171 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
Opportunities and limitations of transition voltage spectroscopy: a theoretical analysis
In molecular charge transport, transition voltage spectroscopy (TVS) holds
the promise that molecular energy levels can be explored at bias voltages lower
than required for resonant tunneling. We investigate the theoretical basis of
this novel tool, using a generic model. In particular, we study the length
dependence of the conducting frontier orbital and of the 'transition voltage'
as a function of length. We show that this dependence is influenced by the
amount of screening of the electrons in the molecule, which determines the
voltage drop to be located at the contacts or across the entire molecule. We
observe that the transition voltage depends significantly on the length, but
that the ratio between the transition voltage and the conducting frontier
orbital is approximately constant only in strongly screening (conjugated)
molecules. Uncertainty about the screening within a molecule thus limits the
predictive power of TVS. We furthermore argue that the relative length
independence of the transition voltage for non-conjugated chains is due to
strong localization of the frontier orbitals on the end groups ensuring binding
of the rods to the metallic contacts. Finally, we investigate the
characteristics of TVS in asymmetric molecular junctions. If a single level
dominates the transport properties, TVS can provide a good estimate for both
the level position and the degree of junction asymmetry. If more levels are
involved the applicability of TVS becomes limited.Comment: 8 pages, 12 figure
Transition Voltage Spectroscopy and the Nature of Vacuum Tunneling
Transition Voltage Spectroscopy (TVS) has been proposed as a tool to analyze
charge transport through molecular junctions. We extend TVS to Au-vacuum-Au
junctions and study the distance dependence of the transition voltage V_t(d)
for clean electrodes in cryogenic vacuum. On the one hand, this allows us to
provide an important reference for V_t(d)-measurements on molecular junctions.
On the other hand, we show that TVS forms a simple and powerful test for vacuum
tunneling models
Influence of Molecular Organization on the Electrical Characteristics of {\pi}-conjugated Self-assembled Monolayers
Two new thiol compounds with {\sigma}-{\pi}-{\sigma} structure were
synthesized and self-assembled on gold substrates. The morphology and the
structural characterization of SAMs assessed by infrared spectroscopy, contact
angle, XPS, electrochemistry and scanning tunneling microscopy (STM) show the
formation of monolayers. SAMs with a terthiophene (3TSH) core as conjugated
system are much better organized compared to those with a naphthalene
carbodiimide (NaphSH) core as demonstrated by the cyclic voltammetry and STM
studies. The surface concentration of 3TSH and NaphSH is respectively three and
six times lower than ordered SAMs of pure alkyl chains. A large number of I/V
characteristics have been studied either by STS measurements on gold substrates
or by C-AFM on gold nanodots. Transition Voltage Spectroscopy (TVS) was used to
clearly identify the transport in these partially organized monolayers. The
chemical nature of the conjugated system, donor for 3TSH and acceptor for
NaphSH, involves an opposite rectification associated to the asymmetrical
coupling of the molecular orbitals and the electrodes. The conductance
histograms show that the 3TSH junctions are less dispersed than those of NaphSH
junctions. This is explained by a better control of the molecular organization
in the molecular junctions.Comment: Full paper with supporting informatio
Conductance statistics from a large array of sub-10 nm molecular junctions
Devices made of few molecules constitute the miniaturization limit that both
inorganic and organic-based electronics aspire to reach. However, integration
of millions of molecular junctions with less than 100 molecules each has been a
long technological challenge requiring well controlled nanometric electrodes.
Here we report molecular junctions fabricated on a large array of sub-10 nm
single crystal Au nanodots electrodes, a new approach that allows us to measure
the conductance of up to a million of junctions in a single conducting Atomic
Force Microscope (C-AFM) image. We observe two peaks of conductance for
alkylthiol molecules. Tunneling decay constant (beta) for alkanethiols, is in
the same range as previous studies. Energy position of molecular orbitals,
obtained by transient voltage spectroscopy, varies from peak to peak, in
correlation with conductance values.Comment: ACS Nano (in press
Molecule-Electrode Interface Energetics in Molecular Junction: a Transition Voltage Spectroscopy Study
We assess the performances of the transition voltage spectroscopy (TVS)
method to determine the energies of the molecular orbitals involved in the
electronic transport though molecular junctions. A large number of various
molecular junctions made with alkyl chains but with different chemical
structure of the electrode-molecule interfaces are studied. In the case of
molecular junctions with clean, unoxidized electrode-molecule interfaces, i.e.
alkylthiols and alkenes directly grafted on Au and hydrogenated Si,
respectively, we measure transition voltages in the range 0.9 - 1.4 V. We
conclude that the TVS method allows estimating the onset of the tail of the
LUMO density of states, at energy located 1.0 - 1.2 eV above the electrode
Fermi energy. For oxidized interfaces (e.g. the same monolayer measured with Hg
or eGaIn drops, or monolayers formed on a slightly oxidized silicon substrate),
lower transition voltages (0.1 - 0.6 V) are systematically measured. These
values are explained by the presence of oxide-related density of states at
energies lower than the HOMO-LUMO of the molecules. As such, the TVS method is
a useful technique to assess the quality of the molecule-electrode interfaces
in molecular junctions.Comment: Accepted for publication in J. Phys. Chem C. One pdf file including
manuscript, figures and supporting informatio
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