Room temperature hydrosilylation of silicon nanocrystals with bifunctional terminal alkenes

Luminescent Silicon Nanocrystals as Metal Ion Sensors

At relatively low concentrations in aqueous solution, Fe3+, Fe2+, Cu2+, and Ni2+ quench the photoluminescence (PL) of the undecenoic acid-capped silicon (Si) nanocrystals. The PL could be restored by adding a chelating agent, such as ethylenediaminetetraacetic acid (EDTA), to remove the ions. Fe3+ and Cu2+ also significantly increase the PL lifetime. Other metal ions, including Cd2+, Mn2+, Pb2+, Zn2+, In3+, K+, and Ca2+, had no effect on the Si nanocrystal PL. The limits of detection (LODs) for Fe3+ and Cu2+ of 370 and 150 nM, respectively, are low enough for metal ion sensing applications.

A Robust Triboelectric Impact Sensor with Carbon Dioxide Precursor-Based Calcium Carbonate Layer for Slap Match Application

As an urgent international challenge, the sudden change in climate due to global warming needs to be addressed in the near future. This can be achieved through a reduction in fossil fuel utilization and through carbon sequestration, which reduces the concentration of CO2 in the atmosphere. In this study, a self-sustainable impact sensor is proposed through implementing a triboelectric nanogenerator with a CaCO3 contact layer fabricated via a CO2 absorption method. The triboelectric polarity of CaCO3 with the location between the polyimide and the paper and the effects of varying the crystal structure are investigated first. The impact sensing characteristics are then confirmed at various input frequencies and under applied forces. Further, the high mechanical strength and strong adherence of CaCO3 on the surface of the device are demonstrated through enhanced durability compared to the unmodified device. For the intended application, the as-fabricated sensor is used to detect the turning state of the paper Ddakji in a slap match game using a supervised learning algorithm based on a support vector machine presenting a high classification accuracy of 95.8%. The robust CaCO3-based triboelectric device can provide an eco-friendly advantage due to its self-powered characteristics for impact sensing and carbon sequestration.

Pt-Coated Silicon Nanoparticles: An Investigation into the Hydrosilylation on Hydrogen-Terminated Silicon Surfaces Using Pt(dvs)

The interaction of hydrogen-terminated silicon nanoparticles (H-SiNPs) with Karstedt's catalyst at various temperatures was investigated. The results indicate that at room temperature, the oxidative addition of Pt(0) onto H-SiNPs is irreversible, and the catalyst is not eliminated from the surface of H-SiNPs, enabling a facile synthesis of Pt-loaded SiNPs that can undergo ligand exchange. The nature of the Pt-on-Si ensemble is characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Reaction conditions that enable effective hydrosilylation are discussed. It is found that higher temperatures favor reductive elimination of the catalyst and hydrosilylation of 1-octene onto the surface of the H-SiNPs.

Si-H Surface Groups Inhibit Methacrylic Polymerization: Thermal Hydrosilylation of Allyl Methacrylate with Silicon Nanoparticles

Hydrogen-terminated silicon nanoparticles (H-SiNPs) inhibit anerobic thermal autopolymerization of methacrylates. When heated to 100 °C under an inert atmosphere, allyl methacrylate (AMA) was stable for at least 95 h in the presence of 1.2 wt % H-SiNPs, exhibiting less than 0.15% conversion, whereas the neat monomer solidified within 24 h (over 10% conversion after 34 h). A mechanism is proposed that is based on H-transfer from SiNPs to the thermally activated methacrylic dimer biradical, quenching autopolymerization. An analysis of SiNPs isolated after heating in AMA reveals the grafting of ester groups. Thermal hydrosilylation offers a facile way to attach an allyl group to the surface of SiNPs.

Water-Soluble Silicon Quantum Dots toward Fluorescence-Guided Photothermal Nanotherapy

We report carboxy-terminated silicon quantum dots (SiQDs) that exhibit high solubility in water due to the high molecular coverage of surface monolayers, bright light emission with high photoluminescence quantum yields (PLQYs), long-term stability in the PL property for monitoring cells, less toxicity to the cells, and a high photothermal response. We prepared water-soluble SiQDs by the thermal hydrosilylation of 10-undecenoic acid on their hydrogen-terminated surfaces, provided by the thermal disproportionation of triethoxysilane hydrolyzed at pH 3 and subsequent hydrofluoric etching. The 10-undecanoic acid-functionalized SiQDs (UA:SiQDs) showed long-term stability in hydrophilic solvents including ethanol and water (pH 7). We assess their interaction with live cells by means of cellular uptake, short-term toxicity, and, for the first time, long-term cytotoxicity. Results show that UA:SiQDs are potential candidates for theranostics, with their good optical properties enabling imaging for more than 18 days and a photothermal response having a 25.1% photothermal conversion efficiency together with the direct evidence of cell death by laser irradiation. UA:SiQDs have low cytotoxicity with full viability of up to 400 μg/mL for the short term and a 50% cell viability value after 14 days of incubation at a 50 μg/mL concentration.

Two-Photon Excitation Spectroscopy of Silicon Quantum Dots and Ramifications for Bio-Imaging

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has potential opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range 1.46 < ℏω < 1.91 eV (wavelength 850 > λ > 650 nm) above the two-photon band gap E_g^(QD)/2, and at a representative photon energy ℏω = 0.99 eV (λ = 1250 nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters d = 1.8 ± 0.2 nm and d = 2.3 ± 0.3 nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals, and the onset of 2PA is observed to be blue shifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bioimaging using 2PE-PL are simulated in various biological tissues and compared to efficiencies of other quantum dots and molecular fluorophores and found to be comparable or superior at greater depths.

Preparation of polymer-based foam for efficient oil-water separation based on surface engineering

The leakages of a large number of organic solvents and oil spills not only cause extensive economic losses, but also destroy the ecological environment. However, there were few studies on the surface engineering of adsorption materials for efficient oil-water separation in complex environments. In this research, through surface engineering, the polymer-based foam exhibited high efficiency oil-water separation performance in different pH environments. Hydrophobic groups were introduced onto the surface of nano-sized SiO₂ particles via hydrolysis and polycondensation reactions, and then the modified SiO₂ was loaded on the foam. After modification, the water contact angle of the modified foam increased from 116.4° to 152.5°, and the oil-water separation performance was obviously enhanced. The oil removal rate of the modified foam remained above 96%. The highest capacity of petroleum diesel was 33.4 g^-1, which was much higher than other similar adsorption materials. In addition, the modified foam maintained good hydrophobicity and oil removal rate in a wide pH range. The efficient oleophilic and hydrophobic foam prepared by combining green physical foaming with surface engineering was expected to be widely used in large-scale organic solvent recovery and oil leakage emergency treatment.

"Turning the dials": controlling synthesis, structure, composition, and surface chemistry to tailor silicon nanoparticle properties

Silicon nanoparticles (SiNPs) can be challenging to prepare with defined size, crystallinity, composition, and surface chemistry. As is the case for any nanomaterial, controlling these parameters is essential if SiNPs are to realize their full potential in areas such as alternative energy generation and storage, sensors, and medical imaging. Numerous teams have explored and established innovative synthesis methods, as well as surface functionalization protocols to control these factors. Furthermore, substantial effort has been expended to understand how the abovementioned parameters influence material properties. In the present review we provide a commentary highlighting the benefits and limitations of available methods for preparing silicon nanoparticles as well as demonstrations of tailoring optical and electronic properties through definition of structure (i.e., crystalline vs. amorphous), composition and surface chemistry. Finally, we highlight potential opportunities for future SiNP studies.

Sonochemical Reaction of Bifunctional Molecules on Silicon (111) Hydride Surface

While the sonochemical grafting of molecules on silicon hydride surface to form stable Si-C bond via hydrosilylation has been previously described, the susceptibility towards nucleophilic functional groups during the sonochemical reaction process remains unclear. In this work, a competitive study between a well-established thermal reaction and sonochemical reaction of nucleophilic molecules (cyclopropylamine and 3-Butyn-1-ol) was performed on p-type silicon hydride (111) surfaces. The nature of surface grafting from these reactions was examined through contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Cyclopropylamine, being a sensitive radical clock, did not experience any ring-opening events. This suggested that either the Si-H may not have undergone homolysis as reported previously under sonochemical reaction or that the interaction to the surface hydride via a lone-pair electron coordination bond was reversible during the process. On the other hand, silicon back-bond breakage and subsequent surface roughening were observed for 3-Butyn-1-ol at high-temperature grafting (≈150 °C). Interestingly, the sonochemical reaction did not produce appreciable topographical changes to surfaces at the nano scale and the further XPS analysis may suggest Si-C formation. This indicated that while a sonochemical reaction may be indifferent towards nucleophilic groups, the surface was more reactive towards unsaturated carbons. To the best of the author's knowledge, this is the first attempt at elucidating the underlying reactivity mechanisms of nucleophilic groups and unsaturated carbon bonds during sonochemical reaction of silicon hydride surfaces.

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Design guidelines for transition metals as interstitial emitters in silicon nanocrystals to tune photoluminescence properties: zinc as biocompatible example

Silicon nanocrystals (Si NCs) are attractive candidates for biomarkers in medical imaging. Building on recent work [McVey et al., J. Chem. Phys. Lett., 2015, 6/9, 1573; McVey et al., Nanoscale, 2018, 15600], we focus on interstitial (i-) doping of Si NCs by transition metals (TMs), and investigate the optoelectronic structure with Zn as example. Carrying out extensive ground and excited state calculations using density functional theory (DFT), we provide insight into the interdependencies of parameters which define photoluminescence (PL) properties as per TM element, their position, and their density within Si NCs of realistic size. For i-Zn in Si NCs, we predict a very high radiation efficiency with a wavelength located well above the range of auto-luminescence originating from human tissue and blood. We derive general guidelines for i-TM doping of Si NCs to arrive at a desired emission wavelength with maximum radiation efficiency. Moving on from this general description, we reveal the concept of using the plasmonic resonance of i-TM dopants in the microwave (μW) spectrum to trigger selective thermal apoptosis of tagged cells in vivo after cell marking, paving the way towards a theragnostics tool with minimum side effects.

Water-soluble silicon nanocrystals as NIR luminescent probes for time-gated biomedical imaging

Luminescent probes based on silicon nanocrystals (SiNCs) have many advantages for bioimaging compared to more conventional quantum dots: abundancy of silicon combined with its biocompatibility; tunability of the emission color of SiNCs in the red and NIR spectral region to gain deeper tissue penetration; long emission lifetimes of SiNCs (hundreds of μs) enabling time-gated acquisitions to avoid background noise caused by tissue autofluorescence and scattered excitation light. Here we report a new three-step synthesis, based on a low temperature thiol-ene click reaction that can afford SiNCs, colloidally stable in water, with preserved bright red and NIR photoluminescence (band maxima at 735 and 945 nm for nanocrystals with diameters of 4 and 5 nm, respectively) and long emission lifetimes. Their luminescence is insensitive to dioxygen and sensitive to pH changes in the physiological range, enabling pH sensing. In vivo studies demonstrated tumor accumulation, 48 hours clearance and a 3-fold improvement of the signal-to-noise ratio compared to steady-state imaging.

Spectral tuning of colloidal Si nanocrystal luminescence by post-laser irradiation in liquid

We report a simple technique to tune the luminescence spectra of blue-emitting colloidal silicon nanocrystals (Si-ncs) to the ultraviolet region via post-laser irradiation.

Fast room-temperature functionalization of silicon nanoparticles using alkyl silanols

We report a fast, easy and efficient method for the functionalization of hydrogen-terminated silicon nanoparticles (H-Si NPs). Using silanol compounds, a range of functionalized Si NPs could be produced in only 1 h reaction time at room temperature.

Critical assessment of wet-chemical oxidation synthesis of silicon quantum dots

The wet-chemical Si QD synthesis by oxidation of magnesium silicide (Mg₂Si) with bromine (Br₂) was revisited.

One-Step Direct Fixation of Atmospheric CO2 by Si-H Surface in Solution

The efficient conversion of carbon dioxide (CO₂) into useful chemicals has important practical significance for environmental protection. Until now, direct fixation of atmospheric CO₂ needs first extraction from the atmosphere, an energy-intensive process. Silicon (or Si-H surface), Earth-abundant, low-cost and non-toxic, is a promising material for heterogeneous CO₂ chemical fixation. Here we report one-step fixing of CO₂ directly from the atmosphere to a paraformaldehyde-like polymer by Si-H surface at room temperature. With the assistance of HF, commercial silicon powder was used as a heterogeneous reducing agent, for converting gaseous CO₂ to a polymer of fluorine substituted polyoxymethylene and hydroxyl substituted polyoxymethylene alternating copolymer (F-POM). Making use of the Si-H surface toward the fixation of atmospheric gaseous CO₂ is a conceptually distinct and commercially interesting strategy for making useful chemicals and environmental protection.

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Atmospheric CO₂ is fixed with HF-treated silicon powders via one-step method

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The product is fluorine substituted polymer (F-POM)

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The fixation process is monitored by in situ infrared studies and mass spectra

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The mechanism on the direct CO₂ fixation by Si-H surface is proposed

Synthesis of PEG-grafted boron doped Si nanocrystals

Silicon nanocrystals are intriguing materials for biomedical imaging applications because of their unique optical properties and biological compatibility. We report a new surface functionalization route to synthesize biological buffer soluble and colloidally stable silicon nanocrystals, which is enabled by surface boron doping. Harnessing the distinctive Lewis acidic boron surface sites, postsynthetic modifications of plasma synthesized boron doped nanocrystals were carried out with polyethylene glycol (PEG-OH) ligands in dimethyl sulfoxide under photochemical conditions. The influence of PEG concentration, PEG molecular weight, and boron doping percentage on the nanocrystal solubility in a biological buffer has been investigated. The boron doping facilitates the surface functionalization via two probable pathways, by providing excellent initial dispersiblity in polar solvents and providing available acidic boron surface sites for bonding. These boron doped silicon nanocrystals have nearly identical absorption features as intrinsic silicon nanocrystals, indicating that they are promising candidates for biological imaging applications.

Aerosol-Phase Synthesis and Processing of Luminescent Silicon Nanocrystals

Silicon quantum dots are attractive materials for luminescent devices and bioimaging applications. For these light-emitting applications, higher photoluminescence efficiency is desired in order to achieve better device performance. Nonthermal plasma synthesis successfully allows for the continuous production of silicon nanocrystals, but postprocessing is necessary to improve photoluminescence quantum yields so that nanocrystals can be used for luminescence applications. In this work, we demonstrate an all-aerosol-phase synthesis and processing route that integrates nonthermal plasma synthesis, plasma-assisted surface functionalization with alkene ligands, and in-flight annealing within one flow stream. Here, luminescent silicon nanocrystals are synthesized and postprocessed on a time scale of only 100 ms, which is orders of magnitude faster than previous synthesis and functionalization schemes. The as-produced silicon nanocrystals have photoluminescence quantum yields exceeding 20%, which is a 5-fold increase compared to previous silicon nanocrystals synthesized with all-aerosol-phase approaches. We attribute the enhanced photoluminescence to the reduced "dark" nanocrystal fraction due to reduction of dangling bond density and desorption of surface silyl species induced by the in-flight annealing. We also demonstrate that the ligand coverage plays a minor role for the photoluminescence properties, but that the nature of the silicon hydride surface groups is a major factor.

Emerging Functions of Nanostructured Porous Silicon-With a Focus on the Emissive Properties of Photons, Electrons, and Ultrasound

Recent topics of application studies on porous silicon (PS) are reviewed here with a focus on the emissive properties of visible light, quasiballistic hot electrons, and acoustic wave. By exposing PS in solvents to pulse laser, size-controlled nc-Si dot colloids can be formed through fragmentation of the PS layer with a considerably higher yield than the conventional techniques such as laser ablation of bulk silicon and sol-gel precursor process. Fabricated colloidal samples show strong visible photoluminescence (~40% in quantum efficiency in the red band). This provides an energy- and cost-effective route for production of nc-Si quantum dots. A multiple-tunneling transport mode through nc-Si dot chain induces efficient quasiballistic hot electron emission from an nc-Si diode. Both the efficiency and the output electron energy dispersion are remarkably improved by using monolayer graphene as a surface electrode. Being a relatively low operating voltage device compatible with silicon planar fabrication process, the emitter is applicable to mask-less parallel lithography under an active matrix drive. It has been demonstrated that the integrated 100 × 100 emitter array is useful for multibeam lithography and that the selected emission pattern is delineated with little distortion. Highly reducing activity of emitted electrons is applicable to liquid-phase thin film deposition of metals (Cu) and semiconductors (Si, Ge, and SiGe). Due to an extremely low thermal conductivity and volumetric heat capacity of nc-Si layer, on the other hand, thermo-acoustic conversion is enhanced to a practical level. A temperature fluctuation produced at the surface of nc-Si layer is quickly transferred into air, and then an acoustic wave is emitted without any mechanical vibrations. The non-resonant and broad-band emissivity with low harmonic distortions makes it possible to use the emitter for generating audible sound under a full digital drive and reproducing complicated ultrasonic communication calls between mice.

XPS Analysis of 2- and 3-Aminothiophenol Grafted on Silicon (111) Hydride Surfaces

Following on from our previous study on the resonance/inductive structures of ethynylaniline, this report examines similar effects arising from resonance structures with aromatic aminothiophenol with dual electron-donating substituents. In brief, 2- and 3-aminothiophenol were thermally grafted on silicon (111) hydride substrate at 130 °C under nonpolar aprotic mesitylene. From the examination of high resolution XPS Si2p, N1s, and S2p spectrum, it was noticed that there was a strong preference of NH₂ over SH to form Si⁻N linkage on the silicon hydride surface for 2-aminothiophenol. However, for 3-aminothiophenol, there was a switch in reactivity of the silicon hydride toward SH group. This was attributed to the antagonistic and cooperative resonance effects for 2- and 3-aminothiophenol, respectively. The data strongly suggested that the net resonance of the benzylic-based compound could have played an important role in the net distribution of negative charge along the benzylic framework and subsequently influenced the outcome of the surface reaction. To the best of the authors' knowledge, this correlation between dual electron-donating substituents and the outcome of the nucleophilic addition toward silicon hydride surfaces has not been described before in literature.

One-Pot Gram-Scale Synthesis of Hydrogen-Terminated Silicon Nanoparticles

Silicon nanoparticles (Si NPs) are highly attractive materials for typical quantum dots functions, such as in light-emitting and bioimaging applications, owing to silicon's intrinsic merits of minimal toxicity, low cost, high abundance, and easy and highly stable functionalization. Especially nonoxidized Si NPs with a covalently bound coating serve well in these respects, given the minimization of surface defects upon hydrosilylation of H-terminated Si NPs. However, to date, methods to obtain such H-terminated Si NPs are still not easy. Herein, we report a new synthetic method to produce size-tunable robust, highly crystalline H-terminated Si NPs (4-9 nm) using microwave irradiation within 5 min at temperatures between 25 and 200 °C and their further covalent functionalization. The key step to obtain highly fluorescent (quantum yield of 7-16%) green-red Si NPs in one simple step is the reduction of triethoxysilane and (+)-sodium l-ascorbate, yielding routinely ∼1 g of H-Si NPs via a highly scalable route in 5-15 min. Subsequent functionalization via hydrosilylation yielded Si NPs with an emission quantum yield of 12-14%. This approach can be used to easily produce high-quality H-Si NPs in gram-scale quantities, which brings the application of functionalized Si NPs significantly closer.

Si nanocrystal solution with stability for one year

Colloidal silicon nanocrystals (SiNCs) are a promising material for next-generation nanostructured devices. High-stability SiNC solutions are required for practical use as well as studies on the properties of SiNC. Here, we show a solution of SiNCs that was stable for one year without aggregation. The stable solution was synthesized by a facile process, i.e., pulsed laser ablation of a Si wafer in isopropyl alcohol (IPA). The long-term stability was due to a large ζ-potential of −50 mV from a SiNC passivation layer composed of oxygen, hydrogen, and alkane groups, according to the results of eight experiments and theoretical calculations. This passivation layer also resulted in good performance as an additive for a conductive polymer film. Namely, a 5-fold enhancement in carrier density was established by the addition of SiNCs into an organic conductive polymer, poly(3-dodecylthiophene), which is useful for solar cells. Furthermore, it was found that fresh (<1 day) and aged (4 months) SiNCs give the same enhancement. The long-term stability was attributed to a great repulsive energy in IPA, whose value was quantified as a function the distance between SiNCs.

A stable nanocrystal for one year without aggregation in a liquid is synthesized by one-step, one-pot, and one-hour process.

Highly Fluorescent Silicon Nanocrystals Stabilized in Water Using Quatsomes

Fluorescent silicon (Si) nanocrystals (2.8 nm diameter) were incorporated into surfactant assemblies of cetyltrimethylammonium bromide (CTAB) and cholesterol, called quatsomes. In water, the quatsome-Si nanocrystal assemblies remain fluorescent and well-dispersed for weeks. In contrast to Si nanocrystals, alkanethiol-capped gold (Au) nanocrystals do not form stable dispersions in water with quatsomes. Cryogenic transmission electron microscopy (cryo-TEM) confirmed that the Si nanocrystal-quatsome structures do not change over the course of several weeks. The long-term stability of the Si nanocrystal-quatsome assemblies, their fluorescence, and biocompatibility makes them attractive candidates for medical applications.

Silicon Nanocrystals with pH-Sensitive Tunable Light Emission from Violet to Blue-Green

We fabricated a silicon nanocrystal (NC) suspension with visible, continuous, tunable light emission with pH sensitivity from violet to blue-green. Transmission electron microscopy (TEM) images and X-ray diffraction (XRD) pattern analysis exhibit the highly crystalline nanoparticles of silicon. Photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra at different pH values, such as 1, 3, 5, 7, 9, and 11, reveal the origins of light emission from the silicon NC suspension, which includes both the quantum confinement effect and surface bonding. The quantum confinement effect dominates the PL origins of silicon NCs, especially determining the tunability and the emission range of PL, while the surface bonding regulates the maximum peak center, full width at half maximum (FWHM), and offsets of PL peaks in response to the changing pH value. The peak fitting of PLE curves reveals one of the divided PLE peaks shifts towards a shorter wavelength when the pH value increases, which implies correspondence with the surface bonding between silicon NCs and hydrogen atoms or hydroxyl groups. The consequent detailed analysis of the PL spectra indicates that the surface bonding results in the transforming of the PL curves towards longer wavelengths with the increasing pH values, which is defined as the pH sensitivity of PL. These results suggest that the present silicon NCs with pH-sensitive tunable light emission could find promising potential applications as optical sources, bio-sensors, etc.

Study of Colloidal Suspensions of Silicon Nanoparticles: Effect of Surface Oxidation on the Photoluminescence Property

Nanosilicon is currently under intensive research owing to its extremely promising properties, particularly in photonics domain. However, the photoluminescence (PL) mechanism of silicon nanocrystal remains unclear. We propose, in this paper, a simple method to investigate the PL properties of silicon nanoparticles by exposing the nanoparticles directly to the solvents. The interaction between the nanoparticles and different solvents allows us to assume that not only the quantum size effect but also the surface defect states play a critical role in the PL mechanism, especially at high energy region. This can be confirmed by the results of Transmission Electron Microscopy, FTIR and Raman Spectroscopy.

Grafting of Ring-Opened Cyclopropylamine Thin Films on Silicon (100) Hydride via UV Photoionization

The grafting of cyclopropylamine onto a silicon (100) hydride (Si-H) surface via a ring-opening mechanism using UV photoionization is described here. In brief, radicals generated from the Si-H surface upon UV irradiation were found to behave in classical hydrogen abstraction theory manner by which the distal amine group was first hydrogen abstracted and the radical propagated down to the cyclopropane moiety. This subsequently liberated the strained bonds of the cyclopropane group and initiated the surface grafting process, producing a thin film approximately 10-15 nm in height. Contact angle measurements also showed that such photoionization irradiation had yielded an extremely hydrophilic surface (∼21.3°) and X-ray photoelectron spectroscopy also confirmed the coupling was through the Si-C linkage. However, when the surface underwent high-temperature hydrosilylation (>160 °C), the reaction proceeded predominantly through the nucleophilic NH₂ group to form a Si-N linkage to the surface. This rendered the surface hydrophobic and hence suggested that the Si-H homolysis model may not be the main process. To the best of our knowledge, this was the first attempt reported in the literature to use photoionization to directly graft cyclopropylamine onto a silicon surface and in due course generate a highly rich NH-terminated surface that was found to be highly bioactive in promoting cell viability on the basis of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide studies.

Heterogeneous reduction of carbon dioxide by hydride-terminated silicon nanocrystals

Silicon constitutes 28% of the earth's mass. Its high abundance, lack of toxicity and low cost coupled with its electrical and optical properties, make silicon unique among the semiconductors for converting sunlight into electricity. In the quest for semiconductors that can make chemicals and fuels from sunlight and carbon dioxide, unfortunately the best performers are invariably made from rare and expensive elements. Here we report the observation that hydride-terminated silicon nanocrystals with average diameter 3.5 nm, denoted ncSi:H, can function as a single component heterogeneous reducing agent for converting gaseous carbon dioxide selectively to carbon monoxide, at a rate of hundreds of μmol h⁻¹ g⁻¹. The large surface area, broadband visible to near infrared light harvesting and reducing power of SiH surface sites of ncSi:H, together play key roles in this conversion. Making use of the reducing power of nanostructured hydrides towards gaseous carbon dioxide is a conceptually distinct and commercially interesting strategy for making fuels directly from sunlight.

Elemental silicon is widely studied for photovoltaic applications. Here, the authors report that hydride-terminated silicon nanocrystals can also function as single component heterogeneous reducing agent for converting gaseous carbon dioxide selectively to carbon monoxide.

Expanding the Repertoire of Molecular Linkages to Silicon: Si-S, Si-Se, and Si-Te Bonds

Silicon is the foundation of the electronics industry and is now the basis for a myriad of new hybrid electronics applications, including sensing, silicon nanoparticle-based imaging and light emission, photonics, and applications in solar fuels, among others. From interfacing of biological materials to molecular electronics, the nature of the chemical bond plays important roles in electrical transport and can have profound effects on the electronics of the underlying silicon itself, affecting its work function, among other things. This work describes the chemistry to produce ≡Si-E bonds (E = S, Se, and Te) through very fast microwave heating (10-15 s) and direct thermal heating (hot plate, 2 min) through the reaction of hydrogen-terminated silicon surfaces with dialkyl or diaryl dichalcogenides. The chemistry produces surface-bound ≡Si-SR, ≡Si-SeR, and ≡Si-TeR groups. Although the interfacing of molecules through ≡Si-SR and ≡Si-SeR bonds is known, to the best of our knowledge, the heavier chalcogenide variant, ≡Si-TeR, has not been described previously. The identity of the surface groups was determined by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and depth profiling with time-of-flight-secondary ionization mass spectrometry (ToF-SIMS). Possible mechanisms are outlined, and the most likely, based upon parallels with well-established molecular literature, involve surface silyl radicals or dangling bonds that react with either the alkyl or aryl dichalcogenide directly, REER, or its homolysis product, the alkyl or aryl chalcogenyl radical, RE· (where E = S, Se, and Te).

Stöber strategy for synthesizing multifluorescent organosilica nanocrystals

We first report the Stöber synthesis of diamond-structured fluorescent organosilica nanocrystals (OSNCs) with finely tunable fluorescence (460–625 nm).

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.

Synthesis and Ligand Exchange of Thiol-Capped Silicon Nanocrystals

Hydride-terminated silicon (Si) nanocrystals were capped with dodecanethiol by a thermally promoted thiolation reaction. Under an inert atmosphere, the thiol-capped nanocrystals exhibit photoluminescence (PL) properties similar to those of alkene-capped Si nanocrystals, including size-tunable emission wavelength, relatively high quantum yields (>10%), and long radiative lifetimes (26-280 μs). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy confirmed that the ligands attach to the nanocrystal surface via covalent Si-S bonds. The thiol-capping layer, however, readily undergoes hydrolysis and severe degradation in the presence of moisture. Dodecanethiol could be exchanged with dodecene by hydrosilylation for enhanced stability.

Controlled Styrene Monolayer Capping of Silicon Nanocrystals by Room Temperature Hydrosilylation

Undecanoic acid facilitates attachment of styrene to hydride-terminated Si nanocrystals at room temperature, avoiding polymerization of styrene, yielding free-standing styrene-terminated Si nanocrystals. The nanocrystals have diamond cubic crystal structure, with photophysical properties similar to typical alkene-capped Si nanocrystals, such as bright photoluminescence with relatively long radiative lifetimes. We propose a reaction mechanism for room temperature styrene addition in which the resonance form of undecanoic acid coordinates to surface Si-H and facilitates H(-) attack at terminal C═C of styrene.

Bandgap Tuning of Silicon Quantum Dots by Surface Functionalization with Conjugated Organic Groups

The quantum confinement and enhanced optical properties of silicon quantum dots (SiQDs) make them attractive as an inexpensive and nontoxic material for a variety of applications such as light emitting technologies (lighting, displays, sensors) and photovoltaics. However, experimental demonstration of these properties and practical application into optoelectronic devices have been limited as SiQDs are generally passivated with covalently bound insulating alkyl chains that limit charge transport. In this work, we show that strategically designed triphenylamine-based surface ligands covalently bonded to the SiQD surface using conjugated vinyl connectivity results in a 70 nm red-shifted photoluminescence relative to their decyl-capped control counterparts. This suggests that electron density from the SiQD is delocalized into the surface ligands to effectively create a larger hybrid QD with possible macroscopic charge transport properties.

Facile production of stable silicon nanoparticles: laser chemistry coupled to in situ stabilization via room temperature hydrosilylation

Stable, alkyl-terminated, light-emitting silicon nanoparticles have been synthesized in a continuous process by laser pyrolysis of a liquid trialkyl-silane precursor selected as a safer alternative to gas silane (SiH4). Stabilization was achieved by in situ reaction using a liquid collection system instead of the usual solid state filtration. The alkene contained in the collection liquid (1-dodecene) reacted with the newly formed silicon nanoparticles in an unusual room-temperature hydrosilylation process. It was achieved by the presence of fluoride species, also produced during laser pyrolysis from the decomposition of sulfur hexafluoride (SF6) selected as a laser sensitizer. This process directly rendered alkyl-passivated silicon nanoparticles with consistent morphology and size (<3 nm), avoiding the use of costly post-synthetic treatments.

Prospects of nanoscience with nanocrystals

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today's strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

SEI-component formation on sub 5 nm sized silicon nanoparticles in Li-ion batteries: the role of electrode preparation, FEC addition and binders

Nanocrystalline silicon electrodes were exposed to long-term cycling in Li-ion batteries under different conditions and their SEI formation was investigated by diverse techniques.

Effective increase in the refractive index of novel transparent silicone hybrid films by introduction of functionalized silicon nanoparticles

The refractive index of novel transparent silicone hybrid films can be effectively enhanced by introduction of functionalized silicon nanoparticles via a facile strategy.

Pseudo-direct bandgap transitions in silicon nanocrystals: effects on optoelectronics and thermoelectrics

While silicon nanostructures are extensively used in electronics, the indirect bandgap of silicon poses challenges for optoelectronic applications like photovoltaics and light emitting diodes (LEDs). Here, we show that size-dependent pseudo-direct bandgap transitions in silicon nanocrystals dominate the interactions between (photoexcited) charge carriers and phonons, and hence the optoelectronic properties of silicon nanocrystals. Direct measurements of the electronic density of states (DOS) for different sized silicon nanocrystals reveal that these pseudo-direct transitions, likely arising from the nanocrystal surface, can couple with the quantum-confined silicon states. Moreover, we demonstrate that since these transitions determine the interactions of charge carriers with phonons, they change the light emission, absorption, charge carrier diffusion and phonon drag (Seebeck coefficient) in nanoscaled silicon semiconductors. Therefore, these results can have important implications for the design of optoelectronics and thermoelectric devices based on nanostructured silicon.

Colloidal silicon quantum dots: from preparation to the modification of self-assembled monolayers (SAMs) for bio-applications

Summarizes recent advances in the preparation, surface modification and bio-applications of silicon quantum dots.

Colloidal luminescent silicon nanorods

Silicon nanorods are grown by trisilane decomposition in hot squalane in the presence of tin (Sn) nanocrystals and dodecylamine. Sn induces solution-liquid-solid nanorod growth with dodecylamine serving as a stabilizing ligand. As-prepared nanorods do not luminesce, but etching with hydrofluoric acid to remove residual surface oxide followed by thermal hydrosilylation with 1-octadecene induces bright photoluminescence with quantum yields of 4-5%. X-ray photoelectron spectroscopy shows that the ligands prevent surface oxidation for months when stored in air.