13 resultados para ZN2 IONS
em Consorci de Serveis Universitaris de Catalunya (CSUC), Spain
Resumo:
Estudi elaborat a partir duna estada al Stony Brook University al juliol del 2006. El RbTiOPO4 (RTP) monocristall s un material d' ptica no lineal molt rellevant i utilitzat en la tecnologia lser actual, qumicament molt estable i amb unes propietats fsiques molt destacades, entre elles destaquen els alts coeficients electro-ptics i l'alt llindar de dany ptic que presenta. En els ltims anys sest utilitzant tecnolgicament en aplicacions d'ptica no lineal en general i electro-ptiques en particular. En alguns casos ja ha substitut, millorant prestacions, a materials tals com el KTP o el LNB(1). Dopant RTP amb ions lantnids (Ln3+) (2-4), el material es converteix en un material lser auto-doblador de freqncia, combinant les seves propietats no lineals amb les de matriu lser. El RTP genera radiaci de segon harmnic (SHG) a partir dun feix fonamental amb longituds dona inferiors a 990 nm, que s el lmit que presenta el KTP.La determinaci de la ubicaci estructural i lestudi de l'entorn local del ions actius lser s de fonamental importncia per a la correcta interpretaci de les propietats espectroscpiques daquest material. Mesures de difracci de neutrons sobre mostra de pols cristall mostren que els ions Nb5+ i Ln3+ noms 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 tendncia a substituir l'i alcal present a l'estructura (10).Desprs dels resultats obtinguts en el present treball a partir de la tcnica EXAFS a la installaci sincrot del Brookhaven National Laboratory/State University of New York (Stony Brook) es pot concloure definitivament que els ions Nb subiquen 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 dabsorci com demissi, del material i per la avaluaci dels parmetres del seu comportament durant l'acci lser.
Resumo:
Degut a la gran demanda tecnolgica, actualment hi ha un gran inters en desenvolupar medis magntics amb entitats ferromagntiques de dimensions nanomtriques. Aquesta demanda promou la investigaci i el desenvolupament de nous materials i processos de fabricaci que permetin controlar duna manera ms precisa les propietats magntiques i estructurals. Entre els mtodes de litografia convencionals (per exemple deposici fsica a travs de mscares, deposici qumica en fase vapor i electrodeposici), recentment sha demostrat que la irradiaci amb ions a travs de mscares prelitografiades, sembla ser un bon mtode per a la fabricaci destructures ferromagntiques de lordre dels nanmetres. Aquesta tcnica pot ser aplicada per aprofitar la transici paramagnticaferromagntica que presenten alguns materials al ser desordenats estructuralment (per exemple FeAl, FePt3, Ni3Sn2). En el treball que es presenta a continuaci sutilitza laliatge Fe60Al40 per a fabricar estructures ferromagntiques embegudes en una matriu paramagntica mitjanant irradiaci amb ions darg a travs duna membrana de polimetil metacrilat (PMMA) prviament litografiada amb feixos delectrons (EBL). La fabricaci daquest sistema t com a objectiu destudiar levoluci de la morfologia i el gruix de PMMA (a partir de SEM i AFM) i del comportament magntic de les estructures fabricades (MFM i MOKE), quan s irradiat consecutivament a diferents energies. Per a completar lestudi shan utilitzat simulacions per a determinar les condicions dirradiaci (TRIM), com per a una millor comprensi dels resultats (simulacions micromagntiques). El contingut de la memria inclou una breu introducci histrica i conceptual sobre el magnetisme. A continuaci sexposen les tcniques necessries per a la fabricaci, preparaci i caracteritzaci de la mostra. Finalment es presenta una discussi dels resultats obtinguts i les conclusions.
Resumo:
En aquest treball de recerca, shan estudiat les dues isoformes de metallotionena CnMT1 i CnMT2 presents en el fong patogen Cryptococcus neoformans. Recentment sha descobert que aquest fong t com a factor de virulncia, els nivells de coure del medi on es troba. Les dues isoformes produdes en medis rics en Zn(II) shan utilitzat per a fer valoracions amb Cu(I) i Cd(II), i sha seguit levoluci dels experiments mitjanant les tcniques DC, UV-vis, i ESI-MS. Sha pogut observar que les dues isoformes tenen preferncia per enllaar Cu(I). Per altra banda tamb sha establert una gran homologia entre les dues seqncies.
Resumo:
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 nm680 nm).
Resumo:
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.
Resumo:
Background: The degree of metal binding specificity in metalloproteins such as metallothioneins (MTs) can be crucial for their functional accuracy. Unlike most other animal species, pulmonate molluscs possess homometallic MT isoforms loaded with Cu+ or Cd2+. They have, so far, been obtained as native metal-MT complexes from snail tissues, where they are involved in the metabolism of the metal ion species bound to the respective isoform. However, it has not as yet been discerned if their specific metal occupation is the result of a rigid control of metal availability, or isoform expression programming in the hosting tissues or of structural differences of the respective peptides determining the coordinative options for the different metal ions. In this study, the Roman snail (Helix pomatia) Cu-loaded and Cd-loaded isoforms (HpCuMT and HpCdMT) were used as model molecules in order t o elucidate the biochemical and evolutionary mechanisms permitting pulmonate MTs to achieve specificity for their cognate metal ion. Results: HpCuMT and HpCdMT were recombinantly synthesized in the presence of Cd2+, Zn2+ or Cu2+ and corresponding metal complexes analysed by electrospray mass spectrometry and circular dichroism (CD) and ultra violet-visible (UV-Vis) spectrophotometry. Both MT isoforms were only able to form unique, homometallic and stable complexes (Cd6-HpCdMT and Cu12-HpCuMT) with their cognate metal ions. Yeast complementation assays demonstrated that the two isoforms assumed metal-specific functions, in agreement with their binding preferences, in heterologous eukaryotic environments. In the snail organism, the functional metal specificity of HpCdMT and HpCuMT was contributed by metal-specific transcription programming and cell-specific expression. Sequence elucidation and phylogenetic analysis of MT isoforms from a number of snail species revealed that they possess an unspecific and two metal-specific MT isoforms, whose metal specificity was achieved exclusively by evolutionary modulation of non-cysteine amino acid positions. Conclusion: The Roman snail HpCdMT and HpCuMT isoforms can thus be regarded as prototypes of isoform families that evolved genuine metal-specificity within pulmonate molluscs. Diversification into these isoforms may have been initiated by gene duplication, followed by speciation and selection towards opposite needs for protecting copper-dominated metabolic pathways from nonessential cadmium. The mechanisms enabling these proteins to be metal-specific could also be relevant for other metalloproteins.
Resumo:
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 103%. 2009 American Institute of Physics. doi:10.1063/1.3213386
Resumo:
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.
Resumo:
A scheme to generate long-range spin-spin interactions between three-level ions in a chain is presented, providing a feasible experimental route to the rich physics of well-known SU(3) models. In particular, we demonstrate different signatures of quantum chaos which can be controlled and observed in experiments with trapped ions.
Resumo:
By exciting at 940 nm, we have characterized the 1.84 m near infrared emission of trivalent thulium ions in Yb3+, Tm3+:KGd WO4 2 single crystals as a function of the dopant concentration and temperature, from 10 K to room temperature. An overall 3H6 Stark splitting of 470 cm1 for the Tm3+ ions in the Yb3+, Tm3+:KGd WO4 2 was obtained. We also studied the blue emission at 476 nm Tm3+ and the near infrared emissions at 1.48 m Tm3+ and 1 m Yb3+ as a function of the dopant concentration. Experimental decay times of the 1G4, 3H4, and 3F4 Tm3+ and 2F5/2 Yb3+ excited states have been measured as a function of Yb3+ and Tm3+ ion concentrations. For the 3F4 3H6 transition of Tm3+ ions, we used the reciprocity method to calculate the maximum emission cross section of 3.07 1020 cm2 at 1.84 m for the polarization parallel to the Nm principal optical direction.
