| 1. |
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| 2. |
- Lin, Yuan-Chih, 1987, et al.
(författare)
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Understanding the Interactions between Vibrational Modes and Excited State Relaxation in Y₃₋ₓCeₓAl₅O₁₂: Design Principles for Phosphors Based on 5d-4f Transitions
- 2018
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 30:6, s. 1865-1877
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Tidskriftsartikel (refereegranskat)abstract
- The oxide garnet Y 3 Al 5 O 12 (YAG), when a few percent of the activator ions Ce 3+ substitutes for Y 3+ , is a luminescent material widely used in phosphor-converted white lighting. However, fundamental questions surrounding the defect chemistry and luminescent per formance of this material remain, especially in regard to the nature and role of vibrational dynamics. Here, we provide a complete phonon assignment of YAG and establish the general spectral trends upon variation of the Ce 3+ dopant concentration and temperature, which are shown to correlate with the macroscopic luminescence properties of Y 3-x Ce x Al 5 O 12 . Increasing the Ce 3+ concentration and/or temperature leads to a red-shift of the emitted light, as a result of increased crystal-field splitting due to a larger tetragonal distortion of the CeO 8 moieties. Decreasing the Ce 3+ concentration or cosubstitution of smaller and/or lighter atoms on the Y sites creates the potential to suppress thermal quenching of luminescence because the frequencies of phonon modes important for nonradiative relaxation mechanisms are upward-shifted and hence less readily activated. It follows that design principles for finding new Ce 3+ -doped oxide phosphors emitting at longer wavelengths require tetragonally distorted environments around the CeO 8 moieties and a sufficiently rigid host structure and/or low activator-ion concentration to avoid thermal quenching of luminescence.
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| 3. |
- Lin, Yuan-Chih, 1987, et al.
(författare)
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Vibrationally induced color shift tuning of photoluminescence in Ce³⁺-doped garnet phosphors
- 2019
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7534 .- 2050-7526. ; 7:41, s. 12926-12934
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Tidskriftsartikel (refereegranskat)abstract
- A critical challenge in the field of phosphor converted white light emitting diodes (pc-WLEDs) pertains to understanding and controlling the variation of emission color with device temperature. Here we, through a combined photoluminescence (PL) and Raman spectroscopy study of the three garnet type phosphors Ce3+-doped Y3Al5O12 (YAG:Ce3+), Ca3Sc2Si3O12 (CSS:Ce3+), and Sr3Y2Ge3O12 (SYG:Ce3+), show that the color of the PL is systematically shifted upon changing the operation temperature of the phosphor. A general trend is observed that the PL exhibits a red-shift as a function of increasing temperature, until the point at which the vibrational modes of the CeO8 moieties, which induce dynamical tetragonal distortions of the CeO8 dodecahedra, are fully activated. Upon further temperature increase, the PL turns to a blue-shift because of a counteracting and predominating effect of thermal lattice expansion that progressively makes the CeO8 dodecahedra more cubal like. Since this behavior is the result of the symmetry relations intrinsic to the garnet structure, the present mechanism can be generally applicable to materials of this type. It thereby provides a route for tuning the PL of this important class of phosphor materials.
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| 4. |
- Svenningsson, Leo, 1990, et al.
(författare)
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Molecular Orientation Distribution of Regenerated Cellulose Fibers Investigated with Polarized Raman Spectroscopy
- 2019
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Ingår i: Macromolecules. - : American Chemical Society (ACS). - 1520-5835 .- 0024-9297. ; 52:10, s. 3918-3924
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Tidskriftsartikel (refereegranskat)abstract
- The molecular orientation distribution of polymeric fibers influences physical properties. We present a novel method of analyzing polarized Raman experiments to determine molecular orientation, which is based on exchanging the Legendre polynomial approach with a wrapped Lorentzian function, as determined from a prescreening of X-ray scattering patterns. This method removes the need for performing right angle scattering experiments while avoiding common approximations. The molecular orientation of regenerated cellulose fibers, using the presented method, is shown to correlate well with X-ray scattering and an analogous experiment using solid-state NMR spectroscopy. Challenges of quantitatively measuring molecular anisotropy occur with semi-crystalline, partially modified, or composite materials. As such, a plethora of techniques, each with a unique chemical selectivity, is paramount for material characterization.
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| 5. |
- Bielecki, Johan, et al.
(författare)
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Structure and dehydration mechanism of the proton conducting oxide Ba2In2O5(H2O)(x)
- 2016
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 4:4, s. 1224-1232
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Tidskriftsartikel (refereegranskat)abstract
- The structure and dehydration mechanism of the proton conducting oxide Ba2In2O5(H2O)(x) are investigated by means of variable temperature (20-600 degrees C) Raman spectroscopy together with thermal gravimetric analysis and inelastic neutron scattering. At room temperature, Ba2In2O5(H2O)(x) is found to be fully hydrated (x = 1) and to have a perovskite-like structure, which dehydrates gradually with increasing temperature and at around 600 degrees C the material is essentially dehydrated (x approximate to 0.2). The dehydrated material exhibits a brownmillerite structure, which is featured by alternating layers of InO6 octahedra and InO4 tetrahedra. The transition from a perovskite-like to a brownmillerite-like structure upon increasing temperature occurs through the formation of an intermediate phase at ca. 370 degrees C, corresponding to a hydration degree of approximately 50%. The structure of the intermediate phase is similar to the structure of the dehydrated material, but with the difference that it exhibits a non-centrosymmetric distortion of the InO6 octahedra that is not present in the dehydrated material. The dehydration process upon heating is a two-stage mechanism; for temperatures below the hydrated-to-intermediate phase transition, dehydration is characterized by a homogenous release of protons over the entire oxide lattice, whereas above the transition a preferential desorption of protons originating in the nominally tetrahedral layers is observed. Furthermore, our spectroscopic results point towards the co-existence of two structural phases, which relate to the two lowest-energy proton configurations in the material. The relative contributions of the two proton configurations depend on how the sample is hydrated.
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| 6. |
- Eklöf-Österberg, Carin, 1987, et al.
(författare)
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Dynamics of Hydride Ions in Metal Hydride-Reduced BaTiO3 Samples Investigated with Quasielastic Neutron Scattering
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:4, s. 2019-2030
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Tidskriftsartikel (refereegranskat)abstract
- Perovskite-type oxyhydrides, BaTiO3-xHx, have been recently shown to exhibit hydride-ion (H-) conductivity at elevated temperatures, but the underlying mechanism of hydride-ion conduction and how it depends on temperature and oxygen vacancy concentration remains unclear. Here, we investigate, through the use of quasielastic neutron scattering techniques, the nature of the hydride-ion dynamics in three metal hydride-reduced BaTiO3 samples that are characterized by the simultaneous presence of hydride ions and oxygen vacancies. Measurements of elastic fixed window scans upon heating reveal the presence of quasielastic scattering due to hydride-ion dynamics for temperatures above ca. 200 K. Analyses of quasielastic spectra measured at low (225 and 250 K) and high (400-700 K) temperature show that the dynamics can be adequately described by established models of jump diffusion. At low temperature, <= 250 K, all of the models feature a characteristic jump distance of about 2.8 angstrom, thus of the order of the distance between neighboring oxygen atoms or oxygen vacancies of the perovskite lattice and a mean residence time between successive jumps of the order of 0.1 ns. At higher temperatures, >400 K, the jump distance increases to about 4 angstrom, thus of the order of the distance between next-nearest neighboring oxygen atoms or oxygen vacancies, with a mean residence time of the order of picoseconds. A diffusion constant D was computed from the data measured at low and high temperatures, respectively, and takes on values of about 0.4 X 10(-6) cm(-2) s(-1) at the lowest applied temperature of 225 K and between ca. 20 X 10(-6) and 100 X 10(-6) cm(-2) s(-1) at temperatures between 400 and 700 K. Activation energies E-a were derived from the measurements at high temperatures and take on values of about 0.1 eV and show a slight increase with increasing oxygen vacancy concentration.
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| 7. |
- Enrichi, Francesco, et al.
(författare)
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Ag nanoaggregates as efficient broadband sensitizers for Tb3+ ions in silica-zirconia ion-exchanged sol-gel glasses and glass-ceramics
- 2018
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Ingår i: Optical materials (Amsterdam). - : Elsevier. - 0925-3467 .- 1873-1252. ; 84, s. 668-674
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we report the study of down-shifting silica-zirconia glass and glass-ceramic films doped by Tb3+ ions and Ag nanoaggregates, which combine the typical spectral properties of the rare-earth-ions with the broadband sensitizing effect of the metal nanostructures. Na-Tb co-doped silica-zirconia samples were obtained by a modified sol-gel route. Dip-coating deposition followed by annealing for solvent evaporation and matrix densification were repeated several times, obtaining a homogeneous crack-free film. A final treatment at 700 °C or 1000 °C was performed to control the nanoscale structural properties of the samples, resulting respectively in a glass (G) or a glass-ceramic (GC), where tetragonal zirconia nanocrystals are surrounded by an amorphous silica matrix. Ag introduction was then achieved by ion-exchange in a molten salt bath, followed by annealing in air to control the migration and aggregation of the metal ions. The comparison of the structural, compositional and optical properties are presented for G and GC samples, providing evidence of highly efficient photoluminescence enhancement in both systems, slightly better in G than in GC samples, with a remarkable increase of the green Tb3+ PL emission at 330 nm excitation: 12 times for G and 8 times for GC samples. Furthermore, after Ag-exchange, the shape of Tb3+ excitation resembles the one of Ag ions/nanoaggregates, with a broad significant absorption in the whole UV-blue spectral region. This broadband enhanced downshifting could find potential applications in lighting devices and in PV solar cells.
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| 8. |
- Filippov, Stanislav, et al.
(författare)
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Analysis of Dihydrogen Bonding in Ammonium Borohydride
- 2019
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:47, s. 28631-28639
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Tidskriftsartikel (refereegranskat)abstract
- The structural and vibrational properties of ammonium borohydride, NH4BH4, have been examined by first-principles density functional theory (DFT) calculations and inelastic neutron scattering (INS). The H disordered crystal structure of NH4BH4 is composed of the tetrahedral complex ions NH4+ and BH4-, which are arranged as in the fcc NaCl structure and linked by intermolecular dihydrogen bonding. Upon cooling, the INS spectra revealed a structural transition between 45 and 40 K. The reversible transition occurs upon heating between 46 and 49 K. In the low-temperature form reorientational dynamics are frozen. The libration modes for BH4- and NH4+ are near 300 and 200 cm(-1), respectively. Upon entering the fcc high-temperature form, NH4+ ions attain fast reorientational dynamics, as indicated in the disappearance of the NH4+ libration band, whereas BH4- ions become significantly mobile only at temperatures above 100 K. The vibrational behavior of BH4- ions in NH4BH4 compares well to the heavier alkali metal borohydrides, NaBH4-CsBH4. DFT calculations revealed a nondirectional nature of the dihydrogen bonding in NH4BH4 with only weak tendency for long-range order. Different rotational configurations of complex ions appear quasi-degenerate, which is reminiscent of glasses.
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| 9. |
- Fluri, A., et al.
(författare)
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Anisotropic Proton and Oxygen Ion Conductivity in Epitaxial Ba2In2O5 Thin Films
- 2017
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:40, s. 21797-21805
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Tidskriftsartikel (refereegranskat)abstract
- Solid oxide oxygen ion and proton conductors are a highly important class of materials for renewable energy conversion devices like solid oxide fuel cells. Ba2In2O5 (BIO) exhibits both oxygen ion and proton conduction, in a dry and humid environment, respectively. In a dry environment, the brownmillerite crystal structure of BIO exhibits an ordered oxygen ion sublattice, which has been speculated to result in anisotropic oxygen ion conduction. The hydrated structure of BIO, however, resembles a perovskite and the protons in it were predicted to be ordered in layers. To complement the significant theoretical and experimental efforts recently reported on the potentially anisotropic conductive properties in BIO, we measure here both the proton and oxygen ion conductivity along different crystallographic directions. Using epitaxial thin films with different crystallographic orientations, the charge transport for both charge carriers is shown to be anisotropic. The anisotropy of the oxygen ion conduction can indeed be explained by the layered structure of the oxygen sublattice of BIO. The anisotropic proton conduction, however, further supports the suggested ordering of the protonic defects in the material. The differences in proton conduction along different crystallographic directions attributed to proton ordering in BIO are of a similar extent as those observed along different crystallographic directions in materials where proton ordering is not present but where protons find preferential conduction pathways through chainlike or layered structures.
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| 10. |
- Jedvik Granhed, Erik, 1979, et al.
(författare)
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Band: Vs. polaron: Vibrational motion and chemical expansion of hydride ions as signatures for the electronic character in oxyhydride barium titanate
- 2019
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 7:27, s. 16211-16221
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Tidskriftsartikel (refereegranskat)abstract
- The oxyhydride phase of barium titanate, BaTiO3-xHx, is a mixed hydride ion and electron conductor. The substitution of oxygen with hydrogen to form a hydride ion is accompanied by donation of an electron to the initially empty titanium 3d conduction band. It is not clear, however, whether the electron forms a delocalized state where it is shared among all titanium ions forming a bandstate, or if it localizes on a titanium ion and forms a bound electron polaron. Here, we investigate polaron formation in this material using density-functional theory (DFT) calculations, where the self-interaction error has been corrected by the DFT + U method and the HSE hybrid functional. While calculated formation energies do not provide a conclusive description of the electronic state, a comparison of the results from first-principles phonon calculations with vibrational spectra measured with inelastic neutron scattering (INS) suggests that the electrons form bandstates in bulk BaTiO3-xHx. This is further supported by comparison of the computed chemical expansion of the involved defect species with experimental data of the lattice expansion in the oxyhydride formation. The oxyhydride phase of barium titanate, BaTiO3-xHx, should thus exhibit metallic-like conductivity.
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