First-principles Study Of The Low-dimensional Metal-π Electron Systems

Low-dimensional metal-π systems exhibit a rich variety of unique physicalphenomena, which make them ideal materials for the applications inmicroelectronictechnologies, information processing and storage, and clean energy. Inthis dissertation, we investigate the geometric, electronic and magnetic properties ofone-dimensional organometallic sandwich molecular wires, two-dimentionalgraphene/metal/SiC(0001) intercalation-like systems and zero-dimensionalmetal-attached fullerene.Complex organometallic sandwich molecular wires (SMWs) are demonstrated tobe a promising source ofhalf-metallic materials because of wide variety of sources andtunable properties. The half-metallicityand ferromagnetism/antiferromagnetism ofcomplex SMWs are found to originate from the couplingbetween d electrons of TM andTM’ atoms via the ligands, which determines the filling ofthe d bands of TMs. Ageneral “competitive charger-transfer” model is proposed to predict thehalf-metallicityin complex TM’-TMnCpn+1SMWs. Furthermore, a series of robusthalf-metallicferromagnets, as well as a narrow gap antiferromagnet, are identified usingab initio calculationscombined with the model analysis. On the other hand, we design aspintronic device by self-assembling molecular magnet on graphene.It is highly desirable to prepare large-scale graphene and integrate it into thecurrent semiconductor technology. We find Lithium intercalation will induce thedecoupling of epitaxial graphene on SiC(0001) and thus recover the character ofquasi-free-standing graphene. The decoupling mechanism and dynamic process ofinsertion are investigated, and the results agree well with the experiments. Whereas intransition metal (TM) intercalated cases, we reveal the unexpected Dirac spectrum instrongly bound graphene systems. The hybridization betweenthe graphene states andthose of the modulated TM on SiC leads to the migration of Dirac fermionfromgraphene to TM d-bands, resulting in d-state Dirac fermions. The Dirac spectrum is,otherwise,isotropic and almost identical to that of the free-standing graphene for bias aslarge as0.6V. We study the feasibility of the Ca decorated carbon or carbon-boron fullerenes andalkali-metal (Li, Na and K) decorated boron fullerenes for hydrogen storage. It is foundthat the metal atoms are strongly attached at the centers of n-membered rings, formingstable metal-fullerene complexes with no metal clustering.The capacity of suchcomplexes in hydrogen storage is depicted by the gravimetric densities and bindingenergies of hydrogen. Furthermore, we discuss the hydrogen binding mechanismandrecyclability of the systems.