Carlos Luna

Electroplating and magnetostructural characterization of multisegmented Co54Ni46/Co85Ni15 nanowires from single electrochemical bath in anodic alumina templates.

Highly hexagonally ordered hard anodic aluminum oxide membranes, which have been modified by a thin cover layer of SiO2 deposited by atomic layer deposition method, were used as templates for the synthesis of electrodeposited magnetic Co-Ni nanowire arrays having diameters of around 180 to 200 nm and made of tens of segments with alternating compositions of Co54Ni46 and Co85Ni15. Each Co-Ni single segment has a mean length of around 290 nm for the Co54Ni46 alloy, whereas the length of the Co85Ni15 segments was around 430 nm. The composition and crystalline structure of each Co-Ni nanowire segment were determined by transmission electron microscopy and selected area electron diffraction techniques. The employed single-bath electrochemical nanowire growth method allows for tuning both the composition and crystalline structure of each individual Co-Ni segment. The room temperature magnetic behavior of the multisegmented Co-Ni nanowire arrays is also studied and correlated with their structural and morphological properties.

Preparation and magnetoresistance behavior of nickel nanoparticles embedded in hydrogenated carbon film

Nickel nanoparticles arrays, growth into hydrogenated amorphous carbon, were prepared by means of RF-plasma enhanced chemical vapor deposition and RF-sputtering co-deposition from acetylene and a nickel target. The resulting nanocomposite films were characterized by X-ray diffraction and atomic force microscopy, and their magnetic responses and magnetoresistance behavior were investigated as a function of the exciting magnetic field and the Ni nanoparticles content, which was conveniently controlled by adjusting the deposition time. These physical properties were explained by a combination of hopping and tunneling effects.

Biosynthesis of silver fine particles and particles decorated with nanoparticles using the extract of Illicium verum (star anise) seeds.

Given the upsurge of new technologies based on nanomaterials, the development of sustainable methods to obtain functional nanostructures has become an imperative task. In this matter, several recent researches have shown that the biodegradable natural antioxidants of several plant extracts can be used simultaneously as reducing and stabilizing agents in the wet chemical synthesis of metallic nanoparticles, opening new opportunities to design greener synthesis. However, the challenge of these new techniques is to produce stable colloidal nanoparticles with controlled particle uniformity, size, shape and aggregation state, in similar manner than the well-established synthetic methods. In the present work, colloidal metallic silver nanoparticles have been synthesized using silver nitrate and extracts of Illicium verum (star anise) seeds at room temperature in a facile one-step procedure. The resulting products were colloidal suspensions of two populations of silver nanoparticles, one of them with particle sizes of few nanometers and the other with particles of tens of nm. Strikingly, the variation of the AgNO3/extract weight ratio in the reaction medium yielded to the variation of the spatial distribution of the nanoparticles: high AgNO3/extract concentration ratios yielded to randomly dispersed particles, whereas for lower AgNO3/extract ratios, the biggest particles appeared coated with the finest nanoparticles. This biosynthesized colloidal system, with controlled particle aggregation states, presents plasmonic and SERS properties with potential applications in molecular sensors and nanophotonic devices.

Synthesis of metallic silver nanoparticles and silver organometallic nanodisks mediated by extracts of Capsicum annuum var. aviculare (piquin) fruits

Silver-based nanostructures were prepared through reduction/oxidation reactions of aqueous silver nitrate solutions mediated by extracts of red fruits of the piquin pepper (Capsicum annuum var. aviculare) at room temperature. Detailed morphological and microstructural studies using X-ray diffraction, conventional and high-resolution transmission electron microscopy and selected area electron diffraction revealed that the product was constituted by three kinds of nanoparticles. One of them was composed of twinned metallic silver nanoparticles with a size of few nanometers. Other kind of particles was ultrafine disk-like single crystals of silver 4,4′-dimethyldiazoaminobenzene, being in our best knowledge the first time that this compound is reported in the form of nanoparticles. Both kinds of nanoparticles experienced processes of self-assembly and subsequent grain growth to form the third kind of nanoparticles. Such resulting nanostructures are monocrystalline and flattened metallic silver nanoparticles that have diameters around tens of nanometers, the [112] direction perpendicular to the particle plane, and are coated by a surface organometallic layer and residues of biomolecules. The ultraviolet-visible spectrum of the biosynthesized product showed a surface plasmon resonance (SPR) extinction band with an absorbance maximum at around 400 nm, thereby confirming the presence of fine Ag particles. Studies carried out by Fourier transform infrared spectroscopy indicated that the principal active compounds responsible of the reduction of the Ag ions are proteins and capsaicin (through the amino groups) and phenolic compounds (through hydroxyl groups).

Magnetization reversal dependence on effective magnetic anisotropy in electroplated Co–Cu nanowire arrays

Arrays of Co(100−x)Cu(x) (0 ≤ x ≤ 27) nanowires with 45 nm of diameter and 18 μm in length have been potentiostatically electrodeposited into the hexagonally self-assembled nanopores of anodic alumina membranes. The structural characterization of Co–Cu nanowires confirms the coexistence of both hcp and fcc crystalline phases, with textures that are strongly affected by the fractional content of Cu. Parallel magnetic studies of the room temperature magnetization process by First Order Reversal Curve (FORC) analysis and the angular dependence of coercivity confirm the presence of two coexisting ferromagnetic phases on intermediate Cu content nanowires, ascribed to a softer magnetic phase for pure Co and a harder magnetic one for the Co–Cu composition alloy, respectively. The temperature dependence of coercivity and remanence reveal a reorientation of the effective magnetic anisotropy with the addition of Cu to the Co–Cu alloy nanowires, being enhanced by the coexistence of the two ferromagnetic phases.

Synthesis of antibacterial silver-based nanodisks and dendritic structures mediated by royal jelly

The one-step preparation of silver nanoparticles and dendritic structures mediated by aqueous royal jelly solutions has been investigated for the first time. It has been found that royal jelly (RJ) is a complex organic matrix that can be simultaneously used as a reducing and stabilizing agent in the chemical synthesis of colloidal silver-based nanostructures from aqueous AgNO3 solutions, without the requirement of additional reagents or heating sources to initiate the oxidation–reduction reactions. The resulting product consisted of very fine single-crystal disks of Ag and silver 4,4′-dimethyldiazoaminobenzene (a triazenic compound). Both kinds of particles tended to coalesce and form supramolecular dendritic structures, the AgNO3/RJ weight ratio chosen in the synthesis being a key parameter to control the crystal growth and the microstructural properties of the Ag nanodisks. Data obtained from Fourier transform infrared and Raman spectroscopy analysis indicated that these nanostructures are coated by RJ biomolecules (residues of proteins and carbohydrates). In vitro biological assays showed that these nanostructures exhibit a promising enhanced antibacterial activity against both Gram-positive and Gram-negative bacteria.

Control of crystallite orientation and size in Fe and FeCo nanoneedles

Uniform magnetic nanoneedles have been prepared by hydrogen reduction of elongated nanoarchitectures. These precursors are as-prepared or cobalt-coated aggregates of highly oriented haematite nanocrystals (∼5 nm). The final materials are flattened nanoneedles formed by chains of assembled Fe or FeCo single-domain nanocrystals. The microstructural properties of such nanoneedles were tailored using renewed and improved synthetic strategies. In this fashion, the needle elongation and composition, the crystallite size (from 15 up to 30 nm), the nanocrystal orientation (with the 〈110〉 or 〈001〉 directions roughly along the long axis of the nanoneedle) and their type of arrangement (single chains, frustrated double chains and double chains) were controlled by modifying the reduction time, the axial ratio of the precursor haematite and the presence of additional coatings of aluminum or yttrium compounds. The values of the coercivity H(C) found for these nanoneedles are compared with the values predicted by the chain of spheres model assuming a symmetric fanning mechanism for magnetization reversal.

Pseudo-monocrystalline properties of cylindrical nanowires confinedly grown by electrodeposition in nanoporous alumina templates

Four different cylindrical nanowire systems (nickel, cobalt, Co23Cu77, and multisegmented Co58Ni42/Co83Ni17 nanowires) with single-crystal-like properties were characterized by transmission electron microscopy and selected-area electron diffraction (SAED) under different tilting angles. Although these nanowires have different chemical compositions, crystalline structures and/or diameters, they exhibit similar behaviors, which are unexpected for ideal single-crystals. All the samples presented SAED patterns that did not experience changes from one zone axis to another when the nanowire was tilted in a wide range of angles, exhibiting related apparent interplanar distances that are dependent on the nanowire inclination, yielding deformed patterns that can be unrecognizable. Moreover, face-centered cubic nanowires presented classically forbidden reflections. These behaviors were explained by considering the characteristics of the measurement technique and the confined template-assisted growth, which force the atoms to be accommodated in a cylindrical volume with nanoscale dimensions, yielding the frustrated formation of stable facets and right angles in the nanowire radial directions, together with the formation of stacking faults.

Study of the microstructure and surface morphology of silver nanolayers obtained by ion-beam deposition

The aim of this research is to fabricate Ag nanoparticle assemblies forming nanolayers with different thicknesses in order to evaluate their physical properties and their 3-D (three-dimensional) surface micromorphology. Such Ag nanolayers were deposited by physical vapor deposition methods under ion-beam irradiation in argon atmosphere at room temperature. X-ray diffraction (XRD) characterizations confirmed the formation of silver nanocrystals. Using surface field parameters, which were derived from atomic force microscopy (AFM) analysis, the surface morphology of the nanolayers were described according ISO 25178-2:2012. The height and slope distribution functions, together with the autocorrelation (ACF), the height–height correlation (HHCF) and the power spectral density (PSDF) functions were determined for the different analyzed samples using linear interpolation in the horizontal direction. All samples had a fractal character with a fractal dimension of 2.54 ± 0.01 independent of the film thickness.

Magnetoresistance of nanocomposite copper/carbon thin films

Nanocomposite thin films made of partially oxidized Cu nanoparticles embedded into hydrogenated amorphous carbon, with different thicknesses and Cu/C ratio, were prepared by means of radio frequency plasma enhanced chemical vapor deposition and radio frequency sputtering using acetylene gas and copper target. The surface roughness was investigated using atomic force microscopy, revealing the fractal geometry of the Cu/carbon thin films at the nanoscale with fractal dimensions around 2.7. In addition, the electrical properties of these films and their dependence on the application of low magnetic fields were explored at room temperature. It was found that when the Cu nanoparticles are separated by gaps, the electrical conduction is governed by tunneling effects. In these conditions, the samples exhibit negative magnetoresistance values, displaying steps in the dependences on the magnetic field. These properties suggest the potential use of these films as magnetic sensors in spintronics.