green emission from electro-mechanically powered elastomeric zinc sulphide composite

Nano Energy (]]]]) ], ]]]–]]]

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Available online at www.sciencedirect.com

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Simultaneous dual-channel blue/green emission from electro-mechanically powered elastomeric zinc sulphide composite

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Soon Moon Jeongn,1, Seongkyu Song1, Hyunmin Kim

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Division of Nano & Energy Convergence Research, DGIST, Daegu 711-873, Republic of Korea 25 27

Received 3 November 2015; received in revised form 30 December 2015; accepted 15 January 2016

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KEYWORDS

Abstract

Mechanoluminescence; Electroluminescence; Color; Dual-channel; Composite

Mechanoluminescent (ML) materials, which luminesce in response to mechanical stimuli, are attractive candidates in developing energy-sustainable technology and are widely used in sensors, ubiquitous light sources, and displays. Metal-doped zinc sulphide (ZnS) is considered a promising ML material because it produces intense and lasting luminescence under repeated mechanical stresses. Previous studies of ZnS luminescence have focused on single-channel emission by electroluminescence (EL) or ML generated by applied electric fields or mechanical stresses, respectively. Here, we report the simultaneous generation of EL and ML from an elastomeric ZnS composite embedded with silver nanowires and demonstrate the independent control of both EL and ML responses. We describe the tuning of the strength and color of the EL/ ML emissions from a single ZnS-based structure by applying combinations of electrical and mechanical excitation forces. We also demonstrate a multi-color-patterned EL/ML emitting display using the ZnS-based composite; this application may provide a basis for the development of new optomechanical displays. & 2016 Published by Elsevier Ltd.

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Introduction Luminescence phenomena occur when the electronic states of a solid are excited by external energy and the excitations are

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Corresponding author. E-mail address: [email protected] (S.M. Jeong). 1 These authors contributed equally to this work.

released as light [1]. One such phenomenon, mechanoluminescence (ML), is defined as the generation of light by the application of a mechanical stress to a substance [2–6]. The demonstration of elastico-ML, in which tensile stresses produce photoemissions [7], has stimulated considerable interest in the study of ML; remarkable advances have been achieved in this field [8–10]. Zinc sulphide (ZnS) is considered a promising MLproducing material because of the intense and durable ML responses it displays under repeated stress applications [11–17].

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http://dx.doi.org/10.1016/j.nanoen.2016.01.012 2211-2855/& 2016 Published by Elsevier Ltd.

Please cite this article as: S.M. Jeong, et al., Simultaneous dual-channel blue/green emission from electro-mechanically powered elastomeric zinc sulphide composite, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2016.01.012

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Recently, a ZnS-microparticle-embedded polydimethylsiloxane (PDMS) composite (ZnS+PDMS) has received increasing attention as an ML-generating source because it shows high durability, designability, and tunability under various mechanical deformations [6,11–13,18–19]. Notably, a PDMS-based structure of thick powder-based ZnS+PDMS films sandwiched between two electrodes exhibits alternating-current electroluminescence (ACEL) characteristics. The PDMS acts as both a dielectric material and a binder. The application of stretchable transparent electrodes [20–22] in such structures facilitates the development of intrinsically stretchable EL devices with good performance under large strain cycles [23]. Thus, the ZnS +PDMS composite can offer simultaneous EL and ML from a single material by combining different excitations of applied electric fields and mechanical stresses. Luminescence is produced from ZnS by introducing lattice defects that deviate from the stoichiometric Zn/S ratio and by doping the material with impurity atoms called activators [24]. Most studies on ZnS ML have reported singleemission spectra with single colors determined by the types of dopants [11–13,16–19]. However, studies of EL have shown tunable emission colors by varying the applied electrical frequency [25–27]. The color of the EL from ZnS + PDMS composites can be changed by varying the electrical frequency applied to the composite. At high excitation frequencies, a blue-colored EL occurs because of the higher contribution from the deepest energy levels, or blue centers. In contrast, at low frequencies, the emission is dominated by energy levels emitting in the green range [13,18–19]. Based on blue EL generation from ZnS + PDMS composites at high-frequency electrical excitations, we have obtained blue-colored ML under harsh mechanical vibration conditions provided by strong winds, while weaker mechanical excitations produced green emissions [13]. This previous work suggested similar behaviors in EL and ML responses to applied electrical or mechanical frequencies. The relationship between EL and ML is not clearly understood, but it is generally accepted that two emission bands of blue and green exist in ZnS phosphors [28–29]. We previously proposed an explanation for ML color shifts [11] employing the mechanism of photoluminescence (PL) [30–33]. The green emission may arise from the transition between impurityinduced shallow donor states and the t2 state of Cu [30–31], whereas the blue emission may relate to the e state [32–33]. Recently, Hao and co-workers provided an alternate explanation, ascribing blue emissions to the donor–acceptor (A) emission process of VS (sulfur vacancies at shallow donor level) and CuZn (Cu + substituted for Zn2 + at acceptor level) and green emissions to D-A pair recombination between AlZn (Al3 + substituted for Zn2 + at donor level) and CuZn [18–19]. No uniform model exists yet concerning the color shift as well as the generation mechanism of ML in ZnS. However, according to previous reports [11–13,18–19], we note that mild mechanical excitation has been reported to emit green ML because it is insufficient to activate blue emission centers, whereas the application of high electrical frequency is sufficient to shift the excitation from the green to blue centers. Here, we demonstrate the successful manipulation of dual-channel EL and ML emissions from a single ZnSembedded PDMS composite by the simultaneous application

of electrical and mechanical inputs. To our knowledge, this work reports the first demonstration of simultaneous EL and ML generation from a single system. The colors of the EL and ML can be independently regulated by controlling the excitation strength of each source. Under high electrical frequencies, the blue EL spectrum is reinforced, whereas the green ML is dominant in low-frequency conditions. We believe that this ML-based stretchable electronic light source could be applied to advanced systems such as wearable multifunctional sensors/displays, biomedical devices, and electronic skin, which are subjected to large body movements [34].

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Materials and methods Materials and EL/ML sample fabrication To fabricate the EL/ML-emitting elastomeric ZnS composite, two Ag nanowire (AgNW)-embedded PDMS plates ( 300 μm thick) were prepared using a transferring process from a glass substrate to PDMS. Initially, AgNW dispersed in ethanol (SLV-NW-35, Blue Nano Inc.) was spin-coated onto a precleaned glass substrate at 500 and 100 rpm for the top and bottom plates, respectively, and subsequently dried at 100 1C for 1 min. Prior to spin-coating, part of the glass was screened with tape to form an electrode with an active emitting area of 1  2 cm2 at the intersection of the bottom and top AgNW strips. Liquid PDMS (Sylgard 184, Dow Corning) with a curing agent at a weight ratio of 10:1 was cast on top of the AgNW film and subsequently cured at 100 1C for 35 min under Al mold pressure for uniform thickness. After peeling away from the glass, the AgNW film was buried at the surface of the cured PDMS to form a conductive and stretchable layer [21]. The electrical conductivity was realized by the two-dimensionally percolated AgNW networks. For the emitting layer (ZnS +PDMS composite), ZnS doped with a Cu-based phosphor (25 μm average particle diameter, Figure S1, Supplementary Materials; (GG64, Global Tungsten & Powders Corp.) and liquid PDMS with a curing agent (weight ratio 10:1) were mixed at a weight ratio of 7:3 and subsequently sandwiched between the previously prepared AgNW-embedded PDMS layers at 100 1C for 35 min. More information regarding the EL and ML characteristics of the ZnS + PDMS composite used here, but arranged in different device architectures, is described in our previous reports [11–13]. The thickness of the ZnS + PDMS composite layer was determined by scanning electron microscopy (SEM, Hitachi SU8220). To fabricate the patterned bottom AgNW electrode, bird- and flower-shaped patterns were created using tape screening and spin-coating at 100 rpm of the AgNW solution. The detailed fabrication process of this device is provided in Figure S2 (Supplementary materials).

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Electro-optical characterization 119 Electro-optical measurements of the ACEL devices were performed by applying a sinusoidal voltage of 150 V amplitude under frequencies of 10, 30, 50, 100, 200, 300, 500, 800, and 1000 Hz from a function generator (AFG3102, Tektronix) with a voltage amplifier (HAS4051, NF Corp.).

Please cite this article as: S.M. Jeong, et al., Simultaneous dual-channel blue/green emission from electro-mechanically powered elastomeric zinc sulphide composite, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2016.01.012

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Simultaneous dual-channel blue/green emission During the voltage application, the current was measured using a current meter (3706A, Keithley) and light emission was collected using the combination of a spectrometer (QE65000, Ocean Optics, Inc.), and a vertically aligned optical fiber equipped with a collimating lens (74-UV, Ocean Optics). Time-dependent optical intensities, obtained by integrating the spectral intensity from 400 nm to 600 nm, were recorded every 8 ms (the integration time limit of the spectrometer). In addition to time-dependent data, all EL, EL/ML, and ML spectra were recorded under 3 s integration times; these were insensitive to intensity and color variations within a few tens of milliseconds. Only the frequencydependent EL and EL/ML intensities were obtained by integrating the spectral intensity (400–600 nm) to include the color variation information. The coordinates of the measured EL, EL/ML, and ML spectra in the International Commission on Illumination (CIE) 1931 color spaces were calculated. The brightness levels of the emissions were measured using a spectroradiometer (PR-670, Photo Research Inc.). Images of the generated light were captured using a digital camera (Canon EOS 70D).

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A schematic of our approach and device is shown in Figure 1a. The ZnS + PDMS composite layer is sandwiched between two AgNW-embedded (average NW diameter and length: 35 nm and 10 μm, respectively) PDMS (AgNW+ PDMS) layers that allow electrical conduction and transmittance under repeated mechanical stresses. The SEM images in

3 Figure 1b show well-laminated PDMS(AgNW + PDMS)/ZnS + PDMS/(AgNW+ PDMS)PDMS layers. Magnified images of the AgNW +PDMS and ZnS + PDMS composite layers are displayed in Figure 1c–e. We constructed two types of AgNW + PDMS layers such that the transmittance of the top thin layer would differ from that of the bottom thick layer, which mainly contributed to the electrical conductivity of the device. The sheet resistance values of the top and bottom layers prior to lamination were 155.2754.5 Ω/sq and 5.571.4 Ω/sq, with transmittance values at 550 nm of 50.5% and 15.7%, respectively (Figure S3, Supplementary materials). Prior to investigating the dual-channel blue/green emission by simultaneous EL/ML, we verified the EL characteristics of the device under mechanical stresses. We used a stretching-releasing (S-R) testing system to apply repetitive mechanical stresses to the ZnS + PDMS composite layer. We fabricated a thin (90-μm-thick) ZnS + PDMS composite emitting layer (EML; SEM image, Figure S4, Supplementary materials) sandwiched between two AgNW-embedded PDMS layers as an EL-dominant elastomeric device. This device exhibited weak ML and is hereafter referred to as the “thinEML device.” Photographs of the device installed in the S-R system are shown in Figure 2a and b. For the mechanical stresses, we used a stretching condition of 40% (elongated distance of 35 mm, Figure 2b) from the released state (initial distance of 25 mm, Figure 2a). To verify the induced EL characteristics of the thin-EML device under repetitive SR motion, we performed AC electric current and optical measurements during a 0.12-s cycle of repetitive stretchingreleasing motion speed ( = 500 cycles per minute (cpm))

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119 Figure 1 (a) Schematic of the device and experimental approach used in this study; the simultaneous emission of dual-channel blue/green EL/ML or solely blue EL can be controlled using a combination of electrical and mechanical excitations. The elastomeric ZnS+PDMS composite is sandwiched between conductive AgNW-embedded PDMS layers. (b) Cross-sectional SEM image of the elastomeric ZnS device. Magnified details of (b) depict (c) the top AgNW +PDMS layer, (d) the ZnS+ PDMS composite, and (e) the bottom AgNW+ PDMS layer. Please cite this article as: S.M. Jeong, et al., Simultaneous dual-channel blue/green emission from electro-mechanically powered elastomeric zinc sulphide composite, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2016.01.012

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Figure 2 Electro-optical characteristics of the thin-EML (90-μm-thick) device under applied 150-V AC electric field at 1 kHz frequency. Photographs of the device installed in S-R system under (a) released and (b) stretched states. Inset photographs depict EL emissions of the released and stretched thin-EML device. (c) Time-dependent optical emission intensity obtained by integrating the spectral intensity in the range from 400 nm to 600 nm (integral over [400, 600]) with 8 ms integrating time (int. time) measurement condition (top), electrical current recorded every 10 ms (middle), and calculated stretching distance (bottom) by the relation of r– r
cos(2πt/T) under an S-R speed of 500 cpm.

(Figure 2c). During the S-R motion, the EL was recorded every 8 ms, corresponding to the integration time limit of the optical detector. The measurements showed that the time-dependent changes in the optical intensity and electric current followed an elongation curve expressed as r r
cos(2πt/T), where r is the radius of the rotating bar, t is the time, and T is the period (Figure S5, Supplementary materials). When the sample was stretched, the EL intensity and current gradually increased; the highest EL intensity was observed at the maximum elongation condition. This result is explained by the decreased thickness of the EML due to stretching, which corresponded to an increased capacitance of the ZnS + PDMS composite layer. The increased capacitance enabled a larger AC current flow through the dielectric layer, resulting in an increased EL intensity. Although the AgNW + PDMS composite resistance was increased with increased stretching distance, the EL intensity and current were more sensitive to changes in the electric field, because the emission intensity is mainly determined by the bias voltage rather than by the conductive components of the current. In addition, the changes in resistance were considerably smaller than the total device resistance. Hence, the behaviors of the EL intensity and current were primarily determined by the stretching distance, which corresponded to the decrease of the EML thickness. Regarding the frequency-dependent electro-optical characteristics, including intensity and color, of the thinEML device under S-R motion, a detailed explanation is provided in Figure S6–S8 (Supplementary materials). Based on the observations for the thin-EML device, we fabricated a corresponding thick-EML device with a ZnS

+ PDMS layer thickness of  150 μm (Figure S9, Supplementary materials) to investigate the simultaneously emitted EL and ML under S-R motion and the application of AC voltage. Compared to the thin device, the thick-EML device exhibited lower EL and higher ML intensities with the decrease of the electric field and an increased number of ML-emitting ZnS particles in the film. A comparison between the measured ML intensities of the thick- and thin-EML devices showed an approximately 10-fold difference (Figure S10, Supplementary Materials). Considering the high EL intensity observed from the thin-EML device, the decrease in EL by a factor of approximately 35 is likely to be more effective in observing simultaneous EL/ML emissions than the increase in ML by increasing the EML thickness. Because of these combined effects, an EL/ML spectrum is emitted from the thick-EML device with S-R motion (Figure 3a). The EL spectrum from the released state is blue, whereas green emissions become dominant after the S-R motion is started (inset of Figure 3a). The measured AC current of the thickEML device showed behavior similar to that of the thin-EML device (Figure S6c, Supplementary materials), with a linear dependence on the frequency and a similar tendency in the low-frequency region (Figure S11, Supplementary materials). However, the EL/ML intensity behavior with increasing frequency differs from that of the thin device (Figure 3b). The S-R case exhibits a higher intensity than the released and stretched static conditions because of the ML generation. Even without S-R motion, the EL shows a stronger saturation tendency in the released and stretched conditions compared to that observed in the thin-EML device (Figure S6a, Supplementary materials). This observation is

Please cite this article as: S.M. Jeong, et al., Simultaneous dual-channel blue/green emission from electro-mechanically powered elastomeric zinc sulphide composite, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2016.01.012

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Simultaneous dual-channel blue/green emission

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Figure 3 Electro-optical characteristics of the thick-EML (150-μm-thick) device under applied 150-V AC electric field for released, stretched, and S-R (500 cpm) conditions. (a) EL (released and stretched) and EL/ML (S-R) spectra (int. time =3 s). Inset: Normalized spectra. b) Optical emission intensity (integral over [400, 600], int. time =3 s) for the released, stretched, and S-R conditions as a function of frequency. (c) Frequency dependence of the EL and EL/ML spectra (int. time =3 s) under released (left) and S-R (right) conditions. (d) Changes in CIE coordinates (x, y) obtained from spectra in (c). Photographic insets depict images of EL (released) and EL/ML (S-R) emissions at 1 kHz frequency applied electric field. (e) Time-resolved intensity of EL/ML (S-R) from the thick-EML device (1 kHz, int. time =8 ms).

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explained by considering the weaker electric field of the thick-EML device: because EL increases exponentially with voltage, the lower voltage creates a smaller number of empty EL centers in the thick film, which are easily filled with increases in frequency. Thus, saturation occurs more rapidly in the thick-EML device. The EL and EL/ML spectra of the released and S-R cases are shown in Figure 3c, demonstrating the differences between

the conditions. The EL-only spectra (released) exhibit bluedominant spectral shapes with increasing frequency, whereas green is dominant in the S-R condition. The CIE coordinates also show color differences between the released and S-R cases with increases in frequency (Figure 3d). In particular, the EL/ML color (bluish-green, S-R) at 1 kHz is clearly different from the EL color (blue, released), as shown in the photographs (insets of Figure 3d). In short, the emission of the

Please cite this article as: S.M. Jeong, et al., Simultaneous dual-channel blue/green emission from electro-mechanically powered elastomeric zinc sulphide composite, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2016.01.012

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Figure 4 (a) Schematic of the ZnS+PDMS composite layer sandwiched between two AgNW-embedded PDMS layers. The bottom AgNW+ PDMS layer is patterned to emit EL in the shape of a bird and a flower. Photographs of the patterned EL, EL/ML and ML images under various electro-mechanical stress conditions: (b) Patterned EL image from the released and stretched device (inset) under an AC electric field of 150 V with 1 kHz frequency. Patterned EL/ML image with ML background under an applied AC electric field with (c) 1 kHz and d) 100 Hz electrical frequency, together with 500 cpm S-R motion. (e) ML image under the same S-R condition (without electric field). (f) Comparison of the EL, EL/ML, and ML spectra (int. time =3 s) and (g) CIE coordinates obtained at the positions noted as EL (A), EL/ML (B), and ML (C) in panels (b), (c), and (e), respectively.

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composite film device changes from blue to bluish-green when the mechanical stress is triggered. We also performed a time-resolved EL/ML intensity measurement during the S-R motion. ML was generated twice during each 500-cpm S-R cycle; EL was enhanced when the device was stretched. Combining these two phenomena, the EL/ML emission intensity exhibits three peaks during each cycle (Figure 3e). The first and second ML peaks occur during stretching and release, respectively. As shown, the time distance between the two ML peaks is asymmetric, because the stress variation speed is largest during the initial deformation. The EL peak is located at the center of the stretchingreleasing transition, reaching a maximum intensity at the maximum elongation. Because the thick-EML device is MLdominant, the EL peak is lower in intensity than the ML peaks. This implies that the EL/ML emission produces a flickering luminescence of differing intensities and colors. However, the intensity and color variations occur within a few tens of milliseconds, which are difficult to distinguish visually. If phosphors with different ML lifetimes were used or different mechanical stresses were applied, it would be possible to control the intensity and color variations, depending on the desired properties of the device. To describe the potential applications of such devices, the discussion of device performance regarding brightness and

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durability is useful. The measured brightness was very low in both the thin- and thick-EML cases, at 1–1.3 cd/m2 and  0.1–0.35 cd/m2, respectively. The low brightness values of the developed devices can be explained as follows. The brightness of EL/ML devices is mainly determined by the EML thickness. It is always desirable to have high brightness for both EL and ML. However, EL and ML brightness values are inversely related. Decreasing the EML thickness increases the EL brightness by increasing the electric field, but simultaneously decreases the ML brightness by decreasing the number of emitting particles, whereas increasing the EML thickness creates the opposite situation. Moreover, the transmittance of the AgNWembedded PDMS film was only 50%, even in the thin AgNW electrode, which was one major obstacle to improving the brightness (Figure S3, Supplementary materials). Hence, to fabricate an EL/ML device with a measurable EL/ML range, the proper EML thickness was around  100 μm in current experimental conditions. We also performed experiments verifying the durability of the thick-EML device, as shown in Figure S12 (Supplementary materials). ML intensity was gradually diminished with increased S-R cycling, whereas the ML/EL intensity was stable for at least 5000 S-R cycles. Because the EML contains a few layers of ML particles and PDMS, the current mechanical stress

Please cite this article as: S.M. Jeong, et al., Simultaneous dual-channel blue/green emission from electro-mechanically powered elastomeric zinc sulphide composite, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2016.01.012

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Simultaneous dual-channel blue/green emission condition can be considered as rather harsh condition for stable ML emission. Detailed information on these behaviors is provided in the Supplementary materials. These inherent problems of brightness and durability may be solved by employing highly efficient EL/ML materials with a better fabrication process for the AgNW electrodes. The use of efficient materials may enable the fabrication of bright and durable EL/ML composites more sensitive to even mild electro-optical stimuli. According to the above findings, we demonstrated a unique display application that used the simultaneous dual-channel blue/green emission of the fabricated devices to visualize a patterned region by independently controlling the applied electromechanical stresses. Figure 4a shows a schematic of the device composed of a ZnS + PDMS composite layer sandwiched between two AgNW-embedded PDMS layers. Simplified shapes of a flower and a bird were patterned using AgNWs on the bottom PDMS layer to create a simultaneous EL/ML-emitting image, whereas only ML was emitted from the no-AgNW region. Patterning the top AgNW-embedded ZnS + PDMS layer was unnecessary, because EL was emitted wherever the electric field was applied between the (patterned) bottom and top AgNWs. The thickness of the ZnS + PDMS layer was carefully tuned to obtain the median value of approximately 125 μm (Figure S13, Supplementary materials) between the EL- and MLdominant thickness values of 90 and 150 μm, respectively, to avoid the weak ML emissions observed from the thin-EML device or the insufficient color differences seen in the thickEML case. To demonstrate patterned imaging in a single-layered ZnS + PDMS composite device, we applied an electromechanical stress to create patterned EL images with simultaneous MLemitting backgrounds. The application of an electric field during cyclic S-R motion was expected to generate the emission of a mixture of EL (blue or green) and ML (green) from the patterned region and only ML (green) from the background region. Figure 4b–e shows various images with different color characteristics, each acquired under independently controlled electric and S-R conditions. With an electric field of 150 V at 1 kHz and no S-R motion, the blue EL-emitting flower and bird images are clearly seen. No short circuit is observed, even when the device is manually stretched to 40% elongation (inset of Figure 4b). When the S-R motion is applied, the patterned images exhibit a skyblue color resulting from the color mixture of the EL and ML emissions; the background ML is green (Figure 4c). By decreasing the EL frequency to 100 Hz, the color change of the patterned region is verified (Figure 4d). Interestingly, although the color of the images is green, similar to the ML background, the images remain visible because the green EL/ML-emitting pattern is brighter than the ML-onlyemitting background. Without the applied electric field, only the green ML is observable, without patterns, because the top PDMS layer is not patterned (Figure 4e). The spectra and corresponding CIE coordinates at the marked positions (EL (A) (Figure 4b), EL/ML (B) (Figure 4c), and ML (C) (Figure 4e)) are shown in Figure 4f–g, respectively. The color difference between EL/ML (B) and ML (C) is attributed to the relatively EL-dominant condition established by the film properties, which inevitably weakens the ML. This drawback could be resolved by using a stronger

7 voltage source to obtain a higher electric field, even in thick ZnS + PDMS films; thus, a broad range of EML thicknesses can be used to produce bright EL/ML images.

Conclusions

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In this study, we demonstrated simultaneous EL/ML emissions from a ZnS+PDMS composite sandwiched between AgNWembedded PDMS layers by applying both electrical fields and mechanical stresses to the composite. We obtained blue EL from an EL-dominant device (thin-EML) with EL intensity and current results showing similar sinusoidal behaviors with changes in the stretching distance of the composite film. During cyclic S-R motion, the highest EL intensity and AC current were obtained at the maximum elongation condition; both recovered the original values after the release of stress. Similar color characteristics were observed with increasing electrical frequency in both the released and S-R conditions. However, when a thickEML device was used, the generation of ML affected the color variation in the S-R condition and green emissions were introduced by the mechanical excitation. Based on these results, we fabricated a patterned EL/ML device to visualize the EL/ML and ML emitting regions. Dual-channel EL/ML light emission from a single ZnS-based material, as demonstrated here, could be advantageous for the development of energy-saving hybrid systems in EL/ML imaging or display applications. If the designed light source could generate an additional light by a different excitation source, this would offer a novel multi-modal functional source. Our findings provide significant information for the emerging research field of stretchable electronics by employing a device structure using ML materials embedded in an intrinsically stretchable ACEL device. We believe that the findings of this study provide important data toward the convergence of research on stretchable EL/ML materials; this data may find applications in many research fields. Elucidating the current challenges in determining excitationsource-dependent blue/green peak generation is an important step for the feasible design of a novel multi-modal functional light source. For EL, we have experimentally verified the increase of the blue peak contribution when the electrical frequency applied to an ACEL device increased [11–13]. We reported similar ML behavior by comparing simple stretching with strong vibrations under wind-driven conditions [13]. For comparison, we performed PL measurements and found similar tendencies in PL emissions; the intensity of the blue peak is gradually increased as the applied UV (365 nm) energy (μW/ cm2) increases (Figure S14, Supplementary materials). Hence, based on the previous EL and ML results and the current PL data, we conclude that the emitted color of the ZnS:Cu-based phosphor used in these studies is highly susceptible to various excitation strengths or frequencies, whether electrical (EL), mechanical (ML), or optical (PL). As mentioned in Section 1, the change of color can be induced by the two emission bands (blue and green) existing in the phosphor. A possible mechanism for this change was suggested in [13]. Unfortunately, even this study fails to establish a detailed mechanism; discovering this mechanism is an important factor for the feasible design of electro-mechanically powered optomechanical displays. However, we note that related research is being performed continuously to resolve this bottleneck. We believe that these ambiguous subjects will be resolved in the near future,

Please cite this article as: S.M. Jeong, et al., Simultaneous dual-channel blue/green emission from electro-mechanically powered elastomeric zinc sulphide composite, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2016.01.012

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Acknowledgments

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This work was supported by the Mid-career Researcher Program (NRF-2014R1A2A2A01003133) through the National Research Foundation of Korea (NRF) grant. This work was also supported by the DGIST R&D Program (15-NB-04) through DGIST, which is funded by the Ministry of Science, ICT and Future Planning (MSIP).

Appendix A.

Supplementary material

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2016.01.012.

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Soon Moon Jeong is currently Senior Researcher at the Division of Nano and Energy Convergence Research of Daegu Gyeongbuk Institute of Science and Technology (DGIST). He obtained his Ph.D. degree in Department of Organic and Polymeric Materials from Tokyo Institute of Technology, Japan in 2008. He worked at Nippon Oil Corporation as a Senior Researcher during 2009–2012. Then he joined Nano & Bio Research Division of DGIST in 2012 and built research team. His current research interests are (a) Development of Mechanoluminescent Devices and (b) Application of Photonic structures in organic optical devices.

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Seongkyu Song is currently Researcher at the Division of Nano and Energy Convergence Research of Daegu Gyeongbuk Institute of Science and Technology (DGIST). He received his M. S degree in Department of Chemical Engineering from Keimyung University, Republic of Korea in 2011. His research interests are Mechanoluminescence and Transparent electrodes related with stretchable devices.

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Hyunmin Kim received his Ph.D. degree with the topic of Nonlinear Optics from nanostructural systems from the department of chemistry in University of California-Irvine in 2009. Then, he moved to National Institute of Standards and Technology in Gaithersburg until 2011 as a guest researcher to delve out the details of a multiplicative-type super-resolution microscopy using optical four-wave mixing. A brief postdoctoral period for 10 month in Korea Research Institute of Chemical Technology regarding time-resolved stimulated Raman systems was critically followed by the permanent position in the one of the newly founded Korean national labs (DGIST) from the year of 2012.

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Please cite this article as: S.M. Jeong, et al., Simultaneous dual-channel blue/green emission from electro-mechanically powered elastomeric zinc sulphide composite, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2016.01.012

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