Hyun Jae Song

The critical effect of solvent geometry on the determination of fullerene (C60) self-assembly into dot, wire and disk structures

Geometrically defined C60 self-assembled disks, wires and dots have been systematically obtained via a solution drop-drying process at room temperature; during this process, we discovered that there is a critical correlation between the geometry of the solvent and the final geometry of the self-assembled C60 structure.

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

Ab initio powder diffraction structure analysis of a host-guest network: short contacts between tetrathiafulvalene molecules in a pore.

Many crystal structures of porous coordination networks have been solved by single-crystal X-ray crystallography, providing detailed molecular structural information on framework, guest arrangement, and even reactive intermediates generated in situ. Such detailed structural analysis facilitated the explosive development of porous coordination networks in the last 15 years. On the other hand, although there are many examples of ab initio powder X-ray diffraction (PXRD) analysis of inorganic materials, organic solids, nonporous coordination networks, and discrete small molecules, there are only few reports of ab initio PXRD analysis of porous coordination networks describing guest behavior (guest exchange, gas adsorption, etc.). This is because ab initio PXRD analysis is considerably more challenging than singlecrystal structure determination. Porous coordination networks usually have large unit cells and often low symmetry that contribute to severe peak overlap which hampers accurate structure determination. By using high-resolution synchrotron PXRD and the simulating annealing method, we have now succeeded in solving the crystal structure of a biporous coordination network having an unprecedented arrangement of tetrathiafulvalene (TTF) molecules and a unit cell larger than 15000 . The PXRD analysis revealed very short intermolecular S···S contacts between TTF molecules (3.370 ) that have a nonplanar shape indicating a neutral form. Those findings agree with solid-state spectroscopic features. A key issue in inducing intriguing physical properties in TTF is how to arrange the molecules. We tried to achieve a unique arrangement of TTF molecules by using the pores of a porous coordination network. We used the previously reported network [(ZnI2)3(1)2(2)]n·xC6H5NO2·yCH3OH (3, where 1 is 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) and 2 is triphenylene; x 4 and y 2) and prepared new isostructural ZnBr2 network 4 (Scheme 1) by instant synthesis. [3b]

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.

Spontaneous electron transfer from C60 to Au ions: oxidation of C60 and hole doping

Spontaneous oxidation of C60 by Au ions was observed. When C60 was guided to make contacts with Au3+ ions in aqueous HAuCl4 solution, electrons were spontaneously transferred from C60 to Au3+ ions, resulting in hole (h+) doped C60 cations and Au nanoparticles on a C60 layer. This spontaneous electron transfer occurs due to the galvanic displacement from C60 to Au3+ ions owing to the energy difference between Fermi energy level of C60 (−4.7 eV) and standard reduction potential of Au3+ ion (+1.002 V). The oxidation of C60 as well as the consequent formation of reduced Au nanoparticles were confirmed by atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemistry. The switch of majority charge carrier type from electron to hole and its stability in air were also confirmed by monitoring I–Vg characteristic curves of a C60 field effect transistor (FET) device before and after the reaction with Au3+ ions.

Carbon Nanotube Schottky Diode via Selective Electrochemical Metal Deposition

A single walled carbon nanotube (SWNT) Schottky diode was fabricated via selective electrochemical metal deposition on a prefabricated SWNT field effect transistor device. By electrochemically depositing Pd on only one of the prepatterned Ti electrodes, asymmetric Ohmic (at Pd-SWNT) and Schottky (at SWNT-Ti) contacts were resolved, resulting in efficient current rectification. The selective electrochemical deposition was performed by electrically isolating two Ti electrodes connected through a SWNT by depleting hole carriers in the SWNT upon the simultaneous application of high positive gate voltage during the deposition process. The successful selective deposition of Pd metals was confirmed by X-ray photoelectron spectroscopy.