Quantitative interpretation of the transition voltages in gold-poly(phenylene) thiol-gold molecular junctions

Abstract

The transition voltage of three different asymmetric Au∕poly(phenylene) thiol∕Au molecular junctions in which the central molecule is either benzene thiol, biphenyl thiol, or terphenyl thiol is investigated by first-principles quantum transport simulations. For all the junctions, the calculated transition voltage at positive polarity is in quantitative agreement with the experimental values and shows weak dependence on alterations of the Au-phenyl contact. When compared to the strong coupling at the Au-S contact, which dominates the alignment of various molecular orbitals with respect to the electrode Fermi level, the coupling at the Au-phenyl contact produces only a weak perturbation. Therefore, variations of the Au-phenyl contact can only have a minor influence on the transition voltage. These findings not only provide an explanation to the uniformity in the transition voltages found for π-conjugated molecules measured with different experimental methods, but also demonstrate the advantage of transition voltage spectroscopy as a tool for determining the positions of molecular levels in molecular devices.

Transition voltage spectroscopy of scanning tunneling microscopy vacuum junctions

“…In an attempt to explain the large discrepancy between the predicted and measured behavior of V t as a function of 1/d, we will consider the effect of an image charge potential as has been suggested by Huisman et al 17 and Trouwborst et al 22 The existence of image charges 6 can have an effect on the tunneling barrier height and width, as has been pointed out in previous studies. 19,24 To incorporate the effect of an image charge in the Simmons model, an extra term has to be added to the effective barrier height.…”

Transition voltages of vacuum-spaced and molecular junctions with Ag and Pt electrodes

“…S3 in the supplementary material). 55,56 When compared with the Au-biphenyl thiol-Au junction with the same molecule-electrode interfaces, 35 the positions of these transmission peaks turn out to be shifted to lower energies. For example, the replacement of gold electrodes with silver ones moves the HOMO-dominated transmission peak from −0.96 eV down to −1.28 eV.…”

“…24 By employing the non-equilibrium Green's function formalism combined with density functional theory (i.e., the NEGF+DFT approach), [25][26][27][28][29][30][31][32][33][34] we have shown that the calculated transition voltages of three different asymmetric Au/poly(phenylene) thiol/Au molecular junctions can be in quantitative agreement with the experimental values at positive polarity. 35 Thus, it is highly desirable to carry out studies beyond the tunneling barrier model in order to investigate the effects of the electrodes' composition and their shape on the transition voltages of molecular junctions. These will enable TVS to become a widely used tool for understanding charge transport mechanism of molecular electronic devices.…”

“…In order to make a comparison with our previous results for Au/poly(phenylene) thiol/Au junctions, 35 here biphenyl thiol is chosen as the central molecule. Similarly to the case of gold, chemisorption always occurs when organosulfur compounds adsorb on Ag and Pt surfaces.…”

Concurrent conductance and transition voltage spectroscopy study of scanning tunneling microscopy vacuum junctions. Does it unravel new physics?

“…4), where the ghost current overcomes the tunneling current. This is the reason why we believe that incorporating the atomistic structure of the electrodes (e.g., within approaches based on the density functional theory 23,24 ) will not change the conclusions of the present study, which is mostly based on the classical barrier picture for charge transport via tunneling in vacuo. Such details can be quite relevant for TVS in other cases: e.g., for vacuum nanogaps of sizes smaller than those (d > 0.8 nm) presently considered 23 or molecular junctions, 24 where through-space processes are negligible.…”