Hee Won Seo

Strained gallium nitride nanowires

Gallium nitride nanowires were synthesized on silicon substrates by chemical vapor deposition using the reaction of gallium and gallium nitride mixture with ammonia. Iron nanoparticles were used as catalysts. The diameter of nanowires is uniform as 25 nm and the lengths are 20–40 μm. The nanowires have single crystalline wurtzite structure with a few stacking faults. A careful examination into x-ray diffraction and Raman scattering data revealed that the separations of the neighboring lattice planes along the growth direction are shorter than those of bulk gallium nitride. The nanowires would experience biaxial compressive stresses in the inward radial direction and the induced tensile uniaxial stresses in the growth direction. The shifts of the band gap due to the stresses have been estimated using the experimental data, showing that the reduction of the band gap due to the tensile stresses can occur more significantly than the increase due to the compressive stresses. The temperature-dependent photolumi...

Single-crystalline gallium nitride nanobelts

Single-crystalline wurtzite gallium nitride nanobelts were synthesized by thermal reaction of gallium, gallium nitride, and ammonia using iron and boron oxide as catalysts. The structure of nanobelts was investigated by high-resolution transmission electron microscopy with electron energy-loss spectroscopy. They have a distinctive triangle tip and thick side edges. The widths are 200–300 nm, the thickness of belt plane is about 1/10 of the width, and the lengths are up to a few tens μm. The growth direction is uniformly perpendicular to the [010] direction.

Triangular gallium nitride nanorods

Gallium nitride nanorods were synthesized by a chemical vapor deposition using the reaction of gallium/gallium nitride with ammonia. All nanorods have, exclusively, a triangle cross section with an average diameter of 50 nm. They consist of single-crystalline wurtzite structure crystal grown with the [010] direction. X-ray diffraction and Raman spectroscopy data suggest no shift of the lattice constants from those of the bulk. Temperature-dependent photoluminescence exhibits the I2 and free-to-bound emission peaks. The present triangular gallium nitride nanorods would be free from the stress, having the band-gap energy of the bulk.

Boron nitride nanotubes synthesized in the temperature range 1000-1200 °C

Abstract Boron nitride nanotubes were synthesized on the iron-deposited alumina substrates by a catalytic reaction of the ball-milled boron and boron nitride powder mixture with ammonia in the temperature range 1000–1200 °C. The diameter of nanotubes is in the range of 40–100 nm. The nanotubes grown below 1100 °C possess exclusively a bamboo-like structure. As the temperature increases to 1200 °C, almost all nanotubes show a cylindrical structure in which the boron nitride sheets are tilted to the tube axis by an angle of about 25°. Electron energy-loss spectroscopy identifies that the ratio of boron and nitrogen is almost one. The Raman scattering peak associated with the E 2g mode shifts to the higher frequency and narrows as the growth temperature increases. The results indicate that the growth temperature can be a crucial growth parameter in controlling the structure and crystallinity of boron nitride nanotubes. On the basis of the structural features, we suggest a base-growth mechanism for both bamboo-like and cylindrical boron nitride nanotubes.

Dielectrophoretic assembly and characterization of individually suspended Ag, GaN, SnO2 and Ga2O3 nanowires

We report the assembly and characterization of individually suspended Ag, GaN, SnO(2), and Ga(2)O(3) nanowires (NWs) using dielectrophoresis. The four kinds of NWs were individually assembled using an experimental approach based on the dielectrophoretic force equation. To freely suspend the individual NWs, we controlled the thickness of the bottom electrode. After depositing a Pt top electrode using a focused ion beam, we investigated the I-V curves of NW devices according to the change in the bottom electrode metal as well as the free suspension height from the insulator. We found that their conductivity for four kinds of NWs was remarkably increased along with the increase in the suspension height, while the gate effect in GaN was reduced.

Two-Dimensional WS2@Nitrogen-Doped Graphite for High-Performance Lithium Ion Batteries: Experiments and Molecular Dynamics Simulations

As promising candidates for anode materials in lithium ion batteries (LIB), two-dimensional tungsten disulfide (WS2) and WS2@(N-doped) graphite composites were synthesized, and their electrochemical properties were comprehensibly studied in conjunction with calculations. The WS2 nanosheets, WS2@graphite, and WS2@N-doped graphite (N-graphite) exhibit outstanding cycling performance with capacities of 633, 780, and 963 mA h g-1, respectively. To understand their lithium storage mechanism, first-principles calculations involving a series of ab initio NVT- NPT molecular dynamics simulations were conducted. The calculated discharge curves for amorphous phase are well matched with the experimental ones, and the capacities reach 620, 743, and 915 mA h g-1 for WS2, WS2@graphite, and WS2@N-graphite, respectively. The large capacities of the two composites can be attributed to the tendency of W and Li atoms to interact with graphite, suppressing the formation of W metal clusters. In the case of WS2@N-graphite, vigorous amorphization of the N-graphite enhances the interaction of W and Li atoms with the fragmented N-graphite in such a way that unfavorable Li-W repulsion is avoided at very early stage of lithiation. As a result, the volume expansion in WS2@graphite and WS2@N-graphite is calculated to be remarkably small (only 6 and 44%, respectively, versus 150% for WS2). Therefore WS2@(N-)graphite composites are expected to be almost free of mechanical pulverization after repeated cycles, which makes them promising and excellent candidates for high-performance LIBs.

Resonance charaterizaion of multi-wall cabon nanotubes

We report the suspended assembly for the fabrication of resonator using multi-wall carbon nanotubes (MWNT). To vibrate a MWNT on plane surface, it should need suspension of a MWNT across trench electrodes. So we propose fabrication method of a MWNT resonator using dielectrophoresis (DEP). By drying the solution during DEP assembly, the capillary force pulls the MWNT toward the bottom substrate, and the MWNT then remains as a deformed structure adhering on the bottom substrate after the solution has dried out. Show successful results which are some SEM images of suspended MWNT. And so, resonances can be detected.

Assembly and Characterization of Nanodevice using Carbon Nanotubes and Nanowires

We report the assembly and characterization of three types of Ag, SnO2, and Ga2O3 nanowires (NWs) and two types of carbon nanotubes (NTs) using dielectrophoresis. The NWs and NTs were individually assembled using an experimental approach based on the dielectrophoretic force equation. After depositing a Pt top electrode using a focused ion beam, we investigated the I-V curves of NW and NT devices.

Fabrication and charaterizaion of resonator using multi-wall cabon nanotubes

We report the suspended assembly for the fabrication of resonator using multi-wall carbon nanotubes (MWNT). To vibrate a MWNT on plane surface, it should need suspension of a MWNT across trench electrodes. So we propose fabrication method of a MWNT resonator using dielectrophoresis (DEP). By drying the solution during DEP assembly, the capillary force pulls the MWNT toward the bottom substrate, and the MWNT then remains as a deformed structure adhering on the bottom substrate after the solution has dried out. In addition, the theoretical and experimental considerations of resonance of fabricated device were discussed.

Assembly technique and charaterizaion for the nanodevice using dielectrophoresis

We report the assembly for the fabrication of nanodevices using nanowires (NWs). The nanowires were individually assembled using an experimental approach based on the dielectrophoretic force equation. After depositing a Pt top electrode using a focused ion beam, we investigated the I-V curves of nanodevices. We used control the thickness of the bottom electrode for the suspension from the bottom electrode. We found that their conductivities in nanodevices were remarkably increased along with the increase of the suspension height.