Theoretical Study On Electronic Structure And Transport Properties For Phosphorene And Its Nanostructures

Phosphorene is a new layered two-dimensional(2D)material after graphene.It has high anisotropic structural and physical properties,with a appropriate di-rect band gap which increases as the number of layers increasing.It also has a high carrier mobility up to magnitude 10~4.Because of these proper physical prop-erties,phosphorene has become one of the most popular candidate materials for the application in nano-optoelectronic devices.However,in experimental prepara-tion and application,we usually need to consider the geometric edge morphology of 2D material samples and their nanosystems.For a 2D material nanostructure,the edge atom configuration qualitatively determines almost all physical proper-ties of the system,especially electrical and magnetic properties.An example that has been extensively studied theoretically and experimentally is graphene nanos-tructures.However,there is still a lack of related research on the nonregular hexagonal structure of 2D black phosphorene.Therefore,establish the theory for the crystallographic characterization on 2D black phosphorene,with which applied to the nanostructures in classifying their edge morphology,is very important for identifying,understanding and applying edge states from the level of atomic or-bitals.Therefore,it has fundamental physical significance and device application prospect.The calculation of the electronic structure and quantum transport for 2D ma-terial nanosystems is also an important topic in itself.The tight-binding(TB)approximation and the density functional theory(DFT)are two commonly used methods.Both of them can be combined with the scattering matrix(SM)or non-equilibrium Green's function(NEGF)to treat transport problems,and each has its advantages and disadvantages.The advantage of the former is that it can be calcu-lated analytically and can quickly deal with nanostructures of large geometric size,while the disadvantage is that the amount of analytical computation is large and can only give qualitative results.Fortunately,there are several specialized software packages that can solve the problem of analytical computation,such as Kwant.In principle,the DFT can accurately calculate arbitrary shapes nanostructures and obtain quantitative results.Although there are mature software packages(such as VASP),the computational resources required for the system increase dramatically as the system size increases.In this thesis,we will study the electronic states and transport properties of a variety of phosphorene nanostructures on the basis of theoretical studies of 2D black phosphorene crystallographic characterization.And we use two calculation methods to intersperse and combine them,focusing on the classification of edge morphology,edge states,and transport properties of phosphorene nanostructures.The main work is described in chapters 3 to 6:1.We have established a theoretical characterization method of black phos-phorene crystallography that does not depend on group theory.The chiral number,the relative angle of the crystal orientation,and the crystal orientation vector are combined to describe the nonregular hexagonal planar lattice of black phospho-rene,and any possible crystal orientation is included.In addition to reproducing the four special crystal orientations previously studied,a new special crystal ori-entation boundary was also found.These five special crystal orientations divide the entire plane into four regions with different characteristics.Then the effect of the two sub-planes in the z direction is taken into account,and it is clear that the black phosphorene nanoribbons(PNRs)that are cut along each crystal direction have both beard and beard boundary morphologies.In addition,whether it has edge states depends on the existence of periodic zigzag atom configurations along the edges.2.We have constructed a z-shaped homojunction based on black PNRs.When the twist angle changes from 0 to 90~o,the hypotenuse of the center zone of the z-junction sweeps the edge morphology of all crystallographic directions,and the conductance is sensitive to it.In specific,when the hypotenuse sweeps across a boundary with edge state,an oscillating conductance platform appears near the Fermi level,otherwise electron transport without the edge state would be hindered.Interestingly,the number of oscillating peaks in the platform exactly matches the number of hypotenuses.This is because the discrete energy levels corresponding to the lone pair of electrons on the serrated edge provide a transmission channel for the electrons.3.There are two major categories of PNRs obtained by cutting along al-l possible crystal directions,with or without edge states,and there are twofold and one degenerate edge states.The PNRs corresponding to the newly discov-ered slope(SL)crystal orientation has a twofold degenerate edge state.Based on this phenomenon,we construct a P N homogeneous junction with both edge and layer degrees of freedom.We found that when the transmission energy windows composed of the same edge-state bands are aligned,there will be a significant con-ductivity peak near the Fermi level,regardless of the degree of freedom.However,when the transmission energy windows composed of different edge-state bands are aligned,the conductance peak near the Fermi level is almost zero for the edge degree,and a visible conductance peak appears for the layer degree and decreases with the increase of vertical electric field.4.We have investigated the mechanical properties of zigzag blue phosphorene nanoribbons(zBluePNRs)with different chemical modifications at the edges.It was found that under the action of in-plane uniaxial strain,the zBluePNR with a width of about 2 nm and double-edge sulfur(S)modification showed a negative Poisson's ratio(NPR)effect.At the same time,the edge sulfur atom modification can also cause spin splitting of the band structure,and the zBluePNR can be transformed from a magnetic metal state to a semi-metal state by applying a sufficiently large strain.