Theoretical Investigation On Charge And Spin Transport Properties Of Graphene And Silicene Nanoribbons

With the minimization of electronic devices,the electrical properties of the devices have attracted more and more attention at nanometer scale and even molecular scale.With the continuous development of experimental technology,including nano-processing technology,self-assembly technology,scanning tunneling microscope technology and break junction method etc.,people are able to make molecular devices on the level of small molecules or molecular groups and even individual molecules.So far,finding the suittable functional molecules or nanomaterials to realize device functions remains a major topic of molecular electronics.In recent years,people have turned their attention to the research of low-dimensional nanomaterials.In particular,graphene,once discovered,immediately opened up the enthusiasm for the study of low-dimensional nanomaterials.Silicene,which belongs to the carbon group as graphene,is considered to be better compatible with modern silicon-based technologies and also attracts much attention.To apply graphene and silicene to the circuit,they are usually cut into nanoribbons.At present,a great deal of work has been done on the research of transport properties of graphene and silicene nanoribbons,but there are still some problems to deserve our discussion.For instance,the zigzag graphene nanoribbons(ZGNRs),the two edge states are spin anti-ferromagneticaly polarized with an opposite spin orientation and zero total magnetic moment.In a way,the applications of pristine ZGNRs seem to be hindered for spintronics,and need to take appropriate measures to the manipulation of the spin configuration in the edge of ZGNRs.Bare silicene nanoribbons are unstable,so it is necessary to modify the edge silicon atoms during the preparation of the silicene nanoribbons.Using a different chemical environment will produce different edge modification situation.But at present,the transport properties of silicene nanoribbon segments for different edge modifications are not yet clear.In addition,it is still a research hot spot for the design of multifunctional molecular devices based on graphene and silicene nanoribbons.In this paper,using density functional theory and non-equilibrium Green's function,we study the charge and spin transport properties of zigzag graphene and silicene nanoribbons,and design the multifunctional molecular device based on graphene nanoribbon.We also control the edge magnetic properties of ZGNRs by self-doping method.In addition,the charge transport properties of zigzag silicene nanoribbons(ZSiNRs)modified with different functional groups are also studied.The specific research contents and results are as follows:1.Spin transport properties in lower n-acene-graphene nanojunctions.We consider an all-carbon junction consisting of n-acene molecules between two ZGNRs electrodes.Initial spin configurations on both electrodes are set to simulate two external magnetic fields,the directions of which can be tuned by setting spin configurations to parallel or antiparallel,respectively.Our theoretical results show that with n-acene molecules ranging from anthracene to hexacene,the spin-polarized electronic states near the Fermi level can be induced by the spin-polarized ZGNR electrodes,which strengthen gradually to facilitate the electronic transport.The dual-orientation spin-filtering and spin-rectifying effect are observed in a wide region of bias voltage.Among them,the spin polarizability can reach nearly 100%,and the rectification ratio exceeds 103.Importantly,an over 8000%giant magnetoresistance is obtained in the low bias range from-0.1V to +0.1V.Moreover,negative differential resistance behaviors are detected in these devices.The potential mechanisms for these intriguing phenomena are proposed.This giant magnetoresistance ratio,spin-filtering and spin-rectifying efficiency are mainly caused by the symmetry mismatch of wave functions of the ZGNR energy bands near the Fermi level.Moreover,NDR effects can be also attributed to the mismatch between electronic states of the left and right electrodes,and are related to molecular eigenstates.These findings would be instructive for the design and synthesis of high-performance graphene-based spin-related devices.2.Role of edge dehydrogenation in magnetization and spin transport of zigzag graphene nanoribbons with line defects.For ZGNRs,the ground state is antiferromagnetic coupling between the two edges,leading to zero total magnetization.In a way,this hinders the applications of pristine ZGNRs for spintronics.The aim of our present work is to study,using first-principles method,the magnetism and spin transport properties of the 558defect-ZGNRs and 57defect-ZGNRs in which 558defect(57defect)locate in one side or in both sides,and to evaluate the impact of edge dehydrogenation.Results show that magnetization can be induced or strengthened obviously in 558defect-ZGNRs unterminated by hydrogen,but not for 57defect-ZGNRs.This is because a spin-polarized a edge state appears near the Fermi level and strengthens spin-splitting of energy bands at bare edges of the 558defect-ZGNRs.Moreover,compared with pristine ZGNRs,the 558defect-ZGNRs with bare edges can realize a transition from antiferromagnetic coupling to ferromagnetic coupling between both edges.In addition,the spin-filter efficiency can be effectively improved in our proposed devices by edge dehydrogenation.Our results demonstrate that the presence of a edge state near the Fermi level plays an important role in controlling spin transport of the graphene-based spintronic devices.3.The electronic transport properties of zigzag silicene nanoribbon slices with edge hydrogenation and oxidation.The edges of silicene nanoribbons(SiNRs)are often chemically functionalized because the edge dangling bonds of pristine SiNRs are unstable.Hydrogen gas are often used in plasma etching to saturate the dangling bond.The H-rich environment can lead to H2-saturated SiNRs.Oxygen(O)or OH chemical groups can be attached to the edges of SiNRs during oxygen plasma etching or oxidative cutting of silicene materials.Therefore,in this paper,we focus first on SiNR slices with various edge hydrogenation and oxidation which are derived from the chemical environments during fabrication of the SiNRs.Using the first-principles approach,we calculate the electronic transport properties of ZSiNRs slices with edge hydrogenation and oxidation sandwiched between H-terminated ZSiNRs electrodes and investigate systematically the impact of different edge functional groups on the transport properties.We focus on two series of SiNR-based molecular devices:one for the functional groups:H,OH,O and H2 locating on both edges of 6ZSiNR slices,and the other for the functional groups:OH,O,H2 locating on one edge and the other edge by saturated H group.These symmetric and asymmetric edge modifications have different impact on the I-V curve characteristics.For the former,the current under bias is suppressed;for the latter,the current is significantly enhanced.It can be attributed to the symmetry mismatch of the ? and ?*subbands under the C2 operation along the central axis for 6ZSiNR slices with symmetric edge modifications.In addition,the current intensity varies with the different edge functional groups,and the comparison of the current magnitude indicates H>OH>O>H2.This is mainly because that the OH group has similar band structure as the like-sp2 hybridization of H edge,and the O group form a band across the Fermi level to provide transmission channel,and while the H2 group can yield band gap to be not conducive to the transport.As a result,these edge functional groups play an important role in the electrical performance of SiNR-based devices,and understanding their mechanism is crucial for practical applications of SiNR-based devices.The thesis includes the following six chapters:The first chapter mainly introduces the research background of molecular electronics,progress in experimental and theoretical studies of molecular devices,transmission characteristics of molecular devices and the structure and properties of graphene and silicene and their nanoribbons.The second chapter is the theoretical method part.We briefly introduce the method of calculating electronic structure-density functional theory,and the method of calculating transport properties of molecular devices-nonequilibrium Green's function method.Simultaneously,Self-consistently solving transport properties of devices using density functional theory and nonequilibrium Green's function method is also introduced.The third to the fifth chapters introduce the specific works and calculation results based on the above theoretical method:In chapter three,we study the spin transport properties in lower n-acene-graphene nanojunctions,and have successfully designed multi-functional molecular devices with giant magnetoresistance effect,dual-orientation spin filtering and spin rectification effects and negative differential resistance effects;In chapter four,we study the role of edge dehydrogenation in magnetization and spin transport of zigzag graphene nanoribbons with line defects,and have successfully achieved the transition from antiferromagnetic coupling to ferromagnetic coupling of the carbon atoms on both sides of the ZGNRs,and have improved the spin polarization of the devices;In the following chapter,we discuss the effects of various edge hydrogenation and oxidation functional groups on the charge transport properties of silicone nanoribbons.The last chapter draws a conclusion and gives a prospect for the whole work of this thesis.