Study On Electrical And Magnetic Properties Of Carbon Electrode Based Molecular Devices

In recent years,research on molecular electronic devices has made significant progress both experimentally and theoretically.Compared with traditional inorganic electronic devices,molecular devices made of organic molecular materials have many advantages,such as light weight,low cost,and large-area production by solution method.Molecular switches,molecular rectifiers,transistors,light-emitting diodes,photoresponsive devices and many other molecular devices have been developed and their performance is constantly improving.Among them,molecular junctions composed of electrode-molecular-electrodes are widely used as the most basic molecular devices for the study of intramolecular charge transport mechanisms.The electrode-molecular interface is a key structure in molecular devices.The metal-organic molecular interface has been an important limiting factor for molecular devices due to the mismatch between metal and molecular properties.This paper focuses on the use of pure carbon electrodes instead of metal electrodes to construct new all-carbon molecular devices.We mainly conducted the following research:One is to prepare a molecular junction device based on a carbon electrode.We used a carbon electrode instead of a conventional metal electrode to obtain a very stable molecular junction device.The bottom electrode is a carbon film obtained by pyrolyzed photoresist film(PPF),the surface is very smooth and the organic molecules can be modified by electrochemical methods.By means of diazo reduction,different organic molecules can be covalently modified on the bottom carbon electrode to form a stable and dense molecular layer.Finally,a top carbon electrode is prepared by thermal evaporation on the molecular layer to form a sandwich structure of the carbon electrodeorganic molecular layer-carbon electrode.By analyzing the IV curve of the carbon molecular junction,it is known that the current in the molecular junction is transmitted by tunneling.The second is to study the effect of electric field on the Fermi level of PPF electrode.In the carbon molecular junction,the current through the molecular junction is the tunneling current,and the decisive factor affecting the tunneling current is the tunneling barrier at the interface between the electrode and the molecular(the difference between the electrode Fermi level and the molecular energy level).If the Fermi level of the electrode can be regulated,then the size of the tunneling barrier is changed,and the current in the molecular junction is changed to achieve active control of the molecular junction.According to the literature,the electronic density of PPF electrodes is very small compared to metals.When an electric field is applied,the induced charge is very likely to change the Fermi level of PPF.We used the Kelvin probe force microscope(KPFM)method to measure the surface work function of the PPF electrode,and studied the change of the Fermi level of the PPF electrode by controlling the gate voltage.The third is to study the electrical properties of the magnetic response of carbon molecular junctions.When the charge is transferred in the organic molecule,due to the hyperfine interaction of the molecules,the charge transfer will respond to the external magnetic field,forming an organic magnetoresistance(OMAR)effect.Compared to conventional metal-electrode organic magnetoresistive devices,our carbon electrode devices are very stable over a wide temperature range,providing a good prerequisite for magnetoresistive testing at low temperatures.We studied the low temperature of a carbon electrode molecular junction and the current change under a magnetic field.After applying a strong magnetic field to the carbon molecular junction,we observed that the resistance of the carbon molecular junction became large and had a positive magnetoresistance characteristic symmetric with respect to the positive and negative magnetic fields under the magnetic field.