Nitrogen doping effects on the structure of graphene

Graphene and nitrogen doped graphene have been prepared by modified Hummers’ method and the following ammonia heat-treatment process, respectively. The effects of N-doping on the structure of graphene have been systematically investigated by various characterization techniques. SEM, TEM, BET, Raman and XRD analysis were used to distinguish the difference of the microstructures; and FT-IR, XPS, especially XANES were performed to elucidate the bonding information such as C-N. The effect of nitrogen doping on the structure of graphene has been obtained. More defects are present on nitrogen doped graphene as elucidated by BET, XRD, Raman, and XANES characterizations. XANES analysis also indicates that the N-doping decreases the surface oxygen-containing groups.     

Perturbed angular correlation study of radiation-induced defects in Rh metal

Radiation-induced defects are studied in cubic rhodium metal, using the local probe technique ‘Time differential perturbed angular correlation’ (TDPAC) at liquid N₂ temperature. Isochronal annealing was done at 300, 1073 and 1473 K temperatures. The irradiated sample showed two quadrupole interaction frequencies at 1150 and 93 MHz. The low frequency disappeared at room-temperature annealing, which was assigned to In trapped at a vacancy, whereas the higher frequency remained up to high temperatures and was attributed to In trapped at Rh–C complexes in the Rh matrix.     

Atomistic lattice simulation of high Tc oxides

Summary A survey is outlined of some of the normal-state crystal properties of high T_c oxides that have been calculated recently by atomistic lattice simulations.     

The influence of aliovalent impurities on the oxidation kinetics of nickel at high temperatures

The influence of chromium and sodium on the nickel oxidation kinetics has been studied as a function of temperature (1373-1673 K) and oxygen activity (10−10⁵ Pa O₂), using microthermogravimetric techniques. It has been shown that the oxidation of Ni-Cr and Ni-Na alloys, like that of pure nickel, follows strictly the parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni_1−yO, it has been found that the oxidation rate of the Ni-Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of double ionized cation vacancies in a Ni_1−yO-Cr₂O₃ solid solution is fixed on a constant level by trivalent chromium ions, substitutionally incorporated into the cation sublattice of this oxide. In the case of the Ni-Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of the doping effect, by assuming that univalent sodium ions dissolve substitutionally in the cation sublattice of nickel oxide.     

Three-dimensional radiative transfer in an isotropically scattering, plane-parallel medium: generalized X- and Y-functions

The topic of this work is the generalized X- and Y-functions of multidimensional radiative transfer. The physical problem considered is spatially varying, collimated radiation incident on the upper boundary of an isotropically scattering, plane-parallel medium. An integral transform is used to reduce the three-dimensional transport equation to a one-dimensional form, and a modified Ambarzumian's method is used to derive coupled, integro-differential equations for the source functions at the boundaries of the medium. The resulting equations are said to be in double-integral form because the integration is over both angular variables. Numerical results are presented to illustrate the computational characteristics of the formulation.     

Stability and instability of nonlinear defect states in the coupled mode equations—Analytical and numerical study

Coupled backward and forward wave amplitudes of an electromagnetic field propagating in a periodic and nonlinear medium at Bragg resonance are governed by the nonlinear coupled mode equations (NLCME). This system of PDEs, similar in structure to the Dirac equations, has gap soliton solutions that travel at any speed between 0 and the speed of light. A recently considered strategy for spatial trapping or capture of gap optical soliton light pulses is based on the appropriate design of localized defects in the periodic structure. Localized defects in the periodic structure give rise to defect modes, which persist as nonlinear defect modes as the amplitude is increased. Soliton trapping is the transfer of incoming soliton energy to nonlinear defect modes. To serve as targets for such energy transfer, nonlinear defect modes must be stable. We therefore investigate the stability of nonlinear defect modes. Resonance among discrete localized modes and radiation modes plays a role in the mechanism for stability and instability, in a manner analogous to the nonlinear Schrödinger/Gross-Pitaevskii (NLS/GP) equation. However, the nature of instabilities and how energy is exchanged among modes is considerably more complicated than for NLS/GP due, in part, to a continuous spectrum of radiation modes which is unbounded above and below. In this paper we (a) establish the instability of branches of nonlinear defect states which, for vanishing amplitude, have a linearization with eigenvalues embedded within the continuous spectrum, (b) numerically compute, using Evans function, the linearized spectrum of nonlinear defect states of an interesting multiparameter family of defects, and (c) perform direct time-dependent numerical simulations in which we observe the exchange of energy among discrete and continuum modes.     

Experimental and theoretical investigations of the electrical properties of undoped and magnesium-doped GaN layers

The ac characteristics of GaN : Mg and undoped GaN layers, grown by MOVPE on sapphire substrates, are measured for a wide range of temperature and bias conditions, in order to investigate the effect of the magnesium-related level on the transport properties. Two peaks, whose height and position depend on the measurement temperature, are observed in the admittance curves (G/ω versus frequency) of the Mg-doped samples, whereas only one peak appears in undoped samples. The study of the frequency dependence of the impedance, with a model including the two metallic Au/GaN junctions, the GaN layer itself, shows that, besides the effect of the differential resistance of the layer which plays a role in both sample types, the presence of a Mg-related deep level contributes to the observed variations of the peaks in the admittance curves of the p-doped samples. Results of a theoretical steady-state and small-signal analysis based on numerical modelling of the Au/GaN/Au heterostructure complete our analysis.     

Time-resolved photoluminescence of delta-doped AlGaAs/GaAs heterostructures

The photoluminescence (PL) at low temperature of three delta-doped AlGaAs/GaAs heterostructures is investigated under continuous and pulsed excitations. The PL under continuous excitations allows the identification of the trapping centres and probes the carrier's transfer to the GaAs channel. The time-resolved photoluminescence (TRPL) inquires about carrier dynamics and gives radiative lifetimes of different levels. The DX level shows two time constants of the intensity decay relating the splitting of the valence band under impurity strains which reduce the crystal symmetry. The two time constants evolve with temperature and exhibit an increase near T=50 K.     

A high temperature powder neutron diffraction structural study of the apatite-type oxide ion conductor, La9.67Si6O26.5

High-resolution neutron powder diffraction studies of the oxide ion conductor La_9.67Si₆O_26.5 are reported for temperatures ranging between 25 and 900 °C. The best fit to the data was obtained for space group P6₃ and there was no evidence for any change in symmetry over the temperature range studied. Interstitial oxide ions are identified lying in sites similar to those predicted by previous computer modelling studies, and in agreement with structural studies on related materials. Furthermore, occupancy of these sites is enhanced by Frenkel-type disorder from neighbouring positions. The results thus add further weight to the interpretation that, in these apatite-type systems, the silicate substructure is important for the accommodation of interstitial oxide ions and their migration.     

The sensitivity of radiative transfer calculations to the changes in the HITRAN database from 1982 to 2004

Over the last quarter century, improvements in the determination of the spectroscopic characteristics of the infrared-active trace species have enhanced our ability to retrieve quantitative distributions of temperatures, clouds, and abundances for various trace species within the Earth's atmosphere. These improvements have also allowed for refinements in the estimates of climatic effects attributed to changes in the Earth's atmospheric composition. Modeling efforts, however, have frequently experienced significant delays in assimilating improved spectroscopic information. Such is the case for highly parameterized models, where considerable effort is typically required to incorporate any revisions. Thus, a line-by-line radiative transfer model has been used to investigate the magnitude of the effects resulting from modifications to the spectroscopic information. Calculations from this line-by-line model have demonstrated that recent modifications to the HITRAN (High Resolution Transmission) line parameters, the continuum formulation, and the CO₂ line-mixing formulation can significantly affect the interpretation of the high spectral resolution radiance and brightness temperature retrievals. For certain moderate-resolution satellite remote sensing channels, modifications to these spectroscopic parameters and formulations have shown the capacity to induce changes in the calculated radiances equivalent to brightness temperature differences of 1-2 K. Model calculations have further shown that modifications of the spectroscopic characteristics tend to have a modest effect on the determination of spectrally integrated radiances, fluxes, and radiative forcing estimates, with the largest differences being of order 1 W m⁻² for the total thermal infrared fluxes, and of order 2-3% of the calculated radiative forcing at the tropopause attributed to the combined doubling of CO₂, N₂O, and CH₄. The results from this investigation are intended to function as a guide to differentiate between cases where older parameterizations provide acceptable results, within specified accuracy bounds, and cases where upgrades to the latest spectroscopic database are necessary.