Multivariate Analysis of Light-Activated SMOX Gas Sensors.

Chemoresistive gas sensors made from SnO2, ZnO, WO3, and In2O3 have been prepared by flame spray pyrolysis. The sensors' response to CO and NO2 in darkness and under illumination at different wavelengths, using commercially available LEDs, was investigated. Operation at room temperature turned out to be impractical due to the condensation of water inside the porous sensing layers and the irreversible changes it caused. Accordingly, for sensors operated at 70 °C, a characterization procedure was developed and proven to deliver consistent data. The resulting data set was so complex that usual univariate data analysis was intricate and, consequently, was investigated by correlation and principal component analysis. The results show that light of different wavelengths affects not only the resistance of each material, both under exposure to the target gases in humidity and in its absence, but also the sensor response to humidity and the target gases. It was found that each of the materials behaves differently under light exposure, and it was possible to identify conditions that need further investigations.

Revealing the local crystallinity of single silicon core-shell nanowires using tip-enhanced Raman spectroscopy.

Tip-enhanced Raman spectroscopy is combined with polarization angle-resolved spectroscopy to investigate the nanometer-scale structural properties of core-shell silicon nanowires (crystalline Si core and amorphous Si shell), which were synthesized by platinum-catalyzed vapor-liquid-solid growth and silicon overcoating by thermal chemical vapor deposition. Local changes in the fraction of crystallinity in these silicon nanowires are characterized at an optical resolution of about 300 nm. Furthermore, we are able to resolve the variations in the intensity ratios of the Raman peaks of crystalline Si and amorphous Si by applying tip-enhanced Raman spectroscopy, at sample positions being 8 nm apart. The local crystallinity revealed using confocal Raman spectroscopy and tip-enhanced Raman spectroscopy agrees well with the high-resolution transmission electron microscopy images. Additionally, the polarizations of Raman scattering and the photoluminescence signal from the tip-sample nanogap are explored by combining polarization angle-resolved emission spectroscopy with tip-enhanced optical spectroscopy. Our work demonstrates the significant potential of resolving local structural properties of Si nanomaterials at the sub-10 nanometer scale using tip-enhanced Raman techniques.

Synthesis and thermoelastic properties of Zr(CN2)2 and Hf(CN2)2.

Two binary transition metal cyanamides, Zr(CN2)2 and Hf(CN2)2, were prepared by solid-state metathesis (SSM) reactions and separately controlled by differential thermoanalysis (DTA). The crystal structure of Hf(CN2)2 was solved and refined from a single-phase crystal powder by X-ray diffraction (XRD) in the space group Pbcn. Zr(CN2)2 was characterized by isotypic indexing. The crystal structure of M(CN2)2 compounds with M = Zr, Hf is closely related to that of LiY(CN2)2, but reveals large cavities due to the absence of lithium ions. Hf(CN2)2 exhibits thermoelastic properties characteristic of a flexible framework material. The calculated phonon energies, elastic tensor, and thermal expansion tensor are presented; the volumetric coefficient of thermal expansion is predicted to be near-zero under ambient conditions (αV = -3.5 × 10-6 K-1).