137 research outputs found
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
First-principles Analysis of Photo-current in Graphene PN Junctions
We report a first principles investigation of photocurrent generation by
graphene PN junctions. The junctions are formed by either chemically doping
with nitrogen and boron atoms, or by controlling gate voltages. Non-equilibrium
Green's function (NEGF) formalism combined with density functional theory (DFT)
is applied to calculate the photo-response function. The graphene PN junctions
show a broad band photo-response including the terahertz range. The dependence
of the response on the angle between the light polarization vector and the PN
interface is determined. Its variation against photon energy is
calculated in the visible range. The essential properties of chemically doped
and gate-controlled PN junctions are similar, but the former shows fingerprints
of dopant distribution.Comment: 7 pages, 6 figure
Ab initio nonequilibrium quantum transport and forces with the real-space projector augmented wave method
We present an efficient implemention of a non-equilibrium Green function
(NEGF) method for self-consistent calculations of electron transport and forces
in nanostructured materials. The electronic structure is described at the level
of density functional theory (DFT) using the projector augmented wave method
(PAW) to describe the ionic cores and an atomic orbital basis set for the
valence electrons. External bias and gate voltages are treated in a
self-consistent manner and the Poisson equation with appropriate boundary
conditions is solved in real space. Contour integration of the Green function
and parallelization over k-points and real space makes the code highly
efficient and applicable to systems containing several hundreds of atoms. The
method is applied to a number of different systems demonstrating the effects of
bias and gate voltages, multiterminal setups, non-equilibrium forces, and spin
transport.Comment: Accepted by Phys.Rev.
First-principles calculation on the transport properties of molecular wires between Au clusters under equilibrium
Based on the matrix Green's function method combined with hybrid
tight-binding / density functional theory, we calculate the conductances of a
series of gold-dithiol molecule-gold junctions including benzenedithiol (BDT),
benzenedimethanethiol (BDMT), hexanedithiol (HDT), octanedithiol (ODT) and
decanedithiol (DDT). An atomically-contacted extended molecule model is used in
our calculation. As an important procedure, we determine the position of the
Fermi level by the energy reference according to the results from ultraviolet
photoelectron spectroscopy (UPS) experiments. After considering the
experimental uncertainty in UPS measurement, the calculated results of
molecular conductances near the Fermi level qualitatively agree with the
experimental values measured by Tao et. al. [{\it Science} 301, 1221 (2003);
{\it J. Am. Chem. Soc.} 125, 16164 (2003); {\it Nano. Lett.} 4, 267 (2004).]Comment: 12 pages,8 figure
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