Kang Ho Park

Maximization of acoustic energy difference between two spaces

There has recently been an increasing interest in the generation of a sound field that is audible in one spatial region and inaudible in an adjacent region. The method proposed here ensures the control of the amplitude and phase of multiple acoustic sources in order to maximize the acoustic energy difference between two adjacent regions while also ensuring that evenly distributed source strengths are used. The performance of the method proposed is evaluated by computer simulations and experiments with real loudspeaker arrays in the shape of a circle and a sphere. The proposed method gives an improvement in the efficiency of radiation into the space in which the sound should be audible, while maintaining the acoustic pressure difference between two acoustic spaces. This is shown to give an improvement of performance compared to the contrast control method previously proposed.

Fabrication of metal nanowire using carbon nanotube as a mask

We report on the fabrication of metal nanowires on an insulating substrate using carbon nanotubes as a new kind of mask material. By irradiating Ar+ ions of 300 eV energy on a nanotube-coated Au/Ti thin layer on a SiO2 substrate, Au/Ti nanowires were successfully formed just underneath the nanotube, indicating that the carbon nanotubes had acted as a good mask against the argon ion bombardment. The Au/Ti wire of a few nanometers in width was frequently observed among the wires of various widths. After the formation of the Au/Ti nanowires, the carbon nanotube on the metal nanowire could be removed by atomic force microscope.

Room temperature observation of single electron tunneling effect in self-assembled metal quantum dots on a semiconductor substrate

We report on the observation of room-temperature single electron tunneling phenomena in a metal-insulator-metal-semiconductor double-junction structure. The nanosized Ag dots were self-assembled on a Sb-terminated Si(100) surface, and the Coulomb gap and staircases were observed in the local current–voltage (I–V) measurements using scanning tunneling microscopy. The I–V characteristics exhibiting the single electron tunneling behavior vary significantly with the variation of the measurement position within the same Ag droplet. These phenomena are well described by the tip-dot(Ag)-Si double-junction picture.

Highly sensitive NO2 sensor array based on undecorated single-walled carbon nanotube monolayer junctions

Using the surface-programmed assembly technique, we have fabricated a uniform array of NO2 sensors based on undecorated single-walled carbon nanotube (SWNT) monolayer junctions. The sensitivity of the SWNT monolayer network-based sensors to NO2 gas was investigated at room temperature. We have found that the response of the gas sensors is inversely proportional to the initial conductance of the SWNT network. This behavior is different from conventional gas sensors based on uniform films, and it may be critical for the calibration of sensors in practical applications. An analytical model based on standard percolation theory is proposed to explain this behavior.

Ag growth on Si(111) with an Sb surfactant investigated by scanning tunneling microscopy

Abstract We have investigated a room-temperature growth mode of ultrathin Ag films on a Si(111) surface with an Sb surfactant using STM in a UHV system. On the Sb-passivated Si surface, small sized islands were formed up to 1.1 ML. Flat Ag islands were dominant at 2.1 ML, coalescing into larger islands at 3.2 ML. Although the initial growth mode of Ag films on the Sb-terminated Si(111) surface was Volmer-Weber (island growth), the films were much more uniform than Ag growth on clean (Si(111) at the higher coverages. From the analysis of STM images of Ag films grown with and without an Sb surfactant, the uniform growth of Ag films using an Sb surfactant appears to be caused by the kinetic effects of Ag on the preadsorbed Sb layer. Our STM results indicated that Sb suppresses the surface diffusion of Ag atoms and increases the Ag-island density. The increased island density is believed to cause coalescence of Ag islands at higher coverages of Ag, resulting in the growth of atomically flat and uniform Ag islands on the Sb surfactant layer.

Interaction of nitrogen with Si(111)-7 × 7 surfaces at elevated temperatures

Abstract The interaction of nitrogen with Si(111)-7×7 surfaces at elevated temperatures ranging from 600 to 900°C was investigated by using a scanning tunneling microscope (STM). Silicon nitride islands in a nanometer scale were formed upon nitrogen exposure at temperatures above 700°C. In the initial stage of nitridation, those islands were formed preferentially at the step edges and then additional ones were formed on the Si terrace with further nitridation. The growth of the silicon nitrides showed a strong preference along the 〈112〉 directions, resulting in triangular islands, and such a trend became more prominent at higher temperatures. Silicon nitride islands showed an 8×8 superstructure that appeared as an ordered arrangement of bright protrusions in a three-fold symmetry. By STM measurements of the surface morphology, we show the influences of the nitridation and post-annealing temperatures on the island size distribution, island density, and total surface coverage. Based on the experimental results, two possible mechanisms of thermal nitridation are suggested.

Self-organization of uniform Ag nano-clusters on Sb-terminated Si(100) surface

Nanometer sized Ag clusters were found to be uniformly formed in the initial stage of Ag growth on Sb-terminated Si(100) surfaces. Due to the saturation of Si dangling bonds by Sb adatoms, Ag clusters were grown on the Sb-terminated Si(100) surface without a Ag wetting layer. We found that the diameters and heights of Ag clusters were confined to a nanometer scale, and the size distribution was quite uniform compared to Ag growth on Sb-terminated Si(111). Those features are considered to result from the separation of Ag clusters by coherently aligned voids in the underlying Sb-terminated Si(100) surface. Tunneling spectroscopy measurements showed that the local conduction properties of Ag clusters gradually changed from semiconducting to metallic as Ag coverage increased.

FABRICATION OF LATERAL SINGLE-ELECTRON TUNNELING STRUCTURES BY FIELD-INDUCED MANIPULATION OF AG NANOCLUSTERS ON A SILICON SURFACE

Nanostructures composed of Ag clusters on an Sb-terminated Si surface were designed in a highly controlled manner and the electric conduction through Ag nanoclusters to the silicon substrate was investigated by using a scanning tunneling microscopy/spectroscopy. It was found that the lateral conduction between neighboring Ag clusters significantly contributed to the tunneling current–voltage characteristics, and the metallic single-electron tunneling structures employing the lateral conduction channels at room temperature can be fabricated via a field-induced manipulation of Ag clusters.

Coulomb staircases by lateral tunneling between adjacent nanoclusters formed on Si surfaces

We have investigated the oscillatory tunneling current–voltage characteristics on metal nanoclusters formed on Sb-terminated Si(100) surfaces by using scanning tunneling microscopy/spectroscopy. Through the systematic investigation on a variety of cluster configuration environments, we suggest that the lateral tunneling between adjacent clusters dominantly contributes to the occurrence of the single electron tunneling phenomena. In the single clusters formed on Si surfaces, we detected only current oscillations, which must be distinguished from Coulomb staircases. Those results strongly suggest that Coulomb staircases should not originate from the direct conduction of electrons through Schottky junction between the single clusters and Si substrates in contrast to other previous reports.

Scanning tunneling microscope study of Sb/Si(111)-53×53 structure

We have investigated Sb overlayer structures on Si(111) with low energy electron diffraction and scanning tunneling microscope (STM). The Sb/Si(111)-53×53 structure was constructed via the desorption of Sb from the saturated 2×1 surface at elevated temperature (700–750 °C). Its atomic structure and formation process were extensively studied with STM images, considering the structural stability. This Sb structure could be described in terms of the site selective replacement of outermost Si atoms with Sb atoms in the dimer-adatom-stacking fault 5×5 structure, which is generated by both the saturation of dangling bonds and strain due to the large lattice mismatch.