Charge Transport with Single Molecules – An Electrochemical Approach

Abstract

After an introduction and brief review of charge transport in nanoscale molecular systems we report on experimental studies in gold | (single) molecule | gold junctions at solid | liquid interfaces employing a scanning tunneling microscopy (STM)-based 'break junction' technique. We demonstrate attempts in developing basic relationships between molecular structure, conductance properties and nanoscale electrochemical concepts based on four case studies from our own work. In experiments with ?, ?-alkanedithiol and biphenyldithiol molecular junctions we address the role of sulfur-gold couplings and molecular conformation, such as gauche defects in the alkyl chains and the torsion angle between two phenyl rings. Combination with quantum chemistry calculations enabled a detailed molecular-level understanding of the electronic structure and transport characteristics of both systems. Employing the concept of 'electrolyte gating' with redox-active molecules, such as thiol-terminated derivatives of viologens (HS-6V6-SH or (HS-6V6)) we demonstrate the construction of symmetric and asymmetric active molecular junctions with transistor- or diode-like behavior upon polarization in an electrochemical environment. The experimental data could be represented quantitatively by the Kutznetsov/Ulstrup model assuming a two-step electron transfer with partial vibration relaxation. Finally, we show that surface-immobilized gold nanoparticles with a diameter of (2.4 ± 0.5) nm exhibit features of locally addressable multi-state electronic switching upon electrolyte gating, which appears to be reminiscent of a sequential charging through several 'oxidation/reduction states'.

Correlation of breaking forces, conductances and geometries of molecular junctions

“…Molecular junctions created by single molecules trapped between two probes enable the creation of nanoscale structures with unique mechanical, electrical, optical and quantum properties123. A common strategy to form molecular junctions is based on the approach and contact of a sharp nanoprobe to a second probe in the presence of the molecules of interest, followed by the subsequent separation of the probes.…”

“…A common strategy to form molecular junctions is based on the approach and contact of a sharp nanoprobe to a second probe in the presence of the molecules of interest, followed by the subsequent separation of the probes. This strategy is widely employed in mechanically controllable break junction (MCBJ) or scanning tunnelling microscopy-based break junction (STMBJ)123 experiments, and in force spectroscopy456 to characterise electrical and mechanical properties, respectively. In particular, conductance studies with small organic molecules revealed unique correlations between molecular structure and junction conductances.…”

Predictive Models for Thermal Behavior of Chemicals with Quantitative Structure‐Property Relationships

“…Therefore, distinctly different families of the conductance can be observed experimentally, even for the same compound entrapped within the junction. DFT‐based calculations revealed that the presence of gauche defects within the alkyl chain results in a significant decrease of the bridge conductance, compared with an all‐trans conformation 86. The model calculations also showed that the conductance depends strongly on the number of sulfur atoms coordinated to the gold atoms 87.…”

Charge transport through perylene bisimide molecular junctions: An electrochemical approach

“…Working in an electrochemical environment, such as an electrified solid liquid interface, offers the opportunity that two potentials can be controlled separately: The bias voltage E bias between the two WEs (WE1 and WE2) as well as the potential drop between each WE and the reference electrode (RE). The electrolyte acts as a gate ensuring a strong coupling to the applied external field (field strength 10 9 V m −1 , gate thickness ∼1 nm) (, Fig. ).…”

“…The idea of an “electrolyte gate” to control charge transport in molecular electronics was introduced by Wrighton and coworkers , Meulenkamp , Schönenberger et al and McEuen et al , and further developed by Tao et al , Ulstrup et al , Lindsay et al , Haiss et al , Vanmaekelbergh et al and us . The electrochemical approach is unique as the measured current represents both, the electrical contact to the external circuit and to the functional state of the tailored (single) molecules and/or clusters.…”