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O Stephan

Publications and source records attributed to O Stephan.

7 recordsLinked to original sources
PubMed2006

Phase segregation in cerium-lanthanum solid solutions.

Electron energy-loss spectroscopy (EELS) in combination with scanning transmission electron microscopy (STEM) reveals that the La enrichment at the surface of cerium-lanthanum solid solutions is an averaged effect and that segregation occurs in a mixed oxide phase. This separation occurs within a crystalline particle, where the dopant-rich phase is located at the surface of the dopant-deficient phase. The limiting structure appears to be a solid solution with a La fraction of x = 0.6 in the bulk and x = 0.75 at the surface. Up to a La fraction of 0.6, this phase will coexist with a lanthanum-type structure in different proportions depending on the dopant amount. STEM-EELS appears to be a powerful technique to clarify the existence of a multiphase system, and it shows that XRF, XPS, and XRD measure averaged results and do not show the phase complexity of the solids.

Journal Article↗
PubMed2006

Defective structure of BN nanotubes: from single vacancies to dislocation lines.

A combination of electron microscopy and theoretical calculations provides new insights into the structure, electronics, and energetics of point defects and vacancy lines in BN single-wall nanotubes (SWNT). We show that the point defects forming under electron irradiation in the BN SWNTs are primarily divacancies. Due to the partially ionic character of the BN bonding, divacancies behave like an associated Schottky pair, with a dissociation energy of around 8 eV. Clustering of multiple vacancies is energetically favorable and leads to extended defects which locally change the nanotube diameter and chirality. Nevertheless these defects do not alter significantly the band gap energy, and all of them have electronic structure similar to that of BN divacancies. We thus conclude that under irradiation BN SWNT may have a very stable alteration of its electronic and optical properties.

Boron Compounds↗
PubMed2001

Porcine endogenous retroviruses (PERVs): generation of specific antibodies, development of an immunoperoxidase assay (IPA) and inhibition by AZT.

Xenotransplantation may be associated with the risk of transmission of microorganisms. In particular, the porcine endogenous retroviruses (PERV) have raised concerns as in vitro experiments show susceptibility of human cells for PERV infection. However, it remains unclear whether PERVs are able to infect transplant recipients in vivo and whether they are pathogenic. It is therefore essential that the risks are evaluated and for this purpose specific and sensitive screening methods for PERVs have to be developed. We generated specific antibodies against all major structural proteins of PERV and developed several assays which allow antibodies against PERV to be detected as indirect evidence of infection. For direct detection of PERV production, reverse transcriptase (RT) assays were used. PCR methods were used to detect provirus integration and the presence of viral mRNA. Here we present an immunoperoxidase assay (IPA), which would allow the detection of viral proteins in infected cells as well as antibodies against PERV in the serum of an infected host. The specificity of the sera used in the assay was determined by several methods, including immunoelectron microscopy, and the sensitivity of the assay was compared with other methods. This IPA was used to detect PERV infection in in vitro experiments for evaluation of the virus host range, for titrating the virus and for testing anti-viral properties of AZT. Using this method it was shown that AZT inhibits replication of PERV. This IPA may be very useful for the surveillance of preclinical and clinical xenotransplantations.

Amino Acid Sequence↗