Magnetic And Optical Properties Of 8-hydroxyquinoline-based Small Molecules

Spintronics is a hot topic in the research fields of electronics and consdensed-matter physics. Classical electronics only considers the charge of the electrons, spintronics considers both the charge and spin characters. Core contents of spintronics include study of spin injection, transport, detection, and manipulation. Due to an increase of the spin degree of freedom on the basis of the charge degree of freedom, spintronics has brought high-density memory, and also a few new physical concepts or phenomena such as spin current, spin valve, spin hall effect and so on. The combination of spintronics and semiconductor materials is expected to produce new types of semiconductor spintronics devices, break through the bottleneck of Moore’s law in traditional microelectronics, and bring a revolution in information technology.Compared with inorganic materials, organic semiconductors are composed of low-weight atoms (such as C, N, O, H, etc.), which leads to a smaller spin-orbit coupling and a weaker hyperfine interaction. And these two roles are the main factor of spin-polarized carrier transport, resulting in a longer spin-relaxation time in organic semiconductors. On the other hand, organic semiconductors are candidates for spintronics because of the "soft" property. But the conductance mismatches in organic semiconductors for the application of spintronics decrease the efficiency of spin injection. Morever, the "ill-defined layer" on the interface of ferromagnetic metal electrodes and organic semiconductors hinders the spin transport in the organic semiconductor layer. Using organic magnetic semiconductors as spin inject electrodes can effectively resolve the problems of conductance mismatches and ill-defined layer. Organic magnetic semiconductors provide a new direction for full organic spin valves, and open a new chapter of organic spintronics.In 2002, Dediu et al. used semi-metal material LSMO as ferromagnetic electrode and sexithenyl (T6) as the transport layer. They reported the spin injection and transport in organic material firstly. In 2004, Xiong et al. reported an organic spin valve with a LSMO/Alq3/Co structure. They observed a 40% magnetoresistance at 11 K. In 2011, Li et al. observed a 0.04% MR response in a V(TCNE)x/Rubrene/V(TCNE)x full organic spin valve using the organic polymer magnet V(TCNE)X as the ferromagnetic electrodes. This is the first report about spintronic devices composed with full organic materials.Organic magnets have become the focus of attention in recent years due to the unique organic characteristics and different magnetic interaction mechanism. The magnetism of usual organic magnets originates in long-range order of the s and p electron spin. The magnetic mechanism is complex and difficult to synthesize, so that the usual organic magnets haven’t been applied in organic spintronics. Organic small molecules are low in weight, cheap, mechanically flexible, and chemically interactive. Recently, some researcher reported room-temperature ferromagnetism in transition metal doped organic small molecules. But there is one of key questions unanswered:the possibility of transition metal cluster can’t be ruled out, and the complexity of magnetism in organic materials requires a further check.8-hydroxyquinoline small molecules have been widely used in OLED, organic solar cell because of their excellent optoelectronic properties. In this dissertation, we studied the structure, electronic structure, and the magnetic properties of nonmagnetic doped 8-hydroxyquinoline complexes by experiments and first-principles density functional theory. The problems of magnetic metal cluster and secondary magnetic phase can be essentially avoided by using nonmagnetic elements. This is not only an efficient way to study the intrinsic property of organic magnetism, but it also provides new ideas for organic ferromagnetic semiconductors. Besides, transition metal 8-hydroxyquinoline small molecules may have intrinsic magnetic properties due to the partially occupied 3d states of transition metal. Organic thin films obtained by thermal evaporation are usually amorphous or microcrystalline, and it’s hard to get an effective ferromagnetic exchange interaction due to the long distances between the local magnetic moments. In this dissertation, we also studied the influence of crystalline structure on the magnetic and optical properties of 8-hydroxyquinoline small molecules. The detailed contents and main results are given below:1. Room temperature ferromagnetic properties of Al-doped bis(8-hydroxyquinoline)cobalt (Coq2) moleculesAl-doped Coq2 films with different proportion were obtain by thermal evaporation. Room-temperature ferromagnetism was observed in Al-doped Coq2 films measured by SQUID. FTIR and XPS spectra showed that the doped Al atom tends to bond with N and O atom of the quinoline ligands. XAFS showed that there is no decomposition and recombination behavior during the co-evaporating process. So the possibility of magnetic Co metal cluster can be excluded. The ferromagnetism may be attributed to the interaction between Al atoms and Np states around LUMO.2. Effect of oxygen on the magnetic property of bis(8-hydroxyquinoline)copper (Cuq2)The magnetic properties of Cuq2 were investigated by experiments and first-principles density functional theory (DFT) calculations. The as-prepared Cuq2 film shows paramagnetic behavior. After annealing in air, room temperature ferromagnetic properties were found in Cuq2 film. The Fourier transform infrared spectroscopy (FTIR) analysis indicates a new vibrational mode related to out of plane O-H bend in the annealed film. DFT calculations show that the energy difference between the ferromagnetic and the antiferromagnetic states is greatly increased after O doping, which may be responsible for the room temperature ferromagnetism in the annealed Cuq2 film.3. Room temperature ferromagnetic properties of dysprosium-doped tris(8-hydroxyquinoline) aluminumRoom temperature ferromagnetism was observed in rare earth dysprosium (Dy)-doped Alq3 organic small molecular films. The Dy-doped Alq3 films were prepared by co-evaporating pure Alq3 and Dy powders on Si substrates in vacuum. The Dy-doped Alq3 films show ferromagnetic behaviours at room temperature. FTIR spectra indicate that the doped Dy atoms prefer to bond with N and O atoms in the Alq3 molecules. The local magnetic moments of isolated Dy-doped Alq3 molecule is 6 μB. The room-temperature ferromagnetism is attributed to the large energy difference between the FM and AFM states.4. Preparation and characterization of magnetic bis(8-hydroxyquinoline)manganese (Mnq2)Crystals of Mnq2 were prepared by using physical vapor deposition (PVD) method. The magnetic properties were investigated by experiments and DFT calculation. The structural properties were investigated by scanning electronic microscopy and X-ray diffraction. The prepared Mnq2 crystals showed paramagnetic properties at room temperature. DFT calculations show that the energy difference between the ferromagnetic and the antiferromagnetic states is 0.1 meV, showing that it is difficult to prepare the ferromagnetic Mnq2.5. Effect of low-temperature annealing on structure and optical properties of amorphous tris(8-hydroxyquinoline) aluminium filmsNeedle-shaped nanostructured Alq3 nanorods have been obtained by annealing the Alq3 amorphous films at 150℃ which is lower than the glass transition temperature (172℃). The growth of Alq3 nanorods is attributed to the migration and stacking of Alq3 molecules which acquire thermal energy from the annealing process. The intensity of the photoluminescence (PL) firstly increases and then decreases as the annealing time increases and the annealing process leads to a spectral blue shift. The optical transmittance spectra of the Alq3 nanorods show a smaller average absorption in the visible range and a wider optical band gap compared with that of the amorphous films.