267 research outputs found
Mössbauer studies of trimethyl and triphenyl tin chloride adsorbed on grafoil
Trimethyl tin chloride [(CH3)3SnCl] and triphenyl tin chloride [(C6H5)3SnCl] adsorbed on grafoil were studied by the Mössbauer resonance in 119Sn. The temperature dependence of the Mössbauer fraction and the anisotropy ratio of the quadrupole transitions in (CH3)3SnCl measured at two different angles supplied information on the structure of the substrate and on the anisotropy of the recoilless fraction. The relative orientation of (C6H5)3SnCl adsorbed on grafoil was found to be with the SnâCl axis perpendicular to the exposed basal planes of grafoil, contrary to (CH3)3SnCl, where the molecules have their symmetry axes parallel to the grafoil plane
Full capacitance matrix of coupled quantum dot arrays: static and dynamical effects
We numerically calculated the full capacitance matrices for both
one-dimensional (1D) and two-dimensional (2D) quantum-dot arrays. We found it
is necessary to use the full capacitance matrix in modeling coupled quantum dot
arrays due to weaker screening in these systems in comparison with arrays of
normal metal tunnel junctions. The static soliton potential distributions in
both 1D and 2D arrays are well approximated by the unscreened (1/r) coulomb
potential, instead of the exponential fall-off expected from the often used
nearest neighbor approximation. The Coulomb potential approximation also
provides a simple expression for the full inverse capacitance matrix of uniform
quantum dot arrays. In terms of dynamics, we compare the current-voltage (I-V)
characteristics of voltage biased 1D arrays using either the full capacitance
matrix or its nearest neighbor approximation. The I-V curves show clear
differences and the differences become more pronounced when larger arrays are
considered.Comment: 8 pages preprint format, 3 PostScript figure
Effect of temperature on pollen tube kinetics and dynamics in sweet cherry, Prunus avium (Rosaceae)
The article is available at:
http://www.amjbot.org/cgi/content/full/91/4/558Prevailing ambient temperature during the reproductive phase is one of several important factors for seed and fruit set in different plant species, and its consequences on reproductive success may increase with global warming. The effect of temperature on pollen performance was evaluated in sweet cherry (Prunus avium L.), comparing as pollen donors two cultivars that differ in their adaptation to temperature. âSunburstâ is a cultivar that originated in Canada with a pedigree of cultivars from Northern Europe, while âCristobalinaâ is a cultivar native to southeast Spain, adapted to warmer conditions. Temperature effects were tested either in controlled-temperature chambers or in the field in a plastic cage. In both genotypes, an increase in temperature reduced pollen germination, but accelerated pollen tube growth. However, a different genotypic response, which reflected the overall adaptation of the pollen donor, was obtained for pollen tube dynamics, expressed as the census of the microgametophyte population that successfully reached the base of the style. While both cultivars performed similarly at 20°C, the microgametophyte population was reduced at 30°C for Sunburst and at 10°C for Cristobalina. These results indicate a differential genotypic response to temperature during the reproductive phase, which could be important in terms of the time needed for a plant species to adapt to rapid temperature changes.A. H. was supported by an AECI and an SIA-DGA fellowship, and financial support for this work was provided by INIA (project grant RTA 01-103).Peer reviewe
Photoluminescence investigations of 2D hole Landau levels in p-type single Al_{x}Ga_{1-x}As/GaAs heterostructures
We study the energy structure of two-dimensional holes in p-type single
Al_{1-x}Ga_{x}As/GaAs heterojunctions under a perpendicular magnetic field.
Photoluminescence measurments with low densities of excitation power reveal
rich spectra containing both free and bound-carrier transitions. The
experimental results are compared with energies of valence-subband Landau
levels calculated using a new numerical procedure and a good agreement is
achieved. Additional lines observed in the energy range of free-carrier
recombinations are attributed to excitonic transitions. We also consider the
role of many-body effects in photoluminescence spectra.Comment: 13 pages, 10 figures, accepted to Physical Review
Delocalization Enhances Conductivity at High Doping Concentrations
Many applications of organic semiconductors require high electrical conductivities and hence high doping levels. Therefore, it is indispensable for effective material design to have an accurate understanding of the underlying transport mechanisms in this regime. In this study, own and literature experimental data that reveal a power-law relation between the conductivity and charge density of strongly p-doped conjugated polymers are combined. This behavior cannot consistently be described with conventional models for charge transport in energetically disordered materials. Here, it is shown that the observations can be explained in terms of a variable range hopping model with an energy-dependent localization length. A tight-binding model is used to quantitatively estimate of the energy-dependent localization length, which is used in an analytical variable range hopping model. In the limit of low charge densities, the model reproduces the well-known Mott variable range hopping behavior, while for high charge densities, the experimentally observed superlinear increase in conductivity with charge density is reproduced. The latter behavior occurs when the Fermi level reaches partially delocalized states. This insight can be anticipated to lead to new strategies to increase the conductivity of organic semiconductors
Field and temperature dependence of the photocurrent in polymer/fullerene bulk heterojunction solar cells
The photocurrent in polymer/fullerene blends is characterized as a function of bias at temperatures ranging from 125 to 300 K. Assuming a constant generation rate and bimolecular recombination, the results are numerically modeled within the drift-diffusion approximation. Bimolecular recombination is found to be a dominant factor in the field dependence of the photocurrent in the entire measured voltage range. Inclusion of field dependent geminate pair dissociation and recombination via the Onsager expressions gives a much stronger field dependence than experimentally observed. From the temperature dependence of the extracted mobilities, we can simultaneously estimate the broadening of the transporting highest occupied and lowest unoccupied molecular orbital levels. ©2005 American Institute of Physic
Charge Transport in Pure and Mixed Phases in Organic Solar Cells
In organic solar cells continuous donor and acceptor networks are consid-
ered necessary for charge extraction, whereas discontinuous neat phases
and molecularly mixed donorâacceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity, and donorâacceptor mixing on charge transport remains only semi- quantitatively described. Here, cosublimed donorâacceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network are studied. Using transient measurements, spanning from sub-picoseconds to micro- seconds photogenerated charge motion is measured in complete photovoltaic devices, to show that even highly diluted donor sites (5.7%â10% molar) in a buckminsterfullerene matrix enable hole transport. Hopping between isolated donor sites can occur by long-range hole tunneling through several buckmin- sterfullerene molecules, over distances of up to â4 nm. Hence, these results question the relevance of âpristineâ phases and whether a continuous interpen- etrating donorâacceptor network is the ideal morphology for charge transport
Reply to Comment on ``Two-dimensional charged-exciton complexes''
We respond to criticisms raised by K. Varga (cond-mat/9802262) and reaffirm
that the results in our original paper obtained using a two-body analytical
method remains valid within the framework of an effective excitonic composite
model. The conceptual model of the excitonic systems as well as the numerical
method based on variational functions utilized by Varga differ significantly
from ours. Hence comparison of binding energies of the charged-biexciton
remains questionable. In this reply, we discuss the shortcomings of modelling
the charged-biexciton as a five-body system and treating excitonic complexes as
atomic systems, as done in Varga's Comment.Comment: 3 pages, no figure
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