Yangdo Kim

The growth mechanism and optical properties of ultralong ZnO nanorod arrays with a high aspect ratio by a preheating hydrothermal method

Well-aligned ultralong ZnO nanorod arrays with a length of 10 microm have been synthesized on glass substrates using a preheating hydrothermal method. The diameter of the nanorods is in the range from 50 to 80 nm, and the aspect ratio and alignment can be simply controlled by varying the preheating time. Based on the evolution of aspect ratio with preheating time, a possible growth mechanism was proposed. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the nanostructures are well oriented with the c-axis perpendicular to the substrate. The photoluminescence (PL) spectrum of the as-grown ZnO nanostructure reveals a near-band-edge (NBE) emission peak and a yellow emission, and the origin of yellow emission was confirmed to be from the absorbed hydroxyl group. The ultralong nanorod arrays made in solution have a desirable diameter, length, density and orientation for ordered nanodevice applications.

Comparison of TiN Films Deposited Using Tetrakisdimethylaminotitanium and Tetrakisdiethylaminotitanium by the Atomic Layer Deposition Method

TiN films were deposited by the atomic layer deposition (ALD) method using either tetrakisdimethylaminotitanium (TDMAT) or tetrakisdiethylaminotitanium (TDEAT) as the Ti precursor and NH3 as the reactant gas. The TiN films deposited using TDMAT showed two saturated TiN film growth regions which were observed in the temperature ranges between 175 and 190°C and between 200 and 210°C. The TiN films deposited using TDEAT showed a saturated growth rate in the temperature range between 275 and 300°C. The growth rates of the TiN films deposited using either TDMAT or TDEAT were about 5 and 1 A/cycle, respectively. The TiN films grown by the ALD method showed relatively low carbon content compared to the TiN films deposited by other conventional chemical vapor deposition and metalorganic chemical vapor deposition methods using the same precursors. The resistivity of the TiN films deposited by the ALD method was below 1,000 µΩcm. The TiN films deposited using TDEAT showed a lower resistivity than the films deposited using TDMAT. The calculated densities of the TiN films deposited using either TDMAT or TDEAT were about 3.55 and 4.38 g/cm3, respectively. This paper presents a comparison of the characteristics of the TiN films deposited using the two different precursors, TDMAT and TDEAT.

Effective load transfer by a chromium carbide nanostructure in a multi-walled carbon nanotube/copper matrix composite.

Multi-walled carbon nanotube (MWCNT) reinforced copper (Cu) matrix composites, which exhibit chromium (Cr) carbide nanostructures at the MWCNT/Cu interface, were prepared through a carbide formation using CuCr alloy powder. The fully densified and oriented MWCNTs dispersed throughout the composites were prepared using spark plasma sintering (SPS) followed by hot extrusion. The tensile strengths of the MWCNT/CuCr composites increased with increasing MWCNTs content, while the tensile strength of MWCNT/Cu composite decreased from that of monolithic Cu. The enhanced tensile strength of the MWCNT/CuCr composites is a result of possible load-transfer mechanisms of the interfacial Cr carbide nanostructures. The multi-wall failure of MWCNTs observed in the fracture surface of the MWCNT/CuCr composites indicates an improvement in the load-bearing capacity of the MWCNTs. This result shows that the Cr carbide nanostructures effectively transferred the tensile load to the MWCNTs during fracture through carbide nanostructure formation in the MWCNT/Cu composite.

Characteristics of TiN Films Deposited by Remote Plasma-Enhanced Atomic Layer Deposition Method

In this paper, we report a remote plasma-enhanced atomic layer deposition (PEALD) method which shows a relatively wide temperature window compared to the conventional ALD process due to the increased reactivity of reactant gas as well as the increased reactivity of active nitrogen radicals in the plasma process. The remote PEALD TiN films we produced showed significantly lower impurity contents than films deposited by other methods such as plasma-enhanced chemical vapor deposition (CVD), metal-organic CVD or other ALD using the same precursor. TiN films deposited by remote PEALD at 250°C showed a resistivity value as low as approximately 300 µΩcm and exhibited excellent conformal deposition with almost above 95% step coverage on a 0.25-µm-wide and 2.5-µm-deep contact hole structure.

Low-Temperature Growth of Carbon Nanotube by Plasma-Enhanced Chemical Vapor Deposition using Nickel Catalyst

Carbon nanotubes (CNTs) were grown on silicon (Si) substrates with a native oxide layer using a nickel (Ni) catalyst by the plasma-enhanced chemical vapor deposition method at 550 and 650°C. Multiwall CNTs with a large inside hollow core were observed with samples deposited on Si substrates with a native oxide layer. However, no CNT growth was observed with samples deposited on Si substrates without a native oxide layer due to the formation of Ni-silicide. The native oxide layer was believed to act as the diffusion barrier and to suppress the formation of Ni-silicide. The typical diameter and length of CNTs ranged from about 30 to 100 nm and 2 to 3 µm, respectively. The size of CNTs was more strongly dependent on the thickness of the Ni layer than on the growth temperature. The surface roughness and amount of surface defects decreased with increasing process temperature. We performed a post hydrogen plasma treatment for the purification of raw CNTs. The post hydrogen plasma treatment successfully removed residual graphite particles and/or metallic impurities without significant damage to the CNTs.

A coupled solidification analysis of materials and cooling roller in direct rolling process

Abstract A computer program has been developed to analyze the two-dimensional unsteady conservation equations for transport phenomena in the pool region with semi-solid materials containing the cooling roll of direct rolling (or twin-roll strip continuous casting). The solidification phenomena of the semi-solid metal are investigated considering phase transformation. The energy equation of the cooling roll is solved simultaneously with the conservation equations of the semi-solid metal to consider heat transfer through the cooling roll. The finite difference method and the finite method are used in the analysis of the pool region and the cooling roll, respectively, to reduce computing time and to improve the accuracy of calculation. The assumption of viscous flow and Darcys flow are used for the calculation of the flow and the temperature fields of the semi-solid metal. In the present study, the influences of the solid fraction and the casting speed are investigated from the points of view of the possibility of fabrication and roll design.

Modifying hydrogen bonding interaction in solvent and dispersion of ZnO nanoparticles: impact on the photovoltaic performance of inverted organic solar cells

We demonstrate that the morphology of ZnO buffer layers for the inverted organic solar cells (IOSC) was improved by controlling the molecular interaction in solvents to disperse ZnO nanoparticles (NPs). Dispersion of ZnO NPs in non-polar solvent usually resulted in turbid or semi-transparent solution. In this study, uniform dispersion of ZnO NPs was achieved through applying mixed solvents composed of both non-polar and polar solvents without the assistance of an organic surfactant. From the analysis based on Hansen solubility parameter theory, the improvement was mainly associated with hydrogen bonding interactions between the two component of solvents. Smooth ZnO buffer layers were deposited using the colloidal solution based on the binary solvent mixture and the photovoltaic performance of the IOSC was highly improved by optimizing solvent mixtures.

Effect of the dehydrogenation speed and Nd content on the microstructure and magnetic properties of HDDR processed Nd-Fe-B magnets

The effect of Nd content and dehydrogenation speed on the microstructure and magnetic properties of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-B magnetic powders was studied. The NdxB6.4Ga0.3Nb0.2Febal (x=12.5–13.5, at.%) mold casting alloys were subjected to HDDR process after homogenization heat treatment. During desorption-recombination stage, dehydrogenation speed and time were systematically changed to control the speed of the desorption-recombination reaction. The higher Nd content resulted in better magnetic properties of the HDDR powder, and this was attributed to the thicker and more uniform Nd-rich phase at grain boundaries. It was also confirmed that the slow dehydrogenation speed could maximize the effect of high Nd content on the magnetic properties of HDDR powder. At the optimized dehydrogenation speed, the coercivity and remanence was 15.3 kOe and 13.0 kG, respectively, at 12.9 at.% Nd content, which resulted in a (BH)max of 36.8 MGOe.

Removal of the Metallorganic Polymer Residues Formed at Via Holes

The removal characteristics of polymer layers formed during the reactive ion etching of via holes with via etch-stopped on a TiN layer (VEST) structure were investigated under various ashing and stripping conditions. A relatively thick sidewall polymer layer containing large amounts of F and Ti due to the reaction between CF x -based etching gas and a sputtered Ti layer were observed, while a comparatively thin polymer layer with negligible F and only a small amount of Ti was formed at the bottom of the via hole. The intruded sunflower-shaped thick polymer layer at the boundary of the via hole was clearly distinguishable after ashing under various conditions. Oxygen-based plasma ashing removed F-containing carbon contaminants but left the metallorganic polymer due to the formation of metal oxides such as TiO x . Additional wet stripping is required to completely remove the metallorganic polymer residues. Nonhydroxylamine (non-HA)-based strippers more effectively removed the polymer layer than HA-based strippers, regardless of the photoresist used. Polymer hardening due to the ashing at elevated temperatures reduced the wet stripping capability. This study shows that a polymer-free via hole can be achieved by using a non-HA-based stripper and Ar dry cleaning and by keeping the ashing temperature low enough to prevent polymer hardening.

Microfabrication and optical properties of highly ordered silver nanostructures

Using thermal evaporation, we fabricated five uniform and regular arrays of Ag nanostructures with different shapes that were based on an anodized aluminum oxide template and analyzed their optical properties. Round-top-shaped structures are obtained readily, whereas to obtain needle-on-round-top-shaped and needle-shaped structures, control of the directionality of evaporation, pore size, length, temperature of the substrate, etc., was required. We then observed optical sensitivity of the nanostructures by using surface-enhanced Raman scattering, and we preliminarily investigated the dependency of Raman signal to the roughness and shape of the nanostructures.