Electroluminescent Pressure-Sensing Displays - ACS Publications

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Electroluminescent Pressure Sensing Displays Seung Won Lee, Sung Hwan Cho, Han Sol Kang, Gwangmook Kim, Jong Sung Kim, Beomjin Jeong, Eui Hyuk Kim, Seunggun Yu, Ihn Hwang, Hyowon Han, Tae Hyun Park, Seok-Heon Jung, Jin-Kyun Lee, Wooyoung Shim, and Cheolmin Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01790 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Applied Materials & Interfaces

Electroluminescent Pressure Sensing Displays Seung Won Lee, †,∇ Sung Hwan Cho, †,∇ Han Sol Kang, † Gwangmook Kim, † Jong Sung Kim, † Beomjin Jeong, † Eui Hyuk Kim, † Seunggun Yu, † Ihn Hwang, † Hyowon Han, † Tae Hyun Park, † Seok-Heon Jung, ‡ Jin Kyun Lee, ‡ Wooyoung Shim† and Cheolmin Park†, * †

Department of Materials Science and Engineering, Yonsei University

Yonsei-ro 50, Seodaemun-gu, Seoul, 03722 (Republic of Korea) ‡

Department of Polymer Science and Engineering, Inha University

Yonghyeon-dong, Nam-gu, Incheon, 22212 (Republic of Korea)

KEYWORDS:

Alternating

current

electroluminescence,

Capacitive

pressure

sensor,

Electroluminescent pressure visualization, Ionic polymer gels, Materials characterization by electroluminescence, Position detection, Pressure monitoring

ACS Paragon Plus Environment

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ABSTRACT

Simultaneous sensing and visualization of pressure provides a useful platform to obtain information about a pressurizing object, but the fabrication of such interactive displays at the single-device

level

remains

challenging.

Here,

we

present

a

pressure

responsive

electroluminescent (EL) display that allows for both sensing and visualization of pressure. Our device

is

based

on

a

two-terminal

capacitor

with

six

constituent

layers:

top

electrode/insulator/hole injection layer/emissive layer/electron transport layer/bottom electrode. Light emission upon exposure to an alternating current field between two electrodes is controlled by the capacitance change of the insulator arising from the pressure applied on top. Besides capacitive pressure sensing, our EL display allows for direct visualization of the static and dynamic information of position, shape, and size of a pressurizing object on a single device platform. Monitoring the pressurized area of an elastomeric hemisphere on a device by EL enables quantitative estimation of the Young’s modulus of the elastomer, offering a new and facile characterization method for the mechanical properties of soft materials.


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INTRODUCTION

Besides quantitative and precise detection of pressure which is among the most important functions in numerous applications including smart windows,1 displays,2 security systems,3 mobile phones,4 and prospective electronic skins (e-skins),5−7 direct visualization of the pressure source while sensing it can further extend the usefulness of a sensor by offering novel functions such as shape and position recognition of a pressurizing object and dynamic motion monitoring of an object on a sensor.8−24 This approach, commonly studied in the area of interactive displays, has been extensively developed. Tunable-coloration/light emission have been successfully demonstrated by combining optical elements with pressure sensors mainly based on the pressure dependent change in either electrical resistance25−29 or capacitance,30−37 allowing the visualization of pressure; examples include organic light emitting diodes (OLEDs),8 electroluminescence,11−15 electrochromic,16−19 and triboelectrification devices.20,21 Considering that the various coloration display elements based on absorbance and reflective modes in general suffer from rather low brightness, slow response time and low light efficiency, the interactive electroluminescent (EL) displays are promising due to their benefits of ultra-thin and high color contrast and efficiency.

In spite of successful direct pressure visualization with EL, previous works still required active circuit elements such as transistors with pixelated arrays of displaying elements to obtain precise spatial information of a pressurizing object.8−10 Ideally, it would be a straightforward strategy to combine two independent EL display and pressure sensing elements without active matrix circuitry, but which can still provide precise spatial information in a non-pixelated manner. This can be done when a pressure sensor is merged into a display for simultaneous pressure-sensitive


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light emission and electrical pressure sensing. We envision that an alternating current (AC)driven electroluminescent (ACEL) device,38−43 where the EL significantly depends upon the capacitance of an insulator in the device, can realize a single-device platform for the direct sensing and visualization of pressure.

Here, we present a single-device level electroluminescent pressure sensing display (EPSD) based on AC operation, which allows for the direct visualization of a pressurizing object while detecting pressure. Our EPSD contains six vertically stacked layers: transparent top electrode/pressure sensitive insulator/hole injection layer (HIL)/emissive layer/electron transport layer (ETL)/bottom electrode sandwiched between two poly(ethylene terephthalate) (PET) films. Unlike active matrix circuitry-based light emission sensors, which are analogous to pixelated displays, our capacitor elements provide important potential advantages. First, in the device layout the pressure is coupled with an electric field that activates EL emission allowing static and dynamic local light emission (e.g., position, shape, and size of objects) as shown in Figure 1a. Second, because a highly compressive, high-dielectric-constant ionic gel34 is employed and coupled with efficient field induced charge injection, there is potential for very low AC voltage operation without sacrificing light intensity, which contrasts with state-of-the art results based on electroluminescent skin11−15 and conventional ACEL devices38−43 (Table S1, Supporting Information). Third, the magnitude of applied pressure can be quantified by sensing elements and is directly related to the intensity of light emission, which makes it a useful characterization platform for exploring the mechanical properties of a soft material such as its Young’s modulus.


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EXPERIMENTAL SECTION

Materials Preparation: Yellow fluorescent polymer (Product: PDY-132), red (Product: SPR001), blue poly(spirobifluorene)-based copolymer (product: SPB-02T), and white (SPW-111) were purchased from Merck Co. MWNTs (Grade: TMC220-10) grown by CVD and purified to over 95 wt% were manufactured at Nano Solution, Inc., Seoul, Korea. Poly(styrene-block-4vinylpyridine) (PS-b-P4VP), a dispersant for MWNTs, was synthesized by Polymer Source, Inc., Dorval, Canada. The PEDOT:PSS (Clevios P VP AI4083) was modified by mixing with Zonyl surfactant (FS-300 fluoro-surfactant from Aldrich) (0.5 wt% with respect to PEDOT:PSS), which

promoted

the

wetting

of

the

PEDOT:PSS

layer

on

an

emission

layer.

Poly(dimethylsiloxane) (PDMS) and cross-linker were purchased from Dow Corning. Poly(vinylidene was

purchased

fluoride-co-trifluoroethylene-co-chlorofluoroethylene) from

PIEZOTECH,

Inc.,

France.

[P(VDF-TrFE-CFE)]

1-Ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]), acetone, indium tin oxide (ITO) deposited on poly(ethylene terephthalate) (PET), trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FOTS), zinc acetylacetonate hydrate (Zn(acac)2), and poly(ethylenimine) (PEI) were purchased from SigmaAldrich. All other materials were purchased from Aldrich and used as received.

Fabrication of topographically patterned ionic gel layer with micropyramids: A micropatterned pyramidal relief mould was fabricated on a 4-inch silicon wafer (with 300-nmthick thermally grown silicon oxide) by photolithography, followed by chemical etching. The arrays of engraved pyramids were developed in P4mm symmetry with square area and height of each pyramid 5 × 5 µm2 and 5 µm, respectively. The micropatterned Si mould was treated by O2


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plasma at 40 W for 3 min prior to the deposition of a FOTS self-assembled monolayer which facilitated the removal of an ionic gel layer from the Si mould. A 15 wt% P(VDF-TrFE-CFE) solution was prepared in acetone. Various amounts of [EMI][TFSA] were added in the solution, followed by vigorous stirring at 75 °C for 30 min. Ionic liquid/polymer mixture solutions were spin-coated on micropatterned Si substrates at 1000 rpm for 60 s and subsequently annealed at 75 °C for 24 h under N2. The moulded ionic gel films were peeled off from the Si moulds.

Fabrication of the electroluminescent pressure sensing Display (EPSD): First, bottom ITO electrodes with the thickness and sheet resistance of 80 nm and 20 Ω cm−2, respectively, were sputtered onto either a glass or a PET substrate. The substrate with ITO electrodes was sequentially cleaned with acetone and 2-propanol (twice each) in an ultrasonic bath for 10 min each, followed by UV treatment for 15 min. Zn(acac)2 was dissolved in anhydrous ethanol (25 mg mL−1) and stirred at 60 °C for 12 h. The precursor solution was subsequently filtered through a poly(tetrafluoro ethylene) filter (pore diameter: 0.45 µm) to remove agglomerates of undissolved precursor. ZnO precursor solution was spin-coated onto the cleaned ITO-coated substrate, followed by thermal annealing in ambient atmosphere at 120 °C for 30 s, giving rise to a uniform 10-nm-thick ZnO film. Subsequently, 0.4 wt% PEI dissolved in 2-methoxyethanol was spin-coated onto the ZnO film followed by thermal annealing at 100 °C for 10 min in ambient atmosphere. An emissive film was prepared by spin-coating a dispersion of MWNTs in toluene (1 wt%) onto the PEI/ZnO layers. MWNTs were efficiently dispersed using PS-b-P4VP and used for enhancing charge injection efficiency.36 A PEDOT:PSS layer was then spin-coated from the modified solution, as described above. A topographically patterned ionic gel layer peeled off from a Si mould was subsequently transferred onto the PEDOT:PSS. Finally, top ITO/PET


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electrodes were carefully placed on the transferred ionic gel layer (Figure S1, Supporting Information). Gold meshes were pasted on both ITO electrodes to create a firm electrical contact for capacitance and electroluminescence measurements.

Characterization Methods: Cross-sectional view of an EPSD was obtained using focused-ionbeam transmission electron microscopy (FIBTEM) (JIB-4601F, JEOL). The luminance and EL spectra of the devices were obtained using a spectroradiometer (Konica CS 2000) and fiber optic spectrometer (StellarNet BLUE-wave). The current–voltage–luminance (I–V–L) characteristics of the devices were measured with a multichannel precision AC power analyzer (ZIMMER Electronics Systems LMG 500). Capacitance measurement was performed with a precision LCR (inductance, capacitance, resistance) meter (Agilent E4980A). The frequency was varied from 100 Hz to 100 kHz. For the capacitance change measurement as a function of pressure, a computer-controlled universal manipulator (Teraleader) was set up with the LCR meter. The vertical spatial and force resolution of the equipment are 1 µm and 10 mN, respectively. The device structure and whole measure systems used to evaluate the capacitance and luminance are following to our previous research papers.23,34 All measurements were performed under ambient conditions in air.


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RESULTS AND DISCUSSION

The structure of an EPSD is shown in Figure 1a. Our device architecture is composed of (i) a pressure-sensitive ionic gel layer with periodic arrays of topographical micropyramids and (ii) a series of layers responsible for efficient field-induced charge injection and subsequent light emission upon application of an AC field between the top and bottom electrodes. Briefly, an ETL of poly(ethylenimine) (PEI)/zinc oxide (ZnO) of thickness ~50 nm, an emissive layer of Super Yellow-multi-walled carbon nanotube (MWNT) composite approximately 40 nm thick, and

a

30-nm-thick

HIL

of

poly(3,4-ethylenedioxythiophene)-poly(styrene

sulfonate)

(PEDOT:PSS) were sequentially spin-coated on a transparent indium tin oxide (ITO) electrode on a PET substrate. A topographically microstructured ionic gel insulator containing solutionblended poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]) was carefully transferred on top of the PEDOT:PSS, followed by mounting a top ITO electrode on a PET substrate (Figure S1, Supporting Information). The topographical pyramids were developed on an approximately 5-µm-thick flat ionic gel layer and each pyramid was 5 µm in width and 5 µm in height,34 as shown by scanning electron microscopy (SEM) (Figure 1b). Three stacked layers beneath the ionic gel layer were visualized by cross-sectional highresolution transmission electron microscopy (TEM), combined with energy dispersive X-ray (EDX) mapping of the constituent atomic elements of the layers as shown in Figure 1c (Figure S2, Supporting Information). An EPSD fabricated 3 × 3 cm2 in area is shown in Figure 1d.


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Figure 1. Device architecture and working principle of the electroluminescent pressure sensing display (EPSD). (a) Illustration of an EPSD suitable for both sensing and directly visualizing pressure. (b) A SEM image of topographically patterned ionic gel layer with micropyramids developed on an approximately 5-µm-thick flat ionic gel layer. (c) A high-resolution crosssectional TEM image of the device. Each layer is artificially colored for clear visualization. (d) An optical photograph of a plane-view EPSD with active area 3 × 3 cm2. (e) Schematic illustration of the working principle of an EPSD for simultaneous sensing and visualization of pressure.

Pressure exerted on the top PET first deforms a mechanically elastic ionic gel layer of arrays


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of pyramids, significantly reducing the initial air gap between the top flat ITO and the arrays of pyramids due to the deformation. The decrease in thickness and the increase in effective dielectric constant (εeff) of the ionic gel/air layer upon application of pressure give rise to large enhancement in capacitance, allowing for highly sensitive capacitive pressure sensing, as schematically illustrated in Figure 1e. It should be noted that the three stacked layers of HIL, emissive layer, and ETL barely changed in both thickness and dielectric constant upon application of pressure due to their glassy properties. The sensitive capacitance change of a topographical ionic gel layer as a function of pressure can be utilized for efficient EL change since (i) the increase in contact area and (ii) the increase the built-in electric field, which are the main driving force for ACEL, can be readily altered by the capacitance of an ionic gel layer. It is anticipated that the increased capacitance of the ionic gel/air region with pressure can (i) enlarge contact area and (ii) enhance the built-in field and result in lowering the charge injection barrier from the bottom electrode to an emission layer during AC operation, giving rise to the increase in EL intensity with pressure schematically illustrated in Figure 1e (Figure S3, Supporting Information). The control of both capacitance and EL intensity with applied pressure in our single device platform makes our EPSD suitable for simultaneous pressure sensing and visualization, offering an approach for providing a variety of additional information of a pressurizing object (besides pressure), as shown next.

Before introducing a topographically patterned ionic gel layer in our AC driven device, we investigated how a flat ionic gel layer affected the light-emitting performance of an ACEL as functions of ionic liquid content and layer thickness. The light-emitting device with flat ionic gel structure is schematically illustrated in the inset of Figure 2a. Luminance–voltage (L-V)


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characteristics in Figure 2a show that the light intensity of an ACEL device at a constant AC voltage was greatly enhanced with an ionic liquid in the ionic gel layer up to 25 wt% with respect to P(VDF-TrFE-CFE) (thickness = 3.5 µm). The threshold voltage decreased with ionic liquid, and a device with an ionic gel layer containing 25 wt% ionic liquid exhibited its threshold voltage of approximately ± 4 V, which was substantially lower than that of an ACEL device with a pure P(VDF-TrFE-CFE) layer (± 14 V). The AC-frequency-dependent device performance was also examined; the results in Figure 2b and c show that all devices with ionic gel layers displayed brightness and power efficiency values higher than that with a pure P(VDF-TrFE-CFE) layer, over the broad range of AC frequencies. The maximum brightness values were approximately 300, 1000, and 1800 cd m−2 at AC voltage of ± 20 V and at frequencies 1, 10, and 100 kHz, respectively, in a device with an ionic gel layer containing 25 wt% ionic liquid (Figure S5, Supporting Information).

Figure 2. Electroluminescent performance of ACELs with flat ionic gel layers. (a) Schematic of an ACEL with a flat ionic gel layer operated under AC field in the inset. Luminance versus


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voltage (L–V) characteristics of devices with ionic gel layers having different polymer:ionic liquid contents. (b) Luminance and (c) power efficiency versus frequency characteristics of devices at constant voltage (14 V). (d) Luminance versus voltage (L–V) characteristics of devices with ionic gel layers containing 25 wt% ionic liquid as a function of the thickness of ionic gel layer between 200 nm and 14 µm. All the devices show their threshold voltages approximately at 4 V. (e) Threshold voltages of the devices as a function of the thickness of flat ionic gel layers from 200 nm to 14 µm. (f) EL spectra of ACELs with different emissive layers (red, yellow, blue and white) measured at 7 V. All devices contain ionic gel layers with 25 wt% ionic liquid. Photographs of the devices with the emission area of 2 × 2 mm2 are shown with different colours in the inset.

The low threshold voltage in an ACEL device with ionic gel layer is attributed to its high specific capacitance of approximately 100 nF cm−2 arising from the spontaneous formation of ion-pair electrical double layers (EDL) under the influence of an electric field (Figure S6, Supporting Information). The high capacitance of an ionic gel layer requires a small AC voltage to accumulate charges on its surface, exerting most of the input AC voltage on the HIL/emissive layer/ETL. The substantially large electric field in these layers results in the lowered threshold voltage and high frequency-dependent light emission relative to devices with no ionic liquid. Device performance was degraded at AC frequencies higher than approximately 10 kHz and AC voltage of ± 14 V, above which ions in an ionic gel layer could not respond to the voltage change due to low ion mobility. Luminance–voltage (L-V) characteristics of ACEL devices were examined as a function of the thickness of ionic gel layer between 200 nm and 14 µm, as shown in Figure 2d. Interestingly, the threshold voltages of the devices did not alter significantly regardless of the thickness of the ionic gel layer as shown in Figure 2e. A device with a 14-µmthick ionic gel layer exhibited threshold voltage of ± 4 V, similar to one with 1-µm-thick ionic gel layer, due to its high capacitance based on ion-pair EDL. The resistance linearly depends on the thickness of the film while the specific capacitance is independent on the film thickness. However, RC time constant of circuit linearly increase with thickness because of capacitance


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dispersion. This means that the polarization of an ionic gel based capacitor can be switched as thickness at high frequencies.44 We analyzed the electrical properties of the ionic gel as a function of thickness. These results clearly suggest that the capacitance of ionic gel layer depended upon film thickness in the operating frequency range of our EPSD and thus electric field depended on the film thickness. (Figure S4, Supporting Information). Our device can be also considered as the series of two impedance elements and the ionic gel has much smaller impedance than electroluminescent part. When an ion gel layer was removed, the light emission occurred with direct carrier injection of holes and electrons from top and bottom electrodes, respectively, giving rise to the performance much higher than that with an ionic gel layer. In contrast, a device with a 14-µm-thick P(VDF-TrFE-CFE) layer without ionic liquid showed threshold voltage of ± 56 V as shown in Figure 2e. As expected, luminance–voltage (LV) characteristics of the devices with flat insulating layer having no ionic liquid are thicknessdependent (Figure S7, Supporting Information). The characteristics of our ACEL devices with high-capacitance ionic gel layers provide great freedom for designing a topographically patterned pressure sensing ionic gel for EPSD, as will be discussed in detail below. Red, yellow, blue, and white light emissions were successfully demonstrated in ACEL devices with each light emissive layer, as shown in Figure 2f. The normalized EL spectra of the ACEL devices in Figure 2f exhibited maximum EL emission wavelengths at 480, 530, and 650 nm for blue, yellow and red colours, respectively. Time-resolved measurement of an ACEL clearly shows that light emission occurred at every negative polarity when driven by sine wave pulse ± 10 V at 1 kHz (Figure S8, Supporting Information). The results support the model of electron injection from the bottom electrode, followed by exciton formation of the electrons with holes injected from the PEDOT:PSS hole injection layer, consistent with our previous results.23,40 The detailed


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light emission mechanism and efficiency of our devices were examined via the characteristics of impedance, resistance, reactance and phase angle (Figure S9, Supporting Information).

Figure 3. Capacitive and electroluminescent pressure sensing. (a) Capacitance change (∆C/C0) and sensitivity as functions of pressure of an EPSD with a topographically patterned ionic gel layer containing 25 wt% ionic liquid. (b) Time-resolved capacitance change response of the device under repetitive mechanical loads with different pressures of 1, 4, 8, 10, 20, 50 and 100 kPa. Both response and relaxation of the capacitance change at ∆ 10 kPa occur in 25 ms, as shown in the inset. (c) The endurance of capacitance change of the device during 7000 cycles of ∆ 5 kPa. (d) EL intensity of an EPSD under different applied pressures from 10 to 150 kPa. (e) EL change (∆L/L0) and sensitivity as a function of pressure of an EPSD. In pressure regimes greater than 60 kPa, the intensity linearly increased with pressure when device was operated at an AC voltage and frequency of ± 7 V and 1 kHz, giving rise to the EL sensitivity of 2.26 kPa−1. The results of a device with a topographically patterned PDMS layer are also shown for comparison. (f) Normalized EL change response of the device under consecutive gentle fingertouch events.

By employing an ionic gel layer with periodic arrays of topographical pyramids in our ACEL device, a novel EPSD was realized which detects pressure through capacitance change and


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simultaneously visualizes the pressure. We investigated the capacitive pressure sensing performances of our EPSD. Drastic capacitance change in an EPSD with a topographically patterned ionic gel layer containing 25 wt% ionic liquid was observed with pressure, as shown in Figure 3a. The ionic gel layer was chosen because it gave rise to the best EL performance in terms of low turn-on voltage and high brightness without significantly harming the capacitive sensing properties. The capacitive sensitivity of the device was evaluated as a function of pressure, defined as Sc = δ(∆C/C0)/δp, where p is the applied pressure, and C and C0 are the capacitances with and without applied pressure, respectively. The sensitivity was averaged with individual sensitivities at every 10 kPa of pressure change. A high sensitivity of 12 kPa−1 was obtained at pressure below 10 kPa and the sensitivity decreased to 4.5 kPa−1 at pressures between 10 and 20 kPa and to 3 kPa−1 at pressures from 20 to 30 kPa. Our EPSD also exhibited fast capacitance response and relaxation times (~25 ms) upon application of pressure, as shown in Figure 3b. In addition, our device showed reliable capacitance response behaviour over the broad range of applied pressures (1 ‒ 100 kPa). The capacitance change barely changed even after 7000 loading–unloading cycles with 5 kPa pressure, as shown in Figure 3c. As previously described, the thin glassy stacked layers of HIL/emissive layer/ETL employed for pressure visualization in our EPSD did not significantly affect the capacitive pressure sensing performance. The great advantage of our EPSD lies in the fact that pressure exerted on the device can be directly visualized in EL over a broad range of pressure upon AC operation. EL spectra of an EPSD containing a yellow emission layer were obtained as a function of pressure; the results in Figure 3d clearly show that light emission intensity increased with the pressure applied. For quantitative analysis of EL response to pressure, EL intensity of each spectrum in the range 530


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– 630 nm was integrated. The plot in Figure 3e shows that EL intensity was rather weakly dependent on pressure less than 50 kPa. At high pressure (greater than 60 kPa), the intensity linearly increased with pressure when the device was operated at AC voltage and frequency of ± 7 V and 1 kHz. The EL sensitivity is defined as SEL = δ(∆L/L0)/δp, where p is the applied pressure, and L and L0 are the integrated EL intensities with and without applied pressure, respectively. EL sensitivity of the EPSD calculated from the slope of the plot in the highpressure regime was approximately 2.26 kPa-1. For comparison, we employed a conventional poly(dimethyl siloxane) (PDMS) rubber pressure sensing layer with the same topographical pyramids as those in the ionic gel layer. Unlike a device with a patterned ionic gel layer, no significant EL signals were observed upon application of pressure because voltage bias was not large enough, as shown in Figure 3e. It should be also noted that the EL sensitivity we obtained was orders of magnitude higher than the light emission sensitivity values of state-of-the art results based on triboelectrification EL18 and capacitive change.31 Interestingly, the sensitivity from capacitive change is highest at low pressures while the sensitivity from EL change is best at high pressures. These can be combined to enable complementary pressure sensing across scales. (Figure S10, Supporting Information). It should be also noted that the sensitivity we examined in our device is widely accepted in the pressure sensor field but may be not appropriate for our device with pyramidal arrays. Detection of the localized forces may be more appropriate through optical and capacitive means for our purpose. Due to the lack of our experimental facility for detecting the localized forces, we just followed the conventional method the previous works adopted for sensitivity measurement.14,20,21 EL sensing in our EPSD was fast and reliable enough to distinguish consecutive gentle finger touch events, as shown in Figure 3f. All the results were confirmed


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with a set of at least 5 sensors to ensure reproducibility of our devices (Figure S11). Our device exhibited good humidity stability under the relative humidity of approximately 90 %.(Figure S11 and S12, Supporting Information).

Figure 4. Direct pressure visualization for static pressure information. (a) Photographs of topographic rubber stamps with heart and star shapes with the same areas. Shape visualization of the two stamps on EPSDs with the same applied pressure. (b) Capacitance change with time when each stamp was sequentially placed on an EPSD. (c) Photographs of various sets of two positions touched by sharp pencil tips with contact area of 1 mm2. The locations of the touched points were clearly visualized in EL. (d) Capacitance change when each two-point touch was sequentially made with time. No significant difference was made in capacitance as there was no pressure difference. (e) Photographs demonstrating EPSD suitable for human motion sensing. The most bent position was visualized in EL while bending fingers. (f) A plot of normalized EL intensity as a function of bending angle defined in the scheme of the inset.

Direct visualization of pressure in our EPSD offers a convenient approach to obtaining information regarding the shape, size, and position of a pressurizing object without pixelated sensing arrays. We designed topographic star and heart rubber stamps with similar areas. When these stamps were gently touched on an EPSD with area 9 cm2, both capacitance and EL


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information were obtained. Figure 4a shows that two different shapes were clearly visualized in EL (the right images of Figure 4a) while no significant difference was made in the capacitance since there was no pressure difference (Figure 4b). Similarly, the positional information of an object was also obtained by EL. Various sets of two positions were chosen and touched with sharp tips of pencils with contact area of approximately 1 mm2 with the same pressures. The locations of the touched points were clearly visualized in EL, allowing for accessing positional information, as shown in Figure 4c. Again, the capacitance change in all 4 sets of touch events was very similar because there was no significant difference in total pressure on the EPSD, as shown in Figure 4d. When mounted on a finger, our EPSD also successfully visualized the spot on a finger with the largest compression (maximum bending) as shown in Figure 4e. EL intensity of an EPSD increased with bending angle as shown in Figure 4f.

Figure 5. Direct pressure visualization for dynamic pressure information. (a) Schematic of dynamic EL visualization for writing motion tracking. (b) Series of photographs visualizing pressure points in EL consecutively captured upon writing at time interval of 0.20 s. Dotted “L” shape line added in the photograph for guidance. (c) A photograph showing a letter “L” in EL produced by 1 s continuous exposure during which the time- and position-dependent EL images


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in (b) were collected. A superimposed photograph showing the complete trajectory of the letter “L”. (d) Schematic of dynamic EL visualization for tracking the position of a 5 g metal ball. (e) Live photograph images of the ball moving with speed of 10 cm s-1 on an EPSD captured by the camcorder in consecutive frames at an interval of 0.10 s. (f) An OM image showing a magnified circular region of (c). Pressurized individual pyramids of approximately 5 µm in size are visualized in EL.

Furthermore, our single EPSD offers a convenient way to obtain dynamic information (spatial pressure information with time) of a pressurizing object through temporal analysis. Dynamic motion of an object was monitored on an EPSD with time, as schematically shown in Figure 5a. For example, handwriting motion using a pressure pen was tracked on the device. A series of images (consecutive frames) was recorded at an interval of 0.20 s, as shown in Figure 5b. Point-by-point light emission on pressurized spots during handwriting was recorded with time, giving rise to a handwritten EL character of the letter “L” with 1 s exposure time, as shown in Figure 5c. In addition, the trace of a rolling ball on a device was monitored in real time as schematically shown in Figure 5d. The series of captured photographs shows that a 5 g weigh metallic ball was successfully monitored at the speed of 10 cm s−1 using EL, as shown in Figure 5e. The spatial resolution of the device was, in principle, approximately 10 µm defined by the periodicity of our pyramidal.20−22 The OM image in Figure 5f showing EL emission from individual pressurized pyramids (~5 µm) confirms the 10 µm spatial resolution of our EPSD.22 The reason that the square regions of our device in optical microscope look bright is due to the light source of the optical microscope. The topographically periodic pyramids of a transparent ionic gel layer become like optical lens capable of focusing light, making these pyramids look bright under optical microscope even without AC field (Figure S13, Supporting Information). Our EPSD was also useful as a handy instrument for characterizing the mechanical properties of soft materials as shown in Figure 6a. As illustrated in Figure 6b, we estimated EL


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arising from an elastomeric hemisphere with known dimensions placed and pressed on an EPSD. By analyzing the circular EL area of the contact surface captured by the camera of a mobile phone, we were able to obtain Young’s modulus of the hemisphere. According to the normal contact Hertz’s theory of an elastic hemisphere subjected to a normal force N in contact with a rigid flat object, an elastic sphere undergoes a local contact surface deflection relative to a distant

parallel plane.45,46 The radius of the contact surface is given by = ( ) , where the constant K is commonly referred to as the effective stiffness and represents the elastic properties of the materials expressed by

=

+

, where E1 and E2 are the Young’s moduli, and ν1 and ν2

are the Poisson’s ratios of the hemisphere and the flat object, respectively. The effective radius of curvature R of the bodies is given by

= + with R1 and R2 being the radii of curvature of

the two bodies. The radius of curvature of the flat object is set to infinity ( = ∞). The E modulus of the rigid object is also usually considered much higher than that of the deformable sphere.


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Figure 6. The EPSD for characterizing mechanical properties of an elastomer. (a) A photograph showing simple handy optical acquisition system for characterizing mechanical properties. (b) Schematic of a hemisphere on an EPSD illustrating the principle used for evaluating Young’s modulus of the hemisphere based on a modified Hertz normal contact model. A contact circle with radius of a is visualized in EL when the hemisphere is pressed with force N. (c) A photograph of a hemisphere with radius of 7 mm on an EPSD. The bottom photograph shows hemispheres prepared with various PDMS:crosslinking agent mixtures (5:1, 10:1, 20:1, and 30:1). (d) Photographs of contact area visualized in EL with applied forces of 0, 1, 5, 10, and 15 N (left to right). (e) Plots of the cube of contact circle radius versus applied force obtained from EL images of the pressurized hemispheres with different stiffness as a function of the mixing ratio of PDMS:crosslinking agent. (f) Young’s moduli of the hemispheres with different stiffness derived from theoretical equation with the slopes of the plots in (e).


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For the successful demonstration, we prepared four PDMS hemispheres with different Young’s moduli by varying the amount of cross-linking agent, as shown in Figure 6c. The highest modulus was expected from a hemisphere with PDMS:crosslinker ratio of 5:1 and the lowest from that with ratio 30:1. When a hemisphere (30:1) was subjected to pressure, the contact EL area increased with pressure, as shown in Figure 6d. Based on the EL patterns of the hemispheres as a function of pressure, volume of contact circle vs. applied force plots were obtained, as shown in Figure 6e. The radii of the contact circles were determined by hue, saturation, and value (HSV) scale image mapping after being processed using MatLab based on a conventional cell-phone application (Figure S14, Supporting Information). The Young’s moduli

of the hemi-spheres were successfully obtained from the slopes of the plots based on E =

=

× "# (Figure S15, Supporting Information). The Young’s modulus values in Figure 6f

were consistent with those in the literature.47,48 It is also noted that the slight difference in the Young's modulus of PDMS from the literature may be due to difference in curing temperature. Further study for improving accuracy is needed for the practical implementations of our technique.

CONCLUSIONS

We demonstrated a single-device level pressure sensing displays capable of detecting pressure through capacitance change and visualizing pressure via the electroluminescence of polymer light-emitting materials. High-performance EL pressure sensing was achieved due to pressuresensitive electric field modification of a topographically patterned ionic gel layer upon AC


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operation. Direct visualization of pressure on our EPSD at a low operation voltage of 4 V allowed acquisition of the spatial information of position, shape, and size of a pressurizing object and dynamic information such as handwriting motion tracking and real-time monitoring of a moving object. Interestingly, our EPSD was conveniently utilized for quantitatively estimating the Young’s modulus of an elastomer, offering a new facile characterization method for the mechanical properties of soft materials.

ASSOCIATED CONTENT Supporting Information. Supporting results are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Office phone: +82-2-2123-2833 Fax:+82-2-312-5375

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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S.W.L and S.H.C contributed equally to this work. This study was supported by a grant from the National Research Foundation of Korea (NRF) funded by Korean government (MEST) (No. 2017R1A2A1A05001160) and by the third stage of the Brain Korea 21 Plus project in 2017. REFERENCES [1] Ge, D.; Lee, E.; Yang, L.; Cho, Y.; Li, M.; Gianola, D. S.; Yang S. A Robust Smart Window: Reversibly Switching from High Transparency to Angle-Independent Structural Color Display. Adv. Mater. 2015, 27, 2489–2495. [2] Morin, S. A.; Shepherd, R. F.; Kwok, S. W.; Stokes, A. A.; Nemiroski, A.; Whitesides, G. M. Camouflage and Display for Soft Machines. Science 2012, 337, 828–832. [3] Trung, T. Q.; Lee, N. -E. Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human Activity Monitoring and Personal Healthcare. Adv. Mater. 2016, 28, 4338–4372. [4] Gao, W.; Emaminejad, S.; Nyein, H. Y. Y.; Challa, S.; Chen, K.; Peck, A.; Fahad, H. M.; Ota, H.; Shiraki, H.; Kiriya, D.; Lien, D. -H.; Brooks, G. A.; Davis, R. W.; Javey, A. FullyIntegrated Wearable Sensor Arrays for Multiplexed in-situ Perspiration Analysis. Nature 2016, 529, 509–514. [5] Kim, C. H.; Lee, H. H.; Oh, K. H.; Sun, J. Y. Highly Stretchable, Transparent Ionic Touch Panel. Science 2016, 353, 682–687. [6] Zang, Y.; Zhang, F.; Di, C. –a.; Zhu, D.; Advances of Flexible Pressure Sensors Toward Artificial Intelligence and Health Care Applications. Mater. Horiz. 2015, 2, 140–156. [7] Chortos, A.; Liu, J.; Bao, Z. Pursuing Prosthetic Electronic Skin. Nat. Mater. 2016, 15, 937– 950. [8] Wang, C.; Hwang, D.; Yu, Z.; Takei, K.; Park, J.; Chen, T.; Ma, B.; Javey, A. UserInteractive Electronic Skin for Instantaneous Pressure Visualization. Nat. Mater. 2013, 12, 899–904.


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