First-Principles Investigation On Spin Transport Properties Of Silicene Nanoribbons

With the continuous development of device's miniaturization and circuit's integration,the traditional silicon-based semiconductor devices are approaching its physical limit.When further reduce the size of the electronic devices reached to nanometer level,the influence of quantum effects will be more and more importance.The further development of electronic technology needs to be based on new materials and new mechanisms.The emergence of nanomaterials has provided possibility to the progress of electronic technology.In 2004,Grahpene,which was successfully prepared in experiment and found that it has many excellent properties,has attracted considerable attentions.However,graphene is a gapless semiconductor,which limits its application in the field of electronic technology.Silicene,a graphene-like monolayer honeycomb structure of silicon,has been successfully synthesized on the experiment in 2012.And an obvious natural advantage of silicene over graphene is its better compatibility and expected integration with current silicon-based nanotechnologies.This implies that it has quite important prospect in application.Compared to graphene,the research of electronic transport properties on silicene was still in the stage of preliminary study,and at the same time,using electron spin degree of freedom to create high speed,high integration,and low power consumption spintronic devices,also become the research hot spot.So use this nanomaterial and its properties,people can design many kinds of new nanodevices with different functions,promote the development of electronic technology.Using nonequilibrium Green's function in combination with the density functional theory,we studied the transport properties of several nanostructures and a series of new electronic phenomena are discovered.Through analysis,we explained the physical mechanism,and discussed their application in nanoelectronics and spintronics.The contents of this thesis are as follows:Chapter 1.Firstly,we introduce the research background and development status of nanoelectronics,some characteristics of nanomaterials and the interesting common phenomena in quantum transport.Then we summarize the research progress and application prospect of low-dimensional materials represented by graphene and silicene,and a simple introduction of spintronics are given.Finally,we introduce the main research contents of this paper.Chapter 2.We introduce the theoretical calculation method used in this dissertation.Density functional theory(DFT)is the most widely used theory in predicting electronic structures of nanomaterials and non-equilibrium Green's function method(NEGF)is a robust tool to deal with non-equilibrium electron transport.Then we introduce the combination of DFT and NEGF to calculate the current integration.Chapter 3.We study the spin-dependent transport properties of two symmetrical edge hydrogenated silicene nanoribbon heterojunctions.The edges of silicene nanoribbon are often chemically functionalized because the edge dangling bonds of pristine silicene nanoribbon are unstable.Hydrogen gas is often used in plasma etching to saturate the dangling bonds.If the silicon atoms at the edges were saturated in different ways,great difference in band structure,edge states,and the stability of ground state could be observed in the silicene nanoribbon.The studies show that the bare and symmetric zigzag silicene nanoribbon have the antiferromagnetic ground state,while the asymmetric zigzag silicene nanoribbon show the ferromagnetic ground state.That is to say,the electronic and magnetic properties of zigzag silicene nanoribbon can be tuned by different forms of edge hydrogenation.In this chapter,we design two different heterojunctions,namely,H1-6ZSiNR-H1/H0-6ZSiNR-H0 and H1-6ZSiNR-H1/H2-6ZSiNR-H2.Our results show that the devices can exhibit multiple spin-resolved transport properties,such as spin filtering effect,negative differential resistance and rectifier behavior.The mechanism for the phenomena are proposed in this work.Chapter 4.This work is based on the previous work.We study the spin-dependent transport properties of three asymmetrical edge hydrogenated silicene nanoribbon heterojunctions.Similar to zigzag graphene nanoribbon,the pristine zigzag silicene nanoribbon also show a symmetry-dependent transport property although they have a buckled structure,where odd and even-width zigzag ribbons are found to have completely different current-voltage characteristics despite the similarity of their band structures.For example,bipolar spin-filter behavior was observed in even-width devices,while the odd ones show a linear metallic conducting behavior that limits their application in spintronic devices.Theoretical studies have demonstrated that the zigzag silicene nanoribbon with asymmetric edge hydrogenation(H2-ZSiNR-H1)are energetically more stable than the symmetric ones and are bipolar magnetic semiconductor.When we design a heterojunction consisting of H2-mZSiNR-H1/H1-nZSiNR-H],where m=5 and n=5,6,7 is the number of silicon dimer lines across the ribbon width.The results show that perfect spin-filter effect,rectification,and negative differential resistance behavior can be found in these devices.This makes the heterojunctions exhibit of great potential application in the design of spintronic nanodevices.Chapter 5.We study the switch effect of dithienylethene-based polymer and also the influence of different electrode configuration on the switch characteristic.Generally,the realization of molecular switch requires two stable conductive states and can reversibly transform between the two conductive states in response to external triggers.If the difference between two states is large,one of them can be seen as "open" state and the other as "closed" state,thus the system can be a good molecular switch.In this chapter,we study the electronic transport behaviors of metal-dithienylethene-metal molecular junctions with different metallic contacts.The study found that the molecular with "closed" and "open" configurations really exist obvious switch effect and can be switched in a reversible manner by redox trigger or illumination.It is also found that the switch effect depends on the electronic properties of two configurations of molecular instead of the contact modes.Chapter 6.Summary and outlook.