First-principles Studies On The Fluorination Of Four Atomic Chains And Fluorinated Graphene

Atomic chains have different physical properties from bulk materials,and their electronic transport properties exhibit new quantum behaviors.Therefore,the quantum transport properties and applications of atomic chains have attracted widespread attention from researchers.It has been found that some non-metallic atom chains have better transport properties than metallic atom chains,such as C and Si.Moreover,the conductivity of Si-chains with different lengths exhibits odd-even oscillations.For this reason,the charge distribution and mechanical behavior of some non-metallic atom chains are discussed and predicted.In addition to the above atomic chain nano-materials,an another interesting nano-material is two-dimensional graphene.The valence band and conduction band of graphene intersect at the Dirac point,which results in a typical zero band gap and hinders its practical application in nano-electronic devices.At present,there are many methods for opening band gap into graphene,such as adsorption atoms,doping methods,and stress.This thesis focuses on the fluorination of pure graphene and the control of band gap under stress.In order to further investigate the electronic properties of non-metallic atom chains,we analyze the electronic properties of carbon chains with different lengths,and explore the way to open the band gap of graphene.In this work,the first-principles calculation method was used to systematically research the electronic properties of monoatomic nanochains and fluorinated graphene,reveal the charge distribution of atomic chains under stress and the variation of density of states and band structures with the degree of fluorination and biaxial stress.The main contents and conclusions are summarized as follows:1.The structural evolution of one-dimensional linear atomic chains(C,Si,S,and P)and the changes in charge distribution of atomic chains during stretching and compression were studied.The results show that: 1)The C and Si atom chains tend to form a stable linear structure.For the first time,it is theoretically predicted that the S and P atoms can form a stable curved chain structure;2)At the beginning of the elastic compression stage,the linear atomic chains of C and Si have metastable transition states,namely there is a bending point at which the binding energy of the atomic chain suddenly decreases and increases with further compression,whereas the S and P atom chains do not have a similar metastable transition state;3)The charge distributions in C,Si,P and S monoatomic chains are not uniform under stretching orbuckling.The charge of carbon chains with different lengths exhibits odd-even oscillations,and the change of mechanical behavior has little effect on the charge distribution of atomic chains.2.The energy band structure and density of states of graphene at different fluorination levels were studied.1)The barrier for a single fluorine atom to pass through the graphene carbon ring was calculated,we found that the F atom is difficult to pass through the carbon ring of graphene because the barrier is too high(18.92 e V);2)Four fluorination configurations of graphene(C: F = 1:1)(Chair,Zigzag,Boat,Armchair)were considered,it was found that the chair-type is the most stable fluorinated configuration.Meanwhile the stability of chair-type fluorinated graphene with different fluorination degrees were disscussed,it was found that double-sided fluorination was more stable than single-sided fluorination,and the higher the degree of fluorination,the more stable the configuration;3)The band structure and density of states for the most stable configuration of fluorinated graphene,double-sided chair-type fluorination under different stresses were calculated.It was found that the effect of double-sided fluorination with a ratio of C:F=1:1 on the electronic properties is more pronounced.The completely opened bandgap is 3.1 eV,which indicates that the fluorinated graphene is a semiconductor with little change of band shape near the Fermi level and a weaker conductivity.When symmetric biaxial stress was applied to the fluorinated graphene,the valence band of fluorinated graphene gradually shifts toward lower energy,while the conduction band gradually shifts toward higher energy.Under the effect of stress and fluorine atoms,the degree of atomic fluctuations in the chair-type fluorinated graphene configuration tends to decrease and the band gap of the fluorinated graphene increases gradually.The combination of stress with fluorination has a more obvious influence on the electronic properties of graphene.