Theoretical Study On The Charge And Spin Transport Properties Of Graphene-boron Nitride Nanoribbons

In 2004,Geim and Novoselov at the University of Manchester isolated the graphene from graphite by mechanical stripping,for the first time.This single-atomic layer thickness material immediately attracted the worldwide’s attention.Monolayer graphene is a true two-dimensional material,which breaks people’s perception that two-dimensional materials can not exist without a 3D base.At the same time,graphene has excellent mechanical,thermal,magnetic and electrical properties,and is the potential of next generation nanoelectronic devices’ candidate materials.Therefore,a large number of scientists are attracted to study graphene and its derivative materials,and Geim and Novoselov are also shared the 2010 Nobel Prize in physics.Graphene has some special magnetic properties,such as high carrier mobility,weak spin-orbit coupling and long spin-scattering time,more and more scientists are attracted to study the magnetic properties of graphene.It is well known that conventional magnetic materials are mainly derived from d or f shell electrons,and perfect carbon materials usually do not exhibit magnetism,but we can make the graphene produce magnetic by modifying or manufacturing defects.In this paper,we study the charge-spin transport properties of graphene-boron nitride nanoribbons by density functional theory and non-equilibrium Green’s functions.The electron spin polarization of the system is controlled by introducing a vacancy on the graphene-boron nitride nanoribbons or applying a gate voltage to produce a spin-polarized current.The main contents of this paper are as follows:1.In this paper,we investigate the electron spin transport properties of the graphene-boron nitride nanoribbons in the presence of vacancies.The conduction band and valence band gap of the zigzag graphene nanoribbons is zero.However,the band gap of the zigzag boron nitride nanoribbons is 4.5 e V,which is a insulator.Therefore,zigzag-type boron nitride nanoribbons replace zigzag-type graphene nanoribbons can open the zero band gap.(ZBNNR)4-(ZGNR)4 and(ZBNNR)5-(ZGNR)3 were choosed from calculating the projection patterns of the different models.The results show that the modelsare in good agreement with our requirements,and then based on these two models,we study the spin transport properties of systems in different vacancies.We investigate the effect of vacancy types(N vacancy and B vacancy)and vacancy positions on electron spin transport properties,respectively.From the results we can not see spin polarization and negative differential resistance phenomena when there is no vacancy.However,with the vacancies in our model,the spin polarization phenomenon and the spin negative differential resistance phenomena appears.When the vacancy is N vacancy,there will be spin negative differential resistance phenomenon occurs;and when the vacancy is B vacancy,there is no negative differential resistance phenomenon.When the N vacancy is at the upper edge of the nanoribbons heterostructure,the spin-down transport channel is the main channel.On the contrary,when the N vacancy is at the lower edge of the nanoribbons heterostructure,the spin-up transport channel is the main channel.It can also be found from the results that the magnitude current value of the N vacancy is much larger than that of the B vacancy.While the vacancy is in the center of the graphene-boron nitride nanoribbons,no matter the vacancy is N vacancy or B vacancy,the magnitude of the current value is larger than that when the vacancy is at the joint position of the graphene and boron nitride nanoribbons.It is expected that the model devices we have studied will be applied in future spintronics and nanoelectronic devices.2.In this paper,the electronic transport properties of(ZBNNR)5-(ZGNR)3-Bv nanoribbons heterostructures under different gate voltages were studied by using the density functional theory and the non-equilibrium Green’s function.When there is no gate voltages,the magnitude current value of this model is very small.It has relatively large turn-on voltage and no spin polarization and negative differential resistance phenomena.It is a typical semiconductor characteristic material.When applied the gate voltages,a significant spin filtering and rectification phenomena occurred.When the applied gate voltage is positive voltage,the transport properties are relatively large adjustment,but the spin polarization phenomenon is weak.From the PDOS spectrum,we can see the electronic state density mainly localized on the C atom,and the electronic state density during entire central scattering region is localized,does not expand,so the magnitude ofcurrent value is relatively small.When the applied gate voltage is the negative gate voltage,there will be a significant spin filter phenomenon,because the negative gate voltage adjust the spin down transport channel current more obvious,but for the spin up transport channel adjustment is not obvious.It can be seen from the PDOS spectrum that the electronic density state is mainly localized on the N atom and the B atom,and the N atoms and B atoms around the vacancies in the central scattering region are extended,as the mainly electronic transport channel,the current is relatively large.When the applied gate voltage is-4 V,the rectification ratio can reach RR = 44 at the bias voltage of 0.3 V for the spin up channel.The high rectification ratio and the spin negative differential resistance indicate that this model can be used as spin filter devices.3.In this paper,the density functional theory and the non-equilibrium Green’s function are used to study the electronic transport properties of single phenalenyl molecular connected to gold electrodes.Phenalenyl molecule is a highly symmetric(D3h)organic radical,there are two different connections between the molecule itself and the gold electrodes,two second-nearest or one second-nearest and one third-nearest.When the central molecules are used second-nearest connected with the gold electrodes,the magnitude of current value will be restrained with the increase of the applied gate voltages,and the current value under the positive and negative bias is symmetrical,and there is no rectification phenomenon.However,when the central molecule is one second-nearest and one third-nearest atom connected to gold electrodes,the magnitude of current value under positive bias is gradually raised with the increase of the applied gate voltages,however,the magnitude of current value under the negative bias is almost a constant resulting in a rectification phenomenon.The system is asymmetric,then it may produce rectification phenomenon,which can manufacture molecular rectifier devices.