Stabilisation of Fe2O3-rich Perovskite Nanophase in Epitaxial Rare-earth Doped BiFeO3 Films.

Researchers have demonstrated that BiFeO3 exhibits ferroelectric hysteresis but none have shown a strong ferromagnetic response in either bulk or thin film without significant structural or compositional modification. When remanent magnetisations are observed in BiFeO3 based thin films, iron oxide second phases are often detected. Using aberration-corrected scanning transmission electron microscopy, atomic resolution electron energy loss spectrum-mapping and quantitative energy dispersive X-ray spectroscopy analysis, we reveal the existence of a new Fe2O3-rich perovskite nanophase, with an approximate formula (Fe0.6Bi0.25Nd0.15)(3+) Fe(3+)O3, formed within epitaxial Ti and Nd doped BiFeO3 perovskite films grown by pulsed laser deposition. The incorporation of Nd and Bi ions on the A-site and coherent growth with the matrix stabilise the Fe2O3-rich perovskite phase and preliminary density functional theory calculations suggest that it should have a ferrimagnetic response. Perovskite-structured Fe2O3 has been reported previously but never conclusively proven when fabricated at high-pressure high-temperature. This work suggests the incorporation of large A-site species may help stabilise perovskite-structured Fe2O3. This finding is therefore significant not only to the thin film but also to the high-pressure community.

Fabrication and luminescence of monolayered boron nitride quantum dots.

Monolayered boron nitride (BN) quantum dots (QDs; lateral size ≈10 nm) are fabricated using a novel method. Unlike monolayered BN sheets, these BN QDs exhibit blue-green luminescence due to defects formed during preparation. This optical behavior adds significant functionality to a material that is already receiving much attention. It is further shown that the QDs are nontoxic to biological cells and well suited to bio-imaging.

Fabrication of luminescent monolayered tungsten dichalcogenides quantum dots with giant spin-valley coupling.

A high yield (>36 wt %) method has been developed of preparing monolayered tungsten dichalcogenide (WS2) quantum dots (QDs) with lateral size ∼8-15 nm from multilayered WS2 flakes. The monolayered WS2 QDs are, like monolayered WS2 sheets, direct semiconductors despite the flake precursors being an indirect semiconductor. However, the QDs have a significantly larger direct transition energy (3.16 eV) compared to the sheets (2.1 eV) and enhanced photoluminescence (PL; quantum yield ∼4%) in the blue-green spectral region at room temperature. UV/vis measurements reveal a giant spin-valley coupling of the monolayered WS2 QDs at around 570 meV, which is larger than that of monolayered WS2 sheets (∼400 meV). This spin-valley coupling was further confirmed by PL as direct transitions from the conduction band minimum to split valence band energy levels, leading to multiple luminescence peaks centered at around 369 (3.36 eV) and 461 nm (2.69 eV, also contributed by a new defect level). The discovery of giant spin-valley coupling and the strong luminescence of the monolayered WS2 QDs make them potentially of interests for the applications in semiconductor-based spintronics, conceptual valley-based electronics, quantum information technology and optoelectronic devices. However, we also demonstrate that the fabricated monolayered WS2 QDs can be a nontoxic fluorescent label for high contrast bioimaging application.

Nanopatterning by ion implantation through nanoporous alumina masks.

The important problem of how to generate lateral order for ion implantation patterning of substrates is solved by using a nanoporous anodic alumina membrane as a mask. Co and Pt implantation is used at two implantation doses. In order to observe the achieved implantation zones free from artifacts, electron transparent thin nitride and oxide films are used as substrates, which allows the quality of pattern transfer from the mask to the thin film to be assessed by plan-view transmission electron microscopy. Characteristic density variations of implanted elements across projected pore-regions of the mask, such as ring and dome shapes, and corresponding variation of cluster size are discussed, and therefore the method also serves as a suitable test bed for ion beam focusing studies by cylindrical or conical pores.

Cationic surface reconstructions on cerium oxide nanocrystals: an aberration-corrected HRTEM study.

Instabilities of nanoscale ceria surface facets are examined on the atomic level. The electron beam and its induced atom migration are proposed as a readily available probe to emulate and quantify functional surface activity, which is crucial for, for example, catalytic performance. In situ phase contrast high-resolution transmission electron microscopy with spherical aberration correction is shown to be the ideal tool to analyze cationic reconstruction. Hydrothermally prepared ceria nanoparticles with particularly enhanced {100} surface exposure are explored. Experimental analysis of cationic reconstruction is supported by molecular dynamics simulations where the Madelung energy is shown to be directly related to the binding energy, which enables one to generate a visual representation of the distribution of "reactive" surface oxygen.