Young-Tae Kim

Organometal Halide Perovskite Artificial Synapses.

Organometal halide perovskite synaptic devices are fabricated; they emulate important working principles of a biological synapse, including excitatory postsynaptic current, paired-pulse facilitation, short-term plasticity, long-term plasticity, and spike-timing dependent plasticity. These properties originate from possible ion migration in the ion-rich perovskite matrix. This work has extensive applicability and practical significance in neuromorphic electronics.

Highly Efficient Light-Emitting Diodes of Colloidal Metal-Halide Perovskite Nanocrystals Beyond Quantum Size

Colloidal metal-halide perovskite quantum dots (QDs) with a dimension less than the exciton Bohr diameter DB (quantum size regime) emerged as promising light emitters due to their spectrally narrow light, facile color tuning, and high photoluminescence quantum efficiency (PLQE). However, their size-sensitive emission wavelength and color purity and low electroluminescence efficiency are still challenging aspects. Here, we demonstrate highly efficient light-emitting diodes (LEDs) based on the colloidal perovskite nanocrystals (NCs) in a dimension > DB (regime beyond quantum size) by using a multifunctional buffer hole injection layer (Buf-HIL). The perovskite NCs with a dimension greater than DB show a size-irrespective high color purity and PLQE by managing the recombination of excitons occurring at surface traps and inside the NCs. The Buf-HIL composed of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and perfluorinated ionomer induces uniform perovskite particle films with complete film coverage and prevents exciton quenching at the PEDOT:PSS/perovskite particle film interface. With these strategies, we achieved a very high PLQE (∼60.5%) in compact perovskite particle films without any complex post-treatments and multilayers and a high current efficiency of 15.5 cd/A in the LEDs of colloidal perovskite NCs, even in a simplified structure, which is the highest efficiency to date in green LEDs that use colloidal organic-inorganic metal-halide perovskite nanoparticles including perovskite QDs and NCs. These results can help to guide development of various light-emitting optoelectronic applications based on perovskite NCs.

Novel approach for observing the asymmetrical evolution and the compositional nonuniformity of laser pulsed atom probe tomography of a single ZnO nanowire

The characterization of ZnO nanowires is crucial for developing nanostructured devices together with related compounds and alloys with an atomic-scale regime. This study describes the effects of laser energy on the atom probe tomography analysis of a single ZnO nanowire with a high aspect ratio, diameters of 80?100 nm and lengths of 10 µm. We observed both an asymmetrical evolution in the apex morphology and the compositional nonuniformities of Zn and O ions with respect to the laser energies ranging from 5 to 40 nJ. When the higher laser illumination exposed to the ZnO nanowires, non-uniform field strength becomes noticeable especially at the laser incident side of the samples. Moreover, we measured the charge state ratios of Zn+ and Zn2+ ions as a function of the applied laser energies. Our results proved important for accurate quantitative characterization and better interpretation for the laser-pulsed atom probe tomography of ZnO-based devices.

Implementing Room-Temperature Multiferroism by Exploiting Hexagonal-Orthorhombic Morphotropic Phase Coexistence in LuFeO3 Thin Films

Room-temperature multiferroism in LuFeO3 (LFO) films is demonstrated by exploiting the orthorhombic-hexagonal (o-h) morphotrophic phase coexistence. The LFO film further reveals a magnetoelectric coupling effect that is not shown in single-phase (h- or o-) LFO. The observed multiferroism is attributed to the combination of sufficient polarization from h-LFO and net magnetization from o-LFO.

Artificial Synapses: Organometal Halide Perovskite Artificial Synapses (Adv. Mater. 28/2016).

A synapse-emulating electronic device based on organometal halide perovskite thin films is described by T.-W. Lee and co-workers on page 5916. The device successfully emulates important characteristics of a biological synapse. This work extends the application of organometal halide perovskites to bioinspired electronic devices, and contributes to the development of neuromorphic electronics.

A correlation between small-molecule dependent nanomorphology and device performance of organic light-emitting diodes with ternary blend emitting layers

The morphology of emitting layers (EMLs) plays a vital role in determining the overall performance of solution processed phosphorescent organic light emitting diodes (PhOLEDs). Herein, the morphology of undoped small molecule binary blend EMLs prepared by blending tris(4-carbazoyle-9-ylphenyl)amine (TCTA) hole transport material (HTM) with a series of electron transport materials (ETMLs) was studied using a transmission electron microscope (TEM). Experimental results show that phase separation of the binary blend EMLs significantly depends on the polarity of the host. The binary blend ELMs were further doped with tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) to form ternary blend EMLs and the resulting morphology was examined using scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS). The results report for the first time the existence of Ir(ppy)3 needle-like aggregates in small molecule ternary blend EMLs. By comparing the size of the aggregates formed in small molecule ternary blend EMLs with those formed in polymer–small molecule blends, our results showed that small molecule blend EMLs exhibit minimal Ir(ppy)3 aggregates with a low aspect ratio in contrast to polymer–small molecule blends. The effect of mixed solvent on the distribution of the aggregates was also examined using a TEM and an atomic force microscope (AFM). The disappearance of the aggregates with varying solvent mixture ratios signifies that solvent mixture is an effective way to control homogeneous distribution of Ir(ppy)3 in the emitting layers of PhOLEDs. This was further evidenced by an improvement in light emitting efficiency and current efficiency of OLED devices fabricated using a mixed solvent.

Fully Elastic Conductive Films from Viscoelastic Composites

We investigated, for the first time, the conditions where a thermoplastic conductive composite can exhibit completely reversible stretchability at high elongational strains (ε = 1.8). We studied a composite of Au nanosheets and a polystyrene-block-polybutadiene-block-polystyrene block copolymer as an example. The composite had an outstandingly low sheet resistance (0.45 Ω/sq). We found that when a thin thermoplastic composite film is placed on a relatively thicker chemically cross-linked elastomer film, it can follow the reversible elastic behavior of the bottom elastomer. Such elasticity comes from the restoration of the block copolymer microstructure. The strong adhesion of the thermoplastic polymer to the metallic fillers is advantageous in the fabrication of mechanically robust, highly conductive, stretchable electrodes. The chemical stability of the Au composite was used to fabricate high luminescence, stretchable electrochemiluminescence displays with a conventional top-bottom electrode setup and with a horizontal electrode setup.

Influence of Laser-Pulse Energy on Field Evaporation of LaAlO 3 in Atom Probe Tomography Analysis

As followed by the design concept of metal-oxide-semiconductor (MOS) devices, the development of dielectric materials with high-k property becomes a crucial for reducing the equivalent oxide thickness. In order to develop the novel dielectric materials, understanding of correlations between electrical properties and chemical nature at metal-oxide interfaces has been an essential topic [1]. To identify this subject, atomic-scale investigation of bulk oxide materials through laser-pulsed atom probe tomography (APT) has been widely performed [2-7]. From the fundamental point of view, we present the influence of laser-pulse energy on APT results of lanthanum aluminum oxide (LaAlO3), termed LAO, especially mass-resolving power (MRP). The LAO is one of the potential candidates for substituting the conventional SiO2, which exhibit many advantages such as a high dielectric constant and large band-gap energy with ranging between 5.8 and 6.6 eV. Analysis were performed using a LAWATAP microscope in laser-pulsing mode of pulse energy varying with systematically 0.03~0.21 μJ at ~100 kHz pulse repetition rate, 0.002 atom/pulse detection rate, and 30 K.

Correlation of Controllable Aggregation with Light-Emitting Property in Polymer Blend Optoelectronic Devices

The control of solution-processed emitting layers in organic-based optoelectronic devices enables cost-effective processing and highly efficient properties. However, a solution-based protocol for emitter fabrication is highly complex, and the link between the device performance and internal nanoscale features as well as three associated fabricating parameters (e.g., the employed solvents, annealing temperatures, and molecular concentration) needs to be understood. Here, this study investigates the influence of the solution-processing parameters on the nanostructure-property relationship in light emitters that consist of iridium complexes doped in polymer. The boiling points and evaporation rates of the selected solvents govern the nanomorphology of molecular aggregation in the as-processed state, and the aggregation is either needle-like, spherical, or even a mixture of needles and spheres. Furthermore, a direct observation via in situ heating microscopy indicates that annealing of emitters containing a needle-type aggregation promotes the associated molecular transport, leading to a substantial reduction in the surface roughness. Consequently, a nearly threefold increase in the current efficiency of the device is induced. These findings have important implications for the tuning of the aggregation of iridium complexes for emitters used in the new evolution of high-performance organic-based optoelectronic devices.