Fenggong Wang's research while affiliated with University of Maryland, College Park and other places

Cu-induced spin polarization in tris(8-hydroxyquinoline) aluminum (Alq3) has been studied by first-principles calculation. The entanglement between an electron transfer from Cu to Alq3 and a structural distortion of the Alq3 molecule gives rise to a strong spin polarization in this originally nonmagnetic molecule Alq3. The calculated total magnetic moment mainly resides on the Alq3 molecule (0.6 μB), while that on Cu is moderate (0.4 μB). The finding of a spin polarization induced by the nonmagnetic element Cu is helpful for understanding of spin-related phenomenon in Alq3 and potentially enabling future magnetic-metal-free organic spintronics.

We report for the first time the controllable growth of tubular nanostructures at the nanoscale of the broadly applied organic drug material, tris(8-hydroxyquinoline)gallium (Gaq3), by an extremely facile approach. The products show a relatively low turn-on field (8.5 V μm-1) and a very high maximum current density (2.3 mA cm-2). The length of the Gaq3 tubes and nanostructures can be readily manipulated and the adopted method is highly transferrable to other organic materials.

Metal-Mqx (M = Al, Ga, Zn, Be, and Ca, x = 2 or 3) 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. Here, we report the structure of Mn-Gaq3 investigated by using first-principle density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy. First, the structures of Mn-Gaq3 were predicted by first-principle DFT calculations. Then, all reasonable structures achieved from the calculations were used to fit the EXAFS spectra. By this method, the structure of Mn-Gaq3 is well obtained. We believe this method is also applicable to other metal-Mqx films.

The manganite La0.67Sr0.33MnO3 (LSMO) thin films were grown on glass substrates in a mixed argon and oxygen atmosphere by using RF magnetron sputtering. The structural characteristics, transport behaviors and magnetic properties of LSMO films were studied by annealing the films in air at 550 and 620°C. The out-of-plane lattice parameter aLSMO contracted after annealing and was close to that of bulk LSMO abulk, indicating that the internal strain was fully relaxed. Nanocrystalline grains were observed in the annealed films. Enhanced saturation magnetization and metal-to-insulator transition temperature (TMI=268 K) were also obtained. Curie temperatures (Tc) of the as-grown films was 340 K with the same as that of annealed at 550°C, but dropped to 315 K when the annealing temperature increased to 620°C, which can be attributed to the oxygen release during annealing in atmosphere.

The magnetic properties of tris(8-hydroxyquinoline)iron (Feq3) are investigated by experiments and first-principles density functional theory. In contrast to the diamagnetic behavior of Alq3, the Feq3 films deposited by vacuum thermal evaporating exhibit paramagnetic behaviors at 5 K. The calculated electronic structure of Feq3 molecule shows clear exchange splitting between the majority and minority spin channels. The total magnetic moment is about 1 μB, which mainly derives from the localized Fe 3d orbital. The paramagnetic behavior observed is ascribed to the small energy difference (1 meV) between ferromagnetic and antiferromagnetic coupling.

Molecular and electronic structures of Mn-doped tris-8-hydroxyquinolinate gallium (Gaq3) were investigated by first-principle density-functional theory (DFT) calculations and Fourier transform-infrared spectroscopy (FTIR) and photoluminescence (PL) spectra measurements. Calculation results show that the lowest unoccupied molecular orbital (LUMO) sets reside mainly on N2, N3 p states, as well as on the p states of their third-nearest-neighbor C atoms. The doped Mn atoms prefer to be located at the ligand center. 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 the 2p states of C and N atoms on pyridyl ring are also spin polarized due to the hybridization with Mn 3d states. The positive charged Mn ions lead to an electron trap site in the energy gap and a red luminescence appears in PL spectra, which may be related to the MLCT excited states.Highlights► Mn-doped tris-8-hydroxyquinolinate gallium thin films are first studied. ► Interaction between Mn atoms and Gaq3 molecule is simulated by the DFT method. ► Electrons deplete from Mn atoms and accumulate on pyridyl ring of Gaq3 molecule. ► Hybridization between 3d and 2p states leads to spin polarization of C, N atoms. ► Red luminescence is found in Mn-doped Gaq3 films.

Large room-temperature (RT) magnetoresistance (MR) and temperature-dependent MR inversion have been observed in tris (8-hydroxyquinoline) aluminum (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 temperatures. 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.

The structural properties of Co-doped tris(8-hydroxyquinoline)aluminum (Alq(3)) have been studied by grazing incidence X-ray absorption fine structure (GIXAFS) and Fourier transform infrared spectroscopy (FTIR). GIXAFS analysis suggests that there are multivalent Co-Alq(3) 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 the meridional (mer) isomer of Alq(3) rather than forming inorganic compounds.

We report a positive spin valve effect—low resistance for parallel electrodes configuration—in organic spin valves fabricated by vacuum thermal evaporation using half-metal perovskite manganites La0.67Sr0.33MnO3 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. The origination mechanism of negative and positive GMR in organic spin valves has been discussed in detail.

The electronic and magnetic properties of undoped MoO2 have been studied using first-principles calculations within both the generalized gradient approximation (GGA) and GGA+U method. The calculated results show that no magnetic moment forms in perfect MoO2. For MoO2 with Mo vacancies, the GGA results show some magnetic moments whereas the GGA+U (U=−1 eV for Mo) results indicate no magnetic moment forms. In the presence of type II O vacancies, both the GGA and GGA+U results show no magnetic moment can form irrespective of the vacancies concentration. Nevertheless, the type I O vacancies always lead to formation of magnetic moments which couple ferromagnetically and should be the main origin of the magnetism in undoped MoO2. The different structural properties and the corresponding charge-density redistribution behaviors of the two inequivalent types of oxygen vacancies are the origin of the different magnetic behaviors. For the magnetic interaction, the Ruderman-Kittel-Kasuya-Yoshida interaction and the superexchange mechanism cooperatively underlie the magnetism.