Youn Sang Kim

Thin‐Film Formation of Imidazolium‐Based Conjugated Polydiacetylenes and Their Application for Sensing Anionic Surfactants

Therehave been many exciting and significant results from theapplication of new imidazolium receptors to many anionictargets. Martnez-Mez et al. recently reported a colorimet-ric method for the selective determination of anionic surfac-tants using imidazolium-containing ionic liquids on a solidsupport.

Low‐Temperature, Solution‐Processed and Alkali Metal Doped ZnO for High‐Performance Thin‐Film Transistors

) and its dep-osition requires a high-cost vacuum process. More importantly, the poor transparency of silicon makes it unsuitable for trans-parent applications, and transparency is one of the key issues for future display technology. Consequently, in a search for alterna-tives for amorphous silicon, considerable interest has focused on metal oxide semiconductors, such as In, Ga, or Zn oxides, as these exhibit high optical transparencies, and have excel-lent electrical properties with high electron mobility, chemical stability, and solution processability. For example, ZnO-based semiconductors have been successfully incorporated into var-ious electronic devices, such as electron transfer layers for solar cells,

All‐Solution‐Processed Flexible Thin Film Piezoelectric Nanogenerator

An all-solution-processed flexible thin film piezoelectric nanogenerator is demonstrated using reactive zinc hydroxo-condensation and a screen-printing method. The highly elastic thin film allows the piezoelectric energy to be generated through the mechanical rolling and muscle stretching of the piezoelectric unit. This flexible all solution-processed nanogenerator is promising for use in future energy harvesters such as wearable human patches and mobile electronics.

Oxidation of silver electrodes induces transition from conventional to inverted photovoltaic characteristics in polymer solar cells

Oxidation of silver top electrodes in polymer solar cells induces the devices to exhibit a transition from conventional to inverted photovoltaic characteristics. As silver oxidizes, its work function increases from 4.3 to 5.0 eV. The silver top electrode thus reverts from collecting electrons to collecting holes during device operation. We have quantified the extent of this transformation and find it to be correlated with the rate of silver oxidation, which we can influence by altering the annealing sequence of the polymer photoactive layer, ultimately varying the grain size of silver in the top electrode.

High‐Power Density Piezoelectric Energy Harvesting Using Radially Strained Ultrathin Trigonal Tellurium Nanowire Assembly

A high-yield solution-processed ultrathin (<10 nm) trigonal tellurium (t-Te) nanowire (NW) is introduced as a new class of piezoelectric nanomaterial with a six-fold higher piezoelectric constant compared to conventional ZnO NWs for a high-volume power-density nanogenerator (NG). While determining the energy-harvesting principle in a NG consisting of t-Te NW, it is theoretically and experimentally found that t-Te NW is piezoelectrically activated only by creating strain in its radial direction, along which it has an asymmetric crystal structure. Based upon this mechanism, a NG with a monolayer consisting of well-aligned t-Te NWs and a power density of 9 mW/cm(3) is fabricated.

An effective energy harvesting method from a natural water motion active transducer

We demonstrated a new water motion active transducer (WMAT) without any external bias-voltage sources or additional processes, which critically limit the use of conventional passive capacitive transducers that convert mechanical motion into electric energy. From a simple structure, we successfully turned on an LED using various kinds of natural water motion. The WMAT, which has wide applicability, has good potential to be a candidate for generating sustainable electric energy.

UV–Visible Spectroscopic Analysis of Electrical Properties in Alkali Metal‐Doped Amorphous Zinc Tin Oxide Thin‐Film Transistors

Solution-processed and alkali metals, such as Li and Na, are introduced in doped amorphous zinc tin oxide (ZTO) semiconductor TFTs, which show better electrical performance, such as improved field effect mobility, than intrinsic amorphous ZTO semiconductor TFTs. Furthermore, by using spectroscopic UV-visible analysis we propose a comprehensive technique for monitoring the improved electrical performance induced by alkali metal doping in terms of the change in optical properties. The change in the optical bandgap supported by the Burstein-Moss theory could successfully show a mobility increase that is related to interstitial doping of alkali metal in ZTO semiconductors.

Layer-by-Layer Growth of Polymer/Quantum Dot Composite Multilayers by Nucleophilic Substitution in Organic Media†

Layer-by-layer (LbL) self-assembly is a versatile and simple methodology for growing polymer and polymer/inorganic nanoparticle hybrid multilayer thin films with controlled chemical composition and thickness on the nanometer scale. Traditional LbL assembly is carried out in aqueous media and is based on the electrostatic attraction between two oppositely charged materials, such as polycations and polyanions. The recent progress in utilizing hydrogen bonding, click chemistry, disulfide bonding, silanization, esterification, urethane linking, amidation, and so forth, for LbL self-assembly has allowed the growth of multilayer thin films in polar solvents, mainly water and/or alcohols. To our knowledge, LbL self-assembly for functional organic/inorganic nanocomposites has not yet been implemented in nonpolar solvents. Herein we report the first success in using a nucleophilic substitution reaction for LbL self-assembly of organic/inorganic multilayers in nonpolar solvents. Based on a nucleophilic substitution reaction between Br and NH2, alternating layers of highly hydrophobic CdSe@ZnS quantum dots (QDs) capped with 2-bromo-2-methylpropionic acid (BMPA) in toluene or hexane and poly(amidoamine) dendrimer (PAMA) in ethanol were deposited to form QD/ PAMA composite multilayer thin films. The resulting thin films exhibited more robust photoluminescence (PL) in air (oxidation) and in the presence of moisture (hydrolysis) than those obtained by electrostatic LbL self-assembly. These results also demonstrate the possibility of LbL growth of patterned films based on nucleophilic substitution with the aid of microcontact printing. Photoluminescent (and electroluminescent) polymer/QD nanocomposite films are quite important in technical applications and may be used as functional components in electronic devices, such as optical thin films, or for biomedical imaging. Nevertheless, there has been limited success in fabricating polymer/QD composite thin films using the LbL self-assembly techniques developed to date, because the PL properties of the embedded QDs are usually poor. Conventional LbL self-assembly techniques are carried out in aqueous or polar media, which means that the QDs, which are produced either directly in aqueous or polar media or obtained through ligand exchange or phase transfer, have poor surface passivation, which makes the PL of the resulting QDs vulnerable either during LbL self-assembly or during the thin film storage. Recent studies have shown that a high packing density of small and hydrophilic thiol ligands reduces the quantum yield of QDs significantly. Kotov et al. reported that the PL intensity of composite multilayer thin films of polyelectrolyte and citrate-stabilized CdSe@CdS QDs was increased by 50–500 times after ambient light irradiation for several days owing to surface oxidization on the QDs with ambient oxygen for 3 days, which was accompanied by a notable blue shift in the PL bands with exposure time. To date, the growth of polymer/QD multilayer thin films that preserve the original PL behavior of the QDs is a significant challenge for LbL self-assembly. To circumvent this challenge, we prepared LbL-assembled highly hydrophobic BMPA-stabilized CdSe@ZnS QDs in nonpolar solvents based on nucleophilic substitution of the terminal Br groups of a BMPA coating with the NH2 groups of PAMA. CdSe@ZnS QDs consisting of 4 nm CdSe cores and 1 nm ZnS shells stabilized by oleic acid were prepared in hexane or toluene according to a reported method. The original oleic acid stabilizer ligands were replaced with BMPA through ligand exchange, leading to BMPA-stabilized CdSe@ZnS QDs, denoted BMPA-QDs (see the Supporting Information, Figure S1). Ligand exchange reduced the quantum yield of the QDs (relative to coumarin 545) from 59 % to 30%. For comparison, the oleic acid stabilizers of the QDs were also replaced with mercapto acetic acid (MAA) through phase transfer (from toluene to aqueous media) to form negatively charged MAA-coated QDs, denoted MAA-QDs (see experimental details in the Supporting Information). In this case, the relative quantum yield of the resulting MAA-QDs at pH 9 was approximately 9 %, which shows that using hydrophilic thiol ligands to replace the original hydrophobic ligands caused a dramatic decrease in the PL quantum yield. [*] B. Lee, Y. Kim, S. Lee, Prof. J. Cho School of Advanced Materials Engineering Kookmin University, Seoul 136-702 (Korea) E-mail: jinhan@kookmin.ac.kr

Facile Synthesis of Oxidation-Resistant Copper Nanowires toward Solution-Processable, Flexible, Foldable, and Free-Standing Electrodes

Oxidation-resistant copper nanowires (Cu NWs) are synthesized by a polyol reduction method. These Cu NWs show excellent oxidation resistance, good dispersibility, and have a low sintering temperature. A Cu NW-based flexible, foldable, and free-standing electrode is fabricated by filtration and a sintering process. The electrode also exhibits high electrical conductivity even bending, folding, and free-standing.

Novel Synthesis, Coating, and Networking of Curved Copper Nanowires for Flexible Transparent Conductive Electrodes

In this work, a whole manufacturing process of the curved copper nanowires (CCNs) based flexible transparent conductive electrode (FTCE) is reported with all solution processes, including synthesis, coating, and networking. The CCNs with high purity and good quality are designed and synthesized by a binary polyol coreduction method. In this reaction, volume ratio and reaction time are the significant factors for the successful synthesis. These nanowires have an average 50 nm in width and 25-40 μm range in length with curved structure and high softness. Furthermore, a meniscus-dragging deposition (MDD) method is used to uniformly coat the well-dispersed CCNs on the glass or polyethylene terephthalate substrate with a simple process. The optoelectrical property of the CCNs thin films is precisely controlled by applying the MDD method. The FTCE is fabricated by networking of CCNs using solvent-dipped annealing method with vacuum-free, transfer-free, and low-temperature conditions. To remove the natural oxide layer, the CCNs thin films are reduced by glycerol or NaBH4 solution at low temperature. As a highly robust FTCE, the CCNs thin film exhibits excellent optoelectrical performance (T = 86.62%, R(s) = 99.14 Ω ◻(-1)), flexibility, and durability (R/R(0) < 1.05 at 2000 bending, 5 mm of bending radius).