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 nano­(bio)­electronics. 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)­molecule 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>_t) 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>_t 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 ω­(<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 ω­(<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 ω­(<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 (M₁–S–(C₆H₄)<i>_n</i>–S–M₂, with 1 ≤ <i>n</i> ≤ 4 and M_1,2 = 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_SAM – 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_h = <i>E</i>_F – <i>E</i>_HOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy <i>E</i>_F of the junction. Where photoelectron spectroscopy data are available, the ε_h 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_h 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_h 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>_t), and ε_h 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 −CH₂– was replaced with O or S, that is, following the general formula HS­(CH₂CH₂X)_<i>n</i>CH₂CH₂SH, where X = CH₂, 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 CH₂ < O < S. This change in resistance is ascribed to the observed identical trend in contact resistance, <i>R</i>_c, 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_h (<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_h 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>_c 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 Oligo­phenylene 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 measurements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 nm², 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