Helena Pardo

Mechanical properties of graphene nanoribbons

Herein, we investigate the structural, electronic and mechanical properties of zigzag graphene nanoribbons in the presence of stress by applying density functional theory within the GGA-PBE (generalized gradient approximation-Perdew-Burke-Ernzerhof) approximation. The uniaxial stress is applied along the periodic direction, allowing a unitary deformation in the range of ± 0.02%. The mechanical properties show a linear response within that range while a nonlinear dependence is found for higher strain. The most relevant results indicate that Youngs modulus is considerable higher than those determined for graphene and carbon nanotubes. The geometrical reconstruction of the C-C bonds at the edges hardens the nanostructure. The features of the electronic structure are not sensitive to strain in this linear elastic regime, suggesting the potential for using carbon nanostructures in nano-electronic devices in the near future.

A new structure in the REBaCuFeO5+δ series: LaBaCuFeO5+δ. Structure and magnetic properties in the La1−xPrxBaCuFeO5+δ system

Abstract The crystal structure of LaBaCuFeO 5+ δ is determined by neutron powder diffraction at 15 K and the structure and magnetic properties of the La 1− x Pr x BaCuFeO 5+ δ system are studied by AC-susceptibility measurements and RT 57 Fe Mossbauer spectroscopy. The best fit using the Rietveld method is obtained for an orthorhombic model in space group I mmm with unit cell parameters a =5.5586(8) A, b =5.5550(9) A and c =7.8155(2) A. The main feature of this model is a disordered structure where the oxygen atom vacancies play an important role. The magnetic susceptibility in the La 1− x Pr x BaCuFeO 5+ δ system is studied and T f , μ eff and Θ are determined. Mossbauer spectra provide information relative to the iron electronic state and environment. They support the hypothesis of the key role that the oxygen atom positions play in the magnetic properties of the system by affecting the superexchange interactions in the network.

Cu(II) complexation with His-Gly and His-Ala. X-ray structure of [Cu(his-gly)2(H2O)2].6H2O

Abstract Two novel Cu(II)–dipeptide complexes containing l -histidyl– l -glycine and l -histidyl– l -alanine as ligands were prepared and characterised by infrared and electronic spectroscopies. EPR measurements were also made in solution and in the solid state. Cyclic voltammetric studies were carried out on aqueous solution. The crystalline structure of the [Cu( l -histidyl– l -glycine) 2 (H 2 O) 2 ]·6H 2 O was determined by X-ray diffractometry. It crystallises in the space group C 2. The Cu(II) is in a tetragonally distorted octahedral coordination, where the basal plane is occupied by N-atoms belonging to the histidine portion. The apical positions are occupied by two molecules of water.

Influence of processing conditions on the crystal structure and magnetic behavior of La0.7Ca0.3MnO3±δ samples

Abstract In this work we report crystallographic structure variations and the related modifications on the magnetic behavior of La 0.7 Ca 0.3 MnO 3± δ introduced by heat-treatments in different synthesis atmospheric conditions. We have prepared polycrystalline ceramic samples using a modified polymeric precursors method, which produces highly homogeneous specimens. The use of argon atmosphere enlarges the crystalline c -axis as detected by Rietveld refinements. As a consequence, an improvement in the magnetic transition temperature T C of the samples was observed. Our results also indicate that different heat-treatment conditions change the magnetic interactions between the ferromagnetic (F) and antiferromagnetic (AF) structures of these systems. Our conclusions rely on the use of AC magnetic susceptibility measurements as the experimental tool for measuring these variations.

Sm(III) complexation with α-amino acids: X-ray crystal structure of [Sm2(Hala)4(H2O)8](ClO4)4(Cl)2

Abstract Sm(III) coordination compounds are currently used as radiotherapeutic agents. Moreover, it is known that tumour cells show enhanced intake of α-amino acids (H n aa). With this in mind, the study of samarium complexes with these ligands has obvious interest. In this work, potentiometric studies of Sm(III) in the presence of glycine, alanine, proline, tryptophane, valine, glutamic acid and cysteine have been carried out. Experiments were performed in aqueous solution ([Sm 3+ ]=5.0–60.0 mM) at 37°C and 0.15 M NaClO 4 . Mono and dinuclear species were detected in the pH range 1.5–5.3. For higher pH values, Sm(OH) 3 competes with the formation of Sm complexes. In addition, Sm(III)–H n aa complexes were isolated and characterized at solid state. The structure of [Sm 2 (Hala) 4 (H 2 O) 8 ](ClO 4 ) 4 (Cl) 2 is reported. In this complex, two Sm centres are joined by four μ-COO bridges. Four molecules of water complete the positions of each eight-coordinate Sm atom.

Sm(III) Complexation with amino acids. Crystal structures of [Sm2(Pro)6(H2O)6](ClO4)6 and [Sm(Asp)(H2O)4]Cl2

Samarium complexes with amino acids have been studied by potentiometric measurements. Experiments were performed in aqueous solution at 37.0 °C and 0.15 M NaClO4, resembling physiological medium. Low stability Sm(III)–amino acid complexes were detected. Complexes with 1 ∶ 1 ligand to metal molar ratios are predominant in the studied pH interval. Other mono- and di-nuclear species can be detected up to pH 6. For higher pH values, hydrolysis to [Sm(OH)]2+ and Sm(OH)3 becomes the main process. Some of the studied complexes were isolated and characterized in the solid state. Single-crystal X-ray structures of [Sm2(Pro)6(H2O)6](ClO4)6 (1) and [Sm(Asp)(H2O)4]Cl2 (2) are reported. In both complexes, an eight coordinate Sm atom is found, surrounded by carboxylate groups and water molecules. In 1, Sm atoms form infinite chains bridged by proline ligands (via carboxylate groups). An additional monodentate amino acid is present in the coordination sphere of each Sm center. In 2, the amino acid also bridges Sm atoms but using α and β carboxylate groups. The overall structure consists of infinite planes in which all carboxylate groups, acting as bidentate ligands, are bonded to Sm atoms.

Current trends in materials for dye sensitized solar cells.

Here, we intend to review those patents related with the technology of dye sensitized solar cells. In particular we discuss patents and papers that enable metal oxide layer to be more controllable and feasible for applications, and new and innovative dyes, sensitizers and electrolytes with promising features. Finally various methods were reviewed for fabricating semiconductor layers and complete DSSC devices focusing on the mass production of photovoltaic cells.

Synthesis, characterization, and magnetic properties of room-temperature nanofluid ferromagnetic graphite

We report the chemical synthesis route, structural characterization, and physical properties of nanofluid magnetic graphite (NFMG) obtained from the previously synthesized bulk organic magnetic graphite (MG) by stabilizing the aqueous ferrofluid suspension with an addition of active cationic surfactant. The measured magnetization-field hysteresis curves along with the temperature dependence of magnetization confirmed room-temperature ferromagnetism in both MG and NFMG samples.

The effect of manganite nanoparticle addition on the low field magnetoresistance of polyaniline

In this report, we studied the effect of the addition of manganite nanoparticles on the microstructure and magnetotransport properties of polyaniline polymers. We showed experimental results on the fractal dimension of the polyaniline–manganite nanocomposites by means of small angle X-ray scattering measurements. A decrease in the number of polarons is observed for the composite with a concentration of 20% manganite nanoparticles. On the other hand, for the same concentration, the presence of a low magnetic field increases the number of polarons, in relation to an enhancement of the negative low field magnetoresistance (LFMR). This enhancement was observed for a critical amount of manganite nanoparticle addition in the whole temperature regime analyzed, as envisaged from the low magnetic field dependence in the polyaniline polaron formation observed using confocal Raman spectroscopy at room temperature.

Influence of oxygen disorder on the magnetic properties of LaBaCuFeO5+δ: an EXAFS and neutron diffraction study

Abstract LaBaCuFeO5+δ (LaBCFO) exhibits an irreversible magnetic behavior below T=68(3) K. The local structure around the Cu and Fe atoms studied by EXAFS reveals distortions in the internal shells of the coordination polyhedra of these atoms. A magnetic model could be satisfactorily refined from neutron powder diffraction data, with a magnetic unit cell with lattice parameters related to the structural constants as a M =3a/ 2 , b M =b/ 2 and cM=c/2. This model introduces the existence of randomly distributed magnetic centers in the structure, in agreement with the irreversibility observed by the DC susceptibility measurements. The oxygen disorder including vacancies and the shift in the position of the non-stoichiometric oxygen atoms produced by the imperfect La/Ba replacement in the packing are considered to be key for the magnetic behavior of the LaBCFO.