Fully Stretchable Optoelectronic Sensors Based on Colloidal Quantum

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,*,‡ and Sungwoo Hwang*,‡,# †

Inorganic Material Laboratory, ‡Device Laboratory, and #Device & System Research Center, Samsung Advanced Institute of Technology, Suwon, Gyeonggi-do 16678, Republic of Korea § Department of Chemical and Biological Engineering, Gachon University, Seongnam, Gyeonggi-do 13120, Republic of Korea ∥ Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea ⊥ School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea S Supporting Information *

ABSTRACT: Flexible and stretchable optoelectronic devices can be potentially applied in displays, biosensors, biomedicine, robotics, and energy generation. The use of nanomaterials with superior optical properties such as quantum dots (QDs) is important in the realization of wearable displays and biomedical devices, but specific structural design as well as selection of materials should preferentially accompany this technology to realize stretchable forms of these devices. Here, we report stretchable optoelectronic sensors manufactured using colloidal QDs and integrated with elastomeric substrates, whose optoelectronic properties are stable under various deformations. A graphene electrode is adopted to ensure extreme bendability of the devices. Ultrathin QD lightemitting diodes and QD photodetectors are transfer-printed onto a prestrained elastomeric layout to form wavy configurations with regular patterns. The layout is mechanically stretchable until the structure is converted to a flat configuration. The emissive and active area itself can be stretched or compressed by buckled structures, which are applicable to wearable electronic devices. We demonstrate that these stretchable optoelectronic sensors can be used for continuous monitoring of blood waves via photoplethysmography signal recording. These and related systems create important and unconventional opportunities for stretchable and foldable optoelectronic devices with health-monitoring capability and, thus, meet the demand for wearable and body-integrated electronics. KEYWORDS: quantum dot, light-emitting diode, photodetector, stretchable electronics, photoplethysmographic sensor

S

optogenetics, and drug delivery, which requires exact specific wavelengths, because of their narrow spectral emission bandwidths as well as their tunable optical and electronic properties by precise size control.13−19 In order to integrate these great advantages of QDs into stretchable optoelectronics, the elements that compose the LEDs and PDs should be deformable. Although the substrates and electrodes can be stretched, when the optoelectronic device integrated with QDs as the active material elongates, the

tretchable electronics have attracted considerable interest because of their wide range of applications such as wearable and skin-mounted electronics. Since they are deformable, they can be attached to flexible, curved, and stretchable surfaces or parts of the body, gloves, and textiles.1−7 Stretchable light-emitting diodes (LEDs) and photodetectors (PDs) as components of stretchable electronic devices are essential in wearable displays and/or wearable healthcare devices.8−10 Stretchable optoelectronic devices integrated onto a soft and curvilinear human body have great potential in performing diagnostic, therapeutic, or surgical functions.10−12 As self-emissive materials for LEDs, quantum dots (QDs) are suitable for biomedicine, including blood oximetry, © 2017 American Chemical Society

Received: March 19, 2017 Accepted: May 23, 2017 Published: May 23, 2017 5992

DOI: 10.1021/acsnano.7b01894 ACS Nano 2017, 11, 5992−6003

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Figure 1. Schematic illustration of the QD-LED structure and the fabrication process to create stretchable devices with a wavy configuration by transfer-printing. (a) The QD-LED was fabricated on a 1.3 μm thick PEN substrate and transfer-printed onto a prestrained elastomeric substrate using a thermal release tape. The LED was encapsulated within an elastomeric superstrate after releasing the prestrain. (b) Perspective scanning electron micrograph of a wavy LED with a tilted angle of 75° formed by a 70% prestrain. (c) Optical image of a wavy LED embedded in Ecoflex and stretched with a strain of 30%. (d) Cross-sectional schematics and SEM and TEM images of the wavy LED mounted on prestrained (70%) thin Ecoflex.

distance between the QDs increases, and cracks occur in the internal structure of the device, which result in a rapid decline of the device performance. Therefore, these issues should be structurally overcome, and various geometrical designs such as buckling, filamentary serpentine, and island bridge structures can be adopted to realize stretchable optoelectronic devices. Among these techniques, a buckled structure of interconnects or devices has been applied to other stretchable electronic devices such as integrated circuits.8,20−22 Direct buckling of entire device stacks, which include an active layer and not an interconnect, is desirable for QD-based electronic devices because it enables stretching and folding of the active area in LEDs and PDs used in stretchable displays and skin-mounted health-monitoring sensors. In this paper, we demonstrate a highly stretchable optoelectronic sensor composed solely of QD active layers with a specific geometrical design of the device that allows high flexibility/stretchability. We used QDs owing to their superior optical properties and graphene as electrodes to provide extreme bendability to the device and low sheet resistance. The stretchable form of the sensor is realized by the buckling

structure, which spontaneously occurs by coupling the ultrathin devices with a prestrained elastomer. The buckled devices are elongated until the vertical displacement has all been converted into a planar strain. We have applied a uniformly wavy structural configuration to the entire device stack of QD-based biosensors with a graphene electrode to realize stretching and folding functions of the emissive/active areas in these devices for applications in wearable biosensors and displays. The devices endure repetitive stretching and compression and show stable electrical and optical properties under various deformations owing to the regular wavy configuration as well as the superior mechanical strength and electrical property of graphene.23−26 These LEDs can be stretched up to a strain of 70% without degradation of device performance and can be folded to a 35 μm bending radius of curvature with high brightness and luminous efficiency, which implies that these thin LEDs can be attached on curved surfaces or stretchable elastic substrates. Stretchable forms of the PDs using PbS QDs as an active layer were accomplished by applying the regular wavy structure to the devices, which can be firmly attached to the skin as an electronic patch. These stretchable optoelectronic 5993


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Figure 2. (a) Raman spectrum of the SLG on the PEN and the bare PEN substrate. (b) Sheet resistance of the SLG, PEDOT film on PEN, PEDOT/graphene film on PDMS, and PEDOT/graphene film on PEN substrate. (c) Optical transmittance of the graphene, PDMS substrate, PEDOT/graphene, and PEDOT/graphene/PEN film. (d) Sheet resistance of the PEDOT/graphene/PEN film on the prestrained PDMS and Ecoflex substrates under increasing strain and relative changes in conductivity of the stretchable PEDOT/graphene/PEN film on PDMS and Ecoflex substrates. The inset shows an optical micrograph of the wavy PEDOT/graphene/PEN film on Ecoflex (scale bar, 300 μm).

(TFB) and TiO2 layers were used as hole and electron transport layers (ETL), respectively. The QD layers were spincoated or transfer-printed on top of the hole transport layer (HTL) based on a previously reported method.27 Cross-linking of the QDs and thermal annealing were performed using a previously reported method.28 An Al electrode was thermally evaporated on top of the ETL to form the cathode. The device was peeled away from the handling substrate and applied to prestrained elastomeric substrates [polydimethylsiloxane (PDMS) and Ecoflex] by a transfer technique using a thermal release tape (Supplementary Methods). Releasing the strain in the elastomer led to surface deformations and formed welldefined wavy layouts to provide fully reversible stretchability/ compressibility. The elastomeric substrates were treated with UV-ozone before lamination for strong adhesion to the devices. This process led to the spontaneous formation of 2D wavy structures, and the QD-LEDs could be stretched as shown in Figure 1b. A thin Ecoflex layer was cast on top to form a passivation layer, which protects the device against oxygen and moisture and reduces the mechanical stress applied on the device, with the assistance of neutral mechanical plane designs for stretching, compression, and bending. Figure 1c shows an optical image of the final stretchable LED in a wavy configuration embedded in thin Ecoflex and being stretched

biosensors can be noninvasively laminated onto the skin to measure photoplethysmography (PPG) signal pulses by illuminating the skin with the LED and absorbing the light transmitted through the skin with the PD. This technology can contribute to the manufacture of wearable/foldable displays and various biomedical sensors for recording biosignals and bioimaging, with the advantages of being waterproof, ultrathin, and stretchable.

RESULTS AND DISCUSSION Figure 1a shows a schematic illustration of the steps for fabricating stretchable LEDs that use graphene and QDs as the anode and the active layer, respectively, by assembling them on prestrained elastomeric substrates via transfer printing of ultrathin devices. For the transparent electrode of the stretchable LED, single-layered graphene (SLG) grown on Cu foil by chemical vapor deposition (CVD) was transferred onto a polyethylene naphthalate (PEN) substrate (1.3 μm or 12 μm thick, Goodfellow) to form the anode for the LEDs (see Methods section). Then, a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Clevios PH 1000) was spin-coated on top of the graphene/PEN substrates with a thickness of 50 nm. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-s-butylphenyl))diphenylamine)] 5994


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Figure 3. Stretchable and foldable QD-LEDs. (a) Optical images of the flexible QD-LEDs before and after detaching the devices from the glass support provided for handling convenience. (b) Optical image of a foldable QD-LED, wrapped around the edge of a PDMS-coated microscope coverslip. The right inset shows a magnified view of the side of the foldable LED. (c) Images of a buckled wavy QD-LED mounted on a prestrained Ecoflex substrate. The inset shows a SEM image of the wavy QD-LED with a top metal cathode (scale bar, 300 μm). (d) Photographs of the stretchable red, green, and blue QD-LEDs at strains of 0% and 70%, biased at 7, 9, and 11 V, respectively. (e) Optical images of the wavy LED under tensile strains ranging from 0% to 70% along the horizontal direction. (f) Surface profiles of the wavy LED with strains of 20% (top panel) and 70% (bottom panel) measured by alpha-step. (g) Average amplitudes and wavelength changes of the wavy LEDs as a function of the strain applied to the elastomer substrate. The blue lines represent the calculation results, and the error bars represent the standard deviation in the wavelengths and amplitudes measured under various strains.

bent up to a bending radius of 35 μm without any crack or damage. Although each QD has an isolated structure, the QD film in this uniform buckled structure endured various deformations. One periodic length of the wavy device is longer than 4000 times the thickness of the QD film. Under maximum tensile strain, the gap between QDs in the outer plane of the wavy structure of the QD film increases by 0.0004−0.0017 nm

by a 30% strain. Figure 1d shows the cross-sectional schematic, scanning electron microscope (SEM), and transmission electron microscope (TEM) images of the wavy device mounted on the prestrained (70%) thin Ecoflex. One periodic length of the wavy device measured 185 μm in an area deposited by a metal cathode and 154 μm in an area without a metal cathode. The device buckled by a prestrain of 70% was 5995


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HL5500PC). The optical transmittance values of the SLG, PDMS (600 μm), graphene/PEDOT, and PEDOT/graphenelayered structure on PEN are shown in Figure 2c. The pristine SLG shows an optical transmittance of 97.7% at a 550 nm wavelength. The transmittance of the resulting graphene/ PEDOT film (50 nm) is greater than 90% in the range of 310− 800 nm wavelengths, and the transmittance at 550 nm is 94%, which makes it suitable for use as a transparent electrode. Figure 2d shows the sheet resistance and the conductivity changes as a function of the strain. The PEDOT/graphene/ PEN film adhered onto the prestretched thin PDMS and Ecoflex substrates with prestrains of 30% and 70%, respectively. After releasing the strain in the elastomeric substrates, the electrodes formed a buckled structure with a regular wavy pattern. The inset in Figure 2d shows an optical micrograph of the wavy PEDOT/graphene/PEN film on thin Ecoflex (∼600 μm) after releasing the 70% prestrain. The wavelength of the wavy structure of the film with the prestrain of 70% was 146 μm. Very little change occurred in the resistance and conductivity in the strain range from 0% to 70%. The conductivity changed from 2504.2 S cm−1 in the pristine state to 2394.1 S cm−1 at 70% strain. The sheet resistance increased by 2.3% and 4.6%, respectively, when stretched up to the prestrain levels of each PDMS and Ecoflex substrate. The active and semiconducting layers and metal cathode were sequentially formed on the anode described above according to a previously reported procedure.27 After the ultrathin LEDs were fabricated, they were transferred onto a prestrained thin PDMS or Ecoflex substrate by a thermal release tape. After releasing the prestrain, the back of the LEDs could be encapsulated within a thin Ecoflex layer. Figure 3a shows the optical images of the flexible LEDs, which were fabricated on 12 μm thick PEN substrates before and after detaching the devices from the PDMS-coated glass supports during operation at 7 and 9 V for the red- and green-emissive LEDs, respectively. After detaching the LEDs, they could be deformed into diverse shapes and showed stable light emission over the entire emissive area in their flexible and even foldable forms, as shown in Figure 3b. The resulting devices were extremely bendable such that they could be folded, as demonstrated in the LED wrapped over the edge of a PDMS-coated microscope coverslip (thickness: ∼100 μm). The optical and electrical properties of the foldable LEDs fabricated on 12 μm thick PEN substrates were stable under a bending radius of 250 μm (Figure S3). Structural configurations that combine ultrathin, flexible geometries such as wavy shapes can impart mechanical stretchability to these devices in concept, which allows reversible deformity between wavy and planar structures.33,34 The stretchable form of the QD-LEDs was achieved by the integration of the LEDs fabricated on a 1.3 μm thick PEN substrate onto a prestrained thin elastomeric substrate. Releasing the prestrain in the elastomer led to the deformation of the LED to uniform and regular wavy patterns on the elastomer surface. Figure 3c shows the optical images of the wavy device created by transfer-printing these LEDs onto a prestrained (70%) Ecoflex substrate followed by the release of the prestrain. The devices maintained sinusoidal shapes during the deformation, as shown in the inset in Figure 3c. The waves have uniform periodic structures with a wavelength of 185 μm and amplitude of 53 μm for a 70% prestrain. Figure 3d shows the optical images of the front side of stretchable LEDs with various levels of strain at unperturbed

depending on the position of the neutral mechanical plane when compared to the planar QD film, which is small enough to be deemed negligible considering the average length of organic aliphatic ligands on the QD surface (∼1 nm). The theoretical calculation is described in detail in the Supplementary Note. Further, QD films are embedded in the semiconducting layers and the elastomeric layers using neutral mechanical plane designs, which results in the minimization of their mechanical stress.20 Therefore, the QD films are able to maintain mechanically and electrically stable conditions without cracks despite the repetitive processes of stretching and compressing. The device fabrication process proceeded by laminating an ultrathin PEN film on the thin PDMS-coated glass substrate for handling. Graphene was transferred onto the PEN without cracks or wrinkles with the assistance of a rigid handling support, which was a PDMS-coated glass (Figure S1). The inset of Figure S1 shows a SEM image of an SLG transferred onto the PEN film. Figure 2a shows the Raman spectra of the transferred graphene on the PEN and the bare PEN substrate. As shown in the inset in Figure 2a, the 2D peak is distinguishable at around 2650 cm−1 and is shifted by 34 cm−1 compared to the measurement corresponding to the graphene/SiO2. However, the G peak is buried by a highintensity peak of PEN at 1580 cm−1. The Raman spectrum measured from the graphene/SiO2 with an I2D/IG value of 2.7 (where I2D and IG represent the intensity of the 2D and G peak, respectively) and the narrow peaks indicate that high-quality SLG was transferred onto the receiver substrates without defects (Figure S2). The sheet resistances of graphene, PEDOT film, and PEDOT/graphene on PEN substrate are shown in Figure 2b. The PEDOT thickness is limited to 50 nm for ensuring high transmittance of LEDs above 95% across the visible range, which is the thickness generally used in QD-LED as a hole-injection layer (HIL).27−29 The sheet resistance of the graphene grown on a Cu foil by CVD was approximately 500 Ω sq−1, and that of the PEDOT film was 180 Ω sq−1. The grain boundaries in graphene have a strong influence on its electrical charge transport.30 In the case of PEDOT/graphene on a PEN substrate, the sheet resistance was significantly reduced compared to the sum of the parallel resistances of the PEDOT/graphene multilayer as well as that of PEDOT alone of the same thickness. Conductive polymer chains covering graphene form electrically conductive connections of graphene grains, which provide ways for smooth charge transport. This makes a graphene/PEDOT stack a good anode even though the thickness of the stack is thin (50 nm).31 In addition, the PEDOT plays an important role in delivering holes from the graphene to the device stack, including an active layer in the vertical direction, through a cascade energy band structure, that allows vigorous hole injection from the graphene and reduction in the sheet resistance. The measured sheet resistance was 78.3 Ω sq−1 for the PEDOT/graphene on PEN and 91.2 Ω sq−1 for the PEDOT/graphene on PDMS. The reason that the PEDOT/graphene on an elastomer such as PDMS has a higher sheet resistance is that cracks in graphene occur due to tensile stress caused by the drying process during graphene transfer and by thermal annealing during PEDOT film formation.32 Therefore, the PEDOT/graphene on PEN has a lower sheet resistance than that on PDMS, as shown in Figure 2b. The sheet resistance of the electrodes was measured by the four-point probe Van der Pauw method at room temperature using the Hall-effect measurement system (Nanometrics 5996


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Figure 4. (a) Current density characteristics of the red-, green-, and blue-emissive QD-LEDs as a function of the voltage. (b) Brightness characteristics of the red-, green-, and blue-emissive QD-LEDs as a function of the voltage. (c) Luminous efficiency of the red, green, and blue QD-LEDs as a function of the current density. (d) Normalized EL spectra of the red, green, and blue QD-LEDs. (e) Brightness and luminous efficiency characteristics of the stretchable red QD-LED with strain. The inset shows optical images of the wavy QD-LED at strains of 0% and 70%. (f) Effect of strain cycles on the current density and brightness of the stretchable red QD-LED. The inset shows schematics and optical images of the wavy QD-LED encapsulated with thin Ecoflex at strains of 0% and 70%.

and stretched states during operation at 7, 9, and 11 V for the red, green, and blue LEDs, respectively. Figure 3e shows the optical microscope images of a metal-cathode side (back side) of the stretchable LEDs with various levels of strain. The surface profiles of the wavy LED were directly measured by alpha-step under various strains, as shown in Figure 3f. As the device was elongated up to a strain of 70%, the device wavelength increased from 185 μm to 307 μm and the amplitude decreased from 53 μm to 6 μm, as shown in Figure 3g. Figure 3g shows the average wavelength and amplitude as a function of the applied strains. The amplitudes were computed from the relationship between the amplitude and the period with a fixed arc length of the sinusoidal wave at various strains, according to the following:

L=

∫0

=

∫0

λ(ε)

r′(t ) r′(t ) dt λ(ε)

(1 + A2 α 2 cos2 αt ) dt

(1)

where L is the arc length, A is the wave amplitude, ε is the strain, λ(ε) is the wavelength, and α is 2π/λ(ε). The detailed theoretical calculation is described in the Supplementary Note (Figure S4). The amplitudes measured by the alpha-step at various strains matched well with those calculated by the contour integrals evaluated from the sinusoidal wave shapes without any parameter fitting. Such well-controlled and highly periodic wavy structures make the stretchable LEDs durable because the applied stresses are not focused at a certain local 5997


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Figure 5. Stretchable PbS QD photodetectors. (a) Schematic illustration of the stretchable PbS QD photodetector structure after releasing prestrain of the elastomeric substrate. (b) J−V characteristics of the QD photodetector in the dark and under illumination of the red QD-LED with a brightness of 1000 cd m−2. (c) The external quantum efficiency (EQE) spectra of the QD photodetector. (d) Current response of the QD photodetector as a function of time with repetitive on/off light illumination from the red QD-LED. Wavelength: 618 nm; irradiation: 1000 cd m−2. (e) Photographs of the stretchable QD photodetectors at strains of 0%, 20%, and 40%.

point but are evenly distributed over a uniform wavy structure during repetitive stretching and compressing. Figure 4a and b show the current density and luminance curves versus the applied bias for the stretchable red-, green-, and blue-emissive LEDs, respectively. The threshold voltages of the red-, green-, and blue-emitting devices were 2.8, 4.2, and 6.0 V, respectively. The operating and turn-on voltages of the LEDs significantly decreased by using the graphene anode. This turnon voltage was much lower than that in previously reported stretchable polymer LEDs.35,36 The peak brightness and luminous efficiency reached 1310 cd m−2 and 2.25 cd A−1, respectively, for the red LEDs, 1240 cd m−2 and 1.75 cd A−1, respectively, for the green LEDs, and 936 cd m−2 and 1.42 cd A−1, respectively, for the blue LEDs, as shown in Figure 4a−c. The stretchable LEDs showed high brightness in spite of the relatively low current density compared to indium tin oxide (ITO)-based LEDs, because of the combination of graphene and HIL with high conductivity (Figure 4a and b). This result implies that the injected electron/hole balance was enhanced in this device structure, and the charge recombination probability increased. Therefore, the maximum luminous efficiency of these LEDs improved by more than 5 times compared to that recorded for the ITO-based QD-LEDs, as shown in Figure 4c.29

Figure 4d shows the EL spectra of the red-, green-, and blueemissive QD-LEDs. The wavelengths at maximum EL peaks for the red, green, and blue LEDs were 618, 520, and 453 nm, respectively, with respective brightness values of 1310, 1240, and 936 cd m−2. These stretchable LEDs showed very high color purity because of the use of QDs as a light-emitting material, and the full-width at half-maximum was 38 nm for the red LED and 35 nm for the green and blue LEDs. Uniform and bright emission was observed over the entire emissive area until the LED was stretched up to a 70% strain. The brightness and luminous efficiency of the red LED as a function of the strain are shown in Figure 4e. Only slight variations in the brightness and luminous efficiencies of the stretchable LEDs were observed with variation in the strain. This can be seen in the result shown in Figure 2d, where the resistance change was only 4.6%. The compression and stretching of this LED was repeated, and the cycling stability of the optical and electrical characteristics was investigated by applying a 70% strain. The current density and brightness were measured after 1, 10, and 100 cycles of stretching and compression, and these are depicted as a function of the number of cycles in Figure 4f. The current density increased by 11.8% after the completion of 100 cycles, which led to reduced 5998


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Figure 6. (a) Skin-mounted photoplethysmographic sensor composed of QD-LEDs during LED operation at 8.4 V and QD photodetectors wrapped around the finger of a subject. (b) Real-time PPG signal pulse wave measured by a stretchable QD photodetector using the stretchable QD-LED and an ITO-based rigid QD-LED as a light source. (c) Real-time record over several pulse periods under illumination from the red QD-LED. (d) Photograph of the epidermal QD-LED attached on a wrist at a driving voltage of 9 V. (e) Optical image of a stretchable red QD-LED mounted on thin Ecoflex, tightly stretched on the tip of a pen. The white arrows indicate the stretching direction. (f) Photograph of a bent QD-LED that uses an encapsulating layer of PDMS (front side) and Ecoflex (back side) for providing waterproof function after immersion into water. The right graph shows the brightness of the QD-LED measured at 8.4 V as a function of immersion time.

and superstrate. This can effectively reduce the generation of internal cracks in the device.20 The LEDs encapsulated with a thin Ecoflex layer showed stable optical and electrical properties after repetitive stretching/release cycles (Figure 4f). The brightness of the encapsulated LED decreased by 12.3% after the completion of 100 cycles, which was significantly less when compared to that of the LED without the passivation layer (29.7%).

luminous efficiency. Repetitive cycling can generate minute cracks in the TiO2 layer, which leads to the increase in leakage current and the current density in the device, and thus, it affects the luminous efficiency of the LED. However, as mentioned earlier, in LEDs that use thin Ecoflex as a passivation layer, the QD layer and the TiO2 layer with a relatively high modulus compared to polymer layers can be placed in the neutral mechanical plane by controlling the thickness of the substrate 5999


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mm) can result in a change in the signal. Therefore, in order to obtain accurate signals, the patient is required to show minimum movement, which subjects the patient to discomfort. However, because our patch-type stretchable LED is firmly fixed on the skin, it can obtain accurate signals regardless of the movement of the subjects; for example, the wrist can be turned up to 180° during measurement without affecting the results. The measured PPG signal pulses according to the brightness at various applied voltages are shown in Figure S5. These devices also have the potential to be attached to various parts of the skin as well as to distal ends such as ears and fingers following a proper design of the device structure and the wireless communication system for application in an integrated patchtype electronic device used for comprehensive diagnostics, as shown in Figure 6d. The epidermal LED attached to the wrist of a person was stretched (∼35%) as the wrist skin area increased according to the joint movement and showed stable operation while the wrist joint was bent outward and stretched repetitively. Figure 6e shows the case of extreme stretching of a biaxially stretchable LED on the tip of a pen. The LED was pulled by the pen in the direction shown by the white arrows with local peak strains of up to ∼70%, which was estimated from the distances between adjacent cells. As a demonstration, biaxially stretchable LEDs were attached on freely deformable substrates such as a balloon or textile and normally operated under various deformations such as swelling, diagonal stretching, crumpling, double folding, and 180° twisting (Figure S6). Encapsulation of these devices with an elastomer provides waterproof function. The LEDs were transferred onto a thin PDMS pad (∼600 μm), and the back (cathode side) of these devices was encapsulated by a thin passivation layer of Ecoflex (∼600 μm). These LEDs showed perfect waterproof characteristics when soaked in water, as shown in Figure 6f, and durability against bending (Figure S7), which guarantees a robust operation of this device inside the body when used in advanced surgical tools or in drug delivery systems.

The extreme bendability and stretchability of these LEDs create various application possibilities. For example, stretchable LEDs attached to a silicone elastomer can be applied to stretchable biosensors by using stretchable PDs in wearable electronic patches. In particular, they are useful for biomedical devices that require specific emissive wavelengths, such as photoplethysmographic sensors, blood oximetry, or drug delivery systems, because these QD-LEDs offer advantages in terms of high performance and stretchability and the emissive spectra are precisely controllable within a narrow spectral bandwidth. Photoconductive PbS QD-PDs are effective in detecting red and near-IR light having a good skin penetration rate. Moreover, they are suitable for fabrication of flexible/ stretchable sensors owing to the ultrathin device structure. QDPDs were fabricated with inverted device structure of graphene/PEDOT/TiO2/PbS/MoO3/Au, and they could be made stretchable using the method applied for fabricating the stretchable QD-LED as shown in Figure 5a. The photocurrent/ dark current ratio measured under illumination of 618 nm wavelength and 1000 cd m−2 brightness from the QD-LED was 1.3 × 104 at 0 V, as shown in Figure 5b. Responsivity and detectivity of the QD-PD measured at 0 V for an illumination wavelength of 633 nm with 150 μW power were 0.13 A W−1 and 6.05 × 109 cm Hz1/2 W−1, respectively. The PbS-based PDs are sensitive in the range of the visible and IR spectra. Figure 5c shows the external quantum efficiency of the QD-PD as a function of wavelength, and the external quantum efficiency (EQE) at a wavelength of 618 nm was 12%. Figure 5d shows the photocurrent density of the QD-PD at an applied bias of −0.1 V, which reacts according to on/off switching of the QDLED with a brightness of 1000 cd m−2. The rise and fall time of the photocurrent were 87 and 129 ms, respectively, which are sufficiently fast to detect the bloodwave changes in real time. Figure 5e shows photographs of the stretchable QD-PDs mounted on the thin Ecoflex under various levels of strain. As a demonstration of the applicability of these devices in biosignal monitoring, we attached our stretchable LEDs and PDs around the tip of a forefinger after the devices were properly stretched, as shown in Figure 6a. The red light emitted from the QD-LED passed through the distal phalangeal tip of the forefinger and was detected by the epidermal PD. The pressure pulse of the cardiac cycle could be determined by measuring the changes in light absorption caused by illumination of the skin. Figure 6b and c show one pulse wave and the real-time record over several pulse periods, respectively. One pulse wave recorded using the stretchable LED and an ITO-based rigid LED was obtained under the same conditions of a luminous intensity of 520 cd m−2 for comparison. In contrast with the rigid device, the stretchable sensor shows two clearly distinguishable peaks: one caused by incoming blood wave ejected by the left ventricular contraction and the other caused by the reflected wave from the venous plexus at the distal end of the body. The peak ratio (P2/P1) and time ratio (t2/t1) for a male test subject were determined as 75% and 19%, respectively. These values and waveforms were very similar to those expected for a healthy male in his late thirties.37 The device adhered tightly to the skin and nail owing to the glutinousness and stretchability of the elastic substrate. Thus, most of the beam passed through the distal phalangeal tip of the forefinger without light leakage. As a result, accurate and clean PPG signal pulses could be measured without electrical signal noise. With the existing tong-type PPG sensor, a small movement of the finger or a small change in its position (∼1

CONCLUSIONS In conclusion, this work has demonstrated flexible, stretchable, and foldable forms of high-performance optoelectronic sensors with regular wavy structures composed of QD-LEDs and QDPDs, which are formed via transfer printing of ultrathin QD devices. The stretchable sensors have an electrically and mechanically stable graphene electrode supported by a conductive polymer, in addition to their advantages of high transmittance and low sheet resistance. During repetitive stretching/compression processes, the electrical and optical properties of the stretchable sensors remained very stable and unaffected by the geometrical change between the buckled and planar structures. Regular wavy patterns naturally formed by prestrain made the device stretchable with evenly distributed stresses, and the stretchable LEDs and PDs showed uniform emission across the entire emissive area and fast photoresponse, respectively, with high efficiency under various strains. The advances in such a device structure and geometrical design provide important and unusual capabilities to optoelectronics based on nanomaterials such as QDs for use in biomedical systems. The experiment to measure PPG signals showed that these stretchable devices are suitable for biomedical applications such as real-time health monitoring with accuracy and convenience. These results suggest that the technology presented here can be utilized not only in foldable and 6000


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ACS Nano wearable displays with high fill factor but also in patch-type biomedical electronic devices.

stretching stage, which was capable of applying uniaxial tensile or compressive strains and controlling the radius of curvature of the device. The EL spectra and current−voltage−luminance (I−V−L) characteristics were recorded using a CS2000 spectroradiometer (Konica Minolta Inc.) coupled with an M6100 voltage and current source measurement unit (McScience Inc.). Transfer Printing of Stretchable QD Devices. The ultrathin QD devices on PEN substrates were peeled away from the PDMS/ glass handling substrates and then transfer-printed onto the prestrained thin PDMS or Ecoflex substrate by using a thermal release tape. To form the stretchable wavy layouts, the device was transferred to an elastomeric substrate such as a thin PDMS or Ecoflex, which is typically prestrained uniaxially or biaxially. To enhance adhesion between the device and the Ecoflex (or PDMS), the surface activation of elastomeric substrates was accelerated by exposure to UV-ozone for 8 min, followed by thermal annealing in an N2 atmosphere at 120 °C for 5 min for LEDs and at 110 °C for 2 min for PDs after applying the devices onto elastomeric substrates. After transfer-printing onto the prestrained Ecoflex (or PDMS), the releasing of the prestrain made the elastomers and ultrathin devices shrink and a regular wavy structure was formed. Stretching/Releasing Test and Measurement. Stretching tests were performed using mechanical bending stages that are capable of applying uniaxial tensile or compressive strains. These stages were directly mounted in electrical probing stations that were coupled with semiconductor parameter analyzers (Agilent, 5155C). Epidermal Photoplethysmographic Sensor. The epidermal optoelectronic sensor was fabricated by attaching a stretchable QDLED and a stretchable QD-PD onto a 300 μm thick Ecoflex film. The epidermal LED was operated using a dc power supply (Tektronix PWS4205). The epidermal PD was connected to an oscilloscope through the customized PCB composed of high-pass and low-pass filters and amplifiers for recording PPG data. A rigid ITO-based QDLED (1 × 1 in.; emissive area of 20 mm2) was used for comparison of the PPG signal pulses obtained with the stretchable QD-LED.

METHODS Synthesis of Quantum Dots. CdSe/CdS/ZnS (core/shell/shell) QDs for the red-emissive QD and CdSe/ZnS QDs for the green- and blue-emissive QDs were synthesized in the laboratory. The detailed synthesis methods of the QDs are described in the Supplementary Methods. PbS QDs with an absorption peak of 1100 nm wavelength were purchased from Evident Technologies Inc. Stretchable QD-LED Fabrication. A solution of 10:1 weight ratio of PDMS (Sylgard 184, Dow Corning) to hardener was spin-coated on glass substrates at 6000 rpm for 60 s to form thin layers of PDMS, which were used as temporary supporting substrates for easy handling. PEN foils (ES361010, ES361015, Goodfellow) with thicknesses of 1.3 and 12 μm were laminated onto thin PDMS/glass substrates by removing the air gap using shears. Graphene was grown by CVD on sheets of Cu foil in a gas mixture of CH4/H2 (3:12 sccm) at a temperature and pressure of 1030 °C and 150 mTorr, respectively. A graphene monolayer was transferred onto the PEN substrates after coating poly(methyl methacrylate) (PMMA A2; Microchem.) with a thickness of 300 nm on top of the graphene and etching away the foil in Cu etchant (a 1 M aqueous solution of FeCl3). This was followed by removal of the PMMA by washing with acetone. A mixture of PEDOT:PSS (PH1000, Clevios), 5 vol % dimethyl sulfoxide (DMSO), and 0.2 vol % Zonyl FS-300 fluorosurfactant (Fluka) was spin-coated on the graphene at 4000 rpm for 60 s and annealed in a vacuum oven at 120 °C for 1 h. This was followed by spin-coating of a mixture of PH1000 and 5 vol % DMSO at 2000 rpm for 60 s and thermal annealing in a vacuum oven at 120 °C for 1 h. TFB dispersed in mxylene with 0.5 wt % was spin-coated on the PEDOT:PSS at 2000 rpm for 30 s and annealed by placing in an oven at 140 °C for 30 min in an N2 glovebox. QD films with a thickness of 40 nm were spin-coated on top of the TFB layer.27 The QD films were cross-linked by 1,7diaminoheptane for high performance28 and annealed at 140 °C for 35 min in an N2 glovebox. A sol−gel TiO2 layer acting as an ETL was spin-coated at 2000 rpm for 30 s and then annealed at 100 °C for 15 min under ambient conditions. An Al electrode as a cathode was thermally evaporated by using a shadow mask on top of the ETL under high vacuum (4 × 10−7 Torr) and performing thermal annealing after metal deposition in a N2 atmosphere at 110 °C for 10 min. Stretchable QD-PD Fabrication. Four-layered graphene was transferred onto the PEN/handling substrates by the repetitive transfer method described previously. A 15 nm thick PEDOT:PSS layer was spin-coated on the graphene and annealed in a vacuum oven at 120 °C for 1 h. A 60 nm thick sol−gel TiO2 layer acting as a hole blocking layer in the inverted PD structure was spin-coated on the graphene/ PEDOT electrode and then annealed at 145 °C for 1 h under ambient conditions. The PbS QDs (Evident Technologies Inc., abs. 1100 nm) were purified by alcohol washing and centrifugation before use. The PbS QDs (50 mg/mL) dispersed in chlorobenzene were spin-coated at 1400 rpm for 40 s. In order to obtain multiple-layered PbS QDs on the preanchored PbS QDs, 1 wt % 1,2-ethanedithiol (EDT, Aldrich)/ acetonitrile (Aldrich) solution was dropped on the PbS QD layer, followed by an interval of 5 min at room temperature, and then spincoated at 2000 rpm for 15 s. The film was spin-washed by clean acetonitrile at 2000 rpm for 15 s. A 260 nm thick PbS QD layer was formed by 13 repetitions of layer-by-layer coating of the PbS QD and the EDT treatment. The PbS QD layer was annealed at 70 °C for 30 min and at 100 °C for 1 min in a N2 atmosphere. A 15 nm thick MoO3 layer acting as an electron blocking layer was deposited on top of the PbS QD layer by thermal evaporation under a pressure of 7 × 10−7 Torr. A 60 nm thick Au electrode as an anode was thermally evaporated under a pressure of 7 × 10−7 Torr. Electrical Measurements and Device Characterization. The sheet resistances of the electrodes were measured by the four-point probe Van der Pauw method at room temperature using the Hall-effect measurement system (Nanometrics HL5500PC). Stretchable QDLEDs and stretchable electrodes were mounted on a mechanical

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01894. Supplementary Notes 1.1−1.3, Supplementary Methods 2.1−2.4, and Figures S1−S7 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail (T.-H. Kim): [email protected]. *E-mail (S. Park): [email protected]. *E-mail (S. Hwang): [email protected]. ORCID

Tae-Ho Kim: 0000-0002-9130-3925 Jaehyun Hur: 0000-0003-1604-6655 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2016R1D1A1B03931903). This work was supported by IBSR006-D1. The authors thank Tae Hyung Kim, M.D., for medical advice on technical questions and Yeon Kyung Kang for helpful discussions and comments. 6001


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ACS Nano

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Fully Stretchable Optoelectronic Sensors Based on Colloidal Quantum

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