A kirigami concept for transparent and stretchable nanofiber networks-based conductors and UV photodetectors

Journal Pre-proof A Kirigami Concept for Transparent and Stretchable Nanofiber Networks-based Conductors and UV Photodetectors Juhyeon Kim, Hyeji Park, Soo-Hwan Jeong

PII:

S1226-086X(19)30531-3

DOI:

https://doi.org/10.1016/j.jiec.2019.10.006

Reference:

JIEC 4812

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

15 August 2019

Revised Date:

2 October 2019

Accepted Date:

3 October 2019

Please cite this article as: Kim J, Park H, Jeong S-Hwan, A Kirigami Concept for Transparent and Stretchable Nanofiber Networks-based Conductors and UV Photodetectors, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.10.006

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A Kirigami Concept for Transparent and Stretchable Nanofiber Networks-based Conductors and UV Photodetectors

Juhyeon Kim1,1 , Hyeji Park1,2,a and Soo-Hwan Jeong1,* 1

Department of Chemical Engineering, Kyungpook National University (KNU), 80, Daehak-

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Ro, Buk-Gu, Daegu 41566, Republic of Korea Advanced Instrumentation Institute, Korea Research Institute of Standards and Science,

These authors contributed equally to this work.

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Corresponding author. E-mail: [email protected]

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Daejeon 34113, Republic of Korea

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Graphical abstract

Abstract 1

To meet the increasing demand for wearable electronics today, fabrication of the stretchable devices becomes crucial. In this respect, a stretchable conductor is an essential component for achieving stretchability of the device. Herein, a stretchable and transparent conductor unit, Aumetallized PVP (Au@PVP) nanofiber network on a kirigami-structured PDMS substrate, was newly developed. By a series of comparative studies, the effectiveness of our strategies to the

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invariant electrical conductivity and high stretchability is convincingly demonstrated. Our stretchable conductor showed high stretchability of 110% without significant change in resistance, ~50% increase. It also exhibited ~80% transparency, as well as excellent durability. To point out its applicability, we fabricated a transparent and stretchable photodetector having

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the same geometry. ZnO nanorod, the 1D transparent metal oxide nanostructure, is used as a

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sensing material owing to its high sensitivity for UV light and large surface to volume ratio. The resulting device showed outstanding ON/OFF ratio of 1020 at its original state and 440

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under 80% strain. Its fast response/reset time, high transparency and stable performance

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indicate the feasibility of the stretchable and transparent optoelectronic device.

Keywords: stretchable, kirigami, transparent, metal nanofiber network, photodetector,

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wearable electronics

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1. Introduction

Recent flexible and stretchable electronics advances have opened great opportunities for future wearable electronics [1,2]. This new class of electronics can maintain original device performance under various mechanical deformations including folding, twisting, bending, and stretching, and hence offer applications for memories [3], electronic skins [4,5], electronic eye

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cameras [6], and wearable health monitoring systems [7,8]. Stretchable conductors are essential building blocks to realize successful stretchable electronics development, because they can serve as electrodes and interconnects in numerous stretchable electronic systems. In particular, optically transparent stretchable conductors could expand their application into stretchable

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touch screens [11], and photodetectors [12,13], etc.

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optoelectronic devices, including stretchable solar cells [9], light-emitting diodes (LEDs) [10],

Metalized electrospun nanofibers have been recently explored for stretchable and

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transparent conductors. They can be synthesized by electrospinning and metal deposition processes, and can easily form conducting pathways at low nanofiber density compared with

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metallic nanowires or metalized electrospun nanofibers due to their high aspect ratio [14], as well as offering better stretchability. Lee et al. [15] demonstrated that networks comprising

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longer metal nanowires exhibited lower strain sensitivity than those from shorter nanowires. Examples of the metalized nanofibers include the forms of nanotroughs [16-19], and metal

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coated polymer nanofibers [20,21]. While most achieve sheet resistance of < 25 Ω sq-1 at 80% transmittance and > 50% stretchability, they still suffer from increased electrical resistance with increasing tensile strain, which makes them difficult to incorporate into stretchable electronic devices. The key to reliable operations under large deformations is minimizing resistance change due to applied tensile strain, particularly for stretchable photodetectors and 3

similar, since photodetectors sense light stimuli using resistance (or current) differences under different illumination conditions. Therefore, we applied the ‘kirigami’ concept to metalized nanofiber network to fabricate high performance stretchable transparent conductors and photodetectors. Kirigami has attracted increasing interest for stretchable electronic applications, including electrodes [22,23], triboelectric nanogenerators [24,25], transistors [26], lithium-ion batteries [27], solar trackers

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[28], etc. Previous studies have shown that kirigami devices exhibit nearly strain invariant electrical conductivity due to the ability of kirigami to accommodate extreme strains. For example, kirigami nanocomposites sustained up to 200% tensile strain without significant conductivity change [22], and Cu-coated kirigami paper also showed negligible resistance

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change (6.34%) at 200% strain [24].

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However, to the best of our knowledge, no previous reports have considered kirigami approaches for metalized nanofiber networks for stretchable electronic devices. Therefore, this

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paper shaped polydimethylsiloxane (PDMS) into kirigami using a proprietary mold, and then transferred Au coated electrospun polyvinylpyrrolidone (PVP) nanofibers onto the kirigami

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PDMS substrate. The resulting kirigami conductor exhibited 5.0-fold lower strain sensitivity compared with nanofiber networks transferred onto pristine PDMS; and excellent durability,

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after 1000 stretching/releasing cycles at 60% maximum strain. A stretchable ultraviolet (UV) photodetector was fabricated by growing ZnO nanorods

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on polymer fiber surfaces to produce a light detecting material, as a proof of concept for stretchable optoelectronics. Combining the nanofiber network and kirigami enabled stretchable electrodes and sensing channels and provided stable performance even at 80% maximum strain and repeated stretching cycles (up to 50 cycles with 80% strain), demonstrating the applicability of the proposed approach for practical optoelectronic devices. 4

2. Materials and methods

2.1. Fabricating stretchable conductors Randomly oriented Au@PVP nanofiber networks were fabricated by sputtering of Au on electrospun PVP nanofibers, as described previously [21]. The kirigami PDMS substrate was prepared using a proprietary mold. We pressed a mixture of PDMS pre-polymer and curing

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agent (Sylgard 184, Dow Corning, USA) at 10:1 weight ratio into the mold and cured at 80°C for 1 h. After curing, the PDMS was carefully peeled off from the mold, and then cut along the kirigami pattern created by the mold. Fig. S1 (Supporting Information, SI) shows the kirigami PDMS key parameters: horizontal spacing, x; vertical spacing, y; cut length, a; and cut width,

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b. The Au@PVP nanofiber network was then transferred onto the kirigami PDMS and hot

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pressed under mild conditions (~12 kPa, 200°C) to increase contact points between nanofibers and the substrate. Nanofibers that bridged the cut, floating in the air, were carefully removed

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using tweezers. Three other conductor types were also prepared for comparison: Au film. Sputtering Au onto pristine PDMS prepared without the mold.

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Kirigami Au film. Sputtering Au onto the kirigami PDMS.

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Au@PVP nanofiber network. Transferring Au@PVP nanofibers onto pristine

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i)

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PDMS substrate.

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2.2. Fabricating stretchable photodetectors Free-standing PVP nanofibers were synthesized in the same way as stretchable

conductors. A ZnO seed layer was partially deposited onto PVP nanofibers using masking tapes by radio-frequency (RF) magnetron sputtering system (KVS-4000L series, Korea Vacuum Tech., Korea) to form ZnO nanorod channels. Then the ZnO coated nanofibers were protected 5

by masking tape and Au layer was deposited onto the PVP nanofibers, forming electrodes. The resulting Au-ZnO-Au nanofiber network was transferred onto kirigami PDMS. A hydrothermal process was performed after transfer to enhance ZnO nanorod growth on the seed layer. The nutrient solution was prepared by adding 3 ml ammonia water (NH4OH, Duksan Pure Chemicals, 25.0~30.0%) to 0.01 M zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Daejung Chemicals & Metals, > 98.0%) solution (100 ml). Then the sample was floated face-down on

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the solution (pH 10.7) at 90°C for 30 min. Finally, the sample was washed in deionized water and dried at room temperature overnight.

2.3. Characterization

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Sample morphologies were characterized by scanning electron microscopy (SEM) (SU-

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8220, Hitachi, Japan), and optical microscopy (BX43, Olympus, Japan). Elemental composition was analyzed with energy dispersive spectroscopy (EDS) in the SEM system.

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Crystallinity of ZnO nanorods grown on nanofiber surfaces were examined with x-ray diffraction (XRD) (D8 Discover, Bruker, Germany). Chemical composition was analyzed by

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X-ray photoelectron spectroscopy (XPS) (NEXSA, Thermo Fisher Scientific, USA). Optical transmittance spectra were recorded by UV–vis spectroscopy (Optizen POP, Mecasys, Korea)

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at 400–1000 nm.

For the stretching test, eutectic gallium-indium (EGaIn, Sigma Aldrich, ≥ 99.99%) liquid

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metal was painted on each end of the sample (1 cm long, 2 cm separation) to reduce contact resistance. Then tensile strain was applied to the sample using computer controlled and motorized linear stretching stage with constant strain rate of 1 mm s-1, and electrical resistance was measured every 0.3 s using a digital multimeter (Agilent 34401A, Agilent Technologies, USA). Current-voltage characteristics (I-V) and time-dependent photoresponse characteristics 6

for the photodetectors were measured using a Keithley 2400 measurement system (USA) under dark and illuminated conditions. UV light (365 nm) was generated using a portable UV lamp

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(VL-4.LC, Vilber Lourmat, France). All measurements were performed at room temperature.

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3. Results and discussion

3.1. Au@PVP nanofiber network characterization Fig. 1a illustrates the proposed fabrication procedure for transparent and stretchable conductors. A PVP nanofiber network is initially produced on a proprietary Cu ring collector (5 cm inner diameter) via electrospinning. Fig. 1b shows a representative SEM image of the

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electrospun PVP nanofiber network. PVP nanofibers (~410 nm average diameter) are randomly oriented due to the ring collector, which has uniform electric field distribution inside the ring [17]. The collected polymer nanofibers were then coated with a thin layer of Au by sputtering, forming an Au-metallized PVP (Au@PVP) nanofiber network. As shown in Fig. 1c, the

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average fiber diameter increased to ~509 nm after metallization. Cross-sectional SEM image

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(Fig. 1d) reveals a core-shell structure resulting from conformal Au sputtering on polymer fiber surface. Average Au shell thickness (~50 nm) was calculated from the increase in nanofiber

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average diameter after Au deposition.

The Au@PVP nanofiber network was transferred onto kirigami PDMS substrate and hot-

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pressed, as illustrated in inset (i) of Fig. 1e, with the detailed kirigami structure presented in Fig. S1 (SI). Finally, the floating nanofibers that bridged the cuts were removed as shown in

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Fig. S2 (SI). Optical images clearly show that nanofibers remained only on the PDMS surface,

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without any breakage or fiber damage after the removal process. Fig. 1e and inset (ii) shows transmittance spectrum and photograph for the resultant

kirigami Au@PVP nanofiber network conductor. The conductor shows relatively high transparency, approaching ~80% transmittance at 550 nm. All Au@PVP nanofiber networks studied in this work had almost the same transmittance (~80%) by controlling the electrospun 8

nanofibers density.

3.2. Electrical performance of stretchable conductors Electrical resistance of the kirigami Au@PVP nanofiber network as stretchable conductors was measured according to applied tensile strain. Fig. 2a shows the relative

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resistance change, ∆𝑅(= 𝑅 − 𝑅0 ) 𝑅0

(1)

with respect to applied tensile strain, where 𝑅 is the measured resistance under strain, and 𝑅0

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is the initial (unstrained) resistance. Results for Au film, kirigami Au film, and Au@PVP

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nanofiber network are also plotted for comparison. Au film exhibited sudden electrical failure before 5% stretching, whereas Au@PVP nanofiber network stretched up to 110% strain

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without a sharp resistance rise, although resistance increased 240%. Thus, the nanofiber network can accommodate applied tensile strain through nanofiber rearrangement and

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reorientation [20,29].

On the other hand, film and nanofiber networks on kirigami substrate exhibited enhanced

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tensile strain tolerance. The kirigami Au film not only withstood 55% tensile strain but showed

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a similar resistance change for the Au@PVP nanofiber network up to 40% strain. This stretchability is more than 11-fold higher than that of Au film sputtered on pristine substrate. The kirigami Au@PVP nanofiber network showed the best performance among the four conductors. As shown in the inset of Fig. 2a, the resistance of kirigami Au@PVP nanofiber network increased 20% at 50% strain and 70% at 110% strain, 3.4–5.0 fold lower than that of 9

the Au@PVP nanofiber network. This result proves that the kirigami cuts are capable of accommodating the applied strain while maintaining conductive pathways.

3.3. Stretching mechanism The PDMS with kirigami shape exhibited distinctly different tensile behavior compared

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with pristine PDMS. Fig. 2b shows that the rectangular kirigami cuts opened along the tensile direction with increasing strain, and the cuts flipped and rotated to align with the applied load, producing out-of-plane buckling. Cut flipping and rotation did not occur when cut length was shorter than the vertical spacing (i.e., a < y), and the kirigami PDMS deformed more closely to

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the cut-free pristine PDMS [30-32]. The kirigami PDMS cut length was much longer than the

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vertical spacing. Therefore, the flip-rotation mechanism enabled high kirigami stretchability. The mechanical stress on kirigami structure can be investigated by numerical simulation.

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We used finite element methods (FEM) to investigate the stress distribution of the kirigami PDMS with applied tensile strain < 80%, solving numerically 2-dimensional linear elastic

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model using COMSOL 5.2®. The 2-dimensional rectangle sheet (2 cm x 1 cm) is fixed at oneend and a load is applied to the other end of the sheet. When 60% tensile deformation was

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simulated, our kirigami structure exhibited excellent stress-accommodation ability, with only

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undergoing local stress. Fig. 2c illustrates that the highest stress was concentrated near the corner of the cuts (red color) because large deformation, such as opening, flipping, and rotation tended to preferentially occur at the cuts, whereas stress was distributed throughout the other regions of the kirigami PDMS. This was consistent with previous observations [22,30,33]. Therefore, kirigami can accommodate high tensile strain, protecting Au film and Au@PVP 10

nanofibers on the PDMS from rupture and disconnection. This could be the reason for low strain sensitivity of kirigami. Kirigami stretchability can be enhanced by adjusting geometric parameters, e.g. ratios of vertical spacing to cut width (y/b) and kirigami width to the sum of cut width and horizontal spacing ((a + y)/(b + x)). The maximum elongation increases as y/b decreases and (a + y)/(b + x) increases [31,32,34]. In other words, decreasing vertical spacing, increasing cut width and

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the number of cuts make the kirigami more stretchable for the same dimensions of the kirigami, and increasing the number of cuts will reduce the load on each cut [35]. Stress at each cut corner could be further reduced by making cut corners circular rather than rectangular [22].

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Fig. S3a,b (SI) show optical microscope images for Au@PVP nanofiber networks on the center of pristine and kirigami PDMS under various strains. Uniaxial tensile strain caused

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severe nanofiber breakage for the pristine PDMS along the stretching direction; whereas

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nanofibers on kirigami PDMS showed no significant disconnection, indicating little strain was imposed on individual fibers. The length of kirigami PDMS increased with increasing applied strain, due to gradual opening of the cuts without significant PDMS elongation. Therefore,

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nanofibers on the kirigami PDMS were subject to significantly less tensile deformation in the stretching direction compared with those on pristine PDMS. Fig. S3c shows nanofiber

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morphologies adjacent to kirigami cut corners under 60% stretching. Despite the corners

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exhibiting high stress concentration in the simulation (Fig. 2c), conducting pathways were maintained.

This comparative study confirms effective strain accommodation enabled by kirigami,

which could be the reason for low strain sensitivity of the kirigami Au@PVP nanofiber network.

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3.4. Feasibility and durability test of the conductor network Section 3.2 confirmed kirigami nanofiber network benefits for enhanced electrical performance and stretchability. We then connected LED circuits to the nanofiber networks for further demonstration of the kirigami Au@PVP nanofiber network stretchability. Fig. 2d shows photographs of LEDs integrated with Au@PVP and kirigami Au@PVP nanofiber networks with respect to tensile strain up to 90%, operated at 3 V. Initial resistances of the network and

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kirigami network were 129 and 176 Ω, respectively, with almost the same transmittance (80%). Although the network provided a higher initial LED brightness than the kirigami network due to lower initial resistance, the LEDs lost the brightness rapidly during stretching. However,

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LED brightness was almost unchanged even under 90% tensile strain for the kirigami network, indicating its low strain sensitivity.

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Fig. 2e shows cyclic stretching/releasing tests for 1000 cycles with 15, 30, 60%

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maximum strains to evaluate the durability of the kirigami Au@PVP nanofiber network. The kirigami Au@PVP nanofiber network exhibited negligible resistance change over 1000 cycles for 15 and 30% strain. However, the peak of the relative resistance change increased up to 20%

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after 1000 cycles for 60% strain. The performance of our kirigami nanofiber network was superior or comparable to those of the previously reported stretchable metal

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nanofiber/nanowire networks [21,36-38]. For example, the resistance of the Cu nanowire

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network/polyurethane conductor increased less than 300% after 1000 stretching cycles of 0 to 10% strain [36]. This desirable durability could be attributed to reversible sliding of the nanofibers adjacent to each other in the network and excellent damage tolerance of kirigami.

3.5. Characterization of the Au-ZnO-Au@PVP nanofiber on the kirigami PDMS substrate 12

Previous outcomes confirmed that combining nanofiber network and kirigami provided an effective method to fabricate stretchable conductors with low strain sensitivity. Therefore, this concept was further adopted to fabricate stretchable photodetectors with reliable performance under large deformation. ZnO is a promising transparent metal oxide with large exciton binding energy (60 meV) and wide bandgap (3.37 eV) at room temperature [39,40]. It is inherently visible-blind and can detect low UV light concentrations [41,42], making it widely

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used in the photodetector applications [43-46]. ZnO one-dimensional (1D) nanostructures including nanorods and nanowires, especially, provide much larger surface to volume ratio compared to ZnO film. Therefore, they normally exhibit higher photoresponse, promoting oxygen adsorption and desorption at the surface [47]. Although ZnO 1D nanostructures have

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been widely studied for fabricating high performance UV photodetectors, few studies have

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been considered stretchable photodetectors.

Fig. 3a describes the process to fabricate electrospun nanofiber network based

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stretchable UV photodetectors. The first step is the same as for the transparent and stretchable conductors, i.e., preparing the electrospun PVP nanofiber network. ZnO seed layer and Au are

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sequentially sputtered over the PVP nanofibers surfaces through masking tapes. Consequently, the highly conductive Au@PVP nanofiber networks constructed the electrodes with the

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ZnO@PVP nanofiber network was located between the electrodes.

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Fig. S4 (SI) presents EDS spectra for three regions: seed layer, (i); seed layer/electrode overlap, (ii); and electrode region, (iii). EDS confirms Zn in the seed layer region, Zn and Au in the overlap region, and Au in the electrode region, proving that ZnO and Au were partially sputtered. The Au-ZnO-Au@PVP nanofiber network was transferred onto kirigami PDMS and ZnO nanorods were prepared over the fibers through subsequent hydrothermal reaction. 13

Fig. 3b indicates that ZnO nanorods were grown on the surface of PVP nanofibers after the hydrothermal reaction, maintaining the network configuration without any disconnection. Fig. 3c shows cross-sectional view of ZnO nanorods@PVP nanofibers. High density nanorods were wrapping the PVP nanofibers. Average ZnO nanorods diameter and length were ~84 and ~628 nm, respectively. Fig. 3d shows the crystal structure of the ZnO nanorods on the polymer surface,

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characterized by XRD. All diffraction peaks correspond to the hexagonal wurtzite ZnO (JCPDS, No. 36-1451). The strong and sharp (002) peak indicates preferred orientation in the c-axis. Fig. 3e points out that the fabricated kirigami photodetector achieved relatively high

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transparency ~72% at 550 nm wavelength.

The high resolution XPS spectrum of Zn 2p is presented in Fig. S5. Two major peaks at

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binding energy 1044.1 and 102.9 eV correspond to Zn 2p3/2 and Zn 2p1/2, respectively. The

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consistent spin-energy separation value between the Zn 2p3/2 peak and the Zn 2p1/2 peak is 23.2 eV. Combined with the XRD analysis (Fig. 3d), these results are also in good agreement with

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those reported for ZnO [48].

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3.6. UV detection mechanism of ZnO

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Fig. 4a shows how UV detection mechanism of ZnO is associated with oxygen adsorption and desorption [49-52]. Oxygen is adsorbed on ZnO surface by capturing free electrons, creating negatively charged oxygen ions [ O2 (g) + 𝑒 − → O− 2 (ad) ], and surface depletion layer with low conductivity near the surface.

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𝐸 (eV) =

ℎ𝑐 1240 = 𝜆 𝜆 (nm)

(2)

Electron and hole pairs are created [ℎν → ℎ+ + 𝑒 − ] under UV light illumination at photon energy above ZnO bandgap (3.37 eV). The photon energy of UV light (3.40 eV) equivalent to its wavelength (365 nm), which is larger than bandgap of the sensing material, can be calculated using Equation (2). The photogenerated holes migrate to the surface and discharge negatively charged oxygen ions to desorb oxygen from the surface [ℎ+ + O− 2 (ad) →

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O2 (g) ] (Fig. 4b), decreasing the surface depletion layer width. Since unpaired electrons gradually accumulate until oxygen adsorption-desorption reaches equilibrium, current increases until saturation. When the light is turned off, electrons and holes quickly recombine,

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and oxygen is adsorbed back onto the ZnO surface, returning the device to its initial state. The

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energy band diagram is shown in Fig. S6 (SI) describing the photodetection mechanism. Considering that ZnO nanorods based on the hydrothermal method has low crystallinity

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and high oxygen defect [53], the major trapping/scattering centers for conduction electrons

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might be the surface defect of ZnO nanorods [54].

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3.7. UV detection performance of the photodetector device against strain Fig. 4c shows I-V characteristics of the device at 0% strain, measured in dark conditions

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and illuminated with 365 nm UV light (13.69 W/m2 effective UV intensity). Fig. S7 (SI) displays higher magnification I-V curve in the dark. The linear I-V relationship indicates good ohmic contacts between ZnO nanorods and Au on PVP nanofiber surfaces. The current significantly increased to 129 nA under UV illumination at 5 V bias voltage, more than four orders of magnitude higher than the dark current (74 pA). This current increase was attributed 15

to the aforementioned generation of photo-excited electron-hole pairs. From the results above, at 5 V bias voltage, the responsivity of our device is measured to be 0.124 mA/W and the device shows good linearity. We then studied kirigami photodetector performance as a stretchable device by uniaxially stretching along the direction of the channel length. The UV light was periodically turned on (for 5 min) and off (for 10 min) at 5 V bias voltage. Fig. 4d shows the dynamic

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photoresponse for the device at various tensile strains between 0 and 80%. The device demonstrated high stretchability and a reversible response even at strain as high as 80%. Fig. 4e plots ON/OFF ratio of 𝐼light (current under UV light) to 𝐼dark (current in dark) with

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respect to applied tensile strain. As shown in Fig. S3, the fibers lost their connectivity under strain at some points. Then the electrical conductivity is lowered (Fig. 2a), and it can cause

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on/off ratio decrease, from 1020 to 440. Nevertheless, these ratios are much higher than

unstrained state [55,56].

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previously reported stretchable photodetectors using ZnO as the light detection materials at

The Au@PVP nanofiber network kirigami electrodes exhibited lower resistance than

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ZnO nanorods channel (107-1010 Ω) and stable resistance change during stretching, as shown

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in Fig. 2a. Therefore, they have little impact on ON/OFF ratio change with strain, and hence

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the decrease in ON/OFF ratio could be related to the increased channel length; 𝐼ph =

𝑃abs 𝑞𝜇𝑉𝜏 ℎ𝜈𝑙 2

(3)

where 𝑃abs is the power absorbed by the device, q is the elementary charge, μ is the carrier mobility, V is the applied voltage, 𝜏 is the photogenerated carrier lifetime, ℎ is the Planck constant, ν is the frequency of the absorbed photon, and 𝑙 is channel length of the device [57]. 16

Equation (3) indicates that photocurrent is inversely proportional to the square of the channel length at constant light power and voltage. Fig. S8 (SI) confirms good agreement with the relationship between the photocurrent and the channel length. Thus, ZnO nanorod channel lengths increase as tensile strain increases, consequently, device photocurrent and ON/OFF ratio decreased. The slight deviation from the perfect fit could be due to contact losses between ZnO nanorods under stretching.

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Response/reset time is another important photodetector characteristic. Reset time is defined as the time for the current to decrease to 1/e (37%) of maximum value after UV light is off [58], and response time is that for the current increase 63% from the dark current.

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Response time increased from 49 to 90 s and the reset time showed increasing trend from 25 to 46 s within 0-80% strain.

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Response and reset processes can be fitted by corresponding empirical equations. The

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response process can be described in exponential form [59,60] as 𝐼 = 𝐼𝑠 [1 − 𝐴1 𝑒 −(𝑡/𝜏1 ) − 𝐴2 𝑒 −(𝑡/𝜏2 ) ]

(4)

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where Is is the saturation current, 𝐴1 and A2 are weighing factors (A1 + A2 = 1), and 𝜏1 and 𝜏2 are time constants. Fig. S9a,b (SI) shows that the experimental data (0 and 80% strain)

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fitted well with Equation (4). Both weighing factors 𝐴1 and 𝐴2 were equal to 0.5 at 0 and

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80% strains. Both fitted time constants 𝜏1 and 𝜏2 increased from 51.7 to 90.7 s with increasing strain. The reset processes can be described by a second-order exponential decay function [61,62] as 𝐼 = 𝐼0 [𝐴3 𝑒 −(𝑡/𝜏3 ) + 𝐴4 𝑒 −(𝑡/𝜏4 ) ]

(5)

where 𝐼0 is the initial current at the switching-off moment, 𝐴3 and 𝐴4 are weighing factors 17

( 𝐴3 + 𝐴4 = 1 ), and τ3 and 𝜏4 are time constants. We can assign 𝜏3 < 𝜏4 so that 𝜏3 represents rapid electron-hole pair recombination, and 𝜏4 represents the slow process attributed to oxygen adsorption and desorption on the ZnO nanorods surfaces. Fig. S9c,d (SI) show experimental data at 0 and 80% strain fitted to Equation (5). The fitted time constants for both fast and slow components increased upon stretching: τ3 increased from 6.98 to 21.7 s and 𝜏4 increased from 53.2 to 133.5 s within 80% strain.

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Increased response time and fitted time constants 𝜏1 and 𝜏2 upon stretching correspond to slower response process, i.e., it takes longer time to reach equilibrium between oxygen desorption and adsorption at the nanorod surface. Previous reports [50,51] have shown

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that photodetector response speed is affected by the oxygen concentrations. Oxygen-deficiency induces slow response processes. When the kirigami device was stretched, out-of-plane

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kirigami PDMS deformation and ZnO nanorods@PVP nanofiber rearrangements would occur.

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Thus, it could reduce oxygen diffusion rate and concentration in the surrounding environment of the ZnO nanorods. The increased time constant for the slow component (𝜏4 ) could be

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attributed to the retarded diffusion and adsorption of oxygen to restore the surface depletion layer because 𝜏4 is the surface-related process. In addition, the reduced oxygen adsorption

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rate could extend the time for holes to be attracted and held at the surface [55]. Consequently, electron-hole recombination rate would become slower and the time constant for the fast

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component (𝜏3 ) would increase. In Table S1 (SI), the comparison of key characteristics for the UV photodetectors is

presented, consisting of on/off ratio, responsivity, response/reset time, voltage bias and stretchability data.

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3.8. Cyclic stability of the photodetector device The cyclic stretching test was carried out to verify device stability and reliability. The device was repeatedly stretched up to 50 cycles with 80% maximum strain, and device photoresponse was investigated under 80% strain for each cycle. Fig. 4f shows that photoresponse curves exhibited nearly identical behavior even under repeated stretching up to

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stretchable photodetectors using the proposed method.

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80%, indicating excellent stability. This confirms the potential of practical realization of

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4. Conclusions Highly stretchable transparent conductors and photodetectors have been fabricated using an electrospun nanofiber network and kirigami substrates. The fabricated kirigami Au@PVP nanofiber networks exhibited 3.4–5.0 fold less resistance increase under stretching compared to the nanofiber network on the pristine substrate with the same transmittance (80% at 550 nm). This low strain sensitivity originated from an effective strain accommodation enabled by both

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rearrangements of the nanofiber network and kirigami. Kirigami conductors showed excellent electrical stability even over 1000 stretching cycles between 0 and 60% strain. For the kirigami UV photodetectors, both the electrodes and UV detection channels were composed of kirigami nanofiber networks. Therefore, the devices

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had good transmittance of 72% and reversible optoelectronic performance under tensile strain

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up to 80%.

Cyclic stability tests showed that the photoresponse behavior was nearly identical after

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repeated stretching up to 50 cycles (maximum strain of 80%), indicating that combining nanofiber network and kirigami could enable stable operations of stretchable electronic devices

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even under high tensile strains. Therefore, this approach could be suitable for development of

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high performance stretchable optoelectronic and sensing devices.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may

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be considered as potential competing interests:

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Figures

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Fig. 1. (a) Schematic illustration of the fabrication procedures for the Au@PVP nanofiber network-based kirigami conductor: (i) fabrication of Au@PVP nanofiber network via electrospinning and Au deposition and (ii) transfer of Au@PVP nanofiber network onto the kirigami PDMS. SEM images of (b) electrospun PVP nanofiber network and (c) Au@PVP nanofiber network after sputtering of Au. (d) Cross-sectional SEM image of Au@PVP nanofibers. (e) Optical transmittance spectrum of the kirigami Au@PVP nanofiber network conductor. The inset (i) and (ii) show the digital photographs of the kirigami PDMS and stretchable conductor with ~80% transmittance, placed on the logo of our institution, respectively. 27

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Fig. 2. (a) Relative change in resistance (Δ𝑅/𝑅0 ) as a function of tensile strain with four types of conductors. Inset shows Au@PVP and kirigami Au@PVP nanofiber network magnification in the range of 0 to 60% strain. (b) Photograph series of kirigami PDMS showing stretching of up to 90% strain. (c) FEM simulation for a single kirigami PDMS element under 80% strain. (d) LED brightness with respect to tensile strain for (i) Au@PVP and (ii) kirigami Au@PVP nanofiber networks. (e) Relative resistance change of the kirigami Au@PVP nanofiber network for 1000 cycles with different maximum strains.

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Fig. 3. (a) Fabrication process for the kirigami photodetector. (b) SEM image of a junction of two ZnO nanorods@PVP nanofibers. Scale bar = 1 μm. Inset shows SEM image for ZnO nanorods@PVP nanofiber network. Scale bar = 5 μm. (c) Cross-sectional SEM image for the ZnO nanorods@PVP nanofibers. (d) XRD image for ZnO nanorods grown on PVP nanofiber surfaces. (e) Optical transmittance spectrum of the photodetector. Inset shows photodetector with ~72% transmittance at the wavelength of 550 nm. 29

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Fig. 4. UV detection mechanism for the stretchable photodetector (a) in dark and (b) under UV illumination (photon energy larger than bandgap) conditions. (c) I-V curves for unstrained photodetector in dark and UV (365 nm) illuminated conditions. (d) Dynamic photoelectric response of the device under 0, 20, 40, 60, and 80% tensile strains measured by periodically switching on and off UV light (365 nm) at 5 V bias voltage. (e) Device ON/OFF ratio with respect to strain. (f) Photodetector cyclic stability at 80% strain for 50 stretching/releasing cycles between 0 and 80% strains. 30

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Supplementary Data

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Fig. S1. Rectangular pattern designed for the kirigami PDMS. Tensile strain is applied to the kirigami at the two ends in the direction indicated by the arrows.

Fig. S2. Illustrations and optical images of Au@PVP nanofiber networks on the kirigami PDMS before and after removing floating nanofibers.

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Fig. S3. Pristine and kirigami PDMS (1st row) observed regions and optical microscope images of Au@PVP nanofiber networks on each region at different tensile strains of 0, 30, and 60%. (2nd-4th rows) (a) pristine PDMS center, (b) kirigami PDMS center, and (c) adjacent to the corner of the kirigami cut.

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Fig. S4. (a) Nanofiber network comprising three regions: (i) seed layer containing ZnO@PVP nanofibers, (ii) seed layer/electrode overlap region containing ZnO@PVP and Au@PVP nanofibers, and (iii) electrode region containing Au@PVP nanofibers. (b) EDS spectra of each region in the nanofiber network.

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Fig. S5. The high-resolution Zn 2p XPS spectrum.

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Fig. S6. Energy band diagram (a) in dark and (b) under UV illumination. 𝜙Au and 𝜙ZnO are work function of Au and ZnO, respectively. UV photon energy is 3.40 eV, larger than ZnO bandgap (𝐸g ), 3.37 eV.

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Fig. S7. Higher magnification I-V curve for unstrained photodetector under dark condition.

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Fig. S8. Photocurrent with respect to channel length.

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Fig. S9. Fitting of the response curves at (a) 0% and (b) 80% strain, and the reset curves at (c) 0% and (d) 80% strain.

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Table S1. Comparison of UV photodetector performances On/off ratio

Responsivity (mA/W)

Response time (s)

Bias (V)

Stretchability (%)

Transparency (%)

Ref.

Au-ZnO-Au@PVP nanofiber

~1020

0.124

49

25

5

80

72

This work

TiO2 nanowire

760

-

<5

-

10

-

[1]

AgxO/TiO2/ITO/Glass

-

~323

0.287

0.361

-1

N/A

> 70

[2]

ZnO nanowire@PI film

-

~1000

9.1

56

10

Flexible

N/A

[3]

PbS quantum dotspoly(3-hexylthiopehene)

310

-

0.16

0.11

-

100

N/A

[4]

AgO, ZnO nanowire

188

-

30.3

5.8

2

100

N/A

[5]

ZnO nanowire@PU fiber

158.2

-

29.8

6.6

3

125

N/A

[6]

Bendable R~5 mm

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Reset time (s)

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Materials and design

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