Frustrated Lewis Pair Chelation as a Vehicle for Low-Temperature Semiconductor Element and Polymer Deposition.

The stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB, i Pr2 P(C6 H4 )BCy2 (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH2 }] and [PB{GeH2 }] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine-borane ligand (PB) after element deposition, and 2) the single-source precursor [PB{SiH2 }] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe2 }] was also prepared, and shown to release poly(dimethylsilane) [SiMe2 ]n upon heating. Overall, this study introduces a "closed loop" deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.

Ensemble Effects in the Temperature-Dependent Photoluminescence of Silicon Nanocrystals.

In this work the temperature-dependent photoluminescence of alkyl-capped silicon nanocrystals with mean diameters of between 3 and 9 nm has been investigated. The nanocrystals were characterized extensively by FTIR, TEM, powder XRD, and X-ray photoelectron spectroscopy prior to low-temperature and time-resolved photoluminescence spectroscopy experiments. The photoluminescence (PL) properties were evaluated in the temperature range of 41-300 K. We found that the well-known temperature-dependent blueshift of the PL maximum decreases with increasing nanocrystal diameter and eventually becomes a redshift for nanocrystal diameters larger than 6 nm. This implies that the observed shifts cannot be explained solely by band-gap widening, as is commonly assumed. We propose that the luminescence of drop-cast silicon nanocrystals is affected by particle ensemble effects, which can explain the otherwise surprising temperature dependence of the luminescence peak.

Endogenous dynamic nuclear polarization NMR of hydride-terminated silicon nanoparticles.

Silicon nanoparticles (SiNPs) are intriguing materials and their properties fascinate the broader scientific community; they are also attractive to the biological and materials science sub-disciplines because of their established biological and environmental compatibility, as well as their far-reaching practical applications. While characterization of the particle nanostructure can be performed using 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy, poor sensitivity due to low Boltzmann population and long acquisition times hinder in-depth studies of these potentially game-changing materials. In this study, we compare two dynamic nuclear polarization (DNP) NMR protocols to boost 29Si sensitivity in hydride-terminated SiNPs. First, we assess a traditional indirect DNP approach, where a nitroxide biradical (AMUPol or bCTbk) is incorporated into a glassing agent and transferred through protons (e- → 1H → 29Si) to enhance the silicon. In this mode, electron paramagnetic resonance (EPR) spectroscopy demonstrated that the hydride-terminated surface was highly reactive with the exogenous biradicals, thus decomposing the radicals within hours and resulting in an enhancement factor, ε, of 3 (TB = 15 s) for the 64 nm SiNP, revealing the surface components. Secondly, direct DNP NMR methods were used to enhance the silicon without the addition of an exogenous radical (i.e., use of dangling bonds as an endogenous radical source). With radical concentrations <1 mM, 29Si enhancements were obtained for the series of SiNPs ranging from 3 to 64 nm. The ability to use direct 29Si DNP transfer (e- → 29Si) shows promise for DNP studies of these inorganic nanomaterials (ε = 6 (TB = 79 min) for 64 nm SiNPs) with highly reactive surfaces, showing the sub-surface and core features. These preliminary findings lay a foundation for future endogenous radical development through tailoring the surface chemistry, targeting further sensitivity gains.

Water-Assisted Transfer Patterning of Nanomaterials.

We introduce a straightforward and cost-effective water-assisted approach to transfer patterns of nanomaterials onto diverse substrates. The transfer method relies on the hydrophobic effect and utilizes a water-soluble polymer film as a carrier to transfer hydrophobic nanomaterials from a patterned source substrate onto a target substrate. Using this approach, nanomaterials are transferred readily from solutions onto surfaces of various shapes and compositions with high fidelity for feature sizes approaching 10 microns.

Functional Bioinorganic Hybrids from Enzymes and Luminescent Silicon-Based Nanoparticles.

This study reports the preparation of functional bioinorganic hybrid materials exhibiting catalytic activity and photoluminescent properties arising from the combination of enzymes and freestanding silicon-based nanoparticles. The hybrid materials reported herein have potential applications in biological sensing/imaging and theranostics, as they combine long-lived silicon-based nanoparticle photoluminescence with substrate-specific enzymatic activity. Thermal hydrosilylation of undecenoic acid and alkene-terminated poly(ethylene oxide) with hydride-terminated silicon nanocrystals afforded nanoparticles functionalized with a mixed surface made up of carboxylic acid and poly(ethylene oxide) moieties. These silicon-based nanoparticles were subsequently conjugated with prototypical enzymes through the carbodiimide-mediated amide coupling reaction in order to form bioinorganic hybrids that display solubility and photostability in phosphate buffer, photoluminescence (λmax = 630 nm), and enzymatic activity. They were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), dynamic light scattering analysis (DLS), photoluminescence spectroscopy, and pertinent enzyme activity assays.

Superhydrophobic Silicon Nanocrystal-Silica Aerogel Hybrid Materials: Synthesis, Properties, and Sensing Application.

Silicon nanocrystals (SiNCs) are abundant and exhibit exquisitely tailorable optoelectronic properties. The incorporation of SiNCs into highly porous and lightweight substrates such as aerogels leads to hybrid materials possessing the attractive features of both materials. This study describes the covalent deposition of SiNCs on and intercalation into silica aerogels, explores the properties, and demonstrates a prototype sensing application of the composite material. SiNCs of different sizes were functionalized with triethoxyvinylsilane (TEVS) via a radical grafting approach and subsequently used for the synthesis of photoluminescent silica hybrids. The resulting SiNC-containing aerogels possess high porosities, SiNC-based size-dependent photoluminescence, transparency, and a superhydrophobic macroscopic surface. The materials were used to examine the photoluminescence response toward low concentrations of 3-nitrotoluene (270 µM), demonstrating their potential as a sensing platform for high-energy materials.

The influence of conjugated alkynyl(aryl) surface groups on the optical properties of silicon nanocrystals: photoluminescence through in-gap states.

Developing new methods, other than size and shape, for controlling the optoelectronic properties of semiconductor nanocrystals is a highly desired target. Here we demonstrate that the photoluminescence (PL) of silicon nanocrystals (SiNCs) can be tuned in the range 685-800 nm solely via surface functionalization with alkynyl(aryl) (phenylacetylene, 2-ethynylnaphthalene, 2-ethynyl-5-hexylthiophene) surface groups. Scanning tunneling microscopy/spectroscopy on single nanocrystals revealed the formation of new in-gap states adjacent to the conduction band edge of the functionalized SiNCs. PL red-shifts were attributed to emission through these in-gap states, which reduce the effective band gap for the electron-hole recombination process. The observed in-gap states can be associated with new interface states formed via (-Si-C≡C-) bonds in combination with conjugated molecules as indicated by ab initio calculations. In contrast to alkynyl(aryl)s, the formation of in-gap states and shifts in PL maximum of the SiNCs were not observed with aryl (phenyl, naphthalene, 2-hexylthiophene) and alkynyl (1-dodecyne) surface groups. These outcomes show that surface functionalization with alkynyl(aryl) molecules is a valuable tool to control the electronic structure and optical properties of SiNCs via tuneable interface states, which may enhance the performance of SiNCs in semiconductor devices.

Phosphorus Pentachloride Initiated Functionalization of Silicon Nanocrystals.

Phosphorus pentachloride (PCl5) has long been used to chlorinate hydrocarbons. It has also been applied in silicon surface chemistry to facilitate alkylation via a two-step halogenation/Grignard route. Here we report a study of the reaction of PCl5 with hydride-terminated silicon nanocrystals (H-SiNCs). An examination of the reaction mechanism has allowed us to establish a functionalization protocol that uses PCl5 as a surface radical initiator to introduce alkyl and alkenyl moieties to the surface of H-SiNCs. The reaction proceeds quickly in a single step, at room temperature and the functionalized silicon nanocrystals retained their morphology and crystallinity. The resulting materials exhibited size-dependent photoluminescence that was approximately 3× as bright as that observed for thermally hydrosilylated SiNCs. Furthermore, the absolute PL quantum yield (AQY) was more than double. The high AQY is expected to enable SiNCs to compete with chalcogenide-based quantum dots in various applications.

Synthesis and Surface Functionalization of Hydride-Terminated Ge Nanocrystals Obtained from the Thermal Treatment of Ge(OH)2.

The synthesis of germanium nanocrystals (GeNCs) with well-defined surface chemistry is of considerable interest because of their potential applications in the optoelectronic, battery, and semiconductor industries. Modifying and tailoring GeNC surface chemistry provides an avenue by which reactivity, environmental compatibility (e.g., solubility, resistance to oxidation), and electronic properties may be tailored. Hydride-terminated GeNCs (H-GeNCs) are of particular interest because the reactivity of surface Ge-H bonds toward alkenes and alkynes via hydrogermylation affords the potential for convenient modification; however, these reactions and their scope have not been widely explored. This report describes a straightforward route for preparing a GeNC/GeO2 composite via disproportionation of heretofore-unexplored Ge(II) oxide-based precursor from which the H-GeNCs were freed by subsequently chemical etching. The H-GeNCs were derivatized using a series of hydrogermylation approaches (i.e., thermally activated, radical-initiated, and borane-catalyzed). The presented findings indicate surface functionalization occurs under all conditions investigated; however the nature of surface species (i.e., monolayers vs multilayers) and surface coverage varies depending upon the conditions employed.

Microwave-assisted synthesis and prototype oxygen reduction electrocatalyst application of N-doped carbon-coated Fe3O4 nanorods.

Fe3O4 nanorods coated with nitrogen-doped mesoporous carbon (ND-Fe3O4@mC) shells of defined thicknesses have been prepared via a new microwave-assisted approach. Microstructural characterization of these ND-Fe3O4@mC structures was performed using x-ray diffraction, x-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy. Following identification, the electrochemical performance of the catalysts was evaluated using linear sweep voltammetry with a rotating disc electrode system. The present investigation reveals enhanced oxygen reduction reaction catalytic activity and the carbon layer thickness influences oxygen diffusion to the active Fe3O4 nanorod core.

Cellulose nanocrystal-derived hollow mesoporous carbon spheres and their application as a metal-free catalyst.

In this contribution, we demonstrate the fabrication of hollow mesoporous carbon spheres (HCSs) derived from cellulose nanocrystals (CNCs). The HCSs were prepared by templating CNCs onto sacrificial silica spheres followed by heat treatment. Mesoporous carbon spheres result from the removal of the silica spheres by etching. The walls of the HCSs are approximately 4 nm thick and are composed of amorphous and graphitic carbon. The catalytic activity of the HCSs was investigated for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by sodium borohydride (NaBH4). The present investigation reveals the outstanding catalytic activity of these particles. The reaction rate followed pseudo-first order kinetics with k value of 4.72 × 10-3 s-1 and activity parameter of 52.2 s-1 g-1, which showed superior performance compared to that of metal nanoparticle and metal nanoparticle-carbon hybrid based catalysts.

Instantaneous Functionalization of Chemically Etched Silicon Nanocrystal Surfaces.

Remarkable advances in surface hydrosilylation reactions of C=C and C=O bonds on hydride-terminated silicon have revolutionized silicon surface functionalization. However, existing methods for functionalizing hydride-terminated Si nanocrystals (H-SiNCs) require long reaction times and elevated temperatures. Herein, we report a room-temperature method for functionalizing H-SiNC surfaces within seconds by stripping outermost atoms on H-SiNC surfaces with xenon difluoride (XeF2 ). Detailed analysis of the reaction byproducts by in situ NMR spectroscopy and GC-MS provided unprecedented insight into NC surface composition and reactivity as well as the complex reaction mechanism of XeF2 activated hydrosilylation.

Alkoxy-Terminated Si Surfaces: A New Reactive Platform for the Functionalization and Derivatization of Silicon Quantum Dots.

Alkoxy-terminated silicon quantum dots (SiQDs) were synthesized via hydrosilylation of aliphatic ketones on hydride-terminated SiQD (H-SiQD) surfaces under microwave-irradiation. Aromatic ketones undergo hydrosilylation on H-SiQD surfaces at room temperature without requiring any catalyst. The alkoxy-terminated SiQDs are soluble in organic solvents, colloidally stable, and show bright and size dependent photoluminescence (PL). The alkoxy-functionalized silicon surfaces were used as reactive platform for further functionalization via unprecedented ligand exchange of the alkoxy-surface groups with alkyl or alkenyl-surface groups in the presence of BH3·THF. Proton nuclear magnetic resonance ((1)H NMR), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) spectroscopy confirmed alkoxy-terminated surfaces and their ligand exchange reactions in the presence of various alkenes and alkynes.

Detection of nitroaromatics in the solid, solution, and vapor phases using silicon quantum dot sensors.

Silicon quantum dots (Si-QDs) represent a well-known QD fluorophore that can emit throughout the visible spectrum depending on the interface structure and surface functional group. Detection of nitroaromatic compounds by monitoring the luminescence response of the sensor material (typically fluorescent polymers) currently forms the basis of new explosives sensing technologies. Freestanding silicon QDs may represent a benign alternative with a high degree of chemical and physical versatility. Here, we investigate dodecyl and amine-terminated Si-QD luminescence response to the presence of nitrobenzene and dinitrotoluene (DNT) in various solid, solution, and vapor forms. For dinitrotoluene vapor the 3σ detection limit was 6 ppb for monomer-terminated QDs. For nitroaromatics dissolved in toluene the detection limit was on the order of 400 nM, corresponding to ∼100 pg of material distributed over ∼1 cm(2) on the sensor surface. Solid traces of nitroaromatics were also easily detectable via a simple 'touch test'. The samples showed minimal interference effects from common contaminants such as water, ethanol, and acetonitrile. The sensor can be as simple and inexpensive as a small circle of filter paper dipped into a QD solution, with a single vial of QDs able to make hundreds of these sensors. Additionally, a trial fiber-optic sensor device was tested by applying the QDs to one end of a 2 × 2 fiber coupler and exposing them to controlled DNT vapor. Finally, the quenching mechanism was explored via luminescence dynamics measurements and is different for blue (amine) and red (dodecyl) fluorescent silicon QDs.

Nanosilver and Nano Zero-Valent Iron Exposure Affects Nutrient Exchange Across the Sediment-Water Interface.

To examine how nanoparticles influence biogeochemical cycles in streams, we studied the acute impact of nanosilver (nAg) and nanoparticulate zero-valent iron (nZVI) exposure on nutrient and oxygen exchange across the sediment-water interface of two streams (agricultural canal and wetland) that differed in their water quality and sediment characteristics. At the agricultural site, nAg increased oxygen consumption and decreased N2 flux rates from that observed in control incubations. nZVI caused sediment-water systems from both streams to go hypoxic within 1.5 h of exposure. N2 flux rates were at least an order of magnitude higher in nZVI treatments as compared to control. Water column nitrate and nitrite concentrations were not impacted by nZVI exposure but total dissolved phosphorus concentrations were higher in cores treated with nZVI. nAg and nZVI exposure to surface water ecosystems can disrupt ecological function across the sediment-water interface.

Silicon Nanocrystals and Silicon-Polymer Hybrids: Synthesis, Surface Engineering, and Applications.

Silicon nanocrystals (Si-NCs) are emerging as an attractive class of quantum dots owing to the natural abundance of silicon in the Earth's crust, their low toxicity compared to many Group II-VI and III-V based quantum dots, compatibility with the existing semiconductor industry infrastructure, and their unique optoelectronic properties. Despite these favorable qualities, Si-NCs have not received the same attention as Group II-VI and III-V quantum dots, because of their lower emission quantum yields, difficulties associated with synthesizing monodisperse particles, and oxidative instability. Recent advancements indicate the surface chemistry of Si-NCs plays a key role in determining many of their properties. This Review summarizes new reports related to engineering Si-NC surfaces, synthesis of Si-NC/polymer hybrids, and their applications in sensing, diodes, catalysis, and batteries.

Grafting Poly(3-hexylthiophene) from Silicon Nanocrystal Surfaces: Synthesis and Properties of a Functional Hybrid Material with Direct Interfacial Contact.

Hybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3-hexylthiophene) (SiNC@P3HT) by applying surface-initiated Kumada catalyst transfer polycondensation (SI-KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor-acceptor interfaces. Systematic characterization provides evidence of a core-shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type-II heterostructure.

Functionalization of hydride-terminated photoluminescent silicon nanocrystals with organolithium reagents.

Hydride-terminated photoluminescent silicon nanocrystals (SiNCs) were functionalized with organolithium compounds. The reaction is proposed to proceed through cleavage of Si - Si bonds and formation of a Si - Li surface species. The method yields colloidally stabilized SiNCs at room temperature with short reaction times. SiNCs with mixed surface functionalities can be prepared in an easy two-step reaction by this method by quenching of the Si - Li group with electrophiles or by addressing free Si - H groups on the surface with a hydrosilylation reaction.

Radical Initiated Hydrosilylation on Silicon Nanocrystal Surfaces: An Evaluation of Functional Group Tolerance and Mechanistic Study.

Hydrosilylation is among the most common methods used for modifying silicon surface chemistry. It provides a wide range of surface functionalities and effective passivation of surface sites. Herein, we report a systematic study of radical initiated hydrosilylation of silicon nanocrystal (SiNC) surfaces using two common radical initiators (i.e., 2,2'-azobis(2-methylpropionitrile) and benzoyl peroxide). Compared to other widely applied hydrosilylation methods (e.g., thermal, photochemical, and catalytic), the radical initiator based approach is particle size independent, requires comparatively low reaction temperatures, and yields monolayer surface passivation after short reaction times. The effects of differing functional groups (i.e., alkene, alkyne, carboxylic acid, and ester) on the radical initiated hydrosilylation are also explored. The results indicate functionalization occurs and results in the formation of monolayer passivated surfaces.

Charge transfer state emission dynamics in blue-emitting functionalized silicon nanocrystals.

We explore the dynamics of blue emission from dodecylamine and ammonia functionalized silicon nanocrystals (Si NCs) with average diameters of ∼3 and ∼6 nm using time-resolved photoluminescence (TRPL) spectroscopy. The Si NCs exhibit nanosecond PL decay dynamics that is independent of NC size and uniform across the emission spectrum. The TRPL measurements reveal complete quenching of core state emission by a charge transfer state that is responsible for the blue PL with a radiative recombination rate of ∼5 × 10(7) s(-1). A detailed picture of the charge transfer state emission dynamics in these functionalized Si NCs is proposed.