Study On The Field Effect And Kinetics In Graphene-Based Single-Molecule Junctions

The miniaturization of transistors is one of the key factors to improving chip performance.However,as the size of transistors decreasing,they face the physical limitations of Moore’s Law and enormous cost consumption,resulting in a slow process of miniaturization.Transistors constructed from a single molecule are expected to become important components of miniaturized chips in the future,because the size of single-molecule transistors can reach sub-nanometer scale,which fundamentally breaks through the physical limits of the development of Moore’s Law and opens up a new direction for chip development.In this paper,an array of graphene nano-point electrodes was prepared by micro-nano processing.A stable single-molecule electronic device was constructed by connecting a single molecule to the graphene nano-electrode pairs through amide covalent bonds.The essential law and the regulation law of the physical-chemical reaction process were studied at single-molecule scale.While avoiding the average effect of the ensemble system,the multifunctional exploration of single-molecule devices is further realized.The research work of this paper is divided into three parts:1.Single molecule field effect transistor modulated by ionic liquid gate.The Ideal FET with practical application value should have the advantages of stability at room temperature and high performance,i.e.,strong grid adjustment ability,high device switch on/off ratio and low power consumption.However,most single-molecule field-effect transistors are implemented at low temperatures,and their performance at room temperature is mediocre.Although previously reported graphene-based single-molecule field-effect transistors can achieve bipolar control at room temperature,due to poor molecular conductivity and electron cloud separation,the efficiency of gate electrodes in molecular orbital control is generally not high,and the switch on/off ratio is poor.Around these key issues,we have successfully constructed graphene-based single-molecule field-effect transistors using porphyrin molecule and octahedral zinc metal complex molecular system in our experiments.The on-state current of the devices is improved by increasing the conjugation degree of the whole molecule and increasing the conductivity by introducing metal atoms.For the transistor using porphyrin,an on/off ratio about4800 and a subthreshold swing about 179 mV/dec are realized.In the experiment,the molecular frontier orbital energy level is effectively controlled by the ionic liquid gate,which improves the performance parameters such as the switch on/off ratio of the device.2.Dynamic monitoring of hydrogen migration in single molecule porphyrin.Free-base porphyrins are dark red crystalline compounds that play an important role in photosynthesis,metal coordination chemistry and other reactions.Two hydrogen atoms can migrate in a framework of four nitrogen sites to produce internal hydrogen migration.Intramolecular hydrogen migration of porphyrins has been extensively studied under scanning tunneling microscopy and in mechanically controlled fracture junction systems.However,the rapid rate of hydrogen migration of porphyrins limits the observation and application of porphyrins in conventional environments.Based on the platform of single-molecule devices with graphene electrodes,we found that porphyrin molecules have obvious bias-dependent conductive bistable and conductive tetrastable switching in the range of liquid nitrogen temperature and room temperature.The activation energy of the reaction fitted by Arrhenius Formula was close to the theoretical activation energy of the hydrogen migration reaction,which verifies that the above phenomenon comes from the hydrogen migration process inside the porphyrin molecule.Furthermore,by simulating the transmission spectra of each hydrogen migration configuration of porphyrin molecule,we assigned each conductive state reasonably and clarified the hydrogen migration rule in graphene-based single molecule porphyrin devices.3.Dynamic monitoring of redox process of metal coordination molecules.Switching between high and low states of devices has a very important application in signal processing and storage.In this part,the high and low conductivity state switching is realized through the metal redox reaction of octahedral molecules coordinated by metal iron.Because the electron and spin density around the metal atom in the octahedral metal coordination molecule are highly localized in electricity and space,it becomes a large quantum dot system;In addition,the metal atom is placed in the center of the molecular skeleton to maximize the interaction between the molecule and the electric field,leading to increasing of the device conductivity.In this paper,the iron complex molecules were connected to graphenebased single molecule devices through amide bonds.The bistable conductivity switching caused by the oxidation-reduction of iron atoms was found in the voltampere characteristic curve and current-time trajectory diagram;The negative differential resistance effect and bias dependence in different devices were attributed to the different angle of "cardan joint" in the molecular structure.