Search results for " Defects"
showing 10 items of 294 documents
Tuning the defect configurations in nematic and smectic liquid crystalline shells
2013
Thin liquid crystalline shells surrounding and surrounded by aqueous phases can be conveniently produced using a nested capillary microfluidic system, as was first demonstrated by Fernandez-Nieves et al. in 2007. By choosing particular combinations of stabilizers in the internal and external phases, different types of alignment, uniform or hybrid, can be ensured within the shell. Here, we investigate shells in the nematic and smectic phases under varying boundary conditions, focusing in particular on textural transformations during phase transitions, on the interaction between topological defects in the director field and inclusions in the liquid crystal (LC), and on the possibility to rel…
Controlling the oxidation processes of Zn nanoparticles produced by pulsed laser ablation in aqueous solution
2016
We used online UV-VIS optical absorption and photoluminescence spectra, acquired during and after pulsed laser ablation of a Zinc plate in aqueous solution, to investigate the effect of the laser repetition rate and liquid environment on the oxidation processes of the produced nanoparticles. A transient Zn/ZnO core-shell structure was revealed by the coexistence of an absorption peak around 5.0 eV due to Zn surface plasmon resonance and of an edge at 3.4 eV coming from wurtzite ZnO. The growth kinetics of ZnO at the various repetition rates, selectively probed by the excitonic emission at 3.3 eV, began immediately at the onset of laser ablation and was largely independent of the repetition …
Luminescence features of nonbridging oxygen hole centres in silica probed by site-selective excitation with tunable laser
2008
Time-resolved photoluminescence at 1.9 eV associated with the nonbridging oxygen hole centre (NBOHC) in silica was investigated under excitation with a ns pulsed laser system, tunable in the visible range. Mapping of the excitation/emission pattern evidences the site-selective excitation of the resonant zero phonon line (ZPL) transition due to its weak coupling with the stretching mode of dangling oxygen. Decay of ZPL follows an exponential law with lifetime of 15.3 μs, which provides a precise measure of the electronic transition probability of a single NBOHC.
Enhanced optical properties of Cd–Mg-co-doped ZnO nanoparticles induced by low crystal structure distortion
2020
Abstract The growth of CdxMg0.125-xZn0.875O nanoparticles with yellow-orange luminescence is achieved up to 2.5 at. % Cd via a modified sol–gel process. X-ray diffraction analysis confirmed that all the nanoparticles have the hexagonal wurtzite structure. It is found that Cd doping has a considerable effect on the crystal size, microstrain, band gap, and photoluminescence of the Mg0·125Zn0·875O structure, originating from a preferred crystallographic orientation along the (101) plane of the wurtzite structure. The shift and broadening of the E2(high) mode observed in the Raman spectra due to growth-induced strain corroborates the small distortion observed in the X-ray diffraction data. The …
Photoluminescence spectral dispersion as a probe of structural inhomogeneity in silica
2011
We perform time-resolved photoluminescence measurements on point defects in amorphous silicon dioxide (silica). In particular, we report data on the decay kinetics of the emission signals of extrinsic oxygen deficient centres of the second type from singlet and directly excited triplet states, and we use them as a probe of structural inhomogeneity. Luminescence activity in sapphire (alpha-Al(2)O(3)) is studied as well and used as a model system to compare the optical properties of defects in silica with those of defects embedded in a crystalline matrix. Only for defects in silica did we observe a variation of the decay lifetimes with emission energy and a time dependence of the first moment…
Inhomogeneous width of oxygen-deficient centers induced by electron irradiation of silica
2009
We report a study of the luminescence activity of oxygen-deficient centers stabilized in as-grown synthetic silica, as compared with the same defects induced by $\ensuremath{\beta}$ irradiation at increasing doses, ranging from $1.2\ifmmode\times\else\texttimes\fi{}{10}^{3}$ to $5\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{kGy}$. We experimentally observe a progressive broadening of the luminescence band with increasing total electron dose released on samples. By analyzing our data within a theoretical model capable of separating homogeneous and inhomogeneous contribution to the total luminescence linewidth, we observe that the increasing of the width is entirely ascribable to t…
Twofold coordinated Ge defects induced by gamma-ray irradiation in Ge-doped SiO2
2008
We report an experimental study by photoluminescence, optical absorption and Electron Paramagnetic Resonance measurements on the effects of exposure of Ge-doped amorphous SiO2 to gamma ray radiation at room temperature. We have evidenced that irradiation at doses of the order of 1 MGy is able to generate Ge-related defects, recognizable from their optical properties as twofold coordinated Ge centers. Until now, such centers, responsible for photosensitivity of Ge-doped SiO2, have been induced only in synthesis procedures of materials. The found result evidences a role played by gamma radiation in generating photosensitive defects and could furnish a novel basis for photosensitive pattern wr…
Photoluminescence transient study of surface defects in ZnO nanorods grown by chemical bath deposition
2015
Two deep level defects (2.25 and 2.03 eV) associated with oxygen vacancies (Vo) were identified in ZnO nanorods (NRs) grown by low cost chemical bath deposition. A transient behaviour in the photoluminescence (PL) intensity of the two Vo states was found to be sensitive to the ambient environment and to NR post-growth treatment. The largest transient was found in samples dried on a hot plate with a PL intensity decay time, in air only, of 23 and 80 s for the 2.25 and 2.03 eV peaks, respectively. Resistance measurements under UV exposure exhibited a transient behaviour in full agreement with the PL transient, indicating a clear role of atmospheric O-2 on the surface defect states. A model fo…
Generation of oxygen deficient point defects in silica by γ and β irradiation
2007
We report an experimental study of the effects of y and β irradiation on the generation of a point defect known as ODC(II) in various types of commercial silica (a-SiO 2 ). The ODC(II) has been detected by means of photoluminescence (PL) spectroscopy measuring the PL band centered at 4.4 eV and excited at 5.0 eV associated to this defect. Our experiments show that ODC(II) are induced in all the investigated materials after irradiation at doses higher than 5 x 10 2 kGy. A good agreement is observed between the efficiencies of generation of ODC(II) under y and β irradiation, enabling a comprehensive study up to the dose of 5 x 10 6 kGy. Two different growth rates, one in the low and one in th…
Relaxation processes of point defects in vitreous silica from femtosecond to nanoseconds
2008
We studied ultrafast relaxation of localized excited states at Ge-related oxygen deficient centers in silica using femtosecond transient-absorption spectroscopy. The relaxation dynamics exhibits a biexponential decay, which we ascribe to the departure from the Frank-Condon region of the first excited singlet state in 240 fs, followed by cooling in ∼10 ps. At later times, a nonexponential relaxation spanning up to 40 ns occurs, which is fitted with an inhomogeneous distribution of nonradiative relaxation rates, following a chi-square distribution with one degree of freedom. This reveals several analogies with phenomena such as neutron reactions, quantum dot blinking, or intramolecular vibrat…