Sadan Ozcan

Non-equilibrium cation distribution and enhanced spin disorder in hollow CoFe2O4 nanoparticles

We present magnetic properties of hollow and solid CoFe(2)O(4) nanoparticles that were obtained by annealing of Co(33)Fe(67)/CoFe(2)O(4) (core/shell) nanoparticles. Hollow nanoparticles were polycrystalline whereas the solid nanoparticles were mostly single crystal. Electronic structure studies were performed by photoemission which revealed that particles with hollow morphology have a higher degree of inversion compared to solid nanoparticles and the bulk counterpart. Electronic structure and the magnetic measurements show that particles have uncompensated spins. Quantitative comparison of saturation magnetization (M(S )), assuming bulk Néel type spin structure with cationic distribution, calculated from quantitative XPS analysis, is presented. The thickness of uncompensated spins is calculated to be significantly large for particles with hollow morphology compared to solid nanoparticles. Both morphologies show a lack of saturation up to 7 T. Moreover magnetic irreversibility exists up to 7 T of cooling fields for the entire temperature range (10-300 K). These effects are due to the large bulk anisotropy constant of CoFe(2)O(4) which is the highest among the cubic spinel ferrites. The effect of the uncompensated spins for hollow nanoparticles was investigated by cooling the sample in large fields of up to 9 T. The magnitude of horizontal shift resulting from the unidirectional anisotropy was more than three times larger than that of solid nanoparticles. As an indication signature of uncompensated spin structure, 11% vertical shift for hollow nanoparticles is observed, whereas solid nanoparticles do not show a similar shift. Deconvolution of the hysteresis response recorded at 300 K reveals the presence of a significant paramagnetic component for particles with hollow morphology which further confirms enhanced spin disorder.

Effects of rapid thermal annealing on the structural and local atomic properties of ZnO: Ge nanocomposite thin films

We have investigated the structural and local atomic properties of Ge nanocrystals (Ge-ncs) embedded ZnO (ZnO: Ge) thin films. The films were deposited by sequential sputtering of ZnO and Ge thin film layers on z-cut quartz substrates followed by an ex-situ rapid thermal annealing (RTA) at 600 °C for 30, 60, and 90 s under forming gas atmosphere. Effects of RTA time on the evolution of Ge-ncs were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), hard x-ray photoelectron spectroscopy (HAXPES), and extended x-ray absorption fine structure (EXAFS). XRD patterns have clearly shown that fcc diamond phase Ge-ncs of sizes ranging between 18 and 27 nm are formed upon RTA and no Ge-oxide peak has been detected. However, cross-section SEM images have clearly revealed that after RTA process, Ge layers form varying size nanoclusters composed of Ge-ncs regions. EXAFS performed at the Ge K-edge to probe the local atomic structure of the Ge-ncs has revealed that as prepared ZnO:Ge possesses G...

Structural, Optic, and Magnetic Investigation of the Synthesized ZnO and Zn

Both ZnO and Zn0.99Co0.01O semiconductors were synthesized through solid state reaction via mechanical milling and thermal treatment. Initially the wurtzite ZnO structures of the synthesized particles were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Since these techniques were unable to identify both contamination atoms and Co distribution, energy dispersive X-ray spectrometry (EDS) was used. EDS showed a successful doping of Co atoms with the atomic ratio of 0.9 ± 0.1%, and also showed a contamination of tungsten (W) atoms, in the atomic ratio of 1.6 ± 0.2% for Zn0.99Co0.01O, and 1.3 ± 0.2% for ZnO. Substitutions of Co+2 ions with Zn+2 host atoms in the ZnO lattice were exposed through X-ray photo spectroscopy (XPS) data of Co 2p electronic energy levels. UV-vis absorption spectroscopy (UV-vis) was also used to prove Co substitutions in the ZnO lattice. This was revealed by a decrease in band gap from 3.25 ± 0.01 eV to 3.03 ± 0.01 eV, and the existence of newly permitted transitions between intra ionic d-d* levels. The ferromagnetic effect of Co doping in ZnO lattice was revealed by the coercivity of ~154±50 Oe and positive Curie-Weiss temperature, 79 ± 1 K. Beside ferromagnetic interactions, the calculated effective Bohr Magnetron (¿eff), 0.32±0.01 ¿B, suggested anti-ferromagnetic interactions due to be less than the theoretical spin based magnetic moment of Co2+ ions, 3.0 ¿B.

_{0.99}

MnxB100-x alloys (x = 30-60) were fabricated by arc melting technique. Structural analysis showed that alloy with Mn55B45 atomic composition has a single-phase orthorhombic FeB-type MnB structure. As-synthesized alloy has a saturation magnetization of 130 emu/g at 300 K, which is the largest value previously reported. Saturation magnetization increases rapidly from 3.5 to 130 emu/g as x increases from 30 to 55. Further increase of the x causes the reduction of the saturation magnetization to 50 emu/g. Magnetic properties of the MnB alloy have been also evaluated by first-principles density functional theory calculations. Electronic structure calculations were performed using full potential linearized augmented plane wave method and a spin polarized generalized gradient approximation. A 1.914 μB magnetization per MnB unit was calculated, which corresponds to 158 emu/g at 0 K. Curie temperature (TC) of the Mn55B45 was measured as 566 K which is in good agreement with the values reported for MnB alloy.

Co

We fabricated UVB filtered TiO₂ MSM photodetectors by the localized surface plasmon resonance effect. A plasmonic filter structure was designed using FDTD simulations. Final filter structure was fabricated with Al nano-cylinders with a 70 nm radius 180 nm period on 360 nm SiO₂film. The spectral response of the TiO₂ MSM photodetector was modified and the UVB response was reduced by approx. 60% with an LSPR structure, resulting in a peak responsivity shift of more than 40 nm. To our knowledge, this is the first published result for the spectral response modification of TiO₂ photodetectors with LSPR technique.