Reversible Halide Exchange Reaction of Organometal Trihalide Perovskite Colloidal Nanocrystals for Full-Range Band Gap Tuning.

In recent years, methylammonium lead halide (MAPbX3, where X = Cl, Br, and I) perovskites have attracted tremendous interest caused by their outstanding photovoltaic performance. Mixed halides have been frequently used as the active layer of solar cells, as a result of their superior physical properties as compared to those of traditionally used pure iodide. Herein, we report a remarkable finding of reversible halide-exchange reactions of MAPbX3, which facilitates the synthesis of a series of mixed halide perovskites. We synthesized MAPbBr3 plate-type nanocrystals (NCs) as a starting material by a novel solution reaction using octylamine as the capping ligand. The synthesis of MAPbBr(3-x)Clx and MAPbBr(3-x)Ix NCs was achieved by the halide exchange reaction of MAPbBr3 with MACl and MAI, respectively, in an isopropyl alcohol solution, demonstrating full-range band gap tuning over a wide range (1.6-3 eV). Moreover, photodetectors were fabricated using these composition-tuned NCs; a strong correlation was observed between the photocurrent and photoluminescence decay time. Among the two mixed halide perovskite series, those with I-rich composition (x = 2), where a sole tetragonal phase exists without the incorporation of a cubic phase, exhibited the highest photoconversion efficiency. To understand the composition-dependent photoconversion efficiency, first-principles density-functional theory calculations were carried out, which predicted many plausible configurations for cubic and tetragonal phase mixed halides.

Magnetic anisotropy and morphology of Fe epitaxial layers grown on MgO/InAs heterostructures.

In-plane magnetic anisotropy and the corresponding morphology of Fe epitaxial layers have been investigated with respect to underlying MgO growth temperature when epitaxial Fe/MgO layers are grown on InAs (001) substrates. Coexistence of three-dimensional Fe islands with strong in-plane textures along <110> and (100) is observed on 4 nm thick MgO layers grown on 200 degrees C, leading to the absence of magnetic anisotropy. Meanwhile, the partially relaxed MgO layers grown above 300 degrees C give rise to two-dimensional Fe layers with cubic magnetic anisotropy. The higher MgO growth temperature accelerates the two-dimensional layer formation of the subsequent Fe as well as the advent of cubic anisotropy by reducing underlying strain within the MgO layer.

Facile synthesis of Pd@Pt core-shell nanocubes with low Pt content via direct seed-mediated growth and their enhanced activity for formic acid oxidation.

Pd@Pt core-shell nanocubes with a partially covered Pt shell on the Pd nanocubes were synthesized by a direct seed-mediated growth method without a washing process. The FAO activity of Pd@Pt 0.4 at% was 4.3 times and 2.2 times higher than that of Pd cubes and commercial Pt/C, respectively.

X-ray photoelectron spectroscopy and first principles calculation of BCN nanotubes.

Multiwalled boron carbonitride (BCN) nanotubes with two different structures were synthesized via thermal chemical vapor deposition; one has 10% C atoms homogeneously doped into BN nanotubes (B0.45C0.1N0.45 NTs), and the other has BN layers sheathed with 5-nm-thick C outerlayers (BN-C NTs). The electronic structures of the B, C, and N atoms were thoroughly probed by synchrotron X-ray photoelectron spectroscopy and the X-ray absorption near-edge structure method. The B0.45C0.1N0.45 NTs contain a significant amount of B-C and C-N bonding with a pyridine-like structure (hole structure), which reduces the pi bonding states of the B and N atoms. From the XPS valence band spectrum, the band gap was estimated to be about 2.8 eV. In the BN-C NTs, the C and BN domains are separated without forming the pyridine-like structure. Using the first principles method, we investigated the relative stabilities and electronic structures of the various isomers of the double-walled (12,0)@(20,0) BCN NTs. The C-outerlayer BN nanotube structure is the most stable isomer, when there exist no defects in the tubes with B/N = 1.0 (i.e., graphite-like structure). In addition, a reasonable model, which is characterized by the motives consisted of three pyridine-like rings around a hollow site, is presented for the local structure of C atoms in the B0.45N0.45C0.1 NTs. A considerable decrease of the band gap due to the 10% C doping was predicted, which was consistent with the experimental results.