Study Of Response To External Field Of Electronic Transport Of Organic Molecular Devices

With the continuous miniaturization of electronic devices and the application andmaturation of manipulation technology in micro scale, such as Scanning Tunneling Microscope(abbreviated as STM), Atomic Force Microscope (abbreviated as AFM) and so on, the molecularelectronics get extensive concern and make enormous progress in recent decades. Due to thedistinctive advantages, organic molecular devices are considered as one of the most promisingmaterials to replace the Si-based semiconductor devices. The scientists desire to design andmanufacture kinds of organic molecular devices corresponding to the conventional electronicdevices. As one of the most important element devices in circuit, molecular field effect transistoris a molecular device of great expectation. Seeking functional molecular devices and realizingthe control of device function with external field has been an untiringly pursued goal. Recently,the field of molecular field effect transistor has experienced rapid development bothexperimentally and theoretically. However, the investigations mostly concentrate on the effort torealize the control and characterization of electronic transport with external field, comprehensiveunderstanding of external electric field effects still needs and then molecular devices responding toexternal field efficiently can be found to reach the practical use.Moreover, most studies are based on homometallic molecular junctions. Constructingheterometallic molecular junctions was once an insuperable impediment. Nevertheless, in2009,Chen et al had overcome the difficulty. They successfully prepared heterometallic molecularjunctions, and investigated their rectification properties. However, relevant theoretical workneeds to do to understand the mechanism of rectification and its influence factors.Based previous studies and existing problems, the first part of this job is investigating theresponse to gate electric field of organic conjugated molecules, by density functional theorycombining elastic scattering Green’s function method. The second one is calculating themechanism and influence factors of the rectifying behavior of heterometallic molecular junctionsusing nonequilibrium Green’s function based on density functional theory. The detail work is asfollowing:Firstly, we chose a series of organic conjugated molecules containing rich π electrons, suchas aromatic compounds, condensed ring and heterocyclic ring systems, to construct moleculardevices and investigate their response to gate electric field. The computational results show thatalthough the aromatic compounds and condensed ring systems are rich of π electrons, advantageous to conduction, their response to gate electric field are not remarkable. Theheterocyclic ring systems are also rich of π electrons, however, the doping of N and S atomsleads to great electric dipole moments parallel the gate direction, which can make noticeableresponse to gate electric field. Especially for2,5-dimethyl-thiophene-dithiol junction, it hasredox centers and relatively large dipole moment parallel gate direction, and can respond to thegate electric field remarkably as well, which consists with experimental conclusion. Also, itscurrent-voltage properties have intense monotonous response to gate external electric field,distinct from those of others. The evolution of eigenvalues can reveal its different current-voltageproperties with gate electric field to some extent. In addition, the coupling energy between themolecules and electrodes can also explain the response of current to gate electric field to acertain extent. By analyzing the evolution of atomic charges with gate electric field, we find thatthe top part of2,5-dimethyl-thiophene-dithiol containing S atom makes the main contribution toits electronic transport, showing an N-channel-metal-oxide-semiconductor-like character. Itsconductance can be controlled to be high or low state by adjusting the polarity and magnitude ofgate electric field. So, it may be a good candidate for molecular devices, such as field-effecttransistors.Secondly, based on the experimental investigation of Chen et al, we built the models ofheterometallic molecular junctions and study their rectification mechanism and influence factors.The results show that the rectifying behavior of heterometallic molecular junctions is derivedfrom the different coupling interactions between the molecule and electrodes. For the molecularjunction of whom the HOMO makes major contribution to the conductance, the internal energylevels of molecule is determined by the Fermi energy of one electrode which couples with themolecule intensely. Thus, the energy levels of the molecule shift differently under positive andnegative bias voltage, resulting in rectification. In addition, we further studied the effect ofdifferent metallic electrodes, terminal groups, chain length of molecules and distance betweentwo electrodes to rectifying behavior. The results show that the rectifying behavior ofheterometallic molecular junctions has no direct relationship with the work function of metallicelectrodes. Instead, it has relationship with the outer-electron structures of metallic electrodes.The metallic electrodes with greatly different electron configurations couple with the molecule inunlike ways, so remarkable rectifying behavior occurs. Conversely, the metallic electrodes withsimilar electron configurations couple with the molecule in similar ways, thus no obviousrectifying behavior emerges. Also, the terminal groups can visibly influence the couplinginteraction between the molecule and electrodes, and then change the current-voltage propertiesof the system to large extent, even its direction of rectification. In our calculated systems, we find that the chain length can not change the electronic properties of systems essentially.Increasing or decreasing the chain length of molecule affects the current weakly. However, toolong or too short chain length is adverse to improvement of rectification ratio. Finally, thevariation of the distance between electrodes can lead to marked change. The molecular junctionwith equilibrium configuration has the most remarkable rectifying behavior. With stretching thejunction by certain distance, the current through molecule increases firstly and then decreasesafterwards, and even presents negative differential conductance behavior.This thesis consists of five chapters. The first chapter is the introduction, where we introducethe organic molecular devices briefly and then emphasize the development and current situationof the response of molecular devices to external field. The second chapter describes theorymethod, including density functional theory to get electronic structures, elastic scattering Green’sfunction method and nonequilibrium Green’s function both of which are used to obtain theelectronic transport properties. Chapter three and four introduce the main work during the studyfor Mater’s degree, covering the investigation of response to gate electric field of moleculardevices and the mechanism and influence factors of rectification of heterometallic molecularjunctions. The last chapter is the overview and prospective of this thesis.