The First Principle Study Of Spin Polarization In Charged Molecule Polythiophene

With the rapid development of computer technology, the first principles calculation basing on density function theory has become an important means to study material performances and were widely used in solid, surface, molecular and biological system research. In this dissertation, we studied the effect in organic micromolecules induced by charge injection using the first principles calculation. Combined charge properties and spin characteristics, organic spintronic studies spin polarized electron injection, transport, detection and spin control. Research about this mainly concern about two types of devices, one is organic device that electrode is made by magnetic metal, the other is organic device without any magnetic element. In2002, Dediu team used LSMO as electrode, six-thiophene(6T) as the intermediate layer to prepare LSMO/6T/LSMO structure device in which the spin polarized injection and transport is realized. In2008, researchers observed magnetic resistance in the structure of ITO/PEDOT/6T/Ca/Al devices by exerting weak magnetic field. Either manetic or non-magnetic electrode is used, magnetic resistance is observed in the organic layer6T. It means the spin polarized electron transport is not related with the electrode’s magnetic performance and we notice that there is charge injection in the two types devices, so the observed magnetic resistance may be releated with organic layer’s characteristics. Organic materials are different from inorganic materials, electron(hole) injection will from local states, such as polaron and bipolaron. Although the spin-orbit coupling is weak in the organic materials, the realations between charge and spin in organic materials will present some unique phenomenon. Recently, calculation found the charge injection in Alq3molecule will lead to spin polarization, the emergency of spin density and the presence of a net magnetic moment. It should be related to the long spin relaxation and giant magnetic resistance found in Alq3. The paper analyzes the causes of the emergency of magnetic moment, and draws conclusion that it is due to the different change of the three Al-N bonds length led by charge injection, which finaly causes molecular structure change. The Alq3molecular structure is complex and the influence led by charge injection is also complex. For6T molecule. It’s worth to study if charge injection causes the similar effect.Polythiophene is a type of conductive polymer which can reach high conductivity at room temperature by doping, is worth of study and has drawn many task group’s attention in the world. In addition, polythiophene and its derivatives also have a series of excellent properties, which have very broad application prospects in electrical conductors, nonlinear optics, thermal color phenomenon, light battery and other aspects. This kind material has become one of matrixs which are used to study the relation between conjugated polymer structure and properties, design new functional conductive polymers. At present, the study of polythiophene and its derivatives mainly concentrated on the synthetic of high performance derivatives, and making clear the relation between polythiophene structure and properties will provide the laboratory development of new thiophene derivatives with theoretical basis and support. Polythiophene only can be conductive on codition of doping, and present research maily concentrated in undoped system, the research about structure and properties of doped polythiophene is little. In this paper, we use thiophene series molecules as research object, study the effect led by charge injection by first principles calculation which basing on density function theory. The detailed research and main results are given below:1. Spin polarization in charged6T moleculeAt first, we study the neutral6T molecule, the result show that it’s nonmagnetic and it is plane molecule with Mirror symmetry. After charge injecting, the bond lengths change, but the symmetry of molecular structure hardly change. With the increase of charge injected quantity, net magnetic moment is found in6T molecule, which increases with the injection of charge quantity increase, but the spin polarized charge quantity is always less than the injected charge quantity. We study the spatial distribution of spin polarized charge which has a certain symmetry, and the distribution of molecular symmetry is closely related to the symmetry of structure. 2. Spin polarization in charged thiophene moleculesWe find netural thiophene molecules are nonmangnetic, their structure has certain symmetry and they are planar molecules by computation. After injecting charge, the atom bonds length change but the symmetry of molecule structure do not alter. When the quantity of injected charge reaches a certain value, the molecule will present magnetic moment, namely there is "critical electric quantity", and different molecules have differernt critical quantities. Comparison of2T,3T,4T,5T,6T molecules’"critical electric quantity", we conclude that the critical quantity is related to molecular length, which increase as the molecular length increase. In addition, the critical quantity is also connected with the structural symmetry, the thiophene molecules which contain odd number of thiophene rings has approximate Mirror symmetric structure and their critical quantity is relatively small. After injecting charge, the quantity of spin polarized charge is smaller than the quantity of charge injected. We study the polarization charge distribution of thiophene molecules, and find it presents certain symmetry which is related with the molecular structure symmetry. It further illustrates the importance of molecular structure, which not only affects the "critical electric quantity", but also affects the distribution of polarization charge.