Chemistry of the Au-Thiol Interface through the Lens of Single-Molecule Flicker Noise Measurements.

Chemistry of the Au-S interface at the nanoscale is one of the most complex systems to study, as the nature and strength of the Au-S bond change under different experimental conditions. In this study, using mechanically controlled break junction technique, we probed the conductance and analyzed Flicker noise for several aliphatic and aromatic thiol derivatives and thioethers. We demonstrate that Flicker noise can be used to unambiguously differentiate between stronger chemisorption (Au-SR) and weaker physisorption (Au-SRR') type interactions. The Flicker noise measurements indicate that the gold rearrangement in chemisorbed Au-SR junctions resembles that of the Au rearrangement in pure Au-Au metal contact breaking, which is independent of the molecular backbone structure and the resulting conductance. In contrast, thioethers showed the formation of a weaker physisorbed Au-SRR' type bond, and the Flicker noise measurement indicates the changes in the Au-anchoring group interface but not the Au-Au rearrangement like that in the Au-SR case. Additionally, by employing single-molecular conductance and Flicker noise analysis, we have probed the interfacial electric field-catalyzed ring-opening reaction of cyclic thioether under mild environmental conditions, which otherwise requires harsh chemical conditions for cleavage of the C-S bond. All of our conductance measurements are complemented by NEGF transport calculations. This study illustrates that the single-molecule conductance, together with the Flicker noise measurements can be used to tune and monitor chemical reactions at the single-molecule level.

Quantum Interference and Contact Effects in the Thermoelectric Performance of Anthracene-Based Molecules.

We report on the single-molecule electronic and thermoelectric properties of strategically chosen anthracene-based molecules with anchor groups capable of binding to noble metal substrates, such as gold and platinum. Specifically, we study the effect of different anchor groups, as well as quantum interference, on the electric conductance and the thermopower of gold/single-molecule/gold junctions and generally find good agreement between theory and experiments. All molecular junctions display transport characteristics consistent with coherent transport and a Fermi alignment approximately in the middle of the highest occupied molecular orbital/lowest unoccupied molecular orbital gap. Single-molecule results are in agreement with previously reported thin-film data, further supporting the notion that molecular design considerations may be translated from the single- to many-molecule devices. For combinations of anchor groups where one binds significantly more strongly to the electrodes than the other, the stronger anchor group appears to dominate the thermoelectric behavior of the molecular junction. For other combinations, the choice of electrode material can determine the sign and magnitude of the thermopower. This finding has important implications for the design of thermoelectric generator devices, where both n- and p-type conductors are required for thermoelectric current generation.

Trusting our machines: validating machine learning models for single-molecule transport experiments.

In this tutorial review, we will describe crucial aspects related to the application of machine learning to help users avoid the most common pitfalls. The examples we present will be based on data from the field of molecular electronics, specifically single-molecule electron transport experiments, but the concepts and problems we explore will be sufficiently general for application in other fields with similar data. In the first part of the tutorial review, we will introduce the field of single-molecule transport, and provide an overview of the most common machine learning algorithms employed. In the second part of the tutorial review, we will show, through examples grounded in single-molecule transport, that the promises of machine learning can only be fulfilled by careful application. We will end the tutorial review with a discussion of where we, as a field, could go from here.

Electrochemical control of the single molecule conductance of a conjugated bis(pyrrolo)tetrathiafulvalene based molecular switch.

As the field of unimolecular electronics develops, there is growing interest in the development of functionalised molecular wires, such as switches, which will allow for more complex molecular-scale circuits. To this end, a three redox state single molecule switch, 1, based on bis(pyrrolo)tetrathiafulvalene (BPTTF) has been designed, synthesised and investigated using scanning tunnelling microscopy break junction (STM-BJ) studies and quantum transport calculations. Oxidising the BPTTF unit increases its conjugation, which was anticipated to increase the molecular conductance of 1. By changing the redox state of 1 electrochemically it was possible to vary the single molecule conductance by more than an order of magnitude (from 10-5.2G0 to 10-3.8G0). Simulations afforded a qualitatively similar trend. An additional, higher conductance feature is present in most traces at junction sizes of around 2.0 nm - further extension affords the switchable lower conductance feature at junction sizes closer to the molecular length (ca. 3.0 nm). Analysis of the conductance traces shows that these two conductance features occur sequentially in nearly all junctions. This behaviour is attributed to an alternative initial junction conformation in which one or more of the BPTTF sulfur atoms acts as an anchoring group. This hypothesis is supported by a computational study of binding conformations and STM-BJ studies on a model compound, 2, with only one thiol anchor. Our results indicate that the redox properties of BPTTF make it an excellent candidate for use in single molecule switches.

Single-molecule detection of dihydroazulene photo-thermal reaction using break junction technique.

Charge transport by tunnelling is one of the most ubiquitous elementary processes in nature. Small structural changes in a molecular junction can lead to significant difference in the single-molecule electronic properties, offering a tremendous opportunity to examine a reaction on the single-molecule scale by monitoring the conductance changes. Here, we explore the potential of the single-molecule break junction technique in the detection of photo-thermal reaction processes of a photochromic dihydroazulene/vinylheptafulvene system. Statistical analysis of the break junction experiments provides a quantitative approach for probing the reaction kinetics and reversibility, including the occurrence of isomerization during the reaction. The product ratios observed when switching the system in the junction does not follow those observed in solution studies (both experiment and theory), suggesting that the junction environment was perturbing the process significantly. This study opens the possibility of using nano-structured environments like molecular junctions to tailor product ratios in chemical reactions.

Mapping the details of contact effect of modulated Au-octanedithiol-Au break junction by force-conductance cross-correlation.

We have measured the force and conductance of Au-octanedithiol-Au junctions using a modified conducting atomic force microscopy break junction technique with sawtooth modulations. Force-conductance two-dimensional cross-correlation histogram (FC-2DCCH) analysis for the single-molecule plateaus is demonstrated. Interestingly, four strong correlated regions appear in FC-2DCCHs consistently when modulations with different amplitudes are applied, in sharp contrast to the results under no modulation. These regions reflect the conductance and force changes during the transition of two molecule/electrode contact configurations. As the modulation amplitude increases, intermediate transition states of the contact configurations are discerned and further confirmed by comparing individual traces. This study unravels the relation between force and conductance hidden in the data of a modulated single-molecule break junction system and provides a fresh understanding of electron transport properties at molecule/electrode interfaces.

Measurement and understanding of single-molecule break junction rectification caused by asymmetric contacts.

The contact effects of single-molecule break junctions on rectification behaviors were experimentally explored by a systematic control of anchoring groups of 1,4-disubstituted benzene molecular junctions. Single-molecule conductance and I-V characteristic measurements reveal a strong correlation between rectifying effects and the asymmetry in contacts. Analysis using energy band models and I-V calculations suggested that the rectification behavior is mainly caused by asymmetric coupling strengths at the two contact interfaces. Fitting of the rectification ratio by a modified Simmons model we developed suggests asymmetry in potential drop across the asymmetric anchoring groups as the mechanism of rectifying I-V behavior. This study provides direct experimental evidence and sheds light on the mechanisms of rectification behavior induced simply by contact asymmetry, which serves as an aid to interpret future single-molecule electronic behavior involved with asymmetric contact conformation.

Force and conductance molecular break junctions with time series crosscorrelation.

Force and conductance, measured across 4,4'-bipyridine simultaneously, are crosscorrelated using a two dimensional (2D) histogram method. The result is a 2D multivariate statistical analysis superior to current one dimensional histogram techniques for exploring significant conductance and force modulations within SMBJs. This method is sensitive enough to crosscorrelate signal modulations between force and conductance traces associated with contact geometry perturbations predicted in literature such as Au-molecule contact twisting and slipping during junction elongation.