Determinaci�� de la ubicaci�� estructural dels ions Yb3+ i Nb3+ en Yb:RTP, Nb:RTP i (Nb,Yb):RTP

Estudi elaborat a partir d���una estada al Stony Brook University al juliol del 2006. El RbTiOPO4 (RTP) monocristal���l�� ��s un material d' ��ptica no lineal molt rellevant i utilitzat en la tecnologia l��ser actual, qu��micament molt estable i amb unes propietats f��siques molt destacades, entre elles destaquen els alts coeficients electro-��ptics i l'alt llindar de dany ��ptic que presenta. En els ��ltims anys s���est�� utilitzant tecnol��gicament en aplicacions d'��ptica no lineal en general i electro-��ptiques en particular. En alguns casos ja ha substitu��t, millorant prestacions, a materials tals com el KTP o el LNB(1). Dopant RTP amb ions lant��nids (Ln3+) (2-4), el material es converteix en un material l��ser auto-doblador de freq����ncia, combinant les seves propietats no lineals amb les de matriu l��ser. El RTP genera radiaci�� de segon harm��nic (SHG) a partir d���un feix fonamental amb longituds d���ona inferiors a 990 nm, que ��s el l��mit que presenta el KTP.La determinaci�� de la ubicaci�� estructural i l���estudi de l'entorn local del ions actius l��ser ��s de fonamental import��ncia per a la correcta interpretaci�� de les propietats espectrosc��piques d���aquest material. Mesures de difracci�� de neutrons sobre mostra de pols cristal���l�� mostren que els ions Nb5+ i Ln3+ nom��s substitueixin posicions de Ti4+ (8-9). Estudis molt recents d'EPR (electron paramagnetic resonance) semblen indicar que quan la concentraci�� d'i�� Ln3+ es baixa, aquest i�� presenta la tend��ncia a substituir l'i�� alcal�� present a l'estructura (10).Despr��s dels resultats obtinguts en el present treball a partir de la t��cnica EXAFS a la instal���laci�� sincrot�� del Brookhaven National Laboratory/State University of New York (Stony Brook) es pot concloure definitivament que els ions Nb s���ubiquen en la posici�� Ti (1) i que els ions Yb3+ es distribueixen paritariament en les dues posicions del Ti (1 i 2). Aquests resultats aporten una valuosa informaci�� per a la correcta interpretaci�� dels espectres, tant d���absorci�� com d���emissi��, del material i per la avaluaci�� dels par��metres del seu comportament durant l'acci�� l��ser.

Current transport and electroluminescence mechanisms in thin SiO2 films containing Si nanocluster-sensitized erbium ions.

We have studied the current transport and electroluminescence properties of metal oxide semiconductor MOS devices in which the oxide layer, which is codoped with silicon nanoclusters and erbium ions, is made by magnetron sputtering. Electrical measurements have allowed us to identify a Poole-Frenkel conduction mechanism. We observe an important contribution of the Si nanoclusters to the conduction in silicon oxide films, and no evidence of Fowler-Nordheim tunneling. The results suggest that the electroluminescence of the erbium ions in these layers is generated by energy transfer from the Si nanoparticles. Finally, we report an electroluminescence power efficiency above 10���3%. �� 2009 American Institute of Physics. doi:10.1063/1.3213386

Structural factors impacting carrier transport and electroluminescence from Si nanocluster-sensitized Er ions.

We present an analysis of factors influencing carrier transport and electroluminescence (EL) at 1.5 ��m from erbium-doped silicon-rich silica (SiOx) layers. The effects of both the active layer thickness and the Si excess content on the electrical excitation of erbium are studied. We demonstrate that when the thickness is decreased from a few hundred to tens of nanometers the conductivity is greatly enhanced. Carrier transport is well described in all cases by a Poole-Frenkel mechanism, while the thickness-dependent current density suggests an evolution of both density and distribution of trapping states induced by Si nanoinclusions. We ascribe this observation to stress-induced effects prevailing in thin films, which inhibit the agglomeration of Si atoms, resulting in a high density of sub-nm Si inclusions that induce traps much shallower than those generated by Si nanoclusters (Si-ncs) formed in thicker films. There is no direct correlation between high conductivity and optimized EL intensity at 1.5 ��m. Our results suggest that the main excitation mechanism governing the EL signal is impact excitation, which gradually becomes more efficient as film thickness increases, thanks to the increased segregation of Si-ncs, which in turn allows more efficient injection of hot electrons into the oxide matrix. Optimization of the EL signal is thus found to be a compromise between conductivity and both number and degree of segregation of Si-ncs, all of which are governed by a combination of excess Si content and sample thickness. This material study has strong implications for many electrically driven devices using Si-ncs or Si-excess mediated EL.

onductivitat t��rmica i ��ndex de refracci�� no lineal dels compostos KRE(WO4)2 (RE=Gd,Y, Yb and Lu)

Projecte de recerca elaborat a partir d���una estada al Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy entre setembre i desembre 2007. Els materials monocristal���lins tungstats dobles de potassi i terra rara, KRE(WO4)2, a partir d'ara KREW, s��n en l'actualitat un material competitiu com a material actiu per sistemes de l��ser d'estat s��lid. Aquests materials monocl��nics s��n f��cils de dopar amb altres densitats de ions lant��nid, Ln3+, i a m��s presenten unes seccions eficaces d'absorci�� i d'emissi��, molt elevades. Dins d���aquesta fam��lia, destaca el KLuW; degut als seus millors resultats com a material l��ser. Durant aquesta estada d���un mes al laboratori Max Born de Berlin, s���han realitzat les mesures de conductivitat t��rmica d���aquest material, per tal de obtenir el seu tensor de segon ordre de conductivitat t��rmica. El bombeig ��ptic dels materials l��ser d���estat s��lid genera calor com a resultat de la termalitzaci�� en els multiplets, de les relaxacions no-radiatives i de les absorcions residuals (defectes, impureses). Per tant, el coneixement de les propietats t��rmiques de qualsevol material actiu ��s essencial pel disseny de la cavitat l��ser i l���avaluaci�� de la funci�� l��ser, especialment en r��gims d���altes pot��ncies(...)

Development of advanced silicon radiation detectors for high energy physics and medical imaging

The 1st chapter of this work presents the different experiments and collaborations in which I am involved during my PhD studies of Physics. Following those descriptions, the 2nd chapter is dedicated to how the radiation affects the silicon sensors, as well as some experimental measurements carried out at CERN (Geneve, Schwitzerland) and IFIC (Valencia, Spain) laboratories. Besides the previous investigation results, this chapter includes the most recent scientific papers appeared in the latest RD50 (Research & Development #50) Status Report, published in January 2007, as well as some others published this year. The 3rd and 4th are dedicated to the simulation of the electrical behavior of solid state detectors. In chapter 3 are reported the results obtained for the illumination of edgeless detectors irradiated at different fluences, in the framework of the TOSTER Collaboration. The 4th chapter reports about simulation design, simulation and fabrication of a novel 3D detector developed at CNM for ions detection in the future ITER fusion reactor. This chapter will be extended with irradiation simulations and experimental measurements in my PhD Thesis.

Metallic and nonmetallic shine in luster: An elastic ion backscattering study

Luster is a metal glass nanocomposite layer first produced in the Middle East in early Islamic times ( 9th AD) made of metal copper or silver nanoparticles embedded in a silica-based glassy matrix. These nanoparticles are produced by ion exchange between Cu+ and Ag+ and alkaline ions from the glassy matrix and further growth in a reducing atmosphere. The most striking property of luster is its capability of reflecting light like a continuous metal layer and it was unexpectedly found to be linked to one single production parameter: the presence of lead in the glassy matrix composition. The purpose of this article is to describe the characteristics and differences of the nanoparticle layers developed on lead rich and lead free glasses. Copper luster layers obtained using the ancient recipes and methods are analyzed by means of elastic ion backscattering spectroscopy associated with other analytical techniques. The depth profile of the different elements is determined, showing that the luster layer formed in lead rich glasses is 5���6 times thinner and 3���4 times Cu richer. Therefore, the metal nanoparticles are more densely packed in the layer and this fact is related to its higher reflectivity. It is shown that lead influences the structure of the metal nanoparticle layer through the change of the precipitation kinetics

Multifunctional ligands for application in Alzheimer���s Disease: molecular modelling and biological evaluation

Projecte de recerca elaborat a partir d���una estada a la University of British Columbia, Canad��, entre 2010 i 2012 La malaltia d'Alzheimer (MA) representa avui la forma m��s comuna de dem��ncia en la poblaci�� envellida. Malgrat fa 100 anys que va ser descoberta, encara avui no existeix cap tractament preventiu i/o curatiu ni cap agent de diagn��stic que permeti valorar quantitativament l'evoluci�� d'aquesta malaltia. L'objectiu en el que s'emmarca aquest treball ��s contribuir a aportar solucions al problema de la manca d'agents terap��utics i de diagnosi, un��vocs i rigorosos, per a la MA. Des del camp de la qu��mica bioinorg��nica ��s f��cil fixar-se en l'excessiva concentraci�� d'ions Zn(II) i Cu(II) en els cervells de malalts de MA, plantejar-se la seva utilitzaci�� com a dianes terap��utica i, en conseq����ncia, cercar agents quelants que evitin la formaci�� de plaques senils o contribueixin a la seva dissoluci��. Si b�� aquest va ser el punt de partida d���aquest projecte, els m��ltiples factors implicats en la patog��nesi de la MA fan que el cl��ssic paradigma d��� ��una mol��cula, una diana�� limiti la capacitat de la mol��cula de combatre aquesta malaltia tan complexa. Per tant, un esfor�� considerable s���ha dedicat al disseny d���agentsmultifuncionals que combatin els m��ltiples factors que caracteritzen el desenvolupament de la MA. En el present treball s���han dissenyat agents multifuncionals inspirats en dos esquelets moleculars ben establers i coneguts en el camp de la qu��mica medicinal: la tioflavina-T (ThT) i la deferiprona (DFP). La utilitzaci�� de t��cniques in silico que inclouen c��lculs farmacocin��tics i modelatge molecular ha estat un proc��s cabdal per a l���avaluaci�� dels millors candidats en base als seg��ents requeriments: (a) compliment de determinades propietats farmacocin��tiques que estableixin el seu possible ��s com a f��rmac (b) hidrofobicitat adequada per travessar la BBB i (c) interacci�� amb el p��ptid A������en soluci��.

Vacancy and interstitial depth profiles in ion-implanted silicon

An experimental method of studying shifts between concentration-versus-depth profiles of vacancy- and interstitial-type defects in ion-implanted silicon is demonstrated. The concept is based on deep level transient spectroscopy measurements utilizing the filling pulse variation technique. The vacancy profile, represented by the vacancy��oxygen center, and the interstitial profile, represented by the interstitial carbon��substitutional carbon pair, are obtained at the same sample temperature by varying the duration of the filling pulse. The effect of the capture in the Debye tail has been extensively studied and taken into account. Thus, the two profiles can be recorded with a high relative depth resolution. Using low doses, point defects have been introduced in lightly doped float zone n-type silicon by implantation with 6.8 MeV boron ions and 680 keV and 1.3 MeV protons at room temperature. The effect of the angle of ion incidence has also been investigated. For all implantation conditions the peak of the interstitial profile is displaced towards larger depths compared to that of the vacancy profile. The amplitude of this displacement increases as the width of the initial point defect distribution increases. This behavior is explained by a simple model where the preferential forward momentum of recoiling silicon atoms and the highly efficient direct recombination of primary point defects are taken into account.

Silicon nanocluster sensitization of erbium ions under low-energy optical excitation

The sensitizing action of amorphous silicon nanoclusters on erbium ions in thin silica films has been studied under low-energy (long wavelength) optical excitation. Profound differences in fast visible and infrared emission dynamics have been found with respect to the high-energy (shortwavelength) case. These findings point out to a strong dependence of the energy transfer process on the optical excitation energy. Total inhibition of energy transfer to erbium states higher than thefirst excited state (4I13/2) has been demonstrated for excitation energy below 1.82 eV (excitation wavelength longer than 680 nm). Direct excitation of erbium ions to the first excited state (4I13/2)has been confirmed to be the dominant energy transfer mechanism over the whole spectral range of optical excitation used (540 nm��680 nm).

Model for the emission of Si+ ions during oxygen bombardment of Si(100) surfaces

The variation in the emission of Si+ ions from ion-beam-induced oxidized silicon surfaces has been studied. The stoichiometry and the electronic structure of the altered layer has been characterized using x-ray photoelectron spectroscopy (XPS). The XPS analysis of the Si 2p core level indicates the strong presence of suboxide chemical states when bombarding at angles of incidence larger than 30 ��. Since the surface stoichiometry or degree of oxidation varies with the angle of incidence, the corresponding valence-band structures also differ among each other. A comparison between experimental measurements and theoretically calculated Si and SiO2 valence bands indicates that the valence bands for the altered layers are formed by a combination of those two. Since Si-Si bonds are present in the suboxide molecules, the top of the respective new valence bands are formed by the corresponding 3p-3p Si-like subbands, which extend up to the Si Fermi level. The changes in stoichiometry and electronic structure have been correlated with the emission of Si+ ions from these surfaces. From the results a general model for the Si+ ion emission is proposed combining the resonant tunneling and local-bond-breaking models.

Energy transfer mechanism and Auger effect in Er3+ coupled silicon nanoparticle samples

We report a spectroscopic study about the energy transfer mechanism among silicon nanoparticles (Si-np), both amorphous and crystalline, and Er ions in a silicon dioxide matrix. From infrared spectroscopic analysis, we have determined that the physics of the transfer mechanism does not depend on the Si-np nature, finding a fast (< 200 ns) energy transfer in both cases, while the amorphous nanoclusters reveal a larger transfer efficiency than the nanocrystals. Moreover, the detailed spectroscopic results in the visible range here reported are essential to understand the physics behind the sensitization effect, whose knowledge assumes a crucial role to enhance the transfer rate and possibly employing the material in optical amplifier devices. Joining the experimental data, performed with pulsed and continuous-wave excitation, we develop a model in which the internal intraband recombination within Si-np is competitive with the transfer process via an Auger electron"recycling" effect. Posing a different light on some detrimental mechanism such as Auger processes, our findings clearly recast the role of Si-np in the sensitization scheme, where they are able to excite very efficiently ions in close proximity to their surface. (C) 2010 American Institute of Physics.

Ba3Yb(BO3)3 single crystals. Growth and spectroscopic characterization

We obtained Ba3Yb(BO3)3 single crystals by the flux method with solutions of the BaB2O4Na2OYb2O3 system. The evolution of the cell parameters with temperature shows a slope change at temperatures near 873 K, which may indicate a phase transition that is not observed by changes appearing in the x-ray powder patterns or by differential thermal analysis (DTA). The evolution of the diffraction patterns with the temperature shows incongruent melting at temperatures higher than 1473 K. DTA indicates that there is incongruent melting and this process is irreversible. Ba3Yb(BO3)3 has a wide transparency window from 247 to 3900 nm. We recorded optical absorption and emission spectra at room and low temperature, and we determined the splitting of Yb3+ ions. We used the reciprocity method to calculate the maximum emission cross section of 0.28 10-20 cm2 at 966 nm. The calculated lifetime of Yb3+ in Ba3Yb(BO3)3 is trad = 2.62 ms, while the measured lifetime is t = 3.80 ms.

Optical-model potential for electron and positron elastic scattering by atoms

An optical-model potential for systematic calculations of elastic scattering of electrons and positrons by atoms and positive ions is proposed. The electrostatic interaction is determined from the Dirac-Hartree-Fock self-consistent atomic electron density. In the case of electron projectiles, the exchange interaction is described by means of the local-approximation of Furness and McCarthy. The correlation-polarization potential is obtained by combining the correlation potential derived from the local density approximation with a long-range polarization interaction, which is represented by means of a Buckingham potential with an empirical energy-dependent cutoff parameter. The absorption potential is obtained from the local-density approximation, using the Born-Ochkur approximation and the Lindhard dielectric function to describe the binary collisions with a free-electron gas. The strength of the absorption potential is adjusted by means of an empirical parameter, which has been determined by fitting available absolute elastic differential cross-section data for noble gases and mercury. The Dirac partial-wave analysis with this optical-model potential provides a realistic description of elastic scattering of electrons and positrons with energies in the range from ~100 eV up to ~5 keV. At higher energies, correlation-polarization and absorption corrections are small and the usual static-exchange approximation is sufficiently accurate for most practical purposes.