Wondong Kim

Controlled two-dimensional distribution of nanoparticles by spin-coating method

We demonstrate that the controlled distribution of nanoparticles can be achieved by employing the spin-coating method. The Co and Ag nanoparticles were uniformly distributed on the Si and SiO2 substrates with this method. The particle density was controllable by varying the concentration of colloids. The spatial distribution of the nanoparticles within the patterned area was also shown to be uniform with small boundary effect, which is favorable for current microelectronics technology. We propose that the spin-coating method can be utilized in developing mass production processes for future nanodevices.

New Mg-based antiperovskites PnNMg3 (Pn=As, Sb)

Abstract We report the structural, transport, and magnetic properties of new Mg-based pnictogen compounds PnNMg 3 (Pn=As, Sb), which are prepared from the reaction of pnictogen and Mg 3 N 2 at 800xa0°C. Both compounds crystallize in the ideal cubic perovskite structure without showing any structural distortion. From the comprehensive characterizations the bonding character of SbNMg 3 is found to be ionic. Both antimony and nitrogen in SbNMg 3 have the unusual oxidation state of 3−. This ionic picture agrees well with the electronic structure obtained by band structure calculation.

Magnetic bubblecade memory based on chiral domain walls

Unidirectional motion of magnetic domain walls is the key concept underlying next-generation domain-wall-mediated memory and logic devices.

A study on development of multi-parametric measure of heart rate variability diagnosing cardiovascular disease

In the present study, we propose a new methodology to develop the multi-parametric measure of heart rate variability (HRV) diagnosing cardiovascular disease using various linear and nonlinear measures of HRV. We recorded electrocardiogram for three recumbent postures; the supine, left lateral, and right lateral postures. Twenty control subjects, 51 patients with angina pectoris and 13 patients with acute coronary syndrome participated in this study. We used several time- and frequency-domain measures of HRV as linear measures. As nonlinear measures, Poincare plots, the fractal scaling measures and complexity estimations were used. To develop the multi-parametric measure of HRV, we used the multiple discriminant analysis method among statistical techniques. Multiple discriminant analysis was used to discriminate among three groups, i.e. control, angina pectoris and acute coronary syndrome. As a result, the multiple discriminant analysis gave 75.0 % of goodness of fit. In addition, the sensitivity and specificity for angina pectoris group are 72.5 % and 81.8 %, respectively. In the case of acute coronary syndrome group, the sensitivity and specificity are 84.6 % and 91.5 %, respectively. When the linear and nonlinear measures of HRV are individually used as a clinical tool to diagnose cardiac autonomic function, there is quite a possibility that the wrong results will be obtained due to each measure has different characteristics. Although our study is a preliminary one, we suggest that the multi-parametric measure, which takes into consideration the whole possible linear and nonlinear measures of HRV, may be helpful to diagnose the cardiovascular disease as a diagnostic supplementary tool.

Acetylene molecules on the Si(0 0 1) surface: room-temperature adsorption and structural modification upon annealing

Abstract Acetylene molecules adsorbed on the Si(0xa00xa01) surface are studied by scanning tunneling microscope and ab initio pseudopotential calculations. In case of room-temperature adsorption, there are basically two different kinds of di-σ bonding geometries at low coverage: (i) on-top and (ii) end-bridge di-σ configurations. The tetra-σ configurations are not found. Theoretical calculations show that the paired end-bridge configuration is dominant at high coverage. Based on these findings, we discuss the results at saturation coverage in the existing literature consistently. Subsequent annealing of the C 2 H 2 /Si(0xa00xa01) surface induces different C-induced surface structures depending on the acetylene coverage. At low coverage, parallel lines of dimer vacancies are observed. As the amount of the adsorbed acetylene increases, the c(4×4) ordered phase replaces the dimer vacancy line phase. Near saturation, clusters are formed and the surface becomes rough.

Unipolar resistive switching in insulating niobium oxide film and probing electroforming induced metallic components

We have observed unipolar-type resistance switching in an ultrathin niobium oxide film. An analysis of the temperature dependence of the resistance switching transport revealed that low-resistance state showed a type of electrical conduction typically observed in metals. The modification in chemical binding states of the film in different resistance states was studied using x-ray photoelectron spectroscopy. The analysis of XPS showed that metallic suboxides NbOδ (δu2009≪u20092), decomposed from some of Nb2O5 and NbO2 components of the film, were created after electroforming process, suggesting that the metallic suboxides are constituting elements of metallic channels in the low resistance state.

Effect of oxygen annealing on the magnetoresistance in La0.7Ca0.3MnO3 epitaxial films

Epitaxially c-axis oriented thin films of La0.7Ca0.3MnO3 (LCMO) have been fabricated on (001) SrTiO3 and (001) LaAlO3 substrates by pulsed laser-ablated deposition technique. The physical properties of as-grown films are correlated with the structural nature of the substrate. The O2-annealed films grown on both substrates exhibit higher metal-insulator (M-I) transition temperatures (TMI) and larger magnetoresistance (MR) ratios than the corresponding as-grown films. On the other hand, the Ar-annealed films display enhanced MR without any noticeable change in TMI. We suggest that increase in TMI is mainly ascribed to oxygen incorporation whereas enhancement of MR is largely associated with microstructural change induced by thermal annealing. Structural and transport data reveal that post-annealing with O2 not only saturates the anion defect sites but also relieves structural strain, resulting in the enhancement of TMI and large MR.

Layer-to-island growth of electrodeposited Cu2O films and filamentary switching in single-channeled grain boundaries

We investigated a growth behavior of highly oriented and columnar grained cuprous oxide (Cu2O) films, which were obtained through a chelate-assisted electrochemical solution approach. It was demonstrated that the electrochemical growth of Cu2O films followed a layer-to-island growth mode with a critical thickness of ∼190u2002nm. The chelating agent induced the layer-growth of flat-surfaced films consisting of single-crystalline planar grains, and influenced the preferred orientation of films maintained within the island-growth mode. In particular, the single-crystalline columnar grains with stable interfaces and diameters of 100–200 nm provided highly localized areas of linear grain boundaries for filamentary resistive switching. We measured different conduction behaviors of flat-surfaced films showing nonswitching Ohmic conduction and unipolar memory switching in as-deposited and annealed films, respectively. These different conduction behaviors were found to originate from the microstructure changes generat...

Valence band structures of the phase change material Ge2Sb2Te5

We report the experimental evidence of significant change of the valence band structure during crystallization of Ge2Sb2Te5 (GST). Amorphous GST, prepared by sputter deposition at room temperature (RT), transforms successively into face-centered-cubic (fcc) and a hexagonal-close-packed (hcp) structures at around 150 and 300°C, respectively, during a stepwise temperature increase from RT to 350°C. During temperature increase, ultraviolet photoemission spectra were in vacuo obtained using synchrotron radiation. The measurement of the amorphous and fcc GST shows that the difference between the maximum valence band edge and the Fermi level reduces by 0.35eV during crystallization. For the fcc to hcp phase transformation, no band gap reduction was observed.

Thermoelastic response of thin metal films and their adjacent materials

A pulsed laser beam applied to a thin metal film is capable of launching an acoustic wave due to thermal expansion. Heat transfer from the thin metal film to adjacent materials can also induce thermal expansion; thus, the properties of these adjacent materials (as well as the thin metal film) should be considered for a complete description of the thermoelastic response. Here, we show that adjacent materials with a small specific heat and large thermal expansion coefficient can generate an enhanced acoustic wave and we demonstrate a three-fold increase in the peak pressure of the generated acoustic wave on substitution of parylene for polydimethylsiloxane.