Sang Hwan Nam

Time-resolved optical emission studies on the laser ablation of a graphite target: The effects of ambient gases

Time-resolved optical emission spectroscopy was employed to examine the temporal properties of a plume formed by laser ablation of a graphite target in He, N2, and Ar background gases. Time-dependent spectroscopic temperatures of electronically excited C2 and CN molecules generated in the plume at different background gases and pressures were derived by simulation of the emission spectra. The rotational temperature of CN molecules was significantly higher than the vibrational temperature. This indicates that they are formed in the gas phase by the chemical reaction C2+N2→CN. At pressure above 0.5Torr, the temperature in each background gas was apparently in the order of Ar>N2>He while there was no such gas dependence at 0.1Torr. The expansion dynamics and cooling rate of the plume turned out to be highly dependent on the background gas and its pressure.

Er3+ photoluminescence from Er-doped amorphous SiOx films prepared by pulsed laser deposition at room temperature: The effects of oxygen concentration

We have fabricated Er-doped amorphous SiOx films by laser ablation of a Si:Er2O3 target in He atmosphere. The photoluminescence intensity at 1.54 μm was highly dependent on the oxygen content in the film, which turned out to be changed significantly by the ambient He pressure. Also, we have adopted time-of-flight quadrupole mass spectroscopy to obtain kinetic energies of ionic species in a plume produced by laser ablation. Si and Er ions do not overlap spatially as they expand toward the Si substrate and Er ions impinge on the preformed SiOx layer.

Fabrication of Nanodot Arrays on Si by Pulsed Laser Deposition Using Anodic Aluminum Oxide Nanopore Membrane as Mask

We demonstrate the fabrication of metal and semiconductor nanodot arrays on Si by pulsed laser deposition using an anodic alumina membrane as a mask. A Hexagonal-close-packed nanopore membrane mask was fabricated by the anodic oxidation of aluminum sheet. Pulsed laser deposition of Ag, Ni, ZnO, and Er-doped Si (Si:Er) nanodots was performed on a Si substrate to which an alumina membrane mask adheres by van der Waals interaction. Highly ordered arrays of Ag, Ni, ZnO, and Si:Er nanodots with average diameters ranging from 55 to 86 nm and a periodicity of ~100 nm were obtained. Also, the time-resolved images of laser-produced plasma plumes were analyzed to examine the dynamical properties of the ejected materials.

Photoinduced evaporation of mass-selected aniline+(water)n (n=4-20) clusters.

Photofragmentation of mass-selected aniline(+)(water)n (An(+)Wn, n=4-20) clusters is investigated over photon energies ranging from 1.65 to 4.66 eV by linear tandem time-of-flight mass spectrometry. The aniline ring turns out to survive irradiation of photons, and most of the absorbed photon energy flows to the hydrogen-bonding networks to be used up for liberation of water molecules. The average number of ejected water molecules measured as a function of photon energy reveals that the loss of water molecules is a photoevaporation process. The distributions of internal energies for parent ions and binding energies of water molecules are estimated from the plots of photofragment branching ratio versus photon energy, which give nice Gaussian fits. Also, density functional theory calculations are performed to obtain optimized structures of isomers for An(+)Wn clusters and binding energies. The authors find that the An(+)W6 cluster has a highly symmetric structure and its binding energy in An(+)W6-->An(+)W5+W stands out. This is in line with the experimental results showing that n=6 is a magic number in the mass distribution and An(+)W6 is relatively stable in metastable decay.

Interaction of ultraviolet laser with a silicon carbide plume produced by laser ablation

Formation mechanisms and expansion dynamics of silicon-carbon mixed cluster ions (SinCm+) in a laser-produced plasma plume produced by laser (λ=1064 nm) ablation of a SiC target in high vacuum were investigated by time-of-flight quadrupole mass spectrometry. Space- and time-resolved laser-plume interaction was examined by illuminating the plume with a time-delayed and line-focused. Ultraviolet laser pulse. Si2C was the dominant photodissociation product of silicon carbide cluster ions. Silicon-carbon mixed clusters turned out to be formed via association of atomic species in a region away from, not in front of, the target surface after a certain delay time.

Density Functional Study of Intradimer Proton Transfers in Hydrated Adenine Dimer Ions, A2+(H2O)n (n = 0-2)

Structures of mono- and dihydrated adenine dimers and their cations were calculated using B3LYP density functional theory with the 6-31+G(d,p) basis set, in order to help understand photofragmentation experiments of hydrated adenine dimers from the energetics point of view. Several important pathways leading to the major fragmentation product, protonated adenine ion (AH(+)), thermodynamically at minimum costs were investigated at the ground-state electronic potential surface of hydrated adenine dimer cations. Our calculations suggest that the proton transfer from one adenine moiety to the other in hydrated dimer ions readily occurs with negligible barriers in normal hydration conditions. In asymmetrically hydrated ions, however, the proton transfer to more hydrated adenine moieties is kinetically hindered due to heightened transition-state barriers, while the other way is still barrierless. Such directional preference in proton transfer may be characterized as a unique dimer ion property, stemming from the difference in basicity of the two nitrogen atoms involved in the double hydrogen bond that would be equivalent without hydration. We also found that dimer cleavage requires about 4 times larger energy than evaporation of individual water molecules, so it is likely that most solvent molecules evaporate before the eventual dimer cleavage when available internal energy is limited.

Clear-cut observation of clearance of sustainable upconverting nanoparticles from lymphatic system of small living mice.

The significance of lymphatic system has gathered great attention for immunotechnology related to cancer metastasis and immunotherapy. To develop innovative immunodiagnostics and immunotherapy in in vivo environments, it is very important to understand excretion pathways and clearance of injected cargoes. Herein, we employed Tm3+-doped upconverting nanoparticles (UCNPs) with versatile advantages suitable for long-term non-invasive in vivo optical imaging and tracking. Transport and retention of the UCNPs in the lymphatic system were evaluated with high-quality NIR-to-NIR upconversion luminescence (UCL) imaging. We obtained their kinetic luminescence profiles for the injection site and sentinel lymph node (SLN) and observed luminescence signals for one month; we also examined UCL images in SLN tissues, organs, and faeces at each time point. We speculate that the injected UCNPs in a footpad of a small mouse are transported rapidly from the lymphatic system to the blood system and then eventually result in an efficient excretion by the hepatobiliary route. These results will support development of novel techniques for SLN biopsy as well as immunotechnology.

Er3+ Photoluminescence from Er-doped Silicon-Rich Silicon Oxide Films Deposited by Laser Ablation of a Si:Er Target in an Oxygen Atmosphere

We have developed a new pulsed laser deposition method for fabricating Er-doped silicon-rich silicon oxide (SRSO:Er) films. A target composed of a pure Si disk and an Er metal strip was ablated by a line-focused laser beam in oxygen atmosphere. The oxygen concentration that determines the relative concentrations of the three phases (Si–Si, SiOx, and SiO2) in the film was easily controlled by varying the ambient oxygen pressure. The photoluminescence intensity at 1.54 µm from Er3+ ions was strongly dependent on the amount of the Si–Si phase in the SRSO:Er films.

Wavelength Conversion Effect-Assisted Dye-Sensitized Solar cells for Enhanced Solar Light Harvesting

In the present study, an enhanced solar light harvesting strategy based on the wavelength conversion effect is suggested for dye-sensitized solar cells (DSSCs). Incorporating β-NaYF4:Yb3+,Er3+ phosphor microcrystals as a component of the DSSCs enables wavelength conversion from infrared to visible light, which maximizes the absorption of visible light by the C106 organometallic dye and produces excess current, thus enhancing the quantum efficiency. It is demonstrated that the photocurrent density generated is increased by the wavelength conversion effect of the phosphor material. The introduction of up-converting β-NaYF4:Yb3+,Er3+ phosphor microcrystals dispersed in an iodide electrolyte and the use of an additional phosphor reflecting film increases the overall solar-to-electrical energy conversion efficiency (η) from 9.02% of reference DSSCs to 9.48 and 10.76%, respectively. DSSCs with phosphors contained in both the iodide electrolyte solution as well as an additional phosphor reflecting film show a highly enhanced overall efficiency of 10.90%. Herein, a strategy for effective wavelength conversion is investigated for enhanced solar light harvesting, and the results of the study contribute the most viable strategy for achieving maximum utilization of the solar spectrum.

Laser Ablation of a Zn Target in Electric Field

The average kinetic energy of Zn+ ions and optical emission intensities from Zn* and Zn+* produced by laser ablation of a Zn target were measured as a function of bias voltage applied to a grid mounted facing the target to elucidate the effects of the external electric field on the laser-produced plasma plume. It turned out that the relative concentration of the ionic species in the plume increased and the crystallinity of the deposited ZnO film improved with increasing the bias voltage.