62 research outputs found
Phonon interference effects in molecular junctions
We study coherent phonon transport through organic, \pi-conjugated molecules.
Using first principles calculations and Green's function methods, we find that
the phonon transmission function in cross-conjugated molecules, like
meta-connected benzene, exhibits destructive quantum interference features very
analogous to those observed theoretically and experimentally for electron
transport in similar molecules. The destructive interference features observed
in four different cross-conjugated molecules significantly reduce the thermal
conductance with respect to linear conjugated analogues. Such control of the
thermal conductance by chemical modifications could be important for
thermoelectric applications of molecular junctions.Comment: 10 pages, 10 figure
Quantifying Transition Voltage Spectroscopy of Molecular Junctions
Transition voltage spectroscopy (TVS) has recently been introduced as a
spectroscopic tool for molecular junctions where it offers the possibility to
probe molecular level energies at relatively low bias voltages. In this work we
perform extensive ab-initio calculations of the non-linear current voltage
relations for a broad class of single-molecule transport junctions in order to
assess the applicability and limitations of TVS. We find, that in order to
fully utilize TVS as a quantitative spectroscopic tool, it is important to
consider asymmetries in the coupling of the molecule to the two electrodes.
When this is taken properly into account, the relation between the transition
voltage and the energy of the molecular orbital closest to the Fermi level
closely follows the trend expected from a simple, analytical model.Comment: 5 pages, 4 figures. To appear in PR
Improving Transition Voltage Spectroscopy of Molecular Junctions
Transition voltage spectroscopy (TVS) is a promising spectroscopic tool for
molecular junctions. The principles in TVS is to find the minimum on a
Fowler-Nordheim plot where is plotted against and relate the
voltage at the minimum, , to the closest molecular level.
Importantly, , is approximately half the voltage required to see a
peak in the curve. Information about the molecular level position can
thus be obtained at relatively low voltages. In this work we show that the
molecular level position can be determined at even lower voltages, by finding the minimum of with .
On the basis of a simple Lorentzian transmission model we analyze theoretical
{\it ab initio} as well as experimental curves and show that the voltage
required to determine the molecular levels can be reduced by as
compared to conventional TVS. As for conventional TVS, the symmetry/asymmetry
of the molecular junction needs to be taken into account in order to gain
quantitative information. We show that the degree of asymmetry may be estimated
from a plot of vs. .Comment: 6 pages, 8 figure
Inelastic vibrational signals in electron transport across graphene nanoconstrictions
We present calculations of the inelastic vibrational signals in the
electrical current through a graphene nanoconstriction. We find that the
inelastic signals are only present when the Fermi-level position is tuned to
electron transmission resonances, thus, providing a fingerprint which can link
an electron transmission resonance to originate from the nanoconstriction. The
calculations are based on a novel first-principles method which includes the
phonon broadening due to coupling with phonons in the electrodes. We find that
the signals are modified due to the strong coupling to the electrodes, however,
still remain as robust fingerprints of the vibrations in the nanoconstriction.
We investigate the effect of including the full self-consistent potential drop
due to finite bias and gate doping on the calculations and find this to be of
minor importance
First-principles method for electron-phonon coupling and electron mobility: Applications to 2D materials
We present density functional theory calculations of the phonon-limited
mobility in n-type monolayer graphene, silicene and MoS. The material
properties, including the electron-phonon interaction, are calculated from
first-principles. We provide a detailed description of the normalized full-band
relaxation time approximation for the linearized Boltzmann transport equation
(BTE) that includes inelastic scattering processes. The bulk electron-phonon
coupling is evaluated by a supercell method. The method employed is fully
numerical and does therefore not require a semi-analytic treatment of part of
the problem and, importantly, it keeps the anisotropy information stored in the
coupling as well as the band structure. In addition, we perform calculations of
the low-field mobility and its dependence on carrier density and temperature to
obtain a better understanding of transport in graphene, silicene and monolayer
MoS. Unlike graphene, the carriers in silicene show strong interaction with
the out-of-plane modes. We find that graphene has more than an order of
magnitude higher mobility compared to silicene. For MoS, we obtain several
orders of magnitude lower mobilities in agreement with other recent theoretical
results. The simulations illustrate the predictive capabilities of the newly
implemented BTE solver applied in simulation tools based on first-principles
and localized basis sets
Controlling the transmission line shape of molecular t-stubs and potential thermoelectric applications
Asymmetric line shapes can occur in the transmission function describing
electron transport in the vicinity of a minimum caused by quantum interference
effects. Such asymmetry can be used to increase the thermoelectric efficiency
of molecular junctions. So far, however, asymmetric line shapes have been only
empirically found for just a few rather complex organic molecules where the
origins of the line shapes relation to molecular structure were not resolved.
In the present work we introduce a method to analyze the structure dependence
of the asymmetry of interference dips from simple two site tight-binding
models, where one site corresponds to a molecular orbital of the wire and
the other to an atomic orbital of a side group, which allows us to
analytically characterize the peak shape in terms of just two parameters. We
assess our scheme with first-principles electron transport calculations for a
variety of {\it t-stub} molecules and also address their suitability for
thermoelectric applications.Comment: 11 pages, 5 figures; J. Chem. Phys., in print (2011
Efficient first-principles calculation of phonon assisted photocurrent in large-scale solar cell devices
We present a straightforward and computationally cheap method to obtain the
phonon-assisted photocurrent in large-scale devices from first-principles
transport calculations. The photocurrent is calculated using nonequilibrium
Green's function with light-matter interaction from the first-order Born
approximation while electron-phonon coupling (EPC) is included through special
thermal displacements (STD). We apply the method to a silicon solar cell device
and demonstrate the impact of including EPC in order to properly describe the
current due to the indirect band-to-band transitions. The first-principles
results are successfully compared to experimental measurements of the
temperature and light intensity dependence of the open-circuit voltage of a
silicon photovoltaic module. Our calculations illustrate the pivotal role
played by EPC in photocurrent modelling to avoid underestimation of the
open-circuit voltage, short-circuit current and maximum power. This work
represents a recipe for computational characterization of future photovoltaic
devices including the combined effects of light-matter interaction,
phonon-assisted tunneling and the device potential at finite bias from the
level of first-principles simulations
Electrochemical control of quantum interference in anthraquinone-based molecular switches
Using first-principles calculations we analyze the electronic transport
properties of a recently proposed anthraquinone based electrochemical switch.
Robust conductance on/off ratios of several orders of magnitude are observed
due to destructive quantum interference present in the anthraquinone, but
absent in the hydroquinone molecular bridge. A simple explanation of the
interference effect is achieved by transforming the frontier molecular orbitals
into localized molecular orbitals thereby obtaining a minimal tight-binding
model describing the transport in the relevant energy range in terms of hopping
via the localized orbitals. The topology of the tight-binding model, which is
dictated by the symmetries of the molecular orbitals, determines the amount of
quantum interference.Comment: 6 pages, 6 figure
Multiterminal single-molecule--graphene-nanoribbon thermoelectric devices with gate-voltage tunable figure of merit ZT
We study thermoelectric devices where a single 18-annulene molecule is
connected to metallic zigzag graphene nanoribbons (ZGNR) via highly transparent
contacts that allow for injection of evanescent wave functions from ZGNRs into
the molecular ring. Their overlap generates a peak in the electronic
transmission, while ZGNRs additionally suppress hole-like contributions to the
thermopower. Thus optimized thermopower, together with suppression of phonon
transport through ZGNR-molecule-ZGNR structure, yield the thermoelectric figure
of merit ZT ~ 0.5 at room temperature and 0.5 < ZT < 2.5 below liquid nitrogen
temperature. Using the nonequilibrium Green function formalism combined with
density functional theory, recently extended to multiterminal devices, we show
how the transmission resonance can also be manipulated by the voltage applied
to a third ZGNR electrode, acting as the top gate covering molecular ring, to
tune the value of ZT.Comment: 5 pages, 4 figures, PDFLaTe
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