Optical properties and frequency upconversion fluorescence in a Tm3+-doped alkali niobium tellurite glass

Optical spectroscopic properties of Tm3+-doped 60TeO(2)-10GeO(2)-10K(2)O-10Li(2)O-10Nb(2)O(5) glass are reported. The absorption spectra were obtained and radiative parameters were determined using the Judd-Ofelt theory. Characteristics of excited states were studied in two sets of experiments. Excitation at 360 nm originates a relatively narrow band emission at 450 nm attributed to transition D-1(2)-->F-3(4) of the Tm3+ ion with photon energy larger than the band-gap energy of the glass matrix. Excitation at 655 nm originates a frequency upconverted emission at 450 nm (D-1(2)-->F-3(4)) and emission at 790 nm (H-3(4)-->H-3(6)). The radiative lifetimes of levels D-1(2) and H-3(4) were measured and the differences between their experimental values and the theoretical predictions are understood as due to the contribution of energy transfer among Tm3+ ions. (C) 2003 American Institute of Physics.

UV-visible absorption characteristics of TEOS-derived and Cr-doped silica sonogels aged in different pH solutions and heat treated up to 600 degrees C

Cr-doped xerogels were obtained by sol-gel process from the acid-catalyzed and ultrasound-stimulated hydrolysis of tetraethoxysilane (TEOS) with addition of CrCl3.6H(2)O in water solution during the liquid step of the process. The gels were aged immersed in different pH solutions for about 30 days, after that they were allowed to dry. The samples were annealed at temperatures ranging from 40 to 600degreesC and analyzed by UV-visible absorption spectroscopy. Cr3+ is the preferable oxidation state of the chromium ion in the gels annealed up to 250-300degreesC, in the case of aging in solutions of pH=5 and 11. A high UV absorption below similar to320 nm, due to the host gel, and different absorption bands, depending on the temperature, due to the chromium ion were observed in the xerogels at temperatures below similar to250degreesC, in the case of aging in solutions of pH=1 and 2. These absorption bands have not been assigned. Above 300degreesC up to 600degreesC, Cr5+, and possibly Cr6+, are the preferable oxidation states of the chromium ion independent of the pH of the aging solution, so the xerogels turn to a yellowish appearance in all cases.

Red, green, and blue upconversion luminescence in ytterbium-sensitized praseodymium-doped lead-cadmium-germanate glass

Blue, green, red, and near-infrared upconversion luminescence in the wavelength region of 480-740 nm in Pr3+/Yb3+-codoped lead-cadmium-germanate glass under 980 nm diode laser excitation, is presented. Upconversion emission peaks around 485, 530, 610, 645, and 725 nm which were ascribed to the P-3(0)-H-3(J) (J = 4, 5, and 6), and P-3(0)-F-3(J) (J = 2, 3, and 4), transitions, respectively, were observed. The population of the praseodymium upper P-3(0) emitting level was accomplished through a combination of ground-state absorption of Yb3+ ions at the F-2(7/2), energy-transfer Yb3+(2F(5/2))-Pr3+(H-3(4)), and excited-state absorption of Pr3+ ions provoking the (1)G(4)-P-3(0) transition. The dependence of the upconversion luminescence upon the Yb3+-concentration and diode laser power, is also examined, in order to subsidize the proposed upconversion excitation mechanism. (C) 2004 Elsevier B,V. All rights reserved.

Intense red upconversion emission in infrared excited holmium-doped PbGeO3-PbF2-CdF2 transparent glass ceramic

Intense red upconversion emission around 650 nm in PbGeO3-PbF2-CdF2 transparent glass ceramic containing beta-PbF2:Ho3+ nanocrystals, is presented. The holmium-doped vitroceramic samples were excited by a 980 nm diode laser source. The 650 nm upconversion signal was assigned to the F-5(5) --> I-5(8) transition of holmium ions. Very low intensity signals around 490 and 540 nm corresponding to the F-5(2,3) --> I-5(8) and S-4(2), F-5(4) --> I-5(8) transitions, respectively, were also detected. The upconversion excitation mechanism was achieved through a combination of stepwise phonon-assisted multiphoton absorption, cross-relaxation processes involving pairs of holmium ions, and excited-state absorption. Using a diode laser pump source around 850 nm green upconversion emission around 540 nm was the observed predominant signal. (C) 2004 Elsevier B.V. All rights reserved.

The influence of crystallization route on the properties of lanthanum-doped Bi4Ti3O12 thin films prepared from polymeric precursors

Pure and lanthanum-doped Bi4Ti3O12 thin films were deposited on Pt/Ti/SiO2/Si substrate using a polymeric precursor solution. The spin-coated films were specular and crack-free and crystalline after annealing at 700 degreesC for 2 h. Crystallinity and morphological evaluation were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Multilayered films obtained using the intermediate-crystalline layer route present a dense microstructure with spherical grains. Films obtained using the intermediate-amorphous layer, present elongated grains around 250 nm in size. The dielectric and ferroelectric properties of the lanthanum-doped Bi4Ti3O12 films are strongly affected by the crystallization route. The hysteresis loops are fully saturated with a remnant polarization and drive voltage of the films, heat-treated by the intermediate-crystalline (P-r = 20.2 muC/cm(2) and V = 1.35 V) and for the film heat-treated by amorphous route (P-r = 22.4 muC/cm(2) and V = 2.99 V). (C) 2004 Elsevier B.V. All rights reserved.

A theoretical analysis of the TiO2/Sn doped (110) surface properties

We have used the periodic quantum-mechanical method with density functional theory at the B3LYP level in order to study TiO2/Sn doped (1 1 0) surfaces and have investigated the structural, electronic and energy band properties of these oxides. Our calculated relaxation directions for TiO2 is the experimental one and is also in agreement with other theoretical results. We also observe for the doped systems relaxation of lattice positions of the atoms. Modification of Sri, O and Ti charges depend on the planes and positions of the substituted atoms. Doping can modify the Fermi levels, energy gaps as well as the localization and composition of both valence and conduction band main components. Doping can also modify the chemical, electronic and optical properties of these oxides surfaces increasing their suitability for use as gas sensors and optoelectronic devices. (c) 2005 Elsevier B.V. All rights reserved.

Sintering of ZnO doped SnO2

Dense SnO2 ceramics were obtained by doping with ZnO concentrations varies from 0.5 to 5.0 mel. The obtained powder was isostaticaly pressed to 210MPa in cylindrical shape and sintered from ambient to 1.500 degrees C using constant heating rate of 10 degrees C/min. Densities above 97% were obtained depending on the ZnO doping concentration. A maximum density was reached with the addition of 2 mol%. It can be concluded from shrinkage data and the observed microstructure obtained by SEM that the solid solution limit for ZnO in SnO2 is about 1.0 mol%. Above this concentration the formation of a second ceramic phase is observed.

Optical properties and energy transfer processes in (Tm3+, Nd3+) doped tungstate fluorophosphate glass

We investigate the linear optical properties and energy transfer processes in tungstate fluorophosphate glass doped with thulium (Tm3+) and neodymium (Nd3+) ions. The linear absorption spectra from 370 to 3000 nm were obtained. Transitions probabilities, radiative lifetimes, and transition branching ratios were determined using the Judd-Ofelt [Phys. Rev. 127, 750 (1962); J. Chem. Phys. 37, 511 (1962)] theory. Frequency up-conversion to the blue region and fluorescence in the infrared were observed upon pulsed excitation in the range of 630-700 nm. The excitation spectra of the luminescence were obtained to understand the origin of the signals. The temporal decay of the fluorescence was measured for different concentrations of the doping ions. Energy transfer rates among the Tm3+ and Nd3+ ions were also determined.

Photoluminescence temperature dependence of doped parabolic quantum wells

Parabolic quantum wells (PQWs) have been studied by temperature dependent photoluminescence (PL). Two kind of samples have been studied. Concerning the undoped sample, the dominant luminescences were the bulk GaAs and the fundamental transition of the PQW. The evolution on temperature of the energy position of both PL emissions follows the well known Varshing formula. For the doped samples strong radiative recombination of the electron gas with photogenerated holes was observed. At low temperature strong Fermi level enhancement occurs in the luminescence as a result of the multi-electron-hole scattering, which is smear out increasing the temperature.

Sintering and crystallite growth of nanocrystalline copper doped tin oxide

The effect of Cu2+ contents and of firing temperature on sintering and crystallite growth of nanocrystalline SnO2 xerogels was analyzed by thermoanalysis (mass loss (TG), linear shrinkage, and differential thermal analysis (DTA)), X-ray powder diffraction (XRPD), and EXAFS (extended X-ray absorption fine structures) measurements. Samples were prepared by two methods: (a) coprecipitation of a colloidal suspension from aqueous solution containing both Sn(IV) and Cu(II) ions and (b) grafting copper(II) species on the surface of tin pride gel. The thermoanalysis has shown that the shrinkage associated with the mass loss decreases by increasing the amount of copper. The EXAFS measurements carried out at the Cu K edge have evidenced the presence of copper in substitutional solid solution for the dried xerogel prepared with 0.7 mol % of copper, while for higher concentration of doping, copper has been observed also at the external surface of crystallites. The solid solution is metastable and copper migrates toward the surface during firing. The XRPD and DTA results have shown a recrystallization process near 320 degrees C, which leads to crystallite growth. The presence of copper segregated near the crystallite surface controls its growth.

Synthesis and luminescence properties of water dispersible Eu3+-doped boehmite nanoparticles

Nanocrystallized boehmite gamma-AlOOH center dot nH(2)O had been synthesized by spray-drying (SD) of a solution of aluminium tri-sec-butoxide peptized by nitric acid. The sub-micronic spherical particles obtained had an average diameter of 500 nm and were built of 100 nm or less platelet-like sub-particles. The average crystallite size calculated from XRD was 1.6 nm following the b axis (i.e. one unit cell) and 3-4 nm perpendicular to b. As a result of the nanometric sizes of crystallites, there was a large surface free for water adsorption and it was found to be n = 1.18 +/- 0.24H(2)O per AlOOH. The SD spheres spontaneously dispersed in water at room temperature and formed stable-over months-suspensions with nanometre-size particles (25-85 nm). Luminescent europium-doped nanocrystallized boehmites AlOOH: Eu (Al0.98Eu0.02OOH center dot nH(2)O) were synthesized the same way by SD and demonstrated the same crystallization properties and morphologies as the undoped powders. It is inferred from the Eu3+ luminescence spectroscopy that partly hydrated europium species are immobilized on the boehmite nanocrystals where they are directly bonded to alpha(OH) groups of the AlOOH surface. The europium coordination is schematically written [Eu3+(OH)(alpha)(H2O)(7-alpha/2)]. The europium-doped boehmite from SD spontaneously dispersed in water: the luminescence spectroscopy proves that most of the Eu3+ ions were detached from the NPs during water dispersion. The AlOOH: Eu nanoparticles were modified by the amine acid asparagine (ASN). The modification aimed to render the NPs compatible for further bio-functionalization. After surface modification, the NPs easily dispersed in water; the luminescence spectra after dispersion prove that the Eu3+ ions were held at the boehmite surface.

Luminescence in semi-crystalline zirconium titanate doped with lanthanum

This paper aims to describe the synthesis of the semi-crystalline and crystalline powder of lanthanum doped with zirconium titanate (65/35), LZT through Pechini method. The analysis done by Raman demonstrated that semi-crystalline phase at 550 degrees C and crystalline phase after 600 degrees C were formed. The XRD pattern shows the ZrTiO4 phase formation demonstrating that La substitutions into the lattice take place. The calcined powder at different temperatures shows a semi-crystalline phase presenting photoluminescence effect when processed at low temperatures. From 300 to 400 degrees C a broadband is observed at 563 nm and 568 nm, respectively. Defects creation such as: Zr3+ center dot Vo(center dot center dot) and Ti3+ - V-O(center dot center dot), Zr and Ti replaced by La with vacancy formation, impurities and imperfections contributed to the photoluminescence effect. However, the main emission is due to a reverse Ti4+ -> O2- or/and Zr4+ -> O2- transition that occur within a regular titanate or zirconate eight-fold coordination [BO8-delta], B = Zr4+, Ti4+. (c) 2006 Elsevier B.V. All rights reserved.

Local and global magnetic properties of Zn1-xCoxO and Mn-doped GaAs thin films

Amorphous and crystalline thin films of Mn-doped(0.5%-10%) GaAs and crystalline thin films of Zn1-xCoxO(x = 3%-20%) were investigated by means of magnetic susceptibility and electron spin resonance (ESR). For the Mn-doped GaAs samples, our results show the absence of ferromagnetic ordering for the amorphous films in the 300 > T > 2 K temperature range, in contrast to the ferromagnetism found in crystalline films for T-C < 110 K. A single ESR line with a temperature independent g-value (g similar to 2) is observed for the amorphous films, and the behavior of this ESR linewidth depends on the level of crystallinity of the film. For the Mn-doped GaAs crystalline films, only a ferromagnetic mode is observed for T < TC when the film is ferromagnetic. Turning now the Zn1-xCoxO films, ferromagnetic loops were observed at room temperature for these films. The magnetization data show an increasing of the saturation magnetization M. as a function of x reaching a maximum value for x approximate to 10%. ESR experiments at T = 300 K in the same films show a strong anisotropic ferromagnetic mode (FMR) for x = 0.10.

A new amperometric biosensor for salicylate based on salicylate hydroxylase immobilized on polipyrrole film doped with hexacyanoferrate

An amperometric biosensor for salicylate detection was developed by immobilizing salicylate hydroxylase via glutaraldehyde onto a polypyrrole film doped with hexacyanoferrate, supported on a glassy carbon electrode surface. The sensor monitors the catechol produced in the enzymatic reaction on the film surface, at an applied potential of 150 mV vs. SCE. A [NADH]/[salicylate] ratio between 2 and 4 gave the best response. The biosensor presented the best performance in a solution with pH=7.4. The response time was about 40 s. A linear range of response was observed for salicylate concentrations between 1.0x10(-5) and 1.0x10(-4) mol l(-1) and the equation adjusted for this curve was I=(-0.04+/-0.01)+(11.4+/-0.2)[salicylate] with a correlation coefficient of 0.999 for n=6. The biosensor retains its activity for at least 10 days despite daily use. The results obtained using the biosensor for salicylate determination, in three different samples of antithermic drugs, presented a good correlation with the standard colorimetric method.

Magnetic behavior of interstitial Fe impurities in divalent Ca, Sr, and Yb hosts

We have used a first-principles real-space approach to investigate the electronic structure and the magnetic behavior of interstitial Fe impurities in divalent Ca, Sr, and Yb hosts. The dependence of the local moment as a function of lattice relaxation around the impurity is obtained and contrasted with that of interstitial Fe in trivalent and tetravalent Zr, Y, Ti, and Sc hosts. The trends obtained for local moment formation at the impurity site an in agreement with experimental time-differential perturbed gamma-ray angular distribution technique observations.