Hyun-Hwi Lee

Secondary ion mass spectrometry without secondary ion emission. Recombinative scattering of hyperthermal Cs+ ions from a Si(111) surface adsorbed with water

Collision of hyperthermal Cs+ ion beams with a Si(111) surface partially covered with water gives rise to emission of CsX+ cluster ions (X is a surface atom or molecule) even when the monomer X+ ions are not produced. The yield for atomic and cluster ion emission is examined as a function of Cs+ collision energy, based on which, we propose that CsX+ species are formed by recombination of the scattered, low kinetic energy Cs+ ions and the gaseous neutral species emanating from a surface. It is also demonstrated that under this condition the secondary neutral flux contains a large fraction of molecular units.

Heavy ion-surface interaction at low energy: scattering of 3–300 eV Cs+, Xe+, and Ar+ from the Si surface

In order to understand the nature of a hyperthermal collision between a heavy projectile and a light atom surface, Cs + , Xe + , and Ar + ion beams are scattered from Si surfaces for collision energies of 3-300 eV. The scattered ions are analyzed for their mass and kinetic energy. The kinetic energy of the scattered ions is much higher than expected from a classical binary collision model, for example, the ratio of scattered to incident ion energy (E s /E i ) is 0.64 for a Cs + -Si(111) collision at an incident energy of 3 eV and a scattering angle of 90°. The E s /E i for Cs + decreases sharply with increasing energy up to 50 eV. For Xe + and Ar + scattering from Si(100), even higher E s /E i values are measured at low collision energy, but this is attributed to preferential neutralization of the slow noble gas ions on the surface and the resulting upshift in their ionic energy distribution. Molecular dynamics classical trajectory simulation of heavy projectile-light atom surface collisions reveals that the lattice atoms collectively respond during a hyperthermal collision, resulting in a drastic E s /E i increase compared to the collision of a light projectile. An important parameter that determines the E s /E i ratio is the time scale of the collision relative to the energy propagation inside a solid.

Self-prevention of instability in a low-power microwave Ar plasma jet for biomedical applications

The behavior of a low-power microwave Ar plasma jet according to the target shape and distance is investigated. The plasma jet shows distinct behavior when it contacts a human finger or grounded metals. No plasma channel and no attraction of the jet to the human finger and metal plate are observed in contrast to low-frequency plasmas. Glow-to-arc transition does not occur even at a very small target distance (<1 mm) between a sharp metal tip and bare electrodes. It is a highly favorable property of the microwave plasma for biomedical applications. Reflection coefficient, current, electric field and electron density are investigated to find the mechanism. This unique phenomenon is caused by the characteristic of microwave frequency systems. A decrease of the target distance induces impedance mismatching leading to the reduction of net input power. It is found that the change in the geometry of the plasma jet is the dominant factor for impedance mismatching. This prevents changes in the discharge regime including glow-to-arc transition, similar to ballast. The mechanism is different from the instability prevention methods including the dielectric barrier in low-frequency systems. Insignificant electric field induced on the metal plate by the impedance mismatching can be the reason for the absence of the plasma channel. Emission intensities of reactive species of the plasma jet are almost uniform regardless of the target distance. Electrical safety and performance can be ensured by the low-power microwave plasma jet.

A crystallographic investigation of GaN nanostructures by reciprocal space mapping in a grazing incidence geometry

Reciprocal space mapping with a two-dimensional (2D) area detector in a grazing incidence geometry was applied to determine crystallographic orientations of GaN nanostructures epitaxially grown on a sapphire substrate. By using both unprojected and projected reciprocal space mapping with a proper coordinate transformation, the crystallographic orientations of GaN nanostructures with respect to that of a substrate were unambiguously determined. In particular, the legs of multipods in the wurtzite phase were found to preferentially nucleate on the sides of tetrahedral cores in the zinc blende phase.

Real-time x-ray scattering study of the initial growth of organic crystals on polymer brushes

We studied the early-stage growth structures of pentacene organic crystals grown on polymer brushes using real-time x-ray scattering techniques. In situ x-ray reflectivity and atomic force microscopy analyses revealed that at temperatures close to the glass transition temperature of polymer brush, the pentacene overlayer on a polymer brush film showed incomplete condensation and 3D island structures from the first monolayer. A growth model based on these observations was used to quantitatively analyze the real-time anti-Bragg x-ray scattering intensities measured during pentacene growth to obtain the time-dependent layer coverage of the individual pentacene monolayers. The extracted total coverage confirmed significant desorption and incomplete condensation in the pentacene films deposited on the polymer brushes. These effects are ascribed to the change in the surface viscoelasticity of the polymer brushes around the glass transition temperature.

Nanostructural analysis of GaN tripods and hexapods grown on c‐plane sapphire

The crystallographic and structural characteristics of GaN tripods and hexapods grown on c-plane sapphire substrates were investigated using synchrotron X-ray scattering and microscopic analysis. The core structure of a GaN hexapod is revealed to be in the zincblende phase with an inversion domain, and a refined crystallographic analysis of tripods and hexapods with synchrotron X-ray scattering shows the existence of the zincblende phase in wurtzite-based protruding nanorods. The atomistic model combined with this crystallographic analysis reveals that the core size of a hexapod is much smaller than the diameters of the protruding nanorods. This refined structural analysis can be utilized in tailoring the opto-electronic characteristics of GaN multipods.

Novel thooth bleaching technique using non-thermal atmospheric-pressure plasmas

Hydrogen peroxide (H2O2) is a widely used tooth bleaching material that is effective and safe. However, the exact mechanism of bleaching action by H202 is not completely understood. One possible mechanism is that H2O2 breaks down to produce oxygen radicals, which attack organic pigment molecules, and causes bleaching. In-office bleaching systems use from 30 to 44% H2O2 bleaching gel and a high-intensity light source. The light source may enhance bleaching by heating the H202 and consequently accelerating bleaching, but this mechanism has not been confirmed yet. In this study, we demonstrate a tooth bleaching procedure that uses room-temperature plasma, instead of a light source in an in-office H202 bleaching system. Extracted human teeth were used in these in vivo experiments. All the teeth were sectioned into two species to be used in two groups; experimental group and control group. The experimental group was treated by 28% of a H202 with a plasma jet for 10 minutes, while control group was treated by H202 alone for the same time. Removal of the tooth surface protein was proved through scanning electron microscope images and Ponceau staining method. We had analyzed the bleaching results by comparing the overall color changes of the teeth photos which were taken before and after these treatments. Combining plasma and H2O2 improved the bleaching efficacy by a factor of three compared to using H2O2 alone. Tooth surface proteins were noticeably removed by plasma treatment. When a piece of tooth was added to a solution of H2O2 as a catalyst, the production of -OH after plasma treatment was 1.9 times greater than when using H2O2 alone. In conclusion, the plasma irradiation enhanced the tooth bleaching effect through the removal of tooth surface proteins and prominently enhanced. OH generation reaction from H2O2.

Study of multilayer structures as soft x-ray mirrors

Molybdenum‐silicon multilayer as soft x‐ray mirrors have been fabricated using a magnetron sputtering system. Their structures have been characterized by x‐ray diffraction (XRD) and computer simulation. Reflectivities at normal incidence have been measured by using monochromatized synchrotron radiation in the 18–24 nm region. A normal incidence reflectivity as high as 40% at 20.8 nm was achieved. Multilayer structural parameters optimized for various soft x‐ray laser wavelengths are also given.