Zhi-Zhan Chen

High-temperature ferromagnetism in (Co, Al)-codoped ZnO powders

Zn0.95−xCo0.05AlxO (x=0,0.01,0.03) powders were prepared from the acetate-derived precursor by the sol-gel route. The structural and magnetic properties of the powders were investigated. X-ray absorption spectroscopy and high-resolution transmission electron microscope analyses indicate that Co2+ substitute for Zn2+ without changing the wurtzite structure. The powder shows paramagnetic behavior at 5K for x=0 sample. For x=0.01 and 0.03, the powders exhibit ferromagnetic behavior at 360K. It was demonstrated experimentally that high-temperature ferromagnetism in Co-doped ZnO powders can be obtained through increasing the carrier concentration which was realized by doping a few percent of Al.

Structures and magnetic properties of (Mn, N)-codoped ZnO thin films

Zn0.96Mn0.04O and Zn0.96Mn0.04O:N thin films with wurtzite structure were grown by inductively coupled plasma enhanced chemical vapor deposition method. Although both samples exhibit ferromagnetism at room temperature, the saturation magnetic moment (1.4μB∕Mn) of the Mn- and N-codoped sample is much larger than that (0.3μB∕Mn) of the N-free one. The x-ray absorption near edge structure analysis reveals that the codoped Mn and N impurities can be substitutionally incorporated into the ZnO host in the Zn0.96Mn0.04O:N thin film. The first-principles calculations suggest that the N substitution for the O site in Mn-doped ZnO can change the interaction of neighboring Mn–Mn pairs from antiferromagnetic to ferromagnetic, and accordingly the effective magnetic moment per Mn is greatly enhanced.

Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy

To understand and improve the applications of titanium-oxide nanoparticles, it is extremely important to perform a detailed investigation of the surface and the interior structural properties of nanocrystalline materials, such as rutile and anatase with diameter of few nanometers. Here, x-ray absorption spectroscopy has been used to identify the local Ti environment and characterize the related electronic structure. We combine experimental results at the Ti K edge in both bulk and nanocrystal samples to determine the lattice distortion via the characteristic pre-edge features and the variation in the multiple-scattering region of the x-ray absorption near-edge structure spectra. The correlation between peak intensities and surface-to-volume ratio of nanoparticles is also discussed.

Large-scale fabrication of silicon carbide hollow spheres

The synthesis of SiC hollow spheres is reported. The shell of the spheres is formed through a solid–gas reaction rather than traditional chemical methods. The sphere size can be modulated from the microscale to the nanoscale, and the shell thickness can be finely tuned by adjusting the experimental parameters. This technique presents a new paradigm in the preparation of hollow spheres. Based on this technique, other carbide hollow spheres could also be synthesized.

Origin of ferromagnetism of (Co,Al)-codoped ZnO from first-principles calculations

Magnetic mechanisms for the (Co,Al)-codoped ZnO, based on the first-principles calculations, are evaluated. Additional electrons induced by Al doping can stabilize the ferromagnetic state in Co-doped ZnO. The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction may be the dominant mechanism when the distance between Co and Al is far away, while the double-exchange interaction may gain an advantage over the RKKY when the distance is near. The multiple mechanisms can explain the relationship between the magnetic performance and the relative substitutional site of Al. The study yields insight into the origin of the ferromagnetism of ZnO-based materials.

Effect of donor localization on the magnetic properties of Zn–Co–O system

The effect of donor localization on the magnetic properties of Zn–Co–O system was investigated by resonant inelastic x-ray scattering and first principle calculations. It was found that the ferromagnetic coupling was induced through charge transfer between Co 3d and donor defect. The charge transfer was dependent on the electron localization of donor defect. The electron localization of oxygen vacancy defect was much stronger than that of donor defect formed by the substitution of Al3+ for Zn2+. In order to elucidate the magnetic mechanism, a series of Zn0.95Co0.05O and Zn0.94Co0.05Al0.01O films were prepared under different oxygen partial pressure (PO2). The magnetic properties of Zn0.95Co0.05O films were dependent on PO2, while Zn0.94Co0.05Al0.01O films showed stable ferromagnetism.

Effect of oxygen partial pressure on the local structure and magnetic properties of Co-doped ZnO films

Zn0.95Co0.05O films were prepared under different oxygen partial pressures (PO2) by inductively coupled plasma enhanced physical vapor deposition. The effect of PO2 on the local structure and magnetic properties was investigated. The x-ray absorption spectroscopy at the Co K-edge, Co L-edge, and O K-edge revealed that the main defects were oxygen vacancies when the films were deposited under very low PO2. The change from room-temperature ferromagnetism to paramagnetism was observed with increasing PO2. It was experimentally demonstrated that the oxygen vacancy defect is absolutely necessary to induce ferromagnetic couplings in Co-doped ZnO films.

Magnetic Properties of Mn-Doped ZnO Nanostructures Synthesized by Chemical Vapor Transport

Mn-doped ZnO nanostructures have been fabricated by chemical vapor transport (CVT). Nanobelts, nanorods and nanowires have different growth directions because of their different growth environments. X-ray diffraction, electron paramagnetic resonance and Raman spectrum methods have been used to identify the substitution of a Zn site with Mn ions. Despite their different morphologies, these nanostructures possess the same magnetic properties. Magnetization was paramagnetic and antiferromagnetic. No ferromagnetism was observed even at T=5 K.

Room-temperature anomalous Hall effect and magnetroresistance in (Ga, Co)-codoped ZnO diluted magnetic semiconductor films*

This paper reports that the (Ga, Co)-codoped ZnO thin films have been grown by inductively coupled plasma enhanced physical vapour deposition. Room-temperature ferromagnetism is observed for the as-grown thin films. The x-ray absorption fine structure characterization reveals that Co2+ and Ga3+ ions substitute for Zn2+ ions in the ZnO lattice and exclude the possibility of extrinsic ferromagnetism origin. The ferromagnetic (Ga, Co)-codoped ZnO thin films exhibit carrier concentration dependent anomalous Hall effect and positive magnetoresistance at room temperature. The mechanism of anomalous Hall effect and magneto-transport in ferromagnetic ZnO-based diluted magnetic semiconductors is discussed.

Room-Temperature Phonon Replica in Band-to-Band Transition of 6H-SiC Analyzed Using Transmission Spectrums

Room-temperature phonon replicas in the band-to-band transition of 6H-SiC were analyzed using transmission spectrums (TSs). By fitting the experimental data with theoretical equations, the energies of two turning points were determined to be Eg+Ep and Eg-Ep. the Eg values were about 2.976 eV for low-resistivity (LR) 6H-SiC and 2.970 eV for semi-insulating (SI) 6H-SiC, while the Ep values were about 56 meV for the former and 63 meV for the latter, which were consistent with the values extracted by the conventional method. The difference between the band gap energies of the LR and SI 6H-SiC samples was due to the Moss–Burstein effect. At room temperature, phonons participating in the band-to-band transition of 6H-SiC were dominated by longitudinal acoustic (LA) phonons with energies of about 56–63 meV.