Increased room temperature ferromagnetism in Co-doped tetrahedral perovskite niobates.

Dilute magnetic semiconductors (DMSs), such as (In, Mn)As and (Ga, Mn)As prototypes, are limited to III-V semiconductors with Curie temperatures (T c) far from room temperature, thereby hindering their wide application. Here, one kind of DMS based on perovskite niobates is reported. BaM x Nb(1-x)O3-δ (M = Fe, Co) powders are prepared by the composite-hydroxide-mediated method. The addition of M elements endows BaM x Nb(1-x)O3-δ with local ferromagnetism. The tetragonal BaCo x Nb(1-x)O3-δ nanocrystals can be obtained by Co doping, which shows strong saturation magnetization (M sat) of 2.22 emu g-1, a remnant magnetization (M r) of 0.084 emu g-1 and a small coercive field (H c) of 167.02 Oe at room temperature. The ab initio calculations indicate that Co doping could lead to a 64% local spin polarization at the Fermi level (E F) with net spin DOS of 0.89 electrons eV-1, this result shows the possibility of maintaining strong ferromagnetism at room temperature. In addition, the trade-off effect between the defect band absorption and ferromagnetic properties of BaM x Nb(1-x)O3-δ is verified experimentally and theoretically.

Polymer-Derived Lightweight SiBCN Ceramic Nanofibers with High Microwave Absorption Performance.

Lightweight SiBCN ceramic nanofibers were prepared by a combination of electrostatic spinning and high-temperature annealing techniques, showing tunable electromagnetic wave absorption. By controlling the annealing temperature, the nanoscale architectures and atomic bonding structures of as-prepared nanofibers could be well regulated. The resulting SiBCN nanofibers ∼300 nm in diameter, which were composed of an amorphous matrix, ß-SiC, and free carbon nanocrystals, were defect-free after annealing at 1600 °C. SiBCN nanofibers annealed at 1600 °C exhibited good microwave absorption, obtaining a minimum reflection coefficient of -56.9 dB at 10.56 GHz, a sample thickness of 2.6 mm with a maximum effective absorption bandwidth of 3.45 GHz, and a maximum dielectric constant of 0.44. Owing to the optimized A + B + C microstructure, SiBCN ceramic nanofibers with satisfying microwave absorption properties endowed the nanofibers with the potential to be used as lightweight, ultrastrong radar wave absorbers applied in military and the commercial market.

Topotactic Growth of Free-Standing Two-Dimensional Perovskite Niobates with Low Symmetry Phase.

Here, we report a novel topotactic method to grow 2D free-standing perovskite using KNbO3 (KN) as a model system. Perovskite KN with monoclinic phase, distorted by as large as ∼6 degrees compared with orthorhombic KN, is obtained from 2D KNbO2 after oxygen-assisted annealing at relatively low temperature (530 °C). Piezoresponse force microscopy (PFM) measurements confirm that the 2D KN sheets show strong spontaneous polarization (Ps) along [101̅]pc direction and a weak in-plane polarization, which is consistent with theoretical predictions. Thickness-dependent stripe domains, with increased surface displacement and PFM phase changes, are observed along the monoclinic tilt direction, indicating the preserved strain in KN induces the variation of nanoscale ferroelectric properties. 2D perovskite KN with low symmetry phase stable at room temperature will provide new opportunities in the exploration of nanoscale information storage devices and better understanding of ferroelectric/ferroelastic phenomena in 2D perovskite oxides.

Solution-Based Synthesis of Layered Two-Dimensional Oxides as Broadband Emitters.

Preparing transition-metal oxides in their two-dimensional (2D) form is the key to exploring their unrevealed low-dimensional properties, such as the p-type transparent superconductivity, topological Mott insulator state, existence of the condensed 2D electron/hole gas, and strain-tunable catalysis. However, existing approaches suffer from the specific constraint techniques and precursors that limit their product types. Here, we report a solution-based method to directly synthesize KNbO2 in 2D by an out-of-the-pot growth process at low temperature, which is observed directly in real time. The developed method can also be applied to other 2D ternary oxide syntheses, including CsNbO2 and composited NaxK1-xNbO2, and it can be extended to the preparation of self-assembled nanofilms. In addition, We demonstrate the emission of broadband photoluminescence (PL, λ ∼ 350-800 nm) from as-synthesized single-crystal 2D KNbO2 sheets down to a single unit cell thickness. The ultra-broadband emission is ascribed to the self-trapped excitation state (STEs) from the in-phase distortion of the NbO6 octahedrons in 2D NbO2- layers. Beyond the broader luminescent range and the robust material thermal stability of niobates, the absence of sample size restrictions and the large aspect ratio of the 2D oxide sheets will provide opportunities in miniaturizing and advancing 2D-materials integrated optoelectronic devices.