María Jose Capitán

Formation of a non-magnetic metallic iron nitride layer on bcc Fe(100)

We report the formation of a stable iron nitride thin film following the exposition of an Fe(001) single crystal to atomic N plasma produced by means of a radio-frequency (rf) discharge source. The obtained phase, enabled by N-atomic diffusion into the iron matrix, has been characterized using spectroscopical and structural techniques. The comparison of the experimental results with first-principles calculations sheds light on the formed structure stability. The result is the formation of a metallic non-magnetic protective coating film on the iron substrate with a zinc blende (ZB) structure ( 00 -FeN). The formed film has high oxidation resistance, metallic character, non-magnetic ground state and good epitaxial growth. Thermal treatment of the iron nitride film shows that, at around 700K, the FeN is decomposed, resulting in bcc Fe as a consequence of the high diffusivity of N in Fe.

Growth of textured adenine thin films to exhibit only chiral faces.

The growth of adenine on Au(111) from the submonolayer up to several microns film thickness is studied by combining STM and surface X-ray diffraction techniques. The study shows that adenine thin films are composed of highly textured crystallites with the reported α-adenine crystal structure in such a way that they only exhibit (001)-faces. Such faces, with chiral p2 symmetry planes, have an average size of 15 nm. Thus, the formed adenine films reported herein, which expose only chiral and biocompatible faces, might find applications such as enantioselective heterogeneous catalysts for the chemical and pharmaceutical industries.

Onset of Chiral Adenine Surface Growth

The structure and stability of adenine crystals and thin layers has been studied by using scanning tunneling microscopy, X-ray diffraction, and density functional theory calculations. We have found that adenine crystals can be grown in two phases that are energetically quasi-degenerate, the structure of which can be described as a pile-up of 2D adenine planes. In each plane, the structure can be described as an aggregation of adenine dimers. Under certain conditions, kinetic effects can favor the growth of the less stable phase. These results have been used to understand the growth of adenine thin films on gold under ultra-high vacuum conditions. We have found that the grown phase corresponds to the α-phase, which is composed of stacked prochiral planes. In this way, the adenine nanocrystals exhibit a surface that is enantiopure. These results could open new insight into the applications of adenine in biological, medical, and enantioselective or pharmaceutical fields.

Relaxation Behavior of Poly(ester carbonate) Block Copolymer Across the Melting Region

The dielectric behavior of a family of block copolymers based on poly(butylene terephthalate) (PBT) and aliphatic polycarbonate (PC), with the composition of PBT varying from 100 wt.-% to 40 wt.-%, has been investigated by broad-band dielectric spectroscopy in the frequency range from 10 -1 to 10 9 Hz. At temperatures above the glass transition, the merging of localγ process and the cooperative β process was studied for PBT and 40/60 and 60/40 PBT-PC copolymers. The experimental data can be satisfactorily analyzed considering a sum of two Havriliak-Negami functions indicating that both β and γ relaxations can be treated as independent processes. Dielectric measurements for the 40/60 copolymer were extended over and above the melting region to characterize the influence of crystal melting on the dynamic behavior. It is shown that the progressive loss of the crystalline lamellar stack microstructure, as characterized by differential scanning calorimetry and small- and wide-angle X-ray scattering, provokes a dramatic change in the dynamic behavior of both the β and γ process.

Anomalous diffraction with soft X-ray synchrotron radiation: DANES from pentakismethylammonium undecachlorodibismuthate at the K absorption edge of chlorine

Soft X-ray diffraction from synchrotron radiation gives access to the use of MAD (multi-wavelength anomalous diffraction) and DAFS / DANES (diffraction anomalous fine structure / diffraction near edge structure) methods with relatively light elements down to Z514 (silicon), including elements such as sulfur and phosphorus essential in life. It also exploits the very strong dispersion of heavy elements (like uranium) both in resonant magnetic scattering and in protein crystallography. A brief review of the technical progress will be given. Recent measurements on the fine structure of anomalous diffraction from a ferroelectric salt, (NH CH ) Bi Cl at wavelengths 33 5 2 11 ˚ near the K-X-ray absorption edge of chlorine have shown that diffraction experiments with 4.4 A photons are becoming feasible. The dispersion of 60 reflections (36 unique reflections) has been measured at 30 energies between 2809 and 2838 eV in steps of 1 eV. The 52 theoretical calculation of the dispersion X-ray diffraction from the chlorobismuthate anion (Bi Cl ) suggests a strong anisotropic of 21 1 anomalous dispersion which is confirmed by our data. The data were collected at room temperature, well below the phase transition at 307 K where the salt looses its ferroelectric property. Future experiments aim at the observation of the dispersion of X-ray diffraction at various temperatures near the temperature of phase transition, where the site-selectivity of DAFS is well suited to follow the role of the central Cl-atom bridging the two Bi atoms. Our present experiments provide an insight into the present technical state of art of soft X-ray diffraction at the beam line ID1 at the European Synchrotron Radiation Facility in 1999 and ways for the improvement of this unique instrument.