Yahui Cheng

Room-temperature ferromagnetism and the scaling relation between magnetization and average granule size in nanocrystalline Zn/ZnO core-shell structures prepared by sputtering.

Ferromagnetism is found in nanocrystalline Zn/ZnO core-shell structures prepared by sputtering pure Zn with subsequent oxidation. The saturation magnetization (M(S)) of the passivated ZnO shells increases with decrease in average particle size (d). The Curie temperature of the samples is above 400 degrees C. It is found that the ferromagnetism has a close relationship with point defects in ZnO shells. The maximum magnetization is estimated to be 28 emu cm(-3) (i.e. 0.14 mu(B) per unit cell) at 300 K, which is over three orders of magnitude larger than that of undoped ZnO nanoparticles or nanorods (10(-3)-10(-2) emu cm(-3)). More importantly, there is a scaling relation of M(s) alpha 1/d(n) (n = 5.20 +/- 0.20) for samples with d <or= 76 nm despite substantial differences in the particle size and shape. The results suggest that defects at the interface of the Zn/ZnO heterostructure make the main magnetic contributions.

Effect of oxygen partial pressure on the ferromagnetism of Cr-doped TiO2 films

Polycrystalline Cr-doped TiO2 films were fabricated by co-sputtering Cr and TiO2 targets in a pure Ar and O2 mixture with various oxygen partial pressures. Ferromagnetism is observed in all samples and the Curie temperatures are well above 390 K. The saturation magnetization of films shows strong dependence on the oxygen growth pressure while keeping a constant Cr concentration. The ferromagnetism is enhanced in the films deposited at lower oxygen pressures which are thus oxygen deficient, indicating an important role of oxygen vacancies in the ferromagnetic origin of Cr-doped TiO2.

Efficient photo-degradation of dyes using CuWO4 nanoparticles with electron sacrificial agents: a combination of experimental and theoretical exploration

CuWO4 is a promising photocatalytic material, responding in the visible light range, to enhance the utilization of solar energy. Here, CuWO4 nanoparticles have been synthesized via a polyol-mediated synthesis method and subsequently characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Vis Spectrophotometery combined with theoretical density functional theory (DFT) calculations. For the as-prepared CuWO4 samples, a strong adsorption capacity for the organic pollutant MB rather than photodegradation has been observed. The first-principle calculation with Heyd–Scuseria–Ernzerhof (HSE) screened coulomb hybrid functional results indicate that localization of hybridization of O 2p-orbitals and Cu 3d-orbitals, large electron effective mass and more positive conduction band edge of CuWO4 lead to low carrier mobility and thus the high recombination of excited carriers. Meanwhile, the optical absorption spectrum of experimental observation is consistent with theoretical calculations of pristine CuWO4, demonstrating few defects inhibiting light absorption. To avoid the high rate of recombination of the excited carriers, electron sacrificial agents (H2O2, Na2S2O8) are utilized to suppress the recombination. The photocatalytic activity is thus largely improved.

Tunable electrical and magnetic properties of half-metallic ZnxFe3−xO4 from first principles

The electrical and magnetic properties of Zn-doped Fe(3)O(4) at different doping concentrations of Zn have been investigated using a density functional method with generalized-gradient approximation corrected for on-site Coulombic interactions. The electronic structure calculation predicts that Zn(x)Fe(3-x)O(4) (0 ≤x≤ 0.875) is half-metallic with a full spin polarization. The hopping carrier concentration of Zn(x)Fe(3-x)O(4) decreases with increasing x, which indicates a distinct increase in the resistivity. The saturation magnetization of Zn(x)Fe(3-x)O(4) increases evidently with increasing x from x = 0 to x = 0.75 (i.e. from 4.0 to 8.3 μ(B)/f.u.) and then decreases rapidly to zero at x = 1. The robust half-metallicity, large tunability of electrical and magnetic properties of a Zn doped Fe(3)O(4) system make it a promising functional material for spintronic applications.

Influence of oxygen partial pressure on the ferromagnetic properties of polycrystalline Cr-doped ZnO films

Polycrystalline Cr-doped ZnO films are prepared by the co-sputtering method. Diamagnetism is observed in the conductive samples deposited in pure Ar. However, ferromagnetism is found in films with the same Cr dopant prepared under different oxygen partial pressures. The magnetization shows a strong dependence on the Cr concentration and, especially, on oxygen pressure. It is found that native point defects, which can be adjusted by the oxygen partial pressure during deposition, play a crucial role in the observed magnetic behaviors. The obtained ferromagnetism can be described by the dopant-donor/acceptor hybridization model, which associates exchange interaction with shallow-bound carriers. These results may help to understand the wide range of experimentally determined magnetic moments and its changes with different metal types and concentrations prepared by different groups and methods.

A class of monolayer metal halogenides MX2: Electronic structures and band alignments

With systematic first principles calculations, a class of monolayer metal halogenides MX2 (M = Mg, Ca, Zn, Cd, Ge, Pb; M = Cl, Br, I) has been proposed. Our study indicates that these monolayer materials are semiconductors with the band gaps ranging from 2.03 eV of ZnI2 to 6.08 eV of MgCl2. Overall, the band gap increases with the increase of the electronegativity of the X atom or the atomic number of the metal M. Meanwhile, the band gaps of monolayer MgX2 (X = Cl, Br) are direct while those of other monolayers are indirect. Based on the band edge curvatures, the derived electron (me) and hole (mh) effective masses of MX2 monolayers are close to their corresponding bulk values except that the me of CdI2 is three times larger and the mh for PbI2 is twice larger. Finally, the band alignments of all the studied MX2 monolayers are provided using the vacuum level as energy reference. These theoretical results may not only introduce the monolayer metal halogenides family MX2 into the emerging two-dimensional ma...

Extraordinary Hall effect and universal scaling in Fex(ZnO)1–x granular thin films at room temperature

On the benefit of the concept of the so-called diluted magnetic oxides, Fex(ZnO)1–x (x = 0.50–0.85) granular thin films with different thickness through 2D and 3D percolation region were prepared by ion beam assisted deposition. All samples are ferromagnetic at room-temperature due to the ferromagnetic nature of the Fe-doped ZnO matrix, which is quite different from the superparamagnetic behavior in the insulator-matrix based granular films. Along with decreasing thickness, the Hall coefficient RS is largely enhanced. The maximum RS reaches 4.27 × 10−7 m3/C in ∼2.8 nm Fe0.6(ZnO)0.4 granular film, which is nearly 9 times larger than the RS (4.64 × 10−8 m3/C) of the ∼50 nm Fe0.6(ZnO)0.4 sample. Meanwhile, the RS could maintain in a wide temperature region from 10 K to 300 K and the Hall sensitivity reaches ∼130 V/AT at room-temperature. The scaling exponential of n = 1.7 ± 0.1 in σxy∼σxxn is observed, fitting well with the recent developed universal scaling theory characterized by n = 1.6 in the dirty limit.

Magnetic properties of fluffy Fe@α-Fe 2 O 3 core-shell nanowires

AbstractNovel fluffy Fe@α-Fe2O3 core-shell nanowires have been synthesized using the chemical reaction of ferrous sulfate and sodium borohydride, as well as the post-annealing process in air. The coercivity of the as-synthesized nanowires is above 684 Oe in the temperature range of 5 to 300 K, which is significantly higher than that of the bulk Fe (approximately 0.9 Oe). Through the annealing process in air, the coercivity and the exchange field are evidently improved. Both the coercivity and the exchange field increase with increasing annealing time (TA) and reach their maximum values of 1,042 and 78 Oe, respectively, at TA = 4 h. The magnetic measurements show that the effective anisotropy is increased with increasing the thickness of theα-Fe2O3 by annealing. The large values of coercivity and exchange field, as well as the high surface area to volume ratio, may make the fluffy Fe@α-Fe2O3 core-shell nanowire a promising candidate for the applications of the magnetic drug delivery, electrochemical energy storage, gas sensors, photocatalysis, and so forth.

Identifying the descriptor governing NO oxidation on mullite Sm(Y, Tb, Gd, Lu)Mn2O5 for diesel exhaust cleaning

The current fast selective catalytic reduction (fast-SCR) technology shows effectiveness in converting diesel engine generated nitrogen oxides (NOx) to environmentally benign nitrogen (N2) with the aid of the precious metal catalyst platinum. Driven by the previous finding of the low-cost mullites great superiority over Pt in terms of NO oxidation, a series of Mn-based oxides Sm(Y, Tb, Gd, Lu)Mn2O5 are synthesized to identify a general descriptor that governs the catalytic performance. Utilizing soft X-ray absorption characterization and molecular orbital theory, here, we show that the catalytic activity difference presents little dependence on the 3d electron occupancy when the A site element is varied (Sm, Tb, Y, Gd, Lu). Instead, strong p–d hybridization between lattice O and octahedral Mn leads to weak bonding strength between external O* and pyramid Mn and essentially increases the catalytic activity for converting NO to NO2.

Room-temperature ferromagnetism in nanocrystalline Cu/Cu2O core-shell structures prepared by magnetron sputtering

Cu/Cu2O core-shell nanoparticles with diameters around 8–9 nm have been fabricated by magnetron sputtering pure Cu targets with subsequent annealing in oxygen. Room-temperature ferromagnetism (FM) was observed in the samples annealed at 150 °C for 10–120 min. The maximum of saturated magnetization is as high as 19.8 emu/cc. The photoluminescence spectra show solid evidence that the FM originates from Cu vacancies in the Cu2O shell of the Cu/Cu2O core-shell nanoparticles. Furthermore, the FM can be modulated by the amount of Cu vacancies through the Cu/Cu2O core-shell interface engineering. Fundamentally, the FM can be understood by the charge-transfer ferromagnetism model based on Stoner theory.