Ilsoo Kim

Synthesis of Si Nanosheets by a Chemical Vapor Deposition Process and Their Blue Emissions

We synthesized free-standing Si nanosheets (NSs) with a thickness of about <2 nm using a chemical vapor deposition process and studied their optical properties. The Si NSs were formed by the formation of frameworks first along six different <110> directions normal to [111], its zone axis, and then by filling the spaces between the frameworks along the <112> directions under high flow rate of processing gas. The Si NSs showed blue emission at 435 nm, and absorbance and photoluminescence (PL) excitation measurements indicate that enhanced direct band transition attributes to the emission. Time-resolved PL measurement, which showed PL emission at 435 nm and a radiative lifetime of 1.346 ns, also indicates the enhanced direct band gap transition in these Si NSs. These outcomes indicate that dimensionality of Si nanostructures may affect the band gap transition and, in turn, the optical properties.

Bacterial Recognition of Silicon Nanowire Arrays

Understanding how living cells interact with nanostructures is integral to a better understanding of the fundamental principles of biology and the development of next-generation biomedical/bioenergy devices. Recent studies have demonstrated that mammalian cells can recognize nanoscale topographies and respond to these structures. From this perspective, there is a growing recognition that nanostructures, along with their specific physicochemical properties, can also be used to regulate the responses and motions of bacterial cells. Here, by utilizing a well-defined silicon nanowire array platform and single-cell imaging, we present direct evidence that Shewanella oneidensis MR-1 can recognize nanoscale structures and that their swimming patterns and initial attachment locations are strongly influenced by the presence of nanowires on a surface. Analyses of bacterial trajectories revealed that MR-1 cells exhibited a confined diffusion mode in the presence of nanowires and showed preferential attachment to the nanowires, whereas a superdiffusion mode was observed in the absence of nanowires. These results demonstrate that nanoscale topography can affect bacterial movement and attachment and play an important role during the early stages of biofilm formation.

Thermal conductivities of Si1−xGex nanowires with different germanium concentrations and diameters

The thermal conductivities of Si1−xGex nanowires (NWs) synthesized with Ge concentrations of 0.0%, 0.4%, 4%, and 9% and different diameters were measured from 40 to 420 K. The thermal conductivity of Si1−xGex NWs decreases as the Ge concentration increases due to alloy scattering. As the diameter of the Si1−xGex NWs decreases, the thermal conductivity decreases due to phonon boundary scattering. However, the thermal conductivity dependency on the diameter of the NWs is not significant. This indicates that alloy scattering should be the dominant scattering mechanism in Si1−xGex NWs. This study should provide a basis for designing efficient thermoelectric devices out of Si1−xGex NWs and Si1−xGex nanocomposites.

Characteristics of the late Quaternary tephra layers in the East/Japan Sea and their new occurrences in western Ulleung Basin sediments

Abstract The lithologic characteristics, stratigraphic relationships, and areal distribution of late Quaternary tephra layers have been determined based on four piston cores, recovered from the western Ulleung Basin of East/Japan Sea. The results show that, using morphological and major element compositional data, volcanic glass shards dispersed in tephra layers are identical to those of fallout deposits of the Ulleung–Oki (ca. 9.3 ka), Aira–Tanzawa (ca. 22 ka), and Ulleung–Yamato (ca. 33 ka). The lapilli tephra layers (Ulleung–Oki and Ulleung–Yamato) originating from Ulleung Island consist predominantly of pumice-type glass shards associated with minor amounts of alkali-feldspar, biotite, and plagioclase. On the other hand, the ash layers (including the Aira–Tanzawa ash) derived from the Japanese islands are mainly composed of bubble-wall and/or plane-type glasses that contain higher SiO 2 and lower Al 2 O 3 than the lapilli tephra layers. The occurrence of tephra layers in the western Ulleung Basin and their stratigraphic correlation extend the distribution of pumice-type glass shards about 50–100 km west of the previously known fallout zone.

Coupling of semiconductor nanowires with neurons and their interfacial structure.

We report on the compatibility of various nanowires with hippocampal neurons and the structural study of the neuron–nanowire interface. Si, Ge, SiGe, and GaN nanowires are compatible with hippocampal neurons due to their native oxide, but ZnO nanowires are toxic to neuron due to a release of Zn ion. The interfaces of fixed Si nanowire and hippocampal neuron, cross-sectional samples, were prepared by focused ion beam and observed by transmission electron microscopy. The results showed that the processes of neuron were adhered well on the nanowire without cleft.

Modification of magnetic properties of epitaxial Co/Cu multilayers by 1 MeV C+ irradiation

Epitaxial [Co/Cu]10 grown on Si (001) has been irradiated by 1 MeV C+ with a dose of 1×1016/cm2. The intrinsic characteristics of epitaxy, such as crystal structure and fourfold magnetic anisotropy, are conserved after ion irradiation. However, the extrinsic magnetic properties are changed such that the coercivity decreases and the squareness of the hysteresis loop is noticeably improved. The intensity of the Cu (200) peak in the x-ray diffraction pattern increases about three times and its half-width decreases after irradiation. It is concluded that the changes of magnetic properties induced by ion irradiation result from the grain growth and the improved crystalline quality.

Neuron-like differentiation of mesenchymal stem cells on silicon nanowires

The behavior of mammalian cells on vertical nanowire (NW) arrays, including cell spreading and the dynamic distribution of focal adhesions and cytoskeletal proteins, has been intensively studied to extend the implications for cellular manipulations in vitro. Prompted by the result that cells on silicon (Si) NWs showed morphological changes and reduced migration rates, we have explored the transition of mesenchymal stem cells into a neuronal lineage by using SiNWs with varying lengths. When human mesenchymal stem cells (hMSCs) were cultured on the longest SiNWs for 3 days, most of the cells exhibited elongated shapes with neurite-like extensions and dot-like focal adhesions that were prominently observed along with actin filaments. Under these circumstances, the cell motility analyzed by live cell imaging was found to decrease due to the presence of SiNWs. In addition, the slowed growth rate, as well as the reduced population of S phase cells, suggested that the cell cycle was likely arrested in response to the differentiation process. Furthermore, we measured the mRNA levels of several lineage-specific markers to confirm that the SiNWs actually induced neuron-like differentiation of the hMSCs while hampering their osteogenic differentiation. Taken together, our results implied that SiNWs were capable of inducing active reorganization of cellular behaviors, collectively guiding the fate of hMSCs into the neural lineage even in the absence of any inducing reagent.

Influence of the calcining temperature on the superconducting transition and critical current of Y-Ba-Cu-oxide compound

Superconducting transition and critical current of a Y‐Ba‐Cu‐oxide compound prepared by ceramic technique have been studied for the samples which have the same nominal composition and sintering conditions, while systematically varying the calcining temperature from 850 to 1000 °C. It was observed that the shape of the superconducting transition in resistivity measurement markedly depends on the calcining temperature. All samples showed metallic behavior in normal state and samples which have lower resistivity showed higher critical current density. The resistivity just above onset temperature decreased with increasing the calcining temperature up to 950 °C, but above 975 °C the opposite behavior was observed. This behavior was discussed in connection with x‐ray diffraction patterns of the samples.

Effects of ion irradiation on epitaxial Cu/Ni/Cu(001) with perpendicular magnetic anisotropy

Epitaxial fcc Cu/Ni(t)/Cu(001) structures (t=20,30,60,90 A) possessing perpendicular magnetic anisotropy were irradiated by 1 MeV C+ with a dose of 2×1016 ions/cm2. The spin reorientation transitions from perpendicular to in-plane magnetization occurred after ion irradiation. X-ray diffraction measurements showed that the crystal structures were conserved, the grain sizes of Cu in the Cu/Ni(60, 90 A)/Cu(001) films increased by 38%, and the strain in the Ni layer was relaxed after irradiation when compared to the as-deposited samples. It is concluded that the spin reorientation transition is caused by the relaxation of the strain in Ni layer, which is initiated by ion irradiation.

Enhanced Neurite Outgrowth by Intracellular Stimulation

Electrical stimulation through direct electrical activation has been widely used to recover the function of neurons, primarily through the extracellular application of thin film electrodes. However, studies using extracellular methods show limited ability to reveal correlations between the cells and the electrical stimulation due to interference from external sources such as membrane capacitance and culture medium. Here, we demonstrate long-term intracellular electrical stimulation of undamaged pheochromocytoma (PC-12) cells by utilizing a vertical nanowire electrode array (VNEA). The VNEA was prepared by synthesizing silicon nanowires on a Si substrate through a vapor-liquid-solid (VLS) mechanism and then fabricating them into electrodes with semiconductor nanodevice processing. PC-12 cells were cultured on the VNEA for 4 days with intracellular electrical stimulation and then a 2-day stabilization period. Periodic scanning via two-photon microscopy confirmed that the electrodes pierced the cells without inducing damage. Electrical stimulation through the VNEA enhances cellular differentiation and neurite outgrowth by about 50% relative to extracellular stimulation under the same conditions. VNEA-mediated stimulation also revealed that cellular differentiation and growth in the cultures were dependent on the potential used to stimulate them. Intracellular stimulation using nanowires could pave the way for controlled cellular differentiation and outgrowth studies in living cells.