Serge Nitsche

Epitaxial growth of β-FeSi2 on silicon (111): a real-time RHEED analysis

Abstract We present a detailed analysis of the solid phase epitaxy (SPE) of β-FeSi 2 on the silicon (111) face. The initial thickness of the iron film epitaxially deposited on the (111) silicon face has been varied from 2 toA. Using a well-defined procedure of annealing (a fixed heating rate d T /d t = 1−4°C), we show that this initial iron thickness is a key parameter which controls the kinetic behaviour of Fe Si intermixing at the interface. Indeed at the same temperature after the same thermal treatment, completely different chemical and crystalline states have been observed at the surface depending essentially upon the original iron film thickness. This kinetic behaviour at the Fe Si interface is experimentally analysed through a quantitative analysis of RHEED patterns. Major facts are: (i) all the low-temperature equilibrium Fe Si phases (bcc Fe(+Si), simple cubic FeSi, β-FeSi 2 ) have been grown epitaxially on the silicon (111) face, (ii) an epitaxial FeSi 2 strained phase has been clearly identified by the RHEED technique. This result is suggested to be an intrinsic consequence of the silicon stress field applied to the distorted fluorite structure of β-FeSi 2 ; (iii) a transient amorphous phase is most probably formed at 300 T e Fe ≈ 30–50A, preceding the formation of the epitaxial β-FeSi 2 phase. The appearance of such an amorphous phase at a definite iron thickness adds new experimental insights into solid state amorphisation which is a typical process occuring at interfaces of transition-metal systems.

Calcium Carbonate Crystals Promote Calcium Oxalate Crystallization by Heterogeneous or Epitaxial Nucleation: Possible Involvement in the Control of Urinary Lithogenesis

A large proportion of urinary stones have calcium oxalate (CaOx) as the major mineral phase. In these stones, CaOx is generally associated with minor amounts of other calcium salts. Several reports showing the presence of calcium carbonate (CaCO3) and calcium phosphate in renal stones suggested that crystals of those salts might be present in the early steps of stone formation. Such crystals might therefore promote CaOx crystallization from supersaturated urine by providing an appropriate substrate for heterogeneous nucleation. That possibility was investigated by seeding a metastable solution of45Ca oxalate with vaterite or calcite crystallites. Accretion of CaOx was monitored by45Ca incorporation. We showed that (1) seeds of vaterite (the hexagonal polymorph of CaCO3) and calcite (the rhomboedric form) could initiate calcium oxalate crystal growth; (2) in the presence of lithostathine, an inhibitor of CaCO3 crystal growth, such accretion was not observed. In addition, scanning electron microscopy demonstrated that growth occurred by epitaxy onto calcite seeds whereas no special orientation was observed onto vaterite. It was concluded that calcium carbonate crystals promote crystallization of calcium oxalate and that inhibitors controlling calcium carbonate crystal formation in Henle’s loop might play an important role in the prevention of calcium oxalate stone formation.A large proportion of urinary stones have calcium oxalate (CaOx) as the major mineral phase. In these stones, CaOx is generally associated with minor amounts of other calcium salts. Several reports showing the presence of calcium carbonate (CaCO3) and calcium phosphate in renal stones suggested that crystals of those salts might be present in the early steps of stone formation. Such crystals might therefore promote CaOx crystallization from supersaturated urine by providing an appropriate substrate for heterogeneous nucleation. That possibility was investigated by seeding a metastable solution of45Ca oxalate with vaterite or calcite crystallites. Accretion of CaOx was monitored by45Ca incorporation. We showed that (1) seeds of vaterite (the hexagonal polymorph of CaCO3) and calcite (the rhomboedric form) could initiate calcium oxalate crystal growth; (2) in the presence of lithostathine, an inhibitor of CaCO3 crystal growth, such accretion was not observed. In addition, scanning electron microscopy demonstrated that growth occurred by epitaxy onto calcite seeds whereas no special orientation was observed onto vaterite. It was concluded that calcium carbonate crystals promote crystallization of calcium oxalate and that inhibitors controlling calcium carbonate crystal formation in Henle’s loop might play an important role in the prevention of calcium oxalate stone formation.

Preferential carbon etching by hydrogen inside hexagonal voids of 6H-SiC(0001)

6H-SiC(0001) samples have been etched in a hot-wall chemical vapor deposition reactor at a hydrogen pressure of 13 mbar at 1800 ° C . The surface morphology and elemental composition have been studied by scanning electron microscopy and micro-Auger analysis. Stoichiometric etching of SiC with equal atomic concentrations of Si and C is found on the flat sections of the surface, but in hexagonal voids of the SiC samples, a selective removal of C, leading to a pure Si surface at the bottom of the voids, is observed. Fast gas diffusion is the main transport mechanism for etching of the flat surface, while Knudsen diffusion becomes important inside the voids. It is proposed that the lower diffusion constant of reaction products containing Si compared to those containing C, leads to a preferential removal of C and a Si enrichment inside the voids.

Hydrogen bioelectrooxidation on gold nanoparticle-based electrodes modified by Aquifex aeolicus hydrogenase: Application to hydrogen/oxygen enzymatic biofuel cells.

For the first time, gold nanoparticle-based electrodes have been used as platforms for efficient immobilization of the [NiFe] hydrogenase from the hyperthermophilic bacterium Aquifex aeolicus. AuNPs were characterized by electronic microscopy, dynamic light scattering and UV-Vis spectroscopy. Two sizes around 20.0±5.3 nm and 37.2±4.3 nm nm were synthesized. After thiol-based functionalization, the AuNPs were proved to allow direct H2 oxidation over a large range of temperatures. A high current density up to 1.85±0.15 mA·cm(-2) was reached at the smallest AuNPs, which is 170 times higher than the one recorded at the bare gold electrode. The catalytic current was especially studied as a function of the AuNP size and amount, and procedure for deposition. A synergetic effect between the AuNP porous deposit and the increase surface area was shown. Compared to previously used nanomaterials such as carbon nanofibers, the covalent grafting of the enzyme on the thiol-modified gold nanoparticles was shown to enhance the stability of the hydrogenase. This bioanode was finally coupled to a biocathode where BOD from Myrothecium verrucaria was immobilized on AuNP-based film. The performance of the so-mounted H2/O2 biofuel cell was evaluated, and a power density of 0.25 mW·cm(-2) was recorded.

Experimental and modeling studies of the link between microstructure and effective diffusivity of cement pastes

ABSTRACT This paper presents a review of complementary experimental and modeling studies of the link between microstructure and diffusivity of cement pastes. These studies were performed in the framework of the workgroup Concrete Engineering Barrier headed by Andra. Knowledge of the link between microstructure and diffusivity is needed to run properly the coupled chemical, transport and mechanical models used to assess the long term behavior of concrete in deep repository conditions. The first part of the paper is devoted to a description of the porous texture of ordinary Portland cement pastes, in particular obtained using Transmission Electronic Microscopy. The second and third parts are respectively dedicated to the presentations of an analytical method and a 3D numerical one, both of them aimed at linking microstructure and diffusivity of cement pastes.

Nanocavity Buffer Induced by Gas Ion Implantation in Silicon Substrate for Strain Relaxation of Heteroepitaxial Si1-xGex/Si Thin Layers

To weight the importance of a nanocavity buffer in a SiGe deposition substrate, some P type (001) FZ Si wafers are implanted (A samples) or not (B samples) at room temperature with 5×10 16 He + cm –2 at 10keV. They are annealed at 700°C for one hour to form a nanocavity layer close to the Si surface. Then, the wafers are carefully chemically cleaned in a clean room to remove both organic and metallic impurities from the surface. They are coated either by 210 nm (A) or 430 nm (B) Si 1−x Ge x (x=0.20±0.02) alloy grown at 575°C for 0.42 hour by low pressure chemical vapor deposition (LP-CVD) with a growth rate of 8 to 17 nm.mn −1 . Both kinds of samples are studied by cross section transmission electron microscopy, X-rays diffraction, Rutherford backscattering, atomic force microscopy and etch pit counts. The association of these techniques demonstrates that the thin SiGe layer which is deposited on sample A is fully relaxed and that the threading dislocation density (estimated to hardly reach 4×10 3 cm −2 ) is at least one order of magnitude lower than what is obtained so far using ion implantation assistance in SiGe layer growth on Silicon. The roughness of the SiGe surface is low enough to stand a further Si epitaxy. Nevertheless, the mechanism involved responsible for the threading dislocation annihilation and/or confinement is still unclear.

Properties of nano-structured cuprous oxide thin films fabricated by thermal oxidation of copper layer

Among the potential photovoltaic devices based on semiconductor oxides as active layer is cuprous oxide (Cu2O). Although the theoretical limit of Cu2O solar cell efficiency is 20%, the best efficiency obtained up to now is only 2%. This is due to a very limited amount of work devoted to this semiconductor and only during last few years this material has been investigated for solar cells applications. In this work we report our results of optical, structural and surface morphology investigations of Cu2O films prepared by thermal oxidation of copper layer. The effects of oxidation temperature and oxygen partial pressure on surface morphology and crystalline structure of Cu2O films were studied. Scanning electron microscope results have shown that Cu2O films have microcrystalline structure with grain size of about 5-15 μm. Analysis of fine structure shows typical lattice spacing of cubic Cu2O structure. X-ray investigations have shown that the films consist of single Cu2O phase without any interstitial phase and have a nano-grain structure. The grains have an average dimensions about (33-41) nm. Optical investigations have shown that the absorption edge of prepared films is due to a direct allowed transition. The value of the optical band gap is 2.08 eV.

Calcium Carbonate Crystals Promote Epitaxial Calcium Oxalate Crystal Growth

Calcium oxalate (CaOx) urolithiasis is the most common urinary stone disease in the Occident. The physiopathology of this kind of stone is not well understood because crystallization occurs in vivo in a very complex environment whose parameters are difficult to control. Calcium oxalate is the major mineral phase of these stones, but it is generally associated with minor amounts of other calcium salts, including carbonate and phosphate.