| Molecular electronics not only meets the growing technological requirements for miniaturization of conventional silicon-based electronic devices,but also provides a reliable platform for exploring the intrinsic properties of materials at the molecular level,and is one of the research focuses.In general,molecular-scale electronics is a strategy to build functional circuits based on the intrinsic properties of a single or several molecules.Compared with silicon-based electronics,the nanoscale molecular size can greatly increase the capacity and processing speed,providing the ability to exceed the integration limits of traditional silicon-based circuits.Second,the diversity of molecular structures,which can be achieved through flexible chemical synthesis,further enriches the functionality of the devices.In addition,individual molecules,as natural zero-dimensional quantum-limited domain systems,may observe novel physical effects that cannot be observed with conventional materials or methods.The subject of this field is the construction,measurement,and understanding of the current-voltage response of circuits,in which molecular systems play an important role as key elements.Based on this research background,we have conducted the following studies using a graphene-based single-molecule device platform.In Chapter 2,we investigate the intramolecular motion behavior of tetraphenylene molecules with Aggregation-Induced Emission（AIE）properties.Dynamic changes in the steric structure of individual molecules,including double bond isomerization,benzene ring rotation,and formation of cyclization intermediates of the tetraphenylene molecule,are fed back through real-time multilevel current signals,and twisting of the biphenyl molecules at both ends of the molecular bridge is observed in a specific temperature range.Further,the kinetic and thermodynamic correlations of the double bond isomerization are investigated by varying the experimental temperature and bias voltage.The visualization and regulation of the intramolecular motion process are realized,and the mechanism behind the special properties of AIE materials is revealed from the perspective of single molecule level.In Chapter 3,based on the graphene-based single-molecule device fabrication technique,a single-molecule field effect transistor device is constructed by introducing a third electrode,which gives the device excellent gate modulation capability.By introducing a molecule with a spiral ring structure into the single-molecule device,the Coulomb blockade and negative differential conductance phenomena caused by destructive quantum interference of the spiro-conjugated molecule are experimentally observed for the first time,and the regulation of the Coulomb blockade and negative differential conductance phenomena is realized by temperature and gate voltage changes.This work demonstrates the inherent existence of destructive quantum interference in the transport through spiro-conjugated systems,enriching the basic structural units for studying quantum interference effects and effective modules in molecular transistors or rectifiers.In Chapter 4,we apply single-molecule electrical techniques to the detection of short-lived species of organic diradicals,and observe the transformation process of closed-shell structures to open-shell singlet and triplet structures using variable temperature and magnetic,confirming that the diradical NTC-2NH2 has a singlet ground state and a thermally excited triplet state as well as the magnetic field can promote the stability of the triplet structure.This provides a new mean to study the properties of short-lived species of open-shell molecules,which is a guideline for key steps in molecular synthesis and material preparation.In Chapter 5,we investigate the spin transport properties of Fe（II）spin-crossover complexes.By applying different electric fields,the splitting energy of the coordination field is affected,which in turn changes the electron configuration of Fe（II）to achieve the single-molecule spin switches.The regulation of high and low spin states is achieved by using bias voltage and magnetic field,and the kinetic and thermodynamic parameters of the high and low spin states switching process is obtained,demonstrating that increasing bias voltages and magnetic fields can promote the stabilization of low spin state.This work develops a new type of single-molecule spin switch,which is of guidance for the fabrication of future nanoelectronic components.In Chapter 6,we construct graphene-based single-molecule devices with porphyrin and cobalt porphyrin complex molecules as the functional core.The hydrogen tautomerization of single porphyrin molecule cavity was monitored using real-time current signals,and the rate of tautomerization was significantly enhanced by increasing bias voltage.In addition,the Kondo effect caused by the strong correlation of 3d electrons in the metal atoms at the center of the cobalt porphyrin molecule was observed,and the transition from the Kondo valley to the Kondo peak was achieved by applying a magnetic field.Using gradient-changing electric fields,conductance switching due to the charge-state and spin-state changes of cobalt atom is achieved.The high and low spin-state switching process of Co（II）is studied by magnetic field-dependent,temperature-dependent,and bias-dependent experiments,and the corresponding thermodynamic and kinetic parameters are obtained.The study of electronic charge properties and spin properties can promote the development of molecular devices with switching,rectification,and storage functions. |
