13 research outputs found
Toward Density-Functional Theory-Based StructureâConductance Relationships in Single Molecule Junctions
A method is presented that allows for the calculation
using density
functional theory (DFT) of the tunneling conductance of single molecule
junctions for thousands of junction structures. With a single scaling
parameter, conductance is evaluated from clusters consisting of the
molecule bonded to one Au atom at each end. Junction geometries are
obtained without any constraints from ab initio molecular dynamics
simulations at room temperature. This method accurately reproduces
standard DFT-based conductance values for several molecular and electrode
structures while reducing the computational cost by a factor of âŒ400Ă,
allowing for the conductance of tens of thousands of geometries to
be computed. When applied to a pair of conjugated molecules, these
large data sets quantify the effect on conductance of molecular structure
or quantum chemical properties. This methodology enables reliable
DFT-based conductance calculations at a negligible computational cost
and opens the way to quantitative structureâconductance relationships
Interface Tuning of Current-Induced Cooling in Molecular Circuits
We
study the effect of the atomistic structure of metalâmolecule
contacts on the current-induced damping and excitation of vibrations
in molecular circuits by means of <i>first-principles</i> calculations. We consider a carbene-based molecule bound to Au electrodes
via three different tip terminations: a tetramer, a pyramid, and a
chainlike structure. The change in the width and position of molecular
levels associated with each of these metalâmolecule structures
under an applied voltage controls the heating and cooling processes.
In blunt tips, where the electronic coupling between molecular and
Au bulk states is strong, the cooling efficiency decreases as a function
of bias which results in the heating of the most active vibrational
modes. On the other hand, in chainlike structures where the coupling
is weak, the cooling rate has a nonmonotonic behavior as a function
of the applied bias and increases sharply beyond a certain voltage.
This results in a current-induced cooling at high bias. These findings
open the way to the efficient removal of excess heat from the junction
through control of the metalâmolecule contact structures
Origin of Vibrational Instabilities in Molecular Wires with Separated Electronic States
Current-induced
heating in molecular junctions stems from the interaction
between tunneling electrons and localized molecular vibrations. If
the electronic excitation of a given vibrational mode exceeds heat
dissipation, a situation known as vibrational instability is established,
which can seriously compromise the integrity of the junction. Using
out of equilibrium first-principles calculations, we demonstrate that
vibrational instabilities can take place in the general case of molecular
wires with separated unoccupied electronic states. From the <i>ab initio</i> results, we derive a model to characterize unstable
vibrational modes and construct a diagram that maps mode stability.
These results generalize previous theoretical work and predict vibrational
instabilities in a new regime
Conductance of Molecular Junctions Formed with Silver Electrodes
We
compare the conductance of a series of amine-terminated oligophenyl
and alkane molecular junctions formed with Ag and Au electrodes using
the scanning tunneling microscope based break-junction technique.
For these molecules that conduct through the highest occupied molecular
orbital, junctions formed with Au electrodes are more conductive than
those formed with Ag electrodes, consistent with the lower work function
for Ag. The measured conductance decays exponentially with molecular
backbone length with a decay constant that is essentially the same
for Ag and Au electrodes. However, the formation and evolution of
molecular junctions upon elongation are very different for these two
metals. Specifically, junctions formed with Ag electrodes sustain
significantly longer elongation when compared with Au due to a difference
in the initial gap opened up when the metal point-contact is broken.
Using this observation and density functional theory calculations
of junction structure and conductance we explain the trends observed
in the single molecule junction conductance. Our work thus opens a
new path to the conductance measurements of a single molecule junction
in Ag electrodes
Highly Conducting Ï-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes
We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct AuâC covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off <i>in situ</i>, resulting in formation of a direct covalent Ï bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated Ï system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the Ï system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G<sub>0</sub> = 2e<sup>2</sup>/h). Junctions formed with methylene-terminated oligophenyls with two to four phenyl units show a 100-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling, as they exhibit an exponential dependence of conductance on oligomer length. In addition, density functional theory based calculations for the AuâxylyleneâAu junction show near-resonant transmission, with a crossover to tunneling for the longer oligomers
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ï-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ï-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ï-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ï-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices
In Situ Formation of NâHeterocyclic Carbene-Bound Single-Molecule Junctions
Self-assembled
monolayers (SAMs) formed using N-heterocyclic carbenes
(NHCs) have recently emerged as thermally and chemically ultrastable
alternatives to those formed from thiols. The rich chemistry and strong
Ï-donating ability of NHCs offer unique prospects for applications
in nanoelectronics, sensing, and electrochemistry. Although stable
in SAMs, free carbenes are notoriously reactive, making their electronic
characterization challenging. Here we report the first investigation
of electron transport across single NHC-bound molecules using the
scanning tunneling microscope-based break junction (STM-BJ) technique.
We develop a series of air-stable metal NHC complexes that can be
electrochemically reduced in situ to form NHCâelectrode contacts,
enabling reliable single-molecule conductance measurements of NHCs
under ambient conditions. Using this approach, we show that the conductance
of an NHC depends on the identity of the single metal atom to which
it is coordinated in the junction. Our observations are supported
by density functional theory (DFT) calculations, which also firmly
establish the contributions of the NHC linker to the junction transport
characteristics. Our work demonstrates a powerful method to probe
electron transfer across NHCâelectrode interfaces; more generally,
it opens the door to the exploitation of surface-bound NHCs in constructing
novel, functionalized electrodes and/or nanoelectronic devices