First-principles Investigations On The Control Of Spin-polarized Transport In Two-dimensional Materials

High spin polarization is a central issue in spintronics,and how to get fully spin-polarized transport and generate pure spin current has been the focus of numerous studies for many years.Due to its many unique properties,the emerging two-dimensional(2D)materials are considered as the ideal materials to replace the existing silicon materials in the electronic devices in the future.Under these two backgrounds,how to combine spintronics with two-dimensional materials,especially how to achieve 100%spin polarization transport and pure spin current in two-dimensional materials,has become the focus issue in the past decade.Zigzag graphene nanoribbons(ZGNR)and SiC nanoribbons are considered as important candidates to achieve these two goals because of their edge states and edge magnetic moment.More importantly,quite a few schemes have been suggested to achieve half-metallicity(a property with only one conducting spin channel)in a zigzag edged graphene nanoribbon,such as electrical field,edge decoration,and B-N co-doping.However,it has been shown that the strength of the electrical field should be extremely high to achieve halfmetallicity in ZGNR,which is almost unavailable in the laboratory.Meanwhile,chemical modification can lead to a dramatic decrease or even the disappearance of the energy difference between the antiferromagnetic(AFM)and ferromagnetic(FM)edge configurations,whereas the half-metallicity can only be obtained from the AFM ground state.Thus,a finite temperature can easily turn the ZGNRs paramagnetic,which makes the half-metallicity practically unobservable.Moreover,there are two mainly schemes or mechanisms for producing pure spin current,such as spin dependent Seebeck effect and photogalvanic effect.In this thesis,we have focused on photogalvanic effect.Up to now,the conditions for obtaining pure spin current are prohibitively strict since it can be achieved only by precisely tuning the photons at specific energy or specific polarization/helicity angle,which is rather difficult practically.Thus,a much more robust scheme to avoid these demanding conditions is highly desired.Aiming at the scientific problem of how to realize fully spin polarization transport and pure spin current of two-dimensional materials,this paper studies the control of the spin polarized transport by using the method combining density functional theory and non-equilibrium Green's function.The main contents are as follows:1.We have studied the doping effects on the electron transport properties in zigzag edged SiC nanoribbons.It is found that a single N atom doped at an edge C site kills all the transmission channels contributed by the C-terminated edge states,leaving only the transmission channels from the Si-terminated edge states conducting.Thus,no matter whether the ribbon is in AFM state or FM state,we always have one spin conducting.This is interesting since we do not need to worry about the switching between the AFM state and FM state due to finite temperature.Thus,it suggests a very important scheme for realizing fully spin polarized transport in zigzag SiC nanoribbons based spintronic devices.2.Zigzag-edged graphene and h-BN nanoribbons with passivated edges in the ground state are either spin-degenerate or spin-unpolarized systems,which are not directly applicable for spin polarized transport.In this work,based on density functional calculations,we demonstrate that the combination of them to form van der Waals(vdW)heterostructures can be adopted to realize fully spin polarized transport.As an example,the ballistic transport properties of a zigzag-edged graphene nanoribbon(ZGNR)with six zigzag carbon chains is studied first with each lead as a vdW heterostructure formed by attaching a h-BN monolayer to each side of the ZGNR with AA-stacking.A greatly decreased transmission gap(0.28 eV)in one spin and a greatly increased transmission gap(0.82 eV)in the other spin is achieved in the transmission function,resulting in fully spin polarized transport at low bias.This arises from the stagger potential imposed by the h-BN layers,which not only exists in the leads,but also extends to the channel region.It changes the energy of the edge states of different spins localized at different sublattices in an opposite way in the whole device.The transmission gap and the threshold voltage can be further modulated by applying a vertical pressure to the vdW leads to tune the strength of the stagger potential or simply by changing the ribbon width.Finally,the increase of the channel length will greatly reduce the magnitude of the transmission around the Fermi level and the transmission gap will eventually recover the value of the band gap of the pristine ZGNR.These findings not only provide a novel way for achieving fully spin polarized transport in graphene,but also demonstrate the great importance of vdW heterostructures in the design of spintronic devices.3.By constructing transport junctions using graphene-based van der Waals(vdW)heterostructures in which a zigzag-edged graphene nanoribbon(ZGNR)is sandwiched between two hexagonal boron-nitride sheets,we computationally demonstrate a new scheme for generating perfect spin-polarized quantum transport in ZGNRs by light irradiation.The mechanism lies in the lift of spin degeneracy of ZGNR induced by the stagger potential it receives from the h-BN sheets and the subsequent possibility of single spin excitation of electrons from the valence band to the conduction band by properly tuning the photon energy.This scheme is rather robust in that we always achieve desirable results irrespective of whether we decrease or increase the interlayer distance by applying compressive or tensile strain vertically to the sheets or shift the h-BN sheets in-plane relative to the graphene nanoribbons.More importantly,this scheme overcomes the long-standing difficulties in traditional ways of using solely electrical field or chemical modification for obtaining half-metallic transport in ZGNRs and thus paves a more feasible way for their application in spintronics.4.We propose a novel idea to generate pure spin current with photogalvanic effect(PGE)by designing devices with spatial inversion symmetry based on two dimensional(2D)materials.Thus far,no research attention on PGE has been reported in systems with spatial inversion symmetry,spatial inversion symmetry is just what has been considered as "useless" and thus never believed to show any PGE,but we will show that spatial inversion symmetry is an ideal source for pure spin current with PGE.We have proposed a new model,where each semi-infinite half of the AGNR is periodically patterned by ferromagnetic equilateral triangle antidots with zigzag edges and the triangle antidots are oppositely directed(like['?')in the two semi-infinite ribbons.This arises from the antisymmetric spin density due to the spatial inversion symmetry of the structure.By extensive first principles calculations,we show that when the central region with spatial inversion symmetry is irradiated by photons with any energy(larger than the threshold,that is spin gap)and any polarization angle,the induced photocurrent of the two spin channels will always flow in opposite directions with exactly equal magnitude,robustly resulting in pure spin current accompanied with no charge current.