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Highly Sensitive Ultraviolet Light Sensor Based on Photoactive Organic Gate Dielectrics with an Azobenzene Derivative Kang Eun Lee,†,⊥ Jea Uk Lee,‡,⊥ Dong Gi Seong,† Moon-Kwang Um,† and Wonoh Lee*,§ †

Composites Research Division, Korea Institute of Materials Science, 797 Changwon-daero, Changwon, Gyungnam 51508, South Korea ‡ C-industry Incubation Research Center, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, South Korea § School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, South Korea S Supporting Information *

ABSTRACT: Photochromic molecules have been recently adopted by many researchers for organic field-effect transistor (OFET) applications, since they offer the opportunity to achieve flexible-type ultraviolet (UV) light sensors with the advantages of organic material-based solution processes. Here, we present the novel usage of an azobenzene derivative in the gate dielectric layer of an OFET for highly sensitive and reliable UV sensing applications. Owing to the large change of capacitance caused by the reversible photoisomerization of azobenzene, the OFET device can modulate efficiently the current signal under UV and visible light. We found that the on-current was greatly amplified upon UV light irradiation with good photoresponsivity and photocurrent ratio and then fully returned to the initial state under visible light. In addition, the device shows a strongly linear relationship with the UV radiation intensity and repetitive on−off response in real-time UV sensing tests, thus being potentially applied in highly sensitive and reliable UV sensors.

1. INTRODUCTION The extent of ultraviolet (UV) light exposure on the earth surface has been continuously increasing due to stratospheric ozone depletion by severe air pollution, which causes harmful diseases such as sunburn, skin cancer, and cataracts.1−3 Public awareness about the dangers of UV radiation has increased the demand for evaluating the intensity of UV light in day-to-day life. So far, photodiode-type UV sensors have been widely used in daily life activities such as sterilization, disinfection, water purification, air quality (UV-index) monitoring, as well as various academic and industrial fields, including military applications.4,5 Unfortunately, most commercialized UV sensors are intrinsically rigid because of the inorganic materials employed, such as zinc oxide or aluminum gallium nitride, and their fabrication methods are complex and involve high production costs. As a consequence, their application in realtime UV monitoring systems, such as portable and wearable devices, which require low costs, high flexibility, and sensitivity, is not easy.6,7 Therefore, solution-processable organic materials © XXXX American Chemical Society

have attracted great attention in the real-time UV sensing research field. Organic field-effect transistors (OFETs) based on organic semiconductors have been widely utilized in various electronics markets, such as radio frequency identification tags, electronic papers, flexible displays, and smart memory/sensor elements, because they have the unique characteristics of flexibility, lightweight, transparency, and low cost.8−11 In addition to the efforts made to improve the electrical properties of OFET devices for practical applications by introducing new organic semiconductors or improving the processing techniques, many research groups have attempted to realize new devices by incorporating various functionalities into conventional OFETs for advanced applications, such as gas sensors, optical switching, memory, and light-emitting devices.12−15 The performance Received: August 20, 2016 Revised: September 26, 2016

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DOI: 10.1021/acs.jpcc.6b08427 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Scheme 1. Schematic Illustration of an OFET-Type UV Sensor Using a Photoactive Gate Dielectric with an Asymmetric Azobenzene Derivative

scarce due to their susceptibility toward phase separation and the crystallization of mixtures of photochromic molecules and commercial dielectric polymers. Among well-known photochromic molecules, spiropyrans have successfully been utilized in gate dielectric hybrid systems with poly(methyl methacrylate) (PMMA),27,28 while reports on azobenzene/PMMA hybrids as bulk gate dielectrics are rare. Therefore, we have focused here on azobenzene as a photoactive gate dielectric material since it can lead to considerable capacitance changes in azobenzene/PMMA hybrid films. Under UV irradiation, the azobenzene/PMMA blending film shows a large increase of capacitance and also a fast recovery upon visible light due to the efficient alignment of the azobenzene dipoles in the hybrid film.29 Note that only capacitance changes are reported in this work.29 Herein, we present an OFET device with an azobenzeneether/PMMA (Azo-e/PMMA) hybrid film as a photoactive gate dielectric layer for UV sensing applications. In order to prevent the aggregation and crystallization of azobenzene molecules in the PMMA matrix film, we synthesized the highly soluble azobenzene derivative (Azo-e) containing asymmetric alkyl chain branches via sequential two-step reactions.29,30 Scheme 1 shows the chemical structure of the synthesized Azoe and its photoinduced isomerization upon UV and visible light. The results show that the Azo-e/PMMA hybrid film exhibits a large capacitance change and its OFET device shows good photoresponsivity and photocurrent ratio toward UV irradiation with respect to visible light. Most interestingly, the device exhibits highly linear sensitivity to the UV light intensity and stable sensing performance upon repetitive switching of the light source.

enhancement of OFET devices can be achieved by integrating chemically functionalized semiconducting organic molecules that can be easily modified to have enhanced electrical properties. Especially, OFET-based photodetectors have received great attention as they can display high sensitivity by signal amplification based on gate-bias control using newly adopted organic semiconductors.16,17 In the case of UV photodetectors based on transistors, many research groups have fabricated devices by incorporating photochromic molecules into OFETs. Photochromic molecules are known to undergo reversible isomerization under lightillumination at specific wavelengths. Typical photochromic molecules that respond to UV light at 365 nm are azobenzene, spiropyran, diarylethene, and stilbene.18 Azobenzene, one of the most used organic chromophores for optical switching, isomerizes from the trans to the cis state under UV radiation (365 nm). When azobenzene is exposed to visible light or heat, the cis structure of azobenzene switches back to the trans conformation. Such reversible photoinduced isomerization characteristic was exploited by introducing a thiol-functionalized biphenyl azobenzene as a self-assembled monolayer (SAM) on a gold (Au) electrode in order to reversibly modulate the charge injection at the interface between a metal electrode and a semiconducting organic channel.19 Since spiropyran can switch back-and-forth between its open and closed forms triggered by UV and visible light, respectively, it has also been used as an efficient optical modulator in an organic channel layer as well as an interface modifier on SAMs.20,21 Diarylethene has been also incorporated into polymeric semiconductors, where its photochromic properties changed the energy levels upon irradiation with either UV or visible light.22 Note that diarylethene has a closed form under UV light, in contrast to spiropyran behavior. Furthermore, πconjugated stilbene, which has the same photoisomerization activity as azobenzene, has been used as an active layer in OFETs and exhibited high optical responses.23 Well-defined gate dielectrics can manipulate the interfacial properties between active and dielectric layers and, eventually, can improve the performance of OFETs.24,25 In addition, a hybrid system of gate dielectrics can facilitate new functionalities such as multisensing to enable the simultaneous and selective detection of different environments by providing tailored device properties. Photochromic molecules have been extensively adopted for tuning the charge transport in semiconducting organic materials.26 However, their use in dielectric layers is still limited to SAMs for interface control between the dielectric and semiconductor layers. Reports on bulk-type gate dielectrics with photochromic materials are

2. EXPERIMENTAL SECTION Synthesis of 4-(4′-Hexylphenyl)azo Phenol (Azo− OH).30 4-Hexylaniline (5.00 g, 28.2 mmol) was dissolved in a mixture of concentrated hydrochloric acid (12 mL) and distilled (DI) water (100 mL). Sodium nitrite (2.92 g, 42.3 mmol) in DI water (10 mL) was added dropwise to the solution at 5 °C. An aqueous solution (200 mL) of phenol (2.65 g, 28.2 mmol), sodium hydroxide (1.13 g, 28.2 mmol), and sodium bicarbonate (11.1 g, 132 mmol) was then added dropwise to the mixture under vigorous stirring. After stirring for 6 h at 5 °C, the mixture was acidified with concentrated hydrochloric acid until pH 3. Then, the mixture was extracted with ethyl acetate (200 mL) three times. The combined organic layer was washed and dried over magnesium sulfate. After evaporation of the solvent, the residue was purified by column chromatography on silica gel with hexane/ethyl acetate (9:1 B


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Figure 1. Synthetic route for 4′-hexyl-phenyl-[4-(propylbutoxy)phenyl]diazene (Azo-e) with asymmetric branched alkyl chains.

Avance) using chloroform-d (CDCl3) as the solvent and tetramethylsilane as the internal reference. Optical microscopy (OM) images were obtained for Azo-e solutions in various organic solvents using a Nikon ECLIPSE LV150N microscope. The Azo-e/PMMA blending film and PVA-coated film were observed by atomic force microscopy (AFM) with a Park Systems NX10 model in tapping mode with height contrast. Optical absorption spectra were recorded on a UV−vis spectrophotometer (Agilent Cary 5000). Capacitance measurements for the Azo-e/PMMA hybrid film were carried out using a Biologic VSP-300 potentiostat at the frequency range of 1− 106 Hz with ac amplitude of 10 mV. Here, the capacitor was fabricated by deposition of a Au electrode (50 nm thick) on the hybrid film coating the ITO glass. As for the electrical properties of the manufactured OFET, the current−voltage characteristics were measured using a MST-5500B probe station with a Keithley 4200-SCS semiconductor parametric analyzer in a N2-filled glovebox. The optoelectronic properties were characterized using laser irradiation with tunable monochromatic UV (365 nm) and visible light (450 nm), where the light intensity was 10 mW cm−2. Furthermore, the dependency on the UV light intensity was also examined at 0.5, 1, 2, and 4 mW cm−2.

v:v), followed by recrystallization from hexane to give yellow platelet crystals (4.4 g, 55.3%). 1H NMR (400 MHz, CDCl3): δ (ppm), 0.90 (t, 3H), 1.27−1.38 (m, 6H), 1.61−1.69 (m, 2H), 2.68 (t, 2H), 5.38 (s, 1H), 6.93 (d, 2H), 7.29 (d, 2H), 7.80 (d, 2H), 7.87 (d, 2H). Synthesis of 4′-Hexylphenyl-[4-(propylbutoxy)phenyl]diazene (Azo-e).29 The synthesized Azo−OH (1.00 g, 3.54 mmol) and potassium carbonate (1.47 g, 10.6 mmol) were dissolved in dimethylformamide (50 mL). Then, 4bromoheptane was added dropwise to the solution at 130 °C. The mixture was then refluxed overnight. The solvent was evaporated under vacuum, and the residue was extracted with ethyl acetate (200 mL) three times. The obtained organic layers were combined, washed with brine, and dried over magnesium sulfate. After evaporation of the solvent, the residue was purified by column chromatography on silica gel with chloroform to afford a red-orange liquid (1.00 g, 74.2%). 1H NMR (400 MHz, CDCl3): δ (ppm), 0.87−0.95 (m, 9H), 1.28− 1.75 (m, 16H), 2.68 (t, 2H), 4.35 (m, 1H), 6.97 (d, 2H), 7.26 (d, 2H), 7.79 (d, 2H), 7.87 (d, 2H). OFET Fabrication. A schematic structure of the OFET-type UV sensor is shown with a gate electrode of indium tin oxide (ITO) glass, source and drain electrodes of Au, a channel layer of poly(3-hexylthiophene) (P3HT), an interlayer of poly(vinyl alcohol) (PVA), and the Azo-e/PMMA hybrid film as the gate dielectric layer (Scheme 1). The OFET device was fabricated on an ITO glass substrate. The ITO glass was cleaned in an ultrasonic bath with isopropyl alcohol for 30 min. A 2:3 (w/w) mixture of Azo-e and PMMA dissolved in chloroform at 10 wt % was spin-coated at 2000 rpm for 60 s on the ITO glass substrate. After the sample was dried under vacuum for 3 h, a PVA solution (3 wt % in DI water) was spin-coated at 1600 rpm for 60 s. Here, a PVA interlayer was introduced to prevent the dielectric layer from being damaged by the P3HT solution. Then, as a channel layer, a P3HT solution dissolved in chloroform (0.6 wt %) was spin-coated on the device. After spin-coating P3HT on the PVA interlayer, 50 nm thick Au layers as the source and drain (S/D) electrodes were deposited on the device using a thermal evaporator with a patterned shadow mask. The thicknesses of the hybrid dielectric layer, the PVA interlayer, and the P3HT active channel layer were 1 μm, 60 nm, and 11 μm, respectively. The overall thickness of the fabricated device was approximately 100 μm. The channel length (L) was determined as 50 μm from the distance between the two parallel S/D Au electrodes, and the channel width (W) was measured to be 1 mm from the length of the deposited Au electrodes. Characterization. The chemical structures of Azo−OH and Azo-e were identified by 1H NMR spectroscopy (Bruker

3. RESULTS AND DISCUSSION Azo-e was synthesized via sequential diazo-coupling and alkylation reactions based on a protocol reported elsewhere,29,30 as shown in Figure 1. Through a diazo-coupling reaction, a hydroxyl functional group was introduced in hexyl azobenzene achieving further alkyl functionalization, and the intermediate compound Azo−OH was obtained as a yellow powder, as confirmed by 1H NMR spectroscopy (Figure S1 in the Supporting Information). Then, the hydroxyl group of Azo−OH was reacted with alkyl bromide through a substitution reaction to afford Azo-e with a branched alkoxy group. The 1H NMR spectrum clearly indicates the successful alkylation with both a branched alkoxy group and a hexyl group. The asymmetrical and branched alkyl chains in Azo-e enhance the solubility of the azobenzene derivative in various organic solvents, such as chloroform, dimethylformamide, toluene, and trichloroethylene (Figure S2 in the Supporting Information) and avoid its aggregation in the PMMA matrix at high chromophore concentration.31 For example, asymmetric Azo-e is well dispersed in chloroform and its film blended with PMMA shows no visible crystallization, which ensures the smooth optical-switching of azobenzene molecules by trans to cis isomerization when irradiated with UV light. Here, 40 wt % of Azo-e was used in the PMMA matrix as the optimal concentration, as determined by investigating the C


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Figure 2. (a) UV−vis absorption spectra and (b) frequency-dependent capacitance of the Azo-e/PMMA blending film upon sequential irradiation of different light (ambient−UV−visible).

smoothness of the hybrid film (Figure S3 in the Supporting Information). Loadings of Azo-e in PMMA over 40% gave a highly rough surface morphology with a poor interface between the active channel layer and the gate electrode, which may lead to unstable and inferior performance OFET devices. Lower amounts of Azo-e in the hybrid film may result in worse optoelectronic performances owing to insufficient photoswitching chromophores. Figure 2a shows the UV−vis absorption spectra of the Azoe/PMMA hybrid film. The results show the reversible trans to cis to trans photoisomerization of azobenzene molecules in the PMMA matrix. The initial configuration of Azo-e before UV illumination is the trans state, indicated by a maximum absorption peak at 365 nm. Upon UV irradiation at 365 nm, the intensity of the trans absorption band (365 nm) decreases and that of the cis band (450 nm) increases, as per the conversion from the trans to the cis configuration of Azo-e. As shown in Figure 2a, the Azo-e/PMMA film became a darker color from light yellow under UV irradiation. After illumination with visible light (450 nm), Azo-e returned to its initial trans state. As a control, no changes were observed in the UV−vis absorption spectra of a pure PMMA film upon sequential UV and visible light irradiation (Figure S4 in the Supporting Information). No differences in color were observed either in the pure PMMA film upon sequential UV and visible light irradiation. The photoswitching properties of azobenzene were also examined by measuring the change of capacitance under the same light conditions. In order to prepare an azobenzene-based capacitor, an Azo-e/PMMA blending solution was spin-coated onto an ITO substrate. A top contact electrode was prepared by thermal deposition of a Au layer on the surface of the Azo-e/ PMMA layer. Figure 2b shows the fully reversible photoswitching properties of the azobenzene-based capacitor, which are entirely consistent with the UV−vis absorption spectra (Figure 2a). No change in the capacitance was observed for the pure PMMA film (Figure S4 in the Supporting Information). Upon exposure to UV light at 365 nm for 30 s, the capacitance increased by about 25% at low frequency (1.0 Hz) and by about 10% at high frequency (1.0 MHz). Then, under illumination with visible light at 450 nm, the capacitance fully recovered the original value. Note that such reversible changes of capacitance occur at all frequency levels, which supports the potential

application of this capacitor in various electronic systems requiring different frequencies. The observed variation in the capacitance is due to the differences in the dipole moment between the trans and cis isomers of Azo-e, where the trans isomer has a planar geometry with a dipole moment of 0.5D and the cis structure has spherical-like isometry with a dipole moment of 3D.31 Such reversible photoswitching properties of the Azo-e/PMMA hybrid can be applied in UV light sensing applications. Note that it has been well-known that the azobenzene has poor thermal stability so that there is limitation to use it solely.32 Even though its hybrid with PMMA exhibits very low glass transition temperature (Tg) of 45 °C,33 the capacitance change of the hybrid upon UV irradiation can be increased due to the accelerated thermal back switching effect (Figure S5 in the Supporting Information). Since the plasticization of PMMA matrix is reduced near or above Tg, the increase of free volume in the hybrid and higher thermal energy can enhance the orientation mobility of photochromic molecules.29 However, such a temperature-dependent capacitance change is undesirable to use as a component in stable UV sensors, because the optical sensor should give consistent signals at a specified light intensity. Therefore, thermally independent azobenzene-based hybrid system should be further investigated, where the capacitance change is insensitive to temperature change. In this specific work, we only focus on the behavior at room temperature to ignore such temperature dependency. Based on these reversible photoswitching characteristics, we fabricated an OFET device using the Azo-e/PMMA hybrid film as a gate dielectric layer, where photochromic azobenzene can alter the capacitance of the gate dielectric under UV light illumination and, therefore, modulate the electrical signal of the OFET. We adopted a typical bottom-gate top-contact architecture with the Azo-e/PMMA hybrid dielectric layer, as illustrated in Scheme 1. ITO glass was used as the transparent gate electrode, and the Azo-e/PMMA hybrid film (1 μm) was spin-coated on the ITO glass. P3HT was chosen as the semiconducting channel layer owing to its high mobility and solution processability for easy incorporation into complex electronic circuitries. A PVA interlayer was introduced to prevent the dielectric layer from being damaged by the organic solvent (chloroform) in the P3HT solution. Note that the PVA D


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Figure 3. (a) Output and (b) transfer characteristics of the azobenzene-based OFET.

Figure 4. (a) Transfer curves of the Azo-e/PMMA hybrid gate dielectric-based phototransistor under UV (30 and 60 s) and visible light (60 s) and (b) time-dependent photoresponse behavior as a drain current at a gate voltage (−80 V).

V, 1.26 × 103, and 1.75 × 10−4 cm2 V−1 s−1, respectively, which show negligible changes compared to those of the PMMA-only dielectric transistor (Vth = −5 V, Ion/Ioff = 1.5 × 103, and μh = 6.9 × 10−4 cm2 V−1 s−1). Also, such electrical parameters are close to those from other previous P3HT-based transistors.35 Therefore, the introduction of Azo-e into a gate dielectric with a PVA interlayer does not change the electrical properties of the polymer-based OFET device. To elucidate the optoelectronic properties of the Azo-e/ PMMA hybrid gate dielectric-based OFET device, transfer curves were measured under light illumination using an UV laser with a fixed intensity of 10 mW cm−2 at a wavelength of 365 nm. As shown in Figure 4, the on-current of the device increased significantly by about 100−5000% and the maximum increase was achieved after 30 s of UV illumination, after which there were no notable current changes, even after 60 s of UV irradiation (Figure 4a). After UV irradiation for 30 s, visible light was applied to the device using a monochromatic laser source with an intensity of 10 mW cm−2 at a wavelength of 450 nm. After 60 s irradiation of visible light, the drain current fully returned to the initial stage (Figure 4b), which is consistent with the results in Figure 2b. Based on this UV and visible light dependency of the OFET device, a real-time photosensing test was performed at VG = −80 V by sequentially turning on UV (15 s) and visible light

interlayer facilitates a smoother surface morphology for the dielectric layer (Figure S6 in the Supporting Information). Figures 3a and b present the electrical characteristics of the fabricated OFET device. As for the output characteristics, the drain current (ID) clearly displays a linear and increasing behavior at low drain voltages (VD), as well as saturation regions at high VD levels for all different gate voltages (VG), suggesting well-constructed gate modulation through a successful ohmic contact between the S/D electrodes and the semiconductor/dielectric layers. The negative voltage modulation represents the standard p-type field-effect action of the P3HT semiconductor. In order to measure the charge carrier (hole) mobility (μh), on/off current ratio (Ion/Ioff), and threshold voltage (Vth), ID was measured while sweeping VG from 0 to −80 V at a scan rate of 50 mV s−1 with a constant VD of −80 V. The transfer curves with a logarithmic scale (left axis) of |ID| and a linear scale (right axis) of |ID|1/2 are plotted in Figure 3b. The hole mobility can be calculated by the linearfitting plot of |ID|1/2 versus VG at the saturation region using the following equation34 ID = μ h

W C i(VG − Vth)2 2L

where Ci is the specific capacitance of the dielectric layer (2.8 nF cm−2). The obtained values of Vth, Ion/Ioff, and μh were −5.0 E


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Figure 5. (a) Transfer characteristics and (b) real-time photoresponse behavior of the Azo-e/PMMA hybrid gate dielectric-based phototransistor under UV and visible light illumination.

Figure 6. (a) Photoresponsivity and photocurrent ratio and (b) UV light intensity dependency of the normalized maximum on-current of the Azo-e/ PMMA hybrid gate dielectric-based phototransistor.

repetitively stable photoswitching performance by introducing photochromic azobenzene molecules into a gate dielectric layer. By introducing asymmetric alkyl branches, the azobenzene derivative became highly soluble in various organic solvents, and its hybrid film with PMMA had smooth surface morphology without significant phase aggregation and crystallization. Furthermore, the use of a PVA interlayer on the azobenzene/PMMA hybrid film enabled the assembly of the semiconducting organic material, which facilitated the development of a high-performance OFET device with good signal amplification. Due to the large changes in the capacitance of the azobenzene/PMMA hybrid, the azobenzene-based OFET device exhibited highly amplified on-current values under UV irradiation and fully recovered its original state upon visible light illumination with good photoresponsivity and photocurrent ratio. In real-time light on−off tests, the manufactured OFET exhibited highly sensitive and repetitive photoswitching behavior. As a quantitatively reliable UV sensor, our device has demonstrated excellent linear sensitivity to the UV radiation intensity. Therefore, the presented azobenzenebased OFET device has great potential in quantitative sensors to precisely measure the UV light intensity.

(45 s) as shown in Figure 5. Note that the normalized oncurrent Inorm in Figure 5b is defined as the ratio between the drain current under UV light illumination (IUV) and the drain current under visible light (Ivis). The device exhibits high UV light sensitivity and stable reversibility of the photoswitching characteristics under repetitive illumination of UV and visible light, supporting its potential application in UV sensors. The UV light sensing performance of the manufactured OFET device was quantitatively examined by investigating the photoresponsivity (R) and photocurrent ratio (P) with respect to the current signal under visible light, which can be calculated as R=

IUV − I vis , Pinc

P=

IUV − I vis I vis

where Pinc is the incident illumination power on the active channel of the device. As shown in Figure 6, the Azo-e/PMMA hybrid gate dielectric-based device exhibits a maximum R value of 6.3 mA W1− and a P value of 69.3. Furthermore, the device exhibits a highly linear relationship of the normalized oncurrent with the UV light intensity (Figure 6b). Therefore, it can be suggested that the azobenzene-based OFET device has great potential in quantitative sensors to measure the UV radiation intensity with high precision.

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ASSOCIATED CONTENT

S Supporting Information *

4. CONCLUSIONS In conclusion, we have successfully fabricated an OFET-type photosensor with high sensitivity to the UV light intensity and

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08427. F


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The Journal of Physical Chemistry C 1

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H NMR spectra of Azo−OH and Azo-e, photographs of Azo-e solutions in various organic solvents, OM image of the Azo-e/PMMA blend film, AFM images of the Azo-e/ PMMA blend film, UV−vis spectrum and capacitance of a pure PMMA film, temperature-dependent capacitance change of the Azo-e/PMMA films, and AFM images of the Azo-e/PMMA films with and without a PVA interlayer (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +82-62-530-1682. Author Contributions ⊥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) (No. 2015R1A2A2A04003160) and was also supported by the Principal Research Program in the Korea Institute of Materials Science (KIMS).

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