Highly aliovalent Na(1+) ions were selected as the dopant to substitute Bi(3+) ions in BiFeO₃ (BFO) and Bi1-xNaxFeO₃ (x = 0, 0.01, 0.03, 0.05) nanoparticles prepared via a facile sol-gel method. Weak ferromagnetism and an obvious exchange bias (EB) phenomenon without field cooling were observed in the samples. To establish the presence of EB in the nanoparticles, training effect (TE) data were analyzed using Binek's model. Moreover, with the increase in Na(1+) content, the band gap was decreased, while interestingly, the leakage current density was significantly reduced and the smallest leakage current density (∼10(-7) A cm(-2)) was observed for the 3% Na-doped BFO. The electrical conduction mechanism of samples was investigated by plotting log J versus log E. Oxygen vacancies decreased with the increase of Na content analyzed through X-ray photoelectron spectroscopy (XPS) measurements. To further explain the decrease of band gap and leakage current density with the increase of Na content, the interplay of oxygen vacancies and holes was analyzed and a phenomenological qualitative model based on the electronic energy band proposed.
These days, opto-electronic functional devices based on three-dimensional lead halide perovskites (LHPs) are emerging. LHPs could be spin-coated to other materials, making it very convenient to combine LHPs with different categories of materials including metals, semiconductors, and polymers and achieve high-level performances. In this review, we will discuss the development in the LHP-based functional devices in recent years. After a brief presentation of the LHP’s properties, we will focus on the functional devices including lasers, photodetectors, and modulators. Then the fabrication of the LHP-based devices will be presented, which is followed by the summary and outlook.
Bi 0.97 Na 0.03 Fe 1Àx Ni x O 3 (x ¼ 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 Bi 0.97 Na 0.03 Fe 1Àx Ni x O 3 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 Bi 0.97 Na 0.03 Fe 1Àx Ni x O 3 samples with x ¼ 0.01, 0.015 and the effect is qualitatively explained by introducing a conducting channel model based on the high-density mobile charges.
Bi0.98Na0.02Fe1−xRuxO3 (x = 0, 0.01, 0.02) samples were prepared via a sol–gel method. The formation of the desired materials was confirmed using X‐ray diffraction. The tetravalent Ru doping could effectively reduce the number of oxygen vacancy, and subsequently reduce the leakage current. Interestingly, the optical band gap was narrowed gradually in our samples, which seems contradictory with the variation in oxygen vacancy. To better understand this abnormal optical property, the effect of bandwidth was analyzed based on the change in Fe–O bond length and Fe–O–Fe bond angle, and the corresponding phenomenological qualitative model was proposed. The magnetization was increased with Ru substitution. In addition, an exchange bias (EB) phenomenon without field cooled process was observed at room temperature for all the samples, which was explained by introducing a core–shell structure model. Moreover, the EB behavior becomes more pronounced at 5 K for 1% Ru‐doped sample.
Multiferroic material as a photovoltaic material has gained considerable attention in recent years. Nanoparticles (NPs) La 0.1 Bi 0.9−x Sr x FeO y (LBSF, x = 0, 0.2, 0.4) with dopant Sr 2+ 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 BiFeO 3 -based multiferroic materials with pure phase by co-doping with La and Sr at the A sites of BiFeO 3 .
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