Seok Min Yoon

Synthesis of Single‐Crystal Tetra(4‐pyridyl)porphyrin Rectangular Nanotubes in the Vapor Phase

Stacking up: One-dimensional single-crystalline rectangular nanotubes (RNTs) of 5,10,15,20-tetra(4-pyridyl)porphyrin (H(2)TPyP, see picture) are synthesized by a vaporization-condensation-recrystallization process. The single-crystal X-ray diffraction and selected-area electron diffraction data reveal that the H(2)TPyP RNTs form by self-stacking of H(2)TPyP units through hydrogen-bonding, H-pi, and pi-pi intermolecular interactions.

p-type semiconducting GeSe combs by a vaporization-condensation-recrystallization (VCR) process.

Metal and semiconductor chalcogenides represented by II–VI and IV–VI compounds have been widely investigated to exploit their unique optical and electrical properties in the fields of infrared optics, photovoltaic cells, and thermoelectric devices. Up to now, most of the studies related to chalcogenides have been concentrated on II–VI compounds because of their excellent quantum-size effects together with their good electrical properties. Moreover, the synthetic methodologies to obtain highly crystalline 0D and 1D nanostructures of these compounds are relatively straightforward. On the contrary, IV–VI chalcogenides, especially germanium selenides (GeSe) that have a narrow bandgap energy (1.08 eV) for p-type semiconducting structures, have been less highlighted in spite of their wide usage in resistive memory cells and glass-forming materials for photonic devices with thin-film structure. Such a low popularity of GeSe seems to be related to the lack of facile synthetic routes to form single-crystalline, low-dimensional GeSe structures, prohibiting in-depth studies about their intrinsic electrical and optical/optoelectronic properties. Note that their congeners, such as lead chalcogenides (PbX, X1⁄4 S, Se, Te) that display thermoelectric behavior with pine tree/hyperbranched structures and germanium telluride (GeTe) nanowires that show phase-changing memory properties, have been successfully demonstrated. Here we report the facile synthesis of single-crystalline germanium monoselenide (GeSe) comb structures via an atmospheric vaporization–condensation–recrystallization (VCR) process using a commercially available single precursor (bulk GeSe powder). The synthesized GeSe combs display a p-type semiconducting transport property as well as photo-switching behavior. The VCR process, a specific form of vapor-mediated self-assembly processes, involves the generation of vapors of the organic/inorganic powder precursors at elevated temperature, followed by condensation of the vapors on a solid substrate at a lower temperature region, from which precursor-specific recrystallizations into various low-dimensional structures are induced. Several unprecedented organic nanoand microstructures including m-aminobenzoic acid helical nanobelts, C60 nanodisks, and porphyrin rectangular nanotubes have been

Optical Waveguiding and Lasing Action in Porphyrin Rectangular Microtube with Subwavelength Wall Thicknesses

Lasing action by planar-, fiber-, or ring-type waveguide has been extensively investigated with different types of microcavities such as thin films, wires, cylindrical tubes, or ribbons. However, the lasing action by sharp bending waveguide, which promises efficient interconnection of amplified light in the photonic circuits, remains unexplored. Here, we report the first observation of microcavity effects in the organic rectangular microtubes (RMTs) with sharp bends (ca. 90°) and subwavelength nanoscale wall thicknesses, based on single crystalline and themostable tetra(4-pyridyl)porphyrin (H(2)TPyP)-RMTs synthesized by the VCR process. A bright tip emission is observed from the sharp bending edges of a single RMT upon laser excitation, demonstrating a clear waveguiding behavior in RMT. The appearance of a peak from the (0-1) band at a threshold tube length and the gradual decrease of its full width at half-maximum (fwhm) suggest that amplification of spontaneous emission (ASE) is developed by stimulated emission along the walls of the RMTs. The ehancement of the ASE peak together with the narrowing of its fhwm over a threshold pump power and the tube size (width and length) dependence of the mode spacing strongly support vibronic lasing action in the RMTs. The stimulated emission by the subwavelength bending waveguide demonstrates that the organic RMTs can be applied as new building blocks for micromanipulation of optical path and amplification in the integrated circuits for efficient photonic devices.

Growth of germanium nanowires using liquid GeCl4 as a precursor: the critical role of Si impurities

Liquid GeCl(4) precursors have been employed to grow into one dimensional Ge nanowires (NWs) via a vapor-liquid-solid (VLS) process, in which Si, supplied as a form of liquid SiCl(4), plays a critical role for the successful formation of Ge NWs.

Autogenerated Air Bubbles for the Spontaneous Formation of Nanostructures

In this paper, a novel mild synthetic route was tentatively designed to achieve both shape and dimension-defined nanostructure such as nanorods, nanowires and nanocircles which were fully characterized by AFM and TEM. Bubbles, which are automatically generated when water is mixed with ethanol by a simple shaking, act as a nanoreactor to produce these nanostructures. Fe containing nanowire, nanorod and nanocircle are successfully prepared by using this method. Autogenerated bubble-mediated formation of nanostructures has advantages over conventional methods in terms of short reaction time, low energy consumption and absence of complicated experimental equipments.