Mech-Electro-Magnetic Properties Of Two-Dimensional Materials And Nanostructures

Two-dimensional nanomaterials have attracted great attention owing to their outstanding properties and potential applications in future nanodevices.From zero-gap graphene to semiconducting black phosphorus and transition metal dichalside,to insulating hexagonal boron nitride(h-BN),to the recently studied topological insulator and so on,knowledge on these two-dimensional materials are further deepened by theoretical research as well as experimental synthesis and characterization.Besides,device designing of these materials and the working mechanism of the device are developing fast.However,a lot of physical phenomenon and laws about these nanomaterials are still not well understood,so further investigation for mech-electro-magnetic coupling of these materials is of great importance in the optimization of properties and function control.In this thesis,using first-principles calculations based on density functional theory,we systematically study the physical properties of stacked graphene and h-BN nanostructures as well as newly predicted two-dimensional nanosheets and nanofilms.By applying external field like strain and electric field,we studied the variation of their structures,electronic and magnetic properties.Ample phenomena from the coupling of strain,electronic and magnetic properties are unveiled.The main contents are listed below:(1)Study on mech-electro-magnetic properties of stacked graphene and h-BN nanostructures.Through first-principles calculations of spin-polarized density functional theory,we have systemically investigated the electronic and magnetic properties of graphene nanoribbons on h-BN substrate.Due to the polarization of h-BN nanoribbons,the spin-degenerated band structure of zigzag graphene nanoribbons becomes generated and can be easily tuned to half-metal by transversely applied field.In contrast to the zigzag nanoribbons,the electronic properties of armchair graphene nanoribbons are hardly changed on the h-BN nanoribbons.When zigzag graphene nanoribbons are sandwiched between two h-BN nanostructures,the electronic and magnetic properties can be effectively modulated by bias and interlayer spacing,resulting in semiconductor to half-metal transition.We contribute the the transition to the enhanced charge polarization of zigzag h-BN nanostructure.In sharp contrast to the half-metallicity of zigzag graphene nanoribbons under transversely applied electric field,the present designation is compatible to the field effect transistor of current semiconducting industry technology and paves a new way for spintronic device of graphene nanostructures.(2)Study on electronic and magnetic modulation of graphene-like two-dimensional materials and nanostructures.Based on first-principles calculations,we theoretically studied the electronic and magnetic properties of the predicted monolayer beryllium sulfide(h-Be S)and boron phosphide sheet(h-BP)with hexagonal structure.Our result shows that h-Be S is a stable wide gap semiconductor.The electronic properties of h-Be S nanoribbons is chirality-dependent Armchair Be S nanoribbons are semiconducting.And the energy gap of bare armchair Be S nanoribbons is almost ribbon width-independent,but it shows sensitivity to transversely applied electric field,exhibiting giant stark effect.The stark coefficient of wide nanoribbons is larger than that of narrow ones and easier to transit from semiconductor to metal.On the other hand,bare zigzag Be S nanoribbons is magnetic metal.The spin-polarized electrons are mainly contributed by the 2p orbital of edge Be atom,exhibiting spin glass state.Particularly,when the outmost Be and S atoms are ferromagnetically ordered,the magnetic moment of the nanoribbons can reach up to 1.15 μB.For monolayer h-BP,its direct energy gap is very close to that of silicon in semiconducting industry.But the estimated effective mass of monolayer h-BP is only half of silicon,so the mobility of carriers in h-BP is much higher,endowing h-BP with potential applications in two-dimensional semiconductors.In addition,external strain can effectively tune the electronic properties of h-BP and the strain-dependent energy gap differs from the red shift in most semiconductors.When the h-BP is semi-hydrogenated,the semiconducting sheet becomes magnetic metal and the spin density is mainly contributed by the atom that doesn’t bond with hydrogen.Besides,the total magnetic moment of the ferromagnetic metal increases linearly as biaxial strain increases.(3)Study on surface magnetism modulation of boron monophosphide nanofilms by external strain and electric field.Cubic boron monophosphide(c-BP)is a nonmagnetictransparent semiconductor.Through first-principles calculations of spin-polarized density functional theory,we have systemically investigated the surface magnetism and magnetoelectric effect in BP nanofilms.The surface magnetism originates from the nonequilibrium occupiation of spin-polized electrons around the Fermi level which is driven by the built-in electric field.In particular,the thickness-dependent surface magnetism exhibits linear magnetoelectric effect with giant coupling coefficient and is strain tunable.Our results also show that when the thickness of the nanofilms is less than 4 atom layers,the nanofilms will optimize to mono-and bi-layer h-BP layers withdirect energy gap.With the development of nanotechnology,cubic BP has been synthesised with controllable size and the prediction on the surface magnetism and linear magnetoelectric effect in BP nanoflims is of practical importance.