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A Mechanoluminescent ZnS:Cu/Rhodamine/SiO/PDMS and Piezoresistive CNT/PDMS Hybrid Sensor: RedLight Emission and a Standardized Strain Quantification Kee-Sun Sohn, Suman Timilsina, Satendra Pal Singh, Jin-Woong Lee, and Ji Sik Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12931 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 3, 2016

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ACS Applied Materials & Interfaces

A Mechanoluminescent ZnS:Cu/Rhodamine/SiO2/PDMS and Piezoresistive CNT/PDMS Hybrid Sensor: Red-Light Emission and a Standardized Strain Quantification Kee-Sun Sohna, Suman Timilsinab, Satendra Pal Singha, Jin-Woong Leea, and Ji Sik Kimb,* a

Nanotechnology & Advanced Materials Engineering, Sejong University, Seoul, 143-747, South

Korea b

School of Nano & Advanced Materials Engineering, Kyungpook National University

Kyeongbuk,742-711, South Korea KEYWORDS: strain sensors, mechanoluminescence, piezoresistive, rhodamine B, ZnS:Cu

ABSTRACT: We developed a hybrid strain sensor by combining mechanoluminescent ZnS:Cu/rhodamine/SiO2/PDMS composites and piezoresistive CNT/PDMS for qualitative and quantitative analysis of onsite strain. The former guarantees a qualitative onsite measure of strain with red light emission via mechanoluminescence (ML) and the latter takes part in accurate quantification of strain through the change in electrical resistance. The PDMS matrix enabled a strain sensing in a wider range of strain, spanning up to several hundred % in comparison to the

ACS Paragon Plus Environment

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conventional rigid matrix composites and ceramic-based ML sensors. Red-light emission would be much more effective for the visualization of strain (or stress) when ML is used as a warning sign in actual applications such as social infrastructure safety diagnosis, emergency guide lighting, and more importantly, in biomedical applications such as in the diagnosis of motility and peristalsis disorders in the gastrointestinal tract. Despite the realization of an efficient redlight-emitting ML in a ZnS:Cu/rhodamine/SiO2/PDMS composite, the quantification and standardization of strain throughout the ML has been far from complete. In this regard, the piezoresistive

CNT/PDMS

compensated

for

this

demerit

of

mechanoluminescent

ZnS:Cu/rhodamine/SiO2/PDMS composites.

1. INTRODUCTION The variety of strain (or stress) sensing systems includes laser photoelasticity, laser-ultrasound, Raman spectroscopy, electrical resistance measurement, and a strain gauge (or wire) attachment technique.1-5 However, such strain sensors operating via expensive instrumentation do not meet the human need for immediate onsite stress sensing. The most dramatic form for human recognition of an external stimulus such as mechanical strain is immediate, onsite visualization. ML that would enable light emission upon mechanical loading would be one of the most promising means for the immediate sensing of onsite strain. Non-destructive ML has been recently realized via two brilliant types of materials, SrAl2O4 (SAO) and ZnS series, with various doping elements. The SAO type includes SrAl2O4:Eu; SrAl2O4:Eu,Dy; and, SrAl2O4:Eu,Dy,Nd.614

The ZnS type involves ZnS:Mn; ZnS:Cu,Al; ZnS:Cu,Mn; and, ZnS:Cu,Cl.15-20 The most

promising level of ML was achieved by the green emissions from both SrAl2O4:Eu and ZnS:Mn. The SAO and ZnS-based ML materials exhibit an emission, color tuning possible from blue to


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red by varying the dopant.21-27 For example, a red emission in ZnS:Mn was obtained through Te substitution at the Mn site.26,27 There are also several other red emitting mechanoluminescent materials such as, Pr3+ activated calcium niobates composed of mCaO·Nb2O5 (m = 1, 2, and 3) and MZnOS:Mn (M = Ba, Ca).28-30 The CaZnOS:Mn2+/epoxy resin ML composite with red emission was used for mechanical stress sensor, that can sense and image multiple mechanical stresses and decipher the stress intensity distribution.30 However, application of these red emitting ML materials for stress sensing are limited to rigid matrix composite only and show no ML with soft matrix composite. The rigid matrix ML restricted the strain sensing within a limited strain range and made it impossible to sense a high strain reaching beyond 100 %, which might be required for an artificial skin application. The PDMS matrix-ML powder-composite on the other hand, would be the most promising for strain sensing in a wider range of strain, spanning up several hundred % in comparison to the conventional rigid matrix composites and ceramic-based ML sensors. Very recently, Jeong et al,31 have introduced PDMS-based ML composites and reported several noteworthy outcomes. They also have reported a new strategy for the color manipulation by physically combining a fluorescent dyes with existing mechanoluminescent materials based on the PDMS matrix. In which the ZnS/PDMS composite was spontaneously diffused with 4(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran

(DCJTB)

and

color manipulation from green to red via DCJTB was achieved. However, the emission color claimed to be red was not true red as per requirements, but an amber color with the emission wavelengths ~600 nm, and no method for quantification of strain through the ML process was introduced.31 In order to overcome these deficiencies, we have introduced rhodamine B dyecoated SiO2 nano-particles (rhodamine/SiO2) as color-converting agents in place of DCJTB and


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hybridized with piezoresistive CNT/PDMS composite sheet for the standardized quantification of strain though an electrical resistance measurement. The rhodamine B dye-coated SiO2 nano-particles (rhodamine/SiO2) were mixed with mechanoluminescent ZnS:Cu micro-particles in a PDMS matrix. The green ML of ZnS:Cu is down-converted to red light by the rhodamine/SiO2, which are well known to be efficiently excited in the green range and subsequently emit red/yellow light when coupled with green phosphors.32,33 However, rhodamine/SiO2 has never been combined with mechanoluminescent materials at mechanical excitations, although such a promising color conversion propensity has been proven in photo-excitations. We eventually constructed a ZnS:Cu/rhodamine/SiO2/PDMS composite by combining the well-known mechanoluminescent ZnS:Cu and a color-converting rhodamine/SiO2. The ZnS:Cu/rhodamine/SiO2/PDMS composite gave a promising red emission under all types of mechanical loading conditions without additional photo-excitations. In parallel with the realization of a red-emitting mechanoluminescent strain sensor, an accurate quantification and standardization should be realized in order to accomplish promising strain sensing, but this requirement has yet to be fulfilled for ML-based strain sensors. For the sake of accurate quantification

and

standardization

of strain

sensing,

we hybridized

both

mechanoluminescent and piezoresistive materials and developed a bilayer hybrid strain sensor consisting of ZnS:Cu/rhodamine/SiO2/PDMS and CNT/PDMS. The piezoresistive function in CNT/PDMS denotes a change in electrical resistance by loading.34-36 To develop a hybridized piezoresistive-mechanoluminescent strain sensor, a CNT/PDMS composite sheet was attached to a ZnS:Cu/rhodamine/SiO2/PDMS composite sheet. The ZnS:Cu/rhodamine/SiO2/PDMS layer takes part in the qualitative, onsite measure of strain on the spot through luminescence, and the CNT/PDMS layer guarantees accurate, quantitative and standardized strain sensing via the


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change in electrical resistance. This hybridization compensates for the demerits of the mechanoluminescent strain sensor that has led to serious problems in the quantification of strain. However, electrical resistance measurement requires complicated instrumentation to constitute an electrical circuit, and, moreover, it requires extra energy in the form of electrical voltage (or current) to be applied to the CNT/PDMS specimen in order to measure the electrical resistance. Therefore,

the

piezoresistive

CNT/PDMS

and

the

mechanoluminescent

ZnS:Cu/rhodamine/SiO2/PDMS are mutually complementary.

2. EXPERIMENTAL SECTION A bilayer composite was synthesized from commercially available ZnS:Cu (LONCO Company Limited), rhodamine B (Sigma-Aldrich), SiO2 (Sigma-Aldrich), CNT (Carbon Nano-material Technology Co. Ltd), and PDMS (Sylgard® 184 Silicone Elastomer). The diameter of the SiO2 was 20 nm; the CNT was multi-walled with a diameter of 20 nm and a length of 5 µm. First, a rhodamine B-coated SiO2 was prepared. Then, 0.1 gm of rhodamine B and 10 gm of SiO2 were combined in 200 ml of methanol and stirred constantly using a magnetic stirrer at 50 oC. The rhodamine B-coated SiO2 was obtained as a red dry powder after the methanol was completely evaporated. To construct a piezoresistive layer, 1 wt % of CNT was mixed with liquid PDMS in a plastic cylindrical container. To form the mechanoluminescent layer, the rhodamine B-coated SiO2 and ZnS:Cu were mixed homogeneously in a ratio of 1.25:1 by wt %, and this homogeneously mixed powder was then dispersed in liquid PDMS at a ratio of 5:1 by wt % in another plastic cylindrical container. In order to homogeneously disperse the nano-particles in PDMS and avoid agglomeration, particularly of the CNT, a few alumina grinding balls with a diameter of 10 mm


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were added and both containers were transferred to a planetary shear mixer and were dispersed at a mixing speed of 400 rpm for 2 hr. A PDMS curing agent was then added at a weight ratio of 1:10 and again the containers were transfered to a planetary shear mixer for 20 min. Finally, the composites were degassed inside a vacuum for 20 min to remove entrapped bubbles. On a glass plate, a 150 mm x 50 mm rectangular mold was prepared using paper tape with a thickness of 0.30 mm. Using the Doctor's Blade Technique, a piezoresistive CNT/PDMS composite was cast. Then the glass plate was heated at 60 oC for 30 min. The CNT/PDMS composite was then partially solidified and a second rectangular mold with a thickness of 1.40 mm constructed from polymer was attached precisely above the first mold with the help of double-sided tape. A rhodamine B-coated SiO2/ZnS:Cu and PDMS composite was then poured onto the mold, and the mechanoluminescent layer was cast following the Doctor's Blade Technique. The glass plate was heated at 60 oC for 2 hours and a completely solidified bilayer composite was produced. A rhodamine B-coated SiO2/ZnS:Cu and PDMS composite was added onto the partially solidified CNT-PDMS composite in order to strengthen the bonding of the bilayer. Since the CNT/PDMS and ZnS:Cu/rhodamine/SiO2/PDMS composites both shared the PDMS matrix, there was no mismatch problems on the interface area, even when a severe strain of as much as several hundred percent was applied. The completely solidified bilayer composite was then cut into a typical sub-size tensile specimen: the gauge length, the width, and the thickness were 25 mm × 6 mm × 1.50 mm, respectively. On the CNT-PDMS face, two copper wires were connected to the specimen with the help of a CNT-PDMS mixture at a distance of 20 mm. Figure S1 in the Supporting Information shows an actual photo of the specimen in detail. In addition, a schematic diagram of the experimental setup is shown in Figure S2 in the Supporting Information.


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3. RESULTS AND DISCUSSION First, we examine the photoluminescence (PL) of every constituent material along with their mixture. The excitation and emission spectra of these materials were monitored inside the PDMS. There was no interference due to the PDMS matrix since the PDMS was optically transparent in the wavelength range adopted for our PL, photoluminescence excitation (PLE) and ML measurements. The PL measurement of bare SiO2 was skipped because it showed no luminescence.

In

particular,

the

emission

spectrum

of

ZnS:Cu/PDMS

and

ZnS:Cu/rhodamine/SiO2/PDMS was monitored as a function of the excitation wavelength. Figure 1 (a) shows the PL spectra for the ZnS:Cu/rhodamine/SiO2/PDMS monitored at various excitation wavelengths from 254 to 365 nm, exhibiting two type of emission peaks. One from the ZnS:Cu (weaker peaks on the shorter wavelength side) and the other from rhodamine-coated SiO2 (stronger peaks on the longer wavelength side). Figure 1 (b) shows the emission spectra of ZnS:Cu/PDMS and rhodamine/SiO2/PDMS separately. The emission spectrum of ZnS:Cu was blue-shifted (marked with blue arrow), and that of the rhodamine-coated SiO2 was red-shifted (marked with red arrow) as the excitation wavelength moved from 254 to 365 nm. Such a mutually

converse

shift

made

the

emission

spectrum

of

the

mixture

of

ZnS:Cu/rhodamine/SiO2/PDMS unchanged with respect to the excitation wavelength. The green emission from ZnS:Cu was not 100% down-converted to the red emission by the rhodaminecoated SiO2. The ZnS:Cu emission in the ZnS:Cu/rhodamine/SiO2/PDMS composite survived, but the spectral shape differed from that of a bare ZnS:Cu emission. As shown in Figure 1 (c), the PLE spectrum of the rhodamine/SiO2/PDMS composite detected at 590 nm provides a reasonable

explanation

for

the

shape

change

of

the

ZnS:Cu

emission

in

the


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ZnS:Cu/rhodamine/SiO2/PDMS composite. The excitation (absorption) band of the rhodaminecoated SiO2 (Figure 1 (c)) dropped dramatically in the left side of the bare ZnS:Cu emission spectrum (Figure 1 (b)), as indicated by a long vertical line, so that only the right side of the ZnS:Cu emission spectrum in the ZnS:Cu/rhodamine/SiO2/PDMS composite (Figure 1 (a)) was decreased dramatically due to the absorption by the rhodamine-coated SiO2, and this made it to appear as blue-shifted. In addition, the ML spectrum for ZnS:Cu/PDMS and ZnS:Cu/rhodamine/SiO2/PDMS was measured under cyclic loading conditions of 1 Hz. The ML spectrum slightly differed from that of the PL spectrum for both ZnS:Cu/PDMS and ZnS:Cu/rhodamine/SiO2/PDMS composites, as shown in Figure 2 (a) and (b). It is very common that the PL spectrum differed from the electroluminescence (EL) and cathodoluminescence (CL) spectra for almost all ZnS-based luminescent materials with various dopants.15-20 In addition, the emission peak wavelength varied with the applied voltage and frequency in the EL and CL situations and also varied with the excitation wavelength and excitation power in the PL situation. It is obvious that different centers and traps would operate in response to the excitation details (PL, EL, CL, ML, input power, frequency, wavelength, etc.).15-20 Such a complicated dopant (and defect) energy level distribution in the band gap constitutes a very complicated recombination process,16,19,20 and the emission photon energy distribution depends on the relative contribution of each recombination process. A detailed discussion on this issue would be outside the scope of the present investigation. We were interested only in a clear experimental finding that the ML peak for ZnS:Cu/PDMS would be located in an energy side that would be lower (longer wavelength) than the PL peak, which would indicate the red emission peak shift in the same way for ZnS:Cu/rhodamine/SiO2/PDMS as well. However, the emission spectrum of ZnS:Cu/PDMS that


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survived in the ZnS:Cu/rhodamine/SiO2/PDMS composite did not show such a prominent distinction between the ML and PL spectra, as shown in Figure 2 (b), which might be due to a strong absorption by rhodamine. Figure 3 (a) shows the load vs. displacement plot for the bilayer hybrid sensor consisting of CNT/PDMS and ZnS:Cu/rhodamine/SiO2/PDMS at three different frequencies. The load nearly coincided with the displacement. The overall load level was very low because of the PDMS matrix's highly stretchable character and the load signal was noisy in contrast to the clearer displacement signal. Accordingly, it would be more convenient to employ the displacement rather than the load as an applied input signal against the response signals such as luminescent intensity and electrical resistance. Also, the expression 'applied elongation' can be used along with 'applied displacement.' In fact, the terms 'load,' 'displacement,' 'stress,' and 'strain' should be treated equally as input signals with no distinction among them from a practical point of view, since they are all in completely linear relationships both in static and dynamic conditions with no time delay. Consequently, it would make no difference what terminology was used as the input signal in this study. It is also noteworthy that the terms stress and strain stand for a sort of macroscale concept but describe neither localized micro- nor nano-scale stresses and strains. A more reliable interpretation for the ZnS:Cu/PDMS composite ML strain sensor has been recently introduced in Ref. 37. The newly suggested ML mechanism differed sharply from the conventional ML mechanism involving a strong load transfer from the matrix to ZnS:Cu powders and stress-driven trap releasing in association with a local piezoelectric field.22-24 Several decisive pieces of experimental evidence were established to substantiate a more reliable mechanism for the ML of the ZnS:Cu/PDMS composite.37 The mechanically driven luminescence in a ZnS:Cu/PDMS composite turned out to originate from triboelectricity-induced


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charges.37 The triboelectricity has recently been attracting a great deal of attention since it turned out to be a very promising concept in view of the energy harvesting.38, 39 The ML detected in response to cyclic elongations revealed that two distinct luminescent peaks were observed corresponding to every single tensile loading cycle. This means that the luminescence was discretely detected only at loading (extension) and deloading (retreat) moments, as shown in Figure 3 (b). This finding is in sharp contrast to the conventional mechanoluminescent response to cyclic loading. Although a two-peak behavior in ZnS:Mn film under compressive load of 500 N looks similar to our observation,21 but it was just a typical loading rate dependent ML in response to the static loading along with an abrupt deloading. We have also reported the exactly the same observation in SAO.40 Figure S3 in the Supporting Information exhibits a systematic comparison between the conventional ML and our case in the dynamic loading condition, leading to a concrete conclusion that the ZnS:Cu/PDMS and ZnS:Cu/rhodamine/SiO2/PDMS composites can never be interpreted by conventional ML. A certain degree of concurrency between the applied cyclic loading and the luminescent response was clearly observed in the conventional ML for the SrAl2O4:Eu2+/Epoxy resin composite.10 However, the ZnS:Cu/PDMS composite obviously showed a clearly separated double-peak response to each loading cycle. This distinction became more prominent when the load (or displacement) vs. luminescent intensity plot, the so-called hysteresis behavior, was examined precisely. A complete distinction in hysteresis behavior between SrAl2O4:Eu2+/Epoxy resin and ZnS:Cu/PDMS was clearly confirmed in Figure S3 (c) and (d) in the Supporting Information. Therefore, such an extraordinary double-peak response in the ZnS:Cu/PDMS composite proved to be a decisive evidence supporting the newly proposed triboelectricity-induced luminescence mechanism.37 There was neither a strong load transfer from the PDMS matrix to ZnS:Cu


10

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particles nor the consecutive ML in response to the stress on the ZnS:Cu particle. Only a soft friction on the interface between the PDMS matrix and the ZnS:Cu particle was responsible for the luminescence. The most decisive evidence to support our newly proposed ML mechanism is the absence of ML when a much harder matrix material was adopted. This was also doublechecked clearly by the absence of ML in the ZnS:Cu/Acryl resin composite.37 It should be noted that the Acryl resin has proven to be a good matrix material for ML,6-14, 37 because a strong load transfer is guaranteed and the Acryl resin is optically transparent in the visible range. The epoxy resin also led to no ML for ZnS:Cu, although it also proved to be a good matrix material for ML for other ML phoshpors.6-14, 37 If the triboelectricity-involved ML mechanism were to be denied, then one should explain the absence of ML in these hard matrixes in advance. In this context, the newly proposed ML mechanism also took effect in the ZnS:Cu/rhodamine/SiO2/PDMS composite. Because of the triboelectricity-involved trait, i.e., the completely separated double-peak response, it would be tricky to reliably quantify and standardize the strain via the mechanically driven luminescence in both ZnS:Cu/PDMS and ZnS:Cu/rhodamine/SiO2/PDMS composites, in contrast to the successful standardized quantification of strain in the case of a conventional mechanoluminescent

SrAl2O4:Eu2+/Epoxy

resin

composite.10

The

quantification

and

standardization problem is the most discouraging drawback of both the green-light-emitting ZnS:Cu/PDMS

and

red-light-emitting

ZnS:Cu/rhodamine/SiO2/PDMS

composites.

A

standardized strain sensor requires a consistent relationship between the applied strain and the luminescent intensity under all types of loading circumstances irrespective of static or dynamic loading and whether it is tensile or compressive.10 As shown in Figure S3, the double-peak response along with an eccentric hysteresis behavior makes it impossible to standardize the


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luminescent intensity-displacement relationship. Another more serious obstacle for standardized quantification is a loading frequency-dependent ML, namely, the luminescent intensity increased with the loading frequency, as shown in Figure 3 (b). Thus, it would not be a good idea to quantify the strain (load, stress, or displacement) using the ML response. CNT/PDMS composites have recently attracted a great deal of attention for their versatility in many applications. 34-36,41-44 One of the most remarkable applications for CNT/PDMS composites is a strain sensor as its electrical resistance varies with the applied strain due to the electrical resistance-strain sensitivity of the CNT network in PDMS.34-36 The interlinked CNT network acts as a conducting agent in the insulated PDMS matrix, and the network can be either slackened or shrunk upon external loading, which thereby leads to a change in electrical resistance. Since the strain has a consistent linear relationship with the measured resistance for CNT/PDMS composites, it is possible to quantify the strain with accuracy by measuring the electrical resistance. The CNT/PDMS composite would be a reliable, standardized strain sensor since a consistent strain-electrical resistance relationship holds whatever loading conditions are adopted. In this regard, we employed a piezoresistive CNT/PDMS part to compensate for the failure in quantification and standardization of the mechanoluminescent ZnS:Cu/rhodamine/SiO2/PDMS sensor. This means that an electrical signal from the piezoresistive CNT/PDMS part was much more useful in accurately measuring the strain rather than using the ML from the ZnS:Cu/Rhodamine/SiO2/PDMS part. In spite of the guaranteed, accurate quantification and standardization for the piezoresistive strain sensor, strain sensing along with visual confirmation through the ML would be more promising than just a simple digit representation, because human visualization of an external stimulus such as strain has a much more dramatic impact. Therefore,


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both the piezoresistive and mechanoluminescent approaches for strain sensing are mutually complementary. The CNT/PDMS part in the bilayer composite plays a very positive role in achieving the standardized quantification of an applied strain. Figure 3 (c) shows the electrical resistance vs. the displacement for a bilayer CNT/PDMS and ZnS:Cu/rhodamine/SiO2/PDMS hybrid strain sensor at three different frequencies. A small shoulder appears in the electrical resistance signal, particularly, in the deloading (retreat) side of every cycle. A similar shoulder has been observed in the deloading side of periodic compressive loading cycle.45 The appearance of this sort of shoulders is indicative of the second harmonic generation and hysteresis behavior, which would make it impracticable to achieve a standardized quantification of strain sensing. In contrast to the deloading part involving the shoulder, a relatively good coincidence between the electrical resistance and the displacement was observed in the loading (elongation) side of every cycle. On the other hand, the ML intensity vs. displace plot (Figure 3 (b)) would never lead to such a coincidence, that is, neither in the loading nor the deloading part. The acceptable overlap observed between the electrical resistance and the displacement in the elongation side of every cycle designates a pseudo-linear relationship between the electrical resistance and the displacement, which is positive for a reliable standardized quantification of strain. Figure 4 shows a clear one-to-one relationship between the electrical resistance and the displacement. The data constituting Figure 4 were extracted from the loading (elongation) part only. What is more interesting in Figure 4 is the consistent level of electrical resistance, depending only upon the displacement level but not being affected by the loading frequency. Figure 4 shows the complete coincidence of three electrical resistance vs. displacement plots obtained at three different frequencies, which confirmed the loading rate independency and thereby made it possible to


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constitute a standardized strain-sensing system. This is a very positive trait that can be used to accomplish a standardized strain-sensing system. On the contrary, we observed an increased mechanoluminescent intensity as the frequency increased even at the same level of displacement amplitude, as shown in Figure 3 (b).

3. CONCLUSION We examined the hybridized CNT/PDMS and ZnS:Cu/rhodamine/SiO2/PDMS sensor in detail in terms of both the piezoresistance and mechanoluminescence by applying cyclic elongation at three different frequencies. The relationship between applied strain and electrical resistance response was examined quantitatively and returned an accurate, standardized strain sensing. Also, a red light-emitting mechanoluminescent sensor was developed that can be much more useful in producing an immediate onsite red-light emission upon applied strain, which would be beneficial as a warning sign in actual applications such as social infrastructure safety diagnosis, emergency guide lighting, and biomedical strain sensors. The mechanoluminescent part of the hybrid strain sensor enables an immediate detection of onsite strain with no high precision for strain measurement, while the piezoresistive part enables an accurate quantification and standardization of strain. The electrical and luminescent signals operate

in

a

mutually

complementary

manner

in

the

CNT/PDMS

and

ZnS:Cu/rhodamine/SiO2/PDMS hybrid sensor, and thereby along with either micro- or nanopatterning this hybrid sensor would enlarge its applicability in many fields such as safety diagnosis, toys, military equipment, biomedical sensors, etc.

ASSOCIATED CONTENT


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Supporting Information. Illustration of a bilayer sample, data acquisition and a comparison of luminescence and hysteresis behaviors under cyclic loading for the conventional ML and ZnS:CuPDMS composite “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2015M3D1A1069705).

ABBREVIATIONS PDMS, polydimethylsiloxane; CNT, carbon nano tube; DCJTB, 4-(dicyanomethylene)-2-tbutyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran, PL, photoluminescenc; EL, electroluminescence; CL, cathodoluminescence; ML, mechanoluminescence

REFERENCES (1) Rosakis, A. J.; Samudrala, O.; Coker, D. Cracks Faster than the Shear Wave Speed. Science 1999, 284, 1337-1340.


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Figure 1. Emission spectra for (a) ZnS:Cu/rhodamine/SiO2/PDMS, (b) ZnS:Cu/PDMS (left) and rhodamine/SiO2/PPDMS

(right) at various excitation wavelengths, and (c) excitation and

emission spectra for rhodamine/SiO2/PDMS.


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Figure 2. PL and ML spectra for (a) ZnS:Cu/PDMS and (b) ZnS:Cu/rhodamine/SiO2/PDMS composite.


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Figure 3. Cyclic response at frequencies of 1, 3, and 6 Hz for (a) Load versus displacement, (b) emission intensity versus displacement, and (c) electrical resistance versus displacement showing a complete coincidence. The left and the bottom axis in each plot shown in black color represents displacement (mm) and time (sec.) respectively.


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Figure 4. Electrical resistance versus displacement at frequencies of 1, 3, and 6 Hz, showing a complete coincidence.


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Table of Contents Graphic


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PDMS and

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