On the nature of various growth facets at the interfaces of gold catalyzed-silicon nanowires.

Modified embedded atom method-molecular dynamics simulations have been performed to define the various growth facets at the interfaces of Au catalyzed-silicon nanowires (SiNWs). The interfacial growth facet of Au/SiNW on c-Si (111) surface has a single, planar interface, that grows parallel to the Si (111) planes. The interfacial growth facet on c-Si (211) surface has an asymmetrical, cone shape. The interface of Au/SiNW consists of two planes--the greater being {111} and the lesser being {100} planes. And finally the interfacial growth facet on c-Si (110) surface has a symmetrical, cone shape. The interface consists of two {111} planes. These findings exhibit good agreement with the previous experimental observation done with cross-sectional high-resolution transmission electron microscopy. We also predict the interfacial growth facet of Au/SiNW with NW direction (100), explaining that such a direction rarely exists due to geometrical limitations. We propose that changes in SiNW direction are caused the existence of various growth facets at the Au/SiNW interfaces.

Thickness dependence of the crystallization of Au/glass ultrathin films.

The crystallization of Au/glass ultrathin films for surface plasmon resonance (SPR) biosensor has been studied using synchrotron X-ray scattering and field emission scanning electron microscope. In films thinner than 30 nm, crystallized Au grains with [111] preferred orientation were formed in the as-deposited amorphous precursor. In film with 58-nm thickness, however, Au (200) powder grains existed on top of the Au (111)-oriented grains near the interface. At the annealing temperature of 400 degrees C, the Au (200) powder grains disappeared, while the Au (111)-oriented grains grew further. The behavior of the surface morphology of Au thin films post-annealed at 400 degrees C was consistent with the thickness dependence of the crystallization. In a 10-nm-thick film, the Au (111)-oriented grains fully crystallized, and then became separated with each other. By increasing to 30-nm film thickness, the Au (111)-oriented grains grew further, coalesced into large columnar-type grains, and showed smooth surface. We suggest that the appropriate thickness of Au/glass thin film for SPR biosensor need over 30-nm-thick, considering smooth surface.

Crystallization of Au-Si/glass thin film: a real-time synchrotron X-ray scattering study.

The crystallization of amorphous, Si-rich, Au28Si72/glass thin film was studied in real-time synchrotron X-ray scattering experiments. The amorphous film crystallizes first into Au and Si phases at a low temperature of 206 degrees C. At annealing temperatures above eutectic temperature (T(E) = 360 degrees C), the Au phase melts while the Si phase rapidly grows further. The crystallized Au28Si72 thin film has nanowire-type grains with 1000-nm-length and 10-nm-diameter. We confirm that the Au liquid phase contributes to the low-temperature crystallization of the Si solid phase for Si-nanowire growth.

Novel fiber optic sensor probe with a pair of highly reflected connectors and a vessel of water absorption material for water leak detection.

The use of a fiber optic quasi-distributed sensing technique for detecting the location and severity of water leakage is suggested. A novel fiber optic sensor probe is devised with a vessel of water absorption material called as water combination soil (WCS) located between two highly reflected connectors: one is a reference connector and the other is a sensing connector. In this study, the sensing output is calculated from the reflected light signals of the two connectors. The first reflected light signal is a reference and the second is a sensing signal which is attenuated by the optical fiber bending loss due to the WCS expansion absorbing water. Also, the bending loss of each sensor probe is determined by referring to the total number of sensor probes and the total power budget of an entire system. We have investigated several probe characteristics to show the design feasibility of the novel fiber sensor probe. The effects of vessel sizes of the probes on the water detection sensitivity are studied. The largest vessel probe provides the highest sensitivity of 0.267 dB/mL, while the smallest shows relatively low sensitivity of 0.067 dB/mL, and unstable response. The sensor probe with a high output value provides a high sensitivity with various detection levels while the number of total installable sensor probes decreases.

Oxidation Behaviors of Aluminum Nanopowders with Different Particle Sizes: A Real-Time Synchrotron X-ray Scattering Study.

We have studied the oxidation behaviors of aluminum (Al) nanopowders with different particle sizes using a real-time synchrotron X-ray scattering during annealing in air. The Al nanopowders with small particle size of 78 nm at room temperature (RT) were a single crystal. The surface of the nanopowders was first oxidized to amorphous Al oxide near 450 °C, and then crystallized to γ-Al2O3 phase at 550 °C. The inside of the nanopowders existed as crystal Al phase at 680 °C, high compared to the melting temperature of Al bulk, 660 °C. In contrast, the Al nanopowders with large particle size of 816 nm at RT have multi grains inside a particle. The surface and grain boundary of the powders were first oxidized to amorphous Al oxide near 470 °C, and then crystallized to γ-Al2O3 phase at 550 °C. The inside of the powders existed as amorphous Al phase at 620 °C, melted at 656 °C, and then oxidized gradually above 656 °C.

Annealing Behaviors of Al and Al-Mg-Si Alloy: A Real-Time Synchrotron X-ray Scattering Study.

We have studied the annealing behaviors of pure Al (Al-1050) and Al-Mg-Si alloy (Al-6061) with plate-type using a real-time synchrotron X-ray scattering in vacuum. At room temperature (RT), the crystal domain size of Al phase in the Al-Mg-Si alloy was small as 70 nm, compared to that in the pure Al, 142 nm. The crystal Al phase in the Al-Mg-Si alloy has more thermal stability than that in the pure Al. The crystal Al phase in the Al-Mg-Si alloy was thermally stable in amount and size up to 250 °C. These are due to the existence of intermetallic crystal Mg2Si phase, which is thermally stable in amount and size up to 250 °C.

Oxidation Behaviors of Co, Cr, and Ni Nanopowders: A Real-Time Synchrotron X-ray Scattering Study.

We have studied the oxidation behaviors of Co, Cr, and Ni nanopowders for applications in biomedicine using a real-time synchrotron X-ray scattering in air. The Co nanopowders have a crystal domain size of 70 nm at room temperature (RT). The surface of the Co nanopowders was oxidized to CoO phase at RT. The interior of the Co nanopowders was rapidly oxidized at above 300 °C and was mostly oxidized above 550 °C. The Cr nanopowders have a crystal domain size of 92 nm at RT. The surface of the Cr nanopowders was oxidized to Cr2O3 phase at RT, but the interior of the nanopowders was present as Cr phase without being oxidized up to 650 °C. On the other hand, the Ni nanopowders have a crystal domain size of 93 nm at RT. The surface of the Ni nanopowders began to oxidize to Ni2O3 phase at 300 °C, and the interior of the nanopowders was mostly oxidized above 600 °C.

Sintering Behaviors of Au Nanopowders with Different Particle Sizes: A Real-Time Synchrotron X-ray Scattering Study.

We have studied the solid-phase sintering behaviors of Au nanopowders with different particle sizes using real-time synchrotron X-ray scattering and field-emission scanning electron microscopy (FESEM) experiments in air. Au powders with large particle size of 173 nm at room temperature showed a defect-free crystal domain size of 47 nm. Most of these powders showed multiple grains within the particle. Solid-phase sintering typically occurred via surface diffusion of Au atoms near the surface of powders at temperatures above 300 °C, Au nanopowders with small particle size of 50 nm at room temperature showed a crystal domain size of 43 nm. Most of these powders showed a single grain inside the particle. Solid-phase sintering occurred simultaneously with volume diffusion of Au atoms near the grain boundary inside powders at temperatures above 250 °C and surface diffusion near the surface of powders at temperatures above 300 °C. Our study revealed that the solid-phase sintering behaviors of Au nanopowders began at temperatures over 250 °C, which is much lower than the melting temperature of 1064 °C. The thermal analysis results were consistent with the results of real-time synchrotron X-ray scattering and FE-SEM.

Average BER analysis of SCM-based free-space optical systems by considering the effect of IM3 with OSSB signals under turbulence channels.

In this paper, we derive the average bit error rate (BER) of subcarrier multiplexing (SCM)-based free space optics (FSO) systems using a dual-drive Mach-Zehnder modulator (DD-MZM) for optical single-sideband (OSSB) signals under atmospheric turbulence channels. In particular, we consider the third-order intermodulation (IM3), a significant performance degradation factor, in the case of high input signal power systems. The derived average BER, as a function of the input signal power and the scintillation index, is employed to determine the optimum number of SCM users upon the designing FSO systems. For instance, when the user number doubles, the input signal power decreases by almost 2 dBm under the log-normal and exponential turbulence channels at a given average BER.

High Characteristic Sr-Ferrite Magnetic Powders Made by New Direct-Spray Roasting.

We have studied the process of new direct-spray roasting; SrCO3 powders were directly added into the 12FeCl2 x 4H2O solution first, and then α-Fe2O3 powders with the particle size of 210-620 nm was made after spray roasting in the low temperature of 400-700 degrees C. The direct-spray roasting enables us to get the uniform mixing of α-Fe2O3 and SrCO3 powders, and to lower the calcination temperature of Sr-ferrite, compared to that of the conventional dry method. Also, the magnetic properties of the powders made by direct-spray roasting are very higher than that made by the dry method. We suggest that the Sr-ferrite powders by the direct-spray roasting act as useful high-Br magnetic powders for high characteristic resin-bonded magnet.

Phase Transformation of Se/(Cu,In,Ga)/Mo/Glass Thin Films: A Real-Time Synchrotron X-ray Scattering Study.

The phase transformation of Se/(Cu,In,Ga)/Mo/glass thin films during annealing in a vacuum on and off state was studied in a real-time synchrotron X-ray scattering experiment. The crystalline CIGS phase is a solid solution of crystalline CIS and CGS phases. The crystalline CIS phase was formed first at lower temperature. By increasing the temperature, the crystalline CIS phase disappeared, while the crystalline Ga2Se3, In2Se3, Cu2In phases grew simultaneously. Finally, the crystalline CIGS phase was formed at higher temperature, while the crystalline Ga2Se3, In2Se3, Cu2In phases disappeared gradually. The behavior of the crystal domain sizes was consistent with the changes of X-ray powder diffraction profiles. The high crystallization temperature of the CIGS phase was attributed to the activation energy barrier for the diffusion of Ga ions into the intermediate CIS phase.