Spatial charge separated two-dimensional/two-dimensional Cu-In2S3/CdS heterojunction for boosting photocatalytic hydrogen production.

Two-dimensional (2D) beta indium sulfide (ß-In2S3) shows great potential in photocatalytic hydrogen production due to its broad-spectrum response, relatively negative conduction band edge, high carrier mobility and low toxicity. However, the high charge recombination rate limits the application of In2S3. Here, we in-situ grew 2D cadmium sulfide (CdS) on the surface of In2S3 doped with copper ions (Cu2+) to construct a heterojunction photocatalyst that suppresses charge recombination. The in-situ grown method and shared sulfur composition were conducive to forming the efficient interface contact between In2S3 and CdS, promoting charge transfer and showing the high spatial charge separation rate, resulting in a hydrogen production rate of 868 µmol g-1h-1. The induced Cu2+ extended the light absorption range and stabilized the photocatalyst. By creating stable 2D/2D heterojunction photocatalysts with high charge separation efficiency, this work opens new possibilities for applying In2S3 materials in photocatalytic hydrogen production.

Supramolecular Radical Electronics.

Supramolecular radical chemistry is an emerging area bridging supramolecular chemistry and radical chemistry, and the integration of radicals into the supramolecular architecture offers a new dimension for tuning their structures and functions. Although various efforts have been devoted to the fabrication of supramolecular junctions, the charge transport characterization through the supramolecular radicals remained unexplored due to the challenges in creating supramolecular radicals at the single-molecule level. Here, we demonstrate the fabrication and charge transport investigation of a supramolecular radical junction using the electrochemical scanning tunneling microscope-based break junction (EC-STM-BJ) technique. We found that the conductance of a supramolecular radical junction was more than 1 order of magnitude higher than that of a supramolecular junction without a radical and even higher than that of a fully conjugated oligophenylenediamine molecule with a similar length. The combined experimental and theoretical investigations revealed that the radical increased the binding energy and decreased the energy gap in the supramolecular radical junction, which leads to the near-resonant transport through the supramolecular radical. Our work demonstrated that the supramolecular radical can provide not only strong binding but also efficient electrical coupling between building blocks, which provides new insights into supramolecular radical chemistry and new materials with supramolecular radicals.

Voltage-driven control of single-molecule keto-enol equilibrium in a two-terminal junction system.

Keto-enol tautomerism, describing an equilibrium involving two tautomers with distinctive structures, provides a promising platform for modulating nanoscale charge transport. However, such equilibria are generally dominated by the keto form, while a high isomerization barrier limits the transformation to the enol form, suggesting a considerable challenge to control the tautomerism. Here, we achieve single-molecule control of a keto-enol equilibrium at room temperature by using a strategy that combines redox control and electric field modulation. Based on the control of charge injection in the single-molecule junction, we could access charged potential energy surfaces with opposite thermodynamic driving forces, i.e., exhibiting a preference for the conducting enol form, while the isomerization barrier is also significantly reduced. Thus, we could selectively obtain desired and stable tautomers, which leads to significant modulation of the single-molecule conductance. This work highlights the concept of single-molecule control of chemical reactions on more than one potential energy surface.

Fano Resonance in Single-Molecule Junctions.

The Fano resonance in single-molecule junctions could be created by interaction with discrete and continuous molecular orbitals and enables effective electron transport modulation between constructive and destructive interference within a small energy range. However, direct observation of Fano resonance remains unexplored because of the disappearance of discrete orbitals by molecule-electrode coupling. We demonstrated the room-temperature observation of Fano resonance from electrochemical gated single-molecule conductance and current-voltage measurements of a para-carbazole anion junction. Theoretical calculations reveal that the negative charge on the nitrogen atom induces a localized HOMO on the molecular center, creating Fano resonance by interfering with the delocalized LUMO on the molecular backbone. Our findings demonstrate that the Fano resonance in electron transport through single-molecule junctions opens pathways for designs of interference-based electronic devices.

Quantum Interference-Controlled Conductance Enhancement in Stacked Graphene-like Dimers.

Stacking interactions are of significant importance in the fields of chemistry, biology, and material optoelectronics because they determine the efficiency of charge transfer between molecules and their quantum states. Previous studies have proven that when two monomers are π-stacked in series to form a dimer, the electrical conductance of the dimer is significantly lower than that of the monomer. Here, we present a strong opposite case that when two anthanthrene monomers are π-stacked to form a dimer in a scanning tunneling microscopic break junction, the conductance increases by as much as 25 in comparison with a monomer, which originates from a room-temperature quantum interference. Remarkably, both theory and experiment consistently reveal that this effect can be reversed by changing the connectivity of external electrodes to the monomer core. These results demonstrate that synthetic control of connectivity to molecular cores can be combined with stacking interactions between their π systems to modify and optimize charge transfer between molecules, opening up a wide variety of potential applications ranging from organic optoelectronics and photovoltaics to nanoelectronics and single-molecule electronics.

The Evolution of the Charge Transport Mechanism in Single-Molecule Break Junctions Revealed by Flicker Noise Analysis.

The electronic noise characterization of single-molecule devices provides insights into the mechanisms of charge transport. In this work, it is reported that flicker noise can serve as an indicator of the time-dependent evolution of charge transport mechanisms in the single-molecule break junction process. By introducing time-frequency analysis, the authors find that flicker noise components of the molecule junction show time evolution behavior in the dynamic break junction process. A further investigation of the power-law dependence of flicker with conductance during the dynamic break junction process reveals that the mechanism of charge transport transits from the through-space transport to the through-bond transport, and is dominated by through-space transport again when the junction is about to rupture. The authors' results provide a flicker noise-based way to characterize the time-dependent evolution of charge transport mechanisms in single-molecule break junctions.

The Characterization of Electronic Noise in the Charge Transport through Single-Molecule Junctions.

With the goal of creating single-molecule devices and integrating them into circuits, the emergence of single-molecule electronics provides various techniques for the fabrication of single-molecule junctions and the investigation of charge transport through such junctions. Among the techniques for characterization of charge transport through molecular junctions, electronic noise characterization is an effective strategy with which issues from molecule-electrode interfaces, mechanisms of charge transport, and changes in junction configurations are studied. Electronic noise analysis in single-molecule junctions can be used to identify molecular conformations and even monitor reaction kinetics. This review summarizes the various types of electronic noise that have been characterized during single-molecule electrical detection, including the functions of random telegraph signal (RTS) noise, flicker noise, shot noise, and their corresponding applications, which provide some guidelines for the future application of these techniques to problems of charge transport through single-molecule junctions.

Electric field-induced switching among multiple conductance pathways in single-molecule junctions.

Here, we report the switching among multiple conductance pathways achieved by sliding the scanning tunneling microscope tip among different binding sites under different electric fields. With an increase in the electric field, high molecular conductance states appear, suggesting the formation of different configurations in single-molecule junctions. The switch can be operated in situ and reversibly, which is also confirmed by the apparent conductance conversion in I-V measurements. Theoretical simulations also agree well with the experimental results, which implies that the electric field enables the possibility to trigger switching in single-molecule junctions.

Preparation and Application of Microelectrodes at the Single-Molecule Scale.

Molecular electronics offers a potential solution for the miniaturization of electronics beyond conventional silicon electronics. A key goal of molecular electronics is to fabricate the single-molecule junction with the functions of electronic units. The term "molecular junction" means a molecular cluster or a single molecule incorporated between two microelectrodes, and electrons are transported across it. The methods of constructing molecular junctions dynamically were developed, such as STM-BJ, AFM-BJ, and MCBJ, providing precise control of the gap and easy measurement of thousands of junctions. Electrodes based on these techniques are commonly called microelectrodes because at least one dimension is on the micron scale. In this manuscript, we summarize the preparation methods of microelectrodes and their application in single-molecule measurements. In addition, we discuss the electrode factor that influences the molecular electrical properties, such as material, curvature radius and cone angle, and further provide a brief prospect of molecular electronics.