T.C. Rojas

GOLD GLYCONANOPARTICLES AS WATER-SOLUBLE POLYVALENT MODELS TO STUDY CARBOHYDRATE INTERACTIONS

Glycosphingolipid clustering and interactions at the cell membrane can be modeled by gold glyconanoparticles prepared with biologically significant oligosaccharides. Such water-soluble gold glyconanoparticles with highly polyvalent carbohydrate displays (see picture, gray hemisphere: gold nanoparticle) have been obtained by a simple and versatile strategy.

Hard nanocomposite Ti–Si–N coatings prepared by DC reactive magnetron sputtering

Abstract Films resulting from Si additions to TiN matrix were prepared with Si contents in the range 0–19 at.%, using a closed field unbalanced DC magnetron sputtering system. Transmission Electron Microscopy (TEM) analyses revealed the nanocrystalline nature of these coatings, confirming the results of grain size evaluation from X-ray diffraction (XRD) patterns. Nanoindentation tests and scratch tests were carried out for the mechanical characterisation. Regarding the results, the samples show hardness values as high as 45 GPa. Best hardness values were found for Si content in the range 4–10 at.%. Almost all samples showed high critical loads for total adhesion failure, generally higher than 80 N, although the critical load for the first adhesion failure was found to be lower than 20 N for several samples. XRD patterns revealed the presence of only one phase that can be assigned to a cubic B1 NaCl structure, typical for TiN, with a lattice parameter of approximately 0.430 nm. The preferential growth, as a function of Si content, changes from a strong (111) orientation at the lowest Si additions to a weak (200) orientation at the highest Si content. Density values in the range 3.0–3.7 g/cm3 were obtained for most of the samples prepared with deposition rates between 0.5 and 1.1 μm/h, although higher density values were obtained for higher Ti deposition rates, with maximum of approximately 4.7 g/cm3 for the case of samples with low Si content.

Boron Compounds as Stabilizers of a Complex Microstructure in a Co‐B‐based Catalyst for NaBH4 Hydrolysis

Co⋅B‐based materials are widely used as catalysts for hydrogen generation through sodium borohydride self‐decomposition. In the mid 1990 s, the aqueous and organic chemistry involved in Co⋅B synthesis and handling was studied. Nevertheless, the exact microstructure of these catalysts has remained unsolved. Herein we present an exhaustive study which shows a new and complete microstructural view of a Co⋅B‐based material together with the chemistry of the cobalt and boron involved. By using nanoscale‐resolution microscopy and spectroscopy techniques, we have elucidated the role of boron compounds as stabilizers in a complex microstructure, which also explains its high catalytic performance and long‐term stability. The catalyst is proposed to be made up of 1–3 nm hcp Co0 nanoparticles embedded in amorphous CoxB (x=1, 2, 3), CoxOy, Co(BO2)2, and B2O3 phases alternatively or all together. All of these amorphous phases protect the nanocrystalline metallic core from growth and oxidation.

Low temperature synthesis of dense SiO2 thin films by ion beam induced chemical vapor deposition

Abstract This paper presents a comparative study of SiO 2 thin films prepared at room temperature by ion beam induced chemical vapor deposition (IBICVD) and plasma enhanced chemical vapor deposition (PECVD) methods. The films are characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), Rutherford backscattering spectroscopy (RBS), electron recoil detection analysis (ERDA), nuclear reaction analysis (NRA), X-ray reflectometry and spectroscopic ellipsometry. While the films prepared by IBICVD are very compact and dense and have a high refractive index ( n =1.48 at λ=550 nm), those prepared by PECVD exhibit a lower refractive index value ( n =1.45 at λ=550 nm), lower density and have a higher surface roughness. The different microstructure and properties of the two sets of films are discussed in relation to the ballistic effects that occur by the action of the highly energetic ion beams (e.g. 400 eV) impinging on the surface of the films prepared by IBICVD.

Study of the thermal stability of carbon nitride thin films prepared by reactive magnetron sputtering

Abstract CN x amorphous films have been prepared by reactive magnetron sputtering in a pure N 2 discharge. The films grown on NaCl have been characterised by Fourier transform infrared spectroscopy (IR), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). C/N atomic ratios have been determined by EELS with values in the range 2.0–1.2 for samples grown under different conditions. The thermal stability of the films upon heating in vacuum was followed ‘in situ’ at the transmission electron microscope by EELS. This study has been completed by a thermogravimetric and mass spectrometer analysis of evolved gases upon heating in nitrogen flow and vacuum, respectively. Under these conditions the films are stable up to 1023 K. Above this temperature the films decompose by elimination of nitrogen remaining a carbonaceous residue. The thermal stability of the films upon annealing in air was studied by following the evolution of the X-ray photoelectron spectroscopy (XPS) peaks during heating in air of films grown on steel. Deconvolution analysis of the XPS spectra allows to determine the evolution of the different type of bonds. In particular pure carbon in the films appears more reactive to oxygen than CN and C–N bonds.

The preparation of metal–polymer composite materials using ultrasound radiation: Part II. Differences in physical properties of cobalt–polymer and iron–polymer composites

Composite materials containing amorphous iron embedded in poly(methylacrylate) or poly(methylmethacrylate) and amorphous cobalt embedded in poly(methylacrylate) were formed using a sonochemical method. The physical and thermal properties of the composite materials were probed. A significant difference in the solubility of the iron‐poly(methylacrylate) and cobalt‐poly(methylacrylate) in various solvents was observed. This difference is accounted for by the stronger interaction existing between the cobalt and the surrounding polymer. For iron‐poly(methylacrylate) this interaction is weakened due to the formation of an iron complex.

Structure of diamondlike carbon films deposited by femtosecond and nanosecond pulsed laser ablation

The characterization of diamondlike carbon (DLC) films is a challenging subject, considering the diversity of carbon-based nanostructures depending on the deposition process. We propose to combine multiwavelength (MW) Raman spectroscopy and electron energy-loss spectroscopy (EELS) to probe the structural disorder and the carbon hybridizations of DLC films deposited by pulsed laser ablation performed either with a nanosecond laser (film labeled ns-DLC), either with a femtosecond laser (film labeled fs-DLC). Such deposition methods allow to reach a rather high carbon sp3 hybridization but with some significant differences in terms of structural disorder and carbonaceous chain configurations. MW Raman investigations, both in the UV and visible range, is a popular and nondestructive way to probe the structural disorder and the carbon hybridizations. EELS allows the determination of the carbon plasmon energy in the low-loss energy region of the spectra, as well as the fine structure of the ionization threshold...

A new bottom-up methodology to produce silicon layers with a closed porosity nanostructure and reduced refractive index

A new approach is presented to produce amorphous porous silicon coatings (a-pSi) with closed porosity by magnetron sputtering of a silicon target. It is shown how the use of He as the process gas at moderated power (50-150 W RF) promotes the formation of closed nanometric pores during the growth of the silicon films. The use of oblique-angle deposition demonstrates the possibility of aligning and orientating the pores in one direction. The control of the deposition power allows the control of the pore size distribution. The films have been characterized by a variety of techniques, including scanning and transmission electron microscopy, electron energy loss spectroscopy, Rutherford back scattering and x-ray photoelectron spectroscopy, showing the incorporation of He into the films (most probably inside the closed pores) and limited surface oxidation of the silicon coating. The ellipsometry measurements show a significant decrease in the refractive index of porous coatings (n(500 nm) = 3.75) in comparison to dense coatings (n(500 nm) = 4.75). The capability of the method to prepare coatings with a tailored refractive index is therefore demonstrated. The versatility of the methodology is shown in this paper by preparing intrinsic or doped silicon and also depositing (under DC or RF discharge) a-pSi films on a variety of substrates, including flexible materials, with good chemical and mechanical stability. The fabrication of multilayers of silicon films of controlled refractive index in a simple (one-target chamber) deposition methodology is also presented.

The role of CN chemical bonding on the tribological behaviour of CNx coatings

Abstract The tribological performance of CN x coatings depends strongly on both the environmental conditions and the nature of the coating, in relation to the deposition process. In this paper, we present and discuss friction results in relation to the nature, crystal structure, chemical composition and hybridization state of CN x coatings prepared by dual ion beam sputtering under various conditions. The films were characterized by infrared spectroscopy, X-ray photoelectron spectroscopy and electron energy loss spectroscopy. By increasing the polarization of the substrate, an increase of the N/C atomic ratio, together with a decrease of the CN/CN and CN/CC bonding ratios are observed. The concentration of CN triple bond is negligible. Reciprocating pin-on-plane friction tests have been carried out in humid ambient air and in ultrahigh vacuum. Steady-state friction in ambient air has been found to be in the range of 0.14–0.25, increasing slightly with the increase of the substrate polarization during deposition. The presence of CN double bond is associated with a lower friction coefficient. The steady-state friction of the film exhibiting the lowest friction in ambient air is near 0.6 in ultrahigh vacuum. The chemical composition of the topmost surfaces both inside and outside the UHV wear tracks of the plane and the pin was investigated by in situ XPS and AES performed at the completion of the friction test in the analytical tribometer. The high friction in UHV is associated with a significant transfer of the iron oxide top layers from the pin as wear particles inside the wear track of the plane, without any CN transfer onto the steel pin.

Preparation, characterization and thermal evolution of oxygen passivated nanocrystalline cobalt

Nanocrystalline cobalt powders have been prepared by the inert gas evaporation method. After preparation the materials were passivated by pure oxygen and air exposure. In the present paper we describe the application of different techniques like transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to achieve an exhaustive chemical and structural characterization of the nanocomposite material in its original state (passivated Co powder) and after thermal treatments in vacuum. After passivation the cobalt fcc nanocrystals (typical sizes 2-20 nm) are covered by an amorphous oxide passivation layer with a short range structural order similar to the cubic (fcc) CoO phase. Upon heating, crystallisation of the oxide layer is observed together with the formation of the Co 3 O 4 spinel phase. In situ XPS experiments allow the determination of the relative amounts of oxide and metal as a function of the oxygen dose. The present study is relevant due to the correlation between microstructure and the unusual magnetic properties of this type of material.