Jiwen Liu

In this work, an investigation of MoO2 nanosheets grown on an SiO2/Si substrate using MoO3 and S powders as the raw materials by chemical vapor deposition was carried out. The HRTEM, XPS, and Raman results revealed that the S-doped MoO2 nanosheets were synthesized at a large scale, and the proportion of S and MoO3 precursors influenced the morphology of the nanosheets. Photoluminescence spectra exhibited fluorescent emissions from the MoO2 nanosheets at room temperature from defects and electron transitions between the valence and conduction bands. UV–visible absorption spectra showed that the bandgap of the MoO2 nanosheets can be controlled by the S doping concentration.

Oriented CuO nanowire films were synthesized on a large scale using simple method of direct heating copper grids in air. The field emission properties of the sample can be enhanced by improving the aspect ratio of the nanowires just through a facile method of controlling the synthesis conditions. Although the density of the nanowires is large enough, the screen effect is not an important factor in this field emission process because few nanowires sticking out above the rest. Benefiting from the unique geometrical and structural features, the CuO nanowire samples show excellent field emission (FE) properties. The FE measurements of CuO nanowire films illustrate that the sample synthesized at 500 °C for 8 h has a comparatively low turn-on field of 0.68 V/μm, a low threshold field of 1.1 V/μm, and a large field enhancement factor β of 16782 (a record high value for CuO nanostructures, to the best of our knowledge), indicating that the samples are promising candidates for field emission applications.

The effects of N-induced acceptor defects on tuning optical, transport, and magnetic properties of In₂O₃ films fabricated by magnetron sputtering technique were investigated systematically by X-ray diffraction, X-ray photoelectron spectroscopy, UV–visible absorbance, Hall effect, film resistivity (ρ) versus temperature, magnetoresistance, and magnetic measurements. Detailed structural analyses reveal that N-doped In₂O₃ films have a cubic bixbyite structure with the substitutional N defect at the O sites of In₂O₃ lattice. The N-doped In₂O₃ films display clear room-temperature ferromagnetic behavior and Mott variable range-hopping transport behavior. With increasing N-doping concentration, the saturated magnetization of the films monotonically increases and the conductivity transforms into p-type. Crossover of negative to positive magnetoresistance and a red shift of the optical band gap E_g are also observed with N-doping. First-principles calculations show that the localized holes induced by N-doping can mediate the magnetic interaction by short-range N₁:p-In:d/p-N₂:p hybridization in N-doped In₂O₃ system. Therefore, the intrinsic ferromagnetic ordering in N-doped In₂O₃ films can be attributed to p–p interaction between N 2p orbitals, which causes a large Zeeman-split effect to suppress the carrier’s hopping path, leading to formation of positive magnetoresistance.

Silicon carbide/copper (SiC/Cu) ultrathin multilayer films were deposited on Si substrates by alternating the use of radio-frequency and direct current magnetron sputtering. It was found that the SiC layers possess an amorphous structure and Cu is incorporated into the SiC layers. The films have a p-type semiconductor characteristic, room-temperature ferromagnetic behavior, and negative magnetoresistance. A carrier concentration of up to 2.12 × 1020 and a saturation magnetization of up to 12.14 emu/cm3 are obtained, and Mott variable range hopping mainly dominates the conduction mechanism of the films. The theoretical fitting for the experimental magnetoresistance curves and the theoretical calculation of the density of states of Cu-doped SiC indicate that the ferromagnetism of the films originates from a carrier-mediated p–d exchange interaction.

The observed room-temperature ferromagnetism for Fe and Cu codoped In₂O₃ has strong correlation with the Fe_In + V_O complex.

The (In0.95−xFexCu0.05)2O3 (x = 0.06, 0.08, 0.15, and 0.20) films prepared by RF-magnetron sputtering were investigated by the combination of x-ray absorption spectroscopy (XAS) at Fe, Cu, and O K-edge. Although the Fe and O K-edge XAS spectra show that the Fe atoms substitute for the In sites of In2O3 lattice for all the films, the Cu K-edge XAS spectra reveal that the codoped Cu atoms are separated to form the Cu metal clusters. After being annealed in air, the Fe atoms are still substitutionally incorporated into the In2O3 lattice, while the Cu atoms form the CuO secondary phases. With the increase of Fe concentration, the bond length RFe-O shortens and the Debye–Waller factor σ2Fe-O increases in the first coordination shell of Fe, which are attributed to the relaxation of oxygen environment around the substitutional Fe ions. The forming of Cu relating secondary phases in the films is due to high ionization energy of Cu atoms, leading that the Cu atoms are energetically much harder to be oxidized to substitute for the In sites of In2O3 lattice than Fe atoms. These results provide new experimental guidance in the preparation of the codoped In2O3 based dilute magnetic oxides.