Theoretische Untersuchungen zur Übergangsspannungs-Spektroskopie (TVS)
Theoretische Chemie: Moleküle, Materialien, Oberflächen
Zusammenfassung der Projektergebnisse
The project aimed at the theoretical investigation of the basis and applications of transition voltage spectroscopy (TVS) in molecular electronics, an experimental method proposed by the group of Prof. C. D. Frisbie. The leading idea of TVS was to devise a method enabling the simple determination of a central parameter that controls the charge transport via tunneling in molecular junctions: the energy offset of the dominant molecular orbital (MO) relative to metallic Fermi level. A series of publications emerging from this project demonstrated and re-emphasized the importance for the charge transport through molecular junctions of the so-called transition voltage Vt, the key quantity of the method (TVS) envisaged. The best example in this sense is perhaps the law of corresponding states for the transport via tunneling, which we first deduced theoretically. It is expressed as a close, appealingly simple mathematical form, which relates the current and bias appropriately rescaled. The units used for this rescaling — and thence the key role played by Vt — are expressed in terms of the transition voltage. This theoretical result has been validated against a rich experimental body of data. It includes but is not limited to current-voltage measurements on two benchmark series (oligophenylenedithiols and alkanedithiols) of molecular junctions fabricated under a conducting probe atomic force microscope CP-AFM) platform, by using three of the most utilized electrodes (gold, silver, and platinum) in molecular electronics, having conductances varying on several orders of magnitudes. Laws of corresponding states, as expression of universal behaviors, could only be expected in cases wherein the preparation/fabrication of the materials under investigation is a wellcontrolled process, and their properties are highly reproducible. Searching for a law of corresponding states in molecular electronics, a field often plagued by strong stochastic fluctuations, seems totally out of place. For this reason, the law of corresponding states demonstrated in this project is a quite unexpected result of universal behavior. Even more noteworthy, this law of corresponding states is free of any empirical parameters needing adjusting to experiment. Not even the celebrated van der Waals law of corresponding states for classical fluids satisfies this condition. Work done within this project emphasized that, for a proper theoretical analysis of the MO offset, theoretical approaches going beyond the conventional density functional theory (DFT) are needed. Such approaches, — which include methods like the outer valence Green’s functions (OVGF), coupled-cluster singles and doubles (CCSD), and Δ-DFT — have been applied within this project. On this basis it was possible to demonstrate that the relevant MO offset can be disentangled in contributions having a clear physical origin, e.g., due to solvents or work function/Fermi level pinning effects. Here we report a detailed analysis of transport and transport-related data in the benchmark (oligophenylenedithiol) series. The usefulness of TVS has also demonstrated in the presence of solvent and intramolecular reorganization effects. In this sense, two examples can be given. First, the evidence for a charge conjugation property in single-molecule junctions based on azurin in electrochemical environment, as a showcase for 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. Second, the unusually strong (ensemble) fluctuations in bipyridinebased junctions due to the unusual intramolecular reorganization, which traces back to the most appealing structural feature of this molecule, namely the twist angle between the two pyridine rings. Last but not least, a number of model-based studies of this project considered several practical aspects on how physical insight into the transport via tunneling at the nanoscale can be gained by including TVS as an important element in transport data analysis.
Projektbezogene Publikationen (Auswahl)
- Important Insight into Electron Transfer in Single-Molecule Junctions Based on Redox Metalloproteins from Transition Voltage Spectroscopy, J. Phys. Chem. C 117 (2013) 25798
I. Bâldea
(Siehe online unter https://doi.org/10.1021/jp408873c) - Transition voltage spectroscopy reveals significant solvent effects on molecular transport and settles an important issue in bipyridine-based junctions, Nanoscale 5 (2013) 9222. (Featured by Reginnovations.)
I. Bâldea
(Siehe online unter https://doi.org/10.1039/c3nr51290h) - A quantum chemical study from a molecular transport perspective: ionization and electron attachment energies for species often used to fabricate single-molecule junctions, Faraday Discuss. 174 (2014) 35
I. Bâldea
(Siehe online unter https://doi.org/10.1039/c4fd00101j) - Single-molecule junctions based on bipyridine: impact of an unusual reorganization on the charge transport, J. Phys. Chem. C 118 (2014) 8676
I. Bâldea
(Siehe online unter https://doi.org/10.1021/jp412675k) - Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function, ACS Nano 9 (2015) 8022
Z. Xie, I. Bâldea, C. Smith, Y. Wu, and C. D. Frisbie
(Siehe online unter https://doi.org/10.1021/acsnano.5b01629) - Uncovering a law of corresponding states for electron tunneling in molecular junctions, Nanoscale 7 (2015) 10465
I. Bâldea, Z. Xie, and C. D. Frisbie
(Siehe online unter https://doi.org/10.1039/c5nr02225h)
