Theoretical Study On The Effect Of Adsorption On The Spin Transport Properties Of Molecular Devices

To keep the trend of the continuous miniaturization of the electronic components in semiconductor chips,a strategy of constructing molecular electronic devices by using molecular as the building blocks is proposed.During the research on functional molecular devices,it is found that molecular spintronic devices can be designed with the electron spin as the information carrier,which has further superior performance in information processing,data storage,signal transmission and energy consumption.By far,a wide variety of functional molecular spintronic devices have been designed,fabricated and investigated theoretically and experimentally.Studies have shown that molecular devices can be regulated in some way to achieve a specific function.Generally,the electronic structure,magnetism and transport properties of molecular devices can be affected by various external factors such as magnetic field,electric field and strain.In addition,some experimental and theoretical studies have shown that the electronic structure and magnetism of the device can be effectively changed by adsorption.For example,molecular spin sensors with gas sensing function and spin filters with spin filtering effect can be obtained through adsorption.Adsorption mainly include physical adsorption and chemisorption,which is a strategy with diverse selectivity and easy to implement.In this thesis,the effects of atomic or molecular chemisorption on the electronic structure,magnetic properties and spin transport properties of molecular devices were studied by using the non-equilibrium Green's function method based on density functional theory,and then high-performance gas sensor and spin filter were designed at the molecular level.The main research contents and results of this thesis are as follows.1.The effect of fluorine atom adsorption on the electrical transport properties of boron nitride nanotubesExploring and regulating the magnetism of low-dimensional materials is beneficial to the design of nanoscale spintronic devices.Experimental and theoretical studies show that fluorination can effectively modulate the electronic structure and magnetism of nanostructures.In this work,the effects of chemical adsorption of single and double fluorine atoms on the electronic structure,magnetic properties and spin transport properties of boron nitride nanotubes were studied.Calculations show that single fluorinated Boron nitride nanotube(SF-BNNT)has ferromagnetic and semi-metallic properties.The six adsorption configurations of double fluorinated Boron nitride nanotubes(DF-BNNTs)show ferromagnetism.However,the electronic structure of DF-BNNT is closely related to the adsorption configuration of the two fluorine atoms,and DF-BNNT can be a semiconductor,a semi-metal or even a conductor.The spin current-voltage curves of the two-probe devices based on SF-BNNT or DF-BNNT indicate that some devices exhibit high spin filtering efficiency.It can be explained by the change of BNNT energy bands caused by the adsorption of fluorine atoms and the overlap change of spin-resolved energy bands of the two electrodes under bias voltages.2.The effect of oxygen adsorption on the spin transport properties of transition metal annulene complexesIn a single-molecule spin device,the core molecule of the device is easy to interact with the surrounding gas molecules due to adsorption,which will affect the spin transport characteristics of the device.In this work,molecular devices were constructed using transition metal-dibenzotetraaza[14]annulene complex(TM(DBTAA),TM=Co,Cu,Ni)as the core molecule and the single-walled carbon nanotubes as electrodes,the effects of O₂ adsorption on the spin transport properties of the devices were investigated.The results show that the spin of the allowed electrons through the Co(DBTAA)device changes after adsorption of O₂,the spin filtering efficiency(SFE)of the Cu(DBTAA)device is improved after adsorption of O₂,and the Ni(DBTAA)device changes from spin degenerate to spin polarized after adsorption of O₂ and the spin filtering efficiency can reach up to 90%.Through further analysis,it is shown that after the adsorption of O₂ molecule,of which the ground state is triplet,the spin down electrons of the devices transfer to the O₂,which leads to the change in the number of spin up and spin down electrons in the devices,and then changes the spin transport characteristics of the devices.This paper is divided into five chapters.In the first chapter,the development of molecular spintronics is reviewed,and some functional molecular devices are briefly introduced.In the second chapter,the theoretical methods for calculating the charge transport properties of molecular devices are introduced.The third chapter studies the effect of single-/double-fluorine-atom adsorption on the electrical transport properties of boron nitride nanotubes and designs molecular spin electronic devices with high SFE.The fourth chapter studies the effect of oxygen adsorption on the spin transport properties of transition metal dibenzotetraaza[14]annulene complex,and finds that oxygen adsorption can effectively modulate the SFE of devices.The fifth chapter summarizes the work and the innovation points following a prospect.