Synthesis of nanoamorphous germanium and its transformation to nanocrystalline germanium

A simple reaction between a mild reducing agent such as a trialkoxysilane and Ge IV species such as germanium tetraalkoxides in a room-temperature water/alcohol solution produces silica-coated ultrasmall (2-3 nm) amorphous germanium nanoparticles (na-Ge/SiO 2). The initial reaction involves the straightforward hydrolysis and condensation of the precursors, Ge(OCH 2CH 3) 4 and (CH 3CH 2O) 3SiH, where the reaction rate depends on the water concentration in the reaction medium. These processes can be further accelerated by adding acid to the reaction medium or carrying out the reaction at higher temperatures. At low water contents (up to 50% water/ethanol) and low acid concentrations, the reaction proceeds as a clear solution, and no precipitation is observed. The initially colorless clear solution progressively changes to pale yellow, yellow, orange, red, and finally dark red as the na-Ge particles grow. Evaporation of the solvent yields a reddish-brown powder/monolith consisting of na-Ge, embedded in an encapsulating amorphous silica matrix, na-Ge/SiO 2. The formation of na-Ge proceeds extremely slowly and follows a first-order dependence on both water concentration and diameter of the na-Ge particles under the reaction conditions used. Annealing of the na-Ge/SiO 2 powder under an inert atmosphere at 600°C produces ultrasmall germanium nanocrystals (nc-Ge) embedded in amorphous silica (nc-Ge/SiO 2). Freestanding, colloidally stable nc-Ge is obtained by chemical etching of the encapsulating silica matrix. Two oxide-based precursors in a water/alcohol solution, one acting as a reducing agent ((CH 3CH 2O) 3SiH) and the other as a germanium source (Ge(OCH 2CH 3) 4), react very slowly to form colloidally stable amorphous nanoparticles of germanium encapsulated by a shell of silica (na-Ge/SiO 2). Evaporation of the solvent yields monoliths of na-Ge/SiO 2, which can be annealed to form nanocrystalline germanium (nc-Ge) embedded in a silica matrix (nc-Ge/SiO 2), from which free-standing, colloidally stable nc-Ge can be obtained by chemical etching of the encapsulating silica matrix. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Vapor swellable colloidal photonic crystals with pressure tunability

Polyferrocenylsilane gel photonic crystals have been reversibly swollen using solvent vapors, and exhibit precise pressure tunability over a wavelength range of greater than 100 nmGeneralitat Valenciana CTDIA/2002/2

Organization of bridging organics in periodic mesoporous organosilicas (PMOs) - Polarization micro-raman spectroscopy

The organization of bridging organics in oriented periodic mesoporous organosilica film (OPMOF) was demonstrated using the polarization micro-Raman spectroscopy (PMRS) in conjunction with powder x-ray diffraction (PXRD) and polarization optical microscopy (POM). The synthesis and the structural characterization of hexagonal symmetry OPMOF containing bridge-bonded ethane, ethene inside the silica channel walls were described. The mesoscale channels were found to run parallel to the surface of the underlying glass substrates as demonstrated by the PXRD measurements. A hexagonal array of channels with glassy silica organosilica walls was the best description of the structure shown by the PMRS measurements of OPMOF

Nanometer-Scale Precision Tuning of 3D Photonic Crystals Made Possible Using Polyelectrolytes with Controlled Short Chain Length and Narrow Polydispersity

Nanometer‐scale tuning of the optical properties of prefabricated photonic crystals is achieved via layer‐by‐layer assembly of polyelectrolytes in the interstitial spaces of the photonic lattice. The key to the approach is using polyelectrolytes with controlled short chain lengths. This ensures they do not block the air voids, thereby maintaining uniform coating and thus precise and reproducible optical tuning.Ministerio de Economía y Competitividad MAT2011-23593, CSD2007-00007Junta de Andalucía FQM3579, FQM5247European Union 30708

From bare metal powders to colloidally stable TCO dispersions and transparent nanoporous conducting metal oxide thin films

A simple, green, robust, widely applicable, multi-gram and cost-effective 'one-pot' synthesis of aqueous dispersions of colloidally stable 3-6 nm TCO NPs using bare metal powder precursors is described, and their utilization for making TCO high surface area nanoporous films is also demonstrated, which speaks well for their usage in a wide range of possible processes and devices. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Green nanochemistry: Metal oxide nanoparticles and porous thin films from bare metal powders

A universal, simple, robust, widely applicable and cost-effective aqueous process is described for a controlled oxidative dissolution process of micrometer-sized metal powders to form high-purity aqueous dispersions of colloidally stable 3-8 nm metal oxide nanoparticles. Their utilization for making single and multilayer optically transparent high-surface-area nanoporous films is demonstrated. This facile synthesis is anticipated to find numerous applications in materials science, engineering, and nanomedicine. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Molecular structures and vibrations of neutral and anionic CuOx (x = 1-3,6) clusters

We report equilibrium geometric structures of CuO2, CuO3, CuO6, and CuO clusters obtained by an all-electron linear combination of atomic orbitals scheme within the density-functional theory with generalized gradient approximation to describe the exchange-correlation effects. The vibrational stability of all clusters is examined on the basis of the vibrational frequencies. A structure with Cs symmetry is found to be the lowest-energy structure for CuO2, while a -shaped structure with C2v symmetry is the most stable structure for CuO3. For the larger CuO6 and CuO clusters, several competitive structures exist with structures containing ozonide units being higher in energy than those with O2 units. The infrared and Raman spectra are calculated for the stable optimal geometries. ~Comment: Uses Revtex4, (Better quality figures can be obtained from authors

Periodic mesoporous hydridosilica-synthesis of an "impossible" material and its thermal transformation into brightly photoluminescent periodic mesoporous nanocrystal silicon-silica composite

There has always been a fascination with "impossible" compounds, ones that do not break any rules of chemical bonding or valence but whose structures are unstable and do not exist. This instability can usually be rationalized in terms of chemical or physical restrictions associated with valence electron shells, multiple bonding, oxidation states, catenation, and the inert pair effect. In the pursuit of these "impossible" materials, appropriate conditions have sometimes been found to overcome these instabilities and synthesize missing compounds, yet for others these tricks have yet to be uncovered and the materials remain elusive. In the scientifically and technologically important field of periodic mesoporous silicas (PMS), one such "impossible" material is periodic mesoporous hydridosilica (meso-HSiO1.5). It is the archetype of a completely interrupted silica open framework material: its pore walls are comprised of a three-connected three-dimensional network that should be so thermodynamically unstable that any mesopores present would immediately collapse upon removal of the mesopore template. In this study we show that meso-HSiO1.5 can be synthesized by template-directed self-assembly of HSi(OEt)3 under aqueous acid-catalyzed conditions and after template extraction remains stable to 300 °C. Above this temperature, bond redistribution reactions initiate a metamorphic transformation which eventually yields periodic mesoporous nanocrystalline silicon-silica, meso-ncSi/SiO2, a nanocomposite material in which brightly photoluminescent silicon nanocrystallites are embedded within a silica matrix throughout the mesostructure. The integration of the properties of silicon nanocrystallinity with silica mesoporosity provides a wealth of new opportunities for emerging nanotechnologies. © 2011 American Chemical Society

Assembling photoluminescent silicon nanocrystals into periodic mesoporous organosilica

A contemporary question in the intensely active field of periodic mesoporous organosilica (PMO) materials is how large a silsesquioxane precursor can be self-assembled under template direction into the pore walls of an ordered mesostructure. An answer to this question is beginning to emerge with the ability to synthesize dendrimer, buckyball, and polyhedral oligomeric silsesquioxane PMOs. In this paper, we further expand the library of large-scale silsesquioxane precursors by demonstrating that photoluminescent nanocrystalline silicon that has been surface-capped with oligo(triethoxysilylethylene), denoted as ncSi:(CH 2CH 2Si(OEt) 3) nH, can be self-assembled into a photoluminescent nanocrystalline silicon periodic mesoporous organosilica (ncSi-PMO). A comprehensive multianalytical characterization of the structural and optical properties of ncSi-PMO demonstrates that the material gainfully combines the photoluminescent properties of nanocrystalline silicon with the porous structure of the PMO. This integration of two functional components makes ncSi-PMO a promising multifunctional material for optoelectronic and biomedical applications. © 2012 American Chemical Society