Fluorescent Silicon Clusters and Nanoparticles

The fluorescence of silicon clusters is reviewed. Atomic clusters of silicon have been at the focus of research for several decades because of the relevance of size effects for material properties, the importance of silicon in electronics and the potential applications in bio-medicine. To date numerous examples of nanostructured forms of fluorescent silicon have been reported. This article introduces the principles and underlying concepts relevant for fluorescence of nanostructured silicon such as excitation, energy relaxation, radiative and non-radiative decay pathways and surface passivation. Experimental methods for the production of silicon clusters are presented. The geometric and electronic properties are reviewed and the implications for the ability to emit fluorescence are discussed. Free and pure silicon clusters produced in molecular beams appear to have properties that are unfavourable for light emission. However, when passivated or embedded in a suitable host, they may emit fluorescence. The current available data show that both quantum confinement and localised transitions, often at the surface, are responsible for fluorescence. By building silicon clusters atom by atom, and by embedding them in shells atom by atom, new insights into the microscopic origins of fluorescence from nanoscale silicon can be expected.Comment: 5 figures, chapter in "Silicon Nanomaterials Sourcebook", editor Klaus D. Sattler, CRC Press, August 201

A QM/MM approach for the study of monolayer-protected gold clusters

We report the development and implementation of hybrid methods that combine quantum mechanics (QM) with molecular mechanics (MM) to theoretically characterize thiolated gold clusters. We use, as training systems, structures such as Au25(SCH2-R)18 and Au38(SCH2-R)24, which can be readily compared with recent crystallographic data. We envision that such an approach will lead to an accurate description of key structural and electronic signatures at a fraction of the cost of a full quantum chemical treatment. As an example, we demonstrate that calculations of the 1H and 13C NMR shielding constants with our proposed QM/MM model maintain the qualitative features of a full DFT calculation, with an order-of-magnitude increase in computational efficiency.Comment: Journal of Materials Science, 201

Impact of Long-Term Application of Domestic Sewage Water on Soil Properties Around Hubli City in Karnataka, India

The effects of application of domestic sewage water for over four decades on physical, chemical and biological properties of soils were studied at three sites in Gabbur, Mavanur and Katnur villages near Hubli city in North Karnataka, India. Long-term use of sewage irrigation improved soil physical properties in terms of decrease in bulk density and dispersion index and increase in aggregate stability and water holding capacity compared to the unirrigated check. Despite long-term irrigation with sewage water with an EC of >1 dS m−1, the EC of soils was low (0.20–0.45 dS m−1). An increase of one-and-half to two times in organic carbon content, available N, P, K and S, in the sewage irrigated soils was observed compared to soils not irrigated with sewage. However, the available Zn, Fe, Cu and B increased only slightly with the exception of Mn which increased substantially. In general there was a decreasing trend of organic carbon, available N, P, K and S with distance away from the stream course. Despite no heavy metals in sewage water, they were detected in soils but not in toxic levels. There were 94, 80 and 60 % more bacteria, fungi and actinomycetes, respectively, in sewage irrigated soils over the non-irrigated soils. Further, the coliform bacteria (E. coli) were observed in soil samples, suggesting fecal contamination

Crystal Growth of Thiol-Stabilized Gold Nanoparticles by Heat-Induced Coalescence

A monolayer of dodecanethiol-stabilized gold nanoparticles changed into two-dimensional and three-dimensional self-organized structures by annealing at 323 K. Subsequent crystal growth of gold nanoparticles occurred. Thiol molecules, although chemisorbed, form relatively unstable bonds with the gold surface; a few thiols desorbed from the surface and oxidized to disulfides at 323 K, because the interaction energy between thiol macromolecules is larger than that between a thiol and a nanoparticle. The gold nanoparticles approached each other and grew into large single or twinned crystals because of the van der Waals attraction and the heat generated by the exothermic formation of disulfides

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