A single atom change turns insulating saturated wires into molecular conductors

Intermolecular Interactions and Quantum Interference Effects in Molecular Junctions

Destructive quantum interference (DQI) leads to a decrease in the conductance of certain well-documented molecules. Experimental observations have revealed both direct and indirect manifestations of DQI, although a comprehensive understanding of the underlying causes of these distinct outcomes remains elusive. In both cases, DQI lowers the conductance, but only the direct case exhibits a characteristic V-shaped dip in differential conductance. Currently, the direct signature has exclusively been observed in monolayers and gated single-molecule systems. In this study, we employ density functional theory to elucidate a plausible explanation for the absence of a direct DQI signature in single molecules. Specifically, we attribute the direct DQI signature to a resonance shift induced by intermolecular interactions, which are absent in the individual molecules. By illustrating the impact of these intermolecular interactions, we emphasize the need for explicit treatment of intermolecular interactions when simulating monolayers.

Gradual Change between Coherent and Incoherent Tunneling Regimes Induced by Polarizable Halide Substituents in Molecular Tunnel Junctions

This paper describes a gradual transition of charge transport across molecular junctions from coherent to incoherent tunneling by increasing the number and polarizability of halide substituents of phenyl-terminated aliphatic monolayers of the form S(CH2)10OPhXn, X = F, Cl, Br, or I; n = 0, 1, 2, 3, or 5. In contrast to earlier work where incoherent tunneling was induced by introducing redox-active groups or increasing the molecular length, we show that increasing the polarizability, while keeping the organization of the monolayer structure unaltered, results in a gradual change in the mechanism of tunneling of charge carriers where the activation energy increased from 23 meV for n = 0 (associated with coherent tunneling) to 257 meV for n = 5 with X = Br (associated with incoherent tunneling). Interestingly, this increase in incoherent tunneling rate with polarizability resulted in an improved molecular diode performance. Our findings suggest an avenue to improve the electronic function of molecular devices by introducing polarizable atoms.

Tuning Overbias Plasmon Energy and Intensity in Molecular Plasmonic Tunneling Junctions by Atomic Polarizability

Plasmon excitation in molecular tunnel junctions is interesting because the plasmonic properties of the device can be, in principle, controlled by varying the chemical structure of the molecules. The plasmon energy of the excited plasmons usually follows the quantum cutoff law, but frequently overbias plasmon energy has been observed, which can be explained by quantum shot noise, multielectron processes, or hot carrier models. So far, clear correlations between molecular structure and the plasmon energy have not been reported. Here, we introduce halogenated molecules (HS(CH2)12X, with X = H, F, Cl, Br, or I) with polarizable terminal atoms as the tunnel barriers and demonstrate molecular control over both the excited plasmon intensity and energy for a given applied voltage. As the polarizability of the terminal atom increases, the tunnel barrier height decreases, resulting in an increase in the tunneling current and the plasmon intensity without changing the tunneling barrier width. We also show that the plasmon energy is controlled by the electrostatic potential drop at the molecule-electrode interface, which depends on the polarizability of the terminal atom and the metal electrode material (Ag, Au, or Pt). Our results give new insights in the relation between molecular structure, electronic structure of the molecular junction, and the plasmonic properties which are important for the development of molecular scale plasmonic-electronic devices.

An artificial synapse based on molecular junctions

Shrinking the size of the electronic synapse to molecular length-scale, for example, an artificial synapse directly fabricated by using individual or monolayer molecules, is important for maximizing the integration density, reducing the energy consumption, and enabling functionalities not easily achieved by other synaptic materials. Here, we show that the conductance of the self-assembled peptide molecule monolayer could be dynamically modulated by placing electrical biases, enabling us to implement basic synaptic functions. Both short-term plasticity (e.g., paired-pulse facilitation) and long-term plasticity (e.g., spike-timing-dependent plasticity) are demonstrated in a single molecular synapse. The dynamic current response is due to a combination of both chemical gating and coordination effects between Ag⁺ and hosting groups within peptides which adjusts the electron hopping rate through the molecular junction. In the end, based on the nonlinearity and short-term synaptic characteristics, the molecular synapses are utilized as reservoirs for waveform recognition with 100% accuracy at a small mask length.

Smart Eutectic Gallium-Indium: From Properties to Applications

Eutectic gallium-indium (EGaIn), a liquid metal with a melting point close to or below room temperature, has attracted extensive attention in recent years due to its excellent properties such as fluidity, high conductivity, thermal conductivity, stretchability, self-healing capability, biocompatibility, and recyclability. These features of EGaIn can be adjusted by changing the experimental condition, and various composite materials with extended properties can be further obtained by mixing EGaIn with other materials. In this review, not only the are unique properties of EGaIn introduced, but also the working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The applications of EGaIn in various fields, such as flexible electronics (sensors, antennas, electronic circuits), molecular electronics (molecular memory, opto-electronic switches, or reconfigurable junctions), energy catalysis (heat management, motors, generators, batteries), biomedical science (drug delivery, tumor therapy, bioimaging and neural interfaces) are reviewed. Finally, a critical discussion of the main challenges for the development of EGaIn-based techniques are discussed, and the potential applications in new fields are prospected.

Interplay between Interfacial Energy, Contact Mechanics, and Capillary Forces in EGaIn Droplets

Eutectic gallium-indium (EGaIn) is increasingly employed as an interfacial conductor material in molecular electronics and wearable healthcare devices owing to its ability to be shaped at room temperature, conductivity, and mechanical stability. Despite this emerging usage, the mechanical and physical mechanisms governing EGaIn interactions with surrounding objects─mainly regulated by surface tension and interfacial adhesion─remain poorly understood. Here, using depth-sensing nanoindentation (DSN) on pristine EGaIn/GaO_x surfaces, we uncover how changes in EGaIn/substrate interfacial energies regulate the adhesive and contact mechanic behaviors, notably the evolution of EGaIn capillary bridges with distinct capillary geometries and pressures. Varying the interfacial energy by subjecting EGaIn to different chemical environments and by functionalizing the tip with chemically distinct self-assembled monolayers (SAMs), we show that the adhesion forces between EGaIn and the solid substrate can be increased by up to 2 orders of magnitude, resulting in about a 60-fold increase in the elongation of capillary bridges. Our data reveal that by deploying molecular junctions with SAMs of different terminal groups, the trends of charge transport rates, the resistance of monolayers, and the contact interactions between EGaIn and monolayers from electrical characterizations are governed by the interfacial energies as well. This study provides a key understanding into the role of interfacial energy on geometrical characteristics of EGaIn capillary bridges, offering insights toward the fabrication of EGaIn junctions in a controlled fashion.

Doping and coupling strength in molecular conductors: polyacetylene as a case study

The doping mechanisms responsible for elevating the currents up to eleven orders of magnitude in semiconducting polymer films are today well characterized. Doping can also improve the performance of nanoscale devices or single molecule conductors, but the mechanism in this case appears to be different, with theoretical studies suggesting that the dopant affects the electronic properties of the junctions. In the present report, multiscale time-dependent DFT transport simulations help clarify the way in which n-type doping can raise the current flowing through a polymer chain connected to a pair of electrodes, with the focus on polyacetylene. In particular, our multiscale methodology offers control over the magnitude of the chemical coupling between the molecule and the electrodes, which allows us to analyze the effect of doping in low and strong coupling regimes. Interestingly, our results establish that the impact of dopants is the highest in weakly coupled devices, while their presence tends to be irrelevant in low-resistance junctions. Our calculations point out that both the equalization of the frontier orbitals with the Fermi level and a small gap between the HOMO and the LUMO must result from doping in order to observe any significant increase of the currents.