Construction And Mechanism Of The Spintronic Devices Based On Chiral Conductive Polyaniline

By introducing the concept of spin into traditional electronics,spintronics studies how to manipulate electronic spins efficiently and precisely to achieve the storage,transmission and processing of high-density information.Organic molecules,due to the weak spin-orbit coupling,have expected long spin relaxation time and spin diffusion lengths.Combined with the desirable properties such as flexibility,low cost and ease of large-area production,organic materials are of particular attraction in spin-polarized transport and the research on all-organic spintronic devices may become a trend.At present,many spintronic devices require a working environment with low temperature and strong magnetic fields,which is not conducive to industrial development.In order to solve this problem and drive further the application of electron spins in magnetic storage,according to the chirality induced spin selectivity(CISS)effect and spin-transport principles,conductive polymers,supramolecular chirality and spintronic devices were integrated into one topic in this thesis.Starting from the synthesis of chiral and achiral conductive polyaniline(PANI)fibers,we studied the structural characteristics and conductive mechanism of chiral polyaniline,fabricated the Au/chiral-PANI/Ni,Au/chiral-PANI/Au,Ni/chiral-PANI/Ni two-terminal spintronic devices and Au/achiral-PANI/Ni devices,completed the performance test and analysis of the above devices and obtained higher spin polarization(SP)and magnetoresistance(MR)at room temperature and low magnetic fields.Besides,the experimental results were verified by theoretical simulation and calculation.The main work is as follows:1.By using the method of chiral dopant(camphorsulfonic acid,CSA)in-situ induced polymerization,we synthesized chiral conductive PANI microfibers,which can be acted as the spin transporter in our chiral devices.Characterized by SEM,TEM,SAED and XRD,the fibers showed an ultra-ordered helical structure with a diameter of about 1?m and a length of about 20?m.The circular dichroisms(CD)of the chiral PANI fibers showed mirror images and the ellipticity was approximately 800 mdeg.After that,the achiral conductive PANI fibers were prepared by achiral p-toluenesulfonic acid(PTSA)as small-molecule dopant and amplified by a subsequent self-assembly process in order to be used in the achiral devices.The SEM showed that the achiral fibers also had a diameter of about 1 ?m and a length of about 20 ?m.2.We fabricated two-terminal lateral spintronic devices in which non-ferromagnetic electrode Au and ferromagnetic electrode Ni are electrically connected by a single chiral-PANI microfiber.The devices'electrical properties and magnetic responses including the spin polarization(SP)and magnetoresistance(MR)were tested.The results showed the high spin-selectivity electron transport in these devices.It was found that the spin polarization decreased as the bias increased but the value was still up to 0.35 at room temperature and a high bias of 5V.In a word,the chiral devices exhibited high spin polarization and magnetoresistance and the spin diffusion length reached to a large scale(a few microns)at room temperature and low magnetic field conditions,showing the devices could be used as effective spin-filters.In addition,when the applied magnetic field was parallel or antiparallel to the axial direction of the single fiber,the devices exhibited high resistance or low resistance depending on the fiber's chirality,a typical property of a spin logic device,suggesting a great application potential for introducing chiral organic materials into spintronic devices.3.Control experiments were performed in which we only change one factor(electrode or chirality)and kept the other parts of the setup unchanged.By investigating their electrical properties and the relevant magnetic response,we found almost no spin selectivity in Au/chiral-PANI/Au,Ni/chiral-PANI/Ni and Au/achiral-PANI/Ni devices,whose I-V curves have no magnetic field orientation dependent splitting.However,we can see clearly obvious spin selectivity and spin filtering ability in electron transmission through the Au/chiral-PANI/Ni devices in comparison with the contrast experiments,suggesting the important role of the chiral ultra-ordered structure and heterogeneous electrodes(a ferromagnetic electrode and a non-ferromagnetic electrode)in detecting the spin signals.4.The first-principles method were used to optimize the model of chiral PANI molecule and then theoretical calculations were performed based on the tight-binding SSH model.The results showed that the electronic transport in chiral PANI had a high spin selectivity and the spin polarization gradually decreased and tended to stabilize when the bias increased.Due to ?-? stacking and spin-orbit coupling,when electrons moved along the long axis of the fibers at a positive bias,right-handed PANI fibers prefered to transfer the spin-down electrons and tended to hinder the spin-up electrons' movement whereas the left-handed PANI fibers were unbeneficial to pass through the spin-down electrons,facilitating the spin-down electrons' transport.If a negative bias was applied to reverse the direction of the electron motion,then right-handed PANI fibers facilitated the spin-up electrons' transport while those left-handed PANI fibers facilitated the spin-down electrons'transport.The theoretical calculation results were consistent with our experimental results and the results of previous research on chirality.