Minju Ying

Collision of hydrogen atom with single-walled carbon nanotube: Adsorption, insertion, and healing

Interaction of hydrogen atom with (5, 5) single-walled carbon nanotube (SWNT) has been studied over the collision energy range from 1 to 30 eV using a molecular dynamics simulation method. In the energy interval of 1–3 eV, the hydrogen atom can be chemisorbed on the outer wall of the SWNT, provided the impact point is near a vertex carbon atom of a hexagon. The lowest incident energy needed for a hydrogen atom to pass through a hexagon ring on the SWNT is estimated to be 14 eV. Hydrogen atoms that enter into the SWNT would either be encapsulated in it to form endohedral H@tube complex, or escape out of it. The hole on the sidewall of the nanotube induced by the collision of hydrogen atom can be healed after relaxation for several picoseconds.

Plasma properties of a laser-ablated aluminum target in air

The optical emission spectra of the plasma generated by a 1.06-μm Nd:YAG laser irradiation of Al target in air was recorded and analyzed in a spatially resolved manner. Electron temperatures and densities in the plasma were obtained using the relative emission intensities and the Stark-broadened linewidths of Al(I) emission lines, respectively. The dependence of the electron density and temperature on the distance from the target surface and on the laser irradiance were manifested. We also discussed how the air takes part in the plasma evolution process and confirmed that the ignition of the air plasma was by the collisions between the energetic electrons and the nitrogen atoms through a cascade avalanche process.

Structures of hydrogen molecules in single-walled carbon nanotubes

Abstract Molecular-dynamics simulations were used to investigate the structures of hydrogen molecules formed within (5,5), (9,0) and (10,10) single-walled carbon nanotubes (SWNT). In the (5,5) and (9,0) SWNTs, H 2 molecules preferably form linear lines and helical curves, showing the strong confinement of small-diameter SWNTs. The structure of H 2 molecules in the (5,5) and (9,0) SWNTs is different, showing the dependence on the helicities of the SWNTs. The accumulation process of H 2 molecules in the (10,10) SWNT is also presented, showing the formation of H 2 molecules cylindrical shells.

Study of the plasma produced from laser ablation of a KTP crystal

Optical emission spectra of the plasma produced by 1.06 mm Nd:YAG laser irradiation of a potassium titanium phosphate (KTP) crystal in vacuum and in air were recorded and analyzed in a spatially resolved manner. Electron temperatures and densities in the plasma in vacuum were determined with the Boltzmann plot method using five Ti(II) emission lines and the Stark-broadened linewidths of neutral K atoms, respectively. The dependence of the electron densities and temperatures on distance from the target surface and on laser irradiance were manifested. In order to see the effect of backing gas, we also performed laser ablation of a KTP crystal in air and compared the results with that of the ablation in vacuum. Gas dynamical parameters of the laser-induced KTP evaporation wave were simulated using a hydrocode developed in our group based on the self-similar solutions of the gas dynamical equations. Moreover, the velocity of the evaporating wave and the vapor pressure were calculated under different laser power densities. # 2002 Elsevier Science B.V. All rights reserved. PACS: 52.50.Jm; 42.62.Fi

Influence of annealing condition on photoluminescence characteristics of AlGaAs/GaAs multiple quantum well

Abstract The energy levels in AlGaAs/GaAs multiple quantum well (MQW) were calculated with transfer matrix method (TMM) and compared with experimental results from photoluminescence (PL) measurements. The comparison shows good agreement. A thin layer of sputtered SiO 2 film was deposited on AlGaAs/GaAs MQW samples and a subsequent rapid thermal annealing (RTA) was carried out. Photoluminescence (PL) measurements showed that the PL characteristics of the wells at different depths beneath the surface of the AlGaAs/GaAs MQW behave differently with the change of RTA condition. The PL intensities of the wells close to the surface and the bottom of the AlGaAs/GaAs MQW are sensitive to the RTA condition, while those of the wells in the middle of the MQW show little variations with the change of RTA condition.

DFT calculation on the energy thresholds of DNA damages under irradiation conditions

Density functional theory (DFT) calculation of various segments of DNA have been performed to aid our understanding of DNA damages under the conditions of irradiation. These damages studied include strand breaks, base releasing from backbone, and amidogen/methyl removing from bases. In the calculation, the effect of the hydrogen bond interaction between bases in the base-pair, as well as, that of the stacking interaction between the base pairs in adjacent turns of the double helix are both considered. In addition, we also evaluated the effect of the biochemical environment, such as sodium counterions and water molecules, on the stabilization of DNA segment. Base release damage is found to have lowest energy threshold. The remove of an atom or a radical from sugar backbone, which may result in single-strand breaks or double-strand breaks, has the highest energy threshold. All the energy thresholds of these DNA damages are determined to be in the range from 4.5 to 8.7 eV.

Emission properties of laser ablation of SnO2: Sb transparent conducting film and KTiOPO4 crystal

Abstract Optical emission spectra of Nd:YAG laser ablation of KTiOPO 4 (KTP) crystal and SnO 2 :Sb transparent conducting thin film were recorded and analyzed in vacuum and in air. The integral intensities of spectral lines from laser-ablated KTP crystal were obtained as functions of distance from the target surface and laser power density in vacuum and in air. The ambient gas effects on pulsed laser ablation of target were discussed. We also performed laser ablation of SnO 2 :Sb transparent conducting thin film in air and the electron temperature and full-width at half-maximum (FWHM) of atomic and ionic spectral lines in the plasma were quantified using Boltzmann plot method and Lorentzian fit, respectively. Integral intensities of atomic and ionic Sn spectral lines were also obtained as functions of distance from the target surface and laser irradiance. The intensity ratio of ionic and atomic Sn spectral lines as a function of laser power density was got which gives some information about the variation of ionization ratio with laser irradiance in the plasma produced by high-power laser.

Electronic stopping powers for fluorine ions in 19F+-implanted tin-oxide films prepared by APCVD

Abstract Electronic stopping powers for 50–180 keV 19F ions in tin-oxide films were obtained by range measurement. Depth profiles of 19F in tin-oxide films were measured by using the 19F(p,αγ)16O resonant nuclear reaction at ER=872.1 keV. A proper convolution calculation method was used to extract the true distribution of fluorine from the experimental excitation yield curves. The electronic stopping powers were derived through fitting the projected range distributions, simulated by using the TRIM/XLL code, to the experimentally measured range distributions. It is shown that the electronic stopping cross-sections obtained in this work agree well with those calculated by using TRIM90 and can be well described by the four-parameter formulae. But they are systematically larger than the results obtained from the TRIM98 code.

The importance of electronic exchange-correlation in non-self-consistent frozen density approximation

The effects of different treatments for the exchange-correlation energy on the accuracy of non-self-consistent frozen density approximation (FDA) are discussed. Local spin density approximation (LSDA) and non-local spin density approximation (NLSDA) are employed, respectively. Corresponding results obtained by using full-self-consistent density functional theory (DFT) are also given for the purpose of comparison. Explicit calculations for hydrogen bonds, covalent bonds and ionic bonds indicate that, comparing with LSDA, NLSDA can improve the accuracy of FDA as well as that of DFT. This improvement attributed to the refinements in the treatment for the electronic exchange-correlation energy may help to extend the application of FDA.

Electronic stopping powers of molybdenum metal for 19F ions at low velocity

Abstract Electronic stopping powers for 80–350 keV 19F ions in molybdenum were obtained by range measurement. Depth profiles of 19F in molybdenum were measured by using the 19F(p,αγ)16O resonant nuclear reaction at ER=872.1 keV. A proper convolution calculation method was used to extract the true distribution of fluorine from the experimental excitation yield curves. The electronic stopping powers were derived through fitting the projected range distributions, simulated by using the TRIM/XLL code, to the experimentally measured range distributions. The electronic stopping cross sections were compared with those obtained from Monte Carlo simulation codes.