Jeung Sun Ahn

Determination of weighted density of states of vibrational modes in Zn-substituted myoglobin

Abstract Site-selective fluorescence spectroscopy has been performed for Zn-substituted myoglobin in the lowest optical absorption band at 4 K. The single-site fluorescence spectrum as well as the distribution of the zero-phonon transition energy have been obtained. Furthermore, the density of states of vibrational modes of myoglobin weighted by the coupling strength between the chromophore and the polypeptide chain has been determined from the analysis of the obtained spectrum. This is the first case in which the weighted density of states of low-frequency vibrational modes has been determined in biological materials without employing any model shape functions. The results are compared with those for dye-doped polymers.

Photoluminescence of confined excitons in nanoscale C60 clusters

Abstract Formation of C60 aggregates has been found in toluene, benzene and carbon disulfide (CS2) solutions by means of photoluminescence spectroscopy. From the detailed investigation of the temperature dependence of luminescence, it has been confirmed that the C60 aggregates are formed at freezing temperature of these solvents in the cooling process. The 0 0 transition energy of their luminescence spectrum has been found to be blue-shifted in comparison with that of C60 crystal. It has been also found that the irradiation of ultraviolet light upon the C60 aggregates in benzene at 265 K transforms them to stable substances under atmosphere, which look like round-shaped nanoscale particles in the high-resolution TEM images.

Designing rigid carbon foams

We use ab initio density functional calculations to study the stability, elastic properties and electronic structure of sp(2) carbon minimal surfaces with negative Gaussian curvature, called schwarzites. We focus on two systems with cubic unit cells containing 152 and 200 carbon atoms, which are metallic and very rigid. The porous schwarzite structure allows for efficient and reversible doping by electron donors and acceptors, making it a promising candidate for the next generation of alkali ion batteries. We identify schwarzite structures that act as arrays of interconnected spin quantum dots or become magnetic when doped. We introduce two interpenetrating schwarzite structures that may find their use as the ultimate super-capacitor.

Glass transition of Zn-substituted myoglobin probed by absorption and site-selective fluorescence spectroscopies

The temperature dependence of absorption and laser-induced fluorescence spectra including the resonance line has been measured for Zn-substituted myoglobin (ZnMb) in the temperature range between 4 and 300 K. The spectral shape observed below 180 K has been found to be well explained by assuming that the site-energy distribution function is temperature independent and is equal to that determined at 4 K. This may indicate that ZnMb is glass-like below 180 K. The spectral shape of the absorption spectrum at temperatures higher than 180 K has been explained by using a configuration-coordinate model, assuming the conformational substates are distributed thermally. This means that ZnMb is liquid-like in this temperature region. As a result, it is concluded that ZnMb undergoes a liquid-glass transition around 180 K.

Glass transition of deoxymyoglobin probed by optical absorption spectroscopy

We have measured the absorption spectrum of horse deoxymyoglobin in glycerol-water mixture around 430 nm in the 130 - 320 K temperature range. The observed asymmetric spectral shape of the Soret band was analyzed using a configuration-coordinate model. The results support the idea that myoglobin is liquid-like at physiological temperatures, but is glass-like below about 250 K. The equilibrium position of the iron atom in the heme group in the electronic excited state was estimated from the determined parameter values.

Universal feature of weighted density of states of vibrational modes in dye-doped polymers

Laser-induced fluorescence spectra have been measured, including narrow resonance fluorescence lines appearing at the same energy as the exciting light, for several dye molecules doped in various polymers at low temperatures. The fluorescence spectra of dye molecules in a single site have been determined by a numerical method using the experimentally obtained site-energy distribution functions. The weighted density of states of low-frequency vibrational modes has been also determined from the single-site fluorescence spectrum obtained for each combination of a guest molecule and a host polymer. Thus determined weighted density of states has been found to depend on host polymers but not on guest molecules. By employing the scales normalized by the peak frequency and intensity, it has been found that there exists a universality in the spectra of the weighted density of states for all the samples examined.

Composition and crystalline properties of TiNi thin films prepared by pulsed laser deposition under vacuum and in ambient Ar gas.

TiNi shape memory alloy thin films were deposited using the pulsed laser deposition under vacuum and in an ambient Ar gas. Our main purpose is to investigate the influences of ambient Ar gas on the composition and the crystallization temperature of TiNi thin films. The deposited films were characterized by energy-dispersive X-ray spectrometry, a surface profiler, and X-ray diffraction at room temperature. In the case of TiNi thin films deposited in an ambient Ar gas, the compositions of the films were found to be very close to the composition of target when the substrate was placed at the shock front. The in-situ crystallization temperature (ca. 400°C) of the TiNi film prepared at the shock front in an ambient Ar gas was found to be lowered by ca. 100°C in comparison with that of a TiNi film prepared under vacuum.

Resonance fluorescence spectra of dye-doped polymers

Abstract Site-selected fluorescence spectra have been measured for Mg-octaethylporphyrin doped in polystyrene and PMMA films at low temperatures by a time-correlated single-photon counting method under CW mode-locked dye-laser excitation into the lowest optical absorption band. A narrow resonance line accompanied by a broad sideband has been observed by rejecting the scattered laser light using a time gate. A single-site fluorescence spectrum has been obtained by the use of saturation effect of the laser-induced fluorescence. The density of phonon states weighted by the electron-phonon coupling coefficient has been determined, for the first time for a dye-polymer system, from the analysis of the obtained spectrum. Temperature dependence of the laser-induced fluorescence spectrum is compared with the simulation based on the weighted phonon-state density.

Direct Synthesis of Carbon Nanotube Field Emitters on Metal Substrate for Open-Type X-ray Source in Medical Imaging

We report the design, fabrication and characterization of a carbon nanotube enabled open-type X-ray system for medical imaging. We directly grew the carbon nanotubes used as electron emitter for electron gun on a non-polished raw metallic rectangular-rounded substrate with an area of 0.1377 cm2 through a plasma enhanced chemical vapor deposition system. The stable field emission properties with triode electrodes after electrical aging treatment showed an anode emission current of 0.63 mA at a gate field of 7.51 V/μm. The 4.5-inch cubic shape open type X-ray system was developed consisting of an X-ray aperture, a vacuum part, an anode high voltage part, and a field emission electron gun including three electrodes with focusing, gate and cathode electrodes. Using this system, we obtained high-resolution X-ray images accelerated at 42–70 kV voltage by digital switching control between emitter and ground electrode.

Carbon Nanotube Field Emitters Synthesized on Metal Alloy Substrate by PECVD for Customized Compact Field Emission Devices to Be Used in X-Ray Source Applications

In this study, a simple, efficient, and economical process is reported for the direct synthesis of carbon nanotube (CNT) field emitters on metal alloy. Given that CNT field emitters can be customized with ease for compact and cold field emission devices, they are promising replacements for thermionic emitters in widely accessible X-ray source electron guns. High performance CNT emitter samples were prepared in optimized plasma conditions through the plasma-enhanced chemical vapor deposition (PECVD) process and subsequently characterized by using a scanning electron microscope, tunneling electron microscope, and Raman spectroscopy. For the cathode current, field emission (FE) characteristics with respective turn on (1 μA/cm2) and threshold (1 mA/cm2) field of 2.84 and 4.05 V/μm were obtained. For a field of 5.24 V/μm, maximum current density of 7 mA/cm2 was achieved and a field enhancement factor β of 2838 was calculated. In addition, the CNT emitters sustained a current density of 6.7 mA/cm2 for 420 min under a field of 5.2 V/μm, confirming good operational stability. Finally, an X-ray generated image of an integrated circuit was taken using the compact field emission device developed herein.