Large reduction in thermal conductivity for SiGe alloy nanowire wrapped with a Ge nanoparticle-embedded SiO2 shell.

We demonstrate silicon germanium (SiGe) alloy nanowires (NWs) with Ge nanoparticles (GeNPs) embedded in a SiO2 shell as a material for decreasing thermal conductivity. During thermal oxidation of SiGe NWs to form SiGe-SiO2 core-shell structures, Ge atoms were diffused into the SiO2 shell to relax the strain in the SiGe core, and agglomerated as a few nanometer-sized particles. This structure leads to a large reduction in thermal conductivity due to the GeNP-phonon interaction, while electrical conductivity is sustained because the core of the SiGe alloy NW provides a current path for the charged carriers. The thermal conductivity of the SiGe alloy NWs wrapped with a GeNP-embedded SiO2 shell is 0.41 W m(-1) K(-1) at 300 K.

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.

Large thermoelectric figure-of-merits from SiGe nanowires by simultaneously measuring electrical and thermal transport properties.

The strongly correlated thermoelectric properties have been a major hurdle for high-performance thermoelectric energy conversion. One possible approach to avoid such correlation is to suppress phonon transport by scattering at the surface of confined nanowire structures. However, phonon characteristic lengths are broad in crystalline solids, which makes nanowires insufficient to fully suppress heat transport. Here, we employed Si-Ge alloy as well as nanowire structures to maximize the depletion of heat-carrying phonons. This results in a thermal conductivity as low as ∼1.2 W/m-K at 450 K, showing a large thermoelectric figure-of-merit (ZT) of ∼0.46 compared with those of SiGe bulks and even ZT over 2 at 800 K theoretically. All thermoelectric properties were "simultaneously" measured from the same nanowires to facilitate accurate ZT measurements. The surface-boundary scattering is prominent when the nanowire diameter is over ∼100 nm, whereas alloying plays a more important role in suppressing phonon transport for smaller ones.

Light-Matter Interactions in Cesium Lead Halide Perovskite Nanowire Lasers.

Light-matter interactions in inorganic perovskite nanolasers are investigated using single-crystalline cesium lead halide (CsPbX3, X = Cl, Br, and I) nanowires synthesized by the chemical vapor transport method. The perovskite nanowires exhibit a uniform growth direction, smooth surfaces, straight end facets, and homogeneous composition distributions. Lasing occurs in the perovskite nanowires at low thresholds (3 µJ/cm(2)) with high quality factors (Q = 1200-1400) under ambient atmospheric environments. The wavelengths of the nanowire lasers are tunable by controlling the stoichiometry of the halide, allowing the lasing of the inorganic perovskite nanowires from blue to red. The unusual spacing of the Fabry-Pérot modes suggests strong light-matter interactions in the reduced mode volume of the nanowires, while the polarization of the lasing indicates that the Fabry-Pérot modes belong to the same fundamental transverse mode. The dispersion curve of the exciton-polariton model suggests that the group refractive index of the polariton is significantly enhanced.

Unexpected Size Effect Observed in ZnO-Au Composite Photocatalysts.

Semiconductor-metal nanocomposites prepared with well-defined gold nanoclusters, such as Au25, Au144, and Au807, showed size-dependent photocatalytic activities for the reduction of nile blue and azobenzene. Whereas the photoreduction of nile blue was directly related with the charge separation and transfer rate from the photoexcited ZnO to gold nanoclusters, the photoreaction of azobenzene showed unexpected size effect with a clear threshold. Mechanistic investigations revealed that the photoreduction of azobenzene proceeded via a proton-coupled electron transfer process. The photocatalytic activity of the ZnO-Au nanocomposites was also dependent on the excitation intensity, demonstrating that the multielectron/multiproton process was controlled by the charge separation and transfer in the nanocomposites.

Thermoelectric Properties of Nanowires with a Graphitic Shell.

A thermoelectric device that can generate electricity from waste heat can play an important role in a global energy solution. However, the strongly correlated thermoelectric properties have remained a major hurdle for the highly efficient conversion of thermoelectric energy. Herein, the electrical and thermal properties of Si and SiO2 nanowires with few-layer graphitic shells are demonstrated; these structures exhibit enhanced electrical properties but no increase in thermal conductivity. The main path of the phonons through the structures is the core nanowire, which has a large cross-sectional area relative to that of the graphitic shell layer. However, the electrical conductivities of the nanowires with shell structures are high because of the good electrical conductivity of the graphitic shell, despite its small cross-sectional area.