1,721,332 research outputs found
Sviluppo di sistemi nanostrutturati per nanomedicina
No full textIl progetto di ricerca ha avuto come tematiche principali lo sviluppo, la caratterizzazione e l’applicazione sperimentale di sistemi nanostrutturati per scopi biomedici. L’attenzione si è focalizzata principalmente su matrici di tipo inorganico (CaF2, SrF2) drogate con ioni lantanidi che conferiscono proprietà ottiche (luminescenza in upconversion ed emissione Stokes) se opportunamente eccitate con radiazione elettromagnetica. La metodologia di sintesi sviluppata (idrotermale) può essere considerata come una “Green reaction” poiché non implica l’utilizzo o lo smaltimento di sostanze inquinanti e le condizioni di lavoro (temperature relativamente basse) non necessitano di consumi energetici elevati. La caratterizzazione di tali sistemi si può suddividere principalmente in due categorie: caratterizzazione ottica e caratterizzazione strutturale/morfologica
"Unusual Ln3+ substitutional defects": The local chemical environment of Pr3+ and Nd3+ in nanocrystalline TiO2 by Ln-K edge EXAFS
No full textThe local chemical environment of the trivalent lanthanide cations in anatase TiO(2) nanopowders doped with 1mol% of Pr or Nd, prepared via a sol-gel technique, has been studied by means of EXAFS at the Pr and Nd-K edge. Titanium dioxide can be considered an "unusual'' host for doping with Ln(3+) ions due to the large mismatch of both charge and ionic radii between the dopant and the host constituent cations. However, it can be demonstrated that the lanthanide ions enter the anatase structure as substitutional defects with respect to Ti, but that the amount of disorder around the substitutional defects is very large. For both Pr(3+) and Nd(3+) ions, the Ln-O and Ln-Ti distances have been found to increase by about 0.45 angstrom, with respect to what is found for the Ti-O and Ti-Ti distances in pure anatase. Valence-bond calculations have been used to validate the Ln-O distances obtained by the EXAFS fitting. Finally, no evidences for oxygen vacancies clustering around the substitutional defects have been found. Luminescence spectroscopy has shown that the lanthanide ions do not segregate in oxide or pyrochlore impurities phases
Optical spectroscopy of zinc metaphosphate glasses activated by Ce3+ and Tb3+ ions
No full textThe photoluminescence of Ce3+ and Tb3+ ions in zinc metaphosphate glasses is investigated. The blue and green emissions of Tb3+ ions are enhanced upon UV excitation through energy transfer from Ce3+ to Tb3+ ions. The efficiency of such an energy transfer was estimated based on spectroscopic data and resulted in being about 20-23%. Spectroscopic data revealed that the energy transfer occurs via a non-radiative process inside Ce3+-Tb3+ clusters formed in the glass. This ion clustering could be useful for the design of efficient conversion phosphors of ultraviolet to blue and green light
Crystal structure and optical spectroscopy of Ca3Ln2Si3O12 (Ln = Gd and Lu) doped with Eu3+
No full textThe luminescence of Eu3+ introduced as impurity in Ca3Gd2Si3O12, and Ca3Lu2Si3O12 has been investigated. According to the crystallographic evidences, three sites for the dopant ion are available with different occupation in the two hosts. In both matrixes, the dominant 5D0 → 7F2 emission line reveals a low symmetry of the Eu3+ crystal sites. In the excitation spectrum of Eu doped Ca3Gd2Si3O12 a Gd3+ → Eu3+ ET process is clearly visible
Local Chemical Environment of Pr3+ Substitutional Defects in Bulk and Nanocrystalline Gd3Ga5O12: A Joint EXAFS and Luminescence Study
No full textA joint approach using EXAFS (extended X-ray absorption fine structure) and luminescence results has been used to obtain information on the local structure and the electron-phonon coupling of the Pr3+ ion in bulk and nanocrystalline gadolinium gallium garnet samples. The nanocrystalline sample is prepared with a combustion synthesis, whereas the bulk sample is prepared by a sintering procedure using the nanocrystalline powders. EXAFS measurements show that the Pr-O distances are similar for the bulk and the nanocrystalline samples but a notable amount of static disorder in the first coordination sphere of the dopant is found to be present in the nanocrystalline sample. From the behavior of the emission line width and shift of a Pr3+ emission band as a function of the temperature, the electron-phonon coupling strength of the Pr3+ ion has been evaluated. The electron-phonon coupling strength results to be notably stronger for the nanocrystalline sample than for the bulk one. This behavior is attributed to the fact that the local surrounding of the Pr3+ ion is softer for the nanocrystalline sample than for the bulk one
Unusual Ln(3+) substitutional defects: The local chemical environment of Eu3+ and Er3+ in nanocrystalline Nb2O5 by Ln-K edge EXAFS
No full textCOUPLING OF ELECTRONS TO INTERMOLECULAR PHONONS IN MOLECULAR CHARGE-TRANSFER DIMERS - A RESONANCE RAMAN-STUDY
No full textWe report resonance Raman scattering RRS spectra and Raman excitation profiles REP of a system containing dimers of identical molecular radical ions measured with laser excitation in resonance with the charge transfer CT transition. A Peierls–Hubbard PH Hamiltonian has been used to model the investigated system and to calculate its optical and RRS properties. Results are reported for two polyoxometallate salts of tetrathiafulvalene TTF, namely TTF2W6O19) and TTF2 Mo6 O19) whose structures contain almost isolated TTF ) 2 dimers. The RRS spectra of TTF2W6O19), measured in resonance with the CT absorption band centered at 832 nm, show 1 three phonon modes located at 55, 90, and 116 cm modes have been associated to the out-of-phase combinations of the translational motions of the two molecules composing the dimer. Such modes are effective in modulating the intradimer transfer integral, thus providing an efficient mechanism for coupling with the electronic system and for enhancement of the scattering intensity at resonance with the CT transition. The REP for the three strongly coupled modes of TTF2W6O19) have been measured with laser excitation wavelengths ranging from 740 to 930 nm. Quantitative analysis of the REP data has been performed based on a perturbative solution of the PH model to second order in the electron-molecular-vibration EMV and electron-intermolecular-phonon EIP interactions. The CT absorption profile and the REP’s have been calculated using a time correlator technique and the model parameters have been optimized in order to fit the experimental REP data. Infrared vibronic absorptions of TTF2W6O19), originated by the EMV coupling, have been measured and independent information on the electronic parameters of the PH model have been derived. This has made the choice of the fitting parameters used for the REP calculations rather unambiguous and has allowed us to obtain, for the first time, reliable experimental estimates of the EIP coupling constants
Investigation of Eu3+ site occupancy in cubic Y2O3 and Lu2O3 nanocrystals
No full textThe distribution of luminescent Eu3+ ions in crystals with the cubic bixbyite-type structure is subject of debate. In this work, the actual occupancy of the two cation sites available for europium in yttria and lutetia nanocrystals with sizes of 10-20 nm has been evaluated by means of Eu-151 Mossbauer spectroscopy. The spectral contribution of the ion at different crystalline sites has been resolved, allowing for the quadrupole splitting. The spectra of the nanocrystalline Y1.8Eu0.2O3 and Lu1.8Eu0.2O3 compounds have been analyzed in detail; the C-3i and C-2 sites are occupied by the europium ion in a statistical way. The hyperfine parameters have been discussed in terms of symmetry and bonding. An increase of the covalence of the Eu-O bond has been found in the series of compounds Gd2O3, Eu2O3, Y2O3 and Lu2O
Spectroscopic investigations of nanostructured LiNbO3 doped with Eu3+
No full textStructural and optical properties of the sol-gel derived nanocrystalline lithium niobate (LiNbO3) powders doped with Eu3+ ions have been studied. In particular, the influence of the sizes of nanoparticles controlled by temperature on the structural and luminescence properties has been investigated. Emission bands corresponding to 5D0 emission became more resolved with increasing nanocrystal size and changed to a typical Eu3+:LiNbO3 single crystal spectrum for nanocrystals having an average size of more than 40 nm. Nonlinear optical properties of nanostructured LiNbO3 have been confirmed by simple observation of second harmonic generation effect (SHG). The possibility of using nanostructured LiNbO3 doped with rare-earth ions as self-doubling elements in integrated optoelectronic devices has been discussed
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