840 research outputs found
Monitoring and modeling infiltration–recharge dynamics of managed aquifer recharge with desalinated seawater
We study the relation between surface infiltration and groundwater
recharge during managed aquifer recharge (MAR) with desalinated seawater in
an infiltration pond, at the Menashe site that overlies the northern part of
the Israeli Coastal Aquifer. We monitor infiltration dynamics at multiple
scales (up to the scale of the entire pond) by measuring the ponding depth,
sediment water content and groundwater levels, using pressure sensors,
single-ring infiltrometers, soil sensors, and observation wells. During a
month (January 2015) of continuous intensive MAR
(2.45  ×  10<sup>6</sup> m<sup>3</sup> discharged to a 10.7 ha area),
groundwater level has risen by 17 m attaining full connection with the pond,
while average infiltration rates declined by almost 2 orders of magnitude
(from  ∼  11 to  ∼  0.4 m d<sup>−1</sup>). This reduction can be
explained solely by the lithology of the unsaturated zone that includes
relatively low-permeability sediments. Clogging processes at the pond-surface
– abundant in many MAR operations – are negated by the high-quality
desalinated seawater (turbidity  ∼  0.2 NTU, total dissolved solids
 ∼  120 mg L<sup>−1</sup>) or negligible compared to the low-permeability
layers. Recharge during infiltration was estimated reasonably well by simple
analytical models, whereas a numerical model was used for estimating
groundwater recharge after the end of infiltration. It was found that a
calibrated numerical model with a one-dimensional representative sediment
profile is able to capture MAR dynamics, including temporal reduction of
infiltration rates, drainage and groundwater recharge. Measured infiltration
rates of an independent MAR event (January 2016) fitted well to those
calculated by the calibrated numerical model, showing the model validity. The
successful quantification methodologies of the temporal groundwater recharge
are useful for MAR practitioners and can serve as an input for groundwater
flow models
Current Profiles of Molecular Nanowires; DFT Green Function Representation
The Liouville-space Green function formalism is used to compute the current
density profile across a single molecule attached to electrodes. Time ordering
is maintained in real, physical, time, avoiding the use of artificial time
loops and backward propagations. Closed expressions for molecular currents,
which only require DFT calculations for the isolated molecule, are derived to
fourth order in the molecule/electrode coupling.Comment: 21 page
Microscopic theory of single-electron tunneling through molecular-assembled metallic nanoparticles
We present a microscopic theory of single-electron tunneling through metallic
nanoparticles connected to the electrodes through molecular bridges. It
combines the theory of electron transport through molecular junctions with the
description of the charging dynamics on the nanoparticles. We apply the theory
to study single-electron tunneling through a gold nanoparticle connected to the
gold electrodes through two representative benzene-based molecules. We
calculate the background charge on the nanoparticle induced by the charge
transfer between the nanoparticle and linker molecules, the capacitance and
resistance of molecular junction using a first-principles based Non-Equilibrium
Green's Function theory. We demonstrate the variety of transport
characteristics that can be achieved through ``engineering'' of the
metal-molecule interaction.Comment: To appear in Phys. Rev.
Modeling transport through single-molecule junctions
Non-equilibrium Green's functions (NEGF) formalism combined with extended
Huckel (EHT) and charging model are used to study electrical conduction through
single-molecule junctions. Analyzed molecular complex is composed of asymmetric
1,4-Bis((2'-para-mercaptophenyl)-ethinyl)-2-acetyl-amino-5-nitro-benzene
molecule symmetrically coupled to two gold electrodes [Reichert et al., Phys.
Rev. Lett. Vol.88 (2002), pp. 176804]. Owing to this model, the accurate values
of the current flowing through such junction can be obtained by utilizing basic
fundamentals and coherently deriving model parameters. Furthermore, the
influence of the charging effect on the transport characteristics is
emphasized. In particular, charging-induced reduction of conductance gap,
charging-induced rectification effect and charging-generated negative value of
the second derivative of the current with respect to voltage are observed and
examined for molecular complex.Comment: 8 pages, 3 figure
Orbital Interaction Mechanisms of Conductance Enhancement and Rectification by Dithiocarboxylate Anchoring Group
We study computationally the electron transport properties of
dithiocarboxylate terminated molecular junctions. Transport properties are
computed self-consistently within density functional theory and nonequilibrium
Green's functions formalism. A microscopic origin of the experimentally
observed current amplification by dithiocarboxylate anchoring groups is
established. For the 4,4'-biphenyl bis(dithiocarboxylate) junction, we find
that the interaction of the lowest unoccupied molecular orbital (LUMO) of the
dithiocarboxylate anchoring group with LUMO and highest occupied molecular
orbital (HOMO) of the biphenyl part results in bonding and antibonding
resonances in the transmission spectrum in the vicinity of the electrode Fermi
energy. A new microscopic mechanism of rectification is predicted based on the
electronic structure of asymmetrical anchoring groups. We show that the peaks
in the transmission spectra of 4'-thiolato-biphenyl-4-dithiocarboxylate
junction respond differently to the applied voltage. Depending upon the origin
of a transmission resonance in the orbital interaction picture, its energy can
be shifted along with the chemical potential of the electrode to which the
molecule is more strongly or more weakly coupled
Electron transport through dipyrimidinyl-diphenyl diblock molecular wire: protonation effect
Recently, rectifying direction inversion has been observed in
dipyrimidinyl-diphenyl (PMPH) diblock molecular wire [J. Am. Chem. Soc. (2005)
127, 10456], and a protonation mechanism was suggested to explain this
interesting phenomena. In this paper, we study the protonation effect on
transport properties of PMPH molecule by first principles calculations. No
significant rectification is found for the pristine diblock molecular wire.
Protonation leads to conductance enhancement and rectification. However, for
all considered junctions with rectifying effect, the preferential current
directions are samely from dipyrimidinyl side to diphenyl side. Effect of
molecule-electrode anchoring geometry is studied, and it is not responsible for
the discrepancy between experiment and theory.Comment: 17 pages, 8 figure
Effect of Thermoelectric Cooling in Nanoscale Junctions
We propose a thermoelectric cooling device based on an atomic-sized junction.
Using first-principles approaches, we investigate the working conditions and
the coefficient of performance (COP) of an atomic-scale electronic refrigerator
where the effects of phonon's thermal current and local heating are included.
It is observed that the functioning of the thermoelectric nano-refrigerator is
restricted to a narrow range of driving voltages. Compared with the bulk
thermoelectric system with the overwhelmingly irreversible Joule heating, the
4-Al atomic refrigerator has a higher efficiency than a bulk thermoelectric
refrigerator with the same due to suppressed local heating via the
quasi-ballistic electron transport and small driving voltages. Quantum nature
due to the size minimization offered by atomic-level control of properties
facilitates electron cooling beyond the expectation of the conventional
thermoelectric device theory.Comment: 8 figure
Efficient energy transfer in light-harvesting systems, I: optimal temperature, reorganization energy, and spatial-temporal correlations
Understanding the mechanisms of efficient and robust energy transfer in
light-harvesting systems provides new insights for the optimal design of
artificial systems. In this paper, we use the Fenna-Matthews-Olson (FMO)
protein complex and phycocyanin 645 (PC 645) to explore the general dependence
on physical parameters that help maximize the efficiency and maintain its
stability. With the Haken-Strobl model, the maximal energy transfer efficiency
(ETE) is achieved under an intermediate optimal value of dephasing rate. To
avoid the infinite temperature assumption in the Haken-Strobl model and the
failure of the Redfield equation in predicting the Forster rate behavior, we
use the generalized Bloch-Redfield (GBR) equation approach to correctly
describe dissipative exciton dynamics and find that maximal ETE can be achieved
under various physical conditions, including temperature, reorganization
energy, and spatial-temporal correlations in noise. We also identify regimes of
reorganization energy where the ETE changes monotonically with temperature or
spatial correlation and therefore cannot be optimized with respect to these two
variables
Highly Conducting pi-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes
We measure electronic conductance through single conjugated molecules bonded
to Au metal electrodes with direct Au-C covalent bonds using the scanning
tunneling microscope based break-junction technique. We start with molecules
terminated with trimethyltin end groups that cleave off in situ resulting in
formation of a direct covalent sigma bond between the carbon backbone and the
gold metal electrodes. The molecular carbon backbone used in this study consist
of a conjugated pi-system that has one terminal methylene group on each end,
which bonds to the electrodes, achieving large electronic coupling of the
electrodes to the pi-system. The junctions formed with the prototypical example
of 1,4-dimethylenebenzene show a conductance approaching one conductance
quantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenyls
with two to four phenyl units show a hundred-fold increase in conductance
compared with junctions formed with amine-linked oligophenyls. The conduction
mechanism for these longer oligophenyls is tunneling as they exhibit an
exponential dependence of conductance with oligomer length. In addition,
density functional theory based calculations for the Au-xylylene-Au junction
show near-resonant transmission with a cross-over to tunneling for the longer
oligomers.Comment: Accepted to the Journal of the American Chemical Society as a
Communication
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