Jieun Ko

Solution-processed amorphous hafnium-lanthanum oxide gate insulator for oxide thin-film transistors

Solution-processed high-K dielectrics for oxide thin-film transistors (TFTs) have been widely studied with the objective of achieving high performance and low-cost TFTs for next-generation displays. In this study, we introduce an amorphous hafnium-lanthanum oxide (HfLaOx) gate insulator with high electrical permittivity which was fabricated by the simple spin-coating method. In particular, the solution-processed HfLaOx dielectric layer, which was achieved by a mixture of two Hf and La metal hydroxide precursors, showed amorphous properties, a low leakage current and a high dielectric constant. The solution-processed HfLaOx dielectric layers showed a breakdown voltage as high as 5 MV cm−1 in strength and a dielectric constant above 22. Based on their implementation as a gate insulator, the solution-processed ZnO/HfLaOx TFTs showed good and stable performances during operation at a low voltage. A mobility of μ = 1.6 cm2 V−1 s−1, an on/off current ratio of 106, and a threshold voltage of 0.0015 V were obtained under a 5 V gate bias. Our results show the possibility of the solution-processed amorphous HfLaOx dielectric layer as a gate insulator for oxide TFTs. We believe that this amorphous HfLaOx dielectric has good potential for next-generation high-performance TFT devices.

Organic Nonvolatile Resistive Switching Memory Based on Molecularly Entrapped Fullerene Derivative within a Diblock Copolymer Nanostructure

Organic nonvolatile resistive switching memory is developed via selective incorporation of fullerene derivatives, [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), into the nanostructure of self-assembled poly(styrene-b-methyl methacrylate) (PS₁₀ -b-PMMA₁₃₀) diblock copolymer. PS₁₀ -b-PMMA₁₃₀ diblock copolymer provides a spatially ordered nanotemplate with a 10-nm PS nanosphere domain surrounded by a PMMA matrix. Spin casting of the blend solution of PS₁₀ -b-PMMA₁₃₀ and PCBM spontaneously forms smooth films without PCBM aggregation in which PCBM molecules are incorporated within a PS nanosphere domain of PS₁₀ -b-PMMA₁₃₀ nanostructure by preferential intermixing propensity of PCBM and PS. Based on the well-defined PS₁₀-b-PMMA ₁₃₀/PCBM nanostructure, resistive random access memory (ReRAM) exhibits significantly improved bipolar-switching behavior with stable and reproducible properties at low operating voltages (RESET at 1.3 V and SET at -1.5 V) under ambient conditions. Finally, flexible memory devices are achieved using a nanostructured PS₁₀ -b-PMMA₁₃₀ /PCBM composite in which no significant degradation of electrical properties is observed before and after bending.

Nanograined thermoelectric Bi2Te2.7Se0.3 with ultralow phonon transport prepared from chemically exfoliated nanoplatelets

Herein, we report on a scalable synthesis of surfactant-free Bi2Te2.7Se0.3 nanocrystals by chemical exfoliation and subsequent spark plasma sintering to fabricate nanostructured thermoelectric bulk materials. The exfoliated n-type Bi2Te2.7Se0.3 nanoplatelets were shown to transform into nanoscroll-type crystals (∼5 nm in diameter, ∼50 nm in length) by ultrasonication. The thermoelectric performance of the Bi2Te2.7Se0.3 nanocrystals was found to be recoverable by minimizing surface oxides by chemical reduction of the exfoliated suspensions. Nanostructured bulk materials, composed of plate-like grains with ∼50 nm thickness, were prepared by sintering of the ultrasonicated sample using a spark plasma sintering technique. The resulting compound showed drastic reduction of lattice thermal conductivity (0.31 W m−1 K−1 @ 400 K) due to enhanced phonon scattering at highly dense grain boundaries without deterioration of the power factor (21.0 × 10−4 W m−1 K−2 @ 400 K). The peak ZT value of the present compound (∼0.8 @400 K) is comparable to that of n-type single crystalline Bi2(Te,Se)3, which is one of the highest among the reported values for n-type materials synthesized by a soft chemical route.

All solid state flexible supercapacitors operating at 4 V with a cross-linked polymer–ionic liquid electrolyte

4 V-operated all solid symmetrical supercapacitors that employ mixtures of various weight compositions with cross-linked poly-4-vinylphenol (c-P4VPh) and 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) electrolytes have been demonstrated and characterized. The values at 1:3, 3.5, 4 and 4.5 (c-P4VPh:EMITFSI) offer free-standing membranes with high ionic conductivity. In the case of 1:3.5, the best specific capacitance (172.44 F g−1 in a single-electrode) and energy density (72.23 W h kg−1) were obtained at symmetrical cells based on porous carbon electrodes. Every prepared SC was reliable over 1000 cycles in the range of 0–4 V. They also have excellent flexibility and maintain capacitance after completing the bending test a thousand times.

A robust ionic liquid–polymer gate insulator for high-performance flexible thin film transistors

Herein, we propose an ionic liquid–polymer dielectric layer for flexible electronics reinforced by a chemical interaction between the polymer matrix (PVP) and the ionic liquid. Due to the robust structures of the cross-linked PVP matrix and hydrogen bonding between the ionic liquid and PVP, the ionic liquid–PVP (IL–PVP) layer exhibited a good mechanical strength when bending up to 1000 times and a stable thermal behaviour up to 300 °C. Furthermore, the IL–PVP dielectric layer showed a high capacitance value of ∼2 μF cm−2 and was operated well as a gate insulator for flexible ZnO thin film transistors with a linear field-effect mobility of ∼3.3 cm2 V−1 s−1 at a gate bias of 3 V.

Self-Healing Polymer Dielectric for a High Capacitance Gate Insulator

Self-healing materials are required for development of various flexible electronic devices to repair cracks and ruptures caused by repetitive bending or folding. Specifically, a self-healing dielectric layer has huge potential to achieve healing electronics without mechanical breakdown in flexible operations. Here, we developed a high performance self-healing dielectric layer with an ionic liquid and catechol-functionalized polymer which exhibited a self-healing ability for both bulk and film states under mild self-healing conditions at 55 °C for 30 min. Due to the sufficient ion mobility of the ionic liquid in the polymer matrix, it had a high capacitance value above 1 μF/cm(2) at 20 Hz. Moreover, zinc oxide (ZnO) thin-film transistors (TFTs) with a self-healing dielectric layer exhibited a high field-effect mobility of 16.1 ± 3.07 cm(2) V(-1) s(-1) at a gate bias of 3 V. Even after repetitive self-healing of the dielectric layer from mechanical breaking, the electrical performance of the TFTs was well-maintained.

Flexible Ionic-Electronic Hybrid Oxide Synaptic TFTs with Programmable Dynamic Plasticity for Brain-Inspired Neuromorphic Computing

Emulation of biological synapses is necessary for future brain-inspired neuromorphic computational systems that could look beyond the standard von Neuman architecture. Here, artificial synapses based on ionic-electronic hybrid oxide-based transistors on rigid and flexible substrates are demonstrated. The flexible transistors reported here depict a high field-effect mobility of ≈9 cm2 V-1 s-1 with good mechanical performance. Comprehensive learning abilities/synaptic rules like paired-pulse facilitation, excitatory and inhibitory postsynaptic currents, spike-time-dependent plasticity, consolidation, superlinear amplification, and dynamic logic are successfully established depicting concurrent processing and memory functionalities with spatiotemporal correlation. The results present a fully solution processable approach to fabricate artificial synapses for next-generation transparent neural circuits.

Enhanced thermoelectric properties of Au nanodot-included Bi2Te3 nanotube composites

Herein, we report on a scalable synthesis of Au nanodot (Au-ND)/Bi2Te3 nanotube (BT-NT) nanocomposites by the bottom-up synthesis of hybrid raw materials and subsequent spark plasma sintering, and their thermoelectric properties were systematically compared with those of Au-doped Bi2Te3 compounds. The Au nanodots were included as seeds and co-crystallized in the crystal growth of BT-NTs, which were well-dispersed in the Bi2Te3 matrix as nanoinclusions (10–20 nm). The thermoelectric performance (ZT) of the Au-ND/BT-NT nanocomposite was found to be enhanced by ∼67%, compared to pristine Bi2Te3 due to electron energy filtering and phonon scattering effects in the presence of embedded Au-NDs. The resulting compound showed an enhanced power factor (23.0 × 10−4 W m−1 K−2 @ 440 K, 27% improvement) and a reduced lattice thermal conductivity (0.47 W m−1 K−1 @ 440 K, 22% reduction). The peak ZT value of the present compound (0.95 @ 480 K) is larger than that of n-type single crystalline Bi2(Te,Se)3, which is one of the highest among the reported values for n-type Bi2Te3-based materials synthesized using a soft chemical route.

Fully Solution-Processed and Foldable Metal-Oxide Thin-Film Transistor

Flexible and foldable thin-film transistors (TFTs) have been widely studied with the objective of achieving high-performance and low-cost flexible TFTs for next-generation displays. In this study, we introduced the fabrication of foldable TFT devices with excellent mechanical stability, high transparency, and high performance by a fully solution process including PI, YOx, In2O3, SWCNTs, IL-PVP, and Ag NWs. The fabricated fully solution-processed TFTs showed a higher transmittance above 86% in the visible range. Additionally, the charge-carrier mobility and Ion/Ioff ratio of them were 7.12 ± 0.43 cm(2)/V·s and 5.53 ± 0.82 × 10(5) at a 3 V low voltage operating, respectively. In particular, the fully solution-processed TFTs showed good electrical characteristics under tensile strain with 1 mm bending and even extreme folding up to a strain of 26.79%. Due to the good compatibility of each component layer, it maintained the charge-carrier mobility over 79% of initial devices after 5,000 cycles of folding test in both the parallel and perpendicular direction with a bending radius of 1 mm. These results show the potential of the fully solution-processed TFTs as flexible TFTs for a next generation devices because of the robust mechanical flexibility, transparency, and high electrical performance of it.

Oligodontia and Curly Hair Occur with Ectodysplasin-A Mutations

Oligodontia is the developmental absence of more than 5 permanent teeth except for the third molar. Familial oligodontia can occur as an isolated form or as part of a genetic syndrome. Mutations in the MSX1, PAX9, AXIN2, EDA, and WNT10A genes have been identified in familial non-syndromic oligodontia. Ectodermal dysplasia is a group of syndromes involving abnormalities of the ectodermal structures and is comprised of more than 150 different forms. Mutations in the ectodysplasin-A (EDA) gene have been associated with X-linked hypohidrotic ectodermal dysplasia, and partial disruption of the EDA signaling pathway has been shown to cause an isolated form of oligodontia. We identified 2 X-linked oligodontia families and performed mutational analysis of the EDA gene. The mutational analysis revealed 2 novel EDA mutations: c.866G>T, p.Arg289Leu and c.1135T>G, p.Phe379Val (reference sequence NM_001399.4). These mutations were perfectly segregated with oligodontia and curly hair within each family and were not found in the 150 control X-chromosomes with the same ethnic background and in the exome variant server. This study broadens the mutational spectrum of the EDA gene and the understanding of X-linked oligodontia with curly hair.