Luminescent properties of Ce in MSO4 (M: Ca, Sr and Ba) and the effect of Na co-doping

The optical properties of Ce³⁺ in CaSO₄, SrSO₄ and BaSO₄ are reported. The Ce³⁺ ion shows 4f⁰5d¹→²F_5/2,²F_7/2 luminescence in all three sulphates. Co-doping with Na⁺ does not change the local surrounding of the Ce³⁺ ion, but enhances the amount of Ce³⁺ ions built in. Under optical excitation, besides the typical Ce³⁺ doublet emission in the ultraviolet spectral region, band emission around 445 nm was observed. This band emission was not assigned to emission from a Ce³⁺ centre, but to emission from an impurity-trapped exciton. Under X-ray excitation, both Ce³⁺ emission and an emission band around 380 nm was observed. This band was assigned to emission from a self-trapped exciton.     

Prediction of anisotropic transport in Nafion containing catalyst layers

Using dissipative particle dynamics we model phase separation within Nafion electrolytes of thickness between 5 and 10 nm containing 20 volume percent water, sandwiched between a carbon catalyst support (CCS) layer and air. The diffusion pathway for protons and water is probed by Monte Carlo trajectory calculations. While varying the interactions between CCS and water, diffusion parallel to the CCS is predicted to be highest and perpendicular diffusion to be lowest for a hydrophilic CCS. This is explained by variations in water density profiles along the perpendicular direction, which act as bottlenecks for diffusion. Increasing the hydrophobic character of the CCS lifts up these bottlenecks.     

Scintillation properties of K2LaCl5 with Ce doping

In this work we present the scintillation properties of K₂LaCl₅ doped with Ce³⁺ concentrations of 0, 0.1, 1 and 10%. Under X-ray excitation the crystals show the efficient Ce³⁺ (5d–4f) luminescence between 340 and 400 nm. Depending on the Ce concentration, the light yields vary between 24,000 and 50,000 photons/MeV (ph/MeV). Experiments with 662 KeV gamma ray excitation show light yields varying between 13,000 and 30,000 ph/MeV. For Ce luminescence a single exponential decay time of approximately 40 ns is expected. However, for K₂LaCl₅:10%Ce, the decay is not single exponential. As a function of the time t, it can be described by a 1/t^1.63 behaviour for large t. With a crystal of K₂LaCl₅:10%Ce we obtained an energy resolution of 5.1% for 662 KeV gamma ray excitation of ¹³⁷Cs.     

Modeling gas permeation through membranes by kinetic Monte Carlo: applications to H2, O2, and N2 in hydrated Nafion®

We present a simulation tool in order to predict gas permeation through heterogeneous, microphase separated structures. The method combines dissipative particle dynamics (DPD) with kinetic Monte Carlo (KMC). Morphologies obtained from DPD are mapped onto a high density grid on which gas diffusion takes place. Required input parameters for the KMC calculations are the gas solubility and gas diffusion constant within each of the pure phase components. Our method was tested and validated for permeation of H(2), O(2), and N(2) gasses through hydrated Nafion membranes at various temperatures and water contents. We predict that membranes that contain an equal volume fraction of water, those with the highest ion exchange capacity exhibit the largest N(2) and O(2) permeation rates. For membranes of the same ion exchange capacity the H(2), O(2), and N(2) and permeability increases approximately linearly with Bragg spacing. We also predict that O(2) gas permeation depends much more on bottleneck phenomena within the phase separated morphologies than H(2) gas permeation. Overall, the calculated H(2) and O(2) permeability is found to be slightly lower than experimental values. This is attributed to the robustness of DPD resulting in ～7% larger Bragg spacing as compared with experiment and∕or increased gas solubility within the polymer phase with water uptake.     

Emission and excitation spectra of ZnO:Ga and ZnO:Ga,N ceramics

The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressing have been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescence spectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, is observed in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and the bandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, E _m (0) = 3.367 ± 0.005 eV, is in agreement with the data for thin zinc oxide films. The luminescence excitation spectra in the range from 3 to 6.5 eV are reported and the mechanism of energy transfer to excitons and luminescence centers is considered.     

Simulation of equivalent weight dependence of Nafion morphologies and predicted trends regarding water diffusion

Microphase separation within hydrated Nafion^® membranes was simulated using Dissipative Particle Dynamics (DPD). Morphologies were obtained at branching densities corresponding with equivalent weights ranging from 800 to 1400 (g/mole SO₃) and water percentage volume contents ([H₂O]) varying between 10% and 30%. All cases showed pronounced microphase separation involving a hydrophobic Teflon phase and a hydrophilic phase in which water is associated with SO₃ groups that are located near the phase boundaries. Pore morphologies were found to depend strongly on water content and branching density. The average pore radius (R_pore) and the distance between the pores (D_cl-cl) were found to increase with water content obeying the relations R_pore = 1.3 + α[H₂O] (nm), and D_cl-cl = 3.2 + β[H₂O] (nm). The values of the expansion coefficients α and β decrease linearly with branching density with α = 5.3 × 10⁻⁵ × (EW-450) and β = 1.3 × 10⁻⁴ × (EW-450) nm/vol%. For decreasing branching density the pores obtain a more spherical character. The consequence of this on water diffusion is estimated by employing Monte Carlo trajectory calculations in which we assume that water movement is confined within the hydrophilic phase and local water mobility to be equal to that of pure water. The estimated diffusion constants increase linearly with branching density (i.e. linear decrease with equivalent weight). Experimental water diffusion constants obtained from literature for Nafion1100 membrane are in good agreement with our calculations. A counterintuitive picture evolves in which smaller pores lead to enhanced water diffusion.     

Experimental study of the 4fn → 4fn and 4fn → 4fn-15d1 transitions of the lanthanide diiodides LnI2 (Ln=Nd, Sm, Eu, Dy, Tm, Yb)

The diffuse reflectance and photoluminescence (PL) spectra of NdI(2), SmI(2), EuI(2), DyI(2), TmI(2) and YbI(2) were measured between 225 and 12?500?nm in order to determine their 4f(n)?→?4f(n-1)5d(1) optical bandgaps. The results were compared with those obtained using an empirical model of the electronic structure of LnI(2). The results can be used to explain the lanthanide valency and crystalline structure changes of other lanthanide diiodides such as PrI(2).     

Fast path and polarization manipulation of telecom wavelength single photons in lithium niobate waveguide devices

We demonstrate fast polarization and path control of photons at 1550?nm in lithium niobate waveguide devices using the electro-optic effect. We show heralded single photon state engineering, quantum interference, fast state preparation of two entangled photons, and feedback control of quantum interference. These results point the way to a single platform that will enable the integration of nonlinear single photon sources and fast reconfigurable circuits for future photonic quantum information science and technology.