Modeling single molecule junction mechanics as a probe of interface bonding

Nuclear Magnetic Resonance Chemical Shift as a Probe for Single-Molecule Charge Transport

Existing modelling tools, developed to aid the design of efficient molecular wires and to better understand their charge-transport behaviour and mechanism, have limitations in accuracy and computational cost. Further research is required to develop faster and more precise methods that can yield information on how charge transport properties are impacted by changes in the chemical structure of a molecular wire. In this study, we report a clear semilogarithmic correlation between charge transport efficiency and nuclear magnetic resonance chemical shifts in multiple series of molecular wires, also accounting for the presence of chemical substituents. The NMR data was used to inform a simple tight-binding model that accurately captures the experimental single-molecule conductance values, especially useful in this case as more sophisticated density functional theory calculations fail due to inherent limitations. Our study demonstrates the potential of NMR spectroscopy as a valuable tool for characterising, rationalising, and gaining additional insights on the charge transport properties of single-molecule junctions.

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.

Probabilistic Approach to Low Strain Rate Atomistic Simulations of Ultimate Tensile Strength of Polymer Crystals

Molecular dynamics simulations of the tensile ultimate properties of polymer crystals require the use of empirical potentials that model bond dissociation. However, fully reactive potentials are computationally expensive such that reactive simulations cannot reach the low strain rates of typical experiments. Here, we present a hybrid approach that uses the simplicity of a classical, nonreactive potential, information from bond dissociation energy calculations, and a probabilistic expression that mimics bond breaking. The approach is demonstrated for poly(p-phenylene terephthalamide) and, with one tunable parameter, the calculated tensile ultimate stress matches that obtained using a fully reactive simulation at high strain rates. Then, the hybrid simulations are run at much lower strain rates where the ultimate tensile stress is strain rate-independent and consistent with the expected experimental range.

Stretch Evolution of Electronic Coupling of the Thiophenyl Anchoring Group with Gold in Mechanically Controllable Break Junctions

The current-voltage characteristics of a single-molecule junction are determined by the electronic coupling Γ between the electronic states of the electrodes and the dominant transport channel(s) of the molecule. Γ is profoundly affected by the choice of the anchoring groups and their binding positions on the tip facets and the tip-tip separation. In this work, mechanically controllable break junction experiments on the N,N'-bis(5-ethynylbenzenethiol-salicylidene)ethylenediamine are presented, in particular, the stretch evolution of Γ with increasing tip-tip separation. The stretch evolution of Γ is characterized by recurring local maxima and can be related to the deformation of the molecule and sliding of the anchoring groups above the tip facets and along the tip edges. A dynamic simulation approach is implemented to model the stretch evolution of Γ, which captures the experimentally observed features remarkably well and establishes a link to the microscopic structure of the single-molecule junction.

Learning Conductance: Gaussian Process Regression for Molecular Electronics

Experimental studies of charge transport through single molecules often rely on break junction setups, where molecular junctions are repeatedly formed and broken while measuring the conductance, leading to a statistical distribution of conductance values. Modeling this experimental situation and the resulting conductance histograms is challenging for theoretical methods, as computations need to capture structural changes in experiments, including the statistics of junction formation and rupture. This type of extensive structural sampling implies that even when evaluating conductance from computationally efficient electronic structure methods, which typically are of reduced accuracy, the evaluation of conductance histograms is too expensive to be a routine task. Highly accurate quantum transport computations are only computationally feasible for a few selected conformations and thus necessarily ignore the rich conformational space probed in experiments. To overcome these limitations, we investigate the potential of machine learning for modeling conductance histograms, in particular by Gaussian process regression. We show that by selecting specific structural parameters as features, Gaussian process regression can be used to efficiently predict the zero-bias conductance from molecular structures, reducing the computational cost of simulating conductance histograms by an order of magnitude. This enables the efficient calculation of conductance histograms even on the basis of computationally expensive first-principles approaches by effectively reducing the number of necessary charge transport calculations, paving the way toward their routine evaluation.

Driven Dynamics of Long-Time Bond-Breaking Events

We present a predict-correct trajectory propagation (PCTP) method for simulating nonequilibrium driven dynamics of the long-time bond-breaking event in an atomic force microscope (AFM). Whereas the parallel replica method can extend the time scale of molecular dynamics (MD) simulations of infrequent-event systems with high parallel efficiency, the second aspect of the time scale difficulty in MD simulations for slow-driven systems, namely, the unphysically high attempt frequency of an infrequent event, cannot be resolved by this method. Here, we take a gold nanojunction under mechanical pulling as a simulation system and demonstrate that the PCTP simulation is capable of capturing key transition dynamics of bond breaking predicted by accurate MD simulations in the activationless regime, such as at cryogenic temperature with high pulling rates. The PCTP algorithm includes three steps: (i) the trajectory prediction of the AFM tip, (ii) the atomic structural relaxation of the nanojunction, and (iii) the trajectory correction of the tip motion predicted by step (i). Application of the PCTP simulation to the bond breaking at AFM experimental pulling rates reveals different rupture mechanisms, depending on temperature and single-atom nanojunction structures, which are consistent with recent AFM bond-breaking observations.

High-Voltage-Assisted Mechanical Stabilization of Single-Molecule Junctions

Resonant tunneling is an efficient mechanism for charge transport through nanoscale conductance junctions due to the relatively high currents involved. However, continuous charging and discharging cycles of the nanoconductor during resonant tunneling often lead to mechanical instability. The realization of efficient nanoscale electronic components therefore depends to a large extent on the ability to mechanically stabilize them during resonant transport. In this work, we focus on single-molecule junctions, demonstrating that their mechanical stability during resonant transport can be increased by increasing the bias voltage. This counter-intuitive effect is attributed to the energy dependence of the molecule-lead coupling densities, which promote the rate of transport-induced cooling of molecular vibrations at higher voltages. The required energy dependence is characteristic of realistic electrodes (such as graphene), which cannot be modeled within the commonly invoked wide-band approximation. Our research provides new guidelines for the design of mechanically stable molecular devices operating in the regime of resonant charge transport and demonstrates these guidelines while considering realistic features of single-molecule junctions.

Dynamic breaking of a single gold bond

While one might assume that the force to break a chemical bond gives a measure of the bond strength, this intuition is misleading. If the force is loaded slowly, thermal fluctuations may break the bond before it is maximally stretched, and the breaking force will be less than the bond can sustain. Conversely, if the force is loaded rapidly it is more likely that the maximum breaking force is measured. Paradoxically, no clear differences in breaking force were observed in experiments on gold nanowires, despite being conducted under very different conditions. Here we explore the breaking behaviour of a single Au-Au bond and show that the breaking force is dependent on the loading rate. We probe the temperature and structural dependencies of breaking and suggest that the paradox can be explained by fast breaking of atomic wires and slow breaking of point contacts giving very similar breaking forces.