From exponential to linear growth in polyelectrolyte multilayers

There exist two types of polyelectrolyte multilayers: those whose thickness increases linearly with the number of deposition steps, which are nicely structured, and those whose thickness increases exponentially, which resembles hydrogels. This simple picture has recently slightly evolved with the finding that some exponentially growing films enter into a linear growth phase after a certain number of deposition steps. In this study, we investigate the buildup process of hyaluronic acid/poly(L-lysine) (HA/PLL) multilayers that constitute one of the best known exponentially growing systems. The films are built by using two deposition methods: the well-known dipping method and the more recent spraying method where the polyelectrolyte solutions are sprayed alternately onto a vertical substrate. The goal of this study is twofold. First, we investigate the influence of the main parameters (i.e., spraying rate and spraying time) of the spraying method on the film growth process. We find that, as for the dipping method, the film thickness first evolves exponentially with the number of deposition steps, and after a given number of deposition steps, it follows a linear evolution. We find that similar behavior is observed with the dipping method. Second, because the spraying method allows the very fine variation of the different parameters of the buildup, we use this method to investigate the exponential-to-linear transition. We find that this transition always takes place after about 12 deposition steps whatever the values of the parameters controlling the deposition process. We discuss our results in light of a model proposed by Hübsch et al. (Hübsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P. Langmuir 2004, 20, 1980-1985) and later by Salom?ki et al. (Salom?ki, M.; Vinokurov, I. A.; Kankare, J. Langmuir 2005, 21, 11232-11240) in which it is assumed that the exponential-to-linear transition is due to a film restructuring that progressively forbids the diffusion of one of the polyelectrolytes constituting the film over part of the film. This "forbidden" zone then grows with the number of deposition steps so that the outer zone of the film that is still concerned with diffusion keeps a constant thickness and moves upward as the total film thickness increases.     

Effect of annealing in an external magnetic field on the magnetic texture of Mo-containing nanocrystalline alloys

The magnetic texture of (Fe_1?x Co_x)₇₆Mo₈Cu₁B₁₅ (x = 0, 0.5) nanocrystalline alloys is studied for different amounts of nanocrystalline grains. The originally amorphous alloys were annealed in external longitudinal and transverse magnetic fields of 0.025 T and 0.8T, respectively. Mössbauer measurements were carried out at room and liquid nitrogen temperatures in order to gain information on the hyperfine interactions and the orientation of the magnetization. The obtained results are compared with those received from zero-field annealed samples. Magneto-optical Kerr effect (MOKE) was applied for the investigation of possible changes at the surface of the x = 0 ribbon as a function of annealing temperature and applied magnetic field. A combination of uniaxial anisotropy, which originates from the shape anisotropy, and four-fold anisotropy, which is a contribution from crystallites of nanometre size embedded in the residual amorphous matrix, is unveiled.     

Amorphous stainless steel coatings prepared by reactive magnetron-sputtering from austenitic stainless steel targets

Stainless steel films were reactively magnetron sputtered in argon/methane gas flow onto oxidized silicon wafers using austenitic stainless-steel targets. The deposited films of about 200 nm thickness were characterized by conversion electron M?ssbauer spectroscopy, magneto–optical Kerr-effect, X-ray diffraction, scanning electron microscopy, Rutherford backscattering spectrometry, atomic force microscopy, corrosion resistance tests, and Raman spectroscopy. These complementary methods were used for a detailed examination of the carburization effects in the sputtered stainless-steel films. The formation of an amorphous and soft ferromagnetic phase in a wide range of the processing parameters was found. Further, the influence of the substrate temperature and of post vacuum-annealing were examined to achieve a comprehensive understanding of the carburization process and phase formation. P. Schaaf’s former affiliation: II. Physikalisches Institut, Universit?t G?ttingen, Friedrich-Hund-Platz 1, 37077 G?ttingen, Germany.     

Experience with a toroidal proportional detector for backscattered Mössbauer ψ-rays and X-rays

Because backscattering geometry in Mössbauer spectroscopy is particularly useful for non-destructive testing interesting technical applications arise. The reported toroidal proportional detector for backscattered Mössbauer ψ-rays and X-rays enables non-destructive testing of technical surfaces since no sample preparation is required. Improvements in assembling, shielding, wire-fixing and gas-filling-in comparison to the first version of the detector-are reported. The performance of the improved detector is demonstrated by measurements of steel surfaces which are modified by plasma nitriding and laser irradiation.     

CXMS study of mild steel laser alloyed with CrB2

Laser alloying of surfaces has attracted a great deal of attention for technical applications. By laser alloying of materials it is possible to improve hardness as well as wear and corrosion resistance of the surface without affecting the bulk material. The surface of a mild steel (C45) substrate was laser-alloyed with chromium-boride CrB₂. The chromium-boride was added to the substrate surface by powder injection during laser surface melting with a high power continuous-wave CO₂-laser. The resulting surface layers were studied by surface Mössbauer measurements. The backscattering geometry of Conversion X-ray Mössbauer Spectroscopy (CXMS) was used to study the phase formation in the laser alloyed surface. The results for the treated surfaces are discussed for different samples.     

Mössbauer Optimization of the Direct Synthesis of β-FeSi2 by Ion Beam Mixing of Fe/Si Bilayers

At present, there is an increasing interest in the iron di-silicide phase -FeSi₂, which is supposed to be a direct band gap semiconductor and one of the most promising materials for silicon-based optoelectronics, e.g., light-emitting devices, solar cells, and photo detectors. But this phase is very difficult to be produced. Here, the successful direct synthesis of this phase by ion beam mixing of Fe/Si bilayers at temperatures in the range of 400 to 600°C is reported. The aim of the experiments was to achieve a complete reaction of the deposited Fe layer with the Si substrate that results in the formation of a pure, single-phased -FeSi₂ surface layer. The obtained silicide layers, their structure and composition are investigated by conversion electron Mössbauer spectroscopy (CEMS), Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD). The fraction of the -FeSi₂ formed is determined by CEMS as function of ion species, energy, fluence and temperature. Complete growth and formation of a single-phased -FeSi₂ layer was achieved by 205 keV Xe ion irradiation at a fluence of 2×10¹⁶ ions/cm² at 600°C.     

Investigation of the Thermal Stability of Laser Nitrided Iron and Stainless Steel by Annealing Treatments

Laser nitriding has revealed to be a very promising and effective treatment to improve the technical properties, like surface hardness and corrosion-wear resistance, of iron and steels. The high nitrogen concentration, the fastness and precision of the treatment and the easy experimental setup make this technique very suitable for applications on industrial scale. Samples of pure iron and austenitic stainless steel have been irradiated with ns laser pulses in the UV radiation range and analyzed by means of Conversion Electron Mössbauer Spectroscopy (CEMS), Resonant Nuclear Reaction Analysis (RNRA), Grazing Incidence X-Ray Diffraction (GXRD) and Microhardness. Mössbauer Spectroscopy, in particular, is capable of detecting the phase composition of the nitrided layer and therefore represents an essential tool for these kind of analysis. The thermal stability of the treated samples have been investigated by subsequent annealings at increasing temperatures in vacuum and in air. For iron samples the annealing treatment at 250°C shows a rather drastic phase transformation from phase (fcc) into (Fe₄N) while a strong depletion of N has been observed for 400°C or higher, regardless of the ambient pressure (atmospheric or vacuum). On the other hand, the stainless steel shows a very good thermal stability up to 500°C, but higher temperatures induce a gradual decrease in the nitrogen concentration which seems to be a common feature for both pure iron and stainless steel. Furthermore, annealing in air leads to the formation of a thin oxide layer on the surface of the iron sample which is easily characterized by Mössbauer spectroscopy.     

Zero-field splitting parameters of mono- and di-azaphenanthrenes

Zero-field splitting parameters of phenanthrene and 26 azaanalogues in their lowest triplet state have been evaluated from EPR spectra; triplet lifetimes axe also given The parameters D of phenanthrene and the monoazaanalogues are in good accord with calculated values. The results are discussed in terms of differences in electron distribution and configuration.     

Mixed hyperfine interaction - A tool to investigate the short range order and the strange magnetic behaviour of amorphous Fe-based binary alloys

The Mössbauer study of the mixed magnetic dipole and electric quadrupole interaction in the paramagnetic state of amorphous Fe?Zr and Fe?Hf alloys is presented. Strong evidence for chemical short range order of the iron-pure alloys is found. The hyperfine parameters of the iron-rich alloys are marked by a complex applied field and temperature dependence, suggesting a not negligible spincorrelation well above T_c.     

Laser hydriding of crystalline and amorphous silicon

Crystalline and ion-beam-amorphized silicon samples were irradiated with a pulsed nanosecond excimer laser in a pure hydrogen atmosphere. The hydrogen concentration was determined via the ¹H(¹⁵N,)¹²C nuclear reaction. In the case of crystalline silicon, hydrogen incorporation into the sample surface was found to be well above the hydrogen solubility limit at thermodynamic equilibrium. The hydrogen depth profiles perfectly matched the damage profiles measured via Rutherford backscattering channeling spectroscopy. For the pre-amorphized silicon samples, the laser treatment resulted in epitaxial recrystallization of the amorphous top layer, but the hydrogen uptake was found to be negligible in that case. The experimental data were compared with the results of thermodynamic simulations of the laser–gas–material interaction. PACS 52.38.Mf; 61.72.Tt; 64.70.Dv