A.C. Yanes

Nanocrystal-size selective spectroscopy in SnO2:Eu3+ semiconductor quantum dots

Glass ceramics of composition 95SiO2-5SnO2 doped with 0.4mol% Eu3+ have been prepared by thermal treatment of sol-gel glasses. The segregated SnO2 nanocrystals present a mean size comparable to the bulk exciton Bohr radius (about 2.4nm), corresponding to a wide band-gap quantum-dot system in an insulator SiO2 glass. A fraction of the Eu3+ ions is incorporated to the SnO2 nanocrystals in the process. In these strong confinement conditions, the energy gap presents a high dependence on the nanocrystal size. Taking advantage of this effect, it has been possible to excite selectively the Eu3+ ions located in the SnO2 nanocrystals, by energy transfer from the host, obtaining emission spectra that depend on the nanocrystal size. The Eu3+ ions environment in small nanocrystals (radius under 2nm) are very distorted, meanwhile they are like crystalline for nanocrystals with a radius of some nanometers.

Cooperative energy transfer in Yb3+–Tb3+ codoped silica sol-gel glasses

Optical properties of Yb3+–Tb3+ codoped silica sol-gel samples have been studied after the gel to glass transition. Different upconversion emissions have been observed under near infrared excitation at about 1 μm. The Tb3+ ions are excited by means of energy transfer processes from Yb3+ ions. The temporal evolution of the blue-green upconversion emissions coming from Tb3+ ions and their dependence on the excitation intensity at about 1 μm has been studied. The experimental results are in good agreement with a cooperative resonant energy transfer mechanism from Yb3+ ions. An efficient backtransfer process is observed from Tb3+ ions towards Yb3+ ions. The upconversion efficiency, which is limited by this backtransfer process, has been obtained and compared with other upconversion results in similar matrix.

Luminescent properties of transparent nanostructured Eu3+ doped SnO2–SiO2 glass-ceramics prepared by the sol–gel method

Nanostructured silica based ceramic samples of composition (100− x)SiO2–xSnO2, doped with 0.4 mol% of Eu3+ and with x from 1 to 10, have been developed after a thermal treatment of precursor sol–gel glasses. A structural analysis has been performed by x-ray diffraction and high resolution transmission electron microscopy. The mean radius of the obtained SnO2 nanocrystals, ranging from 2 to 5 nm, is comparable to the exciton Bohr radius, corresponding to wide bandgap semiconductor quantum dots in an insulator SiO2 glass. A spectroscopy study in terms of emission and excitation spectra has been carried out as a function of SnO2 concentration. Moreover, time-resolved fluorescence measurements have also been performed in order to discern the emission of ions in glassy and nanocrystalline environments. The nanocrystal sizes have been obtained and compared by using the Brus and Scherrer equations.

Structural and dielectric characterization of cadmium tartrate

Polycrystalline samples of dimeric cadmium tartrate, [(Cd,C4H4O6)2H2O)]3H2O [labeled CdT(I)], were studied using impedance measurements and x-ray powder diffraction. The dependence of the real part of the dielectric constant on temperature showed a sharp peak at about 65 °C, revealing a structural phase transition, while the other broad peak in the temperature range (70<T<85 °C) was due to the loss of water molecules. The x-ray powder diffraction patterns at three temperatures (25, 60, and 70 °C) are consistent with three nonequivalent space groups. According to these results, it seems that this compound undergoes two successive phase transitions: P212121→P21→Pnmn, suggesting an intermediate ferroelectric behavior, labeled CdT(II) between a paraelectric CdT(I) and an anhydrous phase, labeled CdT(III).

Increase in the Tb3+ green emission in SiO2–LaF3 nano-glass-ceramics by codoping with Dy3+ ions

95SiO2–5LaF3 sol-gel derived nano-glass-ceramics single doped with 0.1Dy3+ or 0.1Tb3+ mol % and codoped with 0.1Dy3+ and xTb3+ (x=0.1,0.3) mol % were successfully obtained. XRD and HRTEM measurements confirm the precipitation of LaF3 nanocrystals during the ceramming process, with mean size ranging from 10 to 20 nm and increasing with the thermal treatment temperature. About 75% of lanthanide ions are partitioned into LaF3 nanocrystals, as calculated from luminescence decays. The effect of increasing the Tb3+ concentration and also of codoping with Dy3+ in the Tb3+ green emission from the D54 level have been studied. The energy transfer mechanisms between Tb3+ ions and also between Tb3+–Dy3+ ions, which favor the green emission, have been analyzed.

Characterization and thermal and electromagnetic behaviour of gadolinium-doped calcium tartrate crystals grown by the solution technique

Energy dispersive analysis by X-rays (EDAX), X-ray diffraction (XRD), infrared (IR), thermogravimetric (TG), differential thermogravimetric (DTG), differential scanning calorimetric (DSC) and electromagnetic studies carried out on crystalline materials obtained by diffusion of gadolinium and calcium ions through silica gel impregnated with tartaric acid and grown by means of slow cooling between 72°C and 40°C are reported. EDAX confirmed the presence of gadolinium. X-ray diffraction data did not show any appreciable differences between the structures of doped and undoped calcium tartrate. IR in the range of 500–4000 cm−1 and the description of peaks recorded for the material are given. Results of thermal analysis (TG, DTG and DSC) indicated that the material is thermally unstable. Dielectric and magnetic measurements were determined in the frequency range 45 MHz to 2 GHz.

Site-selective spectroscopy in Sm3+-doped sol–gel-derived nano-glass-ceramics containing SnO2 quantum dots

Nano-glass-ceramics of composition 95SiO(2)-5SnO(2) doped with 0.4 Sm(3+) (mol%) were synthesized by the thermal treatment of precursor sol-gel glasses. Structural and luminescence measurements were carried out. The precipitated SnO(2) nanocrystals in the glass matrix constitute a wide bandgap quantum-dot system with size comparable to the bulk exciton Bohr radius. A site-selective excitation, by energy transfer from the semiconductor host, reveals that a fraction of the Sm(3+) ions are incorporated in the SnO(2) nanocrystals, whereas the rest remains in the silica glassy phase. An evolution in the Sm(3+) emission spectra has been observed when the SnO(2) nanocrystals are excited with different UV wavelengths, which has been ascribed to selective excitation of nanocrystal sets with predetermined size.

Chromenes and benzofurans from Ageratina glechonophylla

Abstract A re-examination of the aerial part of Ageratina glechonophylla yielded spathulenol (1,6-dihydroxy-4(14)-eudesmene) as a natural product, six known chromenes, a mixture of three dimeric chromenes, four thymol derivatives and 3-(hydroxymethyl)-3-(acetoxymethyl)-and 3-(isobutyryloxymethyl)-6-methylbenzofuran were also identified in this plant.

Electrical conductivity of doped and undoped calcium tartrate

The electrical conductivity of polycrystalline samples of calcium tartrate tetrahydrate ([CaC4H4O6 2H2O] 2H2O) in pure form and doped with barium and with strontium were studied in the temperature range (65<T<95°C). According to these results, it seems that two types of conduction exist in these compounds, one at low temperature and the other at high temperature, by the way of extrinsic and intrinsic conduction, respectively. This behavior may be attributed to the rotation of the tartrate ions by thermal energy.

Diterpenes and other constituents of Eupatorium salvia

Abstract An ethanol extract of the aerial parts of Eupatorium salvia was re-examined and five labdane-type diterpenes, one sesquiterpene and two flavonoids were obtained. Structures were elucidated by spectroscopic methods and chemical transformations.