Formation of Layer-Structured Black Phosphorus Nanocrystals during High-Speed Rotation of Two-Dimensional Amorphous Ultrathin Films

Sonication and centrifugation of two-dimensional nanosheets have been widely used to produce various layer-structured nanocrystals (LSNCs), but the formation mechanism is not yet clear. A general understanding is that the formation of LSNCs is due to the splitting of crystalline sheets under sonication/centrifugation. However, this has not been supported by experimental evidence. Here we experimentally show that high-speed rotation of amorphous black phosphorus ultrathin film can lead to the regulated formation of massive LSNCs confined in the spatial region of the original film. The probable sizes of these LSNCs are several nanometers depending on the rotation speed. Their volumes show a clear log-normal distribution having a line-width increase with rotation speed. This phenomenon can be explained based on the two-dimensional continuity and momentum equations. Our findings provide insight into formation and mechanisms of ultrathin amorphous films and LSNCs under sonication and centrifugation generally used

Synthesis of Crystalline Pyramidal ε‑FeSi and Morphology- and Size-Dependent Ferromagnetism

Crystalline pyramidal ε-FeSi particles smaller than 1 μm in size with {111} lateral facets are synthesized by a spontaneous chemical vapor deposition method. The nanocrystals initially nucleate from the amorphous film via self-clustering forming a rectangular ε-FeSi (001) terrace as a result of the cubic crystalline structure and subsequent anisotropic accumulation on the terrace produces the pyramidal morphology. Room-temperature ferromagnetism is observed from ε-FeSi particles larger than 250 nm and having the {111} facets. A model is postulated to explain the morphology- and size-dependent ferromagnetism based on the nonuniform Fe atomic arrangement that forms atomic-scale islands on the surface and dipole interaction among these islands in the large enough particles. The morphology- and size-dependent ferromagnetism allows control of the magnetic moments of mesostructures and is important to spintronics and other applications

Strong Facet-Induced and Light-Controlled Room-Temperature Ferromagnetism in Semiconducting β‑FeSi₂ Nanocubes

Crystalline β-FeSi₂ nanocubes with two {100} facets and four {011} lateral facets synthesized by spontaneous one-step chemical vapor deposition exhibit strong room-temperature ferromagnetism with saturation magnetization of 15 emu/g. The room-temperature ferromagnetism is observed from the β-FeSi₂ nanocubes larger than 150 nm with both the {100} and {011} facets. The ferromagnetism is tentatively explained with a simplified model including both the itinerant electrons in surface states and the local moments on Fe atoms near the surfaces. The work demonstrates the transformation from a nonmagnetic semiconductor to a magnetic one by exposing specific facets and the room-temperature ferromagnetism can be manipulated under light irradiation. The semiconducting β-FeSi₂ nanocubes may have large potential in silicon-based spintronic applications

Synergistic WO₃·2H₂O Nanoplates/WS₂ Hybrid Catalysts for High-Efficiency Hydrogen Evolution

Tungsten trioxide dihydrate (WO₃·2H₂O) nanoplates are prepared by <i>in situ</i> anodic oxidation of tungsten disulfide (WS₂) film on carbon fiber paper (CFP). The WO₃·2H₂O/WS₂ hybrid catalyst exhibits excellent synergistic effects which facilitate the kinetics of the hydrogen evolution reaction (HER). The electrochromatic effect takes place via hydrogen intercalation into WO₃·2H₂O. This process is accelerated by the desirable proton diffusion coefficient in the layered WO₃·2H₂O. Hydrogen spillover from WO₃·2H₂O to WS₂ occurs via atomic polarization caused by the electric field of the charges on the planar defect or edge active sites of WS₂. The optimized hybrid catalyst presents a geometrical current density of 100 mA cm^–2 at 152 mV overpotential with a Tafel slope of ∼54 mV per decade, making the materials one of the most active nonprecious metal HER catalysts

Magnetic and Quantum Transport Properties of Small-Sized Transition-Metal-Pentalene Sandwich Cluster

The chemical bonds and magnetic and quantum transport properties of small-sized transition-metal-pentalene sandwich clusters TM_2<i>n</i>Pn_<i>n</i>+1 (TM = V, Cr, Mn, Co, and Ni; <i>n</i> = 1, 2) were investigated by using density functional theory and nonequilibrium Green’s function method. Theoretical results show that TM_2<i>n</i>Pn_<i>n</i>+1 sandwiches have high stabilities. The TM–TM bond order gradually decreases with the increase of 3d electron number of TM atoms and TM_2<i>n</i>Pn_<i>n</i>+1 could exhibit different spin states. With Au as two electrodes, significant spin-filter capability was observed in TM_2<i>n</i>Pn_<i>n</i>+1, and such a filter can be switched on/off by changing the spin state. In addition, giant magnetoresistance was also found in the systems. These interesting quantum transport properties indicate that TM_2<i>n</i>Pn_<i>n</i>+1 sandwiches are promising materials for designing molecular junction with different functions

Ultrathin Amorphous Alumina Nanoparticles with Quantum-Confined Oxygen-Vacancy-Induced Blue Photoluminescence as Fluorescent Biological Labels

Ultrathin alumina nanoparticles (NPs) with an average size of less than 4 nm are produced from porous anodic alumina membranes. The alumina NPs in a suspension produce strong blue tunable photoluminescence (PL) with a high quantum efficiency of ∼15% and Stokes shift as large as 1.0 eV. An obvious blue-shift and diminished line width are observed after storing the suspension in air. The tunable blue PL which is closely related to the oxygen vacancy (OV) defect centers at different depths beneath the surface depends on the NP size. The experimental observations are corroborated by theoretical derivation demonstrating that the electron wave functions of the OV-induced defect levels are extended in space, and quantum confinement takes place when the alumina NP is smaller than the spread of the wave functions. It is thus possible to control the PL behavior by changing the NP size and OV depth distribution and the alumina NPs are experimentally demonstrated to be robust and nontoxic biological probes

Phase-Engineering-Induced Generation and Control of Highly Anisotropic and Robust Excitons in Few-Layer ReS₂

The anisotropic exciton behavior in two-dimensional materials induced by spin–orbit coupling or anisotropic spatial confinement has been exploited in imaging applications. Herein, we propose a new strategy to generate high-energy and robust anisotropic excitons in few-layer ReS₂ nanosheets by phase engineering. This approach overcomes the limitation imposed by the layer thickness, enabling production of visible polarized photoluminescence at room temperature. Ultrasonic chemical exfoliation is implemented to introduce the metallic T phase of ReS₂ into the few-layer semiconducting Td nanosheets. In this configuration, light excitation can readily produce “hot” electrons to tunnel to the Td phase via the metal–​semiconductor interface to enhance the overlap between the wave functions and screened Coulomb interactions. Owing to the strong electron–hole interaction, significant increase in the optical band gap is observed. Highly anisotropic and tightly bound excitons with visible light emission (1.5–2.25 eV) are produced and can be controlled by tailoring the T phase concentration. This novel strategy allows manipulation of polarized optical information and has great potential in optoelectronic devices