1,392 research outputs found
Effect of stretching on the ballistic conductance of Au nanocontacts in presence of CO: a density functional study
CO adsorption on an Au monatomic chain is studied within density functional
theory in nanocontact geometries as a function of the contact stretching. We
compare the bridge and atop adsorption sites of CO, finding that the bridge
site is energetically favored at all strains studied here. Atop adsorption
gives rise to an almost complete suppression of the ballistic conductance of
the nanocontact, while adsorption at the bridge site results in a conductance
value close to 0.6 G0, in agreement with previous experimental data. We show
that only the bridge site can qualitatively account for the evolution of the
conductance as a function of the contact stretching observed in the
experimental conductance traces. The numerical discrepancy between the
theoretical and experimental conductance slopes is rationalized through a
simple model for the elastic response of the metallic leads. We also verify
that our conductance values are not affected by the specific choice of the
nanocontact geometry by comparing two different atomistic models for the tips
Heavy Metal Pollutions: State of the Art and Innovation in Phytoremediation
Mineral nutrition of plants greatly depends on both environmental conditions, particularly of soils, and the genetic background of the plant itself. Being sessile, plants adopted a range of strategies for sensing and responding to nutrient availability to optimize development and growth, as well as to protect their metabolisms from heavy metal toxicity. Such mechanisms, together with the soil environment, meaning the soil microorganisms and their interaction with plant roots, have been extensively studied with the goal of exploiting them to reclaim polluted lands; this approach, defined phytoremediation, will be the subject of this review. The main aspects and innovations in this field are considered, in particular with respect to the selection of efficient plant genotypes, the application of improved cultural strategies, and the symbiotic interaction with soil microorganisms, to manage heavy metal polluted soils
CONTROLLING HEAVY METAL ACCUMULATION IN PLANTS
The present study is aimed at understanding the interactions between plants, microorganisms
and heavy metals. We have considered a natural environment, in Northern France, that shows high
cadmium, zinc and lead pollution. Plants of the metal hyperaccumulator Arabidopsis halleri were
collected together with their rhyzospheric soil. Several bacterial strains were then isolated from this
soil and identified. Among these, a strain of Pseudomonas putida was considered for further
analyses. Proteomic analysis was performed on this strain (called P. putida-Cd001) grown with and
without Cd added to the nutrient medium. The differential patterns of protein expression were
visualized by two dimensional electrophoresis technique and proteins were identified by MS
analysis. Results showed that many different membrane proteins were up-regulated upon Cd
treatment, in particular membrane transporters
The role of ABC1K7 and ABC1K8, two ABC1K kinases of Arabidopsis thaliana
The activity of bc1 complex kinases (ABC1K) belong to a large group of atypical protein kinases found in prokaryotes and eukaryotes. In bacteria and mitochondria, ABC1K kinases are involved in the respiratory pathway, being necessary for the synthesis of the coenzyme Q. In chloroplasts, ABC1K proteins play a role in prenylquinone synthesis and stress responses, but their precise function remains unclear. A functional characterization was carried out for ABC1K7 and ABC1K8, two ABC1K proteins of the Arabidopsis thaliana plastome. The comparison of abc1k7 and abc1k8 mutants, abc1k7/abc1k8 double mutant and wild-type plants revealed a reduction in plastidial ironcontaining proteins of the Cytb6f complex in the mutants. Iron uptake from soil is not hampered in mutant lines, suggesting that ABC1K7 and ABC1K8 affect iron distribution within the chloroplast. Moreover, mutant plants accumulated more ferritin and superoxide, and showed reduced tolerance to reactive oxygen species (ROS). Because ROS take part in abscisic acid (ABA) signaling, we investigated the relation between ABA and ABC1K7 and ABC1K8 and found that both genes were upregulated by ABA treatment, while expression of several ABA-responsive genes resulted affected in mutants. Moreover, analyzing ABA-mediated processes, we determined that germination was more affected by ABA treatment and osmotic and salt stress in the single and double mutants than in wild-type plants. Stomatal aperture was also reduced in the mutants under standard growth conditions and was not further reduced by exogenous ABA application. Furthermore, ABA-induced senescence symptoms were more severe in the leaves of the mutants compared to wild type leaves. Taken together, these data suggest that ABC1K7 and ABC1K8 probably act in signaling pathways that influence responses to ROS production and oxidative stress, such as ABA signaling, probably by influencing the cellular redox state and chloroplast lipid metabolism
Interaction of a CO molecule with a Pt monatomic wire: electronic structure and ballistic conductance
We carry out a first-principles density functional study of the interaction
between a monatomic Pt wire and a CO molecule, comparing the energy of
different adsorption configurations (bridge, on top, substitutional, and tilted
bridge) and discussing the effects of spin-orbit (SO) coupling on the
electronic structure and on the ballistic conductance of two of these systems
(bridge and substitutional). We find that, when the wire is unstrained, the
bridge configuration is energetically favored, while the substitutional
geometry becomes possible only after the breaking of the Pt-Pt bond next to CO.
The interaction can be described by a donation/back-donation process similar to
that occurring when CO adsorbs on transition-metal surfaces, a picture which
remains valid also in presence of SO coupling. The ballistic conductance of the
(tipless) nanowire is not much reduced by the adsorption of the molecule on the
bridge and on-top sites, but shows a significant drop in the substitutional
case. The differences in the electronic structure due to the SO coupling
influence the transmission only at energies far away from the Fermi level so
that fully- and scalar-relativistic conductances do not differ significantly.Comment: 12 pages, 12 figures; figure misplacement and minor syntax issues
fixed, some references updated and correcte
A First-Principles Approach to Insulators in Finite Electric Fields
We describe a method for computing the response of an insulator to a static,
homogeneous electric field. It consists of iteratively minimizing an electric
enthalpy functional expressed in terms of occupied Bloch-like states on a
uniform grid of k points. The functional has equivalent local minima below a
critical field E_c that depends inversely on the density of k points; the
disappearance of the minima at E_c signals the onset of Zener breakdown. We
illustrate the procedure by computing the piezoelectric and nonlinear
dielectric susceptibility tensors of III-V semiconductors.Comment: 4 pages, with 1 postscript figure embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/is_ef/index.htm
Interaction of a CO molecule with a Pt monoatomic chain: the top geometry
Recent experiments showed that the conductance of Pt nanocontacts and
nanowires is measurably reduced by adsorption of CO. We present DFT
calculations of the electronic structure and ballistic conductance of a Pt
monoatomic chain and a CO molecule adsorbed in an on-top position. We find that
the main electronic molecule-chain interaction occurs via the and
orbitals of the molecule, involved in a donation/back-donation
process similar to that of CO on transition-metal surfaces. The ideal ballistic
conductance of the monoatomic chain undergoes a moderate reduction by about 1.0
G_0 (from 4 G_0 to 3.1 G_0) upon adsorption of CO. By repeating all
calculations with and without spin-orbit coupling, no substantial spin-orbit
induced change emerges either in the chain-molecule interaction mechanism or in
the conductance.Comment: 4 pages, 2 figures, in proceedings of Frontiers of Fundamental and
Computational Physic
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