Investigaton On Electromagnetic Properties Of One-dimensional Nanostructures

With the development of science and technology progress,the electronic devices with the smaller volume,more rapid operation,and higher integrated features have been pursued continuously.However,it is difficult to meet these requirements for the devices based on the traditional silicon materials due to the restriction of the various physical principles.Two-dimensional materials,such as graphene,have excellent properties and be considered to be ideal replacements of silicon.But in two-dimensional form,these materials are generally do not have magnetism and can't be applied to spintronics devices directly.Thus,we here make investigations on how to tune the properties of graphene and armchair black phosphorus nanotubes(APNTs)by shape-cutting,edge modification,introducing topologic defects,doping,strain,and applying an external electrical field or magnetic field,and so on.And the first-principles methods are used to calculate their properties.Based on these studies,we can obtain the nanostructures with excellent properties,which might be used for developing spintronics devices.In this paper,we first make an introduction on the research progress of the two-dimensional materials and corresponding one-dimensional material,and a brief description on theoretical methods which is used in our works.Then,we design the “Christmas tree” shaped graphene nanoribbons(CTGNRs)by cutting graphene,and magnetic semiconductors with ferromagnetic(FM)ground states are found on all-size CTGNRs.What's more,for the devices which based on CTGNRs,a dual spin-filtering effect with the perfect(100%)spin polarization,high-performance dual spin diode effect with a rectification ratio about 1010,and a giant magnetoresistance(MR)approaching 1010% can be realized.We also study using fluorine(F)atoms to saturate the edge carbon atoms of CTGNRs.The calculation results show that F-CTGNRs are more stable than H-CTGNRs,and the size effects of F-CTGNRs are more obvious.For example,they can be a spin metal,a spin semiconductor,a spin half-metal or a spin gapless semiconductor completely depending on their geometrical sizes.And also,the transport properties of the device,which constructed by the F-CTGNR with a spin metallic feature exhibit the dual spin-filtering effect with the perfect(100%)spin polarization,high-performance dual spin diode effect with a rectification ratio up to 1010 and a giant magnetoresistace(MR)about 104%.Furthermore,we construct several graphene heterojunctions(GHJs)by polycrystalline graphene with different orders of defects in grain boundaries(GBs).By calculation,we find that GBs can induce significant localized electron states and spin transport with different polarization depends strongly on the orders of defects in GBs.Meanwhile,the transport properties of the GHJs under both P configuration and AP configuration show spin-filtering,spin-rectifying,and giant magnetoresistance effects.By analysis,it is find that the relationship between molecular energy levels and their corresponding wave functions play a key role in these transport properties.Finally,we consider the substitution doping of 3d TM atoms(Fe,Co,and Ni)in the APNTs.The calculated results show that the doping elements and doping positions have an important influence on the magneto-electronic properties of the nanotubes.Such as the doping in the inner layer,only the energy band of Ni-doped nanotube is spin-split and it behaves like half-metal characteristics,while the Fe-and Co-doped one cannot induce a magnetism in a nanotube and features metal and semiconductor,respectively.The doping with three elements in the outer layer can all induce magnetism in nanotube,which shows the characteristics of magnetic semiconductor or spin semiconductor.What's more,we also discuss the effects of external electrical field and strain applied on the nanotube.Overall,we have studied the magneto-electronics properties of several nanostructures and expect that our findings can provide a reference and thinking pathway for follow-up research works,especially for experimental explorations.