Correlated Energy-Level Alignment Effects Determine Substituent-Tuned Single-Molecule Conductance

Abstract

The rational design of single molecule electrical components requires a deep and predictive understanding of structure-function relationships. Here we explore the relationship between chemical substituents and the conductance of metal-single molecule-metal junctions, using functionalized oligophenylenevinylenes as a model system. Using a combination of mechanically controlled break-junction experiments and various levels of theory including non-equilibrium Green's functions, we demonstrate that the connection between gas-phase molecular electronic structure and in-junction molecular conductance is complicated by the involvement of multiple mutually correlated and opposing effects that contribute to energy level alignment in the junction. We propose that these opposing correlations represent powerful new "design principles," because their physical origins make them broadly applicable, and they are capable of predicting the direction and relative magnitude of observed conductance trends. In particular, we show that they are consistent with the observed conductance variability not just within our own experimental results, but also within disparate molecular series reported in literature and, crucially, with the trend in variability across these molecular series, which previous simple models fail to explain. The design principles introduced here can therefore aid in both screening and suggesting novel design strategies for maximizing conductance tunability in single-molecule systems.

Quantifying Image Charge Effects in Molecular Tunnel Junctions Based on Self-Assembled Monolayers of Substituted Oligophenylene Ethynylene Dithiols

“…Importantly, both experiments and theory demonstrate the image potential contribution to ε h to be ∼0.1 eV per contact for OPX, which translates to approximately 25–30% of the total magnitude of ε h for complete, two-contact junctions. Note that this significant effect differs slightly from previous findings in which the image charge in single molecule junctions was found to dominate the energy level alignment. ,,− Our results do not contradict the earlier findings, but rather indicate that image charge effects are system-specific and depend on the size and orientation of molecules with respect to the metal electrodes, as well as the electrode shape and total number of electrodes (up to 3 if there is a gate).…”

“…Given that the total energy level offset ε h trans is ∼0.6 eV, eV image is clearly important as each contact provides ∼−0.08 eV, meaning that the total contribution of eV image to ε h trans is ∼−0.16 eV or 25% of the total offset from E f . One can say that image charge effects are a significant factorthough not the dominant factorin overall energy alignment in OPX junctions. ,,− …”

“…Relating the conductance behavior of molecules to their electronic structure is a central focus of the field of molecular electronics. − However, electronic phenomena at the interface between discrete molecules and metal electrodes generally yield substantial changes in orbital energies with respect to their values in isolated molecules, and thus, understanding these factors is critical. The presence of metal–molecule bond dipoles and metal image potentials are two recognized effects responsible for shifting of orbital energies when a molecule binds to a metallic substrate. − Additionally, for junctions based on molecular assemblies such as self-assembled monolayers (SAMs) rather than single molecules, intermolecular interactions also play a major role in energy level alignment …”

“…Here, our particular focus is on determining the role of the image potential V image on orbital energies in OPX molecular junctions. Ionization energies and electron affinities (EAs) of molecules near metal surfaces can be profoundly affected by the metal’s image potential, ,,− and this in turn can influence the HOMO (or LUMO) alignment with respect to the Fermi level. However, these effects depend on the proximity of the molecule to the metal surface and, crucially, on molecular orientation with respect to the surface. − It is not immediately clear how big the image charge effect will be for a given molecular junction.…”

Molecular Diodes With Tunable Threshold Voltage Based on π‐Extended Tetrathiafulvalene

“…[2] Since then, the performance of molecular diodes, This helped regulate single-molecule conductance. [12] Venkataraman et al showed that the dominant conducting orbitals for thiophene/thiophene-1,1-dioxide oligomers of an equal length can be tuned by varying the electron affinity of the units incorporated into the backbone. [13] Koch et al and Zojer et al have demonstrated the importance of molecular modification for tuning charge transport in organic electronics from the sophisticated study of spectroscopy [14] and theoretical calculations.…”