Theoretical Studies On Spin Polarization Of P Electron And Its Regulation In Low Dimensional Systems

Carbon and boron atoms with p-electrons as valence shell electrons are relatively abundant on the earth and closely related to human production and living.In molecular systems,the spin-polarized behavior of p-electrons is closely related to the design and application of magnetic materials,spintronic devices and so on,thus it has important basic research significance.Also,if the p valence electrons in the system are associated with high-angular f electrons that occupied on the unfilled valence shell,more abundant orbital hybridization and spin-polarized behavior will occur.Therefore,understanding these electron spin-polarized behaviors is an important challenge in fundamental theoretical research.In this case,further achievement on controlling the behaviors of the electrons will greatly promote the new progress in designing molecular or nano-scale device.In this work,based on the quantum first-principles method,we studied the electron spin-polarized effects on one-dimensional carbon nanostructures and zero-dimensional boron fullerenes,and investigated the electronic correlation properties of p-electrons and 5f electrons.A method for the regulation of p electrons spin-polarized for carbon atom was developed,as well as non-equilibrium Green's function combined with density functional theory(NEGF-DFT)for the transport properties of the 5f electron system was developed.The all-boron fullerene B40 is the second non-metallic cage molecule discovered by experiment,and receives extensive attention.As a hollow cage structure,whether B40 fullerene is also a superatom,which has not been solved.Here,by means of density functional theory(DFT)method,B40 is calculated by pure functional PBE and hybrid functionals PBE0,B3 LYP and HSE06,respectively,and similar conclusions are obtained.The results reveal that B40 presents superatomic properties.It not only has superatomic 1S,1P,1D and 1F orbitals,but also has superatomic orbitals 2S,2P,2D and 2F.It should be noticed that the 2F superatomic orbital of B40 is partially occupied,thus adding six electrons to neutral B40 fullerene would form a superatomic structure with fully occupied shells.The computed nucleus-independent chemical shift values suggest that the B406-is of higher electron delocalization than B40 itself.Our work opens up a new perspective for the superatomic physics in boron fullerene family.The above study found that B40 fullerene is one kind of superatom,and one pathway that superatom leading to functional materials is assembly.For this purpose,we used the DFT method to study the interaction between typical boron-based B40 superatoms.The results show that different oligomers constructed by modulating the arrangement of two B40 superatoms result in changes in the electronic structure,but their common feature is retaining partial superatomic properties for monomers.The reason is that the inner shell superatomic orbital of two monomers maintains the electronic localization,while the valence shell superatomic orbitals cannot maintain the original shape due to the bonding and antibonding hybridization.Furthermore,in the case of retaining partial superatomic properties,B40 oligomers could correspondingly achieve a transformation from the insulator to semiconductor.The decrease of the energy band is probably caused by the disappearance of the superatomic orbitals of “the main quantum number” 2.Our findings highlight that the interaction between superatoms could bring about synergy effects differing from the monomers.Thus,our research will help the development of new materials and devices,especially superatoms serving as building blocks.Asymmetry in the electronic structure of low-dimensional carbon nanomaterials is important for designing molecular devices such as directional transport and magnetic switching.We use DFT method to achieve an asymmetric spin distribution in(9,0)or(10,0)carbon nanotubes(CNTs)by capping CNTs with a fullerene hemisphere at one end and saturating the dangling bonds with hydrogen atoms at the other end.The results show that the asymmetric structures lead to obvious asymmetry in the spin distribution along the tube axis direction.More interestingly,the heterogeneity of the spin distribution can be controlled by charging the system.For Cap-(9,0)CNTs,the spins at the two ends are ferromagnetically coupled,while that is antiferromagnetically coupled for neutral Cap-(10,0)CNTs.This might induced by structure,for the fact that the end of cap for Cap-(10,0)CNTs is pentagon and that is a hexagon for Cap-(9,0)CNTs.In contrast,the proportion of the tube axial electronic population of the system does not changed significantly.Further analyses of the electron density difference reveal the loss and gain of charge at both ends.The results provide a strategy for controlling the spin distribution for functional molecular devices through a simple charge adjustment.The spin polarization of CNTs offers a tunable building block for spintronic devices and is also crucial for realizing the properties of carbon-based electronics.However,the effect of chiral CNTs is still unclear.Here,we use DFT method to investigate the spin-polarized effect of a series of typical finite-length chiral CNTs(9,m).The results show that the ground state of the chiral CNTs is also antiferromagnetically coupled,similar to pure zigzag CNTs.The spin density of chiral CNTs(9,m)decreases gradually with the increase in m and vanishes altogether when m is larger than or equal to 6.The armchair edge units on both ends of the(9,m)CNTs exhibit a clear inhibition of spin polarization,allowing a control method of the spin density of(9,m)CNTs by adjusting the number of armchair edge units on the tube end.Furthermore,analysis of the molecular orbitals(MOs)shows that the spin of the ground state for(9,m)CNTs mainly comes from the contributions of the frontier MOs,and the energy gap decreases gradually with reduction of the spin density for chiral CNTs.Our research further develops the study of the spin polarization of CNTs and provides a strategy for controlling the spin polarization of functional molecular devices through chiral vector adjustment.The spin-polarized behavior of p electrons will become more complicated due to the introduction of other high-angle momentum valence electrons,but it also has important potential applications in designing molecular devices,semiconductor electronic devices,etc.We studied the correlation properties of p electrons and 5f valence shell electrons originated from confining actinide atoms in the one-dimensional CNTs.And particularly,a new transport calculation method for this system was developed,based on NEGF-DFT.Firstly,we use DFT method to calculate the structure that actinide atoms embed in the(4,4)or(5,5)CNTs,namely An@(4,4)/(5,5)CNTs(An = Ac,Th,Pa and U).The results show that actinide atoms mainly contribute to the unoccupied MOs and the frontier MOs,and the contribution of 5f electron to frontier orbitals gradually increases with the increasing of the atomic number.Meanwhile,in order to predict the conductivity of such systems,we developed a new research method for transport behavior.The key to this method is to calculate the 5f electron transport properties,using non-equilibrium Green's function formalism combined with density functional theory.And the conductivity of the system will be reflected in the transmission spectrum,realizing the calculated method for 5f electron transport of the actinide atoms.Unexpected,under the CNTs confined conditions,the 5f electrons of actinide atoms are scattered,which suppress the electron transport properties of CNTs.Furthermore,transmission spectra of U@(4,4)/(5,5)CNTs show that bias voltage will induce peaks in transmission spectrum shift left in turn.This study will provide new insights into the research for transport properties of high-angle momentum shell electrons.In summary,the systematic work of this doctoral thesis will play an important role in promoting the research on the spin-polarized effects of p electron in low-dimensional systems.The developed modelling control methods and new transport calculation methods will provide important reference and support for experimental and theoretical research.