Ioan Bâldea

Multistate vibronic interactions in the benzene radical cation. II. Quantum dynamical simulations

The multistate vibronic dynamics in the X 2E1g-Ẽ 2B2u electronic states of the benzene radical cation is investigated theoretically by an ab initio quantum-dynamical approach. The vibronic coupling scheme and the ab initio values of the system parameters are adopted from the previous Paper I. Vibronic line spectra are obtained with the Lanczos procedure. Extensive calculations on wave-packet propagation have been performed with the aid of the multiconfiguration time-dependent Hartree method. Up to five coupled electronic potential energy surfaces and 13 vibrational degrees of freedom have been included in these calculations. As a result, the impact of a third electronic state (X or B) on a strongly coupled manifold (B-C or D-Ẽ states) is quantitatively assessed. It leads to a restructuring of the spectral envelope which is stronger for the B-D-Ẽ than for the X-B-C system. The internal conversion dynamics is characterized by a stepwise transfer of electronic population to the lowest electronic s...

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)n-S-M2, with 1 ≤ n ≤ 4 and M1,2 = Ag, Au, Pt) to examine the impact of molecular length (n) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = ΦSAM - Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current-voltage (I-V) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full I-V curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment εh = EF - EHOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy EF of the junction. Where photoelectron spectroscopy data are available, the εh values agree very well with those determined by TVS. Using a single-level model, which we justify via ab initio quantum chemical calculations at post-density functional theory level and additional UV-visible absorption measurements, we are able to quantitatively reproduce the I-V measurements in the whole bias range investigated (∼1.0-1.5 V) and to understand the behavior of εh 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 εh 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 (R). Correlation among Φ, ΔΦ, R, transition voltage (Vt), and εh and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions.

Interpretation of stochastic events in single-molecule measurements of conductance and transition voltage spectroscopy.

The first simultaneous measurements of transition voltage (V(t)) spectroscopy (TVS) and conductance (G) histograms (Guo et al., J. Am. Chem. Soc. 2011, 133, 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 V(t) and G and reveals that fluctuations in the molecular orbital alignment have a significantly larger impact on G 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.

Revealing molecular orbital gating by transition voltage spectroscopy

Recently, Song et al [Nature 462, 1039 (2009)] employed transition-voltage spectroscopy to demonstrate that the energy

Multistate multimode vibronic dynamics: Entanglement of electronic and vibrational degrees of freedom in the benzene radical cation

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Important issues facing model-based approaches to tunneling transport in molecular junctions

of the highest occupied molecular orbital (HOMO) of single-molecule transistors can be controlled by a gate potential

Sources of negative differential resistance in electric nanotransport

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Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based Molecular Tunnel Junctions: Evidence for Universal Behavior

. To demonstrate the linear dependence

Exceptionally Small Statistical Variations in the Transport Properties of Metal-Molecule-Metal Junctions Composed of 80 Oligophenylene Dithiol Molecules

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Combined Jahn–Teller and Pseudo-Jahn–Teller Effects in the Benzene Radical Cation

, the experimental data have been interpreted by modeling the molecule as an energy barrier spanning the spatial source-drain region of molecular junctions. Since, as shown in this work, that crude model cannot quantitatively describe the measured