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11.
Unusual crystal structure of 12CaO·7Al2O3 is composed by a framework of positively charged nanocages, which enable accommodation of various negative ions (and even electrons) inside these cages. Different filling of cages leads to significant changes in electronic structure and as the result in luminescence properties, as well. Luminescence was studied using time-resolved spectroscopy in VUV in the temperature range from 6 to 300 K. Electron loaded samples exhibit UV luminescence band peaked at ~5 eV. The excitation spectrum of this emission has the onset at the energy gap value of 6.8 eV, and its decay is well described with the sum of two exponential functions with life-times of τ1 = 3.7 ns and τ2 = 29 ns, respectively. Its thermal quenching is well approximated by the sum of two Mott-Seitz type curves with the activation energies of 34 meV and 70 meV. Experimental results indicate that this luminescence is possibly due to radiative decay of two singlet self-trapped exciton states, which hole components are localized on two non-equivalent framework oxygens. 相似文献
12.
Elo Kibena Dr. Margus Marandi Dr. Uno Mäeorg Luna B. Venarusso Dr. Gilberto Maia Dr. Leonard Matisen Dr. Aarne Kasikov Prof. Väino Sammelselg Dr. Kaido Tammeveski 《Chemphyschem》2013,14(5):1043-1054
An electrochemical study of Au electrodes electrografted with azobenzene (AB), Fast Garnet GBC (GBC) and Fast Black K (FBK) diazonium compounds is presented. Electrochemical quartz crystal microbalance, ellipsometry and atomic force microscopy investigations reveal the formation of multilayer films. The elemental composition of the aryl layers is examined by X‐ray photoelectron spectroscopy. The electrochemical measurements reveal a quasi‐reversible voltammogram of the Fe(CN)63?/4? redox couple on bare Au and a sigmoidal shape for the GBC‐ and FBK‐modified Au electrodes, thus demonstrating that electron transfer is blocked due to the surface modification. The electrografted AB layer results in strongest inhibition of the Fe(CN)63?/4? response compared with other aryl layers. The same tendencies are observed for oxygen reduction; however, the blocking effect is not as strong as in the Fe(CN)63?/4? redox system. The electrochemical impedance spectroscopy measurements allowed the calculation of low charge‐transfer rates to the Fe(CN)63? probe for the GBC‐ and FBK‐modified Au electrodes in relation to bare Au. From these measurements it can be concluded that the FBK film is less compact or presents more pinholes than the electrografted GBC layer. 相似文献
13.
A. Lushchik Ch. Lushchik I. Kudryavtseva A. Maaroos F. Savikhin E. Shablonin E. Vasil’chenko 《Optics and Spectroscopy》2011,111(3):434-440
The processes of photon multiplication in insulators have been considered. The luminescence of Tb3+ ions (5
D
3 → 7
F
J
, 5
D
4 → 7
F
J
transitions) upon intracenter excitation, the optical excitation of oxyanions, or the formation of separated electrons and
holes has been studied for CaSO4 doped with Tb3+ and Na+ ions at 6–9 K. An increase in Tb3+ concentration from 0.2 to 4 at % and transition from single Tb3+-Na+ states to centers that contain two or three terbium ions leads to the redistribution of the luminescence intensities in favor
of the 5
D
4 → 7
F
J
transitions and increase in their efficiency due to the possibility of the cooperative 5
D
3 → 5
D
4 and 7
F
6 → 7
F
J
transitions and the 4f
75d
1 → 5
D
3 and 7
F
6 → 5
D
4 transitions in the two- and three-terbium centers. Based on the example of MgO single crystals with highly mobile excitons,
holes, and electrons, the migration of free excitons and holes toward Cr3+ ions in the crystal bulk and their exit from the bulk to the surface have been revealed at 9 K. Surface losses limit the
luminescence quantum yield of MgO:Cr3+, CaSO4:Tb3+, and many other materials. 相似文献
14.
S. A. Dolgov T. K?rner A. Lushchik A. Maaroos S. Nakonechnyi E. Shablonin 《Physics of the Solid State》2011,53(6):1244-1252
The properties of the majority trapped-hole centers in MgO, such as g-factors, positions of absorption and luminescence bands, and temperatures of thermal destruction, have been analyzed with
the emphasis on the observed regular trends and interrelations between the properties of these centers. Particular emphasis
has been placed on the positively charged [Be]+ and [Ca]+ trapped-hole centers, which have a large cross section for recombination with conduction electrons. In these centers, a hole
is localized at an oxygen ion near the impurity Be2+ or Ca2+ ion located at a regular cation site. The generation and transformation of defects due to the recombination of either relaxed
conduction electrons with OH−-containing hole centers or cold and hot electrons with [Be]+ and [Ca]+ centers have been considered. Using the interrelation of the characteristics of hole centers and taking into account that
the recombination emission band revealed at ∼6.8 eV is due to the Ca2+-containing centers that are stable below 50 K, the prospects for the EPR detection of the [Ca]+ center at T < 4.2 K have been discussed. 相似文献
15.
Aarne S. Pohjonen Flyura Djurabekova Antti Kuronen Steven P. Fitzgerald Kai Nordlund 《哲学杂志》2013,93(32):3994-4010
We have analyzed in detail the mechanism leading to tip growth on a surface which operates via nucleation of dislocations on a near-surface void under tensile surface stress. We derived a simplified analytical model describing the relevant physical factors related to the observed linearity between the void radius and the maximum depth of the void for the growth to occur. The model is based on the direct numerical calculation of atomic level stresses in the simulated system. Based on the present model we can estimate this maximum depth for a void of a certain size under a given stress in the size range which is beyond the feasibility of the molecular dynamics simulation method. 相似文献
16.
Diels-Alder adducts of 1,4-diphenyl-1,3-cyclopentadiene and maleic anhydride were investigated by recording the 1H and 13C{1H} NMR spectra of three isomeric diphenylbicyclo[2.2.1]hept-5-ene endo and exo 2,3-dianhydrides. the spectra were recorded in CD2Cl2 and analysed completely. the effect of the endo and exo configuration of the anhydride ring on the chemical shifts of the bridgehead phenyl protons is discussed. the ortho protons of the exo isomers resonate at higher field than those of the endo isomer, and the resonance pattern of the aromatic protons is narrower in the exo than the endo anhydride. the aromatic regions of the spectra are compared with the same regions of the 1H NMR spectra of the earlier investigated addition products of 1,4-di-p-tolyl-1,3-cyclopentadiene and 1-phenyl-4-p-tolyl-1,3-cyclopentadiene with maleic anhydride. Chemical shifts of the bridge protons are explained on the basis of X-ray data of the compounds and MacroModel calculations on the minimum energy conformations. 相似文献
17.
18.
Aarne Pajunen Fernando Cmara Jose M. Dominques‐Vera Enrique Colacio 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(2):e49-e50
In the title compound, [Cu(C10H18N5)(CH4O)]ClO4, four N atoms from the deprotonated ligand derived from bis(3‐aminopropyl)amine and 2‐imidazolecarboxaldehyde are coordinated to the Cu atom. The four N atoms occupy equatorial positions with Cu—N bond distances ranging from 1.998 (2) to 2.046 (3) Å. The methanol O atom occupies one axial position with a Cu—O bond distance of 2.295 (2) Å. 相似文献
19.
S. A. Dolgov V. Isakhanyan T. Krner P. Liblik A. Maaroos S. Nakonechnyi 《Radiation measurements》2004,38(4-6):699-702
Doping of MgO single crystals with Be results in the formation of numerous Be-containing paramagnetic centres, easily detectable by EPR, and creates an intensive luminescence band at 6.2 eV, observable at T<200 K, and gives rise to new thermoluminescence peaks at 147 and 190 K. A paramagnetic centre with a rhombic symmetry that decays at 160 K was identified as a [Be]+ (i.e. O−–Be2+) centre—a hole trapped by O2− lattice ion near a Be2+ perturbing defect. The luminescence excitation and isochronal annealing studies led to the conclusion that the 6.2 eV luminescence arises at the radiative decay of electron excitations near Be2+. These excitations can be created at the recombination of electrons with the holes localised in the [Be]+ centres, at the recombination of holes with the electrons trapped in the Be1+ centre or at a direct excitation of oxygen near the Be2+ ion. 相似文献
20.
Generalized Hermite-Birkhoff interpolation gives a unifying method for constructing exponentially fitted linear multistep methods explicitly. Fitting may be done at any set of given (real) parameters. The construction principle permits study of the convergence and stability properties of these methods. Moreover, it shows how the properties depend on the corresponding properties of classical linear multistep methods with constant coefficients. The theoretical results are illustrated by some examples. 相似文献