε) (4) α(ε) = 2π λ(ε) (5) where t, A(ε), α(ε), L, λ(ε), and λ o denote the displacement of the horizontal axis with respect to the sine curve, amplitude at a given strain, prefactor for periodicity, fixed arc length of one period in a sine curve, periodicity at a given strain, and periodicity at no applied strain, respectively. For a given strain value (ε), because λ o is known (= 185 μm), λ(ε) can be calculated from Eq.(4), from which α can be calculated from Eq.(5). Because L has a fixed value (= 307 μm, measured wavelength), A(ε) can be calculated by numerically solving Eq.(3). For example, at strain (ε) of 10%, λ(ε) = λ o (1+ε) = μm, and α(ε) = 2π/ λ(ε) = μm -1. Then, from Eq.(3), The value of A(ε) is obtained as 58 μm = (1 + A 2 (0.031μm 1 ) 2 cos 2 (0.031μm 1 )t dt 0 2

3 A series of these calculations gives the theoretical values of the amplitude at given strain values, as shown in Fig. S Theoretical Calculation of the Increased Gap between QDs When Bent In the wavy structure of the device formed by a 70% prestrain, assuming that a wrinkled substrate follows the sine curve (A(ε)sin{α(ε)t}) and the neutral mechanical plane is located in the center plane of the QD film (40-nm thick), the increased length (ΔL) of the outer plane of the QD film can be calculated by the following equation for a fixed arc length of half period ((L+ΔL)/2) in the sine curve. r(t) = (t, A(ε)sin {α(ε)t}) (1) r (t) = (1, A(ε)α(ε)cos {α(ε)t}) (2) λ(ε) 0 λ(ε) 0 L + ΔL = r (t) r (t)dt = (1 + A(ε) 2 α(ε) 2 cos 2 {α(ε)t})dt (6) λ(ε) = λ o (1 + ε) (4) α(ε) = 2π λ(ε) (5) where t, A(ε), α(ε), (L+ΔL)/2, λ(ε), and λ o denote the displacement to the horizontal axis with respect to the sine curve, amplitude at a given strain, prefactor for periodicity, fixed arc length of a half period in the sine curve, periodicity at a given strain, and periodicity at no applied strain, respectively. For a 0% strain, because λ o is known (= μm), λ(ε) is equal to λ o from Eq.(4), from which α can be calculated from Eq.(5). Because L is fixed value (= 307 μm, measured wavelength) and A+ΔA is μm (measured amplitude + 20 nm), ΔL can be calculated by numerically solving Eq.(6). 3

4 From Eq.(6), the value of ΔL was obtained as 183 nm. Therefore, the outer plane of the QD film (half period in the sine curve) increased by 91.5 nm. The center-to-center distance between QDs in the in-plane direction was measured as 6.79 nm from the Grazing Incidence Small Angle X-ray Scattering results S1. The number of QDs along the horizontal axis was estimated to be (307 μm/2)/6.79 nm = Therefore, the increased gap between QDs in the outer plane of the sinusoidal QD film for the half period was calculated to be 91.5 nm/ = nm. If the neutral mechanical plane is located on the surface of the PEN substrate, QD film is placed 75 ~115 nm away from the PEN substrate in the out-of-plane direction and the gap between QDs in the outer plane would increase by nm by the formula. These values are extremely small compared with the average length of organic ligands on a QD surface (~1 nm), i.e nm or nm << 1 nm. Therefore, when bent to a bending radius of μm (185 μm/4), the effect of mechanical deformation on the electrical and optoelectronic properties of the QD devices is negligible Characterization of the Transferred Graphene: Raman Spectrum Analysis. To investigate the quality of graphene and to identify a single layer of graphene, the Raman spectrum of the graphene transferred onto SiO 2 was analysed. As shown in Fig. S2, transferred graphene has a high quality because D peak around 1350 cm -1 did not exist, and the G and 2D peaks, located at 1586 and 2684 cm -1, showed sharp and narrow spectral bandwidth with fullwidth at half maximum of 15.2 and 35 cm -1, respectively. The 2D/G value was 2.7, which means that the transferred graphene was an SLG. From the Raman spectrum of the graphene on PEN, the existence of a 2D peak, located at around 2650 cm -1, is a clear evidence of the graphene on 4

5 PEN (inset in Fig. 2b). Compared with the 2D peak position of the Raman spectrum measured from graphene/sio 2, the 2D peak of graphene/pen was shifted by 34 cm -1 due to the interaction with a substrate S2. The G peak of graphene/pen overlapped with the peak of PEN at 1580 cm -1. However, the peak intensity (1580 cm -1 ) of graphene/pen was two times higher than that of bare PEN due to the effect of the G peak of graphene. Therefore, from the Raman spectrum analysis, a high-quality SLG was successfully transferred on the receiver substrates without defects. 2. Supplementary Methods 2.1. Synthesis of Red-Emitting CdSe/CdS/ZnS QDs. The red-light-emitting CdSe/CdS/ZnS core/shell QDs were prepared as follows: To synthesize 0.5 g of CdSe/CdS/ZnS QDs, 1.6 mm of CdO powder (0.206 g, Sigma-Aldrich, %) and 6.4 mm of oleic acid (OA, 1.8 g, Sigma-Aldrich, 95%) were mixed in 40 ml of trioctylamine (TOA, Sigma-Aldrich, 90%). The mixed solution was degassed and heated to 150 C while being rapidly stirred. Then, the temperature was further increased to 300 C under N 2 flow. At 300 C, 0.2 ml of 2.0 M Se (Alfa) in trioctylphosphine (TOP, Strem, 97%) was quickly injected into a Cd-containing reaction mixture. After 90 s, 1.2 mm of n-octanethiol in TOA (210 μl in 6 ml) was injected at a rate of 1 ml min 1 using a syringe pump, and the reaction proceeded for 40 min. A 0.25-M Zn precursor solution was prepared by dissolving 0.92 g of zinc acetate and 2.8 g of OA in 20 ml of TOA at 200 C under N 2 atmosphere; A 16 ml volume of Zn OA solution (at approximately 100 C) was injected into the Cd-containing reaction medium at a rate of 2 ml min 1. Then, 6.4 mm of n-octanethiol in TOA (1.12 ml in 6 ml) were injected at a rate of 1 ml min 1 using a syringe pump. The total reaction time was 2 h. 5

6 The product was purified by repeating the precipitation/redispersion processes to remove the excess shell reagents Synthesis of Green-Emitting CdSe/ZnS QDs The green-light-emitting CdSe/ZnS core/shell QDs were prepared from the reaction between metal-oleate and TOPS and TOPSe. For the preparation of Cd-(oleate) 2 and Zn-(oleate) 2 solution, 0.2 mmol of CdO and 4.0 mmol of Zn(OAc) 2 were added into the reaction solution composed of 6.0 ml of OA and 15.0 ml of 1-ODE in a glove box. The mixture was degassed under vacuum for 2 h at 130 C and then heated to 300 C under Ar atmosphere. At this elevated temperature, 0.2 mmol of TOPSe solution (1 M) was rapidly injected into the solution, followed by the injection of 4 mmol of TOPS solution (1 M) to form ZnS shells on the cores. The reaction mixture was maintained at 300 C for 15 min. The product was purified by repeating the precipitation/redispersion processes to remove the excess shell reagents Synthesis of Blue-Emitting CdSe/ZnS QDs The blue-light-emitting CdSe/ZnS core/shell QDs were prepared using the reaction between metal-oleate and elements S and Se. To prevent rapid growth of QDs and to obtain blue-lightemitting QDs, elements S and Se were employed as anion precursors instead of general precursors such as the TOPSe and TOPS solution. For the preparation of Cd-(oleate) 2 and Zn- (oleate) 2 solution, 1.0 mmol of CdO and 9.0 mmol of Zn(OAc) 2 were added into the reaction solution composed of 8.0 ml of OA and 15.0 ml of 1-ODE in a glove box. The mixture was degassed under vacuum for 2 h at 130 C and then heated to 300 C under Ar atmosphere. At 300 C, 1.8 mmol of S and 0.2 mmol of Se dissolved in 3 ml of 1-ODE were injected into the 6

7 Cd-(oleate) 2 and Zn-(oleate) 2 solution. After 10 min of reaction, 8.0 mmol of TBPS solution (2 M) was slowly injected into the reaction mixture at 300 C without any purification. The reaction mixture was held at the same temperature for 50 min to form thick ZnS shells on the cores. The product was purified by repeating the precipitation/redispersion processes to remove the excess shell reagents Transfer Printing of the LEDs and PDs For the transfer of the device onto the prestrained elastomer substrate, an ultrathin LED or PD on a PDMS/glass support was attached to a thermal release tape by applying a pressure of ~200 kpa using a roller. The device with a thermal release tape was heated in a vacuum oven up to 60 C for 1 h, loading three ceramic wafers (total of g) on top of the thermal release tape for pressing. After the device was detached from the PDMS/glass support, the thermal release tape was laminated with prestrained elastomer substrate which was exposed to UV-ozone for 8 min in advance for strong adhesion. The thermal release tape was slowly removed from the device after mild heating in a N 2 glove box at 120 C for 5 min for LEDs and at 110 C for 2 min for PDs. 7

8 PEN 300 nm Graphene PDMS/glass support 1 cm Figure S1. Photograph of SLG transferred on a PEN substrate attached onto a PDMS/glass support. The inset shows a SEM image of the transferred graphene. 8

9 Single-layer graphene 2D Intensity (a.u.) G Raman shift (cm -1 ) Figure S2. Raman spectrum of SLG transferred onto SiO 2 /Si wafer. 9

10 (a) Current density (ma cm -2 ) Flat Folding (bending radius ~250 μm) Voltage (V) (b) 1400 Brightness (cd/m 2 ) unfolding folding unfolding second-folding Figure S3. Electrical and luminous properties of the green-emissive LED. (a) Current voltage characteristics in a flat and folded state. (b) Brightness of the LED at folding/unfolding cycles. 10

11 70 60 Calculation Amplitude (μm) Strain (%) Strain (%) Wavelength, λ (μm) Ampllitude, A (μm) Figure S4. Calculated amplitude values of the wavy LED as a function of the strain applied to an elastomeric substrate. 11

12 (a) Amplitude, arb.units (b) V 7.6 V 8.4 V 9.2 V 10.0 V Time (s) Brightness Bias (V) (cd/m 2 ) Figure S5. (a) PPG signal pulse wave with LEDs biased at various voltages from 6.8 to 10.0 V. (b) Brightness of the LED corresponding to the bias. 12

13 (a) (b) 200 μm (c) Diagonal stretching (d) Crumpling Stretched (e) Double Folding (f) 180 Twisting Figure S6. (a) Photograph of biaxially stretchable LEDs attached on the surface of a balloon. (b) Photograph of the LEDs during operation at 9 V. The inset shows a SEM image of a biaxially stretchable LED. (c f) Photographs of the stretchable LEDs with various deformations such as diagonal stretching, crumpling, double folding, and 180 twisting. 13

14 Figure S7. Photographs of a bent QD-LED that uses an encapsulating layer of PDMS (front side) and Ecoflex (back side), operating at 8.4 V (scale bar, 1 cm). 14

15 Supplementary References (S1) Kim, T.-H.; Cho, K.-S.; Lee, E. K.; Lee, S. J.; Chae, J.; Kim, J. W.; Kim, D. H.; Kwon, J.-Y.; Amaratunga, G.; Lee, S. Y.; Choi, B.-L.; Kuk, Y.; Kim, J. M.; Kim, K. Full-colour quantum dot displays fabricated by transfer printing. Nat. Photon. 2011, 5, (S2) Bukowska, H.; Meinerzhagen, F.; Akcöltekin, S.; Ochedowski, O.; Neubert, M.; Buck, V.; Schleberger, M. Raman spectra of graphene exfoliated on insulating crystalline substrates. New J. Phys. 2011, 13,