nling Xu

The abnormal electrical and optical properties in Na and Ni codoped BiFeO3 nanoparticles

Bi0.97Na0.03Fe1−xNixO3 (x = 0, 0.005, 0.01, 0.015) nanoparticles are prepared via a sol-gel method. Weak ferromagnetism and exchange bias phenomenon without field cooling are observed in the samples. The oxygen vacancy concentration and leakage current density are increased with increasing the Ni content. However, with the increase of Ni content, the band gap of Bi0.97Na0.03Fe1−xNixO3 nanoparticles first decreases and then increases. To explain the abnormal phenomenon, the interplay of oxygen vacancy donor and hole acceptor is analyzed and a phenomenological qualitative model based on the electronic energy band is proposed. Additionally, the threshold switching behavior appears in Bi0.97Na0.03Fe1−xNixO3 samples with x = 0.01, 0.015 and the effect is qualitatively explained by introducing a conducting channel model based on the high-density mobile charges.

Abnormal variation of band gap in Zn doped Bi0.9La0.1FeO3 nanoparticles: Role of Fe-O-Fe bond angle and Fe-O bond anisotropy

Bi0.9La0.1FeO3 (BLFO) and Bi0.9La0.1Fe0.99Zn0.01O3 (BLFZO) nanoparticles were prepared via a sol-gel method. The oxygen vacancies and holes increase with Zn doping analyzed through X-ray photoelectron spectroscopy, which could contribute to the increase of leakage current density. However, with the increase of the defects (oxygen vacancies and holes), the band gap of BLFZO also is increased. To explain the abnormal phenomenon, the bandwidth of occupied and unoccupied bands was analyzed based on the structural symmetry driven by the Fe-O-Fe bond angle and Fe-O bond anisotropy.

Room temperature threshold switching behaviors of Bi0.9Nd0.1Fe1−xCoxO3 nanoparticles

Many investigations have reported threshold switching (TS) effects in amorphous semiconductors, but it is rarely observed in BiFeO3 nanoparticles. Here, the effects of cobalt doping on the electrical conductivity of Bi0.9Nd0.1Fe1−xCoxO3 (x = 0, 0.01, 0.03, 0.05) nanoparticles were investigated. Leakage current density (J) versus applied voltage (V) curves were measured and it was found that the leakage current increases with increases of the cobalt dopant concentration. With the increase of the maximal applied voltage, the samples with cobalt doping exhibit nearly symmetric TS behaviors at room temperature. With the increase of cobalt dopant concentration, the optical band gap is decreased, and the smallest optical band gap value (∼2.01 eV) is observed for the 3% Co doped BiFeO3 sample. X-ray photoelectron spectroscopy of the samples shows that the cobalt dopant induces an enhancement of the oxygen vacancy concentration. Possible mechanisms for the threshold switching effects are discussed on the basis of a conductive filament model. The mobile charge carriers align to form conducting filaments when the threshold voltage (VTH) is achieved, corresponding to the transition from a high-resistance state to a low-resistance state, and the rupture of the conductive path takes place when the applied voltage is decreased. The leakage current decreases dramatically and eventually returns to the initial high-resistance state.

Template-free synthesis of Nd0.1Bi0.9FeO3 nanotubes with large inner diameter and wasp-waisted hysteresis loop

One-dimensional (1D) nanotubes of Nd0.1Bi0.9FeO3 (NBFO) with an inner diameter of ∼50 nm were synthesized via sol-gel based electrospinning without template assistant. The phases, morphologies, crystalline structures, and magnetic properties of these 1D nanostructures were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy and SQUID, respectively. It was found that the calcination condition plays a crucial role in determining the morphologies and the magnetic properties. Interestingly, these 1D NBFO nanotubes exhibit wasp-waisted magnetic hysteresis with a lower coercivity and larger saturation magnetization, which were prevalent in natural rocks and artificial composite materials. The origin of these wasp-waisted hysteresis loops was discussed.

Effects of (La, Sr) co-doping on electrical conduction and magnetic properties of BiFeO3 nanoparticles*

Multiferroic material as a photovoltaic material has gained considerable attention in recent years. Nanoparticles (NPs) La0.1Bi0.9−xSrxFeOy (LBSF, x = 0, 0.2, 0.4) with dopant Sr2+ ions were synthesized by the sol–gel method. A systematic change in the crystal structure from rhombohedral to tetragonal upon increasing Sr doping was observed. There is an obvious change in the particle size from 180 nm to 50 nm with increasing Sr substitution into LBFO. It was found that Sr doping effectively narrows the band gap from ~ 2.08 eV to ~ 1.94 eV, while it leads to an apparent enhancement in the electrical conductivity of LBSF NPs, making a transition from insulator to semiconductor. This suggests an effective way to modulate the conductivity of BiFeO3-based multiferroic materials with pure phase by co-doping with La and Sr at the A sites of BiFeO3.