Carbon-Based Nanoporous Materials Structural Design and Physical Properties:First-Principles Studies
Author:Chen Shi Chang
Supervisor:chen ke qiu
With the advancement of science and technology,our demand for new functional materials is increasing with each passing day.Carbon,one of the most abundant elements on the Earth,has formed countless allotropes,due to its three flexible hybridizations,namely sp,sp~2 and sp~3.Since the ancient times,carbon materials have been favored by people because of their environmental friendly,light weight,high strength,and fascinating electrical and optical properties,and have been widely used in semiconductor devices,optical devices,batteries,hydrogen storage,and filtration.Carbon-based nanoporous materials have been widely concerned due to their porosity,low mass density and rich mechanical properties,and have been applied in lithium ion batteries,supercapacitors and catalysis.Although many nanoporous carbon materials have been proposed and successfully prepared,they are far from meeting our needs.In this thesis,we design one-and three-dimensional porous carbon foams based on multi-wing graphene nanoribbons,as well as a series of two-dimensional graphyne structures,and reveal the stabilities,electronic properties and the regulatory mechanisms of these structures based on first-principles calculations.Firstly,based on the Zig(1,1)carbon foam,we constructed porous carbon foam nanowires and conducted a density functional theory study.We found that carbon nanowires have higher stability relative to their bulk materials,and form a topological surface protection mechanism.At the same time,the charge concentrates on the surface of the nanowire to form a surface state,which causes the topologically protected metallicity of the carbon foam nanowires.By hydrogenating the surface of the nanowire in different ways,we find that the surface state is destroyed,opening a band Gap of 0.15-1.5 eV.Most surprisingly,when hydrogen is adsorbed on the top side of the nanowire,the metallicity is enhanced.Molecular dynamics simulations have shown that carbon nanowires are extremely stable at room temperature,with a melting point of 2200 K.When the temperature reaches 3700 K,carbon nanowires are fused into irregular multi-walled carbon nanotubes,which provide a new insight of synthesizing multi-walled carbon nanotubes.Secondly,we propose aγ-carbon foam structure based on six-wing graphene nanoribbons,and predict its stability and electronic structure based on the first principle calculations.We found that although the three-wing and six-wing graphene nanoribbons are comparable in formation energy,the binding energy ofγ-carbon foam is higher compared to the conventional carbon foam,which means a better chemical stability;and its dynamics stability and mechanical stability has been verified.The mass density and mechanical properties decrease exponentially with the increase of the pore size of the foam.Most interestingly,these three-dimensional carbon foams well inherit the size-dependent electronic properties of graphene nanoribbons.This provides a new idea for designing three-dimensional structures with the excellent electronic properties of graphene nanoribbons.Next,in order to verify whether the electronic properties of GNRs are inherited in all three-dimensional carbon foam structures,we designed six types of carbon foams based on multi-winged graphene nanoribbons.Based on these structures,porous carbon foam structures with asymmetric and multilayer graphene were designed;and all of these carbon foams were uniformly named.It has been found that the symmetrical carbon foam of single-layered GNRs meets our expectation.In asymmetrical structures,this size-dependent electronic structural characteristic is destroyed;in a multilayer GNRs foam structures,the electron The expectation is strongly depend on the interlayer spacing between GNRs:when the distance is>3.35?(graphene layer spacing in graphite),the idea is achieved;otherwise,it is destroyed.We also systematically analyzed the chemical,kinetic and mechanical stability of these structures and confirmed that these structures are stable;at the same time,it was found by X-ray diffraction spectroscopies that some of the structures can be found in the ultrathin-graphite foam,detonation soot of TNT and diesel oil,and chimney soot,which further confirms the stabilities of these structures.Finally,we propose an orthorhombic R(m,n)L(x,y)-graphyne based on R-graphyne.The phonon spectrum studies indicate that this new type of graphyne structures are dynamically stable.The most interesting is their electronic properties:When(m,n)are even,the valence band and conduction band of graphynes form round-cornered rectangles like low-energy behaviors in the Brillouin zone,accompanied by multiple anisotropic Dirac points.At the same time,with the increase of(m,n),the diameters of the rounded rectangles gradually increase and intersect,leading to different shapes of low-energy behaviors in the first Brillouin zone,and the reconstructions happened in the intersection points.Increasing the length of the acetylene chain in the graphyne will change the radius of the rounded rectangle,but the low energy behavior and the Dirac point are still maintaied;meanwhile,the tensile or compressive stress will not affect the characteristics of this Dirac fermion.However,when any one of(m,n)is an odd number,the system exhibits a metal feature.Our works will provide the theoretical basis for experimental preparation of two-dimensional carbon Dirac materials.