Electrochemical Gating Of Charge Transport Through Single-molecule Junctions Using Mechanically Controllable Break Junction Technique

Manipulating charge transport at the single molecular level has drew considerable attentions because it not only provides a promising technical proposal for miniaturization of the electronic components,but also enables researchers to explore novel phenomena at molecular level that differ from the macroscopic system.Besides,the intrinsic properties at molecular level ean act as the guidance for designing high-performance semiconductor materials.Among the tuning approaches,electrochemical gating is a facile and effective way for manipulating the charge transport through the single-molecule junctions.However,there are still quite a lot of problems which need to be solved.With regard to the device level,one of the challenges is the fabrication of electrochemically integrated molecular devices.Electrochemical gating is mainly based on scanning probing microscope break junction(STMBJ)technique in previous reports.Considering the fact that the tip is separated from the substrate,it can only be used as an investigation tool rather than molecular devices for integration.The other challenge is the performance of molecular devices.On/Off ratio is the key parameter for evaluating the performance of both single-molecule electrochemical switch and single-molecule electrochemical transistor,which need to be improved in future studies.Considering above issues,we had carried out works in three aspects as follows:1.We developed a novel technique for measuring the single-molecule electronic devices upon electrochemically integrated coated tip-bead(ECTB)chip.After improving the mechanically controllable break junction(MCBJ)setup,the electrochemical gating can be successfully employed upon MCBJ techniques.2.We fabricated single-molecule electrochemical switch based on tetrathiafulvalene(TTF)derivative through ECTB chip.The TTF is nonaromatic group due to the non-coplanar structure,which leads to a low molecular conductance at neutral state before electrochemical gating.However,the aromaticity of TTF group can be achieved by electrochemically oxidizing and the molecular conductance will be enhanced significantly.We found that the switch ratio of the single-molecule electrochemical switch based on TTF derivative can reach up to more than two orders of magnitude.3.We realized the experimental manipulation and observation of destructive quantum interference(DQI)under ambient conditions.During the electrochemical gating of thiophene derivative with DQI effect,we found that the molecular conductance can be varied by almost two orders of magnitude with variation of electrode potentials in nonfaradaic region,which is significantly higher than that of thiophene derivative without DQI,indicating the in-situ fine tuning of DQI effect can be realized through electrochemical gating in nonfaradaic region without changing molecular structure.Accordingly,we successfully fabricated the first single-molecule electrochemical transistor device based on DQI with significantly high switch ratio of about 100.More importantly,we found the minimum molecular conductance with the variation of electrode potentials in nonfadadaic region.According to several additional evidences from cyclie voltammetry measurements,current-electrode potential measurements,current-voltage measurements and theoretical calculation,we concluded that this minimum molecular conductance originates from surpassing of the DQI dip at the Fermi energy through electrochemical gating,thus providing direct evidence for charge transport controlled by an anti-resonance arising from DQI at room temperature.