Geon Dae Moon

A New Theranostic System Based on Gold Nanocages and Phase-Change Materials with Unique Features for Photoacoustic Imaging and Controlled Release

This communication reports a new theranostic system with a combination of capabilities to both enhance the contrast of photoacoustic (PA) imaging and control the release of a chemical or biological effector by high-intensity focused ultrasound (HIFU). The fabrication of this system simply involves filling the hollow interiors of gold nanocages with a phase-change material (PCM) such as 1-tetradecanol that has a melting point of 38-39 °C. The PCM can be premixed and thus loaded with a dye, as well as other chemical or biological effectors. When exposed to direct heating or HIFU, the PCM will melt and escape from the interiors of nanocages through small pores on the surface, concurrently releasing the encapsulated molecules into the surrounding medium. We can control the release profile by varying the power of HIFU, the duration of exposure to HIFU, or both.

Chemical transformations in ultrathin chalcogenide nanowires.

We have studied the chemical transformations in ultrathin chalcogenide nanowires with an aim to understand the parameters that control the morphology and crystal structure of the product. Ultrathin Te nanowires were transformed into Ag2Te nanowires with preservation of the single crystallinity. The Ag2Te nanowires were then converted into CdTe, ZnTe, and PbTe using cation-exchange reactions, and the CdTe nanowires were further transformed into PtTe2 nanotubes. On the basis of the solubility products of the ionic solids, the crystal structures of the involved solids, the reaction kinetics, and the reaction conditions for transformations, we were able to reach the following conclusions: (i) The solubility products of ionic solids can be used as a rough criterion to predict if the transformation is thermodynamically favorable or not. (ii) The morphological preservation of reactant nanowires is more sensitive to the change in length rather than the total volume in addition to the lattice matching between the reactant and product nanowires. (iii) The crystal structure resulting from a transformation should be determined by the free energy of formation and the stability of the products. (iv) The transformation involving small volume change or topotactic lattice matching is considered homogeneous along the entire length of the nanowires, preserving both the single crystallinity and the morphology of the reactant nanowires.

Assembled Monolayers of Hydrophilic Particles on Water Surfaces

A facile and quick approach to prepare self-assembled monolayers of water-dispersible particles on the water surface is presented. Particle suspensions in alcohols were dropped on a water reservoir to form long-range ordered monolayers of various particles, including spherical solid particles, soft hydrogel particles, metal nanoparticles, quantum dots, nanowires, single-wall carbon nanotubes (SWCNTs), nanoplates, and nanosheets. A systematic study was conducted on the variables affecting the monolayer assembly: the solubility parameter of spreading solvents, particle concentration, zeta potential of the particles in the suspension, surface tension of the water phase, hardness of the particles, and addition of a salt in the suspension. This method requires no hydrophobic surface treatment of the particles, which is useful to exploit these monolayer films without changing the native properties of the particles. The study highlights a quick 2D colloidal assembly without cracks in the wafer scale as well as transparent conductive thin films made of SWCNTs and graphenes.

Quick, Controlled Synthesis of Ultrathin Bi2Se3 Nanodiscs and Nanosheets

Ultrathin (4-6 nm) single-crystal Bi(2)Se(3) nanodiscs and nanosheets were synthesized through a simple and quick solution process. The growth mechanism was investigated in detail. Crystal seeds grew via 2D self-attachment of small nanoparticles followed by epitaxial recrystallization into single crystals. The lateral dimension of the nanodiscs increased and their shape changed from circles to hexagons as the reaction temperature increased. Positively charged polymer surfactants greatly enlarged the lateral dimension to produce nanosheets with uniform thickness.

Continuous production of uniform poly(3-hexylthiophene) (P3HT) nanofibers by electrospinning and their electrical properties

Uniform regio-regular poly(3-hexylthiophene) nanofibers and their blend nanofibers with poly(e-caprolactone) have been obtained by electrospinning and their electrical properties in single nanofiber field effect transistors have been compared. The key to the success was restricting the precipitation of P3HT at the nozzle tip, therefore preventing the nozzle from being clogged. The field effect mobility of pure P3HT fibers was 0.017 cm2V−1 s−1 which is acceptable for device fabrication. Blend fibers with PCL also showed field effect behaviour with slightly degraded electrical properties by an order or two.

Highly stretchable patterned gold electrodes made of Au nanosheets.

Multilayered Au nanosheets are suggested as a novel class of material for fabricating stretchable electrodes suitable for organic-based electronic devices. The electrodes show no difference in resistivity during repeated stretching cycles of up to ϵ = 40%.

Solution-based synthesis of anisotropic metal chalcogenide nanocrystals and their applications

This article reviews recent advances in solution phase synthesis to generate 1-D and 2-D anisotropic metal chalcogenide (MC) nanostructures with a focus on using different growth mechanisms to control the shapes of the MCs. Four different synthetic approaches have been reviewed: naturally favoured growth due to its intrinsically anisotropic crystal structure, modified anisotropic growth by changing surface energies or utilizing organic templates, oriented attachment of small nanocrystal building blocks to form nanowires or nanosheets, and chemical transformation from existing nanostructures into new species. We discuss current understanding of the thermodynamic and kinetic aspects associated with the mechanisms of forming these anisotropic MC nanostructures. We provide examples of representative applications of anisotropic chalcogenide nanomaterials that are expected to be practically meaningful in the near future. The applications include electrodes for lithium ion batteries, photodetectors, thermoelectric devices, and solar cells. A brief review of other potential applications (oxygen reduction reaction, localized surface plasmon resonance, topological insulator, superconductor) is provided as well. This review ends with discussions on the challenges to be investigated thoroughly in the solution-based synthesis of anisotropic nanomaterials, which includes surface energy control, correcting the nucleation & growth mechanism, removal of the organic surfactant, kinetic study on the chemical transformation, scale-up of production, and eco-friendly synthesis.

Buckling-Assisted Patterning of Multiple Polymers

2010 WILEY-VCH Verlag Gmb Two-dimensional (2D) patterning of polymers has seen a considerable development during the last two decades. The patterns have been mainly fabricated using lithographic methods such as photolithography, soft lithography, capillary force lithography, and lithographically controlled wetting. These techniques are well-established and reliable, but do not allow the patterning of multiple polymers which can greatly diversify the uses of patterned microstructures. So far, we do not have a strategy to generate multicomponent polymer patterns in a simple and inexpensive way. In this study, we utilized buckling to fabricate patterned 2D microstructures comprising multiple polymers. The buckled substrates were used as templates to confine multiple polymers in the trenches of the sinusoidal topology. Unconventional patterns were obtained by taking advantage of localized dewetting of the polymers. We further demonstrated transfer of the polymeric patterns to other substrates. This study is based upon the unique characters of buckling: reversibility and morphology variation. Compressive stress created in a high-modulus layer on an elastic support results in sinusoidal topology during the stress release. Whitesides and coworkers visualized the reversible nature of buckling by cycling between thermal heating and cooling. Recent studies on stretchable electronics have also utilized the reversible nature of buckling. Especially, Rogers and coworkers pioneered the buckling-assisted stretchable thin film transistors made of inorganic semiconductors. In addition, Chiche et al. formed a checkerboard pattern by re-stretching a buckled sample perpendicular to the initial stretching direction. Lin and Yang demonstrated the transition from one-dimensional (1D) ripples to 2D herringbone structures by separately adjusting the time and the strain applied to the elastomeric substrate. Once depositing one polymer in the trenches of a buckled substrate, the capability of the morphological variation was utilized in this study to generate new buckling patterns and second polymer was deposited in the trenches of the new buckling patterns. The multicomponent patterns were transferred to another flat substrate by employing the reversible nature of buckling. This simple process allows for the generation of unique structures over the whole area of the substrate. Figure 1 shows SEM images of the buckled poly(styrene) (PS) layer on a poly(dimethyl siloxane) (PDMS) substrate and the resulting patterns after deposition of other polymers in the trenches of the buckled PS layer. To generate buckling, PS-coated PDMS substrate was heated at 90 8C and then cooled down to room temperature. The mismatch in thermal expansion between the PDMS substrate and the PS layer created a wavy pattern (Fig. 1A). The wavelength andmagnitude were 6.01 0.23mm and 883 7 nm, respectively (Fig. S1 in Supporting Information). As examples of filling in the trenches of the buckled patterns, poly(e-caprolactone) (PCL) solution, polyethylene glycol (PEG), and hydrogel macromer were deposited. PCL was spin-coated on the buckled surface and then annealed at 66 8C which is slightly above the melting point of PCL (Tm 60 8C). In order to track the locations of PCL chains, a fluorescent dye (rhodamine B) was mixed in the PCL solution before spin-coating. The viscous PCL liquid upon heating flew into the trenches to generate continuous PCL lines, as shown in Figure 1B. The thermal annealing did not change the topology of the initial buckling pattern. The fluorescence image appeared blur right after PCL spin-coating, but the sample showed a strong and localized fluorescence image along the trenches of the buckled surface after thermal annealing (Fig. S1 in Supporting Information). We also investigated the distribution of PCL before and after thermal annealing by AFM

Strain-controlled release of molecules from arrayed microcapsules supported on an elastomer substrate.

Over the last two decades, considerable attention has been paid to the controlled release of drugs. Most studies have focused on sustained release from a variety of drug-containing carriers. However, there are many clinical situations that require more than a prolonged, continuous release of drugs. For example, chronopharmacological studies indicate clear temporal or physical dependence of the onset of certain diseases on circadian rhythms. The treatment of such diseases would benefit from smart control over the release pattern of a drug in response to in vivo physiological changes or external stimulations. A fast response of the drug carrier to the stimuli may enable real-time control of the dosage. The concept of stimuli-regulated release has been studied for changes in the pH value or temperature, 12] ultrasound, and electric 14] or magnetic fields, 16] but has not been applied to mechanical strain, although this stimulus is ubiquitous in the body or very simple to apply externally. Strain changes are involved in many processes, such as compression in cartilage and bones, tension in muscles and tendons, and shear force in blood vessels. Strain-controlled release, if possible, could be applied in patches that respond to body motions, without the need for continuous release in vain. It would also be useful for implanted patches that could synchronize with the mechanical motions of organs, muscles, and tendons. This study suggests a concept for the realization of straincontrolled release. We demonstrate the fabrication of arrayed microcapsules supported on an elastomer substrate. The arrayed microcapsules were obtained by using buckled polymer thin films, which can provide stretchability without defects. The stretchable microcapsules were prepared according to the schematic illustration in Figure 1a (for more detail, see the Supporting Information). Heating and

Surfactant-free CuInSe 2 nanocrystals transformed from In 2 Se 3 nanoparticles and their application for a flexible UV photodetector

In(2)Se(3) nanoparticles were synthesized in an aqueous solution without using any surfactant and then chemically transformed into CuInSe(2) nanocrystals. The transformation was thermodynamically favorable and fast. The 93% production yield in mild reaction conditions allowed mass production of the CuInSe(2) nanocrystals. By the virtue of the surface charges, the CuInSe(2) nanocrystals were well dispersed in polar solvents. The surfactant-free nanocrystals enabled the formation of semiconducting CuInSe(2) films on a flexible polymer substrate without any thermal treatment. We took advantage of this to fabricate a flexible UV photodetector. The current and sensitivity of the devices could be improved by utilizing CuInSe(2) nanocrystals annealed at 160 °C in the reaction batch. On bending test, the detection sensitivity remained the same until the bending radius was reduced down to 4 mm. The dynamic response of the film device was stable and reproducible during light illumination (350 nm).