Manuel Ocaña

Photonic crystal properties of packed submicrometric SiO2 spheres

In this letter, we investigate the optical properties of packed monodisperse silica submicron spheres by means of optical transmission measurements. The results are compatible with a three dimensional face centered cubic order in these solid structures. The lattice parameter of these structures, and therefore their optical properties, can be easily tuned through the sphere size (between 200 and 700 nm) thus covering the whole visible and near infrared spectrum.

Nanoparticle-based one-dimensional photonic crystals.

Herein we present a fast, reliable method for building nanoparticle-based 1D photonic crystals in which a periodic modulation of the refractive index is built by alternating different types of nanoparticles and by controlling the level of porosity of each layer. The versatility of the method is further confirmed by building up optically doped photonic crystals in which the opening of transmission windows due to the creation of defect states in the gap is demonstrated. The potential of this new type of structure as a sensing material is illustrated by analyzing the specific color changes induced by the infiltration of solvents of different refractive indexes.

Factors affecting the infrared and Raman spectra of rutile powders

The utility of infrared and Raman spectroscopy in the characterization of powdered samples is illustrated for the case of TiO2 (rutile). Particle shape and state of aggregation of the microcrystals are shown to have a strong effect on the infrared powder spectra as predicted by the Theory of the Average Dielectric Constant. On the other hand, surface effects related to particle size can be observed in the Raman spectra by the appearance of new bands as the surface to volume ratio increases. Finally, it is shown that the nonstoichiometric character of rutile does not have a significant effect in the infrared powder spectra.

Response of Nanoparticle-Based One-Dimensional Photonic Crystals to Ambient Vapor Pressure

Herein we report an analysis of the variation of the optical properties of different nanoparticle-based one-dimensional photonic crystal architectures versus changes in the ambient vapor pressure. Gradual shift of the optical response provides us with information on the sorption properties of these structures and allow us to measure precise adsorption isotherms of these porous multilayers. The potential of nanoparticle-based one-dimensional photonic crystals as base materials for optical sensing devices is demonstrated in this way.

Optical properties of α-Fe2O3 microcrystals in the infrared

Since haematite ( alpha -Fe2O3) microcrystals are available with well defined shape (sphere, spindle and disc) and size homogeneity, the authors have been able to check the validity of the classical theory of absorption of radiation by small particles in the infrared. The agreement between observed and calculated infrared spectra is very good once particle aggregation is taken into consideration. Comparison between calculated and experimental IR spectra also affords a useful method for determination of the relative orientation of the crystallographic axes in the morphological coordinate frame.

An ionic liquid based synthesis method for uniform luminescent lanthanide fluoride nanoparticles

We describe a facile procedure for the synthesis of uniform lanthanide fluoride nanophosphors by homogeneous precipitation in ethylene glycol solutions containing lanthanide precursors and an ionic liquid (1-butyl, 2-methylimidazolium tetrafluoroborate). It is shown that the use of this ionic liquid as a fluoride source, an appropriate choice of the solvent and the lanthanide precursor, and the adjustment of reaction temperature, are essential to obtain uniform nanoparticles. This method is applied to the preparation of pure YF3, EuF3 and TbF3 nanoparticles as well as of Eu-doped YF3 and Tb-doped YF3. In most cases, highly uniform nanoparticles were obtained, the size of which could be tuned in the nanometer range by adjusting the nature and concentration of the starting lanthanide precursor. The luminescent properties of the synthesized materials are also evaluated.

Well-defined colloidal tin(IV) oxide particles

Spherical and rod-like SnO 2 particles of narrow size distribution have been obtained by aging at 100°C acidified tin(IV) chloride solutions in the presence of urea or formamide. It was shown that spherical particles, the x-ray diffraction of which was characteristic of cassiterite, consisted of a large number of much smaller subunits. The rod-like particles had the same structure, but of higher degree of crystallinity. Infrared spectra of these powders were evaluated in terms of the theory of the average dielectric constant (TADC), in order to gain additional information on the particle morphology and the state of aggregation.

OPTICAL CONSTANTS OF TETRAGONAL AND CUBIC ZIRCONIAS IN THE INFRARED

The infrared optical constants of cubic and tetragonal zirconias were determined from the analysis of the reflectance spectra of yttria doped zirconia single crystals and undoped tetragonal zirconia powders, respectively. In the latter case, the reflectance spectrum was analyzed by an effective medium Theory, which has been proven to successfully determine the dielectric properties of small powder particles. The symmetry of the resulting modes of tetragonal zirconia was elucidated by performing theoretical calculations of the vibrational properties.

The variability of the infrared powder spectrum of amorphous SiO2

The assignment of a shoulder at about 1200 cm−1 in the infrared spectrum of a film of amorphous silica to a longitudinal mode is experimentally confirmed. On the basis of this assignment we analyze the influence of particle shape and state of aggregation on the infrared spectra of amorphous SiO2 powders, by using the theory of Average Dielectric Constant. The analysis shows a general tendency of the SiO2 particles to adopt plate-like morphologies. The utility of the infrared spectroscopy in the morphological characterization of powdered materials is demonstrated.

Oxidation state and localization of chromium ions in Cr-doped cassiterite and Cr-doped malayaite

Abstract The oxidation state and the localization of the chromium ions in the ceramic matrix of Cr-doped cassiterite (SnO 2 ) and Cr-doped malayaite (CaSnSiO 5 ) pigments have been investigated through the use of X-ray absorption near-edge (XANES), extended X-ray absorption fine structure (EXAFS), optical absorption and electron spin resonance (ESR) spectroscopies as well as by measurements of the unit cell parameters and the magnetic susceptibility of the pigments. We have found that three types of chromium species are present in the Cr-doped cassiterite pigments; a majority phase consisting of Cr(III) oxide clusters, a small amount of CrO 2 nanoparticles and Cr(IV) ions dissolved in the cassiterite lattice (∼5% of the total Cr amount), which must be responsible for the violet colour of this pigment. In the case of Cr-doped malayaite, most chromium cations are tetravalent forming a solid solution with the malayaite lattice by mainly substituting for Sn(IV) cations in isolated octahedral positions, although a very small amount of Cr(IV) also substitutes for tetrahedral Si(IV).