349. A Necessary Trade-off for
Semiclassical Electrodynamics: Accurate Short-Range Coulomb
Interactions versus the Enforcement of Causality?
T. E. Li, H.-Ta Chen, A. Nitzan, M.
Sukharev and J. E.
Subotnik
PDF
J. Phys. Chem. Lett., 9, 5955−5961 (2018)
We investigate two key representative semiclassical approaches
for propagating resonant energy transfer (RET) between a pair of
electronic two-level systems (donor and acceptor) with coupled
Maxwell. Liouville equations. On the one hand, when the
electromagnetic (EM) field is treated classically and Coulomb
interactions are treated quantum-mechanically, we find that a
quantum. classical mismatch leads to a violation of causality,
i. e., the acceptor can be excited before the retarded EM field
arrives. On the other hand, if we invoke a classical
intermolecular Coulomb operator, we find that the energy
transfer in the near field loses quantitative accuracy compared
with Fo. rster theory, even though causality is strictly obeyed.
Thus, our work raises a fundamental paradox when choosing a
semiclassical electrodynamics algorithm. Namely, which is more
important: Accurate short-range interactions or long-range
causality? Apparently, one cannot have one_fs cake and eat it
too.
348. Universal approach to quantum
thermodynamics in the strong coupling regime
W. Dou, M. A. Ochoa, A. Nitzan and J. E.
Subotnik
PDF
Phys. Rev. B 98, 134306 (2018)
We present a protocol for the study of the dynamics and
thermodynamics of quantum systems strongly coupled to a bath and
subject to an external modulation. Our protocol quantifies the
evolution of the system-bath composite by expanding the full
density matrix as a series in the powers of the modulation rate,
from which the functional form of work, heat, and entropy rates
can be obtained. Under slow driving, thermodynamic laws are
established. The entropy production rate is positive and is
found to be related to the excess work dissipated by friction,
at least up to second order in the driving speed. As an example
of the present methodology, we reproduce the results for the
quantum thermodynamics of the driven resonance level model. We
also emphasize that our formalism is quite general and allows
for electron-electron interactions, which can give rise to
exotic Kondo resonances appearing in thermodynamic quantities.
347. Simultaneous weak measurement of
non-commuting observables: a generalized Arthurs-Kelly
protocol
M. A. Ochoa1, W. Belzig and A.
Nitzan
PDF
Scientific Reports, 8, 15781 (2018)
In contrast to a projective quantum measurement, in a weak
measurement the system is only weakly perturbed while only
partial information on the measured observable is obtained. A
simultaneous measurement of non-commuting observables cannot be
projective, however the strongest possible such measurement can
be defined as providing their values at the smallest uncertainty
limit. Starting with the Arthurs and Kelly (AK) protocol for
such measurement of position and momentum, we derive a
systematic extension to a corresponding weak measurement along
three steps: First, a plausible form of the weak measurement
operator analogous to the Gaussian Kraus operator, often used to
model a weak measurement of a single observable, is obtained by
projecting a naive extension (valid for commuting observable)
onto the corresponding Gabor space. Second, we show that the so
obtained set of measurement operators satisfies the
normalization condition for the probability to obtain given
values of the position and momentum in the weak measurement
operation, namely that this set constitutes a positive operator
valued measure (POVM) in the position-momentum space. Finally,
we show that the so-obtained measurement operator corresponds to
a generalization of the AK measurement protocol in which the
initial detector wavefunctions is suitable broadened.
346. Electronic noise due to temperature
differences in atomic-scale junctions
O. Shein Lumbroso, L. Simine, A. Nitzan,
D. Segal and O.
Tal
PDF
Nature, 562, 242 (2018)
Here we report measurements of a fundamental electronic noise that
is generated by temperature differences across nanoscale
conductors, which we term ‘delta-T noise’. We experimentally
demonstrate this noise in atomic and molecular junctions, and
analyse it theoretically using the Landauer formalism.
345. Upside/Downside statistical
mechanics of nonequilibrium Brownian motion. II. Heat transfer
and energy partitioning of a free particle
G. T. Craven, R. Chen and A.
Nitzan
PDF
J. Chem. Phys. 149, 104103 (2018)
The energy partitioning during activation and relaxation events
under steady-state conditions for a Brownian particle driven by
multiple thermal reservoirs of different local temperatures is
investigated. Specifically, we apply the formalism derived in
Paper I [G. T. Craven and A. Nitzan, J. Chem. Phys. 148, 044101
(2018)] to examine the thermal transport properties of two
sub-ensembles of Brownian processes, distinguished at any given
time by the specification that all the trajectories in each group
have, at that time, energy either above (upside) or below
(downside) a preselected energy threshold. Dynamical properties
describing energy accumulation and release during
activation/relaxation events and relations for upside/downside
energy partitioning between thermal reservoirs are derived. The
implications for heat transport induced by upside and downside
events are discussed.
344. Kinetic Schemes in Open
Interacting Systems
A. Nitzan and M.
Galperin
PDF
J. Phys. Chem. Lett. 9, 4886−4892 (2018)
We discuss utilization of kinetic schemes for description of open
interacting systems, focusing on vibrational energy relaxation for
an oscillator coupled to a nonequilibirum electronic bath.
Standard kinetic equations with constant rate coefficients are
obtained under the assumption of time scale separation between the
system and bath, with the bath dynamics much faster than that of
the system of interest. This assumption may break down in certain
limits, and we show that ignoring this may lead to qualitatively
wrong predictions. Connection with more general, nonequilibrium
Green's function (NEGF) analysis is demonstrated. Our
considerations are illustrated within generic molecular junction
models with electron-vibration coupling.
343. Mixed quantum-classical
electrodynamics: Understanding spontaneous decay and zero-point
energy
T. E. Li, A. Nitzan, M. Sukharev, T.
Martinez, H.-Ta Chen and J. E.
Subotnik
PDF
Phys. Rev. A 97, 032105 (2018)
The dynamics of an electronic two-level system coupled to an
electromagnetic field are simulated explicitly for one- and
three-dimensional systems through semiclassical propagation of the
Maxwell-Liouville equations. We consider three flavors of mixed
quantum-classical dynamics: (i) the classical path approximation
(CPA), (ii) Ehrenfest dynamics, and (iii) symmetrical
quasiclassical (SQC) dynamics. Our findings are as follows: (i)
The CPA fails to recover a consistent description of spontaneous
emission, (ii) a consistent "spontaneous" emission can be obtained
from Ehrenfest dynamics, provided that one starts in an electronic
superposition state, and (iii) spontaneous emission is always
obtained using SQC dynamics. Using the SQC and Ehrenfest
frameworks, we further calculate the dynamics following an
incoming pulse, but here we find very different responses: SQC and
Ehrenfest dynamics deviate sometimes strongly in the calculated
rate of decay of the transient excited state. Nevertheless, our
work confirms the earlier observations by Miller [J. Chem. Phys.
69, 2188 (1978)] that Ehrenfest dynamics can effectively describe
some aspects of spontaneous emission and highlights interesting
possibilities for studying light-matter interactions with
semiclassical mechanics.
342. Upside/Downside statistical mechanics
of nonequilibrium Brownian motion. I. Distributions, moments,
and correlation functions of a free particle
G. T. Craven and A.
Nitzan
PDF
J. Chem. Phys 148, 044101
(2018)
Statistical properties of Brownian motion that arise by
analyzing, separately, trajectories over which the system energy
increases (upside) or decreases (downside) with respect to a
threshold energy level are derived. This selective analysis is
applied to examine transport properties of a nonequilibrium
Brownian process that is coupled to multiple thermal sources
characterized by different temperatures. Distributions, moments,
and correlation functions of a free particle that occur during
upside and downside events are investigated for energy
activation and energy relaxation processes and also for positive
and negative energy fluctuations from the average energy. The
presented results are sufficiently general and can be applied
without modification to the standard Brownian motion. This
article focuses on the mathematical basis of this selective
analysis. In subsequent articles in this series, we apply this
general formalism to processes in which heat transfer between
thermal reservoirs is mediated by activated rate processes that
take place in a system bridging them.
341. Maicol A. Ochoa, N Zimbovskaya and A. Nitzan
Quantum thermodynamics for driven
dissipative bosonic systems
PDF
Physical Review B 97, 085434 (2018) (14
pages)
We investigate two prototypical dissipative bosonic systems
under slow driving and arbitrary system-bath coupling
strength, recovering their dynamic evolution as well as the
heat and work rates, and we verify that thermodynamic laws are
respected. Specifically, we look at the damped harmonic
oscillator and the damped two-level system. For the former, we
study independently the slow time-dependent perturbation in
the oscillator frequency and in the coupling strength. For the
latter, we concentrate on the slow modulation of the energy
gap between the two levels. Importantly, we are able to find
the entropy production rates for each case without explicitly
defining nonequilibrium extensions for the entropy functional.
This analysis also permits the definition of phenomenological
friction coefficients in terms of structural properties of the
system-bath composite.
340. N. A. Zimbovskaya and A. Nitzan
Thermally induced charge current through
long
molecules
PDF
J. Chem. Phys 148, 024303 (2018)
In this work, we theoretically study steady state thermoelectric
transport through a single-molecule junction with a long
chain-like bridge. Electron transmission through the system is
computed using a tight-binding model for the bridge. We analyze
dependences of thermocurrent on the bridge length in unbiased
and biased systems operating within and beyond the linear
response regime. It is shown that the length-dependent
thermocurrent is controlled by the lineshape of electron
transmission in the interval corresponding to the HOMO/LUMO
transport channel. Also, it is demonstrated that electron
interactions with molecular vibrations may significantly affect
the length-dependent thermocurrent.
339.
M. Sukharev and A.
Nitzan
Optics
of exciton-plasmon nanomaterials
PDF
J. Phys.: Cond. Matt. 29, 443003 (2017)
This review provides a
brief introduction to the physics of coupled exciton-plasmon
systems, the
theoretical description and experimental manifestation of such
phenomena,
followed by an account of the state-of-the-art methodology for
the numerical
simulations of such phenomena and supplemented by a number of
FORTRAN codes, by
which the interested reader can introduce himself/herself to the
practice of
such simulations. Applications to CW light scattering as well as
transient
response and relaxation are described. Particular attention is
given to
so-called strong coupling limit, where the hybrid
exciton-plasmon nature of the
system response is strongly expressed. While traditional
descriptions of such
phenomena usually rely on analysis of the electromagnetic
response of
inhomogeneous dielectric environments that individually support
plasmon and exciton
excitations, here we explore also the consequences of a more
detailed
description of the molecular environment in terms of its quantum
density matrix
(applied in a mean field approximation level). Such a
description makes it
possible to account for characteristics that cannot be described
by the
dielectric response model: the effects of dephasing on the
molecular response
on one hand, and nonlinear response on the other. It also
highlights the still
missing important ingredients in the numerical approach, in
particular its
limitation to a classical description of the radiation field and
its reliance
on a mean field description of the many-body molecular system.
We end our
review with an outlook to the near future, where these
limitations will be addressed
and new novel applications of the numerical approach will be
pursued.
Electron-transfer-induced and phononic heat
transport in molecular environments
J. Chem. Phys. 147,
124101
(2017)
PDF
A
unified theory of heat transport in environments
that sustain intersite phononic coupling and electron hopping is
developed. The
heat currents generated by both phononic transport and electron
transfer
between sites characterized by different local temperatures are
calculated and
compared. Using typical molecular parameters we find that the
electron-transfer-induced heat current can be comparable to that
of the
standard phononic transport for donor-acceptor pairs with
efficient
bidirectional electron transfer rates (relatively small
intersite distance and
favorable free-energy difference). In most other situations,
phononic transport
is the dominant heat transfer mechanism.
Electrothermal
transistor effect and cyclic electronic currents in multithermal
charge
transfer networks
Phys. Rev.
Lett. 118,
207201(2017)
PDF
The rate of electron
transfer between a molecular species and a metal, each at a
different local
temperature, is examined theoretically through the
implementation of a
bithermal (characterized by two temperatures) Marcus formalism.
Expressions for
the rate constant and the electronic contribution to a heat
transfer mechanism
which is induced by the temperature gradient between a molecule
and metal are
constructed. The system of coupled dynamical equations
describing the
electronic and thermal currents are derived and examined over
diverse ranges of
reaction geometries and temperature gradients. It is shown that
electron
transfer across the molecule-metal interface is associated with
heat transfer
and that the electron exchange between metal and molecule makes
a distinct
contribution to the interfacial heat conduction even when the
net electronic
current vanishes.
Electron transfer at thermally
heterogeneous molecule-metal interfaces
PDF
J. Chem. Phys. 146, 092305
(2017) (10 pages)
The rate of electron transfer between a molecular
species and a metal, each at a different local temperature, is
examined
theoretically through the implementation of a bithermal
(characterized by two
temperatures) Marcus formalism. Expressions for the rate
constant and the
electronic contribution to a heat transfer mechanism which is
induced by the
temperature gradient between a molecule and metal are
constructed. The system
of coupled dynamical equations describing the electronic and
thermal currents
are derived and examined over diverse ranges of reaction
geometries and
temperature gradients. It is shown that electron transfer across
the
molecule-metal interface is associated with heat transfer and
that the electron
exchange between metal and molecule makes a distinct
contribution to the
interfacial heat conduction even when the net electronic current
vanishes.
Energy
distribution and local fluctuations in strongly coupled open
quantum systems:
The extended resonant level model
Phys. Rev. B 94, 035420 (2016) (7
pages)
PDF
We study the energy
distribution in the extended resonant level model at
equilibrium. Previous
investigations [Phys. Rev. B 89, 161306 (2014); 93, 115318
(2016)] have found,
for a resonant electronic level interacting with a thermal
free-electron
wide-band bath, that the expectation value for the energy of the
interacting
subsystem can be correctly calculated by considering a symmetric
splitting of
the interaction Hamiltonian between the subsystem and the bath.
However, the
general implications of this approach were questioned [Phys.
Rev. B 92, 235440
(2015)]. Here, we show that, already at equilibrium, such
splitting fails to
describe the energy fluctuations, as measured here by the second
and third
central moments (namely, width and skewness) of the energy
distribution.
Furthermore, we find that when the wide-band approximation does
not hold, no
splitting of the system-bath interaction can describe the system
thermodynamics.
We conclude that in general no proper division subsystem of the
Hamiltonian of
the composite system can account for the energy distribution of
the subsystem.
This also implies that the thermodynamic effects due to local
changes in the
subsystem cannot in general be described by such splitting.
Electron
transfer across a thermal
gradient
PDF
PNAS, 113, 9421–9429 (2016)
PNAS Commentary by D.
V. Matyushov www.pnas.org/cgi/doi/10.1073/pnas.1610542113
Charge transfer is a fundamental process that
underlies a multitude of phenomena in chemistry and biology.
Recent advances in
observing and manipulating charge and heat transport at the
nanoscale, and
recently developed techniques for monitoring temperature at high
temporal and
spatial resolution, imply the need for considering electron
transfer across
thermal gradients. Here, a theory is developed for the rate of
electron
transfer and the associated heat transport between
donor-acceptor pairs located
at sites of different temperatures. To this end, through
application of a
generalized multidimensional transition state theory, the
traditional Arrhenius
picture of activation energy as a single point on a free energy
surface is
replaced with a bithermal property that is derived from
statistical weighting
over all configurations where the reactant and product states
are
equienergetic. The flow of energy associated with the electron
transfer process
is also examined, leading to relations between the rate of heat
exchange among
the donor and acceptor sites as functions of the temperature
difference and the
electronic driving bias. In particular, we find that an open
electron transfer
channel contributes to enhanced heat transport between sites
even when they are
in electronic equilibrium. The presented results provide a
unified theory for
charge transport and the associated heat conduction between
sites at different
temperatures.
On
the widths of Stokes lines
in Raman scattering
PDF
J. Chem. Phys. 144, 244114
(2016)
(8 pages)
Within a generic model we analyze the Stokes
linewidth in surface enhanced Raman scattering
Covalently
bonded single-molecule junctions with stable and reversible
photoswitched
conductivity
Science, 352,
1443-45 (2016)
PDF
Science
perspective by C. D. Frisbie VOL 352
ISSUE 6292, page 1394
Through molecular engineering, single diarylethenes
were covalently
sandwiched between graphene electrodes to form stable molecular
conduction
junctions. Our experimental and theoretical studies of these
junctions
consistently show and interpret reversible conductance
photoswitching at room
temperature and stochastic switching between different
conductive states at low
temperature at a single-molecule level. We demonstrate a fully
reversible,
two-mode, single- molecule electrical switch with unprecedented
levels of
accuracy (on/off ratio of ~100), stability (over a year), and
reproducibility
Maximum
efficiency of state-space models of molecular scale engines
PDF
J. Chem. Phys. 145, 014108 (2016)
The performance of
nano-scale energy conversion devices is studied in the framework
of state-space
models where a device is described by a graph comprising states
and transitions
between them represented by nodes and links, respectively.
Particular segments
of this network represent input (driving) and output processes
whose properly
chosen flux ratio provides the energy conversion efficiency.
Simple cyclical
graphs yield Carnot efficiency for the maximum conversion yield.
We give
general proof that opening a link that separate between the two
driving
segments always leads to reduced efficiency. We illustrate these
general result
with simple models of a thermoelectric nanodevice and an organic
photovoltaic
cell. In the latter an intersecting link of the above type
corresponds to
non-radiative carriers recombination and the reduced maximum
efficiency is
manifested as a smaller open-circuit voltage.
Quantum thermodynamics of the driven
resonant level model
PDF
Phys. Rev. B, 93, 115318
(2016)
(14 pages)
We present a consistent
thermodynamic theory for the resonant level model in the
wide-band limit, whose
level energy is driven slowly by an external force. The problem
of defining
"system" and "bath" in the strong-coupling regime is
circumvented
by considering as the system everything that is influenced by
the externally
driven level. The thermodynamic functions that are obtained to
first order
beyond the quasistatic limit fulfill the first and second law
with a positive
entropy production, successfully connect to the forces
experienced by the
external driving, and reproduce the correct weak-coupling limit
of stochastic
thermodynamics.
Plasmon
transmission through excitonic subwavelength gaps
PDF
J. Chem. Phys. 144, 144703 (2016)
(8
pages)
We study the transfer of
electromagnetic energy across a subwavelength gap separating two
co-axial metal
nanorods. In the absence of spacer in the gap separating the
rods, the system
exhibits strong coupling behavior between longitudinal plasmons
in the two
rods. The nature and magnitude of this coupling are studied by
varying various
geometrical parameters. As a function of frequency, the
transmission is
dominated by a split longitudinal plasmon peak. The two hybrid
modes are the
dipole-like "bonding" mode characterized by a peak intensity in
the
gap and a quadrupole-like "antibonding" mode whose amplitude
vanishes
at the gap center. When the length of one rod is varied, this
mode spectrum
exhibits the familiar anti-crossing behavior that depends on the
coupling
strength determined by the gap width. When off-resonant 2-level
emitters are
placed in the gap, almost no effect on the frequency dependent
transmission is
observed. In contrast, when the molecular system is resonant
with the plasmonic
line shape, the transmission is strongly modified, showing
characteristics of
strong exciton-plasmon coupling. Most strongly modified is the
transmission
near the lower frequency "bonding" plasmon mode. The presence of
resonant molecules in the gap affects not only the
molecule-field interaction
but also the spatial distribution of the field intensity and the
electromagnetic energy flux across the junction.
Molecular Electronic
States Near Metal Surfaces at Equilibrium Using Potential of
Mean Force and
Numerical Renormalization Group Methods: Hysteresis Revisited.
J. Chem.
Phys., 144, 074109 (2016) (12
pages)
PDF
We investigate
equilibrium observables for molecules near metals by employing a
potential of
mean force (PMF) that takes level broadening into account.
Through comparison
with exact data, we demonstrate that this PMF approach performs
quite well,
even for cases where molecule-electrode couplings depend on
nuclear position.
As an application, we reexamine the possibility of hysteresis
effects within
the Anderson-Holstein model (i. e., an impurity coupled both to
a metal surface
and a nuclear oscillator). As compared against the standard mean
field approach
by Galperin et al. [Nano Lett. 5, 125 (2005)], our PMF approach
agrees much
better with exact results for average electronic populations
both at zero and
finite temperature; we find, however, that mean field theory can
be very useful
for predicting the onset of dynamical instabilities, metastable
states, and
hysteresis.
Observation and analysis of Fano-like
lineshapes in the Raman spectra of molecules adsorbed at metal
interfaces
Phys. Rev. B 93, 035411
(2016) (6
pages)
PDF
Surface-enhanced Raman scattering (SERS) from
bipyridyl ethylene adsorbed on gold dumbbells shows Fano-like
spectra at high
incident light intensity. This is accompanied by an increased
electronic
temperature, while no vibrational anti-Stokes scattering is
observed. Theory
indicates that interference between vibrational and electronic
Raman scattering
can yield such asymmetric scattering lineshapes. The best fit to
observations
is however obtained by disregarding this coupling and accounting
for the
detailed lineshape of the continuous electronic component of the
SERS.
Nuclear Dynamics at Molecule-Metal
Interfaces: A pseudoparticle perspective PDF
J. Phys. Chem. Lett. 6,
4896-4903 (2015)
We discuss nuclear dynamics at molecule-metal
interfaces including nonequilibrium molecular junctions.
Starting from the
many-body states (pseudoparticle) formulation of the
molecule-metal system in
the molecular vibronic basis, we introduce gradient expansion to
reduce the
adiabatic nuclear dynamics (that is, nuclear dynamics on a
single molecular
potential surface) into its semiclassical form while maintaining
the effect of
the nonadiabatic electronic transitions between different
molecular charge
states. This yields a set of equations for the nuclear dynamics
in the presence
of these nonadiabatic transitions, which reproduce the
surface-hopping
formulation in the limit of small metal-molecule coupling (where
broadening of
the molecular energy levels can be disregarded) and Ehrenfest
dynamics (motion
on the potential of mean force) when information on the
different charging
states is traced out.
Frictional
effects near
a metal surface
PDF
J. Chem. Phys. 143, 054103 (2015) (9
pages)
When a classical master equation (CME) is used to
describe the nonadiabatic dynamics of a molecule at metal
surfaces, we show
that in the regime of reasonably strong molecule-metal
couplings, the CME can
be reduced to a Fokker-Planck equation with an explicit form of
electronic
friction. For a single metal substrate at thermal equilibrium,
the electronic
friction and random force satisfy the fluctuation-dissipation
theorem. When we
investigate the time scale for an electron transfer (ET) event
between the
molecule and metal surface, we find that the ET rates show a
turnover effect
(just as in Kramer's theory) as a function of frictional
damping.
Surface
hopping with a manifold of electronic states, III: transients,
broadening and
the Marcus picture PDF
J. Chem. Phys. 142, 234106
(2015) (8 pages)
In a previous paper [Dou et al., J. Chem. Phys. 142, 084110
(2015)], we
have introduced a surface hopping (SH) approach to deal with the
Anderson-Holstein model. Here, we address some interesting aspects
that have
not been discussed previously, including transient phenomena and
extensions to
arbitrary impurity-bath couplings. In particular, in this paper we
show that
the SH approach captures phonon coherence beyond the secular
approximation, and
that SH rates agree with Marcus theory at steady state. Finally,
we show that,
in cases where the electronic tunneling rate depends on nuclear
position, a
straightforward use of Marcus theory rates yields a useful
starting point for
capturing level broadening. For a simple such model, we find I-V
curves that
exhibit negative differential resistance.
323. K. Kaasbjerg and A.
Nitzan
Theory of light
emission from quantum noise in plasmonic contacts:
above-threshold
We
develop a theoretical framework for the description of light
emission
from plasmonic contacts based on the nonequilibrium Green function
formalism. Our theory establishes a fundamental link between the
nite-frequency quantum noise and AC conductance of the contact
and the
light emission. Calculating the quantum noise to higher orders in
the
electron-plasmon interaction, we identify a plasmon-induced
electron-electron interaction as the source of experimentally
observed
above-threshold light emission from biased STM contacts. Our
ndings
provide important insight into the eect of interactions on the
light
emission from atomic-scale contacts.
322.
W. Dou, A. Nitzan and J. E. Subotnik
A
Surface Hopping Approach For the Anderson-Holstein Model
PDF
We
investigate a simple surface hopping (SH) approach for
modeling a
single impurity level coupled to a single phonon and an
electronic
(metal) bath (i. e., the Anderson-Holstein model). The
phonon degree of
freedom is treated classically with motion along-and hops
between-diabatic potential energy surfaces. The hopping rate
is
determined by the dynamics of the electronic bath (which are
treated
implicitly). For the case of one electronic bath, in the
limit of small
coupling to the bath, SH recovers phonon relaxation to
thermal
equilibrium and yields the correct impurity electron
population (as
compared with numerical renormalization group). For the case
of out of
equilibrium dynamics, SH current-voltage (I-V) curve is
compared with
the quantum master equation (QME) over a range of
parameters, spanning
the quantum region to the classical region. In the limit of
large
temperature, SH and QME agree. Furthermore, we can show
that, in the
limit of low temperature, the QME agrees with real-time path
integral
calculations. As such, the simple procedure described here
should be
useful in many other contexts.
321. A.
Migliore and A. Nitzan
Irreversibility in redox
molecular conduction: single versus double metal-molecule
interfaces
Electrochimica Acta 160,
363–375 (2015)
PDF
In this work we analyze the onset and manifestation of
irreversibility
phenomena in the charge transport at single and double metal-redox
molecule
interfaces, with special emphasis on the role of the nuclear
system reorganization
energy in causing the distortion of cyclic voltammograms in the
first case and
the occurrence of hysteresis phenomena in the second case. Under
physical
conditions for which two states of the molecular system come into
play, effects
of irreversibility increase with the reorganization energy at a
single
interface, while an opposite trend is seen in the conduction
through a
molecular junction. The apparent contradiction between these two
behaviors,
which was raised in a previous work (Migliore, A.;
Nitzan, A., J. Am. Chem. Soc. 2013,
135, 9420-32) is here resolved through
detailed
investigation of the connections between molecule reorganization
energy,
bias-dependent population of the molecular redox site(s), and
threshold voltage
scan rate for the onset of irreversible behavior. Moreover, our
investigation
of the effects of the reorganization energy on the voltammogram
peaks proposes
a strategy for extracting the value of the reorganization energy
of the
molecular system from the experimental behavior.
318. K. B. Whaley, A. A.
Kocherzhenko and A. Nitzan
Coherent and Diffusive
Timescales for Exciton Dissociation in Bulk Heterojuncion
Photovoltaic Cells
J. Phys. Chem. C, 118,
27235−27244
(2014)
PDF
We
study the dynamics of charge separation in bulk heterojunction
organic
photovoltaic systems in light of recent experimental observations
that
this process is characterized by multiple time scales in the range
of
10 fs to 100 ps. Coherent evolution of the excitonic state has
been
suggested to dominate the early stages of the charge separation
process
and diffusion of localized excitons to be dominant at longer
times.
Both of these processes obviously depend on the system morphology,
in
particular on the grain sizes of the donor and acceptor phases.
Here we
analyze these mechanisms and their characteristic time scales,
aiming
to verify the consistency of the proposed mechanisms with the
experimentally observed time scales of charge separation. We
suggest
that the coherent mechanism that dominates the early stage of
charge
separation involves delocalized excitons. These excitons are
formed by
optical excitation of clusters of strongly interacting donor
sites, and
the charge separation rate is determined by the probability that
such
sites lie at the donor-acceptor interface. The (relatively) slow
diffusive rate is estimated from the mean first passage time for a
diffusing exciton to reach the donor grain surface. Our estimates,
based on available exciton diffusion rates and morphology data,
are
consistent with experimental observations.
CB30. A. Nitzan
Beyond molecular conduction:
Optical and thermal effects in molecular junctions
Adv.
Chem. Phys. 157, 135-158
(2014)
PDF
This
paper focuses on two subjects whose study pertains to, but go
beyond, standard
considerations of electronic transport in molecular conduction
junctions.
Thermal effects - heat generation in, and heat conduction
through, such
junctions have direct implications to junction stability.
Optical interactions
are obvious potential tools for junction characterization and
control. An
overview of recent experimental and theoretical studies of these
subjects is
provided.
CB29. M. S. Shishodia, B. D.
Fainberg, and A. Nitzan
Theory of energy transfer interactions
near sphere and nanoshell based plasmonic nanostructure
Theory
of energy transfer interactions between a pair of two level
molecules
in the molecular nanojunction including surface plasmon (SP)
dressed
interaction of plasmonic nanostructure, replicating metallic leads
is
presented. Results on the modification of bare dipolar
interaction,
known to be responsible for molecular energy transfer processes,
in the
proximity of metallic nanosystem are presented. Specifically, the
manuscript includes theoretical investigation of nanosphere (NSP)
monomer, nanoshell (NSH) monomer, and coupled nanosphere pair
(dimer)
based nanosystems. Closed form analytical expressions for NSP and
NSH
structures tailored for molecular nanojunction geometry are
derived in
the theoretical framework of multipole spectral expansion (MSE)
method,
which is straightforwardly extendible to dimers and multimers. The
role
of size and dielectric environment on energy transfer is
investigated
and interpreted. Theory predicts that the monomer and dimer both
enhance the dipolar interaction, yet, dimer geometry is favorable
due
to its spectral tuning potential originated from plasmon
hybridization
and true resemblance with typical molecular nanojunctions.
317. M. Einax, M. Dierl, Philip R. Schiff and
A. Nitzan
Multiple state representation scheme
for organic bulk heterojunction
solar cells: A novel analysis perspective
European Phys. Let., 104,
40002 (2013) (6 pages)
PDF
The
physics of organic bulk heterojunction solar cells is
studied within a
six state model, which is used to analyze the factors that
affect
current-voltage characteristics, powervoltage properties and
efficiency, and their dependence on nonradiative losses,
reorganization
of the nuclear environment, and environmental polarization.
Both
environmental reorganization and polarity are explicitly
taken into
account by incorporating Marcus heterogeneous and
homogeneous electron
transfer rates. The environmental polarity is found to have
a
non-negligible influence both on the stationary current and
on the
overall solar cell performance. For our organic bulk
heterojunction
solar cell operating under steady-state open circuit
condition, we also
find that the open circuit voltage logarithmically decreases
with
increasing nonradiative electron-hole recombination
processes.
316. M. Sukharev, N. Freifeld and A. Nitzan
Numerical calculations of
radiative and non-radiative relaxation of molecules near metal
particles
J. Phys.
Chem. C, 118, 1054
(2014)
The
dependence of the radiative emission and the non-radiative (energy
transfer to the metal) relaxation rates of a molecule near a small
metal particle on the molecule-to-particle distance and on the
molecular orientation is calculated using a numerical solution of
the
Maxwell equations for a model that described the metal as a
dispersive
dielectric particle and the molecule as an oscillating point
dipole.
The emission rate is obtained by evaluating the total oscillating
dipole in the system, while the non-radiative rate is inferred
from the
rate of heat production on the particle. For the distance
dependence of
the non-radiative rate we find, in agreement with experimental
observations, marked deviation from the prediction of the standard
theory of fluorescence resonance energy transfer (FRET). In
departure
from previous interpretations, we find that electromagnetic
retardation
is the main source of this deviation at large molecule-particle
separations. The radiative emission rate reflects the total dipole
induced in the molecule-particle system, and its behavior as
function
of distance and orientation stems mostly from the magnitude of the
oscillating polarization on the metal particle (which, at
resonance, is
strongly affected by plasmon excitation), and from the way this
polarization combines with the molecular dipole to form the total
system dipole.
315.
J. Gersten,
K. Kaasbjerg and A. Nitzan
Induced spin filtering in electron
transmission through chiral molecular
layers adsorbed on metals with strong spin-orbit coupling
J. Chem. Phys. 139,
114111 (2013) (20 pages)
PDF
Recent
observations of considerable spin polarization in photoemission
from
metal surfaces through monolayers of chiral molecules were
followed by
several efforts to rationalize the results as the effect of
spin-orbit
interaction that accompanies electronic motion on helical, or more
generally strongly curved, potential surfaces. In this paper we
(a)
argue, using simple models, that motion in curved force-fields
with the
typical energies used and the characteristic geometry of DNA
cannot
account for such observations; (b) introduce the concept of
induced
spin filtering, whereupon selectivity in the transmission of the
electron orbital angular momentum can induce spin selectivity in
the
transmission process provided there is strong spin-orbit coupling
in
the substrate; and (c) show that the spin polarization in the
tunneling
current as well as the photoemission current from gold covered by
helical adsorbates can be of the observed order of magnitude. Our
results can account for most of the published observations that
involved gold and silver substrates; however, recent results
obtained
with an aluminum substrate can be rationalized within the present
model
only if strong spin-orbit coupling is caused by the built-in
electric
field at the molecule-metal interface.
314.
K.
Kaasbjerg, T.
Novotny and A. Nitzan
Carrier-induced renormalization of
vibrational frequencies in nanoscale
junctions: Signatures of vibrational damping and heating
Phys. Rev. B (Rapid Communication), 88,
201405(R) (2013)
PDF
In
nanoscale junctions the interaction between charge carriers and
the
local vibrations results in renormalization, damping, and heating
of
the vibrational modes. Herewe formulate a nonequilibrium Green's
function based theory to describe such effects. Studying a generic
junction model with an off-resonant electronic level, we find a
strong
bias dependence of the frequency renormalization and vibrational
damping accompanied by pronounced nonlinear vibrational heating in
junctions with intermediate values of the coupling to the leads.
Combining our theory with ab initio calculations, we furthermore
show
that the bias dependence of the Raman shifts and linewidths
observed
experimentally in an oligo(3)-phenylenevinylene (OPV3) junction
[Ward
et al., Nat. Nanotechnol. 6, 33 (2011)] may be explained by a
combination of dynamic carrier screening and molecular charging.
313. A. Migliore
and A. Nitzan
Irreversibility and hysteresis in
redox molecular conduction junctions
J.Am.Chem.Soc, 135, 9420-32
(2013)
312. A.
White, A. Migliore, M. Galperin and A. Nitzan
Quantum
Transport
With Two Interacting Conduction Channels
311.
M. Kornbluth, T. Seideman
and A. Nitzan
Light-induced electronic
non-equilibrium in plasmonic particles
J. Chem. Phys. 138,
174707 (2013) (9 pages)
PDF
We
consider the transient non-equilibrium electronic distribution
that is
created in a metal nanoparticle upon plasmon excitation. Following
light absorption, the created plasmons decohere within a few
femtoseconds, producing uncorrelated electron-hole pairs. The
corresponding non-thermal electronic distribution evolves in
response
to the photo-exciting pulse and to subsequent relaxation
processes.
First, on the femtosecond timescale, the electronic subsystem
relaxes
to a Fermi-Dirac distribution characterized by an electronic
temperature. Next, within picoseconds, thermalization with the
underlying lattice phonons leads to a hot particle in internal
equilibrium that subsequently equilibrates with the environment.
Here
we focus on the early stage of this multistep relaxation process,
and
on the properties of the ensuing non-equilibrium electronic
distribution. We consider the form of this distribution as derived
from
the balance between the optical absorption and the subsequent
relaxation processes, and discuss its implication for (a) heating
of
illuminated plasmonic particles, (b) the possibility to optically
induce current in junctions, and (c) the prospect for experimental
observation of such light-driven transport phenomena.
310.
S. K. Maiti and A. Nitzan
Mobility edge phenomenon in a Hubbard
chain: A mean field
study
Physics
Letters A, 377, 1205-1209 (2013)
PDF
We
show that a tight-binding one-dimensional chain composed of
interacting
and non-interacting atomic sites can exhibit multiple mobility
edges at
different values of carrier energy in presence of external
electric
field. Within a mean field Hartree-Fock approximation we
numerically
calculate two-terminal transport by using Green's function
formalism.
Several cases are analyzed depending on the arrangements of
interacting
and non-interacting atoms in the chain. The analysis may be
helpful in
designing mesoscale switching devices.
309.
M. Galperin† and Abraham Nitzan
Cooperative effects in inelastic
tunneling
J. Phys.Chem. B, 317,
4449-4453 (2013)
PDF
Several
aspects of intermolecular effects in molecular conduction have
been
studied in recent years. These experimental and theoretical
studies,
made on several setups of molecular conduction junctions, have
focused
on the current-voltage characteristic that is usually dominated by
the
elastic transmission properties of such junctions. In this paper,
we
address cooperative intermolecular effects in the inelastic
tunneling
signal calculated for simple generic models of such systems. We
find
that peak heights in the inelastic (d2I/dE2 vs E) spectrum may be
affected by such cooperative effects even when direct
intermolecular
interactions can be disregarded. This finding suggests that
comparing
experimental results to calculations made on single-molecule
junctions
should be done with care.
Polaron formation: Ehrenfest dynamics
vs. exact results
J. Chem. Phys. 138,
044112 (2013)
PDF
We
use a one-dimensional tight binding model with an impurity site
characterized by electronvibration coupling, to describe electron
transfer and localization at zero temperature, aiming to examine
the
process of polaron formation in this system. In particular we
focus on
comparing a semiclassical approach that describes nuclear motion
in
this many vibronic-states system on the Ehrenfest dynamics level
to a
numerically exact fully quantum calculation based on the
Bonca-Trugman
method [J. Bon¡ca and S. A. Trugman, Phys. Rev. Lett. 75, 2566
(1995)].
In both approaches, thermal relaxation in the nuclear subspace is
implemented in equivalent approximate ways: In the Ehrenfest
calculation the uncoupled (to the electronic subsystem) motion of
the
classical (harmonic) oscillator is simply damped as would be
implied by
coupling to a Markovian zero temperature bath. In the quantum
calculation, thermal relaxation is implemented by augmenting the
Liouville equation for the oscillator density matrix with kinetic
terms
that account for the same relaxation. In both cases we calculate
the
probability to trap the electron by forming a polaron and the
probability that it escapes to infinity. Comparing these
calculations,
we find that while both result in similar long time yields for
these
processes, the Ehrenfest-dynamics based calculation fails to
account
for the correct time scale for the polaron formation. This failure
results, as usual, from the fact that at the early stage of
polaron
formation the classical nuclear dynamics takes place on an
unphysical
average potential surface that reflects the distributed electronic
population in the system, while the quantum calculation accounts
fully
for correlations between the electronic and vibrational
subsystems.
Yield
of exciton dissociation in a donor–acceptor photovoltaic junction
Phys.
Chem. Chem. Phys.
14, 14270
(2012)
PDF
A
simple model is constructed to describe dissociation of charge
transfer
excitons in bulk heterojunction solar cells, and its dependence on
the
physical parameters of the system. In bulk heterojunction organic
photovoltaics (OPVs), exciton dissociation occurs almost
exclusively at
the interface between the donor and acceptor, following
one-electron
initial excitation from the HOMO to the LUMO levels of the donor,
and
charge transfer to the acceptor to make a charge-transfer exciton.
After exciton breakup, and neglecting the trapping of individual
carriers, the electron may undergo two processes for decay: one
process
involves the electron and/or hole leaving the interface, and
migrating
to the electrode. This is treated here as the electron moving on a
set
of acceptor sites. The second loss process is radiationless decay
following recombination of the acceptor electron with the donor
cation;
this is treated by adding a relaxation term. These two processes
compete with one another. We model both the exciton breakup and
the
subsequent electron motion. Results depend on tunneling amplitude,
energetics, disorder, Coulomb barriers, and energy level matchups,
particularly the so-called LUMO-LUMO offset.
306.
A. Migliore, P. Schiff and
A. Nitzan
On the relationship
between molecular state and single electron pictures
in simple electrochemical junctions
Phys.
Chem. Chem. Phys.,
14, 13746 -
13753
(2012)
PDF
We
consider a molecular conduction junction that comprises a redox
molecule bridging between metal electrodes, in the limit of weak
coupling and high temperature where electron transport is
dominated by
Marcus electron transfer kinetics. We address the correspondence
between the Marcus description in terms of nuclear potential
energy
surfaces associated with different charging states of the
molecular
bridge, and the single electron description commonly used in
theories
of molecular conduction. The relationship between the energy gap,
reorganization energy and activation energy parameters of the
Marcus
theory and the corresponding energy parameters in the single
electron
description is elucidated. We point out that while transport in
the
normal Marcus regime involves activated (therefore relatively
slow)
transitions between at least two charging states of the molecular
bridge, deep in the inverted regime only one of these states is
locally
stable and transitions into this state are activationless. The
relatively slow rates that characterize the normal Marcus
transport
regime manifest themselves in the appearance of hysteresis in the
system transport behavior as a function of gate or bias potentials
for
relatively slow scan rates of these potentials, but not
bistability in
the junction conduction behavior. We also consider the limit of
fast
solvent reorganization that may reflect the response of the
electronic
environment (electronic polarization of a solvent and of the metal
electrodes) to the changing charging state of the bridge. In this
limit, environmental reorganization appears as renormalization of
the
bridge electronic energy levels. We show that the effect of this
reorganization on the junction conduction properties is not
universal
and depends on the particular bridge charging states that are
involved
in the conduction process.
305. M. Galperin
and A. Nitzan
Molecular optoelectronics: The
interaction of molecular
conduction junctions with light
Phys.
Chem. Chem. Phys.,
14, 9421
- 9438 (2012)
PDF
The
interaction of light with molecular conduction junctions is
attracting
growing interest as a challenging experimental and theoretical
problem
on one hand, and because of its potential application as a
characterization and control tool on the other. It stands at the
interface between two important fields, molecular electronics and
molecular plasmonics and has attracted attention as a challenging
scientific problem with potentially important technological
consequences. Here we review the present state of the art of this
field, focusing on several key phenomena and applications: using
light
as a switching device, using light to control junction transport
in the
adiabatic and non-adiabatic regimes, light generation in biased
junctions and Raman scattering from such systems. This field has
seen
remarkable progress in the past decade, and the growing
availability of
scanning tip configurations that can combine optical and
electrical
probes suggests that further progress towards the goal of
realizing
molecular optoelectronics on the nanoscale is imminent.
304. A. Migliore
and A. Nitzan
On the evaluation of the
Marcus-Hush-Chidsey integral
J.
Electroanal. Chem. 671, 99-101
(2012)
PDF
The
electrochemical rate constant obtained from the Marcus-Hush theory
of
heterogeneous electron transfer is given as a relatively complex
integral. Recently, two apparently different expressions of this
rate
constant in the form of a series of analytical functions appeared
in
the literature. We demonstrate here the equivalence of these
expressions and discuss their different approximations, resulting
from
the two distinct analytical derivations, which have implications
in the
practical calculation of electron transfer rate constants at
electrode
surfaces.
Raman scattering from
molecular conduction junctions:
the charge transfer mechanism
Phys. Rev. B, 85,
115435 (2012) (12
pages)
PDF
We
present a model for the charge transfer contribution to
surface-enhanced Raman spectroscopy (SERS) in a molecular
junction. The
model is a generalization of the equilibrium scheme for SERS
of a
molecule adsorbed on a metal surface [B. N. J. Persson. Chem.
Phys.
Lett. 82, 561 (1981)].We extend the same physical
consideration to a
nonequilibrium situation in a biased molecular junction and to
nonzero
temperatures. Two approaches are considered and compared: a
semiclassical approach appropriate for nonresonance Raman
scattering,
and a quantum approach based on the nonequilibrium Green's
function
method. Nonequilibrium effects on this contribution to SERS
are
demonstrated with numerical examples. It is shown that the
semiclassical approach provides an excellent approximation to
the full
quantum calculation as long as the molecular electronic state
is
outside the Fermi window, that is, as long as the
field-induced charge
transfer is small.
302.
G. Li, M. S. Shishodia, B. D. Fainberg, B. Apter,
M. Oren, A. Nitzan, M. A. Ratner
Compensation
of Coulomb
blocking and energy transfer in the current voltage
characteristic of molecular
conduction junctions
Nano Lettters, 12,
2228-32 (2012)
PDF
We
have studied the influence of both exciton effects and Coulomb
repulsion on current in molecular nanojunctions. We show that
dipolar
energy-transfer interactions between the sites in the wire can at
high
voltage compensate Coulomb blocking for particular relationships
between their values. Tuning this relationship may be achieved by
using
the effect of plasmonic nanostructure on dipolar energy-transfer
interactions.
301. D. Rai, O. Hod
and A. Nitzan
Magnetic Fields Effects on the
Electronic Conduction
Properties of Molecular Ring Structures
Phys. Rev. B, 85, 155440 (2012) (21
pages)
PDF
While
mesoscopic conducting loops are sensitive to external magnetic
fields,
as is pronouncedly exemplified by observations of the
Aharonov-Bohm
(AB) effect in such structures, the small radius of molecular
rings
implies that the field needed to observe the AB periodicity is
unrealistically large. In this paper, we study the effect of
magnetic
field on electronic transport in molecular conduction junctions
involving ring molecules, aiming to identify conditions where
magnetic
field dependence can be realistically observed. We consider
electronic
conduction of molecular ring structures modeled both within the
tight-binding (H"uckel) model and as continuous rings. We also
show
that much of the qualitative behavior of conduction in these
models can
be rationalized in terms of a much simpler junction model based on
a
two-state molecular bridge. Dephasing in these models is affected
by
two common tools: the B"uttiker probe method and coherence damping
within a density matrix formulation. We show that current through
a
benzene ring can be controlled by moderate fields provided that
several
conditions are satisfied: (a) conduction must be dominated by
degenerate (in the free molecule) molecular electronic resonances,
associated with multiple pathways as is often the case with ring
molecules, (b) molecular-leads electronic coupling must be weak so
as
to affect relatively distinct conduction resonances, (c) molecular
binding to the leads must be asymmetric (e. g., for benzene,
connection
in the meta or ortho, but not para, configurations), and (d)
dephasing
has to be small. When these conditions are satisfied, considerable
sensitivity to an imposed magnetic field normal to the molecular
ring
plane is found in benzene and other aromatic molecules.
Interestingly,
in symmetric junctions (e. g., para-connected benzene), the
transmission coefficient can show sensitivity to magnetic field
that is
not reflected in the current-voltage characteristic. The analog of
this
behavior is also found in the continuous ring and the two-level
models.
Although sensitivity to magnetic field is suppressed by dephasing,
quantitative estimates indicate that magnetic field control can be
observed in suitable molecular conduction junctions.
300. M. Galperin
and A. Nitzan
PDF
Raman scattering from biased
molecular conduction junctions: The electronic background and
its temperature
Phys. Rev. B 84, 195325 (2011) (10 pages)
The existence of background
in the surface-enhanced Raman scattering from molecules
adsorbed on metal surfaces has been known since the early
studies about this phenomenon and is usually attributed to
transitions between electronic states of the metal
substrate. This paper reformulates the theory of this
phenomenon in the framework of the nonequilibrium Green
function formalism, which makes it possible to extend it to
the case of Raman scattering from nonequilibrium (biased and
current-carrying) molecular junctions. Following recent
experiments, we address, in particular, the Raman-scattering
measurement of current-induced electronic heating. The Raman
temperature, defined by fitting the ratio between the Stokes
and the anti-Stokes Raman signals to a Boltzmann factor is
compared to another measure of electronic heating obtained
by assuming that, close to the molecule-metal contact, the
electronic distribution is dominated by the transmission
process. We find that the Raman temperatures considerably
exceed this upper bound to the metal-electron heating. In
agreement with this observation, we show that the Raman
temperature reflects the electronic nonequilibrium in the
molecular bridge itself. We also show that the
Raman-temperature concept breaks down at large biases.
Optics of atomic clusters in two
dimensions: rigorous numerical studies
Phys. Rev. A 84, 043802 (2011) (10 pages)
We consider the interaction of electromagnetic radiation of
arbitrary polarization with multilevel atoms in a self-consistent
manner, taking into account both spatial and temporal dependencies
of local fields. This is done by numerically solving the
corresponding system of coupled Maxwell-Liouville equations for
various geometries. In particular, we scrutinize linear optical
properties of nanoscale atomic clusters, demonstrating the
significant role played by collective effects and dephasing. It is
shown that subwavelength atomic clusters exhibit two resonant
modes, one of which is localized slightly below the atomic
transition frequency of an individual atom, while the other is
positioned considerably above it. As an initial exploration of
future applications of this approach, the optical response of
core-shell nanostructures, with a core consisting of silver and a
shell composed of resonant atoms, is examined.
Heterojunction organic photovoltaic
cells as molecular heat engines: A simple model for the
performance analysis
J. Phys. Chem. C, 115, 21396–21401 (2011)
Organic heterojunction
solar cells are analyzed within a minimal model that
includes the essential physical features of such systems.
The dynamical properties of this model, calculated using a
master equation approach, account for the qualitative
behavior of such systems. The model yields explicit results
for currentvoltage behavior as well as performance
characteristics expressed in terms of the thermodynamic
efficiency as well as the power conversion efficiency at
maximum power, making it possible to evaluate the optimal
setup for this device model.
297.
D. Rai, O. Hod and A. Nitzan
PDF
Magnetic Field Control of the Current
through Molecular Ring Junctions
J. Phys. Chem. Lett. 2, 2118–2124 (2011)
Whereas conducting loops are,
in principle, sensitive to external magnetic field, as is
pronouncedly exemplified by the Aharonov-Bohm (AB) effect, the
small radius of molecular rings makes the observation of such
effects challenging. Indeed, the unrealistically large
magnetic field needed to realize the AB effect in molecular
rings has led to a widespread belief that molecular conduction
is insensitive to laboratory realizable fields. Here we
revisit this issue, presenting conditions under which magnetic
field control of molecular ring conduction is realizable with
pronounced effects on the IV characteristics. We find these
conditions to be (a) weakmolecule-lead coupling, implying
relatively distinct conduction resonances, (b) asymmetric
junction structure (e. g., meta- or ortho-connected benzene
rather than a para structure), and (c) minimal dephasing
(implying low temperature) so as to maintain coherence between
multiple pathways of conduction.
296.
A. Migliore and A. Nitzan
Nonlinear charge transport in redox
molecular junctions: a Marcus perspective
ACS Nano, 5, 6669-6685 (2011)
Redox molecular junctions are
molecular conduction junctions that involve more than one
oxidation state of the molecular bridge. This property is
derived from the ability of the molecule to transiently
localize transmitting electrons, implying relatively weak
molecule-leads coupling and, in many cases, the validity of
the Marcus theory of electron transfer. Here we study the
implications of this property on the nonlinear transport
properties of such junctions. We obtain an analytical solution
of the integral equations that describe molecular conduction
in the Marcus kinetic regime and use it in different physical
limits to predict some important features of nonlinear
transport in metalmoleculemetal junctions. In particular,
conduction, rectification, and negative differential
resistance can be obtained in different regimes of interplay
between two different conduction channels associated with
different localization properties of the excess molecular
charge, without specific assumptions about the electronic
structure of the molecular bridge. The predicted behaviors
show temperature dependences typically observed in the
experiment. The validity of the proposed model and ways to
test its predictions and implement the implied control
strategies are discussed.
295. A. Bednorz W. Belzig and A.
Nitzan
PDF
Time correlation functions in quantum
continuous measurement
New J. Phys., 14, 013009(2012) [20 pages]
A continuous projective
measurement of a quantum system often leads to a suppression
of the dynamics, known as the Zeno effect. Alternatively,
generalized nonprojective, so-called `weak' measurements can
be carried out. Such a measurement is parameterized by its
strength parameter that can interpolate continuously between
the ideal strong measurement with no dynamics-the strict Zeno
effect, and a weak measurement characterized by almost free
dynamics but blurry observations. Here we analyze the
stochastic properties of this uncertainty component in the
resulting observation trajectory. The observation uncertainty
results from intrinsic quantum uncertainty, the effect of
measurement on the system (backaction) and detector noise. It
is convenient to separate the latter, system-independent
contribution from the system-dependent uncertainty, and this
paper shows how to accomplish this separation. The
systemdependent uncertainty is found in terms of a
quasi-probability, which, despite its weaker properties, is
shown to satisfy a weak positivity condition. We discuss the
basic properties of this quasi-probability with special
emphasis on its time correlation functions as well as their
relationship to the full correlation functions along the
observation trajectory, and illustrate our general results
with simple examples. We demonstrate a violation of classical
macrorealism using the fourthorder time correlation functions
with respect to the quasi-probability in the two level system.
294.
M.
Galperin and A. Nitzan
Raman scattering and electronic heating
in molecular conduction junctions
J. Phys. Chem. Lett. 2, 2110–2113 (2011)
Surface-enhanced Raman
scattering (SERS) was recently used to monitor
nonequilibrium properties of molecular conduction junctions.
Ward et al. (Nat. Nanotechnol. 2011, 6, 33) have used such
measurements to estimate heating of the molecular vibrations
(indicated by the ratio between Stokes and anti-Stokes Raman
peaks) as well as the electronic metal substrate (inferred
from the corresponding components of the Raman continuum).
The latter observation suggests, contrary to standard
assumptions, significant heating of the metal contacts.
Here, we discuss this observation by advancing a theory of
the electronic Raman scattering background in biased current
carrying molecular junctions and using it to estimate the
electronic heating, as seen in the Raman signal. We reach
the unexpected conclusion that while heating of the
electronic background in Raman scattering from biased
molecular junctions is indeed observed, this does not
necessarily imply an appreciable deviation from thermal
equilibrium in the electronic distributions in the leads.
CB27. A. Landau, L. Kronik and A.
Nitzan
PDF
Molecular conduction
junctions: Intermolecular effects
in Perspectives of Mesoscopic
Physics (World
Scientific, Singapore, 2010).
Intermolecular interactions can affect the conduction properties
of molecular junctions in several ways: Direct and
through-substrate electronic interactions affect the spectral
properties (density of states) of the conducting junction,
intermolecular electrostatic interactions affect the positioning
of molecular electronic energies and thereby the nature of
interface polarization. Such interactions also influence the
screening properties of the junction and consequently the
electrostatic potential profile across the biased junction. Other
consequences include effects on junction mechanical properties
that can be manifested by a different temperature dependence of
conduction for a single molecule and for a molecular layer, as
well as effects on optical response that may be important, for
example, for the junction response associated with light induced
switching. This article discusses some of these effects and their
implications for the performance of molecular junctions.
293.
Circular
Currents in Molecular Wires
PDF
D. Rai, O. Hod and A. Nitzan
J. Phys. Chem C, 114, 20583-20594 (2010)
We consider circular currents driven by voltage bias in
molecular wires with loop substructures studied within simple
tight-binding models. Previous studies of this issue have
focused on specific molecular structures. Here we address
several general issues. First we consider the quantitative
definition of a circular current and adopt a definition that
identifies the circular component of a loop current as the sole
source of the magnetic field induced in the loop. The latter may
be associated with the field at the loop center, with the
magnetic moment associated with this field or with the total
magnetic flux threading the loop. We show that all three
measures yield an identical definition of the loop current.
Second, we study dephasing effects on the loop current and the
associated induced magnetic field. Finally, we consider circular
currents in several molecular structuressbenzene, azulene,
naphthalene, and anthracenesand show that circular currents
occur generically in such structures; can be, in certain voltage
ranges, considerably larger than the net current through the
molecule; and are furthermore quite persistent to normal thermal
dephasing.
292. M. Einax, G. Solomon, W. Dieterich
and A. Nitzan
PDF
Unidirectional hopping
transport of interacting particles on a finite chain
J. Chem. Phys. 133, 054102
(2010) [12 pages]
Particle transport through an open, discrete one-dimensional
channel against a mechanical or chemical bias is analyzed
within a master equation approach. The channel, externally
driven by time-dependent site energies, allows multiple
occupation due to the coupling to reservoirs. Performance
criteria and optimization of active transport in a two-site
channel are discussed as a function of reservoir chemical
potentials, the load potential, interparticle interaction
strength, driving mode, and driving period. Our results,
derived from exact rate equations, are used in addition to
test a previously developed time-dependent density functional
theory, suggesting a wider applicability of that method in
investigations of many particle systems far from equilibrium.
291.
P.Schiff
and A. Nitzan
PDF
Kramers barrier crossing
as a cooling machine
Chemical Physics
The achievement of local cooling is a prominent goal in the
design of functional transport nanojunctions. One generic
mechanism for local cooling is driving a system through a local
uphill potential step. In this paper we examine the
manifestation of this mechanism in the context of the Kramers
barrier crossing problem. For a particle crossing a barrier, the
local effective temperature and the local energy exchange with
the thermal environment are calculated, and the coefficient of
performance of the ensuing cooling process is evaluated.
290.
V.
Ben-Moshe, D. Rai, S. S. Skourtis and A.
Nitzan
PDF
Steady state current transfer
and scattering theory
J. Chem. Phys, 133, 054105 (2010) [9
pages]
The correspondence between the steady-state theory of current
transfer and scattering theory in a system of coupled
tight-binding models of one-dimensional wires is explored. For
weak interwire coupling both calculations give nearly identical
results, except at singular points associated with band edges.
The effect of decoherence in each of these models is studied
using a generalization of the Liouville-von Neuman equation
suitable for steady-state situations. An example of a single
impurity model is studied in detail, leading to a lattice model
of scattering off target that affects both potential scattering
and decoherence. For an impurity level lying inside the energy
band, the transmission coefficient diminishes with increasing
dephasing rate, while the opposite holds for impurity energy
outside the band. The efficiency of current transfer in the
coupled wire system decreases with increasing dephasing.
289.
V.
Ben Moshe, A. Nitzan, S. S. Skourtis and D. Beratan
PDF
Steady-state
theory of current transfer
J. Phys. Chem. C, 114, 8005–8013 (2010)
Current transfer is defined as a charge-transfer process where
the transferred charge carries information about its original
motion. We have recently suggested that such transfer causes the
asymmetry observed in electron transfer induced by circularly
polarized light through helical wires. This paper presents the
steady-state theory of current transfer within a tight binding
model of coupled wires systems. The asymmetry in the system
response to a steady current imposed in a particular direction
on one of the wires is used to define the efficiency of current
transfer.
288. G. Li, B. D. Fainberg, A. Nitzan, S. Kohler and Peter
Hänggi
PDF
Coherent charge transport
through molecular wires: Exciton blocking and current from
electronic excitations in the wire
Phys. Rev. B 81,
165310(2010) [14 pages]
We consider exciton effects
on current in molecular nanojunctions, using a model
comprising a two twolevel sites bridge connecting
free-electron reservoirs. Expanding the density operator in
the many-electron eigenstates of the uncoupled sites, we
obtain a 16x16 density matrix in the bridge subspace whose
dynamics is governed by Liouville equation that takes into
account interactions on the bridge as well as electron
injection and damping to and from the leads. Our consideration
can be considerably simplified by using the pseudospin
description based on the symmetry properties of Lie group
SU(2). We study the influence of the bias voltage, the Coulomb
repulsion, and the energy-transfer interactions on the
steady-state current and, in particular, focus on the effect
of the excitonic interaction between bridge sites. Our
calculations show that in case of noninteracting electrons
this interaction leads to reduction in the current at high
voltage for a homodimer bridge. In other words, we predict the
effect of "exciton" blocking. The effect of exciton blocking
is modified for a heterodimer bridge and disappears for strong
Coulomb repulsion at sites. In the latter case the exciton
type interactions can open new channels for electronic
conduction. In particular, in the case of strong Coulomb
repulsion, conduction exists even when the electronic
connectivity does not exist.
287. M. Einax, M. Körner, P Maass and
A. Nitzan
PDF
Nonlinear hopping transport
in ring systems and open channels
Phys. Chem. Chem. Phys. 12, 645-654 (2010)
We study the nonlinear
hopping transport in one-dimensional rings and open channels.
Analytical results are derived for the stationary current
response to a constant bias without assuming any specific
coupling of the rates to the external fields. It is shown that
anomalous large effective jump lengths, as observed in recent
experiments by taking the ratio of the third-order nonlinear
and the linear conductivity, can occur already in ordered
systems. Rectification effects due to site energy disorder in
ring systems are expected to become irrelevant for large
system sizes. In open channels, in contrast, rectification
effects occur already for disorder in the jump barriers and do
not vanish in the thermodynamic limit. Numerical solutions for
a sinusoidal bias show that the ring system provides a good
description for the transport behavior in the open channel for
intermediate and high frequencies. For low frequencies
temporal variations in the mean particle number have to be
taken into account in the open channel, which cannot be
captured in the more simple ring model.
CB26. B.D. Fainberg, P. Hanggi, S. Kohler and
A. Nitzan
Exciton- and Light-induced
Current in Molecular Nanojunctions
American Institute of Physics
Conference Proceedings 1147, 78-86 (2009)
(ICTOPON-2009 Conference: Transport And Optical Properties Of
Nanomaterials)
In isolated organic molecular chain,
electron-hole pairs (excitons) possess dipole moments
interacting with each other and leading to exciton tunneling
along the molecule. Although this
mechanism does not induce a net charge transfer, it nevertheless
may influence the electrical current, in particular, if the
molecule is subject to laser radiation. We have investigated exciton- and
light-induced current in molecular nanojunctions. Using a model
comprising a two two-level sites bridge connecting free electron
reservoirs we show that the exciton coupling between the sites
of the molecular bridge can markedly effect the source-drain
current through a molecular junction. In some cases when excited
and unexcited states of the sites are coupled differently to the
leads, the contribution from electron-hole excitations can
exceed the Landauer elastic current and dominate the observed
conduction. We have proposed an optical control method using
chirped pulses for enhancing charge transfer in unbiased
junctions where the bridging molecule is characterized by a
strong charge-transfer transition.
285. J. E. Subotnik, T. Hansen, M. A.
Ratner and A. Nitzan
Nonequilibrium steady state
transport via the reduced density matrix operator
J. Chem. Phys. 130,
144105
(2009)
PDF
We present a very simple
model for numerically describing the steady state dynamics of a
system interacting with continua of states representing a bath.
Our model can be applied to equilibrium and nonequilibrium
problems. For a one-state system coupled to two free electron
reservoirs, our results match the Landauer formula for current
traveling through a molecule. More significantly, we can also
predict the nonequilibrium steady state population on a molecule
between two out-of-equilibrium contacts. While the method
presented here is for one-electron Hamiltonians, we outline how
this model may be extended to include electron-electron
interactions and correlations, an approach which suggests a
connection between the conduction problem and the electronic
structure problem.
Cooling mechanisms in molecular
conduction
junctions
Phys. Rev. B 80, 115427 (2009) [12
pages]
PDF
While
heating of a current carrying Ohmic conductors is an obvious
consequence of the diffusive nature of the conduction in such
systems,
current-induced cooling has been recently reported in some
molecular
conduction junctions. In this paper, we demonstrate by simple
models
the possibility of cooling molecular junctions under applied
bias, and
discuss several mechanisms for such an effect. Our model is
characterized by single electron tunneling between electrodes
represented by free electron reservoirs through a system
characterized
by its electron levels, nuclear vibrations and their structures.
We
consider cooling mechanisms resulting from (a) cooling of
one
electrode surface by tunneling-induced depletion of high-energy
electrons; (b) cooling by coherent sub resonance electronic
transport
analogous to atomic laser-induced cooling and (c) the incoherent
analog
of process (b)-cooling by driven activated transport. The
non-equilibrium Green function formulation of junction transport
is
used in the first two cases, while a master equation approach is
applied in the analysis of the third.
Cooperative effects in
molecular conduction II: The Semiconductor−Metal Molecular
Junction
J. Phys. Chem. A 113,
7451–7460 (2009)
PDF
Our recent
calculation of the effect of intermolecular interactions on
molecular conduction (J. Comput. Theor. Nanosci. 2008, 5, 535) is
generalized to molecules adsorbed on a model semiconductor surface
and in a metal-molecule-semiconductor junction. The metal and
semiconductor electrodes are represented by cubic lattices within
generic tight binding models, where the semiconductor two-band
structure is described by using a simple site-alteration property.
A physically motivated choice of parameters for the molecule(s)
and the electrodes completes the model definition. The model
encompasses direct intermolecular interactions as well as
through-metal interactions and can be solved exactly to yield
spectral properties (surface density of states) and transport
characteristics (transmission coefficients and current-voltage
behavior) for single-molecule junctions and molecular layers. The
model is applied to analyzing the effect of intermolecular
interactions on the predicted negative differential resistance in
metal-molecule-semiconductor junctions (recently observed in
scanning tunneling microscopy studies of adsorbates on Si(100)).
Transport in State Space:
Voltage-Dependent Conductance Calculations of
Benzene-1,4-dithiol
Nano Let. 9, 1770-74
(2009)
PDF
We implement a method to study transport in a basis of many-body
molecular states using the nonequilibrium Hubbard Green's function
technique. A well-studied system, a junction consisting of
benzene-dithiol on gold, is the focus of our consideration.
Electronic structure calculations are carried out at the
Hartree-Fock (HF), density functional theory (DFT), and
coupled-cluster singles and doubles (CCSD) levels, and multiple
molecular states are included in the transport calculation. The
conductance calculation yields new information about the transport
mechanism in BDT junctions.
Chiral control of
electron transmission through molecules
Phys. Rev. Let. 101,
238103
(2008)
PDF
277.
B.
Fainberg and A. Nitzan
Decaying Rabi oscillations in quantum-dot tunnelling
junctions
Phys. Status Solidi A 206,
948–951
(2009)
PDF
Motivated by
the experiments by Zrenner et al. [Nature 418,
612 (2002)], we study the influence of relaxation
processes on converting Rabi oscillations in a strongly
biased single-quantum-dot photodiode into deterministic
photocurrents. We show that the behavior of a quantum dot
with different tunnel rates for electron and holes is
qualitatively different from that with the equal tunnel
rates: in the latter case the current shows
attenuating oscillations with the Rabi frequency. In
contrast, for different electrons and holes tunnelling
rates, the frequency of these oscillations diminishes, and
they disappear beyond a definite asymmetry threshold. We
give an analytical solution of the problem and a numerical
example showing a different behaviour of the transferred
charge in the small attenuation limit for equal and
different tunnel rates for electrons and holes.
The
non-linear response of molecular junctions: the polaron
model revisited
J. Phys.: Condens. Matter 20, 374107(2008)
PDF
A polaron model proposed
as a possible mechanism for non-linear conductance
(Galperin et al
2005 Nano Lett. 5
125–30) is revisited with the focus on the differences
between the weak and
strong molecule–lead
coupling cases. Within the one-molecule-level model we
present an
approximate
expression for the electronic Green function
corresponding to the inelastic
transport case, which
in the appropriate limits reduces to expressions
presented previously for
the isolated molecule
and for a molecular junction coupled to a slow
vibration (static limit). The
relevance of
considerations based on the isolated molecule limit to
understanding properties of
molecular junctions
is discussed.
Inelastic transport
in the Coulomb blockade regime within a nonequilibrium
atomic limit
Phys. Rev. B 78,
125320
(2008)
PDF
A method developed by Sandalov et al. Int. J. Quantum Chem.
94, 113 2003 is applied to inelastic
transport in the case of strong correlations on the
molecule, which is relatively weakly coupled to contacts.
The
ability of the approach to deal with the transport in the
language of many-body molecular states as well as to
take into account charge-specific normal modes and
nonadiabatic couplings is stressed. We demonstrate the
capabilities of the technique within simple model
calculations and compare it to previously published
approaches.
Theory of light-induced current in
molecular-tunneling junctions excited with intense
shaped pulses
Phys. Rev. B 76, 245329 (2007)
PDF
A
theory for light-induced current by strong optical pulses
in molecular-tunneling junctions is described. We consider
a molecular bridge represented by its highest occupied and
lowest unoccupied levels, HOMO and LUMO, respectively. We
take into account two types of couplings between the
molecule and the metal leads: electron transfer that gives
rise to net current in the biased junction and energy
transfer between the molecule and electron-hole
excitations in the leads. Using a Markovian approximation,
we derive a closed system of equations for the expectation
values of the relevant variables: populations and
molecular polarization polarization that are binary, and
exciton populations that are tetradic in the annihilation
and creation operators for electrons in the molecular
states. We have proposed an optical control method using
chirped pulses for enhancing charge transfer in unbiased
junctions where the bridging molecule is characterized by
a strong charge-transfer transition. An approximate
analytical solution of the resulting dynamical equation is
supported by a full numerical solution. When energy
transfer between the molecule and electron-hole
excitations in the leads is absent, the optical control
problem for inducing charge transfer with linearly chirped
pulse can be reduced to the Landau-Zener transition to a
decaying level. In the absence of both energy transfer and
optical coupling the Landau theory is recovered. The
proposed control mechanism is potentially useful for
developing novel opto-electronic single-electron devices
with optical gating based on molecular nanojunctions.
264. M.
Galperin, A. Nitzan, and M. A. Ratner
Resonant inelastic
tunneling in molecular junctions
Phys. Rev. B 73,
045314
(2006)
PDF
Within a phonon-assisted resonance level model
we develop a self-consistent procedure for
calculating electron transport currents in
molecular junctions with intermediate to
strong electron-phonon interaction. The scheme
takes into account the mutual influence of the
electron and phonon subsystems. It is based on
the second order cumulant expansion, used to
express the correlation function of
the phonon shift generator in terms of the
phonon momentum Green function. Equation of
motion _EOM_ method is used to obtain an
255. O.Durr, W.Dieterich and A. Nitzan
Coupled ion and network dynamics in polymer electrolytes:
J. Chem. Phys. 121, 12732-39
(2004)
PDF
Monte Carlo
Hysteresis,
switching,
and Negative Differential Resistance in molecular
junctions: A polaron model
Nano Letters
PDF
Within a simple mean-field model (self-consistent Hartree
approximation) we discuss the possibility of polaron
formation on a molecular wire as a mechanism for negative
differential resistance (NDR), switching, and/or
hysteresis in the I- V characteristic of molecular
junctions. This mechanism differs from earlier proposed
mechanisms of charging and conformational change. The
polaron model captures the essential physics and provides
qualitative correspondence with experimental data. The
importance of active redox centers in the molecule is
indicated.
253. M.
Galperin, A. Nitzan, M. A. Ratner and D. R. Stewart
Molecular Transport
Junctions: Asymmetry in Inelastic Tunneling Processes
PDF
Inelastic electron tunneling
spectroscopy (IETS) measurements are usually carried out
in the low-voltage (Ohmic)regime where the elastic
conduction voltage characteristic is symmetric to voltage
inversion. Inelastic features, normally observed in the
second derivative d2I/dV2 are also
symmetric (in fact antisymmetric) in many cases, but
asymmetry is sometimes observed. We show that such
asymmetry can occur due to different energy dependences of
the two contact self-energies. This may be attributed to
differences in contact density of states (different
contact material) or different energy dependence of the
coupling (STM like geometry or asymmetric positioning of
molecular vibrational modes in the junction). The
asymmetry scales with difference between the energy
dependence of these self-energies, and disappears when
this dependence is the same for the two contacts. Our
nonequilibrium Green function approach goes beyond
proposed WKB scattering theory
252.
M. Ratner and A. Nitzan
The chemist's view of
Nanotechnology: Molecular Electronics
ננוטכנולוגיה
בראי הכימיה: אלקטרוניקה מולקולרית
Bulletin of the
Israel Chemical Society, Issue 14, p. 3-13 (Dec
2003) PDF
251. A.W. Ghosh,P.S.
Damle,S. Datta,and A. Nitzan
Molecular
Electronics: Theory and device prospects
MRS Bulletin, 29,
391-395
(2004)
PDF
Understanding current flow through
molecular conductors involves simulating the
250. D.
Segal and A. Nitzan
A spin-Boson thermal
rectifier
PDF
Phys. Rev. Let. 94,
034301 (2005)
Following the observation that inherent anharmonicity is
required, in addition to structural asymmetry, for
rectification of heat transfer in nanojunctions, we analyze
this phenomenon within the simplest anharmonic system –a
spin-boson nanojunction model. We consider two variants of
the model: a linear separable model in which the heat
reservoirs contribute additively and a non-separable model
suitable for a stronger system-bath interaction. Both model
show asymmetric (rectifying) heat conduction when symmetry
is removed by taking different couplings to the heat
reservoirs.
249. M.
Galperin, M. Ratner and A. Nitzan
On lineshapes and linewidths of
vibrational features in inelastic electron tunneling
spectroscopy
Nano Letters, 4, 1605-1611 (2004)
PDF
We addresses the lineshape and linewidths observed in recent
inelastic electron tunneling spectroscopy (IETS)
experiments. The non-equilibrium Green function (NEGF)
formalism is used to analyze the effect of the
electron-phonon interaction on the tunneling spectra. We
find that IETS lineshapes are sensitive to junction
parameters, in particular the position of the bridge
electronic resonance and the molecule-lead coupling that may
be controlled experimentally. Intrinsic IETS linewidths are
found to be dominated by the coupling of molecular
vibrations to electron-hole pairs excitations in the lead(s)
to which the molecule is bonded chemically. While estimated
widths are of similar order of magnitudes as observed in the
recent experiment of Wang et al (Nano Letters, 4, 643
(2004)), one cannot rule out inhomogeneous contribution to
the linewidth in this monolayer experiment.
248. M. Galperin
M. A. Ratner and A. Nitzan
Inelastic electron tunneling
spectroscopy in molecular junctions: Peaks and dips
J. Chem. Phys. 121, 11965-11979
(2004) PDF
We study inelastic electron tunneling through a molecular
junction using the non-equilibrium Green function
(NEGF) formalism. The effect of the mutual influence between
the phonon and the electron subsystems on the electron
tunneling process is considered within a general
self-consistent scheme. Results of this calculation are
compared to those obtained from the simpler Born
approximation and the simplest perturbation theory
approaches, and some shortcomings of the latter are pointed
out. The self-consistent calculation allows also for
evaluating other related quantities such as the power loss
during electron conduction. Regarding the inelastic
spectrum, two types of inelastic contributions are
discussed. Features associated with real and virtual energy
transfer to phonons are usually observed in the second
derivative of the current I with respect to the
voltage when plotted against ?. Signatures of
resonant tunneling driven by an intermediate molecular ion
appear as peaks in the first derivative and may
show phonon sidebands. The dependence of the observed
vibrationally induced lineshapes on the junction
characteristics, and the linewidth associated with these
features are also discussed.
247. Y. Calev,
H. Cohen, G. Cuniberti, A. Nitzan, and D. Porath
A Tight binding description of the STM
image of molecular chains
A tight binding model for scanning tunneling microscopy
images of a molecule adsorbed on a metal surface is
described. The model is similar in spirit to that used to
analyze conduction along molecular wires connecting two
metal leads and makes it possible to relate these two
measurements and the information that may be gleaned from
the corresponding results. In particular, the dependence of
molecular conduction properties along and across a molecular
chain on the chain length, intersite electronic coupling
strength and on thermal and disorder effects are discussed
and contrasted. It is noted that structural or chemical
defects that may affect drastically the conduction along a
molecular chain have a relatively modest influence on
conduction across the molecular wire in the transversal
direction.
246. S. Skourtis
and A. Nitzan
Effects of initial state preparation
on the distance dependence of electron transfer through
molecular bridges and wires
J. Chem. Phys., 119, 6271-6
(2003) PDF
The dependence of electron transfer rates and yields in
bridged molecular systems on the bridge length, and the
dependence of the zero-bias conduction of molecular wires on
wire length are discussed. Both phenomena are controlled by
tunneling across the molecular bridge and are consequently
expected to show exponential decrease with bridge length
that is indeed often observed. Deviations from this
exponential dependence for long bridges, in particular a
crossover to a very weak dependence on bridge length were
recently observed experimentally and discussed theoretically
in terms of thermal relaxation and dephasing on the bridge.
Here we discuss two other factors that potentially affect
the bridge length dependence of these phenomena.
First, in experiments initiated by an initial preparation of
a non-stationary "donor" state the initial energy is not
well defined. A small contribution from initially populated
eigenstates that are of mostly bridge-level character may
dominate transmission for long bridges, resulting in weak or
no bridge-length dependence. Secondly, in steady state
experiments the distribution of initial states (for example
the Fermi distribution at the electrodes in conduction
experiments) will cause deviations from exponential
dependence on wire length because even a small population in
higher energy states will dominate the transmission through
long wires. For the first scenario we show that the
crossover behavior observed for electron transfer in DNA
between G and GGG species separated by AT chains can be
largely reproduced just by initial state effects.
245.
S.Zilberman, T.Becker, F.Mugele, B.N.J.Persson and
A.Nitzan
Dynamics of squeeze-out:Theory and
experiments
J. Chem. Phys., 118, 1160 (2003)
PDF
We consider the dynamics of squeeze-out of a molecularly
thin confined two-dimensional ~2D liquidlike layer.The
squeeze-out is described by a generalized 2D Navier–Stokes
equation which is solved exactly for the limiting case where
the squeeze-out nucleates at the center of the contact area,
and where the ~perpendicular! three-dimensional pressure
profile is Hertzian.We also present numerical results for
the case where the nucleation is off-center.The theoretical
results are in good agreement with recent experimental data
by two of us for octamethylcyclotetrasiloxane.In light of
our theoretical model calculations,we also discuss the
spatially resolved diffusion experiments of Mukhopadhyay et
al.(Phys.Rev.Lett.89,136103 ~2002).Here,we obtain a puzzling
disagreement between theory and experiment which requires
more investigation.
244. P. Graf, M.
G. Kurnikova, R. D. Coalson and A. Nitzan
Comparison
of Dynamic Lattice Monte-Carlo Simulations and Dielectric
Self Energy
Poisson-Nernst-Planck continuum theory for model
ion-channels
PDF
Simulations of ion permeation through narrow model
cylindrical channels are carried out using a dynamic lattice
Monte Carlo (DLMC) algorithm (equivalent to high friction
Langevin dynamics) for the time-evolution of the ions in the
system, based on a careful evaluation of the electrostatic
forces acting upon each particle. To mimic the process
of ion transport through protein channels, the cylindrical
channel is embedded in a dielectric slab (representing a
lipid bilayer membrane). The protein/membrane
structure is taken to be rigid, and the water solvent is
treated as a dielectric continuum. Results of these
simulations are compared to corresponding results obtained
via Poisson-Nernst-Planck (PNP) theory. In the PNP
approach, the mobile ions are treated as a continuous charge
density, and the electrostatic force on each ion is treated
in an approximate fashion. Significant differences
between DLMC and PNP results are found, with the degree of
discrepancy increasing as the radius of the ion channel is
reduced. A major source of error is traced to the
neglect in the effective PNP potential of the Dielectric
Self Energy (DSE), which is due to interaction of each
permeant ion with the dielectrically inhomogeneous
environment provided by the water/channel/membrane
system. When this static single-particle potential is
pre-calculated and added to the effective potential used in
PNP theory, substantial improvement in the quality of the
results for current voltage curves and steady-state
concentrations is obtained. In fact, the results obtained by
this approach, termed Dielectric Self Energy
Poisson-Nernst-Planck (DSEPNP) theory, agree nearly
quantitatively with DLMC simulation results over the entire
range of channel radii (4-12A) studied.
243. D. Segal,
A. Nitzan and P. Hanggi
Thermal conductance through molecular
wires
J.
Chem. Phys. 119, 6840-6855
(2003)
PDF
We consider phononic heat transport through molecular chains
connecting two thermal reservoirs. For relatively short
molecules at normal temperatures we find, using classical
stochastic simulations, that heat conduction is dominated by
the harmonic part of the molecular force-field. We develop a
general theory for the heat conduction through harmonic
chains in 3-dimensions. Our approach uses the standard
formalism that leads to the generalized Langevin equation
for a system coupled to a harmonic heat bath, however the
driving and relaxation terms are considered separately in a
way that leads directly to the steady state response and the
heat current under non-equilibrium driving. A Landauer-type
expression for the heat conduction is obtained, in agreement
with other recent studies. We used this general formalism to
study the heat conduction properties of alkane. We find that
for relatively short (1-30 carbon molecules) the length and
temperature dependence of the molecular heat conduction
results from the balance of three factors: (i) The molecular
frequency spectrum in relation to the frequency cutoff of
the thermal reservoirs, (ii) the degree of localization of
the molecular normal modes and (iii) the molecule-heat
reservoirs coupling. The fact that molecular modes at
different frequency regimes have different localization
properties gives rise to intricate dependence of the heat
conduction on molecular length at different temperatures.
For example, the heat conduction increases with molecular
length for short molecular chains at low temperature.
Isotopically substituted disordered chains are also studied
and their behavior can be traced to the above factors
together with the increased mode localization in disordered
chain and the increase in the density of low frequency modes
associated with heavier mass substitution. Finally, we
compare the heat conduction obtained from this microscopic
calculation to that estimated by considering the molecule as
a cylinder characterized by a macroscopic heat conduction
typical to organic solids. We find that this classical model
overestimates the heat conduction of single alkane molecules
by about an order of magnitude at room temperature.
Implications of the present study to the problem of heating
in electrically conducting molecular junctions are pointed
out.
242. M. Galperin
and A. Nitzan
NEGF-HF method for molecular junction
properties calculations
Annals
of the NY Academy of Science, in
press
PDF
Electron-electron interaction is an essential issue in
predicting the properties of molecular conduction junction.
An accurate treatment requires taking proper account of the
potential distribution across the junction as well as for
the change in electronic structure under the external
voltage drop. Another important point is the necessity to
treat molecules as open quantum systems. In the present work
we address these issues within the framework of the
non-equilibrium Green’s function formalism at the
Hartree-Fock level (NEGF-HF), which permits self-consistent
treatment of the problem. We apply the method to study the
experimentally observed asymmetry in the I/V curves with
respect to polarity of voltage bias in Hg-Au junctions
containing bilayers of alkanethiols of different chain
length. The origin of the effect is suggested to be the
asymmetric behavior of the character of the highest occupied
molecular orbital (HOMO) of the junction at opposite biases,
which leads to different effective barriers for electron
transfer across the junction at opposite signs of the
voltage drop. The calculated potential profile shows the
capacitor-like nature for the junction with the weak link.
241. A. Troisi,
M.A. Ratner and A. Nitzan
A Rate Constant Expression for Charge
Transfer through Fluctuating Bridges
J.
Chem.
Phys.
PDF
A rate constant expression for charge transfer reactions
mediated by flexible bridges is presented as a series of
terms of decreasing importance. The leading term corresponds
to the static limit obtained under the Condon approximation.
Corrections due to finite time fluctuations are evaluated
explicitly, assuming a Gaussian shape of the coupling
autocorrelation function and the Marcus model with a
one-dimensional harmonic thermal bath. The use of this model
for the interpretation of the experimental data is
discussed.
240. A. Nitzan
and Mark Ratner
Electron transport in molecular wire
functions: Models and Mechanisms
Science, 300, 1384-1389 (2003)
PDF
Molecular conductance junctions, in which single molecules
or small groups of molecules act as conductors of electrical
current between two electrodes, have become an area of
intense activity. This overview focuses on theoretical
formulation and interpretation of the problem of molecular
conductance. We consider how to formulate the problem
of molecular conductance junctions, and the calculation of
their current/voltage characteristics. We discuss
different approaches and mechanisms, the relationship
between conductance and the fundamental chemical kinetics of
electron transfer and some of the challenging areas not yet
properly addressed.
239. A. Troisi, M.A. Ratner and A. Nitzan
Vibronic effects in off-resonance
molecular wire conduction
J. Chem. Phys. 118, 6072-6082 (2003)
PDF
A model for the calculation of the inelastic contribution to
the low-bias electron transport in molecular junctions is
presented. It is an extension to the inelastic case of the
Green's function approach to the calculation of the
conduction of such systems. The model is suited for the
calculation in the off resonance regime (where molecular
levels are far from the Fermi energy) and in the low bias
limit, a typical situation encountered in inelastic electron
tunneling measurements. The presentation of a general model
is followed by the introduction of several approximations
that make the calculation feasible for many systems of
interest. Ab initio calculations of the vibronic coupling
that leads to inelastic contribution to the conductance are
performed for several molecules (butadiene, biphenyl,
dipyrrole and dithiophene), representative of possible
molecular wires. The role of inelastic conduction is then
quantified without empirical parameters and the vibrational
modes that dominate the process are identified. The
situations where the inelastic mechanism is particularly
relevant are considered. The limits of this approach for the
resonant case are also discussed.
238. V.
Mujica, A. Nitzan, S. Datta, M. A. Ratner and C. P.
Kubiak
Molecular Wire Junctions: Tuning the
Conductance
J. Phys. Chem., 107, 91-95
(2003) PDF
Junctions consisting of two electrodes and a single molecule
acting as a wire can be investigated by scanning probe
conductance measurements. The conductance for low
temperature, short wire structures can be characterized by
the elastic scattering of the electrons, as described by
general Landauer formulas. Tuning the bridge energy levels
either by field effect transistor (FET)- like behavior
(charging) or by chemical bond formation can raise or lower
the conductance by moving the poles of the molecular Green’s
function. For small molecular structures with particular
HOMO/ LUMO characteristics, functionalization might be a
more effective means than Coulomb charging by external
gates.
237. A. Nitzan
The relationship between electron
transfer rate and molecular conduction. 2. The sequential
hopping case
Israel Journal of Chemistry, 42,
163-166 (2002) PDF
This note discusses the relationship between a given bridge
assisted electron transfer rate and the corresponding zero
bias molecular conduction of the same molecular species, in
the limit where both process occur by sequential hopping. It
follows a previous publication (A. Nitzan,
J. Phys. Chem. A 105, 2677-2679(2001)) in which the
same issue was discussed for coherent tunneling transfer.
236. S.
Pleutin, H. Grabert, G. L. Ingold and A.
Nitzan
The electrostatic potential profile
along a biased molecular wire: A model quantum
mechanical calculation
J. Chem. Phys. 118,
3756-3763
(2003)
PDF
We study the electrostatic potential of a molecular wire
bridging two metallic electrodes in the limit of weak
contacts. With the use of a tight-binding model including a
fully three-dimensional treatment of the electrostatics of
the molecular junction, the potential is shown to be poorly
screened, dropping mostly along the entire molecule. In
addition, we observe pronounced Friedel oscillations that
can be related to the breaking of electron-hole symmetry.
Our results are in semi-quantitative agreement with recent
state-of-the-art ab-initio calculations and point to the
need of a three-dimensional treatment to properly capture
the behavior of the electrostatic potential. Based on these
results, current-voltage curves are calculated within the
Landauer formalism. It is shown that Coulomb interaction
partially compensates the localization of the charges
induced by the electric field and consequently tends to
suppress zones of negative differential resistance.
235.
M. Galperin, A. Nitzan, S. Sek and M. Majda
Asymmetric Electron
Transmission across Asymmetric Alkanethiol Bilayer
Junctions
J. Electroanalytical Chemistry, in
press
PDF
Asymmetric i/V curves with respect to the polarity of the
voltage bias were observed in the Hg-Au junctions containing
bilayers of alkanethiols of different chain length.
Larger current resulted when a negative bias was applied to
the metal carrying a longer chain alkanethiol monolayer.
This behavior is simulated using a single molecule junction
model, within the frameworks of the extended Hückel (EH)
model and the nonequilibrium Green's function formalism at
the Hartree Fock level (NEGF-HF). Qualitative agreement with
the experimental results with respect to the magnitude and
sign of this asymmetry is obtained. On the basis of the
NEGF-HF calculation, the origin of the effect is suggested
to be the asymmetric behavior of the character of the
junction highest occupied molecular orbital (HOMO) at
opposite biases. This change of character leads to different
effective barriers for electron transfer for opposite signs
of the voltage drop across the junction.
234.
J. Lehmann, S. Kohler, P. Hanggi and A. Nitzan
Rectification of
laser-induced electronic transport through molecules
J. Chem. Phys. 118,
3283-3293 (2003)
PDF
We study the in uence of laser radiation on
the electron transport through a molecular wire weakly
coupled to two leads. In the absence of a generalized
parity symmetry, the molecule rectifes the laser induced
current resulting in directed electron transport without
any applied voltage. We consider two generic ways of
dynamical symmetry breaking: mixing of different harmonics
of the laser eld and molecules consisting of asymmetric
groups. For the evaluation of the nonlinear current, a
numerically effcient formalism is derived which is based
upon the Floquet solutions of the driven molecule. This
permits a treatment in the non-adiabatic regime and beyond
linear response.
233.
A. Nitzan, M. Galperin, G-L. Ingold and H. Grabert
On the electrostatic
potential profile in biased molecular wires
J. Chem. Phys., 117, 10837-41 (2002)
PDF
The potential profile across a biased
molecular junction is calculated within the framework of a
simple Thomas-Fermi type screening model. In particular,
the relationship between this profile and the lateral
molecular cross section is examined. We find that a
transition from a linear potential profile to a potential
that drops mainly near the molecule-metal contacts occurs
with increasing cross-section width, in agreement with
numerical quantum calculations.
232. A. Mamonov,
R. D. Coalson, A. Nitzan, and M. G. Kurnikova
The role of the dielectric barrier in
narrow biological channels: a novel composite approach to
modeling single channel currents
Biophysical Journal, 84, 3646-61 (2003)
PDF
A novel composite continuum theory to calculate ion currents
through the channel of known structure that incorporates
dynamic molecular information is proposed. The approach is
applied and tested to predict current through the Gramicidin
A ion channel, a narrow pore in which the applicability of
the conventional continuum theories is uncertain. The
proposed approach utilizes a modified version of
Poisson-Nernst-Planck (PNP) theory, termed Potential
of-Mean-Force-Poisson-Nernst-Planck (PMFPNP) theory, to
compute ion currents. As in standard PNP, ion permeation is
modeled as a continuum drift-diffusion process in a
self-consistent electrostatic potential. In PMFPNP, however,
the free energy of inserting a single ion into the channel
i.e., the potential of mean force (PMF) along the permeation
pathway, is calculated using methods of molecular modeling
at equilibrium. Therefore, the information about the dynamic
relaxation of a protein and other surrounding media is
incorporated into the model of ion permeation, i.e. the
dynamic flexibility of the channel environment is accounted
for via the PMFPNP prescription. The PMF profile of the ion
along the GA channel is obtained by combining an equilibrium
molecular dynamics (MD) simulations to sample dynamic
protein configurations with ion(s) in the channel, with
continuum electrostatics calculation of the free energy. The
diffusion coefficient of a potassium ion within the channel
was also calculated using the MD trajectory. Therefore, no
direct fitting parameters are required in our model.
The results of our study revealed that equilibrium
fluctuations of the GA channel molecule produce significant
electrostatic stabilization of an ion inside the channel.
The dielectric-self energy of the ion remains essentially
unchanged in the course of the MD simulation, indicating
that no substantial changes in the protein geometry occur as
the ion passed through it. Also, the model accounted for the
experimentally observed saturation of ion current with
increase of the electrolyte concentration, in contrast to
the predictions of standard PNP theory.
231. B.N.J.
Persson,
V.N.
Samoilov, S. Zilberman and A. Nitzan
Phenomenology of Squeezing and Sliding
of molecularly thin Xe, CH4 and C16H34 lubrication films
between smooth and rough curved solid surfaces with
long-range elasticity
J. Chem. Phys. 117,
3897-3914
(2002)
PDF
The properties of Xe, CH4 and C16H34 lubricant confined
between two approaching solids are investigated by a model
that accounts for the curvature and elastic properties of
the solid surfaces. We consider both smooth surfaces, and
surfaces with short- scale roughness. In most cases we
observe well defined molecular layers develop in the
lubricant film when the width of the film is of the order of
a few atomic diameters, but in some cases atomic scale
roughness inhibit the formation of these layers, and the
lubricant exhibit liquid- like properties. An external
squeezing- pressure
induces discontinuous, thermally activated changes in the
number n of lubricant layers. We observe that the layering
transition tends to nucleate in disordered or imperfect
regions in the lubrication film. We also present and discuss
results of sliding dynamics for Xe and C16H34 lubrication
films.
230. J. Lehmann,
S. Kohler, P. Hanggi and A. Nitzan
Molecular Wires Acting as Coherent
Quantum Ratchets
Phys.
Rev. Letters, 88, 228305 (2002).
PDF
The effect of laser fields on the
electron transport through a molecular wire being weakly
coupled
to two leads is investigated. The
molecular wire acts as a quantum ratchet if the molecule
is composed of periodically arranged, asymmetric groups.
This setup presents a quantum rectifier with a finite
dc-response in the absence of a static bias. The nonlinear
current is evaluated in closed form within the Floquet
basis of the isolated, driven wire. The current response
reveals multiple current reversals together with a
nonlinear dependence (reflecting avoided quasi-energy
crossings) on both the amplitude and the frequency of the
laser field. The current saturates for long wires at a
nonzero value, while it may change sign upon decreasing
its length.
229. M.
Galperin, A. Nitzan and I. Benjamin
Numerical simulations of electron
tunneling currents in water
J. Phys. Chem. 106, 10790-96
(2002).
PDF
This paper presents results of numerical simulations of
electron tunneling through water that extend our previous
calculations on such systems in several ways. First, a
tip-substrate configuration is used; second, calculations
are carried in the presence of an external potential bias;
third, the image potential that reflects the interaction of
the electron with the mobile metal electrons is taken into
account in the static image approximation. Finally,
all-to-all transmission probability calculations are
performed in order to get an order-of-magnitude estimate of
the current-voltage characteristics of this junction model.
The computed currents are within the range of the few
available experimental observations on scanning tunneling
microscope (STM) currents in water, indicating that our
calculation may have taken into account all the important
physical attributes of such systems. In addition we examine
the effect of the water medium on the spatial distribution
of the tunneling flux. We find that while different water
configurations scatter the tunneling electron in different
ways, on the average the water affected loss of resolution
is rather small in the deep tunneling regime but can be
substantial in energy regimes where the tunneling is
strongly affected by water-supported resonance structures.
228. V. Mujica,
M. Ratner and A. Nitzan
Molecular
rectification:
Why is it so rare?
Chem.
Phys. 281, 147-150( 2002).
PDF
Although conductance measurements of single molecule and few
molecules junctions are currently being reported, there is a
striking rarity of molecular rectification in these reports.
Molecular rectification can be defined as the absence of
inversion symmetry, I(V)=-I(-V), where I and V are the
measured current and applied voltage. In molecular junctions
of the form metal/molecule/metal, there is generally an
absence of structural mirror symmetry. One might then expect
rectification arising from this asymmetrical structure. We
suggest here that molecular rectification in tunneling
junctions is generally dificult to achieve, essentially
because deformation of the structure in the presence of
finite voltage will result in effectively symmetric voltage
profiles for forward and reverse biases.
227. U. Peskin,
M. Galperin and A. Nitzan
Traversal times for resonant tunneling
J.
Phys. Chem. B, 106, 8306-8312 (2002).
PDF
The tunneling time of particle through given barrier is
commonly defined in terms of "internal clocks" which
effectively measure the interaction time with internal
degrees of freedom of the barrier. It is known that this
definition of the time scale for tunneling is not unique in
the sense that it depends on the clock used to define it.
For the case of resonance tunneling, a particular choice
that in the limit of a high/broad square barrier yields the
original result of Büttiker and Landauer (Phys. Rev. Lett.
1982, 49, 1739 ) is correlated to the lifetime of the
resonance state. This is illustrated for analytically
solvable one-dimensional double barrier models and for a
realistic model of electron tunneling through a static water
barrier. The latter calculation constitutes a novel
application of this concept to a 3-dimensional model, and
the observed structure in the energy dependence of the
computed traversal time reflects the existence of transient
tunneling resonances associated with instantaneous water
structures. These models, characterized by the existence of
shape resonances in the barrier, make it possible to examine
different internal clocks that were proposed for measuring
tunneling times in situations where a "clock independent"
intrinsic time scale (the resonance life time) for the
tunneling time exists. It is argued that this time may be
used in order to estimate the relative importance of
dynamical barrier processes that affect the tunneling
probability.
226. M.
Galperin, S. Toledo and A. Nitzan
Numerical computation of tunneling
fluxes
J. Chem. Phys. 117, 10817-26
(2002).
PDF
The computation of tunneling probabilities in three
dimensions is a numerical challenge, because the small
transition probabilities associated with the overlap of
exponentially vanishing wavefunction-tails require large
computational accuracy. In scattering situations arising,
e.g., in electron tunneling in metal-molecule-metal
junctions, this is compounded by the need to provide a
proper truncation procedure at the numerical boundaries of
the computed system and by the need to account for
electrostatic fields and image interactions. This paper
describes a numerical methodology to deal with these
problems. A pseudo-potential that describes the underlying
system is assumed given. Electrostatic fields and image
interactions are evaluated for the given boundary conditions
from numerically solving Laplace and Poisson equations.
Tunneling probabilities are computed using a grid-based
absorbing boundary conditions Green's function method. An
efficient and exact way to implement the absorbing boundary
conditions by using the exact self-energy associated with
separating the scattering system from the rest of the
infinite space is described. This makes it possible to
substantially reduce the size of the grid used in such
calculations. Two applications, an examination of the
possibility to resolve the spatial structure of an electron
wavefunction in an electron cavity by scanning tunneling
microscopy, and a calculation of electron tunneling
probabilities through water, are presented.
225. D. Segal
and A. Nitzan
Heating in current carrying molecular
junctions
J. Chem. Phys. 117, 3915-3927 (2002).
PDF
A framework for estimating heating and expected temperature
rise in current carrying molecular junctions is described.
Our approach is based on applying the Redfield approximation
to a tight binding model for the molecular bridge
supplemented by coupling to a phonon bath. This model, used
previously to study thermal relaxation effects on electron
transfer and conduction in molecular junctions, is extended
and used to evaluate the fraction of available energy, i.e.
of the potential drop, that is released as heat on the
molecular bridge. Classical heat conduction theory is then
applied to estimate the expected temperature rise. For a
reasonable choice of molecular parameters and for junctions
carrying currents in the nA range, we find the temperature
rise to be a modest few degrees. It is argued, however, that
using classical theory to describe heat transport away from
the junction may underestimate the heating effect.
224. D. Segal
and A. Nitzan
Conduction in molecular junctions:
Inelastic effects
Chem. Phys. 281, 235-256 (2002).
PDF
The effect of a thermal environment on
electron (or hole) transfer through molecular bridges and
on the electron conduction properties of such bridges is
studied. Our steady state formalism based on an extension
of the Redfield theory (D. Segal et al, J. Phys. Chem. B
104 (2000) 3817; Chem. Phys. 268 (2001) 315) is extended
in two ways: First, a better description of the weak
coupling limit, which accounts for the asymmetry of the
energy dependence of the quasi-elastic component of the
transmission is employed. Secondly, for strong coupling to
the thermal bath the small polaron transformation is
employed prior to the Redfield expansion. It is shown that
the thermal coupling is mainly characterized by two
physical parameters: The reorganization energy that
measures the coupling strength and the correlation time
(or its inverse - the spectral width) of the thermal bath.
Implications for the observed dependence of the
bridge-length dependence of the transmissions are
discussed. It is argued that in the intermediate regime
between tunneling behavior and site-to-site thermally
induced hopping, the transport properties may depend on
the interplay between the local relaxation rate and the
transmission dynamics.
223. O. Duerr,
T. Volz, W. Dieterich and A. Nitzan
Dynamic percolation theory for
particle diffusion in a polymer network
J. Chem. Phys. 117, 441
(2002).
PDF PS
Tracer diffusion of small molecules through dense systems of
chain polymers is studied within an athermal lattice model,
where hard-core interactions are taken into account by means
of the site exclusion principle. An approximate mapping of
this problem onto dynamic percolation theory is proposed.
This method is shown to yield quantitative results for the
tracer correlation factor of the molecules as a function of
density and chain length provided the non-Poisson character
of temporal renewals in the disorder configurations is
properly taken into account.
222. O. Duerr,
W. Dieterich P. Maas and A. Nitzan
Effective medium theory of conduction
in stretched polymer
electrolytes
J. Phys. Chem. B, 106,
6149-6155 (2002). PDF
Recent experimental observations of anisotropic conductivity
in stretched polymer electrolytes films of the polyethylene
oxide family are discussed. The main experimental
observations, enhancement of the ionic diffusion and
conductivity in the stretch direction and decrease in these
transport coefficients in the normal direction are
interpreted in terms of an effective two-phase model. This
two-phase model is based on the idea that a highly
conducting phase is associated with oriented molecular
structures which are surrounded by poorly conducting
boundary regions. This model is evaluated within the
framework of differential effective medium theory (DEMT).
Under stretching these regions change from spherical to
prolate-spheroidal shapes. The computed dependence of the DC
conductivity tensor and its AC counterpart on the stretch
parameters is in good agreement with experimental results.
221. F. Mugele,
B.N.J. Persson, S. Zilberman, A. Nitzan and M. Salmeron
Frictional Properties of Chain
Alcohols and the Dynamics of Layering Transitions
Tribology Letters, 12,
123-129, (2002).
PDF
We used a surface forces apparatus to investigate layering
transitions and frictional properties of chain alcohol
films. All but the last two monolayers, strongly bound to
each mica surface can be removed by squeezing. Unlike other
systems however, chain alcohol films of undecanol and
octanol were found to retain their bulk- like lubrication
properties down to a thickness of only one (bi) layer. The
transition
where this last molecularly thin liquid layer is expelled
from the gap proceeds in less than one second. From two-
dimensional snapshots of the contact area during the
expulsion process, we find that the boundary line between
the areas of initial and final film thickness bends and
roughens as it moves across the contact area. In the final
state, we frequently find pockets of trapped liquid. Both
the bending and roughening
of the boundary line and the trapped pockets are due to a
dynamic instability that we describe with a two- dimensional
hydrodynamic model. The length scale of the roughening is
determined by an elastic line tension.
220. S.
Zilberman, B.N.J. Persson, A. Nitzan
Theory and Simulations of Squeeze-out
Dynamics in Boundary Lubrication
J. Chem. Phys., 115, 11268
(2001). PDF
The dynamics of expulsion of the last liquidlike monolayer
of molecules confined between two surfaces ~measured
recently for the first time [J. Chem. Phys. 114, 1831
(2001)] has been analyzed by solving the two- dimensional
Navier– Stokes equation combined with kinetic Monte Carlo
simulations. Instabilities in the boundary line of the
expelled film produce a rough boundary for all length scales
above a critical value. The squeeze- out of liquid is shown
to result from the 2D- pressure gradient in the lubrication
film in the contact area. The Monte Carlo simulations agrees
well with experiments, reproducing most qualitative and
quantitative features. In particular it shows the formation
of small islands, which ~in the absence of pinning
mechanism! drift slowly to the periphery of the contact
area. We calculate the drift velocity analytically as a
function of the distance of the island to the periphery of
the contact area. Experiments indicate that some kind of
pinning mechanism prevails, trapping fluid pockets for very
long times. When including such pinning areas in the
simulations, three distinct squeeze phases and time scales
were observed: (1) initial fast squeeze of most of the
fluid; (2) slower squeeze of unpinned fluid pockets; (3)
long term pinning of fluid pockets. We also show that a
distribution of small pinning areas may produce a
synergistic effect, slowing down the second phase of the
squeeze, compared to a small number of big pinning areas.
The paper presents a new stochastic numerical approach to
problems of moving boundaries
which naturally accounts for thermal fluctuations and their
effect in unstable dynamics.
CB21. O. Durr,
P. Pendzig, W. Dieterich, A. Nitzan
Model studies of diffusion in glassy
and polymer ion conductors
Solid State Ionics: Science &
Technology Proc. 6-th Asian Conf. on Solid State Ionics,
edited by B. V. R. Chowdari and S.
Chandra (World Scientific Publ. Co., Singapore, 1998) p.
33 PDF
Ion-conducting glasses and polymer systems show several
characteristic peculiarities in their composition-dependent
diffusion properties and in their dynamic response. First we
give a brief review of the current understanding of the ion
dynamics in network glasses in terms of stochastic theories.
Secondly, a model for PEO(polyethylene-oxide)-based polymer
electrolytes is described. The equation of state is
calculated from the quasichemical approximation. Using this
information together with Monte Carlo simulations of
diffusion at constant volume, we obtain constant-pressure
results for polymer chain and ion diffusion as a function of
ion concentration. This theory allows us to make comparison
with experiments which are carried out at constant pressure.
CB20. O. Durr,
W. Dieterich, A. Nitzan
Charge Transport in Polymer Ion
Conductors: a Monte Carlo Study
NATO Science Series III. Computer and
Systems Sciences, 177, 288-292 (2001).
PDF
Diffusion of ions through a fluctuating polymeric host is
studied both by Monte Carlo simulation of the complete
system dynamics and by dynamic bond percolation (DBP)
theory. Comparison of both methods suggests a
multiscale-like approach for calculating the diffusion
coefficients of the ions
219. O. Duerr,
W. Dieterich and A. Nitzan
Diffusion in polymer electrolytes and
the dynamic percolation model
Solid State Ionics, 149,
125-130 (2002).
PDF
A semi-microscopic description of ionic transport in
polyethylene oxide-type electrolytes is presented, which is
based on discrete stochastic moves of individual rnolecular
units. Diffusion coefficients for ions and for the center of
mass motion of chains are calculated by Monte Carlo
simulation as a function of various model parameters, with
emphasis on the incorporation of pressure effects. Within an
even more coarse-grained description of diffusion in an
athermal system of point-like particles and chains we
provide a quantitative verification of the concept of
dynamic percolation.
218. M. Galperin
and A. Nitzan
Inelastic effects in electron
tunneling through water layers
J. Chem. Phys. 115, 2681-2694 (2001).
PDF
Calculations of tunneling matrix elements associated with
electron transfer through molecular environments are usually
done for given frozen nuclear configurations; the underlying
assumption being that nuclear motions are slow relative to
the timescale of a tunneling event. This paper examines this
issue for the case of electron tunneling through water. The
motivation for this study is a recent calculation [Peskin et
al, J. Chem. Phys. 111, 7558 (1999)] that indicates that
electron tunneling through water may be enhanced by
tunneling resonances in the range of ~1eV below the vacuum
barrier, and finds that the lifetimes of such resonances are
in the 10fs range, same order as OH stretch periods. Our
calculation is based on the
absorbing-boundary-conditions-Green's-function (ABCGF)
method and proceeds in two steps. First we consider the
effect of a single symmetric OH-stretch mode on electron
tunneling in an otherwise frozen water environment and
establish that the inelastic tunneling probability is small
enough to justify an approach based on perturbation theory
limited to single phonon transitions. Next we note that on
the short timescale of a tunneling event, even under
resonance conditions, water nuclear dynamics may be
represented in the instantaneous normal modes picture. We
generalize the ABCGF method to take into account low order
inelastic scattering from a continuum of such harmonic
normal modes. We find that near resonance the total
inelastic transmission probability is of the same order as
the elastic one, and may lead to an additional ~20-40%
enhancement of the overall transmission in the range of up
to 1eV below the vacuum barrier. The absolute energy
exchange is small, of the order of 1% of the incident
electron energy. Surprisingly, we find that the main
contribution to the inelastic transmission is associated
with energy transfer into the rotational-librational range
of the water instantaneous normal mode spectrum.
217. D. G. Wu,
D. Cahen, P. Graf, R. Naaman, A. Nitzan and D. Shvarts
Direct Detection of Low Concentration
NO in Physiological Solutions by a New GaAs-Based
Sensor
Chem. Eur. J. 7, 1743-1749(2001).
PDF
Nitric oxide (NO) acts as a signal molecule in the nervous
system, as a defense against infections, as a regulator of
blood pressure and a gate keeper of blood flow to different
organs. In vivo it is suggested to have a lifetime of few
seconds. Therefore, its direct de-tection at low
concentrations is difficult. We report on a new type of
hybrid organic-semiconductor electronic sensor, which makes
detection of nitric oxide in physiological solution
possible. The mode of action of the device is described to
explain how its elec-trical resistivity changes as a result
of NO binding to a layer of native hemin molecules.
These molecules are self-assembled on a GaAs surface to
which they are attached via a carboxylate binding group. The
new sensor provides a fast and simple method to directly
detect NO at concentrations down to1 mM in physiological
aqueous (pH=7.4) solution at room temperature.
216. S.
Zilberman, B. N. J. Persson, A. Nitzan, F. Mugele, and
M. Salmeron
Boundary Lubrication: Dynamics of
Squeeze-Out
Phys. Rev. E, 63,
055103R-(1-4) (2001).
PDF
The dynamics of the expulsion of the last liquid monolayer
of molecules conned between two surfaces (measured recently
for the rst time) has been analyzed by solving the
two-dimensional Navier-Stokes equation combined with kinetic
Monte Carlo simulations. Instabilities in the boundary line
of the expelledlm were observed. We show that the
instabilities produce a rough boundary for all length scales
above a critical value and a smooth boundary for shorter
lengths. The squeezing out of all but a few trapped islands
of liquid is shown to be the result of the pressure gradient
in the contact area.
215. A. Nitzan
A relationship between electron
transfer rates and molecular conduction.
J. Phys. Chem. A, 105, 2677-9
(2001).
PDF
This note discusses the relationship
between a given intramolecular bridge assisted electron
transfer rate and the corresponding zero bias molecular
conduction of the same molecular species.
214. D. Segal
and A. Nitzan
Steady state quantum mechanics of
thermally relaxing systems.
Chem. Phys.268, 315-335 (2001).
PDF
A theoretical description of quantum mechanical steady
states is developed. Applications for simple quantum
mechanical systems described in terms of coupled level
structures yield a formulation equivalent to time
independent scattering theory. Applications to steady states
of thermally relaxing systems leads to time independent
scattering theory in Liouville space that is equivalent to
the tetradic Green's function formalism. It provides however
a direct route to derive the particular form of the
Liouville equation applicable in steady state situations.
The theory is applied to study the conduction properties in
the super-exchange model of a metal-molecule-metal contact
weakly coupled to the thermal environment. The energy
resolved temperature dependent transmission probability, as
well as its coherent (tunneling) and incoherent (activated)
parts are calculated using the Redfield approximation. These
transmissions have a coherent (tunneling) and an incoherent
(activated) components that depend differently on the energy
gap (or barrier), on the temperature and on the bridge
length. The coherent component is most important at low
temperatures, large energy gaps and small chain lengths. The
incoherent component dominates in the opposite limits. The
overall (integrated) transmission provides a generalization
of the Landauer conduction formula for small junctions in
the presence of thermal relaxation.
213. A. Nitzan
Electron transmission through
molecules and molecular interfaces
Annual Reviews of Physical Chemistry,
52, 681-750 (2001).
PDF(original
version)
PDF
(Published,
somewhat shorter, version)
Electron transmission through
molecules and molecular interfaces has been a subject of
intensive research due to recent interest in electron
transfer phenomena underlying the operation of the
scanning tunneling microscope (STM) on one hand, and in
the transmission properties of molecular bridges between
conducting leads on the other. In these processes the
traditional molecular view of electron transfer between
donor and acceptor species give rise to a novel view of
the molecule as a current carrying conductor, and
observables such as electron transfer rates and yields are
replaced by the conductivities, or more generally by
current-voltage relationships, in molecular junctions.
Such investigations of electrical junctions, in which
single molecules or small molecular assemblies operate as
conductors constitutes a major part of what has become the
active field of molecular electronics.
In this paper I review the
current knowledge and understanding of this field, with
particular emphasis on theoretical issues. Different
approaches to computing the conduction properties of
molecules and molecular assemblies are reviewed, and the
relationships between them are discussed. Following a
detailed discussion of static junctions models, a review
of our current understanding of the role played by
inelastic processes, dephasing and thermal relaxation
effects, is provided. The most important molecular
environment for electron transfer and transmission is
water, and our current theoretical understanding of
electron transmission through water layers is reviewed.
Finally, a brief discussion of overbarrier transmission,
exemplified by photoemission through adsorbed molecular
layers or low energy electron transmission through such
layers is provided. Similarities and differences between
the different systems studied are discussed.
THIS TABLE OF CONTENTS REFERS TO ORIGINAL VERSION:
Abstract | 1 |
1. Introduction | 2 |
2. Theoretical approaches to molecular conduction | 5 |
_____2.1. Standard electron transfer theory | 5 |
_____2.2. Transmission between conducting leads | 9 |
_____2.3. The Landauer Formula | 12 |
_____2.4. Molecular conduction | 15 |
_____2.5. Relation to electron transfer rates | 17 |
_____2.6. Quantum chemical calculations | 19 |
_____2.7. Spatial-grid based pseudopotential approaches | 22 |
_____2.8. Density functional calculations | 24 |
_____2.9. Potential profiles | 26 |
_____2.10. Rectification | 28 |
_____2.11. Carrier-carrier interactions | 28 |
_____2.12. Some open issues | 30 |
3. Dephasing and relaxation effects | 32 |
_____3.1. Tunneling traversal times | 33 |
_____3.2. Nuclear relaxation during electron transmission | 36 |
_____3.3. Thermal interactions in molecular conduction | 40 |
_____3.4. Reduced density matrix approaches | 44 |
4. Electron tunneling through water | 53 |
5. Overbarrier transmission | 62 |
6. Conclusions and outlook | 66 |
====================================
212. M.
Galperin, A. Nitzan and Uri Peskin
Traversal time for electron tunneling
in water
J. Chem. Phys. 114, 9205-8
(2001).
PDF
The traversal time for tunneling is a
measure of the time during which the transmitted particle
can be affected by interactions localized in the barrier.
The Büttiker-Landauer approach, which estimates this time
by imposing an internal clock on the system, has been
applied so far for relatively simple 1-dimensional models.
Here we apply this approach to estimate the traversal time
for electron tunneling through a realistic 3-dimensional
model of a water layer. Observed structure in the energy
dependence of times computed reflects the existence of
transient tunneling resonances associated with
instantaneous water structures.
211. P. Graf and
A. Nitzan, M. G. Kurnikova and R. D. Coalson
A dynamic lattice Monte Carlo model of
ion transport in inhomogeneous dielectric
environments: Method and
implementation
J. Phys. Chem. 104,
12324-12338 (2000).
PDF
ABSTRACT: A dynamic lattice Monte
Carlo (DLMC) simulation approach to the description of ion
transport in dielectric environments is presented.
Conventional approaches using periodic boundary conditions
are inefficient for non-equilibrium situations in
inhomogeneous systems. Instead the simulated system is
embedded in a bigger system that determines the average
electrostatic potential and the ionic concentrations at
its boundaries. Two issues are of special importance:
implementing the given boundary conditions in the
treatment of dynamical processes at and near the
boundaries, and efficient evaluation of ion-ion
interaction in the heterogeneous dielectric medium during
the Monte Carlo simulation. The performance of the method
is checked by comparing numerical results to exact
solutions for simple geometries, and to mean field
(Poisson-Nernst-Planck, PNP) theory in a system where the
latter should provide a reasonable description. Other
examples in which the PNP theory fails in various degrees
are shown and discussed. In particular, PNP cannot
account for the characteristics of ion transport through
narrow membrane channels.
======================================
210. A. A.
Kornyshev and A. Nitzan
Effect of overscreening on the
localization of hydrated electrons
Zeitschrift für Physikalische Chemie,
215, 701 (2001).
PDF
The problem of the ground state of
hydrated electron is revisited with a focus on the effects
due to nonlocal dielectric response of water. The standard
variational analysis is performed. It takes into account -
in addition to nonlocal polarization of nuclear modes -
the electron repulsion from the closed shells of water
molecules, a possibility to form a cavity around the
electron and the interaction of the hydrated electron with
the high frequency electronic degrees of freedom of water.
The classical dielectric continuum limit, shown for a
reference, gives the ground state hydration energy that is
2.5 times smaller than the experimental value. The
situation alters dramatically if one accounts for the
nonlocal dielectric response of water. If one takes
literally the existing MD simulation data for the static
wave-vector dependent dielectric response function (with
an "over-screening" resonance at k ~3A-1), the
hydration energy becomes 3 times larger than the
experimental one. Thus, an over-screening may have a
dramatic effect on the formation of the hydrated electron.
For a reduced height of the “over-screening peak”, the
ground state energy reduces to the measured value. At the
same time, over-screening enhances the localization of
electrons. The undamped resonance gives rise to an
unphysically small localization radius. A reduced
resonance, that provides the correct ground stage energy,
is better in this respect but it still gives very compact
localization: 2/3 of the Bohr radius. It is thus concluded
that either the defect structure of water around the
electron suppresses the resonance, or the models of bulk
water, which predict a high peak in the response function,
are inadequate. The study paves the way to future
molecular or phenomenological multi-order parameter
models, in which the density and polarization of molecular
dipoles and charge density distributions of the solvated
electron were considered on the same footing. Such models
might reveal the reduction of over-screening near the
excess electron.
======================================
209. A. Nitzan
and J. Jortner, J. Wilkie, A. L. Burin and M. A. Ratner
Tunneling Time for Electron Transfer
Reactions
J. Phys. Chem. B, 104,
5661-5665 (2000) PDF
ABSTRACT: The tunneling time for
non-adiabatic electron transfer reactions described within
the superexchange model is estimated using a Büttiker type
internal clock: the electron is taken to posses two
internal spin states that are weakly coupled on the
bridge. By studying the transition probability between
these channels during the tunneling process the traversal
time through the bridge can be estimated. Like the
Büttiker-Landauer result it is linear in the bridge
length, but its dependence on the barrier energy UB
approaches the Büttiker-Landauer form only in the limit of
strong interstate coupling (broad band). In the 'normal'
super-exchange (weak coupling) limit it is inversely
proportional to the barrier energy.
======================================
208. D. Segal,
A. Nitzan, W. B. Davis and M.A. Ratner
Activated conductionin microscopic
molecular junctions
J. Phys. Chem., 104, 2790-2793
(2000).
PDF
ABSTRACT: We analyze the connection
between the electron transfer (ET) rate through a given
molecular bridge, and the conduction of a junction based
on the same bridge between two metals. The Landauer
relation between the conduction of a junction and its
transmission properties is generalized to yield a relation
between conduction and ET rate, including transfer
processes dominated by thermal activation. The relation
between the orders of magnitude of these observables
involves an additional length parameter, of the order of
the range of the donor wavefunction. We find that the
functional dependence of these observables on the bridge
length (N) and on the temperature (T) changes from the
exponential and temperature independent, exp(-?N) for
small N, to algebraic and thermally activated form,
[1/(alpha1+alpha2*N)]*exp(-beta*E/kT), as N increases. An
intermediate range of apparent independence on N exists if
alpha1>>alpha2. This behavior is the analog to the
quantum Kramers (barrier crossing) problem, analyzed with
respect to the barrier length.
======================================
207. D.
Segal, A. Nitzan, W. B. Davis, M.R. Wasielewski and M.A.
Ratner
Electron transfer rates in bridged
molecular systems 2. A steady state analysis of coherent
tunneling and thermal transitions
J. Phys. Chem. B, 104,
3817-3829 (2000).
PDF
ABSTRACT: The effect of dephasing and
relaxation on electron transfer in bridged molecular
systems is investigated using a simple molecular model.
The interaction between the molecular system and the
thermal environment is described on the level of the
Redfield theory, modified when needed for the description
of steady state situations. Noting that transient as well
as steady state measurements are possible in such system,
we discuss the relation between the rates obtained from
these different types of experiments, in particular the
conditions under which these rates are the same. Also, a
formal relation between the steady state rate for electron
transfer across a molecular bridge, and the conductance of
this bridge when placed between two metal contacts is
established. The effect of dephasing and relaxation on the
electron transfer is investigated and new observations are
made with regard to the transition from the superexchange
to the thermal (hopping through bridge) regime of the
transfer process. In particular, the rate is temperature
independent in the superexchange regime and its dependence
on the bridge-length (N) is exponential, . The rate
behaves like beyond a crossover value of N,
where ?E is the energy gap between the donor/acceptor and
the bridge levels, and where ?1 and ?2 are characteristic
times for activation onto the bridge and diffusion in the
bridge, respectively. We find that in typical cases ?1»?2,
and therefore a region of very weak N dependence is
expected before the Ohmic behavior, N-1, is established
for large enough N. In addition, a relatively weak
exponential dependence, exp(-?N), is expected for long
bridges if competing processes capture electrons away from
the bridge sites. Finally, we consider ways to distinguish
experimentally between the thermal and the tunneling
routes.
======================================
206. Uri Peskin,
Ake Edlund, Ilan Bar-On, Misha Galperin and Abraham
Nitzan
Transient Resonance Structures
in Electron Tunneling Through Water
J. Chem. Phys. 111, 7558-66
(1999).
PDF
The mechanism of electrons tunneling through a narrow water barrier between two Pt(100) metal surfaces is studied. Assuming an adiabatic picture in which the water configuration is static on the time scale of the electron motion, the tunneling probabilities are found to increase non-monotonically as a function of incident electron energy. A numerical investigation of single electron scattering wavefunctions suggests that the tunneling is enhanced by resonances, associated with molecular cavities in which the electron is trapped between repulsive oxygen cores. The lifetimes of these resonances are calculated using a novel filter diagonalization scheme, based on a converging high order perturbative expansion of the single-electron Green’s function, and are found to be of order 10 fs. The possibility that transient resonance supporting structures contribute to the enhancement of tunneling through water is discussed.
==============================================
205. A. Nitzan
and I. Benjamin
Electron transmission through
molecular layers: Numerical simulations and theoretical
considerations
Accounts of Chemical Research, 32,
854-861 (1999).
PDF
Electron transmission through molecular layers takes place in all intermolecular electron transfer processes in condensed phases. It can also be monitored directly in STM experiments in liquid environments, in photoemission through adsorbed molecular layers and in low energy electron transmission (LEET) with electron incident from the gas phase on metals covered with such layers. Finally, such processes are encountered in molecular electronic devices, which combine molecular ‘resistors’ with metal or semiconductor contacts. This paper reviews recent computational approaches to electron transmission through water and other environments, emphasizing the strong correlation between layer composition and structure and its transmission properties.
=============================================
204. A. Nitzan
Ultrafast relaxation in water [News
and views commentary]
Nature, 402, 472-475 (1999).
PDF
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203. Misha
Galperin, Dvira Segal and Abraham Nitzan
Perturbation theory approach to
tunneling: Direct and resonance transmission in
super-exchange models
J. Chem. Phys., 111, 1569-1579
(1999).
PDF
In this paper we examine, within simple models, different approaches to computing tunneling probabilities in super-exchange models of electron transfer. The relationship between tunneling calculations that use scattering theory type formalisms and approaches based on standing waves, which are more closely related to electron transfer between bound donor and acceptor states, is established. Transmission probabilities computed by using truncated basis representations are compared to exact analytical or numerical results for 1 and 2-dimensional models. We find that while resonance tunneling is well approximated by truncated basis approaches, computing deep tunneling using such basis sets can lead to large errors. Implications for calculations of bridge assisted electron transfer are discussed.
====================================
202. W.
Dieterich, O. Durr, P. Pendzig, A. Bunde and A. Nitzan
Percolation concepts in Solid State
Ionics
Physica A, 266, 229-237(1999).
PDF
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201. Maria G.
Kurnikova, Rob D. Coalson, Peter Graf and Abraham Nitzan
A Lattice Relaxation Algorithm for 3D
Poisson-Nernst-Planck Theory with Application to Ion
Transport Through the Gramicidin A Channel
Biophys. Journal, 76,
642-656(1999). PDF
Abstract: A lattice relaxation algorithm is developed to
solve the Poisson-
Nernst-Planck (PNP) Equations for ion transport through
arbitrary three
dimensional volumes. Calculations of systems characterized
by simple par-
allel plate and cylindrical pore geometries are presented in
order to calibrate
the accuracy of the method. A study of ion transport through
Gramicidin
A dimer is carried out within this PNP framework. Good
agreement with
experimental measurements is obtained. Strengths and
weaknesses of the
PNP approach are discussed.
=====================================
200. P. Pendzig,
W. Dieterich and A. Nitzan
Monte-Carlo study of diffusion in
polymer electrolytes
J. Non-Crystalline solids, 235,
748(1998).
PDF
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199. O. Durr, P.
Pendzig, W. Dieterich and A. Nitzan
Model studies of diffusion in glassy
and polymer ionic conductors
Solid State Ionics, 'Science and
technology', p. 33-41(1998)
=====================================
198. R. Naaman,
R. Haran, A. Nitzan, D. Evans and M. Galperin
Electron transmission through
molecular layers
J. Phys. Chem. B 102, 3658-3668(1998).
PDF
[Feature Article]
Contents
1. Introduction
2. Experimental Background
3. Theoretical Background
4. Experimental Methods
5. Computational Methods
6. Experimental Results
7. Some Numerical Results
8. Discussion
a. Dimensionality
b. Band structure
effects
abstract
This article discusses general
issues associated with electron transmission through thin
molecular films. On the experimental side we emphasize
recent investigations of photoemission through organized
organic films adsorbed on metal surfaces. Theoretical and
numerical approaches to transmission and tunneling through
such films are discussed. We focus on the relation between
the structure of the film and its transmission properties.
In the experimental work these are controlled by varying
the organic layer, by changing its thickness and by
inducing disorder via thermal heating and by depositing
mixtures of two molecular types. In numerical simulations
of simple model systems we consider the dimensionality of
the process, effect of molecular ordering and the relation
between electronic band structure in the film and its
transmission properties. It is shown that electron
transmission through thin molecular layersconstitutes a
sensitive tool for investigating molecular film properties
in addition to providing a convenient prototype system for
the study of electron transport in molecular electronic
devices.
========================================
197. P. Graf and
Abraham Nitzan
Numerical simulations of solvation in
simple polar fluids: Dependence on the thermodynamic state
below and above the critical point
Chemical Physics, 235,
297-312(1998).
PDF
The phenomenology of electron solvation in polar solvents
is studied by investigating the characteristics of
electron solvation in a model polar solvent, a Stockmayer
liquid characterized by a combination of
and dipolar intermolecular interactions which interacts with
the electron with a combination Lennard-Jones of short
range repulsive and of electrostatic (dipole-charge)
interactions. Energetics and dynamical properties of the
solvated electron are studied as functions of the solvent
density and of solvent molecular parameters which
determine the electron solvent interaction and the solvent
dynamical response. We find that electron localization
in this solvent is caused primarily by the repulsive part of
the electron-solvent interaction. Upon increasing
the solvent molecular dipole from zero, the electron becomes
more localized; however, this effect seems to
saturate at moderate solvent polarities, and further
increase of the polarity changes the ground (and excited)
state energies without affecting strongly the electron size.
In this regime the electron behaves approximately
like a classical charge distribution as far as the
dependence of its solvation energy on the solvent polarity
is
concerned. The dynamical response of the solvent to the
solvated electron is investigated by studying the
solvent- induced fluctuations of the electron’s energy
levels. As expected we find that fluctuations in the
ground and excited state energies are dominated by the
electrostatic part of the electron-solvent interaction,
and their dynamics therefore reflects the solvent rotational
motion. Surprisingly, however, the electrostatic
contributions mostly cancel in the fluctuations of the gap
between the ground and first excited state.
Consequently the gap fluctuations are dominated by the
solvent translational motions. The implications of
these observations on the dynamics of electron solvation are
discussed.
========================================
196. D.G. Evans,
A. Nitzan and M.A. Ratner
Photo-induced electron transfer in
mixed-valence compounds: Beyond the golden rule regime
J. Chem. Phys. 108,
6387-6393(1998).
PDF
The short- time charge transfer evolution following
photoexcitation in mixed valence compounds is
studied using path integral calculations. Due to the large
nonadiabatic coupling, path integral
calculations using direct path summation techniques are
inadequate, and charge transfer dynamics
can only be computed using a transfer matrix technique
developed by Makri and Makarov. The
resulting relaxation is considerably slower than that
predicted by low- order perturbation theory. The
effects of the solvent on the decay process, and the
validity of the golden rule to predict the
dynamics of the decay process are investigated. The effects
of preparing an initial state that is not
a rovibrational state of the acceptor potential energy
surface is also examined. These exact
calculations show that the large electronic mixing gives
rise to very fast oscillations in the electronic
state population as the wave function oscillates coherently
between the donor and acceptor. This is
followed by a slower relaxation induced by the coupling to
the dissipative solvent modes, which
occurs on time scales <100 fs. This information provides
insight into the mechanism for
oscillations observed in time- resolved transient spectra of
these compounds, and suggests
substantial limitations of the golden rule picture.
=======================================
195. D.
Rostkier-Edelstein, P. Graf and A. Nitzan
Computing vibrational energy relaxation for high frequency
modes in condensed phases.
J. Chem. Phys. 107, 10470 (1997).
PDF
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194. W. B. Davis,
M.R. Wasielewski, M.A. Ratner, V. Mujica annd A. Nitzan
Electron transfer rates in bridged molecular systems: A
phenomenological approach to relaxation.
J. Phys. Chem. A101, 6158-6164 (1997).
PDF
=========================================
191.
Benjamin, I.; Evans, D.; Nitzan, A.
Electron
tunneling through water layers: effect of layer structure
and thickness
Journal of Chemical Physics, vol.106 p.
6647-54(1997).
PDF
=======================================
184. Lonergan, M.C.; Nitzan, A.;
Ratner, M.A.; Shriver, D.F.
Dynamically disordered
hopping, glass transition, and polymer electrolytes
Journal of Chemical Physics, vol.103, p. 3253-61(1995).
PDF
183. Vijayadamodar, G.V.; Nitzan,
A.
On the application of
instantaneous normal mode analysis to long time dynamics
of liquids
Journal of Chemical Physics, vol.103, p. 2169-77(1995).
PDF
181. Olender, R.; Nitzan, A.
Solvation dynamics in
dielectric solvents with restricted molecular rotations:
polyethers
Journal of Chemical Physics, vol.102, p. 7180-96(1995).
PDF
180. Lonergan, M.C.; Nitzan, A.;
Ratner, M.A.
Ionic diffusion in dynamically-disordered materials:
motion on a renewing, percolative lattice
Journal of Molecular Liquids, vol.60, p. 269-88(1994)
=============================================
179. A.
D. Hammerich, A. Nitzan and M. A. ratner
Fourier analysis, correlation functions
and non-adiabatic electron transfer: wave- packets and exact
representations,
Theoretica Chimica Acta, 89, 383-399
(1994).
PDF
178.
Knodler, D.; Dieterich, W.; Lonsky, C.; Nitzan, A.
Nonlinear relaxation and solvation dynamics in a Coulomb
lattice gas
Journal of Chemical Physics, vol.102, p. 465-70(1995).
PDF
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================================
174. Rostkier-Edelstein, D.;
Urbakh, M.; Nitzan, A.
Electron tunneling through a dielectric barrier
Journal of Chemical Physics, vol.101, p. 8224-37(1994).
PDF
173.
Nitzan,
A.; Ratner, M.A.
Conduction in polymers: dynamic disorder transport
Journal of Physical Chemistry, vol.98, p.
1765-75(1994) (Feature Article).
PDF
172.
Olender, R.; Nitzan, A.
Lattice theory of solvation and dissociation in
macromolecular fluids. II. Quasichemical approximation
Journal of Chemical Physics, vol.101, p. 2338-49(1994).
PDF
171. G. Makov and A.
Nitzan
SOLVATION
AND
IONIZATION NEAR A DIELECTRIC SURFACE.
PDF
J.
Phys. Chem. 98, 3459 (1994)
Abstract: Solvation and ionization energies of ions and neutral atomic solutes near planar and spherical (cluster) surfaces were calculated using continuum dielectric theory. For highly polarized solvents, the magnitude of the solvation energy at the surface is similar to that in the bulk, while the reorganization energy and the photodetachment energy of an electron from a negative ion are found to be slightly larger for solutes located at the surface. The cluster size dependence of the solvation and ionization energies is found to be determined only by the properties of the solvent, in good agreement with experimental data, and to be insensitive to the location of the solute in the cluster. Implications for the size dependence of the photodetachment energies from negative ions in water and ammonia clusters are discussed. In particular it is suggested that the observed photodetachment energies from small (NH ) clusters are compatible with the dielectric predictions for electron solvation in solid ammonia.
======================================
170. A. Mosyak and A. Nitzan
Electron solvation:
Quantum and classical aspects.
in: Reaction Dynamics in Clusters
and Condensed Phases, edited by B. Pullman, J. Jortner
and R.D. Levine (Kluwer, Amsterdam,1994), p.557.
169.
Neria, E.; Nitzan, A.
Numerical simulations of solvation dynamics in electrolyte
solutions
Journal of Chemical Physics, vol.100, p. 3855-68(1994).
PDF
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======================================
166. M.C.
Longergan, A. Nitzan and M. Ratner
Ionic diffusion in dynamically-disordered
materials: Motion on a renewing percolative lattice.
J. Mol. Liquids, (1994).
=========================================
165. E. Neria and A. Nitzan
Numerical Studies of
Solvation Dynamics in Electrolyte Solutions in
"Ultrafast
Phenomena in Condensed
Molecular Systems
Edited
by
Y Guaduel and P. Rossky, Am. Phys. Soc. (1993)
Recent experimental
works on solvation dynamics in polar solvent containing
added
salt indicate the
existence of a contribution to the solvation process
associated
with the salt ions.
This contribution can not be described by the linearized
Poisson Boltzmann
equation, i.e. the Debye Folkehagen theory. Numerical
simulations on a model
system (a Stockmayer solvent containing spherical ions)
show that the primary
contribution to this dynamics comes from ion-solvent
exchange at the first
solvation shell surrounding the solute. The fast time
scales associated
primarily with the solvent response are only slightly
dependent
on the presence and
properties of the electrolyte in the concentration range
studied.
======================================
164. G. Makov and A. Nitzan
Electronic Properties of Finite Metallic
Systems.
Phys.
Rev.
B, 47, 2301 (1993).
PDF
The relationship
between the surface geometry and certain electronic
properties
is considered for
neutral metal systems. For systems enclosed by surfaces of
constant curvature, the
total energy, the surface energy, and the chemical
potential are found to
depend linearly on the surface curvature. Explicit
expressions are found
for the coefficients of this dependence, in particular for
the curvature energy.
It is shown that for systems with surfaces of varying
curvature a surface
charge distribution is formed (and therefore an electric
potential varying
across the surface), to ensure a constant chemical
potential.
Implications for the
ionization potentials of finite and infinite systems of
finite curvature (e.g.,
spheroids and thin wires) are discussed.
======================================
163. E. Neria and A. Nitzan
Semiclassical Evaluation of Nonadiabatic Rates
in Condensed
Phases.
J.
Chem. Phys, 99, 1109 (1993).
PDF
A procedure for
calculating nonadiabatic transition rates in the
semiclassical
limit is implemented
and tested for models relevant for condensed phase
processes. The method
is based on evaluating the golden rule rate expression
using a quantum
description for the electronic subsystem and a semiclassical
propagation for the
nuclear degrees of freedom, similar to Heller's calculation
of absorption and Raman
spectra. In condensed phase processes, the short
lifetimes of the
relevant correlation functions make it possible to implement
the
procedure within the
frozen Gaussian method. Furthermore, because of the large
density of states
involved, which implies fast dephasing, incoherent
superpositions of
frozen Gaussian trajectories may be used for the evaluation
of
the rate. The method is
tested using two simple exactly soluble models. One of
them, consisting of two
coupled electronic potential surfaces, harmonic and
linear, is also used
for testing and comparing a recently proposed algorithm by
Tully. The other, the
well-known displaced harmonic potentials model, is a
prototype of many
condensed processes. Finally, the method is applied for
calculating the
nonadiabatic raditionless relaxation of the solvated
electron
from its first excited
state to the fully solvated ground state.
======================================
162. M.D. Todd, A. Nitzan and M.A.
Ratner
Electron Transfer via Superexchange; A
Time-Dependent Approach.
J.
Phys. Chem., 97, 29-33 (1993).
PDF
The superexchange
model for the facilitation of electron
transfer between donors
and acceptors by means of intermediate bridges is recast
in a time-dependent
framework. We develop a model in which three electronic
states (donor, bridge,
and acceptor) are considered and vibronic levels are
included on the donor
and acceptor. An explicitly time-dependent formulation, in
which the golden rule
is rewritten in the form of an integral over a
timedependent
correlation function, is used to calculate the rate. Simple
example
calculations for model
three-systems are given; inverted region behaviour and
reasonable values are
obtained. The model is a general one and should be useful
for interpretation of
superexchange-assisted transfer based on the experimental
observations of
frequencies and displacements (such as the information that
can
be obtained with Raman
spectroscopy). The model can be generalized to include
effects such as
dephasing, relaxation, solvent dynamics, and
anharmonicities, as
well as breakdown of
the Condon approximation.
======================================
161 R.
Granek and A. Nitzan
Comment on: Self-consistent theory of
polymer dynamics in melts
J.Chem.Phys. 97,3873-3874(1992).
PDF
====================================================
160. R. Olender and A. Nitzan
Ion Solvation and
Association in Complex Solvents: Theoretical Considerations
Electrochemia
Acta. Vol 37, 1505-1509
(1992).
PDF
We discuss some theoretical issues concerning ion
association in complex hosts.
In particular the
effect of temperature and pressure on ion pairing is
discussed
in relation to recent
experimental observations in polymer ionic conductors.
=================================================
159. N. Liver and A. Nitzan
Redox Properties of Small Semiconductor
Particles.
J.
Phys. Chem, 96, 3366-3373 (1992).
PDF
In this paper we study
the equilibrium properties of the semiconductor particles
of intermediate sizes
(_Debye length) in contact with an electrolyte solution
containing a given
redox pair. We focus on the size dependence of electrical
and
thermodynamical
quantities associated with such particles. The equilibrium
distribution of the
potential and of the charge in the particle and in the
surrounding electrolyte
is obtained analytically in limiting cases and computed
in the general case
using the nonlinear PoissonBoltzmann equation, assuming
Boltzmann statistics
for carrier distributions in the semiconductor. A simple
relation for the
sizedependent redox potential of a semiconductor sphere
characterized by its
radius and charge is proposed and is found to provide a good
approximation for a
broad range of electrolyte concentrations. This leads to an
expression for the
"equilibrium constant" for the semiconductor/electrolyte
system, which relates
the concentrations of the electrolytic redox components to
the concentration,
size, and charge of the semiconductor particles.
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158. G. Makov and A. Nitzan
Association of Ion Pairs in Clusters of
Dielectric Solvents.
J.
Phys. Chem., 96,2965-2967 (1992).
PDF
The association of ion
pairs in clusters is studied in an extended Bjerrum model
using both a Coulombic
potential and a potential of mean force calculated from
molecular
dynamicssimulations. Association is found to increase
rapidly as the
cluster size decreases
until a critical size is reached, below which the ions are
in the paired state.
For NaCl in water this is estimated at 12 water molecules
for contact ion pairs.
It is also found that the short-range details of the
potential are important
and that replacing them with a Coulombic assumption leads
to a large overestimate
of the critical size.
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157. E.
Neria and A. Nitzan
Simulations
of
Solvation Dynamics in Simple Polar Solvents.
J.
Chem. Phys. 96, 5433-5440 (1992).
PDF
The paper describes
the results of computer simulations of charge solvation
dynamics in a Stockmayer solvent (Lennard-Jones spheres with
point dipoles at their centers). The solvent molecules are
characterized by mass and moment of inertia which can be
varied independently, thus providing the possibility to
study the separate effects of the rotational and
translational solvent motions on the solvation process. We
focus on the role played by these degrees of freedom, and on
the contributions of different solvation shells around the
solute to the solvation process in order to check the
validity of recently proposed theories of
solvation dynamics. We
find that even in this structureless solvent, as in the more
structures solvents studied earlier, inertial effects
dominate the solvation process, and dielectric solvation
theories which do not take into account these effects cannot
describe the observed dynamics. The dynamic mean spherical
approximation and generalized diffusion theories cannot
account for the observed dynamics even when solvent
translations are frozen.
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