Exceptional\nField Effect and Negative Differential\nConductance in Spiro-Conjugated Single-Molecule Junctions

The\nadvancement of molecular electronics endeavors to build miniaturized\nelectronic devices using molecules as the key building blocks by harnessing\ntheir internal structures and electronic orbitals. To date, linear\nplanar conjugated or cross-conjugated molecules have been extensively\nemployed in the fabrication of single-molecule devices, benefiting\nfrom their good conductivity and compatibility with electrode architectures.\nHowever, the development of multifunctional single-molecule devices,\nparticularly those with unique charge transport properties, necessitates\na more rigorous selection of molecular materials. Among different\nassortments of molecules suited for the construction of molecular\ncircuits, Spiro-conjugated structures, specifically spirobifluorene\nderivatives, stand out as promising candidates due to their distinctive\nelectronic properties. In this work, we focus on the charge transport\ncharacteristics of Spiro-conjugated molecules sandwiched between graphene\nnanogaps. Experiments reveal significant Coulomb blockade and distinct\nnegative differential conductance effects. Beyond two-terminal device\nmeasurements, solid-state gate electrodes are utilized to create single-molecule\ntransistors, successfully modulating the molecular energy levels to\nachieve an on/off ratio exceeding 1000. This endeavor not only offers\nvaluable insights into the design and fabrication of future practical\nmolecular devices, blessed with enhanced performance and functionality,\nbut also presents a new paradigm for the investigation of fundamental\nphysical phenomena.