Theoretical Investigations on Transition Voltage Spektroscopy (TVS)

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The project is devoted to the theoretical investigation of the Transition Voltage Spectroscopy (TVS). TVS is an experimental tool of molecular electronics proposed recently by Frisbie et al, which aims at determining the relative energetic alignment of the molecular orbitals with respect to metal's Fermi level from the (transition) voltage at the minimum of the Fowler-Nordheim (FN) diagram. The FN diagrams are directly obtained from the measured I-V curves. The orbital energy offset is a key parameter for molecular transport, because it controls the charge transfer efficiency. In close collaboration with two experimental groups, the work planned in this project will attempt to provide a firm theoretical foundation of TVS. To this aim, an approach of molecular transport based on molecular orbitals with a clear physical meaning (as opposed to Kohn-Sham orbitals) is needed. The main methodological goal of this project is to develop and implement an ab initio (not based on Density functional theory (DFT)) approach to molecular transport relying upon the outer valence Green's functions (OVGF), a well-established method of quantum chemistry. It is backed by four decades of work in quantum chemistry, demonstrating that the outer valence region can be accurately described within a single-particle picture. Such an ab initio transport approach based on dressed quasi-particles that incorporate post-HF (Hartree-Fock) electron-electron interaction effects is not only of interest for TVS but also for the molecular transport theory in general. In implementing the OVGF-based transport approach, the computational advantage offered by the Cholesky decomposition will be exploited, which enables calculations for large molecules with high-quality basis sets and reasonable computing cost. The basic TVS idea was to provide experimentalists with a simple tool to directly determine the orbital energy offset without the need to resort to sophisticated transport calculations. This practical goal, at which the project also aims, can be achieved only if sufficiently simple models can be utilized, whose parameters can be extracted from experimental data and validated by microscopic calculations. Realistic models will be proposed that will extend the Newns-Anderson model, which has been employed in recent studies and turned out to successfully explain a series of experimental TVS findings. Extensions envisaged in this project will incorporate, e. g., the effect of anchoring groups and electrodes' work function, whose significant impact on the transition voltage has been demonstrated in recent experimental studies. The impact of stochastic fluctuations on two-dimensional transition-voltage-conductance histograms (that became available due to recent experimental achievements of Tao et al) and of the inner reorganization on TVS for molecules with a floppy degree of freedom will also constitute important topics of investigation.