118 research outputs found
Photoluminescence Stokes shift and exciton fine structure in CdTe nanocrystals
The photoluminescence spectra of spherical CdTe nanocrystals with zincblende
structure are studied by size-selective spectroscopic techniques. We observe a
resonant Stokes shift of 15 meV when the excitation laser energy is tuned to
the red side of the absorption band at 2.236 eV. The experimental data are
analyzed within a symmetry-based tight-binding theory of the exciton spectrum,
which is first shown to account for the size dependence of the fundamental gap
reported previously in the literature. The theoretical Stokes shift presented
as a function of the gap shows a good agreement with the experimental data,
indicating that the measured Stokes shift indeed arises from the electron-hole
exchange interaction.Comment: 8 pages, 4 figures, LaTe
Hole spin dephasing time associated to hyperfine interaction in quantum dots
The spin interaction of a hole confined in a quantum dot with the surrounding
nuclei is described in terms of an effective magnetic field. We show that, in
contrast to the Fermi contact hyperfine interaction for conduction electrons,
the dipole-dipole hyperfine interaction is anisotropic for a hole, for both
pure or mixed hole states. We evaluate the coupling constants of the
hole-nuclear interaction and demonstrate that they are only one order of
magnitude smaller than the coupling constants of the electron-nuclear
interaction. We also study, theoretically, the hole spin dephasing of an
ensemble of quantum dots via the hyperfine interaction in the framework of
frozen fluctuations of the nuclear field, in absence or in presence of an
applied magnetic field. We also discuss experiments which could evidence the
dipole-dipole hyperfine interaction and give information on hole mixing.Comment: 35 pages, 7 figures and 2 table
Determinação de selenometionina por quimiolumiescĂȘncia em castanha do Brasil.
SelĂȘnio Ă© reconhecido como um micronutriente essencial. PorĂ©m, a bioutilidade deste elemento estĂĄ intrĂsecamente dependente da sua forma quĂmica. EspĂ©cies orgĂąnicas de selĂȘnio sĂŁo mais facilmente absorvidas por organismos humanos quando comparado a suas espĂ©cies inorgĂąnicas. Castanhas do Brasil podem ser consideradas uma boa fonte de selĂȘnio, pois apresenta concentraçÔes significativas deste elemento, principalmente na forma de selenometionina.1 Neste trabalho, sistema para anĂĄlise em fluxo para a determinação de selenometionina em amostras de castanha do Brasil empregando quimioluminescĂȘncia foi desenvolvido, utilizando uma cĂ©lula de reação com estrutura em vĂłrtex construĂda com tecnologia LTCC2
Cinética da adsorção de Cd, Co, Cr, Cu, K, Ni e Zn em soluçÔes aquosas usando zeólita natural integrada à tecnologia LTCC.
Materiais naturais, disponĂveis em grandes quantidades e que possam ser empregados como adsorventes de baixo custo para o tratamento de efluentes, vem sendo o alvo de inĂșmeras pesquisas. Quitosana, zeĂłlitas, esponjas naturais e carvĂŁo ativado sĂŁo empregados com sucesso para este propĂłsito1. Dentro deste contexto, o objetivo do trabalho foi investigar a capacidade de remoção de Cd, Co, Cr, Cu, K, Ni e Zn atravĂ©s de uma amostra de zeĂłlita natural integrada a dispositivos cerĂąmicos mediante a tecnologia LTCC
Novel LTCC-potentiometric microfluidic device for biparametric analysis of organic compounds carrying plastic antibodies as ionophores: Application to sulfamethoxazole and trimethoprim
Monitoring organic environmental contaminants is of crucial importance to ensure public health. This requires simple, portable and robust devices to carry out on-site analysis. For this purpose, a low-temperature co-fired ceramics (LTCC) microfluidic potentiometric device (LTCC/ÎŒPOT) was developed for the first time for an organic compound: sulfamethoxazole (SMX).
Sensory materials relied on newly designed plastic antibodies. Solâgel, self-assembling monolayer and molecular-imprinting techniques were merged for this purpose. Silica beads were amine-modified and linked to SMX via glutaraldehyde modification. Condensation polymerization was conducted around SMX to fill the vacant spaces. SMX was removed after, leaving behind imprinted sites of complementary shape. The obtained particles were used as ionophores in plasticized PVC membranes. The most suitable membrane composition was selected in steady-state assays. Its suitability to flow analysis was verified in flow-injection studies with regular tubular electrodes.
The LTCC/ÎŒPOT device integrated a bidimensional mixer, an embedded reference electrode based on Ag/AgCl and an Ag-based contact screen-printed under a micromachined cavity of 600 ÎŒm depth. The sensing membranes were deposited over this contact and acted as indicating electrodes. Under optimum conditions, the SMX sensor displayed slopes of about â58.7 mV/decade in a range from 12.7 to 250 ÎŒg/mL, providing a detection limit of 3.85 ÎŒg/mL and a sampling throughput of 36 samples/h with a reagent consumption of 3.3 mL per sample.
The system was adjusted later to multiple analyte detection by including a second potentiometric cell on the LTCC/ÎŒPOT device. No additional reference electrode was required. This concept was applied to Trimethoprim (TMP), always administered concomitantly with sulphonamide drugs, and tested in fish-farming waters. The biparametric microanalyzer displayed Nernstian behaviour, with average slopes â54.7 (SMX) and +57.8 (TMP) mV/decade. To demonstrate the microanalyzer capabilities for real applications, it was successfully applied to single and simultaneous determination of SMX and TMP in aquaculture waters
Exciton states and optical properties of CdSe nanocrystals
The optical spectra of CdSe nanocrystals up to 55 A in diameter are analyzed
in a wide range of energies from the fine structure of the low-energy
excitations to the so-called high-energy transitions. We apply a symmetry-based
method in two steps. First we take the tight-binding (TB) parameters from the
bulk sp^{3}s^{*} TB model, extended to include the spin-orbit interaction. The
full single-particle spectra are obtained from an exact diagonalization by
using a group-theoretical treatment. The electron-hole interaction is next
introduced: Both the Coulomb (direct) and exchange terms are considered. The
high-energy excitonic transitions are studied by computing the electric dipole
transition probabilities between single-particle states, while the transition
energies are obtained by taking into account the Coulomb interaction. The fine
structure of the lowest excitonic states is analyzed by including the
electron-hole exchange interaction and the wurtzite crystal-field terms in the
exciton Hamiltonian. The latter is diagonalized in the single electron-hole
pair excitation subspace of progressively increasing size until convergence.
The peaks in the theoretical transition spectra are then used to deduce the
resonant and nonresonant Stokes shifts, which are compared with their measured
values in photoluminescence experiments. We find that the final results depend
on the crystal-field term, the relative size of the surface and the degree of
saturation of the dangling bonds. The results show a satisfactory agreement
with the available experimental data.Comment: Revtex, 24 pages, 7 Postscript figure
Measuring spatio-temporal variability of chemical seawater parameters around the Spanish coasts
Regional but not global temperature variability underestimated by climate models at supradecadal timescales
Knowledge of the characteristics of natural climate variability is vital when assessing the range of plausible future climate trajectories in the next decades to centuries. The reliable detection of climate fluctuations on multidecadal to centennial timescales depends on proxy reconstructions and model simulations, as the instrumental record extends back only a few decades in most parts of the world. Systematic comparisons between model-simulated and proxy-based inferences of natural variability, however, often seem contradictory. Locally, simulated temperature variability is consistently smaller on multidecadal and longer timescales than is indicated by proxy-based reconstructions, implying that climate models or proxy interpretations might have deficiencies. In contrast, at global scales, studies found agreement between simulated and proxy reconstructed temperature variations. Here we review the evidence regarding the scale of natural temperature variability during recent millennia. We identify systematic reconstruction deficiencies that may contribute to differing local and global modelâproxy agreement but conclude that they are probably insufficient to resolve such discrepancies. Instead, we argue that regional climate variations persisted for longer timescales than climate models simulating past climate states are able to reproduce. This would imply an underestimation of the regional variability on multidecadal and longer timescales and would bias climate projections and attribution studies. Thus, efforts are needed to improve the simulation of natural variability in climate models accompanied by further refining proxy-based inferences of variability.This study was undertaken by members of CVAS and 2k Network, working groups of the Past Global Changes (PAGES) Global Research association. This is a contribution to the SPACE ERC, STACY and PALMOD projects. The SPACE ERC project has received funding from the European Research Council (ERC) under the European Unionâs Horizon 2020 research and innovation programme (grant agreement no. 716092). STACY has been funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, project no. 395588486). This work has also been supported by the German Federal Ministry of Education and Research (BMBF), through the PalMod project (subprojects 01LP1926B (O.B.), 01LP1926D (M.C.) and 01LP1926C (B.E., P.S. and N.W.)) from the Research for Sustainability initiative (FONA). B.E. is supported by the Heinrich Böll Foundation. E.M.-C. was supported by the PARAMOUR project, funded by the Fonds de la Recherche ScientifiqueâFNRS and the FWO under the Excellence of Science (EOS) programme (grant no. O0100718F, EOS ID no. 30454083). A.H. was supported by a Legacy Grant from the Australian Research Council Centre of Excellence for Australian Biodiversity and Heritage. B.M. was supported by LINKA20102 and the Spanish Ministry of Science and Innovation project CEX2018â000794âS. The work originated from discussions at the CVAS working group of PAGES at a workshop at the Internationales Wissenschaftsforum Heidelberg, which was funded by a Hengstberger Prize. We thank N. Beech, C. Brierley, F. Gonzalez-Rouco and M. MacPartland for comments on earlier drafts of the manuscript. This manuscript uses data provided by the World Climate Research Programmeâs Working Group on Coupled Modelling, which is responsible for CMIP and PMIP. We thank the research groups for producing and kindly making their model outputs, measurements and palaeoclimate reconstructions available to us. Editorial assistance, in the form of language editing and correction, was provided by XpertScientific Editing and Consulting Services. We acknowledge support by the Open Access Publication Funds of Alfred-Wegener-Institut Helmholtz Zentrum fĂŒr Polar- und Meeresforschung.Peer ReviewedPostprint (author's final draft
The Spanish Institute of Oceanography recent updates of data sharing within the framework of international marine data management initiatives.
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