Theoretical Study Of Electronic Structures And Transport Properties Of Carbon-based Spiral/Helical Nanomaterials

Spiral or helical nanomaterials have great potential applications in optical modulators,mechanical sensors and quantum conductances,etc.Suffering from the limitation of nano preparation technology,the researches concerning spiral or helical nanomaterials made no significant progress in recent year,while the synthesis can be elementarily realized through screw dislocation growth and various chemical cyclizations.Fortunately,the rapid development of nano preparation technology provides the possibilities to fabricate the spiral or helical nanomaterials with controllability.Before that,the theoretical investigation of physics in spiral or helical nanomaterials is of great significance.In this thesis,some spiral or helical nanomaterials are designed to theoretically explore their electronic structure and quantum transport properties using the state-of-the-art density functional theory in combination with non-equilibrium Green's function method(DFT-NEGF).The underlying physics of the strain-induced phenomena and phase transitions are declared and disclosed.The potential application of the designed spiral or helical nanomaterials in soft electronics,nano-solenoids and quantum inductances are further highlighted.The investigati ons primarily focus on graphene spirals,graphene helixes,helical heteronanotubes and spiral sulflower systems.Graphene spirals present both planar structure of graphene monolayer and stacking of graphene multilayer.With theoretical analysis,two primar y electronic interaction are defined in graphene spirals,interlayer coupling and intralayer coupling.Remarkably,three strain-induced phase transitions are observed,containing metal—semiconductor,spin degeneracy—spin polarization and indirect bandgap—direct bandgap transition,originating from the construction and destruction of interlayer p_z electron coupling.The strength of interlayer coupling can be enhanced by the periphery modification and the structural width.Correspondingly,the transport mechanism of graphene spirals is dominated by the interlayer tunneling and intralayer transport,which can be tuned effectively by axial strain.Graphene helixes(GHs)constructed by rolling up graphene nanoribbons present ultra-high mechanical flexibility.With the pitch varying from 1 nm to 6 nm,the deformation energy of graphene helixes is very small,down to 0.06 e V.With the analysis of eletronic density of states and band structures,the elecronic structure of graphene helixes keeps?coupling character similar to that of planar graphene nanoribbons.The axial strain egineering can effectively alter the direct band gap of armchair graphene helixes which still obey the 3p,3p+1 and 3p+2 classification rule.The excellent edge state endows zigzag graphene helixes with inherent advantage in the applications of nano-solenoids and quantum inductance.Chirality is the nature of carbon nanotubes,while the electronic states near Fermi level are achiral.The helical states locate far away from Fermi level.In order to induce the helical state around Fermi level,the helical heteronanotubes are constructed.Summarizing all possible arrangements,the rule of inducing helical interface states is concluded.When the number of zigzag carbon chains is less than4,the BN region will provide enough potential barrier to force the delocalized?obitals distributing along the helical interface.Due to the polarization of BN region,the helical heteronanotubes are classified into parallel and antiparallel category.By contrast,the antiparallel systems show better helical interface states,which provide channels for carrier transport to induce helical current.Similar to graphene spirals,the electronic structure of spiral sulflower show s interlayer coupling and intralayer coupling as well,while the interlayer coupling in spiral sulflower is weaker.Driven by the axial strain,the bandgap of spiral sulflower can be tuned effectively from 0.22 e V to 3.60 e V.However,the dielectric response is limited due to the intrisic indirect bandgap.Additionally,the elastic constant of spiral sulflower is around 4.02?4.28 GPa,demonstrating great applications in tunable soft electronics.