The Construction Of Asymmetric Metal Junction And Study Of Single Molecule Rectification Effects

Creating functional electrical devices using individual or ensemble molecules,not only meets the increasing technical demands of the miniaturization of traditional Si-based electronic devices,but also provides an ideal window of exploring the intrinsic properties of materials at the molecular level.Since the early 1970 s,wiring single molecules to two metallic electrodes has been proposed and studied.Among them,molecular rectifier has received much attention,for it has current rectification effect on the molecular scale,which facilitates the flow of current in one bias direction and suppresses current flow in the opposite direction.To date,people have found that the physical asymmetry is a key factor to realize the rectification function in single-molecule electronics.Thus,quite a few molecules with asymmetric anchoring groups or molecular backbones have been designed and investigated to achieve current rectification.However,obtaining such intrinsic structural asymmetries of molecules requires complicated chemical synthesis,and experimental verifications are difficult.An alternative way for constructing these asymmetric molecular junctions is to use asymmetric electrodes.Recently,a few studies have proved the noticeable rectifying effect in self-assembled monolayers at asymmetric molecule-electrode interfaces using metal and semiconductor electrodes.Unfortunately,constructing asymmetric molecular junctions with dissimilar metal electrodes remains a great challenge at a single molecular level due to technical difficulty.Based on the above discussion,we develop a z-piezo pulse-modulated STM-BJ technique to efficiently fabricate asymmetric molecular junction,involving Au,Ag and Pt dissimilar electrodes.We measured the conductance and current-voltage curves of different hybrid junctions under positive and negative bias voltages,and studied their rectifying effects at the single-molecule level.The mechanism of electron transport and rectification caused by electrode asymmetry are discussed.In addition,the influence of the anchoring group on single molecular was also studied.In this thesis,we selected diphenylacetylene and anthraquinone molecules with different number and position of anchoring groups to further explore the regulation of anchoring groups on quantum transport of single molecules.The work is as follows:1.We develop a z-piezo pulse-modulated STM-BJ technique to efficiently fabricate asymmetric molecular junction with Au and Ag dissimilar electrodes.The reliability of asymmetric metal junctions is verified by I(s)technique.At the same time,4,4?-vinylenedipyridine(BPY-EE)was used as the target molecule to systematically investigate the conductance of different metal junctions.We find the conductance value follows the order of symmetric Ag junction < asymmetric Ag and Au junction <symmetric Au junction,which is consistent with the calculated energy-dependent transmission coefficient T(E).The current work provides a feasible way to fabricate hybrid junctions based on asymmetric metal electrodes.At the same time,the electron transport of molecular junction with different metal electrodes are revealed.2.Using the z-piezo pulse-modulated STM-BJ,we further explore the different types of asymmetric molecular junctions,named Ag-para-BT-Au and Pt-BPY-EE-Au hybrid junctions.We have measured the conductance and current-voltage curves of different hybrid junctions under positive and negative bias voltages,then discuss the mechanism of electron transmission and rectification caused by electrode asymmetry.The single-molecule conductance of Ag/BPY-EE/Au decreases by about 70% when reversing the bias voltage from 100 m V to-100 m V,and a clear asymmetric I-V feature 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 T(E)at the two bias voltages.Other asymmetric junctions exhibit similar rectifying behavior.The current work deepens the understanding of electron junctions,and provides a feasible way to design molecular rectifier in the future.3.Employing traditional STM break junction(STM-BJ)technique,diphenylacetylene and anthraquinone based molecules as the target molecules,We study the influence of the number and position of different anchoring groups on molecular electron transport of single molecular junction.The results show that the conductance is the larger for diphenylacetylene molecule with the anchor group in the para position,while the conductance is small for diphenylacetylene molecule with meta position.This effect can be attributed to the influence of quantum interference.At the same time,the conductance increases with the increasing anchoring groups,which may be caused by the increase in electron transport paths.For anthraquinone molecules,no conductivity peak can be detected for anchoring group at 1,8 position,while the obvious conductivity peak can be seen for anchoring group at 2,6 position and 1,5 position.This phenomenon is similar to the results reported in the literature,which proves that destructive quantum interference does not exist for those positions.