Scientific Papers in SCI


Title: Energy transfer efficiency in YF3 nanocrystals: Quantifying the Yb3+ to Tm3+ infrared dynamics
Author(s): Quintanilla, M; Nunez, NO; Cantelar, E; Ocana, M; Cuss, F
Source: Journal of Applied Physics, 113 (2013) 174308 (6 pages)

abstract | fulltext

In this work, we report on the determination of the infrared Yb3+ → Tm3+ energy transfer efficiency in YF3:Yb3+/Tm3+ nanocrystals through the study of Yb3+ dynamics. The obtained results are compared to those previously reported in macrocrystals to analyze possible changes related to size reduction. Luminescence lifetimes are much shorter in the nanoparticles than in bulk samples, a behavior that can be related to Yb3+ → Yb3+ migration and the enhanced surface/volume ratio of the nanoparticles. On the other hand, Yb3+ → Tm3+ energy transfer macroparameter remains unaltered, demonstrating that spectroscopic intrinsic parameters such as radiative and non-radiative probabilities are not affected by size reduction. Finally, a formula that describes Yb3+ lifetime dependence with Yb3+ and Tm3+ concentration is proposed, considering both the effects produced by migration between Yb3+ ions and energy transfer from Yb3+ to Tm3+ ions.

May, 2013 | DOI: 10.1063/1.4803540

Title: Differences in n-type doping efficiency between Al- and Ga-ZnO films
Author(s): Gabas, M; Landa-Canovas, A; Costa-Kramer, JL; Agullo-Rueda, F; Gonzalez-Elipe, AR; Diaz-Carrasco, P; Hernandez-Moro, J; Lorite, I; Herrero, P; Castillero, P; Barranco, A; Ramos-Barrado, JR
Source: Journal of Applied Physics, 113 (2013) 163709 (9 pages)

abstract | fulltext

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices.

April, 2013 | DOI: 10.1063/1.4803063

Title: Synthesis and functionalization of biocompatible Tb:CePO4 nanophosphors with spindle-like shape
Author(s): Rodriguez-Liviano, S; Aparicio, FJ; Becerro, AI; Garcia-Sevillano, J; Cantelar, E; Rivera, S; Hernandez, Y; de la Fuente, JM; Ocana, M
Source: Journal of Nanoparticle Research 15 (2013) 15:1402

abstract | fulltext

Monoclinic Tb:CePO4 nanophosphors with a spindle-like morphology and tailored size (in the nanometer and micrometer range) have been prepared through a very simple procedure, which consists of aging, at low temperature (120 °C), ethylene glycol solutions containing only cerium and terbium acetylacetonates and phosphoric acid, not requiring the addition of surfactants or capping agents. The influence of the heating mode (conventional convection oven or microwave oven) and the Tb doping level on the luminescent, structural and morphological features of the precipitated nanoparticles have also been analyzed. This study showed that microwave-assisted heating resulted in an important beneficial effect on the luminescent properties of these nanophosphors. Finally, a procedure for the functionalization of the Tb:CePO4 nanoparticles with aspartic-dextran is also reported. The functionalized nanospindles presented negligible toxicity for Verocells, which along with theirs excellent luminescent properties, make them suitable for biomedical applications.

January, 2013 | DOI: 10.1007/s11051-012-1402-7

Title: Enhancement of visible light-induced surface photo-activity of nanostructured N–TiO2 thin films modified by ion implantation
Author(s): Romero-Gomez, P; Lopez-Santos, C; Borras, A; Espinos, JP; Palmero, A; Gonzalez-Elipe, AR
Source: Chemical Physics Letters, 582 (2013) 95-99

abstract | fulltext

This work reports the morphological and chemical modifications induced in TiO2 thin films by bombardment with high energy N+ ions at different temperatures and their different photo-activity responses after implantation under visible and UV light illumination. When implanted samples are illuminated with visible light, no dye photo-decolouration takes place despite that light transformed the surfaces from hydrophobic to hydrophilic. In agreement with the Wenzel model of wetting, correlation is found between visible light photo-activity and film morphology. We conclude that the photo-activity response can be separated into shallow and Schottky barrier photo-activity, this latter involving a thicker layer of material.

September, 2013 | DOI: 10.1016/j.cplett.2013.07.025

Title: Allochthonous red pigments used in burial practices at the Copper Age site of Valencina de la Concepción (Sevilla, Spain): characterisation and social dimension
Author(s): Rogerio-Candelera, MA; Herrera, LK; Miller, AZ; Sanjuan, LG; Molina, CM; Wheatley, DW; Justo, A; Saiz-Jimenez, C
Source: Journal of Archaeological Science, 40 (2013) 279-290

abstract | fulltext

The use of red pigments linked to burial practices is widely documented in the Iberian prehistoric record and very often it has been traditionally interpreted as a ritual practice entailing the utilisation of local raw materials (iron oxides). Some research works, nevertheless, have also detected the use of red pigments which can only be interpreted as allochthonous. The red pigments spread over a single inhumation in a monumental Megalithic tomb surrounding Valencina de la Concepción Copper Age settlement was studied by means of X-ray diffraction, field emission scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray microfluorescence, micro-Raman and Fourier transform infrared spectroscopies. This approach allowed characterising the red pigments as cinnabar, mixed with tiny amounts of iron oxides. The presence of cinnabar, a product that was necessarily imported, in a context of an exceptional set of grave goods, suggests that the use of cinnabar was linked not only to ritual but also to practices related to the display of social status.

January, 2013 | DOI: 10.1016/j.jas.2012.08.004

Title: LaPO4:Er microspheres with high NIR luminescent quantum yield
Author(s): Garcia-Sevillano, J; Cantelar, E; Justo, A; Ocana, M; Cusso, F
Source: Materials Chemistry and Physics, 138 (2013) 666-671

abstract | fulltext

Er-doped LaPO4 microspheres have been synthesized by spray pyrolysis and the near infrared (NIR) properties have been characterized. It has been found that, following an adequate post-annealing treatment, the emission properties are remarkably improved. The NIR luminescence intensity is highly enhanced and its decay time increases to a value almost coincident with the reported radiative lifetime, which implies that the quantum yield approaches η ≈ 100%. This improvement in luminescence characteristics is probably related to the suppression of residual OH− radicals, that otherwise act as NIR luminescence quenchers, and to the increase in material's crystallinity.

March, 2013 | DOI: 10.1016/j.matchemphys.2012.12.036

Title: Synthesis and tribological properties of WSex films prepared by magnetron sputtering
Author(s): Dominguez-Meister, S; Justo, A; Sanchez-Lopez, JC
Source: Materials Chemistry and Physics, 142 (2013) 186-194

abstract | fulltext

WSex films with variable Se/W ratio were deposited by non-reactive r.f. magnetron sputtering from WSe2 target changing the applied d.c. pulsed bias conditions and substrate temperature. The structural and chemical properties were measured by cross-sectional scanning electron microscopy (X-SEM), energy dispersive analysis (EDX), X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS). The tribological properties were measured in ambient air (RH = 30–40%) and dry nitrogen by means of a reciprocating ball-on-disk tribometer. A clear correlation was found between the Se/W ratio and the measured friction coefficient displaying values below 0.1 (in ambient air) and 0.03 (in dry N2) for ratios Se/W ≥ 0.6 as determined by electron probe microanalysis (EPMA). The results demonstrated that notable tribological results could be obtained even in ambient air (friction ≤ 0.07 and wear rate ≈10−7 mm3 Nm−1) by controlling the film microstructure and chemical composition. By incorporating carbon, wear and chemical resistance can be gained by formation of non-stoichiometric carbides and/or alloying into the defective WSex hexagonal structure. The existence of a WSe2 rich interfacial layer (either on the ball scar or embedded in the film track) was evidenced by Raman in low friction conditions. The improvement in tribological performance is therefore obtained by means of layered WSex, the formation of gradient composition from metallic W (hard) to WSe2 (lubricant) and carbon incorporation.

October, 2013 | DOI: 10.1016/j.matchemphys.2013.07.004

Title: Segregation to the grain boundaries in YSZ bicrystals: A Molecular Dynamics study
Author(s): Gonzalez-Romero, RL; Melendez, JJ; Gomez-Garcia, D; Cumbrera, FL; Dominguez-Rodriguez, A
Source: Solid State Ionics, 237 (2013) 8-15

abstract | fulltext

A Molecular Dynamics study about the segregation of yttrium at 1500 K to a Σ5 grain boundary in 8 mol% YSZ has been performed. Segregation has been induced by explicitly taking into account the excess energy associated to the elastic misfit effect for yttrium cations located nearby the grain boundary planes. After an initial transient, a steady regime is reached, in which the number of yttrium cations does not increase with time. Accumulation of yttrium cations is accompanied by that of zirconium ones and oxygen vacancies at some distance of the grain boundary planes. The changes in the radial distribution functions for different ionic pairs are discussed, as also the effect of segregation on oxygen diffusion along the grain boundaries and in volume. Finally, the possibility that segregated yttrium located at available free sites at the grain boundaries is pointed out.

April, 2013 | DOI: 10.1016/j.ssi.2013.02.002

Title: Generalized master plots as a straightforward approach for determining the kinetic model: The case of cellulose pyrolysis
Author(s): Sanchez-Jimenez, PE; Perez-Maqueda, LA; Perejon, A; Criado, JM
Source: Thermochimica Acta, 552 (2013) 54-59

abstract | fulltext

The thermal degradation of cellulose is a complex reaction and, despite the large amount of work by many investigators during the last decades, the actual understanding of the thermal decomposition kinetics is still very limited. Thus, while several mechanisms have been proposed to describe the process, the real model has not yet been clearly identified. In this paper, a set of experimental curves recorded under different heating schedules, i.e., linear heating rate, isothermal and constant rate thermal analysis (CRTA), has been analyzed using isoconversional and master plots methodology to discriminate the kinetic model followed by the reaction.

January, 2013 | DOI: 10.1016/j.tca.2012.11.003

Title: Hydration properties of synthetic high-charge micas saturated with different cations: An experimental approach
Author(s): Pavon, E; Castro, MA; Naranjo, M; Orta, MM; Pazos, MC; Alba, MD
Source: American Mineralogist, 98 (2013) 394-400

abstract | fulltext

An understanding of the interaction mechanisms between exchangeable cations and layered silicates is of interest from both a basic and an applied point of view. Among 2:1 phyllosilicates, a new family of swelling high-charge synthetic micas has been shown to be potentially useful as decontaminant. However, the location of the interlayer cations, their acidity and the water structure in the interlayer space of these silicates are still unknown. The aim of this paper was therefore to study the hydration state of the interlayer cations in the interlayer space of high-charge expandable micas and to evaluate the effect that this hydration has on the swelling and acidity behavior of these new materials. To achieve these objectives, three synthetic micas with different charge density total layer charges (ranging between 2 and 4 per unit cell) and with five interlayer cations (Na+, Li+, K+, Mg2+, and Al3+) were synthesized and their hydration state, interlayer space, and acidity analyzed by DTA/TG, XRD, and 1H MAS NMR spectroscopy. The results showed that the hydration state depends on both the layer charge and the nature of the interlayer cation. A high participation of the inner-sphere complexes in the highly charged confined space has been inferred and proposed to induce Brønsted acidity in the solid.

March, 2013 | DOI: 10.2138/​am.2013.4217

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