Investigation On The Adsorption Of Organic Molecule On Ferromagnetic Surface

Organic materials are mainly composed of light elements, such as carbon, hydrogen, oxygen and nitrogen. In practice, the properties of organic materials can be changed by substitution of component elements or modification of molecular structure. Compared with conventional inorganic semiconductors, organic materials are easily fabricated and low cost. At the same time, because of the weak spin-orbit coupling in organic materials, the spin transport properties of organic materials are improved immensely compared with conventional semiconductors present in today’s devices. Commonly, owing to the π electron conjugated system, organic molecules have shown excellent magnetic, optical and electric properties, such as organic magnetic field effect, organic electroluminescence effect and high conductivity. Therefore, organic materials have huge potential and broad perspective of application.Combining molecular electronics with organic spintronics represents one of the most exciting avenues in building future spintronic devices, such as the widely used organic spin valve. Organic spin valve is a layered structure of two ferromagnetic electrodes separated by nonmagnetic spacer and allows spin polarized electrons to, travel through it. The efficiency of organic spin valve depends on the spin transport throughout and the efficiency of spin injection into the nonmagnetic organic spacer.Nowadays, most of the organic light emitting devices (OLED) are fluorescent. That means only the energy of singlet excitons are mainly utilized. However, according to quantum mechanics, the ratio of singlet exciton to triplet exciton is 1:3. What’s worse, the optical and energy loss in devices make the efficiency of OLED lower. How to improve the ratio of singlet exciton is not only a crucial issue but also an avenue in increasing the efficiency of OLED fundamentally. Spin polarized injection through ferromagnetic electrodes could increase the efficiency of OLED effectively. However, the interaction between organic molecules and ferromagnetic surface influences the efficiency directly. Understanding the relevance of interaction will help to tailor the electronic and magnetic structure of such interface and, furthermore, overcome the significant spin loss observed in spintronic devices and improve the efficiency of spin polarized injection. Consequently, an investigation of the interaction between organic molecules and ferromagnetic surface is vitally necessary.Practically, the distance between organic molecules and ferromagnetic surface could be changed by environmental and artificial factors. A research of the relation between distance and spin polarization is demanded. Focused on this difficulty, we investigate the relation between organic spintronic properties and distance based on density functional theory. The results show that the spin polarization is not monotonically relative to distance increase. Both anti-ferromagnetic coupling and charge transfer effect devote to spin polarization of organic molecules. Main contents of this thesis are summarized as follows1.On basis of density functional theory, spin polarization of organic molecules under different distance has been investigated. Density of states and energy of different systems are given and we discover the spin polarization of organic molecule attenuates while distance increasing.2.Both atomic magnetization and net molecular magnetization are shown. According to our calculation, neither atomic magnetization nor molecular magnetization change monotonically as distance increasing.3.Furthermore, to interpret the reason of positive molecular magnetization, we compute the net charge of benzene molecule. As the calculation shows, the benzene is not neutral. What’s more, the net charge decreases as distance grows larger. This result coincides with the phenomenon that the molecular net magnetization attenuates with larger distance.