Duk-Young Jung

Reversible Interpenetration in a Metal-Organic Framework Triggered by Ligand Removal and Addition

Interpenetration is known for the structures of many minerals and ice; most notably for ice, it exists in doubly interpenetrating (VI, VII, and VIII) and non-interpenetrating (Ih) forms with the latter being porous and having nearly half of the density of the former. In synthetic materials, specifically in metal–organic frameworks (MOFs), interpenetration is generally considered undesirable because it reduces porosity. However, on the contrary, many advantageous properties also arise when MOFs are interpenetrated, such as selective guest capture, stepwise gas adsorption, enhanced framework robustness, photoluminescence control, and guest-responsive porosity. Therefore, various strategies have been suggested to control interpenetration during synthesis. However, once these extended network materials are prepared as interpenetrating or non-interpenetrating structures, the degree of interpenetration generally remains unchanged, because numerous chemical bonds must be broken and subsequently reformed in a very concerted way during the process unlike some interlocked coordination compounds in solution (Figure 1a).

One-dimensional copper–pyridinedicarboxylate polymer containing square-planar Cu(II) centers exhibiting antiferromagnetic coupling

Abstract The hydrothermal reaction of Cu(NO 3 ) 2 ·2.5H 2 O with 2,5-pyridinedicarboxylic acid (2,5-PDCH 2 ) led to the formation of aone-dimensional coordination polymer with the empirical formula of Cu(2,5-PDC)(H 2 O) ( 1 ). On the other hand, the hydrothermalreaction of Cu(NO 3 ) 2 ·2.5H 2 O with 2,4-pyridinedicarboxylic acid (2,4-PDCH 2 ) gave a mononuclear Cu(II) compound Cu(2,4-PDC) 2 ( 2 ). The structures of both compounds have been determined by X-ray diffraction. In both compounds, the coordinationsphere of the Cu metal is square-planar. The polymer 1 exhibits strong antiferromagnetic coupling between the paramagneticCu(II) metals in the adjacent layers even in the absence of intervening groups.

Optimum channel thickness in pentacene-based thin-film transistors

We report on the influence of pentacene channel thickness on the field-effect hole mobility in pentacene-based thin-film transistors (TFTs) that employ the top-contact mode for the source/drain electrodes. Our pentacene channel layers were deposited in the thickness range of 16–90 nm by thermal evaporation on 450 nm thick Al2O3+x dielectric films. The TFTs with increasingly thinner pentacene layers displayed correspondingly higher hole mobility, but an optimum thickness was determined to be about 30 nm because the TFTs with pentacene layers thinner than 30 nm exhibited high leakage current in the off-state bias regime. After a proper chemical treatment was performed onto the Al2O3+x gate dielectric, our optimized TFT with a 30 nm thick pentacene channel exhibited high mobility of ∼0.2 cm2/V s with an on/off current ratio of 105.

A New Mussel‐Inspired Polydopamine Sensitizer for Dye‐Sensitized Solar Cells: Controlled Synthesis and Charge Transfer

The efficient electron injection by direct dye-to-TiO(2) charge transfer and strong adhesion of mussel-inspired synthetic polydopamine (PDA) dyes with TiO(2) electrode is demonstrated. Spontaneous self-polymerization of dopamine using dip-coating (DC) and cyclic voltammetry (CV) in basic buffer solution were applied to TiO(2) layers under a nitrogen atmosphere, which offers a facile and reliable synthetic pathway to make the PDA dyes, PDA-DC and PDA-CV, with conformal surface and perform an efficient dye-to-TiO(2) charge transfer. Both synthetic methods led to excellent photovoltaic results and the PDA-DC dye exhibited larger current density and efficiency values than those in the PDA-CV dye. Under simulated AM 1.5 G solar light (100 mW cm(-2)), a PDA-DC dye exhibited a short circuit current density of 5.50 mW cm(-2), corresponding to an overall power conversion efficiency of 1.2 %, which is almost 10 times that of the dopamine dye-sensitized solar cell. The PDA dyes showed strong adhesion with the nanocrystalline TiO(2) electrodes and the interface engineering of a dye-adsorbed TiO(2) surface through the control of the coating methods, reaction times and solution concentration maximized the overall conversion efficiency, resulting in a remarkably high efficiency.

Photoresponse of CsPbBr3 and Cs4PbBr6 Perovskite Single Crystals

High-quality and millimeter-sized perovskite single crystals of CsPbBr3 and Cs4PbBr6 were prepared in organic solvents and studied for correlation between photocurrent generation and photoluminescence (PL) emission. The CsPbBr3 crystals, which have a 3D perovskite structure, showed a highly sensitive photoresponse and poor PL signal. In contrast, Cs4PbBr6 crystals, which have a 0D perovskite structure, exhibited more than 1 order of magnitude higher PL intensity than CsPbBr3, which generated an ultralow photoresponse under illumination. Their contrasting optoelectrical characteristics were attributed to different exciton binding energies, induced by coordination geometry of the [PbBr6]4- octahedron sublattices. This work correlated the local structures of lead in the primitive perovskite and its derivatives to PL spectra as well as photoconductivity.

Orientation-controlled assembly and solvothermal ion-exchange of layered double hydroxide nanocrystals

The orientation-controlled LDH crystals on Si substrates were intercalated by dicarboxylate ions to give the anisotropic layer expansion.

Threshold voltage change due to organic-inorganic interface in pentacene thin-film transistors

We have constructed pentacene-based organic thin-film transistors (OTFTs) using 270nm thick Al2O3+x gate dielectric deposited on indium tin oxide glass. Two different deposition techniques have been employed for the solid pentacene channel of our OTFTs: traditional thermal evaporation (TE) and energetic cluster evaporation (ECE). The TE-deposited pentacene channel appeared superior to the ECE-deposited pentacene in regard to crystallinity and hole mobility. However, the ECE-prepared OTFT showed an earlier turn on with smaller threshold voltage than the TE-prepared OTFT. This threshold voltage shift appeared more prominent with proper chemical treatment on the surface of our Al2O3+x gate oxide.

One-dimensional organometallic molecular wires via assembly of Rh(CO)2Cl(amine): chemical control of interchain distances and optical properties.

The assembly of Rh(CO)(2)Cl(amine) via molecular orbital symmetry interactions, resulting in the formation of one-dimensional molecular wires, has been discovered. The assembly behavior was quite dependent on the type of amine. The interchain distances and optical properties could be controlled by changing the length of the alkyl chain in the amine. We believe that this discovery can be applied not only to preparing more diverse one-dimensional molecular wires via the introduction of predesigned amines but also to gaining a deeper understanding of the physical properties of molecular metals.

Surface Selective Deposition of PMMA on Layered Double Hydroxide Nanocrystals Immobilized on Solid Substrates

2009 WILEY-VCH Verlag Gm Assembly of nanometer-sized particles on various solid substrates has been the focus of intense interest in the development of new integrated functional materials. Layered double hydroxides (LDHs), known as anionic or hydrotalcite-like clays, have been investigated as a multifunctional inorganic material, for example, host materials, catalysts, sorbents, and bioinorganic and polymer–inorganic composites. To date in this area, most of the work performed has been on powder samples in colloidal solutions, where the bulk properties of the randomly assembled nanocrystals predominate over the contribution of the individual ones. LDH particles in the form of powders are considered one of the strongly correlated systems in the field of strong interparticle interactions involving electrostatic forces as well as hydrogen bonding. These hydrophilic ensembles of LDH particles are expected to be less reactive toward incoming reactants, especially organic anions such as carboxylates. In this context, we recently introduced a novel method of controlling the face-to-face assembly of [Mg4Al2(OH)12]CO3 nH2O (MgAl-LDH) nanocrystals on Si substrates in closely packed arrays with a highly-ordered orientation, which can be used for solvothermal anion exchange to give a drastic anisotropic size change that can be observed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Multilayer LDH nanocrystals on solid substrates not only make chemical reactions much more reliable, but also open up a new platform of other useful chemical interfaces difficult to achieve in a bulk system. Motivated by the need to assemble functional nanomaterials based on hybrid thin films, we demonstrate in the present study that the LDH nanocrystal organization method can provide a tunable reactive inorganic interface, depending on the liquid media and the surface characteristics of the applied substrates. We were able to precisely modify the surface potentials of the LDH nanocrystals in colloid solutions by changing the solvents, leading to the well-oriented LDH monolayer films acting as a reactive inorganic interface for the fabrication of polymer–LDH hybrid films. The hydroxide groups of LDH provide a facile route to produce the additional surface modification required to develop nanoscale inorganic composite thin films, such as superhydrophobic and polymer–inorganic hybrid films. Among the existing synthetic approaches to the preparation of polymer–inorganic hybrid nanocomposites, surface-initiated polymerization (SIP) allows for the high affinity of the graft polymer by employing the surface modification of the initiator on the surface of layered inorganic compounds. Graft polymers generated on clay surfaces by SIP have especially attracted considerable interest because of their practical applications involving improved mechanical, thermal, and barrier properties. The density of the grafting surfaces could be adjusted by employing different initiators. However, in most systems based on silicate materials reported to date it has been proved that the grafting polymer films have low polymer densities because of the stepwise generation of the initiator molecules to form a monolayer, the difficulty in introducing initiators and monomers into the clay surfaces, and the occurrence of unnecessary side reactions. Herein, we present a precise control method to increase the grafting polymer density on hydroxyl-rich LDH surfaces by using self-assembled monolayers (SAMs) to create a uniform initiator monolayer, in which we were able to change the area coverage of the assembled LDH nanocrystals on the substrates. To the best of our knowledge, this is the first example of the graft density control of polymer films by adjusting the area coverage of an immobilized LDH monolayer with a highly oriented structure on oxide, metal, and polymer substrates. Specifically, the incorporation of poly(methyl methacrylate) (PMMA) on the immobilized LDH surface provides us with new polymer–LDH hybrid films as well as a nanoscale reaction platform, which is extremely difficult in bulk systems. We investigated the orientation and area coverage of MgAl-LDH depending upon the applied substrates and solvents. Figure 1 shows that the tile-like LDH crystals were bound in parallel to the substrate planes on Si. Protic solvents gave a monolayer of MgAl-LDH with higher coverage of at least 50%, whereas nonprotic solvents such as toluene resulted in double and triple layers in some parts with a lower coverage of about 20%. Additional ultrasonic treatment in clean organic solvents for 30min produced no distinguishable changes in the particle assembly, implying that the adhesion is strong enough to resist the ultrasound-induced vibration. Figure 2a presents the surface coverage ratios, namely the percentage areas covered by the MgAl-LDH nanocrystals with respect to the whole Si surface, which are governed by the degree of attraction of MgAl-LDH to the substrates. Alcohols gave higher values of the lateral packing among the solvents. Most of the alcohols, denoted as Group I, gave ratios of around 70% to 90%.

Anthraquinone sulfonate modified, layered double hydroxide nanosheets for dye-sensitized solar cells.

Dye-sensitized solar cells (DSCs) have been extensively investigated for solar energy conversion by using various combinations of inorganic semiconductors and organic sensitizers because of their low cost, easy production, and high efficiency. For efficient visible-light absorption, various organic dyes have been intensively exploited because of their advantages, such as their high molar extinction coefficients and tunable optical bands, in which they are anchored on mesoscopic TiO2 semiconductors. Recently, inorganic semiconducting materials, such as quantum dots (CdSe, CdS,) and organometal perovskites, have been proposed as inorganic sensitizers in photovoltaic cells to overcome some drawbacks of the organic dyes such as their relatively low heat stability and narrow absorption bands. One possible approach to improving the inherent light-harvesting ability of the organic dyes is to hybridize them with nanosized multifunctional inorganic materials such as layered double hydroxides (LDHs), which can provide a stable chemical environment, higher heat or photostability, and are environmentally friendly. The LDHs, also known as anionic or hydrotalcite-like clays, are useful in new multifunctional systems such as biological carriers, catalysts, and hybrid optical layers. Recently, Duan et al. reported an ultrathin hybrid film consisting of LDH nanosheets and luminescent polyanions, in which the LDH nanosheets induced a welldefined photoluminescence of polymer monolayers that are individually separated by the exfoliated LDH nanosheets. Moreover, the LDHs provide a stable chemical environment to increase the photochemical function and thermal stability of the intercalated organic photochromic dyes. In this study, LDH nanosheets are suggested as the inorganic matrix in an attempt to induce an intense photochromic function of the organic photochromic dyes, anthraquinone sulfonate anion (AQS), that are chemically immobilized on the surface of the LDH nanosheets. Herein, we report a new hybrid light sensitizer for DSCs, in which the AQS anion is selected as the organic sensitizer and the LDH nanosheets as the inorganic host. This is believed to be the first example of hybrid LDH/organic nanosheets used as a light sensitizer in photovoltaic devices. The chemical structure and photochromic behavior of the LDH AQS nanosheets in formamide are shown in Figure 1a. The powder X-ray diffraction (XRD) pattern of LDH AQS microcrystals indicated a well-crystallized rhombohedral hydrotalcite-like, 3R1 phase with lattice parameters of a=3.05 and c=60.0 . An antiparallel arrangement of the AQS would be the best model by assuming that the length of the AQS anion was 12.9 . Notably, a transparent solution was obtained by ultrasound treatment for 10 min, indicating the successful exfoliation of the platelike LDH AQS microcrystals. Typical Tyndall light scattering of the resulting solution demonstrated the presence of exfoliated LDH nanosheets as shown in Figure 1a. Interestingly, the suspension showed a strong photoinduced coloration that was not seen in the AQS–formamide solution. In (4) of Figure 1b, the UV/Vis absorption spectra for the irradiated suspension of the LDH AQS nanosheets show extremely enhanced absorption bands in the range of 400–600 nm, where the absorption bands at 435 and 525 nm are characteristic signals for the reduction state of anthraquinone sulfonate (AQS ) that have a long-term stability in a high pH condition, whereas formamide (the solvent) might be oxidized during the photoreaction. Under continuous irradiation of the exfoliated LDH AQS solution shown in (2) of Figure 1b, the band intensities gradually increased as a function of the irradiation time and then decreased after the light was cut off. The rate constants for photocoloration and decay were 0.072 and 0.059 min , respectively. This may be [a] Dr. J. H. Lee, J. Chang, J.-H. Cha, Prof. D.-Y. Jung, S. S. Kim, Prof. J. M. Kim Department of Chemistry-BK21 and Sungkyunkwan Advanced Institute of Nanotechnology Institute of Basic Sciences, Sungkyunkwan University Suwon, 440-746 (Korea) Fax: (+82)31-290-7075 E-mail : dyjung@skku.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000703.