Dynamics Of Spin Injection And Transport In Organic Polymer

In the past few decades,the potential applications of electronic spin in semiconductor devices have attracted a lot of interest.Different from the classical electronics,spintronics involves both the electronic and spin characters of an electron.Fundamental studies of spintronics include investigations of spin injection,transport and detection in electronic materials,as well as spin manipulation.The goal is to understand the interaction between the spin of electrons and crystalline environments and to make useful devices.Among the fundamental studies of spintronics,magnetic semiconductors and half metals are the key materials.The basic structure of a spintronics device is a semiconductor heterojunction which is used to manipulate the spin injection,transport and detection. Therefore,the spin transport in magnetic semiconductors,half metals and a semiconductor heterojunction is one of the central questions.Recently,several model devices have been schemed out.For example,Datta and Das designed the first spin transistor.Zutic et al. provided a new type of magnetic field-effect transistor.However,the low efficiency of spin injection into semiconductor blocks the practical applications of such devices.The spin polarization of injected current is closely related to the ratio between resistances of the two layers.Therefore,more attentions should be paid to the influence of interfacial effects on the spin injection and spin evolution during the transport process.Compared with conventional semiconductors,soft organic semiconductors are good candidates as the contact materials between magnetic layers.The spin relaxation length is much longer than that of conventional semiconductors due to the weak spin-orbital and hyperfine interactions.The electric,magnetic and optical properties are unique in both the organic molecules and conjugated polymers.Up to now,the electric properties of quasi-one dimensional conducting polymers have been well understood.However,there is a lack of full understanding about the spin proerty of organic polymer,since it is a new field called organic spintronics.In the past few years,there have been extensive theoretical and experimental studies on organic spintronics.Dediu's group first observed the room temperature spin polarization injection into conjugated organic material sexithienyl(T₆) in La_0.7Sr_0.3MnO₃/T₆/La_0.7Sr_0.3MnO₃ planar hybrid junction.The negative magnetic resistance indicates the spin injection in polymer.Xiong et al.have also studied the spin injection and transport in La_0.7Sr_0.3MnO₃/Alq₃/Co organic spin valve.The measured magnetoresistance can be as high as 40%at low temperature.Subsequently,the molecular spin valves and nanowire organic spin valves have been fabricated experimentally.Due to the important influence of interfacial effect on spin injection,much attention has been paid to improve the capability of organic spin devices.Besides,in organic light emitting diodes(OLEDs),spin polarized materials can replace conventional electrodes,which shows that light emitting efficiency can be tuned by spin-dependent method.Besides the experimental investigations,the theoretical researches on spin injection and transport are mainly based on two methods:the macroscopic thermodynamic theory and the microscopic spin-charge coupling model.The former can well describe the spin diffusion of carriers in polymers whereas the latter is suitable for revealing the microscopic mechanism.Although much effort has been devoted to the study of organic spintronics,many questions are still under debate,such as the character of spin carriers in polymers,the influence of interface coupling and conductance between polymers and electrodes on spin injection,spin diffusion of spin carriers in polymers,the impact of spin related interactions on spin transport,etc.It's worthy noting that,due to the strong electron-lattice interactions in conjugated polymers,the charge carriers are self-trapped states rather than ordinary electrons or holes.Considering the unique charge-spin relation of self-trapped states in polylmers,Xie et al.investigated the ground state of the ferromagnetic metal/polymer junction.However, such theoretical model is based on a static picture while the spin injection and transport in polymers is obviously a dynamic nonequilibrium process.Therefore,much is need to know about the spin dynamics.In this paper,we investigated the dynamic process of spin injection and transport in polymers in the framework of the well-known Su-Schrieffer-Heeger(SSH) model and Brazovskii-Kirova(BK) model,which provide the basic quantum characteristic of the system. The detailed research contents include:1.Theoretical research on spin injection in an organic device with a spin polarized self-assembled monolayerTo date,the spin injection has been achieved mainly by introducing the ferromagnetic electrodes in spin devices.Here we suggest another approach to achieve spin injection in an organic device by a spin polarized self-assembled monolayer(SPSAM).Self-assembled monolayers have been widely used to control the charge injection in organic electronic devices since they can change the coupling or barriers between the metal and the polymer for charge injection.However.the introduction of a SPSAM can break the spin degeneracy of the whole system.It was found that the coupling between metal and polymer is spin-dependent due to the involvement of SPSAM.In addition,the coupling between metal and polymer can be tuned by the inserted SPSAM.The spin injection is also tuned correspondingly.It should be mentioned that,the spin-independent manipulation can reduce injected spin polarization. Therefore,to achieve a high spin injection,the spin-independent manipulation should be depressed.In addition,spin carriers transporting in the polymer layer are also discussed.A dynamic picture of spin carriers in polymer is given in the frame work of non-adiabatic approach. During the dynamic transport in an organic device,one electronic state may extend to the whole system,which includes both the electrode and the polymer layer.Contrasting with rigid semiconductors,the electronic state injected into the polymer layer is confined together to form a wave-packet due to strong electron-lattice interactions.A spin carrier with non-integral electronic charge and net spin has been obtained within the polymer side,which is defined as a spin quasi-bipolaron.Beyond the definition of a normal spin polaron or a spinless bipolaron, the spin quasi-bipolaron is a direct proof of spin injection.2.Influence of spin angle between oppositely charged polarons on the yield of excitonsRecently in organic light emitting diodes(OLEDs),ferromagnetic materials can replace conventional electrodes,which is aimed to lift or tune light emitting efficiency by spin-dependent method.When the two ferromagnetic electrodes are parallel or antiparallel arranged,the yield of singlet exciton is 0 and 50%respectly.That is the yield of singlet excitons are greatly influenced by the angle between the polarized orientations of two ferromagnetic electrodes.Since the spin orientation of polarons injected into polymer is directly decided by the ferromagnetic electrode nearby,we studied the influence of spin angle between oppositely charged polarons on the yield of excitons.It was found that in the absence of electron-electron interactions,the yield of total excitons is independent onφ,while the yield of singlet and triplet exciton presents a linear variation with cosφ,whereφis the angle between the spin orientations of polarons.The presence of electron-electron interactions shows a depression on the yield of singlet exciton.However,we found that the formation of singlet excitons from antiparallel spin oriented polaron is not influenced by the electron-electron interactions.3.Spin precession of a polaron in organic polymers induced by a gate voltageThe spin manipulation during the transport process is also the basic issue for the application of organic spin devices.There are mainly two approaches to manipulate the spin of carriers.One is the spin-valve setup,where the output current is governed by the spin-polarization of ferromagnetic contact.Another is the so called spin field effect transistor. The spin-polarized current is injected from a ferromagnetic material and then collected by the second ferromagnetic material on the other side.During the transport process,the spin of a polaron appears periodic precession when it transports in polymer under a gate voltage.The localization of a polaron makes it easy to probe the spin precession dependence on the position of a polaron.It was found that the spin precession takes place only within the highest occupied state of the polaron.Such intralevel spin precession is different from the interlevel spin precession for normal semiconductor.For a localized spin carrier in polymer,the spin precession period length of a polaron is closely related to the strength of spin-orbit interaction. Therefore,by tuning the spin precession with respect to the magnetization direction of the ferromagnetic collector,the transmitted spin-polarized current can be modulated.Besides appling the gate voltage,the spin precession can be also tuned by the magnetic field.When spin precession of the moving polaron is induced by a magnetic field,the periodic time of the spin precession is not dependent on the strength of driving electric fields, while the periodic length is related to the strength of driving electric field.When spin precession of the moving polaron is induced by a gate voltage,things are different.The periodic length of the spin precession is independent of the strength of driving electric fields, while the periodic time is in inverse proportion to the strength of driving electric field. Therefore we can choose appropriate spin control method according to actual need.