Single-molecule photoelectron tunnelling spectroscopy

From Molecular Electronics to Molecular Intelligence

Molecular electronics is a field that explores the ultimate limits of electronic device dimensions by using individual molecules as operable electronic devices. Over the past five decades since the proposal of a molecular rectifier by Aviram and Ratner in 1974 ( Chem. Phys. Lett.1974,29, 277-283), researchers have developed various fabrication and characterization techniques to explore the electrical properties of molecules. With the push of electrical characterizations and data analysis methodologies, the reproducibility issues of the single-molecule conductance measurement have been chiefly resolved, and the origins of conductance variation among different devices have been investigated. Numerous prototypical molecular electronic devices with external physical and chemical stimuli have been demonstrated based on the advances of instrumental and methodological developments. These devices enable functions such as switching, logic computing, and synaptic-like computing. However, as the goal of molecular electronics, how can molecular-based intelligence be achieved through single-molecule electronic devices? At the fiftieth anniversary of molecular electronics, we try to answer this question by summarizing recent progress and providing an outlook on single-molecule electronics. First, we review the fabrication methodologies for molecular junctions, which provide the foundation of molecular electronics. Second, the preliminary efforts of molecular logic devices toward integration circuits are discussed for future potential intelligent applications. Third, some molecular devices with sensing applications through physical and chemical stimuli are introduced, demonstrating phenomena at a single-molecule scale beyond conventional macroscopic devices. From this perspective, we summarize the current challenges and outlook prospects by describing the concepts of "AI for single-molecule electronics" and "single-molecule electronics for AI".

Boron-Doped Single-Molecule van der Waals Diode

Single-molecule diode was the first proposed device in molecular electronics. Despite the great efforts and advances over 50 years, the reported rectification ratios, the most critical parameter of a diode, remain moderate for the single-molecule diode. Herein, we report an approach to achieve a larger rectification ratio by adopting the combined strategies of p-type boron doping, the single-layer graphene nodes, and the van der Waals layer-by-layer architecture. Measured current-voltage curves showed one of the as-fabricated single-molecule diodes hit an unprecedented large rectification ratio of 457 at ±1 V. Break junction operations and spectroscopic measurements revealed the three-atom-thick configuration of the single-molecule diodes. With the experimental and theoretical calculation results, we demonstrated the doped boron atoms induced holes to redistribute the electron density, making the asymmetric coupling at positive and negative biases, and the van der Waals interaction promoted asymmetric coupling and significantly boosted diode performance.

Tracking Noncovalent Interactions of π, π-Hole, and Ion in Molecular Complexes at the Single-Molecule Level

Noncovalent interactions involving aromatic rings, such as π-stacking and π-ion interactions, play an essential role in molecular recognition, assembly, catalysis, and electronics. However, the inherently weak and complex nature of these interactions has made it challenging to study them experimentally, especially with regard to elucidating their properties in solution. Herein, the noncovalent interactions between π and π-hole, π and cation, and π-hole and anion in molecular complexes in nonpolar solution are investigated in situ through single-molecule electrical measurements in combination with theoretical calculations. Specifically, phenyl and pentafluorobenzyl groups serve as π and π-hole sites, respectively, while Li+ and Cl- are employed as the cation and anion. Our findings reveal that, in comparison with homogeneous π···π interactions, heterogeneous π···π-hole and π···cation interactions exhibit greater binding energies, resulting in a longer binding lifetime of the molecular junctions. Meanwhile, π···Li+ and π-hole···Cl- interactions present significantly distinct binding characteristics, with the former being stronger but more flexible than the latter. Furthermore, by changing the molecular components, similar conductivity can be achieved in both molecular dimers or sandwich complexes. These results provide new insights into π- and π-hole-involved noncovalent interactions, offering novel strategies for precise manipulation of molecular assembly, recognition, and molecular device.

Plasmon-Molecule Interactions in Single-Molecule Junctions

Single-molecule optoelectronics offers opportunities for advancing integrated photonics and electronics, which also serves as a tool to elucidate the underlying mechanism of light-matter interaction. Plasmonics, which plays pivotal role in the interaction of photons and matter, have became an emerging area. A comprehensive understanding of the plasmonic excitation and modulation mechanisms within single-molecule junctions (SMJs) lays the foundation for optoelectronic devices. Consequently, this review primarily concentrates on illuminating the fundamental principles of plasmonics within SMJs, delving into their research methods and modulation factors of plasmon-exciton. Moreover, we underscore the interaction phenomena within SMJs, including the enhancement of molecular fluorescence by plasmonics, Fano resonance and Rabi splitting caused by the interaction of plasmon-exciton. Finally, by emphasizing the potential applications of plasmonics within SMJs, such as their roles in optical tweezers, single-photon sources, super-resolution imaging, and chemical reactions, we elucidate the future prospects and current challenges in this domain.