7 research outputs found
Single-Molecule Junctions Based on Bipyridine: Impact of an Unusual Reorganization on Charge Transport
The (4,4′)-bipyridine molecule
(44bpy) has attracted particular
interest in molecular electronics because single-molecule junctions
can be directly formed via nitrogen–gold affinity, obviating
the need of understanding nontrivial invasive effects due to extra
anchoring groups. In a recent study, an apparent conundrum related
to the transport through 44bpy junctions has been resolved by emphasizing
the essential role of the environment (solvent vs ambient conditions).
In the present paper, we demonstrate the robustness of the conclusion
of that study, by introducing intramolecular reorganization as a new
and essential element in the analysis. This extension is necessary
in the light of recent investigations drawing attention to the unusual
character of intramolecular reorganization in 44bpy as a molecule
possessing a floppy, highly anharmonic degree of freedom, which is
strongly and nonlinearly coupled to the molecular orbital dominating
the charge transport. As a further important effect related to the
significant and unusual intramolecular reorganization, we investigate
the excess (shot) noise and find values substantially larger than
in cases of molecular junctions wherein it has been measured so far.
The noise power and Fano factor calculations demonstrate the importance
of energy-dependent transmission, a fact disregarded in the interpretation
of experimental data for nanojunctions and molecular junctions investigated
so far. According to the theoretical results reported here, the intramolecular
reorganization should have a more pronounced overall impact on the
charge transport in 44bpy for bias voltages larger than those explored
in existing experiments, but not much larger to become prohibitive.
These findings should motivate companion experimental investigations
in this direction
Important Insight into Electron Transfer in Single-Molecule Junctions Based on Redox Metalloproteins from Transition Voltage Spectroscopy
In a recent experimental work, results
of the first transition
voltage spectroscopy (TVS) investigation on azurin have been reported.
This forms a great case to better understand the electron transfer
through bacterial redox metalloproteins, a process of fundamental
importance from chemical, physical, and biological perspectives, and
of practical importance for nanoÂ(bio)Âelectronics. In the present paper
we challenge the tentative interpretation put forward in the aforementioned
experimental study and propose a different theoretical interpretation.
To explain the experimental TVS data, we adopt an extended Newns–Anderson
framework, whose accuracy and robustness is demonstrated. We show
that that this framework clearly meets the need to obtain a consistent
description across experiments. Most importantly, the presently proposed
theoretical approach permits unraveling novel aspects on the impact
of the electrochemical scanning microscope environment on the charge
transport through single-(bio)Âmolecule junctions based on redox units.
The usefulness of TVS as a versatile method of investigation, also
able to provide important insight into the charge transport through
metalloproteins, is emphasized
Interpretation of Stochastic Events in Single-Molecule Measurements of Conductance and Transition Voltage Spectroscopy
The first simultaneous measurements of transition voltage
(<i>V</i><sub>t</sub>) spectroscopy (TVS) and conductance
(<i>G</i>) histograms (Guo et al., <i>J. Am. Chem.
Soc.</i> <b>2011</b>, <i>133</i>, 19189) form
a great case
for studying stochastic effects, which are ubiquitous in molecular
junctions. Here an interpretation of those data is proposed that emphasizes
the different physical content of <i>V</i><sub>t</sub> and <i>G</i> and reveals that fluctuations in the molecular orbital
alignment have a significantly larger impact on <i>G</i> than initially claimed. The present study demonstrates the usefulness
of corroborating statistical information on different transport properties
and gives support to TVS as a valuable investigative tool
Vibrational Frequencies of Fractionally Charged Molecular Species: Benchmarking DFT Results against ab Initio Calculations
Recent
advances in nano/molecular electronics and electrochemistry
made it possible to continuously tune the fractional charge <i>q</i> of single molecules and to use vibrational spectroscopic
methods to monitor such changes. Approaches to compute vibrational
frequencies ωÂ(<i>q</i>) of fractionally charged species
based on the density functional theory (DFT) are faced with an important
issue: the basic quantity used in these calculations, the total energy,
should exhibit piecewise linearity with respect to the fractional
charge, but approximate, commonly utilized exchange correlation functionals
do not obey this condition. In this paper, with the aid of a simple
and representative example, we benchmark results for ωÂ(<i>q</i>) obtained within the DFT against ab initio methods, namely,
coupled cluster singles and doubles and also second- and third-order
Møller–Plesset perturbation) expansions. These results
indicate that, in spite of missing the aforementioned piecewise linearity,
DFT-based values ωÂ(<i>q</i>) can reasonably be trusted
Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function
We report the results of an extensive investigation of metal–molecule–metal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M<sub>1</sub>–S–(C<sub>6</sub>H<sub>4</sub>)<i><sub>n</sub></i>–S–M<sub>2</sub>, with 1 ≤ <i>n</i> ≤ 4 and M<sub>1,2</sub> = Ag, Au, Pt) to examine the impact of molecular length (<i>n</i>) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = Φ<sub>SAM</sub> – Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current–voltage (<i>I</i>–<i>V</i>) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full <i>I</i>–<i>V</i> curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment ε<sub>h</sub> = <i>E</i><sub>F</sub> – <i>E</i><sub>HOMO</sub> of the dominant molecular orbital (HOMO) relative to the Fermi energy <i>E</i><sub>F</sub> of the junction. Where photoelectron spectroscopy data are available, the ε<sub>h</sub> values agree very well with those determined by TVS. Using a single-level model, which we justify <i>via ab initio</i> quantum chemical calculations at post-density functional theory level and additional UV–visible absorption measurements, we are able to quantitatively reproduce the <i>I</i>–<i>V</i> measurements in the whole bias range investigated (∼1.0–1.5 V) and to understand the behavior of ε<sub>h</sub> and Γ (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metal–S bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in ε<sub>h</sub> exhibiting a weak dependence with the work function Φ. Both of these parameters play a key role in determining the tunneling attenuation factor (β) and junction resistance (<i>R</i>). Correlation among Φ, ΔΦ, <i>R</i>, transition voltage (<i>V</i><sub>t</sub>), and ε<sub>h</sub> and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions
Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based Molecular Tunnel Junctions: Evidence for Universal Behavior
The
transport properties of molecular junctions based on alkanedithiols
with three different methylene chain lengths were compared with junctions
based on similar chains wherein every third −CH<sub>2</sub>– was replaced with O or S, that is, following the general
formula HSÂ(CH<sub>2</sub>CH<sub>2</sub>X)<sub><i>n</i></sub>CH<sub>2</sub>CH<sub>2</sub>SH, where X = CH<sub>2</sub>, O, or S
and <i>n</i> = 1, 2, or 3. Conducting probe atomic force
microscopy revealed that the low bias resistance of the chains increased
upon substitution in the order CH<sub>2</sub> < O < S. This
change in resistance is ascribed to the observed identical trend in
contact resistance, <i>R</i><sub>c</sub>, whereas the exponential
prefactor β (length sensitivity) was essentially the same for
all chains. Using an established, analytical single-level model, we
computed the effective energy offset ε<sub>h</sub> (<i>i.e.</i>, Fermi level relative to the effective HOMO level)
and the electronic coupling strength Γ from the current–voltage
(<i>I–V</i>) data. The ε<sub>h</sub> values
were only weakly affected by heteroatom substitution, whereas the
interface coupling strength Γ varied by over an order of magnitude.
Consequently, we ascribe the strong variation in <i>R</i><sub>c</sub> to the systematic change in Γ. Quantum chemical
calculations reveal that the HOMO density shifts from the terminal
SH groups for the alkanedithiols to the heteroatoms in the substituted
chains, which provides a plausible explanation for the marked decrease
in Γ for the dithiols with electron-rich heteroatoms. The results
indicate that the electronic coupling and thus the resistance of alkanedithiols
can be tuned by substitution of even a single atom in the middle of
the molecule. Importantly, when appropriately normalized, the experimental <i>I–V</i> curves were accurately simulated over the full
bias range (±1.5 V) using the single-level model with no adjustable
parameters. The data could be collapsed to a single universal curve
predicted by the model, providing clear evidence that the essential
physics is captured by this analytical approach and supporting its
utility for molecular electronics
Exceptionally Small Statistical Variations in the Transport Properties of Metal–Molecule–Metal Junctions Composed of 80 OligoÂphenylene Dithiol Molecules
Strong stochastic
fluctuations witnessed as very broad resistance
(<i>R</i>) histograms with widths comparable to or even
larger than the most probable values characterize many measurements
in the field of molecular electronics, particularly those measurements
based on single molecule junctions at room temperature. Here we show
that molecular junctions containing 80 oligophenylene dithiol molecules
(OPDn, 1 ≤ <i>n</i> ≤ 4) connected in parallel
display small relative statistical deviationsδ<i>R</i>/<i>R</i> ≈ 25% after only ∼200
independent measurementsî—¸and we analyze the sources of these
deviations quantitatively. The junctions are made by conducting probe
atomic force microscopy (CP-AFM) in which an Au-coated tip contacts
a self-assembled monolayer (SAM) of OPDs on Au. Using contact mechanics
and direct measurements of the molecular surface coverage, the tip
radius, tip-SAM adhesion force (<i>F</i>), and sample elastic
modulus (<i>E</i>), we find that the tip-SAM contact area
is approximately 25 nm<sup>2</sup>, corresponding to about 80 molecules
in the junction. Supplementing this information with <i>I–V</i> data and an analytic transport model, we are able to quantitatively
describe the sources of deviations <i>δR</i> in <i>R</i>: namely, <i>δN</i> (deviations in the
number of molecules in the junction), <i>δε</i> (deviations in energetic position of the dominant molecular orbital),
and <i>δΓ</i> (deviations in molecule-electrode
coupling). Our main results are (1) direct determination of <i>N</i>; (2) demonstration that <i>δN</i>/<i>N</i> for CP-AFM junctions is remarkably small (≤2%)
and that the largest contributions to <i>δR</i> are <i>δε</i> and <i>δΓ</i>; (3)
demonstration that δ<i>R</i>/<i>R</i> after
only ∼200 measurements is substantially smaller than most reports
based on >1000 measurements for single molecule break junctions.
Overall,
these results highlight the excellent reproducibility of junctions
composed of tens of parallel molecules, which may be important for
continued efforts to build robust molecular devices