Study On Electronic Transport Properties Of Aromatic Molecules

One of research topics in nanotechnology and molecular electronics is constructing photo-electronic devices with functional single molecule. Molecular devices refer to the devices sized in nanoscale with a specific functionality that are built up by a single or a few molecules. The materials of the devices are different, such as nanowires, nanotubes, nanoparticles, small organic molecules, biological molecules, DNA and so on. In recent years, measurements and studies on electrical properties of single molecule from experiments and theories have played an important role in molecular electronics. The development of social productive forces will be greatly promoted, once the research results were put into application. Aromatic molecules have good electrical properties because of containingπ-electrons which can move freely in the molecular, which are chosen as the prototype candidates. The present work uses density functional theory combined with elastic scattering Green's function theory to study the electronic transport properties of aromatic molecules theoretically.We simulate the electrodes by tetrahedral gold atoms clusters, and molecules are sandwiched between two gold clusters to form an extended molecular system. First, we study the planarity and length impact on the electronic transport properties. We choose 4,4'-diaminobipheny(C12H12N2) and 4,4''-diamino-p-terphenyl(C18H16N2) to study their electronic transport properties. Through the analysis of their geometrical structures, extended molecular I-V curves, electronic transport spectra and molecular orbital electronic distributions, we find that the conductance decreases when the aromatic ring is twisted. The longer the aromatic molecule, the lower the current at the same bias. It means that the appearance of the twist and the increase of molecular length are disadvantaged factors to electronic transport.Secondly, we study the impact from different number of aromatic rings and length of molecules on the electronic transport properties. We choose the aromatic molecules 1,4-diaminobenzene(C6H8N2) , 1,4-diaminonaphthalene(C10H10N2) ,9,10-diaminoanthracene(C14H12N2) , 2,6-diaminonaphthalene(C10H10N2) ,2,6-diaminoanthracene(C14H12N2 ) as our models. Through the analysis of the extended molecular geometrical structures, electronic transport spectra, energy levels, molecular orbital electronic distributions and molecular conductance, we find that as the number of aromatic molecules increases, the energy of highest occupied molecular orbital (HOMO) rises, HOMO-LUMO(lowest unoccupied molecular orbitals) gap decreases, and electron tunneling becomes easy and the set-on voltage is lower. The conductance increases with the increase of the number of the aromatic ring when the molecular length keeps the same. When the molecular length is also increased, however, its conductance decreases. It means that in a certain extent, the influence of molecular length is greater than that of number of aromatic rings on the conductance.Thirdly, we study the isomeric compound of aromatic molecules influence on the electronic transport properties. Four isomers of C10H10N2 are chosen, which are 1,5-diaminonaphthalene , 2,6-diaminonaphthalene , 1,4-diaminonaphthalene , 2,7-diaminonaphthalene.We discuss the molecular structure impact on electronic transport properties, and study the molecular electronic transport spectrum, and compare the local molecular orbital of extended molecular orbital (LUMO + 2). The first electronic tunneling peak is attributed to the molecular orbital LUMO + 2.This paper consists of seven chapters: the first chapter gives a brief introduction of molecular electronics, including research significance and the existing problems. The second chapter introduces the single-particle approximation of multi-particle systems, including Born-Oppenheimer approximation, Hartee-Fock approximation, and density functional theory. The third chapter mainly introduces the elastic scattering green's function method for calculating current-voltage characteristic of molecular junctions. Chapter 4, 5 and 6 contain the simulation results and discussions. Chapter 7 summarizes the research and prospects the future development of molecular electronics.