Structural, vibrational, and optical properties of silicon cluster assembled films

Enhancing the properties of bone China ceramics by treatment with microporous SiO2 nanoparticles

In this study, microporous silicon dioxide nanoparticles (SiO2–NPs) were used to improve the physical, chemical, and mechanical properties of bone China ceramics. Microporous SiO2–NPs were prepared economically from sodium metasilicate (SMS) as a precursor with cetyltrimethylammonium bromide (CTAB) as a surfactant at different concentrations. The prepared SiO2–NPs were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy to confirm the formation of microporous SiO2–NPs. The optimum concentrations of the precursor and surfactant used in the SiO2–NPs synthesis were set to be 1.5% and 2 g/200 ml, with a size range of 7–96 nm. SiO2–NPs prepared at the optimum concentrations were incorporated into bone China at different concentrations to evaluate their effect on flexural strength and elasticity. The bone China prepared using 1% SiO2–NPs (B1) had the highest flexural strength and Young's modulus values. Sample characteristics, including self-cleaning, differential scanning calorimetry, thermogravimetric analysis, bulk density (BD), apparent porosity (AP), and water absorption (WA), were investigated. The results revealed outstanding characteristic such as self-cleaning ability, remarkable increase in AP and WA, and a decrease in BD.

Photoluminescence from silicon nanoparticles embedded in ammonium silicon hexafluoride

Silicon (Si) nanoparticles (NPs) were synthesized by transforming a Si wafer surface to ammonium silicon hexafluoride (ASH) or (NH(4))(2)SiF(6) under acid vapor treatment. Si-NPs which were found to be embedded within the polycrystalline (ASH) layer exhibit a strong green-orange photoluminescence (PL). Differential PL measurements revealed a major double component spectrum consisting of a broad band associated with the ASH-Si wafer interfacial porous oxide layer and a high energy band attributable to Si-NPs embedded in the ASH. The origin of the latter emission can be explained in terms of quantum/spatial confinement effects probably mediated by oxygen related defects in or around Si-NPs. Although Si-NPs are derived from the interface they are much smaller in size than those embedded within the interfacial porous oxide layer (SiO(x), x > 1.5). Transmission electron microscopy (TEM) combined with Raman scattering and Fourier transformed infrared (FTIR) analysis confirmed the presence of Si-NP and Si-O bondings pointing to the role of oxygen related defects in a porous/amorphous structure. The presence of oxygen of up to 4.5 at.% in the (NH(4))(2)SiF(6) layer was confirmed by energy dispersive spectroscopy (EDS) analysis.

Is a highly ionic material still ionic as a nanoparticle?

The evolution of ionicity with size in highly ionic nanoparticles is investigated in small sesquioxide clusters. Representative clusters (Y2O3)N (N < 50) are theoretically analyzed by first-principle calculations within the density functional theory within the local-density approximation (DFT-LDA) framework and compared to experimental results obtained in an ultrahigh vacuum environment. By studying the structural relaxation and the electronic density of states as a function of size, the respective roles of ionicity and covalency are elucidated. For compounds as ionic as rare earth sesquioxides, the highly ionic bond essentially governs and preserves the crystalline structure. Particular attention is paid to the mechanism responsible for the surface relaxation. The role of the ions at the corners and edges appears prominent, especially in reducing the dipole carried by the particles. Eventually, contrary to the observations and computations concerning ionic surfaces, the mean ionicity remains constant as the size is reduced. It emphasizes that the description of highly ionic nanoparticles cannot be directly inferred from knowledge regarding the ionic surface reconstruction.