| Low-dimensional π-electron conjugated systems exhibit a rich variety of unique and practical physical properties, which make them ideal materials for applications in the fields of nano-electronic technologies, energy conversion, information storage and spintronics. The interface coupling plays an important role in their application. These interactions can be weak van der Vaals interaction or through appropriate metal atoms or functional groups which may cause profound effect on their electronic structures and transport properties. Meanwhile, the electron spin polarization and magnetic coupling of π-conjugated system are crucial for the applications in spintronics devices.Quantum transport properties of materials at the atomic scale will affect the ultimate limitation size of function devices. The nonlinear, negative differential conductance, rectifying Ⅰ-Ⅴ characteristics of nano material systems are closely related to the device performance. In this thesis, we performed first principle calculation based on density functional theory (DFT) to study the electronic structure of two typical π-electron conjugated systems, as well as the electron transport properties using non-equilibrium green’s function method. The interface coupling, magnetic coupling and hydrogen modified have been considered to tune the electronic properties and transport properties. The π-electron conjugated systems involved in our works include carbon nanotube, graphene spirals, graphyne and two-dimensional metal-organic frameworks. The aim of this work is to reveal the roles of these couplings in tuning the electronic properties of the π-electron conjugated systems, offering theoretical references for the applications in nanoscaled electronic devices.The Ph.D. Thesis mainly includes the following parts:(1) Single-walled carbon nanotubes (SWNTs) are very attractive for electronic devices applications due to their superior geometry, good mechanical and extraordinary thermal conductivity. These properties make them potential building blocks of carbon-based nano-electronic devices. Using first-principles calculations, we investigate the energetically most favorable coupling patterns and the electronic structures of SWNT monolayers and bilayers formed by super-aligned (5,5) and (7,0) SWNTs. It is found that the (5,5) SWNT monolayer prefers a "face-by-face" stacking pattern with the binding energy of 13.90 meV/atom, whereas the (7,0) SWNT monolayer favors an "edge-by-edge" pattern with the binding energy of 10.82 meV/atom. The (5,5) SWNT arrays are semiconducting with a band gap up to 114 meV for the bilayer, while the (7,0) SWNT arrays are metallic with a tiny overlap between valence and conduction bands. A wide range of conductivities, from 12.5 S/cm to 6600 S/cm, have been reported in CNT films. This is due to the poor electronic tunneling at inter-tube junction. The influence of the metal-(η6-SWNT) interconnects in the electrical conductivity of SWNT film has been reported in recent experiments. Using non-equilibrium Green’s function with density function theory, we performed theoretical calculations on the electron transport properties of (Cr, Li, Au)-SWNT systems. We revealed the roles of transition metal Cr, alkalis metal Li and inert metal Au in improving the electrical conductance of metal-SWNT systems. Our calculated results show that transport properties along the inter-tube direction are strongly dependent on the connecting metal atoms varying over many orders of magnitudes. Gold atoms fail to enhance the electrical conductance of SWNT systems. Meanwhile, negative differential resistances are demonstrated in semiconducting inter-tube models, which would have potential applications in the electronic device. Our results provide a promising way to optimize the performance of SWNT based networks.(2) Tensile strain is quite efficient in tuning the electronic properties of a material. We investigated the electronic structures and electronic transports of graphene spirals in response to external strain. It was found that tensile strain can drastically modify the electronic properties of graphene spirals. When the axial tensile strain exceeds an ultimate strain, the graphene spirals convert from a metal to a magnetic semiconductor with stable ferromagnetic ordering along the helical edges. Stress also can effectively regulate its conductive properties. More interestingly, the negative differential conductance region is sensitive to external strain and moves to the low voltage region with the increase of tensile strain. These interesting properties are quite promising for applications in nanoscaled electronic devices, such as pressure sensor, transistor, high-speed switching data storage and so on.(3) As a typical π-conjugated macrocyclic system, porphyrin (PP) exhibits rich coordination chemistry with various metal ions. We investigated the electronic structure of metal-free 2D PP nanosheet and MOFs composing of PP and transition metal atoms. We found that metal-free 2D PP sheet has a spin-polarized ground state with a local magnetic moment of about 1μB per unit cell. We correlated the spin-polarization with the nonequivalence of two sublattices of the cycloocatetraene (COT) cores. Cr-polyporphyrin (Cr-PP) in particular has stable ferromagnetic ordering with a Curie temperature (Tc) of about 187 K as indicated by the Monte Carlo simulations based on 2D Ising model, which is much higher than that reported in 2D Mn-phthalocyanine framework. The ferromagnetic Cr-PP nanosheet can be tuned to half-metallic by electron doping. These interesting results opens up an avenue for the development of 2D organic nanostructures with stable ferromagnetism and half-metallicity. Another 2D π-conjugated metal organic framework is the metal-bis(dithiolene) with Kagome lattice. We investigated the electronic properties of TM-bis(dithiolene) with TM=Cr, Fe, Co, Ni, using first principles calculations. It was found that Cr-, Fe-and Co-bis(dithiolene) exhibit spin polarized with large splitting energies of 5.39,6.44,6.02eV, respectively. Fe-and Co-bis(dithiolene) exhibit ferromagnetic(FM) ground states and half metal feature. Monto Carlo simulation on the basis of the Ising model suggest that it has a Curie temperature of about 305 K for Co-bis(dithiolene), indicating a potential application in room temperature. Moreover, we predicted a frustration magnetic state with S=3/2 on Kagome lattice vertices in Cr-bis(dithiolene) which is 0.3 eV more stable than its FM state in energy, suggesting a promising candidate for two dimensional spin-liquid materials.(4) The electronic band structures of hydrogenated graphyne were investigates using first-principle calculation. Our result show that the atomic structures and electronic properties of hydrogenated graphynes are very sensitive to the hydrogen coverage. At low coverage, hydrogen atoms favor to chemically adsorb on the sp* hybridization carbon atoms in the chains rather than on those sp hybridization in the hexagons. We also found that the hydrogen atoms adsorbed on graphyne is easier than that on graphene. Furthermore, we discuss the stability of a new hydrocarbon structure which composed of sp3 and sp2 hybridized carbon atoms. |
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