The determination of mineral composition of foods involves, in most cases, the use of long and tedious sample preparation, which consumes acids and reagents and sometimes requires the use of expensive instrumentation. This is the main reason for the search for direct analytical procedures, based on the use of infrared sample spectra and chemometrics, to model the signals in order to determine the presence of essential and trace toxic elements in foods. The state-of-the-art of the research in this field has been established in the present review article from the critical evaluation of articles available in the literature. Chemometric methods employed and their validation, together with a discussion about the different techniques used for signal acquisition, were evaluated for their ability to predict new sample composition.
Thin films of perylene on Si(111) and Si(100) substrates have been investigated using a variety of experimental techniques. We find that the structural and morphological properties as well as the growth modes strongly depend on the preparation parameters. In general, we observe the existence of a relatively weak coupling between perylene and the two single crystal substrates. However, under special preparation conditions, it is possible to obtain a multilayer phase on the Si(111) substrate that is characterized by flat-lying, parallel-oriented molecules, and strong coupling with the substrate in the first layer. This phase has different structural, electronic, and intermolecular bonding properties as compared to the known crystalline phases. On Si(100), by varying the deposition rate between 0.1 and 10 nm/min, it is possible to observe a transition from island growth mode, with large and isolated crystallites, to homogeneous film growth. These findings contribute to the basic knowledge for film engineering. Thus, the film morphology could be designed ranging from the growth of very large single grains suitable for a complete nanodevice to homogenous films for application in large displays.
We successfully grow high-quality hydrogenated amorphous-silicon-germanium alloys (a-SiGe:H) by the hot-wire chemical-vapor deposition (HWCVD) technique using silane and germane gas mixtures. These alloys display electronic properties as good as those grown by the plasma-enhanced chemical-vapor deposition (PECVD) technique, when comparing materials with the same optical bandgaps. However, we grow materials with good electrical properties at high deposition rates—up to 40 Å/s, compared to 1–4 Å/s for PECVD materials. Our alloys exhibit similar trends with increasing Ge content to alloys grown by PECVD. The defect density, the dark conductivity, and the degree of nanostructural heterogeneity (as measured by small-angle X-ray scattering) all increase with increasing germanium content in the alloy. The nanostructural heterogeneity displays a sharp transition between 9 at.% and 14 at.% germanium. PECVD- grown a-SiGe:H alloys exhibit a similar transition at 20 at.% Ge. The photoconductivity and the ambipolar diffusion length of the alloys decrease with increasing germanium content. For a fixed silane-to-germane gas ratio, all material properties improve substantially when increasing substrate temperature (Tsub) from 220°C to 375°C. Increasing Tsub also narrows the optical bandgap and lowers the hydrogen content in the alloys for the same germane-to-silane gas ratio.
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