Stable Silicon Research Articles

Amorphous silicon nanowires combining high nonlinearity, FOM and optical stability

We demonstrate optically stable amorphous silicon nanowires with both high nonlinear figure of merit (FOM) of ~5 and high nonlinearity Re(γ) = 1200W(-1)m(-1). We observe no degradation in these parameters over the entire course of our experiments including systematic study under operation at 2 W coupled peak power (i.e. ~2GW/cm(2)) over timescales of at least an hour.

Silicon isotope constraints on sources and utilization of silicic acid in the northern South China Sea

The stable silicon isotopic composition (δ30Si) of waters and diatoms has increasingly been used to investigate the biogeochemical cycling of Si in the major ocean basins. Here we present the first Si isotope data set from the northern South China Sea (NSCS), a large marginal sea system in the western North Pacific to examine sources and utilization of silicic acid (Si(OH)4). During two cruises in July–August 2009 (summer) and January 2010 (winter), samples for isotope measurements of dissolved Si(OH)4 (δ30SiSi(OH)4) and of biogenic silica (δ30SiBSi) in suspended particles were collected along a transect perpendicular to the coast from the inner shelf to the deep-water slope, as well as at the South East Asian Time-series Study (SEATS) station located in the NSCS basin. Surface δ30SiSi(OH)4 generally increased from values ∼+2.3‰ on the inner shelf to ∼+2.8‰ above the deep basin, suggesting an increasing utilization of dissolved Si(OH)4 reflecting the transition from eutrophic to oligotrophic conditions. The δ30SiBSi values were systematically lower than the corresponding δ30SiSi(OH)4 in the euphotic zone (above 100m) on the shelf and slope. In contrast at station SEATS in the NSCS basin, δ30SiBSi signatures in both seasons were within error equal to δ30SiSi(OH)4 in the surface mixed layer (above 50m) and δ30SiBSi in waters below were significantly higher than the corresponding δ30SiSi(OH)4. By comparing the field data with the Si isotope fractionation revealed by the Rayleigh or the steady state models, we demonstrate the existence of variable Si(OH)4 origins in different areas of the NSCS. Surface waters on the inner shelf were largely fed by nutrients from the Pearl River input. While the primary source of Si(OH)4 for the euphotic zone on the outer shelf and slope was upwelling or vertical mixing from underlying waters, the Si(OH)4 in the surface mixed layer of the NSCS basin might have originated from horizontal mixing with other highly fractionated surface waters. As a consequence, the Si isotope dynamics in the NSCS are largely controlled by variable biological fractionation of Si in waters from different sources with different initial Si isotopic compositions rather than any single source water.

A New View of Microcrystalline Silicon: The Role of Plasma Processing in Achieving a Dense and Stable Absorber Material for Photovoltaic Applications

AbstractTo further lower production costs and increase conversion efficiency of thin‐film silicon solar modules, challenges are the deposition of high‐quality microcrystalline silicon (μc‐Si:H) at an increased rate and on textured substrates that guarantee efficient light trapping. A qualitative model that explains how plasma processes act on the properties of μc‐Si:H and on the related solar cell performance is presented, evidencing the growth of two different material phases. The first phase, which gives signature for bulk defect density, can be obtained at high quality over a wide range of plasma process parameters and dominates cell performance on flat substrates. The second phase, which consists of nanoporous 2D regions, typically appears when the material is grown on substrates with inappropriate roughness, and alters or even dominates the electrical performance of the device. The formation of this second material phase is shown to be highly sensitive to deposition conditions and substrate geometry, especially at high deposition rates. This porous material phase is more prone to the incorporation of contaminants present in the plasma during film deposition and is reported to lead to solar cells with instabilities with respect to humidity exposure and post‐deposition oxidation. It is demonstrated how defective zones influence can be mitigated by the choice of suitable plasma processes and silicon sub‐oxide doped layers, for reaching high efficiency stable thin film silicon solar cells.

Blue-green luminescent silicon nanocrystals fabricated by nanosecond pulsed laser ablation in dimethyl sulfoxide

In this work, stable blue-green luminescent colloidal silicon nanocrystals (SiNCs) are fabricated by nanosecond pulsed laser ablation of a silicon target in dimethyl sulfoxide (DMSO). Transmission electron microscopy and X-ray diffraction analysis have shown the formation of spherical silicon nanocrystals in the colloid with size range of 2-5 nm. Our results show that the DMSO stabilizes the silicon nanocrystals via oxide formations on the nanocrytals surfaces by a simple route of laser ablation and a schematic representation of the process is suggested. The colloid exhibits strong blue luminescent emissions in the spectral range of 455-465 nm when excited at wavelengths near the direct band gap of the silicon nanocrystal. The luminescent emission band shifts to longer wavelengths (green light) if the excitation wavelength increases toward the indirect band gap of the SiNCs. The oxidized SiNCs with quantum confinement effects are shown to be responsible for visible photoluminescence of the colloid. The observed blue-green emission of the colloid makes it a good candidate for display, solid-state lighting and biological luminescent based devices.

Stabilization of Low Valent Silicon Fluorides in the Coordination Sphere of Transition Metals

Silicon(II) fluoride is unstable; therefore, isolation of the stable species is highly challenging and was not successful during the last 45 years. SiF(2) is generally generated in the gas phase at very high temperatures (~1100-1200 °C) and low pressures and readily disproportionates or polymerizes. We accomplished the syntheses of stable silicon(II) fluoride species by coordination of silicon(II) to transition metal carbonyls. Silicon(II) fluoride compounds L(F)Si·M(CO)(5) {M = Cr (4), Mo (5), W(6)} (L = PhC(NtBu)(2)) were prepared by metathesis reaction from the corresponding chloride with Me(3)SnF. However, the chloride derivatives L(Cl)Si·M(CO)(5) {M = Cr (1), Mo (2), W(3)} (L = PhC(NtBu)(2)) were prepared by the treatment of transition metal carbonyls with L(Cl)Si. Direct fluorination of L(Cl)Si with Me(3)SnF resulted in oxidative addition products. Compounds 4-6 are stable at ambient temperature under an inert atmosphere of nitrogen. Compounds 4-6 were characterized by NMR spectroscopy, EI-MS spectrometry, and elemental analysis. The molecular structures of 4 and 6 were unambiguously established by single-crystal X-ray diffraction. Compounds 4 and 6 are the first structurally characterized fluorides, after the discovery of SiF(2) about four and a half decades ago.

Hydrolytically stable octahedral silicon complexes as bioactive scaffolds: application to the design of DNA intercalators

We here introduce octahedral silicon serving as a structural center for the design of hydrolytically stable bioactive complexes as demonstrated with the generation of silicon-based high affinity DNA binders. This proof-of-principle study suggests that octahedral silicon complexes are falsely neglected, promising structural templates for widespread applications in chemical biology and medicinal chemistry.

Orientation of One‐Dimensional Silicon Polymer Films Studied by X‐Ray Absorption Spectroscopy

Molecular orientations for thin films of one‐dimensional silicon polymers grown by vacuum evaporation have been assigned by near‐edge X‐ray absorption fine structure (NEXAFS) using linearly polarized synchrotron radiation. The polymer investigated was polydimethylsilane (PDMS) which is the simplest stable silicon polymer, and one of the candidate materials for one‐dimensional molecular wire. For PDMS films deposited on highly oriented pyrolytic graphite (HOPG), four resonance peaks have been identified in the Si K‐edge NEXAFS spectra. Among these peaks, the intensities of the two peaks lower‐energy at 1842.0 eV and 1843.2 eV were found to be strongly polarization dependent. The peaks are assigned to the resonance excitations from the Si 1s to σ∗ pyz and σ∗ px orbitals localized at the Si–C and Si–Si bonds, respectively. Quantitative evaluation of the polarization dependence of the NEXAFS spectra revealed that the molecules are self‐assembled on HOPG surface, and the backbones of the PDMS are oriented nearly parallel to the surface. The observed orientation is opposite to the previously observed results for PDMS on the other surfaces such as oxide (indium tin oxide) and metal (polycrystalline copper). The flat‐lying feature of PDMS observed only on HOPG surface is attributed to the interaction between CH bonds in PDMS and π orbitals in HOPG surface.

Size-Dependent Absolute Quantum Yields for Size-Separated Colloidally-Stable Silicon Nanocrystals

Size-selective precipitation was used to successfully separate colloidally stable allylbenzene-capped silicon nanocrystals into several visible emitting monodisperse fractions traversing the quantum size effect range of 1-5 nm. This enabled the measurement of the absolute quantum yield and lifetime of photoluminescence of allylbenzene-capped silicon nanocrystals as a function of size. The absolute quantum yield and lifetime are found to monotonically decrease with decreasing nanocrystal size, which implies that nonradiative vibrational and surface defect effects overwhelm spatial confinement effects that favor radiative relaxation. Visible emission absolute quantum yields as high as 43% speak well for the development of "green" silicon nanocrystal color-tunable light emitting diodes that can potentially match the performance of their toxic heavy metal chalcogenide counterparts.

Effect of the electronic structure of metal solvents on silicon whisker growth

We have analyzed and systematized available data on silicon whisker growth with the use of a wide variety of metal solvents. The catalytic properties of metals responsible for stable Si whisker growth are shown to depend on the position of the metal in the Periodic Table, the interaction between the molten metal and silicon, and the number of electrons in the s and (n − 1)d electron orbitals of the metal. The most stable silicon whisker growth is ensured by Cu, Au, Ni, Ag, Pd, and Pt particles.

Cover Picture: Preparation of Highly Stable and Water-Dispersible Silicon Quantum Dots by Using an Organic Peroxide (Chem. Eur. J. 46/2011)

Cover Picture: Preparation of Highly Stable and Water-Dispersible Silicon Quantum Dots by Using an Organic Peroxide (Chem. Eur. J. 46/2011)

Donor–Acceptor-Stabilized Silicon Analogue of an Acid Anhydride

A stable silicon analogue of an acid anhydride {PhC(Bu(t)N)(2)}Si{═O·B(C(6)F(5))(3)}O-Si(H){═O·B(C(6)F(5))(3)}{(NBu(t))(HNBu(t))CPh} (4) with a O═Si-O-Si═O core has been prepared by treating monochlorosilylene PhC(Bu(t)N)(2)SiCl (1) with H(2)O·B(C(6)F(5))(3) in the presence of NHC (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Compound 4 has been characterized by elemental analysis and multinuclear NMR spectroscopic investigations. The molecular structure of 4 has been established by single-crystal X-ray diffraction studies, and DFT calculations support the experimental results.

Preparation of Highly Stable and Water-Dispersible Silicon Quantum Dots by Using an Organic Peroxide

Preparation of Highly Stable and Water-Dispersible Silicon Quantum Dots by Using an Organic Peroxide

Refractive index spectral dependence, Raman spectra, and transmission spectra of high-purity Si28, Si29, Si30, and Sinat single crystals

Precise measurement of the refractive index of stable silicon isotopes $^{28}$Si, $^{29}$Si, $^{30}$Si single crystals with enrichments above 99.9 at.% and a silicon single crystal $^{nat}$Si of natural isotopic composition is performed with the Fourier-transform interference refractometry method from 1.06 to more than 80 mkm with 0.1 cm$^{-1}$ resolution and accuracy of $2 \times 10^{-5} ... 1 \times 10^{-4}$. The oxygen and carbon concentrations in all crystals are within $5 \times 10^{15}$ cm$^{-3}$ and the content of metal impurities is $10^{-5} ... 10^{-6}$ at.%. The peculiar changes of the refractive index in the phonon absorption region of all silicon single crystals are shown. The coefficients of generalized Cauchy dispersion function approximating the experimental refractive index values all over the measuring range are given. The transmission and Raman spectra are also studied.

Preparation and characterization of silicon oil based ferrofluid

Stable silicon oil based ferrofluid was prepared in the present investigation. Silicon oil surfactant ethoxy terminated polydimethylsiloxane was used to modify the Fe 3O 4 nanoparticles. The Fe 3O 4 nanoparticles were firstly coated with a SiO 2 layer by the hydrolysis of tetraethoxysilane. Then using the active hydroxyl groups on the surface of the SiO 2, silicon oil surfactant was covalently grafted onto the Fe 3O 4 nanoparticles surface. The ethoxy terminated polydimethylsiloxane has similar molecular chain structure and good compatibility with that of the carrier liquid, thus ensuring stable dispersion of modified Fe 3O 4 in the carrier silicon oil. The interaction between Fe 3O 4 and the modifier was characterized by IR and XPS. The crystal structure and the magnetic properties of the Fe 3O 4 nanoparticles were determined by XRD and VSM, respectively. The size and morphology of the particles were observed using TEM. The properties of the silicon oil based ferrofluid were characterized by Gouy magnetic balance. The results indicated that the ferrofluid had high magnetism and good stability. The rheological properties and thermostability of the ferrofluid were also investigated.

Colloidally Stable Silicon Nanocrystals with Near‐Infrared Photoluminescence for Biological Fluorescence Imaging

Luminescent silicon nanocrystals (ncSi) are showing great promise as photoluminescent tags for biological fluorescence imaging, with size-dependent emission that can be tuned into the near-infrared biological window and reported lack of toxicity. Here, colloidally stable ncSi with NIR photoluminescence are synthesized from (HSiO1.5)n sol-gel glasses and are used in biological fluorescence imaging. Modifications to the thermal processing conditions of (HSiO1.5)n sol-gel glasses, the development of new ncSi oxide liberation chemistry, and an appropriate alkyl surface passivation scheme lead to the formation of colloidally stable ncSi with photoluminescence centered at 955 nm. Water solubility and biocompatibility are achieved through encapsulation of the hydrophobic alkyl-capped ncSi within PEG-terminated solid lipid nanoparticles. Their applicability to biological imaging is demonstrated with the in-vitro fluorescence labelling of human breast tumor cells.

Exohedral Silicon Fullerenes: Si60Pn60 and Si80Pn60 (Pn = P, As, Sb and Bi)

Based on density functional theory (DFT) calculations, we predict that the icosahedral structures of the silicon fullerenes Si60 and Si80 can be stabilized by 12 exohedral pentagons of group V-A unit Pn5 (Pn = P, As, Sb or Bi). The 12 pentagons can fully passivate the dangling bonds associated with 12 pentagonal Si5 rings on the silicon fullerene cages, thereby resulting in stable exohedral silicon fullerenes Si60Pn60 and Si80Pn60. Properties of the eight Si60Pn60 and Si80Pn60 clusters, including harmonic vibrational frequencies, electron affinity (EA), the HOMO–LUMO gap and NICS values, are computed. We find that all eight Si60Pn60 and Si80Pn60 fullerenes possess relatively large HOMO–LUMO gaps, high electron affinities, and that the Si60Pn60 fullerenes exhibit weak aromaticity. Among eight clusters examined, the exohedral fullerene Ih-Si60P60 possesses the largest cohesive energy per atom. Ab initio molecular dynamics (AIMD) simulation is performed to demonstrate thermal stability of the hollow cage structure of Si60P60 at the room temperature.

ChemInform Abstract: Zwitterionic and Donor‐Stabilized N‐Heterocyclic Silylenes (NHSis) for Metal‐Free Activation of Small Molecules

AbstractReview: 130 refs.

The Diamond Within a Silicon Analog of Cyclobutadiene

The distinctive rhombic structure of the central ring of a stable silicon analog of cyclobutadiene provides new insights into antiaromaticity.

A Planar Rhombic Charge-Separated Tetrasilacyclobutadiene

The cyclobutadiene (CBD) molecule C(4)H(4) deviates from a high-symmetry square geometry to compensate for its antiaromatic electronic structure. Here, we report a CBD silicon analog, Si(4)(EMind)(4) (1), stabilized by the bulky 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl (EMind) groups, obtained as air- and moisture-sensitive orange crystals by the reduction of (EMind)SiBr(3) with three equivalents of lithium naphthalenide. X-ray crystallography reveals a planar and rhombic structure of the Si(4) four-membered ring, with alternating pyramidal and planar configurations at the silicon atoms. The large (29)Si chemical shift differences (Δδ > 350 parts per million) in the solid-state nuclear magnetic resonance spectra suggest a contribution of an alternately charge-separated structure. The rhombic-shaped charge-separated singlet state of compound 1 thus stabilizes its cyclic 4π-electron antiaromaticity in a manner that contrasts sharply with the bond-length alternation, characterizing the rectangular distortion of carbon-based CBD.

Synthesis of Stable Silicon Heterocycles by Reaction of Organic Substrates with a Chlorosilylene [PhC(NtBu)2SiCl

Heteroleptic chlorosilylene (PhC(NtBu)(2)SiCl) (1) reacts with unsaturated organic compounds under oxidative addition. Reactions of 1 with cyclooctatetraene (COT) and a diimine afford [1+4]-cycloaddition products 3 and 6, respectively. In the case of COT, one Si-N bond of the amidinato ligand is cleaved, resulting in tetracoordinate silicon, whereas with a diimine a pentacoordinate silicon is formed. Treatment of 1 with ArN=C=NAr (Ar=2,6-iPr(2)C(6)H(3)) yields silaimine complex 4 with cleavage of one of the C=N bonds. The facile isolation of silaimine complexes is probably due to the kinetic protection afforded by the bulky Ar moiety. When 1 is treated with tert-butyl isocyanate, cleavage of the C=O bond is observed, which leads to formation of the four-membered Si(2)O(2) cycle 5. The same product is formed when 1 is allowed to react with trimethylamine N-oxide. When 1 is treated with diphenyl disulfide, cleavage of the S-S bond occurs to form 7. All products have been characterized by multinuclear NMR spectroscopy, EI mass spectrometry, and elemental analysis. In addition, the molecular structures of 3-6 have been determined by single-crystal X-ray diffraction studies. Collectively, these results suggest that owing to the presence of the lone pair of electrons, the propensity of 1 to undergo oxidative addition is very high.