Efficient surface grafting of luminescent silicon quantum dots by photoinitiated hydrosilylation

We suggest a method for efficient (high-coverage) grafting of organic molecules onto photoluminescent silicon nanoparticles. High coverage grafting was enabled by use of a modified etching process that produces a hydrogen-terminated surface on the nanoparticles with very little residual oxygen and by carefully excluding oxygen during the grafting process. It had not previously been possible to produce such a clean H-terminated surface on free silicon nanoparticles or, subsequently, to produce grafted particles without significant surface oxygen. This allowed us to (1) prepare air-stable green-emitting silicon nanoparticles, (2) prepare stable dispersions of grafted silicon nanoparticles in a variety of organic solvents from which particles can readily be precipitated by addition of nonsolvent, dried, and redispersed, (3) separate these nanoparticles by size (and therefore emission color) using conventional chromatographic methods, (4) protect the particles from chemical attack and photoluminescence quenching, and (5) provide functional groups on the particle surface for further derivatization. We also show, using 1H NMR, that the photoinitiated hydrosilylation reaction does not specifically graft the terminal carbon atom to the surface but that attachment at both the first and second atom occurs.     

Photophysical properties of blue - emitting silicon nanoparticles

Silicon nanoparticles with strong blue photoluminescence were synthesized by electrochemical etching of silicon wafers and ultrasonically removed under N(2) atmosphere in organic solvents to produce colloids. Thermal treatment leads to the formation of colloidal Si particles of 3 ± 1 nm diameter, which upon excitation with 340 - 380 nm light exhibited room temperature luminescence in the range from 400 to 500 nm. The emission and the one- and two-photon excitation spectra of the particles are not sensitive to surface functionalization with methyl 2-methylprop-2-enoate. However, the derivatized particles show higher emission quantum yields in air-saturated suspensions (44%) than the underivatized particles (27%), as well as higher stability of its dispersions.FTIR and XPS spectra indicate a significant surface oxidation of the particles. The Si:O:C ratio at the surface of the derivatized particles estimated from XPS is Si(3)O(6)(C(5)O(2)H(y))(1), with y = 7 - 8. Vibronic spacing is observed in both the emission and excitation spectra. The information obtained from one-photon excitation experiments (emission and excitation spectra, photoluminescence quantum yields, luminescence decay lifetimes and anisotropy correlation lifetimes), as well as from two-photon excitation fluorescence correlation spectroscopy (brightness and diffusion coefficients) and TEM indicate that the blue-emitting particles are monodisperse and ball-shaped. Particle size clearly determines the emission and excitation spectral region, as expected from quantum confinement, but the presence and extent of Si-O species on the silicon networks seem crucial for determining the spectrum features and intensity of emission. The nanoparticles could hold great potential as quantum dots for applications as luminescence sensors in biology and environmental science.     

Surface functionalization of silicon nanoparticles produced by laser-driven pyrolysis of silane followed by HF-HNO3 etching

CO2 laser induced pyrolysis of silane was used to produce silicon nanoparticles with an average diameter as small as 5 nm at high rates (up to 200 mg/h). Etching these particles with a mixture of hydrofluoric acid (HF) and nitric acid (HNO3) reduces their size and passivates their surface such that they exhibit bright visible photoluminescence (PL). This paper describes the attachment of organic molecules to hydrogen-terminated and hydroxyl-terminated surfaces of these nanoparticles. Stable particle dispersions in various solvents were obtained by treatment of hydrogen-terminated surfaces with octadecene or undecylenic acid and by treatment of hydroxyl-terminated surfaces with octadecyltrimethoxysilane. Transmission electron microscopy showed that the surface-functionalized particles were well dispersed and crystalline. FTIR spectroscopy confirmed the expected reactions of the organic molecules with the particle surfaces. Photoluminescence measurements showed that surface treatment significantly stabilized the PL properties of the nanoparticles against degradation. Size selective precipitation was applied to particle dispersions and allowed some narrowing and tuning of the PL spectrum.     

Liquid-liquid phase-transfer of magnetic nanoparticles in organic solvents

We report a novel route for the preparation of well-defined colloidal dispersions of magnetic nanoparticles stabilized by steric repulsion in organic solvents. The usual methods standardly lead to the surfaction of multiparticle aggregates, incompatible with our long-term aim of studying and modeling the influence of magnetic dipolar interactions in colloidal dispersions which are free of aggregates, all other interactions being perfectly defined. A new and reproducible method based on a surfactant-mediated liquid-liquid phase transfer of individually dispersed gamma-Fe(2)O(3) nanoparticles from an aqueous colloidal dispersion to an organic phase is developed. The choice of the reagent and the preparation techniques is discussed. Among several solvent/surfactant pairs, the cyclohexane/dimethyldidodecylammonium bromide (DDAB) system is found to fulfill the colloidal stability criterion: aggregation does not appear, even upon aging. A complete transfer of isolated particles is observed above a threshold in DDAB concentration. The nanoparticle surface is then fully covered with adsorbed DDAB molecules, each surfactant head occupying a surface of 0.57+/-0.05 nm(2). The volume fraction of the cyclohexane-based organosols is easily tunable up to a volume fraction of 12% by modifying the volume ratio of the organic and of the aqueous phases during the liquid-liquid phase transfer.     

Semiconductor nanoparticle/polystyrene latex composite materials

Cadmium sulfide and cadmium selenide/cadmium sulfide core/shell nanoparticles stabilized with poly(cysteine acrylamide) have been bound to polystyrene (PS) latexes by three methods. First, anionic 5 nm diameter CdS particles were electrostatically attached to 130 nm surfactant-free cationic PS latexes to form stable dispersions when the amount of CdS particles was less than 10% of the amount required to form a monolayer on the surface of the PS particles or when the amount of CdS particles exceeded the amount required to form a monolayer on the PS particles. Transmission electron microscopy (TEM) showed nanoparticles on the surface of the latex particles. Fluorescence spectra showed unchanged emission from the nanoparticles. Second, anionic, surfactant-free PS latexes were synthesized in the presence of CdS and CdSe/CdS nanoparticles. TEM showed monodisperse latex particles with trapped nanoparticles. Third, surfactant-stabilized latexes were synthesized by copolymerization of styrene with vinylbenzyl(trimethyl)ammonium chloride electrostatically bound to the CdSe/CdS nanoparticle surface. Brownian motion of the submicroscopic composite particles in water was detected by fluorescence microscopy.     

Highly transparent silica monoliths embedded with high concentration oxide nanoparticles

Silica monoliths embedded with high concentration of γ-Fe₂O₃ or TiO₂ nanoparticles were prepared by a sol–gel procedure designed according to the inherent properties of oxide colloids. In the first step, highly dispersible oxide nanoparticles were produced using an in situ modification sol–gel strategy. Then, these particles were re-dispersed in silicon alkoxide-containing solution to form a stable colloidal solution. The hydrolysis and condensation reactions of alkoxide were catalyzed by an organic base (morpholine). Due to the large molecule size of morpholine, the electric double layer on the surface of colloidal particles was not compressed by the ionized morpholine molecules. The colloidal solution thus remained stable during the gelation process. Through this procedure, oxide nanoparticles could be immobilized homogeneously in the pores of a silica matrix, forming highly transparent and crack-free monoliths.     

Enhancement and stabilization of the photoluminescence from porous silicon prepared by Ag‐assisted electrochemical etching

In this paper, we present the results of studies on the photoluminescence (PL) of porous silicon (PSi) samples obtained by etching with the assistance of silver metal in different ways. If the Si sample, after being coated with a layer of silver nanoparticles, is electrochemically etched, its PL intensity becomes hundreds of times stronger than the PL intensity when it is chemically etched in the similar conditions. The difference in the PL intensities is explained partly by the anodic oxidation of silicon which occurs during the electrochemical etching process. The most obvious evidence that silicon had been oxidized anodically in the electrochemical etching process is the disappearance of the PSi layer and the appearance of the silicon oxide layer with mosaic structure when the anodization current density is large enough. The anodic oxidation has the effect of PSi surface passivation. Because of that, the PL of obtained PSi becomes stronger and more stable with time. Copyright © 2012 John Wiley & Sons, Ltd.     

An effective oxidation route to blue emission CdSe quantum dots

Nearly monodisperse CdSe quantum dots with blue emission are obtained through an oxidation approach, in which CdSe particles can be etched into smaller ones. During the oxidation process, CdSe with yellow emission (546 nm) can be rapidly oxidized to blue emission (466 nm) due to its incompletely crystallized structure. Further oxidation results in the slow blue-shift of the photoluminescence peak to 433nm. The quantum fluorescence efficiency of CdSe with blue emission is about 10%, and surface-trap emission becomes evident when the PL peak of CdSe reaches the blue-violet region, since the surface atom ratio increases. This oxidation route offers a simple and mild way to get extremely small CdSe quantum dots.     

Stabilization of silicon nanoparticles by carbenes

Sodium reduction of a mixture of tetrabromosilane with imidazole ionic liquids in organic solvents gives dispersions of silicon nanoparticles stabilized by carbene ligands. It was shown that the size of silicon nanoclusters depends on the size of substituents at nitrogen atoms of 1,3-dialkylimidazol-2-ylidenes.     

Using colloid lithography to fabricate silicon nanopillar arrays on silicon substrates

In this study, we partially grafted geminal silanol groups in the protecting organic shells on the surfaces of gold nanoparticles (AuNPs) and then assembled the alkyl-AuNP-Si(OH)(4) particles onto the surfaces of silicon (Si) wafers. The density of assembled AuNPs on the Si surface was adjusted by varying the geminal silanol group content on the AuNP surface; at its optimal content, it approached the high assembly density (0.0254 particles/nm(2)) of an AuNP assembled monolayer. Using reactive-ion etching (RIE) with the templates as masks, we transferred the patterned AuNP assemblies to form large-area, size-tunable, Si nanopillar arrays, the assembly density of which was controlled by the dimensions of the AuNPs. Using this colloidal lithography (CL) process, we could generate Si nanopillars having sub-10-nm diameters and high aspect ratios. The water contact angles of the high-aspect-ratio Si nanopillars approached 150°. We used another fabrication process, involving electron beam lithography and oxygen plasma treatment, to generate hydrophilic 200-nm-resolution line patterns on a Si surface to assemble the AuNPs into 200-nm-resolution dense lines for use as an etching mask. Subsequent CL provided a patterned Si nanopillar array having a feature size of 200 nm on the Si surface. Using this approach, it was possible to pattern sub-10-nm Si nanopillar arrays having densities as high as 0.0232 nm(-2).     

Synthesis of metal-complexing latices via polymerization in microemulsion

Microlatex dispersions with ca. 120 m² bipyridine-functionalized surface per g polymer were synthesized via copolymerization in microemulsion using two new comonomers containing bipyridine groups. The resulting polymer particles were used for binding diverse metal ions in water as well as in organic solvents. Binding of the ions usually occurs intramolecularly. The colloidal characteristics of the dispersions are preserved.     

Redispersible and stable amorphous calcium phosphate nanoparticles functionalized by an organic bisphosphate

Although much effort has been focused on the preparation of stable amorphous calcium phosphate （ACP） nanoparticles in aqueous solution, the redispersibility and long-term stability of ACP nanoparticles in aqueous solution remains an unresolved problem. In this work, stable colloidal ACPs were prepared by using an organic bisphosphonate （BP） as a sterically hindered agent in aqueous solution. The harvested calcium phosphate nanoparticles were characterized by inductively coupled plasma atomic emission spectrometry （ICP-AES）, Fourier transform infrared （FTIR）, X-ray diffraction （XRD）, dynamic light scattering （DLS） and transmission electron microscopy （TEM）. ICP-AES, FTIR and XRD results suggested the particles were ACP. DLS and TEM results indicated that the size of the ACP nanoparticles were in the range of 60 nm with a spherical morphology. The resulting calcium phosphate nanoparticles retained its amorphous nature in aqueous solution for at least 6 months at room temperature due to the stabilizing effect of the organic bisphosphonate. Moreover, the surface of the ACP nanoparticles adsorbed with the organic bisphosphate used showed good redispersibility and high colloid stability both in organic and aqueous solutions.     

Long-Term Colloidally Stable Aqueous Dispersions of ≤5 nm Spinel Ferrite Nanoparticles

Applications in biomedicine and ferrofluids, for instance, require long-term colloidally stable, concentrated aqueous dispersions of magnetic, biocompatible nanoparticles. Iron oxide and related spinel ferrite nanoparticles stabilized with organic molecules allow fine-tuning of magnetic properties via cation substitution and water-dispersibility. Here, we synthesize≤5 nm iron oxide and spinel ferrite nanoparticles, capped with citrate, betaine and phosphocholine, in a one-pot strategy. We present a robust approach combining elemental (CHN) and thermal gravimetric analysis (TGA) to quantify the ratio of residual solvent molecules and organic stabilizers on the particle surface, being of particular accuracy for ligands with heteroatoms compared to the solvent. SAXS experiments demonstrate the long-term colloidal stability of our aqueous iron oxide and spinel ferrite nanoparticle dispersions for at least 3 months. By the use of SAXS we approved directly the colloidal stability of the nanoparticle dispersions for high concentrations up to 100 g L⁻¹.     

Electrochemistry and electrogenerated chemiluminescence of a spirobifluorene-based donor (triphenylamine)-acceptor (2,1,3-benzothiadiazole) molecule and its organic nanoparticles

A new D-A-π-A-D molecule (Spiro-BTA) containing two 2,1,3-benzothiadiazole (BTA) as the acceptor (A) and triphenylamine as the donor (D) bridged by a spirobifluorene moiety has been synthesized. The novel D-A molecule shows intense red emission (612 nm) with a high PL quantum yield (Φ(PL) = 0.51) in a solid film. A cyclic voltammogram of Spiro-BTA in 1:2 MeCN:benzene/0.1 M Bu(4)NPF(6) shows two reversible oxidation waves and one reversible reduction wave. The first oxidation wave and reduction wave were assigned as two successive electron transfer peaks separated by ～50 mV related to the oxidation of the two noninteracting donors and the reduction of the two noninteracting acceptors, respectively. Electrogenerated chemiluminescence (ECL) of Spiro-BTA upon cyclic oxidation and reduction in MeCN:benzene 1:2 shows a very bright and stable red emission that could be seen in a well-lit room. Using a reprecipitation method, well-dispersed organic nanoparticles (NPs) of the Spiro-BTA were prepared in aqueous solution. The nanoparticles were analyzed by dynamic light scattering (DLS) and scanning electron microscopy (SEM), yielding a NP size (without surfactant) of 130 ± 20 nm, while with surfactant, 100 ± 20 nm. Bathochromic shifts of absorption spectra (～16 ± 2 nm), as compared to that of the dissolved Spiro-BTA in THF, were observed for both NPs in water and as a thin film. While blue shifts (14 ± 2 nm) were observed for the photoluminescence (PL). The PL intensity of the Spiro-BTA nanoparticles was slightly enhanced (Φ(PL) of nanoparticles in water = 48%) over that of the dissolved Spiro-BTA in THF. The ECL of the organic Spiro-BTA nanoparticles in aqueous solution could be observed upon oxidation with tri-n-propylamine as a coreactant.     

Sterically stabilized polypyrrole–palladium nanocomposite particles synthesized by aqueous chemical oxidative dispersion polymerization

Aqueous chemical oxidative dispersion polymerizations of pyrrole using PdCl₂ oxidant were conducted using water-soluble polymeric colloidal stabilizers in order to synthesize polypyrrole–palladium (PPy–Pd) nanocomposite particles in one step. PPy–Pd nanocomposite particles with number average diameters of approximately 30 nm were successfully obtained as colloidally stable aqueous dispersions, which were stable at least for 7 months, using poly(4-lithium styrene sulfonic acid) colloidal stabilizer. The resulting nanocomposite particles were extensively characterized with respect to particle size, size distribution, colloidal stability, nanomorphology, surface/bulk chemical compositions, and conductivity. X-ray photoelectron spectroscopy indicated the existence of poly(styrene sulfonic acid) colloidal stabilizer on the surface of the nanocomposite particles. Transmission electron microscopy studies confirmed that nanometer-sized Pd nanoparticles were distributed in the PPy matrix.     

Grafting of hyperbranched cyclotriphosphazene polymer onto silica nanoparticle and carbon black surfaces

The surface grafting of hyperbranched cyclotriphosphazene polymer onto silica nanoparticles and carbon black was investigated. The grafting of hyperbranched cyclotriphosphazene polymer onto these surfaces was achieved by the repeated reactions of hexachlorocyclotriphosphazene with hexamethylenediamine from surface amino groups and sodium carboxylate groups, respectively. The percentage of grafting onto silica and carbon black surfaces exceeded 760 and 390%, respectively. However, it proved difficult to achieve the theoretical growth of cyclotriphosphazene polymer from these surfaces because of steric hindrance. The introduction of sulfonic acid groups was successfully achieved by the reaction of terminal chlorophosphazene groups of the hyperbranched polymer‐grafted silica and carbon black with sulfanilic acid. The content of sulfonic acid groups introduced onto silica and carbon black surfaces was 4.98 mmol/g and 5.70 mmol/g, respectively. The sulfonated cyclotriphosphazene polymer‐grafted carbon black was extremely hydrophilic, yielding stable colloidal dispersions in polar solvents. The sulfonated cyclotriphosphazene polymer‐grafted silica and carbon black showed ionic conductivity, with the conductance increasing exponentially with increasing relative humidity and temperature. This study may offer important leads in the application of silica nanoparticles and carbon black in polymeric membranes for fuel cells. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4218–4226, 2008     

Tunable surface-enhanced infrared absorption on au nanofilms on si fabricated by self-assembly and growth of colloidal particles

Au colloids were used to fabricate nanoscale-tunable Au nanofilms on silicon for surface-enhanced IR absorption bases in both ambient and electrochemical environments. This wet process incorporates the self-assembly of colloidal Au monolayer using 3-aminopropyl trimethoxysilane as the organic coupler with subsequent chemical plating in an Au(III)/hydroxylamine solution. FTIR spectroscopy in transmission mode of the probe species SCN- was used to evaluate the apparent surface enhancement in IR absorption of 2D Au colloid arrays and chemically plated Au particles. The nanostructure of Au films was examined by atomic force microscopy. The IR and AFM results show that the apparent surface enhancement factor (1-2 orders of magnitude) increases with increasing sizes and/or contact, and the severe aggregation of Au nanoparticles may cause the bipolar band shape. Cyclic voltammetry on the Au nanofilm obtained by the above nucleation and growth strategy exhibits a feasible electrochemical stability and behavior. In situ ATR-FTIR measurement of p-nitrobenzoic acid adsorption demonstrates that the as-grown Au film yields rather promising surface enhancement as well.     

Cu nanoparticles of low polydispersity synthesized by a double-template method and their stability

Cu nanoparticles of well-defined size and stability were synthesized with the aid of a double-template method. The templates consisted of sodium dodecyl sulfate (SDS) aggregates combined with and wrapped by poly(vinylpyrrolidone) (PVP) chains. Copper sulfate was reduced within the templates resulting in multicrystalline Cu nanoparticles. The size of the particles was uniform. They were capped by PVP–SDS complexes and the shape turned out to be non-spherical. PVP used in the experiments has an average molecular weight of 40,000. In this case, the particle dimensions were essentially determined by the chosen concentration of SDS in the reaction solution. No oxidation of the as-grown copper particles was detected even in the absence of inert gas protection during the synthesis process. When exposed to air at room temperature, Cu nanoparticles capped by PVP–SDS complexes showed much better resistance to oxidation than those without the capping agents. Furthermore, the steric and screening effect of the capping agents permitted the preparation of uniform colloidal dispersions stable over months. The material obtained by this double-template method was found to be very sensitive to the synthesis temperature. At synthesis temperatures above 40 °C, CuO instead of Cu was obtained.     

A novel method to prepare water dispersible poly(benzimidazobenzophenanthroline) (BBL) by partial substitution of chain ends with poly(ethylene oxide)

Poly(benzimidazobenzophenanthroline) (BBL) was prepared according to literature method and modified with poly(ethylene oxide) in a one pot synthesis. After precipitation in aqueous sodium carbonate solution and subsequent purification, aqueous dispersions were prepared by ultrasonication. Particle sizes in the dispersions ranged from few tens of nanometers to several micrometers and most of the particles had sizes of 50–250 nm. Further studies indicated that the colloidal stability is a combined result of steric stabilization caused by excluded volume interactions of PEO chains on particle surface and electrostatic stabilization by the dissociated carboxylic acid groups on the particle surface. The product could be processed into uniform films 20–30 nm in thickness by spin coating onto gold-plated silicon substrates having aminethiol monolayer as the top most layer.     

Solvent dependent friction force response of polystyrene brushes prepared by surface initiated polymerization

Polystyrene (PS) brushes were prepared on oxide passivated silicon by the surface initiated polymerization (SIP) technique. From an AIBN-type free radical initiator, which was silanized and immobilized on silicon wafers, styrene brushes were directly polymerized and grafted from the surface. The formation of the initiator monolayer and, subsequently, the polymer brush on the surface were monitored by X-ray photoelectron spectroscopy (XPS) and ellipsometry. Friction force measurements were performed by atomic force microscopy (AFM), using a 5 microm SiO2 colloidal sphere tip and under systematically varied solvent environments (nonpolar to polar), to demonstrate the dependence of brush lubricity on solvation. The relative uptake of solvents in the PS brush was determined by quartz crystal microbalance (QCM), and it correlates well with friction data. It is surmised that, in poor solvent environments, the polymer brush exists in a collapsed conformation, giving rise to the higher observed friction response.