First-principles Calculations On The Structures And Electronic Structures Of Graphene Allotropes And Their Nanotubes

Because of its bonding flexibility,carbon-based systems show unlimited number of different structures with an equally large variety of physical properties.Moreover,the development of nanomaterial science has provided a broad stage for carbon allotrope materials with different properties and applications.In this dissertation,by employing the first-principles method,two types of new 2D graphene allotropes and their carbon nanotubes are studied.Especially,we constructed the nanotubes the very first time and investigated systematically their geometric and electronic properties using the ab initio calculation method.We have also simulated a nitrogen doped carbon nanotubes cooperatively with experimental research,in order to study their electrocatalytic properties.In chapter 3,the structural and electronic properties of an allotrope of graphene named T graphene and the T graphene nanotubes are studied by the first-principles calculations.T graphene had been reported to reveal Dirac-like fermions and high Fermi velocity similar to graphene in its buckled phase.However,these Dirac fermions were questioned to be "artificial",caused by band folding under the unstable buckling in T graphene.On the basis of the calculations on T graphene,we report first-principles studies on two types of new T graphene nanotubes which are rolled up from the twodimensional planar sheet of T graphene.The computations show that the 'artificial'Dirac fermions in T graphene are turned into reality in the T graphene nanotubes.Two sets of T graphene nanotubes with different diameters were studied.One set of T graphene nanotubes reveals a semi-metallic property and shows an increasing number of Dirac points with the diameters of the nanotubes.Another set of T graphene nanotubes reveals metallic property.Our studies indicate that rolling up the allotropes of graphene could provide a new avenue for developing new semimetal materials with fascinating properties.In chapter 4,a new graphene allotrope named HOT graphene,containing carbon hexagons,octagons,and tetragons,is studied.Corresponding series of nanotubes are also constructed by rolling up the HOT graphene sheets.The first-principles calculations are performed on the geometric and electronic structures of the HOT graphene and the HOT graphene nanotubes.Dirac cone and high Fermi velocity are achieved in such a non-pure hexagonal structure of HOT graphene,implying that the honeycomb structure is not an indispensable condition for the Dirac fermions to exist.In terms of HOT graphene nanotubes,they show distinctive electronic structures depending on their topology.The(0,1)n(n? 3)HOT graphene nanotubes reveal the characteristics of semimetals,while the other set of(1,0)n HOT graphene nanotubes shows adjustable band gaps(0?0.51 eV)with the nanotube size.The competition between the curvature and the BZ zone-folding effects determine the band gaps of the(1,0)n HOT graphene nanotubes.Novel conversion between semimetallicity and semiconductivity arises in ultra-small nanotubes(radius<4 ?,i.e.,n<3).Carbon nanotubes(CNTs)are of great interest for many potential applications because of their extraordinary electronic,mechanical and structural properties.In chapter 5,the catalytic activities of a series of pyrrolic N-doped,graphitic N-doped,pyridinic N-doped,Co cluster confined and pure carbon nanotubes were calculated.Our co-workers proposed a facile,general and high-yield strategy for the oriented formation of CNTs.Here,we performed a series of first-principles DFT calculations to further understand the experiments,as well as extend and supplement the experimental research.Since none of the pure samples of pyrrolic N-doped,graphitic N-doped or pyridinic N-doped carbon nanotubes could be obtained separately in experiments,it is hard for the experimentists to identify the origin of the outstanding catalytic activities in the systems.However,our theoretical calculations have helped solve this problem.We show that the outstanding catalytic activities originate from both the graphitic N and the confined Co cluster.On the basis of the theoretical simulations and the experimental data,the appropriate graphitic N doping and the confined metal nanoparticles in CNTs are shown to increase the densities of states near the Fermi level and reduce the work functions of the systems,hence,efficiently enhance the oxygen reduction activities.