186 research outputs found
From Chemistry to Functionality: Trends for the Length Dependence of the Thermopower in Molecular Junctions
We present a systematic ab-initio study of the length dependence of the
thermopower in molecular junctions. The systems under consideration are small
saturated and conjugated molecular chains of varying length attached to gold
electrodes via a number of different binding groups. Different scenarios are
observed: linearly increasing and decreasing thermopower as function of the
chain length as well as positive and negative values for the contact
thermopower. Also deviation from the linear behaviour is found. The trends can
be explained by details of the transmission, in particular the presence,
position and shape of resonances from gateway states. We find that these
gateway states do not only determine the contact thermopower, but can also have
a large influence on the length-dependence itself. This demonstrates that
simple models for electron transport do not apply in general and that chemical
trends are hard to predict. Furthermore, we discuss the limits of our approach
based on Density Functional Theory and compare to more sophisticated methods
like self-energy corrections and the GW theory
Understanding the length dependence of molecular junction thermopower
Thermopower of molecular junctions is sensitive to details in the junction
and may increase, decrease, or saturate with increasing chain length, depending
on the system. Using McConnell's theory for exponentially suppressed transport
together with a simple and easily interpretable tight binding model, we show
how these different behaviors depend on the molecular backbone and its binding
to the contacts. We distinguish between resonances from binding groups or
undercoordinated electrode atoms, and those from the periodic backbone. It is
demonstrated that while the former gives a length-independent contribution to
the thermopower, possibly changing its sign, the latter determines its length
dependence. This means that the question of which orbitals from the periodic
chain that dominate the transport should not be inferred from the sign of the
thermopower but from its length dependence. We find that the same molecular
backbone can, in principle, show four qualitatively different thermopower
trends depending on the binding group: It can be positive or negative for short
chains, and it can either increase or decrease with length
IETS and quantum interference: propensity rules in the presence of an interference feature
Destructive quantum interference in single molecule electronics is an
intriguing phe- nomenon; however, distinguishing quantum interference effects
from generically low transmission is not trivial. In this paper, we discuss how
quantum interference ef- fects in the transmission lead to either low current
or a particular line shape in current-voltage curves, depending on the position
of the interference feature. Sec- ondly, we consider how inelastic electron
tunneling spectroscopy can be used to probe the presence of an interference
feature by identifying vibrational modes that are se- lectively suppressed when
quantum interference effects dominate. That is, we expand the understanding of
propensity rules in inelastic electron tunneling spectroscopy to molecules with
destructive quantum interference.Comment: 19 pages, 6 figure
Designing -stacked molecular structures to control heat transport through molecular junctions
We propose and analyze a new way of using stacking to design molecular
junctions that either enhance or suppress a phononic heat current, but at the
same time remain conductors for an electric current. Such functionality is
highly desirable in thermoelectric energy converters, as well as in other
electronic components where heat dissipation should be minimized or maximized.
We suggest a molecular design consisting of two masses coupled to each other
with one mass coupled to each lead. By having a small coupling (spring
constant) between the masses, it is possible to either reduce, or perhaps more
surprisingly enhance the phonon conductance. We investigate a simple model
system to identify optimal parameter regimes and then use first principle
calculations to extract model parameters for a number of specific molecular
realizations, confirming that our proposal can indeed be realized using
standard molecular building blocks.Comment: 5 pages + supplemental material, 3 figure
Single-molecule Electronics: Cooling Individual Vibrational Modes by the Tunneling Current
Electronic devices composed of single molecules constitute the ultimate limit
in the continued downscaling of electronic components. A key challenge for
single-molecule electronics is to control the temperature of these junctions.
Controlling heating and cooling effects in individual vibrational modes, can in
principle, be utilized to increase stability of single-molecule junctions under
bias, to pump energy into particular vibrational modes to perform
current-induced reactions or to increase the resolution in inelastic electron
tunneling spectroscopy by controlling the life-times of phonons in a molecule
by suppressing absorption and external dissipation processes. Under bias the
current and the molecule exchange energy, which typically results in heating of
the molecule. However, the opposite process is also possible, where energy is
extracted from the molecule by the tunneling current. Designing a molecular
'heat sink' where a particular vibrational mode funnels heat out of the
molecule and into the leads would be very desirable. It is even possible to
imagine how the vibrational energy of the other vibrational modes could be
funneled into the 'cooling mode', given the right molecular design. Previous
efforts to understand heating and cooling mechanisms in single molecule
junctions, have primarily been concerned with small models, where it is unclear
which molecular systems they correspond to. In this paper, our focus is on
suppressing heating and obtaining current-induced cooling in certain
vibrational modes. Strategies for cooling vibrational modes in single-molecule
junctions are presented, together with atomistic calculations based on those
strategies. Cooling and reduced heating are observed for two different cooling
schemes in calculations of atomistic single-molecule junctions.Comment: 18 pages, 6 figure
Mechanical Tuning of Thermal Transport in a Molecular Junction
Understanding and controlling heat transport in molecular junctions would
provide new routes to design nanoscale coupled electronic and phononic devices.
Using first principles full quantum calculations, we tune thermal conductance
of a molecular junction by mechanically compressing and extending a short
alkane chain connected to graphene leads. We find that the thermal conductance
of the compressed junction drops by half in comparison to the extended
junction, making it possible to turn on and off the heat current. The low
conductance of the off state does not vary by further approaching the leads and
stems from the suppression of the transmission of the in--plane transverse and
longitudinal channels. Furthermore, we show that misalignment of the leads does
not reduce the conductance ratio. These results also contribute to the general
understanding of thermal transport in molecular junctions.Comment: 12 pages, 6 figure
Unidirectional hopping transport of interacting particles on a finite chain
Particle transport through an open, discrete 1-D 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.Comment: 33 pages, 8 figure
Complex band structure and electronic transmission
The function of nano-scale devices critically depends on the choice of
materials. For electron transport junctions it is natural to characterize the
materials by their conductance length dependence, . Theoretical
estimations of are made employing two primary theories: complex band
structure and DFT-NEGF Landauer transport. Both reveal information on
of individual states; i.e. complex Bloch waves and transmission eigenchannels,
respectively. However, it is unclear how the -values of the two
approaches compare. Here, we present calculations of decay constants for the
two most conductive states as determined by complex band structure and standard
DFT-NEGF transport calculations for two molecular and one semi-conductor
junctions. Despite the different nature of the two methods, we find strong
agreement of the calculated decay constants for the molecular junctions while
the semi-conductor junction shows some discrepancies. The results presented
here provide a template for studying the intrinsic, channel resolved length
dependence of the junction through complex band structure of the central
material in the heterogeneous nano-scale junction.Comment: 7 pages, 6 figure
Molecular Realization of a Quantum NAND Tree
The negative-AND (NAND) gate is universal for classical computation making it
an important target for development. A seminal quantum computing algorithm by
Farhi, Goldstone and Gutmann has demonstrated its realization by means of
quantum scattering yielding a quantum algorithm that evaluates the output
faster than any classical algorithm. Here, we derive the NAND outputs
analytically from scattering theory using a tight-binding (TB) model and show
the restrictions on the TB parameters in order to still maintain the NAND gate
function. We map the quantum NAND tree onto a conjugated molecular system, and
compare the NAND output with non-equilibrium Green's function (NEGF) transport
calculations using density functional theory (DFT) and TB Hamiltonians for the
electronic structure. Further, we extend our molecular platform to show other
classical gates that can be realized for quantum computing by scattering on
graphs.Comment: 17 pages, 6 figures, 1 tabl
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