Publications by authors named "Sangsub Kim"

7 Publications

  • Page 1 of 1

Chiral polymer hosts for circularly polarized electroluminescence devices.

Chem Sci 2021 Jul 21;12(25):8668-8681. Epub 2021 May 21.

Division of Chemical Engineering and Materials Science, Graduate Program in System Health Science and Engineering, Ewha Womans University Seoul 03760 Republic of Korea

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral -alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred ()-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The ()-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent ()- and ()-dopants. The magnitude of the Kuhn dissymmetry factor ( ) for the ()-helically-enriched polymer film doped with the ()-dopant was found to be one order of magnitude higher than that of the film doped with the ()-dopant. Photoluminescence dissymmetry factors ( ) of the order of 10 were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of . The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the ()- and ()-dopants exhibit electroluminescence dissymmetry factors ( ) of 1.09 × 10 and -1.02 × 10 at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d1sc02095aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8246120PMC
July 2021

Twist to Boost: Circumventing Quantum Yield and Dissymmetry Factor Trade-Off in Circularly Polarized Luminescence.

Inorg Chem 2021 Jun 24;60(11):7738-7752. Epub 2021 Mar 24.

Division of Chemical Engineering and Materials Science, and Graduate Program in System Health Science and Engineering, Ewha Womans University, Seoul 03760, Republic of Korea.

Circularly polarized luminescence (CPL) enables promising applications in asymmetric photonics. However, the performances of CPL molecules do not yet meet the requirements of these applications. The shortcoming originates from the trade-off in CPL between the photoluminescence quantum yield (PLQY) and the photoluminescence dissymmetry factor (). In this study, we developed a molecular strategy to circumvent this trade-off. Our approach takes advantage of the strong propensity of [Pt(N^C^N)Cl], where the N^C^N ligand is 1-(2-oxazoline)-3-(2-pyridyl)phenylate, to form face-to-face stacks. We introduced chiral substituents, including ()-methyl, ()- and ()-isopropyl, and ()-indanyl groups, into the ligand framework. This asymmetric control induces torsional displacements that give homohelical stacks of the Pt(II) complexes. X-ray single-crystal structure analyses for the ()-isopropyl Pt(II) complex reveal the formation of a homohelical dimer with a Pt···Pt distance of 3.48 Å, which is less than the sum of the van der Waals radii of Pt. This helical stack elicits the metal-metal-to-ligand charge-transfer (MMLCT) transition that exhibits strong chiroptical activity due to the electric transition moment making an acute angle to the magnetic transition moment. The PLQY and values of the MMLCT phosphorescence emission of the ()-isopropyl Pt(II) complex are 0.49 and 8.4 × 10, which are improved by factors of ca. 6 and 4, respectively, relative to the values of the unimolecular emission (PLQY, 0.078; , 2.4 × 10). Our photophysical measurements for the systematically controlled Pt(II) complexes reveal that the CPL amplifications depend on the chiral substituent. Our investigations also indicate that excimers are not responsible for the enhanced chiroptical activity. To demonstrate the effectiveness of our approach, organic electroluminescence devices were fabricated. The MMLCT emission devices were found to exhibit simultaneous enhancements in the external quantum efficiency (EQE, 9.7%) and the electroluminescence dissymmetry factor (, 1.2 × 10) over the unimolecular emission devices (EQE, 5.8%; , 0.3 × 10). These results demonstrate the usefulness of using the chiroptically active MMLCT emission for achieving an amplified CPL.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.1c00070DOI Listing
June 2021

Electrical Characterization of Charge Polarity in AlF₃ Anti-Reflection Layers for Complementary Metal Oxide Semiconductor Image Sensors.

J Nanosci Nanotechnol 2018 09;18(9):6005-6009

College of Information and Communication Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea.

In this study, the charge polarity of aluminum fluoride (AlF3) as a function of varying thickness (tAlF3 = 20, 35, 50, 65, and 80 nm) was discussed. AlF3 films were deposited onto p-Si wafers via electron beam sputtering. Thickness dependent charge polarity and reliability issues under bias-temperature stress conditions were identified using a capacitance-voltage (C-V) characterization method. AlF3 was found to possess negative fixed charges, leading to a C-V curve shift toward the positive gate bias direction as tAlF3 was increased up to 50 nm. On the contrary, the C-V characteristics were dominantly affected by the positive charges of mobile ions and/or fluorine vacancies when tAlF3 was increased to more than 50 nm. Additionally, negative bias temperature stress (1 MV/cm, 473 K for 10 mins) increased insulator trapped charges and decreased interface traps in 20 nm thick AlF3 films. These results could be attributed to positively charged fluorine vacancies introduced by broken Al-F bonds within AlF3 films and the passivation of Si dangling bonds due to broken fluorine ions at the interface, respectively. It was believed that 20 nm thick AlF3 films sufficiently attracted holes from the Si substrate, forming a hole accumulation layer on the surface due to total charge polarity of the AlF3 dielectric being entirely governed by negative fixed charges as the thickness of AlF3 decreased. Based on these results, AlF3 films are proposed for use as an anti-reflection layer to replace HfO2 in CMOS image sensors.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1166/jnn.2018.15599DOI Listing
September 2018

Nonstoichiometric Solution-Processed BaTiO₃ Film for Gate Insulator Applications.

J Nanosci Nanotechnol 2018 09;18(9):5942-5946

College of Information and Communication Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea.

Solution processed barium titanate (BTO) was used to fabricate an Al/BaTiO3/p-Si metal-insulator-semiconductor (MIS) structure, which was used as a gate insulator. Changes in the electrical characteristics of the film were investigated as a function of the film thickness and post deposition annealing conditions. Our results showed that a thickness of 5 layers and an annealing temperature of 650 °C produced the highest electrical performance. BaxTi1-xO3 was altered at x = 0.10, 0.30, 0.50, 0.70, 0.90, and 1.0 to investigate changes in the electrical properties as a function of composition. The highest dielectric constant of 87 was obtained for x = 0.10, while the leakage current density was suppressed as Ba content increased. The lowest leakage current density was 1.34×10-10 A/cm2, which was observed at x = 0.90. The leakage current was related to the resistivity of the film, the interface states, and grain densification. Space charge limited current (SCLC) was the dominant leakage mechanism in BTO films based on leakage current analysis. Although a Ba content of x = 0.90 had the highest trap density, the traps were mainly composed of Ti-vacancies, which acted as strong electron traps and affected the film resistivity. A secondary phase, Ba2TiO4, which was observed in cases of excess Ba, acted as a grain refiner and provided faster densification of the film during the thermal process. The absence of a secondary phase in BaO (x = 1.0) led to the formation of many interface states and degradation in the electrical properties. Overall, the insulator properties of BTO were improved when the composition ratio was x = 0.90.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1166/jnn.2018.15595DOI Listing
September 2018

Optimization of the Solution-Based Indium-Zinc Oxide/Zinc-Tin Oxide Channel Layer for Thin-Film Transistors.

J Nanosci Nanotechnol 2018 09;18(9):5913-5918

College of Information and Communication Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea.

Double stacked indium-zinc oxide (IZO)/zinc-tin oxide (ZTO) active layers were employed in amorphous-oxide-semiconductor thin-film transistors (AOS TFTs). Channel layers of the TFTs were optimized by varying the molarity of ZTO back channel layers (0.05, 0.1, 0.2, 0.3 M) and the electrical properties of IZO/ZTO double stacked TFTs were compared to single IZO and ZTO TFTs with varying the molarity and molar ratio. On the basis of the results, IZO/ZTO (0.1 M) TFTs showed the excellent electrical properties of saturation mobility (13.6 cm2/V·s), on-off ratio (7×106), and subthreshold swing (0.223 V/decade) compared to ZTO (0.1 M) of 0.73 cm2/V · s, 1 × 107, 0.416 V/decade and IZO (0.04 M) of 0.10 cm2/V · s, 5 × 106, 0.60 V/decade, respectively. This may be attributed to diffusing Sn into front layer during annealing process. In addition, with varying molarity of ZTO back channel layer, from 0.1 M to 0.3 M ZTO back channel TFTs, electrical properties and positive bias stability deteriorated with increasing molarity of back channel layer because of increasing total trap states. On the other hand, 0.05 M ZTO back channel TFT had inferior electrical properties than that of 0.1 M ZTO back channel TFT. It was related to back channel effect because of having thin thickness of channel layer. Among these devices, 0.1 M ZTO back channel TFT had a lowest total trap density, outstanding electrical properties and stability. Therefore, we recommended IZO/ZTO (0.1 M) TFT as a promising channel structure for advanced display applications.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1166/jnn.2018.15596DOI Listing
September 2018

Trap Profiling Based on Frequency Varied Charge Pumping Method for Hot Carrier Stressed Thin Gate Oxide Metal Oxide Semiconductors Field Effect Transistors.

J Nanosci Nanotechnol 2016 May;16(5):4851-5

In this study, pulse frequency and reverse bias voltage is modified in charge pumping and advanced technique is presented to extract oxide trap profile in hot carrier stressed thin gate oxide metal oxide semiconductor field effect transistors (MOSFETs). Carrier trapping-detrapping in a gate oxide was analyzed after hot carrier stress and the relationship between trapping depth and frequency was investigated. Hot carrier induced interface traps appears in whole channel area but induced border traps mainly appears in above pinch-off region near drain and gradually decreases toward center of the channel. Thus, hot carrier stress causes interface trap generation in whole channel area while most border trap generation occurs in the drain region under the gate. Ultimately, modified charge pumping method was performed to get trap density distribution of hot carrier stressed MOSFET devices, and the trapping-detrapping mechanism is also analyzed.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1166/jnn.2016.12192DOI Listing
May 2016

Analysis of the Electrical Properties of an Electron Injection Layer in Alq3-Based Organic Light Emitting Diodes.

J Nanosci Nanotechnol 2016 May;16(5):4742-5

We investigated the carrier transfer and luminescence characteristics of organic light emitting diodes (OLEDs) with structure ITO/HAT-CN/NPB/Alq3/Al, ITO/HAT-CN/NPB/Alq3/Liq/Al, and ITO/HAT-CN/NPB/Alq3/LiF/A. The performance of the OLED device is improved by inserting an electron injection layer (EIL), which induces lowering of the electron injection barrier. We also investigated the electrical transport behaviors of p-Si/Alq3/Al, p-Si/Alq3/Liq/Al, and p-Si/Alq3/LiF/Al Schottky diodes, by using current-voltage (L-V) and capacitance-voltage (C-V) characterization methods. The parameters of diode quality factor n and barrier height φ(b) were dependent on the interlayer materials between Alq3 and Al. The barrier heights φ(b) were 0.59, 0.49, and 0.45 eV, respectively, and the diode quality factors n were 1.34, 1.31, and 1.30, respectively, obtained from the I-V characteristics. The built in potentials V(bi) were 0.41, 0.42, and 0.42 eV, respectively, obtained from the C-V characteristics. In this experiment, Liq and LiF thin film layers improved the carrier transport behaviors by increasing electron injection from Al to Alq3, and the LiF schottky diode showed better I-V performance than the Liq schottky diode. We confirmed that a Liq or LiF thin film inter-layer governs electron and hole transport at the Al/Alq3 interface, and has an important role in determining the electrical properties of OLED devices.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1166/jnn.2016.12203DOI Listing
May 2016
-->