The Characteristics Of3D Transition Metal Doped Organic Small Molecule Semiconductors

Spintronics is a hot topic in the research fields of condensed-matter physics and electronics. It makes use of both the charge of the electrons and their spin to design devices. In the last several decades, the microelectronics industry has been able to continually shrink the size of electronic components on silicon chips. However, transistors cannot scale down indefinitely, and they are now so small that further shrinking would compromise performance. The microelectronics industry is therefore looking beyond the classic transistor to secure the future of a new generation of electronics. Semiconductor spintronics is a promising candidate with many advantages including the nonvolatile data storage, the high speed of data processing, the high storage density, and the low energy consumption.Organic spintronics is the interdiscipline of spintronics and organic electronics. It makes uses of organic materials to transport and manipulate spin-polarized signals. Compared with inorganic semiconductors, it is more attractive that organic semiconductors have weaker spin-scattering mechanism. The spin-polarization can be maintained for a very long time. The organic semiconductors are composed of low-weight atoms (such as C, N, O, H, etc.) which lead to a small spin-orbit coupling (spin-orbit coupling scales with Z4, where Z is the atomic number) and long spin-relaxation time. Moreover, organic small molecule semiconductors have a variety of species, rich structure and physical properties, and the characteristics of low-weight, easy processing and mechanically flexible. Especially, their performance can be easily modified by functional group modification, hybridization and doping.In2004, Xiong et al. reported a GMR response of40%in a LSMO/Alq3/Co spin valve. In the following researches, devices based on other organic small molecules and polymers have also shown noticeable MR. In2011, Yoo et al. reported the V(TCNE)x/Rubrene/V(TCNE)x full organic spin valve using the organic polymer magnet V(TCNE)x as the ferromagnetic electrodes. They observed an about0.04%MR response. Although the absolute MR values remain small, it indicates the potential applications of organic magnetic semiconductors in all-organic spintronic devices. Baik et al. found ferromagnetism above room temperature in Co-doped Alq3thin films, impling that the preparation of organic semiconductors with ferromagnetic properties is possible. However, the magnetism in transition metal-doped organic small molecules remains unknown, and the accurate structures, which is the first step to the understanding of the magnetism, of these materials remains unclear.Not only the magnetism in transition metal-doped organic small molecules, the structure investigation of transition metal-organic small molecules are also crucial for the understanding of the spin polarized carriers transport through the transition metal-OSC interfaces (such as Co-Alq3) in spintronic devices. The structure and electronic properties in the interface affect remarkably the spin injection efficiency and MR value of the devices. A well-defined interface of FM/OSC is therefore crucial for designing the devices with high spin injection and detection efficiency. While the current researches mainly focus on the electronic structure in the interface, the reports on the structure of FM/OSC interface are rather rare.To solve the issues listed above, we performed the related experiments and simulations, the detailed contents and main results are given below:1. Study on the properties of LSMO ferromagnetic electrode in organic spintronic devicesLSMO is usually used as spin polarized electrode in organic spin valves. In this work, the Lao.67Sro.33Mn03thin films are grown on glass substrates in a mixed argon and oxygen atmosphere by using RF magnetron sputtering. Annealing effect on structural characteristics, transport behaviors and magnetic properties of LSMO films has been studied. The results indicate that the out-of-plane lattice parameter aLSMO contracts after annealing and is close to that of bulk LSMO abuik, implying that the internal strain arising from the lattice mismatch is almost fully relaxed. Nanocrystalline grains are also observed in the annealed films. The transport measurement indicates that the as-grown LSMO films show insulate-like behavior, while a clear metal-to-insulator transition is observed at268K in the annealed films. The magnetic measurement shows that the Curie temperature of620℃annealed films reaches315K, and the films have the largest saturation magnetization which is approximate to that of bulk materials. 2. Molecular and electronic structures of Mn-doped tris-8-hydroxyquinolinate galliumTris-8-hydroxyquinolinate gallium (Gaq3) is an organic small molecule semiconductor with excellent optoelectronic properties, which exhibits higher electroluminescence yields than equivalent devices based on Alq3. The molecular and electronic properties of Mn-doped Gaq3are investigated by ab-initio DFT cluster model calculations, Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectra (PL). The results show that the lowest unoccupied molecular orbital (LUMO) sets reside mainly on N2, N3p states, as well as on the p states of their third-nearest-neighbor C atoms. The electrons deplete from the Mn atom and accumulate mainly on the C, N atoms within the pyridyl ring of ligand A. The magnetic moment is mainly localized around the d orbital of Mn atom and the2p states of C and N atoms on pyridyl ring are also spin polarized due to the injected electrons. The positive charged Mn ions lead to an electron trap site in the energy gap and a red luminescence also appears in PL spectra which may be related to the MLCT excited states.3. Determination of the Structures of Transition metal-Mq3(M=Al and Ga) by X-ray Absorption Fine Structure SpectroscopyMetal-Mqqx (M=Al, Ga, Zn, Be and Ca, x=2or3) complexes play a key role in organic spintronics and organic optoelectronics. However, the accurate structure determination of these complexes has been a challenge for a long time. We determine the accurate structure of annealed Mn-Gaq3thin films by using synchronic radiation x-ray absorption fine structure (XAFS) experiments with the extended X-ray absorption fine structure (EXAFS) analysis at Mn K-edge performed in IFEFFIT software package. The results show that the Mn atoms interact with Gaq3molecules rather than forming inorganic compounds. The EXAFS results show that the Mn atoms locate at the attraction centers with respect to the N and O atoms in a Gaq3molecule and bond with some C atoms on the pyridyl ring in the other molecule (LUMO sets reside mainly on these C atoms). The nearest Mn-N and Mn-O distances are about2.09A and2.12A, respectively. O K-edge x-ray absorption near-edge structure (XANES) spectra show that a new trap state is introduced in the forbidden energy gap by doped metal atoms.We also investigate the structures of Co-Alq3and Alq3-Co interfaces by variable incidence angles grazing incidence XAFS (VIA-GIXAFS) method. By analyzing the XANES spectra at Co K-edge, it is found that the structures of Co-Alq3interfaces depend strongly upon the order of deposition. The interface Co-Alq3(Alq3is deposited on Co layer) is chemically stable without obvious structure changes. However, in the interface Alq3-Co (Co is deposited on Alq3layer), XANES spectra have large discrepancy with that of Co-Alq3interface. It indicates that Co interacts with Alq3molecule, forming a complicated structure similar with the local atomic structures around Co atoms in Co-doped Alq3thin films.4. Magnetism in Mn-doped Tris(8-hydroxyquinoline)AluminumThe magnetic and electronic properties in Mn-doped Alq3has been studied by ab initio density functional theory calculation. The doped Mn atoms deplete electrons to the adjacent non-metal atoms (C, N, etc.). Mn atoms act as a trap sites, and the local magnetic moments in the system arise mainly from the Mn d orbitals. Mn atoms and the transferred electrons localized on the C, N atoms in quinolate ligands form the states similar with bound magnetic polarons which have antiferromagnetic configuration. Two bound magnetic polarons antiparallel with the same magnetic ion have indirect ferromagnetic exchange interaction, leading to the collective magnetism. It may interpret the ferromagnetism found in the Mn-dopd Alq3thin films.