Development And Application Of Single-molecule Electrical Characterization Technology Based On AC Signals

The development of single-molecule electrical characterization technology has greatly promoted the study on the electrical transport properties and the electric field effects of single-molecule.However,most of the current studies on the electrical properties of single molecules are based on the characterization analysis of DC(Direct Current)signals,and there are relatively few studies on the electrical properties related to AC(Alternate Current)signals.For example,from the perspective of device development and manufacturing,the current research on molecular impedance characteristics is very scarce,and these research reports did not accurately locate the impedance analysis to a single molecule.This situation indicates that the present single-molecule devices we could make will not be able to process complex AC signals,nor will they be able to deal with the impedance effects in the integration process.Meanwhile,due to the restriction of the bandwidth of current amplifiers,present measuring instruments can only reach the time resolution of milliseconds or microseconds,which makes most of the research on single-molecule electronics can only be restricted to the measurement of steady or low speed dynamic processes.And the lack of effective data acquisition and analysis means at faster time scales also makes it difficult to effectively study high-speed dynamic events like configuration conversion,charge transfer or rapid redox reactions at molecular scale.All these problems hinder the comprehensive understanding and effective regulation of the mechanism of single molecule electrical transport.Aiming at these technical challenges,this paper has successfully measured and studied the impedance properties of molecules and the high-speed dynamic process in break junction experiment by building a single-molecule electronic measuring instrument and combining it with the electrical characterization technology based on AC signal.The research content of this thesis are as follows:Firstly,we wrote the instrument control program based on Lab VIEW software development platform and FPGA hardware platform,built the single molecular electronics measuring instrument based on scanning tunneling microscopy break junction technology.And after that,we proved the feasibility and stability of the instrument through a series of molecular tests.Secondly,we constructed a molecular impedance measuring instruction based on dynamic break junction technology by combining AC signal excitation with the completed measurement instrument.The principle is to apply specific frequency AC excitations to the molecule when the molecular junction is formed,and then analyze the phase-frequency response to characterize the impedance characteristics of molecules.Compared with previous studies,this technique can effectively analyze the impedance characteristic of a single molecule,and the experimental process is relatively more flexible.Subsequently,we combined the single-molecule electronics measuring instrument with the RF(Radio Frequency)signal measuring circuit to measure the RF signals in atomic point contacts and molecular junctions.With the help of highfrequency RF signals,the time resolution of the single-molecule electronics measuring instrument was effectively improved.We successfully observed the atomic sliding rearrangement at the instant of point contact fracture of gold atoms with nanosecond temporal resolution.And we also realized the observation of rapid processes such as the vibration and fracture of molecular junction for the first time.