The Preparation And Characteristics Of Organic Small Molecule Spintronic Semiconductor Materials And Devices

As a very promising research area in condensed matter physics, spintronics has attracted a lot of interests during the past few decades. Different from the classical electronics, spintronics involves both the charge and spin characters of an electron. Fundamental studies of spintronics include investigations of spin injection, transport, detection, and manipulation. Its goal is to understand the interactions among the electrical, optical, and magnetic characteristics and to achieve novel spin electronic devices. Spintronics is a significant development of electronics, which has not only led to the emergence of high-density memory, but also resulted in some fundamental physical revolution, such as the spin current, spin valves, spintronic Hall Effect and so on. Spintronics devices include the combinations of magnetic metals or magnetic semiconductors with insulator, semiconductor, conductor, or superconductor. It was found recently that the spin polarization with injected current is closely related to the ratio of the resistances in injecting layer and transport layer. The resistance mismatch in the traditional materials hinders them from achieving high spin injection efficiency. Therefore, more attentions should be paid to the influence of interfacial effects on the spin injection and spin evolution during the transport process.In order to adapt spintronics to conventional microelectronics, plenty of researches have been done on spin in semiconductors. The conventional semiconductors used for devices and integrated circuits do not contain magnetic ions and are nonmagnetic. Moreover, the crystal structures of magnetic materials are usually different from that of the semiconductors used in electronics, which makes them incompatible with each other. Compared with conventional inorganic semiconductors, organic semiconductors have weak spin-orbit interaction, week hyperfine interaction, long spin diffusion length and 'soft'matter property, thus they become potential candidates for spin injection and transport applications in spin valves. In terms of spin injection in organic semiconductor, the use of organic magnetic materials as spin inject electrode is considered as the best way to solve the conductance and lattice mismatch problems in organic spin valves. Therefore, the study of organic magnetic semiconductor materials has become a new hotspot, which will promote the full development of the organic spin valve devices.Organic small molecule semiconductors have a variety of species and rich structural and physical properties. Moreover, their performance can be easily modified by functional group modification, hybridization, and doping. Organic semiconductors may open up a way to cheap, low weight, mechanically flexible and chemically interactive electronics. In the field of organic spintronics, organic small molecule semiconductors are used as the space layer of organic spin valves and the matrix materials for transition metals doping during the preparation of the organic magnetic materials.In 2002, Dediu's group firstly reported the spin injection and transport in organic material. They used half-metal material LaxSr1-xMnO3 (LSMO) as the source of electrons and sexithenyl (T6) as the organic layer. The spin injection has been detected, and the current is found to be spin polarized. In recent years, a lot of experiments have confirmed the spin injection and transport in organic materials. For example, Xiong et al. have observed spin injection and transport in a LSMO/Alq₃/Co organic spin valve in 2004. The measured magnetoresistance can be as high as 40% at low temperature. Majumdar et al. have observed a magnetoresistance of 80% at 5 K and 1.5% at room temperature in a LSMO/polymer/Co organic spin valve. They also found that there is a thin spin-selective tunneling interface between LSMO and the polymer, which improves the spin injection. Yoo et al. have observed magnetoresistance in V(TCNE)x/rubrene/LAO/LSMO organic spin valves where the organic magnets V(TCNE)x were used as the spin injecting electrode.Recently, ferromagnetism was found in Co-doped Alq₃ synthesized by thermal coevaporation of pure Co metal and Alq₃ powders. Clear ferromagnetic behavior with a magnetic moment of～0.33μB/Co was observed in 5%(Co/Al atom ratio=0.5) Co-doped Alq₃. It may open up a way to the organic small molecule magnetic semiconductors.After years of works, researchers have grasped the main characteristics of the organic small molecule spintronic semiconductor materials and devices. However, there are still many issues to be solved, such as ill-defined layer, room temperature magnetoresistance, and the structure of transition metals doped organic small molecule materials. The detailed contents and the main results are given below: 1. Positive magnetoresistance in organic spin valvesPositive spin valve effect-low resistance for parallel electrodes configuration-has been observed in organic spin valves fabricated by vacuum thermal evaporation using half-metal perovskite manganites Lao.67Sro.33Mn03 as the bottom electrode, cobalt as the top electrode, and tris (8-hydroxyquinoline) aluminum (Alq₃) as the organic semiconductor spacer. A positive giant magnetoresistance (GMR) of～12% has been observed at 100 K. We considered that the origin of the negative and positive GMR in LSMO/Alq₃/Co organic spin valve comes from the fact that the spin-resolved electronic structures of Co is related to thickness, interface, crystalline structure, impurity, and other possible factors.2. Origin of the ill-defined layer in organic spin valvesThe origination of ill-defined layer in organic spin valves was investigated by atomic force microscopy (AFM) and Rutherford backscattering (RBS) analysis. It was found that conductive bulges of LSMO film and self-grown pinholes in Alq₃ film other than Co inclusions could lead to the formation of ill-defined layer. The morphology of LSMO substrate had a strong influence on that of Alq₃ film, LSMO/Alq₃ and Alq₃/Co interfaces. Moreover, Alq₃ film with the thickness of 1～4 nm could be barriers, which was explained by small active area and added insulated layer in organic magnetic tunnel junctions. The results of RBS measurement showed that the top FM electrode Co only diffuse into the Alq₃ film less than 20 nm below the suface of the organic layer.3. Alq₃-Co nanocomposites based organic hybrid devicesLarge room-temperature (RT) magnetoresistance (MR) and temperature-dependent MR inversion have been observed in Alq₃-cobalt nanocomposites-based organic-inorganic hybrid devices. Negative MR-high resistance for parallel electrodes configuration-due to magnetization reversal of ferromagnetic (FM) electrodes has been observed at low temperature. As the temperature increases, the MR undergoes a sign change. At room temperature, a positive MR of～9.7% with the resistivity dropping monotonously with increasing magnetic fields has been observed. The RT MR is about two orders of magnitude of that in organic-FM nanocomposites measured with nonmagnetic electrodes. The enhancement of RT MR is attributed to the injection of spin polarized carriers into Alq₃-Co nanocomposites. 4. The structure of Co-doped Alq₃ filmThe structural properties of Co-doped Alq₃ have been studied by grazing incidence X-ray absorption fine structure (GIXAFS) and Fourier transform infrared spectroscopy (FTIR). GIXAFS analysis suggests that there are multi-valences Co-Alq₃ complexes and the doped Co atoms tend to locate at the attraction center with respect to N and O atoms and bond with them. The FTIR spectra indicate that the Co atoms interact with meridional (mer) isomer of Alq₃ rather than form inorganic compounds.5. The magnetic and optical properties of tris-8-hydroxyquinoline iron (Feq₃)The structure, magnetic properties, and photoluminescence (PL) of Feq₃ films are experimentally investigated. The corresponding properties and origins are examined and studied by means of first-principles density functional theory. Our experiments show that in contrast to the nonmagnetic behavior and the green emission of Alq₃ film, the magnetic property and photoluminescence of the Feq₃ film display paramagnetic behavior at 5 K and violet emission (392 nm) at room temperature, respectively. Our calculated electronic structure of Feq₃ molecule indicates clear exchange splitting between the majority and minority spin channels. The total magnetic moment of Feq₃ is about 1μB, which mainly derives from the localized Fe 3d orbital with a little contribution of the neighboring nonmetal (C, N, and O) atoms. The observed paramagnetic behavior is due to the small energy difference (1-2 meV) between ferromagnetic and antiferromagnetic coupling in Feq₃. And the electronic structures indicate the violet emission comes from the transition between LUMO and HOMO.