Pollution physico-chimique et bactériologique d’un écosystème aquatique et ses risques écotoxicologiques: Cas du lac Nokoue au Sud Benin

La protection des écosystèmes aquatiques est indispensable à l’équilibre écologique des espèces halieutiques et à une saine alimentation des populations locales. Le lac Nokoué situé au Sud du Bénin est sous l’influence de plusieurs formes de pollution qui résultent pour la plupart des activités anthropiques. Ce travail se propose de faire une évaluation des diverses formes de pollution que subi aujourd’hui ce lac, afin d’alerter sur les risques écotoxicologiques. Pour atteindre cet objectif, des visites du terrain d’étude, des analyses physico-chimiques et bactériologiques ont été effectuées, des entretiens avec des personnes ressources ont été organisés et des enquêtes de terrain ont été menées auprès des populations locales. Les résultats issus des analyses physico-chimiques et bactériologiques de l’eau du lac Nokoué ont permis d’identifier les sources des divers types de polluants du lac Nokoué. La teneur en oxygène dissous varie de 3,62 mg /L à 2,76 mg/L ; la DBO5 de 29 mg/L à 10 mg/L ; la DCO est de 253 mg/L ; la teneur en ammonium est supérieure à 8 mg/L ; la teneur en nitrite avoisine 0,7 mg /L par endroit ; la teneur en coliformes totaux fluctue de 4000 /100 mL à 6000/100 mL ; la teneur en streptocoques fécaux varie de 500/100mL à 4000/100 mL. Ces résultats montrent que le lac Nokoué est pollué. Des stratégies pour limiter la pollution du lac doivent être mises en place par les décideurs à tous les niveaux afin d’éviter des problèmes d’écosanté aux populations du lac Nokoué, ce qui pourrait constituer aussi un frein pour le développement du pays.Keywords: Pollution, écosystème, lac Nokoué, risques écotoxicologique

Electron-nuclei spin dynamics in II-VI semiconductor quantum dots

We report on the dynamics of optically induced nuclear spin polarization in individual CdTe/ZnTe quantum dots loaded with one electron by modulation doping. The fine structure of the hot trion (charged exciton $X^-$ with an electron in the $P$-shell) is identified in photoluminescence excitation spectra. A negative polarisation rate of the photoluminescence, optical pumping of the resident electron and the built-up of dynamic nuclear spin polarisation (DNSP) are observed in time-resolved optical pumping experiments when the quantum dot is excited at higher energy than the hot trion triplet state. The time and magnetic field dependence of the polarisation rate of the $X^-$ emission allows to probe the dynamics of formation of the DNSP in the optical pumping regime. We demonstrate using time-resolved measurements that the creation of a DNSP at B=0T efficiently prevents longitudinal spin relaxation of the electron caused by fluctuations of the nuclear spin bath. The DNSP is built in the microsecond range at high excitation intensity. A relaxation time of the DNSP in about 10 microseconds is observed at $B=0T$ and significantly increases under a magnetic field of a few milli-Tesla. We discuss mechanisms responsible for the fast initialisation and relaxation of the diluted nuclear spins in this system

Etude dynamique des apports en éléments majeurs et nutritifs des eaux de la lagune de Porto-Novo (Sud Bénin)

Afin dfevaluer le niveau trophique de la lagune qui expliquerait la proliferation des algues sur la lagune, les nutriments (NTK et phosphore total) et la Demande Chimique en Oxygene (DCO) des eaux de la lagunePorto-Novo ont ete determines suivant les quatre saisons de lfannee. Lfetude a egalement permis dfanalyser par chromatographie les elements mineraux, anions et cations, dans les eaux de la lagune. Les resultats ont montre que les concentrations en chlore (variant entre 0,84 et 1180 mg/L) et en sodium (variant entre 35,52 et 7305 mg/L) sont plus elevees au cours de la periode de basses eaux ; les eaux marines arrivent dans la lagune par lfintermediaire du lac Nokoue. Les concentrations obtenues au cours de la periode des basses eaux pour les NTK montrent que le Canal de Totche et la savonnerie de lfIndustrie Beninoise des Corps Gras (IBCG) sont tres riches en NTK. De meme, les concentrations elevees pour le phosphore total et les phosphates sont obtenues au debut de la saison des pluies, la concentration la plus elevee est obtenue a lfIBCG. Les valeurs moyennes annuelles de phosphore total dans les eaux varient de 0,31 mg/L a 6,05 mg/L, elles depassent donc 100 ƒÊgP/L : la lagune est donc hypereutrophe selon lfOrganisation de Cooperation et Developpement Economique (OCDE, 1982). Quant aux valeurs moyennes de la DCO, elles varient de 101 mgO2/L a 142 mgO2/L. Les mesures effectuees au cours des differentes periodes de lfannee montrent que les eaux de ruissellement contribuent beaucoup a lfeutrophisation de la lagune. Ceci montre aussi lfurgence de la mise en place dfun systeme de gestion et de controle des eaux usees avant leur deversement dans la lagune

Control of defect-mediated tunneling barrier heights in ultrathin MgO films

The impact of oxygen vacancies on local tunneling properties across rf-sputtered MgO thin films was investigated by optical absorption spectroscopy and conducting atomic force microscopy. Adding O$_2$ to the Ar plasma during MgO growth alters the oxygen defect populations, leading to improved local tunneling characteristics such as a lower density of current hotspots and a lower tunnel current amplitude. We discuss a defect-based potential landscape across ultrathin MgO barriers.Comment: 4 pages, 4 figure

Porosity of silica Stöber particles determined by spin-echo small angle neutron scattering.

Stöber silica particles are used in a diverse range of applications. Despite their widespread industrial and scientific uses, information on the internal structure of the particles is non-trivial to obtain and is not often reported. In this work we have used spin-echo small angle neutron scattering (SESANS) in conjunction with ultra small angle X-ray scattering (USAXS) and pycnometry to study an aqueous dispersion of Stöber particles. Our results are in agreement with models which propose that Stöber particles have a porous core, with a significant fraction of the pores inaccessible to solvent. For samples prepared from the same master sample in a range of H2O : D2O ratio solutions we were able to model the SESANS results for the solution series assuming monodisperse, smooth surfaced spheres of radius 83 nm with an internal open pore volume fraction of 32% and a closed pore fraction of 10%. Our results are consistent with USAXS measurements. The protocol developed and discussed here shows that the SESANS technique is a powerful way to investigate particles much larger than those studied using conventional small angle scattering methods

Structure of self-organized Fe clusters grown on Au(111) analyzed by Grazing Incidence X-Ray Diffraction

We report a detailed investigation of the first stages of the growth of self-organized Fe clusters on the reconstructed Au(111) surface by grazing incidence X-ray diffraction. Below one monolayer coverage, the Fe clusters are in "local epitaxy" whereas the subsequent layers adopt first a strained fcc lattice and then a partly relaxed bcc(110) phase in a Kurdjumov-Sachs epitaxial relationship. The structural evolution is discussed in relation with the magnetic properties of the Fe clusters.Comment: 7 pages, 6 figures, submitted to Physical Review B September 200

Ferromagnetic semiconductors

The current status and prospects of research on ferromagnetism in semiconductors are reviewed. The question of the origin of ferromagnetism in europium chalcogenides, chromium spinels and, particularly, in diluted magnetic semiconductors is addressed. The nature of electronic states derived from 3d of magnetic impurities is discussed in some details. Results of a quantitative comparison between experimental and theoretical results, notably for Mn-based III-V and II-VI compounds, are presented. This comparison demonstrates that the current theory of the exchange interactions mediated by holes in the valence band describes correctly the values of Curie temperatures T_C magnetic anisotropy, domain structure, and magnetic circular dichroism. On this basis, chemical trends are examined and show to lead to the prediction of semiconductor systems with T_C that may exceed room temperature, an expectation that are being confirmed by recent findings. Results for materials containing magnetic ions other than Mn are also presented emphasizing that the double exchange involving hoping through d states may operate in those systems.Comment: 18 pages, 8 figures; special issue of Semicon. Sci. Technol. on semiconductor spintronic

Direct observation of a highly spin-polarized organic spinterface at room temperature

Organic semiconductors constitute promising candidates toward large-scale electronic circuits that are entirely spintronics-driven. Toward this goal, tunneling magnetoresistance values above 300% at low temperature suggested the presence of highly spin-polarized device interfaces. However, such spinterfaces have not been observed directly, let alone at room temperature. Thanks to experiments and theory on the model spinterface between phthalocyanine molecules and a Co single crystal surface, we clearly evidence a highly efficient spinterface. Spin-polarised direct and inverse photoemission experiments reveal a high degree of spin polarisation at room temperature at this interface. We measured a magnetic moment on the molecule’s nitrogen p orbitals, which substantiates an ab-initio theoretical description of highly spin-polarised charge conduction across the interface due to differing spinterface formation mechanisms in each spin channel. We propose, through this example, a recipe to engineer simple organic-inorganic interfaces with remarkable spintronic properties that can endure well above room temperature

Magnetoresistance through a single molecule

The use of single molecules to design electronic devices is an extremely challenging and fundamentally different approach to further downsizing electronic circuits. Two-terminal molecular devices such as diodes were first predicted [1] and, more recently, measured experimentally [2]. The addition of a gate then enabled the study of molecular transistors [3-5]. In general terms, in order to increase data processing capabilities, one may not only consider the electron's charge but also its spin [6,7]. This concept has been pioneered in giant magnetoresistance (GMR) junctions that consist of thin metallic films [8,9]. Spin transport across molecules, i.e. Molecular Spintronics remains, however, a challenging endeavor. As an important first step in this field, we have performed an experimental and theoretical study on spin transport across a molecular GMR junction consisting of two ferromagnetic electrodes bridged by a single hydrogen phthalocyanine (H2Pc) molecule. We observe that even though H2Pc in itself is nonmagnetic, incorporating it into a molecular junction can enhance the magnetoresistance by one order of magnitude to 52%.Comment: To appear in Nature Nanotechnology. Present version is the first submission to Nature Nanotechnology, from May 18th, 201