Aida Serrano

Designing nanostructured strontium aluminate particles with high luminescence properties

The synthesis of sub-micron phosphorescent particles has been widely studied during the past decade because of the promising industrial application of these materials. A number of matrices have been developed, the most actively researched being strontium aluminate doped with europium (Eu2+) or dysprosium (Dy3+), because of its better stability and longer afterglow than other matrices. However, the powders produced by different synthetic routes have a wide particle size range, between 20 and 100 μm. A method of reducing the particle size has not yet been developed, but becomes important if the practical value of these particles is to be realised. As a means of producing sub-micron particles, in the present study the powders were synthesized by a combustion method followed by a dry grinding process. In optimizing the synthesis, particularly in order to understand the effect of the fuel (urea) on phase formation, it was possible to control the reaction and to achieve high luminescence. In addition, a dry grinding process was developed which decreases the particle size and avoids the presence of moisture during the grinding procedure. A correlation between the presence of secondary phases and Eu2+ content was established, and the phosphors can be treated in a nitrogen–hydrogen atmosphere to increase crystallinity and photoluminescence. The underlying significance of the study lies in evaluating the practical application of the product. It was concluded that such a material might be a promising candidate for replacing micron-sized phosphor particles in a number of areas in the future.

XAS study of Mn, Fe and Cu as indicators of historical glass decay

We present here a study of historic glass decay by means of X-ray absorption spectroscopy (XAS). Transition metal cations incorporated in the glass as chromophores exhibit modifications of their oxidising state and chemical environment as the glass suffers a decay process. These modifications can be monitored by measuring X-ray absorption near edge structure, XANES, and extended X-ray absorption fine structure, EXAFS, spectra around a selected atomic species. We apply the technique here to glasses from different periods ranging from 1st century BC to 18th century, demonstrating that XAS provides an advanced tool for qualitative analysis of glass decay. In particular we have found that it is possible to establish a relationship between the oxidation state of Fe and Cu cations and the decay suffered by the glass. In contrast, our results indicate that the Mn oxidizing state is not directly involved in the glass decay of the studied samples.

Precise Tuning of the Nanostructured Surface leading to the Luminescence Enhancement in SrAl 2 O 4 Based Core/Shell Structure

Intensive research has been focused on the synthesis of long-lasting SrAl2O4:EuDy in luminescent materials field. Traditionally, SrAl2O4:EuDy is synthesized in bulk form by solid state. However, their development remains restrained due to this technique is not compatible with large-scale production, sustainability and nanometer-scale requirements. Despite nano-range particles have been obtained by chemical routes, photoluminescence response decreases and application became unpractical. It remains a challenge to synthesize nonrare-earth (RE) phosphors with high photoluminescence. One major challenge for the luminescent materials community is to devise methods to deliver innovative, sustainable and cost effective solutions for the reduction of RE because of the lack of RE availability. Here, we suggest a solution based on molten salts, obtaining nanosheets or micro/nanostructured SrAl2O4:Eu, Dy particles with core-shell structure, employing only 50% of standard amounts of RE. Core-size and shell thickness and crystallinity can be tuned by post-thermal treatment, through which can be modulated the Eu+2 fraction. We find that our methodology leads the functional features of the analogous micron counterpart. These results can be considered a great achievement to scale-up the process. Besides, the harmful collateral effect of nanotechnology must be addressed by using new safe by design core-shell nanostructures.

Unveiling the role of the hexagonal polymorph on SrAl2O4-based phosphors

In persistent luminescence materials, the SrO–Al2O3 system has been mainly studied due to its chemical stability, higher photoluminescence response and longest green-afterglow times. Specifically, the research has focused on SrAl2O4 doped with europium and dysprosium. SrAl2O4 has two polymorphs: monoclinic polymorph (space group P21) and hexagonal polymorph (space group P6322). Besides, the coexistence of these two phases, monoclinic and hexagonal, appears in almost all the results. However, it is not clear how this coexistence influences optical response. Some authors have reported that only the monoclinic structure exhibits luminescence properties, while another suggests that the hexagonal SrAl2O4 polymorph has a higher emission efficiency than the monoclinic polymorph. Here we report a systematic evaluation of the effects of the stabilization of the hexagonal SrAl2O4 polymorph. We show that an interrelationship between the hexagonal polymorph and phosphorescent properties is the linchpin for the development of good luminescence properties. Remarkably, the stabilization of the hexagonal SrAl2O4 polymorph on the monoclinic–hexagonal polymorphic coexistence appears to be related to the preservation of the nanometric nature of the SrAl2O4-based system. Our results will help to understand the role of the hexagonal polymorph in the polymorphic coexistence on SrAl2O4-based systems and may facilitate the development of luminescent nanometric particles for the design and preparation of new light emitting materials.

Highly Bi-doped Cu thin films with large spin-mixing conductance

Spin Hall effect provides an efficient tool for the conversion of a charge current into a spin current, opening the possibility of producing pure spin currents in non-magnetic materials for the next generation of spintronics devices. In this sense, giant Spin Hall Effect has been recently reported in Cu doped with 0.5% Bi grown by sputtering and larger values are expected for larger Bi doping, according to first principles calculations. In this work we demonstrate the possibility of doping Cu with up to 10% of Bi atoms without evidences of Bi surface segregation or cluster formation, as studied by different microscopic and spectroscopic techniques. In addition, YIG/BiCu structures have been grown, showing a spin mixing conductance larger that the one shown by similar Pt/YIG structures. These results reflects the potentiality of these new materials in spintronics devices.The spin Hall effect (SHE) provides an efficient tool for the production of pure spin currents, essentially for the next generation of spintronics devices. Giant SHE has been reported in Cu doped with 0.5% Bi grown by sputtering, and larger values are predicted for larger Bi doping. In this work, we demonstrate the possibility of doping Cu with up to 10% of Bi atoms without evidence of Bi surface segregation or cluster formation. In addition, YIG/BiCu structures have been grown, showing a spin mixing conductance larger that the one shown by similar Pt/YIG structures, reflecting the potentiality of these new materials.