R. A. Sá Ferreira

Full‐Color Phosphors from Amine‐Functionalized Crosslinked Hybrids Lacking Metal Activator Ions

Sol–gel derived hybrids that contain OCH2CH2 (polyethylene glycol, PEG) repeat units grafted onto a siliceous backbone by urea, –NHC(=O)NH–, or urethane, –NHC(=O)O–, bridges have been prepared. It is demonstrated that the white light PL of these materials results from an unusual convolution of a longer lived emission that originates in the NH groups of the urea/urethane bridges with shorter lived electron–hole recombinations occurring in the nanometer-sized siliceous domains. The PL efficiencies reported here (maximum quantum yields at room temperature of ≈ 0.20 ± 0.02 at a 400 nm excitation wavelength) are in the same range as those for tetramethoxysilane–formic acid, and APTES–acetic acid, sol–gel derived phosphors. The high quantum yields combined with the possibility of tuning the emission to colors across the chromaticity diagram present a wide range of potential applications for these hybrid materials.

Photoluminescence and lattice location of Eu and Pr implanted GaN samples

Rare earth (RE) ions implanted GaN films were studied by optical spectroscopy and RBS techniques. Sharp emission lines due to intra-4f n shell transitions can be observed even at room temperature for the Eu 3+ and Pr 3+ .Th e photoluminescence spectra recorded by the above band gap excitation reveal dominant transitions due to the 5 D0- 7 F1,2,3 lines at 6004, 6211 and 6632 ( A for the Eu 3+ and 3 P0,1- 3 F2,3 at 6450 and 6518 ( A, respectively, for the Pr 3+ . We report on the temperature dependence of the intra-ionic emissions as well as on the lattice site location of the RE detailed angular scans through the / 00 0 1S and / 10 % S axial directions; which indicates that for Pr, complete substitutionality on the Ga sites was achieved while for Eu a Ga displaced site was found. r 2001 Elsevier Science B.V. All rights reserved.

Morphological and conductivity studies of di-ureasil xerogels containing lithium triflate

AbstractSol / gel derived poly(oxyethylene)/siloxane hybrids doped with lithium triflate, LiCF 3 SO ,have been investigated. The hosthybrid matrix of these materials, named di-ureasil and represented by U(600), is composed by a siliceous framework to whichpolyether chains containing 8.5 oxyethylene repeat units are covalently bonded through urea linkages. Xerogel samplesU(600) n LiCF 3 SO 3 with n (where n is the molar ratio of oxyethylene moieties per Li ion) between and 0.1 have beenexamined. X-ray diffraction and differential scanning calorimetry have provided conclusiveevidence that the xerogels analyzed areentirely amorphous. The salt-rich material with n / 1 exhibits the highest conductivity over the whole range of temperature analyzed(e.g. 4.3 6 / 10 and 2.0 / 10 4 V 1 cm , respectively, at 25 and 94 8C). # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Di-ureasils; Li ; X-ray diffraction; Differential scanning calorimetry; Ionic conductivity 1. IntroductionHighly conducting, thin, solvent-free electrolyte filmsmay be produced by dissolving salts in high molecularweight polymers carrying adequate coordinating species,such as ether oxygen atoms or amine nitrogen atoms [1

Sol–gel-derived POE/siliceous hybrids doped with Na+ ions: morphology and ionic conductivity

Abstract Hybrid materials produced by the versatile sol–gel process with a wide sodium triflate, NaCF 3 SO 3 , concentration range have been studied. The host matrix of these xerogels, designated as di-ureasil and represented by U(600), is composed by a siliceous network bonded through covalent bonds to short polyether chains by means of bridging urea groups. The results obtained reveal that at high salt concentration, this system forms crystalline phases. The most conducting ormolyte exhibits a conductivity of 3.6×10 −7 Ω −1 cm −1 at room temperature.

An intra-Nd3+ visible to infrared conversion process in hybrid xerogels

Abstract The luminescence spectra of a series of Nd 3+ -based hybrid xerogels were reported. The organic/inorganic matrix, classed as di-ureasils, is a siliceous network to which short polyether-based chains are grafted by means of urea bridges. The luminescent centers were incorporated as neodymium triflate. These multi-wavelength phosphors present a broad green–blue emission band associated with the hybrid backbone and a series of infrared emissions which were assigned to the 4 F 3/2 → 4 I 9/2–13/2 intra-4 f 3 lines. The visible emission spectra provides strong evidence for the existence of an unusually intra-Nd 3+ conversion process of visible light in infrared luminescence.

Room temperature visible/infrared emission and energy transfer in Nd3+-based organic/inorganic hybrids

The photoluminescence features and the energy transfer processes of Nd3+-based siloxane-poly(oxyethylene) hybrids are reported. The host matrix of these materials, classed as di-ureasils, is formed by a siloxane backbone covalently bonded to polyether chains of two molecular weights by means of urea cross-links. The room-temperature photoluminescence spectra of these xerogels show a wide broad purple-blue-green band (350–570 nm), associated with the emitting centres of the di-ureasil host, and the typical near infrared emission of Nd3+ (700–1400 nm), assigned to the 4F3/2 → 4I9/2,11/2,13/2 transitions. Self-absorptions in the visible range, resonant with intra-4f3 transitions, indicate the existence of an energy conversion mechanism of visible di-ureasil emission into near infrared Nd3+ luminescence. The existence of energy transfer between the di-ureasils emitting centres and the Nd3+ ions is demonstrated calculating the lifetimes of these emitting centres. The efficiency of that energy transfer changes both with the polymer molecular weight and the Nd3+ concentration.

An investigation of the morphological, electrical and optoelectronic properties of short chain Di-ureasils doped with Er3+ ions

Hybrid organic/inorganic xerogels containing Er3+ ions have been prepared by the solgel process. The hybrid framework of these compounds, designated as di-ureasils and represented by U(600), is composed of a siliceous backbone to which polyether chains containing 8.5 oxyethylene repeat units are linked through urea (-NHC(=O)NH-) bridges. The trivalent cations have been incorporated in the matrix as erbium triflate, Er(CF3SO3)3. Compositions with n (where n is the molar ratio of (OCH2CH2) repeat units per Er+ ion) between ∞ and 3 have been studied. The morphology of the materials was characterized by means of X-ray Diffraction and Differential Scanning Calorimetry. Complex Impedance spectroscopy was used to evaluate the levels of conductivity exhibited by the xerogels as a function of temperature. The results obtained clearly demonstrate the amorphous character of the samples analysed. At about 100 °C the most conducting sample is U(600)5Er(CF3SO3)3 (approximately 2×10−5 Scm−1), whereas the U(600)60Er(CF3SO3)3 compound displays the highest conductivity at room temperature (1×10−7 Scm−1). We have shown that concentration effects on the quenching of the 1.53 μm emission intensity (excited at 488 nm) are negligible. We have also shown that the Er3+ ions are active at room temperature.

Excitation energy dependence of luminescent sol-gel organically modified silicates

Abstract The light emission features of a new class of intensely luminescent organic/inorganic materials are reported. These sol-gel-derived hybrids contain short highly solvating oxyethylene units (OCH2CH2) grafted onto a silica network by means of urea cross-links. Three xerogels series designated as ureasils U(600), U(900) and U(2000), have been synthesized incorporating different units of oligopolyoxyethylene chains, approximately 8.5, 15.5 and 40.5, respectively. The emission spectra of these ureasils are characterized by a large broad band between 1.82 and 3.44 eV, with a blue ( 2.6 eV) and a purplish-blue ( 2.8–3.0 eV) component. These bands are associated with planar oxygen-based rich-silicon domains with different numbers of Si O structural units. The emission peak position, EP, of the three ureasils shift to higher energies with the increasing of the excitation energy, EX. For excitation energies between 3.10 and 3.76 eV a linear relation with a slope approximately equal to 0.5 has been found between EP and EX.

Short-chain di-ureasil ormolytes doped with potassium triflate: Phase diagram and conductivity behavior

Di-urea cross-linked poly(oxyethylene)/siloxane hybrids, synthesized by the sol-gel process and containing a wide concentration range of potassium triflate, KCF3SO3, have been analyzed by x-ray diffraction and differential scanning calorimetry. The pseudo-phase diagram proposed has been taken into account in the interpretation of the complex impedance measurements. The xerogels prepared are obtained as transparent, thin monoliths. At room temperature the highest conductivity found was 2 × 10−6 Ω−1 cm−1.

Short chain U(600) di-urea cross-linked poly(oxyethylene)/siloxane ormolytes doped with lanthanum triflate salt

Abstract Promising La 3+ -doped electrolytes based on a hybrid poly(oxyethylene)/siliceous host matrix, U(600), have been produced. The organic and inorganic components of the hybrid structure are covalently bonded through urea linkages. The low molecular weight of the polyether segments of U(600) is thought to be responsible for the total amorphous character and high conductivity at room temperature (1.1×10 −4 S cm −1 ) of these ormolytes.