RESUMO
The fabrication of solid-state single-molecule switches with high on-off conductance ratios has been proposed to advance conventional technology in areas such as molecular electronics. Herein, we employed the scanning tunneling microscope break junction (STM-BJ) technique to modulate conductance in single-molecule junctions using mechanically induced stretching. Compound 1a possesses two dihydrobenzothiophene (DHBT) anchoring groups at the opposite ends linked with rigid alkyne side arms to form a gold-molecule-gold junction, while 1b contains 4-pyridine-anchoring groups. The incorporation of ferrocene into the backbone of each compound allows rotational freedom to the cyclopentadienyl (Cp) rings to give two distinct conductance states (high and low) for each. Various control experiments and suspended junction compression/retraction measurements indicate that these high- and low-conductance plateaus are the results of conformational changes within the junctions (extended and folded states) brought about by mechanically induced stretching. A high-low switching factor of 42 was achieved for 1a, whereas an exceptional conductance ratio in excess of 2 orders of magnitude (205) was observed for 1b. To the best of our knowledge, this is the highest experimental on-off conductance switching ratio for a single-molecule junction exploiting the mechanically induced STM-BJ method. Computational studies indicated that the two disparate conductance states observed for 1a and 1b result from mechanically induced conformational changes due to an interplay between conductance and the dihedral angles associated with the electrode-molecule interfaces. Our study reveals the structure-function relationship that determines conductance in such flexible and dynamic systems and promotes the development of a single-molecule variable resistor with high on-off switching factors.
RESUMO
Fabricating single-molecule junctions with asymmetric metal electrodes is significant for realizing single-molecule diodes, but it remains a big challenge. Herein, we develop a z-piezo pulse-modulated scanning tunneling microscopy break junction (STM-BJ) technique to construct a robust asymmetric junction with different metal electrodes. The asymmetric Ag/BPY-EE/Au single-molecule junctions exhibit a middle conductance value in between those of the two individual symmetric metal electrode junctions, which is consistent with the order of calculated energy-dependent transmission coefficient T(E) of the asymmetric junctions at EF. Furthermore, the single-molecule conductance of Ag/BPY-EE/Au decreases by about 70% when reversing the bias voltage from 100 to -100 mV, and a clear asymmetric I-V feature at the single-molecule level is observed for these junctions. This rectifying behavior could be ascribed to a different interfacial coupling of molecules at the two end electrodes, which is confirmed by the different displacement of T(E) at the two bias voltages. Other asymmetric junctions exhibit similar rectifying behavior. The current work provides a feasible way to fabricate hybrid junctions based on asymmetric metal electrodes and investigate their electron transport toward the design of molecular rectifiers.
