Structural Determination of a Graphite/Hexagonal Boron Nitride Superlattice Observed in the Experiment.

A previous study reported an observed unidentified graphite/hexagonal boron nitride (hBN) superlattice structure in special multilayer heterojunction devices via cross-sectional transmission electron microscopy [Haigh S. J. et al., Nat. Mater. 2012, 11, 764-767]. In this letter, we designed and confirmed two possible graphite/hBN superlattice structures (AA and Ab), which were probably the structures observed by the aforementioned experiment. The formation enthalpies of both structures were negative, indicating that they could be successfully synthesized as the previous experiment reported. The results also showed that both structures possessed dynamic stability and elastic stability. Importantly, the theoretical interlayer distances of AA and Ab were 3.34 and 3.30 Å, respectively, which were consistent with the experimental value of 3.32 Å. The X-ray diffraction patterns and Raman spectra of both structures were simulated to aid in distinguishing them. This study on the atomic structure of the graphite/hBN superlattice lays a foundation for further research and application of this material.

Three metallic BN polymorphs: 1D multi-threaded conduction in a 3D network.

In this paper, three novel metallic sp2/sp3-hybridized boron nitride (BN) polymorphs are proposed by first-principles calculations. One of them, denoted as tP-BN, is predicted based on the evolutionary particle swarm structural search. tP-BN is composed of two interlocked rings forming a tube-like 3D network. The stability and band structure calculations show that tP-BN is metastable and metallic at zero pressure. Calculations for the density of states and electron orbitals confirm that the metallicity originates from the sp2-hybridized B and N atoms, forming 1D linear conductive channels in the 3D network. According to the relationship between the atomic structure and electronic properties, another two 3D metastable metallic sp2/sp3-hybridized BN structures are constructed manually. Electronic property calculations show that both of these structures have 1D conductive channels along different axes. The polymorphs predicted in this study enrich the structures and provide a different picture of the conductive mechanism of BN compounds.

Spin Transport Modulation of 2D Fe3O4 Nanosheets Driven by Verwey Phase Transition.

Realizing spin transport between heavy metal and two-dimensional (2D) magnetic materials at high Curie temperature (TC) is crucial to advanced spintronic information storage technology. Here, environmentally stable 2D nonlayered Fe3O4 nanosheets are successfully synthesized using a reproducible process and found that they exhibit vortex magnetic domains at room temperature. A Verwey phase transition temperature (TV) of ≈110 K is identified for ≈3 nm thick nanosheet through Raman characterization and spin Hall device measurement of the Pt/Fe3O4 bilayer. The anisotropic magnetoresistance ratio decreases near TV, while both the spin Hall magnetoresistance ratio and spin mixing conductance (Gr) increase at TV. As the temperature approaches 112 K, the anomalous Hall effect ratio tends to become zero. The maximum Gr reaches ≈5 × 1015 Ω-1m-2 due to the clean and flat interface between Pt and 2D nanosheet. The observed spin transport behavior in Pt/Fe3O4 spin Hall devices indicates that 2D Fe3O4 nanosheets possess potential for high-power micro spintronic storage devices applications.

Room-temperature ferromagnetic CoSe2nanoplates synthesized by chemical vapor deposition.

Among novel two-dimensional materials, transition metal dichalcogenides (TMDs) with 3dmagnetic elements have been extensively researched owing to their unique magnetic, electric, and photoelectric properties. As an important member of TMDs, CoSe2is an interesting material with controversial magnetic properties, hitherto there are few reports related to the magnetism of CoSe2materials. Here, we report the synthesis of CoSe2nanoplates on Al2O3substrates by chemical vapor deposition (CVD). The CVD-grown CoSe2nanoplates exhibit three typical morphologies (regular hexagonal, hexagonal, and pentagonal shapes) and their lateral sizes and thickness of CoSe2nanoplates can reach up to hundreds of microns and several hundred nanometers, respectively. The electric-transport measurement shows a metallic feature of CoSe2nanoplates. Furthermore, the slanted hysteresis loop and nonzero remnant magnetization of the CoSe2nanoplates confirm the ferromagnetism in the temperature range of 5-400 K. This work provides a novel platform for designing CoSe2-based spintronic devices and studying related magnetic mechanisms.

Universal Phase Transitions of AlB2-Type Transition-Metal Diborides.

High-pressure phase transitions of AlB2-type transition-metal diborides (TMB2; TM = Zr, Sc, Ti, Nb, and Y) were systematically investigated using first-principles calculations. Upon subjecting to pressure, these TMB2 compounds underwent universal phase transitions from an AlB2-type to a new high-pressure phase tP6 structure. The analysis of the atomistic mechanism suggests that the tP6 phases result from atomic layer folds of the AlB2-type parent phases under pressure. Stability studies indicate that the tP6-structured ZrB2, ScB2, and NbB2 are stable and may be observed under high pressure and the tP6-structured TiB2 phase may be recovered at ambient pressure.